diff --git a/api/basic_json/to_bjdata.md b/api/basic_json/to_bjdata.md index f84b41350..1a00910b0 100644 --- a/api/basic_json/to_bjdata.md +++ b/api/basic_json/to_bjdata.md @@ -212,4 +212,6 @@ Linear in the size of the JSON value `j`. that is not valid UTF-8 unchanged, as before; `strict` (the default if [`JSON_STRICT_BINARY_UTF8`](../macros/json_strict_binary_utf8.md) is enabled) throws `type_error.316`. - Throws `type_error.321` for a discarded value since version 3.13.0; previously, a discarded value nested in an - array or object was silently skipped, producing invalid BJData. \ No newline at end of file + array or object was silently skipped, producing invalid BJData. +- Writes unsigned integers wider than 64 bits as high-precision numbers since version 3.13.0; previously, they were + silently truncated to 64 bits. \ No newline at end of file diff --git a/api/basic_json/to_bjdata/index.html b/api/basic_json/to_bjdata/index.html index eaf1bb483..38069fe55 100644 --- a/api/basic_json/to_bjdata/index.html +++ b/api/basic_json/to_bjdata/index.html @@ -103,4 +103,4 @@ } }

Output:

[json.exception.other_error.502] use_type requires use_size = true
-

See also

Version history

\ No newline at end of file +

See also

Version history

\ No newline at end of file diff --git a/api/basic_json/to_bjdata/index.md b/api/basic_json/to_bjdata/index.md index ed4254153..c9733c2be 100644 --- a/api/basic_json/to_bjdata/index.md +++ b/api/basic_json/to_bjdata/index.md @@ -189,3 +189,4 @@ Output: - BJData version parameter (for draft3 binary encoding) added in version 3.12.0. - Added `error_handler` parameter in version 3.13.0 unreleased. Its default, `keep`, writes the bytes of a string or object key that is not valid UTF-8 unchanged, as before; `strict` (the default if [`JSON_STRICT_BINARY_UTF8`](https://json.nlohmann.me/api/macros/json_strict_binary_utf8/index.md) is enabled) throws `type_error.316`. - Throws `type_error.321` for a discarded value since version 3.13.0 unreleased; previously, a discarded value nested in an array or object was silently skipped, producing invalid BJData. +- Writes unsigned integers wider than 64 bits as high-precision numbers since version 3.13.0 unreleased; previously, they were silently truncated to 64 bits. diff --git a/api/basic_json/to_bon8.md b/api/basic_json/to_bon8.md index 086955c50..8e8049db2 100644 --- a/api/basic_json/to_bon8.md +++ b/api/basic_json/to_bon8.md @@ -37,8 +37,9 @@ With (2), the bytes written before the exception remain in the output adapter. ## Exceptions -- Throws [out_of_range.407](../../home/exceptions.md#jsonexceptionout_of_range407) if `j` contains an unsigned integer - above 9223372036854775807, which BON8 cannot represent +- Throws [out_of_range.407](../../home/exceptions.md#jsonexceptionout_of_range407) if `j` contains an integer outside + the range of int64 (an unsigned integer above 9223372036854775807, or, with a number type wider than 64 bits, any + integer beyond int64), which BON8 cannot represent - Throws [type_error.316](../../home/exceptions.md#jsonexceptiontype_error316) if `j` contains a string that is not valid UTF-8 - Throws [type_error.321](../../home/exceptions.md#jsonexceptiontype_error321) if `j` or a value nested in it is diff --git a/api/basic_json/to_bon8/index.html b/api/basic_json/to_bon8/index.html index ef2d477ea..52cbb30a6 100644 --- a/api/basic_json/to_bon8/index.html +++ b/api/basic_json/to_bon8/index.html @@ -4,7 +4,7 @@ // (2) static void to_bon8(const basic_json& j, detail::output_adapter<std::uint8_t> o); static void to_bon8(const basic_json& j, detail::output_adapter<char> o); -

Serializes a given JSON value j to a byte vector using the BON8 (Binary Object Notation 8) serialization format. BON8 is a compact binary serialization format that stores strings as UTF-8 without a length prefix.

  1. Returns a byte vector containing the BON8 serialization.
  2. Writes the BON8 serialization to an output adapter.

The exact mapping and its limitations are described on a dedicated page.

Parameters

j (in)
JSON value to serialize
o (in)
output adapter to write serialization to

Return value

  1. BON8 serialization as a byte vector
  2. (none)

Exception safety

Strong guarantee: if an exception is thrown, there are no changes in the JSON value j, which is never modified. With (2), the bytes written before the exception remain in the output adapter.

Exceptions

Complexity

Linear in the size of the JSON value j.

Examples

Example: serialize a JSON value to BON8

The example shows the serialization of a JSON value to a byte vector in BON8 format.

#include <iostream>
+

Serializes a given JSON value j to a byte vector using the BON8 (Binary Object Notation 8) serialization format. BON8 is a compact binary serialization format that stores strings as UTF-8 without a length prefix.

  1. Returns a byte vector containing the BON8 serialization.
  2. Writes the BON8 serialization to an output adapter.

The exact mapping and its limitations are described on a dedicated page.

Parameters

j (in)
JSON value to serialize
o (in)
output adapter to write serialization to

Return value

  1. BON8 serialization as a byte vector
  2. (none)

Exception safety

Strong guarantee: if an exception is thrown, there are no changes in the JSON value j, which is never modified. With (2), the bytes written before the exception remain in the output adapter.

Exceptions

Complexity

Linear in the size of the JSON value j.

Examples

Example: serialize a JSON value to BON8

The example shows the serialization of a JSON value to a byte vector in BON8 format.

#include <iostream>
 #include <iomanip>
 #include <nlohmann/json.hpp>
 
@@ -50,4 +50,4 @@
     }
 }

Output:

[json.exception.type_error.316] invalid UTF-8 byte at index 0: 0xFF
-

See also

Version history

\ No newline at end of file +

See also

Version history

\ No newline at end of file diff --git a/api/basic_json/to_bon8/index.md b/api/basic_json/to_bon8/index.md index b229a67b3..b45944ccb 100644 --- a/api/basic_json/to_bon8/index.md +++ b/api/basic_json/to_bon8/index.md @@ -33,7 +33,7 @@ Strong guarantee: if an exception is thrown, there are no changes in the JSON va ## Exceptions -- Throws [out_of_range.407](https://json.nlohmann.me/home/exceptions/#jsonexceptionout_of_range407) if `j` contains an unsigned integer above 9223372036854775807, which BON8 cannot represent +- Throws [out_of_range.407](https://json.nlohmann.me/home/exceptions/#jsonexceptionout_of_range407) if `j` contains an integer outside the range of int64 (an unsigned integer above 9223372036854775807, or, with a number type wider than 64 bits, any integer beyond int64), which BON8 cannot represent - Throws [type_error.316](https://json.nlohmann.me/home/exceptions/#jsonexceptiontype_error316) if `j` contains a string that is not valid UTF-8 - Throws [type_error.321](https://json.nlohmann.me/home/exceptions/#jsonexceptiontype_error321) if `j` or a value nested in it is discarded; example: `"cannot serialize discarded value to BON8"` diff --git a/api/basic_json/to_bson.md b/api/basic_json/to_bson.md index 08879e15f..d7b014bd4 100644 --- a/api/basic_json/to_bson.md +++ b/api/basic_json/to_bson.md @@ -47,6 +47,10 @@ Strong guarantee: if an exception is thrown, there are no changes in the JSON va - Throws [`type_error.317`](../../home/exceptions.md#jsonexceptiontype_error317) if the top-level type of the JSON value is not an object; example: `"to serialize to BSON, top-level type must be object, but is string"` +- Throws [`out_of_range.407`](../../home/exceptions.md#jsonexceptionout_of_range407) if `j` contains a signed integer + outside the range of int64 or an unsigned integer outside the range of uint64, which is only possible with a number + type wider than 64 bits; example: + `"integer number 9223372036854775808 cannot be represented by BSON as it does not fit int64"` - Throws [`out_of_range.409`](../../home/exceptions.md#jsonexceptionout_of_range409) if a key in the JSON object contains a null byte (code point U+0000); example: `"BSON key cannot contain code point U+0000 (at byte 2)"` - Throws [`out_of_range.412`](../../home/exceptions.md#jsonexceptionout_of_range412) if the length of a document, array, @@ -166,3 +170,5 @@ pass before anything is written. that is not valid UTF-8 unchanged, as before; `strict` (the default if [`JSON_STRICT_BINARY_UTF8`](../macros/json_strict_binary_utf8.md) is enabled) throws `type_error.316` before anything is written. +- Throws `out_of_range.407` for integers that do not fit 64 bits since version 3.13.0; previously, integers of a + number type wider than 64 bits were silently truncated. diff --git a/api/basic_json/to_bson/index.html b/api/basic_json/to_bson/index.html index e64316430..57c4fdfa3 100644 --- a/api/basic_json/to_bson/index.html +++ b/api/basic_json/to_bson/index.html @@ -7,7 +7,7 @@ const error_handler_t error_handler = error_handler_t::keep); static void to_bson(const basic_json& j, detail::output_adapter<char> o, const error_handler_t error_handler = error_handler_t::keep); -

BSON (Binary JSON) is a binary format in which zero or more ordered key/value pairs are stored as a single entity (a so-called document).

  1. Returns a byte vector containing the BSON serialization.
  2. Writes the BSON serialization to an output adapter.

The exact mapping and its limitations are described on a dedicated page.

Parameters

j (in)
JSON value to serialize
o (in)
output adapter to write serialization to
error_handler (in)
how to treat a string or object key in j that is not valid UTF-8; see error_handler_t. The default, keep, writes the ill-formed bytes to the output as is, as every version of to_bson did before this parameter was added; strict throws; replace/ignore sanitize it the same way dump would. If JSON_STRICT_BINARY_UTF8 is enabled, the default is strict instead.

Return value

  1. BSON serialization as a byte vector
  2. (none)

Exception safety

Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

Exceptions

Complexity

Linear in the size of the JSON value j. The length prefixes of all nested documents and arrays are computed in one pass before anything is written.

Examples

Example: serialize a JSON value to BSON

The example shows the serialization of a JSON value to a byte vector in BSON format.

#include <iostream>
+

BSON (Binary JSON) is a binary format in which zero or more ordered key/value pairs are stored as a single entity (a so-called document).

  1. Returns a byte vector containing the BSON serialization.
  2. Writes the BSON serialization to an output adapter.

The exact mapping and its limitations are described on a dedicated page.

Parameters

j (in)
JSON value to serialize
o (in)
output adapter to write serialization to
error_handler (in)
how to treat a string or object key in j that is not valid UTF-8; see error_handler_t. The default, keep, writes the ill-formed bytes to the output as is, as every version of to_bson did before this parameter was added; strict throws; replace/ignore sanitize it the same way dump would. If JSON_STRICT_BINARY_UTF8 is enabled, the default is strict instead.

Return value

  1. BSON serialization as a byte vector
  2. (none)

Exception safety

Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

Exceptions

Complexity

Linear in the size of the JSON value j. The length prefixes of all nested documents and arrays are computed in one pass before anything is written.

Examples

Example: serialize a JSON value to BSON

The example shows the serialization of a JSON value to a byte vector in BSON format.

#include <iostream>
 #include <iomanip>
 #include <nlohmann/json.hpp>
 
@@ -54,4 +54,4 @@
     }
 }

Output:

[json.exception.out_of_range.409] BSON key cannot contain code point U+0000 (at byte 2)
-

See also

Version history

\ No newline at end of file +

See also

Version history

\ No newline at end of file diff --git a/api/basic_json/to_bson/index.md b/api/basic_json/to_bson/index.md index e6f1234c6..651cdc5c3 100644 --- a/api/basic_json/to_bson/index.md +++ b/api/basic_json/to_bson/index.md @@ -39,6 +39,7 @@ Strong guarantee: if an exception is thrown, there are no changes in the JSON va ## Exceptions - Throws [`type_error.317`](https://json.nlohmann.me/home/exceptions/#jsonexceptiontype_error317) if the top-level type of the JSON value is not an object; example: `"to serialize to BSON, top-level type must be object, but is string"` +- Throws [`out_of_range.407`](https://json.nlohmann.me/home/exceptions/#jsonexceptionout_of_range407) if `j` contains a signed integer outside the range of int64 or an unsigned integer outside the range of uint64, which is only possible with a number type wider than 64 bits; example: `"integer number 9223372036854775808 cannot be represented by BSON as it does not fit int64"` - Throws [`out_of_range.409`](https://json.nlohmann.me/home/exceptions/#jsonexceptionout_of_range409) if a key in the JSON object contains a null byte (code point U+0000); example: `"BSON key cannot contain code point U+0000 (at byte 2)"` - Throws [`out_of_range.412`](https://json.nlohmann.me/home/exceptions/#jsonexceptionout_of_range412) if the length of a document, array, string, or binary value exceeds the range of the 32-bit BSON length field; example: `"BSON length 2147483661 exceeds maximum of 2147483647"` - Throws [`out_of_range.415`](https://json.nlohmann.me/home/exceptions/#jsonexceptionout_of_range415) if the subtype of a binary value exceeds 255, the maximum of the BSON binary subtype; example: `"subtype 70000 is too large for the BSON binary subtype (max 255)"` @@ -139,3 +140,4 @@ Output: - `out_of_range.415` is now detected before anything is written, like the other exceptions above, since version 3.13.0 unreleased. - Throws `type_error.321` for a discarded value nested in `j` since version 3.13.0 unreleased; previously, it was silently skipped, producing a document whose declared size did not match what was actually written. - Added `error_handler` parameter in version 3.13.0 unreleased. Its default, `keep`, writes the bytes of a string or object key that is not valid UTF-8 unchanged, as before; `strict` (the default if [`JSON_STRICT_BINARY_UTF8`](https://json.nlohmann.me/api/macros/json_strict_binary_utf8/index.md) is enabled) throws `type_error.316` before anything is written. +- Throws `out_of_range.407` for integers that do not fit 64 bits since version 3.13.0 unreleased; previously, integers of a number type wider than 64 bits were silently truncated. diff --git a/api/basic_json/to_cbor.md b/api/basic_json/to_cbor.md index c92483b47..7a7f7c373 100644 --- a/api/basic_json/to_cbor.md +++ b/api/basic_json/to_cbor.md @@ -46,6 +46,9 @@ Strong guarantee: if an exception is thrown, there are no changes in the JSON va ## Exceptions +- Throws [`out_of_range.407`](../../home/exceptions.md#jsonexceptionout_of_range407) if `j` contains an integer + outside [-2^64, 2^64-1], which is only possible with a number type wider than 64 bits; example: + `"integer number 18446744073709551616 cannot be represented by CBOR as it does not fit [-2^64, 2^64-1]"` - Throws [type_error.316](../../home/exceptions.md#jsonexceptiontype_error316) if a string or object key in `j` is not valid UTF-8 and `error_handler` is `strict` (the default only if [`JSON_STRICT_BINARY_UTF8`](../macros/json_strict_binary_utf8.md) is enabled) @@ -113,3 +116,5 @@ Linear in the size of the JSON value `j`. [`JSON_STRICT_BINARY_UTF8`](../macros/json_strict_binary_utf8.md) is enabled) throws `type_error.316`. - Throws `type_error.321` for a discarded value since version 3.13.0; previously, a discarded value nested in an array or object was silently skipped, producing invalid CBOR. +- Throws `out_of_range.407` for integers that do not fit 64 bits since version 3.13.0; previously, integers of a + number type wider than 64 bits were silently truncated. diff --git a/api/basic_json/to_cbor/index.html b/api/basic_json/to_cbor/index.html index 28818caf5..495174b9c 100644 --- a/api/basic_json/to_cbor/index.html +++ b/api/basic_json/to_cbor/index.html @@ -7,7 +7,7 @@ const error_handler_t error_handler = error_handler_t::keep); static void to_cbor(const basic_json& j, detail::output_adapter<char> o, const error_handler_t error_handler = error_handler_t::keep); -

Serializes a given JSON value j to a byte vector using the CBOR (Concise Binary Object Representation) serialization format. CBOR is a binary serialization format that aims to be more compact than JSON itself, yet more efficient to parse.

  1. Returns a byte vector containing the CBOR serialization.
  2. Writes the CBOR serialization to an output adapter.

The exact mapping and its limitations are described on a dedicated page.

Parameters

j (in)
JSON value to serialize
o (in)
output adapter to write serialization to
error_handler (in)
how to treat a string or object key in j that is not valid UTF-8; see error_handler_t. The default, keep, writes the ill-formed bytes to the output as is, as every version of to_cbor did before this parameter was added; strict throws; replace/ignore sanitize it the same way dump would. If JSON_STRICT_BINARY_UTF8 is enabled, the default is strict instead.

Return value

  1. CBOR serialization as a byte vector
  2. (none)

Exception safety

Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

Exceptions

Complexity

Linear in the size of the JSON value j.

Examples

Example

The example shows the serialization of a JSON value to a byte vector in CBOR format.

#include <iostream>
+

Serializes a given JSON value j to a byte vector using the CBOR (Concise Binary Object Representation) serialization format. CBOR is a binary serialization format that aims to be more compact than JSON itself, yet more efficient to parse.

  1. Returns a byte vector containing the CBOR serialization.
  2. Writes the CBOR serialization to an output adapter.

The exact mapping and its limitations are described on a dedicated page.

Parameters

j (in)
JSON value to serialize
o (in)
output adapter to write serialization to
error_handler (in)
how to treat a string or object key in j that is not valid UTF-8; see error_handler_t. The default, keep, writes the ill-formed bytes to the output as is, as every version of to_cbor did before this parameter was added; strict throws; replace/ignore sanitize it the same way dump would. If JSON_STRICT_BINARY_UTF8 is enabled, the default is strict instead.

Return value

  1. CBOR serialization as a byte vector
  2. (none)

Exception safety

Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

Exceptions

Complexity

Linear in the size of the JSON value j.

Examples

Example

The example shows the serialization of a JSON value to a byte vector in CBOR format.

#include <iostream>
 #include <iomanip>
 #include <nlohmann/json.hpp>
 
@@ -30,4 +30,4 @@
     std::cout << std::endl;
 }

Output:

0xa2 0x67 0x63 0x6f 0x6d 0x70 0x61 0x63 0x74 0xf5 0x66 0x73 0x63 0x68 0x65 0x6d 0x61 0x00 
-

See also

Version history

\ No newline at end of file +

See also

Version history

\ No newline at end of file diff --git a/api/basic_json/to_cbor/index.md b/api/basic_json/to_cbor/index.md index e3bebf93a..9d4b475f4 100644 --- a/api/basic_json/to_cbor/index.md +++ b/api/basic_json/to_cbor/index.md @@ -38,6 +38,7 @@ Strong guarantee: if an exception is thrown, there are no changes in the JSON va ## Exceptions +- Throws [`out_of_range.407`](https://json.nlohmann.me/home/exceptions/#jsonexceptionout_of_range407) if `j` contains an integer outside [-2^64, 2^64-1], which is only possible with a number type wider than 64 bits; example: `"integer number 18446744073709551616 cannot be represented by CBOR as it does not fit [-2^64, 2^64-1]"` - Throws [type_error.316](https://json.nlohmann.me/home/exceptions/#jsonexceptiontype_error316) if a string or object key in `j` is not valid UTF-8 and `error_handler` is `strict` (the default only if [`JSON_STRICT_BINARY_UTF8`](https://json.nlohmann.me/api/macros/json_strict_binary_utf8/index.md) is enabled) - Throws [type_error.321](https://json.nlohmann.me/home/exceptions/#jsonexceptiontype_error321) if `j` or a value nested in it is discarded; example: `"cannot serialize discarded value to CBOR"` @@ -97,3 +98,4 @@ Output: - Compact representation of floating-point numbers added in version 3.8.0. - Added `error_handler` parameter in version 3.13.0 unreleased. Its default, `keep`, writes the bytes of a string or object key that is not valid UTF-8 unchanged, as before; `strict` (the default if [`JSON_STRICT_BINARY_UTF8`](https://json.nlohmann.me/api/macros/json_strict_binary_utf8/index.md) is enabled) throws `type_error.316`. - Throws `type_error.321` for a discarded value since version 3.13.0 unreleased; previously, a discarded value nested in an array or object was silently skipped, producing invalid CBOR. +- Throws `out_of_range.407` for integers that do not fit 64 bits since version 3.13.0 unreleased; previously, integers of a number type wider than 64 bits were silently truncated. diff --git a/api/basic_json/to_msgpack.md b/api/basic_json/to_msgpack.md index da5069c05..fb1cfb161 100644 --- a/api/basic_json/to_msgpack.md +++ b/api/basic_json/to_msgpack.md @@ -46,6 +46,9 @@ Strong guarantee: if an exception is thrown, there are no changes in the JSON va ## Exceptions +- Throws [`out_of_range.407`](../../home/exceptions.md#jsonexceptionout_of_range407) if `j` contains an integer + outside [-2^63, 2^64-1], which is only possible with a number type wider than 64 bits; example: + `"integer number 18446744073709551616 cannot be represented by MessagePack as it does not fit [-2^63, 2^64-1]"` - Throws [`out_of_range.412`](../../home/exceptions.md#jsonexceptionout_of_range412) if the length of a string, binary value, array, or object exceeds 4294967295, the maximum MessagePack can store; example: `"MessagePack length 4294967296 exceeds maximum of 4294967295"` @@ -156,3 +159,5 @@ Linear in the size of the JSON value `j`. `number_unsigned_t`. - Throws `type_error.321` for a discarded value since version 3.13.0; previously, a discarded value nested in an array or object was silently skipped, producing invalid MessagePack. +- Throws `out_of_range.407` for integers that do not fit 64 bits since version 3.13.0; previously, integers of a + number type wider than 64 bits were silently truncated. diff --git a/api/basic_json/to_msgpack/index.html b/api/basic_json/to_msgpack/index.html index 33a1dd954..ced1337db 100644 --- a/api/basic_json/to_msgpack/index.html +++ b/api/basic_json/to_msgpack/index.html @@ -7,7 +7,7 @@ const error_handler_t error_handler = error_handler_t::keep); static void to_msgpack(const basic_json& j, detail::output_adapter<char> o, const error_handler_t error_handler = error_handler_t::keep); -

Serializes a given JSON value j to a byte vector using the MessagePack serialization format. MessagePack is a binary serialization format that aims to be more compact than JSON itself, yet more efficient to parse.

  1. Returns a byte vector containing the MessagePack serialization.
  2. Writes the MessagePack serialization to an output adapter.

The exact mapping and its limitations are described on a dedicated page.

Parameters

j (in)
JSON value to serialize
o (in)
output adapter to write serialization to
error_handler (in)
how to treat a string or object key in j that is not valid UTF-8; see error_handler_t. The default, keep, writes the ill-formed bytes to the output as is, as every version of to_msgpack did before this parameter was added and as the MessagePack specification allows; strict throws; replace/ignore sanitize it the same way dump would. Unlike the other binary writers, the default stays keep even if JSON_STRICT_BINARY_UTF8 is enabled.

Return value

  1. MessagePack serialization as a byte vector
  2. (none)

Exception safety

Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

Exceptions

Complexity

Linear in the size of the JSON value j.

Examples

Example: serialize a JSON value to MessagePack

The example shows the serialization of a JSON value to a byte vector in MessagePack format.

#include <iostream>
+

Serializes a given JSON value j to a byte vector using the MessagePack serialization format. MessagePack is a binary serialization format that aims to be more compact than JSON itself, yet more efficient to parse.

  1. Returns a byte vector containing the MessagePack serialization.
  2. Writes the MessagePack serialization to an output adapter.

The exact mapping and its limitations are described on a dedicated page.

Parameters

j (in)
JSON value to serialize
o (in)
output adapter to write serialization to
error_handler (in)
how to treat a string or object key in j that is not valid UTF-8; see error_handler_t. The default, keep, writes the ill-formed bytes to the output as is, as every version of to_msgpack did before this parameter was added and as the MessagePack specification allows; strict throws; replace/ignore sanitize it the same way dump would. Unlike the other binary writers, the default stays keep even if JSON_STRICT_BINARY_UTF8 is enabled.

Return value

  1. MessagePack serialization as a byte vector
  2. (none)

Exception safety

Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

Exceptions

Complexity

Linear in the size of the JSON value j.

Examples

Example: serialize a JSON value to MessagePack

The example shows the serialization of a JSON value to a byte vector in MessagePack format.

#include <iostream>
 #include <iomanip>
 #include <nlohmann/json.hpp>
 
@@ -51,4 +51,4 @@
     }
 }

Output:

[json.exception.out_of_range.415] subtype 300 is too large for the MessagePack ext type (max 255)
-

See also

Version history

\ No newline at end of file +

See also

Version history

\ No newline at end of file diff --git a/api/basic_json/to_msgpack/index.md b/api/basic_json/to_msgpack/index.md index 649ce21e6..ec9ebb7bd 100644 --- a/api/basic_json/to_msgpack/index.md +++ b/api/basic_json/to_msgpack/index.md @@ -38,6 +38,7 @@ Strong guarantee: if an exception is thrown, there are no changes in the JSON va ## Exceptions +- Throws [`out_of_range.407`](https://json.nlohmann.me/home/exceptions/#jsonexceptionout_of_range407) if `j` contains an integer outside [-2^63, 2^64-1], which is only possible with a number type wider than 64 bits; example: `"integer number 18446744073709551616 cannot be represented by MessagePack as it does not fit [-2^63, 2^64-1]"` - Throws [`out_of_range.412`](https://json.nlohmann.me/home/exceptions/#jsonexceptionout_of_range412) if the length of a string, binary value, array, or object exceeds 4294967295, the maximum MessagePack can store; example: `"MessagePack length 4294967296 exceeds maximum of 4294967295"` - Throws [`out_of_range.415`](https://json.nlohmann.me/home/exceptions/#jsonexceptionout_of_range415) if the subtype of a binary value exceeds 255, the maximum of the MessagePack ext type; example: `"subtype 70000 is too large for the MessagePack ext type (max 255)"` - Throws [type_error.316](https://json.nlohmann.me/home/exceptions/#jsonexceptiontype_error316) if a string or object key in `j` is not valid UTF-8 and `error_handler` is `strict` @@ -133,3 +134,4 @@ Output: - Added `error_handler` parameter in version 3.13.0 unreleased. Its default, `keep`, writes the bytes of a string or object key that is not valid UTF-8 unchanged, as before. - Fixed in version 3.13.0 unreleased to serialize `number_integer_t`/`number_unsigned_t` pairs of different width correctly; before, integers could be serialized with the wrong value if `number_integer_t` was narrower than `number_unsigned_t`. - Throws `type_error.321` for a discarded value since version 3.13.0 unreleased; previously, a discarded value nested in an array or object was silently skipped, producing invalid MessagePack. +- Throws `out_of_range.407` for integers that do not fit 64 bits since version 3.13.0 unreleased; previously, integers of a number type wider than 64 bits were silently truncated. diff --git a/api/macros/json_disable_unlocked_file_io.md b/api/macros/json_disable_unlocked_file_io.md index aebbea726..6bb9d84a6 100644 --- a/api/macros/json_disable_unlocked_file_io.md +++ b/api/macros/json_disable_unlocked_file_io.md @@ -10,6 +10,9 @@ earlier, instead of locking the file once and reading it with the unlocked stdio It does not change any result: values, exceptions, and the position of the file after parsing are the same either way. Only the speed and the locking of the file differ (see the notes on locking below). +Builds with MemorySanitizer use `std::fgetc` automatically, because the inline `getc_unlocked` of glibc reads the stdio +buffer directly, which glibc (not instrumented) filled, and MemorySanitizer would report it as uninitialized. + ## Default definition The default value is `0`: the file is locked once and read with the unlocked stdio functions where they are available. diff --git a/api/macros/json_disable_unlocked_file_io/index.html b/api/macros/json_disable_unlocked_file_io/index.html index cf6031df8..dc85eec01 100644 --- a/api/macros/json_disable_unlocked_file_io/index.html +++ b/api/macros/json_disable_unlocked_file_io/index.html @@ -1,5 +1,5 @@ JSON_DISABLE_UNLOCKED_FILE_IO - JSON for Modern C++
Skip to content

JSON_DISABLE_UNLOCKED_FILE_IO

#define JSON_DISABLE_UNLOCKED_FILE_IO /* value */
-

When defined to 1, parsing from a FILE* reads every character with std::fgetc, as in version 3.12.0 and earlier, instead of locking the file once and reading it with the unlocked stdio functions.

It does not change any result: values, exceptions, and the position of the file after parsing are the same either way. Only the speed and the locking of the file differ (see the notes on locking below).

Default definition

The default value is 0: the file is locked once and read with the unlocked stdio functions where they are available.

#define JSON_DISABLE_UNLOCKED_FILE_IO 0
+

When defined to 1, parsing from a FILE* reads every character with std::fgetc, as in version 3.12.0 and earlier, instead of locking the file once and reading it with the unlocked stdio functions.

It does not change any result: values, exceptions, and the position of the file after parsing are the same either way. Only the speed and the locking of the file differ (see the notes on locking below).

Builds with MemorySanitizer use std::fgetc automatically, because the inline getc_unlocked of glibc reads the stdio buffer directly, which glibc (not instrumented) filled, and MemorySanitizer would report it as uninitialized.

Default definition

The default value is 0: the file is locked once and read with the unlocked stdio functions where they are available.

#define JSON_DISABLE_UNLOCKED_FILE_IO 0

Notes

Background

The C standard library makes every std::fgetc call thread-safe by locking and unlocking the FILE object. When parsing from a FILE*, the parser reads one character at a time, so this happens for every character of the input.

Instead, the parser locks the file once when parsing starts and unlocks it when parsing ends - also when an exception is thrown - and reads the characters in between without locking:

C library functions
glibc (Linux), Apple (macOS, iOS), Android, FreeBSD, NetBSD, OpenBSD, DragonFly BSD flockfile, getc_unlocked, funlockfile (POSIX)
Microsoft (MSVC, clang-cl) _lock_file, _fgetc_nolock, _unlock_file
others (e.g., musl, MinGW, newlib) std::fgetc, as before

This makes parsing from a FILE* 3 to 4 times as fast with the Apple C library and 1.1 to 1.3 times as fast with glibc, whose locking is cheaper. The binary formats (from_cbor, from_msgpack, ...) read from a FILE* the same way.

Locking

While a value is parsed from a FILE*, the parsing thread holds the file's lock. The lock is recursive, so the parsing thread itself - for instance, a SAX handler or a parser callback - can keep using the file with stdio functions such as std::ftell. Other threads that use the same FILE object wait until the parser is done. Before, they could interleave their reads with the parser's, which made the parsed input unpredictable anyway.

A program in which a SAX handler or parser callback waits for another thread that uses the same FILE object would deadlock. Define the macro to 1 for such programs.

ABI compatibility

The value of this macro is encoded in the namespace (tag _dufio), resulting in distinct symbol names. Translation units compiled with and without it can therefore be linked into the same program without One Definition Rule (ODR) violations, but they cannot exchange instances of library types.

Other inputs

Reading from a std::istream is not affected by this macro; see JSON_DISABLE_STREAM_BULK_SCAN for that input.

Examples

Example: parsing from a FILE* (default)

The file is locked once while it is parsed:

#include <cstdio>
 #include <nlohmann/json.hpp>
 
@@ -23,4 +23,4 @@
     json j = json::parse(f);  // std::fgetc for every character
     std::fclose(f);
 }
-

See also

Version history

  • Added in version 3.13.0 unreleased.
\ No newline at end of file +

See also

Version history

\ No newline at end of file diff --git a/api/macros/json_disable_unlocked_file_io/index.md b/api/macros/json_disable_unlocked_file_io/index.md index 0714044d0..80e3c171b 100644 --- a/api/macros/json_disable_unlocked_file_io/index.md +++ b/api/macros/json_disable_unlocked_file_io/index.md @@ -8,6 +8,8 @@ When defined to `1`, parsing from a `FILE*` reads every character with `std::fge It does not change any result: values, exceptions, and the position of the file after parsing are the same either way. Only the speed and the locking of the file differ (see the notes on locking below). +Builds with MemorySanitizer use `std::fgetc` automatically, because the inline `getc_unlocked` of glibc reads the stdio buffer directly, which glibc (not instrumented) filled, and MemorySanitizer would report it as uninitialized. + ## Default definition The default value is `0`: the file is locked once and read with the unlocked stdio functions where they are available. diff --git a/features/performance.md b/features/performance.md index ad24bd069..a15fbf38b 100644 --- a/features/performance.md +++ b/features/performance.md @@ -20,32 +20,39 @@ below apply on every supported standard. This adapter exposes two optimizations - it can reconstruct already-consumed input on demand for error messages, instead of copying every character as it is read, and -- the lexer can scan ordinary string characters directly out of the buffer, several bytes at a time, rather than one - character (and one function call) at a time. +- the lexer can scan ordinary string characters, numbers, and indentation directly out of the buffer, several bytes at + a time, rather than one character (and one function call) at a time. -A `#!cpp std::istream` (including `#!cpp std::ifstream`) or `#!cpp FILE*`, by contrast, is read through -`input_stream_adapter` or `file_input_adapter`, which read one character (or one block, for binary formats) at a time -and expose neither optimization -- the lexer falls back to the same byte-at-a-time path it uses for any -non-contiguous, general-purpose iterator range. An iterator pair over non-contiguous but random-access storage (e.g. -`#!cpp std::deque::iterator`) gets the first optimization but not the second, since the byte-scanning fast path -additionally requires contiguous storage. +A `#!cpp std::istream` (including `#!cpp std::ifstream`) gets the same bulk scanning: `input_stream_adapter` scans the +characters its stream buffer already holds (the *get area*) and asks the stream buffer for more only when they are used +up. Nothing is read ahead, so the stream is left exactly where it would be after reading one character at a time, and +[`operator>>`](../api/operator_gtgt.md) can still read several values from one stream. A stream buffer without a get +area is read one character at a time. A `#!cpp FILE*` is still read one character at a time, but `file_input_adapter` +locks the file once per parse instead of once per character. See +[input sources and performance](parsing/index.md#input-sources-and-performance) for the full table. According to the +description of [#5828](https://github.com/nlohmann/json/pull/5828), parsing from a stream is within about 10 to 20 % of +parsing an equivalent `#!cpp std::string`. -Practically: if the JSON text is already in memory, or small enough to read into memory, prefer passing a -`#!cpp std::string`, a `#!cpp std::vector`, or a pointer range to `parse` over a `#!cpp std::istream`. For a -file, that means reading it into a string first and then parsing the string, rather than passing a -`#!cpp std::ifstream` directly to `parse` -- the latter never benefits from either optimization: +Inputs that are still read one character at a time: iterator ranges that are not contiguous (e.g. +`#!cpp std::list` or `#!cpp std::istreambuf_iterator`), wide strings, and user-defined adapters. An +iterator pair over non-contiguous but random-access storage (e.g. `#!cpp std::deque::iterator`) gets the first +optimization but not the second, since the byte-scanning fast path requires contiguous storage. + +Practically: a file can be passed directly to `parse`, which also avoids the memory for a copy of the file. Reading it +into a string first is no longer necessary for speed: ```cpp -// gets the contiguous fast paths +// fast: the stream buffer's get area is scanned in bulk std::ifstream f("example.json"); -std::string contents((std::istreambuf_iterator(f)), std::istreambuf_iterator()); -json j = json::parse(contents); - -// does not: input_stream_adapter has no fast path -std::ifstream f2("example.json"); -json j2 = json::parse(f2); +json j = json::parse(f); ``` +Two macros switch the stream optimizations off without changing any result: +[`JSON_DISABLE_STREAM_BULK_SCAN`](../api/macros/json_disable_stream_bulk_scan.md) for stream buffers that do not follow +the usual `#!cpp std::streambuf` contract, and +[`JSON_DISABLE_UNLOCKED_FILE_IO`](../api/macros/json_disable_unlocked_file_io.md) for programs in which another thread +must use the same `#!cpp FILE` object while it is parsed. + For contiguous input with many non-ASCII characters, [`JSON_USE_SIMDUTF`](../api/macros/json_use_simdutf.md) can additionally speed up UTF-8 validation by using the [simdutf](https://github.com/simdutf/simdutf) library instead of the built-in scalar validator; streaming inputs (files, `#!cpp std::istream`, wide strings, user-defined adapters) diff --git a/features/performance/index.html b/features/performance/index.html index 891b2df8b..9ebc58ffc 100644 --- a/features/performance/index.html +++ b/features/performance/index.html @@ -1,12 +1,7 @@ - Performance - JSON for Modern C++

Performance

Speed was never the primary goal of this library. The design goals page says so plainly: "There are certainly faster JSON libraries out there." Intuitive syntax, trivial integration, and thorough testing came first. If a hard real-time budget or the last percent of throughput matters more than convenience, a faster, more specialized library may be a better fit.

That said, how you use this library still makes a measurable difference. This page collects practical, code-verified techniques for reducing time, memory, and compile-time cost -- without repeating the detailed pages it links to.

Parsing input

parse accepts a string, a pair of iterators, a container, a std::istream, or a FILE* (see Parsing). Internally, every input is wrapped in an input adapter, and not all adapters are equally fast.

For inputs backed by contiguous, single-byte memory -- a std::string, a std::vector<char>, a string literal, or a pointer range -- the library uses iterator_input_adapter, wrapped in a raw pointer so the fast paths below apply on every supported standard. This adapter exposes two optimizations the lexer detects at compile time:

  • it can reconstruct already-consumed input on demand for error messages, instead of copying every character as it is read, and
  • the lexer can scan ordinary string characters directly out of the buffer, several bytes at a time, rather than one character (and one function call) at a time.

A std::istream (including std::ifstream) or FILE*, by contrast, is read through input_stream_adapter or file_input_adapter, which read one character (or one block, for binary formats) at a time and expose neither optimization -- the lexer falls back to the same byte-at-a-time path it uses for any non-contiguous, general-purpose iterator range. An iterator pair over non-contiguous but random-access storage (e.g. std::deque<char>::iterator) gets the first optimization but not the second, since the byte-scanning fast path additionally requires contiguous storage.

Practically: if the JSON text is already in memory, or small enough to read into memory, prefer passing a std::string, a std::vector<char>, or a pointer range to parse over a std::istream. For a file, that means reading it into a string first and then parsing the string, rather than passing a std::ifstream directly to parse -- the latter never benefits from either optimization:

// gets the contiguous fast paths
+ Performance - JSON for Modern C++      
 
 
  
 
  
 
 
 
 
  
 
 
 
 
 
  
 
 
 
 
       
Performance
 
Speed was never the primary goal of this library. The design goals page says so plainly: "There are certainly faster JSON libraries out there." Intuitive syntax, trivial integration, and thorough testing came first. If a hard real-time budget or the last percent of throughput matters more than convenience, a faster, more specialized library may be a better fit.
 
That said, how you use this library still makes a measurable difference. This page collects practical, code-verified techniques for reducing time, memory, and compile-time cost -- without repeating the detailed pages it links to.
 
Parsing input
 
parse accepts a string, a pair of iterators, a container, a std::istream, or a FILE* (see Parsing). Internally, every input is wrapped in an input adapter, and not all adapters are equally fast.
 
For inputs backed by contiguous, single-byte memory -- a std::string, a std::vector<char>, a string literal, or a pointer range -- the library uses iterator_input_adapter, wrapped in a raw pointer so the fast paths below apply on every supported standard. This adapter exposes two optimizations the lexer detects at compile time:
 
     
  • it can reconstruct already-consumed input on demand for error messages, instead of copying every character as it is read, and
    •  
  • the lexer can scan ordinary string characters, numbers, and indentation directly out of the buffer, several bytes at a time, rather than one character (and one function call) at a time.
    •  
 
A std::istream (including std::ifstream) gets the same bulk scanning: input_stream_adapter scans the characters its stream buffer already holds (the get area) and asks the stream buffer for more only when they are used up. Nothing is read ahead, so the stream is left exactly where it would be after reading one character at a time, and operator>> can still read several values from one stream. A stream buffer without a get area is read one character at a time. A FILE* is still read one character at a time, but file_input_adapter locks the file once per parse instead of once per character. See input sources and performance for the full table. According to the description of #5828, parsing from a stream is within about 10 to 20 % of parsing an equivalent std::string.
 
Inputs that are still read one character at a time: iterator ranges that are not contiguous (e.g. std::list<char> or std::istreambuf_iterator<char>), wide strings, and user-defined adapters. An iterator pair over non-contiguous but random-access storage (e.g. std::deque<char>::iterator) gets the first optimization but not the second, since the byte-scanning fast path requires contiguous storage.
 
Practically: a file can be passed directly to parse, which also avoids the memory for a copy of the file. Reading it into a string first is no longer necessary for speed:
 
// fast: the stream buffer's get area is scanned in bulk
 std::ifstream f("example.json");
-std::string contents((std::istreambuf_iterator<char>(f)), std::istreambuf_iterator<char>());
-json j = json::parse(contents);
-
-// does not: input_stream_adapter has no fast path
-std::ifstream f2("example.json");
-json j2 = json::parse(f2);
-
 
For contiguous input with many non-ASCII characters, JSON_USE_SIMDUTF can additionally speed up UTF-8 validation by using the simdutf library instead of the built-in scalar validator; streaming inputs (files, std::istream, wide strings, user-defined adapters) always use the scalar path regardless of this macro.
 
Large documents
 
Parsing always produces SAX events internally; parse simply feeds them to a consumer that builds a complete basic_json value tree (a DOM) in memory. For documents too large to comfortably hold as a DOM, three alternatives avoid building it:
 
     
  • Implement the SAX interface directly and pass it to sax_parse; only the parts of the input you choose to keep ever become basic_json values.
    •  
  • Pass a parser callback to parse. This still builds a DOM, but the callback can discard finished elements as soon as they are handled, so memory usage stays bounded by one element (plus the unparsed remainder of the input) instead of the whole document -- see the recipe for streaming a large homogeneous array.
    •  
  • Parse into a json_document (<nlohmann/json_view.hpp>) instead of a basic_json. It keeps the input text and builds a flat index of 16 bytes per value; strings and floating-point numbers are not copied, but read from the text when needed. Read-only views give the familiar element access, and only the parts you materialize() become basic_json values. A document that borrows the text instead of owning a copy needs the text to outlive it; see choosing between json, the SAX interface, and json_view.
    •  
 
If the data is naturally record-oriented, consider JSON Lines instead of one large JSON document: reading and parsing it line by line with std::getline means only one line's value is ever in memory at a time, and a malformed line does not invalidate lines already processed.
 
Binary formats
 
JSON text is not a compact format. If the data is only exchanged between programs (not read by humans), the binary formats -- BJData, BON8, BSON, CBOR, MessagePack, and UBJSON -- encode the same values more compactly, which reduces both the bytes transferred and, for most of them, the work needed to parse them back. The size comparison on that page, measured against minified JSON for four reference documents, shows the effect varies a lot by document shape: CBOR and MessagePack come out at 50.5% of the minified JSON size for the numeric-array-heavy canada.json, but only around 87-88% for the string-heavy jeopardy.json, where there is less numeric data to encode more compactly. BON8 is the most compact option in that comparison for text-heavy documents (63.5%-87.5%), at the cost of an incomplete serializer (no unsigned integers above int64). Which format -- and whether it is worth the loss of human readability at all -- depends on the actual data; see the comparison tables before choosing one.
 
Object type: json vs. ordered_json
 
The default json type stores object keys in a std::map, giving logarithmic-time lookup, insertion, and erasure, at the cost of sorting keys alphabetically rather than preserving insertion order (see Object Order). ordered_json uses nlohmann::ordered_map instead, a std::vector-backed container with no lookup index: every key-based operation is a linear scan, so building an object of n distinct keys costs O(n²) in total -- this applies equally to inserting keys one by one and to parsing an object, since the parser inserts each key as it is read. The measurements on the ordered_map page show this is negligible at typical object sizes (2000 keys: 0.7 ms for json vs. 3.6 ms for ordered_json, a 5x factor) but grows steeply for large, machine-generated objects (16 000 keys: 3.3 ms vs. 181.6 ms, a 54x factor).
 
If insertion order matters and an object routinely has many thousands of keys, ordered_json's quadratic build cost may not be acceptable. The library's ObjectType template parameter can be set to a different container instead: nlohmann::fifo_map keeps insertion order with a real lookup index (avoiding the quadratic cost), while std::unordered_map, boost::unordered_flat_map, absl::flat_hash_map, and similar hash maps trade insertion order for average-case constant-time lookup (through an adapter, since their template argument order does not match what basic_json expects) -- see Object Order for the full list.
 
Avoiding copies
 
     
  • Move instead of copy. Constructing a basic_json from an existing one is linear in its size for the copy constructor but constant for the move constructor. The same applies to assigning a large std::string, std::vector, or other container into a value: pass it as std::move(x) rather than x whenever x is no longer needed afterwards.
    •  
  • Access without copying. get<T>() returns a copy of the stored value converted to T. When a reference or pointer to the value already stored inside the basic_json is enough, get_ref() and get_ptr() access it directly: both pages state, word for word, "No copies are made." -- at the cost of that reference or pointer becoming invalid once the underlying value changes.
    •  
  • Iterate by reference. basic_json::iterator::operator*() returns a reference (an alias for basic_json&), but a range-based for loop with a by-value loop variable (for (auto el : j)) still copies each element, because plain auto drops the reference. Write for (const auto& el : j) (or auto& for a mutable loop), and use items() the same way when the key is needed too -- its own examples use for (auto& el : j.items()).
    •  
  • Construct in place. emplace_back() (arrays, amortized constant time) and emplace() (objects, logarithmic in the size of the container for json) forward their arguments directly to a basic_json constructor, rather than requiring a temporary value to be constructed and then copied or moved in. push_back() has an rvalue overload (push_back(basic_json&&)) for a value that already exists: j.push_back(std::move(value)) moves it in instead of copying it.
    •  
  • Skip the bounds check when it is redundant. at() and operator[] have the same complexity (constant for a valid array index, logarithmic for an object key in json) -- the difference is that at() additionally checks the key or index and throws if it is invalid, while operator[] does not (see unchecked access and checked access). Prefer operator[] when the surrounding code has already established that the access is valid.
    •  
  • Reserve array capacity. basic_json has no public reserve(), but when building a large array incrementally with a known final size, get_ref() exposes the underlying array_t so it can be reserved directly -- see "reserving array capacity" for the one-line recipe.
    •  
 
Serialization
 
dump() with the default indent = -1 selects "the most compact representation" (word for word from the page); any non-negative indent pretty-prints instead, which is more readable but produces more bytes and more work. dump() builds and returns a complete string_t containing the whole serialization. operator<< writes directly to a std::ostream instead, through the same serializer, but without ever materializing that intermediate string -- so if the destination is a stream (a file, or std::cout), os << j; avoids the allocation and copy that os << j.dump(); would incur for large values.
 
Diagnostics overhead
 
Two opt-in macros add diagnostic information to exceptions and to every value, at a cost that is only worth paying while it is in use:
 
     
  • JSON_DIAGNOSTICS adds a JSON Pointer to exception messages, pointing at the value that triggered the exception. Quoting the page directly: "enabling this macro increases the size of every JSON value by one pointer and adds some runtime overhead" -- every value gains a parent pointer that has to be kept up to date as the document is built and modified.
    •  
  • JSON_DIAGNOSTIC_POSITIONS adds start_pos() and end_pos(), the byte offsets a value occupied in its parsed input. Quoting the page: "enabling this macro increases the size of every JSON value by two std::size_t fields and adds slight runtime overhead to parsing, copying JSON value objects, and the generation of error messages for exceptions."
    •  
 
Both default to off. Enable them where better diagnostics are worth the overhead (for example, while validating untrusted input, or in a debug build), and keep them off in a release build that does not need them.
 
Compile time
 
<nlohmann/json_fwd.hpp> forward-declares basic_json, json, ordered_json, json_pointer, and adl_serializer, pulling in only a handful of lightweight standard headers instead of the full json.hpp. A header that only needs to name nlohmann::json -- in a function signature or a class member declaration, for instance -- can include json_fwd.hpp and leave #include <nlohmann/json.hpp> to the source files that actually parse, build, or serialize values, the same way a project would forward-declare any other heavy class to keep it out of widely-included headers:
 
// my_type.hpp
+json j = json::parse(f);
+
 
Two macros switch the stream optimizations off without changing any result: JSON_DISABLE_STREAM_BULK_SCAN for stream buffers that do not follow the usual std::streambuf contract, and JSON_DISABLE_UNLOCKED_FILE_IO for programs in which another thread must use the same FILE object while it is parsed.
 
For contiguous input with many non-ASCII characters, JSON_USE_SIMDUTF can additionally speed up UTF-8 validation by using the simdutf library instead of the built-in scalar validator; streaming inputs (files, std::istream, wide strings, user-defined adapters) always use the scalar path regardless of this macro.
 
Large documents
 
Parsing always produces SAX events internally; parse simply feeds them to a consumer that builds a complete basic_json value tree (a DOM) in memory. For documents too large to comfortably hold as a DOM, three alternatives avoid building it:
 
     
  • Implement the SAX interface directly and pass it to sax_parse; only the parts of the input you choose to keep ever become basic_json values.
    •  
  • Pass a parser callback to parse. This still builds a DOM, but the callback can discard finished elements as soon as they are handled, so memory usage stays bounded by one element (plus the unparsed remainder of the input) instead of the whole document -- see the recipe for streaming a large homogeneous array.
    •  
  • Parse into a json_document (<nlohmann/json_view.hpp>) instead of a basic_json. It keeps the input text and builds a flat index of 16 bytes per value; strings and floating-point numbers are not copied, but read from the text when needed. Read-only views give the familiar element access, and only the parts you materialize() become basic_json values. A document that borrows the text instead of owning a copy needs the text to outlive it; see choosing between json, the SAX interface, and json_view.
    •  
 
If the data is naturally record-oriented, consider JSON Lines instead of one large JSON document: reading and parsing it line by line with std::getline means only one line's value is ever in memory at a time, and a malformed line does not invalidate lines already processed.
 
Binary formats
 
JSON text is not a compact format. If the data is only exchanged between programs (not read by humans), the binary formats -- BJData, BON8, BSON, CBOR, MessagePack, and UBJSON -- encode the same values more compactly, which reduces both the bytes transferred and, for most of them, the work needed to parse them back. The size comparison on that page, measured against minified JSON for four reference documents, shows the effect varies a lot by document shape: CBOR and MessagePack come out at 50.5% of the minified JSON size for the numeric-array-heavy canada.json, but only around 87-88% for the string-heavy jeopardy.json, where there is less numeric data to encode more compactly. BON8 is the most compact option in that comparison for text-heavy documents (63.5%-87.5%), at the cost of an incomplete serializer (no unsigned integers above int64). Which format -- and whether it is worth the loss of human readability at all -- depends on the actual data; see the comparison tables before choosing one.
 
Object type: json vs. ordered_json
 
The default json type stores object keys in a std::map, giving logarithmic-time lookup, insertion, and erasure, at the cost of sorting keys alphabetically rather than preserving insertion order (see Object Order). ordered_json uses nlohmann::ordered_map instead, a std::vector-backed container with no lookup index: every key-based operation is a linear scan, so building an object of n distinct keys costs O(n²) in total -- this applies equally to inserting keys one by one and to parsing an object, since the parser inserts each key as it is read. The measurements on the ordered_map page show this is negligible at typical object sizes (2000 keys: 0.7 ms for json vs. 3.6 ms for ordered_json, a 5x factor) but grows steeply for large, machine-generated objects (16 000 keys: 3.3 ms vs. 181.6 ms, a 54x factor).
 
If insertion order matters and an object routinely has many thousands of keys, ordered_json's quadratic build cost may not be acceptable. The library's ObjectType template parameter can be set to a different container instead: nlohmann::fifo_map keeps insertion order with a real lookup index (avoiding the quadratic cost), while std::unordered_map, boost::unordered_flat_map, absl::flat_hash_map, and similar hash maps trade insertion order for average-case constant-time lookup (through an adapter, since their template argument order does not match what basic_json expects) -- see Object Order for the full list.
 
Avoiding copies
 
     
  • Move instead of copy. Constructing a basic_json from an existing one is linear in its size for the copy constructor but constant for the move constructor. The same applies to assigning a large std::string, std::vector, or other container into a value: pass it as std::move(x) rather than x whenever x is no longer needed afterwards.
    •  
  • Access without copying. get<T>() returns a copy of the stored value converted to T. When a reference or pointer to the value already stored inside the basic_json is enough, get_ref() and get_ptr() access it directly: both pages state, word for word, "No copies are made." -- at the cost of that reference or pointer becoming invalid once the underlying value changes.
    •  
  • Iterate by reference. basic_json::iterator::operator*() returns a reference (an alias for basic_json&), but a range-based for loop with a by-value loop variable (for (auto el : j)) still copies each element, because plain auto drops the reference. Write for (const auto& el : j) (or auto& for a mutable loop), and use items() the same way when the key is needed too -- its own examples use for (auto& el : j.items()).
    •  
  • Construct in place. emplace_back() (arrays, amortized constant time) and emplace() (objects, logarithmic in the size of the container for json) forward their arguments directly to a basic_json constructor, rather than requiring a temporary value to be constructed and then copied or moved in. push_back() has an rvalue overload (push_back(basic_json&&)) for a value that already exists: j.push_back(std::move(value)) moves it in instead of copying it.
    •  
  • Skip the bounds check when it is redundant. at() and operator[] have the same complexity (constant for a valid array index, logarithmic for an object key in json) -- the difference is that at() additionally checks the key or index and throws if it is invalid, while operator[] does not (see unchecked access and checked access). Prefer operator[] when the surrounding code has already established that the access is valid.
    •  
  • Reserve array capacity. basic_json has no public reserve(), but when building a large array incrementally with a known final size, get_ref() exposes the underlying array_t so it can be reserved directly -- see "reserving array capacity" for the one-line recipe.
    •  
 
Serialization
 
dump() with the default indent = -1 selects "the most compact representation" (word for word from the page); any non-negative indent pretty-prints instead, which is more readable but produces more bytes and more work. dump() builds and returns a complete string_t containing the whole serialization. operator<< writes directly to a std::ostream instead, through the same serializer, but without ever materializing that intermediate string -- so if the destination is a stream (a file, or std::cout), os << j; avoids the allocation and copy that os << j.dump(); would incur for large values.
 
Diagnostics overhead
 
Two opt-in macros add diagnostic information to exceptions and to every value, at a cost that is only worth paying while it is in use:
 
     
  • JSON_DIAGNOSTICS adds a JSON Pointer to exception messages, pointing at the value that triggered the exception. Quoting the page directly: "enabling this macro increases the size of every JSON value by one pointer and adds some runtime overhead" -- every value gains a parent pointer that has to be kept up to date as the document is built and modified.
    •  
  • JSON_DIAGNOSTIC_POSITIONS adds start_pos() and end_pos(), the byte offsets a value occupied in its parsed input. Quoting the page: "enabling this macro increases the size of every JSON value by two std::size_t fields and adds slight runtime overhead to parsing, copying JSON value objects, and the generation of error messages for exceptions."
    •  
 
Both default to off. Enable them where better diagnostics are worth the overhead (for example, while validating untrusted input, or in a debug build), and keep them off in a release build that does not need them.
 
Compile time
 
<nlohmann/json_fwd.hpp> forward-declares basic_json, json, ordered_json, json_pointer, and adl_serializer, pulling in only a handful of lightweight standard headers instead of the full json.hpp. A header that only needs to name nlohmann::json -- in a function signature or a class member declaration, for instance -- can include json_fwd.hpp and leave #include <nlohmann/json.hpp> to the source files that actually parse, build, or serialize values, the same way a project would forward-declare any other heavy class to keep it out of widely-included headers:
 
// my_type.hpp
 #include <nlohmann/json_fwd.hpp>
 
 class my_type
@@ -19,4 +14,4 @@
 #include "my_type.hpp"
 
 nlohmann::json my_type::config() const { /* ... */ }
-
 
One caveat: ABI-affecting macros such as JSON_DIAGNOSTICS and JSON_DIAGNOSTIC_POSITIONS are encoded into the library's inline namespace name. Every translation unit -- whether it includes json_fwd.hpp or the full header -- must define them the same way, or linking fails with undefined references instead of a compile error.
 
If I/O support is not needed at all, JSON_NO_IO excludes <cstdio>, <ios>, <iosfwd>, <istream>, and <ostream> outright and drops the std::istream/FILE* parse overloads and operator<< that depend on them (dump() itself is unaffected, since it only returns a string); it exists for environments where those headers are unavailable (such as Intel SGX), and as a side effect those headers are then never processed by the compiler at all.
 
See also
    
 
   
  
 
 
 
 
 
 Copyright © 2013-2026 Niels Lohmann 
 
 
             
 
 
 
 
 
 
 
    
\ No newline at end of file
+

One caveat: ABI-affecting macros such as JSON_DIAGNOSTICS and JSON_DIAGNOSTIC_POSITIONS are encoded into the library's inline namespace name. Every translation unit -- whether it includes json_fwd.hpp or the full header -- must define them the same way, or linking fails with undefined references instead of a compile error.

If I/O support is not needed at all, JSON_NO_IO excludes <cstdio>, <ios>, <iosfwd>, <istream>, and <ostream> outright and drops the std::istream/FILE* parse overloads and operator<< that depend on them (dump() itself is unaffected, since it only returns a string); it exists for environments where those headers are unavailable (such as Intel SGX), and as a side effect those headers are then never processed by the compiler at all.

See also

Copyright © 2013-2026 Niels Lohmann
\ No newline at end of file diff --git a/features/performance/index.md b/features/performance/index.md index fc7e09950..3be56c595 100644 --- a/features/performance/index.md +++ b/features/performance/index.md @@ -11,23 +11,22 @@ That said, how you use this library still makes a measurable difference. This pa For inputs backed by contiguous, single-byte memory -- a `std::string`, a `std::vector`, a string literal, or a pointer range -- the library uses `iterator_input_adapter`, wrapped in a raw pointer so the fast paths below apply on every supported standard. This adapter exposes two optimizations the lexer detects at compile time: - it can reconstruct already-consumed input on demand for error messages, instead of copying every character as it is read, and -- the lexer can scan ordinary string characters directly out of the buffer, several bytes at a time, rather than one character (and one function call) at a time. +- the lexer can scan ordinary string characters, numbers, and indentation directly out of the buffer, several bytes at a time, rather than one character (and one function call) at a time. -A `std::istream` (including `std::ifstream`) or `FILE*`, by contrast, is read through `input_stream_adapter` or `file_input_adapter`, which read one character (or one block, for binary formats) at a time and expose neither optimization -- the lexer falls back to the same byte-at-a-time path it uses for any non-contiguous, general-purpose iterator range. An iterator pair over non-contiguous but random-access storage (e.g. `std::deque::iterator`) gets the first optimization but not the second, since the byte-scanning fast path additionally requires contiguous storage. +A `std::istream` (including `std::ifstream`) gets the same bulk scanning: `input_stream_adapter` scans the characters its stream buffer already holds (the *get area*) and asks the stream buffer for more only when they are used up. Nothing is read ahead, so the stream is left exactly where it would be after reading one character at a time, and [`operator>>`](https://json.nlohmann.me/api/operator_gtgt/index.md) can still read several values from one stream. A stream buffer without a get area is read one character at a time. A `FILE*` is still read one character at a time, but `file_input_adapter` locks the file once per parse instead of once per character. See [input sources and performance](https://json.nlohmann.me/features/parsing/#input-sources-and-performance) for the full table. According to the description of [#5828](https://github.com/nlohmann/json/pull/5828), parsing from a stream is within about 10 to 20 % of parsing an equivalent `std::string`. -Practically: if the JSON text is already in memory, or small enough to read into memory, prefer passing a `std::string`, a `std::vector`, or a pointer range to `parse` over a `std::istream`. For a file, that means reading it into a string first and then parsing the string, rather than passing a `std::ifstream` directly to `parse` -- the latter never benefits from either optimization: +Inputs that are still read one character at a time: iterator ranges that are not contiguous (e.g. `std::list` or `std::istreambuf_iterator`), wide strings, and user-defined adapters. An iterator pair over non-contiguous but random-access storage (e.g. `std::deque::iterator`) gets the first optimization but not the second, since the byte-scanning fast path requires contiguous storage. + +Practically: a file can be passed directly to `parse`, which also avoids the memory for a copy of the file. Reading it into a string first is no longer necessary for speed: ``` -// gets the contiguous fast paths +// fast: the stream buffer's get area is scanned in bulk std::ifstream f("example.json"); -std::string contents((std::istreambuf_iterator(f)), std::istreambuf_iterator()); -json j = json::parse(contents); - -// does not: input_stream_adapter has no fast path -std::ifstream f2("example.json"); -json j2 = json::parse(f2); +json j = json::parse(f); ``` +Two macros switch the stream optimizations off without changing any result: [`JSON_DISABLE_STREAM_BULK_SCAN`](https://json.nlohmann.me/api/macros/json_disable_stream_bulk_scan/index.md) for stream buffers that do not follow the usual `std::streambuf` contract, and [`JSON_DISABLE_UNLOCKED_FILE_IO`](https://json.nlohmann.me/api/macros/json_disable_unlocked_file_io/index.md) for programs in which another thread must use the same `FILE` object while it is parsed. + For contiguous input with many non-ASCII characters, [`JSON_USE_SIMDUTF`](https://json.nlohmann.me/api/macros/json_use_simdutf/index.md) can additionally speed up UTF-8 validation by using the [simdutf](https://github.com/simdutf/simdutf) library instead of the built-in scalar validator; streaming inputs (files, `std::istream`, wide strings, user-defined adapters) always use the scalar path regardless of this macro. ## Large documents diff --git a/home/exceptions.md b/home/exceptions.md index 3b02d97a0..314c3d680 100644 --- a/home/exceptions.md +++ b/home/exceptions.md @@ -1106,9 +1106,19 @@ double-precision number when `number_float_t` is `#!cpp float`. ### json.exception.out_of_range.407 -An unsigned integer number greater than 9223372036854775807 cannot be represented in the binary format. This exception -is only thrown by [`to_bon8`](../api/basic_json/to_bon8.md), as [BON8](../features/binary_formats/bon8.md) has no -unsigned 64-bit integer type. +An integer number cannot be represented by the binary format it is serialized to: + +- [BON8](../features/binary_formats/bon8.md) only stores integers that fit into int64, so + [`to_bon8`](../api/basic_json/to_bon8.md) throws for unsigned integers greater than 9223372036854775807. +- [CBOR](../features/binary_formats/cbor.md), [MessagePack](../features/binary_formats/messagepack.md), and + [BSON](../features/binary_formats/bson.md) store integers in at most 64 bits. With the default number types, every + integer fits, but a [`number_integer_t`](../api/basic_json/number_integer_t.md) or + [`number_unsigned_t`](../api/basic_json/number_unsigned_t.md) wider than 64 bits (e.g., `__int128`) can hold values + outside the range of the format: [-2^64, 2^64-1] for CBOR, [-2^63, 2^64-1] for MessagePack, and the range of int64 + (signed integers) or uint64 (unsigned integers) for BSON. + +[UBJSON](../features/binary_formats/ubjson.md) and [BJData](../features/binary_formats/bjdata.md) never throw this +exception, because they serialize integers beyond 64 bits as high-precision numbers. !!! note "Former occurrences" @@ -1117,11 +1127,17 @@ unsigned 64-bit integer type. - Since version 3.9.0, integer numbers beyond int64 are serialized as high-precision UBJSON numbers. - Since version 3.12.0, integer numbers beyond int64 are serialized as uint64 BSON numbers. -!!! failure "Example message" + Before version 3.13.0, CBOR, MessagePack, and BSON silently truncated integers wider than 64 bits, and BJData + truncated unsigned integers wider than 64 bits. + +!!! failure "Example messages" ``` integer number 18446744073709551615 cannot be represented by BON8 as it does not fit int64 ``` + ``` + integer number 1267650600228229401496703205376 cannot be represented by CBOR as it does not fit [-2^64, 2^64-1] + ``` ### json.exception.out_of_range.408 diff --git a/home/exceptions/index.html b/home/exceptions/index.html index 46392e68e..20f12106c 100644 --- a/home/exceptions/index.html +++ b/home/exceptions/index.html @@ -257,8 +257,9 @@ exception id: 401

json.exception.out_of_range.405

The JSON Patch operations 'remove' and 'move' cannot be applied to the root element of the JSON value. The exception is also thrown when the parent or a reference token of an empty JSON Pointer is requested, for instance by back or pop_back.

Example message

JSON pointer has no parent

json.exception.out_of_range.406

A parsed number could not be stored without changing it to NaN or INF. For the binary formats, this happens when a finite floating-point number does not fit into number_float_t, for example a double-precision number when number_float_t is float.

Example messages

number overflow parsing '10E1000'
 
[json.exception.out_of_range.406] syntax error while parsing CBOR value: number overflow
-

json.exception.out_of_range.407

An unsigned integer number greater than 9223372036854775807 cannot be represented in the binary format. This exception is only thrown by to_bon8, as BON8 has no unsigned 64-bit integer type.

Former occurrences

The UBJSON and BSON writers used to throw this exception as well:

  • Since version 3.9.0, integer numbers beyond int64 are serialized as high-precision UBJSON numbers.
  • Since version 3.12.0, integer numbers beyond int64 are serialized as uint64 BSON numbers.

Example message

integer number 18446744073709551615 cannot be represented by BON8 as it does not fit int64
-

json.exception.out_of_range.408

The size of an array or object in a binary format exceeds the maximal capacity: the size following # for UBJSON/BJData, or the encoded length for CBOR.

The exception is also thrown for a UBJSON array of a type that is encoded by its marker alone (Z, T or F) whose declared count exceeds 1,048,576 (1 << 20). Such an array has no payload, so its count alone decides how much memory is allocated, and a handful of bytes would otherwise describe billions of values. to_ubjson writes longer arrays of these types without the size and type annotation, so any value it produces can still be read back.

Example messages

excessive array size: 8658170730974374167
+

json.exception.out_of_range.407

An integer number cannot be represented by the binary format it is serialized to:

  • BON8 only stores integers that fit into int64, so to_bon8 throws for unsigned integers greater than 9223372036854775807.
  • CBOR, MessagePack, and BSON store integers in at most 64 bits. With the default number types, every integer fits, but a number_integer_t or number_unsigned_t wider than 64 bits (e.g., __int128) can hold values outside the range of the format: [-2^64, 2^64-1] for CBOR, [-2^63, 2^64-1] for MessagePack, and the range of int64 (signed integers) or uint64 (unsigned integers) for BSON.

UBJSON and BJData never throw this exception, because they serialize integers beyond 64 bits as high-precision numbers.

Former occurrences

The UBJSON and BSON writers used to throw this exception as well:

  • Since version 3.9.0, integer numbers beyond int64 are serialized as high-precision UBJSON numbers.
  • Since version 3.12.0, integer numbers beyond int64 are serialized as uint64 BSON numbers.

Before version 3.13.0 unreleased, CBOR, MessagePack, and BSON silently truncated integers wider than 64 bits, and BJData truncated unsigned integers wider than 64 bits.

Example messages

integer number 18446744073709551615 cannot be represented by BON8 as it does not fit int64
+
integer number 1267650600228229401496703205376 cannot be represented by CBOR as it does not fit [-2^64, 2^64-1]
+

json.exception.out_of_range.408

The size of an array or object in a binary format exceeds the maximal capacity: the size following # for UBJSON/BJData, or the encoded length for CBOR.

The exception is also thrown for a UBJSON array of a type that is encoded by its marker alone (Z, T or F) whose declared count exceeds 1,048,576 (1 << 20). Such an array has no payload, so its count alone decides how much memory is allocated, and a handful of bytes would otherwise describe billions of values. to_ubjson writes longer arrays of these types without the size and type annotation, so any value it produces can still be read back.

Example messages

excessive array size: 8658170730974374167
 
[json.exception.out_of_range.408] syntax error while parsing CBOR size: excessive array size
 
[json.exception.out_of_range.408] syntax error while parsing CBOR size: excessive map size
 
[json.exception.out_of_range.408] syntax error while parsing UBJSON size: excessive array size
@@ -308,4 +309,4 @@ array index 18446744073709551616 exceeds size_type
 exception id: 501

json.exception.other_error.501

A JSON Patch operation 'test' failed. The unsuccessful operation is also printed.

Example message

Executing {"op":"test", "path":"/baz", "value":"bar"} on {"baz": "qux"}:

[json.exception.other_error.501] unsuccessful: {"op":"test","path":"/baz","value":"bar"}

json.exception.other_error.502

to_ubjson and to_bjdata were called with use_type = true but use_size = false. UBJSON requires a size marker (#) after a type marker ($).

Example message

[json.exception.other_error.502] use_type requires use_size = true
-

Note

This exception was added in version 3.13.0 unreleased. Before that, debug builds aborted on an assertion and release builds wrote a $ marker without #, which from_ubjson then rejected.

Copyright © 2013-2026 Niels Lohmann
\ No newline at end of file +

Note

This exception was added in version 3.13.0 unreleased. Before that, debug builds aborted on an assertion and release builds wrote a $ marker without #, which from_ubjson then rejected.

Copyright © 2013-2026 Niels Lohmann
\ No newline at end of file diff --git a/home/exceptions/index.md b/home/exceptions/index.md index 14be1d53a..b60954dbb 100644 --- a/home/exceptions/index.md +++ b/home/exceptions/index.md @@ -1078,7 +1078,12 @@ number overflow parsing '10E1000' ### json.exception.out_of_range.407 -An unsigned integer number greater than 9223372036854775807 cannot be represented in the binary format. This exception is only thrown by [`to_bon8`](https://json.nlohmann.me/api/basic_json/to_bon8/index.md), as [BON8](https://json.nlohmann.me/features/binary_formats/bon8/index.md) has no unsigned 64-bit integer type. +An integer number cannot be represented by the binary format it is serialized to: + +- [BON8](https://json.nlohmann.me/features/binary_formats/bon8/index.md) only stores integers that fit into int64, so [`to_bon8`](https://json.nlohmann.me/api/basic_json/to_bon8/index.md) throws for unsigned integers greater than 9223372036854775807. +- [CBOR](https://json.nlohmann.me/features/binary_formats/cbor/index.md), [MessagePack](https://json.nlohmann.me/features/binary_formats/messagepack/index.md), and [BSON](https://json.nlohmann.me/features/binary_formats/bson/index.md) store integers in at most 64 bits. With the default number types, every integer fits, but a [`number_integer_t`](https://json.nlohmann.me/api/basic_json/number_integer_t/index.md) or [`number_unsigned_t`](https://json.nlohmann.me/api/basic_json/number_unsigned_t/index.md) wider than 64 bits (e.g., `__int128`) can hold values outside the range of the format: [-2^64, 2^64-1] for CBOR, [-2^63, 2^64-1] for MessagePack, and the range of int64 (signed integers) or uint64 (unsigned integers) for BSON. + +[UBJSON](https://json.nlohmann.me/features/binary_formats/ubjson/index.md) and [BJData](https://json.nlohmann.me/features/binary_formats/bjdata/index.md) never throw this exception, because they serialize integers beyond 64 bits as high-precision numbers. Former occurrences @@ -1087,12 +1092,18 @@ The UBJSON and BSON writers used to throw this exception as well: - Since version 3.9.0, integer numbers beyond int64 are serialized as high-precision UBJSON numbers. - Since version 3.12.0, integer numbers beyond int64 are serialized as uint64 BSON numbers. -Example message +Before version 3.13.0 unreleased, CBOR, MessagePack, and BSON silently truncated integers wider than 64 bits, and BJData truncated unsigned integers wider than 64 bits. + +Example messages ``` integer number 18446744073709551615 cannot be represented by BON8 as it does not fit int64 ``` +``` +integer number 1267650600228229401496703205376 cannot be represented by CBOR as it does not fit [-2^64, 2^64-1] +``` + ### json.exception.out_of_range.408 The size of an array or object in a [binary format](https://json.nlohmann.me/features/binary_formats/index.md) exceeds the maximal capacity: the size following `#` for [UBJSON](https://json.nlohmann.me/features/binary_formats/ubjson/index.md)/[BJData](https://json.nlohmann.me/features/binary_formats/bjdata/index.md), or the encoded length for [CBOR](https://json.nlohmann.me/features/binary_formats/cbor/index.md). diff --git a/search/search_index.json b/search/search_index.json index 83f629bd0..5cab8aefe 100644 --- a/search/search_index.json +++ b/search/search_index.json @@ -1 +1 @@ -{"config":{"lang":["en"],"separator":"[\\s\\-\\.]","pipeline":["stopWordFilter"],"fields":{"title":{"boost":1000.0},"text":{"boost":1.0},"tags":{"boost":1000000.0}}},"docs":[{"location":"","title":"JSON for Modern C++","text":"

JSON for Modern C++ is a header-only C++11 library that turns JSON into a first-class C++ data type, using the operator magic of modern C++ so that creating, reading, and modifying JSON values feels as natural as it does in languages like Python. The whole library is available as a single header, json.hpp, with no dependencies, no subproject, and no complex build system to set up; a companion header, json_fwd.hpp, provides forward declarations to keep compile times down. See header-only integration for details. It is heavily unit-tested with 100% code coverage, checked with Valgrind and the Clang Sanitizers for memory leaks, and continuously fuzz-tested by Google OSS-Fuzz.

"},{"location":"#quick-start","title":"Quick start","text":"

Add the single header to your project and use the library like this:

#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // parse a JSON string\n    json j = json::parse(R\"({\"happy\": true, \"pi\": 3.141})\");\n\n    // access and modify values\n    j[\"name\"] = \"Niels\";\n    j[\"list\"] = {1, 0, 2};\n\n    // serialize with an indent of 4 spaces\n    std::cout << j.dump(4) << '\\n';\n}\n

Get the library by copying the single header json.hpp from the releases page into a directory nlohmann on your include path, or by installing it with a package manager:

brew install nlohmann-json    # Homebrew\nvcpkg install nlohmann-json   # vcpkg\n
find_package(nlohmann_json 3.12.0 REQUIRED)\ntarget_link_libraries(myproject PRIVATE nlohmann_json::nlohmann_json)\n

See Integration for CMake in detail, all supported package managers (Conan, Meson, Bazel, Conda, and more), and pkg-config.

"},{"location":"#explore-the-documentation","title":"Explore the documentation","text":"

  • Features

    Creating, parsing, accessing, and serializing JSON values, JSON Pointer/Patch, binary formats, and more.

    Features

  • Integration

    Add the library to your project via a single header, CMake, a package manager, or pkg-config.

    Integration

  • API documentation

    The complete reference for basic_json and its member functions, types, and related classes.

    API documentation

  • FAQ

    Answers to common questions and known surprises when using the library.

    FAQ

  • Releases

    What changed in each release, with links to the relevant documentation.

    Releases

  • Community

    The ecosystem, contribution guidelines, governance, and quality assurance around the project.

    Community

Unreleased changes

This documentation is built from the develop branch and may describe changes that are not part of a release yet. Their version numbers are followed by an unreleased badge; see Releases for what shipped in each version.

The library is licensed under the MIT License. The source code, issue tracker, and discussions are on GitHub.

"},{"location":"api/json/","title":"nlohmann::json","text":"
using json = basic_json<>;\n

This type is the default specialization of the basic_json class which uses the standard template types.

"},{"location":"api/json/#examples","title":"Examples","text":"Example

The example below demonstrates how to use the type nlohmann::json.

#include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON object\n    json j =\n    {\n        {\"pi\", 3.141},\n        {\"happy\", true},\n        {\"name\", \"Niels\"},\n        {\"nothing\", nullptr},\n        {\n            \"answer\", {\n                {\"everything\", 42}\n            }\n        },\n        {\"list\", {1, 0, 2}},\n        {\n            \"object\", {\n                {\"currency\", \"USD\"},\n                {\"value\", 42.99}\n            }\n        }\n    };\n\n    // add new values\n    j[\"new\"][\"key\"][\"value\"] = {\"another\", \"list\"};\n\n    // count elements\n    auto s = j.size();\n    j[\"size\"] = s;\n\n    // pretty print with indent of 4 spaces\n    std::cout << std::setw(4) << j << '\\n';\n}\n

Output:

{\n    \"answer\": {\n        \"everything\": 42\n    },\n    \"happy\": true,\n    \"list\": [\n        1,\n        0,\n        2\n    ],\n    \"name\": \"Niels\",\n    \"new\": {\n        \"key\": {\n            \"value\": [\n                \"another\",\n                \"list\"\n            ]\n        }\n    },\n    \"nothing\": null,\n    \"object\": {\n        \"currency\": \"USD\",\n        \"value\": 42.99\n    },\n    \"pi\": 3.141,\n    \"size\": 8\n}\n
"},{"location":"api/json/#see-also","title":"See also","text":"
  • basic_json - the underlying class template
  • ordered_json - specialization that preserves the insertion order of object keys
"},{"location":"api/json/#version-history","title":"Version history","text":"

Since version 1.0.0.

"},{"location":"api/json_document/","title":"nlohmann::json_document","text":"

Defined in header <nlohmann/json_view.hpp>

using json_document = basic_json_document<json>;\n

This type is a basic_json_document of the default json specialization.

"},{"location":"api/json_document/#examples","title":"Examples","text":"Example

The example below demonstrates how to use the type nlohmann::json_document.

#include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // json_document is basic_json_document<nlohmann::json>: same value types\n    // and containers as the ordinary json specialization\n    json_document doc = json_document::parse(R\"({\"pi\": 3.14, \"numbers\": [1, 2, 3]})\");\n\n    std::cout << doc.root().is_object() << '\\n';\n    std::cout << doc.root().materialize().dump() << '\\n';\n}\n

Output:

true\n{\"numbers\":[1,2,3],\"pi\":3.14}\n
"},{"location":"api/json_document/#see-also","title":"See also","text":"
  • json_view - a view of a value of a json_document
  • ordered_json_document - the corresponding document for ordered_json
"},{"location":"api/json_document/#version-history","title":"Version history","text":"

Since version 3.13.0 unreleased.

"},{"location":"api/json_editable_document/","title":"nlohmann::json_editable_document","text":"

Defined in header <nlohmann/json_view.hpp>

using json_editable_document = basic_json_document<json, true>;\n

This type is an editable basic_json_document of the default json specialization: in addition to everything json_document offers, set and push_back change values after parsing, without ever rewriting the source text -- see Edits and Editing a document.

"},{"location":"api/json_editable_document/#examples","title":"Examples","text":"Example

The example below patches two fields of a small configuration document -- changing one and adding another -- and dumps it back out with the member order and the spelling of the untouched number preserved, something a plain json value cannot do.

#include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json = nlohmann::json;\nusing json_editable_document = nlohmann::json_editable_document;\nusing json_editable_view = nlohmann::json_editable_view;\n\nint main()\n{\n    // a configuration file, as it might be read from disk\n    const std::string text = R\"({\"name\": \"cache\", \"host\": \"db1\", \"port\": 6379, \"price\": 19.90})\";\n    std::cout << text << \"\\n\\n\";\n\n    // patch two fields -- \"price\" is never touched\n    json_editable_document doc = json_editable_document::parse(text);\n    doc.set(doc.root(), \"host\", \"db2\");\n    doc.set(doc.root(), \"retries\", 3);\n\n    // member order (the new member at the end) and the untouched number's\n    // exact spelling survive\n    std::cout << doc.root().dump(-1, ' ', false, json_editable_view::number_format::source) << '\\n';\n\n    // the same patch on a plain json value: keys are sorted (object_t is a\n    // std::map), and \"price\" is rewritten even though the patch never\n    // touched it\n    json plain = json::parse(text);\n    plain[\"host\"] = \"db2\";\n    plain[\"retries\"] = 3;\n    std::cout << plain.dump() << '\\n';\n}\n

Output:

{\"name\": \"cache\", \"host\": \"db1\", \"port\": 6379, \"price\": 19.90}\n\n{\"name\":\"cache\",\"host\":\"db2\",\"port\":6379,\"price\":19.90,\"retries\":3}\n{\"host\":\"db2\",\"name\":\"cache\",\"port\":6379,\"price\":19.9,\"retries\":3}\n
"},{"location":"api/json_editable_document/#see-also","title":"See also","text":"
  • json_editable_view - a view of a value of a json_editable_document
  • json_document - the read-only document this type adds edits to
  • ordered_json_editable_document - the corresponding editable document for ordered_json
  • Edits - what an edit guarantees
"},{"location":"api/json_editable_document/#version-history","title":"Version history","text":"

Since version 3.13.0 unreleased.

"},{"location":"api/json_editable_view/","title":"nlohmann::json_editable_view","text":"

Defined in header <nlohmann/json_view.hpp>

using json_editable_view = basic_json_view<json, true>;\n

This type is a basic_json_view of a value of a json_editable_document. It offers the same read-only interface as json_view; what is different is what it can be a view of -- a value that set and push_back can change, with every view still referring to it seeing the change, see Edits.

"},{"location":"api/json_editable_view/#examples","title":"Examples","text":"Example

The example below is the same as json_editable_document's: every view read back out of the document -- doc.root() and the views nested under it -- sees the edits made through set.

#include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json = nlohmann::json;\nusing json_editable_document = nlohmann::json_editable_document;\nusing json_editable_view = nlohmann::json_editable_view;\n\nint main()\n{\n    // a configuration file, as it might be read from disk\n    const std::string text = R\"({\"name\": \"cache\", \"host\": \"db1\", \"port\": 6379, \"price\": 19.90})\";\n    std::cout << text << \"\\n\\n\";\n\n    // patch two fields -- \"price\" is never touched\n    json_editable_document doc = json_editable_document::parse(text);\n    doc.set(doc.root(), \"host\", \"db2\");\n    doc.set(doc.root(), \"retries\", 3);\n\n    // member order (the new member at the end) and the untouched number's\n    // exact spelling survive\n    std::cout << doc.root().dump(-1, ' ', false, json_editable_view::number_format::source) << '\\n';\n\n    // the same patch on a plain json value: keys are sorted (object_t is a\n    // std::map), and \"price\" is rewritten even though the patch never\n    // touched it\n    json plain = json::parse(text);\n    plain[\"host\"] = \"db2\";\n    plain[\"retries\"] = 3;\n    std::cout << plain.dump() << '\\n';\n}\n

Output:

{\"name\": \"cache\", \"host\": \"db1\", \"port\": 6379, \"price\": 19.90}\n\n{\"name\":\"cache\",\"host\":\"db2\",\"port\":6379,\"price\":19.90,\"retries\":3}\n{\"host\":\"db2\",\"name\":\"cache\",\"port\":6379,\"price\":19.9,\"retries\":3}\n
"},{"location":"api/json_editable_view/#see-also","title":"See also","text":"
  • json_editable_document - the document type this view refers into
  • json_view - the corresponding read-only view
  • ordered_json_editable_view - the corresponding view for ordered_json_editable_document
"},{"location":"api/json_editable_view/#version-history","title":"Version history","text":"

Since version 3.13.0 unreleased.

"},{"location":"api/json_view/","title":"nlohmann::json_view","text":"

Defined in header <nlohmann/json_view.hpp>

using json_view = basic_json_view<json>;\n

This type is a basic_json_view of a value of a json_document.

"},{"location":"api/json_view/#examples","title":"Examples","text":"Example

The example below demonstrates how to use the type nlohmann::json_view.

#include <iostream>\n#include <nlohmann/json_view.hpp>\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // json_view is basic_json_view<nlohmann::json>: a read-only handle\n    // returned by json_document::root()\n    nlohmann::json_document doc = nlohmann::json_document::parse(\"[1, 2, 3]\");\n    nlohmann::json_view v = doc.root();\n\n    std::cout << v.is_array() << ' ' << v.size() << '\\n';\n}\n

Output:

true 3\n
"},{"location":"api/json_view/#see-also","title":"See also","text":"
  • json_document - the document type this view refers into
  • ordered_json_view - the corresponding view for ordered_json_document
"},{"location":"api/json_view/#version-history","title":"Version history","text":"

Since version 3.13.0 unreleased.

"},{"location":"api/operator_gtgt/","title":"nlohmann::operator>>(basic_json)","text":"
std::istream& operator>>(std::istream& i, basic_json& j);\n

Deserializes an input stream to a JSON value.

"},{"location":"api/operator_gtgt/#parameters","title":"Parameters","text":"i (in, out) input stream to read a serialized JSON value from j (in, out) JSON value to write the deserialized input to"},{"location":"api/operator_gtgt/#return-value","title":"Return value","text":"

the stream i

"},{"location":"api/operator_gtgt/#exception-safety","title":"Exception safety","text":"

Strong guarantee: if an exception is thrown, there are no changes in j.

"},{"location":"api/operator_gtgt/#exceptions","title":"Exceptions","text":"
  • Throws parse_error.101 in case of an unexpected token, or if i has no stream buffer (i.rdbuf() == nullptr, for instance std::istream(nullptr)).
  • If reading from i reaches the end of the input and eofbit is part of i's exceptions() mask, the std::ios_base::failure thrown by i itself propagates instead of a parse_error, the same as it would for the standard library's own extraction operators.
"},{"location":"api/operator_gtgt/#complexity","title":"Complexity","text":"

Linear in the length of the input. The parser is a predictive LL(1) parser.

"},{"location":"api/operator_gtgt/#notes","title":"Notes","text":"

A UTF-8 byte order mark is silently ignored.

Invalid Unicode escapes and unpaired surrogates in the input are reported as parse_error.101 with a detailed message.

operator>> parses exactly one JSON value, so it can be called repeatedly to read a sequence of concatenated JSON values from the same stream:

json j1, j2;\ninput >> j1;  // parses the first value\ninput >> j2;  // parses the next value\n

A number must be followed by whitespace

A number is only terminated by the character that follows it. That character is read from the stream to detect the end of the number, and it is not put back. When a value that is a number is immediately followed by the next value, the first character of that next value is lost:

std::istringstream input(\"1true\");\njson j1, j2;\ninput >> j1;  // j1 == 1\ninput >> j2;  // throws parse_error.101: the stream now starts at \"rue\"\n

Separating the values with whitespace avoids this, because the character that is eaten is then the separator:

std::istringstream input(\"1 true\");\njson j1, j2;\ninput >> j1;  // j1 == 1\ninput >> j2;  // j2 == true\n

Only numbers are affected. Values ending in a self-delimiting character do not read past themselves, so truefalse, [1][2], {\"a\":1}{\"b\":2}, and \"a\"\"b\" can be read back to back without a separator.

Define JSON_PRECISE_STREAM_POSITION to 1 to leave the terminating character in the stream instead, so that the stream is positioned right after the value for every value type and no separator is needed. This is tracked in #5340.

Note that reading concatenated values does not work for JSON Lines (newline-delimited JSON) input -- see that page for why and for the recommended alternative.

By default, a '\\0' (NUL) byte encountered while reading a value is treated as end of input, rather than as an ordinary (and, outside of a string, invalid) byte; see the FAQ entry for details and the JSON_STRICT_NUL_HANDLING macro to opt into rejecting it instead. Because operator>> only parses a single value and does not require the rest of the stream to be consumed, a NUL byte after a complete value has no effect on operator>> either way; it only matters while a value is still being read.

Deprecation

This function replaces function std::istream& operator<<(basic_json& j, std::istream& i) which has been deprecated in version 3.0.0. It will be removed in version 4.0.0. Please replace calls like j << i; with i >> j;.

See the migration guide for how to update existing code.

"},{"location":"api/operator_gtgt/#examples","title":"Examples","text":"Example

The example below shows how a JSON value is constructed by reading a serialization from a stream.

#include <iostream>\n#include <iomanip>\n#include <sstream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create stream with serialized JSON\n    std::stringstream ss;\n    ss << R\"({\n        \"number\": 23,\n        \"string\": \"Hello, world!\",\n        \"array\": [1, 2, 3, 4, 5],\n        \"boolean\": false,\n        \"null\": null\n    })\";\n\n    // create JSON value and read the serialization from the stream\n    json j;\n    ss >> j;\n\n    // serialize JSON\n    std::cout << std::setw(2) << j << '\\n';\n}\n

Output:

{\n  \"array\": [\n    1,\n    2,\n    3,\n    4,\n    5\n  ],\n  \"boolean\": false,\n  \"null\": null,\n  \"number\": 23,\n  \"string\": \"Hello, world!\"\n}\n
"},{"location":"api/operator_gtgt/#see-also","title":"See also","text":"
  • accept - check if the input is valid JSON
  • parse - deserialize from a compatible input
  • JSON_STRICT_NUL_HANDLING - opt in to rejecting a NUL byte in the input instead of treating it as end of input
  • JSON_PRECISE_STREAM_POSITION - opt in to leaving the stream positioned right after a number
  • JSON_DISABLE_STREAM_BULK_SCAN - read the stream one character at a time
"},{"location":"api/operator_gtgt/#version-history","title":"Version history","text":"
  • Added in version 1.0.0.
  • JSON_STRICT_NUL_HANDLING added in version 3.13.0 unreleased to optionally reject a NUL byte in the input instead of treating it as end of input; planned to become the default in version 4.0.0.
  • JSON_PRECISE_STREAM_POSITION added in version 3.13.0 unreleased to optionally leave the character that terminates a number in the stream; planned to become the default in version 4.0.0.
  • Fixed a null pointer dereference for an std::istream without a stream buffer (now throws parse_error.101), and a crash (std::terminate) when i has eofbit in its exception mask, in version 3.13.0 unreleased.
  • Changed to the strong exception safety guarantee in version 3.13.0 unreleased: j is no longer left with a partially parsed value if parsing throws.
  • Scans the characters in the stream buffer of i in bulk in version 3.13.0 unreleased; the position of i after the call is unchanged (JSON_DISABLE_STREAM_BULK_SCAN restores reading one character at a time).
"},{"location":"api/operator_literal_json/","title":"nlohmann::operator\"\"_json","text":"
json operator \"\"_json(const char* s, std::size_t n);\njson operator \"\"_json(const char8_t* s, std::size_t n);  // since C++20\n

This operator implements a user-defined string literal for JSON objects. It can be used by adding _json to a string literal and returns a json object if no parse error occurred.

It is recommended to bring the operator into scope using any of the following lines: 

using nlohmann::literals::operator \"\"_json;\nusing namespace nlohmann::literals;\nusing namespace nlohmann::json_literals;\nusing namespace nlohmann::literals::json_literals;\nusing namespace nlohmann;\n

This is suggested to ease migration to the next major version release of the library. See JSON_USE_GLOBAL_UDLS and the migration guide for details. The operator is declared in header <nlohmann/json_literals.hpp>, which <nlohmann/json.hpp> includes unless JSON_NO_AUTOMATIC_UDLS is defined.

"},{"location":"api/operator_literal_json/#parameters","title":"Parameters","text":"s (in) a string representation of a JSON object n (in) length of string s"},{"location":"api/operator_literal_json/#return-value","title":"Return value","text":"

json value parsed from s

"},{"location":"api/operator_literal_json/#exceptions","title":"Exceptions","text":"

The function can throw anything that parse(s, s+n) would throw.

"},{"location":"api/operator_literal_json/#complexity","title":"Complexity","text":"

Linear.

"},{"location":"api/operator_literal_json/#examples","title":"Examples","text":"Example

The following code shows how to create JSON values from string literals.

#include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    json j = R\"( {\"hello\": \"world\", \"answer\": 42} )\"_json;\n\n    std::cout << std::setw(2) << j << '\\n';\n}\n

Output:

{\n  \"answer\": 42,\n  \"hello\": \"world\"\n}\n
"},{"location":"api/operator_literal_json/#see-also","title":"See also","text":"
  • Creating JSON values - the article on creating JSON values
  • JSON_NO_AUTOMATIC_UDLS - do not include the user-defined string literals automatically
"},{"location":"api/operator_literal_json/#version-history","title":"Version history","text":"
  • Added in version 1.0.0.
  • Moved to namespace nlohmann::literals::json_literals in 3.11.0.
  • Added char8_t* overload in version 3.13.0 unreleased.
"},{"location":"api/operator_literal_json_pointer/","title":"nlohmann::operator\"\"_json_pointer","text":"
json_pointer operator \"\"_json_pointer(const char* s, std::size_t n);\njson_pointer operator \"\"_json_pointer(const char8_t* s, std::size_t n);  // since C++20\n

This operator implements a user-defined string literal for JSON Pointers. It can be used by adding _json_pointer to a string literal and returns a json_pointer object if no parse error occurred.

It is recommended to bring the operator into scope using any of the following lines: 

using nlohmann::literals::operator \"\"_json_pointer;\nusing namespace nlohmann::literals;\nusing namespace nlohmann::json_literals;\nusing namespace nlohmann::literals::json_literals;\nusing namespace nlohmann;\n
This is suggested to ease migration to the next major version release of the library. See JSON_USE_GLOBAL_UDLS and the migration guide for details. The operator is declared in header <nlohmann/json_literals.hpp>, which <nlohmann/json.hpp> includes unless JSON_NO_AUTOMATIC_UDLS is defined.

"},{"location":"api/operator_literal_json_pointer/#parameters","title":"Parameters","text":"s (in) a string representation of a JSON Pointer n (in) length of string s"},{"location":"api/operator_literal_json_pointer/#return-value","title":"Return value","text":"

json_pointer value parsed from s

"},{"location":"api/operator_literal_json_pointer/#exceptions","title":"Exceptions","text":"

The function can throw anything that json_pointer::json_pointer would throw.

"},{"location":"api/operator_literal_json_pointer/#complexity","title":"Complexity","text":"

Linear.

"},{"location":"api/operator_literal_json_pointer/#examples","title":"Examples","text":"Example

The following code shows how to create JSON Pointers from string literals.

#include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    json j = R\"( {\"hello\": \"world\", \"answer\": 42} )\"_json;\n    auto val = j[\"/hello\"_json_pointer];\n\n    std::cout << std::setw(2) << val << '\\n';\n}\n

Output:

\"world\"\n
"},{"location":"api/operator_literal_json_pointer/#see-also","title":"See also","text":"
  • json_pointer - type to represent JSON Pointers
  • JSON_NO_AUTOMATIC_UDLS - do not include the user-defined string literals automatically
"},{"location":"api/operator_literal_json_pointer/#version-history","title":"Version history","text":"
  • Added in version 2.0.0.
  • Moved to namespace nlohmann::literals::json_literals in 3.11.0.
  • Added char8_t* overload in version 3.13.0 unreleased.
"},{"location":"api/operator_ltlt/","title":"nlohmann::operator<<(basic_json), nlohmann::operator<<(json_pointer)","text":"
std::ostream& operator<<(std::ostream& o, const basic_json& j);      // (1)\n\nstd::ostream& operator<<(std::ostream& o, const json_pointer& ptr);  // (2)\n
  1. Serialize the given JSON value j to the output stream o. The JSON value will be serialized using the dump member function.
    • The indentation of the output can be controlled with the member variable width of the output stream o. For instance, using the manipulator std::setw(4) on o sets the indentation level to 4 and the serialization result is the same as calling dump(4).
    • The indentation character can be controlled with the member variable fill of the output stream o. For instance, the manipulator std::setfill('\\\\t') sets indentation to use a tab character rather than the default space character.
  2. Write a string representation of the given JSON pointer ptr to the output stream o. The string representation is obtained using the to_string member function.
"},{"location":"api/operator_ltlt/#parameters","title":"Parameters","text":"o (in, out) stream to write to j (in) JSON value to serialize ptr (in) JSON pointer to write"},{"location":"api/operator_ltlt/#return-value","title":"Return value","text":"

the stream o

"},{"location":"api/operator_ltlt/#exceptions","title":"Exceptions","text":"
  1. Throws type_error.316 if a string stored inside the JSON value is not UTF-8 encoded. Note that unlike the dump member functions, no error_handler can be set.
  2. None.
"},{"location":"api/operator_ltlt/#complexity","title":"Complexity","text":"

Linear.

"},{"location":"api/operator_ltlt/#notes","title":"Notes","text":"

Deprecation

Function std::ostream& operator<<(std::ostream& o, const basic_json& j) replaces function std::ostream& operator>>(const basic_json& j, std::ostream& o) which has been deprecated in version 3.0.0. It will be removed in version 4.0.0. Please replace calls like j >> o; with o << j;.

See the migration guide for how to update existing code.

"},{"location":"api/operator_ltlt/#examples","title":"Examples","text":"Example: (1) serialize JSON value to stream

The example below shows the serialization with different parameters to width to adjust the indentation level.

#include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n\n    // serialize without indentation\n    std::cout << j_object << \"\\n\\n\";\n    std::cout << j_array << \"\\n\\n\";\n\n    // serialize with indentation\n    std::cout << std::setw(4) << j_object << \"\\n\\n\";\n    std::cout << std::setw(2) << j_array << \"\\n\\n\";\n    std::cout << std::setw(1) << std::setfill('\\t') << j_object << \"\\n\\n\";\n}\n

Output:

{\"one\":1,\"two\":2}\n\n[1,2,4,8,16]\n\n{\n    \"one\": 1,\n    \"two\": 2\n}\n\n[\n  1,\n  2,\n  4,\n  8,\n  16\n]\n\n{\n    \"one\": 1,\n    \"two\": 2\n}\n
Example: (2) write JSON pointer to stream

The example below shows how to write a JSON pointer to a stream.

#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON pointer\n    json::json_pointer ptr(\"/foo/bar/baz\");\n\n    // write string representation to stream\n    std::cout << ptr << std::endl;\n}\n

Output:

/foo/bar/baz\n
"},{"location":"api/operator_ltlt/#see-also","title":"See also","text":"
  • dump - serialize to a JSON-formatted string
  • basic_json_view::operator<< - the corresponding operator for basic_json_view
  • Serialization - the serialization article
"},{"location":"api/operator_ltlt/#version-history","title":"Version history","text":"
  1. Added in version 1.0.0. Added support for indentation character and deprecated std::ostream& operator>>(const basic_json& j, std::ostream& o) in version 3.0.0.
  2. Added in version 3.11.0.
"},{"location":"api/ordered_json/","title":"nlohmann::ordered_json","text":"
using ordered_json = basic_json<ordered_map>;\n

This type preserves the insertion order of object keys.

"},{"location":"api/ordered_json/#iterator-invalidation","title":"Iterator invalidation","text":"

The type is based on ordered_map which in turn uses a std::vector to store object elements. Therefore, adding object elements can yield a reallocation in which case all iterators (including the end() iterator) and all references to the elements are invalidated. Also, any iterator or reference after the insertion point will point to the same index, which is now a different value.

"},{"location":"api/ordered_json/#complexity","title":"Complexity","text":"

ordered_map has no lookup index: every key-based object operation is a linear scan, so building or parsing an object of n keys costs O(n\u00b2) rather than O(n log n). See ordered_map complexity for the per-operation table and for measured numbers.

"},{"location":"api/ordered_json/#examples","title":"Examples","text":"Example

The example below demonstrates how ordered_json preserves the insertion order of object keys.

#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing ordered_json = nlohmann::ordered_json;\n\nint main()\n{\n    ordered_json j;\n    j[\"one\"] = 1;\n    j[\"two\"] = 2;\n    j[\"three\"] = 3;\n\n    std::cout << j.dump(2) << '\\n';\n}\n

Output:

{\n  \"one\": 1,\n  \"two\": 2,\n  \"three\": 3\n}\n
"},{"location":"api/ordered_json/#see-also","title":"See also","text":"
  • ordered_map
  • Object Order
"},{"location":"api/ordered_json/#version-history","title":"Version history","text":"

Since version 3.9.0.

"},{"location":"api/ordered_json_document/","title":"nlohmann::ordered_json_document","text":"

Defined in header <nlohmann/json_view.hpp>

using ordered_json_document = basic_json_document<ordered_json>;\n

This type is a basic_json_document of the ordered_json specialization: materialize() on one of its views preserves the insertion order of object keys, instead of sorting them like json_document does.

"},{"location":"api/ordered_json_document/#examples","title":"Examples","text":"Example

The example below demonstrates how ordered_json_document preserves the insertion order of object keys when materializing.

#include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing ordered_json_document = nlohmann::ordered_json_document;\n\nint main()\n{\n    // ordered_json_document is basic_json_document<nlohmann::ordered_json>:\n    // materialize() preserves the insertion (source) order of object keys,\n    // instead of sorting them like json_document does\n    ordered_json_document doc = ordered_json_document::parse(R\"({\"z\": 1, \"a\": 2, \"m\": 3})\");\n    std::cout << doc.root().materialize().dump() << '\\n';\n}\n

Output:

{\"z\":1,\"a\":2,\"m\":3}\n
"},{"location":"api/ordered_json_document/#see-also","title":"See also","text":"
  • ordered_json_view - a view of a value of an ordered_json_document
  • json_document - the corresponding document for the default json specialization
  • Object Order
"},{"location":"api/ordered_json_document/#version-history","title":"Version history","text":"

Since version 3.13.0 unreleased.

"},{"location":"api/ordered_json_editable_document/","title":"nlohmann::ordered_json_editable_document","text":"

Defined in header <nlohmann/json_view.hpp>

using ordered_json_editable_document = basic_json_document<ordered_json, true>;\n

This type is an editable basic_json_document of the ordered_json specialization: set and push_back change values after parsing, as for json_editable_document, and materialize() preserves the document order of object members -- including members set added -- instead of sorting them like json_editable_document does.

"},{"location":"api/ordered_json_editable_document/#examples","title":"Examples","text":"Example

The example below edits a document with set, then shows that materialize() keeps the member order of the source text (with the new member at the end) for ordered_json_editable_document, where it would sort the members alphabetically for json_editable_document.

#include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing ordered_json_editable_document = nlohmann::ordered_json_editable_document;\n\nint main()\n{\n    // ordered_json_editable_document is basic_json_document<nlohmann::ordered_json, true>\n    ordered_json_editable_document doc = ordered_json_editable_document::parse(R\"({\"z\": 1, \"a\": 2, \"m\": 3})\");\n    doc.set(doc.root(), \"b\", 4); // set() always appends a new member at the end\n\n    // materialize() preserves the document order (with \"b\" at the end),\n    // instead of sorting the keys the way json_editable_document does\n    std::cout << doc.root().materialize().dump() << '\\n';\n}\n

Output:

{\"z\":1,\"a\":2,\"m\":3,\"b\":4}\n
"},{"location":"api/ordered_json_editable_document/#see-also","title":"See also","text":"
  • ordered_json_editable_view - a view of a value of an ordered_json_editable_document
  • ordered_json_document - the read-only document this type adds edits to
  • json_editable_document - the corresponding editable document for the default json specialization
  • Object Order
  • Edits - what an edit guarantees
"},{"location":"api/ordered_json_editable_document/#version-history","title":"Version history","text":"

Since version 3.13.0 unreleased.

"},{"location":"api/ordered_json_editable_view/","title":"nlohmann::ordered_json_editable_view","text":"

Defined in header <nlohmann/json_view.hpp>

using ordered_json_editable_view = basic_json_view<ordered_json, true>;\n

This type is a basic_json_view of a value of an ordered_json_editable_document, the corresponding view for ordered_json_view the way json_editable_view is for json_view.

"},{"location":"api/ordered_json_editable_view/#examples","title":"Examples","text":"Example

The example below is the same as ordered_json_editable_document's: the views set returns see the document's member order preserved on materialize(), unlike for a json_editable_document.

#include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing ordered_json_editable_document = nlohmann::ordered_json_editable_document;\n\nint main()\n{\n    // ordered_json_editable_document is basic_json_document<nlohmann::ordered_json, true>\n    ordered_json_editable_document doc = ordered_json_editable_document::parse(R\"({\"z\": 1, \"a\": 2, \"m\": 3})\");\n    doc.set(doc.root(), \"b\", 4); // set() always appends a new member at the end\n\n    // materialize() preserves the document order (with \"b\" at the end),\n    // instead of sorting the keys the way json_editable_document does\n    std::cout << doc.root().materialize().dump() << '\\n';\n}\n

Output:

{\"z\":1,\"a\":2,\"m\":3,\"b\":4}\n
"},{"location":"api/ordered_json_editable_view/#see-also","title":"See also","text":"
  • ordered_json_editable_document - the document type this view refers into
  • ordered_json_view - the corresponding read-only view
  • json_editable_view - the corresponding view for json_editable_document
"},{"location":"api/ordered_json_editable_view/#version-history","title":"Version history","text":"

Since version 3.13.0 unreleased.

"},{"location":"api/ordered_json_view/","title":"nlohmann::ordered_json_view","text":"

Defined in header <nlohmann/json_view.hpp>

using ordered_json_view = basic_json_view<ordered_json>;\n

This type is a basic_json_view of a value of an ordered_json_document.

"},{"location":"api/ordered_json_view/#examples","title":"Examples","text":"Example

The example below demonstrates how to use the type nlohmann::ordered_json_view.

#include <iostream>\n#include <nlohmann/json_view.hpp>\n\nint main()\n{\n    nlohmann::ordered_json_document doc = nlohmann::ordered_json_document::parse(R\"({\"z\": 1, \"a\": 2})\");\n    nlohmann::ordered_json_view v = doc.root();\n\n    std::cout << std::boolalpha << v.is_object() << ' ' << v.size() << '\\n';\n}\n

Output:

true 2\n
"},{"location":"api/ordered_json_view/#see-also","title":"See also","text":"
  • ordered_json_document - the document type this view refers into
  • json_view - the corresponding view for json_document
"},{"location":"api/ordered_json_view/#version-history","title":"Version history","text":"

Since version 3.13.0 unreleased.

"},{"location":"api/ordered_map/","title":"nlohmann::ordered_map","text":"
template<class Key, class T, class IgnoredLess = std::less<Key>,\n         class Allocator = std::allocator<std::pair<const Key, T>>>\nstruct ordered_map : std::vector<std::pair<const Key, T>, Allocator>;\n

A minimal map-like container that preserves insertion order for use within nlohmann::ordered_json (nlohmann::basic_json<ordered_map>).

"},{"location":"api/ordered_map/#template-parameters","title":"Template parameters","text":"Key key type T mapped type IgnoredLess comparison function (ignored and only added to ensure compatibility with std::map) Allocator allocator type"},{"location":"api/ordered_map/#iterator-invalidation","title":"Iterator invalidation","text":"

The type uses a std::vector to store object elements. Therefore, adding elements can yield a reallocation in which case all iterators (including the end() iterator) and all references to the elements are invalidated.

When the storage grows, the keys are copied and the mapped values are moved to the new storage. A plain std::vector would copy the whole elements instead, because their const keys make them not nothrow move constructible; for ordered_json, this would be a deep copy of every nested value. The values are only copied if T is not default constructible or not nothrow move assignable.

"},{"location":"api/ordered_map/#member-types","title":"Member types","text":"
  • key_type - key type (Key)
  • mapped_type - mapped type (T)
  • Container - base container type (std::vector<std::pair<const Key, T>, Allocator>)
  • iterator
  • const_iterator
  • size_type
  • value_type
  • key_compare - key comparison function 
    std::equal_to<Key>  // until C++14\n\nstd::equal_to<>     // since C++14\n
"},{"location":"api/ordered_map/#member-functions","title":"Member functions","text":"
  • (constructor)
  • (destructor)
  • emplace
  • operator[]
  • at
  • erase
  • count
  • find
  • insert
"},{"location":"api/ordered_map/#exception-safety","title":"Exception safety","text":"

emplace, operator[], and insert(value) have the strong exception guarantee: if an exception is thrown (for instance, because copying a key or allocating memory fails), the contents of the container are unchanged.

"},{"location":"api/ordered_map/#complexity","title":"Complexity","text":"

Because the elements are stored in a std::vector in insertion order, there is no index to look a key up by. Every key-based operation performs a linear scan over the stored elements. With n denoting the number of elements in the container:

Operation Complexity Note emplace O(n) scans for an existing key, then appends (amortized O(1)) operator[] O(n) delegates to emplace (non-const) or at (const) at O(n) throws std::out_of_range if the key is not found find O(n) count O(n) the result is always 0 or 1 erase(key) O(n) scan, then move the remaining elements one position down erase(pos), erase(first, last) O(n) moves all elements after the erased range insert(value) O(n) equivalent to emplace insert(first, last) O((n + m) * m) for m inserted elements

This differs from std::map, where the same operations are O(log n).

Quadratic cost of building large objects

Because every insertion scans all elements inserted so far, building an object of n distinct keys costs O(n\u00b2) in total. This applies to filling an ordered_json object key by key as well as to parsing one, since the parser inserts each key as it is read.

The cost is negligible for the object sizes typically found in configuration files or API payloads, but it grows steeply for machine-generated objects with many thousands of keys. Measured with -O2 -DNDEBUG for parsing a flat object of n keys, relative to nlohmann::json (which uses std::map):

n json ordered_json factor 2000 0.7 ms 3.6 ms 5\u00d7 4000 0.8 ms 14.0 ms 19\u00d7 8000 1.6 ms 67.8 ms 43\u00d7 16 000 3.3 ms 181.6 ms 54\u00d7

If key order matters for objects of that size, consider a container with a lookup index, such as nlohmann::fifo_map (integration), as the object type -- see object order.

"},{"location":"api/ordered_map/#examples","title":"Examples","text":"Example

The example shows the different behavior of std::map and nlohmann::ordered_map.

#include <iostream>\n#include <nlohmann/json.hpp>\n\n// simple output function\ntemplate<typename Map>\nvoid output(const char* prefix, const Map& m)\n{\n    std::cout << prefix << \" = { \";\n    for (auto& element : m)\n    {\n        std::cout << element.first << \":\" << element.second << ' ';\n    }\n    std::cout << \"}\" << std::endl;\n}\n\nint main()\n{\n    // create and fill two maps\n    nlohmann::ordered_map<std::string, std::string> m_ordered;\n    m_ordered[\"one\"] = \"eins\";\n    m_ordered[\"two\"] = \"zwei\";\n    m_ordered[\"three\"] = \"drei\";\n\n    std::map<std::string, std::string> m_std;\n    m_std[\"one\"] = \"eins\";\n    m_std[\"two\"] = \"zwei\";\n    m_std[\"three\"] = \"drei\";\n\n    // output: m_ordered is ordered by insertion order, m_std is ordered by key\n    output(\"m_ordered\", m_ordered);\n    output(\"m_std\", m_std);\n\n    // erase and re-add \"one\" key\n    m_ordered.erase(\"one\");\n    m_ordered[\"one\"] = \"eins\";\n\n    m_std.erase(\"one\");\n    m_std[\"one\"] = \"eins\";\n\n    // output: m_ordered shows newly added key at the end; m_std is again ordered by key\n    output(\"m_ordered\", m_ordered);\n    output(\"m_std\", m_std);\n}\n

Output:

m_ordered = { one:eins two:zwei three:drei }\nm_std = { one:eins three:drei two:zwei }\nm_ordered = { two:zwei three:drei one:eins }\nm_std = { one:eins three:drei two:zwei }\n
"},{"location":"api/ordered_map/#see-also","title":"See also","text":"
  • ordered_json
"},{"location":"api/ordered_map/#version-history","title":"Version history","text":"
  • Added in version 3.9.0 to implement nlohmann::ordered_json.
  • Added key_compare member in version 3.11.0.
  • Changed in version 3.13.0 unreleased: growing the storage moves the mapped values instead of copying them.
"},{"location":"api/adl_serializer/","title":"nlohmann::adl_serializer","text":"
template<typename, typename>\nstruct adl_serializer;\n

Serializer that uses ADL (Argument-Dependent Lookup) to choose to_json/from_json functions from the types' namespaces.

It is implemented similarly to

template<typename ValueType>\nstruct adl_serializer {\n    template<typename BasicJsonType>\n    static void to_json(BasicJsonType& j, const T& value) {\n        // calls the \"to_json\" method in T's namespace\n    }\n\n    template<typename BasicJsonType>\n    static void from_json(const BasicJsonType& j, T& value) {\n        // same thing, but with the \"from_json\" method\n    }\n};\n
"},{"location":"api/adl_serializer/#member-functions","title":"Member functions","text":"
  • from_json - convert a JSON value to any value type
  • to_json - convert any value type to a JSON value
"},{"location":"api/adl_serializer/#version-history","title":"Version history","text":"
  • Added in version 2.1.0.
"},{"location":"api/adl_serializer/from_json/","title":"nlohmann::adl_serializer::from_json","text":"
// (1)\ntemplate<typename BasicJsonType, typename TargetType = ValueType>\nstatic auto from_json(BasicJsonType && j, TargetType& val) noexcept(\n    noexcept(::nlohmann::from_json(std::forward<BasicJsonType>(j), val)))\n-> decltype(::nlohmann::from_json(std::forward<BasicJsonType>(j), val), void())\n\n// (2)\ntemplate<typename BasicJsonType, typename TargetType = ValueType>\nstatic auto from_json(BasicJsonType && j) noexcept(\nnoexcept(::nlohmann::from_json(std::forward<BasicJsonType>(j), detail::identity_tag<TargetType> {})))\n-> decltype(::nlohmann::from_json(std::forward<BasicJsonType>(j), detail::identity_tag<TargetType> {}))\n

This function is usually called by the get() function of the basic_json class (either explicitly or via the conversion operators).

  1. This function is chosen for default-constructible value types.
  2. This function is chosen for value types which are not default-constructible.
"},{"location":"api/adl_serializer/from_json/#parameters","title":"Parameters","text":"j (in) JSON value to read from val (out) value to write to"},{"location":"api/adl_serializer/from_json/#return-value","title":"Return value","text":"
  1. (none) -- the converted value is written to the output parameter val.
  2. the JSON value j converted to TargetType
"},{"location":"api/adl_serializer/from_json/#examples","title":"Examples","text":"Example: (1) Default-constructible type

The example below shows how a from_json function can be implemented for a user-defined type. This function is called by the adl_serializer when get<ns::person>() is called.

#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nnamespace ns\n{\n// a simple struct to model a person\nstruct person\n{\n    std::string name;\n    std::string address;\n    int age;\n};\n} // namespace ns\n\nnamespace ns\n{\nvoid from_json(const json& j, person& p)\n{\n    j.at(\"name\").get_to(p.name);\n    j.at(\"address\").get_to(p.address);\n    j.at(\"age\").get_to(p.age);\n}\n} // namespace ns\n\nint main()\n{\n    json j;\n    j[\"name\"] = \"Ned Flanders\";\n    j[\"address\"] = \"744 Evergreen Terrace\";\n    j[\"age\"] = 60;\n\n    auto p = j.get<ns::person>();\n\n    std::cout << p.name << \" (\" << p.age << \") lives in \" << p.address << std::endl;\n}\n

Output:

Ned Flanders (60) lives in 744 Evergreen Terrace\n
Example: (2) Non-default-constructible type

The example below shows how a from_json is implemented as part of a specialization of the adl_serializer to realize the conversion of a non-default-constructible type.

#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nnamespace ns\n{\n// a simple struct to model a person (not default constructible)\nstruct person\n{\n    person(std::string n, std::string a, int aa)\n        : name(std::move(n)), address(std::move(a)), age(aa)\n    {}\n\n    std::string name;\n    std::string address;\n    int age;\n};\n} // namespace ns\n\nnamespace nlohmann\n{\ntemplate <>\nstruct adl_serializer<ns::person>\n{\n    static ns::person from_json(const json& j)\n    {\n        return {j.at(\"name\"), j.at(\"address\"), j.at(\"age\")};\n    }\n\n    // Here's the catch! You must provide a to_json method! Otherwise, you\n    // will not be able to convert person to json, since you fully\n    // specialized adl_serializer on that type\n    static void to_json(json& j, ns::person p)\n    {\n        j[\"name\"] = p.name;\n        j[\"address\"] = p.address;\n        j[\"age\"] = p.age;\n    }\n};\n} // namespace nlohmann\n\nint main()\n{\n    json j;\n    j[\"name\"] = \"Ned Flanders\";\n    j[\"address\"] = \"744 Evergreen Terrace\";\n    j[\"age\"] = 60;\n\n    auto p = j.get<ns::person>();\n\n    std::cout << p.name << \" (\" << p.age << \") lives in \" << p.address << std::endl;\n}\n

Output:

Ned Flanders (60) lives in 744 Evergreen Terrace\n
"},{"location":"api/adl_serializer/from_json/#see-also","title":"See also","text":"
  • to_json
"},{"location":"api/adl_serializer/from_json/#version-history","title":"Version history","text":"
  • Added in version 2.1.0.
"},{"location":"api/adl_serializer/to_json/","title":"nlohmann::adl_serializer::to_json","text":"
template<typename BasicJsonType, typename TargetType = ValueType>\nstatic auto to_json(BasicJsonType& j, TargetType && val) noexcept(\n    noexcept(::nlohmann::to_json(j, std::forward<TargetType>(val))))\n-> decltype(::nlohmann::to_json(j, std::forward<TargetType>(val)), void())\n

This function is usually called by the constructors of the basic_json class.

"},{"location":"api/adl_serializer/to_json/#parameters","title":"Parameters","text":"j (out) JSON value to write to val (in) value to read from"},{"location":"api/adl_serializer/to_json/#examples","title":"Examples","text":"Example

The example below shows how a to_json function can be implemented for a user-defined type. This function is called by the adl_serializer when the constructor basic_json(ns::person) is called.

#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nnamespace ns\n{\n// a simple struct to model a person\nstruct person\n{\n    std::string name;\n    std::string address;\n    int age;\n};\n} // namespace ns\n\nnamespace ns\n{\nvoid to_json(json& j, const person& p)\n{\n    j = json{ {\"name\", p.name}, {\"address\", p.address}, {\"age\", p.age} };\n}\n} // namespace ns\n\nint main()\n{\n    ns::person p = {\"Ned Flanders\", \"744 Evergreen Terrace\", 60};\n\n    json j = p;\n\n    std::cout << j << std::endl;\n}\n

Output:

{\"address\":\"744 Evergreen Terrace\",\"age\":60,\"name\":\"Ned Flanders\"}\n
"},{"location":"api/adl_serializer/to_json/#see-also","title":"See also","text":"
  • from_json
"},{"location":"api/adl_serializer/to_json/#version-history","title":"Version history","text":"
  • Added in version 2.1.0.
"},{"location":"api/basic_json/","title":"nlohmann::basic_json","text":"

Defined in header <nlohmann/json.hpp>

template<\n    template<typename U, typename V, typename... Args> class ObjectType = std::map,\n    template<typename U, typename... Args> class ArrayType = std::vector,\n    class StringType = std::string,\n    class BooleanType = bool,\n    class NumberIntegerType = std::int64_t,\n    class NumberUnsignedType = std::uint64_t,\n    class NumberFloatType = double,\n    template<typename U> class AllocatorType = std::allocator,\n    template<typename T, typename SFINAE = void> class JSONSerializer = adl_serializer,\n    class BinaryType = std::vector<std::uint8_t>,\n    class CustomBaseClass = void\n>\nclass basic_json;\n
"},{"location":"api/basic_json/#template-parameters","title":"Template parameters","text":"Template parameter Description Derived type ObjectType type for JSON objects object_t ArrayType type for JSON arrays array_t StringType type for JSON strings and object keys string_t BooleanType type for JSON booleans boolean_t NumberIntegerType type for JSON integer numbers number_integer_t NumberUnsignedType type for JSON unsigned integer numbers number_unsigned_t NumberFloatType type for JSON floating-point numbers number_float_t AllocatorType type of the allocator to use JSONSerializer the serializer to resolve internal calls to to_json() and from_json() json_serializer BinaryType type for binary arrays binary_t CustomBaseClass extension point for user code json_base_class_t

The library imposes a number of requirements on these types that are not expressed as C++ concepts, such as the container operations object_t and array_t must provide, or the fact that StringType must be char-based. They are collected in Template Parameter Requirements.

"},{"location":"api/basic_json/#specializations","title":"Specializations","text":"
  • json - default specialization
  • ordered_json - a specialization that maintains the insertion order of object keys
"},{"location":"api/basic_json/#iterator-invalidation","title":"Iterator invalidation","text":"

All operations that add values to an array (push_back , operator+=, emplace_back, insert, and operator[] for a non-existing index) can yield a reallocation, in which case all iterators (including the end() iterator) and all references to the elements are invalidated.

For ordered_json, also all operations that add a value to an object (push_back, operator+=, emplace, insert, update, and operator[] for a non-existing key) can yield a reallocation, in which case all iterators (including the end() iterator) and all references to the elements are invalidated.

"},{"location":"api/basic_json/#requirements","title":"Requirements","text":"

The class satisfies the following concept requirements:

"},{"location":"api/basic_json/#basic","title":"Basic","text":"
  • DefaultConstructible: JSON values can be default-constructed. The result will be a JSON null value.
  • MoveConstructible: A JSON value can be constructed from an rvalue argument.
  • CopyConstructible: A JSON value can be copy-constructed from an lvalue expression.
  • MoveAssignable: A JSON value can be assigned from an rvalue argument.
  • CopyAssignable: A JSON value can be copy-assigned from an lvalue expression.
  • Destructible: JSON values can be destructed.
"},{"location":"api/basic_json/#layout","title":"Layout","text":"
  • StandardLayoutType: JSON values have standard layout: All non-static data members are private and standard layout types, the class has no virtual functions or (virtual) base classes.
"},{"location":"api/basic_json/#library-wide","title":"Library-wide","text":"
  • EqualityComparable: JSON values can be compared with ==, see operator==.
  • LessThanComparable: JSON values can be compared with <, see operator<.
  • Swappable: Any JSON lvalue or rvalue of can be swapped with any lvalue or rvalue of other compatible types, using unqualified function swap.
  • NullablePointer: JSON values can be compared against std::nullptr_t objects which are used to model the null value.
"},{"location":"api/basic_json/#container","title":"Container","text":"
  • Container: JSON values can be used like STL containers and provide iterator access.
  • ReversibleContainer: JSON values can be used like STL containers and provide reverse iterator access.
"},{"location":"api/basic_json/#member-types","title":"Member types","text":"
  • adl_serializer - the default serializer
  • value_t - the JSON type enumeration
  • json_pointer - JSON Pointer implementation
  • json_serializer - type of the serializer to for conversions from/to JSON
  • error_handler_t - type to choose behavior on decoding errors
  • cbor_tag_handler_t - type to choose how to handle CBOR tags
  • initializer_list_t - type for initializer lists of basic_json values
  • input_format_t - type to choose the format to parse
  • json_sax_t - type for SAX events
  • with_object_t, with_array_t, with_string_t, with_boolean_t, with_integers_t, with_float_t, with_allocator_t, with_json_serializer_t, with_binary_t, with_base_class_t - types to create a basic_json type with one (or two) replaced template parameters
"},{"location":"api/basic_json/#exceptions","title":"Exceptions","text":"
  • exception - general exception of the basic_json class
    • parse_error - exception indicating a parse error
    • invalid_iterator - exception indicating errors with iterators
    • type_error - exception indicating executing a member function with a wrong type
    • out_of_range - exception indicating access out of the defined range
    • other_error - exception indicating other library errors
"},{"location":"api/basic_json/#container-types","title":"Container types","text":"Type Definition value_type basic_json reference value_type& const_reference const value_type& difference_type std::ptrdiff_t size_type std::size_t allocator_type AllocatorType<basic_json> pointer std::allocator_traits<allocator_type>::pointer const_pointer std::allocator_traits<allocator_type>::const_pointer iterator LegacyBidirectionalIterator const_iterator constant LegacyBidirectionalIterator reverse_iterator reverse iterator, derived from iterator const_reverse_iterator reverse iterator, derived from const_iterator iteration_proxy helper type for items function"},{"location":"api/basic_json/#json-value-data-types","title":"JSON value data types","text":"
  • array_t - type for arrays
  • binary_t - type for binary arrays
  • boolean_t - type for booleans
  • default_object_comparator_t - default comparator for objects
  • number_float_t - type for numbers (floating-point)
  • number_integer_t - type for numbers (integer)
  • number_unsigned_t - type for numbers (unsigned)
  • object_comparator_t - comparator for objects
  • object_t - type for objects
  • string_t - type for strings
"},{"location":"api/basic_json/#parser-callback","title":"Parser callback","text":"
  • parse_event_t - parser event types
  • parser_callback_t - per-element parser callback type
"},{"location":"api/basic_json/#member-functions","title":"Member functions","text":"
  • (constructor)
  • (destructor)
  • operator= - copy assignment
  • array (static) - explicitly create an array
  • binary (static) - explicitly create a binary array
  • object (static) - explicitly create an object
"},{"location":"api/basic_json/#object-inspection","title":"Object inspection","text":"

Functions to inspect the type of a JSON value.

  • type - return the type of the JSON value
  • operator value_t - return the type of the JSON value
  • type_name - return the type as string
  • is_primitive - return whether the type is primitive
  • is_structured - return whether the type is structured
  • is_null - return whether the value is null
  • is_boolean - return whether the value is a boolean
  • is_number - return whether the value is a number
  • is_number_integer - return whether the value is an integer number
  • is_number_unsigned - return whether the value is an unsigned integer number
  • is_number_float - return whether the value is a floating-point number
  • is_object - return whether the value is an object
  • is_array - return whether the value is an array
  • is_string - return whether the value is a string
  • is_binary - return whether the value is a binary array
  • is_discarded - return whether the value is discarded

Optional functions to access the diagnostic positions.

  • start_pos - return the start position of the value
  • end_pos - return the one past the end position of the value
"},{"location":"api/basic_json/#value-access","title":"Value access","text":"

Direct access to the stored value of a JSON value.

  • get - get a value
  • get_to - get a value and write it to a destination
  • get_ptr - get a pointer value
  • get_ref - get a reference value
  • operator ValueType - get a value
  • get_binary - get a binary value
  • as_base_class - access the custom base class
"},{"location":"api/basic_json/#element-access","title":"Element access","text":"

Access to the JSON value

  • at - access specified element with bounds checking
  • operator[] - access specified element
  • value - access specified object element with default value
  • front - access the first element
  • back - access the last element
"},{"location":"api/basic_json/#lookup","title":"Lookup","text":"
  • find - find an element in a JSON object
  • count - returns the number of occurrences of a key in a JSON object
  • contains - check the existence of an element in a JSON object
"},{"location":"api/basic_json/#iterators","title":"Iterators","text":"
  • begin - returns an iterator to the first element
  • cbegin - returns a const iterator to the first element
  • end - returns an iterator to one past the last element
  • cend - returns a const iterator to one past the last element
  • rbegin - returns an iterator to the reverse-beginning
  • rend - returns an iterator to the reverse-end
  • crbegin - returns a const iterator to the reverse-beginning
  • crend - returns a const iterator to the reverse-end
  • items - wrapper to access iterator member functions in range-based for
"},{"location":"api/basic_json/#capacity","title":"Capacity","text":"
  • empty - checks whether the container is empty
  • size - returns the number of elements
  • max_size - returns the maximum possible number of elements
"},{"location":"api/basic_json/#modifiers","title":"Modifiers","text":"
  • clear - clears the contents
  • push_back - add a value to an array/object
  • operator+= - add a value to an array/object
  • emplace_back - add a value to an array
  • emplace - add a value to an object if a key does not exist
  • erase - remove elements
  • insert - inserts elements
  • update - updates a JSON object from another object, overwriting existing keys
  • swap - exchanges the values
"},{"location":"api/basic_json/#lexicographical-comparison-operators","title":"Lexicographical comparison operators","text":"
  • operator== - comparison: equal
  • operator!= - comparison: not equal
  • operator< - comparison: less than
  • operator> - comparison: greater than
  • operator<= - comparison: less than or equal
  • operator>= - comparison: greater than or equal
  • operator<=> - comparison: 3-way
"},{"location":"api/basic_json/#serialization-dumping","title":"Serialization / Dumping","text":"
  • dump - serialization
"},{"location":"api/basic_json/#deserialization-parsing","title":"Deserialization / Parsing","text":"
  • parse (static) - deserialize from a compatible input
  • accept (static) - check if the input is valid JSON
  • sax_parse (static) - generate SAX events
"},{"location":"api/basic_json/#json-pointer-functions","title":"JSON Pointer functions","text":"
  • flatten - return flattened JSON value
  • unflatten - unflatten a previously flattened JSON value
"},{"location":"api/basic_json/#json-patch-functions","title":"JSON Patch functions","text":"
  • patch - applies a JSON patch
  • patch_inplace - applies a JSON patch in place
  • diff (static) - creates a diff as a JSON patch
"},{"location":"api/basic_json/#json-merge-patch-functions","title":"JSON Merge Patch functions","text":"
  • merge_patch - applies a JSON Merge Patch
  • merge_diff (static) - creates a diff as a JSON Merge Patch
"},{"location":"api/basic_json/#static-functions","title":"Static functions","text":"
  • meta - returns version information on the library
  • get_allocator - returns the allocator associated with the container
"},{"location":"api/basic_json/#binary-formats","title":"Binary formats","text":"
  • from_bjdata (static) - create a JSON value from an input in BJData format
  • from_bon8 (static) - create a JSON value from an input in BON8 format
  • from_bson (static) - create a JSON value from an input in BSON format
  • from_cbor (static) - create a JSON value from an input in CBOR format
  • from_msgpack (static) - create a JSON value from an input in MessagePack format
  • from_ubjson (static) - create a JSON value from an input in UBJSON format
  • to_bjdata (static) - create a BJData serialization of a given JSON value
  • to_bon8 (static) - create a BON8 serialization of a given JSON value
  • to_bson (static) - create a BSON serialization of a given JSON value
  • to_cbor (static) - create a CBOR serialization of a given JSON value
  • to_msgpack (static) - create a MessagePack serialization of a given JSON value
  • to_ubjson (static) - create a UBJSON serialization of a given JSON value
"},{"location":"api/basic_json/#non-member-functions","title":"Non-member functions","text":"
  • operator<<(std::ostream&) - serialize to stream
  • operator>>(std::istream&) - deserialize from stream
  • to_string - user-defined to_string function for JSON values
  • format_as - user-defined format_as function for JSON values (fmt support)
"},{"location":"api/basic_json/#literals","title":"Literals","text":"
  • operator\"\"_json - user-defined string literal for JSON values
"},{"location":"api/basic_json/#helper-classes","title":"Helper classes","text":"
  • std::formatter<basic_json> - make JSON values formattable with std::format
  • std::hash<basic_json> - return a hash value for a JSON object
  • std::swap<basic_json> - exchanges the values of two JSON objects
"},{"location":"api/basic_json/#examples","title":"Examples","text":"Example

The example shows how the library is used.

#include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON object\n    json j =\n    {\n        {\"pi\", 3.141},\n        {\"happy\", true},\n        {\"name\", \"Niels\"},\n        {\"nothing\", nullptr},\n        {\n            \"answer\", {\n                {\"everything\", 42}\n            }\n        },\n        {\"list\", {1, 0, 2}},\n        {\n            \"object\", {\n                {\"currency\", \"USD\"},\n                {\"value\", 42.99}\n            }\n        }\n    };\n\n    // add new values\n    j[\"new\"][\"key\"][\"value\"] = {\"another\", \"list\"};\n\n    // count elements\n    auto s = j.size();\n    j[\"size\"] = s;\n\n    // pretty print with indent of 4 spaces\n    std::cout << std::setw(4) << j << '\\n';\n}\n

Output:

{\n    \"answer\": {\n        \"everything\": 42\n    },\n    \"happy\": true,\n    \"list\": [\n        1,\n        0,\n        2\n    ],\n    \"name\": \"Niels\",\n    \"new\": {\n        \"key\": {\n            \"value\": [\n                \"another\",\n                \"list\"\n            ]\n        }\n    },\n    \"nothing\": null,\n    \"object\": {\n        \"currency\": \"USD\",\n        \"value\": 42.99\n    },\n    \"pi\": 3.141,\n    \"size\": 8\n}\n
"},{"location":"api/basic_json/#see-also","title":"See also","text":"
  • RFC 8259: The JavaScript Object Notation (JSON) Data Interchange Format
"},{"location":"api/basic_json/#version-history","title":"Version history","text":"
  • Added in version 1.0.0.
"},{"location":"api/basic_json/accept/","title":"nlohmann::basic_json::accept","text":"
// (1)\ntemplate<typename InputType>\nstatic bool accept(InputType&& i,\n                   const bool ignore_comments = false,\n                   const bool ignore_trailing_commas = false);\n\n// (2)\ntemplate<typename IteratorType, typename SentinelType = IteratorType>\nstatic bool accept(IteratorType first, SentinelType last,\n                   const bool ignore_comments = false,\n                   const bool ignore_trailing_commas = false);\n

Checks whether the input is valid JSON.

  1. Reads from a compatible input.

  2. Reads from a pair of character iterators, or an iterator and a sentinel of a different type (C++20 ranges support)

    The value_type of the iterator must be an integral type with a size of 1, 2, or 4 bytes, which will be interpreted respectively as UTF-8, UTF-16, and UTF-32. If SentinelType differs from IteratorType, it must be comparable to the iterator type with operator!=.

Unlike the parse() function, this function neither throws an exception in case of invalid JSON input (i.e., a parse error) nor creates diagnostic information.

"},{"location":"api/basic_json/accept/#template-parameters","title":"Template parameters","text":"InputType

A compatible input, for instance:

  • an std::istream object
  • a FILE pointer (throws if null)
  • a C-style array of characters
  • a pointer to a null-terminated string of single byte characters (throws if null)
  • a std::string
  • a container obj for which begin(obj) and end(obj) produce a valid pair of iterators (as found via ADL or member functions, with semantics compatible to std::begin and std::end)
IteratorType

a compatible iterator type, for instance.

  • a pair of std::string::iterator or std::vector<std::uint8_t>::iterator
  • a pair of pointers such as ptr and ptr + len
SentinelType

defaults to IteratorType; may be a different type comparable to IteratorType via operator!=, for instance.

  • a custom sentinel type for C++20 ranges
  • std::default_sentinel_t, when IteratorType is std::counted_iterator
"},{"location":"api/basic_json/accept/#parameters","title":"Parameters","text":"i (in) Input to parse from. ignore_comments (in) whether comments should be ignored and treated like whitespace (true) or yield a parse error (false); (optional, false by default) ignore_trailing_commas (in) whether trailing commas in arrays or objects should be ignored and treated like whitespace (true) or yield a parse error (false); (optional, false by default) first (in) iterator to the start of the character range last (in) iterator to the end of the character range, or a sentinel value that compares equal to the end iterator with operator!="},{"location":"api/basic_json/accept/#return-value","title":"Return value","text":"

Whether the input is valid JSON.

"},{"location":"api/basic_json/accept/#exception-safety","title":"Exception safety","text":"

Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

"},{"location":"api/basic_json/accept/#exceptions","title":"Exceptions","text":"

Throws parse_error.101 in case of an empty input like a null FILE* or char* pointer.

"},{"location":"api/basic_json/accept/#complexity","title":"Complexity","text":"

Linear in the length of the input. The parser is a predictive LL(1) parser.

"},{"location":"api/basic_json/accept/#notes","title":"Notes","text":"

A UTF-8 byte order mark is silently ignored.

By default, a '\\0' (NUL) byte anywhere in the input is treated as end of input, rather than as an ordinary (and, outside of a string, invalid) byte; see the FAQ entry for details and the JSON_STRICT_NUL_HANDLING macro to opt into rejecting it instead.

"},{"location":"api/basic_json/accept/#examples","title":"Examples","text":"Example: (1) reading from a string

The example below demonstrates the accept() function reading from a string.

#include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // a valid JSON text\n    auto valid_text = R\"(\n    {\n        \"numbers\": [1, 2, 3]\n    }\n    )\";\n\n    // an invalid JSON text\n    auto invalid_text = R\"(\n    {\n        \"strings\": [\"extra\", \"comma\", ]\n    }\n    )\";\n\n    std::cout << std::boolalpha\n              << json::accept(valid_text) << ' '\n              << json::accept(invalid_text) << '\\n';\n}\n

Output:

true false\n
Example: (2) reading from an iterator pair

The example below demonstrates the accept() function reading from an iterator pair. Only the first call covers exactly the JSON text; the second one also covers the trailing bytes and is therefore rejected.

#include <iostream>\n#include <vector>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // a buffer containing a JSON text followed by more data\n    std::vector<std::uint8_t> input = {'[', '1', ',', '2', ',', '3', ']', 'o', 't', 'h', 'e', 'r'};\n\n    std::cout << std::boolalpha\n              << json::accept(input.begin(), input.begin() + 7) << ' '\n              << json::accept(input.begin(), input.end()) << '\\n';\n}\n

Output:

true false\n
"},{"location":"api/basic_json/accept/#see-also","title":"See also","text":"
  • parse - deserialize from a compatible input
  • sax_parse - parse input using the SAX interface
  • operator>> - deserialize from stream
  • JSON_STRICT_NUL_HANDLING - opt in to rejecting a NUL byte in the input instead of treating it as end of input
  • JSON_DISABLE_STREAM_BULK_SCAN - read a std::istream one character at a time
  • JSON_DISABLE_UNLOCKED_FILE_IO - read a FILE* with the locking stdio functions
"},{"location":"api/basic_json/accept/#version-history","title":"Version history","text":"
  • Added in version 3.0.0.
  • Ignoring comments via ignore_comments added in version 3.9.0.
  • Changed runtime assertion in case of FILE* null pointers to exception in version 3.12.0.
  • Added ignore_trailing_commas in version 3.13.0 unreleased.
  • Extended container support (1) to include types with lvalue-only ADL begin/end (matching std::begin/std::end semantics) in version 3.13.0 unreleased.
  • Extended overload (2) to accept heterogeneous iterator+sentinel pairs (C++20 ranges support) in version 3.13.0 unreleased.
  • JSON_STRICT_NUL_HANDLING added in version 3.13.0 unreleased to optionally reject a NUL byte in the input instead of treating it as end of input; planned to become the default in version 4.0.0.
  • Parsing from an std::istream scans the characters in its stream buffer in bulk in version 3.13.0 unreleased (JSON_DISABLE_STREAM_BULK_SCAN).
  • Parsing from a FILE* locks the file once instead of once per character in version 3.13.0 unreleased (JSON_DISABLE_UNLOCKED_FILE_IO).

Deprecation

Overload (2) replaces calls to accept with a pair of iterators as their first parameter which has been deprecated in version 3.8.0. This overload will be removed in version 4.0.0. Please replace all calls like accept({ptr, ptr+len}, ...); with accept(ptr, ptr+len, ...);.

You should be warned by your compiler with a -Wdeprecated-declarations warning if you are using a deprecated function.

See the migration guide for how to update existing code.

"},{"location":"api/basic_json/array/","title":"nlohmann::basic_json::array","text":"
static basic_json array(initializer_list_t init = {});\n

Creates a JSON array value from a given initializer list. That is, given a list of values a, b, c, creates the JSON value [a, b, c]. If the initializer list is empty, the empty array [] is created.

"},{"location":"api/basic_json/array/#parameters","title":"Parameters","text":"init (in) initializer list with JSON values to create an array from (optional)"},{"location":"api/basic_json/array/#return-value","title":"Return value","text":"

JSON array value

"},{"location":"api/basic_json/array/#exception-safety","title":"Exception safety","text":"

Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

"},{"location":"api/basic_json/array/#complexity","title":"Complexity","text":"

Linear in the size of init.

"},{"location":"api/basic_json/array/#notes","title":"Notes","text":"

This function is only needed to express two edge cases that cannot be realized with the initializer list constructor (basic_json(initializer_list_t, bool, value_t)). These cases are:

  1. creating an array whose elements are all pairs whose first element is a string -- in this case, the initializer list constructor would create an object, taking the first elements as keys
  2. creating an empty array -- passing the empty initializer list to the initializer list constructor yields an empty object
"},{"location":"api/basic_json/array/#examples","title":"Examples","text":"Example

The following code shows an example for the array function.

#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON arrays\n    json j_no_init_list = json::array();\n    json j_empty_init_list = json::array({});\n    json j_nonempty_init_list = json::array({1, 2, 3, 4});\n    json j_list_of_pairs = json::array({ {\"one\", 1}, {\"two\", 2} });\n\n    // serialize the JSON arrays\n    std::cout << j_no_init_list << '\\n';\n    std::cout << j_empty_init_list << '\\n';\n    std::cout << j_nonempty_init_list << '\\n';\n    std::cout << j_list_of_pairs << '\\n';\n}\n

Output:

[]\n[]\n[1,2,3,4]\n[[\"one\",1],[\"two\",2]]\n
"},{"location":"api/basic_json/array/#see-also","title":"See also","text":"
  • basic_json(initializer_list_t) - create a JSON value from an initializer list
  • object - create a JSON object value from an initializer list
  • Creating JSON values - the article on creating JSON values
"},{"location":"api/basic_json/array/#version-history","title":"Version history","text":"
  • Added in version 1.0.0.
"},{"location":"api/basic_json/array_t/","title":"nlohmann::basic_json::array_t","text":"
using array_t = ArrayType<basic_json, AllocatorType<basic_json>>;\n

The type used to store JSON arrays.

RFC 8259 describes JSON arrays as follows:

An array is an ordered sequence of zero or more values.

To store objects in C++, a type is defined by the template parameters explained below.

"},{"location":"api/basic_json/array_t/#template-parameters","title":"Template parameters","text":"ArrayType container type to store arrays. It must be a vector-like container: the library uses operator[], at(), and resize(), and requires random-access iterators. std::vector and std::deque qualify; std::list does not. See Template Parameter Requirements for the full list of requirements. AllocatorType the allocator to use for objects (e.g., std::allocator)"},{"location":"api/basic_json/array_t/#notes","title":"Notes","text":""},{"location":"api/basic_json/array_t/#default-type","title":"Default type","text":"

With the default values for ArrayType (std::vector) and AllocatorType (std::allocator), the default type for array_t is:

std::vector<\n  basic_json, // value_type\n  std::allocator<basic_json> // allocator_type\n>\n
"},{"location":"api/basic_json/array_t/#limits","title":"Limits","text":"

RFC 8259 specifies:

An implementation may set limits on the maximum depth of nesting.

In this class, the array's limit of nesting is not explicitly constrained. However, a maximum depth of nesting may be introduced by the compiler or runtime environment. A theoretical limit can be queried by calling the max_size function of a JSON array.

"},{"location":"api/basic_json/array_t/#storage","title":"Storage","text":"

Arrays are stored as pointers in a basic_json type. That is, for any access to array values, a pointer of type array_t* must be dereferenced.

"},{"location":"api/basic_json/array_t/#examples","title":"Examples","text":"Example

The following code shows that array_t is by default, a typedef to std::vector<nlohmann::json>.

#include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::boolalpha << std::is_same<std::vector<json>, json::array_t>::value << std::endl;\n}\n

Output:

true\n
"},{"location":"api/basic_json/array_t/#see-also","title":"See also","text":"
  • object_t the type used to store JSON objects
  • binary_t the type used to store binary values
  • is_array checks whether the JSON value is an array
  • max_size returns the maximum possible number of elements
"},{"location":"api/basic_json/array_t/#version-history","title":"Version history","text":"
  • Added in version 1.0.0.
  • Made capacity() optional, so that array types such as std::deque can be used, in version 3.13.0 unreleased.
"},{"location":"api/basic_json/as_base_class/","title":"nlohmann::basic_json::as_base_class","text":"
json_base_class_t& as_base_class() noexcept;\nconst json_base_class_t& as_base_class() const noexcept;\n

Returns a reference to this object as its custom base class json_base_class_t. No copy is made.

Since basic_json derives from json_base_class_t, a member of basic_json hides any member of the custom base class with the same name. This function makes such hidden members accessible again.

"},{"location":"api/basic_json/as_base_class/#return-value","title":"Return value","text":"

reference to this object as json_base_class_t

"},{"location":"api/basic_json/as_base_class/#exception-safety","title":"Exception safety","text":"

No-throw guarantee: this function never throws exceptions.

"},{"location":"api/basic_json/as_base_class/#complexity","title":"Complexity","text":"

Constant.

"},{"location":"api/basic_json/as_base_class/#notes","title":"Notes","text":"

The function is equivalent to static_cast<json_base_class_t&>(j) (or static_cast<const json_base_class_t&>(j)).

"},{"location":"api/basic_json/as_base_class/#examples","title":"Examples","text":"Example

The example shows how to use as_base_class to access members of the custom base class that are hidden by members of basic_json.

#include <iostream>\n#include <nlohmann/json.hpp>\n\nclass base_class_with_hidden_members\n{\n  public:\n    const char* type_name() const noexcept\n    {\n        return \"my_type_name\";\n    }\n\n    std::size_t size() const noexcept\n    {\n        return 42;\n    }\n};\n\nusing json = nlohmann::json::with_base_class_t<base_class_with_hidden_members>;\n\nint main()\n{\n    json j = {1, 2, 3};\n\n    // the members of basic_json hide the members of the base class\n    std::cout << j.type_name() << ' ' << j.size() << '\\n';\n\n    // access the hidden members of the base class\n    std::cout << j.as_base_class().type_name() << ' ' << j.as_base_class().size() << '\\n';\n}\n

Output:

array 3\nmy_type_name 42\n
"},{"location":"api/basic_json/as_base_class/#see-also","title":"See also","text":"
  • json_base_class_t - type of the custom base class
"},{"location":"api/basic_json/as_base_class/#version-history","title":"Version history","text":"
  • Added in version 3.13.0 unreleased.
"},{"location":"api/basic_json/at/","title":"nlohmann::basic_json::at","text":"
// (1)\nreference at(size_type idx);\nconst_reference at(size_type idx) const;\n\n// (2)\nreference at(const typename object_t::key_type& key);\nconst_reference at(const typename object_t::key_type& key) const;\n\n// (3)\ntemplate<typename KeyType>\nreference at(KeyType&& key);\ntemplate<typename KeyType>\nconst_reference at(KeyType&& key) const;\n\n// (4)\nreference at(const json_pointer& ptr);\nconst_reference at(const json_pointer& ptr) const;\n
  1. Returns a reference to the array element at specified location idx, with bounds checking.
  2. Returns a reference to the object element with specified key key, with bounds checking.
  3. See 2. This overload is only available if KeyType is comparable with typename object_t::key_type and typename object_comparator_t::is_transparent denotes a type.
  4. Returns a reference to the element at specified JSON pointer ptr, with bounds checking.
"},{"location":"api/basic_json/at/#template-parameters","title":"Template parameters","text":"KeyType A type for an object key other than json_pointer that is comparable with string_t using object_comparator_t. This can also be a string view (C++17)."},{"location":"api/basic_json/at/#parameters","title":"Parameters","text":"idx (in) index of the element to access key (in) object key of the elements to access ptr (in) JSON pointer to the desired element"},{"location":"api/basic_json/at/#return-value","title":"Return value","text":"
  1. reference to the element at index idx
  2. reference to the element at key key
  3. reference to the element at key key
  4. reference to the element pointed to by ptr
"},{"location":"api/basic_json/at/#exception-safety","title":"Exception safety","text":"

Strong exception safety: if an exception occurs, the original value stays intact.

"},{"location":"api/basic_json/at/#exceptions","title":"Exceptions","text":"
  1. The function can throw the following exceptions:
    • Throws type_error.304 if the JSON value is not an array; in this case, calling at with an index makes no sense. See the example below.
    • Throws out_of_range.401 if the index idx is out of range of the array; that is, idx >= size(). See the example below.
  2. The function can throw the following exceptions:
    • Throws type_error.304 if the JSON value is not an object; in this case, calling at with a key makes no sense. See the example below.
    • Throws out_of_range.403 if the key key is not stored in the object; that is, find(key) == end(). See the example below.
  3. See 2.
  4. The function can throw the following exceptions:
    • Throws parse_error.106 if an array index in the passed JSON pointer ptr begins with '0'. See the example below.
    • Throws parse_error.109 if an array index in the passed JSON pointer ptr is not a number. See the example below.
    • Throws out_of_range.401 if an array index in the passed JSON pointer ptr is out of range. See the example below.
    • Throws out_of_range.402 if the array index '-' is used in the passed JSON pointer ptr. As at provides checked access (and no elements are implicitly inserted), the index '-' is always invalid. See the example below.
    • Throws out_of_range.403 if the JSON pointer describes a key of an object which cannot be found. See the example below.
    • Throws out_of_range.404 if the JSON pointer ptr can not be resolved. See the example below.
    • Throws out_of_range.410 if an array index in the passed JSON pointer ptr exceeds the range of size_type (e.g., on 32-bit platforms).
"},{"location":"api/basic_json/at/#complexity","title":"Complexity","text":"
  1. Constant.
  2. Logarithmic in the size of the container.
  3. Logarithmic in the size of the container.
  4. Logarithmic in the size of the container.
"},{"location":"api/basic_json/at/#notes","title":"Notes","text":"

Deprecation

Overload (4) also accepts a json_pointer whose template argument is a basic_json specialization (e.g., nlohmann::json_pointer<nlohmann::json>) instead of a string type. This is deprecated since version 3.11.0 and will be removed in a future major version; use basic_json::json_pointer (for json, nlohmann::json_pointer<std::string>) instead.

You should be warned by your compiler with a -Wdeprecated-declarations warning if you are using a deprecated function.

See the migration guide for how to update existing code.

"},{"location":"api/basic_json/at/#examples","title":"Examples","text":"Example: (1) access specified array element with bounds checking

The example below shows how array elements can be read and written using at(). It also demonstrates the different exceptions that can be thrown.

#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON array\n    json array = {\"first\", \"2nd\", \"third\", \"fourth\"};\n\n    // output element at index 2 (third element)\n    std::cout << array.at(2) << '\\n';\n\n    // change element at index 1 (second element) to \"second\"\n    array.at(1) = \"second\";\n\n    // output changed array\n    std::cout << array << '\\n';\n\n    // exception type_error.304\n    try\n    {\n        // use at() on a non-array type\n        json str = \"I am a string\";\n        str.at(0) = \"Another string\";\n    }\n    catch (const json::type_error& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n\n    // exception out_of_range.401\n    try\n    {\n        // try to write beyond the array limit\n        array.at(5) = \"sixth\";\n    }\n    catch (const json::out_of_range& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

Output:

\"third\"\n[\"first\",\"second\",\"third\",\"fourth\"]\n[json.exception.type_error.304] cannot use at() with string\n[json.exception.out_of_range.401] array index 5 is out of range\n
Example: (1) access specified array element with bounds checking

The example below shows how array elements can be read using at(). It also demonstrates the different exceptions that can be thrown.

#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON array\n    const json array = {\"first\", \"2nd\", \"third\", \"fourth\"};\n\n    // output element at index 2 (third element)\n    std::cout << array.at(2) << '\\n';\n\n    // exception type_error.304\n    try\n    {\n        // use at() on a non-array type\n        const json str = \"I am a string\";\n        std::cout << str.at(0) << '\\n';\n    }\n    catch (const json::type_error& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n\n    // exception out_of_range.401\n    try\n    {\n        // try to read beyond the array limit\n        std::cout << array.at(5) << '\\n';\n    }\n    catch (const json::out_of_range& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

Output:

\"third\"\n[json.exception.type_error.304] cannot use at() with string\n[json.exception.out_of_range.401] array index 5 is out of range\n
Example: (2) access specified object element with bounds checking

The example below shows how object elements can be read and written using at(). It also demonstrates the different exceptions that can be thrown.

#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON object\n    json object =\n    {\n        {\"the good\", \"il buono\"},\n        {\"the bad\", \"il cattivo\"},\n        {\"the ugly\", \"il brutto\"}\n    };\n\n    // output element with key \"the ugly\"\n    std::cout << object.at(\"the ugly\") << '\\n';\n\n    // change element with key \"the bad\"\n    object.at(\"the bad\") = \"il cattivo\";\n\n    // output changed array\n    std::cout << object << '\\n';\n\n    // exception type_error.304\n    try\n    {\n        // use at() on a non-object type\n        json str = \"I am a string\";\n        str.at(\"the good\") = \"Another string\";\n    }\n    catch (const json::type_error& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n\n    // exception out_of_range.401\n    try\n    {\n        // try to write at a nonexisting key\n        object.at(\"the fast\") = \"il rapido\";\n    }\n    catch (const json::out_of_range& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

Output:

\"il brutto\"\n{\"the bad\":\"il cattivo\",\"the good\":\"il buono\",\"the ugly\":\"il brutto\"}\n[json.exception.type_error.304] cannot use at() with string\n[json.exception.out_of_range.403] key 'the fast' not found\n
Example: (2) access specified object element with bounds checking

The example below shows how object elements can be read using at(). It also demonstrates the different exceptions that can be thrown.

#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON object\n    const json object =\n    {\n        {\"the good\", \"il buono\"},\n        {\"the bad\", \"il cattivo\"},\n        {\"the ugly\", \"il brutto\"}\n    };\n\n    // output element with key \"the ugly\"\n    std::cout << object.at(\"the ugly\") << '\\n';\n\n    // exception type_error.304\n    try\n    {\n        // use at() on a non-object type\n        const json str = \"I am a string\";\n        std::cout << str.at(\"the good\") << '\\n';\n    }\n    catch (const json::type_error& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n\n    // exception out_of_range.401\n    try\n    {\n        // try to read from a nonexisting key\n        std::cout << object.at(\"the fast\") << '\\n';\n    }\n    catch (const json::out_of_range)\n    {\n        std::cout << \"out of range\" << '\\n';\n    }\n}\n

Output:

\"il brutto\"\n[json.exception.type_error.304] cannot use at() with string\nout of range\n
Example: (3) access specified object element using string_view with bounds checking

The example below shows how object elements can be read and written using at(). It also demonstrates the different exceptions that can be thrown.

#include <iostream>\n#include <string_view>\n#include <nlohmann/json.hpp>\n\nusing namespace std::string_view_literals;\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON object\n    json object =\n    {\n        {\"the good\", \"il buono\"},\n        {\"the bad\", \"il cattivo\"},\n        {\"the ugly\", \"il brutto\"}\n    };\n\n    // output element with key \"the ugly\" using string_view\n    std::cout << object.at(\"the ugly\"sv) << '\\n';\n\n    // change element with key \"the bad\" using string_view\n    object.at(\"the bad\"sv) = \"il cattivo\";\n\n    // output changed array\n    std::cout << object << '\\n';\n\n    // exception type_error.304\n    try\n    {\n        // use at() with string_view on a non-object type\n        json str = \"I am a string\";\n        str.at(\"the good\"sv) = \"Another string\";\n    }\n    catch (const json::type_error& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n\n    // exception out_of_range.401\n    try\n    {\n        // try to write at a nonexisting key using string_view\n        object.at(\"the fast\"sv) = \"il rapido\";\n    }\n    catch (const json::out_of_range& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

Output:

\"il brutto\"\n{\"the bad\":\"il cattivo\",\"the good\":\"il buono\",\"the ugly\":\"il brutto\"}\n[json.exception.type_error.304] cannot use at() with string\n[json.exception.out_of_range.403] key 'the fast' not found\n
Example: (3) access specified object element using string_view with bounds checking

The example below shows how object elements can be read using at(). It also demonstrates the different exceptions that can be thrown.

#include <iostream>\n#include <string_view>\n#include <nlohmann/json.hpp>\n\nusing namespace std::string_view_literals;\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON object\n    const json object =\n    {\n        {\"the good\", \"il buono\"},\n        {\"the bad\", \"il cattivo\"},\n        {\"the ugly\", \"il brutto\"}\n    };\n\n    // output element with key \"the ugly\" using string_view\n    std::cout << object.at(\"the ugly\"sv) << '\\n';\n\n    // exception type_error.304\n    try\n    {\n        // use at() with string_view on a non-object type\n        const json str = \"I am a string\";\n        std::cout << str.at(\"the good\"sv) << '\\n';\n    }\n    catch (const json::type_error& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n\n    // exception out_of_range.401\n    try\n    {\n        // try to read from a nonexisting key using string_view\n        std::cout << object.at(\"the fast\"sv) << '\\n';\n    }\n    catch (const json::out_of_range& e)\n    {\n        std::cout << \"out of range\" << '\\n';\n    }\n}\n

Output:

\"il brutto\"\n[json.exception.type_error.304] cannot use at() with string\nout of range\n
Example: (4) access specified element via JSON Pointer

The example below shows how object elements can be read and written using at(). It also demonstrates the different exceptions that can be thrown.

#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // create a JSON value\n    json j =\n    {\n        {\"number\", 1}, {\"string\", \"foo\"}, {\"array\", {1, 2}}\n    };\n\n    // read-only access\n\n    // output element with JSON pointer \"/number\"\n    std::cout << j.at(\"/number\"_json_pointer) << '\\n';\n    // output element with JSON pointer \"/string\"\n    std::cout << j.at(\"/string\"_json_pointer) << '\\n';\n    // output element with JSON pointer \"/array\"\n    std::cout << j.at(\"/array\"_json_pointer) << '\\n';\n    // output element with JSON pointer \"/array/1\"\n    std::cout << j.at(\"/array/1\"_json_pointer) << '\\n';\n\n    // writing access\n\n    // change the string\n    j.at(\"/string\"_json_pointer) = \"bar\";\n    // output the changed string\n    std::cout << j[\"string\"] << '\\n';\n\n    // change an array element\n    j.at(\"/array/1\"_json_pointer) = 21;\n    // output the changed array\n    std::cout << j[\"array\"] << '\\n';\n\n    // out_of_range.106\n    try\n    {\n        // try to use an array index with leading '0'\n        json::reference ref = j.at(\"/array/01\"_json_pointer);\n    }\n    catch (const json::parse_error& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n\n    // out_of_range.109\n    try\n    {\n        // try to use an array index that is not a number\n        json::reference ref = j.at(\"/array/one\"_json_pointer);\n    }\n    catch (const json::parse_error& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n\n    // out_of_range.401\n    try\n    {\n        // try to use an invalid array index\n        json::reference ref = j.at(\"/array/4\"_json_pointer);\n    }\n    catch (const json::out_of_range& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n\n    // out_of_range.402\n    try\n    {\n        // try to use the array index '-'\n        json::reference ref = j.at(\"/array/-\"_json_pointer);\n    }\n    catch (const json::out_of_range& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n\n    // out_of_range.403\n    try\n    {\n        // try to use a JSON pointer to a nonexistent object key\n        json::const_reference ref = j.at(\"/foo\"_json_pointer);\n    }\n    catch (const json::out_of_range& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n\n    // out_of_range.404\n    try\n    {\n        // try to use a JSON pointer that cannot be resolved\n        json::reference ref = j.at(\"/number/foo\"_json_pointer);\n    }\n    catch (const json::out_of_range& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

Output:

1\n\"foo\"\n[1,2]\n2\n\"bar\"\n[1,21]\n[json.exception.parse_error.106] parse error: array index '01' must not begin with '0'\n[json.exception.parse_error.109] parse error: array index 'one' is not a number\n[json.exception.out_of_range.401] array index 4 is out of range\n[json.exception.out_of_range.402] array index '-' (2) is out of range\n[json.exception.out_of_range.403] key 'foo' not found\n[json.exception.out_of_range.404] unresolved reference token 'foo'\n
Example: (4) access specified element via JSON Pointer

The example below shows how object elements can be read using at(). It also demonstrates the different exceptions that can be thrown.

#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // create a JSON value\n    const json j =\n    {\n        {\"number\", 1}, {\"string\", \"foo\"}, {\"array\", {1, 2}}\n    };\n\n    // read-only access\n\n    // output element with JSON pointer \"/number\"\n    std::cout << j.at(\"/number\"_json_pointer) << '\\n';\n    // output element with JSON pointer \"/string\"\n    std::cout << j.at(\"/string\"_json_pointer) << '\\n';\n    // output element with JSON pointer \"/array\"\n    std::cout << j.at(\"/array\"_json_pointer) << '\\n';\n    // output element with JSON pointer \"/array/1\"\n    std::cout << j.at(\"/array/1\"_json_pointer) << '\\n';\n\n    // out_of_range.109\n    try\n    {\n        // try to use an array index that is not a number\n        json::const_reference ref = j.at(\"/array/one\"_json_pointer);\n    }\n    catch (const json::parse_error& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n\n    // out_of_range.401\n    try\n    {\n        // try to use an invalid array index\n        json::const_reference ref = j.at(\"/array/4\"_json_pointer);\n    }\n    catch (const json::out_of_range& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n\n    // out_of_range.402\n    try\n    {\n        // try to use the array index '-'\n        json::const_reference ref = j.at(\"/array/-\"_json_pointer);\n    }\n    catch (const json::out_of_range& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n\n    // out_of_range.403\n    try\n    {\n        // try to use a JSON pointer to a nonexistent object key\n        json::const_reference ref = j.at(\"/foo\"_json_pointer);\n    }\n    catch (const json::out_of_range& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n\n    // out_of_range.404\n    try\n    {\n        // try to use a JSON pointer that cannot be resolved\n        json::const_reference ref = j.at(\"/number/foo\"_json_pointer);\n    }\n    catch (const json::out_of_range& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

Output:

1\n\"foo\"\n[1,2]\n2\n[json.exception.parse_error.109] parse error: array index 'one' is not a number\n[json.exception.out_of_range.401] array index 4 is out of range\n[json.exception.out_of_range.402] array index '-' (2) is out of range\n[json.exception.out_of_range.403] key 'foo' not found\n[json.exception.out_of_range.404] unresolved reference token 'foo'\n
"},{"location":"api/basic_json/at/#see-also","title":"See also","text":"
  • documentation on checked access
  • operator[] for unchecked access by reference
  • value for access with default value
  • basic_json_view::at - the same access on a zero-copy view
"},{"location":"api/basic_json/at/#version-history","title":"Version history","text":"
  1. Added in version 1.0.0.
  2. Added in version 1.0.0.
  3. Added in version 3.11.0. Fixed in version 3.13.0 unreleased to consistently accept std::string_view-convertible keys, as already supported by operator[], value, find, and other lookup functions.
  4. Added in version 2.0.0. Throws parse_error.109 instead of out_of_range.404 for a one-character array index that is not a digit (e.g., /x) in version 3.13.0 unreleased, as it already did for longer ones.
"},{"location":"api/basic_json/back/","title":"nlohmann::basic_json::back","text":"
reference back();\n\nconst_reference back() const;\n

Returns a reference to the last element in the container. For a JSON container c, the expression c.back() is equivalent to

auto tmp = c.end();\n--tmp;\nreturn *tmp;\n
"},{"location":"api/basic_json/back/#return-value","title":"Return value","text":"

In the case of a structured type (array or object), a reference to the last element is returned. In the case of number, string, boolean, or binary values, a reference to the value is returned.

"},{"location":"api/basic_json/back/#exception-safety","title":"Exception safety","text":"

Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

"},{"location":"api/basic_json/back/#exceptions","title":"Exceptions","text":"

If the JSON value is null, exception invalid_iterator.214 is thrown.

"},{"location":"api/basic_json/back/#complexity","title":"Complexity","text":"

Constant.

"},{"location":"api/basic_json/back/#notes","title":"Notes","text":"

Precondition

The array or object must not be empty. Calling back on an empty array or object yields undefined behavior.

"},{"location":"api/basic_json/back/#examples","title":"Examples","text":"Example

The following code shows an example for back().

#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_boolean = true;\n    json j_number_integer = 17;\n    json j_number_float = 23.42;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_object_empty(json::value_t::object);\n    json j_array = {1, 2, 4, 8, 16};\n    json j_array_empty(json::value_t::array);\n    json j_string = \"Hello, world\";\n\n    // call back()\n    std::cout << j_boolean.back() << '\\n';\n    std::cout << j_number_integer.back() << '\\n';\n    std::cout << j_number_float.back() << '\\n';\n    std::cout << j_object.back() << '\\n';\n    //std::cout << j_object_empty.back() << '\\n';  // undefined behavior\n    std::cout << j_array.back() << '\\n';\n    //std::cout << j_array_empty.back() << '\\n';   // undefined behavior\n    std::cout << j_string.back() << '\\n';\n\n    // back() called on a null value\n    try\n    {\n        json j_null;\n        j_null.back();\n    }\n    catch (const json::invalid_iterator& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

Output:

true\n17\n23.42\n2\n16\n\"Hello, world\"\n[json.exception.invalid_iterator.214] cannot get value\n
"},{"location":"api/basic_json/back/#see-also","title":"See also","text":"
  • front to access the first element
  • basic_json_view::back - the same access on a zero-copy view
"},{"location":"api/basic_json/back/#version-history","title":"Version history","text":"
  • Added in version 1.0.0.
  • Adjusted code to return reference to binary values in version 3.8.0.
"},{"location":"api/basic_json/basic_json/","title":"nlohmann::basic_json::basic_json","text":"
// (1)\nbasic_json(const value_t v);\n\n// (2)\nbasic_json(std::nullptr_t = nullptr) noexcept;\n\n// (3)\ntemplate<typename CompatibleType>\nbasic_json(CompatibleType&& val) noexcept(noexcept(\n           JSONSerializer<U>::to_json(std::declval<basic_json_t&>(),\n                                      std::forward<CompatibleType>(val))));\n\n// (4)\ntemplate<typename BasicJsonType>\nbasic_json(const BasicJsonType& val);\n\n// (5)\nbasic_json(initializer_list_t init,\n           bool type_deduction = true,\n           value_t manual_type = value_t::array);\n\n// (6)\nbasic_json(size_type cnt, const basic_json& val);\n\n// (7)\nbasic_json(iterator first, iterator last);\nbasic_json(const_iterator first, const_iterator last);\n\n// (8)\nbasic_json(const basic_json& other);\n\n// (9)\nbasic_json(basic_json&& other) noexcept;\n

  1. Create an empty JSON value with a given type. The value will be default initialized with an empty value which depends on the type:

    Value type initial value null null boolean false string \"\" number 0 object {} array [] binary empty array

    The postcondition of this constructor can be restored by calling clear().

  2. Create a null JSON value. It either takes a null pointer as parameter (explicitly creating null) or no parameter (implicitly creating null). The passed null pointer itself is not read -- it is only used to choose the right constructor.

  3. This is a \"catch all\" constructor for all compatible JSON types; that is, types for which a to_json() method exists. The constructor forwards the parameter val to that method (to json_serializer<U>::to_json method with U = uncvref_t<CompatibleType>, to be exact).

    Template type CompatibleType includes, but is not limited to, the following types:

    • arrays: array_t and all kinds of compatible containers such as std::vector, std::deque, std::list, std::forward_list, std::array, std::valarray, std::set, std::unordered_set, std::multiset, and std::unordered_multiset with a value_type from which a basic_json value can be constructed.
    • objects: object_t and all kinds of compatible associative containers such as std::map, std::unordered_map, std::multimap, and std::unordered_multimap with a key_type compatible to string_t and a value_type from which a basic_json value can be constructed.
    • strings: string_t, string literals, and all compatible string containers can be used.

    Rvalue containers are moved from

    If val is an rvalue of a compatible array-like or object-like container other than array_t/object_t (e.g., json j = std::move(vec); or json j(std::vector<std::string>{...});), the elements are moved into the JSON value instead of copied. Afterwards, the elements of the source container are in a valid but unspecified state (moved-from). Keys of associative containers are const and therefore still copied. Lvalues and const rvalues are copied, and the source is left unchanged.

    • numbers: number_integer_t, number_unsigned_t, number_float_t, and all convertible number types such as int, size_t, int64_t, float or double can be used.
    • boolean: boolean_t / bool can be used.
    • binary: binary_t / std::vector<uint8_t> may be used; unfortunately because string literals cannot be distinguished from binary character arrays by the C++ type system, all types compatible with const char* will be directed to the string constructor instead. This is both for backwards compatibility and due to the fact that a binary type is not a standard JSON type.

    See the examples below.

  4. This is a constructor for existing basic_json types. It does not hijack copy/move constructors, since the parameter has different template arguments than the current ones.

    The constructor tries to convert the internal m_value of the parameter. Each member value (object, array, string, etc.) is serialized via the corresponding to_json() overload. For objects and strings, the conversion requires that the target basic_json type's object_t::key_type (or string_t) be directly constructible from the source type's corresponding member type via is_constructible. If this requirement is not met, the conversion does not fail to compile; instead, it silently falls back to the array-conversion path, which represents objects as arrays of [key, value] pairs and strings as arrays of character codes. This is a known limitation tracked in issue #3425.

  5. Creates a JSON value of type array or object from the passed initializer list init. In case type_deduction is true (default), the type of the JSON value to be created is deducted from the initializer list init according to the following rules:

    1. If the list is empty, an empty JSON object value {} is created.
    2. If the list consists of pairs whose first element is a string, a JSON object value is created where the first elements of the pairs are treated as keys and the second elements are as values.
    3. In all other cases, an array is created.

    The following flowchart also takes into account what happens when type_deduction is false, in which case manual_type decides between object and array, and an object can only be forced if init actually matches rule 2 (or is empty):

    flowchart TD\n    A([\"initializer_list init\"]) --> B{\"empty, or every element is a 2-element<br/>array whose first element is a string?\"}\n    B -->|\"yes\"| C{\"type_deduction\"}\n    B -->|\"no\"| D{\"type_deduction\"}\n    C -->|\"true\"| OBJ[\"create object\"]\n    C -->|\"false\"| E{\"manual_type\"}\n    E -->|\"object\"| OBJ\n    E -->|\"array\"| ARR[\"create array\"]\n    D -->|\"true\"| ARR\n    D -->|\"false\"| F{\"manual_type\"}\n    F -->|\"array\"| ARR\n    F -->|\"object\"| ERR[\"throw type_error.301\"]

    The rules aim to create the best fit between a C++ initializer list and JSON values. The rationale is as follows:

    1. The empty initializer list is written as {} which is exactly an empty JSON object.
    2. C++ has no way of describing mapped types other than to list a list of pairs. As JSON requires that keys must be of type string, rule 2 is the weakest constraint one can pose on initializer lists to interpret them as an object.
    3. In all other cases, the initializer list could not be interpreted as a JSON object type, so interpreting it as a JSON array type is safe.

    With the rules described above, the following JSON values cannot be expressed by an initializer list:

    • the empty array ([]): use array(initializer_list_t) with an empty initializer list in this case
    • arrays whose elements satisfy rule 2: use array(initializer_list_t) with the same initializer list in this case

    Function array() and object() force array and object creation from initializer lists, respectively.

    Brace initialization yields arrays

    Because this constructor takes an initializer_list_t, brace-initializing a json/ordered_json from another json value wraps it in a single-element array rather than copying it:

    json j1 = \"hello\";\njson j2{j1};   // [!] j2 is [\"hello\"], NOT a copy of j1\njson j3(j1);   // j3 is \"hello\" -- parentheses copy as expected\n

    See the FAQ entry on brace initialization for the full explanation, an opt-in macro to change this behavior, and how to explicitly create a single-element array (json::array({value})) if that is what you want.

  6. Constructs a JSON array value by creating cnt copies of a passed value. In case cnt is 0, an empty array is created.

  7. Constructs the JSON value with the contents of the range [first, last). The semantics depend on the different types a JSON value can have:

    • In case of a null type, invalid_iterator.206 is thrown.
    • In case of other primitive types (number, boolean, string, or binary), first must be begin() and last must be end(). In this case, the value is copied. Otherwise, invalid_iterator.204 is thrown.
    • In case of structured types (array, object), the constructor behaves as similar versions for std::vector or std::map; that is, a JSON array or object is constructed from the values in the range.

  8. Creates a copy of a given JSON value.

  9. Move constructor. Constructs a JSON value with the contents of the given value other using move semantics. It \"steals\" the resources from other and leaves it as JSON null value.

"},{"location":"api/basic_json/basic_json/#template-parameters","title":"Template parameters","text":"CompatibleType

a type such that:

  • CompatibleType is not derived from std::istream,
  • CompatibleType is not basic_json (to avoid hijacking copy/move constructors),
  • CompatibleType is not a different basic_json type (i.e. with different template arguments)
  • CompatibleType is not a basic_json nested type (e.g., json_pointer, iterator, etc.)
  • if JSON_DISABLE_TUPLE_REFERENCE_CONVERSION is defined to 1: CompatibleType is not a one-element std::tuple holding a reference to basic_json
  • json_serializer<U> (with U = uncvref_t<CompatibleType>) has a to_json(basic_json_t&, CompatibleType&&) method
BasicJsonType:

a type such that:

  • BasicJsonType is a basic_json type.
  • BasicJsonType has different template arguments than basic_json_t.

Note: For cross-basic_json conversions to produce correct results, the target basic_json's object_t::key_type and string_t must be directly constructible from the source basic_json's corresponding types. See the description of overload (4) above for details on what happens when this requirement is not met.

U: uncvref_t<CompatibleType>"},{"location":"api/basic_json/basic_json/#parameters","title":"Parameters","text":"v (in) the type of the value to create val (in) the value to be forwarded to the respective constructor init (in) initializer list with JSON values type_deduction (in) internal parameter; when set to true, the type of the JSON value is deducted from the initializer list init; when set to false, the type provided via manual_type is forced. This mode is used by the functions array(initializer_list_t) and object(initializer_list_t). manual_type (in) internal parameter; when type_deduction is set to false, the created JSON value will use the provided type (only value_t::array and value_t::object are valid); when type_deduction is set to true, this parameter has no effect cnt (in) the number of JSON copies of val to create first (in) the beginning of the range to copy from (included) last (in) the end of the range to copy from (excluded) other (in) the JSON value to copy/move"},{"location":"api/basic_json/basic_json/#exception-safety","title":"Exception safety","text":"
  1. Strong guarantee: if an exception is thrown, there are no changes to any JSON value.
  2. No-throw guarantee: this constructor never throws exceptions.
  3. Depends on the called constructor. For types directly supported by the library (i.e., all types for which no to_json() function was provided), a strong guarantee holds: if an exception is thrown, there are no changes to any JSON value. Exception: when val is an rvalue array-like or object-like container, its elements are moved one by one; if an exception is thrown in the middle of the conversion (e.g., an allocation failure), the JSON value is unchanged, but the source container may be left partially moved-from.
  4. Depends on the called constructor. For types directly supported by the library (i.e., all types for which no to_json() function was provided), a strong guarantee holds: if an exception is thrown, there are no changes to any JSON value.
  5. Strong guarantee: if an exception is thrown, there are no changes to any JSON value.
  6. Strong guarantee: if an exception is thrown, there are no changes to any JSON value.
  7. Strong guarantee: if an exception is thrown, there are no changes to any JSON value.
  8. Strong guarantee: if an exception is thrown, there are no changes to any JSON value.
  9. No-throw guarantee: this constructor never throws exceptions.
"},{"location":"api/basic_json/basic_json/#exceptions","title":"Exceptions","text":"
  1. (none)
  2. The function does not throw exceptions.
  3. (none)
  4. (none)
  5. The function can throw the following exceptions:
    • Throws type_error.301 if type_deduction is false, manual_type is value_t::object, but init contains an element which is not a pair whose first element is a string. In this case, the constructor could not create an object. If type_deduction would have been true, an array would have been created. See object(initializer_list_t) for an example.
  6. (none)
  7. The function can throw the following exceptions:
    • Throws invalid_iterator.201 if iterators first and last are not compatible (i.e., do not belong to the same JSON value). In this case, the range [first, last) is undefined.
    • Throws invalid_iterator.204 if iterators first and last belong to a primitive type (number, boolean, string, or binary), but first does not point to the first element anymore. In this case, the range [first, last) is undefined. See the example code below.
    • Throws invalid_iterator.206 if iterators first and last belong to a null value. In this case, the range [first, last) is undefined.
  8. (none)
  9. The function does not throw exceptions.
"},{"location":"api/basic_json/basic_json/#complexity","title":"Complexity","text":"
  1. Constant.
  2. Constant.
  3. Usually linear in the size of the passed val, also depending on the implementation of the called to_json() method.
  4. Usually linear in the size of the passed val, also depending on the implementation of the called to_json() method.
  5. Linear in the size of the initializer list init.
  6. Linear in cnt.
  7. Linear in distance between first and last.
  8. Linear in the size of other.
  9. Constant.
"},{"location":"api/basic_json/basic_json/#notes","title":"Notes","text":"

  • Overload 5:

    Empty initializer list

    When used without parentheses around an empty initializer list, basic_json() is called instead of this function, yielding the JSON null value.

  • Overload 4:

    Implicit conversion

    The conversion is implicit unless JSON_USE_IMPLICIT_CONVERSIONS is defined to 0 and BasicJsonType::string_t differs from string_t. In that case, the constructor is explicit, so a JSON value with a different string type is no longer silently converted, for example when it is passed to a function taking const json&. Write json(other) or other.get<json>() instead.

  • Overload 7:

    Preconditions

    • Iterators first and last must be initialized. **This precondition is enforced with a runtime assertion.
    • Range [first, last) is valid. Usually, this precondition cannot be checked efficiently. Only certain edge cases are detected; see the description of the exceptions above. A violation of this precondition yields undefined behavior.

    Runtime assertion

    A precondition is enforced with a runtime assertion.

  • Overload 8:

    Postcondition

    *this == other

  • Overload 9:

    Postconditions

    • *thishas the same value asother` before the call.
    • other is a JSON null value
"},{"location":"api/basic_json/basic_json/#examples","title":"Examples","text":"Example: (1) create an empty value with a given type

The following code shows the constructor for different value_t values.

#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create the different JSON values with default values\n    json j_null(json::value_t::null);\n    json j_boolean(json::value_t::boolean);\n    json j_number_integer(json::value_t::number_integer);\n    json j_number_float(json::value_t::number_float);\n    json j_object(json::value_t::object);\n    json j_array(json::value_t::array);\n    json j_string(json::value_t::string);\n\n    // serialize the JSON values\n    std::cout << j_null << '\\n';\n    std::cout << j_boolean << '\\n';\n    std::cout << j_number_integer << '\\n';\n    std::cout << j_number_float << '\\n';\n    std::cout << j_object << '\\n';\n    std::cout << j_array << '\\n';\n    std::cout << j_string << '\\n';\n}\n

Output:

null\nfalse\n0\n0.0\n{}\n[]\n\"\"\n
Example: (2) create a null object

The following code shows the constructor with and without a null pointer parameter.

#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // implicitly create a JSON null value\n    json j1;\n\n    // explicitly create a JSON null value\n    json j2(nullptr);\n\n    // serialize the JSON null value\n    std::cout << j1 << '\\n' << j2 << '\\n';\n}\n

Output:

null\nnull\n
Example: (3) create a JSON value from compatible types

The following code shows the constructor with several compatible types.

#include <iostream>\n#include <deque>\n#include <list>\n#include <forward_list>\n#include <set>\n#include <unordered_map>\n#include <unordered_set>\n#include <valarray>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // ============\n    // object types\n    // ============\n\n    // create an object from an object_t value\n    json::object_t object_value = { {\"one\", 1}, {\"two\", 2} };\n    json j_object_t(object_value);\n\n    // create an object from std::map\n    std::map<std::string, int> c_map\n    {\n        {\"one\", 1}, {\"two\", 2}, {\"three\", 3}\n    };\n    json j_map(c_map);\n\n    // create an object from std::unordered_map\n    std::unordered_map<const char*, double> c_umap\n    {\n        {\"one\", 1.2}, {\"two\", 2.3}, {\"three\", 3.4}\n    };\n    json j_umap(c_umap);\n\n    // create an object from std::multimap\n    std::multimap<std::string, bool> c_mmap\n    {\n        {\"one\", true}, {\"two\", true}, {\"three\", false}, {\"three\", true}\n    };\n    json j_mmap(c_mmap); // only one entry for key \"three\" is used\n\n    // create an object from std::unordered_multimap\n    std::unordered_multimap<std::string, bool> c_ummap\n    {\n        {\"one\", true}, {\"two\", true}, {\"three\", false}, {\"three\", false}\n    };\n    json j_ummap(c_ummap); // only one entry for key \"three\" is used\n\n    // serialize the JSON objects\n    std::cout << j_object_t << '\\n';\n    std::cout << j_map << '\\n';\n    std::cout << j_umap << '\\n';\n    std::cout << j_mmap << '\\n';\n    std::cout << j_ummap << \"\\n\\n\";\n\n    // ===========\n    // array types\n    // ===========\n\n    // create an array from an array_t value\n    json::array_t array_value = {\"one\", \"two\", 3, 4.5, false};\n    json j_array_t(array_value);\n\n    // create an array from std::vector\n    std::vector<int> c_vector {1, 2, 3, 4};\n    json j_vec(c_vector);\n\n    // create an array from std::valarray\n    std::valarray<short> c_valarray {10, 9, 8, 7};\n    json j_valarray(c_valarray);\n\n    // create an array from std::deque\n    std::deque<double> c_deque {1.2, 2.3, 3.4, 5.6};\n    json j_deque(c_deque);\n\n    // create an array from std::list\n    std::list<bool> c_list {true, true, false, true};\n    json j_list(c_list);\n\n    // create an array from std::forward_list\n    std::forward_list<std::int64_t> c_flist {12345678909876, 23456789098765, 34567890987654, 45678909876543};\n    json j_flist(c_flist);\n\n    // create an array from std::array\n    std::array<unsigned long, 4> c_array {{1, 2, 3, 4}};\n    json j_array(c_array);\n\n    // create an array from std::set\n    std::set<std::string> c_set {\"one\", \"two\", \"three\", \"four\", \"one\"};\n    json j_set(c_set); // only one entry for \"one\" is used\n\n    // create an array from std::unordered_set\n    std::unordered_set<std::string> c_uset {\"one\", \"one\"};\n    json j_uset(c_uset); // only one entry for \"one\" is used\n\n    // create an array from std::multiset\n    std::multiset<std::string> c_mset {\"one\", \"two\", \"one\", \"four\"};\n    json j_mset(c_mset); // both entries for \"one\" are used\n\n    // create an array from std::unordered_multiset\n    std::unordered_multiset<std::string> c_umset {\"one\", \"one\"};\n    json j_umset(c_umset); // both entries for \"one\" are used\n\n    // serialize the JSON arrays\n    std::cout << j_array_t << '\\n';\n    std::cout << j_vec << '\\n';\n    std::cout << j_valarray << '\\n';\n    std::cout << j_deque << '\\n';\n    std::cout << j_list << '\\n';\n    std::cout << j_flist << '\\n';\n    std::cout << j_array << '\\n';\n    std::cout << j_set << '\\n';\n    std::cout << j_uset << '\\n';\n    std::cout << j_mset << '\\n';\n    std::cout << j_umset << \"\\n\\n\";\n\n    // ============\n    // string types\n    // ============\n\n    // create string from a string_t value\n    json::string_t string_value = \"The quick brown fox jumps over the lazy dog.\";\n    json j_string_t(string_value);\n\n    // create a JSON string directly from a string literal\n    json j_string_literal(\"The quick brown fox jumps over the lazy dog.\");\n\n    // create string from std::string\n    std::string s_stdstring = \"The quick brown fox jumps over the lazy dog.\";\n    json j_stdstring(s_stdstring);\n\n    // serialize the JSON strings\n    std::cout << j_string_t << '\\n';\n    std::cout << j_string_literal << '\\n';\n    std::cout << j_stdstring << \"\\n\\n\";\n\n    // ============\n    // number types\n    // ============\n\n    // create a JSON number from number_integer_t\n    json::number_integer_t value_integer_t = -42;\n    json j_integer_t(value_integer_t);\n\n    // create a JSON number from number_unsigned_t\n    json::number_integer_t value_unsigned_t = 17;\n    json j_unsigned_t(value_unsigned_t);\n\n    // create a JSON number from an anonymous enum\n    enum { enum_value = 17 };\n    json j_enum(enum_value);\n\n    // create values of different integer types\n    short n_short = 42;\n    int n_int = -23;\n    long n_long = 1024;\n    int_least32_t n_int_least32_t = -17;\n    uint8_t n_uint8_t = 8;\n\n    // create (integer) JSON numbers\n    json j_short(n_short);\n    json j_int(n_int);\n    json j_long(n_long);\n    json j_int_least32_t(n_int_least32_t);\n    json j_uint8_t(n_uint8_t);\n\n    // create values of different floating-point types\n    json::number_float_t v_ok = 3.141592653589793;\n    json::number_float_t v_nan = NAN;\n    json::number_float_t v_infinity = INFINITY;\n\n    // create values of different floating-point types\n    float n_float = 42.23;\n    float n_float_nan = 1.0f / 0.0f;\n    double n_double = 23.42;\n\n    // create (floating point) JSON numbers\n    json j_ok(v_ok);\n    json j_nan(v_nan);\n    json j_infinity(v_infinity);\n    json j_float(n_float);\n    json j_float_nan(n_float_nan);\n    json j_double(n_double);\n\n    // serialize the JSON numbers\n    std::cout << j_integer_t << '\\n';\n    std::cout << j_unsigned_t << '\\n';\n    std::cout << j_enum << '\\n';\n    std::cout << j_short << '\\n';\n    std::cout << j_int << '\\n';\n    std::cout << j_long << '\\n';\n    std::cout << j_int_least32_t << '\\n';\n    std::cout << j_uint8_t << '\\n';\n    std::cout << j_ok << '\\n';\n    std::cout << j_nan << '\\n';\n    std::cout << j_infinity << '\\n';\n    std::cout << j_float << '\\n';\n    std::cout << j_float_nan << '\\n';\n    std::cout << j_double << \"\\n\\n\";\n\n    // =============\n    // boolean types\n    // =============\n\n    // create boolean values\n    json j_truth = true;\n    json j_falsity = false;\n\n    // serialize the JSON booleans\n    std::cout << j_truth << '\\n';\n    std::cout << j_falsity << '\\n';\n}\n

Output:

{\"one\":1,\"two\":2}\n{\"one\":1,\"three\":3,\"two\":2}\n{\"one\":1.2,\"three\":3.4,\"two\":2.3}\n{\"one\":true,\"three\":false,\"two\":true}\n{\"one\":true,\"three\":false,\"two\":true}\n\n[\"one\",\"two\",3,4.5,false]\n[1,2,3,4]\n[10,9,8,7]\n[1.2,2.3,3.4,5.6]\n[true,true,false,true]\n[12345678909876,23456789098765,34567890987654,45678909876543]\n[1,2,3,4]\n[\"four\",\"one\",\"three\",\"two\"]\n[\"one\"]\n[\"four\",\"one\",\"one\",\"two\"]\n[\"one\",\"one\"]\n\n\"The quick brown fox jumps over the lazy dog.\"\n\"The quick brown fox jumps over the lazy dog.\"\n\"The quick brown fox jumps over the lazy dog.\"\n\n-42\n17\n17\n42\n-23\n1024\n-17\n8\n3.141592653589793\nnull\nnull\n42.22999954223633\nnull\n23.42\n\ntrue\nfalse\n

Note the output is platform-dependent.

Example: (4) create a JSON value from another basic_json specialization

The example below shows how a json value is converted to an ordered_json value and back using the converting constructor. Note how the original insertion order of oj is not restored, because it was already given up when converting to json, whose object_t sorts by key.

#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing ordered_json = nlohmann::ordered_json;\n\nint main()\n{\n    // create an ordered_json value; insertion order is preserved\n    ordered_json oj = {{\"c\", 3}, {\"a\", 1}, {\"b\", 2}};\n\n    // convert to json -- overload (4) is used; keys end up sorted\n    json j(oj);\n\n    // convert back to ordered_json -- the original insertion order is lost,\n    // because it was already given up when converting to json\n    ordered_json oj2(j);\n\n    std::cout << oj << '\\n';\n    std::cout << j << '\\n';\n    std::cout << oj2 << '\\n';\n}\n

Output:

{\"c\":3,\"a\":1,\"b\":2}\n{\"a\":1,\"b\":2,\"c\":3}\n{\"a\":1,\"b\":2,\"c\":3}\n
Example: (5) create a container (array or object) from an initializer list

The example below shows how JSON values are created from initializer lists.

#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_empty_init_list = json({});\n    json j_object = { {\"one\", 1}, {\"two\", 2} };\n    json j_array = {1, 2, 3, 4};\n    json j_nested_object = { {\"one\", {1}}, {\"two\", {1, 2}} };\n    json j_nested_array = { {{1}, \"one\"}, {{1, 2}, \"two\"} };\n\n    // serialize the JSON value\n    std::cout << j_empty_init_list << '\\n';\n    std::cout << j_object << '\\n';\n    std::cout << j_array << '\\n';\n    std::cout << j_nested_object << '\\n';\n    std::cout << j_nested_array << '\\n';\n}\n

Output:

{}\n{\"one\":1,\"two\":2}\n[1,2,3,4]\n{\"one\":[1],\"two\":[1,2]}\n[[[1],\"one\"],[[1,2],\"two\"]]\n
Example: (6) construct an array with count copies of a given value

The following code shows examples for creating arrays with several copies of a given value.

#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create an array by creating copies of a JSON value\n    json value = \"Hello\";\n    json array_0 = json(0, value);\n    json array_1 = json(1, value);\n    json array_5 = json(5, value);\n\n    // serialize the JSON arrays\n    std::cout << array_0 << '\\n';\n    std::cout << array_1 << '\\n';\n    std::cout << array_5 << '\\n';\n}\n

Output:

[]\n[\"Hello\"]\n[\"Hello\",\"Hello\",\"Hello\",\"Hello\",\"Hello\"]\n
Example: (7) construct a JSON container given an iterator range

The example below shows several ways to create JSON values by specifying a subrange with iterators.

#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_array = {\"alpha\", \"bravo\", \"charly\", \"delta\", \"easy\"};\n    json j_number = 42;\n    json j_object = {{\"one\", \"eins\"}, {\"two\", \"zwei\"}};\n\n    // create copies using iterators\n    json j_array_range(j_array.begin() + 1, j_array.end() - 2);\n    json j_number_range(j_number.begin(), j_number.end());\n    json j_object_range(j_object.begin(), j_object.find(\"two\"));\n\n    // serialize the values\n    std::cout << j_array_range << '\\n';\n    std::cout << j_number_range << '\\n';\n    std::cout << j_object_range << '\\n';\n\n    // example for an exception\n    try\n    {\n        json j_invalid(j_number.begin() + 1, j_number.end());\n    }\n    catch (const json::invalid_iterator& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

Output:

[\"bravo\",\"charly\"]\n42\n{\"one\":\"eins\"}\n[json.exception.invalid_iterator.204] iterators out of range\n
Example: (8) copy constructor

The following code shows an example for the copy constructor.

#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON array\n    json j1 = {\"one\", \"two\", 3, 4.5, false};\n\n    // create a copy\n    json j2(j1);\n\n    // serialize the JSON array\n    std::cout << j1 << \" = \" << j2 << '\\n';\n    std::cout << std::boolalpha << (j1 == j2) << '\\n';\n}\n

Output:

[\"one\",\"two\",3,4.5,false] = [\"one\",\"two\",3,4.5,false]\ntrue\n
Example: (9) move constructor

The code below shows the move constructor explicitly called via std::move.

#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON value\n    json a = 23;\n\n    // move contents of a to b\n    json b(std::move(a));\n\n    // serialize the JSON arrays\n    std::cout << a << '\\n';\n    std::cout << b << '\\n';\n}\n

Output:

null\n23\n
"},{"location":"api/basic_json/basic_json/#see-also","title":"See also","text":"
  • array create a JSON array value, forcing array creation from an initializer list even when it looks like an object
  • object create a JSON object value, forcing object creation from an initializer list
  • binary create a JSON binary array value
  • operator= copy assignment operator
  • Creating JSON values - the article on creating JSON values
"},{"location":"api/basic_json/basic_json/#version-history","title":"Version history","text":"
  1. Since version 1.0.0.
  2. Since version 1.0.0.
  3. Since version 2.1.0.
  4. Since version 3.2.0. Explicit for different string types if JSON_USE_IMPLICIT_CONVERSIONS is 0 since version 3.13.0 unreleased.
  5. Since version 1.0.0.
  6. Since version 1.0.0.
  7. Since version 1.0.0. Fixed in version 3.13.0 unreleased to also check the iterator range for binary values; before, a range that did not cover the whole value (such as (end(), end())) was accepted and the whole binary value was copied, unlike the other primitive types.
  8. Since version 1.0.0.
  9. Since version 1.0.0.
"},{"location":"api/basic_json/begin/","title":"nlohmann::basic_json::begin","text":"
iterator begin() noexcept;\nconst_iterator begin() const noexcept;\n

Returns an iterator to the first element.

"},{"location":"api/basic_json/begin/#return-value","title":"Return value","text":"

iterator to the first element

"},{"location":"api/basic_json/begin/#exception-safety","title":"Exception safety","text":"

No-throw guarantee: this member function never throws exceptions.

"},{"location":"api/basic_json/begin/#complexity","title":"Complexity","text":"

Constant.

"},{"location":"api/basic_json/begin/#examples","title":"Examples","text":"Example

The following code shows an example for begin().

#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create an array value\n    json array = {1, 2, 3, 4, 5};\n\n    // get an iterator to the first element\n    json::iterator it = array.begin();\n\n    // serialize the element that the iterator points to\n    std::cout << *it << '\\n';\n}\n

Output:

1\n
"},{"location":"api/basic_json/begin/#see-also","title":"See also","text":"
  • end returns an iterator to one past the last element
  • cbegin returns a const iterator to the first element
  • rbegin returns a reverse iterator to the last element
  • items returns an iteration proxy to access keys and values during range-based for loops
  • Iterators - the article on iterators
  • basic_json_view::begin - the same iteration on a zero-copy view (in document order, not sorted by key)
"},{"location":"api/basic_json/begin/#version-history","title":"Version history","text":"
  • Added in version 1.0.0.
"},{"location":"api/basic_json/binary/","title":"nlohmann::basic_json::binary","text":"
// (1)\nstatic basic_json binary(const typename binary_t::container_type& init);\nstatic basic_json binary(typename binary_t::container_type&& init);\n\n// (2)\nstatic basic_json binary(const typename binary_t::container_type& init,\n                         typename binary_t::subtype_type subtype);\nstatic basic_json binary(typename binary_t::container_type&& init,\n                         typename binary_t::subtype_type subtype);\n
  1. Creates a JSON binary array value from a given binary container.
  2. Creates a JSON binary array value from a given binary container with subtype.

Binary values are part of various binary formats, such as CBOR, MessagePack, and BSON. This constructor is used to create a value for serialization to those formats.

"},{"location":"api/basic_json/binary/#parameters","title":"Parameters","text":"init (in) container containing bytes to use as a binary type subtype (in) subtype to use in CBOR, MessagePack, and BSON"},{"location":"api/basic_json/binary/#return-value","title":"Return value","text":"

JSON binary array value

"},{"location":"api/basic_json/binary/#exception-safety","title":"Exception safety","text":"

Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

"},{"location":"api/basic_json/binary/#complexity","title":"Complexity","text":"

Linear in the size of init; constant for typename binary_t::container_type&& init versions.

"},{"location":"api/basic_json/binary/#notes","title":"Notes","text":"

Note, this function exists because of the difficulty in correctly specifying the correct template overload in the standard value ctor, as both JSON arrays and JSON binary arrays are backed with some form of a std::vector. Because JSON binary arrays are a non-standard extension, it was decided that it would be best to prevent automatic initialization of a binary array type, for backwards compatibility and so it does not happen on accident.

"},{"location":"api/basic_json/binary/#examples","title":"Examples","text":"Example

The following code shows how to create a binary value.

#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a binary vector\n    std::vector<std::uint8_t> vec = {0xCA, 0xFE, 0xBA, 0xBE};\n\n    // create a binary JSON value with subtype 42\n    json j = json::binary(vec, 42);\n\n    // output type and subtype\n    std::cout << \"type: \" << j.type_name() << \", subtype: \" << j.get_binary().subtype() << std::endl;\n}\n

Output:

type: binary, subtype: 42\n
"},{"location":"api/basic_json/binary/#see-also","title":"See also","text":"
  • binary_t type for binary values
  • get_binary get a reference to the stored binary value
  • is_binary return whether the value is binary
  • byte_container_with_subtype container for binary values with subtype
  • Binary Values - the article on binary values
"},{"location":"api/basic_json/binary/#version-history","title":"Version history","text":"
  • Added in version 3.8.0.
  • Changed the type of subtype from std::uint8_t to binary_t::subtype_type (std::uint64_t) in version 3.10.0.
"},{"location":"api/basic_json/binary_t/","title":"nlohmann::basic_json::binary_t","text":"
using binary_t = byte_container_with_subtype<BinaryType>;\n

This type is a type designed to carry binary data that appears in various serialized formats, such as CBOR's Major Type 2, MessagePack's bin, and BSON's generic binary subtype. This type is NOT a part of standard JSON and exists solely for compatibility with these binary types. As such, it is simply defined as an ordered sequence of zero or more byte values.

Additionally, as an implementation detail, the subtype of the binary data is carried around as a std::uint64_t, which is compatible with both of the binary data formats that use binary subtyping, (though the specific numbering is incompatible with each other, and it is up to the user to translate between them). The subtype is added to BinaryType via the helper type byte_container_with_subtype.

CBOR's RFC 8949 describes this type as:

Major type 2: A byte string. The number of bytes in the string is equal to the argument.

MessagePack's documentation on the bin type family describes this type as:

Bin format family stores a byte array in 2, 3, or 5 bytes of extra bytes in addition to the size of the byte array.

BSON's specifications describe several binary types; however, this type is intended to represent the generic binary type which has the description:

Generic binary subtype - This is the most commonly used binary subtype and should be the 'default' for drivers and tools.

None of these impose any limitations on the internal representation other than the basic unit of storage be some type of array whose parts are decomposable into bytes.

The default representation of this binary format is a std::vector<std::uint8_t>, which is a very common way to represent a byte array in modern C++.

"},{"location":"api/basic_json/binary_t/#template-parameters","title":"Template parameters","text":"BinaryType

container type to store arrays

Although not formally expressed as a C++ concept, BinaryType must be default-constructible, copy/move-constructible, and support push_back(), .data(), and .size(), because byte_container_with_subtype derives directly from it. Its value_type must additionally be exactly one byte wide (e.g., std::uint8_t/char/std::byte): the binary serializers (CBOR, MessagePack, BSON, UBJSON) read and write the container's raw bytes via reinterpret_cast, which is only correct for byte-sized elements -- a container like std::vector<std::intptr_t> will not work as BinaryType. The elements must be stored contiguously, and the binary readers additionally require resize() and operator[]. See Template Parameter Requirements for the full list.

"},{"location":"api/basic_json/binary_t/#notes","title":"Notes","text":""},{"location":"api/basic_json/binary_t/#default-type","title":"Default type","text":"

The default values for BinaryType is std::vector<std::uint8_t>.

"},{"location":"api/basic_json/binary_t/#supported-byte-types","title":"Supported byte types","text":"

std::vector<std::uint8_t>, std::vector<char>, and std::vector<std::byte> are supported. Regardless of which of them is configured, dump writes the bytes as the numbers 0..255.

"},{"location":"api/basic_json/binary_t/#custom-binarytype-behavior","title":"Custom BinaryType behavior","text":"

When a custom BinaryType is configured (other than the default std::vector<std::uint8_t>), you can assign values of that type directly to a basic_json instance, and they will automatically be recognized as binary values rather than arrays:

using custom_json = nlohmann::ordered_json::with_binary_t<std::vector<std::byte>>;\n\nstd::vector<std::byte> data{std::byte{1}, std::byte{2}, std::byte{3}};\ncustom_json j = data;  // Creates a binary value, not an array\nassert(j.is_binary());\n\n// Round-tripping works seamlessly\nauto extracted = j.get<std::vector<std::byte>>();\nassert(extracted == data);\n

This automatic type detection is a convenience feature that only applies to custom (non-default) BinaryType configurations. The default nlohmann::json continues to treat std::vector<std::uint8_t> as arrays for backward compatibility.

"},{"location":"api/basic_json/binary_t/#storage","title":"Storage","text":"

Binary Arrays are stored as pointers in a basic_json type. That is, for any access to array values, a pointer of the type binary_t* must be dereferenced.

"},{"location":"api/basic_json/binary_t/#notes-on-subtypes","title":"Notes on subtypes","text":"

  • CBOR

    • Binary values are represented as byte strings. Subtypes are written as tags.

  • MessagePack

    • If a subtype is given and the binary array contains exactly 1, 2, 4, 8, or 16 elements, the fixext family (fixext1, fixext2, fixext4, fixext8) is used. For other sizes, the ext family (ext8, ext16, ext32) is used. The subtype is then added as a signed 8-bit integer.
    • If no subtype is given, the bin family (bin8, bin16, bin32) is used.

  • BSON

    • If a subtype is given, it is used and added as an unsigned 8-bit integer.
    • If no subtype is given, the generic binary subtype 0x00 is used.
"},{"location":"api/basic_json/binary_t/#examples","title":"Examples","text":"Example

The following code shows that binary_t is by default, a typedef to nlohmann::byte_container_with_subtype<std::vector<std::uint8_t>>.

#include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::boolalpha << std::is_same<nlohmann::byte_container_with_subtype<std::vector<std::uint8_t>>, json::binary_t>::value << std::endl;\n}\n

Output:

true\n
"},{"location":"api/basic_json/binary_t/#see-also","title":"See also","text":"
  • byte_container_with_subtype
"},{"location":"api/basic_json/binary_t/#version-history","title":"Version history","text":"
  • Added in version 3.8.0. Changed the type of subtype to std::uint64_t in version 3.10.0.
  • Fixed dump, std::hash, and to_ubjson for byte types that are not integers (e.g., std::byte) in version 3.13.0 unreleased. dump now writes the bytes of a signed byte type (e.g., char) as 0..255 rather than as negative numbers.
"},{"location":"api/basic_json/boolean_t/","title":"nlohmann::basic_json::boolean_t","text":"
using boolean_t = BooleanType;\n

The type used to store JSON booleans.

RFC 8259 implicitly describes a boolean as a type which differentiates the two literals true and false.

To store boolean values in C++, a type is defined by the template parameter BooleanType which chooses the type to use.

"},{"location":"api/basic_json/boolean_t/#template-parameters","title":"Template parameters","text":"BooleanType the type to store booleans. As it is stored directly inside a basic_json value (in a union), it must be a trivially default-constructible, trivially copyable, and trivially destructible type that is convertible to and from bool. See Template Parameter Requirements."},{"location":"api/basic_json/boolean_t/#notes","title":"Notes","text":""},{"location":"api/basic_json/boolean_t/#default-type","title":"Default type","text":"

With the default values for BooleanType (bool), the default type for boolean_t is bool.

"},{"location":"api/basic_json/boolean_t/#storage","title":"Storage","text":"

Boolean values are stored directly inside a basic_json type.

"},{"location":"api/basic_json/boolean_t/#examples","title":"Examples","text":"Example

The following code shows that boolean_t is by default, a typedef to bool.

#include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::boolalpha << std::is_same<bool, json::boolean_t>::value << std::endl;\n}\n

Output:

true\n
"},{"location":"api/basic_json/boolean_t/#see-also","title":"See also","text":"
  • is_boolean checks whether the JSON value is a boolean
"},{"location":"api/basic_json/boolean_t/#version-history","title":"Version history","text":"
  • Added in version 1.0.0.
"},{"location":"api/basic_json/cbegin/","title":"nlohmann::basic_json::cbegin","text":"
const_iterator cbegin() const noexcept;\n

Returns an iterator to the first element.

"},{"location":"api/basic_json/cbegin/#return-value","title":"Return value","text":"

iterator to the first element

"},{"location":"api/basic_json/cbegin/#exception-safety","title":"Exception safety","text":"

No-throw guarantee: this member function never throws exceptions.

"},{"location":"api/basic_json/cbegin/#complexity","title":"Complexity","text":"

Constant.

"},{"location":"api/basic_json/cbegin/#examples","title":"Examples","text":"Example

The following code shows an example for cbegin().

#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create an array value\n    const json array = {1, 2, 3, 4, 5};\n\n    // get an iterator to the first element\n    json::const_iterator it = array.cbegin();\n\n    // serialize the element that the iterator points to\n    std::cout << *it << '\\n';\n}\n

Output:

1\n
"},{"location":"api/basic_json/cbegin/#see-also","title":"See also","text":"
  • begin returns an iterator to the first element
  • cend returns a const iterator to one past the last element
  • crbegin returns a const reverse iterator to the last element
  • Iterators - the article on iterators
  • basic_json_view::cbegin - the same iteration on a zero-copy view
"},{"location":"api/basic_json/cbegin/#version-history","title":"Version history","text":"
  • Added in version 1.0.0.
"},{"location":"api/basic_json/cbor_tag_handler_t/","title":"nlohmann::basic_json::cbor_tag_handler_t","text":"
enum class cbor_tag_handler_t\n{\n    error,\n    ignore,\n    store\n};\n

This enumeration is used in from_cbor and sax_parse to choose how to treat tags:

error report a parse error in case of a tag (the from_cbor overloads throw a parse_error exception by default) ignore ignore tags store store tagged byte strings (for bytes 0xd8..0xdb) as binary values with the tag as subtype; other tagged values are read as if the tag were ignored. If several tags precede a byte string, only the innermost one is stored."},{"location":"api/basic_json/cbor_tag_handler_t/#examples","title":"Examples","text":"Example

The example below shows how the different values of the cbor_tag_handler_t influence the behavior of from_cbor when reading a tagged byte string.

#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // tagged byte string\n    std::vector<std::uint8_t> vec = {{0xd8, 0x42, 0x44, 0xcA, 0xfe, 0xba, 0xbe}};\n\n    // cbor_tag_handler_t::error throws\n    try\n    {\n        auto b_throw_on_tag = json::from_cbor(vec, true, true, json::cbor_tag_handler_t::error);\n    }\n    catch (const json::parse_error& e)\n    {\n        std::cout << e.what() << std::endl;\n    }\n\n    // cbor_tag_handler_t::ignore ignores the tag\n    auto b_ignore_tag = json::from_cbor(vec, true, true, json::cbor_tag_handler_t::ignore);\n    std::cout << b_ignore_tag << std::endl;\n\n    // cbor_tag_handler_t::store stores the tag as binary subtype\n    auto b_store_tag = json::from_cbor(vec, true, true, json::cbor_tag_handler_t::store);\n    std::cout << b_store_tag << std::endl;\n}\n

Output:

[json.exception.parse_error.112] parse error at byte 1: syntax error while parsing CBOR value: invalid byte: 0xD8\n{\"bytes\":[202,254,186,190],\"subtype\":null}\n{\"bytes\":[202,254,186,190],\"subtype\":66}\n
"},{"location":"api/basic_json/cbor_tag_handler_t/#see-also","title":"See also","text":"
  • from_cbor deserializes a JSON value from CBOR
  • input_format_t the enumeration of supported input formats
  • CBOR - the article on the CBOR format
"},{"location":"api/basic_json/cbor_tag_handler_t/#version-history","title":"Version history","text":"
  • Added in version 3.9.0. Added value store in 3.10.0.
"},{"location":"api/basic_json/cend/","title":"nlohmann::basic_json::cend","text":"
const_iterator cend() const noexcept;\n

Returns an iterator to one past the last element.

"},{"location":"api/basic_json/cend/#return-value","title":"Return value","text":"

iterator one past the last element

"},{"location":"api/basic_json/cend/#exception-safety","title":"Exception safety","text":"

No-throw guarantee: this member function never throws exceptions.

"},{"location":"api/basic_json/cend/#complexity","title":"Complexity","text":"

Constant.

"},{"location":"api/basic_json/cend/#examples","title":"Examples","text":"Example

The following code shows an example for cend().

#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create an array value\n    json array = {1, 2, 3, 4, 5};\n\n    // get an iterator to one past the last element\n    json::const_iterator it = array.cend();\n\n    // decrement the iterator to point to the last element\n    --it;\n\n    // serialize the element that the iterator points to\n    std::cout << *it << '\\n';\n}\n

Output:

5\n
"},{"location":"api/basic_json/cend/#see-also","title":"See also","text":"
  • end returns an iterator to one past the last element
  • cbegin returns a const iterator to the first element
  • crend returns a const reverse iterator to one before the first element
  • Iterators - the article on iterators
  • basic_json_view::cend - the same iteration on a zero-copy view
"},{"location":"api/basic_json/cend/#version-history","title":"Version history","text":"
  • Added in version 1.0.0.
"},{"location":"api/basic_json/clear/","title":"nlohmann::basic_json::clear","text":"
void clear() noexcept;\n

Clears the content of a JSON value and resets it to the default value as if basic_json(value_t) would have been called with the current value type from type():

Value type initial value null null boolean false string \"\" number 0 binary An empty byte vector with no subtype object {} array []

Has the same effect as calling

*this = basic_json(type());\n
"},{"location":"api/basic_json/clear/#exception-safety","title":"Exception safety","text":"

No-throw guarantee: this function never throws exceptions.

"},{"location":"api/basic_json/clear/#complexity","title":"Complexity","text":"

Linear in the size of the JSON value.

"},{"location":"api/basic_json/clear/#notes","title":"Notes","text":"

All iterators, pointers, and references related to this container are invalidated.

"},{"location":"api/basic_json/clear/#examples","title":"Examples","text":"Example

The example below shows the effect of clear() to different JSON types.

#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_null;\n    json j_boolean = true;\n    json j_number_integer = 17;\n    json j_number_float = 23.42;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n    json j_string = \"Hello, world\";\n\n    // call clear()\n    j_null.clear();\n    j_boolean.clear();\n    j_number_integer.clear();\n    j_number_float.clear();\n    j_object.clear();\n    j_array.clear();\n    j_string.clear();\n\n    // serialize the cleared values()\n    std::cout << j_null << '\\n';\n    std::cout << j_boolean << '\\n';\n    std::cout << j_number_integer << '\\n';\n    std::cout << j_number_float << '\\n';\n    std::cout << j_object << '\\n';\n    std::cout << j_array << '\\n';\n    std::cout << j_string << '\\n';\n}\n

Output:

null\nfalse\n0\n0.0\n{}\n[]\n\"\"\n
"},{"location":"api/basic_json/clear/#see-also","title":"See also","text":"
  • erase removes elements from a JSON value
  • empty checks whether the JSON value has no elements
"},{"location":"api/basic_json/clear/#version-history","title":"Version history","text":"
  • Added in version 1.0.0.
  • Added support for binary types in version 3.8.0.
  • Fixed in version 3.13.0 unreleased to also clear the subtype of a binary value; before, the subtype was left unchanged.
"},{"location":"api/basic_json/contains/","title":"nlohmann::basic_json::contains","text":"
// (1)\nbool contains(const typename object_t::key_type& key) const;\n\n// (2)\ntemplate<typename KeyType>\nbool contains(KeyType&& key) const;\n\n// (3)\nbool contains(const json_pointer& ptr) const;\n\n// (4)\ntemplate<typename T>\nbool contains(T) const = delete;\n
  1. Check whether an element exists in a JSON object with a key equivalent to key. If the element is not found or the JSON value is not an object, false is returned.
  2. See 1. This overload is only available if KeyType is comparable with typename object_t::key_type and typename object_comparator_t::is_transparent denotes a type.
  3. Check whether the given JSON pointer ptr can be resolved in the current JSON value.
  4. Deleted: this overload is only available if T is an integral type and is declared as deleted, so that a call with an integer key (for example, j.contains(0)) fails to compile. Otherwise, the integer literal 0 would convert to a null const char* and, from there, to the key type, causing undefined behavior at runtime.
"},{"location":"api/basic_json/contains/#template-parameters","title":"Template parameters","text":"KeyType A type for an object key other than json_pointer that is comparable with string_t using object_comparator_t. This can also be a string view (C++17)."},{"location":"api/basic_json/contains/#parameters","title":"Parameters","text":"key (in) key value to check its existence. ptr (in) JSON pointer to check its existence."},{"location":"api/basic_json/contains/#return-value","title":"Return value","text":"
  1. true if an element with specified key exists. If no such element with such a key is found or the JSON value is not an object, false is returned.
  2. See 1.
  3. true if the JSON pointer can be resolved to a stored value, false otherwise.
  4. Deleted; a call with an integral argument does not compile.
"},{"location":"api/basic_json/contains/#exception-safety","title":"Exception safety","text":"

Strong exception safety: if an exception occurs, the original value stays intact.

"},{"location":"api/basic_json/contains/#exceptions","title":"Exceptions","text":"
  1. The function does not throw exceptions.
  2. The function does not throw exceptions.
  3. The function does not throw exceptions.
  4. Deleted; a call with an integral argument does not compile.
"},{"location":"api/basic_json/contains/#complexity","title":"Complexity","text":"

Logarithmic in the size of the JSON object.

"},{"location":"api/basic_json/contains/#notes","title":"Notes","text":"
  • This method always returns false when executed on a JSON type that is not an object.
  • This method can be executed on any JSON value type.
  • Calling this function with an integer argument (for example, contains(0)) does not compile: such an argument would otherwise implicitly convert to a null const char* and, from there, cause undefined behavior when constructing a std::string for the object key. To check for an array element instead, use at, operator[], or compare against size.

Postconditions

If j.contains(x) returns true for a key or JSON pointer x, then it is safe to call j[x].

Deprecation

Overload (3) also accepts a json_pointer whose template argument is a basic_json specialization (e.g., nlohmann::json_pointer<nlohmann::json>) instead of a string type. This is deprecated since version 3.11.0 and will be removed in a future major version; use basic_json::json_pointer (for json, nlohmann::json_pointer<std::string>) instead.

You should be warned by your compiler with a -Wdeprecated-declarations warning if you are using a deprecated function.

See the migration guide for how to update existing code.

"},{"location":"api/basic_json/contains/#examples","title":"Examples","text":"Example: (1) check with key

The example shows how contains() is used.

#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // create some JSON values\n    json j_object = R\"( {\"key\": \"value\"} )\"_json;\n    json j_array = R\"( [1, 2, 3] )\"_json;\n\n    // call contains\n    std::cout << std::boolalpha <<\n              \"j_object contains 'key': \" << j_object.contains(\"key\") << '\\n' <<\n              \"j_object contains 'another': \" << j_object.contains(\"another\") << '\\n' <<\n              \"j_array contains 'key': \" << j_array.contains(\"key\") << std::endl;\n}\n

Output:

j_object contains 'key': true\nj_object contains 'another': false\nj_array contains 'key': false\n
Example: (2) check with key using string_view

The example shows how contains() is used.

#include <iostream>\n#include <string_view>\n#include <nlohmann/json.hpp>\n\nusing namespace std::string_view_literals;\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // create some JSON values\n    json j_object = R\"( {\"key\": \"value\"} )\"_json;\n    json j_array = R\"( [1, 2, 3] )\"_json;\n\n    // call contains\n    std::cout << std::boolalpha <<\n              \"j_object contains 'key': \" << j_object.contains(\"key\"sv) << '\\n' <<\n              \"j_object contains 'another': \" << j_object.contains(\"another\"sv) << '\\n' <<\n              \"j_array contains 'key': \" << j_array.contains(\"key\"sv) << std::endl;\n}\n

Output:

j_object contains 'key': true\nj_object contains 'another': false\nj_array contains 'key': false\n
Example: (3) check with JSON pointer

The example shows how contains() is used.

#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // create a JSON value\n    json j =\n    {\n        {\"number\", 1}, {\"string\", \"foo\"}, {\"array\", {1, 2}}\n    };\n\n    std::cout << std::boolalpha\n              << j.contains(\"/number\"_json_pointer) << '\\n'\n              << j.contains(\"/string\"_json_pointer) << '\\n'\n              << j.contains(\"/array\"_json_pointer) << '\\n'\n              << j.contains(\"/array/1\"_json_pointer) << '\\n'\n              << j.contains(\"/array/-\"_json_pointer) << '\\n'\n              << j.contains(\"/array/4\"_json_pointer) << '\\n'\n              << j.contains(\"/baz\"_json_pointer) << '\\n'\n              // an array index with a leading '0' is not found\n              << j.contains(\"/array/01\"_json_pointer) << '\\n'\n              // an array index that is not a number is not found\n              << j.contains(\"/array/one\"_json_pointer) << std::endl;\n}\n

Output:

true\ntrue\ntrue\ntrue\nfalse\nfalse\nfalse\nfalse\nfalse\n
"},{"location":"api/basic_json/contains/#see-also","title":"See also","text":"
  • find find a value in an object
  • count returns the number of occurrences of a key
  • basic_json_view::contains - the same check on a zero-copy view
"},{"location":"api/basic_json/contains/#version-history","title":"Version history","text":"
  1. Added in version 3.11.0.
  2. Added in version 3.6.0. Extended template KeyType to support comparable types in version 3.11.0. Fixed in version 3.13.0 unreleased to consistently accept std::string_view-convertible keys, as already supported by operator[], at, value, and other lookup functions.
  3. Added in version 3.7.0.
  4. Deleted overloads for integral key types added in version 3.13.0 unreleased to reject such calls at compile time instead of causing undefined behavior at runtime.
"},{"location":"api/basic_json/count/","title":"nlohmann::basic_json::count","text":"
// (1)\nsize_type count(const typename object_t::key_type& key) const;\n\n// (2)\ntemplate<typename KeyType>\nsize_type count(KeyType&& key) const;\n\n// (3)\ntemplate<typename T>\nsize_type count(T) const = delete;\n
  1. Returns the number of elements with key key. If ObjectType is the default std::map type, the return value will always be 0 (key was not found) or 1 (key was found).
  2. See 1. This overload is only available if KeyType is comparable with typename object_t::key_type and typename object_comparator_t::is_transparent denotes a type.
  3. Deleted: this overload is only available if T is an integral type and is declared as deleted, so that a call with an integer key (for example, j.count(0)) fails to compile. Otherwise, the integer literal 0 would convert to a null const char* and, from there, to the key type, causing undefined behavior at runtime.
"},{"location":"api/basic_json/count/#template-parameters","title":"Template parameters","text":"KeyType A type for an object key other than json_pointer that is comparable with string_t using object_comparator_t. This can also be a string view (C++17)."},{"location":"api/basic_json/count/#parameters","title":"Parameters","text":"key (in) key value of the element to count."},{"location":"api/basic_json/count/#return-value","title":"Return value","text":"

Number of elements with key key. If the JSON value is not an object, the return value will be 0.

"},{"location":"api/basic_json/count/#exception-safety","title":"Exception safety","text":"

Strong exception safety: if an exception occurs, the original value stays intact.

"},{"location":"api/basic_json/count/#complexity","title":"Complexity","text":"

Logarithmic in the size of the JSON object.

"},{"location":"api/basic_json/count/#notes","title":"Notes","text":"
  • This method always returns 0 when executed on a JSON type that is not an object.
  • Calling this function with an integer argument (for example, count(0)) does not compile: such an argument would otherwise implicitly convert to a null const char* and, from there, cause undefined behavior when constructing a std::string for the object key. To check for an array element instead, use at, operator[], or compare against size.
"},{"location":"api/basic_json/count/#examples","title":"Examples","text":"Example: (1) count number of elements

The example shows how count() is used.

#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON object\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n\n    // call count()\n    auto count_two = j_object.count(\"two\");\n    auto count_three = j_object.count(\"three\");\n\n    // print values\n    std::cout << \"number of elements with key \\\"two\\\": \" << count_two << '\\n';\n    std::cout << \"number of elements with key \\\"three\\\": \" << count_three << '\\n';\n}\n

Output:

number of elements with key \"two\": 1\nnumber of elements with key \"three\": 0\n
Example: (2) count number of elements using string_view

The example shows how count() is used.

#include <iostream>\n#include <string_view>\n#include <nlohmann/json.hpp>\n\nusing namespace std::string_view_literals;\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON object\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n\n    // call count()\n    auto count_two = j_object.count(\"two\"sv);\n    auto count_three = j_object.count(\"three\"sv);\n\n    // print values\n    std::cout << \"number of elements with key \\\"two\\\": \" << count_two << '\\n';\n    std::cout << \"number of elements with key \\\"three\\\": \" << count_three << '\\n';\n}\n

Output:

number of elements with key \"two\": 1\nnumber of elements with key \"three\": 0\n
"},{"location":"api/basic_json/count/#see-also","title":"See also","text":"
  • find find a value in an object
  • contains checks whether a key exists
  • basic_json_view::count - the same check on a zero-copy view
"},{"location":"api/basic_json/count/#version-history","title":"Version history","text":"
  1. Added in version 3.11.0.
  2. Added in version 1.0.0. Changed parameter key type to KeyType&& in version 3.11.0. Fixed in version 3.13.0 unreleased to consistently accept std::string_view-convertible keys, as already supported by operator[], at, value, and other lookup functions.
  3. Deleted overload for integral key types added in version 3.13.0 unreleased to reject such calls at compile time instead of causing undefined behavior at runtime.
"},{"location":"api/basic_json/crbegin/","title":"nlohmann::basic_json::crbegin","text":"
const_reverse_iterator crbegin() const noexcept;\n

Returns an iterator to the reverse-beginning; that is, the last element.

"},{"location":"api/basic_json/crbegin/#return-value","title":"Return value","text":"

reverse iterator to the last element

"},{"location":"api/basic_json/crbegin/#exception-safety","title":"Exception safety","text":"

No-throw guarantee: this member function never throws exceptions.

"},{"location":"api/basic_json/crbegin/#complexity","title":"Complexity","text":"

Constant.

"},{"location":"api/basic_json/crbegin/#examples","title":"Examples","text":"Example

The following code shows an example for crbegin().

#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create an array value\n    json array = {1, 2, 3, 4, 5};\n\n    // get an iterator to the reverse-beginning\n    json::const_reverse_iterator it = array.crbegin();\n\n    // serialize the element that the iterator points to\n    std::cout << *it << '\\n';\n}\n

Output:

5\n
"},{"location":"api/basic_json/crbegin/#see-also","title":"See also","text":"
  • crend returns a const reverse iterator to one before the first element
  • rbegin returns a reverse iterator to the last element
  • cbegin returns a const iterator to the first element
  • Iterators - the article on iterators
"},{"location":"api/basic_json/crbegin/#version-history","title":"Version history","text":"
  • Added in version 1.0.0.
"},{"location":"api/basic_json/crend/","title":"nlohmann::basic_json::crend","text":"
const_reverse_iterator crend() const noexcept;\n

Returns an iterator to the reverse-end; that is, one before the first element. This element acts as a placeholder, attempting to access it results in undefined behavior.

"},{"location":"api/basic_json/crend/#return-value","title":"Return value","text":"

reverse iterator to the element following the last element

"},{"location":"api/basic_json/crend/#exception-safety","title":"Exception safety","text":"

No-throw guarantee: this member function never throws exceptions.

"},{"location":"api/basic_json/crend/#complexity","title":"Complexity","text":"

Constant.

"},{"location":"api/basic_json/crend/#examples","title":"Examples","text":"Example

The following code shows an example for crend().

#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create an array value\n    json array = {1, 2, 3, 4, 5};\n\n    // get an iterator to the reverse-end\n    json::const_reverse_iterator it = array.crend();\n\n    // increment the iterator to point to the first element\n    --it;\n\n    // serialize the element that the iterator points to\n    std::cout << *it << '\\n';\n}\n

Output:

1\n
"},{"location":"api/basic_json/crend/#see-also","title":"See also","text":"
  • crbegin returns a const reverse iterator to the last element
  • rend returns a reverse iterator to one before the first element
  • cend returns a const iterator to one past the last element
  • Iterators - the article on iterators
"},{"location":"api/basic_json/crend/#version-history","title":"Version history","text":"
  • Added in version 1.0.0.
"},{"location":"api/basic_json/default_object_comparator_t/","title":"nlohmann::basic_json::default_object_comparator_t","text":"
using default_object_comparator_t = std::less<StringType>;  // until C++14\n\nusing default_object_comparator_t = std::less<>;            // since C++14\n

The default comparator used by object_t.

Since C++14 a transparent comparator is used which prevents unnecessary string construction when looking up a key in an object.

The actual comparator used depends on object_t and can be obtained via object_comparator_t.

"},{"location":"api/basic_json/default_object_comparator_t/#examples","title":"Examples","text":"Example

The example below demonstrates the default comparator.

#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::boolalpha\n              << \"one < two : \" << json::default_object_comparator_t{}(\"one\", \"two\") << \"\\n\"\n              << \"three < four : \" << json::default_object_comparator_t{}(\"three\", \"four\") << std::endl;\n}\n

Output:

one < two : true\nthree < four : false\n
"},{"location":"api/basic_json/default_object_comparator_t/#see-also","title":"See also","text":"
  • object_comparator_t the comparator actually used by object_t
  • object_t the type used to store JSON objects
"},{"location":"api/basic_json/default_object_comparator_t/#version-history","title":"Version history","text":"
  • Added in version 3.11.0.
"},{"location":"api/basic_json/diff/","title":"nlohmann::basic_json::diff","text":"
static basic_json diff(const basic_json& source,\n                       const basic_json& target);\n

Creates a JSON Patch so that value source can be changed into the value target by calling patch function.

For two JSON values source and target, the following code always yields true: 

source.patch(diff(source, target)) == target;\n

"},{"location":"api/basic_json/diff/#parameters","title":"Parameters","text":"source (in) JSON value to compare from target (in) JSON value to compare against"},{"location":"api/basic_json/diff/#return-value","title":"Return value","text":"

a JSON patch to convert the source to target

"},{"location":"api/basic_json/diff/#exception-safety","title":"Exception safety","text":"

Strong guarantee: source and target are never modified.

"},{"location":"api/basic_json/diff/#complexity","title":"Complexity","text":"

Linear in the lengths of source and target.

"},{"location":"api/basic_json/diff/#notes","title":"Notes","text":"

Currently, only remove, add, and replace operations are generated.

"},{"location":"api/basic_json/diff/#examples","title":"Examples","text":"Example

The following code shows how a JSON patch is created as a diff for two JSON values.

#include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // the source document\n    json source = R\"(\n        {\n            \"baz\": \"qux\",\n            \"foo\": \"bar\"\n        }\n    )\"_json;\n\n    // the target document\n    json target = R\"(\n        {\n            \"baz\": \"boo\",\n            \"hello\": [\n                \"world\"\n            ]\n        }\n    )\"_json;\n\n    // create the patch\n    json patch = json::diff(source, target);\n\n    // roundtrip\n    json patched_source = source.patch(patch);\n\n    // output patch and roundtrip result\n    std::cout << std::setw(4) << patch << \"\\n\\n\"\n              << std::setw(4) << patched_source << std::endl;\n}\n

Output:

[\n    {\n        \"op\": \"replace\",\n        \"path\": \"/baz\",\n        \"value\": \"boo\"\n    },\n    {\n        \"op\": \"remove\",\n        \"path\": \"/foo\"\n    },\n    {\n        \"op\": \"add\",\n        \"path\": \"/hello\",\n        \"value\": [\n            \"world\"\n        ]\n    }\n]\n\n{\n    \"baz\": \"boo\",\n    \"hello\": [\n        \"world\"\n    ]\n}\n
"},{"location":"api/basic_json/diff/#see-also","title":"See also","text":"
  • RFC 6902 (JSON Patch)
  • patch applies a JSON Patch
  • patch_inplace applies a JSON Patch in place
  • merge_patch applies a JSON Merge Patch
  • merge_diff creates a diff as a JSON Merge Patch
"},{"location":"api/basic_json/diff/#version-history","title":"Version history","text":"
  • Added in version 2.0.0.
"},{"location":"api/basic_json/dump/","title":"nlohmann::basic_json::dump","text":"
string_t dump(const int indent = -1,\n              const char indent_char = ' ',\n              const bool ensure_ascii = false,\n              const error_handler_t error_handler = error_handler_t::strict) const;\n

Serialization function for JSON values. The function tries to mimic Python's json.dumps() function, and currently supports its indent and ensure_ascii parameters.

"},{"location":"api/basic_json/dump/#parameters","title":"Parameters","text":"indent (in) If indent is nonnegative, then array elements and object members will be pretty-printed with that indent level. An indent level of 0 will only insert newlines. -1 (the default) selects the most compact representation. indent_char (in) The character to use for indentation if indent is greater than 0. The default is (space). ensure_ascii (in) If ensure_ascii is true, all non-ASCII characters in the output are escaped with \\uXXXX sequences, and the result consists of ASCII characters only. error_handler (in) how to react on decoding errors; there are four possible values (see error_handler_t: strict (throws an exception in case a decoding error occurs; default), replace (replace invalid UTF-8 sequences with U+FFFD), ignore (ignore invalid UTF-8 sequences during serialization; all valid bytes are copied to the output unchanged, and invalid bytes are dropped), and keep (write the ill-formed bytes to the output as is, without escaping them, even if ensure_ascii is true; the result is then not valid UTF-8, but equals the input bytes exactly, and well-formed characters around the ill-formed bytes are still escaped as usual))."},{"location":"api/basic_json/dump/#return-value","title":"Return value","text":"

string containing the serialization of the JSON value

"},{"location":"api/basic_json/dump/#exception-safety","title":"Exception safety","text":"

Strong guarantee: if an exception is thrown, there are no changes to any JSON value.

"},{"location":"api/basic_json/dump/#exceptions","title":"Exceptions","text":"

Throws type_error.316 if a string stored inside the JSON value is not UTF-8 encoded and error_handler is set to strict

Serializing untrusted input

When serializing values that may contain invalid or untrusted UTF-8 (e.g., bytes taken directly from network input), dump() throws type_error.316 in the default strict mode. To serialize such data without throwing, pass error_handler_t::replace (substitutes U+FFFD) or error_handler_t::ignore. Callers that serialize untrusted input on a crash-sensitive path should either choose a non-strict error handler or wrap dump() in a try/catch.

See the FAQ for details.

"},{"location":"api/basic_json/dump/#complexity","title":"Complexity","text":"

Linear.

"},{"location":"api/basic_json/dump/#notes","title":"Notes","text":"

Floating-point numbers are written with the fewest digits that read back as the same value (for double; see number handling).

Binary values are serialized as an object containing two keys:

  • \"bytes\": an array of bytes as integers
  • \"subtype\": the subtype as integer or null if the binary has no subtype
"},{"location":"api/basic_json/dump/#examples","title":"Examples","text":"Example

The following example shows the effect of different indent, indent_char, and ensure_ascii parameters to the result of the serialization.

#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n    json j_string = \"Hell\u00f6 \ud83d\ude00!\";\n\n    // call dump()\n    std::cout << \"objects:\" << '\\n'\n              << j_object.dump() << \"\\n\\n\"\n              << j_object.dump(-1) << \"\\n\\n\"\n              << j_object.dump(0) << \"\\n\\n\"\n              << j_object.dump(4) << \"\\n\\n\"\n              << j_object.dump(1, '\\t') << \"\\n\\n\";\n\n    std::cout << \"arrays:\" << '\\n'\n              << j_array.dump() << \"\\n\\n\"\n              << j_array.dump(-1) << \"\\n\\n\"\n              << j_array.dump(0) << \"\\n\\n\"\n              << j_array.dump(4) << \"\\n\\n\"\n              << j_array.dump(1, '\\t') << \"\\n\\n\";\n\n    std::cout << \"strings:\" << '\\n'\n              << j_string.dump() << '\\n'\n              << j_string.dump(-1, ' ', true) << '\\n';\n\n    // create JSON value with invalid UTF-8 byte sequence\n    json j_invalid = \"\u00e4\\xA9\u00fc\";\n    try\n    {\n        std::cout << j_invalid.dump() << std::endl;\n    }\n    catch (const json::type_error& e)\n    {\n        std::cout << e.what() << std::endl;\n    }\n\n    std::cout << \"string with replaced invalid characters: \"\n              << j_invalid.dump(-1, ' ', false, json::error_handler_t::replace)\n              << \"\\nstring with ignored invalid characters: \"\n              << j_invalid.dump(-1, ' ', false, json::error_handler_t::ignore)\n              << '\\n';\n}\n

Output:

objects:\n{\"one\":1,\"two\":2}\n\n{\"one\":1,\"two\":2}\n\n{\n\"one\": 1,\n\"two\": 2\n}\n\n{\n    \"one\": 1,\n    \"two\": 2\n}\n\n{\n    \"one\": 1,\n    \"two\": 2\n}\n\narrays:\n[1,2,4,8,16]\n\n[1,2,4,8,16]\n\n[\n1,\n2,\n4,\n8,\n16\n]\n\n[\n    1,\n    2,\n    4,\n    8,\n    16\n]\n\n[\n    1,\n    2,\n    4,\n    8,\n    16\n]\n\nstrings:\n\"Hell\u00f6 \ud83d\ude00!\"\n\"Hell\\u00f6 \\ud83d\\ude00!\"\n[json.exception.type_error.316] invalid UTF-8 byte at index 2: 0xA9\nstring with replaced invalid characters: \"\u00e4\ufffd\u00fc\"\nstring with ignored invalid characters: \"\u00e4\u00fc\"\n
"},{"location":"api/basic_json/dump/#see-also","title":"See also","text":"
  • to_string returns a string representation of a JSON value
  • operator<< serialize to stream
  • basic_json_view::dump the corresponding function of basic_json_view, serializing directly from a flat index without building a basic_json value
  • Serialization - the serialization article
"},{"location":"api/basic_json/dump/#version-history","title":"Version history","text":"
  • Added in version 1.0.0.
  • Indentation character indent_char, option ensure_ascii and exceptions added in version 3.0.0.
  • Error handlers added in version 3.4.0.
  • Serialization of binary values added in version 3.8.0.
  • Error handler keep added in version 3.13.0 unreleased.
  • Doubles are written with the shortest digits (\u017bmij instead of Grisu2) since version 3.13.0 unreleased; about 0.1% of doubles are written differently, most of them with fewer digits.
"},{"location":"api/basic_json/emplace/","title":"nlohmann::basic_json::emplace","text":"
template<class... Args>\nstd::pair<iterator, bool> emplace(Args&& ... args);\n

Inserts a new element into a JSON object constructed in-place with the given args if there is no element with the key in the container. If the function is called on a JSON null value, an empty object is created before appending the value created from args.

"},{"location":"api/basic_json/emplace/#template-parameters","title":"Template parameters","text":"Args compatible types to create a basic_json object"},{"location":"api/basic_json/emplace/#iterator-invalidation","title":"Iterator invalidation","text":"

For ordered_json, adding a value to an object can yield a reallocation, in which case all iterators (including the end() iterator) and all references to the elements are invalidated.

"},{"location":"api/basic_json/emplace/#parameters","title":"Parameters","text":"args (in) arguments to forward to a constructor of basic_json"},{"location":"api/basic_json/emplace/#return-value","title":"Return value","text":"

a pair consisting of an iterator to the inserted element, or the already-existing element if no insertion happened, and a bool denoting whether the insertion took place.

"},{"location":"api/basic_json/emplace/#exception-safety","title":"Exception safety","text":"

Strong guarantee: if an exception is thrown, there are no changes to any JSON value. As an exception, a null value is converted to an empty object before the element is added and keeps that type if adding the element throws.

"},{"location":"api/basic_json/emplace/#exceptions","title":"Exceptions","text":"

Throws type_error.311 when called on a type other than JSON object or null; example: \"cannot use emplace() with number\"

"},{"location":"api/basic_json/emplace/#complexity","title":"Complexity","text":"

Logarithmic in the size of the container, O(log(size())).

"},{"location":"api/basic_json/emplace/#examples","title":"Examples","text":"Example

The example shows how emplace() can be used to add elements to a JSON object. Note how the null value was silently converted to a JSON object. Further note how no value is added if there was already one value stored with the same key.

#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json object = {{\"one\", 1}, {\"two\", 2}};\n    json null;\n\n    // print values\n    std::cout << object << '\\n';\n    std::cout << null << '\\n';\n\n    // add values\n    auto res1 = object.emplace(\"three\", 3);\n    null.emplace(\"A\", \"a\");\n    null.emplace(\"B\", \"b\");\n\n    // the following call will not add an object, because there is already\n    // a value stored at key \"B\"\n    auto res2 = null.emplace(\"B\", \"c\");\n\n    // print values\n    std::cout << object << '\\n';\n    std::cout << *res1.first << \" \" << std::boolalpha << res1.second << '\\n';\n\n    std::cout << null << '\\n';\n    std::cout << *res2.first << \" \" << std::boolalpha << res2.second << '\\n';\n}\n

Output:

{\"one\":1,\"two\":2}\nnull\n{\"one\":1,\"three\":3,\"two\":2}\n3 true\n{\"A\":\"a\",\"B\":\"b\"}\n\"b\" false\n
"},{"location":"api/basic_json/emplace/#see-also","title":"See also","text":"
  • emplace_back add a value to an array
  • insert add values to an array/object
  • Modifying values - the article on modifying values
"},{"location":"api/basic_json/emplace/#version-history","title":"Version history","text":"
  • Since version 2.0.8.
  • Fixed in version 3.13.0 unreleased: for ordered_json, the value could previously only be passed as an rvalue; it can now also be passed as an lvalue or a const lvalue, matching the behavior of json.
"},{"location":"api/basic_json/emplace_back/","title":"nlohmann::basic_json::emplace_back","text":"
template<class... Args>\nreference emplace_back(Args&& ... args);\n

Creates a JSON value from the passed parameters args to the end of the JSON value. If the function is called on a JSON null value, an empty array is created before appending the value created from args.

"},{"location":"api/basic_json/emplace_back/#template-parameters","title":"Template parameters","text":"Args compatible types to create a basic_json object"},{"location":"api/basic_json/emplace_back/#iterator-invalidation","title":"Iterator invalidation","text":"

By adding an element to the end of the array, a reallocation can happen, in which case all iterators (including the end() iterator) and all references to the elements are invalidated. Otherwise, only the end() iterator is invalidated.

"},{"location":"api/basic_json/emplace_back/#parameters","title":"Parameters","text":"args (in) arguments to forward to a constructor of basic_json"},{"location":"api/basic_json/emplace_back/#return-value","title":"Return value","text":"

reference to the inserted element

"},{"location":"api/basic_json/emplace_back/#exception-safety","title":"Exception safety","text":"

Strong guarantee: if an exception is thrown, there are no changes to any JSON value. As an exception, a null value is converted to an empty array before the element is added and keeps that type if adding the element throws.

"},{"location":"api/basic_json/emplace_back/#exceptions","title":"Exceptions","text":"

Throws type_error.311 when called on a type other than JSON array or null; example: \"cannot use emplace_back() with number\"

"},{"location":"api/basic_json/emplace_back/#complexity","title":"Complexity","text":"

Amortized constant.

"},{"location":"api/basic_json/emplace_back/#examples","title":"Examples","text":"Example

The example shows how emplace_back() can be used to add elements to a JSON array. Note how the null value was silently converted to a JSON array.

#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json array = {1, 2, 3, 4, 5};\n    json null;\n\n    // print values\n    std::cout << array << '\\n';\n    std::cout << null << '\\n';\n\n    // add values\n    array.emplace_back(6);\n    null.emplace_back(\"first\");\n    null.emplace_back(3, \"second\");\n\n    // print values\n    std::cout << array << '\\n';\n    std::cout << null << '\\n';\n}\n

Output:

[1,2,3,4,5]\nnull\n[1,2,3,4,5,6]\n[\"first\",[\"second\",\"second\",\"second\"]]\n
"},{"location":"api/basic_json/emplace_back/#see-also","title":"See also","text":"
  • operator+= add a value to an array/object
  • push_back add a value to an array/object
  • Modifying values - the article on modifying values
"},{"location":"api/basic_json/emplace_back/#version-history","title":"Version history","text":"
  • Since version 2.0.8.
  • Returns reference since 3.7.0.
"},{"location":"api/basic_json/empty/","title":"nlohmann::basic_json::empty","text":"
bool empty() const noexcept;\n

Checks if a JSON value has no elements (i.e., whether its size() is 0).

"},{"location":"api/basic_json/empty/#return-value","title":"Return value","text":"

The return value depends on the different types and is defined as follows:

Value type return value null true boolean false string false number false binary false object result of function object_t::empty() array result of function array_t::empty()"},{"location":"api/basic_json/empty/#exception-safety","title":"Exception safety","text":"

No-throw guarantee: this function never throws exceptions.

"},{"location":"api/basic_json/empty/#complexity","title":"Complexity","text":"

Constant, as long as array_t and object_t satisfy the Container concept; that is, their empty() functions have constant complexity.

"},{"location":"api/basic_json/empty/#possible-implementation","title":"Possible implementation","text":"
bool empty() const noexcept\n{\n    return size() == 0;\n}\n
"},{"location":"api/basic_json/empty/#notes","title":"Notes","text":"

This function does not return whether a string stored as JSON value is empty -- it returns whether the JSON container itself is empty which is false in the case of a string.

"},{"location":"api/basic_json/empty/#examples","title":"Examples","text":"Example

The following code uses empty() to check if a JSON object contains any elements.

#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_null;\n    json j_boolean = true;\n    json j_number_integer = 17;\n    json j_number_float = 23.42;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_object_empty(json::value_t::object);\n    json j_array = {1, 2, 4, 8, 16};\n    json j_array_empty(json::value_t::array);\n    json j_string = \"Hello, world\";\n\n    // call empty()\n    std::cout << std::boolalpha;\n    std::cout << j_null.empty() << '\\n';\n    std::cout << j_boolean.empty() << '\\n';\n    std::cout << j_number_integer.empty() << '\\n';\n    std::cout << j_number_float.empty() << '\\n';\n    std::cout << j_object.empty() << '\\n';\n    std::cout << j_object_empty.empty() << '\\n';\n    std::cout << j_array.empty() << '\\n';\n    std::cout << j_array_empty.empty() << '\\n';\n    std::cout << j_string.empty() << '\\n';\n}\n

Output:

true\nfalse\nfalse\nfalse\nfalse\ntrue\nfalse\ntrue\nfalse\n
"},{"location":"api/basic_json/empty/#see-also","title":"See also","text":"
  • size returns the number of elements
  • clear clears the content and resets the value to the default value
  • basic_json_view::empty - the same check on a zero-copy view
"},{"location":"api/basic_json/empty/#version-history","title":"Version history","text":"
  • Added in version 1.0.0.
  • Extended to return false for binary types in version 3.8.0.
"},{"location":"api/basic_json/end/","title":"nlohmann::basic_json::end","text":"
iterator end() noexcept;\nconst_iterator end() const noexcept;\n

Returns an iterator to one past the last element.

"},{"location":"api/basic_json/end/#return-value","title":"Return value","text":"

iterator one past the last element

"},{"location":"api/basic_json/end/#exception-safety","title":"Exception safety","text":"

No-throw guarantee: this member function never throws exceptions.

"},{"location":"api/basic_json/end/#complexity","title":"Complexity","text":"

Constant.

"},{"location":"api/basic_json/end/#examples","title":"Examples","text":"Example

The following code shows an example for end().

#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create an array value\n    json array = {1, 2, 3, 4, 5};\n\n    // get an iterator to one past the last element\n    json::iterator it = array.end();\n\n    // decrement the iterator to point to the last element\n    --it;\n\n    // serialize the element that the iterator points to\n    std::cout << *it << '\\n';\n}\n

Output:

5\n
"},{"location":"api/basic_json/end/#see-also","title":"See also","text":"
  • begin returns an iterator to the first element
  • cend returns a const iterator to one past the last element
  • rend returns a reverse iterator to one before the first element
  • Iterators - the article on iterators
  • basic_json_view::end - the same iteration on a zero-copy view
"},{"location":"api/basic_json/end/#version-history","title":"Version history","text":"
  • Added in version 1.0.0.
"},{"location":"api/basic_json/end_pos/","title":"nlohmann::basic_json::end_pos","text":"
#if JSON_DIAGNOSTIC_POSITIONS\nconstexpr std::size_t end_pos() const noexcept;\n#endif\n

Returns the position immediately following the last character of the JSON string from which the value was parsed from.

JSON type return value object position after the closing } array position after the closing ] string position after the closing \" number position after the last character boolean position after e null position after l"},{"location":"api/basic_json/end_pos/#return-value","title":"Return value","text":"

the position of the character following the last character of the given value in the parsed JSON string, if the value was created by the parse function, or std::string::npos if the value was constructed otherwise

"},{"location":"api/basic_json/end_pos/#exception-safety","title":"Exception safety","text":"

No-throw guarantee: this member function never throws exceptions.

"},{"location":"api/basic_json/end_pos/#complexity","title":"Complexity","text":"

Constant.

"},{"location":"api/basic_json/end_pos/#notes","title":"Notes","text":"

Note

The function is only available if macro JSON_DIAGNOSTIC_POSITIONS has been defined to 1 before including the library header.

Invalidation

The returned positions are only valid as long as the JSON value is not changed. The positions are not updated when the JSON value is changed.

"},{"location":"api/basic_json/end_pos/#examples","title":"Examples","text":"Example 
#include <iostream>\n\n#define JSON_DIAGNOSTIC_POSITIONS 1\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::string json_string = R\"(\n    {\n        \"address\": {\n            \"street\": \"Fake Street\",\n            \"housenumber\": 1\n        }\n    }\n    )\";\n    json j = json::parse(json_string);\n\n    std::cout << \"Root diagnostic positions: \\n\";\n    std::cout << \"\\tstart_pos: \" << j.start_pos() << '\\n';\n    std::cout << \"\\tend_pos:\" << j.end_pos() << \"\\n\";\n    std::cout << \"Original string: \\n\";\n    std::cout << \"{\\n        \\\"address\\\": {\\n            \\\"street\\\": \\\"Fake Street\\\",\\n            \\\"housenumber\\\": 1\\n        }\\n    }\" << \"\\n\";\n    std::cout << \"Parsed string: \\n\";\n    std::cout << json_string.substr(j.start_pos(), j.end_pos() - j.start_pos()) << \"\\n\\n\";\n\n    std::cout << \"address diagnostic positions: \\n\";\n    std::cout << \"\\tstart_pos:\" << j[\"address\"].start_pos() << '\\n';\n    std::cout << \"\\tend_pos:\" << j[\"address\"].end_pos() << \"\\n\\n\";\n    std::cout << \"Original string: \\n\";\n    std::cout << \"{            \\\"street\\\": \\\"Fake Street\\\",\\n            \\\"housenumber\\\": 1\\n        }\" << \"\\n\";\n    std::cout << \"Parsed string: \\n\";\n    std::cout << json_string.substr(j[\"address\"].start_pos(), j[\"address\"].end_pos() - j[\"address\"].start_pos()) << \"\\n\\n\";\n\n    std::cout << \"street diagnostic positions: \\n\";\n    std::cout << \"\\tstart_pos:\" << j[\"address\"][\"street\"].start_pos() << '\\n';\n    std::cout << \"\\tend_pos:\" << j[\"address\"][\"street\"].end_pos() << \"\\n\\n\";\n    std::cout << \"Original string: \\n\";\n    std::cout << \"\\\"Fake Street\\\"\" << \"\\n\";\n    std::cout << \"Parsed string: \\n\";\n    std::cout << json_string.substr(j[\"address\"][\"street\"].start_pos(), j[\"address\"][\"street\"].end_pos() - j[\"address\"][\"street\"].start_pos()) << \"\\n\\n\";\n\n    std::cout << \"housenumber diagnostic positions: \\n\";\n    std::cout << \"\\tstart_pos:\" << j[\"address\"][\"housenumber\"].start_pos() << '\\n';\n    std::cout << \"\\tend_pos:\" << j[\"address\"][\"housenumber\"].end_pos() << \"\\n\\n\";\n    std::cout << \"Original string: \\n\";\n    std::cout << \"1\" << \"\\n\";\n    std::cout << \"Parsed string: \\n\";\n    std::cout << json_string.substr(j[\"address\"][\"housenumber\"].start_pos(), j[\"address\"][\"housenumber\"].end_pos() - j[\"address\"][\"housenumber\"].start_pos()) << \"\\n\\n\";\n}\n

Output:

Root diagnostic positions: \n    start_pos: 5\n    end_pos:109\nOriginal string: \n{\n        \"address\": {\n            \"street\": \"Fake Street\",\n            \"housenumber\": 1\n        }\n    }\nParsed string: \n{\n        \"address\": {\n            \"street\": \"Fake Street\",\n            \"housenumber\": 1\n        }\n    }\n\naddress diagnostic positions: \n    start_pos:26\n    end_pos:103\n\nOriginal string: \n{            \"street\": \"Fake Street\",\n            \"housenumber\": 1\n        }\nParsed string: \n{\n            \"street\": \"Fake Street\",\n            \"housenumber\": 1\n        }\n\nstreet diagnostic positions: \n    start_pos:50\n    end_pos:63\n\nOriginal string: \n\"Fake Street\"\nParsed string: \n\"Fake Street\"\n\nhousenumber diagnostic positions: \n    start_pos:92\n    end_pos:93\n\nOriginal string: \n1\nParsed string: \n1\n

The output shows the start/end positions of all the objects and fields in the JSON string.

"},{"location":"api/basic_json/end_pos/#see-also","title":"See also","text":"
  • start_pos to access the start position
  • JSON_DIAGNOSTIC_POSITIONS for an overview of the diagnostic positions
"},{"location":"api/basic_json/end_pos/#version-history","title":"Version history","text":"
  • Added in version 3.12.0.
"},{"location":"api/basic_json/erase/","title":"nlohmann::basic_json::erase","text":"
// (1)\niterator erase(iterator pos);\nconst_iterator erase(const_iterator pos);\n\n// (2)\niterator erase(iterator first, iterator last);\nconst_iterator erase(const_iterator first, const_iterator last);\n\n// (3)\nsize_type erase(const typename object_t::key_type& key);\n\n// (4)\ntemplate<typename KeyType>\nsize_type erase(KeyType&& key);\n\n// (5)\nvoid erase(const size_type idx);\n

  1. Removes an element from a JSON value specified by iterator pos. The iterator pos must be valid and dereferenceable. Thus, the end() iterator (which is valid, but is not dereferenceable) cannot be used as a value for pos.

    If called on a primitive type other than null, the resulting JSON value will be null.

  2. Remove an element range specified by [first; last) from a JSON value. The iterator first does not need to be dereferenceable if first == last: erasing an empty range is a no-op.

    If called on a primitive type other than null, the resulting JSON value will be null.

  3. Removes an element from a JSON object by key.

  4. See 3. This overload is only available if KeyType is comparable with typename object_t::key_type and typename object_comparator_t::is_transparent denotes a type.

  5. Removes an element from a JSON array by index.

"},{"location":"api/basic_json/erase/#template-parameters","title":"Template parameters","text":"KeyType A type for an object key other than json_pointer that is comparable with string_t using object_comparator_t. This can also be a string view (C++17)."},{"location":"api/basic_json/erase/#parameters","title":"Parameters","text":"pos (in) iterator to the element to remove first (in) iterator to the beginning of the range to remove last (in) iterator past the end of the range to remove key (in) object key of the elements to remove idx (in) array index of the element to remove"},{"location":"api/basic_json/erase/#return-value","title":"Return value","text":"
  1. Iterator following the last removed element. If the iterator pos refers to the last element, the end() iterator is returned.
  2. Iterator following the last removed element. If the iterator last refers to the last element, the end() iterator is returned.
  3. Number of elements removed. If ObjectType is the default std::map type, the return value will always be 0 (key was not found) or 1 (key was found).
  4. See 3.
  5. (none)
"},{"location":"api/basic_json/erase/#exception-safety","title":"Exception safety","text":"

Strong exception safety: if an exception occurs, the original value stays intact.

"},{"location":"api/basic_json/erase/#exceptions","title":"Exceptions","text":"
  1. The function can throw the following exceptions:
    • Throws type_error.307 if called on a null value; example: \"cannot use erase() with null\"
    • Throws invalid_iterator.202 if called on an iterator which does not belong to the current JSON value; example: \"iterator does not fit current value\"
    • Throws invalid_iterator.205 if called on a primitive type with invalid iterator (i.e., any iterator which is not begin()); example: \"iterator out of range\"
  2. The function can throw the following exceptions:
    • Throws type_error.307 if called on a null value; example: \"cannot use erase() with null\"
    • Throws invalid_iterator.203 if called on iterators which does not belong to the current JSON value; example: \"iterators do not fit current value\"
    • Throws invalid_iterator.204 if called on a primitive type with invalid iterators (i.e., if first != begin() and last != end()); example: \"iterators out of range\"
  3. The function can throw the following exceptions:
    • Throws type_error.307 when called on a type other than JSON object; example: \"cannot use erase() with null\"
  4. See 3.
  5. The function can throw the following exceptions:
    • Throws type_error.307 when called on a type other than JSON array; example: \"cannot use erase() with null\"
    • Throws out_of_range.401 when idx >= size(); example: \"array index 17 is out of range\"
"},{"location":"api/basic_json/erase/#complexity","title":"Complexity","text":"
  1. The complexity depends on the type:
    • objects: amortized constant
    • arrays: linear in distance between pos and the end of the container
    • strings and binary: linear in the length of the member
    • other types: constant
  2. The complexity depends on the type:
    • objects: log(size()) + std::distance(first, last)
    • arrays: linear in the distance between first and last, plus linear in the distance between last and end of the container
    • strings and binary: linear in the length of the member
    • other types: constant
  3. log(size()) + count(key)
  4. log(size()) + count(key)
  5. Linear in distance between idx and the end of the container.
"},{"location":"api/basic_json/erase/#notes","title":"Notes","text":"
  1. Invalidates iterators and references at or after the point of the erase, including the end() iterator.
  2. (none)
  3. References and iterators to the erased elements are invalidated. Other references and iterators are not affected.
  4. See 3.
  5. (none)
"},{"location":"api/basic_json/erase/#examples","title":"Examples","text":"Example: (1) remove element given an iterator

The example shows the effect of erase() for different JSON types using an iterator.

#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_boolean = true;\n    json j_number_integer = 17;\n    json j_number_float = 23.42;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n    json j_string = \"Hello, world\";\n\n    // call erase()\n    j_boolean.erase(j_boolean.begin());\n    j_number_integer.erase(j_number_integer.begin());\n    j_number_float.erase(j_number_float.begin());\n    j_object.erase(j_object.find(\"two\"));\n    j_array.erase(j_array.begin() + 2);\n    j_string.erase(j_string.begin());\n\n    // print values\n    std::cout << j_boolean << '\\n';\n    std::cout << j_number_integer << '\\n';\n    std::cout << j_number_float << '\\n';\n    std::cout << j_object << '\\n';\n    std::cout << j_array << '\\n';\n    std::cout << j_string << '\\n';\n}\n

Output:

null\nnull\nnull\n{\"one\":1}\n[1,2,8,16]\nnull\n
Example: (2) remove elements given an iterator range

The example shows the effect of erase() for different JSON types using an iterator range.

#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_boolean = true;\n    json j_number_integer = 17;\n    json j_number_float = 23.42;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n    json j_string = \"Hello, world\";\n\n    // call erase()\n    j_boolean.erase(j_boolean.begin(), j_boolean.end());\n    j_number_integer.erase(j_number_integer.begin(), j_number_integer.end());\n    j_number_float.erase(j_number_float.begin(), j_number_float.end());\n    j_object.erase(j_object.find(\"two\"), j_object.end());\n    j_array.erase(j_array.begin() + 1, j_array.begin() + 3);\n    j_string.erase(j_string.begin(), j_string.end());\n\n    // print values\n    std::cout << j_boolean << '\\n';\n    std::cout << j_number_integer << '\\n';\n    std::cout << j_number_float << '\\n';\n    std::cout << j_object << '\\n';\n    std::cout << j_array << '\\n';\n    std::cout << j_string << '\\n';\n}\n

Output:

null\nnull\nnull\n{\"one\":1}\n[1,8,16]\nnull\n
Example: (3) remove element from a JSON object given a key

The example shows the effect of erase() for different JSON types using an object key.

#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON object\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n\n    // call erase()\n    auto count_one = j_object.erase(\"one\");\n    auto count_three = j_object.erase(\"three\");\n\n    // print values\n    std::cout << j_object << '\\n';\n    std::cout << count_one << \" \" << count_three << '\\n';\n}\n

Output:

{\"two\":2}\n1 0\n
Example: (4) remove element from a JSON object given a key using string_view

The example shows the effect of erase() for different JSON types using an object key.

#include <iostream>\n#include <string_view>\n#include <nlohmann/json.hpp>\n\nusing namespace std::string_view_literals;\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON object\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n\n    // call erase()\n    auto count_one = j_object.erase(\"one\"sv);\n    auto count_three = j_object.erase(\"three\"sv);\n\n    // print values\n    std::cout << j_object << '\\n';\n    std::cout << count_one << \" \" << count_three << '\\n';\n}\n

Output:

{\"two\":2}\n1 0\n
Example: (5) remove element from a JSON array given an index

The example shows the effect of erase() using an array index.

#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON array\n    json j_array = {0, 1, 2, 3, 4, 5};\n\n    // call erase()\n    j_array.erase(2);\n\n    // print values\n    std::cout << j_array << '\\n';\n}\n

Output:

[0,1,3,4,5]\n
"},{"location":"api/basic_json/erase/#see-also","title":"See also","text":"
  • clear clears the contents
  • insert add values to an array/object
  • Modifying values - the article on modifying values
"},{"location":"api/basic_json/erase/#version-history","title":"Version history","text":"
  1. Added in version 1.0.0. Added support for binary types in version 3.8.0.
  2. Added in version 1.0.0. Added support for binary types in version 3.8.0.
  3. Added in version 1.0.0.
  4. Added in version 3.11.0. Fixed in version 3.13.0 unreleased to consistently accept std::string_view-convertible keys, as already supported by operator[], at, value, and other lookup functions.
  5. Added in version 1.0.0.
"},{"location":"api/basic_json/error_handler_t/","title":"nlohmann::basic_json::error_handler_t","text":"
enum class error_handler_t {\n    strict,\n    replace,\n    ignore,\n    keep\n};\n

This enumeration is used to choose how to treat ill-formed UTF-8 in a string value or object key:

  • dump uses it while serializing a basic_json value to text.
  • to_cbor, to_msgpack, to_ubjson, to_bjdata, and to_bson use it while serializing a basic_json value to that binary format. Their default is keep, as no binary writer checked before this parameter was added. CBOR, UBJSON, BJData, and BSON require valid UTF-8, so for these four the default is strict if JSON_STRICT_BINARY_UTF8 is enabled; MessagePack's specification explicitly allows a string to contain ill-formed UTF-8, so to_msgpack stays at keep. to_bon8 does not take this parameter: BON8 always validates, since UTF-8 lead bytes are structural to that format.
  • from_cbor, from_msgpack, from_ubjson, from_bjdata, and from_bson use it while parsing that binary format, to decide whether to check a string value or object key for well-formed UTF-8 at all; by default (keep) they do not, as no binary reader did before this parameter was added. from_bon8 does not take this parameter, for the same reason to_bon8 does not.

Four values are differentiated:

strict throw a type_error/parse_error exception in case of invalid UTF-8 replace replace invalid UTF-8 sequences with U+FFFD (\ufffd REPLACEMENT CHARACTER) ignore ignore invalid UTF-8 sequences; all valid bytes are copied to the output unchanged, and invalid bytes are dropped keep keep invalid UTF-8 sequences unchanged; only meaningful for the binary formats mentioned above, since dump itself must produce text, and keep there writes the ill-formed bytes to the output as is, so the result is then not valid UTF-8 (but still equals the input bytes exactly, including around any well-formed characters, which are still escaped as usual)"},{"location":"api/basic_json/error_handler_t/#examples","title":"Examples","text":"Example

The example below shows how the different values of the error_handler_t influence the behavior of dump when reading serializing an invalid UTF-8 sequence.

#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON value with invalid UTF-8 byte sequence\n    json j_invalid = \"\u00e4\\xA9\u00fc\";\n    try\n    {\n        std::cout << j_invalid.dump() << std::endl;\n    }\n    catch (const json::type_error& e)\n    {\n        std::cout << e.what() << std::endl;\n    }\n\n    std::cout << \"string with replaced invalid characters: \"\n              << j_invalid.dump(-1, ' ', false, json::error_handler_t::replace)\n              << \"\\nstring with ignored invalid characters: \"\n              << j_invalid.dump(-1, ' ', false, json::error_handler_t::ignore)\n              << \"\\nstring with the invalid byte kept as is (\" << j_invalid.dump(-1, ' ', false, json::error_handler_t::keep).size()\n              << \" bytes, not valid UTF-8 itself)\\n\";\n}\n

Output:

[json.exception.type_error.316] invalid UTF-8 byte at index 2: 0xA9\nstring with replaced invalid characters: \"\u00e4\ufffd\u00fc\"\nstring with ignored invalid characters: \"\u00e4\u00fc\"\nstring with the invalid byte kept as is (7 bytes, not valid UTF-8 itself)\n
"},{"location":"api/basic_json/error_handler_t/#see-also","title":"See also","text":"
  • dump serializes a JSON value, with an error_handler_t parameter to configure invalid UTF-8 handling
  • Handling invalid UTF-8 - the article on handling invalid UTF-8
"},{"location":"api/basic_json/error_handler_t/#version-history","title":"Version history","text":"
  • Added in version 3.4.0.
  • Added keep, and made this enumeration apply to the binary readers and writers in addition to dump, in version 3.13.0.
"},{"location":"api/basic_json/exception/","title":"nlohmann::basic_json::exception","text":"
class exception : public std::exception;\n

This class is an extension of std::exception objects with a member id for exception ids. It is used as the base class for all exceptions thrown by the basic_json class. This class can hence be used as \"wildcard\" to catch exceptions, see example below.

classDiagram\n  direction LR\n\n    class std_exception [\"std::exception\"] {\n        <<interface>>\n    }\n\n    class json_exception [\"basic_json::exception\"] {\n        +const int id\n        +const char* what() const\n    }\n\n    class json_parse_error [\"basic_json::parse_error\"] {\n        +const std::size_t byte\n    }\n\n    class json_invalid_iterator [\"basic_json::invalid_iterator\"]\n    class json_type_error [\"basic_json::type_error\"]\n    class json_out_of_range [\"basic_json::out_of_range\"]\n    class json_other_error [\"basic_json::other_error\"]\n\n    std_exception <|-- json_exception\n    json_exception <|-- json_parse_error\n    json_exception <|-- json_invalid_iterator\n    json_exception <|-- json_type_error\n    json_exception <|-- json_out_of_range\n    json_exception <|-- json_other_error\n\n    style json_exception fill:#CCCCFF

Subclasses:

  • parse_error for exceptions indicating a parse error
  • invalid_iterator for exceptions indicating errors with iterators
  • type_error for exceptions indicating executing a member function with a wrong type
  • out_of_range for exceptions indicating access out of the defined range
  • other_error for exceptions indicating other library errors
"},{"location":"api/basic_json/exception/#member-functions","title":"Member functions","text":"
  • what - returns explanatory string
"},{"location":"api/basic_json/exception/#member-variables","title":"Member variables","text":"
  • id - the id of the exception
"},{"location":"api/basic_json/exception/#notes","title":"Notes","text":"

To have nothrow-copy-constructible exceptions, we internally use std::runtime_error which can cope with arbitrary-length error messages. Intermediate strings are built with static functions and then passed to the actual constructor.

"},{"location":"api/basic_json/exception/#examples","title":"Examples","text":"Example

The following code shows how arbitrary library exceptions can be caught.

#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    try\n    {\n        // calling at() for a non-existing key\n        json j = {{\"foo\", \"bar\"}};\n        json k = j.at(\"non-existing\");\n    }\n    catch (const json::exception& e)\n    {\n        // output exception information\n        std::cout << \"message: \" << e.what() << '\\n'\n                  << \"exception id: \" << e.id << std::endl;\n    }\n}\n

Output:

message: [json.exception.out_of_range.403] key 'non-existing' not found\nexception id: 403\n
"},{"location":"api/basic_json/exception/#see-also","title":"See also","text":"

List of exceptions

"},{"location":"api/basic_json/exception/#version-history","title":"Version history","text":"
  • Since version 3.0.0.
"},{"location":"api/basic_json/find/","title":"nlohmann::basic_json::find","text":"
// (1)\niterator find(const typename object_t::key_type& key);\nconst_iterator find(const typename object_t::key_type& key) const;\n\n// (2)\ntemplate<typename KeyType>\niterator find(KeyType&& key);\ntemplate<typename KeyType>\nconst_iterator find(KeyType&& key) const;\n\n// (3)\ntemplate<typename T>\niterator find(T) = delete;\ntemplate<typename T>\nconst_iterator find(T) const = delete;\n
  1. Finds an element in a JSON object with a key equivalent to key. If the element is not found or the JSON value is not an object, end() is returned.
  2. See 1. This overload is only available if KeyType is comparable with typename object_t::key_type and typename object_comparator_t::is_transparent denotes a type.
  3. Deleted: this overload is only available if T is an integral type and is declared as deleted, so that a call with an integer key (for example, j.find(0)) fails to compile. Otherwise, the integer literal 0 would convert to a null const char* and, from there, to the key type, causing undefined behavior at runtime.
"},{"location":"api/basic_json/find/#template-parameters","title":"Template parameters","text":"KeyType A type for an object key other than json_pointer that is comparable with string_t using object_comparator_t. This can also be a string view (C++17)."},{"location":"api/basic_json/find/#parameters","title":"Parameters","text":"key (in) key value of the element to search for."},{"location":"api/basic_json/find/#return-value","title":"Return value","text":"

Iterator to an element with a key equivalent to key. If no such element is found or the JSON value is not an object, a past-the-end iterator (see end()) is returned.

"},{"location":"api/basic_json/find/#exception-safety","title":"Exception safety","text":"

Strong exception safety: if an exception occurs, the original value stays intact.

"},{"location":"api/basic_json/find/#complexity","title":"Complexity","text":"

Logarithmic in the size of the JSON object.

"},{"location":"api/basic_json/find/#notes","title":"Notes","text":"
  • This method always returns end() when executed on a JSON type that is not an object.
  • Calling this function with an integer argument (for example, find(0)) does not compile: such an argument would otherwise implicitly convert to a null const char* and, from there, cause undefined behavior when constructing a std::string for the object key. To access an array element instead, use at, operator[], or compare against size.
"},{"location":"api/basic_json/find/#examples","title":"Examples","text":"Example: (1) find object element by key

The example shows how find() is used.

#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON object\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n\n    // call find\n    auto it_two = j_object.find(\"two\");\n    auto it_three = j_object.find(\"three\");\n\n    // print values\n    std::cout << std::boolalpha;\n    std::cout << \"\\\"two\\\" was found: \" << (it_two != j_object.end()) << '\\n';\n    std::cout << \"value at key \\\"two\\\": \" << *it_two << '\\n';\n    std::cout << \"\\\"three\\\" was found: \" << (it_three != j_object.end()) << '\\n';\n}\n

Output:

\"two\" was found: true\nvalue at key \"two\": 2\n\"three\" was found: false\n
Example: (2) find object element by key using string_view

The example shows how find() is used.

#include <iostream>\n#include <string_view>\n#include <nlohmann/json.hpp>\n\nusing namespace std::string_view_literals;\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON object\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n\n    // call find\n    auto it_two = j_object.find(\"two\"sv);\n    auto it_three = j_object.find(\"three\"sv);\n\n    // print values\n    std::cout << std::boolalpha;\n    std::cout << \"\\\"two\\\" was found: \" << (it_two != j_object.end()) << '\\n';\n    std::cout << \"value at key \\\"two\\\": \" << *it_two << '\\n';\n    std::cout << \"\\\"three\\\" was found: \" << (it_three != j_object.end()) << '\\n';\n}\n

Output:

\"two\" was found: true\nvalue at key \"two\": 2\n\"three\" was found: false\n
"},{"location":"api/basic_json/find/#see-also","title":"See also","text":"
  • count returns the number of occurrences of a key
  • contains checks whether a key exists
  • basic_json_view::find - the same lookup on a zero-copy view (a duplicate key resolves to the first member, whereas parse() keeps the last)
"},{"location":"api/basic_json/find/#version-history","title":"Version history","text":"
  1. Added in version 3.11.0.
  2. Added in version 1.0.0. Changed to support comparable types in version 3.11.0. Fixed in version 3.13.0 unreleased to consistently accept std::string_view-convertible keys, as already supported by operator[], at, value, and other lookup functions.
  3. Deleted overloads for integral key types added in version 3.13.0 unreleased to reject such calls at compile time instead of causing undefined behavior at runtime.
"},{"location":"api/basic_json/flatten/","title":"nlohmann::basic_json::flatten","text":"
basic_json flatten() const;\n

The function creates a JSON object whose keys are JSON pointers (see RFC 6901) and whose values are all primitive (see is_primitive() for more information). The original JSON value can be restored using the unflatten() function.

"},{"location":"api/basic_json/flatten/#return-value","title":"Return value","text":"

an object that maps JSON pointers to primitive values

"},{"location":"api/basic_json/flatten/#exception-safety","title":"Exception safety","text":"

Strong exception safety: if an exception occurs, the original value stays intact.

"},{"location":"api/basic_json/flatten/#complexity","title":"Complexity","text":"

Linear in the size of the JSON value.

"},{"location":"api/basic_json/flatten/#notes","title":"Notes","text":"

Empty objects and arrays are flattened to null and will not be reconstructed correctly by the unflatten() function.

"},{"location":"api/basic_json/flatten/#examples","title":"Examples","text":"Example: flatten a JSON object

The following code shows how a JSON object is flattened to an object whose keys consist of JSON pointers.

#include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON value\n    json j =\n    {\n        {\"pi\", 3.141},\n        {\"happy\", true},\n        {\"name\", \"Niels\"},\n        {\"nothing\", nullptr},\n        {\n            \"answer\", {\n                {\"everything\", 42}\n            }\n        },\n        {\"list\", {1, 0, 2}},\n        {\n            \"object\", {\n                {\"currency\", \"USD\"},\n                {\"value\", 42.99}\n            }\n        }\n    };\n\n    // call flatten()\n    std::cout << std::setw(4) << j.flatten() << '\\n';\n}\n

Output:

{\n    \"/answer/everything\": 42,\n    \"/happy\": true,\n    \"/list/0\": 1,\n    \"/list/1\": 0,\n    \"/list/2\": 2,\n    \"/name\": \"Niels\",\n    \"/nothing\": null,\n    \"/object/currency\": \"USD\",\n    \"/object/value\": 42.99,\n    \"/pi\": 3.141\n}\n
Example: empty objects and arrays are flattened to null

The following code shows that an empty object and an empty array are both flattened to null, and that unflatten() restores them as null rather than as empty containers.

#include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON value with an empty object and an empty array\n    json j =\n    {\n        {\"empty_object\", json::object()},\n        {\"empty_array\", json::array()},\n        {\"name\", \"Niels\"}\n    };\n\n    // call flatten()\n    json flattened = j.flatten();\n    std::cout << std::setw(4) << flattened << \"\\n\\n\";\n\n    // the empty containers cannot be restored by unflatten()\n    std::cout << std::setw(4) << flattened.unflatten() << '\\n';\n}\n

Output:

{\n    \"/empty_array\": null,\n    \"/empty_object\": null,\n    \"/name\": \"Niels\"\n}\n\n{\n    \"empty_array\": null,\n    \"empty_object\": null,\n    \"name\": \"Niels\"\n}\n
"},{"location":"api/basic_json/flatten/#see-also","title":"See also","text":"
  • unflatten the reverse function
"},{"location":"api/basic_json/flatten/#version-history","title":"Version history","text":"
  • Added in version 2.0.0.
"},{"location":"api/basic_json/format_as/","title":"format_as(basic_json)","text":"
template <typename BasicJsonType>\nstd::string format_as(const BasicJsonType& j);\n

This function implements the format_as customization point used by the {fmt} library (fmtlib). It has no dependency on any fmt header and no effect at all unless a caller's translation unit also includes fmt and calls fmt::format/fmt::print on a JSON value.

"},{"location":"api/basic_json/format_as/#template-parameters","title":"Template parameters","text":"BasicJsonType a specialization of basic_json"},{"location":"api/basic_json/format_as/#return-value","title":"Return value","text":"

string containing the serialization of the JSON value (same as dump())

"},{"location":"api/basic_json/format_as/#exception-safety","title":"Exception safety","text":"

Strong guarantee: if an exception is thrown, there are no changes to any JSON value.

"},{"location":"api/basic_json/format_as/#exceptions","title":"Exceptions","text":"

Throws type_error.316 if a string stored inside the JSON value is not UTF-8 encoded

"},{"location":"api/basic_json/format_as/#complexity","title":"Complexity","text":"

Linear.

"},{"location":"api/basic_json/format_as/#possible-implementation","title":"Possible implementation","text":"
template <typename BasicJsonType>\nstd::string format_as(const BasicJsonType& j)\n{\n    return j.dump();\n}\n
"},{"location":"api/basic_json/format_as/#notes","title":"Notes","text":"

Version-dependent effect on fmt

fmt only picks up a format_as overload that returns a std::string in fmt 10.0.0 through 11.0.2. Starting with fmt 11.1.0, fmt restricts automatic format_as pickup to overloads that return an arithmetic type, so this function has no effect there (it is simply unused, not a compile error).

If you use fmt >= 11.1.0, or want the same pretty-print spec support that std::formatter<basic_json> has (\"{:#}\", a width to set the indent such as \"{:2}\"/\"{:#2}\", and fill-and-align to pick the indent character such as \"{:.>#}\"), define your own fmt::formatter specialization mirroring the same logic:

template <>\nstruct fmt::formatter<nlohmann::json>\n{\n    // -1 means compact output (dump()); any value >= 0 means pretty-printed\n    // output with that many spaces (or indent_char) per level.\n    int indent = -1;\n    char indent_char = ' ';\n\n    constexpr auto parse(format_parse_context& ctx) -> format_parse_context::iterator\n    {\n        auto it = ctx.begin();\n        const auto end = ctx.end();\n        constexpr auto is_align = [](char c)\n        {\n            return c == '<' || c == '>' || c == '^';\n        };\n\n        // [[fill] align] - repurposed here to pick a custom indent character\n        if (it != end && it + 1 != end && is_align(it[1]))\n        {\n            indent_char = *it;\n            it += 2;\n        }\n        else if (it != end && is_align(*it))\n        {\n            ++it;\n        }\n\n        // ['#'] - \"alternate form\", used here to request pretty-printing with a\n        // default indent of 4 (overridden by an explicit width below, if given)\n        if (it != end && *it == '#')\n        {\n            indent = 4;\n            ++it;\n        }\n\n        // [width] - repurposed here to pick the indent size; a width without '#'\n        // implies pretty-printing since an indent otherwise has no meaning\n        if (it != end && *it >= '1' && *it <= '9')\n        {\n            indent = 0;\n            while (it != end && *it >= '0' && *it <= '9')\n            {\n                indent = (indent * 10) + (*it - '0');\n                ++it;\n            }\n        }\n\n        if (it != end && *it != '}')\n        {\n            throw fmt::format_error(\"invalid format args for nlohmann::json\");\n        }\n\n        return it;\n    }\n\n    auto format(const nlohmann::json& j, format_context& ctx) const\n    {\n        const auto dumped = j.dump(indent, indent_char);\n        return fmt::format_to(ctx.out(), \"{}\", dumped);\n    }\n};\n

This recipe isn't shipped by the library itself, since doing so would make fmt a build dependency (see the FAQ entry on using JSON values with std::format or fmt for more background) \u2014 but it is compiled and exercised against a real, current fmt release as part of the library's own test suite (tests/fmt_formatter, via CMake FetchContent), so it's kept in sync with std::formatter<basic_json> and verified to actually work, not just illustrative.

"},{"location":"api/basic_json/format_as/#examples","title":"Examples","text":"Example

The following code shows how the library's format_as() function integrates with fmt::format, allowing argument-dependent lookup.

#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON value\n    json j = {{\"one\", 1}, {\"two\", 2}};\n\n    // format_as() is found via argument-dependent lookup, the same way\n    // fmt::format/fmt::print would find it\n    auto j_str = format_as(j);\n\n    std::cout << j_str << std::endl;\n}\n

Output:

{\"one\":1,\"two\":2}\n
"},{"location":"api/basic_json/format_as/#see-also","title":"See also","text":"
  • dump
  • std::formatter - the std::format (C++20) equivalent
  • Serialization - the serialization article
    • "},{"location":"api/basic_json/format_as/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json/from_bjdata/","title":"nlohmann::basic_json::from_bjdata","text":"
    // (1)\ntemplate<typename InputType>\nstatic basic_json from_bjdata(InputType&& i,\n                              const bool strict = true,\n                              const bool allow_exceptions = true,\n                              const error_handler_t error_handler = error_handler_t::keep);\n// (2)\ntemplate<typename IteratorType, typename SentinelType = IteratorType>\nstatic basic_json from_bjdata(IteratorType first, SentinelType last,\n                              const bool strict = true,\n                              const bool allow_exceptions = true,\n                              const error_handler_t error_handler = error_handler_t::keep);\n

    Deserializes a given input to a JSON value using the BJData (Binary JData) serialization format.

    1. Reads from a compatible input.
    2. Reads from an iterator range, or an iterator and a sentinel of a different type (C++20 ranges support).

    The exact mapping and its limitations are described on a dedicated page.

    "},{"location":"api/basic_json/from_bjdata/#template-parameters","title":"Template parameters","text":"InputType

    A compatible input, for instance:

    • an std::istream object
    • a FILE pointer
    • a C-style array of characters
    • a pointer to a null-terminated string of single byte characters
    • a container obj for which begin(obj) and end(obj) produce a valid pair of iterators (as found via ADL or member functions, with semantics compatible to std::begin and std::end)
    IteratorType a compatible iterator type SentinelType

    defaults to IteratorType; may be a different type comparable to IteratorType via operator!=, for instance.

    • a custom sentinel type for C++20 ranges
    • std::default_sentinel_t, when IteratorType is std::counted_iterator
    "},{"location":"api/basic_json/from_bjdata/#parameters","title":"Parameters","text":"i (in) an input in BJData format convertible to an input adapter first (in) iterator to the start of the input last (in) iterator to the end of the input, or a sentinel value that compares equal to the end iterator with operator!= strict (in) whether to expect the input to be consumed until EOF (true by default) allow_exceptions (in) whether to throw exceptions in case of a parse error (optional, true by default) error_handler (in) how to treat a string value or object key that is not valid UTF-8; see error_handler_t. BJData does not require a decoder to reject ill-formed UTF-8, so checking is opt-in: the default, keep, does not check at all, as every binary reader did before this parameter was added; strict checks and throws; replace/ignore sanitize the string the same way dump would"},{"location":"api/basic_json/from_bjdata/#return-value","title":"Return value","text":"

    deserialized JSON value; in case of a parse error and allow_exceptions set to false, the return value will be value_t::discarded. The latter can be checked with is_discarded.

    "},{"location":"api/basic_json/from_bjdata/#exception-safety","title":"Exception safety","text":"

    Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

    "},{"location":"api/basic_json/from_bjdata/#exceptions","title":"Exceptions","text":"
    • Throws parse_error.110 if the given input ends prematurely or the end of the file was not reached when strict was set to true
    • Throws parse_error.112 if a parse error occurs
    • Throws parse_error.113 if a string could not be parsed successfully, or if a string value or object key is not valid UTF-8 and error_handler is strict
    • Throws out_of_range.408 if the size of an optimized container or n-dimensional array cannot be represented by std::size_t
    "},{"location":"api/basic_json/from_bjdata/#complexity","title":"Complexity","text":"

    Linear in the size of the input.

    "},{"location":"api/basic_json/from_bjdata/#examples","title":"Examples","text":"Example

    The example shows the deserialization of a byte vector in BJData format to a JSON value.

    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create byte vector\n    std::vector<std::uint8_t> v = {0x7B, 0x69, 0x07, 0x63, 0x6F, 0x6D, 0x70, 0x61,\n                                   0x63, 0x74, 0x54, 0x69, 0x06, 0x73, 0x63, 0x68,\n                                   0x65, 0x6D, 0x61, 0x69, 0x00, 0x7D\n                                  };\n\n    // deserialize it with BJData\n    json j = json::from_bjdata(v);\n\n    // print the deserialized JSON value\n    std::cout << std::setw(2) << j << std::endl;\n}\n

    Output:

    {\n  \"compact\": true,\n  \"schema\": 0\n}\n
    "},{"location":"api/basic_json/from_bjdata/#see-also","title":"See also","text":"
    • to_bjdata create a BJData serialization of a JSON value
    • from_cbor create a JSON value from an input in CBOR format
    • from_msgpack create a JSON value from an input in MessagePack format
    • from_bson create a JSON value from an input in BSON format
    • from_ubjson create a JSON value from an input in UBJSON format
    • from_bon8 create a JSON value from an input in BON8 format
    "},{"location":"api/basic_json/from_bjdata/#version-history","title":"Version history","text":"
    • Added in version 3.11.0.
    • Extended container support (1) to include types with lvalue-only ADL begin/end (matching std::begin/std::end semantics) in version 3.13.0 unreleased.
    • Extended overload (2) to accept heterogeneous iterator+sentinel pairs (C++20 ranges support) in version 3.13.0 unreleased.
    • Added error_handler parameter in version 3.13.0 unreleased.

    Deprecation

    • Overload (2) replaces calls to from_bjdata with a pointer and a length as first two parameters, which has been deprecated in version 3.13.0 unreleased. In version 4.0.0 unreleased, this overload will be deleted (= delete) rather than removed. Please replace all calls like from_bjdata(ptr, len, ...); with from_bjdata(ptr, ptr+len, ...);.

    You should be warned by your compiler with a -Wdeprecated-declarations warning if you are using a deprecated function.

    "},{"location":"api/basic_json/from_bon8/","title":"nlohmann::basic_json::from_bon8","text":"
    // (1)\ntemplate<typename InputType>\nstatic basic_json from_bon8(InputType&& i,\n                            const bool strict = true,\n                            const bool allow_exceptions = true);\n// (2)\ntemplate<typename IteratorType, typename SentinelType = IteratorType>\nstatic basic_json from_bon8(IteratorType first, SentinelType last,\n                            const bool strict = true,\n                            const bool allow_exceptions = true);\n

    Deserializes a given input to a JSON value using the BON8 (Binary Object Notation 8) serialization format.

    1. Reads from a compatible input.
    2. Reads from an iterator range, or an iterator and a sentinel of a different type (C++20 ranges support).

    The exact mapping and its limitations are described on a dedicated page.

    "},{"location":"api/basic_json/from_bon8/#template-parameters","title":"Template parameters","text":"InputType

    A compatible input, for instance:

    • an std::istream object
    • a FILE pointer
    • a C-style array of characters
    • a pointer to a null-terminated string of single byte characters
    • a container obj for which begin(obj) and end(obj) produce a valid pair of iterators (as found via ADL or member functions, with semantics compatible to std::begin and std::end)
    IteratorType a compatible iterator type SentinelType

    defaults to IteratorType; may be a different type comparable to IteratorType via operator!=, for instance.

    • a custom sentinel type for C++20 ranges
    • std::default_sentinel_t, when IteratorType is std::counted_iterator
    "},{"location":"api/basic_json/from_bon8/#parameters","title":"Parameters","text":"i (in) an input in BON8 format convertible to an input adapter first (in) iterator to the start of the input last (in) iterator to the end of the input, or a sentinel value that compares equal to the end iterator with operator!= strict (in) whether to expect the input to be consumed until EOF (true by default) allow_exceptions (in) whether to throw exceptions in case of a parse error (optional, true by default)"},{"location":"api/basic_json/from_bon8/#return-value","title":"Return value","text":"

    deserialized JSON value; in case of a parse error and allow_exceptions set to false, the return value will be value_t::discarded. The latter can be checked with is_discarded.

    "},{"location":"api/basic_json/from_bon8/#exception-safety","title":"Exception safety","text":"

    Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

    "},{"location":"api/basic_json/from_bon8/#exceptions","title":"Exceptions","text":"
    • Throws parse_error.110 if the given input ends prematurely or the end of the file was not reached when strict was set to true
    • Throws parse_error.112 if a parse error occurs, for instance an invalid byte, a string that is not valid UTF-8, or an object key that is not a string
    "},{"location":"api/basic_json/from_bon8/#complexity","title":"Complexity","text":"

    Linear in the size of the input.

    "},{"location":"api/basic_json/from_bon8/#examples","title":"Examples","text":"Example

    The example shows the deserialization of a byte vector in BON8 format to a JSON value.

    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create byte vector\n    std::vector<std::uint8_t> v = {0x89, 0x63, 0x6f, 0x6d, 0x70, 0x61, 0x63, 0x74,\n                                   0xf9, 0x66, 0x6f, 0x72, 0x6d, 0x61, 0x74, 0xff,\n                                   0x42, 0x4f, 0x4e, 0x38, 0xff, 0x73, 0x63, 0x68,\n                                   0x65, 0x6d, 0x61, 0x90\n                                  };\n\n    // deserialize it with BON8\n    json j = json::from_bon8(v);\n\n    // print the deserialized JSON value\n    std::cout << std::setw(2) << j << std::endl;\n}\n

    Output:

    {\n  \"compact\": true,\n  \"format\": \"BON8\",\n  \"schema\": 0\n}\n
    "},{"location":"api/basic_json/from_bon8/#see-also","title":"See also","text":"
    • to_bon8 create a BON8 serialization of a JSON value
    • from_cbor create a JSON value from an input in CBOR format
    • from_msgpack create a JSON value from an input in MessagePack format
    • from_bson create a JSON value from an input in BSON format
    • from_ubjson create a JSON value from an input in UBJSON format
    • from_bjdata create a JSON value from an input in BJData format
    "},{"location":"api/basic_json/from_bon8/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.

    Deprecation

    • Overload (2) replaces calls to from_bon8 with a pointer and a length as first two parameters, which has been deprecated in version 3.13.0 unreleased. In version 4.0.0 unreleased, this overload will be deleted (= delete) rather than removed. Please replace all calls like from_bon8(ptr, len, ...); with from_bon8(ptr, ptr+len, ...);.

    You should be warned by your compiler with a -Wdeprecated-declarations warning if you are using a deprecated function.

    "},{"location":"api/basic_json/from_bson/","title":"nlohmann::basic_json::from_bson","text":"
    // (1)\ntemplate<typename InputType>\nstatic basic_json from_bson(InputType&& i,\n                            const bool strict = true,\n                            const bool allow_exceptions = true,\n                            const error_handler_t error_handler = error_handler_t::keep);\n// (2)\ntemplate<typename IteratorType, typename SentinelType = IteratorType>\nstatic basic_json from_bson(IteratorType first, SentinelType last,\n                            const bool strict = true,\n                            const bool allow_exceptions = true,\n                            const error_handler_t error_handler = error_handler_t::keep);\n

    Deserializes a given input to a JSON value using the BSON (Binary JSON) serialization format.

    1. Reads from a compatible input.
    2. Reads from an iterator range, or an iterator and a sentinel of a different type (C++20 ranges support).

    The exact mapping and its limitations are described on a dedicated page.

    "},{"location":"api/basic_json/from_bson/#template-parameters","title":"Template parameters","text":"InputType

    A compatible input, for instance:

    • an std::istream object
    • a FILE pointer
    • a C-style array of characters
    • a pointer to a null-terminated string of single byte characters
    • a container obj for which begin(obj) and end(obj) produce a valid pair of iterators (as found via ADL or member functions, with semantics compatible to std::begin and std::end)
    IteratorType a compatible iterator type SentinelType

    defaults to IteratorType; may be a different type comparable to IteratorType via operator!=, for instance.

    • a custom sentinel type for C++20 ranges
    • std::default_sentinel_t, when IteratorType is std::counted_iterator
    "},{"location":"api/basic_json/from_bson/#parameters","title":"Parameters","text":"i (in) an input in BSON format convertible to an input adapter first (in) iterator to the start of the input last (in) iterator to the end of the input, or a sentinel value that compares equal to the end iterator with operator!= strict (in) whether to expect the input to be consumed until EOF (true by default) allow_exceptions (in) whether to throw exceptions in case of a parse error (optional, true by default) error_handler (in) how to treat a string value or object key that is not valid UTF-8; see error_handler_t. BSON does not require a decoder to reject ill-formed UTF-8, so checking is opt-in: the default, keep, does not check at all, as every binary reader did before this parameter was added; strict checks and throws; replace/ignore sanitize the string the same way dump would"},{"location":"api/basic_json/from_bson/#return-value","title":"Return value","text":"

    deserialized JSON value; in case of a parse error and allow_exceptions set to false, the return value will be value_t::discarded. The latter can be checked with is_discarded.

    "},{"location":"api/basic_json/from_bson/#exception-safety","title":"Exception safety","text":"

    Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

    "},{"location":"api/basic_json/from_bson/#exceptions","title":"Exceptions","text":"
    • Throws parse_error.110 if the given input ends prematurely or the end of the input was not reached when strict was set to true
    • Throws parse_error.112 if a parse error occurs (e.g., an invalid string or byte array length)
    • Throws parse_error.114 if an unsupported BSON record type is encountered
    • Throws parse_error.113 if a string value or object key is not valid UTF-8 and error_handler is strict
    "},{"location":"api/basic_json/from_bson/#complexity","title":"Complexity","text":"

    Linear in the size of the input.

    "},{"location":"api/basic_json/from_bson/#examples","title":"Examples","text":"Example

    The example shows the deserialization of a byte vector in BSON format to a JSON value.

    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create byte vector\n    std::vector<std::uint8_t> v = {0x1b, 0x00, 0x00, 0x00, 0x08, 0x63, 0x6f, 0x6d,\n                                   0x70, 0x61, 0x63, 0x74, 0x00, 0x01, 0x10, 0x73,\n                                   0x63, 0x68, 0x65, 0x6d, 0x61, 0x00, 0x00, 0x00,\n                                   0x00, 0x00, 0x00\n                                  };\n\n    // deserialize it with BSON\n    json j = json::from_bson(v);\n\n    // print the deserialized JSON value\n    std::cout << std::setw(2) << j << std::endl;\n}\n

    Output:

    {\n  \"compact\": true,\n  \"schema\": 0\n}\n
    "},{"location":"api/basic_json/from_bson/#see-also","title":"See also","text":"
    • BSON specification
    • to_bson for the analogous serialization
    • from_cbor for the related CBOR format
    • from_msgpack for the related MessagePack format
    • from_ubjson for the related UBJSON format
    • from_bjdata for the related BJData format
    • from_bon8 for the related BON8 format
    "},{"location":"api/basic_json/from_bson/#version-history","title":"Version history","text":"
    • Added in version 3.4.0.
    • Extended container support (1) to include types with lvalue-only ADL begin/end (matching std::begin/std::end semantics) in version 3.13.0 unreleased.
    • Extended overload (2) to accept heterogeneous iterator+sentinel pairs (C++20 ranges support) in version 3.13.0 unreleased.
    • Added error_handler parameter in version 3.13.0 unreleased.

    Deprecation

    • Overload (2) replaces calls to from_bson with a pointer and a length as first two parameters, which has been deprecated in version 3.8.0. In version 4.0.0 unreleased, this overload will be deleted (= delete) rather than removed. Please replace all calls like from_bson(ptr, len, ...); with from_bson(ptr, ptr+len, ...);.
    • Overload (2) replaces calls to from_bson with a pair of iterators as their first parameter, which has been deprecated in version 3.8.0. This overload will be removed in version 4.0.0. Please replace all calls like from_bson({ptr, ptr+len}, ...); with from_bson(ptr, ptr+len, ...);.

    You should be warned by your compiler with a -Wdeprecated-declarations warning if you are using a deprecated function.

    See the migration guide for how to update existing code.

    "},{"location":"api/basic_json/from_cbor/","title":"nlohmann::basic_json::from_cbor","text":"
    // (1)\ntemplate<typename InputType>\nstatic basic_json from_cbor(InputType&& i,\n                            const bool strict = true,\n                            const bool allow_exceptions = true,\n                            const cbor_tag_handler_t tag_handler = cbor_tag_handler_t::error,\n                            const error_handler_t error_handler = error_handler_t::keep);\n\n// (2)\ntemplate<typename IteratorType, typename SentinelType = IteratorType>\nstatic basic_json from_cbor(IteratorType first, SentinelType last,\n                            const bool strict = true,\n                            const bool allow_exceptions = true,\n                            const cbor_tag_handler_t tag_handler = cbor_tag_handler_t::error,\n                            const error_handler_t error_handler = error_handler_t::keep);\n

    Deserializes a given input to a JSON value using the CBOR (Concise Binary Object Representation) serialization format.

    1. Reads from a compatible input.
    2. Reads from an iterator range, or an iterator and a sentinel of a different type (C++20 ranges support).

    The exact mapping and its limitations are described on a dedicated page.

    "},{"location":"api/basic_json/from_cbor/#template-parameters","title":"Template parameters","text":"InputType

    A compatible input, for instance:

    • an std::istream object
    • a FILE pointer
    • a C-style array of characters
    • a pointer to a null-terminated string of single byte characters
    • a container obj for which begin(obj) and end(obj) produce a valid pair of iterators (as found via ADL or member functions, with semantics compatible to std::begin and std::end)
    IteratorType a compatible iterator type SentinelType

    defaults to IteratorType; may be a different type comparable to IteratorType via operator!=, for instance.

    • a custom sentinel type for C++20 ranges
    • std::default_sentinel_t, when IteratorType is std::counted_iterator
    "},{"location":"api/basic_json/from_cbor/#parameters","title":"Parameters","text":"i (in) an input in CBOR format convertible to an input adapter first (in) iterator to the start of the input last (in) iterator to the end of the input, or a sentinel value that compares equal to the end iterator with operator!= strict (in) whether to expect the input to be consumed until EOF (true by default) allow_exceptions (in) whether to throw exceptions in case of a parse error (optional, true by default) tag_handler (in) how to treat CBOR tags (optional, error by default); see cbor_tag_handler_t for more information error_handler (in) how to treat a string value or object key that is not valid UTF-8; see error_handler_t. CBOR does not require a decoder to reject ill-formed UTF-8, so checking is opt-in: the default, keep, does not check at all, as every binary reader did before this parameter was added; strict checks and throws; replace/ignore sanitize the string the same way dump would"},{"location":"api/basic_json/from_cbor/#return-value","title":"Return value","text":"

    deserialized JSON value; in case of a parse error and allow_exceptions set to false, the return value will be value_t::discarded. The latter can be checked with is_discarded.

    "},{"location":"api/basic_json/from_cbor/#exception-safety","title":"Exception safety","text":"

    Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

    "},{"location":"api/basic_json/from_cbor/#exceptions","title":"Exceptions","text":"
    • Throws parse_error.110 if the given input ends prematurely or the end of the file was not reached when strict was set to true
    • Throws parse_error.112 if unsupported features from CBOR were used in the given input or if the input is not valid CBOR
    • Throws parse_error.113 if a map key is not a string (keys of other types are not supported, as JSON object keys are always strings), or if a string value or object key is not valid UTF-8 and error_handler is strict
    "},{"location":"api/basic_json/from_cbor/#complexity","title":"Complexity","text":"

    Linear in the size of the input.

    "},{"location":"api/basic_json/from_cbor/#examples","title":"Examples","text":"Example

    The example shows the deserialization of a byte vector in CBOR format to a JSON value.

    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create byte vector\n    std::vector<std::uint8_t> v = {0xa2, 0x67, 0x63, 0x6f, 0x6d, 0x70, 0x61, 0x63,\n                                   0x74, 0xf5, 0x66, 0x73, 0x63, 0x68, 0x65, 0x6d,\n                                   0x61, 0x00\n                                  };\n\n    // deserialize it with CBOR\n    json j = json::from_cbor(v);\n\n    // print the deserialized JSON value\n    std::cout << std::setw(2) << j << std::endl;\n}\n

    Output:

    {\n  \"compact\": true,\n  \"schema\": 0\n}\n
    "},{"location":"api/basic_json/from_cbor/#see-also","title":"See also","text":"
    • to_cbor create a CBOR serialization of a JSON value
    • from_msgpack create a JSON value from an input in MessagePack format
    • from_bson create a JSON value from an input in BSON format
    • from_ubjson create a JSON value from an input in UBJSON format
    • from_bjdata create a JSON value from an input in BJData format
    • from_bon8 create a JSON value from an input in BON8 format
    "},{"location":"api/basic_json/from_cbor/#version-history","title":"Version history","text":"
    • Added in version 2.0.9.
    • Parameter start_index since version 2.1.1.
    • Changed to consume input adapters, removed start_index parameter, and added strict parameter in version 3.0.0.
    • Added allow_exceptions parameter in version 3.2.0.
    • Added tag_handler parameter in version 3.9.0.
    • Extended container support (1) to include types with lvalue-only ADL begin/end (matching std::begin/std::end semantics) in version 3.13.0 unreleased.
    • Extended overload (2) to accept heterogeneous iterator+sentinel pairs (C++20 ranges support) in version 3.13.0 unreleased.
    • Added error_handler parameter in version 3.13.0 unreleased.

    Deprecation

    • Overload (2) replaces calls to from_cbor with a pointer and a length as first two parameters, which has been deprecated in version 3.8.0. In version 4.0.0 unreleased, this overload will be deleted (= delete) rather than removed. Please replace all calls like from_cbor(ptr, len, ...); with from_cbor(ptr, ptr+len, ...);.
    • Overload (2) replaces calls to from_cbor with a pair of iterators as their first parameter, which has been deprecated in version 3.8.0. This overload will be removed in version 4.0.0. Please replace all calls like from_cbor({ptr, ptr+len}, ...); with from_cbor(ptr, ptr+len, ...);.

    You should be warned by your compiler with a -Wdeprecated-declarations warning if you are using a deprecated function.

    See the migration guide for how to update existing code.

    "},{"location":"api/basic_json/from_msgpack/","title":"nlohmann::basic_json::from_msgpack","text":"
    // (1)\ntemplate<typename InputType>\nstatic basic_json from_msgpack(InputType&& i,\n                               const bool strict = true,\n                               const bool allow_exceptions = true,\n                               const error_handler_t error_handler = error_handler_t::keep);\n// (2)\ntemplate<typename IteratorType, typename SentinelType = IteratorType>\nstatic basic_json from_msgpack(IteratorType first, SentinelType last,\n                               const bool strict = true,\n                               const bool allow_exceptions = true,\n                               const error_handler_t error_handler = error_handler_t::keep);\n

    Deserializes a given input to a JSON value using the MessagePack serialization format.

    1. Reads from a compatible input.
    2. Reads from an iterator range, or an iterator and a sentinel of a different type (C++20 ranges support).

    The exact mapping and its limitations are described on a dedicated page.

    "},{"location":"api/basic_json/from_msgpack/#template-parameters","title":"Template parameters","text":"InputType

    A compatible input, for instance:

    • an std::istream object
    • a FILE pointer
    • a C-style array of characters
    • a pointer to a null-terminated string of single byte characters
    • a container obj for which begin(obj) and end(obj) produce a valid pair of iterators (as found via ADL or member functions, with semantics compatible to std::begin and std::end)
    IteratorType a compatible iterator type SentinelType

    defaults to IteratorType; may be a different type comparable to IteratorType via operator!=, for instance.

    • a custom sentinel type for C++20 ranges
    • std::default_sentinel_t, when IteratorType is std::counted_iterator
    "},{"location":"api/basic_json/from_msgpack/#parameters","title":"Parameters","text":"i (in) an input in MessagePack format convertible to an input adapter first (in) iterator to the start of the input last (in) iterator to the end of the input, or a sentinel value that compares equal to the end iterator with operator!= strict (in) whether to expect the input to be consumed until EOF (true by default) allow_exceptions (in) whether to throw exceptions in case of a parse error (optional, true by default) error_handler (in) how to treat a string value or object key that is not valid UTF-8; see error_handler_t. MessagePack's specification explicitly allows ill-formed UTF-8, so checking is opt-in: the default, keep, does not check at all, as every binary reader did before this parameter was added; strict checks and throws; replace/ignore sanitize the string the same way dump would"},{"location":"api/basic_json/from_msgpack/#return-value","title":"Return value","text":"

    deserialized JSON value; in case of a parse error and allow_exceptions set to false, the return value will be value_t::discarded. The latter can be checked with is_discarded.

    "},{"location":"api/basic_json/from_msgpack/#exception-safety","title":"Exception safety","text":"

    Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

    "},{"location":"api/basic_json/from_msgpack/#exceptions","title":"Exceptions","text":"
    • Throws parse_error.110 if the given input ends prematurely or the end of the file was not reached when strict was set to true
    • Throws parse_error.112 if unsupported features from MessagePack were used in the given input or if the input is not valid MessagePack
    • Throws parse_error.113 if a map key is not a string (keys of other types are not supported, as JSON object keys are always strings), or if a string value or object key is not valid UTF-8 and error_handler is strict
    "},{"location":"api/basic_json/from_msgpack/#complexity","title":"Complexity","text":"

    Linear in the size of the input.

    "},{"location":"api/basic_json/from_msgpack/#examples","title":"Examples","text":"Example

    The example shows the deserialization of a byte vector in MessagePack format to a JSON value.

    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create byte vector\n    std::vector<std::uint8_t> v = {0x82, 0xa7, 0x63, 0x6f, 0x6d, 0x70, 0x61, 0x63,\n                                   0x74, 0xc3, 0xa6, 0x73, 0x63, 0x68, 0x65, 0x6d,\n                                   0x61, 0x00\n                                  };\n\n    // deserialize it with MessagePack\n    json j = json::from_msgpack(v);\n\n    // print the deserialized JSON value\n    std::cout << std::setw(2) << j << std::endl;\n}\n

    Output:

    {\n  \"compact\": true,\n  \"schema\": 0\n}\n
    "},{"location":"api/basic_json/from_msgpack/#see-also","title":"See also","text":"
    • to_msgpack create a MessagePack serialization of a JSON value
    • from_cbor create a JSON value from an input in CBOR format
    • from_bson create a JSON value from an input in BSON format
    • from_ubjson create a JSON value from an input in UBJSON format
    • from_bjdata create a JSON value from an input in BJData format
    • from_bon8 create a JSON value from an input in BON8 format
    "},{"location":"api/basic_json/from_msgpack/#version-history","title":"Version history","text":"
    • Added in version 2.0.9.
    • Parameter start_index since version 2.1.1.
    • Changed to consume input adapters, removed start_index parameter, and added strict parameter in version 3.0.0.
    • Added allow_exceptions parameter in version 3.2.0.
    • Extended container support (1) to include types with lvalue-only ADL begin/end (matching std::begin/std::end semantics) in version 3.13.0 unreleased.
    • Extended overload (2) to accept heterogeneous iterator+sentinel pairs (C++20 ranges support) in version 3.13.0 unreleased.
    • Added error_handler parameter in version 3.13.0 unreleased.

    Deprecation

    • Overload (2) replaces calls to from_msgpack with a pointer and a length as first two parameters, which has been deprecated in version 3.8.0. In version 4.0.0 unreleased, this overload will be deleted (= delete) rather than removed. Please replace all calls like from_msgpack(ptr, len, ...); with from_msgpack(ptr, ptr+len, ...);.
    • Overload (2) replaces calls to from_msgpack with a pair of iterators as their first parameter, which has been deprecated in version 3.8.0. This overload will be removed in version 4.0.0. Please replace all calls like from_msgpack({ptr, ptr+len}, ...); with from_msgpack(ptr, ptr+len, ...);.

    You should be warned by your compiler with a -Wdeprecated-declarations warning if you are using a deprecated function.

    See the migration guide for how to update existing code.

    "},{"location":"api/basic_json/from_ubjson/","title":"nlohmann::basic_json::from_ubjson","text":"
    // (1)\ntemplate<typename InputType>\nstatic basic_json from_ubjson(InputType&& i,\n                              const bool strict = true,\n                              const bool allow_exceptions = true,\n                              const error_handler_t error_handler = error_handler_t::keep);\n// (2)\ntemplate<typename IteratorType, typename SentinelType = IteratorType>\nstatic basic_json from_ubjson(IteratorType first, SentinelType last,\n                              const bool strict = true,\n                              const bool allow_exceptions = true,\n                              const error_handler_t error_handler = error_handler_t::keep);\n

    Deserializes a given input to a JSON value using the UBJSON (Universal Binary JSON) serialization format.

    1. Reads from a compatible input.
    2. Reads from an iterator range, or an iterator and a sentinel of a different type (C++20 ranges support).

    The exact mapping and its limitations are described on a dedicated page.

    "},{"location":"api/basic_json/from_ubjson/#template-parameters","title":"Template parameters","text":"InputType

    A compatible input, for instance:

    • an std::istream object
    • a FILE pointer
    • a C-style array of characters
    • a pointer to a null-terminated string of single byte characters
    • a container obj for which begin(obj) and end(obj) produce a valid pair of iterators (as found via ADL or member functions, with semantics compatible to std::begin and std::end)
    IteratorType a compatible iterator type SentinelType

    defaults to IteratorType; may be a different type comparable to IteratorType via operator!=, for instance.

    • a custom sentinel type for C++20 ranges
    • std::default_sentinel_t, when IteratorType is std::counted_iterator
    "},{"location":"api/basic_json/from_ubjson/#parameters","title":"Parameters","text":"i (in) an input in UBJSON format convertible to an input adapter first (in) iterator to the start of the input last (in) iterator to the end of the input, or a sentinel value that compares equal to the end iterator with operator!= strict (in) whether to expect the input to be consumed until EOF (true by default) allow_exceptions (in) whether to throw exceptions in case of a parse error (optional, true by default) error_handler (in) how to treat a string value or object key that is not valid UTF-8; see error_handler_t. UBJSON does not require a decoder to reject ill-formed UTF-8, so checking is opt-in: the default, keep, does not check at all, as every binary reader did before this parameter was added; strict checks and throws; replace/ignore sanitize the string the same way dump would"},{"location":"api/basic_json/from_ubjson/#return-value","title":"Return value","text":"

    deserialized JSON value; in case of a parse error and allow_exceptions set to false, the return value will be value_t::discarded. The latter can be checked with is_discarded.

    "},{"location":"api/basic_json/from_ubjson/#exception-safety","title":"Exception safety","text":"

    Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

    "},{"location":"api/basic_json/from_ubjson/#exceptions","title":"Exceptions","text":"
    • Throws parse_error.110 if the given input ends prematurely or the end of the file was not reached when strict was set to true
    • Throws parse_error.112 if a parse error occurs
    • Throws parse_error.113 if a string could not be parsed successfully, or if a string value or object key is not valid UTF-8 and error_handler is strict
    • Throws out_of_range.408 if the size of an optimized container or n-dimensional array cannot be represented by std::size_t
    "},{"location":"api/basic_json/from_ubjson/#complexity","title":"Complexity","text":"

    Linear in the size of the input.

    "},{"location":"api/basic_json/from_ubjson/#examples","title":"Examples","text":"Example

    The example shows the deserialization of a byte vector in UBJSON format to a JSON value.

    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create byte vector\n    std::vector<std::uint8_t> v = {0x7B, 0x69, 0x07, 0x63, 0x6F, 0x6D, 0x70, 0x61,\n                                   0x63, 0x74, 0x54, 0x69, 0x06, 0x73, 0x63, 0x68,\n                                   0x65, 0x6D, 0x61, 0x69, 0x00, 0x7D\n                                  };\n\n    // deserialize it with UBJSON\n    json j = json::from_ubjson(v);\n\n    // print the deserialized JSON value\n    std::cout << std::setw(2) << j << std::endl;\n}\n

    Output:

    {\n  \"compact\": true,\n  \"schema\": 0\n}\n
    "},{"location":"api/basic_json/from_ubjson/#see-also","title":"See also","text":"
    • to_ubjson create a UBJSON serialization of a JSON value
    • from_cbor create a JSON value from an input in CBOR format
    • from_msgpack create a JSON value from an input in MessagePack format
    • from_bson create a JSON value from an input in BSON format
    • from_bjdata create a JSON value from an input in BJData format
    • from_bon8 create a JSON value from an input in BON8 format
    "},{"location":"api/basic_json/from_ubjson/#version-history","title":"Version history","text":"
    • Added in version 3.1.0.
    • Added allow_exceptions parameter in version 3.2.0.
    • Extended container support (1) to include types with lvalue-only ADL begin/end (matching std::begin/std::end semantics) in version 3.13.0 unreleased.
    • Extended overload (2) to accept heterogeneous iterator+sentinel pairs (C++20 ranges support) in version 3.13.0 unreleased.
    • Added error_handler parameter in version 3.13.0 unreleased.

    Deprecation

    • Overload (2) replaces calls to from_ubjson with a pointer and a length as first two parameters, which has been deprecated in version 3.8.0. In version 4.0.0 unreleased, this overload will be deleted (= delete) rather than removed. Please replace all calls like from_ubjson(ptr, len, ...); with from_ubjson(ptr, ptr+len, ...);.
    • Overload (2) replaces calls to from_ubjson with a pair of iterators as their first parameter, which has been deprecated in version 3.8.0. This overload will be removed in version 4.0.0. Please replace all calls like from_ubjson({ptr, ptr+len}, ...); with from_ubjson(ptr, ptr+len, ...);.

    You should be warned by your compiler with a -Wdeprecated-declarations warning if you are using a deprecated function.

    See the migration guide for how to update existing code.

    "},{"location":"api/basic_json/front/","title":"nlohmann::basic_json::front","text":"
    reference front();\nconst_reference front() const;\n

    Returns a reference to the first element in the container. For a JSON container c, the expression c.front() is equivalent to *c.begin().

    "},{"location":"api/basic_json/front/#return-value","title":"Return value","text":"

    In the case of a structured type (array or object), a reference to the first element is returned. In the case of number, string, boolean, or binary values, a reference to the value is returned.

    "},{"location":"api/basic_json/front/#exception-safety","title":"Exception safety","text":"

    Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

    "},{"location":"api/basic_json/front/#exceptions","title":"Exceptions","text":"

    If the JSON value is null, exception invalid_iterator.214 is thrown.

    "},{"location":"api/basic_json/front/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json/front/#notes","title":"Notes","text":"

    Precondition

    The array or object must not be empty. Calling front on an empty array or object yields undefined behavior.

    "},{"location":"api/basic_json/front/#examples","title":"Examples","text":"Example

    The following code shows an example for front().

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_null;\n    json j_boolean = true;\n    json j_number_integer = 17;\n    json j_number_float = 23.42;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_object_empty(json::value_t::object);\n    json j_array = {1, 2, 4, 8, 16};\n    json j_array_empty(json::value_t::array);\n    json j_string = \"Hello, world\";\n\n    // call front()\n    //std::cout << j_null.front() << '\\n';          // would throw\n    std::cout << j_boolean.front() << '\\n';\n    std::cout << j_number_integer.front() << '\\n';\n    std::cout << j_number_float.front() << '\\n';\n    std::cout << j_object.front() << '\\n';\n    //std::cout << j_object_empty.front() << '\\n';  // undefined behavior\n    std::cout << j_array.front() << '\\n';\n    //std::cout << j_array_empty.front() << '\\n';   // undefined behavior\n    std::cout << j_string.front() << '\\n';\n}\n

    Output:

    true\n17\n23.42\n1\n1\n\"Hello, world\"\n
    "},{"location":"api/basic_json/front/#see-also","title":"See also","text":"
    • back to access the last element
    • basic_json_view::front - the same access on a zero-copy view (document order, not sorted; duplicate keys are visited)
    "},{"location":"api/basic_json/front/#version-history","title":"Version history","text":"
    • Added in version 1.0.0.
    • Adjusted code to return reference to binary values in version 3.8.0.
    "},{"location":"api/basic_json/get/","title":"nlohmann::basic_json::get","text":"
    // (1)\ntemplate<typename ValueType>\nValueType get() const noexcept(\n    noexcept(JSONSerializer<ValueType>::from_json(\n        std::declval<const basic_json_t&>(), std::declval<ValueType&>())));\n\n// (2)\ntemplate<typename BasicJsonType>\nBasicJsonType get() const;\n\n// (3)\ntemplate<typename PointerType>\nPointerType get() noexcept;\n\ntemplate<typename PointerType>\nconst PointerType get() const noexcept;  // constexpr since C++14\n

    1. Explicit type conversion between the JSON value and a compatible value which is CopyConstructible and DefaultConstructible. The value is converted by calling the json_serializer<ValueType> from_json() method.

      The function is equivalent to executing 

      ValueType ret;\nJSONSerializer<ValueType>::from_json(*this, ret);\nreturn ret;\n

      This overload is chosen if:

      • ValueType is not basic_json,
      • json_serializer<ValueType> has a from_json() method of the form void from_json(const basic_json&, ValueType&), and
      • json_serializer<ValueType> does not have a from_json() method of the form ValueType from_json(const basic_json&)

      If the type is not CopyConstructible and not DefaultConstructible, the value is converted by calling the json_serializer<ValueType> from_json() method.

      The function is then equivalent to executing 

      return JSONSerializer<ValueTypeCV>::from_json(*this);\n

      This overload is chosen if:

      • ValueType is not basic_json and
      • json_serializer<ValueType> has a from_json() method of the form ValueType from_json(const basic_json&)

      If json_serializer<ValueType> has both overloads of from_json(), the latter one is chosen.

    2. Overload for basic_json specializations. The function is equivalent to executing 

      return *this;\n

    3. Explicit pointer access to the internally stored JSON value. No copies are made.

    "},{"location":"api/basic_json/get/#template-parameters","title":"Template parameters","text":"ValueType the value type to return BasicJsonType a specialization of basic_json PointerType pointer type; must be a pointer to array_t, object_t, string_t, boolean_t, number_integer_t, or number_unsigned_t, number_float_t, or binary_t. Other types will not compile."},{"location":"api/basic_json/get/#return-value","title":"Return value","text":"
    1. copy of the JSON value, converted to ValueType
    2. a copy of *this, converted into BasicJsonType
    3. pointer to the internally stored JSON value if the requested pointer type fits to the JSON value; nullptr otherwise
    "},{"location":"api/basic_json/get/#exception-safety","title":"Exception safety","text":"

    Depends on what json_serializer<ValueType> from_json() method throws for overloads (1) and (2); the JSON value itself is never modified, since get() is a const member function. No-throw guarantee for overload (3): this function never throws exceptions.

    "},{"location":"api/basic_json/get/#exceptions","title":"Exceptions","text":"

    Depends on what json_serializer<ValueType> from_json() method throws

    "},{"location":"api/basic_json/get/#complexity","title":"Complexity","text":"

    Depends on the json_serializer<ValueType>::from_json() implementation for overloads (1) and (2); constant for overload (3).

    "},{"location":"api/basic_json/get/#notes","title":"Notes","text":"

    Undefined behavior for pointers

    Writing data to the pointee (overload 3) of the result yields an undefined state.

    Undefined behavior for numeric conversions

    Conversions between numeric types are performed by the corresponding from_json() implementation using the target C++ type. When converting between numeric types, the library does not check whether the source value is representable by the target type.

    If the source value is outside the range of the target type, the behavior is the same as the corresponding C++ conversion. In particular, converting a floating-point value to an integer type that cannot represent the value results in undefined behavior.

    See Number conversion for more information.

    std::optional conversions

    Prior to version 3.13.0 unreleased, get<std::optional<T>>() (and other conversions to std::optional<T>) failed to compile in every configuration, due to an internal implementation bug that made the from_json overload for std::optional unreachable regardless of the JSON_USE_IMPLICIT_CONVERSIONS setting. This has been fixed.

    "},{"location":"api/basic_json/get/#examples","title":"Examples","text":"Example: (1) explicit conversion to compatible types

    The example below shows several conversions from JSON values to other types. There a few things to note: (1) Floating-point numbers can be converted to integers, (2) A JSON array can be converted to a standard std::vector<short>, (3) A JSON object can be converted to C++ associative containers such as std::map<std::string, json>.

    #include <iostream>\n#include <map>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON value with different types\n    json json_types =\n    {\n        {\"boolean\", true},\n        {\n            \"number\", {\n                {\"integer\", 42},\n                {\"floating-point\", 17.23}\n            }\n        },\n        {\"string\", \"Hello, world!\"},\n        {\"array\", {1, 2, 3, 4, 5}},\n        {\"null\", nullptr}\n    };\n\n    // use explicit conversions\n    auto v1 = json_types[\"boolean\"].get<bool>();\n    auto v2 = json_types[\"number\"][\"integer\"].get<int>();\n    auto v3 = json_types[\"number\"][\"integer\"].get<short>();\n    auto v4 = json_types[\"number\"][\"floating-point\"].get<float>();\n    auto v5 = json_types[\"number\"][\"floating-point\"].get<int>();\n    auto v6 = json_types[\"string\"].get<std::string>();\n    auto v7 = json_types[\"array\"].get<std::vector<short>>();\n    auto v8 = json_types.get<std::map<std::string, json>>();\n\n    // print the conversion results\n    std::cout << v1 << '\\n';\n    std::cout << v2 << ' ' << v3 << '\\n';\n    std::cout << v4 << ' ' << v5 << '\\n';\n    std::cout << v6 << '\\n';\n\n    for (auto i : v7)\n    {\n        std::cout << i << ' ';\n    }\n    std::cout << \"\\n\\n\";\n\n    for (auto i : v8)\n    {\n        std::cout << i.first << \": \" << i.second << '\\n';\n    }\n}\n

    Output:

    1\n42 42\n17.23 17\nHello, world!\n1 2 3 4 5 \n\narray: [1,2,3,4,5]\nboolean: true\nnull: null\nnumber: {\"floating-point\":17.23,\"integer\":42}\nstring: \"Hello, world!\"\n
    Example: (2) explicit conversion to another basic_json specialization

    The example below shows how a json value is converted to an ordered_json value using get<BasicJsonType>().

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing ordered_json = nlohmann::ordered_json;\n\nint main()\n{\n    // create a JSON value\n    json j = {{\"one\", 1}, {\"two\", 2}, {\"three\", 3}};\n\n    // convert to a different basic_json specialization\n    ordered_json oj = j.get<ordered_json>();\n\n    std::cout << j << '\\n';\n    std::cout << oj << '\\n';\n}\n

    Output:

    {\"one\":1,\"three\":3,\"two\":2}\n{\"one\":1,\"three\":3,\"two\":2}\n
    Example: (3) explicit pointer access to the stored value

    The example below shows how pointers to internal values of a JSON value can be requested. Note that no type conversions are made and a #cpp nullptr is returned if the value and the requested pointer type does not match.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON number\n    json value = 17;\n\n    // explicitly getting pointers\n    auto p1 = value.get<const json::number_integer_t*>();\n    auto p2 = value.get<json::number_integer_t*>();\n    auto p3 = value.get<json::number_integer_t* const>();\n    auto p4 = value.get<const json::number_integer_t* const>();\n    auto p5 = value.get<json::number_float_t*>();\n\n    // print the pointees\n    std::cout << *p1 << ' ' << *p2 << ' ' << *p3 << ' ' << *p4 << '\\n';\n    std::cout << std::boolalpha << (p5 == nullptr) << '\\n';\n}\n

    Output:

    17 17 17 17\ntrue\n
    "},{"location":"api/basic_json/get/#see-also","title":"See also","text":"
    • get_to convert and write into a passed value
    • get_ptr get a pointer to the stored value
    • get_ref get a reference to the stored value
    • operator ValueType get a value via implicit conversion
    • Converting values - the type conversions article
    • basic_json_view::get - the same conversion on a zero-copy view (many types are converted without ever building a basic_json value)
    "},{"location":"api/basic_json/get/#version-history","title":"Version history","text":"
    1. Since version 2.1.0.
    2. Since version 2.1.0. Extended to work with other specializations of basic_json in version 3.2.0.
    3. Since version 1.0.0.
    "},{"location":"api/basic_json/get_allocator/","title":"nlohmann::basic_json::get_allocator","text":"
    static allocator_type get_allocator();\n

    Returns the allocator associated with the container.

    "},{"location":"api/basic_json/get_allocator/#return-value","title":"Return value","text":"

    associated allocator

    "},{"location":"api/basic_json/get_allocator/#exception-safety","title":"Exception safety","text":"

    Strong guarantee: if an exception is thrown, there are no changes to any JSON value.

    "},{"location":"api/basic_json/get_allocator/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json/get_allocator/#examples","title":"Examples","text":"Example

    The example shows how get_allocator() is used to created json values.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    auto alloc = json::get_allocator();\n    using traits_t = std::allocator_traits<decltype(alloc)>;\n\n    json* j = traits_t::allocate(alloc, 1);\n    traits_t::construct(alloc, j, \"Hello, world!\");\n\n    std::cout << *j << std::endl;\n\n    traits_t::destroy(alloc, j);\n    traits_t::deallocate(alloc, j, 1);\n}\n

    Output:

    \"Hello, world!\"\n
    "},{"location":"api/basic_json/get_allocator/#see-also","title":"See also","text":"
    • basic_json the class template, with AllocatorType as one of its template parameters
    • Template Parameter Requirements - the requirements for AllocatorType
    "},{"location":"api/basic_json/get_allocator/#version-history","title":"Version history","text":"
    • Added in version 1.0.0.
    "},{"location":"api/basic_json/get_binary/","title":"nlohmann::basic_json::get_binary","text":"
    binary_t& get_binary();\n\nconst binary_t& get_binary() const;\n

    Returns a reference to the stored binary value.

    "},{"location":"api/basic_json/get_binary/#return-value","title":"Return value","text":"

    Reference to binary value.

    "},{"location":"api/basic_json/get_binary/#exception-safety","title":"Exception safety","text":"

    Strong exception safety: if an exception occurs, the original value stays intact.

    "},{"location":"api/basic_json/get_binary/#exceptions","title":"Exceptions","text":"

    Throws type_error.302 if the value is not binary

    "},{"location":"api/basic_json/get_binary/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json/get_binary/#examples","title":"Examples","text":"Example

    The following code shows how to query a binary value.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a binary vector\n    std::vector<std::uint8_t> vec = {0xCA, 0xFE, 0xBA, 0xBE};\n\n    // create a binary JSON value with subtype 42\n    json j = json::binary(vec, 42);\n\n    // output type and subtype\n    std::cout << \"type: \" << j.type_name() << \", subtype: \" << j.get_binary().subtype() << std::endl;\n}\n

    Output:

    type: binary, subtype: 42\n
    "},{"location":"api/basic_json/get_binary/#see-also","title":"See also","text":"
    • get get a value (explicit conversion)
    • get_ref get a reference to the stored value
    "},{"location":"api/basic_json/get_binary/#version-history","title":"Version history","text":"
    • Added in version 3.8.0.
    "},{"location":"api/basic_json/get_ptr/","title":"nlohmann::basic_json::get_ptr","text":"
    template<typename PointerType>\nPointerType get_ptr() noexcept;\n\ntemplate<typename PointerType>\nconstexpr const PointerType get_ptr() const noexcept;\n

    Implicit pointer access to the internally stored JSON value. No copies are made.

    "},{"location":"api/basic_json/get_ptr/#template-parameters","title":"Template parameters","text":"PointerType pointer type; must be a pointer to array_t, object_t, string_t, boolean_t, number_integer_t, or number_unsigned_t, number_float_t, or binary_t. Other types will not compile."},{"location":"api/basic_json/get_ptr/#return-value","title":"Return value","text":"

    pointer to the internally stored JSON value if the requested pointer type fits to the JSON value; nullptr otherwise

    "},{"location":"api/basic_json/get_ptr/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this function never throws exceptions.

    "},{"location":"api/basic_json/get_ptr/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json/get_ptr/#notes","title":"Notes","text":"

    Undefined behavior

    The pointer becomes invalid if the underlying JSON object changes.

    Consider the following example code where the pointer ptr changes after the array is resized. As a result, reading or writing to ptr after the array change would be undefined behavior. The address of the first array element changes, because the underlying std::vector is resized after adding a fifth element.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    json j = {1, 2, 3, 4};\n    auto* ptr = j[0].get_ptr<std::int64_t*>();\n    std::cout << \"value at \" << ptr << \" is \" << *ptr << std::endl;\n\n    j.push_back(5);\n\n    ptr = j[0].get_ptr<std::int64_t*>();\n    std::cout << \"value at \" << ptr << \" is \" << *ptr << std::endl;\n}\n

    Output:

    value at 0x6000012fc1c8 is 1\nvalue at 0x6000029fc088 is 1\n
    "},{"location":"api/basic_json/get_ptr/#examples","title":"Examples","text":"Example

    The example below shows how pointers to internal values of a JSON value can be requested. Note that no type conversions are made and a nullptr is returned if the value and the requested pointer type does not match.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON number\n    json value = 17;\n\n    // explicitly getting pointers\n    auto p1 = value.get_ptr<const json::number_integer_t*>();\n    auto p2 = value.get_ptr<json::number_integer_t*>();\n    auto p3 = value.get_ptr<json::number_integer_t* const>();\n    auto p4 = value.get_ptr<const json::number_integer_t* const>();\n    auto p5 = value.get_ptr<json::number_float_t*>();\n\n    // print the pointees\n    std::cout << *p1 << ' ' << *p2 << ' ' << *p3 << ' ' << *p4 << '\\n';\n    std::cout << std::boolalpha << (p5 == nullptr) << '\\n';\n}\n

    Output:

    17 17 17 17\ntrue\n
    "},{"location":"api/basic_json/get_ptr/#see-also","title":"See also","text":"
    • get_ref() get a reference value
    "},{"location":"api/basic_json/get_ptr/#version-history","title":"Version history","text":"
    • Added in version 1.0.0.
    • Extended to binary types in version 3.8.0.
    "},{"location":"api/basic_json/get_ref/","title":"nlohmann::basic_json::get_ref","text":"
    template<typename ReferenceType>\nReferenceType get_ref();\n\ntemplate<typename ReferenceType>\nconst ReferenceType get_ref() const;\n

    Implicit reference access to the internally stored JSON value. No copies are made.

    "},{"location":"api/basic_json/get_ref/#template-parameters","title":"Template parameters","text":"ReferenceType reference type; must be a reference to array_t, object_t, string_t, boolean_t, number_integer_t, or number_unsigned_t, number_float_t, or binary_t. Enforced by a static assertion."},{"location":"api/basic_json/get_ref/#return-value","title":"Return value","text":"

    reference to the internally stored JSON value if the requested reference type fits to the JSON value; throws type_error.303 otherwise

    "},{"location":"api/basic_json/get_ref/#exception-safety","title":"Exception safety","text":"

    Strong exception safety: if an exception occurs, the original value stays intact.

    "},{"location":"api/basic_json/get_ref/#exceptions","title":"Exceptions","text":"

    Throws type_error.303 if the requested reference type does not match the stored JSON value type; example: \"incompatible ReferenceType for get_ref, actual type is binary\".

    "},{"location":"api/basic_json/get_ref/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json/get_ref/#notes","title":"Notes","text":"

    Undefined behavior

    The reference becomes invalid if the underlying JSON object changes.

    "},{"location":"api/basic_json/get_ref/#examples","title":"Examples","text":"Example

    The example shows several calls to get_ref().

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON number\n    json value = 17;\n\n    // explicitly getting references\n    auto r1 = value.get_ref<const json::number_integer_t&>();\n    auto r2 = value.get_ref<json::number_integer_t&>();\n\n    // print the values\n    std::cout << r1 << ' ' << r2 << '\\n';\n\n    // incompatible type throws exception\n    try\n    {\n        auto r3 = value.get_ref<json::number_float_t&>();\n    }\n    catch (const json::type_error& ex)\n    {\n        std::cout << ex.what() << '\\n';\n    }\n}\n

    Output:

    17 17\n[json.exception.type_error.303] incompatible ReferenceType for get_ref, actual type is number\n
    "},{"location":"api/basic_json/get_ref/#see-also","title":"See also","text":"
    • get_ptr() get a pointer value
    • basic_json_view::get_string - the closest counterpart on a zero-copy view: a string without a copy, but as a view rather than a reference to a value that must already exist
    "},{"location":"api/basic_json/get_ref/#version-history","title":"Version history","text":"
    • Added in version 1.1.0.
    • Extended to binary types in version 3.8.0.
    "},{"location":"api/basic_json/get_to/","title":"nlohmann::basic_json::get_to","text":"
    template<typename ValueType>\nValueType& get_to(ValueType& v) const noexcept(\n    noexcept(JSONSerializer<ValueType>::from_json(\n        std::declval<const basic_json_t&>(), v)));\n

    Explicit type conversion between the JSON value and a compatible value. The value is filled into the input parameter by calling the json_serializer<ValueType> from_json() method.

    The function is equivalent to executing 

    ValueType v;\nJSONSerializer<ValueType>::from_json(*this, v);\n

    This overload is chosen if:

    • ValueType is not basic_json,
    • json_serializer<ValueType> has a from_json() method of the form void from_json(const basic_json&, ValueType&)

    v must not be const. Passing a const object is a compile-time error. For types such as arithmetic types, enums, and C arrays, the error is a static_assert that names the problem. For other types, the overload is not viable, and the compiler reports that no matching get_to was found.

    "},{"location":"api/basic_json/get_to/#template-parameters","title":"Template parameters","text":"ValueType the value type to return"},{"location":"api/basic_json/get_to/#return-value","title":"Return value","text":"

    the input parameter, allowing chaining calls

    "},{"location":"api/basic_json/get_to/#exception-safety","title":"Exception safety","text":"

    Depends on what json_serializer<ValueType> from_json() method throws; the JSON value itself is never modified, since get_to() is a const member function.

    "},{"location":"api/basic_json/get_to/#exceptions","title":"Exceptions","text":"

    Depends on what json_serializer<ValueType> from_json() method throws

    "},{"location":"api/basic_json/get_to/#complexity","title":"Complexity","text":"

    Depends on the json_serializer<ValueType>::from_json() implementation.

    "},{"location":"api/basic_json/get_to/#examples","title":"Examples","text":"Example

    The example below shows several conversions from JSON values to other types. There a few things to note: (1) Floating-point numbers can be converted to integers, (2) A JSON array can be converted to a standard std::vector<short>, (3) A JSON object can be converted to C++ associative containers such as std::map<std::string, json>.

    #include <iostream>\n#include <map>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON value with different types\n    json json_types =\n    {\n        {\"boolean\", true},\n        {\n            \"number\", {\n                {\"integer\", 42},\n                {\"floating-point\", 17.23}\n            }\n        },\n        {\"string\", \"Hello, world!\"},\n        {\"array\", {1, 2, 3, 4, 5}},\n        {\"null\", nullptr}\n    };\n\n    bool v1;\n    int v2;\n    short v3;\n    float v4;\n    int v5;\n    std::string v6;\n    std::vector<short> v7;\n    std::map<std::string, json> v8;\n\n    // use explicit conversions\n    json_types[\"boolean\"].get_to(v1);\n    json_types[\"number\"][\"integer\"].get_to(v2);\n    json_types[\"number\"][\"integer\"].get_to(v3);\n    json_types[\"number\"][\"floating-point\"].get_to(v4);\n    json_types[\"number\"][\"floating-point\"].get_to(v5);\n    json_types[\"string\"].get_to(v6);\n    json_types[\"array\"].get_to(v7);\n    json_types.get_to(v8);\n\n    // print the conversion results\n    std::cout << v1 << '\\n';\n    std::cout << v2 << ' ' << v3 << '\\n';\n    std::cout << v4 << ' ' << v5 << '\\n';\n    std::cout << v6 << '\\n';\n\n    for (auto i : v7)\n    {\n        std::cout << i << ' ';\n    }\n    std::cout << \"\\n\\n\";\n\n    for (auto i : v8)\n    {\n        std::cout << i.first << \": \" << i.second << '\\n';\n    }\n}\n

    Output:

    1\n42 42\n17.23 17\nHello, world!\n1 2 3 4 5 \n\narray: [1,2,3,4,5]\nboolean: true\nnull: null\nnumber: {\"floating-point\":17.23,\"integer\":42}\nstring: \"Hello, world!\"\n
    "},{"location":"api/basic_json/get_to/#see-also","title":"See also","text":"
    • get get a value (explicit conversion)
    • get_ref get a reference to the stored value
    • get_ptr get a pointer to the stored value
    • Converting values - the type conversions article
    • basic_json_view::get_to - the same conversion on a zero-copy view
    "},{"location":"api/basic_json/get_to/#version-history","title":"Version history","text":"
    • Since version 3.3.0.
    • Added a static_assert with a clear message for const arguments in version 3.13.0 unreleased.
    "},{"location":"api/basic_json/input_format_t/","title":"nlohmann::basic_json::input_format_t","text":"
    enum class input_format_t {\n    json,\n    cbor,\n    msgpack,\n    ubjson,\n    bson,\n    bjdata,\n    bon8\n};\n

    This enumeration is used in the sax_parse function to choose the input format to parse:

    json JSON (JavaScript Object Notation) cbor CBOR (Concise Binary Object Representation) msgpack MessagePack ubjson UBJSON (Universal Binary JSON) bson BSON (Binary JSON) bjdata BJData (Binary JData) bon8 BON8 (Binary Object Notation 8)"},{"location":"api/basic_json/input_format_t/#examples","title":"Examples","text":"Example

    The example below shows how an input_format_t enum value is passed to sax_parse to set the input format to CBOR.

    #include <iostream>\n#include <iomanip>\n#include <sstream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\n// a simple event consumer that collects string representations of the passed\n// values; note inheriting from json::json_sax_t is not required, but can\n// help not to forget a required function\nclass sax_event_consumer : public json::json_sax_t\n{\n  public:\n    std::vector<std::string> events;\n\n    bool null() override\n    {\n        events.push_back(\"null()\");\n        return true;\n    }\n\n    bool boolean(bool val) override\n    {\n        events.push_back(\"boolean(val=\" + std::string(val ? \"true\" : \"false\") + \")\");\n        return true;\n    }\n\n    bool number_integer(number_integer_t val) override\n    {\n        events.push_back(\"number_integer(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_unsigned(number_unsigned_t val) override\n    {\n        events.push_back(\"number_unsigned(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_float(number_float_t val, const string_t& s) override\n    {\n        events.push_back(\"number_float(val=\" + std::to_string(val) + \", s=\" + s + \")\");\n        return true;\n    }\n\n    bool string(string_t& val) override\n    {\n        events.push_back(\"string(val=\" + val + \")\");\n        return true;\n    }\n\n    bool start_object(std::size_t elements) override\n    {\n        events.push_back(\"start_object(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_object() override\n    {\n        events.push_back(\"end_object()\");\n        return true;\n    }\n\n    bool start_array(std::size_t elements) override\n    {\n        events.push_back(\"start_array(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_array() override\n    {\n        events.push_back(\"end_array()\");\n        return true;\n    }\n\n    bool key(string_t& val) override\n    {\n        events.push_back(\"key(val=\" + val + \")\");\n        return true;\n    }\n\n    bool binary(json::binary_t& val) override\n    {\n        events.push_back(\"binary(val=[...])\");\n        return true;\n    }\n\n    bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex) override\n    {\n        events.push_back(\"parse_error(position=\" + std::to_string(position) + \", last_token=\" + last_token + \",\\n            ex=\" + std::string(ex.what()) + \")\");\n        return false;\n    }\n};\n\nint main()\n{\n    // CBOR byte string\n    std::vector<std::uint8_t> vec = {{0x44, 0xcA, 0xfe, 0xba, 0xbe}};\n\n    // create a SAX event consumer object\n    sax_event_consumer sec;\n\n    // parse CBOR\n    bool result = json::sax_parse(vec, &sec, json::input_format_t::cbor);\n\n    // output the recorded events\n    for (auto& event : sec.events)\n    {\n        std::cout << event << \"\\n\";\n    }\n\n    // output the result of sax_parse\n    std::cout << \"\\nresult: \" << std::boolalpha << result << std::endl;\n}\n

    Output:

    binary(val=[...])\n\nresult: true\n
    "},{"location":"api/basic_json/input_format_t/#see-also","title":"See also","text":"
    • sax_parse generic SAX parse interface, taking an input_format_t to select the input format
    • cbor_tag_handler_t configures how CBOR tags are treated while parsing
    "},{"location":"api/basic_json/input_format_t/#version-history","title":"Version history","text":"
    • Added in version 3.2.0.
    "},{"location":"api/basic_json/insert/","title":"nlohmann::basic_json::insert","text":"
    // (1)\niterator insert(const_iterator pos, const basic_json& val);\niterator insert(const_iterator pos, basic_json&& val);\n\n// (2)\niterator insert(const_iterator pos, size_type cnt, const basic_json& val);\n\n// (3)\niterator insert(const_iterator pos, const_iterator first, const_iterator last);\n\n// (4)\niterator insert(const_iterator pos, initializer_list_t ilist);\n\n// (5)\nvoid insert(const_iterator first, const_iterator last);\n
    1. Inserts element val into an array before iterator pos.
    2. Inserts cnt copies of val into an array before iterator pos.
    3. Inserts elements from range [first, last) into an array before iterator pos.
    4. Inserts elements from initializer list ilist into an array before iterator pos.
    5. Inserts elements from range [first, last) into an object.
    "},{"location":"api/basic_json/insert/#iterator-invalidation","title":"Iterator invalidation","text":"

    For all cases where an element is added to an array, a reallocation can happen, in which case all iterators (including the end() iterator) and all references to the elements are invalidated. Otherwise, only the end() iterator is invalidated. Also, any iterator or reference after the insertion point will point to the same index, which is now a different value.

    For ordered_json, also adding an element to an object can yield a reallocation which again invalidates all iterators and all references. Also, any iterator or reference after the insertion point will point to the same index, which is now a different value.

    "},{"location":"api/basic_json/insert/#parameters","title":"Parameters","text":"pos (in) iterator before which the content will be inserted; may be the end() iterator val (in) value to insert cnt (in) number of copies of val to insert first (in) the start of the range of elements to insert last (in) the end of the range of elements to insert ilist (in) initializer list to insert the values from"},{"location":"api/basic_json/insert/#return-value","title":"Return value","text":"
    1. iterator pointing to the inserted val.
    2. iterator pointing to the first element inserted, or pos if cnt==0
    3. iterator pointing to the first element inserted, or pos if first==last
    4. iterator pointing to the first element inserted, or pos if ilist is empty
    5. (none)
    "},{"location":"api/basic_json/insert/#exception-safety","title":"Exception safety","text":"

    Strong exception safety: if an exception occurs, the original value stays intact.

    "},{"location":"api/basic_json/insert/#exceptions","title":"Exceptions","text":"
    1. The function can throw the following exceptions:
      • Throws type_error.309 if called on JSON values other than arrays; example: \"cannot use insert() with string\"
      • Throws invalid_iterator.202 if called on an iterator which does not belong to the current JSON value; example: \"iterator does not fit current value\"
    2. The function can throw the following exceptions:
      • Throws type_error.309 if called on JSON values other than arrays; example: \"cannot use insert() with string\"
      • Throws invalid_iterator.202 if called on an iterator which does not belong to the current JSON value; example: \"iterator does not fit current value\"
    3. The function can throw the following exceptions:
      • Throws type_error.309 if called on JSON values other than arrays; example: \"cannot use insert() with string\"
      • Throws invalid_iterator.202 if called on an iterator which does not belong to the current JSON value; example: \"iterator does not fit current value\"
      • Throws invalid_iterator.210 if first and last do not belong to the same JSON value; example: \"iterators do not fit\"
      • Throws invalid_iterator.211 if first or last are iterators into container for which insert is called; example: \"passed iterators may not belong to container\"
      • Throws invalid_iterator.202 if first or last do not point to an array; example: \"iterators first and last must point to arrays\"
    4. The function can throw the following exceptions:
      • Throws type_error.309 if called on JSON values other than arrays; example: \"cannot use insert() with string\"
      • Throws invalid_iterator.202 if called on an iterator which does not belong to the current JSON value; example: \"iterator does not fit current value\"
    5. The function can throw the following exceptions:
      • Throws type_error.309 if called on JSON values other than objects; example: \"cannot use insert() with string\"
      • Throws invalid_iterator.202 if first or last do not point to an object; example: \"iterators first and last must point to objects\"
      • Throws invalid_iterator.210 if first and last do not belong to the same JSON value; example: \"iterators do not fit\"
    "},{"location":"api/basic_json/insert/#complexity","title":"Complexity","text":"
    1. Constant plus linear in the distance between pos and end of the container.
    2. Linear in cnt plus linear in the distance between pos and end of the container.
    3. Linear in std::distance(first, last) plus linear in the distance between pos and end of the container.
    4. Linear in ilist.size() plus linear in the distance between pos and end of the container.
    5. O(N*log(size() + N)), where N is the number of elements to insert.
    "},{"location":"api/basic_json/insert/#examples","title":"Examples","text":"Example: (1) insert element into array

    The example shows how insert() is used.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON array\n    json v = {1, 2, 3, 4};\n\n    // insert number 10 before number 3\n    auto new_pos = v.insert(v.begin() + 2, 10);\n\n    // output new array and result of insert call\n    std::cout << *new_pos << '\\n';\n    std::cout << v << '\\n';\n}\n

    Output:

    10\n[1,2,10,3,4]\n
    Example: (2) insert copies of element into array

    The example shows how insert() is used.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON array\n    json v = {1, 2, 3, 4};\n\n    // insert number 7 copies of number 7 before number 3\n    auto new_pos = v.insert(v.begin() + 2, 7, 7);\n\n    // output new array and result of insert call\n    std::cout << *new_pos << '\\n';\n    std::cout << v << '\\n';\n}\n

    Output:

    7\n[1,2,7,7,7,7,7,7,7,3,4]\n
    Example: (3) insert a range of elements into an array

    The example shows how insert() is used.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON array\n    json v = {1, 2, 3, 4};\n\n    // create a JSON array to copy values from\n    json v2 = {\"one\", \"two\", \"three\", \"four\"};\n\n    // insert range from v2 before the end of array v\n    auto new_pos = v.insert(v.end(), v2.begin(), v2.end());\n\n    // output new array and result of insert call\n    std::cout << *new_pos << '\\n';\n    std::cout << v << '\\n';\n}\n

    Output:

    \"one\"\n[1,2,3,4,\"one\",\"two\",\"three\",\"four\"]\n
    Example: (4) insert elements from an initializer list into an array

    The example shows how insert() is used.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON array\n    json v = {1, 2, 3, 4};\n\n    // insert range from v2 before the end of array v\n    auto new_pos = v.insert(v.end(), {7, 8, 9});\n\n    // output new array and result of insert call\n    std::cout << *new_pos << '\\n';\n    std::cout << v << '\\n';\n}\n

    Output:

    7\n[1,2,3,4,7,8,9]\n
    Example: (5) insert a range of elements into an object

    The example shows how insert() is used.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create two JSON objects\n    json j1 = {{\"one\", \"eins\"}, {\"two\", \"zwei\"}};\n    json j2 = {{\"eleven\", \"elf\"}, {\"seventeen\", \"siebzehn\"}};\n\n    // output objects\n    std::cout << j1 << '\\n';\n    std::cout << j2 << '\\n';\n\n    // insert range from j2 to j1\n    j1.insert(j2.begin(), j2.end());\n\n    // output result of insert call\n    std::cout << j1 << '\\n';\n}\n

    Output:

    {\"one\":\"eins\",\"two\":\"zwei\"}\n{\"eleven\":\"elf\",\"seventeen\":\"siebzehn\"}\n{\"eleven\":\"elf\",\"one\":\"eins\",\"seventeen\":\"siebzehn\",\"two\":\"zwei\"}\n
    "},{"location":"api/basic_json/insert/#see-also","title":"See also","text":"
    • emplace add a value to an object
    • emplace_back add a value to an array
    • push_back add a value to an array/object
    • update merges objects
    "},{"location":"api/basic_json/insert/#version-history","title":"Version history","text":"
    1. Added in version 1.0.0.
    2. Added in version 1.0.0.
    3. Added in version 1.0.0.
    4. Added in version 1.0.0. Fixed in version 3.13.0 unreleased to copy the values before inserting; before, an ilist that referred to elements of the array being inserted into could insert wrong values, because the range insert could move from or shift an element before it was copied.
    5. Added in version 3.0.0.
    "},{"location":"api/basic_json/invalid_iterator/","title":"nlohmann::basic_json::invalid_iterator","text":"
    class invalid_iterator : public exception;\n

    This exception is thrown if iterators passed to a library function do not match the expected semantics.

    Exceptions have ids 2xx (see list of iterator errors).

    classDiagram\n  direction LR\n\n    class std_exception [\"std::exception\"] {\n        <<interface>>\n    }\n\n    class json_exception [\"basic_json::exception\"] {\n        +const int id\n        +const char* what() const\n    }\n\n    class json_parse_error [\"basic_json::parse_error\"] {\n        +const std::size_t byte\n    }\n\n    class json_invalid_iterator [\"basic_json::invalid_iterator\"]\n    class json_type_error [\"basic_json::type_error\"]\n    class json_out_of_range [\"basic_json::out_of_range\"]\n    class json_other_error [\"basic_json::other_error\"]\n\n    std_exception <|-- json_exception\n    json_exception <|-- json_parse_error\n    json_exception <|-- json_invalid_iterator\n    json_exception <|-- json_type_error\n    json_exception <|-- json_out_of_range\n    json_exception <|-- json_other_error\n\n    style json_invalid_iterator fill:#CCCCFF
    "},{"location":"api/basic_json/invalid_iterator/#member-functions","title":"Member functions","text":"
    • what - returns explanatory string
    "},{"location":"api/basic_json/invalid_iterator/#member-variables","title":"Member variables","text":"
    • id - the id of the exception
    "},{"location":"api/basic_json/invalid_iterator/#examples","title":"Examples","text":"Example

    The following code shows how a invalid_iterator exception can be caught.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    try\n    {\n        // calling iterator::key() on non-object iterator\n        json j = \"string\";\n        json::iterator it = j.begin();\n        auto k = it.key();\n    }\n    catch (const json::invalid_iterator& e)\n    {\n        // output exception information\n        std::cout << \"message: \" << e.what() << '\\n'\n                  << \"exception id: \" << e.id << std::endl;\n    }\n}\n

    Output:

    message: [json.exception.invalid_iterator.207] cannot use key() for non-object iterators\nexception id: 207\n
    "},{"location":"api/basic_json/invalid_iterator/#see-also","title":"See also","text":"
    • exception for the base class of all exceptions thrown by the library
    • List of iterator errors
    • parse_error for exceptions indicating a parse error
    • type_error for exceptions indicating executing a member function with a wrong type
    • out_of_range for exceptions indicating access out of the defined range
    • other_error for exceptions indicating other library errors
    "},{"location":"api/basic_json/invalid_iterator/#version-history","title":"Version history","text":"
    • Since version 3.0.0.
    "},{"location":"api/basic_json/is_array/","title":"nlohmann::basic_json::is_array","text":"
    constexpr bool is_array() const noexcept;\n

    This function returns true if and only if the JSON value is an array.

    "},{"location":"api/basic_json/is_array/#return-value","title":"Return value","text":"

    true if type is an array, false otherwise.

    "},{"location":"api/basic_json/is_array/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this member function never throws exceptions.

    "},{"location":"api/basic_json/is_array/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json/is_array/#examples","title":"Examples","text":"Example

    The following code exemplifies is_array() for all JSON types.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_null;\n    json j_boolean = true;\n    json j_number_integer = 17;\n    json j_number_unsigned_integer = 12345678987654321u;\n    json j_number_float = 23.42;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n    json j_string = \"Hello, world\";\n    json j_binary = json::binary({1, 2, 3});\n\n    // call is_array()\n    std::cout << std::boolalpha;\n    std::cout << j_null.is_array() << '\\n';\n    std::cout << j_boolean.is_array() << '\\n';\n    std::cout << j_number_integer.is_array() << '\\n';\n    std::cout << j_number_unsigned_integer.is_array() << '\\n';\n    std::cout << j_number_float.is_array() << '\\n';\n    std::cout << j_object.is_array() << '\\n';\n    std::cout << j_array.is_array() << '\\n';\n    std::cout << j_string.is_array() << '\\n';\n    std::cout << j_binary.is_array() << '\\n';\n}\n

    Output:

    false\nfalse\nfalse\nfalse\nfalse\nfalse\ntrue\nfalse\nfalse\n
    "},{"location":"api/basic_json/is_array/#see-also","title":"See also","text":"
    • is_object checks whether the JSON value is an object
    • is_structured checks whether the JSON value is structured (array or object)
    • type returns the type of the JSON value
    • array_t the type used to store JSON arrays
    • basic_json_view::is_array - the same check on a zero-copy view
    "},{"location":"api/basic_json/is_array/#version-history","title":"Version history","text":"
    • Added in version 1.0.0.
    "},{"location":"api/basic_json/is_binary/","title":"nlohmann::basic_json::is_binary","text":"
    constexpr bool is_binary() const noexcept;\n

    This function returns true if and only if the JSON value is a binary array.

    "},{"location":"api/basic_json/is_binary/#return-value","title":"Return value","text":"

    true if type is binary, false otherwise.

    "},{"location":"api/basic_json/is_binary/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this member function never throws exceptions.

    "},{"location":"api/basic_json/is_binary/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json/is_binary/#examples","title":"Examples","text":"Example

    The following code exemplifies is_binary() for all JSON types.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_null;\n    json j_boolean = true;\n    json j_number_integer = 17;\n    json j_number_unsigned_integer = 12345678987654321u;\n    json j_number_float = 23.42;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n    json j_string = \"Hello, world\";\n    json j_binary = json::binary({1, 2, 3});\n\n    // call is_binary()\n    std::cout << std::boolalpha;\n    std::cout << j_null.is_binary() << '\\n';\n    std::cout << j_boolean.is_binary() << '\\n';\n    std::cout << j_number_integer.is_binary() << '\\n';\n    std::cout << j_number_unsigned_integer.is_binary() << '\\n';\n    std::cout << j_number_float.is_binary() << '\\n';\n    std::cout << j_object.is_binary() << '\\n';\n    std::cout << j_array.is_binary() << '\\n';\n    std::cout << j_string.is_binary() << '\\n';\n    std::cout << j_binary.is_binary() << '\\n';\n}\n

    Output:

    false\nfalse\nfalse\nfalse\nfalse\nfalse\nfalse\nfalse\ntrue\n
    "},{"location":"api/basic_json/is_binary/#see-also","title":"See also","text":"
    • is_primitive checks whether the JSON value is primitive
    • binary_t the type used to store binary values
    • get_binary returns a reference to the stored binary value
    • basic_json_view::is_binary - the same check on a zero-copy view
    "},{"location":"api/basic_json/is_binary/#version-history","title":"Version history","text":"
    • Added in version 3.8.0.
    "},{"location":"api/basic_json/is_boolean/","title":"nlohmann::basic_json::is_boolean","text":"
    constexpr bool is_boolean() const noexcept;\n

    This function returns true if and only if the JSON value is true or false.

    "},{"location":"api/basic_json/is_boolean/#return-value","title":"Return value","text":"

    true if type is boolean, false otherwise.

    "},{"location":"api/basic_json/is_boolean/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this member function never throws exceptions.

    "},{"location":"api/basic_json/is_boolean/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json/is_boolean/#examples","title":"Examples","text":"Example

    The following code exemplifies is_boolean() for all JSON types.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_null;\n    json j_boolean = true;\n    json j_number_integer = 17;\n    json j_number_unsigned_integer = 12345678987654321u;\n    json j_number_float = 23.42;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n    json j_string = \"Hello, world\";\n    json j_binary = json::binary({1, 2, 3});\n\n    // call is_boolean()\n    std::cout << std::boolalpha;\n    std::cout << j_null.is_boolean() << '\\n';\n    std::cout << j_boolean.is_boolean() << '\\n';\n    std::cout << j_number_integer.is_boolean() << '\\n';\n    std::cout << j_number_unsigned_integer.is_boolean() << '\\n';\n    std::cout << j_number_float.is_boolean() << '\\n';\n    std::cout << j_object.is_boolean() << '\\n';\n    std::cout << j_array.is_boolean() << '\\n';\n    std::cout << j_string.is_boolean() << '\\n';\n    std::cout << j_binary.is_boolean() << '\\n';\n}\n

    Output:

    false\ntrue\nfalse\nfalse\nfalse\nfalse\nfalse\nfalse\nfalse\n
    "},{"location":"api/basic_json/is_boolean/#see-also","title":"See also","text":"
    • boolean_t the type used to store JSON booleans
    • is_primitive checks whether the JSON value is primitive
    • basic_json_view::is_boolean - the same check on a zero-copy view
    "},{"location":"api/basic_json/is_boolean/#version-history","title":"Version history","text":"
    • Added in version 1.0.0.
    "},{"location":"api/basic_json/is_discarded/","title":"nlohmann::basic_json::is_discarded","text":"
    constexpr bool is_discarded() const noexcept;\n

    This function returns true for a JSON value if either:

    • the value was discarded during parsing with a callback function (see parser_callback_t), or
    • the value is the result of parsing invalid JSON with parameter allow_exceptions set to false; see parse for more information.
    "},{"location":"api/basic_json/is_discarded/#return-value","title":"Return value","text":"

    true if type is discarded, false otherwise.

    "},{"location":"api/basic_json/is_discarded/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this member function never throws exceptions.

    "},{"location":"api/basic_json/is_discarded/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json/is_discarded/#notes","title":"Notes","text":"

    Comparisons

    Discarded values are never compared equal with operator==. That is, checking whether a JSON value j is discarded will only work via:

    j.is_discarded()\n

    because

    j == json::value_t::discarded\n

    will always be false.

    Removal during parsing with callback functions

    When a value is discarded by a callback function (see parser_callback_t) during parsing, then it is removed when it is part of a structured value. For instance, if the second value of an array is discarded, instead of [null, discarded, false], the array [null, false] is returned. If the top-level value itself is discarded by the callback, the parse call returns a null value.

    After a successful parse, this function always returns false: discarded values can only occur during parsing and are either removed when inside a structured value or replaced by null at the top level. The exception is parsing with allow_exceptions set to false: a parse error then yields a discarded value for which this function returns true (see parse).

    "},{"location":"api/basic_json/is_discarded/#examples","title":"Examples","text":"Example: is_discarded() for ordinary JSON values

    The following code exemplifies is_discarded() for all JSON types.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_null;\n    json j_boolean = true;\n    json j_number_integer = 17;\n    json j_number_unsigned_integer = 12345678987654321u;\n    json j_number_float = 23.42;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n    json j_string = \"Hello, world\";\n    json j_binary = json::binary({1, 2, 3});\n\n    // call is_discarded()\n    std::cout << std::boolalpha;\n    std::cout << j_null.is_discarded() << '\\n';\n    std::cout << j_boolean.is_discarded() << '\\n';\n    std::cout << j_number_integer.is_discarded() << '\\n';\n    std::cout << j_number_unsigned_integer.is_discarded() << '\\n';\n    std::cout << j_number_float.is_discarded() << '\\n';\n    std::cout << j_object.is_discarded() << '\\n';\n    std::cout << j_array.is_discarded() << '\\n';\n    std::cout << j_string.is_discarded() << '\\n';\n    std::cout << j_binary.is_discarded() << '\\n';\n}\n

    Output:

    false\nfalse\nfalse\nfalse\nfalse\nfalse\nfalse\nfalse\nfalse\n
    Example: discarded values from parsing

    The following code shows the two situations in which a discarded value can be observed: parsing invalid JSON with allow_exceptions set to false, and a parser callback that discards the top-level value (which is replaced by null and therefore does not remain discarded).

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // parsing invalid JSON without exceptions yields a discarded value\n    json j_invalid = json::parse(\"[1,2,3\", nullptr, false);\n\n    // a callback that discards the top-level value does not leave it\n    // \"discarded\" -- it is replaced by null instead\n    json j_discarded_by_callback = json::parse(\"[1,2,3]\", [](int /*depth*/, json::parse_event_t event, json& /*parsed*/)\n    {\n        return event != json::parse_event_t::array_start;\n    });\n\n    std::cout << std::boolalpha;\n    std::cout << \"j_invalid.is_discarded()               = \" << j_invalid.is_discarded() << '\\n';\n    std::cout << \"j_discarded_by_callback                = \" << j_discarded_by_callback << '\\n';\n    std::cout << \"j_discarded_by_callback.is_discarded() = \" << j_discarded_by_callback.is_discarded() << '\\n';\n}\n

    Output:

    j_invalid.is_discarded()               = true\nj_discarded_by_callback                = null\nj_discarded_by_callback.is_discarded() = false\n
    "},{"location":"api/basic_json/is_discarded/#see-also","title":"See also","text":"
    • basic_json_view::is_discarded - the corresponding check on a zero-copy view, which is true if the view refers to no value
    "},{"location":"api/basic_json/is_discarded/#version-history","title":"Version history","text":"
    • Added in version 1.0.0.
    "},{"location":"api/basic_json/is_null/","title":"nlohmann::basic_json::is_null","text":"
    constexpr bool is_null() const noexcept;\n

    This function returns true if and only if the JSON value is null.

    "},{"location":"api/basic_json/is_null/#return-value","title":"Return value","text":"

    true if type is null, false otherwise.

    "},{"location":"api/basic_json/is_null/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this member function never throws exceptions.

    "},{"location":"api/basic_json/is_null/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json/is_null/#examples","title":"Examples","text":"Example

    The following code exemplifies is_null() for all JSON types.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_null;\n    json j_boolean = true;\n    json j_number_integer = 17;\n    json j_number_unsigned_integer = 12345678987654321u;\n    json j_number_float = 23.42;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n    json j_string = \"Hello, world\";\n    json j_binary = json::binary({1, 2, 3});\n\n    // call is_null()\n    std::cout << std::boolalpha;\n    std::cout << j_null.is_null() << '\\n';\n    std::cout << j_boolean.is_null() << '\\n';\n    std::cout << j_number_integer.is_null() << '\\n';\n    std::cout << j_number_unsigned_integer.is_null() << '\\n';\n    std::cout << j_number_float.is_null() << '\\n';\n    std::cout << j_object.is_null() << '\\n';\n    std::cout << j_array.is_null() << '\\n';\n    std::cout << j_string.is_null() << '\\n';\n    std::cout << j_binary.is_null() << '\\n';\n}\n

    Output:

    true\nfalse\nfalse\nfalse\nfalse\nfalse\nfalse\nfalse\nfalse\n
    "},{"location":"api/basic_json/is_null/#see-also","title":"See also","text":"
    • is_array checks whether the JSON value is an array
    • is_object checks whether the JSON value is an object
    • type returns the type of the JSON value
    • value_t the enumeration of JSON types
    • basic_json_view::is_null - the same check on a zero-copy view
    "},{"location":"api/basic_json/is_null/#version-history","title":"Version history","text":"
    • Added in version 1.0.0.
    "},{"location":"api/basic_json/is_number/","title":"nlohmann::basic_json::is_number","text":"
    constexpr bool is_number() const noexcept;\n

    This function returns true if and only if the JSON value is a number. This includes both integer (signed and unsigned) and floating-point values.

    "},{"location":"api/basic_json/is_number/#return-value","title":"Return value","text":"

    true if type is number (regardless whether integer, unsigned integer, or floating-point), false otherwise.

    "},{"location":"api/basic_json/is_number/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this member function never throws exceptions.

    "},{"location":"api/basic_json/is_number/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json/is_number/#possible-implementation","title":"Possible implementation","text":"
    constexpr bool is_number() const noexcept\n{\n    return is_number_integer() || is_number_float();\n}\n
    "},{"location":"api/basic_json/is_number/#examples","title":"Examples","text":"Example

    The following code exemplifies is_number() for all JSON types.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_null;\n    json j_boolean = true;\n    json j_number_integer = 17;\n    json j_number_unsigned_integer = 12345678987654321u;\n    json j_number_float = 23.42;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n    json j_string = \"Hello, world\";\n    json j_binary = json::binary({1, 2, 3});\n\n    // call is_number()\n    std::cout << std::boolalpha;\n    std::cout << j_null.is_number() << '\\n';\n    std::cout << j_boolean.is_number() << '\\n';\n    std::cout << j_number_integer.is_number() << '\\n';\n    std::cout << j_number_unsigned_integer.is_number() << '\\n';\n    std::cout << j_number_float.is_number() << '\\n';\n    std::cout << j_object.is_number() << '\\n';\n    std::cout << j_array.is_number() << '\\n';\n    std::cout << j_string.is_number() << '\\n';\n    std::cout << j_binary.is_number() << '\\n';\n}\n

    Output:

    false\nfalse\ntrue\ntrue\ntrue\nfalse\nfalse\nfalse\nfalse\n
    "},{"location":"api/basic_json/is_number/#see-also","title":"See also","text":"
    • is_number_integer() check if the value is an integer or unsigned integer number
    • is_number_unsigned() check if the value is an unsigned integer number
    • is_number_float() check if the value is a floating-point number
    • basic_json_view::is_number - the same check on a zero-copy view
    "},{"location":"api/basic_json/is_number/#version-history","title":"Version history","text":"
    • Added in version 1.0.0.
    • Extended to also return true for unsigned integers in 2.0.0.
    "},{"location":"api/basic_json/is_number_float/","title":"nlohmann::basic_json::is_number_float","text":"
    constexpr bool is_number_float() const noexcept;\n

    This function returns true if and only if the JSON value is a floating-point number. This excludes signed and unsigned integer values.

    "},{"location":"api/basic_json/is_number_float/#return-value","title":"Return value","text":"

    true if type is a floating-point number, false otherwise.

    "},{"location":"api/basic_json/is_number_float/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this member function never throws exceptions.

    "},{"location":"api/basic_json/is_number_float/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json/is_number_float/#examples","title":"Examples","text":"Example

    The following code exemplifies is_number_float() for all JSON types.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_null;\n    json j_boolean = true;\n    json j_number_integer = 17;\n    json j_number_unsigned_integer = 12345678987654321u;\n    json j_number_float = 23.42;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n    json j_string = \"Hello, world\";\n    json j_binary = json::binary({1, 2, 3});\n\n    // call is_number_float()\n    std::cout << std::boolalpha;\n    std::cout << j_null.is_number_float() << '\\n';\n    std::cout << j_boolean.is_number_float() << '\\n';\n    std::cout << j_number_integer.is_number_float() << '\\n';\n    std::cout << j_number_unsigned_integer.is_number_float() << '\\n';\n    std::cout << j_number_float.is_number_float() << '\\n';\n    std::cout << j_object.is_number_float() << '\\n';\n    std::cout << j_array.is_number_float() << '\\n';\n    std::cout << j_string.is_number_float() << '\\n';\n    std::cout << j_binary.is_number_float() << '\\n';\n}\n

    Output:

    false\nfalse\nfalse\nfalse\ntrue\nfalse\nfalse\nfalse\nfalse\n
    "},{"location":"api/basic_json/is_number_float/#see-also","title":"See also","text":"
    • is_number() check if the value is a number
    • is_number_integer() check if the value is an integer or unsigned integer number
    • is_number_unsigned() check if the value is an unsigned integer number
    • basic_json_view::is_number_float - the same check on a zero-copy view
    "},{"location":"api/basic_json/is_number_float/#version-history","title":"Version history","text":"
    • Added in version 1.0.0.
    "},{"location":"api/basic_json/is_number_integer/","title":"nlohmann::basic_json::is_number_integer","text":"
    constexpr bool is_number_integer() const noexcept;\n

    This function returns true if and only if the JSON value is a signed or unsigned integer number. This excludes floating-point values.

    "},{"location":"api/basic_json/is_number_integer/#return-value","title":"Return value","text":"

    true if type is an integer or unsigned integer number, false otherwise.

    "},{"location":"api/basic_json/is_number_integer/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this member function never throws exceptions.

    "},{"location":"api/basic_json/is_number_integer/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json/is_number_integer/#examples","title":"Examples","text":"Example

    The following code exemplifies is_number_integer() for all JSON types.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_null;\n    json j_boolean = true;\n    json j_number_integer = 17;\n    json j_number_unsigned_integer = 12345678987654321u;\n    json j_number_float = 23.42;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n    json j_string = \"Hello, world\";\n    json j_binary = json::binary({1, 2, 3});\n\n    // call is_number_integer()\n    std::cout << std::boolalpha;\n    std::cout << j_null.is_number_integer() << '\\n';\n    std::cout << j_boolean.is_number_integer() << '\\n';\n    std::cout << j_number_integer.is_number_integer() << '\\n';\n    std::cout << j_number_unsigned_integer.is_number_integer() << '\\n';\n    std::cout << j_number_float.is_number_integer() << '\\n';\n    std::cout << j_object.is_number_integer() << '\\n';\n    std::cout << j_array.is_number_integer() << '\\n';\n    std::cout << j_string.is_number_integer() << '\\n';\n    std::cout << j_binary.is_number_integer() << '\\n';\n}\n

    Output:

    false\nfalse\ntrue\ntrue\nfalse\nfalse\nfalse\nfalse\nfalse\n
    "},{"location":"api/basic_json/is_number_integer/#see-also","title":"See also","text":"
    • is_number() check if the value is a number
    • is_number_unsigned() check if the value is an unsigned integer number
    • is_number_float() check if the value is a floating-point number
    • basic_json_view::is_number_integer - the same check on a zero-copy view
    "},{"location":"api/basic_json/is_number_integer/#version-history","title":"Version history","text":"
    • Added in version 1.0.0.
    • Extended to also return true for unsigned integers in 2.0.0.
    "},{"location":"api/basic_json/is_number_unsigned/","title":"nlohmann::basic_json::is_number_unsigned","text":"
    constexpr bool is_number_unsigned() const noexcept;\n

    This function returns true if and only if the JSON value is an unsigned integer number. This excludes floating-point and signed integer values.

    If number_unsigned_t is the same type as number_integer_t, every integer is stored as number_integer_t, and this function always returns false.

    "},{"location":"api/basic_json/is_number_unsigned/#return-value","title":"Return value","text":"

    true if type is an unsigned integer number, false otherwise.

    "},{"location":"api/basic_json/is_number_unsigned/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this member function never throws exceptions.

    "},{"location":"api/basic_json/is_number_unsigned/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json/is_number_unsigned/#examples","title":"Examples","text":"Example

    The following code exemplifies is_number_unsigned() for all JSON types.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_null;\n    json j_boolean = true;\n    json j_number_integer = 17;\n    json j_number_unsigned_integer = 12345678987654321u;\n    json j_number_float = 23.42;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n    json j_string = \"Hello, world\";\n    json j_binary = json::binary({1, 2, 3});\n\n    // call is_number_unsigned()\n    std::cout << std::boolalpha;\n    std::cout << j_null.is_number_unsigned() << '\\n';\n    std::cout << j_boolean.is_number_unsigned() << '\\n';\n    std::cout << j_number_integer.is_number_unsigned() << '\\n';\n    std::cout << j_number_unsigned_integer.is_number_unsigned() << '\\n';\n    std::cout << j_number_float.is_number_unsigned() << '\\n';\n    std::cout << j_object.is_number_unsigned() << '\\n';\n    std::cout << j_array.is_number_unsigned() << '\\n';\n    std::cout << j_string.is_number_unsigned() << '\\n';\n    std::cout << j_binary.is_number_unsigned() << '\\n';\n}\n

    Output:

    false\nfalse\nfalse\ntrue\nfalse\nfalse\nfalse\nfalse\nfalse\n
    "},{"location":"api/basic_json/is_number_unsigned/#see-also","title":"See also","text":"
    • is_number() check if the value is a number
    • is_number_integer() check if the value is an integer or unsigned integer number
    • is_number_float() check if the value is a floating-point number
    • basic_json_view::is_number_unsigned - the same check on a zero-copy view
    "},{"location":"api/basic_json/is_number_unsigned/#version-history","title":"Version history","text":"
    • Added in version 2.0.0.
    "},{"location":"api/basic_json/is_object/","title":"nlohmann::basic_json::is_object","text":"
    constexpr bool is_object() const noexcept;\n

    This function returns true if and only if the JSON value is an object.

    "},{"location":"api/basic_json/is_object/#return-value","title":"Return value","text":"

    true if type is an object, false otherwise.

    "},{"location":"api/basic_json/is_object/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this member function never throws exceptions.

    "},{"location":"api/basic_json/is_object/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json/is_object/#examples","title":"Examples","text":"Example

    The following code exemplifies is_object() for all JSON types.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_null;\n    json j_boolean = true;\n    json j_number_integer = 17;\n    json j_number_float = 23.42;\n    json j_number_unsigned_integer = 12345678987654321u;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n    json j_string = \"Hello, world\";\n    json j_binary = json::binary({1, 2, 3});\n\n    // call is_object()\n    std::cout << std::boolalpha;\n    std::cout << j_null.is_object() << '\\n';\n    std::cout << j_boolean.is_object() << '\\n';\n    std::cout << j_number_integer.is_object() << '\\n';\n    std::cout << j_number_unsigned_integer.is_object() << '\\n';\n    std::cout << j_number_float.is_object() << '\\n';\n    std::cout << j_object.is_object() << '\\n';\n    std::cout << j_array.is_object() << '\\n';\n    std::cout << j_string.is_object() << '\\n';\n    std::cout << j_binary.is_object() << '\\n';\n}\n

    Output:

    false\nfalse\nfalse\nfalse\nfalse\ntrue\nfalse\nfalse\nfalse\n
    "},{"location":"api/basic_json/is_object/#see-also","title":"See also","text":"
    • is_array checks whether the JSON value is an array
    • is_structured checks whether the JSON value is structured (array or object)
    • type returns the type of the JSON value
    • object_t the type used to store JSON objects
    • basic_json_view::is_object - the same check on a zero-copy view
    "},{"location":"api/basic_json/is_object/#version-history","title":"Version history","text":"
    • Added in version 1.0.0.
    "},{"location":"api/basic_json/is_primitive/","title":"nlohmann::basic_json::is_primitive","text":"
    constexpr bool is_primitive() const noexcept;\n

    This function returns true if and only if the JSON type is primitive (string, number, boolean, null, binary).

    "},{"location":"api/basic_json/is_primitive/#return-value","title":"Return value","text":"

    true if type is primitive (string, number, boolean, null, or binary), false otherwise.

    "},{"location":"api/basic_json/is_primitive/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this member function never throws exceptions.

    "},{"location":"api/basic_json/is_primitive/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json/is_primitive/#possible-implementation","title":"Possible implementation","text":"
    constexpr bool is_primitive() const noexcept\n{\n    return is_null() || is_string() || is_boolean() || is_number() || is_binary();\n}\n
    "},{"location":"api/basic_json/is_primitive/#notes","title":"Notes","text":"

    The term primitive stems from RFC 8259:

    JSON can represent four primitive types (strings, numbers, booleans, and null) and two structured types (objects and arrays).

    This library extends primitive types to binary types, because binary types are roughly comparable to strings. Hence, is_primitive() returns true for binary values.

    "},{"location":"api/basic_json/is_primitive/#examples","title":"Examples","text":"Example

    The following code exemplifies is_primitive() for all JSON types.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_null;\n    json j_boolean = true;\n    json j_number_integer = 17;\n    json j_number_float = 23.42;\n    json j_number_unsigned_integer = 12345678987654321u;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n    json j_string = \"Hello, world\";\n    json j_binary = json::binary({1, 2, 3});\n\n    // call is_primitive()\n    std::cout << std::boolalpha;\n    std::cout << j_null.is_primitive() << '\\n';\n    std::cout << j_boolean.is_primitive() << '\\n';\n    std::cout << j_number_integer.is_primitive() << '\\n';\n    std::cout << j_number_unsigned_integer.is_primitive() << '\\n';\n    std::cout << j_number_float.is_primitive() << '\\n';\n    std::cout << j_object.is_primitive() << '\\n';\n    std::cout << j_array.is_primitive() << '\\n';\n    std::cout << j_string.is_primitive() << '\\n';\n    std::cout << j_binary.is_primitive() << '\\n';\n}\n

    Output:

    true\ntrue\ntrue\ntrue\ntrue\nfalse\nfalse\ntrue\ntrue\n
    "},{"location":"api/basic_json/is_primitive/#see-also","title":"See also","text":"
    • is_structured() returns whether the JSON value is structured
    • is_null() returns whether the JSON value is null
    • is_string() returns whether the JSON value is a string
    • is_boolean() returns whether the JSON value is a boolean
    • is_number() returns whether the JSON value is a number
    • is_binary() returns whether the JSON value is a binary array
    • basic_json_view::is_primitive - the same check on a zero-copy view
    "},{"location":"api/basic_json/is_primitive/#version-history","title":"Version history","text":"
    • Added in version 1.0.0.
    • Extended to return true for binary types in version 3.8.0.
    "},{"location":"api/basic_json/is_string/","title":"nlohmann::basic_json::is_string","text":"
    constexpr bool is_string() const noexcept;\n

    This function returns true if and only if the JSON value is a string.

    "},{"location":"api/basic_json/is_string/#return-value","title":"Return value","text":"

    true if type is a string, false otherwise.

    "},{"location":"api/basic_json/is_string/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this member function never throws exceptions.

    "},{"location":"api/basic_json/is_string/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json/is_string/#examples","title":"Examples","text":"Example

    The following code exemplifies is_string() for all JSON types.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_null;\n    json j_boolean = true;\n    json j_number_integer = 17;\n    json j_number_float = 23.42;\n    json j_number_unsigned_integer = 12345678987654321u;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n    json j_string = \"Hello, world\";\n    json j_binary = json::binary({1, 2, 3});\n\n    // call is_string()\n    std::cout << std::boolalpha;\n    std::cout << j_null.is_string() << '\\n';\n    std::cout << j_boolean.is_string() << '\\n';\n    std::cout << j_number_integer.is_string() << '\\n';\n    std::cout << j_number_unsigned_integer.is_string() << '\\n';\n    std::cout << j_number_float.is_string() << '\\n';\n    std::cout << j_object.is_string() << '\\n';\n    std::cout << j_array.is_string() << '\\n';\n    std::cout << j_string.is_string() << '\\n';\n    std::cout << j_binary.is_string() << '\\n';\n}\n

    Output:

    false\nfalse\nfalse\nfalse\nfalse\nfalse\nfalse\ntrue\nfalse\n
    "},{"location":"api/basic_json/is_string/#see-also","title":"See also","text":"
    • is_primitive checks whether the JSON value is primitive
    • type returns the type of the JSON value
    • string_t the type used to store JSON strings
    • basic_json_view::is_string - the same check on a zero-copy view
    "},{"location":"api/basic_json/is_string/#version-history","title":"Version history","text":"
    • Added in version 1.0.0.
    "},{"location":"api/basic_json/is_structured/","title":"nlohmann::basic_json::is_structured","text":"
    constexpr bool is_structured() const noexcept;\n

    This function returns true if and only if the JSON type is structured (array or object).

    "},{"location":"api/basic_json/is_structured/#return-value","title":"Return value","text":"

    true if type is structured (array or object), false otherwise.

    "},{"location":"api/basic_json/is_structured/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this member function never throws exceptions.

    "},{"location":"api/basic_json/is_structured/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json/is_structured/#possible-implementation","title":"Possible implementation","text":"
    constexpr bool is_structured() const noexcept\n{\n    return is_array() || is_object();\n}\n
    "},{"location":"api/basic_json/is_structured/#notes","title":"Notes","text":"

    The term structured stems from RFC 8259:

    JSON can represent four primitive types (strings, numbers, booleans, and null) and two structured types (objects and arrays).

    Note that though strings are containers in C++, they are treated as primitive values in JSON.

    "},{"location":"api/basic_json/is_structured/#examples","title":"Examples","text":"Example

    The following code exemplifies is_structured() for all JSON types.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_null;\n    json j_boolean = true;\n    json j_number_integer = 17;\n    json j_number_float = 23.42;\n    json j_number_unsigned_integer = 12345678987654321u;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n    json j_string = \"Hello, world\";\n    json j_binary = json::binary({1, 2, 3});\n\n    // call is_structured()\n    std::cout << std::boolalpha;\n    std::cout << j_null.is_structured() << '\\n';\n    std::cout << j_boolean.is_structured() << '\\n';\n    std::cout << j_number_integer.is_structured() << '\\n';\n    std::cout << j_number_unsigned_integer.is_structured() << '\\n';\n    std::cout << j_number_float.is_structured() << '\\n';\n    std::cout << j_object.is_structured() << '\\n';\n    std::cout << j_array.is_structured() << '\\n';\n    std::cout << j_string.is_structured() << '\\n';\n    std::cout << j_binary.is_structured() << '\\n';\n}\n

    Output:

    false\nfalse\nfalse\nfalse\nfalse\ntrue\ntrue\nfalse\nfalse\n
    "},{"location":"api/basic_json/is_structured/#see-also","title":"See also","text":"
    • is_primitive() returns whether JSON value is primitive
    • is_array() returns whether the value is an array
    • is_object() returns whether the value is an object
    • basic_json_view::is_structured - the same check on a zero-copy view
    "},{"location":"api/basic_json/is_structured/#version-history","title":"Version history","text":"
    • Added in version 1.0.0.
    "},{"location":"api/basic_json/items/","title":"nlohmann::basic_json::items","text":"
    iteration_proxy<iterator> items() noexcept;\niteration_proxy<const_iterator> items() const noexcept;\n

    This function allows accessing iterator::key() and iterator::value() during range-based for loops. In these loops, a reference to the JSON values is returned, so there is no access to the underlying iterator.

    For loop without items() function:

    for (auto it = j_object.begin(); it != j_object.end(); ++it)\n{\n    std::cout << \"key: \" << it.key() << \", value:\" << it.value() << '\\n';\n}\n

    Range-based for loop without items() function:

    for (auto it : j_object)\n{\n    // \"it\" is of type json::reference and has no key() member\n    std::cout << \"value: \" << it << '\\n';\n}\n

    Range-based for loop with items() function:

    for (auto& el : j_object.items())\n{\n    std::cout << \"key: \" << el.key() << \", value:\" << el.value() << '\\n';\n}\n

    The items() function also allows using structured bindings (C++17):

    for (auto& [key, val] : j_object.items())\n{\n    std::cout << \"key: \" << key << \", value:\" << val << '\\n';\n}\n

    If you need to name the type of the dereferenced element explicitly (e.g., to write a standalone function that takes it as a parameter, or to use items() with std::for_each), use decltype:

    using element_type = decltype(*j_object.items().begin());\n

    The per-element type (iteration_proxy_value) lives in the library's internal detail namespace and is intentionally unspecified as a stable, named type -- decltype is the supported way to obtain it, but its exact name/definition may change between versions.

    "},{"location":"api/basic_json/items/#return-value","title":"Return value","text":"

    iteration proxy object wrapping the current value with an interface to use in range-based for loops

    "},{"location":"api/basic_json/items/#exception-safety","title":"Exception safety","text":"

    Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

    "},{"location":"api/basic_json/items/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json/items/#notes","title":"Notes","text":"

    When iterating over an array, key() will return the index of the element as string (see example). For primitive types (e.g., numbers), key() returns an empty string.

    Lifetime issues

    Using items() on temporary objects is dangerous. Make sure the object's lifetime exceeds the iteration. See #2040 for more information.

    "},{"location":"api/basic_json/items/#examples","title":"Examples","text":"Example

    The following code shows an example for items().

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n\n    // example for an object\n    for (auto& x : j_object.items())\n    {\n        std::cout << \"key: \" << x.key() << \", value: \" << x.value() << '\\n';\n    }\n\n    // example for an array\n    for (auto& x : j_array.items())\n    {\n        std::cout << \"key: \" << x.key() << \", value: \" << x.value() << '\\n';\n    }\n}\n

    Output:

    key: one, value: 1\nkey: two, value: 2\nkey: 0, value: 1\nkey: 1, value: 2\nkey: 2, value: 4\nkey: 3, value: 8\nkey: 4, value: 16\n
    "},{"location":"api/basic_json/items/#see-also","title":"See also","text":"
    • begin returns an iterator to the first element
    • end returns an iterator to one past the last element
    • basic_json_view::items - the same range on a zero-copy view (const auto, not const auto&: items are produced on the fly; document order, not sorted; duplicate keys are visited)
    "},{"location":"api/basic_json/items/#version-history","title":"Version history","text":"
    • Added iterator_wrapper in version 3.0.0.
    • Added items and deprecated iterator_wrapper in version 3.1.0.
    • Added structured binding support in version 3.5.0.

    Deprecation

    This function replaces the static function iterator_wrapper which was introduced in version 1.0.0, but has been deprecated in version 3.1.0. Function iterator_wrapper will be removed in version 4.0.0. Please replace all occurrences of iterator_wrapper(j) with j.items().

    You should be warned by your compiler with a -Wdeprecated-declarations warning if you are using a deprecated function.

    See the migration guide for how to update existing code.

    "},{"location":"api/basic_json/json_base_class_t/","title":"nlohmann::basic_json::json_base_class_t","text":"
    using json_base_class_t = detail::json_base_class<CustomBaseClass>;\n

    The base class used to inject custom functionality into each instance of basic_json. Examples of such functionality might be metadata, additional member functions (e.g., visitors), or other application-specific code.

    "},{"location":"api/basic_json/json_base_class_t/#template-parameters","title":"Template parameters","text":"CustomBaseClass the base class to be added to basic_json"},{"location":"api/basic_json/json_base_class_t/#notes","title":"Notes","text":""},{"location":"api/basic_json/json_base_class_t/#default-type","title":"Default type","text":"

    The default value for CustomBaseClass is void. In this case, an empty base class is used and no additional functionality is injected.

    "},{"location":"api/basic_json/json_base_class_t/#limitations","title":"Limitations","text":"

    The type CustomBaseClass has to be a default-constructible, non-final class. basic_json only supports copy/move construction/assignment if CustomBaseClass does so as well. A CustomBaseClass with non-static data members forfeits basic_json's standard layout guarantee. See Template Parameter Requirements.

    "},{"location":"api/basic_json/json_base_class_t/#name-conflicts","title":"Name conflicts","text":"

    Since basic_json derives from CustomBaseClass, members of basic_json hide members of CustomBaseClass with the same name. Hidden members remain accessible via as_base_class or by casting the value to json_base_class_t.

    Avoid generic member names

    Future versions of the library may add members to basic_json that hide members of CustomBaseClass that are accessible today. To reduce the risk of such conflicts, avoid generic names for the members of CustomBaseClass, for instance by using a distinctive prefix.

    "},{"location":"api/basic_json/json_base_class_t/#examples","title":"Examples","text":"Example

    The following code shows how to inject custom data and methods for each node.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nclass visitor_adaptor_with_metadata\n{\n  public:\n    template <class Fnc>\n    void visit(const Fnc& fnc) const;\n\n    int metadata = 42;\n  private:\n    template <class Ptr, class Fnc>\n    void do_visit(const Ptr& ptr, const Fnc& fnc) const;\n};\n\nusing json = nlohmann::json::with_base_class_t<visitor_adaptor_with_metadata>;\n\ntemplate <class Fnc>\nvoid visitor_adaptor_with_metadata::visit(const Fnc& fnc) const\n{\n    do_visit(json::json_pointer{}, fnc);\n}\n\ntemplate <class Ptr, class Fnc>\nvoid visitor_adaptor_with_metadata::do_visit(const Ptr& ptr, const Fnc& fnc) const\n{\n    using value_t = nlohmann::detail::value_t;\n    const json& j = *static_cast<const json*>(this);\n    switch (j.type())\n    {\n        case value_t::object:\n            fnc(ptr, j);\n            for (const auto& entry : j.items())\n            {\n                entry.value().do_visit(ptr / entry.key(), fnc);\n            }\n            break;\n        case value_t::array:\n            fnc(ptr, j);\n            for (std::size_t i = 0; i < j.size(); ++i)\n            {\n                j.at(i).do_visit(ptr / std::to_string(i), fnc);\n            }\n            break;\n        case value_t::null:\n        case value_t::string:\n        case value_t::boolean:\n        case value_t::number_integer:\n        case value_t::number_unsigned:\n        case value_t::number_float:\n        case value_t::binary:\n            fnc(ptr, j);\n            break;\n        case value_t::discarded:\n        default:\n            break;\n    }\n}\n\nint main()\n{\n    // create a json object\n    json j;\n    j[\"null\"];\n    j[\"object\"][\"uint\"] = 1U;\n    j[\"object\"].metadata = 21;\n\n    // visit and output\n    j.visit(\n         [&](const json::json_pointer & p,\n             const json & j)\n    {\n        std::cout << (p.empty() ? std::string{\"/\"} : p.to_string())\n                  << \" - metadata = \" << j.metadata << \" -> \" << j.dump() << '\\n';\n    });\n}\n

    Output:

    / - metadata = 42 -> {\"null\":null,\"object\":{\"uint\":1}}\n/null - metadata = 42 -> null\n/object - metadata = 21 -> {\"uint\":1}\n/object/uint - metadata = 42 -> 1\n
    "},{"location":"api/basic_json/json_base_class_t/#see-also","title":"See also","text":"
    • as_base_class - access the custom base class
    • Template Parameter Requirements - the requirements for CustomBaseClass
    "},{"location":"api/basic_json/json_base_class_t/#version-history","title":"Version history","text":"
    • Added in version 3.11.3.
    • Made a public member type in version 3.13.0 unreleased; it was private before, so it could not be named outside the class.
    "},{"location":"api/basic_json/json_serializer/","title":"nlohmann::basic_json::json_serializer","text":"
    template<typename T, typename SFINAE>\nusing json_serializer = JSONSerializer<T, SFINAE>;\n
    "},{"location":"api/basic_json/json_serializer/#template-parameters","title":"Template parameters","text":"T type to convert; will be used in the to_json/from_json functions SFINAE type to add compile type checks via SFINAE; usually void"},{"location":"api/basic_json/json_serializer/#notes","title":"Notes","text":""},{"location":"api/basic_json/json_serializer/#default-type","title":"Default type","text":"

    The default values for json_serializer is adl_serializer.

    "},{"location":"api/basic_json/json_serializer/#requirements","title":"Requirements","text":"

    A custom serializer must provide static void to_json(basic_json&, T) for every type it serializes, and either static void from_json(const basic_json&, T&) or static T from_json(const basic_json&) for every type it deserializes. See Template Parameter Requirements.

    "},{"location":"api/basic_json/json_serializer/#examples","title":"Examples","text":"Example

    The example below shows how a conversion of a non-default-constructible type is implemented via a specialization of the adl_serializer.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nnamespace ns\n{\n// a simple struct to model a person (not default constructible)\nstruct person\n{\n    person(std::string n, std::string a, int aa)\n        : name(std::move(n)), address(std::move(a)), age(aa)\n    {}\n\n    std::string name;\n    std::string address;\n    int age;\n};\n} // namespace ns\n\nnamespace nlohmann\n{\ntemplate <>\nstruct adl_serializer<ns::person>\n{\n    static ns::person from_json(const json& j)\n    {\n        return {j.at(\"name\"), j.at(\"address\"), j.at(\"age\")};\n    }\n\n    // Here's the catch! You must provide a to_json method! Otherwise, you\n    // will not be able to convert person to json, since you fully\n    // specialized adl_serializer on that type\n    static void to_json(json& j, ns::person p)\n    {\n        j[\"name\"] = p.name;\n        j[\"address\"] = p.address;\n        j[\"age\"] = p.age;\n    }\n};\n} // namespace nlohmann\n\nint main()\n{\n    json j;\n    j[\"name\"] = \"Ned Flanders\";\n    j[\"address\"] = \"744 Evergreen Terrace\";\n    j[\"age\"] = 60;\n\n    auto p = j.get<ns::person>();\n\n    std::cout << p.name << \" (\" << p.age << \") lives in \" << p.address << std::endl;\n}\n

    Output:

    Ned Flanders (60) lives in 744 Evergreen Terrace\n
    "},{"location":"api/basic_json/json_serializer/#see-also","title":"See also","text":"
    • adl_serializer the default json_serializer
    • get explicit type conversion using the json_serializer's from_json() method
    • get_to explicit type conversion into a variable using the json_serializer's from_json() method
    • Arbitrary Type Conversions - the article on converting between JSON values and arbitrary types
    "},{"location":"api/basic_json/json_serializer/#version-history","title":"Version history","text":"
    • Since version 2.0.0.
    "},{"location":"api/basic_json/max_size/","title":"nlohmann::basic_json::max_size","text":"
    size_type max_size() const noexcept;\n

    Returns the maximum number of elements a JSON value is able to hold due to system or library implementation limitations, i.e. std::distance(begin(), end()) for the JSON value.

    "},{"location":"api/basic_json/max_size/#return-value","title":"Return value","text":"

    The return value depends on the different types and is defined as follows:

    Value type return value null 0 (same as size()) boolean 1 (same as size()) string 1 (same as size()) number 1 (same as size()) binary 1 (same as size()) object result of function object_t::max_size() array result of function array_t::max_size()"},{"location":"api/basic_json/max_size/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this function never throws exceptions.

    "},{"location":"api/basic_json/max_size/#complexity","title":"Complexity","text":"

    Constant, as long as array_t and object_t satisfy the Container concept; that is, their max_size() functions have constant complexity.

    "},{"location":"api/basic_json/max_size/#notes","title":"Notes","text":"

    This function does not return the maximal length of a string stored as JSON value -- it returns the maximal number of string elements the JSON value can store which is 1.

    "},{"location":"api/basic_json/max_size/#examples","title":"Examples","text":"Example

    The following code calls max_size() on the different value types.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_null;\n    json j_boolean = true;\n    json j_number_integer = 17;\n    json j_number_float = 23.42;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n    json j_string = \"Hello, world\";\n\n    // call max_size()\n    std::cout << j_null.max_size() << '\\n';\n    std::cout << j_boolean.max_size() << '\\n';\n    std::cout << j_number_integer.max_size() << '\\n';\n    std::cout << j_number_float.max_size() << '\\n';\n    std::cout << j_object.max_size() << '\\n';\n    std::cout << j_array.max_size() << '\\n';\n    std::cout << j_string.max_size() << '\\n';\n}\n

    Output:

    0\n1\n1\n1\n115292150460684697\n576460752303423487\n1\n

    Note the output is platform-dependent.

    "},{"location":"api/basic_json/max_size/#see-also","title":"See also","text":"
    • size returns the number of elements
    • array_t the type used to store JSON arrays
    • object_t the type used to store JSON objects
    "},{"location":"api/basic_json/max_size/#version-history","title":"Version history","text":"
    • Added in version 1.0.0.
    • Extended to return 1 for binary types in version 3.8.0.
    "},{"location":"api/basic_json/merge_diff/","title":"nlohmann::basic_json::merge_diff","text":"
    static basic_json merge_diff(const basic_json& source,\n                             const basic_json& target);\n

    Creates a JSON Merge Patch so that value source can be changed into the value target by calling the merge_patch function. The patch contains only what differs: changed and added members with their new values, and removed members with the value null. Nested objects are compared member by member; any other value that differs, including an array, is replaced as a whole.

    For two JSON values source and target, where target contains no object member whose value is null, the following code always yields true (for ordered_json, see the notes below): 

    basic_json patched = source;\npatched.merge_patch(merge_diff(source, target));\npatched == target;\n

    "},{"location":"api/basic_json/merge_diff/#parameters","title":"Parameters","text":"source (in) JSON value to compare from target (in) JSON value to compare against"},{"location":"api/basic_json/merge_diff/#return-value","title":"Return value","text":"

    a JSON Merge Patch to convert the source to target

    "},{"location":"api/basic_json/merge_diff/#exception-safety","title":"Exception safety","text":"

    Strong guarantee: source and target are never modified.

    "},{"location":"api/basic_json/merge_diff/#complexity","title":"Complexity","text":"

    Linear in the total number of values (including nested ones) in source and target, times the cost of a key-based object operation (looking up a key, or adding one to the patch): logarithmic for json, and linear for ordered_json (see ordered_map complexity). For ordered_json, diffing two objects with n and m members therefore takes O((n+m)\u00b2).

    "},{"location":"api/basic_json/merge_diff/#notes","title":"Notes","text":"
    • A merge patch uses null to remove a member, so it cannot set a member to null. As in RFC 7396, a null member of target is treated as absent: the patch removes the member from source instead of setting it to null. A member that is null in both source and target is unchanged and left out of the patch, so source keeps it.
    • A merge patch cannot reorder object members, and merge_patch appends added members at the end. For ordered_json, the patched value therefore has the same members as target, but not necessarily in the same order, and may compare unequal to it. The patch lists changed and removed members in the order of source, followed by added members in the order of target.
    "},{"location":"api/basic_json/merge_diff/#examples","title":"Examples","text":"Example

    The following code shows how a JSON Merge Patch is created as a diff for two JSON values.

    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // the source document\n    json source = R\"(\n        {\n            \"title\": \"Goodbye!\",\n            \"author\": {\n                \"givenName\": \"John\",\n                \"familyName\": \"Doe\"\n            },\n            \"tags\": [\n                \"example\",\n                \"sample\"\n            ],\n            \"content\": \"This will be unchanged\"\n        }\n    )\"_json;\n\n    // the target document\n    json target = R\"(\n        {\n            \"title\": \"Hello!\",\n            \"author\": {\n                \"givenName\": \"John\"\n            },\n            \"tags\": [\n                \"example\"\n            ],\n            \"content\": \"This will be unchanged\",\n            \"phoneNumber\": \"+01-123-456-7890\"\n        }\n    )\"_json;\n\n    // create the patch\n    json patch = json::merge_diff(source, target);\n\n    // roundtrip\n    json patched_source = source;\n    patched_source.merge_patch(patch);\n\n    // output patch and roundtrip result\n    std::cout << std::setw(4) << patch << \"\\n\\n\"\n              << std::setw(4) << patched_source << std::endl;\n}\n

    Output:

    {\n    \"author\": {\n        \"familyName\": null\n    },\n    \"phoneNumber\": \"+01-123-456-7890\",\n    \"tags\": [\n        \"example\"\n    ],\n    \"title\": \"Hello!\"\n}\n\n{\n    \"author\": {\n        \"givenName\": \"John\"\n    },\n    \"content\": \"This will be unchanged\",\n    \"phoneNumber\": \"+01-123-456-7890\",\n    \"tags\": [\n        \"example\"\n    ],\n    \"title\": \"Hello!\"\n}\n
    "},{"location":"api/basic_json/merge_diff/#see-also","title":"See also","text":"
    • RFC 7396 (JSON Merge Patch)
    • merge_patch applies a JSON Merge Patch
    • diff creates a diff as a JSON Patch
    "},{"location":"api/basic_json/merge_diff/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json/merge_patch/","title":"nlohmann::basic_json::merge_patch","text":"
    void merge_patch(const basic_json& apply_patch);\n

    The merge patch format is primarily intended for use with the HTTP PATCH method as a means of describing a set of modifications to a target resource's content. This function applies a merge patch to the current JSON value.

    The function implements the following algorithm from Section 2 of RFC 7396 (JSON Merge Patch):

    define MergePatch(Target, Patch):\n  if Patch is an Object:\n    if Target is not an Object:\n      Target = {} // Ignore the contents and set it to an empty Object\n    for each Name/Value pair in Patch:\n      if Value is null:\n        if Name exists in Target:\n          remove the Name/Value pair from Target\n      else:\n        Target[Name] = MergePatch(Target[Name], Value)\n    return Target\n  else:\n    return Patch\n

    Thereby, Target is the current object; that is, the patch is applied to the current value.

    "},{"location":"api/basic_json/merge_patch/#parameters","title":"Parameters","text":"apply_patch (in) the patch to apply"},{"location":"api/basic_json/merge_patch/#exception-safety","title":"Exception safety","text":"

    Basic guarantee: if an exception is thrown during the operation, the JSON value may be partially modified.

    "},{"location":"api/basic_json/merge_patch/#complexity","title":"Complexity","text":"

    Linear in the lengths of apply_patch.

    "},{"location":"api/basic_json/merge_patch/#notes","title":"Notes","text":"

    apply_patch may be *this itself or refer to a value contained in *this (for example, a subobject returned by (*this)[key]); it is read as it was when merge_patch() was called, before any modification of *this.

    "},{"location":"api/basic_json/merge_patch/#examples","title":"Examples","text":"Example

    The following code shows how a JSON Merge Patch is applied to a JSON document.

    #include <iostream>\n#include <nlohmann/json.hpp>\n#include <iomanip> // for std::setw\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // the original document\n    json document = R\"({\n                \"title\": \"Goodbye!\",\n                \"author\": {\n                    \"givenName\": \"John\",\n                    \"familyName\": \"Doe\"\n                },\n                \"tags\": [\n                    \"example\",\n                    \"sample\"\n                ],\n                \"content\": \"This will be unchanged\"\n            })\"_json;\n\n    // the patch\n    json patch = R\"({\n                \"title\": \"Hello!\",\n                \"phoneNumber\": \"+01-123-456-7890\",\n                \"author\": {\n                    \"familyName\": null\n                },\n                \"tags\": [\n                    \"example\"\n                ]\n            })\"_json;\n\n    // apply the patch\n    document.merge_patch(patch);\n\n    // output original and patched document\n    std::cout << std::setw(4) << document << std::endl;\n}\n

    Output:

    {\n    \"author\": {\n        \"givenName\": \"John\"\n    },\n    \"content\": \"This will be unchanged\",\n    \"phoneNumber\": \"+01-123-456-7890\",\n    \"tags\": [\n        \"example\"\n    ],\n    \"title\": \"Hello!\"\n}\n
    "},{"location":"api/basic_json/merge_patch/#see-also","title":"See also","text":"
    • RFC 7396 (JSON Merge Patch)
    • patch apply a JSON patch
    • merge_diff creates a diff as a JSON Merge Patch
    "},{"location":"api/basic_json/merge_patch/#version-history","title":"Version history","text":"
    • Added in version 3.0.0.
    • Fixed use of freed or relocated memory when apply_patch is *this or refers to a value contained in *this, in version 3.13.0 unreleased.
    "},{"location":"api/basic_json/meta/","title":"nlohmann::basic_json::meta","text":"
    static basic_json meta();\n

    This function returns a JSON object with information about the library, including the version number and information on the platform and compiler.

    "},{"location":"api/basic_json/meta/#return-value","title":"Return value","text":"

    JSON object holding version information

    key description compiler Information on the used compiler. It is an object with the following keys: c++ (the used C++ standard), family (the compiler family; possible values are clang, icc, gcc, hp, ilecpp, msvc, pgcpp, sunpro, and unknown), and version (the compiler version). copyright The copyright line for the library as string. name The name of the library as string. platform The used platform as string. Possible values are win32, linux, apple, unix, and unknown. url The URL of the project as string. version The version of the library. It is an object with the following keys: major, minor, and patch as defined by Semantic Versioning, and string (the version string)."},{"location":"api/basic_json/meta/#exception-safety","title":"Exception safety","text":"

    Strong guarantee: if an exception is thrown, there are no changes to any JSON value.

    "},{"location":"api/basic_json/meta/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json/meta/#examples","title":"Examples","text":"Example

    The following code shows an example output of the meta() function.

    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // call meta()\n    std::cout << std::setw(4) << json::meta() << '\\n';\n}\n

    Output:

    {\n    \"compiler\": {\n        \"c++\": \"201103\",\n        \"family\": \"gcc\",\n        \"version\": \"12.4.0\"\n    },\n    \"copyright\": \"(C) 2013-2026 Niels Lohmann\",\n    \"name\": \"JSON for Modern C++\",\n    \"platform\": \"apple\",\n    \"url\": \"https://github.com/nlohmann/json\",\n    \"version\": {\n        \"major\": 3,\n        \"minor\": 12,\n        \"patch\": 0,\n        \"string\": \"3.12.0\"\n    }\n}\n

    Note the output is platform-dependent.

    "},{"location":"api/basic_json/meta/#see-also","title":"See also","text":"
    • NLOHMANN_JSON_VERSION_MAJOR/NLOHMANN_JSON_VERSION_MINOR/NLOHMANN_JSON_VERSION_PATCH - library version information
    "},{"location":"api/basic_json/meta/#version-history","title":"Version history","text":"
    • Added in version 2.1.0.
    "},{"location":"api/basic_json/number_float_t/","title":"nlohmann::basic_json::number_float_t","text":"
    using number_float_t = NumberFloatType;\n

    The type used to store JSON numbers (floating-point).

    RFC 8259 describes numbers as follows:

    The representation of numbers is similar to that used in most programming languages. A number is represented in base 10 using decimal digits. It contains an integer component that may be prefixed with an optional minus sign, which may be followed by a fraction part and/or an exponent part. Leading zeros are not allowed. (...) Numeric values that cannot be represented in the grammar below (such as Infinity and NaN) are not permitted.

    This description includes both integer and floating-point numbers. However, C++ allows more precise storage if it is known whether the number is a signed integer, an unsigned integer, or a floating-point number. Therefore, three different types, number_integer_t, number_unsigned_t and number_float_t are used.

    To store floating-point numbers in C++, a type is defined by the template parameter NumberFloatType which chooses the type to use.

    "},{"location":"api/basic_json/number_float_t/#template-parameters","title":"Template parameters","text":"NumberFloatType the type to store floating-point numbers. The type must be float, double, or long double. The parser converts float, double, and a long double that is IEEE 754 binary64 itself. It converts other long double formats with std::from_chars where available, or with std::strtold otherwise. Serialization falls back to std::snprintf. The binary formats additionally require float or double, because they have no encoding for long double. See Template Parameter Requirements."},{"location":"api/basic_json/number_float_t/#notes","title":"Notes","text":""},{"location":"api/basic_json/number_float_t/#default-type","title":"Default type","text":"

    With the default values for NumberFloatType (double), the default type for number_float_t is double.

    "},{"location":"api/basic_json/number_float_t/#default-behavior","title":"Default behavior","text":"
    • The restrictions about leading zeros are not enforced in C++. Instead, leading zeros in floating-point literals will be ignored. Internally, the value will be stored as a decimal number. For instance, the C++ floating-point literal 01.2 will be serialized to 1.2. During deserialization, leading zeros yield an error.
    • Not-a-number (NaN) values will be serialized to null.
    "},{"location":"api/basic_json/number_float_t/#limits","title":"Limits","text":"

    RFC 8259 states:

    This specification allows implementations to set limits on the range and precision of numbers accepted. Since software that implements IEEE 754-2008 binary64 (double precision) numbers is generally available and widely used, good interoperability can be achieved by implementations that expect no more precision or range than these provide, in the sense that implementations will approximate JSON numbers within the expected precision.

    This implementation does exactly follow this approach, as it uses double precision floating-point numbers by default. Note that with the default double, numbers smaller than -1.79769313486232e+308 or greater than 1.79769313486232e+308 are rejected during parsing with out_of_range.406. Non-finite values (NaN and infinity) can only be stored by assigning them directly and are serialized to null.

    During deserialization (from JSON text or any of the binary formats), a finite number that does not fit into number_float_t is rejected with out_of_range.406, for example a double-precision number in a binary format when number_float_t is float.

    "},{"location":"api/basic_json/number_float_t/#storage","title":"Storage","text":"

    Floating-point number values are stored directly inside a basic_json type.

    "},{"location":"api/basic_json/number_float_t/#examples","title":"Examples","text":"Example

    The following code shows that number_float_t is by default, a typedef to double.

    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::boolalpha << std::is_same<double, json::number_float_t>::value << std::endl;\n}\n

    Output:

    true\n
    "},{"location":"api/basic_json/number_float_t/#see-also","title":"See also","text":"
    • number_integer_t the type used to store JSON integer numbers
    • number_unsigned_t the type used to store JSON unsigned integer numbers
    • is_number_float checks whether the JSON value is a floating-point number
    • Number Handling - the article on number handling
    "},{"location":"api/basic_json/number_float_t/#version-history","title":"Version history","text":"
    • Added in version 1.0.0.
    "},{"location":"api/basic_json/number_integer_t/","title":"nlohmann::basic_json::number_integer_t","text":"
    using number_integer_t = NumberIntegerType;\n

    The type used to store JSON numbers (integers).

    RFC 8259 describes numbers as follows:

    The representation of numbers is similar to that used in most programming languages. A number is represented in base 10 using decimal digits. It contains an integer component that may be prefixed with an optional minus sign, which may be followed by a fraction part and/or an exponent part. Leading zeros are not allowed. (...) Numeric values that cannot be represented in the grammar below (such as Infinity and NaN) are not permitted.

    This description includes both integer and floating-point numbers. However, C++ allows more precise storage if it is known whether the number is a signed integer, an unsigned integer, or a floating-point number. Therefore, three different types, number_integer_t, number_unsigned_t and number_float_t are used.

    To store integer numbers in C++, a type is defined by the template parameter NumberIntegerType which chooses the type to use.

    "},{"location":"api/basic_json/number_integer_t/#template-parameters","title":"Template parameters","text":"NumberIntegerType the type to store signed integers. It must be a signed integral type (std::is_integral) with a std::numeric_limits specialization, and it is stored directly inside a basic_json value. See Template Parameter Requirements."},{"location":"api/basic_json/number_integer_t/#notes","title":"Notes","text":""},{"location":"api/basic_json/number_integer_t/#default-type","title":"Default type","text":"

    With the default values for NumberIntegerType (std::int64_t), the default type for number_integer_t is std::int64_t.

    "},{"location":"api/basic_json/number_integer_t/#default-behavior","title":"Default behavior","text":"
    • The restrictions about leading zeros are not enforced in C++. Instead, leading zeros in integer literals lead to an interpretation as an octal number. Internally, the value will be stored as a decimal number. For instance, the C++ integer literal 010 will be serialized to 8. During deserialization, leading zeros yield an error.
    "},{"location":"api/basic_json/number_integer_t/#limits","title":"Limits","text":"

    RFC 8259 specifies:

    An implementation may set limits on the range and precision of numbers.

    When the default type is used, the maximal integer number that can be stored is 9223372036854775807 (INT64_MAX) and the minimal integer number that can be stored is -9223372036854775808 (INT64_MIN). Integer numbers that are out of range will yield over/underflow when used in a constructor. During deserialization (from JSON text or any of the binary formats), too large or small integer numbers will automatically be stored as number_unsigned_t or number_float_t.

    RFC 8259 further states:

    Note that when such software is used, numbers that are integers and are in the range [-253+1, 253-1] are interoperable in the sense that implementations will agree exactly on their numeric values.

    As this range is a subrange of the exactly supported range [INT64_MIN, INT64_MAX], this class's integer type is interoperable.

    "},{"location":"api/basic_json/number_integer_t/#storage","title":"Storage","text":"

    Integer number values are stored directly inside a basic_json type.

    "},{"location":"api/basic_json/number_integer_t/#examples","title":"Examples","text":"Example

    The following code shows that number_integer_t is by default, a typedef to std::int64_t.

    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::boolalpha << std::is_same<std::int64_t, json::number_integer_t>::value << std::endl;\n}\n

    Output:

    true\n
    "},{"location":"api/basic_json/number_integer_t/#see-also","title":"See also","text":"
    • number_unsigned_t the type used to store JSON unsigned integer numbers
    • number_float_t the type used to store JSON floating-point numbers
    • is_number_integer checks whether the JSON value is a signed integer number
    • Number Handling - the article on number handling
    "},{"location":"api/basic_json/number_integer_t/#version-history","title":"Version history","text":"
    • Added in version 1.0.0.
    "},{"location":"api/basic_json/number_unsigned_t/","title":"nlohmann::basic_json::number_unsigned_t","text":"
    using number_unsigned_t = NumberUnsignedType;\n

    The type used to store JSON numbers (unsigned).

    RFC 8259 describes numbers as follows:

    The representation of numbers is similar to that used in most programming languages. A number is represented in base 10 using decimal digits. It contains an integer component that may be prefixed with an optional minus sign, which may be followed by a fraction part and/or an exponent part. Leading zeros are not allowed. (...) Numeric values that cannot be represented in the grammar below (such as Infinity and NaN) are not permitted.

    This description includes both integer and floating-point numbers. However, C++ allows more precise storage if it is known whether the number is a signed integer, an unsigned integer, or a floating-point number. Therefore, three different types, number_integer_t, number_unsigned_t and number_float_t are used.

    To store unsigned integer numbers in C++, a type is defined by the template parameter NumberUnsignedType which chooses the type to use.

    "},{"location":"api/basic_json/number_unsigned_t/#template-parameters","title":"Template parameters","text":"NumberUnsignedType the type to store unsigned integers. It must be an unsigned integral type (std::is_integral) with a std::numeric_limits specialization, and it must be able to represent the absolute value of every number_integer_t value. Alternatively, it can be the same type as number_integer_t; then every integer is stored as number_integer_t. See Template Parameter Requirements."},{"location":"api/basic_json/number_unsigned_t/#notes","title":"Notes","text":""},{"location":"api/basic_json/number_unsigned_t/#default-type","title":"Default type","text":"

    With the default values for NumberUnsignedType (std::uint64_t), the default type for number_unsigned_t is std::uint64_t.

    "},{"location":"api/basic_json/number_unsigned_t/#default-behavior","title":"Default behavior","text":"
    • The restrictions about leading zeros are not enforced in C++. Instead, leading zeros in integer literals lead to an interpretation as an octal number. Internally, the value will be stored as a decimal number. For instance, the C++ integer literal 010 will be serialized to 8. During deserialization, leading zeros yield an error.
    "},{"location":"api/basic_json/number_unsigned_t/#limits","title":"Limits","text":"

    RFC 8259 specifies:

    An implementation may set limits on the range and precision of numbers.

    When the default type is used, the maximal integer number that can be stored is 18446744073709551615 (UINT64_MAX) and the minimal integer number that can be stored is 0. Integer numbers that are out of range will yield over/underflow when used in a constructor. During deserialization (from JSON text or any of the binary formats), too large or small integer numbers will automatically be stored as number_integer_t or number_float_t.

    RFC 8259 further states:

    Note that when such software is used, numbers that are integers and are in the range [-253+1, 253-1] are interoperable in the sense that implementations will agree exactly on their numeric values.

    As this range is a subrange (when considered in conjunction with the number_integer_t type) of the exactly supported range [0, UINT64_MAX], this class's integer type is interoperable.

    "},{"location":"api/basic_json/number_unsigned_t/#storage","title":"Storage","text":"

    Integer number values are stored directly inside a basic_json type.

    "},{"location":"api/basic_json/number_unsigned_t/#examples","title":"Examples","text":"Example

    The following code shows that number_unsigned_t is by default, a typedef to std::uint64_t.

    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::boolalpha << std::is_same<std::uint64_t, json::number_unsigned_t>::value << std::endl;\n}\n

    Output:

    true\n
    "},{"location":"api/basic_json/number_unsigned_t/#see-also","title":"See also","text":"
    • number_integer_t the type used to store JSON integer numbers
    • number_float_t the type used to store JSON floating-point numbers
    • is_number_unsigned checks whether the JSON value is an unsigned integer number
    • Number Handling - the article on number handling
    "},{"location":"api/basic_json/number_unsigned_t/#version-history","title":"Version history","text":"
    • Added in version 2.0.0.
    • Can be the same type as number_integer_t since version 3.13.0 unreleased.
    "},{"location":"api/basic_json/object/","title":"nlohmann::basic_json::object","text":"
    static basic_json object(initializer_list_t init = {});\n

    Creates a JSON object value from a given initializer list. The initializer lists elements must be pairs, and their first elements must be strings. If the initializer list is empty, the empty object {} is created.

    "},{"location":"api/basic_json/object/#parameters","title":"Parameters","text":"init (in) initializer list with JSON values to create an object from (optional)"},{"location":"api/basic_json/object/#return-value","title":"Return value","text":"

    JSON object value

    "},{"location":"api/basic_json/object/#exception-safety","title":"Exception safety","text":"

    Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

    "},{"location":"api/basic_json/object/#exceptions","title":"Exceptions","text":"

    Throws type_error.301 if init is not a list of pairs whose first elements are strings. In this case, no object can be created. When such a value is passed to basic_json(initializer_list_t, bool, value_t), an array would have been created from the passed initializer list init. See the example below.

    "},{"location":"api/basic_json/object/#complexity","title":"Complexity","text":"

    Linear in the size of init.

    "},{"location":"api/basic_json/object/#notes","title":"Notes","text":"

    This function is only added for symmetry reasons. In contrast to the related function array(initializer_list_t), there are no cases that can only be expressed by this function. That is, any initializer list init can also be passed to the initializer list constructor basic_json(initializer_list_t, bool, value_t).

    "},{"location":"api/basic_json/object/#examples","title":"Examples","text":"Example

    The following code shows an example for the object function.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON objects\n    json j_no_init_list = json::object();\n    json j_empty_init_list = json::object({});\n    json j_list_of_pairs = json::object({ {\"one\", 1}, {\"two\", 2} });\n\n    // serialize the JSON objects\n    std::cout << j_no_init_list << '\\n';\n    std::cout << j_empty_init_list << '\\n';\n    std::cout << j_list_of_pairs << '\\n';\n\n    // example for an exception\n    try\n    {\n        // can only create an object from a list of pairs\n        json j_invalid_object = json::object({{ \"one\", 1, 2 }});\n    }\n    catch (const json::type_error& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

    Output:

    {}\n{}\n{\"one\":1,\"two\":2}\n[json.exception.type_error.301] cannot create object from initializer list\n
    "},{"location":"api/basic_json/object/#see-also","title":"See also","text":"
    • basic_json(initializer_list_t) - create a JSON value from an initializer list
    • array - create a JSON array value from an initializer list
    • Creating JSON values - the article on creating JSON values
    "},{"location":"api/basic_json/object/#version-history","title":"Version history","text":"
    • Added in version 1.0.0.
    "},{"location":"api/basic_json/object_comparator_t/","title":"nlohmann::basic_json::object_comparator_t","text":"
    using object_comparator_t = typename object_t::key_compare;\n// or\nusing object_comparator_t = default_object_comparator_t;\n

    The comparator used by object_t. Defined as typename object_t::key_compare if available, and default_object_comparator_t otherwise.

    "},{"location":"api/basic_json/object_comparator_t/#examples","title":"Examples","text":"Example

    The example below demonstrates the used object comparator.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::boolalpha\n              << \"json::object_comparator_t(\\\"one\\\", \\\"two\\\") = \" << json::object_comparator_t{}(\"one\", \"two\") << \"\\n\"\n              << \"json::object_comparator_t(\\\"three\\\", \\\"four\\\") = \" << json::object_comparator_t{}(\"three\", \"four\") << std::endl;\n}\n

    Output:

    json::object_comparator_t(\"one\", \"two\") = true\njson::object_comparator_t(\"three\", \"four\") = false\n
    "},{"location":"api/basic_json/object_comparator_t/#see-also","title":"See also","text":"
    • object_t the type used to store JSON objects
    • default_object_comparator_t the fallback comparator used when object_t has no key_compare member type
    "},{"location":"api/basic_json/object_comparator_t/#version-history","title":"Version history","text":"
    • Added in version 3.0.0.
    • Changed to be conditionally defined as typename object_t::key_compare or default_object_comparator_t in version 3.11.0.
    • Fixed the fallback to default_object_comparator_t, which previously failed to compile for object types without a key_compare member type, in version 3.13.0 unreleased.
    "},{"location":"api/basic_json/object_t/","title":"nlohmann::basic_json::object_t","text":"
    using object_t = ObjectType<StringType,\n                            basic_json,\n                            default_object_comparator_t,\n                            AllocatorType<std::pair<const StringType, basic_json>>>;\n

    The type used to store JSON objects.

    RFC 8259 describes JSON objects as follows:

    An object is an unordered collection of zero or more name/value pairs, where a name is a string and a value is a string, number, boolean, null, object, or array.

    To store objects in C++, a type is defined by the template parameters described below.

    "},{"location":"api/basic_json/object_t/#template-parameters","title":"Template parameters","text":"ObjectType the container to store objects. Its template parameters must have the same order and meaning as those of std::map; in particular, the third parameter is a comparator. std::unordered_map, whose third parameter is a hash function, therefore needs an adapter -- see Template Parameter Requirements for the full list of requirements, an adapter example, and the containers that are known to work. StringType the type of the keys or names (e.g., std::string). The comparison function std::less<StringType> is used to order elements inside the container. AllocatorType the allocator to use for objects (e.g., std::allocator)"},{"location":"api/basic_json/object_t/#notes","title":"Notes","text":""},{"location":"api/basic_json/object_t/#default-type","title":"Default type","text":"

    With the default values for ObjectType (std::map), StringType (std::string), and AllocatorType (std::allocator), the default type for object_t is:

    // until C++14\nstd::map<\n  std::string, // key_type\n  basic_json, // value_type\n  std::less<std::string>, // key_compare\n  std::allocator<std::pair<const std::string, basic_json>> // allocator_type\n>\n\n// since C++14\nstd::map<\n  std::string, // key_type\n  basic_json, // value_type\n  std::less<>, // key_compare\n  std::allocator<std::pair<const std::string, basic_json>> // allocator_type\n>\n

    See default_object_comparator_t for more information.

    "},{"location":"api/basic_json/object_t/#behavior","title":"Behavior","text":"

    The choice of object_t influences the behavior of the JSON class. With the default type, objects have the following behavior:

    • When all names are unique, objects will be interoperable in the sense that all software implementations receiving that object will agree on the name-value mappings.
    • When the names within an object are not unique, it is unspecified which one of the values for a given key will be chosen. For instance, {\"key\": 2, \"key\": 1} could be equal to either {\"key\": 1} or {\"key\": 2}. To reject duplicate keys instead of silently resolving them one way or another, see this parsing recipe.
    • Internally, name/value pairs are stored in lexicographical order of the names. Objects will also be serialized (see dump) in this order. For instance, {\"b\": 1, \"a\": 2} and {\"a\": 2, \"b\": 1} will be stored and serialized as {\"a\": 2, \"b\": 1}.
    • When comparing objects, the order of the name/value pairs is irrelevant. This makes objects interoperable in the sense that they will not be affected by these differences. For instance, {\"b\": 1, \"a\": 2} and {\"a\": 2, \"b\": 1} will be treated as equal.
    "},{"location":"api/basic_json/object_t/#limits","title":"Limits","text":"

    RFC 8259 specifies:

    An implementation may set limits on the maximum depth of nesting.

    In this class, the object's limit of nesting is not explicitly constrained. However, a maximum depth of nesting may be introduced by the compiler or runtime environment. A theoretical limit can be queried by calling the max_size function of a JSON object.

    "},{"location":"api/basic_json/object_t/#storage","title":"Storage","text":"

    Objects are stored as pointers in a basic_json type. That is, for any access to object values, a pointer of type object_t* must be dereferenced.

    "},{"location":"api/basic_json/object_t/#object-key-order","title":"Object key order","text":"

    The order name/value pairs are added to the object are not preserved by the library. Therefore, iterating an object may return name/value pairs in a different order than they were originally stored. In fact, keys will be traversed in alphabetical order as std::map with std::less is used by default. Please note this behavior conforms to RFC 8259, because any order implements the specified \"unordered\" nature of JSON objects.

    "},{"location":"api/basic_json/object_t/#cross-basic_json-conversion-requirements","title":"Cross-basic_json conversion requirements","text":"

    When converting an object from one basic_json specialization to another via the converting constructor (overload 4), the target object_t's key_type must be directly constructible from the source basic_json's string_t type (or more generally, from the source object's key type). If this requirement is not met, the conversion does not fail; instead, the object is silently converted as an array of key-value pairs, which is incorrect. See issue #3425 for details and an example.

    "},{"location":"api/basic_json/object_t/#examples","title":"Examples","text":"Example

    The following code shows that object_t is by default, a typedef to std::map<json::string_t, json>.

    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::boolalpha << std::is_same<std::map<json::string_t, json>, json::object_t>::value << std::endl;\n}\n

    Output:

    true\n
    "},{"location":"api/basic_json/object_t/#see-also","title":"See also","text":"
    • array_t the type used to store JSON arrays
    • string_t the type used to store JSON strings
    • object_comparator_t the comparator used to order object keys
    • Object Order - the article on object key ordering
    "},{"location":"api/basic_json/object_t/#version-history","title":"Version history","text":"
    • Added in version 1.0.0.
    • Allowed object types whose erase(iterator) returns void in version 3.13.0 unreleased.
    "},{"location":"api/basic_json/operator%2B%3D/","title":"nlohmann::basic_json::operator+=","text":"
    // (1)\nreference operator+=(basic_json&& val);\nreference operator+=(const basic_json& val);\n\n// (2)\nreference operator+=(const typename object_t::value_type& val);\n\n// (3)\nreference operator+=(initializer_list_t init);\n

    1. Appends the given element val to the end of the JSON array. If the function is called on a JSON null value, an empty array is created before appending val.

    2. Inserts the given element val to the JSON object. If the function is called on a JSON null value, an empty object is created before inserting val.

    3. This function allows using operator+= with an initializer list. In case

      1. the current value is an object,
      2. the initializer list init contains only two elements, and
      3. the first element of init is a string,

      init is converted into an object element and added using operator+=(const typename object_t::value_type&). Otherwise, init is converted to a JSON value and added using operator+=(basic_json&&).

    "},{"location":"api/basic_json/operator%2B%3D/#iterator-invalidation","title":"Iterator invalidation","text":"

    For all cases where an element is added to an array, a reallocation can happen, in which case all iterators (including the end() iterator) and all references to the elements are invalidated. Otherwise, only the end() iterator is invalidated.

    For ordered_json, also adding an element to an object can yield a reallocation which again invalidates all iterators and all references.

    "},{"location":"api/basic_json/operator%2B%3D/#parameters","title":"Parameters","text":"val (in) the value to add to the JSON array/object init (in) an initializer list"},{"location":"api/basic_json/operator%2B%3D/#return-value","title":"Return value","text":"

    *this

    "},{"location":"api/basic_json/operator%2B%3D/#exception-safety","title":"Exception safety","text":"

    Strong guarantee: if an exception is thrown, there are no changes to any JSON value. As an exception, a null value is converted to an empty array or object before the element is added and keeps that type if adding the element throws.

    "},{"location":"api/basic_json/operator%2B%3D/#exceptions","title":"Exceptions","text":"
    1. Throws type_error.308 when called on a type other than JSON array or null; example: \"cannot use push_back() with number\"
    2. Throws type_error.308 when called on a type other than JSON object or null; example: \"cannot use push_back() with number\"
    3. Throws type_error.308 when called on a type other than JSON array or null; example: \"cannot use push_back() with number\"
    "},{"location":"api/basic_json/operator%2B%3D/#complexity","title":"Complexity","text":"
    1. Amortized constant.
    2. Logarithmic in the size of the container, O(log(size())).
    3. Linear in the size of the initializer list init.
    "},{"location":"api/basic_json/operator%2B%3D/#notes","title":"Notes","text":"

    (3) This function is required to resolve an ambiguous overload error, because pairs like {\"key\", \"value\"} can be both interpreted as object_t::value_type or std::initializer_list<basic_json>, see #235 for more information.

    "},{"location":"api/basic_json/operator%2B%3D/#examples","title":"Examples","text":"Example: (1) add element to array

    The example shows how push_back() and += can be used to add elements to a JSON array. Note how the null value was silently converted to a JSON array.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json array = {1, 2, 3, 4, 5};\n    json null;\n\n    // print values\n    std::cout << array << '\\n';\n    std::cout << null << '\\n';\n\n    // add values\n    array.push_back(6);\n    array += 7;\n    null += \"first\";\n    null += \"second\";\n\n    // print values\n    std::cout << array << '\\n';\n    std::cout << null << '\\n';\n}\n

    Output:

    [1,2,3,4,5]\nnull\n[1,2,3,4,5,6,7]\n[\"first\",\"second\"]\n
    Example: (2) add element to object

    The example shows how push_back() and += can be used to add elements to a JSON object. Note how the null value was silently converted to a JSON object.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json object = {{\"one\", 1}, {\"two\", 2}};\n    json null;\n\n    // print values\n    std::cout << object << '\\n';\n    std::cout << null << '\\n';\n\n    // add values\n    object.push_back(json::object_t::value_type(\"three\", 3));\n    object += json::object_t::value_type(\"four\", 4);\n    null += json::object_t::value_type(\"A\", \"a\");\n    null += json::object_t::value_type(\"B\", \"b\");\n\n    // print values\n    std::cout << object << '\\n';\n    std::cout << null << '\\n';\n}\n

    Output:

    {\"one\":1,\"two\":2}\nnull\n{\"four\":4,\"one\":1,\"three\":3,\"two\":2}\n{\"A\":\"a\",\"B\":\"b\"}\n
    Example: (3) add to object from initializer list

    The example shows how initializer lists are treated as objects when possible.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json object = {{\"one\", 1}, {\"two\", 2}};\n    json null;\n\n    // print values\n    std::cout << object << '\\n';\n    std::cout << null << '\\n';\n\n    // add values:\n    object.push_back({\"three\", 3});  // object is extended\n    object += {\"four\", 4};           // object is extended\n    null.push_back({\"five\", 5});     // null is converted to array\n\n    // print values\n    std::cout << object << '\\n';\n    std::cout << null << '\\n';\n\n    // would throw:\n    //object.push_back({1, 2, 3});\n}\n

    Output:

    {\"one\":1,\"two\":2}\nnull\n{\"four\":4,\"one\":1,\"three\":3,\"two\":2}\n[[\"five\",5]]\n
    "},{"location":"api/basic_json/operator%2B%3D/#see-also","title":"See also","text":"
    • emplace_back add a value to an array
    • push_back add a value to an array/object
    "},{"location":"api/basic_json/operator%2B%3D/#version-history","title":"Version history","text":"
    1. Since version 1.0.0.
    2. Since version 1.0.0.
    3. Since version 2.0.0.
    "},{"location":"api/basic_json/operator%3D/","title":"nlohmann::basic_json::operator=","text":"
    basic_json& operator=(basic_json other) noexcept (\n    std::is_nothrow_move_constructible<value_t>::value &&\n    std::is_nothrow_move_assignable<value_t>::value &&\n    std::is_nothrow_move_constructible<json_value>::value &&\n    std::is_nothrow_move_assignable<json_value>::value &&\n    std::is_nothrow_move_assignable<json_base_class_t>::value\n);\n

    Copy assignment operator. Copies a JSON value via the \"copy and swap\" strategy: It is expressed in terms of the copy constructor, destructor, and the swap() member function.

    "},{"location":"api/basic_json/operator%3D/#parameters","title":"Parameters","text":"other (in) value to copy from"},{"location":"api/basic_json/operator%3D/#exception-safety","title":"Exception safety","text":"

    Strong guarantee: if an exception is thrown while copying other, there are no changes to *this.

    "},{"location":"api/basic_json/operator%3D/#complexity","title":"Complexity","text":"

    Linear.

    "},{"location":"api/basic_json/operator%3D/#examples","title":"Examples","text":"Example

    The code below shows and example for the copy assignment. It creates a copy of value a which is then swapped with b. Finally, the copy of a (which is the null value after the swap) is destroyed.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json a = 23;\n    json b = 42;\n\n    // copy-assign a to b\n    b = a;\n\n    // serialize the JSON arrays\n    std::cout << a << '\\n';\n    std::cout << b << '\\n';\n}\n

    Output:

    23\n23\n
    "},{"location":"api/basic_json/operator%3D/#see-also","title":"See also","text":"
    • basic_json create a JSON value
    • swap exchanges the contents of two JSON values
    "},{"location":"api/basic_json/operator%3D/#version-history","title":"Version history","text":"
    • Added in version 1.0.0.
    "},{"location":"api/basic_json/operator%5B%5D/","title":"nlohmann::basic_json::operator[]","text":"
    // (1)\nreference operator[](size_type idx);\nconst_reference operator[](size_type idx) const;\n\n// (2)\nreference operator[](typename object_t::key_type key);\nconst_reference operator[](const typename object_t::key_type& key) const;\n\n// (3)\ntemplate<typename KeyType>\nreference operator[](KeyType&& key);\ntemplate<typename KeyType>\nconst_reference operator[](KeyType&& key) const;\n\n// (4)\nreference operator[](const json_pointer& ptr);\nconst_reference operator[](const json_pointer& ptr) const;\n
    1. Returns a reference to the array element at specified location idx.
    2. Returns a reference to the object element with specified key key. The non-const qualified overload takes the key by value.
    3. See 2. This overload is only available if KeyType is comparable with typename object_t::key_type and typename object_comparator_t::is_transparent denotes a type.
    4. Returns a reference to the element with specified JSON pointer ptr.
    "},{"location":"api/basic_json/operator%5B%5D/#template-parameters","title":"Template parameters","text":"KeyType A type for an object key other than json_pointer that is comparable with string_t using object_comparator_t. This can also be a string view (C++17)."},{"location":"api/basic_json/operator%5B%5D/#iterator-invalidation","title":"Iterator invalidation","text":"

    For the non-const versions 1. and 4., when passing an array index that does not exist, it is created and filled with a null value before a reference to it is returned. For this, a reallocation can happen, in which case all iterators (including the end() iterator) and all references to the elements are invalidated.

    For ordered_json, also passing an object key to the non-const versions 2., 3., and 4., a reallocation can happen which again invalidates all iterators and all references.

    "},{"location":"api/basic_json/operator%5B%5D/#parameters","title":"Parameters","text":"idx (in) index of the element to access key (in) object key of the element to access ptr (in) JSON pointer to the desired element"},{"location":"api/basic_json/operator%5B%5D/#return-value","title":"Return value","text":"
    1. (const) reference to the element at index idx
    2. (const) reference to the element at key key
    3. (const) reference to the element at key key
    4. (const) reference to the element pointed to by ptr
    "},{"location":"api/basic_json/operator%5B%5D/#exception-safety","title":"Exception safety","text":"

    Strong exception safety: if an exception occurs, the original value stays intact.

    "},{"location":"api/basic_json/operator%5B%5D/#exceptions","title":"Exceptions","text":"
    1. The function can throw the following exceptions:
      • Throws type_error.305 if the JSON value is not an array or null; in that case, using the [] operator with an index makes no sense.
      • Throws std::length_error if idx equals the maximum value of size_type; the array is left unchanged. (This is the one index for which growing the array to hold it cannot be expressed as a size_type size, the same way an oversized resize throws.)
    2. The function can throw the following exceptions:
      • Throws type_error.305 if the JSON value is not an object or null; in that case, using the [] operator with a key makes no sense.
    3. See 2.

    4. The function can throw the following exceptions:

      • Throws parse_error.106 if an array index in the passed JSON pointer ptr begins with '0'.
      • Throws parse_error.109 if an array index in the passed JSON pointer ptr is not a number.
      • Throws out_of_range.402 if the array index '-' is used in the passed JSON pointer ptr for the const version.
      • Throws out_of_range.404 if the JSON pointer ptr can not be resolved.
      • Throws out_of_range.410 if an array index in the passed JSON pointer ptr exceeds the range of size_type (e.g., on 32-bit platforms).

      For the const version, an object key or array index in ptr that does not exist is not reported by an exception, but is undefined behavior (see the notes below). Use at for checked access.

    "},{"location":"api/basic_json/operator%5B%5D/#complexity","title":"Complexity","text":"
    1. Constant if idx is in the range of the array. Otherwise, linear in idx - size().
    2. Logarithmic in the size of the container.
    3. Logarithmic in the size of the container.
    4. Logarithmic in the size of the container.
    "},{"location":"api/basic_json/operator%5B%5D/#notes","title":"Notes","text":"

    Undefined behavior and runtime assertions

    The following cases apply to the const overloads; the non-const overloads instead insert the missing element (see the notes below).

    1. If the element at index idx does not exist, the behavior is undefined and is guarded by a runtime assertion!
    2. If the element with key key does not exist, the behavior is undefined and is guarded by a runtime assertion!
    3. If the JSON pointer ptr refers to an object key or an array index that does not exist, the behavior is undefined and is guarded by a runtime assertion!

    1. The non-const version may add values: If idx is beyond the range of the array (i.e., idx >= size()), then the array is silently filled up with null values to make idx a valid reference to the last stored element. In case the value was null before, it is converted to an array.

    2. If key is not found in the object, then it is silently added to the object and filled with a null value to make key a valid reference. In case the value was null before, it is converted to an object.

    3. See 2.

    4. null values are created in arrays and objects if necessary.

      In particular:

      • If the JSON pointer points to an object key that does not exist, it is created and filled with a null value before a reference to it is returned.
      • If the JSON pointer points to an array index that does not exist, it is created and filled with a null value before a reference to it is returned. All indices between the current maximum and the given index are also filled with null.
      • The special value - is treated as a synonym for the index past the end.

      Creating intermediate levels that don't exist yet

      When the JSON pointer traverses intermediate levels that don't exist at all yet (not just a missing leaf), each missing level is created as an array or an object depending on whether the corresponding pointer token parses as a non-negative integer: a numeric token creates an array, a non-numeric token creates an object. For example, on an initially null value, /foo/0/0/0 creates nested arrays, while /foo/one/one/one creates nested objects. This is not specified by the JSON Pointer RFC; it is this library's own, intentional disambiguation rule. See also JSON Pointer.

    Deprecation

    Overload (4) also accepts a json_pointer whose template argument is a basic_json specialization (e.g., nlohmann::json_pointer<nlohmann::json>) instead of a string type. This is deprecated since version 3.11.0 and will be removed in a future major version; use basic_json::json_pointer (for json, nlohmann::json_pointer<std::string>) instead.

    You should be warned by your compiler with a -Wdeprecated-declarations warning if you are using a deprecated function.

    See the migration guide for how to update existing code.

    "},{"location":"api/basic_json/operator%5B%5D/#examples","title":"Examples","text":"Example: (1) access specified array element

    The example below shows how array elements can be read and written using [] operator. Note the addition of null values.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON array\n    json array = {1, 2, 3, 4, 5};\n\n    // output element at index 3 (fourth element)\n    std::cout << array[3] << '\\n';\n\n    // change last element to 6\n    array[array.size() - 1] = 6;\n\n    // output changed array\n    std::cout << array << '\\n';\n\n    // write beyond array limit\n    array[10] = 11;\n\n    // output changed array\n    std::cout << array << '\\n';\n}\n

    Output:

    4\n[1,2,3,4,6]\n[1,2,3,4,6,null,null,null,null,null,11]\n
    Example: (1) access specified array element (const)

    The example below shows how array elements can be read using the [] operator.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON array\n    const json array = {\"first\", \"2nd\", \"third\", \"fourth\"};\n\n    // output element at index 2 (third element)\n    std::cout << array.at(2) << '\\n';\n}\n

    Output:

    \"third\"\n
    Example: (2) access specified object element

    The example below shows how object elements can be read and written using the [] operator.

    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON object\n    json object =\n    {\n        {\"one\", 1}, {\"two\", 2}, {\"three\", 2.9}\n    };\n\n    // output element with key \"two\"\n    std::cout << object[\"two\"] << \"\\n\\n\";\n\n    // change element with key \"three\"\n    object[\"three\"] = 3;\n\n    // output changed array\n    std::cout << std::setw(4) << object << \"\\n\\n\";\n\n    // mention nonexisting key\n    object[\"four\"];\n\n    // write to nonexisting key\n    object[\"five\"][\"really\"][\"nested\"] = true;\n\n    // output changed object\n    std::cout << std::setw(4) << object << '\\n';\n}\n

    Output:

    2\n\n{\n    \"one\": 1,\n    \"three\": 3,\n    \"two\": 2\n}\n\n{\n    \"five\": {\n        \"really\": {\n            \"nested\": true\n        }\n    },\n    \"four\": null,\n    \"one\": 1,\n    \"three\": 3,\n    \"two\": 2\n}\n
    Example: (2) access specified object element (const)

    The example below shows how object elements can be read using the [] operator.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON object\n    const json object =\n    {\n        {\"one\", 1}, {\"two\", 2}, {\"three\", 2.9}\n    };\n\n    // output element with key \"two\"\n    std::cout << object[\"two\"] << '\\n';\n}\n

    Output:

    2\n
    Example: (3) access specified object element using string_view

    The example below shows how object elements can be read using the [] operator.

    #include <iostream>\n#include <iomanip>\n#include <string_view>\n#include <nlohmann/json.hpp>\n\nusing namespace std::string_view_literals;\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON object\n    json object =\n    {\n        {\"one\", 1}, {\"two\", 2}, {\"three\", 2.9}\n    };\n\n    // output element with key \"two\"\n    std::cout << object[\"two\"sv] << \"\\n\\n\";\n\n    // change element with key \"three\"\n    object[\"three\"sv] = 3;\n\n    // output changed array\n    std::cout << std::setw(4) << object << \"\\n\\n\";\n\n    // mention nonexisting key\n    object[\"four\"sv];\n\n    // write to nonexisting key\n    object[\"five\"sv][\"really\"sv][\"nested\"sv] = true;\n\n    // output changed object\n    std::cout << std::setw(4) << object << '\\n';\n}\n

    Output:

    2\n\n{\n    \"one\": 1,\n    \"three\": 3,\n    \"two\": 2\n}\n\n{\n    \"five\": {\n        \"really\": {\n            \"nested\": true\n        }\n    },\n    \"four\": null,\n    \"one\": 1,\n    \"three\": 3,\n    \"two\": 2\n}\n
    Example: (3) access specified object element using string_view (const)

    The example below shows how object elements can be read using the [] operator.

    #include <iostream>\n#include <string_view>\n#include <nlohmann/json.hpp>\n\nusing namespace std::string_view_literals;\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON object\n    const json object =\n    {\n        {\"one\", 1}, {\"two\", 2}, {\"three\", 2.9}\n    };\n\n    // output element with key \"two\"\n    std::cout << object[\"two\"sv] << '\\n';\n}\n

    Output:

    2\n
    Example: (4) access specified element via JSON Pointer

    The example below shows how values can be read and written using JSON Pointers.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // create a JSON value\n    json j =\n    {\n        {\"number\", 1}, {\"string\", \"foo\"}, {\"array\", {1, 2}}\n    };\n\n    // read-only access\n\n    // output element with JSON pointer \"/number\"\n    std::cout << j[\"/number\"_json_pointer] << '\\n';\n    // output element with JSON pointer \"/string\"\n    std::cout << j[\"/string\"_json_pointer] << '\\n';\n    // output element with JSON pointer \"/array\"\n    std::cout << j[\"/array\"_json_pointer] << '\\n';\n    // output element with JSON pointer \"/array/1\"\n    std::cout << j[\"/array/1\"_json_pointer] << '\\n';\n\n    // writing access\n\n    // change the string\n    j[\"/string\"_json_pointer] = \"bar\";\n    // output the changed string\n    std::cout << j[\"string\"] << '\\n';\n\n    // \"change\" a nonexisting object entry\n    j[\"/boolean\"_json_pointer] = true;\n    // output the changed object\n    std::cout << j << '\\n';\n\n    // change an array element\n    j[\"/array/1\"_json_pointer] = 21;\n    // \"change\" an array element with nonexisting index\n    j[\"/array/4\"_json_pointer] = 44;\n    // output the changed array\n    std::cout << j[\"array\"] << '\\n';\n\n    // \"change\" the array element past the end\n    j[\"/array/-\"_json_pointer] = 55;\n    // output the changed array\n    std::cout << j[\"array\"] << '\\n';\n}\n

    Output:

    1\n\"foo\"\n[1,2]\n2\n\"bar\"\n{\"array\":[1,2],\"boolean\":true,\"number\":1,\"string\":\"bar\"}\n[1,21,null,null,44]\n[1,21,null,null,44,55]\n
    Example: (4) access specified element via JSON Pointer (const)

    The example below shows how values can be read using JSON Pointers.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // create a JSON value\n    const json j =\n    {\n        {\"number\", 1}, {\"string\", \"foo\"}, {\"array\", {1, 2}}\n    };\n\n    // read-only access\n\n    // output element with JSON pointer \"/number\"\n    std::cout << j[\"/number\"_json_pointer] << '\\n';\n    // output element with JSON pointer \"/string\"\n    std::cout << j[\"/string\"_json_pointer] << '\\n';\n    // output element with JSON pointer \"/array\"\n    std::cout << j[\"/array\"_json_pointer] << '\\n';\n    // output element with JSON pointer \"/array/1\"\n    std::cout << j[\"/array/1\"_json_pointer] << '\\n';\n}\n

    Output:

    1\n\"foo\"\n[1,2]\n2\n
    "},{"location":"api/basic_json/operator%5B%5D/#see-also","title":"See also","text":"
    • documentation on unchecked access
    • documentation on runtime assertions
    • see at for access by reference with range checking
    • see value for access with default value
    • basic_json_view::operator[] - the same access on a zero-copy view (a duplicate key resolves to the first member, whereas parse() keeps the last; a missing key gives a discarded view instead of undefined behavior)
    "},{"location":"api/basic_json/operator%5B%5D/#version-history","title":"Version history","text":"
    1. Added in version 1.0.0. Fixed in version 3.13.0 unreleased to throw std::length_error instead of emptying the array and accessing it out of bounds when idx equals the maximum value of size_type. A missing index in the const version is guarded by a runtime assertion since version 3.13.0 unreleased.
    2. Added in version 1.0.0. Added overloads for T* key in version 1.1.0. Removed overloads for T* key (replaced by 3) in version 3.11.0.
    3. Added in version 3.11.0. Fixed in version 3.13.0 unreleased to consistently accept std::string_view-convertible keys, as already supported by at, value, find, and other lookup functions.
    4. Added in version 2.0.0. A missing array index in the const version is guarded by a runtime assertion since version 3.13.0 unreleased. Throws parse_error.109 instead of out_of_range.404 for a one-character array index that is not a digit (e.g., /x) in version 3.13.0 unreleased, as it already did for longer ones.
    "},{"location":"api/basic_json/operator_ValueType/","title":"nlohmann::basic_json::operator ValueType","text":"
    template<typename ValueType>\nJSON_EXPLICIT operator ValueType() const;\n

    Implicit type conversion between the JSON value and a compatible value. The call is realized by calling get(). See Notes for the meaning of JSON_EXPLICIT.

    "},{"location":"api/basic_json/operator_ValueType/#template-parameters","title":"Template parameters","text":"ValueType the value type to return"},{"location":"api/basic_json/operator_ValueType/#return-value","title":"Return value","text":"

    copy of the JSON value, converted to ValueType

    "},{"location":"api/basic_json/operator_ValueType/#exception-safety","title":"Exception safety","text":"

    Depends on what json_serializer<ValueType> from_json() method throws; the JSON value itself is never modified, since operator ValueType() is a const member function that only calls get().

    "},{"location":"api/basic_json/operator_ValueType/#exceptions","title":"Exceptions","text":"

    Depends on what json_serializer<ValueType> from_json() method throws

    "},{"location":"api/basic_json/operator_ValueType/#complexity","title":"Complexity","text":"

    Linear in the size of the JSON value.

    "},{"location":"api/basic_json/operator_ValueType/#notes","title":"Notes","text":"

    Definition of JSON_EXPLICIT

    By default JSON_EXPLICIT is defined to the empty string, so the signature is:

    template<typename ValueType>\noperator ValueType() const;\n

    If JSON_USE_IMPLICIT_CONVERSIONS is set to 0, JSON_EXPLICIT is defined to explicit:

    template<typename ValueType>\nexplicit operator ValueType() const;\n

    That is, implicit conversions can be switched off by defining JSON_USE_IMPLICIT_CONVERSIONS to 0.

    Future behavior change

    Implicit conversions will be switched off by default in the next major release of the library. That is, JSON_EXPLICIT will be set to explicit by default.

    You can prepare existing code by already defining JSON_USE_IMPLICIT_CONVERSIONS to 0 and replace any implicit conversions with calls to get.

    See the migration guide for how to update existing code.

    "},{"location":"api/basic_json/operator_ValueType/#examples","title":"Examples","text":"Example

    The example below shows several conversions from JSON values to other types. There are a few things to note: (1) Floating-point numbers can be converted to integers, (2) A JSON array can be converted to a standard std::vector<short>, (3) A JSON object can be converted to C++ associative containers such as std::map<std::string, json>.

    #include <iostream>\n#include <map>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON value with different types\n    json json_types =\n    {\n        {\"boolean\", true},\n        {\n            \"number\", {\n                {\"integer\", 42},\n                {\"floating-point\", 17.23}\n            }\n        },\n        {\"string\", \"Hello, world!\"},\n        {\"array\", {1, 2, 3, 4, 5}},\n        {\"null\", nullptr}\n    };\n\n    // use implicit conversions\n    bool v1 = json_types[\"boolean\"];\n    int v2 = json_types[\"number\"][\"integer\"];\n    short v3 = json_types[\"number\"][\"integer\"];\n    float v4 = json_types[\"number\"][\"floating-point\"];\n    int v5 = json_types[\"number\"][\"floating-point\"];\n    std::string v6 = json_types[\"string\"];\n    std::vector<short> v7 = json_types[\"array\"];\n    std::map<std::string, json> v8 = json_types;\n\n    // print the conversion results\n    std::cout << v1 << '\\n';\n    std::cout << v2 << ' ' << v3 << '\\n';\n    std::cout << v4 << ' ' << v5 << '\\n';\n    std::cout << v6 << '\\n';\n\n    for (auto i : v7)\n    {\n        std::cout << i << ' ';\n    }\n    std::cout << \"\\n\\n\";\n\n    for (auto i : v8)\n    {\n        std::cout << i.first << \": \" << i.second << '\\n';\n    }\n\n    // example for an exception\n    try\n    {\n        bool v1 = json_types[\"string\"];\n    }\n    catch (const json::type_error& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

    Output:

    1\n42 42\n17.23 17\nHello, world!\n1 2 3 4 5 \n\narray: [1,2,3,4,5]\nboolean: true\nnull: null\nnumber: {\"floating-point\":17.23,\"integer\":42}\nstring: \"Hello, world!\"\n[json.exception.type_error.302] type must be boolean, but is string\n
    "},{"location":"api/basic_json/operator_ValueType/#see-also","title":"See also","text":"
    • get get a value (explicit conversion)
    • Converting values - the type conversions article
    "},{"location":"api/basic_json/operator_ValueType/#version-history","title":"Version history","text":"
    • Since version 1.0.0.
    • Macros JSON_EXPLICIT/JSON_USE_IMPLICIT_CONVERSIONS added in version 3.9.0.
    "},{"location":"api/basic_json/operator_eq/","title":"nlohmann::basic_json::operator==","text":"
    // until C++20\nbool operator==(const_reference lhs, const_reference rhs) noexcept;   // (1)\n\ntemplate<typename ScalarType>\nbool operator==(const_reference lhs, const ScalarType rhs) noexcept(/* see below */); // (2)\n\ntemplate<typename ScalarType>\nbool operator==(ScalarType lhs, const const_reference rhs) noexcept(/* see below */); // (2)\n\n// since C++20\nbool operator==(const_reference rhs) const noexcept;             // (1)\n\ntemplate<typename ScalarType>\nbool operator==(ScalarType rhs) const noexcept(/* see below */); // (2)\n

    1. Compares two JSON values for equality according to the following rules:

      • Two JSON values are equal if (1) neither value is discarded, and (2) they are of the same type and their stored values are the same according to their respective operator==.
      • Integer and floating-point numbers are compared by their exact values, without converting the integer to floating-point. For instance, 9007199254740993 and 9007199254740992.0 are not equal.

    2. Compares a JSON value and a scalar or a scalar and a JSON value for equality by converting the scalar to a JSON value and comparing both JSON values according to 1.

    "},{"location":"api/basic_json/operator_eq/#template-parameters","title":"Template parameters","text":"ScalarType a scalar type according to std::is_scalar<ScalarType>::value"},{"location":"api/basic_json/operator_eq/#parameters","title":"Parameters","text":"lhs (in) first value to consider rhs (in) second value to consider"},{"location":"api/basic_json/operator_eq/#return-value","title":"Return value","text":"

    whether the values lhs/*this and rhs are equal

    "},{"location":"api/basic_json/operator_eq/#exception-safety","title":"Exception safety","text":"
    1. No-throw guarantee: this function never throws exceptions.
    2. No-throw guarantee if converting the scalar to a JSON value cannot throw, as for numbers, Booleans, and nullptr; the function is noexcept exactly in that case. Otherwise, it throws what the conversion throws, for example std::bad_alloc when converting a string, or out_of_range.410 for an enum value not mapped by NLOHMANN_JSON_SERIALIZE_ENUM_STRICT.
    "},{"location":"api/basic_json/operator_eq/#complexity","title":"Complexity","text":"

    Linear.

    "},{"location":"api/basic_json/operator_eq/#notes","title":"Notes","text":"

    Comparing special values

    • NaN values are unordered within the domain of numbers. The following comparisons all yield false:
      1. Comparing a NaN with itself.
      2. Comparing a NaN with another NaN.
      3. Comparing a NaN and any other number.
    • JSON null values are all equal.
    • Discarded values never compare equal to themselves.

    Comparing floating-point numbers

    Floating-point numbers inside JSON values numbers are compared with json::number_float_t::operator== which is double::operator== by default. To compare floating-point while respecting an epsilon, an alternative comparison function could be used, for instance

    template<typename T, typename = typename std::enable_if<std::is_floating_point<T>::value, T>::type>\ninline bool is_same(T a, T b, T epsilon = std::numeric_limits<T>::epsilon()) noexcept\n{\n    return std::abs(a - b) <= epsilon;\n}\n

    Or you can define your own equality function like this:

    bool my_equal(const_reference lhs, const_reference rhs)\n{\n    const auto lhs_type = lhs.type();\n    const auto rhs_type = rhs.type();\n    if (lhs_type == rhs_type)\n    {\n        switch(lhs_type)\n            // self_defined case\n            case value_t::number_float:\n                return std::abs(lhs - rhs) <= std::numeric_limits<float>::epsilon();\n            // other cases remain the same with the original\n            ...\n    }\n...\n}\n

    Comparing different basic_json specializations

    Comparing different basic_json specializations can have surprising effects. For instance, the result of comparing the JSON objects

    {\n   \"version\": 1,\n   \"type\": \"integer\"\n}\n

    and

    {\n   \"type\": \"integer\",\n   \"version\": 1\n}\n

    depends on whether nlohmann::json or nlohmann::ordered_json is used:

    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    nlohmann::json uj1 = {{\"version\", 1}, {\"type\", \"integer\"}};\n    nlohmann::json uj2 = {{\"type\", \"integer\"}, {\"version\", 1}};\n\n    nlohmann::ordered_json oj1 = {{\"version\", 1}, {\"type\", \"integer\"}};\n    nlohmann::ordered_json oj2 = {{\"type\", \"integer\"}, {\"version\", 1}};\n\n    std::cout << std::boolalpha << (uj1 == uj2) << '\\n' << (oj1 == oj2) << std::endl;\n}\n

    Output:

    true\nfalse\n
    "},{"location":"api/basic_json/operator_eq/#examples","title":"Examples","text":"Example: (1) compare JSON values

    The example demonstrates comparing several JSON types.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create several JSON values\n    json array_1 = {1, 2, 3};\n    json array_2 = {1, 2, 4};\n    json object_1 = {{\"A\", \"a\"}, {\"B\", \"b\"}};\n    json object_2 = {{\"B\", \"b\"}, {\"A\", \"a\"}};\n    json number_1 = 17;\n    json number_2 = 17.000000000000001L;\n    json string_1 = \"foo\";\n    json string_2 = \"bar\";\n\n    // output values and comparisons\n    std::cout << std::boolalpha;\n    std::cout << array_1 << \" == \" << array_2 << \" \" << (array_1 == array_2) << '\\n';\n    std::cout << object_1 << \" == \" << object_2 << \" \" << (object_1 == object_2) << '\\n';\n    std::cout << number_1 << \" == \" << number_2 << \" \" << (number_1 == number_2) << '\\n';\n    std::cout << string_1 << \" == \" << string_2 << \" \" << (string_1 == string_2) << '\\n';\n}\n

    Output:

    [1,2,3] == [1,2,4] false\n{\"A\":\"a\",\"B\":\"b\"} == {\"A\":\"a\",\"B\":\"b\"} true\n17 == 17.0 true\n\"foo\" == \"bar\" false\n
    Example: (2) compare JSON values with nullptr

    The example demonstrates comparing several JSON types against the null pointer (JSON null).

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create several JSON values\n    json array = {1, 2, 3};\n    json object = {{\"A\", \"a\"}, {\"B\", \"b\"}};\n    json number = 17;\n    json string = \"foo\";\n    json null;\n\n    // output values and comparisons\n    std::cout << std::boolalpha;\n    std::cout << array << \" == nullptr \" << (array == nullptr) << '\\n';\n    std::cout << object << \" == nullptr \" << (object == nullptr) << '\\n';\n    std::cout << number << \" == nullptr \" << (number == nullptr) << '\\n';\n    std::cout << string << \" == nullptr \" << (string == nullptr) << '\\n';\n    std::cout << null << \" == nullptr \" << (null == nullptr) << '\\n';\n}\n

    Output:

    [1,2,3] == nullptr false\n{\"A\":\"a\",\"B\":\"b\"} == nullptr false\n17 == nullptr false\n\"foo\" == nullptr false\nnull == nullptr true\n
    "},{"location":"api/basic_json/operator_eq/#see-also","title":"See also","text":"
    • operator!= compare for inequality
    • operator<=> comparison: 3-way (C++20)
    • basic_json_view::operator== - the same comparison on a zero-copy view, without building a basic_json value for it
    "},{"location":"api/basic_json/operator_eq/#version-history","title":"Version history","text":"
    1. Added in version 1.0.0. Added C++20 member functions in version 3.11.0.
    2. Added in version 1.0.0. Added C++20 member functions in version 3.11.0. Made conditionally noexcept in version 3.13.0 unreleased; before, a throwing conversion called std::terminate.
    "},{"location":"api/basic_json/operator_ge/","title":"nlohmann::basic_json::operator>=","text":"
    // until C++20\nbool operator>=(const_reference lhs, const_reference rhs) noexcept;   // (1)\n\ntemplate<typename ScalarType>\nbool operator>=(const_reference lhs, const ScalarType rhs) noexcept(/* see below */); // (2)\n\ntemplate<typename ScalarType>\nbool operator>=(ScalarType lhs, const const_reference rhs) noexcept(/* see below */); // (2)\n

    1. Compares whether one JSON value lhs is greater than or equal to another JSON value rhs according to the following rules:

      • The comparison always yields false if (1) either operand is discarded, or (2) either operand is NaN and the other operand is either NaN or any other number.
      • Otherwise, returns the result of !(lhs < rhs) (see operator<).

    2. Compares whether a JSON value is greater than or equal to a scalar or a scalar is greater than or equal to a JSON value by converting the scalar to a JSON value and comparing both JSON values according to 1.

    "},{"location":"api/basic_json/operator_ge/#template-parameters","title":"Template parameters","text":"ScalarType a scalar type according to std::is_scalar<ScalarType>::value"},{"location":"api/basic_json/operator_ge/#parameters","title":"Parameters","text":"lhs (in) first value to consider rhs (in) second value to consider"},{"location":"api/basic_json/operator_ge/#return-value","title":"Return value","text":"

    whether lhs is greater than or equal to rhs

    "},{"location":"api/basic_json/operator_ge/#exception-safety","title":"Exception safety","text":"
    1. No-throw guarantee: this function never throws exceptions.
    2. No-throw guarantee if converting the scalar to a JSON value cannot throw, as for numbers, Booleans, and nullptr; the function is noexcept exactly in that case. Otherwise, it throws what the conversion throws, for example std::bad_alloc when converting a string, or out_of_range.410 for an enum value not mapped by NLOHMANN_JSON_SERIALIZE_ENUM_STRICT.
    "},{"location":"api/basic_json/operator_ge/#complexity","title":"Complexity","text":"

    Linear.

    "},{"location":"api/basic_json/operator_ge/#notes","title":"Notes","text":"

    Comparing NaN

    NaN values are unordered within the domain of numbers. The following comparisons all yield false: 1. Comparing a NaN with itself. 2. Comparing a NaN with another NaN. 3. Comparing a NaN and any other number.

    Operator overload resolution

    Since C++20 overload resolution will consider the rewritten candidate generated from operator<=>.

    Deprecation

    If JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON is defined to 1, the library declares a member bool operator>=(const_reference rhs) const noexcept in C++20 mode to emulate the legacy comparison of discarded values. This member is deprecated since version 3.11.0, together with the legacy comparison behavior.

    See the migration guide for how to update existing code.

    "},{"location":"api/basic_json/operator_ge/#examples","title":"Examples","text":"Example

    The example demonstrates comparing several JSON types.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create several JSON values\n    json array_1 = {1, 2, 3};\n    json array_2 = {1, 2, 4};\n    json object_1 = {{\"A\", \"a\"}, {\"B\", \"b\"}};\n    json object_2 = {{\"B\", \"b\"}, {\"A\", \"a\"}};\n    json number_1 = 17;\n    json number_2 = 17.0000000000001L;\n    json string_1 = \"foo\";\n    json string_2 = \"bar\";\n\n    // output values and comparisons\n    std::cout << std::boolalpha;\n    std::cout << array_1 << \" >= \" << array_2 << \" \" << (array_1 >= array_2) << '\\n';\n    std::cout << object_1 << \" >= \" << object_2 << \" \" << (object_1 >= object_2) << '\\n';\n    std::cout << number_1 << \" >= \" << number_2 << \" \" << (number_1 >= number_2) << '\\n';\n    std::cout << string_1 << \" >= \" << string_2 << \" \" << (string_1 >= string_2) << '\\n';\n}\n

    Output:

    [1,2,3] >= [1,2,4] false\n{\"A\":\"a\",\"B\":\"b\"} >= {\"A\":\"a\",\"B\":\"b\"} true\n17 >= 17.0000000000001 false\n\"foo\" >= \"bar\" true\n
    "},{"location":"api/basic_json/operator_ge/#see-also","title":"See also","text":"
    • operator<=> comparison: 3-way
    "},{"location":"api/basic_json/operator_ge/#version-history","title":"Version history","text":"
    1. Added in version 1.0.0. Conditionally removed since C++20 in version 3.11.0.
    2. Added in version 1.0.0. Conditionally removed since C++20 in version 3.11.0. Made conditionally noexcept in version 3.13.0 unreleased; before, a throwing conversion called std::terminate.
    "},{"location":"api/basic_json/operator_gt/","title":"nlohmann::basic_json::operator>","text":"
    // until C++20\nbool operator>(const_reference lhs, const_reference rhs) noexcept;   // (1)\n\ntemplate<typename ScalarType>\nbool operator>(const_reference lhs, const ScalarType rhs) noexcept(/* see below */); // (2)\n\ntemplate<typename ScalarType>\nbool operator>(ScalarType lhs, const const_reference rhs) noexcept(/* see below */); // (2)\n

    1. Compares whether one JSON value lhs is greater than another JSON value rhs according to the following rules:

      • The comparison always yields false if (1) either operand is discarded, or (2) either operand is NaN and the other operand is either NaN or any other number.
      • Otherwise, returns the result of !(lhs <= rhs) (see operator<=).

    2. Compares whether a JSON value is greater than a scalar or a scalar is greater than a JSON value by converting the scalar to a JSON value and comparing both JSON values according to 1.

    "},{"location":"api/basic_json/operator_gt/#template-parameters","title":"Template parameters","text":"ScalarType a scalar type according to std::is_scalar<ScalarType>::value"},{"location":"api/basic_json/operator_gt/#parameters","title":"Parameters","text":"lhs (in) first value to consider rhs (in) second value to consider"},{"location":"api/basic_json/operator_gt/#return-value","title":"Return value","text":"

    whether lhs is greater than rhs

    "},{"location":"api/basic_json/operator_gt/#exception-safety","title":"Exception safety","text":"
    1. No-throw guarantee: this function never throws exceptions.
    2. No-throw guarantee if converting the scalar to a JSON value cannot throw, as for numbers, Booleans, and nullptr; the function is noexcept exactly in that case. Otherwise, it throws what the conversion throws, for example std::bad_alloc when converting a string, or out_of_range.410 for an enum value not mapped by NLOHMANN_JSON_SERIALIZE_ENUM_STRICT.
    "},{"location":"api/basic_json/operator_gt/#complexity","title":"Complexity","text":"

    Linear.

    "},{"location":"api/basic_json/operator_gt/#notes","title":"Notes","text":"

    Comparing NaN

    NaN values are unordered within the domain of numbers. The following comparisons all yield false: 1. Comparing a NaN with itself. 2. Comparing a NaN with another NaN. 3. Comparing a NaN and any other number.

    Operator overload resolution

    Since C++20 overload resolution will consider the rewritten candidate generated from operator<=>.

    "},{"location":"api/basic_json/operator_gt/#examples","title":"Examples","text":"Example

    The example demonstrates comparing several JSON types.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create several JSON values\n    json array_1 = {1, 2, 3};\n    json array_2 = {1, 2, 4};\n    json object_1 = {{\"A\", \"a\"}, {\"B\", \"b\"}};\n    json object_2 = {{\"B\", \"b\"}, {\"A\", \"a\"}};\n    json number_1 = 17;\n    json number_2 = 17.0000000000001L;\n    json string_1 = \"foo\";\n    json string_2 = \"bar\";\n\n    // output values and comparisons\n    std::cout << std::boolalpha;\n    std::cout << array_1 << \" > \" << array_2 << \" \" << (array_1 > array_2) << '\\n';\n    std::cout << object_1 << \" > \" << object_2 << \" \" << (object_1 > object_2) << '\\n';\n    std::cout << number_1 << \" > \" << number_2 << \" \" << (number_1 > number_2) << '\\n';\n    std::cout << string_1 << \" > \" << string_2 << \" \" << (string_1 > string_2) << '\\n';\n}\n

    Output:

    [1,2,3] > [1,2,4] false\n{\"A\":\"a\",\"B\":\"b\"} > {\"A\":\"a\",\"B\":\"b\"} false\n17 > 17.0000000000001 false\n\"foo\" > \"bar\" true\n
    "},{"location":"api/basic_json/operator_gt/#see-also","title":"See also","text":"
    • operator<=> comparison: 3-way
    "},{"location":"api/basic_json/operator_gt/#version-history","title":"Version history","text":"
    1. Added in version 1.0.0. Conditionally removed since C++20 in version 3.11.0.
    2. Added in version 1.0.0. Conditionally removed since C++20 in version 3.11.0. Made conditionally noexcept in version 3.13.0 unreleased; before, a throwing conversion called std::terminate.
    "},{"location":"api/basic_json/operator_le/","title":"nlohmann::basic_json::operator<=","text":"
    // until C++20\nbool operator<=(const_reference lhs, const_reference rhs) noexcept;   // (1)\n\ntemplate<typename ScalarType>\nbool operator<=(const_reference lhs, const ScalarType rhs) noexcept(/* see below */); // (2)\n\ntemplate<typename ScalarType>\nbool operator<=(ScalarType lhs, const const_reference rhs) noexcept(/* see below */); // (2)\n

    1. Compares whether one JSON value lhs is less than or equal to another JSON value rhs according to the following rules:

      • The comparison always yields false if (1) either operand is discarded, or (2) either operand is NaN and the other operand is either NaN or any other number.
      • Otherwise, returns the result of !(rhs < lhs) (see operator<).

    2. Compares whether a JSON value is less than or equal to a scalar or a scalar is less than or equal to a JSON value by converting the scalar to a JSON value and comparing both JSON values according to 1.

    "},{"location":"api/basic_json/operator_le/#template-parameters","title":"Template parameters","text":"ScalarType a scalar type according to std::is_scalar<ScalarType>::value"},{"location":"api/basic_json/operator_le/#parameters","title":"Parameters","text":"lhs (in) first value to consider rhs (in) second value to consider"},{"location":"api/basic_json/operator_le/#return-value","title":"Return value","text":"

    whether lhs is less than or equal to rhs

    "},{"location":"api/basic_json/operator_le/#exception-safety","title":"Exception safety","text":"
    1. No-throw guarantee: this function never throws exceptions.
    2. No-throw guarantee if converting the scalar to a JSON value cannot throw, as for numbers, Booleans, and nullptr; the function is noexcept exactly in that case. Otherwise, it throws what the conversion throws, for example std::bad_alloc when converting a string, or out_of_range.410 for an enum value not mapped by NLOHMANN_JSON_SERIALIZE_ENUM_STRICT.
    "},{"location":"api/basic_json/operator_le/#complexity","title":"Complexity","text":"

    Linear.

    "},{"location":"api/basic_json/operator_le/#notes","title":"Notes","text":"

    Comparing NaN

    NaN values are unordered within the domain of numbers. The following comparisons all yield false: 1. Comparing a NaN with itself. 2. Comparing a NaN with another NaN. 3. Comparing a NaN and any other number.

    Operator overload resolution

    Since C++20 overload resolution will consider the rewritten candidate generated from operator<=>.

    Deprecation

    If JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON is defined to 1, the library declares a member bool operator<=(const_reference rhs) const noexcept in C++20 mode to emulate the legacy comparison of discarded values. This member is deprecated since version 3.11.0, together with the legacy comparison behavior.

    See the migration guide for how to update existing code.

    "},{"location":"api/basic_json/operator_le/#examples","title":"Examples","text":"Example

    The example demonstrates comparing several JSON types.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create several JSON values\n    json array_1 = {1, 2, 3};\n    json array_2 = {1, 2, 4};\n    json object_1 = {{\"A\", \"a\"}, {\"B\", \"b\"}};\n    json object_2 = {{\"B\", \"b\"}, {\"A\", \"a\"}};\n    json number_1 = 17;\n    json number_2 = 17.0000000000001L;\n    json string_1 = \"foo\";\n    json string_2 = \"bar\";\n\n    // output values and comparisons\n    std::cout << std::boolalpha;\n    std::cout << array_1 << \" <= \" << array_2 << \" \" << (array_1 <= array_2) << '\\n';\n    std::cout << object_1 << \" <= \" << object_2 << \" \" << (object_1 <= object_2) << '\\n';\n    std::cout << number_1 << \" <= \" << number_2 << \" \" << (number_1 <= number_2) << '\\n';\n    std::cout << string_1 << \" <= \" << string_2 << \" \" << (string_1 <= string_2) << '\\n';\n}\n

    Output:

    [1,2,3] <= [1,2,4] true\n{\"A\":\"a\",\"B\":\"b\"} <= {\"A\":\"a\",\"B\":\"b\"} true\n17 <= 17.0000000000001 true\n\"foo\" <= \"bar\" false\n
    "},{"location":"api/basic_json/operator_le/#see-also","title":"See also","text":"
    • operator<=> comparison: 3-way
    "},{"location":"api/basic_json/operator_le/#version-history","title":"Version history","text":"
    1. Added in version 1.0.0. Conditionally removed since C++20 in version 3.11.0.
    2. Added in version 1.0.0. Conditionally removed since C++20 in version 3.11.0. Made conditionally noexcept in version 3.13.0 unreleased; before, a throwing conversion called std::terminate.
    "},{"location":"api/basic_json/operator_lt/","title":"nlohmann::basic_json::operator<","text":"
    // until C++20\nbool operator<(const_reference lhs, const_reference rhs) noexcept;   // (1)\n\ntemplate<typename ScalarType>\nbool operator<(const_reference lhs, const ScalarType rhs) noexcept(/* see below */); // (2)\n\ntemplate<typename ScalarType>\nbool operator<(ScalarType lhs, const const_reference rhs) noexcept(/* see below */); // (2)\n

    1. Compares whether one JSON value lhs is less than another JSON value rhs according to the following rules:

      • If either operand is discarded, the comparison yields false.
      • If both operands have the same type, the values are compared using their respective operator<.
      • Integer and floating-point numbers are compared by their exact values, without converting the integer to floating-point. For instance, 9007199254740992.0 is less than 9007199254740993.
      • In case lhs and rhs have different types, the values are ignored and the order of the types is considered, which is:
        1. null
        2. boolean
        3. number (all types)
        4. object
        5. array
        6. string
        7. binary For instance, any boolean value is considered less than any string.

    2. Compares whether a JSON value is less than a scalar or a scalar is less than a JSON value by converting the scalar to a JSON value and comparing both JSON values according to 1.

    "},{"location":"api/basic_json/operator_lt/#template-parameters","title":"Template parameters","text":"ScalarType a scalar type according to std::is_scalar<ScalarType>::value"},{"location":"api/basic_json/operator_lt/#parameters","title":"Parameters","text":"lhs (in) first value to consider rhs (in) second value to consider"},{"location":"api/basic_json/operator_lt/#return-value","title":"Return value","text":"

    whether lhs is less than rhs

    "},{"location":"api/basic_json/operator_lt/#exception-safety","title":"Exception safety","text":"
    1. No-throw guarantee: this function never throws exceptions.
    2. No-throw guarantee if converting the scalar to a JSON value cannot throw, as for numbers, Booleans, and nullptr; the function is noexcept exactly in that case. Otherwise, it throws what the conversion throws, for example std::bad_alloc when converting a string, or out_of_range.410 for an enum value not mapped by NLOHMANN_JSON_SERIALIZE_ENUM_STRICT.
    "},{"location":"api/basic_json/operator_lt/#complexity","title":"Complexity","text":"

    Linear.

    "},{"location":"api/basic_json/operator_lt/#notes","title":"Notes","text":"

    Comparing NaN

    NaN values are unordered within the domain of numbers. The following comparisons all yield false: 1. Comparing a NaN with itself. 2. Comparing a NaN with another NaN. 3. Comparing a NaN and any other number.

    Operator overload resolution

    Since C++20 overload resolution will consider the rewritten candidate generated from operator<=>.

    "},{"location":"api/basic_json/operator_lt/#examples","title":"Examples","text":"Example

    The example demonstrates comparing several JSON types.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create several JSON values\n    json array_1 = {1, 2, 3};\n    json array_2 = {1, 2, 4};\n    json object_1 = {{\"A\", \"a\"}, {\"B\", \"b\"}};\n    json object_2 = {{\"B\", \"b\"}, {\"A\", \"a\"}};\n    json number_1 = 17;\n    json number_2 = 17.0000000000001L;\n    json string_1 = \"foo\";\n    json string_2 = \"bar\";\n\n    // output values and comparisons\n    std::cout << std::boolalpha;\n    std::cout << array_1 << \" == \" << array_2 << \" \" << (array_1 < array_2) << '\\n';\n    std::cout << object_1 << \" == \" << object_2 << \" \" << (object_1 < object_2) << '\\n';\n    std::cout << number_1 << \" == \" << number_2 << \" \" << (number_1 < number_2) << '\\n';\n    std::cout << string_1 << \" == \" << string_2 << \" \" << (string_1 < string_2) << '\\n';\n}\n

    Output:

    [1,2,3] == [1,2,4] true\n{\"A\":\"a\",\"B\":\"b\"} == {\"A\":\"a\",\"B\":\"b\"} false\n17 == 17.0000000000001 true\n\"foo\" == \"bar\" false\n
    "},{"location":"api/basic_json/operator_lt/#see-also","title":"See also","text":"
    • operator<=> comparison: 3-way
    "},{"location":"api/basic_json/operator_lt/#version-history","title":"Version history","text":"
    1. Added in version 1.0.0. Conditionally removed since C++20 in version 3.11.0.
    2. Added in version 1.0.0. Conditionally removed since C++20 in version 3.11.0. Made conditionally noexcept in version 3.13.0 unreleased; before, a throwing conversion called std::terminate.
    "},{"location":"api/basic_json/operator_ne/","title":"nlohmann::basic_json::operator!=","text":"
    // until C++20\nbool operator!=(const_reference lhs, const_reference rhs) noexcept;   // (1)\n\ntemplate<typename ScalarType>\nbool operator!=(const_reference lhs, const ScalarType rhs) noexcept(/* see below */); // (2)\n\ntemplate<typename ScalarType>\nbool operator!=(ScalarType lhs, const const_reference rhs) noexcept(/* see below */); // (2)\n

    1. Compares two JSON values for inequality. Returns !(lhs == rhs).

      • This means the comparison is simply the logical negation of operator==, including for special values like NaN and discarded.

    2. Compares a JSON value and a scalar or a scalar and a JSON value for inequality by converting the scalar to a JSON value and comparing both JSON values according to 1.

    "},{"location":"api/basic_json/operator_ne/#template-parameters","title":"Template parameters","text":"ScalarType a scalar type according to std::is_scalar<ScalarType>::value"},{"location":"api/basic_json/operator_ne/#parameters","title":"Parameters","text":"lhs (in) first value to consider rhs (in) second value to consider"},{"location":"api/basic_json/operator_ne/#return-value","title":"Return value","text":"

    whether the values lhs/*this and rhs are not equal

    "},{"location":"api/basic_json/operator_ne/#exception-safety","title":"Exception safety","text":"
    1. No-throw guarantee: this function never throws exceptions.
    2. No-throw guarantee if converting the scalar to a JSON value cannot throw, as for numbers, Booleans, and nullptr; the function is noexcept exactly in that case. Otherwise, it throws what the conversion throws, for example std::bad_alloc when converting a string, or out_of_range.410 for an enum value not mapped by NLOHMANN_JSON_SERIALIZE_ENUM_STRICT.
    "},{"location":"api/basic_json/operator_ne/#complexity","title":"Complexity","text":"

    Linear.

    "},{"location":"api/basic_json/operator_ne/#notes","title":"Notes","text":"

    C++20

    Since C++20, basic_json declares no operator!=. The compiler rewrites a != b as !(a == b) using operator==, so the result is the same as described above.

    Comparing NaN and discarded

    Since operator!= is defined as !(a == b), the behavior for special values follows that of operator==:

    • For NaN values: NaN == NaN yields false, so NaN != NaN yields true.
    • For discarded values: discarded == x yields false for any x, so discarded != x yields true.
    "},{"location":"api/basic_json/operator_ne/#examples","title":"Examples","text":"Example: (1) compare JSON values

    The example demonstrates comparing several JSON types.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create several JSON values\n    json array_1 = {1, 2, 3};\n    json array_2 = {1, 2, 4};\n    json object_1 = {{\"A\", \"a\"}, {\"B\", \"b\"}};\n    json object_2 = {{\"B\", \"b\"}, {\"A\", \"a\"}};\n    json number_1 = 17;\n    json number_2 = 17.000000000000001L;\n    json string_1 = \"foo\";\n    json string_2 = \"bar\";\n\n    // output values and comparisons\n    std::cout << std::boolalpha;\n    std::cout << array_1 << \" != \" << array_2 << \" \" << (array_1 != array_2) << '\\n';\n    std::cout << object_1 << \" != \" << object_2 << \" \" << (object_1 != object_2) << '\\n';\n    std::cout << number_1 << \" != \" << number_2 << \" \" << (number_1 != number_2) << '\\n';\n    std::cout << string_1 << \" != \" << string_2 << \" \" << (string_1 != string_2) << '\\n';\n}\n

    Output:

    [1,2,3] != [1,2,4] true\n{\"A\":\"a\",\"B\":\"b\"} != {\"A\":\"a\",\"B\":\"b\"} false\n17 != 17.0 false\n\"foo\" != \"bar\" true\n
    Example: (2) compare JSON values with nullptr

    The example demonstrates comparing several JSON types against the null pointer (JSON null).

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create several JSON values\n    json array = {1, 2, 3};\n    json object = {{\"A\", \"a\"}, {\"B\", \"b\"}};\n    json number = 17;\n    json string = \"foo\";\n    json null;\n\n    // output values and comparisons\n    std::cout << std::boolalpha;\n    std::cout << array << \" != nullptr \" << (array != nullptr) << '\\n';\n    std::cout << object << \" != nullptr \" << (object != nullptr) << '\\n';\n    std::cout << number << \" != nullptr \" << (number != nullptr) << '\\n';\n    std::cout << string << \" != nullptr \" << (string != nullptr) << '\\n';\n    std::cout << null << \" != nullptr \" << (null != nullptr) << '\\n';\n}\n

    Output:

    [1,2,3] != nullptr true\n{\"A\":\"a\",\"B\":\"b\"} != nullptr true\n17 != nullptr true\n\"foo\" != nullptr true\nnull != nullptr false\n
    "},{"location":"api/basic_json/operator_ne/#see-also","title":"See also","text":"
    • operator== comparison: equal
    • operator<=> comparison: 3-way (C++20)
    • basic_json_view::operator!= - the same comparison on a zero-copy view, without building a basic_json value for it
    "},{"location":"api/basic_json/operator_ne/#version-history","title":"Version history","text":"
    1. Added in version 1.0.0. Added a C++20 member function in version 3.11.0. Changed in version 3.13.0 unreleased to remove special-casing for NaN and discarded values; operator!= now consistently means !(a == b). Removed the C++20 member function in version 3.13.0 unreleased; since C++20, the compiler rewrites a != b using operator==.
    2. Added in version 1.0.0. Changed in version 3.13.0 unreleased to remove special-casing for NaN and discarded values; operator!= now consistently means !(a == b). Since C++20, the compiler rewrites a != b using operator==. Made conditionally noexcept in version 3.13.0 unreleased; before, a throwing conversion called std::terminate.
    "},{"location":"api/basic_json/operator_spaceship/","title":"nlohmann::basic_json::operator<=>","text":"
    // since C++20\nstd::partial_ordering operator<=>(const_reference rhs) const noexcept;                   // (1)\n\ntemplate<typename ScalarType>\nstd::partial_ordering operator<=>(const ScalarType rhs) const noexcept(/* see below */); // (2)\n

    1. 3-way compares two JSON values producing a result of type std::partial_ordering according to the following rules:

      • Two JSON values compare with a result of std::partial_ordering::unordered if either value is discarded.
      • If both JSON values are of the same type, the result is produced by 3-way comparing their stored values using their respective operator<=>.
      • Integer and floating-point numbers are 3-way compared by their exact values, without converting the integer to floating-point. For instance, 9007199254740993 compares greater than 9007199254740992.0.
      • Otherwise, yields a result by comparing the type (see value_t).

    2. 3-way compares a JSON value and a scalar or a scalar and a JSON value by converting the scalar to a JSON value and 3-way comparing both JSON values (see 1).

    "},{"location":"api/basic_json/operator_spaceship/#template-parameters","title":"Template parameters","text":"ScalarType a scalar type according to std::is_scalar<ScalarType>::value"},{"location":"api/basic_json/operator_spaceship/#parameters","title":"Parameters","text":"rhs (in) second value to consider"},{"location":"api/basic_json/operator_spaceship/#return-value","title":"Return value","text":"

    the std::partial_ordering of the 3-way comparison of *this and rhs

    "},{"location":"api/basic_json/operator_spaceship/#exception-safety","title":"Exception safety","text":"
    1. No-throw guarantee: this function never throws exceptions.
    2. No-throw guarantee if converting the scalar to a JSON value cannot throw, as for numbers, Booleans, and nullptr; the function is noexcept exactly in that case. Otherwise, it throws what the conversion throws, for example std::bad_alloc when converting a string, or out_of_range.410 for an enum value not mapped by NLOHMANN_JSON_SERIALIZE_ENUM_STRICT.
    "},{"location":"api/basic_json/operator_spaceship/#complexity","title":"Complexity","text":"

    Linear.

    "},{"location":"api/basic_json/operator_spaceship/#notes","title":"Notes","text":"

    Comparing NaN

    • NaN values are unordered within the domain of numbers. The following comparisons all yield std::partial_ordering::unordered:
      1. Comparing a NaN with itself.
      2. Comparing a NaN with another NaN.
      3. Comparing a NaN and any other number.
    "},{"location":"api/basic_json/operator_spaceship/#examples","title":"Examples","text":"Example: (1) comparing JSON values

    The example demonstrates comparing several JSON values.

    #include <compare>\n#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nconst char* to_string(const std::partial_ordering& po)\n{\n    if (std::is_lt(po))\n    {\n        return \"less\";\n    }\n    else if (std::is_gt(po))\n    {\n        return \"greater\";\n    }\n    else if (std::is_eq(po))\n    {\n        return \"equivalent\";\n    }\n    return \"unordered\";\n}\n\nint main()\n{\n    // create several JSON values\n    json array_1 = {1, 2, 3};\n    json array_2 = {1, 2, 4};\n    json object_1 = {{\"A\", \"a\"}, {\"B\", \"b\"}};\n    json object_2 = {{\"B\", \"b\"}, {\"A\", \"a\"}};\n    json number = 17;\n    json string = \"foo\";\n    json discarded = json(json::value_t::discarded);\n\n    // output values and comparisons\n    std::cout << array_1 << \" <=> \" << array_2 << \" := \" << to_string(array_1 <=> array_2) << '\\n'; // *NOPAD*\n    std::cout << object_1 << \" <=> \" << object_2 << \" := \" << to_string(object_1 <=> object_2) << '\\n'; // *NOPAD*\n    std::cout << string << \" <=> \" << number << \" := \" << to_string(string <=> number) << '\\n'; // *NOPAD*\n    std::cout << string << \" <=> \" << discarded << \" := \" << to_string(string <=> discarded) << '\\n'; // *NOPAD*\n}\n

    Output:

    [1,2,3] <=> [1,2,4] := less\n{\"A\":\"a\",\"B\":\"b\"} <=> {\"A\":\"a\",\"B\":\"b\"} := equivalent\n\"foo\" <=> 17 := greater\n\"foo\" <=> <discarded> := unordered\n
    Example: (2) comparing JSON values and scalars

    The example demonstrates comparing several JSON values and scalars.

    #include <compare>\n#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nconst char* to_string(const std::partial_ordering& po)\n{\n    if (std::is_lt(po))\n    {\n        return \"less\";\n    }\n    else if (std::is_gt(po))\n    {\n        return \"greater\";\n    }\n    else if (std::is_eq(po))\n    {\n        return \"equivalent\";\n    }\n    return \"unordered\";\n}\n\nint main()\n{\n    using float_limits = std::numeric_limits<json::number_float_t>;\n    constexpr auto nan = float_limits::quiet_NaN();\n\n    // create several JSON values\n    json boolean = false;\n    json number = 17;\n    json string = \"17\";\n\n    // output values and comparisons\n    std::cout << std::boolalpha << std::fixed;\n    std::cout << boolean << \" <=> \" << true << \" := \" << to_string(boolean <=> true) << '\\n'; // *NOPAD*\n    std::cout << number << \" <=> \" << 17.0 << \" := \" << to_string(number <=> 17.0) << '\\n'; // *NOPAD*\n    std::cout << number << \" <=> \" << nan << \" := \" << to_string(number <=> nan) << '\\n'; // *NOPAD*\n    std::cout << string << \" <=> \" << 17 << \" := \" << to_string(string <=> 17) << '\\n'; // *NOPAD*\n}\n

    Output:

    false <=> true := less\n17 <=> 17.000000 := equivalent\n17 <=> nan := unordered\n\"17\" <=> 17 := greater\n
    "},{"location":"api/basic_json/operator_spaceship/#see-also","title":"See also","text":"
    • operator== - comparison: equal
    • operator!= - comparison: not equal
    • operator< - comparison: less than
    • operator<= - comparison: less than or equal
    • operator> - comparison: greater than
    • operator>= - comparison: greater than or equal
    "},{"location":"api/basic_json/operator_spaceship/#version-history","title":"Version history","text":"
    1. Added in version 3.11.0.
    2. Added in version 3.11.0. Made conditionally noexcept in version 3.13.0 unreleased; before, a throwing conversion called std::terminate.
    "},{"location":"api/basic_json/operator_value_t/","title":"nlohmann::basic_json::operator value_t","text":"
    constexpr operator value_t() const noexcept;\n

    Return the type of the JSON value as a value from the value_t enumeration.

    "},{"location":"api/basic_json/operator_value_t/#return-value","title":"Return value","text":"

    the type of the JSON value

    Value type return value null value_t::null boolean value_t::boolean string value_t::string number (integer) value_t::number_integer number (unsigned integer) value_t::number_unsigned number (floating-point) value_t::number_float object value_t::object array value_t::array binary value_t::binary discarded value_t::discarded"},{"location":"api/basic_json/operator_value_t/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this member function never throws exceptions.

    "},{"location":"api/basic_json/operator_value_t/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json/operator_value_t/#examples","title":"Examples","text":"Example

    The following code exemplifies operator value_t() for all JSON types.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_null;\n    json j_boolean = true;\n    json j_number_integer = -17;\n    json j_number_unsigned = 42u;\n    json j_number_float = 23.42;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n    json j_string = \"Hello, world\";\n\n    // call operator value_t()\n    json::value_t t_null = j_null;\n    json::value_t t_boolean = j_boolean;\n    json::value_t t_number_integer = j_number_integer;\n    json::value_t t_number_unsigned = j_number_unsigned;\n    json::value_t t_number_float = j_number_float;\n    json::value_t t_object = j_object;\n    json::value_t t_array = j_array;\n    json::value_t t_string = j_string;\n\n    // print types\n    std::cout << std::boolalpha;\n    std::cout << (t_null == json::value_t::null) << '\\n';\n    std::cout << (t_boolean == json::value_t::boolean) << '\\n';\n    std::cout << (t_number_integer == json::value_t::number_integer) << '\\n';\n    std::cout << (t_number_unsigned == json::value_t::number_unsigned) << '\\n';\n    std::cout << (t_number_float == json::value_t::number_float) << '\\n';\n    std::cout << (t_object == json::value_t::object) << '\\n';\n    std::cout << (t_array == json::value_t::array) << '\\n';\n    std::cout << (t_string == json::value_t::string) << '\\n';\n}\n

    Output:

    true\ntrue\ntrue\ntrue\ntrue\ntrue\ntrue\ntrue\n
    "},{"location":"api/basic_json/operator_value_t/#see-also","title":"See also","text":"
    • type named member function equivalent to this implicit conversion operator
    • value_t the enumeration of JSON types
    "},{"location":"api/basic_json/operator_value_t/#version-history","title":"Version history","text":"
    • Added in version 1.0.0.
    • Added unsigned integer type in version 2.0.0.
    • Added binary type in version 3.8.0.
    "},{"location":"api/basic_json/other_error/","title":"nlohmann::basic_json::other_error","text":"
    class other_error : public exception;\n

    This exception is thrown in case of errors that cannot be classified with the other exception types.

    Exceptions have ids 5xx (see list of other errors).

    classDiagram\n  direction LR\n\n    class std_exception [\"std::exception\"] {\n        <<interface>>\n    }\n\n    class json_exception [\"basic_json::exception\"] {\n        +const int id\n        +const char* what() const\n    }\n\n    class json_parse_error [\"basic_json::parse_error\"] {\n        +const std::size_t byte\n    }\n\n    class json_invalid_iterator [\"basic_json::invalid_iterator\"]\n    class json_type_error [\"basic_json::type_error\"]\n    class json_out_of_range [\"basic_json::out_of_range\"]\n    class json_other_error [\"basic_json::other_error\"]\n\n    std_exception <|-- json_exception\n    json_exception <|-- json_parse_error\n    json_exception <|-- json_invalid_iterator\n    json_exception <|-- json_type_error\n    json_exception <|-- json_out_of_range\n    json_exception <|-- json_other_error\n\n    style json_other_error fill:#CCCCFF
    "},{"location":"api/basic_json/other_error/#member-functions","title":"Member functions","text":"
    • what - returns explanatory string
    "},{"location":"api/basic_json/other_error/#member-variables","title":"Member variables","text":"
    • id - the id of the exception
    "},{"location":"api/basic_json/other_error/#examples","title":"Examples","text":"Example

    The following code shows how a other_error exception can be caught.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    try\n    {\n        // executing a failing JSON Patch operation\n        json value = R\"({\n            \"best_biscuit\": {\n                \"name\": \"Oreo\"\n            }\n        })\"_json;\n        json patch = R\"([{\n            \"op\": \"test\",\n            \"path\": \"/best_biscuit/name\",\n            \"value\": \"Choco Leibniz\"\n        }])\"_json;\n        value.patch(patch);\n    }\n    catch (const json::other_error& e)\n    {\n        // output exception information\n        std::cout << \"message: \" << e.what() << '\\n'\n                  << \"exception id: \" << e.id << std::endl;\n    }\n}\n

    Output:

    message: [json.exception.other_error.501] unsuccessful: {\"op\":\"test\",\"path\":\"/best_biscuit/name\",\"value\":\"Choco Leibniz\"}\nexception id: 501\n
    "},{"location":"api/basic_json/other_error/#see-also","title":"See also","text":"
    • exception for the base class of all exceptions thrown by the library
    • List of other errors
    • parse_error for exceptions indicating a parse error
    • invalid_iterator for exceptions indicating errors with iterators
    • type_error for exceptions indicating executing a member function with a wrong type
    • out_of_range for exceptions indicating access out of the defined range
    "},{"location":"api/basic_json/other_error/#version-history","title":"Version history","text":"
    • Since version 3.0.0.
    "},{"location":"api/basic_json/out_of_range/","title":"nlohmann::basic_json::out_of_range","text":"
    class out_of_range : public exception;\n

    This exception is thrown in case a library function is called on an input parameter that exceeds the expected range, for instance, in the case of array indices or nonexisting object keys.

    Exceptions have ids 4xx (see list of out-of-range errors).

    classDiagram\n  direction LR\n\n    class std_exception [\"std::exception\"] {\n        <<interface>>\n    }\n\n    class json_exception [\"basic_json::exception\"] {\n        +const int id\n        +const char* what() const\n    }\n\n    class json_parse_error [\"basic_json::parse_error\"] {\n        +const std::size_t byte\n    }\n\n    class json_invalid_iterator [\"basic_json::invalid_iterator\"]\n    class json_type_error [\"basic_json::type_error\"]\n    class json_out_of_range [\"basic_json::out_of_range\"]\n    class json_other_error [\"basic_json::other_error\"]\n\n    std_exception <|-- json_exception\n    json_exception <|-- json_parse_error\n    json_exception <|-- json_invalid_iterator\n    json_exception <|-- json_type_error\n    json_exception <|-- json_out_of_range\n    json_exception <|-- json_other_error\n\n    style json_out_of_range fill:#CCCCFF
    "},{"location":"api/basic_json/out_of_range/#member-functions","title":"Member functions","text":"
    • what - returns explanatory string
    "},{"location":"api/basic_json/out_of_range/#member-variables","title":"Member variables","text":"
    • id - the id of the exception
    "},{"location":"api/basic_json/out_of_range/#examples","title":"Examples","text":"Example

    The following code shows how a out_of_range exception can be caught.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    try\n    {\n        // calling at() for an invalid index\n        json j = {1, 2, 3, 4};\n        j.at(4) = 10;\n    }\n    catch (const json::out_of_range& e)\n    {\n        // output exception information\n        std::cout << \"message: \" << e.what() << '\\n'\n                  << \"exception id: \" << e.id << std::endl;\n    }\n}\n

    Output:

    message: [json.exception.out_of_range.401] array index 4 is out of range\nexception id: 401\n
    "},{"location":"api/basic_json/out_of_range/#see-also","title":"See also","text":"
    • exception for the base class of all exceptions thrown by the library
    • List of out-of-range errors
    • parse_error for exceptions indicating a parse error
    • invalid_iterator for exceptions indicating errors with iterators
    • type_error for exceptions indicating executing a member function with a wrong type
    • other_error for exceptions indicating other library errors
    "},{"location":"api/basic_json/out_of_range/#version-history","title":"Version history","text":"
    • Since version 3.0.0.
    "},{"location":"api/basic_json/parse/","title":"nlohmann::basic_json::parse","text":"
    // (1)\ntemplate<typename InputType>\nstatic basic_json parse(InputType&& i,\n                        const parser_callback_t cb = nullptr,\n                        const bool allow_exceptions = true,\n                        const bool ignore_comments = false,\n                        const bool ignore_trailing_commas = false);\n\n// (2)\ntemplate<typename IteratorType, typename SentinelType = IteratorType>\nstatic basic_json parse(IteratorType first, SentinelType last,\n                        const parser_callback_t cb = nullptr,\n                        const bool allow_exceptions = true,\n                        const bool ignore_comments = false,\n                        const bool ignore_trailing_commas = false);\n
    1. Deserialize from a compatible input.

    2. Deserialize from a pair of character iterators, or an iterator and a sentinel of a different type (C++20 ranges support)

      The value_type of the iterator must be an integral type with size of 1, 2, or 4 bytes, which will be interpreted respectively as UTF-8, UTF-16, and UTF-32. If SentinelType differs from IteratorType, it must be comparable to the iterator type with operator!=.

    "},{"location":"api/basic_json/parse/#template-parameters","title":"Template parameters","text":"InputType

    A compatible input, for instance:

    • an std::istream object
    • a FILE pointer (throws if null)
    • a C-style array of characters
    • a pointer to a null-terminated string of single byte characters (throws if null)
    • a std::string
    • a container obj for which begin(obj) and end(obj) produce a valid pair of iterators (as found via ADL or member functions, with semantics compatible to std::begin and std::end)
    IteratorType

    a compatible iterator type, for instance.

    • a pair of std::string::iterator or std::vector<std::uint8_t>::iterator
    • a pair of pointers such as ptr and ptr + len
    SentinelType

    defaults to IteratorType; may be a different type comparable to IteratorType via operator!=, for instance.

    • a custom sentinel type for C++20 ranges
    • std::default_sentinel_t, when IteratorType is std::counted_iterator
    "},{"location":"api/basic_json/parse/#parameters","title":"Parameters","text":"i (in) Input to parse from. cb (in) a parser callback function of type parser_callback_t which is used to control the deserialization by filtering unwanted values (optional) allow_exceptions (in) whether to throw exceptions in case of a parse error (optional, true by default) ignore_comments (in) whether comments should be ignored and treated like whitespace (true) or yield a parse error (false); (optional, false by default) ignore_trailing_commas (in) whether trailing commas in arrays or objects should be ignored and treated like whitespace (true) or yield a parse error (false); (optional, false by default) first (in) iterator to the start of a character range last (in) iterator to the end of a character range, or a sentinel value that compares equal to the end iterator with operator!="},{"location":"api/basic_json/parse/#return-value","title":"Return value","text":"

    Deserialized JSON value; in case of a parse error and allow_exceptions set to false, the return value will be value_t::discarded. The latter can be checked with is_discarded.

    "},{"location":"api/basic_json/parse/#exception-safety","title":"Exception safety","text":"

    Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

    "},{"location":"api/basic_json/parse/#exceptions","title":"Exceptions","text":"
    • Throws parse_error.101 in case of an unexpected token, or empty input like a null FILE* or char* pointer, or an std::istream without a stream buffer (i.rdbuf() == nullptr, for instance std::istream(nullptr)).
    • If reading from an std::istream reaches the end of the input and eofbit is part of the stream's exceptions() mask, the std::ios_base::failure thrown by the stream itself propagates instead of a parse_error, the same as it would for the standard library's own extraction operators.
    "},{"location":"api/basic_json/parse/#complexity","title":"Complexity","text":"

    Linear in the length of the input. The parser is a predictive LL(1) parser. The complexity can be higher if the parser callback function cb or reading from (1) the input i or (2) the iterator range [first, last] has a super-linear complexity.

    "},{"location":"api/basic_json/parse/#notes","title":"Notes","text":"

    A UTF-8 byte order mark is silently ignored.

    Invalid Unicode escapes and unpaired surrogates in the input are reported as parse_error.101 with a detailed message.

    By default, a '\\0' (NUL) byte anywhere in the input is treated as end of input, rather than as an ordinary (and, outside of a string, invalid) byte; see the FAQ entry for details and the JSON_STRICT_NUL_HANDLING macro to opt into rejecting it instead.

    Performance

    Contiguous inputs (strings, string views, vectors, arrays, pointer ranges) are the fastest to parse. A std::istream comes close: the parser scans the characters its stream buffer holds in bulk, without reading ahead, so the stream is left exactly where it would be after reading one character at a time. A FILE* is read one character at a time, but the file is locked only once, so other threads that use the same FILE object wait until parsing is done. See input sources and performance, JSON_DISABLE_STREAM_BULK_SCAN, and JSON_DISABLE_UNLOCKED_FILE_IO.

    "},{"location":"api/basic_json/parse/#examples","title":"Examples","text":"Example: (1) parse from a character array

    The example below demonstrates the parse() function reading from an array.

    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // a JSON text\n    char text[] = R\"(\n    {\n        \"Image\": {\n            \"Width\":  800,\n            \"Height\": 600,\n            \"Title\":  \"View from 15th Floor\",\n            \"Thumbnail\": {\n                \"Url\":    \"http://www.example.com/image/481989943\",\n                \"Height\": 125,\n                \"Width\":  100\n            },\n            \"Animated\" : false,\n            \"IDs\": [116, 943, 234, 38793]\n        }\n    }\n    )\";\n\n    // parse and serialize JSON\n    json j_complete = json::parse(text);\n    std::cout << std::setw(4) << j_complete << \"\\n\\n\";\n}\n

    Output:

    {\n    \"Image\": {\n        \"Animated\": false,\n        \"Height\": 600,\n        \"IDs\": [\n            116,\n            943,\n            234,\n            38793\n        ],\n        \"Thumbnail\": {\n            \"Height\": 125,\n            \"Url\": \"http://www.example.com/image/481989943\",\n            \"Width\": 100\n        },\n        \"Title\": \"View from 15th Floor\",\n        \"Width\": 800\n    }\n}\n
    Example: (1) parse from a string

    The example below demonstrates the parse() function with and without callback function.

    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // a JSON text\n    auto text = R\"(\n    {\n        \"Image\": {\n            \"Width\":  800,\n            \"Height\": 600,\n            \"Title\":  \"View from 15th Floor\",\n            \"Thumbnail\": {\n                \"Url\":    \"http://www.example.com/image/481989943\",\n                \"Height\": 125,\n                \"Width\":  100\n            },\n            \"Animated\" : false,\n            \"IDs\": [116, 943, 234, 38793]\n        }\n    }\n    )\";\n\n    // parse and serialize JSON\n    json j_complete = json::parse(text);\n    std::cout << std::setw(4) << j_complete << \"\\n\\n\";\n\n    // define parser callback\n    json::parser_callback_t cb = [](int depth, json::parse_event_t event, json & parsed)\n    {\n        // skip object elements with key \"Thumbnail\"\n        if (event == json::parse_event_t::key and parsed == json(\"Thumbnail\"))\n        {\n            return false;\n        }\n        else\n        {\n            return true;\n        }\n    };\n\n    // parse (with callback) and serialize JSON\n    json j_filtered = json::parse(text, cb);\n    std::cout << std::setw(4) << j_filtered << '\\n';\n}\n

    Output:

    {\n    \"Image\": {\n        \"Animated\": false,\n        \"Height\": 600,\n        \"IDs\": [\n            116,\n            943,\n            234,\n            38793\n        ],\n        \"Thumbnail\": {\n            \"Height\": 125,\n            \"Url\": \"http://www.example.com/image/481989943\",\n            \"Width\": 100\n        },\n        \"Title\": \"View from 15th Floor\",\n        \"Width\": 800\n    }\n}\n\n{\n    \"Image\": {\n        \"Animated\": false,\n        \"Height\": 600,\n        \"IDs\": [\n            116,\n            943,\n            234,\n            38793\n        ],\n        \"Title\": \"View from 15th Floor\",\n        \"Width\": 800\n    }\n}\n
    Example: (1) parse from an input stream

    The example below demonstrates the parse() function with and without callback function.

    #include <iostream>\n#include <iomanip>\n#include <sstream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // a JSON text\n    auto text = R\"(\n    {\n        \"Image\": {\n            \"Width\":  800,\n            \"Height\": 600,\n            \"Title\":  \"View from 15th Floor\",\n            \"Thumbnail\": {\n                \"Url\":    \"http://www.example.com/image/481989943\",\n                \"Height\": 125,\n                \"Width\":  100\n            },\n            \"Animated\" : false,\n            \"IDs\": [116, 943, 234, 38793]\n        }\n    }\n    )\";\n\n    // fill a stream with JSON text\n    std::stringstream ss;\n    ss << text;\n\n    // parse and serialize JSON\n    json j_complete = json::parse(ss);\n    std::cout << std::setw(4) << j_complete << \"\\n\\n\";\n\n    // define parser callback\n    json::parser_callback_t cb = [](int depth, json::parse_event_t event, json & parsed)\n    {\n        // skip object elements with key \"Thumbnail\"\n        if (event == json::parse_event_t::key and parsed == json(\"Thumbnail\"))\n        {\n            return false;\n        }\n        else\n        {\n            return true;\n        }\n    };\n\n    // fill a stream with JSON text\n    ss.clear();\n    ss << text;\n\n    // parse (with callback) and serialize JSON\n    json j_filtered = json::parse(ss, cb);\n    std::cout << std::setw(4) << j_filtered << '\\n';\n}\n

    Output:

    {\n    \"Image\": {\n        \"Animated\": false,\n        \"Height\": 600,\n        \"IDs\": [\n            116,\n            943,\n            234,\n            38793\n        ],\n        \"Thumbnail\": {\n            \"Height\": 125,\n            \"Url\": \"http://www.example.com/image/481989943\",\n            \"Width\": 100\n        },\n        \"Title\": \"View from 15th Floor\",\n        \"Width\": 800\n    }\n}\n\n{\n    \"Image\": {\n        \"Animated\": false,\n        \"Height\": 600,\n        \"IDs\": [\n            116,\n            943,\n            234,\n            38793\n        ],\n        \"Title\": \"View from 15th Floor\",\n        \"Width\": 800\n    }\n}\n
    Example: (1) parse from a contiguous container

    The example below demonstrates the parse() function reading from a contiguous container.

    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // a JSON text given as std::vector\n    std::vector<std::uint8_t> text = {'[', '1', ',', '2', ',', '3', ']', '\\0'};\n\n    // parse and serialize JSON\n    json j_complete = json::parse(text);\n    std::cout << std::setw(4) << j_complete << \"\\n\\n\";\n}\n

    Output:

    [\n    1,\n    2,\n    3\n]\n
    Example: (2) parse from a non-null-terminated string

    The example below demonstrates the parse() function reading from a string that is not null-terminated.

    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // a JSON text given as string that is not null-terminated\n    const char* ptr = \"[1,2,3]another value\";\n\n    // parse and serialize JSON\n    json j_complete = json::parse(ptr, ptr + 7);\n    std::cout << std::setw(4) << j_complete << \"\\n\\n\";\n}\n

    Output:

    [\n    1,\n    2,\n    3\n]\n
    Example: (2) parse from an iterator pair

    The example below demonstrates the parse() function reading from an iterator pair.

    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // a JSON text given an input with other values\n    std::vector<std::uint8_t> input = {'[', '1', ',', '2', ',', '3', ']', 'o', 't', 'h', 'e', 'r'};\n\n    // parse and serialize JSON\n    json j_complete = json::parse(input.begin(), input.begin() + 7);\n    std::cout << std::setw(4) << j_complete << \"\\n\\n\";\n}\n

    Output:

    [\n    1,\n    2,\n    3\n]\n
    Example: effect of allow_exceptions parameter

    The example below demonstrates the effect of the allow_exceptions parameter in the parse() function.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // an invalid JSON text\n    std::string text = R\"(\n    {\n        \"key\": \"value without closing quotes\n    }\n    )\";\n\n    // parse with exceptions\n    try\n    {\n        json j = json::parse(text);\n    }\n    catch (const json::parse_error& e)\n    {\n        std::cout << e.what() << std::endl;\n    }\n\n    // parse without exceptions\n    json j = json::parse(text, nullptr, false);\n\n    if (j.is_discarded())\n    {\n        std::cout << \"the input is invalid JSON\" << std::endl;\n    }\n    else\n    {\n        std::cout << \"the input is valid JSON: \" << j << std::endl;\n    }\n}\n

    Output:

    [json.exception.parse_error.101] parse error at line 4, column 0: syntax error while parsing value - invalid string: control character U+000A (LF) must be escaped to \\u000A or \\n; last read: '\"value without closing quotes<U+000A>'\nthe input is invalid JSON\n
    Example: effect of ignore_comments parameter

    The example below demonstrates the effect of the ignore_comments parameter in the parse() function.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::string s = R\"(\n    {\n        // update in 2006: removed Pluto\n        \"planets\": [\"Mercury\", \"Venus\", \"Earth\", \"Mars\",\n                    \"Jupiter\", \"Uranus\", \"Neptune\" /*, \"Pluto\" */]\n    }\n    )\";\n\n    try\n    {\n        json j = json::parse(s);\n    }\n    catch (json::exception& e)\n    {\n        std::cout << e.what() << std::endl;\n    }\n\n    json j = json::parse(s,\n                         /* callback */ nullptr,\n                         /* allow exceptions */ true,\n                         /* ignore_comments */ true);\n    std::cout << j.dump(2) << '\\n';\n}\n

    Output:

    [json.exception.parse_error.101] parse error at line 3, column 9: syntax error while parsing object key - invalid literal; last read: '<U+000A>    {<U+000A>        /'; expected string literal\n{\n  \"planets\": [\n    \"Mercury\",\n    \"Venus\",\n    \"Earth\",\n    \"Mars\",\n    \"Jupiter\",\n    \"Uranus\",\n    \"Neptune\"\n  ]\n}\n
    Example: effect of ignore_trailing_commas parameter

    The example below demonstrates the effect of the ignore_trailing_commas parameter in the parse() function.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::string s = R\"(\n    {\n        \"planets\": [\n            \"Mercury\",\n            \"Venus\",\n            \"Earth\",\n            \"Mars\",\n            \"Jupiter\",\n            \"Uranus\",\n            \"Neptune\",\n        ]\n    }\n    )\";\n\n    try\n    {\n        json j = json::parse(s);\n    }\n    catch (json::exception& e)\n    {\n        std::cout << e.what() << std::endl;\n    }\n\n    json j = json::parse(s,\n                         /* callback */ nullptr,\n                         /* allow exceptions */ true,\n                         /* ignore_comments */ false,\n                         /* ignore_trailing_commas */ true);\n    std::cout << j.dump(2) << '\\n';\n}\n

    Output:

    [json.exception.parse_error.101] parse error at line 11, column 9: syntax error while parsing value - unexpected ']'; expected '[', '{', or a literal\n{\n  \"planets\": [\n    \"Mercury\",\n    \"Venus\",\n    \"Earth\",\n    \"Mars\",\n    \"Jupiter\",\n    \"Uranus\",\n    \"Neptune\"\n  ]\n}\n
    "},{"location":"api/basic_json/parse/#see-also","title":"See also","text":"
    • accept - check if the input is valid JSON
    • sax_parse - parse input using the SAX interface
    • operator>> - deserialize from stream
    • JSON_STRICT_NUL_HANDLING - opt in to rejecting a NUL byte in the input instead of treating it as end of input
    • JSON_DISABLE_STREAM_BULK_SCAN - read a std::istream one character at a time
    • JSON_DISABLE_UNLOCKED_FILE_IO - read a FILE* with the locking stdio functions
    "},{"location":"api/basic_json/parse/#version-history","title":"Version history","text":"
    • Added in version 1.0.0.
    • Overload for contiguous containers (1) added in version 2.0.3.
    • Ignoring comments via ignore_comments added in version 3.9.0.
    • Changed runtime assertion in case of FILE* null pointers to exception in version 3.12.0.
    • Added ignore_trailing_commas in version 3.13.0 unreleased.
    • Extended container support (1) to include types with lvalue-only ADL begin/end (matching std::begin/std::end semantics) in version 3.13.0 unreleased.
    • Extended overload (2) to accept heterogeneous iterator+sentinel pairs (C++20 ranges support) in version 3.13.0 unreleased.
    • JSON_STRICT_NUL_HANDLING added in version 3.13.0 unreleased to optionally reject a NUL byte in the input instead of treating it as end of input; planned to become the default in version 4.0.0.
    • Extended empty-input detection to also cover an std::istream without a stream buffer, and fixed a crash (std::terminate) when parsing from an std::istream with eofbit in its exception mask, in version 3.13.0 unreleased.
    • Parsing from an std::istream scans the characters in its stream buffer in bulk in version 3.13.0 unreleased (JSON_DISABLE_STREAM_BULK_SCAN restores reading one character at a time).
    • Parsing from a FILE* locks the file once instead of once per character in version 3.13.0 unreleased (JSON_DISABLE_UNLOCKED_FILE_IO restores std::fgetc).

    Deprecation

    Overload (2) replaces calls to parse with a pair of iterators as their first parameter which has been deprecated in version 3.8.0. This overload will be removed in version 4.0.0. Please replace all calls like parse({ptr, ptr+len}, ...); with parse(ptr, ptr+len, ...);.

    You should be warned by your compiler with a -Wdeprecated-declarations warning if you are using a deprecated function.

    See the migration guide for how to update existing code.

    "},{"location":"api/basic_json/parse_error/","title":"nlohmann::basic_json::parse_error","text":"
    class parse_error : public exception;\n

    The library throws this exception when a parse error occurs. Parse errors can occur during the deserialization of JSON text, BJData, BON8, BSON, CBOR, MessagePack, UBJSON, as well as when using JSON Patch.

    Member byte holds the byte index of the last read character in the input file (see note below).

    Exceptions have ids 1xx (see list of parse errors).

    classDiagram\n  direction LR\n\n    class std_exception [\"std::exception\"] {\n        <<interface>>\n    }\n\n    class json_exception [\"basic_json::exception\"] {\n        +const int id\n        +const char* what() const\n    }\n\n    class json_parse_error [\"basic_json::parse_error\"] {\n        +const std::size_t byte\n    }\n\n    class json_invalid_iterator [\"basic_json::invalid_iterator\"]\n    class json_type_error [\"basic_json::type_error\"]\n    class json_out_of_range [\"basic_json::out_of_range\"]\n    class json_other_error [\"basic_json::other_error\"]\n\n    std_exception <|-- json_exception\n    json_exception <|-- json_parse_error\n    json_exception <|-- json_invalid_iterator\n    json_exception <|-- json_type_error\n    json_exception <|-- json_out_of_range\n    json_exception <|-- json_other_error\n\n    style json_parse_error fill:#CCCCFF
    "},{"location":"api/basic_json/parse_error/#member-functions","title":"Member functions","text":"
    • what - returns explanatory string
    "},{"location":"api/basic_json/parse_error/#member-variables","title":"Member variables","text":"
    • id - the id of the exception
    • byte - byte index of the parse error
    "},{"location":"api/basic_json/parse_error/#notes","title":"Notes","text":"

    For an input with n bytes, 1 is the index of the first character and n+1 is the index of the terminating null byte or the end of file. This also holds true when reading a byte vector for binary formats.

    "},{"location":"api/basic_json/parse_error/#examples","title":"Examples","text":"Example

    The following code shows how a parse_error exception can be caught.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    try\n    {\n        // parsing input with a syntax error\n        json j = json::parse(\"[1,2,3,]\");\n    }\n    catch (const json::parse_error& e)\n    {\n        // output exception information\n        std::cout << \"message: \" << e.what() << '\\n'\n                  << \"exception id: \" << e.id << '\\n'\n                  << \"byte position of error: \" << e.byte << std::endl;\n    }\n}\n

    Output:

    message: [json.exception.parse_error.101] parse error at line 1, column 8: syntax error while parsing value - unexpected ']'; expected '[', '{', or a literal\nexception id: 101\nbyte position of error: 8\n
    "},{"location":"api/basic_json/parse_error/#see-also","title":"See also","text":"
    • exception for the base class of all exceptions thrown by the library
    • List of parse errors
    • invalid_iterator for exceptions indicating errors with iterators
    • type_error for exceptions indicating executing a member function with a wrong type
    • out_of_range for exceptions indicating access out of the defined range
    • other_error for exceptions indicating other library errors
    "},{"location":"api/basic_json/parse_error/#version-history","title":"Version history","text":"
    • Since version 3.0.0.
    "},{"location":"api/basic_json/parse_event_t/","title":"nlohmann::basic_json::parse_event_t","text":"
    enum class parse_event_t : std::uint8_t {\n    object_start,\n    object_end,\n    array_start,\n    array_end,\n    key,\n    value\n};\n

    The parser callback distinguishes the following events:

    • object_start: the parser read { and started to process a JSON object
    • key: the parser read a key of a value in an object
    • object_end: the parser read } and finished processing a JSON object
    • array_start: the parser read [ and started to process a JSON array
    • array_end: the parser read ] and finished processing a JSON array
    • value: the parser finished reading a JSON value
    "},{"location":"api/basic_json/parse_event_t/#examples","title":"Examples","text":"Example

    The following code parses a small JSON text with a parser callback that reports every event together with its depth and keeps every value (by always returning true).

    #include <iostream>\n#include <string>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\n// translate a parse_event_t to a human-readable name\nstd::string event_name(json::parse_event_t event)\n{\n    switch (event)\n    {\n        case json::parse_event_t::object_start:\n            return \"object_start\";\n        case json::parse_event_t::object_end:\n            return \"object_end\";\n        case json::parse_event_t::array_start:\n            return \"array_start\";\n        case json::parse_event_t::array_end:\n            return \"array_end\";\n        case json::parse_event_t::key:\n            return \"key\";\n        case json::parse_event_t::value:\n            return \"value\";\n        default:\n            return \"unknown\";\n    }\n}\n\nint main()\n{\n    // a small JSON text\n    auto text = R\"({\"pi\": 3.141, \"numbers\": [1, 2]})\";\n\n    // parse the text and report every event together with its depth;\n    // returning true keeps every value unchanged\n    json j = json::parse(text, [](int depth, json::parse_event_t event, json& /*parsed*/)\n    {\n        std::cout << depth << \" \" << event_name(event) << '\\n';\n        return true;\n    });\n\n    // the callback did not change anything, so the parsed value is unaffected\n    std::cout << j << '\\n';\n}\n

    Output:

    0 object_start\n1 key\n1 value\n1 key\n1 array_start\n2 value\n2 value\n1 array_end\n0 object_end\n{\"numbers\":[1,2],\"pi\":3.141}\n
    "},{"location":"api/basic_json/parse_event_t/#see-also","title":"See also","text":"
    • parser_callback_t callback function type for the parser
    • parse deserialize from a compatible input
    "},{"location":"api/basic_json/parse_event_t/#version-history","title":"Version history","text":"
    • Added in version 1.0.0.
    "},{"location":"api/basic_json/parser_callback_t/","title":"nlohmann::basic_json::parser_callback_t","text":"
    template<typename BasicJsonType>\nusing parser_callback_t =\n    std::function<bool(int depth, parse_event_t event, BasicJsonType& parsed)>;\n

    With a parser callback function, the result of parsing a JSON text can be influenced. When passed to parse, it is called on certain events (passed as parse_event_t via parameter event) with a set recursion depth depth and context JSON value parsed. The return value of the callback function is a boolean indicating whether the element that emitted the callback shall be kept or not.

    We distinguish six scenarios (determined by the event type) in which the callback function can be called. The following table describes the values of the parameters depth, event, and parsed.

    parameter event description parameter depth parameter parsed parse_event_t::object_start the parser read { and started to process a JSON object depth of the parent of the JSON object a JSON value with type discarded parse_event_t::key the parser read a key of a value in an object depth of the currently parsed JSON object a JSON string containing the key parse_event_t::object_end the parser read } and finished processing a JSON object depth of the parent of the JSON object the parsed JSON object parse_event_t::array_start the parser read [ and started to process a JSON array depth of the parent of the JSON array a JSON value with type discarded parse_event_t::array_end the parser read ] and finished processing a JSON array depth of the parent of the JSON array the parsed JSON array parse_event_t::value the parser finished reading a JSON value depth of the value the parsed JSON value

    Discarding a value (i.e., returning false) has different effects depending on the context in which function was called:

    • Discarded values in structured types are skipped. That is, the parser will behave as if the discarded value was never read. This holds for every value type and for both kinds of parent: a discarded element is removed from the surrounding array, and a discarded member is removed from the surrounding object together with its key.
    • Arrays and objects can be discarded either at their parse_event_t::array_start/parse_event_t::object_start event or at their parse_event_t::array_end/parse_event_t::object_end event, and both remove the whole value. Discarding it at the start event also means the callback is called neither for the content of the value nor for its matching end event.
    • Discarding a parse_event_t::key event discards the whole object member. The callback is still called for the associated value, but its return value has no further effect.
    • In case a value outside a structured type is skipped, it is replaced with null. This case happens if the top-level element is skipped.
    "},{"location":"api/basic_json/parser_callback_t/#parameters","title":"Parameters","text":"depth (in) the depth of the recursion during parsing event (in) an event of type parse_event_t indicating the context in the callback function has been called parsed (in, out) the current intermediate parse result; note that writing to this value has no effect for parse_event_t::key events"},{"location":"api/basic_json/parser_callback_t/#return-value","title":"Return value","text":"

    Whether the JSON value which called the function during parsing should be kept (true) or not (false). In the latter case, it is skipped completely, or replaced by null if it is the top-level value.

    "},{"location":"api/basic_json/parser_callback_t/#examples","title":"Examples","text":"Example: skip an object key while parsing

    The example below demonstrates the parse() function with and without callback function.

    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // a JSON text\n    auto text = R\"(\n    {\n        \"Image\": {\n            \"Width\":  800,\n            \"Height\": 600,\n            \"Title\":  \"View from 15th Floor\",\n            \"Thumbnail\": {\n                \"Url\":    \"http://www.example.com/image/481989943\",\n                \"Height\": 125,\n                \"Width\":  100\n            },\n            \"Animated\" : false,\n            \"IDs\": [116, 943, 234, 38793]\n        }\n    }\n    )\";\n\n    // parse and serialize JSON\n    json j_complete = json::parse(text);\n    std::cout << std::setw(4) << j_complete << \"\\n\\n\";\n\n    // define parser callback\n    json::parser_callback_t cb = [](int depth, json::parse_event_t event, json & parsed)\n    {\n        // skip object elements with key \"Thumbnail\"\n        if (event == json::parse_event_t::key and parsed == json(\"Thumbnail\"))\n        {\n            return false;\n        }\n        else\n        {\n            return true;\n        }\n    };\n\n    // parse (with callback) and serialize JSON\n    json j_filtered = json::parse(text, cb);\n    std::cout << std::setw(4) << j_filtered << '\\n';\n}\n

    Output:

    {\n    \"Image\": {\n        \"Animated\": false,\n        \"Height\": 600,\n        \"IDs\": [\n            116,\n            943,\n            234,\n            38793\n        ],\n        \"Thumbnail\": {\n            \"Height\": 125,\n            \"Url\": \"http://www.example.com/image/481989943\",\n            \"Width\": 100\n        },\n        \"Title\": \"View from 15th Floor\",\n        \"Width\": 800\n    }\n}\n\n{\n    \"Image\": {\n        \"Animated\": false,\n        \"Height\": 600,\n        \"IDs\": [\n            116,\n            943,\n            234,\n            38793\n        ],\n        \"Title\": \"View from 15th Floor\",\n        \"Width\": 800\n    }\n}\n
    Example: how discarded values are removed

    The example below shows where discarded values are removed. The array and the number are discarded in different ways, but in each case the parse result contains neither the value nor its key.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // a JSON text with an array and a number inside an object\n    auto text = R\"({\"IDs\": [116, 943], \"Width\": 800})\";\n\n    // discard the array when the parser reads its opening bracket\n    json j_array_start = json::parse(text, [](int /*depth*/, json::parse_event_t event, json& /*parsed*/)\n    {\n        return event != json::parse_event_t::array_start;\n    });\n\n    // discard the same array when the parser reads its closing bracket\n    json j_array_end = json::parse(text, [](int /*depth*/, json::parse_event_t event, json& /*parsed*/)\n    {\n        return event != json::parse_event_t::array_end;\n    });\n\n    // discard the number, but keep its key\n    json j_value = json::parse(text, [](int /*depth*/, json::parse_event_t event, json & parsed)\n    {\n        return !(event == json::parse_event_t::value && parsed == json(800));\n    });\n\n    // discard the key of the number\n    json j_key = json::parse(text, [](int /*depth*/, json::parse_event_t event, json & parsed)\n    {\n        return !(event == json::parse_event_t::key && parsed == json(\"Width\"));\n    });\n\n    // discard the top-level object\n    json j_root = json::parse(text, [](int /*depth*/, json::parse_event_t event, json& /*parsed*/)\n    {\n        return event != json::parse_event_t::object_end;\n    });\n\n    // in every case, the discarded value is removed together with its key\n    std::cout << j_array_start << '\\n'\n              << j_array_end << '\\n'\n              << j_value << '\\n'\n              << j_key << '\\n'\n              << j_root << '\\n';\n}\n

    Output:

    {\"Width\":800}\n{\"Width\":800}\n{\"IDs\":[116,943]}\n{\"IDs\":[116,943]}\nnull\n
    "},{"location":"api/basic_json/parser_callback_t/#see-also","title":"See also","text":"
    • parse deserialize from a compatible input
    • parse_event_t enumeration of parser events
    "},{"location":"api/basic_json/parser_callback_t/#version-history","title":"Version history","text":"
    • Added in version 1.0.0.
    • Fixed in version 3.13.0 unreleased to also remove discarded values from a parent object; before, discarding an array or a value stored under an object key left a discarded member behind, which made the parse result serialize to invalid JSON.
    • Fixed in version 3.13.0 unreleased so that discarding an array or object at its start event also hides its content from the callback, as documented above; before, the callback was still called for the content, and the key of every member of a discarded object was kept in memory until the parse ended.
    "},{"location":"api/basic_json/patch/","title":"nlohmann::basic_json::patch","text":"
    basic_json patch(const basic_json& json_patch) const;\n

    JSON Patch defines a JSON document structure for expressing a sequence of operations to apply to a JSON document. With this function, a JSON Patch is applied to the current JSON value by executing all operations from the patch.

    "},{"location":"api/basic_json/patch/#parameters","title":"Parameters","text":"json_patch (in) JSON patch document"},{"location":"api/basic_json/patch/#return-value","title":"Return value","text":"

    patched document

    "},{"location":"api/basic_json/patch/#exception-safety","title":"Exception safety","text":"

    Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

    "},{"location":"api/basic_json/patch/#exceptions","title":"Exceptions","text":"
    • Throws parse_error.104 if the JSON patch does not consist of an array of objects.
    • Throws parse_error.105 if the JSON patch is malformed (e.g., mandatory attributes are missing); example: \"operation 'add' must have member 'path'\".
    • Throws out_of_range.401 if an array index is out of range.
    • Throws parse_error.106 if an array index in a \"path\" or \"from\" member begins with '0'; example: \"array index '01' must not begin with '0'\".
    • Throws parse_error.107 if a \"path\" or \"from\" member is not empty and does not begin with a slash (/); example: \"JSON pointer must be empty or begin with '/' - was: 'a'\".
    • Throws parse_error.108 if a tilde (~) in a \"path\" or \"from\" member is not followed by 0 or 1; example: \"escape character '~' must be followed with '0' or '1'\".
    • Throws parse_error.109 if an array index in a \"path\" or \"from\" member is not a number; example: \"array index 'foo' is not a number\".
    • Throws out_of_range.402 if the array index - is used where an existing element is required (the \"path\" of \"replace\", the \"from\" of \"move\" and \"copy\"); example: \"array index '-' (3) is out of range\".
    • Throws out_of_range.403 if a JSON pointer inside the patch could not be resolved successfully in the current JSON value; example: \"key baz not found\".
    • Throws out_of_range.404 if a reference token of a JSON pointer inside the patch cannot be resolved, e.g., - in a \"remove\" operation or 1a for an array; example: \"unresolved reference token '-'\".
    • Throws out_of_range.405 if JSON pointer has no parent (\"remove\", \"move\")
    • Throws out_of_range.411 if an \"add\" operation's target location has a parent that is neither an object nor an array.
    • Throws out_of_range.413 if a \"remove\" operation's target location has a parent that is neither an object nor an array.
    • Throws out_of_range.414 if a \"move\" operation's \"from\" location is a proper prefix of its \"path\" location.
    • Throws other_error.501 if \"test\" operation was unsuccessful.
    "},{"location":"api/basic_json/patch/#complexity","title":"Complexity","text":"

    Linear in the size of the JSON value and the length of the JSON patch. As usually the patch affects only a fraction of the JSON value, the complexity can usually be neglected.

    "},{"location":"api/basic_json/patch/#notes","title":"Notes","text":"

    The application of a patch is atomic: Either all operations succeed and the patched document is returned or an exception is thrown. In any case, the original value is not changed: the patch is applied to a copy of the value.

    "},{"location":"api/basic_json/patch/#examples","title":"Examples","text":"Example: apply a JSON patch

    The following code shows how a JSON patch is applied to a value.

    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // the original document\n    json doc = R\"(\n        {\n          \"baz\": \"qux\",\n          \"foo\": \"bar\"\n        }\n    )\"_json;\n\n    // the patch\n    json patch = R\"(\n        [\n          { \"op\": \"replace\", \"path\": \"/baz\", \"value\": \"boo\" },\n          { \"op\": \"add\", \"path\": \"/hello\", \"value\": [\"world\"] },\n          { \"op\": \"remove\", \"path\": \"/foo\"}\n        ]\n    )\"_json;\n\n    // apply the patch\n    json patched_doc = doc.patch(patch);\n\n    // output original and patched document\n    std::cout << std::setw(4) << doc << \"\\n\\n\"\n              << std::setw(4) << patched_doc << std::endl;\n}\n

    Output:

    {\n    \"baz\": \"qux\",\n    \"foo\": \"bar\"\n}\n\n{\n    \"baz\": \"boo\",\n    \"hello\": [\n        \"world\"\n    ]\n}\n
    Example: out_of_range.414 exception

    The following code shows how a \"move\" operation whose \"from\" location is a proper prefix of its \"path\" location is rejected, and how the original document is left unchanged because the patch is applied to a copy.

    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // the original document\n    json doc = R\"(\n        {\n          \"a\": { \"b\": 1 }\n        }\n    )\"_json;\n\n    // a patch that tries to move \"/a\" into one of its own children\n    json patch = R\"(\n        [\n          { \"op\": \"move\", \"from\": \"/a\", \"path\": \"/a/b\" }\n        ]\n    )\"_json;\n\n    // exception out_of_range.414\n    try\n    {\n        json patched_doc = doc.patch(patch);\n    }\n    catch (const json::out_of_range& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n\n    // the original document is unchanged\n    std::cout << std::setw(4) << doc << std::endl;\n}\n

    Output:

    [json.exception.out_of_range.414] cannot move value: 'from' path '/a' is a proper prefix of 'path' '/a/b'\n{\n    \"a\": {\n        \"b\": 1\n    }\n}\n
    "},{"location":"api/basic_json/patch/#see-also","title":"See also","text":"
    • RFC 6902 (JSON Patch)
    • RFC 6901 (JSON Pointer)
    • patch_inplace applies a JSON Patch without creating a copy of the document
    • merge_patch applies a JSON Merge Patch
    "},{"location":"api/basic_json/patch/#version-history","title":"Version history","text":"
    • Added in version 2.0.0.
    • Added out_of_range.411 and stopped relying on an internal assertion when an \"add\" operation's target location has a non-object/non-array parent in version 3.13.0 unreleased.
    • Added out_of_range.413 and stopped silently ignoring a \"remove\" operation whose target location has a non-object/non-array parent in version 3.13.0 unreleased.
    • Added out_of_range.414 and rejected a \"move\" operation whose \"from\" location is a proper prefix of its \"path\" location instead of silently producing a corrupted result in version 3.13.0 unreleased.
    • Throws parse_error.109 instead of out_of_range.404 for a one-character array index that is not a digit (e.g., /x) in version 3.13.0 unreleased, as it already did for longer ones.
    "},{"location":"api/basic_json/patch_inplace/","title":"nlohmann::basic_json::patch_inplace","text":"
    void patch_inplace(const basic_json& json_patch);\n

    JSON Patch defines a JSON document structure for expressing a sequence of operations to apply to a JSON document. With this function, a JSON Patch is applied to the current JSON value by executing all operations from the patch. This function applies a JSON patch in place and returns void.

    "},{"location":"api/basic_json/patch_inplace/#parameters","title":"Parameters","text":"json_patch (in) JSON patch document"},{"location":"api/basic_json/patch_inplace/#exception-safety","title":"Exception safety","text":"

    No guarantees, value may be corrupted by an unsuccessful patch operation.

    "},{"location":"api/basic_json/patch_inplace/#exceptions","title":"Exceptions","text":"
    • Throws parse_error.104 if the JSON patch does not consist of an array of objects.
    • Throws parse_error.105 if the JSON patch is malformed (e.g., mandatory attributes are missing); example: \"operation 'add' must have member 'path'\".
    • Throws out_of_range.401 if an array index is out of range.
    • Throws parse_error.106 if an array index in a \"path\" or \"from\" member begins with '0'; example: \"array index '01' must not begin with '0'\".
    • Throws parse_error.107 if a \"path\" or \"from\" member is not empty and does not begin with a slash (/); example: \"JSON pointer must be empty or begin with '/' - was: 'a'\".
    • Throws parse_error.108 if a tilde (~) in a \"path\" or \"from\" member is not followed by 0 or 1; example: \"escape character '~' must be followed with '0' or '1'\".
    • Throws parse_error.109 if an array index in a \"path\" or \"from\" member is not a number; example: \"array index 'foo' is not a number\".
    • Throws out_of_range.402 if the array index - is used where an existing element is required (the \"path\" of \"replace\", the \"from\" of \"move\" and \"copy\"); example: \"array index '-' (3) is out of range\".
    • Throws out_of_range.403 if a JSON pointer inside the patch could not be resolved successfully in the current JSON value; example: \"key baz not found\".
    • Throws out_of_range.404 if a reference token of a JSON pointer inside the patch cannot be resolved, e.g., - in a \"remove\" operation or 1a for an array; example: \"unresolved reference token '-'\".
    • Throws out_of_range.405 if JSON pointer has no parent (\"remove\", \"move\")
    • Throws out_of_range.411 if an \"add\" operation's target location has a parent that is neither an object nor an array.
    • Throws out_of_range.413 if a \"remove\" operation's target location has a parent that is neither an object nor an array.
    • Throws out_of_range.414 if a \"move\" operation's \"from\" location is a proper prefix of its \"path\" location.
    • Throws other_error.501 if \"test\" operation was unsuccessful.
    "},{"location":"api/basic_json/patch_inplace/#complexity","title":"Complexity","text":"

    Linear in the size of the JSON value and the length of the JSON patch. As usually the patch affects only a fraction of the JSON value, the complexity can usually be neglected.

    "},{"location":"api/basic_json/patch_inplace/#notes","title":"Notes","text":"

    Unlike patch, patch_inplace applies the operation \"in place\" and no copy of the JSON value is created. That makes it faster for large documents by avoiding the copy. However, the JSON value might be corrupted if the function throws an exception.

    "},{"location":"api/basic_json/patch_inplace/#examples","title":"Examples","text":"Example: apply a JSON patch in place

    The following code shows how a JSON patch is applied to a value.

    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // the original document\n    json doc = R\"(\n        {\n          \"baz\": \"qux\",\n          \"foo\": \"bar\"\n        }\n    )\"_json;\n\n    // the patch\n    json patch = R\"(\n        [\n          { \"op\": \"replace\", \"path\": \"/baz\", \"value\": \"boo\" },\n          { \"op\": \"add\", \"path\": \"/hello\", \"value\": [\"world\"] },\n          { \"op\": \"remove\", \"path\": \"/foo\"}\n        ]\n    )\"_json;\n\n    // output original document\n    std::cout << \"Before\\n\" << std::setw(4) << doc << std::endl;\n\n    // apply the patch\n    doc.patch_inplace(patch);\n\n    // output patched document\n    std::cout << \"\\nAfter\\n\" << std::setw(4) << doc << std::endl;\n}\n

    Output:

    Before\n{\n    \"baz\": \"qux\",\n    \"foo\": \"bar\"\n}\n\nAfter\n{\n    \"baz\": \"boo\",\n    \"hello\": [\n        \"world\"\n    ]\n}\n
    Example: out_of_range.403 exception with a partially applied patch

    The following code shows a patch whose first operation succeeds and whose second operation fails. Because patch_inplace applies each operation directly to the value, the first operation's effect is still visible after the exception is caught, unlike patch.

    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // the original document\n    json doc = R\"(\n        {\n          \"a\": 1,\n          \"b\": 2\n        }\n    )\"_json;\n\n    // a patch whose second operation fails\n    json patch = R\"(\n        [\n          { \"op\": \"replace\", \"path\": \"/a\", \"value\": 99 },\n          { \"op\": \"remove\", \"path\": \"/nonexistent\" }\n        ]\n    )\"_json;\n\n    // exception out_of_range.403\n    try\n    {\n        doc.patch_inplace(patch);\n    }\n    catch (const json::out_of_range& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n\n    // the first operation has already been applied to doc\n    std::cout << std::setw(4) << doc << std::endl;\n}\n

    Output:

    [json.exception.out_of_range.403] key 'nonexistent' not found\n{\n    \"a\": 99,\n    \"b\": 2\n}\n
    "},{"location":"api/basic_json/patch_inplace/#see-also","title":"See also","text":"
    • RFC 6902 (JSON Patch)
    • RFC 6901 (JSON Pointer)
    • patch applies a JSON Patch
    • merge_patch applies a JSON Merge Patch
    "},{"location":"api/basic_json/patch_inplace/#version-history","title":"Version history","text":"
    • Added in version 3.11.0.
    • Added out_of_range.411 and stopped relying on an internal assertion when an \"add\" operation's target location has a non-object/non-array parent in version 3.13.0 unreleased.
    • Added out_of_range.413 and stopped silently ignoring a \"remove\" operation whose target location has a non-object/non-array parent in version 3.13.0 unreleased.
    • Added out_of_range.414 and rejected a \"move\" operation whose \"from\" location is a proper prefix of its \"path\" location instead of silently producing a corrupted result in version 3.13.0 unreleased.
    • Throws parse_error.109 instead of out_of_range.404 for a one-character array index that is not a digit (e.g., /x) in version 3.13.0 unreleased, as it already did for longer ones.
    "},{"location":"api/basic_json/push_back/","title":"nlohmann::basic_json::push_back","text":"
    // (1)\nvoid push_back(basic_json&& val);\nvoid push_back(const basic_json& val);\n\n// (2)\nvoid push_back(const typename object_t::value_type& val);\n\n// (3)\nvoid push_back(initializer_list_t init);\n

    1. Appends the given element val to the end of the JSON array. If the function is called on a JSON null value, an empty array is created before appending val.

    2. Inserts the given element val to the JSON object. If the function is called on a JSON null value, an empty object is created before inserting val.

    3. This function allows using push_back with an initializer list. In case

      1. the current value is an object,
      2. the initializer list init contains only two elements, and
      3. the first element of init is a string,

      init is converted into an object element and added using push_back(const typename object_t::value_type&). Otherwise, init is converted to a JSON value and added using push_back(basic_json&&).

    "},{"location":"api/basic_json/push_back/#iterator-invalidation","title":"Iterator invalidation","text":"

    For all cases where an element is added to an array, a reallocation can happen, in which case all iterators (including the end() iterator) and all references to the elements are invalidated. Otherwise, only the end() iterator is invalidated.

    For ordered_json, also adding an element to an object can yield a reallocation which again invalidates all iterators and all references.

    "},{"location":"api/basic_json/push_back/#parameters","title":"Parameters","text":"val (in) the value to add to the JSON array/object init (in) an initializer list"},{"location":"api/basic_json/push_back/#exception-safety","title":"Exception safety","text":"

    Strong guarantee: if an exception is thrown, there are no changes to any JSON value. As an exception, a null value is converted to an empty array or object before the element is added and keeps that type if adding the element throws.

    "},{"location":"api/basic_json/push_back/#exceptions","title":"Exceptions","text":"
    1. Throws type_error.308 when called on a type other than JSON array or null; example: \"cannot use push_back() with number\"
    2. Throws type_error.308 when called on a type other than JSON object or null; example: \"cannot use push_back() with number\"
    3. Throws type_error.308 when called on a type other than JSON array or null; example: \"cannot use push_back() with number\"
    "},{"location":"api/basic_json/push_back/#complexity","title":"Complexity","text":"
    1. Amortized constant.
    2. Logarithmic in the size of the container, O(log(size())).
    3. Linear in the size of the initializer list init.
    "},{"location":"api/basic_json/push_back/#notes","title":"Notes","text":"

    (3) This function is required to resolve an ambiguous overload error, because pairs like {\"key\", \"value\"} can be both interpreted as object_t::value_type or std::initializer_list<basic_json>, see #235 for more information.

    "},{"location":"api/basic_json/push_back/#examples","title":"Examples","text":"Example: (1) add element to array

    The example shows how push_back() and += can be used to add elements to a JSON array. Note how the null value was silently converted to a JSON array.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json array = {1, 2, 3, 4, 5};\n    json null;\n\n    // print values\n    std::cout << array << '\\n';\n    std::cout << null << '\\n';\n\n    // add values\n    array.push_back(6);\n    array += 7;\n    null += \"first\";\n    null += \"second\";\n\n    // print values\n    std::cout << array << '\\n';\n    std::cout << null << '\\n';\n}\n

    Output:

    [1,2,3,4,5]\nnull\n[1,2,3,4,5,6,7]\n[\"first\",\"second\"]\n
    Example: (2) add element to object

    The example shows how push_back() and += can be used to add elements to a JSON object. Note how the null value was silently converted to a JSON object.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json object = {{\"one\", 1}, {\"two\", 2}};\n    json null;\n\n    // print values\n    std::cout << object << '\\n';\n    std::cout << null << '\\n';\n\n    // add values\n    object.push_back(json::object_t::value_type(\"three\", 3));\n    object += json::object_t::value_type(\"four\", 4);\n    null += json::object_t::value_type(\"A\", \"a\");\n    null += json::object_t::value_type(\"B\", \"b\");\n\n    // print values\n    std::cout << object << '\\n';\n    std::cout << null << '\\n';\n}\n

    Output:

    {\"one\":1,\"two\":2}\nnull\n{\"four\":4,\"one\":1,\"three\":3,\"two\":2}\n{\"A\":\"a\",\"B\":\"b\"}\n
    Example: (3) add to object from initializer list

    The example shows how initializer lists are treated as objects when possible.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json object = {{\"one\", 1}, {\"two\", 2}};\n    json null;\n\n    // print values\n    std::cout << object << '\\n';\n    std::cout << null << '\\n';\n\n    // add values:\n    object.push_back({\"three\", 3});  // object is extended\n    object += {\"four\", 4};           // object is extended\n    null.push_back({\"five\", 5});     // null is converted to array\n\n    // print values\n    std::cout << object << '\\n';\n    std::cout << null << '\\n';\n\n    // would throw:\n    //object.push_back({1, 2, 3});\n}\n

    Output:

    {\"one\":1,\"two\":2}\nnull\n{\"four\":4,\"one\":1,\"three\":3,\"two\":2}\n[[\"five\",5]]\n
    "},{"location":"api/basic_json/push_back/#see-also","title":"See also","text":"
    • emplace_back add a value to an array
    • operator+= add a value to an array/object
    • Modifying values - the article on modifying values
    "},{"location":"api/basic_json/push_back/#version-history","title":"Version history","text":"
    1. Since version 1.0.0.
    2. Since version 1.0.0.
    3. Since version 2.0.0.
    "},{"location":"api/basic_json/rbegin/","title":"nlohmann::basic_json::rbegin","text":"
    reverse_iterator rbegin() noexcept;\nconst_reverse_iterator rbegin() const noexcept;\n

    Returns an iterator to the reverse-beginning; that is, the last element.

    "},{"location":"api/basic_json/rbegin/#return-value","title":"Return value","text":"

    reverse iterator to the last element

    "},{"location":"api/basic_json/rbegin/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this member function never throws exceptions.

    "},{"location":"api/basic_json/rbegin/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json/rbegin/#examples","title":"Examples","text":"Example

    The following code shows an example for rbegin().

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create an array value\n    json array = {1, 2, 3, 4, 5};\n\n    // get an iterator to the reverse-beginning\n    json::reverse_iterator it = array.rbegin();\n\n    // serialize the element that the iterator points to\n    std::cout << *it << '\\n';\n}\n

    Output:

    5\n
    "},{"location":"api/basic_json/rbegin/#see-also","title":"See also","text":"
    • rend returns a reverse iterator to one before the first element
    • crbegin returns a const reverse iterator to the last element
    • begin returns an iterator to the first element
    • Iterators - the article on iterators
    "},{"location":"api/basic_json/rbegin/#version-history","title":"Version history","text":"
    • Added in version 1.0.0.
    "},{"location":"api/basic_json/rend/","title":"nlohmann::basic_json::rend","text":"
    reverse_iterator rend() noexcept;\nconst_reverse_iterator rend() const noexcept;\n

    Returns an iterator to the reverse-end; that is, one before the first element. This element acts as a placeholder, attempting to access it results in undefined behavior.

    "},{"location":"api/basic_json/rend/#return-value","title":"Return value","text":"

    reverse iterator to the element following the last element

    "},{"location":"api/basic_json/rend/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this member function never throws exceptions.

    "},{"location":"api/basic_json/rend/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json/rend/#examples","title":"Examples","text":"Example

    The following code shows an example for rend().

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create an array value\n    json array = {1, 2, 3, 4, 5};\n\n    // get an iterator to the reverse-end\n    json::reverse_iterator it = array.rend();\n\n    // increment the iterator to point to the first element\n    --it;\n\n    // serialize the element that the iterator points to\n    std::cout << *it << '\\n';\n}\n

    Output:

    1\n
    "},{"location":"api/basic_json/rend/#see-also","title":"See also","text":"
    • rbegin returns a reverse iterator to the last element
    • crend returns a const reverse iterator to one before the first element
    • end returns an iterator to one past the last element
    • Iterators - the article on iterators
    "},{"location":"api/basic_json/rend/#version-history","title":"Version history","text":"
    • Added in version 1.0.0.
    "},{"location":"api/basic_json/sax_parse/","title":"nlohmann::basic_json::sax_parse","text":"
    // (1)\ntemplate <typename InputType, typename SAX>\nstatic bool sax_parse(InputType&& i,\n                      SAX* sax,\n                      input_format_t format = input_format_t::json,\n                      const bool strict = true,\n                      const bool ignore_comments = false,\n                      const bool ignore_trailing_commas = false,\n                      const cbor_tag_handler_t tag_handler = cbor_tag_handler_t::error);\n\n// (2)\ntemplate<class IteratorType, class SAX, class SentinelType = IteratorType>\nstatic bool sax_parse(IteratorType first, SentinelType last,\n                      SAX* sax,\n                      input_format_t format = input_format_t::json,\n                      const bool strict = true,\n                      const bool ignore_comments = false,\n                      const bool ignore_trailing_commas = false,\n                      const cbor_tag_handler_t tag_handler = cbor_tag_handler_t::error);\n

    Read from input and generate SAX events

    1. Read from a compatible input.

    2. Read from a pair of character iterators, or an iterator and a sentinel of a different type (C++20 ranges support).

      The value_type of the iterator must be an integral type with a size of 1, 2, or 4 bytes, which will be interpreted respectively as UTF-8, UTF-16, and UTF-32. If SentinelType differs from IteratorType, it must be comparable to the iterator type with operator!=.

    The SAX event lister must follow the interface of json_sax.

    "},{"location":"api/basic_json/sax_parse/#template-parameters","title":"Template parameters","text":"InputType

    A compatible input, for instance:

    • an std::istream object
    • a FILE pointer
    • a C-style array of characters
    • a pointer to a null-terminated string of single byte characters
    • a container obj for which begin(obj) and end(obj) produce a valid pair of iterators (as found via ADL or member functions, with semantics compatible to std::begin and std::end)
    IteratorType a compatible iterator type for overload (2); a pair of character iterators whose value_type is an integral type with a size of 1, 2, or 4 bytes (interpreted respectively as UTF-8, UTF-16, and UTF-32) SentinelType

    defaults to IteratorType; may be a different type comparable to IteratorType via operator!=, for overload (2), for instance.

    • a custom sentinel type for C++20 ranges
    • std::default_sentinel_t, when IteratorType is std::counted_iterator
    SAX a class fulfilling the SAX event listener interface; see json_sax"},{"location":"api/basic_json/sax_parse/#parameters","title":"Parameters","text":"i (in) Input to parse from sax (in) SAX event listener (must not be null) format (in) the format to parse (JSON, BJData, BON8, BSON, CBOR, MessagePack, or UBJSON) (optional, input_format_t::json by default), see input_format_t for more information strict (in) whether the input has to be consumed completely (optional, true by default); when false and the input is a std::istream, the character that terminates a number is consumed unless JSON_PRECISE_STREAM_POSITION is defined to 1; see operator>> ignore_comments (in) whether comments should be ignored and treated like whitespace (true) or yield a parse error (false); (optional, false by default) ignore_trailing_commas (in) whether trailing commas in arrays or objects should be ignored and treated like whitespace (true) or yield a parse error (false); (optional, false by default) tag_handler (in) how to handle CBOR tags; see cbor_tag_handler_t. Ignored for formats other than CBOR (optional, cbor_tag_handler_t::error by default). first (in) iterator to the start of a character range last (in) iterator to the end of a character range, or a sentinel value that compares equal to the end iterator with operator!="},{"location":"api/basic_json/sax_parse/#return-value","title":"Return value","text":"

    return value of the last processed SAX event

    "},{"location":"api/basic_json/sax_parse/#exception-safety","title":"Exception safety","text":"

    Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

    "},{"location":"api/basic_json/sax_parse/#exceptions","title":"Exceptions","text":"
    • Throws parse_error.101 in case of an unexpected token, or empty input like a null FILE* or char* pointer.
    "},{"location":"api/basic_json/sax_parse/#complexity","title":"Complexity","text":"

    Linear in the length of the input. The parser is a predictive LL(1) parser. The complexity can be higher if the SAX consumer sax has a super-linear complexity.

    "},{"location":"api/basic_json/sax_parse/#notes","title":"Notes","text":"

    A UTF-8 byte order mark is silently ignored.

    When reading from a FILE*, the file is locked while it is parsed. The lock is recursive, so the SAX handler may use the same file with stdio functions, but it must not wait for another thread that uses the same FILE object; see JSON_DISABLE_UNLOCKED_FILE_IO.

    "},{"location":"api/basic_json/sax_parse/#examples","title":"Examples","text":"Example

    The example below demonstrates the sax_parse() function reading from string and processing the events with a user-defined SAX event consumer.

    #include <iostream>\n#include <iomanip>\n#include <sstream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\n// a simple event consumer that collects string representations of the passed\n// values; note inheriting from json::json_sax_t is not required, but can\n// help not to forget a required function\nclass sax_event_consumer : public json::json_sax_t\n{\n  public:\n    std::vector<std::string> events;\n\n    bool null() override\n    {\n        events.push_back(\"null()\");\n        return true;\n    }\n\n    bool boolean(bool val) override\n    {\n        events.push_back(\"boolean(val=\" + std::string(val ? \"true\" : \"false\") + \")\");\n        return true;\n    }\n\n    bool number_integer(number_integer_t val) override\n    {\n        events.push_back(\"number_integer(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_unsigned(number_unsigned_t val) override\n    {\n        events.push_back(\"number_unsigned(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_float(number_float_t val, const string_t& s) override\n    {\n        events.push_back(\"number_float(val=\" + std::to_string(val) + \", s=\" + s + \")\");\n        return true;\n    }\n\n    bool string(string_t& val) override\n    {\n        events.push_back(\"string(val=\" + val + \")\");\n        return true;\n    }\n\n    bool start_object(std::size_t elements) override\n    {\n        events.push_back(\"start_object(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_object() override\n    {\n        events.push_back(\"end_object()\");\n        return true;\n    }\n\n    bool start_array(std::size_t elements) override\n    {\n        events.push_back(\"start_array(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_array() override\n    {\n        events.push_back(\"end_array()\");\n        return true;\n    }\n\n    bool key(string_t& val) override\n    {\n        events.push_back(\"key(val=\" + val + \")\");\n        return true;\n    }\n\n    bool binary(json::binary_t& val) override\n    {\n        events.push_back(\"binary(val=[...])\");\n        return true;\n    }\n\n    bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex) override\n    {\n        events.push_back(\"parse_error(position=\" + std::to_string(position) + \", last_token=\" + last_token + \",\\n            ex=\" + std::string(ex.what()) + \")\");\n        return false;\n    }\n};\n\nint main()\n{\n    // a JSON text\n    auto text = R\"(\n    {\n        \"Image\": {\n            \"Width\":  800,\n            \"Height\": 600,\n            \"Title\":  \"View from 15th Floor\",\n            \"Thumbnail\": {\n                \"Url\":    \"http://www.example.com/image/481989943\",\n                \"Height\": 125,\n                \"Width\":  100\n            },\n            \"Animated\" : false,\n            \"IDs\": [116, 943, 234, -38793],\n            \"DeletionDate\": null,\n            \"Distance\": 12.723374634\n        }\n    }]\n    )\";\n\n    // create a SAX event consumer object\n    sax_event_consumer sec;\n\n    // parse JSON\n    bool result = json::sax_parse(text, &sec);\n\n    // output the recorded events\n    for (auto& event : sec.events)\n    {\n        std::cout << event << \"\\n\";\n    }\n\n    // output the result of sax_parse\n    std::cout << \"\\nresult: \" << std::boolalpha << result << std::endl;\n}\n

    Output:

    start_object(elements=18446744073709551615)\nkey(val=Image)\nstart_object(elements=18446744073709551615)\nkey(val=Width)\nnumber_unsigned(val=800)\nkey(val=Height)\nnumber_unsigned(val=600)\nkey(val=Title)\nstring(val=View from 15th Floor)\nkey(val=Thumbnail)\nstart_object(elements=18446744073709551615)\nkey(val=Url)\nstring(val=http://www.example.com/image/481989943)\nkey(val=Height)\nnumber_unsigned(val=125)\nkey(val=Width)\nnumber_unsigned(val=100)\nend_object()\nkey(val=Animated)\nboolean(val=false)\nkey(val=IDs)\nstart_array(elements=18446744073709551615)\nnumber_unsigned(val=116)\nnumber_unsigned(val=943)\nnumber_unsigned(val=234)\nnumber_integer(val=-38793)\nend_array()\nkey(val=DeletionDate)\nnull()\nkey(val=Distance)\nnumber_float(val=12.723375, s=12.723374634)\nend_object()\nend_object()\nparse_error(position=460, last_token=12.723374634<U+000A>        }<U+000A>    }],\n            ex=[json.exception.parse_error.101] parse error at line 17, column 6: syntax error while parsing value - unexpected ']'; expected end of input)\n\nresult: false\n
    "},{"location":"api/basic_json/sax_parse/#see-also","title":"See also","text":"
    • parse - deserialize from a compatible input
    • accept - check if the input is valid JSON
    • JSON_DISABLE_STREAM_BULK_SCAN - read a std::istream one character at a time
    • JSON_DISABLE_UNLOCKED_FILE_IO - read a FILE* with the locking stdio functions
    "},{"location":"api/basic_json/sax_parse/#version-history","title":"Version history","text":"
    • Added in version 3.2.0.
    • Ignoring comments via ignore_comments added in version 3.9.0.
    • Added ignore_trailing_commas in version 3.13.0 unreleased.
    • Added tag_handler in version 3.13.0 unreleased.
    • Extended container support (1) to include types with lvalue-only ADL begin/end (matching std::begin/std::end semantics) in version 3.13.0 unreleased.
    • Extended overload (2) to accept heterogeneous iterator+sentinel pairs (C++20 ranges support) in version 3.13.0 unreleased.
    • JSON_PRECISE_STREAM_POSITION added in version 3.13.0 unreleased to optionally leave a std::istream positioned right after the parsed value when strict is false.
    • Parsing from an std::istream scans the characters in its stream buffer in bulk in version 3.13.0 unreleased (JSON_DISABLE_STREAM_BULK_SCAN).
    • Parsing from a FILE* locks the file once instead of once per character in version 3.13.0 unreleased (JSON_DISABLE_UNLOCKED_FILE_IO).

    Deprecation

    Overload (2) replaces calls to sax_parse with a pair of iterators as their first parameter which has been deprecated in version 3.8.0. This overload will be removed in version 4.0.0. Please replace all calls like sax_parse({ptr, ptr+len}); with sax_parse(ptr, ptr+len);.

    See the migration guide for how to update existing code.

    "},{"location":"api/basic_json/size/","title":"nlohmann::basic_json::size","text":"
    size_type size() const noexcept;\n

    Returns the number of elements in a JSON value.

    "},{"location":"api/basic_json/size/#return-value","title":"Return value","text":"

    The return value depends on the different types and is defined as follows:

    Value type return value null 0 boolean 1 string 1 number 1 binary 1 object result of function object_t::size() array result of function array_t::size()"},{"location":"api/basic_json/size/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this function never throws exceptions.

    "},{"location":"api/basic_json/size/#complexity","title":"Complexity","text":"

    Constant, as long as array_t and object_t satisfy the Container concept; that is, their size() functions have constant complexity.

    "},{"location":"api/basic_json/size/#notes","title":"Notes","text":"

    This function does not return the length of a string stored as JSON value -- it returns the number of elements in the JSON value which is 1 in the case of a string.

    "},{"location":"api/basic_json/size/#examples","title":"Examples","text":"Example

    The following code calls size() on the different value types.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_null;\n    json j_boolean = true;\n    json j_number_integer = 17;\n    json j_number_float = 23.42;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_object_empty(json::value_t::object);\n    json j_array = {1, 2, 4, 8, 16};\n    json j_array_empty(json::value_t::array);\n    json j_string = \"Hello, world\";\n\n    // call size()\n    std::cout << j_null.size() << '\\n';\n    std::cout << j_boolean.size() << '\\n';\n    std::cout << j_number_integer.size() << '\\n';\n    std::cout << j_number_float.size() << '\\n';\n    std::cout << j_object.size() << '\\n';\n    std::cout << j_object_empty.size() << '\\n';\n    std::cout << j_array.size() << '\\n';\n    std::cout << j_array_empty.size() << '\\n';\n    std::cout << j_string.size() << '\\n';\n}\n

    Output:

    0\n1\n1\n1\n2\n0\n5\n0\n1\n
    "},{"location":"api/basic_json/size/#see-also","title":"See also","text":"
    • empty checks whether the JSON value has no elements
    • max_size returns the maximum possible number of elements
    • basic_json_view::size - the same function on a zero-copy view
    "},{"location":"api/basic_json/size/#version-history","title":"Version history","text":"
    • Added in version 1.0.0.
    • Extended to return 1 for binary types in version 3.8.0.
    "},{"location":"api/basic_json/start_pos/","title":"nlohmann::basic_json::start_pos","text":"
    #if JSON_DIAGNOSTIC_POSITIONS\nconstexpr std::size_t start_pos() const noexcept;\n#endif\n

    Returns the position of the first character in the JSON string from which the value was parsed from.

    JSON type return value object position of the opening { array position of the opening [ string position of the opening \" number position of the first character boolean position of t for true and f for false null position of n"},{"location":"api/basic_json/start_pos/#return-value","title":"Return value","text":"

    the position of the first character of the value in the parsed JSON string, if the value was created by the parse function, or std::string::npos if the value was constructed otherwise

    "},{"location":"api/basic_json/start_pos/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this member function never throws exceptions.

    "},{"location":"api/basic_json/start_pos/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json/start_pos/#notes","title":"Notes","text":"

    Note

    The function is only available if macro JSON_DIAGNOSTIC_POSITIONS has been defined to 1 before including the library header.

    Invalidation

    The returned positions are only valid as long as the JSON value is not changed. The positions are not updated when the JSON value is changed.

    "},{"location":"api/basic_json/start_pos/#examples","title":"Examples","text":"Example 
    #include <iostream>\n\n#define JSON_DIAGNOSTIC_POSITIONS 1\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::string json_string = R\"(\n    {\n        \"address\": {\n            \"street\": \"Fake Street\",\n            \"housenumber\": 1\n        }\n    }\n    )\";\n    json j = json::parse(json_string);\n\n    std::cout << \"Root diagnostic positions: \\n\";\n    std::cout << \"\\tstart_pos: \" << j.start_pos() << '\\n';\n    std::cout << \"\\tend_pos:\" << j.end_pos() << \"\\n\";\n    std::cout << \"Original string: \\n\";\n    std::cout << \"{\\n        \\\"address\\\": {\\n            \\\"street\\\": \\\"Fake Street\\\",\\n            \\\"housenumber\\\": 1\\n        }\\n    }\" << \"\\n\";\n    std::cout << \"Parsed string: \\n\";\n    std::cout << json_string.substr(j.start_pos(), j.end_pos() - j.start_pos()) << \"\\n\\n\";\n\n    std::cout << \"address diagnostic positions: \\n\";\n    std::cout << \"\\tstart_pos:\" << j[\"address\"].start_pos() << '\\n';\n    std::cout << \"\\tend_pos:\" << j[\"address\"].end_pos() << \"\\n\\n\";\n    std::cout << \"Original string: \\n\";\n    std::cout << \"{            \\\"street\\\": \\\"Fake Street\\\",\\n            \\\"housenumber\\\": 1\\n        }\" << \"\\n\";\n    std::cout << \"Parsed string: \\n\";\n    std::cout << json_string.substr(j[\"address\"].start_pos(), j[\"address\"].end_pos() - j[\"address\"].start_pos()) << \"\\n\\n\";\n\n    std::cout << \"street diagnostic positions: \\n\";\n    std::cout << \"\\tstart_pos:\" << j[\"address\"][\"street\"].start_pos() << '\\n';\n    std::cout << \"\\tend_pos:\" << j[\"address\"][\"street\"].end_pos() << \"\\n\\n\";\n    std::cout << \"Original string: \\n\";\n    std::cout << \"\\\"Fake Street\\\"\" << \"\\n\";\n    std::cout << \"Parsed string: \\n\";\n    std::cout << json_string.substr(j[\"address\"][\"street\"].start_pos(), j[\"address\"][\"street\"].end_pos() - j[\"address\"][\"street\"].start_pos()) << \"\\n\\n\";\n\n    std::cout << \"housenumber diagnostic positions: \\n\";\n    std::cout << \"\\tstart_pos:\" << j[\"address\"][\"housenumber\"].start_pos() << '\\n';\n    std::cout << \"\\tend_pos:\" << j[\"address\"][\"housenumber\"].end_pos() << \"\\n\\n\";\n    std::cout << \"Original string: \\n\";\n    std::cout << \"1\" << \"\\n\";\n    std::cout << \"Parsed string: \\n\";\n    std::cout << json_string.substr(j[\"address\"][\"housenumber\"].start_pos(), j[\"address\"][\"housenumber\"].end_pos() - j[\"address\"][\"housenumber\"].start_pos()) << \"\\n\\n\";\n}\n

    Output:

    Root diagnostic positions: \n    start_pos: 5\n    end_pos:109\nOriginal string: \n{\n        \"address\": {\n            \"street\": \"Fake Street\",\n            \"housenumber\": 1\n        }\n    }\nParsed string: \n{\n        \"address\": {\n            \"street\": \"Fake Street\",\n            \"housenumber\": 1\n        }\n    }\n\naddress diagnostic positions: \n    start_pos:26\n    end_pos:103\n\nOriginal string: \n{            \"street\": \"Fake Street\",\n            \"housenumber\": 1\n        }\nParsed string: \n{\n            \"street\": \"Fake Street\",\n            \"housenumber\": 1\n        }\n\nstreet diagnostic positions: \n    start_pos:50\n    end_pos:63\n\nOriginal string: \n\"Fake Street\"\nParsed string: \n\"Fake Street\"\n\nhousenumber diagnostic positions: \n    start_pos:92\n    end_pos:93\n\nOriginal string: \n1\nParsed string: \n1\n

    The output shows the start/end positions of all the objects and fields in the JSON string.

    "},{"location":"api/basic_json/start_pos/#see-also","title":"See also","text":"
    • end_pos to access the end position
    • JSON_DIAGNOSTIC_POSITIONS for an overview of the diagnostic positions
    "},{"location":"api/basic_json/start_pos/#version-history","title":"Version history","text":"
    • Added in version 3.12.0.
    "},{"location":"api/basic_json/std_formatter/","title":"std::formatter<nlohmann::basic_json>","text":"
    namespace std {\n    template <>\n    struct formatter<nlohmann::basic_json, char>;\n}\n

    Specialization to make JSON values formattable with std::format (and the other members of C++20's <format> header, such as std::format_to).

    A subset of the standard format spec grammar is supported, repurposed for JSON pretty-printing; any other spec component (sign, the 0 flag, precision, L, a dynamic width such as \"{:{}}\", or a trailing type character) throws std::format_error:

    • \"{}\" serializes the value the same way as dump() (compact, no whitespace).
    • \"{:#}\" (\"alternate form\") serializes the value the same way as dump(4) (pretty-printed with an indent of 4).
    • A width, with or without \"#\" (e.g. \"{:2}\" or \"{:#2}\"), serializes the value the same way as dump(width) \u2014 a width on its own implies pretty-printing, since an indent size has no meaning for compact output.
    • fill-and-align (e.g. \"{:.>#}\" or \"{:.>3}\") picks a custom indent character, the same way as dump(indent, indent_char). The alignment direction itself ('<', '>', '^') has no separate meaning for JSON values \u2014 only the fill character before it is used, and any of the three directions is accepted.

    This specialization is only available for char-based JSON values and only if the standard library provides <format>, controlled by the JSON_HAS_STD_FORMAT macro.

    "},{"location":"api/basic_json/std_formatter/#examples","title":"Examples","text":"Example

    The example shows how to format JSON values with std::format.

    #include <format>\n#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    json j = {{\"one\", 1}, {\"two\", 2}};\n\n    // compact formatting, like dump()\n    std::cout << std::format(\"{}\", j) << \"\\n\\n\";\n\n    // pretty-printed formatting, like dump(4)\n    std::cout << std::format(\"{:#}\", j) << \"\\n\\n\";\n\n    // a width sets the indent, like dump(2)\n    std::cout << std::format(\"{:2}\", j) << \"\\n\\n\";\n\n    // fill-and-align sets the indent character, like dump(4, '.')\n    std::cout << std::format(\"{:.>#}\", j) << std::endl;\n}\n

    Output:

    {\"one\":1,\"two\":2}\n\n{\n    \"one\": 1,\n    \"two\": 2\n}\n\n{\n  \"one\": 1,\n  \"two\": 2\n}\n\n{\n....\"one\": 1,\n....\"two\": 2\n}\n
    "},{"location":"api/basic_json/std_formatter/#see-also","title":"See also","text":"
    • dump - serialization
    • operator<<(std::ostream&) - serialize to stream
    • format_as - customization point used by fmt::format (fmtlib)
    • Serialization - the serialization article
    "},{"location":"api/basic_json/std_formatter/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json/std_hash/","title":"std::hash<nlohmann::basic_json>","text":"
    namespace std {\n    struct hash<nlohmann::basic_json>;\n}\n

    Return a hash value for a JSON object. The hash function tries to rely on std::hash where possible. Furthermore, the type of the JSON value is taken into account, so null, false, and numbers may hash differently from each other. Numbers that compare equal under operator== always hash equally, regardless of whether they are stored as signed integer, unsigned integer, or floating-point number.

    Numbers are hashed by their value converted to number_float_t. Converting an integer to number_float_t therefore keeps its hash, but converting a floating-point number to an integer type is lossy and can change it: 0.5 converts to 0, which need not have the same hash. Unequal numbers can also share a hash value, for example two large integers that convert to the same number_float_t.

    "},{"location":"api/basic_json/std_hash/#examples","title":"Examples","text":"Example

    The example shows how to calculate hash values for different JSON values.

    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    std::cout << \"hash(null) = \" << std::hash<json> {}(json(nullptr)) << '\\n'\n              << \"hash(false) = \" << std::hash<json> {}(json(false)) << '\\n'\n              << \"hash(0) = \" << std::hash<json> {}(json(0)) << '\\n'\n              << \"hash(0U) = \" << std::hash<json> {}(json(0U)) << '\\n'\n              << \"hash(0.0) = \" << std::hash<json> {}(json(0.0)) << '\\n'\n              << \"hash(\\\"\\\") = \" << std::hash<json> {}(json(\"\")) << '\\n'\n              << \"hash({}) = \" << std::hash<json> {}(json::object()) << '\\n'\n              << \"hash([]) = \" << std::hash<json> {}(json::array()) << '\\n'\n              << \"hash({\\\"hello\\\": \\\"world\\\"}) = \" << std::hash<json> {}(\"{\\\"hello\\\": \\\"world\\\"}\"_json)\n              << std::endl;\n}\n

    Output:

    hash(null) = 2654435769\nhash(false) = 2654436030\nhash(0) = 2654436221\nhash(0U) = 2654436221\nhash(0.0) = 2654436221\nhash(\"\") = 11160318156688833227\nhash({}) = 2654435832\nhash([]) = 2654435899\nhash({\"hello\": \"world\"}) = 3701319991624763853\n

    The hash values shown are examples only. They depend on the platform, the compiler, and the compiler version, and they can change between versions of this library. Do not persist them or rely on specific values.

    "},{"location":"api/basic_json/std_hash/#see-also","title":"See also","text":"
    • operator== compares two JSON values for equality, consistent with equal hash values
    "},{"location":"api/basic_json/std_hash/#version-history","title":"Version history","text":"
    • Added in version 1.0.0.
    • Extended for arbitrary basic_json types in version 3.10.5.
    • Numbers that compare equal hash equally since version 3.13.0 unreleased; before, 0, 0U, and 0.0 had different hash values.
    "},{"location":"api/basic_json/std_swap/","title":"std::swap<basic_json>","text":"
    namespace std {\n    void swap(nlohmann::basic_json& j1, nlohmann::basic_json& j2);\n}\n

    Exchanges the values of two JSON objects.

    "},{"location":"api/basic_json/std_swap/#parameters","title":"Parameters","text":"j1 (in, out) value to be replaced by j2 j2 (in, out) value to be replaced by j1"},{"location":"api/basic_json/std_swap/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this function never throws exceptions.

    "},{"location":"api/basic_json/std_swap/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json/std_swap/#possible-implementation","title":"Possible implementation","text":"
    void swap(nlohmann::basic_json& j1, nlohmann::basic_json& j2)\n{\n    j1.swap(j2);\n}\n
    "},{"location":"api/basic_json/std_swap/#examples","title":"Examples","text":"Example

    The following code shows how two values are swapped with std::swap.

    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j1 = {{\"one\", 1}, {\"two\", 2}};\n    json j2 = {1, 2, 4, 8, 16};\n\n    std::cout << \"j1 = \" << j1 << \" | j2 = \" << j2 << '\\n';\n\n    // swap values\n    std::swap(j1, j2);\n\n    std::cout << \"j1 = \" << j1 << \" | j2 = \" << j2 << std::endl;\n}\n

    Output:

    j1 = {\"one\":1,\"two\":2} | j2 = [1,2,4,8,16]\nj1 = [1,2,4,8,16] | j2 = {\"one\":1,\"two\":2}\n
    "},{"location":"api/basic_json/std_swap/#see-also","title":"See also","text":"
    • swap
    "},{"location":"api/basic_json/std_swap/#version-history","title":"Version history","text":"
    • Added in version 1.0.0.
    • Extended for arbitrary basic_json types in version 3.10.5.
    "},{"location":"api/basic_json/string_t/","title":"nlohmann::basic_json::string_t","text":"
    using string_t = StringType;\n

    The type used to store JSON strings.

    RFC 8259 describes JSON strings as follows:

    A string is a sequence of zero or more Unicode characters.

    To store strings in C++, a type is defined by the template parameter described below. Unicode values are split by the JSON class into byte-sized characters during deserialization.

    "},{"location":"api/basic_json/string_t/#template-parameters","title":"Template parameters","text":"StringType

    the container to store strings (e.g., std::string). Note this container is used for keys/names in objects, see object_t.

    StringType must have a char-compatible value_type: the library relies on UTF-8/char-based storage and processing internally, so std::wstring, std::u16string, and std::u32string are not valid choices for StringType. To work with wide-character data, convert it to/from UTF-8 at the boundary instead -- see the FAQ's wide string handling section for a conversion recipe.

    Beyond the character type, the library expects a substantial part of the std::string interface (contiguous null-terminated data(), substr(), find(), append(), ...). See Template Parameter Requirements for the full list and for the string types that are known to work.

    "},{"location":"api/basic_json/string_t/#notes","title":"Notes","text":""},{"location":"api/basic_json/string_t/#default-type","title":"Default type","text":"

    With the default values for StringType (std::string), the default type for string_t is std::string.

    "},{"location":"api/basic_json/string_t/#encoding","title":"Encoding","text":"

    Strings are stored in UTF-8 encoding. Therefore, functions like std::string::size() or std::string::length() return the number of bytes in the string rather than the number of characters or glyphs.

    "},{"location":"api/basic_json/string_t/#string-comparison","title":"String comparison","text":"

    RFC 8259 states:

    Software implementations are typically required to test names of object members for equality. Implementations that transform the textual representation into sequences of Unicode code units and then perform the comparison numerically, code unit by code unit, are interoperable in the sense that implementations will agree in all cases on equality or inequality of two strings. For example, implementations that compare strings with escaped characters unconverted may incorrectly find that \"a\\\\b\" and \"a\\u005Cb\" are not equal.

    This implementation is interoperable as it does compare strings code unit by code unit.

    "},{"location":"api/basic_json/string_t/#storage","title":"Storage","text":"

    String values are stored as pointers in a basic_json type. That is, for any access to string values, a pointer of type string_t* must be dereferenced.

    "},{"location":"api/basic_json/string_t/#cross-basic_json-conversion-requirements","title":"Cross-basic_json conversion requirements","text":"

    When converting a string value from one basic_json specialization to another via the converting constructor (overload 4), the target string_t must be directly constructible from the source basic_json's string_t type. If this requirement is not met, the conversion does not fail; instead, the string is silently converted as an array of character codes, which is incorrect. See issue #3425 for details and an example.

    "},{"location":"api/basic_json/string_t/#examples","title":"Examples","text":"Example

    The following code shows that string_t is by default, a typedef to std::string.

    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::boolalpha << std::is_same<std::string, json::string_t>::value << std::endl;\n}\n

    Output:

    true\n
    "},{"location":"api/basic_json/string_t/#see-also","title":"See also","text":"
    • object_t the type used to store JSON objects (and their keys, which are also string_t)
    • binary_t the type used to store binary values
    • get_ptr returns a pointer to the stored string value
    "},{"location":"api/basic_json/string_t/#version-history","title":"Version history","text":"
    • Added in version 1.0.0.
    • Removed the requirement that string_t be implicitly convertible from std::string, which the BSON writer and the UBJSON reader relied on, in version 3.13.0 unreleased.
    "},{"location":"api/basic_json/swap/","title":"nlohmann::basic_json::swap","text":"
    // (1)\nvoid swap(reference other) noexcept (\n    std::is_nothrow_move_constructible<value_t>::value &&\n    std::is_nothrow_move_assignable<value_t>::value &&\n    std::is_nothrow_move_constructible<json_value>::value &&\n    std::is_nothrow_move_assignable<json_value>::value &&\n    std::is_nothrow_move_constructible<json_base_class_t>::value &&\n    std::is_nothrow_move_assignable<json_base_class_t>::value\n);\n\n// (2)\nfriend void swap(reference left, reference right) noexcept (\n    std::is_nothrow_move_constructible<value_t>::value &&\n    std::is_nothrow_move_assignable<value_t>::value &&\n    std::is_nothrow_move_constructible<json_value>::value &&\n    std::is_nothrow_move_assignable<json_value>::value &&\n    std::is_nothrow_move_constructible<json_base_class_t>::value &&\n    std::is_nothrow_move_assignable<json_base_class_t>::value\n);\n\n// (3)\nvoid swap(array_t& other);\n\n// (4)\nvoid swap(object_t& other);\n\n// (5)\nvoid swap(string_t& other);\n\n// (6)\nvoid swap(binary_t& other);\n\n// (7)\nvoid swap(typename binary_t::container_type& other);\n
    1. Exchanges the contents of the JSON value with those of other. Does not invoke any move, copy, or swap operations on individual elements. All iterators and references remain valid. The past-the-end iterator is invalidated. If macro JSON_DIAGNOSTIC_POSITIONS is defined to 1, the start_pos()/end_pos() diagnostic positions are exchanged along with the value. The json_base_class_t subobject is exchanged along with the value as well, the same way it is copied or moved by the copy/move constructors and assignment operators.
    2. Exchanges the contents of the JSON value from left with those of right. Does not invoke any move, copy, or swap operations on individual elements. All iterators and references remain valid. The past-the-end iterator is invalidated. Implemented as a friend function callable via ADL. If macro JSON_DIAGNOSTIC_POSITIONS is defined to 1, the start_pos()/end_pos() diagnostic positions are exchanged along with the value. The json_base_class_t subobject is exchanged along with the value as well, the same way it is copied or moved by the copy/move constructors and assignment operators.
    3. Exchanges the contents of a JSON array with those of other. Does not invoke any move, copy, or swap operations on individual elements. All iterators and references remain valid. The past-the-end iterator is invalidated.
    4. Exchanges the contents of a JSON object with those of other. Does not invoke any move, copy, or swap operations on individual elements. All iterators and references remain valid. The past-the-end iterator is invalidated.
    5. Exchanges the contents of a JSON string with those of other. Does not invoke any move, copy, or swap operations on individual elements. All iterators and references remain valid. The past-the-end iterator is invalidated.
    6. Exchanges the contents of a binary value with those of other. Does not invoke any move, copy, or swap operations on individual elements. All iterators and references remain valid. The past-the-end iterator is invalidated.
    7. Exchanges the contents of a binary value with those of other. Does not invoke any move, copy, or swap operations on individual elements. All iterators and references remain valid. The past-the-end iterator is invalidated. Unlike version (6), no binary subtype is involved.
    "},{"location":"api/basic_json/swap/#parameters","title":"Parameters","text":"other (in, out) value to exchange the contents with left (in, out) value to exchange the contents with right (in, out) value to exchange the contents with"},{"location":"api/basic_json/swap/#exception-safety","title":"Exception safety","text":"
    1. No-throw guarantee: this function never throws exceptions.
    2. No-throw guarantee: this function never throws exceptions.
    3. Strong guarantee: if an exception is thrown, there are no changes to any JSON value.
    4. Strong guarantee: if an exception is thrown, there are no changes to any JSON value.
    5. Strong guarantee: if an exception is thrown, there are no changes to any JSON value.
    6. Strong guarantee: if an exception is thrown, there are no changes to any JSON value.
    7. Strong guarantee: if an exception is thrown, there are no changes to any JSON value.
    "},{"location":"api/basic_json/swap/#exceptions","title":"Exceptions","text":"
    1. No-throw guarantee: this function never throws exceptions.
    2. No-throw guarantee: this function never throws exceptions.
    3. Throws type_error.310 if called on JSON values other than arrays; example: \"cannot use swap(array_t&) with boolean\"
    4. Throws type_error.310 if called on JSON values other than objects; example: \"cannot use swap(object_t&) with boolean\"
    5. Throws type_error.310 if called on JSON values other than strings; example: \"cannot use swap(string_t&) with boolean\"
    6. Throws type_error.310 if called on JSON values other than binaries; example: \"cannot use swap(binary_t&) with boolean\"
    7. Throws type_error.310 if called on JSON values other than binaries; example: \"cannot use swap(binary_t::container_type&) with boolean\"
    "},{"location":"api/basic_json/swap/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json/swap/#examples","title":"Examples","text":"Example: (1, 2) swap JSON values

    The example below shows how JSON values can be swapped with swap().

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create two JSON values\n    json j1 = {1, 2, 3, 4, 5};\n    json j2 = {{\"pi\", 3.141592653589793}, {\"e\", 2.718281828459045}};\n\n    // swap the values\n    j1.swap(j2);\n\n    // output the values\n    std::cout << \"j1 = \" << j1 << '\\n';\n    std::cout << \"j2 = \" << j2 << '\\n';\n}\n

    Output:

    j1 = {\"e\":2.718281828459045,\"pi\":3.141592653589793}\nj2 = [1,2,3,4,5]\n
    Example: (3) swap array

    The example below shows how arrays can be swapped with swap().

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON value\n    json value = {{\"array\", {1, 2, 3, 4}}};\n\n    // create an array_t\n    json::array_t array = {\"Snap\", \"Crackle\", \"Pop\"};\n\n    // swap the array stored in the JSON value\n    value[\"array\"].swap(array);\n\n    // output the values\n    std::cout << \"value = \" << value << '\\n';\n    std::cout << \"array = \" << array << '\\n';\n}\n

    Output:

    value = {\"array\":[\"Snap\",\"Crackle\",\"Pop\"]}\narray = [1,2,3,4]\n
    Example: (4) swap object

    The example below shows how objects can be swapped with swap().

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON value\n    json value = { {\"translation\", {{\"one\", \"eins\"}, {\"two\", \"zwei\"}}} };\n\n    // create an object_t\n    json::object_t object = {{\"cow\", \"Kuh\"}, {\"dog\", \"Hund\"}};\n\n    // swap the object stored in the JSON value\n    value[\"translation\"].swap(object);\n\n    // output the values\n    std::cout << \"value = \" << value << '\\n';\n    std::cout << \"object = \" << object << '\\n';\n}\n

    Output:

    value = {\"translation\":{\"cow\":\"Kuh\",\"dog\":\"Hund\"}}\nobject = {\"one\":\"eins\",\"two\":\"zwei\"}\n
    Example: (5) swap string

    The example below shows how strings can be swapped with swap().

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON value\n    json value = { \"the good\", \"the bad\", \"the ugly\" };\n\n    // create string_t\n    json::string_t string = \"the fast\";\n\n    // swap the object stored in the JSON value\n    value[1].swap(string);\n\n    // output the values\n    std::cout << \"value = \" << value << '\\n';\n    std::cout << \"string = \" << string << '\\n';\n}\n

    Output:

    value = [\"the good\",\"the fast\",\"the ugly\"]\nstring = the bad\n
    Example: (6) swap binary

    The example below shows how binary values can be swapped with swap().

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a binary value\n    json value = json::binary({1, 2, 3});\n\n    // create a binary_t\n    json::binary_t binary = {{4, 5, 6}};\n\n    // swap the object stored in the JSON value\n    value.swap(binary);\n\n    // output the values\n    std::cout << \"value = \" << value << '\\n';\n    std::cout << \"binary = \" << json(binary) << '\\n';\n}\n

    Output:

    value = {\"bytes\":[4,5,6],\"subtype\":null}\nbinary = {\"bytes\":[1,2,3],\"subtype\":null}\n
    "},{"location":"api/basic_json/swap/#see-also","title":"See also","text":"
    • std::swap<basic_json>
    • operator= copy assignment
    • basic_json create a JSON value
    "},{"location":"api/basic_json/swap/#version-history","title":"Version history","text":"
    1. Since version 1.0.0. Exchanges the json_base_class_t subobject along with the value since version 3.13.0 unreleased.
    2. Since version 1.0.0. Exchanges the json_base_class_t subobject along with the value since version 3.13.0 unreleased.
    3. Since version 1.0.0.
    4. Since version 1.0.0.
    5. Since version 1.0.0.
    6. Since version 3.8.0.
    7. Since version 3.8.0.
    "},{"location":"api/basic_json/to_bjdata/","title":"nlohmann::basic_json::to_bjdata","text":"
    // (1)\nstatic std::vector<std::uint8_t> to_bjdata(const basic_json& j,\n                                           const bool use_size = false,\n                                           const bool use_type = false,\n                                           const bjdata_version_t version = bjdata_version_t::draft2,\n                                           const error_handler_t error_handler = error_handler_t::keep);\n\n// (2)\nstatic void to_bjdata(const basic_json& j, detail::output_adapter<std::uint8_t> o,\n                      const bool use_size = false, const bool use_type = false,\n                      const bjdata_version_t version = bjdata_version_t::draft2,\n                      const error_handler_t error_handler = error_handler_t::keep);\nstatic void to_bjdata(const basic_json& j, detail::output_adapter<char> o,\n                      const bool use_size = false, const bool use_type = false,\n                      const bjdata_version_t version = bjdata_version_t::draft2,\n                      const error_handler_t error_handler = error_handler_t::keep);\n

    Serializes a given JSON value j to a byte vector using the BJData (Binary JData) serialization format. BJData aims to be more compact than JSON itself, yet more efficient to parse.

    1. Returns a byte vector containing the BJData serialization.
    2. Writes the BJData serialization to an output adapter.

    The exact mapping and its limitations are described on a dedicated page.

    "},{"location":"api/basic_json/to_bjdata/#parameters","title":"Parameters","text":"j (in) JSON value to serialize o (in) output adapter to write serialization to use_size (in) whether to add size annotations to container types; optional, false by default. use_type (in) whether to add type annotations to container types (must be combined with use_size = true); optional, false by default. version (in) which version of BJData to use (see note on \"Binary values\" on BJData); optional, bjdata_version_t::draft2 by default. error_handler (in) how to treat a string or object key in j that is not valid UTF-8; see error_handler_t. The default, keep, writes the ill-formed bytes to the output as is, as every version of to_bjdata did before this parameter was added; strict throws; replace/ignore sanitize it the same way dump would. If JSON_STRICT_BINARY_UTF8 is enabled, the default is strict instead."},{"location":"api/basic_json/to_bjdata/#return-value","title":"Return value","text":"
    1. BJData serialization as byte vector
    2. (none)
    "},{"location":"api/basic_json/to_bjdata/#exception-safety","title":"Exception safety","text":"

    Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

    "},{"location":"api/basic_json/to_bjdata/#exceptions","title":"Exceptions","text":"
    • Throws other_error.502 if use_type is true and use_size is false, and j contains a non-empty array, object, or binary value.
    • Throws type_error.316 if a string or object key in j is not valid UTF-8 and error_handler is strict (the default only if JSON_STRICT_BINARY_UTF8 is enabled)
    • Throws type_error.321 if j or a value nested in it is discarded; example: \"cannot serialize discarded value to BJData\"
    "},{"location":"api/basic_json/to_bjdata/#complexity","title":"Complexity","text":"

    Linear in the size of the JSON value j.

    "},{"location":"api/basic_json/to_bjdata/#examples","title":"Examples","text":"Example: serialize a JSON value to BJData

    The example shows the serialization of a JSON value to a byte vector in BJData format.

    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\n// function to print BJData's diagnostic format\nvoid print_byte(uint8_t byte)\n{\n    if (32 < byte and byte < 128)\n    {\n        std::cout << (char)byte;\n    }\n    else\n    {\n        std::cout << (int)byte;\n    }\n}\n\nint main()\n{\n    // create a JSON value\n    json j = R\"({\"compact\": true, \"schema\": false})\"_json;\n\n    // serialize it to BJData\n    std::vector<std::uint8_t> v = json::to_bjdata(j);\n\n    // print the vector content\n    for (auto& byte : v)\n    {\n        print_byte(byte);\n    }\n    std::cout << std::endl;\n\n    // create an array of numbers\n    json array = {1, 2, 3, 4, 5, 6, 7, 8};\n\n    // serialize it to BJData using default representation\n    std::vector<std::uint8_t> v_array = json::to_bjdata(array);\n    // serialize it to BJData using size optimization\n    std::vector<std::uint8_t> v_array_size = json::to_bjdata(array, true);\n    // serialize it to BJData using type optimization\n    std::vector<std::uint8_t> v_array_size_and_type = json::to_bjdata(array, true, true);\n\n    // print the vector contents\n    for (auto& byte : v_array)\n    {\n        print_byte(byte);\n    }\n    std::cout << std::endl;\n\n    for (auto& byte : v_array_size)\n    {\n        print_byte(byte);\n    }\n    std::cout << std::endl;\n\n    for (auto& byte : v_array_size_and_type)\n    {\n        print_byte(byte);\n    }\n    std::cout << std::endl;\n}\n

    Output:

    {i7compactTi6schemaF}\n[i1i2i3i4i5i6i7i8]\n[#i8i1i2i3i4i5i6i7i8\n[$i#i812345678\n
    Example: other_error.502 exception

    The example shows how requesting type annotations (use_type) without size annotations (use_size) throws an exception, because type-optimized containers can only be read back with a preceding size.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a non-empty JSON array\n    json j = {1, 2, 3};\n\n    // exception other_error.502\n    try\n    {\n        json::to_bjdata(j, false, true);\n    }\n    catch (const json::other_error& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

    Output:

    [json.exception.other_error.502] use_type requires use_size = true\n
    "},{"location":"api/basic_json/to_bjdata/#see-also","title":"See also","text":"
    • from_bjdata create a JSON value from an input in BJData format
    • to_cbor create a CBOR serialization of a JSON value
    • to_msgpack create a MessagePack serialization of a JSON value
    • to_bson create a BSON serialization of a JSON value
    • to_ubjson create a UBJSON serialization of a JSON value
    • to_bon8 create a BON8 serialization of a JSON value
    "},{"location":"api/basic_json/to_bjdata/#version-history","title":"Version history","text":"
    • Added in version 3.11.0.
    • BJData version parameter (for draft3 binary encoding) added in version 3.12.0.
    • Added error_handler parameter in version 3.13.0 unreleased. Its default, keep, writes the bytes of a string or object key that is not valid UTF-8 unchanged, as before; strict (the default if JSON_STRICT_BINARY_UTF8 is enabled) throws type_error.316.
    • Throws type_error.321 for a discarded value since version 3.13.0 unreleased; previously, a discarded value nested in an array or object was silently skipped, producing invalid BJData.
    "},{"location":"api/basic_json/to_bon8/","title":"nlohmann::basic_json::to_bon8","text":"
    // (1)\nstatic std::vector<std::uint8_t> to_bon8(const basic_json& j);\n\n// (2)\nstatic void to_bon8(const basic_json& j, detail::output_adapter<std::uint8_t> o);\nstatic void to_bon8(const basic_json& j, detail::output_adapter<char> o);\n

    Serializes a given JSON value j to a byte vector using the BON8 (Binary Object Notation 8) serialization format. BON8 is a compact binary serialization format that stores strings as UTF-8 without a length prefix.

    1. Returns a byte vector containing the BON8 serialization.
    2. Writes the BON8 serialization to an output adapter.

    The exact mapping and its limitations are described on a dedicated page.

    "},{"location":"api/basic_json/to_bon8/#parameters","title":"Parameters","text":"j (in) JSON value to serialize o (in) output adapter to write serialization to"},{"location":"api/basic_json/to_bon8/#return-value","title":"Return value","text":"
    1. BON8 serialization as a byte vector
    2. (none)
    "},{"location":"api/basic_json/to_bon8/#exception-safety","title":"Exception safety","text":"

    Strong guarantee: if an exception is thrown, there are no changes in the JSON value j, which is never modified. With (2), the bytes written before the exception remain in the output adapter.

    "},{"location":"api/basic_json/to_bon8/#exceptions","title":"Exceptions","text":"
    • Throws out_of_range.407 if j contains an unsigned integer above 9223372036854775807, which BON8 cannot represent
    • Throws type_error.316 if j contains a string that is not valid UTF-8
    • Throws type_error.321 if j or a value nested in it is discarded; example: \"cannot serialize discarded value to BON8\"
    "},{"location":"api/basic_json/to_bon8/#complexity","title":"Complexity","text":"

    Linear in the size of the JSON value j.

    "},{"location":"api/basic_json/to_bon8/#examples","title":"Examples","text":"Example: serialize a JSON value to BON8

    The example shows the serialization of a JSON value to a byte vector in BON8 format.

    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // create a JSON value\n    json j = R\"({\"compact\": true, \"format\": \"BON8\", \"schema\": 0})\"_json;\n\n    // serialize it to BON8\n    std::vector<std::uint8_t> v = json::to_bon8(j);\n\n    // print the vector content\n    for (auto& byte : v)\n    {\n        std::cout << \"0x\" << std::hex << std::setw(2) << std::setfill('0') << (int)byte << \" \";\n    }\n    std::cout << std::endl;\n}\n

    Output:

    0x89 0x63 0x6f 0x6d 0x70 0x61 0x63 0x74 0xf9 0x66 0x6f 0x72 0x6d 0x61 0x74 0xff 0x42 0x4f 0x4e 0x38 0xff 0x73 0x63 0x68 0x65 0x6d 0x61 0x90 \n
    Example: type_error.316 exception

    The example shows how serializing a string that is not valid UTF-8 throws an exception, because BON8 stores strings as UTF-8.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON string that is not valid UTF-8\n    std::string invalid_utf8;\n    invalid_utf8.push_back(static_cast<char>(0xFF));\n    json j = invalid_utf8;\n\n    // exception type_error.316\n    try\n    {\n        json::to_bon8(j);\n    }\n    catch (const json::type_error& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

    Output:

    [json.exception.type_error.316] invalid UTF-8 byte at index 0: 0xFF\n
    "},{"location":"api/basic_json/to_bon8/#see-also","title":"See also","text":"
    • from_bon8 create a JSON value from an input in BON8 format
    • to_cbor create a CBOR serialization of a JSON value
    • to_msgpack create a MessagePack serialization of a JSON value
    • to_bson create a BSON serialization of a JSON value
    • to_ubjson create a UBJSON serialization of a JSON value
    • to_bjdata create a BJData serialization of a JSON value
    "},{"location":"api/basic_json/to_bon8/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json/to_bson/","title":"nlohmann::basic_json::to_bson","text":"
    // (1)\nstatic std::vector<std::uint8_t> to_bson(const basic_json& j,\n                                         const error_handler_t error_handler = error_handler_t::keep);\n\n// (2)\nstatic void to_bson(const basic_json& j, detail::output_adapter<std::uint8_t> o,\n                    const error_handler_t error_handler = error_handler_t::keep);\nstatic void to_bson(const basic_json& j, detail::output_adapter<char> o,\n                    const error_handler_t error_handler = error_handler_t::keep);\n

    BSON (Binary JSON) is a binary format in which zero or more ordered key/value pairs are stored as a single entity (a so-called document).

    1. Returns a byte vector containing the BSON serialization.
    2. Writes the BSON serialization to an output adapter.

    The exact mapping and its limitations are described on a dedicated page.

    "},{"location":"api/basic_json/to_bson/#parameters","title":"Parameters","text":"j (in) JSON value to serialize o (in) output adapter to write serialization to error_handler (in) how to treat a string or object key in j that is not valid UTF-8; see error_handler_t. The default, keep, writes the ill-formed bytes to the output as is, as every version of to_bson did before this parameter was added; strict throws; replace/ignore sanitize it the same way dump would. If JSON_STRICT_BINARY_UTF8 is enabled, the default is strict instead."},{"location":"api/basic_json/to_bson/#return-value","title":"Return value","text":"
    1. BSON serialization as a byte vector
    2. (none)
    "},{"location":"api/basic_json/to_bson/#exception-safety","title":"Exception safety","text":"

    Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

    "},{"location":"api/basic_json/to_bson/#exceptions","title":"Exceptions","text":"
    • Throws type_error.317 if the top-level type of the JSON value is not an object; example: \"to serialize to BSON, top-level type must be object, but is string\"
    • Throws out_of_range.409 if a key in the JSON object contains a null byte (code point U+0000); example: \"BSON key cannot contain code point U+0000 (at byte 2)\"
    • Throws out_of_range.412 if the length of a document, array, string, or binary value exceeds the range of the 32-bit BSON length field; example: \"BSON length 2147483661 exceeds maximum of 2147483647\"
    • Throws out_of_range.415 if the subtype of a binary value exceeds 255, the maximum of the BSON binary subtype; example: \"subtype 70000 is too large for the BSON binary subtype (max 255)\"
    • Throws type_error.316 if a string or object key is not valid UTF-8 and error_handler is strict (the default only if JSON_STRICT_BINARY_UTF8 is enabled)
    • Throws type_error.321 if a value nested in j is discarded (the top-level value itself is covered by type_error.317 above, since it must be an object); example: \"cannot serialize discarded value to BSON\"
    "},{"location":"api/basic_json/to_bson/#complexity","title":"Complexity","text":"

    Linear in the size of the JSON value j. The length prefixes of all nested documents and arrays are computed in one pass before anything is written.

    "},{"location":"api/basic_json/to_bson/#examples","title":"Examples","text":"Example: serialize a JSON value to BSON

    The example shows the serialization of a JSON value to a byte vector in BSON format.

    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // create a JSON value\n    json j = R\"({\"compact\": true, \"schema\": 0})\"_json;\n\n    // serialize it to BSON\n    std::vector<std::uint8_t> v = json::to_bson(j);\n\n    // print the vector content\n    for (auto& byte : v)\n    {\n        std::cout << \"0x\" << std::hex << std::setw(2) << std::setfill('0') << (int)byte << \" \";\n    }\n    std::cout << std::endl;\n}\n

    Output:

    0x1b 0x00 0x00 0x00 0x08 0x63 0x6f 0x6d 0x70 0x61 0x63 0x74 0x00 0x01 0x10 0x73 0x63 0x68 0x65 0x6d 0x61 0x00 0x00 0x00 0x00 0x00 0x00 \n
    Example: out_of_range.409 exception

    The example shows how serializing a JSON object whose key contains a null byte (U+0000) throws an exception, because BSON keys are null-terminated C strings and cannot contain U+0000 themselves.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON object whose key contains a null byte (U+0000)\n    std::string key = \"ab\";\n    key.push_back('\\0');\n    key.push_back('c');\n    json j = {{key, 1}};\n\n    // exception out_of_range.409\n    try\n    {\n        json::to_bson(j);\n    }\n    catch (const json::out_of_range& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

    Output:

    [json.exception.out_of_range.409] BSON key cannot contain code point U+0000 (at byte 2)\n
    "},{"location":"api/basic_json/to_bson/#see-also","title":"See also","text":"
    • from_bson create a JSON value from an input in BSON format
    • to_cbor create a CBOR serialization of a JSON value
    • to_msgpack create a MessagePack serialization of a JSON value
    • to_ubjson create a UBJSON serialization of a JSON value
    • to_bjdata create a BJData serialization of a JSON value
    • to_bon8 create a BON8 serialization of a JSON value
    "},{"location":"api/basic_json/to_bson/#version-history","title":"Version history","text":"
    • Added in version 3.4.0.
    • Throws out_of_range.412 and out_of_range.415 since version 3.13.0 unreleased.
    • Linear in the size of j, and no longer limited by the call stack for deeply nested values, since version 3.13.0 unreleased.
    • out_of_range.415 is now detected before anything is written, like the other exceptions above, since version 3.13.0 unreleased.
    • Throws type_error.321 for a discarded value nested in j since version 3.13.0 unreleased; previously, it was silently skipped, producing a document whose declared size did not match what was actually written.
    • Added error_handler parameter in version 3.13.0 unreleased. Its default, keep, writes the bytes of a string or object key that is not valid UTF-8 unchanged, as before; strict (the default if JSON_STRICT_BINARY_UTF8 is enabled) throws type_error.316 before anything is written.
    "},{"location":"api/basic_json/to_cbor/","title":"nlohmann::basic_json::to_cbor","text":"
    // (1)\nstatic std::vector<std::uint8_t> to_cbor(const basic_json& j,\n                                          const error_handler_t error_handler = error_handler_t::keep);\n\n// (2)\nstatic void to_cbor(const basic_json& j, detail::output_adapter<std::uint8_t> o,\n                     const error_handler_t error_handler = error_handler_t::keep);\nstatic void to_cbor(const basic_json& j, detail::output_adapter<char> o,\n                     const error_handler_t error_handler = error_handler_t::keep);\n

    Serializes a given JSON value j to a byte vector using the CBOR (Concise Binary Object Representation) serialization format. CBOR is a binary serialization format that aims to be more compact than JSON itself, yet more efficient to parse.

    1. Returns a byte vector containing the CBOR serialization.
    2. Writes the CBOR serialization to an output adapter.

    The exact mapping and its limitations are described on a dedicated page.

    "},{"location":"api/basic_json/to_cbor/#parameters","title":"Parameters","text":"j (in) JSON value to serialize o (in) output adapter to write serialization to error_handler (in) how to treat a string or object key in j that is not valid UTF-8; see error_handler_t. The default, keep, writes the ill-formed bytes to the output as is, as every version of to_cbor did before this parameter was added; strict throws; replace/ignore sanitize it the same way dump would. If JSON_STRICT_BINARY_UTF8 is enabled, the default is strict instead."},{"location":"api/basic_json/to_cbor/#return-value","title":"Return value","text":"
    1. CBOR serialization as a byte vector
    2. (none)
    "},{"location":"api/basic_json/to_cbor/#exception-safety","title":"Exception safety","text":"

    Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

    "},{"location":"api/basic_json/to_cbor/#exceptions","title":"Exceptions","text":"
    • Throws type_error.316 if a string or object key in j is not valid UTF-8 and error_handler is strict (the default only if JSON_STRICT_BINARY_UTF8 is enabled)
    • Throws type_error.321 if j or a value nested in it is discarded; example: \"cannot serialize discarded value to CBOR\"
    "},{"location":"api/basic_json/to_cbor/#complexity","title":"Complexity","text":"

    Linear in the size of the JSON value j.

    "},{"location":"api/basic_json/to_cbor/#examples","title":"Examples","text":"Example

    The example shows the serialization of a JSON value to a byte vector in CBOR format.

    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // create a JSON value\n    json j = R\"({\"compact\": true, \"schema\": 0})\"_json;\n\n    // serialize it to CBOR\n    std::vector<std::uint8_t> v = json::to_cbor(j);\n\n    // print the vector content\n    for (auto& byte : v)\n    {\n        std::cout << \"0x\" << std::hex << std::setw(2) << std::setfill('0') << (int)byte << \" \";\n    }\n    std::cout << std::endl;\n}\n

    Output:

    0xa2 0x67 0x63 0x6f 0x6d 0x70 0x61 0x63 0x74 0xf5 0x66 0x73 0x63 0x68 0x65 0x6d 0x61 0x00 \n
    "},{"location":"api/basic_json/to_cbor/#see-also","title":"See also","text":"
    • from_cbor create a JSON value from an input in CBOR format
    • to_msgpack create a MessagePack serialization of a JSON value
    • to_bson create a BSON serialization of a JSON value
    • to_ubjson create a UBJSON serialization of a JSON value
    • to_bjdata create a BJData serialization of a JSON value
    • to_bon8 create a BON8 serialization of a JSON value
    "},{"location":"api/basic_json/to_cbor/#version-history","title":"Version history","text":"
    • Added in version 2.0.9.
    • Compact representation of floating-point numbers added in version 3.8.0.
    • Added error_handler parameter in version 3.13.0 unreleased. Its default, keep, writes the bytes of a string or object key that is not valid UTF-8 unchanged, as before; strict (the default if JSON_STRICT_BINARY_UTF8 is enabled) throws type_error.316.
    • Throws type_error.321 for a discarded value since version 3.13.0 unreleased; previously, a discarded value nested in an array or object was silently skipped, producing invalid CBOR.
    "},{"location":"api/basic_json/to_msgpack/","title":"nlohmann::basic_json::to_msgpack","text":"
    // (1)\nstatic std::vector<std::uint8_t> to_msgpack(const basic_json& j,\n                                            const error_handler_t error_handler = error_handler_t::keep);\n\n// (2)\nstatic void to_msgpack(const basic_json& j, detail::output_adapter<std::uint8_t> o,\n                       const error_handler_t error_handler = error_handler_t::keep);\nstatic void to_msgpack(const basic_json& j, detail::output_adapter<char> o,\n                       const error_handler_t error_handler = error_handler_t::keep);\n

    Serializes a given JSON value j to a byte vector using the MessagePack serialization format. MessagePack is a binary serialization format that aims to be more compact than JSON itself, yet more efficient to parse.

    1. Returns a byte vector containing the MessagePack serialization.
    2. Writes the MessagePack serialization to an output adapter.

    The exact mapping and its limitations are described on a dedicated page.

    "},{"location":"api/basic_json/to_msgpack/#parameters","title":"Parameters","text":"j (in) JSON value to serialize o (in) output adapter to write serialization to error_handler (in) how to treat a string or object key in j that is not valid UTF-8; see error_handler_t. The default, keep, writes the ill-formed bytes to the output as is, as every version of to_msgpack did before this parameter was added and as the MessagePack specification allows; strict throws; replace/ignore sanitize it the same way dump would. Unlike the other binary writers, the default stays keep even if JSON_STRICT_BINARY_UTF8 is enabled."},{"location":"api/basic_json/to_msgpack/#return-value","title":"Return value","text":"
    1. MessagePack serialization as a byte vector
    2. (none)
    "},{"location":"api/basic_json/to_msgpack/#exception-safety","title":"Exception safety","text":"

    Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

    "},{"location":"api/basic_json/to_msgpack/#exceptions","title":"Exceptions","text":"
    • Throws out_of_range.412 if the length of a string, binary value, array, or object exceeds 4294967295, the maximum MessagePack can store; example: \"MessagePack length 4294967296 exceeds maximum of 4294967295\"
    • Throws out_of_range.415 if the subtype of a binary value exceeds 255, the maximum of the MessagePack ext type; example: \"subtype 70000 is too large for the MessagePack ext type (max 255)\"
    • Throws type_error.316 if a string or object key in j is not valid UTF-8 and error_handler is strict
    • Throws type_error.321 if j or a value nested in it is discarded; example: \"cannot serialize discarded value to MessagePack\"
    "},{"location":"api/basic_json/to_msgpack/#complexity","title":"Complexity","text":"

    Linear in the size of the JSON value j.

    "},{"location":"api/basic_json/to_msgpack/#examples","title":"Examples","text":"Example: serialize a JSON value to MessagePack

    The example shows the serialization of a JSON value to a byte vector in MessagePack format.

    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // create a JSON value\n    json j = R\"({\"compact\": true, \"schema\": 0})\"_json;\n\n    // serialize it to MessagePack\n    std::vector<std::uint8_t> v = json::to_msgpack(j);\n\n    // print the vector content\n    for (auto& byte : v)\n    {\n        std::cout << \"0x\" << std::hex << std::setw(2) << std::setfill('0') << (int)byte << \" \";\n    }\n    std::cout << std::endl;\n}\n

    Output:

    0x82 0xa7 0x63 0x6f 0x6d 0x70 0x61 0x63 0x74 0xc3 0xa6 0x73 0x63 0x68 0x65 0x6d 0x61 0x00 \n
    Example: out_of_range.415 exception

    The example shows how serializing a binary value whose subtype exceeds 255 throws an exception, because the MessagePack ext type stores the subtype in a single byte.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON value with a binary subtype that exceeds 255\n    json j = json::binary({1, 2, 3}, 300);\n\n    // exception out_of_range.415\n    try\n    {\n        json::to_msgpack(j);\n    }\n    catch (const json::out_of_range& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

    Output:

    [json.exception.out_of_range.415] subtype 300 is too large for the MessagePack ext type (max 255)\n
    "},{"location":"api/basic_json/to_msgpack/#see-also","title":"See also","text":"
    • from_msgpack create a JSON value from an input in MessagePack format
    • to_cbor create a CBOR serialization of a JSON value
    • to_bson create a BSON serialization of a JSON value
    • to_ubjson create a UBJSON serialization of a JSON value
    • to_bjdata create a BJData serialization of a JSON value
    • to_bon8 create a BON8 serialization of a JSON value
    "},{"location":"api/basic_json/to_msgpack/#version-history","title":"Version history","text":"
    • Added in version 2.0.9.
    • Throws out_of_range.412 and out_of_range.415 since version 3.13.0 unreleased.
    • Added error_handler parameter in version 3.13.0 unreleased. Its default, keep, writes the bytes of a string or object key that is not valid UTF-8 unchanged, as before.
    • Fixed in version 3.13.0 unreleased to serialize number_integer_t/number_unsigned_t pairs of different width correctly; before, integers could be serialized with the wrong value if number_integer_t was narrower than number_unsigned_t.
    • Throws type_error.321 for a discarded value since version 3.13.0 unreleased; previously, a discarded value nested in an array or object was silently skipped, producing invalid MessagePack.
    "},{"location":"api/basic_json/to_string/","title":"to_string(basic_json)","text":"
    template <typename BasicJsonType>\nstd::string to_string(const BasicJsonType& j);\n

    This function implements a user-defined to_string for JSON objects.

    "},{"location":"api/basic_json/to_string/#template-parameters","title":"Template parameters","text":"BasicJsonType a specialization of basic_json whose string_t is convertible to std::string; for other string types, use dump, which returns a string_t"},{"location":"api/basic_json/to_string/#return-value","title":"Return value","text":"

    string containing the serialization of the JSON value

    "},{"location":"api/basic_json/to_string/#exception-safety","title":"Exception safety","text":"

    Strong guarantee: if an exception is thrown, there are no changes to any JSON value.

    "},{"location":"api/basic_json/to_string/#exceptions","title":"Exceptions","text":"

    Throws type_error.316 if a string stored inside the JSON value is not UTF-8 encoded

    "},{"location":"api/basic_json/to_string/#complexity","title":"Complexity","text":"

    Linear.

    "},{"location":"api/basic_json/to_string/#possible-implementation","title":"Possible implementation","text":"
    template <typename BasicJsonType>\nstd::string to_string(const BasicJsonType& j)\n{\n    return j.dump();\n}\n
    "},{"location":"api/basic_json/to_string/#examples","title":"Examples","text":"Example

    The following code shows how the library's to_string() function integrates with others, allowing argument-dependent lookup.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing std::to_string;\n\nint main()\n{\n    // create values\n    json j = {{\"one\", 1}, {\"two\", 2}};\n    int i = 42;\n\n    // use ADL to select best to_string function\n    auto j_str = to_string(j);  // calling nlohmann::to_string\n    auto i_str = to_string(i);  // calling std::to_string\n\n    // serialize without indentation\n    std::cout << j_str << \"\\n\\n\"\n              << i_str << std::endl;\n}\n

    Output:

    {\"one\":1,\"two\":2}\n\n42\n
    "},{"location":"api/basic_json/to_string/#see-also","title":"See also","text":"
    • dump
    • Serialization - the serialization article
    "},{"location":"api/basic_json/to_string/#version-history","title":"Version history","text":"

    Added in version 3.7.0.

    "},{"location":"api/basic_json/to_ubjson/","title":"nlohmann::basic_json::to_ubjson","text":"
    // (1)\nstatic std::vector<std::uint8_t> to_ubjson(const basic_json& j,\n                                           const bool use_size = false,\n                                           const bool use_type = false,\n                                           const error_handler_t error_handler = error_handler_t::keep);\n\n// (2)\nstatic void to_ubjson(const basic_json& j, detail::output_adapter<std::uint8_t> o,\n                      const bool use_size = false, const bool use_type = false,\n                      const error_handler_t error_handler = error_handler_t::keep);\nstatic void to_ubjson(const basic_json& j, detail::output_adapter<char> o,\n                      const bool use_size = false, const bool use_type = false,\n                      const error_handler_t error_handler = error_handler_t::keep);\n

    Serializes a given JSON value j to a byte vector using the UBJSON (Universal Binary JSON) serialization format. UBJSON aims to be more compact than JSON itself, yet more efficient to parse.

    1. Returns a byte vector containing the UBJSON serialization.
    2. Writes the UBJSON serialization to an output adapter.

    The exact mapping and its limitations are described on a dedicated page.

    "},{"location":"api/basic_json/to_ubjson/#parameters","title":"Parameters","text":"j (in) JSON value to serialize o (in) output adapter to write serialization to use_size (in) whether to add size annotations to container types; optional, false by default. use_type (in) whether to add type annotations to container types (must be combined with use_size = true); optional, false by default. error_handler (in) how to treat a string or object key in j that is not valid UTF-8; see error_handler_t. The default, keep, writes the ill-formed bytes to the output as is, as every version of to_ubjson did before this parameter was added; strict throws; replace/ignore sanitize it the same way dump would. If JSON_STRICT_BINARY_UTF8 is enabled, the default is strict instead."},{"location":"api/basic_json/to_ubjson/#return-value","title":"Return value","text":"
    1. UBJSON serialization as a byte vector
    2. (none)
    "},{"location":"api/basic_json/to_ubjson/#exception-safety","title":"Exception safety","text":"

    Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

    "},{"location":"api/basic_json/to_ubjson/#exceptions","title":"Exceptions","text":"
    • Throws other_error.502 if use_type is true and use_size is false, and j contains a non-empty array, object, or binary value.
    • Throws type_error.316 if a string or object key in j is not valid UTF-8 and error_handler is strict (the default only if JSON_STRICT_BINARY_UTF8 is enabled)
    • Throws type_error.321 if j or a value nested in it is discarded; example: \"cannot serialize discarded value to UBJSON\"
    "},{"location":"api/basic_json/to_ubjson/#complexity","title":"Complexity","text":"

    Linear in the size of the JSON value j.

    "},{"location":"api/basic_json/to_ubjson/#examples","title":"Examples","text":"Example: serialize a JSON value to UBJSON

    The example shows the serialization of a JSON value to a byte vector in UBJSON format.

    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\n// function to print UBJSON's diagnostic format\nvoid print_byte(uint8_t byte)\n{\n    if (32 < byte and byte < 128)\n    {\n        std::cout << (char)byte;\n    }\n    else\n    {\n        std::cout << (int)byte;\n    }\n}\n\nint main()\n{\n    // create a JSON value\n    json j = R\"({\"compact\": true, \"schema\": false})\"_json;\n\n    // serialize it to UBJSON\n    std::vector<std::uint8_t> v = json::to_ubjson(j);\n\n    // print the vector content\n    for (auto& byte : v)\n    {\n        print_byte(byte);\n    }\n    std::cout << std::endl;\n\n    // create an array of numbers\n    json array = {1, 2, 3, 4, 5, 6, 7, 8};\n\n    // serialize it to UBJSON using default representation\n    std::vector<std::uint8_t> v_array = json::to_ubjson(array);\n    // serialize it to UBJSON using size optimization\n    std::vector<std::uint8_t> v_array_size = json::to_ubjson(array, true);\n    // serialize it to UBJSON using type optimization\n    std::vector<std::uint8_t> v_array_size_and_type = json::to_ubjson(array, true, true);\n\n    // print the vector contents\n    for (auto& byte : v_array)\n    {\n        print_byte(byte);\n    }\n    std::cout << std::endl;\n\n    for (auto& byte : v_array_size)\n    {\n        print_byte(byte);\n    }\n    std::cout << std::endl;\n\n    for (auto& byte : v_array_size_and_type)\n    {\n        print_byte(byte);\n    }\n    std::cout << std::endl;\n}\n

    Output:

    {i7compactTi6schemaF}\n[i1i2i3i4i5i6i7i8]\n[#i8i1i2i3i4i5i6i7i8\n[$i#i812345678\n
    Example: other_error.502 exception

    The example shows how requesting type annotations (use_type) without size annotations (use_size) throws an exception, because type-optimized containers can only be read back with a preceding size.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a non-empty JSON array\n    json j = {1, 2, 3};\n\n    // exception other_error.502\n    try\n    {\n        json::to_ubjson(j, false, true);\n    }\n    catch (const json::other_error& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

    Output:

    [json.exception.other_error.502] use_type requires use_size = true\n
    "},{"location":"api/basic_json/to_ubjson/#see-also","title":"See also","text":"
    • from_ubjson create a JSON value from an input in UBJSON format
    • to_cbor create a CBOR serialization of a JSON value
    • to_msgpack create a MessagePack serialization of a JSON value
    • to_bson create a BSON serialization of a JSON value
    • to_bjdata create a BJData serialization of a JSON value
    • to_bon8 create a BON8 serialization of a JSON value
    "},{"location":"api/basic_json/to_ubjson/#version-history","title":"Version history","text":"
    • Added in version 3.1.0.
    • Added error_handler parameter in version 3.13.0 unreleased. Its default, keep, writes the bytes of a string or object key that is not valid UTF-8 unchanged, as before; strict (the default if JSON_STRICT_BINARY_UTF8 is enabled) throws type_error.316.
    • Throws type_error.321 for a discarded value since version 3.13.0 unreleased; previously, a discarded value nested in an array or object was silently skipped, producing invalid UBJSON.
    "},{"location":"api/basic_json/type/","title":"nlohmann::basic_json::type","text":"
    constexpr value_t type() const noexcept;\n

    Return the type of the JSON value as a value from the value_t enumeration.

    "},{"location":"api/basic_json/type/#return-value","title":"Return value","text":"

    the type of the JSON value

    Value type return value null value_t::null boolean value_t::boolean string value_t::string number (integer) value_t::number_integer number (unsigned integer) value_t::number_unsigned number (floating-point) value_t::number_float object value_t::object array value_t::array binary value_t::binary discarded value_t::discarded"},{"location":"api/basic_json/type/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this member function never throws exceptions.

    "},{"location":"api/basic_json/type/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json/type/#examples","title":"Examples","text":"Example

    The following code exemplifies type() for all JSON types.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_null;\n    json j_boolean = true;\n    json j_number_integer = -17;\n    json j_number_unsigned = 42u;\n    json j_number_float = 23.42;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n    json j_string = \"Hello, world\";\n\n    // call type()\n    std::cout << std::boolalpha;\n    std::cout << (j_null.type() == json::value_t::null) << '\\n';\n    std::cout << (j_boolean.type() == json::value_t::boolean) << '\\n';\n    std::cout << (j_number_integer.type() == json::value_t::number_integer) << '\\n';\n    std::cout << (j_number_unsigned.type() == json::value_t::number_unsigned) << '\\n';\n    std::cout << (j_number_float.type() == json::value_t::number_float) << '\\n';\n    std::cout << (j_object.type() == json::value_t::object) << '\\n';\n    std::cout << (j_array.type() == json::value_t::array) << '\\n';\n    std::cout << (j_string.type() == json::value_t::string) << '\\n';\n}\n

    Output:

    true\ntrue\ntrue\ntrue\ntrue\ntrue\ntrue\ntrue\n
    "},{"location":"api/basic_json/type/#see-also","title":"See also","text":"
    • operator value_t implicit conversion operator equivalent to this named member function
    • type_name returns the type as a string, for use in error messages
    • value_t the enumeration of JSON types
    • basic_json_view::type - the same function on a zero-copy view
    "},{"location":"api/basic_json/type/#version-history","title":"Version history","text":"
    • Added in version 1.0.0.
    • Added unsigned integer type in version 2.0.0.
    • Added binary type in version 3.8.0.
    "},{"location":"api/basic_json/type_error/","title":"nlohmann::basic_json::type_error","text":"
    class type_error : public exception;\n

    This exception is thrown in case of a type error; that is, a library function is executed on a JSON value whose type does not match the expected semantics.

    Exceptions have ids 3xx (see list of type errors).

    classDiagram\n  direction LR\n\n    class std_exception [\"std::exception\"] {\n        <<interface>>\n    }\n\n    class json_exception [\"basic_json::exception\"] {\n        +const int id\n        +const char* what() const\n    }\n\n    class json_parse_error [\"basic_json::parse_error\"] {\n        +const std::size_t byte\n    }\n\n    class json_invalid_iterator [\"basic_json::invalid_iterator\"]\n    class json_type_error [\"basic_json::type_error\"]\n    class json_out_of_range [\"basic_json::out_of_range\"]\n    class json_other_error [\"basic_json::other_error\"]\n\n    std_exception <|-- json_exception\n    json_exception <|-- json_parse_error\n    json_exception <|-- json_invalid_iterator\n    json_exception <|-- json_type_error\n    json_exception <|-- json_out_of_range\n    json_exception <|-- json_other_error\n\n    style json_type_error fill:#CCCCFF
    "},{"location":"api/basic_json/type_error/#member-functions","title":"Member functions","text":"
    • what - returns explanatory string
    "},{"location":"api/basic_json/type_error/#member-variables","title":"Member variables","text":"
    • id - the id of the exception
    "},{"location":"api/basic_json/type_error/#examples","title":"Examples","text":"Example

    The following code shows how a type_error exception can be caught.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    try\n    {\n        // calling push_back() on a string value\n        json j = \"string\";\n        j.push_back(\"another string\");\n    }\n    catch (const json::type_error& e)\n    {\n        // output exception information\n        std::cout << \"message: \" << e.what() << '\\n'\n                  << \"exception id: \" << e.id << std::endl;\n    }\n}\n

    Output:

    message: [json.exception.type_error.308] cannot use push_back() with string\nexception id: 308\n
    "},{"location":"api/basic_json/type_error/#see-also","title":"See also","text":"
    • exception for the base class of all exceptions thrown by the library
    • List of type errors
    • parse_error for exceptions indicating a parse error
    • invalid_iterator for exceptions indicating errors with iterators
    • out_of_range for exceptions indicating access out of the defined range
    • other_error for exceptions indicating other library errors
    "},{"location":"api/basic_json/type_error/#version-history","title":"Version history","text":"
    • Since version 3.0.0.
    "},{"location":"api/basic_json/type_name/","title":"nlohmann::basic_json::type_name","text":"
    const char* type_name() const noexcept;\n

    Returns the type name as string to be used in error messages -- usually to indicate that a function was called on a wrong JSON type.

    "},{"location":"api/basic_json/type_name/#return-value","title":"Return value","text":"

    a string representation of the type (value_t):

    Value type return value null \"null\" boolean \"boolean\" string \"string\" number (integer, unsigned integer, floating-point) \"number\" object \"object\" array \"array\" binary \"binary\" discarded \"discarded\" invalid (corrupted value) \"invalid\"

    The \\\"invalid\\\" type

    The \"invalid\" return value indicates a corrupted JSON value \u2014 this can occur if an enum value falls outside the range of valid value_t values. This is useful for diagnosing data corruption or internal errors.

    "},{"location":"api/basic_json/type_name/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this member function never throws exceptions.

    "},{"location":"api/basic_json/type_name/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json/type_name/#examples","title":"Examples","text":"Example

    The following code exemplifies type_name() for all JSON types.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_null;\n    json j_boolean = true;\n    json j_number_integer = -17;\n    json j_number_unsigned = 42u;\n    json j_number_float = 23.42;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n    json j_string = \"Hello, world\";\n\n    // call type_name()\n    std::cout << j_null << \" is a \" << j_null.type_name() << '\\n';\n    std::cout << j_boolean << \" is a \" << j_boolean.type_name() << '\\n';\n    std::cout << j_number_integer << \" is a \" << j_number_integer.type_name() << '\\n';\n    std::cout << j_number_unsigned << \" is a \" << j_number_unsigned.type_name() << '\\n';\n    std::cout << j_number_float << \" is a \" << j_number_float.type_name() << '\\n';\n    std::cout << j_object << \" is an \" << j_object.type_name() << '\\n';\n    std::cout << j_array << \" is an \" << j_array.type_name() << '\\n';\n    std::cout << j_string << \" is a \" << j_string.type_name() << '\\n';\n}\n

    Output:

    null is a null\ntrue is a boolean\n-17 is a number\n42 is a number\n23.42 is a number\n{\"one\":1,\"two\":2} is an object\n[1,2,4,8,16] is an array\n\"Hello, world\" is a string\n
    "},{"location":"api/basic_json/type_name/#see-also","title":"See also","text":"
    • type returns the type of the JSON value
    • value_t the enumeration of JSON types
    • basic_json_view::type_name - the same function on a zero-copy view
    "},{"location":"api/basic_json/type_name/#version-history","title":"Version history","text":"
    • Added in version 1.0.0.
    • Part of the public API version since 2.1.0.
    • Changed return value to const char* and added noexcept in version 3.0.0.
    • Added support for binary type in version 3.8.0.
    • Added \"invalid\" return value for corrupted JSON values in version 3.13.0 unreleased.
    "},{"location":"api/basic_json/unflatten/","title":"nlohmann::basic_json::unflatten","text":"
    basic_json unflatten() const;\n

    The function restores the arbitrary nesting of a JSON value that has been flattened before using the flatten() function. The JSON value must meet certain constraints:

    1. The value must be an object.
    2. The keys must be JSON pointers (see RFC 6901)
    3. The mapped values must be primitive JSON types.
    "},{"location":"api/basic_json/unflatten/#return-value","title":"Return value","text":"

    the original JSON from a flattened version

    "},{"location":"api/basic_json/unflatten/#exception-safety","title":"Exception safety","text":"

    Strong exception safety: if an exception occurs, the original value stays intact.

    "},{"location":"api/basic_json/unflatten/#exceptions","title":"Exceptions","text":"

    The function can throw the following exceptions:

    • Throws type_error.314 if value is not an object
    • Throws type_error.315 if object values are not primitive
    • Throws type_error.313 if a key (JSON pointer) leads to a conflicting nesting; example: \"invalid value to unflatten\"
    • Throws parse_error.106 if an array index in a key begins with '0'; example: \"array index '01' must not begin with '0'\"
    • Throws parse_error.107 if a key is not empty and does not begin with a slash (/); example: \"JSON pointer must be empty or begin with '/' - was: 'a'\"
    • Throws parse_error.108 if a tilde (~) in a key is not followed by 0 or 1; example: \"escape character '~' must be followed with '0' or '1'\"
    • Throws parse_error.109 if an array index in a key is not a number; example: \"array index 'one' is not a number\"
    • Throws out_of_range.404 if a level becomes an array (because one of its keys is 0) and another key at that level begins with a digit but is not a valid array index (such as 1a), or is -; example: \"unresolved reference token '-'\"
    "},{"location":"api/basic_json/unflatten/#complexity","title":"Complexity","text":"

    Linear in the size of the JSON value.

    "},{"location":"api/basic_json/unflatten/#notes","title":"Notes","text":"

    Empty objects and arrays are flattened by flatten() to null values and cannot unflattened to their original type.

    A flattened array and a flattened object whose keys are array indices are indistinguishable, because both are described by the same JSON pointers. A value is therefore restored as an array if and only if one of its keys is the reference token 0, and as an object otherwise: {\"2\": 1} is restored unchanged, whereas {\"0\": 1} is restored as [1]. This decision does not depend on the order in which the flattened object is iterated.

    Apart from these two cases, for a JSON value j, the following is always true: j == j.flatten().unflatten().

    "},{"location":"api/basic_json/unflatten/#examples","title":"Examples","text":"Example

    The following code shows how a flattened JSON object is unflattened into the original nested JSON object.

    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON value\n    json j_flattened =\n    {\n        {\"/answer/everything\", 42},\n        {\"/happy\", true},\n        {\"/list/0\", 1},\n        {\"/list/1\", 0},\n        {\"/list/2\", 2},\n        {\"/name\", \"Niels\"},\n        {\"/nothing\", nullptr},\n        {\"/object/currency\", \"USD\"},\n        {\"/object/value\", 42.99},\n        {\"/pi\", 3.141}\n    };\n\n    // call unflatten()\n    std::cout << std::setw(4) << j_flattened.unflatten() << '\\n';\n}\n

    Output:

    {\n    \"answer\": {\n        \"everything\": 42\n    },\n    \"happy\": true,\n    \"list\": [\n        1,\n        0,\n        2\n    ],\n    \"name\": \"Niels\",\n    \"nothing\": null,\n    \"object\": {\n        \"currency\": \"USD\",\n        \"value\": 42.99\n    },\n    \"pi\": 3.141\n}\n
    "},{"location":"api/basic_json/unflatten/#see-also","title":"See also","text":"
    • flatten the reverse function
    "},{"location":"api/basic_json/unflatten/#version-history","title":"Version history","text":"
    • Added in version 2.0.0.
    • Made the array/object decision independent of the object's iteration order in version 3.13.0 unreleased.
    • Throws parse_error.109 instead of out_of_range.404 for a one-character array index that is not a digit (e.g., /x) in version 3.13.0 unreleased, as it already did for longer ones.
    "},{"location":"api/basic_json/update/","title":"nlohmann::basic_json::update","text":"
    // (1)\nvoid update(const_reference j, bool merge_objects = false);\n\n// (2)\nvoid update(const_iterator first, const_iterator last, bool merge_objects = false);\n
    1. Inserts all values from JSON object j.
    2. Inserts all values from range [first, last)

    When merge_objects is false (default), existing keys are overwritten. When merge_objects is true, recursively merges objects with common keys.

    If the JSON value is null, it is implicitly converted to an empty object before the values are inserted.

    The function is motivated by Python's dict.update function.

    "},{"location":"api/basic_json/update/#iterator-invalidation","title":"Iterator invalidation","text":"

    For ordered_json, adding a value to an object can yield a reallocation, in which case all iterators (including the end() iterator) and all references to the elements are invalidated.

    "},{"location":"api/basic_json/update/#parameters","title":"Parameters","text":"j (in) JSON object to read values from merge_objects (in) when true, keys that exist in both objects and whose value in the source is itself an object are merged recursively; all other values are overwritten as usual (default: false) first (in) the beginning of the range of elements to insert last (in) the end of the range of elements to insert"},{"location":"api/basic_json/update/#exception-safety","title":"Exception safety","text":"

    Basic guarantee: if an exception is thrown during the operation, the JSON value may be partially modified.

    "},{"location":"api/basic_json/update/#exceptions","title":"Exceptions","text":"
    1. The function can throw the following exceptions:
      • Throws type_error.312 if called on JSON values other than objects; example: \"cannot use update() with string\"
    2. The function can throw the following exceptions:
      • Throws type_error.312 if called on JSON values other than objects; example: \"cannot use update() with string\"
      • Throws invalid_iterator.210 if first and last do not belong to the same JSON value; example: \"iterators do not fit\"
    "},{"location":"api/basic_json/update/#complexity","title":"Complexity","text":"
    1. O(N*log(size() + N)), where N is the number of elements to insert.
    2. O(N*log(size() + N)), where N is the number of elements to insert.
    "},{"location":"api/basic_json/update/#notes","title":"Notes","text":"

    The argument j (or, for overload (2), the range [first, last)) may be *this itself or refer to a value contained in *this (for example, a subobject returned by (*this)[key]); it is read as it was when update() was called, before any modification of *this.

    "},{"location":"api/basic_json/update/#examples","title":"Examples","text":"Example: (1) update with another object

    The example shows how update() is used.

    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // create two JSON objects\n    json o1 = R\"( {\"color\": \"red\", \"price\": 17.99, \"names\": {\"de\": \"Flugzeug\"}} )\"_json;\n    json o2 = R\"( {\"color\": \"blue\", \"speed\": 100, \"names\": {\"en\": \"plane\"}} )\"_json;\n    json o3 = o1;\n\n    // add all keys from o2 to o1 (updating \"color\", replacing \"names\")\n    o1.update(o2);\n\n    // add all keys from o2 to o1 (updating \"color\", merging \"names\")\n    o3.update(o2, true);\n\n    // output updated object o1 and o3\n    std::cout << std::setw(2) << o1 << '\\n';\n    std::cout << std::setw(2) << o3 << '\\n';\n}\n

    Output:

    {\n  \"color\": \"blue\",\n  \"names\": {\n    \"en\": \"plane\"\n  },\n  \"price\": 17.99,\n  \"speed\": 100\n}\n{\n  \"color\": \"blue\",\n  \"names\": {\n    \"de\": \"Flugzeug\",\n    \"en\": \"plane\"\n  },\n  \"price\": 17.99,\n  \"speed\": 100\n}\n
    Example: (2) update with an iterator range

    The example shows how update() is used.

    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // create two JSON objects\n    json o1 = R\"( {\"color\": \"red\", \"price\": 17.99, \"names\": {\"de\": \"Flugzeug\"}} )\"_json;\n    json o2 = R\"( {\"color\": \"blue\", \"speed\": 100, \"names\": {\"en\": \"plane\"}} )\"_json;\n    json o3 = o1;\n\n    // add all keys from o2 to o1 (updating \"color\", replacing \"names\")\n    o1.update(o2.begin(), o2.end());\n\n    // add all keys from o2 to o1 (updating \"color\", merging \"names\")\n    o3.update(o2.begin(), o2.end(), true);\n\n    // output updated object o1 and o3\n    std::cout << std::setw(2) << o1 << '\\n';\n    std::cout << std::setw(2) << o3 << '\\n';\n}\n

    Output:

    {\n  \"color\": \"blue\",\n  \"names\": {\n    \"en\": \"plane\"\n  },\n  \"price\": 17.99,\n  \"speed\": 100\n}\n{\n  \"color\": \"blue\",\n  \"names\": {\n    \"de\": \"Flugzeug\",\n    \"en\": \"plane\"\n  },\n  \"price\": 17.99,\n  \"speed\": 100\n}\n
    Example: (1) merge user settings into default settings

    One common use case for this function is the handling of user settings. Assume your application can be configured in some aspects:

    {\n    \"color\": \"red\",\n    \"active\": true,\n    \"name\": {\"de\": \"Maus\", \"en\": \"mouse\"}\n}\n

    The user may override the default settings selectively:

    {\n    \"color\": \"blue\",\n    \"name\": {\"es\": \"rat\u00f3n\"},\n}\n

    Then update manages the merging of default settings and user settings:

    auto user_settings = json::parse(\"config.json\");\nauto effective_settings = get_default_settings();\neffective_settings.update(user_settings);\n

    Now effective_settings contains the default settings, but those keys set by the user are overwritten:

    {\n    \"color\": \"blue\",\n    \"active\": true,\n    \"name\": {\"es\": \"rat\u00f3n\"}\n}\n

    Note existing keys were just overwritten. To merge objects, merge_objects setting should be set to true:

    auto user_settings = json::parse(\"config.json\");\nauto effective_settings = get_default_settings();\neffective_settings.update(user_settings, true);\n
    {\n    \"color\": \"blue\",\n    \"active\": true,\n    \"name\": {\"de\": \"Maus\", \"en\": \"mouse\", \"es\": \"rat\u00f3n\"}\n}\n
    "},{"location":"api/basic_json/update/#see-also","title":"See also","text":"
    • insert add values to an array/object
    • merge_patch applies a JSON Merge Patch
    • Modifying values - the article on modifying values
    "},{"location":"api/basic_json/update/#version-history","title":"Version history","text":"
    • Added in version 3.0.0.
    • Added merge_objects parameter in 3.10.5.
    • Fixed use of freed or relocated memory when the argument is *this or refers to a value contained in *this, in version 3.13.0 unreleased.
    "},{"location":"api/basic_json/value/","title":"nlohmann::basic_json::value","text":"
    // (1)\ntemplate<class ValueType>\nValueType value(const typename object_t::key_type& key,\n                ValueType&& default_value) const;\n\n// (2)\ntemplate<class ValueType, class KeyType>\nValueType value(KeyType&& key,\n                ValueType&& default_value) const;\n\n// (3)\ntemplate<class ValueType>\nValueType value(const json_pointer& ptr,\n                const ValueType& default_value) const;\n

    This is equivalent to Python's dict.get(key, default).

    1. Returns either a copy of an object's element at the specified key key or a given default value if no element with key key exists.

      The function is basically equivalent to executing 

      try {\n   return at(key);\n} catch(out_of_range) {\n   return default_value;\n}\n

    2. See 1. This overload is only available if KeyType is comparable with typename object_t::key_type and typename object_comparator_t::is_transparent denotes a type.

    3. Returns either a copy of an object's element at the specified JSON pointer ptr or a given default value if no value at ptr exists.

      The function is basically equivalent to executing 

      try {\n   return at(ptr);\n} catch(out_of_range) {\n   return default_value;\n}\n

    Differences to at and operator[]

    • Unlike at, this function does not throw if the given key/ptr was not found.
    • Unlike operator[], this function does not implicitly add an element to the position defined by key/ptr key. This function is furthermore also applicable to const objects.

    Integer keys

    Calling this function with an integer key argument (for example, value(0, 1)) does not compile in C++11, where object_comparator_t is not transparent: such an argument would otherwise implicitly convert to a null const char* and, from there, cause undefined behavior when constructing a std::string for the object key. To access an array element with a default value, use at together with a try/catch block, or compare against size instead.

    "},{"location":"api/basic_json/value/#template-parameters","title":"Template parameters","text":"KeyType A type for an object key other than json_pointer that is comparable with string_t using object_comparator_t. This can also be a string view (C++17). ValueType type compatible to JSON values, for instance int for JSON integer numbers, bool for JSON booleans, or std::vector types for JSON arrays. Note the type of the expected value at key/ptr and the default value default_value must be compatible."},{"location":"api/basic_json/value/#parameters","title":"Parameters","text":"key (in) key of the element to access default_value (in) the value to return if key/ptr found no value ptr (in) a JSON pointer to the element to access"},{"location":"api/basic_json/value/#return-value","title":"Return value","text":"
    1. copy of the element at key key or default_value if key is not found
    2. copy of the element at key key or default_value if key is not found
    3. copy of the element at JSON Pointer ptr or default_value if no value for ptr is found
    "},{"location":"api/basic_json/value/#exception-safety","title":"Exception safety","text":"

    Strong guarantee: if an exception is thrown, there are no changes to any JSON value.

    "},{"location":"api/basic_json/value/#exceptions","title":"Exceptions","text":"
    1. The function can throw the following exceptions:
      • Throws type_error.302 if default_value does not match the type of the value at key
      • Throws type_error.306 if the JSON value is not an object; in that case, using value() with a key makes no sense.
    2. See 1.
    3. The function can throw the following exceptions:
      • Throws type_error.302 if default_value does not match the type of the value at ptr
      • Throws type_error.306 if the JSON value is not an array or object; in that case, using value() with a JSON pointer makes no sense.
      • Throws parse_error.106 if an array index in the passed JSON pointer ptr begins with '0'.
      • Throws parse_error.109 if an array index in the passed JSON pointer ptr is not a number.
    "},{"location":"api/basic_json/value/#complexity","title":"Complexity","text":"
    1. Logarithmic in the size of the container.
    2. Logarithmic in the size of the container.
    3. Logarithmic in the size of the container.
    "},{"location":"api/basic_json/value/#notes","title":"Notes","text":"

    null members are not missing

    The default value is used only if the key (or JSON Pointer) does not exist. A member that exists but is null is converted like any other value, so j.value(\"k\", 0) throws a type_error.302 if \"k\" is null. See Access with default value for alternatives.

    Return type

    The value function is a template, and the return type of the function is determined by the type of the provided default value unless otherwise specified. This can have unexpected effects. In the example below, we store a 64-bit unsigned integer. We get exactly that value when using operator[]. However, when we call value and provide 0 as default value, then -1 is returned. This occurs, because 0 has type int which overflows when handling the value 18446744073709551615.

    To address this issue, either provide a correctly typed default value or use the template parameter to specify the desired return type. Note that this issue occurs even when a value is stored at the provided key, and the default value is not used as the return value.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    json j = json::parse(R\"({\"uint64\": 18446744073709551615})\");\n\n    std::cout << \"operator[]:                \" << j[\"uint64\"] << '\\n'\n              << \"default value (int):       \" << j.value(\"uint64\", 0) << '\\n'\n              << \"default value (uint64_t):  \" << j.value(\"uint64\", std::uint64_t(0)) << '\\n'\n              << \"explicit return value type: \" << j.value<std::uint64_t>(\"uint64\", 0) << '\\n';\n}\n

    Output:

    operator[]:                18446744073709551615\ndefault value (int):       -1\ndefault value (uint64_t):  18446744073709551615\nexplicit return value type: 18446744073709551615\n

    Deprecation

    Overload (3) also accepts a json_pointer whose template argument is a basic_json specialization (e.g., nlohmann::json_pointer<nlohmann::json>) instead of a string type. This is deprecated since version 3.11.0 and will be removed in a future major version; use basic_json::json_pointer (for json, nlohmann::json_pointer<std::string>) instead.

    You should be warned by your compiler with a -Wdeprecated-declarations warning if you are using a deprecated function.

    See the migration guide for how to update existing code.

    "},{"location":"api/basic_json/value/#examples","title":"Examples","text":"Example: (1) access specified object element with default value

    The example below shows how object elements can be queried with a default value.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON object with different entry types\n    json j =\n    {\n        {\"integer\", 1},\n        {\"floating\", 42.23},\n        {\"string\", \"hello world\"},\n        {\"boolean\", true},\n        {\"object\", {{\"key1\", 1}, {\"key2\", 2}}},\n        {\"array\", {1, 2, 3}}\n    };\n\n    // access existing values\n    int v_integer = j.value(\"integer\", 0);\n    double v_floating = j.value(\"floating\", 47.11);\n\n    // access nonexisting values and rely on default value\n    std::string v_string = j.value(\"nonexisting\", \"oops\");\n    bool v_boolean = j.value(\"nonexisting\", false);\n\n    // output values\n    std::cout << std::boolalpha << v_integer << \" \" << v_floating\n              << \" \" << v_string << \" \" << v_boolean << \"\\n\";\n}\n

    Output:

    1 42.23 oops false\n
    Example: (2) access specified object element using string_view with default value

    The example below shows how object elements can be queried with a default value.

    #include <iostream>\n#include <string_view>\n#include <nlohmann/json.hpp>\n\nusing namespace std::string_view_literals;\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON object with different entry types\n    json j =\n    {\n        {\"integer\", 1},\n        {\"floating\", 42.23},\n        {\"string\", \"hello world\"},\n        {\"boolean\", true},\n        {\"object\", {{\"key1\", 1}, {\"key2\", 2}}},\n        {\"array\", {1, 2, 3}}\n    };\n\n    // access existing values\n    int v_integer = j.value(\"integer\"sv, 0);\n    double v_floating = j.value(\"floating\"sv, 47.11);\n\n    // access nonexisting values and rely on default value\n    std::string v_string = j.value(\"nonexisting\"sv, \"oops\");\n    bool v_boolean = j.value(\"nonexisting\"sv, false);\n\n    // output values\n    std::cout << std::boolalpha << v_integer << \" \" << v_floating\n              << \" \" << v_string << \" \" << v_boolean << \"\\n\";\n}\n

    Output:

    1 42.23 oops false\n
    Example: (3) access specified object element via JSON Pointer with default value

    The example below shows how object elements can be queried with a default value.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // create a JSON object with different entry types\n    json j =\n    {\n        {\"integer\", 1},\n        {\"floating\", 42.23},\n        {\"string\", \"hello world\"},\n        {\"boolean\", true},\n        {\"object\", {{\"key1\", 1}, {\"key2\", 2}}},\n        {\"array\", {1, 2, 3}}\n    };\n\n    // access existing values\n    int v_integer = j.value(\"/integer\"_json_pointer, 0);\n    double v_floating = j.value(\"/floating\"_json_pointer, 47.11);\n\n    // access nonexisting values and rely on default value\n    std::string v_string = j.value(\"/nonexisting\"_json_pointer, \"oops\");\n    bool v_boolean = j.value(\"/nonexisting\"_json_pointer, false);\n\n    // output values\n    std::cout << std::boolalpha << v_integer << \" \" << v_floating\n              << \" \" << v_string << \" \" << v_boolean << \"\\n\";\n}\n

    Output:

    1 42.23 oops false\n
    Example: (1) type_error.302 and type_error.306 exceptions

    The example below shows how value() throws type_error.302 when the default value's type does not match the type of the stored value, and type_error.306 when value() is called on a JSON value that is not an object.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON object with a string value\n    json j = {{\"name\", \"the good\"}};\n\n    // exception type_error.302\n    try\n    {\n        int v = j.value(\"name\", 0);\n        std::cout << v << '\\n';\n    }\n    catch (const json::type_error& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n\n    // exception type_error.306\n    try\n    {\n        json str = \"I am a string\";\n        auto v = str.value(\"name\", 0);\n        std::cout << v << '\\n';\n    }\n    catch (const json::type_error& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

    Output:

    [json.exception.type_error.302] type must be number, but is string\n[json.exception.type_error.306] cannot use value() with string\n
    "},{"location":"api/basic_json/value/#see-also","title":"See also","text":"
    • see at for access by reference with range checking
    • see operator[] for unchecked access by reference
    • basic_json_view::value - the same access on a zero-copy view
    "},{"location":"api/basic_json/value/#version-history","title":"Version history","text":"
    1. Added in version 1.0.0. Changed parameter default_value type from const ValueType& to ValueType&& in version 3.11.0. Deleted overload for integral key types added in version 3.13.0 unreleased to reject such calls at compile time instead of causing undefined behavior at runtime.
    2. Added in version 3.11.0. Made ValueType the first template parameter in version 3.11.2. Fixed in version 3.13.0 unreleased to consistently accept std::string_view-convertible keys, as already supported by operator[], at, find, and other lookup functions.
    3. Added in version 2.0.2. Extended to work with arrays in version 3.13.0 unreleased, including fixing an issue where resolving ptr through an array unexpectedly threw out_of_range instead of returning the resolved element (or default_value, as documented). Throws parse_error.109 instead of returning default_value for a one-character array index that is not a digit (e.g., /x) in version 3.13.0, as it already did for longer ones.
    "},{"location":"api/basic_json/value_t/","title":"nlohmann::basic_json::value_t","text":"
    enum class value_t : std::uint8_t {\n    null,\n    object,\n    array,\n    string,\n    boolean,\n    number_integer,\n    number_unsigned,\n    number_float,\n    binary,\n    discarded\n};\n

    This enumeration collects the different JSON types. It is internally used to distinguish the stored values, and the functions is_null, is_object, is_array, is_string, is_boolean, is_number (with is_number_integer, is_number_unsigned, and is_number_float), is_discarded, is_binary, is_primitive, and is_structured rely on it.

    "},{"location":"api/basic_json/value_t/#notes","title":"Notes","text":"

    Ordering

    The order of types is as follows:

    1. null
    2. boolean
    3. number_integer, number_unsigned, number_float
    4. object
    5. array
    6. string
    7. binary

    discarded is unordered.

    flowchart LR\n    A[null] --> B[boolean]\n    B --> C[\"number_integer / number_unsigned / number_float\"]\n    C --> D[object]\n    D --> E[array]\n    E --> F[string]\n    F --> G[binary]

    Types of numbers

    There are three enumerators for numbers (number_integer, number_unsigned, and number_float) to distinguish between different types of numbers:

    • number_unsigned_t for unsigned integers
    • number_integer_t for signed integers
    • number_float_t for floating-point numbers or to approximate integers which do not fit into the limits of their respective type

    Comparison operators

    operator< and operator<=> (since C++20) are overloaded and compare according to the ordering described above. Until C++20 all other relational and equality operators yield results according to the integer value of each enumerator. Since C++20 some compilers consider the rewritten candidates generated from operator<=> during overload resolution, while others do not. For predictable and portable behavior use:

    • operator< or operator<=> when wanting to compare according to the order described above
    • operator== or operator!= when wanting to compare according to each enumerators integer value
    "},{"location":"api/basic_json/value_t/#examples","title":"Examples","text":"Example

    The following code how type() queries the value_t for all JSON types.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_null;\n    json j_boolean = true;\n    json j_number_integer = -17;\n    json j_number_unsigned = 42u;\n    json j_number_float = 23.42;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n    json j_string = \"Hello, world\";\n\n    // call type()\n    std::cout << std::boolalpha;\n    std::cout << (j_null.type() == json::value_t::null) << '\\n';\n    std::cout << (j_boolean.type() == json::value_t::boolean) << '\\n';\n    std::cout << (j_number_integer.type() == json::value_t::number_integer) << '\\n';\n    std::cout << (j_number_unsigned.type() == json::value_t::number_unsigned) << '\\n';\n    std::cout << (j_number_float.type() == json::value_t::number_float) << '\\n';\n    std::cout << (j_object.type() == json::value_t::object) << '\\n';\n    std::cout << (j_array.type() == json::value_t::array) << '\\n';\n    std::cout << (j_string.type() == json::value_t::string) << '\\n';\n}\n

    Output:

    true\ntrue\ntrue\ntrue\ntrue\ntrue\ntrue\ntrue\n
    "},{"location":"api/basic_json/value_t/#see-also","title":"See also","text":"
    • type return the type of the JSON value
    • type_name return the type as string
    • operator value_t return the type of the JSON value
    • is_primitive return whether the type is primitive
    • is_structured return whether the type is structured
    "},{"location":"api/basic_json/value_t/#version-history","title":"Version history","text":"
    • Added in version 1.0.0.
    • Added unsigned integer type in version 2.0.0.
    • Added binary type in version 3.8.0.
    "},{"location":"api/basic_json/with_t/","title":"nlohmann::basic_json::with_t","text":"

    Member alias templates with_object_t, with_array_t, with_string_t, with_boolean_t, with_integers_t, with_float_t, with_allocator_t, with_json_serializer_t, with_binary_t, and with_base_class_t.

    template<template<typename, typename, typename...> class ObjectType2>\nusing with_object_t = basic_json<ObjectType2, ArrayType, StringType, BooleanType,\n      NumberIntegerType, NumberUnsignedType, NumberFloatType,\n      AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;\n\ntemplate<template<typename, typename...> class ArrayType2>\nusing with_array_t = basic_json<ObjectType, ArrayType2, StringType, BooleanType,\n      NumberIntegerType, NumberUnsignedType, NumberFloatType,\n      AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;\n\ntemplate<class StringType2>\nusing with_string_t = basic_json<ObjectType, ArrayType, StringType2, BooleanType,\n      NumberIntegerType, NumberUnsignedType, NumberFloatType,\n      AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;\n\ntemplate<class BooleanType2>\nusing with_boolean_t = basic_json<ObjectType, ArrayType, StringType, BooleanType2,\n      NumberIntegerType, NumberUnsignedType, NumberFloatType,\n      AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;\n\ntemplate<class NumberIntegerType2, class NumberUnsignedType2>\nusing with_integers_t = basic_json<ObjectType, ArrayType, StringType, BooleanType,\n      NumberIntegerType2, NumberUnsignedType2, NumberFloatType,\n      AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;\n\ntemplate<class NumberFloatType2>\nusing with_float_t = basic_json<ObjectType, ArrayType, StringType, BooleanType,\n      NumberIntegerType, NumberUnsignedType, NumberFloatType2,\n      AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;\n\ntemplate<template<typename> class AllocatorType2>\nusing with_allocator_t = basic_json<ObjectType, ArrayType, StringType, BooleanType,\n      NumberIntegerType, NumberUnsignedType, NumberFloatType,\n      AllocatorType2, JSONSerializer, BinaryType, CustomBaseClass>;\n\ntemplate<template<typename, typename = void> class JSONSerializer2>\nusing with_json_serializer_t = basic_json<ObjectType, ArrayType, StringType, BooleanType,\n      NumberIntegerType, NumberUnsignedType, NumberFloatType,\n      AllocatorType, JSONSerializer2, BinaryType, CustomBaseClass>;\n\ntemplate<class BinaryType2>\nusing with_binary_t = basic_json<ObjectType, ArrayType, StringType, BooleanType,\n      NumberIntegerType, NumberUnsignedType, NumberFloatType,\n      AllocatorType, JSONSerializer, BinaryType2, CustomBaseClass>;\n\ntemplate<class CustomBaseClass2>\nusing with_base_class_t = basic_json<ObjectType, ArrayType, StringType, BooleanType,\n      NumberIntegerType, NumberUnsignedType, NumberFloatType,\n      AllocatorType, JSONSerializer, BinaryType, CustomBaseClass2>;\n

    These member alias templates make it easier to create a basic_json type that is identical to the current type except for one (or, in the case of with_integers_t, two) of its template parameters. Spelling out all 11 template parameters of basic_json just to change a single one is verbose and error-prone; these aliases only require the replacement type(s).

    with_object_t<ObjectType2> replaces ObjectType with_array_t<ArrayType2> replaces ArrayType with_string_t<StringType2> replaces StringType with_boolean_t<BooleanType2> replaces BooleanType with_integers_t<NumberIntegerType2, NumberUnsignedType2> replaces both NumberIntegerType and NumberUnsignedType; the two are combined into a single alias because they are usually changed together (for instance, when switching to fixed-width integer types) with_float_t<NumberFloatType2> replaces NumberFloatType with_allocator_t<AllocatorType2> replaces AllocatorType with_json_serializer_t<JSONSerializer2> replaces JSONSerializer with_binary_t<BinaryType2> replaces BinaryType with_base_class_t<CustomBaseClass2> replaces CustomBaseClass; see also json_base_class_t"},{"location":"api/basic_json/with_t/#notes","title":"Notes","text":"

    All other template parameters are kept unchanged, so the resulting type still uses, for instance, the same ObjectType unless with_object_t itself is used.

    The aliases are members of every basic_json specialization, including ordered_json, and the type they produce is again a basic_json specialization. They can therefore be chained to replace several template parameters at once:

    using my_json = nlohmann::json::with_integers_t<int, unsigned int>::with_float_t<float>;\nusing my_ordered_json = nlohmann::ordered_json::with_string_t<std::wstring>;\n

    The result is the same type as spelling out all template parameters, so the order of the chained aliases does not matter. For instance, nlohmann::json::with_object_t<nlohmann::ordered_map> is nlohmann::ordered_json.

    "},{"location":"api/basic_json/with_t/#examples","title":"Examples","text":"Example

    The following code shows how with_object_t can be used to create a JSON type that stores object elements in a std::map and therefore keeps them sorted by key, unlike the default type which preserves insertion order only when nlohmann::ordered_json is used.

    #include <iostream>\n#include <map>\n#include <nlohmann/json.hpp>\n\n// a JSON type that stores objects in a std::map (which keeps keys sorted)\n// instead of the default ordered associative container\nusing sorted_json = nlohmann::json::with_object_t<std::map>;\n\nint main()\n{\n    sorted_json j;\n    j[\"c\"] = 1;\n    j[\"a\"] = 2;\n    j[\"b\"] = 3;\n\n    // keys are sorted, because std::map is used to store the object\n    std::cout << j.dump() << std::endl;\n}\n

    Output:

    {\"a\":2,\"b\":3,\"c\":1}\n
    "},{"location":"api/basic_json/with_t/#see-also","title":"See also","text":"
    • basic_json - the template parameters that can be replaced
    • json_base_class_t - the type used for CustomBaseClass
    "},{"location":"api/basic_json/with_t/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json/~basic_json/","title":"nlohmann::basic_json::~basic_json","text":"
    ~basic_json() noexcept;\n

    Destroys the JSON value and frees all allocated memory.

    "},{"location":"api/basic_json/~basic_json/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this member function never throws exceptions.

    "},{"location":"api/basic_json/~basic_json/#complexity","title":"Complexity","text":"

    Linear.

    "},{"location":"api/basic_json/~basic_json/#see-also","title":"See also","text":"
    • basic_json constructs a JSON value
    • clear clears the content of a JSON value without destroying it
    "},{"location":"api/basic_json/~basic_json/#version-history","title":"Version history","text":"
    • Added in version 1.0.0.
    "},{"location":"api/basic_json_document/","title":"nlohmann::basic_json_document","text":"

    Defined in header <nlohmann/json_view.hpp>

    template<typename BasicJsonType, bool Editable = false>\nclass basic_json_document;\n

    A parsed JSON text, held as a flat index of its values (16 bytes per value) instead of a tree of BasicJsonType values. Strings and numbers stay in the source text; only strings that contain escapes are decoded, into one buffer owned by the document. basic_json_view is a read-only handle to one value of a basic_json_document; materialize() turns a subtree into the BasicJsonType value that BasicJsonType::parse() would have produced for it.

    A document may borrow the text it was parsed from (the caller's buffer must then outlive the document) or own it (a copy, or an rvalue std::string that was moved in); see owns_source. basic_json_document is move-only: a copy would duplicate the whole index, the decoded-string buffer, and the owned text, and a move is enough in every use we know of. The index, the decoded strings, and the owned text live in one heap block that the document holds through a std::unique_ptr; this is also why views survive a move of the document.

    With Editable == true, the document also offers set, push_back, insert, and erase to change values in place, see Edits below. The source text itself is never written; a read-only document (Editable == false, the default) does not carry any of the bookkeeping edits need, and calling any of them on one fails to compile (static_assert).

    "},{"location":"api/basic_json_document/#template-parameters","title":"Template parameters","text":"BasicJsonType a specialization of basic_json, for instance json or ordered_json. Only 64-bit number_integer_t/number_unsigned_t types are supported; this is checked with a static_assert. Editable whether the document supports set, push_back, insert, and erase (optional, false by default). See Edits below."},{"location":"api/basic_json_document/#specializations","title":"Specializations","text":"
    • json_document - read-only documents of the default specialization json
    • ordered_json_document - read-only documents of ordered_json
    • json_editable_document - editable documents of json
    • ordered_json_editable_document - editable documents of ordered_json
    "},{"location":"api/basic_json_document/#member-types","title":"Member types","text":"
    • view_type - the type of view returned by root() (basic_json_view<BasicJsonType, Editable>)
    • value_t - the JSON type enumeration, see basic_json::value_t
    • image_check - how load() validates an image (full, bounds, none), see image_check
    "},{"location":"api/basic_json_document/#member-functions","title":"Member functions","text":"
    • (constructor)
    "},{"location":"api/basic_json_document/#parsing","title":"Parsing","text":"
    • parse (static) - deserialize from a compatible input, borrowing or owning it as appropriate
    • parse_copy (static) - deserialize a copy of a compatible input
    • accept (static) - check whether the input is valid JSON
    • read - (re-)parse into this document, reusing its memory
    "},{"location":"api/basic_json_document/#access","title":"Access","text":"
    • root - the view of the root value
    • is_discarded - return whether the last parse failed
    • source - the parsed text
    • owns_source - return whether the document holds its own copy of the text
    • node_count - the number of index entries (values plus object keys)
    • memory_usage - the number of bytes held by the document
    • shrink_to_fit - release unused index capacity
    "},{"location":"api/basic_json_document/#images","title":"Images","text":"
    • save - the document as an image that load() reads without parsing
    • load (static) - read an image written by save()
    "},{"location":"api/basic_json_document/#edits","title":"Edits","text":"
    • set - replace a value, or set an object member, an array element, or the value a JSON pointer refers to (Editable documents only)
    • push_back - append to an array (Editable documents only)
    • insert - insert an element into an array before a given position (Editable documents only)
    • erase - remove an object member, an array element, or the value a JSON pointer refers to (Editable documents only)
    "},{"location":"api/basic_json_document/#edits_1","title":"Edits","text":"

    An editable document (Editable == true) can be changed after parsing, with set, push_back, insert, and erase; json_editable_document and ordered_json_editable_document are the corresponding specializations. A few points apply to every edit:

    • The source text is never written, and the parsed index never moves: every value keeps the node it was parsed into, so views taken before an edit stay valid, including root(). New values (and the element sequences of an edited array/object) go to storage owned by the document, allocated on demand.
    • A view keeps referring to the same value. After set replaces the value a view refers to, that view sees the new value; a view of a value that a later edit replaces or drops keeps showing what it last held. An edit of an array or object, however, invalidates the iterators taken over it (its members may now live in a different sequence), and a string obtained with get_string() stays valid even as further edits happen (earlier buffers of edited text are kept alive, not overwritten).
    • Values are accepted three ways: a basic_json_view of any document (read-only or editable; it is copied, nothing is shared with the source document), a BasicJsonType value, or anything BasicJsonType can be constructed from (numbers, strings, bool, nullptr, containers, ...).
    • dump() writes an edited document with members in document order, new members at the end, and, with number_format::source, keeps the spelling of every number that was not itself edited -- see Editing a document for why this matters.
    • read() discards all edits, shrink_to_fit() does not move the node index once there are edits, and memory_usage() includes the memory edits use. source_offset() of a value introduced by an edit is static_cast<std::size_t>(-1), the same value it reports for a decoded string.
    "},{"location":"api/basic_json_document/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_document/accept/","title":"nlohmann::basic_json_document::accept","text":"
    template<typename InputType>\nstatic bool accept(InputType&& input,\n                   const bool ignore_comments = false,\n                   const bool ignore_trailing_commas = false);\n

    Checks whether the input is valid JSON, accepting and rejecting exactly what BasicJsonType::accept() does, with the same options. Unlike parse(), this function never throws an exception for invalid input, and the returned bool is the only result -- no document is returned.

    The result is the same as that of BasicJsonType::accept() by design; the difference is how the check is made. This function runs the document's index builder (and discards the index), while BasicJsonType::accept() runs the SAX parser with a handler that ignores every event. Use this one if you go on to parse the same input with parse() anyway, so that both use the same code; either is fine for a plain yes/no answer.

    "},{"location":"api/basic_json_document/accept/#template-parameters","title":"Template parameters","text":"InputType A compatible input; see parse."},{"location":"api/basic_json_document/accept/#parameters","title":"Parameters","text":"input (in) Input to check. ignore_comments (in) whether comments should be ignored and treated like whitespace (true) or yield a parse error (false); (optional, false by default) ignore_trailing_commas (in) whether trailing commas in arrays or objects should be ignored and treated like whitespace (true) or yield a parse error (false); (optional, false by default)"},{"location":"api/basic_json_document/accept/#return-value","title":"Return value","text":"

    Whether the input is valid JSON.

    "},{"location":"api/basic_json_document/accept/#exception-safety","title":"Exception safety","text":"

    Strong guarantee: this function itself never throws for an invalid input; it can only throw what allocating the input's own copy (for inputs that are always read into a buffer) throws.

    "},{"location":"api/basic_json_document/accept/#complexity","title":"Complexity","text":"

    Linear in the length of the input.

    "},{"location":"api/basic_json_document/accept/#notes","title":"Notes","text":"

    An integer argument that is not a bool where the flags are expected, such as accept(ptr, len), does not compile; see parse.

    "},{"location":"api/basic_json_document/accept/#examples","title":"Examples","text":"Example 
    #include <iostream>\n#include <nlohmann/json.hpp>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // accept() behaves exactly like basic_json::accept(): the same inputs are\n    // accepted or rejected, with the same ignore_comments/ignore_trailing_commas\n    // options\n    std::cout << json_document::accept(R\"({\"a\": 1})\") << '\\n';\n    std::cout << json_document::accept(R\"({\"a\": 1,})\") << '\\n'; // trailing comma: rejected by default\n    std::cout << json_document::accept(R\"({\"a\": 1,})\", false, true) << '\\n'; // ignore_trailing_commas\n\n    std::cout << (json_document::accept(R\"({\"a\": 1})\") == nlohmann::json::accept(R\"({\"a\": 1})\")) << '\\n';\n}\n

    Output:

    true\nfalse\ntrue\ntrue\n
    "},{"location":"api/basic_json_document/accept/#see-also","title":"See also","text":"
    • parse - deserialize from a compatible input
    • BasicJsonType::accept - the corresponding function of basic_json
    "},{"location":"api/basic_json_document/accept/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_document/basic_json_document/","title":"nlohmann::basic_json_document::basic_json_document","text":"
    // (1)\nbasic_json_document() = default;\n\n// (2)\nbasic_json_document(basic_json_document&& other) noexcept = default;\n\n// (3)\nbasic_json_document(const basic_json_document&) = delete;\n
    1. Creates an empty (discarded) document: root() returns a discarded view, and is_discarded() is true.
    2. Move constructor. Takes over other's index and, if owned, its text; other is left as an empty document. Views taken from other before the move remain valid, because the index is heap-allocated independently of the basic_json_document object.
    3. basic_json_document is move-only. Copying is disabled because of its cost: a copy would duplicate the whole index, the decoded-string buffer, and the owned text, while a move is enough in every use we know of.
    "},{"location":"api/basic_json_document/basic_json_document/#parameters","title":"Parameters","text":"other (in) another document to move the index and text from"},{"location":"api/basic_json_document/basic_json_document/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: the default and move constructors never throw exceptions.

    "},{"location":"api/basic_json_document/basic_json_document/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json_document/basic_json_document/#examples","title":"Examples","text":"Example

    The example below shows the default constructor and demonstrates that basic_json_document is move-only.

    #include <iostream>\n#include <utility>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // the default constructor creates an empty (discarded) document\n    json_document empty;\n    std::cout << empty.is_discarded() << '\\n';\n\n    // json_document is move-only: parse() itself returns by value (moved out),\n    // and a document can be moved again, e.g. into a container\n    json_document doc = json_document::parse(R\"({\"a\": 1})\");\n    json_document moved = std::move(doc);\n    std::cout << moved.root().is_object() << '\\n';\n\n    // copying is disabled at compile time:\n    // json_document another = moved; // does not compile\n}\n

    Output:

    true\ntrue\n
    "},{"location":"api/basic_json_document/basic_json_document/#see-also","title":"See also","text":"
    • parse - deserialize from a compatible input
    • is_discarded - return whether the last parse failed
    "},{"location":"api/basic_json_document/basic_json_document/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_document/erase/","title":"nlohmann::basic_json_document::erase","text":"
    // (1)\nstd::size_t erase(view_type object, string_view_t key);\n\n// (2)\ntemplate<typename I>\nvoid erase(view_type array, I idx);\n\n// (3)\nstd::size_t erase(const json_pointer& ptr);\n

    Only an editable document (Editable == true, e.g. json_editable_document) has erase; calling it on a read-only basic_json_document fails to compile (static_assert).

    1. Removes every member of object whose key is key (see Notes on duplicate keys) and returns how many were removed; 0 if object has no member with this key.
    2. Removes the element at index idx of array, which must already exist (idx < array.size()).
    3. Removes the value the JSON pointer ptr refers to, relative to root(). ptr must not be empty (root() itself cannot be erased), and the parent of the target must already exist. The target is removed as in 1. if the parent is an object (returns the number of removed members, 0 or more) and as in 2. if it is an array (always returns 1).
    "},{"location":"api/basic_json_document/erase/#template-parameters","title":"Template parameters","text":"I an integral type other than bool, deduced (overloads taking a bool or a non-integral type for idx do not participate in overload resolution)."},{"location":"api/basic_json_document/erase/#parameters","title":"Parameters","text":"object (in) the object to remove a member of array (in) the array to remove an element of key (in) the key of the member(s) to remove idx (in) the index of the element to remove; a negative value throws (see Exceptions) ptr (in) a JSON pointer to the value to remove, relative to root()"},{"location":"api/basic_json_document/erase/#return-value","title":"Return value","text":"
    1. the number of removed members (0 if object had none with this key)
    2. (nothing)
    3. the number of removed values (0 or more for an object member, always 1 for an array element)
    "},{"location":"api/basic_json_document/erase/#exceptions","title":"Exceptions","text":"
    1. Throws type_error.307 if object is not an object -- the same message BasicJsonType::erase throws for the same type.
    2. Throws type_error.307 if array is not an array. Throws out_of_range.401 if idx is negative, or if idx >= array.size().
    3. Throws out_of_range.405 (\"JSON pointer has no parent\") if ptr is empty. Throws what at throws (overload 3) for resolving ptr's parent. For the last reference token itself: if the parent is an array, throws what 2. throws for an index that is out of range, or, for a token that is not a valid array index, parse_error.106 (a leading '0'), parse_error.109 (not a number), out_of_range.410 (too large for size_type), or out_of_range.404 (an empty token); otherwise (an object, or a primitive value the pointer's parent resolves to) throws what 1. throws.

    Overloads 1. and 2. also throw invalid_iterator.202 (\"view does not belong to this document\") if object/array is a discarded view or a view of a different document. Overload 3. never throws invalid_iterator.202: it takes no view, and it always starts from this document's own root().

    "},{"location":"api/basic_json_document/erase/#complexity","title":"Complexity","text":"
    1. Linear in the number of members of object.
    2. Linear in the number of elements of array at or after idx (they move one slot over).
    3. Linear in the number of reference tokens of ptr and, for each token, in the number of members of the object at that level or the index into the array (as at), plus the complexity of 1. or 2. for the last token.
    "},{"location":"api/basic_json_document/erase/#notes","title":"Notes","text":"

    Duplicate keys

    Overload 1. removes every member with key, not just the first -- unlike set, which assigns the first occurrence and drops the rest. This is why it returns a count rather than a single view: there may be more than one member removed, or none.

    Like set and push_back, erase never moves an element's value: a view still referring to a removed member or element keeps showing what it last held (see Edits) -- it just no longer appears when array/object is read, dumped, or iterated. What does change is the container's sequence of links to its elements: removing an element of array (2.) shifts the links to the elements after it by one, as insert does in the other direction. Every iterator already taken over array/object is invalidated by an erase, not only those at or after the removed element, since the container may now use a different sequence.

    "},{"location":"api/basic_json_document/erase/#examples","title":"Examples","text":"Example

    The example below drops a deprecated field and a decommissioned entry from a configuration document -- using all three overloads -- and shows what stays intact that would not with a plain json/ordered_json value: the order of the fields around the ones removed, and the exact spelling of a number that was never touched.

    #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json = nlohmann::json;\nusing json_editable_document = nlohmann::json_editable_document;\nusing json_editable_view = nlohmann::json_editable_view;\n\nint main()\n{\n    // a deprecated field is dropped from a configuration file, and a\n    // decommissioned replica is removed from the list -- \"price\" keeps its\n    // trailing zero, and the fields around the removed ones keep their order\n    const std::string text = R\"({\n  \"name\": \"cache\",\n  \"legacy_host\": \"db0\",\n  \"host\": \"db1\",\n  \"price\": 19.90,\n  \"replicas\": [\"db2\", \"db3\", \"db4\"]\n})\";\n\n    json_editable_document doc = json_editable_document::parse(text);\n\n    doc.erase(doc.root(), \"legacy_host\");                                      // (1) an object member\n    doc.erase(doc.root()[\"replicas\"], 1);                                      // (2) an array element (\"db3\")\n    const std::size_t removed = doc.erase(json::json_pointer(\"/replicas/0\"));  // (3) via a JSON pointer\n\n    std::cout << removed << '\\n';\n    std::cout << doc.root().dump(2, ' ', false, json_editable_view::number_format::source) << \"\\n\\n\";\n\n    // the same edits on a plain json value: object_t is a std::map, so\n    // parsing already sorted the keys, and dump() rewrites every number to\n    // its shortest form, even \"price\", which was never touched\n    json plain = json::parse(text);\n    plain.erase(\"legacy_host\");\n    plain[\"replicas\"].erase(1);\n    plain[\"replicas\"].erase(0);\n    std::cout << plain.dump(2) << '\\n';\n}\n

    Output:

    1\n{\n  \"name\": \"cache\",\n  \"host\": \"db1\",\n  \"price\": 19.90,\n  \"replicas\": [\n    \"db4\"\n  ]\n}\n\n{\n  \"host\": \"db1\",\n  \"name\": \"cache\",\n  \"price\": 19.9,\n  \"replicas\": [\n    \"db4\"\n  ]\n}\n
    "},{"location":"api/basic_json_document/erase/#see-also","title":"See also","text":"
    • insert - insert an element into an array
    • set - replace a value, or set an object member, an array element, or the value a JSON pointer refers to
    • push_back - append to an array
    • root - the view of the root value, the starting point of overload 3
    • BasicJsonType::erase - the corresponding function of basic_json
    • Edits - what an edit guarantees, for every overload
    "},{"location":"api/basic_json_document/erase/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_document/insert/","title":"nlohmann::basic_json_document::insert","text":"
    template<typename I, typename V>\nview_type insert(view_type array, I idx, V&& value);\n

    Only an editable document (Editable == true, e.g. json_editable_document) has insert; calling it on a read-only basic_json_document fails to compile (static_assert).

    Inserts value into array as a new element before position idx, which must not be past the end (idx <= array.size(); idx == array.size() appends, like push_back). Unlike push_back, a null array does not first become an empty array: array must already be an array.

    value is accepted three ways: a basic_json_view of any document -- read-only or editable, and it does not have to be array's own document -- which is copied so that nothing is shared with the source document afterward; a BasicJsonType value; or anything BasicJsonType can be constructed from (numbers, strings, bool, nullptr, containers, ...).

    "},{"location":"api/basic_json_document/insert/#template-parameters","title":"Template parameters","text":"I an integral type other than bool, deduced (overloads taking a bool or a non-integral type for idx do not participate in overload resolution). V the type of value, deduced; see above for what is accepted."},{"location":"api/basic_json_document/insert/#parameters","title":"Parameters","text":"array (in) the array to insert into idx (in) the position to insert value before; a negative value throws (see Exceptions) value (in) the value to insert"},{"location":"api/basic_json_document/insert/#return-value","title":"Return value","text":"

    a view of the new element, now holding value

    "},{"location":"api/basic_json_document/insert/#exception-safety","title":"Exception safety","text":"

    Basic exception safety: value is fully encoded -- including the checks below -- into storage owned by the document before anything already reachable from root() is touched, so a failure while encoding value (an invalid argument, or std::bad_alloc) leaves the document completely unchanged, other than memory allocated for the encoding that is not reclaimed. A failure of a later allocation -- while array switches from its parsed layout to a growable block, or while that block grows, see Notes -- can still leave array already switched to that layout even though value itself was not inserted.

    "},{"location":"api/basic_json_document/insert/#exceptions","title":"Exceptions","text":"

    Throws type_error.309 if array is not an array -- the same message BasicJsonType::insert throws for the same type; a null array throws this too (see above). Throws out_of_range.401 if idx is negative, or if idx > array.size(). Throws invalid_iterator.202 (\"view does not belong to this document\") if array is a discarded view or a view of a different document. Throws type_error.302 if value is a discarded view or a discarded BasicJsonType value, and type_error.319 if value is (or contains) a binary value -- BasicJsonType can hold one, but a json_document cannot. Throws type_error.316 if value is (or contains) a string that is not valid UTF-8, with the same message BasicJsonType::dump() gives for that string.

    "},{"location":"api/basic_json_document/insert/#complexity","title":"Complexity","text":"

    Linear in the number of elements of array at or after idx (they move one slot over), plus time linear in the size of value to encode it into the document's storage (constant for a scalar, linear in the number of nested values for an array or object): like push_back, the elements of array move to a growable block of links the first time it is inserted into (or set/push_back on), and that block grows in amortized constant time; inserting before the end within that block still shifts every later element.

    "},{"location":"api/basic_json_document/insert/#notes","title":"Notes","text":"

    Like set on a member or an element, insert never moves an existing element's value -- only where array's links to its elements live -- so a view of an existing element of array stays valid across an insert, and keeps referring to the same element even though its index shifts. Any iterator already taken over array is invalidated, since it was walking the old layout. See Edits for what stays valid across an edit in general.

    "},{"location":"api/basic_json_document/insert/#examples","title":"Examples","text":"Example

    The example below inserts a step into the middle of a deployment plan, without touching the steps that come after it, and shows that a view taken before the insert keeps referring to the same element even though its index shifts -- something a plain json/ordered_json array, or its std::vector-based storage, has no equivalent for.

    #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json = nlohmann::json;\nusing json_editable_document = nlohmann::json_editable_document;\nusing json_editable_view = nlohmann::json_editable_view;\n\nint main()\n{\n    // a deployment plan -- \"budget\" is written with a trailing zero that has\n    // no effect on its value\n    const std::string text = R\"({\n  \"release\": \"2026.09\",\n  \"steps\": [\"build\", \"test\", \"deploy\"],\n  \"budget\": 19.90\n})\";\n\n    json_editable_document doc = json_editable_document::parse(text);\n\n    const std::size_t deploy_index = 2;\n    const auto deploy = doc.root()[\"steps\"][deploy_index];  // held across the insert\n\n    doc.insert(doc.root()[\"steps\"], deploy_index, \"smoke-test\"); // insert before \"deploy\"\n\n    // the held view still refers to \"deploy\", even though its index moved\n    // from 2 to 3, and nothing else in the document was touched\n    std::cout << deploy.dump() << '\\n';\n    std::cout << doc.root().dump(2, ' ', false, json_editable_view::number_format::source) << \"\\n\\n\";\n\n    // the same edit on a plain json value: an index held from before the\n    // insert now refers to whatever moved into that slot, and dump()\n    // rewrites \"budget\" to its shortest form even though it was never\n    // touched\n    json plain = json::parse(text);\n    plain[\"steps\"].insert(plain[\"steps\"].begin() + static_cast<std::ptrdiff_t>(deploy_index), \"smoke-test\");\n    std::cout << plain[\"steps\"][deploy_index].dump() << '\\n';\n    std::cout << plain.dump(2) << '\\n';\n}\n

    Output:

    \"deploy\"\n{\n  \"release\": \"2026.09\",\n  \"steps\": [\n    \"build\",\n    \"test\",\n    \"smoke-test\",\n    \"deploy\"\n  ],\n  \"budget\": 19.90\n}\n\n\"smoke-test\"\n{\n  \"budget\": 19.9,\n  \"release\": \"2026.09\",\n  \"steps\": [\n    \"build\",\n    \"test\",\n    \"smoke-test\",\n    \"deploy\"\n  ]\n}\n
    "},{"location":"api/basic_json_document/insert/#see-also","title":"See also","text":"
    • push_back - append to an array
    • erase - remove an object member, an array element, or the value a JSON pointer refers to
    • set - replace a value, or set an object member, an array element, or the value a JSON pointer refers to
    • BasicJsonType::insert - the corresponding function of basic_json
    • Edits - what an edit guarantees, for every overload
    "},{"location":"api/basic_json_document/insert/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_document/is_discarded/","title":"nlohmann::basic_json_document::is_discarded","text":"
    bool is_discarded() const noexcept;\n

    Returns whether the document holds no value, either because it was default-constructed or because the last call to parse(), parse_copy(), or read() failed with allow_exceptions set to false.

    "},{"location":"api/basic_json_document/is_discarded/#return-value","title":"Return value","text":"

    true if the document is discarded, false otherwise.

    "},{"location":"api/basic_json_document/is_discarded/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this function never throws exceptions.

    "},{"location":"api/basic_json_document/is_discarded/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json_document/is_discarded/#notes","title":"Notes","text":"

    When the document is discarded, root() returns a discarded view (its is_discarded() is also true).

    "},{"location":"api/basic_json_document/is_discarded/#examples","title":"Examples","text":"Example 
    #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // with allow_exceptions == false, a parse error produces a discarded\n    // document instead of throwing -- exactly like basic_json::parse()\n    json_document doc = json_document::parse(R\"({\"a\": )\", /* allow_exceptions */ false);\n    std::cout << doc.is_discarded() << '\\n';\n    std::cout << doc.root().is_discarded() << '\\n';\n\n    // a successful parse is never discarded\n    json_document ok = json_document::parse(R\"({\"a\": 1})\", false);\n    std::cout << ok.is_discarded() << '\\n';\n}\n

    Output:

    true\ntrue\nfalse\n
    "},{"location":"api/basic_json_document/is_discarded/#see-also","title":"See also","text":"
    • parse - deserialize from a compatible input
    • is_discarded (basic_json_view) - return whether a view is invalid
    "},{"location":"api/basic_json_document/is_discarded/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_document/load/","title":"nlohmann::basic_json_document::load","text":"
    // (1)\nstatic basic_json_document load(const std::uint8_t* image, std::size_t size,\n                                 const image_check check = image_check::full);\n\n// (2)\nstatic basic_json_document load(const std::vector<std::uint8_t>& image,\n                                 const image_check check = image_check::full);\n\n// (3)\nstatic basic_json_document load(std::vector<std::uint8_t>&& image,\n                                 const image_check check = image_check::full);\n
    1. Reads an image save() wrote, from a pointer and a byte count. The image is borrowed: image must stay alive and unchanged for as long as the returned document, and any view taken from it, is used.
    2. Reads an image from a std::vector. Also borrowed -- equivalent to overload 1 called with image.data() and image.size().
    3. Reads an image, keeping the vector instead of copying it: image is moved into the document (no copy), which then owns it for as long as it needs the text and the decoded strings. owns_source() is true afterward.

    In every overload, the node index is copied into storage the document itself owns -- so that it is properly aligned, and, for an editable document, can be edited -- while the text and the decoded strings stay in image. The hash indexes large objects use for lookup are rebuilt, exactly as after parsing.

    "},{"location":"api/basic_json_document/load/#parameters","title":"Parameters","text":"image (in) the image save() wrote (overloads 1 and 2), or one to take ownership of (overload 3) size (in) the number of bytes at image (overload 1) check (in) how thoroughly to validate image before trusting it; see image_check below (optional, image_check::full by default)"},{"location":"api/basic_json_document/load/#return-value","title":"Return value","text":"

    The document read from the image.

    "},{"location":"api/basic_json_document/load/#exception-safety","title":"Exception safety","text":"

    Overloads 1 and 2 give the strong guarantee: image is only read, never written, so a thrown exception leaves the caller's buffer untouched.

    Overload 3 moves image into the document before validating it, so that a good image is kept without a copy. If loading then fails, the partially built document -- and the vector now inside it -- is discarded along with the exception, and image itself is left empty, not restored to what was passed in. Move a copy in instead, or validate with overload 2 first, if the original vector must survive a failed load.

    "},{"location":"api/basic_json_document/load/#exceptions","title":"Exceptions","text":"

    On a big-endian target, throws type_error.320 -- the same exception save() throws there, since the image format is little-endian only.

    Otherwise throws parse_error.116 if image is not one save() could have written, or fails the requested check:

    message when too short image is nullptr, or size is smaller than the 64-byte header unknown format the header's magic bytes or version do not match, or a reserved header field is not zero sizes out of range the node count, text size, or decoded-string size the header describes does not fit size, or the '\\0' after the text or after the decoded strings is missing the check failed check is not image_check::none, and the image fails it -- see image_check

    Example messages

    [json.exception.parse_error.116] parse error: invalid json_document image: too short\n
    [json.exception.parse_error.116] parse error: invalid json_document image: unknown format\n
    [json.exception.parse_error.116] parse error: invalid json_document image: sizes out of range\n
    [json.exception.parse_error.116] parse error: invalid json_document image: the check failed\n

    "},{"location":"api/basic_json_document/load/#complexity","title":"Complexity","text":"

    Linear in the number of nodes, which are always copied into the document. image_check::none does not read the text or the decoded strings. image_check::bounds reads one byte of the text for each float token (its sign, to check the recorded digits against the token's length), and nothing else of the text or the decoded strings.

    With image_check::full, linear in the size of the image (the nodes, the text, and the decoded strings), plus sorting the ranges of the strings and of the float tokens (at most one per node): the contents of each distinct range are checked once, however many nodes refer to it.

    "},{"location":"api/basic_json_document/load/#notes","title":"Notes","text":""},{"location":"api/basic_json_document/load/#image_check","title":"image_check","text":"
    using image_check = detail::view::image_check;\n\nenum class image_check\n{\n    full,\n    bounds,\n    none\n};\n

    How thoroughly load() validates image before trusting it.

    full (the default) Checks everything the parser itself guarantees: structure and bounds; that every string is valid UTF-8 (and, for a string still in the source text, that it contains no quote, backslash, or control character); and that every number token is well-formed and matches the value stored for it. Strings and float tokens may share a range only if the ranges are identical (as nodes that share a value do), and never overlap otherwise. Reading and serializing a checked image is safe and always produces valid JSON, exactly as for a parsed document. This is the right choice for an image from anything you do not fully control -- a file, a cache shared with other processes, a peer on the network. bounds Checks structure and bounds only: that every offset and count in the node index stays inside the image. So load(), root(), element access, and materialize() can never read outside the image, even on a damaged one. It does not guarantee that the image describes valid JSON: a string that a full check would have rejected can make dump() write invalid UTF-8 or invalid JSON, or throw type_error.316, and a number can read back with a value that does not match how it is spelled. An editable document does not take such a string over: set, push_back and insert throw type_error.316 when they copy it from a view of the loaded document, and leave the editable document unchanged. bounds skips scanning the text and the decoded strings, so it fits a cache your own process just wrote and reads straight back, where damage would mean a bug or a hardware fault rather than adversarial input. none Checks nothing. Reading a damaged image is undefined behavior, so use it only for an image you trust as much as your own memory.

    Lifetime. Overloads 1 and 2 borrow image: it must stay alive and byte-for-byte unchanged for as long as the returned document, and any view taken from it, is used -- exactly like a document parse() borrowed its input for. Overload 3 avoids this by keeping the vector itself; see owns_source.

    Experimental

    The image format is versioned but not yet stable, and may change in an incompatible way before it is declared stable; load() already rejects an image written by a different format version with parse_error.116 (\"unknown format\"). Use images to cache a document within one build of the library, or to hand one to another process running the same build on the same (little-endian) machine -- not as a long-term storage format.

    "},{"location":"api/basic_json_document/load/#examples","title":"Examples","text":"Caching a document, ownership, and a rejected image

    The example below saves a parsed document as an image, checks that load() reproduces the original dump() without parsing, and shows the difference between load(std::move(image)) (owned) and load(image) (borrowed). It then damages one byte of the image and shows image_check::full rejecting it with parse_error.116, while image_check::bounds -- meant for a cache the process already trusts -- still reads it without going out of bounds.

    #include <cstdint>\n#include <iostream>\n#include <string>\n#include <vector>\n#include <nlohmann/json_view.hpp>\n\nusing json = nlohmann::json;\nusing json_document = nlohmann::json_document;\nusing image_check = json_document::image_check;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // the image of a parsed document -- as if read back from a cache file or\n    // received from another process running the same build of the library\n    const std::string text = R\"({\"name\": \"cache\", \"note\": \"caf\\u00e9\", \"replicas\": [\"db2\", \"db3\"]})\";\n    const json_document parsed = json_document::parse(text);\n    const std::vector<std::uint8_t> image = parsed.save();\n\n    // (1)/(2) load() needs no parsing, yet dumps exactly what parsing did\n    const json_document borrowed = json_document::load(image);\n    std::cout << (borrowed.root().dump() == parsed.root().dump()) << '\\n';\n    std::cout << borrowed.owns_source() << '\\n'; // borrowed: still points into `image`\n\n    // (3) load(std::move(image)) keeps the vector instead of copying it\n    std::vector<std::uint8_t> to_move = image;\n    const json_document owned = json_document::load(std::move(to_move));\n    std::cout << owned.owns_source() << '\\n';\n\n    // a damaged image -- the last byte of the decoded string \"note\" holds\n    // (an escape sequence, so it was unescaped into the document's own\n    // buffer), flipped, as storage or transport corruption might do\n    std::vector<std::uint8_t> damaged = image;\n    damaged[damaged.size() - 2] = 0xFF;\n\n    // image_check::full inspects strings and numbers, so it catches the damage\n    try\n    {\n        static_cast<void>(json_document::load(damaged, image_check::full));\n    }\n    catch (const json::parse_error& e)\n    {\n        std::cout << e.id << '\\n';\n    }\n\n    // image_check::bounds only checks structure and bounds, so a cache the\n    // process already trusts loads without the extra scan -- reading a value\n    // the damage did not touch is still safe\n    const json_document trusted = json_document::load(damaged, image_check::bounds);\n    std::cout << trusted.root()[\"name\"].get<std::string>() << '\\n';\n}\n

    Output:

    true\nfalse\ntrue\n116\ncache\n
    "},{"location":"api/basic_json_document/load/#see-also","title":"See also","text":"
    • save - write the document as an image
    • owns_source - return whether the document holds its own copy of the text
    • parse - deserialize from JSON text instead of an image
    • Images - why and when to use images
    "},{"location":"api/basic_json_document/load/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_document/memory_usage/","title":"nlohmann::basic_json_document::memory_usage","text":"
    std::size_t memory_usage() const noexcept;\n

    Returns the number of bytes held by the document: the node index, the decoded-string buffer (for strings that contain escapes), and, for an owned document, its copy of the source text.

    "},{"location":"api/basic_json_document/memory_usage/#return-value","title":"Return value","text":"

    The number of bytes the document holds, 0 for a discarded document.

    "},{"location":"api/basic_json_document/memory_usage/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this function never throws exceptions.

    "},{"location":"api/basic_json_document/memory_usage/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json_document/memory_usage/#notes","title":"Notes","text":"

    The exact value depends on the platform, the allocator, and the library's own layout, and may change between versions; do not rely on it being a specific number, and do not compare it across different builds or platforms. Compare it for the same document over time, or between documents built with the same binary, instead -- for instance to observe the effect of shrink_to_fit().

    "},{"location":"api/basic_json_document/memory_usage/#examples","title":"Examples","text":"Example 
    #include <iostream>\n#include <string>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // long enough that copying it needs a real (heap) allocation, so the\n    // comparison below does not depend on the standard library's small\n    // string optimization threshold\n    const std::string text = std::string(200, ' ') + \"[1, 2, 3, 4, 5]\";\n\n    // memory_usage() is not portable across platforms/allocators/compilers, so\n    // compare it relatively instead of printing the raw byte count\n    json_document borrowed = json_document::parse(text);\n    json_document owned = json_document::parse_copy(text);\n\n    // the owned document additionally stores its own copy of the source text\n    std::cout << (owned.memory_usage() > borrowed.memory_usage()) << '\\n';\n\n    // a document with more values needs a larger index\n    json_document small = json_document::parse(std::string(\"[1]\"));\n    json_document large = json_document::parse(std::string(\"[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]\"));\n    std::cout << (large.memory_usage() > small.memory_usage()) << '\\n';\n}\n

    Output:

    true\ntrue\n
    "},{"location":"api/basic_json_document/memory_usage/#see-also","title":"See also","text":"
    • node_count - the number of index entries
    • shrink_to_fit - release unused index capacity
    "},{"location":"api/basic_json_document/memory_usage/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_document/node_count/","title":"nlohmann::basic_json_document::node_count","text":"
    std::size_t node_count() const noexcept;\n

    Returns the number of entries in the document's flat index.

    "},{"location":"api/basic_json_document/node_count/#return-value","title":"Return value","text":"

    The number of index entries: one per value (of any type, at any nesting depth) plus one per object key. 0 for a discarded document.

    "},{"location":"api/basic_json_document/node_count/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this function never throws exceptions.

    "},{"location":"api/basic_json_document/node_count/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json_document/node_count/#notes","title":"Notes","text":"

    Each index entry is 16 bytes, so node_count() * 16 is the size of the index itself (part, but not all, of memory_usage(), which also counts decoded strings and, for an owned document, the text).

    "},{"location":"api/basic_json_document/node_count/#examples","title":"Examples","text":"Example 
    #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    // node_count() is the size of the flat index: one 16-byte node per value,\n    // plus one per object key (values and keys are all the index stores)\n    json_document scalar = json_document::parse(\"42\");\n    std::cout << scalar.node_count() << '\\n';\n\n    json_document doc = json_document::parse(R\"({\"a\": 1, \"b\": [1, 2]})\");\n    std::cout << doc.node_count() << '\\n';\n}\n

    Output:

    1\n7\n
    "},{"location":"api/basic_json_document/node_count/#see-also","title":"See also","text":"
    • memory_usage - the number of bytes held by the document
    • shrink_to_fit - release unused index capacity
    "},{"location":"api/basic_json_document/node_count/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_document/owns_source/","title":"nlohmann::basic_json_document::owns_source","text":"
    bool owns_source() const noexcept;\n

    Returns whether the document holds its own copy of the parsed text or is borrowing the caller's buffer.

    "},{"location":"api/basic_json_document/owns_source/#return-value","title":"Return value","text":"

    true if the document owns the text returned by source(), false if it borrows it (or if the document is discarded).

    "},{"location":"api/basic_json_document/owns_source/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this function never throws exceptions.

    "},{"location":"api/basic_json_document/owns_source/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json_document/owns_source/#notes","title":"Notes","text":"

    See the ownership table on parse for which inputs are borrowed and which are owned. A borrowed document (owns_source() == false) is only valid while the buffer it was parsed from is still alive.

    "},{"location":"api/basic_json_document/owns_source/#examples","title":"Examples","text":"Example 
    #include <iostream>\n#include <string>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    std::string text = R\"({\"a\": 1})\";\n\n    // borrowed: the document only points into `text`; `text` must outlive it\n    json_document borrowed = json_document::parse(text);\n    std::cout << borrowed.owns_source() << '\\n';\n\n    // owned: parse_copy() always takes its own copy\n    json_document copied = json_document::parse_copy(text);\n    std::cout << copied.owns_source() << '\\n';\n\n    // owned: an rvalue std::string is moved in, not copied, but still owned\n    json_document moved_in = json_document::parse(std::string(text));\n    std::cout << moved_in.owns_source() << '\\n';\n}\n

    Output:

    false\ntrue\ntrue\n
    "},{"location":"api/basic_json_document/owns_source/#see-also","title":"See also","text":"
    • parse - deserialize from a compatible input
    • parse_copy - deserialize a copy of a compatible input, always owned
    • source - the parsed text
    "},{"location":"api/basic_json_document/owns_source/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_document/parse/","title":"nlohmann::basic_json_document::parse","text":"
    // (1)\ntemplate<typename InputType>\nstatic basic_json_document parse(InputType&& input,\n                                 const bool allow_exceptions = true,\n                                 const bool ignore_comments = false,\n                                 const bool ignore_trailing_commas = false);\n\n// (2)\ntemplate<typename IteratorType>\nstatic basic_json_document parse(IteratorType first, IteratorType last,\n                                 const bool allow_exceptions = true,\n                                 const bool ignore_comments = false,\n                                 const bool ignore_trailing_commas = false);\n
    1. Deserialize from a compatible input, borrowing or owning it depending on its value category and type (see Notes).
    2. Deserialize from a pair of input iterators.

    Both overloads accept the same JSON text as BasicJsonType::parse(), with the same ignore_comments/ignore_trailing_commas options, but build a basic_json_document (a flat index into the input) instead of a tree of BasicJsonType values. They take the same inputs as BasicJsonType::parse(). Byte inputs that stay alive can be borrowed; all others (streams, FILE*, wide characters, non-contiguous iterators) are read into a copy that the document owns -- see the Notes. Use parse_copy() to get an owned copy of a borrowable input as well.

    "},{"location":"api/basic_json_document/parse/#template-parameters","title":"Template parameters","text":"InputType

    A compatible input, for instance:

    • a std::string, std::string_view, or a C-style array of single-byte characters
    • a pointer to a null-terminated string of single-byte characters (char, signed char, unsigned char, std::uint8_t)
    • a container for which obj.data() and obj.size() give contiguous single-byte access, e.g. std::vector<char> or std::vector<std::uint8_t>
    • an std::istream object
    • a FILE* pointer
    • wide characters (wchar_t, char16_t, char32_t), which are converted to UTF-8: a wide string object (std::wstring, std::u16string, std::u32string), an array (e.g. a wide string literal), or a pointer to a null-terminated wide string
    IteratorType an input iterator type, for instance a pair of pointers such as ptr and ptr + len, or a pair of std::string::iterator; the iterators of single-byte characters are borrowed or read like the byte inputs above, those of wide characters are converted to UTF-8"},{"location":"api/basic_json_document/parse/#parameters","title":"Parameters","text":"input (in) Input to parse from. allow_exceptions (in) whether to throw exceptions in case of a parse error (optional, true by default) ignore_comments (in) whether comments should be ignored and treated like whitespace (true) or yield a parse error (false); (optional, false by default) ignore_trailing_commas (in) whether trailing commas in arrays or objects should be ignored and treated like whitespace (true) or yield a parse error (false); (optional, false by default) first (in) iterator to the start of a character range last (in) iterator to the end of a character range"},{"location":"api/basic_json_document/parse/#return-value","title":"Return value","text":"

    The parsed document. If allow_exceptions is false and the input is not valid JSON, the returned document is discarded; see is_discarded.

    "},{"location":"api/basic_json_document/parse/#exceptions","title":"Exceptions","text":"

    Throws the same exception BasicJsonType::parse() throws for the same input and options -- the same exception id, message, and position -- because on a failing input the library's own parser is run on the same bytes to produce the diagnostic. This replay runs only for a failing input, and costs about a lexing pass up to the error (its SAX handler builds no values); with allow_exceptions set to false, it is skipped and a discarded document is returned instead. Additionally throws out_of_range.416 if the input is 4294967280 bytes (4 GiB minus 16 bytes) or larger, a size BasicJsonType::parse() does not reject.

    "},{"location":"api/basic_json_document/parse/#complexity","title":"Complexity","text":"

    Linear in the length of the input.

    "},{"location":"api/basic_json_document/parse/#notes","title":"Notes","text":"

    Ownership. Whether the document borrows input or owns a copy of it depends on its value category and type:

    input ownership lvalue byte container (std::string, std::vector<char>, ...), std::string_view, C string, character array borrowed -- input must outlive the document non-const rvalue std::string owned, moved in without a copy other rvalue byte container (including a const rvalue std::string) owned, copied stream, FILE*, wide characters, or anything else read through the general input adapter owned, read into a buffer (a stream is read to its end)

    For overload (2), a pair of pointers to single-byte integers (e.g. const char*, std::uint8_t*) is borrowed. From C++20 on, so is any other contiguous iterator over single bytes, such as std::vector<char>::iterator or std::string::const_iterator. Before C++20 these iterators cannot be told apart from other class-type iterators, so their range is read into an owned buffer, as is any non-contiguous range (e.g. of a std::list<char>).

    See owns_source to check which happened after a call, and the feature page for the reasoning.

    Numbers. As for BasicJsonType::parse(), an integer literal too large for the 64-bit integer type becomes a floating-point value.

    No lengths. An integer argument that is not a bool where the flags are expected -- for example parse(ptr, len) -- does not compile (the overload is deleted). Such a call would convert len to allow_exceptions and read ptr as a null-terminated string, past the end of a buffer that has none. To parse a buffer of a given length, pass a pair of pointers: parse(ptr, ptr + len). The same holds for parse_copy, accept, and read.

    "},{"location":"api/basic_json_document/parse/#examples","title":"Examples","text":"Example: (1) borrowed vs. owned input, and errors identical to BasicJsonType::parse() 
    #include <iostream>\n#include <string>\n#include <nlohmann/json.hpp>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // an lvalue std::string is BORROWED: the document only stores a pointer\n    // into `text`, so `text` must outlive `borrowed`\n    std::string text = R\"({\"count\": 3})\";\n    json_document borrowed = json_document::parse(text);\n    std::cout << borrowed.owns_source() << '\\n'; // false\n\n    // an rvalue std::string is MOVED into the document -- no copy of the text\n    json_document owned = json_document::parse(std::string(R\"({\"count\": 3})\"));\n    std::cout << owned.owns_source() << '\\n'; // true\n\n    // errors are identical to basic_json::parse: same exception id, message,\n    // and position, because the library parser runs on the same bytes on a\n    // failing input\n    try\n    {\n        static_cast<void>(json_document::parse(R\"({\"count\": )\"));\n    }\n    catch (const nlohmann::json::parse_error& e)\n    {\n        std::cout << e.id << '\\n';\n    }\n}\n

    Output:

    false\ntrue\n101\n
    Example: (2) parse an iterator range (no NUL terminator required) 
    #include <iostream>\n#include <vector>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // a JSON value embedded in a larger, non-null-terminated buffer (e.g. a\n    // slice received over the network)\n    std::vector<char> buffer = {'[', '1', ',', '2', ']', 'j', 'u', 'n', 'k'};\n\n    // a pointer pair is BORROWED, exactly like a byte container lvalue: the\n    // document points into the buffer. (From C++20 on, std::vector<char>\n    // iterators are borrowed as well; before, they are copied.)\n    const char* first = buffer.data();\n    json_document doc = json_document::parse(first, first + 5);\n    std::cout << doc.root().is_array() << ' ' << doc.root().size() << '\\n';\n    std::cout << doc.owns_source() << '\\n'; // false: borrowed\n}\n

    Output:

    true 2\nfalse\n
    "},{"location":"api/basic_json_document/parse/#see-also","title":"See also","text":"
    • parse_copy - deserialize a copy of a compatible input
    • accept - check whether the input is valid JSON
    • read - (re-)parse into this document, reusing its memory
    • owns_source - return whether the document holds its own copy of the text
    • load - read a document from an image instead of parsing JSON text
    • BasicJsonType::parse - the corresponding function of basic_json
    "},{"location":"api/basic_json_document/parse/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_document/parse_copy/","title":"nlohmann::basic_json_document::parse_copy","text":"
    template<typename InputType>\nstatic basic_json_document parse_copy(InputType&& input,\n                                      const bool allow_exceptions = true,\n                                      const bool ignore_comments = false,\n                                      const bool ignore_trailing_commas = false);\n

    Deserialize from a compatible input, always taking the document's own copy of it, regardless of the value category or type of input. Unlike parse(), the returned document never depends on input staying alive.

    "},{"location":"api/basic_json_document/parse_copy/#template-parameters","title":"Template parameters","text":"InputType A compatible input; see parse."},{"location":"api/basic_json_document/parse_copy/#parameters","title":"Parameters","text":"input (in) Input to parse from. allow_exceptions (in) whether to throw exceptions in case of a parse error (optional, true by default) ignore_comments (in) whether comments should be ignored and treated like whitespace (true) or yield a parse error (false); (optional, false by default) ignore_trailing_commas (in) whether trailing commas in arrays or objects should be ignored and treated like whitespace (true) or yield a parse error (false); (optional, false by default)"},{"location":"api/basic_json_document/parse_copy/#return-value","title":"Return value","text":"

    The parsed document, with owns_source() true. If allow_exceptions is false and the input is not valid JSON, the returned document is discarded; see is_discarded.

    "},{"location":"api/basic_json_document/parse_copy/#exceptions","title":"Exceptions","text":"

    Same as parse.

    "},{"location":"api/basic_json_document/parse_copy/#complexity","title":"Complexity","text":"

    Linear in the length of the input.

    "},{"location":"api/basic_json_document/parse_copy/#notes","title":"Notes","text":"

    parse_copy() accepts and rejects exactly what parse() does, and classifies numbers the same way; it only differs in that the input is always copied rather than sometimes borrowed. Prefer parse() when the input's lifetime already covers the document's, since it avoids the copy for borrowed inputs.

    An integer argument that is not a bool where the flags are expected, such as parse_copy(ptr, len), does not compile; see parse.

    "},{"location":"api/basic_json_document/parse_copy/#examples","title":"Examples","text":"Example

    The example below returns a document from a function whose local buffer would otherwise not outlive it.

    #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\njson_document parse_from_temporary_buffer()\n{\n    char buffer[] = R\"({\"a\": 1})\";\n    // parse() would borrow `buffer`, which is about to go out of scope;\n    // parse_copy() takes its own copy instead, so the returned document does\n    // not depend on `buffer` afterward\n    return json_document::parse_copy(buffer);\n}\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    json_document doc = parse_from_temporary_buffer();\n    std::cout << doc.owns_source() << '\\n';\n    std::cout << doc.root().is_object() << '\\n';\n}\n

    Output:

    true\ntrue\n
    "},{"location":"api/basic_json_document/parse_copy/#see-also","title":"See also","text":"
    • parse - deserialize from a compatible input, borrowing it where possible
    • owns_source - return whether the document holds its own copy of the text
    "},{"location":"api/basic_json_document/parse_copy/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_document/push_back/","title":"nlohmann::basic_json_document::push_back","text":"
    template<typename V>\nview_type push_back(view_type array, V&& value);\n

    Appends value as a new last element of array. A null array first becomes an empty array, the same way set turns a null object into an empty object.

    value is accepted three ways: a basic_json_view of any document -- read-only or editable, and it does not have to be array's own document -- which is copied so that nothing is shared with the source document afterward; a BasicJsonType value; or anything BasicJsonType can be constructed from (numbers, strings, bool, nullptr, containers, ...).

    Only an editable document (Editable == true, e.g. json_editable_document) has push_back; calling it on a read-only basic_json_document fails to compile (static_assert).

    "},{"location":"api/basic_json_document/push_back/#template-parameters","title":"Template parameters","text":"V the type of value, deduced; see above for what is accepted."},{"location":"api/basic_json_document/push_back/#parameters","title":"Parameters","text":"array (in) the array (or null value) to append to value (in) the value to append"},{"location":"api/basic_json_document/push_back/#return-value","title":"Return value","text":"

    a view of the new last element of array, now holding value

    "},{"location":"api/basic_json_document/push_back/#exception-safety","title":"Exception safety","text":"

    Basic exception safety: value is fully encoded -- including the checks below -- into storage owned by the document before anything already reachable from root() is touched, so a failure while encoding value (an invalid argument, or std::bad_alloc) leaves the document completely unchanged, other than memory allocated for the encoding that is not reclaimed. A failure of a later allocation -- while array switches from its parsed layout to a growable block, or while that block grows, see Notes -- can still leave a partial effect, such as a null array argument already turned into an empty array even though value itself was not appended.

    "},{"location":"api/basic_json_document/push_back/#exceptions","title":"Exceptions","text":"

    Throws type_error.308 if array is neither an array nor null -- the same message BasicJsonType::push_back throws for the same type. Throws invalid_iterator.202 (\"view does not belong to this document\") if array is a discarded view or a view of a different document. Throws type_error.302 if value is a discarded view or a discarded BasicJsonType value, and type_error.319 if value is (or contains) a binary value -- BasicJsonType can hold one, but a json_document cannot. Throws type_error.316 if value is (or contains) a string that is not valid UTF-8, with the same message BasicJsonType::dump() gives for that string.

    "},{"location":"api/basic_json_document/push_back/#complexity","title":"Complexity","text":"

    Amortized constant, plus time linear in the size of value to encode it into the document's storage (constant for a scalar, linear in the number of nested values for an array or object): the elements of array move to a growable block of links the first time it is appended to (or set on), and that block itself grows -- doubling its capacity, so the cost of growing it amortizes to constant per element -- only once it runs out of room. See Notes.

    "},{"location":"api/basic_json_document/push_back/#notes","title":"Notes","text":"

    Like set on a member or an element, push_back never moves an existing element itself -- only where array's links to its elements live -- so a view of an existing element of array stays valid across a push_back, but any iterator already taken over array is invalidated, since it was walking the old layout. See Edits for what stays valid across an edit in general.

    "},{"location":"api/basic_json_document/push_back/#examples","title":"Examples","text":"Example

    The example below appends records to an array one at a time, as they might arrive from a stream of events, without ever building a BasicJsonType value for the array or for the records already in it, and shows that a view taken from an earlier push_back still refers to the same element once later ones have run.

    #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json = nlohmann::json;\nusing json_editable_document = nlohmann::json_editable_document;\n\nint main()\n{\n    // \"events\" starts out null -- the first push_back() turns it into an\n    // array, exactly like set() turns a null object member into an object\n    json_editable_document doc = json_editable_document::parse(R\"({\"source\": \"sensor-1\", \"events\": null})\");\n\n    const auto first = doc.push_back(doc.root()[\"events\"], json{{\"type\", \"start\"}, {\"t\", 0}});\n    for (int t = 1; t <= 3; ++t)\n    {\n        doc.push_back(doc.root()[\"events\"], json{{\"type\", \"tick\"}, {\"t\", t}});\n    }\n\n    // push_back() never moves an existing element: a view taken from an\n    // earlier call still refers to the same element after later ones\n    std::cout << first.dump() << '\\n';\n    std::cout << doc.root()[\"events\"].size() << '\\n';\n    std::cout << doc.root().dump(2) << '\\n';\n}\n

    Output:

    {\"t\":0,\"type\":\"start\"}\n4\n{\n  \"source\": \"sensor-1\",\n  \"events\": [\n    {\n      \"t\": 0,\n      \"type\": \"start\"\n    },\n    {\n      \"t\": 1,\n      \"type\": \"tick\"\n    },\n    {\n      \"t\": 2,\n      \"type\": \"tick\"\n    },\n    {\n      \"t\": 3,\n      \"type\": \"tick\"\n    }\n  ]\n}\n
    "},{"location":"api/basic_json_document/push_back/#see-also","title":"See also","text":"
    • set - replace a value, or set an object member, an array element, or the value a JSON pointer refers to
    • insert - insert an element into an array before a given position
    • erase - remove an object member, an array element, or the value a JSON pointer refers to
    • root - the view of the root value
    • BasicJsonType::push_back - the corresponding function of basic_json
    • Edits - what an edit guarantees, for every overload
    "},{"location":"api/basic_json_document/push_back/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_document/read/","title":"nlohmann::basic_json_document::read","text":"
    template<typename InputType>\nvoid read(InputType&& input,\n         const bool allow_exceptions = true,\n         const bool ignore_comments = false,\n         const bool ignore_trailing_commas = false);\n

    (Re-)parses input into *this, discarding the document's previous value and reusing its memory (the node index, the decoded-string buffer, and, if applicable, the owned copy of the text) rather than allocating a fresh document. parse() is implemented in terms of this function, applied to a default-constructed document.

    "},{"location":"api/basic_json_document/read/#template-parameters","title":"Template parameters","text":"InputType A compatible input; see parse."},{"location":"api/basic_json_document/read/#parameters","title":"Parameters","text":"input (in) Input to parse from. allow_exceptions (in) whether to throw exceptions in case of a parse error (optional, true by default) ignore_comments (in) whether comments should be ignored and treated like whitespace (true) or yield a parse error (false); (optional, false by default) ignore_trailing_commas (in) whether trailing commas in arrays or objects should be ignored and treated like whitespace (true) or yield a parse error (false); (optional, false by default)"},{"location":"api/basic_json_document/read/#exceptions","title":"Exceptions","text":"

    Same as parse.

    "},{"location":"api/basic_json_document/read/#complexity","title":"Complexity","text":"

    Linear in the length of the input.

    "},{"location":"api/basic_json_document/read/#notes","title":"Notes","text":"

    Every view taken from *this before the call -- including the previous root() -- is invalidated, whether or not the new parse succeeds; take fresh views from root() afterward.

    input is borrowed or owned by the same rules as parse(); a document can borrow on one call and own on the next, since ownership is decided freshly each time. An input that is copied (see the table in parse()) is written into the document's previous owned copy, reusing its memory; a moved-in std::string replaces it.

    Reusing a document matters most for large inputs: the operating system provides the memory of a fresh node index one page at a time, and every page costs a page fault the first time it is written. On x86-64 Linux (4 KiB pages), parsing a 55 MB document into a reused document took about 40 % less time than parsing it into a fresh one. Programs that parse many documents of similar size should therefore keep one document and call read().

    An integer argument that is not a bool where the flags are expected, such as read(ptr, len), does not compile; see parse.

    "},{"location":"api/basic_json_document/read/#examples","title":"Examples","text":"Example

    The example below parses a sequence of messages into the same document, reusing its memory instead of allocating a new document for each one.

    #include <iostream>\n#include <string>\n#include <vector>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    std::vector<std::string> messages =\n    {\n        R\"({\"id\": 1})\", R\"({\"id\": 2, \"tag\": \"x\"})\", R\"({\"id\": 3})\"\n    };\n\n    // parse into the same document over and over: its node index and decode\n    // buffer are reused instead of being freed and reallocated for each message\n    json_document doc;\n    std::size_t total = 0;\n    for (const auto& msg : messages)\n    {\n        doc.read(msg);\n        total += doc.root().size();\n    }\n    std::cout << total << '\\n';\n\n    // read() can also change what kind of input is owned/borrowed between calls\n    doc.read(std::string(R\"({\"owned\": true})\"));\n    std::cout << doc.owns_source() << '\\n';\n}\n

    Output:

    4\ntrue\n
    "},{"location":"api/basic_json_document/read/#see-also","title":"See also","text":"
    • parse - deserialize from a compatible input
    • root - the view of the root value
    • load - read a document from an image instead of parsing JSON text
    "},{"location":"api/basic_json_document/read/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_document/root/","title":"nlohmann::basic_json_document::root","text":"
    // (1)\nview_type root() const& noexcept;\n\n// (2)\nview_type root() const&& = delete;\n
    1. Returns a view of the root value of the document.
    2. Deleted: the view of a temporary document would dangle.
    "},{"location":"api/basic_json_document/root/#return-value","title":"Return value","text":"

    A view_type (i.e. basic_json_view<BasicJsonType>) for the root value, or a discarded view if the document is discarded.

    "},{"location":"api/basic_json_document/root/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this function never throws exceptions.

    "},{"location":"api/basic_json_document/root/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json_document/root/#notes","title":"Notes","text":"

    Lifetime. A view refers into the document, so the document must outlive it. root() can therefore only be called on a document that has a name (an lvalue); calling it on a temporary does not compile:

    auto v = json_document::parse(text).root();  // error: the document is destroyed at the end of the statement\n\nauto doc = json_document::parse(text);       // OK: keep the document alive\nauto v = doc.root();\n

    root() is a cheap handle into the document's index, not a copy of anything; call it as often as needed. The returned view is valid under the same conditions as any other view of the document -- see Object inspection -- in particular, it is invalidated by the next read() or shrink_to_fit() on this document. A view can dangle, like a std::string_view; see Ownership and lifetime.

    "},{"location":"api/basic_json_document/root/#examples","title":"Examples","text":"Example 
    #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    json_document doc = json_document::parse(R\"({\"greeting\": \"hi\"})\");\n    std::cout << doc.root().is_object() << '\\n';\n\n    // root() is a cheap handle, not a copy: repeated calls observe the same value\n    std::cout << (doc.root().type() == doc.root().type()) << '\\n';\n\n    // the root of a failed parse (allow_exceptions == false) is discarded\n    json_document failed = json_document::parse(\"{\", false);\n    std::cout << failed.root().is_discarded() << '\\n';\n}\n

    Output:

    true\ntrue\ntrue\n
    "},{"location":"api/basic_json_document/root/#see-also","title":"See also","text":"
    • is_discarded - return whether the last parse failed
    • materialize - build the BasicJsonType value of a subtree
    "},{"location":"api/basic_json_document/root/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_document/save/","title":"nlohmann::basic_json_document::save","text":"
    std::vector<std::uint8_t> save() const;\n

    Writes the document as an image: a byte buffer that load reads back without parsing. The image holds the node index, the source text (plus, for an edited document, the number tokens edits wrote), and the decoded strings (plus the strings edits wrote) -- everything root() needs, with nothing left to parse.

    An edited document is written in its current state, with its values in document order, the way the library's own parser would have produced them for that JSON text: a member set added goes at the end, an erased member leaves no trace, and a float that is not finite (NaN or positive/negative infinity) becomes null, the same substitution dump() makes. The same document always saves to the same bytes -- also across BasicJsonType and Editable, since the image reflects document order and values only, not which specialization produced them.

    "},{"location":"api/basic_json_document/save/#return-value","title":"Return value","text":"

    The image, as a std::vector<std::uint8_t>. Pass it, or a pointer to its data together with its size, to load to read the document back.

    "},{"location":"api/basic_json_document/save/#exception-safety","title":"Exception safety","text":"

    Strong guarantee: save() does not modify *this (it is const), so if it throws, the document is left exactly as it was, and the partially built image is discarded with the exception.

    "},{"location":"api/basic_json_document/save/#exceptions","title":"Exceptions","text":"

    Throws type_error.320 if the document is discarded -- a default-constructed document, or one a failed parse()/ read() with allow_exceptions == false left discarded.

    On a big-endian target, throws type_error.320 with a different message instead: the image format is little-endian only (see Notes).

    Throws out_of_range.416 if the node count, the text, or the decoded strings of the image would individually reach 4 GiB -- the same 32-bit offsets parse() and, for edits, set/push_back are already limited to.

    Example messages

    [json.exception.type_error.320] cannot save a discarded json_document\n
    [json.exception.type_error.320] json_document images need a little-endian target\n
    [json.exception.out_of_range.416] images of 4 GiB or more are not supported by json_document\n

    "},{"location":"api/basic_json_document/save/#complexity","title":"Complexity","text":"

    Linear in the size of the document: the number of nodes, plus the length of the text and the decoded strings that end up in the image.

    "},{"location":"api/basic_json_document/save/#notes","title":"Notes","text":"

    Format. The image begins with a 64-byte header (the magic bytes \"NJVI\", a version number, the node count, and the sizes of the text and the decoded strings, all little-endian), followed by the nodes (16 bytes each), the text and a '\\0', and the decoded strings and a '\\0'. load checks the header, and the sizes it describes, before reading anything else -- see load's Exceptions.

    Experimental

    The image format is versioned but not yet stable: it may change in an incompatible way before it is declared stable. Use images to cache a document within one build of the library, or to hand one to another process running the same build on the same (little-endian) machine -- not as a long-term storage format. Keep the original JSON text if you need to read a saved document back with a future library version.

    Little-endian only. The image is written as raw little-endian bytes, with no byte-swapping. save() (and load) throw type_error.320 on a big-endian target rather than silently produce bytes a big-endian reader could not interpret correctly.

    "},{"location":"api/basic_json_document/save/#examples","title":"Examples","text":"Caching a parsed document as an image

    The example below saves a parsed configuration as an image -- the way a service might cache one to answer later requests without parsing the text again -- and confirms that loading it back gives exactly the same result as parsing did, and that saving is deterministic.

    #include <cstdint>\n#include <iostream>\n#include <string>\n#include <vector>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // a configuration a service parses once and then caches as an image, so\n    // that later requests can load() it instead of parsing the text again\n    const std::string text = R\"({\"name\": \"cache\", \"host\": \"db1\", \"port\": 6379, \"replicas\": [\"db2\", \"db3\"]})\";\n    const json_document config = json_document::parse(text);\n\n    // save() turns the parsed document into a byte buffer: a 64-byte header,\n    // the node index, the source text, and the decoded strings\n    const std::vector<std::uint8_t> image = config.save();\n    std::cout << image.size() << '\\n';\n\n    // the same document always saves to the same bytes\n    std::cout << (image == json_document::parse(text).save()) << '\\n';\n\n    // loading the image back needs no parsing, yet dumps exactly what\n    // parsing the text produced\n    const json_document reloaded = json_document::load(image);\n    std::cout << (reloaded.root().dump() == config.root().dump()) << '\\n';\n}\n

    Output:

    316\ntrue\ntrue\n
    "},{"location":"api/basic_json_document/save/#see-also","title":"See also","text":"
    • load - read an image written by save()
    • owns_source - return whether the document holds its own copy of the text
    • basic_json_view::dump - serialize the document to JSON text instead of an image
    • Images - why and when to use images
    "},{"location":"api/basic_json_document/save/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_document/set/","title":"nlohmann::basic_json_document::set","text":"
    // (1)\ntemplate<typename V>\nview_type set(view_type target, V&& value);\n\n// (2)\ntemplate<typename V>\nview_type set(view_type object, string_view_t key, V&& value);\n\n// (3)\ntemplate<typename I, typename V>\nview_type set(view_type array, I idx, V&& value);\n\n// (4)\ntemplate<typename V>\nview_type set(const json_pointer& ptr, V&& value);\n

    Only an editable document (Editable == true, e.g. json_editable_document) has set; calling it on a read-only basic_json_document fails to compile (static_assert).

    1. Replaces the value target refers to with value.
    2. Sets the member key of the object object to value: assigns it if object already has a member with this key -- the first one, should the key occur more than once, and the later duplicates are then dropped (see the Notes below) -- or appends a new member at the end otherwise. A null object first becomes an empty object.
    3. Assigns value to the element at index idx of the array array, which must already exist (idx < array.size()).
    4. Sets the value the JSON pointer ptr refers to, relative to root(), to value. The parent of the target must already exist: an object member is set as in 2. (added if it does not exist yet), an array element is assigned as in 3., and a last reference token of \"-\", or equal to the size of the array, appends value instead, exactly as push_back would. A null parent becomes what basic_json::operator[] with a JSON pointer makes of it: an array if the last reference token is \"-\" or consists of digits only (for an index beyond 0, the array is first filled with null values up to that index), an object otherwise. An empty ptr sets root() itself, as in 1.

    In every overload, value is accepted three ways: a basic_json_view of any document -- read-only or editable, and it does not have to be target's/object's/array's own document -- which is copied so that nothing is shared with the source document afterward; a BasicJsonType value; or anything BasicJsonType can be constructed from (numbers, strings, bool, nullptr, containers, ...).

    "},{"location":"api/basic_json_document/set/#template-parameters","title":"Template parameters","text":"V the type of value, deduced; see above for what is accepted. I an integral type other than bool, deduced (overloads taking a bool or a non-integral type for idx do not participate in overload resolution)."},{"location":"api/basic_json_document/set/#parameters","title":"Parameters","text":"target (in) the value to replace object (in) the object (or null value) whose member to set array (in) the array whose element to assign key (in) the key of the member to set idx (in) the index of the element to assign; a negative value throws (see Exceptions) ptr (in) a JSON pointer to the value to set, relative to root() value (in) the new value"},{"location":"api/basic_json_document/set/#return-value","title":"Return value","text":"
    1. a view of target, now holding value
    2. a view of the member key of object, now holding value
    3. a view of the element idx of array, now holding value
    4. a view of the value ptr refers to, now holding value
    "},{"location":"api/basic_json_document/set/#exception-safety","title":"Exception safety","text":"

    Basic exception safety: value is fully encoded -- including the checks below -- into storage owned by the document before anything already reachable from root() is touched, so a failure while encoding value (an invalid argument, or std::bad_alloc) leaves the document completely unchanged, other than memory allocated for the encoding that is not reclaimed. A failure of a later allocation -- while an edited array or object switches from its parsed layout to a growable block, see Notes -- can still leave a partial effect, such as a null object/array argument already turned into an empty object/array even though value itself was not linked in. Likewise, a failure of value in 4. leaves a null parent that is set with an index beyond 0 already filled with the null values before the index.

    "},{"location":"api/basic_json_document/set/#exceptions","title":"Exceptions","text":"
    1. Throws type_error.302 if value is a discarded view, or a discarded BasicJsonType value (e.g. BasicJsonType(value_t::discarded)) -- an object or array value, of either kind, is fine and is encoded as a whole subtree.
    2. Throws type_error.305 if object is neither an object nor null -- the same message operator[] throws for a string argument on such a value. Throws type_error.316 if key is not valid UTF-8, with the same message BasicJsonType::dump() gives for that string. Also throws what 1. throws for value.
    3. Throws type_error.305 if array is not an array -- the same message operator[] throws for a numeric argument on such a value. Throws out_of_range.401 if idx is negative, or if idx >= array.size(). Also throws what 1. throws for value.
    4. Throws what at throws (overload 3) for resolving ptr's parent, except that a missing object member or an array index equal to the array's size at the very last reference token is not an error there (it becomes a new member or an appended element) instead of out_of_range.403/out_of_range.402. For the last reference token itself: if the parent is an object (or a primitive value, where it throws type_error.305), throws what 2. throws; if the parent is an array, throws what 3. throws for an index that is out of range, or, for a token that is not a valid array index, parse_error.106 (a leading '0'), parse_error.109 (not a number), out_of_range.410 (too large for size_type), or out_of_range.404 (an empty token); the same errors are thrown for a null parent and a token of digits (the parent is not changed then), and out_of_range.401 if the index is 4294967295 or more. Also throws what 1. throws for value.

    Every overload also throws type_error.319 if value is (or contains) a binary value -- BasicJsonType can hold one, but a json_document cannot -- and invalid_iterator.202 (\"view does not belong to this document\") if target/object/array is a discarded view or a view of a different document (overloads 1-3 only; overload 4 always starts from this document's own root()).

    "},{"location":"api/basic_json_document/set/#complexity","title":"Complexity","text":"
    1. Linear in the size of value (encoding it into the document's storage): constant for a scalar, linear in the number of nested values for an array or object. If target is itself an array or object that spans more than one node in its parent's original, unedited layout, and value is a scalar, replacing it additionally costs time linear in the size of the document, the first time (the parent of target is looked up from root(), and then switches to links) -- see Notes. Once the parent has links, target is replaced in constant time: setting every element of a large array one after the other is linear overall. To avoid the lookup altogether, use 3. (or 2. for an object), which know the parent.
    2. Linear in the number of members of object, to find an existing member with key, plus the complexity of 1. for value.
    3. Constant, plus the complexity of 1. for value.
    4. Linear in the number of reference tokens of ptr and, for each token, in the number of members of the object at that level or the index into the array (as at), plus the complexity of 2. or 3. for the last token; for a null parent and an index, linear in the index.
    "},{"location":"api/basic_json_document/set/#notes","title":"Notes","text":"

    Duplicate keys

    If object already has more than one member with key (2.), the first one is assigned value and every later member with the same key is removed -- so that a lookup, an iteration, and materialize() of object afterward all agree on a single value for key, the same way operator[] already picks the first occurrence of a duplicate key for reading. See the Notes on duplicate keys of operator[].

    Setting a member (2.) or an element (3., through 4.) of an array or object whose elements have not been edited before switches it from its parsed layout to a growable block holding links to its elements; a later push_back or set on the same container reuses that block, growing it (amortized constant time) only once it runs out of room. This never moves an element itself -- only where the container's links to its elements live -- so a view of an element stays valid, but any iterator already taken over the container is invalidated, since it was walking the old layout. See Edits for what stays valid across an edit in general.

    The same switch happens, for the same reason, when overload 1. replaces a multi-node array/object value with a scalar: the parent's element sequence is what has to switch to links, not target itself, because the parent originally stepped over target's whole subtree by its node count, which no longer applies once target is a one-node scalar.

    "},{"location":"api/basic_json_document/set/#examples","title":"Examples","text":"Example: (1)/(2)/(3)/(4) replace a value, set a member, assign an element, set via a JSON pointer

    The example below edits a small configuration document -- replacing a value, adding an object member, assigning an array element, and reaching a field through a JSON pointer -- and shows what dump() preserves that is lost once the same edits are made on a BasicJsonType value instead: the order object members were written in, and the exact spelling of a number that was never touched.

    #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json = nlohmann::json;\nusing json_editable_document = nlohmann::json_editable_document;\nusing json_editable_view = nlohmann::json_editable_view;\n\nint main()\n{\n    // a configuration file, as it might be read from disk -- \"price\" is\n    // written with a trailing zero that has no effect on its value\n    const std::string text = R\"({\n  \"name\": \"cache\",\n  \"host\": \"db1\",\n  \"port\": 6379,\n  \"price\": 19.90,\n  \"replicas\": [\"db2\", \"db3\"],\n  \"timeout\": 30\n})\";\n\n    json_editable_document doc = json_editable_document::parse(text);\n\n    doc.set(doc.root()[\"port\"], 6380);                    // (1) replace a value\n    doc.set(doc.root(), \"region\", \"us-east\");              // (2) add a member\n    doc.set(doc.root()[\"replicas\"], 0, \"db4\");             // (3) assign an element\n    doc.set(json::json_pointer(\"/timeout\"), 45);           // (4) via a JSON pointer\n\n    // members stay in document order (the new one at the end), and a number\n    // that was not itself edited keeps its exact spelling\n    std::cout << doc.root().dump(2, ' ', false, json_editable_view::number_format::source) << \"\\n\\n\";\n\n    // the same edits on a plain json value: object_t is a std::map, so\n    // parsing already sorted the keys, and dump() rewrites every number to\n    // its shortest form, even \"price\", which was never touched\n    json plain = json::parse(text);\n    plain[\"port\"] = 6380;\n    plain[\"region\"] = \"us-east\";\n    plain[\"replicas\"][0] = \"db4\";\n    plain[json::json_pointer(\"/timeout\")] = 45;\n    std::cout << plain.dump(2) << '\\n';\n}\n

    Output:

    {\n  \"name\": \"cache\",\n  \"host\": \"db1\",\n  \"port\": 6380,\n  \"price\": 19.90,\n  \"replicas\": [\n    \"db4\",\n    \"db3\"\n  ],\n  \"timeout\": 45,\n  \"region\": \"us-east\"\n}\n\n{\n  \"host\": \"db1\",\n  \"name\": \"cache\",\n  \"port\": 6380,\n  \"price\": 19.9,\n  \"region\": \"us-east\",\n  \"replicas\": [\n    \"db4\",\n    \"db3\"\n  ],\n  \"timeout\": 45\n}\n
    "},{"location":"api/basic_json_document/set/#see-also","title":"See also","text":"
    • push_back - append to an array
    • insert - insert an element into an array
    • erase - remove an object member, an array element, or the value a JSON pointer refers to
    • root - the view of the root value, the starting point of overload 4
    • basic_json_view::dump - serialize the document, keeping an untouched number's spelling with number_format::source
    • Edits - what an edit guarantees, for every overload
    • Editing a document - why editable documents keep the source text's order and number spelling
    "},{"location":"api/basic_json_document/set/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_document/shrink_to_fit/","title":"nlohmann::basic_json_document::shrink_to_fit","text":"
    void shrink_to_fit();\n

    Releases capacity that is no longer needed, both of the node index and of the buffer for decoded strings (strings that contained escape sequences), e.g. after read() replaced a large document with a much smaller one. Like std::vector::shrink_to_fit(), this is a non-binding request: the library may keep more capacity than strictly necessary.

    "},{"location":"api/basic_json_document/shrink_to_fit/#exception-safety","title":"Exception safety","text":"

    Strong guarantee: if an exception is thrown, there are no changes to the document.

    "},{"location":"api/basic_json_document/shrink_to_fit/#exceptions","title":"Exceptions","text":"

    May throw std::bad_alloc if the reallocation fails; on exception, the document is unchanged.

    "},{"location":"api/basic_json_document/shrink_to_fit/#complexity","title":"Complexity","text":"

    Linear in node_count() plus the length of the decoded strings.

    "},{"location":"api/basic_json_document/shrink_to_fit/#notes","title":"Notes","text":"

    Invalidates views

    Unlike moving the document, shrink_to_fit() invalidates every view taken from this document before the call, including a previously obtained root(): the node index is moved into a new, smaller allocation, and the old one is freed. Take a fresh view from root() after calling this function.

    This is unlike std::vector::shrink_to_fit(), which promises nothing about validity but in practice often leaves iterators alone when it did not need to reallocate; here, an implementation that avoids a reallocation when possible would be an internal optimization only, not a guarantee to rely on.

    "},{"location":"api/basic_json_document/shrink_to_fit/#examples","title":"Examples","text":"Example 
    #include <iostream>\n#include <sstream>\n#include <string>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // build a large array (many nodes), then read a small one into the same\n    // document: the index grown for the large input is still allocated\n    std::ostringstream big;\n    big << '[';\n    for (int i = 0; i < 500; ++i)\n    {\n        if (i != 0)\n        {\n            big << ',';\n        }\n        big << i;\n    }\n    big << ']';\n\n    json_document doc;\n    doc.read(big.str());\n    const std::size_t big_nodes = doc.node_count();\n\n    doc.read(std::string(\"[1]\"));\n    std::cout << (doc.node_count() < big_nodes) << '\\n'; // far fewer live nodes now\n    const std::size_t before = doc.memory_usage();\n\n    // shrink_to_fit() moves the index into a block sized for what is actually\n    // used. This INVALIDATES every view taken from this document before the\n    // call (they point into the old, now-freed block) -- take fresh ones from\n    // root() afterward.\n    doc.shrink_to_fit();\n    const std::size_t after = doc.memory_usage();\n    std::cout << (after <= before) << '\\n';\n\n    // a freshly taken view is valid and correct\n    std::cout << doc.root().is_array() << ' ' << doc.root().size() << '\\n';\n}\n

    Output:

    true\ntrue\ntrue 1\n
    "},{"location":"api/basic_json_document/shrink_to_fit/#see-also","title":"See also","text":"
    • node_count - the number of index entries
    • memory_usage - the number of bytes held by the document
    • root - the view of the root value
    "},{"location":"api/basic_json_document/shrink_to_fit/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_document/source/","title":"nlohmann::basic_json_document::source","text":"
    view_type::string_view_t source() const noexcept;\n

    Returns the parsed text, whether it is borrowed from the caller or owned by the document.

    "},{"location":"api/basic_json_document/source/#return-value","title":"Return value","text":"

    A string_view_t (std::string_view on C++17 and newer) over the parsed text, or an empty one if the document is discarded.

    "},{"location":"api/basic_json_document/source/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this function never throws exceptions.

    "},{"location":"api/basic_json_document/source/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json_document/source/#notes","title":"Notes","text":"

    For a borrowed document, source() points directly into the caller's buffer, so it is only valid while that buffer is; see owns_source.

    "},{"location":"api/basic_json_document/source/#examples","title":"Examples","text":"Example 
    #include <iostream>\n#include <string>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    std::string text = R\"({\"a\": 1})\";\n    json_document doc = json_document::parse(text);\n\n    // source() is the parsed text, whether borrowed or owned\n    std::cout << (doc.source().size() == text.size()) << '\\n';\n    std::cout << std::string(doc.source().data(), doc.source().size()) << '\\n';\n\n    // for a borrowed document, source() points right into the caller's buffer\n    std::cout << (doc.source().data() == text.data()) << '\\n';\n}\n

    Output:

    true\n{\"a\": 1}\ntrue\n
    "},{"location":"api/basic_json_document/source/#see-also","title":"See also","text":"
    • owns_source - return whether the document holds its own copy of the text
    • source_offset - byte offset of a value in the source text
    "},{"location":"api/basic_json_document/source/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_view/","title":"nlohmann::basic_json_view","text":"

    Defined in header <nlohmann/json_view.hpp>

    template<typename BasicJsonType, bool Editable = false>\nclass basic_json_view;\n

    A read-only handle to one value of a basic_json_document: two pointers (a pointer to the document and a pointer into its index), trivially copyable. A view is valid as long as

    • the document is alive,
    • the document has not been re-parsed with read() (or parse() into it) or shrunk with shrink_to_fit() since the view was taken, and
    • if the document borrows its source text, that text is still alive and unmodified (floating-point values and unescaped strings are read from it lazily).

    Moving the document itself does not invalidate its views: the index is heap-allocated independently of the basic_json_document object.

    basic_json_view provides the read-only part of the BasicJsonType interface:

    • the type-inspection functions
    • element access
    • lookup
    • iteration
    • conversion: get<T>(), get_string(), number_token(), and materialize() to build the BasicJsonType value of a subtree on demand
    • comparison: operator== and operator!= compare two views, or a view and a BasicJsonType value, without ever building a BasicJsonType value for a view; no ordering comparison (operator<) is provided

    operator[], at, contains, and value also accept a json_pointer. Unlike basic_json, there are no implicit conversion operators (no operator ValueType()), and no get_ptr() or get_ref(); see get.

    basic_json_view itself is always read-only -- it never has a set or push_back of its own. A view of an editable document (Editable == true) sees every edit made through basic_json_document::set and basic_json_document::push_back: once a value is changed, every view that still refers to it -- including ones taken before the change -- reads the new value. See Edits.

    "},{"location":"api/basic_json_view/#template-parameters","title":"Template parameters","text":"BasicJsonType a specialization of basic_json, matching the basic_json_document the view was taken from. Editable whether the view is of an editable document, matching the basic_json_document it was taken from (optional, false by default). See Edits."},{"location":"api/basic_json_view/#specializations","title":"Specializations","text":"
    • json_view - views of a json_document
    • ordered_json_view - views of an ordered_json_document
    • json_editable_view - views of a json_editable_document
    • ordered_json_editable_view - views of an ordered_json_editable_document
    "},{"location":"api/basic_json_view/#member-types","title":"Member types","text":"
    • value_t - the JSON type enumeration, see basic_json::value_t
    • string_t, number_integer_t, number_unsigned_t, number_float_t, json_pointer - the corresponding member types of BasicJsonType
    • size_type - std::size_t
    • string_view_t - std::string_view on C++17 and newer, a minimal internal substitute otherwise
    • iterator, const_iterator - a forward iterator over the elements of an array or the member values of an object, in document order; both names refer to the same type, since a view is always read-only
    • number_format - how dump() writes numbers
    "},{"location":"api/basic_json_view/#member-functions","title":"Member functions","text":"
    • (constructor)
    "},{"location":"api/basic_json_view/#object-inspection","title":"Object inspection","text":"
    • type - return the type of the value
    • type_name - return the type as string
    • is_null - return whether the value is null
    • is_boolean - return whether the value is a boolean
    • is_number - return whether the value is a number
    • is_number_integer - return whether the value is an integer number
    • is_number_unsigned - return whether the value is an unsigned integer number
    • is_number_float - return whether the value is a floating-point number
    • is_string - return whether the value is a string
    • is_array - return whether the value is an array
    • is_object - return whether the value is an object
    • is_binary - return whether the value is a binary array (always false)
    • is_primitive - return whether the type is primitive
    • is_structured - return whether the type is structured
    • is_discarded - return whether the view is invalid
    "},{"location":"api/basic_json_view/#element-access","title":"Element access","text":"
    • at - access specified element with bounds checking
    • operator[] - access specified element
    • value - access specified element with default value
    • front - access the first element
    • back - access the last element
    "},{"location":"api/basic_json_view/#lookup","title":"Lookup","text":"
    • find - find an element in an object
    • count - returns the number of occurrences of a key in an object
    • contains - check the existence of an element in an object
    "},{"location":"api/basic_json_view/#iterators","title":"Iterators","text":"
    • begin - returns an iterator to the first element
    • cbegin - returns a const iterator to the first element
    • end - returns an iterator to one past the last element
    • cend - returns a const iterator to one past the last element
    • items - wrapper to access iterator member functions in range-based for
    "},{"location":"api/basic_json_view/#capacity","title":"Capacity","text":"
    • size - return the number of elements
    • empty - return whether the value has no elements
    "},{"location":"api/basic_json_view/#conversion","title":"Conversion","text":"
    • get - get a value
    • get_to - get a value and write it to a destination
    • get_string - get a string value without a copy
    • number_token - get a number's token text without a copy
    • materialize - build the BasicJsonType value of this subtree
    "},{"location":"api/basic_json_view/#comparison","title":"Comparison","text":"
    • operator== - comparison: equal
    • operator!= - comparison: not equal
    "},{"location":"api/basic_json_view/#serialization","title":"Serialization","text":"
    • dump - serialize to a JSON-formatted string
    • operator<< - serialize to stream
    "},{"location":"api/basic_json_view/#source-access","title":"Source access","text":"
    • source_offset - byte offset of this value in the document's source text
    "},{"location":"api/basic_json_view/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_view/at/","title":"nlohmann::basic_json_view::at","text":"
    // (1)\nbasic_json_view at(string_view_t key) const;\nbasic_json_view at(const char* key) const;\nbasic_json_view at(const string_t& key) const;\n\n// (2)\nbasic_json_view at(size_type idx) const;\ntemplate<typename IntegerType>\nbasic_json_view at(IntegerType idx) const;\n\n// (3)\nbasic_json_view at(const json_pointer& ptr) const;\n
    1. Returns the value of the object member with key key -- the first one, should the key occur more than once (see Notes on duplicate keys).
    2. Returns the array element at index idx. The template accepts every integer type except bool and std::size_t and forwards to the size_type overload, as for operator[]; a negative idx is out of range.
    3. Returns the value a JSON pointer ptr refers to, starting at this value.
    "},{"location":"api/basic_json_view/at/#parameters","title":"Parameters","text":"key (in) object key of the element to access idx (in) index of the element to access ptr (in) JSON pointer to the element to access"},{"location":"api/basic_json_view/at/#return-value","title":"Return value","text":"
    1. the value of the first member with key key
    2. the element at index idx
    3. the value ptr resolves to, starting at this value
    "},{"location":"api/basic_json_view/at/#exception-safety","title":"Exception safety","text":"

    Strong guarantee: there are no changes to the view or the document it refers to.

    "},{"location":"api/basic_json_view/at/#exceptions","title":"Exceptions","text":"
    1. The function can throw the following exceptions, both with the same message as the corresponding call to BasicJsonType::at:
      • Throws type_error.304 if the value is not an object.
      • Throws out_of_range.403 if no member has key key.
    2. The function can throw the following exceptions, both with the same message as the corresponding call to BasicJsonType::at:
      • Throws type_error.304 if the value is not an array.
      • Throws out_of_range.401 if idx >= size().
    3. The function can throw the following exceptions, all with the same message as the corresponding call to BasicJsonType::at:
      • Throws parse_error.106 if an array index in ptr begins with '0'.
      • Throws parse_error.109 if an array index in ptr is not a number.
      • Throws out_of_range.401 if an array index in ptr is out of range.
      • Throws out_of_range.402 if a reference token is \"-\" at an array -- at never inserts an element, so \"-\" is always invalid.
      • Throws out_of_range.403 if a reference token names an object member that does not exist.
      • Throws out_of_range.404 if ptr cannot be resolved because a reference token is used on a primitive value.

    None of these exceptions carry a JSON_DIAGNOSTICS path: the view has no BasicJsonType value to point at, so the exception is created without one, even if BasicJsonType was built with JSON_DIAGNOSTICS enabled.

    "},{"location":"api/basic_json_view/at/#complexity","title":"Complexity","text":"
    1. Linear in the number of members: as for ordered_json, members are compared one after another, in document order, stopping at the first match. Each comparison first checks the key's length -- already known from the index, without reading the key bytes -- before comparing its content. Objects with 128 or more members get a hash index of their keys while parsing, so that a lookup in them takes constant time on average.
    2. Linear in idx: elements are skipped one at a time from the first one, since they are not a fixed size in the index (unlike BasicJsonType's array, which is random-access).
    3. Linear in the number of reference tokens of ptr and, for each token, in the number of members of the object at that level (as 1.) or the index into the array (as 2.).
    "},{"location":"api/basic_json_view/at/#notes","title":"Notes","text":"

    Duplicate keys: the first member wins

    If the source text repeats a key, this resolves to the first member with it, not to the last one that materialize() and parse() keep. See the Notes on duplicate keys of operator[] and Duplicate keys for the reasons and for how to get the last value.

    Unlike operator[], which returns a discarded view for a missing key or an out-of-range index, at always throws -- exactly as BasicJsonType::at does, and with the same messages, so existing error handling written against BasicJsonType::at keeps working unchanged when switched to a view. This also holds for overload 3: unlike operator[] with a JSON pointer, which returns a discarded view for a missing key or an out-of-range index, at throws for those too (out_of_range.403/out_of_range.401).

    "},{"location":"api/basic_json_view/at/#examples","title":"Examples","text":"Example: (1)/(2) access specified element with bounds checking

    The example below reads required fields out of a service configuration with at, and shows that the exceptions it throws -- for a wrong type and for a missing key -- carry the same messages BasicJsonType::at would produce for the same JSON text.

    #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\nusing json_view = nlohmann::json_view;\n\nint main()\n{\n    // required fields of a service configuration -- at() reports a missing\n    // or wrong-typed field with the very same exception basic_json::at()\n    // would throw for the equivalent nlohmann::json value, so error\n    // handling written against basic_json::at() keeps working unchanged\n    json_document config = json_document::parse(R\"({\"name\": \"cache\", \"port\": \"6379\"})\");\n    const json_view service = config.root();\n\n    std::cout << service.at(\"port\").materialize().dump() << '\\n';\n\n    try\n    {\n        // \"port\" is a string, not an array\n        static_cast<void>(service.at(\"port\").at(0));\n    }\n    catch (const nlohmann::json::type_error& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n\n    try\n    {\n        static_cast<void>(service.at(\"timeout\"));\n    }\n    catch (const nlohmann::json::out_of_range& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

    Output:

    \"6379\"\n[json.exception.type_error.304] cannot use at() with string\n[json.exception.out_of_range.403] key 'timeout' not found\n
    Example: (3) access specified element via JSON pointer with bounds checking

    The example below shows that at with a JSON pointer throws exactly the exceptions, with exactly the messages, that BasicJsonType::at throws for the same pointer and the same document.

    #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\nusing json_pointer = nlohmann::json::json_pointer;\n\nint main()\n{\n    json_document doc = json_document::parse(R\"({\"region\": \"eu\", \"servers\": [\"eu-1\", \"eu-2\"]})\");\n    const auto root = doc.root();\n\n    std::cout << root.at(json_pointer(\"/servers/1\")).materialize().dump() << '\\n';\n\n    // at() throws for every resolution failure -- with the very same\n    // message json::at(ptr) would throw for the same pointer and the same\n    // document\n    try\n    {\n        static_cast<void>(root.at(json_pointer(\"/servers/5\")));\n    }\n    catch (const nlohmann::json::out_of_range& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n\n    try\n    {\n        static_cast<void>(root.at(json_pointer(\"/missing\")));\n    }\n    catch (const nlohmann::json::out_of_range& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

    Output:

    \"eu-2\"\n[json.exception.out_of_range.401] array index 5 is out of range\n[json.exception.out_of_range.403] key 'missing' not found\n
    "},{"location":"api/basic_json_view/at/#see-also","title":"See also","text":"
    • operator[] - access specified element (returns a discarded view instead of throwing)
    • front, back - access the first or last element
    • BasicJsonType::at - the corresponding function of basic_json
    • json_pointer - JSON pointer type used by overload 3
    "},{"location":"api/basic_json_view/at/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_view/back/","title":"nlohmann::basic_json_view::back","text":"
    basic_json_view back() const;\n

    Returns the last element of an array, the last member value of an object, or the value itself if it is primitive (as for BasicJsonType::back(), a primitive value is a range of one element).

    "},{"location":"api/basic_json_view/back/#return-value","title":"Return value","text":"

    The last element or member value. For a primitive value (number, string, boolean), the value itself.

    "},{"location":"api/basic_json_view/back/#exception-safety","title":"Exception safety","text":"

    Strong guarantee: there are no changes to the view or the document it refers to.

    "},{"location":"api/basic_json_view/back/#exceptions","title":"Exceptions","text":"

    Throws invalid_iterator.214 if the view is null or discarded, or if it is an empty array or object.

    This exception does not carry a JSON_DIAGNOSTICS path: the view has no BasicJsonType value to point at, so the exception is created without one, even if BasicJsonType was built with JSON_DIAGNOSTICS enabled.

    "},{"location":"api/basic_json_view/back/#complexity","title":"Complexity","text":"

    Linear in the size of the array or object: unlike front(), which only ever looks at the first element, back() has to walk every element to find where they end, since elements are not a fixed size in the index.

    "},{"location":"api/basic_json_view/back/#notes","title":"Notes","text":"

    Unlike BasicJsonType::back(), which has undefined behavior for an empty array or object, back() throws invalid_iterator.214 for an empty array, an empty object, a null value, or a discarded view.

    "},{"location":"api/basic_json_view/back/#examples","title":"Examples","text":"Example

    The example below reads only the final status of a build log with back(). Even though back() is linear in the number of events (unlike front(), which is constant), it still avoids building a BasicJsonType value for the events that are not needed.

    #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    // the same build log; back() reads only the final status. It is linear\n    // in the number of events (unlike front(), which is constant), but\n    // still far less work than materializing the whole array\n    json_document log = json_document::parse(R\"([\"queued\", \"started\", \"compiling\", \"linking\", \"done\"])\");\n    std::cout << log.root().back().materialize().dump() << '\\n';\n\n    // an empty log -- back() throws instead of the undefined behavior\n    // basic_json::back() has for an empty array\n    json_document empty_log = json_document::parse(\"[]\");\n    try\n    {\n        static_cast<void>(empty_log.root().back());\n    }\n    catch (const nlohmann::json::invalid_iterator& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

    Output:

    \"done\"\n[json.exception.invalid_iterator.214] cannot get value\n
    "},{"location":"api/basic_json_view/back/#see-also","title":"See also","text":"
    • front - access the first element
    • BasicJsonType::back - the corresponding function of basic_json
    "},{"location":"api/basic_json_view/back/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_view/basic_json_view/","title":"nlohmann::basic_json_view::basic_json_view","text":"
    basic_json_view() noexcept = default;\n

    Creates an invalid (discarded) view: type() is value_t::discarded and is_discarded() is true.

    This is the only constructor a caller can use directly. Every other view is obtained from a basic_json_document, via root() or by navigating into a container with operator[], at, front, back, find, or iteration.

    "},{"location":"api/basic_json_view/basic_json_view/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this constructor never throws exceptions.

    "},{"location":"api/basic_json_view/basic_json_view/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json_view/basic_json_view/#notes","title":"Notes","text":"

    basic_json_view is trivially copyable (it holds two pointers), so a default-constructed view can be used as a placeholder for \"no value yet\" and later be assigned a real view.

    "},{"location":"api/basic_json_view/basic_json_view/#examples","title":"Examples","text":"Example

    The example below shows the default constructor and that a basic_json_view is a small, copyable handle.

    #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // the default constructor is the only public one: it creates an invalid\n    // (discarded) view, useful as a \"no value yet\" placeholder\n    nlohmann::json_view v;\n    std::cout << v.is_discarded() << '\\n';\n\n    // views are trivially copyable handles (two pointers); the document owns\n    // the actual data\n    nlohmann::json_view copy = v;\n    std::cout << copy.is_discarded() << '\\n';\n}\n

    Output:

    true\ntrue\n
    "},{"location":"api/basic_json_view/basic_json_view/#see-also","title":"See also","text":"
    • is_discarded - return whether the view is invalid
    • root - the view of a document's root value
    "},{"location":"api/basic_json_view/basic_json_view/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_view/begin/","title":"nlohmann::basic_json_view::begin","text":"
    iterator begin() const noexcept;\n

    Returns an iterator to the first element of an array, or the first member value of an object, in document order -- the order the values appear in the source text, not sorted by key. A primitive value iterates as a range of one element (itself), and a null value or a discarded view iterates as an empty range.

    "},{"location":"api/basic_json_view/begin/#return-value","title":"Return value","text":"

    Iterator to the first element.

    "},{"location":"api/basic_json_view/begin/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this function never throws exceptions.

    "},{"location":"api/basic_json_view/begin/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json_view/begin/#notes","title":"Notes","text":"

    For an object, iteration visits every member, including all occurrences of a duplicate key -- unlike operator[], at, find, and contains, which all resolve to the first member with a given key. See the Notes on duplicate keys of operator[].

    Because objects are iterated in document order rather than sorted by key, the order seen here can differ from what iterating the materialize()d BasicJsonType value would produce: a basic_json object (std::map-backed by default) sorts its keys, while a view does not.

    An ordered_json value also keeps document order, but it keeps only one entry per key: as parse() does, it keeps the last value, at the position of the key's first occurrence. The view, in contrast, visits every member, including the duplicates.

    "},{"location":"api/basic_json_view/begin/#examples","title":"Examples","text":"Example

    The example below iterates a log record's members with begin()/end() and prints them in the order they were written. Materializing the record into a BasicJsonType object and iterating that would instead print the members sorted by key.

    #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    // a log record: the fields matter in the order they were written, e.g.\n    // to reproduce the record as it was logged. A nlohmann::json object\n    // sorts its keys, so materializing and iterating it would instead print\n    // them alphabetically (\"level\", \"message\", \"time\")\n    json_document record = json_document::parse(R\"({\"time\": \"10:00:01\", \"level\": \"info\", \"message\": \"started\"})\");\n\n    for (auto it = record.root().begin(); it != record.root().end(); ++it)\n    {\n        std::cout << it.key() << '=' << it->materialize().dump() << '\\n';\n    }\n}\n

    Output:

    time=\"10:00:01\"\nlevel=\"info\"\nmessage=\"started\"\n
    "},{"location":"api/basic_json_view/begin/#see-also","title":"See also","text":"
    • end - returns an iterator to one past the last element
    • cbegin - returns a const iterator to the first element
    • items - access iterator member functions in range-based for
    • BasicJsonType::begin - the corresponding function of basic_json
    "},{"location":"api/basic_json_view/begin/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_view/cbegin/","title":"nlohmann::basic_json_view::cbegin","text":"
    iterator cbegin() const noexcept;\n

    Returns an iterator to the first element, in document order. Equivalent to begin(): a view is always read-only, so const_iterator and iterator are the same type, and cbegin() exists only so that generic code that expects a cbegin()/cend() pair works with a view too.

    "},{"location":"api/basic_json_view/cbegin/#return-value","title":"Return value","text":"

    Iterator to the first element; identical to what begin() returns.

    "},{"location":"api/basic_json_view/cbegin/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this function never throws exceptions.

    "},{"location":"api/basic_json_view/cbegin/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json_view/cbegin/#examples","title":"Examples","text":"Example

    The example below sums many measurements with std::accumulate, using cbegin()/cend() as the range -- the same way it would for any standard container -- without ever materializing the whole array into a BasicJsonType value.

    #include <iostream>\n#include <numeric>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\nusing json_view = nlohmann::json_view;\n\nint main()\n{\n    // sum many measurements with std::accumulate; cbegin()/cend() (identical\n    // to begin()/end() here -- the view is always read-only) let the view be\n    // used with standard algorithms without ever materializing the whole\n    // array into a nlohmann::json value\n    json_document measurements = json_document::parse(\"[3, 1, 4, 1, 5, 9, 2, 6]\");\n    const auto values = measurements.root();\n\n    const int sum = std::accumulate(values.cbegin(), values.cend(), 0,\n                                    [](int total, const json_view & v)\n    {\n        return total + v.materialize().get<int>();\n    });\n\n    std::cout << sum << '\\n';\n}\n

    Output:

    31\n
    "},{"location":"api/basic_json_view/cbegin/#see-also","title":"See also","text":"
    • begin - returns an iterator to the first element
    • cend - returns a const iterator to one past the last element
    • BasicJsonType::cbegin - the corresponding function of basic_json
    "},{"location":"api/basic_json_view/cbegin/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_view/cend/","title":"nlohmann::basic_json_view::cend","text":"
    iterator cend() const noexcept;\n

    Returns an iterator to one past the last element, in document order. Equivalent to end(): a view is always read-only, so const_iterator and iterator are the same type, and cend() exists only so that generic code that expects a cbegin()/cend() pair works with a view too.

    "},{"location":"api/basic_json_view/cend/#return-value","title":"Return value","text":"

    Iterator one past the last element; identical to what end() returns.

    "},{"location":"api/basic_json_view/cend/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this function never throws exceptions.

    "},{"location":"api/basic_json_view/cend/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json_view/cend/#examples","title":"Examples","text":"Example

    The example below checks that every record of a batch is an object with std::all_of, using cbegin()/cend() as the range -- the same way it would for any standard container -- before materializing any record of the batch.

    #include <algorithm>\n#include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\nusing json_view = nlohmann::json_view;\n\nint main()\n{\n    // check that every element of a (possibly large) batch is an object,\n    // before materializing any of them -- cbegin()/cend() (identical to\n    // begin()/end() here) work as the range for std::all_of like they would\n    // for any standard container\n    json_document batch = json_document::parse(R\"([{\"id\": 1}, {\"id\": 2}, {\"id\": 3}])\");\n    const auto records = batch.root();\n\n    const bool all_objects = std::all_of(records.cbegin(), records.cend(),\n                                         [](const json_view & v)\n    {\n        return v.is_object();\n    });\n\n    std::cout << std::boolalpha << all_objects << '\\n';\n}\n

    Output:

    true\n
    "},{"location":"api/basic_json_view/cend/#see-also","title":"See also","text":"
    • end - returns an iterator to one past the last element
    • cbegin - returns a const iterator to the first element
    • BasicJsonType::cend - the corresponding function of basic_json
    "},{"location":"api/basic_json_view/cend/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_view/contains/","title":"nlohmann::basic_json_view::contains","text":"
    // (1)\nbool contains(string_view_t key) const;\nbool contains(const char* key) const;\nbool contains(const string_t& key) const;\n\n// (2)\nbool contains(const json_pointer& ptr) const;\n
    1. Checks whether the value is an object with a member with key key.
    2. Checks whether a JSON pointer ptr can be resolved, starting at this value.
    "},{"location":"api/basic_json_view/contains/#parameters","title":"Parameters","text":"key (in) key value to check its existence ptr (in) JSON pointer to check its existence"},{"location":"api/basic_json_view/contains/#return-value","title":"Return value","text":"
    1. true if the value is an object and has a member with key key, false otherwise
    2. true if ptr can be resolved to a value starting at this view, false otherwise
    "},{"location":"api/basic_json_view/contains/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this function never throws exceptions.

    "},{"location":"api/basic_json_view/contains/#complexity","title":"Complexity","text":"
    1. Linear in the number of members: as for ordered_json, members are compared one after another, in document order, stopping at the first match. Each comparison first checks the key's length -- already known from the index, without reading the key bytes -- before comparing its content. Objects with 128 or more members get a hash index of their keys while parsing, so that a lookup in them takes constant time on average.
    2. Linear in the number of reference tokens of ptr and, for each token, in the number of members of the object at that level or the index into the array -- as for operator[] and at with a JSON pointer.
    "},{"location":"api/basic_json_view/contains/#notes","title":"Notes","text":"

    Duplicate keys: the first member wins

    If the source text repeats a key, this resolves to the first member with it, not to the last one that materialize() and parse() keep. See the Notes on duplicate keys of operator[] and Duplicate keys for the reasons and for how to get the last value.

    Overload 1 always returns false when the value is not an object -- including a discarded view.

    Postconditions

    If v.contains(key) returns true, then v[key] is not discarded and v.at(key) does not throw. If v.contains(ptr) returns true, then v[ptr] is not discarded and v.at(ptr) does not throw.

    Overload 2 never throws

    Unlike BasicJsonType::contains(const json_pointer&), which can throw for certain malformed pointers (for instance an empty array-index reference token), overload 2 never throws: a missing key, an out-of-range or malformed array index, a \"-\" index, or a reference token used on a primitive all simply make it return false.

    "},{"location":"api/basic_json_view/contains/#examples","title":"Examples","text":"Example: (1) check with key

    The example below counts how many of a batch of records carry an optional retry_of field, using contains() to check without ever materializing a single record of the batch.

    #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    // count how many of many incoming records carry an optional \"retry_of\"\n    // field -- contains() only walks the flat index, so scanning a large\n    // batch like this never builds a single nlohmann::json value\n    json_document batch = json_document::parse(R\"(\n      [\n        {\"id\": 1},\n        {\"id\": 2, \"retry_of\": 1},\n        {\"id\": 3},\n        {\"id\": 4, \"retry_of\": 3}\n      ]\n    )\");\n\n    const auto records = batch.root();\n    std::size_t retries = 0;\n    for (std::size_t i = 0; i < records.size(); ++i)\n    {\n        if (records[i].contains(\"retry_of\"))\n        {\n            ++retries;\n        }\n    }\n    std::cout << retries << \" of \" << records.size() << \" records are retries\\n\";\n}\n

    Output:

    2 of 4 records are retries\n
    Example: (2) check with JSON pointer

    The example below checks an optional, nested field with a JSON pointer, and shows two pointers that contains() resolves to false without throwing.

    #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\nusing json_pointer = nlohmann::json::json_pointer;\n\nint main()\n{\n    // \"retry_of\" is only present on some records, nested a level down\n    // inside \"meta\"\n    json_document batch = json_document::parse(R\"(\n      [\n        {\"id\": 1, \"meta\": {}},\n        {\"id\": 2, \"meta\": {\"retry_of\": 1}}\n      ]\n    )\");\n\n    const auto records = batch.root();\n    const json_pointer retry_of(\"/meta/retry_of\");\n    for (std::size_t i = 0; i < records.size(); ++i)\n    {\n        const auto record = records[i];\n        if (record.contains(retry_of))\n        {\n            std::cout << \"record \" << i << \" is a retry of \" << record[retry_of].get<int>() << '\\n';\n        }\n        else\n        {\n            std::cout << \"record \" << i << \" is original\\n\";\n        }\n    }\n\n    // contains() with a JSON pointer never throws -- not even for a\n    // pointer that indexes into a primitive (\"/0/id/x\") or uses a\n    // malformed array index (\"/01\"), either of which would need a\n    // try/catch with json::contains(ptr)\n    std::cout << std::boolalpha << records.contains(json_pointer(\"/0/id/x\")) << '\\n';\n    std::cout << std::boolalpha << records.contains(json_pointer(\"/01\")) << '\\n';\n}\n

    Output:

    record 0 is original\nrecord 1 is a retry of 1\nfalse\nfalse\n
    "},{"location":"api/basic_json_view/contains/#see-also","title":"See also","text":"
    • find - find a value in an object
    • count - returns the number of occurrences of a key
    • at, operator[] - resolve a JSON pointer and throw, or return a discarded view
    • BasicJsonType::contains - the corresponding function of basic_json
    "},{"location":"api/basic_json_view/contains/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_view/count/","title":"nlohmann::basic_json_view::count","text":"
    size_type count(string_view_t key) const;\nsize_type count(const char* key) const;\nsize_type count(const string_t& key) const;\n

    Returns the number of members with key key if the value is an object, 0 otherwise. Members with a repeated key are all counted.

    "},{"location":"api/basic_json_view/count/#parameters","title":"Parameters","text":"key (in) key value of the element to count"},{"location":"api/basic_json_view/count/#return-value","title":"Return value","text":"

    the number of members of the object with key key (duplicates included), or 0 if the value is not an object or has no such member.

    "},{"location":"api/basic_json_view/count/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this function never throws exceptions.

    "},{"location":"api/basic_json_view/count/#complexity","title":"Complexity","text":"

    Linear in the number of members: unlike the lookup functions, count() does not stop at the first match, because it has to count every member with the key. Members are compared one after another, in document order. Each comparison first checks the key's length -- already known from the index, without reading the key bytes -- before comparing its content. The hash index of the keys of objects with 128 or more members does not help here, since every member is visited.

    "},{"location":"api/basic_json_view/count/#notes","title":"Notes","text":"

    This method always returns 0 when the value is not an object -- including a discarded view.

    If the source text repeats a key, count() returns more than 1 for it, even though the materialized value (like the result of parse()) would have only one member with that key. Hence count(key) > 1 detects duplicate keys. This is unlike the lookup functions (operator[], at, find, contains), which resolve to the first member; see the Notes on duplicate keys of operator[].

    "},{"location":"api/basic_json_view/count/#examples","title":"Examples","text":"Example

    The example below validates that every transaction of a batch carries exactly one amount field, using count() before deciding whether to materialize a transaction at all. It reports a transaction with no amount field and one with a duplicated amount field.

    #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    // validate that every transaction of a batch carries exactly one\n    // \"amount\" field before materializing any of them into a nlohmann::json\n    // value -- count() counts every member with the key\n    json_document batch = json_document::parse(R\"(\n      [\n        {\"id\": 1, \"amount\": 9.99},\n        {\"id\": 2},\n        {\"id\": 3, \"amount\": 5.00, \"amount\": 500.00}\n      ]\n    )\");\n\n    const auto transactions = batch.root();\n    for (std::size_t i = 0; i < transactions.size(); ++i)\n    {\n        const auto transaction = transactions[i];\n        const auto amounts = transaction.count(\"amount\");\n        if (amounts != 1)\n        {\n            std::cout << \"transaction \" << i << \" has \" << amounts << \" \\\"amount\\\" fields\\n\";\n            continue;\n        }\n        std::cout << \"transaction \" << i << \": \" << transaction[\"amount\"].materialize().dump() << '\\n';\n    }\n}\n

    Output:

    transaction 0: 9.99\ntransaction 1 has 0 \"amount\" fields\ntransaction 2 has 2 \"amount\" fields\n
    "},{"location":"api/basic_json_view/count/#see-also","title":"See also","text":"
    • find - find a value in an object
    • contains - checks whether a key exists
    • BasicJsonType::count - the corresponding function of basic_json
    "},{"location":"api/basic_json_view/count/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_view/dump/","title":"nlohmann::basic_json_view::dump","text":"
    string_t dump(const int indent = -1,\n              const char indent_char = ' ',\n              const bool ensure_ascii = false,\n              const number_format numbers = number_format::shortest) const;\n

    Serializes this value (and its subtree) directly from the flat index, without ever building a BasicJsonType value first. With the default numbers == number_format::shortest, the result is the same string BasicJsonType::dump would produce for the value BasicJsonType::parse() builds from the same source text, called with the same indent, indent_char, and ensure_ascii -- except that members of an object appear in document order rather than sorted by key, and every occurrence of a repeated key is written rather than only the last one (see Notes on duplicate keys). For a json_view (whose BasicJsonType is not ordered), this means dump() can print an object's members in a different order than materialize().dump() of the same subtree.

    "},{"location":"api/basic_json_view/dump/#parameters","title":"Parameters","text":"indent (in) If indent is nonnegative, array elements and object members are pretty-printed with that indent level. An indent level of 0 only inserts newlines. -1 (the default) selects the most compact representation. indent_char (in) The character used for indentation if indent is greater than 0. The default is (space). ensure_ascii (in) If ensure_ascii is true, all non-ASCII characters in the output are escaped with \\uXXXX sequences, and the result consists of ASCII characters only. numbers (in) how to write numbers, see number_format: shortest (the default) writes them the way BasicJsonType::dump would; source copies every number exactly as it appears in the source text."},{"location":"api/basic_json_view/dump/#return-value","title":"Return value","text":"

    string containing the serialization of this value, or \"<discarded>\" if the view is discarded.

    "},{"location":"api/basic_json_view/dump/#exception-safety","title":"Exception safety","text":"

    Strong exception safety: if an exception is thrown, there are no changes to the view or the document it refers to.

    "},{"location":"api/basic_json_view/dump/#exceptions","title":"Exceptions","text":"

    May throw std::bad_alloc if allocating the output string fails. Unlike BasicJsonType::dump, there is no error_handler parameter and no type_error.316: the view only ever holds text the parser already validated as UTF-8, so there is nothing to replace or ignore.

    "},{"location":"api/basic_json_view/dump/#complexity","title":"Complexity","text":"

    Linear in the size of the output text.

    "},{"location":"api/basic_json_view/dump/#notes","title":"Notes","text":"

    The walk over the subtree is iterative, so the nesting depth it can write is limited by available memory only, not by the call stack -- as for materialize().

    Strings are escaped by the same rules as BasicJsonType::dump.

    With numbers == number_format::shortest, floats are written with the library's shortest round-trip conversion, exactly as BasicJsonType::dump would (e.g. 1.5, 100.0, 1e+100), and integers are copied from the source text -- already canonical in JSON, so this matches their shortest form too -- except that -0 is written as 0, the way BasicJsonType::parse() reads it.

    number_format::source copies every number exactly as written in the source text instead, with no exception for -0 -- 1.50, 1E2, -0.0, -0, or all digits of an integer literal with more digits than any number type holds (such a literal is itself classified as a float, see What is different) -- something BasicJsonType cannot do, since parsing already reduces every number to its parsed value.

    "},{"location":"api/basic_json_view/dump/#examples","title":"Examples","text":"Example

    The example below forwards a single record out of a larger batch, and re-serializes a configuration file, both without ever building a BasicJsonType value for the surrounding array or for the parts of it that were not needed. It also shows that materialize().dump() of the configuration sorts its keys, where dump() on the view keeps the order they appear in the source text.

    #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\nusing json_view = nlohmann::json_view;\n\nint main()\n{\n    // a large batch of sensor readings -- forward just the one that changed,\n    // without ever building a basic_json value for the batch or for the\n    // readings that are not needed\n    const json_document batch = json_document::parse(R\"(\n      [{\"id\": 1, \"temp\": 21.5}, {\"id\": 2, \"temp\": 87.3}, {\"id\": 3, \"temp\": 21.7}]\n    )\");\n    const json_view readings = batch.root();\n    std::cout << readings[1].dump() << '\\n';\n\n    // a configuration file -- dump() on the view keeps the member order of\n    // the source text; a json value's object_t is std::map, so\n    // materialize().dump() of the very same view sorts the keys instead\n    const json_document config = json_document::parse(\n                                     R\"({\"name\": \"cache\", \"host\": \"db1\", \"port\": 6379, \"timeout\": 30})\");\n    std::cout << config.root().dump(2) << \"\\n\\n\";\n    std::cout << config.root().materialize().dump(2) << '\\n';\n}\n

    Output:

    {\"id\":2,\"temp\":87.3}\n{\n  \"name\": \"cache\",\n  \"host\": \"db1\",\n  \"port\": 6379,\n  \"timeout\": 30\n}\n\n{\n  \"host\": \"db1\",\n  \"name\": \"cache\",\n  \"port\": 6379,\n  \"timeout\": 30\n}\n
    "},{"location":"api/basic_json_view/dump/#see-also","title":"See also","text":"
    • number_format - how dump() writes numbers
    • operator<< - serialize this value to a stream
    • materialize - build a BasicJsonType value, e.g. to use BasicJsonType::dump's error_handler
    • BasicJsonType::dump - the corresponding function of basic_json
    "},{"location":"api/basic_json_view/dump/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_view/empty/","title":"nlohmann::basic_json_view::empty","text":"
    bool empty() const noexcept;\n

    Checks whether size() is 0, as BasicJsonType::empty() would for the same value.

    "},{"location":"api/basic_json_view/empty/#return-value","title":"Return value","text":"

    The return value depends on the type and is defined as follows:

    Value type return value null true discarded true boolean false string false number false object object_t::empty() array array_t::empty()"},{"location":"api/basic_json_view/empty/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this function never throws exceptions.

    "},{"location":"api/basic_json_view/empty/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json_view/empty/#notes","title":"Notes","text":"

    As for BasicJsonType::empty(), this does not return whether a string value is empty -- it is false for any string, regardless of its length.

    "},{"location":"api/basic_json_view/empty/#examples","title":"Examples","text":"Example

    The example below uses size() and empty() to decide whether a parsed message is worth acting on, without materializing it into a BasicJsonType value.

    #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // decide whether a batch is worth processing before building any\n    // nlohmann::json value for it\n    json_document batch = json_document::parse(R\"([1, 2, 3, 4, 5])\");\n    json_document empty_batch = json_document::parse(\"[]\");\n\n    std::cout << batch.root().empty() << ' ' << batch.root().size() << '\\n';\n    std::cout << empty_batch.root().empty() << ' ' << empty_batch.root().size() << '\\n';\n\n    // as for basic_json: null has size 0, every other scalar has size 1\n    json_document n = json_document::parse(\"null\");\n    json_document s = json_document::parse(R\"(\"hi\")\");\n    std::cout << n.root().size() << ' ' << s.root().size() << '\\n';\n}\n

    Output:

    false 5\ntrue 0\n0 1\n
    "},{"location":"api/basic_json_view/empty/#see-also","title":"See also","text":"
    • size - return the number of elements
    • BasicJsonType::empty - the corresponding function of basic_json
    "},{"location":"api/basic_json_view/empty/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_view/end/","title":"nlohmann::basic_json_view::end","text":"
    iterator end() const noexcept;\n

    Returns an iterator to one past the last element of an array, one past the last member value of an object, in document order -- see begin(). A primitive value iterates as a range of one element (itself), and a null value or a discarded view iterates as an empty range (begin() == end()).

    "},{"location":"api/basic_json_view/end/#return-value","title":"Return value","text":"

    Iterator one past the last element.

    "},{"location":"api/basic_json_view/end/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this function never throws exceptions.

    "},{"location":"api/basic_json_view/end/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json_view/end/#notes","title":"Notes","text":"

    iterator is a forward iterator: unlike BasicJsonType::iterator, it cannot be decremented, so there is no way to reach the last element by stepping back from end(). Use back() instead.

    "},{"location":"api/basic_json_view/end/#examples","title":"Examples","text":"Example

    The example below scans a (possibly large) array of readings for the first one over a threshold, stopping the loop at end() as soon as one is found. Only the matching reading, if any, is ever materialized.

    #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    // scan a (possibly large) array of readings for the first one over a\n    // threshold; the loop stops at end() as soon as one is found, and only\n    // the matching reading is ever materialized\n    json_document readings = json_document::parse(\"[12, 18, 25, 31, 9]\");\n    const auto values = readings.root();\n\n    auto it = values.begin();\n    for (; it != values.end(); ++it)\n    {\n        if (it->materialize().get<int>() > 20)\n        {\n            break;\n        }\n    }\n\n    if (it != values.end())\n    {\n        std::cout << \"first reading over 20: \" << it->materialize().dump() << '\\n';\n    }\n}\n

    Output:

    first reading over 20: 25\n
    "},{"location":"api/basic_json_view/end/#see-also","title":"See also","text":"
    • begin - returns an iterator to the first element
    • cend - returns a const iterator to one past the last element
    • BasicJsonType::end - the corresponding function of basic_json
    "},{"location":"api/basic_json_view/end/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_view/find/","title":"nlohmann::basic_json_view::find","text":"
    iterator find(string_view_t key) const;\niterator find(const char* key) const;\niterator find(const string_t& key) const;\n

    Finds a member with key key -- the first one, should the key occur more than once (see Notes on duplicate keys). If the value is not an object, or no member has this key, end() is returned.

    "},{"location":"api/basic_json_view/find/#parameters","title":"Parameters","text":"key (in) key value of the element to search for"},{"location":"api/basic_json_view/find/#return-value","title":"Return value","text":"

    An iterator to the member with key key, or end() if there is none or the value is not an object.

    "},{"location":"api/basic_json_view/find/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this function never throws exceptions.

    "},{"location":"api/basic_json_view/find/#complexity","title":"Complexity","text":"

    Linear in the number of members: as for ordered_json, members are compared one after another, in document order, stopping at the first match. Each comparison first checks the key's length -- already known from the index, without reading the key bytes -- before comparing its content. Objects with 128 or more members get a hash index of their keys while parsing, so that a lookup in them takes constant time on average.

    "},{"location":"api/basic_json_view/find/#notes","title":"Notes","text":"

    Duplicate keys: the first member wins

    If the source text repeats a key, this resolves to the first member with it, not to the last one that materialize() and parse() keep. See the Notes on duplicate keys of operator[] and Duplicate keys for the reasons and for how to get the last value.

    Unlike BasicJsonType::find, which always returns end() for a non-object type, this also does so for a discarded view -- there is no separate \"invalid\" iterator to return.

    "},{"location":"api/basic_json_view/find/#examples","title":"Examples","text":"Example

    The example below scans a batch of events for those that carry an optional user_id field, using find() instead of operator[] (which would throw for the events that are not objects at all) or contains() followed by a second lookup. Events without a match are never materialized.

    #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    // a batch of incoming events; only some carry a \"user_id\" -- find()\n    // locates it without throwing for the events that turn out to not be\n    // objects, and without materializing an event that does not match\n    json_document batch = json_document::parse(R\"(\n      [\n        {\"type\": \"click\", \"user_id\": 42},\n        {\"type\": \"ping\"},\n        {\"type\": \"click\", \"user_id\": 7}\n      ]\n    )\");\n\n    const auto events = batch.root();\n    for (std::size_t i = 0; i < events.size(); ++i)\n    {\n        const auto event = events[i];\n        const auto it = event.find(\"user_id\");\n        if (it != event.end())\n        {\n            std::cout << \"user \" << it->materialize().dump() << '\\n';\n        }\n    }\n}\n

    Output:

    user 42\nuser 7\n
    "},{"location":"api/basic_json_view/find/#see-also","title":"See also","text":"
    • count - returns the number of occurrences of a key
    • contains - checks whether a key exists
    • BasicJsonType::find - the corresponding function of basic_json
    "},{"location":"api/basic_json_view/find/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_view/front/","title":"nlohmann::basic_json_view::front","text":"
    basic_json_view front() const;\n

    Returns the first element of an array, the first member value of an object, or the value itself if it is primitive (as for BasicJsonType::front(), a primitive value is a range of one element).

    "},{"location":"api/basic_json_view/front/#return-value","title":"Return value","text":"

    The first element or member value. For a primitive value (number, string, boolean), the value itself.

    "},{"location":"api/basic_json_view/front/#exception-safety","title":"Exception safety","text":"

    Strong guarantee: there are no changes to the view or the document it refers to.

    "},{"location":"api/basic_json_view/front/#exceptions","title":"Exceptions","text":"

    Throws invalid_iterator.214 if the view is null or discarded, or if it is an empty array or object.

    This exception does not carry a JSON_DIAGNOSTICS path: the view has no BasicJsonType value to point at, so the exception is created without one, even if BasicJsonType was built with JSON_DIAGNOSTICS enabled.

    "},{"location":"api/basic_json_view/front/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json_view/front/#notes","title":"Notes","text":"

    Unlike BasicJsonType::front(), which has undefined behavior for an empty array or object, front() throws invalid_iterator.214 for an empty array, an empty object, a null value, or a discarded view.

    "},{"location":"api/basic_json_view/front/#examples","title":"Examples","text":"Example

    The example below reads only the earliest entry of a build log with front(), without materializing the rest of the (possibly long) log.

    #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    // the build log of a running job; front() reads just the earliest event\n    // without materializing the (possibly long) rest of the log\n    json_document log = json_document::parse(R\"([\"queued\", \"started\", \"compiling\", \"linking\", \"done\"])\");\n    std::cout << log.root().front().materialize().dump() << '\\n';\n\n    // an empty log -- front() throws instead of the undefined behavior\n    // basic_json::front() has for an empty array\n    json_document empty_log = json_document::parse(\"[]\");\n    try\n    {\n        static_cast<void>(empty_log.root().front());\n    }\n    catch (const nlohmann::json::invalid_iterator& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

    Output:

    \"queued\"\n[json.exception.invalid_iterator.214] cannot get value\n
    "},{"location":"api/basic_json_view/front/#see-also","title":"See also","text":"
    • back - access the last element
    • BasicJsonType::front - the corresponding function of basic_json
    "},{"location":"api/basic_json_view/front/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_view/get/","title":"nlohmann::basic_json_view::get","text":"
    template<typename T>\nT get() const;\n

    Converts the value to T.

    For the types below, the conversion works directly on the flat index -- no BasicJsonType value is built for it:

    • bool
    • arithmetic types other than bool (for a boolean, 0/1, exactly as for BasicJsonType::get<T>())
    • std::nullptr_t
    • std::basic_string<char, Traits, Alloc> (including string_t) -- a copy of the string
    • string_view_t -- no copy: the returned view points into the document's source() text, or, for a string that contains escape sequences, into the document's own buffer of decoded strings (see get_string())
    • BasicJsonType -- equivalent to materialize()
    • basic_json_view -- returns *this
    • std::vector<U, A> -- from an array, element by element, each converted with get<U>(); std::vector<basic_json_view> keeps a view of every element instead of a value
    • std::map<K, V, C, A> and std::unordered_map<K, V, H, E, A>, if K is constructible from a (const char*, std::size_t) pair -- from an object, member by member, each value converted with get<V>(); with a repeated key, the last value is kept, as BasicJsonType::parse() (and materialize()) does; std::map<std::string, basic_json_view> keeps views of the members instead of values

    Every other T -- std::list, std::pair, std::array, enumerations, user types with a from_json(), ... -- is converted by materialize().get<T>(): the subtree is built into a real BasicJsonType value first (as BasicJsonType::parse() would), and converted from there exactly as BasicJsonType::get<T>() would convert it.

    "},{"location":"api/basic_json_view/get/#template-parameters","title":"Template parameters","text":"T the type to convert the value to"},{"location":"api/basic_json_view/get/#return-value","title":"Return value","text":"

    the value, converted to T

    "},{"location":"api/basic_json_view/get/#exception-safety","title":"Exception safety","text":"

    Strong guarantee: there are no changes to the view or the document it refers to.

    "},{"location":"api/basic_json_view/get/#exceptions","title":"Exceptions","text":"
    • For the directly-converted types listed above (other than BasicJsonType and basic_json_view, which never throw): throws type_error.302 if the value's type does not match T -- the same exception, with the same message, that BasicJsonType::get<T>() throws for the same JSON type and T.
    • For std::vector<U, A>: throws type_error.302 if the value is not an array; otherwise, whatever converting an element to U throws.
    • For std::map/std::unordered_map: throws type_error.302 if the value is not an object; otherwise, whatever converting a member to the mapped type throws.
    • For every other T: whatever materialize().get<T>() throws -- typically type_error.302, or whatever a user-provided from_json() throws.

    None of the exceptions thrown directly by this function (the first three bullets above) carry a JSON_DIAGNOSTICS path: the view has no BasicJsonType value to point at. An exception thrown while converting through materialize() (the last bullet) is different: it is thrown by a real BasicJsonType value, so it does carry a JSON_DIAGNOSTICS path if BasicJsonType was built with it enabled. That path is relative to the root of the view's subtree, because materialize() builds only the subtree (with parent pointers): it is empty if the failing value is the subtree's root itself, and it never is the path within the whole document.

    "},{"location":"api/basic_json_view/get/#complexity","title":"Complexity","text":"
    • bool, arithmetic types, std::nullptr_t, string_view_t, basic_json_view: constant.
    • std::basic_string<char, Traits, Alloc>: constant, plus one allocation and a copy of the string's bytes.
    • BasicJsonType: linear in the size of the subtree, see materialize().
    • std::vector<U, A>: linear in the number of elements, times the complexity of converting one element to U.
    • std::map/std::unordered_map: linear in the number of members for walking them, plus the container's own insertion cost per member (logarithmic for std::map, amortized constant for std::unordered_map), times the complexity of converting one member to the mapped type.
    • every other T: the cost of materialize() and of destroying the BasicJsonType value afterwards, plus that of BasicJsonType::get<T>().
    "},{"location":"api/basic_json_view/get/#notes","title":"Notes","text":"

    Floating-point values

    A floating-point T is converted from the same digits the lexer would see during BasicJsonType::parse(), using the same conversion, so the result is bit-for-bit identical to BasicJsonType::parse(text).get<T>() for the same source text.

    Duplicate keys

    std::map/std::unordered_map conversions keep the last value of a repeated key, like materialize() and BasicJsonType::parse() do. This is the opposite of operator[]/at/find/contains, which resolve to the first occurrence (see the Notes on duplicate keys).

    No pointers, references, or implicit conversion

    Unlike BasicJsonType, basic_json_view has no stored value anywhere to hand out a pointer or a reference to, so it provides neither get_ptr(), get_ref(), nor operator ValueType(). get_string() (equivalently, get<string_view_t>()) is the zero-copy alternative for strings.

    "},{"location":"api/basic_json_view/get/#examples","title":"Examples","text":"Example

    The example below reads typed fields straight into C++ variables, collects a view of every array element with get<std::vector<basic_json_view>>() instead of a value, and converts a nested object into a user type through its from_json() -- which runs on a BasicJsonType value materialize() builds for just that one member.

    #include <iostream>\n#include <nlohmann/json_view.hpp>\n#include <string>\n#include <vector>\n\nusing json_document = nlohmann::json_document;\nusing json_view = nlohmann::json_view;\n\n// address has no direct conversion in get<T>(), so get<address>() falls back\n// to materialize().get<address>() -- a real nlohmann::json value is built\n// for just this one member, and its own from_json() runs on that\nstruct address\n{\n    std::string city;\n    int zip = 0;\n};\n\nvoid from_json(const nlohmann::json& j, address& a)\n{\n    j.at(\"city\").get_to(a.city);\n    j.at(\"zip\").get_to(a.zip);\n}\n\nint main()\n{\n    json_document doc = json_document::parse(R\"(\n      {\n        \"name\": \"Alice\",\n        \"active\": true,\n        \"orders\": [1, 2, 3],\n        \"address\": {\"city\": \"Berlin\", \"zip\": 10115}\n      }\n    )\");\n    const json_view customer = doc.root();\n\n    // read typed fields straight into C++ variables -- none of these build\n    // a nlohmann::json value\n    const std::string name = customer[\"name\"].get<std::string>();\n    const bool active = customer[\"active\"].get<bool>();\n    std::cout << name << (active ? \" (active)\" : \" (inactive)\") << '\\n';\n\n    // std::vector<json_view> keeps views of the array elements instead of\n    // copies of their values\n    bool first = true;\n    for (const json_view order : customer[\"orders\"].get<std::vector<json_view>>())\n    {\n        std::cout << (first ? \"\" : \" \") << order.get<int>();\n        first = false;\n    }\n    std::cout << '\\n';\n\n    // everything else goes through materialize()\n    const address a = customer[\"address\"].get<address>();\n    std::cout << a.city << ' ' << a.zip << '\\n';\n}\n

    Output:

    Alice (active)\n1 2 3\nBerlin 10115\n
    "},{"location":"api/basic_json_view/get/#see-also","title":"See also","text":"
    • get_to - convert and write into a passed value
    • get_string - the string, without a copy
    • number_token - a number's token text, without a copy
    • materialize - build the BasicJsonType value of this subtree
    • BasicJsonType::get - the corresponding function of basic_json
    "},{"location":"api/basic_json_view/get/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_view/get_string/","title":"nlohmann::basic_json_view::get_string","text":"
    string_view_t get_string() const;\n

    Returns the string value as a string_view_t, without copying it.

    "},{"location":"api/basic_json_view/get_string/#return-value","title":"Return value","text":"

    The string, as a string_view_t that points either into the document's source() text (a string with no escape sequences), or into the document's own buffer of decoded strings (a string that contains escape sequences, such as \"\\n\" or \"\\u00e9\", which had to be decoded once when the document was parsed).

    "},{"location":"api/basic_json_view/get_string/#exception-safety","title":"Exception safety","text":"

    Strong guarantee: there are no changes to the view or the document it refers to.

    "},{"location":"api/basic_json_view/get_string/#exceptions","title":"Exceptions","text":"

    Throws type_error.302 if the value is not a string; example: \"type must be string, but is array\".

    This exception does not carry a JSON_DIAGNOSTICS path: the view has no BasicJsonType value to point at, so the exception is created without one, even if BasicJsonType was built with JSON_DIAGNOSTICS enabled.

    "},{"location":"api/basic_json_view/get_string/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json_view/get_string/#notes","title":"Notes","text":"

    basic_json_view has no BasicJsonType value stored anywhere, so unlike BasicJsonType, it has no get_ref() to hand out a reference to a stored string_t. get_string() (equivalently, get<string_view_t>()) is the zero-copy alternative: BasicJsonType::get_ref<const string_t&>() is its closest counterpart, except that it returns a view instead of a reference to a value that must already exist.

    get_string() borrows the string: nothing is copied, whereas get<std::string>() copies it. For a borrowed document, the returned string_view_t is valid as long as the caller's buffer; for an owned one, as long as the document.

    The returned string_view_t is valid exactly as long as the view that produced it -- see the validity rules of basic_json_view -- and, for a string with no escapes, for as long as the document's source text, which for a borrowed document must remain valid and unmodified for as long as the document exists, since floating-point values and unescaped strings are read from the source lazily.

    "},{"location":"api/basic_json_view/get_string/#examples","title":"Examples","text":"Example

    The example below pulls one field out of a JSON text that stands in for a large API response, and shows that no std::string was allocated for it: the returned view still points inside the original buffer. A field that contains an escape sequence cannot point into the original text -- it was decoded once into the document's own buffer instead -- but still avoids a per-field allocation.

    #include <iostream>\n#include <nlohmann/json_view.hpp>\n#include <string>\n\nusing json_document = nlohmann::json_document;\nusing json_view = nlohmann::json_view;\n\nint main()\n{\n    // a \"large response\" stand-in: only the \"id\" field is ever read out of it\n    const std::string text =\n        R\"({\"id\": \"8f14e45f-ceea-467e-bb92-963f5e3c7a08\", \"note\": \"created via API\\n\", \"payload\": \"...\"})\";\n    const json_document doc = json_document::parse(text);\n    const json_view response = doc.root();\n\n    const json_view::string_view_t id = response[\"id\"].get_string();\n    std::cout << id << '\\n';\n\n    // no std::string was allocated for \"id\": its bytes still live inside\n    // the original buffer, so id's data lies inside [text.data(),\n    // text.data() + text.size())\n    const bool id_in_source = id.data() >= text.data() && id.data() + id.size() <= text.data() + text.size();\n    std::cout << std::boolalpha << id_in_source << '\\n';\n\n    // \"note\" contains an escape sequence ('\\n'), so it was decoded once\n    // into the document's own buffer -- get_string() still avoids a copy\n    // into a new std::string, but the bytes no longer live inside \"text\"\n    const json_view::string_view_t note = response[\"note\"].get_string();\n    const bool note_in_source = note.data() >= text.data() && note.data() + note.size() <= text.data() + text.size();\n    std::cout << std::boolalpha << note_in_source << '\\n';\n}\n

    Output:

    8f14e45f-ceea-467e-bb92-963f5e3c7a08\ntrue\nfalse\n
    "},{"location":"api/basic_json_view/get_string/#see-also","title":"See also","text":"
    • get - convert the value to a given type (get<string_view_t>() is equivalent to this function)
    • number_token - a number's token text, without a copy
    • BasicJsonType::get_ref - the closest counterpart of basic_json
    "},{"location":"api/basic_json_view/get_string/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_view/get_to/","title":"nlohmann::basic_json_view::get_to","text":"
    template<typename T>\nT& get_to(T& v) const;\n

    Converts the value to T and stores it in v, in one of two ways:

    • For the types that get<T>() converts directly -- arithmetic types, bool, std::nullptr_t, string_view_t, std::basic_string, BasicJsonType, basic_json_view, std::vector, and std::map/std::unordered_map with string keys -- v is assigned: v = get<T>().
    • For every other type, the subtree is built into a BasicJsonType value and materialize().get_to(v) is called, i.e. from_json() is invoked with the existing v, exactly as BasicJsonType::get_to() does.
    "},{"location":"api/basic_json_view/get_to/#template-parameters","title":"Template parameters","text":"T the type to convert the value to"},{"location":"api/basic_json_view/get_to/#parameters","title":"Parameters","text":"v (out) the variable to store the converted value in"},{"location":"api/basic_json_view/get_to/#return-value","title":"Return value","text":"

    v, allowing calls to chain

    "},{"location":"api/basic_json_view/get_to/#exception-safety","title":"Exception safety","text":"
    • For the directly converted types: strong exception safety. If an exception is thrown, v is not modified.
    • For every other type: as for BasicJsonType::get_to(). materialize() throws before v is touched, but a from_json() that throws may leave v partly modified.
    "},{"location":"api/basic_json_view/get_to/#exceptions","title":"Exceptions","text":"
    • For the directly converted types: whatever get<T>() throws for the same value and T.
    • For every other type: whatever materialize() and BasicJsonType::get_to() throw.
    "},{"location":"api/basic_json_view/get_to/#complexity","title":"Complexity","text":"
    • For the directly converted types: whatever get<T>() has for the same T.
    • For every other type: that of materialize(), plus that of BasicJsonType::get_to().
    "},{"location":"api/basic_json_view/get_to/#notes","title":"Notes","text":"

    The difference between the two ways matters for a from_json() that sets only the fields present in the JSON: since it runs on the existing v, the other fields of v keep their values, whereas assigning get<T>() would replace them with those of a default-constructed T.

    "},{"location":"api/basic_json_view/get_to/#examples","title":"Examples","text":"Example

    The example below reads several fields of a service configuration directly into existing variables, then uses the returned reference to fold the host/port pair into a single string in the same expression.

    #include <iostream>\n#include <nlohmann/json_view.hpp>\n#include <string>\n\nusing json_document = nlohmann::json_document;\nusing json_view = nlohmann::json_view;\n\nint main()\n{\n    json_document doc = json_document::parse(R\"({\"host\": \"db.example.com\", \"port\": 5432, \"ssl\": true})\");\n    const json_view config = doc.root();\n\n    // get_to() writes directly into existing variables -- handy for filling\n    // in the members of a struct one field at a time, without an\n    // intermediate value from get<T>() for each one\n    std::string host;\n    int port = 0;\n    bool ssl = false;\n    config[\"host\"].get_to(host);\n    config[\"port\"].get_to(port);\n    config[\"ssl\"].get_to(ssl);\n    std::cout << host << ':' << port << (ssl ? \" (tls)\" : \"\") << '\\n';\n\n    // the return value is a reference to the argument, so a call can be\n    // used directly in a larger expression\n    std::string other_host;\n    std::cout << config[\"host\"].get_to(other_host).size() << '\\n';\n}\n

    Output:

    db.example.com:5432 (tls)\n14\n
    "},{"location":"api/basic_json_view/get_to/#see-also","title":"See also","text":"
    • get - convert the value to a given type
    • BasicJsonType::get_to - the corresponding function of basic_json
    "},{"location":"api/basic_json_view/get_to/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_view/is_array/","title":"nlohmann::basic_json_view::is_array","text":"
    bool is_array() const noexcept;\n

    This function returns true if and only if the value is an array.

    "},{"location":"api/basic_json_view/is_array/#return-value","title":"Return value","text":"

    true if the type is an array, false otherwise.

    "},{"location":"api/basic_json_view/is_array/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this function never throws exceptions.

    "},{"location":"api/basic_json_view/is_array/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json_view/is_array/#examples","title":"Examples","text":"Example

    The example below classifies several parsed documents by the type of their root value, without materializing any of them into a BasicJsonType value.

    #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // several incoming messages, one json_document per message. type() and\n    // is_*() only look at the flat index built by parse(); no nlohmann::json\n    // tree exists yet, and none is built unless materialize() is called\n    json_document d_null = json_document::parse(\"null\");\n    json_document d_bool = json_document::parse(\"true\");\n    json_document d_int = json_document::parse(\"-42\");\n    json_document d_unsigned = json_document::parse(\"42\");\n    json_document d_float = json_document::parse(\"4.2\");\n    json_document d_string = json_document::parse(R\"(\"hi\")\");\n    json_document d_array = json_document::parse(\"[1, 2, 3]\");\n    json_document d_object = json_document::parse(R\"({\"a\": 1})\");\n\n    std::cout << d_null.root().is_null() << '\\n';\n    std::cout << d_bool.root().is_boolean() << '\\n';\n    std::cout << d_int.root().is_number() << ' ' << d_int.root().is_number_integer() << '\\n';\n    std::cout << d_unsigned.root().is_number_unsigned() << '\\n';\n    std::cout << d_float.root().is_number_float() << '\\n';\n    std::cout << d_string.root().is_string() << '\\n';\n    std::cout << d_array.root().is_array() << ' ' << d_array.root().is_structured() << '\\n';\n    std::cout << d_object.root().is_object() << ' ' << d_object.root().is_primitive() << '\\n';\n\n    // JSON text can never produce a binary value: is_binary() is always false\n    std::cout << d_array.root().is_binary() << '\\n';\n\n    // a default-constructed view, and the root of a document that failed to\n    // parse without exceptions, are both discarded\n    nlohmann::json_view invalid;\n    json_document failed = json_document::parse(\"not json\", /* allow_exceptions */ false);\n    std::cout << invalid.is_discarded() << '\\n';\n    std::cout << failed.root().is_discarded() << '\\n';\n\n    // type() returns the same value_t enumeration as basic_json::type()\n    std::cout << (d_object.root().type() == nlohmann::json::value_t::object) << '\\n';\n}\n

    Output:

    true\ntrue\ntrue true\ntrue\ntrue\ntrue\ntrue true\ntrue false\nfalse\ntrue\ntrue\ntrue\n
    "},{"location":"api/basic_json_view/is_array/#see-also","title":"See also","text":"
    • is_structured - return whether the type is structured
    • size - the number of elements
    • empty - whether there are no elements
    • BasicJsonType::is_array - the corresponding function of basic_json
    "},{"location":"api/basic_json_view/is_array/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_view/is_binary/","title":"nlohmann::basic_json_view::is_binary","text":"
    bool is_binary() const noexcept;\n

    This function always returns false: a document is only ever parsed from JSON text, never from CBOR or the other binary formats, and a JSON text has no binary values, so a view can never refer to one. The function exists for interface parity with BasicJsonType::is_binary.

    "},{"location":"api/basic_json_view/is_binary/#return-value","title":"Return value","text":"

    false, always.

    "},{"location":"api/basic_json_view/is_binary/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this function never throws exceptions.

    "},{"location":"api/basic_json_view/is_binary/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json_view/is_binary/#examples","title":"Examples","text":"Example

    The example below classifies several parsed documents by the type of their root value, without materializing any of them into a BasicJsonType value.

    #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // several incoming messages, one json_document per message. type() and\n    // is_*() only look at the flat index built by parse(); no nlohmann::json\n    // tree exists yet, and none is built unless materialize() is called\n    json_document d_null = json_document::parse(\"null\");\n    json_document d_bool = json_document::parse(\"true\");\n    json_document d_int = json_document::parse(\"-42\");\n    json_document d_unsigned = json_document::parse(\"42\");\n    json_document d_float = json_document::parse(\"4.2\");\n    json_document d_string = json_document::parse(R\"(\"hi\")\");\n    json_document d_array = json_document::parse(\"[1, 2, 3]\");\n    json_document d_object = json_document::parse(R\"({\"a\": 1})\");\n\n    std::cout << d_null.root().is_null() << '\\n';\n    std::cout << d_bool.root().is_boolean() << '\\n';\n    std::cout << d_int.root().is_number() << ' ' << d_int.root().is_number_integer() << '\\n';\n    std::cout << d_unsigned.root().is_number_unsigned() << '\\n';\n    std::cout << d_float.root().is_number_float() << '\\n';\n    std::cout << d_string.root().is_string() << '\\n';\n    std::cout << d_array.root().is_array() << ' ' << d_array.root().is_structured() << '\\n';\n    std::cout << d_object.root().is_object() << ' ' << d_object.root().is_primitive() << '\\n';\n\n    // JSON text can never produce a binary value: is_binary() is always false\n    std::cout << d_array.root().is_binary() << '\\n';\n\n    // a default-constructed view, and the root of a document that failed to\n    // parse without exceptions, are both discarded\n    nlohmann::json_view invalid;\n    json_document failed = json_document::parse(\"not json\", /* allow_exceptions */ false);\n    std::cout << invalid.is_discarded() << '\\n';\n    std::cout << failed.root().is_discarded() << '\\n';\n\n    // type() returns the same value_t enumeration as basic_json::type()\n    std::cout << (d_object.root().type() == nlohmann::json::value_t::object) << '\\n';\n}\n

    Output:

    true\ntrue\ntrue true\ntrue\ntrue\ntrue\ntrue true\ntrue false\nfalse\ntrue\ntrue\ntrue\n
    "},{"location":"api/basic_json_view/is_binary/#see-also","title":"See also","text":"
    • type - return the type of the value
    • BasicJsonType::is_binary - the corresponding function of basic_json
    "},{"location":"api/basic_json_view/is_binary/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_view/is_boolean/","title":"nlohmann::basic_json_view::is_boolean","text":"
    bool is_boolean() const noexcept;\n

    This function returns true if and only if the value is a boolean.

    "},{"location":"api/basic_json_view/is_boolean/#return-value","title":"Return value","text":"

    true if the type is a boolean, false otherwise.

    "},{"location":"api/basic_json_view/is_boolean/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this function never throws exceptions.

    "},{"location":"api/basic_json_view/is_boolean/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json_view/is_boolean/#examples","title":"Examples","text":"Example

    The example below classifies several parsed documents by the type of their root value, without materializing any of them into a BasicJsonType value.

    #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // several incoming messages, one json_document per message. type() and\n    // is_*() only look at the flat index built by parse(); no nlohmann::json\n    // tree exists yet, and none is built unless materialize() is called\n    json_document d_null = json_document::parse(\"null\");\n    json_document d_bool = json_document::parse(\"true\");\n    json_document d_int = json_document::parse(\"-42\");\n    json_document d_unsigned = json_document::parse(\"42\");\n    json_document d_float = json_document::parse(\"4.2\");\n    json_document d_string = json_document::parse(R\"(\"hi\")\");\n    json_document d_array = json_document::parse(\"[1, 2, 3]\");\n    json_document d_object = json_document::parse(R\"({\"a\": 1})\");\n\n    std::cout << d_null.root().is_null() << '\\n';\n    std::cout << d_bool.root().is_boolean() << '\\n';\n    std::cout << d_int.root().is_number() << ' ' << d_int.root().is_number_integer() << '\\n';\n    std::cout << d_unsigned.root().is_number_unsigned() << '\\n';\n    std::cout << d_float.root().is_number_float() << '\\n';\n    std::cout << d_string.root().is_string() << '\\n';\n    std::cout << d_array.root().is_array() << ' ' << d_array.root().is_structured() << '\\n';\n    std::cout << d_object.root().is_object() << ' ' << d_object.root().is_primitive() << '\\n';\n\n    // JSON text can never produce a binary value: is_binary() is always false\n    std::cout << d_array.root().is_binary() << '\\n';\n\n    // a default-constructed view, and the root of a document that failed to\n    // parse without exceptions, are both discarded\n    nlohmann::json_view invalid;\n    json_document failed = json_document::parse(\"not json\", /* allow_exceptions */ false);\n    std::cout << invalid.is_discarded() << '\\n';\n    std::cout << failed.root().is_discarded() << '\\n';\n\n    // type() returns the same value_t enumeration as basic_json::type()\n    std::cout << (d_object.root().type() == nlohmann::json::value_t::object) << '\\n';\n}\n

    Output:

    true\ntrue\ntrue true\ntrue\ntrue\ntrue\ntrue true\ntrue false\nfalse\ntrue\ntrue\ntrue\n
    "},{"location":"api/basic_json_view/is_boolean/#see-also","title":"See also","text":"
    • type - return the type of the value
    • BasicJsonType::is_boolean - the corresponding function of basic_json
    "},{"location":"api/basic_json_view/is_boolean/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_view/is_discarded/","title":"nlohmann::basic_json_view::is_discarded","text":"
    bool is_discarded() const noexcept;\n

    Returns whether this view is invalid, i.e. does not refer to a value. This is the case for a default-constructed view (see (constructor)), and for root() of a document that is itself discarded -- in particular, the root of a failed parse() with allow_exceptions set to false.

    A discarded view is also what operator[] returns for a missing key, an index out of range, or a JSON pointer that cannot be resolved, and for any access on a view that is itself discarded (so a chain such as v[\"a\"][\"b\"] is safe). at throws instead.

    "},{"location":"api/basic_json_view/is_discarded/#return-value","title":"Return value","text":"

    true if the view or its document is discarded, false otherwise.

    "},{"location":"api/basic_json_view/is_discarded/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this function never throws exceptions.

    "},{"location":"api/basic_json_view/is_discarded/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json_view/is_discarded/#notes","title":"Notes","text":"

    A basic_json_view is not convertible to bool: such a conversion would mean \"refers to a value\", whereas basic_json converts to the bool it holds, so the same code would silently behave differently. Test !v.is_discarded() explicitly.

    "},{"location":"api/basic_json_view/is_discarded/#examples","title":"Examples","text":"Example

    The example below classifies several parsed documents by the type of their root value, without materializing any of them into a BasicJsonType value.

    #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // several incoming messages, one json_document per message. type() and\n    // is_*() only look at the flat index built by parse(); no nlohmann::json\n    // tree exists yet, and none is built unless materialize() is called\n    json_document d_null = json_document::parse(\"null\");\n    json_document d_bool = json_document::parse(\"true\");\n    json_document d_int = json_document::parse(\"-42\");\n    json_document d_unsigned = json_document::parse(\"42\");\n    json_document d_float = json_document::parse(\"4.2\");\n    json_document d_string = json_document::parse(R\"(\"hi\")\");\n    json_document d_array = json_document::parse(\"[1, 2, 3]\");\n    json_document d_object = json_document::parse(R\"({\"a\": 1})\");\n\n    std::cout << d_null.root().is_null() << '\\n';\n    std::cout << d_bool.root().is_boolean() << '\\n';\n    std::cout << d_int.root().is_number() << ' ' << d_int.root().is_number_integer() << '\\n';\n    std::cout << d_unsigned.root().is_number_unsigned() << '\\n';\n    std::cout << d_float.root().is_number_float() << '\\n';\n    std::cout << d_string.root().is_string() << '\\n';\n    std::cout << d_array.root().is_array() << ' ' << d_array.root().is_structured() << '\\n';\n    std::cout << d_object.root().is_object() << ' ' << d_object.root().is_primitive() << '\\n';\n\n    // JSON text can never produce a binary value: is_binary() is always false\n    std::cout << d_array.root().is_binary() << '\\n';\n\n    // a default-constructed view, and the root of a document that failed to\n    // parse without exceptions, are both discarded\n    nlohmann::json_view invalid;\n    json_document failed = json_document::parse(\"not json\", /* allow_exceptions */ false);\n    std::cout << invalid.is_discarded() << '\\n';\n    std::cout << failed.root().is_discarded() << '\\n';\n\n    // type() returns the same value_t enumeration as basic_json::type()\n    std::cout << (d_object.root().type() == nlohmann::json::value_t::object) << '\\n';\n}\n

    Output:

    true\ntrue\ntrue true\ntrue\ntrue\ntrue\ntrue true\ntrue false\nfalse\ntrue\ntrue\ntrue\n
    "},{"location":"api/basic_json_view/is_discarded/#see-also","title":"See also","text":"
    • operator[] - access specified element; yields a discarded view where an element is missing
    • (constructor) - the default constructor creates a discarded view
    • is_discarded (basic_json_document) - return whether the last parse failed
    • BasicJsonType::is_discarded - the corresponding function of basic_json
    "},{"location":"api/basic_json_view/is_discarded/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_view/is_null/","title":"nlohmann::basic_json_view::is_null","text":"
    bool is_null() const noexcept;\n

    This function returns true if and only if the value is null.

    "},{"location":"api/basic_json_view/is_null/#return-value","title":"Return value","text":"

    true if the type is null, false otherwise.

    "},{"location":"api/basic_json_view/is_null/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this function never throws exceptions.

    "},{"location":"api/basic_json_view/is_null/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json_view/is_null/#examples","title":"Examples","text":"Example

    The example below classifies several parsed documents by the type of their root value, without materializing any of them into a BasicJsonType value.

    #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // several incoming messages, one json_document per message. type() and\n    // is_*() only look at the flat index built by parse(); no nlohmann::json\n    // tree exists yet, and none is built unless materialize() is called\n    json_document d_null = json_document::parse(\"null\");\n    json_document d_bool = json_document::parse(\"true\");\n    json_document d_int = json_document::parse(\"-42\");\n    json_document d_unsigned = json_document::parse(\"42\");\n    json_document d_float = json_document::parse(\"4.2\");\n    json_document d_string = json_document::parse(R\"(\"hi\")\");\n    json_document d_array = json_document::parse(\"[1, 2, 3]\");\n    json_document d_object = json_document::parse(R\"({\"a\": 1})\");\n\n    std::cout << d_null.root().is_null() << '\\n';\n    std::cout << d_bool.root().is_boolean() << '\\n';\n    std::cout << d_int.root().is_number() << ' ' << d_int.root().is_number_integer() << '\\n';\n    std::cout << d_unsigned.root().is_number_unsigned() << '\\n';\n    std::cout << d_float.root().is_number_float() << '\\n';\n    std::cout << d_string.root().is_string() << '\\n';\n    std::cout << d_array.root().is_array() << ' ' << d_array.root().is_structured() << '\\n';\n    std::cout << d_object.root().is_object() << ' ' << d_object.root().is_primitive() << '\\n';\n\n    // JSON text can never produce a binary value: is_binary() is always false\n    std::cout << d_array.root().is_binary() << '\\n';\n\n    // a default-constructed view, and the root of a document that failed to\n    // parse without exceptions, are both discarded\n    nlohmann::json_view invalid;\n    json_document failed = json_document::parse(\"not json\", /* allow_exceptions */ false);\n    std::cout << invalid.is_discarded() << '\\n';\n    std::cout << failed.root().is_discarded() << '\\n';\n\n    // type() returns the same value_t enumeration as basic_json::type()\n    std::cout << (d_object.root().type() == nlohmann::json::value_t::object) << '\\n';\n}\n

    Output:

    true\ntrue\ntrue true\ntrue\ntrue\ntrue\ntrue true\ntrue false\nfalse\ntrue\ntrue\ntrue\n
    "},{"location":"api/basic_json_view/is_null/#see-also","title":"See also","text":"
    • type - return the type of the value
    • BasicJsonType::is_null - the corresponding function of basic_json
    "},{"location":"api/basic_json_view/is_null/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_view/is_number/","title":"nlohmann::basic_json_view::is_number","text":"
    bool is_number() const noexcept;\n

    This function returns true if and only if the value is a number, i.e. an integer, unsigned integer, or floating-point value. It is defined as is_number_integer() || is_number_float().

    "},{"location":"api/basic_json_view/is_number/#return-value","title":"Return value","text":"

    true if the type is a number, false otherwise.

    "},{"location":"api/basic_json_view/is_number/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this function never throws exceptions.

    "},{"location":"api/basic_json_view/is_number/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json_view/is_number/#examples","title":"Examples","text":"Example

    The example below classifies several parsed documents by the type of their root value, without materializing any of them into a BasicJsonType value.

    #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // several incoming messages, one json_document per message. type() and\n    // is_*() only look at the flat index built by parse(); no nlohmann::json\n    // tree exists yet, and none is built unless materialize() is called\n    json_document d_null = json_document::parse(\"null\");\n    json_document d_bool = json_document::parse(\"true\");\n    json_document d_int = json_document::parse(\"-42\");\n    json_document d_unsigned = json_document::parse(\"42\");\n    json_document d_float = json_document::parse(\"4.2\");\n    json_document d_string = json_document::parse(R\"(\"hi\")\");\n    json_document d_array = json_document::parse(\"[1, 2, 3]\");\n    json_document d_object = json_document::parse(R\"({\"a\": 1})\");\n\n    std::cout << d_null.root().is_null() << '\\n';\n    std::cout << d_bool.root().is_boolean() << '\\n';\n    std::cout << d_int.root().is_number() << ' ' << d_int.root().is_number_integer() << '\\n';\n    std::cout << d_unsigned.root().is_number_unsigned() << '\\n';\n    std::cout << d_float.root().is_number_float() << '\\n';\n    std::cout << d_string.root().is_string() << '\\n';\n    std::cout << d_array.root().is_array() << ' ' << d_array.root().is_structured() << '\\n';\n    std::cout << d_object.root().is_object() << ' ' << d_object.root().is_primitive() << '\\n';\n\n    // JSON text can never produce a binary value: is_binary() is always false\n    std::cout << d_array.root().is_binary() << '\\n';\n\n    // a default-constructed view, and the root of a document that failed to\n    // parse without exceptions, are both discarded\n    nlohmann::json_view invalid;\n    json_document failed = json_document::parse(\"not json\", /* allow_exceptions */ false);\n    std::cout << invalid.is_discarded() << '\\n';\n    std::cout << failed.root().is_discarded() << '\\n';\n\n    // type() returns the same value_t enumeration as basic_json::type()\n    std::cout << (d_object.root().type() == nlohmann::json::value_t::object) << '\\n';\n}\n

    Output:

    true\ntrue\ntrue true\ntrue\ntrue\ntrue\ntrue true\ntrue false\nfalse\ntrue\ntrue\ntrue\n
    "},{"location":"api/basic_json_view/is_number/#see-also","title":"See also","text":"
    • is_number_integer - return whether the value is an integer or unsigned integer number
    • is_number_unsigned - return whether the value is an unsigned integer number
    • is_number_float - return whether the value is a floating-point number
    • BasicJsonType::is_number - the corresponding function of basic_json
    "},{"location":"api/basic_json_view/is_number/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_view/is_number_float/","title":"nlohmann::basic_json_view::is_number_float","text":"
    bool is_number_float() const noexcept;\n

    This function returns true if and only if the value is a floating-point number.

    "},{"location":"api/basic_json_view/is_number_float/#return-value","title":"Return value","text":"

    true if the type is value_t::number_float, false otherwise.

    "},{"location":"api/basic_json_view/is_number_float/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this function never throws exceptions.

    "},{"location":"api/basic_json_view/is_number_float/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json_view/is_number_float/#notes","title":"Notes","text":"

    As for BasicJsonType::parse, an integer literal that does not fit into the 64-bit integer type is classified as a floating-point number, so is_number_float() can be true even for an integer-looking token in the source text.

    "},{"location":"api/basic_json_view/is_number_float/#examples","title":"Examples","text":"Example

    The example below classifies several parsed documents by the type of their root value, without materializing any of them into a BasicJsonType value.

    #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // several incoming messages, one json_document per message. type() and\n    // is_*() only look at the flat index built by parse(); no nlohmann::json\n    // tree exists yet, and none is built unless materialize() is called\n    json_document d_null = json_document::parse(\"null\");\n    json_document d_bool = json_document::parse(\"true\");\n    json_document d_int = json_document::parse(\"-42\");\n    json_document d_unsigned = json_document::parse(\"42\");\n    json_document d_float = json_document::parse(\"4.2\");\n    json_document d_string = json_document::parse(R\"(\"hi\")\");\n    json_document d_array = json_document::parse(\"[1, 2, 3]\");\n    json_document d_object = json_document::parse(R\"({\"a\": 1})\");\n\n    std::cout << d_null.root().is_null() << '\\n';\n    std::cout << d_bool.root().is_boolean() << '\\n';\n    std::cout << d_int.root().is_number() << ' ' << d_int.root().is_number_integer() << '\\n';\n    std::cout << d_unsigned.root().is_number_unsigned() << '\\n';\n    std::cout << d_float.root().is_number_float() << '\\n';\n    std::cout << d_string.root().is_string() << '\\n';\n    std::cout << d_array.root().is_array() << ' ' << d_array.root().is_structured() << '\\n';\n    std::cout << d_object.root().is_object() << ' ' << d_object.root().is_primitive() << '\\n';\n\n    // JSON text can never produce a binary value: is_binary() is always false\n    std::cout << d_array.root().is_binary() << '\\n';\n\n    // a default-constructed view, and the root of a document that failed to\n    // parse without exceptions, are both discarded\n    nlohmann::json_view invalid;\n    json_document failed = json_document::parse(\"not json\", /* allow_exceptions */ false);\n    std::cout << invalid.is_discarded() << '\\n';\n    std::cout << failed.root().is_discarded() << '\\n';\n\n    // type() returns the same value_t enumeration as basic_json::type()\n    std::cout << (d_object.root().type() == nlohmann::json::value_t::object) << '\\n';\n}\n

    Output:

    true\ntrue\ntrue true\ntrue\ntrue\ntrue\ntrue true\ntrue false\nfalse\ntrue\ntrue\ntrue\n
    "},{"location":"api/basic_json_view/is_number_float/#see-also","title":"See also","text":"
    • is_number - return whether the value is a number
    • BasicJsonType::is_number_float - the corresponding function of basic_json
    "},{"location":"api/basic_json_view/is_number_float/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_view/is_number_integer/","title":"nlohmann::basic_json_view::is_number_integer","text":"
    bool is_number_integer() const noexcept;\n

    This function returns true if and only if the value is an integer or unsigned integer number.

    "},{"location":"api/basic_json_view/is_number_integer/#return-value","title":"Return value","text":"

    true if the type is value_t::number_integer or value_t::number_unsigned, false otherwise.

    "},{"location":"api/basic_json_view/is_number_integer/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this function never throws exceptions.

    "},{"location":"api/basic_json_view/is_number_integer/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json_view/is_number_integer/#notes","title":"Notes","text":"

    As for BasicJsonType::is_number_integer, this includes unsigned integer values; use is_number_unsigned to test for those specifically.

    "},{"location":"api/basic_json_view/is_number_integer/#examples","title":"Examples","text":"Example

    The example below classifies several parsed documents by the type of their root value, without materializing any of them into a BasicJsonType value.

    #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // several incoming messages, one json_document per message. type() and\n    // is_*() only look at the flat index built by parse(); no nlohmann::json\n    // tree exists yet, and none is built unless materialize() is called\n    json_document d_null = json_document::parse(\"null\");\n    json_document d_bool = json_document::parse(\"true\");\n    json_document d_int = json_document::parse(\"-42\");\n    json_document d_unsigned = json_document::parse(\"42\");\n    json_document d_float = json_document::parse(\"4.2\");\n    json_document d_string = json_document::parse(R\"(\"hi\")\");\n    json_document d_array = json_document::parse(\"[1, 2, 3]\");\n    json_document d_object = json_document::parse(R\"({\"a\": 1})\");\n\n    std::cout << d_null.root().is_null() << '\\n';\n    std::cout << d_bool.root().is_boolean() << '\\n';\n    std::cout << d_int.root().is_number() << ' ' << d_int.root().is_number_integer() << '\\n';\n    std::cout << d_unsigned.root().is_number_unsigned() << '\\n';\n    std::cout << d_float.root().is_number_float() << '\\n';\n    std::cout << d_string.root().is_string() << '\\n';\n    std::cout << d_array.root().is_array() << ' ' << d_array.root().is_structured() << '\\n';\n    std::cout << d_object.root().is_object() << ' ' << d_object.root().is_primitive() << '\\n';\n\n    // JSON text can never produce a binary value: is_binary() is always false\n    std::cout << d_array.root().is_binary() << '\\n';\n\n    // a default-constructed view, and the root of a document that failed to\n    // parse without exceptions, are both discarded\n    nlohmann::json_view invalid;\n    json_document failed = json_document::parse(\"not json\", /* allow_exceptions */ false);\n    std::cout << invalid.is_discarded() << '\\n';\n    std::cout << failed.root().is_discarded() << '\\n';\n\n    // type() returns the same value_t enumeration as basic_json::type()\n    std::cout << (d_object.root().type() == nlohmann::json::value_t::object) << '\\n';\n}\n

    Output:

    true\ntrue\ntrue true\ntrue\ntrue\ntrue\ntrue true\ntrue false\nfalse\ntrue\ntrue\ntrue\n
    "},{"location":"api/basic_json_view/is_number_integer/#see-also","title":"See also","text":"
    • is_number - return whether the value is a number
    • is_number_unsigned - return whether the value is an unsigned integer number
    • BasicJsonType::is_number_integer - the corresponding function of basic_json
    "},{"location":"api/basic_json_view/is_number_integer/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_view/is_number_unsigned/","title":"nlohmann::basic_json_view::is_number_unsigned","text":"
    bool is_number_unsigned() const noexcept;\n

    This function returns true if and only if the value is an unsigned integer number.

    "},{"location":"api/basic_json_view/is_number_unsigned/#return-value","title":"Return value","text":"

    true if the type is value_t::number_unsigned, false otherwise.

    "},{"location":"api/basic_json_view/is_number_unsigned/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this function never throws exceptions.

    "},{"location":"api/basic_json_view/is_number_unsigned/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json_view/is_number_unsigned/#examples","title":"Examples","text":"Example

    The example below classifies several parsed documents by the type of their root value, without materializing any of them into a BasicJsonType value.

    #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // several incoming messages, one json_document per message. type() and\n    // is_*() only look at the flat index built by parse(); no nlohmann::json\n    // tree exists yet, and none is built unless materialize() is called\n    json_document d_null = json_document::parse(\"null\");\n    json_document d_bool = json_document::parse(\"true\");\n    json_document d_int = json_document::parse(\"-42\");\n    json_document d_unsigned = json_document::parse(\"42\");\n    json_document d_float = json_document::parse(\"4.2\");\n    json_document d_string = json_document::parse(R\"(\"hi\")\");\n    json_document d_array = json_document::parse(\"[1, 2, 3]\");\n    json_document d_object = json_document::parse(R\"({\"a\": 1})\");\n\n    std::cout << d_null.root().is_null() << '\\n';\n    std::cout << d_bool.root().is_boolean() << '\\n';\n    std::cout << d_int.root().is_number() << ' ' << d_int.root().is_number_integer() << '\\n';\n    std::cout << d_unsigned.root().is_number_unsigned() << '\\n';\n    std::cout << d_float.root().is_number_float() << '\\n';\n    std::cout << d_string.root().is_string() << '\\n';\n    std::cout << d_array.root().is_array() << ' ' << d_array.root().is_structured() << '\\n';\n    std::cout << d_object.root().is_object() << ' ' << d_object.root().is_primitive() << '\\n';\n\n    // JSON text can never produce a binary value: is_binary() is always false\n    std::cout << d_array.root().is_binary() << '\\n';\n\n    // a default-constructed view, and the root of a document that failed to\n    // parse without exceptions, are both discarded\n    nlohmann::json_view invalid;\n    json_document failed = json_document::parse(\"not json\", /* allow_exceptions */ false);\n    std::cout << invalid.is_discarded() << '\\n';\n    std::cout << failed.root().is_discarded() << '\\n';\n\n    // type() returns the same value_t enumeration as basic_json::type()\n    std::cout << (d_object.root().type() == nlohmann::json::value_t::object) << '\\n';\n}\n

    Output:

    true\ntrue\ntrue true\ntrue\ntrue\ntrue\ntrue true\ntrue false\nfalse\ntrue\ntrue\ntrue\n
    "},{"location":"api/basic_json_view/is_number_unsigned/#see-also","title":"See also","text":"
    • is_number_integer - return whether the value is an integer or unsigned integer number
    • BasicJsonType::is_number_unsigned - the corresponding function of basic_json
    "},{"location":"api/basic_json_view/is_number_unsigned/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_view/is_object/","title":"nlohmann::basic_json_view::is_object","text":"
    bool is_object() const noexcept;\n

    This function returns true if and only if the value is an object.

    "},{"location":"api/basic_json_view/is_object/#return-value","title":"Return value","text":"

    true if the type is an object, false otherwise.

    "},{"location":"api/basic_json_view/is_object/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this function never throws exceptions.

    "},{"location":"api/basic_json_view/is_object/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json_view/is_object/#examples","title":"Examples","text":"Example

    The example below classifies several parsed documents by the type of their root value, without materializing any of them into a BasicJsonType value.

    #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // several incoming messages, one json_document per message. type() and\n    // is_*() only look at the flat index built by parse(); no nlohmann::json\n    // tree exists yet, and none is built unless materialize() is called\n    json_document d_null = json_document::parse(\"null\");\n    json_document d_bool = json_document::parse(\"true\");\n    json_document d_int = json_document::parse(\"-42\");\n    json_document d_unsigned = json_document::parse(\"42\");\n    json_document d_float = json_document::parse(\"4.2\");\n    json_document d_string = json_document::parse(R\"(\"hi\")\");\n    json_document d_array = json_document::parse(\"[1, 2, 3]\");\n    json_document d_object = json_document::parse(R\"({\"a\": 1})\");\n\n    std::cout << d_null.root().is_null() << '\\n';\n    std::cout << d_bool.root().is_boolean() << '\\n';\n    std::cout << d_int.root().is_number() << ' ' << d_int.root().is_number_integer() << '\\n';\n    std::cout << d_unsigned.root().is_number_unsigned() << '\\n';\n    std::cout << d_float.root().is_number_float() << '\\n';\n    std::cout << d_string.root().is_string() << '\\n';\n    std::cout << d_array.root().is_array() << ' ' << d_array.root().is_structured() << '\\n';\n    std::cout << d_object.root().is_object() << ' ' << d_object.root().is_primitive() << '\\n';\n\n    // JSON text can never produce a binary value: is_binary() is always false\n    std::cout << d_array.root().is_binary() << '\\n';\n\n    // a default-constructed view, and the root of a document that failed to\n    // parse without exceptions, are both discarded\n    nlohmann::json_view invalid;\n    json_document failed = json_document::parse(\"not json\", /* allow_exceptions */ false);\n    std::cout << invalid.is_discarded() << '\\n';\n    std::cout << failed.root().is_discarded() << '\\n';\n\n    // type() returns the same value_t enumeration as basic_json::type()\n    std::cout << (d_object.root().type() == nlohmann::json::value_t::object) << '\\n';\n}\n

    Output:

    true\ntrue\ntrue true\ntrue\ntrue\ntrue\ntrue true\ntrue false\nfalse\ntrue\ntrue\ntrue\n
    "},{"location":"api/basic_json_view/is_object/#see-also","title":"See also","text":"
    • is_structured - return whether the type is structured
    • size - the number of elements
    • empty - whether there are no elements
    • BasicJsonType::is_object - the corresponding function of basic_json
    "},{"location":"api/basic_json_view/is_object/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_view/is_primitive/","title":"nlohmann::basic_json_view::is_primitive","text":"
    bool is_primitive() const noexcept;\n

    This function returns true if and only if the value is primitive, i.e. null, a boolean, a number, or a string. It is defined as is_null() || is_string() || is_boolean() || is_number().

    "},{"location":"api/basic_json_view/is_primitive/#return-value","title":"Return value","text":"

    true if the type is primitive, false otherwise.

    "},{"location":"api/basic_json_view/is_primitive/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this function never throws exceptions.

    "},{"location":"api/basic_json_view/is_primitive/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json_view/is_primitive/#examples","title":"Examples","text":"Example

    The example below classifies several parsed documents by the type of their root value, without materializing any of them into a BasicJsonType value.

    #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // several incoming messages, one json_document per message. type() and\n    // is_*() only look at the flat index built by parse(); no nlohmann::json\n    // tree exists yet, and none is built unless materialize() is called\n    json_document d_null = json_document::parse(\"null\");\n    json_document d_bool = json_document::parse(\"true\");\n    json_document d_int = json_document::parse(\"-42\");\n    json_document d_unsigned = json_document::parse(\"42\");\n    json_document d_float = json_document::parse(\"4.2\");\n    json_document d_string = json_document::parse(R\"(\"hi\")\");\n    json_document d_array = json_document::parse(\"[1, 2, 3]\");\n    json_document d_object = json_document::parse(R\"({\"a\": 1})\");\n\n    std::cout << d_null.root().is_null() << '\\n';\n    std::cout << d_bool.root().is_boolean() << '\\n';\n    std::cout << d_int.root().is_number() << ' ' << d_int.root().is_number_integer() << '\\n';\n    std::cout << d_unsigned.root().is_number_unsigned() << '\\n';\n    std::cout << d_float.root().is_number_float() << '\\n';\n    std::cout << d_string.root().is_string() << '\\n';\n    std::cout << d_array.root().is_array() << ' ' << d_array.root().is_structured() << '\\n';\n    std::cout << d_object.root().is_object() << ' ' << d_object.root().is_primitive() << '\\n';\n\n    // JSON text can never produce a binary value: is_binary() is always false\n    std::cout << d_array.root().is_binary() << '\\n';\n\n    // a default-constructed view, and the root of a document that failed to\n    // parse without exceptions, are both discarded\n    nlohmann::json_view invalid;\n    json_document failed = json_document::parse(\"not json\", /* allow_exceptions */ false);\n    std::cout << invalid.is_discarded() << '\\n';\n    std::cout << failed.root().is_discarded() << '\\n';\n\n    // type() returns the same value_t enumeration as basic_json::type()\n    std::cout << (d_object.root().type() == nlohmann::json::value_t::object) << '\\n';\n}\n

    Output:

    true\ntrue\ntrue true\ntrue\ntrue\ntrue\ntrue true\ntrue false\nfalse\ntrue\ntrue\ntrue\n
    "},{"location":"api/basic_json_view/is_primitive/#see-also","title":"See also","text":"
    • is_structured - return whether the type is structured (the complement of this function, for a non-discarded view)
    • BasicJsonType::is_primitive - the corresponding function of basic_json
    "},{"location":"api/basic_json_view/is_primitive/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_view/is_string/","title":"nlohmann::basic_json_view::is_string","text":"
    bool is_string() const noexcept;\n

    This function returns true if and only if the value is a string.

    "},{"location":"api/basic_json_view/is_string/#return-value","title":"Return value","text":"

    true if the type is a string, false otherwise.

    "},{"location":"api/basic_json_view/is_string/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this function never throws exceptions.

    "},{"location":"api/basic_json_view/is_string/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json_view/is_string/#examples","title":"Examples","text":"Example

    The example below classifies several parsed documents by the type of their root value, without materializing any of them into a BasicJsonType value.

    #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // several incoming messages, one json_document per message. type() and\n    // is_*() only look at the flat index built by parse(); no nlohmann::json\n    // tree exists yet, and none is built unless materialize() is called\n    json_document d_null = json_document::parse(\"null\");\n    json_document d_bool = json_document::parse(\"true\");\n    json_document d_int = json_document::parse(\"-42\");\n    json_document d_unsigned = json_document::parse(\"42\");\n    json_document d_float = json_document::parse(\"4.2\");\n    json_document d_string = json_document::parse(R\"(\"hi\")\");\n    json_document d_array = json_document::parse(\"[1, 2, 3]\");\n    json_document d_object = json_document::parse(R\"({\"a\": 1})\");\n\n    std::cout << d_null.root().is_null() << '\\n';\n    std::cout << d_bool.root().is_boolean() << '\\n';\n    std::cout << d_int.root().is_number() << ' ' << d_int.root().is_number_integer() << '\\n';\n    std::cout << d_unsigned.root().is_number_unsigned() << '\\n';\n    std::cout << d_float.root().is_number_float() << '\\n';\n    std::cout << d_string.root().is_string() << '\\n';\n    std::cout << d_array.root().is_array() << ' ' << d_array.root().is_structured() << '\\n';\n    std::cout << d_object.root().is_object() << ' ' << d_object.root().is_primitive() << '\\n';\n\n    // JSON text can never produce a binary value: is_binary() is always false\n    std::cout << d_array.root().is_binary() << '\\n';\n\n    // a default-constructed view, and the root of a document that failed to\n    // parse without exceptions, are both discarded\n    nlohmann::json_view invalid;\n    json_document failed = json_document::parse(\"not json\", /* allow_exceptions */ false);\n    std::cout << invalid.is_discarded() << '\\n';\n    std::cout << failed.root().is_discarded() << '\\n';\n\n    // type() returns the same value_t enumeration as basic_json::type()\n    std::cout << (d_object.root().type() == nlohmann::json::value_t::object) << '\\n';\n}\n

    Output:

    true\ntrue\ntrue true\ntrue\ntrue\ntrue\ntrue true\ntrue false\nfalse\ntrue\ntrue\ntrue\n
    "},{"location":"api/basic_json_view/is_string/#see-also","title":"See also","text":"
    • source_offset - byte offset of this value in the source text, which differs for strings: an escaped string has none
    • BasicJsonType::is_string - the corresponding function of basic_json
    "},{"location":"api/basic_json_view/is_string/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_view/is_structured/","title":"nlohmann::basic_json_view::is_structured","text":"
    bool is_structured() const noexcept;\n

    This function returns true if and only if the value is structured, i.e. an array or an object. It is defined as is_array() || is_object().

    "},{"location":"api/basic_json_view/is_structured/#return-value","title":"Return value","text":"

    true if the type is structured, false otherwise.

    "},{"location":"api/basic_json_view/is_structured/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this function never throws exceptions.

    "},{"location":"api/basic_json_view/is_structured/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json_view/is_structured/#examples","title":"Examples","text":"Example

    The example below classifies several parsed documents by the type of their root value, without materializing any of them into a BasicJsonType value.

    #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // several incoming messages, one json_document per message. type() and\n    // is_*() only look at the flat index built by parse(); no nlohmann::json\n    // tree exists yet, and none is built unless materialize() is called\n    json_document d_null = json_document::parse(\"null\");\n    json_document d_bool = json_document::parse(\"true\");\n    json_document d_int = json_document::parse(\"-42\");\n    json_document d_unsigned = json_document::parse(\"42\");\n    json_document d_float = json_document::parse(\"4.2\");\n    json_document d_string = json_document::parse(R\"(\"hi\")\");\n    json_document d_array = json_document::parse(\"[1, 2, 3]\");\n    json_document d_object = json_document::parse(R\"({\"a\": 1})\");\n\n    std::cout << d_null.root().is_null() << '\\n';\n    std::cout << d_bool.root().is_boolean() << '\\n';\n    std::cout << d_int.root().is_number() << ' ' << d_int.root().is_number_integer() << '\\n';\n    std::cout << d_unsigned.root().is_number_unsigned() << '\\n';\n    std::cout << d_float.root().is_number_float() << '\\n';\n    std::cout << d_string.root().is_string() << '\\n';\n    std::cout << d_array.root().is_array() << ' ' << d_array.root().is_structured() << '\\n';\n    std::cout << d_object.root().is_object() << ' ' << d_object.root().is_primitive() << '\\n';\n\n    // JSON text can never produce a binary value: is_binary() is always false\n    std::cout << d_array.root().is_binary() << '\\n';\n\n    // a default-constructed view, and the root of a document that failed to\n    // parse without exceptions, are both discarded\n    nlohmann::json_view invalid;\n    json_document failed = json_document::parse(\"not json\", /* allow_exceptions */ false);\n    std::cout << invalid.is_discarded() << '\\n';\n    std::cout << failed.root().is_discarded() << '\\n';\n\n    // type() returns the same value_t enumeration as basic_json::type()\n    std::cout << (d_object.root().type() == nlohmann::json::value_t::object) << '\\n';\n}\n

    Output:

    true\ntrue\ntrue true\ntrue\ntrue\ntrue\ntrue true\ntrue false\nfalse\ntrue\ntrue\ntrue\n
    "},{"location":"api/basic_json_view/is_structured/#see-also","title":"See also","text":"
    • is_primitive - return whether the type is primitive
    • size, empty - the number of elements, and whether there are none
    • BasicJsonType::is_structured - the corresponding function of basic_json
    "},{"location":"api/basic_json_view/is_structured/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_view/items/","title":"nlohmann::basic_json_view::items","text":"
    /* unspecified */ items() const noexcept;\n

    Returns a range of items -- (key, value) pairs -- for use in range-based for loops. An item's key() is a string_view_t (for an array element, its index converted to a string, as for BasicJsonType::items()), and its value() is a basic_json_view of the element.

    The returned type is not part of the public API and may change between versions; use a range-based for loop (see the example), or decltype(v.items()) if you need to name it.

    for (const auto& item : v.items())\n{\n    std::cout << \"key: \" << item.key() << \", value: \" << item.value() << '\\n';\n}\n

    On C++17, item also supports structured bindings:

    for (const auto [key, value] : v.items())\n{\n    std::cout << \"key: \" << key << \", value: \" << value << '\\n';\n}\n

    Note the const auto (by value), not const auto&: unlike BasicJsonType::items(), whose elements are references into an existing object, a view's item is produced on the fly for each step of the iteration, so there is nothing for a reference to bind to.

    "},{"location":"api/basic_json_view/items/#return-value","title":"Return value","text":"

    A range whose iterators dereference to an item (with key() and value(), see above) and whose begin()/end() are equivalent to basic_json_view::begin()/end(), in document order.

    "},{"location":"api/basic_json_view/items/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this function never throws exceptions.

    "},{"location":"api/basic_json_view/items/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json_view/items/#notes","title":"Notes","text":"

    As for begin()/end(), items() visits every member of an object, including all occurrences of a duplicate key -- unlike operator[], at, find, and contains, which resolve to the first member with a given key. See the Notes on duplicate keys of operator[].

    Lifetime issues

    Unlike BasicJsonType::items() (see #2040), calling items() on a temporary view is safe, for instance for (auto item : v[\"a\"].items()): the returned range holds a copy of the view. What must outlive the loop is the document and, for a borrowed document, the source buffer.

    "},{"location":"api/basic_json_view/items/#examples","title":"Examples","text":"Example

    The example below shows a settings object whose source text records every update to a key as a duplicate member, in the order they happened. items() walks all of them, so the update history is visible, while operator[] only ever sees the first one and materialize() -- like BasicJsonType::parse() -- keeps only the last.

    #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    // a settings object whose source text records every update to a key as\n    // a duplicate member. items() visits all of them, in document order, so\n    // the update history is visible; operator[] only ever sees the first\n    // one, and materialize() -- like basic_json::parse() -- keeps the last\n    json_document updates = json_document::parse(R\"({\"retries\": 1, \"timeout\": 30, \"retries\": 5})\");\n    const auto settings = updates.root();\n\n    for (const auto& item : settings.items())\n    {\n        std::cout << item.key() << '=' << item.value().materialize().dump() << '\\n';\n    }\n\n    std::cout << \"first \\\"retries\\\" seen by operator[]: \" << settings[\"retries\"].materialize().dump() << '\\n';\n    std::cout << \"last \\\"retries\\\" kept by materialize(): \" << settings.materialize()[\"retries\"].dump() << '\\n';\n}\n

    Output:

    retries=1\ntimeout=30\nretries=5\nfirst \"retries\" seen by operator[]: 1\nlast \"retries\" kept by materialize(): 5\n
    "},{"location":"api/basic_json_view/items/#see-also","title":"See also","text":"
    • begin, end - the iterators items() is built on
    • BasicJsonType::items - the corresponding function of basic_json
    "},{"location":"api/basic_json_view/items/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_view/materialize/","title":"nlohmann::basic_json_view::materialize","text":"
    BasicJsonType materialize() const;\n

    Builds the BasicJsonType value of this subtree: the value BasicJsonType::parse() would have produced for the same source text, allocated for the first time by this call.

    "},{"location":"api/basic_json_view/materialize/#return-value","title":"Return value","text":"

    The BasicJsonType value of this subtree, or a discarded BasicJsonType value (BasicJsonType(value_t::discarded)) if the view is discarded.

    "},{"location":"api/basic_json_view/materialize/#exception-safety","title":"Exception safety","text":"

    Strong guarantee: there are no changes to the view or the document it refers to (nothing about either is mutated by this function).

    "},{"location":"api/basic_json_view/materialize/#exceptions","title":"Exceptions","text":"

    May throw std::bad_alloc (via BasicJsonType's allocator) if constructing the result fails.

    "},{"location":"api/basic_json_view/materialize/#complexity","title":"Complexity","text":"

    Linear in the size of the subtree.

    "},{"location":"api/basic_json_view/materialize/#notes","title":"Notes","text":"

    materialize() replays the subtree through the same SAX builder BasicJsonType::parse() uses, so the result matches it exactly -- including, for an object, that a repeated key keeps only its last value. The replay is iterative, so it is not limited by the call stack the way a naive recursive conversion would be; the JSON nesting depth is limited only by available memory, as for BasicJsonType::parse() itself.

    Unlike parsing with JSON_DIAGNOSTIC_POSITIONS enabled, the values produced by materialize() do not carry source positions. The index has the start offset of every value, but not the end of arrays, objects, and escaped strings, which the positions also need.

    Calling materialize() on the same view repeatedly builds a new, independent BasicJsonType value each time; it never caches the result.

    "},{"location":"api/basic_json_view/materialize/#examples","title":"Examples","text":"Example

    The example below skips messages that are not useful -- a discarded value, or an empty array -- using only is_array() and empty(), and calls materialize() only for the messages that are actually used, so no BasicJsonType value (and none of its per-element allocations) is ever built for the skipped ones.

    #include <array>\n#include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    // three incoming messages; skip the ones that are not useful without ever\n    // building a nlohmann::json value for them\n    json_document heartbeat = json_document::parse(\"null\");\n    json_document empty_batch = json_document::parse(\"[]\");\n    json_document batch = json_document::parse(R\"([{\"id\": 1}, {\"id\": 2}, {\"id\": 3}])\");\n    std::array<const json_document*, 3> messages = {{&heartbeat, &empty_batch, &batch}};\n\n    for (const json_document* d : messages)\n    {\n        // is_array()/empty() only look at the flat index: a discarded\n        // heartbeat or an empty batch is never turned into a nlohmann::json\n        // value, so no per-element allocation happens for them\n        if (!d->root().is_array() || d->root().empty())\n        {\n            std::cout << \"skipped\\n\";\n            continue;\n        }\n\n        // materialize() replays the subtree through the same SAX builder\n        // basic_json::parse() uses, so the result is exactly what\n        // basic_json::parse() would have produced for the same text\n        nlohmann::json value = d->root().materialize();\n        std::cout << value.dump() << '\\n';\n    }\n}\n

    Output:

    skipped\nskipped\n[{\"id\":1},{\"id\":2},{\"id\":3}]\n
    "},{"location":"api/basic_json_view/materialize/#see-also","title":"See also","text":"
    • root - the view of a document's root value
    • BasicJsonType::parse - build a BasicJsonType value directly from a JSON text
    • JSON_DIAGNOSTIC_POSITIONS - source positions on parsed values (not produced by materialize())
    "},{"location":"api/basic_json_view/materialize/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_view/number_format/","title":"nlohmann::basic_json_view::number_format","text":"
    enum class number_format {\n    shortest,\n    source\n};\n

    This enumeration is used in dump to choose how numbers are written. Two values are differentiated:

    shortest integers are copied from the source text -- already canonical in JSON -- except that -0 becomes 0, the way BasicJsonType::parse() reads it; floats are written with the library's shortest round-trip conversion, exactly as BasicJsonType::dump() would (e.g. 1.5, 100.0, 1e+100) source every number is copied exactly as it appears in the source text -- 1.50, 1E2, -0, all digits of an integer literal with more digits than any number type holds -- something BasicJsonType cannot do, since parsing already reduces every number to its parsed value"},{"location":"api/basic_json_view/number_format/#examples","title":"Examples","text":"Example

    The example below writes back a price list received from a supplier: with number_format::shortest (the default), a trailing zero and scientific notation are normalized away and a long account number that overflows every number type is rounded, the same way materialize().dump() (or basic_json::dump()) would; number_format::source keeps every number exactly as it was written in the source text instead.

    #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\nusing json_view = nlohmann::json_view;\n\nint main()\n{\n    // a price list received from a supplier feed -- prices and account\n    // numbers must be forwarded exactly, e.g. into an invoice\n    const json_document doc = json_document::parse(R\"(\n      [{\"sku\": \"A1\", \"price\": 19.90, \"account_id\": 12345678901234567890123456},\n       {\"sku\": \"A2\", \"price\": 1E2, \"account_id\": 98765432109876543210987654}]\n    )\");\n    const json_view list = doc.root();\n\n    // number_format::shortest (the default) writes numbers the way\n    // basic_json::dump() would: \"19.90\" becomes \"19.9\", \"1E2\" becomes\n    // \"100.0\", and each account number -- far beyond any 64-bit integer --\n    // is rounded to the nearest double, exactly as materialize().dump()\n    // (or a plain nlohmann::json) would round it\n    std::cout << list.dump() << '\\n';\n\n    // number_format::source copies every number exactly as it was written\n    // in the source text instead -- something basic_json cannot do at all,\n    // since parsing already reduces every number to its parsed value\n    std::cout << list.dump(-1, ' ', false, json_view::number_format::source) << '\\n';\n}\n

    Output:

    [{\"sku\":\"A1\",\"price\":19.9,\"account_id\":1.2345678901234568e+25},{\"sku\":\"A2\",\"price\":100.0,\"account_id\":9.876543210987655e+25}]\n[{\"sku\":\"A1\",\"price\":19.90,\"account_id\":12345678901234567890123456},{\"sku\":\"A2\",\"price\":1E2,\"account_id\":98765432109876543210987654}]\n
    "},{"location":"api/basic_json_view/number_format/#see-also","title":"See also","text":"
    • dump - serialize to a JSON-formatted string
    • number_token - get a single number's token text without dumping the whole value
    • BasicJsonType::error_handler_t - the analogous enumeration for BasicJsonType::dump's decoding-error behavior
    "},{"location":"api/basic_json_view/number_format/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_view/number_token/","title":"nlohmann::basic_json_view::number_token","text":"
    string_view_t number_token() const;\n

    Returns the number exactly as it appears in the source text, without parsing or rounding it.

    "},{"location":"api/basic_json_view/number_token/#return-value","title":"Return value","text":"

    The number's token text, as a string_view_t into the document's source() text -- for example \"1.50\", \"1E2\", \"-0\", or an integer literal with more digits than any number type holds (such as a 30-digit integer, which get<T>() and materialize() can only represent approximately, as a number_float_t).

    "},{"location":"api/basic_json_view/number_token/#exception-safety","title":"Exception safety","text":"

    Strong guarantee: there are no changes to the view or the document it refers to.

    "},{"location":"api/basic_json_view/number_token/#exceptions","title":"Exceptions","text":"

    Throws type_error.302 if the value is not a number; example: \"type must be number, but is string\".

    This exception does not carry a JSON_DIAGNOSTICS path: the view has no BasicJsonType value to point at, so the exception is created without one, even if BasicJsonType was built with JSON_DIAGNOSTICS enabled.

    "},{"location":"api/basic_json_view/number_token/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json_view/number_token/#notes","title":"Notes","text":"

    BasicJsonType has no counterpart to this function: once a number is parsed into number_integer_t, number_unsigned_t, or number_float_t, its original textual form (leading zeros aside, which are already rejected by the grammar; trailing zeros in the fraction; the case and sign of the exponent; ...) is gone. number_token() is useful precisely where that form must survive -- a price or an identifier that must be reproduced exactly, or a number too large for any of BasicJsonType's number types to hold without loss.

    The returned string_view_t always points into the document's source() text -- numbers are never decoded into the document's separate string buffer -- and is valid exactly as long as that text is, see the validity rules of basic_json_view.

    "},{"location":"api/basic_json_view/number_token/#examples","title":"Examples","text":"Example

    The example below keeps a price and an order ID exactly as they were written in an incoming order, where converting them with get<T>() would lose information: the price picks up floating-point rounding, and the order ID -- more digits than a 64-bit integer holds -- can only be approximated as a double once materialize()d.

    #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\nusing json_view = nlohmann::json_view;\n\nint main()\n{\n    // a price and an order id from an incoming order -- both need to be\n    // reproduced exactly, e.g. for an invoice or an audit log\n    json_document doc = json_document::parse(R\"(\n      {\"price\": 19.90, \"order_id\": 1234567890123456789012345, \"quantity\": 3}\n    )\");\n    const json_view order = doc.root();\n\n    // number_token() returns the number exactly as written in the source\n    std::cout << order[\"price\"].number_token() << '\\n';\n    std::cout << order[\"order_id\"].number_token() << '\\n';\n\n    // get<double>() converts it instead -- the exact source text is gone:\n    // \"19.90\" becomes the double closest to 19.9, printed without the\n    // trailing zero, and the 25-digit order id -- far beyond any 64-bit\n    // integer -- can only be approximated as a double\n    std::cout << order[\"price\"].get<double>() << '\\n';\n    std::cout << order.materialize()[\"order_id\"].dump() << '\\n';\n\n    // an ordinary quantity has nothing to lose either way\n    std::cout << order[\"quantity\"].number_token() << \" == \" << order[\"quantity\"].get<int>() << '\\n';\n}\n

    Output:

    19.90\n1234567890123456789012345\n19.9\n1.2345678901234568e+24\n3 == 3\n
    "},{"location":"api/basic_json_view/number_token/#see-also","title":"See also","text":"
    • get - convert the value to a given type
    • get_string - the string, without a copy
    • BasicJsonType::number_integer_t, number_unsigned_t, number_float_t - the number types get<T>() and materialize() convert into
    "},{"location":"api/basic_json_view/number_token/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_view/operator%5B%5D/","title":"nlohmann::basic_json_view::operator[]","text":"
    // (1)\nbasic_json_view operator[](string_view_t key) const;\nbasic_json_view operator[](const char* key) const;\nbasic_json_view operator[](const string_t& key) const;\n\n// (2)\nbasic_json_view operator[](size_type idx) const;\ntemplate<typename IntegerType>\nbasic_json_view operator[](IntegerType idx) const;\n\n// (3)\nbasic_json_view operator[](const json_pointer& ptr) const;\n
    1. Returns the value of the object member with key key -- the first one, should the key occur more than once (see the Notes below) -- or a discarded view if there is no such member.
    2. Returns the array element at index idx, or a discarded view if idx is out of range. The template accepts every integer type except bool and std::size_t (int, unsigned, long, std::int64_t, ...) and forwards to the size_type overload, so that an integer argument is not ambiguous between that overload and 1; a negative idx is out of range.
    3. Returns the value a JSON pointer ptr refers to, starting at this value, or a discarded view wherever resolving it further is not possible without inserting into or extending the document (see Return value and Exceptions below).
    "},{"location":"api/basic_json_view/operator%5B%5D/#parameters","title":"Parameters","text":"key (in) object key of the element to access idx (in) index of the element to access ptr (in) JSON pointer to the element to access"},{"location":"api/basic_json_view/operator%5B%5D/#return-value","title":"Return value","text":"
    1. the value of the first member with key key, or a discarded view if no member has this key (or if this view is discarded)
    2. the element at index idx, or a discarded view if idx >= size() or idx is negative (or if this view is discarded)
    3. the value ptr resolves to, starting at this value, or a discarded view (also if this view is discarded) for exactly the reference tokens where the const overload of BasicJsonType::operator[] invokes undefined behavior for the same pointer and the same document: an object member that does not exist, or an array index that is out of range
    "},{"location":"api/basic_json_view/operator%5B%5D/#exception-safety","title":"Exception safety","text":"

    Strong guarantee: there are no changes to the view or the document it refers to.

    "},{"location":"api/basic_json_view/operator%5B%5D/#exceptions","title":"Exceptions","text":"
    1. Throws type_error.305 if the value is not an object and not discarded -- the same exception, with the same message, that the const overload of BasicJsonType::operator[] throws for a string argument on a non-object value.
    2. Throws type_error.305 if the value is not an array and not discarded -- the same exception, with the same message, that the const overload of BasicJsonType::operator[] throws for a numeric argument on a non-array value.
    3. Throws the same exceptions, with the same messages, that the const overload of BasicJsonType::operator[] throws for the same pointer and the same document:
      • out_of_range.402 if a reference token is \"-\" at an array.
      • out_of_range.404 if a reference token cannot be resolved because it is used on a primitive value.
      • parse_error.106 if an array index in ptr begins with '0'.
      • parse_error.109 if an array index in ptr is not a number.

    None of these exceptions carry a JSON_DIAGNOSTICS path: the view has no BasicJsonType value to point at, so the exception is created without one, even if BasicJsonType was built with JSON_DIAGNOSTICS enabled.

    "},{"location":"api/basic_json_view/operator%5B%5D/#complexity","title":"Complexity","text":"
    1. Linear in the number of members: as for ordered_json, members are compared one after another, in document order, stopping at the first match. Each comparison first checks the key's length -- already known from the index, without reading the key bytes -- before comparing its content, so a key of a different length than key is rejected without touching the source text. Objects with 128 or more members get a hash index of their keys while parsing, so that a lookup in them takes constant time on average.
    2. Linear in idx: elements are skipped one at a time from the first one, since they are not a fixed size in the index (unlike BasicJsonType's array, which is random-access).
    3. Linear in the number of reference tokens of ptr and, for each token, in the number of members of the object at that level (as 1.) or the index into the array (as 2.).
    "},{"location":"api/basic_json_view/operator%5B%5D/#notes","title":"Notes","text":"

    Unlike BasicJsonType::operator[], which is undefined behavior (guarded by a runtime assertion) for a missing key on a const value, this operator returns a safe, testable result for a missing key or an index out of range: a discarded view, which is tested with is_discarded. There is also no non-const overload that inserts a missing key or extends an array -- a view never modifies the document.

    Chained access

    operator[] on a discarded view returns a discarded view and does not throw, so a chain like v[\"a\"][\"b\"][0] is safe even if \"a\" or \"b\" is missing: the first missing step makes the whole result discarded, which is tested once at the end. Type errors on values that are not discarded still throw: a key on an array or a primitive, or an index on an object or a primitive, is type_error.305 as for BasicJsonType. at still throws for a discarded view, as it does for a missing key.

    Duplicate keys: the first member wins

    If the source text has an object with a duplicate key, operator[] (and at, find, contains, value, and JSON pointer resolution) all resolve to the first member with that key, because a lookup can stop as soon as it finds a match. This is different from materialize() (and BasicJsonType::parse()), which replay every member in order and so keep the last value for a repeated key, so v[\"a\"] and v.materialize()[\"a\"] can differ. To get the value parse() would give, use materialize() or iterate the members with items() and keep the last match. The key hash index of a larger object (128 members or more) leads to the first member of a key as well. begin()/end() and items() iterate over all members, including duplicates, in document order. See Duplicate keys and size().

    JSON pointer resolution

    Overload 3 walks ptr one reference token at a time, starting at this value, the same way at and contains do. It only ever returns a discarded view where the const overload of BasicJsonType::operator[] would be undefined behavior for the same pointer -- a missing object member or an out-of-range array index -- and still throws for every other way ptr can fail to resolve. See at for the checked version, which throws in every case instead, and contains for a version that never throws.

    "},{"location":"api/basic_json_view/operator%5B%5D/#examples","title":"Examples","text":"Example: (1)/(2) access specified element

    The example below reads a couple of fields out of a batch of user records without ever materializing a full BasicJsonType value for the batch. operator[] is used both to look up an optional object member and to index into an array -- in both cases, a missing value comes back as a discarded view that can be tested with a plain if, instead of relying on undefined behavior.

    #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    // two user records from a large API response; only the fields that are\n    // actually read are ever touched, and no nlohmann::json tree is built\n    // for the batch\n    json_document batch = json_document::parse(R\"(\n      [\n        {\"name\": \"Alice\", \"email\": \"alice@example.com\", \"tags\": [\"admin\", \"ops\"]},\n        {\"name\": \"Bob\", \"tags\": []}\n      ]\n    )\");\n\n    const auto users = batch.root();\n    for (std::size_t i = 0; i < users.size(); ++i)\n    {\n        const auto user = users[i];\n        std::cout << user[\"name\"].materialize().dump();\n\n        // operator[] on a missing object key gives a discarded view -- test\n        // it with is_discarded(). The const overload of json::operator[]\n        // would instead be undefined behavior (guarded by an assertion) for\n        // a missing key\n        const auto email = user[\"email\"];\n        if (!email.is_discarded())\n        {\n            std::cout << \" <\" << email.materialize().dump() << \">\";\n        }\n\n        // the same holds for an array index past the end: a discarded view,\n        // not undefined behavior\n        const auto first_tag = user[\"tags\"][0];\n        if (!first_tag.is_discarded())\n        {\n            std::cout << \" #\" << first_tag.materialize().dump();\n        }\n\n        std::cout << '\\n';\n    }\n}\n

    Output:

    \"Alice\" <\"alice@example.com\"> #\"admin\"\n\"Bob\"\n
    Example: (3) access specified element via JSON pointer

    The example below reaches straight into one deeply nested field of a large document with a single JSON pointer, without ever building a tree for the rest of it, and shows the discarded-view and throwing outcomes of a pointer that cannot be fully resolved.

    #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\nusing json_pointer = nlohmann::json::json_pointer;\n\nint main()\n{\n    // a larger document; operator[] with a JSON pointer reaches straight to\n    // one deeply nested field, without ever building a tree for the rest\n    json_document doc = json_document::parse(R\"(\n      {\n        \"region\": {\n          \"servers\": [\n            {\"name\": \"eu-1\", \"metrics\": {\"cpu\": 0.42}},\n            {\"name\": \"eu-2\", \"metrics\": {\"cpu\": 0.71}}\n          ]\n        }\n      }\n    )\");\n\n    const auto root = doc.root();\n    std::cout << root[json_pointer(\"/region/servers/1/metrics/cpu\")].materialize().dump() << '\\n';\n\n    // a missing key or an out-of-range index along the path gives a\n    // discarded view, exactly where const json::operator[] would be\n    // undefined behavior for the same pointer\n    const auto missing = root[json_pointer(\"/region/servers/5/metrics/cpu\")];\n    if (!missing.is_discarded())\n    {\n        std::cout << missing.materialize().dump() << '\\n';\n    }\n    else\n    {\n        std::cout << \"no such server\\n\";\n    }\n\n    // indexing into a primitive still throws, as basic_json::operator[]\n    // does for the same pointer\n    try\n    {\n        static_cast<void>(root[json_pointer(\"/region/servers/0/name/x\")]);\n    }\n    catch (const nlohmann::json::out_of_range& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

    Output:

    0.71\nno such server\n[json.exception.out_of_range.404] unresolved reference token 'x'\n
    "},{"location":"api/basic_json_view/operator%5B%5D/#see-also","title":"See also","text":"
    • at - access specified element with bounds checking (throws instead of returning a discarded view)
    • front, back - access the first or last element
    • find, contains - look up a member without throwing
    • BasicJsonType::operator[] - the corresponding function of basic_json
    • json_pointer - JSON pointer type used by overload 3
    "},{"location":"api/basic_json_view/operator%5B%5D/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_view/operator_eq/","title":"nlohmann::basic_json_view::operator==","text":"
    // (1)\nbool operator==(const basic_json_view& lhs, const basic_json_view& rhs);\n\n// (2)\nbool operator==(const basic_json_view& lhs, const BasicJsonType& rhs);\nbool operator==(const BasicJsonType& lhs, const basic_json_view& rhs);\n
    1. Compares two views for equality: whether the values BasicJsonType::parse() would produce for lhs and rhs are equal, according to BasicJsonType's operator==.
    2. Compares a view and a BasicJsonType value for equality, in either order: whether the value parse() would produce for the view and the other operand are equal, according to BasicJsonType's operator==.

    Neither overload builds a BasicJsonType value for a view to do the comparison (see Notes below). Numbers compare by value across their types (1 == 1.0), and an object compares by its members, with duplicate keys resolved exactly as parse() resolves them -- the last value, at the position of the first occurrence of the key. This is not what operator[] and the other lookups return for a duplicate key: they find the first member (see Duplicate keys).

    "},{"location":"api/basic_json_view/operator_eq/#parameters","title":"Parameters","text":"lhs (in) first value to consider rhs (in) second value to consider"},{"location":"api/basic_json_view/operator_eq/#return-value","title":"Return value","text":"

    whether the values lhs and rhs are equal

    "},{"location":"api/basic_json_view/operator_eq/#exception-safety","title":"Exception safety","text":"

    Strong exception safety: if an exception is thrown, there are no changes to either operand, or to the document(s) a view refers to.

    "},{"location":"api/basic_json_view/operator_eq/#exceptions","title":"Exceptions","text":"

    May throw std::bad_alloc. Unlike the other comparison and most other basic_json_view functions, operator== is not noexcept: resolving an object's members needs a temporary array to sort them by key (see Complexity below), and that allocation can fail.

    "},{"location":"api/basic_json_view/operator_eq/#complexity","title":"Complexity","text":"

    Linear in the size of the compared values: every number, string, array element, and object member is visited at most once, and the walk is iterative, so the nesting depth it can compare is limited by available memory only, not by the call stack (as for materialize()). Resolving an object's members takes an additional O(n log n) in the number of members at that level, since they are sorted by key to detect and resolve duplicates before being compared. Two arrays of different size() are rejected without visiting either one's elements.

    "},{"location":"api/basic_json_view/operator_eq/#notes","title":"Notes","text":"

    Only a single number, boolean, or null value is ever materialized into a BasicJsonType, to reuse its operator== -- for numbers, so that values written differently in the source text but equal in value (e.g. an integer and a floating-point literal) still compare equal, following the same rules BasicJsonType does for special values such as NaN. Constructing one of these scalars never allocates. Strings are compared directly, without allocating, either from the source text on both sides or, for overload 2, against BasicJsonType's own string. Arrays and objects are never materialized at all; only their elements or members are visited, one pair at a time.

    How objects are compared

    For a json_view (BasicJsonType::object_t is std::map), members are compared by key, regardless of the order they appear in the source text. For an ordered_json_view (object_t is ordered_map), they are compared in the order they occur, so the very same two objects with their members reordered can compare equal as json_views but not as ordered_json_views. This is exactly how json and ordered_json compare, see \"Comparing different basic_json specializations\".

    Discarded views

    A discarded view compares the same way a discarded BasicJsonType value does, which is governed by JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON: by default, a discarded view is never equal to anything, not even another discarded view.

    No ordering comparison (operator<) is provided for basic_json_view; materialize() is the way to get a BasicJsonType value that supports it.

    "},{"location":"api/basic_json_view/operator_eq/#examples","title":"Examples","text":"Example

    The example below checks whether a newly received configuration differs from the previous one, and whether a received document matches what a test expects -- directly on views, without ever materializing a BasicJsonType value for either side.

    #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\nusing json = nlohmann::json;\n\nint main()\n{\n    // two snapshots of a polled configuration endpoint -- compare them\n    // directly as views, without ever building a nlohmann::json value for\n    // either one\n    const json_document previous = json_document::parse(\n                                       R\"({\"name\": \"cache\", \"port\": 6379, \"timeout\": 30})\");\n    const json_document current = json_document::parse(\n                                      R\"({\"port\": 6379.0, \"timeout\": 30, \"name\": \"cache\"})\");\n\n    // same members, reordered, and 6379 written as a float -- operator==\n    // treats them the same way BasicJsonType::operator== would\n    std::cout << std::boolalpha << (previous.root() == current.root()) << '\\n';\n\n    // an actually changed value is detected the same way\n    const json_document changed = json_document::parse(\n                                      R\"({\"name\": \"cache\", \"port\": 6380, \"timeout\": 30})\");\n    std::cout << (previous.root() == changed.root()) << '\\n';\n\n    // comparing a view directly against an expected json value -- handy in a\n    // test, without materializing the received document at all\n    const json expected = {{\"name\", \"cache\"}, {\"port\", 6379}, {\"timeout\", 30}};\n    std::cout << (previous.root() == expected) << '\\n';\n}\n

    Output:

    true\nfalse\ntrue\n
    "},{"location":"api/basic_json_view/operator_eq/#see-also","title":"See also","text":"
    • operator!= - compare for inequality
    • materialize - build a BasicJsonType value, e.g. to keep comparing after the document is gone
    • BasicJsonType::operator== - the corresponding function of basic_json
    "},{"location":"api/basic_json_view/operator_eq/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_view/operator_ltlt/","title":"nlohmann::basic_json_view::operator<<","text":"
    std::ostream& operator<<(std::ostream& o, const basic_json_view& v);\n

    Not available when JSON_NO_IO is defined.

    Serializes the given view v to the output stream o, using dump -- exactly as operator<<(std::ostream&, const basic_json&) does for a basic_json value.

    • The indentation of the output can be controlled with the member variable width of the output stream o. For instance, using the manipulator std::setw(4) on o sets the indentation level to 4, and the serialization result is the same as calling v.dump(4). A width of 0 or less (the default) selects the most compact representation, as v.dump(-1) does.
    • The indentation character can be controlled with the member variable fill of the output stream o. For instance, the manipulator std::setfill('\\t') sets indentation to use a tab character rather than the default space character.
    • As for basic_json, o's width is reset to 0 after this call, whether or not it was greater than 0 before.

    Numbers are always written as v.dump() writes them by default, i.e. as with number_format::shortest; there is no way to select number_format::source through the stream.

    "},{"location":"api/basic_json_view/operator_ltlt/#parameters","title":"Parameters","text":"o (in, out) stream to write to v (in) view to serialize"},{"location":"api/basic_json_view/operator_ltlt/#return-value","title":"Return value","text":"

    the stream o

    "},{"location":"api/basic_json_view/operator_ltlt/#exceptions","title":"Exceptions","text":"

    May throw std::bad_alloc, propagated from dump. Unlike operator<<(std::ostream&, const basic_json&), there is no UTF-8 decoding step that could throw type_error.316, and no error_handler to choose between -- see the Exceptions of dump.

    "},{"location":"api/basic_json_view/operator_ltlt/#complexity","title":"Complexity","text":"

    Linear, as dump.

    "},{"location":"api/basic_json_view/operator_ltlt/#examples","title":"Examples","text":"Example

    The example below writes one record out of a larger batch straight to a log stream -- compact for a one-line entry, and pretty-printed with std::setw/std::setfill for a readable dump -- without ever building a BasicJsonType value for the record, or for the rest of the batch.

    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\nusing json_view = nlohmann::json_view;\n\nint main()\n{\n    // one order out of a large incoming batch -- write it straight to a log\n    // stream without ever building a basic_json value for it, or for the\n    // rest of the batch\n    const json_document doc = json_document::parse(R\"(\n      [{\"id\": 1, \"item\": \"cable\"}, {\"id\": 2, \"item\": \"adapter\"}]\n    )\");\n    const json_view orders = doc.root();\n\n    // compact, for a one-line log entry\n    std::cout << orders[1] << '\\n';\n\n    // std::setw sets the indentation level, exactly as for basic_json\n    std::cout << std::setw(2) << orders[1] << \"\\n\\n\";\n\n    // std::setfill changes the indentation character\n    std::cout << std::setw(1) << std::setfill('\\t') << orders[1] << '\\n';\n}\n

    Output:

    {\"id\":2,\"item\":\"adapter\"}\n{\n  \"id\": 2,\n  \"item\": \"adapter\"\n}\n\n{\n    \"id\": 2,\n    \"item\": \"adapter\"\n}\n
    "},{"location":"api/basic_json_view/operator_ltlt/#see-also","title":"See also","text":"
    • dump - serialize to a JSON-formatted string
    • operator<<(std::ostream&) - the corresponding operator for basic_json
    • JSON_NO_IO - switch off functions relying on certain C++ I/O headers
    "},{"location":"api/basic_json_view/operator_ltlt/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_view/operator_ne/","title":"nlohmann::basic_json_view::operator!=","text":"
    // (1)\nbool operator!=(const basic_json_view& lhs, const basic_json_view& rhs);\n\n// (2)\nbool operator!=(const basic_json_view& lhs, const BasicJsonType& rhs);\nbool operator!=(const BasicJsonType& lhs, const basic_json_view& rhs);\n
    1. Compares two views for inequality. Returns !(lhs == rhs), see operator==.
    2. Compares a view and a BasicJsonType value for inequality, in either order. Returns !(lhs == rhs) (or, for the reversed order, !(rhs == lhs)), see operator==.

    Since operator!= is defined as the negation of operator==, it follows the same rules for special cases: for instance, since a discarded view is never equal to anything by default (see operator=='s Notes), it is never unequal to anything either -- discarded != discarded is also false, exactly as for a discarded BasicJsonType value.

    "},{"location":"api/basic_json_view/operator_ne/#parameters","title":"Parameters","text":"lhs (in) first value to consider rhs (in) second value to consider"},{"location":"api/basic_json_view/operator_ne/#return-value","title":"Return value","text":"

    whether the values lhs and rhs are not equal

    "},{"location":"api/basic_json_view/operator_ne/#exception-safety","title":"Exception safety","text":"

    Strong exception safety: if an exception is thrown, there are no changes to either operand, or to the document(s) a view refers to.

    "},{"location":"api/basic_json_view/operator_ne/#exceptions","title":"Exceptions","text":"

    May throw std::bad_alloc, propagated from operator==. Unlike most other basic_json_view functions, operator!= is not noexcept.

    "},{"location":"api/basic_json_view/operator_ne/#complexity","title":"Complexity","text":"

    Linear, as operator==.

    "},{"location":"api/basic_json_view/operator_ne/#notes","title":"Notes","text":"

    See the Notes of operator== -- in particular for how an object's members are compared (order matters for ordered_json_view but not for json_view) and for how discarded views compare.

    No ordering comparison (operator<) is provided for basic_json_view; materialize() is the way to get a BasicJsonType value that supports it.

    "},{"location":"api/basic_json_view/operator_ne/#examples","title":"Examples","text":"Example

    The example below asserts, as a test would, that a received document differs from an unwanted value, and shows that -- as for json/ordered_json -- reordering an object's members is detected as a difference for an ordered_json_view but not for a json_view.

    #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\nusing ordered_json_document = nlohmann::ordered_json_document;\nusing json = nlohmann::json;\n\nint main()\n{\n    // assert, as a test would, that a received document differs from an\n    // unwanted shape -- without ever materializing it into a json value just\n    // to compare\n    const json_document received = json_document::parse(\n                                       R\"({\"status\": \"ok\", \"code\": 200})\");\n    const json unwanted = {{\"status\", \"error\"}, {\"code\", 500}};\n    std::cout << std::boolalpha << (received.root() != unwanted) << '\\n';\n\n    // json (std::map) compares object members regardless of order ...\n    const json_document a = json_document::parse(R\"({\"a\": 1, \"b\": 2})\");\n    const json_document b = json_document::parse(R\"({\"b\": 2, \"a\": 1})\");\n    std::cout << (a.root() != b.root()) << '\\n';\n\n    // ... but ordered_json (ordered_map) compares them in the order they\n    // appear, so the very same reordering is detected as a difference\n    const ordered_json_document oa = ordered_json_document::parse(R\"({\"a\": 1, \"b\": 2})\");\n    const ordered_json_document ob = ordered_json_document::parse(R\"({\"b\": 2, \"a\": 1})\");\n    std::cout << (oa.root() != ob.root()) << '\\n';\n}\n

    Output:

    true\nfalse\ntrue\n
    "},{"location":"api/basic_json_view/operator_ne/#see-also","title":"See also","text":"
    • operator== - compare for equality
    • materialize - build a BasicJsonType value, e.g. to keep comparing after the document is gone
    • BasicJsonType::operator!= - the corresponding function of basic_json
    "},{"location":"api/basic_json_view/operator_ne/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_view/size/","title":"nlohmann::basic_json_view::size","text":"
    size_type size() const noexcept;\n

    Returns the number of elements, as BasicJsonType::size() would for the same value.

    "},{"location":"api/basic_json_view/size/#return-value","title":"Return value","text":"

    The return value depends on the type and is defined as follows:

    Value type return value null 0 discarded 0 boolean 1 string 1 number 1 object number of key/value pairs array number of elements"},{"location":"api/basic_json_view/size/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this function never throws exceptions.

    "},{"location":"api/basic_json_view/size/#complexity","title":"Complexity","text":"

    Constant: for an object or array, the element count is stored in the index, not counted on demand.

    "},{"location":"api/basic_json_view/size/#notes","title":"Notes","text":"

    As for BasicJsonType::size(), this does not return the length of a string value -- it is 1 for a string, regardless of its length.

    If the source text has an object with a duplicate key, every occurrence counts towards its size() -- unlike materialize() (and BasicJsonType::parse()), which keeps only the last value for a repeated key. This means v.size() can be larger than v.materialize().size(). See the Notes on duplicate keys of operator[] for why lookups and iteration disagree on how many members there are.

    "},{"location":"api/basic_json_view/size/#examples","title":"Examples","text":"Example

    The example below uses size() and empty() to decide whether a parsed message is worth acting on, without materializing it into a BasicJsonType value.

    #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // decide whether a batch is worth processing before building any\n    // nlohmann::json value for it\n    json_document batch = json_document::parse(R\"([1, 2, 3, 4, 5])\");\n    json_document empty_batch = json_document::parse(\"[]\");\n\n    std::cout << batch.root().empty() << ' ' << batch.root().size() << '\\n';\n    std::cout << empty_batch.root().empty() << ' ' << empty_batch.root().size() << '\\n';\n\n    // as for basic_json: null has size 0, every other scalar has size 1\n    json_document n = json_document::parse(\"null\");\n    json_document s = json_document::parse(R\"(\"hi\")\");\n    std::cout << n.root().size() << ' ' << s.root().size() << '\\n';\n}\n

    Output:

    false 5\ntrue 0\n0 1\n
    "},{"location":"api/basic_json_view/size/#see-also","title":"See also","text":"
    • empty - return whether the value has no elements
    • BasicJsonType::size - the corresponding function of basic_json
    "},{"location":"api/basic_json_view/size/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_view/source_offset/","title":"nlohmann::basic_json_view::source_offset","text":"
    std::size_t source_offset() const noexcept;\n

    Returns the byte offset of this value in the document's source() text, without materializing anything.

    "},{"location":"api/basic_json_view/source_offset/#return-value","title":"Return value","text":"
    • For a string with no escapes, a number, true/false/null, an array, or an object: the byte offset of the first byte of the value's token (for a string: the first byte after the opening quote) in source().
    • static_cast<std::size_t>(-1) for a discarded view, and for a string that contains escapes -- such a string was decoded once into the document's own buffer, so there is no single byte range in source() left to point at.
    "},{"location":"api/basic_json_view/source_offset/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this function never throws exceptions.

    "},{"location":"api/basic_json_view/source_offset/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json_view/source_offset/#notes","title":"Notes","text":"

    This is a raw offset, not a length: the API does not (yet) expose how many bytes the token occupies in the source text, so source_offset() alone is enough to report where a value came from (for an error message, for syntax highlighting, ...) but not to slice its exact text back out of source() for a string, whose token length is not the same as its decoded size().

    "},{"location":"api/basic_json_view/source_offset/#examples","title":"Examples","text":"Example 
    #include <iostream>\n#include <string>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // source_offset() points into source(): useful to report *where* in the\n    // original text a value came from (error messages, syntax highlighting,\n    // forwarding a sub-range verbatim, ...) without materializing it\n    json_document doc = json_document::parse(R\"(  123)\");\n    auto v = doc.root();\n    std::cout << v.source_offset() << ' '\n              << std::string(doc.source().data() + v.source_offset(), 3) << '\\n';\n\n    // a string without escapes also stays in the source text\n    json_document plain = json_document::parse(R\"(\"ab\")\");\n    std::cout << plain.source()[plain.root().source_offset()] << '\\n';\n\n    // a string with escapes is decoded once into the document's own buffer, so\n    // there is no single byte range in source() to point at: source_offset()\n    // returns the \"not applicable\" sentinel\n    json_document escaped = json_document::parse(R\"(\"a\\nb\")\");\n    std::cout << (escaped.root().source_offset() == static_cast<std::size_t>(-1)) << '\\n';\n\n    // a discarded view -- default-constructed, or the root of a failed parse\n    // with allow_exceptions == false -- has no offset either\n    json_document failed = json_document::parse(\"not json\", false);\n    std::cout << (failed.root().source_offset() == static_cast<std::size_t>(-1)) << '\\n';\n}\n

    Output:

    2 123\na\ntrue\ntrue\n
    "},{"location":"api/basic_json_view/source_offset/#see-also","title":"See also","text":"
    • source - the parsed text
    • is_string - return whether the value is a string
    "},{"location":"api/basic_json_view/source_offset/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_view/type/","title":"nlohmann::basic_json_view::type","text":"
    value_t type() const noexcept;\n

    Returns the type of the value this view refers to, as a value from the value_t enumeration -- the same enumeration BasicJsonType::type() uses.

    "},{"location":"api/basic_json_view/type/#return-value","title":"Return value","text":"

    The type of the value; value_t::discarded for a discarded view.

    "},{"location":"api/basic_json_view/type/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this function never throws exceptions.

    "},{"location":"api/basic_json_view/type/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json_view/type/#notes","title":"Notes","text":"

    Unlike BasicJsonType::type(), this function can never return value_t::binary: a JSON text has no binary values, so type() only distinguishes the eight ordinary JSON value types (plus discarded).

    "},{"location":"api/basic_json_view/type/#examples","title":"Examples","text":"Example

    The example below classifies several parsed documents by the type of their root value, without materializing any of them into a BasicJsonType value.

    #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // several incoming messages, one json_document per message. type() and\n    // is_*() only look at the flat index built by parse(); no nlohmann::json\n    // tree exists yet, and none is built unless materialize() is called\n    json_document d_null = json_document::parse(\"null\");\n    json_document d_bool = json_document::parse(\"true\");\n    json_document d_int = json_document::parse(\"-42\");\n    json_document d_unsigned = json_document::parse(\"42\");\n    json_document d_float = json_document::parse(\"4.2\");\n    json_document d_string = json_document::parse(R\"(\"hi\")\");\n    json_document d_array = json_document::parse(\"[1, 2, 3]\");\n    json_document d_object = json_document::parse(R\"({\"a\": 1})\");\n\n    std::cout << d_null.root().is_null() << '\\n';\n    std::cout << d_bool.root().is_boolean() << '\\n';\n    std::cout << d_int.root().is_number() << ' ' << d_int.root().is_number_integer() << '\\n';\n    std::cout << d_unsigned.root().is_number_unsigned() << '\\n';\n    std::cout << d_float.root().is_number_float() << '\\n';\n    std::cout << d_string.root().is_string() << '\\n';\n    std::cout << d_array.root().is_array() << ' ' << d_array.root().is_structured() << '\\n';\n    std::cout << d_object.root().is_object() << ' ' << d_object.root().is_primitive() << '\\n';\n\n    // JSON text can never produce a binary value: is_binary() is always false\n    std::cout << d_array.root().is_binary() << '\\n';\n\n    // a default-constructed view, and the root of a document that failed to\n    // parse without exceptions, are both discarded\n    nlohmann::json_view invalid;\n    json_document failed = json_document::parse(\"not json\", /* allow_exceptions */ false);\n    std::cout << invalid.is_discarded() << '\\n';\n    std::cout << failed.root().is_discarded() << '\\n';\n\n    // type() returns the same value_t enumeration as basic_json::type()\n    std::cout << (d_object.root().type() == nlohmann::json::value_t::object) << '\\n';\n}\n

    Output:

    true\ntrue\ntrue true\ntrue\ntrue\ntrue\ntrue true\ntrue false\nfalse\ntrue\ntrue\ntrue\n
    "},{"location":"api/basic_json_view/type/#see-also","title":"See also","text":"
    • is_null, is_boolean, is_number, is_string, is_array, is_object - type-specific predicates built on type()
    • BasicJsonType::type - the corresponding function of basic_json
    "},{"location":"api/basic_json_view/type/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_view/type_name/","title":"nlohmann::basic_json_view::type_name","text":"
    const char* type_name() const noexcept;\n

    Returns the type name as string to be used in error messages -- usually to indicate that a function was called on a wrong JSON type. Identical to BasicJsonType::type_name(), including the extra \"discarded\" return value for a discarded view (BasicJsonType::type_name() produces the same string for a discarded BasicJsonType value).

    "},{"location":"api/basic_json_view/type_name/#return-value","title":"Return value","text":"

    a string representation of the type (value_t):

    Value type return value null \"null\" boolean \"boolean\" string \"string\" number (integer, unsigned integer, floating-point) \"number\" object \"object\" array \"array\" discarded \"discarded\"

    type_name() never returns \"binary\", since a JSON text has no binary values (see is_binary()); it also never returns \"invalid\", since a view's kind always comes from a value the parser actually produced.

    "},{"location":"api/basic_json_view/type_name/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this function never throws exceptions.

    "},{"location":"api/basic_json_view/type_name/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/basic_json_view/type_name/#examples","title":"Examples","text":"Example

    The example below reports why some parsed messages were rejected, using only type_name() -- no BasicJsonType value is ever built for the ones that are wrong, and the message text matches what BasicJsonType::type_name() would produce for the same value.

    #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\nusing json_view = nlohmann::json_view;\n\nint main()\n{\n    // report why some parsed messages were rejected, using only\n    // type_name() -- no nlohmann::json value is built for the ones that\n    // are wrong\n    json_document good = json_document::parse(R\"({\"id\": 1})\");\n    json_document bad = json_document::parse(\"[1, 2, 3]\");\n    json_document failed = json_document::parse(\"not json\", /* allow_exceptions */ false);\n\n    for (const json_view v :\n            {\n                good.root(), bad.root(), failed.root()\n            })\n    {\n        if (v.is_object())\n        {\n            std::cout << \"ok\\n\";\n        }\n        else\n        {\n            std::cout << \"expected an object, got \" << v.type_name() << '\\n';\n        }\n    }\n}\n

    Output:

    ok\nexpected an object, got array\nexpected an object, got discarded\n
    "},{"location":"api/basic_json_view/type_name/#see-also","title":"See also","text":"
    • type - return the type of the value
    • BasicJsonType::type_name - the corresponding function of basic_json
    "},{"location":"api/basic_json_view/type_name/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/basic_json_view/value/","title":"nlohmann::basic_json_view::value","text":"
    // (1)\ntemplate<typename T>\nT value(string_view_t key, const T& default_value) const;\nstring_t value(string_view_t key, const char* default_value) const;\n\n// (2)\ntemplate<typename T>\nT value(const json_pointer& ptr, const T& default_value) const;\nstring_t value(const json_pointer& ptr, const char* default_value) const;\n
    1. Returns the value of the object member with key key -- the first one, should the key occur more than once (see Notes on duplicate keys) -- converted to T, or default_value if there is no such member.
    2. Returns the value a JSON pointer ptr refers to, starting at this value, converted to T, or default_value if ptr cannot be resolved.

    Both overloads have a dedicated const char* overload, so v.value(key, \"default\") (and the JSON pointer equivalent) deduce string_t, not const char*, for their return type and for the comparison used to pick between key and default_value.

    "},{"location":"api/basic_json_view/value/#template-parameters","title":"Template parameters","text":"T the type to convert the found value to; also the type of default_value"},{"location":"api/basic_json_view/value/#parameters","title":"Parameters","text":"key (in) object key of the element to access ptr (in) JSON pointer to the element to access default_value (in) the value to return if key/ptr resolves to no value"},{"location":"api/basic_json_view/value/#return-value","title":"Return value","text":"
    1. the first member with key key, converted to T, or default_value
    2. the value ptr resolves to, converted to T, or default_value
    "},{"location":"api/basic_json_view/value/#exception-safety","title":"Exception safety","text":"

    Strong guarantee: there are no changes to the view or the document it refers to.

    "},{"location":"api/basic_json_view/value/#exceptions","title":"Exceptions","text":"
    1. Throws type_error.306 if the value is not an object -- the same exception, with the same message, that BasicJsonType::value throws for the same call. If a member with key key is found, throws whatever converting it to T throws (typically type_error.302, with the same message BasicJsonType::value throws for the same mismatch); a missing member never throws.
    2. Throws type_error.306 if this value -- not the value ptr resolves to -- is neither an object nor an array. Throws parse_error.106 or parse_error.109 if ptr contains a malformed array index. If ptr resolves to a value, throws whatever converting it to T throws. Every other way ptr can fail to resolve -- a missing key, an out-of-range or \"-\" array index, an unresolvable token on a primitive -- yields default_value instead of throwing, exactly as BasicJsonType::value catches out_of_range and returns default_value.

    None of these exceptions carry a JSON_DIAGNOSTICS path: the view has no BasicJsonType value to point at, so the exception is created without one, even if BasicJsonType was built with JSON_DIAGNOSTICS enabled.

    "},{"location":"api/basic_json_view/value/#complexity","title":"Complexity","text":"
    1. Linear in the number of members: as for operator[], members are compared one after another, in document order, stopping at the first match. Plus the complexity of converting the found member to T (see get). Objects with 128 or more members get a hash index of their keys while parsing, so that a lookup in them takes constant time on average.
    2. Linear in the number of reference tokens of ptr and, for each token, in the number of members of the object at that level or the index into the array -- as for the operator[] and at overloads that take a JSON pointer. Plus the complexity of converting the resolved value to T.
    "},{"location":"api/basic_json_view/value/#notes","title":"Notes","text":"

    Duplicate keys: the first member wins

    If the source text repeats a key, this resolves to the first member with it, not to the last one that materialize() and parse() keep. See the Notes on duplicate keys of operator[] and Duplicate keys for the reasons and for how to get the last value.

    Differences to at and operator[]

    Unlike at, this function does not throw if key/ptr resolves to no value. Unlike operator[], it never returns a discarded view -- it always returns a T -- and it is available on any view, since it never needs to insert a missing element the way the non-const BasicJsonType::operator[] would.

    Which values can be asked

    As for BasicJsonType::value, the key overload (1) requires an object, and the JSON pointer overload (2) an object or an array.

    "},{"location":"api/basic_json_view/value/#examples","title":"Examples","text":"Example: (1) access specified object element with default value

    The example below reads a couple of optional configuration fields with a default, so that a missing key never needs a try/catch of its own.

    #include <iostream>\n#include <nlohmann/json_view.hpp>\n#include <string>\n\nusing json_document = nlohmann::json_document;\nusing json_view = nlohmann::json_view;\n\nint main()\n{\n    json_document doc = json_document::parse(R\"({\"server\": {\"host\": \"localhost\"}})\");\n    const json_view server = doc.root()[\"server\"];\n\n    // \"port\" is missing -- value() returns the default instead of\n    // throwing, so optional configuration fields never need their own\n    // try/catch\n    std::cout << server.value(\"host\", std::string(\"0.0.0.0\")) << '\\n';\n    std::cout << server.value(\"port\", 8080) << '\\n';\n\n    // a present but wrong-typed default still throws -- value() only\n    // replaces \"not found\", not \"wrong type\", exactly as basic_json::value\n    try\n    {\n        static_cast<void>(server.value(\"host\", 0));\n    }\n    catch (const nlohmann::json::type_error& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

    Output:

    localhost\n8080\n[json.exception.type_error.302] type must be number, but is string\n
    Example: (2) access specified element via JSON pointer with default value

    The example below reads an optional, nested configuration value with a default, given as a JSON pointer.

    #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\nusing json_view = nlohmann::json_view;\nusing json_pointer = nlohmann::json::json_pointer;\n\nint main()\n{\n    json_document doc = json_document::parse(R\"({\"server\": {\"host\": \"localhost\", \"limits\": {\"connections\": 100}}})\");\n    const json_view config = doc.root();\n\n    // a nested, optional setting read with a default -- no exception, even\n    // though \"timeout\" is missing several levels down\n    std::cout << config.value(json_pointer(\"/server/limits/connections\"), 10) << '\\n';\n    std::cout << config.value(json_pointer(\"/server/limits/timeout\"), 30) << '\\n';\n\n    // an out-of-range array index also falls back to the default\n    json_document list_doc = json_document::parse(R\"({\"servers\": [\"a\", \"b\"]})\");\n    std::cout << list_doc.root().value(json_pointer(\"/servers/5\"), std::string(\"none\")) << '\\n';\n}\n

    Output:

    100\n30\nnone\n
    "},{"location":"api/basic_json_view/value/#see-also","title":"See also","text":"
    • at - access specified element with bounds checking (throws instead of returning a default value)
    • operator[] - access specified element (returns a discarded view instead of a default value)
    • BasicJsonType::value - the corresponding function of basic_json
    "},{"location":"api/basic_json_view/value/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/byte_container_with_subtype/","title":"nlohmann::byte_container_with_subtype","text":"
    template<typename BinaryType>\nclass byte_container_with_subtype : public BinaryType;\n

    This type extends the template parameter BinaryType provided to basic_json with a subtype used by BSON and MessagePack. This type exists so that the user does not have to specify a type themselves with a specific naming scheme in order to override the binary type.

    "},{"location":"api/byte_container_with_subtype/#template-parameters","title":"Template parameters","text":"BinaryType container to store bytes (std::vector<std::uint8_t> by default)"},{"location":"api/byte_container_with_subtype/#member-types","title":"Member types","text":"
    • container_type - the type of the underlying container (BinaryType)
    • subtype_type - the type of the subtype (std::uint64_t)
    "},{"location":"api/byte_container_with_subtype/#member-functions","title":"Member functions","text":"
    • (constructor)
    • operator== - comparison: equal
    • operator!= - comparison: not equal
    • set_subtype - sets the binary subtype
    • subtype - return the binary subtype
    • has_subtype - return whether the value has a subtype
    • clear_subtype - clears the binary subtype
    "},{"location":"api/byte_container_with_subtype/#version-history","title":"Version history","text":"
    • Added in version 3.8.0.
    • Changed the type of subtypes to std::uint64_t in 3.10.0.
    "},{"location":"api/byte_container_with_subtype/byte_container_with_subtype/","title":"nlohmann::byte_container_with_subtype::byte_container_with_subtype","text":"
    // (1)\nbyte_container_with_subtype();\n\n// (2)\nbyte_container_with_subtype(const container_type& container);\nbyte_container_with_subtype(container_type&& container);\n\n// (3)\nbyte_container_with_subtype(const container_type& container, subtype_type subtype);\nbyte_container_with_subtype(container_type&& container, subtype_type subtype);\n
    1. Create an empty binary container without a subtype.
    2. Create a binary container without a subtype.
    3. Create a binary container with a subtype.
    "},{"location":"api/byte_container_with_subtype/byte_container_with_subtype/#parameters","title":"Parameters","text":"container (in) binary container subtype (in) subtype"},{"location":"api/byte_container_with_subtype/byte_container_with_subtype/#examples","title":"Examples","text":"Example

    The example below demonstrates how byte containers can be created.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\n// define a byte container based on std::vector\nusing byte_container_with_subtype = nlohmann::byte_container_with_subtype<std::vector<std::uint8_t>>;\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // (1) create empty container\n    auto c1 = byte_container_with_subtype();\n\n    std::vector<std::uint8_t> bytes = {{0xca, 0xfe, 0xba, 0xbe}};\n\n    // (2) create container\n    auto c2 = byte_container_with_subtype(bytes);\n\n    // (3) create container with subtype\n    auto c3 = byte_container_with_subtype(bytes, 42);\n\n    std::cout << json(c1) << \"\\n\" << json(c2) << \"\\n\" << json(c3) << std::endl;\n}\n

    Output:

    {\"bytes\":[],\"subtype\":null}\n{\"bytes\":[202,254,186,190],\"subtype\":null}\n{\"bytes\":[202,254,186,190],\"subtype\":42}\n
    "},{"location":"api/byte_container_with_subtype/byte_container_with_subtype/#see-also","title":"See also","text":"
    • set_subtype sets the binary subtype
    • subtype return the binary subtype
    • has_subtype return whether the value has a subtype
    • binary create a binary JSON value
    • Binary Values - the article on binary values
    "},{"location":"api/byte_container_with_subtype/byte_container_with_subtype/#version-history","title":"Version history","text":"

    Since version 3.8.0.

    "},{"location":"api/byte_container_with_subtype/clear_subtype/","title":"nlohmann::byte_container_with_subtype::clear_subtype","text":"
    void clear_subtype() noexcept;\n

    Clears the binary subtype and flags the value as not having a subtype, which has implications for serialization; for instance, MessagePack will prefer the bin family over the ext family.

    "},{"location":"api/byte_container_with_subtype/clear_subtype/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this function never throws exceptions.

    "},{"location":"api/byte_container_with_subtype/clear_subtype/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/byte_container_with_subtype/clear_subtype/#examples","title":"Examples","text":"Example

    The example below demonstrates how clear_subtype can remove subtypes.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\n// define a byte container based on std::vector\nusing byte_container_with_subtype = nlohmann::byte_container_with_subtype<std::vector<std::uint8_t>>;\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::vector<std::uint8_t> bytes = {{0xca, 0xfe, 0xba, 0xbe}};\n\n    // create container with subtype\n    auto c1 = byte_container_with_subtype(bytes, 42);\n\n    std::cout << \"before calling clear_subtype(): \" << json(c1) << '\\n';\n\n    c1.clear_subtype();\n\n    std::cout << \"after calling clear_subtype(): \" << json(c1) << '\\n';\n}\n

    Output:

    before calling clear_subtype(): {\"bytes\":[202,254,186,190],\"subtype\":42}\nafter calling clear_subtype(): {\"bytes\":[202,254,186,190],\"subtype\":null}\n
    "},{"location":"api/byte_container_with_subtype/clear_subtype/#see-also","title":"See also","text":"
    • set_subtype sets the binary subtype
    • has_subtype return whether the value has a subtype
    • subtype return the binary subtype
    "},{"location":"api/byte_container_with_subtype/clear_subtype/#version-history","title":"Version history","text":"

    Since version 3.8.0.

    "},{"location":"api/byte_container_with_subtype/has_subtype/","title":"nlohmann::byte_container_with_subtype::has_subtype","text":"
    constexpr bool has_subtype() const noexcept;\n

    Returns whether the value has a subtype.

    "},{"location":"api/byte_container_with_subtype/has_subtype/#return-value","title":"Return value","text":"

    whether the value has a subtype

    "},{"location":"api/byte_container_with_subtype/has_subtype/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this function never throws exceptions.

    "},{"location":"api/byte_container_with_subtype/has_subtype/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/byte_container_with_subtype/has_subtype/#examples","title":"Examples","text":"Example

    The example below demonstrates how has_subtype can check whether a subtype was set.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\n// define a byte container based on std::vector\nusing byte_container_with_subtype = nlohmann::byte_container_with_subtype<std::vector<std::uint8_t>>;\n\nint main()\n{\n    std::vector<std::uint8_t> bytes = {{0xca, 0xfe, 0xba, 0xbe}};\n\n    // create container\n    auto c1 = byte_container_with_subtype(bytes);\n\n    // create container with subtype\n    auto c2 = byte_container_with_subtype(bytes, 42);\n\n    std::cout << std::boolalpha << \"c1.has_subtype() = \" << c1.has_subtype()\n              << \"\\nc2.has_subtype() = \" << c2.has_subtype() << std::endl;\n}\n

    Output:

    c1.has_subtype() = false\nc2.has_subtype() = true\n
    "},{"location":"api/byte_container_with_subtype/has_subtype/#see-also","title":"See also","text":"
    • set_subtype sets the binary subtype
    • clear_subtype clears the binary subtype
    • subtype return the binary subtype
    "},{"location":"api/byte_container_with_subtype/has_subtype/#version-history","title":"Version history","text":"

    Since version 3.8.0.

    "},{"location":"api/byte_container_with_subtype/operator_eq/","title":"nlohmann::byte_container_with_subtype::operator==","text":"
    bool operator==(const byte_container_with_subtype& rhs) const;\n

    Compares two byte containers for equality by comparing (1) the underlying binary data (the BinaryType base, compared with BinaryType's own operator==) and (2) the subtype information -- both containers must either have no subtype, or have a subtype and the same subtype value.

    "},{"location":"api/byte_container_with_subtype/operator_eq/#parameters","title":"Parameters","text":"rhs (in) byte container to compare *this with"},{"location":"api/byte_container_with_subtype/operator_eq/#return-value","title":"Return value","text":"

    whether *this and rhs are equal

    "},{"location":"api/byte_container_with_subtype/operator_eq/#complexity","title":"Complexity","text":"

    Linear in the size of the compared containers.

    "},{"location":"api/byte_container_with_subtype/operator_eq/#examples","title":"Examples","text":"Example

    The example below demonstrates comparing byte containers with and without subtypes.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\n// define a byte container based on std::vector\nusing byte_container_with_subtype = nlohmann::byte_container_with_subtype<std::vector<std::uint8_t>>;\n\nint main()\n{\n    std::vector<std::uint8_t> bytes = {{0xca, 0xfe, 0xba, 0xbe}};\n\n    // create containers without and with a subtype\n    auto c1 = byte_container_with_subtype(bytes);\n    auto c2 = byte_container_with_subtype(bytes);\n    auto c3 = byte_container_with_subtype(bytes, 42);\n    auto c4 = byte_container_with_subtype(bytes, 42);\n    auto c5 = byte_container_with_subtype(bytes, 23);\n\n    std::cout << std::boolalpha\n              << \"c1 == c2: \" << (c1 == c2) << '\\n'\n              << \"c1 == c3: \" << (c1 == c3) << '\\n'\n              << \"c3 == c4: \" << (c3 == c4) << '\\n'\n              << \"c3 == c5: \" << (c3 == c5) << std::endl;\n}\n

    Output:

    c1 == c2: true\nc1 == c3: false\nc3 == c4: true\nc3 == c5: false\n
    "},{"location":"api/byte_container_with_subtype/operator_eq/#see-also","title":"See also","text":"
    • operator!= comparison: not equal
    • has_subtype return whether the value has a subtype
    • subtype return the binary subtype
    "},{"location":"api/byte_container_with_subtype/operator_eq/#version-history","title":"Version history","text":"
    • Added in version 3.8.0.
    "},{"location":"api/byte_container_with_subtype/operator_ne/","title":"nlohmann::byte_container_with_subtype::operator!=","text":"
    bool operator!=(const byte_container_with_subtype& rhs) const;\n

    Compares two byte containers for inequality. Returns !(rhs == *this); see operator== for the equality semantics.

    "},{"location":"api/byte_container_with_subtype/operator_ne/#parameters","title":"Parameters","text":"rhs (in) byte container to compare *this with"},{"location":"api/byte_container_with_subtype/operator_ne/#return-value","title":"Return value","text":"

    whether *this and rhs are not equal

    "},{"location":"api/byte_container_with_subtype/operator_ne/#complexity","title":"Complexity","text":"

    Linear in the size of the compared containers.

    "},{"location":"api/byte_container_with_subtype/operator_ne/#examples","title":"Examples","text":"Example

    The example below demonstrates comparing byte containers with and without subtypes.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\n// define a byte container based on std::vector\nusing byte_container_with_subtype = nlohmann::byte_container_with_subtype<std::vector<std::uint8_t>>;\n\nint main()\n{\n    std::vector<std::uint8_t> bytes = {{0xca, 0xfe, 0xba, 0xbe}};\n\n    // create containers without and with a subtype\n    auto c1 = byte_container_with_subtype(bytes);\n    auto c2 = byte_container_with_subtype(bytes);\n    auto c3 = byte_container_with_subtype(bytes, 42);\n    auto c4 = byte_container_with_subtype(bytes, 42);\n    auto c5 = byte_container_with_subtype(bytes, 23);\n\n    std::cout << std::boolalpha\n              << \"c1 != c2: \" << (c1 != c2) << '\\n'\n              << \"c1 != c3: \" << (c1 != c3) << '\\n'\n              << \"c3 != c4: \" << (c3 != c4) << '\\n'\n              << \"c3 != c5: \" << (c3 != c5) << std::endl;\n}\n

    Output:

    c1 != c2: false\nc1 != c3: true\nc3 != c4: false\nc3 != c5: true\n
    "},{"location":"api/byte_container_with_subtype/operator_ne/#see-also","title":"See also","text":"
    • operator== comparison: equal
    • has_subtype return whether the value has a subtype
    • subtype return the binary subtype
    "},{"location":"api/byte_container_with_subtype/operator_ne/#version-history","title":"Version history","text":"
    • Added in version 3.8.0.
    "},{"location":"api/byte_container_with_subtype/set_subtype/","title":"nlohmann::byte_container_with_subtype::set_subtype","text":"
    void set_subtype(subtype_type subtype) noexcept;\n

    Sets the binary subtype of the value, also flags a binary JSON value as having a subtype, which has implications for serialization.

    "},{"location":"api/byte_container_with_subtype/set_subtype/#parameters","title":"Parameters","text":"subtype (in) subtype to set"},{"location":"api/byte_container_with_subtype/set_subtype/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this function never throws exceptions.

    "},{"location":"api/byte_container_with_subtype/set_subtype/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/byte_container_with_subtype/set_subtype/#examples","title":"Examples","text":"Example

    The example below demonstrates how a subtype can be set with set_subtype.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\n// define a byte container based on std::vector\nusing byte_container_with_subtype = nlohmann::byte_container_with_subtype<std::vector<std::uint8_t>>;\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::vector<std::uint8_t> bytes = {{0xca, 0xfe, 0xba, 0xbe}};\n\n    // create container without subtype\n    auto c = byte_container_with_subtype(bytes);\n\n    std::cout << \"before calling set_subtype(42): \" << json(c) << '\\n';\n\n    // set the subtype\n    c.set_subtype(42);\n\n    std::cout << \"after calling set_subtype(42): \" << json(c) << '\\n';\n}\n

    Output:

    before calling set_subtype(42): {\"bytes\":[202,254,186,190],\"subtype\":null}\nafter calling set_subtype(42): {\"bytes\":[202,254,186,190],\"subtype\":42}\n
    "},{"location":"api/byte_container_with_subtype/set_subtype/#see-also","title":"See also","text":"
    • subtype return the binary subtype
    • has_subtype return whether the value has a subtype
    • clear_subtype clears the binary subtype
    "},{"location":"api/byte_container_with_subtype/set_subtype/#version-history","title":"Version history","text":"

    Since version 3.8.0.

    "},{"location":"api/byte_container_with_subtype/subtype/","title":"nlohmann::byte_container_with_subtype::subtype","text":"
    constexpr subtype_type subtype() const noexcept;\n

    Returns the numerical subtype of the value if it has a subtype. If it does not have a subtype, this function will return subtype_type(-1) as a sentinel value.

    "},{"location":"api/byte_container_with_subtype/subtype/#return-value","title":"Return value","text":"

    the numerical subtype of the binary value, or subtype_type(-1) if no subtype is set

    "},{"location":"api/byte_container_with_subtype/subtype/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this function never throws exceptions.

    "},{"location":"api/byte_container_with_subtype/subtype/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/byte_container_with_subtype/subtype/#examples","title":"Examples","text":"Example

    The example below demonstrates how the subtype can be retrieved with subtype. Note how subtype_type(-1) is returned for container c1.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\n// define a byte container based on std::vector\nusing byte_container_with_subtype = nlohmann::byte_container_with_subtype<std::vector<std::uint8_t>>;\n\nint main()\n{\n    std::vector<std::uint8_t> bytes = {{0xca, 0xfe, 0xba, 0xbe}};\n\n    // create container\n    auto c1 = byte_container_with_subtype(bytes);\n\n    // create container with subtype\n    auto c2 = byte_container_with_subtype(bytes, 42);\n\n    std::cout << \"c1.subtype() = \" << c1.subtype()\n              << \"\\nc2.subtype() = \" << c2.subtype() << std::endl;\n\n    // in case no subtype is set, return special value\n    assert(c1.subtype() == static_cast<byte_container_with_subtype::subtype_type>(-1));\n}\n

    Output:

    c1.subtype() = 18446744073709551615\nc2.subtype() = 42\n
    "},{"location":"api/byte_container_with_subtype/subtype/#see-also","title":"See also","text":"
    • set_subtype sets the binary subtype
    • has_subtype return whether the value has a subtype
    • clear_subtype clears the binary subtype
    "},{"location":"api/byte_container_with_subtype/subtype/#version-history","title":"Version history","text":"
    • Added in version 3.8.0
    • Fixed return value to properly return subtype_type(-1) as documented in version 3.10.0.
    "},{"location":"api/json_pointer/","title":"nlohmann::json_pointer","text":"
    template<typename RefStringType>\nclass json_pointer;\n

    A JSON pointer defines a string syntax for identifying a specific value within a JSON document. It can be used with functions at and operator[]. Furthermore, JSON pointers are the base for JSON patches.

    "},{"location":"api/json_pointer/#template-parameters","title":"Template parameters","text":"RefStringType the string type used for the reference tokens making up the JSON pointer

    Deprecation

    For backwards compatibility RefStringType may also be a specialization of basic_json in which case string_t will be deduced as basic_json::string_t. This feature is deprecated and may be removed in a future major version.

    See the migration guide for how to update existing code.

    A JSON pointer is internally a sequence of reference tokens. front, pop_front, and push_front act on the first reference token, whereas back, pop_back, and push_back act on the last one. parent_pointer returns a new JSON pointer with the last reference token removed (like a non-mutating pop_back):

    flowchart LR\n    A[\"a\"] --> B[\"b\"] --> C[\"c\"]\n\n    front[\"front() / pop_front() / push_front()\"] -.-> A\n    back[\"back() / pop_back() / push_back()\"] -.-> C\n    parent[\"parent_pointer() returns /a/b\"] -.-> B

    The diagram shows the reference tokens of the JSON pointer /a/b/c.

    "},{"location":"api/json_pointer/#member-types","title":"Member types","text":"
    • string_t - the string type used for the reference tokens
    "},{"location":"api/json_pointer/#member-functions","title":"Member functions","text":"
    • (constructor)
    • to_string - return a string representation of the JSON pointer
    • operator string_t - return a string representation of the JSON pointer (deprecated)
    • operator== - compare: equal
    • operator!= - compare: not equal
    • operator<=> - compare: 3-way (C++20)
    • operator/= - append to the end of the JSON pointer
    • operator/ - create JSON Pointer by appending
    • parent_pointer - returns the parent of this JSON pointer
    • pop_back - remove the last reference token
    • back - return last reference token
    • push_back - append an unescaped token at the end of the pointer
    • pop_front - remove the first reference token
    • front - return first reference token
    • push_front - append an unescaped token at the start of the pointer
    • empty - return whether the pointer points to the root document
    "},{"location":"api/json_pointer/#literals","title":"Literals","text":"
    • operator\"\"_json_pointer - user-defined string literal for JSON pointers
    "},{"location":"api/json_pointer/#see-also","title":"See also","text":"
    • RFC 6901
    "},{"location":"api/json_pointer/#version-history","title":"Version history","text":"
    • Added in version 2.0.0.
    • Changed template parameter from basic_json to string type in version 3.11.0.
    "},{"location":"api/json_pointer/back/","title":"nlohmann::json_pointer::back","text":"
    const string_t& back() const;\n

    Return the last reference token.

    "},{"location":"api/json_pointer/back/#return-value","title":"Return value","text":"

    Last reference token.

    "},{"location":"api/json_pointer/back/#exception-safety","title":"Exception safety","text":"

    Strong exception safety: if an exception occurs, the original value stays intact.

    "},{"location":"api/json_pointer/back/#exceptions","title":"Exceptions","text":"

    Throws out_of_range.405 if the JSON pointer has no parent.

    "},{"location":"api/json_pointer/back/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/json_pointer/back/#examples","title":"Examples","text":"Example

    The example shows the usage of back.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // different JSON Pointers\n    json::json_pointer ptr1(\"/foo\");\n    json::json_pointer ptr2(\"/foo/0\");\n\n    // call empty()\n    std::cout << \"last reference token of \\\"\" << ptr1 << \"\\\" is \\\"\" << ptr1.back() << \"\\\"\\n\"\n              << \"last reference token of \\\"\" << ptr2 << \"\\\" is \\\"\" << ptr2.back() << \"\\\"\" << std::endl;\n}\n

    Output:

    last reference token of \"/foo\" is \"foo\"\nlast reference token of \"/foo/0\" is \"0\"\n
    "},{"location":"api/json_pointer/back/#see-also","title":"See also","text":"
    • front return first reference token
    • pop_back remove the last reference token
    • push_back append an unescaped token at the end of the pointer
    • parent_pointer returns the parent of this JSON pointer
    "},{"location":"api/json_pointer/back/#version-history","title":"Version history","text":"
    • Added in version 3.6.0.
    • Changed return type to string_t in version 3.11.0.
    "},{"location":"api/json_pointer/empty/","title":"nlohmann::json_pointer::empty","text":"
    bool empty() const noexcept;\n

    Return whether the pointer points to the root document.

    "},{"location":"api/json_pointer/empty/#return-value","title":"Return value","text":"

    true iff the JSON pointer points to the root document.

    "},{"location":"api/json_pointer/empty/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this function never throws exceptions.

    "},{"location":"api/json_pointer/empty/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/json_pointer/empty/#examples","title":"Examples","text":"Example

    The example shows the result of empty for different JSON Pointers.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // different JSON Pointers\n    json::json_pointer ptr0;\n    json::json_pointer ptr1(\"\");\n    json::json_pointer ptr2(\"/foo\");\n    json::json_pointer ptr3(\"/foo/0\");\n\n    // call empty()\n    std::cout << std::boolalpha\n              << \"\\\"\" << ptr0 << \"\\\": \" << ptr0.empty() << '\\n'\n              << \"\\\"\" << ptr1 << \"\\\": \" << ptr1.empty() << '\\n'\n              << \"\\\"\" << ptr2 << \"\\\": \" << ptr2.empty() << '\\n'\n              << \"\\\"\" << ptr3 << \"\\\": \" << ptr3.empty() << std::endl;\n}\n

    Output:

    \"\": true\n\"\": true\n\"/foo\": false\n\"/foo/0\": false\n
    "},{"location":"api/json_pointer/empty/#see-also","title":"See also","text":"
    • front return first reference token
    • back return last reference token
    • to_string return a string representation of the JSON pointer
    "},{"location":"api/json_pointer/empty/#version-history","title":"Version history","text":"

    Added in version 3.6.0.

    "},{"location":"api/json_pointer/front/","title":"nlohmann::json_pointer::front","text":"
    const string_t& front() const;\n

    Return the first reference token.

    "},{"location":"api/json_pointer/front/#return-value","title":"Return value","text":"

    First reference token.

    "},{"location":"api/json_pointer/front/#exception-safety","title":"Exception safety","text":"

    Strong exception safety: if an exception occurs, the original value stays intact.

    "},{"location":"api/json_pointer/front/#exceptions","title":"Exceptions","text":"

    Throws out_of_range.405 if the JSON pointer has no parent.

    "},{"location":"api/json_pointer/front/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/json_pointer/front/#examples","title":"Examples","text":"Example

    The example shows the usage of front.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // different JSON Pointers\n    json::json_pointer ptr1(\"/foo\");\n    json::json_pointer ptr2(\"/foo/0\");\n\n    // call empty()\n    std::cout << \"first reference token of \\\"\" << ptr1 << \"\\\" is \\\"\" << ptr1.front() << \"\\\"\\n\"\n              << \"first reference token of \\\"\" << ptr2 << \"\\\" is \\\"\" << ptr2.front() << \"\\\"\" << std::endl;\n}\n

    Output:

    first reference token of \"/foo\" is \"foo\"\nfirst reference token of \"/foo/0\" is \"foo\"\n
    "},{"location":"api/json_pointer/front/#see-also","title":"See also","text":"
    • back return last reference token
    • pop_front remove the first reference token
    • push_front append an unescaped token at the start of the pointer
    "},{"location":"api/json_pointer/front/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/json_pointer/json_pointer/","title":"nlohmann::json_pointer::json_pointer","text":"
    explicit json_pointer(const string_t& s = \"\");\n

    Create a JSON pointer according to the syntax described in Section 3 of RFC6901.

    "},{"location":"api/json_pointer/json_pointer/#parameters","title":"Parameters","text":"s (in) string representing the JSON pointer; if omitted, the empty string is assumed which references the whole JSON value"},{"location":"api/json_pointer/json_pointer/#exception-safety","title":"Exception safety","text":"

    Strong guarantee: if an exception is thrown, there are no changes to any JSON pointer.

    "},{"location":"api/json_pointer/json_pointer/#exceptions","title":"Exceptions","text":"
    • Throws parse_error.107 if the given JSON pointer s is nonempty and does not begin with a slash (/); see example below.
    • Throws parse_error.108 if a tilde (~) in the given JSON pointer s is not followed by 0 (representing ~) or 1 (representing /); see example below.
    "},{"location":"api/json_pointer/json_pointer/#complexity","title":"Complexity","text":"

    Linear in the length of s.

    "},{"location":"api/json_pointer/json_pointer/#examples","title":"Examples","text":"Example

    The example shows the construction several valid JSON pointers as well as the exceptional behavior.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // correct JSON pointers\n    json::json_pointer p1;\n    json::json_pointer p2(\"\");\n    json::json_pointer p3(\"/\");\n    json::json_pointer p4(\"//\");\n    json::json_pointer p5(\"/foo/bar\");\n    json::json_pointer p6(\"/foo/bar/-\");\n    json::json_pointer p7(\"/foo/~0\");\n    json::json_pointer p8(\"/foo/~1\");\n\n    // error: JSON pointer does not begin with a slash\n    try\n    {\n        json::json_pointer p9(\"foo\");\n    }\n    catch (const json::parse_error& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n\n    // error: JSON pointer uses escape symbol ~ not followed by 0 or 1\n    try\n    {\n        json::json_pointer p10(\"/foo/~\");\n    }\n    catch (const json::parse_error& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n\n    // error: JSON pointer uses escape symbol ~ not followed by 0 or 1\n    try\n    {\n        json::json_pointer p11(\"/foo/~3\");\n    }\n    catch (const json::parse_error& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

    Output:

    [json.exception.parse_error.107] parse error at byte 1: JSON pointer must be empty or begin with '/' - was: 'foo'\n[json.exception.parse_error.108] parse error: escape character '~' must be followed with '0' or '1'\n[json.exception.parse_error.108] parse error: escape character '~' must be followed with '0' or '1'\n
    "},{"location":"api/json_pointer/json_pointer/#see-also","title":"See also","text":"
    • JSON Pointer - the article on JSON Pointer support
    • operator\"\"_json_pointer user-defined string literal for JSON pointers
    "},{"location":"api/json_pointer/json_pointer/#version-history","title":"Version history","text":"
    • Added in version 2.0.0.
    • Changed type of s to string_t in version 3.11.0.
    "},{"location":"api/json_pointer/operator_eq/","title":"nlohmann::json_pointer::operator==","text":"
    // until C++20\ntemplate<typename RefStringTypeLhs, typename RefStringTypeRhs>\nbool operator==(\n    const json_pointer<RefStringTypeLhs>& lhs,\n    const json_pointer<RefStringTypeRhs>& rhs) noexcept;            // (1)\n\ntemplate<typename RefStringTypeLhs, typename StringType>\nbool operator==(\n    const json_pointer<RefStringTypeLhs>& lhs,\n    const StringType& rhs);                                         // (2)\n\ntemplate<typename RefStringTypeRhs, typename StringType>\nbool operator==(\n    const StringType& lhs,\n    const json_pointer<RefStringTypeRhs>& rhs);                     // (2)\n\n// since C++20\ntemplate<typename RefStringTypeRhs>\nbool operator==(\n    const json_pointer<RefStringTypeRhs>& rhs) const noexcept;      // (1)\n\nbool operator==(const string_t& rhs) const;                         // (2)\n

    1. Compares two JSON pointers for equality by comparing their reference tokens.

    2. Compares a JSON pointer and a string or a string and a JSON pointer for equality by converting the string to a JSON pointer and comparing the JSON pointers according to 1.

    "},{"location":"api/json_pointer/operator_eq/#template-parameters","title":"Template parameters","text":"RefStringTypeLhs, RefStringTypeRhs the string type of the left-hand side or right-hand side JSON pointer, respectively StringType the string type derived from the json_pointer operand (json_pointer::string_t)"},{"location":"api/json_pointer/operator_eq/#parameters","title":"Parameters","text":"lhs (in) first value to consider rhs (in) second value to consider"},{"location":"api/json_pointer/operator_eq/#return-value","title":"Return value","text":"

    whether the values lhs/*this and rhs are equal

    "},{"location":"api/json_pointer/operator_eq/#exception-safety","title":"Exception safety","text":"
    1. No-throw guarantee: this function never throws exceptions.
    2. Strong exception safety: if an exception occurs, the original value stays intact.
    "},{"location":"api/json_pointer/operator_eq/#exceptions","title":"Exceptions","text":"
    1. (none)
    2. The function can throw the following exceptions:
    3. Throws parse_error.107 if the given JSON pointer s is nonempty and does not begin with a slash (/); see example below.
    4. Throws parse_error.108 if a tilde (~) in the given JSON pointer s is not followed by 0 (representing ~) or 1 (representing /); see example below.
    "},{"location":"api/json_pointer/operator_eq/#complexity","title":"Complexity","text":"

    Constant if lhs and rhs differ in the number of reference tokens, otherwise linear in the number of reference tokens.

    "},{"location":"api/json_pointer/operator_eq/#notes","title":"Notes","text":"

    Deprecation

    Overload 2 is deprecated and will be removed in a future major version release.

    See the migration guide for how to update existing code.

    "},{"location":"api/json_pointer/operator_eq/#examples","title":"Examples","text":"Example: (1) Comparing JSON pointers

    The example demonstrates comparing JSON pointers.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // different JSON pointers\n    json::json_pointer ptr0;\n    json::json_pointer ptr1(\"\");\n    json::json_pointer ptr2(\"/foo\");\n\n    // compare JSON pointers\n    std::cout << std::boolalpha\n              << \"\\\"\" << ptr0 << \"\\\" == \\\"\" << ptr0 << \"\\\": \" << (ptr0 == ptr0) << '\\n'\n              << \"\\\"\" << ptr0 << \"\\\" == \\\"\" << ptr1 << \"\\\": \" << (ptr0 == ptr1) << '\\n'\n              << \"\\\"\" << ptr1 << \"\\\" == \\\"\" << ptr2 << \"\\\": \" << (ptr1 == ptr2) << '\\n'\n              << \"\\\"\" << ptr2 << \"\\\" == \\\"\" << ptr2 << \"\\\": \" << (ptr2 == ptr2) << std::endl;\n}\n

    Output:

    \"\" == \"\": true\n\"\" == \"\": true\n\"\" == \"/foo\": false\n\"/foo\" == \"/foo\": true\n
    Example: (2) Comparing JSON pointers and strings

    The example demonstrates comparing JSON pointers and strings, and when doing so may raise an exception.

    #include <exception>\n#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // different JSON pointers\n    json::json_pointer ptr0;\n    json::json_pointer ptr1(\"\");\n    json::json_pointer ptr2(\"/foo\");\n\n    // different strings\n    std::string str0(\"\");\n    std::string str1(\"/foo\");\n    std::string str2(\"bar\");\n\n    // compare JSON pointers and strings\n    std::cout << std::boolalpha\n              << \"\\\"\" << ptr0 << \"\\\" == \\\"\" << str0 << \"\\\": \" << (ptr0 == str0) << '\\n'\n              << \"\\\"\" << str0 << \"\\\" == \\\"\" << ptr1 << \"\\\": \" << (str0 == ptr1) << '\\n'\n              << \"\\\"\" << ptr2 << \"\\\" == \\\"\" << str1 << \"\\\": \" << (ptr2 == str1) << std::endl;\n\n    try\n    {\n        std::cout << \"\\\"\" << str2 << \"\\\" == \\\"\" << ptr2 << \"\\\": \" << (str2 == ptr2) << std::endl;\n    }\n    catch (const json::parse_error& ex)\n    {\n        std::cout << ex.what() << std::endl;\n    }\n}\n

    Output:

    \"\" == \"\": true\n\"\" == \"\": true\n\"/foo\" == \"/foo\": true\n\"bar\" == \"/foo\": [json.exception.parse_error.107] parse error at byte 1: JSON pointer must be empty or begin with '/' - was: 'bar'\n
    "},{"location":"api/json_pointer/operator_eq/#see-also","title":"See also","text":"
    • operator!= compare for inequality
    • operator<=> comparison: 3-way (C++20)
    "},{"location":"api/json_pointer/operator_eq/#version-history","title":"Version history","text":"
    1. Added in version 2.1.0. Added C++20 member functions in version 3.11.2.
    2. Added for backward compatibility and deprecated in version 3.11.2.
    "},{"location":"api/json_pointer/operator_ne/","title":"nlohmann::json_pointer::operator!=","text":"
    // until C++20\ntemplate<typename RefStringTypeLhs, typename RefStringTypeRhs>\nbool operator!=(\n    const json_pointer<RefStringTypeLhs>& lhs,\n    const json_pointer<RefStringTypeRhs>& rhs) noexcept;  // (1)\n\ntemplate<typename RefStringTypeLhs, typename StringType>\nbool operator!=(\n    const json_pointer<RefStringTypeLhs>& lhs,\n    const StringType& rhs);                               // (2)\n\ntemplate<typename RefStringTypeRhs, typename StringType>\nbool operator!=(\n    const StringType& lhs,\n    const json_pointer<RefStringTypeRhs>& rhs);           // (2)\n

    1. Compares two JSON pointers for inequality by comparing their reference tokens.

    2. Compares a JSON pointer and a string or a string and a JSON pointer for inequality by converting the string to a JSON pointer and comparing the JSON pointers according to 1.

    "},{"location":"api/json_pointer/operator_ne/#template-parameters","title":"Template parameters","text":"RefStringTypeLhs, RefStringTypeRhs the string type of the left-hand side or right-hand side JSON pointer, respectively StringType the string type derived from the json_pointer operand (json_pointer::string_t)"},{"location":"api/json_pointer/operator_ne/#parameters","title":"Parameters","text":"lhs (in) first value to consider rhs (in) second value to consider"},{"location":"api/json_pointer/operator_ne/#return-value","title":"Return value","text":"

    whether the values lhs/*this and rhs are not equal

    "},{"location":"api/json_pointer/operator_ne/#exception-safety","title":"Exception safety","text":"
    1. No-throw guarantee: this function never throws exceptions.
    2. Strong exception safety: if an exception occurs, the original value stays intact.
    "},{"location":"api/json_pointer/operator_ne/#exceptions","title":"Exceptions","text":"
    1. (none)
    2. The function can throw the following exceptions:
    3. Throws parse_error.107 if the given JSON pointer s is nonempty and does not begin with a slash (/); see example below.
    4. Throws parse_error.108 if a tilde (~) in the given JSON pointer s is not followed by 0 (representing ~) or 1 (representing /); see example below.
    "},{"location":"api/json_pointer/operator_ne/#complexity","title":"Complexity","text":"

    Constant if lhs and rhs differ in the number of reference tokens, otherwise linear in the number of reference tokens.

    "},{"location":"api/json_pointer/operator_ne/#notes","title":"Notes","text":"

    Operator overload resolution

    Since C++20 overload resolution will consider the rewritten candidate generated from operator==.

    Deprecation

    Overload 2 is deprecated and will be removed in a future major version release.

    See the migration guide for how to update existing code.

    "},{"location":"api/json_pointer/operator_ne/#examples","title":"Examples","text":"Example: (1) Comparing JSON pointers

    The example demonstrates comparing JSON pointers.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // different JSON pointers\n    json::json_pointer ptr0;\n    json::json_pointer ptr1(\"\");\n    json::json_pointer ptr2(\"/foo\");\n\n    // compare JSON pointers\n    std::cout << std::boolalpha\n              << \"\\\"\" << ptr0 << \"\\\" != \\\"\" << ptr0 << \"\\\": \" << (ptr0 != ptr0) << '\\n'\n              << \"\\\"\" << ptr0 << \"\\\" != \\\"\" << ptr1 << \"\\\": \" << (ptr0 != ptr1) << '\\n'\n              << \"\\\"\" << ptr1 << \"\\\" != \\\"\" << ptr2 << \"\\\": \" << (ptr1 != ptr2) << '\\n'\n              << \"\\\"\" << ptr2 << \"\\\" != \\\"\" << ptr2 << \"\\\": \" << (ptr2 != ptr2) << std::endl;\n}\n

    Output:

    \"\" != \"\": false\n\"\" != \"\": false\n\"\" != \"/foo\": true\n\"/foo\" != \"/foo\": false\n
    Example: (2) Comparing JSON pointers and strings

    The example demonstrates comparing JSON pointers and strings, and when doing so may raise an exception.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // different JSON pointers\n    json::json_pointer ptr0;\n    json::json_pointer ptr1(\"\");\n    json::json_pointer ptr2(\"/foo\");\n\n    // different strings\n    std::string str0(\"\");\n    std::string str1(\"/foo\");\n    std::string str2(\"bar\");\n\n    // compare JSON pointers and strings\n    std::cout << std::boolalpha\n              << \"\\\"\" << ptr0 << \"\\\" != \\\"\" << str0 << \"\\\": \" << (ptr0 != str0) << '\\n'\n              << \"\\\"\" << str0 << \"\\\" != \\\"\" << ptr1 << \"\\\": \" << (str0 != ptr1) << '\\n'\n              << \"\\\"\" << ptr2 << \"\\\" != \\\"\" << str1 << \"\\\": \" << (ptr2 != str1) << std::endl;\n\n    try\n    {\n        std::cout << \"\\\"\" << str2 << \"\\\" != \\\"\" << ptr2 << \"\\\": \" << (str2 != ptr2) << std::endl;\n    }\n    catch (const json::parse_error& ex)\n    {\n        std::cout << ex.what() << std::endl;\n    }\n}\n

    Output:

    \"\" != \"\": false\n\"\" != \"\": false\n\"/foo\" != \"/foo\": false\n\"bar\" != \"/foo\": [json.exception.parse_error.107] parse error at byte 1: JSON pointer must be empty or begin with '/' - was: 'bar'\n
    "},{"location":"api/json_pointer/operator_ne/#see-also","title":"See also","text":"
    • operator== compare for equality
    • operator<=> comparison: 3-way (C++20)
    "},{"location":"api/json_pointer/operator_ne/#version-history","title":"Version history","text":"
    1. Added in version 2.1.0.
    2. Added for backward compatibility and deprecated in version 3.11.2.
    "},{"location":"api/json_pointer/operator_slash/","title":"nlohmann::json_pointer::operator/","text":"
    // (1)\njson_pointer operator/(const json_pointer& lhs, const json_pointer& rhs);\n\n// (2)\njson_pointer operator/(const json_pointer& lhs, string_t token);\n\n// (3)\njson_pointer operator/(const json_pointer& lhs, std::size_t array_idx);\n
    1. create a new JSON pointer by appending the right JSON pointer at the end of the left JSON pointer
    2. create a new JSON pointer by appending the unescaped token at the end of the JSON pointer
    3. create a new JSON pointer by appending the array-index-token at the end of the JSON pointer
    "},{"location":"api/json_pointer/operator_slash/#parameters","title":"Parameters","text":"lhs (in) JSON pointer rhs (in) JSON pointer to append token (in) reference token to append array_idx (in) array index to append"},{"location":"api/json_pointer/operator_slash/#return-value","title":"Return value","text":"
    1. a new JSON pointer with rhs appended to lhs
    2. a new JSON pointer with unescaped token appended to lhs
    3. a new JSON pointer with array_idx appended to lhs
    "},{"location":"api/json_pointer/operator_slash/#exception-safety","title":"Exception safety","text":"

    Strong guarantee: if an exception is thrown, there are no changes to any JSON pointer. The operands are not modified; a new JSON pointer is built from a copy of lhs.

    "},{"location":"api/json_pointer/operator_slash/#complexity","title":"Complexity","text":"
    1. Linear in the length of lhs and rhs.
    2. Linear in the length of lhs.
    3. Linear in the length of lhs.
    "},{"location":"api/json_pointer/operator_slash/#examples","title":"Examples","text":"Example

    The example shows the usage of operator/.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON pointer\n    json::json_pointer ptr(\"/foo\");\n\n    // append a JSON Pointer\n    std::cout << \"\\\"\" << ptr / json::json_pointer(\"/bar/baz\") << \"\\\"\\n\";\n\n    // append a string\n    std::cout << \"\\\"\" << ptr / \"fob\" << \"\\\"\\n\";\n\n    // append an array index\n    std::cout << \"\\\"\" << ptr / 42 << \"\\\"\" << std::endl;\n}\n

    Output:

    \"/foo/bar/baz\"\n\"/foo/fob\"\n\"/foo/42\"\n
    "},{"location":"api/json_pointer/operator_slash/#see-also","title":"See also","text":"
    • operator/= append to the end of the JSON pointer
    • push_back append an unescaped token at the end of the pointer
    "},{"location":"api/json_pointer/operator_slash/#version-history","title":"Version history","text":"
    1. Added in version 3.6.0.
    2. Added in version 3.6.0. Changed type of token to string_t in version 3.11.0.
    3. Added in version 3.6.0.
    "},{"location":"api/json_pointer/operator_slasheq/","title":"nlohmann::json_pointer::operator/=","text":"
    // (1)\njson_pointer& operator/=(const json_pointer& ptr);\n\n// (2)\njson_pointer& operator/=(string_t token);\n\n// (3)\njson_pointer& operator/=(std::size_t array_idx)\n
    1. append another JSON pointer at the end of this JSON pointer
    2. append an unescaped reference token at the end of this JSON pointer
    3. append an array index at the end of this JSON pointer
    "},{"location":"api/json_pointer/operator_slasheq/#parameters","title":"Parameters","text":"ptr (in) JSON pointer to append token (in) reference token to append array_idx (in) array index to append"},{"location":"api/json_pointer/operator_slasheq/#return-value","title":"Return value","text":"
    1. JSON pointer with ptr appended
    2. JSON pointer with token appended without escaping token
    3. JSON pointer with array_idx appended
    "},{"location":"api/json_pointer/operator_slasheq/#exception-safety","title":"Exception safety","text":"
    1. Basic guarantee: if an exception is thrown (for instance, if copying a reference token fails), the JSON pointer is left in a valid state, but it may contain some of the reference tokens of ptr.
    2. Strong guarantee: if an exception is thrown, there are no changes to the JSON pointer.
    3. Strong guarantee: if an exception is thrown, there are no changes to the JSON pointer.
    "},{"location":"api/json_pointer/operator_slasheq/#complexity","title":"Complexity","text":"
    1. Linear in the length of ptr.
    2. Amortized constant.
    3. Amortized constant.
    "},{"location":"api/json_pointer/operator_slasheq/#examples","title":"Examples","text":"Example

    The example shows the usage of operator/=.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON pointer\n    json::json_pointer ptr(\"/foo\");\n    std::cout << \"\\\"\" << ptr << \"\\\"\\n\";\n\n    // append a JSON Pointer\n    ptr /= json::json_pointer(\"/bar/baz\");\n    std::cout << \"\\\"\" << ptr << \"\\\"\\n\";\n\n    // append a string\n    ptr /= \"fob\";\n    std::cout << \"\\\"\" << ptr << \"\\\"\\n\";\n\n    // append an array index\n    ptr /= 42;\n    std::cout << \"\\\"\" << ptr << \"\\\"\" << std::endl;\n}\n

    Output:

    \"/foo\"\n\"/foo/bar/baz\"\n\"/foo/bar/baz/fob\"\n\"/foo/bar/baz/fob/42\"\n
    "},{"location":"api/json_pointer/operator_slasheq/#see-also","title":"See also","text":"
    • operator/ create JSON Pointer by appending
    • push_back append an unescaped token at the end of the pointer
    "},{"location":"api/json_pointer/operator_slasheq/#version-history","title":"Version history","text":"
    1. Added in version 3.6.0.
    2. Added in version 3.6.0. Changed type of token to string_t in version 3.11.0.
    3. Added in version 3.6.0.
    "},{"location":"api/json_pointer/operator_spaceship/","title":"nlohmann::json_pointer::operator<=>","text":"
    // since C++20\ntemplate<typename RefStringTypeRhs>\nstd::strong_ordering operator<=>(const json_pointer<RefStringTypeRhs>& rhs) const noexcept; // *NOPAD*\n

    3-way compares two JSON pointers by lexicographically comparing their sequences of reference tokens: corresponding reference tokens are compared with string_t's own operator<=>, and the first pair of tokens that differs determines the result. If all corresponding reference tokens compare equal, the JSON pointer with fewer reference tokens is ordered first.

    "},{"location":"api/json_pointer/operator_spaceship/#template-parameters","title":"Template parameters","text":"RefStringTypeRhs the string type of the right-hand side JSON pointer"},{"location":"api/json_pointer/operator_spaceship/#parameters","title":"Parameters","text":"rhs (in) JSON pointer to compare *this with"},{"location":"api/json_pointer/operator_spaceship/#return-value","title":"Return value","text":"

    the std::strong_ordering of the 3-way comparison of *this and rhs

    "},{"location":"api/json_pointer/operator_spaceship/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this function never throws exceptions.

    "},{"location":"api/json_pointer/operator_spaceship/#complexity","title":"Complexity","text":"

    Linear in the number of reference tokens.

    "},{"location":"api/json_pointer/operator_spaceship/#notes","title":"Notes","text":"

    Ordering enables use as an associative container key

    Together with operator==, operator<=> makes json_pointer a LessThanComparable type, so it can be used as the key type of ordered associative containers such as std::map or std::set.

    Before C++20

    Without C++20's three-way comparison, json_pointer provides a non-member operator< instead, which orders JSON pointers the same way. JSON pointers can therefore be used as keys of ordered associative containers with any supported C++ standard.

    "},{"location":"api/json_pointer/operator_spaceship/#examples","title":"Examples","text":"Example

    The example demonstrates 3-way comparing JSON pointers.

    #include <compare>\n#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nconst char* to_string(const std::strong_ordering& so)\n{\n    if (std::is_lt(so))\n    {\n        return \"less\";\n    }\n    else if (std::is_gt(so))\n    {\n        return \"greater\";\n    }\n    return \"equal\";\n}\n\nint main()\n{\n    // different JSON pointers\n    json::json_pointer ptr1(\"/a/b\");\n    json::json_pointer ptr2(\"/a/c\");\n    json::json_pointer ptr3(\"/a/b/c\");\n    json::json_pointer ptr4(\"/a/b\");\n\n    // 3-way compare JSON pointers\n    std::cout << \"\\\"\" << ptr1 << \"\\\" <=> \\\"\" << ptr2 << \"\\\": \" << to_string(ptr1 <=> ptr2) << '\\n' // *NOPAD*\n              << \"\\\"\" << ptr1 << \"\\\" <=> \\\"\" << ptr3 << \"\\\": \" << to_string(ptr1 <=> ptr3) << '\\n' // *NOPAD*\n              << \"\\\"\" << ptr1 << \"\\\" <=> \\\"\" << ptr4 << \"\\\": \" << to_string(ptr1 <=> ptr4) << std::endl; // *NOPAD*\n}\n

    Output:

    \"/a/b\" <=> \"/a/c\": less\n\"/a/b\" <=> \"/a/b/c\": less\n\"/a/b\" <=> \"/a/b\": equal\n
    "},{"location":"api/json_pointer/operator_spaceship/#see-also","title":"See also","text":"
    • operator== compare: equal
    • operator!= compare: not equal
    "},{"location":"api/json_pointer/operator_spaceship/#version-history","title":"Version history","text":"
    • Added in version 3.11.2.
    "},{"location":"api/json_pointer/operator_string_t/","title":"nlohmann::json_pointer::operator string_t","text":"
    operator string_t() const\n

    Return a string representation of the JSON pointer.

    "},{"location":"api/json_pointer/operator_string_t/#return-value","title":"Return value","text":"

    A string representation of the JSON pointer

    "},{"location":"api/json_pointer/operator_string_t/#possible-implementation","title":"Possible implementation","text":"
    operator string_t() const\n{\n    return to_string();\n}\n
    "},{"location":"api/json_pointer/operator_string_t/#notes","title":"Notes","text":"

    Deprecation

    This function is deprecated in favor of to_string and will be removed in a future major version release.

    See the migration guide for how to update existing code.

    "},{"location":"api/json_pointer/operator_string_t/#examples","title":"Examples","text":"Example

    The example shows how JSON Pointers can be implicitly converted to strings.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // different JSON Pointers\n    json::json_pointer ptr1(\"/foo/0\");\n    json::json_pointer ptr2(\"/a~1b\");\n\n    // implicit conversion to string\n    std::string s;\n    s += ptr1;\n    s += \"\\n\";\n    s += ptr2;\n\n    std::cout << s << std::endl;\n}\n

    Output:

    /foo/0\n/a~1b\n
    "},{"location":"api/json_pointer/operator_string_t/#see-also","title":"See also","text":"
    • to_string return a string representation of the JSON pointer
    • string_t - type for strings
    "},{"location":"api/json_pointer/operator_string_t/#version-history","title":"Version history","text":"
    • Since version 2.0.0.
    • Changed type to string_t and deprecated in version 3.11.0.
    "},{"location":"api/json_pointer/parent_pointer/","title":"nlohmann::json_pointer::parent_pointer","text":"
    json_pointer parent_pointer() const;\n

    Returns the parent of this JSON pointer.

    "},{"location":"api/json_pointer/parent_pointer/#return-value","title":"Return value","text":"

    Parent of this JSON pointer; in case this JSON pointer is the root, the root itself is returned.

    "},{"location":"api/json_pointer/parent_pointer/#exception-safety","title":"Exception safety","text":"

    No-throw guarantee: this function never throws exceptions.

    "},{"location":"api/json_pointer/parent_pointer/#complexity","title":"Complexity","text":"

    Linear in the length of the JSON pointer.

    "},{"location":"api/json_pointer/parent_pointer/#examples","title":"Examples","text":"Example

    The example shows the result of parent_pointer for different JSON Pointers.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // different JSON Pointers\n    json::json_pointer ptr1(\"\");\n    json::json_pointer ptr2(\"/foo\");\n    json::json_pointer ptr3(\"/foo/0\");\n\n    // call parent_pointer()\n    std::cout << std::boolalpha\n              << \"parent of \\\"\" << ptr1 << \"\\\" is \\\"\" << ptr1.parent_pointer() << \"\\\"\\n\"\n              << \"parent of \\\"\" << ptr2 << \"\\\" is \\\"\" << ptr2.parent_pointer() << \"\\\"\\n\"\n              << \"parent of \\\"\" << ptr3 << \"\\\" is \\\"\" << ptr3.parent_pointer() << \"\\\"\" << std::endl;\n}\n

    Output:

    parent of \"\" is \"\"\nparent of \"/foo\" is \"\"\nparent of \"/foo/0\" is \"/foo\"\n
    "},{"location":"api/json_pointer/parent_pointer/#see-also","title":"See also","text":"
    • pop_back remove the last reference token
    • back return the last reference token
    "},{"location":"api/json_pointer/parent_pointer/#version-history","title":"Version history","text":"

    Added in version 3.6.0.

    "},{"location":"api/json_pointer/pop_back/","title":"nlohmann::json_pointer::pop_back","text":"
    void pop_back();\n

    Remove the last reference token.

    "},{"location":"api/json_pointer/pop_back/#exception-safety","title":"Exception safety","text":"

    Strong exception safety: if an exception occurs, the original value stays intact.

    "},{"location":"api/json_pointer/pop_back/#exceptions","title":"Exceptions","text":"

    Throws out_of_range.405 if the JSON pointer has no parent.

    "},{"location":"api/json_pointer/pop_back/#complexity","title":"Complexity","text":"

    Constant.

    "},{"location":"api/json_pointer/pop_back/#examples","title":"Examples","text":"Example

    The example shows the usage of pop_back.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create empty JSON Pointer\n    json::json_pointer ptr(\"/foo/bar/baz\");\n    std::cout << \"\\\"\" << ptr << \"\\\"\\n\";\n\n    // call pop_back()\n    ptr.pop_back();\n    std::cout << \"\\\"\" << ptr << \"\\\"\\n\";\n\n    ptr.pop_back();\n    std::cout << \"\\\"\" << ptr << \"\\\"\\n\";\n\n    ptr.pop_back();\n    std::cout << \"\\\"\" << ptr << \"\\\"\\n\";\n}\n

    Output:

    \"/foo/bar/baz\"\n\"/foo/bar\"\n\"/foo\"\n\"\"\n
    "},{"location":"api/json_pointer/pop_back/#see-also","title":"See also","text":"
    • back return last reference token
    • push_back append an unescaped token at the end of the pointer
    • parent_pointer returns the parent of this JSON pointer
    "},{"location":"api/json_pointer/pop_back/#version-history","title":"Version history","text":"

    Added in version 3.6.0.

    "},{"location":"api/json_pointer/pop_front/","title":"nlohmann::json_pointer::pop_front","text":"
    void pop_front();\n

    Remove the first reference token.

    "},{"location":"api/json_pointer/pop_front/#exception-safety","title":"Exception safety","text":"

    Strong exception safety: if an exception occurs, the original value stays intact.

    "},{"location":"api/json_pointer/pop_front/#exceptions","title":"Exceptions","text":"

    Throws out_of_range.405 if the JSON pointer has no parent.

    "},{"location":"api/json_pointer/pop_front/#complexity","title":"Complexity","text":"

    Linear in the number of reference tokens in the json_pointer.

    "},{"location":"api/json_pointer/pop_front/#examples","title":"Examples","text":"Example

    The example shows the usage of pop_front.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create empty JSON Pointer\n    json::json_pointer ptr(\"/foo/bar/baz\");\n    std::cout << \"\\\"\" << ptr << \"\\\"\\n\";\n\n    // call pop_front()\n    ptr.pop_front();\n    std::cout << \"\\\"\" << ptr << \"\\\"\\n\";\n\n    ptr.pop_front();\n    std::cout << \"\\\"\" << ptr << \"\\\"\\n\";\n\n    ptr.pop_front();\n    std::cout << \"\\\"\" << ptr << \"\\\"\\n\";\n}\n

    Output:

    \"/foo/bar/baz\"\n\"/bar/baz\"\n\"/baz\"\n\"\"\n
    "},{"location":"api/json_pointer/pop_front/#see-also","title":"See also","text":"
    • front return first reference token
    • push_front append an unescaped token at the start of the pointer
    "},{"location":"api/json_pointer/pop_front/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/json_pointer/push_back/","title":"nlohmann::json_pointer::push_back","text":"
    void push_back(const string_t& token);\n\nvoid push_back(string_t&& token);\n

    Append an unescaped token at the end of the reference pointer.

    "},{"location":"api/json_pointer/push_back/#parameters","title":"Parameters","text":"token (in) token to add"},{"location":"api/json_pointer/push_back/#exception-safety","title":"Exception safety","text":"

    Strong exception safety: if an exception occurs, the original value stays intact.

    "},{"location":"api/json_pointer/push_back/#complexity","title":"Complexity","text":"

    Amortized constant.

    "},{"location":"api/json_pointer/push_back/#examples","title":"Examples","text":"Example

    The example shows the result of push_back for different JSON Pointers.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create empty JSON Pointer\n    json::json_pointer ptr;\n    std::cout << \"\\\"\" << ptr << \"\\\"\\n\";\n\n    // call push_back()\n    ptr.push_back(\"foo\");\n    std::cout << \"\\\"\" << ptr << \"\\\"\\n\";\n\n    ptr.push_back(\"0\");\n    std::cout << \"\\\"\" << ptr << \"\\\"\\n\";\n\n    ptr.push_back(\"bar\");\n    std::cout << \"\\\"\" << ptr << \"\\\"\\n\";\n}\n

    Output:

    \"\"\n\"/foo\"\n\"/foo/0\"\n\"/foo/0/bar\"\n
    "},{"location":"api/json_pointer/push_back/#see-also","title":"See also","text":"
    • back return last reference token
    • pop_back remove the last reference token
    • operator/= append to the end of the JSON pointer
    • operator/ create JSON Pointer by appending
    "},{"location":"api/json_pointer/push_back/#version-history","title":"Version history","text":"
    • Added in version 3.6.0.
    • Changed type of token to string_t in version 3.11.0.
    "},{"location":"api/json_pointer/push_front/","title":"nlohmann::json_pointer::push_front","text":"
    void push_front(const string_t& token);\n\nvoid push_front(string_t&& token);\n

    Append an unescaped token at the start of the reference pointer.

    "},{"location":"api/json_pointer/push_front/#parameters","title":"Parameters","text":"token (in) token to add"},{"location":"api/json_pointer/push_front/#exception-safety","title":"Exception safety","text":"

    Basic guarantee: if an exception is thrown (for instance, if copying the reference token fails), the JSON pointer is left in a valid state, but its reference tokens may have changed.

    "},{"location":"api/json_pointer/push_front/#complexity","title":"Complexity","text":"

    Linear in the number of reference tokens in the json_pointer.

    "},{"location":"api/json_pointer/push_front/#examples","title":"Examples","text":"Example

    The example shows the result of push_front for different JSON Pointers.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create empty JSON Pointer\n    json::json_pointer ptr;\n    std::cout << \"\\\"\" << ptr << \"\\\"\\n\";\n\n    // call push_front()\n    ptr.push_front(\"foo\");\n    std::cout << \"\\\"\" << ptr << \"\\\"\\n\";\n\n    ptr.push_front(\"0\");\n    std::cout << \"\\\"\" << ptr << \"\\\"\\n\";\n\n    ptr.push_front(\"bar\");\n    std::cout << \"\\\"\" << ptr << \"\\\"\\n\";\n}\n

    Output:

    \"\"\n\"/foo\"\n\"/0/foo\"\n\"/bar/0/foo\"\n
    "},{"location":"api/json_pointer/push_front/#see-also","title":"See also","text":"
    • front return first reference token
    • pop_front remove the first reference token
    "},{"location":"api/json_pointer/push_front/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/json_pointer/string_t/","title":"nlohmann::json_pointer::string_t","text":"
    using string_t = RefStringType;\n

    The string type used for the reference tokens making up the JSON pointer.

    See basic_json::string_t for more information.

    "},{"location":"api/json_pointer/string_t/#examples","title":"Examples","text":"Example

    The example shows the type string_t and its relation to basic_json::string_t.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    json::json_pointer::string_t s = \"This is a string.\";\n\n    std::cout << s << std::endl;\n\n    std::cout << std::boolalpha << std::is_same<json::json_pointer::string_t, json::string_t>::value << std::endl;\n}\n

    Output:

    This is a string.\ntrue\n
    "},{"location":"api/json_pointer/string_t/#see-also","title":"See also","text":"
    • basic_json::string_t type used to store JSON strings
    • to_string return a string representation of the JSON pointer
    "},{"location":"api/json_pointer/string_t/#version-history","title":"Version history","text":"
    • Added in version 3.11.0.
    "},{"location":"api/json_pointer/to_string/","title":"nlohmann::json_pointer::to_string","text":"
    string_t to_string() const;\n

    Return a string representation of the JSON pointer.

    "},{"location":"api/json_pointer/to_string/#return-value","title":"Return value","text":"

    A string representation of the JSON pointer

    "},{"location":"api/json_pointer/to_string/#exception-safety","title":"Exception safety","text":"

    Strong exception safety: if an exception occurs, the original value stays intact.

    "},{"location":"api/json_pointer/to_string/#complexity","title":"Complexity","text":"

    Linear in the total length of the reference tokens.

    "},{"location":"api/json_pointer/to_string/#notes","title":"Notes","text":"

    For each JSON pointer ptr, it holds:

    ptr == json_pointer(ptr.to_string());\n
    "},{"location":"api/json_pointer/to_string/#examples","title":"Examples","text":"Example

    The example shows the result of to_string.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // different JSON Pointers\n    json::json_pointer ptr1(\"\");\n    json::json_pointer ptr2(\"/foo\");\n    json::json_pointer ptr3(\"/foo/0\");\n    json::json_pointer ptr4(\"/\");\n    json::json_pointer ptr5(\"/a~1b\");\n    json::json_pointer ptr6(\"/c%d\");\n    json::json_pointer ptr7(\"/e^f\");\n    json::json_pointer ptr8(\"/g|h\");\n    json::json_pointer ptr9(\"/i\\\\j\");\n    json::json_pointer ptr10(\"/k\\\"l\");\n    json::json_pointer ptr11(\"/ \");\n    json::json_pointer ptr12(\"/m~0n\");\n\n    std::cout << \"\\\"\" << ptr1.to_string() << \"\\\"\\n\"\n              << \"\\\"\" << ptr2.to_string() << \"\\\"\\n\"\n              << \"\\\"\" << ptr3.to_string() << \"\\\"\\n\"\n              << \"\\\"\" << ptr4.to_string() << \"\\\"\\n\"\n              << \"\\\"\" << ptr5.to_string() << \"\\\"\\n\"\n              << \"\\\"\" << ptr6.to_string() << \"\\\"\\n\"\n              << \"\\\"\" << ptr7.to_string() << \"\\\"\\n\"\n              << \"\\\"\" << ptr8.to_string() << \"\\\"\\n\"\n              << \"\\\"\" << ptr9.to_string() << \"\\\"\\n\"\n              << \"\\\"\" << ptr10.to_string() << \"\\\"\\n\"\n              << \"\\\"\" << ptr11.to_string() << \"\\\"\\n\"\n              << \"\\\"\" << ptr12.to_string() << \"\\\"\" << std::endl;\n}\n

    Output:

    \"\"\n\"/foo\"\n\"/foo/0\"\n\"/\"\n\"/a~1b\"\n\"/c%d\"\n\"/e^f\"\n\"/g|h\"\n\"/i\\j\"\n\"/k\"l\"\n\"/ \"\n\"/m~0n\"\n
    "},{"location":"api/json_pointer/to_string/#see-also","title":"See also","text":"
    • operator string_t return a string representation of the JSON pointer (deprecated)
    • operator<< write a JSON pointer to a stream
    "},{"location":"api/json_pointer/to_string/#version-history","title":"Version history","text":"
    • Since version 2.0.0.
    • Changed return type to string_t in version 3.11.0.
    "},{"location":"api/json_sax/","title":"nlohmann::json_sax","text":"
    template<typename BasicJsonType>\nstruct json_sax;\n

    This class describes the SAX interface used by sax_parse. Each function is called in different situations while the input is parsed. The boolean return value informs the parser whether to continue processing the input.

    For instance, parsing the JSON text {\"a\": [1, true]} triggers the following callbacks, in order:

    sequenceDiagram\n    participant P as Parser\n    participant H as SAX handler\n\n    P->>H: start_object(elements)\n    P->>H: key(\"a\")\n    P->>H: start_array(elements)\n    P->>H: number_unsigned(1)\n    P->>H: boolean(true)\n    P->>H: end_array()\n    P->>H: end_object()

    Note elements is passed as std::numeric_limits<std::size_t>::max() (i.e., \"unknown\") for JSON text input; only binary formats such as CBOR or MessagePack may report the actual number of elements in start_object/ start_array. Also note that 1 is reported via number_unsigned rather than number_integer because it has no leading - sign.

    "},{"location":"api/json_sax/#template-parameters","title":"Template parameters","text":"BasicJsonType a specialization of basic_json"},{"location":"api/json_sax/#member-types","title":"Member types","text":"
    • number_integer_t - BasicJsonType's type for numbers (integer)
    • number_unsigned_t - BasicJsonType's type for numbers (unsigned)
    • number_float_t - BasicJsonType's type for numbers (floating-point)
    • string_t - BasicJsonType's type for strings
    • binary_t - BasicJsonType's type for binary arrays
    "},{"location":"api/json_sax/#member-functions","title":"Member functions","text":"
    • binary (virtual) - a binary value was read
    • boolean (virtual) - a boolean value was read
    • end_array (virtual) - the end of an array was read
    • end_object (virtual) - the end of an object was read
    • key (virtual) - an object key was read
    • null (virtual) - a null value was read
    • number_float (virtual) - a floating-point number was read
    • number_integer (virtual) - an integer number was read
    • number_unsigned (virtual) - an unsigned integer number was read
    • parse_error (virtual) - a parse error occurred
    • start_array (virtual) - the beginning of an array was read
    • start_object (virtual) - the beginning of an object was read
    • string (virtual) - a string value was read
    "},{"location":"api/json_sax/#version-history","title":"Version history","text":"
    • Added in version 3.2.0.
    • Support for binary values (binary_t, binary) added in version 3.8.0.
    "},{"location":"api/json_sax/binary/","title":"nlohmann::json_sax::binary","text":"
    virtual bool binary(binary_t& val) = 0;\n

    A binary value was read.

    "},{"location":"api/json_sax/binary/#parameters","title":"Parameters","text":"val (in) binary value"},{"location":"api/json_sax/binary/#return-value","title":"Return value","text":"

    Whether parsing should proceed.

    "},{"location":"api/json_sax/binary/#notes","title":"Notes","text":"

    It is safe to move the passed binary value.

    "},{"location":"api/json_sax/binary/#examples","title":"Examples","text":"Example

    The example below shows how the SAX interface is used.

    #include <iostream>\n#include <iomanip>\n#include <sstream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\n// a simple event consumer that collects string representations of the passed\n// values; note inheriting from json::json_sax_t is not required, but can\n// help not to forget a required function\nclass sax_event_consumer : public json::json_sax_t\n{\n  public:\n    std::vector<std::string> events;\n\n    bool null() override\n    {\n        events.push_back(\"null()\");\n        return true;\n    }\n\n    bool boolean(bool val) override\n    {\n        events.push_back(\"boolean(val=\" + std::string(val ? \"true\" : \"false\") + \")\");\n        return true;\n    }\n\n    bool number_integer(number_integer_t val) override\n    {\n        events.push_back(\"number_integer(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_unsigned(number_unsigned_t val) override\n    {\n        events.push_back(\"number_unsigned(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_float(number_float_t val, const string_t& s) override\n    {\n        events.push_back(\"number_float(val=\" + std::to_string(val) + \", s=\" + s + \")\");\n        return true;\n    }\n\n    bool string(string_t& val) override\n    {\n        events.push_back(\"string(val=\" + val + \")\");\n        return true;\n    }\n\n    bool start_object(std::size_t elements) override\n    {\n        events.push_back(\"start_object(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_object() override\n    {\n        events.push_back(\"end_object()\");\n        return true;\n    }\n\n    bool start_array(std::size_t elements) override\n    {\n        events.push_back(\"start_array(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_array() override\n    {\n        events.push_back(\"end_array()\");\n        return true;\n    }\n\n    bool key(string_t& val) override\n    {\n        events.push_back(\"key(val=\" + val + \")\");\n        return true;\n    }\n\n    bool binary(json::binary_t& val) override\n    {\n        events.push_back(\"binary(val=[...])\");\n        return true;\n    }\n\n    bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex) override\n    {\n        events.push_back(\"parse_error(position=\" + std::to_string(position) + \", last_token=\" + last_token + \",\\n            ex=\" + std::string(ex.what()) + \")\");\n        return false;\n    }\n};\n\nint main()\n{\n    // CBOR byte string\n    std::vector<std::uint8_t> vec = {{0x44, 0xcA, 0xfe, 0xba, 0xbe}};\n\n    // create a SAX event consumer object\n    sax_event_consumer sec;\n\n    // parse CBOR\n    bool result = json::sax_parse(vec, &sec, json::input_format_t::cbor);\n\n    // output the recorded events\n    for (auto& event : sec.events)\n    {\n        std::cout << event << \"\\n\";\n    }\n\n    // output the result of sax_parse\n    std::cout << \"\\nresult: \" << std::boolalpha << result << std::endl;\n}\n

    Output:

    binary(val=[...])\n\nresult: true\n
    "},{"location":"api/json_sax/binary/#see-also","title":"See also","text":"
    • sax_parse - SAX parser
    • SAX Interface - the SAX interface article
    • Binary Values - the article on binary values
    "},{"location":"api/json_sax/binary/#version-history","title":"Version history","text":"
    • Added in version 3.8.0.
    "},{"location":"api/json_sax/boolean/","title":"nlohmann::json_sax::boolean","text":"
    virtual bool boolean(bool val) = 0;\n

    A boolean value was read.

    "},{"location":"api/json_sax/boolean/#parameters","title":"Parameters","text":"val (in) boolean value"},{"location":"api/json_sax/boolean/#return-value","title":"Return value","text":"

    Whether parsing should proceed.

    "},{"location":"api/json_sax/boolean/#examples","title":"Examples","text":"Example

    The example below shows how the SAX interface is used.

    #include <iostream>\n#include <iomanip>\n#include <sstream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\n// a simple event consumer that collects string representations of the passed\n// values; note inheriting from json::json_sax_t is not required, but can\n// help not to forget a required function\nclass sax_event_consumer : public json::json_sax_t\n{\n  public:\n    std::vector<std::string> events;\n\n    bool null() override\n    {\n        events.push_back(\"null()\");\n        return true;\n    }\n\n    bool boolean(bool val) override\n    {\n        events.push_back(\"boolean(val=\" + std::string(val ? \"true\" : \"false\") + \")\");\n        return true;\n    }\n\n    bool number_integer(number_integer_t val) override\n    {\n        events.push_back(\"number_integer(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_unsigned(number_unsigned_t val) override\n    {\n        events.push_back(\"number_unsigned(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_float(number_float_t val, const string_t& s) override\n    {\n        events.push_back(\"number_float(val=\" + std::to_string(val) + \", s=\" + s + \")\");\n        return true;\n    }\n\n    bool string(string_t& val) override\n    {\n        events.push_back(\"string(val=\" + val + \")\");\n        return true;\n    }\n\n    bool start_object(std::size_t elements) override\n    {\n        events.push_back(\"start_object(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_object() override\n    {\n        events.push_back(\"end_object()\");\n        return true;\n    }\n\n    bool start_array(std::size_t elements) override\n    {\n        events.push_back(\"start_array(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_array() override\n    {\n        events.push_back(\"end_array()\");\n        return true;\n    }\n\n    bool key(string_t& val) override\n    {\n        events.push_back(\"key(val=\" + val + \")\");\n        return true;\n    }\n\n    bool binary(json::binary_t& val) override\n    {\n        events.push_back(\"binary(val=[...])\");\n        return true;\n    }\n\n    bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex) override\n    {\n        events.push_back(\"parse_error(position=\" + std::to_string(position) + \", last_token=\" + last_token + \",\\n            ex=\" + std::string(ex.what()) + \")\");\n        return false;\n    }\n};\n\nint main()\n{\n    // a JSON text\n    auto text = R\"(\n    {\n        \"Image\": {\n            \"Width\":  800,\n            \"Height\": 600,\n            \"Title\":  \"View from 15th Floor\",\n            \"Thumbnail\": {\n                \"Url\":    \"http://www.example.com/image/481989943\",\n                \"Height\": 125,\n                \"Width\":  100\n            },\n            \"Animated\" : false,\n            \"IDs\": [116, 943, 234, -38793],\n            \"DeletionDate\": null,\n            \"Distance\": 12.723374634\n        }\n    }]\n    )\";\n\n    // create a SAX event consumer object\n    sax_event_consumer sec;\n\n    // parse JSON\n    bool result = json::sax_parse(text, &sec);\n\n    // output the recorded events\n    for (auto& event : sec.events)\n    {\n        std::cout << event << \"\\n\";\n    }\n\n    // output the result of sax_parse\n    std::cout << \"\\nresult: \" << std::boolalpha << result << std::endl;\n}\n

    Output:

    start_object(elements=18446744073709551615)\nkey(val=Image)\nstart_object(elements=18446744073709551615)\nkey(val=Width)\nnumber_unsigned(val=800)\nkey(val=Height)\nnumber_unsigned(val=600)\nkey(val=Title)\nstring(val=View from 15th Floor)\nkey(val=Thumbnail)\nstart_object(elements=18446744073709551615)\nkey(val=Url)\nstring(val=http://www.example.com/image/481989943)\nkey(val=Height)\nnumber_unsigned(val=125)\nkey(val=Width)\nnumber_unsigned(val=100)\nend_object()\nkey(val=Animated)\nboolean(val=false)\nkey(val=IDs)\nstart_array(elements=18446744073709551615)\nnumber_unsigned(val=116)\nnumber_unsigned(val=943)\nnumber_unsigned(val=234)\nnumber_integer(val=-38793)\nend_array()\nkey(val=DeletionDate)\nnull()\nkey(val=Distance)\nnumber_float(val=12.723375, s=12.723374634)\nend_object()\nend_object()\nparse_error(position=460, last_token=12.723374634<U+000A>        }<U+000A>    }],\n            ex=[json.exception.parse_error.101] parse error at line 17, column 6: syntax error while parsing value - unexpected ']'; expected end of input)\n\nresult: false\n
    "},{"location":"api/json_sax/boolean/#see-also","title":"See also","text":"
    • sax_parse - SAX parser
    • SAX Interface - the SAX interface article
    "},{"location":"api/json_sax/boolean/#version-history","title":"Version history","text":"
    • Added in version 3.2.0.
    "},{"location":"api/json_sax/end_array/","title":"nlohmann::json_sax::end_array","text":"
    virtual bool end_array() = 0;\n

    The end of an array was read.

    "},{"location":"api/json_sax/end_array/#return-value","title":"Return value","text":"

    Whether parsing should proceed.

    "},{"location":"api/json_sax/end_array/#examples","title":"Examples","text":"Example

    The example below shows how the SAX interface is used.

    #include <iostream>\n#include <iomanip>\n#include <sstream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\n// a simple event consumer that collects string representations of the passed\n// values; note inheriting from json::json_sax_t is not required, but can\n// help not to forget a required function\nclass sax_event_consumer : public json::json_sax_t\n{\n  public:\n    std::vector<std::string> events;\n\n    bool null() override\n    {\n        events.push_back(\"null()\");\n        return true;\n    }\n\n    bool boolean(bool val) override\n    {\n        events.push_back(\"boolean(val=\" + std::string(val ? \"true\" : \"false\") + \")\");\n        return true;\n    }\n\n    bool number_integer(number_integer_t val) override\n    {\n        events.push_back(\"number_integer(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_unsigned(number_unsigned_t val) override\n    {\n        events.push_back(\"number_unsigned(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_float(number_float_t val, const string_t& s) override\n    {\n        events.push_back(\"number_float(val=\" + std::to_string(val) + \", s=\" + s + \")\");\n        return true;\n    }\n\n    bool string(string_t& val) override\n    {\n        events.push_back(\"string(val=\" + val + \")\");\n        return true;\n    }\n\n    bool start_object(std::size_t elements) override\n    {\n        events.push_back(\"start_object(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_object() override\n    {\n        events.push_back(\"end_object()\");\n        return true;\n    }\n\n    bool start_array(std::size_t elements) override\n    {\n        events.push_back(\"start_array(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_array() override\n    {\n        events.push_back(\"end_array()\");\n        return true;\n    }\n\n    bool key(string_t& val) override\n    {\n        events.push_back(\"key(val=\" + val + \")\");\n        return true;\n    }\n\n    bool binary(json::binary_t& val) override\n    {\n        events.push_back(\"binary(val=[...])\");\n        return true;\n    }\n\n    bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex) override\n    {\n        events.push_back(\"parse_error(position=\" + std::to_string(position) + \", last_token=\" + last_token + \",\\n            ex=\" + std::string(ex.what()) + \")\");\n        return false;\n    }\n};\n\nint main()\n{\n    // a JSON text\n    auto text = R\"(\n    {\n        \"Image\": {\n            \"Width\":  800,\n            \"Height\": 600,\n            \"Title\":  \"View from 15th Floor\",\n            \"Thumbnail\": {\n                \"Url\":    \"http://www.example.com/image/481989943\",\n                \"Height\": 125,\n                \"Width\":  100\n            },\n            \"Animated\" : false,\n            \"IDs\": [116, 943, 234, -38793],\n            \"DeletionDate\": null,\n            \"Distance\": 12.723374634\n        }\n    }]\n    )\";\n\n    // create a SAX event consumer object\n    sax_event_consumer sec;\n\n    // parse JSON\n    bool result = json::sax_parse(text, &sec);\n\n    // output the recorded events\n    for (auto& event : sec.events)\n    {\n        std::cout << event << \"\\n\";\n    }\n\n    // output the result of sax_parse\n    std::cout << \"\\nresult: \" << std::boolalpha << result << std::endl;\n}\n

    Output:

    start_object(elements=18446744073709551615)\nkey(val=Image)\nstart_object(elements=18446744073709551615)\nkey(val=Width)\nnumber_unsigned(val=800)\nkey(val=Height)\nnumber_unsigned(val=600)\nkey(val=Title)\nstring(val=View from 15th Floor)\nkey(val=Thumbnail)\nstart_object(elements=18446744073709551615)\nkey(val=Url)\nstring(val=http://www.example.com/image/481989943)\nkey(val=Height)\nnumber_unsigned(val=125)\nkey(val=Width)\nnumber_unsigned(val=100)\nend_object()\nkey(val=Animated)\nboolean(val=false)\nkey(val=IDs)\nstart_array(elements=18446744073709551615)\nnumber_unsigned(val=116)\nnumber_unsigned(val=943)\nnumber_unsigned(val=234)\nnumber_integer(val=-38793)\nend_array()\nkey(val=DeletionDate)\nnull()\nkey(val=Distance)\nnumber_float(val=12.723375, s=12.723374634)\nend_object()\nend_object()\nparse_error(position=460, last_token=12.723374634<U+000A>        }<U+000A>    }],\n            ex=[json.exception.parse_error.101] parse error at line 17, column 6: syntax error while parsing value - unexpected ']'; expected end of input)\n\nresult: false\n
    "},{"location":"api/json_sax/end_array/#see-also","title":"See also","text":"
    • start_array - the beginning of an array was read
    • sax_parse - SAX parser
    "},{"location":"api/json_sax/end_array/#version-history","title":"Version history","text":"
    • Added in version 3.2.0.
    "},{"location":"api/json_sax/end_object/","title":"nlohmann::json_sax::end_object","text":"
    virtual bool end_object() = 0;\n

    The end of an object was read.

    "},{"location":"api/json_sax/end_object/#return-value","title":"Return value","text":"

    Whether parsing should proceed.

    "},{"location":"api/json_sax/end_object/#examples","title":"Examples","text":"Example

    The example below shows how the SAX interface is used.

    #include <iostream>\n#include <iomanip>\n#include <sstream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\n// a simple event consumer that collects string representations of the passed\n// values; note inheriting from json::json_sax_t is not required, but can\n// help not to forget a required function\nclass sax_event_consumer : public json::json_sax_t\n{\n  public:\n    std::vector<std::string> events;\n\n    bool null() override\n    {\n        events.push_back(\"null()\");\n        return true;\n    }\n\n    bool boolean(bool val) override\n    {\n        events.push_back(\"boolean(val=\" + std::string(val ? \"true\" : \"false\") + \")\");\n        return true;\n    }\n\n    bool number_integer(number_integer_t val) override\n    {\n        events.push_back(\"number_integer(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_unsigned(number_unsigned_t val) override\n    {\n        events.push_back(\"number_unsigned(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_float(number_float_t val, const string_t& s) override\n    {\n        events.push_back(\"number_float(val=\" + std::to_string(val) + \", s=\" + s + \")\");\n        return true;\n    }\n\n    bool string(string_t& val) override\n    {\n        events.push_back(\"string(val=\" + val + \")\");\n        return true;\n    }\n\n    bool start_object(std::size_t elements) override\n    {\n        events.push_back(\"start_object(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_object() override\n    {\n        events.push_back(\"end_object()\");\n        return true;\n    }\n\n    bool start_array(std::size_t elements) override\n    {\n        events.push_back(\"start_array(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_array() override\n    {\n        events.push_back(\"end_array()\");\n        return true;\n    }\n\n    bool key(string_t& val) override\n    {\n        events.push_back(\"key(val=\" + val + \")\");\n        return true;\n    }\n\n    bool binary(json::binary_t& val) override\n    {\n        events.push_back(\"binary(val=[...])\");\n        return true;\n    }\n\n    bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex) override\n    {\n        events.push_back(\"parse_error(position=\" + std::to_string(position) + \", last_token=\" + last_token + \",\\n            ex=\" + std::string(ex.what()) + \")\");\n        return false;\n    }\n};\n\nint main()\n{\n    // a JSON text\n    auto text = R\"(\n    {\n        \"Image\": {\n            \"Width\":  800,\n            \"Height\": 600,\n            \"Title\":  \"View from 15th Floor\",\n            \"Thumbnail\": {\n                \"Url\":    \"http://www.example.com/image/481989943\",\n                \"Height\": 125,\n                \"Width\":  100\n            },\n            \"Animated\" : false,\n            \"IDs\": [116, 943, 234, -38793],\n            \"DeletionDate\": null,\n            \"Distance\": 12.723374634\n        }\n    }]\n    )\";\n\n    // create a SAX event consumer object\n    sax_event_consumer sec;\n\n    // parse JSON\n    bool result = json::sax_parse(text, &sec);\n\n    // output the recorded events\n    for (auto& event : sec.events)\n    {\n        std::cout << event << \"\\n\";\n    }\n\n    // output the result of sax_parse\n    std::cout << \"\\nresult: \" << std::boolalpha << result << std::endl;\n}\n

    Output:

    start_object(elements=18446744073709551615)\nkey(val=Image)\nstart_object(elements=18446744073709551615)\nkey(val=Width)\nnumber_unsigned(val=800)\nkey(val=Height)\nnumber_unsigned(val=600)\nkey(val=Title)\nstring(val=View from 15th Floor)\nkey(val=Thumbnail)\nstart_object(elements=18446744073709551615)\nkey(val=Url)\nstring(val=http://www.example.com/image/481989943)\nkey(val=Height)\nnumber_unsigned(val=125)\nkey(val=Width)\nnumber_unsigned(val=100)\nend_object()\nkey(val=Animated)\nboolean(val=false)\nkey(val=IDs)\nstart_array(elements=18446744073709551615)\nnumber_unsigned(val=116)\nnumber_unsigned(val=943)\nnumber_unsigned(val=234)\nnumber_integer(val=-38793)\nend_array()\nkey(val=DeletionDate)\nnull()\nkey(val=Distance)\nnumber_float(val=12.723375, s=12.723374634)\nend_object()\nend_object()\nparse_error(position=460, last_token=12.723374634<U+000A>        }<U+000A>    }],\n            ex=[json.exception.parse_error.101] parse error at line 17, column 6: syntax error while parsing value - unexpected ']'; expected end of input)\n\nresult: false\n
    "},{"location":"api/json_sax/end_object/#see-also","title":"See also","text":"
    • start_object - the beginning of an object was read
    • key - an object key was read
    • sax_parse - SAX parser
    "},{"location":"api/json_sax/end_object/#version-history","title":"Version history","text":"
    • Added in version 3.2.0.
    "},{"location":"api/json_sax/key/","title":"nlohmann::json_sax::key","text":"
    virtual bool key(string_t& val) = 0;\n

    An object key was read.

    "},{"location":"api/json_sax/key/#parameters","title":"Parameters","text":"val (in) object key"},{"location":"api/json_sax/key/#return-value","title":"Return value","text":"

    Whether parsing should proceed.

    "},{"location":"api/json_sax/key/#notes","title":"Notes","text":"

    It is safe to move the passed object key value.

    "},{"location":"api/json_sax/key/#examples","title":"Examples","text":"Example

    The example below shows how the SAX interface is used.

    #include <iostream>\n#include <iomanip>\n#include <sstream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\n// a simple event consumer that collects string representations of the passed\n// values; note inheriting from json::json_sax_t is not required, but can\n// help not to forget a required function\nclass sax_event_consumer : public json::json_sax_t\n{\n  public:\n    std::vector<std::string> events;\n\n    bool null() override\n    {\n        events.push_back(\"null()\");\n        return true;\n    }\n\n    bool boolean(bool val) override\n    {\n        events.push_back(\"boolean(val=\" + std::string(val ? \"true\" : \"false\") + \")\");\n        return true;\n    }\n\n    bool number_integer(number_integer_t val) override\n    {\n        events.push_back(\"number_integer(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_unsigned(number_unsigned_t val) override\n    {\n        events.push_back(\"number_unsigned(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_float(number_float_t val, const string_t& s) override\n    {\n        events.push_back(\"number_float(val=\" + std::to_string(val) + \", s=\" + s + \")\");\n        return true;\n    }\n\n    bool string(string_t& val) override\n    {\n        events.push_back(\"string(val=\" + val + \")\");\n        return true;\n    }\n\n    bool start_object(std::size_t elements) override\n    {\n        events.push_back(\"start_object(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_object() override\n    {\n        events.push_back(\"end_object()\");\n        return true;\n    }\n\n    bool start_array(std::size_t elements) override\n    {\n        events.push_back(\"start_array(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_array() override\n    {\n        events.push_back(\"end_array()\");\n        return true;\n    }\n\n    bool key(string_t& val) override\n    {\n        events.push_back(\"key(val=\" + val + \")\");\n        return true;\n    }\n\n    bool binary(json::binary_t& val) override\n    {\n        events.push_back(\"binary(val=[...])\");\n        return true;\n    }\n\n    bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex) override\n    {\n        events.push_back(\"parse_error(position=\" + std::to_string(position) + \", last_token=\" + last_token + \",\\n            ex=\" + std::string(ex.what()) + \")\");\n        return false;\n    }\n};\n\nint main()\n{\n    // a JSON text\n    auto text = R\"(\n    {\n        \"Image\": {\n            \"Width\":  800,\n            \"Height\": 600,\n            \"Title\":  \"View from 15th Floor\",\n            \"Thumbnail\": {\n                \"Url\":    \"http://www.example.com/image/481989943\",\n                \"Height\": 125,\n                \"Width\":  100\n            },\n            \"Animated\" : false,\n            \"IDs\": [116, 943, 234, -38793],\n            \"DeletionDate\": null,\n            \"Distance\": 12.723374634\n        }\n    }]\n    )\";\n\n    // create a SAX event consumer object\n    sax_event_consumer sec;\n\n    // parse JSON\n    bool result = json::sax_parse(text, &sec);\n\n    // output the recorded events\n    for (auto& event : sec.events)\n    {\n        std::cout << event << \"\\n\";\n    }\n\n    // output the result of sax_parse\n    std::cout << \"\\nresult: \" << std::boolalpha << result << std::endl;\n}\n

    Output:

    start_object(elements=18446744073709551615)\nkey(val=Image)\nstart_object(elements=18446744073709551615)\nkey(val=Width)\nnumber_unsigned(val=800)\nkey(val=Height)\nnumber_unsigned(val=600)\nkey(val=Title)\nstring(val=View from 15th Floor)\nkey(val=Thumbnail)\nstart_object(elements=18446744073709551615)\nkey(val=Url)\nstring(val=http://www.example.com/image/481989943)\nkey(val=Height)\nnumber_unsigned(val=125)\nkey(val=Width)\nnumber_unsigned(val=100)\nend_object()\nkey(val=Animated)\nboolean(val=false)\nkey(val=IDs)\nstart_array(elements=18446744073709551615)\nnumber_unsigned(val=116)\nnumber_unsigned(val=943)\nnumber_unsigned(val=234)\nnumber_integer(val=-38793)\nend_array()\nkey(val=DeletionDate)\nnull()\nkey(val=Distance)\nnumber_float(val=12.723375, s=12.723374634)\nend_object()\nend_object()\nparse_error(position=460, last_token=12.723374634<U+000A>        }<U+000A>    }],\n            ex=[json.exception.parse_error.101] parse error at line 17, column 6: syntax error while parsing value - unexpected ']'; expected end of input)\n\nresult: false\n
    "},{"location":"api/json_sax/key/#see-also","title":"See also","text":"
    • start_object - the beginning of an object was read
    • end_object - the end of an object was read
    • sax_parse - SAX parser
    "},{"location":"api/json_sax/key/#version-history","title":"Version history","text":"
    • Added in version 3.2.0.
    "},{"location":"api/json_sax/null/","title":"nlohmann::json_sax::null","text":"
    virtual bool null() = 0;\n

    A null value was read.

    "},{"location":"api/json_sax/null/#return-value","title":"Return value","text":"

    Whether parsing should proceed.

    "},{"location":"api/json_sax/null/#examples","title":"Examples","text":"Example

    The example below shows how the SAX interface is used.

    #include <iostream>\n#include <iomanip>\n#include <sstream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\n// a simple event consumer that collects string representations of the passed\n// values; note inheriting from json::json_sax_t is not required, but can\n// help not to forget a required function\nclass sax_event_consumer : public json::json_sax_t\n{\n  public:\n    std::vector<std::string> events;\n\n    bool null() override\n    {\n        events.push_back(\"null()\");\n        return true;\n    }\n\n    bool boolean(bool val) override\n    {\n        events.push_back(\"boolean(val=\" + std::string(val ? \"true\" : \"false\") + \")\");\n        return true;\n    }\n\n    bool number_integer(number_integer_t val) override\n    {\n        events.push_back(\"number_integer(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_unsigned(number_unsigned_t val) override\n    {\n        events.push_back(\"number_unsigned(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_float(number_float_t val, const string_t& s) override\n    {\n        events.push_back(\"number_float(val=\" + std::to_string(val) + \", s=\" + s + \")\");\n        return true;\n    }\n\n    bool string(string_t& val) override\n    {\n        events.push_back(\"string(val=\" + val + \")\");\n        return true;\n    }\n\n    bool start_object(std::size_t elements) override\n    {\n        events.push_back(\"start_object(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_object() override\n    {\n        events.push_back(\"end_object()\");\n        return true;\n    }\n\n    bool start_array(std::size_t elements) override\n    {\n        events.push_back(\"start_array(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_array() override\n    {\n        events.push_back(\"end_array()\");\n        return true;\n    }\n\n    bool key(string_t& val) override\n    {\n        events.push_back(\"key(val=\" + val + \")\");\n        return true;\n    }\n\n    bool binary(json::binary_t& val) override\n    {\n        events.push_back(\"binary(val=[...])\");\n        return true;\n    }\n\n    bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex) override\n    {\n        events.push_back(\"parse_error(position=\" + std::to_string(position) + \", last_token=\" + last_token + \",\\n            ex=\" + std::string(ex.what()) + \")\");\n        return false;\n    }\n};\n\nint main()\n{\n    // a JSON text\n    auto text = R\"(\n    {\n        \"Image\": {\n            \"Width\":  800,\n            \"Height\": 600,\n            \"Title\":  \"View from 15th Floor\",\n            \"Thumbnail\": {\n                \"Url\":    \"http://www.example.com/image/481989943\",\n                \"Height\": 125,\n                \"Width\":  100\n            },\n            \"Animated\" : false,\n            \"IDs\": [116, 943, 234, -38793],\n            \"DeletionDate\": null,\n            \"Distance\": 12.723374634\n        }\n    }]\n    )\";\n\n    // create a SAX event consumer object\n    sax_event_consumer sec;\n\n    // parse JSON\n    bool result = json::sax_parse(text, &sec);\n\n    // output the recorded events\n    for (auto& event : sec.events)\n    {\n        std::cout << event << \"\\n\";\n    }\n\n    // output the result of sax_parse\n    std::cout << \"\\nresult: \" << std::boolalpha << result << std::endl;\n}\n

    Output:

    start_object(elements=18446744073709551615)\nkey(val=Image)\nstart_object(elements=18446744073709551615)\nkey(val=Width)\nnumber_unsigned(val=800)\nkey(val=Height)\nnumber_unsigned(val=600)\nkey(val=Title)\nstring(val=View from 15th Floor)\nkey(val=Thumbnail)\nstart_object(elements=18446744073709551615)\nkey(val=Url)\nstring(val=http://www.example.com/image/481989943)\nkey(val=Height)\nnumber_unsigned(val=125)\nkey(val=Width)\nnumber_unsigned(val=100)\nend_object()\nkey(val=Animated)\nboolean(val=false)\nkey(val=IDs)\nstart_array(elements=18446744073709551615)\nnumber_unsigned(val=116)\nnumber_unsigned(val=943)\nnumber_unsigned(val=234)\nnumber_integer(val=-38793)\nend_array()\nkey(val=DeletionDate)\nnull()\nkey(val=Distance)\nnumber_float(val=12.723375, s=12.723374634)\nend_object()\nend_object()\nparse_error(position=460, last_token=12.723374634<U+000A>        }<U+000A>    }],\n            ex=[json.exception.parse_error.101] parse error at line 17, column 6: syntax error while parsing value - unexpected ']'; expected end of input)\n\nresult: false\n
    "},{"location":"api/json_sax/null/#see-also","title":"See also","text":"
    • sax_parse - SAX parser
    • SAX Interface - the SAX interface article
    "},{"location":"api/json_sax/null/#version-history","title":"Version history","text":"
    • Added in version 3.2.0.
    "},{"location":"api/json_sax/number_float/","title":"nlohmann::json_sax::number_float","text":"
    virtual bool number_float(number_float_t val, const string_t& s) = 0;\n

    A floating-point number was read.

    "},{"location":"api/json_sax/number_float/#parameters","title":"Parameters","text":"val (in) floating-point value s (in) string representation of the original input"},{"location":"api/json_sax/number_float/#return-value","title":"Return value","text":"

    Whether parsing should proceed.

    "},{"location":"api/json_sax/number_float/#examples","title":"Examples","text":"Example

    The example below shows how the SAX interface is used.

    #include <iostream>\n#include <iomanip>\n#include <sstream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\n// a simple event consumer that collects string representations of the passed\n// values; note inheriting from json::json_sax_t is not required, but can\n// help not to forget a required function\nclass sax_event_consumer : public json::json_sax_t\n{\n  public:\n    std::vector<std::string> events;\n\n    bool null() override\n    {\n        events.push_back(\"null()\");\n        return true;\n    }\n\n    bool boolean(bool val) override\n    {\n        events.push_back(\"boolean(val=\" + std::string(val ? \"true\" : \"false\") + \")\");\n        return true;\n    }\n\n    bool number_integer(number_integer_t val) override\n    {\n        events.push_back(\"number_integer(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_unsigned(number_unsigned_t val) override\n    {\n        events.push_back(\"number_unsigned(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_float(number_float_t val, const string_t& s) override\n    {\n        events.push_back(\"number_float(val=\" + std::to_string(val) + \", s=\" + s + \")\");\n        return true;\n    }\n\n    bool string(string_t& val) override\n    {\n        events.push_back(\"string(val=\" + val + \")\");\n        return true;\n    }\n\n    bool start_object(std::size_t elements) override\n    {\n        events.push_back(\"start_object(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_object() override\n    {\n        events.push_back(\"end_object()\");\n        return true;\n    }\n\n    bool start_array(std::size_t elements) override\n    {\n        events.push_back(\"start_array(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_array() override\n    {\n        events.push_back(\"end_array()\");\n        return true;\n    }\n\n    bool key(string_t& val) override\n    {\n        events.push_back(\"key(val=\" + val + \")\");\n        return true;\n    }\n\n    bool binary(json::binary_t& val) override\n    {\n        events.push_back(\"binary(val=[...])\");\n        return true;\n    }\n\n    bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex) override\n    {\n        events.push_back(\"parse_error(position=\" + std::to_string(position) + \", last_token=\" + last_token + \",\\n            ex=\" + std::string(ex.what()) + \")\");\n        return false;\n    }\n};\n\nint main()\n{\n    // a JSON text\n    auto text = R\"(\n    {\n        \"Image\": {\n            \"Width\":  800,\n            \"Height\": 600,\n            \"Title\":  \"View from 15th Floor\",\n            \"Thumbnail\": {\n                \"Url\":    \"http://www.example.com/image/481989943\",\n                \"Height\": 125,\n                \"Width\":  100\n            },\n            \"Animated\" : false,\n            \"IDs\": [116, 943, 234, -38793],\n            \"DeletionDate\": null,\n            \"Distance\": 12.723374634\n        }\n    }]\n    )\";\n\n    // create a SAX event consumer object\n    sax_event_consumer sec;\n\n    // parse JSON\n    bool result = json::sax_parse(text, &sec);\n\n    // output the recorded events\n    for (auto& event : sec.events)\n    {\n        std::cout << event << \"\\n\";\n    }\n\n    // output the result of sax_parse\n    std::cout << \"\\nresult: \" << std::boolalpha << result << std::endl;\n}\n

    Output:

    start_object(elements=18446744073709551615)\nkey(val=Image)\nstart_object(elements=18446744073709551615)\nkey(val=Width)\nnumber_unsigned(val=800)\nkey(val=Height)\nnumber_unsigned(val=600)\nkey(val=Title)\nstring(val=View from 15th Floor)\nkey(val=Thumbnail)\nstart_object(elements=18446744073709551615)\nkey(val=Url)\nstring(val=http://www.example.com/image/481989943)\nkey(val=Height)\nnumber_unsigned(val=125)\nkey(val=Width)\nnumber_unsigned(val=100)\nend_object()\nkey(val=Animated)\nboolean(val=false)\nkey(val=IDs)\nstart_array(elements=18446744073709551615)\nnumber_unsigned(val=116)\nnumber_unsigned(val=943)\nnumber_unsigned(val=234)\nnumber_integer(val=-38793)\nend_array()\nkey(val=DeletionDate)\nnull()\nkey(val=Distance)\nnumber_float(val=12.723375, s=12.723374634)\nend_object()\nend_object()\nparse_error(position=460, last_token=12.723374634<U+000A>        }<U+000A>    }],\n            ex=[json.exception.parse_error.101] parse error at line 17, column 6: syntax error while parsing value - unexpected ']'; expected end of input)\n\nresult: false\n
    "},{"location":"api/json_sax/number_float/#see-also","title":"See also","text":"
    • number_integer - an integer number was read
    • number_unsigned - an unsigned integer number was read
    • sax_parse - SAX parser
    "},{"location":"api/json_sax/number_float/#version-history","title":"Version history","text":"
    • Added in version 3.2.0.
    "},{"location":"api/json_sax/number_integer/","title":"nlohmann::json_sax::number_integer","text":"
    virtual bool number_integer(number_integer_t val) = 0;\n

    An integer number was read.

    "},{"location":"api/json_sax/number_integer/#parameters","title":"Parameters","text":"val (in) integer value"},{"location":"api/json_sax/number_integer/#return-value","title":"Return value","text":"

    Whether parsing should proceed.

    "},{"location":"api/json_sax/number_integer/#examples","title":"Examples","text":"Example

    The example below shows how the SAX interface is used.

    #include <iostream>\n#include <iomanip>\n#include <sstream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\n// a simple event consumer that collects string representations of the passed\n// values; note inheriting from json::json_sax_t is not required, but can\n// help not to forget a required function\nclass sax_event_consumer : public json::json_sax_t\n{\n  public:\n    std::vector<std::string> events;\n\n    bool null() override\n    {\n        events.push_back(\"null()\");\n        return true;\n    }\n\n    bool boolean(bool val) override\n    {\n        events.push_back(\"boolean(val=\" + std::string(val ? \"true\" : \"false\") + \")\");\n        return true;\n    }\n\n    bool number_integer(number_integer_t val) override\n    {\n        events.push_back(\"number_integer(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_unsigned(number_unsigned_t val) override\n    {\n        events.push_back(\"number_unsigned(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_float(number_float_t val, const string_t& s) override\n    {\n        events.push_back(\"number_float(val=\" + std::to_string(val) + \", s=\" + s + \")\");\n        return true;\n    }\n\n    bool string(string_t& val) override\n    {\n        events.push_back(\"string(val=\" + val + \")\");\n        return true;\n    }\n\n    bool start_object(std::size_t elements) override\n    {\n        events.push_back(\"start_object(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_object() override\n    {\n        events.push_back(\"end_object()\");\n        return true;\n    }\n\n    bool start_array(std::size_t elements) override\n    {\n        events.push_back(\"start_array(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_array() override\n    {\n        events.push_back(\"end_array()\");\n        return true;\n    }\n\n    bool key(string_t& val) override\n    {\n        events.push_back(\"key(val=\" + val + \")\");\n        return true;\n    }\n\n    bool binary(json::binary_t& val) override\n    {\n        events.push_back(\"binary(val=[...])\");\n        return true;\n    }\n\n    bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex) override\n    {\n        events.push_back(\"parse_error(position=\" + std::to_string(position) + \", last_token=\" + last_token + \",\\n            ex=\" + std::string(ex.what()) + \")\");\n        return false;\n    }\n};\n\nint main()\n{\n    // a JSON text\n    auto text = R\"(\n    {\n        \"Image\": {\n            \"Width\":  800,\n            \"Height\": 600,\n            \"Title\":  \"View from 15th Floor\",\n            \"Thumbnail\": {\n                \"Url\":    \"http://www.example.com/image/481989943\",\n                \"Height\": 125,\n                \"Width\":  100\n            },\n            \"Animated\" : false,\n            \"IDs\": [116, 943, 234, -38793],\n            \"DeletionDate\": null,\n            \"Distance\": 12.723374634\n        }\n    }]\n    )\";\n\n    // create a SAX event consumer object\n    sax_event_consumer sec;\n\n    // parse JSON\n    bool result = json::sax_parse(text, &sec);\n\n    // output the recorded events\n    for (auto& event : sec.events)\n    {\n        std::cout << event << \"\\n\";\n    }\n\n    // output the result of sax_parse\n    std::cout << \"\\nresult: \" << std::boolalpha << result << std::endl;\n}\n

    Output:

    start_object(elements=18446744073709551615)\nkey(val=Image)\nstart_object(elements=18446744073709551615)\nkey(val=Width)\nnumber_unsigned(val=800)\nkey(val=Height)\nnumber_unsigned(val=600)\nkey(val=Title)\nstring(val=View from 15th Floor)\nkey(val=Thumbnail)\nstart_object(elements=18446744073709551615)\nkey(val=Url)\nstring(val=http://www.example.com/image/481989943)\nkey(val=Height)\nnumber_unsigned(val=125)\nkey(val=Width)\nnumber_unsigned(val=100)\nend_object()\nkey(val=Animated)\nboolean(val=false)\nkey(val=IDs)\nstart_array(elements=18446744073709551615)\nnumber_unsigned(val=116)\nnumber_unsigned(val=943)\nnumber_unsigned(val=234)\nnumber_integer(val=-38793)\nend_array()\nkey(val=DeletionDate)\nnull()\nkey(val=Distance)\nnumber_float(val=12.723375, s=12.723374634)\nend_object()\nend_object()\nparse_error(position=460, last_token=12.723374634<U+000A>        }<U+000A>    }],\n            ex=[json.exception.parse_error.101] parse error at line 17, column 6: syntax error while parsing value - unexpected ']'; expected end of input)\n\nresult: false\n
    "},{"location":"api/json_sax/number_integer/#see-also","title":"See also","text":"
    • number_unsigned - an unsigned integer number was read
    • number_float - a floating-point number was read
    • sax_parse - SAX parser
    "},{"location":"api/json_sax/number_integer/#version-history","title":"Version history","text":"
    • Added in version 3.2.0.
    "},{"location":"api/json_sax/number_unsigned/","title":"nlohmann::json_sax::number_unsigned","text":"
    virtual bool number_unsigned(number_unsigned_t val) = 0;\n

    An unsigned integer number was read.

    "},{"location":"api/json_sax/number_unsigned/#parameters","title":"Parameters","text":"val (in) unsigned integer value"},{"location":"api/json_sax/number_unsigned/#return-value","title":"Return value","text":"

    Whether parsing should proceed.

    "},{"location":"api/json_sax/number_unsigned/#examples","title":"Examples","text":"Example

    The example below shows how the SAX interface is used.

    #include <iostream>\n#include <iomanip>\n#include <sstream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\n// a simple event consumer that collects string representations of the passed\n// values; note inheriting from json::json_sax_t is not required, but can\n// help not to forget a required function\nclass sax_event_consumer : public json::json_sax_t\n{\n  public:\n    std::vector<std::string> events;\n\n    bool null() override\n    {\n        events.push_back(\"null()\");\n        return true;\n    }\n\n    bool boolean(bool val) override\n    {\n        events.push_back(\"boolean(val=\" + std::string(val ? \"true\" : \"false\") + \")\");\n        return true;\n    }\n\n    bool number_integer(number_integer_t val) override\n    {\n        events.push_back(\"number_integer(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_unsigned(number_unsigned_t val) override\n    {\n        events.push_back(\"number_unsigned(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_float(number_float_t val, const string_t& s) override\n    {\n        events.push_back(\"number_float(val=\" + std::to_string(val) + \", s=\" + s + \")\");\n        return true;\n    }\n\n    bool string(string_t& val) override\n    {\n        events.push_back(\"string(val=\" + val + \")\");\n        return true;\n    }\n\n    bool start_object(std::size_t elements) override\n    {\n        events.push_back(\"start_object(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_object() override\n    {\n        events.push_back(\"end_object()\");\n        return true;\n    }\n\n    bool start_array(std::size_t elements) override\n    {\n        events.push_back(\"start_array(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_array() override\n    {\n        events.push_back(\"end_array()\");\n        return true;\n    }\n\n    bool key(string_t& val) override\n    {\n        events.push_back(\"key(val=\" + val + \")\");\n        return true;\n    }\n\n    bool binary(json::binary_t& val) override\n    {\n        events.push_back(\"binary(val=[...])\");\n        return true;\n    }\n\n    bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex) override\n    {\n        events.push_back(\"parse_error(position=\" + std::to_string(position) + \", last_token=\" + last_token + \",\\n            ex=\" + std::string(ex.what()) + \")\");\n        return false;\n    }\n};\n\nint main()\n{\n    // a JSON text\n    auto text = R\"(\n    {\n        \"Image\": {\n            \"Width\":  800,\n            \"Height\": 600,\n            \"Title\":  \"View from 15th Floor\",\n            \"Thumbnail\": {\n                \"Url\":    \"http://www.example.com/image/481989943\",\n                \"Height\": 125,\n                \"Width\":  100\n            },\n            \"Animated\" : false,\n            \"IDs\": [116, 943, 234, -38793],\n            \"DeletionDate\": null,\n            \"Distance\": 12.723374634\n        }\n    }]\n    )\";\n\n    // create a SAX event consumer object\n    sax_event_consumer sec;\n\n    // parse JSON\n    bool result = json::sax_parse(text, &sec);\n\n    // output the recorded events\n    for (auto& event : sec.events)\n    {\n        std::cout << event << \"\\n\";\n    }\n\n    // output the result of sax_parse\n    std::cout << \"\\nresult: \" << std::boolalpha << result << std::endl;\n}\n

    Output:

    start_object(elements=18446744073709551615)\nkey(val=Image)\nstart_object(elements=18446744073709551615)\nkey(val=Width)\nnumber_unsigned(val=800)\nkey(val=Height)\nnumber_unsigned(val=600)\nkey(val=Title)\nstring(val=View from 15th Floor)\nkey(val=Thumbnail)\nstart_object(elements=18446744073709551615)\nkey(val=Url)\nstring(val=http://www.example.com/image/481989943)\nkey(val=Height)\nnumber_unsigned(val=125)\nkey(val=Width)\nnumber_unsigned(val=100)\nend_object()\nkey(val=Animated)\nboolean(val=false)\nkey(val=IDs)\nstart_array(elements=18446744073709551615)\nnumber_unsigned(val=116)\nnumber_unsigned(val=943)\nnumber_unsigned(val=234)\nnumber_integer(val=-38793)\nend_array()\nkey(val=DeletionDate)\nnull()\nkey(val=Distance)\nnumber_float(val=12.723375, s=12.723374634)\nend_object()\nend_object()\nparse_error(position=460, last_token=12.723374634<U+000A>        }<U+000A>    }],\n            ex=[json.exception.parse_error.101] parse error at line 17, column 6: syntax error while parsing value - unexpected ']'; expected end of input)\n\nresult: false\n
    "},{"location":"api/json_sax/number_unsigned/#see-also","title":"See also","text":"
    • number_integer - an integer number was read
    • number_float - a floating-point number was read
    • sax_parse - SAX parser
    "},{"location":"api/json_sax/number_unsigned/#version-history","title":"Version history","text":"
    • Added in version 3.2.0.
    "},{"location":"api/json_sax/parse_error/","title":"nlohmann::json_sax::parse_error","text":"
    virtual bool parse_error(std::size_t position,\n                         const std::string& last_token,\n                         const detail::exception& ex) = 0;\n

    A parse error occurred.

    "},{"location":"api/json_sax/parse_error/#parameters","title":"Parameters","text":"position (in) the position in the input where the error occurs last_token (in) the last read token ex (in) an exception object describing the error"},{"location":"api/json_sax/parse_error/#return-value","title":"Return value","text":"

    Whether parsing should proceed (must return false).

    "},{"location":"api/json_sax/parse_error/#examples","title":"Examples","text":"Example

    The example below shows how the SAX interface is used.

    #include <iostream>\n#include <iomanip>\n#include <sstream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\n// a simple event consumer that collects string representations of the passed\n// values; note inheriting from json::json_sax_t is not required, but can\n// help not to forget a required function\nclass sax_event_consumer : public json::json_sax_t\n{\n  public:\n    std::vector<std::string> events;\n\n    bool null() override\n    {\n        events.push_back(\"null()\");\n        return true;\n    }\n\n    bool boolean(bool val) override\n    {\n        events.push_back(\"boolean(val=\" + std::string(val ? \"true\" : \"false\") + \")\");\n        return true;\n    }\n\n    bool number_integer(number_integer_t val) override\n    {\n        events.push_back(\"number_integer(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_unsigned(number_unsigned_t val) override\n    {\n        events.push_back(\"number_unsigned(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_float(number_float_t val, const string_t& s) override\n    {\n        events.push_back(\"number_float(val=\" + std::to_string(val) + \", s=\" + s + \")\");\n        return true;\n    }\n\n    bool string(string_t& val) override\n    {\n        events.push_back(\"string(val=\" + val + \")\");\n        return true;\n    }\n\n    bool start_object(std::size_t elements) override\n    {\n        events.push_back(\"start_object(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_object() override\n    {\n        events.push_back(\"end_object()\");\n        return true;\n    }\n\n    bool start_array(std::size_t elements) override\n    {\n        events.push_back(\"start_array(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_array() override\n    {\n        events.push_back(\"end_array()\");\n        return true;\n    }\n\n    bool key(string_t& val) override\n    {\n        events.push_back(\"key(val=\" + val + \")\");\n        return true;\n    }\n\n    bool binary(json::binary_t& val) override\n    {\n        events.push_back(\"binary(val=[...])\");\n        return true;\n    }\n\n    bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex) override\n    {\n        events.push_back(\"parse_error(position=\" + std::to_string(position) + \", last_token=\" + last_token + \",\\n            ex=\" + std::string(ex.what()) + \")\");\n        return false;\n    }\n};\n\nint main()\n{\n    // a JSON text\n    auto text = R\"(\n    {\n        \"Image\": {\n            \"Width\":  800,\n            \"Height\": 600,\n            \"Title\":  \"View from 15th Floor\",\n            \"Thumbnail\": {\n                \"Url\":    \"http://www.example.com/image/481989943\",\n                \"Height\": 125,\n                \"Width\":  100\n            },\n            \"Animated\" : false,\n            \"IDs\": [116, 943, 234, -38793],\n            \"DeletionDate\": null,\n            \"Distance\": 12.723374634\n        }\n    }]\n    )\";\n\n    // create a SAX event consumer object\n    sax_event_consumer sec;\n\n    // parse JSON\n    bool result = json::sax_parse(text, &sec);\n\n    // output the recorded events\n    for (auto& event : sec.events)\n    {\n        std::cout << event << \"\\n\";\n    }\n\n    // output the result of sax_parse\n    std::cout << \"\\nresult: \" << std::boolalpha << result << std::endl;\n}\n

    Output:

    start_object(elements=18446744073709551615)\nkey(val=Image)\nstart_object(elements=18446744073709551615)\nkey(val=Width)\nnumber_unsigned(val=800)\nkey(val=Height)\nnumber_unsigned(val=600)\nkey(val=Title)\nstring(val=View from 15th Floor)\nkey(val=Thumbnail)\nstart_object(elements=18446744073709551615)\nkey(val=Url)\nstring(val=http://www.example.com/image/481989943)\nkey(val=Height)\nnumber_unsigned(val=125)\nkey(val=Width)\nnumber_unsigned(val=100)\nend_object()\nkey(val=Animated)\nboolean(val=false)\nkey(val=IDs)\nstart_array(elements=18446744073709551615)\nnumber_unsigned(val=116)\nnumber_unsigned(val=943)\nnumber_unsigned(val=234)\nnumber_integer(val=-38793)\nend_array()\nkey(val=DeletionDate)\nnull()\nkey(val=Distance)\nnumber_float(val=12.723375, s=12.723374634)\nend_object()\nend_object()\nparse_error(position=460, last_token=12.723374634<U+000A>        }<U+000A>    }],\n            ex=[json.exception.parse_error.101] parse error at line 17, column 6: syntax error while parsing value - unexpected ']'; expected end of input)\n\nresult: false\n
    "},{"location":"api/json_sax/parse_error/#see-also","title":"See also","text":"
    • sax_parse - SAX parser
    • Parsing and Exceptions - the article on handling parse errors without exceptions
    "},{"location":"api/json_sax/parse_error/#version-history","title":"Version history","text":"
    • Added in version 3.2.0.
    "},{"location":"api/json_sax/start_array/","title":"nlohmann::json_sax::start_array","text":"
    virtual bool start_array(std::size_t elements) = 0;\n

    The beginning of an array was read.

    "},{"location":"api/json_sax/start_array/#parameters","title":"Parameters","text":"elements (in) number of array elements, or std::numeric_limits<std::size_t>::max() if unknown"},{"location":"api/json_sax/start_array/#return-value","title":"Return value","text":"

    Whether parsing should proceed.

    "},{"location":"api/json_sax/start_array/#notes","title":"Notes","text":"

    Binary formats may report the number of elements.

    "},{"location":"api/json_sax/start_array/#examples","title":"Examples","text":"Example

    The example below shows how the SAX interface is used.

    #include <iostream>\n#include <iomanip>\n#include <sstream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\n// a simple event consumer that collects string representations of the passed\n// values; note inheriting from json::json_sax_t is not required, but can\n// help not to forget a required function\nclass sax_event_consumer : public json::json_sax_t\n{\n  public:\n    std::vector<std::string> events;\n\n    bool null() override\n    {\n        events.push_back(\"null()\");\n        return true;\n    }\n\n    bool boolean(bool val) override\n    {\n        events.push_back(\"boolean(val=\" + std::string(val ? \"true\" : \"false\") + \")\");\n        return true;\n    }\n\n    bool number_integer(number_integer_t val) override\n    {\n        events.push_back(\"number_integer(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_unsigned(number_unsigned_t val) override\n    {\n        events.push_back(\"number_unsigned(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_float(number_float_t val, const string_t& s) override\n    {\n        events.push_back(\"number_float(val=\" + std::to_string(val) + \", s=\" + s + \")\");\n        return true;\n    }\n\n    bool string(string_t& val) override\n    {\n        events.push_back(\"string(val=\" + val + \")\");\n        return true;\n    }\n\n    bool start_object(std::size_t elements) override\n    {\n        events.push_back(\"start_object(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_object() override\n    {\n        events.push_back(\"end_object()\");\n        return true;\n    }\n\n    bool start_array(std::size_t elements) override\n    {\n        events.push_back(\"start_array(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_array() override\n    {\n        events.push_back(\"end_array()\");\n        return true;\n    }\n\n    bool key(string_t& val) override\n    {\n        events.push_back(\"key(val=\" + val + \")\");\n        return true;\n    }\n\n    bool binary(json::binary_t& val) override\n    {\n        events.push_back(\"binary(val=[...])\");\n        return true;\n    }\n\n    bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex) override\n    {\n        events.push_back(\"parse_error(position=\" + std::to_string(position) + \", last_token=\" + last_token + \",\\n            ex=\" + std::string(ex.what()) + \")\");\n        return false;\n    }\n};\n\nint main()\n{\n    // a JSON text\n    auto text = R\"(\n    {\n        \"Image\": {\n            \"Width\":  800,\n            \"Height\": 600,\n            \"Title\":  \"View from 15th Floor\",\n            \"Thumbnail\": {\n                \"Url\":    \"http://www.example.com/image/481989943\",\n                \"Height\": 125,\n                \"Width\":  100\n            },\n            \"Animated\" : false,\n            \"IDs\": [116, 943, 234, -38793],\n            \"DeletionDate\": null,\n            \"Distance\": 12.723374634\n        }\n    }]\n    )\";\n\n    // create a SAX event consumer object\n    sax_event_consumer sec;\n\n    // parse JSON\n    bool result = json::sax_parse(text, &sec);\n\n    // output the recorded events\n    for (auto& event : sec.events)\n    {\n        std::cout << event << \"\\n\";\n    }\n\n    // output the result of sax_parse\n    std::cout << \"\\nresult: \" << std::boolalpha << result << std::endl;\n}\n

    Output:

    start_object(elements=18446744073709551615)\nkey(val=Image)\nstart_object(elements=18446744073709551615)\nkey(val=Width)\nnumber_unsigned(val=800)\nkey(val=Height)\nnumber_unsigned(val=600)\nkey(val=Title)\nstring(val=View from 15th Floor)\nkey(val=Thumbnail)\nstart_object(elements=18446744073709551615)\nkey(val=Url)\nstring(val=http://www.example.com/image/481989943)\nkey(val=Height)\nnumber_unsigned(val=125)\nkey(val=Width)\nnumber_unsigned(val=100)\nend_object()\nkey(val=Animated)\nboolean(val=false)\nkey(val=IDs)\nstart_array(elements=18446744073709551615)\nnumber_unsigned(val=116)\nnumber_unsigned(val=943)\nnumber_unsigned(val=234)\nnumber_integer(val=-38793)\nend_array()\nkey(val=DeletionDate)\nnull()\nkey(val=Distance)\nnumber_float(val=12.723375, s=12.723374634)\nend_object()\nend_object()\nparse_error(position=460, last_token=12.723374634<U+000A>        }<U+000A>    }],\n            ex=[json.exception.parse_error.101] parse error at line 17, column 6: syntax error while parsing value - unexpected ']'; expected end of input)\n\nresult: false\n
    "},{"location":"api/json_sax/start_array/#see-also","title":"See also","text":"
    • end_array - the end of an array was read
    • sax_parse - SAX parser
    "},{"location":"api/json_sax/start_array/#version-history","title":"Version history","text":"
    • Added in version 3.2.0.
    "},{"location":"api/json_sax/start_object/","title":"nlohmann::json_sax::start_object","text":"
    virtual bool start_object(std::size_t elements) = 0;\n

    The beginning of an object was read.

    "},{"location":"api/json_sax/start_object/#parameters","title":"Parameters","text":"elements (in) number of object elements, or std::numeric_limits<std::size_t>::max() if unknown"},{"location":"api/json_sax/start_object/#return-value","title":"Return value","text":"

    Whether parsing should proceed.

    "},{"location":"api/json_sax/start_object/#notes","title":"Notes","text":"

    Binary formats may report the number of elements.

    "},{"location":"api/json_sax/start_object/#examples","title":"Examples","text":"Example

    The example below shows how the SAX interface is used.

    #include <iostream>\n#include <iomanip>\n#include <sstream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\n// a simple event consumer that collects string representations of the passed\n// values; note inheriting from json::json_sax_t is not required, but can\n// help not to forget a required function\nclass sax_event_consumer : public json::json_sax_t\n{\n  public:\n    std::vector<std::string> events;\n\n    bool null() override\n    {\n        events.push_back(\"null()\");\n        return true;\n    }\n\n    bool boolean(bool val) override\n    {\n        events.push_back(\"boolean(val=\" + std::string(val ? \"true\" : \"false\") + \")\");\n        return true;\n    }\n\n    bool number_integer(number_integer_t val) override\n    {\n        events.push_back(\"number_integer(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_unsigned(number_unsigned_t val) override\n    {\n        events.push_back(\"number_unsigned(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_float(number_float_t val, const string_t& s) override\n    {\n        events.push_back(\"number_float(val=\" + std::to_string(val) + \", s=\" + s + \")\");\n        return true;\n    }\n\n    bool string(string_t& val) override\n    {\n        events.push_back(\"string(val=\" + val + \")\");\n        return true;\n    }\n\n    bool start_object(std::size_t elements) override\n    {\n        events.push_back(\"start_object(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_object() override\n    {\n        events.push_back(\"end_object()\");\n        return true;\n    }\n\n    bool start_array(std::size_t elements) override\n    {\n        events.push_back(\"start_array(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_array() override\n    {\n        events.push_back(\"end_array()\");\n        return true;\n    }\n\n    bool key(string_t& val) override\n    {\n        events.push_back(\"key(val=\" + val + \")\");\n        return true;\n    }\n\n    bool binary(json::binary_t& val) override\n    {\n        events.push_back(\"binary(val=[...])\");\n        return true;\n    }\n\n    bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex) override\n    {\n        events.push_back(\"parse_error(position=\" + std::to_string(position) + \", last_token=\" + last_token + \",\\n            ex=\" + std::string(ex.what()) + \")\");\n        return false;\n    }\n};\n\nint main()\n{\n    // a JSON text\n    auto text = R\"(\n    {\n        \"Image\": {\n            \"Width\":  800,\n            \"Height\": 600,\n            \"Title\":  \"View from 15th Floor\",\n            \"Thumbnail\": {\n                \"Url\":    \"http://www.example.com/image/481989943\",\n                \"Height\": 125,\n                \"Width\":  100\n            },\n            \"Animated\" : false,\n            \"IDs\": [116, 943, 234, -38793],\n            \"DeletionDate\": null,\n            \"Distance\": 12.723374634\n        }\n    }]\n    )\";\n\n    // create a SAX event consumer object\n    sax_event_consumer sec;\n\n    // parse JSON\n    bool result = json::sax_parse(text, &sec);\n\n    // output the recorded events\n    for (auto& event : sec.events)\n    {\n        std::cout << event << \"\\n\";\n    }\n\n    // output the result of sax_parse\n    std::cout << \"\\nresult: \" << std::boolalpha << result << std::endl;\n}\n

    Output:

    start_object(elements=18446744073709551615)\nkey(val=Image)\nstart_object(elements=18446744073709551615)\nkey(val=Width)\nnumber_unsigned(val=800)\nkey(val=Height)\nnumber_unsigned(val=600)\nkey(val=Title)\nstring(val=View from 15th Floor)\nkey(val=Thumbnail)\nstart_object(elements=18446744073709551615)\nkey(val=Url)\nstring(val=http://www.example.com/image/481989943)\nkey(val=Height)\nnumber_unsigned(val=125)\nkey(val=Width)\nnumber_unsigned(val=100)\nend_object()\nkey(val=Animated)\nboolean(val=false)\nkey(val=IDs)\nstart_array(elements=18446744073709551615)\nnumber_unsigned(val=116)\nnumber_unsigned(val=943)\nnumber_unsigned(val=234)\nnumber_integer(val=-38793)\nend_array()\nkey(val=DeletionDate)\nnull()\nkey(val=Distance)\nnumber_float(val=12.723375, s=12.723374634)\nend_object()\nend_object()\nparse_error(position=460, last_token=12.723374634<U+000A>        }<U+000A>    }],\n            ex=[json.exception.parse_error.101] parse error at line 17, column 6: syntax error while parsing value - unexpected ']'; expected end of input)\n\nresult: false\n
    "},{"location":"api/json_sax/start_object/#see-also","title":"See also","text":"
    • end_object - the end of an object was read
    • key - an object key was read
    • sax_parse - SAX parser
    "},{"location":"api/json_sax/start_object/#version-history","title":"Version history","text":"
    • Added in version 3.2.0.
    "},{"location":"api/json_sax/string/","title":"nlohmann::json_sax::string","text":"
    virtual bool string(string_t& val) = 0;\n

    A string value was read.

    "},{"location":"api/json_sax/string/#parameters","title":"Parameters","text":"val (in) string value"},{"location":"api/json_sax/string/#return-value","title":"Return value","text":"

    Whether parsing should proceed.

    "},{"location":"api/json_sax/string/#notes","title":"Notes","text":"

    It is safe to move the passed string value.

    "},{"location":"api/json_sax/string/#examples","title":"Examples","text":"Example

    The example below shows how the SAX interface is used.

    #include <iostream>\n#include <iomanip>\n#include <sstream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\n// a simple event consumer that collects string representations of the passed\n// values; note inheriting from json::json_sax_t is not required, but can\n// help not to forget a required function\nclass sax_event_consumer : public json::json_sax_t\n{\n  public:\n    std::vector<std::string> events;\n\n    bool null() override\n    {\n        events.push_back(\"null()\");\n        return true;\n    }\n\n    bool boolean(bool val) override\n    {\n        events.push_back(\"boolean(val=\" + std::string(val ? \"true\" : \"false\") + \")\");\n        return true;\n    }\n\n    bool number_integer(number_integer_t val) override\n    {\n        events.push_back(\"number_integer(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_unsigned(number_unsigned_t val) override\n    {\n        events.push_back(\"number_unsigned(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_float(number_float_t val, const string_t& s) override\n    {\n        events.push_back(\"number_float(val=\" + std::to_string(val) + \", s=\" + s + \")\");\n        return true;\n    }\n\n    bool string(string_t& val) override\n    {\n        events.push_back(\"string(val=\" + val + \")\");\n        return true;\n    }\n\n    bool start_object(std::size_t elements) override\n    {\n        events.push_back(\"start_object(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_object() override\n    {\n        events.push_back(\"end_object()\");\n        return true;\n    }\n\n    bool start_array(std::size_t elements) override\n    {\n        events.push_back(\"start_array(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_array() override\n    {\n        events.push_back(\"end_array()\");\n        return true;\n    }\n\n    bool key(string_t& val) override\n    {\n        events.push_back(\"key(val=\" + val + \")\");\n        return true;\n    }\n\n    bool binary(json::binary_t& val) override\n    {\n        events.push_back(\"binary(val=[...])\");\n        return true;\n    }\n\n    bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex) override\n    {\n        events.push_back(\"parse_error(position=\" + std::to_string(position) + \", last_token=\" + last_token + \",\\n            ex=\" + std::string(ex.what()) + \")\");\n        return false;\n    }\n};\n\nint main()\n{\n    // a JSON text\n    auto text = R\"(\n    {\n        \"Image\": {\n            \"Width\":  800,\n            \"Height\": 600,\n            \"Title\":  \"View from 15th Floor\",\n            \"Thumbnail\": {\n                \"Url\":    \"http://www.example.com/image/481989943\",\n                \"Height\": 125,\n                \"Width\":  100\n            },\n            \"Animated\" : false,\n            \"IDs\": [116, 943, 234, -38793],\n            \"DeletionDate\": null,\n            \"Distance\": 12.723374634\n        }\n    }]\n    )\";\n\n    // create a SAX event consumer object\n    sax_event_consumer sec;\n\n    // parse JSON\n    bool result = json::sax_parse(text, &sec);\n\n    // output the recorded events\n    for (auto& event : sec.events)\n    {\n        std::cout << event << \"\\n\";\n    }\n\n    // output the result of sax_parse\n    std::cout << \"\\nresult: \" << std::boolalpha << result << std::endl;\n}\n

    Output:

    start_object(elements=18446744073709551615)\nkey(val=Image)\nstart_object(elements=18446744073709551615)\nkey(val=Width)\nnumber_unsigned(val=800)\nkey(val=Height)\nnumber_unsigned(val=600)\nkey(val=Title)\nstring(val=View from 15th Floor)\nkey(val=Thumbnail)\nstart_object(elements=18446744073709551615)\nkey(val=Url)\nstring(val=http://www.example.com/image/481989943)\nkey(val=Height)\nnumber_unsigned(val=125)\nkey(val=Width)\nnumber_unsigned(val=100)\nend_object()\nkey(val=Animated)\nboolean(val=false)\nkey(val=IDs)\nstart_array(elements=18446744073709551615)\nnumber_unsigned(val=116)\nnumber_unsigned(val=943)\nnumber_unsigned(val=234)\nnumber_integer(val=-38793)\nend_array()\nkey(val=DeletionDate)\nnull()\nkey(val=Distance)\nnumber_float(val=12.723375, s=12.723374634)\nend_object()\nend_object()\nparse_error(position=460, last_token=12.723374634<U+000A>        }<U+000A>    }],\n            ex=[json.exception.parse_error.101] parse error at line 17, column 6: syntax error while parsing value - unexpected ']'; expected end of input)\n\nresult: false\n
    "},{"location":"api/json_sax/string/#see-also","title":"See also","text":"
    • sax_parse - SAX parser
    • SAX Interface - the SAX interface article
    "},{"location":"api/json_sax/string/#version-history","title":"Version history","text":"
    • Added in version 3.2.0.
    "},{"location":"api/macros/","title":"Macros","text":"

    Some aspects of the library can be configured by defining preprocessor macros before including the json.hpp header. See also the macro overview page.

    "},{"location":"api/macros/#runtime-assertions","title":"Runtime assertions","text":"
    • JSON_ASSERT(x) - control behavior of runtime assertions
    "},{"location":"api/macros/#exceptions","title":"Exceptions","text":"
    • JSON_CATCH_USER(exception)JSON_THROW_USER(exception)JSON_TRY_USER - control exceptions
    • JSON_DIAGNOSTICS - control extended diagnostics
    • JSON_DIAGNOSTIC_POSITIONS - access positions of elements
    • JSON_NOEXCEPTION - switch off exceptions
    "},{"location":"api/macros/#parsing","title":"Parsing","text":"
    • JSON_DISABLE_STREAM_BULK_SCAN - read a std::istream one character at a time instead of scanning its stream buffer in bulk
    • JSON_DISABLE_UNLOCKED_FILE_IO - read a FILE* with std::fgetc instead of locking it once and using the unlocked stdio functions
    • JSON_PRECISE_STREAM_POSITION - opt in to leaving an input stream positioned right after a parsed number
    • JSON_STRICT_BINARY_UTF8 - opt in to checking strings for valid UTF-8 in the CBOR, UBJSON, BJData, and BSON writers
    • JSON_STRICT_NUL_HANDLING - opt in to rejecting a NUL byte in the input instead of treating it as end of input
    "},{"location":"api/macros/#language-support","title":"Language support","text":"
    • JSON_HAS_CPP_11JSON_HAS_CPP_14JSON_HAS_CPP_17JSON_HAS_CPP_20 - set supported C++ standard
    • JSON_HAS_FILESYSTEMJSON_HAS_EXPERIMENTAL_FILESYSTEM - control std::filesystem support
    • JSON_HAS_RANGE_VIEW_CONVERSION - control construction from std::ranges views
    • JSON_HAS_RANGES - control std::ranges support
    • JSON_HAS_STATIC_RTTI - control RTTI (run time type information) support
    • JSON_HAS_STD_FORMAT - control std::format/std::formatter support
    • JSON_HAS_THREE_WAY_COMPARISON - control 3-way comparison support
    • JSON_NO_AUTOMATIC_UDLS - do not include the user-defined string literals (UDLs) automatically
    • JSON_NO_IO - switch off functions relying on certain C++ I/O headers
    • JSON_NO_THREAD_LOCAL - switch off the use of thread_local storage
    • JSON_SKIP_UNSUPPORTED_COMPILER_CHECK - do not warn about unsupported compilers
    • JSON_USE_GLOBAL_UDLS - place user-defined string literals (UDLs) into the global namespace
    • JSON_USE_SIMDUTF - use the simdutf library to accelerate UTF-8 validation
    • JSON_VIEW_NO_SIMD - use only portable code in the parser of json_view.hpp
    • JSON_VIEW_USE_SSSE3 - validate non-ASCII strings with SSSE3 in the parser of json_view.hpp
    "},{"location":"api/macros/#library-version","title":"Library version","text":"
    • JSON_SKIP_LIBRARY_VERSION_CHECK - skip library version check
    • NLOHMANN_JSON_VERSION_MAJORNLOHMANN_JSON_VERSION_MINORNLOHMANN_JSON_VERSION_PATCH - library version information
    "},{"location":"api/macros/#library-namespace","title":"Library namespace","text":"
    • NLOHMANN_JSON_NAMESPACE - full name of the nlohmann namespace
    • NLOHMANN_JSON_NAMESPACE_BEGINNLOHMANN_JSON_NAMESPACE_END - open and close the library namespace
    • NLOHMANN_JSON_NAMESPACE_NO_VERSION - disable the version component of the inline namespace
    "},{"location":"api/macros/#type-conversions","title":"Type conversions","text":"
    • JSON_BRACE_INIT_COPY_SEMANTICS - opt in to copy/move semantics for single-element brace initialization
    • JSON_DISABLE_ENUM_SERIALIZATION - switch off default serialization/deserialization functions for enums
    • JSON_DISABLE_TUPLE_REFERENCE_CONVERSION - switch off conversion from a one-element tuple of a JSON reference
    • JSON_USE_IMPLICIT_CONVERSIONS - control implicit conversions
    • JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS - opt in to storing maps with enum keys as objects
    "},{"location":"api/macros/#deprecated-functions","title":"Deprecated functions","text":"
    • JSON_DELETE_DEPRECATED_FUNCTIONS - opt in to deleting the deprecated functions ahead of their removal in version 4.0.0
    "},{"location":"api/macros/#comparison-behavior","title":"Comparison behavior","text":"
    • JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON - control comparison of discarded values
    "},{"location":"api/macros/#serializationdeserialization-macros","title":"Serialization/deserialization macros","text":""},{"location":"api/macros/#enums","title":"Enums","text":"
    • NLOHMANN_JSON_SERIALIZE_ENUM - serialize/deserialize an enum
    • NLOHMANN_JSON_SERIALIZE_ENUM_STRICT - serialize/deserialize an enum with exceptions
    "},{"location":"api/macros/#classes-and-structs","title":"Classes and structs","text":"
    • NLOHMANN_DEFINE_TYPE_INTRUSIVE - serialize/deserialize a non-derived class with private members
    • NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT - serialize/deserialize a non-derived class with private members; uses default values
    • NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE - serialize a non-derived class with private members
    • NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE - serialize/deserialize a non-derived class
    • NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT - serialize/deserialize a non-derived class; uses default values

    • NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE - serialize a non-derived class

    • NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE - serialize/deserialize a derived class with private members

    • NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_DEFAULT - serialize/deserialize a derived class with private members; uses default values
    • NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_ONLY_SERIALIZE - serialize a derived class with private members
    • NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE - serialize/deserialize a derived class
    • NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_DEFAULT - serialize/deserialize a derived class; uses default values
    • NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE - serialize a derived class
    • NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_NAMES - serialize/deserialize a non-derived class with private members; uses custom names
    • NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT_WITH_NAMES - serialize/deserialize a non-derived class with private members; uses default values; uses custom names
    • NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE_WITH_NAMES - serialize a non-derived class with private members; uses custom names
    • NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_NAMES - serialize/deserialize a non-derived class; uses custom names
    • NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT_WITH_NAMES - serialize/deserialize a non-derived class; uses default values; uses custom names

    • NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE_WITH_NAMES - serialize a non-derived class; uses custom names

    • NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_NAMES - serialize/deserialize a derived class with private members; uses custom names

    • NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_DEFAULT_WITH_NAMES - serialize/deserialize a derived class with private members; uses default values; uses custom names
    • NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_ONLY_SERIALIZE_WITH_NAMES - serialize a derived class with private members; uses custom names
    • NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_NAMES - serialize/deserialize a derived class; uses custom names
    • NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_DEFAULT_WITH_NAMES - serialize/deserialize a derived class; uses default values; uses custom names
    • NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE_WITH_NAMES - serialize a derived class; uses custom names
    "},{"location":"api/macros/json_assert/","title":"JSON_ASSERT","text":"
    #define JSON_ASSERT(x) /* value */\n

    This macro controls which code is executed for runtime assertions of the library.

    "},{"location":"api/macros/json_assert/#parameters","title":"Parameters","text":"x (in) expression of a scalar type"},{"location":"api/macros/json_assert/#default-definition","title":"Default definition","text":"

    The default value is assert(x).

    #define JSON_ASSERT(x) assert(x)\n

    Therefore, assertions can be switched off by defining NDEBUG.

    "},{"location":"api/macros/json_assert/#notes","title":"Notes","text":"
    • The library uses numerous assertions to guarantee invariants and to abort in case of otherwise undefined behavior (e.g., when calling operator[] with a missing object key on a const object). See page runtime assertions for more information.
    • Defining the macro to code that does not call std::abort may leave the library in an undefined state.
    • The macro is undefined outside the library.
    "},{"location":"api/macros/json_assert/#examples","title":"Examples","text":"Example: default behavior

    The following code will trigger an assertion at runtime:

    #include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    const json j = {{\"key\", \"value\"}};\n    auto v = j[\"missing\"];\n}\n

    Output:

    Assertion failed: (m_value.object->find(key) != m_value.object->end()), function operator[], file json.hpp, line 2144.\n
    Example: user-defined behavior

    The assertion reporting can be changed by defining JSON_ASSERT(x) differently.

    #include <cstdio>\n#include <cstdlib>\n#define JSON_ASSERT(x) if(!(x)){fprintf(stderr, \"assertion error in %s\\n\", __FUNCTION__); std::abort();}\n\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    const json j = {{\"key\", \"value\"}};\n    auto v = j[\"missing\"];\n}\n

    Output:

    assertion error in operator[]\n
    "},{"location":"api/macros/json_assert/#see-also","title":"See also","text":"
    • Runtime Assertions - overview documentation
    "},{"location":"api/macros/json_assert/#version-history","title":"Version history","text":"
    • Added in version 3.9.0.
    "},{"location":"api/macros/json_brace_init_copy_semantics/","title":"JSON_BRACE_INIT_COPY_SEMANTICS","text":"
    #define JSON_BRACE_INIT_COPY_SEMANTICS /* value */\n

    When defined to 1, single-element brace initialization of a basic_json value is treated as a copy/move of the element rather than wrapping it in a single-element array.

    "},{"location":"api/macros/json_brace_init_copy_semantics/#default-definition","title":"Default definition","text":"

    The default value is 0 (disabled \u2014 existing behavior is preserved).

    #define JSON_BRACE_INIT_COPY_SEMANTICS 0\n
    "},{"location":"api/macros/json_brace_init_copy_semantics/#notes","title":"Notes","text":"

    Background

    C++ always prefers the initializer_list constructor over the copy/move constructor for brace initialization. This means that code like

    json obj = {{\"key\", \"value\"}};\njson j{obj};\n

    creates a single-element array [{\"key\":\"value\"}] instead of a copy of obj. This behavior is compiler-dependent for older compilers (GCC wrapped, Clang did not), but starting from Clang 20, both compilers behave the same way.

    Enabling this macro opts into copy/move semantics for this case (see #5074).

    Opt-in only

    This macro must be defined before including <nlohmann/json.hpp>. Defining it after the include has no effect.

    Applies to every single-element list

    The macro does not only affect a single JSON value in braces. Any single-element braced list is treated as its element, so it no longer creates a one-element array:

    json j1 = {1};       // 1, not [1]\njson j2 = {\"text\"};  // \"text\", not [\"text\"]\njson j3 = {{1, 2}};  // [1,2], not [[1,2]]\n

    Code that relies on these producing arrays must use json::array() instead (see below). Lists with more than one element, and a single [string, value] pair written as a braced list, such as {{\"key\", \"value\"}}, which still creates an object, are not affected. This exception is based on how the pair is written, not on the shape of its value: an existing JSON value that happens to be a two-element array with a string as its first element, such as json arr = {\"key\", 42};, is still copied by json j{arr}; rather than turned into an object. The library's own conversions are not affected either: for example, std::tuple<int>{5} still becomes [5].

    ABI compatibility

    The value of this macro is encoded in the namespace (tag _bics), resulting in distinct symbol names. Translation units compiled with and without it can therefore be linked into the same program without One Definition Rule (ODR) violations, but they cannot exchange instances of library types.

    Workaround without the macro

    To explicitly create a single-element array without enabling this macro, use json::array():

    json j = json::array({obj});  // always creates [obj]\n
    "},{"location":"api/macros/json_brace_init_copy_semantics/#examples","title":"Examples","text":"Example: default behavior (macro not defined)

    Without the macro, single-element brace initialization wraps the value in an array:

    #include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    json obj = {{\"key\", \"value\"}};\n\n    json j{obj};\n    // j is [{\"key\":\"value\"}]  -- single-element array, NOT a copy of obj\n}\n
    Example: opt-in copy semantics (macro defined to 1)

    With the macro, single-element brace initialization copies/moves the value:

    #define JSON_BRACE_INIT_COPY_SEMANTICS 1\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    json obj = {{\"key\", \"value\"}};\n\n    json j{obj};\n    // j is {\"key\":\"value\"}  -- copy of obj\n}\n
    "},{"location":"api/macros/json_brace_init_copy_semantics/#see-also","title":"See also","text":"
    • FAQ: Brace initialization yields arrays
    • basic_json(initializer_list_t) - the affected constructor
    "},{"location":"api/macros/json_brace_init_copy_semantics/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    • Planned to become the default (with the macro removed) in version 4.0.0.
    "},{"location":"api/macros/json_delete_deprecated_functions/","title":"JSON_DELETE_DEPRECATED_FUNCTIONS","text":"
    #define JSON_DELETE_DEPRECATED_FUNCTIONS /* value */\n

    When defined to 1, all deprecated functions of the library are declared as deleted (= delete) instead of only being marked as deprecated. Code that still calls one of them no longer compiles. This way, you can find all calls that need to be replaced before version 4.0.0 unreleased removes these functions; the migration guide describes how.

    A deleted function, unlike a removed one, still takes part in overload resolution. A call that would select it therefore fails to compile instead of silently selecting another overload. This matters for the deprecated from_*(ptr, len) overloads of from_cbor, from_msgpack, from_ubjson, from_bjdata, from_bon8, and from_bson: without them, a call like from_cbor(ptr, len) would compile, read ptr as a NUL-terminated string, and convert len to the strict parameter.

    The macro does not affect the deprecated legacy comparison of discarded values, which is controlled by JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON.

    "},{"location":"api/macros/json_delete_deprecated_functions/#default-definition","title":"Default definition","text":"

    The default value is 0 (disabled, the deprecated functions can still be called, and the compiler warns about it).

    #define JSON_DELETE_DEPRECATED_FUNCTIONS 0\n
    "},{"location":"api/macros/json_delete_deprecated_functions/#notes","title":"Notes","text":"

    CMake option

    The macro can also be set with the CMake option JSON_DeleteDeprecatedFunctions (OFF by default).

    Opt-in only

    This macro must be defined before including <nlohmann/json.hpp>. Defining it after the include has no effect. Define it for the whole project to avoid different declarations of the same class in different translation units.

    ABI compatibility

    The macro only turns calls that compile into calls that do not; it does not change the layout or the behavior of any type. Its value is therefore not encoded in the namespace.

    "},{"location":"api/macros/json_delete_deprecated_functions/#examples","title":"Examples","text":"Example: default behavior (macro not defined)

    Without the macro, the deprecated overload is called, and the compiler warns about it:

    #include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    const std::vector<std::uint8_t> v = {0x82, 0x01, 0x02};\n    auto j = json::from_cbor(v.data(), v.size());\n    // warning: 'from_cbor' is deprecated: Since 3.8.0; use from_cbor(ptr, ptr + len)\n}\n
    Example: deleted deprecated functions (macro defined to 1)

    With the macro, the call does not compile:

    #define JSON_DELETE_DEPRECATED_FUNCTIONS 1\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    const std::vector<std::uint8_t> v = {0x82, 0x01, 0x02};\n    auto j = json::from_cbor(v.data(), v.size());\n    // error: call to deleted function 'from_cbor'\n}\n
    "},{"location":"api/macros/json_delete_deprecated_functions/#see-also","title":"See also","text":"
    • Roadmap: removal of deprecated functions - the deprecated functions and the version they were deprecated in
    • Migration guide: replace deprecated functions - how to replace each deprecated function
    "},{"location":"api/macros/json_delete_deprecated_functions/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    • Planned to be removed in version 4.0.0, which removes the deprecated functions. The deprecated from_*(ptr, len) overloads stay deleted in version 4.0.0 unreleased.
    "},{"location":"api/macros/json_diagnostic_positions/","title":"JSON_DIAGNOSTIC_POSITIONS","text":"
    #define JSON_DIAGNOSTIC_POSITIONS /* value */\n

    This macro enables position diagnostics for generated JSON objects.

    When enabled, two new member functions start_pos() and end_pos() are added to basic_json values. If the value was created by calling theparse function, then these functions allow querying the byte positions of the value in the input it was parsed from. In case the value was constructed by other means, std::string::npos is returned.

    start_pos() returns the position of the first character of a given value in the original JSON string, while end_pos() returns the position of the character following the last character. For objects and arrays, the first and last characters correspond to the opening or closing braces/brackets, respectively. For primitive values, the first and last character represents the opening and closing quotes (strings) or the first and last character of the field's numerical or predefined value (true, false, null), respectively.

    JSON type return value start_pos() return value end_pos() object position of the opening { position after the closing } array position of the opening [ position after the closing ] string position of the opening \" position after the closing \" number position of the first character position after the last character boolean position of t for true and f for false position after e null position of n position after l

    Given the above, end_pos()-start_pos() for a JSON value provides the length of the parsed JSON string for that value, including the opening or closing braces, brackets, or quotes.

    Note that enabling this macro increases the size of every JSON value by two std::size_t fields and adds slight runtime overhead to parsing, copying JSON value objects, and the generation of error messages for exceptions. It also causes these values to be reported in those error messages.

    "},{"location":"api/macros/json_diagnostic_positions/#default-definition","title":"Default definition","text":"

    The default value is 0 (position diagnostics are switched off).

    #define JSON_DIAGNOSTIC_POSITIONS 0\n

    When the macro is not defined, the library will define it to its default value.

    "},{"location":"api/macros/json_diagnostic_positions/#notes","title":"Notes","text":"

    CMake option

    Diagnostic positions can also be controlled with the CMake option JSON_Diagnostic_Positions (OFF by default) which defines JSON_DIAGNOSTIC_POSITIONS accordingly.

    Availability

    Diagnostic positions are only available if the value was created by the parse function. The sax_parse function or all other means to create a JSON value do not set the diagnostic positions and start_pos() and end_pos() will only return std::string::npos for these values.

    Invalidation

    The returned positions are only valid as long as the JSON value is not changed. The positions are not updated when the JSON value is changed.

    "},{"location":"api/macros/json_diagnostic_positions/#examples","title":"Examples","text":"Example: retrieving positions 
    #include <iostream>\n\n#define JSON_DIAGNOSTIC_POSITIONS 1\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::string json_string = R\"(\n    {\n        \"address\": {\n            \"street\": \"Fake Street\",\n            \"housenumber\": 1\n        }\n    }\n    )\";\n    json j = json::parse(json_string);\n\n    std::cout << \"Root diagnostic positions: \\n\";\n    std::cout << \"\\tstart_pos: \" << j.start_pos() << '\\n';\n    std::cout << \"\\tend_pos:\" << j.end_pos() << \"\\n\";\n    std::cout << \"Original string: \\n\";\n    std::cout << \"{\\n        \\\"address\\\": {\\n            \\\"street\\\": \\\"Fake Street\\\",\\n            \\\"housenumber\\\": 1\\n        }\\n    }\" << \"\\n\";\n    std::cout << \"Parsed string: \\n\";\n    std::cout << json_string.substr(j.start_pos(), j.end_pos() - j.start_pos()) << \"\\n\\n\";\n\n    std::cout << \"address diagnostic positions: \\n\";\n    std::cout << \"\\tstart_pos:\" << j[\"address\"].start_pos() << '\\n';\n    std::cout << \"\\tend_pos:\" << j[\"address\"].end_pos() << \"\\n\\n\";\n    std::cout << \"Original string: \\n\";\n    std::cout << \"{            \\\"street\\\": \\\"Fake Street\\\",\\n            \\\"housenumber\\\": 1\\n        }\" << \"\\n\";\n    std::cout << \"Parsed string: \\n\";\n    std::cout << json_string.substr(j[\"address\"].start_pos(), j[\"address\"].end_pos() - j[\"address\"].start_pos()) << \"\\n\\n\";\n\n    std::cout << \"street diagnostic positions: \\n\";\n    std::cout << \"\\tstart_pos:\" << j[\"address\"][\"street\"].start_pos() << '\\n';\n    std::cout << \"\\tend_pos:\" << j[\"address\"][\"street\"].end_pos() << \"\\n\\n\";\n    std::cout << \"Original string: \\n\";\n    std::cout << \"\\\"Fake Street\\\"\" << \"\\n\";\n    std::cout << \"Parsed string: \\n\";\n    std::cout << json_string.substr(j[\"address\"][\"street\"].start_pos(), j[\"address\"][\"street\"].end_pos() - j[\"address\"][\"street\"].start_pos()) << \"\\n\\n\";\n\n    std::cout << \"housenumber diagnostic positions: \\n\";\n    std::cout << \"\\tstart_pos:\" << j[\"address\"][\"housenumber\"].start_pos() << '\\n';\n    std::cout << \"\\tend_pos:\" << j[\"address\"][\"housenumber\"].end_pos() << \"\\n\\n\";\n    std::cout << \"Original string: \\n\";\n    std::cout << \"1\" << \"\\n\";\n    std::cout << \"Parsed string: \\n\";\n    std::cout << json_string.substr(j[\"address\"][\"housenumber\"].start_pos(), j[\"address\"][\"housenumber\"].end_pos() - j[\"address\"][\"housenumber\"].start_pos()) << \"\\n\\n\";\n}\n

    Output:

    Root diagnostic positions: \n    start_pos: 5\n    end_pos:109\nOriginal string: \n{\n        \"address\": {\n            \"street\": \"Fake Street\",\n            \"housenumber\": 1\n        }\n    }\nParsed string: \n{\n        \"address\": {\n            \"street\": \"Fake Street\",\n            \"housenumber\": 1\n        }\n    }\n\naddress diagnostic positions: \n    start_pos:26\n    end_pos:103\n\nOriginal string: \n{            \"street\": \"Fake Street\",\n            \"housenumber\": 1\n        }\nParsed string: \n{\n            \"street\": \"Fake Street\",\n            \"housenumber\": 1\n        }\n\nstreet diagnostic positions: \n    start_pos:50\n    end_pos:63\n\nOriginal string: \n\"Fake Street\"\nParsed string: \n\"Fake Street\"\n\nhousenumber diagnostic positions: \n    start_pos:92\n    end_pos:93\n\nOriginal string: \n1\nParsed string: \n1\n

    The output shows the start/end positions of all the objects and fields in the JSON string.

    Example: using only diagnostic positions in exceptions 
    #include <iostream>\n\n#define JSON_DIAGNOSTIC_POSITIONS 1\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\n/* Demonstration of type error exception with diagnostic positions support enabled */\nint main()\n{\n    //Invalid json string - housenumber type must be int instead of string\n    const std::string json_invalid_string = R\"(\n    {\n        \"address\": {\n            \"street\": \"Fake Street\",\n            \"housenumber\": \"1\"\n        }\n    }\n    )\";\n    json j = json::parse(json_invalid_string);\n    try\n    {\n        int housenumber = j[\"address\"][\"housenumber\"];\n        std::cout << housenumber;\n    }\n    catch (const json::exception& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

    Output:

    [json.exception.type_error.302] (bytes 92-95) type must be number, but is string\n
    The output shows the exception with start/end positions only.\n
    Example: using extended diagnostics with positions enabled in exceptions 
    #include <iostream>\n\n#define JSON_DIAGNOSTICS 1\n#define JSON_DIAGNOSTIC_POSITIONS 1\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\n/* Demonstration of type error exception with diagnostic positions support enabled */\nint main()\n{\n    //Invalid json string - housenumber type must be int instead of string\n    const std::string json_invalid_string = R\"(\n    {\n        \"address\": {\n            \"street\": \"Fake Street\",\n            \"housenumber\": \"1\"\n        }\n    }\n    )\";\n    json j = json::parse(json_invalid_string);\n    try\n    {\n        int housenumber = j[\"address\"][\"housenumber\"];\n        std::cout << housenumber;\n    }\n    catch (const json::exception& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

    Output:

    [json.exception.type_error.302] (/address/housenumber) (bytes 92-95) type must be number, but is string\n
    The output shows the exception with diagnostic path info and start/end positions.\n
    "},{"location":"api/macros/json_diagnostic_positions/#see-also","title":"See also","text":"
    • JSON_Diagnostic_Positions - CMake option to control the macro
    • JSON_DIAGNOSTICS - macro to control extended diagnostics
    "},{"location":"api/macros/json_diagnostic_positions/#version-history","title":"Version history","text":"
    • Added in version 3.12.0.
    "},{"location":"api/macros/json_diagnostics/","title":"JSON_DIAGNOSTICS","text":"
    #define JSON_DIAGNOSTICS /* value */\n

    This macro enables extended diagnostics for exception messages. Possible values are 1 to enable or 0 to disable (default).

    When enabled, exception messages contain a JSON Pointer to the JSON value that triggered the exception. Note that enabling this macro increases the size of every JSON value by one pointer and adds some runtime overhead.

    "},{"location":"api/macros/json_diagnostics/#default-definition","title":"Default definition","text":"

    The default value is 0 (extended diagnostics are switched off).

    #define JSON_DIAGNOSTICS 0\n

    When the macro is not defined, the library will define it to its default value.

    "},{"location":"api/macros/json_diagnostics/#notes","title":"Notes","text":"

    ABI compatibility

    As of version 3.11.0, this macro is no longer required to be defined consistently throughout a codebase to avoid One Definition Rule (ODR) violations, as the value of this macro is encoded in the namespace, resulting in distinct symbol names.

    This allows different parts of a codebase to use different versions or configurations of this library without causing improper behavior.

    Where possible, it is still recommended that all code define this the same way for maximum interoperability.

    CMake option

    Diagnostic messages can also be controlled with the CMake option JSON_Diagnostics (OFF by default) which defines JSON_DIAGNOSTICS accordingly. Note this only applies when building the library from source \u2014 see the pre-installed-package caveat on that page.

    "},{"location":"api/macros/json_diagnostics/#examples","title":"Examples","text":"Example: default behavior 
    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    json j;\n    j[\"address\"][\"street\"] = \"Fake Street\";\n    j[\"address\"][\"housenumber\"] = \"12\";\n\n    try\n    {\n        int housenumber = j[\"address\"][\"housenumber\"];\n    }\n    catch (const json::exception& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

    Output:

    [json.exception.type_error.302] type must be number, but is string\n

    This exception can be hard to debug if storing the value \"12\" and accessing it is further apart.

    Example: extended diagnostic messages 
    #include <iostream>\n\n# define JSON_DIAGNOSTICS 1\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    json j;\n    j[\"address\"][\"street\"] = \"Fake Street\";\n    j[\"address\"][\"housenumber\"] = \"12\";\n\n    try\n    {\n        int housenumber = j[\"address\"][\"housenumber\"];\n    }\n    catch (const json::exception& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

    Output:

    [json.exception.type_error.302] (/address/housenumber) type must be number, but is string\n

    Now the exception message contains a JSON Pointer /address/housenumber that indicates which value has the wrong type.

    Example: using only diagnostic positions in exceptions 
    #include <iostream>\n\n#define JSON_DIAGNOSTIC_POSITIONS 1\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\n/* Demonstration of type error exception with diagnostic positions support enabled */\nint main()\n{\n    //Invalid json string - housenumber type must be int instead of string\n    const std::string json_invalid_string = R\"(\n    {\n        \"address\": {\n            \"street\": \"Fake Street\",\n            \"housenumber\": \"1\"\n        }\n    }\n    )\";\n    json j = json::parse(json_invalid_string);\n    try\n    {\n        int housenumber = j[\"address\"][\"housenumber\"];\n        std::cout << housenumber;\n    }\n    catch (const json::exception& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

    Output:

    [json.exception.type_error.302] (bytes 92-95) type must be number, but is string\n
    The output shows the exception with start/end positions only.

    "},{"location":"api/macros/json_diagnostics/#see-also","title":"See also","text":"
    • JSON_Diagnostics - CMake option to control the macro
    • JSON_DIAGNOSTIC_POSITIONS - macro to access positions of elements
    "},{"location":"api/macros/json_diagnostics/#version-history","title":"Version history","text":"
    • Added in version 3.10.0.
    • As of version 3.11.0, the definition is allowed to vary between translation units.
    "},{"location":"api/macros/json_disable_enum_serialization/","title":"JSON_DISABLE_ENUM_SERIALIZATION","text":"
    #define JSON_DISABLE_ENUM_SERIALIZATION /* value */\n

    When defined to 1, default serialization and deserialization functions for enums are excluded and have to be provided by the user, for example, using NLOHMANN_JSON_SERIALIZE_ENUM (see arbitrary type conversions for more details).

    Parsing or serializing an enum will result in a compiler error.

    This works for both unscoped and scoped enums.

    "},{"location":"api/macros/json_disable_enum_serialization/#default-definition","title":"Default definition","text":"

    The default value is 0.

    #define JSON_DISABLE_ENUM_SERIALIZATION 0\n
    "},{"location":"api/macros/json_disable_enum_serialization/#notes","title":"Notes","text":"

    CMake option

    Enum serialization can also be controlled with the CMake option JSON_DisableEnumSerialization (OFF by default) which defines JSON_DISABLE_ENUM_SERIALIZATION accordingly.

    "},{"location":"api/macros/json_disable_enum_serialization/#examples","title":"Examples","text":"Example: Disabled behavior

    The code below forces the library not to create default serialization/deserialization functions from_json and to_json, meaning the code below does not compile.

    #define JSON_DISABLE_ENUM_SERIALIZATION 1\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nenum class Choice\n{\n    first,\n    second,\n};\n\nint main()\n{\n    // normally invokes to_json serialization function but with JSON_DISABLE_ENUM_SERIALIZATION defined, it does not\n    const json j = Choice::first; \n\n    // normally invokes from_json parse function but with JSON_DISABLE_ENUM_SERIALIZATION defined, it does not\n    Choice ch = j.get<Choice>();\n}\n
    Example: Serialize enum macro

    The code below forces the library not to create default serialization/deserialization functions from_json and to_json, but uses NLOHMANN_JSON_SERIALIZE_ENUM to parse and serialize the enum.

    #define JSON_DISABLE_ENUM_SERIALIZATION 1\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nenum class Choice\n{\n    first,\n    second,\n};\n\nNLOHMANN_JSON_SERIALIZE_ENUM(Choice,\n{\n    { Choice::first, \"first\" },\n    { Choice::second, \"second\" },\n})\n\nint main()\n{\n    // uses user-defined to_json function defined by macro\n    const json j = Choice::first; \n\n    // uses user-defined from_json function defined by macro\n    Choice ch = j.get<Choice>();\n}\n
    Example: User-defined serialization/deserialization functions

    The code below forces the library not to create default serialization/deserialization functions from_json and to_json, but uses user-defined functions to parse and serialize the enum.

    #define JSON_DISABLE_ENUM_SERIALIZATION 1\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nenum class Choice\n{\n    first,\n    second,\n};\n\nvoid from_json(const json& j, Choice& ch)\n{\n    auto value = j.get<std::string>();\n    if (value == \"first\")\n    {\n        ch = Choice::first;\n    }\n    else if (value == \"second\")\n    {\n        ch = Choice::second;\n    }\n}\n\nvoid to_json(json& j, const Choice& ch)\n{\n    if (ch == Choice::first)\n    {\n        j = \"first\";\n    }\n    else if (ch == Choice::second)\n    {\n        j = \"second\";\n    }\n}\n\nint main()\n{\n    // uses user-defined to_json function\n    const json j = Choice::first; \n\n    // uses user-defined from_json function\n    Choice ch = j.get<Choice>();\n}\n
    "},{"location":"api/macros/json_disable_enum_serialization/#see-also","title":"See also","text":"
    • JSON_DisableEnumSerialization - CMake option to control the macro
    • NLOHMANN_JSON_SERIALIZE_ENUM - serialize/deserialize an enum
    "},{"location":"api/macros/json_disable_enum_serialization/#version-history","title":"Version history","text":"
    • Added in version 3.11.0.
    "},{"location":"api/macros/json_disable_stream_bulk_scan/","title":"JSON_DISABLE_STREAM_BULK_SCAN","text":"
    #define JSON_DISABLE_STREAM_BULK_SCAN /* value */\n

    When defined to 1, parsing from a std::istream reads the input one character at a time through the stream buffer, as in version 3.12.0 and earlier, instead of scanning the characters that the stream buffer already holds in bulk.

    The macro is an escape hatch for stream buffers that do not follow the usual contract of std::basic_streambuf (see the requirements on the stream buffer in the notes below). It does not change any result: values, exceptions, and the position of the stream after parsing are the same either way. Only the speed differs.

    "},{"location":"api/macros/json_disable_stream_bulk_scan/#default-definition","title":"Default definition","text":"

    The default value is 0: streams are scanned in bulk.

    #define JSON_DISABLE_STREAM_BULK_SCAN 0\n
    "},{"location":"api/macros/json_disable_stream_bulk_scan/#notes","title":"Notes","text":"

    How streams are scanned

    A std::streambuf keeps the part of the input it has read but not yet handed out in its get area, the range from gptr() to egptr(). A std::filebuf, for instance, reads the file in blocks of a few kilobytes into its get area, and a std::stringbuf exposes its whole string as one get area.

    When parsing from a std::istream, the parser scans this get area directly, with the same fast paths that it uses for a std::string: runs of ordinary string characters, numbers, and indentation are recognized several bytes at a time instead of being requested from the stream buffer one character at a time. When the parser reaches the end of the get area, it asks the stream buffer for the next character as before, which makes the stream buffer refill its get area.

    The parser only reads characters that it would also have read one at a time, and it advances the stream buffer exactly over the characters it consumes. Nothing is read ahead, buffered elsewhere, or put back. Therefore, the stream is left exactly where it was left before:

    • operator>> and sax_parse with strict = false can still read several values from one stream, and the stream can still be used with other std::istream functions after a value has been read.
    • With JSON_PRECISE_STREAM_POSITION, the stream is still positioned right after a parsed number.
    • Exception messages, including the line, the column, and the last read token, are unchanged.

    Performance

    Parsing from a std::ifstream or std::istringstream is typically 1.5 to 2 times as fast as before, and reading many small values from one stream with operator>> 1.4 to 4 times as fast. Parsing from a stream is then within about 10 to 20 % of first reading the whole file into a std::string and parsing that.

    A stream buffer without a get area (one that only overrides underflow() and uflow() and hands out every character from there) offers nothing to scan in bulk, so it is read one character at a time as before.

    Requirements on the stream buffer

    The bulk scan relies on the following behavior, which every stream buffer of the standard library and every stream buffer that keeps its input in its get area has:

    • When sgetc() returns a character while gptr() < egptr(), that character is *gptr(); when sbumpc() returns one, it was at gptr() before the call.
    • Characters in the get area are not changed while they are in the get area.
    • underflow() or uflow() either sets up a get area that holds the returned character at gptr() (or at gptr() - 1 after uflow()), or leaves the get area empty.

    A stream buffer that returns characters from uflow() while leaving unrelated characters in its get area, or that changes the characters in its get area in place, does not meet these requirements. Define the macro to 1 for such stream buffers.

    ABI compatibility

    The value of this macro is encoded in the namespace (tag _dsbs), resulting in distinct symbol names. Translation units compiled with and without it can therefore be linked into the same program without One Definition Rule (ODR) violations, but they cannot exchange instances of library types.

    Other inputs

    Strings, string views, vectors, arrays, and pointer or iterator ranges over contiguous memory are scanned in bulk anyway. Reading from a FILE* is not affected by this macro; see JSON_DISABLE_UNLOCKED_FILE_IO for that input.

    "},{"location":"api/macros/json_disable_stream_bulk_scan/#examples","title":"Examples","text":"Example: reading values from a stream (default)

    The stream is scanned in bulk, and the values are read one after the other:

    #include <iostream>\n#include <sstream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::istringstream input(R\"({\"id\": 1} {\"id\": 2} 42 rest)\");\n    json j1, j2, j3;\n    input >> j1 >> j2 >> j3;  // {\"id\":1}, {\"id\":2}, 42\n\n    std::string word;\n    input >> word;            // \"rest\"\n}\n
    Example: reading one character at a time (macro defined to 1)

    With the macro, the same program reads the stream one character at a time, with the same results:

    #define JSON_DISABLE_STREAM_BULK_SCAN 1\n#include <iostream>\n#include <sstream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::istringstream input(R\"({\"id\": 1} {\"id\": 2} 42 rest)\");\n    json j1, j2, j3;\n    input >> j1 >> j2 >> j3;  // {\"id\":1}, {\"id\":2}, 42\n\n    std::string word;\n    input >> word;            // \"rest\"\n}\n
    "},{"location":"api/macros/json_disable_stream_bulk_scan/#see-also","title":"See also","text":"
    • parse - deserialize from a compatible input
    • operator>> - deserialize from stream
    • sax_parse - generate SAX events
    • JSON_PRECISE_STREAM_POSITION - leave an input stream positioned right after a parsed number
    • JSON_DISABLE_UNLOCKED_FILE_IO - read a FILE* with the locking stdio functions
    • Parsing: input sources and performance
    "},{"location":"api/macros/json_disable_stream_bulk_scan/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/macros/json_disable_tuple_reference_conversion/","title":"JSON_DISABLE_TUPLE_REFERENCE_CONVERSION","text":"
    #define JSON_DISABLE_TUPLE_REFERENCE_CONVERSION /* value */\n

    When defined to 1, a basic_json value can no longer be constructed from a one-element std::tuple whose element is a reference to that basic_json type, such as std::tuple<json&>, std::tuple<const json&>, or std::tuple<json&&>. These are the tuples created by std::forward_as_tuple(j).

    "},{"location":"api/macros/json_disable_tuple_reference_conversion/#default-definition","title":"Default definition","text":"

    The default value is 0 (disabled \u2014 existing behavior is preserved).

    #define JSON_DISABLE_TUPLE_REFERENCE_CONVERSION 0\n
    "},{"location":"api/macros/json_disable_tuple_reference_conversion/#notes","title":"Notes","text":"

    Background

    By default, basic_json can be constructed from any std::tuple whose elements can be converted to JSON; the result is an array. This includes std::tuple<json&>, which becomes a one-element array.

    std::tuple only converts another tuple element by element if its element type cannot be constructed from the whole source tuple. Because json can be constructed from std::tuple<json&>, std::tuple instead converts the whole tuple into a single json value. This has two surprising effects:

    json j = true;\n\n// rejected by some standard libraries (e.g., libc++); with others, the\n// reference binds to a temporary that is destroyed right away\nstd::tuple<const json&> t1(std::forward_as_tuple(j));\n\n// compiles, but std::get<0>(t2) is [true], not true\nstd::tuple<json> t2(std::forward_as_tuple(j));\n

    Enabling this macro removes the conversion, so both tuples are converted element by element: std::get<0>(t1) refers to j, and std::get<0>(t2) is a copy of j (see #2226).

    Opt-in only

    This macro must be defined before including <nlohmann/json.hpp>. Defining it after the include has no effect.

    Affected conversions

    Only one-element tuples holding a reference to the same basic_json type are affected. Constructing a JSON value from them no longer compiles:

    json j = true;\njson a = std::forward_as_tuple(j);  // error with the macro enabled\njson b = json::array({j});          // use this instead: [true]\n

    Tuples holding a JSON value (std::make_tuple(j)), tuples with more than one element, and tuples holding references to other types (including other basic_json specializations) are converted to arrays as before.

    CMake option

    This behavior can also be controlled with the CMake option JSON_DisableTupleReferenceConversion (OFF by default) which defines JSON_DISABLE_TUPLE_REFERENCE_CONVERSION accordingly.

    "},{"location":"api/macros/json_disable_tuple_reference_conversion/#examples","title":"Examples","text":"Example: default behavior (macro not defined) 
    #include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    json j = true;\n\n    std::tuple<json> t(std::forward_as_tuple(j));\n    // std::get<0>(t) is [true] -- the whole tuple was converted\n}\n
    Example: conversion disabled (macro defined to 1) 
    #define JSON_DISABLE_TUPLE_REFERENCE_CONVERSION 1\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    json j = true;\n\n    std::tuple<json> t(std::forward_as_tuple(j));\n    // std::get<0>(t) is true -- a copy of j\n\n    std::tuple<const json&> r(std::forward_as_tuple(j));\n    // std::get<0>(r) refers to j\n}\n
    "},{"location":"api/macros/json_disable_tuple_reference_conversion/#see-also","title":"See also","text":"
    • basic_json(CompatibleType&&) - the affected constructor
    • JSON_DisableTupleReferenceConversion - CMake option to control the macro
    "},{"location":"api/macros/json_disable_tuple_reference_conversion/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    • Planned to become the default (with the macro removed) in version 4.0.0.
    "},{"location":"api/macros/json_disable_unlocked_file_io/","title":"JSON_DISABLE_UNLOCKED_FILE_IO","text":"
    #define JSON_DISABLE_UNLOCKED_FILE_IO /* value */\n

    When defined to 1, parsing from a FILE* reads every character with std::fgetc, as in version 3.12.0 and earlier, instead of locking the file once and reading it with the unlocked stdio functions.

    It does not change any result: values, exceptions, and the position of the file after parsing are the same either way. Only the speed and the locking of the file differ (see the notes on locking below).

    "},{"location":"api/macros/json_disable_unlocked_file_io/#default-definition","title":"Default definition","text":"

    The default value is 0: the file is locked once and read with the unlocked stdio functions where they are available.

    #define JSON_DISABLE_UNLOCKED_FILE_IO 0\n
    "},{"location":"api/macros/json_disable_unlocked_file_io/#notes","title":"Notes","text":"

    Background

    The C standard library makes every std::fgetc call thread-safe by locking and unlocking the FILE object. When parsing from a FILE*, the parser reads one character at a time, so this happens for every character of the input.

    Instead, the parser locks the file once when parsing starts and unlocks it when parsing ends - also when an exception is thrown - and reads the characters in between without locking:

    C library functions glibc (Linux), Apple (macOS, iOS), Android, FreeBSD, NetBSD, OpenBSD, DragonFly BSD flockfile, getc_unlocked, funlockfile (POSIX) Microsoft (MSVC, clang-cl) _lock_file, _fgetc_nolock, _unlock_file others (e.g., musl, MinGW, newlib) std::fgetc, as before

    This makes parsing from a FILE* 3 to 4 times as fast with the Apple C library and 1.1 to 1.3 times as fast with glibc, whose locking is cheaper. The binary formats (from_cbor, from_msgpack, ...) read from a FILE* the same way.

    Locking

    While a value is parsed from a FILE*, the parsing thread holds the file's lock. The lock is recursive, so the parsing thread itself - for instance, a SAX handler or a parser callback - can keep using the file with stdio functions such as std::ftell. Other threads that use the same FILE object wait until the parser is done. Before, they could interleave their reads with the parser's, which made the parsed input unpredictable anyway.

    A program in which a SAX handler or parser callback waits for another thread that uses the same FILE object would deadlock. Define the macro to 1 for such programs.

    ABI compatibility

    The value of this macro is encoded in the namespace (tag _dufio), resulting in distinct symbol names. Translation units compiled with and without it can therefore be linked into the same program without One Definition Rule (ODR) violations, but they cannot exchange instances of library types.

    Other inputs

    Reading from a std::istream is not affected by this macro; see JSON_DISABLE_STREAM_BULK_SCAN for that input.

    "},{"location":"api/macros/json_disable_unlocked_file_io/#examples","title":"Examples","text":"Example: parsing from a FILE* (default)

    The file is locked once while it is parsed:

    #include <cstdio>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::FILE* f = std::fopen(\"example.json\", \"rb\");\n    json j = json::parse(f);  // f is locked until parse() returns\n    std::fclose(f);\n}\n
    Example: locking for every character (macro defined to 1)

    With the macro, every character is read with std::fgetc, with the same result:

    #define JSON_DISABLE_UNLOCKED_FILE_IO 1\n#include <cstdio>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::FILE* f = std::fopen(\"example.json\", \"rb\");\n    json j = json::parse(f);  // std::fgetc for every character\n    std::fclose(f);\n}\n
    "},{"location":"api/macros/json_disable_unlocked_file_io/#see-also","title":"See also","text":"
    • parse - deserialize from a compatible input
    • sax_parse - generate SAX events
    • JSON_DISABLE_STREAM_BULK_SCAN - read a std::istream one character at a time
    • Parsing: input sources and performance
    "},{"location":"api/macros/json_disable_unlocked_file_io/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/macros/json_has_cpp_11/","title":"JSON_HAS_CPP_11, JSON_HAS_CPP_14, JSON_HAS_CPP_17, JSON_HAS_CPP_20, JSON_HAS_CPP_23, JSON_HAS_CPP_26","text":"
    #define JSON_HAS_CPP_11\n#define JSON_HAS_CPP_14\n#define JSON_HAS_CPP_17\n#define JSON_HAS_CPP_20\n#define JSON_HAS_CPP_23\n#define JSON_HAS_CPP_26\n

    The library targets C++11, but also supports some features introduced in later C++ versions (e.g., std::string_view support for C++17). For these new features, the library implements some preprocessor checks to determine the C++ standard. By defining any of these symbols, the internal check is overridden and the provided C++ version is unconditionally assumed. This can be helpful for compilers that only implement parts of the standard and would be detected incorrectly.

    "},{"location":"api/macros/json_has_cpp_11/#default-definition","title":"Default definition","text":"

    The default value is detected based on preprocessor macros such as __cplusplus, _HAS_CXX17, or _MSVC_LANG.

    "},{"location":"api/macros/json_has_cpp_11/#notes","title":"Notes","text":"
    • When the C++ standard is detected automatically, JSON_HAS_CPP_11 is always defined. When you override the detection by defining one of these macros manually, the automatic detection is skipped entirely, so you should define all applicable macros (including JSON_HAS_CPP_11) yourself.
    • All macros are undefined outside the library.
    "},{"location":"api/macros/json_has_cpp_11/#examples","title":"Examples","text":"Example

    The code below forces the library to use the C++14 standard:

    #define JSON_HAS_CPP_14 1\n#include <nlohmann/json.hpp>\n\n...\n
    "},{"location":"api/macros/json_has_cpp_11/#see-also","title":"See also","text":"
    • JSON_HAS_FILESYSTEM / JSON_HAS_EXPERIMENTAL_FILESYSTEM - control std::filesystem support
    • JSON_HAS_RANGES - control std::ranges support
    • JSON_HAS_THREE_WAY_COMPARISON - control 3-way comparison support
    • JSON_HAS_STD_FORMAT - control std::format/std::formatter support
    "},{"location":"api/macros/json_has_cpp_11/#version-history","title":"Version history","text":"
    • Added in version 3.10.0.
    • Added JSON_HAS_CPP_23 in version 3.12.0.
    • Added JSON_HAS_CPP_26 in version 3.13.0 unreleased.
    "},{"location":"api/macros/json_has_filesystem/","title":"JSON_HAS_FILESYSTEM / JSON_HAS_EXPERIMENTAL_FILESYSTEM","text":"
    #define JSON_HAS_FILESYSTEM /* value */\n#define JSON_HAS_EXPERIMENTAL_FILESYSTEM /* value */\n

    When compiling with C++17, the library provides conversions from and to std::filesystem::path. As compiler support for filesystem is limited, the library tries to detect whether <filesystem>/std::filesystem (JSON_HAS_FILESYSTEM) or <experimental/filesystem>/std::experimental::filesystem (JSON_HAS_EXPERIMENTAL_FILESYSTEM) should be used. To override the built-in check, define JSON_HAS_FILESYSTEM or JSON_HAS_EXPERIMENTAL_FILESYSTEM to 1.

    "},{"location":"api/macros/json_has_filesystem/#default-definition","title":"Default definition","text":"

    The default value is detected based on the preprocessor macros __cpp_lib_filesystem, __cpp_lib_experimental_filesystem, __has_include(<filesystem>), or __has_include(<experimental/filesystem>).

    Known compiler/stdlib exclusions

    Even when the feature-test macro indicates filesystem support is available, the library disables it on the following broken toolchains:

    • MinGW + GCC 8 \u2014 disabled entirely (broken std::filesystem implementation; MinGW-w64 bug 737)
    • GCC (non-Clang) < 8 \u2014 disabled (no filesystem support)
    • Clang < 7 \u2014 disabled (no filesystem support)
    • MSVC < 19.14 \u2014 disabled (no filesystem support)
    • iOS < 13 \u2014 disabled (no filesystem support)
    • macOS < Catalina (10.15) \u2014 disabled (no filesystem support)

    If JSON_HAS_FILESYSTEM or JSON_HAS_EXPERIMENTAL_FILESYSTEM is 0 despite __cpp_lib_filesystem being defined, one of the exclusions above likely applies to your toolchain.

    "},{"location":"api/macros/json_has_filesystem/#notes","title":"Notes","text":"
    • Note that older compilers or older versions of libstdc++ also require the library stdc++fs to be linked to for filesystem support.
    • Both macros are undefined outside the library.
    "},{"location":"api/macros/json_has_filesystem/#examples","title":"Examples","text":"Example

    The code below forces the library to use the header <experimental/filesystem>.

    #define JSON_HAS_EXPERIMENTAL_FILESYSTEM 1\n#include <nlohmann/json.hpp>\n\n...\n
    "},{"location":"api/macros/json_has_filesystem/#see-also","title":"See also","text":"
    • JSON_HAS_CPP_11 / JSON_HAS_CPP_14 / JSON_HAS_CPP_17 / JSON_HAS_CPP_20 / JSON_HAS_CPP_23 / JSON_HAS_CPP_26 - set supported C++ standard
    "},{"location":"api/macros/json_has_filesystem/#version-history","title":"Version history","text":"
    • Added in version 3.10.5.
    "},{"location":"api/macros/json_has_range_view_conversion/","title":"JSON_HAS_RANGE_VIEW_CONVERSION","text":"
    #define JSON_HAS_RANGE_VIEW_CONVERSION /* value */\n

    This macro indicates whether a JSON array can be constructed directly from a C++20 range view (std::ranges::view), such as the result of std::views::filter or std::views::transform. Possible values are 1 when supported or 0 when unsupported.

    "},{"location":"api/macros/json_has_range_view_conversion/#default-definition","title":"Default definition","text":"

    The default value is 1 if JSON_HAS_RANGES is 1 and the compiler is not MinGW (that is, __MINGW32__ is not defined), and 0 otherwise.

    When the macro is not defined, the library will define it to its default value.

    Known compiler/stdlib exclusions

    • MinGW -- disabled, because its std::ranges support is incomplete (issue #4916).
    • All toolchains for which JSON_HAS_RANGES is disabled.
    "},{"location":"api/macros/json_has_range_view_conversion/#examples","title":"Examples","text":"Example

    The code below forces the library to disable the conversion from range views:

    #define JSON_HAS_RANGE_VIEW_CONVERSION 0\n#include <nlohmann/json.hpp>\n\n...\n
    "},{"location":"api/macros/json_has_range_view_conversion/#see-also","title":"See also","text":"
    • JSON_HAS_RANGES - control std::ranges support
    • Constructing from a C++20 range view - usage of the feature
    "},{"location":"api/macros/json_has_range_view_conversion/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/macros/json_has_ranges/","title":"JSON_HAS_RANGES","text":"
    #define JSON_HAS_RANGES /* value */\n

    This macro indicates whether the standard library has any support for ranges. Implies support for concepts. Possible values are 1 when supported or 0 when unsupported.

    "},{"location":"api/macros/json_has_ranges/#default-definition","title":"Default definition","text":"

    The default value is detected based on the preprocessor macro __cpp_lib_ranges.

    When the macro is not defined, the library will define it to its default value.

    Known compiler/stdlib exclusions

    Even when the feature-test macro __cpp_lib_ranges indicates ranges support is available, the library disables it on the following incomplete or broken toolchains:

    • GCC 11.1.0 \u2014 disabled (the shipped <ranges> header has a syntax error; issue #4440)
    • libstdc++ < 11 \u2014 disabled (incomplete C++20 ranges support; issue #4440)
    • Clang < 16 with libstdc++ \u2014 disabled (incomplete ranges support; issue #5161)
    • libc++ < 160000 \u2014 disabled (incomplete C++20 ranges support; issue #4440)
    • nvcc (CUDA) 12.0.x and 12.1.x \u2014 disabled (the enable_borrowed_range variable-template syntax triggers a parse error under these two toolkit versions; fixed in CUDA 12.2; issue #3907)

    If JSON_HAS_RANGES is 0 despite __cpp_lib_ranges being defined, one of the exclusions above likely applies to your toolchain.

    "},{"location":"api/macros/json_has_ranges/#examples","title":"Examples","text":"Example

    The code below forces the library to enable support for ranges:

    #define JSON_HAS_RANGES 1\n#include <nlohmann/json.hpp>\n\n...\n
    "},{"location":"api/macros/json_has_ranges/#see-also","title":"See also","text":"
    • JSON_HAS_CPP_11 / JSON_HAS_CPP_14 / JSON_HAS_CPP_17 / JSON_HAS_CPP_20 / JSON_HAS_CPP_23 / JSON_HAS_CPP_26 - set supported C++ standard
    • JSON_HAS_RANGE_VIEW_CONVERSION - control construction from std::ranges views
    • JSON_HAS_STD_FORMAT - a similar feature-detection macro, for std::format/std::formatter support
    "},{"location":"api/macros/json_has_ranges/#version-history","title":"Version history","text":"
    • Added in version 3.11.0.
    • Added the exclusions for libstdc++ < 11, Clang < 16 with libstdc++, libc++ < 16, and nvcc (CUDA) 12.0.x and 12.1.x in version 3.13.0 unreleased. Before, only GCC 11.1.0 was excluded.
    "},{"location":"api/macros/json_has_static_rtti/","title":"JSON_HAS_STATIC_RTTI","text":"
    #define JSON_HAS_STATIC_RTTI /* value */\n

    This macro indicates whether the standard library has any support for RTTI (run time type information). Possible values are 1 when supported or 0 when unsupported.

    "},{"location":"api/macros/json_has_static_rtti/#default-definition","title":"Default definition","text":"

    The default value is detected based on the preprocessor macro _HAS_STATIC_RTTI.

    When the macro is not defined, the library will define it to its default value.

    "},{"location":"api/macros/json_has_static_rtti/#examples","title":"Examples","text":"Example

    The code below forces the library to enable support for libraries with RTTI dependence:

    #define JSON_HAS_STATIC_RTTI 1\n#include <nlohmann/json.hpp>\n\n...\n
    "},{"location":"api/macros/json_has_static_rtti/#see-also","title":"See also","text":"
    • operator ValueType - get a value (implicit); on C++17, this macro controls whether std::any is excluded from its candidate types
    "},{"location":"api/macros/json_has_static_rtti/#version-history","title":"Version history","text":"
    • Added in version 3.11.3.
    "},{"location":"api/macros/json_has_std_format/","title":"JSON_HAS_STD_FORMAT","text":"
    #define JSON_HAS_STD_FORMAT /* value */\n

    This macro indicates whether the standard library has support for std::format/std::formatter (that is, the <format> header). Possible values are 1 when supported or 0 when unsupported.

    "},{"location":"api/macros/json_has_std_format/#default-definition","title":"Default definition","text":"

    The default value is detected based on the preprocessor macros JSON_HAS_CPP_20 and __cpp_lib_format.

    When the macro is not defined, the library will define it to its default value.

    "},{"location":"api/macros/json_has_std_format/#notes","title":"Notes","text":"

    Enabled functionality

    When this macro evaluates to 1, the library provides a std::formatter<basic_json> specialization so JSON values can be used directly with std::format.

    "},{"location":"api/macros/json_has_std_format/#examples","title":"Examples","text":"Example

    The code below forces the library to disable support for std::format, even if the standard library would otherwise support it:

    #define JSON_HAS_STD_FORMAT 0\n#include <nlohmann/json.hpp>\n\n...\n
    "},{"location":"api/macros/json_has_std_format/#see-also","title":"See also","text":"
    • std::formatter<basic_json> - format JSON values with std::format
    "},{"location":"api/macros/json_has_std_format/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/macros/json_has_three_way_comparison/","title":"JSON_HAS_THREE_WAY_COMPARISON","text":"
    #define JSON_HAS_THREE_WAY_COMPARISON /* value */\n

    This macro indicates whether the compiler and standard library support 3-way comparison. Possible values are 1 when supported or 0 when unsupported.

    "},{"location":"api/macros/json_has_three_way_comparison/#default-definition","title":"Default definition","text":"

    The default value is detected based on the preprocessor macros __cpp_impl_three_way_comparison and __cpp_lib_three_way_comparison.

    When the macro is not defined, the library will define it to its default value.

    "},{"location":"api/macros/json_has_three_way_comparison/#examples","title":"Examples","text":"Example

    The code below forces the library to use 3-way comparison:

    #define JSON_HAS_THREE_WAY_COMPARISON 1\n#include <nlohmann/json.hpp>\n\n...\n
    "},{"location":"api/macros/json_has_three_way_comparison/#see-also","title":"See also","text":"
    • operator<=> - 3-way compare JSON values
    • operator== - compare JSON pointers for equality
    "},{"location":"api/macros/json_has_three_way_comparison/#version-history","title":"Version history","text":"
    • Added in version 3.11.0.
    "},{"location":"api/macros/json_no_automatic_udls/","title":"JSON_NO_AUTOMATIC_UDLS","text":"
    #define JSON_NO_AUTOMATIC_UDLS\n

    When defined, <nlohmann/json.hpp> does not include <nlohmann/json_literals.hpp>, so the user-defined string literals operator\"\"_json and operator\"\"_json_pointer are not declared. Include <nlohmann/json_literals.hpp> in the files that use them.

    The literals are ordinary inline functions whose bodies call the parser, so every translation unit that includes them instantiates the parser \u2014 even if it never parses anything itself. Defining JSON_NO_AUTOMATIC_UDLS for a whole project avoids this cost in translation units that do not parse (e.g., ones that only define types and conversions or pass json values around) and reduces their compile time.

    "},{"location":"api/macros/json_no_automatic_udls/#default-definition","title":"Default definition","text":"

    By default, JSON_NO_AUTOMATIC_UDLS is not defined, and <nlohmann/json.hpp> includes <nlohmann/json_literals.hpp>.

    #undef JSON_NO_AUTOMATIC_UDLS\n
    "},{"location":"api/macros/json_no_automatic_udls/#notes","title":"Notes","text":"

    Header <nlohmann/json_literals.hpp>

    The header includes <nlohmann/json.hpp> itself and places the literals according to JSON_USE_GLOBAL_UDLS. It is part of the multi-header sources (include/nlohmann) and of the single-header sources (single_include/nlohmann), next to json.hpp.

    C++ modules

    The nlohmann.json module always exports the literals, regardless of this macro.

    "},{"location":"api/macros/json_no_automatic_udls/#examples","title":"Examples","text":"Example

    The code below includes the library without the literals and adds them in a single translation unit.

    // compiled with -DJSON_NO_AUTOMATIC_UDLS for the whole project\n\n// this file uses the literals, so it includes them explicitly\n// (the header includes <nlohmann/json.hpp> itself)\n#include <nlohmann/json_literals.hpp>\n\nint main()\n{\n    auto j = R\"({\"foo\": 42})\"_json;\n    return j.at(\"/foo\"_json_pointer) == 42 ? 0 : 1;\n}\n

    Without the include of <nlohmann/json_literals.hpp>, the code would fail to compile.

    "},{"location":"api/macros/json_no_automatic_udls/#see-also","title":"See also","text":"
    • operator\"\"_json
    • operator\"\"_json_pointer
    • JSON_USE_GLOBAL_UDLS - place user-defined string literals (UDLs) into the global namespace
    • Compile times - options to reduce compile times
    "},{"location":"api/macros/json_no_automatic_udls/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/macros/json_no_io/","title":"JSON_NO_IO","text":"
    #define JSON_NO_IO\n

    When defined, headers <cstdio>, <ios>, <iosfwd>, <istream>, and <ostream> are not included and parse functions relying on these headers are excluded. This is relevant for environments where these I/O functions are disallowed for security reasons (e.g., Intel Software Guard Extensions (SGX)).

    "},{"location":"api/macros/json_no_io/#default-definition","title":"Default definition","text":"

    By default, JSON_NO_IO is not defined.

    #undef JSON_NO_IO\n
    "},{"location":"api/macros/json_no_io/#examples","title":"Examples","text":"Example

    The code below forces the library not to use the headers <cstdio>, <ios>, <iosfwd>, <istream>, and <ostream>.

    #define JSON_NO_IO 1\n#include <nlohmann/json.hpp>\n\n...\n
    "},{"location":"api/macros/json_no_io/#see-also","title":"See also","text":"
    • operator<< - serialize to stream
    • operator>> - deserialize from stream
    "},{"location":"api/macros/json_no_io/#version-history","title":"Version history","text":"
    • Added in version 3.10.0.
    "},{"location":"api/macros/json_no_thread_local/","title":"JSON_NO_THREAD_LOCAL","text":"
    #define JSON_NO_THREAD_LOCAL\n

    When defined, the library does not use thread_local storage. This is relevant for the few environments whose toolchain does not support it.

    Copying a value and comparing two values both descend into the first levels by letting the containers copy or compare themselves, and finish whatever is nested deeper than that without the call stack, so that neither can exhaust the stack however deeply the values are nested. Each counts the levels it has descended into in a thread_local variable, as a counter shared between threads would be raced.

    Without those counters, no descent can be bounded safely, so objects and arrays are copied and compared without the call stack right away. Both keep working exactly as they do otherwise - the same values come out, the same comparisons hold, and deeply nested values are handled just as safely - but both are slower, because the containers no longer copy or compare themselves. Copying the benchmark documents takes 9% (canada.json) to 34% (twitter.json) longer, and comparing two equal ones 10% (citm_catalog.json) to 90% (canada.json) longer.

    "},{"location":"api/macros/json_no_thread_local/#default-definition","title":"Default definition","text":"

    By default, JSON_NO_THREAD_LOCAL is not defined.

    #undef JSON_NO_THREAD_LOCAL\n

    The library defines it by itself for Clang targeting MinGW, which does not survive the thread_local storage: copying a value segfaults there, with both old and current Clang versions, while GCC targeting MinGW is unaffected. Copying and comparing fall back to working without the call stack there, as they do whenever the macro is defined.

    "},{"location":"api/macros/json_no_thread_local/#examples","title":"Examples","text":"Example

    The code below forces the library not to use thread_local storage.

    #define JSON_NO_THREAD_LOCAL 1\n#include <nlohmann/json.hpp>\n\n...\n
    "},{"location":"api/macros/json_no_thread_local/#see-also","title":"See also","text":"
    • FAQ: Thread safety
    "},{"location":"api/macros/json_no_thread_local/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/macros/json_noexception/","title":"JSON_NOEXCEPTION","text":"
    #define JSON_NOEXCEPTION\n

    Exceptions can be switched off by defining the symbol JSON_NOEXCEPTION. When defining JSON_NOEXCEPTION, try is replaced by if (true), catch is replaced by if (false), and throw is replaced by std::abort().

    The same effect is achieved by setting the compiler flag -fno-exceptions.

    "},{"location":"api/macros/json_noexception/#default-definition","title":"Default definition","text":"

    By default, the macro is not defined.

    #undef JSON_NOEXCEPTION\n
    "},{"location":"api/macros/json_noexception/#notes","title":"Notes","text":"

    The explanatory what() string of exceptions is not available for MSVC if exceptions are disabled, see #2824.

    "},{"location":"api/macros/json_noexception/#examples","title":"Examples","text":"Example

    The code below switches off exceptions in the library.

    #define JSON_NOEXCEPTION 1\n#include <nlohmann/json.hpp>\n\n...\n
    "},{"location":"api/macros/json_noexception/#see-also","title":"See also","text":"
    • Switch off exceptions for more information how to switch off exceptions
    "},{"location":"api/macros/json_noexception/#version-history","title":"Version history","text":"

    Added in version 2.1.0.

    "},{"location":"api/macros/json_precise_stream_position/","title":"JSON_PRECISE_STREAM_POSITION","text":"
    #define JSON_PRECISE_STREAM_POSITION /* value */\n

    When defined to 1, operator>> and sax_parse with strict = false leave a std::istream positioned right after the parsed value for every value type. By default, the character that terminates a number is consumed as well.

    The macro only affects reading from a std::istream when the rest of the stream is not required to be consumed. parse, accept, and all other inputs (strings, iterators, containers, FILE*) are never affected.

    "},{"location":"api/macros/json_precise_stream_position/#default-definition","title":"Default definition","text":"

    The default value is 0 (disabled \u2014 existing behavior is preserved).

    #define JSON_PRECISE_STREAM_POSITION 0\n
    "},{"location":"api/macros/json_precise_stream_position/#notes","title":"Notes","text":"

    Background

    A number is the only JSON value whose end can be detected solely by reading the character that follows it. By default, that character is consumed and not put back, so the stream is left one byte too far after a number, and only after a number:

    std::istringstream input(\"1true\");\njson j;\ninput >> j;  // j == 1, but the stream now starts at \"rue\"\n

    With this macro, the character is only looked at and left in the stream, so the stream starts at true. This does not require the stream buffer to support putting a character back.

    This was not changed unconditionally, because code can depend on the consumed character, even unknowingly (see #5340). Both of the following work by default only because the character after each number is swallowed, and behave differently with this macro:

    std::istringstream input(\"1,2,3\");\njson j1, j2, j3;\ninput >> j1 >> j2 >> j3;  // default: 1, 2, 3\n                          // with the macro: throws parse_error.101 at the ','\n
    std::istringstream input(\"42\\nfoo\");\njson j;\nstd::string line;\ninput >> j;\nstd::getline(input, line);  // default: \"foo\"\n                            // with the macro: \"\" (like after reading an int with >>)\n

    In both cases, the behavior with the macro is what you already get today when the value is not a number: \"a\",\"b\" fails at the ,, and std::getline after {} returns an empty string. This macro offers an opt-in path to the consistent behavior ahead of version 4.0.0, where it is planned to become the default.

    Opt-in only

    This macro must be defined before including <nlohmann/json.hpp>. Defining it after the include has no effect.

    ABI compatibility

    The value of this macro is encoded in the namespace (tag _psp), resulting in distinct symbol names. Translation units compiled with and without it can therefore be linked into the same program without One Definition Rule (ODR) violations, but they cannot exchange instances of library types.

    Bulk scanning

    The parser scans the characters in the stream buffer in bulk (see JSON_DISABLE_STREAM_BULK_SCAN), but it never reads ahead. The stream is left at the same position with and without bulk scanning, in both modes of this macro.

    Workaround without the macro

    Separate the values in the stream with whitespace. The character consumed after a number is then the separator, and whitespace before the next value is skipped anyway.

    "},{"location":"api/macros/json_precise_stream_position/#examples","title":"Examples","text":"Example: default behavior (macro not defined)

    Without the macro, the character after a number is consumed:

    #include <iostream>\n#include <sstream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::istringstream input(\"1true\");\n    json j1, j2;\n    input >> j1;  // j1 == 1\n    input >> j2;  // throws parse_error.101: the stream now starts at \"rue\"\n}\n
    Example: opt-in precise stream position (macro defined to 1)

    With the macro, the stream is positioned right after the number:

    #define JSON_PRECISE_STREAM_POSITION 1\n#include <iostream>\n#include <sstream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::istringstream input(\"1true\");\n    json j1, j2;\n    input >> j1;  // j1 == 1\n    input >> j2;  // j2 == true\n}\n
    "},{"location":"api/macros/json_precise_stream_position/#see-also","title":"See also","text":"
    • operator>> - deserialize from stream
    • sax_parse - generate SAX events
    "},{"location":"api/macros/json_precise_stream_position/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    • Planned to become the default (with the macro removed) in version 4.0.0.
    "},{"location":"api/macros/json_skip_library_version_check/","title":"JSON_SKIP_LIBRARY_VERSION_CHECK","text":"
    #define JSON_SKIP_LIBRARY_VERSION_CHECK\n

    When defined, the library will not create a compiler warning when a different version of the library was already included.

    "},{"location":"api/macros/json_skip_library_version_check/#default-definition","title":"Default definition","text":"

    By default, the macro is not defined.

    #undef JSON_SKIP_LIBRARY_VERSION_CHECK\n
    "},{"location":"api/macros/json_skip_library_version_check/#notes","title":"Notes","text":"

    ABI compatibility

    Mixing different library versions in the same code can be a problem as the different versions may not be ABI compatible.

    "},{"location":"api/macros/json_skip_library_version_check/#examples","title":"Examples","text":"Example: switch off the version check

    The code below switches off the warning about including a different version of the library.

    #define JSON_SKIP_LIBRARY_VERSION_CHECK 1\n#include <nlohmann/json.hpp>\n\n...\n
    Example: warning about a different library version

    The following warning will be shown in case a different version of the library was already included:

    Already included a different version of the library!\n
    "},{"location":"api/macros/json_skip_library_version_check/#see-also","title":"See also","text":"
    • NLOHMANN_JSON_VERSION_MAJOR, NLOHMANN_JSON_VERSION_MINOR, NLOHMANN_JSON_VERSION_PATCH - library version information
    "},{"location":"api/macros/json_skip_library_version_check/#version-history","title":"Version history","text":"
    • Added in version 3.11.0.
    "},{"location":"api/macros/json_skip_unsupported_compiler_check/","title":"JSON_SKIP_UNSUPPORTED_COMPILER_CHECK","text":"
    #define JSON_SKIP_UNSUPPORTED_COMPILER_CHECK\n

    When defined, the library will not create a compile error when a known unsupported compiler is detected. This allows using the library with compilers that do not fully support C++11 and may only work if unsupported features are not used.

    "},{"location":"api/macros/json_skip_unsupported_compiler_check/#default-definition","title":"Default definition","text":"

    By default, the macro is not defined.

    #undef JSON_SKIP_UNSUPPORTED_COMPILER_CHECK\n
    "},{"location":"api/macros/json_skip_unsupported_compiler_check/#examples","title":"Examples","text":"Example

    The code below switches off the check whether the compiler is supported.

    #define JSON_SKIP_UNSUPPORTED_COMPILER_CHECK 1\n#include <nlohmann/json.hpp>\n\n...\n
    "},{"location":"api/macros/json_skip_unsupported_compiler_check/#see-also","title":"See also","text":"
    • JSON_HAS_CPP_11 / JSON_HAS_CPP_14 / JSON_HAS_CPP_17 / JSON_HAS_CPP_20 / JSON_HAS_CPP_23 / JSON_HAS_CPP_26 - set supported C++ standard
    "},{"location":"api/macros/json_skip_unsupported_compiler_check/#version-history","title":"Version history","text":"
    • Added in version 3.2.0.
    "},{"location":"api/macros/json_strict_binary_utf8/","title":"JSON_STRICT_BINARY_UTF8","text":"
    #define JSON_STRICT_BINARY_UTF8 /* value */\n

    When defined to 1, the error_handler parameter of the binary writers to_cbor, to_ubjson, to_bjdata, and to_bson defaults to error_handler_t::strict instead of error_handler_t::keep. These writers then check every string value and object key for valid UTF-8 and throw type_error.316 for ill-formed UTF-8, like dump does. Without it, they write the bytes unchanged. An error_handler passed explicitly always takes precedence.

    The macro does not affect:

    • to_msgpack: the MessagePack specification allows a str value to contain bytes that are not valid UTF-8, so its error_handler always defaults to keep.
    • to_bon8: BON8 always checks, because the UTF-8 lead bytes mark where a string ends.
    • The binary readers (from_cbor, from_msgpack, from_ubjson, from_bjdata, from_bson): none of these formats requires a decoder to reject ill-formed UTF-8, so by default they return the bytes unchanged (error_handler_t::keep), independent of this macro. Pass an error_handler argument explicitly to validate or sanitize the strings they read.
    • from_bon8: BON8 always validates, because the UTF-8 lead bytes mark where a string ends.
    "},{"location":"api/macros/json_strict_binary_utf8/#default-definition","title":"Default definition","text":"

    The default value is 0 (disabled, the behavior of version 3.12.0 and earlier is preserved).

    #define JSON_STRICT_BINARY_UTF8 0\n
    "},{"location":"api/macros/json_strict_binary_utf8/#notes","title":"Notes","text":"

    Background

    CBOR, UBJSON, BJData, and BSON all require strings to be UTF-8. Up to version 3.12.0, the writers did not check this, so they could produce output that other decoders reject. Checking by default would break code that stores other encodings (for instance ISO 8859-1) in a string and only ever writes it to a binary format. You can pass error_handler_t::strict to each call, or use this macro to check by default ahead of version 4.0.0, where strict is planned to become the default (see #5529 and #5651).

    Opt-in only

    This macro must be defined before including <nlohmann/json.hpp>. Defining it after the include has no effect.

    ABI compatibility

    The value of this macro is encoded in the namespace (tag _sbu8), resulting in distinct symbol names. Translation units compiled with and without it can therefore be linked into the same program without One Definition Rule (ODR) violations, but they cannot exchange instances of library types.

    "},{"location":"api/macros/json_strict_binary_utf8/#examples","title":"Examples","text":"Example: default behavior (macro not defined)

    Without the macro, the bytes are written unchanged:

    #include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    auto v = json::to_cbor(json(\"\\xFF\"));\n    // v is {0x61, 0xFF}\n}\n
    Example: opt-in check (macro defined to 1)

    With the macro, ill-formed UTF-8 is rejected:

    #define JSON_STRICT_BINARY_UTF8 1\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    auto v = json::to_cbor(json(\"\\xFF\"));\n    // throws type_error.316: invalid UTF-8 byte at index 0: 0xFF\n}\n
    "},{"location":"api/macros/json_strict_binary_utf8/#see-also","title":"See also","text":"
    • to_cbor - create a CBOR serialization of a JSON value
    • to_ubjson - create a UBJSON serialization of a JSON value
    • to_bjdata - create a BJData serialization of a JSON value
    • to_bson - create a BSON serialization of a JSON value
    • error_handler_t - how dump treats ill-formed UTF-8
    "},{"location":"api/macros/json_strict_binary_utf8/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    • Planned to become the default (with the macro removed) in version 4.0.0.
    "},{"location":"api/macros/json_strict_nul_handling/","title":"JSON_STRICT_NUL_HANDLING","text":"
    #define JSON_STRICT_NUL_HANDLING /* value */\n

    When defined to 1, a '\\0' (NUL) byte in JSON text input is rejected with parse_error.101, like any other unexpected byte, instead of being silently treated as end of input.

    The macro only affects the JSON text parser (parse, accept, sax_parse, and operator>>). There are three cases where a NUL byte is still not rejected:

    • The binary formats (from_bjdata, from_bon8, from_bson, from_cbor, from_msgpack, from_ubjson) are never affected: there, 0x00 is ordinary data.
    • A bare const char* pointer has no length of its own, so its length is still determined with strlen(). The first NUL byte therefore still marks the end of the input, and nothing after it is read.
    • One trailing '\\0' at the end of a char, wchar_t, char16_t, char32_t, or (C++20) char8_t array (e.g., a string literal) is trimmed; see the warning below.
    "},{"location":"api/macros/json_strict_nul_handling/#default-definition","title":"Default definition","text":"

    The default value is 0 (disabled \u2014 existing behavior is preserved).

    #define JSON_STRICT_NUL_HANDLING 0\n
    "},{"location":"api/macros/json_strict_nul_handling/#notes","title":"Notes","text":"

    Background

    By default, a '\\0' byte anywhere in the input is treated the same as the real end of the input, rather than as an ordinary (and, outside of a string, invalid) byte. Everything from that byte onward is silently ignored, without a parse error - including further, otherwise well-formed JSON:

    json::parse(std::string(\"123\") + '\\0');          // == 123, no error\njson::parse(std::string(\"123\") + '\\0' + \"true\"); // == 123, the \"true\" is silently ignored too\n

    This falls out of the same convention used when no explicit input length is given at all: parsing from a const char* already stops at the first NUL byte via strlen(), since a bare pointer has no length of its own. The library applies that same NUL-terminated-C-string convention uniformly, rather than only when a length is genuinely unavailable - so a std::string, iterator range, or container whose content happens to include a NUL byte is affected the same way a raw const char* would be (see the FAQ entry for a fuller explanation).

    This was not fixed unconditionally, because doing so is backwards-incompatible for any caller who happens to depend on the current behavior - even unknowingly, for instance because their input already contains trailing padding they never noticed was being discarded (see #5530). This macro instead offers an opt-in path to the corrected behavior ahead of version 4.0.0, where it is planned to become the default.

    Opt-in only

    This macro must be defined before including <nlohmann/json.hpp>. Defining it after the include has no effect.

    Enabling it also changes how an array of a text-literal element type (char, wchar_t, char16_t, char32_t, or, since C++20, char8_t - including a string literal, e.g. json::parse(\"123\") or json::parse(L\"123\")) is read: such an array normally carries a trailing '\\0' contributed by the compiler, not by the source text. With this macro enabled, that one trailing element is trimmed if present so that parsing a string literal keeps working, for any of these character types; every other element in the array - including any '\\0' that is not the very last element - is read as real data and rejected like any other unexpected byte. Arrays of any other element type (unsigned char, std::uint8_t, ...), as used for CBOR or MessagePack, are never affected by this trimming; their full extent - including a genuine trailing 0x00 - is always preserved, in both states of this macro.

    ABI compatibility

    The value of this macro is encoded in the namespace (tag _snul), resulting in distinct symbol names. Translation units compiled with and without it can therefore be linked into the same program without One Definition Rule (ODR) violations, but they cannot exchange instances of library types.

    Workaround without the macro

    To reject a NUL byte without enabling this macro, trim your input yourself before calling parse():

    s.resize(s.find('\\0')); // drop everything from the first NUL onward, if any\njson::parse(s);\n
    "},{"location":"api/macros/json_strict_nul_handling/#examples","title":"Examples","text":"Example: default behavior (macro not defined)

    Without the macro, a NUL byte silently ends parsing at that point:

    #include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    json j = json::parse(std::string(\"123\") + '\\0' + \"true\");\n    // j is 123 -- the '\\0' and everything after it is silently ignored\n}\n
    Example: opt-in strict handling (macro defined to 1)

    With the macro, a NUL byte is rejected like any other unexpected byte:

    #define JSON_STRICT_NUL_HANDLING 1\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    json j = json::parse(std::string(\"123\") + '\\0' + \"true\");\n    // throws parse_error.101 -- the NUL byte is now invalid input,\n    // exactly like any other unexpected trailing byte\n\n    json ok = json::parse(\"123\");\n    // ok is 123 -- parsing from a string literal still works\n}\n
    "},{"location":"api/macros/json_strict_nul_handling/#see-also","title":"See also","text":"
    • JSON_StrictNulHandling - CMake option to control the macro
    • FAQ: NUL bytes in the input
    • parse - deserialize from a compatible input
    • accept - check if the input is valid JSON
    • operator>> - deserialize from stream
    "},{"location":"api/macros/json_strict_nul_handling/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    • Planned to become the default (with the macro removed) in version 4.0.0.
    "},{"location":"api/macros/json_throw_user/","title":"JSON_CATCH_USER, JSON_THROW_USER, JSON_TRY_USER","text":"
    // (1)\n#define JSON_CATCH_USER(exception) /* value */\n// (2)\n#define JSON_THROW_USER(exception) /* value */\n// (3)\n#define JSON_TRY_USER /* value */\n

    Controls how exceptions are handled by the library.

    1. This macro overrides catch calls inside the library. The argument is the type of the exception to catch. The library uses it in a single place: to swallow any exception escaping the parent-pointer check that JSON_DIAGNOSTICS adds to the class invariant. The places where the library catches its own json::out_of_range exceptions use JSON_INTERNAL_CATCH instead, which JSON_CATCH_USER also overrides unless JSON_INTERNAL_CATCH_USER is defined. The macro is always followed by a scope.
    2. This macro overrides throw calls inside the library. The argument is the exception to be thrown. Note that JSON_THROW_USER should leave the current scope (e.g., by throwing or aborting), as continuing after it may yield undefined behavior.
    3. This macro overrides try calls inside the library. It has no arguments and is always followed by a scope.
    "},{"location":"api/macros/json_throw_user/#parameters","title":"Parameters","text":"exception (in) an exception type"},{"location":"api/macros/json_throw_user/#default-definition","title":"Default definition","text":"

    By default, the macros map to their respective C++ keywords:

    #define JSON_CATCH_USER(exception) catch(exception)\n#define JSON_THROW_USER(exception) throw exception\n#define JSON_TRY_USER              try\n

    When exceptions are switched off, the try block is executed unconditionally, and throwing exceptions is replaced by calling std::abort to make reaching the throw branch abort the process.

    #define JSON_THROW_USER(exception) std::abort()\n#define JSON_TRY_USER              if (true)\n#define JSON_CATCH_USER(exception) if (false)\n
    "},{"location":"api/macros/json_throw_user/#examples","title":"Examples","text":"Example

    The code below switches off exceptions and creates a log entry with a detailed error message in case of errors.

    #include <iostream>\n\n#define JSON_TRY_USER if(true)\n#define JSON_CATCH_USER(exception) if(false)\n#define JSON_THROW_USER(exception)                           \\\n    {std::clog << \"Error in \" << __FILE__ << \":\" << __LINE__ \\\n               << \" (function \" << __FUNCTION__ << \") - \"    \\\n               << (exception).what() << std::endl;           \\\n     std::abort();}\n\n#include <nlohmann/json.hpp>\n
    "},{"location":"api/macros/json_throw_user/#see-also","title":"See also","text":"
    • Switch off exceptions for more information how to switch off exceptions
    • JSON_NOEXCEPTION - switch off exceptions
    "},{"location":"api/macros/json_throw_user/#version-history","title":"Version history","text":"
    • Added in version 3.1.0.
    "},{"location":"api/macros/json_use_global_udls/","title":"JSON_USE_GLOBAL_UDLS","text":"
    #define JSON_USE_GLOBAL_UDLS /* value */\n

    When defined to 1, the user-defined string literals (UDLs) are placed into the global namespace instead of nlohmann::literals::json_literals.

    "},{"location":"api/macros/json_use_global_udls/#default-definition","title":"Default definition","text":"

    The default value is 1.

    #define JSON_USE_GLOBAL_UDLS 1\n

    When the macro is not defined, the library behaves as if it were defined to its default value.

    "},{"location":"api/macros/json_use_global_udls/#notes","title":"Notes","text":"

    Future behavior change

    The user-defined string literals will be removed from the global namespace in the next major release of the library.

    To prepare existing code, define JSON_USE_GLOBAL_UDLS to 0 and bring the string literals into scope where needed. Refer to any of the string literals for details.

    See the migration guide for how to update existing code.

    CMake option

    The placement of user-defined string literals can also be controlled with the CMake option JSON_GlobalUDLs (ON by default) which defines JSON_USE_GLOBAL_UDLS accordingly.

    Leaving out the literals

    If JSON_NO_AUTOMATIC_UDLS is defined, the literals are only declared where <nlohmann/json_literals.hpp> is included; this macro then applies to that header.

    "},{"location":"api/macros/json_use_global_udls/#examples","title":"Examples","text":"Example: Default behavior

    The code below shows the default behavior using the _json UDL.

    #include <nlohmann/json.hpp>\n\n#include <iostream>\n\nint main()\n{\n    auto j = \"42\"_json;\n\n    std::cout << j << std::endl;\n}\n

    Output:

    42\n
    Example: Namespaced UDLs

    The code below shows how UDLs need to be brought into scope before using _json when JSON_USE_GLOBAL_UDLS is defined to 0.

    #define JSON_USE_GLOBAL_UDLS 0\n#include <nlohmann/json.hpp>\n\n#include <iostream>\n\nint main()\n{\n    // auto j = \"42\"_json; // This line would fail to compile,\n                           // because the UDLs are not in the global namespace\n\n    // Bring the UDLs into scope\n    using namespace nlohmann::json_literals;\n\n    auto j = \"42\"_json;\n\n    std::cout << j << std::endl;\n}\n

    Output:

    42\n
    "},{"location":"api/macros/json_use_global_udls/#see-also","title":"See also","text":"
    • operator\"\"_json
    • operator\"\"_json_pointer
    • JSON_NO_AUTOMATIC_UDLS - do not include the user-defined string literals automatically
    • JSON_GlobalUDLs - CMake option to control the macro
    "},{"location":"api/macros/json_use_global_udls/#version-history","title":"Version history","text":"
    • Added in version 3.11.0.
    "},{"location":"api/macros/json_use_implicit_conversions/","title":"JSON_USE_IMPLICIT_CONVERSIONS","text":"
    #define JSON_USE_IMPLICIT_CONVERSIONS /* value */\n

    When defined to 0, implicit conversions are switched off. By default, implicit conversions are switched on. The value directly affects operator ValueType and the converting constructor from a basic_json specialization with a different string type (overload 4).

    "},{"location":"api/macros/json_use_implicit_conversions/#default-definition","title":"Default definition","text":"

    By default, implicit conversions are enabled.

    #define JSON_USE_IMPLICIT_CONVERSIONS 1\n
    "},{"location":"api/macros/json_use_implicit_conversions/#notes","title":"Notes","text":"

    Future behavior change

    Implicit conversions will be switched off by default in the next major release of the library.

    You can prepare existing code by already defining JSON_USE_IMPLICIT_CONVERSIONS to 0 and replace any implicit conversions with calls to get.

    See the migration guide for how to update existing code.

    Automatic migration

    The community-maintained clang-tidy check modernize-nlohmann-json-explicit-conversions rewrites implicit conversions into explicit calls to get; for example, int i = j; becomes int i = j.get<int>();. The check is not part of clang-tidy itself, and it does not catch every case (for example, constructing a std::optional from a JSON value), so review the result. See discussion #4610 for how to build and use it.

    CMake option

    Implicit conversions can also be controlled with the CMake option JSON_ImplicitConversions (ON by default) which defines JSON_USE_IMPLICIT_CONVERSIONS accordingly.

    "},{"location":"api/macros/json_use_implicit_conversions/#examples","title":"Examples","text":"Example: implicit and explicit conversions

    This is an example for an implicit conversion:

    json j = \"Hello, world!\";\nstd::string s = j;\n

    When JSON_USE_IMPLICIT_CONVERSIONS is defined to 0, the code above does no longer compile. Instead, it must be written like this:

    json j = \"Hello, world!\";\nauto s = j.get<std::string>();\n
    Example: conversion between basic_json specializations

    A basic_json specialization with a different string type is also no longer converted implicitly when JSON_USE_IMPLICIT_CONVERSIONS is defined to 0:

    using wjson = nlohmann::json::with_string_t<std::wstring>;\n\nvoid load(const nlohmann::json& j);\n\nwjson wj = /* ... */;\nload(wj);                            // error: no implicit conversion\nload(nlohmann::json(wj));            // OK: explicit conversion\nload(wj.get<nlohmann::json>());      // OK: explicit conversion\n

    Specializations that share the same string type, such as json and ordered_json, remain implicitly convertible.

    "},{"location":"api/macros/json_use_implicit_conversions/#see-also","title":"See also","text":"
    • operator ValueType - get a value (implicit)
    • get - get a value (explicit)
    • JSON_ImplicitConversions - CMake option to control the macro
    "},{"location":"api/macros/json_use_implicit_conversions/#version-history","title":"Version history","text":"
    • Added in version 3.9.0.
    • Also affects the conversion between basic_json specializations with different string types since version 3.13.0 unreleased.
    "},{"location":"api/macros/json_use_legacy_discarded_value_comparison/","title":"JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON","text":"
    #define JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON /* value */\n

    This macro enables the (incorrect) legacy comparison behavior of discarded JSON values. Possible values are 1 to enable or 0 to disable (default).

    When enabled, comparisons involving at least one discarded JSON value yield results as follows:

    Operator Result == false != true < false <= true >= true > false

    Otherwise, comparisons involving at least one discarded JSON value yield false, except for !=, which is the negation of == and yields true.

    "},{"location":"api/macros/json_use_legacy_discarded_value_comparison/#default-definition","title":"Default definition","text":"

    The default value is 0.

    #define JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON 0\n

    When the macro is not defined, the library will define it to its default value.

    "},{"location":"api/macros/json_use_legacy_discarded_value_comparison/#notes","title":"Notes","text":"

    Inconsistent behavior in C++20 and beyond

    When targeting C++20 or above, enabling the legacy comparison behavior is strongly discouraged.

    • The 3-way comparison operator (<=>) will always give the correct result (std::partial_ordering::unordered) regardless of the value of JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON.
    • Overloads for the equality and relational operators emulate the legacy behavior.

    Code outside your control may use either 3-way comparison or the equality and relational operators, resulting in inconsistent and unpredictable behavior.

    See operator<=> for more information on 3-way comparison.

    Deprecation

    The legacy comparison behavior is deprecated and may be removed in a future major version release.

    New code should not depend on it and existing code should try to remove or rewrite expressions relying on it.

    See the migration guide for how to update existing code.

    CMake option

    Legacy comparison can also be controlled with the CMake option JSON_LegacyDiscardedValueComparison (OFF by default) which defines JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON accordingly.

    "},{"location":"api/macros/json_use_legacy_discarded_value_comparison/#examples","title":"Examples","text":"Example

    The code below switches on the legacy discarded value comparison behavior in the library.

    #define JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON 1\n#include <nlohmann/json.hpp>\n\n...\n
    "},{"location":"api/macros/json_use_legacy_discarded_value_comparison/#see-also","title":"See also","text":"
    • JSON_LegacyDiscardedValueComparison - CMake option to control the macro
    "},{"location":"api/macros/json_use_legacy_discarded_value_comparison/#version-history","title":"Version history","text":"
    • Added in version 3.11.0.
    • Fixed in version 3.13.0 unreleased so <= and >= also emulate the legacy behavior in C++20 when the JSON value is the right-hand operand of a scalar comparison; before, only the 3-way-comparison-rewritten candidate was found, which yielded false instead of true.
    "},{"location":"api/macros/json_use_objects_for_enum_keyed_maps/","title":"JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS","text":"
    #define JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS /* value */\n

    When defined to 1, maps whose keys are enums (such as std::map<E, T> or std::unordered_map<E, T>) are stored as JSON objects, using the enum's own conversion for the keys. By default, they are stored as arrays of [key, value] pairs.

    "},{"location":"api/macros/json_use_objects_for_enum_keyed_maps/#default-definition","title":"Default definition","text":"

    The default value is 0 (disabled \u2014 existing behavior is preserved).

    #define JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS 0\n
    "},{"location":"api/macros/json_use_objects_for_enum_keyed_maps/#notes","title":"Notes","text":"

    Background

    JSON object keys are strings, so a map is only stored as an object if its keys can be converted to a string type. Enums are not, even if NLOHMANN_JSON_SERIALIZE_ENUM maps them to strings, so a map with enum keys becomes an array of [key, value] pairs:

    [[\"stopped\", \"aa\"], [\"completed\", \"bb\"]]\n

    With this macro, the same map becomes an object (see #4378):

    {\"completed\": \"bb\", \"stopped\": \"aa\"}\n

    Maps with non-unique keys

    Maps that allow duplicate keys, such as std::multimap<E, T> or std::unordered_multimap<E, T>, are not affected by the macro and are still stored as arrays of [key, value] pairs, as an object cannot hold duplicate keys.

    Reading

    Reading is not affected by the macro: a map with enum keys can always be read from both an array of pairs and an object. For the latter, each key is converted to the enum with its from_json function, e.g., the one defined by NLOHMANN_JSON_SERIALIZE_ENUM. Data written without the macro can therefore still be read after enabling it.

    Keys must serialize to distinct strings

    Each key is converted with the enum's to_json function. If a key is not converted to a string (for instance, an enum without NLOHMANN_JSON_SERIALIZE_ENUM, which is stored as an integer, or an enumerator mapped to nullptr), type_error.302 is thrown. If two keys are converted to the same string (for instance, because NLOHMANN_JSON_SERIALIZE_ENUM maps an unlisted enumerator to the first entry), type_error.318 is thrown. In both cases, the target value is not changed.

    Opt-in only

    This macro must be defined before including <nlohmann/json.hpp>. Defining it after the include has no effect.

    ABI compatibility

    The value of this macro is encoded in the namespace (tag _ekmo), resulting in distinct symbol names. Translation units compiled with and without it can therefore be linked into the same program without One Definition Rule (ODR) violations, but they cannot exchange instances of library types.

    "},{"location":"api/macros/json_use_objects_for_enum_keyed_maps/#examples","title":"Examples","text":"Example: default behavior (macro not defined)

    Without the macro, a map with enum keys is stored as an array of pairs:

    #include <map>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nenum TaskState { TS_STOPPED, TS_RUNNING, TS_COMPLETED };\n\nNLOHMANN_JSON_SERIALIZE_ENUM(TaskState, {\n    {TS_STOPPED, \"stopped\"},\n    {TS_RUNNING, \"running\"},\n    {TS_COMPLETED, \"completed\"},\n})\n\nint main()\n{\n    std::map<TaskState, std::string> m = {{TS_STOPPED, \"aa\"}, {TS_COMPLETED, \"bb\"}};\n\n    json j = m;\n    // j is [[\"stopped\",\"aa\"],[\"completed\",\"bb\"]]\n}\n
    Example: objects for enum-keyed maps (macro defined to 1)

    With the macro, the same map is stored as an object:

    #define JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS 1\n#include <map>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nenum TaskState { TS_STOPPED, TS_RUNNING, TS_COMPLETED };\n\nNLOHMANN_JSON_SERIALIZE_ENUM(TaskState, {\n    {TS_STOPPED, \"stopped\"},\n    {TS_RUNNING, \"running\"},\n    {TS_COMPLETED, \"completed\"},\n})\n\nint main()\n{\n    std::map<TaskState, std::string> m = {{TS_STOPPED, \"aa\"}, {TS_COMPLETED, \"bb\"}};\n\n    json j = m;\n    // j is {\"completed\":\"bb\",\"stopped\":\"aa\"}\n\n    auto m2 = j.get<std::map<TaskState, std::string>>();\n    // m2 == m\n}\n
    "},{"location":"api/macros/json_use_objects_for_enum_keyed_maps/#see-also","title":"See also","text":"
    • Specializing enum conversion
    • NLOHMANN_JSON_SERIALIZE_ENUM - serialize/deserialize an enum
    • NLOHMANN_JSON_SERIALIZE_ENUM_STRICT - serialize/deserialize an enum with exceptions
    "},{"location":"api/macros/json_use_objects_for_enum_keyed_maps/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/macros/json_use_simdutf/","title":"JSON_USE_SIMDUTF","text":"
    #define JSON_USE_SIMDUTF\n

    When defined, the parser validates the UTF-8 content of JSON strings that come from a contiguous byte input (std::string, std::vector<char>/<std::uint8_t>, string literals, const char* ranges, \u2026) using the simdutf library instead of the built-in scalar validator. On text with many non-ASCII characters (e.g. CJK or emoji) this can validate several times faster.

    This is an opt-in external dependency. The library itself remains header-only and its behavior is unchanged: the same input is accepted or rejected either way, and every parse error is reported at the same position with the same message (simdutf is only used to fast-path valid runs; anything it flags falls back to the scalar path so the exact diagnostic is preserved). A std::istream uses simdutf for the characters its stream buffer holds (see JSON_DISABLE_STREAM_BULK_SCAN); other streaming inputs (FILE*, wide strings, user-defined adapters) always use the scalar path.

    When JSON_USE_SIMDUTF is defined you must make the simdutf.h header available on the include path and link the simdutf library. When it is not defined, no simdutf header is included and there is no dependency.

    Requires C++17

    simdutf requires C++17 and its header rejects older standards with an #error. The backend is therefore only compiled in from C++17 on. In C++11 and C++14 the macro has no effect and the scalar validator is used, which accepts and rejects exactly the same input -- only throughput differs. Setting the macro project-wide is therefore safe even when some translation units are built with an older standard.

    Define consistently

    The macro selects between two definitions of the same inline validation function. It must therefore be defined identically for every translation unit that includes the library; mixing translation units that define it with ones that do not is an ODR violation. Prefer setting it as a compile definition on the target rather than with #define in individual source files.

    "},{"location":"api/macros/json_use_simdutf/#default-definition","title":"Default definition","text":"

    By default, JSON_USE_SIMDUTF is not defined and the portable C++11 scalar validator is used.

    #undef JSON_USE_SIMDUTF\n
    "},{"location":"api/macros/json_use_simdutf/#examples","title":"Examples","text":"Example

    The code below enables the simdutf backend for UTF-8 validation.

    #define JSON_USE_SIMDUTF 1\n#include <nlohmann/json.hpp>\n\n...\n

    The project must also link against simdutf, e.g. with CMake:

    target_compile_definitions(your_target PRIVATE JSON_USE_SIMDUTF)\ntarget_link_libraries(your_target PRIVATE simdutf::simdutf)\n

    Testing this configuration

    The unit tests can be built against the simdutf backend with the CMake option JSON_TestSimdutf (OFF by default), which fetches simdutf and defines JSON_USE_SIMDUTF for every test target. The ci_test_simdutf target runs the whole test suite in that configuration.

    "},{"location":"api/macros/json_use_simdutf/#see-also","title":"See also","text":"
    • JSON_TestSimdutf - CMake option to build the unit tests against the simdutf backend
    "},{"location":"api/macros/json_use_simdutf/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/macros/json_view_no_simd/","title":"JSON_VIEW_NO_SIMD","text":"
    #define JSON_VIEW_NO_SIMD\n

    When defined, the parser of basic_json_document (<nlohmann/json_view.hpp>) uses only portable C++ to scan strings. By default, it scans long runs of string bytes 16 at a time with NEON on AArch64 (with GCC and Clang) and SSE2 on x86-64, which are part of the baseline instruction sets of these architectures, and validates non-ASCII text with NEON (or SSSE3, see JSON_VIEW_USE_SSSE3).

    The same input is accepted or rejected either way, with the same values, and errors are reported the same way; only the speed differs. The macro exists for platforms whose compilers lack the intrinsics headers, and to test the portable code.

    ABI compatibility

    The macro selects between two definitions of the same inline functions. Whether it is defined is encoded in the namespace (tag _vns), resulting in distinct symbol names. Translation units compiled with and without it can therefore be linked into the same program without One Definition Rule (ODR) violations, but they cannot exchange instances of library types.

    "},{"location":"api/macros/json_view_no_simd/#default-definition","title":"Default definition","text":"

    By default, JSON_VIEW_NO_SIMD is not defined, and the vector code is used where available.

    #undef JSON_VIEW_NO_SIMD\n
    "},{"location":"api/macros/json_view_no_simd/#examples","title":"Examples","text":"Example

    The code below uses the portable string scanning of the view.

    #define JSON_VIEW_NO_SIMD\n#include <nlohmann/json_view.hpp>\n\n...\n
    "},{"location":"api/macros/json_view_no_simd/#see-also","title":"See also","text":"
    • JSON_VIEW_USE_SSSE3 - validate non-ASCII strings with SSSE3 on x86-64
    • json_view - the zero-copy view
    "},{"location":"api/macros/json_view_no_simd/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/macros/json_view_use_ssse3/","title":"JSON_VIEW_USE_SSSE3","text":"
    #define JSON_VIEW_USE_SSSE3\n

    When defined on x86-64, the parser of basic_json_document (<nlohmann/json_view.hpp>) validates non-ASCII text in strings with SSSE3 without asking the CPU first.

    By default, the parser checks once at run time whether the CPU has SSSE3 (all x86-64 CPUs since about 2011 have it) and then validates non-ASCII text 16 bytes at a time, using the \"lookup4\" algorithm of simdjson; on CPUs without SSSE3, it validates one UTF-8 sequence at a time. The vector check is compiled for SSSE3 with a function attribute (GCC 4.9 and later, Clang), so this needs no compiler option. With MSVC, the check uses __cpuid. On AArch64, the vector check uses NEON and is always on.

    Define the macro only together with a compiler option that enables SSSE3 (e.g. -mssse3, or -march= with a CPU that has it), and only for programs that run on such CPUs. It saves the check of the CPU, which costs little. The same input is accepted or rejected either way; only the speed of non-ASCII text differs.

    ABI compatibility

    The macro selects between two definitions of the same inline functions. Whether it is defined is encoded in the namespace (tag _vssse3, on every architecture), resulting in distinct symbol names. Translation units compiled with and without it can therefore be linked into the same program without One Definition Rule (ODR) violations, but they cannot exchange instances of library types.

    "},{"location":"api/macros/json_view_use_ssse3/#default-definition","title":"Default definition","text":"

    By default, JSON_VIEW_USE_SSSE3 is not defined.

    #undef JSON_VIEW_USE_SSSE3\n
    "},{"location":"api/macros/json_view_use_ssse3/#examples","title":"Examples","text":"Example

    With CMake, for a program that only runs on CPUs with SSSE3:

    target_compile_definitions(your_target PRIVATE JSON_VIEW_USE_SSSE3)\ntarget_compile_options(your_target PRIVATE -mssse3)\n
    "},{"location":"api/macros/json_view_use_ssse3/#see-also","title":"See also","text":"
    • JSON_VIEW_NO_SIMD - use only portable code in the view's parser
    • JSON_USE_SIMDUTF - validate UTF-8 with simdutf in basic_json's parser
    "},{"location":"api/macros/json_view_use_ssse3/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/macros/nlohmann_define_derived_type/","title":"NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE, NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_DEFAULT, NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_ONLY_SERIALIZE, NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE, NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_DEFAULT, NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE","text":"NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE, NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_DEFAULT, NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_ONLY_SERIALIZE, NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE, NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_DEFAULT, NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE 
    // (1)\n#define NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE(type, base_type, member...)\n// (2)\n#define NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_DEFAULT(type, base_type, member...)\n// (3)\n#define NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_ONLY_SERIALIZE(type, base_type, member...)\n\n// (4)\n#define NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE(type, base_type, member...)\n// (5)\n#define NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_DEFAULT(type, base_type, member...)\n// (6)\n#define NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE(type, base_type, member...)\n

    These macros can be used to simplify the serialization/deserialization of derived types if you want to use a JSON object as serialization and want to use the member variable names as object keys in that object.

    • Macros 1, 2, and 3 are to be defined inside the class/struct to create code for. Like NLOHMANN_DEFINE_TYPE_INTRUSIVE, they can access private members.
    • Macros 4, 5, and 6 are to be defined outside the class/struct to create code for, but inside its namespace. Like NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE, they cannot access private members.

    The first parameter is the name of the derived class/struct, the second parameter is the name of the base class/struct and all remaining parameters name the members. The base type must be already serializable/deserializable.

    • Macros 1 and 4 will use at during deserialization and will throw out_of_range.403 if a key is missing in the JSON object.
    • Macros 2 and 5 will use value during deserialization and fall back to the default value for the respective type of the member variable if a key in the JSON object is missing. The generated from_json() function default constructs an object and uses its values as the defaults when calling the value function.

    Summary:

    Need access to private members Need only serialization Allow missing values when de-serializing macro NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_DEFAULT NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_ONLY_SERIALIZE NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_DEFAULT NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE"},{"location":"api/macros/nlohmann_define_derived_type/#parameters","title":"Parameters","text":"type (in) name of the type (class, struct) to serialize/deserialize base_type (in) name of the base type (class, struct) type is derived from member (in) name of the member variable to serialize/deserialize; up to 63 members can be given as a comma-separated list, which may also be empty"},{"location":"api/macros/nlohmann_define_derived_type/#default-definition","title":"Default definition","text":"

    Macros 1 and 2 add two friend functions to the class which take care of the serialization and deserialization:

    template<typename BasicJsonType>\nfriend void to_json(BasicJsonType&, const type&);\ntemplate<typename BasicJsonType>\nfriend void from_json(const BasicJsonType&, type&);\n

    Macros 4 and 5 add two functions to the namespace which take care of the serialization and deserialization:

    template<typename BasicJsonType>\nvoid to_json(BasicJsonType&, const type&);\ntemplate<typename BasicJsonType>\nvoid from_json(const BasicJsonType&, type&);\n

    Macros 3 and 6 add one function to the namespace, which takes care of the serialization only:

    template<typename BasicJsonType>\nvoid to_json(BasicJsonType&, const type&);\n

    In first two cases, they call the to_json/from_json functions of the base type before serializing/deserializing the members of the derived type:

    class A { /* ... */ };\nclass B : public A { /* ... */ };\n\ntemplate<typename BasicJsonType>\nvoid to_json(BasicJsonType& j, const B& b) {\n    nlohmann::to_json(j, static_cast<const A&>(b));\n    // ...\n}\n\ntemplate<typename BasicJsonType>\nvoid from_json(const BasicJsonType& j, B& b) {\n    nlohmann::from_json(j, static_cast<A&>(b));\n    // ...\n}\n

    In the third case, only to_json will be called:

    class A { /* ... */ };\nclass B : public A { /* ... */ };\n\ntemplate<typename BasicJsonType>\nvoid to_json(BasicJsonType& j, const B& b) {\n    nlohmann::to_json(j, static_cast<const A&>(b));\n    // ...\n}\n
    "},{"location":"api/macros/nlohmann_define_derived_type/#notes","title":"Notes","text":"

    Prerequisites

    • Macros 1, 2, and 3 have the same prerequisites of NLOHMANN_DEFINE_TYPE_INTRUSIVE.
    • Macros 4, 5, and 6 have the same prerequisites of NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE.
    • Serialization/deserialization of base types must be defined.

    Derived types without own members

    The member list may be empty. The macro then generates a to_json/from_json pair that only delegates to the base type, so type serializes exactly like base_type:

    struct derived : base\n{\n    NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE(derived, base)\n};\n

    The WITH_NAMES variants do not support this.

    Implementation limits

    See Implementation limits for NLOHMANN_DEFINE_TYPE_INTRUSIVE and NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE, respectively.

    "},{"location":"api/macros/nlohmann_define_derived_type/#examples","title":"Examples","text":"Example: (1) NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE

    Consider the following complete example:

    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing nlohmann::json;\n\nclass A\n{\n  private:\n    double Aa = 0.0;\n    double Ab = 0.0;\n\n  public:\n    A() = default;\n    A(double a, double b) : Aa(a), Ab(b) {}\n    NLOHMANN_DEFINE_TYPE_INTRUSIVE(A, Aa, Ab)\n};\n\nclass B : public A\n{\n  private:\n    int Ba = 0;\n    int Bb = 0;\n\n  public:\n    B() = default;\n    B(int a, int b, double aa, double ab) : A(aa, ab), Ba(a), Bb(b) {}\n    NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE(B, A, Ba, Bb)\n};\n\nint main()\n{\n    B example(23, 42, 3.142, 1.777);\n    json example_json = example;\n\n    std::cout << std::setw(4) << example_json << std::endl;\n}\n

    Output:

    {\n    \"Aa\": 3.142,\n    \"Ab\": 1.777,\n    \"Ba\": 23,\n    \"Bb\": 42\n}\n

    Notes:

    • A and B are default-constructible. This is a requirement for using the macro.
    • A has private members and is not a derived class. Hence, macro NLOHMANN_DEFINE_TYPE_INTRUSIVE is used.
    • As B is a derived class, NLOHMANN_DEFINE_TYPE_INTRUSIVE is not applicable, but NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE must be used.
    • The macro NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE is used inside the class use as NLOHMANN_DEFINE_TYPE_INTRUSIVE.
    "},{"location":"api/macros/nlohmann_define_derived_type/#see-also","title":"See also","text":"
    • NLOHMANN_DEFINE_TYPE_INTRUSIVE / NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT / NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE for similar macros that can be defined inside a non-derived type.
    • NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE / NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT / NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE for similar macros that can be defined outside a non-derived type.
    • Arbitrary Type Conversions for an overview.
    "},{"location":"api/macros/nlohmann_define_derived_type/#version-history","title":"Version history","text":"
    1. Added in version 3.12.0.
    2. Added in version 3.12.0.
    3. Added in version 3.12.0.
    4. Added in version 3.12.0.
    5. Added in version 3.12.0.
    6. Added in version 3.12.0.

    All six macros were changed to allow an empty member list in version 3.13.0 unreleased.

    "},{"location":"api/macros/nlohmann_define_type_intrusive/","title":"NLOHMANN_DEFINE_TYPE_INTRUSIVE, NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT, NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE","text":"
    #define NLOHMANN_DEFINE_TYPE_INTRUSIVE(type, member...)              // (1)\n#define NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT(type, member...) // (2)\n#define NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE(type, member...) // (3)\n

    These macros can be used to simplify the serialization/deserialization of types if you want to use a JSON object as serialization and want to use the member variable names as object keys in that object. The macro is to be defined inside the class/struct to create code for. Unlike NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE, it can access private members. The first parameter is the name of the class/struct, and all remaining parameters name the members.

    1. Will use at during deserialization and will throw out_of_range.403 if a key is missing in the JSON object.
    2. Will use value during deserialization and fall back to the default value for the respective type of the member variable if a key in the JSON object is missing. The generated from_json() function default constructs an object and uses its values as the defaults when calling the value function.
    3. Only defines the serialization. Useful in cases when the type does not have a default constructor and only serialization is required.

    Summary:

    Need access to private members Need only serialization Allow missing values when de-serializing macro NLOHMANN_DEFINE_TYPE_INTRUSIVE NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE"},{"location":"api/macros/nlohmann_define_type_intrusive/#parameters","title":"Parameters","text":"type (in) name of the type (class, struct) to serialize/deserialize member (in) name of the member variable to serialize/deserialize; up to 63 members can be given as a comma-separated list, which may also be empty"},{"location":"api/macros/nlohmann_define_type_intrusive/#default-definition","title":"Default definition","text":"

    The macros add two friend functions to the class which take care of the serialization and deserialization:

    template<typename BasicJsonType>\nfriend void to_json(BasicJsonType&, const type&);\ntemplate<typename BasicJsonType>\nfriend void from_json(const BasicJsonType&, type&); // except (3)\n

    See the examples below for the concrete generated code.

    "},{"location":"api/macros/nlohmann_define_type_intrusive/#notes","title":"Notes","text":"

    Prerequisites

    1. The type type must be default constructible (except (3)). See How can I use get() for non-default constructible/non-copyable types? for how to overcome this limitation.
    2. The macro must be used inside the type (class/struct).

    Types without members

    The member list may be empty. The macro then generates a to_json that produces an empty JSON object {}, and a from_json that reads no members:

    struct marker\n{\n    NLOHMANN_DEFINE_TYPE_INTRUSIVE(marker)\n};\n

    The WITH_NAMES variants do not support this.

    Implementation limits

    • The current implementation is limited to at most 63 member variables. If you want to serialize/deserialize types with more than 63 member variables, you need to define the to_json/from_json functions manually.
    • These macros always produce object-style (named-key) JSON, one key per member. There is no macro variant that serializes a struct's members positionally into a JSON array; for that, write to_json/from_json by hand, building/reading a json::array() of the members in order.
    "},{"location":"api/macros/nlohmann_define_type_intrusive/#examples","title":"Examples","text":"Example: (1) NLOHMANN_DEFINE_TYPE_INTRUSIVE

    Consider the following complete example:

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nnamespace ns\n{\nclass person\n{\n  private:\n    std::string name = \"John Doe\";\n    std::string address = \"123 Fake St\";\n    int age = -1;\n\n  public:\n    person() = default;\n    person(std::string name_, std::string address_, int age_)\n        : name(std::move(name_)), address(std::move(address_)), age(age_)\n    {}\n\n    NLOHMANN_DEFINE_TYPE_INTRUSIVE(person, name, address, age)\n};\n} // namespace ns\n\nint main()\n{\n    ns::person p = {\"Ned Flanders\", \"744 Evergreen Terrace\", 60};\n\n    // serialization: person -> json\n    json j = p;\n    std::cout << \"serialization: \" << j << std::endl;\n\n    // deserialization: json -> person\n    json j2 = R\"({\"address\": \"742 Evergreen Terrace\", \"age\": 40, \"name\": \"Homer Simpson\"})\"_json;\n    auto p2 = j2.get<ns::person>();\n\n    // incomplete deserialization:\n    json j3 = R\"({\"address\": \"742 Evergreen Terrace\", \"name\": \"Maggie Simpson\"})\"_json;\n    try\n    {\n        auto p3 = j3.get<ns::person>();\n    }\n    catch (const json::exception& e)\n    {\n        std::cout << \"deserialization failed: \" << e.what() << std::endl;\n    }\n}\n

    Output:

    serialization: {\"address\":\"744 Evergreen Terrace\",\"age\":60,\"name\":\"Ned Flanders\"}\ndeserialization failed: [json.exception.out_of_range.403] key 'age' not found\n

    Notes:

    • ns::person is default-constructible. This is a requirement for using the macro.
    • ns::person has private member variables. This makes NLOHMANN_DEFINE_TYPE_INTRUSIVE applicable, but not NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE.
    • The macro NLOHMANN_DEFINE_TYPE_INTRUSIVE is used inside the class.
    • A missing key \"age\" in the deserialization yields an exception. To fall back to the default value, NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT can be used.

    The macro is equivalent to:

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nnamespace ns\n{\nclass person\n{\n  private:\n    std::string name = \"John Doe\";\n    std::string address = \"123 Fake St\";\n    int age = -1;\n\n  public:\n    person() = default;\n    person(std::string name_, std::string address_, int age_)\n        : name(std::move(name_)), address(std::move(address_)), age(age_)\n    {}\n\n    template<typename BasicJsonType>\n    friend void to_json(BasicJsonType& nlohmann_json_j, const person& nlohmann_json_t)\n    {\n        nlohmann_json_j[\"name\"] = nlohmann_json_t.name;\n        nlohmann_json_j[\"address\"] = nlohmann_json_t.address;\n        nlohmann_json_j[\"age\"] = nlohmann_json_t.age;\n    }\n\n    template<typename BasicJsonType>\n    friend void from_json(const BasicJsonType& nlohmann_json_j, person& nlohmann_json_t)\n    {\n        nlohmann_json_t.name = nlohmann_json_j.at(\"name\");\n        nlohmann_json_t.address = nlohmann_json_j.at(\"address\");\n        nlohmann_json_t.age = nlohmann_json_j.at(\"age\");\n    }\n};\n} // namespace ns\n\nint main()\n{\n    ns::person p = {\"Ned Flanders\", \"744 Evergreen Terrace\", 60};\n\n    // serialization: person -> json\n    json j = p;\n    std::cout << \"serialization: \" << j << std::endl;\n\n    // deserialization: json -> person\n    json j2 = R\"({\"address\": \"742 Evergreen Terrace\", \"age\": 40, \"name\": \"Homer Simpson\"})\"_json;\n    auto p2 = j2.get<ns::person>();\n\n    // incomplete deserialization:\n    json j3 = R\"({\"address\": \"742 Evergreen Terrace\", \"name\": \"Maggie Simpson\"})\"_json;\n    try\n    {\n        auto p3 = j3.get<ns::person>();\n    }\n    catch (const json::exception& e)\n    {\n        std::cout << \"deserialization failed: \" << e.what() << std::endl;\n    }\n}\n
    Example: (2) NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT

    Consider the following complete example:

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nnamespace ns\n{\nclass person\n{\n  private:\n    std::string name = \"John Doe\";\n    std::string address = \"123 Fake St\";\n    int age = -1;\n\n  public:\n    person() = default;\n    person(std::string name_, std::string address_, int age_)\n        : name(std::move(name_)), address(std::move(address_)), age(age_)\n    {}\n\n    NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT(person, name, address, age)\n};\n} // namespace ns\n\nint main()\n{\n    ns::person p = {\"Ned Flanders\", \"744 Evergreen Terrace\", 60};\n\n    // serialization: person -> json\n    json j = p;\n    std::cout << \"serialization: \" << j << std::endl;\n\n    // deserialization: json -> person\n    json j2 = R\"({\"address\": \"742 Evergreen Terrace\", \"age\": 40, \"name\": \"Homer Simpson\"})\"_json;\n    auto p2 = j2.get<ns::person>();\n\n    // incomplete deserialization:\n    json j3 = R\"({\"address\": \"742 Evergreen Terrace\", \"name\": \"Maggie Simpson\"})\"_json;\n    auto p3 = j3.get<ns::person>();\n    std::cout << \"roundtrip: \" << json(p3) << std::endl;\n}\n

    Output:

    serialization: {\"address\":\"744 Evergreen Terrace\",\"age\":60,\"name\":\"Ned Flanders\"}\nroundtrip: {\"address\":\"742 Evergreen Terrace\",\"age\":-1,\"name\":\"Maggie Simpson\"}\n

    Notes:

    • ns::person is default-constructible. This is a requirement for using the macro.
    • ns::person has private member variables. This makes NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT applicable, but not NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT.
    • The macro NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT is used inside the class.
    • A missing key \"age\" in the deserialization does not yield an exception. Instead, the default value -1 is used.

    The macro is equivalent to:

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nnamespace ns\n{\nclass person\n{\n  private:\n    std::string name = \"John Doe\";\n    std::string address = \"123 Fake St\";\n    int age = -1;\n\n  public:\n    person() = default;\n    person(std::string name_, std::string address_, int age_)\n        : name(std::move(name_)), address(std::move(address_)), age(age_)\n    {}\n\n    template<typename BasicJsonType>\n    friend void to_json(BasicJsonType& nlohmann_json_j, const person& nlohmann_json_t)\n    {\n        nlohmann_json_j[\"name\"] = nlohmann_json_t.name;\n        nlohmann_json_j[\"address\"] = nlohmann_json_t.address;\n        nlohmann_json_j[\"age\"] = nlohmann_json_t.age;\n    }\n\n    template<typename BasicJsonType>\n    friend void from_json(const BasicJsonType& nlohmann_json_j, person& nlohmann_json_t)\n    {\n        person nlohmann_json_default_obj;\n        nlohmann_json_t.name = nlohmann_json_j.value(\"name\", nlohmann_json_default_obj.name);\n        nlohmann_json_t.address = nlohmann_json_j.value(\"address\", nlohmann_json_default_obj.address);\n        nlohmann_json_t.age = nlohmann_json_j.value(\"age\", nlohmann_json_default_obj.age);\n    }\n};\n} // namespace ns\n\nint main()\n{\n    ns::person p = {\"Ned Flanders\", \"744 Evergreen Terrace\", 60};\n\n    // serialization: person -> json\n    json j = p;\n    std::cout << \"serialization: \" << j << std::endl;\n\n    // deserialization: json -> person\n    json j2 = R\"({\"address\": \"742 Evergreen Terrace\", \"age\": 40, \"name\": \"Homer Simpson\"})\"_json;\n    auto p2 = j2.get<ns::person>();\n\n    // incomplete deserialization:\n    json j3 = R\"({\"address\": \"742 Evergreen Terrace\", \"name\": \"Maggie Simpson\"})\"_json;\n    auto p3 = j3.get<ns::person>();\n    std::cout << \"roundtrip: \" << json(p3) << std::endl;\n}\n

    Note how a default-initialized person object is used in the from_json to fill missing values.

    Example: (3) NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE

    Consider the following complete example:

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nnamespace ns\n{\nclass person\n{\n  private:\n    std::string name = \"John Doe\";\n    std::string address = \"123 Fake St\";\n    int age = -1;\n\n  public:\n    // No default constructor\n    person(std::string name_, std::string address_, int age_)\n        : name(std::move(name_)), address(std::move(address_)), age(age_)\n    {}\n\n    NLOHMANN_DEFINE_TYPE_INTRUSIVE(person, name, address, age)\n};\n} // namespace ns\n\nint main()\n{\n    ns::person p = {\"Ned Flanders\", \"744 Evergreen Terrace\", 60};\n\n    // serialization: person -> json\n    json j = p;\n    std::cout << \"serialization: \" << j << std::endl;\n}\n

    Output:

    serialization: {\"address\":\"744 Evergreen Terrace\",\"age\":60,\"name\":\"Ned Flanders\"}\n

    Notes:

    • ns::person is non-default-constructible. This allows this macro to be used instead of NLOHMANN_DEFINE_TYPE_INTRUSIVE and NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT.
    • ns::person has private member variables. This makes NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE applicable, but not NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE.
    • The macro NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE is used inside the class.

    The macro is equivalent to:

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nnamespace ns\n{\nclass person\n{\n  private:\n    std::string name = \"John Doe\";\n    std::string address = \"123 Fake St\";\n    int age = -1;\n\n  public:\n    // No default constructor\n    person(std::string name_, std::string address_, int age_)\n        : name(std::move(name_)), address(std::move(address_)), age(age_)\n    {}\n\n    template<typename BasicJsonType>\n    friend void to_json(BasicJsonType& nlohmann_json_j, const person& nlohmann_json_t)\n    {\n        nlohmann_json_j[\"name\"] = nlohmann_json_t.name;\n        nlohmann_json_j[\"address\"] = nlohmann_json_t.address;\n        nlohmann_json_j[\"age\"] = nlohmann_json_t.age;\n    }\n};\n} // namespace ns\n\nint main()\n{\n    ns::person p = {\"Ned Flanders\", \"744 Evergreen Terrace\", 60};\n\n    // serialization: person -> json\n    json j = p;\n    std::cout << \"serialization: \" << j << std::endl;\n}\n
    "},{"location":"api/macros/nlohmann_define_type_intrusive/#see-also","title":"See also","text":"
    • NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE, NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT, NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE for a similar macro that can be defined outside the type.
    • NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE, NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_DEFAULT, NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_ONLY_SERIALIZE, NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE, NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_DEFAULT, NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE for similar macros for derived types
    • Arbitrary Type Conversions for an overview.
    "},{"location":"api/macros/nlohmann_define_type_intrusive/#version-history","title":"Version history","text":"
    1. Added in version 3.9.0.
    2. Added in version 3.11.0.
    3. Added in version 3.11.3.

    All three macros were changed to work with any basic_json specialization (not only nlohmann::json) in version 3.12.0, and to allow an empty member list in version 3.13.0 unreleased.

    "},{"location":"api/macros/nlohmann_define_type_non_intrusive/","title":"NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE, NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT, NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE","text":"
    #define NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(type, member...)              // (1)\n#define NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT(type, member...) // (2)\n#define NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE(type, member...) // (3)\n

    These macros can be used to simplify the serialization/deserialization of types if you want to use a JSON object as serialization and want to use the member variable names as object keys in that object. The macro is to be defined outside the class/struct to create code for, but inside its namespace. Unlike NLOHMANN_DEFINE_TYPE_INTRUSIVE, it cannot access private members. The first parameter is the name of the class/struct, and all remaining parameters name the members.

    1. Will use at during deserialization and will throw out_of_range.403 if a key is missing in the JSON object.
    2. Will use value during deserialization and fall back to the default value for the respective type of the member variable if a key in the JSON object is missing. The generated from_json() function default constructs an object and uses its values as the defaults when calling the value function.
    3. Only defines the serialization. Useful in cases when the type does not have a default constructor and only serialization is required.

    Summary:

    Need access to private members Need only serialization Allow missing values when de-serializing macro NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE"},{"location":"api/macros/nlohmann_define_type_non_intrusive/#parameters","title":"Parameters","text":"type (in) name of the type (class, struct) to serialize/deserialize member (in) name of the (public) member variable to serialize/deserialize; up to 63 members can be given as a comma-separated list, which may also be empty"},{"location":"api/macros/nlohmann_define_type_non_intrusive/#default-definition","title":"Default definition","text":"

    The macros add two functions to the namespace which take care of the serialization and deserialization:

    template<typename BasicJsonType>\nvoid to_json(BasicJsonType&, const type&);\ntemplate<typename BasicJsonType>\nvoid from_json(const BasicJsonType&, type&); // except (3)\n

    See the examples below for the concrete generated code.

    "},{"location":"api/macros/nlohmann_define_type_non_intrusive/#notes","title":"Notes","text":"

    Prerequisites

    1. The type type must be default constructible (except (3). See How can I use get() for non-default constructible/non-copyable types? for how to overcome this limitation.
    2. The macro must be used outside the type (class/struct).
    3. The passed members must be public.

    Types without members

    The member list may be empty. The macro then generates a to_json that produces an empty JSON object {}, and a from_json that reads no members:

    struct marker {};\nNLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(marker)\n

    The WITH_NAMES variants do not support this.

    Implementation limits

    • The current implementation is limited to at most 63 member variables. If you want to serialize/deserialize types with more than 63 member variables, you need to define the to_json/from_json functions manually.
    • These macros always produce object-style (named-key) JSON, one key per member. There is no macro variant that serializes a struct's members positionally into a JSON array; for that, write to_json/from_json by hand, building/reading a json::array() of the members in order.
    "},{"location":"api/macros/nlohmann_define_type_non_intrusive/#examples","title":"Examples","text":"Example: (1) NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE

    Consider the following complete example:

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nnamespace ns\n{\nstruct person\n{\n    std::string name;\n    std::string address;\n    int age;\n};\n\nNLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(person, name, address, age)\n} // namespace ns\n\nint main()\n{\n    ns::person p = {\"Ned Flanders\", \"744 Evergreen Terrace\", 60};\n\n    // serialization: person -> json\n    json j = p;\n    std::cout << \"serialization: \" << j << std::endl;\n\n    // deserialization: json -> person\n    json j2 = R\"({\"address\": \"742 Evergreen Terrace\", \"age\": 40, \"name\": \"Homer Simpson\"})\"_json;\n    auto p2 = j2.get<ns::person>();\n\n    // incomplete deserialization:\n    json j3 = R\"({\"address\": \"742 Evergreen Terrace\", \"name\": \"Maggie Simpson\"})\"_json;\n    try\n    {\n        auto p3 = j3.get<ns::person>();\n    }\n    catch (const json::exception& e)\n    {\n        std::cout << \"deserialization failed: \" << e.what() << std::endl;\n    }\n}\n

    Output:

    serialization: {\"address\":\"744 Evergreen Terrace\",\"age\":60,\"name\":\"Ned Flanders\"}\ndeserialization failed: [json.exception.out_of_range.403] key 'age' not found\n

    Notes:

    • ns::person is default-constructible. This is a requirement for using the macro.
    • ns::person has only public member variables. This makes NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE applicable.
    • The macro NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE is used outside the class, but inside its namespace ns.
    • A missing key \"age\" in the deserialization yields an exception. To fall back to the default value, NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT can be used.

    The macro is equivalent to:

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nnamespace ns\n{\nstruct person\n{\n    std::string name;\n    std::string address;\n    int age;\n};\n\ntemplate<typename BasicJsonType>\nvoid to_json(BasicJsonType& nlohmann_json_j, const person& nlohmann_json_t)\n{\n    nlohmann_json_j[\"name\"] = nlohmann_json_t.name;\n    nlohmann_json_j[\"address\"] = nlohmann_json_t.address;\n    nlohmann_json_j[\"age\"] = nlohmann_json_t.age;\n}\n\ntemplate<typename BasicJsonType>\nvoid from_json(const BasicJsonType& nlohmann_json_j, person& nlohmann_json_t)\n{\n    nlohmann_json_t.name = nlohmann_json_j.at(\"name\");\n    nlohmann_json_t.address = nlohmann_json_j.at(\"address\");\n    nlohmann_json_t.age = nlohmann_json_j.at(\"age\");\n}\n} // namespace ns\n\nint main()\n{\n    ns::person p = {\"Ned Flanders\", \"744 Evergreen Terrace\", 60};\n\n    // serialization: person -> json\n    json j = p;\n    std::cout << \"serialization: \" << j << std::endl;\n\n    // deserialization: json -> person\n    json j2 = R\"({\"address\": \"742 Evergreen Terrace\", \"age\": 40, \"name\": \"Homer Simpson\"})\"_json;\n    auto p2 = j2.get<ns::person>();\n\n    // incomplete deserialization:\n    json j3 = R\"({\"address\": \"742 Evergreen Terrace\", \"name\": \"Maggie Simpson\"})\"_json;\n    try\n    {\n        auto p3 = j3.get<ns::person>();\n    }\n    catch (const json::exception& e)\n    {\n        std::cout << \"deserialization failed: \" << e.what() << std::endl;\n    }\n}\n
    Example: (2) NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT

    Consider the following complete example:

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nnamespace ns\n{\nstruct person\n{\n    std::string name = \"John Doe\";\n    std::string address = \"123 Fake St\";\n    int age = -1;\n\n    person() = default;\n    person(std::string name_, std::string address_, int age_)\n        : name(std::move(name_)), address(std::move(address_)), age(age_)\n    {}\n};\n\nNLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT(person, name, address, age)\n} // namespace ns\n\nint main()\n{\n    ns::person p = {\"Ned Flanders\", \"744 Evergreen Terrace\", 60};\n\n    // serialization: person -> json\n    json j = p;\n    std::cout << \"serialization: \" << j << std::endl;\n\n    // deserialization: json -> person\n    json j2 = R\"({\"address\": \"742 Evergreen Terrace\", \"age\": 40, \"name\": \"Homer Simpson\"})\"_json;\n    auto p2 = j2.get<ns::person>();\n\n    // incomplete deserialization:\n    json j3 = R\"({\"address\": \"742 Evergreen Terrace\", \"name\": \"Maggie Simpson\"})\"_json;\n    auto p3 = j3.get<ns::person>();\n    std::cout << \"roundtrip: \" << json(p3) << std::endl;\n}\n

    Output:

    serialization: {\"address\":\"744 Evergreen Terrace\",\"age\":60,\"name\":\"Ned Flanders\"}\nroundtrip: {\"address\":\"742 Evergreen Terrace\",\"age\":-1,\"name\":\"Maggie Simpson\"}\n

    Notes:

    • ns::person is default-constructible. This is a requirement for using the macro.
    • ns::person has only public member variables. This makes NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT applicable.
    • The macro NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT is used outside the class, but inside its namespace ns.
    • A missing key \"age\" in the deserialization does not yield an exception. Instead, the default value -1 is used.

    The macro is equivalent to:

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nnamespace ns\n{\nstruct person\n{\n    std::string name = \"John Doe\";\n    std::string address = \"123 Fake St\";\n    int age = -1;\n\n    person() = default;\n    person(std::string name_, std::string address_, int age_)\n        : name(std::move(name_)), address(std::move(address_)), age(age_)\n    {}\n};\n\ntemplate<typename BasicJsonType>\nvoid to_json(BasicJsonType& nlohmann_json_j, const person& nlohmann_json_t)\n{\n    nlohmann_json_j[\"name\"] = nlohmann_json_t.name;\n    nlohmann_json_j[\"address\"] = nlohmann_json_t.address;\n    nlohmann_json_j[\"age\"] = nlohmann_json_t.age;\n}\n\ntemplate<typename BasicJsonType>\nvoid from_json(const BasicJsonType& nlohmann_json_j, person& nlohmann_json_t)\n{\n    person nlohmann_json_default_obj;\n    nlohmann_json_t.name = nlohmann_json_j.value(\"name\", nlohmann_json_default_obj.name);\n    nlohmann_json_t.address = nlohmann_json_j.value(\"address\", nlohmann_json_default_obj.address);\n    nlohmann_json_t.age = nlohmann_json_j.value(\"age\", nlohmann_json_default_obj.age);\n}\n} // namespace ns\n\nint main()\n{\n    ns::person p = {\"Ned Flanders\", \"744 Evergreen Terrace\", 60};\n\n    // serialization: person -> json\n    json j = p;\n    std::cout << \"serialization: \" << j << std::endl;\n\n    // deserialization: json -> person\n    json j2 = R\"({\"address\": \"742 Evergreen Terrace\", \"age\": 40, \"name\": \"Homer Simpson\"})\"_json;\n    auto p2 = j2.get<ns::person>();\n\n    // incomplete deserialization:\n    json j3 = R\"({\"address\": \"742 Evergreen Terrace\", \"name\": \"Maggie Simpson\"})\"_json;\n    auto p3 = j3.get<ns::person>();\n    std::cout << \"roundtrip: \" << json(p3) << std::endl;\n}\n

    Note how a default-initialized person object is used in the from_json to fill missing values.

    Example: (3) NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE

    Consider the following complete example:

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nnamespace ns\n{\nstruct person\n{\n    std::string name;\n    std::string address;\n    int age;\n};\n\nNLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE(person, name, address, age)\n} // namespace ns\n\nint main()\n{\n    ns::person p = {\"Ned Flanders\", \"744 Evergreen Terrace\", 60};\n\n    // serialization: person -> json\n    json j = p;\n    std::cout << \"serialization: \" << j << std::endl;\n}\n

    Output:

    serialization: {\"address\":\"744 Evergreen Terrace\",\"age\":60,\"name\":\"Ned Flanders\"}\n

    Notes:

    • ns::person is non-default-constructible. This allows this macro to be used instead of NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE and NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT.
    • ns::person has only public member variables. This makes NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE applicable.
    • The macro NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE is used outside the class, but inside its namespace ns.

    The macro is equivalent to:

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nnamespace ns\n{\nstruct person\n{\n    std::string name;\n    std::string address;\n    int age;\n};\n\ntemplate<typename BasicJsonType>\nvoid to_json(BasicJsonType& nlohmann_json_j, const person& nlohmann_json_t)\n{\n    nlohmann_json_j[\"name\"] = nlohmann_json_t.name;\n    nlohmann_json_j[\"address\"] = nlohmann_json_t.address;\n    nlohmann_json_j[\"age\"] = nlohmann_json_t.age;\n}\n} // namespace ns\n\nint main()\n{\n    ns::person p = {\"Ned Flanders\", \"744 Evergreen Terrace\", 60};\n\n    // serialization: person -> json\n    json j = p;\n    std::cout << \"serialization: \" << j << std::endl;\n}\n
    "},{"location":"api/macros/nlohmann_define_type_non_intrusive/#see-also","title":"See also","text":"
    • NLOHMANN_DEFINE_TYPE_INTRUSIVE, NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT, NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE for a similar macro that can be defined inside the type.
    • NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE, NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_DEFAULT, NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_ONLY_SERIALIZE, NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE, NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_DEFAULT, NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE for similar macros for derived types
    • Arbitrary Type Conversions for an overview.
    "},{"location":"api/macros/nlohmann_define_type_non_intrusive/#version-history","title":"Version history","text":"
    1. Added in version 3.9.0.
    2. Added in version 3.11.0.
    3. Added in version 3.11.3.

    All three macros were changed to work with any basic_json specialization (not only nlohmann::json) in version 3.12.0, and to allow an empty member list in version 3.13.0 unreleased.

    "},{"location":"api/macros/nlohmann_define_type_with_names/","title":"NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_NAMES, NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT_WITH_NAMES, NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE_WITH_NAMES, NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_NAMES, NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT_WITH_NAMES, NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE_WITH_NAMES, NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_NAMES, NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_DEFAULT_WITH_NAMES, NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_ONLY_SERIALIZE_WITH_NAMES, NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_NAMES, NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_DEFAULT_WITH_NAMES, NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE_WITH_NAMES","text":"NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_NAMES, NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT_WITH_NAMES, NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE_WITH_NAMES, NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_NAMES, NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT_WITH_NAMES, NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE_WITH_NAMES, NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_NAMES, NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_DEFAULT_WITH_NAMES, NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_ONLY_SERIALIZE_WITH_NAMES, NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_NAMES, NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_DEFAULT_WITH_NAMES, NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE_WITH_NAMES 
    #define NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_NAMES(type, \"json_member_name\", member...)\n#define NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT_WITH_NAMES(type, \"json_member_name\", member...)\n#define NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE_WITH_NAMES(type, \"json_member_name\", member...)\n#define NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_NAMES(type, \"json_member_name\", member...)\n#define NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT_WITH_NAMES(type, \"json_member_name\", member...)\n#define NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE_WITH_NAMES(type, \"json_member_name\", member...)\n#define NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_NAMES(type, base_type, \"json_member_name\", member...)\n#define NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_DEFAULT_WITH_NAMES(type, base_type, \"json_member_name\", member...)\n#define NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_ONLY_SERIALIZE_WITH_NAMES(type, base_type, \"json_member_name\", member...)\n#define NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_NAMES(type, base_type, \"json_member_name\", member...)\n#define NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_DEFAULT_WITH_NAMES(type, base_type, \"json_member_name\", member...)\n#define NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE_WITH_NAMES(type, base_type, \"json_member_name\", member...)\n

    These macros can be used in case you want to use the custom names for the member variables in the resulting JSON. They behave exactly as their non-WITH_NAMES counterparts, but require an additional parameter for each member variable which will be used in JSON. Both serialization and deserialization will only use the custom names for JSON, the names of the member variables themselves will be ignored.

    Using the named conversion macros will halve the maximum number of member variables from 63 to 31.

    For further information please refer to the corresponding macros without WITH_NAMES.

    "},{"location":"api/macros/nlohmann_define_type_with_names/#parameters","title":"Parameters","text":"type (in) name of the type (class, struct) to serialize/deserialize base_type (in) name of the base type (class, struct) type is derived from (used only in DEFINE_DERIVED_TYPE macros) json_member_name (in) the string that will be used as the name for the next value member (in) name of the member variable to serialize/deserialize"},{"location":"api/macros/nlohmann_define_type_with_names/#examples","title":"Examples","text":"Example: NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_NAMES

    Consider the following complete example:

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nnamespace ns\n{\nstruct person\n{\n    std::string name;\n    std::string address;\n    int age;\n};\n\nNLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_NAMES(person, \"json_name\", name, \"json_address\", address, \"json_age\", age)\n} // namespace ns\n\nint main()\n{\n    ns::person p = {\"Ned Flanders\", \"744 Evergreen Terrace\", 60};\n\n    // serialization: person -> json\n    json j = p;\n    std::cout << \"serialization: \" << j << std::endl;\n\n    // deserialization: json -> person\n    json j2 = R\"({\"json_address\": \"742 Evergreen Terrace\", \"json_age\": 40, \"json_name\": \"Homer Simpson\"})\"_json;\n    auto p2 = j2.template get<ns::person>();\n\n    // incomplete deserialization:\n    json j3 = R\"({\"json_address\": \"742 Evergreen Terrace\", \"json_name\": \"Maggie Simpson\"})\"_json;\n    try\n    {\n        auto p3 = j3.template get<ns::person>();\n    }\n    catch (const json::exception& e)\n    {\n        std::cout << \"deserialization failed: \" << e.what() << std::endl;\n    }\n}\n

    Output:

    serialization: {\"json_address\":\"744 Evergreen Terrace\",\"json_age\":60,\"json_name\":\"Ned Flanders\"}\ndeserialization failed: [json.exception.out_of_range.403] key 'json_age' not found\n

    Notes:

    • ns::person is default-constructible. This is a requirement for using the macro.
    • ns::person has only public member variables. This makes NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_NAMES applicable.
    • The macro NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_NAMES is used outside the class, but inside its namespace ns.
    • A missing key \"age\" in the deserialization yields an exception. To fall back to the default value, NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT_WITH_NAMES can be used.

    The macro is equivalent to:

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nnamespace ns\n{\nstruct person\n{\n    std::string name;\n    std::string address;\n    int age;\n};\n\ntemplate <typename BasicJsonType, nlohmann::detail::enable_if_t<nlohmann::detail::is_basic_json<BasicJsonType>::value, int> = 0>\nvoid to_json(BasicJsonType& nlohmann_json_j, const person& nlohmann_json_t)\n{\n    nlohmann_json_j[\"json_name\"] = nlohmann_json_t.name;\n    nlohmann_json_j[\"json_address\"] = nlohmann_json_t.address;\n    nlohmann_json_j[\"json_age\"] = nlohmann_json_t.age;\n}\n\ntemplate <typename BasicJsonType, nlohmann::detail::enable_if_t<nlohmann::detail::is_basic_json<BasicJsonType>::value, int> = 0>\nvoid from_json(const BasicJsonType& nlohmann_json_j, person& nlohmann_json_t)\n{\n    nlohmann_json_j.at(\"json_name\").get_to(nlohmann_json_t.name);\n    nlohmann_json_j.at(\"json_address\").get_to(nlohmann_json_t.address);\n    nlohmann_json_j.at(\"json_age\").get_to(nlohmann_json_t.age);\n}\n} // namespace ns\n\nint main()\n{\n    ns::person p = {\"Ned Flanders\", \"744 Evergreen Terrace\", 60};\n\n    // serialization: person -> json\n    json j = p;\n    std::cout << \"serialization: \" << j << std::endl;\n\n    // deserialization: json -> person\n    json j2 = R\"({\"json_address\": \"742 Evergreen Terrace\", \"json_age\": 40, \"json_name\": \"Homer Simpson\"})\"_json;\n    auto p2 = j2.template get<ns::person>();\n\n    // incomplete deserialization:\n    json j3 = R\"({\"json_address\": \"742 Evergreen Terrace\", \"json_name\": \"Maggie Simpson\"})\"_json;\n    try\n    {\n        auto p3 = j3.template get<ns::person>();\n    }\n    catch (const json::exception& e)\n    {\n        std::cout << \"deserialization failed: \" << e.what() << std::endl;\n    }\n}\n
    "},{"location":"api/macros/nlohmann_define_type_with_names/#see-also","title":"See also","text":"
    • NLOHMANN_DEFINE_TYPE_INTRUSIVE, NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT, NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE - the macros these variants add custom JSON key names to
    • NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE, NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT, NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE - the macros these variants add custom JSON key names to
    • NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE, NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_DEFAULT, NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_ONLY_SERIALIZE, NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE, NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_DEFAULT, NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE - similar macros for derived types, also available with custom names
    • Arbitrary Type Conversions - overview of type conversion mechanisms
    "},{"location":"api/macros/nlohmann_define_type_with_names/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"api/macros/nlohmann_json_namespace/","title":"NLOHMANN_JSON_NAMESPACE","text":"
    #define NLOHMANN_JSON_NAMESPACE /* value */\n

    This macro evaluates to the full name of the nlohmann namespace.

    "},{"location":"api/macros/nlohmann_json_namespace/#default-definition","title":"Default definition","text":"

    The default value consists of the root namespace (nlohmann) and an inline ABI namespace. See nlohmann Namespace for details.

    When the macro is not defined, the library will define it to its default value. Overriding this value has no effect on the library.

    "},{"location":"api/macros/nlohmann_json_namespace/#examples","title":"Examples","text":"Example

    The example shows how to use NLOHMANN_JSON_NAMESPACE instead of just nlohmann, as well as how to output the value of NLOHMANN_JSON_NAMESPACE.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\n// possible use case: use NLOHMANN_JSON_NAMESPACE instead of nlohmann\nusing json = NLOHMANN_JSON_NAMESPACE::json;\n\n// macro needed to output the NLOHMANN_JSON_NAMESPACE as string literal\n#define Q(x) #x\n#define QUOTE(x) Q(x)\n\nint main()\n{\n    std::cout << QUOTE(NLOHMANN_JSON_NAMESPACE) << std::endl;\n}\n

    Output:

    nlohmann::json_abi_v3_12_0\n
    "},{"location":"api/macros/nlohmann_json_namespace/#see-also","title":"See also","text":"
    • NLOHMANN_JSON_NAMESPACE_BEGIN, NLOHMANN_JSON_NAMESPACE_END
    • NLOHMANN_JSON_NAMESPACE_NO_VERSION
    "},{"location":"api/macros/nlohmann_json_namespace/#version-history","title":"Version history","text":"
    • Added in version 3.11.0. Changed inline namespace name in version 3.11.2. Added the ABI tag _dp in version 3.12.0, and the ABI tags _bics, _psp, _snul, _sbu8, _ekmo, _dsbs, _dufio, _vns, and _vssse3 in version 3.13.0 unreleased; see nlohmann Namespace.
    "},{"location":"api/macros/nlohmann_json_namespace_begin/","title":"NLOHMANN_JSON_NAMESPACE_BEGIN, NLOHMANN_JSON_NAMESPACE_END","text":"
    #define NLOHMANN_JSON_NAMESPACE_BEGIN /* value */  // (1)\n#define NLOHMANN_JSON_NAMESPACE_END   /* value */  // (2)\n

    These macros can be used to open and close the nlohmann namespace. See nlohmann Namespace for details.

    1. Opens the namespace.
    2. Closes the namespace.
    "},{"location":"api/macros/nlohmann_json_namespace_begin/#default-definition","title":"Default definition","text":"

    The default definitions open and close the nlohmann namespace. The precise definition of NLOHMANN_JSON_NAMESPACE_BEGIN varies as described here.

    1. Default definition of NLOHMANN_JSON_NAMESPACE_BEGIN:

      namespace nlohmann\n{\ninline namespace json_abi_v3_12_0\n{\n

    2. Default definition of NLOHMANN_JSON_NAMESPACE_END: 

      }  // namespace json_abi_v3_12_0\n}  // namespace nlohmann\n

    When these macros are not defined, the library will define them to their default definitions.

    "},{"location":"api/macros/nlohmann_json_namespace_begin/#examples","title":"Examples","text":"Example

    The example shows how to use NLOHMANN_JSON_NAMESPACE_BEGIN/NLOHMANN_JSON_NAMESPACE_END from the How do I convert third-party types? page.

    #include <iostream>\n#include <optional>\n#include <nlohmann/json.hpp>\n\n// partial specialization (see https://json.nlohmann.me/features/arbitrary_types/)\nNLOHMANN_JSON_NAMESPACE_BEGIN\ntemplate <typename T>\nstruct adl_serializer<std::optional<T>>\n{\n    static void to_json(json& j, const std::optional<T>& opt)\n    {\n        if (opt == std::nullopt)\n        {\n            j = nullptr;\n        }\n        else\n        {\n            j = *opt;\n        }\n    }\n};\nNLOHMANN_JSON_NAMESPACE_END\n\nint main()\n{\n    std::optional<int> o1 = 1;\n    std::optional<int> o2 = std::nullopt;\n\n    NLOHMANN_JSON_NAMESPACE::json j;\n    j.push_back(o1);\n    j.push_back(o2);\n    std::cout << j << std::endl;\n}\n

    Output:

    [1,null]\n
    "},{"location":"api/macros/nlohmann_json_namespace_begin/#see-also","title":"See also","text":"
    • nlohmann Namespace
    • NLOHMANN_JSON_NAMESPACE
    • NLOHMANN_JSON_NAMESPACE_NO_VERSION
    "},{"location":"api/macros/nlohmann_json_namespace_begin/#version-history","title":"Version history","text":"
    • Added in version 3.11.0. Changed inline namespace name in version 3.11.2.
    "},{"location":"api/macros/nlohmann_json_namespace_no_version/","title":"NLOHMANN_JSON_NAMESPACE_NO_VERSION","text":"
    #define NLOHMANN_JSON_NAMESPACE_NO_VERSION /* value */\n

    If defined to 1, the version component is omitted from the inline namespace. See nlohmann Namespace for details.

    "},{"location":"api/macros/nlohmann_json_namespace_no_version/#default-definition","title":"Default definition","text":"

    The default value is 0.

    #define NLOHMANN_JSON_NAMESPACE_NO_VERSION 0\n

    When the macro is not defined, the library will define it to its default value.

    "},{"location":"api/macros/nlohmann_json_namespace_no_version/#examples","title":"Examples","text":"Example

    The example shows how to use NLOHMANN_JSON_NAMESPACE_NO_VERSION to disable the version component of the inline namespace.

    #include <iostream>\n\n#define NLOHMANN_JSON_NAMESPACE_NO_VERSION 1\n#include <nlohmann/json.hpp>\n\n// macro needed to output the NLOHMANN_JSON_NAMESPACE as string literal\n#define Q(x) #x\n#define QUOTE(x) Q(x)\n\nint main()\n{\n    std::cout << QUOTE(NLOHMANN_JSON_NAMESPACE) << std::endl;\n}\n

    Output:

    nlohmann::json_abi\n
    "},{"location":"api/macros/nlohmann_json_namespace_no_version/#see-also","title":"See also","text":"
    • nlohmann Namespace
    • NLOHMANN_JSON_NAMESPACE
    • NLOHMANN_JSON_NAMESPACE_BEGIN, NLOHMANN_JSON_NAMESPACE_END
    "},{"location":"api/macros/nlohmann_json_namespace_no_version/#version-history","title":"Version history","text":"
    • Added in version 3.11.2.
    "},{"location":"api/macros/nlohmann_json_serialize_enum/","title":"NLOHMANN_JSON_SERIALIZE_ENUM","text":"
    #define NLOHMANN_JSON_SERIALIZE_ENUM(type, conversion...)\n

    By default, enum values are serialized to JSON as integers. In some cases, this could result in undesired behavior. If an enum is modified or re-ordered after data has been serialized to JSON, the later deserialized JSON data may be undefined or a different enum value than was originally intended.

    The NLOHMANN_JSON_SERIALIZE_ENUM allows to define a user-defined serialization for every enumerator.

    "},{"location":"api/macros/nlohmann_json_serialize_enum/#parameters","title":"Parameters","text":"type (in) name of the enum to serialize/deserialize conversion (in) a pair of an enumerator and a JSON serialization; arbitrary pairs can be given as a comma-separated list"},{"location":"api/macros/nlohmann_json_serialize_enum/#default-definition","title":"Default definition","text":"

    The macro adds two functions to the namespace which take care of the serialization and deserialization:

    template<typename BasicJsonType>\ninline void to_json(BasicJsonType& j, const type& e);\ntemplate<typename BasicJsonType>\ninline void from_json(const BasicJsonType& j, type& e);\n
    "},{"location":"api/macros/nlohmann_json_serialize_enum/#notes","title":"Notes","text":"

    Prerequisites

    The macro must be used inside the namespace of the enum.

    Important notes

    • When using get<ENUM_TYPE>(), undefined JSON values will default to the first specified conversion. Select this default pair carefully. See example 1 below.
    • If an enum or JSON value is specified in multiple conversions, the first matching conversion from the top of the list will be returned when converting to or from JSON. See example 2 below.
    • Maps with enum keys (e.g., std::map<ENUM_TYPE, T>) are stored as arrays of [key, value] pairs by default. Define JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS to store them as objects with the converted keys. Such maps can be read from both forms.
    "},{"location":"api/macros/nlohmann_json_serialize_enum/#examples","title":"Examples","text":"Example: Basic usage

    The example shows how NLOHMANN_JSON_SERIALIZE_ENUM can be used to serialize/deserialize both classical enums and C++11 enum classes:

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nnamespace ns\n{\nenum TaskState\n{\n    TS_STOPPED,\n    TS_RUNNING,\n    TS_COMPLETED,\n    TS_INVALID = -1\n};\n\nNLOHMANN_JSON_SERIALIZE_ENUM(TaskState,\n{\n    { TS_INVALID, nullptr },\n    { TS_STOPPED, \"stopped\" },\n    { TS_RUNNING, \"running\" },\n    { TS_COMPLETED, \"completed\" }\n})\n\nenum class Color\n{\n    red, green, blue, unknown\n};\n\nNLOHMANN_JSON_SERIALIZE_ENUM(Color,\n{\n    { Color::unknown, \"unknown\" }, { Color::red, \"red\" },\n    { Color::green, \"green\" }, { Color::blue, \"blue\" }\n})\n} // namespace ns\n\nint main()\n{\n    // serialization\n    json j_stopped = ns::TS_STOPPED;\n    json j_red = ns::Color::red;\n    std::cout << \"ns::TS_STOPPED -> \" << j_stopped\n              << \", ns::Color::red -> \" << j_red << std::endl;\n\n    // deserialization\n    json j_running = \"running\";\n    json j_blue = \"blue\";\n    auto running = j_running.get<ns::TaskState>();\n    auto blue = j_blue.get<ns::Color>();\n    std::cout << j_running << \" -> \" << running\n              << \", \" << j_blue << \" -> \" << static_cast<int>(blue) << std::endl;\n\n    // deserializing undefined JSON value to enum\n    // (where the first map entry above is the default)\n    json j_pi = 3.14;\n    auto invalid = j_pi.get<ns::TaskState>();\n    auto unknown = j_pi.get<ns::Color>();\n    std::cout << j_pi << \" -> \" << invalid << \", \"\n              << j_pi << \" -> \" << static_cast<int>(unknown) << std::endl;\n}\n

    Output:

    ns::TS_STOPPED -> \"stopped\", ns::Color::red -> \"red\"\n\"running\" -> 1, \"blue\" -> 2\n3.14 -> -1, 3.14 -> 3\n
    Example: Multiple conversions for one enumerator

    The example shows how to use multiple conversions for a single enumerator. In the example, Color::red will always be serialized to \"red\", because the first occurring conversion. The second conversion, however, offers an alternative deserialization from \"rot\" to Color::red.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nnamespace ns\n{\nenum class Color\n{\n    red, green, blue, unknown\n};\n\nNLOHMANN_JSON_SERIALIZE_ENUM(Color,\n{\n    { Color::unknown, \"unknown\" }, { Color::red, \"red\" },\n    { Color::green, \"green\" }, { Color::blue, \"blue\" },\n    { Color::red, \"rot\" } // a second conversion for Color::red\n})\n}\n\nint main()\n{\n    // serialization\n    json j_red = ns::Color::red;\n    std::cout << static_cast<int>(ns::Color::red) << \" -> \" << j_red << std::endl;\n\n    // deserialization\n    json j_rot = \"rot\";\n    auto rot = j_rot.get<ns::Color>();\n    auto red = j_red.get<ns::Color>();\n    std::cout << j_rot << \" -> \" << static_cast<int>(rot) << std::endl;\n    std::cout << j_red << \" -> \" << static_cast<int>(red) << std::endl;\n}\n

    Output:

    0 -> \"red\"\n\"rot\" -> 0\n\"red\" -> 0\n
    "},{"location":"api/macros/nlohmann_json_serialize_enum/#see-also","title":"See also","text":"
    • Specializing enum conversion
    • NLOHMANN_JSON_SERIALIZE_ENUM_STRICT
    • JSON_DISABLE_ENUM_SERIALIZATION
    • JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS
    "},{"location":"api/macros/nlohmann_json_serialize_enum/#version-history","title":"Version history","text":"

    Added in version 3.4.0.

    "},{"location":"api/macros/nlohmann_json_serialize_enum_strict/","title":"NLOHMANN_JSON_SERIALIZE_ENUM_STRICT","text":"
    #define NLOHMANN_JSON_SERIALIZE_ENUM_STRICT(type, conversion...)\n

    By default, enum values are serialized to JSON as integers. In some cases, this could result in undesired behavior. If an enum is modified or re-ordered after data has been serialized to JSON, the later deserialized JSON data may be undefined or a different enum value than was originally intended.

    NLOHMANN_JSON_SERIALIZE_ENUM_STRICT allows to define a user-defined serialization for every enumerator that throws an exception on undefined input.

    "},{"location":"api/macros/nlohmann_json_serialize_enum_strict/#parameters","title":"Parameters","text":"type (in) name of the enum to serialize/deserialize conversion (in) a pair of an enumerator and a JSON serialization; arbitrary pairs can be given as a comma-separated list"},{"location":"api/macros/nlohmann_json_serialize_enum_strict/#default-definition","title":"Default definition","text":"

    The macro adds two functions to the namespace which take care of the serialization and deserialization:

    template<typename BasicJsonType>\ninline void to_json(BasicJsonType& j, const type& e);\ntemplate<typename BasicJsonType>\ninline void from_json(const BasicJsonType& j, type& e);\n
    "},{"location":"api/macros/nlohmann_json_serialize_enum_strict/#notes","title":"Notes","text":"

    Prerequisites

    The macro must be used inside the namespace of the enum.

    Important notes

    • Undefined input throws out_of_range.410 in both directions: when serializing an enum value not listed in the conversions, and when deserializing (e.g., via get<ENUM_TYPE>()) a JSON value that matches no conversion; example: \"enum value out of range for <type>\".
    • If an enum or JSON value is specified in multiple conversions, the first matching conversion from the top of the list will be returned when converting to or from JSON. See example 2 below.
    • Maps with enum keys (e.g., std::map<ENUM_TYPE, T>) are stored as arrays of [key, value] pairs by default. Define JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS to store them as objects with the converted keys. Such maps can be read from both forms.
    "},{"location":"api/macros/nlohmann_json_serialize_enum_strict/#examples","title":"Examples","text":"Example: Basic usage

    The example shows how NLOHMANN_JSON_SERIALIZE_ENUM_STRICT can be used to serialize/deserialize both classical enums and C++11 enum classes:

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nnamespace ns\n{\nenum TaskState\n{\n    TS_STOPPED,\n    TS_RUNNING,\n    TS_COMPLETED,\n    TS_INVALID = -1\n};\n\nNLOHMANN_JSON_SERIALIZE_ENUM_STRICT(TaskState,\n{\n    { TS_INVALID, nullptr },\n    { TS_STOPPED, \"stopped\" },\n    { TS_RUNNING, \"running\" },\n    { TS_COMPLETED, \"completed\" }\n})\n\nenum class Color\n{\n    red, green, blue, unknown\n};\n\nNLOHMANN_JSON_SERIALIZE_ENUM_STRICT(Color,\n{\n    { Color::unknown, \"unknown\" }, { Color::red, \"red\" },\n    { Color::green, \"green\" }, { Color::blue, \"blue\" }\n})\n} // namespace ns\n\nint main()\n{\n    // serialization\n    json j_stopped = ns::TS_STOPPED;\n    json j_red = ns::Color::red;\n    std::cout << \"ns::TS_STOPPED -> \" << j_stopped\n              << \", ns::Color::red -> \" << j_red << std::endl;\n\n    // deserialization\n    json j_running = \"running\";\n    json j_blue = \"blue\";\n    auto running = j_running.get<ns::TaskState>();\n    auto blue = j_blue.get<ns::Color>();\n    std::cout << j_running << \" -> \" << running\n              << \", \" << j_blue << \" -> \" << static_cast<int>(blue) << std::endl;\n\n}\n

    Output:

    ns::TS_STOPPED -> \"stopped\", ns::Color::red -> \"red\"\n\"running\" -> 1, \"blue\" -> 2\n
    Example: Multiple conversions for one enumerator

    The example shows how to use multiple conversions for a single enumerator. In the example, Color::red will always be serialized to \"red\", because the first occurring conversion. The second conversion, however, offers an alternative deserialization from \"rot\" to Color::red.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nnamespace ns\n{\nenum class Color\n{\n    red, green, blue, unknown\n};\n\nNLOHMANN_JSON_SERIALIZE_ENUM_STRICT(Color,\n{\n    { Color::unknown, \"unknown\" }, { Color::red, \"red\" },\n    { Color::green, \"green\" }, { Color::blue, \"blue\" },\n    { Color::red, \"rot\" } // a second conversion for Color::red\n})\n}\n\nint main()\n{\n    // serialization\n    json j_red = ns::Color::red;\n    std::cout << static_cast<int>(ns::Color::red) << \" -> \" << j_red << std::endl;\n\n    // deserialization\n    json j_rot = \"rot\";\n    auto rot = j_rot.get<ns::Color>();\n    auto red = j_red.get<ns::Color>();\n    std::cout << j_rot << \" -> \" << static_cast<int>(rot) << std::endl;\n    std::cout << j_red << \" -> \" << static_cast<int>(red) << std::endl;\n}\n

    Output:

    0 -> \"red\"\n\"rot\" -> 0\n\"red\" -> 0\n
    Example: exceptions on invalid serialization

    The example shows how an invalid serialization causes an exception to be thrown. In the example, Color::unknown is not defined in the mapping used to call NLOHMANN_JSON_SERIALIZE_ENUM_STRICT so causes an exception when used to serialize. Similarly, \"what\" does not refer to an enum value so also causes an exception when deserialization is attempted.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nnamespace ns\n{\n\nenum class Color\n{\n    red,\n    green,\n    blue,\n    unknown // not mapped in JSON_SERIALIZE_ENUM_STRICT\n};\n\nNLOHMANN_JSON_SERIALIZE_ENUM_STRICT(Color,\n{\n    {Color::red, \"red\"},\n    {Color::green, \"green\"},\n    {Color::blue, \"blue\"}\n})\n\n} // namespace ns\n\n\nint main()\n{\n    // invalid serialization\n    try\n    {\n        // ns::color::unknown was not mapped in macro\n        json invalid_serialization = ns::Color::unknown;\n    }\n    catch (const json::exception e)\n    {\n        std::cout << \"deserialization failed: \" << e.what() << std::endl;\n    }\n\n    // invalid deserialization\n    try\n    {\n        // what does not map to an enum\n        json invalid_deserialization(\"what\");\n        ns::Color color = invalid_deserialization.get<ns::Color>();\n    }\n    catch (const json::exception e)\n    {\n        std::cout << \"deserialization failed: \" << e.what() << std::endl;\n    }\n\n    return 0;\n}\n

    Output: 

    deserialization failed: [json.exception.out_of_range.410] enum value out of range for Color\ndeserialization failed: [json.exception.out_of_range.410] enum value out of range for Color: \"what\"\n

    "},{"location":"api/macros/nlohmann_json_serialize_enum_strict/#see-also","title":"See also","text":"
    • Specializing enum conversion
    • NLOHMANN_JSON_SERIALIZE_ENUM
    • JSON_DISABLE_ENUM_SERIALIZATION
    • JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS
    "},{"location":"api/macros/nlohmann_json_serialize_enum_strict/#version-history","title":"Version history","text":"

    Added in version 3.13.0 unreleased.

    "},{"location":"api/macros/nlohmann_json_version_major/","title":"NLOHMANN_JSON_VERSION_MAJOR, NLOHMANN_JSON_VERSION_MINOR, NLOHMANN_JSON_VERSION_PATCH","text":"
    #define NLOHMANN_JSON_VERSION_MAJOR /* value */\n#define NLOHMANN_JSON_VERSION_MINOR /* value */\n#define NLOHMANN_JSON_VERSION_PATCH /* value */\n

    These macros are defined by the library and contain the version numbers according to Semantic Versioning 2.0.0.

    "},{"location":"api/macros/nlohmann_json_version_major/#default-definition","title":"Default definition","text":"

    The macros are defined according to the current library version.

    "},{"location":"api/macros/nlohmann_json_version_major/#examples","title":"Examples","text":"Example

    The example below shows how NLOHMANN_JSON_VERSION_MAJOR, NLOHMANN_JSON_VERSION_MINOR, and NLOHMANN_JSON_VERSION_PATCH are defined by the library.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << \"JSON for Modern C++ version \"\n              << NLOHMANN_JSON_VERSION_MAJOR << \".\"\n              << NLOHMANN_JSON_VERSION_MINOR << \".\"\n              << NLOHMANN_JSON_VERSION_PATCH << std::endl;\n}\n

    Output:

    JSON for Modern C++ version 3.12.0\n
    "},{"location":"api/macros/nlohmann_json_version_major/#see-also","title":"See also","text":"
    • meta - returns version information on the library
    • JSON_SKIP_LIBRARY_VERSION_CHECK - skip library version check
    "},{"location":"api/macros/nlohmann_json_version_major/#version-history","title":"Version history","text":"
    • Added in version 3.1.0.
    "},{"location":"community/","title":"Community","text":"
    • Ecosystem - third-party projects built on top of this library
    • Code of Conduct - the rules and norms of this project
    • Contribution Guidelines - guidelines how to contribute to this project
    • Governance - the governance model of this project
    • Quality Assurance - how the quality of this project is assured
    • Roadmap - what the project will and will not do
    • Security Policy - the security policy of the project
    • Assurance Case - why the library meets its security requirements
    "},{"location":"community/assurance_case/","title":"Assurance case","text":"

    This page argues why the library meets its security requirements. It describes the threats the library faces, where the trust boundaries lie, and how the library's design and the quality assurance counter these threats. To report a vulnerability, see the security policy.

    "},{"location":"community/assurance_case/#threat-model","title":"Threat model","text":"

    The library parses, stores, and serializes JSON values in memory. It does not open network connections, does not open files (it only reads from streams or std::FILE* handles that the caller has already opened), does not read environment variables, and does not implement cryptography or handle credentials.

    The primary threat is therefore untrusted input: JSON text or binary data (BJData, BON8, BSON, CBOR, MessagePack, UBJSON) that an attacker controls, passed to parse, accept, sax_parse, or one of the from_* functions such as from_cbor. Such input may try to

    • make the library read or write out of bounds (malformed lengths, truncated input, invalid UTF-8),
    • trigger undefined behavior (integer overflow in sizes or numbers, invalid casts),
    • exhaust memory (huge announced sizes), or
    • exhaust the call stack (deeply nested arrays and objects).
    "},{"location":"community/assurance_case/#trust-boundaries","title":"Trust boundaries","text":"
    • Untrusted: all serialized input read by the parser, the SAX interface, and the binary readers. The library must handle every possible input by either producing a value or throwing a parse_error (or returning false when exceptions are disabled for the call).
    • Trusted: the C++ code that calls the library. Calling a function with violated preconditions, for instance accessing an array with operator[] out of range, is a programming error and not a security boundary. Such preconditions are checked with runtime assertions in debug builds; functions such as at offer checked access with exceptions.
    flowchart LR\n    A[Untrusted input] --> B[Parser]\n    A --> C[SAX interface]\n    A --> D[Binary readers]\n    B --> E[\"Value tree (basic_json)\"]\n    C --> E\n    D --> E\n    E --> F[Trusted caller]
    "},{"location":"community/assurance_case/#secure-design","title":"Secure design","text":"
    • Strict parsing. The parser accepts exactly the JSON grammar of RFC 8259. Extensions such as comments and trailing commas must be enabled explicitly. Invalid UTF-8 is rejected.
    • Errors are reported, not ignored. Malformed input results in a parse_error with the byte position of the error. Binary readers do not trust announced sizes: strings and binary values grow only as bytes are actually read, arrays reserve at most a fixed number of elements up front, and sizes that no container can hold are rejected.
    • Memory is owned by values. Each basic_json value owns its content, and there is no manual memory management in user code. The destructor does not recurse, so destroying a deeply nested value does not exhaust the stack.
    • Bounded recursion. The JSON parser and the binary readers keep their state in explicit stacks instead of recursing per nesting level. Operations that walk a value, such as dump, copying, comparison, hashing, merge_patch, diff, flatten, and the binary writers, recurse only up to a fixed depth and continue with an explicit stack below it. Applications that process untrusted input can limit its nesting depth with a parser callback.
    • Invariants are checked. The class invariant (for instance, that the pointer for the stored type is never null) is checked with runtime assertions throughout the test suite.
    "},{"location":"community/assurance_case/#common-weaknesses","title":"Common weaknesses","text":"

    The following table maps the relevant classes of the Common Weakness Enumeration to the measures that counter them. The measures are described in detail in Quality assurance.

    Weakness Countermeasures Out-of-bounds read/write (CWE-125, CWE-787) bounds checks on all reads from the input; AddressSanitizer and Valgrind on the test suite; OSS-Fuzz Integer overflow (CWE-190) UndefinedBehaviorSanitizer with integer overflow detection; Clang-Tidy; Cppcheck Use after free, double free (CWE-416, CWE-415) ownership of all memory by values; AddressSanitizer and Valgrind; Clang Static Analyzer Memory leaks (CWE-401) Valgrind (Memcheck) on the test suite Uncontrolled recursion (CWE-674) iterative parser, binary readers, and destructor; bounded recursion in value operations; tests with deeply nested inputs Uncontrolled resource consumption (CWE-400) allocations based on announced sizes are capped; OSS-Fuzz with memory limits Undefined behavior in general (CWE-758) UndefinedBehaviorSanitizer; runtime assertions; Clang-Tidy, Cppcheck, Clang Static Analyzer, Infer

    In addition, every line of the library is covered by the unit tests, and all parsers are fuzz-tested around the clock by OSS-Fuzz.

    "},{"location":"community/code_of_conduct/","title":"Contributor Covenant Code of Conduct","text":""},{"location":"community/code_of_conduct/#our-pledge","title":"Our Pledge","text":"

    We as members, contributors, and leaders pledge to make participation in our community a harassment-free experience for everyone, regardless of age, body size, visible or invisible disability, ethnicity, sex characteristics, gender identity and expression, level of experience, education, socio-economic status, nationality, personal appearance, race, caste, color, religion, or sexual identity and orientation.

    We pledge to act and interact in ways that contribute to an open, welcoming, diverse, inclusive, and healthy community.

    "},{"location":"community/code_of_conduct/#our-standards","title":"Our Standards","text":"

    Examples of behavior that contributes to a positive environment for our community include:

    • Demonstrating empathy and kindness toward other people
    • Being respectful of differing opinions, viewpoints, and experiences
    • Giving and gracefully accepting constructive feedback
    • Accepting responsibility and apologizing to those affected by our mistakes, and learning from the experience
    • Focusing on what is best not just for us as individuals, but for the overall community

    Examples of unacceptable behavior include:

    • The use of sexualized language or imagery, and sexual attention or advances of any kind
    • Trolling, insulting or derogatory comments, and personal or political attacks
    • Public or private harassment
    • Publishing others' private information, such as a physical or email address, without their explicit permission
    • Other conduct which could reasonably be considered inappropriate in a professional setting
    "},{"location":"community/code_of_conduct/#enforcement-responsibilities","title":"Enforcement Responsibilities","text":"

    Community leaders are responsible for clarifying and enforcing our standards of acceptable behavior and will take appropriate and fair corrective action in response to any behavior that they deem inappropriate, threatening, offensive, or harmful.

    Community leaders have the right and responsibility to remove, edit, or reject comments, commits, code, wiki edits, issues, and other contributions that are not aligned to this Code of Conduct, and will communicate reasons for moderation decisions when appropriate.

    "},{"location":"community/code_of_conduct/#scope","title":"Scope","text":"

    This Code of Conduct applies within all community spaces, and also applies when an individual is officially representing the community in public spaces. Examples of representing our community include using an official email address, posting via an official social media account, or acting as an appointed representative at an online or offline event.

    "},{"location":"community/code_of_conduct/#enforcement","title":"Enforcement","text":"

    Instances of abusive, harassing, or otherwise unacceptable behavior may be reported to the community leaders responsible for enforcement at mail@nlohmann.me. All complaints will be reviewed and investigated promptly and fairly.

    All community leaders are obligated to respect the privacy and security of the reporter of any incident.

    "},{"location":"community/code_of_conduct/#enforcement-guidelines","title":"Enforcement Guidelines","text":"

    Community leaders will follow these Community Impact Guidelines in determining the consequences for any action they deem in violation of this Code of Conduct:

    "},{"location":"community/code_of_conduct/#1-correction","title":"1. Correction","text":"

    Community Impact: Use of inappropriate language or other behavior deemed unprofessional or unwelcome in the community.

    Consequence: A private, written warning from community leaders, providing clarity around the nature of the violation and an explanation of why the behavior was inappropriate. A public apology may be requested.

    "},{"location":"community/code_of_conduct/#2-warning","title":"2. Warning","text":"

    Community Impact: A violation through a single incident or series of actions.

    Consequence: A warning with consequences for continued behavior. No interaction with the people involved, including unsolicited interaction with those enforcing the Code of Conduct, for a specified period of time. This includes avoiding interactions in community spaces as well as external channels like social media. Violating these terms may lead to a temporary or permanent ban.

    "},{"location":"community/code_of_conduct/#3-temporary-ban","title":"3. Temporary Ban","text":"

    Community Impact: A serious violation of community standards, including sustained inappropriate behavior.

    Consequence: A temporary ban from any sort of interaction or public communication with the community for a specified period of time. No public or private interaction with the people involved, including unsolicited interaction with those enforcing the Code of Conduct, is allowed during this period. Violating these terms may lead to a permanent ban.

    "},{"location":"community/code_of_conduct/#4-permanent-ban","title":"4. Permanent Ban","text":"

    Community Impact: Demonstrating a pattern of violation of community standards, including sustained inappropriate behavior, harassment of an individual, or aggression toward or disparagement of classes of individuals.

    Consequence: A permanent ban from any sort of public interaction within the community.

    "},{"location":"community/code_of_conduct/#attribution","title":"Attribution","text":"

    This Code of Conduct is adapted from the Contributor Covenant, version 2.1, available at https://www.contributor-covenant.org/version/2/1/code_of_conduct.html.

    Community Impact Guidelines were inspired by Mozilla's code of conduct enforcement ladder.

    For answers to common questions about this code of conduct, see the FAQ at https://www.contributor-covenant.org/faq. Translations are available at https://www.contributor-covenant.org/translations.

    "},{"location":"community/contribution_guidelines/","title":"Contribution Guidelines","text":"

    Thank you for your interest in contributing to this project! What began as an exercise to explore the exciting features of C++11 has evolved into a widely used JSON library. I truly appreciate all the contributions from the community, whether it's proposing features, identifying bugs, or fixing mistakes! To ensure that our collaboration is efficient and effective, please follow these guidelines.

    Feel free to discuss or suggest improvements to this document by submitting a pull request.

    "},{"location":"community/contribution_guidelines/#ways-to-contribute","title":"Ways to Contribute","text":"

    There are multiple ways to contribute.

    "},{"location":"community/contribution_guidelines/#reporting-an-issue","title":"Reporting an issue","text":"

    Please create an issue, assuming one does not already exist, and describe your concern. Note you need a GitHub account for this.

    Clearly describe the issue:

    • If it is a bug, please describe how to reproduce it. If possible, attach a complete example which demonstrates the error. Please also state what you expected to happen instead of the error.
    • If you propose a change or addition, try to give an example what the improved code could look like or how to use it.
    • If you found a compilation error, please tell us which compiler (version and operating system) you used and paste the (relevant part of) the error messages to the ticket.

    Please stick to the provided issue template bug report if possible.

    "},{"location":"community/contribution_guidelines/#reporting-a-security-vulnerability","title":"Reporting a security vulnerability","text":"

    You can report a security vulnerability according to our security policy.

    "},{"location":"community/contribution_guidelines/#discussing-a-new-feature","title":"Discussing a new feature","text":"

    For questions, feature or support requests, please open a discussion. If you find a proposed answer satisfactory, please use the \"Mark as answer\" button to make it easier for readers to see what helped and for the community to filter for open questions.

    "},{"location":"community/contribution_guidelines/#proposing-a-fix-or-an-improvement","title":"Proposing a fix or an improvement","text":"

    Join an ongoing discussion or comment on an existing issue before starting to code. This can help to avoid duplicate efforts or other frustration during the later review.

    Create a pull request against the develop branch and follow the pull request template. In particular,

    • describe the changes in detail, both the what and why,
    • reference existing issues where applicable,
    • add tests to maintain 100% test coverage,
    • update the documentation as needed, and
    • ensure the source code is amalgamated.

    We describe all points in detail below.

    All contributions (including pull requests) must agree to the Developer Certificate of Origin (DCO) version 1.1. This is exactly the same one created and used by the Linux kernel developers and posted on http://developercertificate.org/. This is a developer's certification that he or she has the right to submit the patch for inclusion into the project.

    "},{"location":"community/contribution_guidelines/#how-to","title":"How to...","text":""},{"location":"community/contribution_guidelines/#describe-your-changes","title":"Describe your changes","text":"

    This library is primarily maintained as a spare-time project. As such, I cannot make any guarantee how quickly changes are merged and released. Therefore, it is very important to make the review as smooth as possible by explaining not only what you changed, but why. This rationale can be very valuable down the road when improvements or bugs are discussed years later.

    "},{"location":"community/contribution_guidelines/#reference-an-existing-issue","title":"Reference an existing issue","text":"

    Link a pull request to an issue to clarify that a fix is forthcoming and which issue can be closed after merging. Only a few cases (e.g., fixing typos) do not require prior discussions.

    "},{"location":"community/contribution_guidelines/#write-tests","title":"Write tests","text":"

    The library has an extensive test suite that currently covers 100 % of the library's code. These tests are crucial to maintain API stability and give future contributors confidence that they do not accidentally break things. As Titus Winters aptly put it:

    If you liked it, you should have put a test on it.

    "},{"location":"community/contribution_guidelines/#run-the-tests","title":"Run the tests","text":"

    First, ensure the test suite runs before making any changes:

    $ cmake -S. -B build\n$ cmake --build build -j 10\n$ ctest --test-dir build -j 10\n

    The test suite should report:

    100% tests passed, 0 tests failed out of <number of tests>\n
    "},{"location":"community/contribution_guidelines/#add-tests","title":"Add tests","text":"

    The tests are located in tests/src/unit-*.cpp and contain doctest assertions like CHECK. The tests are structured along the features of the library or the nature of the tests. Usually, it should be clear from the context which existing file needs to be extended, and only very few cases require creating new test files.

    When fixing a bug, edit unit-regression3.cpp and add a section referencing the fixed issue. unit-regression2.cpp holds the older tests; the two files exist because a single one grew large enough for the MinGW linker to fail relocating it, so please keep adding to the smaller file rather than growing the larger one.

    "},{"location":"community/contribution_guidelines/#exceptions","title":"Exceptions","text":"

    When you test exceptions, please use CHECK_THROWS_WITH_AS which also takes the what() argument of the thrown exception into account.

    "},{"location":"community/contribution_guidelines/#coverage","title":"Coverage","text":"

    If test coverage decreases, an automatic warning comment will be posted on the pull request. You can access a code coverage report as an artifact to the \u201cUbuntu\u201d workflow.

    "},{"location":"community/contribution_guidelines/#update-the-documentation","title":"Update the documentation","text":"

    The main documentation of the library is generated from the files docs/mkdocs/docs. This folder contains dedicated pages for certain features, a list of all exceptions, and extensive API documentation with details on every public API function.

    Build the documentation locally using:

    make install_venv -C docs/mkdocs\nmake serve -C docs/mkdocs\n

    The documentation will then be available at http://127.0.0.1:8000/. See the documentation of mkdocs and Material for MkDocs for more information.

    Before opening a pull request, check the documentation like the CI does:

    make build -C docs/mkdocs          # strict build: fails on broken links, anchors, and structure problems\nmake check_mermaid -C docs/mkdocs  # checks the Mermaid diagrams (requires Node.js)\n

    The search index of the docset is generated from mkdocs.yml and each page's title (H1) and declaration by docs/docset/generate_docset.py; make build reports API pages that cannot be classified.

    "},{"location":"community/contribution_guidelines/#amalgamate-the-source-code","title":"Amalgamate the source code","text":"

    The single-header files single_include/nlohmann/json.hpp, single_include/nlohmann/json_fwd.hpp, and single_include/nlohmann/json_literals.hpp are generated from the source files in the include/nlohmann directory. Do not edit the files directly; instead, modify the include/nlohmann sources and regenerate the files by executing:

    make amalgamate\n

    Running make amalgamate will also apply automatic formatting to the source files using Artistic Style. This formatting may modify your source files in-place. Be certain to review and commit any changes to avoid unintended formatting diffs in commits.

    If you add, rename, or remove a header in include/nlohmann, also regenerate the header list in BUILD.bazel (requires CMake) by executing:

    make BUILD.bazel\n

    The amalgamation check in CI fails if any of these generated files is out of date.

    "},{"location":"community/contribution_guidelines/#recommended-documentation","title":"Recommended documentation","text":"
    • The library\u2019s README file is an excellent starting point to understand its functionality.
    • The documentation page is the reference documentation of the library.
    • RFC 8259 is the reference for the JavaScript Object Notation (JSON) Data Interchange Format.
    "},{"location":"community/contribution_guidelines/#please-dont","title":"Please don't...","text":"

    Certain contributions are not helpful.

    "},{"location":"community/contribution_guidelines/#break-the-public-api","title":"Break the public API","text":"

    We take pride in the library being used by numerous customers across various industries. They all rely on the guarantees provided by semantic versioning. Please do not change the library such that the public API of the 3.x.y version is broken. This includes:

    • Changing function signatures (altering parameter types, return types, number of parameters) or changing the const-ness of member functions.
    • Removing functions.
    • Renaming functions or classes.
    • Changing exception handling.
    • Changing exception ids.
    • Changing access specifiers.
    • Changing default arguments.

    What is and is not covered by this guarantee is described in the roadmap.

    Although these guidelines may seem restrictive, they are essential for maintaining the library\u2019s utility.

    Breaking changes may be introduced when they are guarded with a feature macro such as JSON_USE_IMPLICIT_CONVERSIONS which allows selectively changing the behavior of the library. In next steps, the current behavior can then be deprecated. Using feature macros then allows users to test their code against the library in the next major release.

    "},{"location":"community/contribution_guidelines/#break-c11-language-conformance","title":"Break C++11 language conformance","text":"

    This library is designed to work with C++11 and later. This means that any supported C++11 compiler should compile the library without problems. Some compilers like GCC 4.7 (and earlier), Clang 3.3 (and earlier), or Microsoft Visual C++ 2013 and earlier are known not to work due to missing or incomplete C++11 support.

    Please do not add features that do not work with the mentioned supported compilers. Please guard features from C++14 and later against the respective JSON_HAS_CPP_14 macros.

    "},{"location":"community/contribution_guidelines/#break-json-conformance","title":"Break JSON conformance","text":"

    Please refrain from proposing changes that would break JSON conformance. If you propose a conformant extension of JSON to be supported by the library, please motivate this extension.

    "},{"location":"community/contribution_guidelines/#wanted","title":"Wanted","text":"

    The following areas really need contribution and are always welcomed:

    • Extending the continuous integration toward more exotic compilers such as Android NDK, Intel's Compiler, or the bleeding-edge versions of Clang.
    • Improving the efficiency of the JSON parser. The current parser is a hand-written iterative parser that keeps an explicit stack instead of recursing, with hand-coded string handling. Speeding up the lexer or the value construction would be appreciated. Parser generators like Bison or ANTLR do not play nice with single-header files -- I really would like to keep the parser inside the json.hpp header.
    • Extending and updating existing benchmarks to include (the most recent version of) this library. Though efficiency is not everything, speed and memory consumption are very important characteristics for C++ developers, so having proper comparisons would be interesting.

    We look forward to your contributions and collaboration to enhance the library!

    "},{"location":"community/ecosystem/","title":"Ecosystem","text":"

    The projects below build on top of nlohmann::json rather than merely using it - schema validators, language bindings, format converters, and similar building blocks. The list is not exhaustive, and is curated rather than automatically generated. If you maintain or know of a project that belongs here, please let me know.

    For products, applications, and organizations that use the library, see Customers instead.

    "},{"location":"community/ecosystem/#schema-validation","title":"Schema validation","text":"
    • json-schema-validator, a JSON Schema (draft 7) validator with human-readable error messages
    "},{"location":"community/ecosystem/#serialization-and-reflection","title":"Serialization and reflection","text":"
    • nlohmann_json_reflect, a reflection extension for (de)serializing nested containers-in-structs-in-containers
    "},{"location":"community/ecosystem/#encodings","title":"Encodings","text":"
    • base-encode-decode, a header-only Base64/32/16/8/4/2 (and DNA/RNA) encoding library, with an adapter that serializes binary data through nlohmann::json
    "},{"location":"community/ecosystem/#language-bindings-and-interop","title":"Language bindings and interop","text":"
    • pybind11_json, a bidirectional type caster between nlohmann::json and Python objects for pybind11 bindings
    • nanobind_json, the same idea for nanobind bindings
    • nlohmann_json_qt, deserialization helpers for Qt types (QString, QUrl, QDateTime, QVector, ...) from nlohmann::json
    • vulkan2json, serialization and deserialization of Vulkan API structs
    "},{"location":"community/ecosystem/#format-converters","title":"Format converters","text":"
    • tojson, a header-only converter between YAML/XML documents and nlohmann::json
    • json2xml, a header-only converter from nlohmann::json to XML for simple configuration documents
    "},{"location":"community/governance/","title":"Governance","text":"

    The governance model for the JSON for Modern C++ project is a Benevolent Dictator for Life (BDFL) structure. As the sole maintainer, Niels Lohmann is responsible for all key aspects of the project. The project governance may evolve as the project grows, but any changes will be documented here and communicated to contributors.

    "},{"location":"community/governance/#overview","title":"Overview","text":"

    This project is led by a benevolent dictator, Niels Lohmann, and managed by the community. That is, the community actively contributes to the day-to-day maintenance of the project, but the general strategic line is drawn by the benevolent dictator. In case of disagreement, they have the last word. It is the benevolent dictator\u2019s job to resolve disputes within the community and to ensure that the project is able to progress in a coordinated way. In turn, it is the community\u2019s job to guide the decisions of the benevolent dictator through active engagement and contribution.

    "},{"location":"community/governance/#roles-and-responsibilities","title":"Roles and responsibilities","text":""},{"location":"community/governance/#benevolent-dictator-project-lead","title":"Benevolent dictator (project lead)","text":"

    Typically, the benevolent dictator, or project lead, is self-appointed. However, because the community always has the ability to fork, this person is fully answerable to the community. The project lead\u2019s role is a difficult one: they set the strategic objectives of the project and communicate these clearly to the community. They also have to understand the community as a whole and strive to satisfy as many conflicting needs as possible, while ensuring that the project survives in the long term.

    In many ways, the role of the benevolent dictator is less about dictatorship and more about diplomacy. The key is to ensure that, as the project expands, the right people are given influence over it and the community rallies behind the vision of the project lead. The lead\u2019s job is then to ensure that the committers (see below) make the right decisions on behalf of the project. Generally speaking, as long as the committers are aligned with the project\u2019s strategy, the project lead will allow them to proceed as they desire.

    "},{"location":"community/governance/#committers","title":"Committers","text":"

    Committers are contributors who have made several valuable contributions to the project and are now relied upon to both write code directly to the repository and screen the contributions of others. In many cases they are programmers but it is also possible that they contribute in a different role. Typically, a committer will focus on a specific aspect of the project, and will bring a level of expertise and understanding that earns them the respect of the community and the project lead. The role of committer is not an official one, it is simply a position that influential members of the community will find themselves in as the project lead looks to them for guidance and support.

    Committers have no authority over the overall direction of the project. However, they do have the ear of the project lead. It is a committer\u2019s job to ensure that the lead is aware of the community\u2019s needs and collective objectives, and to help develop or elicit appropriate contributions to the project. Often, committers are given informal control over their specific areas of responsibility, and are assigned rights to directly modify certain areas of the source code. That is, although committers do not have explicit decision-making authority, they will often find that their actions are synonymous with the decisions made by the lead.

    "},{"location":"community/governance/#contributors","title":"Contributors","text":"

    Contributors are community members who either have no desire to become committers, or have not yet been given the opportunity by the benevolent dictator. They make valuable contributions, such as those outlined in the list below, but generally do not have the authority to make direct changes to the project code. Contributors engage with the project through communication tools, such as email lists, and via reports and patches attached to issues in the issue tracker, as detailed in our community tools document.

    Anyone can become a contributor. There is no expectation of commitment to the project, no specific skill requirements and no selection process. To become a contributor, a community member simply has to perform one or more actions that are beneficial to the project.

    Some contributors will already be engaging with the project as users, but will also find themselves doing one or more of the following:

    • supporting new users (current users often provide the most effective new user support)
    • reporting bugs
    • identifying requirements
    • supplying graphics and web design
    • programming
    • assisting with project infrastructure
    • writing documentation
    • fixing bugs
    • adding features

    As contributors gain experience and familiarity with the project, they may find that the project lead starts relying on them more and more. When this begins to happen, they gradually adopt the role of committer, as described above.

    "},{"location":"community/governance/#users","title":"Users","text":"

    Users are community members who have a need for the project. They are the most important members of the community: without them, the project would have no purpose. Anyone can be a user; there are no specific requirements.

    Users should be encouraged to participate in the life of the project and the community as much as possible. User contributions enable the project team to ensure that they are satisfying the needs of those users. Common user activities include (but are not limited to):

    • evangelising about the project
    • informing developers of project strengths and weaknesses from a new user\u2019s perspective
    • providing moral support (a \u2018thank you\u2019 goes a long way)
    • providing financial support

    Users who continue to engage with the project and its community will often find themselves becoming more and more involved. Such users may then go on to become contributors, as described above.

    "},{"location":"community/governance/#access-to-project-resources","title":"Access to project resources","text":"

    The project's resources are the GitHub repository with its settings, CI workflows and secrets, and the documentation at json.nlohmann.me, which is built and deployed from the repository. Currently, the project lead is the only person with write or admin access to them.

    "},{"location":"community/governance/#granting-access","title":"Granting access","text":"

    Write or admin access is only granted by the project lead, and only to a contributor whose track record in the project the project lead has reviewed first. The role is assigned manually and is the lowest one that is needed for the task. Access is removed when it is no longer needed. GitHub requires two-factor authentication for everyone who can modify the repository.

    "},{"location":"community/governance/#secrets","title":"Secrets","text":"

    The CI workflows mostly use the token that GitHub creates for each workflow run. It is read-only by default, and each workflow requests only the additional permissions it needs. The few other credentials, such as the token for Semgrep, are stored as encrypted GitHub Actions secrets:

    • Only people with admin access can create, change, or delete them. Their values cannot be read back, not even by admins.
    • They are not passed to workflows that run for pull requests from forks.
    • They must never be committed to the repository or printed in logs.
    • They are rotated whenever someone with admin access leaves the project, and immediately if a leak is suspected.
    "},{"location":"community/governance/#support","title":"Support","text":"

    All participants in the community are encouraged to provide support for new users within the project management infrastructure. This support is provided as a way of growing the community. Those seeking support should recognise that all support activity within the project is voluntary and is therefore provided as and when time allows. A user requiring guaranteed response times or results should therefore seek to purchase a support contract from a vendor. (Of course, that vendor should be an active member of the community.) However, for those willing to engage with the project on its own terms, and willing to help support other users, the community support channels are ideal.

    "},{"location":"community/governance/#contribution-process","title":"Contribution Process","text":"

    Anyone can contribute to the project, regardless of their skills, as there are many ways to contribute. For instance, a contributor might be active on the project mailing list and issue tracker, or might supply patches. The various ways of contributing are described in more detail in our roles in open source document.

    The developer mailing list is the most appropriate place for a contributor to ask for help when making their first contribution.

    "},{"location":"community/governance/#decision-making-process","title":"Decision-Making Process","text":"

    The benevolent dictatorship model does not need a formal conflict resolution process, since the project lead\u2019s word is final. If the community chooses to question the wisdom of the actions of a committer, the project lead can review their decisions by checking the email archives, and either uphold or reverse them.

    Source

    The text was taken from http://oss-watch.ac.uk/resources/benevolentdictatorgovernancemodel.

    "},{"location":"community/quality_assurance/","title":"Quality assurance","text":"

    Ensuring quality is paramount for this project, particularly because numerous other projects depend on it. Each commit to the library undergoes rigorous checks against the following requirements, and any violations will result in a failed build.

    "},{"location":"community/quality_assurance/#c-language-compliance-and-compiler-compatibility","title":"C++ language compliance and compiler compatibility","text":"

    Requirement: Compiler support

    Any compiler with complete C++11 support can compile the library without warnings.

    Note: C++20 modules support may hit compiler-specific issues not covered by the general compiler matrix below. See Modules for known issues and workarounds.

    Note: Some modern features (like C++20 ranges or filesystem support) may be disabled on specific broken or incomplete toolchains even when standard feature-test macros indicate support. See JSON_HAS_RANGES, JSON_HAS_RANGE_VIEW_CONVERSION, and JSON_HAS_FILESYSTEM for details on known exclusions.

    • The library is compiled with 50+ different C++ compilers with different operating systems and platforms, including the oldest versions known to compile the library.

      Compilers used in continuous integration Compiler Architecture Operating System CI AppleClang 16.0.0.16000026; Xcode 16 arm64 macOS 15.2 (Sequoia) GitHub AppleClang 16.0.0.16000026; Xcode 16.1 arm64 macOS 15.2 (Sequoia) GitHub AppleClang 16.0.0.16000026; Xcode 16.2 arm64 macOS 15.2 (Sequoia) GitHub AppleClang 17.0.0.17000013; Xcode 16.3 arm64 macOS 15.5 (Sequoia) GitHub AppleClang 17.0.0.17000013; Xcode 16.4 arm64 macOS 15.5 (Sequoia) GitHub AppleClang 17.0.0.17000319; Xcode 26.0.1 arm64 macOS 15.5 (Sequoia) GitHub AppleClang 17.0.0.17000404; Xcode 26.1.1 arm64 macOS 15.7.9 (Sequoia) GitHub AppleClang 17.0.0.17000603; Xcode 26.2 arm64 macOS 15.7.9 (Sequoia) GitHub AppleClang 17.0.0.17000604; Xcode 26.3 arm64 macOS 15.7.9 (Sequoia) GitHub AppleClang 21.0.0.21000099; Xcode 26.4.1 arm64 macOS 26.6.2 (Tahoe) GitHub AppleClang 21.0.0.21000101; Xcode 26.5 arm64 macOS 26.6.2 (Tahoe) GitHub AppleClang 21.0.0.21000101; Xcode 26.6 arm64 macOS 26.6.2 (Tahoe) GitHub Clang 3.4.2 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 3.5.2 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 3.6.2 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 3.7.1 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 3.8.1 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 3.9.1 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 4.0.1 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 5.0.2 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 6.0.1 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 7.1.0 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 8.0.1 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 9.0.1 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 10.0.1 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 11.0.1 with GNU-like command-line x86_64 Windows Server 2022 (Build 20348) GitHub Clang 11.1.0 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 12.0.1 with GNU-like command-line x86_64 Windows Server 2022 (Build 20348) GitHub Clang 12.0.1 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 13.0.1 with GNU-like command-line x86_64 Windows Server 2022 (Build 20348) GitHub Clang 13.0.1 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 14.0.6 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 14.0.6 with GNU-like command-line x86_64 Windows Server 2022 (Build 20348) GitHub Clang 15.0.7 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 15.0.7 with GNU-like command-line x86_64 Windows Server 2022 (Build 20348) GitHub Clang 16.0.6 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 16.0.6 with GNU-like command-line x86_64 Windows Server 2022 (Build 20348) GitHub Clang 17.0.6 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 18.1.8 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 18.1.8 with GNU-like command-line x86_64 Windows Server 2022 (Build 20348) GitHub Clang 19.1.5 with MSVC-like command-line x86_64 Windows Server 2022 (Build 20348) GitHub Clang 19.1.7 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 19.1.7 with GNU-like command-line x86_64 Windows Server 2022 (Build 20348) GitHub Clang 20.1.1 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 20.1.8 with GNU-like command-line x86_64 Windows Server 2022 (Build 20348) GitHub Clang 21.1.8 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 22.1.8 x86_64 Ubuntu 22.04.1 LTS GitHub CUDA 11.8.0 (nvcc) x86_64 Ubuntu 22.04 LTS GitHub CUDA 12.1.1 (nvcc) x86_64 Ubuntu 22.04 LTS GitHub CUDA 12.6.3 (nvcc) x86_64 Ubuntu 22.04 LTS GitHub Emscripten 4.0.6 x86_64 Ubuntu 22.04.1 LTS GitHub GNU 4.8.5 x86_64 Ubuntu 20.04 LTS GitHub GNU 4.9.3 x86_64 Ubuntu 20.04 LTS GitHub GNU 5.5.0 x86_64 Ubuntu 20.04 LTS GitHub GNU 6.4.0 x86_64 Ubuntu 20.04 LTS GitHub GNU 7.5.0 x86_64 Ubuntu 22.04.1 LTS GitHub GNU 8.5.0 x86_64 Ubuntu 22.04.1 LTS GitHub GNU 9.3.0 x86_64 Ubuntu 22.04.1 LTS GitHub GNU 9.4.0 x86_64 Ubuntu 22.04.1 LTS GitHub GNU 9.5.0 x86_64 Ubuntu 22.04.1 LTS GitHub GNU 10.5.0 x86_64 Ubuntu 22.04.1 LTS GitHub GNU 11.4.0 x86_64 Ubuntu 22.04.1 LTS GitHub GNU 11.5.0 x86_64 Ubuntu 22.04.1 LTS GitHub GNU 12.2.0 (MinGW-W64 i686-ucrt-posix-dwarf) x86_64 Windows Server 2022 (Build 20348) GitHub GNU 12.2.0 (MinGW-W64 x86_64-ucrt-posix-seh) x86_64 Windows Server 2022 (Build 20348) GitHub GNU 12.4.0 x86_64 Ubuntu 22.04.1 LTS GitHub GNU 13.3.0 x86_64 Ubuntu 22.04.1 LTS GitHub GNU 14.2.0 x86_64 Ubuntu 22.04.1 LTS GitHub GNU 15.1.0 x86_64 Ubuntu 22.04.1 LTS GitHub GNU 16.2.0 x86_64 Ubuntu 22.04.1 LTS GitHub GNU 16.1.0 arm64 Ubuntu 24.04 GitHub icpc (ICC) 2021.10.0 20230609 x86_64 Ubuntu 22.04 LTS GitHub icpx (Intel oneAPI DPC++/C++) 2025.3.2 x86_64 Ubuntu 24.04 LTS GitHub nvc++ (NVIDIA HPC SDK) 25.5-0 x86_64 Ubuntu 22.04 LTS GitHub MSVC 19.0.24241.7 x86 Windows 8.1 AppVeyor MSVC 19.16.27035.0 x86 Windows-10 (Build 14393) AppVeyor MSVC 19.29.30157.0 x86 Windows-10 (Build 17763) AppVeyor MSVC 19.44.35207.0 arm64 Windows 11 (Build 26200) GitHub MSVC 19.44.35214.0 x86 Windows Server 2022 (Build 20348) GitHub MSVC 19.44.35214.0 x86_64 Windows Server 2022 (Build 20348) GitHub MSVC 19.51.36231.0 x86 Windows Server 2025 (Build 26100) GitHub MSVC 19.51.36231.0 x86_64 Windows Server 2025 (Build 26100) GitHub

    • The library is compiled with all C++ language revisions (C++11, C++14, C++17, C++20, C++23, and C++26) to detect and fix language deprecations early.

    • The library is checked for compiler warnings:

    • On Clang, -Weverything is used with 7 exceptions.

      Clang warnings 
      # Ignored Clang warnings:\n# -Wno-c++98-compat               The library targets C++11.\n# -Wno-c++98-compat-pedantic      The library targets C++11.\n# -Wno-deprecated-declarations    The library contains annotations for deprecated functions.\n# -Wno-padded                     We do not care about padding warnings.\n# -Wno-covered-switch-default     All switches list all cases and a default case.\n# -Wno-c2y-extensions             Clang 22.1 diagnoses __COUNTER__ as a C2y extension, also in\n#                                 C++ mode. The library does not use __COUNTER__; the warnings\n#                                 all come from vendored Doctest (SECTION/TEST_CASE macros).\n# -Wno-unsafe-buffer-usage        Pervasive: the library's own low-level numeric/buffer code\n#                                 (to_chars, serializer, lexer, binary reader/writer, input\n#                                 adapters, json_pointer) plus vendored Doctest itself (~208\n#                                 distinct sites measured 2026-07-08 on clang trunk) all use\n#                                 raw pointer arithmetic / libc string calls by necessity.\n\nset(CLANG_CXXFLAGS\n    -Werror\n    -Weverything\n    -Wno-c++98-compat\n    -Wno-c++98-compat-pedantic\n    -Wno-deprecated-declarations\n    -Wno-padded\n    -Wno-covered-switch-default\n    -Wno-c2y-extensions\n    -Wno-unsafe-buffer-usage\n)\n

    • On GCC, 300+ warnings are enabled with 8 exceptions.

      GCC warnings 
      # Warning flags determined for GCC 16.2.0 with https://github.com/nlohmann/gcc_flags:\n# Ignored GCC warnings:\n# -Wno-abi-tag           We do not care about ABI tags.\n# -Wno-aggregate-return  The library uses aggregate returns.\n# -Wno-long-long         The library uses the long long type to interface with system functions.\n# -Wno-namespaces        The library uses namespaces.\n# -Wno-nrvo              Doctest triggers this warning.\n# -Wno-padded            We do not care about padding warnings.\n# -Wno-system-headers    We do not care about warnings in system headers.\n# -Wno-templates         The library uses templates.\n\nset(GCC_CXXFLAGS\n    -pedantic\n    -Werror\n    --all-warnings\n    --extra-warnings\n    -W\n    -WNSObject-attribute\n    -Wabbreviated-auto-in-template-arg\n    -Wabi\n    -Wno-abi-tag\n    -Waddress\n    -Waddress-of-packed-member\n    -Wno-aggregate-return\n    -Waggressive-loop-optimizations\n    -Waligned-new=all\n    -Wall\n    -Walloc-size\n    -Walloc-zero\n    -Walloca\n    -Wanalyzer-allocation-size\n    -Wanalyzer-deref-before-check\n    -Wanalyzer-double-fclose\n    -Wanalyzer-double-free\n    -Wanalyzer-exposure-through-output-file\n    -Wanalyzer-exposure-through-uninit-copy\n    -Wanalyzer-fd-access-mode-mismatch\n    -Wanalyzer-fd-double-close\n    -Wanalyzer-fd-leak\n    -Wanalyzer-fd-phase-mismatch\n    -Wanalyzer-fd-type-mismatch\n    -Wanalyzer-fd-use-after-close\n    -Wanalyzer-fd-use-without-check\n    -Wanalyzer-file-leak\n    -Wanalyzer-free-of-non-heap\n    -Wanalyzer-imprecise-fp-arithmetic\n    -Wanalyzer-infinite-loop\n    -Wanalyzer-infinite-recursion\n    -Wanalyzer-jump-through-null\n    -Wanalyzer-malloc-leak\n    -Wanalyzer-mismatching-deallocation\n    -Wanalyzer-null-argument\n    -Wanalyzer-null-dereference\n    -Wanalyzer-out-of-bounds\n    -Wanalyzer-overlapping-buffers\n    -Wanalyzer-possible-null-argument\n    -Wanalyzer-possible-null-dereference\n    -Wanalyzer-putenv-of-auto-var\n    -Wanalyzer-shift-count-negative\n    -Wanalyzer-shift-count-overflow\n    -Wanalyzer-stale-setjmp-buffer\n    -Wanalyzer-symbol-too-complex\n    -Wanalyzer-tainted-allocation-size\n    -Wanalyzer-tainted-array-index\n    -Wanalyzer-tainted-assertion\n    -Wanalyzer-tainted-divisor\n    -Wanalyzer-tainted-offset\n    -Wanalyzer-tainted-size\n    -Wanalyzer-throw-of-unexpected-type\n    -Wanalyzer-too-complex\n    -Wanalyzer-undefined-behavior-ptrdiff\n    -Wanalyzer-undefined-behavior-strtok\n    -Wanalyzer-unsafe-call-within-signal-handler\n    -Wanalyzer-use-after-free\n    -Wanalyzer-use-of-pointer-in-stale-stack-frame\n    -Wanalyzer-use-of-uninitialized-value\n    -Wanalyzer-va-arg-type-mismatch\n    -Wanalyzer-va-list-exhausted\n    -Wanalyzer-va-list-leak\n    -Wanalyzer-va-list-use-after-va-end\n    -Wanalyzer-write-to-const\n    -Wanalyzer-write-to-string-literal\n    -Warith-conversion\n    -Warray-bounds=2\n    -Warray-compare\n    -Warray-parameter\n    -Warray-parameter=2\n    -Wattribute-alias=2\n    -Wattribute-warning\n    -Wattributes\n    -Wauto-profile\n    -Wbidi-chars=any\n    -Wbool-compare\n    -Wbool-operation\n    -Wbuiltin-declaration-mismatch\n    -Wbuiltin-macro-redefined\n    -Wc++0x-compat\n    -Wc++11-compat\n    -Wc++11-extensions\n    -Wc++14-compat\n    -Wc++14-extensions\n    -Wc++17-compat\n    -Wc++17-extensions\n    -Wc++1z-compat\n    -Wc++20-compat\n    -Wc++20-extensions\n    -Wc++23-extensions\n    -Wc++26-compat\n    -Wc++26-extensions\n    -Wc++2a-compat\n    -Wcalloc-transposed-args\n    -Wcannot-profile\n    -Wcast-align\n    -Wcast-align=strict\n    -Wcast-function-type\n    -Wcast-qual\n    -Wcast-user-defined\n    -Wcatch-value=3\n    -Wchanges-meaning\n    -Wchar-subscripts\n    -Wclass-conversion\n    -Wclass-memaccess\n    -Wclobbered\n    -Wcomma-subscript\n    -Wcomment\n    -Wcomments\n    -Wcomplain-wrong-lang\n    -Wconditionally-supported\n    -Wconversion\n    -Wconversion-null\n    -Wcoverage-invalid-line-number\n    -Wcoverage-mismatch\n    -Wcoverage-too-many-conditions\n    -Wcoverage-too-many-paths\n    -Wcpp\n    -Wctad-maybe-unsupported\n    -Wctor-dtor-privacy\n    -Wdangling-else\n    -Wdangling-pointer=2\n    -Wdangling-reference\n    -Wdate-time\n    -Wdefaulted-function-deleted\n    -Wdelete-incomplete\n    -Wdelete-non-virtual-dtor\n    -Wdeprecated\n    -Wdeprecated-copy\n    -Wdeprecated-copy-dtor\n    -Wdeprecated-declarations\n    -Wdeprecated-enum-enum-conversion\n    -Wdeprecated-enum-float-conversion\n    -Wdeprecated-literal-operator\n    -Wdeprecated-openmp\n    -Wdeprecated-variadic-comma-omission\n    -Wdisabled-optimization\n    -Wdiv-by-zero\n    -Wdouble-promotion\n    -Wduplicated-branches\n    -Wduplicated-cond\n    -Weffc++\n    -Welaborated-enum-base\n    -Wempty-body\n    -Wendif-labels\n    -Wenum-compare\n    -Wenum-conversion\n    -Wexceptions\n    -Wexpansion-to-defined\n    -Wexpose-global-module-tu-local\n    -Wexternal-tu-local\n    -Wextra\n    -Wextra-semi\n    -Wflex-array-member-not-at-end\n    -Wfloat-conversion\n    -Wfloat-equal\n    -Wformat-diag\n    -Wformat-overflow=2\n    -Wformat-signedness\n    -Wformat-truncation=2\n    -Wformat=2\n    -Wframe-address\n    -Wfree-nonheap-object\n    -Wglobal-module\n    -Whardened\n    -Wheader-guard\n    -Whsa\n    -Wif-not-aligned\n    -Wignored-attributes\n    -Wignored-qualifiers\n    -Wimplicit-fallthrough=5\n    -Winaccessible-base\n    -Winfinite-recursion\n    -Winherited-variadic-ctor\n    -Winit-list-lifetime\n    -Winit-self\n    -Winline\n    -Wint-in-bool-context\n    -Wint-to-pointer-cast\n    -Winterference-size\n    -Winvalid-constexpr\n    -Winvalid-imported-macros\n    -Winvalid-memory-model\n    -Winvalid-offsetof\n    -Winvalid-pch\n    -Winvalid-utf8\n    -Wkeyword-macro\n    -Wleading-whitespace=spaces\n    -Wliteral-suffix\n    -Wlogical-not-parentheses\n    -Wlogical-op\n    -Wno-long-long\n    -Wlto-type-mismatch\n    -Wmain\n    -Wmaybe-musttail-local-addr\n    -Wmaybe-uninitialized\n    -Wmemset-elt-size\n    -Wmemset-transposed-args\n    -Wmisleading-indentation\n    -Wmismatched-dealloc\n    -Wmismatched-new-delete\n    -Wmismatched-tags\n    -Wmissing-attributes\n    -Wmissing-braces\n    -Wmissing-declarations\n    -Wmissing-field-initializers\n    -Wmissing-include-dirs\n    -Wmissing-profile\n    -Wmissing-requires\n    -Wmissing-template-keyword\n    -Wmultichar\n    -Wmultiple-inheritance\n    -Wmultistatement-macros\n    -Wmusttail-local-addr\n    -Wno-namespaces\n    -Wnarrowing\n    -Wnoexcept\n    -Wnoexcept-type\n    -Wnon-c-typedef-for-linkage\n    -Wnon-template-friend\n    -Wnon-virtual-dtor\n    -Wnonnull\n    -Wnonnull-compare\n    -Wnormalized=nfkc\n    -Wno-nrvo\n    -Wnull-dereference\n    -Wodr\n    -Wold-style-cast\n    -Wopenacc-parallelism\n    -Wopenmp\n    -Wopenmp-simd\n    -Woverflow\n    -Woverlength-strings\n    -Woverloaded-virtual=2\n    -Wpacked\n    -Wpacked-bitfield-compat\n    -Wpacked-not-aligned\n    -Wno-padded\n    -Wparentheses\n    -Wpedantic\n    -Wpessimizing-move\n    -Wplacement-new=2\n    -Wpmf-conversions\n    -Wpointer-arith\n    -Wpointer-compare\n    -Wpragma-once-outside-header\n    -Wpragmas\n    -Wprio-ctor-dtor\n    -Wpsabi\n    -Wrange-loop-construct\n    -Wredundant-decls\n    -Wredundant-move\n    -Wredundant-tags\n    -Wregister\n    -Wreorder\n    -Wrestrict\n    -Wreturn-local-addr\n    -Wreturn-type\n    -Wscalar-storage-order\n    -Wself-move\n    -Wsequence-point\n    -Wsfinae-incomplete\n    -Wsfinae-incomplete=2\n    -Wshadow=compatible-local\n    -Wshadow=global\n    -Wshadow=local\n    -Wshift-count-negative\n    -Wshift-count-overflow\n    -Wshift-negative-value\n    -Wshift-overflow=2\n    -Wsign-compare\n    -Wsign-conversion\n    -Wsign-promo\n    -Wsized-deallocation\n    -Wsizeof-array-argument\n    -Wsizeof-array-div\n    -Wsizeof-pointer-div\n    -Wsizeof-pointer-memaccess\n    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    "},{"location":"community/quality_assurance/#c-standard-library-compliance","title":"C++ standard library compliance","text":"

    Requirement: No prerequisites

    The library has no prerequisites other than the Standard Template Library (STL).

    • The library is compiled and tested with both libc++ and libstdc++ to detect subtle differences or incompatibilities.
    • The code checked with Include What You Use (IWYU) that all required standard headers are included.
    • On Windows, the library is compiled with <Windows.h> being included to detect and avoid common bugs (see unit-windows_h.cpp).
    • The library is compiled with Qt's keyword macros (slots, signals, emit, foreach, forever) being defined to make sure it does not use them as identifiers (see unit-qt_keywords.cpp).
    • The library is compiled with exceptions disabled to support alternative means of error handling.
    "},{"location":"community/quality_assurance/#stable-public-api","title":"Stable public API","text":"

    Requirement: Stable public API

    Any change to the library does not break the public API.

    • All public API functions are tested with a variety of arguments.
    • The library is compiled and tested with different template arguments for number, string, array, and object types.
    • Unit tests cover all lines of the code base.
    • Every exception of the library is thrown in the test suite, and the error messages and exception ids are checked.

    Requirement: Complete documentation

    The public API is extensively documented.

    • Every public API function has a dedicated page in the API reference documentation with a self-contained code example.
    • All examples in the documentation are tested, and changes in their output are treated as an error.
    "},{"location":"community/quality_assurance/#robust-input-processing","title":"Robust input processing","text":"

    Requirement: Standards compliance

    The library is compliant to JSON as defined in RFC 8259.

    • The lexer is tested with all valid Unicode code points and all prefixes of all invalid Unicode code points.
    • The parser is tested against extensive correctness suites for JSON compliance.
    • In addition, the library is continuously fuzz-tested at OSS-Fuzz where the library is checked against billions of inputs.
    • Every crash reported by OSS-Fuzz is fixed together with a unit test that reproduces it, and the fix references the OSS-Fuzz issue. The round-trip checks of the fuzzer drivers are also part of the unit tests. See the fuzz testing documentation.
    "},{"location":"community/quality_assurance/#static-analysis","title":"Static analysis","text":"

    Requirement: State-of-the-art code analysis

    The code is checked with state-of-the-art static code analysis tools.

    • The code is checked with the latest Clang-Tidy.

      Clang-Tidy configuration (.clang-tidy) 
      # portability-avoid-pragma-once: kept disabled on purpose. #pragma once is accepted\n# by every supported compiler, and tools/amalgamate/amalgamate.py strips it from\n# single_include, so there is nothing left to fix here.\n\nChecks: '*,\n\n         -altera-id-dependent-backward-branch,\n         -altera-struct-pack-align,\n         -altera-unroll-loops,\n         -android-cloexec-fopen,\n         -boost-use-ranges,\n         -bugprone-easily-swappable-parameters,\n         -cert-err58-cpp,\n         -clang-analyzer-webkit.NoUncountedMemberChecker,\n         -concurrency-mt-unsafe,\n         -cppcoreguidelines-avoid-const-or-ref-data-members,\n         -cppcoreguidelines-avoid-do-while,\n         -cppcoreguidelines-avoid-goto,\n         -cppcoreguidelines-avoid-magic-numbers,\n         -cppcoreguidelines-avoid-non-const-global-variables,\n         -cppcoreguidelines-macro-usage,\n         -cppcoreguidelines-pro-bounds-avoid-unchecked-container-access,\n         -cppcoreguidelines-pro-bounds-array-to-pointer-decay,\n         -cppcoreguidelines-pro-bounds-constant-array-index,\n         -cppcoreguidelines-pro-bounds-pointer-arithmetic,\n         -cppcoreguidelines-pro-type-reinterpret-cast,\n         -cppcoreguidelines-pro-type-union-access,\n         -cppcoreguidelines-rvalue-reference-param-not-moved,\n         -cppcoreguidelines-virtual-class-destructor,\n         -fuchsia-default-arguments-calls,\n         -fuchsia-default-arguments-declarations,\n         -fuchsia-overloaded-operator,\n         -google-explicit-constructor,\n         -google-readability-function-size,\n         -google-runtime-float,\n         -google-runtime-int,\n         -hicpp-avoid-goto,\n         -hicpp-explicit-conversions,\n         -hicpp-function-size,\n         -hicpp-no-array-decay,\n         -hicpp-no-assembler,\n         -hicpp-signed-bitwise,\n         -hicpp-uppercase-literal-suffix,\n         -llvm-header-guard,\n         -llvm-include-order,\n         -llvm-prefer-static-over-anonymous-namespace,\n         -llvm-use-ranges,\n         -llvmlibc-*,\n         -misc-use-anonymous-namespace,\n         -misc-confusable-identifiers,\n         -misc-include-cleaner,\n         -misc-no-recursion,\n         -misc-non-private-member-variables-in-classes,\n         -modernize-concat-nested-namespaces,\n         -modernize-type-traits,\n         -modernize-use-constraints,\n         -modernize-use-designated-initializers,\n         -modernize-use-nodiscard,\n         -modernize-use-ranges,\n         -modernize-use-std-numbers,\n         -modernize-use-trailing-return-type,\n         -performance-enum-size,\n         -portability-avoid-pragma-once,\n         -readability-function-cognitive-complexity,\n         -readability-function-size,\n         -readability-identifier-length,\n         -readability-magic-numbers,\n         -readability-redundant-access-specifiers,\n         -readability-redundant-parentheses,\n         -readability-redundant-typename,\n         -readability-simplify-boolean-expr,\n         -readability-uppercase-literal-suffix,\n         -readability-use-concise-preprocessor-directives'\n\nCheckOptions:\n  - key: hicpp-special-member-functions.AllowSoleDefaultDtor\n    value: 1\n  # clang-tidy 22.1 extended this check to classes and enums; the test files\n  # define many such helper types at namespace scope, which is harmless\n  - key: misc-use-internal-linkage.AnalyzeTypes\n    value: false\n\nWarningsAsErrors: '*'\n\nHeaderFilterRegex: '.*hpp$'\n

    • The code is checked with the latest Cppcheck with all warnings enabled.

    • The code is checked with the latest Clang Static Analyzer with 89 enabled rules.
    • The code is checked with Infer.
    • The code is checked with Codacy.
    "},{"location":"community/quality_assurance/#dynamic-analysis","title":"Dynamic analysis","text":"

    Requirement: Correctness

    The library is checked for memory correctness and absence of undefined behavior.

    • The test suite is executed with enabled runtime assertions to check invariants and preconditions of functions to detect undefined behavior.
    • The test suite is executed with Valgrind (Memcheck) to detect memory leaks.
    • The test suite is executed with Sanitizers (address sanitizer, undefined behavior sanitizer, integer overflow detection, nullability violations).
    "},{"location":"community/quality_assurance/#dependencies","title":"Dependencies","text":"

    Requirement: No vulnerable dependencies

    The library has no dependencies besides the C++ standard library. The tools used to build, test, and document it are kept free of known vulnerabilities.

    • GitHub Actions are pinned to a commit hash, and the Python packages used by the documentation and the tools are pinned to exact versions.
    • Dependabot checks these dependencies daily and proposes updates as pull requests.
    • Every pull request is checked with the dependency review action. A pull request that adds a dependency with a known vulnerability of any severity fails this check and is not merged.
    • Vulnerability alerts for dependencies are fixed or dismissed with a documented reason before the next release. No release is made while such an alert is open.
    • Third-party code included in the repository for testing, such as doctest, is updated manually.
    "},{"location":"community/quality_assurance/#style-check","title":"Style check","text":"

    Requirement: Common code style

    A common code style is used throughout all code files of the library.

    • The code is formatted with Artistic Style (astyle) against a style configuration that is also enforced in the CI.

      Astyle configuration (tools/astyle/.astylerc) 
      # Configuration for Artistic Style\n# see https://astyle.sourceforge.net/astyle.html\n\n#######################\n# Brace Style Options #\n#######################\n\n# use Allman style for braces\n--style=allman\n\n###############\n# Tab Options #\n###############\n\n# indent using 4 spaces\n--indent=spaces=4\n\n#######################\n# Indentation Options #\n#######################\n\n# indent access modifiers one half indent\n--indent-modifiers\n\n# indent switch cases to the switch block\n--indent-switches\n\n# indent preprocessor blocks\n--indent-preproc-block\n\n# indent preprocessor defines\n--indent-preproc-define\n\n# indent C++ comments\n--indent-col1-comments\n\n###################\n# Padding Options #\n###################\n\n# insert space padding around operators\n--pad-oper\n\n# insert space between if/for/while... and the following parentheses\n--pad-header\n\n# attach the pointer to the variable type (left)\n--align-pointer=type\n\n# attach the reference to the variable type (left)\n--align-reference=type\n\n######################\n# Formatting Options #\n######################\n\n# add braces to unbraced one line conditional statements\n--add-braces\n\n# convert tabs to spaces\n--convert-tabs\n\n# closes whitespace between the ending angle brackets of template definitions\n--close-templates\n\n#################\n# Other Options #\n#################\n\n# do not create backup files\n--suffix=none\n\n# preserve the original file date\n--preserve-date\n\n# display only the files that have been formatted\n--formatted\n\n# for the linux (LF) line end style\n--lineend=linux\n

    • The code style is checked with cpplint with 61 enabled rules.

    "},{"location":"community/quality_assurance/#simple-integration","title":"Simple integration","text":"

    Requirement: Single header

    The library can be used by adding a single header to a C++ project.

    • An amalgamation script is used to check if the source code is exposed as a self-contained single-header file.
    • The test suite is checked against the amalgamated source file as well as the individual source file.

    Requirement: CMake as primary development tool

    All library functions are exposed and usable by CMake.

    • All library options are exposed as CMake options and tested.
    • The library is tested against relevant CMake versions:
    • CMake 3.5 (the earliest supported)
    • CMake 3.31.6 (the latest 3.x release)
    • CMake 4.0.0 (a very recent release)
    "},{"location":"community/roadmap/","title":"Roadmap","text":"

    This page describes what the project intends to do, and what it does not intend to do, over the next year. Concrete work items are tracked in the GitHub milestones and the issue tracker.

    "},{"location":"community/roadmap/#what-the-project-will-do","title":"What the project will do","text":"
    • Keep the C++11 baseline. The library will continue to compile with every supported C++11 compiler. Features of later standards are only used when they are guarded by the JSON_HAS_CPP_* macros.
    • Stay conformant to JSON. The parser and serializer follow RFC 8259. Extensions such as comments or trailing commas remain opt-in.
    • Keep the 3.x public API stable. Releases follow semantic versioning. Changes that would break existing code are only added behind a feature macro, so users can opt in and test their code before a next major release, see Version 4.0.
    • Support a broad range of compilers and platforms. The CI keeps testing old and new versions of GCC, Clang, MSVC, and other compilers on Linux, macOS, and Windows.
    • Keep the quality assurance up. Every change keeps the test coverage at 100%, passes the static and dynamic analysis, and is fuzz-tested by OSS-Fuzz, see Quality assurance.
    • Harden the library against hostile input. Handling deeply nested values without exhausting the call stack is ongoing work.
    • Fix bugs and security issues reported through the issue tracker and the security policy.
    "},{"location":"community/roadmap/#what-the-project-will-not-do","title":"What the project will not do","text":"
    • Break the public API of version 3.x. See API stability for what this covers.
    • Require a newer C++ standard than C++11.
    • Break JSON conformance or enable non-standard extensions by default.
    • Add dependencies or require a build step. The library remains header-only, and the single header json.hpp remains a complete distribution.
    • Trade simplicity for speed or memory efficiency. Performance improvements are welcome, but the library is not meant to compete with the fastest JSON libraries, see Design goals.
    "},{"location":"community/roadmap/#api-stability","title":"API stability","text":"

    Releases follow semantic versioning: a minor or patch release of version 3.x does not break code that uses the public API, unless that code opts in to a change with a macro as described below. In particular, a 3.x release does not:

    • make breaking changes to the signature of a function: the types or order of its existing parameters, its return type, its noexcept or constexpr specifier, or the const-ness of a member function. New parameters may be added if they have a default value;
    • remove or rename a function or class, or change the template parameters of a public class template;
    • change which exceptions a function throws, or the exception ids;
    • change access specifiers, or change or remove existing default arguments. New default arguments may be added;
    • change the JSON type that a valid input parses to, or the text that dump() produces for a valid value;
    • accept input that was rejected before, or reject input that was accepted before;
    • change the order in which the keys of an object are iterated. The default type sorts keys, and ordered_json keeps insertion order;
    • change when iterators, pointers, or references are invalidated, or the state of a moved-from basic_json;
    • add or remove implicit conversions from basic_json;
    • change how to_json and from_json functions are found, or the behavior of adl_serializer;
    • add pure virtual functions to the json_sax interface;
    • remove, rename, renumber, or add enumerators of value_t;
    • remove or rename a documented macro, CMake option, CMake target, or header, or change what a documented macro does.

    Exceptions to these rules, for instance when fixing a bug requires changing the exception a function throws, are documented in the release notes.

    The following are not part of the public API and may change in any release, including patch releases:

    • The text of exception messages returned by what(). Use the exception id to tell errors apart.
    • The ABI, including sizeof(basic_json) and the memory layout of its values. The versioned inline namespace turns mixing versions into a link error.
    • The hash values returned by std::hash for basic_json. Numbers that compare equal still hash equally.
    • Everything in namespace nlohmann::detail, and macros and type traits that are not documented in the API reference.

    Breaking changes are only added behind a macro whose default keeps the 3.x behavior. See Version 4.0 and the macro overview.

    "},{"location":"community/roadmap/#version-40","title":"Version 4.0","text":"

    There is no release date for version 4.0 yet. Proposals that need a major version, for instance stricter type conversions, are collected in issue #3453.

    Not final

    The plan for version 4.0 described below is not final and may still change: macros may be added to or removed from the list, and planned defaults may be revised. Any such change will be documented on this page.

    "},{"location":"community/roadmap/#trying-out-40-today","title":"Trying out 4.0 today","text":"

    Version 4.0 will not be developed on a separate branch. Instead, every breaking change is first added to a 3.x release behind a macro whose default keeps the 3.x behavior. Version 4.0 then switches the defaults and removes the macros. Version 4.0 is therefore the sum of these macros: you can try it on the 3.x release train today by defining each macro to its 4.0 value and fixing what no longer compiles or behaves differently. Once your code works with all of them, it is ready for version 4.0.

    The following macros guard changes that are planned to become the default in version 4.0:

    Macro 3.x default 4.0 behavior CMake option Added JSON_USE_IMPLICIT_CONVERSIONS 1 0: no implicit conversions from basic_json to other types; use get instead JSON_ImplicitConversions 3.9.0 JSON_USE_GLOBAL_UDLS 1 0: the string literals _json and _json_pointer are only available in namespace nlohmann::literals JSON_GlobalUDLs 3.11.0 JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON 0 removed: the deprecated legacy comparison of discarded values can no longer be enabled JSON_LegacyDiscardedValueComparison 3.11.0 JSON_BRACE_INIT_COPY_SEMANTICS 0 1: single-element brace initialization such as json j{obj}; copies the element instead of creating an array \u2013 3.13.0 JSON_PRECISE_STREAM_POSITION 0 1: reading from a stream does not consume the character after a number \u2013 3.13.0 JSON_STRICT_NUL_HANDLING 0 1: a NUL byte in the input is a parse error instead of the end of input JSON_StrictNulHandling 3.13.0 JSON_STRICT_BINARY_UTF8 0 1: to_cbor, to_ubjson, to_bjdata, and to_bson throw for strings that are not valid UTF-8 by default JSON_StrictBinaryUTF8 3.13.0 JSON_DISABLE_TUPLE_REFERENCE_CONVERSION 0 1: a basic_json value can no longer be created from a one-element tuple of a reference to it, such as std::forward_as_tuple(j) JSON_DisableTupleReferenceConversion 3.13.0 JSON_DELETE_DEPRECATED_FUNCTIONS 0 removed: the deprecated functions are removed (see below); the from_*(ptr, len) overloads stay deleted JSON_DeleteDeprecatedFunctions 3.13.0

    For example, the following makes a 3.x release behave like version 4.0 with respect to these changes:

    #define JSON_USE_IMPLICIT_CONVERSIONS 0\n#define JSON_USE_GLOBAL_UDLS 0\n#define JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON 0\n#define JSON_BRACE_INIT_COPY_SEMANTICS 1\n#define JSON_PRECISE_STREAM_POSITION 1\n#define JSON_STRICT_NUL_HANDLING 1\n#define JSON_STRICT_BINARY_UTF8 1\n#define JSON_DISABLE_TUPLE_REFERENCE_CONVERSION 1\n#define JSON_DELETE_DEPRECATED_FUNCTIONS 1\n#include <nlohmann/json.hpp>\n

    The macros must be defined before the library header is included; setting them once in the build system is the easiest way to achieve this.

    "},{"location":"community/roadmap/#removal-of-deprecated-functions","title":"Removal of deprecated functions","text":"

    Version 4.0 will remove all deprecated functions. Compiling with deprecation warnings enabled shows which of them your code still uses. Defining JSON_DELETE_DEPRECATED_FUNCTIONS to 1 turns these warnings into errors, as the deprecated functions are then deleted. The migration guide shows how to replace each of them.

    The from_* overloads taking a pointer and a length are not removed in version 4.0, but stay deleted. Without them, a call like from_cbor(ptr, len) would still compile: it would read ptr as a NUL-terminated string and convert len to the strict parameter.

    Deprecated Since Migration operator<<(basic_json&, std::istream&) 3.0.0 Parsing operator>>(const basic_json&, std::ostream&) 3.0.0 Miscellaneous functions iterator_wrapper 3.1.0 Miscellaneous functions parse, accept, and sax_parse with an initializer list {ptr, len} or {first, last} 3.8.0 Parsing from_bson, from_cbor, from_msgpack, and from_ubjson with (ptr, len) or an initializer list 3.8.0 Parsing json_pointer::operator string_t 3.11.0 JSON Pointers json_pointer with a basic_json type as template argument, and the overloads of value, contains, operator[], and at accepting such a pointer 3.11.0 JSON Pointers Comparing a json_pointer with a string via operator== or operator!= 3.11.2 JSON Pointers from_bjdata and from_bon8 with (ptr, len) 3.13.0 Parsing

    The deprecated legacy comparison of discarded values is controlled by a macro and therefore listed in the table above.

    New breaking changes will follow the same path: they are added to these tables when they land in a 3.x release.

    "},{"location":"community/security_policy/","title":"Security Policy","text":""},{"location":"community/security_policy/#reporting-a-vulnerability","title":"Reporting a Vulnerability","text":"

    We value the security of our users and appreciate your efforts to responsibly disclose vulnerabilities. If you have identified a security vulnerability in this repository, please use the GitHub Security Advisory \"Report a Vulnerability\" tab.

    Until it is published, this draft security advisory will only be visible to the maintainers of this project. Other users and teams may be added once the advisory is created.

    We will send a first response within 14 days, indicating the next steps in handling your report. After the initial reply to your report, we will keep you informed of the progress towards a fix and full announcement and may ask for additional information or guidance.

    For vulnerabilities in third-party dependencies or modules, please report them directly to the respective maintainers.

    "},{"location":"community/security_policy/#disclosure-and-credit","title":"Disclosure and credit","text":"

    Once a fix is released, we publish the security advisory and list the fixed vulnerability in the release notes. We credit the reporter in both, unless they ask not to be named.

    "},{"location":"community/security_policy/#supported-versions","title":"Supported versions","text":"

    Security fixes are made on the develop branch and shipped with the next release. Only the latest release receives security fixes; they are not backported to older releases. A release stops receiving security fixes when the next release is published, so please update to the latest release to get them.

    "},{"location":"community/security_policy/#unofficial-packages","title":"Unofficial packages","text":"

    This project does not publish an official npm package. The npm package nlohmann-json (or similarly named packages) is not maintained or endorsed by this project. See the package managers documentation for supported integration options.

    "},{"location":"community/security_policy/#additional-resources","title":"Additional Resources","text":"
    • Explore security-related topics and contribute to tools and projects through GitHub Security Lab.
    • Learn more about responsible disclosure and reporting vulnerabilities in GitHub at About coordinated disclosure of security vulnerabilities.

    We sincerely thank you for contributing to the security and integrity of this project!

    "},{"location":"features/","title":"Features","text":"

    This section describes the features of the library in detail. If you are new to the library, the pages below are roughly ordered along a typical workflow: create or parse a value, access and modify it, convert it to and from your own C++ types, and finally serialize it again.

    "},{"location":"features/#creating-and-reading-values","title":"Creating and reading values","text":"
    • Creating JSON values \u2014 build values from literals, initializer lists, and STL containers, and understand the {} vs. [] ambiguity.
    • Parsing \u2014 read a JSON value from a string, file, or stream, including JSON Lines, callbacks, the SAX interface, error handling, and parsing untrusted input.
    • Zero-copy JSON views \u2014 read a JSON text through a flat index instead of building a json tree; strings and numbers stay in the input and are only decoded when needed.
    • Editable documents \u2014 modify a parsed document without building a json tree.
    • Images \u2014 save a parsed document and load it again without parsing it.
    • Comments and trailing commas \u2014 opt-in relaxations of the JSON grammar.
    "},{"location":"features/#accessing-and-modifying-values","title":"Accessing and modifying values","text":"
    • Element access \u2014 unchecked (operator[]), checked (at), and access with a default value.
    • JSON Pointer \u2014 address values deep inside a document with RFC 6901 pointers.
    • Iterators \u2014 traverse arrays and objects.
    • Modifying values \u2014 add, update, merge, and remove elements.
    • JSON Patch and Diff and JSON Merge Patch \u2014 apply and compute structured changes.
    "},{"location":"features/#converting-to-and-from-c-types","title":"Converting to and from C++ types","text":"
    • Converting values \u2014 get values out with get/get_to, and understand implicit conversions.
    • Arbitrary types conversions \u2014 teach the library about your own structs and classes.
    • Specializing enum conversion \u2014 map enums to strings instead of integers.
    "},{"location":"features/#serializing-values","title":"Serializing values","text":"
    • Serialization \u2014 turn a value back into JSON text with dump, including pretty-printing and handling of non-ASCII and invalid UTF-8.
    • Binary formats \u2014 encode values more compactly as BJData, BON8, BSON, CBOR, MessagePack, or UBJSON.
    • Binary values \u2014 store and exchange raw byte sequences.
    "},{"location":"features/#how-values-are-stored-and-configured","title":"How values are stored and configured","text":"
    • Types and number handling \u2014 how JSON types map to C++ types and how numbers are treated.
    • Template parameter requirements \u2014 what a type passed as one of basic_json's template parameters has to provide.
    • Object order \u2014 keep insertion order with ordered_json.
    • Performance \u2014 practical advice on parsing, memory use, serialization, and compile times.
    • Runtime assertions, supported macros, the nlohmann namespace, and C++ modules \u2014 build-time and runtime configuration.

    Looking for a specific function?

    This section gives conceptual overviews. For the precise signature, parameters, and return value of a function, see the API Documentation.

    "},{"location":"features/arbitrary_types/","title":"Arbitrary Type Conversions","text":"

    Every type can be serialized in JSON, not just STL containers and scalar types. Usually, you would do something along those lines:

    namespace ns {\n    // a simple struct to model a person\n    struct person {\n        std::string name;\n        std::string address;\n        int age;\n    };\n} // namespace ns\n\nns::person p = {\"Ned Flanders\", \"744 Evergreen Terrace\", 60};\n\n// convert to JSON: copy each value into the JSON object\njson j;\nj[\"name\"] = p.name;\nj[\"address\"] = p.address;\nj[\"age\"] = p.age;\n\n// ...\n\n// convert from JSON: copy each value from the JSON object\nns::person p {\n    j[\"name\"].get<std::string>(),\n    j[\"address\"].get<std::string>(),\n    j[\"age\"].get<int>()\n};\n

    It works, but that's quite a lot of boilerplate... Fortunately, there's a better way:

    // create a person\nns::person p {\"Ned Flanders\", \"744 Evergreen Terrace\", 60};\n\n// conversion: person -> json\njson j = p;\n\nstd::cout << j << std::endl;\n// {\"address\":\"744 Evergreen Terrace\",\"age\":60,\"name\":\"Ned Flanders\"}\n\n// conversion: json -> person\nauto p2 = j.get<ns::person>();\n\n// that's it\nassert(p == p2);\n
    "},{"location":"features/arbitrary_types/#basic-usage","title":"Basic usage","text":"

    To make this work with one of your types, you only need to provide two functions:

    using json = nlohmann::json;\n\nnamespace ns {\n    void to_json(json& j, const person& p) {\n        j = json{ {\"name\", p.name}, {\"address\", p.address}, {\"age\", p.age} };\n    }\n\n    void from_json(const json& j, person& p) {\n        j.at(\"name\").get_to(p.name);\n        j.at(\"address\").get_to(p.address);\n        j.at(\"age\").get_to(p.age);\n    }\n} // namespace ns\n

    That's all! When calling the json constructor with your type, your custom to_json method will be automatically called. Likewise, when calling get<your_type>() or get_to(your_type&), the from_json method will be called.

    Some important things:

    • Those methods MUST be in your type's namespace (which can be the global namespace), or the library will not be able to locate them (in this example, they are in namespace ns, where person is defined).
    • Those methods MUST be available (e.g., proper headers must be included) everywhere you use these conversions. Look at #1108 for errors that may occur otherwise.
    • When using get<your_type>(), your_type MUST be DefaultConstructible. (There is a way to bypass this requirement described later.)
    • In function from_json, use function at() to access the object values rather than operator[]. In case a key does not exist, at throws an exception that you can handle, whereas operator[] exhibits undefined behavior.
    • You do not need to add serializers or deserializers for STL types like std::vector: the library already implements these.
    • If you control the type, consider defining to_json/from_json as friend functions inside the class (\"hidden friends\"). Argument-dependent lookup then only finds them for your type, which also avoids a GCC < 11 compilation error.
    Example: serialize a person to JSON with to_json 
    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nnamespace ns\n{\n// a simple struct to model a person\nstruct person\n{\n    std::string name;\n    std::string address;\n    int age;\n};\n} // namespace ns\n\nnamespace ns\n{\nvoid to_json(json& j, const person& p)\n{\n    j = json{ {\"name\", p.name}, {\"address\", p.address}, {\"age\", p.age} };\n}\n} // namespace ns\n\nint main()\n{\n    ns::person p = {\"Ned Flanders\", \"744 Evergreen Terrace\", 60};\n\n    json j = p;\n\n    std::cout << j << std::endl;\n}\n

    Output:

    {\"address\":\"744 Evergreen Terrace\",\"age\":60,\"name\":\"Ned Flanders\"}\n
    Example: deserialize a person from JSON with from_json 
    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nnamespace ns\n{\n// a simple struct to model a person\nstruct person\n{\n    std::string name;\n    std::string address;\n    int age;\n};\n} // namespace ns\n\nnamespace ns\n{\nvoid from_json(const json& j, person& p)\n{\n    j.at(\"name\").get_to(p.name);\n    j.at(\"address\").get_to(p.address);\n    j.at(\"age\").get_to(p.age);\n}\n} // namespace ns\n\nint main()\n{\n    json j;\n    j[\"name\"] = \"Ned Flanders\";\n    j[\"address\"] = \"744 Evergreen Terrace\";\n    j[\"age\"] = 60;\n\n    auto p = j.get<ns::person>();\n\n    std::cout << p.name << \" (\" << p.age << \") lives in \" << p.address << std::endl;\n}\n

    Output:

    Ned Flanders (60) lives in 744 Evergreen Terrace\n
    "},{"location":"features/arbitrary_types/#simplify-your-life-with-macros","title":"Simplify your life with macros","text":"

    If you just want to serialize/deserialize some structs, the to_json/from_json functions can be a lot of boilerplate.

    There are several macros to make your life easier as long as you want to use a JSON object as serialization. The macros are following the naming pattern, and you can choose the macro based on the needed features:

    • All the macros start with NLOHMANN_DEFINE.
    • If you want a macro for the derived object, use the DERIVED_TYPE variant, otherwise use TYPE.
      • The DERIVED_TYPE variant requires an additional parameter of a base type, which should have the to_json/from_json functions defined. For instance, with a macro of its own.
    • If you need access to the private fields use INTRUSIVE variant, otherwise use NON_INTRUSIVE.
      • The INTRUSIVE macro should be defined inside the target class/struct, NON_INTRUSIVE should be defined within the same namespace.
    • If you want to deserialize the incomplete JSONs, use the WITH_DEFAULTS variant, which will use the default values for the member variables absent in JSON, the variant without WITH_DEFAULTS will raise an exception.
    • If you do not need deserialization at all and only interested in to_json function, you can use the ONLY_SERIALIZE variant.
    • If you want to use the custom JSON names for member variables, use WITH_NAMES variant, otherwise the JSON name of the variable will be the same as its regular name.

    For all the macros, the first parameter is the name of the class/struct. The DERIVED_TYPE macros require a second parameter of a base class. All the remaining parameters name the member variables. The WITH_NAMES macros require a JSON name before each of the variables.

    flowchart TD\n    A[\"choosing a NLOHMANN_DEFINE_* macro\"] --> B{\"adding fields to a base class?\"}\n    B -->|\"yes\"| C[\"...DERIVED_TYPE...\"]\n    B -->|\"no\"| D[\"...TYPE...\"]\n    C --> E{\"need access to private members?\"}\n    D --> E\n    E -->|\"yes\"| F[\"...INTRUSIVE... (used inside the class)\"]\n    E -->|\"no\"| G[\"...NON_INTRUSIVE... (used in the namespace)\"]\n    F --> H{\"only serializing, never parsing back?\"}\n    G --> H\n    H -->|\"yes\"| I[\"...ONLY_SERIALIZE\"]\n    H -->|\"no\"| J{\"allow missing keys when parsing?\"}\n    J -->|\"yes\"| K[\"...WITH_DEFAULT\"]\n    J -->|\"no\"| L[\"plain (missing keys throw)\"]\n    I --> M{\"need custom JSON key names?\"}\n    K --> M\n    L --> M\n    M -->|\"yes\"| N[\"...WITH_NAMES\"]\n    M -->|\"no\"| O[\"done\"]
    Need access to private members Need only serialization Allow missing values when de-serializing macro NLOHMANN_DEFINE_TYPE_INTRUSIVE NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE

    For derived classes and structs, use the following macros

    Need access to private members Need only serialization Allow missing values when de-serializing macro NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_DEFAULT NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_ONLY_SERIALIZE NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_DEFAULT NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE

    Implementation limits

    • The current macro implementations are limited to at most 63 member variables. If you want to serialize/deserialize types with more than 63 member variables, you need to define the to_json/from_json functions manually.
    • For the WITH_NAMES variants the limit is halved to 31 member variables.
    Example: using the NLOHMANN_DEFINE_TYPE_* macros

    The to_json/from_json functions for the person struct above can be created with:

    namespace ns {\n    NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(person, name, address, age)\n}\n

    If you want to inherit the person struct and add a field to it, it can be done with:

    namespace ns {\n    struct person_derived : person {\n        std::string email;\n    };\n\n    NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE(person_derived, person, email)\n}\n

    Here is another example with private members, where NLOHMANN_DEFINE_TYPE_INTRUSIVE is needed:

    namespace ns {\n    class address {\n      private:\n        std::string street;\n        int housenumber;\n        int postcode;\n\n      public:\n        NLOHMANN_DEFINE_TYPE_INTRUSIVE(address, street, housenumber, postcode)\n    };\n}\n

    Or in case if you use some naming convention that you do not want to expose to JSON:

    namespace ns {\n    class address {\n      private:\n        std::string m_street;\n        int m_housenumber;\n        int m_postcode;\n\n      public:\n        NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_NAMES(address, \"street\", m_street,\n                                                           \"housenumber\", m_housenumber,\n                                                           \"postcode\", m_postcode)\n    };\n}\n

    Overriding conversions for natively-supported types

    The library already provides built-in to_json/from_json conversions for STL containers such as std::vector, std::array, and std::map. Defining your own free-function to_json/from_json overload for one of these container types directly (instead of for your own type) can conflict with the built-in overload during overload resolution, producing compiler errors (\"no matching overloaded function\", \"call is ambiguous\") that vary by compiler and library version. If you need different conversion behavior for a container type the library already handles, wrap it in your own type (or use adl_serializer specialization, as shown above for boost::optional) instead of trying to re-specialize to_json/from_json for the container type itself.

    Raw C-style arrays

    Members declared as raw C-style arrays (e.g., char buf[1024]) do not round-trip safely through NLOHMANN_DEFINE_TYPE_* macros or the default (de)serializers: to_json serializes any char array as a JSON string (matching the std::string-constructible overload), but the from_json overload for fixed-size arrays expects a JSON array and iterates it element-wise, which fails with a type_error when given a string. Use std::string, std::array<char, N>, or a manually written to_json/from_json pair for such members instead.

    Macros and nlohmann::ordered_json

    The NLOHMANN_DEFINE_TYPE_*/NLOHMANN_DEFINE_DERIVED_TYPE_* macros are generic over any basic_json specialization, including nlohmann::ordered_json. Simply use ordered_json as the target type and members are serialized in declaration order -- no separate macro or extra code is needed.

    namespace ns {\n    NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(person, name, address, age)\n}\n\nns::person p{\"Ned Flanders\", \"744 Evergreen Terrace\", 60};\nnlohmann::ordered_json j = p; // keys appear in declaration order: name, address, age\n

    Zero-member types

    All 12 NLOHMANN_DEFINE_TYPE_*/NLOHMANN_DEFINE_DERIVED_TYPE_* macros (excluding the WITH_NAMES variants) also accept types with no member variables to serialize, producing/accepting an empty JSON object {} (or, for the derived-type macros, just the base class's own JSON representation):

    namespace ns {\n    struct marker {\n        bool operator==(const marker&) const { return true; }\n        NLOHMANN_DEFINE_TYPE_INTRUSIVE(marker)\n    };\n}\n\nns::marker m{};\nnlohmann::json j = m; // {}\n

    No macro for non-default-constructible types

    There is currently no NLOHMANN_DEFINE_TYPE_*-style macro for types that are not DefaultConstructible. This is not an intentional omission of documentation -- no such macro exists yet; see How can I use get() for non-default constructible/non-copyable types? for the manual pattern to use instead.

    "},{"location":"features/arbitrary_types/#how-do-i-convert-third-party-types","title":"How do I convert third-party types?","text":"

    This requires a bit more advanced technique. But first, let us see how this conversion mechanism works:

    flowchart LR\n    A[\"construct json j = t, or call j.get() for T\"] --> B[\"JSONSerializer for T: to_json / from_json\"]\n    B -->|\"default JSONSerializer\"| C[\"adl_serializer for T: to_json / from_json\"]\n    C -->|\"unqualified call, found via ADL\"| D[\"free to_json(j, t) / from_json(j, t) in T's namespace\"]\n    B -->|\"user specialization replaces the default\"| E[\"user's adl_serializer specialization for T\"]

    The library uses JSON Serializers to convert types to JSON. The default serializer for nlohmann::json is nlohmann::adl_serializer (ADL means Argument-Dependent Lookup).

    It is implemented like this (simplified):

    template <typename T>\nstruct adl_serializer {\n    static void to_json(json& j, const T& value) {\n        // calls the \"to_json\" method in T's namespace\n    }\n\n    static void from_json(const json& j, T& value) {\n        // same thing, but with the \"from_json\" method\n    }\n};\n

    This serializer works fine when you have control over the type's namespace. However, what about boost::optional or std::filesystem::path (C++17)? Hijacking the boost namespace is pretty bad, and it's illegal to add something other than template specializations to std...

    To solve this, you need to add a specialization of adl_serializer to the nlohmann namespace, here's an example:

    // partial specialization (full specialization works too)\nNLOHMANN_JSON_NAMESPACE_BEGIN\ntemplate <typename T>\nstruct adl_serializer<boost::optional<T>> {\n    static void to_json(json& j, const boost::optional<T>& opt) {\n        if (opt == boost::none) {\n            j = nullptr;\n        } else {\n            j = *opt; // this will call adl_serializer<T>::to_json which will\n                      // find the free function to_json in T's namespace!\n        }\n    }\n\n    static void from_json(const json& j, boost::optional<T>& opt) {\n        if (j.is_null()) {\n            opt = boost::none;\n        } else {\n            opt = j.get<T>(); // same as above, but with\n                              // adl_serializer<T>::from_json\n        }\n    }\n};\nNLOHMANN_JSON_NAMESPACE_END\n

    std::optional needs no serializer

    Since version 3.12.0, std::optional<T> is supported out of the box when compiling with C++17 (std::nullopt is converted to and from null). Do not write an adl_serializer for it; this pattern is only needed for types such as boost::optional or for custom semantics. See Conversions and Omitting a field when serializing std::optional.

    ABI compatibility

    Use NLOHMANN_JSON_NAMESPACE_BEGIN and NLOHMANN_JSON_NAMESPACE_END instead of namespace nlohmann { } in code which may be linked with different versions of this library.

    "},{"location":"features/arbitrary_types/#how-can-i-use-get-for-non-default-constructiblenon-copyable-types","title":"How can I use get() for non-default constructible/non-copyable types?","text":"

    For a type that is not DefaultConstructible but is otherwise an ordinary value type, specialize adl_serializer with a from_json overload that returns the value instead of writing into a reference:

    Example: get() for a non-default-constructible type 
    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nnamespace ns\n{\n// a simple struct to model a person (not default constructible)\nstruct person\n{\n    person(std::string n, std::string a, int aa)\n        : name(std::move(n)), address(std::move(a)), age(aa)\n    {}\n\n    std::string name;\n    std::string address;\n    int age;\n};\n} // namespace ns\n\nnamespace nlohmann\n{\ntemplate <>\nstruct adl_serializer<ns::person>\n{\n    static ns::person from_json(const json& j)\n    {\n        return {j.at(\"name\"), j.at(\"address\"), j.at(\"age\")};\n    }\n\n    // Here's the catch! You must provide a to_json method! Otherwise, you\n    // will not be able to convert person to json, since you fully\n    // specialized adl_serializer on that type\n    static void to_json(json& j, ns::person p)\n    {\n        j[\"name\"] = p.name;\n        j[\"address\"] = p.address;\n        j[\"age\"] = p.age;\n    }\n};\n} // namespace nlohmann\n\nint main()\n{\n    json j;\n    j[\"name\"] = \"Ned Flanders\";\n    j[\"address\"] = \"744 Evergreen Terrace\";\n    j[\"age\"] = 60;\n\n    auto p = j.get<ns::person>();\n\n    std::cout << p.name << \" (\" << p.age << \") lives in \" << p.address << std::endl;\n}\n

    Output:

    Ned Flanders (60) lives in 744 Evergreen Terrace\n

    The same technique also works if your type is not copyable, as long as it is MoveConstructible:

    struct move_only_type {\n    move_only_type() = delete;\n    move_only_type(int ii): i(ii) {}\n    move_only_type(const move_only_type&) = delete;\n    move_only_type(move_only_type&&) = default;\n\n    int i;\n};\n\nnamespace nlohmann {\n    template <>\n    struct adl_serializer<move_only_type> {\n        // note: the return type is no longer 'void', and the method only takes\n        // one argument\n        static move_only_type from_json(const json& j) {\n            return {j.get<int>()};\n        }\n\n        // Here's the catch! You must provide a to_json method! Otherwise, you\n        // will not be able to convert move_only_type to json, since you fully\n        // specialized adl_serializer on that type\n        static void to_json(json& j, move_only_type t) {\n            j = t.i;\n        }\n    };\n}\n
    "},{"location":"features/arbitrary_types/#why-cant-i-convert-tofrom-stdany","title":"Why can't I convert to/from std::any?","text":"

    std::any is intentionally excluded from get<T>()/generic conversion support, so get<std::any>() and containers like std::map<std::string, std::any> fail to compile by design -- there is no way to know, from a json value alone, which concrete type to store inside the std::any. To work with heterogeneous JSON values, dispatch on the value's type manually and construct the std::any (or extract from it) yourself:

    std::any value_to_any(const json& j) {\n    if (j.is_boolean())      { return j.get<bool>(); }\n    if (j.is_number_integer()) { return j.get<int>(); }\n    if (j.is_number_float()) { return j.get<double>(); }\n    if (j.is_string())       { return j.get<std::string>(); }\n    // ... handle other types (arrays, objects) as needed for your use case\n    return {};\n}\n\njson any_to_json(const std::any& a) {\n    if (a.type() == typeid(bool))        { return std::any_cast<bool>(a); }\n    if (a.type() == typeid(int))         { return std::any_cast<int>(a); }\n    if (a.type() == typeid(double))      { return std::any_cast<double>(a); }\n    if (a.type() == typeid(std::string)) { return std::any_cast<std::string>(a); }\n    return nullptr;\n}\n
    "},{"location":"features/arbitrary_types/#why-does-serializing-a-stdmapstdunordered_map-with-non-string-keys-produce-an-array","title":"Why does serializing a std::map/std::unordered_map with non-string keys produce an array?","text":"

    A std::map/std::unordered_map whose key type is not string-like (e.g., std::map<int, std::string>) cannot be serialized as a JSON object, because JSON object keys must be strings. See Converting maps with non-string keys in the types article for what the library does instead.

    "},{"location":"features/arbitrary_types/#why-does-stdwstring-convert-or-dump-incorrectly","title":"Why does std::wstring convert or dump incorrectly?","text":"

    The library assumes UTF-8 encoding internally, so std::wstring is not supported out of the box -- see the FAQ entry on wide string handling for why, and for a UTF-8 conversion recipe.

    "},{"location":"features/arbitrary_types/#can-i-write-my-own-serializer-advanced-use","title":"Can I write my own serializer? (Advanced use)","text":"

    Yes. You might want to take a look at unit-udt.cpp in the test suite, to see a few examples.

    If you write your own serializer, you will need to do a few things:

    • use a different basic_json alias than nlohmann::json (the last template parameter of basic_json is the JSONSerializer)
    • use your basic_json alias (or a template parameter) in all your to_json/from_json methods
    • use nlohmann::to_json and nlohmann::from_json when you need ADL

    Here is an example, without simplifications, that only accepts types with a size <= 32, and uses ADL.

    // You should use void as a second template argument\n// if you don't need compile-time checks on T\ntemplate<typename T, typename SFINAE = typename std::enable_if<sizeof(T) <= 32>::type>\nstruct less_than_32_serializer {\n    template <typename BasicJsonType>\n    static void to_json(BasicJsonType& j, T value) {\n        // we want to use ADL, and call the correct to_json overload\n        using nlohmann::to_json; // this method is called by adl_serializer,\n                                 // this is where the magic happens\n        to_json(j, value);\n    }\n\n    template <typename BasicJsonType>\n    static void from_json(const BasicJsonType& j, T& value) {\n        // same thing here\n        using nlohmann::from_json;\n        from_json(j, value);\n    }\n};\n

    Be very careful when reimplementing your serializer, you can stack overflow if you don't pay attention:

    template <typename T, typename = void>\nstruct bad_serializer\n{\n    template <typename BasicJsonType>\n    static void to_json(BasicJsonType& j, const T& value) {\n      // this calls BasicJsonType::json_serializer<T>::to_json(j, value);\n      // if BasicJsonType::json_serializer == bad_serializer ... oops!\n      j = value;\n    }\n\n    template <typename BasicJsonType>\n    static void from_json(const BasicJsonType& j, T& value) {\n      // this calls BasicJsonType::json_serializer<T>::from_json(j, value);\n      // if BasicJsonType::json_serializer == bad_serializer ... oops!\n      value = j.template get<T>(); // oops!\n    }\n};\n
    "},{"location":"features/arbitrary_types/#see-also","title":"See also","text":"
    • Converting values - the general overview of get/get_to and implicit conversions
    • Specializing enum conversion - map enums to JSON strings instead of integers
    • Supported macros - reference for NLOHMANN_DEFINE_TYPE_* and related macros
    • adl_serializer - the default JSONSerializer used in the conversion dispatch
    "},{"location":"features/assertions/","title":"Runtime Assertions","text":"

    The code contains numerous debug assertions to ensure class invariants are valid or to detect undefined behavior. Whereas the former class invariants are nothing to be concerned with, the latter checks for undefined behavior are to detect bugs in client code.

    "},{"location":"features/assertions/#switch-off-runtime-assertions","title":"Switch off runtime assertions","text":"

    Runtime assertions can be switched off by defining the preprocessor macro NDEBUG (see the documentation of assert) which is the default for release builds.

    "},{"location":"features/assertions/#change-assertion-behavior","title":"Change assertion behavior","text":"

    The behavior of runtime assertions can be changed by defining macro JSON_ASSERT(x) before including the json.hpp header.

    "},{"location":"features/assertions/#function-with-runtime-assertions","title":"Function with runtime assertions","text":""},{"location":"features/assertions/#unchecked-access-to-a-const-value","title":"Unchecked access to a const value","text":"

    Function operator[] implements unchecked access for arrays and objects. Whereas a missing element is added in the case of non-const values, accessing a const value with a missing object key or an invalid array index is undefined behavior (think of a dereferenced null pointer) and yields a runtime assertion. This also applies to a JSON pointer that refers to a missing key or an invalid index.

    If you are not sure whether an element exists, use checked access with the at function or call the contains function before.

    See also the documentation on element access.

    Example: missing object key

    The following code will trigger an assertion at runtime:

    #include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    const json j = {{\"key\", \"value\"}};\n    auto v = j[\"missing\"];\n}\n

    Output:

    Assertion failed: (it != m_data.m_value.object->end()), function operator[], file json.hpp, line 28795.\n
    Example 2: Invalid array index in a JSON pointer

    The following code will trigger an assertion at runtime:

    #include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    const json j = {{\"array\", {1, 2, 3}}};\n    auto v = j[\"/array/5\"_json_pointer];\n}\n

    Output:

    Assertion failed: (idx < m_data.m_value.array->size()), function operator[], file json.hpp, line 28758.\n
    "},{"location":"features/assertions/#constructing-from-an-uninitialized-iterator-range","title":"Constructing from an uninitialized iterator range","text":"

    Constructing a JSON value from an iterator range (see constructor) with an uninitialized iterator is undefined behavior and yields a runtime assertion.

    Example: uninitialized iterator range

    The following code will trigger an assertion at runtime:

    #include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    json::iterator it1, it2;\n    json j(it1, it2);\n}\n

    Output:

    Assertion failed: (m_object != nullptr), function operator++, file iter_impl.hpp, line 368.\n
    "},{"location":"features/assertions/#operations-on-uninitialized-iterators","title":"Operations on uninitialized iterators","text":"

    Any operation on uninitialized iterators (i.e., iterators that are not associated with any JSON value) is undefined behavior and yields a runtime assertion.

    Example: uninitialized iterator

    The following code will trigger an assertion at runtime:

    #include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n  json::iterator it;\n  ++it;\n}\n

    Output:

    Assertion failed: (m_object != nullptr), function operator++, file iter_impl.hpp, line 368.\n
    "},{"location":"features/assertions/#changes","title":"Changes","text":""},{"location":"features/assertions/#reading-from-a-null-file-or-char-pointer","title":"Reading from a null FILE or char pointer","text":"

    Reading from a null FILE or char pointer in C++ is undefined behavior. Until version 3.12.0, this library asserted that the pointer was not nullptr using a runtime assertion. If assertions were disabled, this would result in undefined behavior. Since version 3.12.0, this library checks for nullptr and throws a parse_error.101 to prevent the undefined behavior.

    Example: reading from null pointer

    The following code will trigger an assertion at runtime:

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::FILE* f = std::fopen(\"nonexistent_file.json\", \"r\");\n    try {\n        json j = json::parse(f);\n    } catch (std::exception& e) {\n        std::cerr << e.what() << std::endl;\n    }\n}\n

    Output:

    [json.exception.parse_error.101] parse error: attempting to parse an empty input; check that your input string or stream contains the expected JSON\n
    "},{"location":"features/assertions/#see-also","title":"See also","text":"
    • JSON_ASSERT - control behavior of runtime assertions
    "},{"location":"features/binary_values/","title":"Binary Values","text":"

    The library implements several binary formats that encode JSON in an efficient way. Most of these formats support binary values; that is, values that have semantics defined outside the library and only define a sequence of bytes to be stored.

    JSON itself does not have a binary value. As such, binary values are an extension that this library implements to store values received by a binary format. Binary values are never created by the JSON parser and are only part of a serialized JSON text if they have been created manually or via a binary format.

    "},{"location":"features/binary_values/#api-for-binary-values","title":"API for binary values","text":"
    classDiagram\n\nclass binary_t [\"json::binary_t\"] {\n    +void set_subtype(std::uint64_t subtype)\n    +void clear_subtype()\n    +std::uint64_t subtype() const\n    +bool has_subtype() const\n}\n\nclass vector [\"std::vector<uint8_t>\"]\n\nvector <|-- binary_t

    By default, binary values are stored as std::vector<std::uint8_t>. This type can be changed by providing a template parameter to the basic_json type. To store binary subtypes, the storage type is extended and exposed as json::binary_t:

    auto binary = json::binary_t({0xCA, 0xFE, 0xBA, 0xBE});\nauto binary_with_subtype = json::binary_t({0xCA, 0xFE, 0xBA, 0xBE}, 42);\n

    There are several convenience functions to check and set the subtype:

    binary.has_subtype();                   // returns false\nbinary_with_subtype.has_subtype();      // returns true\n\nbinary_with_subtype.clear_subtype();\nbinary_with_subtype.has_subtype();      // returns false\n\nbinary_with_subtype.set_subtype(42);\nbinary.set_subtype(23);\n\nbinary.subtype();                       // returns 23\n

    As json::binary_t is subclassing std::vector<std::uint8_t>, all member functions are available:

    binary.size();  // returns 4\nbinary[1];      // returns 0xFE\n

    JSON values can be constructed from json::binary_t:

    json j = binary;\n

    Binary values are primitive values just like numbers or strings, as reflected by is_binary() and is_primitive():

    j.is_binary();    // returns true\nj.is_primitive(); // returns true\n

    Given a binary JSON value, the binary_t can be accessed by reference via get_binary():

    j.get_binary().has_subtype();  // returns true\nj.get_binary().size();         // returns 4\n

    For convenience, binary JSON values can be constructed via json::binary:

    auto j2 = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 23);\nauto j3 = json::binary({0xCA, 0xFE, 0xBA, 0xBE});\n\nj2 == j;                        // returns true\nj3.get_binary().has_subtype();  // returns false\nj3.get_binary().subtype();      // returns std::uint64_t(-1) as j3 has no subtype\n
    "},{"location":"features/binary_values/#serialization","title":"Serialization","text":"

    Binary values are serialized differently according to the formats.

    "},{"location":"features/binary_values/#json","title":"JSON","text":"

    JSON does not have a binary type, and this library does not introduce a new type as this would break conformance. Instead, binary values are serialized as an object with two keys: bytes holds an array of integers, and subtype is an integer or null.

    Example: serialize a binary value to JSON

    Code:

    // create a binary value of subtype 42\njson j;\nj[\"binary\"] = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 42);\n\n// serialize to standard output\nstd::cout << j.dump(2) << std::endl;\n

    Output:

    {\n  \"binary\": {\n    \"bytes\": [202, 254, 186, 190],\n    \"subtype\": 42\n  }\n}\n

    No roundtrip for binary values

    The JSON parser will not parse the objects generated by binary values back to binary values. This is by design to remain standards compliant. Serializing binary values to JSON is only implemented for debugging purposes.

    "},{"location":"features/binary_values/#bjdata","title":"BJData","text":"

    BJData neither supports binary values nor subtypes and proposes to serialize binary values as an array of uint8 values. The library implements this translation.

    Example: serialize a binary value to BJData

    Code:

    // create a binary value of subtype 42 (will be ignored in BJData)\njson j;\nj[\"binary\"] = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 42);\n\n// convert to BJData\nauto v = json::to_bjdata(j);      \n

    v is a std::vector<std::uint8_t> with the following 20 elements:

    0x7B                                             // '{'\n    0x69 0x06                                    // i 6 (length of the key)\n    0x62 0x69 0x6E 0x61 0x72 0x79                // \"binary\"\n    0x5B                                         // '['\n        0x55 0xCA 0x55 0xFE 0x55 0xBA 0x55 0xBE  // content (each byte prefixed with 'U')\n    0x5D                                         // ']'\n0x7D                                             // '}'\n

    The following code uses the type and size optimization for BJData:

    // convert to BJData using the size and type optimization\nauto v = json::to_bjdata(j, true, true);\n

    The resulting vector has 22 elements; the optimization is not effective for examples with few values:

    0x7B                                // '{'\n    0x23 0x69 0x01                  // '#' 'i' type of the array elements: unsigned integers\n    0x69 0x06                       // i 6 (length of the key)\n    0x62 0x69 0x6E 0x61 0x72 0x79   // \"binary\"\n    0x5B                            // '[' array\n        0x24 0x55                   // '$' 'U' type of the array elements: unsigned integers\n        0x23 0x69 0x04              // '#' i 4 number of array elements\n        0xCA 0xFE 0xBA 0xBE         // content\n

    Note that subtype (42) is not serialized and that BJData has no binary type, and deserializing v would yield the following value:

    {\n  \"binary\": [202, 254, 186, 190]\n}\n
    "},{"location":"features/binary_values/#bon8","title":"BON8","text":"

    BON8 neither supports binary values nor subtypes. The library serializes binary values as an array of integers.

    Example: serialize a binary value to BON8

    Code:

    // create a binary value of subtype 42 (will be ignored in BON8)\njson j;\nj[\"binary\"] = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 42);\n\n// convert to BON8\nauto v = json::to_bon8(j);\n

    v is a std::vector<std::uint8_t> with the following 16 elements:

    0x87                                     // object with 1 member\n    0x62 0x69 0x6E 0x61 0x72 0x79        // \"binary\"\n    0x84                                 // array with 4 elements\n        0xC3 0x22 0xC3 0x56 0xC3 0x12 0xC3 0x16  // content (each byte as a 2-byte integer)\n

    Note that the subtype is lost, and deserializing v would yield the following value:

    {\n  \"binary\": [202, 254, 186, 190]\n}\n
    "},{"location":"features/binary_values/#bson","title":"BSON","text":"

    BSON supports binary values and subtypes. If a subtype is given, it is used and added as an unsigned 8-bit integer. If no subtype is given, the generic binary subtype 0x00 is used.

    Example: serialize a binary value to BSON

    Code:

    // create a binary value of subtype 42\njson j;\nj[\"binary\"] = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 42);\n\n// convert to BSON\nauto v = json::to_bson(j);      \n

    v is a std::vector<std::uint8_t> with the following 22 elements:

    0x16 0x00 0x00 0x00                         // number of bytes in the document\n    0x05                                    // binary value\n        0x62 0x69 0x6E 0x61 0x72 0x79 0x00  // key \"binary\" + null byte\n        0x04 0x00 0x00 0x00                 // number of bytes\n        0x2a                                // subtype\n        0xCA 0xFE 0xBA 0xBE                 // content\n0x00                                        // end of the document\n

    Note that the serialization preserves the subtype, and deserializing v would yield the following value:

    {\n  \"binary\": {\n    \"bytes\": [202, 254, 186, 190],\n    \"subtype\": 42\n  }\n}\n
    "},{"location":"features/binary_values/#cbor","title":"CBOR","text":"

    CBOR supports binary values and subtypes. Any binary value will be serialized as a byte string; a subtype is serialized as a tag preceding it. When parsing, tags are only read back as subtypes with cbor_tag_handler_t::store (see from_cbor). The library will choose the smallest representation using the length of the byte array.

    Example: serialize a binary value to CBOR

    Code:

    // create a binary value of subtype 42\njson j;\nj[\"binary\"] = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 42);\n\n// convert to CBOR\nauto v = json::to_cbor(j);      \n

    v is a std::vector<std::uint8_t> with the following 15 elements:

    0xA1                                   // map(1)\n    0x66                               // text(6)\n        0x62 0x69 0x6E 0x61 0x72 0x79  // \"binary\"\n    0xD8 0x2A                          // tag(42)\n    0x44                               // bytes(4)\n        0xCA 0xFE 0xBA 0xBE            // content\n

    Note that the subtype is serialized as tag. However, parsing tagged values yield a parse error unless json::cbor_tag_handler_t::ignore or json::cbor_tag_handler_t::store is passed to json::from_cbor (see cbor_tag_handler_t).

    {\n  \"binary\": {\n    \"bytes\": [202, 254, 186, 190],\n    \"subtype\": null\n  }\n}\n
    "},{"location":"features/binary_values/#messagepack","title":"MessagePack","text":"

    MessagePack supports binary values and subtypes. If a subtype is given, the ext family is used. The library will choose the smallest representation among fixext1, fixext2, fixext4, fixext8, ext8, ext16, and ext32. The subtype is then added as a signed 8-bit integer.

    If no subtype is given, the bin family (bin8, bin16, bin32) is used.

    Example: serialize a binary value to MessagePack

    Code:

    // create a binary value of subtype 42\njson j;\nj[\"binary\"] = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 42);\n\n// convert to MessagePack\nauto v = json::to_msgpack(j);      \n

    v is a std::vector<std::uint8_t> with the following 14 elements:

    0x81                                   // fixmap1\n    0xA6                               // fixstr6\n        0x62 0x69 0x6E 0x61 0x72 0x79  // \"binary\"\n    0xD6                               // fixext4\n        0x2A                           // subtype\n        0xCA 0xFE 0xBA 0xBE            // content\n

    Note that the serialization preserves the subtype, and deserializing v would yield the following value:

    {\n  \"binary\": {\n    \"bytes\": [202, 254, 186, 190],\n    \"subtype\": 42\n  }\n}\n
    "},{"location":"features/binary_values/#ubjson","title":"UBJSON","text":"

    UBJSON neither supports binary values nor subtypes and proposes to serialize binary values as an array of uint8 values. The library implements this translation.

    Example: serialize a binary value to UBJSON

    Code:

    // create a binary value of subtype 42 (will be ignored in UBJSON)\njson j;\nj[\"binary\"] = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 42);\n\n// convert to UBJSON\nauto v = json::to_ubjson(j);      \n

    v is a std::vector<std::uint8_t> with the following 20 elements:

    0x7B                                             // '{'\n    0x69 0x06                                    // i 6 (length of the key)\n    0x62 0x69 0x6E 0x61 0x72 0x79                // \"binary\"\n    0x5B                                         // '['\n        0x55 0xCA 0x55 0xFE 0x55 0xBA 0x55 0xBE  // content (each byte prefixed with 'U')\n    0x5D                                         // ']'\n0x7D                                             // '}'\n

    The following code uses the type and size optimization for UBJSON:

    // convert to UBJSON using the size and type optimization\nauto v = json::to_ubjson(j, true, true);\n

    The resulting vector has 23 elements; the optimization is not effective for examples with few values:

    0x7B                                // '{'\n    0x24                            // '$' type of the object elements\n    0x5B                            // '[' array\n    0x23 0x69 0x01                  // '#' i 1 number of object elements\n    0x69 0x06                       // i 6 (length of the key)\n    0x62 0x69 0x6E 0x61 0x72 0x79   // \"binary\"\n        0x24 0x55                   // '$' 'U' type of the array elements: unsigned integers\n        0x23 0x69 0x04              // '#' i 4 number of array elements\n        0xCA 0xFE 0xBA 0xBE         // content\n

    Note that subtype (42) is not serialized and that UBJSON has no binary type, and deserializing v would yield the following value:

    {\n  \"binary\": [202, 254, 186, 190]\n}\n
    "},{"location":"features/comments/","title":"Comments","text":"

    This library does not support comments by default. It does so for three reasons:

    1. Comments are not part of the JSON specification. You may argue that // or /* */ are allowed in JavaScript, but JSON is not JavaScript.

    2. This was not an oversight: Douglas Crockford wrote on this in May 2012:

      I removed comments from JSON because I saw people were using them to hold parsing directives, a practice which would have destroyed interoperability. I know that the lack of comments makes some people sad, but it shouldn't.

      Suppose you are using JSON to keep configuration files, which you would like to annotate. Go ahead and insert all the comments you like. Then pipe it through JSMin before handing it to your JSON parser.

    3. It is dangerous for interoperability if some libraries add comment support while others do not. Please check The Harmful Consequences of the Robustness Principle on this.

    However, you can set parameter ignore_comments to true in the parse function to ignore // or /* */ comments. Comments will then be treated as whitespace. Combined with ignore_trailing_commas (also a parse parameter), this covers what is commonly referred to as JSONC (JSON with Comments, as used e.g. by Visual Studio Code's .jsonc files) -- comments and trailing commas, nothing more. This is a different, smaller extension than JSON5, which additionally allows unquoted keys, single-quoted strings, and other syntax changes that this library does not support.

    For more information, see JSON With Commas and Comments (JWCC).

    Example

    Consider the following JSON with comments.

    {\n    // update in 2006: removed Pluto\n    \"planets\": [\"Mercury\", \"Venus\", \"Earth\", \"Mars\",\n                \"Jupiter\", \"Uranus\", \"Neptune\" /*, \"Pluto\" */]\n}\n

    When calling parse without additional argument, a parse error exception is thrown. If ignore_comments is set to true, the comments are ignored during parsing:

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::string s = R\"(\n    {\n        // update in 2006: removed Pluto\n        \"planets\": [\"Mercury\", \"Venus\", \"Earth\", \"Mars\",\n                    \"Jupiter\", \"Uranus\", \"Neptune\" /*, \"Pluto\" */]\n    }\n    )\";\n\n    try\n    {\n        json j = json::parse(s);\n    }\n    catch (json::exception& e)\n    {\n        std::cout << e.what() << std::endl;\n    }\n\n    json j = json::parse(s,\n                         /* callback */ nullptr,\n                         /* allow exceptions */ true,\n                         /* ignore_comments */ true);\n    std::cout << j.dump(2) << '\\n';\n}\n

    Output:

    [json.exception.parse_error.101] parse error at line 3, column 9: syntax error while parsing object key - invalid literal; last read: '<U+000A>    {<U+000A>        /'; expected string literal\n{\n  \"planets\": [\n    \"Mercury\",\n    \"Venus\",\n    \"Earth\",\n    \"Mars\",\n    \"Jupiter\",\n    \"Uranus\",\n    \"Neptune\"\n  ]\n}\n
    "},{"location":"features/conversions/","title":"Converting values","text":"

    A basic_json value stores JSON data, but most of the time you want to move that data into ordinary C++ types (an int, a std::string, a std::vector, or one of your own structs) and back. This page describes how these conversions work.

    "},{"location":"features/conversions/#getting-values-out","title":"Getting values out","text":"

    The get function template returns a copy of the stored value converted to the requested type:

    json j = R\"({\"name\": \"Mary\", \"age\": 42, \"hobbies\": [\"hiking\", \"reading\"]})\"_json;\n\nauto name = j[\"name\"].get<std::string>();               // \"Mary\"\nauto age  = j[\"age\"].get<int>();                        // 42\nauto hobbies = j[\"hobbies\"].get<std::vector<std::string>>();  // {\"hiking\", \"reading\"}\n

    Getting a string without quotes

    A frequent point of confusion: use get, not dump, to read a string value. j[\"name\"].get<std::string>() yields Mary, whereas j[\"name\"].dump() yields the JSON text \"Mary\" (with quotes), because dump always produces a JSON text.

    Alternatively, get_to writes into an existing variable and deduces the target type, which avoids repeating it:

    Example 
    #include <iostream>\n#include <map>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON value with different types\n    json json_types =\n    {\n        {\"boolean\", true},\n        {\n            \"number\", {\n                {\"integer\", 42},\n                {\"floating-point\", 17.23}\n            }\n        },\n        {\"string\", \"Hello, world!\"},\n        {\"array\", {1, 2, 3, 4, 5}},\n        {\"null\", nullptr}\n    };\n\n    bool v1;\n    int v2;\n    short v3;\n    float v4;\n    int v5;\n    std::string v6;\n    std::vector<short> v7;\n    std::map<std::string, json> v8;\n\n    // use explicit conversions\n    json_types[\"boolean\"].get_to(v1);\n    json_types[\"number\"][\"integer\"].get_to(v2);\n    json_types[\"number\"][\"integer\"].get_to(v3);\n    json_types[\"number\"][\"floating-point\"].get_to(v4);\n    json_types[\"number\"][\"floating-point\"].get_to(v5);\n    json_types[\"string\"].get_to(v6);\n    json_types[\"array\"].get_to(v7);\n    json_types.get_to(v8);\n\n    // print the conversion results\n    std::cout << v1 << '\\n';\n    std::cout << v2 << ' ' << v3 << '\\n';\n    std::cout << v4 << ' ' << v5 << '\\n';\n    std::cout << v6 << '\\n';\n\n    for (auto i : v7)\n    {\n        std::cout << i << ' ';\n    }\n    std::cout << \"\\n\\n\";\n\n    for (auto i : v8)\n    {\n        std::cout << i.first << \": \" << i.second << '\\n';\n    }\n}\n

    Output:

    1\n42 42\n17.23 17\nHello, world!\n1 2 3 4 5 \n\narray: [1,2,3,4,5]\nboolean: true\nnull: null\nnumber: {\"floating-point\":17.23,\"integer\":42}\nstring: \"Hello, world!\"\n

    The library already knows how to convert to and from the scalar types and the STL containers (such as std::vector, std::map, std::array, std::optional, and many more). Converting a JSON object back to a std::map or a JSON array back to a std::vector therefore works without any extra code:

    json j = {{\"one\", 1}, {\"two\", 2}};\nauto m = j.get<std::map<std::string, int>>();  // {{\"one\", 1}, {\"two\", 2}}\n

    std::pair and std::tuple are also supported, converting positionally to and from a JSON array:

    json j = {1.0, \"hello\", 42};\nauto t = j.get<std::tuple<double, std::string, int>>();  // {1.0, \"hello\", 42}\n

    Serializing a std::pair/std::tuple whose every element is a string-keyed pair

    When every element of a std::pair or std::tuple is itself a two-element array whose first element is a string (for example std::pair<std::string, int>), serializing it produces a JSON object instead of the expected array:

    using kv = std::pair<std::string, int>;\njson j = std::pair<kv, kv>{{\"a\", 1}, {\"b\", 2}};  // {\"a\":1,\"b\":2}, not [[\"a\",1],[\"b\",2]]\n

    This is a consequence of the brace-initializer object-detection rule: the same rule that lets json{{\"a\", 1}, {\"b\", 2}} create an object also fires here. The resulting object cannot be read back into the original type (get<std::pair<kv, kv>>() throws type_error.302), and duplicate keys collapse into one, losing elements. This only affects std::pair/std::tuple themselves; a std::vector<std::pair<std::string, int>>, or a pair/tuple with at least one element that is not a string-keyed pair, serializes to an array as expected. To force an array, build one explicitly from the elements with array:

    std::pair<kv, kv> p{{\"a\", 1}, {\"b\", 2}};\njson a = json::array({p.first, p.second});  // [[\"a\",1],[\"b\",2]]\n

    Extracting references into a tuple

    A tuple type may also hold references (e.g. std::tuple<double&, std::string&>) to avoid copying: get then returns a tuple of references pointing directly at the elements stored inside the basic_json array, rather than a tuple of copies:

    json j = {1.0, \"hello\"};\nauto refs = j.get<std::tuple<double&, std::string&>>();\nstd::get<1>(refs) = \"world\";  // modifies j[1] in place\n

    A referenced element must name the type the library actually stores \u2014 one of boolean_t, number_integer_t, number_unsigned_t, number_float_t, string_t, binary_t, array_t, or object_t. There is nothing else to refer to, so a reference to any other type is a compile error even when a conversion would exist: std::tuple<int&> is rejected, because the library stores a number_integer_t (std::int64_t by default) and not an int. This restriction applies only to reference elements \u2014 a plain std::tuple<int> converts by value as usual.

    "},{"location":"features/conversions/#implicit-conversions","title":"Implicit conversions","text":"

    By default, a JSON value implicitly converts to a compatible C++ type, so the explicit get call can often be omitted:

    json j = \"Hello\";\nstd::string s = j;   // implicit conversion, same as j.get<std::string>()\n

    Implicit conversions are convenient but can be surprising (for example, in overload resolution or with auto). They can be disabled by defining JSON_USE_IMPLICIT_CONVERSIONS to 0, which forces the explicit get form and can catch unintended conversions at compile time.

    Conversions do not range-check numbers

    Just like C++ itself, the get family performs numeric conversions without range checks \u2014 retrieving a floating-point value as an integer truncates it, and narrowing conversions may overflow. See number conversion for details and how to guard against it.

    std::optional direct construction from JSON null throws

    Constructing or assigning std::optional<T> directly from a JSON value does not correctly produce std::nullopt for a JSON null:

    json j_null;\nstd::optional<std::string> opt = j_null;  // \u274c throws type_error 302\n

    This is due to C++ language rules: std::optional<T> has its own converting constructor that is chosen over basic_json::operator T() when both are viable. Use get<std::optional<T>>() or get_to() instead:

    auto opt = j_null.get<std::optional<std::string>>();  // \u2705 std::nullopt\nj_null.get_to(opt);                                     // \u2705 std::nullopt\n

    static_cast and get<std::optional<T>>() are not guaranteed equivalent

    operator ValueType() (used by static_cast and implicit conversions) intentionally excludes std::optional<T> from delegating to get<T>(), to avoid a constructor ambiguity with std::optional<T>'s own converting constructor from basic_json. As a result, static_cast<std::optional<T>>(json_value) goes through std::optional<T>'s own converting constructor rather than through get<std::optional<T>>(), which can behave differently -- for example, with a custom adl_serializer<std::optional<T>> specialization. Prefer get<std::optional<T>>()/get_to() over static_cast for optional types.

    Converting to a fixed-size destination does not check the array size

    Some destination types have a size that is fixed by their C++ type rather than by the JSON value: std::pair<A, B>, std::tuple<Ts...>, std::array<T, N>, C arrays T[N], and std::map/std::unordered_map with a non-string key type (which is read from an array of two-element arrays). All of them read exactly as many elements as they need via at and never compare the JSON array's size to that number. The two mismatch directions therefore behave differently:

    • The JSON array has too many elements: the surplus is silently discarded, and no exception is thrown.
    • The JSON array has too few elements: at throws out_of_range.401 for the first missing index -- an out-of-range error, not a type_error, even though the cause is a shape mismatch.
    json j = {1, 2, 3, 4, 5};\n\nauto a = j.get<std::array<int, 3>>();       // {1, 2, 3} -- elements 4 and 5 silently dropped\nauto p = j.get<std::pair<int, int>>();      // (1, 2)    -- elements 3, 4, and 5 silently dropped\n\njson k = {1};\nauto q = k.get<std::pair<int, int>>();      // \u274c throws out_of_range.401\n

    If a size mismatch is an error in your application, check the size yourself before converting.

    "},{"location":"features/conversions/#omitting-a-field-when-serializing-stdoptional","title":"Omitting a field when serializing std::optional","text":"

    By default, to_json for std::optional<T> writes either the value or null -- there is no built-in way to make a field disappear from the serialized object entirely when the std::optional is std::nullopt. Because a specialization of adl_serializer<std::optional<T>> only controls how the value is converted (it cannot prevent the containing object's to_json from inserting the key in the first place), omission has to be implemented in the containing type's to_json:

    struct person {\n    std::string name;\n    std::optional<int> age;\n};\n\nvoid to_json(json& j, const person& p) {\n    j = json{{\"name\", p.name}};\n    if (p.age) {\n        j[\"age\"] = *p.age;  // key is only inserted when the optional has a value\n    }\n}\n
    "},{"location":"features/conversions/#putting-values-in","title":"Putting values in","text":"

    The reverse direction works the same way: assigning or constructing a json from a C++ value converts it to JSON.

    std::vector<int> numbers = {1, 2, 3};\njson j = numbers;   // [1,2,3]\n

    Constructing from a C++20 range view

    A json array can also be constructed directly from a C++20 range view (std::ranges::view), such as the result of std::views::filter or std::views::transform -- no intermediate container is needed:

    std::vector<int> nums{1, 2, 37, 42, 21};\nauto filtered = nums | std::views::filter([](int i) { return i > 10; });\njson j(filtered);   // [37,42,21]\n

    This requires JSON_HAS_RANGES to be enabled and is unavailable on MinGW due to incomplete C++20 ranges support there; see JSON_HAS_RANGE_VIEW_CONVERSION.

    "},{"location":"features/conversions/#your-own-types","title":"Your own types","text":"

    The conversions above are built in for standard types. To make the same syntax work for your own types, provide to_json/from_json functions (or use one of the convenience macros). This is described in detail on the arbitrary types conversions page. Enums can be mapped to strings as described in specializing enum conversion.

    "},{"location":"features/conversions/#see-also","title":"See also","text":"
    • get - get a copy converted to a given type
    • get_to - convert into an existing variable
    • get_ref / get_ptr - access the stored value without copying
    • Arbitrary types conversions - support your own types
    • JSON_USE_IMPLICIT_CONVERSIONS - toggle implicit conversions
    "},{"location":"features/creating_values/","title":"Creating JSON values","text":"

    There are several ways to create a JSON value in memory. This page gives an overview; to read a value from JSON text instead, see parsing.

    "},{"location":"features/creating_values/#from-c-values","title":"From C++ values","text":"

    Any value of a supported C++ type can be assigned to or used to construct a json:

    json j_number  = 42;\njson j_float   = 3.141;\njson j_string  = \"Hello\";\njson j_boolean = true;\njson j_null    = nullptr;\njson j_vector  = std::vector<int>{1, 2, 3};   // array\n

    See converting values for the full set of supported types.

    "},{"location":"features/creating_values/#with-initializer-lists","title":"With initializer lists","text":"

    Objects and arrays can be written concisely with brace-enclosed initializer lists:

    // an array\njson array = {1, 2, 3, 4};\n\n// an object (a list of key/value pairs)\njson object = {\n    {\"pi\", 3.141},\n    {\"happy\", true},\n    {\"name\", \"Niels\"},\n    {\"nothing\", nullptr},\n    {\"list\", {1, 0, 2}},\n    {\"object\", {{\"currency\", \"USD\"}, {\"value\", 42.99}}}\n};\n

    The library decides between an array and an object based on the content: a list whose elements are all two-element lists with a string as the first element is treated as an object, everything else as an array.

    Ambiguous cases: {} vs. []

    Because the same {} syntax is used for both arrays and objects, some cases are ambiguous. To force a particular type, use the explicit factory functions json::array and json::object:

    json empty_array_explicit = json::array();               // []\njson empty_object_explicit = json::object();             // {}\n\n// a JSON array with one object, not an object with one member\njson array_of_objects = json::array({{\"key\", \"value\"}}); // [{\"key\":\"value\"}]\n

    Related to this, single-element brace initialization such as json j{value}; wraps the element in a single-element array by default, and its behavior even differs between compilers. See the FAQ for details and the opt-in JSON_BRACE_INIT_COPY_SEMANTICS macro.

    "},{"location":"features/creating_values/#building-incrementally","title":"Building incrementally","text":"

    A value can also be built up piece by piece. Accessing a non-existing object key or array index with operator[] creates the element on the fly:

    json j;                    // null\nj[\"answer\"][\"everything\"] = 42;   // becomes an object\nj[\"list\"] = {1, 0, 2};\nj[\"list\"].push_back(3);           // [1,0,2,3]\n

    See modifying values for push_back, emplace, and related functions.

    "},{"location":"features/creating_values/#with-the-_json-literal","title":"With the _json literal","text":"

    The _json user-defined literal parses a string at the call site and is a convenient way to write a JSON value inline:

    Example 
    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    json j = R\"( {\"hello\": \"world\", \"answer\": 42} )\"_json;\n\n    std::cout << std::setw(2) << j << '\\n';\n}\n

    Output:

    {\n  \"answer\": 42,\n  \"hello\": \"world\"\n}\n

    Note this parses the string, so \"42\"_json is the number 42, whereas json(\"42\") is the JSON string \"42\".

    "},{"location":"features/creating_values/#see-also","title":"See also","text":"
    • basic_json constructors - all ways to construct a value
    • array / object - force array or object type
    • operator\"\"_json - the _json literal
    • Converting values - which C++ types can be used
    • Parsing - create a value from JSON text
    "},{"location":"features/enum_conversion/","title":"Specializing enum conversion","text":"

    By default, enum values are serialized to JSON as integers. In some cases, this could result in undesired behavior. If the integer values of any enum values are changed after data using those enum values has been serialized to JSON, then deserializing that JSON would result in a different enum value being restored, or the value not being found at all.

    It is possible to more precisely specify how a given enum is mapped to and from JSON as shown below:

    // example enum type declaration\nenum TaskState {\n    TS_STOPPED,\n    TS_RUNNING,\n    TS_COMPLETED,\n    TS_INVALID=-1,\n};\n\n// map TaskState values to JSON as strings\nNLOHMANN_JSON_SERIALIZE_ENUM( TaskState, {\n    {TS_INVALID, nullptr},\n    {TS_STOPPED, \"stopped\"},\n    {TS_RUNNING, \"running\"},\n    {TS_COMPLETED, \"completed\"},\n})\n

    The NLOHMANN_JSON_SERIALIZE_ENUM() macro declares a set of to_json() / from_json() functions for type TaskState while avoiding repetition and boilerplate serialization code.

    "},{"location":"features/enum_conversion/#usage","title":"Usage","text":"

    Serialization converts an enum value to its mapped string, deserialization does the reverse, and an unrecognized JSON value deserializes to the first pair in the map:

    // enum to JSON as string\njson j = TS_STOPPED;\nassert(j == \"stopped\");\n\n// json string to enum\njson j3 = \"running\";\nassert(j3.get<TaskState>() == TS_RUNNING);\n\n// undefined json value to enum (where the first map entry above is the default)\njson jPi = 3.14;\nassert(jPi.get<TaskState>() == TS_INVALID );\n
    Example: serializing/deserializing enums, including a second enum type 
    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nnamespace ns\n{\nenum TaskState\n{\n    TS_STOPPED,\n    TS_RUNNING,\n    TS_COMPLETED,\n    TS_INVALID = -1\n};\n\nNLOHMANN_JSON_SERIALIZE_ENUM(TaskState,\n{\n    { TS_INVALID, nullptr },\n    { TS_STOPPED, \"stopped\" },\n    { TS_RUNNING, \"running\" },\n    { TS_COMPLETED, \"completed\" }\n})\n\nenum class Color\n{\n    red, green, blue, unknown\n};\n\nNLOHMANN_JSON_SERIALIZE_ENUM(Color,\n{\n    { Color::unknown, \"unknown\" }, { Color::red, \"red\" },\n    { Color::green, \"green\" }, { Color::blue, \"blue\" }\n})\n} // namespace ns\n\nint main()\n{\n    // serialization\n    json j_stopped = ns::TS_STOPPED;\n    json j_red = ns::Color::red;\n    std::cout << \"ns::TS_STOPPED -> \" << j_stopped\n              << \", ns::Color::red -> \" << j_red << std::endl;\n\n    // deserialization\n    json j_running = \"running\";\n    json j_blue = \"blue\";\n    auto running = j_running.get<ns::TaskState>();\n    auto blue = j_blue.get<ns::Color>();\n    std::cout << j_running << \" -> \" << running\n              << \", \" << j_blue << \" -> \" << static_cast<int>(blue) << std::endl;\n\n    // deserializing undefined JSON value to enum\n    // (where the first map entry above is the default)\n    json j_pi = 3.14;\n    auto invalid = j_pi.get<ns::TaskState>();\n    auto unknown = j_pi.get<ns::Color>();\n    std::cout << j_pi << \" -> \" << invalid << \", \"\n              << j_pi << \" -> \" << static_cast<int>(unknown) << std::endl;\n}\n

    Output:

    ns::TS_STOPPED -> \"stopped\", ns::Color::red -> \"red\"\n\"running\" -> 1, \"blue\" -> 2\n3.14 -> -1, 3.14 -> 3\n
    "},{"location":"features/enum_conversion/#maps-with-enum-keys","title":"Maps with enum keys","text":"

    By default, maps with enum keys, such as std::map<TaskState, std::string>, are stored as arrays of [key, value] pairs, because JSON object keys must be strings. Define JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS before including the library to store them as objects, with the keys converted by the enum's to_json() function:

    std::map<TaskState, std::string> m = {{TS_STOPPED, \"aa\"}, {TS_COMPLETED, \"bb\"}};\n\njson j = m;\n// default:                                   [[\"stopped\",\"aa\"],[\"completed\",\"bb\"]]\n// with JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS: {\"completed\":\"bb\",\"stopped\":\"aa\"}\n

    Either form can be read back, with or without the macro.

    "},{"location":"features/enum_conversion/#notes","title":"Notes","text":"

    Just as in Arbitrary Type Conversions above,

    • NLOHMANN_JSON_SERIALIZE_ENUM() MUST be declared in your enum type's namespace (which can be the global namespace), or the library will not be able to locate it, and it will default to integer serialization.
    • It MUST be available (e.g., proper headers must be included) everywhere you use the conversions.

    Other Important points:

    • When using get<ENUM_TYPE>(), undefined JSON values will default to the first pair specified in your map. Select this default pair carefully. If you desire an exception in this circumstance use NLOHMANN_JSON_SERIALIZE_ENUM_STRICT() which behaves identically except for throwing an out_of_range.410 exception on unrecognized values, both when serializing an enum value not listed in the map and when deserializing a JSON value that matches none of the map's entries.
    • If an enum or JSON value is specified more than once in your map, the first matching occurrence from the top of the map will be returned when converting to or from JSON.
    • To disable the default serialization of enumerators as integers and force a compiler error instead, see JSON_DISABLE_ENUM_SERIALIZATION.
    Example: NLOHMANN_JSON_SERIALIZE_ENUM_STRICT throwing on unrecognized values 
    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nnamespace ns\n{\n\nenum class Color\n{\n    red,\n    green,\n    blue,\n    unknown // not mapped in JSON_SERIALIZE_ENUM_STRICT\n};\n\nNLOHMANN_JSON_SERIALIZE_ENUM_STRICT(Color,\n{\n    {Color::red, \"red\"},\n    {Color::green, \"green\"},\n    {Color::blue, \"blue\"}\n})\n\n} // namespace ns\n\n\nint main()\n{\n    // invalid serialization\n    try\n    {\n        // ns::color::unknown was not mapped in macro\n        json invalid_serialization = ns::Color::unknown;\n    }\n    catch (const json::exception e)\n    {\n        std::cout << \"deserialization failed: \" << e.what() << std::endl;\n    }\n\n    // invalid deserialization\n    try\n    {\n        // what does not map to an enum\n        json invalid_deserialization(\"what\");\n        ns::Color color = invalid_deserialization.get<ns::Color>();\n    }\n    catch (const json::exception e)\n    {\n        std::cout << \"deserialization failed: \" << e.what() << std::endl;\n    }\n\n    return 0;\n}\n

    Output:

    deserialization failed: [json.exception.out_of_range.410] enum value out of range for Color\ndeserialization failed: [json.exception.out_of_range.410] enum value out of range for Color: \"what\"\n
    "},{"location":"features/iterators/","title":"Iterators","text":""},{"location":"features/iterators/#overview","title":"Overview","text":"

    A basic_json value is a container and allows access via iterators. Depending on the value type, basic_json stores zero or more values.

    As for other containers, begin() returns an iterator to the first value and end() returns an iterator to the value following the last value. The latter iterator is a placeholder and cannot be dereferenced. In case of null values, empty arrays, or empty objects, begin() will return end().

    "},{"location":"features/iterators/#iteration-order-for-objects","title":"Iteration order for objects","text":"

    When iterating over objects, values are ordered with respect to the object_comparator_t type which defaults to std::less. See the types documentation for more information.

    Example: iteration order of object values 
    // create JSON object {\"one\": 1, \"two\": 2, \"three\": 3}\njson j;\nj[\"one\"] = 1;\nj[\"two\"] = 2;\nj[\"three\"] = 3;\n\nfor (auto it = j.begin(); it != j.end(); ++it)\n{\n    std::cout << *it << std::endl;\n}\n

    Output:

    1\n3\n2\n

    The reason for the order is the lexicographic ordering of the object keys \"one\", \"three\", \"two\".

    "},{"location":"features/iterators/#access-object-keys-during-iteration","title":"Access object keys during iteration","text":"

    The JSON iterators have two member functions, key() and value() to access the object key and stored value, respectively. When calling key() on a non-object iterator, an invalid_iterator.207 exception is thrown.

    Example: access object keys with key() and value() 
    // create JSON object {\"one\": 1, \"two\": 2, \"three\": 3}\njson j;\nj[\"one\"] = 1;\nj[\"two\"] = 2;\nj[\"three\"] = 3;\n\nfor (auto it = j.begin(); it != j.end(); ++it)\n{\n    std::cout << it.key() << \" : \" << it.value() << std::endl;\n}\n

    Output:

    one : 1\nthree : 3\ntwo : 2\n
    "},{"location":"features/iterators/#range-based-for-loops","title":"Range-based for loops","text":"

    C++11 allows using range-based for loops to iterate over a container.

    for (auto it : j_object)\n{\n    // \"it\" is of type json::reference and has no key() member\n    std::cout << \"value: \" << it << '\\n';\n}\n

    For this reason, the items() function allows accessing iterator::key() and iterator::value() during range-based for loops. In these loops, a reference to the JSON values is returned, so there is no access to the underlying iterator.

    for (auto& el : j_object.items())\n{\n    std::cout << \"key: \" << el.key() << \", value:\" << el.value() << '\\n';\n}\n

    The items() function also allows using structured bindings (C++17):

    for (auto& [key, val] : j_object.items())\n{\n    std::cout << \"key: \" << key << \", value:\" << val << '\\n';\n}\n

    Note

    When iterating over an array, key() will return the index of the element as string. For primitive types (e.g., numbers), key() returns an empty string.

    Warning

    Using items() on temporary objects is dangerous. Make sure the object's lifetime exceeds the iteration. See #2040 for more information.

    "},{"location":"features/iterators/#reverse-iteration-order","title":"Reverse iteration order","text":"

    rbegin() and rend() return iterators in the reverse sequence.

    Example: reverse iteration with rbegin() and rend() 
    json j = {1, 2, 3, 4};\n\nfor (auto it = j.rbegin(); it != j.rend(); ++it)\n{\n    std::cout << *it << std::endl;\n}\n

    Output:

    4\n3\n2\n1\n
    "},{"location":"features/iterators/#iterating-strings-and-binary-values","title":"Iterating strings and binary values","text":"

    Note that \"value\" means a JSON value in this setting, not values stored in the underlying containers. That is, *begin() returns the complete string or binary array and is also safe if the underlying string or binary array is empty.

    Example: iterate over a string value 
    json j = \"Hello, world\";\nfor (auto it = j.begin(); it != j.end(); ++it)\n{\n    std::cout << *it << std::endl;\n}\n

    Output:

    \"Hello, world\"\n
    "},{"location":"features/iterators/#iterator-invalidation","title":"Iterator invalidation","text":"Operations invalidated iterators clear all"},{"location":"features/json_patch/","title":"JSON Patch and Diff","text":""},{"location":"features/json_patch/#patches","title":"Patches","text":"

    JSON Patch (RFC 6902) defines a JSON document structure for expressing a sequence of operations to apply to a JSON document. Operations address locations in the document using JSON Pointer paths. With the patch function, a JSON Patch is applied to the current JSON value by executing all operations from the patch, yielding the patched document as a new value.

    Applying a patch without copying

    patch leaves the original value unchanged and returns the patched result as a copy. If the document is large and the original value is no longer needed, patch_inplace applies the same operations in place instead.

    Example: apply a JSON Patch

    The following code shows how a JSON patch is applied to a value.

    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // the original document\n    json doc = R\"(\n        {\n          \"baz\": \"qux\",\n          \"foo\": \"bar\"\n        }\n    )\"_json;\n\n    // the patch\n    json patch = R\"(\n        [\n          { \"op\": \"replace\", \"path\": \"/baz\", \"value\": \"boo\" },\n          { \"op\": \"add\", \"path\": \"/hello\", \"value\": [\"world\"] },\n          { \"op\": \"remove\", \"path\": \"/foo\"}\n        ]\n    )\"_json;\n\n    // apply the patch\n    json patched_doc = doc.patch(patch);\n\n    // output original and patched document\n    std::cout << std::setw(4) << doc << \"\\n\\n\"\n              << std::setw(4) << patched_doc << std::endl;\n}\n

    Output:

    {\n    \"baz\": \"qux\",\n    \"foo\": \"bar\"\n}\n\n{\n    \"baz\": \"boo\",\n    \"hello\": [\n        \"world\"\n    ]\n}\n
    "},{"location":"features/json_patch/#diff","title":"Diff","text":"

    The library can also calculate a JSON patch (i.e., a diff) given two JSON values with the diff function.

    flowchart LR\n    S[\"source\"] -->|\"diff(source, target)\"| P[\"patch\"]\n    S -->|\"source.patch(patch)\"| T[\"target\"]\n    P -.->|\"applied to source, yields\"| T

    Invariant

    For two JSON values source and target, the following code yields always true:

    source.patch(diff(source, target)) == target;\n
    Example: create a JSON Patch from the difference of two values

    The following code shows how a JSON patch is created as a diff for two JSON values.

    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // the source document\n    json source = R\"(\n        {\n            \"baz\": \"qux\",\n            \"foo\": \"bar\"\n        }\n    )\"_json;\n\n    // the target document\n    json target = R\"(\n        {\n            \"baz\": \"boo\",\n            \"hello\": [\n                \"world\"\n            ]\n        }\n    )\"_json;\n\n    // create the patch\n    json patch = json::diff(source, target);\n\n    // roundtrip\n    json patched_source = source.patch(patch);\n\n    // output patch and roundtrip result\n    std::cout << std::setw(4) << patch << \"\\n\\n\"\n              << std::setw(4) << patched_source << std::endl;\n}\n

    Output:

    [\n    {\n        \"op\": \"replace\",\n        \"path\": \"/baz\",\n        \"value\": \"boo\"\n    },\n    {\n        \"op\": \"remove\",\n        \"path\": \"/foo\"\n    },\n    {\n        \"op\": \"add\",\n        \"path\": \"/hello\",\n        \"value\": [\n            \"world\"\n        ]\n    }\n]\n\n{\n    \"baz\": \"boo\",\n    \"hello\": [\n        \"world\"\n    ]\n}\n
    "},{"location":"features/json_patch/#see-also","title":"See also","text":"
    • JSON Pointer - the addressing scheme used for patch paths
    • JSON Merge Patch - a simpler, less expressive alternative patch format
    • patch - apply a JSON Patch, returning the result as a copy
    • patch_inplace - apply a JSON Patch without copying
    • diff - compute a JSON Patch from two values
    "},{"location":"features/json_pointer/","title":"JSON Pointer","text":""},{"location":"features/json_pointer/#introduction","title":"Introduction","text":"

    The library supports JSON Pointer (RFC 6901) as an alternative means to address structured values. A JSON Pointer is a string that identifies a specific value within a JSON document.

    Consider the following JSON document

    {\n    \"array\": [\"A\", \"B\", \"C\"],\n    \"nested\": {\n        \"one\": 1,\n        \"two\": 2,\n        \"three\": [true, false]\n    }\n}\n

    Then every value inside the JSON document can be identified as follows:

    JSON Pointer JSON value `` {\"array\":[\"A\",\"B\",\"C\"],\"nested\":{\"one\":1,\"two\":2,\"three\":[true,false]}} /array [\"A\",\"B\",\"C\"] /array/0 A /array/1 B /array/2 C /nested {\"one\":1,\"two\":2,\"three\":[true,false]} /nested/one 1 /nested/two 2 /nested/three [true,false] /nested/three/0 true /nested/three/1 false

    Note / does not identify the root (i.e., the whole document), but an object entry with empty key \"\". See RFC 6901 for more information.

    "},{"location":"features/json_pointer/#json-pointer-creation","title":"JSON Pointer creation","text":"

    JSON Pointers can be created from a string:

    json::json_pointer p(\"/nested/one\");\n

    Furthermore, a user-defined string literal can be used to achieve the same result:

    auto p = \"/nested/one\"_json_pointer;\n

    The escaping rules of RFC 6901 are implemented. See the constructor documentation for more information.

    "},{"location":"features/json_pointer/#value-access","title":"Value access","text":"

    JSON Pointers can be used in the at, operator[], and value functions just like object keys or array indices.

    // the JSON value from above\nauto j = json::parse(R\"({\n    \"array\": [\"A\", \"B\", \"C\"],\n    \"nested\": {\n        \"one\": 1,\n        \"two\": 2,\n        \"three\": [true, false]\n    }\n})\");\n\n// access values\nauto val = j[\"\"_json_pointer];                              // {\"array\":[\"A\",\"B\",\"C\"],...}\nauto val1 = j[\"/nested/one\"_json_pointer];                  // 1\nauto val2 = j.at(json::json_pointer(\"/nested/three/1\"));    // false\nauto val3 = j.value(json::json_pointer(\"/nested/four\"), 0); // 0\n

    To read a value with a fallback, use value with a JSON Pointer; to test for existence, use contains. Neither needs intermediate checks, see Nested values.

    Creating intermediate levels that don't exist

    See the operator[] notes for how array vs. object is decided when a pointer creates intermediate levels that don't exist yet.

    "},{"location":"features/json_pointer/#flatten-unflatten","title":"Flatten / unflatten","text":"

    The library implements a function flatten to convert any JSON document into a JSON object where each key is a JSON Pointer and each value is a primitive JSON value (i.e., a string, boolean, number, or null).

    // the JSON value from above\nauto j = json::parse(R\"({\n    \"array\": [\"A\", \"B\", \"C\"],\n    \"nested\": {\n        \"one\": 1,\n        \"two\": 2,\n        \"three\": [true, false]\n    }\n})\");\n\n// create flattened value\nauto j_flat = j.flatten();\n

    The resulting value j_flat is:

    {\n  \"/array/0\": \"A\",\n  \"/array/1\": \"B\",\n  \"/array/2\": \"C\",\n  \"/nested/one\": 1,\n  \"/nested/two\": 2,\n  \"/nested/three/0\": true,\n  \"/nested/three/1\": false\n}\n

    The reverse function, unflatten recreates the original value.

    auto j_original = j_flat.unflatten();\n
    "},{"location":"features/json_pointer/#see-also","title":"See also","text":"
    • Class json_pointer
    • Functions value, contains, and at accept JSON Pointers; see Nested values
    • Function flatten
    • Function unflatten
    • JSON Patch - paths inside a patch are JSON Pointers
    • JSON Merge Patch - an alternative patch format that does not use JSON Pointer
    "},{"location":"features/json_view/","title":"Zero-copy JSON views","text":"

    <nlohmann/json_view.hpp> adds a read-only, non-owning way to look at a parsed JSON text, as an alternative to building a basic_json tree with parse().

    "},{"location":"features/json_view/#the-problem","title":"The problem","text":"

    basic_json::parse() builds a tree of basic_json values: one allocation for every array and object, and every string copied into its own std::string. That is the right trade-off when the program goes on to read and write the value freely, but it does more work than necessary when only a small part of a large JSON text is actually needed, or when the same text is parsed over and over (many small messages, for instance) and most of the resulting tree is thrown away almost immediately.

    "},{"location":"features/json_view/#the-idea","title":"The idea","text":"

    basic_json_document::parse() parses the same JSON grammar, with the same options, but instead of a tree it builds a flat index of the values it found: one 16-byte entry per value (and one per object key), in document order. Integers are converted while parsing and stored in their entry. Floating-point numbers and strings are referenced in place in the source text: a float is converted when it is read, and only strings that contain escape sequences are decoded, into one shared buffer owned by the document.

    basic_json_view is a small, trivially copyable handle (two pointers) into that index. It gives you the read-only, type-inspection part of the basic_json interface -- type() and the is_*() predicates, size()/empty() -- as well as element access (operator[], at, front/back), lookup (find, contains, count), and iteration (begin/end, items) -- without ever allocating a basic_json value. When you do need an actual basic_json value for a subtree, materialize() builds exactly the one parse() would have produced for it.

    "},{"location":"features/json_view/#how-to-use-it","title":"How to use it","text":"

    Include <nlohmann/json_view.hpp> in addition to (or instead of) <nlohmann/json.hpp>. Parse into a json_document, inspect its root(), and materialize() when you need a real value. materialize() works on any view, not just the root, so you can turn just the subtree you need into a value:

    Example: parse a document, inspect its root, and materialize it 
    #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // json_document is basic_json_document<nlohmann::json>: same value types\n    // and containers as the ordinary json specialization\n    json_document doc = json_document::parse(R\"({\"pi\": 3.14, \"numbers\": [1, 2, 3]})\");\n\n    std::cout << doc.root().is_object() << '\\n';\n    std::cout << doc.root().materialize().dump() << '\\n';\n}\n

    Output:

    true\n{\"numbers\":[1,2,3],\"pi\":3.14}\n

    ordered_json_document is the equivalent for ordered_json, just as ordered_json is to json.

    "},{"location":"features/json_view/#ownership-and-lifetime","title":"Ownership and lifetime","text":"

    A document either borrows the text it was parsed from, or owns its own copy of it; call owns_source() to find out which happened. parse() decides this from the value category and type of its argument (an lvalue std::string is borrowed; an rvalue std::string is moved in, owned without a copy; a stream is read into an owned buffer; and so on -- see parse's Notes for the full table). parse_copy() always owns a copy, regardless of the input.

    A borrowed document depends on your buffer

    If a document borrows its text, that text must outlive the document (and every view taken from it) and must stay unmodified for as long as the document exists, since floating-point values and unescaped strings are read from it lazily. Reading or writing through a view after the underlying buffer is gone is undefined behavior, exactly as it would be for a dangling std::string_view.

    A view is valid only while all of the following hold:

    • the document is alive,
    • the document has not been re-parsed since the view was taken (with read() or parse() into it),
    • the document has not had shrink_to_fit() called on it since, and
    • if the document borrows its source text, that text is still alive and unmodified.

    Moving the basic_json_document<> itself is fine and does not invalidate its views: the index is a separate heap allocation that keeps its address across the move. Take a fresh view from root() whenever any of the other conditions above was not met.

    A view can dangle, exactly like a std::string_view: nothing stops you from using it after its document is gone. This is deliberate. A view is two pointers and trivially copyable; making it safe (a std::shared_ptr in the document and a std::weak_ptr in each view) would make it larger, add reference counting to every copy, and a lock() to every access -- exactly the costs the view exists to avoid. What is caught is the obvious case: because a view dies with its document, root() is not callable on a temporary document: auto v = json_document::parse(text).root(); does not compile. Give the document a name first.

    Example: borrowed and owned documents, and when views become invalid 
    #include <iostream>\n#include <string>\n#include <utility>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // BORROWED: doc only points into `text`; `text` must outlive `doc`\n    std::string text = R\"({\"a\": 1})\";\n    json_document doc = json_document::parse(text);\n    std::cout << doc.owns_source() << '\\n'; // false\n\n    // a view is valid as long as the document is alive, has not been\n    // re-parsed (read()) or shrunk (shrink_to_fit()), and -- if borrowed --\n    // the source text is alive\n    nlohmann::json_view v = doc.root();\n    std::cout << v.is_object() << '\\n';\n\n    // re-parsing the SAME document invalidates views taken before the call;\n    // `v` above must not be used after this line\n    doc.read(R\"([1, 2, 3])\");\n    v = doc.root(); // take a fresh view instead\n    std::cout << v.is_array() << '\\n';\n\n    // moving the document does not invalidate views: the node index is\n    // heap-allocated and does not move with the document object\n    json_document moved = std::move(doc);\n    std::cout << v.is_array() << '\\n';\n}\n

    Output:

    false\ntrue\ntrue\ntrue\n
    "},{"location":"features/json_view/#what-is-the-same-as-parse","title":"What is the same as parse()","text":"
    • Accept/reject. accept() and parse() accept and reject exactly the same inputs as basic_json::accept()/basic_json::parse(), with the same ignore_comments and ignore_trailing_commas options.
    • Errors. A failing parse throws the same exception -- the same id, message, and position -- because on failure the library's own parser is run on the same bytes to produce the diagnostic. Valid input never pays for this, and the replay builds no values: it costs about a lexing pass up to the error. allow_exceptions == false skips the replay and gives a discarded document instead of throwing, just as it gives a discarded value for basic_json::parse().
    • Number classification. An integer literal that does not fit into the 64-bit integer type becomes a floating-point value, exactly as it does for basic_json::parse().
    • Macros. JSON_STRICT_NUL_HANDLING and JSON_NOEXCEPTION/JSON_THROW_USER behave the same way they do for <nlohmann/json.hpp>. These are the macros that change what the parser accepts or how it reports errors. The others either do not apply (JSON_DIAGNOSTICS needs parent pointers in basic_json values) or only affect the values materialize() produces, through BasicJsonType.
    "},{"location":"features/json_view/#what-is-different","title":"What is different","text":"
    • Only 64-bit integers. basic_json_document<BasicJsonType> requires BasicJsonType::number_integer_t and number_unsigned_t to both be 64 bits wide; this is a compile-time static_assert.
    • A 4 GiB input limit. An input of 4294967280 bytes (4 GiB minus 16 bytes) or more throws out_of_range.416, a limit basic_json::parse() does not have. It keeps an index entry at 16 bytes: offsets and counts are 32 bits wide. The limit is 16 bytes below 2^32, because the parser reads in 16-byte steps, and a position one step past the end must still fit.
    • A stream is always read to its end. There is no partial/streaming read of an std::istream: the stream is read to its end into a buffer the document owns, and parsing starts after that.
    • No source positions on materialize(). Even with JSON_DIAGNOSTIC_POSITIONS enabled, materialize() does not set them. The index has the start offset of every value, but not the end of arrays, objects, and escaped strings, which the positions also need.
    • Objects iterate in document order. begin()/ end() and items() visit an object's members in the order they appear in the source text. basic_json's default object_t is a std::map, which sorts by key, so iterating a materialize()d value can print members in a different order than iterating the view they came from.
    • Chained access is safe. operator[] with a missing key, an index out of range, or an unresolvable JSON pointer returns a discarded view, and operator[] on a discarded view returns a discarded view without throwing: v[\"a\"][\"b\"][0] can be tested once at the end. Type errors on values that exist (a key on an array, an index on an object) still throw, and at throws for every missing value.
    • Duplicate keys: lookups find the first member, parse() keeps the last. See Duplicate keys below.
    • No JSON_DIAGNOSTICS path. Exceptions thrown by basic_json_view's own element access and lookup functions never carry the JSON Pointer path JSON_DIAGNOSTICS would otherwise add: the view has no basic_json value to point at, so the exception is created without one, regardless of how BasicJsonType was built.
    • Ordering comparisons are not provided by basic_json_view -- there is no operator<. operator== and operator!= are provided, though: two views, or a view and a BasicJsonType value, compare equal exactly when materialize() or parse() would produce equal values for them, without ever building a tree to do it. For ordering, too, materialize() is the way to get a value you can compare.
    "},{"location":"features/json_view/#duplicate-keys","title":"Duplicate keys","text":"

    A lookup in a view and in the parsed value can give different answers

    If an object in the source text repeats a key, operator[], at, find, value, contains, and JSON pointer resolution all return the first member with that key. basic_json::parse() instead keeps the last value of a repeated key, so for {\"a\":1,\"a\":2}, view[\"a\"] is 1 while parse(text)[\"a\"] is 2. count counts every member with the key, so it is 2 here.

    begin()/end() and items() visit every occurrence, in document order, and size() counts all of them, so view.size() can be larger than view.materialize().size().

    Why the first? A lookup can stop as soon as it finds a match. Returning the last member would force every lookup to scan all members of the object, even when the key is found at the very first one: this made lookups in small objects 1.6 to 3.4 times slower. The hash index of the keys of objects with 128 members or more leads to the first member of a key as well. Other zero-copy parsers that index the source text, such as yyjson and simdjson, also return the first member. RFC 8259 only says that names within an object SHOULD be unique and that the behavior of a receiver that sees duplicates is unpredictable, so neither choice is wrong.

    What stays the same as parse()? materialize() and get<std::map<...>>() replay every member in order, so they keep the last value exactly like basic_json::parse() (at the position of the first occurrence of the key, for an ordered_json).

    How do I get the value parse() would give? Either call view.materialize() and look the key up in the result, or iterate the members and keep the last match:

    // the last member with the key \"a\", as parse() would keep it\njson_view last;\nfor (const auto item : view.items())\n{\n    if (item.key() == \"a\")\n    {\n        last = item.value();\n    }\n}\n

    If you cannot trust the source text to have unique keys, check size() against the number of distinct keys, or reject duplicates when iterating.

    "},{"location":"features/json_view/#getting-values-out-without-copying","title":"Getting values out without copying","text":"

    get<T>() converts many T directly from the flat index, without ever building a basic_json value for the conversion: bool, arithmetic types, std::nullptr_t, std::string/other std::basic_strings (copied once), basic_json/ordered_json (via materialize()), basic_json_view itself, std::vector<U>, and std::map/std::unordered_map with string-like keys. Every other type -- std::list, std::pair, std::array, enumerations, user types with a from_json() -- goes through materialize().get<T>() instead: the subtree is built into a real basic_json value first, exactly as parse() would, and converted from there.

    Two conversions never copy at all:

    • get_string() (equivalently, get<string_view_t>()) returns a string as a string_view_t pointing into the document's source() text -- or, for a string that contains escape sequences, into the document's own buffer of decoded strings -- instead of allocating a new std::string.
    • number_token() returns a number exactly as it was written in the source, e.g. \"1.50\", \"1E2\", or an integer with more digits than any number type holds, instead of rounding it into a double/int64_t the way get<T>() (and basic_json::parse()) would.

    Both results are only valid as long as the view -- and, for a string with no escapes, the borrowed source text -- is.

    "},{"location":"features/json_view/#writing-a-view-back","title":"Writing a view back","text":"

    dump() serializes a view directly from the flat index, without ever building a basic_json value. An object's members are written in document order, not sorted by key, and every occurrence of a repeated key is written, not only the last one -- the same two ways iteration already differs from a materialize()d value, see Duplicate keys. materialize().dump() gives a different result in both respects for a json_view.

    By default, numbers are written the way basic_json::dump() would. number_format::source instead copies every number exactly as it was written in the source text -- a price like 19.90, a long order or account ID with more digits than any number type holds, or a high-precision coordinate -- something basic_json cannot do at all, since parsing already reduces a number to its parsed double/int64_t value.

    operator<< writes a view to a stream the way basic_json's does, using the stream's width/fill for indentation.

    "},{"location":"features/json_view/#editing-a-document","title":"Editing a document","text":"

    Everything above is read-only: a json_document/json_view lets you look at a parsed text without copying it, but not change it. basic_json_document<BasicJsonType, true> -- more conveniently spelled json_editable_document or ordered_json_editable_document -- also lets you set a value, push_back onto or insert into an array, and erase an object member or an array element, still without ever building a basic_json tree for parts you do not touch.

    Editable defaults to false, so json_document/ordered_json_document are unaffected -- they carry none of the bookkeeping edits need, and calling set/push_back/insert/erase on one is a compile error, not a runtime one.

    "},{"location":"features/json_view/#why-editing-without-reformatting","title":"Why: editing without reformatting","text":"

    The 19.90 price from above is exactly the kind of value that makes editing a json or ordered_json value in place lossy. Say you parse a configuration file, patch one field, and write it back:

    • json re-sorts every key on the way in (object_t is a std::map) and rewrites every number to its shortest round-trip form on the way out -- a one-field patch turns into a diff that reorders the whole file and rewrites 19.90 to 19.9.
    • ordered_json keeps the key order, but still rewrites every number the same way: parsing has already reduced it to a double/int64_t, and there is no way back to how it was spelled in the source text.

    An editable document keeps both. dump() of an edited document writes members in document order -- a member set added goes at the end, exactly where it was inserted, and an erased member simply leaves a gap: everything around it keeps its place -- and number_format::source keeps the exact spelling of every number an edit did not itself touch; a number an edit did touch is written the way BasicJsonType::dump() would write it, since there is no source spelling for a brand new value.

    Example: patch a configuration, keeping member order and number spellings 
    #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json = nlohmann::json;\nusing json_editable_document = nlohmann::json_editable_document;\nusing json_editable_view = nlohmann::json_editable_view;\n\nint main()\n{\n    // a configuration file, as it might be read from disk\n    const std::string text = R\"({\"name\": \"cache\", \"host\": \"db1\", \"port\": 6379, \"price\": 19.90})\";\n    std::cout << text << \"\\n\\n\";\n\n    // patch two fields -- \"price\" is never touched\n    json_editable_document doc = json_editable_document::parse(text);\n    doc.set(doc.root(), \"host\", \"db2\");\n    doc.set(doc.root(), \"retries\", 3);\n\n    // member order (the new member at the end) and the untouched number's\n    // exact spelling survive\n    std::cout << doc.root().dump(-1, ' ', false, json_editable_view::number_format::source) << '\\n';\n\n    // the same patch on a plain json value: keys are sorted (object_t is a\n    // std::map), and \"price\" is rewritten even though the patch never\n    // touched it\n    json plain = json::parse(text);\n    plain[\"host\"] = \"db2\";\n    plain[\"retries\"] = 3;\n    std::cout << plain.dump() << '\\n';\n}\n

    Output:

    {\"name\": \"cache\", \"host\": \"db1\", \"port\": 6379, \"price\": 19.90}\n\n{\"name\":\"cache\",\"host\":\"db2\",\"port\":6379,\"price\":19.90,\"retries\":3}\n{\"host\":\"db2\",\"name\":\"cache\",\"port\":6379,\"price\":19.9,\"retries\":3}\n
    "},{"location":"features/json_view/#what-stays-valid-and-what-an-edit-costs","title":"What stays valid, and what an edit costs","text":"

    The source text itself is never written, and the parsed index never moves -- a value keeps the node it was parsed into for as long as it is not itself replaced. So every view taken before an edit, including a previously obtained root(), stays valid and, if it still refers to the edited value, sees the edit; a view of a value a later edit drops or replaces just keeps showing what it last held. New values go to storage the document allocates and owns on demand. The one thing an edit does invalidate is the iterators taken over an edited array or object: the first time one of its elements is set, appended to, inserted into, or erased, its elements move from the parsed, fixed layout to a growable block of links so that push_back can later grow it in amortized constant time -- existing elements are not touched, but an iterator that was walking the old layout no longer matches. A string obtained with get_string() is unaffected either way and stays valid across further edits. See basic_json_document's Edits for the details, and set's Exception safety for what an edit guarantees if it throws (the basic guarantee, not the strong one dump() and the read-only functions provide). How edits are kept in the index is described in the architecture overview.

    "},{"location":"features/json_view/#images","title":"Images","text":"

    save() writes a document as an image: a byte buffer that load() reads back into a document without parsing -- no lexing, no building the node index, nothing but copying the nodes and pointing the text and the decoded strings at the image. Where parse_copy() still has to scan the whole input, load() turns that scan into a copy of the node index alone.

    Why. A document that is parsed once and then read many times -- a configuration loaded at startup, a template rendered on every request, a large reference dataset a worker process needs in memory -- pays for parsing once but can amortize save()'s cost across every later load. That makes images useful for a cache: save a document the first time it is parsed (to a file, a shared-memory segment, an in-process cache), and load() it on every later use instead of parsing the source text again. They are just as useful for handing a parsed document to another process (or a forked worker) running the same build of the library, since load() turns the transfer into a copy of the node index plus pointers into the received bytes, not a re-parse.

    Choosing a check. load() takes an image_check that trades validation against speed: image_check::full (the default) checks everything the parser itself guarantees, so a checked image is exactly as safe to read and serialize as a freshly parsed document -- the right choice whenever the image did not come straight from this process's own save(), such as a file or a network peer. image_check::bounds only checks structure and bounds -- cheaper, since it skips scanning the text and the decoded strings -- and fits a cache the process trusts, one it wrote and reads back itself. image_check::none skips validation entirely, for an image trusted as much as the process's own memory. See load()'s Notes for exactly what each level does and does not guarantee.

    Example: cache a parsed configuration as an image 
    #include <cstdint>\n#include <iostream>\n#include <string>\n#include <vector>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // a configuration a service parses once and then caches as an image, so\n    // that later requests can load() it instead of parsing the text again\n    const std::string text = R\"({\"name\": \"cache\", \"host\": \"db1\", \"port\": 6379, \"replicas\": [\"db2\", \"db3\"]})\";\n    const json_document config = json_document::parse(text);\n\n    // save() turns the parsed document into a byte buffer: a 64-byte header,\n    // the node index, the source text, and the decoded strings\n    const std::vector<std::uint8_t> image = config.save();\n    std::cout << image.size() << '\\n';\n\n    // the same document always saves to the same bytes\n    std::cout << (image == json_document::parse(text).save()) << '\\n';\n\n    // loading the image back needs no parsing, yet dumps exactly what\n    // parsing the text produced\n    const json_document reloaded = json_document::load(image);\n    std::cout << (reloaded.root().dump() == config.root().dump()) << '\\n';\n}\n

    Output:

    316\ntrue\ntrue\n

    Experimental

    The image format is versioned but not yet stable, and may change in an incompatible way before it is declared stable. It is little-endian only, and tied to the library build that wrote it -- use it to cache a document or to hand one to another process running the same build, not as a long-term storage format; keep the original JSON text if a saved document needs to be readable by a future library version.

    The idea of a document you can read without parsing comes from zero-copy formats such as FlatBuffers and YaFF; the check load() runs follows the idea of FlatBuffers' Verifier. No code is taken from either.

    "},{"location":"features/json_view/#choosing-between-json-ordered_json-the-sax-interface-and-json_view","title":"Choosing between json, ordered_json, the SAX interface, and json_view","text":"json / ordered_json SAX interface json_document / json_view json_editable_document / json_editable_view Ownership owns every value owns nothing; you decide what to keep, in your handler borrows or owns the text; the index is always owned by the document same as json_document; edits go to storage the document owns Mutability freely mutable not applicable (a one-shot event stream) read-only set/push_back/insert/erase edit in place; the source text is never rewritten What you get a full tree you can read, write, and keep as long as you like a sequence of callbacks; whatever your handler builds from them a flat index plus, on demand, materialize()d json/ordered_json values for the parts you actually use the same, plus dump() of an edited document that keeps the member order and, with number_format::source, the spelling of every untouched number Typical use general-purpose JSON handling: config, request/response bodies you build or modify, anything you hold onto validating or projecting a text into your own data structure without ever holding the whole thing as JSON large or high-volume input where you only need part of it, or need it repeatedly, and can keep the source text (or a copy) alive for as long as the document lives a document you read, patch a few fields of, and write back -- a configuration file, for instance -- where the rest of it should come back exactly as it was Caching/reload not applicable -- re-parse, or roll your own serialization not applicable save()/load(): cache the parsed index as an image and reload it without parsing same, saving the document's current -- possibly edited -- state"},{"location":"features/json_view/#version-history","title":"Version history","text":"
    • Added in version 3.13.0 unreleased.
    "},{"location":"features/macros/","title":"Supported Macros","text":"

    Some aspects of the library can be configured by defining preprocessor macros before including the json.hpp header. See also the API documentation for macros for examples and more information.

    "},{"location":"features/macros/#json_assertx","title":"JSON_ASSERT(x)","text":"

    This macro controls which code is executed for runtime assertions of the library.

    See full documentation of JSON_ASSERT(x).

    "},{"location":"features/macros/#json_brace_init_copy_semantics","title":"JSON_BRACE_INIT_COPY_SEMANTICS","text":"

    When defined to 1, single-element brace initialization of a basic_json value (e.g., json j{value};) is treated as a copy/move of the element rather than wrapping it in a single-element array. The default value is 0, which preserves the existing behavior.

    See full documentation of JSON_BRACE_INIT_COPY_SEMANTICS.

    "},{"location":"features/macros/#json_catch_userexception","title":"JSON_CATCH_USER(exception)","text":"

    This macro overrides catch calls inside the library.

    See full documentation of JSON_CATCH_USER(exception).

    "},{"location":"features/macros/#json_delete_deprecated_functions","title":"JSON_DELETE_DEPRECATED_FUNCTIONS","text":"

    When defined to 1, all deprecated functions are declared as deleted instead of only being marked as deprecated, so code that still calls them no longer compiles. This way, you can find all calls that need to be replaced before version 4.0.0 removes these functions.

    The macro can also be set with the CMake option JSON_DeleteDeprecatedFunctions (OFF by default).

    See full documentation of JSON_DELETE_DEPRECATED_FUNCTIONS.

    "},{"location":"features/macros/#json_diagnostics","title":"JSON_DIAGNOSTICS","text":"

    This macro enables extended diagnostics for exception messages. Possible values are 1 to enable or 0 to disable (default).

    When enabled, exception messages contain a JSON Pointer to the JSON value that triggered the exception, see Extended diagnostic messages for an example. Note that enabling this macro increases the size of every JSON value by one pointer and adds some runtime overhead.

    The diagnostics messages can also be controlled with the CMake option JSON_Diagnostics (OFF by default) which sets JSON_DIAGNOSTICS accordingly.

    See full documentation of JSON_DIAGNOSTICS.

    "},{"location":"features/macros/#json_diagnostic_positions","title":"JSON_DIAGNOSTIC_POSITIONS","text":"

    When enabled, two new member functions start_pos() and end_pos() are added to basic_json values. If the value was created by calling theparse function, then these functions allow querying the byte positions of the value in the input it was parsed from. The byte positions are also used in exceptions to help locate errors.

    The diagnostics positions can also be controlled with the CMake option JSON_Diagnostic_Positions (OFF by default) which sets JSON_DIAGNOSTIC_POSITIONS accordingly.

    See full documentation of JSON_DIAGNOSTIC_POSITIONS

    "},{"location":"features/macros/#json_has_cpp_11-json_has_cpp_14-json_has_cpp_17-json_has_cpp_20-json_has_cpp_23-json_has_cpp_26","title":"JSON_HAS_CPP_11, JSON_HAS_CPP_14, JSON_HAS_CPP_17, JSON_HAS_CPP_20, JSON_HAS_CPP_23, JSON_HAS_CPP_26","text":"

    The library targets C++11, but also supports some features introduced in later C++ versions (e.g., std::string_view support for C++17). For these new features, the library implements some preprocessor checks to determine the C++ standard. By defining any of these symbols, the internal check is overridden and the provided C++ version is unconditionally assumed. This can be helpful for compilers that only implement parts of the standard and would be detected incorrectly.

    See full documentation of JSON_HAS_CPP_11, JSON_HAS_CPP_14, JSON_HAS_CPP_17, JSON_HAS_CPP_20, JSON_HAS_CPP_23, and JSON_HAS_CPP_26.

    "},{"location":"features/macros/#json_has_filesystem-json_has_experimental_filesystem","title":"JSON_HAS_FILESYSTEM, JSON_HAS_EXPERIMENTAL_FILESYSTEM","text":"

    When compiling with C++17, the library provides conversions from and to std::filesystem::path. As compiler support for filesystem is limited, the library tries to detect whether <filesystem>/std::filesystem (JSON_HAS_FILESYSTEM) or <experimental/filesystem>/std::experimental::filesystem (JSON_HAS_EXPERIMENTAL_FILESYSTEM) should be used. To override the built-in check, define JSON_HAS_FILESYSTEM or JSON_HAS_EXPERIMENTAL_FILESYSTEM to 1.

    See full documentation of JSON_HAS_FILESYSTEM and JSON_HAS_EXPERIMENTAL_FILESYSTEM.

    "},{"location":"features/macros/#json_has_ranges","title":"JSON_HAS_RANGES","text":"

    The library uses std::ranges (and concepts) where available, for example, to parse from C++20 ranges and to construct JSON arrays from range views. The library detects whether the standard library supports ranges and disables the support on toolchains with an incomplete implementation. To override the built-in check, define JSON_HAS_RANGES to 1 or 0.

    See full documentation of JSON_HAS_RANGES.

    "},{"location":"features/macros/#json_has_range_view_conversion","title":"JSON_HAS_RANGE_VIEW_CONVERSION","text":"

    When JSON_HAS_RANGES is enabled (and the compiler is not MinGW), a JSON array can be constructed directly from a C++20 range view such as std::views::filter(...). To override the built-in check, define JSON_HAS_RANGE_VIEW_CONVERSION to 1 or 0.

    See full documentation of JSON_HAS_RANGE_VIEW_CONVERSION.

    "},{"location":"features/macros/#json_has_static_rtti","title":"JSON_HAS_STATIC_RTTI","text":"

    The library detects whether the compiler supports run time type information (RTTI), which it needs, for instance, to exclude std::any from the candidate types of the implicit conversion on C++17. To override the built-in check, define JSON_HAS_STATIC_RTTI to 1 or 0.

    See full documentation of JSON_HAS_STATIC_RTTI.

    "},{"location":"features/macros/#json_has_std_format","title":"JSON_HAS_STD_FORMAT","text":"

    When compiling with C++20 and a standard library that provides <format>, the library provides a std::formatter specialization for JSON values. To override the built-in check, define JSON_HAS_STD_FORMAT to 1 or 0.

    See full documentation of JSON_HAS_STD_FORMAT.

    "},{"location":"features/macros/#json_has_three_way_comparison","title":"JSON_HAS_THREE_WAY_COMPARISON","text":"

    When the compiler and standard library support 3-way comparison (the spaceship operator <=>), the library provides it for JSON values. To override the built-in check, define JSON_HAS_THREE_WAY_COMPARISON to 1 or 0.

    See full documentation of JSON_HAS_THREE_WAY_COMPARISON.

    "},{"location":"features/macros/#json_noexception","title":"JSON_NOEXCEPTION","text":"

    Exceptions can be switched off by defining the symbol JSON_NOEXCEPTION.

    See full documentation of JSON_NOEXCEPTION.

    "},{"location":"features/macros/#json_disable_enum_serialization","title":"JSON_DISABLE_ENUM_SERIALIZATION","text":"

    When defined, default parse and serialize functions for enums are excluded and have to be provided by the user, for example, using NLOHMANN_JSON_SERIALIZE_ENUM.

    See full documentation of JSON_DISABLE_ENUM_SERIALIZATION.

    "},{"location":"features/macros/#json_disable_stream_bulk_scan","title":"JSON_DISABLE_STREAM_BULK_SCAN","text":"

    When defined to 1, parsing from a std::istream reads one character at a time instead of scanning the characters that the stream buffer holds in bulk. Results are the same either way; the macro is an escape hatch for stream buffers that do not follow the usual std::streambuf contract. The default value is 0.

    See full documentation of JSON_DISABLE_STREAM_BULK_SCAN.

    "},{"location":"features/macros/#json_disable_tuple_reference_conversion","title":"JSON_DISABLE_TUPLE_REFERENCE_CONVERSION","text":"

    When defined to 1, a JSON value can no longer be created from a one-element std::tuple holding a reference to a JSON value, such as the result of std::forward_as_tuple(j). This lets std::tuple convert such tuples element-wise. This is planned to become the default in version 4.0.0.

    See full documentation of JSON_DISABLE_TUPLE_REFERENCE_CONVERSION.

    "},{"location":"features/macros/#json_disable_unlocked_file_io","title":"JSON_DISABLE_UNLOCKED_FILE_IO","text":"

    When defined to 1, parsing from a FILE* reads every character with std::fgetc, which locks the file each time, instead of locking the file once and reading it with the unlocked stdio functions. Results are the same either way. The default value is 0.

    See full documentation of JSON_DISABLE_UNLOCKED_FILE_IO.

    "},{"location":"features/macros/#json_no_automatic_udls","title":"JSON_NO_AUTOMATIC_UDLS","text":"

    When defined, <nlohmann/json.hpp> does not include <nlohmann/json_literals.hpp> with the user-defined string literals operator\"\"_json and operator\"\"_json_pointer. This reduces the compile time of translation units that do not use them, because the literals instantiate the parser in every translation unit that includes them. Include <nlohmann/json_literals.hpp> where the literals are needed.

    See full documentation of JSON_NO_AUTOMATIC_UDLS.

    "},{"location":"features/macros/#json_no_io","title":"JSON_NO_IO","text":"

    When defined, headers <cstdio>, <ios>, <iosfwd>, <istream>, and <ostream> are not included and parse functions relying on these headers are excluded. This is relevant for environment where these I/O functions are disallowed for security reasons (e.g., Intel Software Guard Extensions (SGX)).

    See full documentation of JSON_NO_IO.

    "},{"location":"features/macros/#json_no_thread_local","title":"JSON_NO_THREAD_LOCAL","text":"

    When defined, the library does not use thread_local storage. Copying a value and comparing two values then always avoid the call stack rather than descending into a bounded number of levels first, which is slower but yields the same values and the same comparisons.

    See full documentation of JSON_NO_THREAD_LOCAL.

    "},{"location":"features/macros/#json_precise_stream_position","title":"JSON_PRECISE_STREAM_POSITION","text":"

    When defined to 1, operator>> and non-strict sax_parse leave an input stream positioned right after the parsed value, instead of also consuming the character that terminates a number. The default value is 0, which preserves the existing behavior; this is planned to become the default in version 4.0.0.

    See full documentation of JSON_PRECISE_STREAM_POSITION.

    "},{"location":"features/macros/#json_skip_library_version_check","title":"JSON_SKIP_LIBRARY_VERSION_CHECK","text":"

    When defined, the library will not create a compiler warning when a different version of the library was already included.

    See full documentation of JSON_SKIP_LIBRARY_VERSION_CHECK.

    "},{"location":"features/macros/#json_skip_unsupported_compiler_check","title":"JSON_SKIP_UNSUPPORTED_COMPILER_CHECK","text":"

    When defined, the library will not create a compile error when a known unsupported compiler is detected. This allows using the library with compilers that do not fully support C++11 and may only work if unsupported features are not used.

    See full documentation of JSON_SKIP_UNSUPPORTED_COMPILER_CHECK.

    "},{"location":"features/macros/#json_strict_binary_utf8","title":"JSON_STRICT_BINARY_UTF8","text":"

    When defined to 1, to_cbor, to_ubjson, to_bjdata, and to_bson throw type_error.316 for a string value or object key that is not valid UTF-8. The default value is 0, which writes the bytes unchanged as before version 3.13.0 unreleased; this is planned to become the default in version 4.0.0.

    The check can also be enabled with the CMake option JSON_StrictBinaryUTF8 (OFF by default) which sets JSON_STRICT_BINARY_UTF8 accordingly.

    See full documentation of JSON_STRICT_BINARY_UTF8.

    "},{"location":"features/macros/#json_strict_nul_handling","title":"JSON_STRICT_NUL_HANDLING","text":"

    When defined to 1, a '\\0' (NUL) byte anywhere in the input is rejected with parse_error.101, like any other unexpected byte, instead of being silently treated as end of input (see the FAQ entry for background). The default value is 0, which preserves the existing behavior; this is planned to become the default in version 4.0.0.

    The strict handling can also be enabled with the CMake option JSON_StrictNulHandling (OFF by default) which sets JSON_STRICT_NUL_HANDLING accordingly.

    See full documentation of JSON_STRICT_NUL_HANDLING.

    "},{"location":"features/macros/#json_throw_userexception","title":"JSON_THROW_USER(exception)","text":"

    This macro overrides throw calls inside the library. The argument is the exception to be thrown.

    See full documentation of JSON_THROW_USER(exception).

    "},{"location":"features/macros/#json_try_user","title":"JSON_TRY_USER","text":"

    This macro overrides try calls inside the library.

    See full documentation of JSON_TRY_USER.

    "},{"location":"features/macros/#json_use_implicit_conversions","title":"JSON_USE_IMPLICIT_CONVERSIONS","text":"

    When defined to 0, implicit conversions are switched off. By default, implicit conversions are switched on.

    See full documentation of JSON_USE_IMPLICIT_CONVERSIONS.

    "},{"location":"features/macros/#json_use_global_udls","title":"JSON_USE_GLOBAL_UDLS","text":"

    When defined to 1 (default), the user-defined string literals operator\"\"_json and operator\"\"_json_pointer are placed into the global namespace instead of nlohmann::literals::json_literals.

    See full documentation of JSON_USE_GLOBAL_UDLS.

    "},{"location":"features/macros/#json_use_legacy_discarded_value_comparison","title":"JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON","text":"

    When defined to 1, the library restores the legacy behavior in which comparing a discarded value with <= or >= yields true instead of false. This behavior is deprecated and switched off (0) by default.

    See full documentation of JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON.

    "},{"location":"features/macros/#json_use_objects_for_enum_keyed_maps","title":"JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS","text":"

    When defined to 1, maps with enum keys (e.g., std::map<E, T>) are stored as objects, using the enum's conversion for the keys, instead of arrays of [key, value] pairs. It is switched off (0) by default.

    See full documentation of JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS.

    "},{"location":"features/macros/#json_use_simdutf","title":"JSON_USE_SIMDUTF","text":"

    When defined, UTF-8 validation of JSON strings read from contiguous byte input is delegated to the simdutf library instead of the built-in scalar validator. This is an opt-in external dependency and is not defined by default.

    See full documentation of JSON_USE_SIMDUTF.

    "},{"location":"features/macros/#nlohmann_define_type_-nlohmann_define_derived_type_","title":"NLOHMANN_DEFINE_TYPE_*(...), NLOHMANN_DEFINE_DERIVED_TYPE_*(...)","text":"

    The library defines 12 macros to simplify the serialization/deserialization of types. See the page on arbitrary type conversion for a detailed discussion.

    "},{"location":"features/macros/#nlohmann_json_namespace-nlohmann_json_namespace_begin-nlohmann_json_namespace_end-nlohmann_json_namespace_no_version","title":"NLOHMANN_JSON_NAMESPACE, NLOHMANN_JSON_NAMESPACE_BEGIN, NLOHMANN_JSON_NAMESPACE_END, NLOHMANN_JSON_NAMESPACE_NO_VERSION","text":"

    These macros relate to the versioned, inline nlohmann namespace:

    • NLOHMANN_JSON_NAMESPACE evaluates to the full name of the nlohmann namespace (including the inline ABI namespace).
    • NLOHMANN_JSON_NAMESPACE_BEGIN / NLOHMANN_JSON_NAMESPACE_END open and close the namespace (for example, to add specializations).
    • NLOHMANN_JSON_NAMESPACE_NO_VERSION, when defined to 1, omits the version component from the inline namespace.

    See the nlohmann Namespace page, and the full documentation of NLOHMANN_JSON_NAMESPACE, NLOHMANN_JSON_NAMESPACE_BEGIN / NLOHMANN_JSON_NAMESPACE_END, and NLOHMANN_JSON_NAMESPACE_NO_VERSION.

    "},{"location":"features/macros/#nlohmann_json_serialize_enumtype","title":"NLOHMANN_JSON_SERIALIZE_ENUM(type, ...)","text":"

    This macro simplifies the serialization/deserialization of enum types. See Specializing enum conversion for more information.

    See full documentation of NLOHMANN_JSON_SERIALIZE_ENUM.

    A strict variant NLOHMANN_JSON_SERIALIZE_ENUM_STRICT throws an exception on undefined input instead of falling back to the first mapping.

    "},{"location":"features/macros/#nlohmann_json_version_major-nlohmann_json_version_minor-nlohmann_json_version_patch","title":"NLOHMANN_JSON_VERSION_MAJOR, NLOHMANN_JSON_VERSION_MINOR, NLOHMANN_JSON_VERSION_PATCH","text":"

    These macros are defined by the library and contain the version numbers according to Semantic Versioning 2.0.0.

    See full documentation of NLOHMANN_JSON_VERSION_MAJOR, NLOHMANN_JSON_VERSION_MINOR, and NLOHMANN_JSON_VERSION_PATCH.

    "},{"location":"features/merge_patch/","title":"JSON Merge Patch","text":"

    The library supports JSON Merge Patch (RFC 7396) as a patch format. The merge patch format is primarily intended for use with the HTTP PATCH method as a means of describing a set of modifications to a target resource's content. This function applies a merge patch to the current JSON value.

    Instead of using JSON Pointer to specify values to be manipulated, it describes the changes using a syntax that closely mimics the document being modified. Unlike JSON Patch, a JSON Merge Patch cannot express every kind of change (e.g., it cannot reorder array elements or remove a specific array element), but it is easier to read and write for object-shaped documents.

    The function merge_diff creates a merge patch that changes one JSON value into another.

    Not a general deep merge

    A merge patch is not a general deep merge: a null value in the patch deletes the key from the target. To merge two objects recursively (e.g., defaults and user settings), use update with merge_objects set to true; see Merging objects.

    Example

    The following code shows how a JSON Merge Patch is applied to a JSON document.

    #include <iostream>\n#include <nlohmann/json.hpp>\n#include <iomanip> // for std::setw\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // the original document\n    json document = R\"({\n                \"title\": \"Goodbye!\",\n                \"author\": {\n                    \"givenName\": \"John\",\n                    \"familyName\": \"Doe\"\n                },\n                \"tags\": [\n                    \"example\",\n                    \"sample\"\n                ],\n                \"content\": \"This will be unchanged\"\n            })\"_json;\n\n    // the patch\n    json patch = R\"({\n                \"title\": \"Hello!\",\n                \"phoneNumber\": \"+01-123-456-7890\",\n                \"author\": {\n                    \"familyName\": null\n                },\n                \"tags\": [\n                    \"example\"\n                ]\n            })\"_json;\n\n    // apply the patch\n    document.merge_patch(patch);\n\n    // output original and patched document\n    std::cout << std::setw(4) << document << std::endl;\n}\n

    Output:

    {\n    \"author\": {\n        \"givenName\": \"John\"\n    },\n    \"content\": \"This will be unchanged\",\n    \"phoneNumber\": \"+01-123-456-7890\",\n    \"tags\": [\n        \"example\"\n    ],\n    \"title\": \"Hello!\"\n}\n
    "},{"location":"features/merge_patch/#see-also","title":"See also","text":"
    • JSON Patch and Diff - a more expressive alternative that describes a sequence of operations
    • JSON Pointer - the addressing scheme used by JSON Patch
    • Function merge_patch
    • Function merge_diff
    • Function update - merge objects, optionally recursively
    "},{"location":"features/modifying_values/","title":"Modifying values","text":"

    Once a JSON value exists, its content can be changed: elements can be added, replaced, merged, and removed. This page gives an overview of the available operations. For read access, see element access.

    "},{"location":"features/modifying_values/#adding-to-arrays","title":"Adding to arrays","text":"

    New elements are appended to an array with push_back or constructed in place with emplace_back. If the value is null, it is converted to an array first, so these functions can also be used to build an array from scratch.

    json j;                 // null\nj.push_back(1);         // [1]\nj.push_back(2);         // [1,2]\nj.emplace_back(3);      // [1,2,3]\n\n// operator+= is a shorthand for push_back\nj += 4;                 // [1,2,3,4]\n
    "},{"location":"features/modifying_values/#adding-to-objects","title":"Adding to objects","text":"

    The most common way to add or replace a member is operator[], which inserts the key if it does not exist yet:

    json j;\nj[\"name\"] = \"Mary\";     // {\"name\":\"Mary\"}\nj[\"name\"] = \"John\";     // {\"name\":\"John\"}  (replaced)\n

    emplace inserts a member only if the key is not already present, and reports whether the insertion happened \u2014 useful for \"add if absent\" semantics.

    "},{"location":"features/modifying_values/#merging-objects","title":"Merging objects","text":"

    To merge one object into another, update copies all members from another object (similar to Python's dict.update). This is the idiomatic way to combine two objects. It has two modes:

    • By default, the merge is shallow: existing keys are overwritten, even if both values are objects.
    • With merge_objects = #!cpp true, keys whose values are objects in both JSON values are merged recursively. Everything else is overwritten. In particular, arrays are replaced, not concatenated.
    Example 
    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // create two JSON objects\n    json o1 = R\"( {\"color\": \"red\", \"price\": 17.99, \"names\": {\"de\": \"Flugzeug\"}} )\"_json;\n    json o2 = R\"( {\"color\": \"blue\", \"speed\": 100, \"names\": {\"en\": \"plane\"}} )\"_json;\n    json o3 = o1;\n\n    // add all keys from o2 to o1 (updating \"color\", replacing \"names\")\n    o1.update(o2);\n\n    // add all keys from o2 to o1 (updating \"color\", merging \"names\")\n    o3.update(o2, true);\n\n    // output updated object o1 and o3\n    std::cout << std::setw(2) << o1 << '\\n';\n    std::cout << std::setw(2) << o3 << '\\n';\n}\n

    Output:

    {\n  \"color\": \"blue\",\n  \"names\": {\n    \"en\": \"plane\"\n  },\n  \"price\": 17.99,\n  \"speed\": 100\n}\n{\n  \"color\": \"blue\",\n  \"names\": {\n    \"de\": \"Flugzeug\",\n    \"en\": \"plane\"\n  },\n  \"price\": 17.99,\n  \"speed\": 100\n}\n

    A common use of the recursive mode is combining defaults with user settings. Nested defaults that the user did not set are kept:

    json defaults = {{\"log\", {{\"level\", \"info\"}, {\"file\", \"app.log\"}}}, {\"retries\", 3}};\njson user_settings = {{\"log\", {{\"level\", \"debug\"}}}};\n\njson config = defaults;\nconfig.update(user_settings, true);\n// {\"log\":{\"file\":\"app.log\",\"level\":\"debug\"},\"retries\":3}\n

    JSON Merge Patch (RFC 7396) also merges objects recursively, but it is a different tool: a null in the patch means \"remove this key\". It is meant for applying merge patch documents (e.g., received via HTTP PATCH). To merge configuration-like objects, use update(..., true). To apply a sequence of well-defined edit operations, see JSON Patch.

    "},{"location":"features/modifying_values/#removing-elements","title":"Removing elements","text":"

    Elements are removed with erase, which accepts an object key, an array index, or an iterator. clear empties a value while keeping its type, and operator[] combined with assignment can overwrite a value entirely.

    json j = {{\"a\", 1}, {\"b\", 2}, {\"c\", 3}};\nj.erase(\"b\");           // {\"a\":1,\"c\":3}\n\njson a = {1, 2, 3, 4};\na.erase(1);             // [1,3,4]  (erase by index)\n
    "},{"location":"features/modifying_values/#see-also","title":"See also","text":"
    • push_back / emplace_back - append to an array
    • emplace - insert into an object if the key is absent
    • update - merge objects (shallow, or recursive with merge_objects)
    • merge_patch - apply an RFC 7396 merge patch
    • erase / clear - remove elements
    • JSON Patch and Diff and JSON Merge Patch - structured modifications
    "},{"location":"features/modules/","title":"Modules","text":"

    This library has experimental support for C++ modules, introduced in C++20. The library can be imported by writing import nlohmann.json; instead of #include <nlohmann/json.hpp>.

    Please be aware that the module is experimental and a full test is outstanding, and the exported symbols are subject to change.

    "},{"location":"features/modules/#requirements","title":"Requirements","text":"

    The nlohmann.json module requires that the build system is configured to build and resolve modules when imported. Obviously, as modules were introduced in C++20, this feature can only be used in C++20 and subsequent versions.

    To enable building the nlohmann.json module (which is not done by default), the macro NLOHMANN_JSON_BUILD_MODULES must be passed to the build system.

    "},{"location":"features/modules/#example","title":"Example","text":"

    When using modules rather than headers, the previous example for creating a json object through a JSON file, would instead be: 

    import std;\nimport nlohmann.json;\n\nusing json = nlohmann::json;\n\n// ...\n\nstd::ifstream f(\"example.json\");\njson data = json::parse(f);\n

    "},{"location":"features/modules/#modules-do-not-export-macros","title":"Modules do not export macros","text":"

    It should be noted that as modules do not export macros, the nlohmann.json module will not export any macros.

    "},{"location":"features/modules/#exported-symbols","title":"Exported symbols","text":"

    Only the following symbols are exported from nlohmann.json:

    • nlohmann::adl_serializer
    • nlohmann::basic_json
    • nlohmann::basic_json_document
    • nlohmann::basic_json_view
    • nlohmann::json
    • nlohmann::json_document
    • nlohmann::json_pointer
    • nlohmann::json_view
    • nlohmann::ordered_json
    • nlohmann::ordered_json_document
    • nlohmann::ordered_json_view
    • nlohmann::ordered_map
    • nlohmann::to_string
    • nlohmann::literals::json_literals::operator\"\"_json
    • nlohmann::literals::json_literals::operator\"\"_json_pointer

    The module always exports the two user-defined string literals, even if JSON_NO_AUTOMATIC_UDLS is defined when building it.

    Additionally, the following nlohmann::detail symbols are exported, solely to work around an MSVC compilation issue (#3970). They are implementation details, not part of the public API, and should not be used directly:

    • nlohmann::detail::json_sax_dom_callback_parser
    • nlohmann::detail::unknown_size
    "},{"location":"features/modules/#known-issues","title":"Known issues","text":"

    C++20 modules support is exercised in CI against current GCC and Clang on Ubuntu, and the default MSVC toolset on Windows Server 2022 \u2014 there is no documented minimum compiler version, unlike feature-test-macro-gated features such as JSON_HAS_RANGES.

    Known compiler issues

    • GCC may emit \"redefinition\" errors when #include <nlohmann/json.hpp> appears in a module preamble together with other imports. This is an upstream GCC bug, not yet resolved as of GCC 16. Workarounds: include nlohmann/json.hpp before other #includes, use import nlohmann.json; instead, or upgrade GCC. (issue #5103)
    • MSVC could fail with C2039: 'json_sax_dom_callback_parser' is not a member of ... detail; fixed by exporting the required internal symbols from json.cppm (see Exported symbols above). (issue #3970)

    If you hit a different module-related build failure, search existing issues before filing a new one.

    "},{"location":"features/namespace/","title":"nlohmann Namespace","text":"

    The 3.11.0 release introduced an inline namespace to allow different parts of a codebase to safely use different versions of the JSON library as long as they never exchange instances of library types.

    "},{"location":"features/namespace/#structure","title":"Structure","text":"

    The complete default namespace name is derived as follows:

    • The root namespace is always nlohmann.
    • The inline namespace starts with json_abi and is followed by several optional ABI tags according to the value of these ABI-affecting macros, in order:
      • JSON_DIAGNOSTICS defined non-zero appends _diag.
      • JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON defined non-zero appends _ldvcmp.
      • JSON_DIAGNOSTIC_POSITIONS defined non-zero appends _dp.
      • JSON_BRACE_INIT_COPY_SEMANTICS defined non-zero appends _bics.
      • JSON_PRECISE_STREAM_POSITION defined non-zero appends _psp.
      • JSON_STRICT_NUL_HANDLING defined non-zero appends _snul.
      • JSON_STRICT_BINARY_UTF8 defined non-zero appends _sbu8.
      • JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS defined non-zero appends _ekmo.
      • JSON_DISABLE_STREAM_BULK_SCAN defined non-zero appends _dsbs.
      • JSON_DISABLE_UNLOCKED_FILE_IO defined non-zero appends _dufio.
      • JSON_VIEW_NO_SIMD defined appends _vns.
      • JSON_VIEW_USE_SSSE3 defined appends _vssse3.
    • The inline namespace ends with the suffix _v followed by the 3 components of the version number separated by underscores. To omit the version component, see Disabling the version component below.

    For example, the namespace name for version 3.11.2 with JSON_DIAGNOSTICS defined to 1 is:

    nlohmann::json_abi_diag_v3_11_2\n
    "},{"location":"features/namespace/#purpose","title":"Purpose","text":"

    Several incompatibilities have been observed. Amongst the most common ones is linking code compiled with different definitions of JSON_DIAGNOSTICS. This is illustrated in the diagram below.

    graph\n    json[\"<strong>nlohmann_json (v3.10.5)</strong><br>JSON_DIAGNOSTICS=0\"]\n    json_diag[\"<strong>nlohmann_json (v3.10.5)</strong><br>JSON_DIAGNOSTICS=1\"]\n    library[\"<strong>some library</strong>\"]\n    app[\"<strong>application</strong>\"]\n\n    library --> json\n    app --> json_diag\n    app --> library

    In releases prior to 3.11.0, mixing any version of the JSON library with different JSON_DIAGNOSTICS settings would result in a crashing application. If some_library never passes instances of JSON library types to the application, this scenario became safe in version 3.11.0 and above due to the inline namespace yielding distinct symbol names.

    "},{"location":"features/namespace/#limitations","title":"Limitations","text":"

    Neither the compiler nor the linker will issue as much as a warning when translation units \u2013 intended to be linked together and that include different versions and/or configurations of the JSON library \u2013 exchange and use library types.

    There is an exception when forward declarations are used (i.e., when including json_fwd.hpp) in which case the linker may complain about undefined references.

    "},{"location":"features/namespace/#disabling-the-version-component","title":"Disabling the version component","text":"

    Different versions are not necessarily ABI-incompatible, but the project does not actively track changes in the ABI and recommends that all parts of a codebase exchanging library types be built with the same version. Users can, at their own risk, disable the version component of the inline namespace, allowing different versions \u2013 but not configurations \u2013 to be used in cases where the linker would otherwise output undefined reference errors.

    To do so, define NLOHMANN_JSON_NAMESPACE_NO_VERSION to 1.

    This applies to version 3.11.2 and above only; versions 3.11.0 and 3.11.1 can apply the technique described in the next section to emulate the effect of the NLOHMANN_JSON_NAMESPACE_NO_VERSION macro.

    Use at your own risk

    Disabling the namespace version component and mixing ABI-incompatible versions will result in crashes or incorrect behavior. You have been warned!

    "},{"location":"features/namespace/#disabling-the-inline-namespace-completely","title":"Disabling the inline namespace completely","text":"

    When interoperability with code using a pre-3.11.0 version of the library is required, users can, at their own risk restore the old namespace layout by redefining NLOHMANN_JSON_NAMESPACE_BEGIN, NLOHMANN_JSON_NAMESPACE_END as follows:

    #define NLOHMANN_JSON_NAMESPACE_BEGIN  namespace nlohmann {\n#define NLOHMANN_JSON_NAMESPACE_END    }\n

    Use at your own risk

    Overriding the namespace and mixing ABI-incompatible versions will result in crashes or incorrect behavior. You have been warned!

    "},{"location":"features/namespace/#version-history","title":"Version history","text":"
    • Introduced inline namespace (json_v3_11_0[_abi-tag]*) in version 3.11.0.
    • Changed structure of inline namespace in version 3.11.2.
    • Added ABI tag _dp (JSON_DIAGNOSTIC_POSITIONS) in version 3.12.0.
    • Added ABI tags _bics (JSON_BRACE_INIT_COPY_SEMANTICS), _psp (JSON_PRECISE_STREAM_POSITION), _snul (JSON_STRICT_NUL_HANDLING), _sbu8 (JSON_STRICT_BINARY_UTF8), _ekmo (JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS), _dsbs (JSON_DISABLE_STREAM_BULK_SCAN), _dufio (JSON_DISABLE_UNLOCKED_FILE_IO), _vns (JSON_VIEW_NO_SIMD), and _vssse3 (JSON_VIEW_USE_SSSE3) in version 3.13.0 unreleased.
    "},{"location":"features/object_order/","title":"Object Order","text":"

    The JSON standard defines objects as \"an unordered collection of zero or more name/value pairs\". As such, an implementation does not need to preserve any specific order of object keys.

    "},{"location":"features/object_order/#default-behavior-sort-keys","title":"Default behavior: sort keys","text":"

    The default type nlohmann::json uses a std::map to store JSON objects, and thus stores object keys sorted alphabetically.

    Example: json sorts object keys 
    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    json j;\n    j[\"one\"] = 1;\n    j[\"two\"] = 2;\n    j[\"three\"] = 3;\n\n    std::cout << j.dump(2) << '\\n';\n}\n

    Output:

    {\n  \"one\": 1,\n  \"three\": 3,\n  \"two\": 2\n}\n
    "},{"location":"features/object_order/#alternative-behavior-preserve-insertion-order","title":"Alternative behavior: preserve insertion order","text":"

    If you do want to preserve the insertion order, you can use the type nlohmann::ordered_json.

    Example: ordered_json preserves insertion order 
    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing ordered_json = nlohmann::ordered_json;\n\nint main()\n{\n    ordered_json j;\n    j[\"one\"] = 1;\n    j[\"two\"] = 2;\n    j[\"three\"] = 3;\n\n    std::cout << j.dump(2) << '\\n';\n}\n

    Output:

    {\n  \"one\": 1,\n  \"two\": 2,\n  \"three\": 3\n}\n

    Alternatively, nlohmann::fifo_map also preserves the insertion order and, unlike ordered_map, keeps a lookup index, so it does not have the quadratic cost described below. It is used through a small adapter (integration).

    If the order does not matter and you only want faster lookup, boost::unordered_flat_map, absl::flat_hash_map, absl::node_hash_map, and several other hash maps work through an adapter that restores the template argument order basic_json expects; see Template Parameter Requirements. Note these are unordered, not insertion-ordered.

    tsl::ordered_map cannot be used: its iterators expose the mapped value as const, while basic_json needs to modify it in place.

    The ordered_map behind nlohmann::ordered_json is deliberately minimal and has no lookup index, so every key access is a linear scan and building an object of n keys costs O(n\u00b2). This is unnoticeable at typical object sizes but becomes significant for objects with many thousands of keys; see ordered_map complexity. The alternatives above keep a lookup index and do not have this cost.

    "},{"location":"features/object_order/#notes-on-parsing","title":"Notes on parsing","text":"

    Note that you also need to call the right parse function when reading from a file. Assume file input.json contains the JSON object above:

    {\n  \"one\": 1,\n  \"two\": 2,\n  \"three\": 3\n}\n

    Right way

    The following code correctly calls the parse function from nlohmann::ordered_json:

    std::ifstream i(\"input.json\");\nauto j = nlohmann::ordered_json::parse(i);\nstd::cout << j.dump(2) << std::endl;\n

    The output will be:

    {\n  \"one\": 1,\n  \"two\": 2,\n  \"three\": 3\n}\n
    Wrong way

    The following code incorrectly calls the parse function from nlohmann::json which does not preserve the insertion order, but sorts object keys. Assigning the result to nlohmann::ordered_json compiles, but does not restore the order from the input file.

    std::ifstream i(\"input.json\");\nnlohmann::ordered_json j = nlohmann::json::parse(i);\nstd::cout << j.dump(2) << std::endl;\n

    The output will be:

    {\n  \"one\": 1,\n  \"three\": 3,\n  \"two\": 2\n}\n
    "},{"location":"features/performance/","title":"Performance","text":"

    Speed was never the primary goal of this library. The design goals page says so plainly: \"There are certainly faster JSON libraries out there.\" Intuitive syntax, trivial integration, and thorough testing came first. If a hard real-time budget or the last percent of throughput matters more than convenience, a faster, more specialized library may be a better fit.

    That said, how you use this library still makes a measurable difference. This page collects practical, code-verified techniques for reducing time, memory, and compile-time cost -- without repeating the detailed pages it links to.

    "},{"location":"features/performance/#parsing-input","title":"Parsing input","text":"

    parse accepts a string, a pair of iterators, a container, a std::istream, or a FILE* (see Parsing). Internally, every input is wrapped in an input adapter, and not all adapters are equally fast.

    For inputs backed by contiguous, single-byte memory -- a std::string, a std::vector<char>, a string literal, or a pointer range -- the library uses iterator_input_adapter, wrapped in a raw pointer so the fast paths below apply on every supported standard. This adapter exposes two optimizations the lexer detects at compile time:

    • it can reconstruct already-consumed input on demand for error messages, instead of copying every character as it is read, and
    • the lexer can scan ordinary string characters directly out of the buffer, several bytes at a time, rather than one character (and one function call) at a time.

    A std::istream (including std::ifstream) or FILE*, by contrast, is read through input_stream_adapter or file_input_adapter, which read one character (or one block, for binary formats) at a time and expose neither optimization -- the lexer falls back to the same byte-at-a-time path it uses for any non-contiguous, general-purpose iterator range. An iterator pair over non-contiguous but random-access storage (e.g. std::deque<char>::iterator) gets the first optimization but not the second, since the byte-scanning fast path additionally requires contiguous storage.

    Practically: if the JSON text is already in memory, or small enough to read into memory, prefer passing a std::string, a std::vector<char>, or a pointer range to parse over a std::istream. For a file, that means reading it into a string first and then parsing the string, rather than passing a std::ifstream directly to parse -- the latter never benefits from either optimization:

    // gets the contiguous fast paths\nstd::ifstream f(\"example.json\");\nstd::string contents((std::istreambuf_iterator<char>(f)), std::istreambuf_iterator<char>());\njson j = json::parse(contents);\n\n// does not: input_stream_adapter has no fast path\nstd::ifstream f2(\"example.json\");\njson j2 = json::parse(f2);\n

    For contiguous input with many non-ASCII characters, JSON_USE_SIMDUTF can additionally speed up UTF-8 validation by using the simdutf library instead of the built-in scalar validator; streaming inputs (files, std::istream, wide strings, user-defined adapters) always use the scalar path regardless of this macro.

    "},{"location":"features/performance/#large-documents","title":"Large documents","text":"

    Parsing always produces SAX events internally; parse simply feeds them to a consumer that builds a complete basic_json value tree (a DOM) in memory. For documents too large to comfortably hold as a DOM, three alternatives avoid building it:

    • Implement the SAX interface directly and pass it to sax_parse; only the parts of the input you choose to keep ever become basic_json values.
    • Pass a parser callback to parse. This still builds a DOM, but the callback can discard finished elements as soon as they are handled, so memory usage stays bounded by one element (plus the unparsed remainder of the input) instead of the whole document -- see the recipe for streaming a large homogeneous array.
    • Parse into a json_document (<nlohmann/json_view.hpp>) instead of a basic_json. It keeps the input text and builds a flat index of 16 bytes per value; strings and floating-point numbers are not copied, but read from the text when needed. Read-only views give the familiar element access, and only the parts you materialize() become basic_json values. A document that borrows the text instead of owning a copy needs the text to outlive it; see choosing between json, the SAX interface, and json_view.

    If the data is naturally record-oriented, consider JSON Lines instead of one large JSON document: reading and parsing it line by line with std::getline means only one line's value is ever in memory at a time, and a malformed line does not invalidate lines already processed.

    "},{"location":"features/performance/#binary-formats","title":"Binary formats","text":"

    JSON text is not a compact format. If the data is only exchanged between programs (not read by humans), the binary formats -- BJData, BON8, BSON, CBOR, MessagePack, and UBJSON -- encode the same values more compactly, which reduces both the bytes transferred and, for most of them, the work needed to parse them back. The size comparison on that page, measured against minified JSON for four reference documents, shows the effect varies a lot by document shape: CBOR and MessagePack come out at 50.5% of the minified JSON size for the numeric-array-heavy canada.json, but only around 87-88% for the string-heavy jeopardy.json, where there is less numeric data to encode more compactly. BON8 is the most compact option in that comparison for text-heavy documents (63.5%-87.5%), at the cost of an incomplete serializer (no unsigned integers above int64). Which format -- and whether it is worth the loss of human readability at all -- depends on the actual data; see the comparison tables before choosing one.

    "},{"location":"features/performance/#object-type-json-vs-ordered_json","title":"Object type: json vs. ordered_json","text":"

    The default json type stores object keys in a std::map, giving logarithmic-time lookup, insertion, and erasure, at the cost of sorting keys alphabetically rather than preserving insertion order (see Object Order). ordered_json uses nlohmann::ordered_map instead, a std::vector-backed container with no lookup index: every key-based operation is a linear scan, so building an object of n distinct keys costs O(n\u00b2) in total -- this applies equally to inserting keys one by one and to parsing an object, since the parser inserts each key as it is read. The measurements on the ordered_map page show this is negligible at typical object sizes (2000 keys: 0.7 ms for json vs. 3.6 ms for ordered_json, a 5x factor) but grows steeply for large, machine-generated objects (16 000 keys: 3.3 ms vs. 181.6 ms, a 54x factor).

    If insertion order matters and an object routinely has many thousands of keys, ordered_json's quadratic build cost may not be acceptable. The library's ObjectType template parameter can be set to a different container instead: nlohmann::fifo_map keeps insertion order with a real lookup index (avoiding the quadratic cost), while std::unordered_map, boost::unordered_flat_map, absl::flat_hash_map, and similar hash maps trade insertion order for average-case constant-time lookup (through an adapter, since their template argument order does not match what basic_json expects) -- see Object Order for the full list.

    "},{"location":"features/performance/#avoiding-copies","title":"Avoiding copies","text":"
    • Move instead of copy. Constructing a basic_json from an existing one is linear in its size for the copy constructor but constant for the move constructor. The same applies to assigning a large std::string, std::vector, or other container into a value: pass it as std::move(x) rather than x whenever x is no longer needed afterwards.
    • Access without copying. get<T>() returns a copy of the stored value converted to T. When a reference or pointer to the value already stored inside the basic_json is enough, get_ref() and get_ptr() access it directly: both pages state, word for word, \"No copies are made.\" -- at the cost of that reference or pointer becoming invalid once the underlying value changes.
    • Iterate by reference. basic_json::iterator::operator*() returns a reference (an alias for basic_json&), but a range-based for loop with a by-value loop variable (for (auto el : j)) still copies each element, because plain auto drops the reference. Write for (const auto& el : j) (or auto& for a mutable loop), and use items() the same way when the key is needed too -- its own examples use for (auto& el : j.items()).
    • Construct in place. emplace_back() (arrays, amortized constant time) and emplace() (objects, logarithmic in the size of the container for json) forward their arguments directly to a basic_json constructor, rather than requiring a temporary value to be constructed and then copied or moved in. push_back() has an rvalue overload (push_back(basic_json&&)) for a value that already exists: j.push_back(std::move(value)) moves it in instead of copying it.
    • Skip the bounds check when it is redundant. at() and operator[] have the same complexity (constant for a valid array index, logarithmic for an object key in json) -- the difference is that at() additionally checks the key or index and throws if it is invalid, while operator[] does not (see unchecked access and checked access). Prefer operator[] when the surrounding code has already established that the access is valid.
    • Reserve array capacity. basic_json has no public reserve(), but when building a large array incrementally with a known final size, get_ref() exposes the underlying array_t so it can be reserved directly -- see \"reserving array capacity\" for the one-line recipe.
    "},{"location":"features/performance/#serialization","title":"Serialization","text":"

    dump() with the default indent = -1 selects \"the most compact representation\" (word for word from the page); any non-negative indent pretty-prints instead, which is more readable but produces more bytes and more work. dump() builds and returns a complete string_t containing the whole serialization. operator<< writes directly to a std::ostream instead, through the same serializer, but without ever materializing that intermediate string -- so if the destination is a stream (a file, or std::cout), os << j; avoids the allocation and copy that os << j.dump(); would incur for large values.

    "},{"location":"features/performance/#diagnostics-overhead","title":"Diagnostics overhead","text":"

    Two opt-in macros add diagnostic information to exceptions and to every value, at a cost that is only worth paying while it is in use:

    • JSON_DIAGNOSTICS adds a JSON Pointer to exception messages, pointing at the value that triggered the exception. Quoting the page directly: \"enabling this macro increases the size of every JSON value by one pointer and adds some runtime overhead\" -- every value gains a parent pointer that has to be kept up to date as the document is built and modified.
    • JSON_DIAGNOSTIC_POSITIONS adds start_pos() and end_pos(), the byte offsets a value occupied in its parsed input. Quoting the page: \"enabling this macro increases the size of every JSON value by two std::size_t fields and adds slight runtime overhead to parsing, copying JSON value objects, and the generation of error messages for exceptions.\"

    Both default to off. Enable them where better diagnostics are worth the overhead (for example, while validating untrusted input, or in a debug build), and keep them off in a release build that does not need them.

    "},{"location":"features/performance/#compile-time","title":"Compile time","text":"

    <nlohmann/json_fwd.hpp> forward-declares basic_json, json, ordered_json, json_pointer, and adl_serializer, pulling in only a handful of lightweight standard headers instead of the full json.hpp. A header that only needs to name nlohmann::json -- in a function signature or a class member declaration, for instance -- can include json_fwd.hpp and leave #include <nlohmann/json.hpp> to the source files that actually parse, build, or serialize values, the same way a project would forward-declare any other heavy class to keep it out of widely-included headers:

    // my_type.hpp\n#include <nlohmann/json_fwd.hpp>\n\nclass my_type\n{\n    nlohmann::json config() const;\n};\n\n// my_type.cpp\n#include <nlohmann/json.hpp>\n#include \"my_type.hpp\"\n\nnlohmann::json my_type::config() const { /* ... */ }\n

    One caveat: ABI-affecting macros such as JSON_DIAGNOSTICS and JSON_DIAGNOSTIC_POSITIONS are encoded into the library's inline namespace name. Every translation unit -- whether it includes json_fwd.hpp or the full header -- must define them the same way, or linking fails with undefined references instead of a compile error.

    If I/O support is not needed at all, JSON_NO_IO excludes <cstdio>, <ios>, <iosfwd>, <istream>, and <ostream> outright and drops the std::istream/FILE* parse overloads and operator<< that depend on them (dump() itself is unaffected, since it only returns a string); it exists for environments where those headers are unavailable (such as Intel SGX), and as a side effect those headers are then never processed by the compiler at all.

    "},{"location":"features/performance/#see-also","title":"See also","text":"
    • Design goals - why this library does not optimize for speed first
    • Architecture - how input adapters, the lexer, and the serializer fit together
    • Parsing - the available parsing functions and inputs
    • SAX interface - parse without building a DOM
    • Zero-copy JSON views - parse into a flat index of the text and read it without building a DOM
    • Binary formats - compact alternatives to JSON text
    • Object Order - json vs. ordered_json and other ObjectType choices
    • Template Parameter Requirements - custom container and allocator types
    • Supported macros - overview of all configuration macros, including the diagnostics ones above
    "},{"location":"features/serialization/","title":"Serialization","text":"

    Serialization is the process of turning a JSON value back into JSON text. It is the counterpart to parsing. The central function is dump, which returns the JSON text as a string.

    json j = {{\"pi\", 3.141}, {\"happy\", true}};\n\nstd::string s = j.dump();   // {\"happy\":true,\"pi\":3.141}\n

    To write a value directly to a stream (for example, a file or std::cout), the operator<< is provided:

    std::cout << j << std::endl;\n

    String, not raw value

    dump always returns a JSON text. Serializing a JSON string therefore includes the surrounding quotes and escapes special characters. To obtain the contained string value without quotes, use get<std::string>() instead of dump. See the converting values page.

    "},{"location":"features/serialization/#pretty-printing","title":"Pretty-printing","text":"

    By default, dump produces the most compact representation without any superfluous whitespace. Passing a non-negative indent argument pretty-prints the output with the given number of spaces per level:

    Example: pretty-print JSON values with dump() 
    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n    json j_string = \"Hell\u00f6 \ud83d\ude00!\";\n\n    // call dump()\n    std::cout << \"objects:\" << '\\n'\n              << j_object.dump() << \"\\n\\n\"\n              << j_object.dump(-1) << \"\\n\\n\"\n              << j_object.dump(0) << \"\\n\\n\"\n              << j_object.dump(4) << \"\\n\\n\"\n              << j_object.dump(1, '\\t') << \"\\n\\n\";\n\n    std::cout << \"arrays:\" << '\\n'\n              << j_array.dump() << \"\\n\\n\"\n              << j_array.dump(-1) << \"\\n\\n\"\n              << j_array.dump(0) << \"\\n\\n\"\n              << j_array.dump(4) << \"\\n\\n\"\n              << j_array.dump(1, '\\t') << \"\\n\\n\";\n\n    std::cout << \"strings:\" << '\\n'\n              << j_string.dump() << '\\n'\n              << j_string.dump(-1, ' ', true) << '\\n';\n\n    // create JSON value with invalid UTF-8 byte sequence\n    json j_invalid = \"\u00e4\\xA9\u00fc\";\n    try\n    {\n        std::cout << j_invalid.dump() << std::endl;\n    }\n    catch (const json::type_error& e)\n    {\n        std::cout << e.what() << std::endl;\n    }\n\n    std::cout << \"string with replaced invalid characters: \"\n              << j_invalid.dump(-1, ' ', false, json::error_handler_t::replace)\n              << \"\\nstring with ignored invalid characters: \"\n              << j_invalid.dump(-1, ' ', false, json::error_handler_t::ignore)\n              << '\\n';\n}\n

    Output:

    objects:\n{\"one\":1,\"two\":2}\n\n{\"one\":1,\"two\":2}\n\n{\n\"one\": 1,\n\"two\": 2\n}\n\n{\n    \"one\": 1,\n    \"two\": 2\n}\n\n{\n    \"one\": 1,\n    \"two\": 2\n}\n\narrays:\n[1,2,4,8,16]\n\n[1,2,4,8,16]\n\n[\n1,\n2,\n4,\n8,\n16\n]\n\n[\n    1,\n    2,\n    4,\n    8,\n    16\n]\n\n[\n    1,\n    2,\n    4,\n    8,\n    16\n]\n\nstrings:\n\"Hell\u00f6 \ud83d\ude00!\"\n\"Hell\\u00f6 \\ud83d\\ude00!\"\n[json.exception.type_error.316] invalid UTF-8 byte at index 2: 0xA9\nstring with replaced invalid characters: \"\u00e4\ufffd\u00fc\"\nstring with ignored invalid characters: \"\u00e4\u00fc\"\n

    The indentation character can be changed with the second argument (e.g., a tab '\\t'). An indent of 0 inserts newlines but no leading spaces, and the default of -1 selects the compact single-line form.

    "},{"location":"features/serialization/#non-ascii-characters","title":"Non-ASCII characters","text":"

    Strings are stored and serialized as UTF-8 (see types). By default, dump copies valid non-ASCII characters as-is. Setting the third argument ensure_ascii to true escapes all non-ASCII characters with \\uXXXX sequences, so that the output contains only ASCII characters:

    json j = \"\u82f9\u679c\";\nj.dump();               // \"\u82f9\u679c\"\nj.dump(-1, ' ', true);  // \"\\u82f9\\u679c\"\n
    "},{"location":"features/serialization/#handling-invalid-utf-8","title":"Handling invalid UTF-8","text":"

    If a string contains invalid UTF-8 sequences (for example, because it holds data in another encoding such as Latin-1), serialization fails by default. The fourth argument of dump selects an error_handler:

    • strict (default) \u2014 throw a type_error.316 exception.
    • replace \u2014 replace invalid bytes with the Unicode replacement character U+FFFD (\ufffd).
    • ignore \u2014 silently drop invalid bytes.
    • keep \u2014 copy invalid bytes to the output unchanged; the result is not valid UTF-8.
    Example: serialize invalid UTF-8 with different error handlers 
    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON value with invalid UTF-8 byte sequence\n    json j_invalid = \"\u00e4\\xA9\u00fc\";\n    try\n    {\n        std::cout << j_invalid.dump() << std::endl;\n    }\n    catch (const json::type_error& e)\n    {\n        std::cout << e.what() << std::endl;\n    }\n\n    std::cout << \"string with replaced invalid characters: \"\n              << j_invalid.dump(-1, ' ', false, json::error_handler_t::replace)\n              << \"\\nstring with ignored invalid characters: \"\n              << j_invalid.dump(-1, ' ', false, json::error_handler_t::ignore)\n              << \"\\nstring with the invalid byte kept as is (\" << j_invalid.dump(-1, ' ', false, json::error_handler_t::keep).size()\n              << \" bytes, not valid UTF-8 itself)\\n\";\n}\n

    Output:

    [json.exception.type_error.316] invalid UTF-8 byte at index 2: 0xA9\nstring with replaced invalid characters: \"\u00e4\ufffd\u00fc\"\nstring with ignored invalid characters: \"\u00e4\u00fc\"\nstring with the invalid byte kept as is (7 bytes, not valid UTF-8 itself)\n

    Avoiding invalid UTF-8

    The best fix is to ensure that all strings are UTF-8 encoded before storing them. See the FAQ on non-ASCII characters for how to convert wide or Latin-1 strings.

    "},{"location":"features/serialization/#numbers-nan-and-binary-values","title":"Numbers, NaN, and binary values","text":"
    • Numbers are serialized with enough precision to round-trip; see number serialization.
    • NaN and infinity cannot be represented in JSON and are serialized as null; see NaN handling. The binary formats can preserve them.
    • Binary values have no JSON representation and are serialized as a helper object for debugging only; see binary values.
    "},{"location":"features/serialization/#using-stdformat-stdprint-and-fmt","title":"Using std::format, std::print, and fmt","text":"

    Since version 3.12.0, JSON values can be formatted directly with C++20's std::format whenever the standard library provides the <format> header (controlled by JSON_HAS_STD_FORMAT). This is enabled by the std::formatter<basic_json> specialization, which also makes JSON values work with std::format_to and with C++23's std::print/std::println:

    std::print(\"{}\", j);      // compact, like j.dump()\nstd::print(\"{:2}\", j);    // pretty-printed with indent 2 (like j.dump(2))\nstd::println(\"{:#}\", j);  // pretty-printed with the default indent\n

    The format spec mirrors the dump parameters: \"{:#}\" pretty-prints, a width such as \"{:2}\" sets the indent, and a fill-and-align prefix such as \"{:.>#}\" sets the indent character.

    For the {fmt} library, the library ships a format_as helper. Note its behavior depends on the fmt version; see the FAQ entry for the details and a recipe for a full fmt::formatter specialization.

    "},{"location":"features/serialization/#serializing-to-other-formats","title":"Serializing to other formats","text":"

    Besides JSON text, a value can also be serialized to the more compact binary formats (BJData, BON8, BSON, CBOR, MessagePack, UBJSON).

    "},{"location":"features/serialization/#see-also","title":"See also","text":"
    • dump - serialize to a JSON-formatted string
    • operator<< - serialize to a stream
    • to_string - user-defined-conversion helper
    • std::formatter<basic_json> - use JSON values with std::format and std::print
    • format_as - use JSON values with the {fmt} library
    • Parsing - the reverse operation
    "},{"location":"features/trailing_commas/","title":"Trailing Commas","text":"

    Like comments, this library does not support trailing commas in arrays and objects by default.

    You can set parameter ignore_trailing_commas to true in the parse function to allow trailing commas in arrays and objects. Note that a single comma as the only content of the array or object ([,] or {,}) is not allowed, and multiple trailing commas ([1,,]) are not allowed either.

    This library does not add trailing commas when serializing JSON data.

    For more information, see JSON With Commas and Comments (JWCC).

    Example

    Consider the following JSON with trailing commas.

    {\n    \"planets\": [\n        \"Mercury\",\n        \"Venus\",\n        \"Earth\",\n        \"Mars\",\n        \"Jupiter\",\n        \"Uranus\",\n        \"Neptune\",\n    ]\n}\n

    When calling parse without additional argument, a parse error exception is thrown. If ignore_trailing_commas is set to true, the trailing commas are ignored during parsing:

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::string s = R\"(\n    {\n        \"planets\": [\n            \"Mercury\",\n            \"Venus\",\n            \"Earth\",\n            \"Mars\",\n            \"Jupiter\",\n            \"Uranus\",\n            \"Neptune\",\n        ]\n    }\n    )\";\n\n    try\n    {\n        json j = json::parse(s);\n    }\n    catch (json::exception& e)\n    {\n        std::cout << e.what() << std::endl;\n    }\n\n    json j = json::parse(s,\n                         /* callback */ nullptr,\n                         /* allow exceptions */ true,\n                         /* ignore_comments */ false,\n                         /* ignore_trailing_commas */ true);\n    std::cout << j.dump(2) << '\\n';\n}\n

    Output:

    [json.exception.parse_error.101] parse error at line 11, column 9: syntax error while parsing value - unexpected ']'; expected '[', '{', or a literal\n{\n  \"planets\": [\n    \"Mercury\",\n    \"Venus\",\n    \"Earth\",\n    \"Mars\",\n    \"Jupiter\",\n    \"Uranus\",\n    \"Neptune\"\n  ]\n}\n
    "},{"location":"features/binary_formats/","title":"Binary Formats","text":"

    Though JSON is a ubiquitous data format, it is not a very compact format suitable for data exchange, for instance, over a network. Hence, the library supports

    • BJData (Binary JData),
    • BON8 (Binary Object Notation 8),
    • BSON (Binary JSON),
    • CBOR (Concise Binary Object Representation),
    • MessagePack, and
    • UBJSON (Universal Binary JSON)

    to efficiently encode JSON values to byte vectors and to decode such vectors.

    "},{"location":"features/binary_formats/#comparison","title":"Comparison","text":""},{"location":"features/binary_formats/#completeness","title":"Completeness","text":"Format Serialization Deserialization BJData complete complete BON8 incomplete: no unsigned integers above int64 complete BSON incomplete: top-level value must be an object incomplete, but all JSON types are supported CBOR complete incomplete, but all JSON types are supported MessagePack complete complete UBJSON complete complete"},{"location":"features/binary_formats/#binary-values","title":"Binary values","text":"Format Binary values Binary subtypes BJData not supported not supported BON8 not supported not supported BSON supported supported CBOR supported supported MessagePack supported supported UBJSON not supported not supported

    See binary values for more information.

    "},{"location":"features/binary_formats/#sizes","title":"Sizes","text":"Format canada.json twitter.json citm_catalog.json jeopardy.json BJData 53.2 % 91.1 % 78.1 % 96.6 % BJData (size) 58.6 % 92.1 % 86.7 % 97.4 % BJData (size+type) 58.6 % 92.1 % 86.5 % 97.4 % BON8 50.5 % 83.8 % 63.5 % 87.5 % BSON 85.8 % 95.2 % 95.8 % 106.7 % CBOR 50.5 % 86.3 % 68.4 % 88.0 % MessagePack 50.5 % 86.0 % 68.5 % 87.9 % UBJSON 53.2 % 91.3 % 78.2 % 96.6 % UBJSON (size) 58.6 % 92.3 % 86.8 % 97.4 % UBJSON (size+type) 55.9 % 92.3 % 85.0 % 95.0 %

    Sizes compared to minified JSON value.

    "},{"location":"features/binary_formats/bjdata/","title":"BJData","text":"

    The BJData format was derived from and improved upon Universal Binary JSON(UBJSON) specification (Draft 12). Specifically, it introduces an optimized array container for efficient storage of N-dimensional packed arrays (ND-arrays); it also adds 5 new type markers - [u] - uint16, [m] - uint32, [M] - uint64, [h] - float16 and [B] - byte - to unambiguously map common binary numeric types; furthermore, it uses little-endian (LE) to store all numerics instead of big-endian (BE) as in UBJSON to avoid unnecessary conversions on commonly available platforms.

    Compared to other binary JSON-like formats such as MessagePack and CBOR, both BJData and UBJSON demonstrate a rare combination of being both binary and quasi-human-readable. This is because all semantic elements in BJData and UBJSON, including the data-type markers and name/string types, are directly human-readable. Data stored in the BJData/UBJSON format is not only compact in size, fast to read/write, but also can be directly searched or read using simple processing.

    References

    • BJData Specification
    "},{"location":"features/binary_formats/bjdata/#serialization","title":"Serialization","text":"

    The library uses the following mapping from JSON values types to BJData types according to the BJData specification:

    JSON value type value/range BJData type marker null null null Z boolean true true T boolean false false F number_integer -9223372036854775808..-2147483649 int64 L number_integer -2147483648..-32769 int32 l number_integer -32768..-129 int16 I number_integer -128..127 int8 i number_integer 128..255 uint8 U number_integer 256..32767 int16 I number_integer 32768..65535 uint16 u number_integer 65536..2147483647 int32 l number_integer 2147483648..4294967295 uint32 m number_integer 4294967296..9223372036854775807 int64 L number_integer 9223372036854775808..18446744073709551615 uint64 M number_unsigned 0..127 int8 i number_unsigned 128..255 uint8 U number_unsigned 256..32767 int16 I number_unsigned 32768..65535 uint16 u number_unsigned 65536..2147483647 int32 l number_unsigned 2147483648..4294967295 uint32 m number_unsigned 4294967296..9223372036854775807 int64 L number_unsigned 9223372036854775808..18446744073709551615 uint64 M number_float any value float64 D string with shortest length indicator string S array see notes on optimized format/ND-array array [ object see notes on optimized format map { binary see notes on binary values array [$B

    Complete mapping

    The mapping is complete in the sense that any JSON value type can be converted to a BJData value.

    Any BJData output created by to_bjdata can be successfully parsed by from_bjdata.

    Size constraints

    The following values can not be converted to a BJData value:

    • strings with more than 18446744073709551615 bytes, i.e., 264-1 bytes (theoretical)

    UTF-8 validation of string values and object keys

    BJData strings must use UTF-8 encoding. By default (the error_handler parameter left at keep), to_bjdata() writes the bytes of string values and object keys unchanged, even if they are not valid UTF-8. With error_handler_t::strict, it throws type_error.316 for ill-formed UTF-8 instead; replace/ignore sanitize the string. JSON_STRICT_BINARY_UTF8 makes strict the default.

    Unused BJData markers

    The following markers are not used in the conversion:

    • Z: no-op values are not created.
    • C: single-byte strings are serialized with S markers.

    NaN/infinity handling

    If NaN or Infinity are stored inside a JSON number, they are serialized properly. This behavior differs from the dump() function which serializes NaN or Infinity to null.

    Endianness

    A breaking difference between BJData and UBJSON is the endianness of numerical values. In BJData, all numerical data types (integers UiuImlML and floating-point values hdD) are stored in the little-endian (LE) byte order as opposed to big-endian as used by UBJSON. Adopting LE to store numeric records avoids unnecessary byte swapping on most modern computers where LE is used as the default byte order.

    Optimized formats

    Optimized formats for containers are supported via two parameters of to_bjdata:

    • Parameter use_size adds size information to the beginning of a container and removes the closing marker.
    • Parameter use_type further checks whether all elements of a container have the same type and adds the type marker to the beginning of the container. The use_type parameter must only be used together with use_size = true.

    Note that use_size = true alone may result in larger representations - the benefit of this parameter is that the receiving side is immediately informed of the number of elements in the container.

    ND-array optimized format

    BJData extends UBJSON's optimized array size marker to support ND-arrays of uniform numerical data types (referred to as packed arrays). For example, the 2-D uint8 integer array [[1,2],[3,4],[5,6]], stored as nested optimized array in UBJSON [ [$U#i2 1 2 [$U#i2 3 4 [$U#i2 5 6 ], can be further compressed in BJData to [$U#[$i#i2 2 3 1 2 3 4 5 6 or [$U#[i2 i3] 1 2 3 4 5 6.

    To maintain type and size information, ND-arrays are converted to JSON objects following the annotated array format (defined in the JData specification (Draft 3)), when parsed using from_bjdata. For example, the above 2-D uint8 array can be parsed and accessed as

    {\n    \"_ArrayType_\": \"uint8\",\n    \"_ArraySize_\": [2,3],\n    \"_ArrayData_\": [1,2,3,4,5,6]\n}\n

    Likewise, when a JSON object in the above form is serialized using to_bjdata, it is automatically converted into a compact BJData ND-array.

    When parsing, an ND-array whose dimension vector is empty, contains a single integer, contains two integers with the first being 1, or contains a 0 is returned as a regular (possibly empty) array rather than an annotated object.

    An object is only converted if the annotation describes a packed array that is parsed back into the same annotated object; otherwise it is serialized as a regular JSON object, so the annotation is never lost in a round trip. This requires all of the following:

    • \"_ArrayType_\" is one of uint8, int8, uint16, int16, uint32, int32, uint64, int64, single, double, char, or byte,
    • \"_ArraySize_\" is an array, since the dimensions are written as the ND-array header's length,
    • \"_ArraySize_\" has at least two entries and is not a 1\u00d7N row vector (first entry 1), since other shapes are parsed back as a regular array,
    • every entry of \"_ArraySize_\" is a positive integer, and their product is representable as a std::size_t,
    • \"_ArrayData_\" is an array holding exactly that many elements, and
    • every element of \"_ArrayData_\" is a number of the kind named by \"_ArrayType_\": for the integer types, a value that fits the named width; for double, any value; for single, a value that survives narrowing to float and back without change (for instance, 0.1 does not, since it is not exactly representable as float).

    An annotated object is always read back with its keys in the order shown above, \"_ArrayType_\", \"_ArraySize_\", \"_ArrayData_\", regardless of the order the ND-array's header stores them in on the wire. This matters for ordered_json, whose comparison takes key order into account.

    The current version of this library does not yet support automatic detection of and conversion from a nested JSON array input to a BJData ND-array.

    Restrictions in optimized data types for arrays and objects

    Due to diminished space saving, hampered readability, and increased security risks, in BJData, the allowed data types following the $ marker in an optimized array and object container are restricted to non-zero-fixed-length data types. Therefore, the valid optimized type markers can only be one of UiuImlMLhdDCB. This also means other variable ([{SH) or zero-length types (TFN) can not be used in an optimized array or object in BJData.

    Binary values

    BJData provides a dedicated B marker (defined in the BJData specification (Draft 3)) that is used in optimized arrays to designate binary data. This means that, unlike UBJSON, binary data can be both serialized and deserialized.

    To preserve compatibility with BJData Draft 2, the Draft 3 optimized binary array must be explicitly enabled using the version parameter of to_bjdata.

    In Draft2 mode (default), if the JSON data contains the binary type, the value stored as a list of integers, as suggested by the BJData documentation. In particular, this means that the serialization and the deserialization of JSON containing binary values into BJData and back will result in a different JSON object.

    Example: serialize JSON values to BJData, with and without size/type optimization 
    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\n// function to print BJData's diagnostic format\nvoid print_byte(uint8_t byte)\n{\n    if (32 < byte and byte < 128)\n    {\n        std::cout << (char)byte;\n    }\n    else\n    {\n        std::cout << (int)byte;\n    }\n}\n\nint main()\n{\n    // create a JSON value\n    json j = R\"({\"compact\": true, \"schema\": false})\"_json;\n\n    // serialize it to BJData\n    std::vector<std::uint8_t> v = json::to_bjdata(j);\n\n    // print the vector content\n    for (auto& byte : v)\n    {\n        print_byte(byte);\n    }\n    std::cout << std::endl;\n\n    // create an array of numbers\n    json array = {1, 2, 3, 4, 5, 6, 7, 8};\n\n    // serialize it to BJData using default representation\n    std::vector<std::uint8_t> v_array = json::to_bjdata(array);\n    // serialize it to BJData using size optimization\n    std::vector<std::uint8_t> v_array_size = json::to_bjdata(array, true);\n    // serialize it to BJData using type optimization\n    std::vector<std::uint8_t> v_array_size_and_type = json::to_bjdata(array, true, true);\n\n    // print the vector contents\n    for (auto& byte : v_array)\n    {\n        print_byte(byte);\n    }\n    std::cout << std::endl;\n\n    for (auto& byte : v_array_size)\n    {\n        print_byte(byte);\n    }\n    std::cout << std::endl;\n\n    for (auto& byte : v_array_size_and_type)\n    {\n        print_byte(byte);\n    }\n    std::cout << std::endl;\n}\n

    Output:

    {i7compactTi6schemaF}\n[i1i2i3i4i5i6i7i8]\n[#i8i1i2i3i4i5i6i7i8\n[$i#i812345678\n
    "},{"location":"features/binary_formats/bjdata/#deserialization","title":"Deserialization","text":"

    The library maps BJData types to JSON value types as follows:

    BJData type JSON value type marker no-op no value, next value is read N null null Z false false F true true T float16 number_float h float32 number_float d float64 number_float D uint8 number_unsigned U int8 number_integer i uint16 number_unsigned u int16 number_integer I uint32 number_unsigned m int32 number_integer l uint64 number_unsigned M int64 number_integer L byte number_unsigned B string string S char string C array array (optimized values are supported) [ ND-array object (in JData annotated array format) [$.#[. object object (optimized values are supported) { binary binary (strongly-typed byte array) [$B

    Complete mapping

    The mapping is complete in the sense that any BJData value can be converted to a JSON value.

    Ill-formed UTF-8 in string values and object keys

    BJData strings must use UTF-8 encoding, but checking it on read is opt-in: with the error_handler parameter left at keep (the default), from_bjdata() accepts a string value or object key whose bytes are not valid UTF-8 and hands them back unchanged. Passing error_handler_t::strict makes from_bjdata() check and throw parse_error.113 for ill-formed UTF-8, and replace/ignore sanitize the string instead of keeping it. However, dump() still requires valid UTF-8 and throws type_error.316 for a value read with the default keep handler, unless an error handler is passed that replaces or ignores the ill-formed bytes. to_bjdata()'s own error_handler parameter defaults to keep (see above), so such a value is written back unchanged.

    Round trips

    A value returned by from_bjdata can be serialized with to_bjdata using any combination of options and parsed back into an equal value, and serializing that value again with the same options produces the same bytes. The exception is binary values: they are only written as an optimized binary array ([$B) if Draft 3 is enabled and both use_size and use_type are set. Otherwise, they are written as arrays of integers and parsed back as such (see the notes on binary values above), and serializing such an array again may choose different, but equally valid, type markers. The bytes can then differ, but parsing them again yields the same value.

    Example: deserialize a JSON value from BJData 
    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create byte vector\n    std::vector<std::uint8_t> v = {0x7B, 0x69, 0x07, 0x63, 0x6F, 0x6D, 0x70, 0x61,\n                                   0x63, 0x74, 0x54, 0x69, 0x06, 0x73, 0x63, 0x68,\n                                   0x65, 0x6D, 0x61, 0x69, 0x00, 0x7D\n                                  };\n\n    // deserialize it with BJData\n    json j = json::from_bjdata(v);\n\n    // print the deserialized JSON value\n    std::cout << std::setw(2) << j << std::endl;\n}\n

    Output:

    {\n  \"compact\": true,\n  \"schema\": 0\n}\n
    "},{"location":"features/binary_formats/bon8/","title":"BON8","text":"

    BON8 (Binary Object Notation 8) is a compact binary serialization format for JSON values. It uses the byte values that cannot begin a UTF-8 character as type markers, so strings are stored as plain UTF-8 without a length prefix: a string ends at the first byte that cannot continue it. Integers from -10 to 39, true, false, null, and the floating-point values -1.0, 0.0, and 1.0 take a single byte, and arrays and objects with up to four elements need no terminator.

    References

    • BON8 specification
    • Reference implementation in HikoGUI
    "},{"location":"features/binary_formats/bon8/#serialization","title":"Serialization","text":"

    The library uses the following mapping from JSON values types to BON8 types according to the BON8 specification:

    JSON value type value/range BON8 type first byte null null null 0xFA boolean true true 0xF9 boolean false false 0xF8 number_integer -9223372036854775808..-2147483649 int64 0x8D number_integer -2147483648..-33818507 int32 0x8C number_integer -33818506..-264075 4-byte negative integer 0xF0..0xF7 number_integer -264074..-1931 3-byte negative integer 0xE0..0xEF number_integer -1930..-11 2-byte negative integer 0xC2..0xDF number_integer -10..-1 1-byte negative integer 0xB8..0xC1 number_integer 0..39 1-byte positive integer 0x90..0xB7 number_integer 40..3879 2-byte positive integer 0xC2..0xDF number_integer 3880..528167 3-byte positive integer 0xE0..0xEF number_integer 528168..67637031 4-byte positive integer 0xF0..0xF7 number_integer 67637032..2147483647 int32 0x8C number_integer 2147483648..9223372036854775807 int64 0x8D number_unsigned 0..39 1-byte positive integer 0x90..0xB7 number_unsigned 40..3879 2-byte positive integer 0xC2..0xDF number_unsigned 3880..528167 3-byte positive integer 0xE0..0xEF number_unsigned 528168..67637031 4-byte positive integer 0xF0..0xF7 number_unsigned 67637032..2147483647 int32 0x8C number_unsigned 2147483648..9223372036854775807 int64 0x8D number_float -1.0 -1.0 0xFB number_float 0.0 0.0 0xFC number_float 1.0 1.0 0xFD number_float any other value representable by a float binary32 0x8E number_float any value NOT representable by a float binary64 0x8F string empty end of string 0xFF string non-empty UTF-8 string 0x00..0x7F, 0xC2..0xF4 array size: 0..4 array with count 0x80..0x84 array size: 5 or more array (terminated by 0xFE) 0x85 object size: 0..4 object with count 0x86..0x8A object size: 5 or more object (terminated by 0xFE) 0x8B binary size: 0..4 array with count 0x80..0x84 binary size: 5 or more array (terminated by 0xFE) 0x85

    An integer that takes 2 to 4 bytes starts with a UTF-8 lead byte (0xC2..0xF7) that is followed by a byte that cannot continue a UTF-8 character: 0x00..0x7F for positive and 0xC0..0xFF for negative integers. A string is terminated by 0xFF only if it is empty, if another string follows it, or if nothing follows it in the message; otherwise, the byte after it ends it: the first byte of the next value, or the 0xFE that ends an array or object. For example, [\"e\"] is serialized as 0x81 0x65 0xFF, but [1,2,3,4,\"e\"] as 0x85 0x91 0x92 0x93 0x94 0x65 0xFE.

    Complete mapping

    Except for the values listed below, any JSON value can be converted to a BON8 value.

    Any BON8 output created by to_bon8 can be successfully parsed by from_bon8.

    Unsupported values

    The following values can not be converted to a BON8 value:

    • unsigned integers above 9223372036854775807, because BON8 has no unsigned 64-bit integer type (out_of_range.407)
    • strings that are not valid UTF-8, because the end of a string is determined from its encoding (type_error.316)

    NaN/infinity handling

    -0.0, Infinity, and -Infinity are serialized as binary32 (type 0x8E, 5 bytes total). NaN is serialized as the binary32 value 0x7F800001 that the specification recommends. This is in contrast to the dump function which serializes NaN or Infinity to null.

    Binary values

    BON8 has no binary type. Binary values are serialized as arrays of integers (0..255), so they are read back as arrays. The subtype is not serialized.

    Canonical representation

    The output follows the specification's canonical representation rules: every value uses the shortest encoding, floating-point numbers use binary32 whenever that loses no precision, and object keys are sorted by their UTF-8 code units. There are two exceptions:

    • Strings are not normalized to Unicode Normalization Form C (NFC).
    • Object keys are written in the order of the object type, which is sorted for json, but not for ordered_json.
    Example: serialize a JSON value to BON8 
    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // create a JSON value\n    json j = R\"({\"compact\": true, \"format\": \"BON8\", \"schema\": 0})\"_json;\n\n    // serialize it to BON8\n    std::vector<std::uint8_t> v = json::to_bon8(j);\n\n    // print the vector content\n    for (auto& byte : v)\n    {\n        std::cout << \"0x\" << std::hex << std::setw(2) << std::setfill('0') << (int)byte << \" \";\n    }\n    std::cout << std::endl;\n}\n

    Output:

    0x89 0x63 0x6f 0x6d 0x70 0x61 0x63 0x74 0xf9 0x66 0x6f 0x72 0x6d 0x61 0x74 0xff 0x42 0x4f 0x4e 0x38 0xff 0x73 0x63 0x68 0x65 0x6d 0x61 0x90 \n
    "},{"location":"features/binary_formats/bon8/#deserialization","title":"Deserialization","text":"

    The library maps BON8 types to JSON value types as follows:

    BON8 type JSON value type first byte UTF-8 string string 0x00..0x7F array with count array 0x80..0x84 array (terminated by 0xFE) array 0x85 object with count object 0x86..0x8A object (terminated by 0xFE) object 0x8B int32 number_unsigned or number_integer 0x8C int64 number_unsigned or number_integer 0x8D binary32 number_float 0x8E binary64 number_float 0x8F 1-byte positive integer number_unsigned 0x90..0xB7 1-byte negative integer number_integer 0xB8..0xC1 UTF-8 string string 0xC2..0xF4, followed by 0x80..0xBF 2- to 4-byte positive integer number_unsigned 0xC2..0xF7, followed by 0x00..0x7F 2- to 4-byte negative integer number_integer 0xC2..0xF7, followed by 0xC0..0xFF false false 0xF8 true true 0xF9 null null 0xFA -1.0 number_float 0xFB 0.0 number_float 0xFC 1.0 number_float 0xFD empty string string 0xFF

    Non-negative integers are read as number_unsigned, negative integers as number_integer.

    Info

    Values that do not use the canonical representation, such as integers with a longer encoding than necessary, arrays and objects with up to four elements that are terminated by 0xFE, unsorted object keys, or a 0xFF after a string that would also end without it, are accepted. A second 0xFF is not a terminator but an empty string.

    Strings must be valid UTF-8, and a string at the very end of a message must be terminated by 0xFF.

    Info

    Any BON8 output created by to_bon8 can be successfully parsed by from_bon8.

    Example: deserialize a JSON value from BON8 
    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create byte vector\n    std::vector<std::uint8_t> v = {0x89, 0x63, 0x6f, 0x6d, 0x70, 0x61, 0x63, 0x74,\n                                   0xf9, 0x66, 0x6f, 0x72, 0x6d, 0x61, 0x74, 0xff,\n                                   0x42, 0x4f, 0x4e, 0x38, 0xff, 0x73, 0x63, 0x68,\n                                   0x65, 0x6d, 0x61, 0x90\n                                  };\n\n    // deserialize it with BON8\n    json j = json::from_bon8(v);\n\n    // print the deserialized JSON value\n    std::cout << std::setw(2) << j << std::endl;\n}\n

    Output:

    {\n  \"compact\": true,\n  \"format\": \"BON8\",\n  \"schema\": 0\n}\n
    "},{"location":"features/binary_formats/bson/","title":"BSON","text":"

    BSON, short for Binary JSON, is a binary-encoded serialization of JSON-like documents. Like JSON, BSON supports the embedding of documents and arrays within other documents and arrays. BSON also contains extensions that allow representation of data types that are not part of the JSON spec. For example, BSON has a Date type and a BinData type.

    References

    • BSON Website - the main source on BSON
    • BSON Specification - the specification
    "},{"location":"features/binary_formats/bson/#serialization","title":"Serialization","text":"

    The library uses the following mapping from JSON values types to BSON types:

    JSON value type value/range BSON type marker null null null 0x0A boolean true, false boolean 0x08 number_integer -9223372036854775808..-2147483649 int64 0x12 number_integer -2147483648..2147483647 int32 0x10 number_integer 2147483648..9223372036854775807 int64 0x12 number_unsigned 0..2147483647 int32 0x10 number_unsigned 2147483648..9223372036854775807 int64 0x12 number_unsigned 9223372036854775808..18446744073709551615 uint64 0x11 number_float any value double 0x01 string any value string 0x02 array any value document 0x04 object any value document 0x03 binary any value binary 0x05

    Incomplete mapping

    The mapping is incomplete, since only JSON-objects (and things contained therein) can be serialized to BSON. Also, keys may not contain U+0000, since they are serialized a zero-terminated c-strings.

    BSON type 0x11 interoperability

    The BSON specification defines type 0x11 as a Timestamp. This library uses marker 0x11 when serializing number_unsigned values in the range 9223372036854775808..18446744073709551615. Other BSON implementations may therefore interpret these values as Timestamps instead of unsigned integers.

    Binary values without a subtype

    BSON requires every binary value to have a subtype. If a binary value has no subtype, this library serializes it with the generic subtype 0x00. After deserialization, has_subtype() returns true and subtype() returns 0. As a result, serializing and deserializing a JSON object containing such a value produces a different JSON object, even though the binary data is unchanged.

    Example: serialize a JSON value to BSON 
    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // create a JSON value\n    json j = R\"({\"compact\": true, \"schema\": 0})\"_json;\n\n    // serialize it to BSON\n    std::vector<std::uint8_t> v = json::to_bson(j);\n\n    // print the vector content\n    for (auto& byte : v)\n    {\n        std::cout << \"0x\" << std::hex << std::setw(2) << std::setfill('0') << (int)byte << \" \";\n    }\n    std::cout << std::endl;\n}\n

    Output:

    0x1b 0x00 0x00 0x00 0x08 0x63 0x6f 0x6d 0x70 0x61 0x63 0x74 0x00 0x01 0x10 0x73 0x63 0x68 0x65 0x6d 0x61 0x00 0x00 0x00 0x00 0x00 0x00 \n
    "},{"location":"features/binary_formats/bson/#deserialization","title":"Deserialization","text":"

    The library maps BSON record types to JSON value types as follows:

    BSON type BSON marker byte JSON value type double 0x01 number_float string 0x02 string document 0x03 object array 0x04 array binary 0x05 binary undefined 0x06 unsupported ObjectId 0x07 unsupported boolean 0x08 boolean UTC Date-Time 0x09 unsupported null 0x0A null Regular Expr. 0x0B unsupported DB Pointer 0x0C unsupported JavaScript Code 0x0D unsupported Symbol 0x0E unsupported JavaScript Code w/ scope 0x0F unsupported int32 0x10 number_integer uint64(Timestamp) 0x11 number_unsigned int64 0x12 number_integer 128-bit decimal float 0x13 unsupported Max Key 0x7F unsupported Min Key 0xFF unsupported

    Incomplete mapping

    The mapping is incomplete. The unsupported mappings are indicated in the table above.

    Handling of BSON type 0x11

    This library deserializes BSON type 0x11 (Timestamp) as a number_unsigned value. The 64-bit value is preserved, but the Timestamp type information is not.

    Lenient BSON input handling

    The BSON reader is lenient in a few areas where the BSON specification is more restrictive:

    • array element keys are not checked against the required decimal sequence (0, 1, 2, ...),
    • any non-zero byte is accepted as true for the boolean type, and
    • the payload for binary subtype 0x02 is returned as-is, including its inner length prefix.

    If BSON input must be validated for strict specification compliance, validate it separately before passing it to from_bson().

    Ill-formed UTF-8 in string values

    The BSON specification requires string values (type 0x02) to be valid UTF-8, but this is not required of a decoder, so checking is opt-in: with the error_handler parameter left at keep (the default), from_bson() accepts a string value whose bytes are not valid UTF-8 and hands them back unchanged. Passing error_handler_t::strict makes from_bson() check and throw parse_error.113 for ill-formed UTF-8, and replace/ignore sanitize the string instead of keeping it. However, dump() still requires valid UTF-8 and throws type_error.316 for a value read with the default keep handler, unless an error handler is passed that replaces or ignores the ill-formed bytes. to_bson()'s own error_handler parameter defaults to keep, so such a string value or element (key) name is written unchanged; with strict (the default if JSON_STRICT_BINARY_UTF8 is enabled), it throws the same exception instead. Element (key) names are never validated on read, since they are read byte-by-byte as a C string. binary values (type 0x05) are unaffected, since they are not required to hold text.

    Example: deserialize a JSON value from BSON 
    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create byte vector\n    std::vector<std::uint8_t> v = {0x1b, 0x00, 0x00, 0x00, 0x08, 0x63, 0x6f, 0x6d,\n                                   0x70, 0x61, 0x63, 0x74, 0x00, 0x01, 0x10, 0x73,\n                                   0x63, 0x68, 0x65, 0x6d, 0x61, 0x00, 0x00, 0x00,\n                                   0x00, 0x00, 0x00\n                                  };\n\n    // deserialize it with BSON\n    json j = json::from_bson(v);\n\n    // print the deserialized JSON value\n    std::cout << std::setw(2) << j << std::endl;\n}\n

    Output:

    {\n  \"compact\": true,\n  \"schema\": 0\n}\n
    "},{"location":"features/binary_formats/cbor/","title":"CBOR","text":"

    The Concise Binary Object Representation (CBOR) is a data format whose design goals include the possibility of extremely small code sizes, fairly small message size, and extensibility without the need for version negotiation.

    References

    • CBOR Website - the main source on CBOR
    • CBOR Playground - an interactive webpage to translate between JSON and CBOR
    • RFC 8949 - the CBOR specification
    "},{"location":"features/binary_formats/cbor/#serialization","title":"Serialization","text":"

    The library uses the following mapping from JSON values types to CBOR types according to the CBOR specification (RFC 8949):

    JSON value type value/range CBOR type first byte null null Null 0xF6 boolean true True 0xF5 boolean false False 0xF4 number_integer -9223372036854775808..-2147483649 Negative integer (8 bytes follow) 0x3B number_integer -2147483648..-32769 Negative integer (4 bytes follow) 0x3A number_integer -32768..-129 Negative integer (2 bytes follow) 0x39 number_integer -128..-25 Negative integer (1 byte follow) 0x38 number_integer -24..-1 Negative integer 0x20..0x37 number_integer 0..23 Integer 0x00..0x17 number_integer 24..255 Unsigned integer (1 byte follow) 0x18 number_integer 256..65535 Unsigned integer (2 bytes follow) 0x19 number_integer 65536..4294967295 Unsigned integer (4 bytes follow) 0x1A number_integer 4294967296..18446744073709551615 Unsigned integer (8 bytes follow) 0x1B number_unsigned 0..23 Integer 0x00..0x17 number_unsigned 24..255 Unsigned integer (1 byte follow) 0x18 number_unsigned 256..65535 Unsigned integer (2 bytes follow) 0x19 number_unsigned 65536..4294967295 Unsigned integer (4 bytes follow) 0x1A number_unsigned 4294967296..18446744073709551615 Unsigned integer (8 bytes follow) 0x1B number_float any value representable by a float Single-Precision Float 0xFA number_float any value NOT representable by a float Double-Precision Float 0xFB string length: 0..23 UTF-8 string 0x60..0x77 string length: 24..255 UTF-8 string (1 byte follow) 0x78 string length: 256..65535 UTF-8 string (2 bytes follow) 0x79 string length: 65536..4294967295 UTF-8 string (4 bytes follow) 0x7A string length: 4294967296..18446744073709551615 UTF-8 string (8 bytes follow) 0x7B array size: 0..23 array 0x80..0x97 array size: 24..255 array (1 byte follow) 0x98 array size: 256..65535 array (2 bytes follow) 0x99 array size: 65536..4294967295 array (4 bytes follow) 0x9A array size: 4294967296..18446744073709551615 array (8 bytes follow) 0x9B object size: 0..23 map 0xA0..0xB7 object size: 24..255 map (1 byte follow) 0xB8 object size: 256..65535 map (2 bytes follow) 0xB9 object size: 65536..4294967295 map (4 bytes follow) 0xBA object size: 4294967296..18446744073709551615 map (8 bytes follow) 0xBB binary size: 0..23 byte string 0x40..0x57 binary size: 24..255 byte string (1 byte follow) 0x58 binary size: 256..65535 byte string (2 bytes follow) 0x59 binary size: 65536..4294967295 byte string (4 bytes follow) 0x5A binary size: 4294967296..18446744073709551615 byte string (8 bytes follow) 0x5B

    Binary values with subtype are mapped to tagged values (0xD8..0xDB) depending on the subtype, followed by a byte string, see \"binary\" cells in the table above.

    Complete mapping

    The mapping is complete in the sense that any JSON value type can be converted to a CBOR value.

    NaN/infinity handling

    NaN, Infinity, and -Infinity are serialized as a CBOR half-precision float (type 0xF9, 3 bytes total): NaN as 0xF9 0x7E 0x00, Infinity as 0xF9 0x7C 0x00, and -Infinity as 0xF9 0xFC 0x00. This behavior differs from the normal JSON serialization which serializes NaN or Infinity to null.

    Note

    Prior to version 3.13.0 unreleased, NaN and Infinity were instead serialized as a CBOR double-precision float (type 0xFB, 9 bytes total), because the check used to select a smaller encoding compared magnitudes with NaN, which is always false and caused the intended half-precision path to be skipped.

    Unused CBOR types

    The following CBOR types are not used in the conversion:

    • UTF-8 strings terminated by \"break\" (0x7F)
    • arrays terminated by \"break\" (0x9F)
    • maps terminated by \"break\" (0xBF)
    • byte strings terminated by \"break\" (0x5F)
    • date/time (0xC0..0xC1)
    • bignum (0xC2..0xC3)
    • decimal fraction (0xC4)
    • bigfloat (0xC5)
    • expected conversions (0xD5..0xD7)
    • simple values (0xE0..0xF3, 0xF8)
    • undefined (0xF7)
    • half-precision floats (0xF9)
    • break (0xFF)

    Tagged items

    Binary subtypes will be serialized as tagged items. See binary values for an example.

    Example: serialize a JSON value to CBOR 
    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // create a JSON value\n    json j = R\"({\"compact\": true, \"schema\": 0})\"_json;\n\n    // serialize it to CBOR\n    std::vector<std::uint8_t> v = json::to_cbor(j);\n\n    // print the vector content\n    for (auto& byte : v)\n    {\n        std::cout << \"0x\" << std::hex << std::setw(2) << std::setfill('0') << (int)byte << \" \";\n    }\n    std::cout << std::endl;\n}\n

    Output:

    0xa2 0x67 0x63 0x6f 0x6d 0x70 0x61 0x63 0x74 0xf5 0x66 0x73 0x63 0x68 0x65 0x6d 0x61 0x00 \n
    "},{"location":"features/binary_formats/cbor/#deserialization","title":"Deserialization","text":"

    The library maps CBOR types to JSON value types as follows:

    CBOR type JSON value type first byte Integer number_unsigned 0x00..0x17 Unsigned integer number_unsigned 0x18 Unsigned integer number_unsigned 0x19 Unsigned integer number_unsigned 0x1A Unsigned integer number_unsigned 0x1B Negative integer number_integer 0x20..0x37 Negative integer number_integer 0x38 Negative integer number_integer 0x39 Negative integer number_integer 0x3A Negative integer number_integer 0x3B Byte string binary 0x40..0x57 Byte string binary 0x58 Byte string binary 0x59 Byte string binary 0x5A Byte string binary 0x5B Byte string binary 0x5F UTF-8 string string 0x60..0x77 UTF-8 string string 0x78 UTF-8 string string 0x79 UTF-8 string string 0x7A UTF-8 string string 0x7B UTF-8 string string 0x7F array array 0x80..0x97 array array 0x98 array array 0x99 array array 0x9A array array 0x9B array array 0x9F map object 0xA0..0xB7 map object 0xB8 map object 0xB9 map object 0xBA map object 0xBB map object 0xBF False false 0xF4 True true 0xF5 Null null 0xF6 Half-Precision Float number_float 0xF9 Single-Precision Float number_float 0xFA Double-Precision Float number_float 0xFB

    Indefinite-length UTF-8 strings (0x7F) and byte strings (0x5F) are supported. Each chunk must be a definite-length string of the same major type, as required by RFC 8949, Section 3.2.3.

    Incomplete mapping

    The mapping is incomplete in the sense that not all CBOR types can be converted to a JSON value. The following CBOR types are not supported and will yield parse errors:

    • simple values (0xE0..0xF3, 0xF8)
    • undefined (0xF7)

    Tagged items (0xC0..0xDB) are not interpreted either; see the note on tagged items below.

    Negative integer overflow

    CBOR negative integers (major type 1) are decoded as -1 - n. If the encoded magnitude n is too large for the result to fit into number_integer_t (std::int64_t by default), the result is stored as number_float_t, like a too small integer in JSON text. For example, -18446744073709551616 (0x3B followed by eight 0xFF bytes) is stored as -1.8446744073709552e+19.

    Object keys

    CBOR allows map keys of any type, whereas JSON only allows strings as keys in object values. Therefore, CBOR maps with keys other than text strings (major type 3) are rejected with a parse_error.113 exception (or, with allow_exceptions set to false, a discarded value) naming the type of the key that was found, for instance:

    [json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR object key: only string keys are supported, but found an unsigned integer; last byte: 0x01\n

    This applies to the SAX interface as well, as the key is read before it is passed on. This is a deliberate restriction of the library's JSON value model, not an oversight: formats built on CBOR maps with integer keys, such as COSE (RFC 9052) or CWT (RFC 8392), cannot be read with this library and need a general-purpose CBOR library instead.

    Ill-formed UTF-8 in text strings

    RFC 8949, Section 3.1 requires CBOR text strings (major type 3) to be valid UTF-8, but leaves it up to the decoder whether to enforce this, so checking is opt-in: with the error_handler parameter left at keep (the default), from_cbor() accepts a text string (object keys included) whose bytes are not valid UTF-8 and hands them back unchanged. Passing error_handler_t::strict makes from_cbor() check and throw parse_error.113 for ill-formed UTF-8, and replace/ignore sanitize the string instead of keeping it. However, dump() still requires valid UTF-8 and throws type_error.316 for a value read with the default keep handler, unless an error handler is passed that replaces or ignores the ill-formed bytes. to_cbor()'s own error_handler parameter defaults to keep, so such a value is written back unchanged; with strict (the default if JSON_STRICT_BINARY_UTF8 is enabled), it throws the same exception instead. Byte strings (major type 2) are unaffected, since they are not required to hold text.

    Tagged items

    Tagged items (0xC0..0xDB) will throw a parse error by default. They can be ignored by passing cbor_tag_handler_t::ignore to function from_cbor, in which case the tag is skipped and the enclosed data item is parsed on its own. Passing cbor_tag_handler_t::store to function from_cbor stores tagged byte strings (for bytes 0xd8..0xdb) as binary values with the tag as subtype; other tagged values are read as if the tag were ignored. If several tags precede a byte string, only the innermost one is stored. Note that no tag is ever interpreted: for instance, a text string tagged with tag 0 (date/time) stays a string.

    Example: deserialize a JSON value from CBOR 
    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create byte vector\n    std::vector<std::uint8_t> v = {0xa2, 0x67, 0x63, 0x6f, 0x6d, 0x70, 0x61, 0x63,\n                                   0x74, 0xf5, 0x66, 0x73, 0x63, 0x68, 0x65, 0x6d,\n                                   0x61, 0x00\n                                  };\n\n    // deserialize it with CBOR\n    json j = json::from_cbor(v);\n\n    // print the deserialized JSON value\n    std::cout << std::setw(2) << j << std::endl;\n}\n

    Output:

    {\n  \"compact\": true,\n  \"schema\": 0\n}\n
    "},{"location":"features/binary_formats/messagepack/","title":"MessagePack","text":"

    MessagePack is an efficient binary serialization format. It lets you exchange data among multiple languages like JSON. But it's faster and smaller. Small integers are encoded into a single byte, and typical short strings require only one extra byte in addition to the strings themselves.

    References

    • MessagePack website
    • MessagePack specification
    "},{"location":"features/binary_formats/messagepack/#serialization","title":"Serialization","text":"

    The library uses the following mapping from JSON values types to MessagePack types according to the MessagePack specification:

    JSON value type value/range MessagePack type first byte null null nil 0xC0 boolean true true 0xC3 boolean false false 0xC2 number_integer -9223372036854775808..-2147483649 int64 0xD3 number_integer -2147483648..-32769 int32 0xD2 number_integer -32768..-129 int16 0xD1 number_integer -128..-33 int8 0xD0 number_integer -32..-1 negative fixint 0xE0..0xFF number_integer 0..127 positive fixint 0x00..0x7F number_integer 128..255 uint 8 0xCC number_integer 256..65535 uint 16 0xCD number_integer 65536..4294967295 uint 32 0xCE number_integer 4294967296..18446744073709551615 uint 64 0xCF number_unsigned 0..127 positive fixint 0x00..0x7F number_unsigned 128..255 uint 8 0xCC number_unsigned 256..65535 uint 16 0xCD number_unsigned 65536..4294967295 uint 32 0xCE number_unsigned 4294967296..18446744073709551615 uint 64 0xCF number_float any value representable by a float float 32 0xCA number_float any value NOT representable by a float float 64 0xCB string length: 0..31 fixstr 0xA0..0xBF string length: 32..255 str 8 0xD9 string length: 256..65535 str 16 0xDA string length: 65536..4294967295 str 32 0xDB array size: 0..15 fixarray 0x90..0x9F array size: 16..65535 array 16 0xDC array size: 65536..4294967295 array 32 0xDD object size: 0..15 fix map 0x80..0x8F object size: 16..65535 map 16 0xDE object size: 65536..4294967295 map 32 0xDF binary size: 0..255 bin 8 0xC4 binary size: 256..65535 bin 16 0xC5 binary size: 65536..4294967295 bin 32 0xC6

    Complete mapping

    The mapping is complete in the sense that any JSON value type can be converted to a MessagePack value.

    Any MessagePack output created by to_msgpack can be successfully parsed by from_msgpack.

    Size constraints

    The following values can not be converted to a MessagePack value:

    • strings with more than 4294967295 bytes
    • byte strings with more than 4294967295 bytes
    • arrays with more than 4294967295 elements
    • objects with more than 4294967295 elements

    Serializing such a value throws out_of_range.412.

    NaN/infinity handling

    NaN, Infinity, and -Infinity are serialized as a MessagePack float 32 (type 0xCA, 5 bytes total), regardless of magnitude, in contrast to the dump function which serializes NaN or Infinity to null.

    Note

    Prior to version 3.13.0 unreleased, NaN and Infinity were instead serialized as a MessagePack float 64 (type 0xCB, 9 bytes total), because the check used to select the smaller float 32 encoding compared magnitudes with NaN, which is always false and caused the float 32 path to be skipped.

    Example: serialize a JSON value to MessagePack 
    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // create a JSON value\n    json j = R\"({\"compact\": true, \"schema\": 0})\"_json;\n\n    // serialize it to MessagePack\n    std::vector<std::uint8_t> v = json::to_msgpack(j);\n\n    // print the vector content\n    for (auto& byte : v)\n    {\n        std::cout << \"0x\" << std::hex << std::setw(2) << std::setfill('0') << (int)byte << \" \";\n    }\n    std::cout << std::endl;\n}\n

    Output:

    0x82 0xa7 0x63 0x6f 0x6d 0x70 0x61 0x63 0x74 0xc3 0xa6 0x73 0x63 0x68 0x65 0x6d 0x61 0x00 \n
    "},{"location":"features/binary_formats/messagepack/#deserialization","title":"Deserialization","text":"

    The library maps MessagePack types to JSON value types as follows:

    MessagePack type JSON value type first byte positive fixint number_unsigned 0x00..0x7F fixmap object 0x80..0x8F fixarray array 0x90..0x9F fixstr string 0xA0..0xBF nil null 0xC0 false false 0xC2 true true 0xC3 float 32 number_float 0xCA float 64 number_float 0xCB uint 8 number_unsigned 0xCC uint 16 number_unsigned 0xCD uint 32 number_unsigned 0xCE uint 64 number_unsigned 0xCF int 8 number_integer 0xD0 int 16 number_integer 0xD1 int 32 number_integer 0xD2 int 64 number_integer 0xD3 str 8 string 0xD9 str 16 string 0xDA str 32 string 0xDB array 16 array 0xDC array 32 array 0xDD map 16 object 0xDE map 32 object 0xDF bin 8 binary 0xC4 bin 16 binary 0xC5 bin 32 binary 0xC6 ext 8 binary 0xC7 ext 16 binary 0xC8 ext 32 binary 0xC9 fixext 1 binary 0xD4 fixext 2 binary 0xD5 fixext 4 binary 0xD6 fixext 8 binary 0xD7 fixext 16 binary 0xD8 negative fixint number_integer 0xE0-0xFF

    Info

    Any MessagePack output created by to_msgpack can be successfully parsed by from_msgpack.

    Object keys

    MessagePack allows map keys of any type, whereas JSON only allows strings as keys in object values. Like the JSON-compatible profile sketched in the MessagePack specification, this library restricts map keys to str values. Maps with keys of any other type are rejected with a parse_error.113 exception (or, with allow_exceptions set to false, a discarded value) naming the type of the key that was found, for instance:

    [json.exception.parse_error.113] parse error at byte 2: syntax error while parsing MessagePack object key: only string keys are supported, but found nil; last byte: 0xC0\n

    This applies to the SAX interface as well, as the key is read before it is passed on. Such input needs a general-purpose MessagePack library instead.

    Ill-formed UTF-8 in string values

    The MessagePack specification explicitly allows a str value (fixstr, str 8, str 16, str 32) to contain a byte sequence that is not valid UTF-8, and expects a deserializer to hand the original bytes back unchanged. This library follows that by default: with its error_handler parameter left at keep (the default), from_msgpack() reads str bytes (object keys included) as-is, without validating them, so such a value round-trips through from_msgpack(to_msgpack(j)) byte for byte. Passing error_handler_t::strict makes from_msgpack() check anyway and throw parse_error.113 for ill-formed UTF-8, and replace/ignore sanitize the string instead of keeping it. to_msgpack() also writes str bytes as-is by default, since the specification permits it; its error_handler parameter can be set to strict to throw type_error.316 instead, or to replace/ignore to sanitize the string, for instance for a decoder that rejects ill-formed UTF-8. However, dump() still requires valid UTF-8 and throws type_error.316 for a value read this way with the default keep handler, unless an error handler is passed that replaces or ignores the ill-formed bytes.

    Example: deserialize a JSON value from MessagePack 
    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create byte vector\n    std::vector<std::uint8_t> v = {0x82, 0xa7, 0x63, 0x6f, 0x6d, 0x70, 0x61, 0x63,\n                                   0x74, 0xc3, 0xa6, 0x73, 0x63, 0x68, 0x65, 0x6d,\n                                   0x61, 0x00\n                                  };\n\n    // deserialize it with MessagePack\n    json j = json::from_msgpack(v);\n\n    // print the deserialized JSON value\n    std::cout << std::setw(2) << j << std::endl;\n}\n

    Output:

    {\n  \"compact\": true,\n  \"schema\": 0\n}\n
    "},{"location":"features/binary_formats/ubjson/","title":"UBJSON","text":"

    Universal Binary JSON (UBJSON) is a binary form directly imitating JSON, but requiring fewer bytes of data. It aims to achieve the generality of JSON, combined with being much easier to process than JSON.

    References

    • UBJSON Website
    "},{"location":"features/binary_formats/ubjson/#serialization","title":"Serialization","text":"

    The library uses the following mapping from JSON values types to UBJSON types according to the UBJSON specification:

    JSON value type value/range UBJSON type marker null null null Z boolean true true T boolean false false F number_integer -9223372036854775808..-2147483649 int64 L number_integer -2147483648..-32769 int32 l number_integer -32768..-129 int16 I number_integer -128..127 int8 i number_integer 128..255 uint8 U number_integer 256..32767 int16 I number_integer 32768..2147483647 int32 l number_integer 2147483648..9223372036854775807 int64 L number_unsigned 0..127 int8 i number_unsigned 128..255 uint8 U number_unsigned 256..32767 int16 I number_unsigned 32768..2147483647 int32 l number_unsigned 2147483648..9223372036854775807 int64 L number_unsigned 9223372036854775808..18446744073709551615 high-precision H number_float any value float64 D string with shortest length indicator string S array see notes on optimized format array [ object see notes on optimized format map {

    Complete mapping

    The mapping is complete in the sense that any JSON value type can be converted to a UBJSON value.

    Any UBJSON output created by to_ubjson can be successfully parsed by from_ubjson.

    Size constraints

    The following values can not be converted to a UBJSON value:

    • strings with more than 9223372036854775807 bytes (theoretical)

    UTF-8 validation of string values and object keys

    UBJSON's required string encoding is UTF-8. By default (the error_handler parameter left at keep), to_ubjson() writes the bytes of string values and object keys unchanged, even if they are not valid UTF-8. With error_handler_t::strict, it throws type_error.316 for ill-formed UTF-8 instead; replace/ignore sanitize the string. JSON_STRICT_BINARY_UTF8 makes strict the default.

    Unused UBJSON markers

    The following markers are not used in the conversion:

    • Z: no-op values are not created.
    • C: single-byte strings are serialized with S markers.

    NaN/infinity handling

    If NaN or Infinity are stored inside a JSON number, they are serialized properly. This behavior differs from the dump() function which serializes NaN or Infinity to null.

    Optimized formats

    The optimized formats for containers are supported: Parameter use_size adds size information to the beginning of a container and removes the closing marker. Parameter use_type further checks whether all elements of a container have the same type and adds the type marker to the beginning of the container. The use_type parameter must only be used together with use_size = true.

    Note that use_size = true alone may result in larger representations - the benefit of this parameter is that the receiving side is immediately informed on the number of elements of the container.

    An array whose type marker is Z (null), T (true) or F (false) stores no payload at all, because the marker already is the value. Its declared count is therefore the only thing that decides how much memory the receiving side allocates, and a handful of bytes can describe billions of elements. from_ubjson rejects such an array with out_of_range.408 when the count exceeds 1,048,576 (1 << 20), and to_ubjson writes longer arrays of these types without the annotation, so any value it produces can be read back.

    Binary values

    If the JSON data contains the binary type, the value stored is a list of integers, as suggested by the UBJSON documentation. In particular, this means that serialization and the deserialization of a JSON containing binary values into UBJSON and back will result in a different JSON object.

    Example: serialize JSON values to UBJSON, with and without size/type optimization 
    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\n// function to print UBJSON's diagnostic format\nvoid print_byte(uint8_t byte)\n{\n    if (32 < byte and byte < 128)\n    {\n        std::cout << (char)byte;\n    }\n    else\n    {\n        std::cout << (int)byte;\n    }\n}\n\nint main()\n{\n    // create a JSON value\n    json j = R\"({\"compact\": true, \"schema\": false})\"_json;\n\n    // serialize it to UBJSON\n    std::vector<std::uint8_t> v = json::to_ubjson(j);\n\n    // print the vector content\n    for (auto& byte : v)\n    {\n        print_byte(byte);\n    }\n    std::cout << std::endl;\n\n    // create an array of numbers\n    json array = {1, 2, 3, 4, 5, 6, 7, 8};\n\n    // serialize it to UBJSON using default representation\n    std::vector<std::uint8_t> v_array = json::to_ubjson(array);\n    // serialize it to UBJSON using size optimization\n    std::vector<std::uint8_t> v_array_size = json::to_ubjson(array, true);\n    // serialize it to UBJSON using type optimization\n    std::vector<std::uint8_t> v_array_size_and_type = json::to_ubjson(array, true, true);\n\n    // print the vector contents\n    for (auto& byte : v_array)\n    {\n        print_byte(byte);\n    }\n    std::cout << std::endl;\n\n    for (auto& byte : v_array_size)\n    {\n        print_byte(byte);\n    }\n    std::cout << std::endl;\n\n    for (auto& byte : v_array_size_and_type)\n    {\n        print_byte(byte);\n    }\n    std::cout << std::endl;\n}\n

    Output:

    {i7compactTi6schemaF}\n[i1i2i3i4i5i6i7i8]\n[#i8i1i2i3i4i5i6i7i8\n[$i#i812345678\n
    "},{"location":"features/binary_formats/ubjson/#deserialization","title":"Deserialization","text":"

    The library maps UBJSON types to JSON value types as follows:

    UBJSON type JSON value type marker no-op no value, next value is read N null null Z false false F true true T float32 number_float d float64 number_float D uint8 number_unsigned U int8 number_integer i int16 number_integer I int32 number_integer l int64 number_integer L string string S char string C array array (optimized values are supported) [ object object (optimized values are supported) {

    Complete mapping

    The mapping is complete in the sense that any UBJSON value can be converted to a JSON value.

    Ill-formed UTF-8 in string values and object keys

    UBJSON's required string encoding is UTF-8, but checking it on read is opt-in: with the error_handler parameter left at keep (the default), from_ubjson() accepts a string value or object key whose bytes are not valid UTF-8 and hands them back unchanged. Passing error_handler_t::strict makes from_ubjson() check and throw parse_error.113 for ill-formed UTF-8, and replace/ignore sanitize the string instead of keeping it. However, dump() still requires valid UTF-8 and throws type_error.316 for a value read with the default keep handler, unless an error handler is passed that replaces or ignores the ill-formed bytes. to_ubjson()'s own error_handler parameter defaults to keep (see above), so such a value is written back unchanged.

    Example: deserialize a JSON value from UBJSON 
    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create byte vector\n    std::vector<std::uint8_t> v = {0x7B, 0x69, 0x07, 0x63, 0x6F, 0x6D, 0x70, 0x61,\n                                   0x63, 0x74, 0x54, 0x69, 0x06, 0x73, 0x63, 0x68,\n                                   0x65, 0x6D, 0x61, 0x69, 0x00, 0x7D\n                                  };\n\n    // deserialize it with UBJSON\n    json j = json::from_ubjson(v);\n\n    // print the deserialized JSON value\n    std::cout << std::setw(2) << j << std::endl;\n}\n

    Output:

    {\n  \"compact\": true,\n  \"schema\": 0\n}\n
    "},{"location":"features/element_access/","title":"Element Access","text":"

    There are many ways elements in a JSON value can be accessed:

    • unchecked access via operator[]
    • checked access via at
    • access with default value via value
    • iterators
    • JSON pointers

    Testing whether a key or index exists before accessing it is also possible, with contains or find (which returns an iterator to the value, or end() if it is not found).

    flowchart TD\n    A[\"accessing a value\"] --> B{\"must it exist?\"}\n    B -->|\"yes, missing is an error\"| C[\"at() -- throws\"]\n    B -->|\"yes, but checking is my job\"| D[\"operator[] -- unchecked\"]\n    B -->|\"no, a fallback is fine\"| E[\"value() -- default value\"]\n    A --> F{\"just testing first?\"}\n    F -->|\"yes\"| G[\"contains() / find()\"]
    "},{"location":"features/element_access/checked_access/","title":"Checked access: at","text":""},{"location":"features/element_access/checked_access/#overview","title":"Overview","text":"

    The at member function performs checked access; that is, it returns a reference to the desired value if it exists and throws a basic_json::out_of_range exception otherwise.

    Example: read access

    Consider the following JSON value:

    {\n    \"name\": \"Mary Smith\",\n    \"age\": 42,\n    \"hobbies\": [\"hiking\", \"reading\"]\n}\n

    Assume the value is parsed to a json variable j.

    expression value j {\"name\": \"Mary Smith\", \"age\": 42, \"hobbies\": [\"hiking\", \"reading\"]} j.at(\"name\") \"Mary Smith\" j.at(\"age\") 42 j.at(\"hobbies\") [\"hiking\", \"reading\"] j.at(\"hobbies\").at(0) \"hiking\" j.at(\"hobbies\").at(1) \"reading\"

    The return value is a reference, so it can be used to modify the original value.

    Example: write access 
    j.at(\"name\") = \"John Smith\";\n

    This code produces the following JSON value:

    {\n    \"name\": \"John Smith\",\n    \"age\": 42,\n    \"hobbies\": [\"hiking\", \"reading\"]\n}\n

    When accessing an invalid index (i.e., an index greater than or equal to the array size) or the passed object key is non-existing, an exception is thrown.

    Example: access via invalid index or missing key 
    j.at(\"hobbies\").at(3) = \"cooking\";\n

    This code produces the following exception:

    [json.exception.out_of_range.401] array index 3 is out of range\n

    When extended diagnostic messages are enabled by defining JSON_DIAGNOSTICS, the exception further gives information where the key or index is missing or out of range.

    [json.exception.out_of_range.401] (/hobbies) array index 3 is out of range\n
    "},{"location":"features/element_access/checked_access/#notes","title":"Notes","text":"

    Exceptions

    • at can only be used with objects (with a string argument) or with arrays (with a numeric argument). For other types, a basic_json::type_error is thrown.
    • basic_json::out_of_range exception exceptions are thrown if the provided key is not found in an object or the provided index is invalid.
    "},{"location":"features/element_access/checked_access/#summary","title":"Summary","text":"scenario non-const value const value access to existing object key reference to existing value is returned const reference to existing value is returned access to valid array index reference to existing value is returned const reference to existing value is returned access to non-existing object key basic_json::out_of_range exception is thrown basic_json::out_of_range exception is thrown access to invalid array index basic_json::out_of_range exception is thrown basic_json::out_of_range exception is thrown"},{"location":"features/element_access/default_value/","title":"Access with default value: value","text":""},{"location":"features/element_access/default_value/#overview","title":"Overview","text":"

    In many situations, such as configuration files, missing values are not exceptional, but may be treated as if a default value was present. For this case, use value(key, default_value) which takes the key you want to access and a default value in case there is no value stored with that key. This is equivalent to Python's dict.get(key, default).

    "},{"location":"features/element_access/default_value/#example","title":"Example","text":"Example

    Consider the following JSON value:

    {\n    \"logOutput\": \"result.log\",\n    \"append\": true\n}\n

    Assume the value is parsed to a json variable j.

    expression value j {\"logOutput\": \"result.log\", \"append\": true} j.value(\"logOutput\", \"logfile.log\") \"result.log\" j.value(\"append\", true) true j.value(\"append\", false) true j.value(\"logLevel\", \"verbose\") \"verbose\""},{"location":"features/element_access/default_value/#nested-values","title":"Nested values","text":"

    To read a value deep inside a document, pass a JSON Pointer instead of a key. The default value is returned if the value at the pointer does not exist, including the case that an intermediate key is missing. There is no need to check each level with contains first.

    json j = {{\"server\", {{\"port\", 8080}}}, {\"list\", {10, 20}}};\n\nint port = j.value(\"/server/port\"_json_pointer, 80);        // 8080\nint timeout = j.value(\"/server/limits/timeout\"_json_pointer, 30); // 30 (missing intermediate key)\nint second = j.value(\"/list/1\"_json_pointer, 0);            // 20 (numeric tokens index arrays)\nint third = j.value(\"/list/5\"_json_pointer, 0);             // 0 (index out of range)\n\nbool has_port = j.contains(\"/server/port\"_json_pointer);    // true\nbool has_host = j.contains(\"/server/host/name\"_json_pointer); // false\n

    If the path is only available as a dotted string such as \"server.port\", do not build the pointer by concatenating \"/\" and the parts: keys containing / or ~ would be misinterpreted. Append each part as a reference token with operator/= instead. It escapes the token for you.

    json::json_pointer to_pointer(const std::string& dotted)\n{\n    json::json_pointer ptr;\n    std::istringstream in(dotted);\n    for (std::string token; std::getline(in, token, '.');)\n    {\n        ptr /= token;\n    }\n    return ptr;\n}\n\nint port = j.value(to_pointer(\"server.port\"), 80);          // 8080\nint first = j.value(to_pointer(\"list.0\"), 0);               // 10\n

    The key \"a/b\" yields the pointer \"/a~1b\"; the dot-splitting itself is up to the caller, so keys containing . need a different separator.

    "},{"location":"features/element_access/default_value/#notes","title":"Notes","text":"

    Exceptions

    • With string keys, value can only be used with objects. For other types, a basic_json::type_error is thrown.
    • With JSON Pointers, value can be used with both objects and arrays. For other types (null, boolean, number, string), a basic_json::type_error is thrown.

    null and mistyped members are not missing

    value returns the default value only if the key is absent. If the member exists, it is converted to the type of the default value, even if it is null. For {\"k\": null}, the call j.value(\"k\", 0) throws a basic_json::type_error, and so does a member of another type such as a string where a number is expected. The same holds for JSON Pointers.

    To treat null like a missing value, check for it explicitly:

    int n = (j.contains(\"k\") && !j[\"k\"].is_null()) ? j[\"k\"].get<int>() : 0;\n

    With C++17, get<std::optional<T>>() maps null to an empty optional. As at throws out_of_range for an absent key, use find to cover both cases:

    std::optional<int> n;                        // empty if \"k\" is absent or null\nif (const auto it = j.find(\"k\"); it != j.end())\n{\n    n = it->get<std::optional<int>>();       // still throws for a non-number such as \"text\"\n}\n

    Return type

    The value function is a template, and the return type of the function is determined by the type of the provided default value unless otherwise specified. This can have unexpected effects. In the example below, we store a 64-bit unsigned integer. We get exactly that value when using operator[]. However, when we call value and provide 0 as default value, then -1 is returned. This occurs, because 0 has type int which overflows when handling the value 18446744073709551615.

    To address this issue, either provide a correctly typed default value or use the template parameter to specify the desired return type. Note that this issue occurs even when a value is stored at the provided key, and the default value is not used as the return value.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    json j = json::parse(R\"({\"uint64\": 18446744073709551615})\");\n\n    std::cout << \"operator[]:                \" << j[\"uint64\"] << '\\n'\n              << \"default value (int):       \" << j.value(\"uint64\", 0) << '\\n'\n              << \"default value (uint64_t):  \" << j.value(\"uint64\", std::uint64_t(0)) << '\\n'\n              << \"explicit return value type: \" << j.value<std::uint64_t>(\"uint64\", 0) << '\\n';\n}\n

    Output:

    operator[]:                18446744073709551615\ndefault value (int):       -1\ndefault value (uint64_t):  18446744073709551615\nexplicit return value type: 18446744073709551615\n
    "},{"location":"features/element_access/default_value/#see-also","title":"See also","text":"
    • value for access with default value
    • documentation on checked access
    • documentation on JSON Pointer
    • contains to check whether a key or JSON Pointer exists
    • json_pointer::operator/= to build a pointer token by token
    "},{"location":"features/element_access/unchecked_access/","title":"Unchecked access: operator[]","text":""},{"location":"features/element_access/unchecked_access/#overview","title":"Overview","text":"

    Elements in a JSON object and a JSON array can be accessed via operator[] similar to a std::map and a std::vector, respectively.

    Example: read access

    Consider the following JSON value:

    {\n    \"name\": \"Mary Smith\",\n    \"age\": 42,\n    \"hobbies\": [\"hiking\", \"reading\"]\n}\n

    Assume the value is parsed to a json variable j.

    expression value j {\"name\": \"Mary Smith\", \"age\": 42, \"hobbies\": [\"hiking\", \"reading\"]} j[\"name\"] \"Mary Smith\" j[\"age\"] 42 j[\"hobbies\"] [\"hiking\", \"reading\"] j[\"hobbies\"][0] \"hiking\" j[\"hobbies\"][1] \"reading\"

    The return value is a reference, so it can modify the original value. In case the passed object key is non-existing, a null value is inserted which can immediately be overwritten.

    Example: write access 
    j[\"name\"] = \"John Smith\";\nj[\"maidenName\"] = \"Jones\";\n

    This code produces the following JSON value:

    {\n    \"name\": \"John Smith\",\n    \"maidenName\": \"Jones\",\n    \"age\": 42,\n    \"hobbies\": [\"hiking\", \"reading\"]\n}\n

    When accessing an invalid index (i.e., an index greater than or equal to the array size), the JSON array is resized such that the passed index is the new maximal index. Intermediate values are filled with null.

    Example: filling up arrays with null values 
    j[\"hobbies\"][0] = \"running\";\nj[\"hobbies\"][3] = \"cooking\";\n

    This code produces the following JSON value:

    {\n    \"name\": \"John Smith\",\n    \"maidenName\": \"Jones\",\n    \"age\": 42,\n    \"hobbies\": [\"running\", \"reading\", null, \"cooking\"]\n}\n
    "},{"location":"features/element_access/unchecked_access/#notes","title":"Notes","text":"

    Design rationale

    The library behaves differently to std::vector and std::map:

    • std::vector::operator[] never inserts a new element.
    • std::map::operator[] is not available for const values.

    The type json wraps all JSON value types. It would be impossible to remove operator[] for const objects. At the same time, inserting elements for non-const objects is really convenient as it avoids awkward insert calls. To this end, we decided to have an inserting non-const behavior for both arrays and objects.

    Info

    The access is unchecked. In case the passed object key does not exist or the passed array index is invalid, no exception is thrown.

    Danger

    • It is undefined behavior to access a const object with a non-existing key.
    • It is undefined behavior to access a const array with an invalid index.
    • In debug mode, an assertion will fire in both cases. You can disable assertions by defining the preprocessor symbol NDEBUG or redefine the macro JSON_ASSERT(x). See the documentation on runtime assertions for more information.

    Exceptions

    operator[] can only be used with objects (with a string argument) or with arrays (with a numeric argument). For other types, a basic_json::type_error is thrown.

    "},{"location":"features/element_access/unchecked_access/#performance-reserving-array-capacity","title":"Performance: reserving array capacity","text":"

    There is no public reserve(count) member on basic_json for pre-allocating array capacity. If you are building a large array incrementally (e.g., via repeated push_back()) and know its final size ahead of time, you can reserve capacity via get_ref() to access the underlying array_t directly:

    json j = json::array();\nj.get_ref<json::array_t&>().reserve(1000);\nfor (int i = 0; i < 1000; ++i) {\n    j.push_back(i);\n}\n
    "},{"location":"features/element_access/unchecked_access/#summary","title":"Summary","text":"scenario non-const value const value access to existing object key reference to existing value is returned const reference to existing value is returned access to valid array index reference to existing value is returned const reference to existing value is returned access to non-existing object key reference to newly inserted null value is returned undefined behavior; runtime assertion in debug mode access to invalid array index reference to newly inserted null value is returned; any index between previous maximal index and passed index are filled with null undefined behavior; runtime assertion in debug mode"},{"location":"features/parsing/","title":"Parsing","text":"

    This library can create a JSON value from a wide range of inputs. This page gives an overview of the available parsing functions and how they behave; the linked pages go into more detail.

    flowchart LR\n    I[\"JSON input\"] --> P[\"parse()\"]\n    I --> S[\"sax_parse()\"]\n    I --> A[\"accept()\"]\n    P -->|\"optional parser callback filters values\"| D[\"basic_json value (DOM)\"]\n    S --> H[\"events delivered to a user SAX handler\"]\n    A --> V[\"bool: is the input valid JSON?\"]
    "},{"location":"features/parsing/#input","title":"Input","text":"

    The parse function reads a JSON value from an input. The input can be

    • a string (std::string, C string, or string literal),
    • a std::istream (e.g., an std::ifstream reading from a file),
    • a FILE* pointer,
    • a pair of iterators over a contiguous range (e.g., a std::vector<std::uint8_t>), or
    • a contiguous container.
    // parse from a string\njson j = json::parse(R\"({\"happy\": true, \"pi\": 3.141})\");\n\n// parse from a file\nstd::ifstream f(\"example.json\");\njson data = json::parse(f);\n

    The input must be encoded in UTF-8; other encodings are not supported. A single input may contain only one JSON value. Inputs consisting of multiple values separated by newlines are handled by the JSON Lines format.

    By default, the library rejects comments and trailing commas. Both can be enabled with parameters of the parse function \u2014 see comments and trailing commas.

    "},{"location":"features/parsing/#strictness-and-trailing-data","title":"Strictness and trailing data","text":"

    parse reads a single JSON value and requires the whole input to be consumed: any non-whitespace data after the value is reported as a parse error. Use it when you want to guarantee that an input is exactly one complete JSON document.

    operator>> follows relaxed std::istream semantics instead: it parses one JSON value and leaves the stream positioned right after it, without requiring the rest of the stream to be consumed. This is what makes it possible to read several concatenated values from the same stream, but it also means that \"a valid document followed by trailing bytes\" is accepted rather than rejected. If you are validating conformance, or need to reject any input that is not exactly one JSON document, prefer parse.

    When using operator>> to read several concatenated values this way, a value that is a number must be followed by whitespace, because operator>> consumes the character that terminates a number, unless JSON_PRECISE_STREAM_POSITION is defined to 1 \u2014 see the operator>> notes for details and examples.

    "},{"location":"features/parsing/#input-sources-and-performance","title":"Input sources and performance","text":"

    All inputs produce the same values and the same exceptions, but they are read in different ways, which makes some of them faster than others:

    input how it is read strings, string views, string literals, contiguous containers, pointer or iterator ranges over contiguous memory the whole input is scanned in bulk: runs of string characters, numbers, and indentation are recognized several bytes at a time std::istream (std::ifstream, std::istringstream, ...) the characters the stream buffer holds are scanned in bulk like a string; the stream buffer is asked for more when they are used up FILE* one character at a time, with the file locked once for the whole parse other iterator ranges (e.g., std::list<char>, std::istreambuf_iterator<char>), wide strings one character at a time

    Parsing a file from a std::ifstream is therefore only about 10 to 20 % slower than reading the whole file into a std::string first and parsing that, and it needs no memory for a copy of the file. The stream is never read ahead: after parsing, it is positioned exactly as if every character had been read one at a time, so operator>> can still read several values from one stream (see also JSON Lines).

    Two macros switch these optimizations off without changing any result: JSON_DISABLE_STREAM_BULK_SCAN for stream buffers that do not follow the usual std::streambuf contract, and JSON_DISABLE_UNLOCKED_FILE_IO for programs in which another thread must use the same FILE object while it is parsed.

    "},{"location":"features/parsing/#sax-vs-dom-parsing","title":"SAX vs. DOM parsing","text":"

    The library offers two parsing models:

    • DOM parsing (the default): the complete input is read and stored as an in-memory basic_json value that can be traversed and modified freely. This is what parse does, and it is the right choice for most use cases.
    • SAX parsing: instead of building a value, the parser reports events (such as \"a string was read\" or \"an object started\") to a handler that you implement. This avoids building the full value in memory and is useful for very large inputs or when you only need to extract parts of the input. See the SAX interface for details and sax_parse for the API.

    You can influence a DOM parse without switching to the SAX interface by passing a parser callback, which is called during parsing and can, for example, discard parts of the input.

    "},{"location":"features/parsing/#exceptions","title":"Exceptions","text":"

    When the input is not valid JSON, the parse function throws an exception by default. If exceptions are undesired or unavailable, the parser can instead return a discarded value, or accept can be used to only check whether an input is valid JSON. See parsing and exceptions for the available options.

    "},{"location":"features/parsing/#see-also","title":"See also","text":"
    • parse - deserialize from a compatible input
    • accept - check if the input is valid JSON
    • sax_parse - generate SAX events
    • JSON Lines - parse newline-delimited JSON
    • parser callbacks - influence the parsing by a callback function
    • SAX interface - implement a custom SAX handler
    • parsing and exceptions - control error handling
    • parsing untrusted input - what to consider when parsing input from untrusted sources
    "},{"location":"features/parsing/json_lines/","title":"JSON Lines","text":"

    The JSON Lines format is a text format of newline-delimited JSON. In particular:

    1. The input must be UTF-8 encoded.
    2. Every line must be a valid JSON value.
    3. The line separator must be \\n. As \\r is silently ignored, \\r\\n is also supported.
    4. The final character may be \\n, but is not required to be one.

    JSON Text example

    {\"name\": \"Gilbert\", \"wins\": [[\"straight\", \"7\u2663\"], [\"one pair\", \"10\u2665\"]]}\n{\"name\": \"Alexa\", \"wins\": [[\"two pair\", \"4\u2660\"], [\"two pair\", \"9\u2660\"]]}\n{\"name\": \"May\", \"wins\": []}\n{\"name\": \"Deloise\", \"wins\": [[\"three of a kind\", \"5\u2663\"]]}\n

    JSON Lines input with more than one value is treated as invalid JSON by the parse or accept functions. To process it line by line, functions like std::getline can be used:

    Example: Parse JSON Text input line by line

    The example below demonstrates how JSON Lines can be processed.

    #include <sstream>\n#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // JSON Lines (see https://jsonlines.org)\n    std::stringstream input;\n    input << R\"({\"name\": \"Gilbert\", \"wins\": [[\"straight\", \"7\u2663\"], [\"one pair\", \"10\u2665\"]]}\n{\"name\": \"Alexa\", \"wins\": [[\"two pair\", \"4\u2660\"], [\"two pair\", \"9\u2660\"]]}\n{\"name\": \"May\", \"wins\": []}\n{\"name\": \"Deloise\", \"wins\": [[\"three of a kind\", \"5\u2663\"]]}\n)\";\n\n    std::string line;\n    while (std::getline(input, line))\n    {\n        std::cout << json::parse(line) << std::endl;\n    }\n}\n

    Output:

    {\"name\":\"Gilbert\",\"wins\":[[\"straight\",\"7\u2663\"],[\"one pair\",\"10\u2665\"]]}\n{\"name\":\"Alexa\",\"wins\":[[\"two pair\",\"4\u2660\"],[\"two pair\",\"9\u2660\"]]}\n{\"name\":\"May\",\"wins\":[]}\n{\"name\":\"Deloise\",\"wins\":[[\"three of a kind\",\"5\u2663\"]]}\n

    Note

    Using operator>> like

    json j;\nwhile (input >> j)\n{\n    std::cout << j << std::endl;\n}\n

    with a JSON Lines input does not work, because the parser will try to parse one value after the last one and throw a parse_error.101 exception. The same happens for a stream of concatenated (non-newline-delimited) JSON values: operator>> reads them one at a time, but the loop above throws after the last value. To read either format with operator>>, check for the end of the stream before each read:

    json j;\nwhile (input >> std::ws && input.peek() != std::char_traits<char>::eof())\n{\n    input >> j;\n    std::cout << j << std::endl;\n}\n

    A value that is a number must be followed by whitespace -- see the notes of operator>> for details.

    "},{"location":"features/parsing/parse_exceptions/","title":"Parsing and Exceptions","text":"

    When the input is not valid JSON, an exception of type parse_error is thrown. This exception contains the position in the input where the error occurred, together with a diagnostic message and the last read input token. The exceptions page contains a list of examples for parse error exceptions. In case you process untrusted input, always enclose your code with a try/catch block, like

    json j;\ntry\n{\n    j = json::parse(my_input);\n}\ncatch (json::parse_error& ex)\n{\n    std::cerr << \"parse error at byte \" << ex.byte << std::endl;\n}\n

    In case exceptions are undesired or not supported by the environment, there are different ways to proceed:

    "},{"location":"features/parsing/parse_exceptions/#switch-off-exceptions","title":"Switch off exceptions","text":"

    The parse() function accepts a bool parameter allow_exceptions which controls whether an exception is thrown when a parse error occurs (true, default) or whether a discarded value should be returned (false).

    json j = json::parse(my_input, nullptr, false);\nif (j.is_discarded())\n{\n    std::cerr << \"parse error\" << std::endl;\n}\n

    Note there is no diagnostic information available in this scenario.

    "},{"location":"features/parsing/parse_exceptions/#use-accept-function","title":"Use accept() function","text":"

    Alternatively, function accept() can be used which does not return a json value, but a bool indicating whether the input is valid JSON.

    if (!json::accept(my_input))\n{\n    std::cerr << \"parse error\" << std::endl;\n}\n

    Again, there is no diagnostic information available.

    "},{"location":"features/parsing/parse_exceptions/#user-defined-sax-interface","title":"User-defined SAX interface","text":"

    Finally, you can implement the SAX interface and decide what should happen in case of a parse error.

    This function has the following interface:

    bool parse_error(std::size_t position,\n                 const std::string& last_token,\n                 const json::exception& ex);\n

    The return value indicates whether the parsing should continue, so the function should usually return false.

    Example: report parse errors without exceptions

    The example derives from the library's DOM parser and overrides parse_error to print the error instead of throwing. Note the DOM parser is an implementation detail (nlohmann::detail) and may change between releases; see Do not use the detail namespace.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\n// a DOM parser that reports parse errors instead of throwing\nclass sax_no_exception : public nlohmann::detail::json_sax_dom_parser<json, nlohmann::detail::string_input_adapter_type>\n{\n  public:\n    explicit sax_no_exception(json& j)\n        : nlohmann::detail::json_sax_dom_parser<json, nlohmann::detail::string_input_adapter_type>(j, false)\n    {}\n\n    bool parse_error(std::size_t position,\n                     const std::string& last_token,\n                     const json::exception& ex)\n    {\n        std::cout << \"parse error at input byte \" << position << \"\\n\"\n                  << ex.what() << \"\\n\"\n                  << \"last read: \\\"\" << last_token << \"\\\"\"\n                  << std::endl;\n        return false;\n    }\n};\n\nint main()\n{\n    std::string myinput = \"[1,2,3,]\";\n\n    json result;\n    sax_no_exception sax(result);\n\n    bool parse_result = json::sax_parse(myinput, &sax);\n    if (!parse_result)\n    {\n        std::cout << \"parsing unsuccessful!\" << std::endl;\n    }\n\n    std::cout << \"parsed value: \" << result << std::endl;\n}\n

    Output:

    parse error at input byte 8\n[json.exception.parse_error.101] parse error at line 1, column 8: syntax error while parsing value - unexpected ']'; expected '[', '{', or a literal\nlast read: \"3,]\"\nparsing unsuccessful!\nparsed value: [1,2,3]\n
    "},{"location":"features/parsing/parser_callbacks/","title":"Parser Callbacks","text":""},{"location":"features/parsing/parser_callbacks/#overview","title":"Overview","text":"

    With a parser callback function, the result of parsing a JSON text can be influenced. When passed to parse, it is called on certain events (passed as parse_event_t via parameter event) with a set recursion depth depth and context JSON value parsed. The return value of the callback function is a boolean indicating whether the element that emitted the callback shall be kept or not.

    The type of the callback function is:

    template<typename BasicJsonType>\nusing parser_callback_t =\n    std::function<bool(int depth, parse_event_t event, BasicJsonType& parsed)>;\n
    "},{"location":"features/parsing/parser_callbacks/#callback-event-types","title":"Callback event types","text":"

    We distinguish six scenarios (determined by the event type) in which the callback function can be called. The following table describes the values of the parameters depth, event, and parsed.

    parameter event description parameter depth parameter parsed parse_event_t::object_start the parser read { and started to process a JSON object depth of the parent of the JSON object a JSON value with type discarded parse_event_t::key the parser read a key of a value in an object depth of the currently parsed JSON object a JSON string containing the key parse_event_t::object_end the parser read } and finished processing a JSON object depth of the parent of the JSON object the parsed JSON object parse_event_t::array_start the parser read [ and started to process a JSON array depth of the parent of the JSON array a JSON value with type discarded parse_event_t::array_end the parser read ] and finished processing a JSON array depth of the parent of the JSON array the parsed JSON array parse_event_t::value the parser finished reading a JSON value depth of the value the parsed JSON value Example: sequence of callback events

    When parsing the following JSON text,

    {\n    \"name\": \"Berlin\",\n    \"location\": [\n        52.519444,\n        13.406667\n    ]\n}\n

    these calls are made to the callback function:

    event depth parsed object_start 0 discarded key 1 \"name\" value 1 \"Berlin\" key 1 \"location\" array_start 1 discarded value 2 52.519444 value 2 13.406667 array_end 1 [52.519444,13.406667] object_end 0 {\"location\":[52.519444,13.406667],\"name\":\"Berlin\"}

    No built-in nesting depth limit

    The library has no built-in limit on recursion/nesting depth while parsing. A parser callback can only discard content it has already parsed (by returning false); it cannot make parsing fail once a nesting limit is exceeded partway through reading a deeply nested value. If you need to reject over-deep untrusted input outright, track depth in a callback and throw from it once your limit is exceeded (a thrown exception propagates out of parse() as usual).

    "},{"location":"features/parsing/parser_callbacks/#return-value","title":"Return value","text":"

    Discarding a value (i.e., returning false) has different effects depending on the context in which the function was called:

    • Discarded values in structured types are skipped. That is, the parser will behave as if the discarded value was never read.
    • In case a value outside a structured type is skipped, it is replaced with null. This case happens if the top-level element is skipped.
    Example: skip an object key while parsing

    The example below demonstrates the parse() function with and without callback function.

    #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // a JSON text\n    auto text = R\"(\n    {\n        \"Image\": {\n            \"Width\":  800,\n            \"Height\": 600,\n            \"Title\":  \"View from 15th Floor\",\n            \"Thumbnail\": {\n                \"Url\":    \"http://www.example.com/image/481989943\",\n                \"Height\": 125,\n                \"Width\":  100\n            },\n            \"Animated\" : false,\n            \"IDs\": [116, 943, 234, 38793]\n        }\n    }\n    )\";\n\n    // parse and serialize JSON\n    json j_complete = json::parse(text);\n    std::cout << std::setw(4) << j_complete << \"\\n\\n\";\n\n    // define parser callback\n    json::parser_callback_t cb = [](int depth, json::parse_event_t event, json & parsed)\n    {\n        // skip object elements with key \"Thumbnail\"\n        if (event == json::parse_event_t::key and parsed == json(\"Thumbnail\"))\n        {\n            return false;\n        }\n        else\n        {\n            return true;\n        }\n    };\n\n    // parse (with callback) and serialize JSON\n    json j_filtered = json::parse(text, cb);\n    std::cout << std::setw(4) << j_filtered << '\\n';\n}\n

    Output:

    {\n    \"Image\": {\n        \"Animated\": false,\n        \"Height\": 600,\n        \"IDs\": [\n            116,\n            943,\n            234,\n            38793\n        ],\n        \"Thumbnail\": {\n            \"Height\": 125,\n            \"Url\": \"http://www.example.com/image/481989943\",\n            \"Width\": 100\n        },\n        \"Title\": \"View from 15th Floor\",\n        \"Width\": 800\n    }\n}\n\n{\n    \"Image\": {\n        \"Animated\": false,\n        \"Height\": 600,\n        \"IDs\": [\n            116,\n            943,\n            234,\n            38793\n        ],\n        \"Title\": \"View from 15th Floor\",\n        \"Width\": 800\n    }\n}\n
    "},{"location":"features/parsing/parser_callbacks/#recipe-rejecting-duplicate-object-keys","title":"Recipe: rejecting duplicate object keys","text":"

    The JSON specification leaves the handling of objects with repeated keys up to the implementation. As described in object_t, it is unspecified which value for a repeated key ends up in the resulting json value -- once parsing has produced that value, the duplicate is already gone, because object storage maps each key to a single value. If duplicate keys should instead be treated as an error, a parser callback can detect them while the object is still being read, before that ambiguity ever applies.

    Example: reject duplicate object keys 
    #include <iostream>\n#include <nlohmann/json.hpp>\n#include <stdexcept>\n#include <string>\n#include <unordered_set>\n#include <vector>\n\nusing json = nlohmann::json;\n\njson parse_strict(const std::string& input)\n{\n    // one key set per nesting depth, reused across sibling objects\n    std::vector<std::unordered_set<std::string>> keys;\n\n    auto reject_duplicate_keys = [&](int depth, json::parse_event_t event, json & parsed)\n    {\n        if (event == json::parse_event_t::object_start)\n        {\n            // keys of this object are reported at depth+1 (see the event table above)\n            const auto child_depth = static_cast<std::size_t>(depth) + 1;\n            if (keys.size() <= child_depth)\n            {\n                keys.resize(child_depth + 1);\n            }\n            keys[child_depth].clear();\n            return true;\n        }\n\n        if (event == json::parse_event_t::key)\n        {\n            auto& seen = keys[static_cast<std::size_t>(depth)];\n            const auto& key = parsed.get_ref<const std::string&>();\n            if (!seen.insert(key).second)\n            {\n                throw std::runtime_error(\"duplicate JSON object key: \" + key);\n            }\n            return true;\n        }\n\n        return true;\n    };\n\n    return json::parse(input, reject_duplicate_keys);\n}\n\nint main()\n{\n    // parsing succeeds when all keys are unique\n    json j = parse_strict(R\"({\"one\": 1, \"two\": 2})\");\n    std::cout << j << '\\n';\n\n    // parsing throws when a key is repeated\n    try\n    {\n        parse_strict(R\"({\"one\": 1, \"one\": 2})\");\n    }\n    catch (const std::exception& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

    Output:

    {\"one\":1,\"two\":2}\nduplicate JSON object key: one\n

    This approach has three limitations:

    • The depth-indexed bookkeeping must account for the fact that object_start reports the depth of the parent of the object, while the key events inside that object are reported one depth deeper (see the event table above); it is easy to get this off by one for nested objects.
    • The thrown exception cannot carry a parse_error-style byte offset, because position tracking only exists inside the parser and lexer, not at the callback layer.
    • The exception only names the repeated key, not where it occurs in the document. Reporting its full path requires maintaining a stack of the enclosing keys and array indices in the callback as well.

    A SAX interface does not lift the position limitation: its key function receives no position either -- only parse_error is passed the byte position.

    "},{"location":"features/parsing/parser_callbacks/#recipe-streaming-a-large-homogeneous-array","title":"Recipe: streaming a large homogeneous array","text":"

    A common use case is a huge top-level array of many similarly-shaped objects, too large to hold entirely in memory as a json value. A parser callback can hand off each completed element to a user function and then discard it, so memory usage stays bounded by a single element (plus the not-yet-parsed tail of the input) rather than the whole document. Since the top-level array's array_start/array_end are reported at depth == 0 (its parent is the document root), the object elements it contains are reported at depth == 1:

    Example: stream a large top-level array 
    std::ifstream input(\"large_array.json\");\n\nauto callback = [](int depth, json::parse_event_t event, json& parsed) -> bool {\n    if (depth == 1 && event == json::parse_event_t::object_end) {\n        handle_element(parsed); // process the element, e.g. write it elsewhere\n        return false;           // discard it -- frees its memory before the next one is parsed\n    }\n    return true; // keep everything else, including the (by then empty) top-level array\n};\n\njson::parse(input, callback);\n

    If the array's elements are scalars or nested arrays instead of objects, check for parse_event_t::value or parse_event_t::array_end at depth == 1 instead. The same approach works for a top-level object of many homogeneous values by checking object_end/value events at depth == 1 there too.

    "},{"location":"features/parsing/parser_callbacks/#recipe-max-nesting-depth-via-a-callback","title":"Recipe: max nesting depth via a callback","text":"

    Since there is no built-in nesting-depth limit (see the note above), a callback can enforce one manually by tracking the maximum depth seen and throwing once it is exceeded:

    Example: limit the nesting depth 
    constexpr int max_depth = 32;\n\nauto callback = [](int depth, json::parse_event_t /*event*/, json& /*parsed*/) -> bool {\n    if (depth > max_depth) {\n        throw std::runtime_error(\"maximum nesting depth exceeded\");\n    }\n    return true;\n};\n\njson::parse(input, callback);\n
    "},{"location":"features/parsing/sax_interface/","title":"SAX Interface","text":"

    The library uses a SAX-like interface with the following functions:

    classDiagram\n\nclass sax_t [\"json::sax_t\"] {\n    <<interface>>\n    +bool null()*\n\n    +bool boolean(bool val)*\n\n    +bool number_integer(number_integer_t val)*\n    +bool number_unsigned(number_unsigned_t val)*\n\n    +bool number_float(number_float_t val, const string_t& s)*\n\n    +bool string(string_t& val)*\n    +bool binary(binary_t& val)*\n\n    +bool start_object(std::size_t elements)*\n    +bool end_object()*\n    +bool start_array(std::size_t elements)*\n    +bool end_array()*\n    +bool key(string_t& val)*\n\n    +bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex)*\n}
    // called when null is parsed\nbool null();\n\n// called when a boolean is parsed; value is passed\nbool boolean(bool val);\n\n// called when a signed or unsigned integer number is parsed; value is passed\nbool number_integer(number_integer_t val);\nbool number_unsigned(number_unsigned_t val);\n\n// called when a floating-point number is parsed; value and original string is passed\nbool number_float(number_float_t val, const string_t& s);\n\n// called when a string is parsed; value is passed and can be safely moved away\nbool string(string_t& val);\n// called when a binary value is parsed; value is passed and can be safely moved away\nbool binary(binary_t& val);\n\n// called when an object or array begins or ends, resp. The number of elements is passed (or -1 if not known)\nbool start_object(std::size_t elements);\nbool end_object();\nbool start_array(std::size_t elements);\nbool end_array();\n// called when an object key is parsed; value is passed and can be safely moved away\nbool key(string_t& val);\n\n// called when a parse error occurs; byte position, the last token, and an exception is passed\nbool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex);\n

    The return value of each function determines whether parsing should proceed.

    To implement your own SAX handler, proceed as follows:

    1. Implement the SAX interface in a class. You can use class nlohmann::json_sax<json> as base class, but you can also use any class where the functions described above are implemented and public.
    2. Create an object of your SAX interface class, e.g. my_sax.
    3. Call bool json::sax_parse(input, &my_sax); where the first parameter can be any input like a string or an input stream and the second parameter is a pointer to your SAX interface.

    Note the sax_parse function only returns a bool indicating the result of the last executed SAX event. It does not return json value - it is up to you to decide what to do with the SAX events. Furthermore, no exceptions are thrown in case of a parse error - it is up to you what to do with the exception object passed to your parse_error implementation. Internally, the SAX interface is used for the DOM parser (class json_sax_dom_parser) as well as the acceptor (json_sax_acceptor), see file json_sax.hpp.

    "},{"location":"features/parsing/sax_interface/#see-also","title":"See also","text":"
    • json_sax - documentation of the SAX interface
    • sax_parse - SAX parser
    "},{"location":"features/parsing/untrusted_input/","title":"Parsing Untrusted Input","text":"

    This page is for applications that parse JSON -- or one of the supported binary formats (BJData, BON8, BSON, CBOR, MessagePack, UBJSON) -- from a source they do not fully control, such as a network connection, an uploaded file, or another process. It summarizes what the library already does for such input and what remains the caller's responsibility, linking to the pages that cover each aspect in detail rather than repeating them.

    For the project's threat model and the countermeasures behind these behaviors, see the assurance case; to report a vulnerability, see the security policy.

    "},{"location":"features/parsing/untrusted_input/#errors-without-exceptions","title":"Errors without exceptions","text":"

    By default, parse() throws a parse_error (for instance parse_error.101 for a syntax error) when the input is not valid. If your environment cannot use exceptions for untrusted input, the library offers several alternatives; see Parsing and exceptions for the full comparison:

    • Pass false as the third argument to parse() to get a discarded value (checked with is_discarded()) instead of a thrown exception, with no diagnostic information.
    • Use accept() to only check whether the input is valid JSON, without building a value.
    • Implement the SAX interface and override parse_error() to react to an error yourself, with the byte position and the exception that would otherwise have been thrown; see the example that overrides it to print instead of throw.

    If exceptions are unavailable entirely (-fno-exceptions, or JSON_NOEXCEPTION defined), every throw in the library becomes a call to std::abort() -- there is no way to recover from a parse error of untrusted input in that configuration; see Switch off exceptions for the details and for overriding this with JSON_THROW_USER.

    "},{"location":"features/parsing/untrusted_input/#nesting-depth","title":"Nesting depth","text":"

    The JSON parser and the binary readers are iterative: they keep the containers they are currently inside of on a heap-allocated stack instead of calling themselves once per nesting level, so the native call stack does not grow with the nesting depth of the input. A deeply nested document is therefore bounded by available memory, not by the call stack, however deeply it is nested.

    No built-in depth limit while parsing

    Neither the parser nor the binary readers impose a limit on how deep the input may nest. An attacker can still exhaust memory (though not the call stack) with a sufficiently deep document. If you need to reject over-deep untrusted input outright, track the depth yourself, either with a parser callback for the JSON parser, or by counting start_object/start_array and end_object/end_array calls in a SAX handler (for the JSON parser or a binary format alike) and throwing once your limit is exceeded.

    Once a value has been parsed, operations that walk it recursively -- serializing it with dump, hashing it, copying it, comparing two values with ==, <, or (in C++20) <=>, merging with update, applying a merge_patch, creating a patch with diff or merge_diff, flatten, and the binary writers (to_cbor, to_msgpack, ...) -- descend at most 128 levels on the call stack and continue below that with an explicit stack instead, so none of them can exhaust the stack either, however deeply the value is nested. Destroying a value (its destructor) never recurses at all, regardless of nesting depth, for the same reason.

    "},{"location":"features/parsing/untrusted_input/#input-size","title":"Input size","text":"

    The library does not limit the overall size of a JSON text; a value nested or wide enough will use memory proportional to the input. If you parse untrusted input of unbounded size, check the size of the file or stream yourself before -- or while -- handing it to parse().

    For the binary formats, an announced size is never trusted outright:

    • Reading a string or binary value copies the input in bounded 4096-byte chunks and grows the result as bytes are actually consumed, rather than allocating the announced length up front -- a truncated input runs out of bytes (reported as a parse error) instead of triggering an oversized allocation.
    • When an array announces its number of elements and the array container supports reserve() (as std::vector, the default, does), the library reserves storage for at most 16384 of them upfront, regardless of how large the announced count is; further elements still grow the container normally as they are read.
    • An announced array or object size that exceeds what the target container could ever hold (its max_size()) is rejected immediately as out_of_range.408, without attempting to allocate anything.
    "},{"location":"features/parsing/untrusted_input/#strings","title":"Strings","text":"

    Invalid UTF-8 is rejected while parsing, not just while serializing:

    • In JSON text, an ill-formed UTF-8 byte in a string is a parse_error.101 (\"invalid string: ill-formed UTF-8 byte\").
    • In a binary format, a string that is not valid UTF-8 is a parse_error.113.

    A '\\0' (NUL) byte inside a quoted JSON string is always rejected (it must be escaped as \\u0000). A NUL byte outside of a string is different: by default it is silently treated as the end of the input, so trailing bytes after it -- including further, otherwise well-formed JSON -- are silently ignored rather than rejected. Since untrusted input that happens to embed a NUL is a way to make part of it disappear without a parse error, see the FAQ entry and consider defining JSON_STRICT_NUL_HANDLING to 1 to reject a NUL byte like any other unexpected byte instead.

    Parsing is not the only place invalid UTF-8 matters: a string that reached a json value some other way (for example, constructed by application code, or, before JSON_STRICT_NUL_HANDLING existed, read from a binary format that does not validate strings) still has to round-trip back to JSON text. By default, dump() throws type_error.316 if the string is not valid UTF-8; passing error_handler_t::replace or error_handler_t::ignore avoids the exception instead of crashing an application that forgot to catch it. See Handling invalid UTF-8 for the options and an example.

    "},{"location":"features/parsing/untrusted_input/#duplicate-object-keys","title":"Duplicate object keys","text":"

    The JSON specification leaves the handling of repeated keys in an object up to the implementation, and this library does too: as described in object_t, it is unspecified which of the values for a repeated key ends up in the parsed object. If your application must reject duplicate keys instead of silently resolving them one way or another, see the parser callback recipe for rejecting duplicate keys.

    "},{"location":"features/parsing/untrusted_input/#numbers","title":"Numbers","text":"

    A number whose value cannot be represented -- for instance 1E1000, which overflows double -- is rejected while parsing as out_of_range.406 rather than silently becoming infinity. An integer that is syntactically valid but does not fit the 64-bit integer types is not rejected; it is instead stored as a double, which may lose precision for very large values. See number limits for the exact ranges and an example.

    "},{"location":"features/parsing/untrusted_input/#comments-and-trailing-commas","title":"Comments and trailing commas","text":"

    Both comments and trailing commas are rejected by default, matching the JSON specification; they must be explicitly enabled per call with the ignore_comments and ignore_trailing_commas parameters of parse() or accept(). Do not enable either for input whose conformance you cannot otherwise control, since interoperability with strictly conforming JSON consumers is exactly what the default rejects.

    "},{"location":"features/parsing/untrusted_input/#checklist","title":"Checklist","text":"
    • Wrap parsing in a try/catch block, or use allow_exceptions=false/accept() if your environment cannot use exceptions; never let JSON_NOEXCEPTION's abort() be the first time you think about error handling.
    • If the input's nesting depth matters to you, enforce your own limit with a parser callback or a SAX handler; the library bounds the call stack but not memory use.
    • Bound the size of the input itself before parsing, independent of the library's own bounded allocations for binary format lengths.
    • Decide up front how you want a string with invalid UTF-8 -- from any source, not only parsing -- to be serialized (strict, replace, or ignore), rather than discovering it from an uncaught type_error.316.
    • If a stray NUL byte silently truncating trailing input is a problem for your input format, define JSON_STRICT_NUL_HANDLING.
    • Decide whether duplicate object keys should be an error for your application, and add a callback if so.
    • Do not enable ignore_comments or ignore_trailing_commas for input that must be strictly conforming JSON.

    For the broader design rationale and how it is tested (fuzzing, sanitizers, static analysis), see the assurance case and quality assurance. To report a security issue in the library itself, follow the security policy.

    "},{"location":"features/parsing/untrusted_input/#see-also","title":"See also","text":"
    • Parsing - overview of the parsing functions
    • Parsing and exceptions - error handling without exceptions
    • Parser callbacks - depth limits, duplicate-key rejection, and streaming recipes
    • SAX interface - implement a custom handler with access to parse errors and positions
    • Serialization - handling invalid UTF-8 when dumping
    • Number handling - number ranges and overflow behavior
    • Assurance case - the library's threat model and countermeasures
    • Security policy - how to report a vulnerability
    "},{"location":"features/types/","title":"Types","text":"

    This page gives an overview of how JSON values are stored and how this can be configured.

    "},{"location":"features/types/#overview","title":"Overview","text":"

    By default, JSON values are stored as follows:

    JSON type C++ type object std::map<std::string, basic_json> array std::vector<basic_json> null std::nullptr_t string std::string boolean bool number std::int64_t, std::uint64_t, and double

    Note there are three different types for numbers - when parsing JSON text, the best fitting type is chosen.

    "},{"location":"features/types/#storage","title":"Storage","text":"
    classDiagram\n\nclass value_t {\n    <<enumeration>>\n    null\n    object\n    array\n    string\n    boolean\n    number_integer\n    number_unsigned\n    number_float\n    binary\n    discarded\n}\n\nclass json_value {\n    <<union>>\n    object_t* object\n    array_t* array\n    string_t* string\n    binary_t* binary\n    boolean_t boolean\n    number_integer_t number_integer\n    number_unsigned_t number_unsigned\n    number_float_t number_float\n}\n\nclass basic_json {\n    -value_t m_type\n    -json_value m_value\n    +typedef object_t\n    +typedef array_t\n    +typedef binary_t\n    +typedef boolean_t\n    +typedef number_integer_t\n    +typedef number_unsigned_t\n    +typedef number_float_t\n}\n\nbasic_json .. json_value\nbasic_json .. value_t
    "},{"location":"features/types/#template-arguments","title":"Template arguments","text":"

    The data types to store a JSON value are derived from the template arguments passed to class basic_json:

    template<\n    template<typename U, typename V, typename... Args> class ObjectType = std::map,\n    template<typename U, typename... Args> class ArrayType = std::vector,\n    class StringType = std::string,\n    class BooleanType = bool,\n    class NumberIntegerType = std::int64_t,\n    class NumberUnsignedType = std::uint64_t,\n    class NumberFloatType = double,\n    template<typename U> class AllocatorType = std::allocator,\n    template<typename T, typename SFINAE = void> class JSONSerializer = adl_serializer,\n    class BinaryType = std::vector<std::uint8_t>,\n    class CustomBaseClass = void\n>\nclass basic_json;\n

    Type json is an alias for basic_json<> and uses the default types.

    From the template arguments, the following types are derived:

    using object_comparator_t = std::less<>;\nusing object_t = ObjectType<StringType, basic_json, object_comparator_t,\n                   AllocatorType<std::pair<const StringType, basic_json>>>;\n\nusing array_t = ArrayType<basic_json, AllocatorType<basic_json>>;\n\nusing string_t = StringType;\n\nusing boolean_t = BooleanType;\n\nusing number_integer_t = NumberIntegerType;\nusing number_unsigned_t = NumberUnsignedType;\nusing number_float_t = NumberFloatType;\n\nusing binary_t = nlohmann::byte_container_with_subtype<BinaryType>;\n

    Not every type can be passed for these template arguments: the library uses the resulting types in ways that imply a number of requirements, for instance that StringType is char-based or that ArrayType is vector-like. These requirements are collected in Template Parameter Requirements.

    "},{"location":"features/types/#objects","title":"Objects","text":"

    RFC 8259 describes JSON objects as follows:

    An object is an unordered collection of zero or more name/value pairs, where a name is a string and a value is a string, number, boolean, null, object, or array.

    "},{"location":"features/types/#default-type","title":"Default type","text":"

    With the default values for ObjectType (std::map), StringType (std::string), and AllocatorType (std::allocator), the default type for object_t is:

    std::map<\n  std::string, // key_type\n  basic_json, // value_type\n  std::less<>, // key_compare\n  std::allocator<std::pair<const std::string, basic_json>> // allocator_type\n>\n
    "},{"location":"features/types/#behavior","title":"Behavior","text":"

    The choice of object_t influences the behavior of the JSON class. With the default type, objects have the following behavior:

    • When all names are unique, objects will be interoperable in the sense that all software implementations receiving that object will agree on the name-value mappings.
    • When the names within an object are not unique, it is unspecified which one of the values for a given key will be chosen. For instance, {\"key\": 2, \"key\": 1} could be equal to either {\"key\": 1} or {\"key\": 2}. To reject duplicate keys instead of silently resolving them one way or another, see this parsing recipe.
    • Internally, name/value pairs are stored in lexicographical order of the names. Objects will also be serialized (see dump) in this order. For instance, both {\"b\": 1, \"a\": 2} and {\"a\": 2, \"b\": 1} will be stored and serialized as {\"a\": 2, \"b\": 1}.
    • When comparing objects, the order of the name/value pairs is irrelevant. This makes objects interoperable in the sense that they will not be affected by these differences. For instance, {\"b\": 1, \"a\": 2} and {\"a\": 2, \"b\": 1} will be treated as equal.
    "},{"location":"features/types/#key-order","title":"Key order","text":"

    The order in which name/value pairs are added to the object is not preserved by the library. Therefore, iterating an object may return name/value pairs in a different order than they were originally stored. In fact, keys will be traversed in alphabetical order as std::map with std::less is used by default. Please note this behavior conforms to RFC 8259, because any order implements the specified \"unordered\" nature of JSON objects.

    "},{"location":"features/types/#limits","title":"Limits","text":"

    RFC 8259 specifies:

    An implementation may set limits on the maximum depth of nesting.

    In this class, the object's limit of nesting is not explicitly constrained. However, a maximum depth of nesting may be introduced by the compiler or runtime environment. A theoretical limit can be queried by calling the max_size function of a JSON object.

    "},{"location":"features/types/#storage_1","title":"Storage","text":"

    Objects are stored as pointers in a basic_json type. That is, for any access to object values, a pointer of type object_t* must be dereferenced.

    "},{"location":"features/types/#converting-maps-with-non-string-keys","title":"Converting maps with non-string keys","text":"

    A std::map/std::unordered_map whose key type is not string-like (e.g., std::map<int, std::string>) is converted to a JSON array of 2-element [key, value] arrays rather than a JSON object, because JSON object keys must be strings:

    std::map<int, std::string> m{{1, \"one\"}, {2, \"two\"}};\njson j = m;\n// j is [[1,\"one\"],[2,\"two\"]], not {\"1\":\"one\",\"2\":\"two\"}\n
    "},{"location":"features/types/#arrays","title":"Arrays","text":"

    RFC 8259 describes JSON arrays as follows:

    An array is an ordered sequence of zero or more values.

    "},{"location":"features/types/#default-type_1","title":"Default type","text":"

    With the default values for ArrayType (std::vector) and AllocatorType (std::allocator), the default type for array_t is:

    std::vector<\n  basic_json, // value_type\n  std::allocator<basic_json> // allocator_type\n>\n
    "},{"location":"features/types/#limits_1","title":"Limits","text":"

    RFC 8259 specifies:

    An implementation may set limits on the maximum depth of nesting.

    In this class, the array's limit of nesting is not explicitly constrained. However, a maximum depth of nesting may be introduced by the compiler or runtime environment. A theoretical limit can be queried by calling the max_size function of a JSON array.

    "},{"location":"features/types/#storage_2","title":"Storage","text":"

    Arrays are stored as pointers in a basic_json type. That is, for any access to array values, a pointer of type array_t* must be dereferenced.

    "},{"location":"features/types/#strings","title":"Strings","text":"

    RFC 8259 describes JSON strings as follows:

    A string is a sequence of zero or more Unicode characters.

    Unicode values are split by the JSON class into byte-sized characters during deserialization.

    "},{"location":"features/types/#default-type_2","title":"Default type","text":"

    With the default values for StringType (std::string), the default type for string_t is std::string.

    "},{"location":"features/types/#encoding","title":"Encoding","text":"

    Strings are stored in UTF-8 encoding. Therefore, functions like std::string::size() or std::string::length() return the number of bytes in the string rather than the number of characters or glyphs.

    "},{"location":"features/types/#string-comparison","title":"String comparison","text":"

    RFC 8259 states:

    Software implementations are typically required to test names of object members for equality. Implementations that transform the textual representation into sequences of Unicode code units and then perform the comparison numerically, code unit by code unit are interoperable in the sense that implementations will agree in all cases on equality or inequality of two strings. For example, implementations that compare strings with escaped characters unconverted may incorrectly find that \"a\\\\b\" and \"a\\u005Cb\" are not equal.

    This implementation is interoperable as it does compare strings code unit by code unit.

    "},{"location":"features/types/#storage_3","title":"Storage","text":"

    String values are stored as pointers in a basic_json type. That is, for any access to string values, a pointer of type string_t* must be dereferenced.

    "},{"location":"features/types/#booleans","title":"Booleans","text":"

    RFC 8259 implicitly describes a boolean as a type which differentiates the two literals true and false.

    "},{"location":"features/types/#default-type_3","title":"Default type","text":"

    With the default values for BooleanType (bool), the default type for boolean_t is bool.

    "},{"location":"features/types/#storage_4","title":"Storage","text":"

    Boolean values are stored directly inside a basic_json type.

    "},{"location":"features/types/#numbers","title":"Numbers","text":"

    See the number handling article for a detailed discussion on how numbers are handled by this library.

    RFC 8259 describes numbers as follows:

    The representation of numbers is similar to that used in most programming languages. A number is represented in base 10 using decimal digits. It contains an integer component that may be prefixed with an optional minus sign, which may be followed by a fraction part and/or an exponent part. Leading zeros are not allowed. (...) Numeric values that cannot be represented in the grammar below (such as Infinity and NaN) are not permitted.

    This description includes both integer and floating-point numbers. However, C++ allows more precise storage if it is known whether the number is a signed integer, an unsigned integer, or a floating-point number. Therefore, three different types, number_integer_t, number_unsigned_t, and number_float_t are used.

    "},{"location":"features/types/#default-types","title":"Default types","text":"

    With the default values for NumberIntegerType (std::int64_t), the default type for number_integer_t is std::int64_t. With the default values for NumberUnsignedType (std::uint64_t), the default type for number_unsigned_t is std::uint64_t. With the default values for NumberFloatType (double), the default type for number_float_t is double.

    "},{"location":"features/types/#default-behavior","title":"Default behavior","text":"
    • The restrictions about leading zeros are not enforced in C++. Instead, leading zeros in integer literals lead to an interpretation as an octal number. Internally, the value will be stored as a decimal number. For instance, the C++ integer literal 010 will be serialized to 8. During deserialization, leading zeros yield an error.
    • Not-a-number (NaN) values will be serialized to null.
    "},{"location":"features/types/#limits_2","title":"Limits","text":"

    RFC 8259 specifies:

    An implementation may set limits on the range and precision of numbers.

    When the default type is used, the maximal integer number that can be stored is 9223372036854775807 (INT64_MAX) and the minimal integer number that can be stored is -9223372036854775808 (INT64_MIN). Integer numbers that are out of range will yield over/underflow when used in a constructor. During deserialization, too large or small integer numbers will automatically be stored as number_unsigned_t or number_float_t.

    When the default type is used, the maximal unsigned integer number that can be stored is 18446744073709551615 (UINT64_MAX) and the minimal integer number that can be stored is 0. Integer numbers that are out of range will yield over/underflow when used in a constructor. During deserialization, too large or small integer numbers will automatically be stored as number_integer_t or number_float_t.

    RFC 8259 further states:

    Note that when such software is used, numbers that are integers and are in the range [-253+1, 253-1] are interoperable in the sense that implementations will agree exactly on their numeric values.

    As this range is a subrange of the exactly supported range [INT64_MIN, INT64_MAX], this class's integer type is interoperable.

    RFC 8259 states:

    This specification allows implementations to set limits on the range and precision of numbers accepted. Since software that implements IEEE 754-2008 binary64 (double precision) numbers is generally available and widely used, good interoperability can be achieved by implementations that expect no more precision or range than these provide, in the sense that implementations will approximate JSON numbers within the expected precision.

    This implementation does exactly follow this approach, as it uses double precision floating-point numbers by default (see NumberFloatType above). Note that with the default double, numbers smaller than -1.79769313486232e+308 or greater than 1.79769313486232e+308 are rejected during parsing with out_of_range.406. Non-finite values (NaN and infinity) can only be stored by assigning them directly and are serialized to null.

    "},{"location":"features/types/#storage_5","title":"Storage","text":"

    Integer number values, unsigned integer number values, and floating-point number values are stored directly inside a basic_json type.

    "},{"location":"features/types/number_handling/","title":"Number Handling","text":"

    This document describes how the library is handling numbers.

    "},{"location":"features/types/number_handling/#background","title":"Background","text":"

    This section briefly summarizes how the JSON specification describes how numbers should be handled.

    "},{"location":"features/types/number_handling/#json-number-syntax","title":"JSON number syntax","text":"

    JSON defines the syntax of numbers as follows:

    RFC 8259, Section 6

    The representation of numbers is similar to that used in most programming languages. A number is represented in base 10 using decimal digits. It contains an integer component that may be prefixed with an optional minus sign, which may be followed by a fraction part and/or an exponent part. Leading zeros are not allowed.

    A fraction part is a decimal point followed by one or more digits.

    An exponent part begins with the letter E in uppercase or lowercase, which may be followed by a plus or minus sign. The E and optional sign are followed by one or more digits.

    The following railroad diagram from json.org visualizes the number syntax:

    "},{"location":"features/types/number_handling/#number-interoperability","title":"Number interoperability","text":"

    On number interoperability, the following remarks are made:

    RFC 8259, Section 6

    This specification allows implementations to set limits on the range and precision of numbers accepted. Since software that implements IEEE 754 binary64 (double precision) numbers [IEEE754] is generally available and widely used, good interoperability can be achieved by implementations that expect no more precision or range than these provide, in the sense that implementations will approximate JSON numbers within the expected precision. A JSON number such as 1E400 or 3.141592653589793238462643383279 may indicate potential interoperability problems, since it suggests that the software that created it expects receiving software to have greater capabilities for numeric magnitude and precision than is widely available.

    Note that when such software is used, numbers that are integers and are in the range [-253+1, 253-1] are interoperable in the sense that implementations will agree exactly on their numeric values.

    "},{"location":"features/types/number_handling/#library-implementation","title":"Library implementation","text":"

    This section describes how this library implements the above number specification.

    "},{"location":"features/types/number_handling/#number-storage","title":"Number storage","text":"

    In the default json type, numbers are stored as std::uint64_t, std::int64_t, and double, respectively. Thereby, std::uint64_t and std::int64_t are used only if they can store the number without loss of precision. If this is impossible (e.g., if the number is too large), the number is stored as double.

    Positive integers are stored as std::uint64_t, while negative integers are stored as std::int64_t. This distinction is determined at parse time: if the JSON number has a leading minus sign, it uses signed integer storage; otherwise, it uses unsigned integer storage.

    flowchart TD\n    A[\"number literal\"] --> B{\"has a fraction (.) or exponent (e/E)?\"}\n    B -->|\"yes\"| F[\"number_float_t\"]\n    B -->|\"no\"| C{\"has a leading minus sign?\"}\n    C -->|\"yes\"| D[\"try number_integer_t\"]\n    C -->|\"no\"| E[\"try number_unsigned_t\"]\n    D -->|\"overflow\"| F\n    E -->|\"overflow\"| F

    Notes

    • Numbers with a decimal digit or scientific notation are always stored as double.
    • The number types can be changed, see Template number types.
    • The library converts integers and floating-point numbers itself, independent of the locale. Floating-point numbers are correctly rounded (to nearest, ties to even). Only a long double that is not IEEE 754 binary64 (e.g., the 80-bit x87 format) is converted with std::from_chars where available, or else with std::strtold. For that call, the library temporarily replaces the . with the decimal point of the current locale (which may be longer than one byte, e.g., in fa_IR.UTF-8), so the result does not depend on the locale either. Changing the locale in another thread during parsing is undefined behavior of the C library, though. Before version 3.13.0 unreleased, the conversion was realized by std::strtoull, std::strtoll, and std::strtod, respectively.

    Examples

    • Integer -12345678912345789123456789 is smaller than INT64_MIN and will be stored as floating-point number -1.2345678912345788e+25.
    • Integer 1E3 will be stored as floating-point number 1000.0.
    "},{"location":"features/types/number_handling/#number-limits","title":"Number limits","text":"
    • Any 64-bit signed or unsigned integer can be stored without loss of precision.
    • Numbers exceeding the limits of double (i.e., numbers whose rounded value is not satisfying std::isfinite such as 1E400) will throw exception json.exception.out_of_range.406 during parsing. Numbers too small for double (such as 1E-400) become zero, with the sign of the number.
    • Floating-point numbers are rounded to the next number representable as double. For instance 3.141592653589793238462643383279 is stored as 0x400921fb54442d18. This is the same behavior as the code double x = 3.141592653589793238462643383279;.

    Interoperability

    • The library is interoperable with respect to the specification, because its supported range [-263, 264-1] is larger than the described range [-253+1, 253-1].
    • All integers outside the range [-263, 264-1], as well as floating-point numbers are stored as double. This also concurs with the specification above.
    "},{"location":"features/types/number_handling/#zeros","title":"Zeros","text":"

    The JSON number grammar allows for different ways to express zero, and this library will store zeros differently:

    Literal Stored value and type Serialization 0 std::uint64_t(0) 0 -0 std::int64_t(0) 0 0.0 double(0.0) 0.0 -0.0 double(-0.0) -0.0 0E0 double(0.0) 0.0 -0E0 double(-0.0) -0.0

    That is, -0 is stored as a signed integer, but the serialization does not reproduce the -.

    "},{"location":"features/types/number_handling/#number-serialization","title":"Number serialization","text":"
    • Integer numbers are serialized as is; that is, no scientific notation is used.
    • Floating-point numbers are serialized with the fewest digits that read back as the same value (the closest such digits if there are several), in the layout of the %g printf modifier: 1.5, 100.0, 1e+100. Doubles are converted with the algorithm of \u017bmij, floats with Grisu2, which can write more digits than necessary.

    Notes regarding precision of floating-point numbers

    As described above, floating-point numbers are rounded to the nearest double and serialized with the shortest representation to allow round-tripping. This can yield confusing examples:

    • The serialization can have fewer decimal places than the input: 2555.5599999999999 will be serialized as 2555.56. The reverse can also be true.
    • The serialization can be in scientific notation even if the input is not: 0.0000972439793401814 will be serialized as 9.72439793401814e-05. The reverse can also be true: 12345E-5 will be serialized as 0.12345.
    • Conversions from float to double can also introduce rounding errors: 
      float f = 0.3;\njson j = f;\nstd::cout << j << '\\n';\n
      yields 0.30000001192092896.

    All examples here can be reproduced by passing the original double value to

    std::printf(\"%.*g\\n\", std::numeric_limits<double>::max_digits10, double_value);\n
    "},{"location":"features/types/number_handling/#nan-handling","title":"NaN handling","text":"

    NaN (not-a-number) cannot be expressed with the number syntax described above and are in fact explicitly excluded:

    RFC 8259, Section 6

    Numeric values that cannot be represented in the grammar below (such as Infinity and NaN) are not permitted.

    That is, there is no way to parse a NaN value. However, assignments can store NaN values in a JSON value.

    This library serializes NaN values as null. This corresponds to the behavior of JavaScript's JSON.stringify function.

    Example

    The following example shows how a NaN value is stored in a json value.

    int main()\n{\n    double val = std::numeric_limits<double>::quiet_NaN();\n    std::cout << \"val=\" << val << std::endl;\n    json j = val;\n    std::cout << \"j=\" << j.dump() << std::endl;\n    val = j;\n    std::cout << \"val=\" << val << std::endl;\n}\n

    output:

    val=nan\nj=null\nval=nan\n
    "},{"location":"features/types/number_handling/#number-comparison","title":"Number comparison","text":"

    Floating-point numbers inside JSON values are compared with json::number_float_t::operator== which is double::operator== by default.

    Alternative comparison functions

    To compare floating-point while respecting an epsilon, an alternative comparison function could be used, for instance

    template<typename T, typename = typename std::enable_if<std::is_floating_point<T>::value, T>::type>\ninline bool is_same(T a, T b, T epsilon = std::numeric_limits<T>::epsilon()) noexcept\n{\n    return std::abs(a - b) <= epsilon;\n}\n
    Or you can self-define an operator equal function like this:

    bool my_equal(const_reference lhs, const_reference rhs)\n{\n    const auto lhs_type = lhs.type();\n    const auto rhs_type = rhs.type();\n    if (lhs_type == rhs_type)\n    {\n        switch(lhs_type)\n        {\n            // self_defined case\n            case value_t::number_float:\n                return std::abs(lhs - rhs) <= std::numeric_limits<float>::epsilon();\n\n            // other cases remain the same with the original\n            ...\n        }\n    }\n    ...\n}\n

    (see #703 for more information.)

    Note

    NaN values never compare equal to themselves or to other NaN values. See #514.

    "},{"location":"features/types/number_handling/#number-conversion","title":"Number conversion","text":"

    Just like the C++ language itself, the get family of functions allows conversions between unsigned and signed integers, and between integers and floating-point values. This behavior may be surprising.

    Unconditional number conversions

    double d = 42.3;                          // non-integer double value 42.3\njson jd = d;                              // stores double value 42.3\nstd::int64_t i = jd.get<std::int64_t>();  // now i==42; no warning or error is produced\n

    Note the last line with throw a json.exception.type_error.302 exception if jd is not a numerical type, for instance a string.

    Numeric conversions are performed according to the corresponding C++ conversion rules. The library does not perform range checks when converting between numeric types.

    In particular, conversions from floating-point values to integer types, or conversions to integer types with a smaller range than the stored value, may produce implementation-defined or undefined behavior if the source value cannot be represented by the target type.

    Applications requiring checked conversions should inspect the stored number type with is_number_float(), is_number_integer(), is_number_unsigned(), or type(), and perform explicit range checks before converting to a narrower type.

    The rationale is twofold:

    1. JSON does not define a number type or precision (see above).
    2. C++ also allows silently converting between number types.

    Conditional number conversion

    The code above can be solved by explicitly checking the nature of the value with members such as is_number_integer() or is_number_unsigned():

    // check if jd is really integer-valued\nif (jd.is_number_integer())\n{\n    // if so, do the conversion and use i\n    std::int64_t i = jd.get<std::int64_t>();\n    // ...\n}\nelse\n{\n    // otherwise, take appropriate action\n    // ...\n}\n

    Note this approach also has the advantage that it can react on non-numerical JSON value types such as strings.

    (Example taken from #777.)

    "},{"location":"features/types/number_handling/#determine-number-types","title":"Determine number types","text":"

    As the example in Number conversion shows, there are different functions to determine the type of the stored number:

    • is_number() returns true for any number type
    • is_number_integer() returns true for signed and unsigned integers
    • is_number_unsigned() returns true for unsigned integers only
    • is_number_float() returns true for floating-point numbers
    • type_name() returns \"number\" for any number type
    • type() returns a different enumerator of value_t for all number types
    function unsigned integer signed integer floating-point string is_number() true true true false is_number_integer() true true false false is_number_unsigned() true false false false is_number_float() false false true false type_name() \"number\" \"number\" \"number\" \"string\" type() number_unsigned number_integer number_float string"},{"location":"features/types/number_handling/#template-number-types","title":"Template number types","text":"

    The number types can be changed with template parameters.

    position number type default type possible values 5 signed integers std::int64_t std::int32_t, std::int16_t, etc. 6 unsigned integers std::uint64_t std::uint32_t, std::uint16_t, etc. 7 floating-point double float, long double

    Constraints on number types

    • The type for signed integers must be convertible from long long. The type for floating-point numbers is used in case of overflow.
    • The type for unsigned integers must be convertible from unsigned long long. The type for floating-point numbers is used in case of overflow.
    • The type for unsigned integers can also be the same signed type as the type for signed integers (e.g., std::int64_t for both). Then every integer is stored as a signed integer, and is_number_unsigned() is always false, see Template Parameter Requirements.
    • The type for floating-point numbers must be distinct from the integer types.
    • Only double, float, and long double are supported for floating-point numbers.

    Example

    A basic_json type that uses long double as floating-point type.

    using json_ld = nlohmann::json::with_float_t<long double>;\n

    Note values should then be parsed with json_ld::parse rather than json::parse as the latter would parse floating-point values to double before then converting them to long double.

    "},{"location":"features/types/template_parameters/","title":"Template Parameter Requirements","text":"

    Class basic_json is configurable through eleven template parameters. The library never formally states what a type passed for one of these parameters has to provide -- the requirements are implied by the way the library uses the resulting object_t, array_t, string_t, etc. This page collects these requirements so they do not have to be discovered by trial and error. Each section lists the concrete types that are known to work for that parameter and the ones that do not, checked against Boost 1.83, Abseil 20250127.0, Folly, EASTL 3.21, ankerl::unordered_dense, phmap, gtl, robin_hood, tsl::ordered_map, and Qt 6.

    To change a single template parameter and keep the others, use the member alias templates with_*_t; for instance, nlohmann::json::with_float_t<long double> is json with long double as number_float_t.

    "},{"location":"features/types/template_parameters/#how-to-read-this-page","title":"How to read this page","text":"

    Requirements are split into two groups:

    • Always required -- needed to instantiate basic_json at all, or needed by functions that virtually every program uses (construction, element access, dump).
    • Required for ... -- only needed when a particular part of the API is instantiated. Member function templates are only instantiated when they are used, so a type may be perfectly usable even though it does not satisfy these requirements, as long as the corresponding functions are never called.

    Requirements are not checked

    Three requirements are checked with a static_assert: the array iterator category, the width of BinaryType's value_type, and NumberUnsignedType being at least as wide as NumberIntegerType. The rest are not diagnosed with dedicated error messages, and violating most of them results in a compiler error somewhere inside the library. Four violations are not caught at compile time at all:

    • A StringType whose data() is not null-terminated compiles and silently misparses numbers stored as a long double that is not IEEE 754 binary64 (e.g., the 80-bit x87 format), because the lexer hands the buffer to std::strtold.
    • A stateful AllocatorType compiles and silently ignores its state: allocation, deallocation, and get_allocator() each use a different default-constructed instance.
    • The two cross-specialization conversions below. These abort on an assertion in a normal build, and only fail silently under NDEBUG.
    "},{"location":"features/types/template_parameters/#overview","title":"Overview","text":"Template parameter Default Notable substitutes ObjectType std::map nlohmann::ordered_map, Abseil hash maps ArrayType std::vector std::deque StringType std::string std::string-like types over char BooleanType bool none worth using NumberIntegerType std::int64_t any signed integer type NumberUnsignedType std::uint64_t any unsigned integer type at least as wide as NumberIntegerType, or NumberIntegerType itself NumberFloatType double float (long double: no binary formats) AllocatorType std::allocator stateless allocators JSONSerializer adl_serializer serializers with the same interface BinaryType std::vector<std::uint8_t> std::vector<char> CustomBaseClass void any default-constructible class

    Third-party containers and incomplete types

    object_t is instantiated inside the definition of basic_json -- it is probed for a key_compare member to form object_comparator_t -- i.e. while basic_json is still an incomplete type. std::map is required by the standard to support incomplete mapped types; most third-party maps are not, and inspecting the mapped type at class scope (for instance with std::is_trivially_move_assignable) makes them unusable as ObjectType, no matter how their template arguments are adapted. This rules out absl::btree_map, phmap::btree_map, gtl::btree_map, robin_hood::unordered_node_map, folly::F14FastMap, and eastl::hash_map.

    array_t is only named in the class definition and is not instantiated until basic_json is complete, so an ArrayType that inspects its value type at class scope is generally fine -- boost::container::small_vector and static_vector both reject incomplete value types yet work here. absl::InlinedVector is the exception: the std::is_trivially_move_assignable<basic_json> it evaluates while instantiating itself re-enters the library's own trait machinery mid-instantiation.

    Folly requires C++20

    Folly's headers use consteval and std::type_identity, so any basic_json specialization that names a Folly type has to be compiled as C++20 or later, whatever the rest of the library supports.

    "},{"location":"features/types/template_parameters/#objecttype","title":"ObjectType","text":"

    ObjectType is instantiated as

    using object_t = ObjectType<StringType,                                    // key_type\n                            basic_json,                                    // mapped_type\n                            default_object_comparator_t,                   // key_compare\n                            AllocatorType<std::pair<const StringType,\n                                                    basic_json>>>;         // allocator_type\n

    i.e., the template arguments follow the order and meaning of std::map.

    "},{"location":"features/types/template_parameters/#always-required","title":"Always required","text":"
    • The template must be usable with four type arguments in the order shown above. The third argument is a comparator; containers that expect something else in this position (e.g., a hash function) need an alias template or wrapper -- see Notes.
    • An optional member type key_compare. If it is present it becomes object_comparator_t; otherwise default_object_comparator_t is used.
    • Member types key_type, mapped_type, value_type, and iterator.
    • value_type must behave like std::pair<const key_type, mapped_type>; the library accesses .first and .second on it.
    • iterator must be default-constructible and satisfy LegacyBidirectionalIterator. The type returned by cbegin()/cend() must satisfy the same requirements.
    • Constructors: default, copy, move, and from an iterator range (first, last).
    • Member functions begin(), end(), cbegin(), cend(), empty(), size(), max_size(), clear(), find(key), count(key), emplace(key, value), insert(value_type), insert(first, last), operator[](key), erase(iterator), and erase(first, last). erase(iterator) may return the following iterator or void; in the latter case the library computes the successor itself, before erasing.
    • erase(key) is optional: if the container does not provide one, the library falls back to find(key) followed by erase(iterator).
    • at(key) is required only by to_ubjson and to_bjdata, but every container tried here provides it.
    • emplace and insert(value_type) must return std::pair<iterator, bool> and must have unique-key semantics; multimaps cannot be used.
    • The type must be swappable (via std::swap or an ADL swap).
    • The comparison operators == and <; !=, <=, >, and >= are derived from them. Where the library uses three-way comparison (C++20), == and <=> are required instead -- the six two-way operators do not satisfy it. They implement basic_json's comparison operators.
    "},{"location":"features/types/template_parameters/#required-for-heterogeneous-key-lookup","title":"Required for heterogeneous key lookup","text":"

    The overloads of at, operator[], find, contains, count, erase, and value that accept a key type other than object_t::key_type require

    • a transparent comparator, i.e. object_comparator_t has a member type is_transparent (this is why the default comparator is std::less<> since C++14), and
    • corresponding heterogeneous find, count, erase, and operator[] overloads on the container.
    "},{"location":"features/types/template_parameters/#notes","title":"Notes","text":""},{"location":"features/types/template_parameters/#stdunordered_map-needs-an-adapter","title":"std::unordered_map needs an adapter","text":"

    std::unordered_map cannot be passed directly: its third template parameter is a hash function, but basic_json passes a comparator in that position. An alias template or wrapper that restores the expected argument order makes it usable:

    template<class Key, class T, class IgnoredCompare, class Allocator>\nstruct unordered_map_object\n    : std::unordered_map<Key, T, std::hash<Key>, std::equal_to<Key>, Allocator>\n{\n    using base_t = std::unordered_map<Key, T, std::hash<Key>, std::equal_to<Key>, Allocator>;\n    using base_t::base_t;\n};\n\nusing unordered_json = nlohmann::json::with_object_t<unordered_map_object>;\n

    Whether std::unordered_map can be instantiated at all depends on the standard library: object_t is formed while basic_json is still incomplete (see the warning above), and libstdc++ 9 needs the size of the mapped type to instantiate the hash map's node type, so the adapter does not compile there. Newer libstdc++ versions, and the hash maps listed below, do not have that problem.

    The adapter above works verbatim for Abseil's, Boost's, phmap's and gtl's hash maps, which all place the hash function third and take a std::pair<const Key, T> allocator fifth. Two need a different adapter:

    • ankerl::unordered_dense expects an allocator over std::pair<Key, T> (non-const key), so the allocator has to be rebound to that or dropped.
    • robin_hood's fifth parameter is the non-type MaxLoadFactor100, so its adapter must drop the allocator entirely.

    None of these hash maps defines key_compare, so all of them additionally rely on object_comparator_t falling back to default_object_comparator_t; see object_comparator_t.

    "},{"location":"features/types/template_parameters/#abseil-hash-maps","title":"Abseil hash maps","text":"

    absl::flat_hash_map and absl::node_hash_map tolerate an incomplete value type, but they take a hash function as their third template argument. The same adapter as for std::unordered_map makes them usable:

    template<class Key, class T, class IgnoredCompare, class Allocator>\nstruct flat_hash_object\n    : absl::flat_hash_map<Key, T, absl::Hash<Key>, std::equal_to<Key>, Allocator>\n{\n    using base_t = absl::flat_hash_map<Key, T, absl::Hash<Key>, std::equal_to<Key>, Allocator>;\n    using base_t::base_t;\n};\n\nusing flat_hash_json = nlohmann::json::with_object_t<flat_hash_object>;\n

    absl::node_hash_map keeps references to the mapped values valid across insertions; absl::flat_hash_map does not, which makes it behave like ordered_json with respect to iterator invalidation. Both expose a capacity() member function, so JSON_DIAGNOSTICS treats them conservatively and keeps the parent pointers correct either way.

    "},{"location":"features/types/template_parameters/#iteration-order","title":"Iteration order","text":"

    The library never relies on the container's iteration order for correctness; it does determine the order in which object keys are serialized by dump and visited by items. See Object Order.

    "},{"location":"features/types/template_parameters/#capacity-marks-a-container-as-insertion-ordered","title":"capacity() marks a container as insertion-ordered","text":"

    With JSON_DIAGNOSTICS enabled, the library detects insertion-ordered maps by probing for a capacity() member function (nlohmann::ordered_map inherits it from std::vector) and refreshes all parent pointers after every insertion. An ObjectType that happens to have a capacity() member is therefore treated conservatively -- this is correct, but slower.

    "},{"location":"features/types/template_parameters/#key-order-and-duplicate-keys","title":"Key order and duplicate keys","text":"

    The library does not sort or de-duplicate keys itself; the behavior described in object_t is entirely the behavior of the chosen container.

    Reference implementation

    docs/mkdocs/docs/examples/custom_object_type.hpp wraps a private std::map and satisfies every requirement above. It does not define key_compare, so object_comparator_t falls back to default_object_comparator_t -- a good starting point for a custom ObjectType.

    #pragma once\n\n#include <map>\n#include <utility>\n\n// A minimal, self-contained ObjectType built around a private std::map.\n// key_compare is deliberately not exposed: when an ObjectType has no\n// key_compare member, the library falls back to its own default comparator.\n// See https://json.nlohmann.me/features/types/template_parameters/#objecttype\ntemplate<class Key, class T, class Compare, class Allocator>\nclass custom_object_type\n{\n    using map_t = std::map<Key, T, Compare, Allocator>;\n    map_t data_;\n\n  public:\n    using key_type = typename map_t::key_type;\n    using mapped_type = typename map_t::mapped_type;\n    using value_type = typename map_t::value_type;\n    using size_type = typename map_t::size_type;\n    using iterator = typename map_t::iterator;\n    using const_iterator = typename map_t::const_iterator;\n\n    custom_object_type() = default;\n    custom_object_type(const custom_object_type&) = default;\n    custom_object_type(custom_object_type&&) = default;\n    custom_object_type& operator=(const custom_object_type&) = default;\n    custom_object_type& operator=(custom_object_type&&) = default;\n\n    template<class InputIt>\n    custom_object_type(InputIt first, InputIt last) : data_(first, last) {}\n\n    iterator begin()\n    {\n        return data_.begin();\n    }\n    iterator end()\n    {\n        return data_.end();\n    }\n    const_iterator begin() const\n    {\n        return data_.begin();\n    }\n    const_iterator end() const\n    {\n        return data_.end();\n    }\n    const_iterator cbegin() const\n    {\n        return data_.cbegin();\n    }\n    const_iterator cend() const\n    {\n        return data_.cend();\n    }\n\n    bool empty() const\n    {\n        return data_.empty();\n    }\n    size_type size() const\n    {\n        return data_.size();\n    }\n    size_type max_size() const\n    {\n        return data_.max_size();\n    }\n    void clear()\n    {\n        data_.clear();\n    }\n\n    iterator find(const key_type& key)\n    {\n        return data_.find(key);\n    }\n    const_iterator find(const key_type& key) const\n    {\n        return data_.find(key);\n    }\n    size_type count(const key_type& key) const\n    {\n        return data_.count(key);\n    }\n\n    std::pair<iterator, bool> emplace(const key_type& key, const mapped_type& value)\n    {\n        return data_.emplace(key, value);\n    }\n\n    std::pair<iterator, bool> insert(const value_type& value)\n    {\n        return data_.insert(value);\n    }\n\n    template<class InputIt>\n    void insert(InputIt first, InputIt last)\n    {\n        data_.insert(first, last);\n    }\n\n    mapped_type& operator[](const key_type& key)\n    {\n        return data_[key];\n    }\n\n    mapped_type& at(const key_type& key)\n    {\n        return data_.at(key);\n    }\n    const mapped_type& at(const key_type& key) const\n    {\n        return data_.at(key);\n    }\n\n    iterator erase(iterator pos)\n    {\n        return data_.erase(pos);\n    }\n    iterator erase(iterator first, iterator last)\n    {\n        return data_.erase(first, last);\n    }\n    size_type erase(const key_type& key)\n    {\n        return data_.erase(key);\n    }\n\n    void swap(custom_object_type& other)\n    {\n        data_.swap(other.data_);\n    }\n\n    friend bool operator==(const custom_object_type& lhs, const custom_object_type& rhs)\n    {\n        return lhs.data_ == rhs.data_;\n    }\n    friend bool operator<(const custom_object_type& lhs, const custom_object_type& rhs)\n    {\n        return lhs.data_ < rhs.data_;\n    }\n};\n
    Example: use the custom ObjectType 
    #include <iostream>\n#include <type_traits>\n\n#include <nlohmann/json.hpp>\n\n#include \"custom_object_type.hpp\"\n\nusing custom_json = nlohmann::json::with_object_t<custom_object_type>;\n\nint main()\n{\n    custom_json j;\n    j[\"pi\"] = 3.141;\n    j[\"happy\"] = true;\n    j[\"list\"] = {1, 2, 3};\n\n    std::cout << j.dump(2) << std::endl;\n    std::cout << std::boolalpha << (custom_json::parse(j.dump()) == j) << std::endl;\n\n    // custom_object_type has no key_compare member, so object_comparator_t\n    // falls back to its default\n    std::cout << std::boolalpha\n              << std::is_same<custom_json::object_comparator_t, custom_json::default_object_comparator_t>::value\n              << std::endl;\n}\n

    Output:

    {\n  \"happy\": true,\n  \"list\": [\n    1,\n    2,\n    3\n  ],\n  \"pi\": 3.141\n}\ntrue\ntrue\n
    "},{"location":"features/types/template_parameters/#compatible-containers","title":"Compatible containers","text":"Container Notes std::map (default) nlohmann::ordered_map used by ordered_json; keeps insertion order nlohmann::fifo_map keeps insertion order; adapter puts fifo_map_compare in the comparator slot boost::container::map, boost::container::flat_map no adapter needed std::unordered_map through the adapter above; not with libstdc++ 9, see the note boost::unordered_map, boost::unordered_flat_map, boost::unordered_node_map through the adapter above absl::flat_hash_map, absl::node_hash_map through the adapter above; flat_hash_map moves mapped values on rehash phmap::flat_hash_map, phmap::node_hash_map, gtl::flat_hash_map through the adapter above ankerl::unordered_dense::map and segmented_map adapter must rebind or drop the allocator robin_hood::unordered_flat_map adapter must drop the allocator folly::F14NodeMap through the adapter above; requires C++20, see the note above folly::sorted_vector_map alias must drop the allocator, whose value type it disagrees on"},{"location":"features/types/template_parameters/#containers-that-cannot-be-used","title":"Containers that cannot be used","text":"Container Reason absl::btree_map, phmap::btree_map, gtl::btree_map require a complete mapped type robin_hood::unordered_node_map, folly::F14FastMap, eastl::hash_map require a complete mapped type eastl::map EASTL iterators do not work with std::iterator_traits tsl::ordered_map its iterators expose the mapped value as const QMap no value_type member type QHash its value_type is the mapped type rather than a key/value pair, and its iterators dereference to the mapped value std::multimap, std::unordered_multimap emplace does not return std::pair<iterator, bool>"},{"location":"features/types/template_parameters/#arraytype","title":"ArrayType","text":"

    ArrayType is instantiated as

    using array_t = ArrayType<basic_json, AllocatorType<basic_json>>;\n
    "},{"location":"features/types/template_parameters/#always-required_1","title":"Always required","text":"
    • The template must be usable with two type arguments (value type and allocator).
    • Member types value_type and iterator.
    • Constructors: default, copy, and move; and from an iterator range (first, last).
    • Member functions begin(), end(), cbegin(), cend(), empty(), size(), max_size(), clear(), operator[](size_type), back(), push_back(), emplace_back(), pop_back(), resize(), insert() (single element, count, and range), erase(pos), and erase(first, last). basic_json::insert(pos, initializer_list) goes through the range overload, so no initializer-list insert is needed. at(size_type) is not required: basic_json::at(size_type) checks the index itself and then uses operator[].
    • iterator must be default-constructible, and it as well as the type returned by cbegin()/cend() must satisfy LegacyRandomAccessIterator. A static_assert only checks for LegacyBidirectionalIterator, but dump (cend() - 1), erase(idx) (begin() + idx), and the random-access operations of basic_json::iterator require random access.
    • The comparison operators, as for ObjectType: == and <, or == and <=> under C++20.
    "},{"location":"features/types/template_parameters/#required-for-individual-functions","title":"Required for individual functions","text":"
    • A member type value_type, for to_bson of an array.
    • A constructor from (count, value), for basic_json(size_type, const basic_json&).
    • Swappability, via std::swap or an ADL swap, for swap(array_t&).

    capacity() is optional

    With JSON_DIAGNOSTICS enabled, the library reads array_t::capacity() to find out whether adding an element reallocated the array and moved its elements, which would invalidate the parent pointers. An array type without a capacity() member function is handled conservatively: the parent pointers of all elements are refreshed after every insertion, which makes adding n elements cost O(*n*\u00b2). Only diagnostics builds pay this; without them capacity() is never called.

    Reference implementation

    docs/mkdocs/docs/examples/custom_array_type.hpp wraps a private std::vector and satisfies every requirement above -- a good starting point for a custom ArrayType.

    #pragma once\n\n#include <memory>\n#include <utility>\n#include <vector>\n\n// A minimal, self-contained ArrayType built around a private std::vector.\n// See https://json.nlohmann.me/features/types/template_parameters/#arraytype\ntemplate<class T, class Allocator = std::allocator<T>>\nclass custom_array_type\n{\n    using vector_t = std::vector<T, Allocator>;\n    vector_t data_;\n\n  public:\n    using value_type = typename vector_t::value_type;\n    using size_type = typename vector_t::size_type;\n    using iterator = typename vector_t::iterator;\n    using const_iterator = typename vector_t::const_iterator;\n\n    custom_array_type() = default;\n    custom_array_type(const custom_array_type&) = default;\n    custom_array_type(custom_array_type&&) = default;\n    custom_array_type& operator=(const custom_array_type&) = default;\n    custom_array_type& operator=(custom_array_type&&) = default;\n\n    template<class InputIt>\n    custom_array_type(InputIt first, InputIt last) : data_(first, last) {}\n\n    custom_array_type(size_type count, const T& value) : data_(count, value) {}\n\n    iterator begin()\n    {\n        return data_.begin();\n    }\n    iterator end()\n    {\n        return data_.end();\n    }\n    const_iterator begin() const\n    {\n        return data_.begin();\n    }\n    const_iterator end() const\n    {\n        return data_.end();\n    }\n    const_iterator cbegin() const\n    {\n        return data_.cbegin();\n    }\n    const_iterator cend() const\n    {\n        return data_.cend();\n    }\n\n    bool empty() const\n    {\n        return data_.empty();\n    }\n    size_type size() const\n    {\n        return data_.size();\n    }\n    size_type max_size() const\n    {\n        return data_.max_size();\n    }\n    void clear()\n    {\n        data_.clear();\n    }\n    void resize(size_type n)\n    {\n        data_.resize(n);\n    }\n\n    T& operator[](size_type pos)\n    {\n        return data_[pos];\n    }\n    const T& operator[](size_type pos) const\n    {\n        return data_[pos];\n    }\n\n    T& back()\n    {\n        return data_.back();\n    }\n    const T& back() const\n    {\n        return data_.back();\n    }\n\n    void push_back(const T& value)\n    {\n        data_.push_back(value);\n    }\n    void push_back(T&& value)\n    {\n        data_.push_back(std::move(value));\n    }\n\n    template<class... Args>\n    void emplace_back(Args&& ... args)\n    {\n        data_.emplace_back(std::forward<Args>(args)...);\n    }\n\n    void pop_back()\n    {\n        data_.pop_back();\n    }\n\n    iterator insert(const_iterator pos, const T& value)\n    {\n        return data_.insert(pos, value);\n    }\n    iterator insert(const_iterator pos, size_type count, const T& value)\n    {\n        return data_.insert(pos, count, value);\n    }\n    template<class InputIt>\n    iterator insert(const_iterator pos, InputIt first, InputIt last)\n    {\n        return data_.insert(pos, first, last);\n    }\n\n    iterator erase(const_iterator pos)\n    {\n        return data_.erase(pos);\n    }\n    iterator erase(const_iterator first, const_iterator last)\n    {\n        return data_.erase(first, last);\n    }\n\n    void swap(custom_array_type& other)\n    {\n        data_.swap(other.data_);\n    }\n\n    friend bool operator==(const custom_array_type& lhs, const custom_array_type& rhs)\n    {\n        return lhs.data_ == rhs.data_;\n    }\n    friend bool operator<(const custom_array_type& lhs, const custom_array_type& rhs)\n    {\n        return lhs.data_ < rhs.data_;\n    }\n};\n
    Example: use the custom ArrayType 
    #include <iostream>\n\n#include <nlohmann/json.hpp>\n\n#include \"custom_array_type.hpp\"\n\nusing custom_json = nlohmann::json::with_array_t<custom_array_type>;\n\nint main()\n{\n    custom_json j = custom_json::array();\n    j.push_back(1);\n    j.push_back(2);\n    j.push_back(3);\n\n    std::cout << j.dump() << std::endl;\n    std::cout << std::boolalpha << (custom_json::parse(j.dump()) == j) << std::endl;\n}\n

    Output:

    [1,2,3]\ntrue\n
    "},{"location":"features/types/template_parameters/#compatible-containers_1","title":"Compatible containers","text":"Container Notes std::vector (default) std::deque references survive appends, but not insertions elsewhere; see the capacity() note above std::pmr::vector through an alias, as the allocator comes from AllocatorType instead boost::container::vector, deque, devector boost::container::stable_vector the only one tried that keeps references valid across every insertion boost::container::small_vector, folly::small_vector through an alias that fixes the inline capacity boost::container::static_vector through the same kind of alias, for arrays that stay within the fixed capacity folly::fbvector requires C++20, see the note above"},{"location":"features/types/template_parameters/#containers-that-cannot-be-used_1","title":"Containers that cannot be used","text":"Container Reason std::list no operator[], and no random-access iterators eastl::vector, QList, QVector no max_size(); they handle the incomplete value type fine absl::InlinedVector requires a complete value type, see the note above absl::FixedArray the size is fixed at construction, so resize, push_back, insert and erase are missing"},{"location":"features/types/template_parameters/#stringtype","title":"StringType","text":"

    StringType is used both for JSON string values and for the keys of JSON objects (string_t and object_t::key_type).

    "},{"location":"features/types/template_parameters/#always-required_2","title":"Always required","text":"
    • A member type value_type that is one byte wide and char-compatible. The library stores and processes UTF-8 encoded char data and passes data() to functions that take a const char*, such as std::strtold (only used to parse a long double that is not IEEE 754 binary64, see NumberFloatType). std::wstring, std::u16string, and std::u32string are not valid choices; see the FAQ on wide string handling.
    • Constructors: default, copy, move, from const char* (which must not be explicit), from (const char*, size_type), and from (size_type, char); and copy or move assignment.
    • Member functions size(), clear(), resize(n, c), data(), push_back(char), and operator[] (const and non-const, returning references). c_str() and back() are not required.
    • data() must return a pointer to a contiguous, null-terminated buffer. float, double, and a long double that is IEEE 754 binary64 are converted by the library itself and do not depend on this. For any other NumberFloatType (a long double of another format), the parser falls back to std::strtold when std::from_chars is not available or declines the token, and std::strtold reads up to the null character. A type whose data() is not null-terminated does not fail to compile; with such a NumberFloatType it can silently misparse floating-point numbers.
    • append(const char*, size_type), used by dump, and append(const StringType&), used by the CBOR reader for indefinite-length strings. The library's internal string concatenation additionally has to append a char and a const char*; for each it selects between append(arg), operator+=, append(first, last), and append(data, size).
    • The comparison operator == against another StringType, and < for use as a key of the chosen ObjectType (with the default comparator, std::less<> must be able to compare two StringType values, and a StringType with the key types used for lookup). != is never applied to a StringType, and == against const char* is resolved by the implicit const char* constructor.
    "},{"location":"features/types/template_parameters/#required-for-the-binary-formats","title":"Required for the binary formats","text":"
    • resize(n), used by the readers to make room for a block of bytes.
    • Non-const operator[], into which the readers std::memcpy those bytes. A non-const data() would serve just as well, but std::string has only had one since C++17, and the library still supports C++11.
    "},{"location":"features/types/template_parameters/#required-for-json-pointer-flatten-and-diff","title":"Required for JSON Pointer, flatten, and diff","text":"
    • A static member npos and the member function find_first_of(char, size_type) -- together with data(), reserve(n), and append(const char*, size_type) they implement the escaping and unescaping of reference tokens described in RFC 6901. Neither find(const StringType&, size_type), nor substr(pos, count), nor replace(pos, count, const StringType&) is required.
    • empty().
    • begin() and end() -- used by operator[](const json_pointer&) to decide whether a reference token denotes an array index.
    "},{"location":"features/types/template_parameters/#required-for-other-functionality","title":"Required for other functionality","text":"Functionality Additional requirement diff, items, std::hash conversion of a std::size_t to StringType: either assignability from the result of std::to_string, or an ADL overload void int_to_string(StringType&, std::size_t) operator/(std::size_t) the same conversion of a std::size_t to StringType as diff, items, and std::hash above std::hash<basic_json> additionally a specialization of std::hash<StringType> to_bson find(value_type) and npos parse from a string_t the input adapters must accept it; otherwise pass a character range operator<<(std::ostream&, const json_pointer&) streamability to std::ostream to_string conversion of StringType to std::string (the function returns a std::string) exception messages data() and size(), or begin() and end()"},{"location":"features/types/template_parameters/#compatible-types","title":"Compatible types","text":"Type Notes std::string (default) std::basic_string with a custom stateless allocator std::pmr::string see the warning below before relying on the memory resource boost::container::string needs a user-supplied std::hash specialization (Boost provides boost::hash instead) folly::fbstring requires C++20, see the note above eastl::string needs a user-supplied std::hash and an ADL int_to_string (it is not assignable from a std::string); parse does not accept it directly -- pass a character range or a std::string a custom string class in a user-defined namespace if the requirements above are met"},{"location":"features/types/template_parameters/#types-that-cannot-be-used","title":"Types that cannot be used","text":"Type Reason std::wstring, std::u16string, std::u32string the character type is not one byte wide std::u8string one byte wide, but char8_t is not char-compatible absl::Cord no value_type, and the storage is not contiguous QString no append(const char*, size_type); its QChar is also two bytes wide, though that is never diagnosed

    A std::pmr::string mostly does not use the memory resource you choose

    basic_json cannot be given an allocator or a memory resource. AllocatorType is default-constructed at every allocation and has to be stateless (see AllocatorType), and string values the library creates are constructed with their own default allocator. So:

    • Every string the library itself produces -- from parse, from dump, or by default construction -- allocates from std::pmr::get_default_resource().
    • Copying an arena-backed string into a value silently drops its memory resource: the copy lands on the default resource, because std::pmr::polymorphic_allocator does not propagate on copy construction. Nothing warns about this.
    • Moving one in does keep it, and later growth still allocates from that arena -- but it does not survive a copy of the enclosing basic_json.
    • Passing std::pmr::polymorphic_allocator as AllocatorType does not work around any of this; it does not compile.

    Apart from moving a string in, the only way to redirect these allocations is the process-global std::pmr::set_default_resource().

    Reference implementation

    docs/mkdocs/docs/examples/custom_string_type.hpp wraps a private std::string and satisfies every requirement above -- a good starting point for a custom StringType. The unit test tests/src/unit-alt-string.cpp contains a more thorough variant, alt_string, exercised against a larger part of the API.

    #pragma once\n\n#include <cstddef>\n#include <ostream>\n#include <string>\n\n// A minimal, self-contained StringType built around a private std::string.\n// Wraps rather than inherits, so it exposes exactly what the library needs\n// and nothing more of std::string's interface.\n//\n// Covers the \"Always required\" members, the extras needed for the binary\n// formats, JSON Pointer / flatten / unflatten, and the int_to_string overload\n// needed for diff and items. Extending it further (e.g. for\n// std::hash<basic_json> or to_bson) is a matter of adding the extra members\n// listed in the \"Required for other functionality\" table.\n//\n// See https://json.nlohmann.me/features/types/template_parameters/#stringtype\nclass custom_string_type\n{\n    std::string data_;\n\n  public:\n    using value_type = char;\n    using size_type = std::string::size_type;\n    using iterator = std::string::iterator;\n    using const_iterator = std::string::const_iterator;\n\n    static constexpr size_type npos = std::string::npos;\n\n    custom_string_type() = default;\n    custom_string_type(const custom_string_type&) = default;\n    custom_string_type(custom_string_type&&) = default;\n    custom_string_type& operator=(const custom_string_type&) = default;\n    custom_string_type& operator=(custom_string_type&&) = default;\n\n    // not explicit: the library relies on being able to hand it a string literal\n    custom_string_type(const char* s) : data_(s) {}\n    custom_string_type(const char* s, size_type count) : data_(s, count) {}\n    custom_string_type(size_type count, char ch) : data_(count, ch) {}\n\n    size_type size() const\n    {\n        return data_.size();\n    }\n    bool empty() const\n    {\n        return data_.empty();\n    }\n    void clear()\n    {\n        data_.clear();\n    }\n    void resize(size_type n)\n    {\n        data_.resize(n);\n    }\n    void resize(size_type n, char c)\n    {\n        data_.resize(n, c);\n    }\n    void reserve(size_type n)\n    {\n        data_.reserve(n);\n    }\n\n    // must stay null-terminated -- the parser hands this to std::strtoull &\n    // friends; std::string::data() has guaranteed that since C++11\n    const char* data() const\n    {\n        return data_.data();\n    }\n\n    void push_back(char c)\n    {\n        data_.push_back(c);\n    }\n\n    char& operator[](size_type pos)\n    {\n        return data_[pos];\n    }\n    char operator[](size_type pos) const\n    {\n        return data_[pos];\n    }\n\n    custom_string_type& append(const char* s, size_type count)\n    {\n        data_.append(s, count);\n        return *this;\n    }\n    custom_string_type& append(const custom_string_type& other)\n    {\n        data_.append(other.data_);\n        return *this;\n    }\n    custom_string_type& operator+=(char c)\n    {\n        data_.push_back(c);\n        return *this;\n    }\n\n    size_type find_first_of(char c, size_type pos = 0) const\n    {\n        return data_.find_first_of(c, pos);\n    }\n\n    iterator begin()\n    {\n        return data_.begin();\n    }\n    iterator end()\n    {\n        return data_.end();\n    }\n    const_iterator begin() const\n    {\n        return data_.begin();\n    }\n    const_iterator end() const\n    {\n        return data_.end();\n    }\n\n    // found by ADL; converts array indices to keys in diff and items\n    friend void int_to_string(custom_string_type& target, std::size_t value)\n    {\n        target.data_ = std::to_string(value);\n    }\n\n    friend bool operator==(const custom_string_type& lhs, const custom_string_type& rhs)\n    {\n        return lhs.data_ == rhs.data_;\n    }\n    friend bool operator<(const custom_string_type& lhs, const custom_string_type& rhs)\n    {\n        return lhs.data_ < rhs.data_;\n    }\n\n    // not required by the library itself, but dump() returns a custom_string_type\n    // and this makes `std::cout << j.dump()` work as expected\n    friend std::ostream& operator<<(std::ostream& os, const custom_string_type& s)\n    {\n        return os << s.data_;\n    }\n};\n
    Example: use the custom StringType 
    #include <iostream>\n\n#include <nlohmann/json.hpp>\n\n#include \"custom_string_type.hpp\"\n\nusing custom_json = nlohmann::json::with_string_t<custom_string_type>;\n\nint main()\n{\n    custom_json j;\n    j[\"pi\"] = 3.141;\n    j[\"happy\"] = true;\n    j[\"list\"] = {1, 2, 3};\n\n    std::cout << j.dump(2) << std::endl;\n    std::cout << std::boolalpha << (custom_json::parse(j.dump()) == j) << std::endl;\n}\n

    Output:

    {\n  \"happy\": true,\n  \"list\": [\n    1,\n    2,\n    3\n  ],\n  \"pi\": 3.141\n}\ntrue\n
    "},{"location":"features/types/template_parameters/#booleantype","title":"BooleanType","text":"

    boolean_t is stored directly inside basic_json, as a member of an anonymous union.

    "},{"location":"features/types/template_parameters/#always-required_3","title":"Always required","text":"
    • A literal type that is trivially default-constructible, trivially copyable, and trivially destructible; otherwise the union's special member functions are deleted.
    • Implicitly convertible from bool -- an explicit constructor is not enough, because the to_json overload for a custom BooleanType is constrained on std::is_convertible -- and contextually convertible to bool (here an explicit operator bool is fine).
    • Comparison operators ==, !=, <, <=, >, >= (or <=>).
    • Convertible from and to bool through the serializer, because get<bool>() is used internally.

    There is little reason to use anything other than bool here.

    "},{"location":"features/types/template_parameters/#compatible-types_1","title":"Compatible types","text":"

    bool is the only usable choice. Another trivially copyable type that is implicitly convertible to and from bool -- std::uint8_t, say -- does compile, and JSON booleans still round-trip, but the type then serves as both boolean_t and an ordinary integer: basic_json can no longer be constructed or assigned from a std::uint8_t at all (the boolean and unsigned-integer to_json overloads become ambiguous), and get<std::uint8_t>() on a number throws type_error.302 instead of returning the value.

    "},{"location":"features/types/template_parameters/#numberintegertype-and-numberunsignedtype","title":"NumberIntegerType and NumberUnsignedType","text":"

    Both types are stored directly inside basic_json's union.

    "},{"location":"features/types/template_parameters/#always-required_4","title":"Always required","text":"
    • std::is_integral must be satisfied: NumberIntegerType must be a signed integer type, NumberUnsignedType an unsigned integer type or the same type as NumberIntegerType (see below). Class types are not supported -- among others, the constructors taking integer values are constrained on std::is_integral.
    • Trivially default-constructible, trivially copyable, and trivially destructible (union member).
    • std::numeric_limits must be specialized for both types.
    • An unsigned NumberUnsignedType must be able to represent the absolute value of every NumberIntegerType value; serialization of negative numbers converts the value to NumberUnsignedType. A static_assert requires it to be at least as wide as NumberIntegerType, which is what that amounts to for the standard integer types.
    • Both types must fit into the internal 64-character number buffer used by dump, which is the case for all standard integer types.
    • std::hash<basic_json> additionally requires std::hash specializations.
    "},{"location":"features/types/template_parameters/#notes_1","title":"Notes","text":"

    The number types influence what the parser accepts: an integer literal that does not round-trip through the chosen type is stored as number_float_t instead. Choosing types narrower than 64 bits therefore silently changes parse results rather than raising an error. See Number Handling for details.

    "},{"location":"features/types/template_parameters/#same-type-for-both","title":"Same type for both","text":"

    NumberUnsignedType may be the same type as NumberIntegerType, e.g. nlohmann::json::with_integers_t<std::int64_t, std::int64_t> (see with_integers_t), for code that only wants to handle one integer type. Such a basic_json stores every integer as number_integer_t:

    • is_number_unsigned() is always false, and type() never returns value_t::number_unsigned. Constructing a value from value_t::number_unsigned gives a value_t::number_integer value.
    • Values of unsigned C++ types (e.g., 5u) are converted to NumberIntegerType, so values above its maximum wrap around, as values outside the range of a narrower NumberIntegerType already do.
    • The parsers store a non-negative integer above the maximum of NumberIntegerType (e.g., 18446744073709551615 for std::int64_t) as number_float_t, like any other integer that does not fit.
    • Converting from a basic_json with distinct types turns its unsigned values into number_integer_t values.
    "},{"location":"features/types/template_parameters/#compatible-types_2","title":"Compatible types","text":"Type pair Support std::int64_t / std::uint64_t (default) full std::int32_t / std::uint32_t, long long / unsigned long long full; narrower types change which literals the parser can represent any other pair of standard signed/unsigned integer types full the same signed integer type for both, e.g. std::int64_t / std::int64_t full; every integer is stored as number_integer_t, see above class types, enumerations not usable; std::is_integral must hold bool, number_float_t, or the type of another member of the union not usable; std::is_integral<bool> is in fact true, but the get_impl_ptr overloads for boolean_t, number_integer_t, number_unsigned_t and number_float_t would collide"},{"location":"features/types/template_parameters/#numberfloattype","title":"NumberFloatType","text":"

    number_float_t is stored directly inside basic_json's union.

    "},{"location":"features/types/template_parameters/#always-required_5","title":"Always required","text":"
    • Trivially default-constructible, trivially copyable, and trivially destructible (union member).
    • std::numeric_limits must be specialized; max_digits10 is used to size the conversion.
    • std::isfinite must be applicable to the type.
    "},{"location":"features/types/template_parameters/#required-for-parsing-and-serialization","title":"Required for parsing and serialization","text":"

    NumberFloatType must be one of float, double, or long double:

    • The parser converts number literals to float, double, and a long double that is IEEE 754 binary64 itself; other long double formats are converted with std::from_chars where available, or with std::strtold. The library provides overloads for exactly these three types.
    • dump falls back to std::snprintf with the %g and %Lg conversion specifiers, for which the library likewise provides only double and long double overloads (float is promoted to double).

    If std::numeric_limits<NumberFloatType> describes an IEEE 754 binary64 number, dump uses the algorithm of \u017bmij, which produces the shortest representation that round-trips. For IEEE 754 binary32 numbers, it uses Grisu2, which produces a short representation that round-trips. Otherwise the snprintf fallback with max_digits10 digits is used.

    "},{"location":"features/types/template_parameters/#required-for-the-binary-formats_1","title":"Required for the binary formats","text":"

    NumberFloatType must be float or double. The writers for CBOR, MessagePack, UBJSON, BJData, BON8, and BSON map a floating-point value onto an IEEE 754 binary32 or binary64 field and have no encoding for long double.

    "},{"location":"features/types/template_parameters/#compatible-types_3","title":"Compatible types","text":"Type Support double (default) full; shortest round-trip output through \u017bmij float full; short round-trip output through Grisu2 long double dump and parse only; the binary format writers do not compile, as they only handle IEEE 754 binary32 and binary64 any other type not usable"},{"location":"features/types/template_parameters/#allocatortype","title":"AllocatorType","text":"

    AllocatorType is instantiated with one argument, for each of object_t, array_t, string_t, binary_t, basic_json, std::pair<const StringType, basic_json>, and std::pair<StringType, basic_json>.

    AllocatorType is not the only allocator a basic_json uses. It allocates the JSON values themselves, but most temporary storage is allocated with std::allocator. This includes the parser's stacks and the stacks that process deeply nested values without recursion.

    "},{"location":"features/types/template_parameters/#always-required_6","title":"Always required","text":"
    • The template must be usable with exactly one type argument. The library instantiates AllocatorType<T> directly and never uses std::allocator_traits<...>::rebind_alloc.
    • It must satisfy the Allocator named requirement so that std::allocator_traits can be used with it.
    • It must be default-constructible and stateless. Objects are allocated with a default-constructed allocator and deallocated with a different default-constructed allocator, and get_allocator() returns a default-constructed instance. Allocators carrying state are not supported, so there is no way to tell a basic_json where to allocate from; see the note under StringType for what that means in practice. A stateful allocator is not diagnosed: it compiles and silently ignores the state.
    • It must support incomplete types: AllocatorType<basic_json> is instantiated inside the definition of basic_json itself.
    • std::allocator_traits<AllocatorType<basic_json>>::pointer becomes basic_json::pointer, and iterators are constructed from raw basic_json* values. The pointer type must therefore be a plain pointer; fancy pointers are not supported.
    "},{"location":"features/types/template_parameters/#compatible-types_4","title":"Compatible types","text":"Type Support std::allocator (default) full a custom stateless allocator template full stateful allocators, e.g. std::pmr::polymorphic_allocator not usable; see the requirements above"},{"location":"features/types/template_parameters/#jsonserializer","title":"JSONSerializer","text":"

    JSONSerializer is instantiated as JSONSerializer<T, void> and defaults to adl_serializer.

    "},{"location":"features/types/template_parameters/#always-required_7","title":"Always required","text":"
    • The template must accept two type arguments. It does not have to give the second one a default -- basic_json declares the parameter as template<typename T, typename SFINAE = void> class JSONSerializer, so uses such as JSONSerializer<T> inside the library supply void themselves. The second parameter exists so that partial specializations can be constrained by SFINAE.
    • For every type T that is converted to a JSON value, a static member function static void to_json(basic_json&, T) must exist.
    • For every type T that is converted from a JSON value, either static void from_json(const basic_json&, T&) or static T from_json(const basic_json&) must exist. The latter form is required for types that are not default-constructible; see Arbitrary Types Conversions.
    • To support the converting constructor between different basic_json specializations, to_json must be available for boolean_t, number_integer_t, number_unsigned_t, number_float_t, string_t, object_t, array_t, and binary_t of the source specialization.
    "},{"location":"features/types/template_parameters/#compatible-types_5","title":"Compatible types","text":"Type Support nlohmann::adl_serializer (default) full a class template deriving from adl_serializer full; the usual way to change behavior while keeping the defaults an unrelated template with the same interface full, but it has to handle every type the library converts"},{"location":"features/types/template_parameters/#binarytype","title":"BinaryType","text":"

    BinaryType is not a JSON type; it is used for the byte strings of the binary formats. It is wrapped as

    using binary_t = nlohmann::byte_container_with_subtype<BinaryType>;\n
    "},{"location":"features/types/template_parameters/#always-required_8","title":"Always required","text":"
    • A non-final class type -- byte_container_with_subtype derives from it publicly.
    • A member type value_type that is exactly one byte wide (e.g., std::uint8_t, char, or std::byte). Readers and writers reinterpret the container's storage as raw bytes, so a wider value_type is rejected with a static_assert.
    • Contiguous storage: the binary readers std::memcpy into &binary[n], the writers reinterpret_cast data(). data() + n would do for the readers too, but they share one helper with StringType, whose non-const data() is C++17 and later only.
    • Default-constructible, copy-constructible, and move-constructible.
    • Member functions size(), empty(), data(), resize(), operator[], back(), begin(), end(), cbegin(), and cend() with random-access iterators, and insert(pos, first, last), which the CBOR reader uses to join the chunks of an indefinite-length byte string. push_back() is not required.
    • Comparison operators: == is used by byte_container_with_subtype, the relational operators by basic_json's comparison operators.
    "},{"location":"features/types/template_parameters/#required-for-individual-functions_1","title":"Required for individual functions","text":"
    • clear(), for basic_json::clear().

    max_size(), at(), reserve(), erase(), pop_back(), and emplace_back() are not used at all.

    See binary_t for how a non-default BinaryType changes the meaning of assigning such a container to a basic_json value.

    Reference implementation

    docs/mkdocs/docs/examples/custom_binary_type.hpp wraps a private std::vector<std::uint8_t> and satisfies every requirement above -- a good starting point for a custom BinaryType.

    #pragma once\n\n#include <cstdint>\n#include <initializer_list>\n#include <vector>\n\n// A minimal, self-contained BinaryType built around a private std::vector.\n// See https://json.nlohmann.me/features/types/template_parameters/#binarytype\nclass custom_binary_type\n{\n    using vector_t = std::vector<std::uint8_t>;\n    vector_t data_;\n\n  public:\n    using value_type = vector_t::value_type;\n    using size_type = vector_t::size_type;\n    using iterator = vector_t::iterator;\n    using const_iterator = vector_t::const_iterator;\n\n    custom_binary_type() = default;\n    custom_binary_type(const custom_binary_type&) = default;\n    custom_binary_type(custom_binary_type&&) = default;\n    custom_binary_type& operator=(const custom_binary_type&) = default;\n    custom_binary_type& operator=(custom_binary_type&&) = default;\n\n    template<class InputIt>\n    custom_binary_type(InputIt first, InputIt last) : data_(first, last) {}\n\n    // so basic_json::binary({0x01, 0x02}) can build one directly\n    custom_binary_type(std::initializer_list<std::uint8_t> init) : data_(init) {}\n\n    size_type size() const\n    {\n        return data_.size();\n    }\n    bool empty() const\n    {\n        return data_.empty();\n    }\n    void clear()\n    {\n        data_.clear();\n    }\n    void resize(size_type n)\n    {\n        data_.resize(n);\n    }\n\n    // read-only is enough: the writers only ever read from a binary value\n    const std::uint8_t* data() const\n    {\n        return data_.data();\n    }\n\n    std::uint8_t& operator[](size_type pos)\n    {\n        return data_[pos];\n    }\n    std::uint8_t operator[](size_type pos) const\n    {\n        return data_[pos];\n    }\n\n    std::uint8_t& back()\n    {\n        return data_.back();\n    }\n    std::uint8_t back() const\n    {\n        return data_.back();\n    }\n\n    iterator begin()\n    {\n        return data_.begin();\n    }\n    iterator end()\n    {\n        return data_.end();\n    }\n    const_iterator begin() const\n    {\n        return data_.begin();\n    }\n    const_iterator end() const\n    {\n        return data_.end();\n    }\n    const_iterator cbegin() const\n    {\n        return data_.cbegin();\n    }\n    const_iterator cend() const\n    {\n        return data_.cend();\n    }\n\n    template<class InputIt>\n    iterator insert(const_iterator pos, InputIt first, InputIt last)\n    {\n        return data_.insert(pos, first, last);\n    }\n\n    friend bool operator==(const custom_binary_type& lhs, const custom_binary_type& rhs)\n    {\n        return lhs.data_ == rhs.data_;\n    }\n    friend bool operator<(const custom_binary_type& lhs, const custom_binary_type& rhs)\n    {\n        return lhs.data_ < rhs.data_;\n    }\n};\n
    Example: use the custom BinaryType 
    #include <iostream>\n\n#include <nlohmann/json.hpp>\n\n#include \"custom_binary_type.hpp\"\n\nusing custom_json = nlohmann::json::with_binary_t<custom_binary_type>;\n\nint main()\n{\n    const auto j = custom_json::binary({0x01, 0x02, 0x03});\n\n    std::cout << j.dump() << std::endl;\n    std::cout << std::boolalpha << (custom_json::from_cbor(custom_json::to_cbor(j)) == j) << std::endl;\n}\n

    Output:

    {\"bytes\":[1,2,3],\"subtype\":null}\ntrue\n
    "},{"location":"features/types/template_parameters/#compatible-containers_2","title":"Compatible containers","text":"Container Notes std::vector<std::uint8_t> (default) std::vector<char>, std::vector<std::byte> dump() writes the bytes as 0..255 whichever is used boost::container::vector<std::uint8_t>, boost::container::small_vector<std::uint8_t, N> absl::InlinedVector<std::uint8_t, N> usable here, unlike as an ArrayType, because the value type is complete eastl::vector<std::uint8_t> usable here, unlike as an ArrayType, because max_size() is not needed folly::fbvector<std::uint8_t> requires C++20, see the note above"},{"location":"features/types/template_parameters/#containers-that-cannot-be-used_2","title":"Containers that cannot be used","text":"Container Reason QByteArray no empty() (it spells that isEmpty()); its insert takes an index rather than an iterator; and it converts to string_t, which makes to_json ambiguous between a string and a binary value std::string binary_t::container_type and string_t would be the same type, so the two swap overloads collide and basic_json cannot be instantiated at all std::deque<std::uint8_t> storage is not contiguous, so there is no data() containers whose value_type is wider than one byte see above -- accepted by the compiler, wrong at runtime"},{"location":"features/types/template_parameters/#custombaseclass","title":"CustomBaseClass","text":"

    CustomBaseClass is an extension point: unless it is void (the default, which selects the empty nlohmann::json_default_base), basic_json publicly derives from it.

    "},{"location":"features/types/template_parameters/#always-required_9","title":"Always required","text":"
    • A non-final, default-constructible class type.
    • basic_json is copy-/move-constructible and copy-/move-assignable only if CustomBaseClass is.
    "},{"location":"features/types/template_parameters/#notes_2","title":"Notes","text":"

    basic_json is documented to be a StandardLayoutType. Because basic_json has non-static data members of its own, a CustomBaseClass with non-static data members forfeits this guarantee.

    Note the namespace of CustomBaseClass becomes an associated namespace of basic_json for the purpose of argument-dependent lookup.

    See json_base_class_t for an example.

    "},{"location":"features/types/template_parameters/#compatible-types_6","title":"Compatible types","text":"Type Support void (default) an empty base class is used; no effect on basic_json any default-constructible, non-final class full; see json_base_class_t"},{"location":"features/types/template_parameters/#cross-specialization-conversions","title":"Cross-specialization conversions","text":"

    Converting a value from one basic_json specialization into another (see the converting constructor) imposes two additional requirements that are not diagnosed at compile time. With assertions enabled they abort on the JSON_ASSERT at the end of the converting constructor; under NDEBUG they fail silently at runtime:

    • The target string_t must be directly constructible from the source string_t. Otherwise the string is converted to an array of character codes.
    • The target object_t::key_type must be directly constructible from the source object's key type. Otherwise the object is converted to an array of key/value pairs.

    See issue #3425, string_t, and object_t.

    "},{"location":"features/types/template_parameters/#see-also","title":"See also","text":"
    • Types -- overview of how JSON values are stored
    • Number Handling -- how the number types affect parsing and serialization
    • Object Order -- using an insertion-ordered ObjectType
    • basic_json -- API documentation of the class template
    "},{"location":"home/architecture/","title":"Architecture","text":"

    This page gives a high-level overview of the library's architecture. It should help new contributors to get an idea of the used concepts and where to make changes.

    "},{"location":"home/architecture/#overview","title":"Overview","text":"

    The library is built around a single class template, nlohmann::basic_json. A basic_json value is a node in a tree of JSON values. All other components either create such a tree from an input (parsing), write a tree to an output (serialization), or give access to it (iterators, JSON Pointer, conversions).

    flowchart LR\n    input[/\"input<br>(string, stream,<br>iterator range, file)\"/]\n    ia[\"input adapter\"]\n    lexer[\"lexer\"]\n    parser[\"parser\"]\n    breader[\"binary_reader\"]\n    sax[\"SAX interface\"]\n    value[(\"basic_json<br>value tree\")]\n    serializer[\"serializer\"]\n    bwriter[\"binary_writer\"]\n    oa[\"output adapter\"]\n    output[/\"output<br>(string, stream,<br>vector)\"/]\n\n    input --> ia\n    ia --> lexer --> parser --> sax\n    ia --> breader --> sax\n    sax --> value\n    value --> serializer --> oa\n    value --> bwriter --> oa\n    oa --> output
    • JSON text is read by an input adapter, tokenized by the lexer, and turned into SAX events by the parser.
    • Binary formats (BJData, BON8, BSON, CBOR, MessagePack, UBJSON) are read by an input adapter and turned into the same SAX events by the binary_reader.
    • A SAX consumer receives the events. The one used by parse builds a basic_json value tree.
    • The serializer (JSON text) or the binary_writer (binary formats) writes a value tree to an output adapter.
    "},{"location":"home/architecture/#source-layout","title":"Source layout","text":"

    The public headers are in include/nlohmann:

    • json.hpp defines class basic_json.
    • json_fwd.hpp contains forward declarations.
    • adl_serializer.hpp, byte_container_with_subtype.hpp, and ordered_map.hpp define adl_serializer, byte_container_with_subtype, and ordered_map.
    • json_view.hpp is a separate, optional header that defines basic_json_document and basic_json_view, a flat-index, read-only, non-owning way to look at a parsed JSON text; see Zero-copy JSON views. It builds on json.hpp internals (it requires the same library version) and has its own detail/view/ subdirectory.

    Everything else lives in detail/ and namespace nlohmann::detail, which is not part of the public API. Paths below are relative to include/nlohmann.

    Component Location Value type enumeration detail/value_t.hpp Input adapters detail/input/input_adapters.hpp Lexer detail/input/lexer.hpp, detail/input/number_parse.hpp, detail/input/string_scan.hpp Parser detail/input/parser.hpp SAX interface and DOM builders detail/input/json_sax.hpp Binary format readers detail/input/binary_reader.hpp JSON serializer detail/output/serializer.hpp, detail/conversions/to_chars.hpp Binary format writers detail/output/binary_writer.hpp Output adapters detail/output/output_adapters.hpp Iterators detail/iterators/ Conversions from/to arbitrary types detail/conversions/from_json.hpp, detail/conversions/to_json.hpp JSON Pointer detail/json_pointer.hpp Exceptions detail/exceptions.hpp Type traits and C++ feature backports detail/meta/ Macros detail/macro_scope.hpp, detail/macro_unscope.hpp, detail/abi_macros.hpp

    The single-header version single_include/nlohmann/json.hpp is generated from these files with make amalgamate and must not be edited by hand. The same command also generates single_include/nlohmann/json_view.hpp from json_view.hpp and detail/view/.

    "},{"location":"home/architecture/#template-parameters","title":"Template parameters","text":"

    basic_json is parameterized by the types it uses to store values and to convert from and to other types:

    Template parameter Default Used for ObjectType std::map objects, see object_t ArrayType std::vector arrays, see array_t StringType std::string strings and object keys, see string_t BooleanType bool Booleans, see boolean_t NumberIntegerType std::int64_t signed integers, see number_integer_t NumberUnsignedType std::uint64_t unsigned integers, see number_unsigned_t NumberFloatType double floating-point numbers, see number_float_t AllocatorType std::allocator allocating objects, arrays, strings, and binary values JSONSerializer adl_serializer conversions from/to other types, see adl_serializer BinaryType std::vector<std::uint8_t> binary values, see binary_t CustomBaseClass void an optional base class, see json_base_class_t

    The library provides two specializations:

    • json uses all default template arguments.
    • ordered_json uses ordered_map as ObjectType to keep the insertion order of object keys.

    The requirements on the template arguments are listed in Template Parameter Requirements.

    "},{"location":"home/architecture/#value-storage","title":"Value storage","text":"

    Each basic_json value stores its content as a tagged union: an enumeration value_t names the type of the value, and a union json_value holds the value itself. Both are members of the nested struct data, which is the only data member m_data of basic_json:

    struct data\n{\n    /// the type of the current element\n    value_t m_type = value_t::null;\n\n    /// the value of the current element\n    json_value m_value = {};\n};\n\ndata m_data = {};\n

    with

    enum class value_t : std::uint8_t\n{\n    null,             ///< null value\n    object,           ///< object (unordered set of name/value pairs)\n    array,            ///< array (ordered collection of values)\n    string,           ///< string value\n    boolean,          ///< boolean value\n    number_integer,   ///< number value (signed integer)\n    number_unsigned,  ///< number value (unsigned integer)\n    number_float,     ///< number value (floating-point)\n    binary,           ///< binary array (ordered collection of bytes)\n    discarded         ///< discarded by the parser callback function\n};\n\nunion json_value {\n  /// object (stored with pointer to save storage)\n  object_t *object;\n  /// array (stored with pointer to save storage)\n  array_t *array;\n  /// string (stored with pointer to save storage)\n  string_t *string;\n  /// binary (stored with pointer to save storage)\n  binary_t *binary;\n  /// boolean\n  boolean_t boolean;\n  /// number (integer)\n  number_integer_t number_integer;\n  /// number (unsigned integer)\n  number_unsigned_t number_unsigned;\n  /// number (floating-point)\n  number_float_t number_float;\n};\n

    Objects, arrays, strings, and binary values are allocated on the heap with AllocatorType, and the union only stores a pointer to them. This keeps a basic_json value small: one pointer-sized union and one byte for the type. The class maintains the invariant that the pointer matching m_type is never null; assert_invariant() checks it with runtime assertions.

    "},{"location":"home/architecture/#node-index-of-json-views","title":"Node index of JSON views","text":"

    A basic_json_document (see Zero-copy JSON views) does not build a tree of values. Its parser (detail/view/builder.hpp) writes a flat array of 16-byte nodes, one per value and one per object key, in document order. A basic_json_view is a pointer to the document and a pointer to one node. The layout is struct node in detail/view/node.hpp (the numbers are bit offsets, 32 bits per row):

    packet-beta\n  0-7: \"kind\"\n  8-15: \"flags\"\n  16-31: \"extra\"\n  32-63: \"off\"\n  64-95: \"len\"\n  96-127: \"next\"
    Bytes Field Type Contents 0 kind uint8_t the type, numbered as value_t: 0 null, 1 object, 2 array, 3 string, 4 boolean, 5 signed integer, 6 unsigned integer, 7 float; 10 for a link (see below). There is no node for a discarded value: a discarded view has none 1 flags uint8_t bits 0-1: where a string's bytes (or a number's token) are (0: the source text, 1: the buffer of decoded strings, for strings with escapes, 2: the edit buffer); bit 2: the value of a boolean; bit 3: moved, bit 4: new, bit 5: linked (see below) 2-3 extra uint16_t numbers: the number of integer digits (low byte) and fraction digits (high byte), 255 for more; objects: the number of their key hash index (1-based), or 0; otherwise 0 4-7 off uint32_t where the value starts: the first byte after a string's opening quote (or its position in the buffer of decoded strings), the first byte of a number or literal, the bracket of an array or object 8-11 len uint32_t strings: the length after decoding; floats and literals: the length of the token; arrays and objects: the number of elements 12-15 next uint32_t arrays and objects: the number of nodes of the subtree, including the node itself
    • Integers keep their converted value in bytes 8-15 instead of len and next; the length of their token follows from the number of digits in extra (and the sign). Non-negative integers are unsigned integers, as with parse.
    • Floats keep only their token. The digit layout in extra lets the conversion read the digits without scanning the token again, and only when the value is read.
    • Object members are the node of the key (a string) followed by the nodes of the value.
    • Navigation needs no pointers: the elements of an array or object follow its node, and the node after a value's subtree is next nodes further for an array or object, and the next node otherwise (document_data::after). Views step from element to element this way and skip whole subtrees in constant time.
    • Offsets are 32 bits wide, so a document is limited to 4294967279 bytes, 4 GiB minus 16 bytes (a margin below 2^32 for positions one scanner step past the end of the text; out_of_range.416).
    • Large objects (128 members or more) get a hash index for their keys after parsing (detail/view/object_index.hpp): an open-addressing table whose slots hold the distance from the object's node to a key's node, so that a lookup does not compare every key. Of duplicate keys the table leads to the first, as a linear search does. The object's extra holds the number of its table. Only 65,535 tables fit into extra; objects beyond them are searched linearly.

    For example, {\"a\": [1, 2.5]} becomes five nodes. Each node's elements follow it, and next leads from an array or object past its subtree:

    flowchart LR\n    n0[\"0: object<br>len 1, next 5\"]\n    n1[\"1: key a\"]\n    n2[\"2: array<br>len 2, next 3\"]\n    n3[\"3: unsigned integer 1\"]\n    n4[\"4: float 2.5\"]\n    e([\"end\"])\n    n0 --> n1 --> n2 --> n3 --> n4 --> e\n    n0 -. next .-> e\n    n2 -. next .-> e
    Node kind extra off len next 0 1 (object) 0 0 1 5 1 3 (string) 0 2 1 0 2 2 (array) 0 6 2 3 3 6 (unsigned integer) 0x0001 7 - - 4 7 (float) 0x0101 10 3 0

    All flags are 0. The integer's bytes 8-15 hold its value, 1; its extra says it has one digit. The float's extra says it has one integer and one fraction digit, and its len is that of the token 2.5.

    Editable documents (json_editable_document, detail/view/edit.hpp and detail/view/edit_storage.hpp) never write the source text and never move or resize the parsed index, so views stay valid while the document is edited:

    • A new scalar is written over its node. Its text (a string, or the token of a number as dump() writes it) goes to the edit buffer, which flags bits 0-1 then name.
    • An array or object whose elements change gets the flag moved (bit 3): its elements then live in a separate sequence (a header node, then the entries), whose number is in off. The entries are links (kind 10), whose bytes 8-15 hold the address of the value's node, so values never move.
    • A node written by an edit gets the flag new (bit 4): it has no position in the source text.
    • A value that an entry of such a sequence links to gets the flag linked (bit 5): its extent in the parsed layout (next) no longer matters, so a scalar can replace an array or object there in place.
    • Views of read-only documents compile without any of this: how views walk the index is a template parameter (navigation<Editable>).

    Images (save and load, detail/view/image.hpp) store the nodes as they are: a 64-byte header (the magic bytes NJVI, a format version, the sizes, and reserved bytes that must be zero), the nodes, the text, and the decoded strings. An edited document is first written in document order, as the parser would have written it (without links), and the numbers of the hash indexes are cleared, since load rebuilds the indexes. So a change of the node layout is a change of the image format: it must raise image_version, and load then rejects images of other versions (parse_error.116) instead of misreading them.

    "},{"location":"home/architecture/#input-adapters","title":"Input adapters","text":"

    Input is read via input adapters that abstract a source. Every input adapter provides this interface:

    /// the type of the characters in the input\nusing char_type = ...;\n\n/// read a single character; returns std::char_traits<char_type>::eof() at the end of the input\ntypename std::char_traits<char_type>::int_type get_character();\n\n/// read up to count * sizeof(T) bytes into dest and return the number of bytes read\n/// (used by the binary readers)\ntemplate<class T>\nstd::size_t get_elements(T* dest, std::size_t count = 1);\n

    The lexer detects two optional extensions at compile time. Only iterator_input_adapter provides them, and only for random-access input of single-byte characters:

    • supports_seek, get_consumed_count(), and copy_consumed_range() let the lexer reconstruct already consumed input for error messages instead of copying every character it reads. With get_read_count() and count_consumed_lines(), the lexer also derives the read position from the adapter and counts lines only when an error needs them, instead of counting characters and lines on every read.
    • supports_bulk_scan, bulk_data(), bulk_remaining(), and bulk_skip() let the lexer scan strings directly in contiguous memory, several bytes at a time, and skip the spaces that indent a line in 8-byte steps.

    The function input_adapter picks the right adapter for the argument passed to parse, accept, sax_parse, or the from_* functions:

    • iterator_input_adapter reads from an iterator range, which also covers strings, containers, and pointers.
    • wide_string_input_adapter reads from ranges of wchar_t, char16_t, or char32_t and converts them to UTF-8. It cannot be used for binary formats; its get_elements() throws.
    • input_stream_adapter reads from a std::istream.
    • file_input_adapter reads from a std::FILE*.
    "},{"location":"home/architecture/#sax-interface","title":"SAX interface","text":"

    The parser does not build values itself. It reports what it reads as events to a SAX consumer, which implements the interface json_sax: null, boolean, number_integer, number_unsigned, number_float, string, binary, start_object, key, end_object, start_array, end_array, and parse_error.

    The library comes with two consumers in detail/input/json_sax.hpp:

    • json_sax_dom_parser builds a basic_json value tree. parse uses it.
    • json_sax_dom_callback_parser does the same, but calls a parser callback for each event, which can skip values. parse uses it when a callback is given.

    The binary_reader emits the same events for binary formats, so sax_parse works with a user-defined consumer for JSON and for all binary formats alike.

    "},{"location":"home/architecture/#output-adapters","title":"Output adapters","text":"

    Output is written via output adapters:

    void write_character(CharType c);\n\nvoid write_characters(const CharType* s, std::size_t length);\n

    The serializer (used by dump and operator<<) and the binary_writer (used by the to_* functions) write to one of these adapters:

    • output_vector_adapter appends to a std::vector.
    • output_stream_adapter writes to a std::ostream.
    • output_string_adapter appends to a string.
    "},{"location":"home/architecture/#value-conversion","title":"Value conversion","text":"

    Values are converted from and to other types with the JSONSerializer template parameter. The default, adl_serializer, calls the free functions

    template<class T>\nvoid to_json(basic_json& j, const T& t);\n\ntemplate<class T>\nvoid from_json(const basic_json& j, T& t);\n

    found by argument-dependent lookup. The library defines them for standard types in detail/conversions; users add them for their own types, see Arbitrary Type Conversions. The serialization macros generate these functions.

    "},{"location":"home/architecture/#additional-features","title":"Additional features","text":"
    • JSON Pointer (class json_pointer) addresses values inside a tree. It is also the basis of JSON Patch.
    • Binary formats are read by binary_reader and written by binary_writer.
    • A custom base class can add members to every basic_json value.
    • Serialization macros generate to_json and from_json functions for user-defined types.
    "},{"location":"home/architecture/#details-namespace","title":"Details namespace","text":"

    Namespace nlohmann::detail contains all implementation details. It is not part of the public API and may change in any release. Besides the components above, it contains:

    • type traits to detect the capabilities of user-defined types (detail/meta/type_traits.hpp),
    • backports of C++14/17 features to C++11 (detail/meta/cpp_future.hpp), and
    • helpers such as string_concat and string_escape.
    "},{"location":"home/customers/","title":"Customers","text":"

    The library is used in multiple projects, applications, operating systems, etc. The list below is not exhaustive, but the result of an internet search. If you know further customers of the library, please let me know.

    "},{"location":"home/customers/#space-exploration","title":"Space Exploration","text":"
    • Peregrine Lunar Lander Flight 01 - The library was used for payload management in the Peregrine Moon Lander, developed by Astrobotic Technology and launched as part of NASA's Commercial Lunar Payload Services (CLPS) program. After six days in orbit, the spacecraft was intentionally redirected into Earth's atmosphere, where it burned up over the Pacific Ocean on January 18, 2024.
    • NASA Unsteady Pressure-Sensitive Paint Processing, NASA software for processing high-speed video recordings of wind tunnel tests on launch vehicle and aircraft models
    • Terma TEMU, an emulator of spacecraft on-board computers used to develop and validate flight software for European space missions
    "},{"location":"home/customers/#automotive","title":"Automotive","text":"
    • Alexa Auto SDK, a software development kit enabling the integration of Alexa into automotive systems
    • Apollo, a framework for building autonomous driving systems
    • Automotive Grade Linux (AGL), a collaborative open-source platform for automotive software development
    • Autoware, an open-source software stack for autonomous driving built on ROS 2
    • Eclipse S-CORE, an open-source software platform for the software-defined vehicle backed by major automotive manufacturers and suppliers
    • Genesis Motor (infotainment), a luxury automotive brand
    • Hyundai (infotainment), a global automotive brand
    • Kia (infotainment), a global automotive brand
    • Mercedes-Benz Operating System (MB.OS), a core component of the vehicle software ecosystem from Mercedes-Benz
    • NVIDIA DRIVE OS, the operating system and DriveWorks SDK powering NVIDIA's platform for autonomous vehicles
    • Rivian (infotainment), an electric vehicle manufacturer
    • Suzuki (infotainment), a global automotive and motorcycle manufacturer
    "},{"location":"home/customers/#gaming-and-entertainment","title":"Gaming and Entertainment","text":"
    • Anno 117: Pax Romana, a city-building strategy game set in the Roman Empire
    • Assassin's Creed: Mirage, a stealth-action game set in the Middle East, focusing on the journey of a young assassin with classic parkour and stealth mechanics
    • Battlefield 6, a military first-person shooter known for its large-scale multiplayer battles
    • Battlefield: REDSEC, a free-to-play battle royale experience set in the Battlefield universe
    • BioMenace: Remastered, a remaster of the classic side-scrolling platform shooter
    • Chasm: The Rift, a first-person shooter blending horror and adventure, where players navigate dark realms and battle monsters
    • College Football 25, a college football simulation game featuring gameplay that mimics real-life college teams and competitions
    • College Football 26, a college football simulation game featuring licensed teams and stadiums
    • College Football 27, the latest installment of the college football simulation series
    • Concepts, a digital sketching app designed for creative professionals, offering flexible drawing tools for illustration, design, and brainstorming
    • Depthkit, a tool for creating and capturing volumetric video, enabling immersive 3D experiences and interactive content
    • Dune: Awakening, an open-world survival MMO set on the desert planet Arrakis
    • EA Sports FC 25, an association football simulation with club, career, and online modes
    • EA Sports FC 26, the latest installment of the association football simulation series
    • EA Sports UFC 6, a mixed martial arts fighting simulation
    • FiveM, a modification framework for Grand Theft Auto V that powers custom multiplayer servers
    • FLUX:: Immersive, a suite of professional audio processing and immersive mixing plugins used in music and post-production
    • IMG.LY, a platform offering creative tools and SDKs for integrating advanced image and video editing in applications
    • immersivetech, a technology company focused on immersive experiences, providing tools and solutions for virtual and augmented reality applications
    • Kodi, a home theater and media center application
    • LOOT, a tool for optimizing the load order of game plugins, commonly used in The Elder Scrolls and Fallout series
    • LunaTranslator, a real-time translation tool for visual novels
    • MaaAssistantArknights, an automation assistant for the mobile game Arknights
    • Madden NFL 25, a sports simulation game capturing the excitement of American football with realistic gameplay and team management features
    • Madden NFL 26, an American football simulation with franchise and team management modes
    • Madden NFL 27, the latest installment of the American football simulation series
    • Marne, an unofficial private server platform for hosting custom Battlefield 1 game experiences
    • Minecraft, a popular sandbox video game
    • Mumble, a low-latency, open-source voice chat application widely used by gaming communities
    • NHL 22, a hockey simulation game offering realistic gameplay, team management, and various modes to enhance the hockey experience
    • OBS Studio, a free and open-source suite for video recording and live streaming
    • OpenRCT2, an open source re-implementation of RollerCoaster Tycoon 2
    • Pixelpart, a 2D animation and video compositing software that allows users to create animated graphics and visual effects with a focus on simplicity and ease of use
    • Razer Cortex, a gaming performance optimizer and system booster designed to enhance the gaming experience
    • Red Dead Redemption II, an open-world action-adventure game following an outlaw's story in the late 1800s, emphasizing deep storytelling and immersive gameplay
    • RetroArch, a frontend for emulators, game engines, and media players built on the libretro API
    • shadPS4, a PlayStation 4 emulator for Windows, Linux and macOS
    • skate., a free-to-play skateboarding game set in an open world
    • Snapchat, a multimedia messaging and augmented reality app for communication and entertainment
    • Steel Century Groove, an action game released in 2026
    • Sunshine, a self-hosted game streaming host compatible with Moonlight clients
    • Tactics Ogre: Reborn, a tactical role-playing game featuring strategic battles and deep storytelling elements
    • Throne and Liberty, an MMORPG that offers an expansive fantasy world with dynamic gameplay and immersive storytelling
    • Unity Vivox, a communication service that enables voice and text chat functionality in multiplayer games developed with Unity
    • xemu, an emulator of the original Xbox console
    • Zool: Redimensioned, a modern reimagining of the classic platformer featuring fast-paced gameplay and vibrant environments
    "},{"location":"home/customers/#consumer-electronics","title":"Consumer Electronics","text":"
    • Audinate, a provider of networked audio solutions specializing in Dante technology, which facilitates high-quality digital audio transport over IP networks
    • Canon CanoScan LIDE, a series of flatbed scanners offering high-resolution image scanning for home and office use
    • Canon PIXMA Printers, a line of all-in-one inkjet printers known for high-quality printing and wireless connectivity
    • Cisco Webex Desk Camera, a video camera designed for professional-quality video conferencing and remote collaboration
    • DJI Edge SDK, the reference applications for DJI's Edge SDK, used to build edge computing services on DJI drone docks
    • Elgato Stream Deck, a family of programmable control surfaces for content creators and their plugin ecosystem
    • Instagrid, a manufacturer of portable, high-performance battery systems for professional mobile power supply
    • iRobot, a manufacturer of autonomous home robots including the Roomba vacuum cleaner range
    • Logitech Logi Bolt, the management application for Logitech's secure wireless connectivity technology
    • Novitus, a manufacturer of fiscal cash registers and point-of-sale devices
    • Philips Hue Personal Wireless Lighting, a smart lighting system for customizable and wireless home illumination
    • Ray-Ban Meta Smart glasses, a pair of smart glasses designed for capturing photos and videos with integrated connectivity and social features
    • Razer Synapse, a unified configuration software enabling hardware customization for Razer devices
    • Sharp Professional Displays, a range of large-format interactive displays for business and education
    • Siemens SINEMA Remote Connect, a remote connectivity solution for monitoring and managing industrial networks and devices securely
    • Skydio, a manufacturer of autonomous drones for inspection, public safety, and defense applications
    • Sony PlayStation 4, a gaming console developed by Sony that offers a wide range of games and multimedia entertainment features
    • Sony Spatial Reality Display, a glasses-free stereoscopic 3D display and its plugins for Blender, 3ds Max, and ZBrush
    • Sony Virtual Webcam Driver for Remote Camera, a software driver that enables the use of Sony cameras as virtual webcams for video conferencing and streaming
    • Yamaha Clavinova, a series of digital pianos combining acoustic piano feel with digital sound technology
    "},{"location":"home/customers/#operating-systems-and-platforms","title":"Operating Systems and Platforms","text":"
    • Apple iOS and macOS, a family of operating systems developed by Apple, including iOS for mobile devices and macOS for desktop computers
    • Chromium, the open-source browser project that Google Chrome, Microsoft Edge, and many other browsers are built on, where the library is used as data container for on-device model execution
    • Google Fuchsia, an open-source operating system developed by Google, designed to be secure, updatable, and adaptable across various devices
    • LG webOS, a Linux-based operating system used in LG smart TVs, signage, and embedded devices
    • Microsoft Azure Linux, a Linux distribution developed by Microsoft for Azure infrastructure and edge workloads
    • OpenHarmony, an open-source operating system for smart devices and the foundation of HarmonyOS
    • SerenityOS, an open-source operating system that aims to provide a simple and beautiful user experience with a focus on simplicity and elegance
    • Windows Subsystem for Linux, a compatibility layer that runs Linux environments natively on Windows
    • Yocto, a Linux-based build system for creating custom operating systems and software distributions, tailored for embedded devices and IoT applications
    "},{"location":"home/customers/#development-tools-and-ides","title":"Development Tools and IDEs","text":"
    • Accentize SpectralBalance, an adaptive speech analysis tool designed to enhance audio quality by optimizing frequency balance in recordings
    • Airbus Ghidralligator, a Ghidra-based emulator from Airbus CyberSecurity used to fuzz and analyse embedded firmware
    • Apache brpc, an industrial-grade remote procedure call framework for C++
    • Arm Compiler for Linux, a software development toolchain for compiling and optimizing applications on Arm-based Linux systems
    • BBEdit, a professional text and code editor for macOS
    • CoderPad, a collaborative coding platform that enables real-time code interviews and assessments for developers; the library is included in every CoderPad instance and can be accessed with a simple #include \"json.hpp\"
    • Codon, an ahead-of-time compiler for a Python-like language
    • Compiler Explorer, a web-based tool that allows users to write, compile, and visualize the assembly output of code in various programming languages; the library is readily available and accessible with the directive #include <nlohmann/json.hpp>.
    • Flutter, a UI toolkit for building natively compiled applications for mobile, web, and desktop from a single codebase
    • Fraunhofer VVenC, a fast and efficient encoder for the Versatile Video Coding (H.266/VVC) standard
    • GitHub CodeQL, a code analysis tool used for identifying security vulnerabilities and bugs in software through semantic queries
    • GoPro ngfx, a low-level graphics abstraction and profiling framework developed by GoPro
    • gRPC, a high-performance universal remote procedure call framework
    • Hex-Rays, a reverse engineering toolset for analyzing and decompiling binaries, primarily used for security research and vulnerability analysis
    • ImHex, a hex editor designed for reverse engineering, providing advanced features for data analysis and manipulation
    • Intel GITS, a tool for capturing and replaying graphics API calls for debugging and performance analysis
    • Intel GPA Framework, a suite of cross-platform tools for capturing, analyzing, and optimizing graphics applications across different APIs
    • Intopix, a provider of advanced image processing and compression solutions used in software development and AV workflows
    • Java SE, the core Java platform that provides the libraries and runtime needed to build and run general-purpose Java applications
    • Meta Yoga, a layout engine that facilitates flexible and efficient user interface design across multiple platforms
    • MKVToolNix, a set of tools for creating, editing, and inspecting MKV (Matroska) multimedia container files
    • MRTech IFF SDK, an image processing SDK for machine vision applications with GPU-accelerated pipelines
    • Nix, a purely functional package manager
    • Notepad++, a free source code editor that supports various programming languages
    • NVIDIA Nsight Compute, a performance analysis tool for CUDA applications that provides detailed insights into GPU performance metrics
    • openFrameworks, a community-developed C++ toolkit for creative coding
    • OpenRGB, an open source RGB lighting control that doesn't depend on manufacturer software
    • OpenTelemetry C++, a library for collecting and exporting observability data in C++, enabling developers to implement distributed tracing and metrics in their application
    • Oracle GraalVM, a high-performance JDK distribution with ahead-of-time compilation and polyglot runtime support
    • Philips amp-cucumber-cpp-runner, a behaviour-driven development test runner for embedded C++ software developed at Philips
    • Qt Creator, an IDE for developing applications using the Qt application framework
    • Qt for MCUs, a graphics framework for building fluid user interfaces on microcontrollers
    • React Native, a framework for building native mobile applications using React
    • Scanbot SDK, a software development kit (SDK) that provides tools for integrating advanced document scanning and barcode scanning capabilities into applications
    • STMicroelectronics TouchGFX, a graphical user interface framework shipped with STM32 microcontrollers for building embedded HMIs
    • swagger-codegen, a template-driven engine that generates API clients and server stubs from an OpenAPI specification
    • Swoole, a coroutine-based concurrency engine for PHP
    • Tracy Profiler, a real-time frame profiler for games and other applications
    • WasmEdge, a lightweight WebAssembly runtime for edge and cloud workloads
    • x64dbg, an open source user mode debugger for Windows, aimed at reverse engineering and malware analysis
    "},{"location":"home/customers/#machine-learning-and-ai","title":"Machine Learning and AI","text":"
    • Alibaba MNN, a lightweight deep learning inference engine for mobile and embedded devices
    • AMD Gaia, an open-source framework for running generative AI applications locally on AMD hardware
    • AMD Vitis AI (VAIP), the execution provider stack that runs AI models on AMD Ryzen AI and adaptive computing devices
    • Apple Core ML Tools, a set of tools for converting and configuring machine learning models for deployment in Apple's Core ML framework
    • Avular Mobile Robotics, a platform for developing and deploying mobile robotics solutions
    • FunASR, a speech recognition toolkit for training and deploying end-to-end models
    • Google gemma.cpp, a lightweight C++ inference engine designed for running AI models from the Gemma family
    • Google Magenta The Infinite Crate, an open-source generative AI plugin for digital audio workstations from Google's Magenta research team
    • GPT4All, a desktop application for running local large language models on consumer hardware
    • Huawei MindSpore, a deep learning framework for training and inference across device, edge, and cloud
    • KTransformers, a framework for heterogeneous large language model inference
    • llama.cpp, a C++ library designed for efficient inference of large language models (LLMs), enabling streamlined integration into applications
    • LocalAI, a self-hosted inference engine that exposes local models through an OpenAI-compatible API
    • MLX, an array framework for machine learning on Apple Silicon
    • Mozilla llamafile, a tool designed for distributing and executing large language models (LLMs) efficiently using a single file format
    • NVIDIA ACE, a suite of real-time AI solutions designed for the development of interactive avatars and digital human applications, enabling scalable and sophisticated user interactions
    • NVIDIA Instant NGP, an implementation of instant neural graphics primitives for rapid scene reconstruction
    • NVIDIA TensorRT, an SDK for high-performance deep learning inference, including its TensorRT-LLM extension for large language models
    • NVIDIA TensorRT-LLM, a toolkit for optimizing and serving large language model inference on GPUs
    • ONNX Runtime, a cross-platform inference and training accelerator for machine learning models
    • OpenVINO, Intel's toolkit for optimizing and deploying deep learning inference across CPUs, GPUs, and NPUs
    • PaddleOCR, an optical character recognition toolkit that turns documents and images into structured data
    • PaddlePaddle, a deep learning framework for distributed training and inference
    • Peer, a platform offering personalized AI assistants for interactive learning and creative collaboration
    • PyTorch, a machine learning framework for building and training neural networks, widely used in research and production
    • Qualcomm AI Engine Direct, a toolchain for building and running generative AI applications on Snapdragon devices
    • sherpa-onnx, a speech toolkit for on-device recognition, synthesis and speaker diarization
    • stable-diffusion.cpp, a C++ implementation of the Stable Diffusion image generation model
    • TanvasTouch, a software development kit (SDK) that enables developers to create tactile experiences on touchscreens, allowing users to feel textures and physical sensations in a digital environment
    • TensorFlow, a machine learning framework that facilitates the development and training of models, supporting data serialization and efficient data exchange between components
    • whisper.cpp, a C++ implementation of OpenAI's Whisper automatic speech recognition model
    "},{"location":"home/customers/#scientific-research-and-analysis","title":"Scientific Research and Analysis","text":"
    • BLACK, a bounded linear temporal logic (LTL) satisfiability checker
    • CERN ALICE O2, the online-offline computing framework of the ALICE heavy-ion experiment at the Large Hadron Collider
    • CERN Atlas Athena, a software framework used in the ATLAS experiment at the Large Hadron Collider (LHC) for performance monitoring
    • CERN CMSSW, the offline software framework of the CMS experiment at the Large Hadron Collider
    • CERN Gaudi, the event-processing framework used by the LHCb and ATLAS experiments at the Large Hadron Collider
    • ICU, the International Components for Unicode, a mature library for software globalization and multilingual support
    • KAMERA, a platform for synchronized data collection and real-time deep learning to map marine species like polar bears and seals, aiding Arctic ecosystem research
    • KiCad, a free and open-source software suite for electronic design automation
    • LLNL ROSE, a compiler infrastructure from Lawrence Livermore National Laboratory for building source-to-source program analysis and transformation tools
    • Maple, a symbolic and numeric computing environment for advanced mathematical modeling and analysis
    • MeVisLab, a software framework for medical image processing and visualization.
    • MITK, the Medical Imaging Interaction Toolkit, a framework for developing interactive medical image processing software
    • OpenPMD API, a versatile programming interface for accessing and managing scientific data, designed to facilitate the efficient storage, retrieval, and sharing of simulation data across various applications and platforms
    • ORNL DataFed, a federated scientific data management system developed at Oak Ridge National Laboratory
    • ParaView, an open-source tool for large-scale data visualization and analysis across various scientific domains
    • QGIS, a free and open-source geographic information system (GIS) application that allows users to create, edit, visualize, and analyze geospatial data across a variety of formats
    • Sandia InterSpec, spectral radiation analysis software from Sandia National Laboratories for identifying radioactive isotopes
    • VolView, a lightweight application for interactive visualization and analysis of 3D medical imaging data.
    • VTK, a software library for 3D computer graphics, image processing, and visualization
    "},{"location":"home/customers/#business-and-productivity-software","title":"Business and Productivity Software","text":"
    • ArcGIS PRO, a desktop geographic information system (GIS) application developed by Esri for mapping and spatial analysis
    • Autodesk Desktop, a software platform developed by Autodesk for creating and managing desktop applications and services
    • Check Point, a cybersecurity company specializing in threat prevention and network security solutions, offering a range of products designed to protect enterprises from cyber threats and ensure data integrity
    • EasyEffects, an audio effects processor for PipeWire offering limiting, compression and equalization
    • espanso, a cross-platform text expander
    • Karabiner-Elements, a keyboard customizer for macOS
    • MacType, a font rendering engine for Windows
    • magicplan, a mobile application for creating floor plans and interior designs using augmented reality
    • Microsoft Office for Mac, a suite of productivity applications developed by Microsoft for macOS, including tools for word processing, spreadsheets, and presentations
    • Microsoft Teams, a team collaboration application offering workspace chat and video conferencing, file storage, and integration of proprietary and third-party applications and services
    • MuseScore, a free and open-source music notation and composition application
    • NanaZip, a 7-Zip derivative built for modern Windows
    • Nexthink Infinity, a digital employee experience management platform for monitoring and improving IT performance
    • Sophos Connect Client, a secure VPN client from Sophos that allows remote users to connect to their corporate network, ensuring secure access to resources and data
    • Stonebranch, a cloud-based cybersecurity solution that integrates backup, disaster recovery, and cybersecurity features to protect data and ensure business continuity for organizations
    • Tablecruncher, a data analysis tool that allows users to import, analyze, and visualize spreadsheet data, offering interactive features for better insights and decision-making
    • VNote, a Markdown-based note-taking application written in C++
    "},{"location":"home/customers/#databases-and-big-data","title":"Databases and Big Data","text":"
    • ADIOS2, a data management framework designed for high-performance input and output operations
    • Apache Doris, a real-time analytical database for high-concurrency queries
    • Claris FileMaker Server, the server platform hosting FileMaker custom apps and databases, developed by Apple subsidiary Claris
    • ClickHouse, a column-oriented database management system for real-time analytical queries
    • Cribl Stream, a real-time data processing platform that enables organizations to collect, route, and transform observability data, enhancing visibility and insights into their systems
    • DB Browser for SQLite, a visual open-source tool for creating, designing, and editing SQLite database files
    • Manticore Search, a database for search, offering full-text and vector queries
    • Milvus, a cloud-native vector database built for embedding similarity search
    • MongoDB, a general-purpose document database
    • MySQL Connector/C++, a C++ library for connecting and interacting with MySQL databases
    • MySQL NDB Cluster, a distributed database system that provides high availability and scalability for MySQL databases
    • MySQL Shell, an advanced client and code editor for interacting with MySQL servers, supporting SQL, Python, and JavaScript
    • PrestoDB, a distributed SQL query engine designed for large-scale data analytics, originally developed by Facebook
    • ROOT Data Analysis Framework, an open-source data analysis framework widely used in high-energy physics and other fields for data processing and visualization
    • Typesense, an open source typo-tolerant search engine
    • Vearch, a distributed vector database developed at JD.com for similarity search and retrieval-augmented generation
    • WiredTiger, a high-performance storage engine for databases, offering support for compression, concurrency, and checkpointing
    "},{"location":"home/customers/#simulation-and-modeling","title":"Simulation and Modeling","text":"
    • Adobe Lagrange, a geometry processing library developed by Adobe for mesh manipulation and analysis
    • Arcturus HoloSuite, a software toolset for capturing, editing, and streaming volumetric video, featuring advanced compression technologies for high-quality 3D content creation
    • azul, a fast and efficient 3D city model viewer designed for visualizing urban environments and spatial data
    • Bambu Studio, a slicing and print management application for Bambu Lab 3D printers
    • Blender, a free and open-source 3D creation suite for modeling, animation, rendering, and more
    • cpplot, a library for creating interactive graphs and charts in C++, which can be viewed in web browsers
    • Foundry Nuke, a powerful node-based digital compositing and visual effects application used in film and television post-production
    • FreeCAD, a free and open-source parametric 3D CAD modeler for product design and engineering
    • GAMS, a high-performance mathematical modeling system for optimization and decision support
    • Keysight WirelessPro, a simulation platform for 5G, 5G-Advanced, and 6G cellular network research
    • Kitware SMTK, a software toolkit for managing simulation models and workflows in scientific and engineering applications
    • M-Star, a computational fluid dynamics software for simulating and analyzing fluid flow
    • MapleSim CAD Toolbox, a software extension for MapleSim that integrates CAD models, allowing users to import, manipulate, and analyze 3D CAD data within the MapleSim environment for enhanced modeling and simulation
    • Microsoft AirSim, a simulator for autonomous vehicles and drones built on Unreal Engine
    • NVIDIA Omniverse, a platform for 3D content creation and collaboration that enables real-time simulations and interactive experiences across various industries
    • OpenSCAD, a script-driven solid 3D CAD modeller
    • OrcaSlicer, an open-source slicer supporting a wide range of consumer 3D printers
    • Pixar Renderman, a photorealistic 3D rendering software developed by Pixar, widely used in the film industry for creating high-quality visual effects and animations
    • PrusaSlicer, the slicing software developed by Prusa Research for its 3D printers
    • ROS - Robot Operating System, a set of software libraries and tools that assist in developing robot applications
    • UBS, a multinational financial services and banking company
    "},{"location":"home/customers/#enterprise-and-cloud-applications","title":"Enterprise and Cloud Applications","text":"
    • Acronis Cyber Protect Cloud, an all-in-one data protection solution that combines backup, disaster recovery, and cybersecurity to safeguard business data from threats like ransomware
    • Baereos, a backup solution that provides data protection and recovery options for various environments, including physical and virtual systems
    • Bitdefender Home Scanner, a tool from Bitdefender that scans devices for malware and security threats, providing a safeguard against potential online dangers
    • Cisco MLS++, an implementation of the Messaging Layer Security protocol for end-to-end encrypted group messaging
    • Citrix Provisioning, a solution that streamlines the delivery of virtual desktops and applications by allowing administrators to manage and provision resources efficiently across multiple environments
    • Citrix Virtual Apps and Desktops, a solution from Citrix that delivers virtual apps and desktops
    • CyberArk, a security solution that specializes in privileged access management, enabling organizations to control and monitor access to critical systems and data, thereby enhancing overall cybersecurity posture
    • Deutsche Telekom sysrepo-plugins, a collection of YANG datastore plugins used to manage network devices
    • Egnyte Desktop, a secure cloud storage solution designed for businesses, enabling file sharing, collaboration, and data management across teams while ensuring compliance and data protection
    • Elster, a digital platform developed by German tax authorities for secure and efficient electronic tax filing and management using secunet protect4use
    • Envoy, a cloud-native edge and service proxy that forms the data plane of many service meshes
    • Ethereum Solidity, a high-level, object-oriented programming language designed for implementing smart contracts on the Ethereum platform
    • gVisor, an application kernel that provides a secure sandbox for running untrusted containers
    • IBM Storage Virtualize, the software powering IBM FlashSystem enterprise storage arrays
    • Inciga, a monitoring tool for IT infrastructure, designed to provide insights into system performance and availability through customizable dashboards and alerts
    • Intel Accelerator Management Daemon for VMware ESXi, a management tool designed for monitoring and controlling Intel hardware accelerators within VMware ESXi environments, optimizing performance and resource allocation
    • Juniper Identity Management Service
    • Meta FBOSS, the software stack that controls the network switches in Meta's data centers
    • Microsoft Azure IoT SDK, a collection of tools and libraries to help developers connect, build, and deploy Internet of Things (IoT) solutions on the Azure cloud platform
    • Microsoft Confidential Consortium Framework, a framework for building secure, highly available applications on trusted execution environments
    • Microsoft WinGet, a command-line utility included in the Windows Package Manager
    • Mitsubishi Electric SECS/GEM, the semiconductor equipment communication software running on Mitsubishi Electric C Controller and C intelligent function modules
    • Moxa, a provider of industrial networking, computing, and automation infrastructure
    • plexusAV, a high-performance AV-over-IP transceiver device capable of video encoding and decoding using the IPMX standard
    • Pointr, a platform for indoor positioning and navigation solutions, offering tools and SDKs for developers to create location-based applications
    • secunet protect4use, a secure, passwordless multifactor authentication solution that transforms smartphones into digital keyrings, ensuring high security for online services and digital identities
    • Sencore MRD 7000, a professional multi-channel receiver and decoder supporting UHD and HD stream decoding
    • Siemens SINEC, a family of network management and infrastructure services for industrial networks
    • Toshiba Industrial Servers, the FS20000R series of industrial servers for factory automation and control systems
    • Wazuh, a security platform for threat detection, integrity monitoring and incident response
    • ZeroTier, a software-defined networking service that creates virtual Ethernet networks
    "},{"location":"home/debugging/","title":"Debugging","text":"

    This page collects the library's built-in debugger integrations and other debugging-related features. They are not linked from a single place elsewhere in the docs, so are collected here.

    "},{"location":"home/debugging/#visual-studio-natvis","title":"Visual Studio (natvis)","text":"

    The repository ships nlohmann_json.natvis at its root, a Natvis file that gives json/ordered_json values a friendly, key/value debugger view instead of showing raw internal fields, when debugging with the MSVC debug engine (cppvsdbg) in Visual Studio or VS Code.

    Debug engines that wrap LLDB instead of the MSVC debug engine (for example, codelldb in VS Code) only have partial/experimental Natvis support, and commonly fall back to showing raw internal fields even with the .natvis file present. Switching to cppvsdbg where available, or checking your debug extension's own Natvis support/version, are the next things to try if this happens. There is currently no bundled LLDB-native pretty-printer script in this repository.

    "},{"location":"home/debugging/#gdb","title":"GDB","text":"

    The repository ships a GDB Python pretty printer under tools/gdb_pretty_printer, with its own usage instructions in that directory's README.md.

    "},{"location":"home/debugging/#extended-exception-diagnostics","title":"Extended exception diagnostics","text":"

    Defining JSON_DIAGNOSTICS before including the library augments type_error/out_of_range-style exceptions with a JSON Pointer to the offending value, which can help pinpoint where in a large document a runtime error occurred. This only applies to exceptions thrown after a value exists (e.g. during element access); parse errors, which happen before any value exists to point at, are not covered by this mechanism -- see Parsing and exceptions for how parse errors report their own location instead.

    "},{"location":"home/design_goals/","title":"Design goals","text":"

    There are myriads of JSON libraries out there, and each may even have its reason to exist. Our class had these design goals:

    • Intuitive syntax. In languages such as Python, JSON feels like a first-class data type. We used all the operator magic of modern C++ to achieve the same feeling in your code.

    • Trivial integration. Our whole code consists of a single header file json.hpp. That's it. No library, no subproject, no dependencies, no complex build system. The class is written in vanilla C++11. All in all, everything should require no adjustment of your compiler flags or project settings.

    • Serious testing. Our class is heavily unit-tested and covers 100% of the code, including all exceptional behavior. Furthermore, we checked with Valgrind and the Clang Sanitizers that there are no memory leaks. Google OSS-Fuzz additionally runs fuzz tests against all parsers 24/7, effectively executing billions of tests so far. To maintain high quality, the project is following the OpenSSF Best Practices.

    Other aspects were not so important to us:

    • Memory efficiency. Each JSON object has an overhead of one pointer (the maximal size of a union) and one enumeration element (1 byte). The default generalization uses the following C++ data types: std::string for strings, int64_t, uint64_t or double for numbers, std::map for objects, std::vector for arrays, and bool for Booleans. However, you can template the generalized class basic_json to your needs.

    • Speed. There are certainly faster JSON libraries out there. However, if your goal is to speed up your development by adding JSON support with a single header, then this library is the way to go. If you know how to use a std::vector or std::map, you are already set.

    See the contribution guidelines for more information.

    "},{"location":"home/exceptions/","title":"Exceptions","text":""},{"location":"home/exceptions/#overview","title":"Overview","text":""},{"location":"home/exceptions/#base-type","title":"Base type","text":"

    All exceptions inherit from class json::exception (which in turn inherits from std::exception). It is used as the base class for all exceptions thrown by the basic_json class. This class can hence be used as \"wildcard\" to catch exceptions.

    classDiagram\n  direction LR\n    class `std::exception` {\n        <<interface>>\n    }\n\n    class `json::exception` {\n        +const int id\n        +const char* what() const\n    }\n\n    class `json::parse_error` {\n        +const std::size_t byte\n    }\n\n    class `json::invalid_iterator`\n    class `json::type_error`\n    class `json::out_of_range`\n    class `json::other_error`\n\n    `std::exception` <|-- `json::exception`\n    `json::exception` <|-- `json::parse_error`\n    `json::exception` <|-- `json::invalid_iterator`\n    `json::exception` <|-- `json::type_error`\n    `json::exception` <|-- `json::out_of_range`\n    `json::exception` <|-- `json::other_error`
    "},{"location":"home/exceptions/#switch-off-exceptions","title":"Switch off exceptions","text":"

    Exceptions are used widely within the library. They can, however, be switched off with either using the compiler flag -fno-exceptions or by defining the symbol JSON_NOEXCEPTION. In this case, exceptions are replaced by abort() calls. You can further control this behavior by defining JSON_THROW_USER (overriding throw), JSON_TRY_USER (overriding try), and JSON_CATCH_USER (overriding catch).

    Note that JSON_THROW_USER should leave the current scope (e.g., by throwing or aborting), as continuing after it may yield undefined behavior.

    Example: switch off exceptions and log errors before aborting

    The code below switches off exceptions and creates a log entry with a detailed error message in case of errors.

    #include <iostream>\n\n#define JSON_TRY_USER if(true)\n#define JSON_CATCH_USER(exception) if(false)\n#define JSON_THROW_USER(exception)                           \\\n    {std::clog << \"Error in \" << __FILE__ << \":\" << __LINE__ \\\n               << \" (function \" << __FUNCTION__ << \") - \"    \\\n               << (exception).what() << std::endl;           \\\n     std::abort();}\n\n#include <nlohmann/json.hpp>\n

    Note the explanatory what() string of exceptions is not available for MSVC if exceptions are disabled, see #2824.

    See documentation of JSON_TRY_USER, JSON_CATCH_USER and JSON_THROW_USER for more information.

    "},{"location":"home/exceptions/#extended-diagnostic-messages","title":"Extended diagnostic messages","text":"

    Exceptions in the library are thrown in the local context of the JSON value they are detected. This makes detailed diagnostics messages, and hence debugging, difficult.

    Example: standard diagnostic message 
    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    json j;\n    j[\"address\"][\"street\"] = \"Fake Street\";\n    j[\"address\"][\"housenumber\"] = \"12\";\n\n    try\n    {\n        int housenumber = j[\"address\"][\"housenumber\"];\n    }\n    catch (const json::exception& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

    Output:

    [json.exception.type_error.302] type must be number, but is string\n

    This exception can be hard to debug if storing the value \"12\" and accessing it is further apart.

    To create better diagnostics messages, each JSON value needs a pointer to its parent value such that a global context (i.e., a path from the root value to the value that led to the exception) can be created. That global context is provided as JSON Pointer.

    As this global context comes at the price of storing one additional pointer per JSON value and runtime overhead to maintain the parent relation, extended diagnostics are disabled by default. They can, however, be enabled by defining the preprocessor symbol JSON_DIAGNOSTICS to 1 before including json.hpp.

    Example: extended diagnostic message with JSON_DIAGNOSTICS 
    #include <iostream>\n\n# define JSON_DIAGNOSTICS 1\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    json j;\n    j[\"address\"][\"street\"] = \"Fake Street\";\n    j[\"address\"][\"housenumber\"] = \"12\";\n\n    try\n    {\n        int housenumber = j[\"address\"][\"housenumber\"];\n    }\n    catch (const json::exception& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

    Output:

    [json.exception.type_error.302] (/address/housenumber) type must be number, but is string\n

    Now the exception message contains a JSON Pointer /address/housenumber that indicates which value has the wrong type.

    See documentation of JSON_DIAGNOSTICS for more information.

    "},{"location":"home/exceptions/#parse-errors","title":"Parse errors","text":"

    The library throws this exception when a parse error occurs. Parse errors can occur during the deserialization of JSON text or of one of the binary formats (BJData, BON8, BSON, CBOR, MessagePack, UBJSON), as well as when using JSON Patch.

    Exceptions have ids 1xx.

    Byte index

    Member byte holds the byte index of the last read character in the input file.

    For an input with n bytes, 1 is the index of the first character and n+1 is the index of the terminating null byte or the end of file. This also holds true when reading a byte vector (CBOR or MessagePack).

    Example: catch a parse_error exception

    The following code shows how a parse_error exception can be caught.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    try\n    {\n        // parsing input with a syntax error\n        json j = json::parse(\"[1,2,3,]\");\n    }\n    catch (const json::parse_error& e)\n    {\n        // output exception information\n        std::cout << \"message: \" << e.what() << '\\n'\n                  << \"exception id: \" << e.id << '\\n'\n                  << \"byte position of error: \" << e.byte << std::endl;\n    }\n}\n

    Output:

    message: [json.exception.parse_error.101] parse error at line 1, column 8: syntax error while parsing value - unexpected ']'; expected '[', '{', or a literal\nexception id: 101\nbyte position of error: 8\n
    "},{"location":"home/exceptions/#jsonexceptionparse_error101","title":"json.exception.parse_error.101","text":"

    This error indicates a syntax error while deserializing a JSON text. The error message describes that an unexpected token (character) was encountered, and the member byte indicates the error position.

    Example message

    Input ended prematurely:

    [json.exception.parse_error.101] parse error at 2: unexpected end of input; expected string literal\n

    No input:

    [json.exception.parse_error.101] parse error at line 1, column 1: attempting to parse an empty input; check that your input string or stream contains the expected JSON\n

    Control character was not escaped:

    [json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid string: control character U+0009 (HT) must be escaped to \\u0009 or \\\\; last read: '\"<U+0009>'\"\n

    String was not closed:

    [json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid string: missing closing quote; last read: '\"'\n

    Invalid number format:

    [json.exception.parse_error.101] parse error at line 1, column 3: syntax error while parsing value - invalid number; expected '+', '-', or digit after exponent; last read: '1E'\n

    \\u was not be followed by four hex digits:

    [json.exception.parse_error.101] parse error at line 1, column 6: syntax error while parsing value - invalid string: '\\u' must be followed by 4 hex digits; last read: '\"\\u01\"'\n

    Invalid UTF-8 surrogate pair:

    [json.exception.parse_error.101] parse error at line 1, column 13: syntax error while parsing value - invalid string: surrogate U+DC00..U+DFFF must follow U+D800..U+DBFF; last read: '\"\\uD7FF\\uDC00'\"\n

    Invalid UTF-8 byte:

    [json.exception.parse_error.101] parse error at line 3, column 24: syntax error while parsing value - invalid string: ill-formed UTF-8 byte; last read: '\"vous \\352t'\n

    Tip

    • Make sure the input is correctly read. Try to write the input to standard output to check if, for instance, the input file was successfully opened.
    • Paste the input to a JSON validator like https://jsonlint.com or a tool like jq.
    "},{"location":"home/exceptions/#jsonexceptionparse_error102","title":"json.exception.parse_error.102","text":"

    JSON uses the \\uxxxx format to describe Unicode characters. Code points above 0xFFFF are split into two \\uxxxx entries (\"surrogate pairs\"). This error indicates that the surrogate pair is incomplete or contains an invalid code point.

    Example message

    parse error at 14: missing or wrong low surrogate\n

    Note

    This exception is not used any more. Instead json.exception.parse_error.101 with a more detailed description is used.

    "},{"location":"home/exceptions/#jsonexceptionparse_error103","title":"json.exception.parse_error.103","text":"

    Unicode supports code points up to 0x10FFFF. Code points above 0x10FFFF are invalid.

    Example message

    parse error: code points above 0x10FFFF are invalid\n

    Note

    This exception is not used any more. Instead json.exception.parse_error.101 with a more detailed description is used.

    "},{"location":"home/exceptions/#jsonexceptionparse_error104","title":"json.exception.parse_error.104","text":"

    RFC 6902 requires a JSON Patch document to be a JSON document that represents an array of objects.

    Example message

    [json.exception.parse_error.104] parse error: JSON patch must be an array of objects\n
    "},{"location":"home/exceptions/#jsonexceptionparse_error105","title":"json.exception.parse_error.105","text":"

    An operation of a JSON Patch document must contain exactly one \"op\" member, whose value indicates the operation to perform. Its value must be one of \"add\", \"remove\", \"replace\", \"move\", \"copy\", or \"test\"; other values are errors.

    Example message

    [json.exception.parse_error.105] parse error: operation 'add' must have member 'value'\n
    [json.exception.parse_error.105] parse error: operation 'copy' must have string member 'from'\n
    [json.exception.parse_error.105] parse error: operation value 'foo' is invalid\n

    "},{"location":"home/exceptions/#jsonexceptionparse_error106","title":"json.exception.parse_error.106","text":"

    An array index in a JSON Pointer (RFC 6901) may be 0 or any number without a leading 0.

    Example message

    [json.exception.parse_error.106] parse error: array index '01' must not begin with '0'\n
    "},{"location":"home/exceptions/#jsonexceptionparse_error107","title":"json.exception.parse_error.107","text":"

    A JSON Pointer must be a Unicode string containing a sequence of zero or more reference tokens, each prefixed by a / character.

    Example message

    [json.exception.parse_error.107] parse error at byte 1: JSON pointer must be empty or begin with '/' - was: 'foo'\n
    "},{"location":"home/exceptions/#jsonexceptionparse_error108","title":"json.exception.parse_error.108","text":"

    In a JSON Pointer, only ~0 and ~1 are valid escape sequences.

    Example message

    [json.exception.parse_error.108] parse error: escape character '~' must be followed with '0' or '1'\n
    "},{"location":"home/exceptions/#jsonexceptionparse_error109","title":"json.exception.parse_error.109","text":"

    A JSON Pointer array index must be a number. This exception is thrown for an array index that does not begin with a digit, except for - and the empty reference token, which throw out_of_range.404 where they cannot be resolved.

    Example messages

    [json.exception.parse_error.109] parse error: array index 'one' is not a number\n
    [json.exception.parse_error.109] parse error: array index '+1' is not a number\n

    Note

    Before version 3.13.0 unreleased, a one-character array index that is not a digit (e.g., x) threw out_of_range.404 instead.

    "},{"location":"home/exceptions/#jsonexceptionparse_error110","title":"json.exception.parse_error.110","text":"

    When parsing a binary format, the byte vector ends before the complete value has been read.

    Example messages

    [json.exception.parse_error.110] parse error at byte 5: syntax error while parsing CBOR string: unexpected end of input\n
    [json.exception.parse_error.110] parse error at byte 2: syntax error while parsing UBJSON value: expected end of input; last byte: 0x5A\n
    [json.exception.parse_error.110] parse error at byte 8: syntax error while parsing BSON number: unexpected end of input\n

    "},{"location":"home/exceptions/#jsonexceptionparse_error112","title":"json.exception.parse_error.112","text":"

    An unexpected byte was read in a binary format or length information is invalid (BSON).

    Example messages

    [json.exception.parse_error.112] parse error at byte 1: syntax error while parsing CBOR value: invalid byte: 0x1C\n
    [json.exception.parse_error.112] parse error at byte 1: syntax error while parsing MessagePack value: invalid byte: 0xC1\n
    [json.exception.parse_error.112] parse error at byte 4: syntax error while parsing BJData size: expected '#' after type information; last byte: 0x02\n
    [json.exception.parse_error.112] parse error at byte 4: syntax error while parsing UBJSON size: expected '#' after type information; last byte: 0x02\n
    [json.exception.parse_error.112] parse error at byte 10: syntax error while parsing BSON string: string length must be at least 1, is -2147483648\n
    [json.exception.parse_error.112] parse error at byte 15: syntax error while parsing BSON binary: byte array length cannot be negative, is -1\n
    [json.exception.parse_error.112] parse error at byte 5: syntax error while parsing BSON document: document size 6 does not match the number of bytes read (5)\n

    "},{"location":"home/exceptions/#jsonexceptionparse_error113","title":"json.exception.parse_error.113","text":"

    A string could not be read from a binary format: either a value that is not a string was read where one was required (for instance as a map key), the string's length specification is invalid, or the string's bytes are not valid UTF-8 and the error_handler parameter of the corresponding from_* function is set to strict. By default (error_handler_t::keep), the bytes of a string are not checked for valid UTF-8 on read; see the ill-formed UTF-8 notes on the individual binary format pages for how such a string is handled depending on error_handler.

    CBOR and MessagePack allow map keys of any type, but JSON object keys are always strings. Maps with keys of any other type (for instance integers or null) are therefore not supported; see the notes on CBOR and MessagePack.

    Example messages

    [json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR object key: only string keys are supported, but found an unsigned integer; last byte: 0x01\n
    [json.exception.parse_error.113] parse error at byte 2: syntax error while parsing MessagePack object key: only string keys are supported, but found nil; last byte: 0xC0\n
    [json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR string: expected length specification (0x60-0x7B) or indefinite string type (0x7F); last byte: 0x7C\n
    [json.exception.parse_error.113] parse error at byte 2: syntax error while parsing UBJSON char: byte after 'C' must be in range 0x00..0x7F; last byte: 0x82\n
    [json.exception.parse_error.113] parse error at byte 3: syntax error while parsing BJData string: string length must not be negative\n
    [json.exception.parse_error.113] parse error at byte 3: syntax error while parsing CBOR string: invalid string: ill-formed UTF-8 byte\n

    "},{"location":"home/exceptions/#jsonexceptionparse_error114","title":"json.exception.parse_error.114","text":"

    The parsing of the corresponding BSON record type is not implemented (yet).

    Example message

    [json.exception.parse_error.114] parse error at byte 5: Unsupported BSON record type 0xFF\n
    "},{"location":"home/exceptions/#jsonexceptionparse_error115","title":"json.exception.parse_error.115","text":"

    A UBJSON high-precision number could not be parsed.

    Example message

    [json.exception.parse_error.115] parse error at byte 5: syntax error while parsing UBJSON high-precision number: invalid number text: 1A\n
    "},{"location":"home/exceptions/#jsonexceptionparse_error116","title":"json.exception.parse_error.116","text":"

    basic_json_document::load() rejected an image: either the bytes are not one save() could have written (too short, an unknown magic number or format version, or sizes that do not fit the buffer), or they are, but fail the requested image_check.

    Example message

    [json.exception.parse_error.116] parse error: invalid json_document image: the check failed\n

    Note

    This exception was added in version 3.13.0 unreleased, together with images.

    "},{"location":"home/exceptions/#iterator-errors","title":"Iterator errors","text":"

    This exception is thrown if iterators passed to a library function do not match the expected semantics.

    Exceptions have ids 2xx.

    Example: catch an invalid_iterator exception

    The following code shows how an invalid_iterator exception can be caught.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    try\n    {\n        // calling iterator::key() on non-object iterator\n        json j = \"string\";\n        json::iterator it = j.begin();\n        auto k = it.key();\n    }\n    catch (const json::invalid_iterator& e)\n    {\n        // output exception information\n        std::cout << \"message: \" << e.what() << '\\n'\n                  << \"exception id: \" << e.id << std::endl;\n    }\n}\n

    Output:

    message: [json.exception.invalid_iterator.207] cannot use key() for non-object iterators\nexception id: 207\n
    "},{"location":"home/exceptions/#jsonexceptioninvalid_iterator201","title":"json.exception.invalid_iterator.201","text":"

    The iterators passed to constructor basic_json(InputIT first, InputIT last) are not compatible, meaning they do not belong to the same container. Therefore, the range (first, last) is invalid.

    Example message

    [json.exception.invalid_iterator.201] iterators are not compatible\n
    "},{"location":"home/exceptions/#jsonexceptioninvalid_iterator202","title":"json.exception.invalid_iterator.202","text":"

    In the erase or insert function, the passed iterator pos does not belong to the JSON value for which the function was called. It hence does not define a valid position for the deletion/insertion.

    Example messages

    [json.exception.invalid_iterator.202] iterator does not fit current value\n
    [json.exception.invalid_iterator.202] iterators first and last must point to objects\n

    "},{"location":"home/exceptions/#jsonexceptioninvalid_iterator203","title":"json.exception.invalid_iterator.203","text":"

    Either iterator passed to function erase(IteratorType first, IteratorType last) does not belong to the JSON value from which values shall be erased. It hence does not define a valid range to delete values from.

    Example message

    [json.exception.invalid_iterator.203] iterators do not fit current value\n
    "},{"location":"home/exceptions/#jsonexceptioninvalid_iterator204","title":"json.exception.invalid_iterator.204","text":"

    When an iterator range for a primitive type (number, boolean, or string) is passed to a constructor or an erase function, this range has to be exactly (begin(), end()), because this is the only way the single stored value is expressed. All other ranges are invalid.

    Example message

    [json.exception.invalid_iterator.204] iterators out of range\n
    "},{"location":"home/exceptions/#jsonexceptioninvalid_iterator205","title":"json.exception.invalid_iterator.205","text":"

    When an iterator for a primitive type (number, boolean, or string) is passed to an erase function, the iterator has to be the begin() iterator, because it is the only way to address the stored value. All other iterators are invalid.

    Example message

    [json.exception.invalid_iterator.205] iterator out of range\n
    "},{"location":"home/exceptions/#jsonexceptioninvalid_iterator206","title":"json.exception.invalid_iterator.206","text":"

    The iterators passed to constructor basic_json(InputIT first, InputIT last) belong to a JSON null value and hence to not define a valid range.

    Example message

    [json.exception.invalid_iterator.206] cannot construct with iterators from null\n
    "},{"location":"home/exceptions/#jsonexceptioninvalid_iterator207","title":"json.exception.invalid_iterator.207","text":"

    The key() member function can only be used on iterators belonging to a JSON object, because other types do not have a concept of a key.

    Example message

    [json.exception.invalid_iterator.207] cannot use key() for non-object iterators\n
    "},{"location":"home/exceptions/#jsonexceptioninvalid_iterator208","title":"json.exception.invalid_iterator.208","text":"

    The operator[] to specify a concrete offset cannot be used on iterators belonging to a JSON object, because JSON objects are unordered.

    Example message

    [json.exception.invalid_iterator.208] cannot use operator[] for object iterators\n
    "},{"location":"home/exceptions/#jsonexceptioninvalid_iterator209","title":"json.exception.invalid_iterator.209","text":"

    The offset operators (+, -, +=, -=) cannot be used on iterators belonging to a JSON object, because JSON objects are unordered.

    Example message

    [json.exception.invalid_iterator.209] cannot use offsets with object iterators\n
    "},{"location":"home/exceptions/#jsonexceptioninvalid_iterator210","title":"json.exception.invalid_iterator.210","text":"

    The iterator range passed to the insert function is not compatible, meaning they do not belong to the same container. Therefore, the range (first, last) is invalid.

    Example message

    [json.exception.invalid_iterator.210] iterators do not fit\n
    "},{"location":"home/exceptions/#jsonexceptioninvalid_iterator211","title":"json.exception.invalid_iterator.211","text":"

    The iterator range passed to the insert function must not be a subrange of the container to insert to.

    Example message

    [json.exception.invalid_iterator.211] passed iterators may not belong to container\n
    "},{"location":"home/exceptions/#jsonexceptioninvalid_iterator212","title":"json.exception.invalid_iterator.212","text":"

    When two iterators are compared, they must belong to the same container.

    Example message

    [json.exception.invalid_iterator.212] cannot compare iterators of different containers\n
    "},{"location":"home/exceptions/#jsonexceptioninvalid_iterator213","title":"json.exception.invalid_iterator.213","text":"

    The order of object iterators cannot be compared, because JSON objects are unordered.

    Example message

    [json.exception.invalid_iterator.213] cannot compare order of object iterators\n
    "},{"location":"home/exceptions/#jsonexceptioninvalid_iterator214","title":"json.exception.invalid_iterator.214","text":"

    Cannot retrieve value from iterator: The iterator either refers to a null value, or it refers to a primitive type (number, boolean, or string), but does not match the iterator returned by begin().

    Example message

    [json.exception.invalid_iterator.214] cannot get value\n
    "},{"location":"home/exceptions/#type-errors","title":"Type errors","text":"

    This exception is thrown in case of a type error; that is, a library function is executed on a JSON value whose type does not match the expected semantics.

    Exceptions have ids 3xx.

    Example: catch a type_error exception

    The following code shows how a type_error exception can be caught.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    try\n    {\n        // calling push_back() on a string value\n        json j = \"string\";\n        j.push_back(\"another string\");\n    }\n    catch (const json::type_error& e)\n    {\n        // output exception information\n        std::cout << \"message: \" << e.what() << '\\n'\n                  << \"exception id: \" << e.id << std::endl;\n    }\n}\n

    Output:

    message: [json.exception.type_error.308] cannot use push_back() with string\nexception id: 308\n
    "},{"location":"home/exceptions/#jsonexceptiontype_error301","title":"json.exception.type_error.301","text":"

    To create an object from an initializer list, the initializer list must consist only of a list of pairs whose first element is a string. When this constraint is violated, an array is created instead.

    Example message

    [json.exception.type_error.301] cannot create object from initializer list\n
    "},{"location":"home/exceptions/#jsonexceptiontype_error302","title":"json.exception.type_error.302","text":"

    During implicit or explicit value conversion, the JSON type must be compatible with the target type. For instance, a JSON string can only be converted into string types, but not into numbers or boolean types.

    Example messages

    [json.exception.type_error.302] type must be object, but is null\n
    [json.exception.type_error.302] type must be string, but is object\n

    This exception is also thrown with JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS if a key of a map with enum keys is not converted to a string, for instance, because the enum is stored as an integer.

    "},{"location":"home/exceptions/#jsonexceptiontype_error303","title":"json.exception.type_error.303","text":"

    To retrieve a reference to a value stored in a basic_json object with get_ref, the type of the reference must match the value type. For instance, for a JSON array, the ReferenceType must be array_t &.

    Example messages

    [json.exception.type_error.303] incompatible ReferenceType for get_ref, actual type is object\n
    [json.exception.type_error.303] incompatible ReferenceType for get_ref, actual type is number\"\n

    "},{"location":"home/exceptions/#jsonexceptiontype_error304","title":"json.exception.type_error.304","text":"

    The at() member functions can only be executed for certain JSON types.

    Example messages

    [json.exception.type_error.304] cannot use at() with string\n
    [json.exception.type_error.304] cannot use at() with number\n

    "},{"location":"home/exceptions/#jsonexceptiontype_error305","title":"json.exception.type_error.305","text":"

    The operator[] member functions can only be executed for certain JSON types.

    Example messages

    [json.exception.type_error.305] cannot use operator[] with a string argument with array\n
    [json.exception.type_error.305] cannot use operator[] with a numeric argument with object\n

    "},{"location":"home/exceptions/#jsonexceptiontype_error306","title":"json.exception.type_error.306","text":"

    The value() member functions can only be executed for certain JSON types.

    Example message

    [json.exception.type_error.306] cannot use value() with number\n
    "},{"location":"home/exceptions/#jsonexceptiontype_error307","title":"json.exception.type_error.307","text":"

    The erase() member functions can only be executed for certain JSON types.

    Example message

    [json.exception.type_error.307] cannot use erase() with string\n
    "},{"location":"home/exceptions/#jsonexceptiontype_error308","title":"json.exception.type_error.308","text":"

    The push_back() and operator+= member functions can only be executed for certain JSON types.

    Example message

    [json.exception.type_error.308] cannot use push_back() with string\n
    "},{"location":"home/exceptions/#jsonexceptiontype_error309","title":"json.exception.type_error.309","text":"

    The insert() member functions can only be executed for certain JSON types.

    Example messages

    [json.exception.type_error.309] cannot use insert() with array\n
    [json.exception.type_error.309] cannot use insert() with number\n

    "},{"location":"home/exceptions/#jsonexceptiontype_error310","title":"json.exception.type_error.310","text":"

    The swap() member functions can only be executed for certain JSON types.

    Example message

    [json.exception.type_error.310] cannot use swap() with number\n
    "},{"location":"home/exceptions/#jsonexceptiontype_error311","title":"json.exception.type_error.311","text":"

    The emplace() and emplace_back() member functions can only be executed for certain JSON types.

    Example messages

    [json.exception.type_error.311] cannot use emplace() with number\n
    [json.exception.type_error.311] cannot use emplace_back() with number\n

    "},{"location":"home/exceptions/#jsonexceptiontype_error312","title":"json.exception.type_error.312","text":"

    The update() member functions can only be executed for certain JSON types.

    Example message

    [json.exception.type_error.312] cannot use update() with array\n
    "},{"location":"home/exceptions/#jsonexceptiontype_error313","title":"json.exception.type_error.313","text":"

    The unflatten() function converts an object whose keys are JSON Pointers back into an arbitrary nested JSON value. The JSON Pointers must not overlap, because then the resulting value would not be well-defined.

    Example message

    [json.exception.type_error.313] invalid value to unflatten\n
    "},{"location":"home/exceptions/#jsonexceptiontype_error314","title":"json.exception.type_error.314","text":"

    The unflatten() function only works for an object whose keys are JSON Pointers.

    Example message

    Calling unflatten() on an array [1,2,3]:

    [json.exception.type_error.314] only objects can be unflattened\n
    "},{"location":"home/exceptions/#jsonexceptiontype_error315","title":"json.exception.type_error.315","text":"

    The unflatten() function only works for an object whose keys are JSON Pointers and whose values are primitive.

    Example message

    Calling unflatten() on an object {\"/1\", [1,2,3]}:

    [json.exception.type_error.315] values in object must be primitive\n
    "},{"location":"home/exceptions/#jsonexceptiontype_error316","title":"json.exception.type_error.316","text":"

    The dump() function only works with UTF-8 encoded strings; that is, if you assign a std::string to a JSON value, make sure it is UTF-8 encoded. See the FAQ entry on serializing untrusted or invalid UTF-8 for background and the recommended fix.

    The binary writers to_cbor(), to_ubjson(), to_bjdata(), and to_bson() throw this exception as well for a string value or object key that is not valid UTF-8 if their error_handler is strict (the default if JSON_STRICT_BINARY_UTF8 is enabled). So does to_msgpack() if error_handler_t::strict is passed.

    Example message

    Calling dump() on a JSON value containing an ISO 8859-1 encoded string: 

    [json.exception.type_error.316] invalid UTF-8 byte at index 15: 0x6F\n

    Tip

    • Store the source file with UTF-8 encoding.
    • Pass an error handler as last parameter to the dump() function to avoid this exception:
      • json::error_handler_t::replace will replace invalid bytes sequences with U+FFFD
      • json::error_handler_t::ignore will silently ignore invalid byte sequences
      • json::error_handler_t::keep will copy invalid byte sequences to the output unchanged
    "},{"location":"home/exceptions/#jsonexceptiontype_error317","title":"json.exception.type_error.317","text":"

    The dynamic type of the object cannot be represented in the requested serialization format (e.g., a raw true or null JSON object cannot be serialized to BSON)

    Example messages

    Serializing null to BSON: 

    [json.exception.type_error.317] to serialize to BSON, top-level type must be object, but is null\n
    Serializing [1,2,3] to BSON: 
    [json.exception.type_error.317] to serialize to BSON, top-level type must be object, but is array\n

    Tip

    Encapsulate the JSON value in an object. That is, instead of serializing true, serialize {\"value\": true}

    "},{"location":"home/exceptions/#jsonexceptiontype_error318","title":"json.exception.type_error.318","text":"

    With JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS, a map with enum keys is stored as an object. This exception is thrown if two of its keys are converted to the same string, so one of the entries would be lost. This happens, for instance, if NLOHMANN_JSON_SERIALIZE_ENUM does not list an enumerator and it is therefore converted like the first listed one.

    Example message

    [json.exception.type_error.318] duplicate object key 'red'\n
    "},{"location":"home/exceptions/#jsonexceptiontype_error319","title":"json.exception.type_error.319","text":"

    basic_json_document::set, basic_json_document::push_back, and basic_json_document::insert can store any basic_json value except a binary one: a json_document has no representation for binary values, which only ever arise from parsing a binary format or from an explicit json::binary value, not from JSON text.

    Example message

    [json.exception.type_error.319] cannot store a binary value in a json_document\n

    Note

    This exception was added in version 3.13.0 unreleased, together with editable json_documents.

    "},{"location":"home/exceptions/#jsonexceptiontype_error320","title":"json.exception.type_error.320","text":"

    basic_json_document::save() cannot write an image of a discarded document. save() and load() also throw this exception on a big-endian target, since the image format is little-endian only.

    Example messages

    [json.exception.type_error.320] cannot save a discarded json_document\n
    [json.exception.type_error.320] json_document images need a little-endian target\n

    Note

    This exception was added in version 3.13.0 unreleased, together with images.

    "},{"location":"home/exceptions/#jsonexceptiontype_error321","title":"json.exception.type_error.321","text":"

    A discarded value (one created by parse() with a callback that returns false for the value, or by default-constructing a basic_json with value_t::discarded) was passed to a binary serialization function, either directly or nested in an array or object. There is no way to represent a discarded value in CBOR, MessagePack, UBJSON, BJData, BSON, or BON8.

    Example message

    Serializing [1, 2] to CBOR, where the second element was discarded by a parser callback: 

    [json.exception.type_error.321] cannot serialize discarded value to CBOR\n

    "},{"location":"home/exceptions/#out-of-range","title":"Out of range","text":"

    This exception is thrown in case a library function is called on an input parameter that exceeds the expected range, for instance, in the case of array indices or nonexisting object keys.

    Exceptions have ids 4xx.

    Example: catch an out_of_range exception

    The following code shows how an out_of_range exception can be caught.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    try\n    {\n        // calling at() for an invalid index\n        json j = {1, 2, 3, 4};\n        j.at(4) = 10;\n    }\n    catch (const json::out_of_range& e)\n    {\n        // output exception information\n        std::cout << \"message: \" << e.what() << '\\n'\n                  << \"exception id: \" << e.id << std::endl;\n    }\n}\n

    Output:

    message: [json.exception.out_of_range.401] array index 4 is out of range\nexception id: 401\n
    "},{"location":"home/exceptions/#jsonexceptionout_of_range401","title":"json.exception.out_of_range.401","text":"

    The provided array index i is larger than size-1.

    Example message

    array index 3 is out of range\n
    "},{"location":"home/exceptions/#jsonexceptionout_of_range402","title":"json.exception.out_of_range.402","text":"

    The special array index - in a JSON Pointer never describes a valid element of the array, but the index past the end. That is, it can only be used to add elements at this position, but not to read it.

    Example message

    array index '-' (3) is out of range\n
    "},{"location":"home/exceptions/#jsonexceptionout_of_range403","title":"json.exception.out_of_range.403","text":"

    The provided key was not found in the JSON object.

    Example message

    key 'foo' not found\n
    "},{"location":"home/exceptions/#jsonexceptionout_of_range404","title":"json.exception.out_of_range.404","text":"

    A reference token in a JSON Pointer could not be resolved, for instance an array index that begins with a digit but contains other characters (e.g., 1a), or - where it cannot be used.

    Example message

    unresolved reference token 'foo'\n
    "},{"location":"home/exceptions/#jsonexceptionout_of_range405","title":"json.exception.out_of_range.405","text":"

    The JSON Patch operations 'remove' and 'move' cannot be applied to the root element of the JSON value. The exception is also thrown when the parent or a reference token of an empty JSON Pointer is requested, for instance by back or pop_back.

    Example message

    JSON pointer has no parent\n
    "},{"location":"home/exceptions/#jsonexceptionout_of_range406","title":"json.exception.out_of_range.406","text":"

    A parsed number could not be stored without changing it to NaN or INF. For the binary formats, this happens when a finite floating-point number does not fit into number_float_t, for example a double-precision number when number_float_t is float.

    Example messages

    number overflow parsing '10E1000'\n
    [json.exception.out_of_range.406] syntax error while parsing CBOR value: number overflow\n

    "},{"location":"home/exceptions/#jsonexceptionout_of_range407","title":"json.exception.out_of_range.407","text":"

    An unsigned integer number greater than 9223372036854775807 cannot be represented in the binary format. This exception is only thrown by to_bon8, as BON8 has no unsigned 64-bit integer type.

    Former occurrences

    The UBJSON and BSON writers used to throw this exception as well:

    • Since version 3.9.0, integer numbers beyond int64 are serialized as high-precision UBJSON numbers.
    • Since version 3.12.0, integer numbers beyond int64 are serialized as uint64 BSON numbers.

    Example message

    integer number 18446744073709551615 cannot be represented by BON8 as it does not fit int64\n
    "},{"location":"home/exceptions/#jsonexceptionout_of_range408","title":"json.exception.out_of_range.408","text":"

    The size of an array or object in a binary format exceeds the maximal capacity: the size following # for UBJSON/BJData, or the encoded length for CBOR.

    The exception is also thrown for a UBJSON array of a type that is encoded by its marker alone (Z, T or F) whose declared count exceeds 1,048,576 (1 << 20). Such an array has no payload, so its count alone decides how much memory is allocated, and a handful of bytes would otherwise describe billions of values. to_ubjson writes longer arrays of these types without the size and type annotation, so any value it produces can still be read back.

    Example messages

    excessive array size: 8658170730974374167\n
    [json.exception.out_of_range.408] syntax error while parsing CBOR size: excessive array size\n
    [json.exception.out_of_range.408] syntax error while parsing CBOR size: excessive map size\n
    [json.exception.out_of_range.408] syntax error while parsing UBJSON size: excessive array size\n

    "},{"location":"home/exceptions/#jsonexceptionout_of_range409","title":"json.exception.out_of_range.409","text":"

    Key identifiers to be serialized to BSON cannot contain code point U+0000, since the key is stored as zero-terminated c-string.

    Example message

    BSON key cannot contain code point U+0000 (at byte 2)\n
    "},{"location":"home/exceptions/#jsonexceptionout_of_range410","title":"json.exception.out_of_range.410","text":"

    This exception is thrown when an undefined value is used with NLOHMANN_JSON_SERIALIZE_ENUM_STRICT, or when an array index in a JSON pointer exceeds the range of size_type (e.g., on 32-bit platforms).

    Example message

    enum value out of range\narray index 18446744073709551616 exceeds size_type\n
    "},{"location":"home/exceptions/#jsonexceptionout_of_range411","title":"json.exception.out_of_range.411","text":"

    A JSON Patch add operation cannot be applied because the target location's parent is neither an object nor an array. Per RFC 6902, an add target must reference a member of an existing object or an element of an existing array; a primitive value (string, number, boolean, etc.) cannot receive a new member or element.

    Example message

    cannot add value: the JSON Patch 'add' target's parent is of type string, but must be an object or array\n

    Note

    This exception was added in version 3.13.0 unreleased. Before that, this situation hit an internal assertion (aborting the program in debug builds) or was silently ignored when assertions were disabled.

    "},{"location":"home/exceptions/#jsonexceptionout_of_range412","title":"json.exception.out_of_range.412","text":"

    BSON stores the length of documents, arrays, strings, and binary values in a signed 32-bit integer, and MessagePack stores the length of strings, binary values, arrays, and objects in at most an unsigned 32-bit integer. This exception is thrown when a value is too large to be described by such a length field.

    Example messages

    BSON length 2147483661 exceeds maximum of 2147483647\n
    MessagePack length 4294967296 exceeds maximum of 4294967295\n

    Note

    This exception was added in version 3.13.0 unreleased. Before that, the BSON length was silently truncated, and to_bson produced documents with negative length prefixes that from_bson rejected; to_msgpack wrote such a value without any length, producing output that could not be read back.

    "},{"location":"home/exceptions/#jsonexceptionout_of_range413","title":"json.exception.out_of_range.413","text":"

    A JSON Patch remove operation cannot be applied because the target location's parent is neither an object nor an array. Per RFC 6902, a remove target must reference a member of an existing object or an element of an existing array; a primitive value (string, number, boolean, etc.) or null has no members or elements to remove.

    Example message

    cannot remove value: the JSON Patch 'remove' target's parent is of type number, but must be an object or array\n

    Note

    This exception was added in version 3.13.0 unreleased. Before that, this situation was silently ignored (the remove operation had no effect).

    "},{"location":"home/exceptions/#jsonexceptionout_of_range414","title":"json.exception.out_of_range.414","text":"

    A JSON Patch move operation's \"from\" location is a proper prefix of its \"path\" location. Per RFC 6902 (section 4.4), a location cannot be moved into one of its own children.

    Example message

    cannot move value: 'from' path '/0' is a proper prefix of 'path' '/0/0'\n

    Note

    This exception was added in version 3.13.0 unreleased. Before that, this situation could succeed with a corrupted result: for an array target, removing the \"from\" element before the \"add\" step shifted subsequent indices, so \"path\" silently re-resolved to a different element than intended.

    "},{"location":"home/exceptions/#jsonexceptionout_of_range415","title":"json.exception.out_of_range.415","text":"

    MessagePack's ext type and BSON's binary subtype are each stored in a single byte. This exception is thrown when serializing a byte_container_with_subtype whose subtype exceeds 255.

    Example message

    [json.exception.out_of_range.415] subtype 70000 is too large for the MessagePack ext type (max 255)\n

    Note

    This exception was added in version 3.13.0 unreleased. Before that, subtypes above 255 were silently truncated modulo 256 instead of raising an error.

    "},{"location":"home/exceptions/#jsonexceptionout_of_range416","title":"json.exception.out_of_range.416","text":"

    basic_json_document::parse() and the other parsing functions of basic_json_document index a value's position in the source text in 32 bits, so they do not support an input of 4294967280 bytes (4 GiB minus 16 bytes) or more. The same 32-bit limit applies to an editable document's own storage: set and push_back throw this exception once the strings and number tokens written by edits reach 4 GiB in total, or once more than 4294967295 arrays/objects have had an element set or appended to them. The same limit applies to an image: save() throws it if the node count, the text, or the decoded strings it would write would individually reach 4 GiB.

    Example messages

    [json.exception.out_of_range.416] input of 4294967280 bytes or more is not supported by json_document\n
    [json.exception.out_of_range.416] edits of 4 GiB or more are not supported by json_document\n
    [json.exception.out_of_range.416] images of 4 GiB or more are not supported by json_document\n

    Note

    This exception was added in version 3.13.0 unreleased, together with <nlohmann/json_view.hpp>. basic_json::parse() has no such limit.

    "},{"location":"home/exceptions/#further-exceptions","title":"Further exceptions","text":"

    This exception is thrown in case of errors that cannot be classified with the other exception types.

    Exceptions have ids 5xx.

    Example: catch an other_error exception

    The following code shows how an other_error exception can be caught.

    #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    try\n    {\n        // executing a failing JSON Patch operation\n        json value = R\"({\n            \"best_biscuit\": {\n                \"name\": \"Oreo\"\n            }\n        })\"_json;\n        json patch = R\"([{\n            \"op\": \"test\",\n            \"path\": \"/best_biscuit/name\",\n            \"value\": \"Choco Leibniz\"\n        }])\"_json;\n        value.patch(patch);\n    }\n    catch (const json::other_error& e)\n    {\n        // output exception information\n        std::cout << \"message: \" << e.what() << '\\n'\n                  << \"exception id: \" << e.id << std::endl;\n    }\n}\n

    Output:

    message: [json.exception.other_error.501] unsuccessful: {\"op\":\"test\",\"path\":\"/best_biscuit/name\",\"value\":\"Choco Leibniz\"}\nexception id: 501\n
    "},{"location":"home/exceptions/#jsonexceptionother_error501","title":"json.exception.other_error.501","text":"

    A JSON Patch operation 'test' failed. The unsuccessful operation is also printed.

    Example message

    Executing {\"op\":\"test\", \"path\":\"/baz\", \"value\":\"bar\"} on {\"baz\": \"qux\"}:

    [json.exception.other_error.501] unsuccessful: {\"op\":\"test\",\"path\":\"/baz\",\"value\":\"bar\"}\n
    "},{"location":"home/exceptions/#jsonexceptionother_error502","title":"json.exception.other_error.502","text":"

    to_ubjson and to_bjdata were called with use_type = true but use_size = false. UBJSON requires a size marker (#) after a type marker ($).

    Example message

    [json.exception.other_error.502] use_type requires use_size = true\n

    Note

    This exception was added in version 3.13.0 unreleased. Before that, debug builds aborted on an assertion and release builds wrote a $ marker without #, which from_ubjson then rejected.

    "},{"location":"home/faq/","title":"Frequently Asked Questions (FAQ)","text":""},{"location":"home/faq/#known-bugs","title":"Known bugs","text":""},{"location":"home/faq/#brace-initialization-yields-arrays","title":"Brace initialization yields arrays","text":"

    Question

    Why does

    json j{true};\n

    and

    json j(true);\n

    yield different results ([true] vs. true)?

    This is a known issue, and -- even worse -- the behavior differs between GCC and Clang. The \"culprit\" for this is the library's constructor overloads for initializer lists to allow syntax like

    json array = {1, 2, 3, 4};\n

    for arrays and

    json object = {{\"one\", 1}, {\"two\", 2}}; \n

    for objects.

    Tip

    To avoid any confusion and ensure portable code, do not use brace initialization with the types basic_json, json, or ordered_json unless you want to create an object or array as shown in the examples above.

    To explicitly create a single-element array, use json::array({value}):

    json j = json::array({true});  // [true]\n

    Opt-in copy semantics (since version 3.13.0 unreleased)

    If you define JSON_BRACE_INIT_COPY_SEMANTICS to 1 before including the library, single-element brace initialization is treated as copy/move instead of creating a single-element array:

    #define JSON_BRACE_INIT_COPY_SEMANTICS 1\n#include <nlohmann/json.hpp>\n\njson obj = {{\"key\", \"value\"}};\njson j{obj};   // -> {\"key\":\"value\"}  (copy, not array)\n

    Without the macro (default behavior), json j{obj} creates [{\"key\":\"value\"}]. This opt-in macro fixes issue #5074 while preserving backwards compatibility for existing code.

    "},{"location":"home/faq/#limitations","title":"Limitations","text":""},{"location":"home/faq/#relaxed-parsing","title":"Relaxed parsing","text":"

    Question

    Can you add an option to ignore trailing commas?

    Yes, trailing commas in arrays and objects can be allowed by setting parameter ignore_trailing_commas of parse to true; see trailing commas. They are rejected by default, and the library never adds them when serializing.

    "},{"location":"home/faq/#parse-errors-reading-non-ascii-characters","title":"Parse errors reading non-ASCII characters","text":"

    Questions

    • Why is the parser complaining about a Chinese character?
    • Does the library support Unicode?
    • I get an exception [json.exception.parse_error.101] parse error at line 1, column 53: syntax error while parsing value - invalid string: ill-formed UTF-8 byte; last read: '\"Test\u00e9$')\"

    The library supports Unicode input as follows:

    • Only UTF-8 encoded input is supported, which is the default encoding for JSON, according to RFC 8259.
    • std::u16string and std::u32string can be parsed, assuming UTF-16 and UTF-32 encoding, respectively. These encodings are not supported when reading from files or other input containers.
    • Other encodings such as Latin-1 or ISO 8859-1 are not supported and will yield parse or serialization errors.
    • The library will not replace Unicode noncharacters.
    • Invalid surrogates (e.g., incomplete pairs such as \\uDEAD) will yield parse errors.
    • The strings stored in the library are UTF-8 encoded. When using the default string type (std::string), note that its length/size functions return the number of stored bytes rather than the number of characters or glyphs.
    • When you store strings with different encodings in the library, calling dump() may throw an exception unless json::error_handler_t::replace, json::error_handler_t::ignore, or json::error_handler_t::keep are used as error handlers.

    In most cases, the parser is right to complain, because the input is not UTF-8 encoded. This is especially true for Microsoft Windows, where Latin-1 or ISO 8859-1 is often the standard encoding.

    "},{"location":"home/faq/#nul-bytes-in-the-input","title":"NUL bytes in the input","text":"

    Questions

    • Why does json::parse() silently ignore part of my input?
    • Why does a std::string/buffer with extra data after the JSON text parse without error, while a similar-looking string with extra text does not?

    A '\\0' (NUL) byte anywhere in the input is treated the same as the real end of the input, rather than as an ordinary (and, outside of a string, invalid) byte. Everything from that byte onward is silently ignored, without a parse error \u2014 including further, otherwise well-formed JSON:

    json::parse(std::string(\"123\") + '\\0');          // == 123, no error\njson::parse(std::string(\"123\") + '\\0' + \"true\"); // == 123, the \"true\" is silently ignored too\n

    This is different from any other unexpected trailing byte, which does raise parse_error.101:

    json::parse(\"123x\"); // throws parse_error.101: unexpected additional data\n

    This falls out of the same convention used when no explicit input length is given at all: json::parse(const char*) already stops at the first NUL byte via strlen(), since a bare pointer has no length of its own. The library applies that same NUL-terminated-C-string convention uniformly, rather than only when a length is genuinely unavailable \u2014 so a std::string, iterator range, or container whose content happens to include a NUL byte is affected the same way a raw const char* would be.

    If your input may contain a trailing or embedded NUL that is not meant to signal the end of the JSON text \u2014 for instance, a fixed-size, zero-padded buffer \u2014 trim it yourself before calling parse(), since the library will otherwise silently stop there instead of raising an error:

    s.resize(s.find('\\0')); // drop everything from the first NUL onward, if any\njson::parse(s);\n

    Opt-in strict handling (since version 3.13.0 unreleased)

    Manually trimming every input is easy to forget. If you define JSON_STRICT_NUL_HANDLING to 1 before including the library, a '\\0' byte is instead rejected like any other unexpected byte and raises parse_error.101, instead of being treated as end of input:

    #define JSON_STRICT_NUL_HANDLING 1\n#include <nlohmann/json.hpp>\n\njson::parse(std::string(\"123\") + '\\0'); // throws parse_error.101 instead of silently returning 123\n

    This macro defaults to 0 (disabled, preserving the behavior described above) to avoid breaking existing code that may depend on it, even unknowingly; it is planned to become the default in version 4.0.0. See its documentation for details, including how it also affects char arrays such as string literals.

    Note that this is unrelated to an unescaped NUL byte occurring inside a quoted JSON string, which is a different, already-invalid case and is correctly rejected either way:

    json::parse(std::string(\"\\\"\") + '\\0' + \"\\\"\"); // throws parse_error.101: control character U+0000 (NUL) must be escaped to \\u0000\n
    "},{"location":"home/faq/#wide-string-handling","title":"Wide string handling","text":"

    Question

    Why are wide strings (e.g., std::wstring) dumped as arrays of numbers?

    As described above, the library assumes UTF-8 as encoding. To store a wide string, you need to change the encoding.

    Example

    #include <codecvt> // codecvt_utf8\n#include <locale>  // wstring_convert\n\n// encoding function\nstd::string to_utf8(std::wstring& wide_string)\n{\n    static std::wstring_convert<std::codecvt_utf8<wchar_t>> utf8_conv;\n    return utf8_conv.to_bytes(wide_string);\n}\n\njson j;\nstd::wstring ws = L\"\u8ecaB1234 \u3053\u3093\u306b\u3061\u306f\";\n\nj[\"original\"] = ws;\nj[\"encoded\"] = to_utf8(ws);\n\nstd::cout << j << std::endl;\n

    The result is:

    {\n  \"encoded\": \"\u8ecaB1234 \u3053\u3093\u306b\u3061\u306f\",\n  \"original\": [36554, 66, 49, 50, 51, 52, 32, 12371, 12435, 12395, 12385, 12399]\n}\n
    "},{"location":"home/faq/#usage","title":"Usage","text":""},{"location":"home/faq/#thread-safety","title":"Thread safety","text":"

    Question

    Is basic_json thread-safe?

    No. basic_json provides no built-in synchronization, the same as std::map or std::vector. Concurrent reads of the same value from multiple threads are safe, as are concurrent (non-overlapping) accesses to independent json objects. However, any concurrent write to a json object -- or a concurrent read while another thread writes to the same object -- is a data race and requires external synchronization (e.g., a std::mutex) by the caller.

    "},{"location":"home/faq/#schema-validation","title":"Schema validation","text":"

    Question

    Does this library support JSON Schema validation?

    Not directly, but the companion project json-schema-validator builds JSON Schema (draft 7; draft 4 in its older, now-superseded 1.x releases) validation on top of this library and is a common recommendation for this use case.

    "},{"location":"home/faq/#exceptions","title":"Exceptions","text":""},{"location":"home/faq/#parsing-without-exceptions","title":"Parsing without exceptions","text":"

    Question

    Is it possible to indicate a parse error without throwing an exception?

    Yes, see Parsing and exceptions.

    "},{"location":"home/faq/#key-name-in-exceptions","title":"Key name in exceptions","text":"

    Question

    Can I get the key of the object item that caused an exception?

    Yes, you can. Please define the symbol JSON_DIAGNOSTICS to get extended diagnostics messages.

    "},{"location":"home/faq/#serialization-issues","title":"Serialization issues","text":""},{"location":"home/faq/#number-precision","title":"Number precision","text":"

    Question

    • It seems that precision is lost when serializing a double.
    • Can I change the precision for floating-point serialization?

    The library serializes floating-point numbers with the shortest representation that parses back to the same value, using at most std::numeric_limits<number_float_t>::max_digits10 (17 for IEEE doubles) significant digits. This guarantees that value -> string -> value round-trips. The reverse direction, string -> value -> string, is only guaranteed for numbers with at most std::numeric_limits<number_float_t>::digits10 (15 for IEEE doubles) significant digits; a number with more digits may not be representable exactly and is then serialized as the closest double.

    cppreference.com

    The value of std::numeric_limits<T>::max_digits10 is the number of base-10 digits that are necessary to uniquely represent all distinct values of the type T, such as necessary for serialization/deserialization to text.

    Tip

    The website https://float.exposed gives a good insight into the internal storage of floating-point numbers.

    See this section on the library's number handling for more information.

    "},{"location":"home/faq/#serializing-untrusted-or-invalid-utf-8","title":"Serializing untrusted or invalid UTF-8","text":"

    Questions

    • Why does dump() throw when I serialize data that came from the network?
    • Is CVE-2024-34363 a vulnerability in this library?

    Crashes reported against this library that stem from an uncaught type_error.316 while serializing unvalidated input (e.g., CVE-2024-34363) are a usage issue, not a library vulnerability: dump() throws in its default strict mode because RFC 8259 requires JSON text to be valid UTF-8.

    The recommended pattern is to pass a non-strict error_handler or to handle the exception:

    // replace invalid sequences with U+FFFD instead of throwing\nconst auto s = j.dump(-1, ' ', false, json::error_handler_t::replace);\n
    "},{"location":"home/faq/#using-json-values-with-stdformat-or-fmt","title":"Using JSON values with std::format or fmt","text":"

    Question

    • Can I use std::format(\"{}\", j) on a JSON value?
    • Can I use fmt::format(\"{}\", j) or fmt::print(\"{}\", j) (the {fmt} library) on a JSON value?

    std::format works out of the box since version 3.13.0 unreleased, as long as the standard library provides <format> (see JSON_HAS_STD_FORMAT); see std::formatter<basic_json> for details, including the \"{:#}\" pretty-print spec, indent widths (\"{:2}\"), and custom indent characters (\"{:.>#}\").

    For fmt, the library ships format_as, a small customization point fmt looks for via argument-dependent lookup. It only has an effect on fmt 10.0.0 through 11.0.2 \u2014 from fmt 11.1.0 onwards, fmt no longer picks up a format_as overload that returns a std::string. On such versions (or any version, if you also want the same \"{:#}\"/width/fill-and-align spec support that std::formatter<basic_json> has), define your own fmt::formatter specialization; see format_as for a recipe that mirrors it.

    If you get ambiguous-overload errors when passing a JSON value to fmt::format/fmt::print without any fmt::formatter<json> specialization in scope, that's fmt picking up basic_json's implicit operator ValueType() conversion operator (see #964 and #958); disabling it via JSON_USE_IMPLICIT_CONVERSIONS 0 avoids the ambiguity.

    "},{"location":"home/faq/#compilation-issues","title":"Compilation issues","text":""},{"location":"home/faq/#android-sdk","title":"Android SDK","text":"

    Question

    Why does the code not compile with Android SDK?

    Since NDK r18 (2018), GCC and the gnustl/stlport C++ libraries have been removed from the Android NDK; Clang and libc++ are now the only compiler and C++ library, and they support C++11 and later out of the box. With a current NDK, no special configuration is needed to use this library.

    Only very old NDKs (before r18), which defaulted to GCC and gnustl, lacked C++11 library features such as std::to_string. If you run into this, update to a current NDK.

    "},{"location":"home/faq/#incomplete-detector-type-with-gcc-11","title":"Incomplete detector type with GCC < 11","text":"

    Question

    Why does GCC 10 or older fail with invalid use of incomplete type 'struct nlohmann::detail::detector<..., to_json_function, ...>' for a type that holds an optional member?

    This happens with GCC 10 and older in C++11/C++14 mode when all of these hold:

    • a class Holder has an optional<Dummy> member (e.g., boost::optional),
    • Dummy has a constructor taking a json value, and
    • to_json for Holder is a free function in the namespace of Dummy.
    class Dummy {\n  public:\n    explicit Dummy(const nlohmann::json& j);\n};\n\nclass Holder {\n    boost::optional<Dummy> d;\n};\n\nvoid to_json(nlohmann::json& j, const Holder& h);  // triggers the error\n

    To decide whether Dummy is copyable, the compiler checks whether a Dummy can be converted to json. That check looks up to_json via argument-dependent lookup, finds the unrelated to_json for Holder, and eventually asks again whether Dummy is copyable. GCC before version 11 turns this cycle into a hard error; GCC 11 and later, Clang, and C++17 mode compile the code. The same error shows up without this library whenever a constrained converting constructor is involved, so the library can't avoid it.

    To work around this, define to_json (and from_json) as a hidden friend inside the class. That way, argument-dependent lookup only finds it for Holder:

    class Holder {\n    boost::optional<Dummy> d;\n\n    friend void to_json(nlohmann::json& j, const Holder& h) { /* ... */ }\n};\n

    The NLOHMANN_DEFINE_TYPE_INTRUSIVE macros define hidden friends as well. See #3669 for details.

    "},{"location":"home/faq/#missing-stl-function","title":"Missing STL function","text":"

    Questions

    • Why do I get a compilation error 'to_string' is not a member of 'std' (or similarly, for strtod or strtof)?
    • Why does the code not compile with MinGW or Android SDK?

    This is not an issue with the code, but rather with the compiler itself. On Android, use a current NDK (see above). For MinGW, please refer to this site and this discussion for information on how to fix this bug.

    "},{"location":"home/license/","title":"License","text":"

    The class is licensed under the MIT License:

    Copyright \u00a9 2013-2026 Niels Lohmann

    Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the \u201cSoftware\u201d), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

    The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

    THE SOFTWARE IS PROVIDED \u201cAS IS\u201d, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

    The class contains the UTF-8 Decoder from Bjoern Hoehrmann which is licensed under the MIT License (see above). Copyright \u00a9 2008-2009 Bj\u00f6rn Hoehrmann bjoern@hoehrmann.de

    The class contains a slightly modified version of the Grisu2 algorithm from Florian Loitsch which is licensed under the MIT License (see above). Copyright \u00a9 2009 Florian Loitsch

    The class contains a port of the shortest double-to-decimal conversion of \u017bmij by Victor Zverovich, including the conversion of the digits to text by Xiang JunBo and the SIMD instruction sequence of Dougall Johnson, which is licensed under the MIT License (see above). Copyright \u00a9 2025 Victor Zverovich

    The class contains a copy of Hedley from Evan Nemerson which is licensed as CC0-1.0.

    The class contains an adapted version of the Eisel-Lemire algorithm, its table of powers of five, and its digit comparison for long numbers from fast_float by Daniel Lemire and contributors, which is available under the MIT License (used here), the Apache 2.0 License, and the Boost Software License. Copyright \u00a9 2021 The fast_float authors

    The view's parser (<nlohmann/json_view.hpp>) contains techniques and code adapted from yyjson by YaoYuan, which is licensed under the MIT License (see above): table-driven decoding of \\u escapes and fixed-offset unrolled checks.

    The view's parser (<nlohmann/json_view.hpp>) validates non-ASCII strings with the vector UTF-8 check of simdjson by Daniel Lemire, Geoff Langdale, John Keiser, and contributors (its \"lookup4\" algorithm and tables, after J. Keiser and D. Lemire, \"Validating UTF-8 In Less Than One Instruction Per Byte\", 2021), which is available under the MIT License (used here) and the Apache 2.0 License. Copyright \u00a9 2018-2025 The simdjson authors

    "},{"location":"home/releases/","title":"Releases","text":"

    This page summarizes the notable changes of every release and links to the relevant documentation. The complete release notes \u2014 including all changes, the download files, and their checksums \u2014 are published on the GitHub releases page.

    Unreleased changes

    This documentation is built from the develop branch and may describe changes that are not part of a release yet. Their version numbers are followed by an unreleased badge.

    "},{"location":"home/releases/#unreleased","title":"Unreleased","text":"
    • Converting an rvalue container to JSON (json j = std::move(vec);, json j = std::map<std::string, std::string>{...};) now moves the elements instead of copying them (e.g., std::vector<std::string> with 100,000 strings: 4.7 ms to 2.3 ms). Two consequences: the source container is left in a valid but unspecified (moved-from) state, and if an exception is thrown during the conversion, the source may be partially moved-from, so the strong exception guarantee no longer holds for the source of an rvalue conversion (the JSON value itself is unaffected). Lvalues and const objects are still copied. See basic_json.
    "},{"location":"home/releases/#v3120-2025-04-11","title":"v3.12.0 (2025-04-11)","text":"

    Fixes bugs found in 3.11.3 and adds several features. All changes are backward-compatible.

    • Adds diagnostic byte positions via JSON_DIAGNOSTIC_POSITIONS, exposed through the new start_pos and end_pos member functions.
    • Makes the conversion macros templated (so they also work with ordered_json) and adds NLOHMANN_DEFINE_DERIVED_TYPE for derived classes.
    • Adds std::optional support (C++17) and lets patch, diff, and flatten work with arbitrary string types.
    • Extends the binary formats: BJData draft 3 and unsigned 64-bit integers for BSON.
    • Adds multidimensional C-array conversion and UTF-8 encoded std::filesystem::path conversions, and raises the minimum CMake version to 3.5 and supports CMake 4.0.

    Full release notes.

    "},{"location":"home/releases/#v3113-2023-11-28","title":"v3.11.3 (2023-11-28)","text":"

    Adds features and fixes bugs found in 3.11.2. All changes are backward-compatible.

    • Adds a custom base class as a node customization point.
    • Adds serialization-only conversion macros (NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE and NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE) and a clearer parse error for empty input.
    • Adds Bazel and Swift Package Manager build support.
    • Fixes custom allocators, a memory leak in adl_serializer's to_json, initializer-list construction when size_type is not int, and many compiler warnings.

    Full release notes.

    "},{"location":"home/releases/#v3112-2022-08-12","title":"v3.11.2 (2022-08-12)","text":"

    Fixes bugs found in 3.11.1 and restructures the namespace. All changes are backward-compatible.

    • Fixes the value function (broken for strings, size types, and nullptr in 3.11.0) and makes json_fwd.hpp self-contained.
    • Restores using json_pointer as a key in associative containers and comparing it with strings.
    • Restructures the inline namespace and allows disabling the version component, and avoids heap allocations in the BJData parser.

    Full release notes.

    "},{"location":"home/releases/#v3111-2022-08-01","title":"v3.11.1 (2022-08-01)","text":"

    Fixes a regression from 3.11.0. All changes are backward-compatible.

    • Restores the global user-defined string literals operator\"\"_json and operator\"\"_json_pointer, which 3.11.0 had moved into a namespace by default.

    Full release notes.

    "},{"location":"home/releases/#v3110-2022-08-01","title":"v3.11.0 (2022-08-01)","text":"

    One of the largest releases ever. All changes are backward-compatible.

    • Allows std::string_view as object keys in at, operator[], value, erase, find, contains, and count.
    • Adds the BJData binary format (the fifth supported format).
    • Improves C++20 support, including operator<=> and <ranges>-compatible iterators.
    • Adds a versioned, ABI-tagged inline namespace (NLOHMANN_JSON_NAMESPACE) and the option to move the UDLs out of the global namespace (JSON_USE_GLOBAL_UDLS).
    • Adds patch_inplace, default values for the conversion macros, and an option to disable enum serialization (JSON_DISABLE_ENUM_SERIALIZATION).

    This release introduced a UDL regression that was fixed in 3.11.1. Full release notes.

    "},{"location":"home/releases/#v3105-2022-01-03","title":"v3.10.5 (2022-01-03)","text":"

    Bug-fix release. All changes are backward-compatible.

    • Guards the std::filesystem conversions behind compiler-support checks (JSON_HAS_FILESYSTEM), which can be set to 0 to disable them altogether.

    Full release notes.

    "},{"location":"home/releases/#v3104-2021-10-16","title":"v3.10.4 (2021-10-16)","text":"

    Fixes regressions introduced in 3.10.0. All changes are backward-compatible.

    • Fixes the std::filesystem::path conversion (which could trigger a stack overflow and broke compilation on Windows).
    • Fixes compilation for types with an explicit defaulted constructor and for code relying on the return values of std::find and std::remove.

    Full release notes.

    "},{"location":"home/releases/#v3103-2021-10-08","title":"v3.10.3 (2021-10-08)","text":"

    Fixes more regressions from 3.10.0. All changes are backward-compatible.

    • Fixes extended-diagnostics assertions triggered by update and by inserting into arrays.
    • Supports custom allocators when writing binary formats into a std::vector, and allows conversion from types that only provide begin()/end().

    Full release notes.

    "},{"location":"home/releases/#v3102-2021-08-26","title":"v3.10.2 (2021-08-26)","text":"

    Re-release of 3.10.1, whose Git tag pointed at the wrong commit due to a bug in the release script. All changes are backward-compatible. Full release notes.

    "},{"location":"home/releases/#v3101-2021-08-24","title":"v3.10.1 (2021-08-24)","text":"

    Fixes a regression from 3.10.0. All changes are backward-compatible.

    • Fixes an extended-diagnostics assertion triggered when used with ordered_json, and hardens the GDB pretty-printer.

    Full release notes.

    "},{"location":"home/releases/#v3100-2021-08-17","title":"v3.10.0 (2021-08-17)","text":"

    Feature release. All changes are backward-compatible.

    • Adds extended diagnostic messages (JSON_DIAGNOSTICS) that prepend a JSON pointer to exception messages to pinpoint the offending value.
    • Adds a GDB pretty-printer and a cbor_tag_handler_t store option to keep CBOR tags as binary subtypes.
    • Supports containers with non-default-constructible types and parsing from std::byte.
    • Adds JSON_NO_IO to exclude the I/O headers and the JSON_HAS_CPP_* macros to override the detected C++ standard.

    Full release notes.

    "},{"location":"home/releases/#v391-2020-08-06","title":"v3.9.1 (2020-08-06)","text":"

    Fixes two regressions from 3.9.0. All changes are backward-compatible.

    • Accepts consecutive comments and completes the ordered_json interface (e.g. ordered_json::parse).

    Full release notes.

    "},{"location":"home/releases/#v390-2020-07-27","title":"v3.9.0 (2020-07-27)","text":"

    Feature release adding four long-requested features. All changes are backward-compatible.

    • Optional comment parsing in parse via the ignore_comments parameter.
    • ordered_json to preserve the insertion order of object keys.
    • An option to switch off implicit conversions.
    • The NLOHMANN_DEFINE_TYPE_* convenience macros, plus high-precision-number support for UBJSON and CBOR tag handling.

    Full release notes.

    "},{"location":"home/releases/#v380-2020-06-14","title":"v3.8.0 (2020-06-14)","text":"

    Feature release. All changes are backward-compatible.

    • Introduces a binary value type that is read from and written to CBOR, BSON, and MessagePack, and can be shared between formats.
    • Generalizes the input adapters to read from any LegacyInputIterator container (3\u201310 % faster parsing).
    • Fixes contains for JSON pointers and makes the binary from_cbor/from_msgpack/etc. functions respect allow_exceptions.

    Full release notes.

    "},{"location":"home/releases/#v373-2019-11-17","title":"v3.7.3 (2019-11-17)","text":"

    Fixes a regression from 3.7.2 that could yield quadratic complexity in destructor calls. All changes are backward-compatible. Full release notes.

    "},{"location":"home/releases/#v372-2019-11-10","title":"v3.7.2 (2019-11-10)","text":"

    Fixes a stack overflow for deeply nested input by making the destructor iterative; parsing is now bounded only by available memory. All changes are backward-compatible. Full release notes.

    "},{"location":"home/releases/#v371-2019-11-06","title":"v3.7.1 (2019-11-06)","text":"

    Bug-fix release. All changes are backward-compatible.

    • Fixes a segmentation fault when serializing the std::int64_t minimum value and fixes contains for JSON pointers.
    • Allows items with a custom string type and makes json_pointer::back const.

    Full release notes.

    "},{"location":"home/releases/#v370-2019-07-28","title":"v3.7.0 (2019-07-28)","text":"

    Convenience features and house-keeping. All changes are backward-compatible.

    • Adds a contains overload that checks a JSON pointer without throwing, a generic to_string, and a return value for emplace_back.

    Full release notes.

    "},{"location":"home/releases/#v361-2019-03-20","title":"v3.6.1 (2019-03-20)","text":"

    Fixes a regression (GCC 7/8 compilation) and a <Windows.h> build error introduced in 3.6.0. All changes are backward-compatible. Full release notes.

    "},{"location":"home/releases/#v360-2019-03-20","title":"v3.6.0 (2019-03-20)","text":"

    Feature release. All changes are backward-compatible.

    • Reworks the JSON pointer interface (operator/, push_back, parent_pointer, \u2026).
    • Adds a contains function to test for an object key and greatly improves the performance of integer serialization.

    Full release notes.

    "},{"location":"home/releases/#v350-2018-12-22","title":"v3.5.0 (2018-12-22)","text":"

    Feature release. All changes are backward-compatible.

    • Adds structured-binding support via the items function and reading from FILE* in the parse function.
    • Fixes the eofbit handling on input streams and a bug in the BSON SAX parser.

    Full release notes.

    "},{"location":"home/releases/#v340-2018-10-30","title":"v3.4.0 (2018-10-30)","text":"

    Feature release. All changes are backward-compatible.

    • Adds BSON read/write support.
    • Adds configurable Unicode error handlers to dump (throw, replace with U+FFFD, or ignore) and the NLOHMANN_JSON_SERIALIZE_ENUM macro for enum conversion.
    • Improves parse-error messages with line/column positions and context.

    Full release notes.

    "},{"location":"home/releases/#v330-2018-10-05","title":"v3.3.0 (2018-10-05)","text":"

    Feature release. All changes are backward-compatible.

    • Adds GCC 4.8 support, the get_to function, and an overhauled and documented CMake integration.

    Full release notes.

    "},{"location":"home/releases/#v320-2018-08-20","title":"v3.2.0 (2018-08-20)","text":"

    Feature release. All changes are backward-compatible.

    • Adds a SAX interface and a non-recursive parser.
    • Adds parsing from wide-string types (std::wstring, std::u16string, std::u32string) and std::string_view (C++17), and round-tripping of std::map/std::unordered_map with non-string keys.

    Full release notes.

    "},{"location":"home/releases/#v312-2018-03-14","title":"v3.1.2 (2018-03-14)","text":"

    Bug-fix release. All changes are backward-compatible.

    • Fixes a memory leak in the parser callback and adds user-defined string-type support to the parser and serializer.

    Full release notes.

    "},{"location":"home/releases/#v311-2018-02-13","title":"v3.1.1 (2018-02-13)","text":"

    Bug-fix release. All changes are backward-compatible.

    • Fixes parsing of indefinite-length CBOR strings, a user-defined conversion to vector types, and overflow detection for UBJSON containers.

    Full release notes.

    "},{"location":"home/releases/#v310-2018-02-01","title":"v3.1.0 (2018-02-01)","text":"

    Feature release. All changes are backward-compatible.

    • Adds UBJSON read/write support and JSON Merge Patch via merge_patch.
    • Switches to the Grisu2 algorithm for short, round-trippable floating-point output, and splits the header into multiple files with a forward-declaration header.

    Full release notes.

    "},{"location":"home/releases/#v301-2017-12-29","title":"v3.0.1 (2017-12-29)","text":"

    Fixes small issues in the JSON Pointer and JSON Patch implementations (invalid \"copy\" targets and non-integer array indices). All changes are backward-compatible. Full release notes.

    "},{"location":"home/releases/#v300-2017-12-17","title":"v3.0.0 (2017-12-17)","text":"

    First 3.x release \u2014 a major release with breaking changes (see the migration guide).

    • Introduces user-defined exceptions (json::exception and subtypes, each with an identifier).
    • Adds a non-throwing accept function and an allow_exceptions flag for parse, and an update function to merge objects.
    • Adds streaming for CBOR and MessagePack and allows storing NaN/infinity.
    • Non-UTF-8 strings now throw on serialization, and the iterator category changed to bidirectional.

    Full release notes.

    "},{"location":"home/releases/#v211-2017-02-25","title":"v2.1.1 (2017-02-25)","text":"

    Bug-fix release. All changes are backward-compatible.

    • Makes number parsing and serialization locale-independent with correct floating-point round-tripping; released files are now GPG-signed.

    Full release notes.

    "},{"location":"home/releases/#v210-2017-01-28","title":"v2.1.0 (2017-01-28)","text":"

    Feature release. All changes are backward-compatible.

    • Adds conversions from and to arbitrary user-defined types via to_json/from_json, the meta function, and the option to switch off exceptions (JSON_NOEXCEPTION).

    Full release notes.

    "},{"location":"home/releases/#v2010-2017-01-02","title":"v2.0.10 (2017-01-02)","text":"

    Fixes several security-relevant bugs in the CBOR and MessagePack parsers found by OSS-Fuzz. All changes are backward-compatible. Full release notes.

    "},{"location":"home/releases/#v209-2016-12-16","title":"v2.0.9 (2016-12-16)","text":"

    Adds the CBOR and MessagePack binary formats. All changes are backward-compatible. Full release notes.

    "},{"location":"home/releases/#v208-2016-12-02","title":"v2.0.8 (2016-12-02)","text":"

    Adds the emplace and emplace_back functions and improves parsing and serialization performance. All changes are backward-compatible. Full release notes.

    "},{"location":"home/releases/#v207-2016-11-02","title":"v2.0.7 (2016-11-02)","text":"

    Fixes several parser bugs found through the \"Parsing JSON is a Minefield\" study (short files, encoding detection, surrogate pairs). All changes are backward-compatible. Full release notes.

    "},{"location":"home/releases/#v206-2016-10-15","title":"v2.0.6 (2016-10-15)","text":"

    Fixes operator[] for JSON pointers so that it creates missing values like the other overloads. All changes are backward-compatible. Full release notes.

    "},{"location":"home/releases/#v205-2016-09-14","title":"v2.0.5 (2016-09-14)","text":"

    Fixes a remaining stream end-of-file detection bug in the parser. All changes are backward-compatible. Full release notes.

    "},{"location":"home/releases/#v204-2016-09-11","title":"v2.0.4 (2016-09-11)","text":"

    Fixes stream end-of-file detection in the parser. All changes are backward-compatible. Full release notes.

    "},{"location":"home/releases/#v203-2016-08-31","title":"v2.0.3 (2016-08-31)","text":"

    Generalizes the parser to accept any contiguous sequence of one-byte elements and deprecates the input-stream constructor in favor of the parse function. All changes are backward-compatible. Full release notes.

    "},{"location":"home/releases/#v202-2016-07-31","title":"v2.0.2 (2016-07-31)","text":"

    Overhauls the parser (now rejecting unescaped control characters), tightens the class invariants, and cleans up the code. All changes are backward-compatible. Full release notes.

    "},{"location":"home/releases/#v201-2016-06-28","title":"v2.0.1 (2016-06-28)","text":"

    Fixes a performance regression in the dump function by adjusting the stream locale once per serialization. All changes are backward-compatible. Full release notes.

    "},{"location":"home/releases/#v200-2016-06-24","title":"v2.0.0 (2016-06-24)","text":"

    Feature release with a minor (potentially non-backward-compatible) API change from added noexcept and constexpr specifiers.

    • Adds JSON Pointer support in at and operator[], plus flatten and unflatten.
    • Adds JSON Patch via diff and patch, unsigned 64-bit integer support, and locale-independent serialization.

    Full release notes.

    "},{"location":"home/releases/#v110-2016-01-24","title":"v1.1.0 (2016-01-24)","text":"

    Bug-fix and feature release. All changes are backward-compatible.

    • Improves floating-point round-tripping, adds a get_ref accessor for stored values, and introduces runtime assertions.

    Full release notes.

    "},{"location":"home/releases/#v100-2015-12-28","title":"v1.0.0 (2015-12-28)","text":"

    First official release. Full release notes.

    "},{"location":"home/releases/#see-also","title":"See also","text":"
    • Migration Guide \u2014 how to future-proof your code for the next major version and replace deprecated functions.
    "},{"location":"home/sponsors/","title":"Sponsors","text":"

    You can sponsor this library at GitHub Sponsors.

    "},{"location":"home/sponsors/#priority-sponsor","title":"Priority Sponsor","text":"
    • Martti Laine
    • Paul Harrington
    "},{"location":"home/sponsors/#named-sponsors","title":"Named Sponsors","text":"
    • Michael Hartmann
    • Stefan Hagen
    • Steve Sperandeo
    • Robert Jefe Lindst\u00e4dt
    • Steve Wagner

    Thanks everyone!

    "},{"location":"integration/","title":"Integration","text":"

    There are several ways to add this header-only library to a C++ project. The following flowchart summarizes how to pick one:

    flowchart TD\n    A[Add the library to a C++ project] --> B{Already using CMake?}\n    B -- no --> C{Using pkg-config or plain Makefiles?}\n    C -- yes --> D[pkg-config]\n    C -- no --> E[Copy the single header]\n    B -- yes --> F{Library installed system-wide?}\n    F -- yes --> G[\"find_package()\"]\n    F -- no --> H{Use a package manager?}\n    H -- yes --> I[Package manager]\n    H -- no --> J[\"add_subdirectory() or FetchContent\"]
    • Copy the single header, as described below \u2014 no build-system integration required.
    • CMake: use find_package() if the library is already installed, add_subdirectory() to embed the source tree, or FetchContent to download it at configure time; see CMake.
    • Package managers: install the library with a package manager such as Homebrew, Conan, or vcpkg; see Package Managers.
    • pkg-config: if you use bare Makefiles instead of CMake, pkg-config can supply the include flags for an already-installed library.

    Once the library is integrated, see the Migration Guide for how to keep your code future-proof across releases.

    "},{"location":"integration/#header-only","title":"Header only","text":"

    json.hpp is the single required file in single_include/nlohmann or released here. You need to add

    #include <nlohmann/json.hpp>\n\n// for convenience\nusing json = nlohmann::json;\n

    to the files you want to process JSON and set the necessary switches to enable C++11 (e.g., -std=c++11 for GCC and Clang).

    You can further use file single_include/nlohmann/json_fwd.hpp for forward declarations (see Compile times), and file single_include/nlohmann/json_literals.hpp for the user-defined string literals if you define JSON_NO_AUTOMATIC_UDLS.

    For the read-only, non-owning basic_json_document/basic_json_view types, additionally include single_include/nlohmann/json_view.hpp; see Zero-copy JSON views.

    "},{"location":"integration/cmake/","title":"CMake","text":""},{"location":"integration/cmake/#integration","title":"Integration","text":"

    You can use the nlohmann_json::nlohmann_json interface target in CMake. This target populates the appropriate usage requirements for INTERFACE_INCLUDE_DIRECTORIES to point to the appropriate include directories and INTERFACE_COMPILE_FEATURES for the necessary C++11 flags. Most package managers that provide a CMake package configuration for this library expose this same target.

    "},{"location":"integration/cmake/#external","title":"External","text":"

    To use this library from a CMake project, you can locate it directly with find_package() and use the namespaced imported target from the generated package configuration:

    Example

    CMakeLists.txt
    cmake_minimum_required(VERSION 3.5)\nproject(ExampleProject LANGUAGES CXX)\n\nfind_package(nlohmann_json 3.12.0 REQUIRED)\n\nadd_executable(example example.cpp)\ntarget_link_libraries(example PRIVATE nlohmann_json::nlohmann_json)\n

    The package configuration file, nlohmann_jsonConfig.cmake, can be used either from an install tree or directly out of the build tree.

    "},{"location":"integration/cmake/#embedded","title":"Embedded","text":"

    To embed the library directly into an existing CMake project, place the entire source tree in a subdirectory and call add_subdirectory() in your CMakeLists.txt file.

    Example

    CMakeLists.txt
    cmake_minimum_required(VERSION 3.5)\nproject(ExampleProject LANGUAGES CXX)\n\n# If you only include this third party in PRIVATE source files, you do not need to install it\n# when your main project gets installed.\nset(JSON_Install OFF CACHE INTERNAL \"\")\n\nadd_subdirectory(nlohmann_json)\n\nadd_executable(example example.cpp)\ntarget_link_libraries(example PRIVATE nlohmann_json::nlohmann_json)\n

    Note

    Do not use include(nlohmann_json/CMakeLists.txt), since that carries with it unintended consequences that will break the build. It is generally discouraged (although not necessarily well documented as such) to use include(...) for pulling in other CMake projects anyways.

    "},{"location":"integration/cmake/#supporting-both","title":"Supporting Both","text":"

    To allow your project to support either an externally supplied or an embedded JSON library, you can use a pattern akin to the following.

    Example

    CMakeLists.txt
    project(ExampleProject LANGUAGES CXX)\n\noption(EXAMPLE_USE_EXTERNAL_JSON \"Use an external JSON library\" OFF)\n\nadd_subdirectory(thirdparty)\n\nadd_executable(example example.cpp)\n\n# Note that the namespaced target will always be available regardless of the import method\ntarget_link_libraries(example PRIVATE nlohmann_json::nlohmann_json)\n
    thirdparty/CMakeLists.txt
    if(EXAMPLE_USE_EXTERNAL_JSON)\n    find_package(nlohmann_json 3.12.0 REQUIRED)\nelse()\n    set(JSON_BuildTests OFF CACHE INTERNAL \"\")\n    add_subdirectory(nlohmann_json)\nendif()\n

    thirdparty/nlohmann_json is then a complete copy of this source tree.

    "},{"location":"integration/cmake/#fetchcontent","title":"FetchContent","text":"

    Since CMake v3.14, FetchContent can be used to automatically download a release as a dependency at configure time.

    Example

    CMakeLists.txt
    cmake_minimum_required(VERSION 3.14)\nproject(ExampleProject LANGUAGES CXX)\n\ninclude(FetchContent)\n\nFetchContent_Declare(json URL https://github.com/nlohmann/json/releases/download/v3.12.0/json.tar.xz)\nFetchContent_MakeAvailable(json)\n\nadd_executable(example example.cpp)\ntarget_link_libraries(example PRIVATE nlohmann_json::nlohmann_json)\n

    Note

    It is recommended to use the URL approach described above which is supported as of version 3.10.0. It is also possible to pass the Git repository like

    FetchContent_Declare(json\n    GIT_REPOSITORY https://github.com/nlohmann/json\n    GIT_TAG v3.12.0\n)\n

    However, the repository https://github.com/nlohmann/json download size is quite large.

    "},{"location":"integration/cmake/#cmake-options","title":"CMake Options","text":""},{"location":"integration/cmake/#json_buildtests","title":"JSON_BuildTests","text":"

    Build the unit tests when BUILD_TESTING is enabled. This option is ON by default if the library's CMake project is the top project and the tests directory exists (the release archive json.tar.xz does not contain it). That is, when integrating the library as described above, the test suite is not built unless explicitly switched on with this option.

    "},{"location":"integration/cmake/#json_ci","title":"JSON_CI","text":"

    Enable CI build targets. The exact targets are used during the several CI steps and are subject to change without notice. This option is OFF by default.

    "},{"location":"integration/cmake/#json_deletedeprecatedfunctions","title":"JSON_DeleteDeprecatedFunctions","text":"

    Delete the deprecated functions instead of only deprecating them by defining the macro JSON_DELETE_DEPRECATED_FUNCTIONS. This option is OFF by default.

    "},{"location":"integration/cmake/#json_diagnostics","title":"JSON_Diagnostics","text":"

    Enable extended diagnostic messages by defining macro JSON_DIAGNOSTICS. This option is OFF by default.

    Does not apply to a pre-installed package

    This option only takes effect when building nlohmann/json from source as part of your own CMake project (e.g. via FetchContent or add_subdirectory). It has no effect on a package that was already built and installed elsewhere (Homebrew, vcpkg, a system package, etc.) \u2014 the resulting compile definition is baked into the exported nlohmann_jsonTargets.cmake at install time, and set(JSON_Diagnostics ON) before find_package() does not change it (verified against the Homebrew-installed package: the exported target still carries a fixed $<$<BOOL:OFF>:JSON_DIAGNOSTICS=1>, regardless of any variable set in the consuming project).

    To enable extended diagnostics for a pre-installed package, override the imported target's property directly after find_package():

    find_package(nlohmann_json REQUIRED)\nset_target_properties(nlohmann_json::nlohmann_json PROPERTIES\n    INTERFACE_COMPILE_DEFINITIONS \"JSON_DIAGNOSTICS=1\")\n

    This only works cleanly when your project is the sole consumer of that imported target. If nlohmann_json is pulled in from more than one place in your dependency graph with different JSON_DIAGNOSTICS values, you may see a \"JSON_DIAGNOSTICS\" redefined compiler error, since conflicting -D flags can end up on the same compile command line.

    "},{"location":"integration/cmake/#json_diagnostic_positions","title":"JSON_Diagnostic_Positions","text":"

    Enable position diagnostics by defining macro JSON_DIAGNOSTIC_POSITIONS. This option is OFF by default.

    "},{"location":"integration/cmake/#json_disableenumserialization","title":"JSON_DisableEnumSerialization","text":"

    Disable default enum serialization by defining the macro JSON_DISABLE_ENUM_SERIALIZATION. This option is OFF by default.

    "},{"location":"integration/cmake/#json_disabletuplereferenceconversion","title":"JSON_DisableTupleReferenceConversion","text":"

    Disable the conversion from a one-element std::tuple holding a reference to a JSON value by defining the macro JSON_DISABLE_TUPLE_REFERENCE_CONVERSION. This option is OFF by default.

    "},{"location":"integration/cmake/#json_fasttests","title":"JSON_FastTests","text":"

    Skip expensive/slow test suites. This option is OFF by default. Depends on JSON_BuildTests.

    "},{"location":"integration/cmake/#json_globaludls","title":"JSON_GlobalUDLs","text":"

    Place user-defined string literals in the global namespace by defining the macro JSON_USE_GLOBAL_UDLS. This option is ON by default; see the migration guide for how to prepare code for the next major release, where the literals are removed from the global namespace.

    "},{"location":"integration/cmake/#json_implicitconversions","title":"JSON_ImplicitConversions","text":"

    Enable implicit conversions by defining macro JSON_USE_IMPLICIT_CONVERSIONS. This option is ON by default; see the migration guide for how to prepare code for the next major release, where implicit conversions are switched off by default.

    "},{"location":"integration/cmake/#json_install","title":"JSON_Install","text":"

    Install CMake targets during install step. This option is ON by default if the library's CMake project is the top project. Installing also generates a pkg-config file for tools that rely on pkg-config instead of CMake.

    "},{"location":"integration/cmake/#json_legacydiscardedvaluecomparison","title":"JSON_LegacyDiscardedValueComparison","text":"

    Enable the (incorrect) legacy comparison behavior of discarded JSON values by defining macro JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON. This option is OFF by default.

    "},{"location":"integration/cmake/#json_multipleheaders","title":"JSON_MultipleHeaders","text":"

    Use the non-amalgamated version of the library. This option is ON by default.

    "},{"location":"integration/cmake/#json_systeminclude","title":"JSON_SystemInclude","text":"

    Treat the library headers like system headers (i.e., adding SYSTEM to the target_include_directories call) to check for this library by tools like Clang-Tidy. This option is OFF by default.

    "},{"location":"integration/cmake/#json_strictbinaryutf8","title":"JSON_StrictBinaryUTF8","text":"

    Check string values and object keys for valid UTF-8 in the CBOR, UBJSON, BJData, and BSON writers, by defining the macro JSON_STRICT_BINARY_UTF8. This option is OFF by default.

    "},{"location":"integration/cmake/#json_strictnulhandling","title":"JSON_StrictNulHandling","text":"

    Reject a '\\0' (NUL) byte in the input instead of treating it as end of input, by defining the macro JSON_STRICT_NUL_HANDLING. This option is OFF by default.

    "},{"location":"integration/cmake/#json_testsimdutf","title":"JSON_TestSimdutf","text":"

    Build the unit tests against the simdutf UTF-8 validation backend by defining JSON_USE_SIMDUTF for every test target. simdutf is fetched during configuration; its version is set by the cache variable JSON_SIMDUTF_VERSION. This option is OFF by default. Depends on JSON_BuildTests.

    "},{"location":"integration/cmake/#json_valgrind","title":"JSON_Valgrind","text":"

    Execute the test suite with Valgrind. This option is OFF by default. Depends on JSON_BuildTests.

    "},{"location":"integration/cmake/#nlohmann_json_build_modules","title":"NLOHMANN_JSON_BUILD_MODULES","text":"

    Build the experimental C++ module nlohmann.json (requires CMake 3.28 or later and C++20). This option is OFF by default.

    A consuming project must link the dedicated nlohmann_json_modules CMake target (not just nlohmann_json::nlohmann_json) for import nlohmann.json; to resolve:

    set(NLOHMANN_JSON_BUILD_MODULES ON)\nadd_subdirectory(path/to/json)\n\nadd_executable(myproject main.cpp)\ntarget_link_libraries(myproject PRIVATE nlohmann_json_modules)\ntarget_compile_definitions(myproject PRIVATE NLOHMANN_JSON_BUILD_MODULES)\n
    "},{"location":"integration/compile_times/","title":"Compile times","text":"

    The library is header-only and makes heavy use of templates, so every translation unit that includes <nlohmann/json.hpp> pays for parsing the header and instantiating what it uses. This page lists the options to reduce that cost, ordered by how much they typically save.

    Measurements

    The numbers below are medians of nine runs compiling a single translation unit with -std=c++17 -c against the single-header version, with Apple clang and GCC 16 on macOS (Apple silicon). They show the order of magnitude to expect; measure your own code before and after a change.

    "},{"location":"integration/compile_times/#include-json_fwdhpp-in-headers","title":"Include json_fwd.hpp in headers","text":"

    Header files that only need to name the json type \u2014 for function declarations, members held by pointer or reference, or friend declarations \u2014 can include <nlohmann/json_fwd.hpp> instead of <nlohmann/json.hpp>. It only forward-declares basic_json, json_pointer, adl_serializer, and ordered_map, and defines the aliases json and ordered_json. The translation units that actually use the values then include <nlohmann/json.hpp>.

    person.hpp
    #pragma once\n#include <nlohmann/json_fwd.hpp>\n\nstruct person;\nvoid to_json(nlohmann::json& j, const person& p);\nvoid from_json(const nlohmann::json& j, person& p);\n
    person.cpp
    #include \"person.hpp\"\n#include <nlohmann/json.hpp>\n\nvoid to_json(nlohmann::json& j, const person& p) { /* ... */ }\nvoid from_json(const nlohmann::json& j, person& p) { /* ... */ }\n
    Compiler json.hpp (-O0) json_fwd.hpp (-O0) Change Apple clang 704 ms 329 ms \u221253% GCC 16 779 ms 242 ms \u221269%

    This is the most effective option, because it avoids the full header in every translation unit that includes your headers.

    "},{"location":"integration/compile_times/#opt-out-of-the-automatic-user-defined-string-literals","title":"Opt out of the automatic user-defined string literals","text":"

    The user-defined string literals operator\"\"_json and operator\"\"_json_pointer are ordinary inline functions whose bodies call the parser. As <nlohmann/json.hpp> includes them by default, every translation unit instantiates the parser, even if it never parses anything itself.

    Define JSON_NO_AUTOMATIC_UDLS for the whole project and include <nlohmann/json_literals.hpp> instead of <nlohmann/json.hpp> in the files that use the literals (it includes <nlohmann/json.hpp> itself):

    target_compile_definitions(my_target PRIVATE JSON_NO_AUTOMATIC_UDLS)\n
    #include <nlohmann/json_literals.hpp> // only where \"...\"_json is used; includes <nlohmann/json.hpp>\n

    The saving applies to translation units that do not parse JSON, for example ones that define types and their conversions or only pass json values around:

    Compiler Translation unit Default (-O0 / -O2) JSON_NO_AUTOMATIC_UDLS (-O0 / -O2) Change Apple clang model 776 ms / 846 ms 629 ms / 692 ms \u221219% / \u221218% GCC 16 model 1022 ms / 1120 ms 882 ms / 965 ms \u221214% / \u221214% Apple clang parsing 992 ms / 1815 ms 1006 ms / 1823 ms +1% / 0% GCC 16 parsing 2018 ms / 3420 ms 1990 ms / 3454 ms \u22121% / +1%

    Translation units that include only the header save up to a third. Translation units that parse anyway instantiate the parser regardless and see no difference.

    "},{"location":"integration/compile_times/#instantiate-basic_json-once","title":"Instantiate basic_json once","text":"

    Each translation unit instantiates the member functions of nlohmann::json it uses. An explicit instantiation declaration tells the compiler that the non-template members are instantiated elsewhere, so it can skip them:

    json_instance.hpp
    #pragma once\n#include <nlohmann/json.hpp>\n\nextern template class nlohmann::basic_json<>;\n
    json_instance.cpp
    #include \"json_instance.hpp\"\n\ntemplate class nlohmann::basic_json<>;\n

    Include json_instance.hpp instead of <nlohmann/json.hpp> and compile and link json_instance.cpp once.

    Compiler Translation unit Default (-O0 / -O2) extern template (-O0 / -O2) Change Apple clang parsing 992 ms / 1815 ms 953 ms / 1625 ms \u22124% / \u221210% GCC 16 parsing 2018 ms / 3420 ms 1522 ms / 2728 ms \u221225% / \u221220% Apple clang json_instance.cpp \u2014 2166 ms / 4660 ms \u2014 GCC 16 json_instance.cpp \u2014 5085 ms / 10616 ms \u2014

    Notes:

    • The saving grows with the number of translation units that use json, while the instantiation translation unit is compiled only once (and is rarely recompiled, as it does not depend on your code).
    • Member function templates (such as get<T>(), parse(InputType&&), or value(key, default)) are not covered by the explicit instantiation and are still instantiated where they are used.
    • The declaration covers exactly nlohmann::json. Add the same lines for nlohmann::ordered_json (nlohmann::basic_json<nlohmann::ordered_map>) or your own basic_json specializations if you use them.
    "},{"location":"integration/compile_times/#use-c20-modules","title":"Use C++20 modules","text":"

    With a toolchain that supports named modules, import nlohmann.json; compiles the library once into a module and avoids parsing the header in every translation unit. See Modules for requirements and known issues. Module support is experimental and currently depends heavily on the compiler version.

    "},{"location":"integration/compile_times/#use-precompiled-headers","title":"Use precompiled headers","text":"

    Build systems can precompile <nlohmann/json.hpp> together with other stable headers, for example with CMake's target_precompile_headers:

    target_precompile_headers(my_target PRIVATE <nlohmann/json.hpp>)\n

    This removes the cost of parsing the header, but not of instantiating templates in each translation unit, so it combines well with the options above.

    "},{"location":"integration/compile_times/#options-without-effect-on-compile-times","title":"Options without effect on compile times","text":"

    Some configuration macros change what the library declares, but do not measurably change compile times:

    Macro Apple clang, model (-O0 / -O2) GCC 16, model (-O0 / -O2) default 776 ms / 846 ms 1022 ms / 1120 ms JSON_NO_IO 764 ms / 836 ms 1022 ms / 1117 ms JSON_USE_GLOBAL_UDLS=0 763 ms / 852 ms 1019 ms / 1106 ms

    JSON_USE_GLOBAL_UDLS only controls where the literals are declared; to avoid their cost, use JSON_NO_AUTOMATIC_UDLS instead.

    "},{"location":"integration/compile_times/#see-also","title":"See also","text":"
    • JSON_NO_AUTOMATIC_UDLS - do not include the user-defined string literals automatically
    • Modules - C++20 module support
    • Header only - including the library
    "},{"location":"integration/migration_guide/","title":"Migration Guide","text":"

    This page collects some guidelines on how to future-proof your code for future versions of this library. For how to add the library to your project in the first place, see Integration, CMake, or Package Managers. The roadmap lists what will change in version 4.0, including the macros that let you try its behavior with a 3.x release; this page describes how to adjust your code.

    "},{"location":"integration/migration_guide/#replace-deprecated-functions","title":"Replace deprecated functions","text":"

    The following functions have been deprecated and will be removed in the next major version (i.e., 4.0.0), see the roadmap for an overview. All deprecations are annotated with HEDLEY_DEPRECATED_FOR to report which function to use instead.

    Find all calls of deprecated functions

    Define JSON_DELETE_DEPRECATED_FUNCTIONS to 1 (or set the CMake option JSON_DeleteDeprecatedFunctions) to delete all deprecated functions. Every remaining call then fails to compile, even if deprecation warnings are disabled, so your code is ready for version 4.0.0 unreleased once it compiles with the macro.

    "},{"location":"integration/migration_guide/#parsing","title":"Parsing","text":"

    • Function friend std::istream& operator<<(basic_json&, std::istream&) is deprecated since 3.0.0. Please use friend std::istream& operator>>(std::istream&, basic_json&) instead.

      DeprecatedFuture-proof 
      nlohmann::json j;\nstd::stringstream ss(\"[1,2,3]\");\nj << ss;\n
      nlohmann::json j;\nstd::stringstream ss(\"[1,2,3]\");\nss >> j;\n

    • Passing iterator pairs or pointer/length pairs to parsing functions (parse, accept, sax_parse, from_cbor, from_msgpack, from_ubjson, and from_bson) via initializer lists is deprecated since 3.8.0. Instead, pass two iterators; for instance, call from_cbor(ptr, ptr+len) instead of from_cbor({ptr, len}). Likewise, passing a pointer and a length as two separate arguments to from_cbor, from_msgpack, from_ubjson, and from_bson is deprecated since 3.8.0, and to from_bjdata and from_bon8 since 3.13.0; call from_cbor(ptr, ptr+len) instead of from_cbor(ptr, len). These overloads will not be removed in version 4.0.0, but deleted, so a call like from_cbor(ptr, len) cannot compile and convert len to the strict parameter.

      DeprecatedFuture-proof 
      const char* s = \"[1,2,3]\";\nbool ok = nlohmann::json::accept({s, s + std::strlen(s)});\n
      const char* s = \"[1,2,3]\";\nbool ok = nlohmann::json::accept(s, s + std::strlen(s));\n
    "},{"location":"integration/migration_guide/#json-pointers","title":"JSON Pointers","text":"

    • Comparing JSON Pointers with strings via operator== and operator!= is deprecated since 3.11.2. To compare a json_pointer p with a string s, convert s to a json_pointer first and use json_pointer::operator== or json_pointer::operator!=.

      DeprecatedFuture-proof 
      nlohmann::json::json_pointer lhs(\"/foo/bar/1\");\nassert(lhs == \"/foo/bar/1\");\n
      nlohmann::json::json_pointer lhs(\"/foo/bar/1\");\nassert(lhs == nlohmann::json::json_pointer(\"/foo/bar/1\"));\n

    • The implicit conversion from JSON Pointers to string (json_pointer::operator string_t) is deprecated since 3.11.0. Use json_pointer::to_string instead.

      DeprecatedFuture-proof 
      nlohmann::json::json_pointer ptr(\"/foo/bar/1\");\nstd::string s = ptr;\n
      nlohmann::json::json_pointer ptr(\"/foo/bar/1\");\nstd::string s = ptr.to_string();\n

    • Passing a basic_json specialization as template parameter RefStringType to json_pointer is deprecated since 3.11.0. The string type can now be directly provided. This also applies to passing such a JSON pointer to at, contains, operator[], and value.

      DeprecatedFuture-proof 
      using my_json = nlohmann::json::with_string_t<my_string_type>;\nnlohmann::json_pointer<my_json> ptr(\"/foo/bar/1\");\n
      nlohmann::json_pointer<my_string_type> ptr(\"/foo/bar/1\");\n

      Thereby, my_json::json_pointer is an alias for nlohmann::json_pointer<my_string_type>; in general, basic_json::json_pointer is always an alias to the json_pointer with the appropriate string type for all specializations of basic_json.

    "},{"location":"integration/migration_guide/#miscellaneous-functions","title":"Miscellaneous functions","text":"

    • The function iterator_wrapper is deprecated since 3.1.0. Please use the member function items instead.

      DeprecatedFuture-proof 
      for (auto &x : nlohmann::json::iterator_wrapper(j))\n{\n    std::cout << x.key() << \":\" << x.value() << std::endl;\n}\n
      for (auto &x : j.items())\n{\n    std::cout << x.key() << \":\" << x.value() << std::endl;\n}\n

    • Function friend std::ostream& operator>>(const basic_json&, std::ostream&) is deprecated since 3.0.0. Please use friend operator<<(std::ostream&, const basic_json&) instead.

      DeprecatedFuture-proof 
      j >> std::cout;\n
      std::cout << j;\n

    • The legacy comparison behavior for discarded values is deprecated since 3.11.0. It is already disabled by default and can still be enabled by defining JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON to 1.

      DeprecatedFuture-proof 
      #define JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON 1\n#include <nlohmann/json.hpp>\n
      #include <nlohmann/json.hpp>\n
    "},{"location":"integration/migration_guide/#replace-implicit-conversions","title":"Replace implicit conversions","text":"

    Implicit conversions via operator ValueType will be switched off by default in the next major release of the library.

    You can prepare existing code by already defining JSON_USE_IMPLICIT_CONVERSIONS to 0 and replace any implicit conversions with calls to get, get_to, get_ref, or get_ptr.

    Automatic migration

    The community-maintained clang-tidy check modernize-nlohmann-json-explicit-conversions rewrites most implicit conversions into calls to get. It is not part of clang-tidy itself; see discussion #4610 for how to build and use it.

    DeprecatedFuture-proofFuture-proof (alternative) 
    nlohmann::json j = \"Hello, world!\";\nstd::string s = j;\n
    nlohmann::json j = \"Hello, world!\";\nauto s = j.get<std::string>();\n
    nlohmann::json j = \"Hello, world!\";\nstd::string s;\nj.get_to(s);\n
    "},{"location":"integration/migration_guide/#import-namespace-literals-for-udls","title":"Import namespace literals for UDLs","text":"

    The user-defined string literals operator\"\"_json and operator\"\"_json_pointer will be removed from the global namespace in the next major release of the library.

    DeprecatedFuture-proof 
    nlohmann::json j = \"[1,2,3]\"_json;\n
    using namespace nlohmann::literals;\nnlohmann::json j = \"[1,2,3]\"_json;\n

    To prepare existing code, define JSON_USE_GLOBAL_UDLS to 0 and bring the string literals into scope where needed.

    "},{"location":"integration/migration_guide/#do-not-hard-code-the-complete-library-namespace","title":"Do not hard-code the complete library namespace","text":"

    The nlohmann namespace contains a sub-namespace to avoid problems when different versions or configurations of the library are used in the same project. Always use nlohmann as namespace or, when the exact version and configuration is relevant, use macro NLOHMANN_JSON_NAMESPACE to denote the namespace.

    DangerousFuture-proofFuture-proof (alternative) 
    void to_json(nlohmann::json_abi_v3_11_2::json& j, const person& p)\n{\n    j[\"age\"] = p.age;\n}\n
    void to_json(nlohmann::json& j, const person& p)\n{\n    j[\"age\"] = p.age;\n}\n
    void to_json(NLOHMANN_JSON_NAMESPACE::json& j, const person& p)\n{\n    j[\"age\"] = p.age;\n}\n
    "},{"location":"integration/migration_guide/#do-not-use-the-detail-namespace","title":"Do not use the detail namespace","text":"

    The nlohmann::detail namespace is not part of the public API of the library and can change in any version without an announcement. Do not rely on any function or type in the detail namespace.

    "},{"location":"integration/package_managers/","title":"Package Managers","text":"

    Homebrew nlohmann-json Meson nlohmann_json Bazel nlohmann_json Conan nlohmann_json Spack nlohmann-json Hunter nlohmann_json vcpkg nlohmann-json cget nlohmann/json Swift Package Manager nlohmann/json NuGet nlohmann.json Conda nlohmann_json MacPorts nlohmann-json CPM.cmake gh:nlohmann/json xmake nlohmann_json

    "},{"location":"integration/package_managers/#running-example","title":"Running example","text":"

    Throughout this page, we will describe how to compile the example file example.cpp below.

    #include <nlohmann/json.hpp>\n#include <iostream>\n#include <iomanip>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::setw(4) << json::meta() << std::endl;\n}\n

    When executed, this program should create output similar to

    {\n    \"compiler\": {\n        \"c++\": \"201103\",\n        \"family\": \"gcc\",\n        \"version\": \"12.4.0\"\n    },\n    \"copyright\": \"(C) 2013-2026 Niels Lohmann\",\n    \"name\": \"JSON for Modern C++\",\n    \"platform\": \"apple\",\n    \"url\": \"https://github.com/nlohmann/json\",\n    \"version\": {\n        \"major\": 3,\n        \"minor\": 12,\n        \"patch\": 0,\n        \"string\": \"3.12.0\"\n    }\n}\n

    Many of the package managers below install a CMake package configuration that exposes the same nlohmann_json::nlohmann_json interface target described in CMake; their CMake examples below link against that target.

    "},{"location":"integration/package_managers/#homebrew","title":"Homebrew","text":"

    Summary

    formula: nlohmann-json

    • Available versions: current version and development version (with --HEAD parameter)
    • The formula is updated with every release.
    • Maintainer: Niels Lohmann
    • File issues at the Homebrew issue tracker
    • Homebrew website

    If you are using Homebrew, you can install the library with

    brew install nlohmann-json\n

    The header can be used directly in your code or via CMake.

    Example: Raw compilation

    1. Create the following file:

      example.cpp
      #include <nlohmann/json.hpp>\n#include <iostream>\n#include <iomanip>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::setw(4) << json::meta() << std::endl;\n}\n

    2. Install the package:

      brew install nlohmann-json\n

    3. Compile the code and pass the Homebrew prefix to the include path such that the library can be found:

      c++ example.cpp -I$(brew --prefix nlohmann-json)/include -std=c++11 -o example\n
    Example: CMake

    1. Create the following files:

      example.cpp
      #include <nlohmann/json.hpp>\n#include <iostream>\n#include <iomanip>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::setw(4) << json::meta() << std::endl;\n}\n
      CMakeLists.txt
      cmake_minimum_required(VERSION 3.15)\nproject(json_example)\n\nfind_package(nlohmann_json CONFIG REQUIRED)\n\nadd_executable(json_example example.cpp)\ntarget_link_libraries(json_example PRIVATE nlohmann_json::nlohmann_json)\n

    2. Install the package:

      brew install nlohmann-json\n

    3. Compile the code and pass the Homebrew prefix to CMake to find installed packages via find_package:

      CMAKE_PREFIX_PATH=$(brew --prefix) cmake -S . -B build\ncmake --build build\n
    "},{"location":"integration/package_managers/#meson","title":"Meson","text":"

    Summary

    wrap: nlohmann_json

    • Available versions: current version and select older versions (see WrapDB)
    • The package is updated automatically from file meson.build.
    • File issues at the library issue tracker
    • Meson website

    If you are using the Meson Build System, add this source tree as a meson subproject. You may also use the include.zip published in this project's Releases to reduce the size of the vendored source tree. Alternatively, you can get a wrap file by downloading it from Meson WrapDB, or use

    meson wrap install nlohmann_json\n

    Please see the Meson project for any issues regarding the packaging.

    The provided meson.build can also be used as an alternative to CMake for installing nlohmann_json system-wide in which case a pkg-config file and the CMake package config files are installed. To use it, have your build system require the nlohmann_json pkg-config dependency, or use find_package(nlohmann_json) in CMake. In Meson, it is preferred to use the dependency() object with a subproject fallback, rather than using the subproject directly.

    The options that change the library's configuration are available in Meson as well, named like the CMake options without the JSON_ prefix: MultipleHeaders, GlobalUDLs, ImplicitConversions, DisableEnumSerialization, DisableTupleReferenceConversion, Diagnostics, Diagnostic_Positions, LegacyDiscardedValueComparison, StrictNulHandling, StrictBinaryUTF8, and DeleteDeprecatedFunctions. They have the same defaults as in CMake, except that MultipleHeaders is false. Set them with -D when setting up the build, or with the subproject name as prefix when the library is used as a subproject:

    meson setup build -Dnlohmann_json:Diagnostics=true\n

    The resulting compile definitions are part of the Meson dependency, the pkg-config file, and the CMake target.

    Example: Wrap

    1. Create the following files:

      meson.build
      project('json_example', 'cpp',\n  version: '1.0',\n  default_options: ['cpp_std=c++11']\n)\n\ndependency_json = dependency('nlohmann_json', required: true)\n\nexecutable('json_example',\n  sources: ['example.cpp'],\n  dependencies: [dependency_json],\n  install: true\n)\n
      example.cpp
      #include <nlohmann/json.hpp>\n#include <iostream>\n#include <iomanip>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::setw(4) << json::meta() << std::endl;\n}\n

    2. Use the Meson WrapDB to fetch the nlohmann/json wrap:

      mkdir subprojects\nmeson wrap install nlohmann_json\n

    3. Build:

      meson setup build\nmeson compile -C build\n
    "},{"location":"integration/package_managers/#bazel","title":"Bazel","text":"

    Summary

    use bazel_dep, git_override, or local_path_override

    • Any version, that is available via Bazel Central Registry
    • File issues at the library issue tracker
    • Bazel website

    This repository provides a Bazel MODULE.bazel and a corresponding BUILD.bazel file. Therefore, this repository can be referenced within a MODULE.bazel by rules such as archive_override, git_override, or local_path_override. To use the library, you need to depend on the target @nlohmann_json//:json (i.e., via deps attribute).

    Example: Bazel module with bazel_dep

    1. Create the following files:

      BUILD
      cc_binary(\n    name = \"main\",\n    srcs = [\"example.cpp\"],\n    deps = [\"@nlohmann_json//:json\"],\n)\n
      MODULE.bazel
      bazel_dep(name = \"nlohmann_json\", version = \"3.12.0.bcr.2\")\n
      example.cpp
      #include <nlohmann/json.hpp>\n#include <iostream>\n#include <iomanip>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::setw(4) << json::meta() << std::endl;\n}\n

    2. Build and run:

      bazel build //:main\nbazel run //:main\n
    "},{"location":"integration/package_managers/#conan","title":"Conan","text":"

    Summary

    recipe: nlohmann_json

    • Available versions: current version and older versions (see Conan Center)
    • The package is updated automatically via this recipe.
    • File issues at the Conan Center issue tracker
    • Conan website

    If you are using Conan to manage your dependencies, merely add nlohmann_json/x.y.z to your conanfile's requires, where x.y.z is the release version you want to use.

    Example: CMake with the Conan toolchain

    1. Create the following files:

      Conanfile.txt
      [requires]\nnlohmann_json/3.12.0\n\n[generators]\nCMakeToolchain\nCMakeDeps\n
      CMakeLists.txt
      cmake_minimum_required(VERSION 3.15)\nproject(json_example)\n\nfind_package(nlohmann_json REQUIRED)\n\nadd_executable(json_example example.cpp)\ntarget_link_libraries(json_example PRIVATE nlohmann_json::nlohmann_json)\n
      example.cpp
      #include <nlohmann/json.hpp>\n#include <iostream>\n#include <iomanip>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::setw(4) << json::meta() << std::endl;\n}\n

    2. Call Conan:

      conan install . --output-folder=build --build=missing\n

    3. Build:

      cmake -S . -B build -DCMAKE_TOOLCHAIN_FILE=\"conan_toolchain.cmake\" -DCMAKE_BUILD_TYPE=Release\ncmake --build build\n
    "},{"location":"integration/package_managers/#spack","title":"Spack","text":"

    Summary

    package: nlohmann-json

    • Available versions: current version and older versions (see Spack package)
    • The package is updated with every release.
    • Maintainer: Axel Huebl
    • File issues at the Spack issue tracker
    • Spack website

    If you are using Spack to manage your dependencies, you can use the nlohmann-json package via

    spack install nlohmann-json\n

    Please see the Spack project for any issues regarding the packaging.

    Example: CMake with a Spack-installed package

    1. Create the following files:

      CMakeLists.txt
      cmake_minimum_required(VERSION 3.15)\nproject(json_example)\n\nfind_package(nlohmann_json REQUIRED)\n\nadd_executable(json_example example.cpp)\ntarget_link_libraries(json_example PRIVATE nlohmann_json::nlohmann_json)\n
      example.cpp
      #include <nlohmann/json.hpp>\n#include <iostream>\n#include <iomanip>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::setw(4) << json::meta() << std::endl;\n}\n

    2. Install the library:

      spack install nlohmann-json\n

    3. Load the environment for your Spack-installed packages:

      spack load nlohmann-json\n

    4. Build the project with CMake:

      cmake -S . -B build -DCMAKE_PREFIX_PATH=$(spack location -i nlohmann-json)\ncmake --build build\n
    "},{"location":"integration/package_managers/#hunter","title":"Hunter","text":"

    Summary

    package: nlohmann_json

    • Available versions: current version and older versions (see Hunter package)
    • The package is updated with every release.
    • File issues at the Hunter issue tracker
    • Hunter website

    If you are using Hunter on your project for external dependencies, then you can use the nlohmann_json package via

    hunter_add_package(nlohmann_json)\n

    Please see the Hunter project for any issues regarding the packaging.

    Example: CMake with HunterGate

    1. Create the following files:

      CMakeLists.txt
      cmake_minimum_required(VERSION 3.15)\n\ninclude(\"cmake/HunterGate.cmake\")\nHunterGate(\n    URL \"https://github.com/cpp-pm/hunter/archive/v0.26.12.tar.gz\"\n    SHA1 \"6498c5d0dec25d7fffb2b0574ebaf265179b894d\"\n)\n\nproject(json_example)\n\nhunter_add_package(nlohmann_json)\nfind_package(nlohmann_json CONFIG REQUIRED)\n\nadd_executable(json_example example.cpp)\ntarget_link_libraries(json_example PRIVATE nlohmann_json::nlohmann_json)\n
      example.cpp
      #include <nlohmann/json.hpp>\n#include <iostream>\n#include <iomanip>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::setw(4) << json::meta() << std::endl;\n}\n

    2. Download required files

      mkdir cmake\nwget https://raw.githubusercontent.com/cpp-pm/gate/master/cmake/HunterGate.cmake -O cmake/HunterGate.cmake\n

    3. Build the project with CMake:

      cmake -S . -B build\ncmake --build build\n
    "},{"location":"integration/package_managers/#vcpkg","title":"vcpkg","text":"

    Summary

    package: nlohmann-json

    • Available versions: current version
    • The package is updated with every release.
    • File issues at the vcpkg issue tracker
    • vcpkg website

    If you are using vcpkg on your project for external dependencies, then you can install the nlohmann-json package with

    vcpkg install nlohmann-json\n

    and follow the then displayed descriptions. Please see the vcpkg project for any issues regarding the packaging.

    Example: CMake with the vcpkg toolchain

    1. Create the following files:

      CMakeLists.txt
      cmake_minimum_required(VERSION 3.15)\nproject(json_example)\n\nfind_package(nlohmann_json CONFIG REQUIRED)\n\nadd_executable(json_example example.cpp)\ntarget_link_libraries(json_example PRIVATE nlohmann_json::nlohmann_json)\n
      example.cpp
      #include <nlohmann/json.hpp>\n#include <iostream>\n#include <iomanip>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::setw(4) << json::meta() << std::endl;\n}\n

    2. Install package:

      vcpkg install nlohmann-json\n

    3. Build:

      cmake -S . -B build -DCMAKE_TOOLCHAIN_FILE=$VCPKG_ROOT/scripts/buildsystems/vcpkg.cmake\ncmake --build build\n
    "},{"location":"integration/package_managers/#cget","title":"cget","text":"

    Summary

    package: nlohmann/json

    • Available versions: current version and older versions
    • The package is updated with every release.
    • File issues at the cget issue tracker
    • cget website

    If you are using cget, you can install the latest master version with

    cget install nlohmann/json\n

    A specific version can be installed with cget install nlohmann/json@v3.12.0. Also, the multiple header version can be installed by adding the -DJSON_MultipleHeaders=ON flag (i.e., cget install nlohmann/json -DJSON_MultipleHeaders=ON).

    Example: CMake with the cget toolchain

    1. Create the following files:

      CMakeLists.txt
      cmake_minimum_required(VERSION 3.15)\nproject(json_example)\n\nfind_package(nlohmann_json REQUIRED)\n\nadd_executable(json_example example.cpp)\ntarget_link_libraries(json_example PRIVATE nlohmann_json::nlohmann_json)\n
      example.cpp
      #include <nlohmann/json.hpp>\n#include <iostream>\n#include <iomanip>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::setw(4) << json::meta() << std::endl;\n}\n

    2. Initialize cget

      cget init\n

    3. Install the library

      cget install nlohmann/json\n

    4. Build

      cmake -S . -B build -DCMAKE_TOOLCHAIN_FILE=cget/cget/cget.cmake\ncmake --build build\n
    "},{"location":"integration/package_managers/#swift-package-manager","title":"Swift Package Manager","text":"

    Summary

    package: nlohmann/json

    • Available versions: current version and older versions
    • The package is updated with every release.
    • File issues at the library issue tracker
    • Xcode documentation

    If you are using the Swift Package Manager, add this repository as a package dependency and depend on its json product:

    dependencies: [\n    .package(url: \"https://github.com/nlohmann/json\", from: \"3.12.0\")\n],\ntargets: [\n    .target(name: \"MyTarget\", dependencies: [.product(name: \"json\", package: \"json\")])\n]\n

    The library's own Package.swift publishes single_include/nlohmann (not single_include) as the public headers directory, so include the header without the nlohmann/ prefix:

    #include <json.hpp>\n
    Example: a minimal executable package

    1. Create the following files (the source file goes into Sources/json_example/, following Swift Package Manager's directory layout convention):

      Package.swift
      // swift-tools-version: 5.9\nimport PackageDescription\n\nlet package = Package(\n    name: \"json_example\",\n    dependencies: [\n        .package(url: \"https://github.com/nlohmann/json\", from: \"3.12.0\")\n    ],\n    targets: [\n        .executableTarget(\n            name: \"json_example\",\n            dependencies: [\n                .product(name: \"json\", package: \"json\")\n            ]\n        )\n    ]\n)\n
      Sources/json_example/example.cpp
      #include <json.hpp>\n#include <iostream>\n#include <iomanip>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::setw(4) << json::meta() << std::endl;\n}\n

    2. Build and run:

      swift run --build-system native\n

    Warning

    On some toolchains, swift run/swift build fail to link an executable target against the header-only json product with an error such as Build input file cannot be found: '.../json.o', because the product itself has no compiled sources; see #4650 and the upstream Swift Package Manager issue. Passing --build-system native (shown above) selects Swift Package Manager's legacy build system, which does not have this problem; depending on the library from a library target instead of an executable is not affected either.

    You can also add the dependency from within Xcode via File \u2192 Add Package Dependencies\u2026 and the same repository URL; see Apple's documentation.

    "},{"location":"integration/package_managers/#nuget","title":"NuGet","text":"

    Summary

    package: nlohmann.json

    • Available versions: current and previous versions
    • The package is updated with every release.
    • Maintainer: Hani Kaabi
    • File issues at the maintainer's issue tracker
    • NuGet website

    If you are using NuGet, you can use the package nlohmann.json with

    dotnet add package nlohmann.json\n

    NuGet integrates with C++ projects through MSBuild, so it is mainly useful for Visual Studio/MSBuild projects; using it as a dependency from other build systems, such as CMake, is possible but more cumbersome than the other package managers on this page.

    Example: Visual Studio project

    1. Right-click the project (any C++ project) in \"Solution Explorer\" and select \"Manage NuGet Packages\u2026\"

    2. Switch to the \"Browse\" tab.

    3. Search for nlohmann.json, select it, and click \"Install\".

    4. #include <nlohmann/json.hpp> in your code and build the project. The package's build/native/nlohmann.json.targets file adds $(MSBuildThisFileDirectory)include to the project's AdditionalIncludeDirectories, so no further include path configuration is needed.

    For further details, see the original discussion this section is based on.

    "},{"location":"integration/package_managers/#conda","title":"Conda","text":"

    Summary

    package: nlohmann_json

    • Available versions: current and previous versions
    • The package is updated with every release.
    • File issues at the feedstock's issue tracker
    • Conda documentation

    If you are using conda, you can use the package nlohmann_json from conda-forge executing

    conda install -c conda-forge nlohmann_json\n
    Example: Raw compilation

    1. Create the following file:

      example.cpp
      #include <nlohmann/json.hpp>\n#include <iostream>\n#include <iomanip>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::setw(4) << json::meta() << std::endl;\n}\n

    2. Create and activate an environment json:

      conda create -n json\nconda activate json\n

    3. Install the package:

      conda install -c conda-forge nlohmann_json\n

    4. Build the code:

      g++ -std=c++11 -I$(conda info --base)/envs/json/include example.cpp -o example\n
    "},{"location":"integration/package_managers/#msys2","title":"MSYS2","text":"

    Summary

    package: mingw-w64-nlohmann-json

    • The package is updated automatically.
    • File issues at the MINGW-packages issue tracker
    • MSYS2 website

    If you are using MSYS2, you can use the mingw-w64-nlohmann-json package; type pacman -S mingw-w64-i686-nlohmann-json or pacman -S mingw-w64-x86_64-nlohmann-json for installation.

    Example: Raw compilation

    1. Create the following file:

      example.cpp
      #include <nlohmann/json.hpp>\n#include <iostream>\n#include <iomanip>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::setw(4) << json::meta() << std::endl;\n}\n

    2. Install the package (from an MSYS2 MinGW 64-bit shell):

      pacman -S mingw-w64-x86_64-nlohmann-json\n

    3. Compile the code:

      g++ example.cpp -std=c++11 -o example\n
    "},{"location":"integration/package_managers/#macports","title":"MacPorts","text":"

    Summary

    port: nlohmann-json

    • Available versions: current version
    • The port is updated with every release.
    • File issues at the MacPorts issue tracker
    • MacPorts website

    If you are using MacPorts, execute 

    sudo port install nlohmann-json\n

    to install the nlohmann-json package.

    Example: Raw compilation

    1. Create the following file:

      example.cpp
      #include <nlohmann/json.hpp>\n#include <iostream>\n#include <iomanip>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::setw(4) << json::meta() << std::endl;\n}\n

    2. Install the package:

      sudo port install nlohmann-json\n

    3. Compile the code and pass the MacPorts prefix to the include path such that the library can be found:

      c++ example.cpp -I/opt/local/include -std=c++11 -o example\n
    Example: CMake

    1. Create the following files:

      example.cpp
      #include <nlohmann/json.hpp>\n#include <iostream>\n#include <iomanip>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::setw(4) << json::meta() << std::endl;\n}\n
      CMakeLists.txt
      cmake_minimum_required(VERSION 3.15)\nproject(json_example)\n\nfind_package(nlohmann_json CONFIG REQUIRED)\n\nadd_executable(json_example example.cpp)\ntarget_link_libraries(json_example PRIVATE nlohmann_json::nlohmann_json)\n

    2. Install the package:

      sudo port install nlohmann-json\n

    3. Compile the code:

      cmake -S . -B build\ncmake --build build\n
    "},{"location":"integration/package_managers/#build2","title":"build2","text":"

    Summary

    package: nlohmann-json library target: nlohmann-json%lib{json} available in package repositories: - cppget.org (recommended) - package's sources (for advanced users)

    • Available versions: current version and older versions since 3.7.3 (see cppget.org)
    • The package is maintained and published by the build2 community in this repository.
    • File issues at the package source repository
    • build2 website

    Note: build2 should not be considered as a standalone package-manager. It is a build-system + package manager + project manager, a set of tools that work hand-in-hand. build2-based projects do not rely on existing CMake scripts and the build scripts defining the project's targets are specific to build2.

    To use this package in an existing build2 project, the general steps are:

    1. Make the package available to download from a package repository that provides it.

      Your project's repositories.manifest specifies where the package manager will try to acquire packages by default. Make sure one of the repositories specified in this file provides nlohmann-json package. The recommended open-source repository is cppget.org.

      If the project has been created using bdep new, cppget.org is already specified in repositories.manifest but commented, just uncomment these lines: 

      :\nrole: prerequisite\nlocation: https://pkg.cppget.org/1/stable\n

    2. Add this package as dependency of your project.

      In your project's manifest add the dependency to the package using depends: nlohmann-json. You could also add some version constraints. For example, to depend on the latest version available: 

      depends: nlohmann-json\n

    3. Add this library as dependency of your target that uses it.

      In the buildfile defining the target that will use this library:

      - import the target `lib{json}` from the `nlohmann-json` package, for example:\n    ```\n    import nljson = nlohmann-json%lib{json}\n    ```\n\n- then add the library's target as requirement for your target using it, for example:\n    ```\n    exe{example} : ... $nljson\n    ```\n

    4. Use the library in your project's code and build it.

      At this point, assuming your project is initialized in a build-configuration, any b or bdep update command that will update/build the project will also acquire the missing dependency automatically, then build it and link it with your target.

      If you just want to synchronize dependencies for all your configurations to download the ones you just added: 

      bdep sync -af\n

      Example: from scratch, using build2's bdep new command

      1. Create a new executable project \"example\" (see bdep new command details for the various options to create a new project):

        bdep new example\n

      2. Edit these files by replacing their content:

        • example/repositories.manifest: Enable acquiring packages from https://cppget.org by uncommenting the related lines:

          project's `repositories.manifest`
          : 1\nsummary: example project repository\n\n:\nrole: prerequisite\nlocation: https://pkg.cppget.org/1/stable\n#trust: ...\n\n#:\n#role: prerequisite\n#location: https://git.build2.org/hello/libhello.git\n

        • example/manifest: Add the latest version of the nlohmann-json package as dependency to the project:

          project's `manifest`
          name: example\nversion: 0.1.0-a.0.z\nlanguage: c++\nsummary: example C++ executable\nlicense: other: proprietary ; Not free/open source.\ndescription-file: README.md\nurl: https://example.org/example\nemail: your@emailprovider.com\n#build-error-email: your@emailprovider.com\ndepends: * build2 >= 0.16.0\ndepends: * bpkg >= 0.16.0\n#depends: libhello ^1.0.0\n\ndepends: nlohmann-json\n

        • example/example/buildfile: import the library's target to be used as requirement for building the executable target exe{example}:

          project's `buildfile`
          libs =\nimport libs = nlohmann-json%lib{json}\n\nexe{example}: {hxx ixx txx cxx}{**} $libs testscript\n\ncxx.poptions =+ \"-I$out_root\" \"-I$src_root\"\n

        • example/example/example.cxx: bdep new generates a \"hello world\" by default, replace it by this:

          example.cxx
          #include <nlohmann/json.hpp>\n#include <iostream>\n#include <iomanip>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::setw(4) << json::meta() << std::endl;\n}\n

        • example/example/testscript: (optional) if you want to be able to test that executable's output is correct using b test:

          `testscript` checking the output of the program
          : json basics\n:\n$* >>~/EOO/\n{\n    \"compiler\": {\n/        \"c\\+\\+\": \"\\d+\",/\n/        \"family\": \"[\\w\\d]+\",/\n/        \"version\": \\d+/\n    },\n/    \"copyright\": \"\\(C\\) 2013-\\d+ Niels Lohmann\",/\n    \"name\": \"JSON for Modern C++\",\n/    \"platform\": \"[\\w\\d]+\",/\n    \"url\": \"https://github.com/nlohmann/json\",\n    \"version\": {\n        \"major\": 3,\n/        \"minor\": \\d+,/\n/        \"patch\": \\d+,/\n/        \"string\": \"3\\.\\d+\\.\\d+\"/\n    }\n}\nEOO\n

      3. Initialize the project in a default C/C++ build configuration directory, then build and test:

        cd example/\n\n# create default C/C++ build configuration in ../example-myconfig/, initialize the project in it (downloads it's dependencies in it too)\nbdep init -C @myconfig cc\n\n# build only,\nb\n\n# or build and test the executable's output, will only work if the `testscript` is correct\nb test\n
      "},{"location":"integration/package_managers/#cpmcmake","title":"CPM.cmake","text":"

      Summary

      package: gh:nlohmann/json

      • Available versions: current and previous versions
      • The package is updated with every release.
      • File issues at the CPM.cmake issue tracker
      • CPM.cmake website

      If you are using CPM.cmake, add the CPM.cmake script and the following snippet to your CMake project:

      CPMAddPackage(\"gh:nlohmann/json@3.12.0\")\n
      Example: CMake with CPMAddPackage

      1. Create the following files:

        example.cpp
        #include <nlohmann/json.hpp>\n#include <iostream>\n#include <iomanip>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::setw(4) << json::meta() << std::endl;\n}\n
        CMakeLists.txt
        cmake_minimum_required(VERSION 3.15)\nproject(json_example)\n\ninclude(${CMAKE_SOURCE_DIR}/cmake/CPM.cmake)\n\nCPMAddPackage(\"gh:nlohmann/json@3.12.0\")\n\nadd_executable(json_example example.cpp)\ntarget_link_libraries(json_example PRIVATE nlohmann_json::nlohmann_json)\n

      2. Download CPM.cmake

        mkdir -p cmake\nwget -O cmake/CPM.cmake https://github.com/cpm-cmake/CPM.cmake/releases/latest/download/get_cpm.cmake\n

      3. Build

        cmake -S . -B build\ncmake --build build\n
      "},{"location":"integration/package_managers/#xmake","title":"xmake","text":"

      Summary

      package: nlohmann_json

      • Available versions: current and previous versions
      • The package is updated with every release.
      • File issues at the xmake issue tracker
      • xmake website
      Example: xmake project

      1. Create the following files:

        example.cpp
        #include <nlohmann/json.hpp>\n#include <iostream>\n#include <iomanip>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::setw(4) << json::meta() << std::endl;\n}\n
        xmake.lua
        add_requires(\"nlohmann_json\")\n\nadd_rules(\"mode.debug\", \"mode.release\")\ntarget(\"xm\")\n    set_kind(\"binary\")\n    add_files(\"example.cpp\")\n    add_packages(\"nlohmann_json\")\n    set_languages(\"cxx11\")\n

      2. Build

        xmake\n

      3. Run

        xmake run\n
      "},{"location":"integration/package_managers/#other-package-managers","title":"Other package managers","text":"

      The library is also contained in many other package repositories; Repology tracks the packaged versions across repositories.

      "},{"location":"integration/package_managers/#buckaroo","title":"Buckaroo","text":"

      If you are using Buckaroo, you can install this library's module with buckaroo add github.com/buckaroo-pm/nlohmann-json. There is a demo repo here.

      Warning

      The module is outdated as the respective repository has not been updated in years.

      "},{"location":"integration/package_managers/#cocoapods","title":"CocoaPods","text":"

      If you are using CocoaPods, you can use the library by adding pod \"nlohmann_json\", '~>3.1.2' to your podfile (see an example). Please file issues at the repository, as its issue tracker is no longer reachable.

      Warning

      The module is outdated as the respective pod has not been updated in years.

      "},{"location":"integration/package_managers/#npm","title":"npm","text":"

      This project does not publish an official npm package. The npm package nlohmann-json (or similarly named packages) is not maintained or endorsed by this project. Use one of the package managers listed above, or integrate the single header directly.

      "},{"location":"integration/package_managers/#esp-idf-and-platformio","title":"ESP-IDF and PlatformIO","text":"

      There is no official package published to the ESP-IDF Component Registry or the PlatformIO Registry. A community-maintained fork, Johboh/nlohmann-json, publishes this library to both registries on each new release and can be used as an unofficial component/package for ESP-IDF and PlatformIO projects. As the library is header-only, it can otherwise be used directly by adding its include/ directory to your component's/project's include paths, like any other integration method described on this page.

      "},{"location":"integration/pkg-config/","title":"Pkg-config","text":"

      If you are using bare Makefiles, you can use pkg-config to generate the include flags that point to where the library is installed:

      pkg-config nlohmann_json --cflags\n

      A pkg-config file is installed by CMake (when the JSON_Install option is enabled, which is the default for a top-level build) as well as by several package managers.

      Users of the Meson build system will also be able to use a system-wide library, which will be found by pkg-config:

      json = dependency('nlohmann_json', required: true)\n
      "}]} \ No newline at end of file +{"config":{"lang":["en"],"separator":"[\\s\\-\\.]","pipeline":["stopWordFilter"],"fields":{"title":{"boost":1000.0},"text":{"boost":1.0},"tags":{"boost":1000000.0}}},"docs":[{"location":"","title":"JSON for Modern C++","text":"

      JSON for Modern C++ is a header-only C++11 library that turns JSON into a first-class C++ data type, using the operator magic of modern C++ so that creating, reading, and modifying JSON values feels as natural as it does in languages like Python. The whole library is available as a single header, json.hpp, with no dependencies, no subproject, and no complex build system to set up; a companion header, json_fwd.hpp, provides forward declarations to keep compile times down. See header-only integration for details. It is heavily unit-tested with 100% code coverage, checked with Valgrind and the Clang Sanitizers for memory leaks, and continuously fuzz-tested by Google OSS-Fuzz.

      "},{"location":"#quick-start","title":"Quick start","text":"

      Add the single header to your project and use the library like this:

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // parse a JSON string\n    json j = json::parse(R\"({\"happy\": true, \"pi\": 3.141})\");\n\n    // access and modify values\n    j[\"name\"] = \"Niels\";\n    j[\"list\"] = {1, 0, 2};\n\n    // serialize with an indent of 4 spaces\n    std::cout << j.dump(4) << '\\n';\n}\n

      Get the library by copying the single header json.hpp from the releases page into a directory nlohmann on your include path, or by installing it with a package manager:

      brew install nlohmann-json    # Homebrew\nvcpkg install nlohmann-json   # vcpkg\n
      find_package(nlohmann_json 3.12.0 REQUIRED)\ntarget_link_libraries(myproject PRIVATE nlohmann_json::nlohmann_json)\n

      See Integration for CMake in detail, all supported package managers (Conan, Meson, Bazel, Conda, and more), and pkg-config.

      "},{"location":"#explore-the-documentation","title":"Explore the documentation","text":"

      • Features

        Creating, parsing, accessing, and serializing JSON values, JSON Pointer/Patch, binary formats, and more.

        Features

      • Integration

        Add the library to your project via a single header, CMake, a package manager, or pkg-config.

        Integration

      • API documentation

        The complete reference for basic_json and its member functions, types, and related classes.

        API documentation

      • FAQ

        Answers to common questions and known surprises when using the library.

        FAQ

      • Releases

        What changed in each release, with links to the relevant documentation.

        Releases

      • Community

        The ecosystem, contribution guidelines, governance, and quality assurance around the project.

        Community

      Unreleased changes

      This documentation is built from the develop branch and may describe changes that are not part of a release yet. Their version numbers are followed by an unreleased badge; see Releases for what shipped in each version.

      The library is licensed under the MIT License. The source code, issue tracker, and discussions are on GitHub.

      "},{"location":"api/json/","title":"nlohmann::json","text":"
      using json = basic_json<>;\n

      This type is the default specialization of the basic_json class which uses the standard template types.

      "},{"location":"api/json/#examples","title":"Examples","text":"Example

      The example below demonstrates how to use the type nlohmann::json.

      #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON object\n    json j =\n    {\n        {\"pi\", 3.141},\n        {\"happy\", true},\n        {\"name\", \"Niels\"},\n        {\"nothing\", nullptr},\n        {\n            \"answer\", {\n                {\"everything\", 42}\n            }\n        },\n        {\"list\", {1, 0, 2}},\n        {\n            \"object\", {\n                {\"currency\", \"USD\"},\n                {\"value\", 42.99}\n            }\n        }\n    };\n\n    // add new values\n    j[\"new\"][\"key\"][\"value\"] = {\"another\", \"list\"};\n\n    // count elements\n    auto s = j.size();\n    j[\"size\"] = s;\n\n    // pretty print with indent of 4 spaces\n    std::cout << std::setw(4) << j << '\\n';\n}\n

      Output:

      {\n    \"answer\": {\n        \"everything\": 42\n    },\n    \"happy\": true,\n    \"list\": [\n        1,\n        0,\n        2\n    ],\n    \"name\": \"Niels\",\n    \"new\": {\n        \"key\": {\n            \"value\": [\n                \"another\",\n                \"list\"\n            ]\n        }\n    },\n    \"nothing\": null,\n    \"object\": {\n        \"currency\": \"USD\",\n        \"value\": 42.99\n    },\n    \"pi\": 3.141,\n    \"size\": 8\n}\n
      "},{"location":"api/json/#see-also","title":"See also","text":"
      • basic_json - the underlying class template
      • ordered_json - specialization that preserves the insertion order of object keys
      "},{"location":"api/json/#version-history","title":"Version history","text":"

      Since version 1.0.0.

      "},{"location":"api/json_document/","title":"nlohmann::json_document","text":"

      Defined in header <nlohmann/json_view.hpp>

      using json_document = basic_json_document<json>;\n

      This type is a basic_json_document of the default json specialization.

      "},{"location":"api/json_document/#examples","title":"Examples","text":"Example

      The example below demonstrates how to use the type nlohmann::json_document.

      #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // json_document is basic_json_document<nlohmann::json>: same value types\n    // and containers as the ordinary json specialization\n    json_document doc = json_document::parse(R\"({\"pi\": 3.14, \"numbers\": [1, 2, 3]})\");\n\n    std::cout << doc.root().is_object() << '\\n';\n    std::cout << doc.root().materialize().dump() << '\\n';\n}\n

      Output:

      true\n{\"numbers\":[1,2,3],\"pi\":3.14}\n
      "},{"location":"api/json_document/#see-also","title":"See also","text":"
      • json_view - a view of a value of a json_document
      • ordered_json_document - the corresponding document for ordered_json
      "},{"location":"api/json_document/#version-history","title":"Version history","text":"

      Since version 3.13.0 unreleased.

      "},{"location":"api/json_editable_document/","title":"nlohmann::json_editable_document","text":"

      Defined in header <nlohmann/json_view.hpp>

      using json_editable_document = basic_json_document<json, true>;\n

      This type is an editable basic_json_document of the default json specialization: in addition to everything json_document offers, set and push_back change values after parsing, without ever rewriting the source text -- see Edits and Editing a document.

      "},{"location":"api/json_editable_document/#examples","title":"Examples","text":"Example

      The example below patches two fields of a small configuration document -- changing one and adding another -- and dumps it back out with the member order and the spelling of the untouched number preserved, something a plain json value cannot do.

      #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json = nlohmann::json;\nusing json_editable_document = nlohmann::json_editable_document;\nusing json_editable_view = nlohmann::json_editable_view;\n\nint main()\n{\n    // a configuration file, as it might be read from disk\n    const std::string text = R\"({\"name\": \"cache\", \"host\": \"db1\", \"port\": 6379, \"price\": 19.90})\";\n    std::cout << text << \"\\n\\n\";\n\n    // patch two fields -- \"price\" is never touched\n    json_editable_document doc = json_editable_document::parse(text);\n    doc.set(doc.root(), \"host\", \"db2\");\n    doc.set(doc.root(), \"retries\", 3);\n\n    // member order (the new member at the end) and the untouched number's\n    // exact spelling survive\n    std::cout << doc.root().dump(-1, ' ', false, json_editable_view::number_format::source) << '\\n';\n\n    // the same patch on a plain json value: keys are sorted (object_t is a\n    // std::map), and \"price\" is rewritten even though the patch never\n    // touched it\n    json plain = json::parse(text);\n    plain[\"host\"] = \"db2\";\n    plain[\"retries\"] = 3;\n    std::cout << plain.dump() << '\\n';\n}\n

      Output:

      {\"name\": \"cache\", \"host\": \"db1\", \"port\": 6379, \"price\": 19.90}\n\n{\"name\":\"cache\",\"host\":\"db2\",\"port\":6379,\"price\":19.90,\"retries\":3}\n{\"host\":\"db2\",\"name\":\"cache\",\"port\":6379,\"price\":19.9,\"retries\":3}\n
      "},{"location":"api/json_editable_document/#see-also","title":"See also","text":"
      • json_editable_view - a view of a value of a json_editable_document
      • json_document - the read-only document this type adds edits to
      • ordered_json_editable_document - the corresponding editable document for ordered_json
      • Edits - what an edit guarantees
      "},{"location":"api/json_editable_document/#version-history","title":"Version history","text":"

      Since version 3.13.0 unreleased.

      "},{"location":"api/json_editable_view/","title":"nlohmann::json_editable_view","text":"

      Defined in header <nlohmann/json_view.hpp>

      using json_editable_view = basic_json_view<json, true>;\n

      This type is a basic_json_view of a value of a json_editable_document. It offers the same read-only interface as json_view; what is different is what it can be a view of -- a value that set and push_back can change, with every view still referring to it seeing the change, see Edits.

      "},{"location":"api/json_editable_view/#examples","title":"Examples","text":"Example

      The example below is the same as json_editable_document's: every view read back out of the document -- doc.root() and the views nested under it -- sees the edits made through set.

      #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json = nlohmann::json;\nusing json_editable_document = nlohmann::json_editable_document;\nusing json_editable_view = nlohmann::json_editable_view;\n\nint main()\n{\n    // a configuration file, as it might be read from disk\n    const std::string text = R\"({\"name\": \"cache\", \"host\": \"db1\", \"port\": 6379, \"price\": 19.90})\";\n    std::cout << text << \"\\n\\n\";\n\n    // patch two fields -- \"price\" is never touched\n    json_editable_document doc = json_editable_document::parse(text);\n    doc.set(doc.root(), \"host\", \"db2\");\n    doc.set(doc.root(), \"retries\", 3);\n\n    // member order (the new member at the end) and the untouched number's\n    // exact spelling survive\n    std::cout << doc.root().dump(-1, ' ', false, json_editable_view::number_format::source) << '\\n';\n\n    // the same patch on a plain json value: keys are sorted (object_t is a\n    // std::map), and \"price\" is rewritten even though the patch never\n    // touched it\n    json plain = json::parse(text);\n    plain[\"host\"] = \"db2\";\n    plain[\"retries\"] = 3;\n    std::cout << plain.dump() << '\\n';\n}\n

      Output:

      {\"name\": \"cache\", \"host\": \"db1\", \"port\": 6379, \"price\": 19.90}\n\n{\"name\":\"cache\",\"host\":\"db2\",\"port\":6379,\"price\":19.90,\"retries\":3}\n{\"host\":\"db2\",\"name\":\"cache\",\"port\":6379,\"price\":19.9,\"retries\":3}\n
      "},{"location":"api/json_editable_view/#see-also","title":"See also","text":"
      • json_editable_document - the document type this view refers into
      • json_view - the corresponding read-only view
      • ordered_json_editable_view - the corresponding view for ordered_json_editable_document
      "},{"location":"api/json_editable_view/#version-history","title":"Version history","text":"

      Since version 3.13.0 unreleased.

      "},{"location":"api/json_view/","title":"nlohmann::json_view","text":"

      Defined in header <nlohmann/json_view.hpp>

      using json_view = basic_json_view<json>;\n

      This type is a basic_json_view of a value of a json_document.

      "},{"location":"api/json_view/#examples","title":"Examples","text":"Example

      The example below demonstrates how to use the type nlohmann::json_view.

      #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // json_view is basic_json_view<nlohmann::json>: a read-only handle\n    // returned by json_document::root()\n    nlohmann::json_document doc = nlohmann::json_document::parse(\"[1, 2, 3]\");\n    nlohmann::json_view v = doc.root();\n\n    std::cout << v.is_array() << ' ' << v.size() << '\\n';\n}\n

      Output:

      true 3\n
      "},{"location":"api/json_view/#see-also","title":"See also","text":"
      • json_document - the document type this view refers into
      • ordered_json_view - the corresponding view for ordered_json_document
      "},{"location":"api/json_view/#version-history","title":"Version history","text":"

      Since version 3.13.0 unreleased.

      "},{"location":"api/operator_gtgt/","title":"nlohmann::operator>>(basic_json)","text":"
      std::istream& operator>>(std::istream& i, basic_json& j);\n

      Deserializes an input stream to a JSON value.

      "},{"location":"api/operator_gtgt/#parameters","title":"Parameters","text":"i (in, out) input stream to read a serialized JSON value from j (in, out) JSON value to write the deserialized input to"},{"location":"api/operator_gtgt/#return-value","title":"Return value","text":"

      the stream i

      "},{"location":"api/operator_gtgt/#exception-safety","title":"Exception safety","text":"

      Strong guarantee: if an exception is thrown, there are no changes in j.

      "},{"location":"api/operator_gtgt/#exceptions","title":"Exceptions","text":"
      • Throws parse_error.101 in case of an unexpected token, or if i has no stream buffer (i.rdbuf() == nullptr, for instance std::istream(nullptr)).
      • If reading from i reaches the end of the input and eofbit is part of i's exceptions() mask, the std::ios_base::failure thrown by i itself propagates instead of a parse_error, the same as it would for the standard library's own extraction operators.
      "},{"location":"api/operator_gtgt/#complexity","title":"Complexity","text":"

      Linear in the length of the input. The parser is a predictive LL(1) parser.

      "},{"location":"api/operator_gtgt/#notes","title":"Notes","text":"

      A UTF-8 byte order mark is silently ignored.

      Invalid Unicode escapes and unpaired surrogates in the input are reported as parse_error.101 with a detailed message.

      operator>> parses exactly one JSON value, so it can be called repeatedly to read a sequence of concatenated JSON values from the same stream:

      json j1, j2;\ninput >> j1;  // parses the first value\ninput >> j2;  // parses the next value\n

      A number must be followed by whitespace

      A number is only terminated by the character that follows it. That character is read from the stream to detect the end of the number, and it is not put back. When a value that is a number is immediately followed by the next value, the first character of that next value is lost:

      std::istringstream input(\"1true\");\njson j1, j2;\ninput >> j1;  // j1 == 1\ninput >> j2;  // throws parse_error.101: the stream now starts at \"rue\"\n

      Separating the values with whitespace avoids this, because the character that is eaten is then the separator:

      std::istringstream input(\"1 true\");\njson j1, j2;\ninput >> j1;  // j1 == 1\ninput >> j2;  // j2 == true\n

      Only numbers are affected. Values ending in a self-delimiting character do not read past themselves, so truefalse, [1][2], {\"a\":1}{\"b\":2}, and \"a\"\"b\" can be read back to back without a separator.

      Define JSON_PRECISE_STREAM_POSITION to 1 to leave the terminating character in the stream instead, so that the stream is positioned right after the value for every value type and no separator is needed. This is tracked in #5340.

      Note that reading concatenated values does not work for JSON Lines (newline-delimited JSON) input -- see that page for why and for the recommended alternative.

      By default, a '\\0' (NUL) byte encountered while reading a value is treated as end of input, rather than as an ordinary (and, outside of a string, invalid) byte; see the FAQ entry for details and the JSON_STRICT_NUL_HANDLING macro to opt into rejecting it instead. Because operator>> only parses a single value and does not require the rest of the stream to be consumed, a NUL byte after a complete value has no effect on operator>> either way; it only matters while a value is still being read.

      Deprecation

      This function replaces function std::istream& operator<<(basic_json& j, std::istream& i) which has been deprecated in version 3.0.0. It will be removed in version 4.0.0. Please replace calls like j << i; with i >> j;.

      See the migration guide for how to update existing code.

      "},{"location":"api/operator_gtgt/#examples","title":"Examples","text":"Example

      The example below shows how a JSON value is constructed by reading a serialization from a stream.

      #include <iostream>\n#include <iomanip>\n#include <sstream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create stream with serialized JSON\n    std::stringstream ss;\n    ss << R\"({\n        \"number\": 23,\n        \"string\": \"Hello, world!\",\n        \"array\": [1, 2, 3, 4, 5],\n        \"boolean\": false,\n        \"null\": null\n    })\";\n\n    // create JSON value and read the serialization from the stream\n    json j;\n    ss >> j;\n\n    // serialize JSON\n    std::cout << std::setw(2) << j << '\\n';\n}\n

      Output:

      {\n  \"array\": [\n    1,\n    2,\n    3,\n    4,\n    5\n  ],\n  \"boolean\": false,\n  \"null\": null,\n  \"number\": 23,\n  \"string\": \"Hello, world!\"\n}\n
      "},{"location":"api/operator_gtgt/#see-also","title":"See also","text":"
      • accept - check if the input is valid JSON
      • parse - deserialize from a compatible input
      • JSON_STRICT_NUL_HANDLING - opt in to rejecting a NUL byte in the input instead of treating it as end of input
      • JSON_PRECISE_STREAM_POSITION - opt in to leaving the stream positioned right after a number
      • JSON_DISABLE_STREAM_BULK_SCAN - read the stream one character at a time
      "},{"location":"api/operator_gtgt/#version-history","title":"Version history","text":"
      • Added in version 1.0.0.
      • JSON_STRICT_NUL_HANDLING added in version 3.13.0 unreleased to optionally reject a NUL byte in the input instead of treating it as end of input; planned to become the default in version 4.0.0.
      • JSON_PRECISE_STREAM_POSITION added in version 3.13.0 unreleased to optionally leave the character that terminates a number in the stream; planned to become the default in version 4.0.0.
      • Fixed a null pointer dereference for an std::istream without a stream buffer (now throws parse_error.101), and a crash (std::terminate) when i has eofbit in its exception mask, in version 3.13.0 unreleased.
      • Changed to the strong exception safety guarantee in version 3.13.0 unreleased: j is no longer left with a partially parsed value if parsing throws.
      • Scans the characters in the stream buffer of i in bulk in version 3.13.0 unreleased; the position of i after the call is unchanged (JSON_DISABLE_STREAM_BULK_SCAN restores reading one character at a time).
      "},{"location":"api/operator_literal_json/","title":"nlohmann::operator\"\"_json","text":"
      json operator \"\"_json(const char* s, std::size_t n);\njson operator \"\"_json(const char8_t* s, std::size_t n);  // since C++20\n

      This operator implements a user-defined string literal for JSON objects. It can be used by adding _json to a string literal and returns a json object if no parse error occurred.

      It is recommended to bring the operator into scope using any of the following lines: 

      using nlohmann::literals::operator \"\"_json;\nusing namespace nlohmann::literals;\nusing namespace nlohmann::json_literals;\nusing namespace nlohmann::literals::json_literals;\nusing namespace nlohmann;\n

      This is suggested to ease migration to the next major version release of the library. See JSON_USE_GLOBAL_UDLS and the migration guide for details. The operator is declared in header <nlohmann/json_literals.hpp>, which <nlohmann/json.hpp> includes unless JSON_NO_AUTOMATIC_UDLS is defined.

      "},{"location":"api/operator_literal_json/#parameters","title":"Parameters","text":"s (in) a string representation of a JSON object n (in) length of string s"},{"location":"api/operator_literal_json/#return-value","title":"Return value","text":"

      json value parsed from s

      "},{"location":"api/operator_literal_json/#exceptions","title":"Exceptions","text":"

      The function can throw anything that parse(s, s+n) would throw.

      "},{"location":"api/operator_literal_json/#complexity","title":"Complexity","text":"

      Linear.

      "},{"location":"api/operator_literal_json/#examples","title":"Examples","text":"Example

      The following code shows how to create JSON values from string literals.

      #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    json j = R\"( {\"hello\": \"world\", \"answer\": 42} )\"_json;\n\n    std::cout << std::setw(2) << j << '\\n';\n}\n

      Output:

      {\n  \"answer\": 42,\n  \"hello\": \"world\"\n}\n
      "},{"location":"api/operator_literal_json/#see-also","title":"See also","text":"
      • Creating JSON values - the article on creating JSON values
      • JSON_NO_AUTOMATIC_UDLS - do not include the user-defined string literals automatically
      "},{"location":"api/operator_literal_json/#version-history","title":"Version history","text":"
      • Added in version 1.0.0.
      • Moved to namespace nlohmann::literals::json_literals in 3.11.0.
      • Added char8_t* overload in version 3.13.0 unreleased.
      "},{"location":"api/operator_literal_json_pointer/","title":"nlohmann::operator\"\"_json_pointer","text":"
      json_pointer operator \"\"_json_pointer(const char* s, std::size_t n);\njson_pointer operator \"\"_json_pointer(const char8_t* s, std::size_t n);  // since C++20\n

      This operator implements a user-defined string literal for JSON Pointers. It can be used by adding _json_pointer to a string literal and returns a json_pointer object if no parse error occurred.

      It is recommended to bring the operator into scope using any of the following lines: 

      using nlohmann::literals::operator \"\"_json_pointer;\nusing namespace nlohmann::literals;\nusing namespace nlohmann::json_literals;\nusing namespace nlohmann::literals::json_literals;\nusing namespace nlohmann;\n
      This is suggested to ease migration to the next major version release of the library. See JSON_USE_GLOBAL_UDLS and the migration guide for details. The operator is declared in header <nlohmann/json_literals.hpp>, which <nlohmann/json.hpp> includes unless JSON_NO_AUTOMATIC_UDLS is defined.

      "},{"location":"api/operator_literal_json_pointer/#parameters","title":"Parameters","text":"s (in) a string representation of a JSON Pointer n (in) length of string s"},{"location":"api/operator_literal_json_pointer/#return-value","title":"Return value","text":"

      json_pointer value parsed from s

      "},{"location":"api/operator_literal_json_pointer/#exceptions","title":"Exceptions","text":"

      The function can throw anything that json_pointer::json_pointer would throw.

      "},{"location":"api/operator_literal_json_pointer/#complexity","title":"Complexity","text":"

      Linear.

      "},{"location":"api/operator_literal_json_pointer/#examples","title":"Examples","text":"Example

      The following code shows how to create JSON Pointers from string literals.

      #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    json j = R\"( {\"hello\": \"world\", \"answer\": 42} )\"_json;\n    auto val = j[\"/hello\"_json_pointer];\n\n    std::cout << std::setw(2) << val << '\\n';\n}\n

      Output:

      \"world\"\n
      "},{"location":"api/operator_literal_json_pointer/#see-also","title":"See also","text":"
      • json_pointer - type to represent JSON Pointers
      • JSON_NO_AUTOMATIC_UDLS - do not include the user-defined string literals automatically
      "},{"location":"api/operator_literal_json_pointer/#version-history","title":"Version history","text":"
      • Added in version 2.0.0.
      • Moved to namespace nlohmann::literals::json_literals in 3.11.0.
      • Added char8_t* overload in version 3.13.0 unreleased.
      "},{"location":"api/operator_ltlt/","title":"nlohmann::operator<<(basic_json), nlohmann::operator<<(json_pointer)","text":"
      std::ostream& operator<<(std::ostream& o, const basic_json& j);      // (1)\n\nstd::ostream& operator<<(std::ostream& o, const json_pointer& ptr);  // (2)\n
      1. Serialize the given JSON value j to the output stream o. The JSON value will be serialized using the dump member function.
        • The indentation of the output can be controlled with the member variable width of the output stream o. For instance, using the manipulator std::setw(4) on o sets the indentation level to 4 and the serialization result is the same as calling dump(4).
        • The indentation character can be controlled with the member variable fill of the output stream o. For instance, the manipulator std::setfill('\\\\t') sets indentation to use a tab character rather than the default space character.
      2. Write a string representation of the given JSON pointer ptr to the output stream o. The string representation is obtained using the to_string member function.
      "},{"location":"api/operator_ltlt/#parameters","title":"Parameters","text":"o (in, out) stream to write to j (in) JSON value to serialize ptr (in) JSON pointer to write"},{"location":"api/operator_ltlt/#return-value","title":"Return value","text":"

      the stream o

      "},{"location":"api/operator_ltlt/#exceptions","title":"Exceptions","text":"
      1. Throws type_error.316 if a string stored inside the JSON value is not UTF-8 encoded. Note that unlike the dump member functions, no error_handler can be set.
      2. None.
      "},{"location":"api/operator_ltlt/#complexity","title":"Complexity","text":"

      Linear.

      "},{"location":"api/operator_ltlt/#notes","title":"Notes","text":"

      Deprecation

      Function std::ostream& operator<<(std::ostream& o, const basic_json& j) replaces function std::ostream& operator>>(const basic_json& j, std::ostream& o) which has been deprecated in version 3.0.0. It will be removed in version 4.0.0. Please replace calls like j >> o; with o << j;.

      See the migration guide for how to update existing code.

      "},{"location":"api/operator_ltlt/#examples","title":"Examples","text":"Example: (1) serialize JSON value to stream

      The example below shows the serialization with different parameters to width to adjust the indentation level.

      #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n\n    // serialize without indentation\n    std::cout << j_object << \"\\n\\n\";\n    std::cout << j_array << \"\\n\\n\";\n\n    // serialize with indentation\n    std::cout << std::setw(4) << j_object << \"\\n\\n\";\n    std::cout << std::setw(2) << j_array << \"\\n\\n\";\n    std::cout << std::setw(1) << std::setfill('\\t') << j_object << \"\\n\\n\";\n}\n

      Output:

      {\"one\":1,\"two\":2}\n\n[1,2,4,8,16]\n\n{\n    \"one\": 1,\n    \"two\": 2\n}\n\n[\n  1,\n  2,\n  4,\n  8,\n  16\n]\n\n{\n    \"one\": 1,\n    \"two\": 2\n}\n
      Example: (2) write JSON pointer to stream

      The example below shows how to write a JSON pointer to a stream.

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON pointer\n    json::json_pointer ptr(\"/foo/bar/baz\");\n\n    // write string representation to stream\n    std::cout << ptr << std::endl;\n}\n

      Output:

      /foo/bar/baz\n
      "},{"location":"api/operator_ltlt/#see-also","title":"See also","text":"
      • dump - serialize to a JSON-formatted string
      • basic_json_view::operator<< - the corresponding operator for basic_json_view
      • Serialization - the serialization article
      "},{"location":"api/operator_ltlt/#version-history","title":"Version history","text":"
      1. Added in version 1.0.0. Added support for indentation character and deprecated std::ostream& operator>>(const basic_json& j, std::ostream& o) in version 3.0.0.
      2. Added in version 3.11.0.
      "},{"location":"api/ordered_json/","title":"nlohmann::ordered_json","text":"
      using ordered_json = basic_json<ordered_map>;\n

      This type preserves the insertion order of object keys.

      "},{"location":"api/ordered_json/#iterator-invalidation","title":"Iterator invalidation","text":"

      The type is based on ordered_map which in turn uses a std::vector to store object elements. Therefore, adding object elements can yield a reallocation in which case all iterators (including the end() iterator) and all references to the elements are invalidated. Also, any iterator or reference after the insertion point will point to the same index, which is now a different value.

      "},{"location":"api/ordered_json/#complexity","title":"Complexity","text":"

      ordered_map has no lookup index: every key-based object operation is a linear scan, so building or parsing an object of n keys costs O(n\u00b2) rather than O(n log n). See ordered_map complexity for the per-operation table and for measured numbers.

      "},{"location":"api/ordered_json/#examples","title":"Examples","text":"Example

      The example below demonstrates how ordered_json preserves the insertion order of object keys.

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing ordered_json = nlohmann::ordered_json;\n\nint main()\n{\n    ordered_json j;\n    j[\"one\"] = 1;\n    j[\"two\"] = 2;\n    j[\"three\"] = 3;\n\n    std::cout << j.dump(2) << '\\n';\n}\n

      Output:

      {\n  \"one\": 1,\n  \"two\": 2,\n  \"three\": 3\n}\n
      "},{"location":"api/ordered_json/#see-also","title":"See also","text":"
      • ordered_map
      • Object Order
      "},{"location":"api/ordered_json/#version-history","title":"Version history","text":"

      Since version 3.9.0.

      "},{"location":"api/ordered_json_document/","title":"nlohmann::ordered_json_document","text":"

      Defined in header <nlohmann/json_view.hpp>

      using ordered_json_document = basic_json_document<ordered_json>;\n

      This type is a basic_json_document of the ordered_json specialization: materialize() on one of its views preserves the insertion order of object keys, instead of sorting them like json_document does.

      "},{"location":"api/ordered_json_document/#examples","title":"Examples","text":"Example

      The example below demonstrates how ordered_json_document preserves the insertion order of object keys when materializing.

      #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing ordered_json_document = nlohmann::ordered_json_document;\n\nint main()\n{\n    // ordered_json_document is basic_json_document<nlohmann::ordered_json>:\n    // materialize() preserves the insertion (source) order of object keys,\n    // instead of sorting them like json_document does\n    ordered_json_document doc = ordered_json_document::parse(R\"({\"z\": 1, \"a\": 2, \"m\": 3})\");\n    std::cout << doc.root().materialize().dump() << '\\n';\n}\n

      Output:

      {\"z\":1,\"a\":2,\"m\":3}\n
      "},{"location":"api/ordered_json_document/#see-also","title":"See also","text":"
      • ordered_json_view - a view of a value of an ordered_json_document
      • json_document - the corresponding document for the default json specialization
      • Object Order
      "},{"location":"api/ordered_json_document/#version-history","title":"Version history","text":"

      Since version 3.13.0 unreleased.

      "},{"location":"api/ordered_json_editable_document/","title":"nlohmann::ordered_json_editable_document","text":"

      Defined in header <nlohmann/json_view.hpp>

      using ordered_json_editable_document = basic_json_document<ordered_json, true>;\n

      This type is an editable basic_json_document of the ordered_json specialization: set and push_back change values after parsing, as for json_editable_document, and materialize() preserves the document order of object members -- including members set added -- instead of sorting them like json_editable_document does.

      "},{"location":"api/ordered_json_editable_document/#examples","title":"Examples","text":"Example

      The example below edits a document with set, then shows that materialize() keeps the member order of the source text (with the new member at the end) for ordered_json_editable_document, where it would sort the members alphabetically for json_editable_document.

      #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing ordered_json_editable_document = nlohmann::ordered_json_editable_document;\n\nint main()\n{\n    // ordered_json_editable_document is basic_json_document<nlohmann::ordered_json, true>\n    ordered_json_editable_document doc = ordered_json_editable_document::parse(R\"({\"z\": 1, \"a\": 2, \"m\": 3})\");\n    doc.set(doc.root(), \"b\", 4); // set() always appends a new member at the end\n\n    // materialize() preserves the document order (with \"b\" at the end),\n    // instead of sorting the keys the way json_editable_document does\n    std::cout << doc.root().materialize().dump() << '\\n';\n}\n

      Output:

      {\"z\":1,\"a\":2,\"m\":3,\"b\":4}\n
      "},{"location":"api/ordered_json_editable_document/#see-also","title":"See also","text":"
      • ordered_json_editable_view - a view of a value of an ordered_json_editable_document
      • ordered_json_document - the read-only document this type adds edits to
      • json_editable_document - the corresponding editable document for the default json specialization
      • Object Order
      • Edits - what an edit guarantees
      "},{"location":"api/ordered_json_editable_document/#version-history","title":"Version history","text":"

      Since version 3.13.0 unreleased.

      "},{"location":"api/ordered_json_editable_view/","title":"nlohmann::ordered_json_editable_view","text":"

      Defined in header <nlohmann/json_view.hpp>

      using ordered_json_editable_view = basic_json_view<ordered_json, true>;\n

      This type is a basic_json_view of a value of an ordered_json_editable_document, the corresponding view for ordered_json_view the way json_editable_view is for json_view.

      "},{"location":"api/ordered_json_editable_view/#examples","title":"Examples","text":"Example

      The example below is the same as ordered_json_editable_document's: the views set returns see the document's member order preserved on materialize(), unlike for a json_editable_document.

      #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing ordered_json_editable_document = nlohmann::ordered_json_editable_document;\n\nint main()\n{\n    // ordered_json_editable_document is basic_json_document<nlohmann::ordered_json, true>\n    ordered_json_editable_document doc = ordered_json_editable_document::parse(R\"({\"z\": 1, \"a\": 2, \"m\": 3})\");\n    doc.set(doc.root(), \"b\", 4); // set() always appends a new member at the end\n\n    // materialize() preserves the document order (with \"b\" at the end),\n    // instead of sorting the keys the way json_editable_document does\n    std::cout << doc.root().materialize().dump() << '\\n';\n}\n

      Output:

      {\"z\":1,\"a\":2,\"m\":3,\"b\":4}\n
      "},{"location":"api/ordered_json_editable_view/#see-also","title":"See also","text":"
      • ordered_json_editable_document - the document type this view refers into
      • ordered_json_view - the corresponding read-only view
      • json_editable_view - the corresponding view for json_editable_document
      "},{"location":"api/ordered_json_editable_view/#version-history","title":"Version history","text":"

      Since version 3.13.0 unreleased.

      "},{"location":"api/ordered_json_view/","title":"nlohmann::ordered_json_view","text":"

      Defined in header <nlohmann/json_view.hpp>

      using ordered_json_view = basic_json_view<ordered_json>;\n

      This type is a basic_json_view of a value of an ordered_json_document.

      "},{"location":"api/ordered_json_view/#examples","title":"Examples","text":"Example

      The example below demonstrates how to use the type nlohmann::ordered_json_view.

      #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nint main()\n{\n    nlohmann::ordered_json_document doc = nlohmann::ordered_json_document::parse(R\"({\"z\": 1, \"a\": 2})\");\n    nlohmann::ordered_json_view v = doc.root();\n\n    std::cout << std::boolalpha << v.is_object() << ' ' << v.size() << '\\n';\n}\n

      Output:

      true 2\n
      "},{"location":"api/ordered_json_view/#see-also","title":"See also","text":"
      • ordered_json_document - the document type this view refers into
      • json_view - the corresponding view for json_document
      "},{"location":"api/ordered_json_view/#version-history","title":"Version history","text":"

      Since version 3.13.0 unreleased.

      "},{"location":"api/ordered_map/","title":"nlohmann::ordered_map","text":"
      template<class Key, class T, class IgnoredLess = std::less<Key>,\n         class Allocator = std::allocator<std::pair<const Key, T>>>\nstruct ordered_map : std::vector<std::pair<const Key, T>, Allocator>;\n

      A minimal map-like container that preserves insertion order for use within nlohmann::ordered_json (nlohmann::basic_json<ordered_map>).

      "},{"location":"api/ordered_map/#template-parameters","title":"Template parameters","text":"Key key type T mapped type IgnoredLess comparison function (ignored and only added to ensure compatibility with std::map) Allocator allocator type"},{"location":"api/ordered_map/#iterator-invalidation","title":"Iterator invalidation","text":"

      The type uses a std::vector to store object elements. Therefore, adding elements can yield a reallocation in which case all iterators (including the end() iterator) and all references to the elements are invalidated.

      When the storage grows, the keys are copied and the mapped values are moved to the new storage. A plain std::vector would copy the whole elements instead, because their const keys make them not nothrow move constructible; for ordered_json, this would be a deep copy of every nested value. The values are only copied if T is not default constructible or not nothrow move assignable.

      "},{"location":"api/ordered_map/#member-types","title":"Member types","text":"
      • key_type - key type (Key)
      • mapped_type - mapped type (T)
      • Container - base container type (std::vector<std::pair<const Key, T>, Allocator>)
      • iterator
      • const_iterator
      • size_type
      • value_type
      • key_compare - key comparison function 
        std::equal_to<Key>  // until C++14\n\nstd::equal_to<>     // since C++14\n
      "},{"location":"api/ordered_map/#member-functions","title":"Member functions","text":"
      • (constructor)
      • (destructor)
      • emplace
      • operator[]
      • at
      • erase
      • count
      • find
      • insert
      "},{"location":"api/ordered_map/#exception-safety","title":"Exception safety","text":"

      emplace, operator[], and insert(value) have the strong exception guarantee: if an exception is thrown (for instance, because copying a key or allocating memory fails), the contents of the container are unchanged.

      "},{"location":"api/ordered_map/#complexity","title":"Complexity","text":"

      Because the elements are stored in a std::vector in insertion order, there is no index to look a key up by. Every key-based operation performs a linear scan over the stored elements. With n denoting the number of elements in the container:

      Operation Complexity Note emplace O(n) scans for an existing key, then appends (amortized O(1)) operator[] O(n) delegates to emplace (non-const) or at (const) at O(n) throws std::out_of_range if the key is not found find O(n) count O(n) the result is always 0 or 1 erase(key) O(n) scan, then move the remaining elements one position down erase(pos), erase(first, last) O(n) moves all elements after the erased range insert(value) O(n) equivalent to emplace insert(first, last) O((n + m) * m) for m inserted elements

      This differs from std::map, where the same operations are O(log n).

      Quadratic cost of building large objects

      Because every insertion scans all elements inserted so far, building an object of n distinct keys costs O(n\u00b2) in total. This applies to filling an ordered_json object key by key as well as to parsing one, since the parser inserts each key as it is read.

      The cost is negligible for the object sizes typically found in configuration files or API payloads, but it grows steeply for machine-generated objects with many thousands of keys. Measured with -O2 -DNDEBUG for parsing a flat object of n keys, relative to nlohmann::json (which uses std::map):

      n json ordered_json factor 2000 0.7 ms 3.6 ms 5\u00d7 4000 0.8 ms 14.0 ms 19\u00d7 8000 1.6 ms 67.8 ms 43\u00d7 16 000 3.3 ms 181.6 ms 54\u00d7

      If key order matters for objects of that size, consider a container with a lookup index, such as nlohmann::fifo_map (integration), as the object type -- see object order.

      "},{"location":"api/ordered_map/#examples","title":"Examples","text":"Example

      The example shows the different behavior of std::map and nlohmann::ordered_map.

      #include <iostream>\n#include <nlohmann/json.hpp>\n\n// simple output function\ntemplate<typename Map>\nvoid output(const char* prefix, const Map& m)\n{\n    std::cout << prefix << \" = { \";\n    for (auto& element : m)\n    {\n        std::cout << element.first << \":\" << element.second << ' ';\n    }\n    std::cout << \"}\" << std::endl;\n}\n\nint main()\n{\n    // create and fill two maps\n    nlohmann::ordered_map<std::string, std::string> m_ordered;\n    m_ordered[\"one\"] = \"eins\";\n    m_ordered[\"two\"] = \"zwei\";\n    m_ordered[\"three\"] = \"drei\";\n\n    std::map<std::string, std::string> m_std;\n    m_std[\"one\"] = \"eins\";\n    m_std[\"two\"] = \"zwei\";\n    m_std[\"three\"] = \"drei\";\n\n    // output: m_ordered is ordered by insertion order, m_std is ordered by key\n    output(\"m_ordered\", m_ordered);\n    output(\"m_std\", m_std);\n\n    // erase and re-add \"one\" key\n    m_ordered.erase(\"one\");\n    m_ordered[\"one\"] = \"eins\";\n\n    m_std.erase(\"one\");\n    m_std[\"one\"] = \"eins\";\n\n    // output: m_ordered shows newly added key at the end; m_std is again ordered by key\n    output(\"m_ordered\", m_ordered);\n    output(\"m_std\", m_std);\n}\n

      Output:

      m_ordered = { one:eins two:zwei three:drei }\nm_std = { one:eins three:drei two:zwei }\nm_ordered = { two:zwei three:drei one:eins }\nm_std = { one:eins three:drei two:zwei }\n
      "},{"location":"api/ordered_map/#see-also","title":"See also","text":"
      • ordered_json
      "},{"location":"api/ordered_map/#version-history","title":"Version history","text":"
      • Added in version 3.9.0 to implement nlohmann::ordered_json.
      • Added key_compare member in version 3.11.0.
      • Changed in version 3.13.0 unreleased: growing the storage moves the mapped values instead of copying them.
      "},{"location":"api/adl_serializer/","title":"nlohmann::adl_serializer","text":"
      template<typename, typename>\nstruct adl_serializer;\n

      Serializer that uses ADL (Argument-Dependent Lookup) to choose to_json/from_json functions from the types' namespaces.

      It is implemented similarly to

      template<typename ValueType>\nstruct adl_serializer {\n    template<typename BasicJsonType>\n    static void to_json(BasicJsonType& j, const T& value) {\n        // calls the \"to_json\" method in T's namespace\n    }\n\n    template<typename BasicJsonType>\n    static void from_json(const BasicJsonType& j, T& value) {\n        // same thing, but with the \"from_json\" method\n    }\n};\n
      "},{"location":"api/adl_serializer/#member-functions","title":"Member functions","text":"
      • from_json - convert a JSON value to any value type
      • to_json - convert any value type to a JSON value
      "},{"location":"api/adl_serializer/#version-history","title":"Version history","text":"
      • Added in version 2.1.0.
      "},{"location":"api/adl_serializer/from_json/","title":"nlohmann::adl_serializer::from_json","text":"
      // (1)\ntemplate<typename BasicJsonType, typename TargetType = ValueType>\nstatic auto from_json(BasicJsonType && j, TargetType& val) noexcept(\n    noexcept(::nlohmann::from_json(std::forward<BasicJsonType>(j), val)))\n-> decltype(::nlohmann::from_json(std::forward<BasicJsonType>(j), val), void())\n\n// (2)\ntemplate<typename BasicJsonType, typename TargetType = ValueType>\nstatic auto from_json(BasicJsonType && j) noexcept(\nnoexcept(::nlohmann::from_json(std::forward<BasicJsonType>(j), detail::identity_tag<TargetType> {})))\n-> decltype(::nlohmann::from_json(std::forward<BasicJsonType>(j), detail::identity_tag<TargetType> {}))\n

      This function is usually called by the get() function of the basic_json class (either explicitly or via the conversion operators).

      1. This function is chosen for default-constructible value types.
      2. This function is chosen for value types which are not default-constructible.
      "},{"location":"api/adl_serializer/from_json/#parameters","title":"Parameters","text":"j (in) JSON value to read from val (out) value to write to"},{"location":"api/adl_serializer/from_json/#return-value","title":"Return value","text":"
      1. (none) -- the converted value is written to the output parameter val.
      2. the JSON value j converted to TargetType
      "},{"location":"api/adl_serializer/from_json/#examples","title":"Examples","text":"Example: (1) Default-constructible type

      The example below shows how a from_json function can be implemented for a user-defined type. This function is called by the adl_serializer when get<ns::person>() is called.

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nnamespace ns\n{\n// a simple struct to model a person\nstruct person\n{\n    std::string name;\n    std::string address;\n    int age;\n};\n} // namespace ns\n\nnamespace ns\n{\nvoid from_json(const json& j, person& p)\n{\n    j.at(\"name\").get_to(p.name);\n    j.at(\"address\").get_to(p.address);\n    j.at(\"age\").get_to(p.age);\n}\n} // namespace ns\n\nint main()\n{\n    json j;\n    j[\"name\"] = \"Ned Flanders\";\n    j[\"address\"] = \"744 Evergreen Terrace\";\n    j[\"age\"] = 60;\n\n    auto p = j.get<ns::person>();\n\n    std::cout << p.name << \" (\" << p.age << \") lives in \" << p.address << std::endl;\n}\n

      Output:

      Ned Flanders (60) lives in 744 Evergreen Terrace\n
      Example: (2) Non-default-constructible type

      The example below shows how a from_json is implemented as part of a specialization of the adl_serializer to realize the conversion of a non-default-constructible type.

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nnamespace ns\n{\n// a simple struct to model a person (not default constructible)\nstruct person\n{\n    person(std::string n, std::string a, int aa)\n        : name(std::move(n)), address(std::move(a)), age(aa)\n    {}\n\n    std::string name;\n    std::string address;\n    int age;\n};\n} // namespace ns\n\nnamespace nlohmann\n{\ntemplate <>\nstruct adl_serializer<ns::person>\n{\n    static ns::person from_json(const json& j)\n    {\n        return {j.at(\"name\"), j.at(\"address\"), j.at(\"age\")};\n    }\n\n    // Here's the catch! You must provide a to_json method! Otherwise, you\n    // will not be able to convert person to json, since you fully\n    // specialized adl_serializer on that type\n    static void to_json(json& j, ns::person p)\n    {\n        j[\"name\"] = p.name;\n        j[\"address\"] = p.address;\n        j[\"age\"] = p.age;\n    }\n};\n} // namespace nlohmann\n\nint main()\n{\n    json j;\n    j[\"name\"] = \"Ned Flanders\";\n    j[\"address\"] = \"744 Evergreen Terrace\";\n    j[\"age\"] = 60;\n\n    auto p = j.get<ns::person>();\n\n    std::cout << p.name << \" (\" << p.age << \") lives in \" << p.address << std::endl;\n}\n

      Output:

      Ned Flanders (60) lives in 744 Evergreen Terrace\n
      "},{"location":"api/adl_serializer/from_json/#see-also","title":"See also","text":"
      • to_json
      "},{"location":"api/adl_serializer/from_json/#version-history","title":"Version history","text":"
      • Added in version 2.1.0.
      "},{"location":"api/adl_serializer/to_json/","title":"nlohmann::adl_serializer::to_json","text":"
      template<typename BasicJsonType, typename TargetType = ValueType>\nstatic auto to_json(BasicJsonType& j, TargetType && val) noexcept(\n    noexcept(::nlohmann::to_json(j, std::forward<TargetType>(val))))\n-> decltype(::nlohmann::to_json(j, std::forward<TargetType>(val)), void())\n

      This function is usually called by the constructors of the basic_json class.

      "},{"location":"api/adl_serializer/to_json/#parameters","title":"Parameters","text":"j (out) JSON value to write to val (in) value to read from"},{"location":"api/adl_serializer/to_json/#examples","title":"Examples","text":"Example

      The example below shows how a to_json function can be implemented for a user-defined type. This function is called by the adl_serializer when the constructor basic_json(ns::person) is called.

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nnamespace ns\n{\n// a simple struct to model a person\nstruct person\n{\n    std::string name;\n    std::string address;\n    int age;\n};\n} // namespace ns\n\nnamespace ns\n{\nvoid to_json(json& j, const person& p)\n{\n    j = json{ {\"name\", p.name}, {\"address\", p.address}, {\"age\", p.age} };\n}\n} // namespace ns\n\nint main()\n{\n    ns::person p = {\"Ned Flanders\", \"744 Evergreen Terrace\", 60};\n\n    json j = p;\n\n    std::cout << j << std::endl;\n}\n

      Output:

      {\"address\":\"744 Evergreen Terrace\",\"age\":60,\"name\":\"Ned Flanders\"}\n
      "},{"location":"api/adl_serializer/to_json/#see-also","title":"See also","text":"
      • from_json
      "},{"location":"api/adl_serializer/to_json/#version-history","title":"Version history","text":"
      • Added in version 2.1.0.
      "},{"location":"api/basic_json/","title":"nlohmann::basic_json","text":"

      Defined in header <nlohmann/json.hpp>

      template<\n    template<typename U, typename V, typename... Args> class ObjectType = std::map,\n    template<typename U, typename... Args> class ArrayType = std::vector,\n    class StringType = std::string,\n    class BooleanType = bool,\n    class NumberIntegerType = std::int64_t,\n    class NumberUnsignedType = std::uint64_t,\n    class NumberFloatType = double,\n    template<typename U> class AllocatorType = std::allocator,\n    template<typename T, typename SFINAE = void> class JSONSerializer = adl_serializer,\n    class BinaryType = std::vector<std::uint8_t>,\n    class CustomBaseClass = void\n>\nclass basic_json;\n
      "},{"location":"api/basic_json/#template-parameters","title":"Template parameters","text":"Template parameter Description Derived type ObjectType type for JSON objects object_t ArrayType type for JSON arrays array_t StringType type for JSON strings and object keys string_t BooleanType type for JSON booleans boolean_t NumberIntegerType type for JSON integer numbers number_integer_t NumberUnsignedType type for JSON unsigned integer numbers number_unsigned_t NumberFloatType type for JSON floating-point numbers number_float_t AllocatorType type of the allocator to use JSONSerializer the serializer to resolve internal calls to to_json() and from_json() json_serializer BinaryType type for binary arrays binary_t CustomBaseClass extension point for user code json_base_class_t

      The library imposes a number of requirements on these types that are not expressed as C++ concepts, such as the container operations object_t and array_t must provide, or the fact that StringType must be char-based. They are collected in Template Parameter Requirements.

      "},{"location":"api/basic_json/#specializations","title":"Specializations","text":"
      • json - default specialization
      • ordered_json - a specialization that maintains the insertion order of object keys
      "},{"location":"api/basic_json/#iterator-invalidation","title":"Iterator invalidation","text":"

      All operations that add values to an array (push_back , operator+=, emplace_back, insert, and operator[] for a non-existing index) can yield a reallocation, in which case all iterators (including the end() iterator) and all references to the elements are invalidated.

      For ordered_json, also all operations that add a value to an object (push_back, operator+=, emplace, insert, update, and operator[] for a non-existing key) can yield a reallocation, in which case all iterators (including the end() iterator) and all references to the elements are invalidated.

      "},{"location":"api/basic_json/#requirements","title":"Requirements","text":"

      The class satisfies the following concept requirements:

      "},{"location":"api/basic_json/#basic","title":"Basic","text":"
      • DefaultConstructible: JSON values can be default-constructed. The result will be a JSON null value.
      • MoveConstructible: A JSON value can be constructed from an rvalue argument.
      • CopyConstructible: A JSON value can be copy-constructed from an lvalue expression.
      • MoveAssignable: A JSON value can be assigned from an rvalue argument.
      • CopyAssignable: A JSON value can be copy-assigned from an lvalue expression.
      • Destructible: JSON values can be destructed.
      "},{"location":"api/basic_json/#layout","title":"Layout","text":"
      • StandardLayoutType: JSON values have standard layout: All non-static data members are private and standard layout types, the class has no virtual functions or (virtual) base classes.
      "},{"location":"api/basic_json/#library-wide","title":"Library-wide","text":"
      • EqualityComparable: JSON values can be compared with ==, see operator==.
      • LessThanComparable: JSON values can be compared with <, see operator<.
      • Swappable: Any JSON lvalue or rvalue of can be swapped with any lvalue or rvalue of other compatible types, using unqualified function swap.
      • NullablePointer: JSON values can be compared against std::nullptr_t objects which are used to model the null value.
      "},{"location":"api/basic_json/#container","title":"Container","text":"
      • Container: JSON values can be used like STL containers and provide iterator access.
      • ReversibleContainer: JSON values can be used like STL containers and provide reverse iterator access.
      "},{"location":"api/basic_json/#member-types","title":"Member types","text":"
      • adl_serializer - the default serializer
      • value_t - the JSON type enumeration
      • json_pointer - JSON Pointer implementation
      • json_serializer - type of the serializer to for conversions from/to JSON
      • error_handler_t - type to choose behavior on decoding errors
      • cbor_tag_handler_t - type to choose how to handle CBOR tags
      • initializer_list_t - type for initializer lists of basic_json values
      • input_format_t - type to choose the format to parse
      • json_sax_t - type for SAX events
      • with_object_t, with_array_t, with_string_t, with_boolean_t, with_integers_t, with_float_t, with_allocator_t, with_json_serializer_t, with_binary_t, with_base_class_t - types to create a basic_json type with one (or two) replaced template parameters
      "},{"location":"api/basic_json/#exceptions","title":"Exceptions","text":"
      • exception - general exception of the basic_json class
        • parse_error - exception indicating a parse error
        • invalid_iterator - exception indicating errors with iterators
        • type_error - exception indicating executing a member function with a wrong type
        • out_of_range - exception indicating access out of the defined range
        • other_error - exception indicating other library errors
      "},{"location":"api/basic_json/#container-types","title":"Container types","text":"Type Definition value_type basic_json reference value_type& const_reference const value_type& difference_type std::ptrdiff_t size_type std::size_t allocator_type AllocatorType<basic_json> pointer std::allocator_traits<allocator_type>::pointer const_pointer std::allocator_traits<allocator_type>::const_pointer iterator LegacyBidirectionalIterator const_iterator constant LegacyBidirectionalIterator reverse_iterator reverse iterator, derived from iterator const_reverse_iterator reverse iterator, derived from const_iterator iteration_proxy helper type for items function"},{"location":"api/basic_json/#json-value-data-types","title":"JSON value data types","text":"
      • array_t - type for arrays
      • binary_t - type for binary arrays
      • boolean_t - type for booleans
      • default_object_comparator_t - default comparator for objects
      • number_float_t - type for numbers (floating-point)
      • number_integer_t - type for numbers (integer)
      • number_unsigned_t - type for numbers (unsigned)
      • object_comparator_t - comparator for objects
      • object_t - type for objects
      • string_t - type for strings
      "},{"location":"api/basic_json/#parser-callback","title":"Parser callback","text":"
      • parse_event_t - parser event types
      • parser_callback_t - per-element parser callback type
      "},{"location":"api/basic_json/#member-functions","title":"Member functions","text":"
      • (constructor)
      • (destructor)
      • operator= - copy assignment
      • array (static) - explicitly create an array
      • binary (static) - explicitly create a binary array
      • object (static) - explicitly create an object
      "},{"location":"api/basic_json/#object-inspection","title":"Object inspection","text":"

      Functions to inspect the type of a JSON value.

      • type - return the type of the JSON value
      • operator value_t - return the type of the JSON value
      • type_name - return the type as string
      • is_primitive - return whether the type is primitive
      • is_structured - return whether the type is structured
      • is_null - return whether the value is null
      • is_boolean - return whether the value is a boolean
      • is_number - return whether the value is a number
      • is_number_integer - return whether the value is an integer number
      • is_number_unsigned - return whether the value is an unsigned integer number
      • is_number_float - return whether the value is a floating-point number
      • is_object - return whether the value is an object
      • is_array - return whether the value is an array
      • is_string - return whether the value is a string
      • is_binary - return whether the value is a binary array
      • is_discarded - return whether the value is discarded

      Optional functions to access the diagnostic positions.

      • start_pos - return the start position of the value
      • end_pos - return the one past the end position of the value
      "},{"location":"api/basic_json/#value-access","title":"Value access","text":"

      Direct access to the stored value of a JSON value.

      • get - get a value
      • get_to - get a value and write it to a destination
      • get_ptr - get a pointer value
      • get_ref - get a reference value
      • operator ValueType - get a value
      • get_binary - get a binary value
      • as_base_class - access the custom base class
      "},{"location":"api/basic_json/#element-access","title":"Element access","text":"

      Access to the JSON value

      • at - access specified element with bounds checking
      • operator[] - access specified element
      • value - access specified object element with default value
      • front - access the first element
      • back - access the last element
      "},{"location":"api/basic_json/#lookup","title":"Lookup","text":"
      • find - find an element in a JSON object
      • count - returns the number of occurrences of a key in a JSON object
      • contains - check the existence of an element in a JSON object
      "},{"location":"api/basic_json/#iterators","title":"Iterators","text":"
      • begin - returns an iterator to the first element
      • cbegin - returns a const iterator to the first element
      • end - returns an iterator to one past the last element
      • cend - returns a const iterator to one past the last element
      • rbegin - returns an iterator to the reverse-beginning
      • rend - returns an iterator to the reverse-end
      • crbegin - returns a const iterator to the reverse-beginning
      • crend - returns a const iterator to the reverse-end
      • items - wrapper to access iterator member functions in range-based for
      "},{"location":"api/basic_json/#capacity","title":"Capacity","text":"
      • empty - checks whether the container is empty
      • size - returns the number of elements
      • max_size - returns the maximum possible number of elements
      "},{"location":"api/basic_json/#modifiers","title":"Modifiers","text":"
      • clear - clears the contents
      • push_back - add a value to an array/object
      • operator+= - add a value to an array/object
      • emplace_back - add a value to an array
      • emplace - add a value to an object if a key does not exist
      • erase - remove elements
      • insert - inserts elements
      • update - updates a JSON object from another object, overwriting existing keys
      • swap - exchanges the values
      "},{"location":"api/basic_json/#lexicographical-comparison-operators","title":"Lexicographical comparison operators","text":"
      • operator== - comparison: equal
      • operator!= - comparison: not equal
      • operator< - comparison: less than
      • operator> - comparison: greater than
      • operator<= - comparison: less than or equal
      • operator>= - comparison: greater than or equal
      • operator<=> - comparison: 3-way
      "},{"location":"api/basic_json/#serialization-dumping","title":"Serialization / Dumping","text":"
      • dump - serialization
      "},{"location":"api/basic_json/#deserialization-parsing","title":"Deserialization / Parsing","text":"
      • parse (static) - deserialize from a compatible input
      • accept (static) - check if the input is valid JSON
      • sax_parse (static) - generate SAX events
      "},{"location":"api/basic_json/#json-pointer-functions","title":"JSON Pointer functions","text":"
      • flatten - return flattened JSON value
      • unflatten - unflatten a previously flattened JSON value
      "},{"location":"api/basic_json/#json-patch-functions","title":"JSON Patch functions","text":"
      • patch - applies a JSON patch
      • patch_inplace - applies a JSON patch in place
      • diff (static) - creates a diff as a JSON patch
      "},{"location":"api/basic_json/#json-merge-patch-functions","title":"JSON Merge Patch functions","text":"
      • merge_patch - applies a JSON Merge Patch
      • merge_diff (static) - creates a diff as a JSON Merge Patch
      "},{"location":"api/basic_json/#static-functions","title":"Static functions","text":"
      • meta - returns version information on the library
      • get_allocator - returns the allocator associated with the container
      "},{"location":"api/basic_json/#binary-formats","title":"Binary formats","text":"
      • from_bjdata (static) - create a JSON value from an input in BJData format
      • from_bon8 (static) - create a JSON value from an input in BON8 format
      • from_bson (static) - create a JSON value from an input in BSON format
      • from_cbor (static) - create a JSON value from an input in CBOR format
      • from_msgpack (static) - create a JSON value from an input in MessagePack format
      • from_ubjson (static) - create a JSON value from an input in UBJSON format
      • to_bjdata (static) - create a BJData serialization of a given JSON value
      • to_bon8 (static) - create a BON8 serialization of a given JSON value
      • to_bson (static) - create a BSON serialization of a given JSON value
      • to_cbor (static) - create a CBOR serialization of a given JSON value
      • to_msgpack (static) - create a MessagePack serialization of a given JSON value
      • to_ubjson (static) - create a UBJSON serialization of a given JSON value
      "},{"location":"api/basic_json/#non-member-functions","title":"Non-member functions","text":"
      • operator<<(std::ostream&) - serialize to stream
      • operator>>(std::istream&) - deserialize from stream
      • to_string - user-defined to_string function for JSON values
      • format_as - user-defined format_as function for JSON values (fmt support)
      "},{"location":"api/basic_json/#literals","title":"Literals","text":"
      • operator\"\"_json - user-defined string literal for JSON values
      "},{"location":"api/basic_json/#helper-classes","title":"Helper classes","text":"
      • std::formatter<basic_json> - make JSON values formattable with std::format
      • std::hash<basic_json> - return a hash value for a JSON object
      • std::swap<basic_json> - exchanges the values of two JSON objects
      "},{"location":"api/basic_json/#examples","title":"Examples","text":"Example

      The example shows how the library is used.

      #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON object\n    json j =\n    {\n        {\"pi\", 3.141},\n        {\"happy\", true},\n        {\"name\", \"Niels\"},\n        {\"nothing\", nullptr},\n        {\n            \"answer\", {\n                {\"everything\", 42}\n            }\n        },\n        {\"list\", {1, 0, 2}},\n        {\n            \"object\", {\n                {\"currency\", \"USD\"},\n                {\"value\", 42.99}\n            }\n        }\n    };\n\n    // add new values\n    j[\"new\"][\"key\"][\"value\"] = {\"another\", \"list\"};\n\n    // count elements\n    auto s = j.size();\n    j[\"size\"] = s;\n\n    // pretty print with indent of 4 spaces\n    std::cout << std::setw(4) << j << '\\n';\n}\n

      Output:

      {\n    \"answer\": {\n        \"everything\": 42\n    },\n    \"happy\": true,\n    \"list\": [\n        1,\n        0,\n        2\n    ],\n    \"name\": \"Niels\",\n    \"new\": {\n        \"key\": {\n            \"value\": [\n                \"another\",\n                \"list\"\n            ]\n        }\n    },\n    \"nothing\": null,\n    \"object\": {\n        \"currency\": \"USD\",\n        \"value\": 42.99\n    },\n    \"pi\": 3.141,\n    \"size\": 8\n}\n
      "},{"location":"api/basic_json/#see-also","title":"See also","text":"
      • RFC 8259: The JavaScript Object Notation (JSON) Data Interchange Format
      "},{"location":"api/basic_json/#version-history","title":"Version history","text":"
      • Added in version 1.0.0.
      "},{"location":"api/basic_json/accept/","title":"nlohmann::basic_json::accept","text":"
      // (1)\ntemplate<typename InputType>\nstatic bool accept(InputType&& i,\n                   const bool ignore_comments = false,\n                   const bool ignore_trailing_commas = false);\n\n// (2)\ntemplate<typename IteratorType, typename SentinelType = IteratorType>\nstatic bool accept(IteratorType first, SentinelType last,\n                   const bool ignore_comments = false,\n                   const bool ignore_trailing_commas = false);\n

      Checks whether the input is valid JSON.

      1. Reads from a compatible input.

      2. Reads from a pair of character iterators, or an iterator and a sentinel of a different type (C++20 ranges support)

        The value_type of the iterator must be an integral type with a size of 1, 2, or 4 bytes, which will be interpreted respectively as UTF-8, UTF-16, and UTF-32. If SentinelType differs from IteratorType, it must be comparable to the iterator type with operator!=.

      Unlike the parse() function, this function neither throws an exception in case of invalid JSON input (i.e., a parse error) nor creates diagnostic information.

      "},{"location":"api/basic_json/accept/#template-parameters","title":"Template parameters","text":"InputType

      A compatible input, for instance:

      • an std::istream object
      • a FILE pointer (throws if null)
      • a C-style array of characters
      • a pointer to a null-terminated string of single byte characters (throws if null)
      • a std::string
      • a container obj for which begin(obj) and end(obj) produce a valid pair of iterators (as found via ADL or member functions, with semantics compatible to std::begin and std::end)
      IteratorType

      a compatible iterator type, for instance.

      • a pair of std::string::iterator or std::vector<std::uint8_t>::iterator
      • a pair of pointers such as ptr and ptr + len
      SentinelType

      defaults to IteratorType; may be a different type comparable to IteratorType via operator!=, for instance.

      • a custom sentinel type for C++20 ranges
      • std::default_sentinel_t, when IteratorType is std::counted_iterator
      "},{"location":"api/basic_json/accept/#parameters","title":"Parameters","text":"i (in) Input to parse from. ignore_comments (in) whether comments should be ignored and treated like whitespace (true) or yield a parse error (false); (optional, false by default) ignore_trailing_commas (in) whether trailing commas in arrays or objects should be ignored and treated like whitespace (true) or yield a parse error (false); (optional, false by default) first (in) iterator to the start of the character range last (in) iterator to the end of the character range, or a sentinel value that compares equal to the end iterator with operator!="},{"location":"api/basic_json/accept/#return-value","title":"Return value","text":"

      Whether the input is valid JSON.

      "},{"location":"api/basic_json/accept/#exception-safety","title":"Exception safety","text":"

      Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

      "},{"location":"api/basic_json/accept/#exceptions","title":"Exceptions","text":"

      Throws parse_error.101 in case of an empty input like a null FILE* or char* pointer.

      "},{"location":"api/basic_json/accept/#complexity","title":"Complexity","text":"

      Linear in the length of the input. The parser is a predictive LL(1) parser.

      "},{"location":"api/basic_json/accept/#notes","title":"Notes","text":"

      A UTF-8 byte order mark is silently ignored.

      By default, a '\\0' (NUL) byte anywhere in the input is treated as end of input, rather than as an ordinary (and, outside of a string, invalid) byte; see the FAQ entry for details and the JSON_STRICT_NUL_HANDLING macro to opt into rejecting it instead.

      "},{"location":"api/basic_json/accept/#examples","title":"Examples","text":"Example: (1) reading from a string

      The example below demonstrates the accept() function reading from a string.

      #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // a valid JSON text\n    auto valid_text = R\"(\n    {\n        \"numbers\": [1, 2, 3]\n    }\n    )\";\n\n    // an invalid JSON text\n    auto invalid_text = R\"(\n    {\n        \"strings\": [\"extra\", \"comma\", ]\n    }\n    )\";\n\n    std::cout << std::boolalpha\n              << json::accept(valid_text) << ' '\n              << json::accept(invalid_text) << '\\n';\n}\n

      Output:

      true false\n
      Example: (2) reading from an iterator pair

      The example below demonstrates the accept() function reading from an iterator pair. Only the first call covers exactly the JSON text; the second one also covers the trailing bytes and is therefore rejected.

      #include <iostream>\n#include <vector>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // a buffer containing a JSON text followed by more data\n    std::vector<std::uint8_t> input = {'[', '1', ',', '2', ',', '3', ']', 'o', 't', 'h', 'e', 'r'};\n\n    std::cout << std::boolalpha\n              << json::accept(input.begin(), input.begin() + 7) << ' '\n              << json::accept(input.begin(), input.end()) << '\\n';\n}\n

      Output:

      true false\n
      "},{"location":"api/basic_json/accept/#see-also","title":"See also","text":"
      • parse - deserialize from a compatible input
      • sax_parse - parse input using the SAX interface
      • operator>> - deserialize from stream
      • JSON_STRICT_NUL_HANDLING - opt in to rejecting a NUL byte in the input instead of treating it as end of input
      • JSON_DISABLE_STREAM_BULK_SCAN - read a std::istream one character at a time
      • JSON_DISABLE_UNLOCKED_FILE_IO - read a FILE* with the locking stdio functions
      "},{"location":"api/basic_json/accept/#version-history","title":"Version history","text":"
      • Added in version 3.0.0.
      • Ignoring comments via ignore_comments added in version 3.9.0.
      • Changed runtime assertion in case of FILE* null pointers to exception in version 3.12.0.
      • Added ignore_trailing_commas in version 3.13.0 unreleased.
      • Extended container support (1) to include types with lvalue-only ADL begin/end (matching std::begin/std::end semantics) in version 3.13.0 unreleased.
      • Extended overload (2) to accept heterogeneous iterator+sentinel pairs (C++20 ranges support) in version 3.13.0 unreleased.
      • JSON_STRICT_NUL_HANDLING added in version 3.13.0 unreleased to optionally reject a NUL byte in the input instead of treating it as end of input; planned to become the default in version 4.0.0.
      • Parsing from an std::istream scans the characters in its stream buffer in bulk in version 3.13.0 unreleased (JSON_DISABLE_STREAM_BULK_SCAN).
      • Parsing from a FILE* locks the file once instead of once per character in version 3.13.0 unreleased (JSON_DISABLE_UNLOCKED_FILE_IO).

      Deprecation

      Overload (2) replaces calls to accept with a pair of iterators as their first parameter which has been deprecated in version 3.8.0. This overload will be removed in version 4.0.0. Please replace all calls like accept({ptr, ptr+len}, ...); with accept(ptr, ptr+len, ...);.

      You should be warned by your compiler with a -Wdeprecated-declarations warning if you are using a deprecated function.

      See the migration guide for how to update existing code.

      "},{"location":"api/basic_json/array/","title":"nlohmann::basic_json::array","text":"
      static basic_json array(initializer_list_t init = {});\n

      Creates a JSON array value from a given initializer list. That is, given a list of values a, b, c, creates the JSON value [a, b, c]. If the initializer list is empty, the empty array [] is created.

      "},{"location":"api/basic_json/array/#parameters","title":"Parameters","text":"init (in) initializer list with JSON values to create an array from (optional)"},{"location":"api/basic_json/array/#return-value","title":"Return value","text":"

      JSON array value

      "},{"location":"api/basic_json/array/#exception-safety","title":"Exception safety","text":"

      Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

      "},{"location":"api/basic_json/array/#complexity","title":"Complexity","text":"

      Linear in the size of init.

      "},{"location":"api/basic_json/array/#notes","title":"Notes","text":"

      This function is only needed to express two edge cases that cannot be realized with the initializer list constructor (basic_json(initializer_list_t, bool, value_t)). These cases are:

      1. creating an array whose elements are all pairs whose first element is a string -- in this case, the initializer list constructor would create an object, taking the first elements as keys
      2. creating an empty array -- passing the empty initializer list to the initializer list constructor yields an empty object
      "},{"location":"api/basic_json/array/#examples","title":"Examples","text":"Example

      The following code shows an example for the array function.

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON arrays\n    json j_no_init_list = json::array();\n    json j_empty_init_list = json::array({});\n    json j_nonempty_init_list = json::array({1, 2, 3, 4});\n    json j_list_of_pairs = json::array({ {\"one\", 1}, {\"two\", 2} });\n\n    // serialize the JSON arrays\n    std::cout << j_no_init_list << '\\n';\n    std::cout << j_empty_init_list << '\\n';\n    std::cout << j_nonempty_init_list << '\\n';\n    std::cout << j_list_of_pairs << '\\n';\n}\n

      Output:

      []\n[]\n[1,2,3,4]\n[[\"one\",1],[\"two\",2]]\n
      "},{"location":"api/basic_json/array/#see-also","title":"See also","text":"
      • basic_json(initializer_list_t) - create a JSON value from an initializer list
      • object - create a JSON object value from an initializer list
      • Creating JSON values - the article on creating JSON values
      "},{"location":"api/basic_json/array/#version-history","title":"Version history","text":"
      • Added in version 1.0.0.
      "},{"location":"api/basic_json/array_t/","title":"nlohmann::basic_json::array_t","text":"
      using array_t = ArrayType<basic_json, AllocatorType<basic_json>>;\n

      The type used to store JSON arrays.

      RFC 8259 describes JSON arrays as follows:

      An array is an ordered sequence of zero or more values.

      To store objects in C++, a type is defined by the template parameters explained below.

      "},{"location":"api/basic_json/array_t/#template-parameters","title":"Template parameters","text":"ArrayType container type to store arrays. It must be a vector-like container: the library uses operator[], at(), and resize(), and requires random-access iterators. std::vector and std::deque qualify; std::list does not. See Template Parameter Requirements for the full list of requirements. AllocatorType the allocator to use for objects (e.g., std::allocator)"},{"location":"api/basic_json/array_t/#notes","title":"Notes","text":""},{"location":"api/basic_json/array_t/#default-type","title":"Default type","text":"

      With the default values for ArrayType (std::vector) and AllocatorType (std::allocator), the default type for array_t is:

      std::vector<\n  basic_json, // value_type\n  std::allocator<basic_json> // allocator_type\n>\n
      "},{"location":"api/basic_json/array_t/#limits","title":"Limits","text":"

      RFC 8259 specifies:

      An implementation may set limits on the maximum depth of nesting.

      In this class, the array's limit of nesting is not explicitly constrained. However, a maximum depth of nesting may be introduced by the compiler or runtime environment. A theoretical limit can be queried by calling the max_size function of a JSON array.

      "},{"location":"api/basic_json/array_t/#storage","title":"Storage","text":"

      Arrays are stored as pointers in a basic_json type. That is, for any access to array values, a pointer of type array_t* must be dereferenced.

      "},{"location":"api/basic_json/array_t/#examples","title":"Examples","text":"Example

      The following code shows that array_t is by default, a typedef to std::vector<nlohmann::json>.

      #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::boolalpha << std::is_same<std::vector<json>, json::array_t>::value << std::endl;\n}\n

      Output:

      true\n
      "},{"location":"api/basic_json/array_t/#see-also","title":"See also","text":"
      • object_t the type used to store JSON objects
      • binary_t the type used to store binary values
      • is_array checks whether the JSON value is an array
      • max_size returns the maximum possible number of elements
      "},{"location":"api/basic_json/array_t/#version-history","title":"Version history","text":"
      • Added in version 1.0.0.
      • Made capacity() optional, so that array types such as std::deque can be used, in version 3.13.0 unreleased.
      "},{"location":"api/basic_json/as_base_class/","title":"nlohmann::basic_json::as_base_class","text":"
      json_base_class_t& as_base_class() noexcept;\nconst json_base_class_t& as_base_class() const noexcept;\n

      Returns a reference to this object as its custom base class json_base_class_t. No copy is made.

      Since basic_json derives from json_base_class_t, a member of basic_json hides any member of the custom base class with the same name. This function makes such hidden members accessible again.

      "},{"location":"api/basic_json/as_base_class/#return-value","title":"Return value","text":"

      reference to this object as json_base_class_t

      "},{"location":"api/basic_json/as_base_class/#exception-safety","title":"Exception safety","text":"

      No-throw guarantee: this function never throws exceptions.

      "},{"location":"api/basic_json/as_base_class/#complexity","title":"Complexity","text":"

      Constant.

      "},{"location":"api/basic_json/as_base_class/#notes","title":"Notes","text":"

      The function is equivalent to static_cast<json_base_class_t&>(j) (or static_cast<const json_base_class_t&>(j)).

      "},{"location":"api/basic_json/as_base_class/#examples","title":"Examples","text":"Example

      The example shows how to use as_base_class to access members of the custom base class that are hidden by members of basic_json.

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nclass base_class_with_hidden_members\n{\n  public:\n    const char* type_name() const noexcept\n    {\n        return \"my_type_name\";\n    }\n\n    std::size_t size() const noexcept\n    {\n        return 42;\n    }\n};\n\nusing json = nlohmann::json::with_base_class_t<base_class_with_hidden_members>;\n\nint main()\n{\n    json j = {1, 2, 3};\n\n    // the members of basic_json hide the members of the base class\n    std::cout << j.type_name() << ' ' << j.size() << '\\n';\n\n    // access the hidden members of the base class\n    std::cout << j.as_base_class().type_name() << ' ' << j.as_base_class().size() << '\\n';\n}\n

      Output:

      array 3\nmy_type_name 42\n
      "},{"location":"api/basic_json/as_base_class/#see-also","title":"See also","text":"
      • json_base_class_t - type of the custom base class
      "},{"location":"api/basic_json/as_base_class/#version-history","title":"Version history","text":"
      • Added in version 3.13.0 unreleased.
      "},{"location":"api/basic_json/at/","title":"nlohmann::basic_json::at","text":"
      // (1)\nreference at(size_type idx);\nconst_reference at(size_type idx) const;\n\n// (2)\nreference at(const typename object_t::key_type& key);\nconst_reference at(const typename object_t::key_type& key) const;\n\n// (3)\ntemplate<typename KeyType>\nreference at(KeyType&& key);\ntemplate<typename KeyType>\nconst_reference at(KeyType&& key) const;\n\n// (4)\nreference at(const json_pointer& ptr);\nconst_reference at(const json_pointer& ptr) const;\n
      1. Returns a reference to the array element at specified location idx, with bounds checking.
      2. Returns a reference to the object element with specified key key, with bounds checking.
      3. See 2. This overload is only available if KeyType is comparable with typename object_t::key_type and typename object_comparator_t::is_transparent denotes a type.
      4. Returns a reference to the element at specified JSON pointer ptr, with bounds checking.
      "},{"location":"api/basic_json/at/#template-parameters","title":"Template parameters","text":"KeyType A type for an object key other than json_pointer that is comparable with string_t using object_comparator_t. This can also be a string view (C++17)."},{"location":"api/basic_json/at/#parameters","title":"Parameters","text":"idx (in) index of the element to access key (in) object key of the elements to access ptr (in) JSON pointer to the desired element"},{"location":"api/basic_json/at/#return-value","title":"Return value","text":"
      1. reference to the element at index idx
      2. reference to the element at key key
      3. reference to the element at key key
      4. reference to the element pointed to by ptr
      "},{"location":"api/basic_json/at/#exception-safety","title":"Exception safety","text":"

      Strong exception safety: if an exception occurs, the original value stays intact.

      "},{"location":"api/basic_json/at/#exceptions","title":"Exceptions","text":"
      1. The function can throw the following exceptions:
        • Throws type_error.304 if the JSON value is not an array; in this case, calling at with an index makes no sense. See the example below.
        • Throws out_of_range.401 if the index idx is out of range of the array; that is, idx >= size(). See the example below.
      2. The function can throw the following exceptions:
        • Throws type_error.304 if the JSON value is not an object; in this case, calling at with a key makes no sense. See the example below.
        • Throws out_of_range.403 if the key key is not stored in the object; that is, find(key) == end(). See the example below.
      3. See 2.
      4. The function can throw the following exceptions:
        • Throws parse_error.106 if an array index in the passed JSON pointer ptr begins with '0'. See the example below.
        • Throws parse_error.109 if an array index in the passed JSON pointer ptr is not a number. See the example below.
        • Throws out_of_range.401 if an array index in the passed JSON pointer ptr is out of range. See the example below.
        • Throws out_of_range.402 if the array index '-' is used in the passed JSON pointer ptr. As at provides checked access (and no elements are implicitly inserted), the index '-' is always invalid. See the example below.
        • Throws out_of_range.403 if the JSON pointer describes a key of an object which cannot be found. See the example below.
        • Throws out_of_range.404 if the JSON pointer ptr can not be resolved. See the example below.
        • Throws out_of_range.410 if an array index in the passed JSON pointer ptr exceeds the range of size_type (e.g., on 32-bit platforms).
      "},{"location":"api/basic_json/at/#complexity","title":"Complexity","text":"
      1. Constant.
      2. Logarithmic in the size of the container.
      3. Logarithmic in the size of the container.
      4. Logarithmic in the size of the container.
      "},{"location":"api/basic_json/at/#notes","title":"Notes","text":"

      Deprecation

      Overload (4) also accepts a json_pointer whose template argument is a basic_json specialization (e.g., nlohmann::json_pointer<nlohmann::json>) instead of a string type. This is deprecated since version 3.11.0 and will be removed in a future major version; use basic_json::json_pointer (for json, nlohmann::json_pointer<std::string>) instead.

      You should be warned by your compiler with a -Wdeprecated-declarations warning if you are using a deprecated function.

      See the migration guide for how to update existing code.

      "},{"location":"api/basic_json/at/#examples","title":"Examples","text":"Example: (1) access specified array element with bounds checking

      The example below shows how array elements can be read and written using at(). It also demonstrates the different exceptions that can be thrown.

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON array\n    json array = {\"first\", \"2nd\", \"third\", \"fourth\"};\n\n    // output element at index 2 (third element)\n    std::cout << array.at(2) << '\\n';\n\n    // change element at index 1 (second element) to \"second\"\n    array.at(1) = \"second\";\n\n    // output changed array\n    std::cout << array << '\\n';\n\n    // exception type_error.304\n    try\n    {\n        // use at() on a non-array type\n        json str = \"I am a string\";\n        str.at(0) = \"Another string\";\n    }\n    catch (const json::type_error& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n\n    // exception out_of_range.401\n    try\n    {\n        // try to write beyond the array limit\n        array.at(5) = \"sixth\";\n    }\n    catch (const json::out_of_range& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

      Output:

      \"third\"\n[\"first\",\"second\",\"third\",\"fourth\"]\n[json.exception.type_error.304] cannot use at() with string\n[json.exception.out_of_range.401] array index 5 is out of range\n
      Example: (1) access specified array element with bounds checking

      The example below shows how array elements can be read using at(). It also demonstrates the different exceptions that can be thrown.

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON array\n    const json array = {\"first\", \"2nd\", \"third\", \"fourth\"};\n\n    // output element at index 2 (third element)\n    std::cout << array.at(2) << '\\n';\n\n    // exception type_error.304\n    try\n    {\n        // use at() on a non-array type\n        const json str = \"I am a string\";\n        std::cout << str.at(0) << '\\n';\n    }\n    catch (const json::type_error& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n\n    // exception out_of_range.401\n    try\n    {\n        // try to read beyond the array limit\n        std::cout << array.at(5) << '\\n';\n    }\n    catch (const json::out_of_range& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

      Output:

      \"third\"\n[json.exception.type_error.304] cannot use at() with string\n[json.exception.out_of_range.401] array index 5 is out of range\n
      Example: (2) access specified object element with bounds checking

      The example below shows how object elements can be read and written using at(). It also demonstrates the different exceptions that can be thrown.

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON object\n    json object =\n    {\n        {\"the good\", \"il buono\"},\n        {\"the bad\", \"il cattivo\"},\n        {\"the ugly\", \"il brutto\"}\n    };\n\n    // output element with key \"the ugly\"\n    std::cout << object.at(\"the ugly\") << '\\n';\n\n    // change element with key \"the bad\"\n    object.at(\"the bad\") = \"il cattivo\";\n\n    // output changed array\n    std::cout << object << '\\n';\n\n    // exception type_error.304\n    try\n    {\n        // use at() on a non-object type\n        json str = \"I am a string\";\n        str.at(\"the good\") = \"Another string\";\n    }\n    catch (const json::type_error& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n\n    // exception out_of_range.401\n    try\n    {\n        // try to write at a nonexisting key\n        object.at(\"the fast\") = \"il rapido\";\n    }\n    catch (const json::out_of_range& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

      Output:

      \"il brutto\"\n{\"the bad\":\"il cattivo\",\"the good\":\"il buono\",\"the ugly\":\"il brutto\"}\n[json.exception.type_error.304] cannot use at() with string\n[json.exception.out_of_range.403] key 'the fast' not found\n
      Example: (2) access specified object element with bounds checking

      The example below shows how object elements can be read using at(). It also demonstrates the different exceptions that can be thrown.

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON object\n    const json object =\n    {\n        {\"the good\", \"il buono\"},\n        {\"the bad\", \"il cattivo\"},\n        {\"the ugly\", \"il brutto\"}\n    };\n\n    // output element with key \"the ugly\"\n    std::cout << object.at(\"the ugly\") << '\\n';\n\n    // exception type_error.304\n    try\n    {\n        // use at() on a non-object type\n        const json str = \"I am a string\";\n        std::cout << str.at(\"the good\") << '\\n';\n    }\n    catch (const json::type_error& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n\n    // exception out_of_range.401\n    try\n    {\n        // try to read from a nonexisting key\n        std::cout << object.at(\"the fast\") << '\\n';\n    }\n    catch (const json::out_of_range)\n    {\n        std::cout << \"out of range\" << '\\n';\n    }\n}\n

      Output:

      \"il brutto\"\n[json.exception.type_error.304] cannot use at() with string\nout of range\n
      Example: (3) access specified object element using string_view with bounds checking

      The example below shows how object elements can be read and written using at(). It also demonstrates the different exceptions that can be thrown.

      #include <iostream>\n#include <string_view>\n#include <nlohmann/json.hpp>\n\nusing namespace std::string_view_literals;\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON object\n    json object =\n    {\n        {\"the good\", \"il buono\"},\n        {\"the bad\", \"il cattivo\"},\n        {\"the ugly\", \"il brutto\"}\n    };\n\n    // output element with key \"the ugly\" using string_view\n    std::cout << object.at(\"the ugly\"sv) << '\\n';\n\n    // change element with key \"the bad\" using string_view\n    object.at(\"the bad\"sv) = \"il cattivo\";\n\n    // output changed array\n    std::cout << object << '\\n';\n\n    // exception type_error.304\n    try\n    {\n        // use at() with string_view on a non-object type\n        json str = \"I am a string\";\n        str.at(\"the good\"sv) = \"Another string\";\n    }\n    catch (const json::type_error& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n\n    // exception out_of_range.401\n    try\n    {\n        // try to write at a nonexisting key using string_view\n        object.at(\"the fast\"sv) = \"il rapido\";\n    }\n    catch (const json::out_of_range& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

      Output:

      \"il brutto\"\n{\"the bad\":\"il cattivo\",\"the good\":\"il buono\",\"the ugly\":\"il brutto\"}\n[json.exception.type_error.304] cannot use at() with string\n[json.exception.out_of_range.403] key 'the fast' not found\n
      Example: (3) access specified object element using string_view with bounds checking

      The example below shows how object elements can be read using at(). It also demonstrates the different exceptions that can be thrown.

      #include <iostream>\n#include <string_view>\n#include <nlohmann/json.hpp>\n\nusing namespace std::string_view_literals;\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON object\n    const json object =\n    {\n        {\"the good\", \"il buono\"},\n        {\"the bad\", \"il cattivo\"},\n        {\"the ugly\", \"il brutto\"}\n    };\n\n    // output element with key \"the ugly\" using string_view\n    std::cout << object.at(\"the ugly\"sv) << '\\n';\n\n    // exception type_error.304\n    try\n    {\n        // use at() with string_view on a non-object type\n        const json str = \"I am a string\";\n        std::cout << str.at(\"the good\"sv) << '\\n';\n    }\n    catch (const json::type_error& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n\n    // exception out_of_range.401\n    try\n    {\n        // try to read from a nonexisting key using string_view\n        std::cout << object.at(\"the fast\"sv) << '\\n';\n    }\n    catch (const json::out_of_range& e)\n    {\n        std::cout << \"out of range\" << '\\n';\n    }\n}\n

      Output:

      \"il brutto\"\n[json.exception.type_error.304] cannot use at() with string\nout of range\n
      Example: (4) access specified element via JSON Pointer

      The example below shows how object elements can be read and written using at(). It also demonstrates the different exceptions that can be thrown.

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // create a JSON value\n    json j =\n    {\n        {\"number\", 1}, {\"string\", \"foo\"}, {\"array\", {1, 2}}\n    };\n\n    // read-only access\n\n    // output element with JSON pointer \"/number\"\n    std::cout << j.at(\"/number\"_json_pointer) << '\\n';\n    // output element with JSON pointer \"/string\"\n    std::cout << j.at(\"/string\"_json_pointer) << '\\n';\n    // output element with JSON pointer \"/array\"\n    std::cout << j.at(\"/array\"_json_pointer) << '\\n';\n    // output element with JSON pointer \"/array/1\"\n    std::cout << j.at(\"/array/1\"_json_pointer) << '\\n';\n\n    // writing access\n\n    // change the string\n    j.at(\"/string\"_json_pointer) = \"bar\";\n    // output the changed string\n    std::cout << j[\"string\"] << '\\n';\n\n    // change an array element\n    j.at(\"/array/1\"_json_pointer) = 21;\n    // output the changed array\n    std::cout << j[\"array\"] << '\\n';\n\n    // out_of_range.106\n    try\n    {\n        // try to use an array index with leading '0'\n        json::reference ref = j.at(\"/array/01\"_json_pointer);\n    }\n    catch (const json::parse_error& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n\n    // out_of_range.109\n    try\n    {\n        // try to use an array index that is not a number\n        json::reference ref = j.at(\"/array/one\"_json_pointer);\n    }\n    catch (const json::parse_error& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n\n    // out_of_range.401\n    try\n    {\n        // try to use an invalid array index\n        json::reference ref = j.at(\"/array/4\"_json_pointer);\n    }\n    catch (const json::out_of_range& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n\n    // out_of_range.402\n    try\n    {\n        // try to use the array index '-'\n        json::reference ref = j.at(\"/array/-\"_json_pointer);\n    }\n    catch (const json::out_of_range& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n\n    // out_of_range.403\n    try\n    {\n        // try to use a JSON pointer to a nonexistent object key\n        json::const_reference ref = j.at(\"/foo\"_json_pointer);\n    }\n    catch (const json::out_of_range& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n\n    // out_of_range.404\n    try\n    {\n        // try to use a JSON pointer that cannot be resolved\n        json::reference ref = j.at(\"/number/foo\"_json_pointer);\n    }\n    catch (const json::out_of_range& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

      Output:

      1\n\"foo\"\n[1,2]\n2\n\"bar\"\n[1,21]\n[json.exception.parse_error.106] parse error: array index '01' must not begin with '0'\n[json.exception.parse_error.109] parse error: array index 'one' is not a number\n[json.exception.out_of_range.401] array index 4 is out of range\n[json.exception.out_of_range.402] array index '-' (2) is out of range\n[json.exception.out_of_range.403] key 'foo' not found\n[json.exception.out_of_range.404] unresolved reference token 'foo'\n
      Example: (4) access specified element via JSON Pointer

      The example below shows how object elements can be read using at(). It also demonstrates the different exceptions that can be thrown.

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // create a JSON value\n    const json j =\n    {\n        {\"number\", 1}, {\"string\", \"foo\"}, {\"array\", {1, 2}}\n    };\n\n    // read-only access\n\n    // output element with JSON pointer \"/number\"\n    std::cout << j.at(\"/number\"_json_pointer) << '\\n';\n    // output element with JSON pointer \"/string\"\n    std::cout << j.at(\"/string\"_json_pointer) << '\\n';\n    // output element with JSON pointer \"/array\"\n    std::cout << j.at(\"/array\"_json_pointer) << '\\n';\n    // output element with JSON pointer \"/array/1\"\n    std::cout << j.at(\"/array/1\"_json_pointer) << '\\n';\n\n    // out_of_range.109\n    try\n    {\n        // try to use an array index that is not a number\n        json::const_reference ref = j.at(\"/array/one\"_json_pointer);\n    }\n    catch (const json::parse_error& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n\n    // out_of_range.401\n    try\n    {\n        // try to use an invalid array index\n        json::const_reference ref = j.at(\"/array/4\"_json_pointer);\n    }\n    catch (const json::out_of_range& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n\n    // out_of_range.402\n    try\n    {\n        // try to use the array index '-'\n        json::const_reference ref = j.at(\"/array/-\"_json_pointer);\n    }\n    catch (const json::out_of_range& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n\n    // out_of_range.403\n    try\n    {\n        // try to use a JSON pointer to a nonexistent object key\n        json::const_reference ref = j.at(\"/foo\"_json_pointer);\n    }\n    catch (const json::out_of_range& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n\n    // out_of_range.404\n    try\n    {\n        // try to use a JSON pointer that cannot be resolved\n        json::const_reference ref = j.at(\"/number/foo\"_json_pointer);\n    }\n    catch (const json::out_of_range& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

      Output:

      1\n\"foo\"\n[1,2]\n2\n[json.exception.parse_error.109] parse error: array index 'one' is not a number\n[json.exception.out_of_range.401] array index 4 is out of range\n[json.exception.out_of_range.402] array index '-' (2) is out of range\n[json.exception.out_of_range.403] key 'foo' not found\n[json.exception.out_of_range.404] unresolved reference token 'foo'\n
      "},{"location":"api/basic_json/at/#see-also","title":"See also","text":"
      • documentation on checked access
      • operator[] for unchecked access by reference
      • value for access with default value
      • basic_json_view::at - the same access on a zero-copy view
      "},{"location":"api/basic_json/at/#version-history","title":"Version history","text":"
      1. Added in version 1.0.0.
      2. Added in version 1.0.0.
      3. Added in version 3.11.0. Fixed in version 3.13.0 unreleased to consistently accept std::string_view-convertible keys, as already supported by operator[], value, find, and other lookup functions.
      4. Added in version 2.0.0. Throws parse_error.109 instead of out_of_range.404 for a one-character array index that is not a digit (e.g., /x) in version 3.13.0 unreleased, as it already did for longer ones.
      "},{"location":"api/basic_json/back/","title":"nlohmann::basic_json::back","text":"
      reference back();\n\nconst_reference back() const;\n

      Returns a reference to the last element in the container. For a JSON container c, the expression c.back() is equivalent to

      auto tmp = c.end();\n--tmp;\nreturn *tmp;\n
      "},{"location":"api/basic_json/back/#return-value","title":"Return value","text":"

      In the case of a structured type (array or object), a reference to the last element is returned. In the case of number, string, boolean, or binary values, a reference to the value is returned.

      "},{"location":"api/basic_json/back/#exception-safety","title":"Exception safety","text":"

      Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

      "},{"location":"api/basic_json/back/#exceptions","title":"Exceptions","text":"

      If the JSON value is null, exception invalid_iterator.214 is thrown.

      "},{"location":"api/basic_json/back/#complexity","title":"Complexity","text":"

      Constant.

      "},{"location":"api/basic_json/back/#notes","title":"Notes","text":"

      Precondition

      The array or object must not be empty. Calling back on an empty array or object yields undefined behavior.

      "},{"location":"api/basic_json/back/#examples","title":"Examples","text":"Example

      The following code shows an example for back().

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_boolean = true;\n    json j_number_integer = 17;\n    json j_number_float = 23.42;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_object_empty(json::value_t::object);\n    json j_array = {1, 2, 4, 8, 16};\n    json j_array_empty(json::value_t::array);\n    json j_string = \"Hello, world\";\n\n    // call back()\n    std::cout << j_boolean.back() << '\\n';\n    std::cout << j_number_integer.back() << '\\n';\n    std::cout << j_number_float.back() << '\\n';\n    std::cout << j_object.back() << '\\n';\n    //std::cout << j_object_empty.back() << '\\n';  // undefined behavior\n    std::cout << j_array.back() << '\\n';\n    //std::cout << j_array_empty.back() << '\\n';   // undefined behavior\n    std::cout << j_string.back() << '\\n';\n\n    // back() called on a null value\n    try\n    {\n        json j_null;\n        j_null.back();\n    }\n    catch (const json::invalid_iterator& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

      Output:

      true\n17\n23.42\n2\n16\n\"Hello, world\"\n[json.exception.invalid_iterator.214] cannot get value\n
      "},{"location":"api/basic_json/back/#see-also","title":"See also","text":"
      • front to access the first element
      • basic_json_view::back - the same access on a zero-copy view
      "},{"location":"api/basic_json/back/#version-history","title":"Version history","text":"
      • Added in version 1.0.0.
      • Adjusted code to return reference to binary values in version 3.8.0.
      "},{"location":"api/basic_json/basic_json/","title":"nlohmann::basic_json::basic_json","text":"
      // (1)\nbasic_json(const value_t v);\n\n// (2)\nbasic_json(std::nullptr_t = nullptr) noexcept;\n\n// (3)\ntemplate<typename CompatibleType>\nbasic_json(CompatibleType&& val) noexcept(noexcept(\n           JSONSerializer<U>::to_json(std::declval<basic_json_t&>(),\n                                      std::forward<CompatibleType>(val))));\n\n// (4)\ntemplate<typename BasicJsonType>\nbasic_json(const BasicJsonType& val);\n\n// (5)\nbasic_json(initializer_list_t init,\n           bool type_deduction = true,\n           value_t manual_type = value_t::array);\n\n// (6)\nbasic_json(size_type cnt, const basic_json& val);\n\n// (7)\nbasic_json(iterator first, iterator last);\nbasic_json(const_iterator first, const_iterator last);\n\n// (8)\nbasic_json(const basic_json& other);\n\n// (9)\nbasic_json(basic_json&& other) noexcept;\n

      1. Create an empty JSON value with a given type. The value will be default initialized with an empty value which depends on the type:

        Value type initial value null null boolean false string \"\" number 0 object {} array [] binary empty array

        The postcondition of this constructor can be restored by calling clear().

      2. Create a null JSON value. It either takes a null pointer as parameter (explicitly creating null) or no parameter (implicitly creating null). The passed null pointer itself is not read -- it is only used to choose the right constructor.

      3. This is a \"catch all\" constructor for all compatible JSON types; that is, types for which a to_json() method exists. The constructor forwards the parameter val to that method (to json_serializer<U>::to_json method with U = uncvref_t<CompatibleType>, to be exact).

        Template type CompatibleType includes, but is not limited to, the following types:

        • arrays: array_t and all kinds of compatible containers such as std::vector, std::deque, std::list, std::forward_list, std::array, std::valarray, std::set, std::unordered_set, std::multiset, and std::unordered_multiset with a value_type from which a basic_json value can be constructed.
        • objects: object_t and all kinds of compatible associative containers such as std::map, std::unordered_map, std::multimap, and std::unordered_multimap with a key_type compatible to string_t and a value_type from which a basic_json value can be constructed.
        • strings: string_t, string literals, and all compatible string containers can be used.

        Rvalue containers are moved from

        If val is an rvalue of a compatible array-like or object-like container other than array_t/object_t (e.g., json j = std::move(vec); or json j(std::vector<std::string>{...});), the elements are moved into the JSON value instead of copied. Afterwards, the elements of the source container are in a valid but unspecified state (moved-from). Keys of associative containers are const and therefore still copied. Lvalues and const rvalues are copied, and the source is left unchanged.

        • numbers: number_integer_t, number_unsigned_t, number_float_t, and all convertible number types such as int, size_t, int64_t, float or double can be used.
        • boolean: boolean_t / bool can be used.
        • binary: binary_t / std::vector<uint8_t> may be used; unfortunately because string literals cannot be distinguished from binary character arrays by the C++ type system, all types compatible with const char* will be directed to the string constructor instead. This is both for backwards compatibility and due to the fact that a binary type is not a standard JSON type.

        See the examples below.

      4. This is a constructor for existing basic_json types. It does not hijack copy/move constructors, since the parameter has different template arguments than the current ones.

        The constructor tries to convert the internal m_value of the parameter. Each member value (object, array, string, etc.) is serialized via the corresponding to_json() overload. For objects and strings, the conversion requires that the target basic_json type's object_t::key_type (or string_t) be directly constructible from the source type's corresponding member type via is_constructible. If this requirement is not met, the conversion does not fail to compile; instead, it silently falls back to the array-conversion path, which represents objects as arrays of [key, value] pairs and strings as arrays of character codes. This is a known limitation tracked in issue #3425.

      5. Creates a JSON value of type array or object from the passed initializer list init. In case type_deduction is true (default), the type of the JSON value to be created is deducted from the initializer list init according to the following rules:

        1. If the list is empty, an empty JSON object value {} is created.
        2. If the list consists of pairs whose first element is a string, a JSON object value is created where the first elements of the pairs are treated as keys and the second elements are as values.
        3. In all other cases, an array is created.

        The following flowchart also takes into account what happens when type_deduction is false, in which case manual_type decides between object and array, and an object can only be forced if init actually matches rule 2 (or is empty):

        flowchart TD\n    A([\"initializer_list init\"]) --> B{\"empty, or every element is a 2-element<br/>array whose first element is a string?\"}\n    B -->|\"yes\"| C{\"type_deduction\"}\n    B -->|\"no\"| D{\"type_deduction\"}\n    C -->|\"true\"| OBJ[\"create object\"]\n    C -->|\"false\"| E{\"manual_type\"}\n    E -->|\"object\"| OBJ\n    E -->|\"array\"| ARR[\"create array\"]\n    D -->|\"true\"| ARR\n    D -->|\"false\"| F{\"manual_type\"}\n    F -->|\"array\"| ARR\n    F -->|\"object\"| ERR[\"throw type_error.301\"]

        The rules aim to create the best fit between a C++ initializer list and JSON values. The rationale is as follows:

        1. The empty initializer list is written as {} which is exactly an empty JSON object.
        2. C++ has no way of describing mapped types other than to list a list of pairs. As JSON requires that keys must be of type string, rule 2 is the weakest constraint one can pose on initializer lists to interpret them as an object.
        3. In all other cases, the initializer list could not be interpreted as a JSON object type, so interpreting it as a JSON array type is safe.

        With the rules described above, the following JSON values cannot be expressed by an initializer list:

        • the empty array ([]): use array(initializer_list_t) with an empty initializer list in this case
        • arrays whose elements satisfy rule 2: use array(initializer_list_t) with the same initializer list in this case

        Function array() and object() force array and object creation from initializer lists, respectively.

        Brace initialization yields arrays

        Because this constructor takes an initializer_list_t, brace-initializing a json/ordered_json from another json value wraps it in a single-element array rather than copying it:

        json j1 = \"hello\";\njson j2{j1};   // [!] j2 is [\"hello\"], NOT a copy of j1\njson j3(j1);   // j3 is \"hello\" -- parentheses copy as expected\n

        See the FAQ entry on brace initialization for the full explanation, an opt-in macro to change this behavior, and how to explicitly create a single-element array (json::array({value})) if that is what you want.

      6. Constructs a JSON array value by creating cnt copies of a passed value. In case cnt is 0, an empty array is created.

      7. Constructs the JSON value with the contents of the range [first, last). The semantics depend on the different types a JSON value can have:

        • In case of a null type, invalid_iterator.206 is thrown.
        • In case of other primitive types (number, boolean, string, or binary), first must be begin() and last must be end(). In this case, the value is copied. Otherwise, invalid_iterator.204 is thrown.
        • In case of structured types (array, object), the constructor behaves as similar versions for std::vector or std::map; that is, a JSON array or object is constructed from the values in the range.

      8. Creates a copy of a given JSON value.

      9. Move constructor. Constructs a JSON value with the contents of the given value other using move semantics. It \"steals\" the resources from other and leaves it as JSON null value.

      "},{"location":"api/basic_json/basic_json/#template-parameters","title":"Template parameters","text":"CompatibleType

      a type such that:

      • CompatibleType is not derived from std::istream,
      • CompatibleType is not basic_json (to avoid hijacking copy/move constructors),
      • CompatibleType is not a different basic_json type (i.e. with different template arguments)
      • CompatibleType is not a basic_json nested type (e.g., json_pointer, iterator, etc.)
      • if JSON_DISABLE_TUPLE_REFERENCE_CONVERSION is defined to 1: CompatibleType is not a one-element std::tuple holding a reference to basic_json
      • json_serializer<U> (with U = uncvref_t<CompatibleType>) has a to_json(basic_json_t&, CompatibleType&&) method
      BasicJsonType:

      a type such that:

      • BasicJsonType is a basic_json type.
      • BasicJsonType has different template arguments than basic_json_t.

      Note: For cross-basic_json conversions to produce correct results, the target basic_json's object_t::key_type and string_t must be directly constructible from the source basic_json's corresponding types. See the description of overload (4) above for details on what happens when this requirement is not met.

      U: uncvref_t<CompatibleType>"},{"location":"api/basic_json/basic_json/#parameters","title":"Parameters","text":"v (in) the type of the value to create val (in) the value to be forwarded to the respective constructor init (in) initializer list with JSON values type_deduction (in) internal parameter; when set to true, the type of the JSON value is deducted from the initializer list init; when set to false, the type provided via manual_type is forced. This mode is used by the functions array(initializer_list_t) and object(initializer_list_t). manual_type (in) internal parameter; when type_deduction is set to false, the created JSON value will use the provided type (only value_t::array and value_t::object are valid); when type_deduction is set to true, this parameter has no effect cnt (in) the number of JSON copies of val to create first (in) the beginning of the range to copy from (included) last (in) the end of the range to copy from (excluded) other (in) the JSON value to copy/move"},{"location":"api/basic_json/basic_json/#exception-safety","title":"Exception safety","text":"
      1. Strong guarantee: if an exception is thrown, there are no changes to any JSON value.
      2. No-throw guarantee: this constructor never throws exceptions.
      3. Depends on the called constructor. For types directly supported by the library (i.e., all types for which no to_json() function was provided), a strong guarantee holds: if an exception is thrown, there are no changes to any JSON value. Exception: when val is an rvalue array-like or object-like container, its elements are moved one by one; if an exception is thrown in the middle of the conversion (e.g., an allocation failure), the JSON value is unchanged, but the source container may be left partially moved-from.
      4. Depends on the called constructor. For types directly supported by the library (i.e., all types for which no to_json() function was provided), a strong guarantee holds: if an exception is thrown, there are no changes to any JSON value.
      5. Strong guarantee: if an exception is thrown, there are no changes to any JSON value.
      6. Strong guarantee: if an exception is thrown, there are no changes to any JSON value.
      7. Strong guarantee: if an exception is thrown, there are no changes to any JSON value.
      8. Strong guarantee: if an exception is thrown, there are no changes to any JSON value.
      9. No-throw guarantee: this constructor never throws exceptions.
      "},{"location":"api/basic_json/basic_json/#exceptions","title":"Exceptions","text":"
      1. (none)
      2. The function does not throw exceptions.
      3. (none)
      4. (none)
      5. The function can throw the following exceptions:
        • Throws type_error.301 if type_deduction is false, manual_type is value_t::object, but init contains an element which is not a pair whose first element is a string. In this case, the constructor could not create an object. If type_deduction would have been true, an array would have been created. See object(initializer_list_t) for an example.
      6. (none)
      7. The function can throw the following exceptions:
        • Throws invalid_iterator.201 if iterators first and last are not compatible (i.e., do not belong to the same JSON value). In this case, the range [first, last) is undefined.
        • Throws invalid_iterator.204 if iterators first and last belong to a primitive type (number, boolean, string, or binary), but first does not point to the first element anymore. In this case, the range [first, last) is undefined. See the example code below.
        • Throws invalid_iterator.206 if iterators first and last belong to a null value. In this case, the range [first, last) is undefined.
      8. (none)
      9. The function does not throw exceptions.
      "},{"location":"api/basic_json/basic_json/#complexity","title":"Complexity","text":"
      1. Constant.
      2. Constant.
      3. Usually linear in the size of the passed val, also depending on the implementation of the called to_json() method.
      4. Usually linear in the size of the passed val, also depending on the implementation of the called to_json() method.
      5. Linear in the size of the initializer list init.
      6. Linear in cnt.
      7. Linear in distance between first and last.
      8. Linear in the size of other.
      9. Constant.
      "},{"location":"api/basic_json/basic_json/#notes","title":"Notes","text":"

      • Overload 5:

        Empty initializer list

        When used without parentheses around an empty initializer list, basic_json() is called instead of this function, yielding the JSON null value.

      • Overload 4:

        Implicit conversion

        The conversion is implicit unless JSON_USE_IMPLICIT_CONVERSIONS is defined to 0 and BasicJsonType::string_t differs from string_t. In that case, the constructor is explicit, so a JSON value with a different string type is no longer silently converted, for example when it is passed to a function taking const json&. Write json(other) or other.get<json>() instead.

      • Overload 7:

        Preconditions

        • Iterators first and last must be initialized. **This precondition is enforced with a runtime assertion.
        • Range [first, last) is valid. Usually, this precondition cannot be checked efficiently. Only certain edge cases are detected; see the description of the exceptions above. A violation of this precondition yields undefined behavior.

        Runtime assertion

        A precondition is enforced with a runtime assertion.

      • Overload 8:

        Postcondition

        *this == other

      • Overload 9:

        Postconditions

        • *thishas the same value asother` before the call.
        • other is a JSON null value
      "},{"location":"api/basic_json/basic_json/#examples","title":"Examples","text":"Example: (1) create an empty value with a given type

      The following code shows the constructor for different value_t values.

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create the different JSON values with default values\n    json j_null(json::value_t::null);\n    json j_boolean(json::value_t::boolean);\n    json j_number_integer(json::value_t::number_integer);\n    json j_number_float(json::value_t::number_float);\n    json j_object(json::value_t::object);\n    json j_array(json::value_t::array);\n    json j_string(json::value_t::string);\n\n    // serialize the JSON values\n    std::cout << j_null << '\\n';\n    std::cout << j_boolean << '\\n';\n    std::cout << j_number_integer << '\\n';\n    std::cout << j_number_float << '\\n';\n    std::cout << j_object << '\\n';\n    std::cout << j_array << '\\n';\n    std::cout << j_string << '\\n';\n}\n

      Output:

      null\nfalse\n0\n0.0\n{}\n[]\n\"\"\n
      Example: (2) create a null object

      The following code shows the constructor with and without a null pointer parameter.

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // implicitly create a JSON null value\n    json j1;\n\n    // explicitly create a JSON null value\n    json j2(nullptr);\n\n    // serialize the JSON null value\n    std::cout << j1 << '\\n' << j2 << '\\n';\n}\n

      Output:

      null\nnull\n
      Example: (3) create a JSON value from compatible types

      The following code shows the constructor with several compatible types.

      #include <iostream>\n#include <deque>\n#include <list>\n#include <forward_list>\n#include <set>\n#include <unordered_map>\n#include <unordered_set>\n#include <valarray>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // ============\n    // object types\n    // ============\n\n    // create an object from an object_t value\n    json::object_t object_value = { {\"one\", 1}, {\"two\", 2} };\n    json j_object_t(object_value);\n\n    // create an object from std::map\n    std::map<std::string, int> c_map\n    {\n        {\"one\", 1}, {\"two\", 2}, {\"three\", 3}\n    };\n    json j_map(c_map);\n\n    // create an object from std::unordered_map\n    std::unordered_map<const char*, double> c_umap\n    {\n        {\"one\", 1.2}, {\"two\", 2.3}, {\"three\", 3.4}\n    };\n    json j_umap(c_umap);\n\n    // create an object from std::multimap\n    std::multimap<std::string, bool> c_mmap\n    {\n        {\"one\", true}, {\"two\", true}, {\"three\", false}, {\"three\", true}\n    };\n    json j_mmap(c_mmap); // only one entry for key \"three\" is used\n\n    // create an object from std::unordered_multimap\n    std::unordered_multimap<std::string, bool> c_ummap\n    {\n        {\"one\", true}, {\"two\", true}, {\"three\", false}, {\"three\", false}\n    };\n    json j_ummap(c_ummap); // only one entry for key \"three\" is used\n\n    // serialize the JSON objects\n    std::cout << j_object_t << '\\n';\n    std::cout << j_map << '\\n';\n    std::cout << j_umap << '\\n';\n    std::cout << j_mmap << '\\n';\n    std::cout << j_ummap << \"\\n\\n\";\n\n    // ===========\n    // array types\n    // ===========\n\n    // create an array from an array_t value\n    json::array_t array_value = {\"one\", \"two\", 3, 4.5, false};\n    json j_array_t(array_value);\n\n    // create an array from std::vector\n    std::vector<int> c_vector {1, 2, 3, 4};\n    json j_vec(c_vector);\n\n    // create an array from std::valarray\n    std::valarray<short> c_valarray {10, 9, 8, 7};\n    json j_valarray(c_valarray);\n\n    // create an array from std::deque\n    std::deque<double> c_deque {1.2, 2.3, 3.4, 5.6};\n    json j_deque(c_deque);\n\n    // create an array from std::list\n    std::list<bool> c_list {true, true, false, true};\n    json j_list(c_list);\n\n    // create an array from std::forward_list\n    std::forward_list<std::int64_t> c_flist {12345678909876, 23456789098765, 34567890987654, 45678909876543};\n    json j_flist(c_flist);\n\n    // create an array from std::array\n    std::array<unsigned long, 4> c_array {{1, 2, 3, 4}};\n    json j_array(c_array);\n\n    // create an array from std::set\n    std::set<std::string> c_set {\"one\", \"two\", \"three\", \"four\", \"one\"};\n    json j_set(c_set); // only one entry for \"one\" is used\n\n    // create an array from std::unordered_set\n    std::unordered_set<std::string> c_uset {\"one\", \"one\"};\n    json j_uset(c_uset); // only one entry for \"one\" is used\n\n    // create an array from std::multiset\n    std::multiset<std::string> c_mset {\"one\", \"two\", \"one\", \"four\"};\n    json j_mset(c_mset); // both entries for \"one\" are used\n\n    // create an array from std::unordered_multiset\n    std::unordered_multiset<std::string> c_umset {\"one\", \"one\"};\n    json j_umset(c_umset); // both entries for \"one\" are used\n\n    // serialize the JSON arrays\n    std::cout << j_array_t << '\\n';\n    std::cout << j_vec << '\\n';\n    std::cout << j_valarray << '\\n';\n    std::cout << j_deque << '\\n';\n    std::cout << j_list << '\\n';\n    std::cout << j_flist << '\\n';\n    std::cout << j_array << '\\n';\n    std::cout << j_set << '\\n';\n    std::cout << j_uset << '\\n';\n    std::cout << j_mset << '\\n';\n    std::cout << j_umset << \"\\n\\n\";\n\n    // ============\n    // string types\n    // ============\n\n    // create string from a string_t value\n    json::string_t string_value = \"The quick brown fox jumps over the lazy dog.\";\n    json j_string_t(string_value);\n\n    // create a JSON string directly from a string literal\n    json j_string_literal(\"The quick brown fox jumps over the lazy dog.\");\n\n    // create string from std::string\n    std::string s_stdstring = \"The quick brown fox jumps over the lazy dog.\";\n    json j_stdstring(s_stdstring);\n\n    // serialize the JSON strings\n    std::cout << j_string_t << '\\n';\n    std::cout << j_string_literal << '\\n';\n    std::cout << j_stdstring << \"\\n\\n\";\n\n    // ============\n    // number types\n    // ============\n\n    // create a JSON number from number_integer_t\n    json::number_integer_t value_integer_t = -42;\n    json j_integer_t(value_integer_t);\n\n    // create a JSON number from number_unsigned_t\n    json::number_integer_t value_unsigned_t = 17;\n    json j_unsigned_t(value_unsigned_t);\n\n    // create a JSON number from an anonymous enum\n    enum { enum_value = 17 };\n    json j_enum(enum_value);\n\n    // create values of different integer types\n    short n_short = 42;\n    int n_int = -23;\n    long n_long = 1024;\n    int_least32_t n_int_least32_t = -17;\n    uint8_t n_uint8_t = 8;\n\n    // create (integer) JSON numbers\n    json j_short(n_short);\n    json j_int(n_int);\n    json j_long(n_long);\n    json j_int_least32_t(n_int_least32_t);\n    json j_uint8_t(n_uint8_t);\n\n    // create values of different floating-point types\n    json::number_float_t v_ok = 3.141592653589793;\n    json::number_float_t v_nan = NAN;\n    json::number_float_t v_infinity = INFINITY;\n\n    // create values of different floating-point types\n    float n_float = 42.23;\n    float n_float_nan = 1.0f / 0.0f;\n    double n_double = 23.42;\n\n    // create (floating point) JSON numbers\n    json j_ok(v_ok);\n    json j_nan(v_nan);\n    json j_infinity(v_infinity);\n    json j_float(n_float);\n    json j_float_nan(n_float_nan);\n    json j_double(n_double);\n\n    // serialize the JSON numbers\n    std::cout << j_integer_t << '\\n';\n    std::cout << j_unsigned_t << '\\n';\n    std::cout << j_enum << '\\n';\n    std::cout << j_short << '\\n';\n    std::cout << j_int << '\\n';\n    std::cout << j_long << '\\n';\n    std::cout << j_int_least32_t << '\\n';\n    std::cout << j_uint8_t << '\\n';\n    std::cout << j_ok << '\\n';\n    std::cout << j_nan << '\\n';\n    std::cout << j_infinity << '\\n';\n    std::cout << j_float << '\\n';\n    std::cout << j_float_nan << '\\n';\n    std::cout << j_double << \"\\n\\n\";\n\n    // =============\n    // boolean types\n    // =============\n\n    // create boolean values\n    json j_truth = true;\n    json j_falsity = false;\n\n    // serialize the JSON booleans\n    std::cout << j_truth << '\\n';\n    std::cout << j_falsity << '\\n';\n}\n

      Output:

      {\"one\":1,\"two\":2}\n{\"one\":1,\"three\":3,\"two\":2}\n{\"one\":1.2,\"three\":3.4,\"two\":2.3}\n{\"one\":true,\"three\":false,\"two\":true}\n{\"one\":true,\"three\":false,\"two\":true}\n\n[\"one\",\"two\",3,4.5,false]\n[1,2,3,4]\n[10,9,8,7]\n[1.2,2.3,3.4,5.6]\n[true,true,false,true]\n[12345678909876,23456789098765,34567890987654,45678909876543]\n[1,2,3,4]\n[\"four\",\"one\",\"three\",\"two\"]\n[\"one\"]\n[\"four\",\"one\",\"one\",\"two\"]\n[\"one\",\"one\"]\n\n\"The quick brown fox jumps over the lazy dog.\"\n\"The quick brown fox jumps over the lazy dog.\"\n\"The quick brown fox jumps over the lazy dog.\"\n\n-42\n17\n17\n42\n-23\n1024\n-17\n8\n3.141592653589793\nnull\nnull\n42.22999954223633\nnull\n23.42\n\ntrue\nfalse\n

      Note the output is platform-dependent.

      Example: (4) create a JSON value from another basic_json specialization

      The example below shows how a json value is converted to an ordered_json value and back using the converting constructor. Note how the original insertion order of oj is not restored, because it was already given up when converting to json, whose object_t sorts by key.

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing ordered_json = nlohmann::ordered_json;\n\nint main()\n{\n    // create an ordered_json value; insertion order is preserved\n    ordered_json oj = {{\"c\", 3}, {\"a\", 1}, {\"b\", 2}};\n\n    // convert to json -- overload (4) is used; keys end up sorted\n    json j(oj);\n\n    // convert back to ordered_json -- the original insertion order is lost,\n    // because it was already given up when converting to json\n    ordered_json oj2(j);\n\n    std::cout << oj << '\\n';\n    std::cout << j << '\\n';\n    std::cout << oj2 << '\\n';\n}\n

      Output:

      {\"c\":3,\"a\":1,\"b\":2}\n{\"a\":1,\"b\":2,\"c\":3}\n{\"a\":1,\"b\":2,\"c\":3}\n
      Example: (5) create a container (array or object) from an initializer list

      The example below shows how JSON values are created from initializer lists.

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_empty_init_list = json({});\n    json j_object = { {\"one\", 1}, {\"two\", 2} };\n    json j_array = {1, 2, 3, 4};\n    json j_nested_object = { {\"one\", {1}}, {\"two\", {1, 2}} };\n    json j_nested_array = { {{1}, \"one\"}, {{1, 2}, \"two\"} };\n\n    // serialize the JSON value\n    std::cout << j_empty_init_list << '\\n';\n    std::cout << j_object << '\\n';\n    std::cout << j_array << '\\n';\n    std::cout << j_nested_object << '\\n';\n    std::cout << j_nested_array << '\\n';\n}\n

      Output:

      {}\n{\"one\":1,\"two\":2}\n[1,2,3,4]\n{\"one\":[1],\"two\":[1,2]}\n[[[1],\"one\"],[[1,2],\"two\"]]\n
      Example: (6) construct an array with count copies of a given value

      The following code shows examples for creating arrays with several copies of a given value.

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create an array by creating copies of a JSON value\n    json value = \"Hello\";\n    json array_0 = json(0, value);\n    json array_1 = json(1, value);\n    json array_5 = json(5, value);\n\n    // serialize the JSON arrays\n    std::cout << array_0 << '\\n';\n    std::cout << array_1 << '\\n';\n    std::cout << array_5 << '\\n';\n}\n

      Output:

      []\n[\"Hello\"]\n[\"Hello\",\"Hello\",\"Hello\",\"Hello\",\"Hello\"]\n
      Example: (7) construct a JSON container given an iterator range

      The example below shows several ways to create JSON values by specifying a subrange with iterators.

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_array = {\"alpha\", \"bravo\", \"charly\", \"delta\", \"easy\"};\n    json j_number = 42;\n    json j_object = {{\"one\", \"eins\"}, {\"two\", \"zwei\"}};\n\n    // create copies using iterators\n    json j_array_range(j_array.begin() + 1, j_array.end() - 2);\n    json j_number_range(j_number.begin(), j_number.end());\n    json j_object_range(j_object.begin(), j_object.find(\"two\"));\n\n    // serialize the values\n    std::cout << j_array_range << '\\n';\n    std::cout << j_number_range << '\\n';\n    std::cout << j_object_range << '\\n';\n\n    // example for an exception\n    try\n    {\n        json j_invalid(j_number.begin() + 1, j_number.end());\n    }\n    catch (const json::invalid_iterator& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

      Output:

      [\"bravo\",\"charly\"]\n42\n{\"one\":\"eins\"}\n[json.exception.invalid_iterator.204] iterators out of range\n
      Example: (8) copy constructor

      The following code shows an example for the copy constructor.

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON array\n    json j1 = {\"one\", \"two\", 3, 4.5, false};\n\n    // create a copy\n    json j2(j1);\n\n    // serialize the JSON array\n    std::cout << j1 << \" = \" << j2 << '\\n';\n    std::cout << std::boolalpha << (j1 == j2) << '\\n';\n}\n

      Output:

      [\"one\",\"two\",3,4.5,false] = [\"one\",\"two\",3,4.5,false]\ntrue\n
      Example: (9) move constructor

      The code below shows the move constructor explicitly called via std::move.

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON value\n    json a = 23;\n\n    // move contents of a to b\n    json b(std::move(a));\n\n    // serialize the JSON arrays\n    std::cout << a << '\\n';\n    std::cout << b << '\\n';\n}\n

      Output:

      null\n23\n
      "},{"location":"api/basic_json/basic_json/#see-also","title":"See also","text":"
      • array create a JSON array value, forcing array creation from an initializer list even when it looks like an object
      • object create a JSON object value, forcing object creation from an initializer list
      • binary create a JSON binary array value
      • operator= copy assignment operator
      • Creating JSON values - the article on creating JSON values
      "},{"location":"api/basic_json/basic_json/#version-history","title":"Version history","text":"
      1. Since version 1.0.0.
      2. Since version 1.0.0.
      3. Since version 2.1.0.
      4. Since version 3.2.0. Explicit for different string types if JSON_USE_IMPLICIT_CONVERSIONS is 0 since version 3.13.0 unreleased.
      5. Since version 1.0.0.
      6. Since version 1.0.0.
      7. Since version 1.0.0. Fixed in version 3.13.0 unreleased to also check the iterator range for binary values; before, a range that did not cover the whole value (such as (end(), end())) was accepted and the whole binary value was copied, unlike the other primitive types.
      8. Since version 1.0.0.
      9. Since version 1.0.0.
      "},{"location":"api/basic_json/begin/","title":"nlohmann::basic_json::begin","text":"
      iterator begin() noexcept;\nconst_iterator begin() const noexcept;\n

      Returns an iterator to the first element.

      "},{"location":"api/basic_json/begin/#return-value","title":"Return value","text":"

      iterator to the first element

      "},{"location":"api/basic_json/begin/#exception-safety","title":"Exception safety","text":"

      No-throw guarantee: this member function never throws exceptions.

      "},{"location":"api/basic_json/begin/#complexity","title":"Complexity","text":"

      Constant.

      "},{"location":"api/basic_json/begin/#examples","title":"Examples","text":"Example

      The following code shows an example for begin().

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create an array value\n    json array = {1, 2, 3, 4, 5};\n\n    // get an iterator to the first element\n    json::iterator it = array.begin();\n\n    // serialize the element that the iterator points to\n    std::cout << *it << '\\n';\n}\n

      Output:

      1\n
      "},{"location":"api/basic_json/begin/#see-also","title":"See also","text":"
      • end returns an iterator to one past the last element
      • cbegin returns a const iterator to the first element
      • rbegin returns a reverse iterator to the last element
      • items returns an iteration proxy to access keys and values during range-based for loops
      • Iterators - the article on iterators
      • basic_json_view::begin - the same iteration on a zero-copy view (in document order, not sorted by key)
      "},{"location":"api/basic_json/begin/#version-history","title":"Version history","text":"
      • Added in version 1.0.0.
      "},{"location":"api/basic_json/binary/","title":"nlohmann::basic_json::binary","text":"
      // (1)\nstatic basic_json binary(const typename binary_t::container_type& init);\nstatic basic_json binary(typename binary_t::container_type&& init);\n\n// (2)\nstatic basic_json binary(const typename binary_t::container_type& init,\n                         typename binary_t::subtype_type subtype);\nstatic basic_json binary(typename binary_t::container_type&& init,\n                         typename binary_t::subtype_type subtype);\n
      1. Creates a JSON binary array value from a given binary container.
      2. Creates a JSON binary array value from a given binary container with subtype.

      Binary values are part of various binary formats, such as CBOR, MessagePack, and BSON. This constructor is used to create a value for serialization to those formats.

      "},{"location":"api/basic_json/binary/#parameters","title":"Parameters","text":"init (in) container containing bytes to use as a binary type subtype (in) subtype to use in CBOR, MessagePack, and BSON"},{"location":"api/basic_json/binary/#return-value","title":"Return value","text":"

      JSON binary array value

      "},{"location":"api/basic_json/binary/#exception-safety","title":"Exception safety","text":"

      Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

      "},{"location":"api/basic_json/binary/#complexity","title":"Complexity","text":"

      Linear in the size of init; constant for typename binary_t::container_type&& init versions.

      "},{"location":"api/basic_json/binary/#notes","title":"Notes","text":"

      Note, this function exists because of the difficulty in correctly specifying the correct template overload in the standard value ctor, as both JSON arrays and JSON binary arrays are backed with some form of a std::vector. Because JSON binary arrays are a non-standard extension, it was decided that it would be best to prevent automatic initialization of a binary array type, for backwards compatibility and so it does not happen on accident.

      "},{"location":"api/basic_json/binary/#examples","title":"Examples","text":"Example

      The following code shows how to create a binary value.

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a binary vector\n    std::vector<std::uint8_t> vec = {0xCA, 0xFE, 0xBA, 0xBE};\n\n    // create a binary JSON value with subtype 42\n    json j = json::binary(vec, 42);\n\n    // output type and subtype\n    std::cout << \"type: \" << j.type_name() << \", subtype: \" << j.get_binary().subtype() << std::endl;\n}\n

      Output:

      type: binary, subtype: 42\n
      "},{"location":"api/basic_json/binary/#see-also","title":"See also","text":"
      • binary_t type for binary values
      • get_binary get a reference to the stored binary value
      • is_binary return whether the value is binary
      • byte_container_with_subtype container for binary values with subtype
      • Binary Values - the article on binary values
      "},{"location":"api/basic_json/binary/#version-history","title":"Version history","text":"
      • Added in version 3.8.0.
      • Changed the type of subtype from std::uint8_t to binary_t::subtype_type (std::uint64_t) in version 3.10.0.
      "},{"location":"api/basic_json/binary_t/","title":"nlohmann::basic_json::binary_t","text":"
      using binary_t = byte_container_with_subtype<BinaryType>;\n

      This type is a type designed to carry binary data that appears in various serialized formats, such as CBOR's Major Type 2, MessagePack's bin, and BSON's generic binary subtype. This type is NOT a part of standard JSON and exists solely for compatibility with these binary types. As such, it is simply defined as an ordered sequence of zero or more byte values.

      Additionally, as an implementation detail, the subtype of the binary data is carried around as a std::uint64_t, which is compatible with both of the binary data formats that use binary subtyping, (though the specific numbering is incompatible with each other, and it is up to the user to translate between them). The subtype is added to BinaryType via the helper type byte_container_with_subtype.

      CBOR's RFC 8949 describes this type as:

      Major type 2: A byte string. The number of bytes in the string is equal to the argument.

      MessagePack's documentation on the bin type family describes this type as:

      Bin format family stores a byte array in 2, 3, or 5 bytes of extra bytes in addition to the size of the byte array.

      BSON's specifications describe several binary types; however, this type is intended to represent the generic binary type which has the description:

      Generic binary subtype - This is the most commonly used binary subtype and should be the 'default' for drivers and tools.

      None of these impose any limitations on the internal representation other than the basic unit of storage be some type of array whose parts are decomposable into bytes.

      The default representation of this binary format is a std::vector<std::uint8_t>, which is a very common way to represent a byte array in modern C++.

      "},{"location":"api/basic_json/binary_t/#template-parameters","title":"Template parameters","text":"BinaryType

      container type to store arrays

      Although not formally expressed as a C++ concept, BinaryType must be default-constructible, copy/move-constructible, and support push_back(), .data(), and .size(), because byte_container_with_subtype derives directly from it. Its value_type must additionally be exactly one byte wide (e.g., std::uint8_t/char/std::byte): the binary serializers (CBOR, MessagePack, BSON, UBJSON) read and write the container's raw bytes via reinterpret_cast, which is only correct for byte-sized elements -- a container like std::vector<std::intptr_t> will not work as BinaryType. The elements must be stored contiguously, and the binary readers additionally require resize() and operator[]. See Template Parameter Requirements for the full list.

      "},{"location":"api/basic_json/binary_t/#notes","title":"Notes","text":""},{"location":"api/basic_json/binary_t/#default-type","title":"Default type","text":"

      The default values for BinaryType is std::vector<std::uint8_t>.

      "},{"location":"api/basic_json/binary_t/#supported-byte-types","title":"Supported byte types","text":"

      std::vector<std::uint8_t>, std::vector<char>, and std::vector<std::byte> are supported. Regardless of which of them is configured, dump writes the bytes as the numbers 0..255.

      "},{"location":"api/basic_json/binary_t/#custom-binarytype-behavior","title":"Custom BinaryType behavior","text":"

      When a custom BinaryType is configured (other than the default std::vector<std::uint8_t>), you can assign values of that type directly to a basic_json instance, and they will automatically be recognized as binary values rather than arrays:

      using custom_json = nlohmann::ordered_json::with_binary_t<std::vector<std::byte>>;\n\nstd::vector<std::byte> data{std::byte{1}, std::byte{2}, std::byte{3}};\ncustom_json j = data;  // Creates a binary value, not an array\nassert(j.is_binary());\n\n// Round-tripping works seamlessly\nauto extracted = j.get<std::vector<std::byte>>();\nassert(extracted == data);\n

      This automatic type detection is a convenience feature that only applies to custom (non-default) BinaryType configurations. The default nlohmann::json continues to treat std::vector<std::uint8_t> as arrays for backward compatibility.

      "},{"location":"api/basic_json/binary_t/#storage","title":"Storage","text":"

      Binary Arrays are stored as pointers in a basic_json type. That is, for any access to array values, a pointer of the type binary_t* must be dereferenced.

      "},{"location":"api/basic_json/binary_t/#notes-on-subtypes","title":"Notes on subtypes","text":"

      • CBOR

        • Binary values are represented as byte strings. Subtypes are written as tags.

      • MessagePack

        • If a subtype is given and the binary array contains exactly 1, 2, 4, 8, or 16 elements, the fixext family (fixext1, fixext2, fixext4, fixext8) is used. For other sizes, the ext family (ext8, ext16, ext32) is used. The subtype is then added as a signed 8-bit integer.
        • If no subtype is given, the bin family (bin8, bin16, bin32) is used.

      • BSON

        • If a subtype is given, it is used and added as an unsigned 8-bit integer.
        • If no subtype is given, the generic binary subtype 0x00 is used.
      "},{"location":"api/basic_json/binary_t/#examples","title":"Examples","text":"Example

      The following code shows that binary_t is by default, a typedef to nlohmann::byte_container_with_subtype<std::vector<std::uint8_t>>.

      #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::boolalpha << std::is_same<nlohmann::byte_container_with_subtype<std::vector<std::uint8_t>>, json::binary_t>::value << std::endl;\n}\n

      Output:

      true\n
      "},{"location":"api/basic_json/binary_t/#see-also","title":"See also","text":"
      • byte_container_with_subtype
      "},{"location":"api/basic_json/binary_t/#version-history","title":"Version history","text":"
      • Added in version 3.8.0. Changed the type of subtype to std::uint64_t in version 3.10.0.
      • Fixed dump, std::hash, and to_ubjson for byte types that are not integers (e.g., std::byte) in version 3.13.0 unreleased. dump now writes the bytes of a signed byte type (e.g., char) as 0..255 rather than as negative numbers.
      "},{"location":"api/basic_json/boolean_t/","title":"nlohmann::basic_json::boolean_t","text":"
      using boolean_t = BooleanType;\n

      The type used to store JSON booleans.

      RFC 8259 implicitly describes a boolean as a type which differentiates the two literals true and false.

      To store boolean values in C++, a type is defined by the template parameter BooleanType which chooses the type to use.

      "},{"location":"api/basic_json/boolean_t/#template-parameters","title":"Template parameters","text":"BooleanType the type to store booleans. As it is stored directly inside a basic_json value (in a union), it must be a trivially default-constructible, trivially copyable, and trivially destructible type that is convertible to and from bool. See Template Parameter Requirements."},{"location":"api/basic_json/boolean_t/#notes","title":"Notes","text":""},{"location":"api/basic_json/boolean_t/#default-type","title":"Default type","text":"

      With the default values for BooleanType (bool), the default type for boolean_t is bool.

      "},{"location":"api/basic_json/boolean_t/#storage","title":"Storage","text":"

      Boolean values are stored directly inside a basic_json type.

      "},{"location":"api/basic_json/boolean_t/#examples","title":"Examples","text":"Example

      The following code shows that boolean_t is by default, a typedef to bool.

      #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::boolalpha << std::is_same<bool, json::boolean_t>::value << std::endl;\n}\n

      Output:

      true\n
      "},{"location":"api/basic_json/boolean_t/#see-also","title":"See also","text":"
      • is_boolean checks whether the JSON value is a boolean
      "},{"location":"api/basic_json/boolean_t/#version-history","title":"Version history","text":"
      • Added in version 1.0.0.
      "},{"location":"api/basic_json/cbegin/","title":"nlohmann::basic_json::cbegin","text":"
      const_iterator cbegin() const noexcept;\n

      Returns an iterator to the first element.

      "},{"location":"api/basic_json/cbegin/#return-value","title":"Return value","text":"

      iterator to the first element

      "},{"location":"api/basic_json/cbegin/#exception-safety","title":"Exception safety","text":"

      No-throw guarantee: this member function never throws exceptions.

      "},{"location":"api/basic_json/cbegin/#complexity","title":"Complexity","text":"

      Constant.

      "},{"location":"api/basic_json/cbegin/#examples","title":"Examples","text":"Example

      The following code shows an example for cbegin().

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create an array value\n    const json array = {1, 2, 3, 4, 5};\n\n    // get an iterator to the first element\n    json::const_iterator it = array.cbegin();\n\n    // serialize the element that the iterator points to\n    std::cout << *it << '\\n';\n}\n

      Output:

      1\n
      "},{"location":"api/basic_json/cbegin/#see-also","title":"See also","text":"
      • begin returns an iterator to the first element
      • cend returns a const iterator to one past the last element
      • crbegin returns a const reverse iterator to the last element
      • Iterators - the article on iterators
      • basic_json_view::cbegin - the same iteration on a zero-copy view
      "},{"location":"api/basic_json/cbegin/#version-history","title":"Version history","text":"
      • Added in version 1.0.0.
      "},{"location":"api/basic_json/cbor_tag_handler_t/","title":"nlohmann::basic_json::cbor_tag_handler_t","text":"
      enum class cbor_tag_handler_t\n{\n    error,\n    ignore,\n    store\n};\n

      This enumeration is used in from_cbor and sax_parse to choose how to treat tags:

      error report a parse error in case of a tag (the from_cbor overloads throw a parse_error exception by default) ignore ignore tags store store tagged byte strings (for bytes 0xd8..0xdb) as binary values with the tag as subtype; other tagged values are read as if the tag were ignored. If several tags precede a byte string, only the innermost one is stored."},{"location":"api/basic_json/cbor_tag_handler_t/#examples","title":"Examples","text":"Example

      The example below shows how the different values of the cbor_tag_handler_t influence the behavior of from_cbor when reading a tagged byte string.

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // tagged byte string\n    std::vector<std::uint8_t> vec = {{0xd8, 0x42, 0x44, 0xcA, 0xfe, 0xba, 0xbe}};\n\n    // cbor_tag_handler_t::error throws\n    try\n    {\n        auto b_throw_on_tag = json::from_cbor(vec, true, true, json::cbor_tag_handler_t::error);\n    }\n    catch (const json::parse_error& e)\n    {\n        std::cout << e.what() << std::endl;\n    }\n\n    // cbor_tag_handler_t::ignore ignores the tag\n    auto b_ignore_tag = json::from_cbor(vec, true, true, json::cbor_tag_handler_t::ignore);\n    std::cout << b_ignore_tag << std::endl;\n\n    // cbor_tag_handler_t::store stores the tag as binary subtype\n    auto b_store_tag = json::from_cbor(vec, true, true, json::cbor_tag_handler_t::store);\n    std::cout << b_store_tag << std::endl;\n}\n

      Output:

      [json.exception.parse_error.112] parse error at byte 1: syntax error while parsing CBOR value: invalid byte: 0xD8\n{\"bytes\":[202,254,186,190],\"subtype\":null}\n{\"bytes\":[202,254,186,190],\"subtype\":66}\n
      "},{"location":"api/basic_json/cbor_tag_handler_t/#see-also","title":"See also","text":"
      • from_cbor deserializes a JSON value from CBOR
      • input_format_t the enumeration of supported input formats
      • CBOR - the article on the CBOR format
      "},{"location":"api/basic_json/cbor_tag_handler_t/#version-history","title":"Version history","text":"
      • Added in version 3.9.0. Added value store in 3.10.0.
      "},{"location":"api/basic_json/cend/","title":"nlohmann::basic_json::cend","text":"
      const_iterator cend() const noexcept;\n

      Returns an iterator to one past the last element.

      "},{"location":"api/basic_json/cend/#return-value","title":"Return value","text":"

      iterator one past the last element

      "},{"location":"api/basic_json/cend/#exception-safety","title":"Exception safety","text":"

      No-throw guarantee: this member function never throws exceptions.

      "},{"location":"api/basic_json/cend/#complexity","title":"Complexity","text":"

      Constant.

      "},{"location":"api/basic_json/cend/#examples","title":"Examples","text":"Example

      The following code shows an example for cend().

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create an array value\n    json array = {1, 2, 3, 4, 5};\n\n    // get an iterator to one past the last element\n    json::const_iterator it = array.cend();\n\n    // decrement the iterator to point to the last element\n    --it;\n\n    // serialize the element that the iterator points to\n    std::cout << *it << '\\n';\n}\n

      Output:

      5\n
      "},{"location":"api/basic_json/cend/#see-also","title":"See also","text":"
      • end returns an iterator to one past the last element
      • cbegin returns a const iterator to the first element
      • crend returns a const reverse iterator to one before the first element
      • Iterators - the article on iterators
      • basic_json_view::cend - the same iteration on a zero-copy view
      "},{"location":"api/basic_json/cend/#version-history","title":"Version history","text":"
      • Added in version 1.0.0.
      "},{"location":"api/basic_json/clear/","title":"nlohmann::basic_json::clear","text":"
      void clear() noexcept;\n

      Clears the content of a JSON value and resets it to the default value as if basic_json(value_t) would have been called with the current value type from type():

      Value type initial value null null boolean false string \"\" number 0 binary An empty byte vector with no subtype object {} array []

      Has the same effect as calling

      *this = basic_json(type());\n
      "},{"location":"api/basic_json/clear/#exception-safety","title":"Exception safety","text":"

      No-throw guarantee: this function never throws exceptions.

      "},{"location":"api/basic_json/clear/#complexity","title":"Complexity","text":"

      Linear in the size of the JSON value.

      "},{"location":"api/basic_json/clear/#notes","title":"Notes","text":"

      All iterators, pointers, and references related to this container are invalidated.

      "},{"location":"api/basic_json/clear/#examples","title":"Examples","text":"Example

      The example below shows the effect of clear() to different JSON types.

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_null;\n    json j_boolean = true;\n    json j_number_integer = 17;\n    json j_number_float = 23.42;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n    json j_string = \"Hello, world\";\n\n    // call clear()\n    j_null.clear();\n    j_boolean.clear();\n    j_number_integer.clear();\n    j_number_float.clear();\n    j_object.clear();\n    j_array.clear();\n    j_string.clear();\n\n    // serialize the cleared values()\n    std::cout << j_null << '\\n';\n    std::cout << j_boolean << '\\n';\n    std::cout << j_number_integer << '\\n';\n    std::cout << j_number_float << '\\n';\n    std::cout << j_object << '\\n';\n    std::cout << j_array << '\\n';\n    std::cout << j_string << '\\n';\n}\n

      Output:

      null\nfalse\n0\n0.0\n{}\n[]\n\"\"\n
      "},{"location":"api/basic_json/clear/#see-also","title":"See also","text":"
      • erase removes elements from a JSON value
      • empty checks whether the JSON value has no elements
      "},{"location":"api/basic_json/clear/#version-history","title":"Version history","text":"
      • Added in version 1.0.0.
      • Added support for binary types in version 3.8.0.
      • Fixed in version 3.13.0 unreleased to also clear the subtype of a binary value; before, the subtype was left unchanged.
      "},{"location":"api/basic_json/contains/","title":"nlohmann::basic_json::contains","text":"
      // (1)\nbool contains(const typename object_t::key_type& key) const;\n\n// (2)\ntemplate<typename KeyType>\nbool contains(KeyType&& key) const;\n\n// (3)\nbool contains(const json_pointer& ptr) const;\n\n// (4)\ntemplate<typename T>\nbool contains(T) const = delete;\n
      1. Check whether an element exists in a JSON object with a key equivalent to key. If the element is not found or the JSON value is not an object, false is returned.
      2. See 1. This overload is only available if KeyType is comparable with typename object_t::key_type and typename object_comparator_t::is_transparent denotes a type.
      3. Check whether the given JSON pointer ptr can be resolved in the current JSON value.
      4. Deleted: this overload is only available if T is an integral type and is declared as deleted, so that a call with an integer key (for example, j.contains(0)) fails to compile. Otherwise, the integer literal 0 would convert to a null const char* and, from there, to the key type, causing undefined behavior at runtime.
      "},{"location":"api/basic_json/contains/#template-parameters","title":"Template parameters","text":"KeyType A type for an object key other than json_pointer that is comparable with string_t using object_comparator_t. This can also be a string view (C++17)."},{"location":"api/basic_json/contains/#parameters","title":"Parameters","text":"key (in) key value to check its existence. ptr (in) JSON pointer to check its existence."},{"location":"api/basic_json/contains/#return-value","title":"Return value","text":"
      1. true if an element with specified key exists. If no such element with such a key is found or the JSON value is not an object, false is returned.
      2. See 1.
      3. true if the JSON pointer can be resolved to a stored value, false otherwise.
      4. Deleted; a call with an integral argument does not compile.
      "},{"location":"api/basic_json/contains/#exception-safety","title":"Exception safety","text":"

      Strong exception safety: if an exception occurs, the original value stays intact.

      "},{"location":"api/basic_json/contains/#exceptions","title":"Exceptions","text":"
      1. The function does not throw exceptions.
      2. The function does not throw exceptions.
      3. The function does not throw exceptions.
      4. Deleted; a call with an integral argument does not compile.
      "},{"location":"api/basic_json/contains/#complexity","title":"Complexity","text":"

      Logarithmic in the size of the JSON object.

      "},{"location":"api/basic_json/contains/#notes","title":"Notes","text":"
      • This method always returns false when executed on a JSON type that is not an object.
      • This method can be executed on any JSON value type.
      • Calling this function with an integer argument (for example, contains(0)) does not compile: such an argument would otherwise implicitly convert to a null const char* and, from there, cause undefined behavior when constructing a std::string for the object key. To check for an array element instead, use at, operator[], or compare against size.

      Postconditions

      If j.contains(x) returns true for a key or JSON pointer x, then it is safe to call j[x].

      Deprecation

      Overload (3) also accepts a json_pointer whose template argument is a basic_json specialization (e.g., nlohmann::json_pointer<nlohmann::json>) instead of a string type. This is deprecated since version 3.11.0 and will be removed in a future major version; use basic_json::json_pointer (for json, nlohmann::json_pointer<std::string>) instead.

      You should be warned by your compiler with a -Wdeprecated-declarations warning if you are using a deprecated function.

      See the migration guide for how to update existing code.

      "},{"location":"api/basic_json/contains/#examples","title":"Examples","text":"Example: (1) check with key

      The example shows how contains() is used.

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // create some JSON values\n    json j_object = R\"( {\"key\": \"value\"} )\"_json;\n    json j_array = R\"( [1, 2, 3] )\"_json;\n\n    // call contains\n    std::cout << std::boolalpha <<\n              \"j_object contains 'key': \" << j_object.contains(\"key\") << '\\n' <<\n              \"j_object contains 'another': \" << j_object.contains(\"another\") << '\\n' <<\n              \"j_array contains 'key': \" << j_array.contains(\"key\") << std::endl;\n}\n

      Output:

      j_object contains 'key': true\nj_object contains 'another': false\nj_array contains 'key': false\n
      Example: (2) check with key using string_view

      The example shows how contains() is used.

      #include <iostream>\n#include <string_view>\n#include <nlohmann/json.hpp>\n\nusing namespace std::string_view_literals;\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // create some JSON values\n    json j_object = R\"( {\"key\": \"value\"} )\"_json;\n    json j_array = R\"( [1, 2, 3] )\"_json;\n\n    // call contains\n    std::cout << std::boolalpha <<\n              \"j_object contains 'key': \" << j_object.contains(\"key\"sv) << '\\n' <<\n              \"j_object contains 'another': \" << j_object.contains(\"another\"sv) << '\\n' <<\n              \"j_array contains 'key': \" << j_array.contains(\"key\"sv) << std::endl;\n}\n

      Output:

      j_object contains 'key': true\nj_object contains 'another': false\nj_array contains 'key': false\n
      Example: (3) check with JSON pointer

      The example shows how contains() is used.

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // create a JSON value\n    json j =\n    {\n        {\"number\", 1}, {\"string\", \"foo\"}, {\"array\", {1, 2}}\n    };\n\n    std::cout << std::boolalpha\n              << j.contains(\"/number\"_json_pointer) << '\\n'\n              << j.contains(\"/string\"_json_pointer) << '\\n'\n              << j.contains(\"/array\"_json_pointer) << '\\n'\n              << j.contains(\"/array/1\"_json_pointer) << '\\n'\n              << j.contains(\"/array/-\"_json_pointer) << '\\n'\n              << j.contains(\"/array/4\"_json_pointer) << '\\n'\n              << j.contains(\"/baz\"_json_pointer) << '\\n'\n              // an array index with a leading '0' is not found\n              << j.contains(\"/array/01\"_json_pointer) << '\\n'\n              // an array index that is not a number is not found\n              << j.contains(\"/array/one\"_json_pointer) << std::endl;\n}\n

      Output:

      true\ntrue\ntrue\ntrue\nfalse\nfalse\nfalse\nfalse\nfalse\n
      "},{"location":"api/basic_json/contains/#see-also","title":"See also","text":"
      • find find a value in an object
      • count returns the number of occurrences of a key
      • basic_json_view::contains - the same check on a zero-copy view
      "},{"location":"api/basic_json/contains/#version-history","title":"Version history","text":"
      1. Added in version 3.11.0.
      2. Added in version 3.6.0. Extended template KeyType to support comparable types in version 3.11.0. Fixed in version 3.13.0 unreleased to consistently accept std::string_view-convertible keys, as already supported by operator[], at, value, and other lookup functions.
      3. Added in version 3.7.0.
      4. Deleted overloads for integral key types added in version 3.13.0 unreleased to reject such calls at compile time instead of causing undefined behavior at runtime.
      "},{"location":"api/basic_json/count/","title":"nlohmann::basic_json::count","text":"
      // (1)\nsize_type count(const typename object_t::key_type& key) const;\n\n// (2)\ntemplate<typename KeyType>\nsize_type count(KeyType&& key) const;\n\n// (3)\ntemplate<typename T>\nsize_type count(T) const = delete;\n
      1. Returns the number of elements with key key. If ObjectType is the default std::map type, the return value will always be 0 (key was not found) or 1 (key was found).
      2. See 1. This overload is only available if KeyType is comparable with typename object_t::key_type and typename object_comparator_t::is_transparent denotes a type.
      3. Deleted: this overload is only available if T is an integral type and is declared as deleted, so that a call with an integer key (for example, j.count(0)) fails to compile. Otherwise, the integer literal 0 would convert to a null const char* and, from there, to the key type, causing undefined behavior at runtime.
      "},{"location":"api/basic_json/count/#template-parameters","title":"Template parameters","text":"KeyType A type for an object key other than json_pointer that is comparable with string_t using object_comparator_t. This can also be a string view (C++17)."},{"location":"api/basic_json/count/#parameters","title":"Parameters","text":"key (in) key value of the element to count."},{"location":"api/basic_json/count/#return-value","title":"Return value","text":"

      Number of elements with key key. If the JSON value is not an object, the return value will be 0.

      "},{"location":"api/basic_json/count/#exception-safety","title":"Exception safety","text":"

      Strong exception safety: if an exception occurs, the original value stays intact.

      "},{"location":"api/basic_json/count/#complexity","title":"Complexity","text":"

      Logarithmic in the size of the JSON object.

      "},{"location":"api/basic_json/count/#notes","title":"Notes","text":"
      • This method always returns 0 when executed on a JSON type that is not an object.
      • Calling this function with an integer argument (for example, count(0)) does not compile: such an argument would otherwise implicitly convert to a null const char* and, from there, cause undefined behavior when constructing a std::string for the object key. To check for an array element instead, use at, operator[], or compare against size.
      "},{"location":"api/basic_json/count/#examples","title":"Examples","text":"Example: (1) count number of elements

      The example shows how count() is used.

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON object\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n\n    // call count()\n    auto count_two = j_object.count(\"two\");\n    auto count_three = j_object.count(\"three\");\n\n    // print values\n    std::cout << \"number of elements with key \\\"two\\\": \" << count_two << '\\n';\n    std::cout << \"number of elements with key \\\"three\\\": \" << count_three << '\\n';\n}\n

      Output:

      number of elements with key \"two\": 1\nnumber of elements with key \"three\": 0\n
      Example: (2) count number of elements using string_view

      The example shows how count() is used.

      #include <iostream>\n#include <string_view>\n#include <nlohmann/json.hpp>\n\nusing namespace std::string_view_literals;\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON object\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n\n    // call count()\n    auto count_two = j_object.count(\"two\"sv);\n    auto count_three = j_object.count(\"three\"sv);\n\n    // print values\n    std::cout << \"number of elements with key \\\"two\\\": \" << count_two << '\\n';\n    std::cout << \"number of elements with key \\\"three\\\": \" << count_three << '\\n';\n}\n

      Output:

      number of elements with key \"two\": 1\nnumber of elements with key \"three\": 0\n
      "},{"location":"api/basic_json/count/#see-also","title":"See also","text":"
      • find find a value in an object
      • contains checks whether a key exists
      • basic_json_view::count - the same check on a zero-copy view
      "},{"location":"api/basic_json/count/#version-history","title":"Version history","text":"
      1. Added in version 3.11.0.
      2. Added in version 1.0.0. Changed parameter key type to KeyType&& in version 3.11.0. Fixed in version 3.13.0 unreleased to consistently accept std::string_view-convertible keys, as already supported by operator[], at, value, and other lookup functions.
      3. Deleted overload for integral key types added in version 3.13.0 unreleased to reject such calls at compile time instead of causing undefined behavior at runtime.
      "},{"location":"api/basic_json/crbegin/","title":"nlohmann::basic_json::crbegin","text":"
      const_reverse_iterator crbegin() const noexcept;\n

      Returns an iterator to the reverse-beginning; that is, the last element.

      "},{"location":"api/basic_json/crbegin/#return-value","title":"Return value","text":"

      reverse iterator to the last element

      "},{"location":"api/basic_json/crbegin/#exception-safety","title":"Exception safety","text":"

      No-throw guarantee: this member function never throws exceptions.

      "},{"location":"api/basic_json/crbegin/#complexity","title":"Complexity","text":"

      Constant.

      "},{"location":"api/basic_json/crbegin/#examples","title":"Examples","text":"Example

      The following code shows an example for crbegin().

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create an array value\n    json array = {1, 2, 3, 4, 5};\n\n    // get an iterator to the reverse-beginning\n    json::const_reverse_iterator it = array.crbegin();\n\n    // serialize the element that the iterator points to\n    std::cout << *it << '\\n';\n}\n

      Output:

      5\n
      "},{"location":"api/basic_json/crbegin/#see-also","title":"See also","text":"
      • crend returns a const reverse iterator to one before the first element
      • rbegin returns a reverse iterator to the last element
      • cbegin returns a const iterator to the first element
      • Iterators - the article on iterators
      "},{"location":"api/basic_json/crbegin/#version-history","title":"Version history","text":"
      • Added in version 1.0.0.
      "},{"location":"api/basic_json/crend/","title":"nlohmann::basic_json::crend","text":"
      const_reverse_iterator crend() const noexcept;\n

      Returns an iterator to the reverse-end; that is, one before the first element. This element acts as a placeholder, attempting to access it results in undefined behavior.

      "},{"location":"api/basic_json/crend/#return-value","title":"Return value","text":"

      reverse iterator to the element following the last element

      "},{"location":"api/basic_json/crend/#exception-safety","title":"Exception safety","text":"

      No-throw guarantee: this member function never throws exceptions.

      "},{"location":"api/basic_json/crend/#complexity","title":"Complexity","text":"

      Constant.

      "},{"location":"api/basic_json/crend/#examples","title":"Examples","text":"Example

      The following code shows an example for crend().

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create an array value\n    json array = {1, 2, 3, 4, 5};\n\n    // get an iterator to the reverse-end\n    json::const_reverse_iterator it = array.crend();\n\n    // increment the iterator to point to the first element\n    --it;\n\n    // serialize the element that the iterator points to\n    std::cout << *it << '\\n';\n}\n

      Output:

      1\n
      "},{"location":"api/basic_json/crend/#see-also","title":"See also","text":"
      • crbegin returns a const reverse iterator to the last element
      • rend returns a reverse iterator to one before the first element
      • cend returns a const iterator to one past the last element
      • Iterators - the article on iterators
      "},{"location":"api/basic_json/crend/#version-history","title":"Version history","text":"
      • Added in version 1.0.0.
      "},{"location":"api/basic_json/default_object_comparator_t/","title":"nlohmann::basic_json::default_object_comparator_t","text":"
      using default_object_comparator_t = std::less<StringType>;  // until C++14\n\nusing default_object_comparator_t = std::less<>;            // since C++14\n

      The default comparator used by object_t.

      Since C++14 a transparent comparator is used which prevents unnecessary string construction when looking up a key in an object.

      The actual comparator used depends on object_t and can be obtained via object_comparator_t.

      "},{"location":"api/basic_json/default_object_comparator_t/#examples","title":"Examples","text":"Example

      The example below demonstrates the default comparator.

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::boolalpha\n              << \"one < two : \" << json::default_object_comparator_t{}(\"one\", \"two\") << \"\\n\"\n              << \"three < four : \" << json::default_object_comparator_t{}(\"three\", \"four\") << std::endl;\n}\n

      Output:

      one < two : true\nthree < four : false\n
      "},{"location":"api/basic_json/default_object_comparator_t/#see-also","title":"See also","text":"
      • object_comparator_t the comparator actually used by object_t
      • object_t the type used to store JSON objects
      "},{"location":"api/basic_json/default_object_comparator_t/#version-history","title":"Version history","text":"
      • Added in version 3.11.0.
      "},{"location":"api/basic_json/diff/","title":"nlohmann::basic_json::diff","text":"
      static basic_json diff(const basic_json& source,\n                       const basic_json& target);\n

      Creates a JSON Patch so that value source can be changed into the value target by calling patch function.

      For two JSON values source and target, the following code always yields true: 

      source.patch(diff(source, target)) == target;\n

      "},{"location":"api/basic_json/diff/#parameters","title":"Parameters","text":"source (in) JSON value to compare from target (in) JSON value to compare against"},{"location":"api/basic_json/diff/#return-value","title":"Return value","text":"

      a JSON patch to convert the source to target

      "},{"location":"api/basic_json/diff/#exception-safety","title":"Exception safety","text":"

      Strong guarantee: source and target are never modified.

      "},{"location":"api/basic_json/diff/#complexity","title":"Complexity","text":"

      Linear in the lengths of source and target.

      "},{"location":"api/basic_json/diff/#notes","title":"Notes","text":"

      Currently, only remove, add, and replace operations are generated.

      "},{"location":"api/basic_json/diff/#examples","title":"Examples","text":"Example

      The following code shows how a JSON patch is created as a diff for two JSON values.

      #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // the source document\n    json source = R\"(\n        {\n            \"baz\": \"qux\",\n            \"foo\": \"bar\"\n        }\n    )\"_json;\n\n    // the target document\n    json target = R\"(\n        {\n            \"baz\": \"boo\",\n            \"hello\": [\n                \"world\"\n            ]\n        }\n    )\"_json;\n\n    // create the patch\n    json patch = json::diff(source, target);\n\n    // roundtrip\n    json patched_source = source.patch(patch);\n\n    // output patch and roundtrip result\n    std::cout << std::setw(4) << patch << \"\\n\\n\"\n              << std::setw(4) << patched_source << std::endl;\n}\n

      Output:

      [\n    {\n        \"op\": \"replace\",\n        \"path\": \"/baz\",\n        \"value\": \"boo\"\n    },\n    {\n        \"op\": \"remove\",\n        \"path\": \"/foo\"\n    },\n    {\n        \"op\": \"add\",\n        \"path\": \"/hello\",\n        \"value\": [\n            \"world\"\n        ]\n    }\n]\n\n{\n    \"baz\": \"boo\",\n    \"hello\": [\n        \"world\"\n    ]\n}\n
      "},{"location":"api/basic_json/diff/#see-also","title":"See also","text":"
      • RFC 6902 (JSON Patch)
      • patch applies a JSON Patch
      • patch_inplace applies a JSON Patch in place
      • merge_patch applies a JSON Merge Patch
      • merge_diff creates a diff as a JSON Merge Patch
      "},{"location":"api/basic_json/diff/#version-history","title":"Version history","text":"
      • Added in version 2.0.0.
      "},{"location":"api/basic_json/dump/","title":"nlohmann::basic_json::dump","text":"
      string_t dump(const int indent = -1,\n              const char indent_char = ' ',\n              const bool ensure_ascii = false,\n              const error_handler_t error_handler = error_handler_t::strict) const;\n

      Serialization function for JSON values. The function tries to mimic Python's json.dumps() function, and currently supports its indent and ensure_ascii parameters.

      "},{"location":"api/basic_json/dump/#parameters","title":"Parameters","text":"indent (in) If indent is nonnegative, then array elements and object members will be pretty-printed with that indent level. An indent level of 0 will only insert newlines. -1 (the default) selects the most compact representation. indent_char (in) The character to use for indentation if indent is greater than 0. The default is (space). ensure_ascii (in) If ensure_ascii is true, all non-ASCII characters in the output are escaped with \\uXXXX sequences, and the result consists of ASCII characters only. error_handler (in) how to react on decoding errors; there are four possible values (see error_handler_t: strict (throws an exception in case a decoding error occurs; default), replace (replace invalid UTF-8 sequences with U+FFFD), ignore (ignore invalid UTF-8 sequences during serialization; all valid bytes are copied to the output unchanged, and invalid bytes are dropped), and keep (write the ill-formed bytes to the output as is, without escaping them, even if ensure_ascii is true; the result is then not valid UTF-8, but equals the input bytes exactly, and well-formed characters around the ill-formed bytes are still escaped as usual))."},{"location":"api/basic_json/dump/#return-value","title":"Return value","text":"

      string containing the serialization of the JSON value

      "},{"location":"api/basic_json/dump/#exception-safety","title":"Exception safety","text":"

      Strong guarantee: if an exception is thrown, there are no changes to any JSON value.

      "},{"location":"api/basic_json/dump/#exceptions","title":"Exceptions","text":"

      Throws type_error.316 if a string stored inside the JSON value is not UTF-8 encoded and error_handler is set to strict

      Serializing untrusted input

      When serializing values that may contain invalid or untrusted UTF-8 (e.g., bytes taken directly from network input), dump() throws type_error.316 in the default strict mode. To serialize such data without throwing, pass error_handler_t::replace (substitutes U+FFFD) or error_handler_t::ignore. Callers that serialize untrusted input on a crash-sensitive path should either choose a non-strict error handler or wrap dump() in a try/catch.

      See the FAQ for details.

      "},{"location":"api/basic_json/dump/#complexity","title":"Complexity","text":"

      Linear.

      "},{"location":"api/basic_json/dump/#notes","title":"Notes","text":"

      Floating-point numbers are written with the fewest digits that read back as the same value (for double; see number handling).

      Binary values are serialized as an object containing two keys:

      • \"bytes\": an array of bytes as integers
      • \"subtype\": the subtype as integer or null if the binary has no subtype
      "},{"location":"api/basic_json/dump/#examples","title":"Examples","text":"Example

      The following example shows the effect of different indent, indent_char, and ensure_ascii parameters to the result of the serialization.

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n    json j_string = \"Hell\u00f6 \ud83d\ude00!\";\n\n    // call dump()\n    std::cout << \"objects:\" << '\\n'\n              << j_object.dump() << \"\\n\\n\"\n              << j_object.dump(-1) << \"\\n\\n\"\n              << j_object.dump(0) << \"\\n\\n\"\n              << j_object.dump(4) << \"\\n\\n\"\n              << j_object.dump(1, '\\t') << \"\\n\\n\";\n\n    std::cout << \"arrays:\" << '\\n'\n              << j_array.dump() << \"\\n\\n\"\n              << j_array.dump(-1) << \"\\n\\n\"\n              << j_array.dump(0) << \"\\n\\n\"\n              << j_array.dump(4) << \"\\n\\n\"\n              << j_array.dump(1, '\\t') << \"\\n\\n\";\n\n    std::cout << \"strings:\" << '\\n'\n              << j_string.dump() << '\\n'\n              << j_string.dump(-1, ' ', true) << '\\n';\n\n    // create JSON value with invalid UTF-8 byte sequence\n    json j_invalid = \"\u00e4\\xA9\u00fc\";\n    try\n    {\n        std::cout << j_invalid.dump() << std::endl;\n    }\n    catch (const json::type_error& e)\n    {\n        std::cout << e.what() << std::endl;\n    }\n\n    std::cout << \"string with replaced invalid characters: \"\n              << j_invalid.dump(-1, ' ', false, json::error_handler_t::replace)\n              << \"\\nstring with ignored invalid characters: \"\n              << j_invalid.dump(-1, ' ', false, json::error_handler_t::ignore)\n              << '\\n';\n}\n

      Output:

      objects:\n{\"one\":1,\"two\":2}\n\n{\"one\":1,\"two\":2}\n\n{\n\"one\": 1,\n\"two\": 2\n}\n\n{\n    \"one\": 1,\n    \"two\": 2\n}\n\n{\n    \"one\": 1,\n    \"two\": 2\n}\n\narrays:\n[1,2,4,8,16]\n\n[1,2,4,8,16]\n\n[\n1,\n2,\n4,\n8,\n16\n]\n\n[\n    1,\n    2,\n    4,\n    8,\n    16\n]\n\n[\n    1,\n    2,\n    4,\n    8,\n    16\n]\n\nstrings:\n\"Hell\u00f6 \ud83d\ude00!\"\n\"Hell\\u00f6 \\ud83d\\ude00!\"\n[json.exception.type_error.316] invalid UTF-8 byte at index 2: 0xA9\nstring with replaced invalid characters: \"\u00e4\ufffd\u00fc\"\nstring with ignored invalid characters: \"\u00e4\u00fc\"\n
      "},{"location":"api/basic_json/dump/#see-also","title":"See also","text":"
      • to_string returns a string representation of a JSON value
      • operator<< serialize to stream
      • basic_json_view::dump the corresponding function of basic_json_view, serializing directly from a flat index without building a basic_json value
      • Serialization - the serialization article
      "},{"location":"api/basic_json/dump/#version-history","title":"Version history","text":"
      • Added in version 1.0.0.
      • Indentation character indent_char, option ensure_ascii and exceptions added in version 3.0.0.
      • Error handlers added in version 3.4.0.
      • Serialization of binary values added in version 3.8.0.
      • Error handler keep added in version 3.13.0 unreleased.
      • Doubles are written with the shortest digits (\u017bmij instead of Grisu2) since version 3.13.0 unreleased; about 0.1% of doubles are written differently, most of them with fewer digits.
      "},{"location":"api/basic_json/emplace/","title":"nlohmann::basic_json::emplace","text":"
      template<class... Args>\nstd::pair<iterator, bool> emplace(Args&& ... args);\n

      Inserts a new element into a JSON object constructed in-place with the given args if there is no element with the key in the container. If the function is called on a JSON null value, an empty object is created before appending the value created from args.

      "},{"location":"api/basic_json/emplace/#template-parameters","title":"Template parameters","text":"Args compatible types to create a basic_json object"},{"location":"api/basic_json/emplace/#iterator-invalidation","title":"Iterator invalidation","text":"

      For ordered_json, adding a value to an object can yield a reallocation, in which case all iterators (including the end() iterator) and all references to the elements are invalidated.

      "},{"location":"api/basic_json/emplace/#parameters","title":"Parameters","text":"args (in) arguments to forward to a constructor of basic_json"},{"location":"api/basic_json/emplace/#return-value","title":"Return value","text":"

      a pair consisting of an iterator to the inserted element, or the already-existing element if no insertion happened, and a bool denoting whether the insertion took place.

      "},{"location":"api/basic_json/emplace/#exception-safety","title":"Exception safety","text":"

      Strong guarantee: if an exception is thrown, there are no changes to any JSON value. As an exception, a null value is converted to an empty object before the element is added and keeps that type if adding the element throws.

      "},{"location":"api/basic_json/emplace/#exceptions","title":"Exceptions","text":"

      Throws type_error.311 when called on a type other than JSON object or null; example: \"cannot use emplace() with number\"

      "},{"location":"api/basic_json/emplace/#complexity","title":"Complexity","text":"

      Logarithmic in the size of the container, O(log(size())).

      "},{"location":"api/basic_json/emplace/#examples","title":"Examples","text":"Example

      The example shows how emplace() can be used to add elements to a JSON object. Note how the null value was silently converted to a JSON object. Further note how no value is added if there was already one value stored with the same key.

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json object = {{\"one\", 1}, {\"two\", 2}};\n    json null;\n\n    // print values\n    std::cout << object << '\\n';\n    std::cout << null << '\\n';\n\n    // add values\n    auto res1 = object.emplace(\"three\", 3);\n    null.emplace(\"A\", \"a\");\n    null.emplace(\"B\", \"b\");\n\n    // the following call will not add an object, because there is already\n    // a value stored at key \"B\"\n    auto res2 = null.emplace(\"B\", \"c\");\n\n    // print values\n    std::cout << object << '\\n';\n    std::cout << *res1.first << \" \" << std::boolalpha << res1.second << '\\n';\n\n    std::cout << null << '\\n';\n    std::cout << *res2.first << \" \" << std::boolalpha << res2.second << '\\n';\n}\n

      Output:

      {\"one\":1,\"two\":2}\nnull\n{\"one\":1,\"three\":3,\"two\":2}\n3 true\n{\"A\":\"a\",\"B\":\"b\"}\n\"b\" false\n
      "},{"location":"api/basic_json/emplace/#see-also","title":"See also","text":"
      • emplace_back add a value to an array
      • insert add values to an array/object
      • Modifying values - the article on modifying values
      "},{"location":"api/basic_json/emplace/#version-history","title":"Version history","text":"
      • Since version 2.0.8.
      • Fixed in version 3.13.0 unreleased: for ordered_json, the value could previously only be passed as an rvalue; it can now also be passed as an lvalue or a const lvalue, matching the behavior of json.
      "},{"location":"api/basic_json/emplace_back/","title":"nlohmann::basic_json::emplace_back","text":"
      template<class... Args>\nreference emplace_back(Args&& ... args);\n

      Creates a JSON value from the passed parameters args to the end of the JSON value. If the function is called on a JSON null value, an empty array is created before appending the value created from args.

      "},{"location":"api/basic_json/emplace_back/#template-parameters","title":"Template parameters","text":"Args compatible types to create a basic_json object"},{"location":"api/basic_json/emplace_back/#iterator-invalidation","title":"Iterator invalidation","text":"

      By adding an element to the end of the array, a reallocation can happen, in which case all iterators (including the end() iterator) and all references to the elements are invalidated. Otherwise, only the end() iterator is invalidated.

      "},{"location":"api/basic_json/emplace_back/#parameters","title":"Parameters","text":"args (in) arguments to forward to a constructor of basic_json"},{"location":"api/basic_json/emplace_back/#return-value","title":"Return value","text":"

      reference to the inserted element

      "},{"location":"api/basic_json/emplace_back/#exception-safety","title":"Exception safety","text":"

      Strong guarantee: if an exception is thrown, there are no changes to any JSON value. As an exception, a null value is converted to an empty array before the element is added and keeps that type if adding the element throws.

      "},{"location":"api/basic_json/emplace_back/#exceptions","title":"Exceptions","text":"

      Throws type_error.311 when called on a type other than JSON array or null; example: \"cannot use emplace_back() with number\"

      "},{"location":"api/basic_json/emplace_back/#complexity","title":"Complexity","text":"

      Amortized constant.

      "},{"location":"api/basic_json/emplace_back/#examples","title":"Examples","text":"Example

      The example shows how emplace_back() can be used to add elements to a JSON array. Note how the null value was silently converted to a JSON array.

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json array = {1, 2, 3, 4, 5};\n    json null;\n\n    // print values\n    std::cout << array << '\\n';\n    std::cout << null << '\\n';\n\n    // add values\n    array.emplace_back(6);\n    null.emplace_back(\"first\");\n    null.emplace_back(3, \"second\");\n\n    // print values\n    std::cout << array << '\\n';\n    std::cout << null << '\\n';\n}\n

      Output:

      [1,2,3,4,5]\nnull\n[1,2,3,4,5,6]\n[\"first\",[\"second\",\"second\",\"second\"]]\n
      "},{"location":"api/basic_json/emplace_back/#see-also","title":"See also","text":"
      • operator+= add a value to an array/object
      • push_back add a value to an array/object
      • Modifying values - the article on modifying values
      "},{"location":"api/basic_json/emplace_back/#version-history","title":"Version history","text":"
      • Since version 2.0.8.
      • Returns reference since 3.7.0.
      "},{"location":"api/basic_json/empty/","title":"nlohmann::basic_json::empty","text":"
      bool empty() const noexcept;\n

      Checks if a JSON value has no elements (i.e., whether its size() is 0).

      "},{"location":"api/basic_json/empty/#return-value","title":"Return value","text":"

      The return value depends on the different types and is defined as follows:

      Value type return value null true boolean false string false number false binary false object result of function object_t::empty() array result of function array_t::empty()"},{"location":"api/basic_json/empty/#exception-safety","title":"Exception safety","text":"

      No-throw guarantee: this function never throws exceptions.

      "},{"location":"api/basic_json/empty/#complexity","title":"Complexity","text":"

      Constant, as long as array_t and object_t satisfy the Container concept; that is, their empty() functions have constant complexity.

      "},{"location":"api/basic_json/empty/#possible-implementation","title":"Possible implementation","text":"
      bool empty() const noexcept\n{\n    return size() == 0;\n}\n
      "},{"location":"api/basic_json/empty/#notes","title":"Notes","text":"

      This function does not return whether a string stored as JSON value is empty -- it returns whether the JSON container itself is empty which is false in the case of a string.

      "},{"location":"api/basic_json/empty/#examples","title":"Examples","text":"Example

      The following code uses empty() to check if a JSON object contains any elements.

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_null;\n    json j_boolean = true;\n    json j_number_integer = 17;\n    json j_number_float = 23.42;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_object_empty(json::value_t::object);\n    json j_array = {1, 2, 4, 8, 16};\n    json j_array_empty(json::value_t::array);\n    json j_string = \"Hello, world\";\n\n    // call empty()\n    std::cout << std::boolalpha;\n    std::cout << j_null.empty() << '\\n';\n    std::cout << j_boolean.empty() << '\\n';\n    std::cout << j_number_integer.empty() << '\\n';\n    std::cout << j_number_float.empty() << '\\n';\n    std::cout << j_object.empty() << '\\n';\n    std::cout << j_object_empty.empty() << '\\n';\n    std::cout << j_array.empty() << '\\n';\n    std::cout << j_array_empty.empty() << '\\n';\n    std::cout << j_string.empty() << '\\n';\n}\n

      Output:

      true\nfalse\nfalse\nfalse\nfalse\ntrue\nfalse\ntrue\nfalse\n
      "},{"location":"api/basic_json/empty/#see-also","title":"See also","text":"
      • size returns the number of elements
      • clear clears the content and resets the value to the default value
      • basic_json_view::empty - the same check on a zero-copy view
      "},{"location":"api/basic_json/empty/#version-history","title":"Version history","text":"
      • Added in version 1.0.0.
      • Extended to return false for binary types in version 3.8.0.
      "},{"location":"api/basic_json/end/","title":"nlohmann::basic_json::end","text":"
      iterator end() noexcept;\nconst_iterator end() const noexcept;\n

      Returns an iterator to one past the last element.

      "},{"location":"api/basic_json/end/#return-value","title":"Return value","text":"

      iterator one past the last element

      "},{"location":"api/basic_json/end/#exception-safety","title":"Exception safety","text":"

      No-throw guarantee: this member function never throws exceptions.

      "},{"location":"api/basic_json/end/#complexity","title":"Complexity","text":"

      Constant.

      "},{"location":"api/basic_json/end/#examples","title":"Examples","text":"Example

      The following code shows an example for end().

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create an array value\n    json array = {1, 2, 3, 4, 5};\n\n    // get an iterator to one past the last element\n    json::iterator it = array.end();\n\n    // decrement the iterator to point to the last element\n    --it;\n\n    // serialize the element that the iterator points to\n    std::cout << *it << '\\n';\n}\n

      Output:

      5\n
      "},{"location":"api/basic_json/end/#see-also","title":"See also","text":"
      • begin returns an iterator to the first element
      • cend returns a const iterator to one past the last element
      • rend returns a reverse iterator to one before the first element
      • Iterators - the article on iterators
      • basic_json_view::end - the same iteration on a zero-copy view
      "},{"location":"api/basic_json/end/#version-history","title":"Version history","text":"
      • Added in version 1.0.0.
      "},{"location":"api/basic_json/end_pos/","title":"nlohmann::basic_json::end_pos","text":"
      #if JSON_DIAGNOSTIC_POSITIONS\nconstexpr std::size_t end_pos() const noexcept;\n#endif\n

      Returns the position immediately following the last character of the JSON string from which the value was parsed from.

      JSON type return value object position after the closing } array position after the closing ] string position after the closing \" number position after the last character boolean position after e null position after l"},{"location":"api/basic_json/end_pos/#return-value","title":"Return value","text":"

      the position of the character following the last character of the given value in the parsed JSON string, if the value was created by the parse function, or std::string::npos if the value was constructed otherwise

      "},{"location":"api/basic_json/end_pos/#exception-safety","title":"Exception safety","text":"

      No-throw guarantee: this member function never throws exceptions.

      "},{"location":"api/basic_json/end_pos/#complexity","title":"Complexity","text":"

      Constant.

      "},{"location":"api/basic_json/end_pos/#notes","title":"Notes","text":"

      Note

      The function is only available if macro JSON_DIAGNOSTIC_POSITIONS has been defined to 1 before including the library header.

      Invalidation

      The returned positions are only valid as long as the JSON value is not changed. The positions are not updated when the JSON value is changed.

      "},{"location":"api/basic_json/end_pos/#examples","title":"Examples","text":"Example 
      #include <iostream>\n\n#define JSON_DIAGNOSTIC_POSITIONS 1\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::string json_string = R\"(\n    {\n        \"address\": {\n            \"street\": \"Fake Street\",\n            \"housenumber\": 1\n        }\n    }\n    )\";\n    json j = json::parse(json_string);\n\n    std::cout << \"Root diagnostic positions: \\n\";\n    std::cout << \"\\tstart_pos: \" << j.start_pos() << '\\n';\n    std::cout << \"\\tend_pos:\" << j.end_pos() << \"\\n\";\n    std::cout << \"Original string: \\n\";\n    std::cout << \"{\\n        \\\"address\\\": {\\n            \\\"street\\\": \\\"Fake Street\\\",\\n            \\\"housenumber\\\": 1\\n        }\\n    }\" << \"\\n\";\n    std::cout << \"Parsed string: \\n\";\n    std::cout << json_string.substr(j.start_pos(), j.end_pos() - j.start_pos()) << \"\\n\\n\";\n\n    std::cout << \"address diagnostic positions: \\n\";\n    std::cout << \"\\tstart_pos:\" << j[\"address\"].start_pos() << '\\n';\n    std::cout << \"\\tend_pos:\" << j[\"address\"].end_pos() << \"\\n\\n\";\n    std::cout << \"Original string: \\n\";\n    std::cout << \"{            \\\"street\\\": \\\"Fake Street\\\",\\n            \\\"housenumber\\\": 1\\n        }\" << \"\\n\";\n    std::cout << \"Parsed string: \\n\";\n    std::cout << json_string.substr(j[\"address\"].start_pos(), j[\"address\"].end_pos() - j[\"address\"].start_pos()) << \"\\n\\n\";\n\n    std::cout << \"street diagnostic positions: \\n\";\n    std::cout << \"\\tstart_pos:\" << j[\"address\"][\"street\"].start_pos() << '\\n';\n    std::cout << \"\\tend_pos:\" << j[\"address\"][\"street\"].end_pos() << \"\\n\\n\";\n    std::cout << \"Original string: \\n\";\n    std::cout << \"\\\"Fake Street\\\"\" << \"\\n\";\n    std::cout << \"Parsed string: \\n\";\n    std::cout << json_string.substr(j[\"address\"][\"street\"].start_pos(), j[\"address\"][\"street\"].end_pos() - j[\"address\"][\"street\"].start_pos()) << \"\\n\\n\";\n\n    std::cout << \"housenumber diagnostic positions: \\n\";\n    std::cout << \"\\tstart_pos:\" << j[\"address\"][\"housenumber\"].start_pos() << '\\n';\n    std::cout << \"\\tend_pos:\" << j[\"address\"][\"housenumber\"].end_pos() << \"\\n\\n\";\n    std::cout << \"Original string: \\n\";\n    std::cout << \"1\" << \"\\n\";\n    std::cout << \"Parsed string: \\n\";\n    std::cout << json_string.substr(j[\"address\"][\"housenumber\"].start_pos(), j[\"address\"][\"housenumber\"].end_pos() - j[\"address\"][\"housenumber\"].start_pos()) << \"\\n\\n\";\n}\n

      Output:

      Root diagnostic positions: \n    start_pos: 5\n    end_pos:109\nOriginal string: \n{\n        \"address\": {\n            \"street\": \"Fake Street\",\n            \"housenumber\": 1\n        }\n    }\nParsed string: \n{\n        \"address\": {\n            \"street\": \"Fake Street\",\n            \"housenumber\": 1\n        }\n    }\n\naddress diagnostic positions: \n    start_pos:26\n    end_pos:103\n\nOriginal string: \n{            \"street\": \"Fake Street\",\n            \"housenumber\": 1\n        }\nParsed string: \n{\n            \"street\": \"Fake Street\",\n            \"housenumber\": 1\n        }\n\nstreet diagnostic positions: \n    start_pos:50\n    end_pos:63\n\nOriginal string: \n\"Fake Street\"\nParsed string: \n\"Fake Street\"\n\nhousenumber diagnostic positions: \n    start_pos:92\n    end_pos:93\n\nOriginal string: \n1\nParsed string: \n1\n

      The output shows the start/end positions of all the objects and fields in the JSON string.

      "},{"location":"api/basic_json/end_pos/#see-also","title":"See also","text":"
      • start_pos to access the start position
      • JSON_DIAGNOSTIC_POSITIONS for an overview of the diagnostic positions
      "},{"location":"api/basic_json/end_pos/#version-history","title":"Version history","text":"
      • Added in version 3.12.0.
      "},{"location":"api/basic_json/erase/","title":"nlohmann::basic_json::erase","text":"
      // (1)\niterator erase(iterator pos);\nconst_iterator erase(const_iterator pos);\n\n// (2)\niterator erase(iterator first, iterator last);\nconst_iterator erase(const_iterator first, const_iterator last);\n\n// (3)\nsize_type erase(const typename object_t::key_type& key);\n\n// (4)\ntemplate<typename KeyType>\nsize_type erase(KeyType&& key);\n\n// (5)\nvoid erase(const size_type idx);\n

      1. Removes an element from a JSON value specified by iterator pos. The iterator pos must be valid and dereferenceable. Thus, the end() iterator (which is valid, but is not dereferenceable) cannot be used as a value for pos.

        If called on a primitive type other than null, the resulting JSON value will be null.

      2. Remove an element range specified by [first; last) from a JSON value. The iterator first does not need to be dereferenceable if first == last: erasing an empty range is a no-op.

        If called on a primitive type other than null, the resulting JSON value will be null.

      3. Removes an element from a JSON object by key.

      4. See 3. This overload is only available if KeyType is comparable with typename object_t::key_type and typename object_comparator_t::is_transparent denotes a type.

      5. Removes an element from a JSON array by index.

      "},{"location":"api/basic_json/erase/#template-parameters","title":"Template parameters","text":"KeyType A type for an object key other than json_pointer that is comparable with string_t using object_comparator_t. This can also be a string view (C++17)."},{"location":"api/basic_json/erase/#parameters","title":"Parameters","text":"pos (in) iterator to the element to remove first (in) iterator to the beginning of the range to remove last (in) iterator past the end of the range to remove key (in) object key of the elements to remove idx (in) array index of the element to remove"},{"location":"api/basic_json/erase/#return-value","title":"Return value","text":"
      1. Iterator following the last removed element. If the iterator pos refers to the last element, the end() iterator is returned.
      2. Iterator following the last removed element. If the iterator last refers to the last element, the end() iterator is returned.
      3. Number of elements removed. If ObjectType is the default std::map type, the return value will always be 0 (key was not found) or 1 (key was found).
      4. See 3.
      5. (none)
      "},{"location":"api/basic_json/erase/#exception-safety","title":"Exception safety","text":"

      Strong exception safety: if an exception occurs, the original value stays intact.

      "},{"location":"api/basic_json/erase/#exceptions","title":"Exceptions","text":"
      1. The function can throw the following exceptions:
        • Throws type_error.307 if called on a null value; example: \"cannot use erase() with null\"
        • Throws invalid_iterator.202 if called on an iterator which does not belong to the current JSON value; example: \"iterator does not fit current value\"
        • Throws invalid_iterator.205 if called on a primitive type with invalid iterator (i.e., any iterator which is not begin()); example: \"iterator out of range\"
      2. The function can throw the following exceptions:
        • Throws type_error.307 if called on a null value; example: \"cannot use erase() with null\"
        • Throws invalid_iterator.203 if called on iterators which does not belong to the current JSON value; example: \"iterators do not fit current value\"
        • Throws invalid_iterator.204 if called on a primitive type with invalid iterators (i.e., if first != begin() and last != end()); example: \"iterators out of range\"
      3. The function can throw the following exceptions:
        • Throws type_error.307 when called on a type other than JSON object; example: \"cannot use erase() with null\"
      4. See 3.
      5. The function can throw the following exceptions:
        • Throws type_error.307 when called on a type other than JSON array; example: \"cannot use erase() with null\"
        • Throws out_of_range.401 when idx >= size(); example: \"array index 17 is out of range\"
      "},{"location":"api/basic_json/erase/#complexity","title":"Complexity","text":"
      1. The complexity depends on the type:
        • objects: amortized constant
        • arrays: linear in distance between pos and the end of the container
        • strings and binary: linear in the length of the member
        • other types: constant
      2. The complexity depends on the type:
        • objects: log(size()) + std::distance(first, last)
        • arrays: linear in the distance between first and last, plus linear in the distance between last and end of the container
        • strings and binary: linear in the length of the member
        • other types: constant
      3. log(size()) + count(key)
      4. log(size()) + count(key)
      5. Linear in distance between idx and the end of the container.
      "},{"location":"api/basic_json/erase/#notes","title":"Notes","text":"
      1. Invalidates iterators and references at or after the point of the erase, including the end() iterator.
      2. (none)
      3. References and iterators to the erased elements are invalidated. Other references and iterators are not affected.
      4. See 3.
      5. (none)
      "},{"location":"api/basic_json/erase/#examples","title":"Examples","text":"Example: (1) remove element given an iterator

      The example shows the effect of erase() for different JSON types using an iterator.

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_boolean = true;\n    json j_number_integer = 17;\n    json j_number_float = 23.42;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n    json j_string = \"Hello, world\";\n\n    // call erase()\n    j_boolean.erase(j_boolean.begin());\n    j_number_integer.erase(j_number_integer.begin());\n    j_number_float.erase(j_number_float.begin());\n    j_object.erase(j_object.find(\"two\"));\n    j_array.erase(j_array.begin() + 2);\n    j_string.erase(j_string.begin());\n\n    // print values\n    std::cout << j_boolean << '\\n';\n    std::cout << j_number_integer << '\\n';\n    std::cout << j_number_float << '\\n';\n    std::cout << j_object << '\\n';\n    std::cout << j_array << '\\n';\n    std::cout << j_string << '\\n';\n}\n

      Output:

      null\nnull\nnull\n{\"one\":1}\n[1,2,8,16]\nnull\n
      Example: (2) remove elements given an iterator range

      The example shows the effect of erase() for different JSON types using an iterator range.

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_boolean = true;\n    json j_number_integer = 17;\n    json j_number_float = 23.42;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n    json j_string = \"Hello, world\";\n\n    // call erase()\n    j_boolean.erase(j_boolean.begin(), j_boolean.end());\n    j_number_integer.erase(j_number_integer.begin(), j_number_integer.end());\n    j_number_float.erase(j_number_float.begin(), j_number_float.end());\n    j_object.erase(j_object.find(\"two\"), j_object.end());\n    j_array.erase(j_array.begin() + 1, j_array.begin() + 3);\n    j_string.erase(j_string.begin(), j_string.end());\n\n    // print values\n    std::cout << j_boolean << '\\n';\n    std::cout << j_number_integer << '\\n';\n    std::cout << j_number_float << '\\n';\n    std::cout << j_object << '\\n';\n    std::cout << j_array << '\\n';\n    std::cout << j_string << '\\n';\n}\n

      Output:

      null\nnull\nnull\n{\"one\":1}\n[1,8,16]\nnull\n
      Example: (3) remove element from a JSON object given a key

      The example shows the effect of erase() for different JSON types using an object key.

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON object\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n\n    // call erase()\n    auto count_one = j_object.erase(\"one\");\n    auto count_three = j_object.erase(\"three\");\n\n    // print values\n    std::cout << j_object << '\\n';\n    std::cout << count_one << \" \" << count_three << '\\n';\n}\n

      Output:

      {\"two\":2}\n1 0\n
      Example: (4) remove element from a JSON object given a key using string_view

      The example shows the effect of erase() for different JSON types using an object key.

      #include <iostream>\n#include <string_view>\n#include <nlohmann/json.hpp>\n\nusing namespace std::string_view_literals;\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON object\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n\n    // call erase()\n    auto count_one = j_object.erase(\"one\"sv);\n    auto count_three = j_object.erase(\"three\"sv);\n\n    // print values\n    std::cout << j_object << '\\n';\n    std::cout << count_one << \" \" << count_three << '\\n';\n}\n

      Output:

      {\"two\":2}\n1 0\n
      Example: (5) remove element from a JSON array given an index

      The example shows the effect of erase() using an array index.

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON array\n    json j_array = {0, 1, 2, 3, 4, 5};\n\n    // call erase()\n    j_array.erase(2);\n\n    // print values\n    std::cout << j_array << '\\n';\n}\n

      Output:

      [0,1,3,4,5]\n
      "},{"location":"api/basic_json/erase/#see-also","title":"See also","text":"
      • clear clears the contents
      • insert add values to an array/object
      • Modifying values - the article on modifying values
      "},{"location":"api/basic_json/erase/#version-history","title":"Version history","text":"
      1. Added in version 1.0.0. Added support for binary types in version 3.8.0.
      2. Added in version 1.0.0. Added support for binary types in version 3.8.0.
      3. Added in version 1.0.0.
      4. Added in version 3.11.0. Fixed in version 3.13.0 unreleased to consistently accept std::string_view-convertible keys, as already supported by operator[], at, value, and other lookup functions.
      5. Added in version 1.0.0.
      "},{"location":"api/basic_json/error_handler_t/","title":"nlohmann::basic_json::error_handler_t","text":"
      enum class error_handler_t {\n    strict,\n    replace,\n    ignore,\n    keep\n};\n

      This enumeration is used to choose how to treat ill-formed UTF-8 in a string value or object key:

      • dump uses it while serializing a basic_json value to text.
      • to_cbor, to_msgpack, to_ubjson, to_bjdata, and to_bson use it while serializing a basic_json value to that binary format. Their default is keep, as no binary writer checked before this parameter was added. CBOR, UBJSON, BJData, and BSON require valid UTF-8, so for these four the default is strict if JSON_STRICT_BINARY_UTF8 is enabled; MessagePack's specification explicitly allows a string to contain ill-formed UTF-8, so to_msgpack stays at keep. to_bon8 does not take this parameter: BON8 always validates, since UTF-8 lead bytes are structural to that format.
      • from_cbor, from_msgpack, from_ubjson, from_bjdata, and from_bson use it while parsing that binary format, to decide whether to check a string value or object key for well-formed UTF-8 at all; by default (keep) they do not, as no binary reader did before this parameter was added. from_bon8 does not take this parameter, for the same reason to_bon8 does not.

      Four values are differentiated:

      strict throw a type_error/parse_error exception in case of invalid UTF-8 replace replace invalid UTF-8 sequences with U+FFFD (\ufffd REPLACEMENT CHARACTER) ignore ignore invalid UTF-8 sequences; all valid bytes are copied to the output unchanged, and invalid bytes are dropped keep keep invalid UTF-8 sequences unchanged; only meaningful for the binary formats mentioned above, since dump itself must produce text, and keep there writes the ill-formed bytes to the output as is, so the result is then not valid UTF-8 (but still equals the input bytes exactly, including around any well-formed characters, which are still escaped as usual)"},{"location":"api/basic_json/error_handler_t/#examples","title":"Examples","text":"Example

      The example below shows how the different values of the error_handler_t influence the behavior of dump when reading serializing an invalid UTF-8 sequence.

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON value with invalid UTF-8 byte sequence\n    json j_invalid = \"\u00e4\\xA9\u00fc\";\n    try\n    {\n        std::cout << j_invalid.dump() << std::endl;\n    }\n    catch (const json::type_error& e)\n    {\n        std::cout << e.what() << std::endl;\n    }\n\n    std::cout << \"string with replaced invalid characters: \"\n              << j_invalid.dump(-1, ' ', false, json::error_handler_t::replace)\n              << \"\\nstring with ignored invalid characters: \"\n              << j_invalid.dump(-1, ' ', false, json::error_handler_t::ignore)\n              << \"\\nstring with the invalid byte kept as is (\" << j_invalid.dump(-1, ' ', false, json::error_handler_t::keep).size()\n              << \" bytes, not valid UTF-8 itself)\\n\";\n}\n

      Output:

      [json.exception.type_error.316] invalid UTF-8 byte at index 2: 0xA9\nstring with replaced invalid characters: \"\u00e4\ufffd\u00fc\"\nstring with ignored invalid characters: \"\u00e4\u00fc\"\nstring with the invalid byte kept as is (7 bytes, not valid UTF-8 itself)\n
      "},{"location":"api/basic_json/error_handler_t/#see-also","title":"See also","text":"
      • dump serializes a JSON value, with an error_handler_t parameter to configure invalid UTF-8 handling
      • Handling invalid UTF-8 - the article on handling invalid UTF-8
      "},{"location":"api/basic_json/error_handler_t/#version-history","title":"Version history","text":"
      • Added in version 3.4.0.
      • Added keep, and made this enumeration apply to the binary readers and writers in addition to dump, in version 3.13.0.
      "},{"location":"api/basic_json/exception/","title":"nlohmann::basic_json::exception","text":"
      class exception : public std::exception;\n

      This class is an extension of std::exception objects with a member id for exception ids. It is used as the base class for all exceptions thrown by the basic_json class. This class can hence be used as \"wildcard\" to catch exceptions, see example below.

      classDiagram\n  direction LR\n\n    class std_exception [\"std::exception\"] {\n        <<interface>>\n    }\n\n    class json_exception [\"basic_json::exception\"] {\n        +const int id\n        +const char* what() const\n    }\n\n    class json_parse_error [\"basic_json::parse_error\"] {\n        +const std::size_t byte\n    }\n\n    class json_invalid_iterator [\"basic_json::invalid_iterator\"]\n    class json_type_error [\"basic_json::type_error\"]\n    class json_out_of_range [\"basic_json::out_of_range\"]\n    class json_other_error [\"basic_json::other_error\"]\n\n    std_exception <|-- json_exception\n    json_exception <|-- json_parse_error\n    json_exception <|-- json_invalid_iterator\n    json_exception <|-- json_type_error\n    json_exception <|-- json_out_of_range\n    json_exception <|-- json_other_error\n\n    style json_exception fill:#CCCCFF

      Subclasses:

      • parse_error for exceptions indicating a parse error
      • invalid_iterator for exceptions indicating errors with iterators
      • type_error for exceptions indicating executing a member function with a wrong type
      • out_of_range for exceptions indicating access out of the defined range
      • other_error for exceptions indicating other library errors
      "},{"location":"api/basic_json/exception/#member-functions","title":"Member functions","text":"
      • what - returns explanatory string
      "},{"location":"api/basic_json/exception/#member-variables","title":"Member variables","text":"
      • id - the id of the exception
      "},{"location":"api/basic_json/exception/#notes","title":"Notes","text":"

      To have nothrow-copy-constructible exceptions, we internally use std::runtime_error which can cope with arbitrary-length error messages. Intermediate strings are built with static functions and then passed to the actual constructor.

      "},{"location":"api/basic_json/exception/#examples","title":"Examples","text":"Example

      The following code shows how arbitrary library exceptions can be caught.

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    try\n    {\n        // calling at() for a non-existing key\n        json j = {{\"foo\", \"bar\"}};\n        json k = j.at(\"non-existing\");\n    }\n    catch (const json::exception& e)\n    {\n        // output exception information\n        std::cout << \"message: \" << e.what() << '\\n'\n                  << \"exception id: \" << e.id << std::endl;\n    }\n}\n

      Output:

      message: [json.exception.out_of_range.403] key 'non-existing' not found\nexception id: 403\n
      "},{"location":"api/basic_json/exception/#see-also","title":"See also","text":"

      List of exceptions

      "},{"location":"api/basic_json/exception/#version-history","title":"Version history","text":"
      • Since version 3.0.0.
      "},{"location":"api/basic_json/find/","title":"nlohmann::basic_json::find","text":"
      // (1)\niterator find(const typename object_t::key_type& key);\nconst_iterator find(const typename object_t::key_type& key) const;\n\n// (2)\ntemplate<typename KeyType>\niterator find(KeyType&& key);\ntemplate<typename KeyType>\nconst_iterator find(KeyType&& key) const;\n\n// (3)\ntemplate<typename T>\niterator find(T) = delete;\ntemplate<typename T>\nconst_iterator find(T) const = delete;\n
      1. Finds an element in a JSON object with a key equivalent to key. If the element is not found or the JSON value is not an object, end() is returned.
      2. See 1. This overload is only available if KeyType is comparable with typename object_t::key_type and typename object_comparator_t::is_transparent denotes a type.
      3. Deleted: this overload is only available if T is an integral type and is declared as deleted, so that a call with an integer key (for example, j.find(0)) fails to compile. Otherwise, the integer literal 0 would convert to a null const char* and, from there, to the key type, causing undefined behavior at runtime.
      "},{"location":"api/basic_json/find/#template-parameters","title":"Template parameters","text":"KeyType A type for an object key other than json_pointer that is comparable with string_t using object_comparator_t. This can also be a string view (C++17)."},{"location":"api/basic_json/find/#parameters","title":"Parameters","text":"key (in) key value of the element to search for."},{"location":"api/basic_json/find/#return-value","title":"Return value","text":"

      Iterator to an element with a key equivalent to key. If no such element is found or the JSON value is not an object, a past-the-end iterator (see end()) is returned.

      "},{"location":"api/basic_json/find/#exception-safety","title":"Exception safety","text":"

      Strong exception safety: if an exception occurs, the original value stays intact.

      "},{"location":"api/basic_json/find/#complexity","title":"Complexity","text":"

      Logarithmic in the size of the JSON object.

      "},{"location":"api/basic_json/find/#notes","title":"Notes","text":"
      • This method always returns end() when executed on a JSON type that is not an object.
      • Calling this function with an integer argument (for example, find(0)) does not compile: such an argument would otherwise implicitly convert to a null const char* and, from there, cause undefined behavior when constructing a std::string for the object key. To access an array element instead, use at, operator[], or compare against size.
      "},{"location":"api/basic_json/find/#examples","title":"Examples","text":"Example: (1) find object element by key

      The example shows how find() is used.

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON object\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n\n    // call find\n    auto it_two = j_object.find(\"two\");\n    auto it_three = j_object.find(\"three\");\n\n    // print values\n    std::cout << std::boolalpha;\n    std::cout << \"\\\"two\\\" was found: \" << (it_two != j_object.end()) << '\\n';\n    std::cout << \"value at key \\\"two\\\": \" << *it_two << '\\n';\n    std::cout << \"\\\"three\\\" was found: \" << (it_three != j_object.end()) << '\\n';\n}\n

      Output:

      \"two\" was found: true\nvalue at key \"two\": 2\n\"three\" was found: false\n
      Example: (2) find object element by key using string_view

      The example shows how find() is used.

      #include <iostream>\n#include <string_view>\n#include <nlohmann/json.hpp>\n\nusing namespace std::string_view_literals;\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON object\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n\n    // call find\n    auto it_two = j_object.find(\"two\"sv);\n    auto it_three = j_object.find(\"three\"sv);\n\n    // print values\n    std::cout << std::boolalpha;\n    std::cout << \"\\\"two\\\" was found: \" << (it_two != j_object.end()) << '\\n';\n    std::cout << \"value at key \\\"two\\\": \" << *it_two << '\\n';\n    std::cout << \"\\\"three\\\" was found: \" << (it_three != j_object.end()) << '\\n';\n}\n

      Output:

      \"two\" was found: true\nvalue at key \"two\": 2\n\"three\" was found: false\n
      "},{"location":"api/basic_json/find/#see-also","title":"See also","text":"
      • count returns the number of occurrences of a key
      • contains checks whether a key exists
      • basic_json_view::find - the same lookup on a zero-copy view (a duplicate key resolves to the first member, whereas parse() keeps the last)
      "},{"location":"api/basic_json/find/#version-history","title":"Version history","text":"
      1. Added in version 3.11.0.
      2. Added in version 1.0.0. Changed to support comparable types in version 3.11.0. Fixed in version 3.13.0 unreleased to consistently accept std::string_view-convertible keys, as already supported by operator[], at, value, and other lookup functions.
      3. Deleted overloads for integral key types added in version 3.13.0 unreleased to reject such calls at compile time instead of causing undefined behavior at runtime.
      "},{"location":"api/basic_json/flatten/","title":"nlohmann::basic_json::flatten","text":"
      basic_json flatten() const;\n

      The function creates a JSON object whose keys are JSON pointers (see RFC 6901) and whose values are all primitive (see is_primitive() for more information). The original JSON value can be restored using the unflatten() function.

      "},{"location":"api/basic_json/flatten/#return-value","title":"Return value","text":"

      an object that maps JSON pointers to primitive values

      "},{"location":"api/basic_json/flatten/#exception-safety","title":"Exception safety","text":"

      Strong exception safety: if an exception occurs, the original value stays intact.

      "},{"location":"api/basic_json/flatten/#complexity","title":"Complexity","text":"

      Linear in the size of the JSON value.

      "},{"location":"api/basic_json/flatten/#notes","title":"Notes","text":"

      Empty objects and arrays are flattened to null and will not be reconstructed correctly by the unflatten() function.

      "},{"location":"api/basic_json/flatten/#examples","title":"Examples","text":"Example: flatten a JSON object

      The following code shows how a JSON object is flattened to an object whose keys consist of JSON pointers.

      #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON value\n    json j =\n    {\n        {\"pi\", 3.141},\n        {\"happy\", true},\n        {\"name\", \"Niels\"},\n        {\"nothing\", nullptr},\n        {\n            \"answer\", {\n                {\"everything\", 42}\n            }\n        },\n        {\"list\", {1, 0, 2}},\n        {\n            \"object\", {\n                {\"currency\", \"USD\"},\n                {\"value\", 42.99}\n            }\n        }\n    };\n\n    // call flatten()\n    std::cout << std::setw(4) << j.flatten() << '\\n';\n}\n

      Output:

      {\n    \"/answer/everything\": 42,\n    \"/happy\": true,\n    \"/list/0\": 1,\n    \"/list/1\": 0,\n    \"/list/2\": 2,\n    \"/name\": \"Niels\",\n    \"/nothing\": null,\n    \"/object/currency\": \"USD\",\n    \"/object/value\": 42.99,\n    \"/pi\": 3.141\n}\n
      Example: empty objects and arrays are flattened to null

      The following code shows that an empty object and an empty array are both flattened to null, and that unflatten() restores them as null rather than as empty containers.

      #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON value with an empty object and an empty array\n    json j =\n    {\n        {\"empty_object\", json::object()},\n        {\"empty_array\", json::array()},\n        {\"name\", \"Niels\"}\n    };\n\n    // call flatten()\n    json flattened = j.flatten();\n    std::cout << std::setw(4) << flattened << \"\\n\\n\";\n\n    // the empty containers cannot be restored by unflatten()\n    std::cout << std::setw(4) << flattened.unflatten() << '\\n';\n}\n

      Output:

      {\n    \"/empty_array\": null,\n    \"/empty_object\": null,\n    \"/name\": \"Niels\"\n}\n\n{\n    \"empty_array\": null,\n    \"empty_object\": null,\n    \"name\": \"Niels\"\n}\n
      "},{"location":"api/basic_json/flatten/#see-also","title":"See also","text":"
      • unflatten the reverse function
      "},{"location":"api/basic_json/flatten/#version-history","title":"Version history","text":"
      • Added in version 2.0.0.
      "},{"location":"api/basic_json/format_as/","title":"format_as(basic_json)","text":"
      template <typename BasicJsonType>\nstd::string format_as(const BasicJsonType& j);\n

      This function implements the format_as customization point used by the {fmt} library (fmtlib). It has no dependency on any fmt header and no effect at all unless a caller's translation unit also includes fmt and calls fmt::format/fmt::print on a JSON value.

      "},{"location":"api/basic_json/format_as/#template-parameters","title":"Template parameters","text":"BasicJsonType a specialization of basic_json"},{"location":"api/basic_json/format_as/#return-value","title":"Return value","text":"

      string containing the serialization of the JSON value (same as dump())

      "},{"location":"api/basic_json/format_as/#exception-safety","title":"Exception safety","text":"

      Strong guarantee: if an exception is thrown, there are no changes to any JSON value.

      "},{"location":"api/basic_json/format_as/#exceptions","title":"Exceptions","text":"

      Throws type_error.316 if a string stored inside the JSON value is not UTF-8 encoded

      "},{"location":"api/basic_json/format_as/#complexity","title":"Complexity","text":"

      Linear.

      "},{"location":"api/basic_json/format_as/#possible-implementation","title":"Possible implementation","text":"
      template <typename BasicJsonType>\nstd::string format_as(const BasicJsonType& j)\n{\n    return j.dump();\n}\n
      "},{"location":"api/basic_json/format_as/#notes","title":"Notes","text":"

      Version-dependent effect on fmt

      fmt only picks up a format_as overload that returns a std::string in fmt 10.0.0 through 11.0.2. Starting with fmt 11.1.0, fmt restricts automatic format_as pickup to overloads that return an arithmetic type, so this function has no effect there (it is simply unused, not a compile error).

      If you use fmt >= 11.1.0, or want the same pretty-print spec support that std::formatter<basic_json> has (\"{:#}\", a width to set the indent such as \"{:2}\"/\"{:#2}\", and fill-and-align to pick the indent character such as \"{:.>#}\"), define your own fmt::formatter specialization mirroring the same logic:

      template <>\nstruct fmt::formatter<nlohmann::json>\n{\n    // -1 means compact output (dump()); any value >= 0 means pretty-printed\n    // output with that many spaces (or indent_char) per level.\n    int indent = -1;\n    char indent_char = ' ';\n\n    constexpr auto parse(format_parse_context& ctx) -> format_parse_context::iterator\n    {\n        auto it = ctx.begin();\n        const auto end = ctx.end();\n        constexpr auto is_align = [](char c)\n        {\n            return c == '<' || c == '>' || c == '^';\n        };\n\n        // [[fill] align] - repurposed here to pick a custom indent character\n        if (it != end && it + 1 != end && is_align(it[1]))\n        {\n            indent_char = *it;\n            it += 2;\n        }\n        else if (it != end && is_align(*it))\n        {\n            ++it;\n        }\n\n        // ['#'] - \"alternate form\", used here to request pretty-printing with a\n        // default indent of 4 (overridden by an explicit width below, if given)\n        if (it != end && *it == '#')\n        {\n            indent = 4;\n            ++it;\n        }\n\n        // [width] - repurposed here to pick the indent size; a width without '#'\n        // implies pretty-printing since an indent otherwise has no meaning\n        if (it != end && *it >= '1' && *it <= '9')\n        {\n            indent = 0;\n            while (it != end && *it >= '0' && *it <= '9')\n            {\n                indent = (indent * 10) + (*it - '0');\n                ++it;\n            }\n        }\n\n        if (it != end && *it != '}')\n        {\n            throw fmt::format_error(\"invalid format args for nlohmann::json\");\n        }\n\n        return it;\n    }\n\n    auto format(const nlohmann::json& j, format_context& ctx) const\n    {\n        const auto dumped = j.dump(indent, indent_char);\n        return fmt::format_to(ctx.out(), \"{}\", dumped);\n    }\n};\n

      This recipe isn't shipped by the library itself, since doing so would make fmt a build dependency (see the FAQ entry on using JSON values with std::format or fmt for more background) \u2014 but it is compiled and exercised against a real, current fmt release as part of the library's own test suite (tests/fmt_formatter, via CMake FetchContent), so it's kept in sync with std::formatter<basic_json> and verified to actually work, not just illustrative.

      "},{"location":"api/basic_json/format_as/#examples","title":"Examples","text":"Example

      The following code shows how the library's format_as() function integrates with fmt::format, allowing argument-dependent lookup.

      #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON value\n    json j = {{\"one\", 1}, {\"two\", 2}};\n\n    // format_as() is found via argument-dependent lookup, the same way\n    // fmt::format/fmt::print would find it\n    auto j_str = format_as(j);\n\n    std::cout << j_str << std::endl;\n}\n

      Output:

      {\"one\":1,\"two\":2}\n
      "},{"location":"api/basic_json/format_as/#see-also","title":"See also","text":"
      • dump
      • std::formatter - the std::format (C++20) equivalent
      • Serialization - the serialization article
        • "},{"location":"api/basic_json/format_as/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json/from_bjdata/","title":"nlohmann::basic_json::from_bjdata","text":"
        // (1)\ntemplate<typename InputType>\nstatic basic_json from_bjdata(InputType&& i,\n                              const bool strict = true,\n                              const bool allow_exceptions = true,\n                              const error_handler_t error_handler = error_handler_t::keep);\n// (2)\ntemplate<typename IteratorType, typename SentinelType = IteratorType>\nstatic basic_json from_bjdata(IteratorType first, SentinelType last,\n                              const bool strict = true,\n                              const bool allow_exceptions = true,\n                              const error_handler_t error_handler = error_handler_t::keep);\n

        Deserializes a given input to a JSON value using the BJData (Binary JData) serialization format.

        1. Reads from a compatible input.
        2. Reads from an iterator range, or an iterator and a sentinel of a different type (C++20 ranges support).

        The exact mapping and its limitations are described on a dedicated page.

        "},{"location":"api/basic_json/from_bjdata/#template-parameters","title":"Template parameters","text":"InputType

        A compatible input, for instance:

        • an std::istream object
        • a FILE pointer
        • a C-style array of characters
        • a pointer to a null-terminated string of single byte characters
        • a container obj for which begin(obj) and end(obj) produce a valid pair of iterators (as found via ADL or member functions, with semantics compatible to std::begin and std::end)
        IteratorType a compatible iterator type SentinelType

        defaults to IteratorType; may be a different type comparable to IteratorType via operator!=, for instance.

        • a custom sentinel type for C++20 ranges
        • std::default_sentinel_t, when IteratorType is std::counted_iterator
        "},{"location":"api/basic_json/from_bjdata/#parameters","title":"Parameters","text":"i (in) an input in BJData format convertible to an input adapter first (in) iterator to the start of the input last (in) iterator to the end of the input, or a sentinel value that compares equal to the end iterator with operator!= strict (in) whether to expect the input to be consumed until EOF (true by default) allow_exceptions (in) whether to throw exceptions in case of a parse error (optional, true by default) error_handler (in) how to treat a string value or object key that is not valid UTF-8; see error_handler_t. BJData does not require a decoder to reject ill-formed UTF-8, so checking is opt-in: the default, keep, does not check at all, as every binary reader did before this parameter was added; strict checks and throws; replace/ignore sanitize the string the same way dump would"},{"location":"api/basic_json/from_bjdata/#return-value","title":"Return value","text":"

        deserialized JSON value; in case of a parse error and allow_exceptions set to false, the return value will be value_t::discarded. The latter can be checked with is_discarded.

        "},{"location":"api/basic_json/from_bjdata/#exception-safety","title":"Exception safety","text":"

        Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

        "},{"location":"api/basic_json/from_bjdata/#exceptions","title":"Exceptions","text":"
        • Throws parse_error.110 if the given input ends prematurely or the end of the file was not reached when strict was set to true
        • Throws parse_error.112 if a parse error occurs
        • Throws parse_error.113 if a string could not be parsed successfully, or if a string value or object key is not valid UTF-8 and error_handler is strict
        • Throws out_of_range.408 if the size of an optimized container or n-dimensional array cannot be represented by std::size_t
        "},{"location":"api/basic_json/from_bjdata/#complexity","title":"Complexity","text":"

        Linear in the size of the input.

        "},{"location":"api/basic_json/from_bjdata/#examples","title":"Examples","text":"Example

        The example shows the deserialization of a byte vector in BJData format to a JSON value.

        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create byte vector\n    std::vector<std::uint8_t> v = {0x7B, 0x69, 0x07, 0x63, 0x6F, 0x6D, 0x70, 0x61,\n                                   0x63, 0x74, 0x54, 0x69, 0x06, 0x73, 0x63, 0x68,\n                                   0x65, 0x6D, 0x61, 0x69, 0x00, 0x7D\n                                  };\n\n    // deserialize it with BJData\n    json j = json::from_bjdata(v);\n\n    // print the deserialized JSON value\n    std::cout << std::setw(2) << j << std::endl;\n}\n

        Output:

        {\n  \"compact\": true,\n  \"schema\": 0\n}\n
        "},{"location":"api/basic_json/from_bjdata/#see-also","title":"See also","text":"
        • to_bjdata create a BJData serialization of a JSON value
        • from_cbor create a JSON value from an input in CBOR format
        • from_msgpack create a JSON value from an input in MessagePack format
        • from_bson create a JSON value from an input in BSON format
        • from_ubjson create a JSON value from an input in UBJSON format
        • from_bon8 create a JSON value from an input in BON8 format
        "},{"location":"api/basic_json/from_bjdata/#version-history","title":"Version history","text":"
        • Added in version 3.11.0.
        • Extended container support (1) to include types with lvalue-only ADL begin/end (matching std::begin/std::end semantics) in version 3.13.0 unreleased.
        • Extended overload (2) to accept heterogeneous iterator+sentinel pairs (C++20 ranges support) in version 3.13.0 unreleased.
        • Added error_handler parameter in version 3.13.0 unreleased.

        Deprecation

        • Overload (2) replaces calls to from_bjdata with a pointer and a length as first two parameters, which has been deprecated in version 3.13.0 unreleased. In version 4.0.0 unreleased, this overload will be deleted (= delete) rather than removed. Please replace all calls like from_bjdata(ptr, len, ...); with from_bjdata(ptr, ptr+len, ...);.

        You should be warned by your compiler with a -Wdeprecated-declarations warning if you are using a deprecated function.

        "},{"location":"api/basic_json/from_bon8/","title":"nlohmann::basic_json::from_bon8","text":"
        // (1)\ntemplate<typename InputType>\nstatic basic_json from_bon8(InputType&& i,\n                            const bool strict = true,\n                            const bool allow_exceptions = true);\n// (2)\ntemplate<typename IteratorType, typename SentinelType = IteratorType>\nstatic basic_json from_bon8(IteratorType first, SentinelType last,\n                            const bool strict = true,\n                            const bool allow_exceptions = true);\n

        Deserializes a given input to a JSON value using the BON8 (Binary Object Notation 8) serialization format.

        1. Reads from a compatible input.
        2. Reads from an iterator range, or an iterator and a sentinel of a different type (C++20 ranges support).

        The exact mapping and its limitations are described on a dedicated page.

        "},{"location":"api/basic_json/from_bon8/#template-parameters","title":"Template parameters","text":"InputType

        A compatible input, for instance:

        • an std::istream object
        • a FILE pointer
        • a C-style array of characters
        • a pointer to a null-terminated string of single byte characters
        • a container obj for which begin(obj) and end(obj) produce a valid pair of iterators (as found via ADL or member functions, with semantics compatible to std::begin and std::end)
        IteratorType a compatible iterator type SentinelType

        defaults to IteratorType; may be a different type comparable to IteratorType via operator!=, for instance.

        • a custom sentinel type for C++20 ranges
        • std::default_sentinel_t, when IteratorType is std::counted_iterator
        "},{"location":"api/basic_json/from_bon8/#parameters","title":"Parameters","text":"i (in) an input in BON8 format convertible to an input adapter first (in) iterator to the start of the input last (in) iterator to the end of the input, or a sentinel value that compares equal to the end iterator with operator!= strict (in) whether to expect the input to be consumed until EOF (true by default) allow_exceptions (in) whether to throw exceptions in case of a parse error (optional, true by default)"},{"location":"api/basic_json/from_bon8/#return-value","title":"Return value","text":"

        deserialized JSON value; in case of a parse error and allow_exceptions set to false, the return value will be value_t::discarded. The latter can be checked with is_discarded.

        "},{"location":"api/basic_json/from_bon8/#exception-safety","title":"Exception safety","text":"

        Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

        "},{"location":"api/basic_json/from_bon8/#exceptions","title":"Exceptions","text":"
        • Throws parse_error.110 if the given input ends prematurely or the end of the file was not reached when strict was set to true
        • Throws parse_error.112 if a parse error occurs, for instance an invalid byte, a string that is not valid UTF-8, or an object key that is not a string
        "},{"location":"api/basic_json/from_bon8/#complexity","title":"Complexity","text":"

        Linear in the size of the input.

        "},{"location":"api/basic_json/from_bon8/#examples","title":"Examples","text":"Example

        The example shows the deserialization of a byte vector in BON8 format to a JSON value.

        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create byte vector\n    std::vector<std::uint8_t> v = {0x89, 0x63, 0x6f, 0x6d, 0x70, 0x61, 0x63, 0x74,\n                                   0xf9, 0x66, 0x6f, 0x72, 0x6d, 0x61, 0x74, 0xff,\n                                   0x42, 0x4f, 0x4e, 0x38, 0xff, 0x73, 0x63, 0x68,\n                                   0x65, 0x6d, 0x61, 0x90\n                                  };\n\n    // deserialize it with BON8\n    json j = json::from_bon8(v);\n\n    // print the deserialized JSON value\n    std::cout << std::setw(2) << j << std::endl;\n}\n

        Output:

        {\n  \"compact\": true,\n  \"format\": \"BON8\",\n  \"schema\": 0\n}\n
        "},{"location":"api/basic_json/from_bon8/#see-also","title":"See also","text":"
        • to_bon8 create a BON8 serialization of a JSON value
        • from_cbor create a JSON value from an input in CBOR format
        • from_msgpack create a JSON value from an input in MessagePack format
        • from_bson create a JSON value from an input in BSON format
        • from_ubjson create a JSON value from an input in UBJSON format
        • from_bjdata create a JSON value from an input in BJData format
        "},{"location":"api/basic_json/from_bon8/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.

        Deprecation

        • Overload (2) replaces calls to from_bon8 with a pointer and a length as first two parameters, which has been deprecated in version 3.13.0 unreleased. In version 4.0.0 unreleased, this overload will be deleted (= delete) rather than removed. Please replace all calls like from_bon8(ptr, len, ...); with from_bon8(ptr, ptr+len, ...);.

        You should be warned by your compiler with a -Wdeprecated-declarations warning if you are using a deprecated function.

        "},{"location":"api/basic_json/from_bson/","title":"nlohmann::basic_json::from_bson","text":"
        // (1)\ntemplate<typename InputType>\nstatic basic_json from_bson(InputType&& i,\n                            const bool strict = true,\n                            const bool allow_exceptions = true,\n                            const error_handler_t error_handler = error_handler_t::keep);\n// (2)\ntemplate<typename IteratorType, typename SentinelType = IteratorType>\nstatic basic_json from_bson(IteratorType first, SentinelType last,\n                            const bool strict = true,\n                            const bool allow_exceptions = true,\n                            const error_handler_t error_handler = error_handler_t::keep);\n

        Deserializes a given input to a JSON value using the BSON (Binary JSON) serialization format.

        1. Reads from a compatible input.
        2. Reads from an iterator range, or an iterator and a sentinel of a different type (C++20 ranges support).

        The exact mapping and its limitations are described on a dedicated page.

        "},{"location":"api/basic_json/from_bson/#template-parameters","title":"Template parameters","text":"InputType

        A compatible input, for instance:

        • an std::istream object
        • a FILE pointer
        • a C-style array of characters
        • a pointer to a null-terminated string of single byte characters
        • a container obj for which begin(obj) and end(obj) produce a valid pair of iterators (as found via ADL or member functions, with semantics compatible to std::begin and std::end)
        IteratorType a compatible iterator type SentinelType

        defaults to IteratorType; may be a different type comparable to IteratorType via operator!=, for instance.

        • a custom sentinel type for C++20 ranges
        • std::default_sentinel_t, when IteratorType is std::counted_iterator
        "},{"location":"api/basic_json/from_bson/#parameters","title":"Parameters","text":"i (in) an input in BSON format convertible to an input adapter first (in) iterator to the start of the input last (in) iterator to the end of the input, or a sentinel value that compares equal to the end iterator with operator!= strict (in) whether to expect the input to be consumed until EOF (true by default) allow_exceptions (in) whether to throw exceptions in case of a parse error (optional, true by default) error_handler (in) how to treat a string value or object key that is not valid UTF-8; see error_handler_t. BSON does not require a decoder to reject ill-formed UTF-8, so checking is opt-in: the default, keep, does not check at all, as every binary reader did before this parameter was added; strict checks and throws; replace/ignore sanitize the string the same way dump would"},{"location":"api/basic_json/from_bson/#return-value","title":"Return value","text":"

        deserialized JSON value; in case of a parse error and allow_exceptions set to false, the return value will be value_t::discarded. The latter can be checked with is_discarded.

        "},{"location":"api/basic_json/from_bson/#exception-safety","title":"Exception safety","text":"

        Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

        "},{"location":"api/basic_json/from_bson/#exceptions","title":"Exceptions","text":"
        • Throws parse_error.110 if the given input ends prematurely or the end of the input was not reached when strict was set to true
        • Throws parse_error.112 if a parse error occurs (e.g., an invalid string or byte array length)
        • Throws parse_error.114 if an unsupported BSON record type is encountered
        • Throws parse_error.113 if a string value or object key is not valid UTF-8 and error_handler is strict
        "},{"location":"api/basic_json/from_bson/#complexity","title":"Complexity","text":"

        Linear in the size of the input.

        "},{"location":"api/basic_json/from_bson/#examples","title":"Examples","text":"Example

        The example shows the deserialization of a byte vector in BSON format to a JSON value.

        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create byte vector\n    std::vector<std::uint8_t> v = {0x1b, 0x00, 0x00, 0x00, 0x08, 0x63, 0x6f, 0x6d,\n                                   0x70, 0x61, 0x63, 0x74, 0x00, 0x01, 0x10, 0x73,\n                                   0x63, 0x68, 0x65, 0x6d, 0x61, 0x00, 0x00, 0x00,\n                                   0x00, 0x00, 0x00\n                                  };\n\n    // deserialize it with BSON\n    json j = json::from_bson(v);\n\n    // print the deserialized JSON value\n    std::cout << std::setw(2) << j << std::endl;\n}\n

        Output:

        {\n  \"compact\": true,\n  \"schema\": 0\n}\n
        "},{"location":"api/basic_json/from_bson/#see-also","title":"See also","text":"
        • BSON specification
        • to_bson for the analogous serialization
        • from_cbor for the related CBOR format
        • from_msgpack for the related MessagePack format
        • from_ubjson for the related UBJSON format
        • from_bjdata for the related BJData format
        • from_bon8 for the related BON8 format
        "},{"location":"api/basic_json/from_bson/#version-history","title":"Version history","text":"
        • Added in version 3.4.0.
        • Extended container support (1) to include types with lvalue-only ADL begin/end (matching std::begin/std::end semantics) in version 3.13.0 unreleased.
        • Extended overload (2) to accept heterogeneous iterator+sentinel pairs (C++20 ranges support) in version 3.13.0 unreleased.
        • Added error_handler parameter in version 3.13.0 unreleased.

        Deprecation

        • Overload (2) replaces calls to from_bson with a pointer and a length as first two parameters, which has been deprecated in version 3.8.0. In version 4.0.0 unreleased, this overload will be deleted (= delete) rather than removed. Please replace all calls like from_bson(ptr, len, ...); with from_bson(ptr, ptr+len, ...);.
        • Overload (2) replaces calls to from_bson with a pair of iterators as their first parameter, which has been deprecated in version 3.8.0. This overload will be removed in version 4.0.0. Please replace all calls like from_bson({ptr, ptr+len}, ...); with from_bson(ptr, ptr+len, ...);.

        You should be warned by your compiler with a -Wdeprecated-declarations warning if you are using a deprecated function.

        See the migration guide for how to update existing code.

        "},{"location":"api/basic_json/from_cbor/","title":"nlohmann::basic_json::from_cbor","text":"
        // (1)\ntemplate<typename InputType>\nstatic basic_json from_cbor(InputType&& i,\n                            const bool strict = true,\n                            const bool allow_exceptions = true,\n                            const cbor_tag_handler_t tag_handler = cbor_tag_handler_t::error,\n                            const error_handler_t error_handler = error_handler_t::keep);\n\n// (2)\ntemplate<typename IteratorType, typename SentinelType = IteratorType>\nstatic basic_json from_cbor(IteratorType first, SentinelType last,\n                            const bool strict = true,\n                            const bool allow_exceptions = true,\n                            const cbor_tag_handler_t tag_handler = cbor_tag_handler_t::error,\n                            const error_handler_t error_handler = error_handler_t::keep);\n

        Deserializes a given input to a JSON value using the CBOR (Concise Binary Object Representation) serialization format.

        1. Reads from a compatible input.
        2. Reads from an iterator range, or an iterator and a sentinel of a different type (C++20 ranges support).

        The exact mapping and its limitations are described on a dedicated page.

        "},{"location":"api/basic_json/from_cbor/#template-parameters","title":"Template parameters","text":"InputType

        A compatible input, for instance:

        • an std::istream object
        • a FILE pointer
        • a C-style array of characters
        • a pointer to a null-terminated string of single byte characters
        • a container obj for which begin(obj) and end(obj) produce a valid pair of iterators (as found via ADL or member functions, with semantics compatible to std::begin and std::end)
        IteratorType a compatible iterator type SentinelType

        defaults to IteratorType; may be a different type comparable to IteratorType via operator!=, for instance.

        • a custom sentinel type for C++20 ranges
        • std::default_sentinel_t, when IteratorType is std::counted_iterator
        "},{"location":"api/basic_json/from_cbor/#parameters","title":"Parameters","text":"i (in) an input in CBOR format convertible to an input adapter first (in) iterator to the start of the input last (in) iterator to the end of the input, or a sentinel value that compares equal to the end iterator with operator!= strict (in) whether to expect the input to be consumed until EOF (true by default) allow_exceptions (in) whether to throw exceptions in case of a parse error (optional, true by default) tag_handler (in) how to treat CBOR tags (optional, error by default); see cbor_tag_handler_t for more information error_handler (in) how to treat a string value or object key that is not valid UTF-8; see error_handler_t. CBOR does not require a decoder to reject ill-formed UTF-8, so checking is opt-in: the default, keep, does not check at all, as every binary reader did before this parameter was added; strict checks and throws; replace/ignore sanitize the string the same way dump would"},{"location":"api/basic_json/from_cbor/#return-value","title":"Return value","text":"

        deserialized JSON value; in case of a parse error and allow_exceptions set to false, the return value will be value_t::discarded. The latter can be checked with is_discarded.

        "},{"location":"api/basic_json/from_cbor/#exception-safety","title":"Exception safety","text":"

        Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

        "},{"location":"api/basic_json/from_cbor/#exceptions","title":"Exceptions","text":"
        • Throws parse_error.110 if the given input ends prematurely or the end of the file was not reached when strict was set to true
        • Throws parse_error.112 if unsupported features from CBOR were used in the given input or if the input is not valid CBOR
        • Throws parse_error.113 if a map key is not a string (keys of other types are not supported, as JSON object keys are always strings), or if a string value or object key is not valid UTF-8 and error_handler is strict
        "},{"location":"api/basic_json/from_cbor/#complexity","title":"Complexity","text":"

        Linear in the size of the input.

        "},{"location":"api/basic_json/from_cbor/#examples","title":"Examples","text":"Example

        The example shows the deserialization of a byte vector in CBOR format to a JSON value.

        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create byte vector\n    std::vector<std::uint8_t> v = {0xa2, 0x67, 0x63, 0x6f, 0x6d, 0x70, 0x61, 0x63,\n                                   0x74, 0xf5, 0x66, 0x73, 0x63, 0x68, 0x65, 0x6d,\n                                   0x61, 0x00\n                                  };\n\n    // deserialize it with CBOR\n    json j = json::from_cbor(v);\n\n    // print the deserialized JSON value\n    std::cout << std::setw(2) << j << std::endl;\n}\n

        Output:

        {\n  \"compact\": true,\n  \"schema\": 0\n}\n
        "},{"location":"api/basic_json/from_cbor/#see-also","title":"See also","text":"
        • to_cbor create a CBOR serialization of a JSON value
        • from_msgpack create a JSON value from an input in MessagePack format
        • from_bson create a JSON value from an input in BSON format
        • from_ubjson create a JSON value from an input in UBJSON format
        • from_bjdata create a JSON value from an input in BJData format
        • from_bon8 create a JSON value from an input in BON8 format
        "},{"location":"api/basic_json/from_cbor/#version-history","title":"Version history","text":"
        • Added in version 2.0.9.
        • Parameter start_index since version 2.1.1.
        • Changed to consume input adapters, removed start_index parameter, and added strict parameter in version 3.0.0.
        • Added allow_exceptions parameter in version 3.2.0.
        • Added tag_handler parameter in version 3.9.0.
        • Extended container support (1) to include types with lvalue-only ADL begin/end (matching std::begin/std::end semantics) in version 3.13.0 unreleased.
        • Extended overload (2) to accept heterogeneous iterator+sentinel pairs (C++20 ranges support) in version 3.13.0 unreleased.
        • Added error_handler parameter in version 3.13.0 unreleased.

        Deprecation

        • Overload (2) replaces calls to from_cbor with a pointer and a length as first two parameters, which has been deprecated in version 3.8.0. In version 4.0.0 unreleased, this overload will be deleted (= delete) rather than removed. Please replace all calls like from_cbor(ptr, len, ...); with from_cbor(ptr, ptr+len, ...);.
        • Overload (2) replaces calls to from_cbor with a pair of iterators as their first parameter, which has been deprecated in version 3.8.0. This overload will be removed in version 4.0.0. Please replace all calls like from_cbor({ptr, ptr+len}, ...); with from_cbor(ptr, ptr+len, ...);.

        You should be warned by your compiler with a -Wdeprecated-declarations warning if you are using a deprecated function.

        See the migration guide for how to update existing code.

        "},{"location":"api/basic_json/from_msgpack/","title":"nlohmann::basic_json::from_msgpack","text":"
        // (1)\ntemplate<typename InputType>\nstatic basic_json from_msgpack(InputType&& i,\n                               const bool strict = true,\n                               const bool allow_exceptions = true,\n                               const error_handler_t error_handler = error_handler_t::keep);\n// (2)\ntemplate<typename IteratorType, typename SentinelType = IteratorType>\nstatic basic_json from_msgpack(IteratorType first, SentinelType last,\n                               const bool strict = true,\n                               const bool allow_exceptions = true,\n                               const error_handler_t error_handler = error_handler_t::keep);\n

        Deserializes a given input to a JSON value using the MessagePack serialization format.

        1. Reads from a compatible input.
        2. Reads from an iterator range, or an iterator and a sentinel of a different type (C++20 ranges support).

        The exact mapping and its limitations are described on a dedicated page.

        "},{"location":"api/basic_json/from_msgpack/#template-parameters","title":"Template parameters","text":"InputType

        A compatible input, for instance:

        • an std::istream object
        • a FILE pointer
        • a C-style array of characters
        • a pointer to a null-terminated string of single byte characters
        • a container obj for which begin(obj) and end(obj) produce a valid pair of iterators (as found via ADL or member functions, with semantics compatible to std::begin and std::end)
        IteratorType a compatible iterator type SentinelType

        defaults to IteratorType; may be a different type comparable to IteratorType via operator!=, for instance.

        • a custom sentinel type for C++20 ranges
        • std::default_sentinel_t, when IteratorType is std::counted_iterator
        "},{"location":"api/basic_json/from_msgpack/#parameters","title":"Parameters","text":"i (in) an input in MessagePack format convertible to an input adapter first (in) iterator to the start of the input last (in) iterator to the end of the input, or a sentinel value that compares equal to the end iterator with operator!= strict (in) whether to expect the input to be consumed until EOF (true by default) allow_exceptions (in) whether to throw exceptions in case of a parse error (optional, true by default) error_handler (in) how to treat a string value or object key that is not valid UTF-8; see error_handler_t. MessagePack's specification explicitly allows ill-formed UTF-8, so checking is opt-in: the default, keep, does not check at all, as every binary reader did before this parameter was added; strict checks and throws; replace/ignore sanitize the string the same way dump would"},{"location":"api/basic_json/from_msgpack/#return-value","title":"Return value","text":"

        deserialized JSON value; in case of a parse error and allow_exceptions set to false, the return value will be value_t::discarded. The latter can be checked with is_discarded.

        "},{"location":"api/basic_json/from_msgpack/#exception-safety","title":"Exception safety","text":"

        Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

        "},{"location":"api/basic_json/from_msgpack/#exceptions","title":"Exceptions","text":"
        • Throws parse_error.110 if the given input ends prematurely or the end of the file was not reached when strict was set to true
        • Throws parse_error.112 if unsupported features from MessagePack were used in the given input or if the input is not valid MessagePack
        • Throws parse_error.113 if a map key is not a string (keys of other types are not supported, as JSON object keys are always strings), or if a string value or object key is not valid UTF-8 and error_handler is strict
        "},{"location":"api/basic_json/from_msgpack/#complexity","title":"Complexity","text":"

        Linear in the size of the input.

        "},{"location":"api/basic_json/from_msgpack/#examples","title":"Examples","text":"Example

        The example shows the deserialization of a byte vector in MessagePack format to a JSON value.

        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create byte vector\n    std::vector<std::uint8_t> v = {0x82, 0xa7, 0x63, 0x6f, 0x6d, 0x70, 0x61, 0x63,\n                                   0x74, 0xc3, 0xa6, 0x73, 0x63, 0x68, 0x65, 0x6d,\n                                   0x61, 0x00\n                                  };\n\n    // deserialize it with MessagePack\n    json j = json::from_msgpack(v);\n\n    // print the deserialized JSON value\n    std::cout << std::setw(2) << j << std::endl;\n}\n

        Output:

        {\n  \"compact\": true,\n  \"schema\": 0\n}\n
        "},{"location":"api/basic_json/from_msgpack/#see-also","title":"See also","text":"
        • to_msgpack create a MessagePack serialization of a JSON value
        • from_cbor create a JSON value from an input in CBOR format
        • from_bson create a JSON value from an input in BSON format
        • from_ubjson create a JSON value from an input in UBJSON format
        • from_bjdata create a JSON value from an input in BJData format
        • from_bon8 create a JSON value from an input in BON8 format
        "},{"location":"api/basic_json/from_msgpack/#version-history","title":"Version history","text":"
        • Added in version 2.0.9.
        • Parameter start_index since version 2.1.1.
        • Changed to consume input adapters, removed start_index parameter, and added strict parameter in version 3.0.0.
        • Added allow_exceptions parameter in version 3.2.0.
        • Extended container support (1) to include types with lvalue-only ADL begin/end (matching std::begin/std::end semantics) in version 3.13.0 unreleased.
        • Extended overload (2) to accept heterogeneous iterator+sentinel pairs (C++20 ranges support) in version 3.13.0 unreleased.
        • Added error_handler parameter in version 3.13.0 unreleased.

        Deprecation

        • Overload (2) replaces calls to from_msgpack with a pointer and a length as first two parameters, which has been deprecated in version 3.8.0. In version 4.0.0 unreleased, this overload will be deleted (= delete) rather than removed. Please replace all calls like from_msgpack(ptr, len, ...); with from_msgpack(ptr, ptr+len, ...);.
        • Overload (2) replaces calls to from_msgpack with a pair of iterators as their first parameter, which has been deprecated in version 3.8.0. This overload will be removed in version 4.0.0. Please replace all calls like from_msgpack({ptr, ptr+len}, ...); with from_msgpack(ptr, ptr+len, ...);.

        You should be warned by your compiler with a -Wdeprecated-declarations warning if you are using a deprecated function.

        See the migration guide for how to update existing code.

        "},{"location":"api/basic_json/from_ubjson/","title":"nlohmann::basic_json::from_ubjson","text":"
        // (1)\ntemplate<typename InputType>\nstatic basic_json from_ubjson(InputType&& i,\n                              const bool strict = true,\n                              const bool allow_exceptions = true,\n                              const error_handler_t error_handler = error_handler_t::keep);\n// (2)\ntemplate<typename IteratorType, typename SentinelType = IteratorType>\nstatic basic_json from_ubjson(IteratorType first, SentinelType last,\n                              const bool strict = true,\n                              const bool allow_exceptions = true,\n                              const error_handler_t error_handler = error_handler_t::keep);\n

        Deserializes a given input to a JSON value using the UBJSON (Universal Binary JSON) serialization format.

        1. Reads from a compatible input.
        2. Reads from an iterator range, or an iterator and a sentinel of a different type (C++20 ranges support).

        The exact mapping and its limitations are described on a dedicated page.

        "},{"location":"api/basic_json/from_ubjson/#template-parameters","title":"Template parameters","text":"InputType

        A compatible input, for instance:

        • an std::istream object
        • a FILE pointer
        • a C-style array of characters
        • a pointer to a null-terminated string of single byte characters
        • a container obj for which begin(obj) and end(obj) produce a valid pair of iterators (as found via ADL or member functions, with semantics compatible to std::begin and std::end)
        IteratorType a compatible iterator type SentinelType

        defaults to IteratorType; may be a different type comparable to IteratorType via operator!=, for instance.

        • a custom sentinel type for C++20 ranges
        • std::default_sentinel_t, when IteratorType is std::counted_iterator
        "},{"location":"api/basic_json/from_ubjson/#parameters","title":"Parameters","text":"i (in) an input in UBJSON format convertible to an input adapter first (in) iterator to the start of the input last (in) iterator to the end of the input, or a sentinel value that compares equal to the end iterator with operator!= strict (in) whether to expect the input to be consumed until EOF (true by default) allow_exceptions (in) whether to throw exceptions in case of a parse error (optional, true by default) error_handler (in) how to treat a string value or object key that is not valid UTF-8; see error_handler_t. UBJSON does not require a decoder to reject ill-formed UTF-8, so checking is opt-in: the default, keep, does not check at all, as every binary reader did before this parameter was added; strict checks and throws; replace/ignore sanitize the string the same way dump would"},{"location":"api/basic_json/from_ubjson/#return-value","title":"Return value","text":"

        deserialized JSON value; in case of a parse error and allow_exceptions set to false, the return value will be value_t::discarded. The latter can be checked with is_discarded.

        "},{"location":"api/basic_json/from_ubjson/#exception-safety","title":"Exception safety","text":"

        Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

        "},{"location":"api/basic_json/from_ubjson/#exceptions","title":"Exceptions","text":"
        • Throws parse_error.110 if the given input ends prematurely or the end of the file was not reached when strict was set to true
        • Throws parse_error.112 if a parse error occurs
        • Throws parse_error.113 if a string could not be parsed successfully, or if a string value or object key is not valid UTF-8 and error_handler is strict
        • Throws out_of_range.408 if the size of an optimized container or n-dimensional array cannot be represented by std::size_t
        "},{"location":"api/basic_json/from_ubjson/#complexity","title":"Complexity","text":"

        Linear in the size of the input.

        "},{"location":"api/basic_json/from_ubjson/#examples","title":"Examples","text":"Example

        The example shows the deserialization of a byte vector in UBJSON format to a JSON value.

        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create byte vector\n    std::vector<std::uint8_t> v = {0x7B, 0x69, 0x07, 0x63, 0x6F, 0x6D, 0x70, 0x61,\n                                   0x63, 0x74, 0x54, 0x69, 0x06, 0x73, 0x63, 0x68,\n                                   0x65, 0x6D, 0x61, 0x69, 0x00, 0x7D\n                                  };\n\n    // deserialize it with UBJSON\n    json j = json::from_ubjson(v);\n\n    // print the deserialized JSON value\n    std::cout << std::setw(2) << j << std::endl;\n}\n

        Output:

        {\n  \"compact\": true,\n  \"schema\": 0\n}\n
        "},{"location":"api/basic_json/from_ubjson/#see-also","title":"See also","text":"
        • to_ubjson create a UBJSON serialization of a JSON value
        • from_cbor create a JSON value from an input in CBOR format
        • from_msgpack create a JSON value from an input in MessagePack format
        • from_bson create a JSON value from an input in BSON format
        • from_bjdata create a JSON value from an input in BJData format
        • from_bon8 create a JSON value from an input in BON8 format
        "},{"location":"api/basic_json/from_ubjson/#version-history","title":"Version history","text":"
        • Added in version 3.1.0.
        • Added allow_exceptions parameter in version 3.2.0.
        • Extended container support (1) to include types with lvalue-only ADL begin/end (matching std::begin/std::end semantics) in version 3.13.0 unreleased.
        • Extended overload (2) to accept heterogeneous iterator+sentinel pairs (C++20 ranges support) in version 3.13.0 unreleased.
        • Added error_handler parameter in version 3.13.0 unreleased.

        Deprecation

        • Overload (2) replaces calls to from_ubjson with a pointer and a length as first two parameters, which has been deprecated in version 3.8.0. In version 4.0.0 unreleased, this overload will be deleted (= delete) rather than removed. Please replace all calls like from_ubjson(ptr, len, ...); with from_ubjson(ptr, ptr+len, ...);.
        • Overload (2) replaces calls to from_ubjson with a pair of iterators as their first parameter, which has been deprecated in version 3.8.0. This overload will be removed in version 4.0.0. Please replace all calls like from_ubjson({ptr, ptr+len}, ...); with from_ubjson(ptr, ptr+len, ...);.

        You should be warned by your compiler with a -Wdeprecated-declarations warning if you are using a deprecated function.

        See the migration guide for how to update existing code.

        "},{"location":"api/basic_json/front/","title":"nlohmann::basic_json::front","text":"
        reference front();\nconst_reference front() const;\n

        Returns a reference to the first element in the container. For a JSON container c, the expression c.front() is equivalent to *c.begin().

        "},{"location":"api/basic_json/front/#return-value","title":"Return value","text":"

        In the case of a structured type (array or object), a reference to the first element is returned. In the case of number, string, boolean, or binary values, a reference to the value is returned.

        "},{"location":"api/basic_json/front/#exception-safety","title":"Exception safety","text":"

        Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

        "},{"location":"api/basic_json/front/#exceptions","title":"Exceptions","text":"

        If the JSON value is null, exception invalid_iterator.214 is thrown.

        "},{"location":"api/basic_json/front/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json/front/#notes","title":"Notes","text":"

        Precondition

        The array or object must not be empty. Calling front on an empty array or object yields undefined behavior.

        "},{"location":"api/basic_json/front/#examples","title":"Examples","text":"Example

        The following code shows an example for front().

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_null;\n    json j_boolean = true;\n    json j_number_integer = 17;\n    json j_number_float = 23.42;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_object_empty(json::value_t::object);\n    json j_array = {1, 2, 4, 8, 16};\n    json j_array_empty(json::value_t::array);\n    json j_string = \"Hello, world\";\n\n    // call front()\n    //std::cout << j_null.front() << '\\n';          // would throw\n    std::cout << j_boolean.front() << '\\n';\n    std::cout << j_number_integer.front() << '\\n';\n    std::cout << j_number_float.front() << '\\n';\n    std::cout << j_object.front() << '\\n';\n    //std::cout << j_object_empty.front() << '\\n';  // undefined behavior\n    std::cout << j_array.front() << '\\n';\n    //std::cout << j_array_empty.front() << '\\n';   // undefined behavior\n    std::cout << j_string.front() << '\\n';\n}\n

        Output:

        true\n17\n23.42\n1\n1\n\"Hello, world\"\n
        "},{"location":"api/basic_json/front/#see-also","title":"See also","text":"
        • back to access the last element
        • basic_json_view::front - the same access on a zero-copy view (document order, not sorted; duplicate keys are visited)
        "},{"location":"api/basic_json/front/#version-history","title":"Version history","text":"
        • Added in version 1.0.0.
        • Adjusted code to return reference to binary values in version 3.8.0.
        "},{"location":"api/basic_json/get/","title":"nlohmann::basic_json::get","text":"
        // (1)\ntemplate<typename ValueType>\nValueType get() const noexcept(\n    noexcept(JSONSerializer<ValueType>::from_json(\n        std::declval<const basic_json_t&>(), std::declval<ValueType&>())));\n\n// (2)\ntemplate<typename BasicJsonType>\nBasicJsonType get() const;\n\n// (3)\ntemplate<typename PointerType>\nPointerType get() noexcept;\n\ntemplate<typename PointerType>\nconst PointerType get() const noexcept;  // constexpr since C++14\n

        1. Explicit type conversion between the JSON value and a compatible value which is CopyConstructible and DefaultConstructible. The value is converted by calling the json_serializer<ValueType> from_json() method.

          The function is equivalent to executing 

          ValueType ret;\nJSONSerializer<ValueType>::from_json(*this, ret);\nreturn ret;\n

          This overload is chosen if:

          • ValueType is not basic_json,
          • json_serializer<ValueType> has a from_json() method of the form void from_json(const basic_json&, ValueType&), and
          • json_serializer<ValueType> does not have a from_json() method of the form ValueType from_json(const basic_json&)

          If the type is not CopyConstructible and not DefaultConstructible, the value is converted by calling the json_serializer<ValueType> from_json() method.

          The function is then equivalent to executing 

          return JSONSerializer<ValueTypeCV>::from_json(*this);\n

          This overload is chosen if:

          • ValueType is not basic_json and
          • json_serializer<ValueType> has a from_json() method of the form ValueType from_json(const basic_json&)

          If json_serializer<ValueType> has both overloads of from_json(), the latter one is chosen.

        2. Overload for basic_json specializations. The function is equivalent to executing 

          return *this;\n

        3. Explicit pointer access to the internally stored JSON value. No copies are made.

        "},{"location":"api/basic_json/get/#template-parameters","title":"Template parameters","text":"ValueType the value type to return BasicJsonType a specialization of basic_json PointerType pointer type; must be a pointer to array_t, object_t, string_t, boolean_t, number_integer_t, or number_unsigned_t, number_float_t, or binary_t. Other types will not compile."},{"location":"api/basic_json/get/#return-value","title":"Return value","text":"
        1. copy of the JSON value, converted to ValueType
        2. a copy of *this, converted into BasicJsonType
        3. pointer to the internally stored JSON value if the requested pointer type fits to the JSON value; nullptr otherwise
        "},{"location":"api/basic_json/get/#exception-safety","title":"Exception safety","text":"

        Depends on what json_serializer<ValueType> from_json() method throws for overloads (1) and (2); the JSON value itself is never modified, since get() is a const member function. No-throw guarantee for overload (3): this function never throws exceptions.

        "},{"location":"api/basic_json/get/#exceptions","title":"Exceptions","text":"

        Depends on what json_serializer<ValueType> from_json() method throws

        "},{"location":"api/basic_json/get/#complexity","title":"Complexity","text":"

        Depends on the json_serializer<ValueType>::from_json() implementation for overloads (1) and (2); constant for overload (3).

        "},{"location":"api/basic_json/get/#notes","title":"Notes","text":"

        Undefined behavior for pointers

        Writing data to the pointee (overload 3) of the result yields an undefined state.

        Undefined behavior for numeric conversions

        Conversions between numeric types are performed by the corresponding from_json() implementation using the target C++ type. When converting between numeric types, the library does not check whether the source value is representable by the target type.

        If the source value is outside the range of the target type, the behavior is the same as the corresponding C++ conversion. In particular, converting a floating-point value to an integer type that cannot represent the value results in undefined behavior.

        See Number conversion for more information.

        std::optional conversions

        Prior to version 3.13.0 unreleased, get<std::optional<T>>() (and other conversions to std::optional<T>) failed to compile in every configuration, due to an internal implementation bug that made the from_json overload for std::optional unreachable regardless of the JSON_USE_IMPLICIT_CONVERSIONS setting. This has been fixed.

        "},{"location":"api/basic_json/get/#examples","title":"Examples","text":"Example: (1) explicit conversion to compatible types

        The example below shows several conversions from JSON values to other types. There a few things to note: (1) Floating-point numbers can be converted to integers, (2) A JSON array can be converted to a standard std::vector<short>, (3) A JSON object can be converted to C++ associative containers such as std::map<std::string, json>.

        #include <iostream>\n#include <map>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON value with different types\n    json json_types =\n    {\n        {\"boolean\", true},\n        {\n            \"number\", {\n                {\"integer\", 42},\n                {\"floating-point\", 17.23}\n            }\n        },\n        {\"string\", \"Hello, world!\"},\n        {\"array\", {1, 2, 3, 4, 5}},\n        {\"null\", nullptr}\n    };\n\n    // use explicit conversions\n    auto v1 = json_types[\"boolean\"].get<bool>();\n    auto v2 = json_types[\"number\"][\"integer\"].get<int>();\n    auto v3 = json_types[\"number\"][\"integer\"].get<short>();\n    auto v4 = json_types[\"number\"][\"floating-point\"].get<float>();\n    auto v5 = json_types[\"number\"][\"floating-point\"].get<int>();\n    auto v6 = json_types[\"string\"].get<std::string>();\n    auto v7 = json_types[\"array\"].get<std::vector<short>>();\n    auto v8 = json_types.get<std::map<std::string, json>>();\n\n    // print the conversion results\n    std::cout << v1 << '\\n';\n    std::cout << v2 << ' ' << v3 << '\\n';\n    std::cout << v4 << ' ' << v5 << '\\n';\n    std::cout << v6 << '\\n';\n\n    for (auto i : v7)\n    {\n        std::cout << i << ' ';\n    }\n    std::cout << \"\\n\\n\";\n\n    for (auto i : v8)\n    {\n        std::cout << i.first << \": \" << i.second << '\\n';\n    }\n}\n

        Output:

        1\n42 42\n17.23 17\nHello, world!\n1 2 3 4 5 \n\narray: [1,2,3,4,5]\nboolean: true\nnull: null\nnumber: {\"floating-point\":17.23,\"integer\":42}\nstring: \"Hello, world!\"\n
        Example: (2) explicit conversion to another basic_json specialization

        The example below shows how a json value is converted to an ordered_json value using get<BasicJsonType>().

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing ordered_json = nlohmann::ordered_json;\n\nint main()\n{\n    // create a JSON value\n    json j = {{\"one\", 1}, {\"two\", 2}, {\"three\", 3}};\n\n    // convert to a different basic_json specialization\n    ordered_json oj = j.get<ordered_json>();\n\n    std::cout << j << '\\n';\n    std::cout << oj << '\\n';\n}\n

        Output:

        {\"one\":1,\"three\":3,\"two\":2}\n{\"one\":1,\"three\":3,\"two\":2}\n
        Example: (3) explicit pointer access to the stored value

        The example below shows how pointers to internal values of a JSON value can be requested. Note that no type conversions are made and a #cpp nullptr is returned if the value and the requested pointer type does not match.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON number\n    json value = 17;\n\n    // explicitly getting pointers\n    auto p1 = value.get<const json::number_integer_t*>();\n    auto p2 = value.get<json::number_integer_t*>();\n    auto p3 = value.get<json::number_integer_t* const>();\n    auto p4 = value.get<const json::number_integer_t* const>();\n    auto p5 = value.get<json::number_float_t*>();\n\n    // print the pointees\n    std::cout << *p1 << ' ' << *p2 << ' ' << *p3 << ' ' << *p4 << '\\n';\n    std::cout << std::boolalpha << (p5 == nullptr) << '\\n';\n}\n

        Output:

        17 17 17 17\ntrue\n
        "},{"location":"api/basic_json/get/#see-also","title":"See also","text":"
        • get_to convert and write into a passed value
        • get_ptr get a pointer to the stored value
        • get_ref get a reference to the stored value
        • operator ValueType get a value via implicit conversion
        • Converting values - the type conversions article
        • basic_json_view::get - the same conversion on a zero-copy view (many types are converted without ever building a basic_json value)
        "},{"location":"api/basic_json/get/#version-history","title":"Version history","text":"
        1. Since version 2.1.0.
        2. Since version 2.1.0. Extended to work with other specializations of basic_json in version 3.2.0.
        3. Since version 1.0.0.
        "},{"location":"api/basic_json/get_allocator/","title":"nlohmann::basic_json::get_allocator","text":"
        static allocator_type get_allocator();\n

        Returns the allocator associated with the container.

        "},{"location":"api/basic_json/get_allocator/#return-value","title":"Return value","text":"

        associated allocator

        "},{"location":"api/basic_json/get_allocator/#exception-safety","title":"Exception safety","text":"

        Strong guarantee: if an exception is thrown, there are no changes to any JSON value.

        "},{"location":"api/basic_json/get_allocator/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json/get_allocator/#examples","title":"Examples","text":"Example

        The example shows how get_allocator() is used to created json values.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    auto alloc = json::get_allocator();\n    using traits_t = std::allocator_traits<decltype(alloc)>;\n\n    json* j = traits_t::allocate(alloc, 1);\n    traits_t::construct(alloc, j, \"Hello, world!\");\n\n    std::cout << *j << std::endl;\n\n    traits_t::destroy(alloc, j);\n    traits_t::deallocate(alloc, j, 1);\n}\n

        Output:

        \"Hello, world!\"\n
        "},{"location":"api/basic_json/get_allocator/#see-also","title":"See also","text":"
        • basic_json the class template, with AllocatorType as one of its template parameters
        • Template Parameter Requirements - the requirements for AllocatorType
        "},{"location":"api/basic_json/get_allocator/#version-history","title":"Version history","text":"
        • Added in version 1.0.0.
        "},{"location":"api/basic_json/get_binary/","title":"nlohmann::basic_json::get_binary","text":"
        binary_t& get_binary();\n\nconst binary_t& get_binary() const;\n

        Returns a reference to the stored binary value.

        "},{"location":"api/basic_json/get_binary/#return-value","title":"Return value","text":"

        Reference to binary value.

        "},{"location":"api/basic_json/get_binary/#exception-safety","title":"Exception safety","text":"

        Strong exception safety: if an exception occurs, the original value stays intact.

        "},{"location":"api/basic_json/get_binary/#exceptions","title":"Exceptions","text":"

        Throws type_error.302 if the value is not binary

        "},{"location":"api/basic_json/get_binary/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json/get_binary/#examples","title":"Examples","text":"Example

        The following code shows how to query a binary value.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a binary vector\n    std::vector<std::uint8_t> vec = {0xCA, 0xFE, 0xBA, 0xBE};\n\n    // create a binary JSON value with subtype 42\n    json j = json::binary(vec, 42);\n\n    // output type and subtype\n    std::cout << \"type: \" << j.type_name() << \", subtype: \" << j.get_binary().subtype() << std::endl;\n}\n

        Output:

        type: binary, subtype: 42\n
        "},{"location":"api/basic_json/get_binary/#see-also","title":"See also","text":"
        • get get a value (explicit conversion)
        • get_ref get a reference to the stored value
        "},{"location":"api/basic_json/get_binary/#version-history","title":"Version history","text":"
        • Added in version 3.8.0.
        "},{"location":"api/basic_json/get_ptr/","title":"nlohmann::basic_json::get_ptr","text":"
        template<typename PointerType>\nPointerType get_ptr() noexcept;\n\ntemplate<typename PointerType>\nconstexpr const PointerType get_ptr() const noexcept;\n

        Implicit pointer access to the internally stored JSON value. No copies are made.

        "},{"location":"api/basic_json/get_ptr/#template-parameters","title":"Template parameters","text":"PointerType pointer type; must be a pointer to array_t, object_t, string_t, boolean_t, number_integer_t, or number_unsigned_t, number_float_t, or binary_t. Other types will not compile."},{"location":"api/basic_json/get_ptr/#return-value","title":"Return value","text":"

        pointer to the internally stored JSON value if the requested pointer type fits to the JSON value; nullptr otherwise

        "},{"location":"api/basic_json/get_ptr/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this function never throws exceptions.

        "},{"location":"api/basic_json/get_ptr/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json/get_ptr/#notes","title":"Notes","text":"

        Undefined behavior

        The pointer becomes invalid if the underlying JSON object changes.

        Consider the following example code where the pointer ptr changes after the array is resized. As a result, reading or writing to ptr after the array change would be undefined behavior. The address of the first array element changes, because the underlying std::vector is resized after adding a fifth element.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    json j = {1, 2, 3, 4};\n    auto* ptr = j[0].get_ptr<std::int64_t*>();\n    std::cout << \"value at \" << ptr << \" is \" << *ptr << std::endl;\n\n    j.push_back(5);\n\n    ptr = j[0].get_ptr<std::int64_t*>();\n    std::cout << \"value at \" << ptr << \" is \" << *ptr << std::endl;\n}\n

        Output:

        value at 0x6000012fc1c8 is 1\nvalue at 0x6000029fc088 is 1\n
        "},{"location":"api/basic_json/get_ptr/#examples","title":"Examples","text":"Example

        The example below shows how pointers to internal values of a JSON value can be requested. Note that no type conversions are made and a nullptr is returned if the value and the requested pointer type does not match.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON number\n    json value = 17;\n\n    // explicitly getting pointers\n    auto p1 = value.get_ptr<const json::number_integer_t*>();\n    auto p2 = value.get_ptr<json::number_integer_t*>();\n    auto p3 = value.get_ptr<json::number_integer_t* const>();\n    auto p4 = value.get_ptr<const json::number_integer_t* const>();\n    auto p5 = value.get_ptr<json::number_float_t*>();\n\n    // print the pointees\n    std::cout << *p1 << ' ' << *p2 << ' ' << *p3 << ' ' << *p4 << '\\n';\n    std::cout << std::boolalpha << (p5 == nullptr) << '\\n';\n}\n

        Output:

        17 17 17 17\ntrue\n
        "},{"location":"api/basic_json/get_ptr/#see-also","title":"See also","text":"
        • get_ref() get a reference value
        "},{"location":"api/basic_json/get_ptr/#version-history","title":"Version history","text":"
        • Added in version 1.0.0.
        • Extended to binary types in version 3.8.0.
        "},{"location":"api/basic_json/get_ref/","title":"nlohmann::basic_json::get_ref","text":"
        template<typename ReferenceType>\nReferenceType get_ref();\n\ntemplate<typename ReferenceType>\nconst ReferenceType get_ref() const;\n

        Implicit reference access to the internally stored JSON value. No copies are made.

        "},{"location":"api/basic_json/get_ref/#template-parameters","title":"Template parameters","text":"ReferenceType reference type; must be a reference to array_t, object_t, string_t, boolean_t, number_integer_t, or number_unsigned_t, number_float_t, or binary_t. Enforced by a static assertion."},{"location":"api/basic_json/get_ref/#return-value","title":"Return value","text":"

        reference to the internally stored JSON value if the requested reference type fits to the JSON value; throws type_error.303 otherwise

        "},{"location":"api/basic_json/get_ref/#exception-safety","title":"Exception safety","text":"

        Strong exception safety: if an exception occurs, the original value stays intact.

        "},{"location":"api/basic_json/get_ref/#exceptions","title":"Exceptions","text":"

        Throws type_error.303 if the requested reference type does not match the stored JSON value type; example: \"incompatible ReferenceType for get_ref, actual type is binary\".

        "},{"location":"api/basic_json/get_ref/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json/get_ref/#notes","title":"Notes","text":"

        Undefined behavior

        The reference becomes invalid if the underlying JSON object changes.

        "},{"location":"api/basic_json/get_ref/#examples","title":"Examples","text":"Example

        The example shows several calls to get_ref().

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON number\n    json value = 17;\n\n    // explicitly getting references\n    auto r1 = value.get_ref<const json::number_integer_t&>();\n    auto r2 = value.get_ref<json::number_integer_t&>();\n\n    // print the values\n    std::cout << r1 << ' ' << r2 << '\\n';\n\n    // incompatible type throws exception\n    try\n    {\n        auto r3 = value.get_ref<json::number_float_t&>();\n    }\n    catch (const json::type_error& ex)\n    {\n        std::cout << ex.what() << '\\n';\n    }\n}\n

        Output:

        17 17\n[json.exception.type_error.303] incompatible ReferenceType for get_ref, actual type is number\n
        "},{"location":"api/basic_json/get_ref/#see-also","title":"See also","text":"
        • get_ptr() get a pointer value
        • basic_json_view::get_string - the closest counterpart on a zero-copy view: a string without a copy, but as a view rather than a reference to a value that must already exist
        "},{"location":"api/basic_json/get_ref/#version-history","title":"Version history","text":"
        • Added in version 1.1.0.
        • Extended to binary types in version 3.8.0.
        "},{"location":"api/basic_json/get_to/","title":"nlohmann::basic_json::get_to","text":"
        template<typename ValueType>\nValueType& get_to(ValueType& v) const noexcept(\n    noexcept(JSONSerializer<ValueType>::from_json(\n        std::declval<const basic_json_t&>(), v)));\n

        Explicit type conversion between the JSON value and a compatible value. The value is filled into the input parameter by calling the json_serializer<ValueType> from_json() method.

        The function is equivalent to executing 

        ValueType v;\nJSONSerializer<ValueType>::from_json(*this, v);\n

        This overload is chosen if:

        • ValueType is not basic_json,
        • json_serializer<ValueType> has a from_json() method of the form void from_json(const basic_json&, ValueType&)

        v must not be const. Passing a const object is a compile-time error. For types such as arithmetic types, enums, and C arrays, the error is a static_assert that names the problem. For other types, the overload is not viable, and the compiler reports that no matching get_to was found.

        "},{"location":"api/basic_json/get_to/#template-parameters","title":"Template parameters","text":"ValueType the value type to return"},{"location":"api/basic_json/get_to/#return-value","title":"Return value","text":"

        the input parameter, allowing chaining calls

        "},{"location":"api/basic_json/get_to/#exception-safety","title":"Exception safety","text":"

        Depends on what json_serializer<ValueType> from_json() method throws; the JSON value itself is never modified, since get_to() is a const member function.

        "},{"location":"api/basic_json/get_to/#exceptions","title":"Exceptions","text":"

        Depends on what json_serializer<ValueType> from_json() method throws

        "},{"location":"api/basic_json/get_to/#complexity","title":"Complexity","text":"

        Depends on the json_serializer<ValueType>::from_json() implementation.

        "},{"location":"api/basic_json/get_to/#examples","title":"Examples","text":"Example

        The example below shows several conversions from JSON values to other types. There a few things to note: (1) Floating-point numbers can be converted to integers, (2) A JSON array can be converted to a standard std::vector<short>, (3) A JSON object can be converted to C++ associative containers such as std::map<std::string, json>.

        #include <iostream>\n#include <map>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON value with different types\n    json json_types =\n    {\n        {\"boolean\", true},\n        {\n            \"number\", {\n                {\"integer\", 42},\n                {\"floating-point\", 17.23}\n            }\n        },\n        {\"string\", \"Hello, world!\"},\n        {\"array\", {1, 2, 3, 4, 5}},\n        {\"null\", nullptr}\n    };\n\n    bool v1;\n    int v2;\n    short v3;\n    float v4;\n    int v5;\n    std::string v6;\n    std::vector<short> v7;\n    std::map<std::string, json> v8;\n\n    // use explicit conversions\n    json_types[\"boolean\"].get_to(v1);\n    json_types[\"number\"][\"integer\"].get_to(v2);\n    json_types[\"number\"][\"integer\"].get_to(v3);\n    json_types[\"number\"][\"floating-point\"].get_to(v4);\n    json_types[\"number\"][\"floating-point\"].get_to(v5);\n    json_types[\"string\"].get_to(v6);\n    json_types[\"array\"].get_to(v7);\n    json_types.get_to(v8);\n\n    // print the conversion results\n    std::cout << v1 << '\\n';\n    std::cout << v2 << ' ' << v3 << '\\n';\n    std::cout << v4 << ' ' << v5 << '\\n';\n    std::cout << v6 << '\\n';\n\n    for (auto i : v7)\n    {\n        std::cout << i << ' ';\n    }\n    std::cout << \"\\n\\n\";\n\n    for (auto i : v8)\n    {\n        std::cout << i.first << \": \" << i.second << '\\n';\n    }\n}\n

        Output:

        1\n42 42\n17.23 17\nHello, world!\n1 2 3 4 5 \n\narray: [1,2,3,4,5]\nboolean: true\nnull: null\nnumber: {\"floating-point\":17.23,\"integer\":42}\nstring: \"Hello, world!\"\n
        "},{"location":"api/basic_json/get_to/#see-also","title":"See also","text":"
        • get get a value (explicit conversion)
        • get_ref get a reference to the stored value
        • get_ptr get a pointer to the stored value
        • Converting values - the type conversions article
        • basic_json_view::get_to - the same conversion on a zero-copy view
        "},{"location":"api/basic_json/get_to/#version-history","title":"Version history","text":"
        • Since version 3.3.0.
        • Added a static_assert with a clear message for const arguments in version 3.13.0 unreleased.
        "},{"location":"api/basic_json/input_format_t/","title":"nlohmann::basic_json::input_format_t","text":"
        enum class input_format_t {\n    json,\n    cbor,\n    msgpack,\n    ubjson,\n    bson,\n    bjdata,\n    bon8\n};\n

        This enumeration is used in the sax_parse function to choose the input format to parse:

        json JSON (JavaScript Object Notation) cbor CBOR (Concise Binary Object Representation) msgpack MessagePack ubjson UBJSON (Universal Binary JSON) bson BSON (Binary JSON) bjdata BJData (Binary JData) bon8 BON8 (Binary Object Notation 8)"},{"location":"api/basic_json/input_format_t/#examples","title":"Examples","text":"Example

        The example below shows how an input_format_t enum value is passed to sax_parse to set the input format to CBOR.

        #include <iostream>\n#include <iomanip>\n#include <sstream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\n// a simple event consumer that collects string representations of the passed\n// values; note inheriting from json::json_sax_t is not required, but can\n// help not to forget a required function\nclass sax_event_consumer : public json::json_sax_t\n{\n  public:\n    std::vector<std::string> events;\n\n    bool null() override\n    {\n        events.push_back(\"null()\");\n        return true;\n    }\n\n    bool boolean(bool val) override\n    {\n        events.push_back(\"boolean(val=\" + std::string(val ? \"true\" : \"false\") + \")\");\n        return true;\n    }\n\n    bool number_integer(number_integer_t val) override\n    {\n        events.push_back(\"number_integer(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_unsigned(number_unsigned_t val) override\n    {\n        events.push_back(\"number_unsigned(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_float(number_float_t val, const string_t& s) override\n    {\n        events.push_back(\"number_float(val=\" + std::to_string(val) + \", s=\" + s + \")\");\n        return true;\n    }\n\n    bool string(string_t& val) override\n    {\n        events.push_back(\"string(val=\" + val + \")\");\n        return true;\n    }\n\n    bool start_object(std::size_t elements) override\n    {\n        events.push_back(\"start_object(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_object() override\n    {\n        events.push_back(\"end_object()\");\n        return true;\n    }\n\n    bool start_array(std::size_t elements) override\n    {\n        events.push_back(\"start_array(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_array() override\n    {\n        events.push_back(\"end_array()\");\n        return true;\n    }\n\n    bool key(string_t& val) override\n    {\n        events.push_back(\"key(val=\" + val + \")\");\n        return true;\n    }\n\n    bool binary(json::binary_t& val) override\n    {\n        events.push_back(\"binary(val=[...])\");\n        return true;\n    }\n\n    bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex) override\n    {\n        events.push_back(\"parse_error(position=\" + std::to_string(position) + \", last_token=\" + last_token + \",\\n            ex=\" + std::string(ex.what()) + \")\");\n        return false;\n    }\n};\n\nint main()\n{\n    // CBOR byte string\n    std::vector<std::uint8_t> vec = {{0x44, 0xcA, 0xfe, 0xba, 0xbe}};\n\n    // create a SAX event consumer object\n    sax_event_consumer sec;\n\n    // parse CBOR\n    bool result = json::sax_parse(vec, &sec, json::input_format_t::cbor);\n\n    // output the recorded events\n    for (auto& event : sec.events)\n    {\n        std::cout << event << \"\\n\";\n    }\n\n    // output the result of sax_parse\n    std::cout << \"\\nresult: \" << std::boolalpha << result << std::endl;\n}\n

        Output:

        binary(val=[...])\n\nresult: true\n
        "},{"location":"api/basic_json/input_format_t/#see-also","title":"See also","text":"
        • sax_parse generic SAX parse interface, taking an input_format_t to select the input format
        • cbor_tag_handler_t configures how CBOR tags are treated while parsing
        "},{"location":"api/basic_json/input_format_t/#version-history","title":"Version history","text":"
        • Added in version 3.2.0.
        "},{"location":"api/basic_json/insert/","title":"nlohmann::basic_json::insert","text":"
        // (1)\niterator insert(const_iterator pos, const basic_json& val);\niterator insert(const_iterator pos, basic_json&& val);\n\n// (2)\niterator insert(const_iterator pos, size_type cnt, const basic_json& val);\n\n// (3)\niterator insert(const_iterator pos, const_iterator first, const_iterator last);\n\n// (4)\niterator insert(const_iterator pos, initializer_list_t ilist);\n\n// (5)\nvoid insert(const_iterator first, const_iterator last);\n
        1. Inserts element val into an array before iterator pos.
        2. Inserts cnt copies of val into an array before iterator pos.
        3. Inserts elements from range [first, last) into an array before iterator pos.
        4. Inserts elements from initializer list ilist into an array before iterator pos.
        5. Inserts elements from range [first, last) into an object.
        "},{"location":"api/basic_json/insert/#iterator-invalidation","title":"Iterator invalidation","text":"

        For all cases where an element is added to an array, a reallocation can happen, in which case all iterators (including the end() iterator) and all references to the elements are invalidated. Otherwise, only the end() iterator is invalidated. Also, any iterator or reference after the insertion point will point to the same index, which is now a different value.

        For ordered_json, also adding an element to an object can yield a reallocation which again invalidates all iterators and all references. Also, any iterator or reference after the insertion point will point to the same index, which is now a different value.

        "},{"location":"api/basic_json/insert/#parameters","title":"Parameters","text":"pos (in) iterator before which the content will be inserted; may be the end() iterator val (in) value to insert cnt (in) number of copies of val to insert first (in) the start of the range of elements to insert last (in) the end of the range of elements to insert ilist (in) initializer list to insert the values from"},{"location":"api/basic_json/insert/#return-value","title":"Return value","text":"
        1. iterator pointing to the inserted val.
        2. iterator pointing to the first element inserted, or pos if cnt==0
        3. iterator pointing to the first element inserted, or pos if first==last
        4. iterator pointing to the first element inserted, or pos if ilist is empty
        5. (none)
        "},{"location":"api/basic_json/insert/#exception-safety","title":"Exception safety","text":"

        Strong exception safety: if an exception occurs, the original value stays intact.

        "},{"location":"api/basic_json/insert/#exceptions","title":"Exceptions","text":"
        1. The function can throw the following exceptions:
          • Throws type_error.309 if called on JSON values other than arrays; example: \"cannot use insert() with string\"
          • Throws invalid_iterator.202 if called on an iterator which does not belong to the current JSON value; example: \"iterator does not fit current value\"
        2. The function can throw the following exceptions:
          • Throws type_error.309 if called on JSON values other than arrays; example: \"cannot use insert() with string\"
          • Throws invalid_iterator.202 if called on an iterator which does not belong to the current JSON value; example: \"iterator does not fit current value\"
        3. The function can throw the following exceptions:
          • Throws type_error.309 if called on JSON values other than arrays; example: \"cannot use insert() with string\"
          • Throws invalid_iterator.202 if called on an iterator which does not belong to the current JSON value; example: \"iterator does not fit current value\"
          • Throws invalid_iterator.210 if first and last do not belong to the same JSON value; example: \"iterators do not fit\"
          • Throws invalid_iterator.211 if first or last are iterators into container for which insert is called; example: \"passed iterators may not belong to container\"
          • Throws invalid_iterator.202 if first or last do not point to an array; example: \"iterators first and last must point to arrays\"
        4. The function can throw the following exceptions:
          • Throws type_error.309 if called on JSON values other than arrays; example: \"cannot use insert() with string\"
          • Throws invalid_iterator.202 if called on an iterator which does not belong to the current JSON value; example: \"iterator does not fit current value\"
        5. The function can throw the following exceptions:
          • Throws type_error.309 if called on JSON values other than objects; example: \"cannot use insert() with string\"
          • Throws invalid_iterator.202 if first or last do not point to an object; example: \"iterators first and last must point to objects\"
          • Throws invalid_iterator.210 if first and last do not belong to the same JSON value; example: \"iterators do not fit\"
        "},{"location":"api/basic_json/insert/#complexity","title":"Complexity","text":"
        1. Constant plus linear in the distance between pos and end of the container.
        2. Linear in cnt plus linear in the distance between pos and end of the container.
        3. Linear in std::distance(first, last) plus linear in the distance between pos and end of the container.
        4. Linear in ilist.size() plus linear in the distance between pos and end of the container.
        5. O(N*log(size() + N)), where N is the number of elements to insert.
        "},{"location":"api/basic_json/insert/#examples","title":"Examples","text":"Example: (1) insert element into array

        The example shows how insert() is used.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON array\n    json v = {1, 2, 3, 4};\n\n    // insert number 10 before number 3\n    auto new_pos = v.insert(v.begin() + 2, 10);\n\n    // output new array and result of insert call\n    std::cout << *new_pos << '\\n';\n    std::cout << v << '\\n';\n}\n

        Output:

        10\n[1,2,10,3,4]\n
        Example: (2) insert copies of element into array

        The example shows how insert() is used.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON array\n    json v = {1, 2, 3, 4};\n\n    // insert number 7 copies of number 7 before number 3\n    auto new_pos = v.insert(v.begin() + 2, 7, 7);\n\n    // output new array and result of insert call\n    std::cout << *new_pos << '\\n';\n    std::cout << v << '\\n';\n}\n

        Output:

        7\n[1,2,7,7,7,7,7,7,7,3,4]\n
        Example: (3) insert a range of elements into an array

        The example shows how insert() is used.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON array\n    json v = {1, 2, 3, 4};\n\n    // create a JSON array to copy values from\n    json v2 = {\"one\", \"two\", \"three\", \"four\"};\n\n    // insert range from v2 before the end of array v\n    auto new_pos = v.insert(v.end(), v2.begin(), v2.end());\n\n    // output new array and result of insert call\n    std::cout << *new_pos << '\\n';\n    std::cout << v << '\\n';\n}\n

        Output:

        \"one\"\n[1,2,3,4,\"one\",\"two\",\"three\",\"four\"]\n
        Example: (4) insert elements from an initializer list into an array

        The example shows how insert() is used.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON array\n    json v = {1, 2, 3, 4};\n\n    // insert range from v2 before the end of array v\n    auto new_pos = v.insert(v.end(), {7, 8, 9});\n\n    // output new array and result of insert call\n    std::cout << *new_pos << '\\n';\n    std::cout << v << '\\n';\n}\n

        Output:

        7\n[1,2,3,4,7,8,9]\n
        Example: (5) insert a range of elements into an object

        The example shows how insert() is used.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create two JSON objects\n    json j1 = {{\"one\", \"eins\"}, {\"two\", \"zwei\"}};\n    json j2 = {{\"eleven\", \"elf\"}, {\"seventeen\", \"siebzehn\"}};\n\n    // output objects\n    std::cout << j1 << '\\n';\n    std::cout << j2 << '\\n';\n\n    // insert range from j2 to j1\n    j1.insert(j2.begin(), j2.end());\n\n    // output result of insert call\n    std::cout << j1 << '\\n';\n}\n

        Output:

        {\"one\":\"eins\",\"two\":\"zwei\"}\n{\"eleven\":\"elf\",\"seventeen\":\"siebzehn\"}\n{\"eleven\":\"elf\",\"one\":\"eins\",\"seventeen\":\"siebzehn\",\"two\":\"zwei\"}\n
        "},{"location":"api/basic_json/insert/#see-also","title":"See also","text":"
        • emplace add a value to an object
        • emplace_back add a value to an array
        • push_back add a value to an array/object
        • update merges objects
        "},{"location":"api/basic_json/insert/#version-history","title":"Version history","text":"
        1. Added in version 1.0.0.
        2. Added in version 1.0.0.
        3. Added in version 1.0.0.
        4. Added in version 1.0.0. Fixed in version 3.13.0 unreleased to copy the values before inserting; before, an ilist that referred to elements of the array being inserted into could insert wrong values, because the range insert could move from or shift an element before it was copied.
        5. Added in version 3.0.0.
        "},{"location":"api/basic_json/invalid_iterator/","title":"nlohmann::basic_json::invalid_iterator","text":"
        class invalid_iterator : public exception;\n

        This exception is thrown if iterators passed to a library function do not match the expected semantics.

        Exceptions have ids 2xx (see list of iterator errors).

        classDiagram\n  direction LR\n\n    class std_exception [\"std::exception\"] {\n        <<interface>>\n    }\n\n    class json_exception [\"basic_json::exception\"] {\n        +const int id\n        +const char* what() const\n    }\n\n    class json_parse_error [\"basic_json::parse_error\"] {\n        +const std::size_t byte\n    }\n\n    class json_invalid_iterator [\"basic_json::invalid_iterator\"]\n    class json_type_error [\"basic_json::type_error\"]\n    class json_out_of_range [\"basic_json::out_of_range\"]\n    class json_other_error [\"basic_json::other_error\"]\n\n    std_exception <|-- json_exception\n    json_exception <|-- json_parse_error\n    json_exception <|-- json_invalid_iterator\n    json_exception <|-- json_type_error\n    json_exception <|-- json_out_of_range\n    json_exception <|-- json_other_error\n\n    style json_invalid_iterator fill:#CCCCFF
        "},{"location":"api/basic_json/invalid_iterator/#member-functions","title":"Member functions","text":"
        • what - returns explanatory string
        "},{"location":"api/basic_json/invalid_iterator/#member-variables","title":"Member variables","text":"
        • id - the id of the exception
        "},{"location":"api/basic_json/invalid_iterator/#examples","title":"Examples","text":"Example

        The following code shows how a invalid_iterator exception can be caught.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    try\n    {\n        // calling iterator::key() on non-object iterator\n        json j = \"string\";\n        json::iterator it = j.begin();\n        auto k = it.key();\n    }\n    catch (const json::invalid_iterator& e)\n    {\n        // output exception information\n        std::cout << \"message: \" << e.what() << '\\n'\n                  << \"exception id: \" << e.id << std::endl;\n    }\n}\n

        Output:

        message: [json.exception.invalid_iterator.207] cannot use key() for non-object iterators\nexception id: 207\n
        "},{"location":"api/basic_json/invalid_iterator/#see-also","title":"See also","text":"
        • exception for the base class of all exceptions thrown by the library
        • List of iterator errors
        • parse_error for exceptions indicating a parse error
        • type_error for exceptions indicating executing a member function with a wrong type
        • out_of_range for exceptions indicating access out of the defined range
        • other_error for exceptions indicating other library errors
        "},{"location":"api/basic_json/invalid_iterator/#version-history","title":"Version history","text":"
        • Since version 3.0.0.
        "},{"location":"api/basic_json/is_array/","title":"nlohmann::basic_json::is_array","text":"
        constexpr bool is_array() const noexcept;\n

        This function returns true if and only if the JSON value is an array.

        "},{"location":"api/basic_json/is_array/#return-value","title":"Return value","text":"

        true if type is an array, false otherwise.

        "},{"location":"api/basic_json/is_array/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this member function never throws exceptions.

        "},{"location":"api/basic_json/is_array/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json/is_array/#examples","title":"Examples","text":"Example

        The following code exemplifies is_array() for all JSON types.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_null;\n    json j_boolean = true;\n    json j_number_integer = 17;\n    json j_number_unsigned_integer = 12345678987654321u;\n    json j_number_float = 23.42;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n    json j_string = \"Hello, world\";\n    json j_binary = json::binary({1, 2, 3});\n\n    // call is_array()\n    std::cout << std::boolalpha;\n    std::cout << j_null.is_array() << '\\n';\n    std::cout << j_boolean.is_array() << '\\n';\n    std::cout << j_number_integer.is_array() << '\\n';\n    std::cout << j_number_unsigned_integer.is_array() << '\\n';\n    std::cout << j_number_float.is_array() << '\\n';\n    std::cout << j_object.is_array() << '\\n';\n    std::cout << j_array.is_array() << '\\n';\n    std::cout << j_string.is_array() << '\\n';\n    std::cout << j_binary.is_array() << '\\n';\n}\n

        Output:

        false\nfalse\nfalse\nfalse\nfalse\nfalse\ntrue\nfalse\nfalse\n
        "},{"location":"api/basic_json/is_array/#see-also","title":"See also","text":"
        • is_object checks whether the JSON value is an object
        • is_structured checks whether the JSON value is structured (array or object)
        • type returns the type of the JSON value
        • array_t the type used to store JSON arrays
        • basic_json_view::is_array - the same check on a zero-copy view
        "},{"location":"api/basic_json/is_array/#version-history","title":"Version history","text":"
        • Added in version 1.0.0.
        "},{"location":"api/basic_json/is_binary/","title":"nlohmann::basic_json::is_binary","text":"
        constexpr bool is_binary() const noexcept;\n

        This function returns true if and only if the JSON value is a binary array.

        "},{"location":"api/basic_json/is_binary/#return-value","title":"Return value","text":"

        true if type is binary, false otherwise.

        "},{"location":"api/basic_json/is_binary/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this member function never throws exceptions.

        "},{"location":"api/basic_json/is_binary/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json/is_binary/#examples","title":"Examples","text":"Example

        The following code exemplifies is_binary() for all JSON types.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_null;\n    json j_boolean = true;\n    json j_number_integer = 17;\n    json j_number_unsigned_integer = 12345678987654321u;\n    json j_number_float = 23.42;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n    json j_string = \"Hello, world\";\n    json j_binary = json::binary({1, 2, 3});\n\n    // call is_binary()\n    std::cout << std::boolalpha;\n    std::cout << j_null.is_binary() << '\\n';\n    std::cout << j_boolean.is_binary() << '\\n';\n    std::cout << j_number_integer.is_binary() << '\\n';\n    std::cout << j_number_unsigned_integer.is_binary() << '\\n';\n    std::cout << j_number_float.is_binary() << '\\n';\n    std::cout << j_object.is_binary() << '\\n';\n    std::cout << j_array.is_binary() << '\\n';\n    std::cout << j_string.is_binary() << '\\n';\n    std::cout << j_binary.is_binary() << '\\n';\n}\n

        Output:

        false\nfalse\nfalse\nfalse\nfalse\nfalse\nfalse\nfalse\ntrue\n
        "},{"location":"api/basic_json/is_binary/#see-also","title":"See also","text":"
        • is_primitive checks whether the JSON value is primitive
        • binary_t the type used to store binary values
        • get_binary returns a reference to the stored binary value
        • basic_json_view::is_binary - the same check on a zero-copy view
        "},{"location":"api/basic_json/is_binary/#version-history","title":"Version history","text":"
        • Added in version 3.8.0.
        "},{"location":"api/basic_json/is_boolean/","title":"nlohmann::basic_json::is_boolean","text":"
        constexpr bool is_boolean() const noexcept;\n

        This function returns true if and only if the JSON value is true or false.

        "},{"location":"api/basic_json/is_boolean/#return-value","title":"Return value","text":"

        true if type is boolean, false otherwise.

        "},{"location":"api/basic_json/is_boolean/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this member function never throws exceptions.

        "},{"location":"api/basic_json/is_boolean/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json/is_boolean/#examples","title":"Examples","text":"Example

        The following code exemplifies is_boolean() for all JSON types.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_null;\n    json j_boolean = true;\n    json j_number_integer = 17;\n    json j_number_unsigned_integer = 12345678987654321u;\n    json j_number_float = 23.42;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n    json j_string = \"Hello, world\";\n    json j_binary = json::binary({1, 2, 3});\n\n    // call is_boolean()\n    std::cout << std::boolalpha;\n    std::cout << j_null.is_boolean() << '\\n';\n    std::cout << j_boolean.is_boolean() << '\\n';\n    std::cout << j_number_integer.is_boolean() << '\\n';\n    std::cout << j_number_unsigned_integer.is_boolean() << '\\n';\n    std::cout << j_number_float.is_boolean() << '\\n';\n    std::cout << j_object.is_boolean() << '\\n';\n    std::cout << j_array.is_boolean() << '\\n';\n    std::cout << j_string.is_boolean() << '\\n';\n    std::cout << j_binary.is_boolean() << '\\n';\n}\n

        Output:

        false\ntrue\nfalse\nfalse\nfalse\nfalse\nfalse\nfalse\nfalse\n
        "},{"location":"api/basic_json/is_boolean/#see-also","title":"See also","text":"
        • boolean_t the type used to store JSON booleans
        • is_primitive checks whether the JSON value is primitive
        • basic_json_view::is_boolean - the same check on a zero-copy view
        "},{"location":"api/basic_json/is_boolean/#version-history","title":"Version history","text":"
        • Added in version 1.0.0.
        "},{"location":"api/basic_json/is_discarded/","title":"nlohmann::basic_json::is_discarded","text":"
        constexpr bool is_discarded() const noexcept;\n

        This function returns true for a JSON value if either:

        • the value was discarded during parsing with a callback function (see parser_callback_t), or
        • the value is the result of parsing invalid JSON with parameter allow_exceptions set to false; see parse for more information.
        "},{"location":"api/basic_json/is_discarded/#return-value","title":"Return value","text":"

        true if type is discarded, false otherwise.

        "},{"location":"api/basic_json/is_discarded/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this member function never throws exceptions.

        "},{"location":"api/basic_json/is_discarded/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json/is_discarded/#notes","title":"Notes","text":"

        Comparisons

        Discarded values are never compared equal with operator==. That is, checking whether a JSON value j is discarded will only work via:

        j.is_discarded()\n

        because

        j == json::value_t::discarded\n

        will always be false.

        Removal during parsing with callback functions

        When a value is discarded by a callback function (see parser_callback_t) during parsing, then it is removed when it is part of a structured value. For instance, if the second value of an array is discarded, instead of [null, discarded, false], the array [null, false] is returned. If the top-level value itself is discarded by the callback, the parse call returns a null value.

        After a successful parse, this function always returns false: discarded values can only occur during parsing and are either removed when inside a structured value or replaced by null at the top level. The exception is parsing with allow_exceptions set to false: a parse error then yields a discarded value for which this function returns true (see parse).

        "},{"location":"api/basic_json/is_discarded/#examples","title":"Examples","text":"Example: is_discarded() for ordinary JSON values

        The following code exemplifies is_discarded() for all JSON types.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_null;\n    json j_boolean = true;\n    json j_number_integer = 17;\n    json j_number_unsigned_integer = 12345678987654321u;\n    json j_number_float = 23.42;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n    json j_string = \"Hello, world\";\n    json j_binary = json::binary({1, 2, 3});\n\n    // call is_discarded()\n    std::cout << std::boolalpha;\n    std::cout << j_null.is_discarded() << '\\n';\n    std::cout << j_boolean.is_discarded() << '\\n';\n    std::cout << j_number_integer.is_discarded() << '\\n';\n    std::cout << j_number_unsigned_integer.is_discarded() << '\\n';\n    std::cout << j_number_float.is_discarded() << '\\n';\n    std::cout << j_object.is_discarded() << '\\n';\n    std::cout << j_array.is_discarded() << '\\n';\n    std::cout << j_string.is_discarded() << '\\n';\n    std::cout << j_binary.is_discarded() << '\\n';\n}\n

        Output:

        false\nfalse\nfalse\nfalse\nfalse\nfalse\nfalse\nfalse\nfalse\n
        Example: discarded values from parsing

        The following code shows the two situations in which a discarded value can be observed: parsing invalid JSON with allow_exceptions set to false, and a parser callback that discards the top-level value (which is replaced by null and therefore does not remain discarded).

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // parsing invalid JSON without exceptions yields a discarded value\n    json j_invalid = json::parse(\"[1,2,3\", nullptr, false);\n\n    // a callback that discards the top-level value does not leave it\n    // \"discarded\" -- it is replaced by null instead\n    json j_discarded_by_callback = json::parse(\"[1,2,3]\", [](int /*depth*/, json::parse_event_t event, json& /*parsed*/)\n    {\n        return event != json::parse_event_t::array_start;\n    });\n\n    std::cout << std::boolalpha;\n    std::cout << \"j_invalid.is_discarded()               = \" << j_invalid.is_discarded() << '\\n';\n    std::cout << \"j_discarded_by_callback                = \" << j_discarded_by_callback << '\\n';\n    std::cout << \"j_discarded_by_callback.is_discarded() = \" << j_discarded_by_callback.is_discarded() << '\\n';\n}\n

        Output:

        j_invalid.is_discarded()               = true\nj_discarded_by_callback                = null\nj_discarded_by_callback.is_discarded() = false\n
        "},{"location":"api/basic_json/is_discarded/#see-also","title":"See also","text":"
        • basic_json_view::is_discarded - the corresponding check on a zero-copy view, which is true if the view refers to no value
        "},{"location":"api/basic_json/is_discarded/#version-history","title":"Version history","text":"
        • Added in version 1.0.0.
        "},{"location":"api/basic_json/is_null/","title":"nlohmann::basic_json::is_null","text":"
        constexpr bool is_null() const noexcept;\n

        This function returns true if and only if the JSON value is null.

        "},{"location":"api/basic_json/is_null/#return-value","title":"Return value","text":"

        true if type is null, false otherwise.

        "},{"location":"api/basic_json/is_null/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this member function never throws exceptions.

        "},{"location":"api/basic_json/is_null/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json/is_null/#examples","title":"Examples","text":"Example

        The following code exemplifies is_null() for all JSON types.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_null;\n    json j_boolean = true;\n    json j_number_integer = 17;\n    json j_number_unsigned_integer = 12345678987654321u;\n    json j_number_float = 23.42;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n    json j_string = \"Hello, world\";\n    json j_binary = json::binary({1, 2, 3});\n\n    // call is_null()\n    std::cout << std::boolalpha;\n    std::cout << j_null.is_null() << '\\n';\n    std::cout << j_boolean.is_null() << '\\n';\n    std::cout << j_number_integer.is_null() << '\\n';\n    std::cout << j_number_unsigned_integer.is_null() << '\\n';\n    std::cout << j_number_float.is_null() << '\\n';\n    std::cout << j_object.is_null() << '\\n';\n    std::cout << j_array.is_null() << '\\n';\n    std::cout << j_string.is_null() << '\\n';\n    std::cout << j_binary.is_null() << '\\n';\n}\n

        Output:

        true\nfalse\nfalse\nfalse\nfalse\nfalse\nfalse\nfalse\nfalse\n
        "},{"location":"api/basic_json/is_null/#see-also","title":"See also","text":"
        • is_array checks whether the JSON value is an array
        • is_object checks whether the JSON value is an object
        • type returns the type of the JSON value
        • value_t the enumeration of JSON types
        • basic_json_view::is_null - the same check on a zero-copy view
        "},{"location":"api/basic_json/is_null/#version-history","title":"Version history","text":"
        • Added in version 1.0.0.
        "},{"location":"api/basic_json/is_number/","title":"nlohmann::basic_json::is_number","text":"
        constexpr bool is_number() const noexcept;\n

        This function returns true if and only if the JSON value is a number. This includes both integer (signed and unsigned) and floating-point values.

        "},{"location":"api/basic_json/is_number/#return-value","title":"Return value","text":"

        true if type is number (regardless whether integer, unsigned integer, or floating-point), false otherwise.

        "},{"location":"api/basic_json/is_number/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this member function never throws exceptions.

        "},{"location":"api/basic_json/is_number/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json/is_number/#possible-implementation","title":"Possible implementation","text":"
        constexpr bool is_number() const noexcept\n{\n    return is_number_integer() || is_number_float();\n}\n
        "},{"location":"api/basic_json/is_number/#examples","title":"Examples","text":"Example

        The following code exemplifies is_number() for all JSON types.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_null;\n    json j_boolean = true;\n    json j_number_integer = 17;\n    json j_number_unsigned_integer = 12345678987654321u;\n    json j_number_float = 23.42;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n    json j_string = \"Hello, world\";\n    json j_binary = json::binary({1, 2, 3});\n\n    // call is_number()\n    std::cout << std::boolalpha;\n    std::cout << j_null.is_number() << '\\n';\n    std::cout << j_boolean.is_number() << '\\n';\n    std::cout << j_number_integer.is_number() << '\\n';\n    std::cout << j_number_unsigned_integer.is_number() << '\\n';\n    std::cout << j_number_float.is_number() << '\\n';\n    std::cout << j_object.is_number() << '\\n';\n    std::cout << j_array.is_number() << '\\n';\n    std::cout << j_string.is_number() << '\\n';\n    std::cout << j_binary.is_number() << '\\n';\n}\n

        Output:

        false\nfalse\ntrue\ntrue\ntrue\nfalse\nfalse\nfalse\nfalse\n
        "},{"location":"api/basic_json/is_number/#see-also","title":"See also","text":"
        • is_number_integer() check if the value is an integer or unsigned integer number
        • is_number_unsigned() check if the value is an unsigned integer number
        • is_number_float() check if the value is a floating-point number
        • basic_json_view::is_number - the same check on a zero-copy view
        "},{"location":"api/basic_json/is_number/#version-history","title":"Version history","text":"
        • Added in version 1.0.0.
        • Extended to also return true for unsigned integers in 2.0.0.
        "},{"location":"api/basic_json/is_number_float/","title":"nlohmann::basic_json::is_number_float","text":"
        constexpr bool is_number_float() const noexcept;\n

        This function returns true if and only if the JSON value is a floating-point number. This excludes signed and unsigned integer values.

        "},{"location":"api/basic_json/is_number_float/#return-value","title":"Return value","text":"

        true if type is a floating-point number, false otherwise.

        "},{"location":"api/basic_json/is_number_float/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this member function never throws exceptions.

        "},{"location":"api/basic_json/is_number_float/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json/is_number_float/#examples","title":"Examples","text":"Example

        The following code exemplifies is_number_float() for all JSON types.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_null;\n    json j_boolean = true;\n    json j_number_integer = 17;\n    json j_number_unsigned_integer = 12345678987654321u;\n    json j_number_float = 23.42;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n    json j_string = \"Hello, world\";\n    json j_binary = json::binary({1, 2, 3});\n\n    // call is_number_float()\n    std::cout << std::boolalpha;\n    std::cout << j_null.is_number_float() << '\\n';\n    std::cout << j_boolean.is_number_float() << '\\n';\n    std::cout << j_number_integer.is_number_float() << '\\n';\n    std::cout << j_number_unsigned_integer.is_number_float() << '\\n';\n    std::cout << j_number_float.is_number_float() << '\\n';\n    std::cout << j_object.is_number_float() << '\\n';\n    std::cout << j_array.is_number_float() << '\\n';\n    std::cout << j_string.is_number_float() << '\\n';\n    std::cout << j_binary.is_number_float() << '\\n';\n}\n

        Output:

        false\nfalse\nfalse\nfalse\ntrue\nfalse\nfalse\nfalse\nfalse\n
        "},{"location":"api/basic_json/is_number_float/#see-also","title":"See also","text":"
        • is_number() check if the value is a number
        • is_number_integer() check if the value is an integer or unsigned integer number
        • is_number_unsigned() check if the value is an unsigned integer number
        • basic_json_view::is_number_float - the same check on a zero-copy view
        "},{"location":"api/basic_json/is_number_float/#version-history","title":"Version history","text":"
        • Added in version 1.0.0.
        "},{"location":"api/basic_json/is_number_integer/","title":"nlohmann::basic_json::is_number_integer","text":"
        constexpr bool is_number_integer() const noexcept;\n

        This function returns true if and only if the JSON value is a signed or unsigned integer number. This excludes floating-point values.

        "},{"location":"api/basic_json/is_number_integer/#return-value","title":"Return value","text":"

        true if type is an integer or unsigned integer number, false otherwise.

        "},{"location":"api/basic_json/is_number_integer/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this member function never throws exceptions.

        "},{"location":"api/basic_json/is_number_integer/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json/is_number_integer/#examples","title":"Examples","text":"Example

        The following code exemplifies is_number_integer() for all JSON types.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_null;\n    json j_boolean = true;\n    json j_number_integer = 17;\n    json j_number_unsigned_integer = 12345678987654321u;\n    json j_number_float = 23.42;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n    json j_string = \"Hello, world\";\n    json j_binary = json::binary({1, 2, 3});\n\n    // call is_number_integer()\n    std::cout << std::boolalpha;\n    std::cout << j_null.is_number_integer() << '\\n';\n    std::cout << j_boolean.is_number_integer() << '\\n';\n    std::cout << j_number_integer.is_number_integer() << '\\n';\n    std::cout << j_number_unsigned_integer.is_number_integer() << '\\n';\n    std::cout << j_number_float.is_number_integer() << '\\n';\n    std::cout << j_object.is_number_integer() << '\\n';\n    std::cout << j_array.is_number_integer() << '\\n';\n    std::cout << j_string.is_number_integer() << '\\n';\n    std::cout << j_binary.is_number_integer() << '\\n';\n}\n

        Output:

        false\nfalse\ntrue\ntrue\nfalse\nfalse\nfalse\nfalse\nfalse\n
        "},{"location":"api/basic_json/is_number_integer/#see-also","title":"See also","text":"
        • is_number() check if the value is a number
        • is_number_unsigned() check if the value is an unsigned integer number
        • is_number_float() check if the value is a floating-point number
        • basic_json_view::is_number_integer - the same check on a zero-copy view
        "},{"location":"api/basic_json/is_number_integer/#version-history","title":"Version history","text":"
        • Added in version 1.0.0.
        • Extended to also return true for unsigned integers in 2.0.0.
        "},{"location":"api/basic_json/is_number_unsigned/","title":"nlohmann::basic_json::is_number_unsigned","text":"
        constexpr bool is_number_unsigned() const noexcept;\n

        This function returns true if and only if the JSON value is an unsigned integer number. This excludes floating-point and signed integer values.

        If number_unsigned_t is the same type as number_integer_t, every integer is stored as number_integer_t, and this function always returns false.

        "},{"location":"api/basic_json/is_number_unsigned/#return-value","title":"Return value","text":"

        true if type is an unsigned integer number, false otherwise.

        "},{"location":"api/basic_json/is_number_unsigned/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this member function never throws exceptions.

        "},{"location":"api/basic_json/is_number_unsigned/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json/is_number_unsigned/#examples","title":"Examples","text":"Example

        The following code exemplifies is_number_unsigned() for all JSON types.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_null;\n    json j_boolean = true;\n    json j_number_integer = 17;\n    json j_number_unsigned_integer = 12345678987654321u;\n    json j_number_float = 23.42;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n    json j_string = \"Hello, world\";\n    json j_binary = json::binary({1, 2, 3});\n\n    // call is_number_unsigned()\n    std::cout << std::boolalpha;\n    std::cout << j_null.is_number_unsigned() << '\\n';\n    std::cout << j_boolean.is_number_unsigned() << '\\n';\n    std::cout << j_number_integer.is_number_unsigned() << '\\n';\n    std::cout << j_number_unsigned_integer.is_number_unsigned() << '\\n';\n    std::cout << j_number_float.is_number_unsigned() << '\\n';\n    std::cout << j_object.is_number_unsigned() << '\\n';\n    std::cout << j_array.is_number_unsigned() << '\\n';\n    std::cout << j_string.is_number_unsigned() << '\\n';\n    std::cout << j_binary.is_number_unsigned() << '\\n';\n}\n

        Output:

        false\nfalse\nfalse\ntrue\nfalse\nfalse\nfalse\nfalse\nfalse\n
        "},{"location":"api/basic_json/is_number_unsigned/#see-also","title":"See also","text":"
        • is_number() check if the value is a number
        • is_number_integer() check if the value is an integer or unsigned integer number
        • is_number_float() check if the value is a floating-point number
        • basic_json_view::is_number_unsigned - the same check on a zero-copy view
        "},{"location":"api/basic_json/is_number_unsigned/#version-history","title":"Version history","text":"
        • Added in version 2.0.0.
        "},{"location":"api/basic_json/is_object/","title":"nlohmann::basic_json::is_object","text":"
        constexpr bool is_object() const noexcept;\n

        This function returns true if and only if the JSON value is an object.

        "},{"location":"api/basic_json/is_object/#return-value","title":"Return value","text":"

        true if type is an object, false otherwise.

        "},{"location":"api/basic_json/is_object/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this member function never throws exceptions.

        "},{"location":"api/basic_json/is_object/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json/is_object/#examples","title":"Examples","text":"Example

        The following code exemplifies is_object() for all JSON types.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_null;\n    json j_boolean = true;\n    json j_number_integer = 17;\n    json j_number_float = 23.42;\n    json j_number_unsigned_integer = 12345678987654321u;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n    json j_string = \"Hello, world\";\n    json j_binary = json::binary({1, 2, 3});\n\n    // call is_object()\n    std::cout << std::boolalpha;\n    std::cout << j_null.is_object() << '\\n';\n    std::cout << j_boolean.is_object() << '\\n';\n    std::cout << j_number_integer.is_object() << '\\n';\n    std::cout << j_number_unsigned_integer.is_object() << '\\n';\n    std::cout << j_number_float.is_object() << '\\n';\n    std::cout << j_object.is_object() << '\\n';\n    std::cout << j_array.is_object() << '\\n';\n    std::cout << j_string.is_object() << '\\n';\n    std::cout << j_binary.is_object() << '\\n';\n}\n

        Output:

        false\nfalse\nfalse\nfalse\nfalse\ntrue\nfalse\nfalse\nfalse\n
        "},{"location":"api/basic_json/is_object/#see-also","title":"See also","text":"
        • is_array checks whether the JSON value is an array
        • is_structured checks whether the JSON value is structured (array or object)
        • type returns the type of the JSON value
        • object_t the type used to store JSON objects
        • basic_json_view::is_object - the same check on a zero-copy view
        "},{"location":"api/basic_json/is_object/#version-history","title":"Version history","text":"
        • Added in version 1.0.0.
        "},{"location":"api/basic_json/is_primitive/","title":"nlohmann::basic_json::is_primitive","text":"
        constexpr bool is_primitive() const noexcept;\n

        This function returns true if and only if the JSON type is primitive (string, number, boolean, null, binary).

        "},{"location":"api/basic_json/is_primitive/#return-value","title":"Return value","text":"

        true if type is primitive (string, number, boolean, null, or binary), false otherwise.

        "},{"location":"api/basic_json/is_primitive/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this member function never throws exceptions.

        "},{"location":"api/basic_json/is_primitive/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json/is_primitive/#possible-implementation","title":"Possible implementation","text":"
        constexpr bool is_primitive() const noexcept\n{\n    return is_null() || is_string() || is_boolean() || is_number() || is_binary();\n}\n
        "},{"location":"api/basic_json/is_primitive/#notes","title":"Notes","text":"

        The term primitive stems from RFC 8259:

        JSON can represent four primitive types (strings, numbers, booleans, and null) and two structured types (objects and arrays).

        This library extends primitive types to binary types, because binary types are roughly comparable to strings. Hence, is_primitive() returns true for binary values.

        "},{"location":"api/basic_json/is_primitive/#examples","title":"Examples","text":"Example

        The following code exemplifies is_primitive() for all JSON types.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_null;\n    json j_boolean = true;\n    json j_number_integer = 17;\n    json j_number_float = 23.42;\n    json j_number_unsigned_integer = 12345678987654321u;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n    json j_string = \"Hello, world\";\n    json j_binary = json::binary({1, 2, 3});\n\n    // call is_primitive()\n    std::cout << std::boolalpha;\n    std::cout << j_null.is_primitive() << '\\n';\n    std::cout << j_boolean.is_primitive() << '\\n';\n    std::cout << j_number_integer.is_primitive() << '\\n';\n    std::cout << j_number_unsigned_integer.is_primitive() << '\\n';\n    std::cout << j_number_float.is_primitive() << '\\n';\n    std::cout << j_object.is_primitive() << '\\n';\n    std::cout << j_array.is_primitive() << '\\n';\n    std::cout << j_string.is_primitive() << '\\n';\n    std::cout << j_binary.is_primitive() << '\\n';\n}\n

        Output:

        true\ntrue\ntrue\ntrue\ntrue\nfalse\nfalse\ntrue\ntrue\n
        "},{"location":"api/basic_json/is_primitive/#see-also","title":"See also","text":"
        • is_structured() returns whether the JSON value is structured
        • is_null() returns whether the JSON value is null
        • is_string() returns whether the JSON value is a string
        • is_boolean() returns whether the JSON value is a boolean
        • is_number() returns whether the JSON value is a number
        • is_binary() returns whether the JSON value is a binary array
        • basic_json_view::is_primitive - the same check on a zero-copy view
        "},{"location":"api/basic_json/is_primitive/#version-history","title":"Version history","text":"
        • Added in version 1.0.0.
        • Extended to return true for binary types in version 3.8.0.
        "},{"location":"api/basic_json/is_string/","title":"nlohmann::basic_json::is_string","text":"
        constexpr bool is_string() const noexcept;\n

        This function returns true if and only if the JSON value is a string.

        "},{"location":"api/basic_json/is_string/#return-value","title":"Return value","text":"

        true if type is a string, false otherwise.

        "},{"location":"api/basic_json/is_string/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this member function never throws exceptions.

        "},{"location":"api/basic_json/is_string/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json/is_string/#examples","title":"Examples","text":"Example

        The following code exemplifies is_string() for all JSON types.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_null;\n    json j_boolean = true;\n    json j_number_integer = 17;\n    json j_number_float = 23.42;\n    json j_number_unsigned_integer = 12345678987654321u;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n    json j_string = \"Hello, world\";\n    json j_binary = json::binary({1, 2, 3});\n\n    // call is_string()\n    std::cout << std::boolalpha;\n    std::cout << j_null.is_string() << '\\n';\n    std::cout << j_boolean.is_string() << '\\n';\n    std::cout << j_number_integer.is_string() << '\\n';\n    std::cout << j_number_unsigned_integer.is_string() << '\\n';\n    std::cout << j_number_float.is_string() << '\\n';\n    std::cout << j_object.is_string() << '\\n';\n    std::cout << j_array.is_string() << '\\n';\n    std::cout << j_string.is_string() << '\\n';\n    std::cout << j_binary.is_string() << '\\n';\n}\n

        Output:

        false\nfalse\nfalse\nfalse\nfalse\nfalse\nfalse\ntrue\nfalse\n
        "},{"location":"api/basic_json/is_string/#see-also","title":"See also","text":"
        • is_primitive checks whether the JSON value is primitive
        • type returns the type of the JSON value
        • string_t the type used to store JSON strings
        • basic_json_view::is_string - the same check on a zero-copy view
        "},{"location":"api/basic_json/is_string/#version-history","title":"Version history","text":"
        • Added in version 1.0.0.
        "},{"location":"api/basic_json/is_structured/","title":"nlohmann::basic_json::is_structured","text":"
        constexpr bool is_structured() const noexcept;\n

        This function returns true if and only if the JSON type is structured (array or object).

        "},{"location":"api/basic_json/is_structured/#return-value","title":"Return value","text":"

        true if type is structured (array or object), false otherwise.

        "},{"location":"api/basic_json/is_structured/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this member function never throws exceptions.

        "},{"location":"api/basic_json/is_structured/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json/is_structured/#possible-implementation","title":"Possible implementation","text":"
        constexpr bool is_structured() const noexcept\n{\n    return is_array() || is_object();\n}\n
        "},{"location":"api/basic_json/is_structured/#notes","title":"Notes","text":"

        The term structured stems from RFC 8259:

        JSON can represent four primitive types (strings, numbers, booleans, and null) and two structured types (objects and arrays).

        Note that though strings are containers in C++, they are treated as primitive values in JSON.

        "},{"location":"api/basic_json/is_structured/#examples","title":"Examples","text":"Example

        The following code exemplifies is_structured() for all JSON types.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_null;\n    json j_boolean = true;\n    json j_number_integer = 17;\n    json j_number_float = 23.42;\n    json j_number_unsigned_integer = 12345678987654321u;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n    json j_string = \"Hello, world\";\n    json j_binary = json::binary({1, 2, 3});\n\n    // call is_structured()\n    std::cout << std::boolalpha;\n    std::cout << j_null.is_structured() << '\\n';\n    std::cout << j_boolean.is_structured() << '\\n';\n    std::cout << j_number_integer.is_structured() << '\\n';\n    std::cout << j_number_unsigned_integer.is_structured() << '\\n';\n    std::cout << j_number_float.is_structured() << '\\n';\n    std::cout << j_object.is_structured() << '\\n';\n    std::cout << j_array.is_structured() << '\\n';\n    std::cout << j_string.is_structured() << '\\n';\n    std::cout << j_binary.is_structured() << '\\n';\n}\n

        Output:

        false\nfalse\nfalse\nfalse\nfalse\ntrue\ntrue\nfalse\nfalse\n
        "},{"location":"api/basic_json/is_structured/#see-also","title":"See also","text":"
        • is_primitive() returns whether JSON value is primitive
        • is_array() returns whether the value is an array
        • is_object() returns whether the value is an object
        • basic_json_view::is_structured - the same check on a zero-copy view
        "},{"location":"api/basic_json/is_structured/#version-history","title":"Version history","text":"
        • Added in version 1.0.0.
        "},{"location":"api/basic_json/items/","title":"nlohmann::basic_json::items","text":"
        iteration_proxy<iterator> items() noexcept;\niteration_proxy<const_iterator> items() const noexcept;\n

        This function allows accessing iterator::key() and iterator::value() during range-based for loops. In these loops, a reference to the JSON values is returned, so there is no access to the underlying iterator.

        For loop without items() function:

        for (auto it = j_object.begin(); it != j_object.end(); ++it)\n{\n    std::cout << \"key: \" << it.key() << \", value:\" << it.value() << '\\n';\n}\n

        Range-based for loop without items() function:

        for (auto it : j_object)\n{\n    // \"it\" is of type json::reference and has no key() member\n    std::cout << \"value: \" << it << '\\n';\n}\n

        Range-based for loop with items() function:

        for (auto& el : j_object.items())\n{\n    std::cout << \"key: \" << el.key() << \", value:\" << el.value() << '\\n';\n}\n

        The items() function also allows using structured bindings (C++17):

        for (auto& [key, val] : j_object.items())\n{\n    std::cout << \"key: \" << key << \", value:\" << val << '\\n';\n}\n

        If you need to name the type of the dereferenced element explicitly (e.g., to write a standalone function that takes it as a parameter, or to use items() with std::for_each), use decltype:

        using element_type = decltype(*j_object.items().begin());\n

        The per-element type (iteration_proxy_value) lives in the library's internal detail namespace and is intentionally unspecified as a stable, named type -- decltype is the supported way to obtain it, but its exact name/definition may change between versions.

        "},{"location":"api/basic_json/items/#return-value","title":"Return value","text":"

        iteration proxy object wrapping the current value with an interface to use in range-based for loops

        "},{"location":"api/basic_json/items/#exception-safety","title":"Exception safety","text":"

        Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

        "},{"location":"api/basic_json/items/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json/items/#notes","title":"Notes","text":"

        When iterating over an array, key() will return the index of the element as string (see example). For primitive types (e.g., numbers), key() returns an empty string.

        Lifetime issues

        Using items() on temporary objects is dangerous. Make sure the object's lifetime exceeds the iteration. See #2040 for more information.

        "},{"location":"api/basic_json/items/#examples","title":"Examples","text":"Example

        The following code shows an example for items().

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n\n    // example for an object\n    for (auto& x : j_object.items())\n    {\n        std::cout << \"key: \" << x.key() << \", value: \" << x.value() << '\\n';\n    }\n\n    // example for an array\n    for (auto& x : j_array.items())\n    {\n        std::cout << \"key: \" << x.key() << \", value: \" << x.value() << '\\n';\n    }\n}\n

        Output:

        key: one, value: 1\nkey: two, value: 2\nkey: 0, value: 1\nkey: 1, value: 2\nkey: 2, value: 4\nkey: 3, value: 8\nkey: 4, value: 16\n
        "},{"location":"api/basic_json/items/#see-also","title":"See also","text":"
        • begin returns an iterator to the first element
        • end returns an iterator to one past the last element
        • basic_json_view::items - the same range on a zero-copy view (const auto, not const auto&: items are produced on the fly; document order, not sorted; duplicate keys are visited)
        "},{"location":"api/basic_json/items/#version-history","title":"Version history","text":"
        • Added iterator_wrapper in version 3.0.0.
        • Added items and deprecated iterator_wrapper in version 3.1.0.
        • Added structured binding support in version 3.5.0.

        Deprecation

        This function replaces the static function iterator_wrapper which was introduced in version 1.0.0, but has been deprecated in version 3.1.0. Function iterator_wrapper will be removed in version 4.0.0. Please replace all occurrences of iterator_wrapper(j) with j.items().

        You should be warned by your compiler with a -Wdeprecated-declarations warning if you are using a deprecated function.

        See the migration guide for how to update existing code.

        "},{"location":"api/basic_json/json_base_class_t/","title":"nlohmann::basic_json::json_base_class_t","text":"
        using json_base_class_t = detail::json_base_class<CustomBaseClass>;\n

        The base class used to inject custom functionality into each instance of basic_json. Examples of such functionality might be metadata, additional member functions (e.g., visitors), or other application-specific code.

        "},{"location":"api/basic_json/json_base_class_t/#template-parameters","title":"Template parameters","text":"CustomBaseClass the base class to be added to basic_json"},{"location":"api/basic_json/json_base_class_t/#notes","title":"Notes","text":""},{"location":"api/basic_json/json_base_class_t/#default-type","title":"Default type","text":"

        The default value for CustomBaseClass is void. In this case, an empty base class is used and no additional functionality is injected.

        "},{"location":"api/basic_json/json_base_class_t/#limitations","title":"Limitations","text":"

        The type CustomBaseClass has to be a default-constructible, non-final class. basic_json only supports copy/move construction/assignment if CustomBaseClass does so as well. A CustomBaseClass with non-static data members forfeits basic_json's standard layout guarantee. See Template Parameter Requirements.

        "},{"location":"api/basic_json/json_base_class_t/#name-conflicts","title":"Name conflicts","text":"

        Since basic_json derives from CustomBaseClass, members of basic_json hide members of CustomBaseClass with the same name. Hidden members remain accessible via as_base_class or by casting the value to json_base_class_t.

        Avoid generic member names

        Future versions of the library may add members to basic_json that hide members of CustomBaseClass that are accessible today. To reduce the risk of such conflicts, avoid generic names for the members of CustomBaseClass, for instance by using a distinctive prefix.

        "},{"location":"api/basic_json/json_base_class_t/#examples","title":"Examples","text":"Example

        The following code shows how to inject custom data and methods for each node.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nclass visitor_adaptor_with_metadata\n{\n  public:\n    template <class Fnc>\n    void visit(const Fnc& fnc) const;\n\n    int metadata = 42;\n  private:\n    template <class Ptr, class Fnc>\n    void do_visit(const Ptr& ptr, const Fnc& fnc) const;\n};\n\nusing json = nlohmann::json::with_base_class_t<visitor_adaptor_with_metadata>;\n\ntemplate <class Fnc>\nvoid visitor_adaptor_with_metadata::visit(const Fnc& fnc) const\n{\n    do_visit(json::json_pointer{}, fnc);\n}\n\ntemplate <class Ptr, class Fnc>\nvoid visitor_adaptor_with_metadata::do_visit(const Ptr& ptr, const Fnc& fnc) const\n{\n    using value_t = nlohmann::detail::value_t;\n    const json& j = *static_cast<const json*>(this);\n    switch (j.type())\n    {\n        case value_t::object:\n            fnc(ptr, j);\n            for (const auto& entry : j.items())\n            {\n                entry.value().do_visit(ptr / entry.key(), fnc);\n            }\n            break;\n        case value_t::array:\n            fnc(ptr, j);\n            for (std::size_t i = 0; i < j.size(); ++i)\n            {\n                j.at(i).do_visit(ptr / std::to_string(i), fnc);\n            }\n            break;\n        case value_t::null:\n        case value_t::string:\n        case value_t::boolean:\n        case value_t::number_integer:\n        case value_t::number_unsigned:\n        case value_t::number_float:\n        case value_t::binary:\n            fnc(ptr, j);\n            break;\n        case value_t::discarded:\n        default:\n            break;\n    }\n}\n\nint main()\n{\n    // create a json object\n    json j;\n    j[\"null\"];\n    j[\"object\"][\"uint\"] = 1U;\n    j[\"object\"].metadata = 21;\n\n    // visit and output\n    j.visit(\n         [&](const json::json_pointer & p,\n             const json & j)\n    {\n        std::cout << (p.empty() ? std::string{\"/\"} : p.to_string())\n                  << \" - metadata = \" << j.metadata << \" -> \" << j.dump() << '\\n';\n    });\n}\n

        Output:

        / - metadata = 42 -> {\"null\":null,\"object\":{\"uint\":1}}\n/null - metadata = 42 -> null\n/object - metadata = 21 -> {\"uint\":1}\n/object/uint - metadata = 42 -> 1\n
        "},{"location":"api/basic_json/json_base_class_t/#see-also","title":"See also","text":"
        • as_base_class - access the custom base class
        • Template Parameter Requirements - the requirements for CustomBaseClass
        "},{"location":"api/basic_json/json_base_class_t/#version-history","title":"Version history","text":"
        • Added in version 3.11.3.
        • Made a public member type in version 3.13.0 unreleased; it was private before, so it could not be named outside the class.
        "},{"location":"api/basic_json/json_serializer/","title":"nlohmann::basic_json::json_serializer","text":"
        template<typename T, typename SFINAE>\nusing json_serializer = JSONSerializer<T, SFINAE>;\n
        "},{"location":"api/basic_json/json_serializer/#template-parameters","title":"Template parameters","text":"T type to convert; will be used in the to_json/from_json functions SFINAE type to add compile type checks via SFINAE; usually void"},{"location":"api/basic_json/json_serializer/#notes","title":"Notes","text":""},{"location":"api/basic_json/json_serializer/#default-type","title":"Default type","text":"

        The default values for json_serializer is adl_serializer.

        "},{"location":"api/basic_json/json_serializer/#requirements","title":"Requirements","text":"

        A custom serializer must provide static void to_json(basic_json&, T) for every type it serializes, and either static void from_json(const basic_json&, T&) or static T from_json(const basic_json&) for every type it deserializes. See Template Parameter Requirements.

        "},{"location":"api/basic_json/json_serializer/#examples","title":"Examples","text":"Example

        The example below shows how a conversion of a non-default-constructible type is implemented via a specialization of the adl_serializer.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nnamespace ns\n{\n// a simple struct to model a person (not default constructible)\nstruct person\n{\n    person(std::string n, std::string a, int aa)\n        : name(std::move(n)), address(std::move(a)), age(aa)\n    {}\n\n    std::string name;\n    std::string address;\n    int age;\n};\n} // namespace ns\n\nnamespace nlohmann\n{\ntemplate <>\nstruct adl_serializer<ns::person>\n{\n    static ns::person from_json(const json& j)\n    {\n        return {j.at(\"name\"), j.at(\"address\"), j.at(\"age\")};\n    }\n\n    // Here's the catch! You must provide a to_json method! Otherwise, you\n    // will not be able to convert person to json, since you fully\n    // specialized adl_serializer on that type\n    static void to_json(json& j, ns::person p)\n    {\n        j[\"name\"] = p.name;\n        j[\"address\"] = p.address;\n        j[\"age\"] = p.age;\n    }\n};\n} // namespace nlohmann\n\nint main()\n{\n    json j;\n    j[\"name\"] = \"Ned Flanders\";\n    j[\"address\"] = \"744 Evergreen Terrace\";\n    j[\"age\"] = 60;\n\n    auto p = j.get<ns::person>();\n\n    std::cout << p.name << \" (\" << p.age << \") lives in \" << p.address << std::endl;\n}\n

        Output:

        Ned Flanders (60) lives in 744 Evergreen Terrace\n
        "},{"location":"api/basic_json/json_serializer/#see-also","title":"See also","text":"
        • adl_serializer the default json_serializer
        • get explicit type conversion using the json_serializer's from_json() method
        • get_to explicit type conversion into a variable using the json_serializer's from_json() method
        • Arbitrary Type Conversions - the article on converting between JSON values and arbitrary types
        "},{"location":"api/basic_json/json_serializer/#version-history","title":"Version history","text":"
        • Since version 2.0.0.
        "},{"location":"api/basic_json/max_size/","title":"nlohmann::basic_json::max_size","text":"
        size_type max_size() const noexcept;\n

        Returns the maximum number of elements a JSON value is able to hold due to system or library implementation limitations, i.e. std::distance(begin(), end()) for the JSON value.

        "},{"location":"api/basic_json/max_size/#return-value","title":"Return value","text":"

        The return value depends on the different types and is defined as follows:

        Value type return value null 0 (same as size()) boolean 1 (same as size()) string 1 (same as size()) number 1 (same as size()) binary 1 (same as size()) object result of function object_t::max_size() array result of function array_t::max_size()"},{"location":"api/basic_json/max_size/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this function never throws exceptions.

        "},{"location":"api/basic_json/max_size/#complexity","title":"Complexity","text":"

        Constant, as long as array_t and object_t satisfy the Container concept; that is, their max_size() functions have constant complexity.

        "},{"location":"api/basic_json/max_size/#notes","title":"Notes","text":"

        This function does not return the maximal length of a string stored as JSON value -- it returns the maximal number of string elements the JSON value can store which is 1.

        "},{"location":"api/basic_json/max_size/#examples","title":"Examples","text":"Example

        The following code calls max_size() on the different value types.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_null;\n    json j_boolean = true;\n    json j_number_integer = 17;\n    json j_number_float = 23.42;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n    json j_string = \"Hello, world\";\n\n    // call max_size()\n    std::cout << j_null.max_size() << '\\n';\n    std::cout << j_boolean.max_size() << '\\n';\n    std::cout << j_number_integer.max_size() << '\\n';\n    std::cout << j_number_float.max_size() << '\\n';\n    std::cout << j_object.max_size() << '\\n';\n    std::cout << j_array.max_size() << '\\n';\n    std::cout << j_string.max_size() << '\\n';\n}\n

        Output:

        0\n1\n1\n1\n115292150460684697\n576460752303423487\n1\n

        Note the output is platform-dependent.

        "},{"location":"api/basic_json/max_size/#see-also","title":"See also","text":"
        • size returns the number of elements
        • array_t the type used to store JSON arrays
        • object_t the type used to store JSON objects
        "},{"location":"api/basic_json/max_size/#version-history","title":"Version history","text":"
        • Added in version 1.0.0.
        • Extended to return 1 for binary types in version 3.8.0.
        "},{"location":"api/basic_json/merge_diff/","title":"nlohmann::basic_json::merge_diff","text":"
        static basic_json merge_diff(const basic_json& source,\n                             const basic_json& target);\n

        Creates a JSON Merge Patch so that value source can be changed into the value target by calling the merge_patch function. The patch contains only what differs: changed and added members with their new values, and removed members with the value null. Nested objects are compared member by member; any other value that differs, including an array, is replaced as a whole.

        For two JSON values source and target, where target contains no object member whose value is null, the following code always yields true (for ordered_json, see the notes below): 

        basic_json patched = source;\npatched.merge_patch(merge_diff(source, target));\npatched == target;\n

        "},{"location":"api/basic_json/merge_diff/#parameters","title":"Parameters","text":"source (in) JSON value to compare from target (in) JSON value to compare against"},{"location":"api/basic_json/merge_diff/#return-value","title":"Return value","text":"

        a JSON Merge Patch to convert the source to target

        "},{"location":"api/basic_json/merge_diff/#exception-safety","title":"Exception safety","text":"

        Strong guarantee: source and target are never modified.

        "},{"location":"api/basic_json/merge_diff/#complexity","title":"Complexity","text":"

        Linear in the total number of values (including nested ones) in source and target, times the cost of a key-based object operation (looking up a key, or adding one to the patch): logarithmic for json, and linear for ordered_json (see ordered_map complexity). For ordered_json, diffing two objects with n and m members therefore takes O((n+m)\u00b2).

        "},{"location":"api/basic_json/merge_diff/#notes","title":"Notes","text":"
        • A merge patch uses null to remove a member, so it cannot set a member to null. As in RFC 7396, a null member of target is treated as absent: the patch removes the member from source instead of setting it to null. A member that is null in both source and target is unchanged and left out of the patch, so source keeps it.
        • A merge patch cannot reorder object members, and merge_patch appends added members at the end. For ordered_json, the patched value therefore has the same members as target, but not necessarily in the same order, and may compare unequal to it. The patch lists changed and removed members in the order of source, followed by added members in the order of target.
        "},{"location":"api/basic_json/merge_diff/#examples","title":"Examples","text":"Example

        The following code shows how a JSON Merge Patch is created as a diff for two JSON values.

        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // the source document\n    json source = R\"(\n        {\n            \"title\": \"Goodbye!\",\n            \"author\": {\n                \"givenName\": \"John\",\n                \"familyName\": \"Doe\"\n            },\n            \"tags\": [\n                \"example\",\n                \"sample\"\n            ],\n            \"content\": \"This will be unchanged\"\n        }\n    )\"_json;\n\n    // the target document\n    json target = R\"(\n        {\n            \"title\": \"Hello!\",\n            \"author\": {\n                \"givenName\": \"John\"\n            },\n            \"tags\": [\n                \"example\"\n            ],\n            \"content\": \"This will be unchanged\",\n            \"phoneNumber\": \"+01-123-456-7890\"\n        }\n    )\"_json;\n\n    // create the patch\n    json patch = json::merge_diff(source, target);\n\n    // roundtrip\n    json patched_source = source;\n    patched_source.merge_patch(patch);\n\n    // output patch and roundtrip result\n    std::cout << std::setw(4) << patch << \"\\n\\n\"\n              << std::setw(4) << patched_source << std::endl;\n}\n

        Output:

        {\n    \"author\": {\n        \"familyName\": null\n    },\n    \"phoneNumber\": \"+01-123-456-7890\",\n    \"tags\": [\n        \"example\"\n    ],\n    \"title\": \"Hello!\"\n}\n\n{\n    \"author\": {\n        \"givenName\": \"John\"\n    },\n    \"content\": \"This will be unchanged\",\n    \"phoneNumber\": \"+01-123-456-7890\",\n    \"tags\": [\n        \"example\"\n    ],\n    \"title\": \"Hello!\"\n}\n
        "},{"location":"api/basic_json/merge_diff/#see-also","title":"See also","text":"
        • RFC 7396 (JSON Merge Patch)
        • merge_patch applies a JSON Merge Patch
        • diff creates a diff as a JSON Patch
        "},{"location":"api/basic_json/merge_diff/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json/merge_patch/","title":"nlohmann::basic_json::merge_patch","text":"
        void merge_patch(const basic_json& apply_patch);\n

        The merge patch format is primarily intended for use with the HTTP PATCH method as a means of describing a set of modifications to a target resource's content. This function applies a merge patch to the current JSON value.

        The function implements the following algorithm from Section 2 of RFC 7396 (JSON Merge Patch):

        define MergePatch(Target, Patch):\n  if Patch is an Object:\n    if Target is not an Object:\n      Target = {} // Ignore the contents and set it to an empty Object\n    for each Name/Value pair in Patch:\n      if Value is null:\n        if Name exists in Target:\n          remove the Name/Value pair from Target\n      else:\n        Target[Name] = MergePatch(Target[Name], Value)\n    return Target\n  else:\n    return Patch\n

        Thereby, Target is the current object; that is, the patch is applied to the current value.

        "},{"location":"api/basic_json/merge_patch/#parameters","title":"Parameters","text":"apply_patch (in) the patch to apply"},{"location":"api/basic_json/merge_patch/#exception-safety","title":"Exception safety","text":"

        Basic guarantee: if an exception is thrown during the operation, the JSON value may be partially modified.

        "},{"location":"api/basic_json/merge_patch/#complexity","title":"Complexity","text":"

        Linear in the lengths of apply_patch.

        "},{"location":"api/basic_json/merge_patch/#notes","title":"Notes","text":"

        apply_patch may be *this itself or refer to a value contained in *this (for example, a subobject returned by (*this)[key]); it is read as it was when merge_patch() was called, before any modification of *this.

        "},{"location":"api/basic_json/merge_patch/#examples","title":"Examples","text":"Example

        The following code shows how a JSON Merge Patch is applied to a JSON document.

        #include <iostream>\n#include <nlohmann/json.hpp>\n#include <iomanip> // for std::setw\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // the original document\n    json document = R\"({\n                \"title\": \"Goodbye!\",\n                \"author\": {\n                    \"givenName\": \"John\",\n                    \"familyName\": \"Doe\"\n                },\n                \"tags\": [\n                    \"example\",\n                    \"sample\"\n                ],\n                \"content\": \"This will be unchanged\"\n            })\"_json;\n\n    // the patch\n    json patch = R\"({\n                \"title\": \"Hello!\",\n                \"phoneNumber\": \"+01-123-456-7890\",\n                \"author\": {\n                    \"familyName\": null\n                },\n                \"tags\": [\n                    \"example\"\n                ]\n            })\"_json;\n\n    // apply the patch\n    document.merge_patch(patch);\n\n    // output original and patched document\n    std::cout << std::setw(4) << document << std::endl;\n}\n

        Output:

        {\n    \"author\": {\n        \"givenName\": \"John\"\n    },\n    \"content\": \"This will be unchanged\",\n    \"phoneNumber\": \"+01-123-456-7890\",\n    \"tags\": [\n        \"example\"\n    ],\n    \"title\": \"Hello!\"\n}\n
        "},{"location":"api/basic_json/merge_patch/#see-also","title":"See also","text":"
        • RFC 7396 (JSON Merge Patch)
        • patch apply a JSON patch
        • merge_diff creates a diff as a JSON Merge Patch
        "},{"location":"api/basic_json/merge_patch/#version-history","title":"Version history","text":"
        • Added in version 3.0.0.
        • Fixed use of freed or relocated memory when apply_patch is *this or refers to a value contained in *this, in version 3.13.0 unreleased.
        "},{"location":"api/basic_json/meta/","title":"nlohmann::basic_json::meta","text":"
        static basic_json meta();\n

        This function returns a JSON object with information about the library, including the version number and information on the platform and compiler.

        "},{"location":"api/basic_json/meta/#return-value","title":"Return value","text":"

        JSON object holding version information

        key description compiler Information on the used compiler. It is an object with the following keys: c++ (the used C++ standard), family (the compiler family; possible values are clang, icc, gcc, hp, ilecpp, msvc, pgcpp, sunpro, and unknown), and version (the compiler version). copyright The copyright line for the library as string. name The name of the library as string. platform The used platform as string. Possible values are win32, linux, apple, unix, and unknown. url The URL of the project as string. version The version of the library. It is an object with the following keys: major, minor, and patch as defined by Semantic Versioning, and string (the version string)."},{"location":"api/basic_json/meta/#exception-safety","title":"Exception safety","text":"

        Strong guarantee: if an exception is thrown, there are no changes to any JSON value.

        "},{"location":"api/basic_json/meta/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json/meta/#examples","title":"Examples","text":"Example

        The following code shows an example output of the meta() function.

        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // call meta()\n    std::cout << std::setw(4) << json::meta() << '\\n';\n}\n

        Output:

        {\n    \"compiler\": {\n        \"c++\": \"201103\",\n        \"family\": \"gcc\",\n        \"version\": \"12.4.0\"\n    },\n    \"copyright\": \"(C) 2013-2026 Niels Lohmann\",\n    \"name\": \"JSON for Modern C++\",\n    \"platform\": \"apple\",\n    \"url\": \"https://github.com/nlohmann/json\",\n    \"version\": {\n        \"major\": 3,\n        \"minor\": 12,\n        \"patch\": 0,\n        \"string\": \"3.12.0\"\n    }\n}\n

        Note the output is platform-dependent.

        "},{"location":"api/basic_json/meta/#see-also","title":"See also","text":"
        • NLOHMANN_JSON_VERSION_MAJOR/NLOHMANN_JSON_VERSION_MINOR/NLOHMANN_JSON_VERSION_PATCH - library version information
        "},{"location":"api/basic_json/meta/#version-history","title":"Version history","text":"
        • Added in version 2.1.0.
        "},{"location":"api/basic_json/number_float_t/","title":"nlohmann::basic_json::number_float_t","text":"
        using number_float_t = NumberFloatType;\n

        The type used to store JSON numbers (floating-point).

        RFC 8259 describes numbers as follows:

        The representation of numbers is similar to that used in most programming languages. A number is represented in base 10 using decimal digits. It contains an integer component that may be prefixed with an optional minus sign, which may be followed by a fraction part and/or an exponent part. Leading zeros are not allowed. (...) Numeric values that cannot be represented in the grammar below (such as Infinity and NaN) are not permitted.

        This description includes both integer and floating-point numbers. However, C++ allows more precise storage if it is known whether the number is a signed integer, an unsigned integer, or a floating-point number. Therefore, three different types, number_integer_t, number_unsigned_t and number_float_t are used.

        To store floating-point numbers in C++, a type is defined by the template parameter NumberFloatType which chooses the type to use.

        "},{"location":"api/basic_json/number_float_t/#template-parameters","title":"Template parameters","text":"NumberFloatType the type to store floating-point numbers. The type must be float, double, or long double. The parser converts float, double, and a long double that is IEEE 754 binary64 itself. It converts other long double formats with std::from_chars where available, or with std::strtold otherwise. Serialization falls back to std::snprintf. The binary formats additionally require float or double, because they have no encoding for long double. See Template Parameter Requirements."},{"location":"api/basic_json/number_float_t/#notes","title":"Notes","text":""},{"location":"api/basic_json/number_float_t/#default-type","title":"Default type","text":"

        With the default values for NumberFloatType (double), the default type for number_float_t is double.

        "},{"location":"api/basic_json/number_float_t/#default-behavior","title":"Default behavior","text":"
        • The restrictions about leading zeros are not enforced in C++. Instead, leading zeros in floating-point literals will be ignored. Internally, the value will be stored as a decimal number. For instance, the C++ floating-point literal 01.2 will be serialized to 1.2. During deserialization, leading zeros yield an error.
        • Not-a-number (NaN) values will be serialized to null.
        "},{"location":"api/basic_json/number_float_t/#limits","title":"Limits","text":"

        RFC 8259 states:

        This specification allows implementations to set limits on the range and precision of numbers accepted. Since software that implements IEEE 754-2008 binary64 (double precision) numbers is generally available and widely used, good interoperability can be achieved by implementations that expect no more precision or range than these provide, in the sense that implementations will approximate JSON numbers within the expected precision.

        This implementation does exactly follow this approach, as it uses double precision floating-point numbers by default. Note that with the default double, numbers smaller than -1.79769313486232e+308 or greater than 1.79769313486232e+308 are rejected during parsing with out_of_range.406. Non-finite values (NaN and infinity) can only be stored by assigning them directly and are serialized to null.

        During deserialization (from JSON text or any of the binary formats), a finite number that does not fit into number_float_t is rejected with out_of_range.406, for example a double-precision number in a binary format when number_float_t is float.

        "},{"location":"api/basic_json/number_float_t/#storage","title":"Storage","text":"

        Floating-point number values are stored directly inside a basic_json type.

        "},{"location":"api/basic_json/number_float_t/#examples","title":"Examples","text":"Example

        The following code shows that number_float_t is by default, a typedef to double.

        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::boolalpha << std::is_same<double, json::number_float_t>::value << std::endl;\n}\n

        Output:

        true\n
        "},{"location":"api/basic_json/number_float_t/#see-also","title":"See also","text":"
        • number_integer_t the type used to store JSON integer numbers
        • number_unsigned_t the type used to store JSON unsigned integer numbers
        • is_number_float checks whether the JSON value is a floating-point number
        • Number Handling - the article on number handling
        "},{"location":"api/basic_json/number_float_t/#version-history","title":"Version history","text":"
        • Added in version 1.0.0.
        "},{"location":"api/basic_json/number_integer_t/","title":"nlohmann::basic_json::number_integer_t","text":"
        using number_integer_t = NumberIntegerType;\n

        The type used to store JSON numbers (integers).

        RFC 8259 describes numbers as follows:

        The representation of numbers is similar to that used in most programming languages. A number is represented in base 10 using decimal digits. It contains an integer component that may be prefixed with an optional minus sign, which may be followed by a fraction part and/or an exponent part. Leading zeros are not allowed. (...) Numeric values that cannot be represented in the grammar below (such as Infinity and NaN) are not permitted.

        This description includes both integer and floating-point numbers. However, C++ allows more precise storage if it is known whether the number is a signed integer, an unsigned integer, or a floating-point number. Therefore, three different types, number_integer_t, number_unsigned_t and number_float_t are used.

        To store integer numbers in C++, a type is defined by the template parameter NumberIntegerType which chooses the type to use.

        "},{"location":"api/basic_json/number_integer_t/#template-parameters","title":"Template parameters","text":"NumberIntegerType the type to store signed integers. It must be a signed integral type (std::is_integral) with a std::numeric_limits specialization, and it is stored directly inside a basic_json value. See Template Parameter Requirements."},{"location":"api/basic_json/number_integer_t/#notes","title":"Notes","text":""},{"location":"api/basic_json/number_integer_t/#default-type","title":"Default type","text":"

        With the default values for NumberIntegerType (std::int64_t), the default type for number_integer_t is std::int64_t.

        "},{"location":"api/basic_json/number_integer_t/#default-behavior","title":"Default behavior","text":"
        • The restrictions about leading zeros are not enforced in C++. Instead, leading zeros in integer literals lead to an interpretation as an octal number. Internally, the value will be stored as a decimal number. For instance, the C++ integer literal 010 will be serialized to 8. During deserialization, leading zeros yield an error.
        "},{"location":"api/basic_json/number_integer_t/#limits","title":"Limits","text":"

        RFC 8259 specifies:

        An implementation may set limits on the range and precision of numbers.

        When the default type is used, the maximal integer number that can be stored is 9223372036854775807 (INT64_MAX) and the minimal integer number that can be stored is -9223372036854775808 (INT64_MIN). Integer numbers that are out of range will yield over/underflow when used in a constructor. During deserialization (from JSON text or any of the binary formats), too large or small integer numbers will automatically be stored as number_unsigned_t or number_float_t.

        RFC 8259 further states:

        Note that when such software is used, numbers that are integers and are in the range [-253+1, 253-1] are interoperable in the sense that implementations will agree exactly on their numeric values.

        As this range is a subrange of the exactly supported range [INT64_MIN, INT64_MAX], this class's integer type is interoperable.

        "},{"location":"api/basic_json/number_integer_t/#storage","title":"Storage","text":"

        Integer number values are stored directly inside a basic_json type.

        "},{"location":"api/basic_json/number_integer_t/#examples","title":"Examples","text":"Example

        The following code shows that number_integer_t is by default, a typedef to std::int64_t.

        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::boolalpha << std::is_same<std::int64_t, json::number_integer_t>::value << std::endl;\n}\n

        Output:

        true\n
        "},{"location":"api/basic_json/number_integer_t/#see-also","title":"See also","text":"
        • number_unsigned_t the type used to store JSON unsigned integer numbers
        • number_float_t the type used to store JSON floating-point numbers
        • is_number_integer checks whether the JSON value is a signed integer number
        • Number Handling - the article on number handling
        "},{"location":"api/basic_json/number_integer_t/#version-history","title":"Version history","text":"
        • Added in version 1.0.0.
        "},{"location":"api/basic_json/number_unsigned_t/","title":"nlohmann::basic_json::number_unsigned_t","text":"
        using number_unsigned_t = NumberUnsignedType;\n

        The type used to store JSON numbers (unsigned).

        RFC 8259 describes numbers as follows:

        The representation of numbers is similar to that used in most programming languages. A number is represented in base 10 using decimal digits. It contains an integer component that may be prefixed with an optional minus sign, which may be followed by a fraction part and/or an exponent part. Leading zeros are not allowed. (...) Numeric values that cannot be represented in the grammar below (such as Infinity and NaN) are not permitted.

        This description includes both integer and floating-point numbers. However, C++ allows more precise storage if it is known whether the number is a signed integer, an unsigned integer, or a floating-point number. Therefore, three different types, number_integer_t, number_unsigned_t and number_float_t are used.

        To store unsigned integer numbers in C++, a type is defined by the template parameter NumberUnsignedType which chooses the type to use.

        "},{"location":"api/basic_json/number_unsigned_t/#template-parameters","title":"Template parameters","text":"NumberUnsignedType the type to store unsigned integers. It must be an unsigned integral type (std::is_integral) with a std::numeric_limits specialization, and it must be able to represent the absolute value of every number_integer_t value. Alternatively, it can be the same type as number_integer_t; then every integer is stored as number_integer_t. See Template Parameter Requirements."},{"location":"api/basic_json/number_unsigned_t/#notes","title":"Notes","text":""},{"location":"api/basic_json/number_unsigned_t/#default-type","title":"Default type","text":"

        With the default values for NumberUnsignedType (std::uint64_t), the default type for number_unsigned_t is std::uint64_t.

        "},{"location":"api/basic_json/number_unsigned_t/#default-behavior","title":"Default behavior","text":"
        • The restrictions about leading zeros are not enforced in C++. Instead, leading zeros in integer literals lead to an interpretation as an octal number. Internally, the value will be stored as a decimal number. For instance, the C++ integer literal 010 will be serialized to 8. During deserialization, leading zeros yield an error.
        "},{"location":"api/basic_json/number_unsigned_t/#limits","title":"Limits","text":"

        RFC 8259 specifies:

        An implementation may set limits on the range and precision of numbers.

        When the default type is used, the maximal integer number that can be stored is 18446744073709551615 (UINT64_MAX) and the minimal integer number that can be stored is 0. Integer numbers that are out of range will yield over/underflow when used in a constructor. During deserialization (from JSON text or any of the binary formats), too large or small integer numbers will automatically be stored as number_integer_t or number_float_t.

        RFC 8259 further states:

        Note that when such software is used, numbers that are integers and are in the range [-253+1, 253-1] are interoperable in the sense that implementations will agree exactly on their numeric values.

        As this range is a subrange (when considered in conjunction with the number_integer_t type) of the exactly supported range [0, UINT64_MAX], this class's integer type is interoperable.

        "},{"location":"api/basic_json/number_unsigned_t/#storage","title":"Storage","text":"

        Integer number values are stored directly inside a basic_json type.

        "},{"location":"api/basic_json/number_unsigned_t/#examples","title":"Examples","text":"Example

        The following code shows that number_unsigned_t is by default, a typedef to std::uint64_t.

        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::boolalpha << std::is_same<std::uint64_t, json::number_unsigned_t>::value << std::endl;\n}\n

        Output:

        true\n
        "},{"location":"api/basic_json/number_unsigned_t/#see-also","title":"See also","text":"
        • number_integer_t the type used to store JSON integer numbers
        • number_float_t the type used to store JSON floating-point numbers
        • is_number_unsigned checks whether the JSON value is an unsigned integer number
        • Number Handling - the article on number handling
        "},{"location":"api/basic_json/number_unsigned_t/#version-history","title":"Version history","text":"
        • Added in version 2.0.0.
        • Can be the same type as number_integer_t since version 3.13.0 unreleased.
        "},{"location":"api/basic_json/object/","title":"nlohmann::basic_json::object","text":"
        static basic_json object(initializer_list_t init = {});\n

        Creates a JSON object value from a given initializer list. The initializer lists elements must be pairs, and their first elements must be strings. If the initializer list is empty, the empty object {} is created.

        "},{"location":"api/basic_json/object/#parameters","title":"Parameters","text":"init (in) initializer list with JSON values to create an object from (optional)"},{"location":"api/basic_json/object/#return-value","title":"Return value","text":"

        JSON object value

        "},{"location":"api/basic_json/object/#exception-safety","title":"Exception safety","text":"

        Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

        "},{"location":"api/basic_json/object/#exceptions","title":"Exceptions","text":"

        Throws type_error.301 if init is not a list of pairs whose first elements are strings. In this case, no object can be created. When such a value is passed to basic_json(initializer_list_t, bool, value_t), an array would have been created from the passed initializer list init. See the example below.

        "},{"location":"api/basic_json/object/#complexity","title":"Complexity","text":"

        Linear in the size of init.

        "},{"location":"api/basic_json/object/#notes","title":"Notes","text":"

        This function is only added for symmetry reasons. In contrast to the related function array(initializer_list_t), there are no cases that can only be expressed by this function. That is, any initializer list init can also be passed to the initializer list constructor basic_json(initializer_list_t, bool, value_t).

        "},{"location":"api/basic_json/object/#examples","title":"Examples","text":"Example

        The following code shows an example for the object function.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON objects\n    json j_no_init_list = json::object();\n    json j_empty_init_list = json::object({});\n    json j_list_of_pairs = json::object({ {\"one\", 1}, {\"two\", 2} });\n\n    // serialize the JSON objects\n    std::cout << j_no_init_list << '\\n';\n    std::cout << j_empty_init_list << '\\n';\n    std::cout << j_list_of_pairs << '\\n';\n\n    // example for an exception\n    try\n    {\n        // can only create an object from a list of pairs\n        json j_invalid_object = json::object({{ \"one\", 1, 2 }});\n    }\n    catch (const json::type_error& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

        Output:

        {}\n{}\n{\"one\":1,\"two\":2}\n[json.exception.type_error.301] cannot create object from initializer list\n
        "},{"location":"api/basic_json/object/#see-also","title":"See also","text":"
        • basic_json(initializer_list_t) - create a JSON value from an initializer list
        • array - create a JSON array value from an initializer list
        • Creating JSON values - the article on creating JSON values
        "},{"location":"api/basic_json/object/#version-history","title":"Version history","text":"
        • Added in version 1.0.0.
        "},{"location":"api/basic_json/object_comparator_t/","title":"nlohmann::basic_json::object_comparator_t","text":"
        using object_comparator_t = typename object_t::key_compare;\n// or\nusing object_comparator_t = default_object_comparator_t;\n

        The comparator used by object_t. Defined as typename object_t::key_compare if available, and default_object_comparator_t otherwise.

        "},{"location":"api/basic_json/object_comparator_t/#examples","title":"Examples","text":"Example

        The example below demonstrates the used object comparator.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::boolalpha\n              << \"json::object_comparator_t(\\\"one\\\", \\\"two\\\") = \" << json::object_comparator_t{}(\"one\", \"two\") << \"\\n\"\n              << \"json::object_comparator_t(\\\"three\\\", \\\"four\\\") = \" << json::object_comparator_t{}(\"three\", \"four\") << std::endl;\n}\n

        Output:

        json::object_comparator_t(\"one\", \"two\") = true\njson::object_comparator_t(\"three\", \"four\") = false\n
        "},{"location":"api/basic_json/object_comparator_t/#see-also","title":"See also","text":"
        • object_t the type used to store JSON objects
        • default_object_comparator_t the fallback comparator used when object_t has no key_compare member type
        "},{"location":"api/basic_json/object_comparator_t/#version-history","title":"Version history","text":"
        • Added in version 3.0.0.
        • Changed to be conditionally defined as typename object_t::key_compare or default_object_comparator_t in version 3.11.0.
        • Fixed the fallback to default_object_comparator_t, which previously failed to compile for object types without a key_compare member type, in version 3.13.0 unreleased.
        "},{"location":"api/basic_json/object_t/","title":"nlohmann::basic_json::object_t","text":"
        using object_t = ObjectType<StringType,\n                            basic_json,\n                            default_object_comparator_t,\n                            AllocatorType<std::pair<const StringType, basic_json>>>;\n

        The type used to store JSON objects.

        RFC 8259 describes JSON objects as follows:

        An object is an unordered collection of zero or more name/value pairs, where a name is a string and a value is a string, number, boolean, null, object, or array.

        To store objects in C++, a type is defined by the template parameters described below.

        "},{"location":"api/basic_json/object_t/#template-parameters","title":"Template parameters","text":"ObjectType the container to store objects. Its template parameters must have the same order and meaning as those of std::map; in particular, the third parameter is a comparator. std::unordered_map, whose third parameter is a hash function, therefore needs an adapter -- see Template Parameter Requirements for the full list of requirements, an adapter example, and the containers that are known to work. StringType the type of the keys or names (e.g., std::string). The comparison function std::less<StringType> is used to order elements inside the container. AllocatorType the allocator to use for objects (e.g., std::allocator)"},{"location":"api/basic_json/object_t/#notes","title":"Notes","text":""},{"location":"api/basic_json/object_t/#default-type","title":"Default type","text":"

        With the default values for ObjectType (std::map), StringType (std::string), and AllocatorType (std::allocator), the default type for object_t is:

        // until C++14\nstd::map<\n  std::string, // key_type\n  basic_json, // value_type\n  std::less<std::string>, // key_compare\n  std::allocator<std::pair<const std::string, basic_json>> // allocator_type\n>\n\n// since C++14\nstd::map<\n  std::string, // key_type\n  basic_json, // value_type\n  std::less<>, // key_compare\n  std::allocator<std::pair<const std::string, basic_json>> // allocator_type\n>\n

        See default_object_comparator_t for more information.

        "},{"location":"api/basic_json/object_t/#behavior","title":"Behavior","text":"

        The choice of object_t influences the behavior of the JSON class. With the default type, objects have the following behavior:

        • When all names are unique, objects will be interoperable in the sense that all software implementations receiving that object will agree on the name-value mappings.
        • When the names within an object are not unique, it is unspecified which one of the values for a given key will be chosen. For instance, {\"key\": 2, \"key\": 1} could be equal to either {\"key\": 1} or {\"key\": 2}. To reject duplicate keys instead of silently resolving them one way or another, see this parsing recipe.
        • Internally, name/value pairs are stored in lexicographical order of the names. Objects will also be serialized (see dump) in this order. For instance, {\"b\": 1, \"a\": 2} and {\"a\": 2, \"b\": 1} will be stored and serialized as {\"a\": 2, \"b\": 1}.
        • When comparing objects, the order of the name/value pairs is irrelevant. This makes objects interoperable in the sense that they will not be affected by these differences. For instance, {\"b\": 1, \"a\": 2} and {\"a\": 2, \"b\": 1} will be treated as equal.
        "},{"location":"api/basic_json/object_t/#limits","title":"Limits","text":"

        RFC 8259 specifies:

        An implementation may set limits on the maximum depth of nesting.

        In this class, the object's limit of nesting is not explicitly constrained. However, a maximum depth of nesting may be introduced by the compiler or runtime environment. A theoretical limit can be queried by calling the max_size function of a JSON object.

        "},{"location":"api/basic_json/object_t/#storage","title":"Storage","text":"

        Objects are stored as pointers in a basic_json type. That is, for any access to object values, a pointer of type object_t* must be dereferenced.

        "},{"location":"api/basic_json/object_t/#object-key-order","title":"Object key order","text":"

        The order name/value pairs are added to the object are not preserved by the library. Therefore, iterating an object may return name/value pairs in a different order than they were originally stored. In fact, keys will be traversed in alphabetical order as std::map with std::less is used by default. Please note this behavior conforms to RFC 8259, because any order implements the specified \"unordered\" nature of JSON objects.

        "},{"location":"api/basic_json/object_t/#cross-basic_json-conversion-requirements","title":"Cross-basic_json conversion requirements","text":"

        When converting an object from one basic_json specialization to another via the converting constructor (overload 4), the target object_t's key_type must be directly constructible from the source basic_json's string_t type (or more generally, from the source object's key type). If this requirement is not met, the conversion does not fail; instead, the object is silently converted as an array of key-value pairs, which is incorrect. See issue #3425 for details and an example.

        "},{"location":"api/basic_json/object_t/#examples","title":"Examples","text":"Example

        The following code shows that object_t is by default, a typedef to std::map<json::string_t, json>.

        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::boolalpha << std::is_same<std::map<json::string_t, json>, json::object_t>::value << std::endl;\n}\n

        Output:

        true\n
        "},{"location":"api/basic_json/object_t/#see-also","title":"See also","text":"
        • array_t the type used to store JSON arrays
        • string_t the type used to store JSON strings
        • object_comparator_t the comparator used to order object keys
        • Object Order - the article on object key ordering
        "},{"location":"api/basic_json/object_t/#version-history","title":"Version history","text":"
        • Added in version 1.0.0.
        • Allowed object types whose erase(iterator) returns void in version 3.13.0 unreleased.
        "},{"location":"api/basic_json/operator%2B%3D/","title":"nlohmann::basic_json::operator+=","text":"
        // (1)\nreference operator+=(basic_json&& val);\nreference operator+=(const basic_json& val);\n\n// (2)\nreference operator+=(const typename object_t::value_type& val);\n\n// (3)\nreference operator+=(initializer_list_t init);\n

        1. Appends the given element val to the end of the JSON array. If the function is called on a JSON null value, an empty array is created before appending val.

        2. Inserts the given element val to the JSON object. If the function is called on a JSON null value, an empty object is created before inserting val.

        3. This function allows using operator+= with an initializer list. In case

          1. the current value is an object,
          2. the initializer list init contains only two elements, and
          3. the first element of init is a string,

          init is converted into an object element and added using operator+=(const typename object_t::value_type&). Otherwise, init is converted to a JSON value and added using operator+=(basic_json&&).

        "},{"location":"api/basic_json/operator%2B%3D/#iterator-invalidation","title":"Iterator invalidation","text":"

        For all cases where an element is added to an array, a reallocation can happen, in which case all iterators (including the end() iterator) and all references to the elements are invalidated. Otherwise, only the end() iterator is invalidated.

        For ordered_json, also adding an element to an object can yield a reallocation which again invalidates all iterators and all references.

        "},{"location":"api/basic_json/operator%2B%3D/#parameters","title":"Parameters","text":"val (in) the value to add to the JSON array/object init (in) an initializer list"},{"location":"api/basic_json/operator%2B%3D/#return-value","title":"Return value","text":"

        *this

        "},{"location":"api/basic_json/operator%2B%3D/#exception-safety","title":"Exception safety","text":"

        Strong guarantee: if an exception is thrown, there are no changes to any JSON value. As an exception, a null value is converted to an empty array or object before the element is added and keeps that type if adding the element throws.

        "},{"location":"api/basic_json/operator%2B%3D/#exceptions","title":"Exceptions","text":"
        1. Throws type_error.308 when called on a type other than JSON array or null; example: \"cannot use push_back() with number\"
        2. Throws type_error.308 when called on a type other than JSON object or null; example: \"cannot use push_back() with number\"
        3. Throws type_error.308 when called on a type other than JSON array or null; example: \"cannot use push_back() with number\"
        "},{"location":"api/basic_json/operator%2B%3D/#complexity","title":"Complexity","text":"
        1. Amortized constant.
        2. Logarithmic in the size of the container, O(log(size())).
        3. Linear in the size of the initializer list init.
        "},{"location":"api/basic_json/operator%2B%3D/#notes","title":"Notes","text":"

        (3) This function is required to resolve an ambiguous overload error, because pairs like {\"key\", \"value\"} can be both interpreted as object_t::value_type or std::initializer_list<basic_json>, see #235 for more information.

        "},{"location":"api/basic_json/operator%2B%3D/#examples","title":"Examples","text":"Example: (1) add element to array

        The example shows how push_back() and += can be used to add elements to a JSON array. Note how the null value was silently converted to a JSON array.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json array = {1, 2, 3, 4, 5};\n    json null;\n\n    // print values\n    std::cout << array << '\\n';\n    std::cout << null << '\\n';\n\n    // add values\n    array.push_back(6);\n    array += 7;\n    null += \"first\";\n    null += \"second\";\n\n    // print values\n    std::cout << array << '\\n';\n    std::cout << null << '\\n';\n}\n

        Output:

        [1,2,3,4,5]\nnull\n[1,2,3,4,5,6,7]\n[\"first\",\"second\"]\n
        Example: (2) add element to object

        The example shows how push_back() and += can be used to add elements to a JSON object. Note how the null value was silently converted to a JSON object.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json object = {{\"one\", 1}, {\"two\", 2}};\n    json null;\n\n    // print values\n    std::cout << object << '\\n';\n    std::cout << null << '\\n';\n\n    // add values\n    object.push_back(json::object_t::value_type(\"three\", 3));\n    object += json::object_t::value_type(\"four\", 4);\n    null += json::object_t::value_type(\"A\", \"a\");\n    null += json::object_t::value_type(\"B\", \"b\");\n\n    // print values\n    std::cout << object << '\\n';\n    std::cout << null << '\\n';\n}\n

        Output:

        {\"one\":1,\"two\":2}\nnull\n{\"four\":4,\"one\":1,\"three\":3,\"two\":2}\n{\"A\":\"a\",\"B\":\"b\"}\n
        Example: (3) add to object from initializer list

        The example shows how initializer lists are treated as objects when possible.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json object = {{\"one\", 1}, {\"two\", 2}};\n    json null;\n\n    // print values\n    std::cout << object << '\\n';\n    std::cout << null << '\\n';\n\n    // add values:\n    object.push_back({\"three\", 3});  // object is extended\n    object += {\"four\", 4};           // object is extended\n    null.push_back({\"five\", 5});     // null is converted to array\n\n    // print values\n    std::cout << object << '\\n';\n    std::cout << null << '\\n';\n\n    // would throw:\n    //object.push_back({1, 2, 3});\n}\n

        Output:

        {\"one\":1,\"two\":2}\nnull\n{\"four\":4,\"one\":1,\"three\":3,\"two\":2}\n[[\"five\",5]]\n
        "},{"location":"api/basic_json/operator%2B%3D/#see-also","title":"See also","text":"
        • emplace_back add a value to an array
        • push_back add a value to an array/object
        "},{"location":"api/basic_json/operator%2B%3D/#version-history","title":"Version history","text":"
        1. Since version 1.0.0.
        2. Since version 1.0.0.
        3. Since version 2.0.0.
        "},{"location":"api/basic_json/operator%3D/","title":"nlohmann::basic_json::operator=","text":"
        basic_json& operator=(basic_json other) noexcept (\n    std::is_nothrow_move_constructible<value_t>::value &&\n    std::is_nothrow_move_assignable<value_t>::value &&\n    std::is_nothrow_move_constructible<json_value>::value &&\n    std::is_nothrow_move_assignable<json_value>::value &&\n    std::is_nothrow_move_assignable<json_base_class_t>::value\n);\n

        Copy assignment operator. Copies a JSON value via the \"copy and swap\" strategy: It is expressed in terms of the copy constructor, destructor, and the swap() member function.

        "},{"location":"api/basic_json/operator%3D/#parameters","title":"Parameters","text":"other (in) value to copy from"},{"location":"api/basic_json/operator%3D/#exception-safety","title":"Exception safety","text":"

        Strong guarantee: if an exception is thrown while copying other, there are no changes to *this.

        "},{"location":"api/basic_json/operator%3D/#complexity","title":"Complexity","text":"

        Linear.

        "},{"location":"api/basic_json/operator%3D/#examples","title":"Examples","text":"Example

        The code below shows and example for the copy assignment. It creates a copy of value a which is then swapped with b. Finally, the copy of a (which is the null value after the swap) is destroyed.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json a = 23;\n    json b = 42;\n\n    // copy-assign a to b\n    b = a;\n\n    // serialize the JSON arrays\n    std::cout << a << '\\n';\n    std::cout << b << '\\n';\n}\n

        Output:

        23\n23\n
        "},{"location":"api/basic_json/operator%3D/#see-also","title":"See also","text":"
        • basic_json create a JSON value
        • swap exchanges the contents of two JSON values
        "},{"location":"api/basic_json/operator%3D/#version-history","title":"Version history","text":"
        • Added in version 1.0.0.
        "},{"location":"api/basic_json/operator%5B%5D/","title":"nlohmann::basic_json::operator[]","text":"
        // (1)\nreference operator[](size_type idx);\nconst_reference operator[](size_type idx) const;\n\n// (2)\nreference operator[](typename object_t::key_type key);\nconst_reference operator[](const typename object_t::key_type& key) const;\n\n// (3)\ntemplate<typename KeyType>\nreference operator[](KeyType&& key);\ntemplate<typename KeyType>\nconst_reference operator[](KeyType&& key) const;\n\n// (4)\nreference operator[](const json_pointer& ptr);\nconst_reference operator[](const json_pointer& ptr) const;\n
        1. Returns a reference to the array element at specified location idx.
        2. Returns a reference to the object element with specified key key. The non-const qualified overload takes the key by value.
        3. See 2. This overload is only available if KeyType is comparable with typename object_t::key_type and typename object_comparator_t::is_transparent denotes a type.
        4. Returns a reference to the element with specified JSON pointer ptr.
        "},{"location":"api/basic_json/operator%5B%5D/#template-parameters","title":"Template parameters","text":"KeyType A type for an object key other than json_pointer that is comparable with string_t using object_comparator_t. This can also be a string view (C++17)."},{"location":"api/basic_json/operator%5B%5D/#iterator-invalidation","title":"Iterator invalidation","text":"

        For the non-const versions 1. and 4., when passing an array index that does not exist, it is created and filled with a null value before a reference to it is returned. For this, a reallocation can happen, in which case all iterators (including the end() iterator) and all references to the elements are invalidated.

        For ordered_json, also passing an object key to the non-const versions 2., 3., and 4., a reallocation can happen which again invalidates all iterators and all references.

        "},{"location":"api/basic_json/operator%5B%5D/#parameters","title":"Parameters","text":"idx (in) index of the element to access key (in) object key of the element to access ptr (in) JSON pointer to the desired element"},{"location":"api/basic_json/operator%5B%5D/#return-value","title":"Return value","text":"
        1. (const) reference to the element at index idx
        2. (const) reference to the element at key key
        3. (const) reference to the element at key key
        4. (const) reference to the element pointed to by ptr
        "},{"location":"api/basic_json/operator%5B%5D/#exception-safety","title":"Exception safety","text":"

        Strong exception safety: if an exception occurs, the original value stays intact.

        "},{"location":"api/basic_json/operator%5B%5D/#exceptions","title":"Exceptions","text":"
        1. The function can throw the following exceptions:
          • Throws type_error.305 if the JSON value is not an array or null; in that case, using the [] operator with an index makes no sense.
          • Throws std::length_error if idx equals the maximum value of size_type; the array is left unchanged. (This is the one index for which growing the array to hold it cannot be expressed as a size_type size, the same way an oversized resize throws.)
        2. The function can throw the following exceptions:
          • Throws type_error.305 if the JSON value is not an object or null; in that case, using the [] operator with a key makes no sense.
        3. See 2.

        4. The function can throw the following exceptions:

          • Throws parse_error.106 if an array index in the passed JSON pointer ptr begins with '0'.
          • Throws parse_error.109 if an array index in the passed JSON pointer ptr is not a number.
          • Throws out_of_range.402 if the array index '-' is used in the passed JSON pointer ptr for the const version.
          • Throws out_of_range.404 if the JSON pointer ptr can not be resolved.
          • Throws out_of_range.410 if an array index in the passed JSON pointer ptr exceeds the range of size_type (e.g., on 32-bit platforms).

          For the const version, an object key or array index in ptr that does not exist is not reported by an exception, but is undefined behavior (see the notes below). Use at for checked access.

        "},{"location":"api/basic_json/operator%5B%5D/#complexity","title":"Complexity","text":"
        1. Constant if idx is in the range of the array. Otherwise, linear in idx - size().
        2. Logarithmic in the size of the container.
        3. Logarithmic in the size of the container.
        4. Logarithmic in the size of the container.
        "},{"location":"api/basic_json/operator%5B%5D/#notes","title":"Notes","text":"

        Undefined behavior and runtime assertions

        The following cases apply to the const overloads; the non-const overloads instead insert the missing element (see the notes below).

        1. If the element at index idx does not exist, the behavior is undefined and is guarded by a runtime assertion!
        2. If the element with key key does not exist, the behavior is undefined and is guarded by a runtime assertion!
        3. If the JSON pointer ptr refers to an object key or an array index that does not exist, the behavior is undefined and is guarded by a runtime assertion!

        1. The non-const version may add values: If idx is beyond the range of the array (i.e., idx >= size()), then the array is silently filled up with null values to make idx a valid reference to the last stored element. In case the value was null before, it is converted to an array.

        2. If key is not found in the object, then it is silently added to the object and filled with a null value to make key a valid reference. In case the value was null before, it is converted to an object.

        3. See 2.

        4. null values are created in arrays and objects if necessary.

          In particular:

          • If the JSON pointer points to an object key that does not exist, it is created and filled with a null value before a reference to it is returned.
          • If the JSON pointer points to an array index that does not exist, it is created and filled with a null value before a reference to it is returned. All indices between the current maximum and the given index are also filled with null.
          • The special value - is treated as a synonym for the index past the end.

          Creating intermediate levels that don't exist yet

          When the JSON pointer traverses intermediate levels that don't exist at all yet (not just a missing leaf), each missing level is created as an array or an object depending on whether the corresponding pointer token parses as a non-negative integer: a numeric token creates an array, a non-numeric token creates an object. For example, on an initially null value, /foo/0/0/0 creates nested arrays, while /foo/one/one/one creates nested objects. This is not specified by the JSON Pointer RFC; it is this library's own, intentional disambiguation rule. See also JSON Pointer.

        Deprecation

        Overload (4) also accepts a json_pointer whose template argument is a basic_json specialization (e.g., nlohmann::json_pointer<nlohmann::json>) instead of a string type. This is deprecated since version 3.11.0 and will be removed in a future major version; use basic_json::json_pointer (for json, nlohmann::json_pointer<std::string>) instead.

        You should be warned by your compiler with a -Wdeprecated-declarations warning if you are using a deprecated function.

        See the migration guide for how to update existing code.

        "},{"location":"api/basic_json/operator%5B%5D/#examples","title":"Examples","text":"Example: (1) access specified array element

        The example below shows how array elements can be read and written using [] operator. Note the addition of null values.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON array\n    json array = {1, 2, 3, 4, 5};\n\n    // output element at index 3 (fourth element)\n    std::cout << array[3] << '\\n';\n\n    // change last element to 6\n    array[array.size() - 1] = 6;\n\n    // output changed array\n    std::cout << array << '\\n';\n\n    // write beyond array limit\n    array[10] = 11;\n\n    // output changed array\n    std::cout << array << '\\n';\n}\n

        Output:

        4\n[1,2,3,4,6]\n[1,2,3,4,6,null,null,null,null,null,11]\n
        Example: (1) access specified array element (const)

        The example below shows how array elements can be read using the [] operator.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON array\n    const json array = {\"first\", \"2nd\", \"third\", \"fourth\"};\n\n    // output element at index 2 (third element)\n    std::cout << array.at(2) << '\\n';\n}\n

        Output:

        \"third\"\n
        Example: (2) access specified object element

        The example below shows how object elements can be read and written using the [] operator.

        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON object\n    json object =\n    {\n        {\"one\", 1}, {\"two\", 2}, {\"three\", 2.9}\n    };\n\n    // output element with key \"two\"\n    std::cout << object[\"two\"] << \"\\n\\n\";\n\n    // change element with key \"three\"\n    object[\"three\"] = 3;\n\n    // output changed array\n    std::cout << std::setw(4) << object << \"\\n\\n\";\n\n    // mention nonexisting key\n    object[\"four\"];\n\n    // write to nonexisting key\n    object[\"five\"][\"really\"][\"nested\"] = true;\n\n    // output changed object\n    std::cout << std::setw(4) << object << '\\n';\n}\n

        Output:

        2\n\n{\n    \"one\": 1,\n    \"three\": 3,\n    \"two\": 2\n}\n\n{\n    \"five\": {\n        \"really\": {\n            \"nested\": true\n        }\n    },\n    \"four\": null,\n    \"one\": 1,\n    \"three\": 3,\n    \"two\": 2\n}\n
        Example: (2) access specified object element (const)

        The example below shows how object elements can be read using the [] operator.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON object\n    const json object =\n    {\n        {\"one\", 1}, {\"two\", 2}, {\"three\", 2.9}\n    };\n\n    // output element with key \"two\"\n    std::cout << object[\"two\"] << '\\n';\n}\n

        Output:

        2\n
        Example: (3) access specified object element using string_view

        The example below shows how object elements can be read using the [] operator.

        #include <iostream>\n#include <iomanip>\n#include <string_view>\n#include <nlohmann/json.hpp>\n\nusing namespace std::string_view_literals;\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON object\n    json object =\n    {\n        {\"one\", 1}, {\"two\", 2}, {\"three\", 2.9}\n    };\n\n    // output element with key \"two\"\n    std::cout << object[\"two\"sv] << \"\\n\\n\";\n\n    // change element with key \"three\"\n    object[\"three\"sv] = 3;\n\n    // output changed array\n    std::cout << std::setw(4) << object << \"\\n\\n\";\n\n    // mention nonexisting key\n    object[\"four\"sv];\n\n    // write to nonexisting key\n    object[\"five\"sv][\"really\"sv][\"nested\"sv] = true;\n\n    // output changed object\n    std::cout << std::setw(4) << object << '\\n';\n}\n

        Output:

        2\n\n{\n    \"one\": 1,\n    \"three\": 3,\n    \"two\": 2\n}\n\n{\n    \"five\": {\n        \"really\": {\n            \"nested\": true\n        }\n    },\n    \"four\": null,\n    \"one\": 1,\n    \"three\": 3,\n    \"two\": 2\n}\n
        Example: (3) access specified object element using string_view (const)

        The example below shows how object elements can be read using the [] operator.

        #include <iostream>\n#include <string_view>\n#include <nlohmann/json.hpp>\n\nusing namespace std::string_view_literals;\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON object\n    const json object =\n    {\n        {\"one\", 1}, {\"two\", 2}, {\"three\", 2.9}\n    };\n\n    // output element with key \"two\"\n    std::cout << object[\"two\"sv] << '\\n';\n}\n

        Output:

        2\n
        Example: (4) access specified element via JSON Pointer

        The example below shows how values can be read and written using JSON Pointers.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // create a JSON value\n    json j =\n    {\n        {\"number\", 1}, {\"string\", \"foo\"}, {\"array\", {1, 2}}\n    };\n\n    // read-only access\n\n    // output element with JSON pointer \"/number\"\n    std::cout << j[\"/number\"_json_pointer] << '\\n';\n    // output element with JSON pointer \"/string\"\n    std::cout << j[\"/string\"_json_pointer] << '\\n';\n    // output element with JSON pointer \"/array\"\n    std::cout << j[\"/array\"_json_pointer] << '\\n';\n    // output element with JSON pointer \"/array/1\"\n    std::cout << j[\"/array/1\"_json_pointer] << '\\n';\n\n    // writing access\n\n    // change the string\n    j[\"/string\"_json_pointer] = \"bar\";\n    // output the changed string\n    std::cout << j[\"string\"] << '\\n';\n\n    // \"change\" a nonexisting object entry\n    j[\"/boolean\"_json_pointer] = true;\n    // output the changed object\n    std::cout << j << '\\n';\n\n    // change an array element\n    j[\"/array/1\"_json_pointer] = 21;\n    // \"change\" an array element with nonexisting index\n    j[\"/array/4\"_json_pointer] = 44;\n    // output the changed array\n    std::cout << j[\"array\"] << '\\n';\n\n    // \"change\" the array element past the end\n    j[\"/array/-\"_json_pointer] = 55;\n    // output the changed array\n    std::cout << j[\"array\"] << '\\n';\n}\n

        Output:

        1\n\"foo\"\n[1,2]\n2\n\"bar\"\n{\"array\":[1,2],\"boolean\":true,\"number\":1,\"string\":\"bar\"}\n[1,21,null,null,44]\n[1,21,null,null,44,55]\n
        Example: (4) access specified element via JSON Pointer (const)

        The example below shows how values can be read using JSON Pointers.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // create a JSON value\n    const json j =\n    {\n        {\"number\", 1}, {\"string\", \"foo\"}, {\"array\", {1, 2}}\n    };\n\n    // read-only access\n\n    // output element with JSON pointer \"/number\"\n    std::cout << j[\"/number\"_json_pointer] << '\\n';\n    // output element with JSON pointer \"/string\"\n    std::cout << j[\"/string\"_json_pointer] << '\\n';\n    // output element with JSON pointer \"/array\"\n    std::cout << j[\"/array\"_json_pointer] << '\\n';\n    // output element with JSON pointer \"/array/1\"\n    std::cout << j[\"/array/1\"_json_pointer] << '\\n';\n}\n

        Output:

        1\n\"foo\"\n[1,2]\n2\n
        "},{"location":"api/basic_json/operator%5B%5D/#see-also","title":"See also","text":"
        • documentation on unchecked access
        • documentation on runtime assertions
        • see at for access by reference with range checking
        • see value for access with default value
        • basic_json_view::operator[] - the same access on a zero-copy view (a duplicate key resolves to the first member, whereas parse() keeps the last; a missing key gives a discarded view instead of undefined behavior)
        "},{"location":"api/basic_json/operator%5B%5D/#version-history","title":"Version history","text":"
        1. Added in version 1.0.0. Fixed in version 3.13.0 unreleased to throw std::length_error instead of emptying the array and accessing it out of bounds when idx equals the maximum value of size_type. A missing index in the const version is guarded by a runtime assertion since version 3.13.0 unreleased.
        2. Added in version 1.0.0. Added overloads for T* key in version 1.1.0. Removed overloads for T* key (replaced by 3) in version 3.11.0.
        3. Added in version 3.11.0. Fixed in version 3.13.0 unreleased to consistently accept std::string_view-convertible keys, as already supported by at, value, find, and other lookup functions.
        4. Added in version 2.0.0. A missing array index in the const version is guarded by a runtime assertion since version 3.13.0 unreleased. Throws parse_error.109 instead of out_of_range.404 for a one-character array index that is not a digit (e.g., /x) in version 3.13.0 unreleased, as it already did for longer ones.
        "},{"location":"api/basic_json/operator_ValueType/","title":"nlohmann::basic_json::operator ValueType","text":"
        template<typename ValueType>\nJSON_EXPLICIT operator ValueType() const;\n

        Implicit type conversion between the JSON value and a compatible value. The call is realized by calling get(). See Notes for the meaning of JSON_EXPLICIT.

        "},{"location":"api/basic_json/operator_ValueType/#template-parameters","title":"Template parameters","text":"ValueType the value type to return"},{"location":"api/basic_json/operator_ValueType/#return-value","title":"Return value","text":"

        copy of the JSON value, converted to ValueType

        "},{"location":"api/basic_json/operator_ValueType/#exception-safety","title":"Exception safety","text":"

        Depends on what json_serializer<ValueType> from_json() method throws; the JSON value itself is never modified, since operator ValueType() is a const member function that only calls get().

        "},{"location":"api/basic_json/operator_ValueType/#exceptions","title":"Exceptions","text":"

        Depends on what json_serializer<ValueType> from_json() method throws

        "},{"location":"api/basic_json/operator_ValueType/#complexity","title":"Complexity","text":"

        Linear in the size of the JSON value.

        "},{"location":"api/basic_json/operator_ValueType/#notes","title":"Notes","text":"

        Definition of JSON_EXPLICIT

        By default JSON_EXPLICIT is defined to the empty string, so the signature is:

        template<typename ValueType>\noperator ValueType() const;\n

        If JSON_USE_IMPLICIT_CONVERSIONS is set to 0, JSON_EXPLICIT is defined to explicit:

        template<typename ValueType>\nexplicit operator ValueType() const;\n

        That is, implicit conversions can be switched off by defining JSON_USE_IMPLICIT_CONVERSIONS to 0.

        Future behavior change

        Implicit conversions will be switched off by default in the next major release of the library. That is, JSON_EXPLICIT will be set to explicit by default.

        You can prepare existing code by already defining JSON_USE_IMPLICIT_CONVERSIONS to 0 and replace any implicit conversions with calls to get.

        See the migration guide for how to update existing code.

        "},{"location":"api/basic_json/operator_ValueType/#examples","title":"Examples","text":"Example

        The example below shows several conversions from JSON values to other types. There are a few things to note: (1) Floating-point numbers can be converted to integers, (2) A JSON array can be converted to a standard std::vector<short>, (3) A JSON object can be converted to C++ associative containers such as std::map<std::string, json>.

        #include <iostream>\n#include <map>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON value with different types\n    json json_types =\n    {\n        {\"boolean\", true},\n        {\n            \"number\", {\n                {\"integer\", 42},\n                {\"floating-point\", 17.23}\n            }\n        },\n        {\"string\", \"Hello, world!\"},\n        {\"array\", {1, 2, 3, 4, 5}},\n        {\"null\", nullptr}\n    };\n\n    // use implicit conversions\n    bool v1 = json_types[\"boolean\"];\n    int v2 = json_types[\"number\"][\"integer\"];\n    short v3 = json_types[\"number\"][\"integer\"];\n    float v4 = json_types[\"number\"][\"floating-point\"];\n    int v5 = json_types[\"number\"][\"floating-point\"];\n    std::string v6 = json_types[\"string\"];\n    std::vector<short> v7 = json_types[\"array\"];\n    std::map<std::string, json> v8 = json_types;\n\n    // print the conversion results\n    std::cout << v1 << '\\n';\n    std::cout << v2 << ' ' << v3 << '\\n';\n    std::cout << v4 << ' ' << v5 << '\\n';\n    std::cout << v6 << '\\n';\n\n    for (auto i : v7)\n    {\n        std::cout << i << ' ';\n    }\n    std::cout << \"\\n\\n\";\n\n    for (auto i : v8)\n    {\n        std::cout << i.first << \": \" << i.second << '\\n';\n    }\n\n    // example for an exception\n    try\n    {\n        bool v1 = json_types[\"string\"];\n    }\n    catch (const json::type_error& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

        Output:

        1\n42 42\n17.23 17\nHello, world!\n1 2 3 4 5 \n\narray: [1,2,3,4,5]\nboolean: true\nnull: null\nnumber: {\"floating-point\":17.23,\"integer\":42}\nstring: \"Hello, world!\"\n[json.exception.type_error.302] type must be boolean, but is string\n
        "},{"location":"api/basic_json/operator_ValueType/#see-also","title":"See also","text":"
        • get get a value (explicit conversion)
        • Converting values - the type conversions article
        "},{"location":"api/basic_json/operator_ValueType/#version-history","title":"Version history","text":"
        • Since version 1.0.0.
        • Macros JSON_EXPLICIT/JSON_USE_IMPLICIT_CONVERSIONS added in version 3.9.0.
        "},{"location":"api/basic_json/operator_eq/","title":"nlohmann::basic_json::operator==","text":"
        // until C++20\nbool operator==(const_reference lhs, const_reference rhs) noexcept;   // (1)\n\ntemplate<typename ScalarType>\nbool operator==(const_reference lhs, const ScalarType rhs) noexcept(/* see below */); // (2)\n\ntemplate<typename ScalarType>\nbool operator==(ScalarType lhs, const const_reference rhs) noexcept(/* see below */); // (2)\n\n// since C++20\nbool operator==(const_reference rhs) const noexcept;             // (1)\n\ntemplate<typename ScalarType>\nbool operator==(ScalarType rhs) const noexcept(/* see below */); // (2)\n

        1. Compares two JSON values for equality according to the following rules:

          • Two JSON values are equal if (1) neither value is discarded, and (2) they are of the same type and their stored values are the same according to their respective operator==.
          • Integer and floating-point numbers are compared by their exact values, without converting the integer to floating-point. For instance, 9007199254740993 and 9007199254740992.0 are not equal.

        2. Compares a JSON value and a scalar or a scalar and a JSON value for equality by converting the scalar to a JSON value and comparing both JSON values according to 1.

        "},{"location":"api/basic_json/operator_eq/#template-parameters","title":"Template parameters","text":"ScalarType a scalar type according to std::is_scalar<ScalarType>::value"},{"location":"api/basic_json/operator_eq/#parameters","title":"Parameters","text":"lhs (in) first value to consider rhs (in) second value to consider"},{"location":"api/basic_json/operator_eq/#return-value","title":"Return value","text":"

        whether the values lhs/*this and rhs are equal

        "},{"location":"api/basic_json/operator_eq/#exception-safety","title":"Exception safety","text":"
        1. No-throw guarantee: this function never throws exceptions.
        2. No-throw guarantee if converting the scalar to a JSON value cannot throw, as for numbers, Booleans, and nullptr; the function is noexcept exactly in that case. Otherwise, it throws what the conversion throws, for example std::bad_alloc when converting a string, or out_of_range.410 for an enum value not mapped by NLOHMANN_JSON_SERIALIZE_ENUM_STRICT.
        "},{"location":"api/basic_json/operator_eq/#complexity","title":"Complexity","text":"

        Linear.

        "},{"location":"api/basic_json/operator_eq/#notes","title":"Notes","text":"

        Comparing special values

        • NaN values are unordered within the domain of numbers. The following comparisons all yield false:
          1. Comparing a NaN with itself.
          2. Comparing a NaN with another NaN.
          3. Comparing a NaN and any other number.
        • JSON null values are all equal.
        • Discarded values never compare equal to themselves.

        Comparing floating-point numbers

        Floating-point numbers inside JSON values numbers are compared with json::number_float_t::operator== which is double::operator== by default. To compare floating-point while respecting an epsilon, an alternative comparison function could be used, for instance

        template<typename T, typename = typename std::enable_if<std::is_floating_point<T>::value, T>::type>\ninline bool is_same(T a, T b, T epsilon = std::numeric_limits<T>::epsilon()) noexcept\n{\n    return std::abs(a - b) <= epsilon;\n}\n

        Or you can define your own equality function like this:

        bool my_equal(const_reference lhs, const_reference rhs)\n{\n    const auto lhs_type = lhs.type();\n    const auto rhs_type = rhs.type();\n    if (lhs_type == rhs_type)\n    {\n        switch(lhs_type)\n            // self_defined case\n            case value_t::number_float:\n                return std::abs(lhs - rhs) <= std::numeric_limits<float>::epsilon();\n            // other cases remain the same with the original\n            ...\n    }\n...\n}\n

        Comparing different basic_json specializations

        Comparing different basic_json specializations can have surprising effects. For instance, the result of comparing the JSON objects

        {\n   \"version\": 1,\n   \"type\": \"integer\"\n}\n

        and

        {\n   \"type\": \"integer\",\n   \"version\": 1\n}\n

        depends on whether nlohmann::json or nlohmann::ordered_json is used:

        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    nlohmann::json uj1 = {{\"version\", 1}, {\"type\", \"integer\"}};\n    nlohmann::json uj2 = {{\"type\", \"integer\"}, {\"version\", 1}};\n\n    nlohmann::ordered_json oj1 = {{\"version\", 1}, {\"type\", \"integer\"}};\n    nlohmann::ordered_json oj2 = {{\"type\", \"integer\"}, {\"version\", 1}};\n\n    std::cout << std::boolalpha << (uj1 == uj2) << '\\n' << (oj1 == oj2) << std::endl;\n}\n

        Output:

        true\nfalse\n
        "},{"location":"api/basic_json/operator_eq/#examples","title":"Examples","text":"Example: (1) compare JSON values

        The example demonstrates comparing several JSON types.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create several JSON values\n    json array_1 = {1, 2, 3};\n    json array_2 = {1, 2, 4};\n    json object_1 = {{\"A\", \"a\"}, {\"B\", \"b\"}};\n    json object_2 = {{\"B\", \"b\"}, {\"A\", \"a\"}};\n    json number_1 = 17;\n    json number_2 = 17.000000000000001L;\n    json string_1 = \"foo\";\n    json string_2 = \"bar\";\n\n    // output values and comparisons\n    std::cout << std::boolalpha;\n    std::cout << array_1 << \" == \" << array_2 << \" \" << (array_1 == array_2) << '\\n';\n    std::cout << object_1 << \" == \" << object_2 << \" \" << (object_1 == object_2) << '\\n';\n    std::cout << number_1 << \" == \" << number_2 << \" \" << (number_1 == number_2) << '\\n';\n    std::cout << string_1 << \" == \" << string_2 << \" \" << (string_1 == string_2) << '\\n';\n}\n

        Output:

        [1,2,3] == [1,2,4] false\n{\"A\":\"a\",\"B\":\"b\"} == {\"A\":\"a\",\"B\":\"b\"} true\n17 == 17.0 true\n\"foo\" == \"bar\" false\n
        Example: (2) compare JSON values with nullptr

        The example demonstrates comparing several JSON types against the null pointer (JSON null).

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create several JSON values\n    json array = {1, 2, 3};\n    json object = {{\"A\", \"a\"}, {\"B\", \"b\"}};\n    json number = 17;\n    json string = \"foo\";\n    json null;\n\n    // output values and comparisons\n    std::cout << std::boolalpha;\n    std::cout << array << \" == nullptr \" << (array == nullptr) << '\\n';\n    std::cout << object << \" == nullptr \" << (object == nullptr) << '\\n';\n    std::cout << number << \" == nullptr \" << (number == nullptr) << '\\n';\n    std::cout << string << \" == nullptr \" << (string == nullptr) << '\\n';\n    std::cout << null << \" == nullptr \" << (null == nullptr) << '\\n';\n}\n

        Output:

        [1,2,3] == nullptr false\n{\"A\":\"a\",\"B\":\"b\"} == nullptr false\n17 == nullptr false\n\"foo\" == nullptr false\nnull == nullptr true\n
        "},{"location":"api/basic_json/operator_eq/#see-also","title":"See also","text":"
        • operator!= compare for inequality
        • operator<=> comparison: 3-way (C++20)
        • basic_json_view::operator== - the same comparison on a zero-copy view, without building a basic_json value for it
        "},{"location":"api/basic_json/operator_eq/#version-history","title":"Version history","text":"
        1. Added in version 1.0.0. Added C++20 member functions in version 3.11.0.
        2. Added in version 1.0.0. Added C++20 member functions in version 3.11.0. Made conditionally noexcept in version 3.13.0 unreleased; before, a throwing conversion called std::terminate.
        "},{"location":"api/basic_json/operator_ge/","title":"nlohmann::basic_json::operator>=","text":"
        // until C++20\nbool operator>=(const_reference lhs, const_reference rhs) noexcept;   // (1)\n\ntemplate<typename ScalarType>\nbool operator>=(const_reference lhs, const ScalarType rhs) noexcept(/* see below */); // (2)\n\ntemplate<typename ScalarType>\nbool operator>=(ScalarType lhs, const const_reference rhs) noexcept(/* see below */); // (2)\n

        1. Compares whether one JSON value lhs is greater than or equal to another JSON value rhs according to the following rules:

          • The comparison always yields false if (1) either operand is discarded, or (2) either operand is NaN and the other operand is either NaN or any other number.
          • Otherwise, returns the result of !(lhs < rhs) (see operator<).

        2. Compares whether a JSON value is greater than or equal to a scalar or a scalar is greater than or equal to a JSON value by converting the scalar to a JSON value and comparing both JSON values according to 1.

        "},{"location":"api/basic_json/operator_ge/#template-parameters","title":"Template parameters","text":"ScalarType a scalar type according to std::is_scalar<ScalarType>::value"},{"location":"api/basic_json/operator_ge/#parameters","title":"Parameters","text":"lhs (in) first value to consider rhs (in) second value to consider"},{"location":"api/basic_json/operator_ge/#return-value","title":"Return value","text":"

        whether lhs is greater than or equal to rhs

        "},{"location":"api/basic_json/operator_ge/#exception-safety","title":"Exception safety","text":"
        1. No-throw guarantee: this function never throws exceptions.
        2. No-throw guarantee if converting the scalar to a JSON value cannot throw, as for numbers, Booleans, and nullptr; the function is noexcept exactly in that case. Otherwise, it throws what the conversion throws, for example std::bad_alloc when converting a string, or out_of_range.410 for an enum value not mapped by NLOHMANN_JSON_SERIALIZE_ENUM_STRICT.
        "},{"location":"api/basic_json/operator_ge/#complexity","title":"Complexity","text":"

        Linear.

        "},{"location":"api/basic_json/operator_ge/#notes","title":"Notes","text":"

        Comparing NaN

        NaN values are unordered within the domain of numbers. The following comparisons all yield false: 1. Comparing a NaN with itself. 2. Comparing a NaN with another NaN. 3. Comparing a NaN and any other number.

        Operator overload resolution

        Since C++20 overload resolution will consider the rewritten candidate generated from operator<=>.

        Deprecation

        If JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON is defined to 1, the library declares a member bool operator>=(const_reference rhs) const noexcept in C++20 mode to emulate the legacy comparison of discarded values. This member is deprecated since version 3.11.0, together with the legacy comparison behavior.

        See the migration guide for how to update existing code.

        "},{"location":"api/basic_json/operator_ge/#examples","title":"Examples","text":"Example

        The example demonstrates comparing several JSON types.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create several JSON values\n    json array_1 = {1, 2, 3};\n    json array_2 = {1, 2, 4};\n    json object_1 = {{\"A\", \"a\"}, {\"B\", \"b\"}};\n    json object_2 = {{\"B\", \"b\"}, {\"A\", \"a\"}};\n    json number_1 = 17;\n    json number_2 = 17.0000000000001L;\n    json string_1 = \"foo\";\n    json string_2 = \"bar\";\n\n    // output values and comparisons\n    std::cout << std::boolalpha;\n    std::cout << array_1 << \" >= \" << array_2 << \" \" << (array_1 >= array_2) << '\\n';\n    std::cout << object_1 << \" >= \" << object_2 << \" \" << (object_1 >= object_2) << '\\n';\n    std::cout << number_1 << \" >= \" << number_2 << \" \" << (number_1 >= number_2) << '\\n';\n    std::cout << string_1 << \" >= \" << string_2 << \" \" << (string_1 >= string_2) << '\\n';\n}\n

        Output:

        [1,2,3] >= [1,2,4] false\n{\"A\":\"a\",\"B\":\"b\"} >= {\"A\":\"a\",\"B\":\"b\"} true\n17 >= 17.0000000000001 false\n\"foo\" >= \"bar\" true\n
        "},{"location":"api/basic_json/operator_ge/#see-also","title":"See also","text":"
        • operator<=> comparison: 3-way
        "},{"location":"api/basic_json/operator_ge/#version-history","title":"Version history","text":"
        1. Added in version 1.0.0. Conditionally removed since C++20 in version 3.11.0.
        2. Added in version 1.0.0. Conditionally removed since C++20 in version 3.11.0. Made conditionally noexcept in version 3.13.0 unreleased; before, a throwing conversion called std::terminate.
        "},{"location":"api/basic_json/operator_gt/","title":"nlohmann::basic_json::operator>","text":"
        // until C++20\nbool operator>(const_reference lhs, const_reference rhs) noexcept;   // (1)\n\ntemplate<typename ScalarType>\nbool operator>(const_reference lhs, const ScalarType rhs) noexcept(/* see below */); // (2)\n\ntemplate<typename ScalarType>\nbool operator>(ScalarType lhs, const const_reference rhs) noexcept(/* see below */); // (2)\n

        1. Compares whether one JSON value lhs is greater than another JSON value rhs according to the following rules:

          • The comparison always yields false if (1) either operand is discarded, or (2) either operand is NaN and the other operand is either NaN or any other number.
          • Otherwise, returns the result of !(lhs <= rhs) (see operator<=).

        2. Compares whether a JSON value is greater than a scalar or a scalar is greater than a JSON value by converting the scalar to a JSON value and comparing both JSON values according to 1.

        "},{"location":"api/basic_json/operator_gt/#template-parameters","title":"Template parameters","text":"ScalarType a scalar type according to std::is_scalar<ScalarType>::value"},{"location":"api/basic_json/operator_gt/#parameters","title":"Parameters","text":"lhs (in) first value to consider rhs (in) second value to consider"},{"location":"api/basic_json/operator_gt/#return-value","title":"Return value","text":"

        whether lhs is greater than rhs

        "},{"location":"api/basic_json/operator_gt/#exception-safety","title":"Exception safety","text":"
        1. No-throw guarantee: this function never throws exceptions.
        2. No-throw guarantee if converting the scalar to a JSON value cannot throw, as for numbers, Booleans, and nullptr; the function is noexcept exactly in that case. Otherwise, it throws what the conversion throws, for example std::bad_alloc when converting a string, or out_of_range.410 for an enum value not mapped by NLOHMANN_JSON_SERIALIZE_ENUM_STRICT.
        "},{"location":"api/basic_json/operator_gt/#complexity","title":"Complexity","text":"

        Linear.

        "},{"location":"api/basic_json/operator_gt/#notes","title":"Notes","text":"

        Comparing NaN

        NaN values are unordered within the domain of numbers. The following comparisons all yield false: 1. Comparing a NaN with itself. 2. Comparing a NaN with another NaN. 3. Comparing a NaN and any other number.

        Operator overload resolution

        Since C++20 overload resolution will consider the rewritten candidate generated from operator<=>.

        "},{"location":"api/basic_json/operator_gt/#examples","title":"Examples","text":"Example

        The example demonstrates comparing several JSON types.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create several JSON values\n    json array_1 = {1, 2, 3};\n    json array_2 = {1, 2, 4};\n    json object_1 = {{\"A\", \"a\"}, {\"B\", \"b\"}};\n    json object_2 = {{\"B\", \"b\"}, {\"A\", \"a\"}};\n    json number_1 = 17;\n    json number_2 = 17.0000000000001L;\n    json string_1 = \"foo\";\n    json string_2 = \"bar\";\n\n    // output values and comparisons\n    std::cout << std::boolalpha;\n    std::cout << array_1 << \" > \" << array_2 << \" \" << (array_1 > array_2) << '\\n';\n    std::cout << object_1 << \" > \" << object_2 << \" \" << (object_1 > object_2) << '\\n';\n    std::cout << number_1 << \" > \" << number_2 << \" \" << (number_1 > number_2) << '\\n';\n    std::cout << string_1 << \" > \" << string_2 << \" \" << (string_1 > string_2) << '\\n';\n}\n

        Output:

        [1,2,3] > [1,2,4] false\n{\"A\":\"a\",\"B\":\"b\"} > {\"A\":\"a\",\"B\":\"b\"} false\n17 > 17.0000000000001 false\n\"foo\" > \"bar\" true\n
        "},{"location":"api/basic_json/operator_gt/#see-also","title":"See also","text":"
        • operator<=> comparison: 3-way
        "},{"location":"api/basic_json/operator_gt/#version-history","title":"Version history","text":"
        1. Added in version 1.0.0. Conditionally removed since C++20 in version 3.11.0.
        2. Added in version 1.0.0. Conditionally removed since C++20 in version 3.11.0. Made conditionally noexcept in version 3.13.0 unreleased; before, a throwing conversion called std::terminate.
        "},{"location":"api/basic_json/operator_le/","title":"nlohmann::basic_json::operator<=","text":"
        // until C++20\nbool operator<=(const_reference lhs, const_reference rhs) noexcept;   // (1)\n\ntemplate<typename ScalarType>\nbool operator<=(const_reference lhs, const ScalarType rhs) noexcept(/* see below */); // (2)\n\ntemplate<typename ScalarType>\nbool operator<=(ScalarType lhs, const const_reference rhs) noexcept(/* see below */); // (2)\n

        1. Compares whether one JSON value lhs is less than or equal to another JSON value rhs according to the following rules:

          • The comparison always yields false if (1) either operand is discarded, or (2) either operand is NaN and the other operand is either NaN or any other number.
          • Otherwise, returns the result of !(rhs < lhs) (see operator<).

        2. Compares whether a JSON value is less than or equal to a scalar or a scalar is less than or equal to a JSON value by converting the scalar to a JSON value and comparing both JSON values according to 1.

        "},{"location":"api/basic_json/operator_le/#template-parameters","title":"Template parameters","text":"ScalarType a scalar type according to std::is_scalar<ScalarType>::value"},{"location":"api/basic_json/operator_le/#parameters","title":"Parameters","text":"lhs (in) first value to consider rhs (in) second value to consider"},{"location":"api/basic_json/operator_le/#return-value","title":"Return value","text":"

        whether lhs is less than or equal to rhs

        "},{"location":"api/basic_json/operator_le/#exception-safety","title":"Exception safety","text":"
        1. No-throw guarantee: this function never throws exceptions.
        2. No-throw guarantee if converting the scalar to a JSON value cannot throw, as for numbers, Booleans, and nullptr; the function is noexcept exactly in that case. Otherwise, it throws what the conversion throws, for example std::bad_alloc when converting a string, or out_of_range.410 for an enum value not mapped by NLOHMANN_JSON_SERIALIZE_ENUM_STRICT.
        "},{"location":"api/basic_json/operator_le/#complexity","title":"Complexity","text":"

        Linear.

        "},{"location":"api/basic_json/operator_le/#notes","title":"Notes","text":"

        Comparing NaN

        NaN values are unordered within the domain of numbers. The following comparisons all yield false: 1. Comparing a NaN with itself. 2. Comparing a NaN with another NaN. 3. Comparing a NaN and any other number.

        Operator overload resolution

        Since C++20 overload resolution will consider the rewritten candidate generated from operator<=>.

        Deprecation

        If JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON is defined to 1, the library declares a member bool operator<=(const_reference rhs) const noexcept in C++20 mode to emulate the legacy comparison of discarded values. This member is deprecated since version 3.11.0, together with the legacy comparison behavior.

        See the migration guide for how to update existing code.

        "},{"location":"api/basic_json/operator_le/#examples","title":"Examples","text":"Example

        The example demonstrates comparing several JSON types.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create several JSON values\n    json array_1 = {1, 2, 3};\n    json array_2 = {1, 2, 4};\n    json object_1 = {{\"A\", \"a\"}, {\"B\", \"b\"}};\n    json object_2 = {{\"B\", \"b\"}, {\"A\", \"a\"}};\n    json number_1 = 17;\n    json number_2 = 17.0000000000001L;\n    json string_1 = \"foo\";\n    json string_2 = \"bar\";\n\n    // output values and comparisons\n    std::cout << std::boolalpha;\n    std::cout << array_1 << \" <= \" << array_2 << \" \" << (array_1 <= array_2) << '\\n';\n    std::cout << object_1 << \" <= \" << object_2 << \" \" << (object_1 <= object_2) << '\\n';\n    std::cout << number_1 << \" <= \" << number_2 << \" \" << (number_1 <= number_2) << '\\n';\n    std::cout << string_1 << \" <= \" << string_2 << \" \" << (string_1 <= string_2) << '\\n';\n}\n

        Output:

        [1,2,3] <= [1,2,4] true\n{\"A\":\"a\",\"B\":\"b\"} <= {\"A\":\"a\",\"B\":\"b\"} true\n17 <= 17.0000000000001 true\n\"foo\" <= \"bar\" false\n
        "},{"location":"api/basic_json/operator_le/#see-also","title":"See also","text":"
        • operator<=> comparison: 3-way
        "},{"location":"api/basic_json/operator_le/#version-history","title":"Version history","text":"
        1. Added in version 1.0.0. Conditionally removed since C++20 in version 3.11.0.
        2. Added in version 1.0.0. Conditionally removed since C++20 in version 3.11.0. Made conditionally noexcept in version 3.13.0 unreleased; before, a throwing conversion called std::terminate.
        "},{"location":"api/basic_json/operator_lt/","title":"nlohmann::basic_json::operator<","text":"
        // until C++20\nbool operator<(const_reference lhs, const_reference rhs) noexcept;   // (1)\n\ntemplate<typename ScalarType>\nbool operator<(const_reference lhs, const ScalarType rhs) noexcept(/* see below */); // (2)\n\ntemplate<typename ScalarType>\nbool operator<(ScalarType lhs, const const_reference rhs) noexcept(/* see below */); // (2)\n

        1. Compares whether one JSON value lhs is less than another JSON value rhs according to the following rules:

          • If either operand is discarded, the comparison yields false.
          • If both operands have the same type, the values are compared using their respective operator<.
          • Integer and floating-point numbers are compared by their exact values, without converting the integer to floating-point. For instance, 9007199254740992.0 is less than 9007199254740993.
          • In case lhs and rhs have different types, the values are ignored and the order of the types is considered, which is:
            1. null
            2. boolean
            3. number (all types)
            4. object
            5. array
            6. string
            7. binary For instance, any boolean value is considered less than any string.

        2. Compares whether a JSON value is less than a scalar or a scalar is less than a JSON value by converting the scalar to a JSON value and comparing both JSON values according to 1.

        "},{"location":"api/basic_json/operator_lt/#template-parameters","title":"Template parameters","text":"ScalarType a scalar type according to std::is_scalar<ScalarType>::value"},{"location":"api/basic_json/operator_lt/#parameters","title":"Parameters","text":"lhs (in) first value to consider rhs (in) second value to consider"},{"location":"api/basic_json/operator_lt/#return-value","title":"Return value","text":"

        whether lhs is less than rhs

        "},{"location":"api/basic_json/operator_lt/#exception-safety","title":"Exception safety","text":"
        1. No-throw guarantee: this function never throws exceptions.
        2. No-throw guarantee if converting the scalar to a JSON value cannot throw, as for numbers, Booleans, and nullptr; the function is noexcept exactly in that case. Otherwise, it throws what the conversion throws, for example std::bad_alloc when converting a string, or out_of_range.410 for an enum value not mapped by NLOHMANN_JSON_SERIALIZE_ENUM_STRICT.
        "},{"location":"api/basic_json/operator_lt/#complexity","title":"Complexity","text":"

        Linear.

        "},{"location":"api/basic_json/operator_lt/#notes","title":"Notes","text":"

        Comparing NaN

        NaN values are unordered within the domain of numbers. The following comparisons all yield false: 1. Comparing a NaN with itself. 2. Comparing a NaN with another NaN. 3. Comparing a NaN and any other number.

        Operator overload resolution

        Since C++20 overload resolution will consider the rewritten candidate generated from operator<=>.

        "},{"location":"api/basic_json/operator_lt/#examples","title":"Examples","text":"Example

        The example demonstrates comparing several JSON types.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create several JSON values\n    json array_1 = {1, 2, 3};\n    json array_2 = {1, 2, 4};\n    json object_1 = {{\"A\", \"a\"}, {\"B\", \"b\"}};\n    json object_2 = {{\"B\", \"b\"}, {\"A\", \"a\"}};\n    json number_1 = 17;\n    json number_2 = 17.0000000000001L;\n    json string_1 = \"foo\";\n    json string_2 = \"bar\";\n\n    // output values and comparisons\n    std::cout << std::boolalpha;\n    std::cout << array_1 << \" == \" << array_2 << \" \" << (array_1 < array_2) << '\\n';\n    std::cout << object_1 << \" == \" << object_2 << \" \" << (object_1 < object_2) << '\\n';\n    std::cout << number_1 << \" == \" << number_2 << \" \" << (number_1 < number_2) << '\\n';\n    std::cout << string_1 << \" == \" << string_2 << \" \" << (string_1 < string_2) << '\\n';\n}\n

        Output:

        [1,2,3] == [1,2,4] true\n{\"A\":\"a\",\"B\":\"b\"} == {\"A\":\"a\",\"B\":\"b\"} false\n17 == 17.0000000000001 true\n\"foo\" == \"bar\" false\n
        "},{"location":"api/basic_json/operator_lt/#see-also","title":"See also","text":"
        • operator<=> comparison: 3-way
        "},{"location":"api/basic_json/operator_lt/#version-history","title":"Version history","text":"
        1. Added in version 1.0.0. Conditionally removed since C++20 in version 3.11.0.
        2. Added in version 1.0.0. Conditionally removed since C++20 in version 3.11.0. Made conditionally noexcept in version 3.13.0 unreleased; before, a throwing conversion called std::terminate.
        "},{"location":"api/basic_json/operator_ne/","title":"nlohmann::basic_json::operator!=","text":"
        // until C++20\nbool operator!=(const_reference lhs, const_reference rhs) noexcept;   // (1)\n\ntemplate<typename ScalarType>\nbool operator!=(const_reference lhs, const ScalarType rhs) noexcept(/* see below */); // (2)\n\ntemplate<typename ScalarType>\nbool operator!=(ScalarType lhs, const const_reference rhs) noexcept(/* see below */); // (2)\n

        1. Compares two JSON values for inequality. Returns !(lhs == rhs).

          • This means the comparison is simply the logical negation of operator==, including for special values like NaN and discarded.

        2. Compares a JSON value and a scalar or a scalar and a JSON value for inequality by converting the scalar to a JSON value and comparing both JSON values according to 1.

        "},{"location":"api/basic_json/operator_ne/#template-parameters","title":"Template parameters","text":"ScalarType a scalar type according to std::is_scalar<ScalarType>::value"},{"location":"api/basic_json/operator_ne/#parameters","title":"Parameters","text":"lhs (in) first value to consider rhs (in) second value to consider"},{"location":"api/basic_json/operator_ne/#return-value","title":"Return value","text":"

        whether the values lhs/*this and rhs are not equal

        "},{"location":"api/basic_json/operator_ne/#exception-safety","title":"Exception safety","text":"
        1. No-throw guarantee: this function never throws exceptions.
        2. No-throw guarantee if converting the scalar to a JSON value cannot throw, as for numbers, Booleans, and nullptr; the function is noexcept exactly in that case. Otherwise, it throws what the conversion throws, for example std::bad_alloc when converting a string, or out_of_range.410 for an enum value not mapped by NLOHMANN_JSON_SERIALIZE_ENUM_STRICT.
        "},{"location":"api/basic_json/operator_ne/#complexity","title":"Complexity","text":"

        Linear.

        "},{"location":"api/basic_json/operator_ne/#notes","title":"Notes","text":"

        C++20

        Since C++20, basic_json declares no operator!=. The compiler rewrites a != b as !(a == b) using operator==, so the result is the same as described above.

        Comparing NaN and discarded

        Since operator!= is defined as !(a == b), the behavior for special values follows that of operator==:

        • For NaN values: NaN == NaN yields false, so NaN != NaN yields true.
        • For discarded values: discarded == x yields false for any x, so discarded != x yields true.
        "},{"location":"api/basic_json/operator_ne/#examples","title":"Examples","text":"Example: (1) compare JSON values

        The example demonstrates comparing several JSON types.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create several JSON values\n    json array_1 = {1, 2, 3};\n    json array_2 = {1, 2, 4};\n    json object_1 = {{\"A\", \"a\"}, {\"B\", \"b\"}};\n    json object_2 = {{\"B\", \"b\"}, {\"A\", \"a\"}};\n    json number_1 = 17;\n    json number_2 = 17.000000000000001L;\n    json string_1 = \"foo\";\n    json string_2 = \"bar\";\n\n    // output values and comparisons\n    std::cout << std::boolalpha;\n    std::cout << array_1 << \" != \" << array_2 << \" \" << (array_1 != array_2) << '\\n';\n    std::cout << object_1 << \" != \" << object_2 << \" \" << (object_1 != object_2) << '\\n';\n    std::cout << number_1 << \" != \" << number_2 << \" \" << (number_1 != number_2) << '\\n';\n    std::cout << string_1 << \" != \" << string_2 << \" \" << (string_1 != string_2) << '\\n';\n}\n

        Output:

        [1,2,3] != [1,2,4] true\n{\"A\":\"a\",\"B\":\"b\"} != {\"A\":\"a\",\"B\":\"b\"} false\n17 != 17.0 false\n\"foo\" != \"bar\" true\n
        Example: (2) compare JSON values with nullptr

        The example demonstrates comparing several JSON types against the null pointer (JSON null).

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create several JSON values\n    json array = {1, 2, 3};\n    json object = {{\"A\", \"a\"}, {\"B\", \"b\"}};\n    json number = 17;\n    json string = \"foo\";\n    json null;\n\n    // output values and comparisons\n    std::cout << std::boolalpha;\n    std::cout << array << \" != nullptr \" << (array != nullptr) << '\\n';\n    std::cout << object << \" != nullptr \" << (object != nullptr) << '\\n';\n    std::cout << number << \" != nullptr \" << (number != nullptr) << '\\n';\n    std::cout << string << \" != nullptr \" << (string != nullptr) << '\\n';\n    std::cout << null << \" != nullptr \" << (null != nullptr) << '\\n';\n}\n

        Output:

        [1,2,3] != nullptr true\n{\"A\":\"a\",\"B\":\"b\"} != nullptr true\n17 != nullptr true\n\"foo\" != nullptr true\nnull != nullptr false\n
        "},{"location":"api/basic_json/operator_ne/#see-also","title":"See also","text":"
        • operator== comparison: equal
        • operator<=> comparison: 3-way (C++20)
        • basic_json_view::operator!= - the same comparison on a zero-copy view, without building a basic_json value for it
        "},{"location":"api/basic_json/operator_ne/#version-history","title":"Version history","text":"
        1. Added in version 1.0.0. Added a C++20 member function in version 3.11.0. Changed in version 3.13.0 unreleased to remove special-casing for NaN and discarded values; operator!= now consistently means !(a == b). Removed the C++20 member function in version 3.13.0 unreleased; since C++20, the compiler rewrites a != b using operator==.
        2. Added in version 1.0.0. Changed in version 3.13.0 unreleased to remove special-casing for NaN and discarded values; operator!= now consistently means !(a == b). Since C++20, the compiler rewrites a != b using operator==. Made conditionally noexcept in version 3.13.0 unreleased; before, a throwing conversion called std::terminate.
        "},{"location":"api/basic_json/operator_spaceship/","title":"nlohmann::basic_json::operator<=>","text":"
        // since C++20\nstd::partial_ordering operator<=>(const_reference rhs) const noexcept;                   // (1)\n\ntemplate<typename ScalarType>\nstd::partial_ordering operator<=>(const ScalarType rhs) const noexcept(/* see below */); // (2)\n

        1. 3-way compares two JSON values producing a result of type std::partial_ordering according to the following rules:

          • Two JSON values compare with a result of std::partial_ordering::unordered if either value is discarded.
          • If both JSON values are of the same type, the result is produced by 3-way comparing their stored values using their respective operator<=>.
          • Integer and floating-point numbers are 3-way compared by their exact values, without converting the integer to floating-point. For instance, 9007199254740993 compares greater than 9007199254740992.0.
          • Otherwise, yields a result by comparing the type (see value_t).

        2. 3-way compares a JSON value and a scalar or a scalar and a JSON value by converting the scalar to a JSON value and 3-way comparing both JSON values (see 1).

        "},{"location":"api/basic_json/operator_spaceship/#template-parameters","title":"Template parameters","text":"ScalarType a scalar type according to std::is_scalar<ScalarType>::value"},{"location":"api/basic_json/operator_spaceship/#parameters","title":"Parameters","text":"rhs (in) second value to consider"},{"location":"api/basic_json/operator_spaceship/#return-value","title":"Return value","text":"

        the std::partial_ordering of the 3-way comparison of *this and rhs

        "},{"location":"api/basic_json/operator_spaceship/#exception-safety","title":"Exception safety","text":"
        1. No-throw guarantee: this function never throws exceptions.
        2. No-throw guarantee if converting the scalar to a JSON value cannot throw, as for numbers, Booleans, and nullptr; the function is noexcept exactly in that case. Otherwise, it throws what the conversion throws, for example std::bad_alloc when converting a string, or out_of_range.410 for an enum value not mapped by NLOHMANN_JSON_SERIALIZE_ENUM_STRICT.
        "},{"location":"api/basic_json/operator_spaceship/#complexity","title":"Complexity","text":"

        Linear.

        "},{"location":"api/basic_json/operator_spaceship/#notes","title":"Notes","text":"

        Comparing NaN

        • NaN values are unordered within the domain of numbers. The following comparisons all yield std::partial_ordering::unordered:
          1. Comparing a NaN with itself.
          2. Comparing a NaN with another NaN.
          3. Comparing a NaN and any other number.
        "},{"location":"api/basic_json/operator_spaceship/#examples","title":"Examples","text":"Example: (1) comparing JSON values

        The example demonstrates comparing several JSON values.

        #include <compare>\n#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nconst char* to_string(const std::partial_ordering& po)\n{\n    if (std::is_lt(po))\n    {\n        return \"less\";\n    }\n    else if (std::is_gt(po))\n    {\n        return \"greater\";\n    }\n    else if (std::is_eq(po))\n    {\n        return \"equivalent\";\n    }\n    return \"unordered\";\n}\n\nint main()\n{\n    // create several JSON values\n    json array_1 = {1, 2, 3};\n    json array_2 = {1, 2, 4};\n    json object_1 = {{\"A\", \"a\"}, {\"B\", \"b\"}};\n    json object_2 = {{\"B\", \"b\"}, {\"A\", \"a\"}};\n    json number = 17;\n    json string = \"foo\";\n    json discarded = json(json::value_t::discarded);\n\n    // output values and comparisons\n    std::cout << array_1 << \" <=> \" << array_2 << \" := \" << to_string(array_1 <=> array_2) << '\\n'; // *NOPAD*\n    std::cout << object_1 << \" <=> \" << object_2 << \" := \" << to_string(object_1 <=> object_2) << '\\n'; // *NOPAD*\n    std::cout << string << \" <=> \" << number << \" := \" << to_string(string <=> number) << '\\n'; // *NOPAD*\n    std::cout << string << \" <=> \" << discarded << \" := \" << to_string(string <=> discarded) << '\\n'; // *NOPAD*\n}\n

        Output:

        [1,2,3] <=> [1,2,4] := less\n{\"A\":\"a\",\"B\":\"b\"} <=> {\"A\":\"a\",\"B\":\"b\"} := equivalent\n\"foo\" <=> 17 := greater\n\"foo\" <=> <discarded> := unordered\n
        Example: (2) comparing JSON values and scalars

        The example demonstrates comparing several JSON values and scalars.

        #include <compare>\n#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nconst char* to_string(const std::partial_ordering& po)\n{\n    if (std::is_lt(po))\n    {\n        return \"less\";\n    }\n    else if (std::is_gt(po))\n    {\n        return \"greater\";\n    }\n    else if (std::is_eq(po))\n    {\n        return \"equivalent\";\n    }\n    return \"unordered\";\n}\n\nint main()\n{\n    using float_limits = std::numeric_limits<json::number_float_t>;\n    constexpr auto nan = float_limits::quiet_NaN();\n\n    // create several JSON values\n    json boolean = false;\n    json number = 17;\n    json string = \"17\";\n\n    // output values and comparisons\n    std::cout << std::boolalpha << std::fixed;\n    std::cout << boolean << \" <=> \" << true << \" := \" << to_string(boolean <=> true) << '\\n'; // *NOPAD*\n    std::cout << number << \" <=> \" << 17.0 << \" := \" << to_string(number <=> 17.0) << '\\n'; // *NOPAD*\n    std::cout << number << \" <=> \" << nan << \" := \" << to_string(number <=> nan) << '\\n'; // *NOPAD*\n    std::cout << string << \" <=> \" << 17 << \" := \" << to_string(string <=> 17) << '\\n'; // *NOPAD*\n}\n

        Output:

        false <=> true := less\n17 <=> 17.000000 := equivalent\n17 <=> nan := unordered\n\"17\" <=> 17 := greater\n
        "},{"location":"api/basic_json/operator_spaceship/#see-also","title":"See also","text":"
        • operator== - comparison: equal
        • operator!= - comparison: not equal
        • operator< - comparison: less than
        • operator<= - comparison: less than or equal
        • operator> - comparison: greater than
        • operator>= - comparison: greater than or equal
        "},{"location":"api/basic_json/operator_spaceship/#version-history","title":"Version history","text":"
        1. Added in version 3.11.0.
        2. Added in version 3.11.0. Made conditionally noexcept in version 3.13.0 unreleased; before, a throwing conversion called std::terminate.
        "},{"location":"api/basic_json/operator_value_t/","title":"nlohmann::basic_json::operator value_t","text":"
        constexpr operator value_t() const noexcept;\n

        Return the type of the JSON value as a value from the value_t enumeration.

        "},{"location":"api/basic_json/operator_value_t/#return-value","title":"Return value","text":"

        the type of the JSON value

        Value type return value null value_t::null boolean value_t::boolean string value_t::string number (integer) value_t::number_integer number (unsigned integer) value_t::number_unsigned number (floating-point) value_t::number_float object value_t::object array value_t::array binary value_t::binary discarded value_t::discarded"},{"location":"api/basic_json/operator_value_t/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this member function never throws exceptions.

        "},{"location":"api/basic_json/operator_value_t/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json/operator_value_t/#examples","title":"Examples","text":"Example

        The following code exemplifies operator value_t() for all JSON types.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_null;\n    json j_boolean = true;\n    json j_number_integer = -17;\n    json j_number_unsigned = 42u;\n    json j_number_float = 23.42;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n    json j_string = \"Hello, world\";\n\n    // call operator value_t()\n    json::value_t t_null = j_null;\n    json::value_t t_boolean = j_boolean;\n    json::value_t t_number_integer = j_number_integer;\n    json::value_t t_number_unsigned = j_number_unsigned;\n    json::value_t t_number_float = j_number_float;\n    json::value_t t_object = j_object;\n    json::value_t t_array = j_array;\n    json::value_t t_string = j_string;\n\n    // print types\n    std::cout << std::boolalpha;\n    std::cout << (t_null == json::value_t::null) << '\\n';\n    std::cout << (t_boolean == json::value_t::boolean) << '\\n';\n    std::cout << (t_number_integer == json::value_t::number_integer) << '\\n';\n    std::cout << (t_number_unsigned == json::value_t::number_unsigned) << '\\n';\n    std::cout << (t_number_float == json::value_t::number_float) << '\\n';\n    std::cout << (t_object == json::value_t::object) << '\\n';\n    std::cout << (t_array == json::value_t::array) << '\\n';\n    std::cout << (t_string == json::value_t::string) << '\\n';\n}\n

        Output:

        true\ntrue\ntrue\ntrue\ntrue\ntrue\ntrue\ntrue\n
        "},{"location":"api/basic_json/operator_value_t/#see-also","title":"See also","text":"
        • type named member function equivalent to this implicit conversion operator
        • value_t the enumeration of JSON types
        "},{"location":"api/basic_json/operator_value_t/#version-history","title":"Version history","text":"
        • Added in version 1.0.0.
        • Added unsigned integer type in version 2.0.0.
        • Added binary type in version 3.8.0.
        "},{"location":"api/basic_json/other_error/","title":"nlohmann::basic_json::other_error","text":"
        class other_error : public exception;\n

        This exception is thrown in case of errors that cannot be classified with the other exception types.

        Exceptions have ids 5xx (see list of other errors).

        classDiagram\n  direction LR\n\n    class std_exception [\"std::exception\"] {\n        <<interface>>\n    }\n\n    class json_exception [\"basic_json::exception\"] {\n        +const int id\n        +const char* what() const\n    }\n\n    class json_parse_error [\"basic_json::parse_error\"] {\n        +const std::size_t byte\n    }\n\n    class json_invalid_iterator [\"basic_json::invalid_iterator\"]\n    class json_type_error [\"basic_json::type_error\"]\n    class json_out_of_range [\"basic_json::out_of_range\"]\n    class json_other_error [\"basic_json::other_error\"]\n\n    std_exception <|-- json_exception\n    json_exception <|-- json_parse_error\n    json_exception <|-- json_invalid_iterator\n    json_exception <|-- json_type_error\n    json_exception <|-- json_out_of_range\n    json_exception <|-- json_other_error\n\n    style json_other_error fill:#CCCCFF
        "},{"location":"api/basic_json/other_error/#member-functions","title":"Member functions","text":"
        • what - returns explanatory string
        "},{"location":"api/basic_json/other_error/#member-variables","title":"Member variables","text":"
        • id - the id of the exception
        "},{"location":"api/basic_json/other_error/#examples","title":"Examples","text":"Example

        The following code shows how a other_error exception can be caught.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    try\n    {\n        // executing a failing JSON Patch operation\n        json value = R\"({\n            \"best_biscuit\": {\n                \"name\": \"Oreo\"\n            }\n        })\"_json;\n        json patch = R\"([{\n            \"op\": \"test\",\n            \"path\": \"/best_biscuit/name\",\n            \"value\": \"Choco Leibniz\"\n        }])\"_json;\n        value.patch(patch);\n    }\n    catch (const json::other_error& e)\n    {\n        // output exception information\n        std::cout << \"message: \" << e.what() << '\\n'\n                  << \"exception id: \" << e.id << std::endl;\n    }\n}\n

        Output:

        message: [json.exception.other_error.501] unsuccessful: {\"op\":\"test\",\"path\":\"/best_biscuit/name\",\"value\":\"Choco Leibniz\"}\nexception id: 501\n
        "},{"location":"api/basic_json/other_error/#see-also","title":"See also","text":"
        • exception for the base class of all exceptions thrown by the library
        • List of other errors
        • parse_error for exceptions indicating a parse error
        • invalid_iterator for exceptions indicating errors with iterators
        • type_error for exceptions indicating executing a member function with a wrong type
        • out_of_range for exceptions indicating access out of the defined range
        "},{"location":"api/basic_json/other_error/#version-history","title":"Version history","text":"
        • Since version 3.0.0.
        "},{"location":"api/basic_json/out_of_range/","title":"nlohmann::basic_json::out_of_range","text":"
        class out_of_range : public exception;\n

        This exception is thrown in case a library function is called on an input parameter that exceeds the expected range, for instance, in the case of array indices or nonexisting object keys.

        Exceptions have ids 4xx (see list of out-of-range errors).

        classDiagram\n  direction LR\n\n    class std_exception [\"std::exception\"] {\n        <<interface>>\n    }\n\n    class json_exception [\"basic_json::exception\"] {\n        +const int id\n        +const char* what() const\n    }\n\n    class json_parse_error [\"basic_json::parse_error\"] {\n        +const std::size_t byte\n    }\n\n    class json_invalid_iterator [\"basic_json::invalid_iterator\"]\n    class json_type_error [\"basic_json::type_error\"]\n    class json_out_of_range [\"basic_json::out_of_range\"]\n    class json_other_error [\"basic_json::other_error\"]\n\n    std_exception <|-- json_exception\n    json_exception <|-- json_parse_error\n    json_exception <|-- json_invalid_iterator\n    json_exception <|-- json_type_error\n    json_exception <|-- json_out_of_range\n    json_exception <|-- json_other_error\n\n    style json_out_of_range fill:#CCCCFF
        "},{"location":"api/basic_json/out_of_range/#member-functions","title":"Member functions","text":"
        • what - returns explanatory string
        "},{"location":"api/basic_json/out_of_range/#member-variables","title":"Member variables","text":"
        • id - the id of the exception
        "},{"location":"api/basic_json/out_of_range/#examples","title":"Examples","text":"Example

        The following code shows how a out_of_range exception can be caught.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    try\n    {\n        // calling at() for an invalid index\n        json j = {1, 2, 3, 4};\n        j.at(4) = 10;\n    }\n    catch (const json::out_of_range& e)\n    {\n        // output exception information\n        std::cout << \"message: \" << e.what() << '\\n'\n                  << \"exception id: \" << e.id << std::endl;\n    }\n}\n

        Output:

        message: [json.exception.out_of_range.401] array index 4 is out of range\nexception id: 401\n
        "},{"location":"api/basic_json/out_of_range/#see-also","title":"See also","text":"
        • exception for the base class of all exceptions thrown by the library
        • List of out-of-range errors
        • parse_error for exceptions indicating a parse error
        • invalid_iterator for exceptions indicating errors with iterators
        • type_error for exceptions indicating executing a member function with a wrong type
        • other_error for exceptions indicating other library errors
        "},{"location":"api/basic_json/out_of_range/#version-history","title":"Version history","text":"
        • Since version 3.0.0.
        "},{"location":"api/basic_json/parse/","title":"nlohmann::basic_json::parse","text":"
        // (1)\ntemplate<typename InputType>\nstatic basic_json parse(InputType&& i,\n                        const parser_callback_t cb = nullptr,\n                        const bool allow_exceptions = true,\n                        const bool ignore_comments = false,\n                        const bool ignore_trailing_commas = false);\n\n// (2)\ntemplate<typename IteratorType, typename SentinelType = IteratorType>\nstatic basic_json parse(IteratorType first, SentinelType last,\n                        const parser_callback_t cb = nullptr,\n                        const bool allow_exceptions = true,\n                        const bool ignore_comments = false,\n                        const bool ignore_trailing_commas = false);\n
        1. Deserialize from a compatible input.

        2. Deserialize from a pair of character iterators, or an iterator and a sentinel of a different type (C++20 ranges support)

          The value_type of the iterator must be an integral type with size of 1, 2, or 4 bytes, which will be interpreted respectively as UTF-8, UTF-16, and UTF-32. If SentinelType differs from IteratorType, it must be comparable to the iterator type with operator!=.

        "},{"location":"api/basic_json/parse/#template-parameters","title":"Template parameters","text":"InputType

        A compatible input, for instance:

        • an std::istream object
        • a FILE pointer (throws if null)
        • a C-style array of characters
        • a pointer to a null-terminated string of single byte characters (throws if null)
        • a std::string
        • a container obj for which begin(obj) and end(obj) produce a valid pair of iterators (as found via ADL or member functions, with semantics compatible to std::begin and std::end)
        IteratorType

        a compatible iterator type, for instance.

        • a pair of std::string::iterator or std::vector<std::uint8_t>::iterator
        • a pair of pointers such as ptr and ptr + len
        SentinelType

        defaults to IteratorType; may be a different type comparable to IteratorType via operator!=, for instance.

        • a custom sentinel type for C++20 ranges
        • std::default_sentinel_t, when IteratorType is std::counted_iterator
        "},{"location":"api/basic_json/parse/#parameters","title":"Parameters","text":"i (in) Input to parse from. cb (in) a parser callback function of type parser_callback_t which is used to control the deserialization by filtering unwanted values (optional) allow_exceptions (in) whether to throw exceptions in case of a parse error (optional, true by default) ignore_comments (in) whether comments should be ignored and treated like whitespace (true) or yield a parse error (false); (optional, false by default) ignore_trailing_commas (in) whether trailing commas in arrays or objects should be ignored and treated like whitespace (true) or yield a parse error (false); (optional, false by default) first (in) iterator to the start of a character range last (in) iterator to the end of a character range, or a sentinel value that compares equal to the end iterator with operator!="},{"location":"api/basic_json/parse/#return-value","title":"Return value","text":"

        Deserialized JSON value; in case of a parse error and allow_exceptions set to false, the return value will be value_t::discarded. The latter can be checked with is_discarded.

        "},{"location":"api/basic_json/parse/#exception-safety","title":"Exception safety","text":"

        Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

        "},{"location":"api/basic_json/parse/#exceptions","title":"Exceptions","text":"
        • Throws parse_error.101 in case of an unexpected token, or empty input like a null FILE* or char* pointer, or an std::istream without a stream buffer (i.rdbuf() == nullptr, for instance std::istream(nullptr)).
        • If reading from an std::istream reaches the end of the input and eofbit is part of the stream's exceptions() mask, the std::ios_base::failure thrown by the stream itself propagates instead of a parse_error, the same as it would for the standard library's own extraction operators.
        "},{"location":"api/basic_json/parse/#complexity","title":"Complexity","text":"

        Linear in the length of the input. The parser is a predictive LL(1) parser. The complexity can be higher if the parser callback function cb or reading from (1) the input i or (2) the iterator range [first, last] has a super-linear complexity.

        "},{"location":"api/basic_json/parse/#notes","title":"Notes","text":"

        A UTF-8 byte order mark is silently ignored.

        Invalid Unicode escapes and unpaired surrogates in the input are reported as parse_error.101 with a detailed message.

        By default, a '\\0' (NUL) byte anywhere in the input is treated as end of input, rather than as an ordinary (and, outside of a string, invalid) byte; see the FAQ entry for details and the JSON_STRICT_NUL_HANDLING macro to opt into rejecting it instead.

        Performance

        Contiguous inputs (strings, string views, vectors, arrays, pointer ranges) are the fastest to parse. A std::istream comes close: the parser scans the characters its stream buffer holds in bulk, without reading ahead, so the stream is left exactly where it would be after reading one character at a time. A FILE* is read one character at a time, but the file is locked only once, so other threads that use the same FILE object wait until parsing is done. See input sources and performance, JSON_DISABLE_STREAM_BULK_SCAN, and JSON_DISABLE_UNLOCKED_FILE_IO.

        "},{"location":"api/basic_json/parse/#examples","title":"Examples","text":"Example: (1) parse from a character array

        The example below demonstrates the parse() function reading from an array.

        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // a JSON text\n    char text[] = R\"(\n    {\n        \"Image\": {\n            \"Width\":  800,\n            \"Height\": 600,\n            \"Title\":  \"View from 15th Floor\",\n            \"Thumbnail\": {\n                \"Url\":    \"http://www.example.com/image/481989943\",\n                \"Height\": 125,\n                \"Width\":  100\n            },\n            \"Animated\" : false,\n            \"IDs\": [116, 943, 234, 38793]\n        }\n    }\n    )\";\n\n    // parse and serialize JSON\n    json j_complete = json::parse(text);\n    std::cout << std::setw(4) << j_complete << \"\\n\\n\";\n}\n

        Output:

        {\n    \"Image\": {\n        \"Animated\": false,\n        \"Height\": 600,\n        \"IDs\": [\n            116,\n            943,\n            234,\n            38793\n        ],\n        \"Thumbnail\": {\n            \"Height\": 125,\n            \"Url\": \"http://www.example.com/image/481989943\",\n            \"Width\": 100\n        },\n        \"Title\": \"View from 15th Floor\",\n        \"Width\": 800\n    }\n}\n
        Example: (1) parse from a string

        The example below demonstrates the parse() function with and without callback function.

        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // a JSON text\n    auto text = R\"(\n    {\n        \"Image\": {\n            \"Width\":  800,\n            \"Height\": 600,\n            \"Title\":  \"View from 15th Floor\",\n            \"Thumbnail\": {\n                \"Url\":    \"http://www.example.com/image/481989943\",\n                \"Height\": 125,\n                \"Width\":  100\n            },\n            \"Animated\" : false,\n            \"IDs\": [116, 943, 234, 38793]\n        }\n    }\n    )\";\n\n    // parse and serialize JSON\n    json j_complete = json::parse(text);\n    std::cout << std::setw(4) << j_complete << \"\\n\\n\";\n\n    // define parser callback\n    json::parser_callback_t cb = [](int depth, json::parse_event_t event, json & parsed)\n    {\n        // skip object elements with key \"Thumbnail\"\n        if (event == json::parse_event_t::key and parsed == json(\"Thumbnail\"))\n        {\n            return false;\n        }\n        else\n        {\n            return true;\n        }\n    };\n\n    // parse (with callback) and serialize JSON\n    json j_filtered = json::parse(text, cb);\n    std::cout << std::setw(4) << j_filtered << '\\n';\n}\n

        Output:

        {\n    \"Image\": {\n        \"Animated\": false,\n        \"Height\": 600,\n        \"IDs\": [\n            116,\n            943,\n            234,\n            38793\n        ],\n        \"Thumbnail\": {\n            \"Height\": 125,\n            \"Url\": \"http://www.example.com/image/481989943\",\n            \"Width\": 100\n        },\n        \"Title\": \"View from 15th Floor\",\n        \"Width\": 800\n    }\n}\n\n{\n    \"Image\": {\n        \"Animated\": false,\n        \"Height\": 600,\n        \"IDs\": [\n            116,\n            943,\n            234,\n            38793\n        ],\n        \"Title\": \"View from 15th Floor\",\n        \"Width\": 800\n    }\n}\n
        Example: (1) parse from an input stream

        The example below demonstrates the parse() function with and without callback function.

        #include <iostream>\n#include <iomanip>\n#include <sstream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // a JSON text\n    auto text = R\"(\n    {\n        \"Image\": {\n            \"Width\":  800,\n            \"Height\": 600,\n            \"Title\":  \"View from 15th Floor\",\n            \"Thumbnail\": {\n                \"Url\":    \"http://www.example.com/image/481989943\",\n                \"Height\": 125,\n                \"Width\":  100\n            },\n            \"Animated\" : false,\n            \"IDs\": [116, 943, 234, 38793]\n        }\n    }\n    )\";\n\n    // fill a stream with JSON text\n    std::stringstream ss;\n    ss << text;\n\n    // parse and serialize JSON\n    json j_complete = json::parse(ss);\n    std::cout << std::setw(4) << j_complete << \"\\n\\n\";\n\n    // define parser callback\n    json::parser_callback_t cb = [](int depth, json::parse_event_t event, json & parsed)\n    {\n        // skip object elements with key \"Thumbnail\"\n        if (event == json::parse_event_t::key and parsed == json(\"Thumbnail\"))\n        {\n            return false;\n        }\n        else\n        {\n            return true;\n        }\n    };\n\n    // fill a stream with JSON text\n    ss.clear();\n    ss << text;\n\n    // parse (with callback) and serialize JSON\n    json j_filtered = json::parse(ss, cb);\n    std::cout << std::setw(4) << j_filtered << '\\n';\n}\n

        Output:

        {\n    \"Image\": {\n        \"Animated\": false,\n        \"Height\": 600,\n        \"IDs\": [\n            116,\n            943,\n            234,\n            38793\n        ],\n        \"Thumbnail\": {\n            \"Height\": 125,\n            \"Url\": \"http://www.example.com/image/481989943\",\n            \"Width\": 100\n        },\n        \"Title\": \"View from 15th Floor\",\n        \"Width\": 800\n    }\n}\n\n{\n    \"Image\": {\n        \"Animated\": false,\n        \"Height\": 600,\n        \"IDs\": [\n            116,\n            943,\n            234,\n            38793\n        ],\n        \"Title\": \"View from 15th Floor\",\n        \"Width\": 800\n    }\n}\n
        Example: (1) parse from a contiguous container

        The example below demonstrates the parse() function reading from a contiguous container.

        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // a JSON text given as std::vector\n    std::vector<std::uint8_t> text = {'[', '1', ',', '2', ',', '3', ']', '\\0'};\n\n    // parse and serialize JSON\n    json j_complete = json::parse(text);\n    std::cout << std::setw(4) << j_complete << \"\\n\\n\";\n}\n

        Output:

        [\n    1,\n    2,\n    3\n]\n
        Example: (2) parse from a non-null-terminated string

        The example below demonstrates the parse() function reading from a string that is not null-terminated.

        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // a JSON text given as string that is not null-terminated\n    const char* ptr = \"[1,2,3]another value\";\n\n    // parse and serialize JSON\n    json j_complete = json::parse(ptr, ptr + 7);\n    std::cout << std::setw(4) << j_complete << \"\\n\\n\";\n}\n

        Output:

        [\n    1,\n    2,\n    3\n]\n
        Example: (2) parse from an iterator pair

        The example below demonstrates the parse() function reading from an iterator pair.

        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // a JSON text given an input with other values\n    std::vector<std::uint8_t> input = {'[', '1', ',', '2', ',', '3', ']', 'o', 't', 'h', 'e', 'r'};\n\n    // parse and serialize JSON\n    json j_complete = json::parse(input.begin(), input.begin() + 7);\n    std::cout << std::setw(4) << j_complete << \"\\n\\n\";\n}\n

        Output:

        [\n    1,\n    2,\n    3\n]\n
        Example: effect of allow_exceptions parameter

        The example below demonstrates the effect of the allow_exceptions parameter in the parse() function.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // an invalid JSON text\n    std::string text = R\"(\n    {\n        \"key\": \"value without closing quotes\n    }\n    )\";\n\n    // parse with exceptions\n    try\n    {\n        json j = json::parse(text);\n    }\n    catch (const json::parse_error& e)\n    {\n        std::cout << e.what() << std::endl;\n    }\n\n    // parse without exceptions\n    json j = json::parse(text, nullptr, false);\n\n    if (j.is_discarded())\n    {\n        std::cout << \"the input is invalid JSON\" << std::endl;\n    }\n    else\n    {\n        std::cout << \"the input is valid JSON: \" << j << std::endl;\n    }\n}\n

        Output:

        [json.exception.parse_error.101] parse error at line 4, column 0: syntax error while parsing value - invalid string: control character U+000A (LF) must be escaped to \\u000A or \\n; last read: '\"value without closing quotes<U+000A>'\nthe input is invalid JSON\n
        Example: effect of ignore_comments parameter

        The example below demonstrates the effect of the ignore_comments parameter in the parse() function.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::string s = R\"(\n    {\n        // update in 2006: removed Pluto\n        \"planets\": [\"Mercury\", \"Venus\", \"Earth\", \"Mars\",\n                    \"Jupiter\", \"Uranus\", \"Neptune\" /*, \"Pluto\" */]\n    }\n    )\";\n\n    try\n    {\n        json j = json::parse(s);\n    }\n    catch (json::exception& e)\n    {\n        std::cout << e.what() << std::endl;\n    }\n\n    json j = json::parse(s,\n                         /* callback */ nullptr,\n                         /* allow exceptions */ true,\n                         /* ignore_comments */ true);\n    std::cout << j.dump(2) << '\\n';\n}\n

        Output:

        [json.exception.parse_error.101] parse error at line 3, column 9: syntax error while parsing object key - invalid literal; last read: '<U+000A>    {<U+000A>        /'; expected string literal\n{\n  \"planets\": [\n    \"Mercury\",\n    \"Venus\",\n    \"Earth\",\n    \"Mars\",\n    \"Jupiter\",\n    \"Uranus\",\n    \"Neptune\"\n  ]\n}\n
        Example: effect of ignore_trailing_commas parameter

        The example below demonstrates the effect of the ignore_trailing_commas parameter in the parse() function.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::string s = R\"(\n    {\n        \"planets\": [\n            \"Mercury\",\n            \"Venus\",\n            \"Earth\",\n            \"Mars\",\n            \"Jupiter\",\n            \"Uranus\",\n            \"Neptune\",\n        ]\n    }\n    )\";\n\n    try\n    {\n        json j = json::parse(s);\n    }\n    catch (json::exception& e)\n    {\n        std::cout << e.what() << std::endl;\n    }\n\n    json j = json::parse(s,\n                         /* callback */ nullptr,\n                         /* allow exceptions */ true,\n                         /* ignore_comments */ false,\n                         /* ignore_trailing_commas */ true);\n    std::cout << j.dump(2) << '\\n';\n}\n

        Output:

        [json.exception.parse_error.101] parse error at line 11, column 9: syntax error while parsing value - unexpected ']'; expected '[', '{', or a literal\n{\n  \"planets\": [\n    \"Mercury\",\n    \"Venus\",\n    \"Earth\",\n    \"Mars\",\n    \"Jupiter\",\n    \"Uranus\",\n    \"Neptune\"\n  ]\n}\n
        "},{"location":"api/basic_json/parse/#see-also","title":"See also","text":"
        • accept - check if the input is valid JSON
        • sax_parse - parse input using the SAX interface
        • operator>> - deserialize from stream
        • JSON_STRICT_NUL_HANDLING - opt in to rejecting a NUL byte in the input instead of treating it as end of input
        • JSON_DISABLE_STREAM_BULK_SCAN - read a std::istream one character at a time
        • JSON_DISABLE_UNLOCKED_FILE_IO - read a FILE* with the locking stdio functions
        "},{"location":"api/basic_json/parse/#version-history","title":"Version history","text":"
        • Added in version 1.0.0.
        • Overload for contiguous containers (1) added in version 2.0.3.
        • Ignoring comments via ignore_comments added in version 3.9.0.
        • Changed runtime assertion in case of FILE* null pointers to exception in version 3.12.0.
        • Added ignore_trailing_commas in version 3.13.0 unreleased.
        • Extended container support (1) to include types with lvalue-only ADL begin/end (matching std::begin/std::end semantics) in version 3.13.0 unreleased.
        • Extended overload (2) to accept heterogeneous iterator+sentinel pairs (C++20 ranges support) in version 3.13.0 unreleased.
        • JSON_STRICT_NUL_HANDLING added in version 3.13.0 unreleased to optionally reject a NUL byte in the input instead of treating it as end of input; planned to become the default in version 4.0.0.
        • Extended empty-input detection to also cover an std::istream without a stream buffer, and fixed a crash (std::terminate) when parsing from an std::istream with eofbit in its exception mask, in version 3.13.0 unreleased.
        • Parsing from an std::istream scans the characters in its stream buffer in bulk in version 3.13.0 unreleased (JSON_DISABLE_STREAM_BULK_SCAN restores reading one character at a time).
        • Parsing from a FILE* locks the file once instead of once per character in version 3.13.0 unreleased (JSON_DISABLE_UNLOCKED_FILE_IO restores std::fgetc).

        Deprecation

        Overload (2) replaces calls to parse with a pair of iterators as their first parameter which has been deprecated in version 3.8.0. This overload will be removed in version 4.0.0. Please replace all calls like parse({ptr, ptr+len}, ...); with parse(ptr, ptr+len, ...);.

        You should be warned by your compiler with a -Wdeprecated-declarations warning if you are using a deprecated function.

        See the migration guide for how to update existing code.

        "},{"location":"api/basic_json/parse_error/","title":"nlohmann::basic_json::parse_error","text":"
        class parse_error : public exception;\n

        The library throws this exception when a parse error occurs. Parse errors can occur during the deserialization of JSON text, BJData, BON8, BSON, CBOR, MessagePack, UBJSON, as well as when using JSON Patch.

        Member byte holds the byte index of the last read character in the input file (see note below).

        Exceptions have ids 1xx (see list of parse errors).

        classDiagram\n  direction LR\n\n    class std_exception [\"std::exception\"] {\n        <<interface>>\n    }\n\n    class json_exception [\"basic_json::exception\"] {\n        +const int id\n        +const char* what() const\n    }\n\n    class json_parse_error [\"basic_json::parse_error\"] {\n        +const std::size_t byte\n    }\n\n    class json_invalid_iterator [\"basic_json::invalid_iterator\"]\n    class json_type_error [\"basic_json::type_error\"]\n    class json_out_of_range [\"basic_json::out_of_range\"]\n    class json_other_error [\"basic_json::other_error\"]\n\n    std_exception <|-- json_exception\n    json_exception <|-- json_parse_error\n    json_exception <|-- json_invalid_iterator\n    json_exception <|-- json_type_error\n    json_exception <|-- json_out_of_range\n    json_exception <|-- json_other_error\n\n    style json_parse_error fill:#CCCCFF
        "},{"location":"api/basic_json/parse_error/#member-functions","title":"Member functions","text":"
        • what - returns explanatory string
        "},{"location":"api/basic_json/parse_error/#member-variables","title":"Member variables","text":"
        • id - the id of the exception
        • byte - byte index of the parse error
        "},{"location":"api/basic_json/parse_error/#notes","title":"Notes","text":"

        For an input with n bytes, 1 is the index of the first character and n+1 is the index of the terminating null byte or the end of file. This also holds true when reading a byte vector for binary formats.

        "},{"location":"api/basic_json/parse_error/#examples","title":"Examples","text":"Example

        The following code shows how a parse_error exception can be caught.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    try\n    {\n        // parsing input with a syntax error\n        json j = json::parse(\"[1,2,3,]\");\n    }\n    catch (const json::parse_error& e)\n    {\n        // output exception information\n        std::cout << \"message: \" << e.what() << '\\n'\n                  << \"exception id: \" << e.id << '\\n'\n                  << \"byte position of error: \" << e.byte << std::endl;\n    }\n}\n

        Output:

        message: [json.exception.parse_error.101] parse error at line 1, column 8: syntax error while parsing value - unexpected ']'; expected '[', '{', or a literal\nexception id: 101\nbyte position of error: 8\n
        "},{"location":"api/basic_json/parse_error/#see-also","title":"See also","text":"
        • exception for the base class of all exceptions thrown by the library
        • List of parse errors
        • invalid_iterator for exceptions indicating errors with iterators
        • type_error for exceptions indicating executing a member function with a wrong type
        • out_of_range for exceptions indicating access out of the defined range
        • other_error for exceptions indicating other library errors
        "},{"location":"api/basic_json/parse_error/#version-history","title":"Version history","text":"
        • Since version 3.0.0.
        "},{"location":"api/basic_json/parse_event_t/","title":"nlohmann::basic_json::parse_event_t","text":"
        enum class parse_event_t : std::uint8_t {\n    object_start,\n    object_end,\n    array_start,\n    array_end,\n    key,\n    value\n};\n

        The parser callback distinguishes the following events:

        • object_start: the parser read { and started to process a JSON object
        • key: the parser read a key of a value in an object
        • object_end: the parser read } and finished processing a JSON object
        • array_start: the parser read [ and started to process a JSON array
        • array_end: the parser read ] and finished processing a JSON array
        • value: the parser finished reading a JSON value
        "},{"location":"api/basic_json/parse_event_t/#examples","title":"Examples","text":"Example

        The following code parses a small JSON text with a parser callback that reports every event together with its depth and keeps every value (by always returning true).

        #include <iostream>\n#include <string>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\n// translate a parse_event_t to a human-readable name\nstd::string event_name(json::parse_event_t event)\n{\n    switch (event)\n    {\n        case json::parse_event_t::object_start:\n            return \"object_start\";\n        case json::parse_event_t::object_end:\n            return \"object_end\";\n        case json::parse_event_t::array_start:\n            return \"array_start\";\n        case json::parse_event_t::array_end:\n            return \"array_end\";\n        case json::parse_event_t::key:\n            return \"key\";\n        case json::parse_event_t::value:\n            return \"value\";\n        default:\n            return \"unknown\";\n    }\n}\n\nint main()\n{\n    // a small JSON text\n    auto text = R\"({\"pi\": 3.141, \"numbers\": [1, 2]})\";\n\n    // parse the text and report every event together with its depth;\n    // returning true keeps every value unchanged\n    json j = json::parse(text, [](int depth, json::parse_event_t event, json& /*parsed*/)\n    {\n        std::cout << depth << \" \" << event_name(event) << '\\n';\n        return true;\n    });\n\n    // the callback did not change anything, so the parsed value is unaffected\n    std::cout << j << '\\n';\n}\n

        Output:

        0 object_start\n1 key\n1 value\n1 key\n1 array_start\n2 value\n2 value\n1 array_end\n0 object_end\n{\"numbers\":[1,2],\"pi\":3.141}\n
        "},{"location":"api/basic_json/parse_event_t/#see-also","title":"See also","text":"
        • parser_callback_t callback function type for the parser
        • parse deserialize from a compatible input
        "},{"location":"api/basic_json/parse_event_t/#version-history","title":"Version history","text":"
        • Added in version 1.0.0.
        "},{"location":"api/basic_json/parser_callback_t/","title":"nlohmann::basic_json::parser_callback_t","text":"
        template<typename BasicJsonType>\nusing parser_callback_t =\n    std::function<bool(int depth, parse_event_t event, BasicJsonType& parsed)>;\n

        With a parser callback function, the result of parsing a JSON text can be influenced. When passed to parse, it is called on certain events (passed as parse_event_t via parameter event) with a set recursion depth depth and context JSON value parsed. The return value of the callback function is a boolean indicating whether the element that emitted the callback shall be kept or not.

        We distinguish six scenarios (determined by the event type) in which the callback function can be called. The following table describes the values of the parameters depth, event, and parsed.

        parameter event description parameter depth parameter parsed parse_event_t::object_start the parser read { and started to process a JSON object depth of the parent of the JSON object a JSON value with type discarded parse_event_t::key the parser read a key of a value in an object depth of the currently parsed JSON object a JSON string containing the key parse_event_t::object_end the parser read } and finished processing a JSON object depth of the parent of the JSON object the parsed JSON object parse_event_t::array_start the parser read [ and started to process a JSON array depth of the parent of the JSON array a JSON value with type discarded parse_event_t::array_end the parser read ] and finished processing a JSON array depth of the parent of the JSON array the parsed JSON array parse_event_t::value the parser finished reading a JSON value depth of the value the parsed JSON value

        Discarding a value (i.e., returning false) has different effects depending on the context in which function was called:

        • Discarded values in structured types are skipped. That is, the parser will behave as if the discarded value was never read. This holds for every value type and for both kinds of parent: a discarded element is removed from the surrounding array, and a discarded member is removed from the surrounding object together with its key.
        • Arrays and objects can be discarded either at their parse_event_t::array_start/parse_event_t::object_start event or at their parse_event_t::array_end/parse_event_t::object_end event, and both remove the whole value. Discarding it at the start event also means the callback is called neither for the content of the value nor for its matching end event.
        • Discarding a parse_event_t::key event discards the whole object member. The callback is still called for the associated value, but its return value has no further effect.
        • In case a value outside a structured type is skipped, it is replaced with null. This case happens if the top-level element is skipped.
        "},{"location":"api/basic_json/parser_callback_t/#parameters","title":"Parameters","text":"depth (in) the depth of the recursion during parsing event (in) an event of type parse_event_t indicating the context in the callback function has been called parsed (in, out) the current intermediate parse result; note that writing to this value has no effect for parse_event_t::key events"},{"location":"api/basic_json/parser_callback_t/#return-value","title":"Return value","text":"

        Whether the JSON value which called the function during parsing should be kept (true) or not (false). In the latter case, it is skipped completely, or replaced by null if it is the top-level value.

        "},{"location":"api/basic_json/parser_callback_t/#examples","title":"Examples","text":"Example: skip an object key while parsing

        The example below demonstrates the parse() function with and without callback function.

        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // a JSON text\n    auto text = R\"(\n    {\n        \"Image\": {\n            \"Width\":  800,\n            \"Height\": 600,\n            \"Title\":  \"View from 15th Floor\",\n            \"Thumbnail\": {\n                \"Url\":    \"http://www.example.com/image/481989943\",\n                \"Height\": 125,\n                \"Width\":  100\n            },\n            \"Animated\" : false,\n            \"IDs\": [116, 943, 234, 38793]\n        }\n    }\n    )\";\n\n    // parse and serialize JSON\n    json j_complete = json::parse(text);\n    std::cout << std::setw(4) << j_complete << \"\\n\\n\";\n\n    // define parser callback\n    json::parser_callback_t cb = [](int depth, json::parse_event_t event, json & parsed)\n    {\n        // skip object elements with key \"Thumbnail\"\n        if (event == json::parse_event_t::key and parsed == json(\"Thumbnail\"))\n        {\n            return false;\n        }\n        else\n        {\n            return true;\n        }\n    };\n\n    // parse (with callback) and serialize JSON\n    json j_filtered = json::parse(text, cb);\n    std::cout << std::setw(4) << j_filtered << '\\n';\n}\n

        Output:

        {\n    \"Image\": {\n        \"Animated\": false,\n        \"Height\": 600,\n        \"IDs\": [\n            116,\n            943,\n            234,\n            38793\n        ],\n        \"Thumbnail\": {\n            \"Height\": 125,\n            \"Url\": \"http://www.example.com/image/481989943\",\n            \"Width\": 100\n        },\n        \"Title\": \"View from 15th Floor\",\n        \"Width\": 800\n    }\n}\n\n{\n    \"Image\": {\n        \"Animated\": false,\n        \"Height\": 600,\n        \"IDs\": [\n            116,\n            943,\n            234,\n            38793\n        ],\n        \"Title\": \"View from 15th Floor\",\n        \"Width\": 800\n    }\n}\n
        Example: how discarded values are removed

        The example below shows where discarded values are removed. The array and the number are discarded in different ways, but in each case the parse result contains neither the value nor its key.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // a JSON text with an array and a number inside an object\n    auto text = R\"({\"IDs\": [116, 943], \"Width\": 800})\";\n\n    // discard the array when the parser reads its opening bracket\n    json j_array_start = json::parse(text, [](int /*depth*/, json::parse_event_t event, json& /*parsed*/)\n    {\n        return event != json::parse_event_t::array_start;\n    });\n\n    // discard the same array when the parser reads its closing bracket\n    json j_array_end = json::parse(text, [](int /*depth*/, json::parse_event_t event, json& /*parsed*/)\n    {\n        return event != json::parse_event_t::array_end;\n    });\n\n    // discard the number, but keep its key\n    json j_value = json::parse(text, [](int /*depth*/, json::parse_event_t event, json & parsed)\n    {\n        return !(event == json::parse_event_t::value && parsed == json(800));\n    });\n\n    // discard the key of the number\n    json j_key = json::parse(text, [](int /*depth*/, json::parse_event_t event, json & parsed)\n    {\n        return !(event == json::parse_event_t::key && parsed == json(\"Width\"));\n    });\n\n    // discard the top-level object\n    json j_root = json::parse(text, [](int /*depth*/, json::parse_event_t event, json& /*parsed*/)\n    {\n        return event != json::parse_event_t::object_end;\n    });\n\n    // in every case, the discarded value is removed together with its key\n    std::cout << j_array_start << '\\n'\n              << j_array_end << '\\n'\n              << j_value << '\\n'\n              << j_key << '\\n'\n              << j_root << '\\n';\n}\n

        Output:

        {\"Width\":800}\n{\"Width\":800}\n{\"IDs\":[116,943]}\n{\"IDs\":[116,943]}\nnull\n
        "},{"location":"api/basic_json/parser_callback_t/#see-also","title":"See also","text":"
        • parse deserialize from a compatible input
        • parse_event_t enumeration of parser events
        "},{"location":"api/basic_json/parser_callback_t/#version-history","title":"Version history","text":"
        • Added in version 1.0.0.
        • Fixed in version 3.13.0 unreleased to also remove discarded values from a parent object; before, discarding an array or a value stored under an object key left a discarded member behind, which made the parse result serialize to invalid JSON.
        • Fixed in version 3.13.0 unreleased so that discarding an array or object at its start event also hides its content from the callback, as documented above; before, the callback was still called for the content, and the key of every member of a discarded object was kept in memory until the parse ended.
        "},{"location":"api/basic_json/patch/","title":"nlohmann::basic_json::patch","text":"
        basic_json patch(const basic_json& json_patch) const;\n

        JSON Patch defines a JSON document structure for expressing a sequence of operations to apply to a JSON document. With this function, a JSON Patch is applied to the current JSON value by executing all operations from the patch.

        "},{"location":"api/basic_json/patch/#parameters","title":"Parameters","text":"json_patch (in) JSON patch document"},{"location":"api/basic_json/patch/#return-value","title":"Return value","text":"

        patched document

        "},{"location":"api/basic_json/patch/#exception-safety","title":"Exception safety","text":"

        Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

        "},{"location":"api/basic_json/patch/#exceptions","title":"Exceptions","text":"
        • Throws parse_error.104 if the JSON patch does not consist of an array of objects.
        • Throws parse_error.105 if the JSON patch is malformed (e.g., mandatory attributes are missing); example: \"operation 'add' must have member 'path'\".
        • Throws out_of_range.401 if an array index is out of range.
        • Throws parse_error.106 if an array index in a \"path\" or \"from\" member begins with '0'; example: \"array index '01' must not begin with '0'\".
        • Throws parse_error.107 if a \"path\" or \"from\" member is not empty and does not begin with a slash (/); example: \"JSON pointer must be empty or begin with '/' - was: 'a'\".
        • Throws parse_error.108 if a tilde (~) in a \"path\" or \"from\" member is not followed by 0 or 1; example: \"escape character '~' must be followed with '0' or '1'\".
        • Throws parse_error.109 if an array index in a \"path\" or \"from\" member is not a number; example: \"array index 'foo' is not a number\".
        • Throws out_of_range.402 if the array index - is used where an existing element is required (the \"path\" of \"replace\", the \"from\" of \"move\" and \"copy\"); example: \"array index '-' (3) is out of range\".
        • Throws out_of_range.403 if a JSON pointer inside the patch could not be resolved successfully in the current JSON value; example: \"key baz not found\".
        • Throws out_of_range.404 if a reference token of a JSON pointer inside the patch cannot be resolved, e.g., - in a \"remove\" operation or 1a for an array; example: \"unresolved reference token '-'\".
        • Throws out_of_range.405 if JSON pointer has no parent (\"remove\", \"move\")
        • Throws out_of_range.411 if an \"add\" operation's target location has a parent that is neither an object nor an array.
        • Throws out_of_range.413 if a \"remove\" operation's target location has a parent that is neither an object nor an array.
        • Throws out_of_range.414 if a \"move\" operation's \"from\" location is a proper prefix of its \"path\" location.
        • Throws other_error.501 if \"test\" operation was unsuccessful.
        "},{"location":"api/basic_json/patch/#complexity","title":"Complexity","text":"

        Linear in the size of the JSON value and the length of the JSON patch. As usually the patch affects only a fraction of the JSON value, the complexity can usually be neglected.

        "},{"location":"api/basic_json/patch/#notes","title":"Notes","text":"

        The application of a patch is atomic: Either all operations succeed and the patched document is returned or an exception is thrown. In any case, the original value is not changed: the patch is applied to a copy of the value.

        "},{"location":"api/basic_json/patch/#examples","title":"Examples","text":"Example: apply a JSON patch

        The following code shows how a JSON patch is applied to a value.

        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // the original document\n    json doc = R\"(\n        {\n          \"baz\": \"qux\",\n          \"foo\": \"bar\"\n        }\n    )\"_json;\n\n    // the patch\n    json patch = R\"(\n        [\n          { \"op\": \"replace\", \"path\": \"/baz\", \"value\": \"boo\" },\n          { \"op\": \"add\", \"path\": \"/hello\", \"value\": [\"world\"] },\n          { \"op\": \"remove\", \"path\": \"/foo\"}\n        ]\n    )\"_json;\n\n    // apply the patch\n    json patched_doc = doc.patch(patch);\n\n    // output original and patched document\n    std::cout << std::setw(4) << doc << \"\\n\\n\"\n              << std::setw(4) << patched_doc << std::endl;\n}\n

        Output:

        {\n    \"baz\": \"qux\",\n    \"foo\": \"bar\"\n}\n\n{\n    \"baz\": \"boo\",\n    \"hello\": [\n        \"world\"\n    ]\n}\n
        Example: out_of_range.414 exception

        The following code shows how a \"move\" operation whose \"from\" location is a proper prefix of its \"path\" location is rejected, and how the original document is left unchanged because the patch is applied to a copy.

        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // the original document\n    json doc = R\"(\n        {\n          \"a\": { \"b\": 1 }\n        }\n    )\"_json;\n\n    // a patch that tries to move \"/a\" into one of its own children\n    json patch = R\"(\n        [\n          { \"op\": \"move\", \"from\": \"/a\", \"path\": \"/a/b\" }\n        ]\n    )\"_json;\n\n    // exception out_of_range.414\n    try\n    {\n        json patched_doc = doc.patch(patch);\n    }\n    catch (const json::out_of_range& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n\n    // the original document is unchanged\n    std::cout << std::setw(4) << doc << std::endl;\n}\n

        Output:

        [json.exception.out_of_range.414] cannot move value: 'from' path '/a' is a proper prefix of 'path' '/a/b'\n{\n    \"a\": {\n        \"b\": 1\n    }\n}\n
        "},{"location":"api/basic_json/patch/#see-also","title":"See also","text":"
        • RFC 6902 (JSON Patch)
        • RFC 6901 (JSON Pointer)
        • patch_inplace applies a JSON Patch without creating a copy of the document
        • merge_patch applies a JSON Merge Patch
        "},{"location":"api/basic_json/patch/#version-history","title":"Version history","text":"
        • Added in version 2.0.0.
        • Added out_of_range.411 and stopped relying on an internal assertion when an \"add\" operation's target location has a non-object/non-array parent in version 3.13.0 unreleased.
        • Added out_of_range.413 and stopped silently ignoring a \"remove\" operation whose target location has a non-object/non-array parent in version 3.13.0 unreleased.
        • Added out_of_range.414 and rejected a \"move\" operation whose \"from\" location is a proper prefix of its \"path\" location instead of silently producing a corrupted result in version 3.13.0 unreleased.
        • Throws parse_error.109 instead of out_of_range.404 for a one-character array index that is not a digit (e.g., /x) in version 3.13.0 unreleased, as it already did for longer ones.
        "},{"location":"api/basic_json/patch_inplace/","title":"nlohmann::basic_json::patch_inplace","text":"
        void patch_inplace(const basic_json& json_patch);\n

        JSON Patch defines a JSON document structure for expressing a sequence of operations to apply to a JSON document. With this function, a JSON Patch is applied to the current JSON value by executing all operations from the patch. This function applies a JSON patch in place and returns void.

        "},{"location":"api/basic_json/patch_inplace/#parameters","title":"Parameters","text":"json_patch (in) JSON patch document"},{"location":"api/basic_json/patch_inplace/#exception-safety","title":"Exception safety","text":"

        No guarantees, value may be corrupted by an unsuccessful patch operation.

        "},{"location":"api/basic_json/patch_inplace/#exceptions","title":"Exceptions","text":"
        • Throws parse_error.104 if the JSON patch does not consist of an array of objects.
        • Throws parse_error.105 if the JSON patch is malformed (e.g., mandatory attributes are missing); example: \"operation 'add' must have member 'path'\".
        • Throws out_of_range.401 if an array index is out of range.
        • Throws parse_error.106 if an array index in a \"path\" or \"from\" member begins with '0'; example: \"array index '01' must not begin with '0'\".
        • Throws parse_error.107 if a \"path\" or \"from\" member is not empty and does not begin with a slash (/); example: \"JSON pointer must be empty or begin with '/' - was: 'a'\".
        • Throws parse_error.108 if a tilde (~) in a \"path\" or \"from\" member is not followed by 0 or 1; example: \"escape character '~' must be followed with '0' or '1'\".
        • Throws parse_error.109 if an array index in a \"path\" or \"from\" member is not a number; example: \"array index 'foo' is not a number\".
        • Throws out_of_range.402 if the array index - is used where an existing element is required (the \"path\" of \"replace\", the \"from\" of \"move\" and \"copy\"); example: \"array index '-' (3) is out of range\".
        • Throws out_of_range.403 if a JSON pointer inside the patch could not be resolved successfully in the current JSON value; example: \"key baz not found\".
        • Throws out_of_range.404 if a reference token of a JSON pointer inside the patch cannot be resolved, e.g., - in a \"remove\" operation or 1a for an array; example: \"unresolved reference token '-'\".
        • Throws out_of_range.405 if JSON pointer has no parent (\"remove\", \"move\")
        • Throws out_of_range.411 if an \"add\" operation's target location has a parent that is neither an object nor an array.
        • Throws out_of_range.413 if a \"remove\" operation's target location has a parent that is neither an object nor an array.
        • Throws out_of_range.414 if a \"move\" operation's \"from\" location is a proper prefix of its \"path\" location.
        • Throws other_error.501 if \"test\" operation was unsuccessful.
        "},{"location":"api/basic_json/patch_inplace/#complexity","title":"Complexity","text":"

        Linear in the size of the JSON value and the length of the JSON patch. As usually the patch affects only a fraction of the JSON value, the complexity can usually be neglected.

        "},{"location":"api/basic_json/patch_inplace/#notes","title":"Notes","text":"

        Unlike patch, patch_inplace applies the operation \"in place\" and no copy of the JSON value is created. That makes it faster for large documents by avoiding the copy. However, the JSON value might be corrupted if the function throws an exception.

        "},{"location":"api/basic_json/patch_inplace/#examples","title":"Examples","text":"Example: apply a JSON patch in place

        The following code shows how a JSON patch is applied to a value.

        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // the original document\n    json doc = R\"(\n        {\n          \"baz\": \"qux\",\n          \"foo\": \"bar\"\n        }\n    )\"_json;\n\n    // the patch\n    json patch = R\"(\n        [\n          { \"op\": \"replace\", \"path\": \"/baz\", \"value\": \"boo\" },\n          { \"op\": \"add\", \"path\": \"/hello\", \"value\": [\"world\"] },\n          { \"op\": \"remove\", \"path\": \"/foo\"}\n        ]\n    )\"_json;\n\n    // output original document\n    std::cout << \"Before\\n\" << std::setw(4) << doc << std::endl;\n\n    // apply the patch\n    doc.patch_inplace(patch);\n\n    // output patched document\n    std::cout << \"\\nAfter\\n\" << std::setw(4) << doc << std::endl;\n}\n

        Output:

        Before\n{\n    \"baz\": \"qux\",\n    \"foo\": \"bar\"\n}\n\nAfter\n{\n    \"baz\": \"boo\",\n    \"hello\": [\n        \"world\"\n    ]\n}\n
        Example: out_of_range.403 exception with a partially applied patch

        The following code shows a patch whose first operation succeeds and whose second operation fails. Because patch_inplace applies each operation directly to the value, the first operation's effect is still visible after the exception is caught, unlike patch.

        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // the original document\n    json doc = R\"(\n        {\n          \"a\": 1,\n          \"b\": 2\n        }\n    )\"_json;\n\n    // a patch whose second operation fails\n    json patch = R\"(\n        [\n          { \"op\": \"replace\", \"path\": \"/a\", \"value\": 99 },\n          { \"op\": \"remove\", \"path\": \"/nonexistent\" }\n        ]\n    )\"_json;\n\n    // exception out_of_range.403\n    try\n    {\n        doc.patch_inplace(patch);\n    }\n    catch (const json::out_of_range& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n\n    // the first operation has already been applied to doc\n    std::cout << std::setw(4) << doc << std::endl;\n}\n

        Output:

        [json.exception.out_of_range.403] key 'nonexistent' not found\n{\n    \"a\": 99,\n    \"b\": 2\n}\n
        "},{"location":"api/basic_json/patch_inplace/#see-also","title":"See also","text":"
        • RFC 6902 (JSON Patch)
        • RFC 6901 (JSON Pointer)
        • patch applies a JSON Patch
        • merge_patch applies a JSON Merge Patch
        "},{"location":"api/basic_json/patch_inplace/#version-history","title":"Version history","text":"
        • Added in version 3.11.0.
        • Added out_of_range.411 and stopped relying on an internal assertion when an \"add\" operation's target location has a non-object/non-array parent in version 3.13.0 unreleased.
        • Added out_of_range.413 and stopped silently ignoring a \"remove\" operation whose target location has a non-object/non-array parent in version 3.13.0 unreleased.
        • Added out_of_range.414 and rejected a \"move\" operation whose \"from\" location is a proper prefix of its \"path\" location instead of silently producing a corrupted result in version 3.13.0 unreleased.
        • Throws parse_error.109 instead of out_of_range.404 for a one-character array index that is not a digit (e.g., /x) in version 3.13.0 unreleased, as it already did for longer ones.
        "},{"location":"api/basic_json/push_back/","title":"nlohmann::basic_json::push_back","text":"
        // (1)\nvoid push_back(basic_json&& val);\nvoid push_back(const basic_json& val);\n\n// (2)\nvoid push_back(const typename object_t::value_type& val);\n\n// (3)\nvoid push_back(initializer_list_t init);\n

        1. Appends the given element val to the end of the JSON array. If the function is called on a JSON null value, an empty array is created before appending val.

        2. Inserts the given element val to the JSON object. If the function is called on a JSON null value, an empty object is created before inserting val.

        3. This function allows using push_back with an initializer list. In case

          1. the current value is an object,
          2. the initializer list init contains only two elements, and
          3. the first element of init is a string,

          init is converted into an object element and added using push_back(const typename object_t::value_type&). Otherwise, init is converted to a JSON value and added using push_back(basic_json&&).

        "},{"location":"api/basic_json/push_back/#iterator-invalidation","title":"Iterator invalidation","text":"

        For all cases where an element is added to an array, a reallocation can happen, in which case all iterators (including the end() iterator) and all references to the elements are invalidated. Otherwise, only the end() iterator is invalidated.

        For ordered_json, also adding an element to an object can yield a reallocation which again invalidates all iterators and all references.

        "},{"location":"api/basic_json/push_back/#parameters","title":"Parameters","text":"val (in) the value to add to the JSON array/object init (in) an initializer list"},{"location":"api/basic_json/push_back/#exception-safety","title":"Exception safety","text":"

        Strong guarantee: if an exception is thrown, there are no changes to any JSON value. As an exception, a null value is converted to an empty array or object before the element is added and keeps that type if adding the element throws.

        "},{"location":"api/basic_json/push_back/#exceptions","title":"Exceptions","text":"
        1. Throws type_error.308 when called on a type other than JSON array or null; example: \"cannot use push_back() with number\"
        2. Throws type_error.308 when called on a type other than JSON object or null; example: \"cannot use push_back() with number\"
        3. Throws type_error.308 when called on a type other than JSON array or null; example: \"cannot use push_back() with number\"
        "},{"location":"api/basic_json/push_back/#complexity","title":"Complexity","text":"
        1. Amortized constant.
        2. Logarithmic in the size of the container, O(log(size())).
        3. Linear in the size of the initializer list init.
        "},{"location":"api/basic_json/push_back/#notes","title":"Notes","text":"

        (3) This function is required to resolve an ambiguous overload error, because pairs like {\"key\", \"value\"} can be both interpreted as object_t::value_type or std::initializer_list<basic_json>, see #235 for more information.

        "},{"location":"api/basic_json/push_back/#examples","title":"Examples","text":"Example: (1) add element to array

        The example shows how push_back() and += can be used to add elements to a JSON array. Note how the null value was silently converted to a JSON array.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json array = {1, 2, 3, 4, 5};\n    json null;\n\n    // print values\n    std::cout << array << '\\n';\n    std::cout << null << '\\n';\n\n    // add values\n    array.push_back(6);\n    array += 7;\n    null += \"first\";\n    null += \"second\";\n\n    // print values\n    std::cout << array << '\\n';\n    std::cout << null << '\\n';\n}\n

        Output:

        [1,2,3,4,5]\nnull\n[1,2,3,4,5,6,7]\n[\"first\",\"second\"]\n
        Example: (2) add element to object

        The example shows how push_back() and += can be used to add elements to a JSON object. Note how the null value was silently converted to a JSON object.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json object = {{\"one\", 1}, {\"two\", 2}};\n    json null;\n\n    // print values\n    std::cout << object << '\\n';\n    std::cout << null << '\\n';\n\n    // add values\n    object.push_back(json::object_t::value_type(\"three\", 3));\n    object += json::object_t::value_type(\"four\", 4);\n    null += json::object_t::value_type(\"A\", \"a\");\n    null += json::object_t::value_type(\"B\", \"b\");\n\n    // print values\n    std::cout << object << '\\n';\n    std::cout << null << '\\n';\n}\n

        Output:

        {\"one\":1,\"two\":2}\nnull\n{\"four\":4,\"one\":1,\"three\":3,\"two\":2}\n{\"A\":\"a\",\"B\":\"b\"}\n
        Example: (3) add to object from initializer list

        The example shows how initializer lists are treated as objects when possible.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json object = {{\"one\", 1}, {\"two\", 2}};\n    json null;\n\n    // print values\n    std::cout << object << '\\n';\n    std::cout << null << '\\n';\n\n    // add values:\n    object.push_back({\"three\", 3});  // object is extended\n    object += {\"four\", 4};           // object is extended\n    null.push_back({\"five\", 5});     // null is converted to array\n\n    // print values\n    std::cout << object << '\\n';\n    std::cout << null << '\\n';\n\n    // would throw:\n    //object.push_back({1, 2, 3});\n}\n

        Output:

        {\"one\":1,\"two\":2}\nnull\n{\"four\":4,\"one\":1,\"three\":3,\"two\":2}\n[[\"five\",5]]\n
        "},{"location":"api/basic_json/push_back/#see-also","title":"See also","text":"
        • emplace_back add a value to an array
        • operator+= add a value to an array/object
        • Modifying values - the article on modifying values
        "},{"location":"api/basic_json/push_back/#version-history","title":"Version history","text":"
        1. Since version 1.0.0.
        2. Since version 1.0.0.
        3. Since version 2.0.0.
        "},{"location":"api/basic_json/rbegin/","title":"nlohmann::basic_json::rbegin","text":"
        reverse_iterator rbegin() noexcept;\nconst_reverse_iterator rbegin() const noexcept;\n

        Returns an iterator to the reverse-beginning; that is, the last element.

        "},{"location":"api/basic_json/rbegin/#return-value","title":"Return value","text":"

        reverse iterator to the last element

        "},{"location":"api/basic_json/rbegin/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this member function never throws exceptions.

        "},{"location":"api/basic_json/rbegin/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json/rbegin/#examples","title":"Examples","text":"Example

        The following code shows an example for rbegin().

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create an array value\n    json array = {1, 2, 3, 4, 5};\n\n    // get an iterator to the reverse-beginning\n    json::reverse_iterator it = array.rbegin();\n\n    // serialize the element that the iterator points to\n    std::cout << *it << '\\n';\n}\n

        Output:

        5\n
        "},{"location":"api/basic_json/rbegin/#see-also","title":"See also","text":"
        • rend returns a reverse iterator to one before the first element
        • crbegin returns a const reverse iterator to the last element
        • begin returns an iterator to the first element
        • Iterators - the article on iterators
        "},{"location":"api/basic_json/rbegin/#version-history","title":"Version history","text":"
        • Added in version 1.0.0.
        "},{"location":"api/basic_json/rend/","title":"nlohmann::basic_json::rend","text":"
        reverse_iterator rend() noexcept;\nconst_reverse_iterator rend() const noexcept;\n

        Returns an iterator to the reverse-end; that is, one before the first element. This element acts as a placeholder, attempting to access it results in undefined behavior.

        "},{"location":"api/basic_json/rend/#return-value","title":"Return value","text":"

        reverse iterator to the element following the last element

        "},{"location":"api/basic_json/rend/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this member function never throws exceptions.

        "},{"location":"api/basic_json/rend/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json/rend/#examples","title":"Examples","text":"Example

        The following code shows an example for rend().

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create an array value\n    json array = {1, 2, 3, 4, 5};\n\n    // get an iterator to the reverse-end\n    json::reverse_iterator it = array.rend();\n\n    // increment the iterator to point to the first element\n    --it;\n\n    // serialize the element that the iterator points to\n    std::cout << *it << '\\n';\n}\n

        Output:

        1\n
        "},{"location":"api/basic_json/rend/#see-also","title":"See also","text":"
        • rbegin returns a reverse iterator to the last element
        • crend returns a const reverse iterator to one before the first element
        • end returns an iterator to one past the last element
        • Iterators - the article on iterators
        "},{"location":"api/basic_json/rend/#version-history","title":"Version history","text":"
        • Added in version 1.0.0.
        "},{"location":"api/basic_json/sax_parse/","title":"nlohmann::basic_json::sax_parse","text":"
        // (1)\ntemplate <typename InputType, typename SAX>\nstatic bool sax_parse(InputType&& i,\n                      SAX* sax,\n                      input_format_t format = input_format_t::json,\n                      const bool strict = true,\n                      const bool ignore_comments = false,\n                      const bool ignore_trailing_commas = false,\n                      const cbor_tag_handler_t tag_handler = cbor_tag_handler_t::error);\n\n// (2)\ntemplate<class IteratorType, class SAX, class SentinelType = IteratorType>\nstatic bool sax_parse(IteratorType first, SentinelType last,\n                      SAX* sax,\n                      input_format_t format = input_format_t::json,\n                      const bool strict = true,\n                      const bool ignore_comments = false,\n                      const bool ignore_trailing_commas = false,\n                      const cbor_tag_handler_t tag_handler = cbor_tag_handler_t::error);\n

        Read from input and generate SAX events

        1. Read from a compatible input.

        2. Read from a pair of character iterators, or an iterator and a sentinel of a different type (C++20 ranges support).

          The value_type of the iterator must be an integral type with a size of 1, 2, or 4 bytes, which will be interpreted respectively as UTF-8, UTF-16, and UTF-32. If SentinelType differs from IteratorType, it must be comparable to the iterator type with operator!=.

        The SAX event lister must follow the interface of json_sax.

        "},{"location":"api/basic_json/sax_parse/#template-parameters","title":"Template parameters","text":"InputType

        A compatible input, for instance:

        • an std::istream object
        • a FILE pointer
        • a C-style array of characters
        • a pointer to a null-terminated string of single byte characters
        • a container obj for which begin(obj) and end(obj) produce a valid pair of iterators (as found via ADL or member functions, with semantics compatible to std::begin and std::end)
        IteratorType a compatible iterator type for overload (2); a pair of character iterators whose value_type is an integral type with a size of 1, 2, or 4 bytes (interpreted respectively as UTF-8, UTF-16, and UTF-32) SentinelType

        defaults to IteratorType; may be a different type comparable to IteratorType via operator!=, for overload (2), for instance.

        • a custom sentinel type for C++20 ranges
        • std::default_sentinel_t, when IteratorType is std::counted_iterator
        SAX a class fulfilling the SAX event listener interface; see json_sax"},{"location":"api/basic_json/sax_parse/#parameters","title":"Parameters","text":"i (in) Input to parse from sax (in) SAX event listener (must not be null) format (in) the format to parse (JSON, BJData, BON8, BSON, CBOR, MessagePack, or UBJSON) (optional, input_format_t::json by default), see input_format_t for more information strict (in) whether the input has to be consumed completely (optional, true by default); when false and the input is a std::istream, the character that terminates a number is consumed unless JSON_PRECISE_STREAM_POSITION is defined to 1; see operator>> ignore_comments (in) whether comments should be ignored and treated like whitespace (true) or yield a parse error (false); (optional, false by default) ignore_trailing_commas (in) whether trailing commas in arrays or objects should be ignored and treated like whitespace (true) or yield a parse error (false); (optional, false by default) tag_handler (in) how to handle CBOR tags; see cbor_tag_handler_t. Ignored for formats other than CBOR (optional, cbor_tag_handler_t::error by default). first (in) iterator to the start of a character range last (in) iterator to the end of a character range, or a sentinel value that compares equal to the end iterator with operator!="},{"location":"api/basic_json/sax_parse/#return-value","title":"Return value","text":"

        return value of the last processed SAX event

        "},{"location":"api/basic_json/sax_parse/#exception-safety","title":"Exception safety","text":"

        Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

        "},{"location":"api/basic_json/sax_parse/#exceptions","title":"Exceptions","text":"
        • Throws parse_error.101 in case of an unexpected token, or empty input like a null FILE* or char* pointer.
        "},{"location":"api/basic_json/sax_parse/#complexity","title":"Complexity","text":"

        Linear in the length of the input. The parser is a predictive LL(1) parser. The complexity can be higher if the SAX consumer sax has a super-linear complexity.

        "},{"location":"api/basic_json/sax_parse/#notes","title":"Notes","text":"

        A UTF-8 byte order mark is silently ignored.

        When reading from a FILE*, the file is locked while it is parsed. The lock is recursive, so the SAX handler may use the same file with stdio functions, but it must not wait for another thread that uses the same FILE object; see JSON_DISABLE_UNLOCKED_FILE_IO.

        "},{"location":"api/basic_json/sax_parse/#examples","title":"Examples","text":"Example

        The example below demonstrates the sax_parse() function reading from string and processing the events with a user-defined SAX event consumer.

        #include <iostream>\n#include <iomanip>\n#include <sstream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\n// a simple event consumer that collects string representations of the passed\n// values; note inheriting from json::json_sax_t is not required, but can\n// help not to forget a required function\nclass sax_event_consumer : public json::json_sax_t\n{\n  public:\n    std::vector<std::string> events;\n\n    bool null() override\n    {\n        events.push_back(\"null()\");\n        return true;\n    }\n\n    bool boolean(bool val) override\n    {\n        events.push_back(\"boolean(val=\" + std::string(val ? \"true\" : \"false\") + \")\");\n        return true;\n    }\n\n    bool number_integer(number_integer_t val) override\n    {\n        events.push_back(\"number_integer(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_unsigned(number_unsigned_t val) override\n    {\n        events.push_back(\"number_unsigned(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_float(number_float_t val, const string_t& s) override\n    {\n        events.push_back(\"number_float(val=\" + std::to_string(val) + \", s=\" + s + \")\");\n        return true;\n    }\n\n    bool string(string_t& val) override\n    {\n        events.push_back(\"string(val=\" + val + \")\");\n        return true;\n    }\n\n    bool start_object(std::size_t elements) override\n    {\n        events.push_back(\"start_object(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_object() override\n    {\n        events.push_back(\"end_object()\");\n        return true;\n    }\n\n    bool start_array(std::size_t elements) override\n    {\n        events.push_back(\"start_array(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_array() override\n    {\n        events.push_back(\"end_array()\");\n        return true;\n    }\n\n    bool key(string_t& val) override\n    {\n        events.push_back(\"key(val=\" + val + \")\");\n        return true;\n    }\n\n    bool binary(json::binary_t& val) override\n    {\n        events.push_back(\"binary(val=[...])\");\n        return true;\n    }\n\n    bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex) override\n    {\n        events.push_back(\"parse_error(position=\" + std::to_string(position) + \", last_token=\" + last_token + \",\\n            ex=\" + std::string(ex.what()) + \")\");\n        return false;\n    }\n};\n\nint main()\n{\n    // a JSON text\n    auto text = R\"(\n    {\n        \"Image\": {\n            \"Width\":  800,\n            \"Height\": 600,\n            \"Title\":  \"View from 15th Floor\",\n            \"Thumbnail\": {\n                \"Url\":    \"http://www.example.com/image/481989943\",\n                \"Height\": 125,\n                \"Width\":  100\n            },\n            \"Animated\" : false,\n            \"IDs\": [116, 943, 234, -38793],\n            \"DeletionDate\": null,\n            \"Distance\": 12.723374634\n        }\n    }]\n    )\";\n\n    // create a SAX event consumer object\n    sax_event_consumer sec;\n\n    // parse JSON\n    bool result = json::sax_parse(text, &sec);\n\n    // output the recorded events\n    for (auto& event : sec.events)\n    {\n        std::cout << event << \"\\n\";\n    }\n\n    // output the result of sax_parse\n    std::cout << \"\\nresult: \" << std::boolalpha << result << std::endl;\n}\n

        Output:

        start_object(elements=18446744073709551615)\nkey(val=Image)\nstart_object(elements=18446744073709551615)\nkey(val=Width)\nnumber_unsigned(val=800)\nkey(val=Height)\nnumber_unsigned(val=600)\nkey(val=Title)\nstring(val=View from 15th Floor)\nkey(val=Thumbnail)\nstart_object(elements=18446744073709551615)\nkey(val=Url)\nstring(val=http://www.example.com/image/481989943)\nkey(val=Height)\nnumber_unsigned(val=125)\nkey(val=Width)\nnumber_unsigned(val=100)\nend_object()\nkey(val=Animated)\nboolean(val=false)\nkey(val=IDs)\nstart_array(elements=18446744073709551615)\nnumber_unsigned(val=116)\nnumber_unsigned(val=943)\nnumber_unsigned(val=234)\nnumber_integer(val=-38793)\nend_array()\nkey(val=DeletionDate)\nnull()\nkey(val=Distance)\nnumber_float(val=12.723375, s=12.723374634)\nend_object()\nend_object()\nparse_error(position=460, last_token=12.723374634<U+000A>        }<U+000A>    }],\n            ex=[json.exception.parse_error.101] parse error at line 17, column 6: syntax error while parsing value - unexpected ']'; expected end of input)\n\nresult: false\n
        "},{"location":"api/basic_json/sax_parse/#see-also","title":"See also","text":"
        • parse - deserialize from a compatible input
        • accept - check if the input is valid JSON
        • JSON_DISABLE_STREAM_BULK_SCAN - read a std::istream one character at a time
        • JSON_DISABLE_UNLOCKED_FILE_IO - read a FILE* with the locking stdio functions
        "},{"location":"api/basic_json/sax_parse/#version-history","title":"Version history","text":"
        • Added in version 3.2.0.
        • Ignoring comments via ignore_comments added in version 3.9.0.
        • Added ignore_trailing_commas in version 3.13.0 unreleased.
        • Added tag_handler in version 3.13.0 unreleased.
        • Extended container support (1) to include types with lvalue-only ADL begin/end (matching std::begin/std::end semantics) in version 3.13.0 unreleased.
        • Extended overload (2) to accept heterogeneous iterator+sentinel pairs (C++20 ranges support) in version 3.13.0 unreleased.
        • JSON_PRECISE_STREAM_POSITION added in version 3.13.0 unreleased to optionally leave a std::istream positioned right after the parsed value when strict is false.
        • Parsing from an std::istream scans the characters in its stream buffer in bulk in version 3.13.0 unreleased (JSON_DISABLE_STREAM_BULK_SCAN).
        • Parsing from a FILE* locks the file once instead of once per character in version 3.13.0 unreleased (JSON_DISABLE_UNLOCKED_FILE_IO).

        Deprecation

        Overload (2) replaces calls to sax_parse with a pair of iterators as their first parameter which has been deprecated in version 3.8.0. This overload will be removed in version 4.0.0. Please replace all calls like sax_parse({ptr, ptr+len}); with sax_parse(ptr, ptr+len);.

        See the migration guide for how to update existing code.

        "},{"location":"api/basic_json/size/","title":"nlohmann::basic_json::size","text":"
        size_type size() const noexcept;\n

        Returns the number of elements in a JSON value.

        "},{"location":"api/basic_json/size/#return-value","title":"Return value","text":"

        The return value depends on the different types and is defined as follows:

        Value type return value null 0 boolean 1 string 1 number 1 binary 1 object result of function object_t::size() array result of function array_t::size()"},{"location":"api/basic_json/size/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this function never throws exceptions.

        "},{"location":"api/basic_json/size/#complexity","title":"Complexity","text":"

        Constant, as long as array_t and object_t satisfy the Container concept; that is, their size() functions have constant complexity.

        "},{"location":"api/basic_json/size/#notes","title":"Notes","text":"

        This function does not return the length of a string stored as JSON value -- it returns the number of elements in the JSON value which is 1 in the case of a string.

        "},{"location":"api/basic_json/size/#examples","title":"Examples","text":"Example

        The following code calls size() on the different value types.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_null;\n    json j_boolean = true;\n    json j_number_integer = 17;\n    json j_number_float = 23.42;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_object_empty(json::value_t::object);\n    json j_array = {1, 2, 4, 8, 16};\n    json j_array_empty(json::value_t::array);\n    json j_string = \"Hello, world\";\n\n    // call size()\n    std::cout << j_null.size() << '\\n';\n    std::cout << j_boolean.size() << '\\n';\n    std::cout << j_number_integer.size() << '\\n';\n    std::cout << j_number_float.size() << '\\n';\n    std::cout << j_object.size() << '\\n';\n    std::cout << j_object_empty.size() << '\\n';\n    std::cout << j_array.size() << '\\n';\n    std::cout << j_array_empty.size() << '\\n';\n    std::cout << j_string.size() << '\\n';\n}\n

        Output:

        0\n1\n1\n1\n2\n0\n5\n0\n1\n
        "},{"location":"api/basic_json/size/#see-also","title":"See also","text":"
        • empty checks whether the JSON value has no elements
        • max_size returns the maximum possible number of elements
        • basic_json_view::size - the same function on a zero-copy view
        "},{"location":"api/basic_json/size/#version-history","title":"Version history","text":"
        • Added in version 1.0.0.
        • Extended to return 1 for binary types in version 3.8.0.
        "},{"location":"api/basic_json/start_pos/","title":"nlohmann::basic_json::start_pos","text":"
        #if JSON_DIAGNOSTIC_POSITIONS\nconstexpr std::size_t start_pos() const noexcept;\n#endif\n

        Returns the position of the first character in the JSON string from which the value was parsed from.

        JSON type return value object position of the opening { array position of the opening [ string position of the opening \" number position of the first character boolean position of t for true and f for false null position of n"},{"location":"api/basic_json/start_pos/#return-value","title":"Return value","text":"

        the position of the first character of the value in the parsed JSON string, if the value was created by the parse function, or std::string::npos if the value was constructed otherwise

        "},{"location":"api/basic_json/start_pos/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this member function never throws exceptions.

        "},{"location":"api/basic_json/start_pos/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json/start_pos/#notes","title":"Notes","text":"

        Note

        The function is only available if macro JSON_DIAGNOSTIC_POSITIONS has been defined to 1 before including the library header.

        Invalidation

        The returned positions are only valid as long as the JSON value is not changed. The positions are not updated when the JSON value is changed.

        "},{"location":"api/basic_json/start_pos/#examples","title":"Examples","text":"Example 
        #include <iostream>\n\n#define JSON_DIAGNOSTIC_POSITIONS 1\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::string json_string = R\"(\n    {\n        \"address\": {\n            \"street\": \"Fake Street\",\n            \"housenumber\": 1\n        }\n    }\n    )\";\n    json j = json::parse(json_string);\n\n    std::cout << \"Root diagnostic positions: \\n\";\n    std::cout << \"\\tstart_pos: \" << j.start_pos() << '\\n';\n    std::cout << \"\\tend_pos:\" << j.end_pos() << \"\\n\";\n    std::cout << \"Original string: \\n\";\n    std::cout << \"{\\n        \\\"address\\\": {\\n            \\\"street\\\": \\\"Fake Street\\\",\\n            \\\"housenumber\\\": 1\\n        }\\n    }\" << \"\\n\";\n    std::cout << \"Parsed string: \\n\";\n    std::cout << json_string.substr(j.start_pos(), j.end_pos() - j.start_pos()) << \"\\n\\n\";\n\n    std::cout << \"address diagnostic positions: \\n\";\n    std::cout << \"\\tstart_pos:\" << j[\"address\"].start_pos() << '\\n';\n    std::cout << \"\\tend_pos:\" << j[\"address\"].end_pos() << \"\\n\\n\";\n    std::cout << \"Original string: \\n\";\n    std::cout << \"{            \\\"street\\\": \\\"Fake Street\\\",\\n            \\\"housenumber\\\": 1\\n        }\" << \"\\n\";\n    std::cout << \"Parsed string: \\n\";\n    std::cout << json_string.substr(j[\"address\"].start_pos(), j[\"address\"].end_pos() - j[\"address\"].start_pos()) << \"\\n\\n\";\n\n    std::cout << \"street diagnostic positions: \\n\";\n    std::cout << \"\\tstart_pos:\" << j[\"address\"][\"street\"].start_pos() << '\\n';\n    std::cout << \"\\tend_pos:\" << j[\"address\"][\"street\"].end_pos() << \"\\n\\n\";\n    std::cout << \"Original string: \\n\";\n    std::cout << \"\\\"Fake Street\\\"\" << \"\\n\";\n    std::cout << \"Parsed string: \\n\";\n    std::cout << json_string.substr(j[\"address\"][\"street\"].start_pos(), j[\"address\"][\"street\"].end_pos() - j[\"address\"][\"street\"].start_pos()) << \"\\n\\n\";\n\n    std::cout << \"housenumber diagnostic positions: \\n\";\n    std::cout << \"\\tstart_pos:\" << j[\"address\"][\"housenumber\"].start_pos() << '\\n';\n    std::cout << \"\\tend_pos:\" << j[\"address\"][\"housenumber\"].end_pos() << \"\\n\\n\";\n    std::cout << \"Original string: \\n\";\n    std::cout << \"1\" << \"\\n\";\n    std::cout << \"Parsed string: \\n\";\n    std::cout << json_string.substr(j[\"address\"][\"housenumber\"].start_pos(), j[\"address\"][\"housenumber\"].end_pos() - j[\"address\"][\"housenumber\"].start_pos()) << \"\\n\\n\";\n}\n

        Output:

        Root diagnostic positions: \n    start_pos: 5\n    end_pos:109\nOriginal string: \n{\n        \"address\": {\n            \"street\": \"Fake Street\",\n            \"housenumber\": 1\n        }\n    }\nParsed string: \n{\n        \"address\": {\n            \"street\": \"Fake Street\",\n            \"housenumber\": 1\n        }\n    }\n\naddress diagnostic positions: \n    start_pos:26\n    end_pos:103\n\nOriginal string: \n{            \"street\": \"Fake Street\",\n            \"housenumber\": 1\n        }\nParsed string: \n{\n            \"street\": \"Fake Street\",\n            \"housenumber\": 1\n        }\n\nstreet diagnostic positions: \n    start_pos:50\n    end_pos:63\n\nOriginal string: \n\"Fake Street\"\nParsed string: \n\"Fake Street\"\n\nhousenumber diagnostic positions: \n    start_pos:92\n    end_pos:93\n\nOriginal string: \n1\nParsed string: \n1\n

        The output shows the start/end positions of all the objects and fields in the JSON string.

        "},{"location":"api/basic_json/start_pos/#see-also","title":"See also","text":"
        • end_pos to access the end position
        • JSON_DIAGNOSTIC_POSITIONS for an overview of the diagnostic positions
        "},{"location":"api/basic_json/start_pos/#version-history","title":"Version history","text":"
        • Added in version 3.12.0.
        "},{"location":"api/basic_json/std_formatter/","title":"std::formatter<nlohmann::basic_json>","text":"
        namespace std {\n    template <>\n    struct formatter<nlohmann::basic_json, char>;\n}\n

        Specialization to make JSON values formattable with std::format (and the other members of C++20's <format> header, such as std::format_to).

        A subset of the standard format spec grammar is supported, repurposed for JSON pretty-printing; any other spec component (sign, the 0 flag, precision, L, a dynamic width such as \"{:{}}\", or a trailing type character) throws std::format_error:

        • \"{}\" serializes the value the same way as dump() (compact, no whitespace).
        • \"{:#}\" (\"alternate form\") serializes the value the same way as dump(4) (pretty-printed with an indent of 4).
        • A width, with or without \"#\" (e.g. \"{:2}\" or \"{:#2}\"), serializes the value the same way as dump(width) \u2014 a width on its own implies pretty-printing, since an indent size has no meaning for compact output.
        • fill-and-align (e.g. \"{:.>#}\" or \"{:.>3}\") picks a custom indent character, the same way as dump(indent, indent_char). The alignment direction itself ('<', '>', '^') has no separate meaning for JSON values \u2014 only the fill character before it is used, and any of the three directions is accepted.

        This specialization is only available for char-based JSON values and only if the standard library provides <format>, controlled by the JSON_HAS_STD_FORMAT macro.

        "},{"location":"api/basic_json/std_formatter/#examples","title":"Examples","text":"Example

        The example shows how to format JSON values with std::format.

        #include <format>\n#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    json j = {{\"one\", 1}, {\"two\", 2}};\n\n    // compact formatting, like dump()\n    std::cout << std::format(\"{}\", j) << \"\\n\\n\";\n\n    // pretty-printed formatting, like dump(4)\n    std::cout << std::format(\"{:#}\", j) << \"\\n\\n\";\n\n    // a width sets the indent, like dump(2)\n    std::cout << std::format(\"{:2}\", j) << \"\\n\\n\";\n\n    // fill-and-align sets the indent character, like dump(4, '.')\n    std::cout << std::format(\"{:.>#}\", j) << std::endl;\n}\n

        Output:

        {\"one\":1,\"two\":2}\n\n{\n    \"one\": 1,\n    \"two\": 2\n}\n\n{\n  \"one\": 1,\n  \"two\": 2\n}\n\n{\n....\"one\": 1,\n....\"two\": 2\n}\n
        "},{"location":"api/basic_json/std_formatter/#see-also","title":"See also","text":"
        • dump - serialization
        • operator<<(std::ostream&) - serialize to stream
        • format_as - customization point used by fmt::format (fmtlib)
        • Serialization - the serialization article
        "},{"location":"api/basic_json/std_formatter/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json/std_hash/","title":"std::hash<nlohmann::basic_json>","text":"
        namespace std {\n    struct hash<nlohmann::basic_json>;\n}\n

        Return a hash value for a JSON object. The hash function tries to rely on std::hash where possible. Furthermore, the type of the JSON value is taken into account, so null, false, and numbers may hash differently from each other. Numbers that compare equal under operator== always hash equally, regardless of whether they are stored as signed integer, unsigned integer, or floating-point number.

        Numbers are hashed by their value converted to number_float_t. Converting an integer to number_float_t therefore keeps its hash, but converting a floating-point number to an integer type is lossy and can change it: 0.5 converts to 0, which need not have the same hash. Unequal numbers can also share a hash value, for example two large integers that convert to the same number_float_t.

        "},{"location":"api/basic_json/std_hash/#examples","title":"Examples","text":"Example

        The example shows how to calculate hash values for different JSON values.

        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    std::cout << \"hash(null) = \" << std::hash<json> {}(json(nullptr)) << '\\n'\n              << \"hash(false) = \" << std::hash<json> {}(json(false)) << '\\n'\n              << \"hash(0) = \" << std::hash<json> {}(json(0)) << '\\n'\n              << \"hash(0U) = \" << std::hash<json> {}(json(0U)) << '\\n'\n              << \"hash(0.0) = \" << std::hash<json> {}(json(0.0)) << '\\n'\n              << \"hash(\\\"\\\") = \" << std::hash<json> {}(json(\"\")) << '\\n'\n              << \"hash({}) = \" << std::hash<json> {}(json::object()) << '\\n'\n              << \"hash([]) = \" << std::hash<json> {}(json::array()) << '\\n'\n              << \"hash({\\\"hello\\\": \\\"world\\\"}) = \" << std::hash<json> {}(\"{\\\"hello\\\": \\\"world\\\"}\"_json)\n              << std::endl;\n}\n

        Output:

        hash(null) = 2654435769\nhash(false) = 2654436030\nhash(0) = 2654436221\nhash(0U) = 2654436221\nhash(0.0) = 2654436221\nhash(\"\") = 11160318156688833227\nhash({}) = 2654435832\nhash([]) = 2654435899\nhash({\"hello\": \"world\"}) = 3701319991624763853\n

        The hash values shown are examples only. They depend on the platform, the compiler, and the compiler version, and they can change between versions of this library. Do not persist them or rely on specific values.

        "},{"location":"api/basic_json/std_hash/#see-also","title":"See also","text":"
        • operator== compares two JSON values for equality, consistent with equal hash values
        "},{"location":"api/basic_json/std_hash/#version-history","title":"Version history","text":"
        • Added in version 1.0.0.
        • Extended for arbitrary basic_json types in version 3.10.5.
        • Numbers that compare equal hash equally since version 3.13.0 unreleased; before, 0, 0U, and 0.0 had different hash values.
        "},{"location":"api/basic_json/std_swap/","title":"std::swap<basic_json>","text":"
        namespace std {\n    void swap(nlohmann::basic_json& j1, nlohmann::basic_json& j2);\n}\n

        Exchanges the values of two JSON objects.

        "},{"location":"api/basic_json/std_swap/#parameters","title":"Parameters","text":"j1 (in, out) value to be replaced by j2 j2 (in, out) value to be replaced by j1"},{"location":"api/basic_json/std_swap/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this function never throws exceptions.

        "},{"location":"api/basic_json/std_swap/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json/std_swap/#possible-implementation","title":"Possible implementation","text":"
        void swap(nlohmann::basic_json& j1, nlohmann::basic_json& j2)\n{\n    j1.swap(j2);\n}\n
        "},{"location":"api/basic_json/std_swap/#examples","title":"Examples","text":"Example

        The following code shows how two values are swapped with std::swap.

        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j1 = {{\"one\", 1}, {\"two\", 2}};\n    json j2 = {1, 2, 4, 8, 16};\n\n    std::cout << \"j1 = \" << j1 << \" | j2 = \" << j2 << '\\n';\n\n    // swap values\n    std::swap(j1, j2);\n\n    std::cout << \"j1 = \" << j1 << \" | j2 = \" << j2 << std::endl;\n}\n

        Output:

        j1 = {\"one\":1,\"two\":2} | j2 = [1,2,4,8,16]\nj1 = [1,2,4,8,16] | j2 = {\"one\":1,\"two\":2}\n
        "},{"location":"api/basic_json/std_swap/#see-also","title":"See also","text":"
        • swap
        "},{"location":"api/basic_json/std_swap/#version-history","title":"Version history","text":"
        • Added in version 1.0.0.
        • Extended for arbitrary basic_json types in version 3.10.5.
        "},{"location":"api/basic_json/string_t/","title":"nlohmann::basic_json::string_t","text":"
        using string_t = StringType;\n

        The type used to store JSON strings.

        RFC 8259 describes JSON strings as follows:

        A string is a sequence of zero or more Unicode characters.

        To store strings in C++, a type is defined by the template parameter described below. Unicode values are split by the JSON class into byte-sized characters during deserialization.

        "},{"location":"api/basic_json/string_t/#template-parameters","title":"Template parameters","text":"StringType

        the container to store strings (e.g., std::string). Note this container is used for keys/names in objects, see object_t.

        StringType must have a char-compatible value_type: the library relies on UTF-8/char-based storage and processing internally, so std::wstring, std::u16string, and std::u32string are not valid choices for StringType. To work with wide-character data, convert it to/from UTF-8 at the boundary instead -- see the FAQ's wide string handling section for a conversion recipe.

        Beyond the character type, the library expects a substantial part of the std::string interface (contiguous null-terminated data(), substr(), find(), append(), ...). See Template Parameter Requirements for the full list and for the string types that are known to work.

        "},{"location":"api/basic_json/string_t/#notes","title":"Notes","text":""},{"location":"api/basic_json/string_t/#default-type","title":"Default type","text":"

        With the default values for StringType (std::string), the default type for string_t is std::string.

        "},{"location":"api/basic_json/string_t/#encoding","title":"Encoding","text":"

        Strings are stored in UTF-8 encoding. Therefore, functions like std::string::size() or std::string::length() return the number of bytes in the string rather than the number of characters or glyphs.

        "},{"location":"api/basic_json/string_t/#string-comparison","title":"String comparison","text":"

        RFC 8259 states:

        Software implementations are typically required to test names of object members for equality. Implementations that transform the textual representation into sequences of Unicode code units and then perform the comparison numerically, code unit by code unit, are interoperable in the sense that implementations will agree in all cases on equality or inequality of two strings. For example, implementations that compare strings with escaped characters unconverted may incorrectly find that \"a\\\\b\" and \"a\\u005Cb\" are not equal.

        This implementation is interoperable as it does compare strings code unit by code unit.

        "},{"location":"api/basic_json/string_t/#storage","title":"Storage","text":"

        String values are stored as pointers in a basic_json type. That is, for any access to string values, a pointer of type string_t* must be dereferenced.

        "},{"location":"api/basic_json/string_t/#cross-basic_json-conversion-requirements","title":"Cross-basic_json conversion requirements","text":"

        When converting a string value from one basic_json specialization to another via the converting constructor (overload 4), the target string_t must be directly constructible from the source basic_json's string_t type. If this requirement is not met, the conversion does not fail; instead, the string is silently converted as an array of character codes, which is incorrect. See issue #3425 for details and an example.

        "},{"location":"api/basic_json/string_t/#examples","title":"Examples","text":"Example

        The following code shows that string_t is by default, a typedef to std::string.

        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::boolalpha << std::is_same<std::string, json::string_t>::value << std::endl;\n}\n

        Output:

        true\n
        "},{"location":"api/basic_json/string_t/#see-also","title":"See also","text":"
        • object_t the type used to store JSON objects (and their keys, which are also string_t)
        • binary_t the type used to store binary values
        • get_ptr returns a pointer to the stored string value
        "},{"location":"api/basic_json/string_t/#version-history","title":"Version history","text":"
        • Added in version 1.0.0.
        • Removed the requirement that string_t be implicitly convertible from std::string, which the BSON writer and the UBJSON reader relied on, in version 3.13.0 unreleased.
        "},{"location":"api/basic_json/swap/","title":"nlohmann::basic_json::swap","text":"
        // (1)\nvoid swap(reference other) noexcept (\n    std::is_nothrow_move_constructible<value_t>::value &&\n    std::is_nothrow_move_assignable<value_t>::value &&\n    std::is_nothrow_move_constructible<json_value>::value &&\n    std::is_nothrow_move_assignable<json_value>::value &&\n    std::is_nothrow_move_constructible<json_base_class_t>::value &&\n    std::is_nothrow_move_assignable<json_base_class_t>::value\n);\n\n// (2)\nfriend void swap(reference left, reference right) noexcept (\n    std::is_nothrow_move_constructible<value_t>::value &&\n    std::is_nothrow_move_assignable<value_t>::value &&\n    std::is_nothrow_move_constructible<json_value>::value &&\n    std::is_nothrow_move_assignable<json_value>::value &&\n    std::is_nothrow_move_constructible<json_base_class_t>::value &&\n    std::is_nothrow_move_assignable<json_base_class_t>::value\n);\n\n// (3)\nvoid swap(array_t& other);\n\n// (4)\nvoid swap(object_t& other);\n\n// (5)\nvoid swap(string_t& other);\n\n// (6)\nvoid swap(binary_t& other);\n\n// (7)\nvoid swap(typename binary_t::container_type& other);\n
        1. Exchanges the contents of the JSON value with those of other. Does not invoke any move, copy, or swap operations on individual elements. All iterators and references remain valid. The past-the-end iterator is invalidated. If macro JSON_DIAGNOSTIC_POSITIONS is defined to 1, the start_pos()/end_pos() diagnostic positions are exchanged along with the value. The json_base_class_t subobject is exchanged along with the value as well, the same way it is copied or moved by the copy/move constructors and assignment operators.
        2. Exchanges the contents of the JSON value from left with those of right. Does not invoke any move, copy, or swap operations on individual elements. All iterators and references remain valid. The past-the-end iterator is invalidated. Implemented as a friend function callable via ADL. If macro JSON_DIAGNOSTIC_POSITIONS is defined to 1, the start_pos()/end_pos() diagnostic positions are exchanged along with the value. The json_base_class_t subobject is exchanged along with the value as well, the same way it is copied or moved by the copy/move constructors and assignment operators.
        3. Exchanges the contents of a JSON array with those of other. Does not invoke any move, copy, or swap operations on individual elements. All iterators and references remain valid. The past-the-end iterator is invalidated.
        4. Exchanges the contents of a JSON object with those of other. Does not invoke any move, copy, or swap operations on individual elements. All iterators and references remain valid. The past-the-end iterator is invalidated.
        5. Exchanges the contents of a JSON string with those of other. Does not invoke any move, copy, or swap operations on individual elements. All iterators and references remain valid. The past-the-end iterator is invalidated.
        6. Exchanges the contents of a binary value with those of other. Does not invoke any move, copy, or swap operations on individual elements. All iterators and references remain valid. The past-the-end iterator is invalidated.
        7. Exchanges the contents of a binary value with those of other. Does not invoke any move, copy, or swap operations on individual elements. All iterators and references remain valid. The past-the-end iterator is invalidated. Unlike version (6), no binary subtype is involved.
        "},{"location":"api/basic_json/swap/#parameters","title":"Parameters","text":"other (in, out) value to exchange the contents with left (in, out) value to exchange the contents with right (in, out) value to exchange the contents with"},{"location":"api/basic_json/swap/#exception-safety","title":"Exception safety","text":"
        1. No-throw guarantee: this function never throws exceptions.
        2. No-throw guarantee: this function never throws exceptions.
        3. Strong guarantee: if an exception is thrown, there are no changes to any JSON value.
        4. Strong guarantee: if an exception is thrown, there are no changes to any JSON value.
        5. Strong guarantee: if an exception is thrown, there are no changes to any JSON value.
        6. Strong guarantee: if an exception is thrown, there are no changes to any JSON value.
        7. Strong guarantee: if an exception is thrown, there are no changes to any JSON value.
        "},{"location":"api/basic_json/swap/#exceptions","title":"Exceptions","text":"
        1. No-throw guarantee: this function never throws exceptions.
        2. No-throw guarantee: this function never throws exceptions.
        3. Throws type_error.310 if called on JSON values other than arrays; example: \"cannot use swap(array_t&) with boolean\"
        4. Throws type_error.310 if called on JSON values other than objects; example: \"cannot use swap(object_t&) with boolean\"
        5. Throws type_error.310 if called on JSON values other than strings; example: \"cannot use swap(string_t&) with boolean\"
        6. Throws type_error.310 if called on JSON values other than binaries; example: \"cannot use swap(binary_t&) with boolean\"
        7. Throws type_error.310 if called on JSON values other than binaries; example: \"cannot use swap(binary_t::container_type&) with boolean\"
        "},{"location":"api/basic_json/swap/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json/swap/#examples","title":"Examples","text":"Example: (1, 2) swap JSON values

        The example below shows how JSON values can be swapped with swap().

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create two JSON values\n    json j1 = {1, 2, 3, 4, 5};\n    json j2 = {{\"pi\", 3.141592653589793}, {\"e\", 2.718281828459045}};\n\n    // swap the values\n    j1.swap(j2);\n\n    // output the values\n    std::cout << \"j1 = \" << j1 << '\\n';\n    std::cout << \"j2 = \" << j2 << '\\n';\n}\n

        Output:

        j1 = {\"e\":2.718281828459045,\"pi\":3.141592653589793}\nj2 = [1,2,3,4,5]\n
        Example: (3) swap array

        The example below shows how arrays can be swapped with swap().

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON value\n    json value = {{\"array\", {1, 2, 3, 4}}};\n\n    // create an array_t\n    json::array_t array = {\"Snap\", \"Crackle\", \"Pop\"};\n\n    // swap the array stored in the JSON value\n    value[\"array\"].swap(array);\n\n    // output the values\n    std::cout << \"value = \" << value << '\\n';\n    std::cout << \"array = \" << array << '\\n';\n}\n

        Output:

        value = {\"array\":[\"Snap\",\"Crackle\",\"Pop\"]}\narray = [1,2,3,4]\n
        Example: (4) swap object

        The example below shows how objects can be swapped with swap().

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON value\n    json value = { {\"translation\", {{\"one\", \"eins\"}, {\"two\", \"zwei\"}}} };\n\n    // create an object_t\n    json::object_t object = {{\"cow\", \"Kuh\"}, {\"dog\", \"Hund\"}};\n\n    // swap the object stored in the JSON value\n    value[\"translation\"].swap(object);\n\n    // output the values\n    std::cout << \"value = \" << value << '\\n';\n    std::cout << \"object = \" << object << '\\n';\n}\n

        Output:

        value = {\"translation\":{\"cow\":\"Kuh\",\"dog\":\"Hund\"}}\nobject = {\"one\":\"eins\",\"two\":\"zwei\"}\n
        Example: (5) swap string

        The example below shows how strings can be swapped with swap().

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON value\n    json value = { \"the good\", \"the bad\", \"the ugly\" };\n\n    // create string_t\n    json::string_t string = \"the fast\";\n\n    // swap the object stored in the JSON value\n    value[1].swap(string);\n\n    // output the values\n    std::cout << \"value = \" << value << '\\n';\n    std::cout << \"string = \" << string << '\\n';\n}\n

        Output:

        value = [\"the good\",\"the fast\",\"the ugly\"]\nstring = the bad\n
        Example: (6) swap binary

        The example below shows how binary values can be swapped with swap().

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a binary value\n    json value = json::binary({1, 2, 3});\n\n    // create a binary_t\n    json::binary_t binary = {{4, 5, 6}};\n\n    // swap the object stored in the JSON value\n    value.swap(binary);\n\n    // output the values\n    std::cout << \"value = \" << value << '\\n';\n    std::cout << \"binary = \" << json(binary) << '\\n';\n}\n

        Output:

        value = {\"bytes\":[4,5,6],\"subtype\":null}\nbinary = {\"bytes\":[1,2,3],\"subtype\":null}\n
        "},{"location":"api/basic_json/swap/#see-also","title":"See also","text":"
        • std::swap<basic_json>
        • operator= copy assignment
        • basic_json create a JSON value
        "},{"location":"api/basic_json/swap/#version-history","title":"Version history","text":"
        1. Since version 1.0.0. Exchanges the json_base_class_t subobject along with the value since version 3.13.0 unreleased.
        2. Since version 1.0.0. Exchanges the json_base_class_t subobject along with the value since version 3.13.0 unreleased.
        3. Since version 1.0.0.
        4. Since version 1.0.0.
        5. Since version 1.0.0.
        6. Since version 3.8.0.
        7. Since version 3.8.0.
        "},{"location":"api/basic_json/to_bjdata/","title":"nlohmann::basic_json::to_bjdata","text":"
        // (1)\nstatic std::vector<std::uint8_t> to_bjdata(const basic_json& j,\n                                           const bool use_size = false,\n                                           const bool use_type = false,\n                                           const bjdata_version_t version = bjdata_version_t::draft2,\n                                           const error_handler_t error_handler = error_handler_t::keep);\n\n// (2)\nstatic void to_bjdata(const basic_json& j, detail::output_adapter<std::uint8_t> o,\n                      const bool use_size = false, const bool use_type = false,\n                      const bjdata_version_t version = bjdata_version_t::draft2,\n                      const error_handler_t error_handler = error_handler_t::keep);\nstatic void to_bjdata(const basic_json& j, detail::output_adapter<char> o,\n                      const bool use_size = false, const bool use_type = false,\n                      const bjdata_version_t version = bjdata_version_t::draft2,\n                      const error_handler_t error_handler = error_handler_t::keep);\n

        Serializes a given JSON value j to a byte vector using the BJData (Binary JData) serialization format. BJData aims to be more compact than JSON itself, yet more efficient to parse.

        1. Returns a byte vector containing the BJData serialization.
        2. Writes the BJData serialization to an output adapter.

        The exact mapping and its limitations are described on a dedicated page.

        "},{"location":"api/basic_json/to_bjdata/#parameters","title":"Parameters","text":"j (in) JSON value to serialize o (in) output adapter to write serialization to use_size (in) whether to add size annotations to container types; optional, false by default. use_type (in) whether to add type annotations to container types (must be combined with use_size = true); optional, false by default. version (in) which version of BJData to use (see note on \"Binary values\" on BJData); optional, bjdata_version_t::draft2 by default. error_handler (in) how to treat a string or object key in j that is not valid UTF-8; see error_handler_t. The default, keep, writes the ill-formed bytes to the output as is, as every version of to_bjdata did before this parameter was added; strict throws; replace/ignore sanitize it the same way dump would. If JSON_STRICT_BINARY_UTF8 is enabled, the default is strict instead."},{"location":"api/basic_json/to_bjdata/#return-value","title":"Return value","text":"
        1. BJData serialization as byte vector
        2. (none)
        "},{"location":"api/basic_json/to_bjdata/#exception-safety","title":"Exception safety","text":"

        Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

        "},{"location":"api/basic_json/to_bjdata/#exceptions","title":"Exceptions","text":"
        • Throws other_error.502 if use_type is true and use_size is false, and j contains a non-empty array, object, or binary value.
        • Throws type_error.316 if a string or object key in j is not valid UTF-8 and error_handler is strict (the default only if JSON_STRICT_BINARY_UTF8 is enabled)
        • Throws type_error.321 if j or a value nested in it is discarded; example: \"cannot serialize discarded value to BJData\"
        "},{"location":"api/basic_json/to_bjdata/#complexity","title":"Complexity","text":"

        Linear in the size of the JSON value j.

        "},{"location":"api/basic_json/to_bjdata/#examples","title":"Examples","text":"Example: serialize a JSON value to BJData

        The example shows the serialization of a JSON value to a byte vector in BJData format.

        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\n// function to print BJData's diagnostic format\nvoid print_byte(uint8_t byte)\n{\n    if (32 < byte and byte < 128)\n    {\n        std::cout << (char)byte;\n    }\n    else\n    {\n        std::cout << (int)byte;\n    }\n}\n\nint main()\n{\n    // create a JSON value\n    json j = R\"({\"compact\": true, \"schema\": false})\"_json;\n\n    // serialize it to BJData\n    std::vector<std::uint8_t> v = json::to_bjdata(j);\n\n    // print the vector content\n    for (auto& byte : v)\n    {\n        print_byte(byte);\n    }\n    std::cout << std::endl;\n\n    // create an array of numbers\n    json array = {1, 2, 3, 4, 5, 6, 7, 8};\n\n    // serialize it to BJData using default representation\n    std::vector<std::uint8_t> v_array = json::to_bjdata(array);\n    // serialize it to BJData using size optimization\n    std::vector<std::uint8_t> v_array_size = json::to_bjdata(array, true);\n    // serialize it to BJData using type optimization\n    std::vector<std::uint8_t> v_array_size_and_type = json::to_bjdata(array, true, true);\n\n    // print the vector contents\n    for (auto& byte : v_array)\n    {\n        print_byte(byte);\n    }\n    std::cout << std::endl;\n\n    for (auto& byte : v_array_size)\n    {\n        print_byte(byte);\n    }\n    std::cout << std::endl;\n\n    for (auto& byte : v_array_size_and_type)\n    {\n        print_byte(byte);\n    }\n    std::cout << std::endl;\n}\n

        Output:

        {i7compactTi6schemaF}\n[i1i2i3i4i5i6i7i8]\n[#i8i1i2i3i4i5i6i7i8\n[$i#i812345678\n
        Example: other_error.502 exception

        The example shows how requesting type annotations (use_type) without size annotations (use_size) throws an exception, because type-optimized containers can only be read back with a preceding size.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a non-empty JSON array\n    json j = {1, 2, 3};\n\n    // exception other_error.502\n    try\n    {\n        json::to_bjdata(j, false, true);\n    }\n    catch (const json::other_error& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

        Output:

        [json.exception.other_error.502] use_type requires use_size = true\n
        "},{"location":"api/basic_json/to_bjdata/#see-also","title":"See also","text":"
        • from_bjdata create a JSON value from an input in BJData format
        • to_cbor create a CBOR serialization of a JSON value
        • to_msgpack create a MessagePack serialization of a JSON value
        • to_bson create a BSON serialization of a JSON value
        • to_ubjson create a UBJSON serialization of a JSON value
        • to_bon8 create a BON8 serialization of a JSON value
        "},{"location":"api/basic_json/to_bjdata/#version-history","title":"Version history","text":"
        • Added in version 3.11.0.
        • BJData version parameter (for draft3 binary encoding) added in version 3.12.0.
        • Added error_handler parameter in version 3.13.0 unreleased. Its default, keep, writes the bytes of a string or object key that is not valid UTF-8 unchanged, as before; strict (the default if JSON_STRICT_BINARY_UTF8 is enabled) throws type_error.316.
        • Throws type_error.321 for a discarded value since version 3.13.0 unreleased; previously, a discarded value nested in an array or object was silently skipped, producing invalid BJData.
        • Writes unsigned integers wider than 64 bits as high-precision numbers since version 3.13.0 unreleased; previously, they were silently truncated to 64 bits.
        "},{"location":"api/basic_json/to_bon8/","title":"nlohmann::basic_json::to_bon8","text":"
        // (1)\nstatic std::vector<std::uint8_t> to_bon8(const basic_json& j);\n\n// (2)\nstatic void to_bon8(const basic_json& j, detail::output_adapter<std::uint8_t> o);\nstatic void to_bon8(const basic_json& j, detail::output_adapter<char> o);\n

        Serializes a given JSON value j to a byte vector using the BON8 (Binary Object Notation 8) serialization format. BON8 is a compact binary serialization format that stores strings as UTF-8 without a length prefix.

        1. Returns a byte vector containing the BON8 serialization.
        2. Writes the BON8 serialization to an output adapter.

        The exact mapping and its limitations are described on a dedicated page.

        "},{"location":"api/basic_json/to_bon8/#parameters","title":"Parameters","text":"j (in) JSON value to serialize o (in) output adapter to write serialization to"},{"location":"api/basic_json/to_bon8/#return-value","title":"Return value","text":"
        1. BON8 serialization as a byte vector
        2. (none)
        "},{"location":"api/basic_json/to_bon8/#exception-safety","title":"Exception safety","text":"

        Strong guarantee: if an exception is thrown, there are no changes in the JSON value j, which is never modified. With (2), the bytes written before the exception remain in the output adapter.

        "},{"location":"api/basic_json/to_bon8/#exceptions","title":"Exceptions","text":"
        • Throws out_of_range.407 if j contains an integer outside the range of int64 (an unsigned integer above 9223372036854775807, or, with a number type wider than 64 bits, any integer beyond int64), which BON8 cannot represent
        • Throws type_error.316 if j contains a string that is not valid UTF-8
        • Throws type_error.321 if j or a value nested in it is discarded; example: \"cannot serialize discarded value to BON8\"
        "},{"location":"api/basic_json/to_bon8/#complexity","title":"Complexity","text":"

        Linear in the size of the JSON value j.

        "},{"location":"api/basic_json/to_bon8/#examples","title":"Examples","text":"Example: serialize a JSON value to BON8

        The example shows the serialization of a JSON value to a byte vector in BON8 format.

        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // create a JSON value\n    json j = R\"({\"compact\": true, \"format\": \"BON8\", \"schema\": 0})\"_json;\n\n    // serialize it to BON8\n    std::vector<std::uint8_t> v = json::to_bon8(j);\n\n    // print the vector content\n    for (auto& byte : v)\n    {\n        std::cout << \"0x\" << std::hex << std::setw(2) << std::setfill('0') << (int)byte << \" \";\n    }\n    std::cout << std::endl;\n}\n

        Output:

        0x89 0x63 0x6f 0x6d 0x70 0x61 0x63 0x74 0xf9 0x66 0x6f 0x72 0x6d 0x61 0x74 0xff 0x42 0x4f 0x4e 0x38 0xff 0x73 0x63 0x68 0x65 0x6d 0x61 0x90 \n
        Example: type_error.316 exception

        The example shows how serializing a string that is not valid UTF-8 throws an exception, because BON8 stores strings as UTF-8.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON string that is not valid UTF-8\n    std::string invalid_utf8;\n    invalid_utf8.push_back(static_cast<char>(0xFF));\n    json j = invalid_utf8;\n\n    // exception type_error.316\n    try\n    {\n        json::to_bon8(j);\n    }\n    catch (const json::type_error& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

        Output:

        [json.exception.type_error.316] invalid UTF-8 byte at index 0: 0xFF\n
        "},{"location":"api/basic_json/to_bon8/#see-also","title":"See also","text":"
        • from_bon8 create a JSON value from an input in BON8 format
        • to_cbor create a CBOR serialization of a JSON value
        • to_msgpack create a MessagePack serialization of a JSON value
        • to_bson create a BSON serialization of a JSON value
        • to_ubjson create a UBJSON serialization of a JSON value
        • to_bjdata create a BJData serialization of a JSON value
        "},{"location":"api/basic_json/to_bon8/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json/to_bson/","title":"nlohmann::basic_json::to_bson","text":"
        // (1)\nstatic std::vector<std::uint8_t> to_bson(const basic_json& j,\n                                         const error_handler_t error_handler = error_handler_t::keep);\n\n// (2)\nstatic void to_bson(const basic_json& j, detail::output_adapter<std::uint8_t> o,\n                    const error_handler_t error_handler = error_handler_t::keep);\nstatic void to_bson(const basic_json& j, detail::output_adapter<char> o,\n                    const error_handler_t error_handler = error_handler_t::keep);\n

        BSON (Binary JSON) is a binary format in which zero or more ordered key/value pairs are stored as a single entity (a so-called document).

        1. Returns a byte vector containing the BSON serialization.
        2. Writes the BSON serialization to an output adapter.

        The exact mapping and its limitations are described on a dedicated page.

        "},{"location":"api/basic_json/to_bson/#parameters","title":"Parameters","text":"j (in) JSON value to serialize o (in) output adapter to write serialization to error_handler (in) how to treat a string or object key in j that is not valid UTF-8; see error_handler_t. The default, keep, writes the ill-formed bytes to the output as is, as every version of to_bson did before this parameter was added; strict throws; replace/ignore sanitize it the same way dump would. If JSON_STRICT_BINARY_UTF8 is enabled, the default is strict instead."},{"location":"api/basic_json/to_bson/#return-value","title":"Return value","text":"
        1. BSON serialization as a byte vector
        2. (none)
        "},{"location":"api/basic_json/to_bson/#exception-safety","title":"Exception safety","text":"

        Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

        "},{"location":"api/basic_json/to_bson/#exceptions","title":"Exceptions","text":"
        • Throws type_error.317 if the top-level type of the JSON value is not an object; example: \"to serialize to BSON, top-level type must be object, but is string\"
        • Throws out_of_range.407 if j contains a signed integer outside the range of int64 or an unsigned integer outside the range of uint64, which is only possible with a number type wider than 64 bits; example: \"integer number 9223372036854775808 cannot be represented by BSON as it does not fit int64\"
        • Throws out_of_range.409 if a key in the JSON object contains a null byte (code point U+0000); example: \"BSON key cannot contain code point U+0000 (at byte 2)\"
        • Throws out_of_range.412 if the length of a document, array, string, or binary value exceeds the range of the 32-bit BSON length field; example: \"BSON length 2147483661 exceeds maximum of 2147483647\"
        • Throws out_of_range.415 if the subtype of a binary value exceeds 255, the maximum of the BSON binary subtype; example: \"subtype 70000 is too large for the BSON binary subtype (max 255)\"
        • Throws type_error.316 if a string or object key is not valid UTF-8 and error_handler is strict (the default only if JSON_STRICT_BINARY_UTF8 is enabled)
        • Throws type_error.321 if a value nested in j is discarded (the top-level value itself is covered by type_error.317 above, since it must be an object); example: \"cannot serialize discarded value to BSON\"
        "},{"location":"api/basic_json/to_bson/#complexity","title":"Complexity","text":"

        Linear in the size of the JSON value j. The length prefixes of all nested documents and arrays are computed in one pass before anything is written.

        "},{"location":"api/basic_json/to_bson/#examples","title":"Examples","text":"Example: serialize a JSON value to BSON

        The example shows the serialization of a JSON value to a byte vector in BSON format.

        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // create a JSON value\n    json j = R\"({\"compact\": true, \"schema\": 0})\"_json;\n\n    // serialize it to BSON\n    std::vector<std::uint8_t> v = json::to_bson(j);\n\n    // print the vector content\n    for (auto& byte : v)\n    {\n        std::cout << \"0x\" << std::hex << std::setw(2) << std::setfill('0') << (int)byte << \" \";\n    }\n    std::cout << std::endl;\n}\n

        Output:

        0x1b 0x00 0x00 0x00 0x08 0x63 0x6f 0x6d 0x70 0x61 0x63 0x74 0x00 0x01 0x10 0x73 0x63 0x68 0x65 0x6d 0x61 0x00 0x00 0x00 0x00 0x00 0x00 \n
        Example: out_of_range.409 exception

        The example shows how serializing a JSON object whose key contains a null byte (U+0000) throws an exception, because BSON keys are null-terminated C strings and cannot contain U+0000 themselves.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON object whose key contains a null byte (U+0000)\n    std::string key = \"ab\";\n    key.push_back('\\0');\n    key.push_back('c');\n    json j = {{key, 1}};\n\n    // exception out_of_range.409\n    try\n    {\n        json::to_bson(j);\n    }\n    catch (const json::out_of_range& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

        Output:

        [json.exception.out_of_range.409] BSON key cannot contain code point U+0000 (at byte 2)\n
        "},{"location":"api/basic_json/to_bson/#see-also","title":"See also","text":"
        • from_bson create a JSON value from an input in BSON format
        • to_cbor create a CBOR serialization of a JSON value
        • to_msgpack create a MessagePack serialization of a JSON value
        • to_ubjson create a UBJSON serialization of a JSON value
        • to_bjdata create a BJData serialization of a JSON value
        • to_bon8 create a BON8 serialization of a JSON value
        "},{"location":"api/basic_json/to_bson/#version-history","title":"Version history","text":"
        • Added in version 3.4.0.
        • Throws out_of_range.412 and out_of_range.415 since version 3.13.0 unreleased.
        • Linear in the size of j, and no longer limited by the call stack for deeply nested values, since version 3.13.0 unreleased.
        • out_of_range.415 is now detected before anything is written, like the other exceptions above, since version 3.13.0 unreleased.
        • Throws type_error.321 for a discarded value nested in j since version 3.13.0 unreleased; previously, it was silently skipped, producing a document whose declared size did not match what was actually written.
        • Added error_handler parameter in version 3.13.0 unreleased. Its default, keep, writes the bytes of a string or object key that is not valid UTF-8 unchanged, as before; strict (the default if JSON_STRICT_BINARY_UTF8 is enabled) throws type_error.316 before anything is written.
        • Throws out_of_range.407 for integers that do not fit 64 bits since version 3.13.0 unreleased; previously, integers of a number type wider than 64 bits were silently truncated.
        "},{"location":"api/basic_json/to_cbor/","title":"nlohmann::basic_json::to_cbor","text":"
        // (1)\nstatic std::vector<std::uint8_t> to_cbor(const basic_json& j,\n                                          const error_handler_t error_handler = error_handler_t::keep);\n\n// (2)\nstatic void to_cbor(const basic_json& j, detail::output_adapter<std::uint8_t> o,\n                     const error_handler_t error_handler = error_handler_t::keep);\nstatic void to_cbor(const basic_json& j, detail::output_adapter<char> o,\n                     const error_handler_t error_handler = error_handler_t::keep);\n

        Serializes a given JSON value j to a byte vector using the CBOR (Concise Binary Object Representation) serialization format. CBOR is a binary serialization format that aims to be more compact than JSON itself, yet more efficient to parse.

        1. Returns a byte vector containing the CBOR serialization.
        2. Writes the CBOR serialization to an output adapter.

        The exact mapping and its limitations are described on a dedicated page.

        "},{"location":"api/basic_json/to_cbor/#parameters","title":"Parameters","text":"j (in) JSON value to serialize o (in) output adapter to write serialization to error_handler (in) how to treat a string or object key in j that is not valid UTF-8; see error_handler_t. The default, keep, writes the ill-formed bytes to the output as is, as every version of to_cbor did before this parameter was added; strict throws; replace/ignore sanitize it the same way dump would. If JSON_STRICT_BINARY_UTF8 is enabled, the default is strict instead."},{"location":"api/basic_json/to_cbor/#return-value","title":"Return value","text":"
        1. CBOR serialization as a byte vector
        2. (none)
        "},{"location":"api/basic_json/to_cbor/#exception-safety","title":"Exception safety","text":"

        Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

        "},{"location":"api/basic_json/to_cbor/#exceptions","title":"Exceptions","text":"
        • Throws out_of_range.407 if j contains an integer outside [-2^64, 2^64-1], which is only possible with a number type wider than 64 bits; example: \"integer number 18446744073709551616 cannot be represented by CBOR as it does not fit [-2^64, 2^64-1]\"
        • Throws type_error.316 if a string or object key in j is not valid UTF-8 and error_handler is strict (the default only if JSON_STRICT_BINARY_UTF8 is enabled)
        • Throws type_error.321 if j or a value nested in it is discarded; example: \"cannot serialize discarded value to CBOR\"
        "},{"location":"api/basic_json/to_cbor/#complexity","title":"Complexity","text":"

        Linear in the size of the JSON value j.

        "},{"location":"api/basic_json/to_cbor/#examples","title":"Examples","text":"Example

        The example shows the serialization of a JSON value to a byte vector in CBOR format.

        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // create a JSON value\n    json j = R\"({\"compact\": true, \"schema\": 0})\"_json;\n\n    // serialize it to CBOR\n    std::vector<std::uint8_t> v = json::to_cbor(j);\n\n    // print the vector content\n    for (auto& byte : v)\n    {\n        std::cout << \"0x\" << std::hex << std::setw(2) << std::setfill('0') << (int)byte << \" \";\n    }\n    std::cout << std::endl;\n}\n

        Output:

        0xa2 0x67 0x63 0x6f 0x6d 0x70 0x61 0x63 0x74 0xf5 0x66 0x73 0x63 0x68 0x65 0x6d 0x61 0x00 \n
        "},{"location":"api/basic_json/to_cbor/#see-also","title":"See also","text":"
        • from_cbor create a JSON value from an input in CBOR format
        • to_msgpack create a MessagePack serialization of a JSON value
        • to_bson create a BSON serialization of a JSON value
        • to_ubjson create a UBJSON serialization of a JSON value
        • to_bjdata create a BJData serialization of a JSON value
        • to_bon8 create a BON8 serialization of a JSON value
        "},{"location":"api/basic_json/to_cbor/#version-history","title":"Version history","text":"
        • Added in version 2.0.9.
        • Compact representation of floating-point numbers added in version 3.8.0.
        • Added error_handler parameter in version 3.13.0 unreleased. Its default, keep, writes the bytes of a string or object key that is not valid UTF-8 unchanged, as before; strict (the default if JSON_STRICT_BINARY_UTF8 is enabled) throws type_error.316.
        • Throws type_error.321 for a discarded value since version 3.13.0 unreleased; previously, a discarded value nested in an array or object was silently skipped, producing invalid CBOR.
        • Throws out_of_range.407 for integers that do not fit 64 bits since version 3.13.0 unreleased; previously, integers of a number type wider than 64 bits were silently truncated.
        "},{"location":"api/basic_json/to_msgpack/","title":"nlohmann::basic_json::to_msgpack","text":"
        // (1)\nstatic std::vector<std::uint8_t> to_msgpack(const basic_json& j,\n                                            const error_handler_t error_handler = error_handler_t::keep);\n\n// (2)\nstatic void to_msgpack(const basic_json& j, detail::output_adapter<std::uint8_t> o,\n                       const error_handler_t error_handler = error_handler_t::keep);\nstatic void to_msgpack(const basic_json& j, detail::output_adapter<char> o,\n                       const error_handler_t error_handler = error_handler_t::keep);\n

        Serializes a given JSON value j to a byte vector using the MessagePack serialization format. MessagePack is a binary serialization format that aims to be more compact than JSON itself, yet more efficient to parse.

        1. Returns a byte vector containing the MessagePack serialization.
        2. Writes the MessagePack serialization to an output adapter.

        The exact mapping and its limitations are described on a dedicated page.

        "},{"location":"api/basic_json/to_msgpack/#parameters","title":"Parameters","text":"j (in) JSON value to serialize o (in) output adapter to write serialization to error_handler (in) how to treat a string or object key in j that is not valid UTF-8; see error_handler_t. The default, keep, writes the ill-formed bytes to the output as is, as every version of to_msgpack did before this parameter was added and as the MessagePack specification allows; strict throws; replace/ignore sanitize it the same way dump would. Unlike the other binary writers, the default stays keep even if JSON_STRICT_BINARY_UTF8 is enabled."},{"location":"api/basic_json/to_msgpack/#return-value","title":"Return value","text":"
        1. MessagePack serialization as a byte vector
        2. (none)
        "},{"location":"api/basic_json/to_msgpack/#exception-safety","title":"Exception safety","text":"

        Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

        "},{"location":"api/basic_json/to_msgpack/#exceptions","title":"Exceptions","text":"
        • Throws out_of_range.407 if j contains an integer outside [-2^63, 2^64-1], which is only possible with a number type wider than 64 bits; example: \"integer number 18446744073709551616 cannot be represented by MessagePack as it does not fit [-2^63, 2^64-1]\"
        • Throws out_of_range.412 if the length of a string, binary value, array, or object exceeds 4294967295, the maximum MessagePack can store; example: \"MessagePack length 4294967296 exceeds maximum of 4294967295\"
        • Throws out_of_range.415 if the subtype of a binary value exceeds 255, the maximum of the MessagePack ext type; example: \"subtype 70000 is too large for the MessagePack ext type (max 255)\"
        • Throws type_error.316 if a string or object key in j is not valid UTF-8 and error_handler is strict
        • Throws type_error.321 if j or a value nested in it is discarded; example: \"cannot serialize discarded value to MessagePack\"
        "},{"location":"api/basic_json/to_msgpack/#complexity","title":"Complexity","text":"

        Linear in the size of the JSON value j.

        "},{"location":"api/basic_json/to_msgpack/#examples","title":"Examples","text":"Example: serialize a JSON value to MessagePack

        The example shows the serialization of a JSON value to a byte vector in MessagePack format.

        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // create a JSON value\n    json j = R\"({\"compact\": true, \"schema\": 0})\"_json;\n\n    // serialize it to MessagePack\n    std::vector<std::uint8_t> v = json::to_msgpack(j);\n\n    // print the vector content\n    for (auto& byte : v)\n    {\n        std::cout << \"0x\" << std::hex << std::setw(2) << std::setfill('0') << (int)byte << \" \";\n    }\n    std::cout << std::endl;\n}\n

        Output:

        0x82 0xa7 0x63 0x6f 0x6d 0x70 0x61 0x63 0x74 0xc3 0xa6 0x73 0x63 0x68 0x65 0x6d 0x61 0x00 \n
        Example: out_of_range.415 exception

        The example shows how serializing a binary value whose subtype exceeds 255 throws an exception, because the MessagePack ext type stores the subtype in a single byte.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON value with a binary subtype that exceeds 255\n    json j = json::binary({1, 2, 3}, 300);\n\n    // exception out_of_range.415\n    try\n    {\n        json::to_msgpack(j);\n    }\n    catch (const json::out_of_range& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

        Output:

        [json.exception.out_of_range.415] subtype 300 is too large for the MessagePack ext type (max 255)\n
        "},{"location":"api/basic_json/to_msgpack/#see-also","title":"See also","text":"
        • from_msgpack create a JSON value from an input in MessagePack format
        • to_cbor create a CBOR serialization of a JSON value
        • to_bson create a BSON serialization of a JSON value
        • to_ubjson create a UBJSON serialization of a JSON value
        • to_bjdata create a BJData serialization of a JSON value
        • to_bon8 create a BON8 serialization of a JSON value
        "},{"location":"api/basic_json/to_msgpack/#version-history","title":"Version history","text":"
        • Added in version 2.0.9.
        • Throws out_of_range.412 and out_of_range.415 since version 3.13.0 unreleased.
        • Added error_handler parameter in version 3.13.0 unreleased. Its default, keep, writes the bytes of a string or object key that is not valid UTF-8 unchanged, as before.
        • Fixed in version 3.13.0 unreleased to serialize number_integer_t/number_unsigned_t pairs of different width correctly; before, integers could be serialized with the wrong value if number_integer_t was narrower than number_unsigned_t.
        • Throws type_error.321 for a discarded value since version 3.13.0 unreleased; previously, a discarded value nested in an array or object was silently skipped, producing invalid MessagePack.
        • Throws out_of_range.407 for integers that do not fit 64 bits since version 3.13.0 unreleased; previously, integers of a number type wider than 64 bits were silently truncated.
        "},{"location":"api/basic_json/to_string/","title":"to_string(basic_json)","text":"
        template <typename BasicJsonType>\nstd::string to_string(const BasicJsonType& j);\n

        This function implements a user-defined to_string for JSON objects.

        "},{"location":"api/basic_json/to_string/#template-parameters","title":"Template parameters","text":"BasicJsonType a specialization of basic_json whose string_t is convertible to std::string; for other string types, use dump, which returns a string_t"},{"location":"api/basic_json/to_string/#return-value","title":"Return value","text":"

        string containing the serialization of the JSON value

        "},{"location":"api/basic_json/to_string/#exception-safety","title":"Exception safety","text":"

        Strong guarantee: if an exception is thrown, there are no changes to any JSON value.

        "},{"location":"api/basic_json/to_string/#exceptions","title":"Exceptions","text":"

        Throws type_error.316 if a string stored inside the JSON value is not UTF-8 encoded

        "},{"location":"api/basic_json/to_string/#complexity","title":"Complexity","text":"

        Linear.

        "},{"location":"api/basic_json/to_string/#possible-implementation","title":"Possible implementation","text":"
        template <typename BasicJsonType>\nstd::string to_string(const BasicJsonType& j)\n{\n    return j.dump();\n}\n
        "},{"location":"api/basic_json/to_string/#examples","title":"Examples","text":"Example

        The following code shows how the library's to_string() function integrates with others, allowing argument-dependent lookup.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing std::to_string;\n\nint main()\n{\n    // create values\n    json j = {{\"one\", 1}, {\"two\", 2}};\n    int i = 42;\n\n    // use ADL to select best to_string function\n    auto j_str = to_string(j);  // calling nlohmann::to_string\n    auto i_str = to_string(i);  // calling std::to_string\n\n    // serialize without indentation\n    std::cout << j_str << \"\\n\\n\"\n              << i_str << std::endl;\n}\n

        Output:

        {\"one\":1,\"two\":2}\n\n42\n
        "},{"location":"api/basic_json/to_string/#see-also","title":"See also","text":"
        • dump
        • Serialization - the serialization article
        "},{"location":"api/basic_json/to_string/#version-history","title":"Version history","text":"

        Added in version 3.7.0.

        "},{"location":"api/basic_json/to_ubjson/","title":"nlohmann::basic_json::to_ubjson","text":"
        // (1)\nstatic std::vector<std::uint8_t> to_ubjson(const basic_json& j,\n                                           const bool use_size = false,\n                                           const bool use_type = false,\n                                           const error_handler_t error_handler = error_handler_t::keep);\n\n// (2)\nstatic void to_ubjson(const basic_json& j, detail::output_adapter<std::uint8_t> o,\n                      const bool use_size = false, const bool use_type = false,\n                      const error_handler_t error_handler = error_handler_t::keep);\nstatic void to_ubjson(const basic_json& j, detail::output_adapter<char> o,\n                      const bool use_size = false, const bool use_type = false,\n                      const error_handler_t error_handler = error_handler_t::keep);\n

        Serializes a given JSON value j to a byte vector using the UBJSON (Universal Binary JSON) serialization format. UBJSON aims to be more compact than JSON itself, yet more efficient to parse.

        1. Returns a byte vector containing the UBJSON serialization.
        2. Writes the UBJSON serialization to an output adapter.

        The exact mapping and its limitations are described on a dedicated page.

        "},{"location":"api/basic_json/to_ubjson/#parameters","title":"Parameters","text":"j (in) JSON value to serialize o (in) output adapter to write serialization to use_size (in) whether to add size annotations to container types; optional, false by default. use_type (in) whether to add type annotations to container types (must be combined with use_size = true); optional, false by default. error_handler (in) how to treat a string or object key in j that is not valid UTF-8; see error_handler_t. The default, keep, writes the ill-formed bytes to the output as is, as every version of to_ubjson did before this parameter was added; strict throws; replace/ignore sanitize it the same way dump would. If JSON_STRICT_BINARY_UTF8 is enabled, the default is strict instead."},{"location":"api/basic_json/to_ubjson/#return-value","title":"Return value","text":"
        1. UBJSON serialization as a byte vector
        2. (none)
        "},{"location":"api/basic_json/to_ubjson/#exception-safety","title":"Exception safety","text":"

        Strong guarantee: if an exception is thrown, there are no changes in the JSON value.

        "},{"location":"api/basic_json/to_ubjson/#exceptions","title":"Exceptions","text":"
        • Throws other_error.502 if use_type is true and use_size is false, and j contains a non-empty array, object, or binary value.
        • Throws type_error.316 if a string or object key in j is not valid UTF-8 and error_handler is strict (the default only if JSON_STRICT_BINARY_UTF8 is enabled)
        • Throws type_error.321 if j or a value nested in it is discarded; example: \"cannot serialize discarded value to UBJSON\"
        "},{"location":"api/basic_json/to_ubjson/#complexity","title":"Complexity","text":"

        Linear in the size of the JSON value j.

        "},{"location":"api/basic_json/to_ubjson/#examples","title":"Examples","text":"Example: serialize a JSON value to UBJSON

        The example shows the serialization of a JSON value to a byte vector in UBJSON format.

        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\n// function to print UBJSON's diagnostic format\nvoid print_byte(uint8_t byte)\n{\n    if (32 < byte and byte < 128)\n    {\n        std::cout << (char)byte;\n    }\n    else\n    {\n        std::cout << (int)byte;\n    }\n}\n\nint main()\n{\n    // create a JSON value\n    json j = R\"({\"compact\": true, \"schema\": false})\"_json;\n\n    // serialize it to UBJSON\n    std::vector<std::uint8_t> v = json::to_ubjson(j);\n\n    // print the vector content\n    for (auto& byte : v)\n    {\n        print_byte(byte);\n    }\n    std::cout << std::endl;\n\n    // create an array of numbers\n    json array = {1, 2, 3, 4, 5, 6, 7, 8};\n\n    // serialize it to UBJSON using default representation\n    std::vector<std::uint8_t> v_array = json::to_ubjson(array);\n    // serialize it to UBJSON using size optimization\n    std::vector<std::uint8_t> v_array_size = json::to_ubjson(array, true);\n    // serialize it to UBJSON using type optimization\n    std::vector<std::uint8_t> v_array_size_and_type = json::to_ubjson(array, true, true);\n\n    // print the vector contents\n    for (auto& byte : v_array)\n    {\n        print_byte(byte);\n    }\n    std::cout << std::endl;\n\n    for (auto& byte : v_array_size)\n    {\n        print_byte(byte);\n    }\n    std::cout << std::endl;\n\n    for (auto& byte : v_array_size_and_type)\n    {\n        print_byte(byte);\n    }\n    std::cout << std::endl;\n}\n

        Output:

        {i7compactTi6schemaF}\n[i1i2i3i4i5i6i7i8]\n[#i8i1i2i3i4i5i6i7i8\n[$i#i812345678\n
        Example: other_error.502 exception

        The example shows how requesting type annotations (use_type) without size annotations (use_size) throws an exception, because type-optimized containers can only be read back with a preceding size.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a non-empty JSON array\n    json j = {1, 2, 3};\n\n    // exception other_error.502\n    try\n    {\n        json::to_ubjson(j, false, true);\n    }\n    catch (const json::other_error& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

        Output:

        [json.exception.other_error.502] use_type requires use_size = true\n
        "},{"location":"api/basic_json/to_ubjson/#see-also","title":"See also","text":"
        • from_ubjson create a JSON value from an input in UBJSON format
        • to_cbor create a CBOR serialization of a JSON value
        • to_msgpack create a MessagePack serialization of a JSON value
        • to_bson create a BSON serialization of a JSON value
        • to_bjdata create a BJData serialization of a JSON value
        • to_bon8 create a BON8 serialization of a JSON value
        "},{"location":"api/basic_json/to_ubjson/#version-history","title":"Version history","text":"
        • Added in version 3.1.0.
        • Added error_handler parameter in version 3.13.0 unreleased. Its default, keep, writes the bytes of a string or object key that is not valid UTF-8 unchanged, as before; strict (the default if JSON_STRICT_BINARY_UTF8 is enabled) throws type_error.316.
        • Throws type_error.321 for a discarded value since version 3.13.0 unreleased; previously, a discarded value nested in an array or object was silently skipped, producing invalid UBJSON.
        "},{"location":"api/basic_json/type/","title":"nlohmann::basic_json::type","text":"
        constexpr value_t type() const noexcept;\n

        Return the type of the JSON value as a value from the value_t enumeration.

        "},{"location":"api/basic_json/type/#return-value","title":"Return value","text":"

        the type of the JSON value

        Value type return value null value_t::null boolean value_t::boolean string value_t::string number (integer) value_t::number_integer number (unsigned integer) value_t::number_unsigned number (floating-point) value_t::number_float object value_t::object array value_t::array binary value_t::binary discarded value_t::discarded"},{"location":"api/basic_json/type/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this member function never throws exceptions.

        "},{"location":"api/basic_json/type/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json/type/#examples","title":"Examples","text":"Example

        The following code exemplifies type() for all JSON types.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_null;\n    json j_boolean = true;\n    json j_number_integer = -17;\n    json j_number_unsigned = 42u;\n    json j_number_float = 23.42;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n    json j_string = \"Hello, world\";\n\n    // call type()\n    std::cout << std::boolalpha;\n    std::cout << (j_null.type() == json::value_t::null) << '\\n';\n    std::cout << (j_boolean.type() == json::value_t::boolean) << '\\n';\n    std::cout << (j_number_integer.type() == json::value_t::number_integer) << '\\n';\n    std::cout << (j_number_unsigned.type() == json::value_t::number_unsigned) << '\\n';\n    std::cout << (j_number_float.type() == json::value_t::number_float) << '\\n';\n    std::cout << (j_object.type() == json::value_t::object) << '\\n';\n    std::cout << (j_array.type() == json::value_t::array) << '\\n';\n    std::cout << (j_string.type() == json::value_t::string) << '\\n';\n}\n

        Output:

        true\ntrue\ntrue\ntrue\ntrue\ntrue\ntrue\ntrue\n
        "},{"location":"api/basic_json/type/#see-also","title":"See also","text":"
        • operator value_t implicit conversion operator equivalent to this named member function
        • type_name returns the type as a string, for use in error messages
        • value_t the enumeration of JSON types
        • basic_json_view::type - the same function on a zero-copy view
        "},{"location":"api/basic_json/type/#version-history","title":"Version history","text":"
        • Added in version 1.0.0.
        • Added unsigned integer type in version 2.0.0.
        • Added binary type in version 3.8.0.
        "},{"location":"api/basic_json/type_error/","title":"nlohmann::basic_json::type_error","text":"
        class type_error : public exception;\n

        This exception is thrown in case of a type error; that is, a library function is executed on a JSON value whose type does not match the expected semantics.

        Exceptions have ids 3xx (see list of type errors).

        classDiagram\n  direction LR\n\n    class std_exception [\"std::exception\"] {\n        <<interface>>\n    }\n\n    class json_exception [\"basic_json::exception\"] {\n        +const int id\n        +const char* what() const\n    }\n\n    class json_parse_error [\"basic_json::parse_error\"] {\n        +const std::size_t byte\n    }\n\n    class json_invalid_iterator [\"basic_json::invalid_iterator\"]\n    class json_type_error [\"basic_json::type_error\"]\n    class json_out_of_range [\"basic_json::out_of_range\"]\n    class json_other_error [\"basic_json::other_error\"]\n\n    std_exception <|-- json_exception\n    json_exception <|-- json_parse_error\n    json_exception <|-- json_invalid_iterator\n    json_exception <|-- json_type_error\n    json_exception <|-- json_out_of_range\n    json_exception <|-- json_other_error\n\n    style json_type_error fill:#CCCCFF
        "},{"location":"api/basic_json/type_error/#member-functions","title":"Member functions","text":"
        • what - returns explanatory string
        "},{"location":"api/basic_json/type_error/#member-variables","title":"Member variables","text":"
        • id - the id of the exception
        "},{"location":"api/basic_json/type_error/#examples","title":"Examples","text":"Example

        The following code shows how a type_error exception can be caught.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    try\n    {\n        // calling push_back() on a string value\n        json j = \"string\";\n        j.push_back(\"another string\");\n    }\n    catch (const json::type_error& e)\n    {\n        // output exception information\n        std::cout << \"message: \" << e.what() << '\\n'\n                  << \"exception id: \" << e.id << std::endl;\n    }\n}\n

        Output:

        message: [json.exception.type_error.308] cannot use push_back() with string\nexception id: 308\n
        "},{"location":"api/basic_json/type_error/#see-also","title":"See also","text":"
        • exception for the base class of all exceptions thrown by the library
        • List of type errors
        • parse_error for exceptions indicating a parse error
        • invalid_iterator for exceptions indicating errors with iterators
        • out_of_range for exceptions indicating access out of the defined range
        • other_error for exceptions indicating other library errors
        "},{"location":"api/basic_json/type_error/#version-history","title":"Version history","text":"
        • Since version 3.0.0.
        "},{"location":"api/basic_json/type_name/","title":"nlohmann::basic_json::type_name","text":"
        const char* type_name() const noexcept;\n

        Returns the type name as string to be used in error messages -- usually to indicate that a function was called on a wrong JSON type.

        "},{"location":"api/basic_json/type_name/#return-value","title":"Return value","text":"

        a string representation of the type (value_t):

        Value type return value null \"null\" boolean \"boolean\" string \"string\" number (integer, unsigned integer, floating-point) \"number\" object \"object\" array \"array\" binary \"binary\" discarded \"discarded\" invalid (corrupted value) \"invalid\"

        The \\\"invalid\\\" type

        The \"invalid\" return value indicates a corrupted JSON value \u2014 this can occur if an enum value falls outside the range of valid value_t values. This is useful for diagnosing data corruption or internal errors.

        "},{"location":"api/basic_json/type_name/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this member function never throws exceptions.

        "},{"location":"api/basic_json/type_name/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json/type_name/#examples","title":"Examples","text":"Example

        The following code exemplifies type_name() for all JSON types.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_null;\n    json j_boolean = true;\n    json j_number_integer = -17;\n    json j_number_unsigned = 42u;\n    json j_number_float = 23.42;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n    json j_string = \"Hello, world\";\n\n    // call type_name()\n    std::cout << j_null << \" is a \" << j_null.type_name() << '\\n';\n    std::cout << j_boolean << \" is a \" << j_boolean.type_name() << '\\n';\n    std::cout << j_number_integer << \" is a \" << j_number_integer.type_name() << '\\n';\n    std::cout << j_number_unsigned << \" is a \" << j_number_unsigned.type_name() << '\\n';\n    std::cout << j_number_float << \" is a \" << j_number_float.type_name() << '\\n';\n    std::cout << j_object << \" is an \" << j_object.type_name() << '\\n';\n    std::cout << j_array << \" is an \" << j_array.type_name() << '\\n';\n    std::cout << j_string << \" is a \" << j_string.type_name() << '\\n';\n}\n

        Output:

        null is a null\ntrue is a boolean\n-17 is a number\n42 is a number\n23.42 is a number\n{\"one\":1,\"two\":2} is an object\n[1,2,4,8,16] is an array\n\"Hello, world\" is a string\n
        "},{"location":"api/basic_json/type_name/#see-also","title":"See also","text":"
        • type returns the type of the JSON value
        • value_t the enumeration of JSON types
        • basic_json_view::type_name - the same function on a zero-copy view
        "},{"location":"api/basic_json/type_name/#version-history","title":"Version history","text":"
        • Added in version 1.0.0.
        • Part of the public API version since 2.1.0.
        • Changed return value to const char* and added noexcept in version 3.0.0.
        • Added support for binary type in version 3.8.0.
        • Added \"invalid\" return value for corrupted JSON values in version 3.13.0 unreleased.
        "},{"location":"api/basic_json/unflatten/","title":"nlohmann::basic_json::unflatten","text":"
        basic_json unflatten() const;\n

        The function restores the arbitrary nesting of a JSON value that has been flattened before using the flatten() function. The JSON value must meet certain constraints:

        1. The value must be an object.
        2. The keys must be JSON pointers (see RFC 6901)
        3. The mapped values must be primitive JSON types.
        "},{"location":"api/basic_json/unflatten/#return-value","title":"Return value","text":"

        the original JSON from a flattened version

        "},{"location":"api/basic_json/unflatten/#exception-safety","title":"Exception safety","text":"

        Strong exception safety: if an exception occurs, the original value stays intact.

        "},{"location":"api/basic_json/unflatten/#exceptions","title":"Exceptions","text":"

        The function can throw the following exceptions:

        • Throws type_error.314 if value is not an object
        • Throws type_error.315 if object values are not primitive
        • Throws type_error.313 if a key (JSON pointer) leads to a conflicting nesting; example: \"invalid value to unflatten\"
        • Throws parse_error.106 if an array index in a key begins with '0'; example: \"array index '01' must not begin with '0'\"
        • Throws parse_error.107 if a key is not empty and does not begin with a slash (/); example: \"JSON pointer must be empty or begin with '/' - was: 'a'\"
        • Throws parse_error.108 if a tilde (~) in a key is not followed by 0 or 1; example: \"escape character '~' must be followed with '0' or '1'\"
        • Throws parse_error.109 if an array index in a key is not a number; example: \"array index 'one' is not a number\"
        • Throws out_of_range.404 if a level becomes an array (because one of its keys is 0) and another key at that level begins with a digit but is not a valid array index (such as 1a), or is -; example: \"unresolved reference token '-'\"
        "},{"location":"api/basic_json/unflatten/#complexity","title":"Complexity","text":"

        Linear in the size of the JSON value.

        "},{"location":"api/basic_json/unflatten/#notes","title":"Notes","text":"

        Empty objects and arrays are flattened by flatten() to null values and cannot unflattened to their original type.

        A flattened array and a flattened object whose keys are array indices are indistinguishable, because both are described by the same JSON pointers. A value is therefore restored as an array if and only if one of its keys is the reference token 0, and as an object otherwise: {\"2\": 1} is restored unchanged, whereas {\"0\": 1} is restored as [1]. This decision does not depend on the order in which the flattened object is iterated.

        Apart from these two cases, for a JSON value j, the following is always true: j == j.flatten().unflatten().

        "},{"location":"api/basic_json/unflatten/#examples","title":"Examples","text":"Example

        The following code shows how a flattened JSON object is unflattened into the original nested JSON object.

        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON value\n    json j_flattened =\n    {\n        {\"/answer/everything\", 42},\n        {\"/happy\", true},\n        {\"/list/0\", 1},\n        {\"/list/1\", 0},\n        {\"/list/2\", 2},\n        {\"/name\", \"Niels\"},\n        {\"/nothing\", nullptr},\n        {\"/object/currency\", \"USD\"},\n        {\"/object/value\", 42.99},\n        {\"/pi\", 3.141}\n    };\n\n    // call unflatten()\n    std::cout << std::setw(4) << j_flattened.unflatten() << '\\n';\n}\n

        Output:

        {\n    \"answer\": {\n        \"everything\": 42\n    },\n    \"happy\": true,\n    \"list\": [\n        1,\n        0,\n        2\n    ],\n    \"name\": \"Niels\",\n    \"nothing\": null,\n    \"object\": {\n        \"currency\": \"USD\",\n        \"value\": 42.99\n    },\n    \"pi\": 3.141\n}\n
        "},{"location":"api/basic_json/unflatten/#see-also","title":"See also","text":"
        • flatten the reverse function
        "},{"location":"api/basic_json/unflatten/#version-history","title":"Version history","text":"
        • Added in version 2.0.0.
        • Made the array/object decision independent of the object's iteration order in version 3.13.0 unreleased.
        • Throws parse_error.109 instead of out_of_range.404 for a one-character array index that is not a digit (e.g., /x) in version 3.13.0 unreleased, as it already did for longer ones.
        "},{"location":"api/basic_json/update/","title":"nlohmann::basic_json::update","text":"
        // (1)\nvoid update(const_reference j, bool merge_objects = false);\n\n// (2)\nvoid update(const_iterator first, const_iterator last, bool merge_objects = false);\n
        1. Inserts all values from JSON object j.
        2. Inserts all values from range [first, last)

        When merge_objects is false (default), existing keys are overwritten. When merge_objects is true, recursively merges objects with common keys.

        If the JSON value is null, it is implicitly converted to an empty object before the values are inserted.

        The function is motivated by Python's dict.update function.

        "},{"location":"api/basic_json/update/#iterator-invalidation","title":"Iterator invalidation","text":"

        For ordered_json, adding a value to an object can yield a reallocation, in which case all iterators (including the end() iterator) and all references to the elements are invalidated.

        "},{"location":"api/basic_json/update/#parameters","title":"Parameters","text":"j (in) JSON object to read values from merge_objects (in) when true, keys that exist in both objects and whose value in the source is itself an object are merged recursively; all other values are overwritten as usual (default: false) first (in) the beginning of the range of elements to insert last (in) the end of the range of elements to insert"},{"location":"api/basic_json/update/#exception-safety","title":"Exception safety","text":"

        Basic guarantee: if an exception is thrown during the operation, the JSON value may be partially modified.

        "},{"location":"api/basic_json/update/#exceptions","title":"Exceptions","text":"
        1. The function can throw the following exceptions:
          • Throws type_error.312 if called on JSON values other than objects; example: \"cannot use update() with string\"
        2. The function can throw the following exceptions:
          • Throws type_error.312 if called on JSON values other than objects; example: \"cannot use update() with string\"
          • Throws invalid_iterator.210 if first and last do not belong to the same JSON value; example: \"iterators do not fit\"
        "},{"location":"api/basic_json/update/#complexity","title":"Complexity","text":"
        1. O(N*log(size() + N)), where N is the number of elements to insert.
        2. O(N*log(size() + N)), where N is the number of elements to insert.
        "},{"location":"api/basic_json/update/#notes","title":"Notes","text":"

        The argument j (or, for overload (2), the range [first, last)) may be *this itself or refer to a value contained in *this (for example, a subobject returned by (*this)[key]); it is read as it was when update() was called, before any modification of *this.

        "},{"location":"api/basic_json/update/#examples","title":"Examples","text":"Example: (1) update with another object

        The example shows how update() is used.

        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // create two JSON objects\n    json o1 = R\"( {\"color\": \"red\", \"price\": 17.99, \"names\": {\"de\": \"Flugzeug\"}} )\"_json;\n    json o2 = R\"( {\"color\": \"blue\", \"speed\": 100, \"names\": {\"en\": \"plane\"}} )\"_json;\n    json o3 = o1;\n\n    // add all keys from o2 to o1 (updating \"color\", replacing \"names\")\n    o1.update(o2);\n\n    // add all keys from o2 to o1 (updating \"color\", merging \"names\")\n    o3.update(o2, true);\n\n    // output updated object o1 and o3\n    std::cout << std::setw(2) << o1 << '\\n';\n    std::cout << std::setw(2) << o3 << '\\n';\n}\n

        Output:

        {\n  \"color\": \"blue\",\n  \"names\": {\n    \"en\": \"plane\"\n  },\n  \"price\": 17.99,\n  \"speed\": 100\n}\n{\n  \"color\": \"blue\",\n  \"names\": {\n    \"de\": \"Flugzeug\",\n    \"en\": \"plane\"\n  },\n  \"price\": 17.99,\n  \"speed\": 100\n}\n
        Example: (2) update with an iterator range

        The example shows how update() is used.

        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // create two JSON objects\n    json o1 = R\"( {\"color\": \"red\", \"price\": 17.99, \"names\": {\"de\": \"Flugzeug\"}} )\"_json;\n    json o2 = R\"( {\"color\": \"blue\", \"speed\": 100, \"names\": {\"en\": \"plane\"}} )\"_json;\n    json o3 = o1;\n\n    // add all keys from o2 to o1 (updating \"color\", replacing \"names\")\n    o1.update(o2.begin(), o2.end());\n\n    // add all keys from o2 to o1 (updating \"color\", merging \"names\")\n    o3.update(o2.begin(), o2.end(), true);\n\n    // output updated object o1 and o3\n    std::cout << std::setw(2) << o1 << '\\n';\n    std::cout << std::setw(2) << o3 << '\\n';\n}\n

        Output:

        {\n  \"color\": \"blue\",\n  \"names\": {\n    \"en\": \"plane\"\n  },\n  \"price\": 17.99,\n  \"speed\": 100\n}\n{\n  \"color\": \"blue\",\n  \"names\": {\n    \"de\": \"Flugzeug\",\n    \"en\": \"plane\"\n  },\n  \"price\": 17.99,\n  \"speed\": 100\n}\n
        Example: (1) merge user settings into default settings

        One common use case for this function is the handling of user settings. Assume your application can be configured in some aspects:

        {\n    \"color\": \"red\",\n    \"active\": true,\n    \"name\": {\"de\": \"Maus\", \"en\": \"mouse\"}\n}\n

        The user may override the default settings selectively:

        {\n    \"color\": \"blue\",\n    \"name\": {\"es\": \"rat\u00f3n\"},\n}\n

        Then update manages the merging of default settings and user settings:

        auto user_settings = json::parse(\"config.json\");\nauto effective_settings = get_default_settings();\neffective_settings.update(user_settings);\n

        Now effective_settings contains the default settings, but those keys set by the user are overwritten:

        {\n    \"color\": \"blue\",\n    \"active\": true,\n    \"name\": {\"es\": \"rat\u00f3n\"}\n}\n

        Note existing keys were just overwritten. To merge objects, merge_objects setting should be set to true:

        auto user_settings = json::parse(\"config.json\");\nauto effective_settings = get_default_settings();\neffective_settings.update(user_settings, true);\n
        {\n    \"color\": \"blue\",\n    \"active\": true,\n    \"name\": {\"de\": \"Maus\", \"en\": \"mouse\", \"es\": \"rat\u00f3n\"}\n}\n
        "},{"location":"api/basic_json/update/#see-also","title":"See also","text":"
        • insert add values to an array/object
        • merge_patch applies a JSON Merge Patch
        • Modifying values - the article on modifying values
        "},{"location":"api/basic_json/update/#version-history","title":"Version history","text":"
        • Added in version 3.0.0.
        • Added merge_objects parameter in 3.10.5.
        • Fixed use of freed or relocated memory when the argument is *this or refers to a value contained in *this, in version 3.13.0 unreleased.
        "},{"location":"api/basic_json/value/","title":"nlohmann::basic_json::value","text":"
        // (1)\ntemplate<class ValueType>\nValueType value(const typename object_t::key_type& key,\n                ValueType&& default_value) const;\n\n// (2)\ntemplate<class ValueType, class KeyType>\nValueType value(KeyType&& key,\n                ValueType&& default_value) const;\n\n// (3)\ntemplate<class ValueType>\nValueType value(const json_pointer& ptr,\n                const ValueType& default_value) const;\n

        This is equivalent to Python's dict.get(key, default).

        1. Returns either a copy of an object's element at the specified key key or a given default value if no element with key key exists.

          The function is basically equivalent to executing 

          try {\n   return at(key);\n} catch(out_of_range) {\n   return default_value;\n}\n

        2. See 1. This overload is only available if KeyType is comparable with typename object_t::key_type and typename object_comparator_t::is_transparent denotes a type.

        3. Returns either a copy of an object's element at the specified JSON pointer ptr or a given default value if no value at ptr exists.

          The function is basically equivalent to executing 

          try {\n   return at(ptr);\n} catch(out_of_range) {\n   return default_value;\n}\n

        Differences to at and operator[]

        • Unlike at, this function does not throw if the given key/ptr was not found.
        • Unlike operator[], this function does not implicitly add an element to the position defined by key/ptr key. This function is furthermore also applicable to const objects.

        Integer keys

        Calling this function with an integer key argument (for example, value(0, 1)) does not compile in C++11, where object_comparator_t is not transparent: such an argument would otherwise implicitly convert to a null const char* and, from there, cause undefined behavior when constructing a std::string for the object key. To access an array element with a default value, use at together with a try/catch block, or compare against size instead.

        "},{"location":"api/basic_json/value/#template-parameters","title":"Template parameters","text":"KeyType A type for an object key other than json_pointer that is comparable with string_t using object_comparator_t. This can also be a string view (C++17). ValueType type compatible to JSON values, for instance int for JSON integer numbers, bool for JSON booleans, or std::vector types for JSON arrays. Note the type of the expected value at key/ptr and the default value default_value must be compatible."},{"location":"api/basic_json/value/#parameters","title":"Parameters","text":"key (in) key of the element to access default_value (in) the value to return if key/ptr found no value ptr (in) a JSON pointer to the element to access"},{"location":"api/basic_json/value/#return-value","title":"Return value","text":"
        1. copy of the element at key key or default_value if key is not found
        2. copy of the element at key key or default_value if key is not found
        3. copy of the element at JSON Pointer ptr or default_value if no value for ptr is found
        "},{"location":"api/basic_json/value/#exception-safety","title":"Exception safety","text":"

        Strong guarantee: if an exception is thrown, there are no changes to any JSON value.

        "},{"location":"api/basic_json/value/#exceptions","title":"Exceptions","text":"
        1. The function can throw the following exceptions:
          • Throws type_error.302 if default_value does not match the type of the value at key
          • Throws type_error.306 if the JSON value is not an object; in that case, using value() with a key makes no sense.
        2. See 1.
        3. The function can throw the following exceptions:
          • Throws type_error.302 if default_value does not match the type of the value at ptr
          • Throws type_error.306 if the JSON value is not an array or object; in that case, using value() with a JSON pointer makes no sense.
          • Throws parse_error.106 if an array index in the passed JSON pointer ptr begins with '0'.
          • Throws parse_error.109 if an array index in the passed JSON pointer ptr is not a number.
        "},{"location":"api/basic_json/value/#complexity","title":"Complexity","text":"
        1. Logarithmic in the size of the container.
        2. Logarithmic in the size of the container.
        3. Logarithmic in the size of the container.
        "},{"location":"api/basic_json/value/#notes","title":"Notes","text":"

        null members are not missing

        The default value is used only if the key (or JSON Pointer) does not exist. A member that exists but is null is converted like any other value, so j.value(\"k\", 0) throws a type_error.302 if \"k\" is null. See Access with default value for alternatives.

        Return type

        The value function is a template, and the return type of the function is determined by the type of the provided default value unless otherwise specified. This can have unexpected effects. In the example below, we store a 64-bit unsigned integer. We get exactly that value when using operator[]. However, when we call value and provide 0 as default value, then -1 is returned. This occurs, because 0 has type int which overflows when handling the value 18446744073709551615.

        To address this issue, either provide a correctly typed default value or use the template parameter to specify the desired return type. Note that this issue occurs even when a value is stored at the provided key, and the default value is not used as the return value.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    json j = json::parse(R\"({\"uint64\": 18446744073709551615})\");\n\n    std::cout << \"operator[]:                \" << j[\"uint64\"] << '\\n'\n              << \"default value (int):       \" << j.value(\"uint64\", 0) << '\\n'\n              << \"default value (uint64_t):  \" << j.value(\"uint64\", std::uint64_t(0)) << '\\n'\n              << \"explicit return value type: \" << j.value<std::uint64_t>(\"uint64\", 0) << '\\n';\n}\n

        Output:

        operator[]:                18446744073709551615\ndefault value (int):       -1\ndefault value (uint64_t):  18446744073709551615\nexplicit return value type: 18446744073709551615\n

        Deprecation

        Overload (3) also accepts a json_pointer whose template argument is a basic_json specialization (e.g., nlohmann::json_pointer<nlohmann::json>) instead of a string type. This is deprecated since version 3.11.0 and will be removed in a future major version; use basic_json::json_pointer (for json, nlohmann::json_pointer<std::string>) instead.

        You should be warned by your compiler with a -Wdeprecated-declarations warning if you are using a deprecated function.

        See the migration guide for how to update existing code.

        "},{"location":"api/basic_json/value/#examples","title":"Examples","text":"Example: (1) access specified object element with default value

        The example below shows how object elements can be queried with a default value.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON object with different entry types\n    json j =\n    {\n        {\"integer\", 1},\n        {\"floating\", 42.23},\n        {\"string\", \"hello world\"},\n        {\"boolean\", true},\n        {\"object\", {{\"key1\", 1}, {\"key2\", 2}}},\n        {\"array\", {1, 2, 3}}\n    };\n\n    // access existing values\n    int v_integer = j.value(\"integer\", 0);\n    double v_floating = j.value(\"floating\", 47.11);\n\n    // access nonexisting values and rely on default value\n    std::string v_string = j.value(\"nonexisting\", \"oops\");\n    bool v_boolean = j.value(\"nonexisting\", false);\n\n    // output values\n    std::cout << std::boolalpha << v_integer << \" \" << v_floating\n              << \" \" << v_string << \" \" << v_boolean << \"\\n\";\n}\n

        Output:

        1 42.23 oops false\n
        Example: (2) access specified object element using string_view with default value

        The example below shows how object elements can be queried with a default value.

        #include <iostream>\n#include <string_view>\n#include <nlohmann/json.hpp>\n\nusing namespace std::string_view_literals;\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON object with different entry types\n    json j =\n    {\n        {\"integer\", 1},\n        {\"floating\", 42.23},\n        {\"string\", \"hello world\"},\n        {\"boolean\", true},\n        {\"object\", {{\"key1\", 1}, {\"key2\", 2}}},\n        {\"array\", {1, 2, 3}}\n    };\n\n    // access existing values\n    int v_integer = j.value(\"integer\"sv, 0);\n    double v_floating = j.value(\"floating\"sv, 47.11);\n\n    // access nonexisting values and rely on default value\n    std::string v_string = j.value(\"nonexisting\"sv, \"oops\");\n    bool v_boolean = j.value(\"nonexisting\"sv, false);\n\n    // output values\n    std::cout << std::boolalpha << v_integer << \" \" << v_floating\n              << \" \" << v_string << \" \" << v_boolean << \"\\n\";\n}\n

        Output:

        1 42.23 oops false\n
        Example: (3) access specified object element via JSON Pointer with default value

        The example below shows how object elements can be queried with a default value.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // create a JSON object with different entry types\n    json j =\n    {\n        {\"integer\", 1},\n        {\"floating\", 42.23},\n        {\"string\", \"hello world\"},\n        {\"boolean\", true},\n        {\"object\", {{\"key1\", 1}, {\"key2\", 2}}},\n        {\"array\", {1, 2, 3}}\n    };\n\n    // access existing values\n    int v_integer = j.value(\"/integer\"_json_pointer, 0);\n    double v_floating = j.value(\"/floating\"_json_pointer, 47.11);\n\n    // access nonexisting values and rely on default value\n    std::string v_string = j.value(\"/nonexisting\"_json_pointer, \"oops\");\n    bool v_boolean = j.value(\"/nonexisting\"_json_pointer, false);\n\n    // output values\n    std::cout << std::boolalpha << v_integer << \" \" << v_floating\n              << \" \" << v_string << \" \" << v_boolean << \"\\n\";\n}\n

        Output:

        1 42.23 oops false\n
        Example: (1) type_error.302 and type_error.306 exceptions

        The example below shows how value() throws type_error.302 when the default value's type does not match the type of the stored value, and type_error.306 when value() is called on a JSON value that is not an object.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON object with a string value\n    json j = {{\"name\", \"the good\"}};\n\n    // exception type_error.302\n    try\n    {\n        int v = j.value(\"name\", 0);\n        std::cout << v << '\\n';\n    }\n    catch (const json::type_error& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n\n    // exception type_error.306\n    try\n    {\n        json str = \"I am a string\";\n        auto v = str.value(\"name\", 0);\n        std::cout << v << '\\n';\n    }\n    catch (const json::type_error& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

        Output:

        [json.exception.type_error.302] type must be number, but is string\n[json.exception.type_error.306] cannot use value() with string\n
        "},{"location":"api/basic_json/value/#see-also","title":"See also","text":"
        • see at for access by reference with range checking
        • see operator[] for unchecked access by reference
        • basic_json_view::value - the same access on a zero-copy view
        "},{"location":"api/basic_json/value/#version-history","title":"Version history","text":"
        1. Added in version 1.0.0. Changed parameter default_value type from const ValueType& to ValueType&& in version 3.11.0. Deleted overload for integral key types added in version 3.13.0 unreleased to reject such calls at compile time instead of causing undefined behavior at runtime.
        2. Added in version 3.11.0. Made ValueType the first template parameter in version 3.11.2. Fixed in version 3.13.0 unreleased to consistently accept std::string_view-convertible keys, as already supported by operator[], at, find, and other lookup functions.
        3. Added in version 2.0.2. Extended to work with arrays in version 3.13.0 unreleased, including fixing an issue where resolving ptr through an array unexpectedly threw out_of_range instead of returning the resolved element (or default_value, as documented). Throws parse_error.109 instead of returning default_value for a one-character array index that is not a digit (e.g., /x) in version 3.13.0, as it already did for longer ones.
        "},{"location":"api/basic_json/value_t/","title":"nlohmann::basic_json::value_t","text":"
        enum class value_t : std::uint8_t {\n    null,\n    object,\n    array,\n    string,\n    boolean,\n    number_integer,\n    number_unsigned,\n    number_float,\n    binary,\n    discarded\n};\n

        This enumeration collects the different JSON types. It is internally used to distinguish the stored values, and the functions is_null, is_object, is_array, is_string, is_boolean, is_number (with is_number_integer, is_number_unsigned, and is_number_float), is_discarded, is_binary, is_primitive, and is_structured rely on it.

        "},{"location":"api/basic_json/value_t/#notes","title":"Notes","text":"

        Ordering

        The order of types is as follows:

        1. null
        2. boolean
        3. number_integer, number_unsigned, number_float
        4. object
        5. array
        6. string
        7. binary

        discarded is unordered.

        flowchart LR\n    A[null] --> B[boolean]\n    B --> C[\"number_integer / number_unsigned / number_float\"]\n    C --> D[object]\n    D --> E[array]\n    E --> F[string]\n    F --> G[binary]

        Types of numbers

        There are three enumerators for numbers (number_integer, number_unsigned, and number_float) to distinguish between different types of numbers:

        • number_unsigned_t for unsigned integers
        • number_integer_t for signed integers
        • number_float_t for floating-point numbers or to approximate integers which do not fit into the limits of their respective type

        Comparison operators

        operator< and operator<=> (since C++20) are overloaded and compare according to the ordering described above. Until C++20 all other relational and equality operators yield results according to the integer value of each enumerator. Since C++20 some compilers consider the rewritten candidates generated from operator<=> during overload resolution, while others do not. For predictable and portable behavior use:

        • operator< or operator<=> when wanting to compare according to the order described above
        • operator== or operator!= when wanting to compare according to each enumerators integer value
        "},{"location":"api/basic_json/value_t/#examples","title":"Examples","text":"Example

        The following code how type() queries the value_t for all JSON types.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_null;\n    json j_boolean = true;\n    json j_number_integer = -17;\n    json j_number_unsigned = 42u;\n    json j_number_float = 23.42;\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n    json j_string = \"Hello, world\";\n\n    // call type()\n    std::cout << std::boolalpha;\n    std::cout << (j_null.type() == json::value_t::null) << '\\n';\n    std::cout << (j_boolean.type() == json::value_t::boolean) << '\\n';\n    std::cout << (j_number_integer.type() == json::value_t::number_integer) << '\\n';\n    std::cout << (j_number_unsigned.type() == json::value_t::number_unsigned) << '\\n';\n    std::cout << (j_number_float.type() == json::value_t::number_float) << '\\n';\n    std::cout << (j_object.type() == json::value_t::object) << '\\n';\n    std::cout << (j_array.type() == json::value_t::array) << '\\n';\n    std::cout << (j_string.type() == json::value_t::string) << '\\n';\n}\n

        Output:

        true\ntrue\ntrue\ntrue\ntrue\ntrue\ntrue\ntrue\n
        "},{"location":"api/basic_json/value_t/#see-also","title":"See also","text":"
        • type return the type of the JSON value
        • type_name return the type as string
        • operator value_t return the type of the JSON value
        • is_primitive return whether the type is primitive
        • is_structured return whether the type is structured
        "},{"location":"api/basic_json/value_t/#version-history","title":"Version history","text":"
        • Added in version 1.0.0.
        • Added unsigned integer type in version 2.0.0.
        • Added binary type in version 3.8.0.
        "},{"location":"api/basic_json/with_t/","title":"nlohmann::basic_json::with_t","text":"

        Member alias templates with_object_t, with_array_t, with_string_t, with_boolean_t, with_integers_t, with_float_t, with_allocator_t, with_json_serializer_t, with_binary_t, and with_base_class_t.

        template<template<typename, typename, typename...> class ObjectType2>\nusing with_object_t = basic_json<ObjectType2, ArrayType, StringType, BooleanType,\n      NumberIntegerType, NumberUnsignedType, NumberFloatType,\n      AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;\n\ntemplate<template<typename, typename...> class ArrayType2>\nusing with_array_t = basic_json<ObjectType, ArrayType2, StringType, BooleanType,\n      NumberIntegerType, NumberUnsignedType, NumberFloatType,\n      AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;\n\ntemplate<class StringType2>\nusing with_string_t = basic_json<ObjectType, ArrayType, StringType2, BooleanType,\n      NumberIntegerType, NumberUnsignedType, NumberFloatType,\n      AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;\n\ntemplate<class BooleanType2>\nusing with_boolean_t = basic_json<ObjectType, ArrayType, StringType, BooleanType2,\n      NumberIntegerType, NumberUnsignedType, NumberFloatType,\n      AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;\n\ntemplate<class NumberIntegerType2, class NumberUnsignedType2>\nusing with_integers_t = basic_json<ObjectType, ArrayType, StringType, BooleanType,\n      NumberIntegerType2, NumberUnsignedType2, NumberFloatType,\n      AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;\n\ntemplate<class NumberFloatType2>\nusing with_float_t = basic_json<ObjectType, ArrayType, StringType, BooleanType,\n      NumberIntegerType, NumberUnsignedType, NumberFloatType2,\n      AllocatorType, JSONSerializer, BinaryType, CustomBaseClass>;\n\ntemplate<template<typename> class AllocatorType2>\nusing with_allocator_t = basic_json<ObjectType, ArrayType, StringType, BooleanType,\n      NumberIntegerType, NumberUnsignedType, NumberFloatType,\n      AllocatorType2, JSONSerializer, BinaryType, CustomBaseClass>;\n\ntemplate<template<typename, typename = void> class JSONSerializer2>\nusing with_json_serializer_t = basic_json<ObjectType, ArrayType, StringType, BooleanType,\n      NumberIntegerType, NumberUnsignedType, NumberFloatType,\n      AllocatorType, JSONSerializer2, BinaryType, CustomBaseClass>;\n\ntemplate<class BinaryType2>\nusing with_binary_t = basic_json<ObjectType, ArrayType, StringType, BooleanType,\n      NumberIntegerType, NumberUnsignedType, NumberFloatType,\n      AllocatorType, JSONSerializer, BinaryType2, CustomBaseClass>;\n\ntemplate<class CustomBaseClass2>\nusing with_base_class_t = basic_json<ObjectType, ArrayType, StringType, BooleanType,\n      NumberIntegerType, NumberUnsignedType, NumberFloatType,\n      AllocatorType, JSONSerializer, BinaryType, CustomBaseClass2>;\n

        These member alias templates make it easier to create a basic_json type that is identical to the current type except for one (or, in the case of with_integers_t, two) of its template parameters. Spelling out all 11 template parameters of basic_json just to change a single one is verbose and error-prone; these aliases only require the replacement type(s).

        with_object_t<ObjectType2> replaces ObjectType with_array_t<ArrayType2> replaces ArrayType with_string_t<StringType2> replaces StringType with_boolean_t<BooleanType2> replaces BooleanType with_integers_t<NumberIntegerType2, NumberUnsignedType2> replaces both NumberIntegerType and NumberUnsignedType; the two are combined into a single alias because they are usually changed together (for instance, when switching to fixed-width integer types) with_float_t<NumberFloatType2> replaces NumberFloatType with_allocator_t<AllocatorType2> replaces AllocatorType with_json_serializer_t<JSONSerializer2> replaces JSONSerializer with_binary_t<BinaryType2> replaces BinaryType with_base_class_t<CustomBaseClass2> replaces CustomBaseClass; see also json_base_class_t"},{"location":"api/basic_json/with_t/#notes","title":"Notes","text":"

        All other template parameters are kept unchanged, so the resulting type still uses, for instance, the same ObjectType unless with_object_t itself is used.

        The aliases are members of every basic_json specialization, including ordered_json, and the type they produce is again a basic_json specialization. They can therefore be chained to replace several template parameters at once:

        using my_json = nlohmann::json::with_integers_t<int, unsigned int>::with_float_t<float>;\nusing my_ordered_json = nlohmann::ordered_json::with_string_t<std::wstring>;\n

        The result is the same type as spelling out all template parameters, so the order of the chained aliases does not matter. For instance, nlohmann::json::with_object_t<nlohmann::ordered_map> is nlohmann::ordered_json.

        "},{"location":"api/basic_json/with_t/#examples","title":"Examples","text":"Example

        The following code shows how with_object_t can be used to create a JSON type that stores object elements in a std::map and therefore keeps them sorted by key, unlike the default type which preserves insertion order only when nlohmann::ordered_json is used.

        #include <iostream>\n#include <map>\n#include <nlohmann/json.hpp>\n\n// a JSON type that stores objects in a std::map (which keeps keys sorted)\n// instead of the default ordered associative container\nusing sorted_json = nlohmann::json::with_object_t<std::map>;\n\nint main()\n{\n    sorted_json j;\n    j[\"c\"] = 1;\n    j[\"a\"] = 2;\n    j[\"b\"] = 3;\n\n    // keys are sorted, because std::map is used to store the object\n    std::cout << j.dump() << std::endl;\n}\n

        Output:

        {\"a\":2,\"b\":3,\"c\":1}\n
        "},{"location":"api/basic_json/with_t/#see-also","title":"See also","text":"
        • basic_json - the template parameters that can be replaced
        • json_base_class_t - the type used for CustomBaseClass
        "},{"location":"api/basic_json/with_t/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json/~basic_json/","title":"nlohmann::basic_json::~basic_json","text":"
        ~basic_json() noexcept;\n

        Destroys the JSON value and frees all allocated memory.

        "},{"location":"api/basic_json/~basic_json/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this member function never throws exceptions.

        "},{"location":"api/basic_json/~basic_json/#complexity","title":"Complexity","text":"

        Linear.

        "},{"location":"api/basic_json/~basic_json/#see-also","title":"See also","text":"
        • basic_json constructs a JSON value
        • clear clears the content of a JSON value without destroying it
        "},{"location":"api/basic_json/~basic_json/#version-history","title":"Version history","text":"
        • Added in version 1.0.0.
        "},{"location":"api/basic_json_document/","title":"nlohmann::basic_json_document","text":"

        Defined in header <nlohmann/json_view.hpp>

        template<typename BasicJsonType, bool Editable = false>\nclass basic_json_document;\n

        A parsed JSON text, held as a flat index of its values (16 bytes per value) instead of a tree of BasicJsonType values. Strings and numbers stay in the source text; only strings that contain escapes are decoded, into one buffer owned by the document. basic_json_view is a read-only handle to one value of a basic_json_document; materialize() turns a subtree into the BasicJsonType value that BasicJsonType::parse() would have produced for it.

        A document may borrow the text it was parsed from (the caller's buffer must then outlive the document) or own it (a copy, or an rvalue std::string that was moved in); see owns_source. basic_json_document is move-only: a copy would duplicate the whole index, the decoded-string buffer, and the owned text, and a move is enough in every use we know of. The index, the decoded strings, and the owned text live in one heap block that the document holds through a std::unique_ptr; this is also why views survive a move of the document.

        With Editable == true, the document also offers set, push_back, insert, and erase to change values in place, see Edits below. The source text itself is never written; a read-only document (Editable == false, the default) does not carry any of the bookkeeping edits need, and calling any of them on one fails to compile (static_assert).

        "},{"location":"api/basic_json_document/#template-parameters","title":"Template parameters","text":"BasicJsonType a specialization of basic_json, for instance json or ordered_json. Only 64-bit number_integer_t/number_unsigned_t types are supported; this is checked with a static_assert. Editable whether the document supports set, push_back, insert, and erase (optional, false by default). See Edits below."},{"location":"api/basic_json_document/#specializations","title":"Specializations","text":"
        • json_document - read-only documents of the default specialization json
        • ordered_json_document - read-only documents of ordered_json
        • json_editable_document - editable documents of json
        • ordered_json_editable_document - editable documents of ordered_json
        "},{"location":"api/basic_json_document/#member-types","title":"Member types","text":"
        • view_type - the type of view returned by root() (basic_json_view<BasicJsonType, Editable>)
        • value_t - the JSON type enumeration, see basic_json::value_t
        • image_check - how load() validates an image (full, bounds, none), see image_check
        "},{"location":"api/basic_json_document/#member-functions","title":"Member functions","text":"
        • (constructor)
        "},{"location":"api/basic_json_document/#parsing","title":"Parsing","text":"
        • parse (static) - deserialize from a compatible input, borrowing or owning it as appropriate
        • parse_copy (static) - deserialize a copy of a compatible input
        • accept (static) - check whether the input is valid JSON
        • read - (re-)parse into this document, reusing its memory
        "},{"location":"api/basic_json_document/#access","title":"Access","text":"
        • root - the view of the root value
        • is_discarded - return whether the last parse failed
        • source - the parsed text
        • owns_source - return whether the document holds its own copy of the text
        • node_count - the number of index entries (values plus object keys)
        • memory_usage - the number of bytes held by the document
        • shrink_to_fit - release unused index capacity
        "},{"location":"api/basic_json_document/#images","title":"Images","text":"
        • save - the document as an image that load() reads without parsing
        • load (static) - read an image written by save()
        "},{"location":"api/basic_json_document/#edits","title":"Edits","text":"
        • set - replace a value, or set an object member, an array element, or the value a JSON pointer refers to (Editable documents only)
        • push_back - append to an array (Editable documents only)
        • insert - insert an element into an array before a given position (Editable documents only)
        • erase - remove an object member, an array element, or the value a JSON pointer refers to (Editable documents only)
        "},{"location":"api/basic_json_document/#edits_1","title":"Edits","text":"

        An editable document (Editable == true) can be changed after parsing, with set, push_back, insert, and erase; json_editable_document and ordered_json_editable_document are the corresponding specializations. A few points apply to every edit:

        • The source text is never written, and the parsed index never moves: every value keeps the node it was parsed into, so views taken before an edit stay valid, including root(). New values (and the element sequences of an edited array/object) go to storage owned by the document, allocated on demand.
        • A view keeps referring to the same value. After set replaces the value a view refers to, that view sees the new value; a view of a value that a later edit replaces or drops keeps showing what it last held. An edit of an array or object, however, invalidates the iterators taken over it (its members may now live in a different sequence), and a string obtained with get_string() stays valid even as further edits happen (earlier buffers of edited text are kept alive, not overwritten).
        • Values are accepted three ways: a basic_json_view of any document (read-only or editable; it is copied, nothing is shared with the source document), a BasicJsonType value, or anything BasicJsonType can be constructed from (numbers, strings, bool, nullptr, containers, ...).
        • dump() writes an edited document with members in document order, new members at the end, and, with number_format::source, keeps the spelling of every number that was not itself edited -- see Editing a document for why this matters.
        • read() discards all edits, shrink_to_fit() does not move the node index once there are edits, and memory_usage() includes the memory edits use. source_offset() of a value introduced by an edit is static_cast<std::size_t>(-1), the same value it reports for a decoded string.
        "},{"location":"api/basic_json_document/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_document/accept/","title":"nlohmann::basic_json_document::accept","text":"
        template<typename InputType>\nstatic bool accept(InputType&& input,\n                   const bool ignore_comments = false,\n                   const bool ignore_trailing_commas = false);\n

        Checks whether the input is valid JSON, accepting and rejecting exactly what BasicJsonType::accept() does, with the same options. Unlike parse(), this function never throws an exception for invalid input, and the returned bool is the only result -- no document is returned.

        The result is the same as that of BasicJsonType::accept() by design; the difference is how the check is made. This function runs the document's index builder (and discards the index), while BasicJsonType::accept() runs the SAX parser with a handler that ignores every event. Use this one if you go on to parse the same input with parse() anyway, so that both use the same code; either is fine for a plain yes/no answer.

        "},{"location":"api/basic_json_document/accept/#template-parameters","title":"Template parameters","text":"InputType A compatible input; see parse."},{"location":"api/basic_json_document/accept/#parameters","title":"Parameters","text":"input (in) Input to check. ignore_comments (in) whether comments should be ignored and treated like whitespace (true) or yield a parse error (false); (optional, false by default) ignore_trailing_commas (in) whether trailing commas in arrays or objects should be ignored and treated like whitespace (true) or yield a parse error (false); (optional, false by default)"},{"location":"api/basic_json_document/accept/#return-value","title":"Return value","text":"

        Whether the input is valid JSON.

        "},{"location":"api/basic_json_document/accept/#exception-safety","title":"Exception safety","text":"

        Strong guarantee: this function itself never throws for an invalid input; it can only throw what allocating the input's own copy (for inputs that are always read into a buffer) throws.

        "},{"location":"api/basic_json_document/accept/#complexity","title":"Complexity","text":"

        Linear in the length of the input.

        "},{"location":"api/basic_json_document/accept/#notes","title":"Notes","text":"

        An integer argument that is not a bool where the flags are expected, such as accept(ptr, len), does not compile; see parse.

        "},{"location":"api/basic_json_document/accept/#examples","title":"Examples","text":"Example 
        #include <iostream>\n#include <nlohmann/json.hpp>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // accept() behaves exactly like basic_json::accept(): the same inputs are\n    // accepted or rejected, with the same ignore_comments/ignore_trailing_commas\n    // options\n    std::cout << json_document::accept(R\"({\"a\": 1})\") << '\\n';\n    std::cout << json_document::accept(R\"({\"a\": 1,})\") << '\\n'; // trailing comma: rejected by default\n    std::cout << json_document::accept(R\"({\"a\": 1,})\", false, true) << '\\n'; // ignore_trailing_commas\n\n    std::cout << (json_document::accept(R\"({\"a\": 1})\") == nlohmann::json::accept(R\"({\"a\": 1})\")) << '\\n';\n}\n

        Output:

        true\nfalse\ntrue\ntrue\n
        "},{"location":"api/basic_json_document/accept/#see-also","title":"See also","text":"
        • parse - deserialize from a compatible input
        • BasicJsonType::accept - the corresponding function of basic_json
        "},{"location":"api/basic_json_document/accept/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_document/basic_json_document/","title":"nlohmann::basic_json_document::basic_json_document","text":"
        // (1)\nbasic_json_document() = default;\n\n// (2)\nbasic_json_document(basic_json_document&& other) noexcept = default;\n\n// (3)\nbasic_json_document(const basic_json_document&) = delete;\n
        1. Creates an empty (discarded) document: root() returns a discarded view, and is_discarded() is true.
        2. Move constructor. Takes over other's index and, if owned, its text; other is left as an empty document. Views taken from other before the move remain valid, because the index is heap-allocated independently of the basic_json_document object.
        3. basic_json_document is move-only. Copying is disabled because of its cost: a copy would duplicate the whole index, the decoded-string buffer, and the owned text, while a move is enough in every use we know of.
        "},{"location":"api/basic_json_document/basic_json_document/#parameters","title":"Parameters","text":"other (in) another document to move the index and text from"},{"location":"api/basic_json_document/basic_json_document/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: the default and move constructors never throw exceptions.

        "},{"location":"api/basic_json_document/basic_json_document/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json_document/basic_json_document/#examples","title":"Examples","text":"Example

        The example below shows the default constructor and demonstrates that basic_json_document is move-only.

        #include <iostream>\n#include <utility>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // the default constructor creates an empty (discarded) document\n    json_document empty;\n    std::cout << empty.is_discarded() << '\\n';\n\n    // json_document is move-only: parse() itself returns by value (moved out),\n    // and a document can be moved again, e.g. into a container\n    json_document doc = json_document::parse(R\"({\"a\": 1})\");\n    json_document moved = std::move(doc);\n    std::cout << moved.root().is_object() << '\\n';\n\n    // copying is disabled at compile time:\n    // json_document another = moved; // does not compile\n}\n

        Output:

        true\ntrue\n
        "},{"location":"api/basic_json_document/basic_json_document/#see-also","title":"See also","text":"
        • parse - deserialize from a compatible input
        • is_discarded - return whether the last parse failed
        "},{"location":"api/basic_json_document/basic_json_document/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_document/erase/","title":"nlohmann::basic_json_document::erase","text":"
        // (1)\nstd::size_t erase(view_type object, string_view_t key);\n\n// (2)\ntemplate<typename I>\nvoid erase(view_type array, I idx);\n\n// (3)\nstd::size_t erase(const json_pointer& ptr);\n

        Only an editable document (Editable == true, e.g. json_editable_document) has erase; calling it on a read-only basic_json_document fails to compile (static_assert).

        1. Removes every member of object whose key is key (see Notes on duplicate keys) and returns how many were removed; 0 if object has no member with this key.
        2. Removes the element at index idx of array, which must already exist (idx < array.size()).
        3. Removes the value the JSON pointer ptr refers to, relative to root(). ptr must not be empty (root() itself cannot be erased), and the parent of the target must already exist. The target is removed as in 1. if the parent is an object (returns the number of removed members, 0 or more) and as in 2. if it is an array (always returns 1).
        "},{"location":"api/basic_json_document/erase/#template-parameters","title":"Template parameters","text":"I an integral type other than bool, deduced (overloads taking a bool or a non-integral type for idx do not participate in overload resolution)."},{"location":"api/basic_json_document/erase/#parameters","title":"Parameters","text":"object (in) the object to remove a member of array (in) the array to remove an element of key (in) the key of the member(s) to remove idx (in) the index of the element to remove; a negative value throws (see Exceptions) ptr (in) a JSON pointer to the value to remove, relative to root()"},{"location":"api/basic_json_document/erase/#return-value","title":"Return value","text":"
        1. the number of removed members (0 if object had none with this key)
        2. (nothing)
        3. the number of removed values (0 or more for an object member, always 1 for an array element)
        "},{"location":"api/basic_json_document/erase/#exceptions","title":"Exceptions","text":"
        1. Throws type_error.307 if object is not an object -- the same message BasicJsonType::erase throws for the same type.
        2. Throws type_error.307 if array is not an array. Throws out_of_range.401 if idx is negative, or if idx >= array.size().
        3. Throws out_of_range.405 (\"JSON pointer has no parent\") if ptr is empty. Throws what at throws (overload 3) for resolving ptr's parent. For the last reference token itself: if the parent is an array, throws what 2. throws for an index that is out of range, or, for a token that is not a valid array index, parse_error.106 (a leading '0'), parse_error.109 (not a number), out_of_range.410 (too large for size_type), or out_of_range.404 (an empty token); otherwise (an object, or a primitive value the pointer's parent resolves to) throws what 1. throws.

        Overloads 1. and 2. also throw invalid_iterator.202 (\"view does not belong to this document\") if object/array is a discarded view or a view of a different document. Overload 3. never throws invalid_iterator.202: it takes no view, and it always starts from this document's own root().

        "},{"location":"api/basic_json_document/erase/#complexity","title":"Complexity","text":"
        1. Linear in the number of members of object.
        2. Linear in the number of elements of array at or after idx (they move one slot over).
        3. Linear in the number of reference tokens of ptr and, for each token, in the number of members of the object at that level or the index into the array (as at), plus the complexity of 1. or 2. for the last token.
        "},{"location":"api/basic_json_document/erase/#notes","title":"Notes","text":"

        Duplicate keys

        Overload 1. removes every member with key, not just the first -- unlike set, which assigns the first occurrence and drops the rest. This is why it returns a count rather than a single view: there may be more than one member removed, or none.

        Like set and push_back, erase never moves an element's value: a view still referring to a removed member or element keeps showing what it last held (see Edits) -- it just no longer appears when array/object is read, dumped, or iterated. What does change is the container's sequence of links to its elements: removing an element of array (2.) shifts the links to the elements after it by one, as insert does in the other direction. Every iterator already taken over array/object is invalidated by an erase, not only those at or after the removed element, since the container may now use a different sequence.

        "},{"location":"api/basic_json_document/erase/#examples","title":"Examples","text":"Example

        The example below drops a deprecated field and a decommissioned entry from a configuration document -- using all three overloads -- and shows what stays intact that would not with a plain json/ordered_json value: the order of the fields around the ones removed, and the exact spelling of a number that was never touched.

        #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json = nlohmann::json;\nusing json_editable_document = nlohmann::json_editable_document;\nusing json_editable_view = nlohmann::json_editable_view;\n\nint main()\n{\n    // a deprecated field is dropped from a configuration file, and a\n    // decommissioned replica is removed from the list -- \"price\" keeps its\n    // trailing zero, and the fields around the removed ones keep their order\n    const std::string text = R\"({\n  \"name\": \"cache\",\n  \"legacy_host\": \"db0\",\n  \"host\": \"db1\",\n  \"price\": 19.90,\n  \"replicas\": [\"db2\", \"db3\", \"db4\"]\n})\";\n\n    json_editable_document doc = json_editable_document::parse(text);\n\n    doc.erase(doc.root(), \"legacy_host\");                                      // (1) an object member\n    doc.erase(doc.root()[\"replicas\"], 1);                                      // (2) an array element (\"db3\")\n    const std::size_t removed = doc.erase(json::json_pointer(\"/replicas/0\"));  // (3) via a JSON pointer\n\n    std::cout << removed << '\\n';\n    std::cout << doc.root().dump(2, ' ', false, json_editable_view::number_format::source) << \"\\n\\n\";\n\n    // the same edits on a plain json value: object_t is a std::map, so\n    // parsing already sorted the keys, and dump() rewrites every number to\n    // its shortest form, even \"price\", which was never touched\n    json plain = json::parse(text);\n    plain.erase(\"legacy_host\");\n    plain[\"replicas\"].erase(1);\n    plain[\"replicas\"].erase(0);\n    std::cout << plain.dump(2) << '\\n';\n}\n

        Output:

        1\n{\n  \"name\": \"cache\",\n  \"host\": \"db1\",\n  \"price\": 19.90,\n  \"replicas\": [\n    \"db4\"\n  ]\n}\n\n{\n  \"host\": \"db1\",\n  \"name\": \"cache\",\n  \"price\": 19.9,\n  \"replicas\": [\n    \"db4\"\n  ]\n}\n
        "},{"location":"api/basic_json_document/erase/#see-also","title":"See also","text":"
        • insert - insert an element into an array
        • set - replace a value, or set an object member, an array element, or the value a JSON pointer refers to
        • push_back - append to an array
        • root - the view of the root value, the starting point of overload 3
        • BasicJsonType::erase - the corresponding function of basic_json
        • Edits - what an edit guarantees, for every overload
        "},{"location":"api/basic_json_document/erase/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_document/insert/","title":"nlohmann::basic_json_document::insert","text":"
        template<typename I, typename V>\nview_type insert(view_type array, I idx, V&& value);\n

        Only an editable document (Editable == true, e.g. json_editable_document) has insert; calling it on a read-only basic_json_document fails to compile (static_assert).

        Inserts value into array as a new element before position idx, which must not be past the end (idx <= array.size(); idx == array.size() appends, like push_back). Unlike push_back, a null array does not first become an empty array: array must already be an array.

        value is accepted three ways: a basic_json_view of any document -- read-only or editable, and it does not have to be array's own document -- which is copied so that nothing is shared with the source document afterward; a BasicJsonType value; or anything BasicJsonType can be constructed from (numbers, strings, bool, nullptr, containers, ...).

        "},{"location":"api/basic_json_document/insert/#template-parameters","title":"Template parameters","text":"I an integral type other than bool, deduced (overloads taking a bool or a non-integral type for idx do not participate in overload resolution). V the type of value, deduced; see above for what is accepted."},{"location":"api/basic_json_document/insert/#parameters","title":"Parameters","text":"array (in) the array to insert into idx (in) the position to insert value before; a negative value throws (see Exceptions) value (in) the value to insert"},{"location":"api/basic_json_document/insert/#return-value","title":"Return value","text":"

        a view of the new element, now holding value

        "},{"location":"api/basic_json_document/insert/#exception-safety","title":"Exception safety","text":"

        Basic exception safety: value is fully encoded -- including the checks below -- into storage owned by the document before anything already reachable from root() is touched, so a failure while encoding value (an invalid argument, or std::bad_alloc) leaves the document completely unchanged, other than memory allocated for the encoding that is not reclaimed. A failure of a later allocation -- while array switches from its parsed layout to a growable block, or while that block grows, see Notes -- can still leave array already switched to that layout even though value itself was not inserted.

        "},{"location":"api/basic_json_document/insert/#exceptions","title":"Exceptions","text":"

        Throws type_error.309 if array is not an array -- the same message BasicJsonType::insert throws for the same type; a null array throws this too (see above). Throws out_of_range.401 if idx is negative, or if idx > array.size(). Throws invalid_iterator.202 (\"view does not belong to this document\") if array is a discarded view or a view of a different document. Throws type_error.302 if value is a discarded view or a discarded BasicJsonType value, and type_error.319 if value is (or contains) a binary value -- BasicJsonType can hold one, but a json_document cannot. Throws type_error.316 if value is (or contains) a string that is not valid UTF-8, with the same message BasicJsonType::dump() gives for that string.

        "},{"location":"api/basic_json_document/insert/#complexity","title":"Complexity","text":"

        Linear in the number of elements of array at or after idx (they move one slot over), plus time linear in the size of value to encode it into the document's storage (constant for a scalar, linear in the number of nested values for an array or object): like push_back, the elements of array move to a growable block of links the first time it is inserted into (or set/push_back on), and that block grows in amortized constant time; inserting before the end within that block still shifts every later element.

        "},{"location":"api/basic_json_document/insert/#notes","title":"Notes","text":"

        Like set on a member or an element, insert never moves an existing element's value -- only where array's links to its elements live -- so a view of an existing element of array stays valid across an insert, and keeps referring to the same element even though its index shifts. Any iterator already taken over array is invalidated, since it was walking the old layout. See Edits for what stays valid across an edit in general.

        "},{"location":"api/basic_json_document/insert/#examples","title":"Examples","text":"Example

        The example below inserts a step into the middle of a deployment plan, without touching the steps that come after it, and shows that a view taken before the insert keeps referring to the same element even though its index shifts -- something a plain json/ordered_json array, or its std::vector-based storage, has no equivalent for.

        #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json = nlohmann::json;\nusing json_editable_document = nlohmann::json_editable_document;\nusing json_editable_view = nlohmann::json_editable_view;\n\nint main()\n{\n    // a deployment plan -- \"budget\" is written with a trailing zero that has\n    // no effect on its value\n    const std::string text = R\"({\n  \"release\": \"2026.09\",\n  \"steps\": [\"build\", \"test\", \"deploy\"],\n  \"budget\": 19.90\n})\";\n\n    json_editable_document doc = json_editable_document::parse(text);\n\n    const std::size_t deploy_index = 2;\n    const auto deploy = doc.root()[\"steps\"][deploy_index];  // held across the insert\n\n    doc.insert(doc.root()[\"steps\"], deploy_index, \"smoke-test\"); // insert before \"deploy\"\n\n    // the held view still refers to \"deploy\", even though its index moved\n    // from 2 to 3, and nothing else in the document was touched\n    std::cout << deploy.dump() << '\\n';\n    std::cout << doc.root().dump(2, ' ', false, json_editable_view::number_format::source) << \"\\n\\n\";\n\n    // the same edit on a plain json value: an index held from before the\n    // insert now refers to whatever moved into that slot, and dump()\n    // rewrites \"budget\" to its shortest form even though it was never\n    // touched\n    json plain = json::parse(text);\n    plain[\"steps\"].insert(plain[\"steps\"].begin() + static_cast<std::ptrdiff_t>(deploy_index), \"smoke-test\");\n    std::cout << plain[\"steps\"][deploy_index].dump() << '\\n';\n    std::cout << plain.dump(2) << '\\n';\n}\n

        Output:

        \"deploy\"\n{\n  \"release\": \"2026.09\",\n  \"steps\": [\n    \"build\",\n    \"test\",\n    \"smoke-test\",\n    \"deploy\"\n  ],\n  \"budget\": 19.90\n}\n\n\"smoke-test\"\n{\n  \"budget\": 19.9,\n  \"release\": \"2026.09\",\n  \"steps\": [\n    \"build\",\n    \"test\",\n    \"smoke-test\",\n    \"deploy\"\n  ]\n}\n
        "},{"location":"api/basic_json_document/insert/#see-also","title":"See also","text":"
        • push_back - append to an array
        • erase - remove an object member, an array element, or the value a JSON pointer refers to
        • set - replace a value, or set an object member, an array element, or the value a JSON pointer refers to
        • BasicJsonType::insert - the corresponding function of basic_json
        • Edits - what an edit guarantees, for every overload
        "},{"location":"api/basic_json_document/insert/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_document/is_discarded/","title":"nlohmann::basic_json_document::is_discarded","text":"
        bool is_discarded() const noexcept;\n

        Returns whether the document holds no value, either because it was default-constructed or because the last call to parse(), parse_copy(), or read() failed with allow_exceptions set to false.

        "},{"location":"api/basic_json_document/is_discarded/#return-value","title":"Return value","text":"

        true if the document is discarded, false otherwise.

        "},{"location":"api/basic_json_document/is_discarded/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this function never throws exceptions.

        "},{"location":"api/basic_json_document/is_discarded/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json_document/is_discarded/#notes","title":"Notes","text":"

        When the document is discarded, root() returns a discarded view (its is_discarded() is also true).

        "},{"location":"api/basic_json_document/is_discarded/#examples","title":"Examples","text":"Example 
        #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // with allow_exceptions == false, a parse error produces a discarded\n    // document instead of throwing -- exactly like basic_json::parse()\n    json_document doc = json_document::parse(R\"({\"a\": )\", /* allow_exceptions */ false);\n    std::cout << doc.is_discarded() << '\\n';\n    std::cout << doc.root().is_discarded() << '\\n';\n\n    // a successful parse is never discarded\n    json_document ok = json_document::parse(R\"({\"a\": 1})\", false);\n    std::cout << ok.is_discarded() << '\\n';\n}\n

        Output:

        true\ntrue\nfalse\n
        "},{"location":"api/basic_json_document/is_discarded/#see-also","title":"See also","text":"
        • parse - deserialize from a compatible input
        • is_discarded (basic_json_view) - return whether a view is invalid
        "},{"location":"api/basic_json_document/is_discarded/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_document/load/","title":"nlohmann::basic_json_document::load","text":"
        // (1)\nstatic basic_json_document load(const std::uint8_t* image, std::size_t size,\n                                 const image_check check = image_check::full);\n\n// (2)\nstatic basic_json_document load(const std::vector<std::uint8_t>& image,\n                                 const image_check check = image_check::full);\n\n// (3)\nstatic basic_json_document load(std::vector<std::uint8_t>&& image,\n                                 const image_check check = image_check::full);\n
        1. Reads an image save() wrote, from a pointer and a byte count. The image is borrowed: image must stay alive and unchanged for as long as the returned document, and any view taken from it, is used.
        2. Reads an image from a std::vector. Also borrowed -- equivalent to overload 1 called with image.data() and image.size().
        3. Reads an image, keeping the vector instead of copying it: image is moved into the document (no copy), which then owns it for as long as it needs the text and the decoded strings. owns_source() is true afterward.

        In every overload, the node index is copied into storage the document itself owns -- so that it is properly aligned, and, for an editable document, can be edited -- while the text and the decoded strings stay in image. The hash indexes large objects use for lookup are rebuilt, exactly as after parsing.

        "},{"location":"api/basic_json_document/load/#parameters","title":"Parameters","text":"image (in) the image save() wrote (overloads 1 and 2), or one to take ownership of (overload 3) size (in) the number of bytes at image (overload 1) check (in) how thoroughly to validate image before trusting it; see image_check below (optional, image_check::full by default)"},{"location":"api/basic_json_document/load/#return-value","title":"Return value","text":"

        The document read from the image.

        "},{"location":"api/basic_json_document/load/#exception-safety","title":"Exception safety","text":"

        Overloads 1 and 2 give the strong guarantee: image is only read, never written, so a thrown exception leaves the caller's buffer untouched.

        Overload 3 moves image into the document before validating it, so that a good image is kept without a copy. If loading then fails, the partially built document -- and the vector now inside it -- is discarded along with the exception, and image itself is left empty, not restored to what was passed in. Move a copy in instead, or validate with overload 2 first, if the original vector must survive a failed load.

        "},{"location":"api/basic_json_document/load/#exceptions","title":"Exceptions","text":"

        On a big-endian target, throws type_error.320 -- the same exception save() throws there, since the image format is little-endian only.

        Otherwise throws parse_error.116 if image is not one save() could have written, or fails the requested check:

        message when too short image is nullptr, or size is smaller than the 64-byte header unknown format the header's magic bytes or version do not match, or a reserved header field is not zero sizes out of range the node count, text size, or decoded-string size the header describes does not fit size, or the '\\0' after the text or after the decoded strings is missing the check failed check is not image_check::none, and the image fails it -- see image_check

        Example messages

        [json.exception.parse_error.116] parse error: invalid json_document image: too short\n
        [json.exception.parse_error.116] parse error: invalid json_document image: unknown format\n
        [json.exception.parse_error.116] parse error: invalid json_document image: sizes out of range\n
        [json.exception.parse_error.116] parse error: invalid json_document image: the check failed\n

        "},{"location":"api/basic_json_document/load/#complexity","title":"Complexity","text":"

        Linear in the number of nodes, which are always copied into the document. image_check::none does not read the text or the decoded strings. image_check::bounds reads one byte of the text for each float token (its sign, to check the recorded digits against the token's length), and nothing else of the text or the decoded strings.

        With image_check::full, linear in the size of the image (the nodes, the text, and the decoded strings), plus sorting the ranges of the strings and of the float tokens (at most one per node): the contents of each distinct range are checked once, however many nodes refer to it.

        "},{"location":"api/basic_json_document/load/#notes","title":"Notes","text":""},{"location":"api/basic_json_document/load/#image_check","title":"image_check","text":"
        using image_check = detail::view::image_check;\n\nenum class image_check\n{\n    full,\n    bounds,\n    none\n};\n

        How thoroughly load() validates image before trusting it.

        full (the default) Checks everything the parser itself guarantees: structure and bounds; that every string is valid UTF-8 (and, for a string still in the source text, that it contains no quote, backslash, or control character); and that every number token is well-formed and matches the value stored for it. Strings and float tokens may share a range only if the ranges are identical (as nodes that share a value do), and never overlap otherwise. Reading and serializing a checked image is safe and always produces valid JSON, exactly as for a parsed document. This is the right choice for an image from anything you do not fully control -- a file, a cache shared with other processes, a peer on the network. bounds Checks structure and bounds only: that every offset and count in the node index stays inside the image. So load(), root(), element access, and materialize() can never read outside the image, even on a damaged one. It does not guarantee that the image describes valid JSON: a string that a full check would have rejected can make dump() write invalid UTF-8 or invalid JSON, or throw type_error.316, and a number can read back with a value that does not match how it is spelled. An editable document does not take such a string over: set, push_back and insert throw type_error.316 when they copy it from a view of the loaded document, and leave the editable document unchanged. bounds skips scanning the text and the decoded strings, so it fits a cache your own process just wrote and reads straight back, where damage would mean a bug or a hardware fault rather than adversarial input. none Checks nothing. Reading a damaged image is undefined behavior, so use it only for an image you trust as much as your own memory.

        Lifetime. Overloads 1 and 2 borrow image: it must stay alive and byte-for-byte unchanged for as long as the returned document, and any view taken from it, is used -- exactly like a document parse() borrowed its input for. Overload 3 avoids this by keeping the vector itself; see owns_source.

        Experimental

        The image format is versioned but not yet stable, and may change in an incompatible way before it is declared stable; load() already rejects an image written by a different format version with parse_error.116 (\"unknown format\"). Use images to cache a document within one build of the library, or to hand one to another process running the same build on the same (little-endian) machine -- not as a long-term storage format.

        "},{"location":"api/basic_json_document/load/#examples","title":"Examples","text":"Caching a document, ownership, and a rejected image

        The example below saves a parsed document as an image, checks that load() reproduces the original dump() without parsing, and shows the difference between load(std::move(image)) (owned) and load(image) (borrowed). It then damages one byte of the image and shows image_check::full rejecting it with parse_error.116, while image_check::bounds -- meant for a cache the process already trusts -- still reads it without going out of bounds.

        #include <cstdint>\n#include <iostream>\n#include <string>\n#include <vector>\n#include <nlohmann/json_view.hpp>\n\nusing json = nlohmann::json;\nusing json_document = nlohmann::json_document;\nusing image_check = json_document::image_check;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // the image of a parsed document -- as if read back from a cache file or\n    // received from another process running the same build of the library\n    const std::string text = R\"({\"name\": \"cache\", \"note\": \"caf\\u00e9\", \"replicas\": [\"db2\", \"db3\"]})\";\n    const json_document parsed = json_document::parse(text);\n    const std::vector<std::uint8_t> image = parsed.save();\n\n    // (1)/(2) load() needs no parsing, yet dumps exactly what parsing did\n    const json_document borrowed = json_document::load(image);\n    std::cout << (borrowed.root().dump() == parsed.root().dump()) << '\\n';\n    std::cout << borrowed.owns_source() << '\\n'; // borrowed: still points into `image`\n\n    // (3) load(std::move(image)) keeps the vector instead of copying it\n    std::vector<std::uint8_t> to_move = image;\n    const json_document owned = json_document::load(std::move(to_move));\n    std::cout << owned.owns_source() << '\\n';\n\n    // a damaged image -- the last byte of the decoded string \"note\" holds\n    // (an escape sequence, so it was unescaped into the document's own\n    // buffer), flipped, as storage or transport corruption might do\n    std::vector<std::uint8_t> damaged = image;\n    damaged[damaged.size() - 2] = 0xFF;\n\n    // image_check::full inspects strings and numbers, so it catches the damage\n    try\n    {\n        static_cast<void>(json_document::load(damaged, image_check::full));\n    }\n    catch (const json::parse_error& e)\n    {\n        std::cout << e.id << '\\n';\n    }\n\n    // image_check::bounds only checks structure and bounds, so a cache the\n    // process already trusts loads without the extra scan -- reading a value\n    // the damage did not touch is still safe\n    const json_document trusted = json_document::load(damaged, image_check::bounds);\n    std::cout << trusted.root()[\"name\"].get<std::string>() << '\\n';\n}\n

        Output:

        true\nfalse\ntrue\n116\ncache\n
        "},{"location":"api/basic_json_document/load/#see-also","title":"See also","text":"
        • save - write the document as an image
        • owns_source - return whether the document holds its own copy of the text
        • parse - deserialize from JSON text instead of an image
        • Images - why and when to use images
        "},{"location":"api/basic_json_document/load/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_document/memory_usage/","title":"nlohmann::basic_json_document::memory_usage","text":"
        std::size_t memory_usage() const noexcept;\n

        Returns the number of bytes held by the document: the node index, the decoded-string buffer (for strings that contain escapes), and, for an owned document, its copy of the source text.

        "},{"location":"api/basic_json_document/memory_usage/#return-value","title":"Return value","text":"

        The number of bytes the document holds, 0 for a discarded document.

        "},{"location":"api/basic_json_document/memory_usage/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this function never throws exceptions.

        "},{"location":"api/basic_json_document/memory_usage/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json_document/memory_usage/#notes","title":"Notes","text":"

        The exact value depends on the platform, the allocator, and the library's own layout, and may change between versions; do not rely on it being a specific number, and do not compare it across different builds or platforms. Compare it for the same document over time, or between documents built with the same binary, instead -- for instance to observe the effect of shrink_to_fit().

        "},{"location":"api/basic_json_document/memory_usage/#examples","title":"Examples","text":"Example 
        #include <iostream>\n#include <string>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // long enough that copying it needs a real (heap) allocation, so the\n    // comparison below does not depend on the standard library's small\n    // string optimization threshold\n    const std::string text = std::string(200, ' ') + \"[1, 2, 3, 4, 5]\";\n\n    // memory_usage() is not portable across platforms/allocators/compilers, so\n    // compare it relatively instead of printing the raw byte count\n    json_document borrowed = json_document::parse(text);\n    json_document owned = json_document::parse_copy(text);\n\n    // the owned document additionally stores its own copy of the source text\n    std::cout << (owned.memory_usage() > borrowed.memory_usage()) << '\\n';\n\n    // a document with more values needs a larger index\n    json_document small = json_document::parse(std::string(\"[1]\"));\n    json_document large = json_document::parse(std::string(\"[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]\"));\n    std::cout << (large.memory_usage() > small.memory_usage()) << '\\n';\n}\n

        Output:

        true\ntrue\n
        "},{"location":"api/basic_json_document/memory_usage/#see-also","title":"See also","text":"
        • node_count - the number of index entries
        • shrink_to_fit - release unused index capacity
        "},{"location":"api/basic_json_document/memory_usage/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_document/node_count/","title":"nlohmann::basic_json_document::node_count","text":"
        std::size_t node_count() const noexcept;\n

        Returns the number of entries in the document's flat index.

        "},{"location":"api/basic_json_document/node_count/#return-value","title":"Return value","text":"

        The number of index entries: one per value (of any type, at any nesting depth) plus one per object key. 0 for a discarded document.

        "},{"location":"api/basic_json_document/node_count/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this function never throws exceptions.

        "},{"location":"api/basic_json_document/node_count/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json_document/node_count/#notes","title":"Notes","text":"

        Each index entry is 16 bytes, so node_count() * 16 is the size of the index itself (part, but not all, of memory_usage(), which also counts decoded strings and, for an owned document, the text).

        "},{"location":"api/basic_json_document/node_count/#examples","title":"Examples","text":"Example 
        #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    // node_count() is the size of the flat index: one 16-byte node per value,\n    // plus one per object key (values and keys are all the index stores)\n    json_document scalar = json_document::parse(\"42\");\n    std::cout << scalar.node_count() << '\\n';\n\n    json_document doc = json_document::parse(R\"({\"a\": 1, \"b\": [1, 2]})\");\n    std::cout << doc.node_count() << '\\n';\n}\n

        Output:

        1\n7\n
        "},{"location":"api/basic_json_document/node_count/#see-also","title":"See also","text":"
        • memory_usage - the number of bytes held by the document
        • shrink_to_fit - release unused index capacity
        "},{"location":"api/basic_json_document/node_count/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_document/owns_source/","title":"nlohmann::basic_json_document::owns_source","text":"
        bool owns_source() const noexcept;\n

        Returns whether the document holds its own copy of the parsed text or is borrowing the caller's buffer.

        "},{"location":"api/basic_json_document/owns_source/#return-value","title":"Return value","text":"

        true if the document owns the text returned by source(), false if it borrows it (or if the document is discarded).

        "},{"location":"api/basic_json_document/owns_source/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this function never throws exceptions.

        "},{"location":"api/basic_json_document/owns_source/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json_document/owns_source/#notes","title":"Notes","text":"

        See the ownership table on parse for which inputs are borrowed and which are owned. A borrowed document (owns_source() == false) is only valid while the buffer it was parsed from is still alive.

        "},{"location":"api/basic_json_document/owns_source/#examples","title":"Examples","text":"Example 
        #include <iostream>\n#include <string>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    std::string text = R\"({\"a\": 1})\";\n\n    // borrowed: the document only points into `text`; `text` must outlive it\n    json_document borrowed = json_document::parse(text);\n    std::cout << borrowed.owns_source() << '\\n';\n\n    // owned: parse_copy() always takes its own copy\n    json_document copied = json_document::parse_copy(text);\n    std::cout << copied.owns_source() << '\\n';\n\n    // owned: an rvalue std::string is moved in, not copied, but still owned\n    json_document moved_in = json_document::parse(std::string(text));\n    std::cout << moved_in.owns_source() << '\\n';\n}\n

        Output:

        false\ntrue\ntrue\n
        "},{"location":"api/basic_json_document/owns_source/#see-also","title":"See also","text":"
        • parse - deserialize from a compatible input
        • parse_copy - deserialize a copy of a compatible input, always owned
        • source - the parsed text
        "},{"location":"api/basic_json_document/owns_source/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_document/parse/","title":"nlohmann::basic_json_document::parse","text":"
        // (1)\ntemplate<typename InputType>\nstatic basic_json_document parse(InputType&& input,\n                                 const bool allow_exceptions = true,\n                                 const bool ignore_comments = false,\n                                 const bool ignore_trailing_commas = false);\n\n// (2)\ntemplate<typename IteratorType>\nstatic basic_json_document parse(IteratorType first, IteratorType last,\n                                 const bool allow_exceptions = true,\n                                 const bool ignore_comments = false,\n                                 const bool ignore_trailing_commas = false);\n
        1. Deserialize from a compatible input, borrowing or owning it depending on its value category and type (see Notes).
        2. Deserialize from a pair of input iterators.

        Both overloads accept the same JSON text as BasicJsonType::parse(), with the same ignore_comments/ignore_trailing_commas options, but build a basic_json_document (a flat index into the input) instead of a tree of BasicJsonType values. They take the same inputs as BasicJsonType::parse(). Byte inputs that stay alive can be borrowed; all others (streams, FILE*, wide characters, non-contiguous iterators) are read into a copy that the document owns -- see the Notes. Use parse_copy() to get an owned copy of a borrowable input as well.

        "},{"location":"api/basic_json_document/parse/#template-parameters","title":"Template parameters","text":"InputType

        A compatible input, for instance:

        • a std::string, std::string_view, or a C-style array of single-byte characters
        • a pointer to a null-terminated string of single-byte characters (char, signed char, unsigned char, std::uint8_t)
        • a container for which obj.data() and obj.size() give contiguous single-byte access, e.g. std::vector<char> or std::vector<std::uint8_t>
        • an std::istream object
        • a FILE* pointer
        • wide characters (wchar_t, char16_t, char32_t), which are converted to UTF-8: a wide string object (std::wstring, std::u16string, std::u32string), an array (e.g. a wide string literal), or a pointer to a null-terminated wide string
        IteratorType an input iterator type, for instance a pair of pointers such as ptr and ptr + len, or a pair of std::string::iterator; the iterators of single-byte characters are borrowed or read like the byte inputs above, those of wide characters are converted to UTF-8"},{"location":"api/basic_json_document/parse/#parameters","title":"Parameters","text":"input (in) Input to parse from. allow_exceptions (in) whether to throw exceptions in case of a parse error (optional, true by default) ignore_comments (in) whether comments should be ignored and treated like whitespace (true) or yield a parse error (false); (optional, false by default) ignore_trailing_commas (in) whether trailing commas in arrays or objects should be ignored and treated like whitespace (true) or yield a parse error (false); (optional, false by default) first (in) iterator to the start of a character range last (in) iterator to the end of a character range"},{"location":"api/basic_json_document/parse/#return-value","title":"Return value","text":"

        The parsed document. If allow_exceptions is false and the input is not valid JSON, the returned document is discarded; see is_discarded.

        "},{"location":"api/basic_json_document/parse/#exceptions","title":"Exceptions","text":"

        Throws the same exception BasicJsonType::parse() throws for the same input and options -- the same exception id, message, and position -- because on a failing input the library's own parser is run on the same bytes to produce the diagnostic. This replay runs only for a failing input, and costs about a lexing pass up to the error (its SAX handler builds no values); with allow_exceptions set to false, it is skipped and a discarded document is returned instead. Additionally throws out_of_range.416 if the input is 4294967280 bytes (4 GiB minus 16 bytes) or larger, a size BasicJsonType::parse() does not reject.

        "},{"location":"api/basic_json_document/parse/#complexity","title":"Complexity","text":"

        Linear in the length of the input.

        "},{"location":"api/basic_json_document/parse/#notes","title":"Notes","text":"

        Ownership. Whether the document borrows input or owns a copy of it depends on its value category and type:

        input ownership lvalue byte container (std::string, std::vector<char>, ...), std::string_view, C string, character array borrowed -- input must outlive the document non-const rvalue std::string owned, moved in without a copy other rvalue byte container (including a const rvalue std::string) owned, copied stream, FILE*, wide characters, or anything else read through the general input adapter owned, read into a buffer (a stream is read to its end)

        For overload (2), a pair of pointers to single-byte integers (e.g. const char*, std::uint8_t*) is borrowed. From C++20 on, so is any other contiguous iterator over single bytes, such as std::vector<char>::iterator or std::string::const_iterator. Before C++20 these iterators cannot be told apart from other class-type iterators, so their range is read into an owned buffer, as is any non-contiguous range (e.g. of a std::list<char>).

        See owns_source to check which happened after a call, and the feature page for the reasoning.

        Numbers. As for BasicJsonType::parse(), an integer literal too large for the 64-bit integer type becomes a floating-point value.

        No lengths. An integer argument that is not a bool where the flags are expected -- for example parse(ptr, len) -- does not compile (the overload is deleted). Such a call would convert len to allow_exceptions and read ptr as a null-terminated string, past the end of a buffer that has none. To parse a buffer of a given length, pass a pair of pointers: parse(ptr, ptr + len). The same holds for parse_copy, accept, and read.

        "},{"location":"api/basic_json_document/parse/#examples","title":"Examples","text":"Example: (1) borrowed vs. owned input, and errors identical to BasicJsonType::parse() 
        #include <iostream>\n#include <string>\n#include <nlohmann/json.hpp>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // an lvalue std::string is BORROWED: the document only stores a pointer\n    // into `text`, so `text` must outlive `borrowed`\n    std::string text = R\"({\"count\": 3})\";\n    json_document borrowed = json_document::parse(text);\n    std::cout << borrowed.owns_source() << '\\n'; // false\n\n    // an rvalue std::string is MOVED into the document -- no copy of the text\n    json_document owned = json_document::parse(std::string(R\"({\"count\": 3})\"));\n    std::cout << owned.owns_source() << '\\n'; // true\n\n    // errors are identical to basic_json::parse: same exception id, message,\n    // and position, because the library parser runs on the same bytes on a\n    // failing input\n    try\n    {\n        static_cast<void>(json_document::parse(R\"({\"count\": )\"));\n    }\n    catch (const nlohmann::json::parse_error& e)\n    {\n        std::cout << e.id << '\\n';\n    }\n}\n

        Output:

        false\ntrue\n101\n
        Example: (2) parse an iterator range (no NUL terminator required) 
        #include <iostream>\n#include <vector>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // a JSON value embedded in a larger, non-null-terminated buffer (e.g. a\n    // slice received over the network)\n    std::vector<char> buffer = {'[', '1', ',', '2', ']', 'j', 'u', 'n', 'k'};\n\n    // a pointer pair is BORROWED, exactly like a byte container lvalue: the\n    // document points into the buffer. (From C++20 on, std::vector<char>\n    // iterators are borrowed as well; before, they are copied.)\n    const char* first = buffer.data();\n    json_document doc = json_document::parse(first, first + 5);\n    std::cout << doc.root().is_array() << ' ' << doc.root().size() << '\\n';\n    std::cout << doc.owns_source() << '\\n'; // false: borrowed\n}\n

        Output:

        true 2\nfalse\n
        "},{"location":"api/basic_json_document/parse/#see-also","title":"See also","text":"
        • parse_copy - deserialize a copy of a compatible input
        • accept - check whether the input is valid JSON
        • read - (re-)parse into this document, reusing its memory
        • owns_source - return whether the document holds its own copy of the text
        • load - read a document from an image instead of parsing JSON text
        • BasicJsonType::parse - the corresponding function of basic_json
        "},{"location":"api/basic_json_document/parse/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_document/parse_copy/","title":"nlohmann::basic_json_document::parse_copy","text":"
        template<typename InputType>\nstatic basic_json_document parse_copy(InputType&& input,\n                                      const bool allow_exceptions = true,\n                                      const bool ignore_comments = false,\n                                      const bool ignore_trailing_commas = false);\n

        Deserialize from a compatible input, always taking the document's own copy of it, regardless of the value category or type of input. Unlike parse(), the returned document never depends on input staying alive.

        "},{"location":"api/basic_json_document/parse_copy/#template-parameters","title":"Template parameters","text":"InputType A compatible input; see parse."},{"location":"api/basic_json_document/parse_copy/#parameters","title":"Parameters","text":"input (in) Input to parse from. allow_exceptions (in) whether to throw exceptions in case of a parse error (optional, true by default) ignore_comments (in) whether comments should be ignored and treated like whitespace (true) or yield a parse error (false); (optional, false by default) ignore_trailing_commas (in) whether trailing commas in arrays or objects should be ignored and treated like whitespace (true) or yield a parse error (false); (optional, false by default)"},{"location":"api/basic_json_document/parse_copy/#return-value","title":"Return value","text":"

        The parsed document, with owns_source() true. If allow_exceptions is false and the input is not valid JSON, the returned document is discarded; see is_discarded.

        "},{"location":"api/basic_json_document/parse_copy/#exceptions","title":"Exceptions","text":"

        Same as parse.

        "},{"location":"api/basic_json_document/parse_copy/#complexity","title":"Complexity","text":"

        Linear in the length of the input.

        "},{"location":"api/basic_json_document/parse_copy/#notes","title":"Notes","text":"

        parse_copy() accepts and rejects exactly what parse() does, and classifies numbers the same way; it only differs in that the input is always copied rather than sometimes borrowed. Prefer parse() when the input's lifetime already covers the document's, since it avoids the copy for borrowed inputs.

        An integer argument that is not a bool where the flags are expected, such as parse_copy(ptr, len), does not compile; see parse.

        "},{"location":"api/basic_json_document/parse_copy/#examples","title":"Examples","text":"Example

        The example below returns a document from a function whose local buffer would otherwise not outlive it.

        #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\njson_document parse_from_temporary_buffer()\n{\n    char buffer[] = R\"({\"a\": 1})\";\n    // parse() would borrow `buffer`, which is about to go out of scope;\n    // parse_copy() takes its own copy instead, so the returned document does\n    // not depend on `buffer` afterward\n    return json_document::parse_copy(buffer);\n}\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    json_document doc = parse_from_temporary_buffer();\n    std::cout << doc.owns_source() << '\\n';\n    std::cout << doc.root().is_object() << '\\n';\n}\n

        Output:

        true\ntrue\n
        "},{"location":"api/basic_json_document/parse_copy/#see-also","title":"See also","text":"
        • parse - deserialize from a compatible input, borrowing it where possible
        • owns_source - return whether the document holds its own copy of the text
        "},{"location":"api/basic_json_document/parse_copy/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_document/push_back/","title":"nlohmann::basic_json_document::push_back","text":"
        template<typename V>\nview_type push_back(view_type array, V&& value);\n

        Appends value as a new last element of array. A null array first becomes an empty array, the same way set turns a null object into an empty object.

        value is accepted three ways: a basic_json_view of any document -- read-only or editable, and it does not have to be array's own document -- which is copied so that nothing is shared with the source document afterward; a BasicJsonType value; or anything BasicJsonType can be constructed from (numbers, strings, bool, nullptr, containers, ...).

        Only an editable document (Editable == true, e.g. json_editable_document) has push_back; calling it on a read-only basic_json_document fails to compile (static_assert).

        "},{"location":"api/basic_json_document/push_back/#template-parameters","title":"Template parameters","text":"V the type of value, deduced; see above for what is accepted."},{"location":"api/basic_json_document/push_back/#parameters","title":"Parameters","text":"array (in) the array (or null value) to append to value (in) the value to append"},{"location":"api/basic_json_document/push_back/#return-value","title":"Return value","text":"

        a view of the new last element of array, now holding value

        "},{"location":"api/basic_json_document/push_back/#exception-safety","title":"Exception safety","text":"

        Basic exception safety: value is fully encoded -- including the checks below -- into storage owned by the document before anything already reachable from root() is touched, so a failure while encoding value (an invalid argument, or std::bad_alloc) leaves the document completely unchanged, other than memory allocated for the encoding that is not reclaimed. A failure of a later allocation -- while array switches from its parsed layout to a growable block, or while that block grows, see Notes -- can still leave a partial effect, such as a null array argument already turned into an empty array even though value itself was not appended.

        "},{"location":"api/basic_json_document/push_back/#exceptions","title":"Exceptions","text":"

        Throws type_error.308 if array is neither an array nor null -- the same message BasicJsonType::push_back throws for the same type. Throws invalid_iterator.202 (\"view does not belong to this document\") if array is a discarded view or a view of a different document. Throws type_error.302 if value is a discarded view or a discarded BasicJsonType value, and type_error.319 if value is (or contains) a binary value -- BasicJsonType can hold one, but a json_document cannot. Throws type_error.316 if value is (or contains) a string that is not valid UTF-8, with the same message BasicJsonType::dump() gives for that string.

        "},{"location":"api/basic_json_document/push_back/#complexity","title":"Complexity","text":"

        Amortized constant, plus time linear in the size of value to encode it into the document's storage (constant for a scalar, linear in the number of nested values for an array or object): the elements of array move to a growable block of links the first time it is appended to (or set on), and that block itself grows -- doubling its capacity, so the cost of growing it amortizes to constant per element -- only once it runs out of room. See Notes.

        "},{"location":"api/basic_json_document/push_back/#notes","title":"Notes","text":"

        Like set on a member or an element, push_back never moves an existing element itself -- only where array's links to its elements live -- so a view of an existing element of array stays valid across a push_back, but any iterator already taken over array is invalidated, since it was walking the old layout. See Edits for what stays valid across an edit in general.

        "},{"location":"api/basic_json_document/push_back/#examples","title":"Examples","text":"Example

        The example below appends records to an array one at a time, as they might arrive from a stream of events, without ever building a BasicJsonType value for the array or for the records already in it, and shows that a view taken from an earlier push_back still refers to the same element once later ones have run.

        #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json = nlohmann::json;\nusing json_editable_document = nlohmann::json_editable_document;\n\nint main()\n{\n    // \"events\" starts out null -- the first push_back() turns it into an\n    // array, exactly like set() turns a null object member into an object\n    json_editable_document doc = json_editable_document::parse(R\"({\"source\": \"sensor-1\", \"events\": null})\");\n\n    const auto first = doc.push_back(doc.root()[\"events\"], json{{\"type\", \"start\"}, {\"t\", 0}});\n    for (int t = 1; t <= 3; ++t)\n    {\n        doc.push_back(doc.root()[\"events\"], json{{\"type\", \"tick\"}, {\"t\", t}});\n    }\n\n    // push_back() never moves an existing element: a view taken from an\n    // earlier call still refers to the same element after later ones\n    std::cout << first.dump() << '\\n';\n    std::cout << doc.root()[\"events\"].size() << '\\n';\n    std::cout << doc.root().dump(2) << '\\n';\n}\n

        Output:

        {\"t\":0,\"type\":\"start\"}\n4\n{\n  \"source\": \"sensor-1\",\n  \"events\": [\n    {\n      \"t\": 0,\n      \"type\": \"start\"\n    },\n    {\n      \"t\": 1,\n      \"type\": \"tick\"\n    },\n    {\n      \"t\": 2,\n      \"type\": \"tick\"\n    },\n    {\n      \"t\": 3,\n      \"type\": \"tick\"\n    }\n  ]\n}\n
        "},{"location":"api/basic_json_document/push_back/#see-also","title":"See also","text":"
        • set - replace a value, or set an object member, an array element, or the value a JSON pointer refers to
        • insert - insert an element into an array before a given position
        • erase - remove an object member, an array element, or the value a JSON pointer refers to
        • root - the view of the root value
        • BasicJsonType::push_back - the corresponding function of basic_json
        • Edits - what an edit guarantees, for every overload
        "},{"location":"api/basic_json_document/push_back/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_document/read/","title":"nlohmann::basic_json_document::read","text":"
        template<typename InputType>\nvoid read(InputType&& input,\n         const bool allow_exceptions = true,\n         const bool ignore_comments = false,\n         const bool ignore_trailing_commas = false);\n

        (Re-)parses input into *this, discarding the document's previous value and reusing its memory (the node index, the decoded-string buffer, and, if applicable, the owned copy of the text) rather than allocating a fresh document. parse() is implemented in terms of this function, applied to a default-constructed document.

        "},{"location":"api/basic_json_document/read/#template-parameters","title":"Template parameters","text":"InputType A compatible input; see parse."},{"location":"api/basic_json_document/read/#parameters","title":"Parameters","text":"input (in) Input to parse from. allow_exceptions (in) whether to throw exceptions in case of a parse error (optional, true by default) ignore_comments (in) whether comments should be ignored and treated like whitespace (true) or yield a parse error (false); (optional, false by default) ignore_trailing_commas (in) whether trailing commas in arrays or objects should be ignored and treated like whitespace (true) or yield a parse error (false); (optional, false by default)"},{"location":"api/basic_json_document/read/#exceptions","title":"Exceptions","text":"

        Same as parse.

        "},{"location":"api/basic_json_document/read/#complexity","title":"Complexity","text":"

        Linear in the length of the input.

        "},{"location":"api/basic_json_document/read/#notes","title":"Notes","text":"

        Every view taken from *this before the call -- including the previous root() -- is invalidated, whether or not the new parse succeeds; take fresh views from root() afterward.

        input is borrowed or owned by the same rules as parse(); a document can borrow on one call and own on the next, since ownership is decided freshly each time. An input that is copied (see the table in parse()) is written into the document's previous owned copy, reusing its memory; a moved-in std::string replaces it.

        Reusing a document matters most for large inputs: the operating system provides the memory of a fresh node index one page at a time, and every page costs a page fault the first time it is written. On x86-64 Linux (4 KiB pages), parsing a 55 MB document into a reused document took about 40 % less time than parsing it into a fresh one. Programs that parse many documents of similar size should therefore keep one document and call read().

        An integer argument that is not a bool where the flags are expected, such as read(ptr, len), does not compile; see parse.

        "},{"location":"api/basic_json_document/read/#examples","title":"Examples","text":"Example

        The example below parses a sequence of messages into the same document, reusing its memory instead of allocating a new document for each one.

        #include <iostream>\n#include <string>\n#include <vector>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    std::vector<std::string> messages =\n    {\n        R\"({\"id\": 1})\", R\"({\"id\": 2, \"tag\": \"x\"})\", R\"({\"id\": 3})\"\n    };\n\n    // parse into the same document over and over: its node index and decode\n    // buffer are reused instead of being freed and reallocated for each message\n    json_document doc;\n    std::size_t total = 0;\n    for (const auto& msg : messages)\n    {\n        doc.read(msg);\n        total += doc.root().size();\n    }\n    std::cout << total << '\\n';\n\n    // read() can also change what kind of input is owned/borrowed between calls\n    doc.read(std::string(R\"({\"owned\": true})\"));\n    std::cout << doc.owns_source() << '\\n';\n}\n

        Output:

        4\ntrue\n
        "},{"location":"api/basic_json_document/read/#see-also","title":"See also","text":"
        • parse - deserialize from a compatible input
        • root - the view of the root value
        • load - read a document from an image instead of parsing JSON text
        "},{"location":"api/basic_json_document/read/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_document/root/","title":"nlohmann::basic_json_document::root","text":"
        // (1)\nview_type root() const& noexcept;\n\n// (2)\nview_type root() const&& = delete;\n
        1. Returns a view of the root value of the document.
        2. Deleted: the view of a temporary document would dangle.
        "},{"location":"api/basic_json_document/root/#return-value","title":"Return value","text":"

        A view_type (i.e. basic_json_view<BasicJsonType>) for the root value, or a discarded view if the document is discarded.

        "},{"location":"api/basic_json_document/root/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this function never throws exceptions.

        "},{"location":"api/basic_json_document/root/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json_document/root/#notes","title":"Notes","text":"

        Lifetime. A view refers into the document, so the document must outlive it. root() can therefore only be called on a document that has a name (an lvalue); calling it on a temporary does not compile:

        auto v = json_document::parse(text).root();  // error: the document is destroyed at the end of the statement\n\nauto doc = json_document::parse(text);       // OK: keep the document alive\nauto v = doc.root();\n

        root() is a cheap handle into the document's index, not a copy of anything; call it as often as needed. The returned view is valid under the same conditions as any other view of the document -- see Object inspection -- in particular, it is invalidated by the next read() or shrink_to_fit() on this document. A view can dangle, like a std::string_view; see Ownership and lifetime.

        "},{"location":"api/basic_json_document/root/#examples","title":"Examples","text":"Example 
        #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    json_document doc = json_document::parse(R\"({\"greeting\": \"hi\"})\");\n    std::cout << doc.root().is_object() << '\\n';\n\n    // root() is a cheap handle, not a copy: repeated calls observe the same value\n    std::cout << (doc.root().type() == doc.root().type()) << '\\n';\n\n    // the root of a failed parse (allow_exceptions == false) is discarded\n    json_document failed = json_document::parse(\"{\", false);\n    std::cout << failed.root().is_discarded() << '\\n';\n}\n

        Output:

        true\ntrue\ntrue\n
        "},{"location":"api/basic_json_document/root/#see-also","title":"See also","text":"
        • is_discarded - return whether the last parse failed
        • materialize - build the BasicJsonType value of a subtree
        "},{"location":"api/basic_json_document/root/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_document/save/","title":"nlohmann::basic_json_document::save","text":"
        std::vector<std::uint8_t> save() const;\n

        Writes the document as an image: a byte buffer that load reads back without parsing. The image holds the node index, the source text (plus, for an edited document, the number tokens edits wrote), and the decoded strings (plus the strings edits wrote) -- everything root() needs, with nothing left to parse.

        An edited document is written in its current state, with its values in document order, the way the library's own parser would have produced them for that JSON text: a member set added goes at the end, an erased member leaves no trace, and a float that is not finite (NaN or positive/negative infinity) becomes null, the same substitution dump() makes. The same document always saves to the same bytes -- also across BasicJsonType and Editable, since the image reflects document order and values only, not which specialization produced them.

        "},{"location":"api/basic_json_document/save/#return-value","title":"Return value","text":"

        The image, as a std::vector<std::uint8_t>. Pass it, or a pointer to its data together with its size, to load to read the document back.

        "},{"location":"api/basic_json_document/save/#exception-safety","title":"Exception safety","text":"

        Strong guarantee: save() does not modify *this (it is const), so if it throws, the document is left exactly as it was, and the partially built image is discarded with the exception.

        "},{"location":"api/basic_json_document/save/#exceptions","title":"Exceptions","text":"

        Throws type_error.320 if the document is discarded -- a default-constructed document, or one a failed parse()/ read() with allow_exceptions == false left discarded.

        On a big-endian target, throws type_error.320 with a different message instead: the image format is little-endian only (see Notes).

        Throws out_of_range.416 if the node count, the text, or the decoded strings of the image would individually reach 4 GiB -- the same 32-bit offsets parse() and, for edits, set/push_back are already limited to.

        Example messages

        [json.exception.type_error.320] cannot save a discarded json_document\n
        [json.exception.type_error.320] json_document images need a little-endian target\n
        [json.exception.out_of_range.416] images of 4 GiB or more are not supported by json_document\n

        "},{"location":"api/basic_json_document/save/#complexity","title":"Complexity","text":"

        Linear in the size of the document: the number of nodes, plus the length of the text and the decoded strings that end up in the image.

        "},{"location":"api/basic_json_document/save/#notes","title":"Notes","text":"

        Format. The image begins with a 64-byte header (the magic bytes \"NJVI\", a version number, the node count, and the sizes of the text and the decoded strings, all little-endian), followed by the nodes (16 bytes each), the text and a '\\0', and the decoded strings and a '\\0'. load checks the header, and the sizes it describes, before reading anything else -- see load's Exceptions.

        Experimental

        The image format is versioned but not yet stable: it may change in an incompatible way before it is declared stable. Use images to cache a document within one build of the library, or to hand one to another process running the same build on the same (little-endian) machine -- not as a long-term storage format. Keep the original JSON text if you need to read a saved document back with a future library version.

        Little-endian only. The image is written as raw little-endian bytes, with no byte-swapping. save() (and load) throw type_error.320 on a big-endian target rather than silently produce bytes a big-endian reader could not interpret correctly.

        "},{"location":"api/basic_json_document/save/#examples","title":"Examples","text":"Caching a parsed document as an image

        The example below saves a parsed configuration as an image -- the way a service might cache one to answer later requests without parsing the text again -- and confirms that loading it back gives exactly the same result as parsing did, and that saving is deterministic.

        #include <cstdint>\n#include <iostream>\n#include <string>\n#include <vector>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // a configuration a service parses once and then caches as an image, so\n    // that later requests can load() it instead of parsing the text again\n    const std::string text = R\"({\"name\": \"cache\", \"host\": \"db1\", \"port\": 6379, \"replicas\": [\"db2\", \"db3\"]})\";\n    const json_document config = json_document::parse(text);\n\n    // save() turns the parsed document into a byte buffer: a 64-byte header,\n    // the node index, the source text, and the decoded strings\n    const std::vector<std::uint8_t> image = config.save();\n    std::cout << image.size() << '\\n';\n\n    // the same document always saves to the same bytes\n    std::cout << (image == json_document::parse(text).save()) << '\\n';\n\n    // loading the image back needs no parsing, yet dumps exactly what\n    // parsing the text produced\n    const json_document reloaded = json_document::load(image);\n    std::cout << (reloaded.root().dump() == config.root().dump()) << '\\n';\n}\n

        Output:

        316\ntrue\ntrue\n
        "},{"location":"api/basic_json_document/save/#see-also","title":"See also","text":"
        • load - read an image written by save()
        • owns_source - return whether the document holds its own copy of the text
        • basic_json_view::dump - serialize the document to JSON text instead of an image
        • Images - why and when to use images
        "},{"location":"api/basic_json_document/save/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_document/set/","title":"nlohmann::basic_json_document::set","text":"
        // (1)\ntemplate<typename V>\nview_type set(view_type target, V&& value);\n\n// (2)\ntemplate<typename V>\nview_type set(view_type object, string_view_t key, V&& value);\n\n// (3)\ntemplate<typename I, typename V>\nview_type set(view_type array, I idx, V&& value);\n\n// (4)\ntemplate<typename V>\nview_type set(const json_pointer& ptr, V&& value);\n

        Only an editable document (Editable == true, e.g. json_editable_document) has set; calling it on a read-only basic_json_document fails to compile (static_assert).

        1. Replaces the value target refers to with value.
        2. Sets the member key of the object object to value: assigns it if object already has a member with this key -- the first one, should the key occur more than once, and the later duplicates are then dropped (see the Notes below) -- or appends a new member at the end otherwise. A null object first becomes an empty object.
        3. Assigns value to the element at index idx of the array array, which must already exist (idx < array.size()).
        4. Sets the value the JSON pointer ptr refers to, relative to root(), to value. The parent of the target must already exist: an object member is set as in 2. (added if it does not exist yet), an array element is assigned as in 3., and a last reference token of \"-\", or equal to the size of the array, appends value instead, exactly as push_back would. A null parent becomes what basic_json::operator[] with a JSON pointer makes of it: an array if the last reference token is \"-\" or consists of digits only (for an index beyond 0, the array is first filled with null values up to that index), an object otherwise. An empty ptr sets root() itself, as in 1.

        In every overload, value is accepted three ways: a basic_json_view of any document -- read-only or editable, and it does not have to be target's/object's/array's own document -- which is copied so that nothing is shared with the source document afterward; a BasicJsonType value; or anything BasicJsonType can be constructed from (numbers, strings, bool, nullptr, containers, ...).

        "},{"location":"api/basic_json_document/set/#template-parameters","title":"Template parameters","text":"V the type of value, deduced; see above for what is accepted. I an integral type other than bool, deduced (overloads taking a bool or a non-integral type for idx do not participate in overload resolution)."},{"location":"api/basic_json_document/set/#parameters","title":"Parameters","text":"target (in) the value to replace object (in) the object (or null value) whose member to set array (in) the array whose element to assign key (in) the key of the member to set idx (in) the index of the element to assign; a negative value throws (see Exceptions) ptr (in) a JSON pointer to the value to set, relative to root() value (in) the new value"},{"location":"api/basic_json_document/set/#return-value","title":"Return value","text":"
        1. a view of target, now holding value
        2. a view of the member key of object, now holding value
        3. a view of the element idx of array, now holding value
        4. a view of the value ptr refers to, now holding value
        "},{"location":"api/basic_json_document/set/#exception-safety","title":"Exception safety","text":"

        Basic exception safety: value is fully encoded -- including the checks below -- into storage owned by the document before anything already reachable from root() is touched, so a failure while encoding value (an invalid argument, or std::bad_alloc) leaves the document completely unchanged, other than memory allocated for the encoding that is not reclaimed. A failure of a later allocation -- while an edited array or object switches from its parsed layout to a growable block, see Notes -- can still leave a partial effect, such as a null object/array argument already turned into an empty object/array even though value itself was not linked in. Likewise, a failure of value in 4. leaves a null parent that is set with an index beyond 0 already filled with the null values before the index.

        "},{"location":"api/basic_json_document/set/#exceptions","title":"Exceptions","text":"
        1. Throws type_error.302 if value is a discarded view, or a discarded BasicJsonType value (e.g. BasicJsonType(value_t::discarded)) -- an object or array value, of either kind, is fine and is encoded as a whole subtree.
        2. Throws type_error.305 if object is neither an object nor null -- the same message operator[] throws for a string argument on such a value. Throws type_error.316 if key is not valid UTF-8, with the same message BasicJsonType::dump() gives for that string. Also throws what 1. throws for value.
        3. Throws type_error.305 if array is not an array -- the same message operator[] throws for a numeric argument on such a value. Throws out_of_range.401 if idx is negative, or if idx >= array.size(). Also throws what 1. throws for value.
        4. Throws what at throws (overload 3) for resolving ptr's parent, except that a missing object member or an array index equal to the array's size at the very last reference token is not an error there (it becomes a new member or an appended element) instead of out_of_range.403/out_of_range.402. For the last reference token itself: if the parent is an object (or a primitive value, where it throws type_error.305), throws what 2. throws; if the parent is an array, throws what 3. throws for an index that is out of range, or, for a token that is not a valid array index, parse_error.106 (a leading '0'), parse_error.109 (not a number), out_of_range.410 (too large for size_type), or out_of_range.404 (an empty token); the same errors are thrown for a null parent and a token of digits (the parent is not changed then), and out_of_range.401 if the index is 4294967295 or more. Also throws what 1. throws for value.

        Every overload also throws type_error.319 if value is (or contains) a binary value -- BasicJsonType can hold one, but a json_document cannot -- and invalid_iterator.202 (\"view does not belong to this document\") if target/object/array is a discarded view or a view of a different document (overloads 1-3 only; overload 4 always starts from this document's own root()).

        "},{"location":"api/basic_json_document/set/#complexity","title":"Complexity","text":"
        1. Linear in the size of value (encoding it into the document's storage): constant for a scalar, linear in the number of nested values for an array or object. If target is itself an array or object that spans more than one node in its parent's original, unedited layout, and value is a scalar, replacing it additionally costs time linear in the size of the document, the first time (the parent of target is looked up from root(), and then switches to links) -- see Notes. Once the parent has links, target is replaced in constant time: setting every element of a large array one after the other is linear overall. To avoid the lookup altogether, use 3. (or 2. for an object), which know the parent.
        2. Linear in the number of members of object, to find an existing member with key, plus the complexity of 1. for value.
        3. Constant, plus the complexity of 1. for value.
        4. Linear in the number of reference tokens of ptr and, for each token, in the number of members of the object at that level or the index into the array (as at), plus the complexity of 2. or 3. for the last token; for a null parent and an index, linear in the index.
        "},{"location":"api/basic_json_document/set/#notes","title":"Notes","text":"

        Duplicate keys

        If object already has more than one member with key (2.), the first one is assigned value and every later member with the same key is removed -- so that a lookup, an iteration, and materialize() of object afterward all agree on a single value for key, the same way operator[] already picks the first occurrence of a duplicate key for reading. See the Notes on duplicate keys of operator[].

        Setting a member (2.) or an element (3., through 4.) of an array or object whose elements have not been edited before switches it from its parsed layout to a growable block holding links to its elements; a later push_back or set on the same container reuses that block, growing it (amortized constant time) only once it runs out of room. This never moves an element itself -- only where the container's links to its elements live -- so a view of an element stays valid, but any iterator already taken over the container is invalidated, since it was walking the old layout. See Edits for what stays valid across an edit in general.

        The same switch happens, for the same reason, when overload 1. replaces a multi-node array/object value with a scalar: the parent's element sequence is what has to switch to links, not target itself, because the parent originally stepped over target's whole subtree by its node count, which no longer applies once target is a one-node scalar.

        "},{"location":"api/basic_json_document/set/#examples","title":"Examples","text":"Example: (1)/(2)/(3)/(4) replace a value, set a member, assign an element, set via a JSON pointer

        The example below edits a small configuration document -- replacing a value, adding an object member, assigning an array element, and reaching a field through a JSON pointer -- and shows what dump() preserves that is lost once the same edits are made on a BasicJsonType value instead: the order object members were written in, and the exact spelling of a number that was never touched.

        #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json = nlohmann::json;\nusing json_editable_document = nlohmann::json_editable_document;\nusing json_editable_view = nlohmann::json_editable_view;\n\nint main()\n{\n    // a configuration file, as it might be read from disk -- \"price\" is\n    // written with a trailing zero that has no effect on its value\n    const std::string text = R\"({\n  \"name\": \"cache\",\n  \"host\": \"db1\",\n  \"port\": 6379,\n  \"price\": 19.90,\n  \"replicas\": [\"db2\", \"db3\"],\n  \"timeout\": 30\n})\";\n\n    json_editable_document doc = json_editable_document::parse(text);\n\n    doc.set(doc.root()[\"port\"], 6380);                    // (1) replace a value\n    doc.set(doc.root(), \"region\", \"us-east\");              // (2) add a member\n    doc.set(doc.root()[\"replicas\"], 0, \"db4\");             // (3) assign an element\n    doc.set(json::json_pointer(\"/timeout\"), 45);           // (4) via a JSON pointer\n\n    // members stay in document order (the new one at the end), and a number\n    // that was not itself edited keeps its exact spelling\n    std::cout << doc.root().dump(2, ' ', false, json_editable_view::number_format::source) << \"\\n\\n\";\n\n    // the same edits on a plain json value: object_t is a std::map, so\n    // parsing already sorted the keys, and dump() rewrites every number to\n    // its shortest form, even \"price\", which was never touched\n    json plain = json::parse(text);\n    plain[\"port\"] = 6380;\n    plain[\"region\"] = \"us-east\";\n    plain[\"replicas\"][0] = \"db4\";\n    plain[json::json_pointer(\"/timeout\")] = 45;\n    std::cout << plain.dump(2) << '\\n';\n}\n

        Output:

        {\n  \"name\": \"cache\",\n  \"host\": \"db1\",\n  \"port\": 6380,\n  \"price\": 19.90,\n  \"replicas\": [\n    \"db4\",\n    \"db3\"\n  ],\n  \"timeout\": 45,\n  \"region\": \"us-east\"\n}\n\n{\n  \"host\": \"db1\",\n  \"name\": \"cache\",\n  \"port\": 6380,\n  \"price\": 19.9,\n  \"region\": \"us-east\",\n  \"replicas\": [\n    \"db4\",\n    \"db3\"\n  ],\n  \"timeout\": 45\n}\n
        "},{"location":"api/basic_json_document/set/#see-also","title":"See also","text":"
        • push_back - append to an array
        • insert - insert an element into an array
        • erase - remove an object member, an array element, or the value a JSON pointer refers to
        • root - the view of the root value, the starting point of overload 4
        • basic_json_view::dump - serialize the document, keeping an untouched number's spelling with number_format::source
        • Edits - what an edit guarantees, for every overload
        • Editing a document - why editable documents keep the source text's order and number spelling
        "},{"location":"api/basic_json_document/set/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_document/shrink_to_fit/","title":"nlohmann::basic_json_document::shrink_to_fit","text":"
        void shrink_to_fit();\n

        Releases capacity that is no longer needed, both of the node index and of the buffer for decoded strings (strings that contained escape sequences), e.g. after read() replaced a large document with a much smaller one. Like std::vector::shrink_to_fit(), this is a non-binding request: the library may keep more capacity than strictly necessary.

        "},{"location":"api/basic_json_document/shrink_to_fit/#exception-safety","title":"Exception safety","text":"

        Strong guarantee: if an exception is thrown, there are no changes to the document.

        "},{"location":"api/basic_json_document/shrink_to_fit/#exceptions","title":"Exceptions","text":"

        May throw std::bad_alloc if the reallocation fails; on exception, the document is unchanged.

        "},{"location":"api/basic_json_document/shrink_to_fit/#complexity","title":"Complexity","text":"

        Linear in node_count() plus the length of the decoded strings.

        "},{"location":"api/basic_json_document/shrink_to_fit/#notes","title":"Notes","text":"

        Invalidates views

        Unlike moving the document, shrink_to_fit() invalidates every view taken from this document before the call, including a previously obtained root(): the node index is moved into a new, smaller allocation, and the old one is freed. Take a fresh view from root() after calling this function.

        This is unlike std::vector::shrink_to_fit(), which promises nothing about validity but in practice often leaves iterators alone when it did not need to reallocate; here, an implementation that avoids a reallocation when possible would be an internal optimization only, not a guarantee to rely on.

        "},{"location":"api/basic_json_document/shrink_to_fit/#examples","title":"Examples","text":"Example 
        #include <iostream>\n#include <sstream>\n#include <string>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // build a large array (many nodes), then read a small one into the same\n    // document: the index grown for the large input is still allocated\n    std::ostringstream big;\n    big << '[';\n    for (int i = 0; i < 500; ++i)\n    {\n        if (i != 0)\n        {\n            big << ',';\n        }\n        big << i;\n    }\n    big << ']';\n\n    json_document doc;\n    doc.read(big.str());\n    const std::size_t big_nodes = doc.node_count();\n\n    doc.read(std::string(\"[1]\"));\n    std::cout << (doc.node_count() < big_nodes) << '\\n'; // far fewer live nodes now\n    const std::size_t before = doc.memory_usage();\n\n    // shrink_to_fit() moves the index into a block sized for what is actually\n    // used. This INVALIDATES every view taken from this document before the\n    // call (they point into the old, now-freed block) -- take fresh ones from\n    // root() afterward.\n    doc.shrink_to_fit();\n    const std::size_t after = doc.memory_usage();\n    std::cout << (after <= before) << '\\n';\n\n    // a freshly taken view is valid and correct\n    std::cout << doc.root().is_array() << ' ' << doc.root().size() << '\\n';\n}\n

        Output:

        true\ntrue\ntrue 1\n
        "},{"location":"api/basic_json_document/shrink_to_fit/#see-also","title":"See also","text":"
        • node_count - the number of index entries
        • memory_usage - the number of bytes held by the document
        • root - the view of the root value
        "},{"location":"api/basic_json_document/shrink_to_fit/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_document/source/","title":"nlohmann::basic_json_document::source","text":"
        view_type::string_view_t source() const noexcept;\n

        Returns the parsed text, whether it is borrowed from the caller or owned by the document.

        "},{"location":"api/basic_json_document/source/#return-value","title":"Return value","text":"

        A string_view_t (std::string_view on C++17 and newer) over the parsed text, or an empty one if the document is discarded.

        "},{"location":"api/basic_json_document/source/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this function never throws exceptions.

        "},{"location":"api/basic_json_document/source/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json_document/source/#notes","title":"Notes","text":"

        For a borrowed document, source() points directly into the caller's buffer, so it is only valid while that buffer is; see owns_source.

        "},{"location":"api/basic_json_document/source/#examples","title":"Examples","text":"Example 
        #include <iostream>\n#include <string>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    std::string text = R\"({\"a\": 1})\";\n    json_document doc = json_document::parse(text);\n\n    // source() is the parsed text, whether borrowed or owned\n    std::cout << (doc.source().size() == text.size()) << '\\n';\n    std::cout << std::string(doc.source().data(), doc.source().size()) << '\\n';\n\n    // for a borrowed document, source() points right into the caller's buffer\n    std::cout << (doc.source().data() == text.data()) << '\\n';\n}\n

        Output:

        true\n{\"a\": 1}\ntrue\n
        "},{"location":"api/basic_json_document/source/#see-also","title":"See also","text":"
        • owns_source - return whether the document holds its own copy of the text
        • source_offset - byte offset of a value in the source text
        "},{"location":"api/basic_json_document/source/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_view/","title":"nlohmann::basic_json_view","text":"

        Defined in header <nlohmann/json_view.hpp>

        template<typename BasicJsonType, bool Editable = false>\nclass basic_json_view;\n

        A read-only handle to one value of a basic_json_document: two pointers (a pointer to the document and a pointer into its index), trivially copyable. A view is valid as long as

        • the document is alive,
        • the document has not been re-parsed with read() (or parse() into it) or shrunk with shrink_to_fit() since the view was taken, and
        • if the document borrows its source text, that text is still alive and unmodified (floating-point values and unescaped strings are read from it lazily).

        Moving the document itself does not invalidate its views: the index is heap-allocated independently of the basic_json_document object.

        basic_json_view provides the read-only part of the BasicJsonType interface:

        • the type-inspection functions
        • element access
        • lookup
        • iteration
        • conversion: get<T>(), get_string(), number_token(), and materialize() to build the BasicJsonType value of a subtree on demand
        • comparison: operator== and operator!= compare two views, or a view and a BasicJsonType value, without ever building a BasicJsonType value for a view; no ordering comparison (operator<) is provided

        operator[], at, contains, and value also accept a json_pointer. Unlike basic_json, there are no implicit conversion operators (no operator ValueType()), and no get_ptr() or get_ref(); see get.

        basic_json_view itself is always read-only -- it never has a set or push_back of its own. A view of an editable document (Editable == true) sees every edit made through basic_json_document::set and basic_json_document::push_back: once a value is changed, every view that still refers to it -- including ones taken before the change -- reads the new value. See Edits.

        "},{"location":"api/basic_json_view/#template-parameters","title":"Template parameters","text":"BasicJsonType a specialization of basic_json, matching the basic_json_document the view was taken from. Editable whether the view is of an editable document, matching the basic_json_document it was taken from (optional, false by default). See Edits."},{"location":"api/basic_json_view/#specializations","title":"Specializations","text":"
        • json_view - views of a json_document
        • ordered_json_view - views of an ordered_json_document
        • json_editable_view - views of a json_editable_document
        • ordered_json_editable_view - views of an ordered_json_editable_document
        "},{"location":"api/basic_json_view/#member-types","title":"Member types","text":"
        • value_t - the JSON type enumeration, see basic_json::value_t
        • string_t, number_integer_t, number_unsigned_t, number_float_t, json_pointer - the corresponding member types of BasicJsonType
        • size_type - std::size_t
        • string_view_t - std::string_view on C++17 and newer, a minimal internal substitute otherwise
        • iterator, const_iterator - a forward iterator over the elements of an array or the member values of an object, in document order; both names refer to the same type, since a view is always read-only
        • number_format - how dump() writes numbers
        "},{"location":"api/basic_json_view/#member-functions","title":"Member functions","text":"
        • (constructor)
        "},{"location":"api/basic_json_view/#object-inspection","title":"Object inspection","text":"
        • type - return the type of the value
        • type_name - return the type as string
        • is_null - return whether the value is null
        • is_boolean - return whether the value is a boolean
        • is_number - return whether the value is a number
        • is_number_integer - return whether the value is an integer number
        • is_number_unsigned - return whether the value is an unsigned integer number
        • is_number_float - return whether the value is a floating-point number
        • is_string - return whether the value is a string
        • is_array - return whether the value is an array
        • is_object - return whether the value is an object
        • is_binary - return whether the value is a binary array (always false)
        • is_primitive - return whether the type is primitive
        • is_structured - return whether the type is structured
        • is_discarded - return whether the view is invalid
        "},{"location":"api/basic_json_view/#element-access","title":"Element access","text":"
        • at - access specified element with bounds checking
        • operator[] - access specified element
        • value - access specified element with default value
        • front - access the first element
        • back - access the last element
        "},{"location":"api/basic_json_view/#lookup","title":"Lookup","text":"
        • find - find an element in an object
        • count - returns the number of occurrences of a key in an object
        • contains - check the existence of an element in an object
        "},{"location":"api/basic_json_view/#iterators","title":"Iterators","text":"
        • begin - returns an iterator to the first element
        • cbegin - returns a const iterator to the first element
        • end - returns an iterator to one past the last element
        • cend - returns a const iterator to one past the last element
        • items - wrapper to access iterator member functions in range-based for
        "},{"location":"api/basic_json_view/#capacity","title":"Capacity","text":"
        • size - return the number of elements
        • empty - return whether the value has no elements
        "},{"location":"api/basic_json_view/#conversion","title":"Conversion","text":"
        • get - get a value
        • get_to - get a value and write it to a destination
        • get_string - get a string value without a copy
        • number_token - get a number's token text without a copy
        • materialize - build the BasicJsonType value of this subtree
        "},{"location":"api/basic_json_view/#comparison","title":"Comparison","text":"
        • operator== - comparison: equal
        • operator!= - comparison: not equal
        "},{"location":"api/basic_json_view/#serialization","title":"Serialization","text":"
        • dump - serialize to a JSON-formatted string
        • operator<< - serialize to stream
        "},{"location":"api/basic_json_view/#source-access","title":"Source access","text":"
        • source_offset - byte offset of this value in the document's source text
        "},{"location":"api/basic_json_view/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_view/at/","title":"nlohmann::basic_json_view::at","text":"
        // (1)\nbasic_json_view at(string_view_t key) const;\nbasic_json_view at(const char* key) const;\nbasic_json_view at(const string_t& key) const;\n\n// (2)\nbasic_json_view at(size_type idx) const;\ntemplate<typename IntegerType>\nbasic_json_view at(IntegerType idx) const;\n\n// (3)\nbasic_json_view at(const json_pointer& ptr) const;\n
        1. Returns the value of the object member with key key -- the first one, should the key occur more than once (see Notes on duplicate keys).
        2. Returns the array element at index idx. The template accepts every integer type except bool and std::size_t and forwards to the size_type overload, as for operator[]; a negative idx is out of range.
        3. Returns the value a JSON pointer ptr refers to, starting at this value.
        "},{"location":"api/basic_json_view/at/#parameters","title":"Parameters","text":"key (in) object key of the element to access idx (in) index of the element to access ptr (in) JSON pointer to the element to access"},{"location":"api/basic_json_view/at/#return-value","title":"Return value","text":"
        1. the value of the first member with key key
        2. the element at index idx
        3. the value ptr resolves to, starting at this value
        "},{"location":"api/basic_json_view/at/#exception-safety","title":"Exception safety","text":"

        Strong guarantee: there are no changes to the view or the document it refers to.

        "},{"location":"api/basic_json_view/at/#exceptions","title":"Exceptions","text":"
        1. The function can throw the following exceptions, both with the same message as the corresponding call to BasicJsonType::at:
          • Throws type_error.304 if the value is not an object.
          • Throws out_of_range.403 if no member has key key.
        2. The function can throw the following exceptions, both with the same message as the corresponding call to BasicJsonType::at:
          • Throws type_error.304 if the value is not an array.
          • Throws out_of_range.401 if idx >= size().
        3. The function can throw the following exceptions, all with the same message as the corresponding call to BasicJsonType::at:
          • Throws parse_error.106 if an array index in ptr begins with '0'.
          • Throws parse_error.109 if an array index in ptr is not a number.
          • Throws out_of_range.401 if an array index in ptr is out of range.
          • Throws out_of_range.402 if a reference token is \"-\" at an array -- at never inserts an element, so \"-\" is always invalid.
          • Throws out_of_range.403 if a reference token names an object member that does not exist.
          • Throws out_of_range.404 if ptr cannot be resolved because a reference token is used on a primitive value.

        None of these exceptions carry a JSON_DIAGNOSTICS path: the view has no BasicJsonType value to point at, so the exception is created without one, even if BasicJsonType was built with JSON_DIAGNOSTICS enabled.

        "},{"location":"api/basic_json_view/at/#complexity","title":"Complexity","text":"
        1. Linear in the number of members: as for ordered_json, members are compared one after another, in document order, stopping at the first match. Each comparison first checks the key's length -- already known from the index, without reading the key bytes -- before comparing its content. Objects with 128 or more members get a hash index of their keys while parsing, so that a lookup in them takes constant time on average.
        2. Linear in idx: elements are skipped one at a time from the first one, since they are not a fixed size in the index (unlike BasicJsonType's array, which is random-access).
        3. Linear in the number of reference tokens of ptr and, for each token, in the number of members of the object at that level (as 1.) or the index into the array (as 2.).
        "},{"location":"api/basic_json_view/at/#notes","title":"Notes","text":"

        Duplicate keys: the first member wins

        If the source text repeats a key, this resolves to the first member with it, not to the last one that materialize() and parse() keep. See the Notes on duplicate keys of operator[] and Duplicate keys for the reasons and for how to get the last value.

        Unlike operator[], which returns a discarded view for a missing key or an out-of-range index, at always throws -- exactly as BasicJsonType::at does, and with the same messages, so existing error handling written against BasicJsonType::at keeps working unchanged when switched to a view. This also holds for overload 3: unlike operator[] with a JSON pointer, which returns a discarded view for a missing key or an out-of-range index, at throws for those too (out_of_range.403/out_of_range.401).

        "},{"location":"api/basic_json_view/at/#examples","title":"Examples","text":"Example: (1)/(2) access specified element with bounds checking

        The example below reads required fields out of a service configuration with at, and shows that the exceptions it throws -- for a wrong type and for a missing key -- carry the same messages BasicJsonType::at would produce for the same JSON text.

        #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\nusing json_view = nlohmann::json_view;\n\nint main()\n{\n    // required fields of a service configuration -- at() reports a missing\n    // or wrong-typed field with the very same exception basic_json::at()\n    // would throw for the equivalent nlohmann::json value, so error\n    // handling written against basic_json::at() keeps working unchanged\n    json_document config = json_document::parse(R\"({\"name\": \"cache\", \"port\": \"6379\"})\");\n    const json_view service = config.root();\n\n    std::cout << service.at(\"port\").materialize().dump() << '\\n';\n\n    try\n    {\n        // \"port\" is a string, not an array\n        static_cast<void>(service.at(\"port\").at(0));\n    }\n    catch (const nlohmann::json::type_error& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n\n    try\n    {\n        static_cast<void>(service.at(\"timeout\"));\n    }\n    catch (const nlohmann::json::out_of_range& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

        Output:

        \"6379\"\n[json.exception.type_error.304] cannot use at() with string\n[json.exception.out_of_range.403] key 'timeout' not found\n
        Example: (3) access specified element via JSON pointer with bounds checking

        The example below shows that at with a JSON pointer throws exactly the exceptions, with exactly the messages, that BasicJsonType::at throws for the same pointer and the same document.

        #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\nusing json_pointer = nlohmann::json::json_pointer;\n\nint main()\n{\n    json_document doc = json_document::parse(R\"({\"region\": \"eu\", \"servers\": [\"eu-1\", \"eu-2\"]})\");\n    const auto root = doc.root();\n\n    std::cout << root.at(json_pointer(\"/servers/1\")).materialize().dump() << '\\n';\n\n    // at() throws for every resolution failure -- with the very same\n    // message json::at(ptr) would throw for the same pointer and the same\n    // document\n    try\n    {\n        static_cast<void>(root.at(json_pointer(\"/servers/5\")));\n    }\n    catch (const nlohmann::json::out_of_range& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n\n    try\n    {\n        static_cast<void>(root.at(json_pointer(\"/missing\")));\n    }\n    catch (const nlohmann::json::out_of_range& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

        Output:

        \"eu-2\"\n[json.exception.out_of_range.401] array index 5 is out of range\n[json.exception.out_of_range.403] key 'missing' not found\n
        "},{"location":"api/basic_json_view/at/#see-also","title":"See also","text":"
        • operator[] - access specified element (returns a discarded view instead of throwing)
        • front, back - access the first or last element
        • BasicJsonType::at - the corresponding function of basic_json
        • json_pointer - JSON pointer type used by overload 3
        "},{"location":"api/basic_json_view/at/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_view/back/","title":"nlohmann::basic_json_view::back","text":"
        basic_json_view back() const;\n

        Returns the last element of an array, the last member value of an object, or the value itself if it is primitive (as for BasicJsonType::back(), a primitive value is a range of one element).

        "},{"location":"api/basic_json_view/back/#return-value","title":"Return value","text":"

        The last element or member value. For a primitive value (number, string, boolean), the value itself.

        "},{"location":"api/basic_json_view/back/#exception-safety","title":"Exception safety","text":"

        Strong guarantee: there are no changes to the view or the document it refers to.

        "},{"location":"api/basic_json_view/back/#exceptions","title":"Exceptions","text":"

        Throws invalid_iterator.214 if the view is null or discarded, or if it is an empty array or object.

        This exception does not carry a JSON_DIAGNOSTICS path: the view has no BasicJsonType value to point at, so the exception is created without one, even if BasicJsonType was built with JSON_DIAGNOSTICS enabled.

        "},{"location":"api/basic_json_view/back/#complexity","title":"Complexity","text":"

        Linear in the size of the array or object: unlike front(), which only ever looks at the first element, back() has to walk every element to find where they end, since elements are not a fixed size in the index.

        "},{"location":"api/basic_json_view/back/#notes","title":"Notes","text":"

        Unlike BasicJsonType::back(), which has undefined behavior for an empty array or object, back() throws invalid_iterator.214 for an empty array, an empty object, a null value, or a discarded view.

        "},{"location":"api/basic_json_view/back/#examples","title":"Examples","text":"Example

        The example below reads only the final status of a build log with back(). Even though back() is linear in the number of events (unlike front(), which is constant), it still avoids building a BasicJsonType value for the events that are not needed.

        #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    // the same build log; back() reads only the final status. It is linear\n    // in the number of events (unlike front(), which is constant), but\n    // still far less work than materializing the whole array\n    json_document log = json_document::parse(R\"([\"queued\", \"started\", \"compiling\", \"linking\", \"done\"])\");\n    std::cout << log.root().back().materialize().dump() << '\\n';\n\n    // an empty log -- back() throws instead of the undefined behavior\n    // basic_json::back() has for an empty array\n    json_document empty_log = json_document::parse(\"[]\");\n    try\n    {\n        static_cast<void>(empty_log.root().back());\n    }\n    catch (const nlohmann::json::invalid_iterator& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

        Output:

        \"done\"\n[json.exception.invalid_iterator.214] cannot get value\n
        "},{"location":"api/basic_json_view/back/#see-also","title":"See also","text":"
        • front - access the first element
        • BasicJsonType::back - the corresponding function of basic_json
        "},{"location":"api/basic_json_view/back/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_view/basic_json_view/","title":"nlohmann::basic_json_view::basic_json_view","text":"
        basic_json_view() noexcept = default;\n

        Creates an invalid (discarded) view: type() is value_t::discarded and is_discarded() is true.

        This is the only constructor a caller can use directly. Every other view is obtained from a basic_json_document, via root() or by navigating into a container with operator[], at, front, back, find, or iteration.

        "},{"location":"api/basic_json_view/basic_json_view/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this constructor never throws exceptions.

        "},{"location":"api/basic_json_view/basic_json_view/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json_view/basic_json_view/#notes","title":"Notes","text":"

        basic_json_view is trivially copyable (it holds two pointers), so a default-constructed view can be used as a placeholder for \"no value yet\" and later be assigned a real view.

        "},{"location":"api/basic_json_view/basic_json_view/#examples","title":"Examples","text":"Example

        The example below shows the default constructor and that a basic_json_view is a small, copyable handle.

        #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // the default constructor is the only public one: it creates an invalid\n    // (discarded) view, useful as a \"no value yet\" placeholder\n    nlohmann::json_view v;\n    std::cout << v.is_discarded() << '\\n';\n\n    // views are trivially copyable handles (two pointers); the document owns\n    // the actual data\n    nlohmann::json_view copy = v;\n    std::cout << copy.is_discarded() << '\\n';\n}\n

        Output:

        true\ntrue\n
        "},{"location":"api/basic_json_view/basic_json_view/#see-also","title":"See also","text":"
        • is_discarded - return whether the view is invalid
        • root - the view of a document's root value
        "},{"location":"api/basic_json_view/basic_json_view/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_view/begin/","title":"nlohmann::basic_json_view::begin","text":"
        iterator begin() const noexcept;\n

        Returns an iterator to the first element of an array, or the first member value of an object, in document order -- the order the values appear in the source text, not sorted by key. A primitive value iterates as a range of one element (itself), and a null value or a discarded view iterates as an empty range.

        "},{"location":"api/basic_json_view/begin/#return-value","title":"Return value","text":"

        Iterator to the first element.

        "},{"location":"api/basic_json_view/begin/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this function never throws exceptions.

        "},{"location":"api/basic_json_view/begin/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json_view/begin/#notes","title":"Notes","text":"

        For an object, iteration visits every member, including all occurrences of a duplicate key -- unlike operator[], at, find, and contains, which all resolve to the first member with a given key. See the Notes on duplicate keys of operator[].

        Because objects are iterated in document order rather than sorted by key, the order seen here can differ from what iterating the materialize()d BasicJsonType value would produce: a basic_json object (std::map-backed by default) sorts its keys, while a view does not.

        An ordered_json value also keeps document order, but it keeps only one entry per key: as parse() does, it keeps the last value, at the position of the key's first occurrence. The view, in contrast, visits every member, including the duplicates.

        "},{"location":"api/basic_json_view/begin/#examples","title":"Examples","text":"Example

        The example below iterates a log record's members with begin()/end() and prints them in the order they were written. Materializing the record into a BasicJsonType object and iterating that would instead print the members sorted by key.

        #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    // a log record: the fields matter in the order they were written, e.g.\n    // to reproduce the record as it was logged. A nlohmann::json object\n    // sorts its keys, so materializing and iterating it would instead print\n    // them alphabetically (\"level\", \"message\", \"time\")\n    json_document record = json_document::parse(R\"({\"time\": \"10:00:01\", \"level\": \"info\", \"message\": \"started\"})\");\n\n    for (auto it = record.root().begin(); it != record.root().end(); ++it)\n    {\n        std::cout << it.key() << '=' << it->materialize().dump() << '\\n';\n    }\n}\n

        Output:

        time=\"10:00:01\"\nlevel=\"info\"\nmessage=\"started\"\n
        "},{"location":"api/basic_json_view/begin/#see-also","title":"See also","text":"
        • end - returns an iterator to one past the last element
        • cbegin - returns a const iterator to the first element
        • items - access iterator member functions in range-based for
        • BasicJsonType::begin - the corresponding function of basic_json
        "},{"location":"api/basic_json_view/begin/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_view/cbegin/","title":"nlohmann::basic_json_view::cbegin","text":"
        iterator cbegin() const noexcept;\n

        Returns an iterator to the first element, in document order. Equivalent to begin(): a view is always read-only, so const_iterator and iterator are the same type, and cbegin() exists only so that generic code that expects a cbegin()/cend() pair works with a view too.

        "},{"location":"api/basic_json_view/cbegin/#return-value","title":"Return value","text":"

        Iterator to the first element; identical to what begin() returns.

        "},{"location":"api/basic_json_view/cbegin/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this function never throws exceptions.

        "},{"location":"api/basic_json_view/cbegin/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json_view/cbegin/#examples","title":"Examples","text":"Example

        The example below sums many measurements with std::accumulate, using cbegin()/cend() as the range -- the same way it would for any standard container -- without ever materializing the whole array into a BasicJsonType value.

        #include <iostream>\n#include <numeric>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\nusing json_view = nlohmann::json_view;\n\nint main()\n{\n    // sum many measurements with std::accumulate; cbegin()/cend() (identical\n    // to begin()/end() here -- the view is always read-only) let the view be\n    // used with standard algorithms without ever materializing the whole\n    // array into a nlohmann::json value\n    json_document measurements = json_document::parse(\"[3, 1, 4, 1, 5, 9, 2, 6]\");\n    const auto values = measurements.root();\n\n    const int sum = std::accumulate(values.cbegin(), values.cend(), 0,\n                                    [](int total, const json_view & v)\n    {\n        return total + v.materialize().get<int>();\n    });\n\n    std::cout << sum << '\\n';\n}\n

        Output:

        31\n
        "},{"location":"api/basic_json_view/cbegin/#see-also","title":"See also","text":"
        • begin - returns an iterator to the first element
        • cend - returns a const iterator to one past the last element
        • BasicJsonType::cbegin - the corresponding function of basic_json
        "},{"location":"api/basic_json_view/cbegin/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_view/cend/","title":"nlohmann::basic_json_view::cend","text":"
        iterator cend() const noexcept;\n

        Returns an iterator to one past the last element, in document order. Equivalent to end(): a view is always read-only, so const_iterator and iterator are the same type, and cend() exists only so that generic code that expects a cbegin()/cend() pair works with a view too.

        "},{"location":"api/basic_json_view/cend/#return-value","title":"Return value","text":"

        Iterator one past the last element; identical to what end() returns.

        "},{"location":"api/basic_json_view/cend/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this function never throws exceptions.

        "},{"location":"api/basic_json_view/cend/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json_view/cend/#examples","title":"Examples","text":"Example

        The example below checks that every record of a batch is an object with std::all_of, using cbegin()/cend() as the range -- the same way it would for any standard container -- before materializing any record of the batch.

        #include <algorithm>\n#include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\nusing json_view = nlohmann::json_view;\n\nint main()\n{\n    // check that every element of a (possibly large) batch is an object,\n    // before materializing any of them -- cbegin()/cend() (identical to\n    // begin()/end() here) work as the range for std::all_of like they would\n    // for any standard container\n    json_document batch = json_document::parse(R\"([{\"id\": 1}, {\"id\": 2}, {\"id\": 3}])\");\n    const auto records = batch.root();\n\n    const bool all_objects = std::all_of(records.cbegin(), records.cend(),\n                                         [](const json_view & v)\n    {\n        return v.is_object();\n    });\n\n    std::cout << std::boolalpha << all_objects << '\\n';\n}\n

        Output:

        true\n
        "},{"location":"api/basic_json_view/cend/#see-also","title":"See also","text":"
        • end - returns an iterator to one past the last element
        • cbegin - returns a const iterator to the first element
        • BasicJsonType::cend - the corresponding function of basic_json
        "},{"location":"api/basic_json_view/cend/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_view/contains/","title":"nlohmann::basic_json_view::contains","text":"
        // (1)\nbool contains(string_view_t key) const;\nbool contains(const char* key) const;\nbool contains(const string_t& key) const;\n\n// (2)\nbool contains(const json_pointer& ptr) const;\n
        1. Checks whether the value is an object with a member with key key.
        2. Checks whether a JSON pointer ptr can be resolved, starting at this value.
        "},{"location":"api/basic_json_view/contains/#parameters","title":"Parameters","text":"key (in) key value to check its existence ptr (in) JSON pointer to check its existence"},{"location":"api/basic_json_view/contains/#return-value","title":"Return value","text":"
        1. true if the value is an object and has a member with key key, false otherwise
        2. true if ptr can be resolved to a value starting at this view, false otherwise
        "},{"location":"api/basic_json_view/contains/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this function never throws exceptions.

        "},{"location":"api/basic_json_view/contains/#complexity","title":"Complexity","text":"
        1. Linear in the number of members: as for ordered_json, members are compared one after another, in document order, stopping at the first match. Each comparison first checks the key's length -- already known from the index, without reading the key bytes -- before comparing its content. Objects with 128 or more members get a hash index of their keys while parsing, so that a lookup in them takes constant time on average.
        2. Linear in the number of reference tokens of ptr and, for each token, in the number of members of the object at that level or the index into the array -- as for operator[] and at with a JSON pointer.
        "},{"location":"api/basic_json_view/contains/#notes","title":"Notes","text":"

        Duplicate keys: the first member wins

        If the source text repeats a key, this resolves to the first member with it, not to the last one that materialize() and parse() keep. See the Notes on duplicate keys of operator[] and Duplicate keys for the reasons and for how to get the last value.

        Overload 1 always returns false when the value is not an object -- including a discarded view.

        Postconditions

        If v.contains(key) returns true, then v[key] is not discarded and v.at(key) does not throw. If v.contains(ptr) returns true, then v[ptr] is not discarded and v.at(ptr) does not throw.

        Overload 2 never throws

        Unlike BasicJsonType::contains(const json_pointer&), which can throw for certain malformed pointers (for instance an empty array-index reference token), overload 2 never throws: a missing key, an out-of-range or malformed array index, a \"-\" index, or a reference token used on a primitive all simply make it return false.

        "},{"location":"api/basic_json_view/contains/#examples","title":"Examples","text":"Example: (1) check with key

        The example below counts how many of a batch of records carry an optional retry_of field, using contains() to check without ever materializing a single record of the batch.

        #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    // count how many of many incoming records carry an optional \"retry_of\"\n    // field -- contains() only walks the flat index, so scanning a large\n    // batch like this never builds a single nlohmann::json value\n    json_document batch = json_document::parse(R\"(\n      [\n        {\"id\": 1},\n        {\"id\": 2, \"retry_of\": 1},\n        {\"id\": 3},\n        {\"id\": 4, \"retry_of\": 3}\n      ]\n    )\");\n\n    const auto records = batch.root();\n    std::size_t retries = 0;\n    for (std::size_t i = 0; i < records.size(); ++i)\n    {\n        if (records[i].contains(\"retry_of\"))\n        {\n            ++retries;\n        }\n    }\n    std::cout << retries << \" of \" << records.size() << \" records are retries\\n\";\n}\n

        Output:

        2 of 4 records are retries\n
        Example: (2) check with JSON pointer

        The example below checks an optional, nested field with a JSON pointer, and shows two pointers that contains() resolves to false without throwing.

        #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\nusing json_pointer = nlohmann::json::json_pointer;\n\nint main()\n{\n    // \"retry_of\" is only present on some records, nested a level down\n    // inside \"meta\"\n    json_document batch = json_document::parse(R\"(\n      [\n        {\"id\": 1, \"meta\": {}},\n        {\"id\": 2, \"meta\": {\"retry_of\": 1}}\n      ]\n    )\");\n\n    const auto records = batch.root();\n    const json_pointer retry_of(\"/meta/retry_of\");\n    for (std::size_t i = 0; i < records.size(); ++i)\n    {\n        const auto record = records[i];\n        if (record.contains(retry_of))\n        {\n            std::cout << \"record \" << i << \" is a retry of \" << record[retry_of].get<int>() << '\\n';\n        }\n        else\n        {\n            std::cout << \"record \" << i << \" is original\\n\";\n        }\n    }\n\n    // contains() with a JSON pointer never throws -- not even for a\n    // pointer that indexes into a primitive (\"/0/id/x\") or uses a\n    // malformed array index (\"/01\"), either of which would need a\n    // try/catch with json::contains(ptr)\n    std::cout << std::boolalpha << records.contains(json_pointer(\"/0/id/x\")) << '\\n';\n    std::cout << std::boolalpha << records.contains(json_pointer(\"/01\")) << '\\n';\n}\n

        Output:

        record 0 is original\nrecord 1 is a retry of 1\nfalse\nfalse\n
        "},{"location":"api/basic_json_view/contains/#see-also","title":"See also","text":"
        • find - find a value in an object
        • count - returns the number of occurrences of a key
        • at, operator[] - resolve a JSON pointer and throw, or return a discarded view
        • BasicJsonType::contains - the corresponding function of basic_json
        "},{"location":"api/basic_json_view/contains/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_view/count/","title":"nlohmann::basic_json_view::count","text":"
        size_type count(string_view_t key) const;\nsize_type count(const char* key) const;\nsize_type count(const string_t& key) const;\n

        Returns the number of members with key key if the value is an object, 0 otherwise. Members with a repeated key are all counted.

        "},{"location":"api/basic_json_view/count/#parameters","title":"Parameters","text":"key (in) key value of the element to count"},{"location":"api/basic_json_view/count/#return-value","title":"Return value","text":"

        the number of members of the object with key key (duplicates included), or 0 if the value is not an object or has no such member.

        "},{"location":"api/basic_json_view/count/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this function never throws exceptions.

        "},{"location":"api/basic_json_view/count/#complexity","title":"Complexity","text":"

        Linear in the number of members: unlike the lookup functions, count() does not stop at the first match, because it has to count every member with the key. Members are compared one after another, in document order. Each comparison first checks the key's length -- already known from the index, without reading the key bytes -- before comparing its content. The hash index of the keys of objects with 128 or more members does not help here, since every member is visited.

        "},{"location":"api/basic_json_view/count/#notes","title":"Notes","text":"

        This method always returns 0 when the value is not an object -- including a discarded view.

        If the source text repeats a key, count() returns more than 1 for it, even though the materialized value (like the result of parse()) would have only one member with that key. Hence count(key) > 1 detects duplicate keys. This is unlike the lookup functions (operator[], at, find, contains), which resolve to the first member; see the Notes on duplicate keys of operator[].

        "},{"location":"api/basic_json_view/count/#examples","title":"Examples","text":"Example

        The example below validates that every transaction of a batch carries exactly one amount field, using count() before deciding whether to materialize a transaction at all. It reports a transaction with no amount field and one with a duplicated amount field.

        #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    // validate that every transaction of a batch carries exactly one\n    // \"amount\" field before materializing any of them into a nlohmann::json\n    // value -- count() counts every member with the key\n    json_document batch = json_document::parse(R\"(\n      [\n        {\"id\": 1, \"amount\": 9.99},\n        {\"id\": 2},\n        {\"id\": 3, \"amount\": 5.00, \"amount\": 500.00}\n      ]\n    )\");\n\n    const auto transactions = batch.root();\n    for (std::size_t i = 0; i < transactions.size(); ++i)\n    {\n        const auto transaction = transactions[i];\n        const auto amounts = transaction.count(\"amount\");\n        if (amounts != 1)\n        {\n            std::cout << \"transaction \" << i << \" has \" << amounts << \" \\\"amount\\\" fields\\n\";\n            continue;\n        }\n        std::cout << \"transaction \" << i << \": \" << transaction[\"amount\"].materialize().dump() << '\\n';\n    }\n}\n

        Output:

        transaction 0: 9.99\ntransaction 1 has 0 \"amount\" fields\ntransaction 2 has 2 \"amount\" fields\n
        "},{"location":"api/basic_json_view/count/#see-also","title":"See also","text":"
        • find - find a value in an object
        • contains - checks whether a key exists
        • BasicJsonType::count - the corresponding function of basic_json
        "},{"location":"api/basic_json_view/count/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_view/dump/","title":"nlohmann::basic_json_view::dump","text":"
        string_t dump(const int indent = -1,\n              const char indent_char = ' ',\n              const bool ensure_ascii = false,\n              const number_format numbers = number_format::shortest) const;\n

        Serializes this value (and its subtree) directly from the flat index, without ever building a BasicJsonType value first. With the default numbers == number_format::shortest, the result is the same string BasicJsonType::dump would produce for the value BasicJsonType::parse() builds from the same source text, called with the same indent, indent_char, and ensure_ascii -- except that members of an object appear in document order rather than sorted by key, and every occurrence of a repeated key is written rather than only the last one (see Notes on duplicate keys). For a json_view (whose BasicJsonType is not ordered), this means dump() can print an object's members in a different order than materialize().dump() of the same subtree.

        "},{"location":"api/basic_json_view/dump/#parameters","title":"Parameters","text":"indent (in) If indent is nonnegative, array elements and object members are pretty-printed with that indent level. An indent level of 0 only inserts newlines. -1 (the default) selects the most compact representation. indent_char (in) The character used for indentation if indent is greater than 0. The default is (space). ensure_ascii (in) If ensure_ascii is true, all non-ASCII characters in the output are escaped with \\uXXXX sequences, and the result consists of ASCII characters only. numbers (in) how to write numbers, see number_format: shortest (the default) writes them the way BasicJsonType::dump would; source copies every number exactly as it appears in the source text."},{"location":"api/basic_json_view/dump/#return-value","title":"Return value","text":"

        string containing the serialization of this value, or \"<discarded>\" if the view is discarded.

        "},{"location":"api/basic_json_view/dump/#exception-safety","title":"Exception safety","text":"

        Strong exception safety: if an exception is thrown, there are no changes to the view or the document it refers to.

        "},{"location":"api/basic_json_view/dump/#exceptions","title":"Exceptions","text":"

        May throw std::bad_alloc if allocating the output string fails. Unlike BasicJsonType::dump, there is no error_handler parameter and no type_error.316: the view only ever holds text the parser already validated as UTF-8, so there is nothing to replace or ignore.

        "},{"location":"api/basic_json_view/dump/#complexity","title":"Complexity","text":"

        Linear in the size of the output text.

        "},{"location":"api/basic_json_view/dump/#notes","title":"Notes","text":"

        The walk over the subtree is iterative, so the nesting depth it can write is limited by available memory only, not by the call stack -- as for materialize().

        Strings are escaped by the same rules as BasicJsonType::dump.

        With numbers == number_format::shortest, floats are written with the library's shortest round-trip conversion, exactly as BasicJsonType::dump would (e.g. 1.5, 100.0, 1e+100), and integers are copied from the source text -- already canonical in JSON, so this matches their shortest form too -- except that -0 is written as 0, the way BasicJsonType::parse() reads it.

        number_format::source copies every number exactly as written in the source text instead, with no exception for -0 -- 1.50, 1E2, -0.0, -0, or all digits of an integer literal with more digits than any number type holds (such a literal is itself classified as a float, see What is different) -- something BasicJsonType cannot do, since parsing already reduces every number to its parsed value.

        "},{"location":"api/basic_json_view/dump/#examples","title":"Examples","text":"Example

        The example below forwards a single record out of a larger batch, and re-serializes a configuration file, both without ever building a BasicJsonType value for the surrounding array or for the parts of it that were not needed. It also shows that materialize().dump() of the configuration sorts its keys, where dump() on the view keeps the order they appear in the source text.

        #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\nusing json_view = nlohmann::json_view;\n\nint main()\n{\n    // a large batch of sensor readings -- forward just the one that changed,\n    // without ever building a basic_json value for the batch or for the\n    // readings that are not needed\n    const json_document batch = json_document::parse(R\"(\n      [{\"id\": 1, \"temp\": 21.5}, {\"id\": 2, \"temp\": 87.3}, {\"id\": 3, \"temp\": 21.7}]\n    )\");\n    const json_view readings = batch.root();\n    std::cout << readings[1].dump() << '\\n';\n\n    // a configuration file -- dump() on the view keeps the member order of\n    // the source text; a json value's object_t is std::map, so\n    // materialize().dump() of the very same view sorts the keys instead\n    const json_document config = json_document::parse(\n                                     R\"({\"name\": \"cache\", \"host\": \"db1\", \"port\": 6379, \"timeout\": 30})\");\n    std::cout << config.root().dump(2) << \"\\n\\n\";\n    std::cout << config.root().materialize().dump(2) << '\\n';\n}\n

        Output:

        {\"id\":2,\"temp\":87.3}\n{\n  \"name\": \"cache\",\n  \"host\": \"db1\",\n  \"port\": 6379,\n  \"timeout\": 30\n}\n\n{\n  \"host\": \"db1\",\n  \"name\": \"cache\",\n  \"port\": 6379,\n  \"timeout\": 30\n}\n
        "},{"location":"api/basic_json_view/dump/#see-also","title":"See also","text":"
        • number_format - how dump() writes numbers
        • operator<< - serialize this value to a stream
        • materialize - build a BasicJsonType value, e.g. to use BasicJsonType::dump's error_handler
        • BasicJsonType::dump - the corresponding function of basic_json
        "},{"location":"api/basic_json_view/dump/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_view/empty/","title":"nlohmann::basic_json_view::empty","text":"
        bool empty() const noexcept;\n

        Checks whether size() is 0, as BasicJsonType::empty() would for the same value.

        "},{"location":"api/basic_json_view/empty/#return-value","title":"Return value","text":"

        The return value depends on the type and is defined as follows:

        Value type return value null true discarded true boolean false string false number false object object_t::empty() array array_t::empty()"},{"location":"api/basic_json_view/empty/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this function never throws exceptions.

        "},{"location":"api/basic_json_view/empty/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json_view/empty/#notes","title":"Notes","text":"

        As for BasicJsonType::empty(), this does not return whether a string value is empty -- it is false for any string, regardless of its length.

        "},{"location":"api/basic_json_view/empty/#examples","title":"Examples","text":"Example

        The example below uses size() and empty() to decide whether a parsed message is worth acting on, without materializing it into a BasicJsonType value.

        #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // decide whether a batch is worth processing before building any\n    // nlohmann::json value for it\n    json_document batch = json_document::parse(R\"([1, 2, 3, 4, 5])\");\n    json_document empty_batch = json_document::parse(\"[]\");\n\n    std::cout << batch.root().empty() << ' ' << batch.root().size() << '\\n';\n    std::cout << empty_batch.root().empty() << ' ' << empty_batch.root().size() << '\\n';\n\n    // as for basic_json: null has size 0, every other scalar has size 1\n    json_document n = json_document::parse(\"null\");\n    json_document s = json_document::parse(R\"(\"hi\")\");\n    std::cout << n.root().size() << ' ' << s.root().size() << '\\n';\n}\n

        Output:

        false 5\ntrue 0\n0 1\n
        "},{"location":"api/basic_json_view/empty/#see-also","title":"See also","text":"
        • size - return the number of elements
        • BasicJsonType::empty - the corresponding function of basic_json
        "},{"location":"api/basic_json_view/empty/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_view/end/","title":"nlohmann::basic_json_view::end","text":"
        iterator end() const noexcept;\n

        Returns an iterator to one past the last element of an array, one past the last member value of an object, in document order -- see begin(). A primitive value iterates as a range of one element (itself), and a null value or a discarded view iterates as an empty range (begin() == end()).

        "},{"location":"api/basic_json_view/end/#return-value","title":"Return value","text":"

        Iterator one past the last element.

        "},{"location":"api/basic_json_view/end/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this function never throws exceptions.

        "},{"location":"api/basic_json_view/end/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json_view/end/#notes","title":"Notes","text":"

        iterator is a forward iterator: unlike BasicJsonType::iterator, it cannot be decremented, so there is no way to reach the last element by stepping back from end(). Use back() instead.

        "},{"location":"api/basic_json_view/end/#examples","title":"Examples","text":"Example

        The example below scans a (possibly large) array of readings for the first one over a threshold, stopping the loop at end() as soon as one is found. Only the matching reading, if any, is ever materialized.

        #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    // scan a (possibly large) array of readings for the first one over a\n    // threshold; the loop stops at end() as soon as one is found, and only\n    // the matching reading is ever materialized\n    json_document readings = json_document::parse(\"[12, 18, 25, 31, 9]\");\n    const auto values = readings.root();\n\n    auto it = values.begin();\n    for (; it != values.end(); ++it)\n    {\n        if (it->materialize().get<int>() > 20)\n        {\n            break;\n        }\n    }\n\n    if (it != values.end())\n    {\n        std::cout << \"first reading over 20: \" << it->materialize().dump() << '\\n';\n    }\n}\n

        Output:

        first reading over 20: 25\n
        "},{"location":"api/basic_json_view/end/#see-also","title":"See also","text":"
        • begin - returns an iterator to the first element
        • cend - returns a const iterator to one past the last element
        • BasicJsonType::end - the corresponding function of basic_json
        "},{"location":"api/basic_json_view/end/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_view/find/","title":"nlohmann::basic_json_view::find","text":"
        iterator find(string_view_t key) const;\niterator find(const char* key) const;\niterator find(const string_t& key) const;\n

        Finds a member with key key -- the first one, should the key occur more than once (see Notes on duplicate keys). If the value is not an object, or no member has this key, end() is returned.

        "},{"location":"api/basic_json_view/find/#parameters","title":"Parameters","text":"key (in) key value of the element to search for"},{"location":"api/basic_json_view/find/#return-value","title":"Return value","text":"

        An iterator to the member with key key, or end() if there is none or the value is not an object.

        "},{"location":"api/basic_json_view/find/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this function never throws exceptions.

        "},{"location":"api/basic_json_view/find/#complexity","title":"Complexity","text":"

        Linear in the number of members: as for ordered_json, members are compared one after another, in document order, stopping at the first match. Each comparison first checks the key's length -- already known from the index, without reading the key bytes -- before comparing its content. Objects with 128 or more members get a hash index of their keys while parsing, so that a lookup in them takes constant time on average.

        "},{"location":"api/basic_json_view/find/#notes","title":"Notes","text":"

        Duplicate keys: the first member wins

        If the source text repeats a key, this resolves to the first member with it, not to the last one that materialize() and parse() keep. See the Notes on duplicate keys of operator[] and Duplicate keys for the reasons and for how to get the last value.

        Unlike BasicJsonType::find, which always returns end() for a non-object type, this also does so for a discarded view -- there is no separate \"invalid\" iterator to return.

        "},{"location":"api/basic_json_view/find/#examples","title":"Examples","text":"Example

        The example below scans a batch of events for those that carry an optional user_id field, using find() instead of operator[] (which would throw for the events that are not objects at all) or contains() followed by a second lookup. Events without a match are never materialized.

        #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    // a batch of incoming events; only some carry a \"user_id\" -- find()\n    // locates it without throwing for the events that turn out to not be\n    // objects, and without materializing an event that does not match\n    json_document batch = json_document::parse(R\"(\n      [\n        {\"type\": \"click\", \"user_id\": 42},\n        {\"type\": \"ping\"},\n        {\"type\": \"click\", \"user_id\": 7}\n      ]\n    )\");\n\n    const auto events = batch.root();\n    for (std::size_t i = 0; i < events.size(); ++i)\n    {\n        const auto event = events[i];\n        const auto it = event.find(\"user_id\");\n        if (it != event.end())\n        {\n            std::cout << \"user \" << it->materialize().dump() << '\\n';\n        }\n    }\n}\n

        Output:

        user 42\nuser 7\n
        "},{"location":"api/basic_json_view/find/#see-also","title":"See also","text":"
        • count - returns the number of occurrences of a key
        • contains - checks whether a key exists
        • BasicJsonType::find - the corresponding function of basic_json
        "},{"location":"api/basic_json_view/find/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_view/front/","title":"nlohmann::basic_json_view::front","text":"
        basic_json_view front() const;\n

        Returns the first element of an array, the first member value of an object, or the value itself if it is primitive (as for BasicJsonType::front(), a primitive value is a range of one element).

        "},{"location":"api/basic_json_view/front/#return-value","title":"Return value","text":"

        The first element or member value. For a primitive value (number, string, boolean), the value itself.

        "},{"location":"api/basic_json_view/front/#exception-safety","title":"Exception safety","text":"

        Strong guarantee: there are no changes to the view or the document it refers to.

        "},{"location":"api/basic_json_view/front/#exceptions","title":"Exceptions","text":"

        Throws invalid_iterator.214 if the view is null or discarded, or if it is an empty array or object.

        This exception does not carry a JSON_DIAGNOSTICS path: the view has no BasicJsonType value to point at, so the exception is created without one, even if BasicJsonType was built with JSON_DIAGNOSTICS enabled.

        "},{"location":"api/basic_json_view/front/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json_view/front/#notes","title":"Notes","text":"

        Unlike BasicJsonType::front(), which has undefined behavior for an empty array or object, front() throws invalid_iterator.214 for an empty array, an empty object, a null value, or a discarded view.

        "},{"location":"api/basic_json_view/front/#examples","title":"Examples","text":"Example

        The example below reads only the earliest entry of a build log with front(), without materializing the rest of the (possibly long) log.

        #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    // the build log of a running job; front() reads just the earliest event\n    // without materializing the (possibly long) rest of the log\n    json_document log = json_document::parse(R\"([\"queued\", \"started\", \"compiling\", \"linking\", \"done\"])\");\n    std::cout << log.root().front().materialize().dump() << '\\n';\n\n    // an empty log -- front() throws instead of the undefined behavior\n    // basic_json::front() has for an empty array\n    json_document empty_log = json_document::parse(\"[]\");\n    try\n    {\n        static_cast<void>(empty_log.root().front());\n    }\n    catch (const nlohmann::json::invalid_iterator& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

        Output:

        \"queued\"\n[json.exception.invalid_iterator.214] cannot get value\n
        "},{"location":"api/basic_json_view/front/#see-also","title":"See also","text":"
        • back - access the last element
        • BasicJsonType::front - the corresponding function of basic_json
        "},{"location":"api/basic_json_view/front/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_view/get/","title":"nlohmann::basic_json_view::get","text":"
        template<typename T>\nT get() const;\n

        Converts the value to T.

        For the types below, the conversion works directly on the flat index -- no BasicJsonType value is built for it:

        • bool
        • arithmetic types other than bool (for a boolean, 0/1, exactly as for BasicJsonType::get<T>())
        • std::nullptr_t
        • std::basic_string<char, Traits, Alloc> (including string_t) -- a copy of the string
        • string_view_t -- no copy: the returned view points into the document's source() text, or, for a string that contains escape sequences, into the document's own buffer of decoded strings (see get_string())
        • BasicJsonType -- equivalent to materialize()
        • basic_json_view -- returns *this
        • std::vector<U, A> -- from an array, element by element, each converted with get<U>(); std::vector<basic_json_view> keeps a view of every element instead of a value
        • std::map<K, V, C, A> and std::unordered_map<K, V, H, E, A>, if K is constructible from a (const char*, std::size_t) pair -- from an object, member by member, each value converted with get<V>(); with a repeated key, the last value is kept, as BasicJsonType::parse() (and materialize()) does; std::map<std::string, basic_json_view> keeps views of the members instead of values

        Every other T -- std::list, std::pair, std::array, enumerations, user types with a from_json(), ... -- is converted by materialize().get<T>(): the subtree is built into a real BasicJsonType value first (as BasicJsonType::parse() would), and converted from there exactly as BasicJsonType::get<T>() would convert it.

        "},{"location":"api/basic_json_view/get/#template-parameters","title":"Template parameters","text":"T the type to convert the value to"},{"location":"api/basic_json_view/get/#return-value","title":"Return value","text":"

        the value, converted to T

        "},{"location":"api/basic_json_view/get/#exception-safety","title":"Exception safety","text":"

        Strong guarantee: there are no changes to the view or the document it refers to.

        "},{"location":"api/basic_json_view/get/#exceptions","title":"Exceptions","text":"
        • For the directly-converted types listed above (other than BasicJsonType and basic_json_view, which never throw): throws type_error.302 if the value's type does not match T -- the same exception, with the same message, that BasicJsonType::get<T>() throws for the same JSON type and T.
        • For std::vector<U, A>: throws type_error.302 if the value is not an array; otherwise, whatever converting an element to U throws.
        • For std::map/std::unordered_map: throws type_error.302 if the value is not an object; otherwise, whatever converting a member to the mapped type throws.
        • For every other T: whatever materialize().get<T>() throws -- typically type_error.302, or whatever a user-provided from_json() throws.

        None of the exceptions thrown directly by this function (the first three bullets above) carry a JSON_DIAGNOSTICS path: the view has no BasicJsonType value to point at. An exception thrown while converting through materialize() (the last bullet) is different: it is thrown by a real BasicJsonType value, so it does carry a JSON_DIAGNOSTICS path if BasicJsonType was built with it enabled. That path is relative to the root of the view's subtree, because materialize() builds only the subtree (with parent pointers): it is empty if the failing value is the subtree's root itself, and it never is the path within the whole document.

        "},{"location":"api/basic_json_view/get/#complexity","title":"Complexity","text":"
        • bool, arithmetic types, std::nullptr_t, string_view_t, basic_json_view: constant.
        • std::basic_string<char, Traits, Alloc>: constant, plus one allocation and a copy of the string's bytes.
        • BasicJsonType: linear in the size of the subtree, see materialize().
        • std::vector<U, A>: linear in the number of elements, times the complexity of converting one element to U.
        • std::map/std::unordered_map: linear in the number of members for walking them, plus the container's own insertion cost per member (logarithmic for std::map, amortized constant for std::unordered_map), times the complexity of converting one member to the mapped type.
        • every other T: the cost of materialize() and of destroying the BasicJsonType value afterwards, plus that of BasicJsonType::get<T>().
        "},{"location":"api/basic_json_view/get/#notes","title":"Notes","text":"

        Floating-point values

        A floating-point T is converted from the same digits the lexer would see during BasicJsonType::parse(), using the same conversion, so the result is bit-for-bit identical to BasicJsonType::parse(text).get<T>() for the same source text.

        Duplicate keys

        std::map/std::unordered_map conversions keep the last value of a repeated key, like materialize() and BasicJsonType::parse() do. This is the opposite of operator[]/at/find/contains, which resolve to the first occurrence (see the Notes on duplicate keys).

        No pointers, references, or implicit conversion

        Unlike BasicJsonType, basic_json_view has no stored value anywhere to hand out a pointer or a reference to, so it provides neither get_ptr(), get_ref(), nor operator ValueType(). get_string() (equivalently, get<string_view_t>()) is the zero-copy alternative for strings.

        "},{"location":"api/basic_json_view/get/#examples","title":"Examples","text":"Example

        The example below reads typed fields straight into C++ variables, collects a view of every array element with get<std::vector<basic_json_view>>() instead of a value, and converts a nested object into a user type through its from_json() -- which runs on a BasicJsonType value materialize() builds for just that one member.

        #include <iostream>\n#include <nlohmann/json_view.hpp>\n#include <string>\n#include <vector>\n\nusing json_document = nlohmann::json_document;\nusing json_view = nlohmann::json_view;\n\n// address has no direct conversion in get<T>(), so get<address>() falls back\n// to materialize().get<address>() -- a real nlohmann::json value is built\n// for just this one member, and its own from_json() runs on that\nstruct address\n{\n    std::string city;\n    int zip = 0;\n};\n\nvoid from_json(const nlohmann::json& j, address& a)\n{\n    j.at(\"city\").get_to(a.city);\n    j.at(\"zip\").get_to(a.zip);\n}\n\nint main()\n{\n    json_document doc = json_document::parse(R\"(\n      {\n        \"name\": \"Alice\",\n        \"active\": true,\n        \"orders\": [1, 2, 3],\n        \"address\": {\"city\": \"Berlin\", \"zip\": 10115}\n      }\n    )\");\n    const json_view customer = doc.root();\n\n    // read typed fields straight into C++ variables -- none of these build\n    // a nlohmann::json value\n    const std::string name = customer[\"name\"].get<std::string>();\n    const bool active = customer[\"active\"].get<bool>();\n    std::cout << name << (active ? \" (active)\" : \" (inactive)\") << '\\n';\n\n    // std::vector<json_view> keeps views of the array elements instead of\n    // copies of their values\n    bool first = true;\n    for (const json_view order : customer[\"orders\"].get<std::vector<json_view>>())\n    {\n        std::cout << (first ? \"\" : \" \") << order.get<int>();\n        first = false;\n    }\n    std::cout << '\\n';\n\n    // everything else goes through materialize()\n    const address a = customer[\"address\"].get<address>();\n    std::cout << a.city << ' ' << a.zip << '\\n';\n}\n

        Output:

        Alice (active)\n1 2 3\nBerlin 10115\n
        "},{"location":"api/basic_json_view/get/#see-also","title":"See also","text":"
        • get_to - convert and write into a passed value
        • get_string - the string, without a copy
        • number_token - a number's token text, without a copy
        • materialize - build the BasicJsonType value of this subtree
        • BasicJsonType::get - the corresponding function of basic_json
        "},{"location":"api/basic_json_view/get/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_view/get_string/","title":"nlohmann::basic_json_view::get_string","text":"
        string_view_t get_string() const;\n

        Returns the string value as a string_view_t, without copying it.

        "},{"location":"api/basic_json_view/get_string/#return-value","title":"Return value","text":"

        The string, as a string_view_t that points either into the document's source() text (a string with no escape sequences), or into the document's own buffer of decoded strings (a string that contains escape sequences, such as \"\\n\" or \"\\u00e9\", which had to be decoded once when the document was parsed).

        "},{"location":"api/basic_json_view/get_string/#exception-safety","title":"Exception safety","text":"

        Strong guarantee: there are no changes to the view or the document it refers to.

        "},{"location":"api/basic_json_view/get_string/#exceptions","title":"Exceptions","text":"

        Throws type_error.302 if the value is not a string; example: \"type must be string, but is array\".

        This exception does not carry a JSON_DIAGNOSTICS path: the view has no BasicJsonType value to point at, so the exception is created without one, even if BasicJsonType was built with JSON_DIAGNOSTICS enabled.

        "},{"location":"api/basic_json_view/get_string/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json_view/get_string/#notes","title":"Notes","text":"

        basic_json_view has no BasicJsonType value stored anywhere, so unlike BasicJsonType, it has no get_ref() to hand out a reference to a stored string_t. get_string() (equivalently, get<string_view_t>()) is the zero-copy alternative: BasicJsonType::get_ref<const string_t&>() is its closest counterpart, except that it returns a view instead of a reference to a value that must already exist.

        get_string() borrows the string: nothing is copied, whereas get<std::string>() copies it. For a borrowed document, the returned string_view_t is valid as long as the caller's buffer; for an owned one, as long as the document.

        The returned string_view_t is valid exactly as long as the view that produced it -- see the validity rules of basic_json_view -- and, for a string with no escapes, for as long as the document's source text, which for a borrowed document must remain valid and unmodified for as long as the document exists, since floating-point values and unescaped strings are read from the source lazily.

        "},{"location":"api/basic_json_view/get_string/#examples","title":"Examples","text":"Example

        The example below pulls one field out of a JSON text that stands in for a large API response, and shows that no std::string was allocated for it: the returned view still points inside the original buffer. A field that contains an escape sequence cannot point into the original text -- it was decoded once into the document's own buffer instead -- but still avoids a per-field allocation.

        #include <iostream>\n#include <nlohmann/json_view.hpp>\n#include <string>\n\nusing json_document = nlohmann::json_document;\nusing json_view = nlohmann::json_view;\n\nint main()\n{\n    // a \"large response\" stand-in: only the \"id\" field is ever read out of it\n    const std::string text =\n        R\"({\"id\": \"8f14e45f-ceea-467e-bb92-963f5e3c7a08\", \"note\": \"created via API\\n\", \"payload\": \"...\"})\";\n    const json_document doc = json_document::parse(text);\n    const json_view response = doc.root();\n\n    const json_view::string_view_t id = response[\"id\"].get_string();\n    std::cout << id << '\\n';\n\n    // no std::string was allocated for \"id\": its bytes still live inside\n    // the original buffer, so id's data lies inside [text.data(),\n    // text.data() + text.size())\n    const bool id_in_source = id.data() >= text.data() && id.data() + id.size() <= text.data() + text.size();\n    std::cout << std::boolalpha << id_in_source << '\\n';\n\n    // \"note\" contains an escape sequence ('\\n'), so it was decoded once\n    // into the document's own buffer -- get_string() still avoids a copy\n    // into a new std::string, but the bytes no longer live inside \"text\"\n    const json_view::string_view_t note = response[\"note\"].get_string();\n    const bool note_in_source = note.data() >= text.data() && note.data() + note.size() <= text.data() + text.size();\n    std::cout << std::boolalpha << note_in_source << '\\n';\n}\n

        Output:

        8f14e45f-ceea-467e-bb92-963f5e3c7a08\ntrue\nfalse\n
        "},{"location":"api/basic_json_view/get_string/#see-also","title":"See also","text":"
        • get - convert the value to a given type (get<string_view_t>() is equivalent to this function)
        • number_token - a number's token text, without a copy
        • BasicJsonType::get_ref - the closest counterpart of basic_json
        "},{"location":"api/basic_json_view/get_string/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_view/get_to/","title":"nlohmann::basic_json_view::get_to","text":"
        template<typename T>\nT& get_to(T& v) const;\n

        Converts the value to T and stores it in v, in one of two ways:

        • For the types that get<T>() converts directly -- arithmetic types, bool, std::nullptr_t, string_view_t, std::basic_string, BasicJsonType, basic_json_view, std::vector, and std::map/std::unordered_map with string keys -- v is assigned: v = get<T>().
        • For every other type, the subtree is built into a BasicJsonType value and materialize().get_to(v) is called, i.e. from_json() is invoked with the existing v, exactly as BasicJsonType::get_to() does.
        "},{"location":"api/basic_json_view/get_to/#template-parameters","title":"Template parameters","text":"T the type to convert the value to"},{"location":"api/basic_json_view/get_to/#parameters","title":"Parameters","text":"v (out) the variable to store the converted value in"},{"location":"api/basic_json_view/get_to/#return-value","title":"Return value","text":"

        v, allowing calls to chain

        "},{"location":"api/basic_json_view/get_to/#exception-safety","title":"Exception safety","text":"
        • For the directly converted types: strong exception safety. If an exception is thrown, v is not modified.
        • For every other type: as for BasicJsonType::get_to(). materialize() throws before v is touched, but a from_json() that throws may leave v partly modified.
        "},{"location":"api/basic_json_view/get_to/#exceptions","title":"Exceptions","text":"
        • For the directly converted types: whatever get<T>() throws for the same value and T.
        • For every other type: whatever materialize() and BasicJsonType::get_to() throw.
        "},{"location":"api/basic_json_view/get_to/#complexity","title":"Complexity","text":"
        • For the directly converted types: whatever get<T>() has for the same T.
        • For every other type: that of materialize(), plus that of BasicJsonType::get_to().
        "},{"location":"api/basic_json_view/get_to/#notes","title":"Notes","text":"

        The difference between the two ways matters for a from_json() that sets only the fields present in the JSON: since it runs on the existing v, the other fields of v keep their values, whereas assigning get<T>() would replace them with those of a default-constructed T.

        "},{"location":"api/basic_json_view/get_to/#examples","title":"Examples","text":"Example

        The example below reads several fields of a service configuration directly into existing variables, then uses the returned reference to fold the host/port pair into a single string in the same expression.

        #include <iostream>\n#include <nlohmann/json_view.hpp>\n#include <string>\n\nusing json_document = nlohmann::json_document;\nusing json_view = nlohmann::json_view;\n\nint main()\n{\n    json_document doc = json_document::parse(R\"({\"host\": \"db.example.com\", \"port\": 5432, \"ssl\": true})\");\n    const json_view config = doc.root();\n\n    // get_to() writes directly into existing variables -- handy for filling\n    // in the members of a struct one field at a time, without an\n    // intermediate value from get<T>() for each one\n    std::string host;\n    int port = 0;\n    bool ssl = false;\n    config[\"host\"].get_to(host);\n    config[\"port\"].get_to(port);\n    config[\"ssl\"].get_to(ssl);\n    std::cout << host << ':' << port << (ssl ? \" (tls)\" : \"\") << '\\n';\n\n    // the return value is a reference to the argument, so a call can be\n    // used directly in a larger expression\n    std::string other_host;\n    std::cout << config[\"host\"].get_to(other_host).size() << '\\n';\n}\n

        Output:

        db.example.com:5432 (tls)\n14\n
        "},{"location":"api/basic_json_view/get_to/#see-also","title":"See also","text":"
        • get - convert the value to a given type
        • BasicJsonType::get_to - the corresponding function of basic_json
        "},{"location":"api/basic_json_view/get_to/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_view/is_array/","title":"nlohmann::basic_json_view::is_array","text":"
        bool is_array() const noexcept;\n

        This function returns true if and only if the value is an array.

        "},{"location":"api/basic_json_view/is_array/#return-value","title":"Return value","text":"

        true if the type is an array, false otherwise.

        "},{"location":"api/basic_json_view/is_array/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this function never throws exceptions.

        "},{"location":"api/basic_json_view/is_array/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json_view/is_array/#examples","title":"Examples","text":"Example

        The example below classifies several parsed documents by the type of their root value, without materializing any of them into a BasicJsonType value.

        #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // several incoming messages, one json_document per message. type() and\n    // is_*() only look at the flat index built by parse(); no nlohmann::json\n    // tree exists yet, and none is built unless materialize() is called\n    json_document d_null = json_document::parse(\"null\");\n    json_document d_bool = json_document::parse(\"true\");\n    json_document d_int = json_document::parse(\"-42\");\n    json_document d_unsigned = json_document::parse(\"42\");\n    json_document d_float = json_document::parse(\"4.2\");\n    json_document d_string = json_document::parse(R\"(\"hi\")\");\n    json_document d_array = json_document::parse(\"[1, 2, 3]\");\n    json_document d_object = json_document::parse(R\"({\"a\": 1})\");\n\n    std::cout << d_null.root().is_null() << '\\n';\n    std::cout << d_bool.root().is_boolean() << '\\n';\n    std::cout << d_int.root().is_number() << ' ' << d_int.root().is_number_integer() << '\\n';\n    std::cout << d_unsigned.root().is_number_unsigned() << '\\n';\n    std::cout << d_float.root().is_number_float() << '\\n';\n    std::cout << d_string.root().is_string() << '\\n';\n    std::cout << d_array.root().is_array() << ' ' << d_array.root().is_structured() << '\\n';\n    std::cout << d_object.root().is_object() << ' ' << d_object.root().is_primitive() << '\\n';\n\n    // JSON text can never produce a binary value: is_binary() is always false\n    std::cout << d_array.root().is_binary() << '\\n';\n\n    // a default-constructed view, and the root of a document that failed to\n    // parse without exceptions, are both discarded\n    nlohmann::json_view invalid;\n    json_document failed = json_document::parse(\"not json\", /* allow_exceptions */ false);\n    std::cout << invalid.is_discarded() << '\\n';\n    std::cout << failed.root().is_discarded() << '\\n';\n\n    // type() returns the same value_t enumeration as basic_json::type()\n    std::cout << (d_object.root().type() == nlohmann::json::value_t::object) << '\\n';\n}\n

        Output:

        true\ntrue\ntrue true\ntrue\ntrue\ntrue\ntrue true\ntrue false\nfalse\ntrue\ntrue\ntrue\n
        "},{"location":"api/basic_json_view/is_array/#see-also","title":"See also","text":"
        • is_structured - return whether the type is structured
        • size - the number of elements
        • empty - whether there are no elements
        • BasicJsonType::is_array - the corresponding function of basic_json
        "},{"location":"api/basic_json_view/is_array/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_view/is_binary/","title":"nlohmann::basic_json_view::is_binary","text":"
        bool is_binary() const noexcept;\n

        This function always returns false: a document is only ever parsed from JSON text, never from CBOR or the other binary formats, and a JSON text has no binary values, so a view can never refer to one. The function exists for interface parity with BasicJsonType::is_binary.

        "},{"location":"api/basic_json_view/is_binary/#return-value","title":"Return value","text":"

        false, always.

        "},{"location":"api/basic_json_view/is_binary/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this function never throws exceptions.

        "},{"location":"api/basic_json_view/is_binary/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json_view/is_binary/#examples","title":"Examples","text":"Example

        The example below classifies several parsed documents by the type of their root value, without materializing any of them into a BasicJsonType value.

        #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // several incoming messages, one json_document per message. type() and\n    // is_*() only look at the flat index built by parse(); no nlohmann::json\n    // tree exists yet, and none is built unless materialize() is called\n    json_document d_null = json_document::parse(\"null\");\n    json_document d_bool = json_document::parse(\"true\");\n    json_document d_int = json_document::parse(\"-42\");\n    json_document d_unsigned = json_document::parse(\"42\");\n    json_document d_float = json_document::parse(\"4.2\");\n    json_document d_string = json_document::parse(R\"(\"hi\")\");\n    json_document d_array = json_document::parse(\"[1, 2, 3]\");\n    json_document d_object = json_document::parse(R\"({\"a\": 1})\");\n\n    std::cout << d_null.root().is_null() << '\\n';\n    std::cout << d_bool.root().is_boolean() << '\\n';\n    std::cout << d_int.root().is_number() << ' ' << d_int.root().is_number_integer() << '\\n';\n    std::cout << d_unsigned.root().is_number_unsigned() << '\\n';\n    std::cout << d_float.root().is_number_float() << '\\n';\n    std::cout << d_string.root().is_string() << '\\n';\n    std::cout << d_array.root().is_array() << ' ' << d_array.root().is_structured() << '\\n';\n    std::cout << d_object.root().is_object() << ' ' << d_object.root().is_primitive() << '\\n';\n\n    // JSON text can never produce a binary value: is_binary() is always false\n    std::cout << d_array.root().is_binary() << '\\n';\n\n    // a default-constructed view, and the root of a document that failed to\n    // parse without exceptions, are both discarded\n    nlohmann::json_view invalid;\n    json_document failed = json_document::parse(\"not json\", /* allow_exceptions */ false);\n    std::cout << invalid.is_discarded() << '\\n';\n    std::cout << failed.root().is_discarded() << '\\n';\n\n    // type() returns the same value_t enumeration as basic_json::type()\n    std::cout << (d_object.root().type() == nlohmann::json::value_t::object) << '\\n';\n}\n

        Output:

        true\ntrue\ntrue true\ntrue\ntrue\ntrue\ntrue true\ntrue false\nfalse\ntrue\ntrue\ntrue\n
        "},{"location":"api/basic_json_view/is_binary/#see-also","title":"See also","text":"
        • type - return the type of the value
        • BasicJsonType::is_binary - the corresponding function of basic_json
        "},{"location":"api/basic_json_view/is_binary/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_view/is_boolean/","title":"nlohmann::basic_json_view::is_boolean","text":"
        bool is_boolean() const noexcept;\n

        This function returns true if and only if the value is a boolean.

        "},{"location":"api/basic_json_view/is_boolean/#return-value","title":"Return value","text":"

        true if the type is a boolean, false otherwise.

        "},{"location":"api/basic_json_view/is_boolean/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this function never throws exceptions.

        "},{"location":"api/basic_json_view/is_boolean/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json_view/is_boolean/#examples","title":"Examples","text":"Example

        The example below classifies several parsed documents by the type of their root value, without materializing any of them into a BasicJsonType value.

        #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // several incoming messages, one json_document per message. type() and\n    // is_*() only look at the flat index built by parse(); no nlohmann::json\n    // tree exists yet, and none is built unless materialize() is called\n    json_document d_null = json_document::parse(\"null\");\n    json_document d_bool = json_document::parse(\"true\");\n    json_document d_int = json_document::parse(\"-42\");\n    json_document d_unsigned = json_document::parse(\"42\");\n    json_document d_float = json_document::parse(\"4.2\");\n    json_document d_string = json_document::parse(R\"(\"hi\")\");\n    json_document d_array = json_document::parse(\"[1, 2, 3]\");\n    json_document d_object = json_document::parse(R\"({\"a\": 1})\");\n\n    std::cout << d_null.root().is_null() << '\\n';\n    std::cout << d_bool.root().is_boolean() << '\\n';\n    std::cout << d_int.root().is_number() << ' ' << d_int.root().is_number_integer() << '\\n';\n    std::cout << d_unsigned.root().is_number_unsigned() << '\\n';\n    std::cout << d_float.root().is_number_float() << '\\n';\n    std::cout << d_string.root().is_string() << '\\n';\n    std::cout << d_array.root().is_array() << ' ' << d_array.root().is_structured() << '\\n';\n    std::cout << d_object.root().is_object() << ' ' << d_object.root().is_primitive() << '\\n';\n\n    // JSON text can never produce a binary value: is_binary() is always false\n    std::cout << d_array.root().is_binary() << '\\n';\n\n    // a default-constructed view, and the root of a document that failed to\n    // parse without exceptions, are both discarded\n    nlohmann::json_view invalid;\n    json_document failed = json_document::parse(\"not json\", /* allow_exceptions */ false);\n    std::cout << invalid.is_discarded() << '\\n';\n    std::cout << failed.root().is_discarded() << '\\n';\n\n    // type() returns the same value_t enumeration as basic_json::type()\n    std::cout << (d_object.root().type() == nlohmann::json::value_t::object) << '\\n';\n}\n

        Output:

        true\ntrue\ntrue true\ntrue\ntrue\ntrue\ntrue true\ntrue false\nfalse\ntrue\ntrue\ntrue\n
        "},{"location":"api/basic_json_view/is_boolean/#see-also","title":"See also","text":"
        • type - return the type of the value
        • BasicJsonType::is_boolean - the corresponding function of basic_json
        "},{"location":"api/basic_json_view/is_boolean/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_view/is_discarded/","title":"nlohmann::basic_json_view::is_discarded","text":"
        bool is_discarded() const noexcept;\n

        Returns whether this view is invalid, i.e. does not refer to a value. This is the case for a default-constructed view (see (constructor)), and for root() of a document that is itself discarded -- in particular, the root of a failed parse() with allow_exceptions set to false.

        A discarded view is also what operator[] returns for a missing key, an index out of range, or a JSON pointer that cannot be resolved, and for any access on a view that is itself discarded (so a chain such as v[\"a\"][\"b\"] is safe). at throws instead.

        "},{"location":"api/basic_json_view/is_discarded/#return-value","title":"Return value","text":"

        true if the view or its document is discarded, false otherwise.

        "},{"location":"api/basic_json_view/is_discarded/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this function never throws exceptions.

        "},{"location":"api/basic_json_view/is_discarded/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json_view/is_discarded/#notes","title":"Notes","text":"

        A basic_json_view is not convertible to bool: such a conversion would mean \"refers to a value\", whereas basic_json converts to the bool it holds, so the same code would silently behave differently. Test !v.is_discarded() explicitly.

        "},{"location":"api/basic_json_view/is_discarded/#examples","title":"Examples","text":"Example

        The example below classifies several parsed documents by the type of their root value, without materializing any of them into a BasicJsonType value.

        #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // several incoming messages, one json_document per message. type() and\n    // is_*() only look at the flat index built by parse(); no nlohmann::json\n    // tree exists yet, and none is built unless materialize() is called\n    json_document d_null = json_document::parse(\"null\");\n    json_document d_bool = json_document::parse(\"true\");\n    json_document d_int = json_document::parse(\"-42\");\n    json_document d_unsigned = json_document::parse(\"42\");\n    json_document d_float = json_document::parse(\"4.2\");\n    json_document d_string = json_document::parse(R\"(\"hi\")\");\n    json_document d_array = json_document::parse(\"[1, 2, 3]\");\n    json_document d_object = json_document::parse(R\"({\"a\": 1})\");\n\n    std::cout << d_null.root().is_null() << '\\n';\n    std::cout << d_bool.root().is_boolean() << '\\n';\n    std::cout << d_int.root().is_number() << ' ' << d_int.root().is_number_integer() << '\\n';\n    std::cout << d_unsigned.root().is_number_unsigned() << '\\n';\n    std::cout << d_float.root().is_number_float() << '\\n';\n    std::cout << d_string.root().is_string() << '\\n';\n    std::cout << d_array.root().is_array() << ' ' << d_array.root().is_structured() << '\\n';\n    std::cout << d_object.root().is_object() << ' ' << d_object.root().is_primitive() << '\\n';\n\n    // JSON text can never produce a binary value: is_binary() is always false\n    std::cout << d_array.root().is_binary() << '\\n';\n\n    // a default-constructed view, and the root of a document that failed to\n    // parse without exceptions, are both discarded\n    nlohmann::json_view invalid;\n    json_document failed = json_document::parse(\"not json\", /* allow_exceptions */ false);\n    std::cout << invalid.is_discarded() << '\\n';\n    std::cout << failed.root().is_discarded() << '\\n';\n\n    // type() returns the same value_t enumeration as basic_json::type()\n    std::cout << (d_object.root().type() == nlohmann::json::value_t::object) << '\\n';\n}\n

        Output:

        true\ntrue\ntrue true\ntrue\ntrue\ntrue\ntrue true\ntrue false\nfalse\ntrue\ntrue\ntrue\n
        "},{"location":"api/basic_json_view/is_discarded/#see-also","title":"See also","text":"
        • operator[] - access specified element; yields a discarded view where an element is missing
        • (constructor) - the default constructor creates a discarded view
        • is_discarded (basic_json_document) - return whether the last parse failed
        • BasicJsonType::is_discarded - the corresponding function of basic_json
        "},{"location":"api/basic_json_view/is_discarded/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_view/is_null/","title":"nlohmann::basic_json_view::is_null","text":"
        bool is_null() const noexcept;\n

        This function returns true if and only if the value is null.

        "},{"location":"api/basic_json_view/is_null/#return-value","title":"Return value","text":"

        true if the type is null, false otherwise.

        "},{"location":"api/basic_json_view/is_null/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this function never throws exceptions.

        "},{"location":"api/basic_json_view/is_null/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json_view/is_null/#examples","title":"Examples","text":"Example

        The example below classifies several parsed documents by the type of their root value, without materializing any of them into a BasicJsonType value.

        #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // several incoming messages, one json_document per message. type() and\n    // is_*() only look at the flat index built by parse(); no nlohmann::json\n    // tree exists yet, and none is built unless materialize() is called\n    json_document d_null = json_document::parse(\"null\");\n    json_document d_bool = json_document::parse(\"true\");\n    json_document d_int = json_document::parse(\"-42\");\n    json_document d_unsigned = json_document::parse(\"42\");\n    json_document d_float = json_document::parse(\"4.2\");\n    json_document d_string = json_document::parse(R\"(\"hi\")\");\n    json_document d_array = json_document::parse(\"[1, 2, 3]\");\n    json_document d_object = json_document::parse(R\"({\"a\": 1})\");\n\n    std::cout << d_null.root().is_null() << '\\n';\n    std::cout << d_bool.root().is_boolean() << '\\n';\n    std::cout << d_int.root().is_number() << ' ' << d_int.root().is_number_integer() << '\\n';\n    std::cout << d_unsigned.root().is_number_unsigned() << '\\n';\n    std::cout << d_float.root().is_number_float() << '\\n';\n    std::cout << d_string.root().is_string() << '\\n';\n    std::cout << d_array.root().is_array() << ' ' << d_array.root().is_structured() << '\\n';\n    std::cout << d_object.root().is_object() << ' ' << d_object.root().is_primitive() << '\\n';\n\n    // JSON text can never produce a binary value: is_binary() is always false\n    std::cout << d_array.root().is_binary() << '\\n';\n\n    // a default-constructed view, and the root of a document that failed to\n    // parse without exceptions, are both discarded\n    nlohmann::json_view invalid;\n    json_document failed = json_document::parse(\"not json\", /* allow_exceptions */ false);\n    std::cout << invalid.is_discarded() << '\\n';\n    std::cout << failed.root().is_discarded() << '\\n';\n\n    // type() returns the same value_t enumeration as basic_json::type()\n    std::cout << (d_object.root().type() == nlohmann::json::value_t::object) << '\\n';\n}\n

        Output:

        true\ntrue\ntrue true\ntrue\ntrue\ntrue\ntrue true\ntrue false\nfalse\ntrue\ntrue\ntrue\n
        "},{"location":"api/basic_json_view/is_null/#see-also","title":"See also","text":"
        • type - return the type of the value
        • BasicJsonType::is_null - the corresponding function of basic_json
        "},{"location":"api/basic_json_view/is_null/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_view/is_number/","title":"nlohmann::basic_json_view::is_number","text":"
        bool is_number() const noexcept;\n

        This function returns true if and only if the value is a number, i.e. an integer, unsigned integer, or floating-point value. It is defined as is_number_integer() || is_number_float().

        "},{"location":"api/basic_json_view/is_number/#return-value","title":"Return value","text":"

        true if the type is a number, false otherwise.

        "},{"location":"api/basic_json_view/is_number/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this function never throws exceptions.

        "},{"location":"api/basic_json_view/is_number/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json_view/is_number/#examples","title":"Examples","text":"Example

        The example below classifies several parsed documents by the type of their root value, without materializing any of them into a BasicJsonType value.

        #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // several incoming messages, one json_document per message. type() and\n    // is_*() only look at the flat index built by parse(); no nlohmann::json\n    // tree exists yet, and none is built unless materialize() is called\n    json_document d_null = json_document::parse(\"null\");\n    json_document d_bool = json_document::parse(\"true\");\n    json_document d_int = json_document::parse(\"-42\");\n    json_document d_unsigned = json_document::parse(\"42\");\n    json_document d_float = json_document::parse(\"4.2\");\n    json_document d_string = json_document::parse(R\"(\"hi\")\");\n    json_document d_array = json_document::parse(\"[1, 2, 3]\");\n    json_document d_object = json_document::parse(R\"({\"a\": 1})\");\n\n    std::cout << d_null.root().is_null() << '\\n';\n    std::cout << d_bool.root().is_boolean() << '\\n';\n    std::cout << d_int.root().is_number() << ' ' << d_int.root().is_number_integer() << '\\n';\n    std::cout << d_unsigned.root().is_number_unsigned() << '\\n';\n    std::cout << d_float.root().is_number_float() << '\\n';\n    std::cout << d_string.root().is_string() << '\\n';\n    std::cout << d_array.root().is_array() << ' ' << d_array.root().is_structured() << '\\n';\n    std::cout << d_object.root().is_object() << ' ' << d_object.root().is_primitive() << '\\n';\n\n    // JSON text can never produce a binary value: is_binary() is always false\n    std::cout << d_array.root().is_binary() << '\\n';\n\n    // a default-constructed view, and the root of a document that failed to\n    // parse without exceptions, are both discarded\n    nlohmann::json_view invalid;\n    json_document failed = json_document::parse(\"not json\", /* allow_exceptions */ false);\n    std::cout << invalid.is_discarded() << '\\n';\n    std::cout << failed.root().is_discarded() << '\\n';\n\n    // type() returns the same value_t enumeration as basic_json::type()\n    std::cout << (d_object.root().type() == nlohmann::json::value_t::object) << '\\n';\n}\n

        Output:

        true\ntrue\ntrue true\ntrue\ntrue\ntrue\ntrue true\ntrue false\nfalse\ntrue\ntrue\ntrue\n
        "},{"location":"api/basic_json_view/is_number/#see-also","title":"See also","text":"
        • is_number_integer - return whether the value is an integer or unsigned integer number
        • is_number_unsigned - return whether the value is an unsigned integer number
        • is_number_float - return whether the value is a floating-point number
        • BasicJsonType::is_number - the corresponding function of basic_json
        "},{"location":"api/basic_json_view/is_number/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_view/is_number_float/","title":"nlohmann::basic_json_view::is_number_float","text":"
        bool is_number_float() const noexcept;\n

        This function returns true if and only if the value is a floating-point number.

        "},{"location":"api/basic_json_view/is_number_float/#return-value","title":"Return value","text":"

        true if the type is value_t::number_float, false otherwise.

        "},{"location":"api/basic_json_view/is_number_float/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this function never throws exceptions.

        "},{"location":"api/basic_json_view/is_number_float/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json_view/is_number_float/#notes","title":"Notes","text":"

        As for BasicJsonType::parse, an integer literal that does not fit into the 64-bit integer type is classified as a floating-point number, so is_number_float() can be true even for an integer-looking token in the source text.

        "},{"location":"api/basic_json_view/is_number_float/#examples","title":"Examples","text":"Example

        The example below classifies several parsed documents by the type of their root value, without materializing any of them into a BasicJsonType value.

        #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // several incoming messages, one json_document per message. type() and\n    // is_*() only look at the flat index built by parse(); no nlohmann::json\n    // tree exists yet, and none is built unless materialize() is called\n    json_document d_null = json_document::parse(\"null\");\n    json_document d_bool = json_document::parse(\"true\");\n    json_document d_int = json_document::parse(\"-42\");\n    json_document d_unsigned = json_document::parse(\"42\");\n    json_document d_float = json_document::parse(\"4.2\");\n    json_document d_string = json_document::parse(R\"(\"hi\")\");\n    json_document d_array = json_document::parse(\"[1, 2, 3]\");\n    json_document d_object = json_document::parse(R\"({\"a\": 1})\");\n\n    std::cout << d_null.root().is_null() << '\\n';\n    std::cout << d_bool.root().is_boolean() << '\\n';\n    std::cout << d_int.root().is_number() << ' ' << d_int.root().is_number_integer() << '\\n';\n    std::cout << d_unsigned.root().is_number_unsigned() << '\\n';\n    std::cout << d_float.root().is_number_float() << '\\n';\n    std::cout << d_string.root().is_string() << '\\n';\n    std::cout << d_array.root().is_array() << ' ' << d_array.root().is_structured() << '\\n';\n    std::cout << d_object.root().is_object() << ' ' << d_object.root().is_primitive() << '\\n';\n\n    // JSON text can never produce a binary value: is_binary() is always false\n    std::cout << d_array.root().is_binary() << '\\n';\n\n    // a default-constructed view, and the root of a document that failed to\n    // parse without exceptions, are both discarded\n    nlohmann::json_view invalid;\n    json_document failed = json_document::parse(\"not json\", /* allow_exceptions */ false);\n    std::cout << invalid.is_discarded() << '\\n';\n    std::cout << failed.root().is_discarded() << '\\n';\n\n    // type() returns the same value_t enumeration as basic_json::type()\n    std::cout << (d_object.root().type() == nlohmann::json::value_t::object) << '\\n';\n}\n

        Output:

        true\ntrue\ntrue true\ntrue\ntrue\ntrue\ntrue true\ntrue false\nfalse\ntrue\ntrue\ntrue\n
        "},{"location":"api/basic_json_view/is_number_float/#see-also","title":"See also","text":"
        • is_number - return whether the value is a number
        • BasicJsonType::is_number_float - the corresponding function of basic_json
        "},{"location":"api/basic_json_view/is_number_float/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_view/is_number_integer/","title":"nlohmann::basic_json_view::is_number_integer","text":"
        bool is_number_integer() const noexcept;\n

        This function returns true if and only if the value is an integer or unsigned integer number.

        "},{"location":"api/basic_json_view/is_number_integer/#return-value","title":"Return value","text":"

        true if the type is value_t::number_integer or value_t::number_unsigned, false otherwise.

        "},{"location":"api/basic_json_view/is_number_integer/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this function never throws exceptions.

        "},{"location":"api/basic_json_view/is_number_integer/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json_view/is_number_integer/#notes","title":"Notes","text":"

        As for BasicJsonType::is_number_integer, this includes unsigned integer values; use is_number_unsigned to test for those specifically.

        "},{"location":"api/basic_json_view/is_number_integer/#examples","title":"Examples","text":"Example

        The example below classifies several parsed documents by the type of their root value, without materializing any of them into a BasicJsonType value.

        #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // several incoming messages, one json_document per message. type() and\n    // is_*() only look at the flat index built by parse(); no nlohmann::json\n    // tree exists yet, and none is built unless materialize() is called\n    json_document d_null = json_document::parse(\"null\");\n    json_document d_bool = json_document::parse(\"true\");\n    json_document d_int = json_document::parse(\"-42\");\n    json_document d_unsigned = json_document::parse(\"42\");\n    json_document d_float = json_document::parse(\"4.2\");\n    json_document d_string = json_document::parse(R\"(\"hi\")\");\n    json_document d_array = json_document::parse(\"[1, 2, 3]\");\n    json_document d_object = json_document::parse(R\"({\"a\": 1})\");\n\n    std::cout << d_null.root().is_null() << '\\n';\n    std::cout << d_bool.root().is_boolean() << '\\n';\n    std::cout << d_int.root().is_number() << ' ' << d_int.root().is_number_integer() << '\\n';\n    std::cout << d_unsigned.root().is_number_unsigned() << '\\n';\n    std::cout << d_float.root().is_number_float() << '\\n';\n    std::cout << d_string.root().is_string() << '\\n';\n    std::cout << d_array.root().is_array() << ' ' << d_array.root().is_structured() << '\\n';\n    std::cout << d_object.root().is_object() << ' ' << d_object.root().is_primitive() << '\\n';\n\n    // JSON text can never produce a binary value: is_binary() is always false\n    std::cout << d_array.root().is_binary() << '\\n';\n\n    // a default-constructed view, and the root of a document that failed to\n    // parse without exceptions, are both discarded\n    nlohmann::json_view invalid;\n    json_document failed = json_document::parse(\"not json\", /* allow_exceptions */ false);\n    std::cout << invalid.is_discarded() << '\\n';\n    std::cout << failed.root().is_discarded() << '\\n';\n\n    // type() returns the same value_t enumeration as basic_json::type()\n    std::cout << (d_object.root().type() == nlohmann::json::value_t::object) << '\\n';\n}\n

        Output:

        true\ntrue\ntrue true\ntrue\ntrue\ntrue\ntrue true\ntrue false\nfalse\ntrue\ntrue\ntrue\n
        "},{"location":"api/basic_json_view/is_number_integer/#see-also","title":"See also","text":"
        • is_number - return whether the value is a number
        • is_number_unsigned - return whether the value is an unsigned integer number
        • BasicJsonType::is_number_integer - the corresponding function of basic_json
        "},{"location":"api/basic_json_view/is_number_integer/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_view/is_number_unsigned/","title":"nlohmann::basic_json_view::is_number_unsigned","text":"
        bool is_number_unsigned() const noexcept;\n

        This function returns true if and only if the value is an unsigned integer number.

        "},{"location":"api/basic_json_view/is_number_unsigned/#return-value","title":"Return value","text":"

        true if the type is value_t::number_unsigned, false otherwise.

        "},{"location":"api/basic_json_view/is_number_unsigned/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this function never throws exceptions.

        "},{"location":"api/basic_json_view/is_number_unsigned/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json_view/is_number_unsigned/#examples","title":"Examples","text":"Example

        The example below classifies several parsed documents by the type of their root value, without materializing any of them into a BasicJsonType value.

        #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // several incoming messages, one json_document per message. type() and\n    // is_*() only look at the flat index built by parse(); no nlohmann::json\n    // tree exists yet, and none is built unless materialize() is called\n    json_document d_null = json_document::parse(\"null\");\n    json_document d_bool = json_document::parse(\"true\");\n    json_document d_int = json_document::parse(\"-42\");\n    json_document d_unsigned = json_document::parse(\"42\");\n    json_document d_float = json_document::parse(\"4.2\");\n    json_document d_string = json_document::parse(R\"(\"hi\")\");\n    json_document d_array = json_document::parse(\"[1, 2, 3]\");\n    json_document d_object = json_document::parse(R\"({\"a\": 1})\");\n\n    std::cout << d_null.root().is_null() << '\\n';\n    std::cout << d_bool.root().is_boolean() << '\\n';\n    std::cout << d_int.root().is_number() << ' ' << d_int.root().is_number_integer() << '\\n';\n    std::cout << d_unsigned.root().is_number_unsigned() << '\\n';\n    std::cout << d_float.root().is_number_float() << '\\n';\n    std::cout << d_string.root().is_string() << '\\n';\n    std::cout << d_array.root().is_array() << ' ' << d_array.root().is_structured() << '\\n';\n    std::cout << d_object.root().is_object() << ' ' << d_object.root().is_primitive() << '\\n';\n\n    // JSON text can never produce a binary value: is_binary() is always false\n    std::cout << d_array.root().is_binary() << '\\n';\n\n    // a default-constructed view, and the root of a document that failed to\n    // parse without exceptions, are both discarded\n    nlohmann::json_view invalid;\n    json_document failed = json_document::parse(\"not json\", /* allow_exceptions */ false);\n    std::cout << invalid.is_discarded() << '\\n';\n    std::cout << failed.root().is_discarded() << '\\n';\n\n    // type() returns the same value_t enumeration as basic_json::type()\n    std::cout << (d_object.root().type() == nlohmann::json::value_t::object) << '\\n';\n}\n

        Output:

        true\ntrue\ntrue true\ntrue\ntrue\ntrue\ntrue true\ntrue false\nfalse\ntrue\ntrue\ntrue\n
        "},{"location":"api/basic_json_view/is_number_unsigned/#see-also","title":"See also","text":"
        • is_number_integer - return whether the value is an integer or unsigned integer number
        • BasicJsonType::is_number_unsigned - the corresponding function of basic_json
        "},{"location":"api/basic_json_view/is_number_unsigned/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_view/is_object/","title":"nlohmann::basic_json_view::is_object","text":"
        bool is_object() const noexcept;\n

        This function returns true if and only if the value is an object.

        "},{"location":"api/basic_json_view/is_object/#return-value","title":"Return value","text":"

        true if the type is an object, false otherwise.

        "},{"location":"api/basic_json_view/is_object/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this function never throws exceptions.

        "},{"location":"api/basic_json_view/is_object/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json_view/is_object/#examples","title":"Examples","text":"Example

        The example below classifies several parsed documents by the type of their root value, without materializing any of them into a BasicJsonType value.

        #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // several incoming messages, one json_document per message. type() and\n    // is_*() only look at the flat index built by parse(); no nlohmann::json\n    // tree exists yet, and none is built unless materialize() is called\n    json_document d_null = json_document::parse(\"null\");\n    json_document d_bool = json_document::parse(\"true\");\n    json_document d_int = json_document::parse(\"-42\");\n    json_document d_unsigned = json_document::parse(\"42\");\n    json_document d_float = json_document::parse(\"4.2\");\n    json_document d_string = json_document::parse(R\"(\"hi\")\");\n    json_document d_array = json_document::parse(\"[1, 2, 3]\");\n    json_document d_object = json_document::parse(R\"({\"a\": 1})\");\n\n    std::cout << d_null.root().is_null() << '\\n';\n    std::cout << d_bool.root().is_boolean() << '\\n';\n    std::cout << d_int.root().is_number() << ' ' << d_int.root().is_number_integer() << '\\n';\n    std::cout << d_unsigned.root().is_number_unsigned() << '\\n';\n    std::cout << d_float.root().is_number_float() << '\\n';\n    std::cout << d_string.root().is_string() << '\\n';\n    std::cout << d_array.root().is_array() << ' ' << d_array.root().is_structured() << '\\n';\n    std::cout << d_object.root().is_object() << ' ' << d_object.root().is_primitive() << '\\n';\n\n    // JSON text can never produce a binary value: is_binary() is always false\n    std::cout << d_array.root().is_binary() << '\\n';\n\n    // a default-constructed view, and the root of a document that failed to\n    // parse without exceptions, are both discarded\n    nlohmann::json_view invalid;\n    json_document failed = json_document::parse(\"not json\", /* allow_exceptions */ false);\n    std::cout << invalid.is_discarded() << '\\n';\n    std::cout << failed.root().is_discarded() << '\\n';\n\n    // type() returns the same value_t enumeration as basic_json::type()\n    std::cout << (d_object.root().type() == nlohmann::json::value_t::object) << '\\n';\n}\n

        Output:

        true\ntrue\ntrue true\ntrue\ntrue\ntrue\ntrue true\ntrue false\nfalse\ntrue\ntrue\ntrue\n
        "},{"location":"api/basic_json_view/is_object/#see-also","title":"See also","text":"
        • is_structured - return whether the type is structured
        • size - the number of elements
        • empty - whether there are no elements
        • BasicJsonType::is_object - the corresponding function of basic_json
        "},{"location":"api/basic_json_view/is_object/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_view/is_primitive/","title":"nlohmann::basic_json_view::is_primitive","text":"
        bool is_primitive() const noexcept;\n

        This function returns true if and only if the value is primitive, i.e. null, a boolean, a number, or a string. It is defined as is_null() || is_string() || is_boolean() || is_number().

        "},{"location":"api/basic_json_view/is_primitive/#return-value","title":"Return value","text":"

        true if the type is primitive, false otherwise.

        "},{"location":"api/basic_json_view/is_primitive/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this function never throws exceptions.

        "},{"location":"api/basic_json_view/is_primitive/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json_view/is_primitive/#examples","title":"Examples","text":"Example

        The example below classifies several parsed documents by the type of their root value, without materializing any of them into a BasicJsonType value.

        #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // several incoming messages, one json_document per message. type() and\n    // is_*() only look at the flat index built by parse(); no nlohmann::json\n    // tree exists yet, and none is built unless materialize() is called\n    json_document d_null = json_document::parse(\"null\");\n    json_document d_bool = json_document::parse(\"true\");\n    json_document d_int = json_document::parse(\"-42\");\n    json_document d_unsigned = json_document::parse(\"42\");\n    json_document d_float = json_document::parse(\"4.2\");\n    json_document d_string = json_document::parse(R\"(\"hi\")\");\n    json_document d_array = json_document::parse(\"[1, 2, 3]\");\n    json_document d_object = json_document::parse(R\"({\"a\": 1})\");\n\n    std::cout << d_null.root().is_null() << '\\n';\n    std::cout << d_bool.root().is_boolean() << '\\n';\n    std::cout << d_int.root().is_number() << ' ' << d_int.root().is_number_integer() << '\\n';\n    std::cout << d_unsigned.root().is_number_unsigned() << '\\n';\n    std::cout << d_float.root().is_number_float() << '\\n';\n    std::cout << d_string.root().is_string() << '\\n';\n    std::cout << d_array.root().is_array() << ' ' << d_array.root().is_structured() << '\\n';\n    std::cout << d_object.root().is_object() << ' ' << d_object.root().is_primitive() << '\\n';\n\n    // JSON text can never produce a binary value: is_binary() is always false\n    std::cout << d_array.root().is_binary() << '\\n';\n\n    // a default-constructed view, and the root of a document that failed to\n    // parse without exceptions, are both discarded\n    nlohmann::json_view invalid;\n    json_document failed = json_document::parse(\"not json\", /* allow_exceptions */ false);\n    std::cout << invalid.is_discarded() << '\\n';\n    std::cout << failed.root().is_discarded() << '\\n';\n\n    // type() returns the same value_t enumeration as basic_json::type()\n    std::cout << (d_object.root().type() == nlohmann::json::value_t::object) << '\\n';\n}\n

        Output:

        true\ntrue\ntrue true\ntrue\ntrue\ntrue\ntrue true\ntrue false\nfalse\ntrue\ntrue\ntrue\n
        "},{"location":"api/basic_json_view/is_primitive/#see-also","title":"See also","text":"
        • is_structured - return whether the type is structured (the complement of this function, for a non-discarded view)
        • BasicJsonType::is_primitive - the corresponding function of basic_json
        "},{"location":"api/basic_json_view/is_primitive/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_view/is_string/","title":"nlohmann::basic_json_view::is_string","text":"
        bool is_string() const noexcept;\n

        This function returns true if and only if the value is a string.

        "},{"location":"api/basic_json_view/is_string/#return-value","title":"Return value","text":"

        true if the type is a string, false otherwise.

        "},{"location":"api/basic_json_view/is_string/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this function never throws exceptions.

        "},{"location":"api/basic_json_view/is_string/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json_view/is_string/#examples","title":"Examples","text":"Example

        The example below classifies several parsed documents by the type of their root value, without materializing any of them into a BasicJsonType value.

        #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // several incoming messages, one json_document per message. type() and\n    // is_*() only look at the flat index built by parse(); no nlohmann::json\n    // tree exists yet, and none is built unless materialize() is called\n    json_document d_null = json_document::parse(\"null\");\n    json_document d_bool = json_document::parse(\"true\");\n    json_document d_int = json_document::parse(\"-42\");\n    json_document d_unsigned = json_document::parse(\"42\");\n    json_document d_float = json_document::parse(\"4.2\");\n    json_document d_string = json_document::parse(R\"(\"hi\")\");\n    json_document d_array = json_document::parse(\"[1, 2, 3]\");\n    json_document d_object = json_document::parse(R\"({\"a\": 1})\");\n\n    std::cout << d_null.root().is_null() << '\\n';\n    std::cout << d_bool.root().is_boolean() << '\\n';\n    std::cout << d_int.root().is_number() << ' ' << d_int.root().is_number_integer() << '\\n';\n    std::cout << d_unsigned.root().is_number_unsigned() << '\\n';\n    std::cout << d_float.root().is_number_float() << '\\n';\n    std::cout << d_string.root().is_string() << '\\n';\n    std::cout << d_array.root().is_array() << ' ' << d_array.root().is_structured() << '\\n';\n    std::cout << d_object.root().is_object() << ' ' << d_object.root().is_primitive() << '\\n';\n\n    // JSON text can never produce a binary value: is_binary() is always false\n    std::cout << d_array.root().is_binary() << '\\n';\n\n    // a default-constructed view, and the root of a document that failed to\n    // parse without exceptions, are both discarded\n    nlohmann::json_view invalid;\n    json_document failed = json_document::parse(\"not json\", /* allow_exceptions */ false);\n    std::cout << invalid.is_discarded() << '\\n';\n    std::cout << failed.root().is_discarded() << '\\n';\n\n    // type() returns the same value_t enumeration as basic_json::type()\n    std::cout << (d_object.root().type() == nlohmann::json::value_t::object) << '\\n';\n}\n

        Output:

        true\ntrue\ntrue true\ntrue\ntrue\ntrue\ntrue true\ntrue false\nfalse\ntrue\ntrue\ntrue\n
        "},{"location":"api/basic_json_view/is_string/#see-also","title":"See also","text":"
        • source_offset - byte offset of this value in the source text, which differs for strings: an escaped string has none
        • BasicJsonType::is_string - the corresponding function of basic_json
        "},{"location":"api/basic_json_view/is_string/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_view/is_structured/","title":"nlohmann::basic_json_view::is_structured","text":"
        bool is_structured() const noexcept;\n

        This function returns true if and only if the value is structured, i.e. an array or an object. It is defined as is_array() || is_object().

        "},{"location":"api/basic_json_view/is_structured/#return-value","title":"Return value","text":"

        true if the type is structured, false otherwise.

        "},{"location":"api/basic_json_view/is_structured/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this function never throws exceptions.

        "},{"location":"api/basic_json_view/is_structured/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json_view/is_structured/#examples","title":"Examples","text":"Example

        The example below classifies several parsed documents by the type of their root value, without materializing any of them into a BasicJsonType value.

        #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // several incoming messages, one json_document per message. type() and\n    // is_*() only look at the flat index built by parse(); no nlohmann::json\n    // tree exists yet, and none is built unless materialize() is called\n    json_document d_null = json_document::parse(\"null\");\n    json_document d_bool = json_document::parse(\"true\");\n    json_document d_int = json_document::parse(\"-42\");\n    json_document d_unsigned = json_document::parse(\"42\");\n    json_document d_float = json_document::parse(\"4.2\");\n    json_document d_string = json_document::parse(R\"(\"hi\")\");\n    json_document d_array = json_document::parse(\"[1, 2, 3]\");\n    json_document d_object = json_document::parse(R\"({\"a\": 1})\");\n\n    std::cout << d_null.root().is_null() << '\\n';\n    std::cout << d_bool.root().is_boolean() << '\\n';\n    std::cout << d_int.root().is_number() << ' ' << d_int.root().is_number_integer() << '\\n';\n    std::cout << d_unsigned.root().is_number_unsigned() << '\\n';\n    std::cout << d_float.root().is_number_float() << '\\n';\n    std::cout << d_string.root().is_string() << '\\n';\n    std::cout << d_array.root().is_array() << ' ' << d_array.root().is_structured() << '\\n';\n    std::cout << d_object.root().is_object() << ' ' << d_object.root().is_primitive() << '\\n';\n\n    // JSON text can never produce a binary value: is_binary() is always false\n    std::cout << d_array.root().is_binary() << '\\n';\n\n    // a default-constructed view, and the root of a document that failed to\n    // parse without exceptions, are both discarded\n    nlohmann::json_view invalid;\n    json_document failed = json_document::parse(\"not json\", /* allow_exceptions */ false);\n    std::cout << invalid.is_discarded() << '\\n';\n    std::cout << failed.root().is_discarded() << '\\n';\n\n    // type() returns the same value_t enumeration as basic_json::type()\n    std::cout << (d_object.root().type() == nlohmann::json::value_t::object) << '\\n';\n}\n

        Output:

        true\ntrue\ntrue true\ntrue\ntrue\ntrue\ntrue true\ntrue false\nfalse\ntrue\ntrue\ntrue\n
        "},{"location":"api/basic_json_view/is_structured/#see-also","title":"See also","text":"
        • is_primitive - return whether the type is primitive
        • size, empty - the number of elements, and whether there are none
        • BasicJsonType::is_structured - the corresponding function of basic_json
        "},{"location":"api/basic_json_view/is_structured/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_view/items/","title":"nlohmann::basic_json_view::items","text":"
        /* unspecified */ items() const noexcept;\n

        Returns a range of items -- (key, value) pairs -- for use in range-based for loops. An item's key() is a string_view_t (for an array element, its index converted to a string, as for BasicJsonType::items()), and its value() is a basic_json_view of the element.

        The returned type is not part of the public API and may change between versions; use a range-based for loop (see the example), or decltype(v.items()) if you need to name it.

        for (const auto& item : v.items())\n{\n    std::cout << \"key: \" << item.key() << \", value: \" << item.value() << '\\n';\n}\n

        On C++17, item also supports structured bindings:

        for (const auto [key, value] : v.items())\n{\n    std::cout << \"key: \" << key << \", value: \" << value << '\\n';\n}\n

        Note the const auto (by value), not const auto&: unlike BasicJsonType::items(), whose elements are references into an existing object, a view's item is produced on the fly for each step of the iteration, so there is nothing for a reference to bind to.

        "},{"location":"api/basic_json_view/items/#return-value","title":"Return value","text":"

        A range whose iterators dereference to an item (with key() and value(), see above) and whose begin()/end() are equivalent to basic_json_view::begin()/end(), in document order.

        "},{"location":"api/basic_json_view/items/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this function never throws exceptions.

        "},{"location":"api/basic_json_view/items/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json_view/items/#notes","title":"Notes","text":"

        As for begin()/end(), items() visits every member of an object, including all occurrences of a duplicate key -- unlike operator[], at, find, and contains, which resolve to the first member with a given key. See the Notes on duplicate keys of operator[].

        Lifetime issues

        Unlike BasicJsonType::items() (see #2040), calling items() on a temporary view is safe, for instance for (auto item : v[\"a\"].items()): the returned range holds a copy of the view. What must outlive the loop is the document and, for a borrowed document, the source buffer.

        "},{"location":"api/basic_json_view/items/#examples","title":"Examples","text":"Example

        The example below shows a settings object whose source text records every update to a key as a duplicate member, in the order they happened. items() walks all of them, so the update history is visible, while operator[] only ever sees the first one and materialize() -- like BasicJsonType::parse() -- keeps only the last.

        #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    // a settings object whose source text records every update to a key as\n    // a duplicate member. items() visits all of them, in document order, so\n    // the update history is visible; operator[] only ever sees the first\n    // one, and materialize() -- like basic_json::parse() -- keeps the last\n    json_document updates = json_document::parse(R\"({\"retries\": 1, \"timeout\": 30, \"retries\": 5})\");\n    const auto settings = updates.root();\n\n    for (const auto& item : settings.items())\n    {\n        std::cout << item.key() << '=' << item.value().materialize().dump() << '\\n';\n    }\n\n    std::cout << \"first \\\"retries\\\" seen by operator[]: \" << settings[\"retries\"].materialize().dump() << '\\n';\n    std::cout << \"last \\\"retries\\\" kept by materialize(): \" << settings.materialize()[\"retries\"].dump() << '\\n';\n}\n

        Output:

        retries=1\ntimeout=30\nretries=5\nfirst \"retries\" seen by operator[]: 1\nlast \"retries\" kept by materialize(): 5\n
        "},{"location":"api/basic_json_view/items/#see-also","title":"See also","text":"
        • begin, end - the iterators items() is built on
        • BasicJsonType::items - the corresponding function of basic_json
        "},{"location":"api/basic_json_view/items/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_view/materialize/","title":"nlohmann::basic_json_view::materialize","text":"
        BasicJsonType materialize() const;\n

        Builds the BasicJsonType value of this subtree: the value BasicJsonType::parse() would have produced for the same source text, allocated for the first time by this call.

        "},{"location":"api/basic_json_view/materialize/#return-value","title":"Return value","text":"

        The BasicJsonType value of this subtree, or a discarded BasicJsonType value (BasicJsonType(value_t::discarded)) if the view is discarded.

        "},{"location":"api/basic_json_view/materialize/#exception-safety","title":"Exception safety","text":"

        Strong guarantee: there are no changes to the view or the document it refers to (nothing about either is mutated by this function).

        "},{"location":"api/basic_json_view/materialize/#exceptions","title":"Exceptions","text":"

        May throw std::bad_alloc (via BasicJsonType's allocator) if constructing the result fails.

        "},{"location":"api/basic_json_view/materialize/#complexity","title":"Complexity","text":"

        Linear in the size of the subtree.

        "},{"location":"api/basic_json_view/materialize/#notes","title":"Notes","text":"

        materialize() replays the subtree through the same SAX builder BasicJsonType::parse() uses, so the result matches it exactly -- including, for an object, that a repeated key keeps only its last value. The replay is iterative, so it is not limited by the call stack the way a naive recursive conversion would be; the JSON nesting depth is limited only by available memory, as for BasicJsonType::parse() itself.

        Unlike parsing with JSON_DIAGNOSTIC_POSITIONS enabled, the values produced by materialize() do not carry source positions. The index has the start offset of every value, but not the end of arrays, objects, and escaped strings, which the positions also need.

        Calling materialize() on the same view repeatedly builds a new, independent BasicJsonType value each time; it never caches the result.

        "},{"location":"api/basic_json_view/materialize/#examples","title":"Examples","text":"Example

        The example below skips messages that are not useful -- a discarded value, or an empty array -- using only is_array() and empty(), and calls materialize() only for the messages that are actually used, so no BasicJsonType value (and none of its per-element allocations) is ever built for the skipped ones.

        #include <array>\n#include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    // three incoming messages; skip the ones that are not useful without ever\n    // building a nlohmann::json value for them\n    json_document heartbeat = json_document::parse(\"null\");\n    json_document empty_batch = json_document::parse(\"[]\");\n    json_document batch = json_document::parse(R\"([{\"id\": 1}, {\"id\": 2}, {\"id\": 3}])\");\n    std::array<const json_document*, 3> messages = {{&heartbeat, &empty_batch, &batch}};\n\n    for (const json_document* d : messages)\n    {\n        // is_array()/empty() only look at the flat index: a discarded\n        // heartbeat or an empty batch is never turned into a nlohmann::json\n        // value, so no per-element allocation happens for them\n        if (!d->root().is_array() || d->root().empty())\n        {\n            std::cout << \"skipped\\n\";\n            continue;\n        }\n\n        // materialize() replays the subtree through the same SAX builder\n        // basic_json::parse() uses, so the result is exactly what\n        // basic_json::parse() would have produced for the same text\n        nlohmann::json value = d->root().materialize();\n        std::cout << value.dump() << '\\n';\n    }\n}\n

        Output:

        skipped\nskipped\n[{\"id\":1},{\"id\":2},{\"id\":3}]\n
        "},{"location":"api/basic_json_view/materialize/#see-also","title":"See also","text":"
        • root - the view of a document's root value
        • BasicJsonType::parse - build a BasicJsonType value directly from a JSON text
        • JSON_DIAGNOSTIC_POSITIONS - source positions on parsed values (not produced by materialize())
        "},{"location":"api/basic_json_view/materialize/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_view/number_format/","title":"nlohmann::basic_json_view::number_format","text":"
        enum class number_format {\n    shortest,\n    source\n};\n

        This enumeration is used in dump to choose how numbers are written. Two values are differentiated:

        shortest integers are copied from the source text -- already canonical in JSON -- except that -0 becomes 0, the way BasicJsonType::parse() reads it; floats are written with the library's shortest round-trip conversion, exactly as BasicJsonType::dump() would (e.g. 1.5, 100.0, 1e+100) source every number is copied exactly as it appears in the source text -- 1.50, 1E2, -0, all digits of an integer literal with more digits than any number type holds -- something BasicJsonType cannot do, since parsing already reduces every number to its parsed value"},{"location":"api/basic_json_view/number_format/#examples","title":"Examples","text":"Example

        The example below writes back a price list received from a supplier: with number_format::shortest (the default), a trailing zero and scientific notation are normalized away and a long account number that overflows every number type is rounded, the same way materialize().dump() (or basic_json::dump()) would; number_format::source keeps every number exactly as it was written in the source text instead.

        #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\nusing json_view = nlohmann::json_view;\n\nint main()\n{\n    // a price list received from a supplier feed -- prices and account\n    // numbers must be forwarded exactly, e.g. into an invoice\n    const json_document doc = json_document::parse(R\"(\n      [{\"sku\": \"A1\", \"price\": 19.90, \"account_id\": 12345678901234567890123456},\n       {\"sku\": \"A2\", \"price\": 1E2, \"account_id\": 98765432109876543210987654}]\n    )\");\n    const json_view list = doc.root();\n\n    // number_format::shortest (the default) writes numbers the way\n    // basic_json::dump() would: \"19.90\" becomes \"19.9\", \"1E2\" becomes\n    // \"100.0\", and each account number -- far beyond any 64-bit integer --\n    // is rounded to the nearest double, exactly as materialize().dump()\n    // (or a plain nlohmann::json) would round it\n    std::cout << list.dump() << '\\n';\n\n    // number_format::source copies every number exactly as it was written\n    // in the source text instead -- something basic_json cannot do at all,\n    // since parsing already reduces every number to its parsed value\n    std::cout << list.dump(-1, ' ', false, json_view::number_format::source) << '\\n';\n}\n

        Output:

        [{\"sku\":\"A1\",\"price\":19.9,\"account_id\":1.2345678901234568e+25},{\"sku\":\"A2\",\"price\":100.0,\"account_id\":9.876543210987655e+25}]\n[{\"sku\":\"A1\",\"price\":19.90,\"account_id\":12345678901234567890123456},{\"sku\":\"A2\",\"price\":1E2,\"account_id\":98765432109876543210987654}]\n
        "},{"location":"api/basic_json_view/number_format/#see-also","title":"See also","text":"
        • dump - serialize to a JSON-formatted string
        • number_token - get a single number's token text without dumping the whole value
        • BasicJsonType::error_handler_t - the analogous enumeration for BasicJsonType::dump's decoding-error behavior
        "},{"location":"api/basic_json_view/number_format/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_view/number_token/","title":"nlohmann::basic_json_view::number_token","text":"
        string_view_t number_token() const;\n

        Returns the number exactly as it appears in the source text, without parsing or rounding it.

        "},{"location":"api/basic_json_view/number_token/#return-value","title":"Return value","text":"

        The number's token text, as a string_view_t into the document's source() text -- for example \"1.50\", \"1E2\", \"-0\", or an integer literal with more digits than any number type holds (such as a 30-digit integer, which get<T>() and materialize() can only represent approximately, as a number_float_t).

        "},{"location":"api/basic_json_view/number_token/#exception-safety","title":"Exception safety","text":"

        Strong guarantee: there are no changes to the view or the document it refers to.

        "},{"location":"api/basic_json_view/number_token/#exceptions","title":"Exceptions","text":"

        Throws type_error.302 if the value is not a number; example: \"type must be number, but is string\".

        This exception does not carry a JSON_DIAGNOSTICS path: the view has no BasicJsonType value to point at, so the exception is created without one, even if BasicJsonType was built with JSON_DIAGNOSTICS enabled.

        "},{"location":"api/basic_json_view/number_token/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json_view/number_token/#notes","title":"Notes","text":"

        BasicJsonType has no counterpart to this function: once a number is parsed into number_integer_t, number_unsigned_t, or number_float_t, its original textual form (leading zeros aside, which are already rejected by the grammar; trailing zeros in the fraction; the case and sign of the exponent; ...) is gone. number_token() is useful precisely where that form must survive -- a price or an identifier that must be reproduced exactly, or a number too large for any of BasicJsonType's number types to hold without loss.

        The returned string_view_t always points into the document's source() text -- numbers are never decoded into the document's separate string buffer -- and is valid exactly as long as that text is, see the validity rules of basic_json_view.

        "},{"location":"api/basic_json_view/number_token/#examples","title":"Examples","text":"Example

        The example below keeps a price and an order ID exactly as they were written in an incoming order, where converting them with get<T>() would lose information: the price picks up floating-point rounding, and the order ID -- more digits than a 64-bit integer holds -- can only be approximated as a double once materialize()d.

        #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\nusing json_view = nlohmann::json_view;\n\nint main()\n{\n    // a price and an order id from an incoming order -- both need to be\n    // reproduced exactly, e.g. for an invoice or an audit log\n    json_document doc = json_document::parse(R\"(\n      {\"price\": 19.90, \"order_id\": 1234567890123456789012345, \"quantity\": 3}\n    )\");\n    const json_view order = doc.root();\n\n    // number_token() returns the number exactly as written in the source\n    std::cout << order[\"price\"].number_token() << '\\n';\n    std::cout << order[\"order_id\"].number_token() << '\\n';\n\n    // get<double>() converts it instead -- the exact source text is gone:\n    // \"19.90\" becomes the double closest to 19.9, printed without the\n    // trailing zero, and the 25-digit order id -- far beyond any 64-bit\n    // integer -- can only be approximated as a double\n    std::cout << order[\"price\"].get<double>() << '\\n';\n    std::cout << order.materialize()[\"order_id\"].dump() << '\\n';\n\n    // an ordinary quantity has nothing to lose either way\n    std::cout << order[\"quantity\"].number_token() << \" == \" << order[\"quantity\"].get<int>() << '\\n';\n}\n

        Output:

        19.90\n1234567890123456789012345\n19.9\n1.2345678901234568e+24\n3 == 3\n
        "},{"location":"api/basic_json_view/number_token/#see-also","title":"See also","text":"
        • get - convert the value to a given type
        • get_string - the string, without a copy
        • BasicJsonType::number_integer_t, number_unsigned_t, number_float_t - the number types get<T>() and materialize() convert into
        "},{"location":"api/basic_json_view/number_token/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_view/operator%5B%5D/","title":"nlohmann::basic_json_view::operator[]","text":"
        // (1)\nbasic_json_view operator[](string_view_t key) const;\nbasic_json_view operator[](const char* key) const;\nbasic_json_view operator[](const string_t& key) const;\n\n// (2)\nbasic_json_view operator[](size_type idx) const;\ntemplate<typename IntegerType>\nbasic_json_view operator[](IntegerType idx) const;\n\n// (3)\nbasic_json_view operator[](const json_pointer& ptr) const;\n
        1. Returns the value of the object member with key key -- the first one, should the key occur more than once (see the Notes below) -- or a discarded view if there is no such member.
        2. Returns the array element at index idx, or a discarded view if idx is out of range. The template accepts every integer type except bool and std::size_t (int, unsigned, long, std::int64_t, ...) and forwards to the size_type overload, so that an integer argument is not ambiguous between that overload and 1; a negative idx is out of range.
        3. Returns the value a JSON pointer ptr refers to, starting at this value, or a discarded view wherever resolving it further is not possible without inserting into or extending the document (see Return value and Exceptions below).
        "},{"location":"api/basic_json_view/operator%5B%5D/#parameters","title":"Parameters","text":"key (in) object key of the element to access idx (in) index of the element to access ptr (in) JSON pointer to the element to access"},{"location":"api/basic_json_view/operator%5B%5D/#return-value","title":"Return value","text":"
        1. the value of the first member with key key, or a discarded view if no member has this key (or if this view is discarded)
        2. the element at index idx, or a discarded view if idx >= size() or idx is negative (or if this view is discarded)
        3. the value ptr resolves to, starting at this value, or a discarded view (also if this view is discarded) for exactly the reference tokens where the const overload of BasicJsonType::operator[] invokes undefined behavior for the same pointer and the same document: an object member that does not exist, or an array index that is out of range
        "},{"location":"api/basic_json_view/operator%5B%5D/#exception-safety","title":"Exception safety","text":"

        Strong guarantee: there are no changes to the view or the document it refers to.

        "},{"location":"api/basic_json_view/operator%5B%5D/#exceptions","title":"Exceptions","text":"
        1. Throws type_error.305 if the value is not an object and not discarded -- the same exception, with the same message, that the const overload of BasicJsonType::operator[] throws for a string argument on a non-object value.
        2. Throws type_error.305 if the value is not an array and not discarded -- the same exception, with the same message, that the const overload of BasicJsonType::operator[] throws for a numeric argument on a non-array value.
        3. Throws the same exceptions, with the same messages, that the const overload of BasicJsonType::operator[] throws for the same pointer and the same document:
          • out_of_range.402 if a reference token is \"-\" at an array.
          • out_of_range.404 if a reference token cannot be resolved because it is used on a primitive value.
          • parse_error.106 if an array index in ptr begins with '0'.
          • parse_error.109 if an array index in ptr is not a number.

        None of these exceptions carry a JSON_DIAGNOSTICS path: the view has no BasicJsonType value to point at, so the exception is created without one, even if BasicJsonType was built with JSON_DIAGNOSTICS enabled.

        "},{"location":"api/basic_json_view/operator%5B%5D/#complexity","title":"Complexity","text":"
        1. Linear in the number of members: as for ordered_json, members are compared one after another, in document order, stopping at the first match. Each comparison first checks the key's length -- already known from the index, without reading the key bytes -- before comparing its content, so a key of a different length than key is rejected without touching the source text. Objects with 128 or more members get a hash index of their keys while parsing, so that a lookup in them takes constant time on average.
        2. Linear in idx: elements are skipped one at a time from the first one, since they are not a fixed size in the index (unlike BasicJsonType's array, which is random-access).
        3. Linear in the number of reference tokens of ptr and, for each token, in the number of members of the object at that level (as 1.) or the index into the array (as 2.).
        "},{"location":"api/basic_json_view/operator%5B%5D/#notes","title":"Notes","text":"

        Unlike BasicJsonType::operator[], which is undefined behavior (guarded by a runtime assertion) for a missing key on a const value, this operator returns a safe, testable result for a missing key or an index out of range: a discarded view, which is tested with is_discarded. There is also no non-const overload that inserts a missing key or extends an array -- a view never modifies the document.

        Chained access

        operator[] on a discarded view returns a discarded view and does not throw, so a chain like v[\"a\"][\"b\"][0] is safe even if \"a\" or \"b\" is missing: the first missing step makes the whole result discarded, which is tested once at the end. Type errors on values that are not discarded still throw: a key on an array or a primitive, or an index on an object or a primitive, is type_error.305 as for BasicJsonType. at still throws for a discarded view, as it does for a missing key.

        Duplicate keys: the first member wins

        If the source text has an object with a duplicate key, operator[] (and at, find, contains, value, and JSON pointer resolution) all resolve to the first member with that key, because a lookup can stop as soon as it finds a match. This is different from materialize() (and BasicJsonType::parse()), which replay every member in order and so keep the last value for a repeated key, so v[\"a\"] and v.materialize()[\"a\"] can differ. To get the value parse() would give, use materialize() or iterate the members with items() and keep the last match. The key hash index of a larger object (128 members or more) leads to the first member of a key as well. begin()/end() and items() iterate over all members, including duplicates, in document order. See Duplicate keys and size().

        JSON pointer resolution

        Overload 3 walks ptr one reference token at a time, starting at this value, the same way at and contains do. It only ever returns a discarded view where the const overload of BasicJsonType::operator[] would be undefined behavior for the same pointer -- a missing object member or an out-of-range array index -- and still throws for every other way ptr can fail to resolve. See at for the checked version, which throws in every case instead, and contains for a version that never throws.

        "},{"location":"api/basic_json_view/operator%5B%5D/#examples","title":"Examples","text":"Example: (1)/(2) access specified element

        The example below reads a couple of fields out of a batch of user records without ever materializing a full BasicJsonType value for the batch. operator[] is used both to look up an optional object member and to index into an array -- in both cases, a missing value comes back as a discarded view that can be tested with a plain if, instead of relying on undefined behavior.

        #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    // two user records from a large API response; only the fields that are\n    // actually read are ever touched, and no nlohmann::json tree is built\n    // for the batch\n    json_document batch = json_document::parse(R\"(\n      [\n        {\"name\": \"Alice\", \"email\": \"alice@example.com\", \"tags\": [\"admin\", \"ops\"]},\n        {\"name\": \"Bob\", \"tags\": []}\n      ]\n    )\");\n\n    const auto users = batch.root();\n    for (std::size_t i = 0; i < users.size(); ++i)\n    {\n        const auto user = users[i];\n        std::cout << user[\"name\"].materialize().dump();\n\n        // operator[] on a missing object key gives a discarded view -- test\n        // it with is_discarded(). The const overload of json::operator[]\n        // would instead be undefined behavior (guarded by an assertion) for\n        // a missing key\n        const auto email = user[\"email\"];\n        if (!email.is_discarded())\n        {\n            std::cout << \" <\" << email.materialize().dump() << \">\";\n        }\n\n        // the same holds for an array index past the end: a discarded view,\n        // not undefined behavior\n        const auto first_tag = user[\"tags\"][0];\n        if (!first_tag.is_discarded())\n        {\n            std::cout << \" #\" << first_tag.materialize().dump();\n        }\n\n        std::cout << '\\n';\n    }\n}\n

        Output:

        \"Alice\" <\"alice@example.com\"> #\"admin\"\n\"Bob\"\n
        Example: (3) access specified element via JSON pointer

        The example below reaches straight into one deeply nested field of a large document with a single JSON pointer, without ever building a tree for the rest of it, and shows the discarded-view and throwing outcomes of a pointer that cannot be fully resolved.

        #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\nusing json_pointer = nlohmann::json::json_pointer;\n\nint main()\n{\n    // a larger document; operator[] with a JSON pointer reaches straight to\n    // one deeply nested field, without ever building a tree for the rest\n    json_document doc = json_document::parse(R\"(\n      {\n        \"region\": {\n          \"servers\": [\n            {\"name\": \"eu-1\", \"metrics\": {\"cpu\": 0.42}},\n            {\"name\": \"eu-2\", \"metrics\": {\"cpu\": 0.71}}\n          ]\n        }\n      }\n    )\");\n\n    const auto root = doc.root();\n    std::cout << root[json_pointer(\"/region/servers/1/metrics/cpu\")].materialize().dump() << '\\n';\n\n    // a missing key or an out-of-range index along the path gives a\n    // discarded view, exactly where const json::operator[] would be\n    // undefined behavior for the same pointer\n    const auto missing = root[json_pointer(\"/region/servers/5/metrics/cpu\")];\n    if (!missing.is_discarded())\n    {\n        std::cout << missing.materialize().dump() << '\\n';\n    }\n    else\n    {\n        std::cout << \"no such server\\n\";\n    }\n\n    // indexing into a primitive still throws, as basic_json::operator[]\n    // does for the same pointer\n    try\n    {\n        static_cast<void>(root[json_pointer(\"/region/servers/0/name/x\")]);\n    }\n    catch (const nlohmann::json::out_of_range& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

        Output:

        0.71\nno such server\n[json.exception.out_of_range.404] unresolved reference token 'x'\n
        "},{"location":"api/basic_json_view/operator%5B%5D/#see-also","title":"See also","text":"
        • at - access specified element with bounds checking (throws instead of returning a discarded view)
        • front, back - access the first or last element
        • find, contains - look up a member without throwing
        • BasicJsonType::operator[] - the corresponding function of basic_json
        • json_pointer - JSON pointer type used by overload 3
        "},{"location":"api/basic_json_view/operator%5B%5D/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_view/operator_eq/","title":"nlohmann::basic_json_view::operator==","text":"
        // (1)\nbool operator==(const basic_json_view& lhs, const basic_json_view& rhs);\n\n// (2)\nbool operator==(const basic_json_view& lhs, const BasicJsonType& rhs);\nbool operator==(const BasicJsonType& lhs, const basic_json_view& rhs);\n
        1. Compares two views for equality: whether the values BasicJsonType::parse() would produce for lhs and rhs are equal, according to BasicJsonType's operator==.
        2. Compares a view and a BasicJsonType value for equality, in either order: whether the value parse() would produce for the view and the other operand are equal, according to BasicJsonType's operator==.

        Neither overload builds a BasicJsonType value for a view to do the comparison (see Notes below). Numbers compare by value across their types (1 == 1.0), and an object compares by its members, with duplicate keys resolved exactly as parse() resolves them -- the last value, at the position of the first occurrence of the key. This is not what operator[] and the other lookups return for a duplicate key: they find the first member (see Duplicate keys).

        "},{"location":"api/basic_json_view/operator_eq/#parameters","title":"Parameters","text":"lhs (in) first value to consider rhs (in) second value to consider"},{"location":"api/basic_json_view/operator_eq/#return-value","title":"Return value","text":"

        whether the values lhs and rhs are equal

        "},{"location":"api/basic_json_view/operator_eq/#exception-safety","title":"Exception safety","text":"

        Strong exception safety: if an exception is thrown, there are no changes to either operand, or to the document(s) a view refers to.

        "},{"location":"api/basic_json_view/operator_eq/#exceptions","title":"Exceptions","text":"

        May throw std::bad_alloc. Unlike the other comparison and most other basic_json_view functions, operator== is not noexcept: resolving an object's members needs a temporary array to sort them by key (see Complexity below), and that allocation can fail.

        "},{"location":"api/basic_json_view/operator_eq/#complexity","title":"Complexity","text":"

        Linear in the size of the compared values: every number, string, array element, and object member is visited at most once, and the walk is iterative, so the nesting depth it can compare is limited by available memory only, not by the call stack (as for materialize()). Resolving an object's members takes an additional O(n log n) in the number of members at that level, since they are sorted by key to detect and resolve duplicates before being compared. Two arrays of different size() are rejected without visiting either one's elements.

        "},{"location":"api/basic_json_view/operator_eq/#notes","title":"Notes","text":"

        Only a single number, boolean, or null value is ever materialized into a BasicJsonType, to reuse its operator== -- for numbers, so that values written differently in the source text but equal in value (e.g. an integer and a floating-point literal) still compare equal, following the same rules BasicJsonType does for special values such as NaN. Constructing one of these scalars never allocates. Strings are compared directly, without allocating, either from the source text on both sides or, for overload 2, against BasicJsonType's own string. Arrays and objects are never materialized at all; only their elements or members are visited, one pair at a time.

        How objects are compared

        For a json_view (BasicJsonType::object_t is std::map), members are compared by key, regardless of the order they appear in the source text. For an ordered_json_view (object_t is ordered_map), they are compared in the order they occur, so the very same two objects with their members reordered can compare equal as json_views but not as ordered_json_views. This is exactly how json and ordered_json compare, see \"Comparing different basic_json specializations\".

        Discarded views

        A discarded view compares the same way a discarded BasicJsonType value does, which is governed by JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON: by default, a discarded view is never equal to anything, not even another discarded view.

        No ordering comparison (operator<) is provided for basic_json_view; materialize() is the way to get a BasicJsonType value that supports it.

        "},{"location":"api/basic_json_view/operator_eq/#examples","title":"Examples","text":"Example

        The example below checks whether a newly received configuration differs from the previous one, and whether a received document matches what a test expects -- directly on views, without ever materializing a BasicJsonType value for either side.

        #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\nusing json = nlohmann::json;\n\nint main()\n{\n    // two snapshots of a polled configuration endpoint -- compare them\n    // directly as views, without ever building a nlohmann::json value for\n    // either one\n    const json_document previous = json_document::parse(\n                                       R\"({\"name\": \"cache\", \"port\": 6379, \"timeout\": 30})\");\n    const json_document current = json_document::parse(\n                                      R\"({\"port\": 6379.0, \"timeout\": 30, \"name\": \"cache\"})\");\n\n    // same members, reordered, and 6379 written as a float -- operator==\n    // treats them the same way BasicJsonType::operator== would\n    std::cout << std::boolalpha << (previous.root() == current.root()) << '\\n';\n\n    // an actually changed value is detected the same way\n    const json_document changed = json_document::parse(\n                                      R\"({\"name\": \"cache\", \"port\": 6380, \"timeout\": 30})\");\n    std::cout << (previous.root() == changed.root()) << '\\n';\n\n    // comparing a view directly against an expected json value -- handy in a\n    // test, without materializing the received document at all\n    const json expected = {{\"name\", \"cache\"}, {\"port\", 6379}, {\"timeout\", 30}};\n    std::cout << (previous.root() == expected) << '\\n';\n}\n

        Output:

        true\nfalse\ntrue\n
        "},{"location":"api/basic_json_view/operator_eq/#see-also","title":"See also","text":"
        • operator!= - compare for inequality
        • materialize - build a BasicJsonType value, e.g. to keep comparing after the document is gone
        • BasicJsonType::operator== - the corresponding function of basic_json
        "},{"location":"api/basic_json_view/operator_eq/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_view/operator_ltlt/","title":"nlohmann::basic_json_view::operator<<","text":"
        std::ostream& operator<<(std::ostream& o, const basic_json_view& v);\n

        Not available when JSON_NO_IO is defined.

        Serializes the given view v to the output stream o, using dump -- exactly as operator<<(std::ostream&, const basic_json&) does for a basic_json value.

        • The indentation of the output can be controlled with the member variable width of the output stream o. For instance, using the manipulator std::setw(4) on o sets the indentation level to 4, and the serialization result is the same as calling v.dump(4). A width of 0 or less (the default) selects the most compact representation, as v.dump(-1) does.
        • The indentation character can be controlled with the member variable fill of the output stream o. For instance, the manipulator std::setfill('\\t') sets indentation to use a tab character rather than the default space character.
        • As for basic_json, o's width is reset to 0 after this call, whether or not it was greater than 0 before.

        Numbers are always written as v.dump() writes them by default, i.e. as with number_format::shortest; there is no way to select number_format::source through the stream.

        "},{"location":"api/basic_json_view/operator_ltlt/#parameters","title":"Parameters","text":"o (in, out) stream to write to v (in) view to serialize"},{"location":"api/basic_json_view/operator_ltlt/#return-value","title":"Return value","text":"

        the stream o

        "},{"location":"api/basic_json_view/operator_ltlt/#exceptions","title":"Exceptions","text":"

        May throw std::bad_alloc, propagated from dump. Unlike operator<<(std::ostream&, const basic_json&), there is no UTF-8 decoding step that could throw type_error.316, and no error_handler to choose between -- see the Exceptions of dump.

        "},{"location":"api/basic_json_view/operator_ltlt/#complexity","title":"Complexity","text":"

        Linear, as dump.

        "},{"location":"api/basic_json_view/operator_ltlt/#examples","title":"Examples","text":"Example

        The example below writes one record out of a larger batch straight to a log stream -- compact for a one-line entry, and pretty-printed with std::setw/std::setfill for a readable dump -- without ever building a BasicJsonType value for the record, or for the rest of the batch.

        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\nusing json_view = nlohmann::json_view;\n\nint main()\n{\n    // one order out of a large incoming batch -- write it straight to a log\n    // stream without ever building a basic_json value for it, or for the\n    // rest of the batch\n    const json_document doc = json_document::parse(R\"(\n      [{\"id\": 1, \"item\": \"cable\"}, {\"id\": 2, \"item\": \"adapter\"}]\n    )\");\n    const json_view orders = doc.root();\n\n    // compact, for a one-line log entry\n    std::cout << orders[1] << '\\n';\n\n    // std::setw sets the indentation level, exactly as for basic_json\n    std::cout << std::setw(2) << orders[1] << \"\\n\\n\";\n\n    // std::setfill changes the indentation character\n    std::cout << std::setw(1) << std::setfill('\\t') << orders[1] << '\\n';\n}\n

        Output:

        {\"id\":2,\"item\":\"adapter\"}\n{\n  \"id\": 2,\n  \"item\": \"adapter\"\n}\n\n{\n    \"id\": 2,\n    \"item\": \"adapter\"\n}\n
        "},{"location":"api/basic_json_view/operator_ltlt/#see-also","title":"See also","text":"
        • dump - serialize to a JSON-formatted string
        • operator<<(std::ostream&) - the corresponding operator for basic_json
        • JSON_NO_IO - switch off functions relying on certain C++ I/O headers
        "},{"location":"api/basic_json_view/operator_ltlt/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_view/operator_ne/","title":"nlohmann::basic_json_view::operator!=","text":"
        // (1)\nbool operator!=(const basic_json_view& lhs, const basic_json_view& rhs);\n\n// (2)\nbool operator!=(const basic_json_view& lhs, const BasicJsonType& rhs);\nbool operator!=(const BasicJsonType& lhs, const basic_json_view& rhs);\n
        1. Compares two views for inequality. Returns !(lhs == rhs), see operator==.
        2. Compares a view and a BasicJsonType value for inequality, in either order. Returns !(lhs == rhs) (or, for the reversed order, !(rhs == lhs)), see operator==.

        Since operator!= is defined as the negation of operator==, it follows the same rules for special cases: for instance, since a discarded view is never equal to anything by default (see operator=='s Notes), it is never unequal to anything either -- discarded != discarded is also false, exactly as for a discarded BasicJsonType value.

        "},{"location":"api/basic_json_view/operator_ne/#parameters","title":"Parameters","text":"lhs (in) first value to consider rhs (in) second value to consider"},{"location":"api/basic_json_view/operator_ne/#return-value","title":"Return value","text":"

        whether the values lhs and rhs are not equal

        "},{"location":"api/basic_json_view/operator_ne/#exception-safety","title":"Exception safety","text":"

        Strong exception safety: if an exception is thrown, there are no changes to either operand, or to the document(s) a view refers to.

        "},{"location":"api/basic_json_view/operator_ne/#exceptions","title":"Exceptions","text":"

        May throw std::bad_alloc, propagated from operator==. Unlike most other basic_json_view functions, operator!= is not noexcept.

        "},{"location":"api/basic_json_view/operator_ne/#complexity","title":"Complexity","text":"

        Linear, as operator==.

        "},{"location":"api/basic_json_view/operator_ne/#notes","title":"Notes","text":"

        See the Notes of operator== -- in particular for how an object's members are compared (order matters for ordered_json_view but not for json_view) and for how discarded views compare.

        No ordering comparison (operator<) is provided for basic_json_view; materialize() is the way to get a BasicJsonType value that supports it.

        "},{"location":"api/basic_json_view/operator_ne/#examples","title":"Examples","text":"Example

        The example below asserts, as a test would, that a received document differs from an unwanted value, and shows that -- as for json/ordered_json -- reordering an object's members is detected as a difference for an ordered_json_view but not for a json_view.

        #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\nusing ordered_json_document = nlohmann::ordered_json_document;\nusing json = nlohmann::json;\n\nint main()\n{\n    // assert, as a test would, that a received document differs from an\n    // unwanted shape -- without ever materializing it into a json value just\n    // to compare\n    const json_document received = json_document::parse(\n                                       R\"({\"status\": \"ok\", \"code\": 200})\");\n    const json unwanted = {{\"status\", \"error\"}, {\"code\", 500}};\n    std::cout << std::boolalpha << (received.root() != unwanted) << '\\n';\n\n    // json (std::map) compares object members regardless of order ...\n    const json_document a = json_document::parse(R\"({\"a\": 1, \"b\": 2})\");\n    const json_document b = json_document::parse(R\"({\"b\": 2, \"a\": 1})\");\n    std::cout << (a.root() != b.root()) << '\\n';\n\n    // ... but ordered_json (ordered_map) compares them in the order they\n    // appear, so the very same reordering is detected as a difference\n    const ordered_json_document oa = ordered_json_document::parse(R\"({\"a\": 1, \"b\": 2})\");\n    const ordered_json_document ob = ordered_json_document::parse(R\"({\"b\": 2, \"a\": 1})\");\n    std::cout << (oa.root() != ob.root()) << '\\n';\n}\n

        Output:

        true\nfalse\ntrue\n
        "},{"location":"api/basic_json_view/operator_ne/#see-also","title":"See also","text":"
        • operator== - compare for equality
        • materialize - build a BasicJsonType value, e.g. to keep comparing after the document is gone
        • BasicJsonType::operator!= - the corresponding function of basic_json
        "},{"location":"api/basic_json_view/operator_ne/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_view/size/","title":"nlohmann::basic_json_view::size","text":"
        size_type size() const noexcept;\n

        Returns the number of elements, as BasicJsonType::size() would for the same value.

        "},{"location":"api/basic_json_view/size/#return-value","title":"Return value","text":"

        The return value depends on the type and is defined as follows:

        Value type return value null 0 discarded 0 boolean 1 string 1 number 1 object number of key/value pairs array number of elements"},{"location":"api/basic_json_view/size/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this function never throws exceptions.

        "},{"location":"api/basic_json_view/size/#complexity","title":"Complexity","text":"

        Constant: for an object or array, the element count is stored in the index, not counted on demand.

        "},{"location":"api/basic_json_view/size/#notes","title":"Notes","text":"

        As for BasicJsonType::size(), this does not return the length of a string value -- it is 1 for a string, regardless of its length.

        If the source text has an object with a duplicate key, every occurrence counts towards its size() -- unlike materialize() (and BasicJsonType::parse()), which keeps only the last value for a repeated key. This means v.size() can be larger than v.materialize().size(). See the Notes on duplicate keys of operator[] for why lookups and iteration disagree on how many members there are.

        "},{"location":"api/basic_json_view/size/#examples","title":"Examples","text":"Example

        The example below uses size() and empty() to decide whether a parsed message is worth acting on, without materializing it into a BasicJsonType value.

        #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // decide whether a batch is worth processing before building any\n    // nlohmann::json value for it\n    json_document batch = json_document::parse(R\"([1, 2, 3, 4, 5])\");\n    json_document empty_batch = json_document::parse(\"[]\");\n\n    std::cout << batch.root().empty() << ' ' << batch.root().size() << '\\n';\n    std::cout << empty_batch.root().empty() << ' ' << empty_batch.root().size() << '\\n';\n\n    // as for basic_json: null has size 0, every other scalar has size 1\n    json_document n = json_document::parse(\"null\");\n    json_document s = json_document::parse(R\"(\"hi\")\");\n    std::cout << n.root().size() << ' ' << s.root().size() << '\\n';\n}\n

        Output:

        false 5\ntrue 0\n0 1\n
        "},{"location":"api/basic_json_view/size/#see-also","title":"See also","text":"
        • empty - return whether the value has no elements
        • BasicJsonType::size - the corresponding function of basic_json
        "},{"location":"api/basic_json_view/size/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_view/source_offset/","title":"nlohmann::basic_json_view::source_offset","text":"
        std::size_t source_offset() const noexcept;\n

        Returns the byte offset of this value in the document's source() text, without materializing anything.

        "},{"location":"api/basic_json_view/source_offset/#return-value","title":"Return value","text":"
        • For a string with no escapes, a number, true/false/null, an array, or an object: the byte offset of the first byte of the value's token (for a string: the first byte after the opening quote) in source().
        • static_cast<std::size_t>(-1) for a discarded view, and for a string that contains escapes -- such a string was decoded once into the document's own buffer, so there is no single byte range in source() left to point at.
        "},{"location":"api/basic_json_view/source_offset/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this function never throws exceptions.

        "},{"location":"api/basic_json_view/source_offset/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json_view/source_offset/#notes","title":"Notes","text":"

        This is a raw offset, not a length: the API does not (yet) expose how many bytes the token occupies in the source text, so source_offset() alone is enough to report where a value came from (for an error message, for syntax highlighting, ...) but not to slice its exact text back out of source() for a string, whose token length is not the same as its decoded size().

        "},{"location":"api/basic_json_view/source_offset/#examples","title":"Examples","text":"Example 
        #include <iostream>\n#include <string>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // source_offset() points into source(): useful to report *where* in the\n    // original text a value came from (error messages, syntax highlighting,\n    // forwarding a sub-range verbatim, ...) without materializing it\n    json_document doc = json_document::parse(R\"(  123)\");\n    auto v = doc.root();\n    std::cout << v.source_offset() << ' '\n              << std::string(doc.source().data() + v.source_offset(), 3) << '\\n';\n\n    // a string without escapes also stays in the source text\n    json_document plain = json_document::parse(R\"(\"ab\")\");\n    std::cout << plain.source()[plain.root().source_offset()] << '\\n';\n\n    // a string with escapes is decoded once into the document's own buffer, so\n    // there is no single byte range in source() to point at: source_offset()\n    // returns the \"not applicable\" sentinel\n    json_document escaped = json_document::parse(R\"(\"a\\nb\")\");\n    std::cout << (escaped.root().source_offset() == static_cast<std::size_t>(-1)) << '\\n';\n\n    // a discarded view -- default-constructed, or the root of a failed parse\n    // with allow_exceptions == false -- has no offset either\n    json_document failed = json_document::parse(\"not json\", false);\n    std::cout << (failed.root().source_offset() == static_cast<std::size_t>(-1)) << '\\n';\n}\n

        Output:

        2 123\na\ntrue\ntrue\n
        "},{"location":"api/basic_json_view/source_offset/#see-also","title":"See also","text":"
        • source - the parsed text
        • is_string - return whether the value is a string
        "},{"location":"api/basic_json_view/source_offset/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_view/type/","title":"nlohmann::basic_json_view::type","text":"
        value_t type() const noexcept;\n

        Returns the type of the value this view refers to, as a value from the value_t enumeration -- the same enumeration BasicJsonType::type() uses.

        "},{"location":"api/basic_json_view/type/#return-value","title":"Return value","text":"

        The type of the value; value_t::discarded for a discarded view.

        "},{"location":"api/basic_json_view/type/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this function never throws exceptions.

        "},{"location":"api/basic_json_view/type/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json_view/type/#notes","title":"Notes","text":"

        Unlike BasicJsonType::type(), this function can never return value_t::binary: a JSON text has no binary values, so type() only distinguishes the eight ordinary JSON value types (plus discarded).

        "},{"location":"api/basic_json_view/type/#examples","title":"Examples","text":"Example

        The example below classifies several parsed documents by the type of their root value, without materializing any of them into a BasicJsonType value.

        #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // several incoming messages, one json_document per message. type() and\n    // is_*() only look at the flat index built by parse(); no nlohmann::json\n    // tree exists yet, and none is built unless materialize() is called\n    json_document d_null = json_document::parse(\"null\");\n    json_document d_bool = json_document::parse(\"true\");\n    json_document d_int = json_document::parse(\"-42\");\n    json_document d_unsigned = json_document::parse(\"42\");\n    json_document d_float = json_document::parse(\"4.2\");\n    json_document d_string = json_document::parse(R\"(\"hi\")\");\n    json_document d_array = json_document::parse(\"[1, 2, 3]\");\n    json_document d_object = json_document::parse(R\"({\"a\": 1})\");\n\n    std::cout << d_null.root().is_null() << '\\n';\n    std::cout << d_bool.root().is_boolean() << '\\n';\n    std::cout << d_int.root().is_number() << ' ' << d_int.root().is_number_integer() << '\\n';\n    std::cout << d_unsigned.root().is_number_unsigned() << '\\n';\n    std::cout << d_float.root().is_number_float() << '\\n';\n    std::cout << d_string.root().is_string() << '\\n';\n    std::cout << d_array.root().is_array() << ' ' << d_array.root().is_structured() << '\\n';\n    std::cout << d_object.root().is_object() << ' ' << d_object.root().is_primitive() << '\\n';\n\n    // JSON text can never produce a binary value: is_binary() is always false\n    std::cout << d_array.root().is_binary() << '\\n';\n\n    // a default-constructed view, and the root of a document that failed to\n    // parse without exceptions, are both discarded\n    nlohmann::json_view invalid;\n    json_document failed = json_document::parse(\"not json\", /* allow_exceptions */ false);\n    std::cout << invalid.is_discarded() << '\\n';\n    std::cout << failed.root().is_discarded() << '\\n';\n\n    // type() returns the same value_t enumeration as basic_json::type()\n    std::cout << (d_object.root().type() == nlohmann::json::value_t::object) << '\\n';\n}\n

        Output:

        true\ntrue\ntrue true\ntrue\ntrue\ntrue\ntrue true\ntrue false\nfalse\ntrue\ntrue\ntrue\n
        "},{"location":"api/basic_json_view/type/#see-also","title":"See also","text":"
        • is_null, is_boolean, is_number, is_string, is_array, is_object - type-specific predicates built on type()
        • BasicJsonType::type - the corresponding function of basic_json
        "},{"location":"api/basic_json_view/type/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_view/type_name/","title":"nlohmann::basic_json_view::type_name","text":"
        const char* type_name() const noexcept;\n

        Returns the type name as string to be used in error messages -- usually to indicate that a function was called on a wrong JSON type. Identical to BasicJsonType::type_name(), including the extra \"discarded\" return value for a discarded view (BasicJsonType::type_name() produces the same string for a discarded BasicJsonType value).

        "},{"location":"api/basic_json_view/type_name/#return-value","title":"Return value","text":"

        a string representation of the type (value_t):

        Value type return value null \"null\" boolean \"boolean\" string \"string\" number (integer, unsigned integer, floating-point) \"number\" object \"object\" array \"array\" discarded \"discarded\"

        type_name() never returns \"binary\", since a JSON text has no binary values (see is_binary()); it also never returns \"invalid\", since a view's kind always comes from a value the parser actually produced.

        "},{"location":"api/basic_json_view/type_name/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this function never throws exceptions.

        "},{"location":"api/basic_json_view/type_name/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/basic_json_view/type_name/#examples","title":"Examples","text":"Example

        The example below reports why some parsed messages were rejected, using only type_name() -- no BasicJsonType value is ever built for the ones that are wrong, and the message text matches what BasicJsonType::type_name() would produce for the same value.

        #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\nusing json_view = nlohmann::json_view;\n\nint main()\n{\n    // report why some parsed messages were rejected, using only\n    // type_name() -- no nlohmann::json value is built for the ones that\n    // are wrong\n    json_document good = json_document::parse(R\"({\"id\": 1})\");\n    json_document bad = json_document::parse(\"[1, 2, 3]\");\n    json_document failed = json_document::parse(\"not json\", /* allow_exceptions */ false);\n\n    for (const json_view v :\n            {\n                good.root(), bad.root(), failed.root()\n            })\n    {\n        if (v.is_object())\n        {\n            std::cout << \"ok\\n\";\n        }\n        else\n        {\n            std::cout << \"expected an object, got \" << v.type_name() << '\\n';\n        }\n    }\n}\n

        Output:

        ok\nexpected an object, got array\nexpected an object, got discarded\n
        "},{"location":"api/basic_json_view/type_name/#see-also","title":"See also","text":"
        • type - return the type of the value
        • BasicJsonType::type_name - the corresponding function of basic_json
        "},{"location":"api/basic_json_view/type_name/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/basic_json_view/value/","title":"nlohmann::basic_json_view::value","text":"
        // (1)\ntemplate<typename T>\nT value(string_view_t key, const T& default_value) const;\nstring_t value(string_view_t key, const char* default_value) const;\n\n// (2)\ntemplate<typename T>\nT value(const json_pointer& ptr, const T& default_value) const;\nstring_t value(const json_pointer& ptr, const char* default_value) const;\n
        1. Returns the value of the object member with key key -- the first one, should the key occur more than once (see Notes on duplicate keys) -- converted to T, or default_value if there is no such member.
        2. Returns the value a JSON pointer ptr refers to, starting at this value, converted to T, or default_value if ptr cannot be resolved.

        Both overloads have a dedicated const char* overload, so v.value(key, \"default\") (and the JSON pointer equivalent) deduce string_t, not const char*, for their return type and for the comparison used to pick between key and default_value.

        "},{"location":"api/basic_json_view/value/#template-parameters","title":"Template parameters","text":"T the type to convert the found value to; also the type of default_value"},{"location":"api/basic_json_view/value/#parameters","title":"Parameters","text":"key (in) object key of the element to access ptr (in) JSON pointer to the element to access default_value (in) the value to return if key/ptr resolves to no value"},{"location":"api/basic_json_view/value/#return-value","title":"Return value","text":"
        1. the first member with key key, converted to T, or default_value
        2. the value ptr resolves to, converted to T, or default_value
        "},{"location":"api/basic_json_view/value/#exception-safety","title":"Exception safety","text":"

        Strong guarantee: there are no changes to the view or the document it refers to.

        "},{"location":"api/basic_json_view/value/#exceptions","title":"Exceptions","text":"
        1. Throws type_error.306 if the value is not an object -- the same exception, with the same message, that BasicJsonType::value throws for the same call. If a member with key key is found, throws whatever converting it to T throws (typically type_error.302, with the same message BasicJsonType::value throws for the same mismatch); a missing member never throws.
        2. Throws type_error.306 if this value -- not the value ptr resolves to -- is neither an object nor an array. Throws parse_error.106 or parse_error.109 if ptr contains a malformed array index. If ptr resolves to a value, throws whatever converting it to T throws. Every other way ptr can fail to resolve -- a missing key, an out-of-range or \"-\" array index, an unresolvable token on a primitive -- yields default_value instead of throwing, exactly as BasicJsonType::value catches out_of_range and returns default_value.

        None of these exceptions carry a JSON_DIAGNOSTICS path: the view has no BasicJsonType value to point at, so the exception is created without one, even if BasicJsonType was built with JSON_DIAGNOSTICS enabled.

        "},{"location":"api/basic_json_view/value/#complexity","title":"Complexity","text":"
        1. Linear in the number of members: as for operator[], members are compared one after another, in document order, stopping at the first match. Plus the complexity of converting the found member to T (see get). Objects with 128 or more members get a hash index of their keys while parsing, so that a lookup in them takes constant time on average.
        2. Linear in the number of reference tokens of ptr and, for each token, in the number of members of the object at that level or the index into the array -- as for the operator[] and at overloads that take a JSON pointer. Plus the complexity of converting the resolved value to T.
        "},{"location":"api/basic_json_view/value/#notes","title":"Notes","text":"

        Duplicate keys: the first member wins

        If the source text repeats a key, this resolves to the first member with it, not to the last one that materialize() and parse() keep. See the Notes on duplicate keys of operator[] and Duplicate keys for the reasons and for how to get the last value.

        Differences to at and operator[]

        Unlike at, this function does not throw if key/ptr resolves to no value. Unlike operator[], it never returns a discarded view -- it always returns a T -- and it is available on any view, since it never needs to insert a missing element the way the non-const BasicJsonType::operator[] would.

        Which values can be asked

        As for BasicJsonType::value, the key overload (1) requires an object, and the JSON pointer overload (2) an object or an array.

        "},{"location":"api/basic_json_view/value/#examples","title":"Examples","text":"Example: (1) access specified object element with default value

        The example below reads a couple of optional configuration fields with a default, so that a missing key never needs a try/catch of its own.

        #include <iostream>\n#include <nlohmann/json_view.hpp>\n#include <string>\n\nusing json_document = nlohmann::json_document;\nusing json_view = nlohmann::json_view;\n\nint main()\n{\n    json_document doc = json_document::parse(R\"({\"server\": {\"host\": \"localhost\"}})\");\n    const json_view server = doc.root()[\"server\"];\n\n    // \"port\" is missing -- value() returns the default instead of\n    // throwing, so optional configuration fields never need their own\n    // try/catch\n    std::cout << server.value(\"host\", std::string(\"0.0.0.0\")) << '\\n';\n    std::cout << server.value(\"port\", 8080) << '\\n';\n\n    // a present but wrong-typed default still throws -- value() only\n    // replaces \"not found\", not \"wrong type\", exactly as basic_json::value\n    try\n    {\n        static_cast<void>(server.value(\"host\", 0));\n    }\n    catch (const nlohmann::json::type_error& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

        Output:

        localhost\n8080\n[json.exception.type_error.302] type must be number, but is string\n
        Example: (2) access specified element via JSON pointer with default value

        The example below reads an optional, nested configuration value with a default, given as a JSON pointer.

        #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\nusing json_view = nlohmann::json_view;\nusing json_pointer = nlohmann::json::json_pointer;\n\nint main()\n{\n    json_document doc = json_document::parse(R\"({\"server\": {\"host\": \"localhost\", \"limits\": {\"connections\": 100}}})\");\n    const json_view config = doc.root();\n\n    // a nested, optional setting read with a default -- no exception, even\n    // though \"timeout\" is missing several levels down\n    std::cout << config.value(json_pointer(\"/server/limits/connections\"), 10) << '\\n';\n    std::cout << config.value(json_pointer(\"/server/limits/timeout\"), 30) << '\\n';\n\n    // an out-of-range array index also falls back to the default\n    json_document list_doc = json_document::parse(R\"({\"servers\": [\"a\", \"b\"]})\");\n    std::cout << list_doc.root().value(json_pointer(\"/servers/5\"), std::string(\"none\")) << '\\n';\n}\n

        Output:

        100\n30\nnone\n
        "},{"location":"api/basic_json_view/value/#see-also","title":"See also","text":"
        • at - access specified element with bounds checking (throws instead of returning a default value)
        • operator[] - access specified element (returns a discarded view instead of a default value)
        • BasicJsonType::value - the corresponding function of basic_json
        "},{"location":"api/basic_json_view/value/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/byte_container_with_subtype/","title":"nlohmann::byte_container_with_subtype","text":"
        template<typename BinaryType>\nclass byte_container_with_subtype : public BinaryType;\n

        This type extends the template parameter BinaryType provided to basic_json with a subtype used by BSON and MessagePack. This type exists so that the user does not have to specify a type themselves with a specific naming scheme in order to override the binary type.

        "},{"location":"api/byte_container_with_subtype/#template-parameters","title":"Template parameters","text":"BinaryType container to store bytes (std::vector<std::uint8_t> by default)"},{"location":"api/byte_container_with_subtype/#member-types","title":"Member types","text":"
        • container_type - the type of the underlying container (BinaryType)
        • subtype_type - the type of the subtype (std::uint64_t)
        "},{"location":"api/byte_container_with_subtype/#member-functions","title":"Member functions","text":"
        • (constructor)
        • operator== - comparison: equal
        • operator!= - comparison: not equal
        • set_subtype - sets the binary subtype
        • subtype - return the binary subtype
        • has_subtype - return whether the value has a subtype
        • clear_subtype - clears the binary subtype
        "},{"location":"api/byte_container_with_subtype/#version-history","title":"Version history","text":"
        • Added in version 3.8.0.
        • Changed the type of subtypes to std::uint64_t in 3.10.0.
        "},{"location":"api/byte_container_with_subtype/byte_container_with_subtype/","title":"nlohmann::byte_container_with_subtype::byte_container_with_subtype","text":"
        // (1)\nbyte_container_with_subtype();\n\n// (2)\nbyte_container_with_subtype(const container_type& container);\nbyte_container_with_subtype(container_type&& container);\n\n// (3)\nbyte_container_with_subtype(const container_type& container, subtype_type subtype);\nbyte_container_with_subtype(container_type&& container, subtype_type subtype);\n
        1. Create an empty binary container without a subtype.
        2. Create a binary container without a subtype.
        3. Create a binary container with a subtype.
        "},{"location":"api/byte_container_with_subtype/byte_container_with_subtype/#parameters","title":"Parameters","text":"container (in) binary container subtype (in) subtype"},{"location":"api/byte_container_with_subtype/byte_container_with_subtype/#examples","title":"Examples","text":"Example

        The example below demonstrates how byte containers can be created.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\n// define a byte container based on std::vector\nusing byte_container_with_subtype = nlohmann::byte_container_with_subtype<std::vector<std::uint8_t>>;\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // (1) create empty container\n    auto c1 = byte_container_with_subtype();\n\n    std::vector<std::uint8_t> bytes = {{0xca, 0xfe, 0xba, 0xbe}};\n\n    // (2) create container\n    auto c2 = byte_container_with_subtype(bytes);\n\n    // (3) create container with subtype\n    auto c3 = byte_container_with_subtype(bytes, 42);\n\n    std::cout << json(c1) << \"\\n\" << json(c2) << \"\\n\" << json(c3) << std::endl;\n}\n

        Output:

        {\"bytes\":[],\"subtype\":null}\n{\"bytes\":[202,254,186,190],\"subtype\":null}\n{\"bytes\":[202,254,186,190],\"subtype\":42}\n
        "},{"location":"api/byte_container_with_subtype/byte_container_with_subtype/#see-also","title":"See also","text":"
        • set_subtype sets the binary subtype
        • subtype return the binary subtype
        • has_subtype return whether the value has a subtype
        • binary create a binary JSON value
        • Binary Values - the article on binary values
        "},{"location":"api/byte_container_with_subtype/byte_container_with_subtype/#version-history","title":"Version history","text":"

        Since version 3.8.0.

        "},{"location":"api/byte_container_with_subtype/clear_subtype/","title":"nlohmann::byte_container_with_subtype::clear_subtype","text":"
        void clear_subtype() noexcept;\n

        Clears the binary subtype and flags the value as not having a subtype, which has implications for serialization; for instance, MessagePack will prefer the bin family over the ext family.

        "},{"location":"api/byte_container_with_subtype/clear_subtype/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this function never throws exceptions.

        "},{"location":"api/byte_container_with_subtype/clear_subtype/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/byte_container_with_subtype/clear_subtype/#examples","title":"Examples","text":"Example

        The example below demonstrates how clear_subtype can remove subtypes.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\n// define a byte container based on std::vector\nusing byte_container_with_subtype = nlohmann::byte_container_with_subtype<std::vector<std::uint8_t>>;\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::vector<std::uint8_t> bytes = {{0xca, 0xfe, 0xba, 0xbe}};\n\n    // create container with subtype\n    auto c1 = byte_container_with_subtype(bytes, 42);\n\n    std::cout << \"before calling clear_subtype(): \" << json(c1) << '\\n';\n\n    c1.clear_subtype();\n\n    std::cout << \"after calling clear_subtype(): \" << json(c1) << '\\n';\n}\n

        Output:

        before calling clear_subtype(): {\"bytes\":[202,254,186,190],\"subtype\":42}\nafter calling clear_subtype(): {\"bytes\":[202,254,186,190],\"subtype\":null}\n
        "},{"location":"api/byte_container_with_subtype/clear_subtype/#see-also","title":"See also","text":"
        • set_subtype sets the binary subtype
        • has_subtype return whether the value has a subtype
        • subtype return the binary subtype
        "},{"location":"api/byte_container_with_subtype/clear_subtype/#version-history","title":"Version history","text":"

        Since version 3.8.0.

        "},{"location":"api/byte_container_with_subtype/has_subtype/","title":"nlohmann::byte_container_with_subtype::has_subtype","text":"
        constexpr bool has_subtype() const noexcept;\n

        Returns whether the value has a subtype.

        "},{"location":"api/byte_container_with_subtype/has_subtype/#return-value","title":"Return value","text":"

        whether the value has a subtype

        "},{"location":"api/byte_container_with_subtype/has_subtype/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this function never throws exceptions.

        "},{"location":"api/byte_container_with_subtype/has_subtype/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/byte_container_with_subtype/has_subtype/#examples","title":"Examples","text":"Example

        The example below demonstrates how has_subtype can check whether a subtype was set.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\n// define a byte container based on std::vector\nusing byte_container_with_subtype = nlohmann::byte_container_with_subtype<std::vector<std::uint8_t>>;\n\nint main()\n{\n    std::vector<std::uint8_t> bytes = {{0xca, 0xfe, 0xba, 0xbe}};\n\n    // create container\n    auto c1 = byte_container_with_subtype(bytes);\n\n    // create container with subtype\n    auto c2 = byte_container_with_subtype(bytes, 42);\n\n    std::cout << std::boolalpha << \"c1.has_subtype() = \" << c1.has_subtype()\n              << \"\\nc2.has_subtype() = \" << c2.has_subtype() << std::endl;\n}\n

        Output:

        c1.has_subtype() = false\nc2.has_subtype() = true\n
        "},{"location":"api/byte_container_with_subtype/has_subtype/#see-also","title":"See also","text":"
        • set_subtype sets the binary subtype
        • clear_subtype clears the binary subtype
        • subtype return the binary subtype
        "},{"location":"api/byte_container_with_subtype/has_subtype/#version-history","title":"Version history","text":"

        Since version 3.8.0.

        "},{"location":"api/byte_container_with_subtype/operator_eq/","title":"nlohmann::byte_container_with_subtype::operator==","text":"
        bool operator==(const byte_container_with_subtype& rhs) const;\n

        Compares two byte containers for equality by comparing (1) the underlying binary data (the BinaryType base, compared with BinaryType's own operator==) and (2) the subtype information -- both containers must either have no subtype, or have a subtype and the same subtype value.

        "},{"location":"api/byte_container_with_subtype/operator_eq/#parameters","title":"Parameters","text":"rhs (in) byte container to compare *this with"},{"location":"api/byte_container_with_subtype/operator_eq/#return-value","title":"Return value","text":"

        whether *this and rhs are equal

        "},{"location":"api/byte_container_with_subtype/operator_eq/#complexity","title":"Complexity","text":"

        Linear in the size of the compared containers.

        "},{"location":"api/byte_container_with_subtype/operator_eq/#examples","title":"Examples","text":"Example

        The example below demonstrates comparing byte containers with and without subtypes.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\n// define a byte container based on std::vector\nusing byte_container_with_subtype = nlohmann::byte_container_with_subtype<std::vector<std::uint8_t>>;\n\nint main()\n{\n    std::vector<std::uint8_t> bytes = {{0xca, 0xfe, 0xba, 0xbe}};\n\n    // create containers without and with a subtype\n    auto c1 = byte_container_with_subtype(bytes);\n    auto c2 = byte_container_with_subtype(bytes);\n    auto c3 = byte_container_with_subtype(bytes, 42);\n    auto c4 = byte_container_with_subtype(bytes, 42);\n    auto c5 = byte_container_with_subtype(bytes, 23);\n\n    std::cout << std::boolalpha\n              << \"c1 == c2: \" << (c1 == c2) << '\\n'\n              << \"c1 == c3: \" << (c1 == c3) << '\\n'\n              << \"c3 == c4: \" << (c3 == c4) << '\\n'\n              << \"c3 == c5: \" << (c3 == c5) << std::endl;\n}\n

        Output:

        c1 == c2: true\nc1 == c3: false\nc3 == c4: true\nc3 == c5: false\n
        "},{"location":"api/byte_container_with_subtype/operator_eq/#see-also","title":"See also","text":"
        • operator!= comparison: not equal
        • has_subtype return whether the value has a subtype
        • subtype return the binary subtype
        "},{"location":"api/byte_container_with_subtype/operator_eq/#version-history","title":"Version history","text":"
        • Added in version 3.8.0.
        "},{"location":"api/byte_container_with_subtype/operator_ne/","title":"nlohmann::byte_container_with_subtype::operator!=","text":"
        bool operator!=(const byte_container_with_subtype& rhs) const;\n

        Compares two byte containers for inequality. Returns !(rhs == *this); see operator== for the equality semantics.

        "},{"location":"api/byte_container_with_subtype/operator_ne/#parameters","title":"Parameters","text":"rhs (in) byte container to compare *this with"},{"location":"api/byte_container_with_subtype/operator_ne/#return-value","title":"Return value","text":"

        whether *this and rhs are not equal

        "},{"location":"api/byte_container_with_subtype/operator_ne/#complexity","title":"Complexity","text":"

        Linear in the size of the compared containers.

        "},{"location":"api/byte_container_with_subtype/operator_ne/#examples","title":"Examples","text":"Example

        The example below demonstrates comparing byte containers with and without subtypes.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\n// define a byte container based on std::vector\nusing byte_container_with_subtype = nlohmann::byte_container_with_subtype<std::vector<std::uint8_t>>;\n\nint main()\n{\n    std::vector<std::uint8_t> bytes = {{0xca, 0xfe, 0xba, 0xbe}};\n\n    // create containers without and with a subtype\n    auto c1 = byte_container_with_subtype(bytes);\n    auto c2 = byte_container_with_subtype(bytes);\n    auto c3 = byte_container_with_subtype(bytes, 42);\n    auto c4 = byte_container_with_subtype(bytes, 42);\n    auto c5 = byte_container_with_subtype(bytes, 23);\n\n    std::cout << std::boolalpha\n              << \"c1 != c2: \" << (c1 != c2) << '\\n'\n              << \"c1 != c3: \" << (c1 != c3) << '\\n'\n              << \"c3 != c4: \" << (c3 != c4) << '\\n'\n              << \"c3 != c5: \" << (c3 != c5) << std::endl;\n}\n

        Output:

        c1 != c2: false\nc1 != c3: true\nc3 != c4: false\nc3 != c5: true\n
        "},{"location":"api/byte_container_with_subtype/operator_ne/#see-also","title":"See also","text":"
        • operator== comparison: equal
        • has_subtype return whether the value has a subtype
        • subtype return the binary subtype
        "},{"location":"api/byte_container_with_subtype/operator_ne/#version-history","title":"Version history","text":"
        • Added in version 3.8.0.
        "},{"location":"api/byte_container_with_subtype/set_subtype/","title":"nlohmann::byte_container_with_subtype::set_subtype","text":"
        void set_subtype(subtype_type subtype) noexcept;\n

        Sets the binary subtype of the value, also flags a binary JSON value as having a subtype, which has implications for serialization.

        "},{"location":"api/byte_container_with_subtype/set_subtype/#parameters","title":"Parameters","text":"subtype (in) subtype to set"},{"location":"api/byte_container_with_subtype/set_subtype/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this function never throws exceptions.

        "},{"location":"api/byte_container_with_subtype/set_subtype/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/byte_container_with_subtype/set_subtype/#examples","title":"Examples","text":"Example

        The example below demonstrates how a subtype can be set with set_subtype.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\n// define a byte container based on std::vector\nusing byte_container_with_subtype = nlohmann::byte_container_with_subtype<std::vector<std::uint8_t>>;\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::vector<std::uint8_t> bytes = {{0xca, 0xfe, 0xba, 0xbe}};\n\n    // create container without subtype\n    auto c = byte_container_with_subtype(bytes);\n\n    std::cout << \"before calling set_subtype(42): \" << json(c) << '\\n';\n\n    // set the subtype\n    c.set_subtype(42);\n\n    std::cout << \"after calling set_subtype(42): \" << json(c) << '\\n';\n}\n

        Output:

        before calling set_subtype(42): {\"bytes\":[202,254,186,190],\"subtype\":null}\nafter calling set_subtype(42): {\"bytes\":[202,254,186,190],\"subtype\":42}\n
        "},{"location":"api/byte_container_with_subtype/set_subtype/#see-also","title":"See also","text":"
        • subtype return the binary subtype
        • has_subtype return whether the value has a subtype
        • clear_subtype clears the binary subtype
        "},{"location":"api/byte_container_with_subtype/set_subtype/#version-history","title":"Version history","text":"

        Since version 3.8.0.

        "},{"location":"api/byte_container_with_subtype/subtype/","title":"nlohmann::byte_container_with_subtype::subtype","text":"
        constexpr subtype_type subtype() const noexcept;\n

        Returns the numerical subtype of the value if it has a subtype. If it does not have a subtype, this function will return subtype_type(-1) as a sentinel value.

        "},{"location":"api/byte_container_with_subtype/subtype/#return-value","title":"Return value","text":"

        the numerical subtype of the binary value, or subtype_type(-1) if no subtype is set

        "},{"location":"api/byte_container_with_subtype/subtype/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this function never throws exceptions.

        "},{"location":"api/byte_container_with_subtype/subtype/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/byte_container_with_subtype/subtype/#examples","title":"Examples","text":"Example

        The example below demonstrates how the subtype can be retrieved with subtype. Note how subtype_type(-1) is returned for container c1.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\n// define a byte container based on std::vector\nusing byte_container_with_subtype = nlohmann::byte_container_with_subtype<std::vector<std::uint8_t>>;\n\nint main()\n{\n    std::vector<std::uint8_t> bytes = {{0xca, 0xfe, 0xba, 0xbe}};\n\n    // create container\n    auto c1 = byte_container_with_subtype(bytes);\n\n    // create container with subtype\n    auto c2 = byte_container_with_subtype(bytes, 42);\n\n    std::cout << \"c1.subtype() = \" << c1.subtype()\n              << \"\\nc2.subtype() = \" << c2.subtype() << std::endl;\n\n    // in case no subtype is set, return special value\n    assert(c1.subtype() == static_cast<byte_container_with_subtype::subtype_type>(-1));\n}\n

        Output:

        c1.subtype() = 18446744073709551615\nc2.subtype() = 42\n
        "},{"location":"api/byte_container_with_subtype/subtype/#see-also","title":"See also","text":"
        • set_subtype sets the binary subtype
        • has_subtype return whether the value has a subtype
        • clear_subtype clears the binary subtype
        "},{"location":"api/byte_container_with_subtype/subtype/#version-history","title":"Version history","text":"
        • Added in version 3.8.0
        • Fixed return value to properly return subtype_type(-1) as documented in version 3.10.0.
        "},{"location":"api/json_pointer/","title":"nlohmann::json_pointer","text":"
        template<typename RefStringType>\nclass json_pointer;\n

        A JSON pointer defines a string syntax for identifying a specific value within a JSON document. It can be used with functions at and operator[]. Furthermore, JSON pointers are the base for JSON patches.

        "},{"location":"api/json_pointer/#template-parameters","title":"Template parameters","text":"RefStringType the string type used for the reference tokens making up the JSON pointer

        Deprecation

        For backwards compatibility RefStringType may also be a specialization of basic_json in which case string_t will be deduced as basic_json::string_t. This feature is deprecated and may be removed in a future major version.

        See the migration guide for how to update existing code.

        A JSON pointer is internally a sequence of reference tokens. front, pop_front, and push_front act on the first reference token, whereas back, pop_back, and push_back act on the last one. parent_pointer returns a new JSON pointer with the last reference token removed (like a non-mutating pop_back):

        flowchart LR\n    A[\"a\"] --> B[\"b\"] --> C[\"c\"]\n\n    front[\"front() / pop_front() / push_front()\"] -.-> A\n    back[\"back() / pop_back() / push_back()\"] -.-> C\n    parent[\"parent_pointer() returns /a/b\"] -.-> B

        The diagram shows the reference tokens of the JSON pointer /a/b/c.

        "},{"location":"api/json_pointer/#member-types","title":"Member types","text":"
        • string_t - the string type used for the reference tokens
        "},{"location":"api/json_pointer/#member-functions","title":"Member functions","text":"
        • (constructor)
        • to_string - return a string representation of the JSON pointer
        • operator string_t - return a string representation of the JSON pointer (deprecated)
        • operator== - compare: equal
        • operator!= - compare: not equal
        • operator<=> - compare: 3-way (C++20)
        • operator/= - append to the end of the JSON pointer
        • operator/ - create JSON Pointer by appending
        • parent_pointer - returns the parent of this JSON pointer
        • pop_back - remove the last reference token
        • back - return last reference token
        • push_back - append an unescaped token at the end of the pointer
        • pop_front - remove the first reference token
        • front - return first reference token
        • push_front - append an unescaped token at the start of the pointer
        • empty - return whether the pointer points to the root document
        "},{"location":"api/json_pointer/#literals","title":"Literals","text":"
        • operator\"\"_json_pointer - user-defined string literal for JSON pointers
        "},{"location":"api/json_pointer/#see-also","title":"See also","text":"
        • RFC 6901
        "},{"location":"api/json_pointer/#version-history","title":"Version history","text":"
        • Added in version 2.0.0.
        • Changed template parameter from basic_json to string type in version 3.11.0.
        "},{"location":"api/json_pointer/back/","title":"nlohmann::json_pointer::back","text":"
        const string_t& back() const;\n

        Return the last reference token.

        "},{"location":"api/json_pointer/back/#return-value","title":"Return value","text":"

        Last reference token.

        "},{"location":"api/json_pointer/back/#exception-safety","title":"Exception safety","text":"

        Strong exception safety: if an exception occurs, the original value stays intact.

        "},{"location":"api/json_pointer/back/#exceptions","title":"Exceptions","text":"

        Throws out_of_range.405 if the JSON pointer has no parent.

        "},{"location":"api/json_pointer/back/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/json_pointer/back/#examples","title":"Examples","text":"Example

        The example shows the usage of back.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // different JSON Pointers\n    json::json_pointer ptr1(\"/foo\");\n    json::json_pointer ptr2(\"/foo/0\");\n\n    // call empty()\n    std::cout << \"last reference token of \\\"\" << ptr1 << \"\\\" is \\\"\" << ptr1.back() << \"\\\"\\n\"\n              << \"last reference token of \\\"\" << ptr2 << \"\\\" is \\\"\" << ptr2.back() << \"\\\"\" << std::endl;\n}\n

        Output:

        last reference token of \"/foo\" is \"foo\"\nlast reference token of \"/foo/0\" is \"0\"\n
        "},{"location":"api/json_pointer/back/#see-also","title":"See also","text":"
        • front return first reference token
        • pop_back remove the last reference token
        • push_back append an unescaped token at the end of the pointer
        • parent_pointer returns the parent of this JSON pointer
        "},{"location":"api/json_pointer/back/#version-history","title":"Version history","text":"
        • Added in version 3.6.0.
        • Changed return type to string_t in version 3.11.0.
        "},{"location":"api/json_pointer/empty/","title":"nlohmann::json_pointer::empty","text":"
        bool empty() const noexcept;\n

        Return whether the pointer points to the root document.

        "},{"location":"api/json_pointer/empty/#return-value","title":"Return value","text":"

        true iff the JSON pointer points to the root document.

        "},{"location":"api/json_pointer/empty/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this function never throws exceptions.

        "},{"location":"api/json_pointer/empty/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/json_pointer/empty/#examples","title":"Examples","text":"Example

        The example shows the result of empty for different JSON Pointers.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // different JSON Pointers\n    json::json_pointer ptr0;\n    json::json_pointer ptr1(\"\");\n    json::json_pointer ptr2(\"/foo\");\n    json::json_pointer ptr3(\"/foo/0\");\n\n    // call empty()\n    std::cout << std::boolalpha\n              << \"\\\"\" << ptr0 << \"\\\": \" << ptr0.empty() << '\\n'\n              << \"\\\"\" << ptr1 << \"\\\": \" << ptr1.empty() << '\\n'\n              << \"\\\"\" << ptr2 << \"\\\": \" << ptr2.empty() << '\\n'\n              << \"\\\"\" << ptr3 << \"\\\": \" << ptr3.empty() << std::endl;\n}\n

        Output:

        \"\": true\n\"\": true\n\"/foo\": false\n\"/foo/0\": false\n
        "},{"location":"api/json_pointer/empty/#see-also","title":"See also","text":"
        • front return first reference token
        • back return last reference token
        • to_string return a string representation of the JSON pointer
        "},{"location":"api/json_pointer/empty/#version-history","title":"Version history","text":"

        Added in version 3.6.0.

        "},{"location":"api/json_pointer/front/","title":"nlohmann::json_pointer::front","text":"
        const string_t& front() const;\n

        Return the first reference token.

        "},{"location":"api/json_pointer/front/#return-value","title":"Return value","text":"

        First reference token.

        "},{"location":"api/json_pointer/front/#exception-safety","title":"Exception safety","text":"

        Strong exception safety: if an exception occurs, the original value stays intact.

        "},{"location":"api/json_pointer/front/#exceptions","title":"Exceptions","text":"

        Throws out_of_range.405 if the JSON pointer has no parent.

        "},{"location":"api/json_pointer/front/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/json_pointer/front/#examples","title":"Examples","text":"Example

        The example shows the usage of front.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // different JSON Pointers\n    json::json_pointer ptr1(\"/foo\");\n    json::json_pointer ptr2(\"/foo/0\");\n\n    // call empty()\n    std::cout << \"first reference token of \\\"\" << ptr1 << \"\\\" is \\\"\" << ptr1.front() << \"\\\"\\n\"\n              << \"first reference token of \\\"\" << ptr2 << \"\\\" is \\\"\" << ptr2.front() << \"\\\"\" << std::endl;\n}\n

        Output:

        first reference token of \"/foo\" is \"foo\"\nfirst reference token of \"/foo/0\" is \"foo\"\n
        "},{"location":"api/json_pointer/front/#see-also","title":"See also","text":"
        • back return last reference token
        • pop_front remove the first reference token
        • push_front append an unescaped token at the start of the pointer
        "},{"location":"api/json_pointer/front/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/json_pointer/json_pointer/","title":"nlohmann::json_pointer::json_pointer","text":"
        explicit json_pointer(const string_t& s = \"\");\n

        Create a JSON pointer according to the syntax described in Section 3 of RFC6901.

        "},{"location":"api/json_pointer/json_pointer/#parameters","title":"Parameters","text":"s (in) string representing the JSON pointer; if omitted, the empty string is assumed which references the whole JSON value"},{"location":"api/json_pointer/json_pointer/#exception-safety","title":"Exception safety","text":"

        Strong guarantee: if an exception is thrown, there are no changes to any JSON pointer.

        "},{"location":"api/json_pointer/json_pointer/#exceptions","title":"Exceptions","text":"
        • Throws parse_error.107 if the given JSON pointer s is nonempty and does not begin with a slash (/); see example below.
        • Throws parse_error.108 if a tilde (~) in the given JSON pointer s is not followed by 0 (representing ~) or 1 (representing /); see example below.
        "},{"location":"api/json_pointer/json_pointer/#complexity","title":"Complexity","text":"

        Linear in the length of s.

        "},{"location":"api/json_pointer/json_pointer/#examples","title":"Examples","text":"Example

        The example shows the construction several valid JSON pointers as well as the exceptional behavior.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // correct JSON pointers\n    json::json_pointer p1;\n    json::json_pointer p2(\"\");\n    json::json_pointer p3(\"/\");\n    json::json_pointer p4(\"//\");\n    json::json_pointer p5(\"/foo/bar\");\n    json::json_pointer p6(\"/foo/bar/-\");\n    json::json_pointer p7(\"/foo/~0\");\n    json::json_pointer p8(\"/foo/~1\");\n\n    // error: JSON pointer does not begin with a slash\n    try\n    {\n        json::json_pointer p9(\"foo\");\n    }\n    catch (const json::parse_error& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n\n    // error: JSON pointer uses escape symbol ~ not followed by 0 or 1\n    try\n    {\n        json::json_pointer p10(\"/foo/~\");\n    }\n    catch (const json::parse_error& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n\n    // error: JSON pointer uses escape symbol ~ not followed by 0 or 1\n    try\n    {\n        json::json_pointer p11(\"/foo/~3\");\n    }\n    catch (const json::parse_error& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

        Output:

        [json.exception.parse_error.107] parse error at byte 1: JSON pointer must be empty or begin with '/' - was: 'foo'\n[json.exception.parse_error.108] parse error: escape character '~' must be followed with '0' or '1'\n[json.exception.parse_error.108] parse error: escape character '~' must be followed with '0' or '1'\n
        "},{"location":"api/json_pointer/json_pointer/#see-also","title":"See also","text":"
        • JSON Pointer - the article on JSON Pointer support
        • operator\"\"_json_pointer user-defined string literal for JSON pointers
        "},{"location":"api/json_pointer/json_pointer/#version-history","title":"Version history","text":"
        • Added in version 2.0.0.
        • Changed type of s to string_t in version 3.11.0.
        "},{"location":"api/json_pointer/operator_eq/","title":"nlohmann::json_pointer::operator==","text":"
        // until C++20\ntemplate<typename RefStringTypeLhs, typename RefStringTypeRhs>\nbool operator==(\n    const json_pointer<RefStringTypeLhs>& lhs,\n    const json_pointer<RefStringTypeRhs>& rhs) noexcept;            // (1)\n\ntemplate<typename RefStringTypeLhs, typename StringType>\nbool operator==(\n    const json_pointer<RefStringTypeLhs>& lhs,\n    const StringType& rhs);                                         // (2)\n\ntemplate<typename RefStringTypeRhs, typename StringType>\nbool operator==(\n    const StringType& lhs,\n    const json_pointer<RefStringTypeRhs>& rhs);                     // (2)\n\n// since C++20\ntemplate<typename RefStringTypeRhs>\nbool operator==(\n    const json_pointer<RefStringTypeRhs>& rhs) const noexcept;      // (1)\n\nbool operator==(const string_t& rhs) const;                         // (2)\n

        1. Compares two JSON pointers for equality by comparing their reference tokens.

        2. Compares a JSON pointer and a string or a string and a JSON pointer for equality by converting the string to a JSON pointer and comparing the JSON pointers according to 1.

        "},{"location":"api/json_pointer/operator_eq/#template-parameters","title":"Template parameters","text":"RefStringTypeLhs, RefStringTypeRhs the string type of the left-hand side or right-hand side JSON pointer, respectively StringType the string type derived from the json_pointer operand (json_pointer::string_t)"},{"location":"api/json_pointer/operator_eq/#parameters","title":"Parameters","text":"lhs (in) first value to consider rhs (in) second value to consider"},{"location":"api/json_pointer/operator_eq/#return-value","title":"Return value","text":"

        whether the values lhs/*this and rhs are equal

        "},{"location":"api/json_pointer/operator_eq/#exception-safety","title":"Exception safety","text":"
        1. No-throw guarantee: this function never throws exceptions.
        2. Strong exception safety: if an exception occurs, the original value stays intact.
        "},{"location":"api/json_pointer/operator_eq/#exceptions","title":"Exceptions","text":"
        1. (none)
        2. The function can throw the following exceptions:
        3. Throws parse_error.107 if the given JSON pointer s is nonempty and does not begin with a slash (/); see example below.
        4. Throws parse_error.108 if a tilde (~) in the given JSON pointer s is not followed by 0 (representing ~) or 1 (representing /); see example below.
        "},{"location":"api/json_pointer/operator_eq/#complexity","title":"Complexity","text":"

        Constant if lhs and rhs differ in the number of reference tokens, otherwise linear in the number of reference tokens.

        "},{"location":"api/json_pointer/operator_eq/#notes","title":"Notes","text":"

        Deprecation

        Overload 2 is deprecated and will be removed in a future major version release.

        See the migration guide for how to update existing code.

        "},{"location":"api/json_pointer/operator_eq/#examples","title":"Examples","text":"Example: (1) Comparing JSON pointers

        The example demonstrates comparing JSON pointers.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // different JSON pointers\n    json::json_pointer ptr0;\n    json::json_pointer ptr1(\"\");\n    json::json_pointer ptr2(\"/foo\");\n\n    // compare JSON pointers\n    std::cout << std::boolalpha\n              << \"\\\"\" << ptr0 << \"\\\" == \\\"\" << ptr0 << \"\\\": \" << (ptr0 == ptr0) << '\\n'\n              << \"\\\"\" << ptr0 << \"\\\" == \\\"\" << ptr1 << \"\\\": \" << (ptr0 == ptr1) << '\\n'\n              << \"\\\"\" << ptr1 << \"\\\" == \\\"\" << ptr2 << \"\\\": \" << (ptr1 == ptr2) << '\\n'\n              << \"\\\"\" << ptr2 << \"\\\" == \\\"\" << ptr2 << \"\\\": \" << (ptr2 == ptr2) << std::endl;\n}\n

        Output:

        \"\" == \"\": true\n\"\" == \"\": true\n\"\" == \"/foo\": false\n\"/foo\" == \"/foo\": true\n
        Example: (2) Comparing JSON pointers and strings

        The example demonstrates comparing JSON pointers and strings, and when doing so may raise an exception.

        #include <exception>\n#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // different JSON pointers\n    json::json_pointer ptr0;\n    json::json_pointer ptr1(\"\");\n    json::json_pointer ptr2(\"/foo\");\n\n    // different strings\n    std::string str0(\"\");\n    std::string str1(\"/foo\");\n    std::string str2(\"bar\");\n\n    // compare JSON pointers and strings\n    std::cout << std::boolalpha\n              << \"\\\"\" << ptr0 << \"\\\" == \\\"\" << str0 << \"\\\": \" << (ptr0 == str0) << '\\n'\n              << \"\\\"\" << str0 << \"\\\" == \\\"\" << ptr1 << \"\\\": \" << (str0 == ptr1) << '\\n'\n              << \"\\\"\" << ptr2 << \"\\\" == \\\"\" << str1 << \"\\\": \" << (ptr2 == str1) << std::endl;\n\n    try\n    {\n        std::cout << \"\\\"\" << str2 << \"\\\" == \\\"\" << ptr2 << \"\\\": \" << (str2 == ptr2) << std::endl;\n    }\n    catch (const json::parse_error& ex)\n    {\n        std::cout << ex.what() << std::endl;\n    }\n}\n

        Output:

        \"\" == \"\": true\n\"\" == \"\": true\n\"/foo\" == \"/foo\": true\n\"bar\" == \"/foo\": [json.exception.parse_error.107] parse error at byte 1: JSON pointer must be empty or begin with '/' - was: 'bar'\n
        "},{"location":"api/json_pointer/operator_eq/#see-also","title":"See also","text":"
        • operator!= compare for inequality
        • operator<=> comparison: 3-way (C++20)
        "},{"location":"api/json_pointer/operator_eq/#version-history","title":"Version history","text":"
        1. Added in version 2.1.0. Added C++20 member functions in version 3.11.2.
        2. Added for backward compatibility and deprecated in version 3.11.2.
        "},{"location":"api/json_pointer/operator_ne/","title":"nlohmann::json_pointer::operator!=","text":"
        // until C++20\ntemplate<typename RefStringTypeLhs, typename RefStringTypeRhs>\nbool operator!=(\n    const json_pointer<RefStringTypeLhs>& lhs,\n    const json_pointer<RefStringTypeRhs>& rhs) noexcept;  // (1)\n\ntemplate<typename RefStringTypeLhs, typename StringType>\nbool operator!=(\n    const json_pointer<RefStringTypeLhs>& lhs,\n    const StringType& rhs);                               // (2)\n\ntemplate<typename RefStringTypeRhs, typename StringType>\nbool operator!=(\n    const StringType& lhs,\n    const json_pointer<RefStringTypeRhs>& rhs);           // (2)\n

        1. Compares two JSON pointers for inequality by comparing their reference tokens.

        2. Compares a JSON pointer and a string or a string and a JSON pointer for inequality by converting the string to a JSON pointer and comparing the JSON pointers according to 1.

        "},{"location":"api/json_pointer/operator_ne/#template-parameters","title":"Template parameters","text":"RefStringTypeLhs, RefStringTypeRhs the string type of the left-hand side or right-hand side JSON pointer, respectively StringType the string type derived from the json_pointer operand (json_pointer::string_t)"},{"location":"api/json_pointer/operator_ne/#parameters","title":"Parameters","text":"lhs (in) first value to consider rhs (in) second value to consider"},{"location":"api/json_pointer/operator_ne/#return-value","title":"Return value","text":"

        whether the values lhs/*this and rhs are not equal

        "},{"location":"api/json_pointer/operator_ne/#exception-safety","title":"Exception safety","text":"
        1. No-throw guarantee: this function never throws exceptions.
        2. Strong exception safety: if an exception occurs, the original value stays intact.
        "},{"location":"api/json_pointer/operator_ne/#exceptions","title":"Exceptions","text":"
        1. (none)
        2. The function can throw the following exceptions:
        3. Throws parse_error.107 if the given JSON pointer s is nonempty and does not begin with a slash (/); see example below.
        4. Throws parse_error.108 if a tilde (~) in the given JSON pointer s is not followed by 0 (representing ~) or 1 (representing /); see example below.
        "},{"location":"api/json_pointer/operator_ne/#complexity","title":"Complexity","text":"

        Constant if lhs and rhs differ in the number of reference tokens, otherwise linear in the number of reference tokens.

        "},{"location":"api/json_pointer/operator_ne/#notes","title":"Notes","text":"

        Operator overload resolution

        Since C++20 overload resolution will consider the rewritten candidate generated from operator==.

        Deprecation

        Overload 2 is deprecated and will be removed in a future major version release.

        See the migration guide for how to update existing code.

        "},{"location":"api/json_pointer/operator_ne/#examples","title":"Examples","text":"Example: (1) Comparing JSON pointers

        The example demonstrates comparing JSON pointers.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // different JSON pointers\n    json::json_pointer ptr0;\n    json::json_pointer ptr1(\"\");\n    json::json_pointer ptr2(\"/foo\");\n\n    // compare JSON pointers\n    std::cout << std::boolalpha\n              << \"\\\"\" << ptr0 << \"\\\" != \\\"\" << ptr0 << \"\\\": \" << (ptr0 != ptr0) << '\\n'\n              << \"\\\"\" << ptr0 << \"\\\" != \\\"\" << ptr1 << \"\\\": \" << (ptr0 != ptr1) << '\\n'\n              << \"\\\"\" << ptr1 << \"\\\" != \\\"\" << ptr2 << \"\\\": \" << (ptr1 != ptr2) << '\\n'\n              << \"\\\"\" << ptr2 << \"\\\" != \\\"\" << ptr2 << \"\\\": \" << (ptr2 != ptr2) << std::endl;\n}\n

        Output:

        \"\" != \"\": false\n\"\" != \"\": false\n\"\" != \"/foo\": true\n\"/foo\" != \"/foo\": false\n
        Example: (2) Comparing JSON pointers and strings

        The example demonstrates comparing JSON pointers and strings, and when doing so may raise an exception.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // different JSON pointers\n    json::json_pointer ptr0;\n    json::json_pointer ptr1(\"\");\n    json::json_pointer ptr2(\"/foo\");\n\n    // different strings\n    std::string str0(\"\");\n    std::string str1(\"/foo\");\n    std::string str2(\"bar\");\n\n    // compare JSON pointers and strings\n    std::cout << std::boolalpha\n              << \"\\\"\" << ptr0 << \"\\\" != \\\"\" << str0 << \"\\\": \" << (ptr0 != str0) << '\\n'\n              << \"\\\"\" << str0 << \"\\\" != \\\"\" << ptr1 << \"\\\": \" << (str0 != ptr1) << '\\n'\n              << \"\\\"\" << ptr2 << \"\\\" != \\\"\" << str1 << \"\\\": \" << (ptr2 != str1) << std::endl;\n\n    try\n    {\n        std::cout << \"\\\"\" << str2 << \"\\\" != \\\"\" << ptr2 << \"\\\": \" << (str2 != ptr2) << std::endl;\n    }\n    catch (const json::parse_error& ex)\n    {\n        std::cout << ex.what() << std::endl;\n    }\n}\n

        Output:

        \"\" != \"\": false\n\"\" != \"\": false\n\"/foo\" != \"/foo\": false\n\"bar\" != \"/foo\": [json.exception.parse_error.107] parse error at byte 1: JSON pointer must be empty or begin with '/' - was: 'bar'\n
        "},{"location":"api/json_pointer/operator_ne/#see-also","title":"See also","text":"
        • operator== compare for equality
        • operator<=> comparison: 3-way (C++20)
        "},{"location":"api/json_pointer/operator_ne/#version-history","title":"Version history","text":"
        1. Added in version 2.1.0.
        2. Added for backward compatibility and deprecated in version 3.11.2.
        "},{"location":"api/json_pointer/operator_slash/","title":"nlohmann::json_pointer::operator/","text":"
        // (1)\njson_pointer operator/(const json_pointer& lhs, const json_pointer& rhs);\n\n// (2)\njson_pointer operator/(const json_pointer& lhs, string_t token);\n\n// (3)\njson_pointer operator/(const json_pointer& lhs, std::size_t array_idx);\n
        1. create a new JSON pointer by appending the right JSON pointer at the end of the left JSON pointer
        2. create a new JSON pointer by appending the unescaped token at the end of the JSON pointer
        3. create a new JSON pointer by appending the array-index-token at the end of the JSON pointer
        "},{"location":"api/json_pointer/operator_slash/#parameters","title":"Parameters","text":"lhs (in) JSON pointer rhs (in) JSON pointer to append token (in) reference token to append array_idx (in) array index to append"},{"location":"api/json_pointer/operator_slash/#return-value","title":"Return value","text":"
        1. a new JSON pointer with rhs appended to lhs
        2. a new JSON pointer with unescaped token appended to lhs
        3. a new JSON pointer with array_idx appended to lhs
        "},{"location":"api/json_pointer/operator_slash/#exception-safety","title":"Exception safety","text":"

        Strong guarantee: if an exception is thrown, there are no changes to any JSON pointer. The operands are not modified; a new JSON pointer is built from a copy of lhs.

        "},{"location":"api/json_pointer/operator_slash/#complexity","title":"Complexity","text":"
        1. Linear in the length of lhs and rhs.
        2. Linear in the length of lhs.
        3. Linear in the length of lhs.
        "},{"location":"api/json_pointer/operator_slash/#examples","title":"Examples","text":"Example

        The example shows the usage of operator/.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON pointer\n    json::json_pointer ptr(\"/foo\");\n\n    // append a JSON Pointer\n    std::cout << \"\\\"\" << ptr / json::json_pointer(\"/bar/baz\") << \"\\\"\\n\";\n\n    // append a string\n    std::cout << \"\\\"\" << ptr / \"fob\" << \"\\\"\\n\";\n\n    // append an array index\n    std::cout << \"\\\"\" << ptr / 42 << \"\\\"\" << std::endl;\n}\n

        Output:

        \"/foo/bar/baz\"\n\"/foo/fob\"\n\"/foo/42\"\n
        "},{"location":"api/json_pointer/operator_slash/#see-also","title":"See also","text":"
        • operator/= append to the end of the JSON pointer
        • push_back append an unescaped token at the end of the pointer
        "},{"location":"api/json_pointer/operator_slash/#version-history","title":"Version history","text":"
        1. Added in version 3.6.0.
        2. Added in version 3.6.0. Changed type of token to string_t in version 3.11.0.
        3. Added in version 3.6.0.
        "},{"location":"api/json_pointer/operator_slasheq/","title":"nlohmann::json_pointer::operator/=","text":"
        // (1)\njson_pointer& operator/=(const json_pointer& ptr);\n\n// (2)\njson_pointer& operator/=(string_t token);\n\n// (3)\njson_pointer& operator/=(std::size_t array_idx)\n
        1. append another JSON pointer at the end of this JSON pointer
        2. append an unescaped reference token at the end of this JSON pointer
        3. append an array index at the end of this JSON pointer
        "},{"location":"api/json_pointer/operator_slasheq/#parameters","title":"Parameters","text":"ptr (in) JSON pointer to append token (in) reference token to append array_idx (in) array index to append"},{"location":"api/json_pointer/operator_slasheq/#return-value","title":"Return value","text":"
        1. JSON pointer with ptr appended
        2. JSON pointer with token appended without escaping token
        3. JSON pointer with array_idx appended
        "},{"location":"api/json_pointer/operator_slasheq/#exception-safety","title":"Exception safety","text":"
        1. Basic guarantee: if an exception is thrown (for instance, if copying a reference token fails), the JSON pointer is left in a valid state, but it may contain some of the reference tokens of ptr.
        2. Strong guarantee: if an exception is thrown, there are no changes to the JSON pointer.
        3. Strong guarantee: if an exception is thrown, there are no changes to the JSON pointer.
        "},{"location":"api/json_pointer/operator_slasheq/#complexity","title":"Complexity","text":"
        1. Linear in the length of ptr.
        2. Amortized constant.
        3. Amortized constant.
        "},{"location":"api/json_pointer/operator_slasheq/#examples","title":"Examples","text":"Example

        The example shows the usage of operator/=.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON pointer\n    json::json_pointer ptr(\"/foo\");\n    std::cout << \"\\\"\" << ptr << \"\\\"\\n\";\n\n    // append a JSON Pointer\n    ptr /= json::json_pointer(\"/bar/baz\");\n    std::cout << \"\\\"\" << ptr << \"\\\"\\n\";\n\n    // append a string\n    ptr /= \"fob\";\n    std::cout << \"\\\"\" << ptr << \"\\\"\\n\";\n\n    // append an array index\n    ptr /= 42;\n    std::cout << \"\\\"\" << ptr << \"\\\"\" << std::endl;\n}\n

        Output:

        \"/foo\"\n\"/foo/bar/baz\"\n\"/foo/bar/baz/fob\"\n\"/foo/bar/baz/fob/42\"\n
        "},{"location":"api/json_pointer/operator_slasheq/#see-also","title":"See also","text":"
        • operator/ create JSON Pointer by appending
        • push_back append an unescaped token at the end of the pointer
        "},{"location":"api/json_pointer/operator_slasheq/#version-history","title":"Version history","text":"
        1. Added in version 3.6.0.
        2. Added in version 3.6.0. Changed type of token to string_t in version 3.11.0.
        3. Added in version 3.6.0.
        "},{"location":"api/json_pointer/operator_spaceship/","title":"nlohmann::json_pointer::operator<=>","text":"
        // since C++20\ntemplate<typename RefStringTypeRhs>\nstd::strong_ordering operator<=>(const json_pointer<RefStringTypeRhs>& rhs) const noexcept; // *NOPAD*\n

        3-way compares two JSON pointers by lexicographically comparing their sequences of reference tokens: corresponding reference tokens are compared with string_t's own operator<=>, and the first pair of tokens that differs determines the result. If all corresponding reference tokens compare equal, the JSON pointer with fewer reference tokens is ordered first.

        "},{"location":"api/json_pointer/operator_spaceship/#template-parameters","title":"Template parameters","text":"RefStringTypeRhs the string type of the right-hand side JSON pointer"},{"location":"api/json_pointer/operator_spaceship/#parameters","title":"Parameters","text":"rhs (in) JSON pointer to compare *this with"},{"location":"api/json_pointer/operator_spaceship/#return-value","title":"Return value","text":"

        the std::strong_ordering of the 3-way comparison of *this and rhs

        "},{"location":"api/json_pointer/operator_spaceship/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this function never throws exceptions.

        "},{"location":"api/json_pointer/operator_spaceship/#complexity","title":"Complexity","text":"

        Linear in the number of reference tokens.

        "},{"location":"api/json_pointer/operator_spaceship/#notes","title":"Notes","text":"

        Ordering enables use as an associative container key

        Together with operator==, operator<=> makes json_pointer a LessThanComparable type, so it can be used as the key type of ordered associative containers such as std::map or std::set.

        Before C++20

        Without C++20's three-way comparison, json_pointer provides a non-member operator< instead, which orders JSON pointers the same way. JSON pointers can therefore be used as keys of ordered associative containers with any supported C++ standard.

        "},{"location":"api/json_pointer/operator_spaceship/#examples","title":"Examples","text":"Example

        The example demonstrates 3-way comparing JSON pointers.

        #include <compare>\n#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nconst char* to_string(const std::strong_ordering& so)\n{\n    if (std::is_lt(so))\n    {\n        return \"less\";\n    }\n    else if (std::is_gt(so))\n    {\n        return \"greater\";\n    }\n    return \"equal\";\n}\n\nint main()\n{\n    // different JSON pointers\n    json::json_pointer ptr1(\"/a/b\");\n    json::json_pointer ptr2(\"/a/c\");\n    json::json_pointer ptr3(\"/a/b/c\");\n    json::json_pointer ptr4(\"/a/b\");\n\n    // 3-way compare JSON pointers\n    std::cout << \"\\\"\" << ptr1 << \"\\\" <=> \\\"\" << ptr2 << \"\\\": \" << to_string(ptr1 <=> ptr2) << '\\n' // *NOPAD*\n              << \"\\\"\" << ptr1 << \"\\\" <=> \\\"\" << ptr3 << \"\\\": \" << to_string(ptr1 <=> ptr3) << '\\n' // *NOPAD*\n              << \"\\\"\" << ptr1 << \"\\\" <=> \\\"\" << ptr4 << \"\\\": \" << to_string(ptr1 <=> ptr4) << std::endl; // *NOPAD*\n}\n

        Output:

        \"/a/b\" <=> \"/a/c\": less\n\"/a/b\" <=> \"/a/b/c\": less\n\"/a/b\" <=> \"/a/b\": equal\n
        "},{"location":"api/json_pointer/operator_spaceship/#see-also","title":"See also","text":"
        • operator== compare: equal
        • operator!= compare: not equal
        "},{"location":"api/json_pointer/operator_spaceship/#version-history","title":"Version history","text":"
        • Added in version 3.11.2.
        "},{"location":"api/json_pointer/operator_string_t/","title":"nlohmann::json_pointer::operator string_t","text":"
        operator string_t() const\n

        Return a string representation of the JSON pointer.

        "},{"location":"api/json_pointer/operator_string_t/#return-value","title":"Return value","text":"

        A string representation of the JSON pointer

        "},{"location":"api/json_pointer/operator_string_t/#possible-implementation","title":"Possible implementation","text":"
        operator string_t() const\n{\n    return to_string();\n}\n
        "},{"location":"api/json_pointer/operator_string_t/#notes","title":"Notes","text":"

        Deprecation

        This function is deprecated in favor of to_string and will be removed in a future major version release.

        See the migration guide for how to update existing code.

        "},{"location":"api/json_pointer/operator_string_t/#examples","title":"Examples","text":"Example

        The example shows how JSON Pointers can be implicitly converted to strings.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // different JSON Pointers\n    json::json_pointer ptr1(\"/foo/0\");\n    json::json_pointer ptr2(\"/a~1b\");\n\n    // implicit conversion to string\n    std::string s;\n    s += ptr1;\n    s += \"\\n\";\n    s += ptr2;\n\n    std::cout << s << std::endl;\n}\n

        Output:

        /foo/0\n/a~1b\n
        "},{"location":"api/json_pointer/operator_string_t/#see-also","title":"See also","text":"
        • to_string return a string representation of the JSON pointer
        • string_t - type for strings
        "},{"location":"api/json_pointer/operator_string_t/#version-history","title":"Version history","text":"
        • Since version 2.0.0.
        • Changed type to string_t and deprecated in version 3.11.0.
        "},{"location":"api/json_pointer/parent_pointer/","title":"nlohmann::json_pointer::parent_pointer","text":"
        json_pointer parent_pointer() const;\n

        Returns the parent of this JSON pointer.

        "},{"location":"api/json_pointer/parent_pointer/#return-value","title":"Return value","text":"

        Parent of this JSON pointer; in case this JSON pointer is the root, the root itself is returned.

        "},{"location":"api/json_pointer/parent_pointer/#exception-safety","title":"Exception safety","text":"

        No-throw guarantee: this function never throws exceptions.

        "},{"location":"api/json_pointer/parent_pointer/#complexity","title":"Complexity","text":"

        Linear in the length of the JSON pointer.

        "},{"location":"api/json_pointer/parent_pointer/#examples","title":"Examples","text":"Example

        The example shows the result of parent_pointer for different JSON Pointers.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // different JSON Pointers\n    json::json_pointer ptr1(\"\");\n    json::json_pointer ptr2(\"/foo\");\n    json::json_pointer ptr3(\"/foo/0\");\n\n    // call parent_pointer()\n    std::cout << std::boolalpha\n              << \"parent of \\\"\" << ptr1 << \"\\\" is \\\"\" << ptr1.parent_pointer() << \"\\\"\\n\"\n              << \"parent of \\\"\" << ptr2 << \"\\\" is \\\"\" << ptr2.parent_pointer() << \"\\\"\\n\"\n              << \"parent of \\\"\" << ptr3 << \"\\\" is \\\"\" << ptr3.parent_pointer() << \"\\\"\" << std::endl;\n}\n

        Output:

        parent of \"\" is \"\"\nparent of \"/foo\" is \"\"\nparent of \"/foo/0\" is \"/foo\"\n
        "},{"location":"api/json_pointer/parent_pointer/#see-also","title":"See also","text":"
        • pop_back remove the last reference token
        • back return the last reference token
        "},{"location":"api/json_pointer/parent_pointer/#version-history","title":"Version history","text":"

        Added in version 3.6.0.

        "},{"location":"api/json_pointer/pop_back/","title":"nlohmann::json_pointer::pop_back","text":"
        void pop_back();\n

        Remove the last reference token.

        "},{"location":"api/json_pointer/pop_back/#exception-safety","title":"Exception safety","text":"

        Strong exception safety: if an exception occurs, the original value stays intact.

        "},{"location":"api/json_pointer/pop_back/#exceptions","title":"Exceptions","text":"

        Throws out_of_range.405 if the JSON pointer has no parent.

        "},{"location":"api/json_pointer/pop_back/#complexity","title":"Complexity","text":"

        Constant.

        "},{"location":"api/json_pointer/pop_back/#examples","title":"Examples","text":"Example

        The example shows the usage of pop_back.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create empty JSON Pointer\n    json::json_pointer ptr(\"/foo/bar/baz\");\n    std::cout << \"\\\"\" << ptr << \"\\\"\\n\";\n\n    // call pop_back()\n    ptr.pop_back();\n    std::cout << \"\\\"\" << ptr << \"\\\"\\n\";\n\n    ptr.pop_back();\n    std::cout << \"\\\"\" << ptr << \"\\\"\\n\";\n\n    ptr.pop_back();\n    std::cout << \"\\\"\" << ptr << \"\\\"\\n\";\n}\n

        Output:

        \"/foo/bar/baz\"\n\"/foo/bar\"\n\"/foo\"\n\"\"\n
        "},{"location":"api/json_pointer/pop_back/#see-also","title":"See also","text":"
        • back return last reference token
        • push_back append an unescaped token at the end of the pointer
        • parent_pointer returns the parent of this JSON pointer
        "},{"location":"api/json_pointer/pop_back/#version-history","title":"Version history","text":"

        Added in version 3.6.0.

        "},{"location":"api/json_pointer/pop_front/","title":"nlohmann::json_pointer::pop_front","text":"
        void pop_front();\n

        Remove the first reference token.

        "},{"location":"api/json_pointer/pop_front/#exception-safety","title":"Exception safety","text":"

        Strong exception safety: if an exception occurs, the original value stays intact.

        "},{"location":"api/json_pointer/pop_front/#exceptions","title":"Exceptions","text":"

        Throws out_of_range.405 if the JSON pointer has no parent.

        "},{"location":"api/json_pointer/pop_front/#complexity","title":"Complexity","text":"

        Linear in the number of reference tokens in the json_pointer.

        "},{"location":"api/json_pointer/pop_front/#examples","title":"Examples","text":"Example

        The example shows the usage of pop_front.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create empty JSON Pointer\n    json::json_pointer ptr(\"/foo/bar/baz\");\n    std::cout << \"\\\"\" << ptr << \"\\\"\\n\";\n\n    // call pop_front()\n    ptr.pop_front();\n    std::cout << \"\\\"\" << ptr << \"\\\"\\n\";\n\n    ptr.pop_front();\n    std::cout << \"\\\"\" << ptr << \"\\\"\\n\";\n\n    ptr.pop_front();\n    std::cout << \"\\\"\" << ptr << \"\\\"\\n\";\n}\n

        Output:

        \"/foo/bar/baz\"\n\"/bar/baz\"\n\"/baz\"\n\"\"\n
        "},{"location":"api/json_pointer/pop_front/#see-also","title":"See also","text":"
        • front return first reference token
        • push_front append an unescaped token at the start of the pointer
        "},{"location":"api/json_pointer/pop_front/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/json_pointer/push_back/","title":"nlohmann::json_pointer::push_back","text":"
        void push_back(const string_t& token);\n\nvoid push_back(string_t&& token);\n

        Append an unescaped token at the end of the reference pointer.

        "},{"location":"api/json_pointer/push_back/#parameters","title":"Parameters","text":"token (in) token to add"},{"location":"api/json_pointer/push_back/#exception-safety","title":"Exception safety","text":"

        Strong exception safety: if an exception occurs, the original value stays intact.

        "},{"location":"api/json_pointer/push_back/#complexity","title":"Complexity","text":"

        Amortized constant.

        "},{"location":"api/json_pointer/push_back/#examples","title":"Examples","text":"Example

        The example shows the result of push_back for different JSON Pointers.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create empty JSON Pointer\n    json::json_pointer ptr;\n    std::cout << \"\\\"\" << ptr << \"\\\"\\n\";\n\n    // call push_back()\n    ptr.push_back(\"foo\");\n    std::cout << \"\\\"\" << ptr << \"\\\"\\n\";\n\n    ptr.push_back(\"0\");\n    std::cout << \"\\\"\" << ptr << \"\\\"\\n\";\n\n    ptr.push_back(\"bar\");\n    std::cout << \"\\\"\" << ptr << \"\\\"\\n\";\n}\n

        Output:

        \"\"\n\"/foo\"\n\"/foo/0\"\n\"/foo/0/bar\"\n
        "},{"location":"api/json_pointer/push_back/#see-also","title":"See also","text":"
        • back return last reference token
        • pop_back remove the last reference token
        • operator/= append to the end of the JSON pointer
        • operator/ create JSON Pointer by appending
        "},{"location":"api/json_pointer/push_back/#version-history","title":"Version history","text":"
        • Added in version 3.6.0.
        • Changed type of token to string_t in version 3.11.0.
        "},{"location":"api/json_pointer/push_front/","title":"nlohmann::json_pointer::push_front","text":"
        void push_front(const string_t& token);\n\nvoid push_front(string_t&& token);\n

        Append an unescaped token at the start of the reference pointer.

        "},{"location":"api/json_pointer/push_front/#parameters","title":"Parameters","text":"token (in) token to add"},{"location":"api/json_pointer/push_front/#exception-safety","title":"Exception safety","text":"

        Basic guarantee: if an exception is thrown (for instance, if copying the reference token fails), the JSON pointer is left in a valid state, but its reference tokens may have changed.

        "},{"location":"api/json_pointer/push_front/#complexity","title":"Complexity","text":"

        Linear in the number of reference tokens in the json_pointer.

        "},{"location":"api/json_pointer/push_front/#examples","title":"Examples","text":"Example

        The example shows the result of push_front for different JSON Pointers.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create empty JSON Pointer\n    json::json_pointer ptr;\n    std::cout << \"\\\"\" << ptr << \"\\\"\\n\";\n\n    // call push_front()\n    ptr.push_front(\"foo\");\n    std::cout << \"\\\"\" << ptr << \"\\\"\\n\";\n\n    ptr.push_front(\"0\");\n    std::cout << \"\\\"\" << ptr << \"\\\"\\n\";\n\n    ptr.push_front(\"bar\");\n    std::cout << \"\\\"\" << ptr << \"\\\"\\n\";\n}\n

        Output:

        \"\"\n\"/foo\"\n\"/0/foo\"\n\"/bar/0/foo\"\n
        "},{"location":"api/json_pointer/push_front/#see-also","title":"See also","text":"
        • front return first reference token
        • pop_front remove the first reference token
        "},{"location":"api/json_pointer/push_front/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/json_pointer/string_t/","title":"nlohmann::json_pointer::string_t","text":"
        using string_t = RefStringType;\n

        The string type used for the reference tokens making up the JSON pointer.

        See basic_json::string_t for more information.

        "},{"location":"api/json_pointer/string_t/#examples","title":"Examples","text":"Example

        The example shows the type string_t and its relation to basic_json::string_t.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    json::json_pointer::string_t s = \"This is a string.\";\n\n    std::cout << s << std::endl;\n\n    std::cout << std::boolalpha << std::is_same<json::json_pointer::string_t, json::string_t>::value << std::endl;\n}\n

        Output:

        This is a string.\ntrue\n
        "},{"location":"api/json_pointer/string_t/#see-also","title":"See also","text":"
        • basic_json::string_t type used to store JSON strings
        • to_string return a string representation of the JSON pointer
        "},{"location":"api/json_pointer/string_t/#version-history","title":"Version history","text":"
        • Added in version 3.11.0.
        "},{"location":"api/json_pointer/to_string/","title":"nlohmann::json_pointer::to_string","text":"
        string_t to_string() const;\n

        Return a string representation of the JSON pointer.

        "},{"location":"api/json_pointer/to_string/#return-value","title":"Return value","text":"

        A string representation of the JSON pointer

        "},{"location":"api/json_pointer/to_string/#exception-safety","title":"Exception safety","text":"

        Strong exception safety: if an exception occurs, the original value stays intact.

        "},{"location":"api/json_pointer/to_string/#complexity","title":"Complexity","text":"

        Linear in the total length of the reference tokens.

        "},{"location":"api/json_pointer/to_string/#notes","title":"Notes","text":"

        For each JSON pointer ptr, it holds:

        ptr == json_pointer(ptr.to_string());\n
        "},{"location":"api/json_pointer/to_string/#examples","title":"Examples","text":"Example

        The example shows the result of to_string.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // different JSON Pointers\n    json::json_pointer ptr1(\"\");\n    json::json_pointer ptr2(\"/foo\");\n    json::json_pointer ptr3(\"/foo/0\");\n    json::json_pointer ptr4(\"/\");\n    json::json_pointer ptr5(\"/a~1b\");\n    json::json_pointer ptr6(\"/c%d\");\n    json::json_pointer ptr7(\"/e^f\");\n    json::json_pointer ptr8(\"/g|h\");\n    json::json_pointer ptr9(\"/i\\\\j\");\n    json::json_pointer ptr10(\"/k\\\"l\");\n    json::json_pointer ptr11(\"/ \");\n    json::json_pointer ptr12(\"/m~0n\");\n\n    std::cout << \"\\\"\" << ptr1.to_string() << \"\\\"\\n\"\n              << \"\\\"\" << ptr2.to_string() << \"\\\"\\n\"\n              << \"\\\"\" << ptr3.to_string() << \"\\\"\\n\"\n              << \"\\\"\" << ptr4.to_string() << \"\\\"\\n\"\n              << \"\\\"\" << ptr5.to_string() << \"\\\"\\n\"\n              << \"\\\"\" << ptr6.to_string() << \"\\\"\\n\"\n              << \"\\\"\" << ptr7.to_string() << \"\\\"\\n\"\n              << \"\\\"\" << ptr8.to_string() << \"\\\"\\n\"\n              << \"\\\"\" << ptr9.to_string() << \"\\\"\\n\"\n              << \"\\\"\" << ptr10.to_string() << \"\\\"\\n\"\n              << \"\\\"\" << ptr11.to_string() << \"\\\"\\n\"\n              << \"\\\"\" << ptr12.to_string() << \"\\\"\" << std::endl;\n}\n

        Output:

        \"\"\n\"/foo\"\n\"/foo/0\"\n\"/\"\n\"/a~1b\"\n\"/c%d\"\n\"/e^f\"\n\"/g|h\"\n\"/i\\j\"\n\"/k\"l\"\n\"/ \"\n\"/m~0n\"\n
        "},{"location":"api/json_pointer/to_string/#see-also","title":"See also","text":"
        • operator string_t return a string representation of the JSON pointer (deprecated)
        • operator<< write a JSON pointer to a stream
        "},{"location":"api/json_pointer/to_string/#version-history","title":"Version history","text":"
        • Since version 2.0.0.
        • Changed return type to string_t in version 3.11.0.
        "},{"location":"api/json_sax/","title":"nlohmann::json_sax","text":"
        template<typename BasicJsonType>\nstruct json_sax;\n

        This class describes the SAX interface used by sax_parse. Each function is called in different situations while the input is parsed. The boolean return value informs the parser whether to continue processing the input.

        For instance, parsing the JSON text {\"a\": [1, true]} triggers the following callbacks, in order:

        sequenceDiagram\n    participant P as Parser\n    participant H as SAX handler\n\n    P->>H: start_object(elements)\n    P->>H: key(\"a\")\n    P->>H: start_array(elements)\n    P->>H: number_unsigned(1)\n    P->>H: boolean(true)\n    P->>H: end_array()\n    P->>H: end_object()

        Note elements is passed as std::numeric_limits<std::size_t>::max() (i.e., \"unknown\") for JSON text input; only binary formats such as CBOR or MessagePack may report the actual number of elements in start_object/ start_array. Also note that 1 is reported via number_unsigned rather than number_integer because it has no leading - sign.

        "},{"location":"api/json_sax/#template-parameters","title":"Template parameters","text":"BasicJsonType a specialization of basic_json"},{"location":"api/json_sax/#member-types","title":"Member types","text":"
        • number_integer_t - BasicJsonType's type for numbers (integer)
        • number_unsigned_t - BasicJsonType's type for numbers (unsigned)
        • number_float_t - BasicJsonType's type for numbers (floating-point)
        • string_t - BasicJsonType's type for strings
        • binary_t - BasicJsonType's type for binary arrays
        "},{"location":"api/json_sax/#member-functions","title":"Member functions","text":"
        • binary (virtual) - a binary value was read
        • boolean (virtual) - a boolean value was read
        • end_array (virtual) - the end of an array was read
        • end_object (virtual) - the end of an object was read
        • key (virtual) - an object key was read
        • null (virtual) - a null value was read
        • number_float (virtual) - a floating-point number was read
        • number_integer (virtual) - an integer number was read
        • number_unsigned (virtual) - an unsigned integer number was read
        • parse_error (virtual) - a parse error occurred
        • start_array (virtual) - the beginning of an array was read
        • start_object (virtual) - the beginning of an object was read
        • string (virtual) - a string value was read
        "},{"location":"api/json_sax/#version-history","title":"Version history","text":"
        • Added in version 3.2.0.
        • Support for binary values (binary_t, binary) added in version 3.8.0.
        "},{"location":"api/json_sax/binary/","title":"nlohmann::json_sax::binary","text":"
        virtual bool binary(binary_t& val) = 0;\n

        A binary value was read.

        "},{"location":"api/json_sax/binary/#parameters","title":"Parameters","text":"val (in) binary value"},{"location":"api/json_sax/binary/#return-value","title":"Return value","text":"

        Whether parsing should proceed.

        "},{"location":"api/json_sax/binary/#notes","title":"Notes","text":"

        It is safe to move the passed binary value.

        "},{"location":"api/json_sax/binary/#examples","title":"Examples","text":"Example

        The example below shows how the SAX interface is used.

        #include <iostream>\n#include <iomanip>\n#include <sstream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\n// a simple event consumer that collects string representations of the passed\n// values; note inheriting from json::json_sax_t is not required, but can\n// help not to forget a required function\nclass sax_event_consumer : public json::json_sax_t\n{\n  public:\n    std::vector<std::string> events;\n\n    bool null() override\n    {\n        events.push_back(\"null()\");\n        return true;\n    }\n\n    bool boolean(bool val) override\n    {\n        events.push_back(\"boolean(val=\" + std::string(val ? \"true\" : \"false\") + \")\");\n        return true;\n    }\n\n    bool number_integer(number_integer_t val) override\n    {\n        events.push_back(\"number_integer(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_unsigned(number_unsigned_t val) override\n    {\n        events.push_back(\"number_unsigned(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_float(number_float_t val, const string_t& s) override\n    {\n        events.push_back(\"number_float(val=\" + std::to_string(val) + \", s=\" + s + \")\");\n        return true;\n    }\n\n    bool string(string_t& val) override\n    {\n        events.push_back(\"string(val=\" + val + \")\");\n        return true;\n    }\n\n    bool start_object(std::size_t elements) override\n    {\n        events.push_back(\"start_object(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_object() override\n    {\n        events.push_back(\"end_object()\");\n        return true;\n    }\n\n    bool start_array(std::size_t elements) override\n    {\n        events.push_back(\"start_array(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_array() override\n    {\n        events.push_back(\"end_array()\");\n        return true;\n    }\n\n    bool key(string_t& val) override\n    {\n        events.push_back(\"key(val=\" + val + \")\");\n        return true;\n    }\n\n    bool binary(json::binary_t& val) override\n    {\n        events.push_back(\"binary(val=[...])\");\n        return true;\n    }\n\n    bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex) override\n    {\n        events.push_back(\"parse_error(position=\" + std::to_string(position) + \", last_token=\" + last_token + \",\\n            ex=\" + std::string(ex.what()) + \")\");\n        return false;\n    }\n};\n\nint main()\n{\n    // CBOR byte string\n    std::vector<std::uint8_t> vec = {{0x44, 0xcA, 0xfe, 0xba, 0xbe}};\n\n    // create a SAX event consumer object\n    sax_event_consumer sec;\n\n    // parse CBOR\n    bool result = json::sax_parse(vec, &sec, json::input_format_t::cbor);\n\n    // output the recorded events\n    for (auto& event : sec.events)\n    {\n        std::cout << event << \"\\n\";\n    }\n\n    // output the result of sax_parse\n    std::cout << \"\\nresult: \" << std::boolalpha << result << std::endl;\n}\n

        Output:

        binary(val=[...])\n\nresult: true\n
        "},{"location":"api/json_sax/binary/#see-also","title":"See also","text":"
        • sax_parse - SAX parser
        • SAX Interface - the SAX interface article
        • Binary Values - the article on binary values
        "},{"location":"api/json_sax/binary/#version-history","title":"Version history","text":"
        • Added in version 3.8.0.
        "},{"location":"api/json_sax/boolean/","title":"nlohmann::json_sax::boolean","text":"
        virtual bool boolean(bool val) = 0;\n

        A boolean value was read.

        "},{"location":"api/json_sax/boolean/#parameters","title":"Parameters","text":"val (in) boolean value"},{"location":"api/json_sax/boolean/#return-value","title":"Return value","text":"

        Whether parsing should proceed.

        "},{"location":"api/json_sax/boolean/#examples","title":"Examples","text":"Example

        The example below shows how the SAX interface is used.

        #include <iostream>\n#include <iomanip>\n#include <sstream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\n// a simple event consumer that collects string representations of the passed\n// values; note inheriting from json::json_sax_t is not required, but can\n// help not to forget a required function\nclass sax_event_consumer : public json::json_sax_t\n{\n  public:\n    std::vector<std::string> events;\n\n    bool null() override\n    {\n        events.push_back(\"null()\");\n        return true;\n    }\n\n    bool boolean(bool val) override\n    {\n        events.push_back(\"boolean(val=\" + std::string(val ? \"true\" : \"false\") + \")\");\n        return true;\n    }\n\n    bool number_integer(number_integer_t val) override\n    {\n        events.push_back(\"number_integer(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_unsigned(number_unsigned_t val) override\n    {\n        events.push_back(\"number_unsigned(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_float(number_float_t val, const string_t& s) override\n    {\n        events.push_back(\"number_float(val=\" + std::to_string(val) + \", s=\" + s + \")\");\n        return true;\n    }\n\n    bool string(string_t& val) override\n    {\n        events.push_back(\"string(val=\" + val + \")\");\n        return true;\n    }\n\n    bool start_object(std::size_t elements) override\n    {\n        events.push_back(\"start_object(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_object() override\n    {\n        events.push_back(\"end_object()\");\n        return true;\n    }\n\n    bool start_array(std::size_t elements) override\n    {\n        events.push_back(\"start_array(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_array() override\n    {\n        events.push_back(\"end_array()\");\n        return true;\n    }\n\n    bool key(string_t& val) override\n    {\n        events.push_back(\"key(val=\" + val + \")\");\n        return true;\n    }\n\n    bool binary(json::binary_t& val) override\n    {\n        events.push_back(\"binary(val=[...])\");\n        return true;\n    }\n\n    bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex) override\n    {\n        events.push_back(\"parse_error(position=\" + std::to_string(position) + \", last_token=\" + last_token + \",\\n            ex=\" + std::string(ex.what()) + \")\");\n        return false;\n    }\n};\n\nint main()\n{\n    // a JSON text\n    auto text = R\"(\n    {\n        \"Image\": {\n            \"Width\":  800,\n            \"Height\": 600,\n            \"Title\":  \"View from 15th Floor\",\n            \"Thumbnail\": {\n                \"Url\":    \"http://www.example.com/image/481989943\",\n                \"Height\": 125,\n                \"Width\":  100\n            },\n            \"Animated\" : false,\n            \"IDs\": [116, 943, 234, -38793],\n            \"DeletionDate\": null,\n            \"Distance\": 12.723374634\n        }\n    }]\n    )\";\n\n    // create a SAX event consumer object\n    sax_event_consumer sec;\n\n    // parse JSON\n    bool result = json::sax_parse(text, &sec);\n\n    // output the recorded events\n    for (auto& event : sec.events)\n    {\n        std::cout << event << \"\\n\";\n    }\n\n    // output the result of sax_parse\n    std::cout << \"\\nresult: \" << std::boolalpha << result << std::endl;\n}\n

        Output:

        start_object(elements=18446744073709551615)\nkey(val=Image)\nstart_object(elements=18446744073709551615)\nkey(val=Width)\nnumber_unsigned(val=800)\nkey(val=Height)\nnumber_unsigned(val=600)\nkey(val=Title)\nstring(val=View from 15th Floor)\nkey(val=Thumbnail)\nstart_object(elements=18446744073709551615)\nkey(val=Url)\nstring(val=http://www.example.com/image/481989943)\nkey(val=Height)\nnumber_unsigned(val=125)\nkey(val=Width)\nnumber_unsigned(val=100)\nend_object()\nkey(val=Animated)\nboolean(val=false)\nkey(val=IDs)\nstart_array(elements=18446744073709551615)\nnumber_unsigned(val=116)\nnumber_unsigned(val=943)\nnumber_unsigned(val=234)\nnumber_integer(val=-38793)\nend_array()\nkey(val=DeletionDate)\nnull()\nkey(val=Distance)\nnumber_float(val=12.723375, s=12.723374634)\nend_object()\nend_object()\nparse_error(position=460, last_token=12.723374634<U+000A>        }<U+000A>    }],\n            ex=[json.exception.parse_error.101] parse error at line 17, column 6: syntax error while parsing value - unexpected ']'; expected end of input)\n\nresult: false\n
        "},{"location":"api/json_sax/boolean/#see-also","title":"See also","text":"
        • sax_parse - SAX parser
        • SAX Interface - the SAX interface article
        "},{"location":"api/json_sax/boolean/#version-history","title":"Version history","text":"
        • Added in version 3.2.0.
        "},{"location":"api/json_sax/end_array/","title":"nlohmann::json_sax::end_array","text":"
        virtual bool end_array() = 0;\n

        The end of an array was read.

        "},{"location":"api/json_sax/end_array/#return-value","title":"Return value","text":"

        Whether parsing should proceed.

        "},{"location":"api/json_sax/end_array/#examples","title":"Examples","text":"Example

        The example below shows how the SAX interface is used.

        #include <iostream>\n#include <iomanip>\n#include <sstream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\n// a simple event consumer that collects string representations of the passed\n// values; note inheriting from json::json_sax_t is not required, but can\n// help not to forget a required function\nclass sax_event_consumer : public json::json_sax_t\n{\n  public:\n    std::vector<std::string> events;\n\n    bool null() override\n    {\n        events.push_back(\"null()\");\n        return true;\n    }\n\n    bool boolean(bool val) override\n    {\n        events.push_back(\"boolean(val=\" + std::string(val ? \"true\" : \"false\") + \")\");\n        return true;\n    }\n\n    bool number_integer(number_integer_t val) override\n    {\n        events.push_back(\"number_integer(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_unsigned(number_unsigned_t val) override\n    {\n        events.push_back(\"number_unsigned(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_float(number_float_t val, const string_t& s) override\n    {\n        events.push_back(\"number_float(val=\" + std::to_string(val) + \", s=\" + s + \")\");\n        return true;\n    }\n\n    bool string(string_t& val) override\n    {\n        events.push_back(\"string(val=\" + val + \")\");\n        return true;\n    }\n\n    bool start_object(std::size_t elements) override\n    {\n        events.push_back(\"start_object(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_object() override\n    {\n        events.push_back(\"end_object()\");\n        return true;\n    }\n\n    bool start_array(std::size_t elements) override\n    {\n        events.push_back(\"start_array(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_array() override\n    {\n        events.push_back(\"end_array()\");\n        return true;\n    }\n\n    bool key(string_t& val) override\n    {\n        events.push_back(\"key(val=\" + val + \")\");\n        return true;\n    }\n\n    bool binary(json::binary_t& val) override\n    {\n        events.push_back(\"binary(val=[...])\");\n        return true;\n    }\n\n    bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex) override\n    {\n        events.push_back(\"parse_error(position=\" + std::to_string(position) + \", last_token=\" + last_token + \",\\n            ex=\" + std::string(ex.what()) + \")\");\n        return false;\n    }\n};\n\nint main()\n{\n    // a JSON text\n    auto text = R\"(\n    {\n        \"Image\": {\n            \"Width\":  800,\n            \"Height\": 600,\n            \"Title\":  \"View from 15th Floor\",\n            \"Thumbnail\": {\n                \"Url\":    \"http://www.example.com/image/481989943\",\n                \"Height\": 125,\n                \"Width\":  100\n            },\n            \"Animated\" : false,\n            \"IDs\": [116, 943, 234, -38793],\n            \"DeletionDate\": null,\n            \"Distance\": 12.723374634\n        }\n    }]\n    )\";\n\n    // create a SAX event consumer object\n    sax_event_consumer sec;\n\n    // parse JSON\n    bool result = json::sax_parse(text, &sec);\n\n    // output the recorded events\n    for (auto& event : sec.events)\n    {\n        std::cout << event << \"\\n\";\n    }\n\n    // output the result of sax_parse\n    std::cout << \"\\nresult: \" << std::boolalpha << result << std::endl;\n}\n

        Output:

        start_object(elements=18446744073709551615)\nkey(val=Image)\nstart_object(elements=18446744073709551615)\nkey(val=Width)\nnumber_unsigned(val=800)\nkey(val=Height)\nnumber_unsigned(val=600)\nkey(val=Title)\nstring(val=View from 15th Floor)\nkey(val=Thumbnail)\nstart_object(elements=18446744073709551615)\nkey(val=Url)\nstring(val=http://www.example.com/image/481989943)\nkey(val=Height)\nnumber_unsigned(val=125)\nkey(val=Width)\nnumber_unsigned(val=100)\nend_object()\nkey(val=Animated)\nboolean(val=false)\nkey(val=IDs)\nstart_array(elements=18446744073709551615)\nnumber_unsigned(val=116)\nnumber_unsigned(val=943)\nnumber_unsigned(val=234)\nnumber_integer(val=-38793)\nend_array()\nkey(val=DeletionDate)\nnull()\nkey(val=Distance)\nnumber_float(val=12.723375, s=12.723374634)\nend_object()\nend_object()\nparse_error(position=460, last_token=12.723374634<U+000A>        }<U+000A>    }],\n            ex=[json.exception.parse_error.101] parse error at line 17, column 6: syntax error while parsing value - unexpected ']'; expected end of input)\n\nresult: false\n
        "},{"location":"api/json_sax/end_array/#see-also","title":"See also","text":"
        • start_array - the beginning of an array was read
        • sax_parse - SAX parser
        "},{"location":"api/json_sax/end_array/#version-history","title":"Version history","text":"
        • Added in version 3.2.0.
        "},{"location":"api/json_sax/end_object/","title":"nlohmann::json_sax::end_object","text":"
        virtual bool end_object() = 0;\n

        The end of an object was read.

        "},{"location":"api/json_sax/end_object/#return-value","title":"Return value","text":"

        Whether parsing should proceed.

        "},{"location":"api/json_sax/end_object/#examples","title":"Examples","text":"Example

        The example below shows how the SAX interface is used.

        #include <iostream>\n#include <iomanip>\n#include <sstream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\n// a simple event consumer that collects string representations of the passed\n// values; note inheriting from json::json_sax_t is not required, but can\n// help not to forget a required function\nclass sax_event_consumer : public json::json_sax_t\n{\n  public:\n    std::vector<std::string> events;\n\n    bool null() override\n    {\n        events.push_back(\"null()\");\n        return true;\n    }\n\n    bool boolean(bool val) override\n    {\n        events.push_back(\"boolean(val=\" + std::string(val ? \"true\" : \"false\") + \")\");\n        return true;\n    }\n\n    bool number_integer(number_integer_t val) override\n    {\n        events.push_back(\"number_integer(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_unsigned(number_unsigned_t val) override\n    {\n        events.push_back(\"number_unsigned(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_float(number_float_t val, const string_t& s) override\n    {\n        events.push_back(\"number_float(val=\" + std::to_string(val) + \", s=\" + s + \")\");\n        return true;\n    }\n\n    bool string(string_t& val) override\n    {\n        events.push_back(\"string(val=\" + val + \")\");\n        return true;\n    }\n\n    bool start_object(std::size_t elements) override\n    {\n        events.push_back(\"start_object(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_object() override\n    {\n        events.push_back(\"end_object()\");\n        return true;\n    }\n\n    bool start_array(std::size_t elements) override\n    {\n        events.push_back(\"start_array(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_array() override\n    {\n        events.push_back(\"end_array()\");\n        return true;\n    }\n\n    bool key(string_t& val) override\n    {\n        events.push_back(\"key(val=\" + val + \")\");\n        return true;\n    }\n\n    bool binary(json::binary_t& val) override\n    {\n        events.push_back(\"binary(val=[...])\");\n        return true;\n    }\n\n    bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex) override\n    {\n        events.push_back(\"parse_error(position=\" + std::to_string(position) + \", last_token=\" + last_token + \",\\n            ex=\" + std::string(ex.what()) + \")\");\n        return false;\n    }\n};\n\nint main()\n{\n    // a JSON text\n    auto text = R\"(\n    {\n        \"Image\": {\n            \"Width\":  800,\n            \"Height\": 600,\n            \"Title\":  \"View from 15th Floor\",\n            \"Thumbnail\": {\n                \"Url\":    \"http://www.example.com/image/481989943\",\n                \"Height\": 125,\n                \"Width\":  100\n            },\n            \"Animated\" : false,\n            \"IDs\": [116, 943, 234, -38793],\n            \"DeletionDate\": null,\n            \"Distance\": 12.723374634\n        }\n    }]\n    )\";\n\n    // create a SAX event consumer object\n    sax_event_consumer sec;\n\n    // parse JSON\n    bool result = json::sax_parse(text, &sec);\n\n    // output the recorded events\n    for (auto& event : sec.events)\n    {\n        std::cout << event << \"\\n\";\n    }\n\n    // output the result of sax_parse\n    std::cout << \"\\nresult: \" << std::boolalpha << result << std::endl;\n}\n

        Output:

        start_object(elements=18446744073709551615)\nkey(val=Image)\nstart_object(elements=18446744073709551615)\nkey(val=Width)\nnumber_unsigned(val=800)\nkey(val=Height)\nnumber_unsigned(val=600)\nkey(val=Title)\nstring(val=View from 15th Floor)\nkey(val=Thumbnail)\nstart_object(elements=18446744073709551615)\nkey(val=Url)\nstring(val=http://www.example.com/image/481989943)\nkey(val=Height)\nnumber_unsigned(val=125)\nkey(val=Width)\nnumber_unsigned(val=100)\nend_object()\nkey(val=Animated)\nboolean(val=false)\nkey(val=IDs)\nstart_array(elements=18446744073709551615)\nnumber_unsigned(val=116)\nnumber_unsigned(val=943)\nnumber_unsigned(val=234)\nnumber_integer(val=-38793)\nend_array()\nkey(val=DeletionDate)\nnull()\nkey(val=Distance)\nnumber_float(val=12.723375, s=12.723374634)\nend_object()\nend_object()\nparse_error(position=460, last_token=12.723374634<U+000A>        }<U+000A>    }],\n            ex=[json.exception.parse_error.101] parse error at line 17, column 6: syntax error while parsing value - unexpected ']'; expected end of input)\n\nresult: false\n
        "},{"location":"api/json_sax/end_object/#see-also","title":"See also","text":"
        • start_object - the beginning of an object was read
        • key - an object key was read
        • sax_parse - SAX parser
        "},{"location":"api/json_sax/end_object/#version-history","title":"Version history","text":"
        • Added in version 3.2.0.
        "},{"location":"api/json_sax/key/","title":"nlohmann::json_sax::key","text":"
        virtual bool key(string_t& val) = 0;\n

        An object key was read.

        "},{"location":"api/json_sax/key/#parameters","title":"Parameters","text":"val (in) object key"},{"location":"api/json_sax/key/#return-value","title":"Return value","text":"

        Whether parsing should proceed.

        "},{"location":"api/json_sax/key/#notes","title":"Notes","text":"

        It is safe to move the passed object key value.

        "},{"location":"api/json_sax/key/#examples","title":"Examples","text":"Example

        The example below shows how the SAX interface is used.

        #include <iostream>\n#include <iomanip>\n#include <sstream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\n// a simple event consumer that collects string representations of the passed\n// values; note inheriting from json::json_sax_t is not required, but can\n// help not to forget a required function\nclass sax_event_consumer : public json::json_sax_t\n{\n  public:\n    std::vector<std::string> events;\n\n    bool null() override\n    {\n        events.push_back(\"null()\");\n        return true;\n    }\n\n    bool boolean(bool val) override\n    {\n        events.push_back(\"boolean(val=\" + std::string(val ? \"true\" : \"false\") + \")\");\n        return true;\n    }\n\n    bool number_integer(number_integer_t val) override\n    {\n        events.push_back(\"number_integer(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_unsigned(number_unsigned_t val) override\n    {\n        events.push_back(\"number_unsigned(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_float(number_float_t val, const string_t& s) override\n    {\n        events.push_back(\"number_float(val=\" + std::to_string(val) + \", s=\" + s + \")\");\n        return true;\n    }\n\n    bool string(string_t& val) override\n    {\n        events.push_back(\"string(val=\" + val + \")\");\n        return true;\n    }\n\n    bool start_object(std::size_t elements) override\n    {\n        events.push_back(\"start_object(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_object() override\n    {\n        events.push_back(\"end_object()\");\n        return true;\n    }\n\n    bool start_array(std::size_t elements) override\n    {\n        events.push_back(\"start_array(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_array() override\n    {\n        events.push_back(\"end_array()\");\n        return true;\n    }\n\n    bool key(string_t& val) override\n    {\n        events.push_back(\"key(val=\" + val + \")\");\n        return true;\n    }\n\n    bool binary(json::binary_t& val) override\n    {\n        events.push_back(\"binary(val=[...])\");\n        return true;\n    }\n\n    bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex) override\n    {\n        events.push_back(\"parse_error(position=\" + std::to_string(position) + \", last_token=\" + last_token + \",\\n            ex=\" + std::string(ex.what()) + \")\");\n        return false;\n    }\n};\n\nint main()\n{\n    // a JSON text\n    auto text = R\"(\n    {\n        \"Image\": {\n            \"Width\":  800,\n            \"Height\": 600,\n            \"Title\":  \"View from 15th Floor\",\n            \"Thumbnail\": {\n                \"Url\":    \"http://www.example.com/image/481989943\",\n                \"Height\": 125,\n                \"Width\":  100\n            },\n            \"Animated\" : false,\n            \"IDs\": [116, 943, 234, -38793],\n            \"DeletionDate\": null,\n            \"Distance\": 12.723374634\n        }\n    }]\n    )\";\n\n    // create a SAX event consumer object\n    sax_event_consumer sec;\n\n    // parse JSON\n    bool result = json::sax_parse(text, &sec);\n\n    // output the recorded events\n    for (auto& event : sec.events)\n    {\n        std::cout << event << \"\\n\";\n    }\n\n    // output the result of sax_parse\n    std::cout << \"\\nresult: \" << std::boolalpha << result << std::endl;\n}\n

        Output:

        start_object(elements=18446744073709551615)\nkey(val=Image)\nstart_object(elements=18446744073709551615)\nkey(val=Width)\nnumber_unsigned(val=800)\nkey(val=Height)\nnumber_unsigned(val=600)\nkey(val=Title)\nstring(val=View from 15th Floor)\nkey(val=Thumbnail)\nstart_object(elements=18446744073709551615)\nkey(val=Url)\nstring(val=http://www.example.com/image/481989943)\nkey(val=Height)\nnumber_unsigned(val=125)\nkey(val=Width)\nnumber_unsigned(val=100)\nend_object()\nkey(val=Animated)\nboolean(val=false)\nkey(val=IDs)\nstart_array(elements=18446744073709551615)\nnumber_unsigned(val=116)\nnumber_unsigned(val=943)\nnumber_unsigned(val=234)\nnumber_integer(val=-38793)\nend_array()\nkey(val=DeletionDate)\nnull()\nkey(val=Distance)\nnumber_float(val=12.723375, s=12.723374634)\nend_object()\nend_object()\nparse_error(position=460, last_token=12.723374634<U+000A>        }<U+000A>    }],\n            ex=[json.exception.parse_error.101] parse error at line 17, column 6: syntax error while parsing value - unexpected ']'; expected end of input)\n\nresult: false\n
        "},{"location":"api/json_sax/key/#see-also","title":"See also","text":"
        • start_object - the beginning of an object was read
        • end_object - the end of an object was read
        • sax_parse - SAX parser
        "},{"location":"api/json_sax/key/#version-history","title":"Version history","text":"
        • Added in version 3.2.0.
        "},{"location":"api/json_sax/null/","title":"nlohmann::json_sax::null","text":"
        virtual bool null() = 0;\n

        A null value was read.

        "},{"location":"api/json_sax/null/#return-value","title":"Return value","text":"

        Whether parsing should proceed.

        "},{"location":"api/json_sax/null/#examples","title":"Examples","text":"Example

        The example below shows how the SAX interface is used.

        #include <iostream>\n#include <iomanip>\n#include <sstream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\n// a simple event consumer that collects string representations of the passed\n// values; note inheriting from json::json_sax_t is not required, but can\n// help not to forget a required function\nclass sax_event_consumer : public json::json_sax_t\n{\n  public:\n    std::vector<std::string> events;\n\n    bool null() override\n    {\n        events.push_back(\"null()\");\n        return true;\n    }\n\n    bool boolean(bool val) override\n    {\n        events.push_back(\"boolean(val=\" + std::string(val ? \"true\" : \"false\") + \")\");\n        return true;\n    }\n\n    bool number_integer(number_integer_t val) override\n    {\n        events.push_back(\"number_integer(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_unsigned(number_unsigned_t val) override\n    {\n        events.push_back(\"number_unsigned(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_float(number_float_t val, const string_t& s) override\n    {\n        events.push_back(\"number_float(val=\" + std::to_string(val) + \", s=\" + s + \")\");\n        return true;\n    }\n\n    bool string(string_t& val) override\n    {\n        events.push_back(\"string(val=\" + val + \")\");\n        return true;\n    }\n\n    bool start_object(std::size_t elements) override\n    {\n        events.push_back(\"start_object(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_object() override\n    {\n        events.push_back(\"end_object()\");\n        return true;\n    }\n\n    bool start_array(std::size_t elements) override\n    {\n        events.push_back(\"start_array(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_array() override\n    {\n        events.push_back(\"end_array()\");\n        return true;\n    }\n\n    bool key(string_t& val) override\n    {\n        events.push_back(\"key(val=\" + val + \")\");\n        return true;\n    }\n\n    bool binary(json::binary_t& val) override\n    {\n        events.push_back(\"binary(val=[...])\");\n        return true;\n    }\n\n    bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex) override\n    {\n        events.push_back(\"parse_error(position=\" + std::to_string(position) + \", last_token=\" + last_token + \",\\n            ex=\" + std::string(ex.what()) + \")\");\n        return false;\n    }\n};\n\nint main()\n{\n    // a JSON text\n    auto text = R\"(\n    {\n        \"Image\": {\n            \"Width\":  800,\n            \"Height\": 600,\n            \"Title\":  \"View from 15th Floor\",\n            \"Thumbnail\": {\n                \"Url\":    \"http://www.example.com/image/481989943\",\n                \"Height\": 125,\n                \"Width\":  100\n            },\n            \"Animated\" : false,\n            \"IDs\": [116, 943, 234, -38793],\n            \"DeletionDate\": null,\n            \"Distance\": 12.723374634\n        }\n    }]\n    )\";\n\n    // create a SAX event consumer object\n    sax_event_consumer sec;\n\n    // parse JSON\n    bool result = json::sax_parse(text, &sec);\n\n    // output the recorded events\n    for (auto& event : sec.events)\n    {\n        std::cout << event << \"\\n\";\n    }\n\n    // output the result of sax_parse\n    std::cout << \"\\nresult: \" << std::boolalpha << result << std::endl;\n}\n

        Output:

        start_object(elements=18446744073709551615)\nkey(val=Image)\nstart_object(elements=18446744073709551615)\nkey(val=Width)\nnumber_unsigned(val=800)\nkey(val=Height)\nnumber_unsigned(val=600)\nkey(val=Title)\nstring(val=View from 15th Floor)\nkey(val=Thumbnail)\nstart_object(elements=18446744073709551615)\nkey(val=Url)\nstring(val=http://www.example.com/image/481989943)\nkey(val=Height)\nnumber_unsigned(val=125)\nkey(val=Width)\nnumber_unsigned(val=100)\nend_object()\nkey(val=Animated)\nboolean(val=false)\nkey(val=IDs)\nstart_array(elements=18446744073709551615)\nnumber_unsigned(val=116)\nnumber_unsigned(val=943)\nnumber_unsigned(val=234)\nnumber_integer(val=-38793)\nend_array()\nkey(val=DeletionDate)\nnull()\nkey(val=Distance)\nnumber_float(val=12.723375, s=12.723374634)\nend_object()\nend_object()\nparse_error(position=460, last_token=12.723374634<U+000A>        }<U+000A>    }],\n            ex=[json.exception.parse_error.101] parse error at line 17, column 6: syntax error while parsing value - unexpected ']'; expected end of input)\n\nresult: false\n
        "},{"location":"api/json_sax/null/#see-also","title":"See also","text":"
        • sax_parse - SAX parser
        • SAX Interface - the SAX interface article
        "},{"location":"api/json_sax/null/#version-history","title":"Version history","text":"
        • Added in version 3.2.0.
        "},{"location":"api/json_sax/number_float/","title":"nlohmann::json_sax::number_float","text":"
        virtual bool number_float(number_float_t val, const string_t& s) = 0;\n

        A floating-point number was read.

        "},{"location":"api/json_sax/number_float/#parameters","title":"Parameters","text":"val (in) floating-point value s (in) string representation of the original input"},{"location":"api/json_sax/number_float/#return-value","title":"Return value","text":"

        Whether parsing should proceed.

        "},{"location":"api/json_sax/number_float/#examples","title":"Examples","text":"Example

        The example below shows how the SAX interface is used.

        #include <iostream>\n#include <iomanip>\n#include <sstream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\n// a simple event consumer that collects string representations of the passed\n// values; note inheriting from json::json_sax_t is not required, but can\n// help not to forget a required function\nclass sax_event_consumer : public json::json_sax_t\n{\n  public:\n    std::vector<std::string> events;\n\n    bool null() override\n    {\n        events.push_back(\"null()\");\n        return true;\n    }\n\n    bool boolean(bool val) override\n    {\n        events.push_back(\"boolean(val=\" + std::string(val ? \"true\" : \"false\") + \")\");\n        return true;\n    }\n\n    bool number_integer(number_integer_t val) override\n    {\n        events.push_back(\"number_integer(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_unsigned(number_unsigned_t val) override\n    {\n        events.push_back(\"number_unsigned(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_float(number_float_t val, const string_t& s) override\n    {\n        events.push_back(\"number_float(val=\" + std::to_string(val) + \", s=\" + s + \")\");\n        return true;\n    }\n\n    bool string(string_t& val) override\n    {\n        events.push_back(\"string(val=\" + val + \")\");\n        return true;\n    }\n\n    bool start_object(std::size_t elements) override\n    {\n        events.push_back(\"start_object(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_object() override\n    {\n        events.push_back(\"end_object()\");\n        return true;\n    }\n\n    bool start_array(std::size_t elements) override\n    {\n        events.push_back(\"start_array(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_array() override\n    {\n        events.push_back(\"end_array()\");\n        return true;\n    }\n\n    bool key(string_t& val) override\n    {\n        events.push_back(\"key(val=\" + val + \")\");\n        return true;\n    }\n\n    bool binary(json::binary_t& val) override\n    {\n        events.push_back(\"binary(val=[...])\");\n        return true;\n    }\n\n    bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex) override\n    {\n        events.push_back(\"parse_error(position=\" + std::to_string(position) + \", last_token=\" + last_token + \",\\n            ex=\" + std::string(ex.what()) + \")\");\n        return false;\n    }\n};\n\nint main()\n{\n    // a JSON text\n    auto text = R\"(\n    {\n        \"Image\": {\n            \"Width\":  800,\n            \"Height\": 600,\n            \"Title\":  \"View from 15th Floor\",\n            \"Thumbnail\": {\n                \"Url\":    \"http://www.example.com/image/481989943\",\n                \"Height\": 125,\n                \"Width\":  100\n            },\n            \"Animated\" : false,\n            \"IDs\": [116, 943, 234, -38793],\n            \"DeletionDate\": null,\n            \"Distance\": 12.723374634\n        }\n    }]\n    )\";\n\n    // create a SAX event consumer object\n    sax_event_consumer sec;\n\n    // parse JSON\n    bool result = json::sax_parse(text, &sec);\n\n    // output the recorded events\n    for (auto& event : sec.events)\n    {\n        std::cout << event << \"\\n\";\n    }\n\n    // output the result of sax_parse\n    std::cout << \"\\nresult: \" << std::boolalpha << result << std::endl;\n}\n

        Output:

        start_object(elements=18446744073709551615)\nkey(val=Image)\nstart_object(elements=18446744073709551615)\nkey(val=Width)\nnumber_unsigned(val=800)\nkey(val=Height)\nnumber_unsigned(val=600)\nkey(val=Title)\nstring(val=View from 15th Floor)\nkey(val=Thumbnail)\nstart_object(elements=18446744073709551615)\nkey(val=Url)\nstring(val=http://www.example.com/image/481989943)\nkey(val=Height)\nnumber_unsigned(val=125)\nkey(val=Width)\nnumber_unsigned(val=100)\nend_object()\nkey(val=Animated)\nboolean(val=false)\nkey(val=IDs)\nstart_array(elements=18446744073709551615)\nnumber_unsigned(val=116)\nnumber_unsigned(val=943)\nnumber_unsigned(val=234)\nnumber_integer(val=-38793)\nend_array()\nkey(val=DeletionDate)\nnull()\nkey(val=Distance)\nnumber_float(val=12.723375, s=12.723374634)\nend_object()\nend_object()\nparse_error(position=460, last_token=12.723374634<U+000A>        }<U+000A>    }],\n            ex=[json.exception.parse_error.101] parse error at line 17, column 6: syntax error while parsing value - unexpected ']'; expected end of input)\n\nresult: false\n
        "},{"location":"api/json_sax/number_float/#see-also","title":"See also","text":"
        • number_integer - an integer number was read
        • number_unsigned - an unsigned integer number was read
        • sax_parse - SAX parser
        "},{"location":"api/json_sax/number_float/#version-history","title":"Version history","text":"
        • Added in version 3.2.0.
        "},{"location":"api/json_sax/number_integer/","title":"nlohmann::json_sax::number_integer","text":"
        virtual bool number_integer(number_integer_t val) = 0;\n

        An integer number was read.

        "},{"location":"api/json_sax/number_integer/#parameters","title":"Parameters","text":"val (in) integer value"},{"location":"api/json_sax/number_integer/#return-value","title":"Return value","text":"

        Whether parsing should proceed.

        "},{"location":"api/json_sax/number_integer/#examples","title":"Examples","text":"Example

        The example below shows how the SAX interface is used.

        #include <iostream>\n#include <iomanip>\n#include <sstream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\n// a simple event consumer that collects string representations of the passed\n// values; note inheriting from json::json_sax_t is not required, but can\n// help not to forget a required function\nclass sax_event_consumer : public json::json_sax_t\n{\n  public:\n    std::vector<std::string> events;\n\n    bool null() override\n    {\n        events.push_back(\"null()\");\n        return true;\n    }\n\n    bool boolean(bool val) override\n    {\n        events.push_back(\"boolean(val=\" + std::string(val ? \"true\" : \"false\") + \")\");\n        return true;\n    }\n\n    bool number_integer(number_integer_t val) override\n    {\n        events.push_back(\"number_integer(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_unsigned(number_unsigned_t val) override\n    {\n        events.push_back(\"number_unsigned(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_float(number_float_t val, const string_t& s) override\n    {\n        events.push_back(\"number_float(val=\" + std::to_string(val) + \", s=\" + s + \")\");\n        return true;\n    }\n\n    bool string(string_t& val) override\n    {\n        events.push_back(\"string(val=\" + val + \")\");\n        return true;\n    }\n\n    bool start_object(std::size_t elements) override\n    {\n        events.push_back(\"start_object(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_object() override\n    {\n        events.push_back(\"end_object()\");\n        return true;\n    }\n\n    bool start_array(std::size_t elements) override\n    {\n        events.push_back(\"start_array(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_array() override\n    {\n        events.push_back(\"end_array()\");\n        return true;\n    }\n\n    bool key(string_t& val) override\n    {\n        events.push_back(\"key(val=\" + val + \")\");\n        return true;\n    }\n\n    bool binary(json::binary_t& val) override\n    {\n        events.push_back(\"binary(val=[...])\");\n        return true;\n    }\n\n    bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex) override\n    {\n        events.push_back(\"parse_error(position=\" + std::to_string(position) + \", last_token=\" + last_token + \",\\n            ex=\" + std::string(ex.what()) + \")\");\n        return false;\n    }\n};\n\nint main()\n{\n    // a JSON text\n    auto text = R\"(\n    {\n        \"Image\": {\n            \"Width\":  800,\n            \"Height\": 600,\n            \"Title\":  \"View from 15th Floor\",\n            \"Thumbnail\": {\n                \"Url\":    \"http://www.example.com/image/481989943\",\n                \"Height\": 125,\n                \"Width\":  100\n            },\n            \"Animated\" : false,\n            \"IDs\": [116, 943, 234, -38793],\n            \"DeletionDate\": null,\n            \"Distance\": 12.723374634\n        }\n    }]\n    )\";\n\n    // create a SAX event consumer object\n    sax_event_consumer sec;\n\n    // parse JSON\n    bool result = json::sax_parse(text, &sec);\n\n    // output the recorded events\n    for (auto& event : sec.events)\n    {\n        std::cout << event << \"\\n\";\n    }\n\n    // output the result of sax_parse\n    std::cout << \"\\nresult: \" << std::boolalpha << result << std::endl;\n}\n

        Output:

        start_object(elements=18446744073709551615)\nkey(val=Image)\nstart_object(elements=18446744073709551615)\nkey(val=Width)\nnumber_unsigned(val=800)\nkey(val=Height)\nnumber_unsigned(val=600)\nkey(val=Title)\nstring(val=View from 15th Floor)\nkey(val=Thumbnail)\nstart_object(elements=18446744073709551615)\nkey(val=Url)\nstring(val=http://www.example.com/image/481989943)\nkey(val=Height)\nnumber_unsigned(val=125)\nkey(val=Width)\nnumber_unsigned(val=100)\nend_object()\nkey(val=Animated)\nboolean(val=false)\nkey(val=IDs)\nstart_array(elements=18446744073709551615)\nnumber_unsigned(val=116)\nnumber_unsigned(val=943)\nnumber_unsigned(val=234)\nnumber_integer(val=-38793)\nend_array()\nkey(val=DeletionDate)\nnull()\nkey(val=Distance)\nnumber_float(val=12.723375, s=12.723374634)\nend_object()\nend_object()\nparse_error(position=460, last_token=12.723374634<U+000A>        }<U+000A>    }],\n            ex=[json.exception.parse_error.101] parse error at line 17, column 6: syntax error while parsing value - unexpected ']'; expected end of input)\n\nresult: false\n
        "},{"location":"api/json_sax/number_integer/#see-also","title":"See also","text":"
        • number_unsigned - an unsigned integer number was read
        • number_float - a floating-point number was read
        • sax_parse - SAX parser
        "},{"location":"api/json_sax/number_integer/#version-history","title":"Version history","text":"
        • Added in version 3.2.0.
        "},{"location":"api/json_sax/number_unsigned/","title":"nlohmann::json_sax::number_unsigned","text":"
        virtual bool number_unsigned(number_unsigned_t val) = 0;\n

        An unsigned integer number was read.

        "},{"location":"api/json_sax/number_unsigned/#parameters","title":"Parameters","text":"val (in) unsigned integer value"},{"location":"api/json_sax/number_unsigned/#return-value","title":"Return value","text":"

        Whether parsing should proceed.

        "},{"location":"api/json_sax/number_unsigned/#examples","title":"Examples","text":"Example

        The example below shows how the SAX interface is used.

        #include <iostream>\n#include <iomanip>\n#include <sstream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\n// a simple event consumer that collects string representations of the passed\n// values; note inheriting from json::json_sax_t is not required, but can\n// help not to forget a required function\nclass sax_event_consumer : public json::json_sax_t\n{\n  public:\n    std::vector<std::string> events;\n\n    bool null() override\n    {\n        events.push_back(\"null()\");\n        return true;\n    }\n\n    bool boolean(bool val) override\n    {\n        events.push_back(\"boolean(val=\" + std::string(val ? \"true\" : \"false\") + \")\");\n        return true;\n    }\n\n    bool number_integer(number_integer_t val) override\n    {\n        events.push_back(\"number_integer(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_unsigned(number_unsigned_t val) override\n    {\n        events.push_back(\"number_unsigned(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_float(number_float_t val, const string_t& s) override\n    {\n        events.push_back(\"number_float(val=\" + std::to_string(val) + \", s=\" + s + \")\");\n        return true;\n    }\n\n    bool string(string_t& val) override\n    {\n        events.push_back(\"string(val=\" + val + \")\");\n        return true;\n    }\n\n    bool start_object(std::size_t elements) override\n    {\n        events.push_back(\"start_object(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_object() override\n    {\n        events.push_back(\"end_object()\");\n        return true;\n    }\n\n    bool start_array(std::size_t elements) override\n    {\n        events.push_back(\"start_array(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_array() override\n    {\n        events.push_back(\"end_array()\");\n        return true;\n    }\n\n    bool key(string_t& val) override\n    {\n        events.push_back(\"key(val=\" + val + \")\");\n        return true;\n    }\n\n    bool binary(json::binary_t& val) override\n    {\n        events.push_back(\"binary(val=[...])\");\n        return true;\n    }\n\n    bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex) override\n    {\n        events.push_back(\"parse_error(position=\" + std::to_string(position) + \", last_token=\" + last_token + \",\\n            ex=\" + std::string(ex.what()) + \")\");\n        return false;\n    }\n};\n\nint main()\n{\n    // a JSON text\n    auto text = R\"(\n    {\n        \"Image\": {\n            \"Width\":  800,\n            \"Height\": 600,\n            \"Title\":  \"View from 15th Floor\",\n            \"Thumbnail\": {\n                \"Url\":    \"http://www.example.com/image/481989943\",\n                \"Height\": 125,\n                \"Width\":  100\n            },\n            \"Animated\" : false,\n            \"IDs\": [116, 943, 234, -38793],\n            \"DeletionDate\": null,\n            \"Distance\": 12.723374634\n        }\n    }]\n    )\";\n\n    // create a SAX event consumer object\n    sax_event_consumer sec;\n\n    // parse JSON\n    bool result = json::sax_parse(text, &sec);\n\n    // output the recorded events\n    for (auto& event : sec.events)\n    {\n        std::cout << event << \"\\n\";\n    }\n\n    // output the result of sax_parse\n    std::cout << \"\\nresult: \" << std::boolalpha << result << std::endl;\n}\n

        Output:

        start_object(elements=18446744073709551615)\nkey(val=Image)\nstart_object(elements=18446744073709551615)\nkey(val=Width)\nnumber_unsigned(val=800)\nkey(val=Height)\nnumber_unsigned(val=600)\nkey(val=Title)\nstring(val=View from 15th Floor)\nkey(val=Thumbnail)\nstart_object(elements=18446744073709551615)\nkey(val=Url)\nstring(val=http://www.example.com/image/481989943)\nkey(val=Height)\nnumber_unsigned(val=125)\nkey(val=Width)\nnumber_unsigned(val=100)\nend_object()\nkey(val=Animated)\nboolean(val=false)\nkey(val=IDs)\nstart_array(elements=18446744073709551615)\nnumber_unsigned(val=116)\nnumber_unsigned(val=943)\nnumber_unsigned(val=234)\nnumber_integer(val=-38793)\nend_array()\nkey(val=DeletionDate)\nnull()\nkey(val=Distance)\nnumber_float(val=12.723375, s=12.723374634)\nend_object()\nend_object()\nparse_error(position=460, last_token=12.723374634<U+000A>        }<U+000A>    }],\n            ex=[json.exception.parse_error.101] parse error at line 17, column 6: syntax error while parsing value - unexpected ']'; expected end of input)\n\nresult: false\n
        "},{"location":"api/json_sax/number_unsigned/#see-also","title":"See also","text":"
        • number_integer - an integer number was read
        • number_float - a floating-point number was read
        • sax_parse - SAX parser
        "},{"location":"api/json_sax/number_unsigned/#version-history","title":"Version history","text":"
        • Added in version 3.2.0.
        "},{"location":"api/json_sax/parse_error/","title":"nlohmann::json_sax::parse_error","text":"
        virtual bool parse_error(std::size_t position,\n                         const std::string& last_token,\n                         const detail::exception& ex) = 0;\n

        A parse error occurred.

        "},{"location":"api/json_sax/parse_error/#parameters","title":"Parameters","text":"position (in) the position in the input where the error occurs last_token (in) the last read token ex (in) an exception object describing the error"},{"location":"api/json_sax/parse_error/#return-value","title":"Return value","text":"

        Whether parsing should proceed (must return false).

        "},{"location":"api/json_sax/parse_error/#examples","title":"Examples","text":"Example

        The example below shows how the SAX interface is used.

        #include <iostream>\n#include <iomanip>\n#include <sstream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\n// a simple event consumer that collects string representations of the passed\n// values; note inheriting from json::json_sax_t is not required, but can\n// help not to forget a required function\nclass sax_event_consumer : public json::json_sax_t\n{\n  public:\n    std::vector<std::string> events;\n\n    bool null() override\n    {\n        events.push_back(\"null()\");\n        return true;\n    }\n\n    bool boolean(bool val) override\n    {\n        events.push_back(\"boolean(val=\" + std::string(val ? \"true\" : \"false\") + \")\");\n        return true;\n    }\n\n    bool number_integer(number_integer_t val) override\n    {\n        events.push_back(\"number_integer(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_unsigned(number_unsigned_t val) override\n    {\n        events.push_back(\"number_unsigned(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_float(number_float_t val, const string_t& s) override\n    {\n        events.push_back(\"number_float(val=\" + std::to_string(val) + \", s=\" + s + \")\");\n        return true;\n    }\n\n    bool string(string_t& val) override\n    {\n        events.push_back(\"string(val=\" + val + \")\");\n        return true;\n    }\n\n    bool start_object(std::size_t elements) override\n    {\n        events.push_back(\"start_object(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_object() override\n    {\n        events.push_back(\"end_object()\");\n        return true;\n    }\n\n    bool start_array(std::size_t elements) override\n    {\n        events.push_back(\"start_array(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_array() override\n    {\n        events.push_back(\"end_array()\");\n        return true;\n    }\n\n    bool key(string_t& val) override\n    {\n        events.push_back(\"key(val=\" + val + \")\");\n        return true;\n    }\n\n    bool binary(json::binary_t& val) override\n    {\n        events.push_back(\"binary(val=[...])\");\n        return true;\n    }\n\n    bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex) override\n    {\n        events.push_back(\"parse_error(position=\" + std::to_string(position) + \", last_token=\" + last_token + \",\\n            ex=\" + std::string(ex.what()) + \")\");\n        return false;\n    }\n};\n\nint main()\n{\n    // a JSON text\n    auto text = R\"(\n    {\n        \"Image\": {\n            \"Width\":  800,\n            \"Height\": 600,\n            \"Title\":  \"View from 15th Floor\",\n            \"Thumbnail\": {\n                \"Url\":    \"http://www.example.com/image/481989943\",\n                \"Height\": 125,\n                \"Width\":  100\n            },\n            \"Animated\" : false,\n            \"IDs\": [116, 943, 234, -38793],\n            \"DeletionDate\": null,\n            \"Distance\": 12.723374634\n        }\n    }]\n    )\";\n\n    // create a SAX event consumer object\n    sax_event_consumer sec;\n\n    // parse JSON\n    bool result = json::sax_parse(text, &sec);\n\n    // output the recorded events\n    for (auto& event : sec.events)\n    {\n        std::cout << event << \"\\n\";\n    }\n\n    // output the result of sax_parse\n    std::cout << \"\\nresult: \" << std::boolalpha << result << std::endl;\n}\n

        Output:

        start_object(elements=18446744073709551615)\nkey(val=Image)\nstart_object(elements=18446744073709551615)\nkey(val=Width)\nnumber_unsigned(val=800)\nkey(val=Height)\nnumber_unsigned(val=600)\nkey(val=Title)\nstring(val=View from 15th Floor)\nkey(val=Thumbnail)\nstart_object(elements=18446744073709551615)\nkey(val=Url)\nstring(val=http://www.example.com/image/481989943)\nkey(val=Height)\nnumber_unsigned(val=125)\nkey(val=Width)\nnumber_unsigned(val=100)\nend_object()\nkey(val=Animated)\nboolean(val=false)\nkey(val=IDs)\nstart_array(elements=18446744073709551615)\nnumber_unsigned(val=116)\nnumber_unsigned(val=943)\nnumber_unsigned(val=234)\nnumber_integer(val=-38793)\nend_array()\nkey(val=DeletionDate)\nnull()\nkey(val=Distance)\nnumber_float(val=12.723375, s=12.723374634)\nend_object()\nend_object()\nparse_error(position=460, last_token=12.723374634<U+000A>        }<U+000A>    }],\n            ex=[json.exception.parse_error.101] parse error at line 17, column 6: syntax error while parsing value - unexpected ']'; expected end of input)\n\nresult: false\n
        "},{"location":"api/json_sax/parse_error/#see-also","title":"See also","text":"
        • sax_parse - SAX parser
        • Parsing and Exceptions - the article on handling parse errors without exceptions
        "},{"location":"api/json_sax/parse_error/#version-history","title":"Version history","text":"
        • Added in version 3.2.0.
        "},{"location":"api/json_sax/start_array/","title":"nlohmann::json_sax::start_array","text":"
        virtual bool start_array(std::size_t elements) = 0;\n

        The beginning of an array was read.

        "},{"location":"api/json_sax/start_array/#parameters","title":"Parameters","text":"elements (in) number of array elements, or std::numeric_limits<std::size_t>::max() if unknown"},{"location":"api/json_sax/start_array/#return-value","title":"Return value","text":"

        Whether parsing should proceed.

        "},{"location":"api/json_sax/start_array/#notes","title":"Notes","text":"

        Binary formats may report the number of elements.

        "},{"location":"api/json_sax/start_array/#examples","title":"Examples","text":"Example

        The example below shows how the SAX interface is used.

        #include <iostream>\n#include <iomanip>\n#include <sstream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\n// a simple event consumer that collects string representations of the passed\n// values; note inheriting from json::json_sax_t is not required, but can\n// help not to forget a required function\nclass sax_event_consumer : public json::json_sax_t\n{\n  public:\n    std::vector<std::string> events;\n\n    bool null() override\n    {\n        events.push_back(\"null()\");\n        return true;\n    }\n\n    bool boolean(bool val) override\n    {\n        events.push_back(\"boolean(val=\" + std::string(val ? \"true\" : \"false\") + \")\");\n        return true;\n    }\n\n    bool number_integer(number_integer_t val) override\n    {\n        events.push_back(\"number_integer(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_unsigned(number_unsigned_t val) override\n    {\n        events.push_back(\"number_unsigned(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_float(number_float_t val, const string_t& s) override\n    {\n        events.push_back(\"number_float(val=\" + std::to_string(val) + \", s=\" + s + \")\");\n        return true;\n    }\n\n    bool string(string_t& val) override\n    {\n        events.push_back(\"string(val=\" + val + \")\");\n        return true;\n    }\n\n    bool start_object(std::size_t elements) override\n    {\n        events.push_back(\"start_object(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_object() override\n    {\n        events.push_back(\"end_object()\");\n        return true;\n    }\n\n    bool start_array(std::size_t elements) override\n    {\n        events.push_back(\"start_array(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_array() override\n    {\n        events.push_back(\"end_array()\");\n        return true;\n    }\n\n    bool key(string_t& val) override\n    {\n        events.push_back(\"key(val=\" + val + \")\");\n        return true;\n    }\n\n    bool binary(json::binary_t& val) override\n    {\n        events.push_back(\"binary(val=[...])\");\n        return true;\n    }\n\n    bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex) override\n    {\n        events.push_back(\"parse_error(position=\" + std::to_string(position) + \", last_token=\" + last_token + \",\\n            ex=\" + std::string(ex.what()) + \")\");\n        return false;\n    }\n};\n\nint main()\n{\n    // a JSON text\n    auto text = R\"(\n    {\n        \"Image\": {\n            \"Width\":  800,\n            \"Height\": 600,\n            \"Title\":  \"View from 15th Floor\",\n            \"Thumbnail\": {\n                \"Url\":    \"http://www.example.com/image/481989943\",\n                \"Height\": 125,\n                \"Width\":  100\n            },\n            \"Animated\" : false,\n            \"IDs\": [116, 943, 234, -38793],\n            \"DeletionDate\": null,\n            \"Distance\": 12.723374634\n        }\n    }]\n    )\";\n\n    // create a SAX event consumer object\n    sax_event_consumer sec;\n\n    // parse JSON\n    bool result = json::sax_parse(text, &sec);\n\n    // output the recorded events\n    for (auto& event : sec.events)\n    {\n        std::cout << event << \"\\n\";\n    }\n\n    // output the result of sax_parse\n    std::cout << \"\\nresult: \" << std::boolalpha << result << std::endl;\n}\n

        Output:

        start_object(elements=18446744073709551615)\nkey(val=Image)\nstart_object(elements=18446744073709551615)\nkey(val=Width)\nnumber_unsigned(val=800)\nkey(val=Height)\nnumber_unsigned(val=600)\nkey(val=Title)\nstring(val=View from 15th Floor)\nkey(val=Thumbnail)\nstart_object(elements=18446744073709551615)\nkey(val=Url)\nstring(val=http://www.example.com/image/481989943)\nkey(val=Height)\nnumber_unsigned(val=125)\nkey(val=Width)\nnumber_unsigned(val=100)\nend_object()\nkey(val=Animated)\nboolean(val=false)\nkey(val=IDs)\nstart_array(elements=18446744073709551615)\nnumber_unsigned(val=116)\nnumber_unsigned(val=943)\nnumber_unsigned(val=234)\nnumber_integer(val=-38793)\nend_array()\nkey(val=DeletionDate)\nnull()\nkey(val=Distance)\nnumber_float(val=12.723375, s=12.723374634)\nend_object()\nend_object()\nparse_error(position=460, last_token=12.723374634<U+000A>        }<U+000A>    }],\n            ex=[json.exception.parse_error.101] parse error at line 17, column 6: syntax error while parsing value - unexpected ']'; expected end of input)\n\nresult: false\n
        "},{"location":"api/json_sax/start_array/#see-also","title":"See also","text":"
        • end_array - the end of an array was read
        • sax_parse - SAX parser
        "},{"location":"api/json_sax/start_array/#version-history","title":"Version history","text":"
        • Added in version 3.2.0.
        "},{"location":"api/json_sax/start_object/","title":"nlohmann::json_sax::start_object","text":"
        virtual bool start_object(std::size_t elements) = 0;\n

        The beginning of an object was read.

        "},{"location":"api/json_sax/start_object/#parameters","title":"Parameters","text":"elements (in) number of object elements, or std::numeric_limits<std::size_t>::max() if unknown"},{"location":"api/json_sax/start_object/#return-value","title":"Return value","text":"

        Whether parsing should proceed.

        "},{"location":"api/json_sax/start_object/#notes","title":"Notes","text":"

        Binary formats may report the number of elements.

        "},{"location":"api/json_sax/start_object/#examples","title":"Examples","text":"Example

        The example below shows how the SAX interface is used.

        #include <iostream>\n#include <iomanip>\n#include <sstream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\n// a simple event consumer that collects string representations of the passed\n// values; note inheriting from json::json_sax_t is not required, but can\n// help not to forget a required function\nclass sax_event_consumer : public json::json_sax_t\n{\n  public:\n    std::vector<std::string> events;\n\n    bool null() override\n    {\n        events.push_back(\"null()\");\n        return true;\n    }\n\n    bool boolean(bool val) override\n    {\n        events.push_back(\"boolean(val=\" + std::string(val ? \"true\" : \"false\") + \")\");\n        return true;\n    }\n\n    bool number_integer(number_integer_t val) override\n    {\n        events.push_back(\"number_integer(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_unsigned(number_unsigned_t val) override\n    {\n        events.push_back(\"number_unsigned(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_float(number_float_t val, const string_t& s) override\n    {\n        events.push_back(\"number_float(val=\" + std::to_string(val) + \", s=\" + s + \")\");\n        return true;\n    }\n\n    bool string(string_t& val) override\n    {\n        events.push_back(\"string(val=\" + val + \")\");\n        return true;\n    }\n\n    bool start_object(std::size_t elements) override\n    {\n        events.push_back(\"start_object(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_object() override\n    {\n        events.push_back(\"end_object()\");\n        return true;\n    }\n\n    bool start_array(std::size_t elements) override\n    {\n        events.push_back(\"start_array(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_array() override\n    {\n        events.push_back(\"end_array()\");\n        return true;\n    }\n\n    bool key(string_t& val) override\n    {\n        events.push_back(\"key(val=\" + val + \")\");\n        return true;\n    }\n\n    bool binary(json::binary_t& val) override\n    {\n        events.push_back(\"binary(val=[...])\");\n        return true;\n    }\n\n    bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex) override\n    {\n        events.push_back(\"parse_error(position=\" + std::to_string(position) + \", last_token=\" + last_token + \",\\n            ex=\" + std::string(ex.what()) + \")\");\n        return false;\n    }\n};\n\nint main()\n{\n    // a JSON text\n    auto text = R\"(\n    {\n        \"Image\": {\n            \"Width\":  800,\n            \"Height\": 600,\n            \"Title\":  \"View from 15th Floor\",\n            \"Thumbnail\": {\n                \"Url\":    \"http://www.example.com/image/481989943\",\n                \"Height\": 125,\n                \"Width\":  100\n            },\n            \"Animated\" : false,\n            \"IDs\": [116, 943, 234, -38793],\n            \"DeletionDate\": null,\n            \"Distance\": 12.723374634\n        }\n    }]\n    )\";\n\n    // create a SAX event consumer object\n    sax_event_consumer sec;\n\n    // parse JSON\n    bool result = json::sax_parse(text, &sec);\n\n    // output the recorded events\n    for (auto& event : sec.events)\n    {\n        std::cout << event << \"\\n\";\n    }\n\n    // output the result of sax_parse\n    std::cout << \"\\nresult: \" << std::boolalpha << result << std::endl;\n}\n

        Output:

        start_object(elements=18446744073709551615)\nkey(val=Image)\nstart_object(elements=18446744073709551615)\nkey(val=Width)\nnumber_unsigned(val=800)\nkey(val=Height)\nnumber_unsigned(val=600)\nkey(val=Title)\nstring(val=View from 15th Floor)\nkey(val=Thumbnail)\nstart_object(elements=18446744073709551615)\nkey(val=Url)\nstring(val=http://www.example.com/image/481989943)\nkey(val=Height)\nnumber_unsigned(val=125)\nkey(val=Width)\nnumber_unsigned(val=100)\nend_object()\nkey(val=Animated)\nboolean(val=false)\nkey(val=IDs)\nstart_array(elements=18446744073709551615)\nnumber_unsigned(val=116)\nnumber_unsigned(val=943)\nnumber_unsigned(val=234)\nnumber_integer(val=-38793)\nend_array()\nkey(val=DeletionDate)\nnull()\nkey(val=Distance)\nnumber_float(val=12.723375, s=12.723374634)\nend_object()\nend_object()\nparse_error(position=460, last_token=12.723374634<U+000A>        }<U+000A>    }],\n            ex=[json.exception.parse_error.101] parse error at line 17, column 6: syntax error while parsing value - unexpected ']'; expected end of input)\n\nresult: false\n
        "},{"location":"api/json_sax/start_object/#see-also","title":"See also","text":"
        • end_object - the end of an object was read
        • key - an object key was read
        • sax_parse - SAX parser
        "},{"location":"api/json_sax/start_object/#version-history","title":"Version history","text":"
        • Added in version 3.2.0.
        "},{"location":"api/json_sax/string/","title":"nlohmann::json_sax::string","text":"
        virtual bool string(string_t& val) = 0;\n

        A string value was read.

        "},{"location":"api/json_sax/string/#parameters","title":"Parameters","text":"val (in) string value"},{"location":"api/json_sax/string/#return-value","title":"Return value","text":"

        Whether parsing should proceed.

        "},{"location":"api/json_sax/string/#notes","title":"Notes","text":"

        It is safe to move the passed string value.

        "},{"location":"api/json_sax/string/#examples","title":"Examples","text":"Example

        The example below shows how the SAX interface is used.

        #include <iostream>\n#include <iomanip>\n#include <sstream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\n// a simple event consumer that collects string representations of the passed\n// values; note inheriting from json::json_sax_t is not required, but can\n// help not to forget a required function\nclass sax_event_consumer : public json::json_sax_t\n{\n  public:\n    std::vector<std::string> events;\n\n    bool null() override\n    {\n        events.push_back(\"null()\");\n        return true;\n    }\n\n    bool boolean(bool val) override\n    {\n        events.push_back(\"boolean(val=\" + std::string(val ? \"true\" : \"false\") + \")\");\n        return true;\n    }\n\n    bool number_integer(number_integer_t val) override\n    {\n        events.push_back(\"number_integer(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_unsigned(number_unsigned_t val) override\n    {\n        events.push_back(\"number_unsigned(val=\" + std::to_string(val) + \")\");\n        return true;\n    }\n\n    bool number_float(number_float_t val, const string_t& s) override\n    {\n        events.push_back(\"number_float(val=\" + std::to_string(val) + \", s=\" + s + \")\");\n        return true;\n    }\n\n    bool string(string_t& val) override\n    {\n        events.push_back(\"string(val=\" + val + \")\");\n        return true;\n    }\n\n    bool start_object(std::size_t elements) override\n    {\n        events.push_back(\"start_object(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_object() override\n    {\n        events.push_back(\"end_object()\");\n        return true;\n    }\n\n    bool start_array(std::size_t elements) override\n    {\n        events.push_back(\"start_array(elements=\" + std::to_string(elements) + \")\");\n        return true;\n    }\n\n    bool end_array() override\n    {\n        events.push_back(\"end_array()\");\n        return true;\n    }\n\n    bool key(string_t& val) override\n    {\n        events.push_back(\"key(val=\" + val + \")\");\n        return true;\n    }\n\n    bool binary(json::binary_t& val) override\n    {\n        events.push_back(\"binary(val=[...])\");\n        return true;\n    }\n\n    bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex) override\n    {\n        events.push_back(\"parse_error(position=\" + std::to_string(position) + \", last_token=\" + last_token + \",\\n            ex=\" + std::string(ex.what()) + \")\");\n        return false;\n    }\n};\n\nint main()\n{\n    // a JSON text\n    auto text = R\"(\n    {\n        \"Image\": {\n            \"Width\":  800,\n            \"Height\": 600,\n            \"Title\":  \"View from 15th Floor\",\n            \"Thumbnail\": {\n                \"Url\":    \"http://www.example.com/image/481989943\",\n                \"Height\": 125,\n                \"Width\":  100\n            },\n            \"Animated\" : false,\n            \"IDs\": [116, 943, 234, -38793],\n            \"DeletionDate\": null,\n            \"Distance\": 12.723374634\n        }\n    }]\n    )\";\n\n    // create a SAX event consumer object\n    sax_event_consumer sec;\n\n    // parse JSON\n    bool result = json::sax_parse(text, &sec);\n\n    // output the recorded events\n    for (auto& event : sec.events)\n    {\n        std::cout << event << \"\\n\";\n    }\n\n    // output the result of sax_parse\n    std::cout << \"\\nresult: \" << std::boolalpha << result << std::endl;\n}\n

        Output:

        start_object(elements=18446744073709551615)\nkey(val=Image)\nstart_object(elements=18446744073709551615)\nkey(val=Width)\nnumber_unsigned(val=800)\nkey(val=Height)\nnumber_unsigned(val=600)\nkey(val=Title)\nstring(val=View from 15th Floor)\nkey(val=Thumbnail)\nstart_object(elements=18446744073709551615)\nkey(val=Url)\nstring(val=http://www.example.com/image/481989943)\nkey(val=Height)\nnumber_unsigned(val=125)\nkey(val=Width)\nnumber_unsigned(val=100)\nend_object()\nkey(val=Animated)\nboolean(val=false)\nkey(val=IDs)\nstart_array(elements=18446744073709551615)\nnumber_unsigned(val=116)\nnumber_unsigned(val=943)\nnumber_unsigned(val=234)\nnumber_integer(val=-38793)\nend_array()\nkey(val=DeletionDate)\nnull()\nkey(val=Distance)\nnumber_float(val=12.723375, s=12.723374634)\nend_object()\nend_object()\nparse_error(position=460, last_token=12.723374634<U+000A>        }<U+000A>    }],\n            ex=[json.exception.parse_error.101] parse error at line 17, column 6: syntax error while parsing value - unexpected ']'; expected end of input)\n\nresult: false\n
        "},{"location":"api/json_sax/string/#see-also","title":"See also","text":"
        • sax_parse - SAX parser
        • SAX Interface - the SAX interface article
        "},{"location":"api/json_sax/string/#version-history","title":"Version history","text":"
        • Added in version 3.2.0.
        "},{"location":"api/macros/","title":"Macros","text":"

        Some aspects of the library can be configured by defining preprocessor macros before including the json.hpp header. See also the macro overview page.

        "},{"location":"api/macros/#runtime-assertions","title":"Runtime assertions","text":"
        • JSON_ASSERT(x) - control behavior of runtime assertions
        "},{"location":"api/macros/#exceptions","title":"Exceptions","text":"
        • JSON_CATCH_USER(exception)JSON_THROW_USER(exception)JSON_TRY_USER - control exceptions
        • JSON_DIAGNOSTICS - control extended diagnostics
        • JSON_DIAGNOSTIC_POSITIONS - access positions of elements
        • JSON_NOEXCEPTION - switch off exceptions
        "},{"location":"api/macros/#parsing","title":"Parsing","text":"
        • JSON_DISABLE_STREAM_BULK_SCAN - read a std::istream one character at a time instead of scanning its stream buffer in bulk
        • JSON_DISABLE_UNLOCKED_FILE_IO - read a FILE* with std::fgetc instead of locking it once and using the unlocked stdio functions
        • JSON_PRECISE_STREAM_POSITION - opt in to leaving an input stream positioned right after a parsed number
        • JSON_STRICT_BINARY_UTF8 - opt in to checking strings for valid UTF-8 in the CBOR, UBJSON, BJData, and BSON writers
        • JSON_STRICT_NUL_HANDLING - opt in to rejecting a NUL byte in the input instead of treating it as end of input
        "},{"location":"api/macros/#language-support","title":"Language support","text":"
        • JSON_HAS_CPP_11JSON_HAS_CPP_14JSON_HAS_CPP_17JSON_HAS_CPP_20 - set supported C++ standard
        • JSON_HAS_FILESYSTEMJSON_HAS_EXPERIMENTAL_FILESYSTEM - control std::filesystem support
        • JSON_HAS_RANGE_VIEW_CONVERSION - control construction from std::ranges views
        • JSON_HAS_RANGES - control std::ranges support
        • JSON_HAS_STATIC_RTTI - control RTTI (run time type information) support
        • JSON_HAS_STD_FORMAT - control std::format/std::formatter support
        • JSON_HAS_THREE_WAY_COMPARISON - control 3-way comparison support
        • JSON_NO_AUTOMATIC_UDLS - do not include the user-defined string literals (UDLs) automatically
        • JSON_NO_IO - switch off functions relying on certain C++ I/O headers
        • JSON_NO_THREAD_LOCAL - switch off the use of thread_local storage
        • JSON_SKIP_UNSUPPORTED_COMPILER_CHECK - do not warn about unsupported compilers
        • JSON_USE_GLOBAL_UDLS - place user-defined string literals (UDLs) into the global namespace
        • JSON_USE_SIMDUTF - use the simdutf library to accelerate UTF-8 validation
        • JSON_VIEW_NO_SIMD - use only portable code in the parser of json_view.hpp
        • JSON_VIEW_USE_SSSE3 - validate non-ASCII strings with SSSE3 in the parser of json_view.hpp
        "},{"location":"api/macros/#library-version","title":"Library version","text":"
        • JSON_SKIP_LIBRARY_VERSION_CHECK - skip library version check
        • NLOHMANN_JSON_VERSION_MAJORNLOHMANN_JSON_VERSION_MINORNLOHMANN_JSON_VERSION_PATCH - library version information
        "},{"location":"api/macros/#library-namespace","title":"Library namespace","text":"
        • NLOHMANN_JSON_NAMESPACE - full name of the nlohmann namespace
        • NLOHMANN_JSON_NAMESPACE_BEGINNLOHMANN_JSON_NAMESPACE_END - open and close the library namespace
        • NLOHMANN_JSON_NAMESPACE_NO_VERSION - disable the version component of the inline namespace
        "},{"location":"api/macros/#type-conversions","title":"Type conversions","text":"
        • JSON_BRACE_INIT_COPY_SEMANTICS - opt in to copy/move semantics for single-element brace initialization
        • JSON_DISABLE_ENUM_SERIALIZATION - switch off default serialization/deserialization functions for enums
        • JSON_DISABLE_TUPLE_REFERENCE_CONVERSION - switch off conversion from a one-element tuple of a JSON reference
        • JSON_USE_IMPLICIT_CONVERSIONS - control implicit conversions
        • JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS - opt in to storing maps with enum keys as objects
        "},{"location":"api/macros/#deprecated-functions","title":"Deprecated functions","text":"
        • JSON_DELETE_DEPRECATED_FUNCTIONS - opt in to deleting the deprecated functions ahead of their removal in version 4.0.0
        "},{"location":"api/macros/#comparison-behavior","title":"Comparison behavior","text":"
        • JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON - control comparison of discarded values
        "},{"location":"api/macros/#serializationdeserialization-macros","title":"Serialization/deserialization macros","text":""},{"location":"api/macros/#enums","title":"Enums","text":"
        • NLOHMANN_JSON_SERIALIZE_ENUM - serialize/deserialize an enum
        • NLOHMANN_JSON_SERIALIZE_ENUM_STRICT - serialize/deserialize an enum with exceptions
        "},{"location":"api/macros/#classes-and-structs","title":"Classes and structs","text":"
        • NLOHMANN_DEFINE_TYPE_INTRUSIVE - serialize/deserialize a non-derived class with private members
        • NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT - serialize/deserialize a non-derived class with private members; uses default values
        • NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE - serialize a non-derived class with private members
        • NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE - serialize/deserialize a non-derived class
        • NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT - serialize/deserialize a non-derived class; uses default values

        • NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE - serialize a non-derived class

        • NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE - serialize/deserialize a derived class with private members

        • NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_DEFAULT - serialize/deserialize a derived class with private members; uses default values
        • NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_ONLY_SERIALIZE - serialize a derived class with private members
        • NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE - serialize/deserialize a derived class
        • NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_DEFAULT - serialize/deserialize a derived class; uses default values
        • NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE - serialize a derived class
        • NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_NAMES - serialize/deserialize a non-derived class with private members; uses custom names
        • NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT_WITH_NAMES - serialize/deserialize a non-derived class with private members; uses default values; uses custom names
        • NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE_WITH_NAMES - serialize a non-derived class with private members; uses custom names
        • NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_NAMES - serialize/deserialize a non-derived class; uses custom names
        • NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT_WITH_NAMES - serialize/deserialize a non-derived class; uses default values; uses custom names

        • NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE_WITH_NAMES - serialize a non-derived class; uses custom names

        • NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_NAMES - serialize/deserialize a derived class with private members; uses custom names

        • NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_DEFAULT_WITH_NAMES - serialize/deserialize a derived class with private members; uses default values; uses custom names
        • NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_ONLY_SERIALIZE_WITH_NAMES - serialize a derived class with private members; uses custom names
        • NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_NAMES - serialize/deserialize a derived class; uses custom names
        • NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_DEFAULT_WITH_NAMES - serialize/deserialize a derived class; uses default values; uses custom names
        • NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE_WITH_NAMES - serialize a derived class; uses custom names
        "},{"location":"api/macros/json_assert/","title":"JSON_ASSERT","text":"
        #define JSON_ASSERT(x) /* value */\n

        This macro controls which code is executed for runtime assertions of the library.

        "},{"location":"api/macros/json_assert/#parameters","title":"Parameters","text":"x (in) expression of a scalar type"},{"location":"api/macros/json_assert/#default-definition","title":"Default definition","text":"

        The default value is assert(x).

        #define JSON_ASSERT(x) assert(x)\n

        Therefore, assertions can be switched off by defining NDEBUG.

        "},{"location":"api/macros/json_assert/#notes","title":"Notes","text":"
        • The library uses numerous assertions to guarantee invariants and to abort in case of otherwise undefined behavior (e.g., when calling operator[] with a missing object key on a const object). See page runtime assertions for more information.
        • Defining the macro to code that does not call std::abort may leave the library in an undefined state.
        • The macro is undefined outside the library.
        "},{"location":"api/macros/json_assert/#examples","title":"Examples","text":"Example: default behavior

        The following code will trigger an assertion at runtime:

        #include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    const json j = {{\"key\", \"value\"}};\n    auto v = j[\"missing\"];\n}\n

        Output:

        Assertion failed: (m_value.object->find(key) != m_value.object->end()), function operator[], file json.hpp, line 2144.\n
        Example: user-defined behavior

        The assertion reporting can be changed by defining JSON_ASSERT(x) differently.

        #include <cstdio>\n#include <cstdlib>\n#define JSON_ASSERT(x) if(!(x)){fprintf(stderr, \"assertion error in %s\\n\", __FUNCTION__); std::abort();}\n\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    const json j = {{\"key\", \"value\"}};\n    auto v = j[\"missing\"];\n}\n

        Output:

        assertion error in operator[]\n
        "},{"location":"api/macros/json_assert/#see-also","title":"See also","text":"
        • Runtime Assertions - overview documentation
        "},{"location":"api/macros/json_assert/#version-history","title":"Version history","text":"
        • Added in version 3.9.0.
        "},{"location":"api/macros/json_brace_init_copy_semantics/","title":"JSON_BRACE_INIT_COPY_SEMANTICS","text":"
        #define JSON_BRACE_INIT_COPY_SEMANTICS /* value */\n

        When defined to 1, single-element brace initialization of a basic_json value is treated as a copy/move of the element rather than wrapping it in a single-element array.

        "},{"location":"api/macros/json_brace_init_copy_semantics/#default-definition","title":"Default definition","text":"

        The default value is 0 (disabled \u2014 existing behavior is preserved).

        #define JSON_BRACE_INIT_COPY_SEMANTICS 0\n
        "},{"location":"api/macros/json_brace_init_copy_semantics/#notes","title":"Notes","text":"

        Background

        C++ always prefers the initializer_list constructor over the copy/move constructor for brace initialization. This means that code like

        json obj = {{\"key\", \"value\"}};\njson j{obj};\n

        creates a single-element array [{\"key\":\"value\"}] instead of a copy of obj. This behavior is compiler-dependent for older compilers (GCC wrapped, Clang did not), but starting from Clang 20, both compilers behave the same way.

        Enabling this macro opts into copy/move semantics for this case (see #5074).

        Opt-in only

        This macro must be defined before including <nlohmann/json.hpp>. Defining it after the include has no effect.

        Applies to every single-element list

        The macro does not only affect a single JSON value in braces. Any single-element braced list is treated as its element, so it no longer creates a one-element array:

        json j1 = {1};       // 1, not [1]\njson j2 = {\"text\"};  // \"text\", not [\"text\"]\njson j3 = {{1, 2}};  // [1,2], not [[1,2]]\n

        Code that relies on these producing arrays must use json::array() instead (see below). Lists with more than one element, and a single [string, value] pair written as a braced list, such as {{\"key\", \"value\"}}, which still creates an object, are not affected. This exception is based on how the pair is written, not on the shape of its value: an existing JSON value that happens to be a two-element array with a string as its first element, such as json arr = {\"key\", 42};, is still copied by json j{arr}; rather than turned into an object. The library's own conversions are not affected either: for example, std::tuple<int>{5} still becomes [5].

        ABI compatibility

        The value of this macro is encoded in the namespace (tag _bics), resulting in distinct symbol names. Translation units compiled with and without it can therefore be linked into the same program without One Definition Rule (ODR) violations, but they cannot exchange instances of library types.

        Workaround without the macro

        To explicitly create a single-element array without enabling this macro, use json::array():

        json j = json::array({obj});  // always creates [obj]\n
        "},{"location":"api/macros/json_brace_init_copy_semantics/#examples","title":"Examples","text":"Example: default behavior (macro not defined)

        Without the macro, single-element brace initialization wraps the value in an array:

        #include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    json obj = {{\"key\", \"value\"}};\n\n    json j{obj};\n    // j is [{\"key\":\"value\"}]  -- single-element array, NOT a copy of obj\n}\n
        Example: opt-in copy semantics (macro defined to 1)

        With the macro, single-element brace initialization copies/moves the value:

        #define JSON_BRACE_INIT_COPY_SEMANTICS 1\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    json obj = {{\"key\", \"value\"}};\n\n    json j{obj};\n    // j is {\"key\":\"value\"}  -- copy of obj\n}\n
        "},{"location":"api/macros/json_brace_init_copy_semantics/#see-also","title":"See also","text":"
        • FAQ: Brace initialization yields arrays
        • basic_json(initializer_list_t) - the affected constructor
        "},{"location":"api/macros/json_brace_init_copy_semantics/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        • Planned to become the default (with the macro removed) in version 4.0.0.
        "},{"location":"api/macros/json_delete_deprecated_functions/","title":"JSON_DELETE_DEPRECATED_FUNCTIONS","text":"
        #define JSON_DELETE_DEPRECATED_FUNCTIONS /* value */\n

        When defined to 1, all deprecated functions of the library are declared as deleted (= delete) instead of only being marked as deprecated. Code that still calls one of them no longer compiles. This way, you can find all calls that need to be replaced before version 4.0.0 unreleased removes these functions; the migration guide describes how.

        A deleted function, unlike a removed one, still takes part in overload resolution. A call that would select it therefore fails to compile instead of silently selecting another overload. This matters for the deprecated from_*(ptr, len) overloads of from_cbor, from_msgpack, from_ubjson, from_bjdata, from_bon8, and from_bson: without them, a call like from_cbor(ptr, len) would compile, read ptr as a NUL-terminated string, and convert len to the strict parameter.

        The macro does not affect the deprecated legacy comparison of discarded values, which is controlled by JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON.

        "},{"location":"api/macros/json_delete_deprecated_functions/#default-definition","title":"Default definition","text":"

        The default value is 0 (disabled, the deprecated functions can still be called, and the compiler warns about it).

        #define JSON_DELETE_DEPRECATED_FUNCTIONS 0\n
        "},{"location":"api/macros/json_delete_deprecated_functions/#notes","title":"Notes","text":"

        CMake option

        The macro can also be set with the CMake option JSON_DeleteDeprecatedFunctions (OFF by default).

        Opt-in only

        This macro must be defined before including <nlohmann/json.hpp>. Defining it after the include has no effect. Define it for the whole project to avoid different declarations of the same class in different translation units.

        ABI compatibility

        The macro only turns calls that compile into calls that do not; it does not change the layout or the behavior of any type. Its value is therefore not encoded in the namespace.

        "},{"location":"api/macros/json_delete_deprecated_functions/#examples","title":"Examples","text":"Example: default behavior (macro not defined)

        Without the macro, the deprecated overload is called, and the compiler warns about it:

        #include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    const std::vector<std::uint8_t> v = {0x82, 0x01, 0x02};\n    auto j = json::from_cbor(v.data(), v.size());\n    // warning: 'from_cbor' is deprecated: Since 3.8.0; use from_cbor(ptr, ptr + len)\n}\n
        Example: deleted deprecated functions (macro defined to 1)

        With the macro, the call does not compile:

        #define JSON_DELETE_DEPRECATED_FUNCTIONS 1\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    const std::vector<std::uint8_t> v = {0x82, 0x01, 0x02};\n    auto j = json::from_cbor(v.data(), v.size());\n    // error: call to deleted function 'from_cbor'\n}\n
        "},{"location":"api/macros/json_delete_deprecated_functions/#see-also","title":"See also","text":"
        • Roadmap: removal of deprecated functions - the deprecated functions and the version they were deprecated in
        • Migration guide: replace deprecated functions - how to replace each deprecated function
        "},{"location":"api/macros/json_delete_deprecated_functions/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        • Planned to be removed in version 4.0.0, which removes the deprecated functions. The deprecated from_*(ptr, len) overloads stay deleted in version 4.0.0 unreleased.
        "},{"location":"api/macros/json_diagnostic_positions/","title":"JSON_DIAGNOSTIC_POSITIONS","text":"
        #define JSON_DIAGNOSTIC_POSITIONS /* value */\n

        This macro enables position diagnostics for generated JSON objects.

        When enabled, two new member functions start_pos() and end_pos() are added to basic_json values. If the value was created by calling theparse function, then these functions allow querying the byte positions of the value in the input it was parsed from. In case the value was constructed by other means, std::string::npos is returned.

        start_pos() returns the position of the first character of a given value in the original JSON string, while end_pos() returns the position of the character following the last character. For objects and arrays, the first and last characters correspond to the opening or closing braces/brackets, respectively. For primitive values, the first and last character represents the opening and closing quotes (strings) or the first and last character of the field's numerical or predefined value (true, false, null), respectively.

        JSON type return value start_pos() return value end_pos() object position of the opening { position after the closing } array position of the opening [ position after the closing ] string position of the opening \" position after the closing \" number position of the first character position after the last character boolean position of t for true and f for false position after e null position of n position after l

        Given the above, end_pos()-start_pos() for a JSON value provides the length of the parsed JSON string for that value, including the opening or closing braces, brackets, or quotes.

        Note that enabling this macro increases the size of every JSON value by two std::size_t fields and adds slight runtime overhead to parsing, copying JSON value objects, and the generation of error messages for exceptions. It also causes these values to be reported in those error messages.

        "},{"location":"api/macros/json_diagnostic_positions/#default-definition","title":"Default definition","text":"

        The default value is 0 (position diagnostics are switched off).

        #define JSON_DIAGNOSTIC_POSITIONS 0\n

        When the macro is not defined, the library will define it to its default value.

        "},{"location":"api/macros/json_diagnostic_positions/#notes","title":"Notes","text":"

        CMake option

        Diagnostic positions can also be controlled with the CMake option JSON_Diagnostic_Positions (OFF by default) which defines JSON_DIAGNOSTIC_POSITIONS accordingly.

        Availability

        Diagnostic positions are only available if the value was created by the parse function. The sax_parse function or all other means to create a JSON value do not set the diagnostic positions and start_pos() and end_pos() will only return std::string::npos for these values.

        Invalidation

        The returned positions are only valid as long as the JSON value is not changed. The positions are not updated when the JSON value is changed.

        "},{"location":"api/macros/json_diagnostic_positions/#examples","title":"Examples","text":"Example: retrieving positions 
        #include <iostream>\n\n#define JSON_DIAGNOSTIC_POSITIONS 1\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::string json_string = R\"(\n    {\n        \"address\": {\n            \"street\": \"Fake Street\",\n            \"housenumber\": 1\n        }\n    }\n    )\";\n    json j = json::parse(json_string);\n\n    std::cout << \"Root diagnostic positions: \\n\";\n    std::cout << \"\\tstart_pos: \" << j.start_pos() << '\\n';\n    std::cout << \"\\tend_pos:\" << j.end_pos() << \"\\n\";\n    std::cout << \"Original string: \\n\";\n    std::cout << \"{\\n        \\\"address\\\": {\\n            \\\"street\\\": \\\"Fake Street\\\",\\n            \\\"housenumber\\\": 1\\n        }\\n    }\" << \"\\n\";\n    std::cout << \"Parsed string: \\n\";\n    std::cout << json_string.substr(j.start_pos(), j.end_pos() - j.start_pos()) << \"\\n\\n\";\n\n    std::cout << \"address diagnostic positions: \\n\";\n    std::cout << \"\\tstart_pos:\" << j[\"address\"].start_pos() << '\\n';\n    std::cout << \"\\tend_pos:\" << j[\"address\"].end_pos() << \"\\n\\n\";\n    std::cout << \"Original string: \\n\";\n    std::cout << \"{            \\\"street\\\": \\\"Fake Street\\\",\\n            \\\"housenumber\\\": 1\\n        }\" << \"\\n\";\n    std::cout << \"Parsed string: \\n\";\n    std::cout << json_string.substr(j[\"address\"].start_pos(), j[\"address\"].end_pos() - j[\"address\"].start_pos()) << \"\\n\\n\";\n\n    std::cout << \"street diagnostic positions: \\n\";\n    std::cout << \"\\tstart_pos:\" << j[\"address\"][\"street\"].start_pos() << '\\n';\n    std::cout << \"\\tend_pos:\" << j[\"address\"][\"street\"].end_pos() << \"\\n\\n\";\n    std::cout << \"Original string: \\n\";\n    std::cout << \"\\\"Fake Street\\\"\" << \"\\n\";\n    std::cout << \"Parsed string: \\n\";\n    std::cout << json_string.substr(j[\"address\"][\"street\"].start_pos(), j[\"address\"][\"street\"].end_pos() - j[\"address\"][\"street\"].start_pos()) << \"\\n\\n\";\n\n    std::cout << \"housenumber diagnostic positions: \\n\";\n    std::cout << \"\\tstart_pos:\" << j[\"address\"][\"housenumber\"].start_pos() << '\\n';\n    std::cout << \"\\tend_pos:\" << j[\"address\"][\"housenumber\"].end_pos() << \"\\n\\n\";\n    std::cout << \"Original string: \\n\";\n    std::cout << \"1\" << \"\\n\";\n    std::cout << \"Parsed string: \\n\";\n    std::cout << json_string.substr(j[\"address\"][\"housenumber\"].start_pos(), j[\"address\"][\"housenumber\"].end_pos() - j[\"address\"][\"housenumber\"].start_pos()) << \"\\n\\n\";\n}\n

        Output:

        Root diagnostic positions: \n    start_pos: 5\n    end_pos:109\nOriginal string: \n{\n        \"address\": {\n            \"street\": \"Fake Street\",\n            \"housenumber\": 1\n        }\n    }\nParsed string: \n{\n        \"address\": {\n            \"street\": \"Fake Street\",\n            \"housenumber\": 1\n        }\n    }\n\naddress diagnostic positions: \n    start_pos:26\n    end_pos:103\n\nOriginal string: \n{            \"street\": \"Fake Street\",\n            \"housenumber\": 1\n        }\nParsed string: \n{\n            \"street\": \"Fake Street\",\n            \"housenumber\": 1\n        }\n\nstreet diagnostic positions: \n    start_pos:50\n    end_pos:63\n\nOriginal string: \n\"Fake Street\"\nParsed string: \n\"Fake Street\"\n\nhousenumber diagnostic positions: \n    start_pos:92\n    end_pos:93\n\nOriginal string: \n1\nParsed string: \n1\n

        The output shows the start/end positions of all the objects and fields in the JSON string.

        Example: using only diagnostic positions in exceptions 
        #include <iostream>\n\n#define JSON_DIAGNOSTIC_POSITIONS 1\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\n/* Demonstration of type error exception with diagnostic positions support enabled */\nint main()\n{\n    //Invalid json string - housenumber type must be int instead of string\n    const std::string json_invalid_string = R\"(\n    {\n        \"address\": {\n            \"street\": \"Fake Street\",\n            \"housenumber\": \"1\"\n        }\n    }\n    )\";\n    json j = json::parse(json_invalid_string);\n    try\n    {\n        int housenumber = j[\"address\"][\"housenumber\"];\n        std::cout << housenumber;\n    }\n    catch (const json::exception& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

        Output:

        [json.exception.type_error.302] (bytes 92-95) type must be number, but is string\n
        The output shows the exception with start/end positions only.\n
        Example: using extended diagnostics with positions enabled in exceptions 
        #include <iostream>\n\n#define JSON_DIAGNOSTICS 1\n#define JSON_DIAGNOSTIC_POSITIONS 1\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\n/* Demonstration of type error exception with diagnostic positions support enabled */\nint main()\n{\n    //Invalid json string - housenumber type must be int instead of string\n    const std::string json_invalid_string = R\"(\n    {\n        \"address\": {\n            \"street\": \"Fake Street\",\n            \"housenumber\": \"1\"\n        }\n    }\n    )\";\n    json j = json::parse(json_invalid_string);\n    try\n    {\n        int housenumber = j[\"address\"][\"housenumber\"];\n        std::cout << housenumber;\n    }\n    catch (const json::exception& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

        Output:

        [json.exception.type_error.302] (/address/housenumber) (bytes 92-95) type must be number, but is string\n
        The output shows the exception with diagnostic path info and start/end positions.\n
        "},{"location":"api/macros/json_diagnostic_positions/#see-also","title":"See also","text":"
        • JSON_Diagnostic_Positions - CMake option to control the macro
        • JSON_DIAGNOSTICS - macro to control extended diagnostics
        "},{"location":"api/macros/json_diagnostic_positions/#version-history","title":"Version history","text":"
        • Added in version 3.12.0.
        "},{"location":"api/macros/json_diagnostics/","title":"JSON_DIAGNOSTICS","text":"
        #define JSON_DIAGNOSTICS /* value */\n

        This macro enables extended diagnostics for exception messages. Possible values are 1 to enable or 0 to disable (default).

        When enabled, exception messages contain a JSON Pointer to the JSON value that triggered the exception. Note that enabling this macro increases the size of every JSON value by one pointer and adds some runtime overhead.

        "},{"location":"api/macros/json_diagnostics/#default-definition","title":"Default definition","text":"

        The default value is 0 (extended diagnostics are switched off).

        #define JSON_DIAGNOSTICS 0\n

        When the macro is not defined, the library will define it to its default value.

        "},{"location":"api/macros/json_diagnostics/#notes","title":"Notes","text":"

        ABI compatibility

        As of version 3.11.0, this macro is no longer required to be defined consistently throughout a codebase to avoid One Definition Rule (ODR) violations, as the value of this macro is encoded in the namespace, resulting in distinct symbol names.

        This allows different parts of a codebase to use different versions or configurations of this library without causing improper behavior.

        Where possible, it is still recommended that all code define this the same way for maximum interoperability.

        CMake option

        Diagnostic messages can also be controlled with the CMake option JSON_Diagnostics (OFF by default) which defines JSON_DIAGNOSTICS accordingly. Note this only applies when building the library from source \u2014 see the pre-installed-package caveat on that page.

        "},{"location":"api/macros/json_diagnostics/#examples","title":"Examples","text":"Example: default behavior 
        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    json j;\n    j[\"address\"][\"street\"] = \"Fake Street\";\n    j[\"address\"][\"housenumber\"] = \"12\";\n\n    try\n    {\n        int housenumber = j[\"address\"][\"housenumber\"];\n    }\n    catch (const json::exception& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

        Output:

        [json.exception.type_error.302] type must be number, but is string\n

        This exception can be hard to debug if storing the value \"12\" and accessing it is further apart.

        Example: extended diagnostic messages 
        #include <iostream>\n\n# define JSON_DIAGNOSTICS 1\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    json j;\n    j[\"address\"][\"street\"] = \"Fake Street\";\n    j[\"address\"][\"housenumber\"] = \"12\";\n\n    try\n    {\n        int housenumber = j[\"address\"][\"housenumber\"];\n    }\n    catch (const json::exception& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

        Output:

        [json.exception.type_error.302] (/address/housenumber) type must be number, but is string\n

        Now the exception message contains a JSON Pointer /address/housenumber that indicates which value has the wrong type.

        Example: using only diagnostic positions in exceptions 
        #include <iostream>\n\n#define JSON_DIAGNOSTIC_POSITIONS 1\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\n/* Demonstration of type error exception with diagnostic positions support enabled */\nint main()\n{\n    //Invalid json string - housenumber type must be int instead of string\n    const std::string json_invalid_string = R\"(\n    {\n        \"address\": {\n            \"street\": \"Fake Street\",\n            \"housenumber\": \"1\"\n        }\n    }\n    )\";\n    json j = json::parse(json_invalid_string);\n    try\n    {\n        int housenumber = j[\"address\"][\"housenumber\"];\n        std::cout << housenumber;\n    }\n    catch (const json::exception& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

        Output:

        [json.exception.type_error.302] (bytes 92-95) type must be number, but is string\n
        The output shows the exception with start/end positions only.

        "},{"location":"api/macros/json_diagnostics/#see-also","title":"See also","text":"
        • JSON_Diagnostics - CMake option to control the macro
        • JSON_DIAGNOSTIC_POSITIONS - macro to access positions of elements
        "},{"location":"api/macros/json_diagnostics/#version-history","title":"Version history","text":"
        • Added in version 3.10.0.
        • As of version 3.11.0, the definition is allowed to vary between translation units.
        "},{"location":"api/macros/json_disable_enum_serialization/","title":"JSON_DISABLE_ENUM_SERIALIZATION","text":"
        #define JSON_DISABLE_ENUM_SERIALIZATION /* value */\n

        When defined to 1, default serialization and deserialization functions for enums are excluded and have to be provided by the user, for example, using NLOHMANN_JSON_SERIALIZE_ENUM (see arbitrary type conversions for more details).

        Parsing or serializing an enum will result in a compiler error.

        This works for both unscoped and scoped enums.

        "},{"location":"api/macros/json_disable_enum_serialization/#default-definition","title":"Default definition","text":"

        The default value is 0.

        #define JSON_DISABLE_ENUM_SERIALIZATION 0\n
        "},{"location":"api/macros/json_disable_enum_serialization/#notes","title":"Notes","text":"

        CMake option

        Enum serialization can also be controlled with the CMake option JSON_DisableEnumSerialization (OFF by default) which defines JSON_DISABLE_ENUM_SERIALIZATION accordingly.

        "},{"location":"api/macros/json_disable_enum_serialization/#examples","title":"Examples","text":"Example: Disabled behavior

        The code below forces the library not to create default serialization/deserialization functions from_json and to_json, meaning the code below does not compile.

        #define JSON_DISABLE_ENUM_SERIALIZATION 1\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nenum class Choice\n{\n    first,\n    second,\n};\n\nint main()\n{\n    // normally invokes to_json serialization function but with JSON_DISABLE_ENUM_SERIALIZATION defined, it does not\n    const json j = Choice::first; \n\n    // normally invokes from_json parse function but with JSON_DISABLE_ENUM_SERIALIZATION defined, it does not\n    Choice ch = j.get<Choice>();\n}\n
        Example: Serialize enum macro

        The code below forces the library not to create default serialization/deserialization functions from_json and to_json, but uses NLOHMANN_JSON_SERIALIZE_ENUM to parse and serialize the enum.

        #define JSON_DISABLE_ENUM_SERIALIZATION 1\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nenum class Choice\n{\n    first,\n    second,\n};\n\nNLOHMANN_JSON_SERIALIZE_ENUM(Choice,\n{\n    { Choice::first, \"first\" },\n    { Choice::second, \"second\" },\n})\n\nint main()\n{\n    // uses user-defined to_json function defined by macro\n    const json j = Choice::first; \n\n    // uses user-defined from_json function defined by macro\n    Choice ch = j.get<Choice>();\n}\n
        Example: User-defined serialization/deserialization functions

        The code below forces the library not to create default serialization/deserialization functions from_json and to_json, but uses user-defined functions to parse and serialize the enum.

        #define JSON_DISABLE_ENUM_SERIALIZATION 1\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nenum class Choice\n{\n    first,\n    second,\n};\n\nvoid from_json(const json& j, Choice& ch)\n{\n    auto value = j.get<std::string>();\n    if (value == \"first\")\n    {\n        ch = Choice::first;\n    }\n    else if (value == \"second\")\n    {\n        ch = Choice::second;\n    }\n}\n\nvoid to_json(json& j, const Choice& ch)\n{\n    if (ch == Choice::first)\n    {\n        j = \"first\";\n    }\n    else if (ch == Choice::second)\n    {\n        j = \"second\";\n    }\n}\n\nint main()\n{\n    // uses user-defined to_json function\n    const json j = Choice::first; \n\n    // uses user-defined from_json function\n    Choice ch = j.get<Choice>();\n}\n
        "},{"location":"api/macros/json_disable_enum_serialization/#see-also","title":"See also","text":"
        • JSON_DisableEnumSerialization - CMake option to control the macro
        • NLOHMANN_JSON_SERIALIZE_ENUM - serialize/deserialize an enum
        "},{"location":"api/macros/json_disable_enum_serialization/#version-history","title":"Version history","text":"
        • Added in version 3.11.0.
        "},{"location":"api/macros/json_disable_stream_bulk_scan/","title":"JSON_DISABLE_STREAM_BULK_SCAN","text":"
        #define JSON_DISABLE_STREAM_BULK_SCAN /* value */\n

        When defined to 1, parsing from a std::istream reads the input one character at a time through the stream buffer, as in version 3.12.0 and earlier, instead of scanning the characters that the stream buffer already holds in bulk.

        The macro is an escape hatch for stream buffers that do not follow the usual contract of std::basic_streambuf (see the requirements on the stream buffer in the notes below). It does not change any result: values, exceptions, and the position of the stream after parsing are the same either way. Only the speed differs.

        "},{"location":"api/macros/json_disable_stream_bulk_scan/#default-definition","title":"Default definition","text":"

        The default value is 0: streams are scanned in bulk.

        #define JSON_DISABLE_STREAM_BULK_SCAN 0\n
        "},{"location":"api/macros/json_disable_stream_bulk_scan/#notes","title":"Notes","text":"

        How streams are scanned

        A std::streambuf keeps the part of the input it has read but not yet handed out in its get area, the range from gptr() to egptr(). A std::filebuf, for instance, reads the file in blocks of a few kilobytes into its get area, and a std::stringbuf exposes its whole string as one get area.

        When parsing from a std::istream, the parser scans this get area directly, with the same fast paths that it uses for a std::string: runs of ordinary string characters, numbers, and indentation are recognized several bytes at a time instead of being requested from the stream buffer one character at a time. When the parser reaches the end of the get area, it asks the stream buffer for the next character as before, which makes the stream buffer refill its get area.

        The parser only reads characters that it would also have read one at a time, and it advances the stream buffer exactly over the characters it consumes. Nothing is read ahead, buffered elsewhere, or put back. Therefore, the stream is left exactly where it was left before:

        • operator>> and sax_parse with strict = false can still read several values from one stream, and the stream can still be used with other std::istream functions after a value has been read.
        • With JSON_PRECISE_STREAM_POSITION, the stream is still positioned right after a parsed number.
        • Exception messages, including the line, the column, and the last read token, are unchanged.

        Performance

        Parsing from a std::ifstream or std::istringstream is typically 1.5 to 2 times as fast as before, and reading many small values from one stream with operator>> 1.4 to 4 times as fast. Parsing from a stream is then within about 10 to 20 % of first reading the whole file into a std::string and parsing that.

        A stream buffer without a get area (one that only overrides underflow() and uflow() and hands out every character from there) offers nothing to scan in bulk, so it is read one character at a time as before.

        Requirements on the stream buffer

        The bulk scan relies on the following behavior, which every stream buffer of the standard library and every stream buffer that keeps its input in its get area has:

        • When sgetc() returns a character while gptr() < egptr(), that character is *gptr(); when sbumpc() returns one, it was at gptr() before the call.
        • Characters in the get area are not changed while they are in the get area.
        • underflow() or uflow() either sets up a get area that holds the returned character at gptr() (or at gptr() - 1 after uflow()), or leaves the get area empty.

        A stream buffer that returns characters from uflow() while leaving unrelated characters in its get area, or that changes the characters in its get area in place, does not meet these requirements. Define the macro to 1 for such stream buffers.

        ABI compatibility

        The value of this macro is encoded in the namespace (tag _dsbs), resulting in distinct symbol names. Translation units compiled with and without it can therefore be linked into the same program without One Definition Rule (ODR) violations, but they cannot exchange instances of library types.

        Other inputs

        Strings, string views, vectors, arrays, and pointer or iterator ranges over contiguous memory are scanned in bulk anyway. Reading from a FILE* is not affected by this macro; see JSON_DISABLE_UNLOCKED_FILE_IO for that input.

        "},{"location":"api/macros/json_disable_stream_bulk_scan/#examples","title":"Examples","text":"Example: reading values from a stream (default)

        The stream is scanned in bulk, and the values are read one after the other:

        #include <iostream>\n#include <sstream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::istringstream input(R\"({\"id\": 1} {\"id\": 2} 42 rest)\");\n    json j1, j2, j3;\n    input >> j1 >> j2 >> j3;  // {\"id\":1}, {\"id\":2}, 42\n\n    std::string word;\n    input >> word;            // \"rest\"\n}\n
        Example: reading one character at a time (macro defined to 1)

        With the macro, the same program reads the stream one character at a time, with the same results:

        #define JSON_DISABLE_STREAM_BULK_SCAN 1\n#include <iostream>\n#include <sstream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::istringstream input(R\"({\"id\": 1} {\"id\": 2} 42 rest)\");\n    json j1, j2, j3;\n    input >> j1 >> j2 >> j3;  // {\"id\":1}, {\"id\":2}, 42\n\n    std::string word;\n    input >> word;            // \"rest\"\n}\n
        "},{"location":"api/macros/json_disable_stream_bulk_scan/#see-also","title":"See also","text":"
        • parse - deserialize from a compatible input
        • operator>> - deserialize from stream
        • sax_parse - generate SAX events
        • JSON_PRECISE_STREAM_POSITION - leave an input stream positioned right after a parsed number
        • JSON_DISABLE_UNLOCKED_FILE_IO - read a FILE* with the locking stdio functions
        • Parsing: input sources and performance
        "},{"location":"api/macros/json_disable_stream_bulk_scan/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/macros/json_disable_tuple_reference_conversion/","title":"JSON_DISABLE_TUPLE_REFERENCE_CONVERSION","text":"
        #define JSON_DISABLE_TUPLE_REFERENCE_CONVERSION /* value */\n

        When defined to 1, a basic_json value can no longer be constructed from a one-element std::tuple whose element is a reference to that basic_json type, such as std::tuple<json&>, std::tuple<const json&>, or std::tuple<json&&>. These are the tuples created by std::forward_as_tuple(j).

        "},{"location":"api/macros/json_disable_tuple_reference_conversion/#default-definition","title":"Default definition","text":"

        The default value is 0 (disabled \u2014 existing behavior is preserved).

        #define JSON_DISABLE_TUPLE_REFERENCE_CONVERSION 0\n
        "},{"location":"api/macros/json_disable_tuple_reference_conversion/#notes","title":"Notes","text":"

        Background

        By default, basic_json can be constructed from any std::tuple whose elements can be converted to JSON; the result is an array. This includes std::tuple<json&>, which becomes a one-element array.

        std::tuple only converts another tuple element by element if its element type cannot be constructed from the whole source tuple. Because json can be constructed from std::tuple<json&>, std::tuple instead converts the whole tuple into a single json value. This has two surprising effects:

        json j = true;\n\n// rejected by some standard libraries (e.g., libc++); with others, the\n// reference binds to a temporary that is destroyed right away\nstd::tuple<const json&> t1(std::forward_as_tuple(j));\n\n// compiles, but std::get<0>(t2) is [true], not true\nstd::tuple<json> t2(std::forward_as_tuple(j));\n

        Enabling this macro removes the conversion, so both tuples are converted element by element: std::get<0>(t1) refers to j, and std::get<0>(t2) is a copy of j (see #2226).

        Opt-in only

        This macro must be defined before including <nlohmann/json.hpp>. Defining it after the include has no effect.

        Affected conversions

        Only one-element tuples holding a reference to the same basic_json type are affected. Constructing a JSON value from them no longer compiles:

        json j = true;\njson a = std::forward_as_tuple(j);  // error with the macro enabled\njson b = json::array({j});          // use this instead: [true]\n

        Tuples holding a JSON value (std::make_tuple(j)), tuples with more than one element, and tuples holding references to other types (including other basic_json specializations) are converted to arrays as before.

        CMake option

        This behavior can also be controlled with the CMake option JSON_DisableTupleReferenceConversion (OFF by default) which defines JSON_DISABLE_TUPLE_REFERENCE_CONVERSION accordingly.

        "},{"location":"api/macros/json_disable_tuple_reference_conversion/#examples","title":"Examples","text":"Example: default behavior (macro not defined) 
        #include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    json j = true;\n\n    std::tuple<json> t(std::forward_as_tuple(j));\n    // std::get<0>(t) is [true] -- the whole tuple was converted\n}\n
        Example: conversion disabled (macro defined to 1) 
        #define JSON_DISABLE_TUPLE_REFERENCE_CONVERSION 1\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    json j = true;\n\n    std::tuple<json> t(std::forward_as_tuple(j));\n    // std::get<0>(t) is true -- a copy of j\n\n    std::tuple<const json&> r(std::forward_as_tuple(j));\n    // std::get<0>(r) refers to j\n}\n
        "},{"location":"api/macros/json_disable_tuple_reference_conversion/#see-also","title":"See also","text":"
        • basic_json(CompatibleType&&) - the affected constructor
        • JSON_DisableTupleReferenceConversion - CMake option to control the macro
        "},{"location":"api/macros/json_disable_tuple_reference_conversion/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        • Planned to become the default (with the macro removed) in version 4.0.0.
        "},{"location":"api/macros/json_disable_unlocked_file_io/","title":"JSON_DISABLE_UNLOCKED_FILE_IO","text":"
        #define JSON_DISABLE_UNLOCKED_FILE_IO /* value */\n

        When defined to 1, parsing from a FILE* reads every character with std::fgetc, as in version 3.12.0 and earlier, instead of locking the file once and reading it with the unlocked stdio functions.

        It does not change any result: values, exceptions, and the position of the file after parsing are the same either way. Only the speed and the locking of the file differ (see the notes on locking below).

        Builds with MemorySanitizer use std::fgetc automatically, because the inline getc_unlocked of glibc reads the stdio buffer directly, which glibc (not instrumented) filled, and MemorySanitizer would report it as uninitialized.

        "},{"location":"api/macros/json_disable_unlocked_file_io/#default-definition","title":"Default definition","text":"

        The default value is 0: the file is locked once and read with the unlocked stdio functions where they are available.

        #define JSON_DISABLE_UNLOCKED_FILE_IO 0\n
        "},{"location":"api/macros/json_disable_unlocked_file_io/#notes","title":"Notes","text":"

        Background

        The C standard library makes every std::fgetc call thread-safe by locking and unlocking the FILE object. When parsing from a FILE*, the parser reads one character at a time, so this happens for every character of the input.

        Instead, the parser locks the file once when parsing starts and unlocks it when parsing ends - also when an exception is thrown - and reads the characters in between without locking:

        C library functions glibc (Linux), Apple (macOS, iOS), Android, FreeBSD, NetBSD, OpenBSD, DragonFly BSD flockfile, getc_unlocked, funlockfile (POSIX) Microsoft (MSVC, clang-cl) _lock_file, _fgetc_nolock, _unlock_file others (e.g., musl, MinGW, newlib) std::fgetc, as before

        This makes parsing from a FILE* 3 to 4 times as fast with the Apple C library and 1.1 to 1.3 times as fast with glibc, whose locking is cheaper. The binary formats (from_cbor, from_msgpack, ...) read from a FILE* the same way.

        Locking

        While a value is parsed from a FILE*, the parsing thread holds the file's lock. The lock is recursive, so the parsing thread itself - for instance, a SAX handler or a parser callback - can keep using the file with stdio functions such as std::ftell. Other threads that use the same FILE object wait until the parser is done. Before, they could interleave their reads with the parser's, which made the parsed input unpredictable anyway.

        A program in which a SAX handler or parser callback waits for another thread that uses the same FILE object would deadlock. Define the macro to 1 for such programs.

        ABI compatibility

        The value of this macro is encoded in the namespace (tag _dufio), resulting in distinct symbol names. Translation units compiled with and without it can therefore be linked into the same program without One Definition Rule (ODR) violations, but they cannot exchange instances of library types.

        Other inputs

        Reading from a std::istream is not affected by this macro; see JSON_DISABLE_STREAM_BULK_SCAN for that input.

        "},{"location":"api/macros/json_disable_unlocked_file_io/#examples","title":"Examples","text":"Example: parsing from a FILE* (default)

        The file is locked once while it is parsed:

        #include <cstdio>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::FILE* f = std::fopen(\"example.json\", \"rb\");\n    json j = json::parse(f);  // f is locked until parse() returns\n    std::fclose(f);\n}\n
        Example: locking for every character (macro defined to 1)

        With the macro, every character is read with std::fgetc, with the same result:

        #define JSON_DISABLE_UNLOCKED_FILE_IO 1\n#include <cstdio>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::FILE* f = std::fopen(\"example.json\", \"rb\");\n    json j = json::parse(f);  // std::fgetc for every character\n    std::fclose(f);\n}\n
        "},{"location":"api/macros/json_disable_unlocked_file_io/#see-also","title":"See also","text":"
        • parse - deserialize from a compatible input
        • sax_parse - generate SAX events
        • JSON_DISABLE_STREAM_BULK_SCAN - read a std::istream one character at a time
        • Parsing: input sources and performance
        "},{"location":"api/macros/json_disable_unlocked_file_io/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/macros/json_has_cpp_11/","title":"JSON_HAS_CPP_11, JSON_HAS_CPP_14, JSON_HAS_CPP_17, JSON_HAS_CPP_20, JSON_HAS_CPP_23, JSON_HAS_CPP_26","text":"
        #define JSON_HAS_CPP_11\n#define JSON_HAS_CPP_14\n#define JSON_HAS_CPP_17\n#define JSON_HAS_CPP_20\n#define JSON_HAS_CPP_23\n#define JSON_HAS_CPP_26\n

        The library targets C++11, but also supports some features introduced in later C++ versions (e.g., std::string_view support for C++17). For these new features, the library implements some preprocessor checks to determine the C++ standard. By defining any of these symbols, the internal check is overridden and the provided C++ version is unconditionally assumed. This can be helpful for compilers that only implement parts of the standard and would be detected incorrectly.

        "},{"location":"api/macros/json_has_cpp_11/#default-definition","title":"Default definition","text":"

        The default value is detected based on preprocessor macros such as __cplusplus, _HAS_CXX17, or _MSVC_LANG.

        "},{"location":"api/macros/json_has_cpp_11/#notes","title":"Notes","text":"
        • When the C++ standard is detected automatically, JSON_HAS_CPP_11 is always defined. When you override the detection by defining one of these macros manually, the automatic detection is skipped entirely, so you should define all applicable macros (including JSON_HAS_CPP_11) yourself.
        • All macros are undefined outside the library.
        "},{"location":"api/macros/json_has_cpp_11/#examples","title":"Examples","text":"Example

        The code below forces the library to use the C++14 standard:

        #define JSON_HAS_CPP_14 1\n#include <nlohmann/json.hpp>\n\n...\n
        "},{"location":"api/macros/json_has_cpp_11/#see-also","title":"See also","text":"
        • JSON_HAS_FILESYSTEM / JSON_HAS_EXPERIMENTAL_FILESYSTEM - control std::filesystem support
        • JSON_HAS_RANGES - control std::ranges support
        • JSON_HAS_THREE_WAY_COMPARISON - control 3-way comparison support
        • JSON_HAS_STD_FORMAT - control std::format/std::formatter support
        "},{"location":"api/macros/json_has_cpp_11/#version-history","title":"Version history","text":"
        • Added in version 3.10.0.
        • Added JSON_HAS_CPP_23 in version 3.12.0.
        • Added JSON_HAS_CPP_26 in version 3.13.0 unreleased.
        "},{"location":"api/macros/json_has_filesystem/","title":"JSON_HAS_FILESYSTEM / JSON_HAS_EXPERIMENTAL_FILESYSTEM","text":"
        #define JSON_HAS_FILESYSTEM /* value */\n#define JSON_HAS_EXPERIMENTAL_FILESYSTEM /* value */\n

        When compiling with C++17, the library provides conversions from and to std::filesystem::path. As compiler support for filesystem is limited, the library tries to detect whether <filesystem>/std::filesystem (JSON_HAS_FILESYSTEM) or <experimental/filesystem>/std::experimental::filesystem (JSON_HAS_EXPERIMENTAL_FILESYSTEM) should be used. To override the built-in check, define JSON_HAS_FILESYSTEM or JSON_HAS_EXPERIMENTAL_FILESYSTEM to 1.

        "},{"location":"api/macros/json_has_filesystem/#default-definition","title":"Default definition","text":"

        The default value is detected based on the preprocessor macros __cpp_lib_filesystem, __cpp_lib_experimental_filesystem, __has_include(<filesystem>), or __has_include(<experimental/filesystem>).

        Known compiler/stdlib exclusions

        Even when the feature-test macro indicates filesystem support is available, the library disables it on the following broken toolchains:

        • MinGW + GCC 8 \u2014 disabled entirely (broken std::filesystem implementation; MinGW-w64 bug 737)
        • GCC (non-Clang) < 8 \u2014 disabled (no filesystem support)
        • Clang < 7 \u2014 disabled (no filesystem support)
        • MSVC < 19.14 \u2014 disabled (no filesystem support)
        • iOS < 13 \u2014 disabled (no filesystem support)
        • macOS < Catalina (10.15) \u2014 disabled (no filesystem support)

        If JSON_HAS_FILESYSTEM or JSON_HAS_EXPERIMENTAL_FILESYSTEM is 0 despite __cpp_lib_filesystem being defined, one of the exclusions above likely applies to your toolchain.

        "},{"location":"api/macros/json_has_filesystem/#notes","title":"Notes","text":"
        • Note that older compilers or older versions of libstdc++ also require the library stdc++fs to be linked to for filesystem support.
        • Both macros are undefined outside the library.
        "},{"location":"api/macros/json_has_filesystem/#examples","title":"Examples","text":"Example

        The code below forces the library to use the header <experimental/filesystem>.

        #define JSON_HAS_EXPERIMENTAL_FILESYSTEM 1\n#include <nlohmann/json.hpp>\n\n...\n
        "},{"location":"api/macros/json_has_filesystem/#see-also","title":"See also","text":"
        • JSON_HAS_CPP_11 / JSON_HAS_CPP_14 / JSON_HAS_CPP_17 / JSON_HAS_CPP_20 / JSON_HAS_CPP_23 / JSON_HAS_CPP_26 - set supported C++ standard
        "},{"location":"api/macros/json_has_filesystem/#version-history","title":"Version history","text":"
        • Added in version 3.10.5.
        "},{"location":"api/macros/json_has_range_view_conversion/","title":"JSON_HAS_RANGE_VIEW_CONVERSION","text":"
        #define JSON_HAS_RANGE_VIEW_CONVERSION /* value */\n

        This macro indicates whether a JSON array can be constructed directly from a C++20 range view (std::ranges::view), such as the result of std::views::filter or std::views::transform. Possible values are 1 when supported or 0 when unsupported.

        "},{"location":"api/macros/json_has_range_view_conversion/#default-definition","title":"Default definition","text":"

        The default value is 1 if JSON_HAS_RANGES is 1 and the compiler is not MinGW (that is, __MINGW32__ is not defined), and 0 otherwise.

        When the macro is not defined, the library will define it to its default value.

        Known compiler/stdlib exclusions

        • MinGW -- disabled, because its std::ranges support is incomplete (issue #4916).
        • All toolchains for which JSON_HAS_RANGES is disabled.
        "},{"location":"api/macros/json_has_range_view_conversion/#examples","title":"Examples","text":"Example

        The code below forces the library to disable the conversion from range views:

        #define JSON_HAS_RANGE_VIEW_CONVERSION 0\n#include <nlohmann/json.hpp>\n\n...\n
        "},{"location":"api/macros/json_has_range_view_conversion/#see-also","title":"See also","text":"
        • JSON_HAS_RANGES - control std::ranges support
        • Constructing from a C++20 range view - usage of the feature
        "},{"location":"api/macros/json_has_range_view_conversion/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/macros/json_has_ranges/","title":"JSON_HAS_RANGES","text":"
        #define JSON_HAS_RANGES /* value */\n

        This macro indicates whether the standard library has any support for ranges. Implies support for concepts. Possible values are 1 when supported or 0 when unsupported.

        "},{"location":"api/macros/json_has_ranges/#default-definition","title":"Default definition","text":"

        The default value is detected based on the preprocessor macro __cpp_lib_ranges.

        When the macro is not defined, the library will define it to its default value.

        Known compiler/stdlib exclusions

        Even when the feature-test macro __cpp_lib_ranges indicates ranges support is available, the library disables it on the following incomplete or broken toolchains:

        • GCC 11.1.0 \u2014 disabled (the shipped <ranges> header has a syntax error; issue #4440)
        • libstdc++ < 11 \u2014 disabled (incomplete C++20 ranges support; issue #4440)
        • Clang < 16 with libstdc++ \u2014 disabled (incomplete ranges support; issue #5161)
        • libc++ < 160000 \u2014 disabled (incomplete C++20 ranges support; issue #4440)
        • nvcc (CUDA) 12.0.x and 12.1.x \u2014 disabled (the enable_borrowed_range variable-template syntax triggers a parse error under these two toolkit versions; fixed in CUDA 12.2; issue #3907)

        If JSON_HAS_RANGES is 0 despite __cpp_lib_ranges being defined, one of the exclusions above likely applies to your toolchain.

        "},{"location":"api/macros/json_has_ranges/#examples","title":"Examples","text":"Example

        The code below forces the library to enable support for ranges:

        #define JSON_HAS_RANGES 1\n#include <nlohmann/json.hpp>\n\n...\n
        "},{"location":"api/macros/json_has_ranges/#see-also","title":"See also","text":"
        • JSON_HAS_CPP_11 / JSON_HAS_CPP_14 / JSON_HAS_CPP_17 / JSON_HAS_CPP_20 / JSON_HAS_CPP_23 / JSON_HAS_CPP_26 - set supported C++ standard
        • JSON_HAS_RANGE_VIEW_CONVERSION - control construction from std::ranges views
        • JSON_HAS_STD_FORMAT - a similar feature-detection macro, for std::format/std::formatter support
        "},{"location":"api/macros/json_has_ranges/#version-history","title":"Version history","text":"
        • Added in version 3.11.0.
        • Added the exclusions for libstdc++ < 11, Clang < 16 with libstdc++, libc++ < 16, and nvcc (CUDA) 12.0.x and 12.1.x in version 3.13.0 unreleased. Before, only GCC 11.1.0 was excluded.
        "},{"location":"api/macros/json_has_static_rtti/","title":"JSON_HAS_STATIC_RTTI","text":"
        #define JSON_HAS_STATIC_RTTI /* value */\n

        This macro indicates whether the standard library has any support for RTTI (run time type information). Possible values are 1 when supported or 0 when unsupported.

        "},{"location":"api/macros/json_has_static_rtti/#default-definition","title":"Default definition","text":"

        The default value is detected based on the preprocessor macro _HAS_STATIC_RTTI.

        When the macro is not defined, the library will define it to its default value.

        "},{"location":"api/macros/json_has_static_rtti/#examples","title":"Examples","text":"Example

        The code below forces the library to enable support for libraries with RTTI dependence:

        #define JSON_HAS_STATIC_RTTI 1\n#include <nlohmann/json.hpp>\n\n...\n
        "},{"location":"api/macros/json_has_static_rtti/#see-also","title":"See also","text":"
        • operator ValueType - get a value (implicit); on C++17, this macro controls whether std::any is excluded from its candidate types
        "},{"location":"api/macros/json_has_static_rtti/#version-history","title":"Version history","text":"
        • Added in version 3.11.3.
        "},{"location":"api/macros/json_has_std_format/","title":"JSON_HAS_STD_FORMAT","text":"
        #define JSON_HAS_STD_FORMAT /* value */\n

        This macro indicates whether the standard library has support for std::format/std::formatter (that is, the <format> header). Possible values are 1 when supported or 0 when unsupported.

        "},{"location":"api/macros/json_has_std_format/#default-definition","title":"Default definition","text":"

        The default value is detected based on the preprocessor macros JSON_HAS_CPP_20 and __cpp_lib_format.

        When the macro is not defined, the library will define it to its default value.

        "},{"location":"api/macros/json_has_std_format/#notes","title":"Notes","text":"

        Enabled functionality

        When this macro evaluates to 1, the library provides a std::formatter<basic_json> specialization so JSON values can be used directly with std::format.

        "},{"location":"api/macros/json_has_std_format/#examples","title":"Examples","text":"Example

        The code below forces the library to disable support for std::format, even if the standard library would otherwise support it:

        #define JSON_HAS_STD_FORMAT 0\n#include <nlohmann/json.hpp>\n\n...\n
        "},{"location":"api/macros/json_has_std_format/#see-also","title":"See also","text":"
        • std::formatter<basic_json> - format JSON values with std::format
        "},{"location":"api/macros/json_has_std_format/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/macros/json_has_three_way_comparison/","title":"JSON_HAS_THREE_WAY_COMPARISON","text":"
        #define JSON_HAS_THREE_WAY_COMPARISON /* value */\n

        This macro indicates whether the compiler and standard library support 3-way comparison. Possible values are 1 when supported or 0 when unsupported.

        "},{"location":"api/macros/json_has_three_way_comparison/#default-definition","title":"Default definition","text":"

        The default value is detected based on the preprocessor macros __cpp_impl_three_way_comparison and __cpp_lib_three_way_comparison.

        When the macro is not defined, the library will define it to its default value.

        "},{"location":"api/macros/json_has_three_way_comparison/#examples","title":"Examples","text":"Example

        The code below forces the library to use 3-way comparison:

        #define JSON_HAS_THREE_WAY_COMPARISON 1\n#include <nlohmann/json.hpp>\n\n...\n
        "},{"location":"api/macros/json_has_three_way_comparison/#see-also","title":"See also","text":"
        • operator<=> - 3-way compare JSON values
        • operator== - compare JSON pointers for equality
        "},{"location":"api/macros/json_has_three_way_comparison/#version-history","title":"Version history","text":"
        • Added in version 3.11.0.
        "},{"location":"api/macros/json_no_automatic_udls/","title":"JSON_NO_AUTOMATIC_UDLS","text":"
        #define JSON_NO_AUTOMATIC_UDLS\n

        When defined, <nlohmann/json.hpp> does not include <nlohmann/json_literals.hpp>, so the user-defined string literals operator\"\"_json and operator\"\"_json_pointer are not declared. Include <nlohmann/json_literals.hpp> in the files that use them.

        The literals are ordinary inline functions whose bodies call the parser, so every translation unit that includes them instantiates the parser \u2014 even if it never parses anything itself. Defining JSON_NO_AUTOMATIC_UDLS for a whole project avoids this cost in translation units that do not parse (e.g., ones that only define types and conversions or pass json values around) and reduces their compile time.

        "},{"location":"api/macros/json_no_automatic_udls/#default-definition","title":"Default definition","text":"

        By default, JSON_NO_AUTOMATIC_UDLS is not defined, and <nlohmann/json.hpp> includes <nlohmann/json_literals.hpp>.

        #undef JSON_NO_AUTOMATIC_UDLS\n
        "},{"location":"api/macros/json_no_automatic_udls/#notes","title":"Notes","text":"

        Header <nlohmann/json_literals.hpp>

        The header includes <nlohmann/json.hpp> itself and places the literals according to JSON_USE_GLOBAL_UDLS. It is part of the multi-header sources (include/nlohmann) and of the single-header sources (single_include/nlohmann), next to json.hpp.

        C++ modules

        The nlohmann.json module always exports the literals, regardless of this macro.

        "},{"location":"api/macros/json_no_automatic_udls/#examples","title":"Examples","text":"Example

        The code below includes the library without the literals and adds them in a single translation unit.

        // compiled with -DJSON_NO_AUTOMATIC_UDLS for the whole project\n\n// this file uses the literals, so it includes them explicitly\n// (the header includes <nlohmann/json.hpp> itself)\n#include <nlohmann/json_literals.hpp>\n\nint main()\n{\n    auto j = R\"({\"foo\": 42})\"_json;\n    return j.at(\"/foo\"_json_pointer) == 42 ? 0 : 1;\n}\n

        Without the include of <nlohmann/json_literals.hpp>, the code would fail to compile.

        "},{"location":"api/macros/json_no_automatic_udls/#see-also","title":"See also","text":"
        • operator\"\"_json
        • operator\"\"_json_pointer
        • JSON_USE_GLOBAL_UDLS - place user-defined string literals (UDLs) into the global namespace
        • Compile times - options to reduce compile times
        "},{"location":"api/macros/json_no_automatic_udls/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/macros/json_no_io/","title":"JSON_NO_IO","text":"
        #define JSON_NO_IO\n

        When defined, headers <cstdio>, <ios>, <iosfwd>, <istream>, and <ostream> are not included and parse functions relying on these headers are excluded. This is relevant for environments where these I/O functions are disallowed for security reasons (e.g., Intel Software Guard Extensions (SGX)).

        "},{"location":"api/macros/json_no_io/#default-definition","title":"Default definition","text":"

        By default, JSON_NO_IO is not defined.

        #undef JSON_NO_IO\n
        "},{"location":"api/macros/json_no_io/#examples","title":"Examples","text":"Example

        The code below forces the library not to use the headers <cstdio>, <ios>, <iosfwd>, <istream>, and <ostream>.

        #define JSON_NO_IO 1\n#include <nlohmann/json.hpp>\n\n...\n
        "},{"location":"api/macros/json_no_io/#see-also","title":"See also","text":"
        • operator<< - serialize to stream
        • operator>> - deserialize from stream
        "},{"location":"api/macros/json_no_io/#version-history","title":"Version history","text":"
        • Added in version 3.10.0.
        "},{"location":"api/macros/json_no_thread_local/","title":"JSON_NO_THREAD_LOCAL","text":"
        #define JSON_NO_THREAD_LOCAL\n

        When defined, the library does not use thread_local storage. This is relevant for the few environments whose toolchain does not support it.

        Copying a value and comparing two values both descend into the first levels by letting the containers copy or compare themselves, and finish whatever is nested deeper than that without the call stack, so that neither can exhaust the stack however deeply the values are nested. Each counts the levels it has descended into in a thread_local variable, as a counter shared between threads would be raced.

        Without those counters, no descent can be bounded safely, so objects and arrays are copied and compared without the call stack right away. Both keep working exactly as they do otherwise - the same values come out, the same comparisons hold, and deeply nested values are handled just as safely - but both are slower, because the containers no longer copy or compare themselves. Copying the benchmark documents takes 9% (canada.json) to 34% (twitter.json) longer, and comparing two equal ones 10% (citm_catalog.json) to 90% (canada.json) longer.

        "},{"location":"api/macros/json_no_thread_local/#default-definition","title":"Default definition","text":"

        By default, JSON_NO_THREAD_LOCAL is not defined.

        #undef JSON_NO_THREAD_LOCAL\n

        The library defines it by itself for Clang targeting MinGW, which does not survive the thread_local storage: copying a value segfaults there, with both old and current Clang versions, while GCC targeting MinGW is unaffected. Copying and comparing fall back to working without the call stack there, as they do whenever the macro is defined.

        "},{"location":"api/macros/json_no_thread_local/#examples","title":"Examples","text":"Example

        The code below forces the library not to use thread_local storage.

        #define JSON_NO_THREAD_LOCAL 1\n#include <nlohmann/json.hpp>\n\n...\n
        "},{"location":"api/macros/json_no_thread_local/#see-also","title":"See also","text":"
        • FAQ: Thread safety
        "},{"location":"api/macros/json_no_thread_local/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/macros/json_noexception/","title":"JSON_NOEXCEPTION","text":"
        #define JSON_NOEXCEPTION\n

        Exceptions can be switched off by defining the symbol JSON_NOEXCEPTION. When defining JSON_NOEXCEPTION, try is replaced by if (true), catch is replaced by if (false), and throw is replaced by std::abort().

        The same effect is achieved by setting the compiler flag -fno-exceptions.

        "},{"location":"api/macros/json_noexception/#default-definition","title":"Default definition","text":"

        By default, the macro is not defined.

        #undef JSON_NOEXCEPTION\n
        "},{"location":"api/macros/json_noexception/#notes","title":"Notes","text":"

        The explanatory what() string of exceptions is not available for MSVC if exceptions are disabled, see #2824.

        "},{"location":"api/macros/json_noexception/#examples","title":"Examples","text":"Example

        The code below switches off exceptions in the library.

        #define JSON_NOEXCEPTION 1\n#include <nlohmann/json.hpp>\n\n...\n
        "},{"location":"api/macros/json_noexception/#see-also","title":"See also","text":"
        • Switch off exceptions for more information how to switch off exceptions
        "},{"location":"api/macros/json_noexception/#version-history","title":"Version history","text":"

        Added in version 2.1.0.

        "},{"location":"api/macros/json_precise_stream_position/","title":"JSON_PRECISE_STREAM_POSITION","text":"
        #define JSON_PRECISE_STREAM_POSITION /* value */\n

        When defined to 1, operator>> and sax_parse with strict = false leave a std::istream positioned right after the parsed value for every value type. By default, the character that terminates a number is consumed as well.

        The macro only affects reading from a std::istream when the rest of the stream is not required to be consumed. parse, accept, and all other inputs (strings, iterators, containers, FILE*) are never affected.

        "},{"location":"api/macros/json_precise_stream_position/#default-definition","title":"Default definition","text":"

        The default value is 0 (disabled \u2014 existing behavior is preserved).

        #define JSON_PRECISE_STREAM_POSITION 0\n
        "},{"location":"api/macros/json_precise_stream_position/#notes","title":"Notes","text":"

        Background

        A number is the only JSON value whose end can be detected solely by reading the character that follows it. By default, that character is consumed and not put back, so the stream is left one byte too far after a number, and only after a number:

        std::istringstream input(\"1true\");\njson j;\ninput >> j;  // j == 1, but the stream now starts at \"rue\"\n

        With this macro, the character is only looked at and left in the stream, so the stream starts at true. This does not require the stream buffer to support putting a character back.

        This was not changed unconditionally, because code can depend on the consumed character, even unknowingly (see #5340). Both of the following work by default only because the character after each number is swallowed, and behave differently with this macro:

        std::istringstream input(\"1,2,3\");\njson j1, j2, j3;\ninput >> j1 >> j2 >> j3;  // default: 1, 2, 3\n                          // with the macro: throws parse_error.101 at the ','\n
        std::istringstream input(\"42\\nfoo\");\njson j;\nstd::string line;\ninput >> j;\nstd::getline(input, line);  // default: \"foo\"\n                            // with the macro: \"\" (like after reading an int with >>)\n

        In both cases, the behavior with the macro is what you already get today when the value is not a number: \"a\",\"b\" fails at the ,, and std::getline after {} returns an empty string. This macro offers an opt-in path to the consistent behavior ahead of version 4.0.0, where it is planned to become the default.

        Opt-in only

        This macro must be defined before including <nlohmann/json.hpp>. Defining it after the include has no effect.

        ABI compatibility

        The value of this macro is encoded in the namespace (tag _psp), resulting in distinct symbol names. Translation units compiled with and without it can therefore be linked into the same program without One Definition Rule (ODR) violations, but they cannot exchange instances of library types.

        Bulk scanning

        The parser scans the characters in the stream buffer in bulk (see JSON_DISABLE_STREAM_BULK_SCAN), but it never reads ahead. The stream is left at the same position with and without bulk scanning, in both modes of this macro.

        Workaround without the macro

        Separate the values in the stream with whitespace. The character consumed after a number is then the separator, and whitespace before the next value is skipped anyway.

        "},{"location":"api/macros/json_precise_stream_position/#examples","title":"Examples","text":"Example: default behavior (macro not defined)

        Without the macro, the character after a number is consumed:

        #include <iostream>\n#include <sstream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::istringstream input(\"1true\");\n    json j1, j2;\n    input >> j1;  // j1 == 1\n    input >> j2;  // throws parse_error.101: the stream now starts at \"rue\"\n}\n
        Example: opt-in precise stream position (macro defined to 1)

        With the macro, the stream is positioned right after the number:

        #define JSON_PRECISE_STREAM_POSITION 1\n#include <iostream>\n#include <sstream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::istringstream input(\"1true\");\n    json j1, j2;\n    input >> j1;  // j1 == 1\n    input >> j2;  // j2 == true\n}\n
        "},{"location":"api/macros/json_precise_stream_position/#see-also","title":"See also","text":"
        • operator>> - deserialize from stream
        • sax_parse - generate SAX events
        "},{"location":"api/macros/json_precise_stream_position/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        • Planned to become the default (with the macro removed) in version 4.0.0.
        "},{"location":"api/macros/json_skip_library_version_check/","title":"JSON_SKIP_LIBRARY_VERSION_CHECK","text":"
        #define JSON_SKIP_LIBRARY_VERSION_CHECK\n

        When defined, the library will not create a compiler warning when a different version of the library was already included.

        "},{"location":"api/macros/json_skip_library_version_check/#default-definition","title":"Default definition","text":"

        By default, the macro is not defined.

        #undef JSON_SKIP_LIBRARY_VERSION_CHECK\n
        "},{"location":"api/macros/json_skip_library_version_check/#notes","title":"Notes","text":"

        ABI compatibility

        Mixing different library versions in the same code can be a problem as the different versions may not be ABI compatible.

        "},{"location":"api/macros/json_skip_library_version_check/#examples","title":"Examples","text":"Example: switch off the version check

        The code below switches off the warning about including a different version of the library.

        #define JSON_SKIP_LIBRARY_VERSION_CHECK 1\n#include <nlohmann/json.hpp>\n\n...\n
        Example: warning about a different library version

        The following warning will be shown in case a different version of the library was already included:

        Already included a different version of the library!\n
        "},{"location":"api/macros/json_skip_library_version_check/#see-also","title":"See also","text":"
        • NLOHMANN_JSON_VERSION_MAJOR, NLOHMANN_JSON_VERSION_MINOR, NLOHMANN_JSON_VERSION_PATCH - library version information
        "},{"location":"api/macros/json_skip_library_version_check/#version-history","title":"Version history","text":"
        • Added in version 3.11.0.
        "},{"location":"api/macros/json_skip_unsupported_compiler_check/","title":"JSON_SKIP_UNSUPPORTED_COMPILER_CHECK","text":"
        #define JSON_SKIP_UNSUPPORTED_COMPILER_CHECK\n

        When defined, the library will not create a compile error when a known unsupported compiler is detected. This allows using the library with compilers that do not fully support C++11 and may only work if unsupported features are not used.

        "},{"location":"api/macros/json_skip_unsupported_compiler_check/#default-definition","title":"Default definition","text":"

        By default, the macro is not defined.

        #undef JSON_SKIP_UNSUPPORTED_COMPILER_CHECK\n
        "},{"location":"api/macros/json_skip_unsupported_compiler_check/#examples","title":"Examples","text":"Example

        The code below switches off the check whether the compiler is supported.

        #define JSON_SKIP_UNSUPPORTED_COMPILER_CHECK 1\n#include <nlohmann/json.hpp>\n\n...\n
        "},{"location":"api/macros/json_skip_unsupported_compiler_check/#see-also","title":"See also","text":"
        • JSON_HAS_CPP_11 / JSON_HAS_CPP_14 / JSON_HAS_CPP_17 / JSON_HAS_CPP_20 / JSON_HAS_CPP_23 / JSON_HAS_CPP_26 - set supported C++ standard
        "},{"location":"api/macros/json_skip_unsupported_compiler_check/#version-history","title":"Version history","text":"
        • Added in version 3.2.0.
        "},{"location":"api/macros/json_strict_binary_utf8/","title":"JSON_STRICT_BINARY_UTF8","text":"
        #define JSON_STRICT_BINARY_UTF8 /* value */\n

        When defined to 1, the error_handler parameter of the binary writers to_cbor, to_ubjson, to_bjdata, and to_bson defaults to error_handler_t::strict instead of error_handler_t::keep. These writers then check every string value and object key for valid UTF-8 and throw type_error.316 for ill-formed UTF-8, like dump does. Without it, they write the bytes unchanged. An error_handler passed explicitly always takes precedence.

        The macro does not affect:

        • to_msgpack: the MessagePack specification allows a str value to contain bytes that are not valid UTF-8, so its error_handler always defaults to keep.
        • to_bon8: BON8 always checks, because the UTF-8 lead bytes mark where a string ends.
        • The binary readers (from_cbor, from_msgpack, from_ubjson, from_bjdata, from_bson): none of these formats requires a decoder to reject ill-formed UTF-8, so by default they return the bytes unchanged (error_handler_t::keep), independent of this macro. Pass an error_handler argument explicitly to validate or sanitize the strings they read.
        • from_bon8: BON8 always validates, because the UTF-8 lead bytes mark where a string ends.
        "},{"location":"api/macros/json_strict_binary_utf8/#default-definition","title":"Default definition","text":"

        The default value is 0 (disabled, the behavior of version 3.12.0 and earlier is preserved).

        #define JSON_STRICT_BINARY_UTF8 0\n
        "},{"location":"api/macros/json_strict_binary_utf8/#notes","title":"Notes","text":"

        Background

        CBOR, UBJSON, BJData, and BSON all require strings to be UTF-8. Up to version 3.12.0, the writers did not check this, so they could produce output that other decoders reject. Checking by default would break code that stores other encodings (for instance ISO 8859-1) in a string and only ever writes it to a binary format. You can pass error_handler_t::strict to each call, or use this macro to check by default ahead of version 4.0.0, where strict is planned to become the default (see #5529 and #5651).

        Opt-in only

        This macro must be defined before including <nlohmann/json.hpp>. Defining it after the include has no effect.

        ABI compatibility

        The value of this macro is encoded in the namespace (tag _sbu8), resulting in distinct symbol names. Translation units compiled with and without it can therefore be linked into the same program without One Definition Rule (ODR) violations, but they cannot exchange instances of library types.

        "},{"location":"api/macros/json_strict_binary_utf8/#examples","title":"Examples","text":"Example: default behavior (macro not defined)

        Without the macro, the bytes are written unchanged:

        #include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    auto v = json::to_cbor(json(\"\\xFF\"));\n    // v is {0x61, 0xFF}\n}\n
        Example: opt-in check (macro defined to 1)

        With the macro, ill-formed UTF-8 is rejected:

        #define JSON_STRICT_BINARY_UTF8 1\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    auto v = json::to_cbor(json(\"\\xFF\"));\n    // throws type_error.316: invalid UTF-8 byte at index 0: 0xFF\n}\n
        "},{"location":"api/macros/json_strict_binary_utf8/#see-also","title":"See also","text":"
        • to_cbor - create a CBOR serialization of a JSON value
        • to_ubjson - create a UBJSON serialization of a JSON value
        • to_bjdata - create a BJData serialization of a JSON value
        • to_bson - create a BSON serialization of a JSON value
        • error_handler_t - how dump treats ill-formed UTF-8
        "},{"location":"api/macros/json_strict_binary_utf8/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        • Planned to become the default (with the macro removed) in version 4.0.0.
        "},{"location":"api/macros/json_strict_nul_handling/","title":"JSON_STRICT_NUL_HANDLING","text":"
        #define JSON_STRICT_NUL_HANDLING /* value */\n

        When defined to 1, a '\\0' (NUL) byte in JSON text input is rejected with parse_error.101, like any other unexpected byte, instead of being silently treated as end of input.

        The macro only affects the JSON text parser (parse, accept, sax_parse, and operator>>). There are three cases where a NUL byte is still not rejected:

        • The binary formats (from_bjdata, from_bon8, from_bson, from_cbor, from_msgpack, from_ubjson) are never affected: there, 0x00 is ordinary data.
        • A bare const char* pointer has no length of its own, so its length is still determined with strlen(). The first NUL byte therefore still marks the end of the input, and nothing after it is read.
        • One trailing '\\0' at the end of a char, wchar_t, char16_t, char32_t, or (C++20) char8_t array (e.g., a string literal) is trimmed; see the warning below.
        "},{"location":"api/macros/json_strict_nul_handling/#default-definition","title":"Default definition","text":"

        The default value is 0 (disabled \u2014 existing behavior is preserved).

        #define JSON_STRICT_NUL_HANDLING 0\n
        "},{"location":"api/macros/json_strict_nul_handling/#notes","title":"Notes","text":"

        Background

        By default, a '\\0' byte anywhere in the input is treated the same as the real end of the input, rather than as an ordinary (and, outside of a string, invalid) byte. Everything from that byte onward is silently ignored, without a parse error - including further, otherwise well-formed JSON:

        json::parse(std::string(\"123\") + '\\0');          // == 123, no error\njson::parse(std::string(\"123\") + '\\0' + \"true\"); // == 123, the \"true\" is silently ignored too\n

        This falls out of the same convention used when no explicit input length is given at all: parsing from a const char* already stops at the first NUL byte via strlen(), since a bare pointer has no length of its own. The library applies that same NUL-terminated-C-string convention uniformly, rather than only when a length is genuinely unavailable - so a std::string, iterator range, or container whose content happens to include a NUL byte is affected the same way a raw const char* would be (see the FAQ entry for a fuller explanation).

        This was not fixed unconditionally, because doing so is backwards-incompatible for any caller who happens to depend on the current behavior - even unknowingly, for instance because their input already contains trailing padding they never noticed was being discarded (see #5530). This macro instead offers an opt-in path to the corrected behavior ahead of version 4.0.0, where it is planned to become the default.

        Opt-in only

        This macro must be defined before including <nlohmann/json.hpp>. Defining it after the include has no effect.

        Enabling it also changes how an array of a text-literal element type (char, wchar_t, char16_t, char32_t, or, since C++20, char8_t - including a string literal, e.g. json::parse(\"123\") or json::parse(L\"123\")) is read: such an array normally carries a trailing '\\0' contributed by the compiler, not by the source text. With this macro enabled, that one trailing element is trimmed if present so that parsing a string literal keeps working, for any of these character types; every other element in the array - including any '\\0' that is not the very last element - is read as real data and rejected like any other unexpected byte. Arrays of any other element type (unsigned char, std::uint8_t, ...), as used for CBOR or MessagePack, are never affected by this trimming; their full extent - including a genuine trailing 0x00 - is always preserved, in both states of this macro.

        ABI compatibility

        The value of this macro is encoded in the namespace (tag _snul), resulting in distinct symbol names. Translation units compiled with and without it can therefore be linked into the same program without One Definition Rule (ODR) violations, but they cannot exchange instances of library types.

        Workaround without the macro

        To reject a NUL byte without enabling this macro, trim your input yourself before calling parse():

        s.resize(s.find('\\0')); // drop everything from the first NUL onward, if any\njson::parse(s);\n
        "},{"location":"api/macros/json_strict_nul_handling/#examples","title":"Examples","text":"Example: default behavior (macro not defined)

        Without the macro, a NUL byte silently ends parsing at that point:

        #include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    json j = json::parse(std::string(\"123\") + '\\0' + \"true\");\n    // j is 123 -- the '\\0' and everything after it is silently ignored\n}\n
        Example: opt-in strict handling (macro defined to 1)

        With the macro, a NUL byte is rejected like any other unexpected byte:

        #define JSON_STRICT_NUL_HANDLING 1\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    json j = json::parse(std::string(\"123\") + '\\0' + \"true\");\n    // throws parse_error.101 -- the NUL byte is now invalid input,\n    // exactly like any other unexpected trailing byte\n\n    json ok = json::parse(\"123\");\n    // ok is 123 -- parsing from a string literal still works\n}\n
        "},{"location":"api/macros/json_strict_nul_handling/#see-also","title":"See also","text":"
        • JSON_StrictNulHandling - CMake option to control the macro
        • FAQ: NUL bytes in the input
        • parse - deserialize from a compatible input
        • accept - check if the input is valid JSON
        • operator>> - deserialize from stream
        "},{"location":"api/macros/json_strict_nul_handling/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        • Planned to become the default (with the macro removed) in version 4.0.0.
        "},{"location":"api/macros/json_throw_user/","title":"JSON_CATCH_USER, JSON_THROW_USER, JSON_TRY_USER","text":"
        // (1)\n#define JSON_CATCH_USER(exception) /* value */\n// (2)\n#define JSON_THROW_USER(exception) /* value */\n// (3)\n#define JSON_TRY_USER /* value */\n

        Controls how exceptions are handled by the library.

        1. This macro overrides catch calls inside the library. The argument is the type of the exception to catch. The library uses it in a single place: to swallow any exception escaping the parent-pointer check that JSON_DIAGNOSTICS adds to the class invariant. The places where the library catches its own json::out_of_range exceptions use JSON_INTERNAL_CATCH instead, which JSON_CATCH_USER also overrides unless JSON_INTERNAL_CATCH_USER is defined. The macro is always followed by a scope.
        2. This macro overrides throw calls inside the library. The argument is the exception to be thrown. Note that JSON_THROW_USER should leave the current scope (e.g., by throwing or aborting), as continuing after it may yield undefined behavior.
        3. This macro overrides try calls inside the library. It has no arguments and is always followed by a scope.
        "},{"location":"api/macros/json_throw_user/#parameters","title":"Parameters","text":"exception (in) an exception type"},{"location":"api/macros/json_throw_user/#default-definition","title":"Default definition","text":"

        By default, the macros map to their respective C++ keywords:

        #define JSON_CATCH_USER(exception) catch(exception)\n#define JSON_THROW_USER(exception) throw exception\n#define JSON_TRY_USER              try\n

        When exceptions are switched off, the try block is executed unconditionally, and throwing exceptions is replaced by calling std::abort to make reaching the throw branch abort the process.

        #define JSON_THROW_USER(exception) std::abort()\n#define JSON_TRY_USER              if (true)\n#define JSON_CATCH_USER(exception) if (false)\n
        "},{"location":"api/macros/json_throw_user/#examples","title":"Examples","text":"Example

        The code below switches off exceptions and creates a log entry with a detailed error message in case of errors.

        #include <iostream>\n\n#define JSON_TRY_USER if(true)\n#define JSON_CATCH_USER(exception) if(false)\n#define JSON_THROW_USER(exception)                           \\\n    {std::clog << \"Error in \" << __FILE__ << \":\" << __LINE__ \\\n               << \" (function \" << __FUNCTION__ << \") - \"    \\\n               << (exception).what() << std::endl;           \\\n     std::abort();}\n\n#include <nlohmann/json.hpp>\n
        "},{"location":"api/macros/json_throw_user/#see-also","title":"See also","text":"
        • Switch off exceptions for more information how to switch off exceptions
        • JSON_NOEXCEPTION - switch off exceptions
        "},{"location":"api/macros/json_throw_user/#version-history","title":"Version history","text":"
        • Added in version 3.1.0.
        "},{"location":"api/macros/json_use_global_udls/","title":"JSON_USE_GLOBAL_UDLS","text":"
        #define JSON_USE_GLOBAL_UDLS /* value */\n

        When defined to 1, the user-defined string literals (UDLs) are placed into the global namespace instead of nlohmann::literals::json_literals.

        "},{"location":"api/macros/json_use_global_udls/#default-definition","title":"Default definition","text":"

        The default value is 1.

        #define JSON_USE_GLOBAL_UDLS 1\n

        When the macro is not defined, the library behaves as if it were defined to its default value.

        "},{"location":"api/macros/json_use_global_udls/#notes","title":"Notes","text":"

        Future behavior change

        The user-defined string literals will be removed from the global namespace in the next major release of the library.

        To prepare existing code, define JSON_USE_GLOBAL_UDLS to 0 and bring the string literals into scope where needed. Refer to any of the string literals for details.

        See the migration guide for how to update existing code.

        CMake option

        The placement of user-defined string literals can also be controlled with the CMake option JSON_GlobalUDLs (ON by default) which defines JSON_USE_GLOBAL_UDLS accordingly.

        Leaving out the literals

        If JSON_NO_AUTOMATIC_UDLS is defined, the literals are only declared where <nlohmann/json_literals.hpp> is included; this macro then applies to that header.

        "},{"location":"api/macros/json_use_global_udls/#examples","title":"Examples","text":"Example: Default behavior

        The code below shows the default behavior using the _json UDL.

        #include <nlohmann/json.hpp>\n\n#include <iostream>\n\nint main()\n{\n    auto j = \"42\"_json;\n\n    std::cout << j << std::endl;\n}\n

        Output:

        42\n
        Example: Namespaced UDLs

        The code below shows how UDLs need to be brought into scope before using _json when JSON_USE_GLOBAL_UDLS is defined to 0.

        #define JSON_USE_GLOBAL_UDLS 0\n#include <nlohmann/json.hpp>\n\n#include <iostream>\n\nint main()\n{\n    // auto j = \"42\"_json; // This line would fail to compile,\n                           // because the UDLs are not in the global namespace\n\n    // Bring the UDLs into scope\n    using namespace nlohmann::json_literals;\n\n    auto j = \"42\"_json;\n\n    std::cout << j << std::endl;\n}\n

        Output:

        42\n
        "},{"location":"api/macros/json_use_global_udls/#see-also","title":"See also","text":"
        • operator\"\"_json
        • operator\"\"_json_pointer
        • JSON_NO_AUTOMATIC_UDLS - do not include the user-defined string literals automatically
        • JSON_GlobalUDLs - CMake option to control the macro
        "},{"location":"api/macros/json_use_global_udls/#version-history","title":"Version history","text":"
        • Added in version 3.11.0.
        "},{"location":"api/macros/json_use_implicit_conversions/","title":"JSON_USE_IMPLICIT_CONVERSIONS","text":"
        #define JSON_USE_IMPLICIT_CONVERSIONS /* value */\n

        When defined to 0, implicit conversions are switched off. By default, implicit conversions are switched on. The value directly affects operator ValueType and the converting constructor from a basic_json specialization with a different string type (overload 4).

        "},{"location":"api/macros/json_use_implicit_conversions/#default-definition","title":"Default definition","text":"

        By default, implicit conversions are enabled.

        #define JSON_USE_IMPLICIT_CONVERSIONS 1\n
        "},{"location":"api/macros/json_use_implicit_conversions/#notes","title":"Notes","text":"

        Future behavior change

        Implicit conversions will be switched off by default in the next major release of the library.

        You can prepare existing code by already defining JSON_USE_IMPLICIT_CONVERSIONS to 0 and replace any implicit conversions with calls to get.

        See the migration guide for how to update existing code.

        Automatic migration

        The community-maintained clang-tidy check modernize-nlohmann-json-explicit-conversions rewrites implicit conversions into explicit calls to get; for example, int i = j; becomes int i = j.get<int>();. The check is not part of clang-tidy itself, and it does not catch every case (for example, constructing a std::optional from a JSON value), so review the result. See discussion #4610 for how to build and use it.

        CMake option

        Implicit conversions can also be controlled with the CMake option JSON_ImplicitConversions (ON by default) which defines JSON_USE_IMPLICIT_CONVERSIONS accordingly.

        "},{"location":"api/macros/json_use_implicit_conversions/#examples","title":"Examples","text":"Example: implicit and explicit conversions

        This is an example for an implicit conversion:

        json j = \"Hello, world!\";\nstd::string s = j;\n

        When JSON_USE_IMPLICIT_CONVERSIONS is defined to 0, the code above does no longer compile. Instead, it must be written like this:

        json j = \"Hello, world!\";\nauto s = j.get<std::string>();\n
        Example: conversion between basic_json specializations

        A basic_json specialization with a different string type is also no longer converted implicitly when JSON_USE_IMPLICIT_CONVERSIONS is defined to 0:

        using wjson = nlohmann::json::with_string_t<std::wstring>;\n\nvoid load(const nlohmann::json& j);\n\nwjson wj = /* ... */;\nload(wj);                            // error: no implicit conversion\nload(nlohmann::json(wj));            // OK: explicit conversion\nload(wj.get<nlohmann::json>());      // OK: explicit conversion\n

        Specializations that share the same string type, such as json and ordered_json, remain implicitly convertible.

        "},{"location":"api/macros/json_use_implicit_conversions/#see-also","title":"See also","text":"
        • operator ValueType - get a value (implicit)
        • get - get a value (explicit)
        • JSON_ImplicitConversions - CMake option to control the macro
        "},{"location":"api/macros/json_use_implicit_conversions/#version-history","title":"Version history","text":"
        • Added in version 3.9.0.
        • Also affects the conversion between basic_json specializations with different string types since version 3.13.0 unreleased.
        "},{"location":"api/macros/json_use_legacy_discarded_value_comparison/","title":"JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON","text":"
        #define JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON /* value */\n

        This macro enables the (incorrect) legacy comparison behavior of discarded JSON values. Possible values are 1 to enable or 0 to disable (default).

        When enabled, comparisons involving at least one discarded JSON value yield results as follows:

        Operator Result == false != true < false <= true >= true > false

        Otherwise, comparisons involving at least one discarded JSON value yield false, except for !=, which is the negation of == and yields true.

        "},{"location":"api/macros/json_use_legacy_discarded_value_comparison/#default-definition","title":"Default definition","text":"

        The default value is 0.

        #define JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON 0\n

        When the macro is not defined, the library will define it to its default value.

        "},{"location":"api/macros/json_use_legacy_discarded_value_comparison/#notes","title":"Notes","text":"

        Inconsistent behavior in C++20 and beyond

        When targeting C++20 or above, enabling the legacy comparison behavior is strongly discouraged.

        • The 3-way comparison operator (<=>) will always give the correct result (std::partial_ordering::unordered) regardless of the value of JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON.
        • Overloads for the equality and relational operators emulate the legacy behavior.

        Code outside your control may use either 3-way comparison or the equality and relational operators, resulting in inconsistent and unpredictable behavior.

        See operator<=> for more information on 3-way comparison.

        Deprecation

        The legacy comparison behavior is deprecated and may be removed in a future major version release.

        New code should not depend on it and existing code should try to remove or rewrite expressions relying on it.

        See the migration guide for how to update existing code.

        CMake option

        Legacy comparison can also be controlled with the CMake option JSON_LegacyDiscardedValueComparison (OFF by default) which defines JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON accordingly.

        "},{"location":"api/macros/json_use_legacy_discarded_value_comparison/#examples","title":"Examples","text":"Example

        The code below switches on the legacy discarded value comparison behavior in the library.

        #define JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON 1\n#include <nlohmann/json.hpp>\n\n...\n
        "},{"location":"api/macros/json_use_legacy_discarded_value_comparison/#see-also","title":"See also","text":"
        • JSON_LegacyDiscardedValueComparison - CMake option to control the macro
        "},{"location":"api/macros/json_use_legacy_discarded_value_comparison/#version-history","title":"Version history","text":"
        • Added in version 3.11.0.
        • Fixed in version 3.13.0 unreleased so <= and >= also emulate the legacy behavior in C++20 when the JSON value is the right-hand operand of a scalar comparison; before, only the 3-way-comparison-rewritten candidate was found, which yielded false instead of true.
        "},{"location":"api/macros/json_use_objects_for_enum_keyed_maps/","title":"JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS","text":"
        #define JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS /* value */\n

        When defined to 1, maps whose keys are enums (such as std::map<E, T> or std::unordered_map<E, T>) are stored as JSON objects, using the enum's own conversion for the keys. By default, they are stored as arrays of [key, value] pairs.

        "},{"location":"api/macros/json_use_objects_for_enum_keyed_maps/#default-definition","title":"Default definition","text":"

        The default value is 0 (disabled \u2014 existing behavior is preserved).

        #define JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS 0\n
        "},{"location":"api/macros/json_use_objects_for_enum_keyed_maps/#notes","title":"Notes","text":"

        Background

        JSON object keys are strings, so a map is only stored as an object if its keys can be converted to a string type. Enums are not, even if NLOHMANN_JSON_SERIALIZE_ENUM maps them to strings, so a map with enum keys becomes an array of [key, value] pairs:

        [[\"stopped\", \"aa\"], [\"completed\", \"bb\"]]\n

        With this macro, the same map becomes an object (see #4378):

        {\"completed\": \"bb\", \"stopped\": \"aa\"}\n

        Maps with non-unique keys

        Maps that allow duplicate keys, such as std::multimap<E, T> or std::unordered_multimap<E, T>, are not affected by the macro and are still stored as arrays of [key, value] pairs, as an object cannot hold duplicate keys.

        Reading

        Reading is not affected by the macro: a map with enum keys can always be read from both an array of pairs and an object. For the latter, each key is converted to the enum with its from_json function, e.g., the one defined by NLOHMANN_JSON_SERIALIZE_ENUM. Data written without the macro can therefore still be read after enabling it.

        Keys must serialize to distinct strings

        Each key is converted with the enum's to_json function. If a key is not converted to a string (for instance, an enum without NLOHMANN_JSON_SERIALIZE_ENUM, which is stored as an integer, or an enumerator mapped to nullptr), type_error.302 is thrown. If two keys are converted to the same string (for instance, because NLOHMANN_JSON_SERIALIZE_ENUM maps an unlisted enumerator to the first entry), type_error.318 is thrown. In both cases, the target value is not changed.

        Opt-in only

        This macro must be defined before including <nlohmann/json.hpp>. Defining it after the include has no effect.

        ABI compatibility

        The value of this macro is encoded in the namespace (tag _ekmo), resulting in distinct symbol names. Translation units compiled with and without it can therefore be linked into the same program without One Definition Rule (ODR) violations, but they cannot exchange instances of library types.

        "},{"location":"api/macros/json_use_objects_for_enum_keyed_maps/#examples","title":"Examples","text":"Example: default behavior (macro not defined)

        Without the macro, a map with enum keys is stored as an array of pairs:

        #include <map>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nenum TaskState { TS_STOPPED, TS_RUNNING, TS_COMPLETED };\n\nNLOHMANN_JSON_SERIALIZE_ENUM(TaskState, {\n    {TS_STOPPED, \"stopped\"},\n    {TS_RUNNING, \"running\"},\n    {TS_COMPLETED, \"completed\"},\n})\n\nint main()\n{\n    std::map<TaskState, std::string> m = {{TS_STOPPED, \"aa\"}, {TS_COMPLETED, \"bb\"}};\n\n    json j = m;\n    // j is [[\"stopped\",\"aa\"],[\"completed\",\"bb\"]]\n}\n
        Example: objects for enum-keyed maps (macro defined to 1)

        With the macro, the same map is stored as an object:

        #define JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS 1\n#include <map>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nenum TaskState { TS_STOPPED, TS_RUNNING, TS_COMPLETED };\n\nNLOHMANN_JSON_SERIALIZE_ENUM(TaskState, {\n    {TS_STOPPED, \"stopped\"},\n    {TS_RUNNING, \"running\"},\n    {TS_COMPLETED, \"completed\"},\n})\n\nint main()\n{\n    std::map<TaskState, std::string> m = {{TS_STOPPED, \"aa\"}, {TS_COMPLETED, \"bb\"}};\n\n    json j = m;\n    // j is {\"completed\":\"bb\",\"stopped\":\"aa\"}\n\n    auto m2 = j.get<std::map<TaskState, std::string>>();\n    // m2 == m\n}\n
        "},{"location":"api/macros/json_use_objects_for_enum_keyed_maps/#see-also","title":"See also","text":"
        • Specializing enum conversion
        • NLOHMANN_JSON_SERIALIZE_ENUM - serialize/deserialize an enum
        • NLOHMANN_JSON_SERIALIZE_ENUM_STRICT - serialize/deserialize an enum with exceptions
        "},{"location":"api/macros/json_use_objects_for_enum_keyed_maps/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/macros/json_use_simdutf/","title":"JSON_USE_SIMDUTF","text":"
        #define JSON_USE_SIMDUTF\n

        When defined, the parser validates the UTF-8 content of JSON strings that come from a contiguous byte input (std::string, std::vector<char>/<std::uint8_t>, string literals, const char* ranges, \u2026) using the simdutf library instead of the built-in scalar validator. On text with many non-ASCII characters (e.g. CJK or emoji) this can validate several times faster.

        This is an opt-in external dependency. The library itself remains header-only and its behavior is unchanged: the same input is accepted or rejected either way, and every parse error is reported at the same position with the same message (simdutf is only used to fast-path valid runs; anything it flags falls back to the scalar path so the exact diagnostic is preserved). A std::istream uses simdutf for the characters its stream buffer holds (see JSON_DISABLE_STREAM_BULK_SCAN); other streaming inputs (FILE*, wide strings, user-defined adapters) always use the scalar path.

        When JSON_USE_SIMDUTF is defined you must make the simdutf.h header available on the include path and link the simdutf library. When it is not defined, no simdutf header is included and there is no dependency.

        Requires C++17

        simdutf requires C++17 and its header rejects older standards with an #error. The backend is therefore only compiled in from C++17 on. In C++11 and C++14 the macro has no effect and the scalar validator is used, which accepts and rejects exactly the same input -- only throughput differs. Setting the macro project-wide is therefore safe even when some translation units are built with an older standard.

        Define consistently

        The macro selects between two definitions of the same inline validation function. It must therefore be defined identically for every translation unit that includes the library; mixing translation units that define it with ones that do not is an ODR violation. Prefer setting it as a compile definition on the target rather than with #define in individual source files.

        "},{"location":"api/macros/json_use_simdutf/#default-definition","title":"Default definition","text":"

        By default, JSON_USE_SIMDUTF is not defined and the portable C++11 scalar validator is used.

        #undef JSON_USE_SIMDUTF\n
        "},{"location":"api/macros/json_use_simdutf/#examples","title":"Examples","text":"Example

        The code below enables the simdutf backend for UTF-8 validation.

        #define JSON_USE_SIMDUTF 1\n#include <nlohmann/json.hpp>\n\n...\n

        The project must also link against simdutf, e.g. with CMake:

        target_compile_definitions(your_target PRIVATE JSON_USE_SIMDUTF)\ntarget_link_libraries(your_target PRIVATE simdutf::simdutf)\n

        Testing this configuration

        The unit tests can be built against the simdutf backend with the CMake option JSON_TestSimdutf (OFF by default), which fetches simdutf and defines JSON_USE_SIMDUTF for every test target. The ci_test_simdutf target runs the whole test suite in that configuration.

        "},{"location":"api/macros/json_use_simdutf/#see-also","title":"See also","text":"
        • JSON_TestSimdutf - CMake option to build the unit tests against the simdutf backend
        "},{"location":"api/macros/json_use_simdutf/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/macros/json_view_no_simd/","title":"JSON_VIEW_NO_SIMD","text":"
        #define JSON_VIEW_NO_SIMD\n

        When defined, the parser of basic_json_document (<nlohmann/json_view.hpp>) uses only portable C++ to scan strings. By default, it scans long runs of string bytes 16 at a time with NEON on AArch64 (with GCC and Clang) and SSE2 on x86-64, which are part of the baseline instruction sets of these architectures, and validates non-ASCII text with NEON (or SSSE3, see JSON_VIEW_USE_SSSE3).

        The same input is accepted or rejected either way, with the same values, and errors are reported the same way; only the speed differs. The macro exists for platforms whose compilers lack the intrinsics headers, and to test the portable code.

        ABI compatibility

        The macro selects between two definitions of the same inline functions. Whether it is defined is encoded in the namespace (tag _vns), resulting in distinct symbol names. Translation units compiled with and without it can therefore be linked into the same program without One Definition Rule (ODR) violations, but they cannot exchange instances of library types.

        "},{"location":"api/macros/json_view_no_simd/#default-definition","title":"Default definition","text":"

        By default, JSON_VIEW_NO_SIMD is not defined, and the vector code is used where available.

        #undef JSON_VIEW_NO_SIMD\n
        "},{"location":"api/macros/json_view_no_simd/#examples","title":"Examples","text":"Example

        The code below uses the portable string scanning of the view.

        #define JSON_VIEW_NO_SIMD\n#include <nlohmann/json_view.hpp>\n\n...\n
        "},{"location":"api/macros/json_view_no_simd/#see-also","title":"See also","text":"
        • JSON_VIEW_USE_SSSE3 - validate non-ASCII strings with SSSE3 on x86-64
        • json_view - the zero-copy view
        "},{"location":"api/macros/json_view_no_simd/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/macros/json_view_use_ssse3/","title":"JSON_VIEW_USE_SSSE3","text":"
        #define JSON_VIEW_USE_SSSE3\n

        When defined on x86-64, the parser of basic_json_document (<nlohmann/json_view.hpp>) validates non-ASCII text in strings with SSSE3 without asking the CPU first.

        By default, the parser checks once at run time whether the CPU has SSSE3 (all x86-64 CPUs since about 2011 have it) and then validates non-ASCII text 16 bytes at a time, using the \"lookup4\" algorithm of simdjson; on CPUs without SSSE3, it validates one UTF-8 sequence at a time. The vector check is compiled for SSSE3 with a function attribute (GCC 4.9 and later, Clang), so this needs no compiler option. With MSVC, the check uses __cpuid. On AArch64, the vector check uses NEON and is always on.

        Define the macro only together with a compiler option that enables SSSE3 (e.g. -mssse3, or -march= with a CPU that has it), and only for programs that run on such CPUs. It saves the check of the CPU, which costs little. The same input is accepted or rejected either way; only the speed of non-ASCII text differs.

        ABI compatibility

        The macro selects between two definitions of the same inline functions. Whether it is defined is encoded in the namespace (tag _vssse3, on every architecture), resulting in distinct symbol names. Translation units compiled with and without it can therefore be linked into the same program without One Definition Rule (ODR) violations, but they cannot exchange instances of library types.

        "},{"location":"api/macros/json_view_use_ssse3/#default-definition","title":"Default definition","text":"

        By default, JSON_VIEW_USE_SSSE3 is not defined.

        #undef JSON_VIEW_USE_SSSE3\n
        "},{"location":"api/macros/json_view_use_ssse3/#examples","title":"Examples","text":"Example

        With CMake, for a program that only runs on CPUs with SSSE3:

        target_compile_definitions(your_target PRIVATE JSON_VIEW_USE_SSSE3)\ntarget_compile_options(your_target PRIVATE -mssse3)\n
        "},{"location":"api/macros/json_view_use_ssse3/#see-also","title":"See also","text":"
        • JSON_VIEW_NO_SIMD - use only portable code in the view's parser
        • JSON_USE_SIMDUTF - validate UTF-8 with simdutf in basic_json's parser
        "},{"location":"api/macros/json_view_use_ssse3/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/macros/nlohmann_define_derived_type/","title":"NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE, NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_DEFAULT, NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_ONLY_SERIALIZE, NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE, NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_DEFAULT, NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE","text":"NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE, NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_DEFAULT, NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_ONLY_SERIALIZE, NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE, NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_DEFAULT, NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE 
        // (1)\n#define NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE(type, base_type, member...)\n// (2)\n#define NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_DEFAULT(type, base_type, member...)\n// (3)\n#define NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_ONLY_SERIALIZE(type, base_type, member...)\n\n// (4)\n#define NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE(type, base_type, member...)\n// (5)\n#define NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_DEFAULT(type, base_type, member...)\n// (6)\n#define NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE(type, base_type, member...)\n

        These macros can be used to simplify the serialization/deserialization of derived types if you want to use a JSON object as serialization and want to use the member variable names as object keys in that object.

        • Macros 1, 2, and 3 are to be defined inside the class/struct to create code for. Like NLOHMANN_DEFINE_TYPE_INTRUSIVE, they can access private members.
        • Macros 4, 5, and 6 are to be defined outside the class/struct to create code for, but inside its namespace. Like NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE, they cannot access private members.

        The first parameter is the name of the derived class/struct, the second parameter is the name of the base class/struct and all remaining parameters name the members. The base type must be already serializable/deserializable.

        • Macros 1 and 4 will use at during deserialization and will throw out_of_range.403 if a key is missing in the JSON object.
        • Macros 2 and 5 will use value during deserialization and fall back to the default value for the respective type of the member variable if a key in the JSON object is missing. The generated from_json() function default constructs an object and uses its values as the defaults when calling the value function.

        Summary:

        Need access to private members Need only serialization Allow missing values when de-serializing macro NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_DEFAULT NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_ONLY_SERIALIZE NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_DEFAULT NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE"},{"location":"api/macros/nlohmann_define_derived_type/#parameters","title":"Parameters","text":"type (in) name of the type (class, struct) to serialize/deserialize base_type (in) name of the base type (class, struct) type is derived from member (in) name of the member variable to serialize/deserialize; up to 63 members can be given as a comma-separated list, which may also be empty"},{"location":"api/macros/nlohmann_define_derived_type/#default-definition","title":"Default definition","text":"

        Macros 1 and 2 add two friend functions to the class which take care of the serialization and deserialization:

        template<typename BasicJsonType>\nfriend void to_json(BasicJsonType&, const type&);\ntemplate<typename BasicJsonType>\nfriend void from_json(const BasicJsonType&, type&);\n

        Macros 4 and 5 add two functions to the namespace which take care of the serialization and deserialization:

        template<typename BasicJsonType>\nvoid to_json(BasicJsonType&, const type&);\ntemplate<typename BasicJsonType>\nvoid from_json(const BasicJsonType&, type&);\n

        Macros 3 and 6 add one function to the namespace, which takes care of the serialization only:

        template<typename BasicJsonType>\nvoid to_json(BasicJsonType&, const type&);\n

        In first two cases, they call the to_json/from_json functions of the base type before serializing/deserializing the members of the derived type:

        class A { /* ... */ };\nclass B : public A { /* ... */ };\n\ntemplate<typename BasicJsonType>\nvoid to_json(BasicJsonType& j, const B& b) {\n    nlohmann::to_json(j, static_cast<const A&>(b));\n    // ...\n}\n\ntemplate<typename BasicJsonType>\nvoid from_json(const BasicJsonType& j, B& b) {\n    nlohmann::from_json(j, static_cast<A&>(b));\n    // ...\n}\n

        In the third case, only to_json will be called:

        class A { /* ... */ };\nclass B : public A { /* ... */ };\n\ntemplate<typename BasicJsonType>\nvoid to_json(BasicJsonType& j, const B& b) {\n    nlohmann::to_json(j, static_cast<const A&>(b));\n    // ...\n}\n
        "},{"location":"api/macros/nlohmann_define_derived_type/#notes","title":"Notes","text":"

        Prerequisites

        • Macros 1, 2, and 3 have the same prerequisites of NLOHMANN_DEFINE_TYPE_INTRUSIVE.
        • Macros 4, 5, and 6 have the same prerequisites of NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE.
        • Serialization/deserialization of base types must be defined.

        Derived types without own members

        The member list may be empty. The macro then generates a to_json/from_json pair that only delegates to the base type, so type serializes exactly like base_type:

        struct derived : base\n{\n    NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE(derived, base)\n};\n

        The WITH_NAMES variants do not support this.

        Implementation limits

        See Implementation limits for NLOHMANN_DEFINE_TYPE_INTRUSIVE and NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE, respectively.

        "},{"location":"api/macros/nlohmann_define_derived_type/#examples","title":"Examples","text":"Example: (1) NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE

        Consider the following complete example:

        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing nlohmann::json;\n\nclass A\n{\n  private:\n    double Aa = 0.0;\n    double Ab = 0.0;\n\n  public:\n    A() = default;\n    A(double a, double b) : Aa(a), Ab(b) {}\n    NLOHMANN_DEFINE_TYPE_INTRUSIVE(A, Aa, Ab)\n};\n\nclass B : public A\n{\n  private:\n    int Ba = 0;\n    int Bb = 0;\n\n  public:\n    B() = default;\n    B(int a, int b, double aa, double ab) : A(aa, ab), Ba(a), Bb(b) {}\n    NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE(B, A, Ba, Bb)\n};\n\nint main()\n{\n    B example(23, 42, 3.142, 1.777);\n    json example_json = example;\n\n    std::cout << std::setw(4) << example_json << std::endl;\n}\n

        Output:

        {\n    \"Aa\": 3.142,\n    \"Ab\": 1.777,\n    \"Ba\": 23,\n    \"Bb\": 42\n}\n

        Notes:

        • A and B are default-constructible. This is a requirement for using the macro.
        • A has private members and is not a derived class. Hence, macro NLOHMANN_DEFINE_TYPE_INTRUSIVE is used.
        • As B is a derived class, NLOHMANN_DEFINE_TYPE_INTRUSIVE is not applicable, but NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE must be used.
        • The macro NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE is used inside the class use as NLOHMANN_DEFINE_TYPE_INTRUSIVE.
        "},{"location":"api/macros/nlohmann_define_derived_type/#see-also","title":"See also","text":"
        • NLOHMANN_DEFINE_TYPE_INTRUSIVE / NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT / NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE for similar macros that can be defined inside a non-derived type.
        • NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE / NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT / NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE for similar macros that can be defined outside a non-derived type.
        • Arbitrary Type Conversions for an overview.
        "},{"location":"api/macros/nlohmann_define_derived_type/#version-history","title":"Version history","text":"
        1. Added in version 3.12.0.
        2. Added in version 3.12.0.
        3. Added in version 3.12.0.
        4. Added in version 3.12.0.
        5. Added in version 3.12.0.
        6. Added in version 3.12.0.

        All six macros were changed to allow an empty member list in version 3.13.0 unreleased.

        "},{"location":"api/macros/nlohmann_define_type_intrusive/","title":"NLOHMANN_DEFINE_TYPE_INTRUSIVE, NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT, NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE","text":"
        #define NLOHMANN_DEFINE_TYPE_INTRUSIVE(type, member...)              // (1)\n#define NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT(type, member...) // (2)\n#define NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE(type, member...) // (3)\n

        These macros can be used to simplify the serialization/deserialization of types if you want to use a JSON object as serialization and want to use the member variable names as object keys in that object. The macro is to be defined inside the class/struct to create code for. Unlike NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE, it can access private members. The first parameter is the name of the class/struct, and all remaining parameters name the members.

        1. Will use at during deserialization and will throw out_of_range.403 if a key is missing in the JSON object.
        2. Will use value during deserialization and fall back to the default value for the respective type of the member variable if a key in the JSON object is missing. The generated from_json() function default constructs an object and uses its values as the defaults when calling the value function.
        3. Only defines the serialization. Useful in cases when the type does not have a default constructor and only serialization is required.

        Summary:

        Need access to private members Need only serialization Allow missing values when de-serializing macro NLOHMANN_DEFINE_TYPE_INTRUSIVE NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE"},{"location":"api/macros/nlohmann_define_type_intrusive/#parameters","title":"Parameters","text":"type (in) name of the type (class, struct) to serialize/deserialize member (in) name of the member variable to serialize/deserialize; up to 63 members can be given as a comma-separated list, which may also be empty"},{"location":"api/macros/nlohmann_define_type_intrusive/#default-definition","title":"Default definition","text":"

        The macros add two friend functions to the class which take care of the serialization and deserialization:

        template<typename BasicJsonType>\nfriend void to_json(BasicJsonType&, const type&);\ntemplate<typename BasicJsonType>\nfriend void from_json(const BasicJsonType&, type&); // except (3)\n

        See the examples below for the concrete generated code.

        "},{"location":"api/macros/nlohmann_define_type_intrusive/#notes","title":"Notes","text":"

        Prerequisites

        1. The type type must be default constructible (except (3)). See How can I use get() for non-default constructible/non-copyable types? for how to overcome this limitation.
        2. The macro must be used inside the type (class/struct).

        Types without members

        The member list may be empty. The macro then generates a to_json that produces an empty JSON object {}, and a from_json that reads no members:

        struct marker\n{\n    NLOHMANN_DEFINE_TYPE_INTRUSIVE(marker)\n};\n

        The WITH_NAMES variants do not support this.

        Implementation limits

        • The current implementation is limited to at most 63 member variables. If you want to serialize/deserialize types with more than 63 member variables, you need to define the to_json/from_json functions manually.
        • These macros always produce object-style (named-key) JSON, one key per member. There is no macro variant that serializes a struct's members positionally into a JSON array; for that, write to_json/from_json by hand, building/reading a json::array() of the members in order.
        "},{"location":"api/macros/nlohmann_define_type_intrusive/#examples","title":"Examples","text":"Example: (1) NLOHMANN_DEFINE_TYPE_INTRUSIVE

        Consider the following complete example:

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nnamespace ns\n{\nclass person\n{\n  private:\n    std::string name = \"John Doe\";\n    std::string address = \"123 Fake St\";\n    int age = -1;\n\n  public:\n    person() = default;\n    person(std::string name_, std::string address_, int age_)\n        : name(std::move(name_)), address(std::move(address_)), age(age_)\n    {}\n\n    NLOHMANN_DEFINE_TYPE_INTRUSIVE(person, name, address, age)\n};\n} // namespace ns\n\nint main()\n{\n    ns::person p = {\"Ned Flanders\", \"744 Evergreen Terrace\", 60};\n\n    // serialization: person -> json\n    json j = p;\n    std::cout << \"serialization: \" << j << std::endl;\n\n    // deserialization: json -> person\n    json j2 = R\"({\"address\": \"742 Evergreen Terrace\", \"age\": 40, \"name\": \"Homer Simpson\"})\"_json;\n    auto p2 = j2.get<ns::person>();\n\n    // incomplete deserialization:\n    json j3 = R\"({\"address\": \"742 Evergreen Terrace\", \"name\": \"Maggie Simpson\"})\"_json;\n    try\n    {\n        auto p3 = j3.get<ns::person>();\n    }\n    catch (const json::exception& e)\n    {\n        std::cout << \"deserialization failed: \" << e.what() << std::endl;\n    }\n}\n

        Output:

        serialization: {\"address\":\"744 Evergreen Terrace\",\"age\":60,\"name\":\"Ned Flanders\"}\ndeserialization failed: [json.exception.out_of_range.403] key 'age' not found\n

        Notes:

        • ns::person is default-constructible. This is a requirement for using the macro.
        • ns::person has private member variables. This makes NLOHMANN_DEFINE_TYPE_INTRUSIVE applicable, but not NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE.
        • The macro NLOHMANN_DEFINE_TYPE_INTRUSIVE is used inside the class.
        • A missing key \"age\" in the deserialization yields an exception. To fall back to the default value, NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT can be used.

        The macro is equivalent to:

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nnamespace ns\n{\nclass person\n{\n  private:\n    std::string name = \"John Doe\";\n    std::string address = \"123 Fake St\";\n    int age = -1;\n\n  public:\n    person() = default;\n    person(std::string name_, std::string address_, int age_)\n        : name(std::move(name_)), address(std::move(address_)), age(age_)\n    {}\n\n    template<typename BasicJsonType>\n    friend void to_json(BasicJsonType& nlohmann_json_j, const person& nlohmann_json_t)\n    {\n        nlohmann_json_j[\"name\"] = nlohmann_json_t.name;\n        nlohmann_json_j[\"address\"] = nlohmann_json_t.address;\n        nlohmann_json_j[\"age\"] = nlohmann_json_t.age;\n    }\n\n    template<typename BasicJsonType>\n    friend void from_json(const BasicJsonType& nlohmann_json_j, person& nlohmann_json_t)\n    {\n        nlohmann_json_t.name = nlohmann_json_j.at(\"name\");\n        nlohmann_json_t.address = nlohmann_json_j.at(\"address\");\n        nlohmann_json_t.age = nlohmann_json_j.at(\"age\");\n    }\n};\n} // namespace ns\n\nint main()\n{\n    ns::person p = {\"Ned Flanders\", \"744 Evergreen Terrace\", 60};\n\n    // serialization: person -> json\n    json j = p;\n    std::cout << \"serialization: \" << j << std::endl;\n\n    // deserialization: json -> person\n    json j2 = R\"({\"address\": \"742 Evergreen Terrace\", \"age\": 40, \"name\": \"Homer Simpson\"})\"_json;\n    auto p2 = j2.get<ns::person>();\n\n    // incomplete deserialization:\n    json j3 = R\"({\"address\": \"742 Evergreen Terrace\", \"name\": \"Maggie Simpson\"})\"_json;\n    try\n    {\n        auto p3 = j3.get<ns::person>();\n    }\n    catch (const json::exception& e)\n    {\n        std::cout << \"deserialization failed: \" << e.what() << std::endl;\n    }\n}\n
        Example: (2) NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT

        Consider the following complete example:

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nnamespace ns\n{\nclass person\n{\n  private:\n    std::string name = \"John Doe\";\n    std::string address = \"123 Fake St\";\n    int age = -1;\n\n  public:\n    person() = default;\n    person(std::string name_, std::string address_, int age_)\n        : name(std::move(name_)), address(std::move(address_)), age(age_)\n    {}\n\n    NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT(person, name, address, age)\n};\n} // namespace ns\n\nint main()\n{\n    ns::person p = {\"Ned Flanders\", \"744 Evergreen Terrace\", 60};\n\n    // serialization: person -> json\n    json j = p;\n    std::cout << \"serialization: \" << j << std::endl;\n\n    // deserialization: json -> person\n    json j2 = R\"({\"address\": \"742 Evergreen Terrace\", \"age\": 40, \"name\": \"Homer Simpson\"})\"_json;\n    auto p2 = j2.get<ns::person>();\n\n    // incomplete deserialization:\n    json j3 = R\"({\"address\": \"742 Evergreen Terrace\", \"name\": \"Maggie Simpson\"})\"_json;\n    auto p3 = j3.get<ns::person>();\n    std::cout << \"roundtrip: \" << json(p3) << std::endl;\n}\n

        Output:

        serialization: {\"address\":\"744 Evergreen Terrace\",\"age\":60,\"name\":\"Ned Flanders\"}\nroundtrip: {\"address\":\"742 Evergreen Terrace\",\"age\":-1,\"name\":\"Maggie Simpson\"}\n

        Notes:

        • ns::person is default-constructible. This is a requirement for using the macro.
        • ns::person has private member variables. This makes NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT applicable, but not NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT.
        • The macro NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT is used inside the class.
        • A missing key \"age\" in the deserialization does not yield an exception. Instead, the default value -1 is used.

        The macro is equivalent to:

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nnamespace ns\n{\nclass person\n{\n  private:\n    std::string name = \"John Doe\";\n    std::string address = \"123 Fake St\";\n    int age = -1;\n\n  public:\n    person() = default;\n    person(std::string name_, std::string address_, int age_)\n        : name(std::move(name_)), address(std::move(address_)), age(age_)\n    {}\n\n    template<typename BasicJsonType>\n    friend void to_json(BasicJsonType& nlohmann_json_j, const person& nlohmann_json_t)\n    {\n        nlohmann_json_j[\"name\"] = nlohmann_json_t.name;\n        nlohmann_json_j[\"address\"] = nlohmann_json_t.address;\n        nlohmann_json_j[\"age\"] = nlohmann_json_t.age;\n    }\n\n    template<typename BasicJsonType>\n    friend void from_json(const BasicJsonType& nlohmann_json_j, person& nlohmann_json_t)\n    {\n        person nlohmann_json_default_obj;\n        nlohmann_json_t.name = nlohmann_json_j.value(\"name\", nlohmann_json_default_obj.name);\n        nlohmann_json_t.address = nlohmann_json_j.value(\"address\", nlohmann_json_default_obj.address);\n        nlohmann_json_t.age = nlohmann_json_j.value(\"age\", nlohmann_json_default_obj.age);\n    }\n};\n} // namespace ns\n\nint main()\n{\n    ns::person p = {\"Ned Flanders\", \"744 Evergreen Terrace\", 60};\n\n    // serialization: person -> json\n    json j = p;\n    std::cout << \"serialization: \" << j << std::endl;\n\n    // deserialization: json -> person\n    json j2 = R\"({\"address\": \"742 Evergreen Terrace\", \"age\": 40, \"name\": \"Homer Simpson\"})\"_json;\n    auto p2 = j2.get<ns::person>();\n\n    // incomplete deserialization:\n    json j3 = R\"({\"address\": \"742 Evergreen Terrace\", \"name\": \"Maggie Simpson\"})\"_json;\n    auto p3 = j3.get<ns::person>();\n    std::cout << \"roundtrip: \" << json(p3) << std::endl;\n}\n

        Note how a default-initialized person object is used in the from_json to fill missing values.

        Example: (3) NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE

        Consider the following complete example:

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nnamespace ns\n{\nclass person\n{\n  private:\n    std::string name = \"John Doe\";\n    std::string address = \"123 Fake St\";\n    int age = -1;\n\n  public:\n    // No default constructor\n    person(std::string name_, std::string address_, int age_)\n        : name(std::move(name_)), address(std::move(address_)), age(age_)\n    {}\n\n    NLOHMANN_DEFINE_TYPE_INTRUSIVE(person, name, address, age)\n};\n} // namespace ns\n\nint main()\n{\n    ns::person p = {\"Ned Flanders\", \"744 Evergreen Terrace\", 60};\n\n    // serialization: person -> json\n    json j = p;\n    std::cout << \"serialization: \" << j << std::endl;\n}\n

        Output:

        serialization: {\"address\":\"744 Evergreen Terrace\",\"age\":60,\"name\":\"Ned Flanders\"}\n

        Notes:

        • ns::person is non-default-constructible. This allows this macro to be used instead of NLOHMANN_DEFINE_TYPE_INTRUSIVE and NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT.
        • ns::person has private member variables. This makes NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE applicable, but not NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE.
        • The macro NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE is used inside the class.

        The macro is equivalent to:

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nnamespace ns\n{\nclass person\n{\n  private:\n    std::string name = \"John Doe\";\n    std::string address = \"123 Fake St\";\n    int age = -1;\n\n  public:\n    // No default constructor\n    person(std::string name_, std::string address_, int age_)\n        : name(std::move(name_)), address(std::move(address_)), age(age_)\n    {}\n\n    template<typename BasicJsonType>\n    friend void to_json(BasicJsonType& nlohmann_json_j, const person& nlohmann_json_t)\n    {\n        nlohmann_json_j[\"name\"] = nlohmann_json_t.name;\n        nlohmann_json_j[\"address\"] = nlohmann_json_t.address;\n        nlohmann_json_j[\"age\"] = nlohmann_json_t.age;\n    }\n};\n} // namespace ns\n\nint main()\n{\n    ns::person p = {\"Ned Flanders\", \"744 Evergreen Terrace\", 60};\n\n    // serialization: person -> json\n    json j = p;\n    std::cout << \"serialization: \" << j << std::endl;\n}\n
        "},{"location":"api/macros/nlohmann_define_type_intrusive/#see-also","title":"See also","text":"
        • NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE, NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT, NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE for a similar macro that can be defined outside the type.
        • NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE, NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_DEFAULT, NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_ONLY_SERIALIZE, NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE, NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_DEFAULT, NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE for similar macros for derived types
        • Arbitrary Type Conversions for an overview.
        "},{"location":"api/macros/nlohmann_define_type_intrusive/#version-history","title":"Version history","text":"
        1. Added in version 3.9.0.
        2. Added in version 3.11.0.
        3. Added in version 3.11.3.

        All three macros were changed to work with any basic_json specialization (not only nlohmann::json) in version 3.12.0, and to allow an empty member list in version 3.13.0 unreleased.

        "},{"location":"api/macros/nlohmann_define_type_non_intrusive/","title":"NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE, NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT, NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE","text":"
        #define NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(type, member...)              // (1)\n#define NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT(type, member...) // (2)\n#define NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE(type, member...) // (3)\n

        These macros can be used to simplify the serialization/deserialization of types if you want to use a JSON object as serialization and want to use the member variable names as object keys in that object. The macro is to be defined outside the class/struct to create code for, but inside its namespace. Unlike NLOHMANN_DEFINE_TYPE_INTRUSIVE, it cannot access private members. The first parameter is the name of the class/struct, and all remaining parameters name the members.

        1. Will use at during deserialization and will throw out_of_range.403 if a key is missing in the JSON object.
        2. Will use value during deserialization and fall back to the default value for the respective type of the member variable if a key in the JSON object is missing. The generated from_json() function default constructs an object and uses its values as the defaults when calling the value function.
        3. Only defines the serialization. Useful in cases when the type does not have a default constructor and only serialization is required.

        Summary:

        Need access to private members Need only serialization Allow missing values when de-serializing macro NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE"},{"location":"api/macros/nlohmann_define_type_non_intrusive/#parameters","title":"Parameters","text":"type (in) name of the type (class, struct) to serialize/deserialize member (in) name of the (public) member variable to serialize/deserialize; up to 63 members can be given as a comma-separated list, which may also be empty"},{"location":"api/macros/nlohmann_define_type_non_intrusive/#default-definition","title":"Default definition","text":"

        The macros add two functions to the namespace which take care of the serialization and deserialization:

        template<typename BasicJsonType>\nvoid to_json(BasicJsonType&, const type&);\ntemplate<typename BasicJsonType>\nvoid from_json(const BasicJsonType&, type&); // except (3)\n

        See the examples below for the concrete generated code.

        "},{"location":"api/macros/nlohmann_define_type_non_intrusive/#notes","title":"Notes","text":"

        Prerequisites

        1. The type type must be default constructible (except (3). See How can I use get() for non-default constructible/non-copyable types? for how to overcome this limitation.
        2. The macro must be used outside the type (class/struct).
        3. The passed members must be public.

        Types without members

        The member list may be empty. The macro then generates a to_json that produces an empty JSON object {}, and a from_json that reads no members:

        struct marker {};\nNLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(marker)\n

        The WITH_NAMES variants do not support this.

        Implementation limits

        • The current implementation is limited to at most 63 member variables. If you want to serialize/deserialize types with more than 63 member variables, you need to define the to_json/from_json functions manually.
        • These macros always produce object-style (named-key) JSON, one key per member. There is no macro variant that serializes a struct's members positionally into a JSON array; for that, write to_json/from_json by hand, building/reading a json::array() of the members in order.
        "},{"location":"api/macros/nlohmann_define_type_non_intrusive/#examples","title":"Examples","text":"Example: (1) NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE

        Consider the following complete example:

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nnamespace ns\n{\nstruct person\n{\n    std::string name;\n    std::string address;\n    int age;\n};\n\nNLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(person, name, address, age)\n} // namespace ns\n\nint main()\n{\n    ns::person p = {\"Ned Flanders\", \"744 Evergreen Terrace\", 60};\n\n    // serialization: person -> json\n    json j = p;\n    std::cout << \"serialization: \" << j << std::endl;\n\n    // deserialization: json -> person\n    json j2 = R\"({\"address\": \"742 Evergreen Terrace\", \"age\": 40, \"name\": \"Homer Simpson\"})\"_json;\n    auto p2 = j2.get<ns::person>();\n\n    // incomplete deserialization:\n    json j3 = R\"({\"address\": \"742 Evergreen Terrace\", \"name\": \"Maggie Simpson\"})\"_json;\n    try\n    {\n        auto p3 = j3.get<ns::person>();\n    }\n    catch (const json::exception& e)\n    {\n        std::cout << \"deserialization failed: \" << e.what() << std::endl;\n    }\n}\n

        Output:

        serialization: {\"address\":\"744 Evergreen Terrace\",\"age\":60,\"name\":\"Ned Flanders\"}\ndeserialization failed: [json.exception.out_of_range.403] key 'age' not found\n

        Notes:

        • ns::person is default-constructible. This is a requirement for using the macro.
        • ns::person has only public member variables. This makes NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE applicable.
        • The macro NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE is used outside the class, but inside its namespace ns.
        • A missing key \"age\" in the deserialization yields an exception. To fall back to the default value, NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT can be used.

        The macro is equivalent to:

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nnamespace ns\n{\nstruct person\n{\n    std::string name;\n    std::string address;\n    int age;\n};\n\ntemplate<typename BasicJsonType>\nvoid to_json(BasicJsonType& nlohmann_json_j, const person& nlohmann_json_t)\n{\n    nlohmann_json_j[\"name\"] = nlohmann_json_t.name;\n    nlohmann_json_j[\"address\"] = nlohmann_json_t.address;\n    nlohmann_json_j[\"age\"] = nlohmann_json_t.age;\n}\n\ntemplate<typename BasicJsonType>\nvoid from_json(const BasicJsonType& nlohmann_json_j, person& nlohmann_json_t)\n{\n    nlohmann_json_t.name = nlohmann_json_j.at(\"name\");\n    nlohmann_json_t.address = nlohmann_json_j.at(\"address\");\n    nlohmann_json_t.age = nlohmann_json_j.at(\"age\");\n}\n} // namespace ns\n\nint main()\n{\n    ns::person p = {\"Ned Flanders\", \"744 Evergreen Terrace\", 60};\n\n    // serialization: person -> json\n    json j = p;\n    std::cout << \"serialization: \" << j << std::endl;\n\n    // deserialization: json -> person\n    json j2 = R\"({\"address\": \"742 Evergreen Terrace\", \"age\": 40, \"name\": \"Homer Simpson\"})\"_json;\n    auto p2 = j2.get<ns::person>();\n\n    // incomplete deserialization:\n    json j3 = R\"({\"address\": \"742 Evergreen Terrace\", \"name\": \"Maggie Simpson\"})\"_json;\n    try\n    {\n        auto p3 = j3.get<ns::person>();\n    }\n    catch (const json::exception& e)\n    {\n        std::cout << \"deserialization failed: \" << e.what() << std::endl;\n    }\n}\n
        Example: (2) NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT

        Consider the following complete example:

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nnamespace ns\n{\nstruct person\n{\n    std::string name = \"John Doe\";\n    std::string address = \"123 Fake St\";\n    int age = -1;\n\n    person() = default;\n    person(std::string name_, std::string address_, int age_)\n        : name(std::move(name_)), address(std::move(address_)), age(age_)\n    {}\n};\n\nNLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT(person, name, address, age)\n} // namespace ns\n\nint main()\n{\n    ns::person p = {\"Ned Flanders\", \"744 Evergreen Terrace\", 60};\n\n    // serialization: person -> json\n    json j = p;\n    std::cout << \"serialization: \" << j << std::endl;\n\n    // deserialization: json -> person\n    json j2 = R\"({\"address\": \"742 Evergreen Terrace\", \"age\": 40, \"name\": \"Homer Simpson\"})\"_json;\n    auto p2 = j2.get<ns::person>();\n\n    // incomplete deserialization:\n    json j3 = R\"({\"address\": \"742 Evergreen Terrace\", \"name\": \"Maggie Simpson\"})\"_json;\n    auto p3 = j3.get<ns::person>();\n    std::cout << \"roundtrip: \" << json(p3) << std::endl;\n}\n

        Output:

        serialization: {\"address\":\"744 Evergreen Terrace\",\"age\":60,\"name\":\"Ned Flanders\"}\nroundtrip: {\"address\":\"742 Evergreen Terrace\",\"age\":-1,\"name\":\"Maggie Simpson\"}\n

        Notes:

        • ns::person is default-constructible. This is a requirement for using the macro.
        • ns::person has only public member variables. This makes NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT applicable.
        • The macro NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT is used outside the class, but inside its namespace ns.
        • A missing key \"age\" in the deserialization does not yield an exception. Instead, the default value -1 is used.

        The macro is equivalent to:

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nnamespace ns\n{\nstruct person\n{\n    std::string name = \"John Doe\";\n    std::string address = \"123 Fake St\";\n    int age = -1;\n\n    person() = default;\n    person(std::string name_, std::string address_, int age_)\n        : name(std::move(name_)), address(std::move(address_)), age(age_)\n    {}\n};\n\ntemplate<typename BasicJsonType>\nvoid to_json(BasicJsonType& nlohmann_json_j, const person& nlohmann_json_t)\n{\n    nlohmann_json_j[\"name\"] = nlohmann_json_t.name;\n    nlohmann_json_j[\"address\"] = nlohmann_json_t.address;\n    nlohmann_json_j[\"age\"] = nlohmann_json_t.age;\n}\n\ntemplate<typename BasicJsonType>\nvoid from_json(const BasicJsonType& nlohmann_json_j, person& nlohmann_json_t)\n{\n    person nlohmann_json_default_obj;\n    nlohmann_json_t.name = nlohmann_json_j.value(\"name\", nlohmann_json_default_obj.name);\n    nlohmann_json_t.address = nlohmann_json_j.value(\"address\", nlohmann_json_default_obj.address);\n    nlohmann_json_t.age = nlohmann_json_j.value(\"age\", nlohmann_json_default_obj.age);\n}\n} // namespace ns\n\nint main()\n{\n    ns::person p = {\"Ned Flanders\", \"744 Evergreen Terrace\", 60};\n\n    // serialization: person -> json\n    json j = p;\n    std::cout << \"serialization: \" << j << std::endl;\n\n    // deserialization: json -> person\n    json j2 = R\"({\"address\": \"742 Evergreen Terrace\", \"age\": 40, \"name\": \"Homer Simpson\"})\"_json;\n    auto p2 = j2.get<ns::person>();\n\n    // incomplete deserialization:\n    json j3 = R\"({\"address\": \"742 Evergreen Terrace\", \"name\": \"Maggie Simpson\"})\"_json;\n    auto p3 = j3.get<ns::person>();\n    std::cout << \"roundtrip: \" << json(p3) << std::endl;\n}\n

        Note how a default-initialized person object is used in the from_json to fill missing values.

        Example: (3) NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE

        Consider the following complete example:

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nnamespace ns\n{\nstruct person\n{\n    std::string name;\n    std::string address;\n    int age;\n};\n\nNLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE(person, name, address, age)\n} // namespace ns\n\nint main()\n{\n    ns::person p = {\"Ned Flanders\", \"744 Evergreen Terrace\", 60};\n\n    // serialization: person -> json\n    json j = p;\n    std::cout << \"serialization: \" << j << std::endl;\n}\n

        Output:

        serialization: {\"address\":\"744 Evergreen Terrace\",\"age\":60,\"name\":\"Ned Flanders\"}\n

        Notes:

        • ns::person is non-default-constructible. This allows this macro to be used instead of NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE and NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT.
        • ns::person has only public member variables. This makes NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE applicable.
        • The macro NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE is used outside the class, but inside its namespace ns.

        The macro is equivalent to:

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nnamespace ns\n{\nstruct person\n{\n    std::string name;\n    std::string address;\n    int age;\n};\n\ntemplate<typename BasicJsonType>\nvoid to_json(BasicJsonType& nlohmann_json_j, const person& nlohmann_json_t)\n{\n    nlohmann_json_j[\"name\"] = nlohmann_json_t.name;\n    nlohmann_json_j[\"address\"] = nlohmann_json_t.address;\n    nlohmann_json_j[\"age\"] = nlohmann_json_t.age;\n}\n} // namespace ns\n\nint main()\n{\n    ns::person p = {\"Ned Flanders\", \"744 Evergreen Terrace\", 60};\n\n    // serialization: person -> json\n    json j = p;\n    std::cout << \"serialization: \" << j << std::endl;\n}\n
        "},{"location":"api/macros/nlohmann_define_type_non_intrusive/#see-also","title":"See also","text":"
        • NLOHMANN_DEFINE_TYPE_INTRUSIVE, NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT, NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE for a similar macro that can be defined inside the type.
        • NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE, NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_DEFAULT, NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_ONLY_SERIALIZE, NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE, NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_DEFAULT, NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE for similar macros for derived types
        • Arbitrary Type Conversions for an overview.
        "},{"location":"api/macros/nlohmann_define_type_non_intrusive/#version-history","title":"Version history","text":"
        1. Added in version 3.9.0.
        2. Added in version 3.11.0.
        3. Added in version 3.11.3.

        All three macros were changed to work with any basic_json specialization (not only nlohmann::json) in version 3.12.0, and to allow an empty member list in version 3.13.0 unreleased.

        "},{"location":"api/macros/nlohmann_define_type_with_names/","title":"NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_NAMES, NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT_WITH_NAMES, NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE_WITH_NAMES, NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_NAMES, NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT_WITH_NAMES, NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE_WITH_NAMES, NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_NAMES, NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_DEFAULT_WITH_NAMES, NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_ONLY_SERIALIZE_WITH_NAMES, NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_NAMES, NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_DEFAULT_WITH_NAMES, NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE_WITH_NAMES","text":"NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_NAMES, NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT_WITH_NAMES, NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE_WITH_NAMES, NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_NAMES, NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT_WITH_NAMES, NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE_WITH_NAMES, NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_NAMES, NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_DEFAULT_WITH_NAMES, NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_ONLY_SERIALIZE_WITH_NAMES, NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_NAMES, NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_DEFAULT_WITH_NAMES, NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE_WITH_NAMES 
        #define NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_NAMES(type, \"json_member_name\", member...)\n#define NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT_WITH_NAMES(type, \"json_member_name\", member...)\n#define NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE_WITH_NAMES(type, \"json_member_name\", member...)\n#define NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_NAMES(type, \"json_member_name\", member...)\n#define NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT_WITH_NAMES(type, \"json_member_name\", member...)\n#define NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE_WITH_NAMES(type, \"json_member_name\", member...)\n#define NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_NAMES(type, base_type, \"json_member_name\", member...)\n#define NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_DEFAULT_WITH_NAMES(type, base_type, \"json_member_name\", member...)\n#define NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_ONLY_SERIALIZE_WITH_NAMES(type, base_type, \"json_member_name\", member...)\n#define NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_NAMES(type, base_type, \"json_member_name\", member...)\n#define NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_DEFAULT_WITH_NAMES(type, base_type, \"json_member_name\", member...)\n#define NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE_WITH_NAMES(type, base_type, \"json_member_name\", member...)\n

        These macros can be used in case you want to use the custom names for the member variables in the resulting JSON. They behave exactly as their non-WITH_NAMES counterparts, but require an additional parameter for each member variable which will be used in JSON. Both serialization and deserialization will only use the custom names for JSON, the names of the member variables themselves will be ignored.

        Using the named conversion macros will halve the maximum number of member variables from 63 to 31.

        For further information please refer to the corresponding macros without WITH_NAMES.

        "},{"location":"api/macros/nlohmann_define_type_with_names/#parameters","title":"Parameters","text":"type (in) name of the type (class, struct) to serialize/deserialize base_type (in) name of the base type (class, struct) type is derived from (used only in DEFINE_DERIVED_TYPE macros) json_member_name (in) the string that will be used as the name for the next value member (in) name of the member variable to serialize/deserialize"},{"location":"api/macros/nlohmann_define_type_with_names/#examples","title":"Examples","text":"Example: NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_NAMES

        Consider the following complete example:

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nnamespace ns\n{\nstruct person\n{\n    std::string name;\n    std::string address;\n    int age;\n};\n\nNLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_NAMES(person, \"json_name\", name, \"json_address\", address, \"json_age\", age)\n} // namespace ns\n\nint main()\n{\n    ns::person p = {\"Ned Flanders\", \"744 Evergreen Terrace\", 60};\n\n    // serialization: person -> json\n    json j = p;\n    std::cout << \"serialization: \" << j << std::endl;\n\n    // deserialization: json -> person\n    json j2 = R\"({\"json_address\": \"742 Evergreen Terrace\", \"json_age\": 40, \"json_name\": \"Homer Simpson\"})\"_json;\n    auto p2 = j2.template get<ns::person>();\n\n    // incomplete deserialization:\n    json j3 = R\"({\"json_address\": \"742 Evergreen Terrace\", \"json_name\": \"Maggie Simpson\"})\"_json;\n    try\n    {\n        auto p3 = j3.template get<ns::person>();\n    }\n    catch (const json::exception& e)\n    {\n        std::cout << \"deserialization failed: \" << e.what() << std::endl;\n    }\n}\n

        Output:

        serialization: {\"json_address\":\"744 Evergreen Terrace\",\"json_age\":60,\"json_name\":\"Ned Flanders\"}\ndeserialization failed: [json.exception.out_of_range.403] key 'json_age' not found\n

        Notes:

        • ns::person is default-constructible. This is a requirement for using the macro.
        • ns::person has only public member variables. This makes NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_NAMES applicable.
        • The macro NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_NAMES is used outside the class, but inside its namespace ns.
        • A missing key \"age\" in the deserialization yields an exception. To fall back to the default value, NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT_WITH_NAMES can be used.

        The macro is equivalent to:

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nnamespace ns\n{\nstruct person\n{\n    std::string name;\n    std::string address;\n    int age;\n};\n\ntemplate <typename BasicJsonType, nlohmann::detail::enable_if_t<nlohmann::detail::is_basic_json<BasicJsonType>::value, int> = 0>\nvoid to_json(BasicJsonType& nlohmann_json_j, const person& nlohmann_json_t)\n{\n    nlohmann_json_j[\"json_name\"] = nlohmann_json_t.name;\n    nlohmann_json_j[\"json_address\"] = nlohmann_json_t.address;\n    nlohmann_json_j[\"json_age\"] = nlohmann_json_t.age;\n}\n\ntemplate <typename BasicJsonType, nlohmann::detail::enable_if_t<nlohmann::detail::is_basic_json<BasicJsonType>::value, int> = 0>\nvoid from_json(const BasicJsonType& nlohmann_json_j, person& nlohmann_json_t)\n{\n    nlohmann_json_j.at(\"json_name\").get_to(nlohmann_json_t.name);\n    nlohmann_json_j.at(\"json_address\").get_to(nlohmann_json_t.address);\n    nlohmann_json_j.at(\"json_age\").get_to(nlohmann_json_t.age);\n}\n} // namespace ns\n\nint main()\n{\n    ns::person p = {\"Ned Flanders\", \"744 Evergreen Terrace\", 60};\n\n    // serialization: person -> json\n    json j = p;\n    std::cout << \"serialization: \" << j << std::endl;\n\n    // deserialization: json -> person\n    json j2 = R\"({\"json_address\": \"742 Evergreen Terrace\", \"json_age\": 40, \"json_name\": \"Homer Simpson\"})\"_json;\n    auto p2 = j2.template get<ns::person>();\n\n    // incomplete deserialization:\n    json j3 = R\"({\"json_address\": \"742 Evergreen Terrace\", \"json_name\": \"Maggie Simpson\"})\"_json;\n    try\n    {\n        auto p3 = j3.template get<ns::person>();\n    }\n    catch (const json::exception& e)\n    {\n        std::cout << \"deserialization failed: \" << e.what() << std::endl;\n    }\n}\n
        "},{"location":"api/macros/nlohmann_define_type_with_names/#see-also","title":"See also","text":"
        • NLOHMANN_DEFINE_TYPE_INTRUSIVE, NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT, NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE - the macros these variants add custom JSON key names to
        • NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE, NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT, NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE - the macros these variants add custom JSON key names to
        • NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE, NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_DEFAULT, NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_ONLY_SERIALIZE, NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE, NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_DEFAULT, NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE - similar macros for derived types, also available with custom names
        • Arbitrary Type Conversions - overview of type conversion mechanisms
        "},{"location":"api/macros/nlohmann_define_type_with_names/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"api/macros/nlohmann_json_namespace/","title":"NLOHMANN_JSON_NAMESPACE","text":"
        #define NLOHMANN_JSON_NAMESPACE /* value */\n

        This macro evaluates to the full name of the nlohmann namespace.

        "},{"location":"api/macros/nlohmann_json_namespace/#default-definition","title":"Default definition","text":"

        The default value consists of the root namespace (nlohmann) and an inline ABI namespace. See nlohmann Namespace for details.

        When the macro is not defined, the library will define it to its default value. Overriding this value has no effect on the library.

        "},{"location":"api/macros/nlohmann_json_namespace/#examples","title":"Examples","text":"Example

        The example shows how to use NLOHMANN_JSON_NAMESPACE instead of just nlohmann, as well as how to output the value of NLOHMANN_JSON_NAMESPACE.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\n// possible use case: use NLOHMANN_JSON_NAMESPACE instead of nlohmann\nusing json = NLOHMANN_JSON_NAMESPACE::json;\n\n// macro needed to output the NLOHMANN_JSON_NAMESPACE as string literal\n#define Q(x) #x\n#define QUOTE(x) Q(x)\n\nint main()\n{\n    std::cout << QUOTE(NLOHMANN_JSON_NAMESPACE) << std::endl;\n}\n

        Output:

        nlohmann::json_abi_v3_12_0\n
        "},{"location":"api/macros/nlohmann_json_namespace/#see-also","title":"See also","text":"
        • NLOHMANN_JSON_NAMESPACE_BEGIN, NLOHMANN_JSON_NAMESPACE_END
        • NLOHMANN_JSON_NAMESPACE_NO_VERSION
        "},{"location":"api/macros/nlohmann_json_namespace/#version-history","title":"Version history","text":"
        • Added in version 3.11.0. Changed inline namespace name in version 3.11.2. Added the ABI tag _dp in version 3.12.0, and the ABI tags _bics, _psp, _snul, _sbu8, _ekmo, _dsbs, _dufio, _vns, and _vssse3 in version 3.13.0 unreleased; see nlohmann Namespace.
        "},{"location":"api/macros/nlohmann_json_namespace_begin/","title":"NLOHMANN_JSON_NAMESPACE_BEGIN, NLOHMANN_JSON_NAMESPACE_END","text":"
        #define NLOHMANN_JSON_NAMESPACE_BEGIN /* value */  // (1)\n#define NLOHMANN_JSON_NAMESPACE_END   /* value */  // (2)\n

        These macros can be used to open and close the nlohmann namespace. See nlohmann Namespace for details.

        1. Opens the namespace.
        2. Closes the namespace.
        "},{"location":"api/macros/nlohmann_json_namespace_begin/#default-definition","title":"Default definition","text":"

        The default definitions open and close the nlohmann namespace. The precise definition of NLOHMANN_JSON_NAMESPACE_BEGIN varies as described here.

        1. Default definition of NLOHMANN_JSON_NAMESPACE_BEGIN:

          namespace nlohmann\n{\ninline namespace json_abi_v3_12_0\n{\n

        2. Default definition of NLOHMANN_JSON_NAMESPACE_END: 

          }  // namespace json_abi_v3_12_0\n}  // namespace nlohmann\n

        When these macros are not defined, the library will define them to their default definitions.

        "},{"location":"api/macros/nlohmann_json_namespace_begin/#examples","title":"Examples","text":"Example

        The example shows how to use NLOHMANN_JSON_NAMESPACE_BEGIN/NLOHMANN_JSON_NAMESPACE_END from the How do I convert third-party types? page.

        #include <iostream>\n#include <optional>\n#include <nlohmann/json.hpp>\n\n// partial specialization (see https://json.nlohmann.me/features/arbitrary_types/)\nNLOHMANN_JSON_NAMESPACE_BEGIN\ntemplate <typename T>\nstruct adl_serializer<std::optional<T>>\n{\n    static void to_json(json& j, const std::optional<T>& opt)\n    {\n        if (opt == std::nullopt)\n        {\n            j = nullptr;\n        }\n        else\n        {\n            j = *opt;\n        }\n    }\n};\nNLOHMANN_JSON_NAMESPACE_END\n\nint main()\n{\n    std::optional<int> o1 = 1;\n    std::optional<int> o2 = std::nullopt;\n\n    NLOHMANN_JSON_NAMESPACE::json j;\n    j.push_back(o1);\n    j.push_back(o2);\n    std::cout << j << std::endl;\n}\n

        Output:

        [1,null]\n
        "},{"location":"api/macros/nlohmann_json_namespace_begin/#see-also","title":"See also","text":"
        • nlohmann Namespace
        • NLOHMANN_JSON_NAMESPACE
        • NLOHMANN_JSON_NAMESPACE_NO_VERSION
        "},{"location":"api/macros/nlohmann_json_namespace_begin/#version-history","title":"Version history","text":"
        • Added in version 3.11.0. Changed inline namespace name in version 3.11.2.
        "},{"location":"api/macros/nlohmann_json_namespace_no_version/","title":"NLOHMANN_JSON_NAMESPACE_NO_VERSION","text":"
        #define NLOHMANN_JSON_NAMESPACE_NO_VERSION /* value */\n

        If defined to 1, the version component is omitted from the inline namespace. See nlohmann Namespace for details.

        "},{"location":"api/macros/nlohmann_json_namespace_no_version/#default-definition","title":"Default definition","text":"

        The default value is 0.

        #define NLOHMANN_JSON_NAMESPACE_NO_VERSION 0\n

        When the macro is not defined, the library will define it to its default value.

        "},{"location":"api/macros/nlohmann_json_namespace_no_version/#examples","title":"Examples","text":"Example

        The example shows how to use NLOHMANN_JSON_NAMESPACE_NO_VERSION to disable the version component of the inline namespace.

        #include <iostream>\n\n#define NLOHMANN_JSON_NAMESPACE_NO_VERSION 1\n#include <nlohmann/json.hpp>\n\n// macro needed to output the NLOHMANN_JSON_NAMESPACE as string literal\n#define Q(x) #x\n#define QUOTE(x) Q(x)\n\nint main()\n{\n    std::cout << QUOTE(NLOHMANN_JSON_NAMESPACE) << std::endl;\n}\n

        Output:

        nlohmann::json_abi\n
        "},{"location":"api/macros/nlohmann_json_namespace_no_version/#see-also","title":"See also","text":"
        • nlohmann Namespace
        • NLOHMANN_JSON_NAMESPACE
        • NLOHMANN_JSON_NAMESPACE_BEGIN, NLOHMANN_JSON_NAMESPACE_END
        "},{"location":"api/macros/nlohmann_json_namespace_no_version/#version-history","title":"Version history","text":"
        • Added in version 3.11.2.
        "},{"location":"api/macros/nlohmann_json_serialize_enum/","title":"NLOHMANN_JSON_SERIALIZE_ENUM","text":"
        #define NLOHMANN_JSON_SERIALIZE_ENUM(type, conversion...)\n

        By default, enum values are serialized to JSON as integers. In some cases, this could result in undesired behavior. If an enum is modified or re-ordered after data has been serialized to JSON, the later deserialized JSON data may be undefined or a different enum value than was originally intended.

        The NLOHMANN_JSON_SERIALIZE_ENUM allows to define a user-defined serialization for every enumerator.

        "},{"location":"api/macros/nlohmann_json_serialize_enum/#parameters","title":"Parameters","text":"type (in) name of the enum to serialize/deserialize conversion (in) a pair of an enumerator and a JSON serialization; arbitrary pairs can be given as a comma-separated list"},{"location":"api/macros/nlohmann_json_serialize_enum/#default-definition","title":"Default definition","text":"

        The macro adds two functions to the namespace which take care of the serialization and deserialization:

        template<typename BasicJsonType>\ninline void to_json(BasicJsonType& j, const type& e);\ntemplate<typename BasicJsonType>\ninline void from_json(const BasicJsonType& j, type& e);\n
        "},{"location":"api/macros/nlohmann_json_serialize_enum/#notes","title":"Notes","text":"

        Prerequisites

        The macro must be used inside the namespace of the enum.

        Important notes

        • When using get<ENUM_TYPE>(), undefined JSON values will default to the first specified conversion. Select this default pair carefully. See example 1 below.
        • If an enum or JSON value is specified in multiple conversions, the first matching conversion from the top of the list will be returned when converting to or from JSON. See example 2 below.
        • Maps with enum keys (e.g., std::map<ENUM_TYPE, T>) are stored as arrays of [key, value] pairs by default. Define JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS to store them as objects with the converted keys. Such maps can be read from both forms.
        "},{"location":"api/macros/nlohmann_json_serialize_enum/#examples","title":"Examples","text":"Example: Basic usage

        The example shows how NLOHMANN_JSON_SERIALIZE_ENUM can be used to serialize/deserialize both classical enums and C++11 enum classes:

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nnamespace ns\n{\nenum TaskState\n{\n    TS_STOPPED,\n    TS_RUNNING,\n    TS_COMPLETED,\n    TS_INVALID = -1\n};\n\nNLOHMANN_JSON_SERIALIZE_ENUM(TaskState,\n{\n    { TS_INVALID, nullptr },\n    { TS_STOPPED, \"stopped\" },\n    { TS_RUNNING, \"running\" },\n    { TS_COMPLETED, \"completed\" }\n})\n\nenum class Color\n{\n    red, green, blue, unknown\n};\n\nNLOHMANN_JSON_SERIALIZE_ENUM(Color,\n{\n    { Color::unknown, \"unknown\" }, { Color::red, \"red\" },\n    { Color::green, \"green\" }, { Color::blue, \"blue\" }\n})\n} // namespace ns\n\nint main()\n{\n    // serialization\n    json j_stopped = ns::TS_STOPPED;\n    json j_red = ns::Color::red;\n    std::cout << \"ns::TS_STOPPED -> \" << j_stopped\n              << \", ns::Color::red -> \" << j_red << std::endl;\n\n    // deserialization\n    json j_running = \"running\";\n    json j_blue = \"blue\";\n    auto running = j_running.get<ns::TaskState>();\n    auto blue = j_blue.get<ns::Color>();\n    std::cout << j_running << \" -> \" << running\n              << \", \" << j_blue << \" -> \" << static_cast<int>(blue) << std::endl;\n\n    // deserializing undefined JSON value to enum\n    // (where the first map entry above is the default)\n    json j_pi = 3.14;\n    auto invalid = j_pi.get<ns::TaskState>();\n    auto unknown = j_pi.get<ns::Color>();\n    std::cout << j_pi << \" -> \" << invalid << \", \"\n              << j_pi << \" -> \" << static_cast<int>(unknown) << std::endl;\n}\n

        Output:

        ns::TS_STOPPED -> \"stopped\", ns::Color::red -> \"red\"\n\"running\" -> 1, \"blue\" -> 2\n3.14 -> -1, 3.14 -> 3\n
        Example: Multiple conversions for one enumerator

        The example shows how to use multiple conversions for a single enumerator. In the example, Color::red will always be serialized to \"red\", because the first occurring conversion. The second conversion, however, offers an alternative deserialization from \"rot\" to Color::red.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nnamespace ns\n{\nenum class Color\n{\n    red, green, blue, unknown\n};\n\nNLOHMANN_JSON_SERIALIZE_ENUM(Color,\n{\n    { Color::unknown, \"unknown\" }, { Color::red, \"red\" },\n    { Color::green, \"green\" }, { Color::blue, \"blue\" },\n    { Color::red, \"rot\" } // a second conversion for Color::red\n})\n}\n\nint main()\n{\n    // serialization\n    json j_red = ns::Color::red;\n    std::cout << static_cast<int>(ns::Color::red) << \" -> \" << j_red << std::endl;\n\n    // deserialization\n    json j_rot = \"rot\";\n    auto rot = j_rot.get<ns::Color>();\n    auto red = j_red.get<ns::Color>();\n    std::cout << j_rot << \" -> \" << static_cast<int>(rot) << std::endl;\n    std::cout << j_red << \" -> \" << static_cast<int>(red) << std::endl;\n}\n

        Output:

        0 -> \"red\"\n\"rot\" -> 0\n\"red\" -> 0\n
        "},{"location":"api/macros/nlohmann_json_serialize_enum/#see-also","title":"See also","text":"
        • Specializing enum conversion
        • NLOHMANN_JSON_SERIALIZE_ENUM_STRICT
        • JSON_DISABLE_ENUM_SERIALIZATION
        • JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS
        "},{"location":"api/macros/nlohmann_json_serialize_enum/#version-history","title":"Version history","text":"

        Added in version 3.4.0.

        "},{"location":"api/macros/nlohmann_json_serialize_enum_strict/","title":"NLOHMANN_JSON_SERIALIZE_ENUM_STRICT","text":"
        #define NLOHMANN_JSON_SERIALIZE_ENUM_STRICT(type, conversion...)\n

        By default, enum values are serialized to JSON as integers. In some cases, this could result in undesired behavior. If an enum is modified or re-ordered after data has been serialized to JSON, the later deserialized JSON data may be undefined or a different enum value than was originally intended.

        NLOHMANN_JSON_SERIALIZE_ENUM_STRICT allows to define a user-defined serialization for every enumerator that throws an exception on undefined input.

        "},{"location":"api/macros/nlohmann_json_serialize_enum_strict/#parameters","title":"Parameters","text":"type (in) name of the enum to serialize/deserialize conversion (in) a pair of an enumerator and a JSON serialization; arbitrary pairs can be given as a comma-separated list"},{"location":"api/macros/nlohmann_json_serialize_enum_strict/#default-definition","title":"Default definition","text":"

        The macro adds two functions to the namespace which take care of the serialization and deserialization:

        template<typename BasicJsonType>\ninline void to_json(BasicJsonType& j, const type& e);\ntemplate<typename BasicJsonType>\ninline void from_json(const BasicJsonType& j, type& e);\n
        "},{"location":"api/macros/nlohmann_json_serialize_enum_strict/#notes","title":"Notes","text":"

        Prerequisites

        The macro must be used inside the namespace of the enum.

        Important notes

        • Undefined input throws out_of_range.410 in both directions: when serializing an enum value not listed in the conversions, and when deserializing (e.g., via get<ENUM_TYPE>()) a JSON value that matches no conversion; example: \"enum value out of range for <type>\".
        • If an enum or JSON value is specified in multiple conversions, the first matching conversion from the top of the list will be returned when converting to or from JSON. See example 2 below.
        • Maps with enum keys (e.g., std::map<ENUM_TYPE, T>) are stored as arrays of [key, value] pairs by default. Define JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS to store them as objects with the converted keys. Such maps can be read from both forms.
        "},{"location":"api/macros/nlohmann_json_serialize_enum_strict/#examples","title":"Examples","text":"Example: Basic usage

        The example shows how NLOHMANN_JSON_SERIALIZE_ENUM_STRICT can be used to serialize/deserialize both classical enums and C++11 enum classes:

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nnamespace ns\n{\nenum TaskState\n{\n    TS_STOPPED,\n    TS_RUNNING,\n    TS_COMPLETED,\n    TS_INVALID = -1\n};\n\nNLOHMANN_JSON_SERIALIZE_ENUM_STRICT(TaskState,\n{\n    { TS_INVALID, nullptr },\n    { TS_STOPPED, \"stopped\" },\n    { TS_RUNNING, \"running\" },\n    { TS_COMPLETED, \"completed\" }\n})\n\nenum class Color\n{\n    red, green, blue, unknown\n};\n\nNLOHMANN_JSON_SERIALIZE_ENUM_STRICT(Color,\n{\n    { Color::unknown, \"unknown\" }, { Color::red, \"red\" },\n    { Color::green, \"green\" }, { Color::blue, \"blue\" }\n})\n} // namespace ns\n\nint main()\n{\n    // serialization\n    json j_stopped = ns::TS_STOPPED;\n    json j_red = ns::Color::red;\n    std::cout << \"ns::TS_STOPPED -> \" << j_stopped\n              << \", ns::Color::red -> \" << j_red << std::endl;\n\n    // deserialization\n    json j_running = \"running\";\n    json j_blue = \"blue\";\n    auto running = j_running.get<ns::TaskState>();\n    auto blue = j_blue.get<ns::Color>();\n    std::cout << j_running << \" -> \" << running\n              << \", \" << j_blue << \" -> \" << static_cast<int>(blue) << std::endl;\n\n}\n

        Output:

        ns::TS_STOPPED -> \"stopped\", ns::Color::red -> \"red\"\n\"running\" -> 1, \"blue\" -> 2\n
        Example: Multiple conversions for one enumerator

        The example shows how to use multiple conversions for a single enumerator. In the example, Color::red will always be serialized to \"red\", because the first occurring conversion. The second conversion, however, offers an alternative deserialization from \"rot\" to Color::red.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nnamespace ns\n{\nenum class Color\n{\n    red, green, blue, unknown\n};\n\nNLOHMANN_JSON_SERIALIZE_ENUM_STRICT(Color,\n{\n    { Color::unknown, \"unknown\" }, { Color::red, \"red\" },\n    { Color::green, \"green\" }, { Color::blue, \"blue\" },\n    { Color::red, \"rot\" } // a second conversion for Color::red\n})\n}\n\nint main()\n{\n    // serialization\n    json j_red = ns::Color::red;\n    std::cout << static_cast<int>(ns::Color::red) << \" -> \" << j_red << std::endl;\n\n    // deserialization\n    json j_rot = \"rot\";\n    auto rot = j_rot.get<ns::Color>();\n    auto red = j_red.get<ns::Color>();\n    std::cout << j_rot << \" -> \" << static_cast<int>(rot) << std::endl;\n    std::cout << j_red << \" -> \" << static_cast<int>(red) << std::endl;\n}\n

        Output:

        0 -> \"red\"\n\"rot\" -> 0\n\"red\" -> 0\n
        Example: exceptions on invalid serialization

        The example shows how an invalid serialization causes an exception to be thrown. In the example, Color::unknown is not defined in the mapping used to call NLOHMANN_JSON_SERIALIZE_ENUM_STRICT so causes an exception when used to serialize. Similarly, \"what\" does not refer to an enum value so also causes an exception when deserialization is attempted.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nnamespace ns\n{\n\nenum class Color\n{\n    red,\n    green,\n    blue,\n    unknown // not mapped in JSON_SERIALIZE_ENUM_STRICT\n};\n\nNLOHMANN_JSON_SERIALIZE_ENUM_STRICT(Color,\n{\n    {Color::red, \"red\"},\n    {Color::green, \"green\"},\n    {Color::blue, \"blue\"}\n})\n\n} // namespace ns\n\n\nint main()\n{\n    // invalid serialization\n    try\n    {\n        // ns::color::unknown was not mapped in macro\n        json invalid_serialization = ns::Color::unknown;\n    }\n    catch (const json::exception e)\n    {\n        std::cout << \"deserialization failed: \" << e.what() << std::endl;\n    }\n\n    // invalid deserialization\n    try\n    {\n        // what does not map to an enum\n        json invalid_deserialization(\"what\");\n        ns::Color color = invalid_deserialization.get<ns::Color>();\n    }\n    catch (const json::exception e)\n    {\n        std::cout << \"deserialization failed: \" << e.what() << std::endl;\n    }\n\n    return 0;\n}\n

        Output: 

        deserialization failed: [json.exception.out_of_range.410] enum value out of range for Color\ndeserialization failed: [json.exception.out_of_range.410] enum value out of range for Color: \"what\"\n

        "},{"location":"api/macros/nlohmann_json_serialize_enum_strict/#see-also","title":"See also","text":"
        • Specializing enum conversion
        • NLOHMANN_JSON_SERIALIZE_ENUM
        • JSON_DISABLE_ENUM_SERIALIZATION
        • JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS
        "},{"location":"api/macros/nlohmann_json_serialize_enum_strict/#version-history","title":"Version history","text":"

        Added in version 3.13.0 unreleased.

        "},{"location":"api/macros/nlohmann_json_version_major/","title":"NLOHMANN_JSON_VERSION_MAJOR, NLOHMANN_JSON_VERSION_MINOR, NLOHMANN_JSON_VERSION_PATCH","text":"
        #define NLOHMANN_JSON_VERSION_MAJOR /* value */\n#define NLOHMANN_JSON_VERSION_MINOR /* value */\n#define NLOHMANN_JSON_VERSION_PATCH /* value */\n

        These macros are defined by the library and contain the version numbers according to Semantic Versioning 2.0.0.

        "},{"location":"api/macros/nlohmann_json_version_major/#default-definition","title":"Default definition","text":"

        The macros are defined according to the current library version.

        "},{"location":"api/macros/nlohmann_json_version_major/#examples","title":"Examples","text":"Example

        The example below shows how NLOHMANN_JSON_VERSION_MAJOR, NLOHMANN_JSON_VERSION_MINOR, and NLOHMANN_JSON_VERSION_PATCH are defined by the library.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << \"JSON for Modern C++ version \"\n              << NLOHMANN_JSON_VERSION_MAJOR << \".\"\n              << NLOHMANN_JSON_VERSION_MINOR << \".\"\n              << NLOHMANN_JSON_VERSION_PATCH << std::endl;\n}\n

        Output:

        JSON for Modern C++ version 3.12.0\n
        "},{"location":"api/macros/nlohmann_json_version_major/#see-also","title":"See also","text":"
        • meta - returns version information on the library
        • JSON_SKIP_LIBRARY_VERSION_CHECK - skip library version check
        "},{"location":"api/macros/nlohmann_json_version_major/#version-history","title":"Version history","text":"
        • Added in version 3.1.0.
        "},{"location":"community/","title":"Community","text":"
        • Ecosystem - third-party projects built on top of this library
        • Code of Conduct - the rules and norms of this project
        • Contribution Guidelines - guidelines how to contribute to this project
        • Governance - the governance model of this project
        • Quality Assurance - how the quality of this project is assured
        • Roadmap - what the project will and will not do
        • Security Policy - the security policy of the project
        • Assurance Case - why the library meets its security requirements
        "},{"location":"community/assurance_case/","title":"Assurance case","text":"

        This page argues why the library meets its security requirements. It describes the threats the library faces, where the trust boundaries lie, and how the library's design and the quality assurance counter these threats. To report a vulnerability, see the security policy.

        "},{"location":"community/assurance_case/#threat-model","title":"Threat model","text":"

        The library parses, stores, and serializes JSON values in memory. It does not open network connections, does not open files (it only reads from streams or std::FILE* handles that the caller has already opened), does not read environment variables, and does not implement cryptography or handle credentials.

        The primary threat is therefore untrusted input: JSON text or binary data (BJData, BON8, BSON, CBOR, MessagePack, UBJSON) that an attacker controls, passed to parse, accept, sax_parse, or one of the from_* functions such as from_cbor. Such input may try to

        • make the library read or write out of bounds (malformed lengths, truncated input, invalid UTF-8),
        • trigger undefined behavior (integer overflow in sizes or numbers, invalid casts),
        • exhaust memory (huge announced sizes), or
        • exhaust the call stack (deeply nested arrays and objects).
        "},{"location":"community/assurance_case/#trust-boundaries","title":"Trust boundaries","text":"
        • Untrusted: all serialized input read by the parser, the SAX interface, and the binary readers. The library must handle every possible input by either producing a value or throwing a parse_error (or returning false when exceptions are disabled for the call).
        • Trusted: the C++ code that calls the library. Calling a function with violated preconditions, for instance accessing an array with operator[] out of range, is a programming error and not a security boundary. Such preconditions are checked with runtime assertions in debug builds; functions such as at offer checked access with exceptions.
        flowchart LR\n    A[Untrusted input] --> B[Parser]\n    A --> C[SAX interface]\n    A --> D[Binary readers]\n    B --> E[\"Value tree (basic_json)\"]\n    C --> E\n    D --> E\n    E --> F[Trusted caller]
        "},{"location":"community/assurance_case/#secure-design","title":"Secure design","text":"
        • Strict parsing. The parser accepts exactly the JSON grammar of RFC 8259. Extensions such as comments and trailing commas must be enabled explicitly. Invalid UTF-8 is rejected.
        • Errors are reported, not ignored. Malformed input results in a parse_error with the byte position of the error. Binary readers do not trust announced sizes: strings and binary values grow only as bytes are actually read, arrays reserve at most a fixed number of elements up front, and sizes that no container can hold are rejected.
        • Memory is owned by values. Each basic_json value owns its content, and there is no manual memory management in user code. The destructor does not recurse, so destroying a deeply nested value does not exhaust the stack.
        • Bounded recursion. The JSON parser and the binary readers keep their state in explicit stacks instead of recursing per nesting level. Operations that walk a value, such as dump, copying, comparison, hashing, merge_patch, diff, flatten, and the binary writers, recurse only up to a fixed depth and continue with an explicit stack below it. Applications that process untrusted input can limit its nesting depth with a parser callback.
        • Invariants are checked. The class invariant (for instance, that the pointer for the stored type is never null) is checked with runtime assertions throughout the test suite.
        "},{"location":"community/assurance_case/#common-weaknesses","title":"Common weaknesses","text":"

        The following table maps the relevant classes of the Common Weakness Enumeration to the measures that counter them. The measures are described in detail in Quality assurance.

        Weakness Countermeasures Out-of-bounds read/write (CWE-125, CWE-787) bounds checks on all reads from the input; AddressSanitizer and Valgrind on the test suite; OSS-Fuzz Integer overflow (CWE-190) UndefinedBehaviorSanitizer with integer overflow detection; Clang-Tidy; Cppcheck Use after free, double free (CWE-416, CWE-415) ownership of all memory by values; AddressSanitizer and Valgrind; Clang Static Analyzer Memory leaks (CWE-401) Valgrind (Memcheck) on the test suite Uncontrolled recursion (CWE-674) iterative parser, binary readers, and destructor; bounded recursion in value operations; tests with deeply nested inputs Uncontrolled resource consumption (CWE-400) allocations based on announced sizes are capped; OSS-Fuzz with memory limits Undefined behavior in general (CWE-758) UndefinedBehaviorSanitizer; runtime assertions; Clang-Tidy, Cppcheck, Clang Static Analyzer, Infer

        In addition, every line of the library is covered by the unit tests, and all parsers are fuzz-tested around the clock by OSS-Fuzz.

        "},{"location":"community/code_of_conduct/","title":"Contributor Covenant Code of Conduct","text":""},{"location":"community/code_of_conduct/#our-pledge","title":"Our Pledge","text":"

        We as members, contributors, and leaders pledge to make participation in our community a harassment-free experience for everyone, regardless of age, body size, visible or invisible disability, ethnicity, sex characteristics, gender identity and expression, level of experience, education, socio-economic status, nationality, personal appearance, race, caste, color, religion, or sexual identity and orientation.

        We pledge to act and interact in ways that contribute to an open, welcoming, diverse, inclusive, and healthy community.

        "},{"location":"community/code_of_conduct/#our-standards","title":"Our Standards","text":"

        Examples of behavior that contributes to a positive environment for our community include:

        • Demonstrating empathy and kindness toward other people
        • Being respectful of differing opinions, viewpoints, and experiences
        • Giving and gracefully accepting constructive feedback
        • Accepting responsibility and apologizing to those affected by our mistakes, and learning from the experience
        • Focusing on what is best not just for us as individuals, but for the overall community

        Examples of unacceptable behavior include:

        • The use of sexualized language or imagery, and sexual attention or advances of any kind
        • Trolling, insulting or derogatory comments, and personal or political attacks
        • Public or private harassment
        • Publishing others' private information, such as a physical or email address, without their explicit permission
        • Other conduct which could reasonably be considered inappropriate in a professional setting
        "},{"location":"community/code_of_conduct/#enforcement-responsibilities","title":"Enforcement Responsibilities","text":"

        Community leaders are responsible for clarifying and enforcing our standards of acceptable behavior and will take appropriate and fair corrective action in response to any behavior that they deem inappropriate, threatening, offensive, or harmful.

        Community leaders have the right and responsibility to remove, edit, or reject comments, commits, code, wiki edits, issues, and other contributions that are not aligned to this Code of Conduct, and will communicate reasons for moderation decisions when appropriate.

        "},{"location":"community/code_of_conduct/#scope","title":"Scope","text":"

        This Code of Conduct applies within all community spaces, and also applies when an individual is officially representing the community in public spaces. Examples of representing our community include using an official email address, posting via an official social media account, or acting as an appointed representative at an online or offline event.

        "},{"location":"community/code_of_conduct/#enforcement","title":"Enforcement","text":"

        Instances of abusive, harassing, or otherwise unacceptable behavior may be reported to the community leaders responsible for enforcement at mail@nlohmann.me. All complaints will be reviewed and investigated promptly and fairly.

        All community leaders are obligated to respect the privacy and security of the reporter of any incident.

        "},{"location":"community/code_of_conduct/#enforcement-guidelines","title":"Enforcement Guidelines","text":"

        Community leaders will follow these Community Impact Guidelines in determining the consequences for any action they deem in violation of this Code of Conduct:

        "},{"location":"community/code_of_conduct/#1-correction","title":"1. Correction","text":"

        Community Impact: Use of inappropriate language or other behavior deemed unprofessional or unwelcome in the community.

        Consequence: A private, written warning from community leaders, providing clarity around the nature of the violation and an explanation of why the behavior was inappropriate. A public apology may be requested.

        "},{"location":"community/code_of_conduct/#2-warning","title":"2. Warning","text":"

        Community Impact: A violation through a single incident or series of actions.

        Consequence: A warning with consequences for continued behavior. No interaction with the people involved, including unsolicited interaction with those enforcing the Code of Conduct, for a specified period of time. This includes avoiding interactions in community spaces as well as external channels like social media. Violating these terms may lead to a temporary or permanent ban.

        "},{"location":"community/code_of_conduct/#3-temporary-ban","title":"3. Temporary Ban","text":"

        Community Impact: A serious violation of community standards, including sustained inappropriate behavior.

        Consequence: A temporary ban from any sort of interaction or public communication with the community for a specified period of time. No public or private interaction with the people involved, including unsolicited interaction with those enforcing the Code of Conduct, is allowed during this period. Violating these terms may lead to a permanent ban.

        "},{"location":"community/code_of_conduct/#4-permanent-ban","title":"4. Permanent Ban","text":"

        Community Impact: Demonstrating a pattern of violation of community standards, including sustained inappropriate behavior, harassment of an individual, or aggression toward or disparagement of classes of individuals.

        Consequence: A permanent ban from any sort of public interaction within the community.

        "},{"location":"community/code_of_conduct/#attribution","title":"Attribution","text":"

        This Code of Conduct is adapted from the Contributor Covenant, version 2.1, available at https://www.contributor-covenant.org/version/2/1/code_of_conduct.html.

        Community Impact Guidelines were inspired by Mozilla's code of conduct enforcement ladder.

        For answers to common questions about this code of conduct, see the FAQ at https://www.contributor-covenant.org/faq. Translations are available at https://www.contributor-covenant.org/translations.

        "},{"location":"community/contribution_guidelines/","title":"Contribution Guidelines","text":"

        Thank you for your interest in contributing to this project! What began as an exercise to explore the exciting features of C++11 has evolved into a widely used JSON library. I truly appreciate all the contributions from the community, whether it's proposing features, identifying bugs, or fixing mistakes! To ensure that our collaboration is efficient and effective, please follow these guidelines.

        Feel free to discuss or suggest improvements to this document by submitting a pull request.

        "},{"location":"community/contribution_guidelines/#ways-to-contribute","title":"Ways to Contribute","text":"

        There are multiple ways to contribute.

        "},{"location":"community/contribution_guidelines/#reporting-an-issue","title":"Reporting an issue","text":"

        Please create an issue, assuming one does not already exist, and describe your concern. Note you need a GitHub account for this.

        Clearly describe the issue:

        • If it is a bug, please describe how to reproduce it. If possible, attach a complete example which demonstrates the error. Please also state what you expected to happen instead of the error.
        • If you propose a change or addition, try to give an example what the improved code could look like or how to use it.
        • If you found a compilation error, please tell us which compiler (version and operating system) you used and paste the (relevant part of) the error messages to the ticket.

        Please stick to the provided issue template bug report if possible.

        "},{"location":"community/contribution_guidelines/#reporting-a-security-vulnerability","title":"Reporting a security vulnerability","text":"

        You can report a security vulnerability according to our security policy.

        "},{"location":"community/contribution_guidelines/#discussing-a-new-feature","title":"Discussing a new feature","text":"

        For questions, feature or support requests, please open a discussion. If you find a proposed answer satisfactory, please use the \"Mark as answer\" button to make it easier for readers to see what helped and for the community to filter for open questions.

        "},{"location":"community/contribution_guidelines/#proposing-a-fix-or-an-improvement","title":"Proposing a fix or an improvement","text":"

        Join an ongoing discussion or comment on an existing issue before starting to code. This can help to avoid duplicate efforts or other frustration during the later review.

        Create a pull request against the develop branch and follow the pull request template. In particular,

        • describe the changes in detail, both the what and why,
        • reference existing issues where applicable,
        • add tests to maintain 100% test coverage,
        • update the documentation as needed, and
        • ensure the source code is amalgamated.

        We describe all points in detail below.

        All contributions (including pull requests) must agree to the Developer Certificate of Origin (DCO) version 1.1. This is exactly the same one created and used by the Linux kernel developers and posted on http://developercertificate.org/. This is a developer's certification that he or she has the right to submit the patch for inclusion into the project.

        "},{"location":"community/contribution_guidelines/#how-to","title":"How to...","text":""},{"location":"community/contribution_guidelines/#describe-your-changes","title":"Describe your changes","text":"

        This library is primarily maintained as a spare-time project. As such, I cannot make any guarantee how quickly changes are merged and released. Therefore, it is very important to make the review as smooth as possible by explaining not only what you changed, but why. This rationale can be very valuable down the road when improvements or bugs are discussed years later.

        "},{"location":"community/contribution_guidelines/#reference-an-existing-issue","title":"Reference an existing issue","text":"

        Link a pull request to an issue to clarify that a fix is forthcoming and which issue can be closed after merging. Only a few cases (e.g., fixing typos) do not require prior discussions.

        "},{"location":"community/contribution_guidelines/#write-tests","title":"Write tests","text":"

        The library has an extensive test suite that currently covers 100 % of the library's code. These tests are crucial to maintain API stability and give future contributors confidence that they do not accidentally break things. As Titus Winters aptly put it:

        If you liked it, you should have put a test on it.

        "},{"location":"community/contribution_guidelines/#run-the-tests","title":"Run the tests","text":"

        First, ensure the test suite runs before making any changes:

        $ cmake -S. -B build\n$ cmake --build build -j 10\n$ ctest --test-dir build -j 10\n

        The test suite should report:

        100% tests passed, 0 tests failed out of <number of tests>\n
        "},{"location":"community/contribution_guidelines/#add-tests","title":"Add tests","text":"

        The tests are located in tests/src/unit-*.cpp and contain doctest assertions like CHECK. The tests are structured along the features of the library or the nature of the tests. Usually, it should be clear from the context which existing file needs to be extended, and only very few cases require creating new test files.

        When fixing a bug, edit unit-regression3.cpp and add a section referencing the fixed issue. unit-regression2.cpp holds the older tests; the two files exist because a single one grew large enough for the MinGW linker to fail relocating it, so please keep adding to the smaller file rather than growing the larger one.

        "},{"location":"community/contribution_guidelines/#exceptions","title":"Exceptions","text":"

        When you test exceptions, please use CHECK_THROWS_WITH_AS which also takes the what() argument of the thrown exception into account.

        "},{"location":"community/contribution_guidelines/#coverage","title":"Coverage","text":"

        If test coverage decreases, an automatic warning comment will be posted on the pull request. You can access a code coverage report as an artifact to the \u201cUbuntu\u201d workflow.

        "},{"location":"community/contribution_guidelines/#update-the-documentation","title":"Update the documentation","text":"

        The main documentation of the library is generated from the files docs/mkdocs/docs. This folder contains dedicated pages for certain features, a list of all exceptions, and extensive API documentation with details on every public API function.

        Build the documentation locally using:

        make install_venv -C docs/mkdocs\nmake serve -C docs/mkdocs\n

        The documentation will then be available at http://127.0.0.1:8000/. See the documentation of mkdocs and Material for MkDocs for more information.

        Before opening a pull request, check the documentation like the CI does:

        make build -C docs/mkdocs          # strict build: fails on broken links, anchors, and structure problems\nmake check_mermaid -C docs/mkdocs  # checks the Mermaid diagrams (requires Node.js)\n

        The search index of the docset is generated from mkdocs.yml and each page's title (H1) and declaration by docs/docset/generate_docset.py; make build reports API pages that cannot be classified.

        "},{"location":"community/contribution_guidelines/#amalgamate-the-source-code","title":"Amalgamate the source code","text":"

        The single-header files single_include/nlohmann/json.hpp, single_include/nlohmann/json_fwd.hpp, and single_include/nlohmann/json_literals.hpp are generated from the source files in the include/nlohmann directory. Do not edit the files directly; instead, modify the include/nlohmann sources and regenerate the files by executing:

        make amalgamate\n

        Running make amalgamate will also apply automatic formatting to the source files using Artistic Style. This formatting may modify your source files in-place. Be certain to review and commit any changes to avoid unintended formatting diffs in commits.

        If you add, rename, or remove a header in include/nlohmann, also regenerate the header list in BUILD.bazel (requires CMake) by executing:

        make BUILD.bazel\n

        The amalgamation check in CI fails if any of these generated files is out of date.

        "},{"location":"community/contribution_guidelines/#recommended-documentation","title":"Recommended documentation","text":"
        • The library\u2019s README file is an excellent starting point to understand its functionality.
        • The documentation page is the reference documentation of the library.
        • RFC 8259 is the reference for the JavaScript Object Notation (JSON) Data Interchange Format.
        "},{"location":"community/contribution_guidelines/#please-dont","title":"Please don't...","text":"

        Certain contributions are not helpful.

        "},{"location":"community/contribution_guidelines/#break-the-public-api","title":"Break the public API","text":"

        We take pride in the library being used by numerous customers across various industries. They all rely on the guarantees provided by semantic versioning. Please do not change the library such that the public API of the 3.x.y version is broken. This includes:

        • Changing function signatures (altering parameter types, return types, number of parameters) or changing the const-ness of member functions.
        • Removing functions.
        • Renaming functions or classes.
        • Changing exception handling.
        • Changing exception ids.
        • Changing access specifiers.
        • Changing default arguments.

        What is and is not covered by this guarantee is described in the roadmap.

        Although these guidelines may seem restrictive, they are essential for maintaining the library\u2019s utility.

        Breaking changes may be introduced when they are guarded with a feature macro such as JSON_USE_IMPLICIT_CONVERSIONS which allows selectively changing the behavior of the library. In next steps, the current behavior can then be deprecated. Using feature macros then allows users to test their code against the library in the next major release.

        "},{"location":"community/contribution_guidelines/#break-c11-language-conformance","title":"Break C++11 language conformance","text":"

        This library is designed to work with C++11 and later. This means that any supported C++11 compiler should compile the library without problems. Some compilers like GCC 4.7 (and earlier), Clang 3.3 (and earlier), or Microsoft Visual C++ 2013 and earlier are known not to work due to missing or incomplete C++11 support.

        Please do not add features that do not work with the mentioned supported compilers. Please guard features from C++14 and later against the respective JSON_HAS_CPP_14 macros.

        "},{"location":"community/contribution_guidelines/#break-json-conformance","title":"Break JSON conformance","text":"

        Please refrain from proposing changes that would break JSON conformance. If you propose a conformant extension of JSON to be supported by the library, please motivate this extension.

        "},{"location":"community/contribution_guidelines/#wanted","title":"Wanted","text":"

        The following areas really need contribution and are always welcomed:

        • Extending the continuous integration toward more exotic compilers such as Android NDK, Intel's Compiler, or the bleeding-edge versions of Clang.
        • Improving the efficiency of the JSON parser. The current parser is a hand-written iterative parser that keeps an explicit stack instead of recursing, with hand-coded string handling. Speeding up the lexer or the value construction would be appreciated. Parser generators like Bison or ANTLR do not play nice with single-header files -- I really would like to keep the parser inside the json.hpp header.
        • Extending and updating existing benchmarks to include (the most recent version of) this library. Though efficiency is not everything, speed and memory consumption are very important characteristics for C++ developers, so having proper comparisons would be interesting.

        We look forward to your contributions and collaboration to enhance the library!

        "},{"location":"community/ecosystem/","title":"Ecosystem","text":"

        The projects below build on top of nlohmann::json rather than merely using it - schema validators, language bindings, format converters, and similar building blocks. The list is not exhaustive, and is curated rather than automatically generated. If you maintain or know of a project that belongs here, please let me know.

        For products, applications, and organizations that use the library, see Customers instead.

        "},{"location":"community/ecosystem/#schema-validation","title":"Schema validation","text":"
        • json-schema-validator, a JSON Schema (draft 7) validator with human-readable error messages
        "},{"location":"community/ecosystem/#serialization-and-reflection","title":"Serialization and reflection","text":"
        • nlohmann_json_reflect, a reflection extension for (de)serializing nested containers-in-structs-in-containers
        "},{"location":"community/ecosystem/#encodings","title":"Encodings","text":"
        • base-encode-decode, a header-only Base64/32/16/8/4/2 (and DNA/RNA) encoding library, with an adapter that serializes binary data through nlohmann::json
        "},{"location":"community/ecosystem/#language-bindings-and-interop","title":"Language bindings and interop","text":"
        • pybind11_json, a bidirectional type caster between nlohmann::json and Python objects for pybind11 bindings
        • nanobind_json, the same idea for nanobind bindings
        • nlohmann_json_qt, deserialization helpers for Qt types (QString, QUrl, QDateTime, QVector, ...) from nlohmann::json
        • vulkan2json, serialization and deserialization of Vulkan API structs
        "},{"location":"community/ecosystem/#format-converters","title":"Format converters","text":"
        • tojson, a header-only converter between YAML/XML documents and nlohmann::json
        • json2xml, a header-only converter from nlohmann::json to XML for simple configuration documents
        "},{"location":"community/governance/","title":"Governance","text":"

        The governance model for the JSON for Modern C++ project is a Benevolent Dictator for Life (BDFL) structure. As the sole maintainer, Niels Lohmann is responsible for all key aspects of the project. The project governance may evolve as the project grows, but any changes will be documented here and communicated to contributors.

        "},{"location":"community/governance/#overview","title":"Overview","text":"

        This project is led by a benevolent dictator, Niels Lohmann, and managed by the community. That is, the community actively contributes to the day-to-day maintenance of the project, but the general strategic line is drawn by the benevolent dictator. In case of disagreement, they have the last word. It is the benevolent dictator\u2019s job to resolve disputes within the community and to ensure that the project is able to progress in a coordinated way. In turn, it is the community\u2019s job to guide the decisions of the benevolent dictator through active engagement and contribution.

        "},{"location":"community/governance/#roles-and-responsibilities","title":"Roles and responsibilities","text":""},{"location":"community/governance/#benevolent-dictator-project-lead","title":"Benevolent dictator (project lead)","text":"

        Typically, the benevolent dictator, or project lead, is self-appointed. However, because the community always has the ability to fork, this person is fully answerable to the community. The project lead\u2019s role is a difficult one: they set the strategic objectives of the project and communicate these clearly to the community. They also have to understand the community as a whole and strive to satisfy as many conflicting needs as possible, while ensuring that the project survives in the long term.

        In many ways, the role of the benevolent dictator is less about dictatorship and more about diplomacy. The key is to ensure that, as the project expands, the right people are given influence over it and the community rallies behind the vision of the project lead. The lead\u2019s job is then to ensure that the committers (see below) make the right decisions on behalf of the project. Generally speaking, as long as the committers are aligned with the project\u2019s strategy, the project lead will allow them to proceed as they desire.

        "},{"location":"community/governance/#committers","title":"Committers","text":"

        Committers are contributors who have made several valuable contributions to the project and are now relied upon to both write code directly to the repository and screen the contributions of others. In many cases they are programmers but it is also possible that they contribute in a different role. Typically, a committer will focus on a specific aspect of the project, and will bring a level of expertise and understanding that earns them the respect of the community and the project lead. The role of committer is not an official one, it is simply a position that influential members of the community will find themselves in as the project lead looks to them for guidance and support.

        Committers have no authority over the overall direction of the project. However, they do have the ear of the project lead. It is a committer\u2019s job to ensure that the lead is aware of the community\u2019s needs and collective objectives, and to help develop or elicit appropriate contributions to the project. Often, committers are given informal control over their specific areas of responsibility, and are assigned rights to directly modify certain areas of the source code. That is, although committers do not have explicit decision-making authority, they will often find that their actions are synonymous with the decisions made by the lead.

        "},{"location":"community/governance/#contributors","title":"Contributors","text":"

        Contributors are community members who either have no desire to become committers, or have not yet been given the opportunity by the benevolent dictator. They make valuable contributions, such as those outlined in the list below, but generally do not have the authority to make direct changes to the project code. Contributors engage with the project through communication tools, such as email lists, and via reports and patches attached to issues in the issue tracker, as detailed in our community tools document.

        Anyone can become a contributor. There is no expectation of commitment to the project, no specific skill requirements and no selection process. To become a contributor, a community member simply has to perform one or more actions that are beneficial to the project.

        Some contributors will already be engaging with the project as users, but will also find themselves doing one or more of the following:

        • supporting new users (current users often provide the most effective new user support)
        • reporting bugs
        • identifying requirements
        • supplying graphics and web design
        • programming
        • assisting with project infrastructure
        • writing documentation
        • fixing bugs
        • adding features

        As contributors gain experience and familiarity with the project, they may find that the project lead starts relying on them more and more. When this begins to happen, they gradually adopt the role of committer, as described above.

        "},{"location":"community/governance/#users","title":"Users","text":"

        Users are community members who have a need for the project. They are the most important members of the community: without them, the project would have no purpose. Anyone can be a user; there are no specific requirements.

        Users should be encouraged to participate in the life of the project and the community as much as possible. User contributions enable the project team to ensure that they are satisfying the needs of those users. Common user activities include (but are not limited to):

        • evangelising about the project
        • informing developers of project strengths and weaknesses from a new user\u2019s perspective
        • providing moral support (a \u2018thank you\u2019 goes a long way)
        • providing financial support

        Users who continue to engage with the project and its community will often find themselves becoming more and more involved. Such users may then go on to become contributors, as described above.

        "},{"location":"community/governance/#access-to-project-resources","title":"Access to project resources","text":"

        The project's resources are the GitHub repository with its settings, CI workflows and secrets, and the documentation at json.nlohmann.me, which is built and deployed from the repository. Currently, the project lead is the only person with write or admin access to them.

        "},{"location":"community/governance/#granting-access","title":"Granting access","text":"

        Write or admin access is only granted by the project lead, and only to a contributor whose track record in the project the project lead has reviewed first. The role is assigned manually and is the lowest one that is needed for the task. Access is removed when it is no longer needed. GitHub requires two-factor authentication for everyone who can modify the repository.

        "},{"location":"community/governance/#secrets","title":"Secrets","text":"

        The CI workflows mostly use the token that GitHub creates for each workflow run. It is read-only by default, and each workflow requests only the additional permissions it needs. The few other credentials, such as the token for Semgrep, are stored as encrypted GitHub Actions secrets:

        • Only people with admin access can create, change, or delete them. Their values cannot be read back, not even by admins.
        • They are not passed to workflows that run for pull requests from forks.
        • They must never be committed to the repository or printed in logs.
        • They are rotated whenever someone with admin access leaves the project, and immediately if a leak is suspected.
        "},{"location":"community/governance/#support","title":"Support","text":"

        All participants in the community are encouraged to provide support for new users within the project management infrastructure. This support is provided as a way of growing the community. Those seeking support should recognise that all support activity within the project is voluntary and is therefore provided as and when time allows. A user requiring guaranteed response times or results should therefore seek to purchase a support contract from a vendor. (Of course, that vendor should be an active member of the community.) However, for those willing to engage with the project on its own terms, and willing to help support other users, the community support channels are ideal.

        "},{"location":"community/governance/#contribution-process","title":"Contribution Process","text":"

        Anyone can contribute to the project, regardless of their skills, as there are many ways to contribute. For instance, a contributor might be active on the project mailing list and issue tracker, or might supply patches. The various ways of contributing are described in more detail in our roles in open source document.

        The developer mailing list is the most appropriate place for a contributor to ask for help when making their first contribution.

        "},{"location":"community/governance/#decision-making-process","title":"Decision-Making Process","text":"

        The benevolent dictatorship model does not need a formal conflict resolution process, since the project lead\u2019s word is final. If the community chooses to question the wisdom of the actions of a committer, the project lead can review their decisions by checking the email archives, and either uphold or reverse them.

        Source

        The text was taken from http://oss-watch.ac.uk/resources/benevolentdictatorgovernancemodel.

        "},{"location":"community/quality_assurance/","title":"Quality assurance","text":"

        Ensuring quality is paramount for this project, particularly because numerous other projects depend on it. Each commit to the library undergoes rigorous checks against the following requirements, and any violations will result in a failed build.

        "},{"location":"community/quality_assurance/#c-language-compliance-and-compiler-compatibility","title":"C++ language compliance and compiler compatibility","text":"

        Requirement: Compiler support

        Any compiler with complete C++11 support can compile the library without warnings.

        Note: C++20 modules support may hit compiler-specific issues not covered by the general compiler matrix below. See Modules for known issues and workarounds.

        Note: Some modern features (like C++20 ranges or filesystem support) may be disabled on specific broken or incomplete toolchains even when standard feature-test macros indicate support. See JSON_HAS_RANGES, JSON_HAS_RANGE_VIEW_CONVERSION, and JSON_HAS_FILESYSTEM for details on known exclusions.

        • The library is compiled with 50+ different C++ compilers with different operating systems and platforms, including the oldest versions known to compile the library.

          Compilers used in continuous integration Compiler Architecture Operating System CI AppleClang 16.0.0.16000026; Xcode 16 arm64 macOS 15.2 (Sequoia) GitHub AppleClang 16.0.0.16000026; Xcode 16.1 arm64 macOS 15.2 (Sequoia) GitHub AppleClang 16.0.0.16000026; Xcode 16.2 arm64 macOS 15.2 (Sequoia) GitHub AppleClang 17.0.0.17000013; Xcode 16.3 arm64 macOS 15.5 (Sequoia) GitHub AppleClang 17.0.0.17000013; Xcode 16.4 arm64 macOS 15.5 (Sequoia) GitHub AppleClang 17.0.0.17000319; Xcode 26.0.1 arm64 macOS 15.5 (Sequoia) GitHub AppleClang 17.0.0.17000404; Xcode 26.1.1 arm64 macOS 15.7.9 (Sequoia) GitHub AppleClang 17.0.0.17000603; Xcode 26.2 arm64 macOS 15.7.9 (Sequoia) GitHub AppleClang 17.0.0.17000604; Xcode 26.3 arm64 macOS 15.7.9 (Sequoia) GitHub AppleClang 21.0.0.21000099; Xcode 26.4.1 arm64 macOS 26.6.2 (Tahoe) GitHub AppleClang 21.0.0.21000101; Xcode 26.5 arm64 macOS 26.6.2 (Tahoe) GitHub AppleClang 21.0.0.21000101; Xcode 26.6 arm64 macOS 26.6.2 (Tahoe) GitHub Clang 3.4.2 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 3.5.2 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 3.6.2 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 3.7.1 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 3.8.1 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 3.9.1 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 4.0.1 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 5.0.2 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 6.0.1 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 7.1.0 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 8.0.1 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 9.0.1 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 10.0.1 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 11.0.1 with GNU-like command-line x86_64 Windows Server 2022 (Build 20348) GitHub Clang 11.1.0 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 12.0.1 with GNU-like command-line x86_64 Windows Server 2022 (Build 20348) GitHub Clang 12.0.1 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 13.0.1 with GNU-like command-line x86_64 Windows Server 2022 (Build 20348) GitHub Clang 13.0.1 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 14.0.6 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 14.0.6 with GNU-like command-line x86_64 Windows Server 2022 (Build 20348) GitHub Clang 15.0.7 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 15.0.7 with GNU-like command-line x86_64 Windows Server 2022 (Build 20348) GitHub Clang 16.0.6 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 16.0.6 with GNU-like command-line x86_64 Windows Server 2022 (Build 20348) GitHub Clang 17.0.6 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 18.1.8 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 18.1.8 with GNU-like command-line x86_64 Windows Server 2022 (Build 20348) GitHub Clang 19.1.5 with MSVC-like command-line x86_64 Windows Server 2022 (Build 20348) GitHub Clang 19.1.7 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 19.1.7 with GNU-like command-line x86_64 Windows Server 2022 (Build 20348) GitHub Clang 20.1.1 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 20.1.8 with GNU-like command-line x86_64 Windows Server 2022 (Build 20348) GitHub Clang 21.1.8 x86_64 Ubuntu 22.04.1 LTS GitHub Clang 22.1.8 x86_64 Ubuntu 22.04.1 LTS GitHub CUDA 11.8.0 (nvcc) x86_64 Ubuntu 22.04 LTS GitHub CUDA 12.1.1 (nvcc) x86_64 Ubuntu 22.04 LTS GitHub CUDA 12.6.3 (nvcc) x86_64 Ubuntu 22.04 LTS GitHub Emscripten 4.0.6 x86_64 Ubuntu 22.04.1 LTS GitHub GNU 4.8.5 x86_64 Ubuntu 20.04 LTS GitHub GNU 4.9.3 x86_64 Ubuntu 20.04 LTS GitHub GNU 5.5.0 x86_64 Ubuntu 20.04 LTS GitHub GNU 6.4.0 x86_64 Ubuntu 20.04 LTS GitHub GNU 7.5.0 x86_64 Ubuntu 22.04.1 LTS GitHub GNU 8.5.0 x86_64 Ubuntu 22.04.1 LTS GitHub GNU 9.3.0 x86_64 Ubuntu 22.04.1 LTS GitHub GNU 9.4.0 x86_64 Ubuntu 22.04.1 LTS GitHub GNU 9.5.0 x86_64 Ubuntu 22.04.1 LTS GitHub GNU 10.5.0 x86_64 Ubuntu 22.04.1 LTS GitHub GNU 11.4.0 x86_64 Ubuntu 22.04.1 LTS GitHub GNU 11.5.0 x86_64 Ubuntu 22.04.1 LTS GitHub GNU 12.2.0 (MinGW-W64 i686-ucrt-posix-dwarf) x86_64 Windows Server 2022 (Build 20348) GitHub GNU 12.2.0 (MinGW-W64 x86_64-ucrt-posix-seh) x86_64 Windows Server 2022 (Build 20348) GitHub GNU 12.4.0 x86_64 Ubuntu 22.04.1 LTS GitHub GNU 13.3.0 x86_64 Ubuntu 22.04.1 LTS GitHub GNU 14.2.0 x86_64 Ubuntu 22.04.1 LTS GitHub GNU 15.1.0 x86_64 Ubuntu 22.04.1 LTS GitHub GNU 16.2.0 x86_64 Ubuntu 22.04.1 LTS GitHub GNU 16.1.0 arm64 Ubuntu 24.04 GitHub icpc (ICC) 2021.10.0 20230609 x86_64 Ubuntu 22.04 LTS GitHub icpx (Intel oneAPI DPC++/C++) 2025.3.2 x86_64 Ubuntu 24.04 LTS GitHub nvc++ (NVIDIA HPC SDK) 25.5-0 x86_64 Ubuntu 22.04 LTS GitHub MSVC 19.0.24241.7 x86 Windows 8.1 AppVeyor MSVC 19.16.27035.0 x86 Windows-10 (Build 14393) AppVeyor MSVC 19.29.30157.0 x86 Windows-10 (Build 17763) AppVeyor MSVC 19.44.35207.0 arm64 Windows 11 (Build 26200) GitHub MSVC 19.44.35214.0 x86 Windows Server 2022 (Build 20348) GitHub MSVC 19.44.35214.0 x86_64 Windows Server 2022 (Build 20348) GitHub MSVC 19.51.36231.0 x86 Windows Server 2025 (Build 26100) GitHub MSVC 19.51.36231.0 x86_64 Windows Server 2025 (Build 26100) GitHub

        • The library is compiled with all C++ language revisions (C++11, C++14, C++17, C++20, C++23, and C++26) to detect and fix language deprecations early.

        • The library is checked for compiler warnings:

        • On Clang, -Weverything is used with 7 exceptions.

          Clang warnings 
          # Ignored Clang warnings:\n# -Wno-c++98-compat               The library targets C++11.\n# -Wno-c++98-compat-pedantic      The library targets C++11.\n# -Wno-deprecated-declarations    The library contains annotations for deprecated functions.\n# -Wno-padded                     We do not care about padding warnings.\n# -Wno-covered-switch-default     All switches list all cases and a default case.\n# -Wno-c2y-extensions             Clang 22.1 diagnoses __COUNTER__ as a C2y extension, also in\n#                                 C++ mode. The library does not use __COUNTER__; the warnings\n#                                 all come from vendored Doctest (SECTION/TEST_CASE macros).\n# -Wno-unsafe-buffer-usage        Pervasive: the library's own low-level numeric/buffer code\n#                                 (to_chars, serializer, lexer, binary reader/writer, input\n#                                 adapters, json_pointer) plus vendored Doctest itself (~208\n#                                 distinct sites measured 2026-07-08 on clang trunk) all use\n#                                 raw pointer arithmetic / libc string calls by necessity.\n\nset(CLANG_CXXFLAGS\n    -Werror\n    -Weverything\n    -Wno-c++98-compat\n    -Wno-c++98-compat-pedantic\n    -Wno-deprecated-declarations\n    -Wno-padded\n    -Wno-covered-switch-default\n    -Wno-c2y-extensions\n    -Wno-unsafe-buffer-usage\n)\n

        • On GCC, 300+ warnings are enabled with 8 exceptions.

          GCC warnings 
          # Warning flags determined for GCC 16.2.0 with https://github.com/nlohmann/gcc_flags:\n# Ignored GCC warnings:\n# -Wno-abi-tag           We do not care about ABI tags.\n# -Wno-aggregate-return  The library uses aggregate returns.\n# -Wno-long-long         The library uses the long long type to interface with system functions.\n# -Wno-namespaces        The library uses namespaces.\n# -Wno-nrvo              Doctest triggers this warning.\n# -Wno-padded            We do not care about padding warnings.\n# -Wno-system-headers    We do not care about warnings in system headers.\n# -Wno-templates         The library uses templates.\n\nset(GCC_CXXFLAGS\n    -pedantic\n    -Werror\n    --all-warnings\n    --extra-warnings\n    -W\n    -WNSObject-attribute\n    -Wabbreviated-auto-in-template-arg\n    -Wabi\n    -Wno-abi-tag\n    -Waddress\n    -Waddress-of-packed-member\n    -Wno-aggregate-return\n    -Waggressive-loop-optimizations\n    -Waligned-new=all\n    -Wall\n    -Walloc-size\n    -Walloc-zero\n    -Walloca\n    -Wanalyzer-allocation-size\n    -Wanalyzer-deref-before-check\n    -Wanalyzer-double-fclose\n    -Wanalyzer-double-free\n    -Wanalyzer-exposure-through-output-file\n    -Wanalyzer-exposure-through-uninit-copy\n    -Wanalyzer-fd-access-mode-mismatch\n    -Wanalyzer-fd-double-close\n    -Wanalyzer-fd-leak\n    -Wanalyzer-fd-phase-mismatch\n    -Wanalyzer-fd-type-mismatch\n    -Wanalyzer-fd-use-after-close\n    -Wanalyzer-fd-use-without-check\n    -Wanalyzer-file-leak\n    -Wanalyzer-free-of-non-heap\n    -Wanalyzer-imprecise-fp-arithmetic\n    -Wanalyzer-infinite-loop\n    -Wanalyzer-infinite-recursion\n    -Wanalyzer-jump-through-null\n    -Wanalyzer-malloc-leak\n    -Wanalyzer-mismatching-deallocation\n    -Wanalyzer-null-argument\n    -Wanalyzer-null-dereference\n    -Wanalyzer-out-of-bounds\n    -Wanalyzer-overlapping-buffers\n    -Wanalyzer-possible-null-argument\n    -Wanalyzer-possible-null-dereference\n    -Wanalyzer-putenv-of-auto-var\n    -Wanalyzer-shift-count-negative\n    -Wanalyzer-shift-count-overflow\n    -Wanalyzer-stale-setjmp-buffer\n    -Wanalyzer-symbol-too-complex\n    -Wanalyzer-tainted-allocation-size\n    -Wanalyzer-tainted-array-index\n    -Wanalyzer-tainted-assertion\n    -Wanalyzer-tainted-divisor\n    -Wanalyzer-tainted-offset\n    -Wanalyzer-tainted-size\n    -Wanalyzer-throw-of-unexpected-type\n    -Wanalyzer-too-complex\n    -Wanalyzer-undefined-behavior-ptrdiff\n    -Wanalyzer-undefined-behavior-strtok\n    -Wanalyzer-unsafe-call-within-signal-handler\n    -Wanalyzer-use-after-free\n    -Wanalyzer-use-of-pointer-in-stale-stack-frame\n    -Wanalyzer-use-of-uninitialized-value\n    -Wanalyzer-va-arg-type-mismatch\n    -Wanalyzer-va-list-exhausted\n    -Wanalyzer-va-list-leak\n    -Wanalyzer-va-list-use-after-va-end\n    -Wanalyzer-write-to-const\n    -Wanalyzer-write-to-string-literal\n    -Warith-conversion\n    -Warray-bounds=2\n    -Warray-compare\n    -Warray-parameter\n    -Warray-parameter=2\n    -Wattribute-alias=2\n    -Wattribute-warning\n    -Wattributes\n    -Wauto-profile\n    -Wbidi-chars=any\n    -Wbool-compare\n    -Wbool-operation\n    -Wbuiltin-declaration-mismatch\n    -Wbuiltin-macro-redefined\n    -Wc++0x-compat\n    -Wc++11-compat\n    -Wc++11-extensions\n    -Wc++14-compat\n    -Wc++14-extensions\n    -Wc++17-compat\n    -Wc++17-extensions\n    -Wc++1z-compat\n    -Wc++20-compat\n    -Wc++20-extensions\n    -Wc++23-extensions\n    -Wc++26-compat\n    -Wc++26-extensions\n    -Wc++2a-compat\n    -Wcalloc-transposed-args\n    -Wcannot-profile\n    -Wcast-align\n    -Wcast-align=strict\n    -Wcast-function-type\n    -Wcast-qual\n    -Wcast-user-defined\n    -Wcatch-value=3\n    -Wchanges-meaning\n    -Wchar-subscripts\n    -Wclass-conversion\n    -Wclass-memaccess\n    -Wclobbered\n    -Wcomma-subscript\n    -Wcomment\n    -Wcomments\n    -Wcomplain-wrong-lang\n    -Wconditionally-supported\n    -Wconversion\n    -Wconversion-null\n    -Wcoverage-invalid-line-number\n    -Wcoverage-mismatch\n    -Wcoverage-too-many-conditions\n    -Wcoverage-too-many-paths\n    -Wcpp\n    -Wctad-maybe-unsupported\n    -Wctor-dtor-privacy\n    -Wdangling-else\n    -Wdangling-pointer=2\n    -Wdangling-reference\n    -Wdate-time\n    -Wdefaulted-function-deleted\n    -Wdelete-incomplete\n    -Wdelete-non-virtual-dtor\n    -Wdeprecated\n    -Wdeprecated-copy\n    -Wdeprecated-copy-dtor\n    -Wdeprecated-declarations\n    -Wdeprecated-enum-enum-conversion\n    -Wdeprecated-enum-float-conversion\n    -Wdeprecated-literal-operator\n    -Wdeprecated-openmp\n    -Wdeprecated-variadic-comma-omission\n    -Wdisabled-optimization\n    -Wdiv-by-zero\n    -Wdouble-promotion\n    -Wduplicated-branches\n    -Wduplicated-cond\n    -Weffc++\n    -Welaborated-enum-base\n    -Wempty-body\n    -Wendif-labels\n    -Wenum-compare\n    -Wenum-conversion\n    -Wexceptions\n    -Wexpansion-to-defined\n    -Wexpose-global-module-tu-local\n    -Wexternal-tu-local\n    -Wextra\n    -Wextra-semi\n    -Wflex-array-member-not-at-end\n    -Wfloat-conversion\n    -Wfloat-equal\n    -Wformat-diag\n    -Wformat-overflow=2\n    -Wformat-signedness\n    -Wformat-truncation=2\n    -Wformat=2\n    -Wframe-address\n    -Wfree-nonheap-object\n    -Wglobal-module\n    -Whardened\n    -Wheader-guard\n    -Whsa\n    -Wif-not-aligned\n    -Wignored-attributes\n    -Wignored-qualifiers\n    -Wimplicit-fallthrough=5\n    -Winaccessible-base\n    -Winfinite-recursion\n    -Winherited-variadic-ctor\n    -Winit-list-lifetime\n    -Winit-self\n    -Winline\n    -Wint-in-bool-context\n    -Wint-to-pointer-cast\n    -Winterference-size\n    -Winvalid-constexpr\n    -Winvalid-imported-macros\n    -Winvalid-memory-model\n    -Winvalid-offsetof\n    -Winvalid-pch\n    -Winvalid-utf8\n    -Wkeyword-macro\n    -Wleading-whitespace=spaces\n    -Wliteral-suffix\n    -Wlogical-not-parentheses\n    -Wlogical-op\n    -Wno-long-long\n    -Wlto-type-mismatch\n    -Wmain\n    -Wmaybe-musttail-local-addr\n    -Wmaybe-uninitialized\n    -Wmemset-elt-size\n    -Wmemset-transposed-args\n    -Wmisleading-indentation\n    -Wmismatched-dealloc\n    -Wmismatched-new-delete\n    -Wmismatched-tags\n    -Wmissing-attributes\n    -Wmissing-braces\n    -Wmissing-declarations\n    -Wmissing-field-initializers\n    -Wmissing-include-dirs\n    -Wmissing-profile\n    -Wmissing-requires\n    -Wmissing-template-keyword\n    -Wmultichar\n    -Wmultiple-inheritance\n    -Wmultistatement-macros\n    -Wmusttail-local-addr\n    -Wno-namespaces\n    -Wnarrowing\n    -Wnoexcept\n    -Wnoexcept-type\n    -Wnon-c-typedef-for-linkage\n    -Wnon-template-friend\n    -Wnon-virtual-dtor\n    -Wnonnull\n    -Wnonnull-compare\n    -Wnormalized=nfkc\n    -Wno-nrvo\n    -Wnull-dereference\n    -Wodr\n    -Wold-style-cast\n    -Wopenacc-parallelism\n    -Wopenmp\n    -Wopenmp-simd\n    -Woverflow\n    -Woverlength-strings\n    -Woverloaded-virtual=2\n    -Wpacked\n    -Wpacked-bitfield-compat\n    -Wpacked-not-aligned\n    -Wno-padded\n    -Wparentheses\n    -Wpedantic\n    -Wpessimizing-move\n    -Wplacement-new=2\n    -Wpmf-conversions\n    -Wpointer-arith\n    -Wpointer-compare\n    -Wpragma-once-outside-header\n    -Wpragmas\n    -Wprio-ctor-dtor\n    -Wpsabi\n    -Wrange-loop-construct\n    -Wredundant-decls\n    -Wredundant-move\n    -Wredundant-tags\n    -Wregister\n    -Wreorder\n    -Wrestrict\n    -Wreturn-local-addr\n    -Wreturn-type\n    -Wscalar-storage-order\n    -Wself-move\n    -Wsequence-point\n    -Wsfinae-incomplete\n    -Wsfinae-incomplete=2\n    -Wshadow=compatible-local\n    -Wshadow=global\n    -Wshadow=local\n    -Wshift-count-negative\n    -Wshift-count-overflow\n    -Wshift-negative-value\n    -Wshift-overflow=2\n    -Wsign-compare\n    -Wsign-conversion\n    -Wsign-promo\n    -Wsized-deallocation\n    -Wsizeof-array-argument\n    -Wsizeof-array-div\n    -Wsizeof-pointer-div\n    -Wsizeof-pointer-memaccess\n    -Wstack-protector\n    -Wstrict-aliasing\n    -Wstrict-aliasing=3\n    -Wstrict-null-sentinel\n    -Wstrict-overflow\n    -Wstrict-overflow=5\n    -Wstring-compare\n    -Wstringop-overflow\n    -Wstringop-overflow=4\n    -Wstringop-overread\n    -Wstringop-truncation\n    -Wsubobject-linkage\n    -Wsuggest-attribute=cold\n    -Wsuggest-attribute=const\n    -Wsuggest-attribute=format\n    -Wsuggest-attribute=malloc\n    -Wsuggest-attribute=noreturn\n    -Wsuggest-attribute=pure\n    -Wsuggest-attribute=returns_nonnull\n    -Wsuggest-final-methods\n    -Wsuggest-final-types\n    -Wsuggest-override\n    -Wswitch\n    -Wswitch-bool\n    -Wswitch-default\n    -Wswitch-enum\n    -Wswitch-outside-range\n    -Wswitch-unreachable\n    -Wsync-nand\n    -Wsynth\n    -Wno-system-headers\n    -Wtautological-compare\n    -Wtemplate-body\n    -Wtemplate-id-cdtor\n    -Wtemplate-names-tu-local\n    -Wno-templates\n    -Wterminate\n    -Wtrailing-whitespace\n    -Wtrampolines\n    -Wtrigraphs\n    -Wtrivial-auto-var-init\n    -Wtsan\n    -Wtype-limits\n    -Wundef\n    -Wunicode\n    -Wuninitialized\n    -Wunknown-pragmas\n    -Wunreachable-code\n    -Wunsafe-loop-optimizations\n    -Wunused\n    -Wunused-but-set-parameter=3\n    -Wunused-but-set-variable=3\n    -Wunused-const-variable=2\n    -Wunused-function\n    -Wunused-label\n    -Wunused-local-typedefs\n    -Wunused-macros\n    -Wunused-parameter\n    -Wunused-result\n    -Wunused-value\n    -Wunused-variable\n    -Wuse-after-free=3\n    -Wuseless-cast\n    -Wvarargs\n    -Wvariadic-macros\n    -Wvector-operation-performance\n    -Wvexing-parse\n    -Wvirtual-inheritance\n    -Wvirtual-move-assign\n    -Wvla\n    -Wvla-parameter\n    -Wvolatile\n    -Wvolatile-register-var\n    -Wwrite-strings\n    -Wxor-used-as-pow\n    -Wzero-as-null-pointer-constant\n    -Wzero-length-bounds\n)\n
        "},{"location":"community/quality_assurance/#c-standard-library-compliance","title":"C++ standard library compliance","text":"

        Requirement: No prerequisites

        The library has no prerequisites other than the Standard Template Library (STL).

        • The library is compiled and tested with both libc++ and libstdc++ to detect subtle differences or incompatibilities.
        • The code checked with Include What You Use (IWYU) that all required standard headers are included.
        • On Windows, the library is compiled with <Windows.h> being included to detect and avoid common bugs (see unit-windows_h.cpp).
        • The library is compiled with Qt's keyword macros (slots, signals, emit, foreach, forever) being defined to make sure it does not use them as identifiers (see unit-qt_keywords.cpp).
        • The library is compiled with exceptions disabled to support alternative means of error handling.
        "},{"location":"community/quality_assurance/#stable-public-api","title":"Stable public API","text":"

        Requirement: Stable public API

        Any change to the library does not break the public API.

        • All public API functions are tested with a variety of arguments.
        • The library is compiled and tested with different template arguments for number, string, array, and object types.
        • Unit tests cover all lines of the code base.
        • Every exception of the library is thrown in the test suite, and the error messages and exception ids are checked.

        Requirement: Complete documentation

        The public API is extensively documented.

        • Every public API function has a dedicated page in the API reference documentation with a self-contained code example.
        • All examples in the documentation are tested, and changes in their output are treated as an error.
        "},{"location":"community/quality_assurance/#robust-input-processing","title":"Robust input processing","text":"

        Requirement: Standards compliance

        The library is compliant to JSON as defined in RFC 8259.

        • The lexer is tested with all valid Unicode code points and all prefixes of all invalid Unicode code points.
        • The parser is tested against extensive correctness suites for JSON compliance.
        • In addition, the library is continuously fuzz-tested at OSS-Fuzz where the library is checked against billions of inputs.
        • Every crash reported by OSS-Fuzz is fixed together with a unit test that reproduces it, and the fix references the OSS-Fuzz issue. The round-trip checks of the fuzzer drivers are also part of the unit tests. See the fuzz testing documentation.
        "},{"location":"community/quality_assurance/#static-analysis","title":"Static analysis","text":"

        Requirement: State-of-the-art code analysis

        The code is checked with state-of-the-art static code analysis tools.

        • The code is checked with the latest Clang-Tidy.

          Clang-Tidy configuration (.clang-tidy) 
          # portability-avoid-pragma-once: kept disabled on purpose. #pragma once is accepted\n# by every supported compiler, and tools/amalgamate/amalgamate.py strips it from\n# single_include, so there is nothing left to fix here.\n\nChecks: '*,\n\n         -altera-id-dependent-backward-branch,\n         -altera-struct-pack-align,\n         -altera-unroll-loops,\n         -android-cloexec-fopen,\n         -boost-use-ranges,\n         -bugprone-easily-swappable-parameters,\n         -cert-err58-cpp,\n         -clang-analyzer-webkit.NoUncountedMemberChecker,\n         -concurrency-mt-unsafe,\n         -cppcoreguidelines-avoid-const-or-ref-data-members,\n         -cppcoreguidelines-avoid-do-while,\n         -cppcoreguidelines-avoid-goto,\n         -cppcoreguidelines-avoid-magic-numbers,\n         -cppcoreguidelines-avoid-non-const-global-variables,\n         -cppcoreguidelines-macro-usage,\n         -cppcoreguidelines-pro-bounds-avoid-unchecked-container-access,\n         -cppcoreguidelines-pro-bounds-array-to-pointer-decay,\n         -cppcoreguidelines-pro-bounds-constant-array-index,\n         -cppcoreguidelines-pro-bounds-pointer-arithmetic,\n         -cppcoreguidelines-pro-type-reinterpret-cast,\n         -cppcoreguidelines-pro-type-union-access,\n         -cppcoreguidelines-rvalue-reference-param-not-moved,\n         -cppcoreguidelines-virtual-class-destructor,\n         -fuchsia-default-arguments-calls,\n         -fuchsia-default-arguments-declarations,\n         -fuchsia-overloaded-operator,\n         -google-explicit-constructor,\n         -google-readability-function-size,\n         -google-runtime-float,\n         -google-runtime-int,\n         -hicpp-avoid-goto,\n         -hicpp-explicit-conversions,\n         -hicpp-function-size,\n         -hicpp-no-array-decay,\n         -hicpp-no-assembler,\n         -hicpp-signed-bitwise,\n         -hicpp-uppercase-literal-suffix,\n         -llvm-header-guard,\n         -llvm-include-order,\n         -llvm-prefer-static-over-anonymous-namespace,\n         -llvm-use-ranges,\n         -llvmlibc-*,\n         -misc-use-anonymous-namespace,\n         -misc-confusable-identifiers,\n         -misc-include-cleaner,\n         -misc-no-recursion,\n         -misc-non-private-member-variables-in-classes,\n         -modernize-concat-nested-namespaces,\n         -modernize-type-traits,\n         -modernize-use-constraints,\n         -modernize-use-designated-initializers,\n         -modernize-use-nodiscard,\n         -modernize-use-ranges,\n         -modernize-use-std-numbers,\n         -modernize-use-trailing-return-type,\n         -performance-enum-size,\n         -portability-avoid-pragma-once,\n         -readability-function-cognitive-complexity,\n         -readability-function-size,\n         -readability-identifier-length,\n         -readability-magic-numbers,\n         -readability-redundant-access-specifiers,\n         -readability-redundant-parentheses,\n         -readability-redundant-typename,\n         -readability-simplify-boolean-expr,\n         -readability-uppercase-literal-suffix,\n         -readability-use-concise-preprocessor-directives'\n\nCheckOptions:\n  - key: hicpp-special-member-functions.AllowSoleDefaultDtor\n    value: 1\n  # clang-tidy 22.1 extended this check to classes and enums; the test files\n  # define many such helper types at namespace scope, which is harmless\n  - key: misc-use-internal-linkage.AnalyzeTypes\n    value: false\n\nWarningsAsErrors: '*'\n\nHeaderFilterRegex: '.*hpp$'\n

        • The code is checked with the latest Cppcheck with all warnings enabled.

        • The code is checked with the latest Clang Static Analyzer with 89 enabled rules.
        • The code is checked with Infer.
        • The code is checked with Codacy.
        "},{"location":"community/quality_assurance/#dynamic-analysis","title":"Dynamic analysis","text":"

        Requirement: Correctness

        The library is checked for memory correctness and absence of undefined behavior.

        • The test suite is executed with enabled runtime assertions to check invariants and preconditions of functions to detect undefined behavior.
        • The test suite is executed with Valgrind (Memcheck) to detect memory leaks.
        • The test suite is executed with Sanitizers (address sanitizer, undefined behavior sanitizer, integer overflow detection, nullability violations).
        "},{"location":"community/quality_assurance/#dependencies","title":"Dependencies","text":"

        Requirement: No vulnerable dependencies

        The library has no dependencies besides the C++ standard library. The tools used to build, test, and document it are kept free of known vulnerabilities.

        • GitHub Actions are pinned to a commit hash, and the Python packages used by the documentation and the tools are pinned to exact versions.
        • Dependabot checks these dependencies daily and proposes updates as pull requests.
        • Every pull request is checked with the dependency review action. A pull request that adds a dependency with a known vulnerability of any severity fails this check and is not merged.
        • Vulnerability alerts for dependencies are fixed or dismissed with a documented reason before the next release. No release is made while such an alert is open.
        • Third-party code included in the repository for testing, such as doctest, is updated manually.
        "},{"location":"community/quality_assurance/#style-check","title":"Style check","text":"

        Requirement: Common code style

        A common code style is used throughout all code files of the library.

        • The code is formatted with Artistic Style (astyle) against a style configuration that is also enforced in the CI.

          Astyle configuration (tools/astyle/.astylerc) 
          # Configuration for Artistic Style\n# see https://astyle.sourceforge.net/astyle.html\n\n#######################\n# Brace Style Options #\n#######################\n\n# use Allman style for braces\n--style=allman\n\n###############\n# Tab Options #\n###############\n\n# indent using 4 spaces\n--indent=spaces=4\n\n#######################\n# Indentation Options #\n#######################\n\n# indent access modifiers one half indent\n--indent-modifiers\n\n# indent switch cases to the switch block\n--indent-switches\n\n# indent preprocessor blocks\n--indent-preproc-block\n\n# indent preprocessor defines\n--indent-preproc-define\n\n# indent C++ comments\n--indent-col1-comments\n\n###################\n# Padding Options #\n###################\n\n# insert space padding around operators\n--pad-oper\n\n# insert space between if/for/while... and the following parentheses\n--pad-header\n\n# attach the pointer to the variable type (left)\n--align-pointer=type\n\n# attach the reference to the variable type (left)\n--align-reference=type\n\n######################\n# Formatting Options #\n######################\n\n# add braces to unbraced one line conditional statements\n--add-braces\n\n# convert tabs to spaces\n--convert-tabs\n\n# closes whitespace between the ending angle brackets of template definitions\n--close-templates\n\n#################\n# Other Options #\n#################\n\n# do not create backup files\n--suffix=none\n\n# preserve the original file date\n--preserve-date\n\n# display only the files that have been formatted\n--formatted\n\n# for the linux (LF) line end style\n--lineend=linux\n

        • The code style is checked with cpplint with 61 enabled rules.

        "},{"location":"community/quality_assurance/#simple-integration","title":"Simple integration","text":"

        Requirement: Single header

        The library can be used by adding a single header to a C++ project.

        • An amalgamation script is used to check if the source code is exposed as a self-contained single-header file.
        • The test suite is checked against the amalgamated source file as well as the individual source file.

        Requirement: CMake as primary development tool

        All library functions are exposed and usable by CMake.

        • All library options are exposed as CMake options and tested.
        • The library is tested against relevant CMake versions:
        • CMake 3.5 (the earliest supported)
        • CMake 3.31.6 (the latest 3.x release)
        • CMake 4.0.0 (a very recent release)
        "},{"location":"community/roadmap/","title":"Roadmap","text":"

        This page describes what the project intends to do, and what it does not intend to do, over the next year. Concrete work items are tracked in the GitHub milestones and the issue tracker.

        "},{"location":"community/roadmap/#what-the-project-will-do","title":"What the project will do","text":"
        • Keep the C++11 baseline. The library will continue to compile with every supported C++11 compiler. Features of later standards are only used when they are guarded by the JSON_HAS_CPP_* macros.
        • Stay conformant to JSON. The parser and serializer follow RFC 8259. Extensions such as comments or trailing commas remain opt-in.
        • Keep the 3.x public API stable. Releases follow semantic versioning. Changes that would break existing code are only added behind a feature macro, so users can opt in and test their code before a next major release, see Version 4.0.
        • Support a broad range of compilers and platforms. The CI keeps testing old and new versions of GCC, Clang, MSVC, and other compilers on Linux, macOS, and Windows.
        • Keep the quality assurance up. Every change keeps the test coverage at 100%, passes the static and dynamic analysis, and is fuzz-tested by OSS-Fuzz, see Quality assurance.
        • Harden the library against hostile input. Handling deeply nested values without exhausting the call stack is ongoing work.
        • Fix bugs and security issues reported through the issue tracker and the security policy.
        "},{"location":"community/roadmap/#what-the-project-will-not-do","title":"What the project will not do","text":"
        • Break the public API of version 3.x. See API stability for what this covers.
        • Require a newer C++ standard than C++11.
        • Break JSON conformance or enable non-standard extensions by default.
        • Add dependencies or require a build step. The library remains header-only, and the single header json.hpp remains a complete distribution.
        • Trade simplicity for speed or memory efficiency. Performance improvements are welcome, but the library is not meant to compete with the fastest JSON libraries, see Design goals.
        "},{"location":"community/roadmap/#api-stability","title":"API stability","text":"

        Releases follow semantic versioning: a minor or patch release of version 3.x does not break code that uses the public API, unless that code opts in to a change with a macro as described below. In particular, a 3.x release does not:

        • make breaking changes to the signature of a function: the types or order of its existing parameters, its return type, its noexcept or constexpr specifier, or the const-ness of a member function. New parameters may be added if they have a default value;
        • remove or rename a function or class, or change the template parameters of a public class template;
        • change which exceptions a function throws, or the exception ids;
        • change access specifiers, or change or remove existing default arguments. New default arguments may be added;
        • change the JSON type that a valid input parses to, or the text that dump() produces for a valid value;
        • accept input that was rejected before, or reject input that was accepted before;
        • change the order in which the keys of an object are iterated. The default type sorts keys, and ordered_json keeps insertion order;
        • change when iterators, pointers, or references are invalidated, or the state of a moved-from basic_json;
        • add or remove implicit conversions from basic_json;
        • change how to_json and from_json functions are found, or the behavior of adl_serializer;
        • add pure virtual functions to the json_sax interface;
        • remove, rename, renumber, or add enumerators of value_t;
        • remove or rename a documented macro, CMake option, CMake target, or header, or change what a documented macro does.

        Exceptions to these rules, for instance when fixing a bug requires changing the exception a function throws, are documented in the release notes.

        The following are not part of the public API and may change in any release, including patch releases:

        • The text of exception messages returned by what(). Use the exception id to tell errors apart.
        • The ABI, including sizeof(basic_json) and the memory layout of its values. The versioned inline namespace turns mixing versions into a link error.
        • The hash values returned by std::hash for basic_json. Numbers that compare equal still hash equally.
        • Everything in namespace nlohmann::detail, and macros and type traits that are not documented in the API reference.

        Breaking changes are only added behind a macro whose default keeps the 3.x behavior. See Version 4.0 and the macro overview.

        "},{"location":"community/roadmap/#version-40","title":"Version 4.0","text":"

        There is no release date for version 4.0 yet. Proposals that need a major version, for instance stricter type conversions, are collected in issue #3453.

        Not final

        The plan for version 4.0 described below is not final and may still change: macros may be added to or removed from the list, and planned defaults may be revised. Any such change will be documented on this page.

        "},{"location":"community/roadmap/#trying-out-40-today","title":"Trying out 4.0 today","text":"

        Version 4.0 will not be developed on a separate branch. Instead, every breaking change is first added to a 3.x release behind a macro whose default keeps the 3.x behavior. Version 4.0 then switches the defaults and removes the macros. Version 4.0 is therefore the sum of these macros: you can try it on the 3.x release train today by defining each macro to its 4.0 value and fixing what no longer compiles or behaves differently. Once your code works with all of them, it is ready for version 4.0.

        The following macros guard changes that are planned to become the default in version 4.0:

        Macro 3.x default 4.0 behavior CMake option Added JSON_USE_IMPLICIT_CONVERSIONS 1 0: no implicit conversions from basic_json to other types; use get instead JSON_ImplicitConversions 3.9.0 JSON_USE_GLOBAL_UDLS 1 0: the string literals _json and _json_pointer are only available in namespace nlohmann::literals JSON_GlobalUDLs 3.11.0 JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON 0 removed: the deprecated legacy comparison of discarded values can no longer be enabled JSON_LegacyDiscardedValueComparison 3.11.0 JSON_BRACE_INIT_COPY_SEMANTICS 0 1: single-element brace initialization such as json j{obj}; copies the element instead of creating an array \u2013 3.13.0 JSON_PRECISE_STREAM_POSITION 0 1: reading from a stream does not consume the character after a number \u2013 3.13.0 JSON_STRICT_NUL_HANDLING 0 1: a NUL byte in the input is a parse error instead of the end of input JSON_StrictNulHandling 3.13.0 JSON_STRICT_BINARY_UTF8 0 1: to_cbor, to_ubjson, to_bjdata, and to_bson throw for strings that are not valid UTF-8 by default JSON_StrictBinaryUTF8 3.13.0 JSON_DISABLE_TUPLE_REFERENCE_CONVERSION 0 1: a basic_json value can no longer be created from a one-element tuple of a reference to it, such as std::forward_as_tuple(j) JSON_DisableTupleReferenceConversion 3.13.0 JSON_DELETE_DEPRECATED_FUNCTIONS 0 removed: the deprecated functions are removed (see below); the from_*(ptr, len) overloads stay deleted JSON_DeleteDeprecatedFunctions 3.13.0

        For example, the following makes a 3.x release behave like version 4.0 with respect to these changes:

        #define JSON_USE_IMPLICIT_CONVERSIONS 0\n#define JSON_USE_GLOBAL_UDLS 0\n#define JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON 0\n#define JSON_BRACE_INIT_COPY_SEMANTICS 1\n#define JSON_PRECISE_STREAM_POSITION 1\n#define JSON_STRICT_NUL_HANDLING 1\n#define JSON_STRICT_BINARY_UTF8 1\n#define JSON_DISABLE_TUPLE_REFERENCE_CONVERSION 1\n#define JSON_DELETE_DEPRECATED_FUNCTIONS 1\n#include <nlohmann/json.hpp>\n

        The macros must be defined before the library header is included; setting them once in the build system is the easiest way to achieve this.

        "},{"location":"community/roadmap/#removal-of-deprecated-functions","title":"Removal of deprecated functions","text":"

        Version 4.0 will remove all deprecated functions. Compiling with deprecation warnings enabled shows which of them your code still uses. Defining JSON_DELETE_DEPRECATED_FUNCTIONS to 1 turns these warnings into errors, as the deprecated functions are then deleted. The migration guide shows how to replace each of them.

        The from_* overloads taking a pointer and a length are not removed in version 4.0, but stay deleted. Without them, a call like from_cbor(ptr, len) would still compile: it would read ptr as a NUL-terminated string and convert len to the strict parameter.

        Deprecated Since Migration operator<<(basic_json&, std::istream&) 3.0.0 Parsing operator>>(const basic_json&, std::ostream&) 3.0.0 Miscellaneous functions iterator_wrapper 3.1.0 Miscellaneous functions parse, accept, and sax_parse with an initializer list {ptr, len} or {first, last} 3.8.0 Parsing from_bson, from_cbor, from_msgpack, and from_ubjson with (ptr, len) or an initializer list 3.8.0 Parsing json_pointer::operator string_t 3.11.0 JSON Pointers json_pointer with a basic_json type as template argument, and the overloads of value, contains, operator[], and at accepting such a pointer 3.11.0 JSON Pointers Comparing a json_pointer with a string via operator== or operator!= 3.11.2 JSON Pointers from_bjdata and from_bon8 with (ptr, len) 3.13.0 Parsing

        The deprecated legacy comparison of discarded values is controlled by a macro and therefore listed in the table above.

        New breaking changes will follow the same path: they are added to these tables when they land in a 3.x release.

        "},{"location":"community/security_policy/","title":"Security Policy","text":""},{"location":"community/security_policy/#reporting-a-vulnerability","title":"Reporting a Vulnerability","text":"

        We value the security of our users and appreciate your efforts to responsibly disclose vulnerabilities. If you have identified a security vulnerability in this repository, please use the GitHub Security Advisory \"Report a Vulnerability\" tab.

        Until it is published, this draft security advisory will only be visible to the maintainers of this project. Other users and teams may be added once the advisory is created.

        We will send a first response within 14 days, indicating the next steps in handling your report. After the initial reply to your report, we will keep you informed of the progress towards a fix and full announcement and may ask for additional information or guidance.

        For vulnerabilities in third-party dependencies or modules, please report them directly to the respective maintainers.

        "},{"location":"community/security_policy/#disclosure-and-credit","title":"Disclosure and credit","text":"

        Once a fix is released, we publish the security advisory and list the fixed vulnerability in the release notes. We credit the reporter in both, unless they ask not to be named.

        "},{"location":"community/security_policy/#supported-versions","title":"Supported versions","text":"

        Security fixes are made on the develop branch and shipped with the next release. Only the latest release receives security fixes; they are not backported to older releases. A release stops receiving security fixes when the next release is published, so please update to the latest release to get them.

        "},{"location":"community/security_policy/#unofficial-packages","title":"Unofficial packages","text":"

        This project does not publish an official npm package. The npm package nlohmann-json (or similarly named packages) is not maintained or endorsed by this project. See the package managers documentation for supported integration options.

        "},{"location":"community/security_policy/#additional-resources","title":"Additional Resources","text":"
        • Explore security-related topics and contribute to tools and projects through GitHub Security Lab.
        • Learn more about responsible disclosure and reporting vulnerabilities in GitHub at About coordinated disclosure of security vulnerabilities.

        We sincerely thank you for contributing to the security and integrity of this project!

        "},{"location":"features/","title":"Features","text":"

        This section describes the features of the library in detail. If you are new to the library, the pages below are roughly ordered along a typical workflow: create or parse a value, access and modify it, convert it to and from your own C++ types, and finally serialize it again.

        "},{"location":"features/#creating-and-reading-values","title":"Creating and reading values","text":"
        • Creating JSON values \u2014 build values from literals, initializer lists, and STL containers, and understand the {} vs. [] ambiguity.
        • Parsing \u2014 read a JSON value from a string, file, or stream, including JSON Lines, callbacks, the SAX interface, error handling, and parsing untrusted input.
        • Zero-copy JSON views \u2014 read a JSON text through a flat index instead of building a json tree; strings and numbers stay in the input and are only decoded when needed.
        • Editable documents \u2014 modify a parsed document without building a json tree.
        • Images \u2014 save a parsed document and load it again without parsing it.
        • Comments and trailing commas \u2014 opt-in relaxations of the JSON grammar.
        "},{"location":"features/#accessing-and-modifying-values","title":"Accessing and modifying values","text":"
        • Element access \u2014 unchecked (operator[]), checked (at), and access with a default value.
        • JSON Pointer \u2014 address values deep inside a document with RFC 6901 pointers.
        • Iterators \u2014 traverse arrays and objects.
        • Modifying values \u2014 add, update, merge, and remove elements.
        • JSON Patch and Diff and JSON Merge Patch \u2014 apply and compute structured changes.
        "},{"location":"features/#converting-to-and-from-c-types","title":"Converting to and from C++ types","text":"
        • Converting values \u2014 get values out with get/get_to, and understand implicit conversions.
        • Arbitrary types conversions \u2014 teach the library about your own structs and classes.
        • Specializing enum conversion \u2014 map enums to strings instead of integers.
        "},{"location":"features/#serializing-values","title":"Serializing values","text":"
        • Serialization \u2014 turn a value back into JSON text with dump, including pretty-printing and handling of non-ASCII and invalid UTF-8.
        • Binary formats \u2014 encode values more compactly as BJData, BON8, BSON, CBOR, MessagePack, or UBJSON.
        • Binary values \u2014 store and exchange raw byte sequences.
        "},{"location":"features/#how-values-are-stored-and-configured","title":"How values are stored and configured","text":"
        • Types and number handling \u2014 how JSON types map to C++ types and how numbers are treated.
        • Template parameter requirements \u2014 what a type passed as one of basic_json's template parameters has to provide.
        • Object order \u2014 keep insertion order with ordered_json.
        • Performance \u2014 practical advice on parsing, memory use, serialization, and compile times.
        • Runtime assertions, supported macros, the nlohmann namespace, and C++ modules \u2014 build-time and runtime configuration.

        Looking for a specific function?

        This section gives conceptual overviews. For the precise signature, parameters, and return value of a function, see the API Documentation.

        "},{"location":"features/arbitrary_types/","title":"Arbitrary Type Conversions","text":"

        Every type can be serialized in JSON, not just STL containers and scalar types. Usually, you would do something along those lines:

        namespace ns {\n    // a simple struct to model a person\n    struct person {\n        std::string name;\n        std::string address;\n        int age;\n    };\n} // namespace ns\n\nns::person p = {\"Ned Flanders\", \"744 Evergreen Terrace\", 60};\n\n// convert to JSON: copy each value into the JSON object\njson j;\nj[\"name\"] = p.name;\nj[\"address\"] = p.address;\nj[\"age\"] = p.age;\n\n// ...\n\n// convert from JSON: copy each value from the JSON object\nns::person p {\n    j[\"name\"].get<std::string>(),\n    j[\"address\"].get<std::string>(),\n    j[\"age\"].get<int>()\n};\n

        It works, but that's quite a lot of boilerplate... Fortunately, there's a better way:

        // create a person\nns::person p {\"Ned Flanders\", \"744 Evergreen Terrace\", 60};\n\n// conversion: person -> json\njson j = p;\n\nstd::cout << j << std::endl;\n// {\"address\":\"744 Evergreen Terrace\",\"age\":60,\"name\":\"Ned Flanders\"}\n\n// conversion: json -> person\nauto p2 = j.get<ns::person>();\n\n// that's it\nassert(p == p2);\n
        "},{"location":"features/arbitrary_types/#basic-usage","title":"Basic usage","text":"

        To make this work with one of your types, you only need to provide two functions:

        using json = nlohmann::json;\n\nnamespace ns {\n    void to_json(json& j, const person& p) {\n        j = json{ {\"name\", p.name}, {\"address\", p.address}, {\"age\", p.age} };\n    }\n\n    void from_json(const json& j, person& p) {\n        j.at(\"name\").get_to(p.name);\n        j.at(\"address\").get_to(p.address);\n        j.at(\"age\").get_to(p.age);\n    }\n} // namespace ns\n

        That's all! When calling the json constructor with your type, your custom to_json method will be automatically called. Likewise, when calling get<your_type>() or get_to(your_type&), the from_json method will be called.

        Some important things:

        • Those methods MUST be in your type's namespace (which can be the global namespace), or the library will not be able to locate them (in this example, they are in namespace ns, where person is defined).
        • Those methods MUST be available (e.g., proper headers must be included) everywhere you use these conversions. Look at #1108 for errors that may occur otherwise.
        • When using get<your_type>(), your_type MUST be DefaultConstructible. (There is a way to bypass this requirement described later.)
        • In function from_json, use function at() to access the object values rather than operator[]. In case a key does not exist, at throws an exception that you can handle, whereas operator[] exhibits undefined behavior.
        • You do not need to add serializers or deserializers for STL types like std::vector: the library already implements these.
        • If you control the type, consider defining to_json/from_json as friend functions inside the class (\"hidden friends\"). Argument-dependent lookup then only finds them for your type, which also avoids a GCC < 11 compilation error.
        Example: serialize a person to JSON with to_json 
        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nnamespace ns\n{\n// a simple struct to model a person\nstruct person\n{\n    std::string name;\n    std::string address;\n    int age;\n};\n} // namespace ns\n\nnamespace ns\n{\nvoid to_json(json& j, const person& p)\n{\n    j = json{ {\"name\", p.name}, {\"address\", p.address}, {\"age\", p.age} };\n}\n} // namespace ns\n\nint main()\n{\n    ns::person p = {\"Ned Flanders\", \"744 Evergreen Terrace\", 60};\n\n    json j = p;\n\n    std::cout << j << std::endl;\n}\n

        Output:

        {\"address\":\"744 Evergreen Terrace\",\"age\":60,\"name\":\"Ned Flanders\"}\n
        Example: deserialize a person from JSON with from_json 
        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nnamespace ns\n{\n// a simple struct to model a person\nstruct person\n{\n    std::string name;\n    std::string address;\n    int age;\n};\n} // namespace ns\n\nnamespace ns\n{\nvoid from_json(const json& j, person& p)\n{\n    j.at(\"name\").get_to(p.name);\n    j.at(\"address\").get_to(p.address);\n    j.at(\"age\").get_to(p.age);\n}\n} // namespace ns\n\nint main()\n{\n    json j;\n    j[\"name\"] = \"Ned Flanders\";\n    j[\"address\"] = \"744 Evergreen Terrace\";\n    j[\"age\"] = 60;\n\n    auto p = j.get<ns::person>();\n\n    std::cout << p.name << \" (\" << p.age << \") lives in \" << p.address << std::endl;\n}\n

        Output:

        Ned Flanders (60) lives in 744 Evergreen Terrace\n
        "},{"location":"features/arbitrary_types/#simplify-your-life-with-macros","title":"Simplify your life with macros","text":"

        If you just want to serialize/deserialize some structs, the to_json/from_json functions can be a lot of boilerplate.

        There are several macros to make your life easier as long as you want to use a JSON object as serialization. The macros are following the naming pattern, and you can choose the macro based on the needed features:

        • All the macros start with NLOHMANN_DEFINE.
        • If you want a macro for the derived object, use the DERIVED_TYPE variant, otherwise use TYPE.
          • The DERIVED_TYPE variant requires an additional parameter of a base type, which should have the to_json/from_json functions defined. For instance, with a macro of its own.
        • If you need access to the private fields use INTRUSIVE variant, otherwise use NON_INTRUSIVE.
          • The INTRUSIVE macro should be defined inside the target class/struct, NON_INTRUSIVE should be defined within the same namespace.
        • If you want to deserialize the incomplete JSONs, use the WITH_DEFAULTS variant, which will use the default values for the member variables absent in JSON, the variant without WITH_DEFAULTS will raise an exception.
        • If you do not need deserialization at all and only interested in to_json function, you can use the ONLY_SERIALIZE variant.
        • If you want to use the custom JSON names for member variables, use WITH_NAMES variant, otherwise the JSON name of the variable will be the same as its regular name.

        For all the macros, the first parameter is the name of the class/struct. The DERIVED_TYPE macros require a second parameter of a base class. All the remaining parameters name the member variables. The WITH_NAMES macros require a JSON name before each of the variables.

        flowchart TD\n    A[\"choosing a NLOHMANN_DEFINE_* macro\"] --> B{\"adding fields to a base class?\"}\n    B -->|\"yes\"| C[\"...DERIVED_TYPE...\"]\n    B -->|\"no\"| D[\"...TYPE...\"]\n    C --> E{\"need access to private members?\"}\n    D --> E\n    E -->|\"yes\"| F[\"...INTRUSIVE... (used inside the class)\"]\n    E -->|\"no\"| G[\"...NON_INTRUSIVE... (used in the namespace)\"]\n    F --> H{\"only serializing, never parsing back?\"}\n    G --> H\n    H -->|\"yes\"| I[\"...ONLY_SERIALIZE\"]\n    H -->|\"no\"| J{\"allow missing keys when parsing?\"}\n    J -->|\"yes\"| K[\"...WITH_DEFAULT\"]\n    J -->|\"no\"| L[\"plain (missing keys throw)\"]\n    I --> M{\"need custom JSON key names?\"}\n    K --> M\n    L --> M\n    M -->|\"yes\"| N[\"...WITH_NAMES\"]\n    M -->|\"no\"| O[\"done\"]
        Need access to private members Need only serialization Allow missing values when de-serializing macro NLOHMANN_DEFINE_TYPE_INTRUSIVE NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_DEFAULT NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_WITH_DEFAULT NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE

        For derived classes and structs, use the following macros

        Need access to private members Need only serialization Allow missing values when de-serializing macro NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_WITH_DEFAULT NLOHMANN_DEFINE_DERIVED_TYPE_INTRUSIVE_ONLY_SERIALIZE NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_WITH_DEFAULT NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE

        Implementation limits

        • The current macro implementations are limited to at most 63 member variables. If you want to serialize/deserialize types with more than 63 member variables, you need to define the to_json/from_json functions manually.
        • For the WITH_NAMES variants the limit is halved to 31 member variables.
        Example: using the NLOHMANN_DEFINE_TYPE_* macros

        The to_json/from_json functions for the person struct above can be created with:

        namespace ns {\n    NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(person, name, address, age)\n}\n

        If you want to inherit the person struct and add a field to it, it can be done with:

        namespace ns {\n    struct person_derived : person {\n        std::string email;\n    };\n\n    NLOHMANN_DEFINE_DERIVED_TYPE_NON_INTRUSIVE(person_derived, person, email)\n}\n

        Here is another example with private members, where NLOHMANN_DEFINE_TYPE_INTRUSIVE is needed:

        namespace ns {\n    class address {\n      private:\n        std::string street;\n        int housenumber;\n        int postcode;\n\n      public:\n        NLOHMANN_DEFINE_TYPE_INTRUSIVE(address, street, housenumber, postcode)\n    };\n}\n

        Or in case if you use some naming convention that you do not want to expose to JSON:

        namespace ns {\n    class address {\n      private:\n        std::string m_street;\n        int m_housenumber;\n        int m_postcode;\n\n      public:\n        NLOHMANN_DEFINE_TYPE_INTRUSIVE_WITH_NAMES(address, \"street\", m_street,\n                                                           \"housenumber\", m_housenumber,\n                                                           \"postcode\", m_postcode)\n    };\n}\n

        Overriding conversions for natively-supported types

        The library already provides built-in to_json/from_json conversions for STL containers such as std::vector, std::array, and std::map. Defining your own free-function to_json/from_json overload for one of these container types directly (instead of for your own type) can conflict with the built-in overload during overload resolution, producing compiler errors (\"no matching overloaded function\", \"call is ambiguous\") that vary by compiler and library version. If you need different conversion behavior for a container type the library already handles, wrap it in your own type (or use adl_serializer specialization, as shown above for boost::optional) instead of trying to re-specialize to_json/from_json for the container type itself.

        Raw C-style arrays

        Members declared as raw C-style arrays (e.g., char buf[1024]) do not round-trip safely through NLOHMANN_DEFINE_TYPE_* macros or the default (de)serializers: to_json serializes any char array as a JSON string (matching the std::string-constructible overload), but the from_json overload for fixed-size arrays expects a JSON array and iterates it element-wise, which fails with a type_error when given a string. Use std::string, std::array<char, N>, or a manually written to_json/from_json pair for such members instead.

        Macros and nlohmann::ordered_json

        The NLOHMANN_DEFINE_TYPE_*/NLOHMANN_DEFINE_DERIVED_TYPE_* macros are generic over any basic_json specialization, including nlohmann::ordered_json. Simply use ordered_json as the target type and members are serialized in declaration order -- no separate macro or extra code is needed.

        namespace ns {\n    NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(person, name, address, age)\n}\n\nns::person p{\"Ned Flanders\", \"744 Evergreen Terrace\", 60};\nnlohmann::ordered_json j = p; // keys appear in declaration order: name, address, age\n

        Zero-member types

        All 12 NLOHMANN_DEFINE_TYPE_*/NLOHMANN_DEFINE_DERIVED_TYPE_* macros (excluding the WITH_NAMES variants) also accept types with no member variables to serialize, producing/accepting an empty JSON object {} (or, for the derived-type macros, just the base class's own JSON representation):

        namespace ns {\n    struct marker {\n        bool operator==(const marker&) const { return true; }\n        NLOHMANN_DEFINE_TYPE_INTRUSIVE(marker)\n    };\n}\n\nns::marker m{};\nnlohmann::json j = m; // {}\n

        No macro for non-default-constructible types

        There is currently no NLOHMANN_DEFINE_TYPE_*-style macro for types that are not DefaultConstructible. This is not an intentional omission of documentation -- no such macro exists yet; see How can I use get() for non-default constructible/non-copyable types? for the manual pattern to use instead.

        "},{"location":"features/arbitrary_types/#how-do-i-convert-third-party-types","title":"How do I convert third-party types?","text":"

        This requires a bit more advanced technique. But first, let us see how this conversion mechanism works:

        flowchart LR\n    A[\"construct json j = t, or call j.get() for T\"] --> B[\"JSONSerializer for T: to_json / from_json\"]\n    B -->|\"default JSONSerializer\"| C[\"adl_serializer for T: to_json / from_json\"]\n    C -->|\"unqualified call, found via ADL\"| D[\"free to_json(j, t) / from_json(j, t) in T's namespace\"]\n    B -->|\"user specialization replaces the default\"| E[\"user's adl_serializer specialization for T\"]

        The library uses JSON Serializers to convert types to JSON. The default serializer for nlohmann::json is nlohmann::adl_serializer (ADL means Argument-Dependent Lookup).

        It is implemented like this (simplified):

        template <typename T>\nstruct adl_serializer {\n    static void to_json(json& j, const T& value) {\n        // calls the \"to_json\" method in T's namespace\n    }\n\n    static void from_json(const json& j, T& value) {\n        // same thing, but with the \"from_json\" method\n    }\n};\n

        This serializer works fine when you have control over the type's namespace. However, what about boost::optional or std::filesystem::path (C++17)? Hijacking the boost namespace is pretty bad, and it's illegal to add something other than template specializations to std...

        To solve this, you need to add a specialization of adl_serializer to the nlohmann namespace, here's an example:

        // partial specialization (full specialization works too)\nNLOHMANN_JSON_NAMESPACE_BEGIN\ntemplate <typename T>\nstruct adl_serializer<boost::optional<T>> {\n    static void to_json(json& j, const boost::optional<T>& opt) {\n        if (opt == boost::none) {\n            j = nullptr;\n        } else {\n            j = *opt; // this will call adl_serializer<T>::to_json which will\n                      // find the free function to_json in T's namespace!\n        }\n    }\n\n    static void from_json(const json& j, boost::optional<T>& opt) {\n        if (j.is_null()) {\n            opt = boost::none;\n        } else {\n            opt = j.get<T>(); // same as above, but with\n                              // adl_serializer<T>::from_json\n        }\n    }\n};\nNLOHMANN_JSON_NAMESPACE_END\n

        std::optional needs no serializer

        Since version 3.12.0, std::optional<T> is supported out of the box when compiling with C++17 (std::nullopt is converted to and from null). Do not write an adl_serializer for it; this pattern is only needed for types such as boost::optional or for custom semantics. See Conversions and Omitting a field when serializing std::optional.

        ABI compatibility

        Use NLOHMANN_JSON_NAMESPACE_BEGIN and NLOHMANN_JSON_NAMESPACE_END instead of namespace nlohmann { } in code which may be linked with different versions of this library.

        "},{"location":"features/arbitrary_types/#how-can-i-use-get-for-non-default-constructiblenon-copyable-types","title":"How can I use get() for non-default constructible/non-copyable types?","text":"

        For a type that is not DefaultConstructible but is otherwise an ordinary value type, specialize adl_serializer with a from_json overload that returns the value instead of writing into a reference:

        Example: get() for a non-default-constructible type 
        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nnamespace ns\n{\n// a simple struct to model a person (not default constructible)\nstruct person\n{\n    person(std::string n, std::string a, int aa)\n        : name(std::move(n)), address(std::move(a)), age(aa)\n    {}\n\n    std::string name;\n    std::string address;\n    int age;\n};\n} // namespace ns\n\nnamespace nlohmann\n{\ntemplate <>\nstruct adl_serializer<ns::person>\n{\n    static ns::person from_json(const json& j)\n    {\n        return {j.at(\"name\"), j.at(\"address\"), j.at(\"age\")};\n    }\n\n    // Here's the catch! You must provide a to_json method! Otherwise, you\n    // will not be able to convert person to json, since you fully\n    // specialized adl_serializer on that type\n    static void to_json(json& j, ns::person p)\n    {\n        j[\"name\"] = p.name;\n        j[\"address\"] = p.address;\n        j[\"age\"] = p.age;\n    }\n};\n} // namespace nlohmann\n\nint main()\n{\n    json j;\n    j[\"name\"] = \"Ned Flanders\";\n    j[\"address\"] = \"744 Evergreen Terrace\";\n    j[\"age\"] = 60;\n\n    auto p = j.get<ns::person>();\n\n    std::cout << p.name << \" (\" << p.age << \") lives in \" << p.address << std::endl;\n}\n

        Output:

        Ned Flanders (60) lives in 744 Evergreen Terrace\n

        The same technique also works if your type is not copyable, as long as it is MoveConstructible:

        struct move_only_type {\n    move_only_type() = delete;\n    move_only_type(int ii): i(ii) {}\n    move_only_type(const move_only_type&) = delete;\n    move_only_type(move_only_type&&) = default;\n\n    int i;\n};\n\nnamespace nlohmann {\n    template <>\n    struct adl_serializer<move_only_type> {\n        // note: the return type is no longer 'void', and the method only takes\n        // one argument\n        static move_only_type from_json(const json& j) {\n            return {j.get<int>()};\n        }\n\n        // Here's the catch! You must provide a to_json method! Otherwise, you\n        // will not be able to convert move_only_type to json, since you fully\n        // specialized adl_serializer on that type\n        static void to_json(json& j, move_only_type t) {\n            j = t.i;\n        }\n    };\n}\n
        "},{"location":"features/arbitrary_types/#why-cant-i-convert-tofrom-stdany","title":"Why can't I convert to/from std::any?","text":"

        std::any is intentionally excluded from get<T>()/generic conversion support, so get<std::any>() and containers like std::map<std::string, std::any> fail to compile by design -- there is no way to know, from a json value alone, which concrete type to store inside the std::any. To work with heterogeneous JSON values, dispatch on the value's type manually and construct the std::any (or extract from it) yourself:

        std::any value_to_any(const json& j) {\n    if (j.is_boolean())      { return j.get<bool>(); }\n    if (j.is_number_integer()) { return j.get<int>(); }\n    if (j.is_number_float()) { return j.get<double>(); }\n    if (j.is_string())       { return j.get<std::string>(); }\n    // ... handle other types (arrays, objects) as needed for your use case\n    return {};\n}\n\njson any_to_json(const std::any& a) {\n    if (a.type() == typeid(bool))        { return std::any_cast<bool>(a); }\n    if (a.type() == typeid(int))         { return std::any_cast<int>(a); }\n    if (a.type() == typeid(double))      { return std::any_cast<double>(a); }\n    if (a.type() == typeid(std::string)) { return std::any_cast<std::string>(a); }\n    return nullptr;\n}\n
        "},{"location":"features/arbitrary_types/#why-does-serializing-a-stdmapstdunordered_map-with-non-string-keys-produce-an-array","title":"Why does serializing a std::map/std::unordered_map with non-string keys produce an array?","text":"

        A std::map/std::unordered_map whose key type is not string-like (e.g., std::map<int, std::string>) cannot be serialized as a JSON object, because JSON object keys must be strings. See Converting maps with non-string keys in the types article for what the library does instead.

        "},{"location":"features/arbitrary_types/#why-does-stdwstring-convert-or-dump-incorrectly","title":"Why does std::wstring convert or dump incorrectly?","text":"

        The library assumes UTF-8 encoding internally, so std::wstring is not supported out of the box -- see the FAQ entry on wide string handling for why, and for a UTF-8 conversion recipe.

        "},{"location":"features/arbitrary_types/#can-i-write-my-own-serializer-advanced-use","title":"Can I write my own serializer? (Advanced use)","text":"

        Yes. You might want to take a look at unit-udt.cpp in the test suite, to see a few examples.

        If you write your own serializer, you will need to do a few things:

        • use a different basic_json alias than nlohmann::json (the last template parameter of basic_json is the JSONSerializer)
        • use your basic_json alias (or a template parameter) in all your to_json/from_json methods
        • use nlohmann::to_json and nlohmann::from_json when you need ADL

        Here is an example, without simplifications, that only accepts types with a size <= 32, and uses ADL.

        // You should use void as a second template argument\n// if you don't need compile-time checks on T\ntemplate<typename T, typename SFINAE = typename std::enable_if<sizeof(T) <= 32>::type>\nstruct less_than_32_serializer {\n    template <typename BasicJsonType>\n    static void to_json(BasicJsonType& j, T value) {\n        // we want to use ADL, and call the correct to_json overload\n        using nlohmann::to_json; // this method is called by adl_serializer,\n                                 // this is where the magic happens\n        to_json(j, value);\n    }\n\n    template <typename BasicJsonType>\n    static void from_json(const BasicJsonType& j, T& value) {\n        // same thing here\n        using nlohmann::from_json;\n        from_json(j, value);\n    }\n};\n

        Be very careful when reimplementing your serializer, you can stack overflow if you don't pay attention:

        template <typename T, typename = void>\nstruct bad_serializer\n{\n    template <typename BasicJsonType>\n    static void to_json(BasicJsonType& j, const T& value) {\n      // this calls BasicJsonType::json_serializer<T>::to_json(j, value);\n      // if BasicJsonType::json_serializer == bad_serializer ... oops!\n      j = value;\n    }\n\n    template <typename BasicJsonType>\n    static void from_json(const BasicJsonType& j, T& value) {\n      // this calls BasicJsonType::json_serializer<T>::from_json(j, value);\n      // if BasicJsonType::json_serializer == bad_serializer ... oops!\n      value = j.template get<T>(); // oops!\n    }\n};\n
        "},{"location":"features/arbitrary_types/#see-also","title":"See also","text":"
        • Converting values - the general overview of get/get_to and implicit conversions
        • Specializing enum conversion - map enums to JSON strings instead of integers
        • Supported macros - reference for NLOHMANN_DEFINE_TYPE_* and related macros
        • adl_serializer - the default JSONSerializer used in the conversion dispatch
        "},{"location":"features/assertions/","title":"Runtime Assertions","text":"

        The code contains numerous debug assertions to ensure class invariants are valid or to detect undefined behavior. Whereas the former class invariants are nothing to be concerned with, the latter checks for undefined behavior are to detect bugs in client code.

        "},{"location":"features/assertions/#switch-off-runtime-assertions","title":"Switch off runtime assertions","text":"

        Runtime assertions can be switched off by defining the preprocessor macro NDEBUG (see the documentation of assert) which is the default for release builds.

        "},{"location":"features/assertions/#change-assertion-behavior","title":"Change assertion behavior","text":"

        The behavior of runtime assertions can be changed by defining macro JSON_ASSERT(x) before including the json.hpp header.

        "},{"location":"features/assertions/#function-with-runtime-assertions","title":"Function with runtime assertions","text":""},{"location":"features/assertions/#unchecked-access-to-a-const-value","title":"Unchecked access to a const value","text":"

        Function operator[] implements unchecked access for arrays and objects. Whereas a missing element is added in the case of non-const values, accessing a const value with a missing object key or an invalid array index is undefined behavior (think of a dereferenced null pointer) and yields a runtime assertion. This also applies to a JSON pointer that refers to a missing key or an invalid index.

        If you are not sure whether an element exists, use checked access with the at function or call the contains function before.

        See also the documentation on element access.

        Example: missing object key

        The following code will trigger an assertion at runtime:

        #include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    const json j = {{\"key\", \"value\"}};\n    auto v = j[\"missing\"];\n}\n

        Output:

        Assertion failed: (it != m_data.m_value.object->end()), function operator[], file json.hpp, line 28795.\n
        Example 2: Invalid array index in a JSON pointer

        The following code will trigger an assertion at runtime:

        #include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    const json j = {{\"array\", {1, 2, 3}}};\n    auto v = j[\"/array/5\"_json_pointer];\n}\n

        Output:

        Assertion failed: (idx < m_data.m_value.array->size()), function operator[], file json.hpp, line 28758.\n
        "},{"location":"features/assertions/#constructing-from-an-uninitialized-iterator-range","title":"Constructing from an uninitialized iterator range","text":"

        Constructing a JSON value from an iterator range (see constructor) with an uninitialized iterator is undefined behavior and yields a runtime assertion.

        Example: uninitialized iterator range

        The following code will trigger an assertion at runtime:

        #include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    json::iterator it1, it2;\n    json j(it1, it2);\n}\n

        Output:

        Assertion failed: (m_object != nullptr), function operator++, file iter_impl.hpp, line 368.\n
        "},{"location":"features/assertions/#operations-on-uninitialized-iterators","title":"Operations on uninitialized iterators","text":"

        Any operation on uninitialized iterators (i.e., iterators that are not associated with any JSON value) is undefined behavior and yields a runtime assertion.

        Example: uninitialized iterator

        The following code will trigger an assertion at runtime:

        #include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n  json::iterator it;\n  ++it;\n}\n

        Output:

        Assertion failed: (m_object != nullptr), function operator++, file iter_impl.hpp, line 368.\n
        "},{"location":"features/assertions/#changes","title":"Changes","text":""},{"location":"features/assertions/#reading-from-a-null-file-or-char-pointer","title":"Reading from a null FILE or char pointer","text":"

        Reading from a null FILE or char pointer in C++ is undefined behavior. Until version 3.12.0, this library asserted that the pointer was not nullptr using a runtime assertion. If assertions were disabled, this would result in undefined behavior. Since version 3.12.0, this library checks for nullptr and throws a parse_error.101 to prevent the undefined behavior.

        Example: reading from null pointer

        The following code will trigger an assertion at runtime:

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::FILE* f = std::fopen(\"nonexistent_file.json\", \"r\");\n    try {\n        json j = json::parse(f);\n    } catch (std::exception& e) {\n        std::cerr << e.what() << std::endl;\n    }\n}\n

        Output:

        [json.exception.parse_error.101] parse error: attempting to parse an empty input; check that your input string or stream contains the expected JSON\n
        "},{"location":"features/assertions/#see-also","title":"See also","text":"
        • JSON_ASSERT - control behavior of runtime assertions
        "},{"location":"features/binary_values/","title":"Binary Values","text":"

        The library implements several binary formats that encode JSON in an efficient way. Most of these formats support binary values; that is, values that have semantics defined outside the library and only define a sequence of bytes to be stored.

        JSON itself does not have a binary value. As such, binary values are an extension that this library implements to store values received by a binary format. Binary values are never created by the JSON parser and are only part of a serialized JSON text if they have been created manually or via a binary format.

        "},{"location":"features/binary_values/#api-for-binary-values","title":"API for binary values","text":"
        classDiagram\n\nclass binary_t [\"json::binary_t\"] {\n    +void set_subtype(std::uint64_t subtype)\n    +void clear_subtype()\n    +std::uint64_t subtype() const\n    +bool has_subtype() const\n}\n\nclass vector [\"std::vector<uint8_t>\"]\n\nvector <|-- binary_t

        By default, binary values are stored as std::vector<std::uint8_t>. This type can be changed by providing a template parameter to the basic_json type. To store binary subtypes, the storage type is extended and exposed as json::binary_t:

        auto binary = json::binary_t({0xCA, 0xFE, 0xBA, 0xBE});\nauto binary_with_subtype = json::binary_t({0xCA, 0xFE, 0xBA, 0xBE}, 42);\n

        There are several convenience functions to check and set the subtype:

        binary.has_subtype();                   // returns false\nbinary_with_subtype.has_subtype();      // returns true\n\nbinary_with_subtype.clear_subtype();\nbinary_with_subtype.has_subtype();      // returns false\n\nbinary_with_subtype.set_subtype(42);\nbinary.set_subtype(23);\n\nbinary.subtype();                       // returns 23\n

        As json::binary_t is subclassing std::vector<std::uint8_t>, all member functions are available:

        binary.size();  // returns 4\nbinary[1];      // returns 0xFE\n

        JSON values can be constructed from json::binary_t:

        json j = binary;\n

        Binary values are primitive values just like numbers or strings, as reflected by is_binary() and is_primitive():

        j.is_binary();    // returns true\nj.is_primitive(); // returns true\n

        Given a binary JSON value, the binary_t can be accessed by reference via get_binary():

        j.get_binary().has_subtype();  // returns true\nj.get_binary().size();         // returns 4\n

        For convenience, binary JSON values can be constructed via json::binary:

        auto j2 = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 23);\nauto j3 = json::binary({0xCA, 0xFE, 0xBA, 0xBE});\n\nj2 == j;                        // returns true\nj3.get_binary().has_subtype();  // returns false\nj3.get_binary().subtype();      // returns std::uint64_t(-1) as j3 has no subtype\n
        "},{"location":"features/binary_values/#serialization","title":"Serialization","text":"

        Binary values are serialized differently according to the formats.

        "},{"location":"features/binary_values/#json","title":"JSON","text":"

        JSON does not have a binary type, and this library does not introduce a new type as this would break conformance. Instead, binary values are serialized as an object with two keys: bytes holds an array of integers, and subtype is an integer or null.

        Example: serialize a binary value to JSON

        Code:

        // create a binary value of subtype 42\njson j;\nj[\"binary\"] = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 42);\n\n// serialize to standard output\nstd::cout << j.dump(2) << std::endl;\n

        Output:

        {\n  \"binary\": {\n    \"bytes\": [202, 254, 186, 190],\n    \"subtype\": 42\n  }\n}\n

        No roundtrip for binary values

        The JSON parser will not parse the objects generated by binary values back to binary values. This is by design to remain standards compliant. Serializing binary values to JSON is only implemented for debugging purposes.

        "},{"location":"features/binary_values/#bjdata","title":"BJData","text":"

        BJData neither supports binary values nor subtypes and proposes to serialize binary values as an array of uint8 values. The library implements this translation.

        Example: serialize a binary value to BJData

        Code:

        // create a binary value of subtype 42 (will be ignored in BJData)\njson j;\nj[\"binary\"] = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 42);\n\n// convert to BJData\nauto v = json::to_bjdata(j);      \n

        v is a std::vector<std::uint8_t> with the following 20 elements:

        0x7B                                             // '{'\n    0x69 0x06                                    // i 6 (length of the key)\n    0x62 0x69 0x6E 0x61 0x72 0x79                // \"binary\"\n    0x5B                                         // '['\n        0x55 0xCA 0x55 0xFE 0x55 0xBA 0x55 0xBE  // content (each byte prefixed with 'U')\n    0x5D                                         // ']'\n0x7D                                             // '}'\n

        The following code uses the type and size optimization for BJData:

        // convert to BJData using the size and type optimization\nauto v = json::to_bjdata(j, true, true);\n

        The resulting vector has 22 elements; the optimization is not effective for examples with few values:

        0x7B                                // '{'\n    0x23 0x69 0x01                  // '#' 'i' type of the array elements: unsigned integers\n    0x69 0x06                       // i 6 (length of the key)\n    0x62 0x69 0x6E 0x61 0x72 0x79   // \"binary\"\n    0x5B                            // '[' array\n        0x24 0x55                   // '$' 'U' type of the array elements: unsigned integers\n        0x23 0x69 0x04              // '#' i 4 number of array elements\n        0xCA 0xFE 0xBA 0xBE         // content\n

        Note that subtype (42) is not serialized and that BJData has no binary type, and deserializing v would yield the following value:

        {\n  \"binary\": [202, 254, 186, 190]\n}\n
        "},{"location":"features/binary_values/#bon8","title":"BON8","text":"

        BON8 neither supports binary values nor subtypes. The library serializes binary values as an array of integers.

        Example: serialize a binary value to BON8

        Code:

        // create a binary value of subtype 42 (will be ignored in BON8)\njson j;\nj[\"binary\"] = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 42);\n\n// convert to BON8\nauto v = json::to_bon8(j);\n

        v is a std::vector<std::uint8_t> with the following 16 elements:

        0x87                                     // object with 1 member\n    0x62 0x69 0x6E 0x61 0x72 0x79        // \"binary\"\n    0x84                                 // array with 4 elements\n        0xC3 0x22 0xC3 0x56 0xC3 0x12 0xC3 0x16  // content (each byte as a 2-byte integer)\n

        Note that the subtype is lost, and deserializing v would yield the following value:

        {\n  \"binary\": [202, 254, 186, 190]\n}\n
        "},{"location":"features/binary_values/#bson","title":"BSON","text":"

        BSON supports binary values and subtypes. If a subtype is given, it is used and added as an unsigned 8-bit integer. If no subtype is given, the generic binary subtype 0x00 is used.

        Example: serialize a binary value to BSON

        Code:

        // create a binary value of subtype 42\njson j;\nj[\"binary\"] = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 42);\n\n// convert to BSON\nauto v = json::to_bson(j);      \n

        v is a std::vector<std::uint8_t> with the following 22 elements:

        0x16 0x00 0x00 0x00                         // number of bytes in the document\n    0x05                                    // binary value\n        0x62 0x69 0x6E 0x61 0x72 0x79 0x00  // key \"binary\" + null byte\n        0x04 0x00 0x00 0x00                 // number of bytes\n        0x2a                                // subtype\n        0xCA 0xFE 0xBA 0xBE                 // content\n0x00                                        // end of the document\n

        Note that the serialization preserves the subtype, and deserializing v would yield the following value:

        {\n  \"binary\": {\n    \"bytes\": [202, 254, 186, 190],\n    \"subtype\": 42\n  }\n}\n
        "},{"location":"features/binary_values/#cbor","title":"CBOR","text":"

        CBOR supports binary values and subtypes. Any binary value will be serialized as a byte string; a subtype is serialized as a tag preceding it. When parsing, tags are only read back as subtypes with cbor_tag_handler_t::store (see from_cbor). The library will choose the smallest representation using the length of the byte array.

        Example: serialize a binary value to CBOR

        Code:

        // create a binary value of subtype 42\njson j;\nj[\"binary\"] = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 42);\n\n// convert to CBOR\nauto v = json::to_cbor(j);      \n

        v is a std::vector<std::uint8_t> with the following 15 elements:

        0xA1                                   // map(1)\n    0x66                               // text(6)\n        0x62 0x69 0x6E 0x61 0x72 0x79  // \"binary\"\n    0xD8 0x2A                          // tag(42)\n    0x44                               // bytes(4)\n        0xCA 0xFE 0xBA 0xBE            // content\n

        Note that the subtype is serialized as tag. However, parsing tagged values yield a parse error unless json::cbor_tag_handler_t::ignore or json::cbor_tag_handler_t::store is passed to json::from_cbor (see cbor_tag_handler_t).

        {\n  \"binary\": {\n    \"bytes\": [202, 254, 186, 190],\n    \"subtype\": null\n  }\n}\n
        "},{"location":"features/binary_values/#messagepack","title":"MessagePack","text":"

        MessagePack supports binary values and subtypes. If a subtype is given, the ext family is used. The library will choose the smallest representation among fixext1, fixext2, fixext4, fixext8, ext8, ext16, and ext32. The subtype is then added as a signed 8-bit integer.

        If no subtype is given, the bin family (bin8, bin16, bin32) is used.

        Example: serialize a binary value to MessagePack

        Code:

        // create a binary value of subtype 42\njson j;\nj[\"binary\"] = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 42);\n\n// convert to MessagePack\nauto v = json::to_msgpack(j);      \n

        v is a std::vector<std::uint8_t> with the following 14 elements:

        0x81                                   // fixmap1\n    0xA6                               // fixstr6\n        0x62 0x69 0x6E 0x61 0x72 0x79  // \"binary\"\n    0xD6                               // fixext4\n        0x2A                           // subtype\n        0xCA 0xFE 0xBA 0xBE            // content\n

        Note that the serialization preserves the subtype, and deserializing v would yield the following value:

        {\n  \"binary\": {\n    \"bytes\": [202, 254, 186, 190],\n    \"subtype\": 42\n  }\n}\n
        "},{"location":"features/binary_values/#ubjson","title":"UBJSON","text":"

        UBJSON neither supports binary values nor subtypes and proposes to serialize binary values as an array of uint8 values. The library implements this translation.

        Example: serialize a binary value to UBJSON

        Code:

        // create a binary value of subtype 42 (will be ignored in UBJSON)\njson j;\nj[\"binary\"] = json::binary({0xCA, 0xFE, 0xBA, 0xBE}, 42);\n\n// convert to UBJSON\nauto v = json::to_ubjson(j);      \n

        v is a std::vector<std::uint8_t> with the following 20 elements:

        0x7B                                             // '{'\n    0x69 0x06                                    // i 6 (length of the key)\n    0x62 0x69 0x6E 0x61 0x72 0x79                // \"binary\"\n    0x5B                                         // '['\n        0x55 0xCA 0x55 0xFE 0x55 0xBA 0x55 0xBE  // content (each byte prefixed with 'U')\n    0x5D                                         // ']'\n0x7D                                             // '}'\n

        The following code uses the type and size optimization for UBJSON:

        // convert to UBJSON using the size and type optimization\nauto v = json::to_ubjson(j, true, true);\n

        The resulting vector has 23 elements; the optimization is not effective for examples with few values:

        0x7B                                // '{'\n    0x24                            // '$' type of the object elements\n    0x5B                            // '[' array\n    0x23 0x69 0x01                  // '#' i 1 number of object elements\n    0x69 0x06                       // i 6 (length of the key)\n    0x62 0x69 0x6E 0x61 0x72 0x79   // \"binary\"\n        0x24 0x55                   // '$' 'U' type of the array elements: unsigned integers\n        0x23 0x69 0x04              // '#' i 4 number of array elements\n        0xCA 0xFE 0xBA 0xBE         // content\n

        Note that subtype (42) is not serialized and that UBJSON has no binary type, and deserializing v would yield the following value:

        {\n  \"binary\": [202, 254, 186, 190]\n}\n
        "},{"location":"features/comments/","title":"Comments","text":"

        This library does not support comments by default. It does so for three reasons:

        1. Comments are not part of the JSON specification. You may argue that // or /* */ are allowed in JavaScript, but JSON is not JavaScript.

        2. This was not an oversight: Douglas Crockford wrote on this in May 2012:

          I removed comments from JSON because I saw people were using them to hold parsing directives, a practice which would have destroyed interoperability. I know that the lack of comments makes some people sad, but it shouldn't.

          Suppose you are using JSON to keep configuration files, which you would like to annotate. Go ahead and insert all the comments you like. Then pipe it through JSMin before handing it to your JSON parser.

        3. It is dangerous for interoperability if some libraries add comment support while others do not. Please check The Harmful Consequences of the Robustness Principle on this.

        However, you can set parameter ignore_comments to true in the parse function to ignore // or /* */ comments. Comments will then be treated as whitespace. Combined with ignore_trailing_commas (also a parse parameter), this covers what is commonly referred to as JSONC (JSON with Comments, as used e.g. by Visual Studio Code's .jsonc files) -- comments and trailing commas, nothing more. This is a different, smaller extension than JSON5, which additionally allows unquoted keys, single-quoted strings, and other syntax changes that this library does not support.

        For more information, see JSON With Commas and Comments (JWCC).

        Example

        Consider the following JSON with comments.

        {\n    // update in 2006: removed Pluto\n    \"planets\": [\"Mercury\", \"Venus\", \"Earth\", \"Mars\",\n                \"Jupiter\", \"Uranus\", \"Neptune\" /*, \"Pluto\" */]\n}\n

        When calling parse without additional argument, a parse error exception is thrown. If ignore_comments is set to true, the comments are ignored during parsing:

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::string s = R\"(\n    {\n        // update in 2006: removed Pluto\n        \"planets\": [\"Mercury\", \"Venus\", \"Earth\", \"Mars\",\n                    \"Jupiter\", \"Uranus\", \"Neptune\" /*, \"Pluto\" */]\n    }\n    )\";\n\n    try\n    {\n        json j = json::parse(s);\n    }\n    catch (json::exception& e)\n    {\n        std::cout << e.what() << std::endl;\n    }\n\n    json j = json::parse(s,\n                         /* callback */ nullptr,\n                         /* allow exceptions */ true,\n                         /* ignore_comments */ true);\n    std::cout << j.dump(2) << '\\n';\n}\n

        Output:

        [json.exception.parse_error.101] parse error at line 3, column 9: syntax error while parsing object key - invalid literal; last read: '<U+000A>    {<U+000A>        /'; expected string literal\n{\n  \"planets\": [\n    \"Mercury\",\n    \"Venus\",\n    \"Earth\",\n    \"Mars\",\n    \"Jupiter\",\n    \"Uranus\",\n    \"Neptune\"\n  ]\n}\n
        "},{"location":"features/conversions/","title":"Converting values","text":"

        A basic_json value stores JSON data, but most of the time you want to move that data into ordinary C++ types (an int, a std::string, a std::vector, or one of your own structs) and back. This page describes how these conversions work.

        "},{"location":"features/conversions/#getting-values-out","title":"Getting values out","text":"

        The get function template returns a copy of the stored value converted to the requested type:

        json j = R\"({\"name\": \"Mary\", \"age\": 42, \"hobbies\": [\"hiking\", \"reading\"]})\"_json;\n\nauto name = j[\"name\"].get<std::string>();               // \"Mary\"\nauto age  = j[\"age\"].get<int>();                        // 42\nauto hobbies = j[\"hobbies\"].get<std::vector<std::string>>();  // {\"hiking\", \"reading\"}\n

        Getting a string without quotes

        A frequent point of confusion: use get, not dump, to read a string value. j[\"name\"].get<std::string>() yields Mary, whereas j[\"name\"].dump() yields the JSON text \"Mary\" (with quotes), because dump always produces a JSON text.

        Alternatively, get_to writes into an existing variable and deduces the target type, which avoids repeating it:

        Example 
        #include <iostream>\n#include <map>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create a JSON value with different types\n    json json_types =\n    {\n        {\"boolean\", true},\n        {\n            \"number\", {\n                {\"integer\", 42},\n                {\"floating-point\", 17.23}\n            }\n        },\n        {\"string\", \"Hello, world!\"},\n        {\"array\", {1, 2, 3, 4, 5}},\n        {\"null\", nullptr}\n    };\n\n    bool v1;\n    int v2;\n    short v3;\n    float v4;\n    int v5;\n    std::string v6;\n    std::vector<short> v7;\n    std::map<std::string, json> v8;\n\n    // use explicit conversions\n    json_types[\"boolean\"].get_to(v1);\n    json_types[\"number\"][\"integer\"].get_to(v2);\n    json_types[\"number\"][\"integer\"].get_to(v3);\n    json_types[\"number\"][\"floating-point\"].get_to(v4);\n    json_types[\"number\"][\"floating-point\"].get_to(v5);\n    json_types[\"string\"].get_to(v6);\n    json_types[\"array\"].get_to(v7);\n    json_types.get_to(v8);\n\n    // print the conversion results\n    std::cout << v1 << '\\n';\n    std::cout << v2 << ' ' << v3 << '\\n';\n    std::cout << v4 << ' ' << v5 << '\\n';\n    std::cout << v6 << '\\n';\n\n    for (auto i : v7)\n    {\n        std::cout << i << ' ';\n    }\n    std::cout << \"\\n\\n\";\n\n    for (auto i : v8)\n    {\n        std::cout << i.first << \": \" << i.second << '\\n';\n    }\n}\n

        Output:

        1\n42 42\n17.23 17\nHello, world!\n1 2 3 4 5 \n\narray: [1,2,3,4,5]\nboolean: true\nnull: null\nnumber: {\"floating-point\":17.23,\"integer\":42}\nstring: \"Hello, world!\"\n

        The library already knows how to convert to and from the scalar types and the STL containers (such as std::vector, std::map, std::array, std::optional, and many more). Converting a JSON object back to a std::map or a JSON array back to a std::vector therefore works without any extra code:

        json j = {{\"one\", 1}, {\"two\", 2}};\nauto m = j.get<std::map<std::string, int>>();  // {{\"one\", 1}, {\"two\", 2}}\n

        std::pair and std::tuple are also supported, converting positionally to and from a JSON array:

        json j = {1.0, \"hello\", 42};\nauto t = j.get<std::tuple<double, std::string, int>>();  // {1.0, \"hello\", 42}\n

        Serializing a std::pair/std::tuple whose every element is a string-keyed pair

        When every element of a std::pair or std::tuple is itself a two-element array whose first element is a string (for example std::pair<std::string, int>), serializing it produces a JSON object instead of the expected array:

        using kv = std::pair<std::string, int>;\njson j = std::pair<kv, kv>{{\"a\", 1}, {\"b\", 2}};  // {\"a\":1,\"b\":2}, not [[\"a\",1],[\"b\",2]]\n

        This is a consequence of the brace-initializer object-detection rule: the same rule that lets json{{\"a\", 1}, {\"b\", 2}} create an object also fires here. The resulting object cannot be read back into the original type (get<std::pair<kv, kv>>() throws type_error.302), and duplicate keys collapse into one, losing elements. This only affects std::pair/std::tuple themselves; a std::vector<std::pair<std::string, int>>, or a pair/tuple with at least one element that is not a string-keyed pair, serializes to an array as expected. To force an array, build one explicitly from the elements with array:

        std::pair<kv, kv> p{{\"a\", 1}, {\"b\", 2}};\njson a = json::array({p.first, p.second});  // [[\"a\",1],[\"b\",2]]\n

        Extracting references into a tuple

        A tuple type may also hold references (e.g. std::tuple<double&, std::string&>) to avoid copying: get then returns a tuple of references pointing directly at the elements stored inside the basic_json array, rather than a tuple of copies:

        json j = {1.0, \"hello\"};\nauto refs = j.get<std::tuple<double&, std::string&>>();\nstd::get<1>(refs) = \"world\";  // modifies j[1] in place\n

        A referenced element must name the type the library actually stores \u2014 one of boolean_t, number_integer_t, number_unsigned_t, number_float_t, string_t, binary_t, array_t, or object_t. There is nothing else to refer to, so a reference to any other type is a compile error even when a conversion would exist: std::tuple<int&> is rejected, because the library stores a number_integer_t (std::int64_t by default) and not an int. This restriction applies only to reference elements \u2014 a plain std::tuple<int> converts by value as usual.

        "},{"location":"features/conversions/#implicit-conversions","title":"Implicit conversions","text":"

        By default, a JSON value implicitly converts to a compatible C++ type, so the explicit get call can often be omitted:

        json j = \"Hello\";\nstd::string s = j;   // implicit conversion, same as j.get<std::string>()\n

        Implicit conversions are convenient but can be surprising (for example, in overload resolution or with auto). They can be disabled by defining JSON_USE_IMPLICIT_CONVERSIONS to 0, which forces the explicit get form and can catch unintended conversions at compile time.

        Conversions do not range-check numbers

        Just like C++ itself, the get family performs numeric conversions without range checks \u2014 retrieving a floating-point value as an integer truncates it, and narrowing conversions may overflow. See number conversion for details and how to guard against it.

        std::optional direct construction from JSON null throws

        Constructing or assigning std::optional<T> directly from a JSON value does not correctly produce std::nullopt for a JSON null:

        json j_null;\nstd::optional<std::string> opt = j_null;  // \u274c throws type_error 302\n

        This is due to C++ language rules: std::optional<T> has its own converting constructor that is chosen over basic_json::operator T() when both are viable. Use get<std::optional<T>>() or get_to() instead:

        auto opt = j_null.get<std::optional<std::string>>();  // \u2705 std::nullopt\nj_null.get_to(opt);                                     // \u2705 std::nullopt\n

        static_cast and get<std::optional<T>>() are not guaranteed equivalent

        operator ValueType() (used by static_cast and implicit conversions) intentionally excludes std::optional<T> from delegating to get<T>(), to avoid a constructor ambiguity with std::optional<T>'s own converting constructor from basic_json. As a result, static_cast<std::optional<T>>(json_value) goes through std::optional<T>'s own converting constructor rather than through get<std::optional<T>>(), which can behave differently -- for example, with a custom adl_serializer<std::optional<T>> specialization. Prefer get<std::optional<T>>()/get_to() over static_cast for optional types.

        Converting to a fixed-size destination does not check the array size

        Some destination types have a size that is fixed by their C++ type rather than by the JSON value: std::pair<A, B>, std::tuple<Ts...>, std::array<T, N>, C arrays T[N], and std::map/std::unordered_map with a non-string key type (which is read from an array of two-element arrays). All of them read exactly as many elements as they need via at and never compare the JSON array's size to that number. The two mismatch directions therefore behave differently:

        • The JSON array has too many elements: the surplus is silently discarded, and no exception is thrown.
        • The JSON array has too few elements: at throws out_of_range.401 for the first missing index -- an out-of-range error, not a type_error, even though the cause is a shape mismatch.
        json j = {1, 2, 3, 4, 5};\n\nauto a = j.get<std::array<int, 3>>();       // {1, 2, 3} -- elements 4 and 5 silently dropped\nauto p = j.get<std::pair<int, int>>();      // (1, 2)    -- elements 3, 4, and 5 silently dropped\n\njson k = {1};\nauto q = k.get<std::pair<int, int>>();      // \u274c throws out_of_range.401\n

        If a size mismatch is an error in your application, check the size yourself before converting.

        "},{"location":"features/conversions/#omitting-a-field-when-serializing-stdoptional","title":"Omitting a field when serializing std::optional","text":"

        By default, to_json for std::optional<T> writes either the value or null -- there is no built-in way to make a field disappear from the serialized object entirely when the std::optional is std::nullopt. Because a specialization of adl_serializer<std::optional<T>> only controls how the value is converted (it cannot prevent the containing object's to_json from inserting the key in the first place), omission has to be implemented in the containing type's to_json:

        struct person {\n    std::string name;\n    std::optional<int> age;\n};\n\nvoid to_json(json& j, const person& p) {\n    j = json{{\"name\", p.name}};\n    if (p.age) {\n        j[\"age\"] = *p.age;  // key is only inserted when the optional has a value\n    }\n}\n
        "},{"location":"features/conversions/#putting-values-in","title":"Putting values in","text":"

        The reverse direction works the same way: assigning or constructing a json from a C++ value converts it to JSON.

        std::vector<int> numbers = {1, 2, 3};\njson j = numbers;   // [1,2,3]\n

        Constructing from a C++20 range view

        A json array can also be constructed directly from a C++20 range view (std::ranges::view), such as the result of std::views::filter or std::views::transform -- no intermediate container is needed:

        std::vector<int> nums{1, 2, 37, 42, 21};\nauto filtered = nums | std::views::filter([](int i) { return i > 10; });\njson j(filtered);   // [37,42,21]\n

        This requires JSON_HAS_RANGES to be enabled and is unavailable on MinGW due to incomplete C++20 ranges support there; see JSON_HAS_RANGE_VIEW_CONVERSION.

        "},{"location":"features/conversions/#your-own-types","title":"Your own types","text":"

        The conversions above are built in for standard types. To make the same syntax work for your own types, provide to_json/from_json functions (or use one of the convenience macros). This is described in detail on the arbitrary types conversions page. Enums can be mapped to strings as described in specializing enum conversion.

        "},{"location":"features/conversions/#see-also","title":"See also","text":"
        • get - get a copy converted to a given type
        • get_to - convert into an existing variable
        • get_ref / get_ptr - access the stored value without copying
        • Arbitrary types conversions - support your own types
        • JSON_USE_IMPLICIT_CONVERSIONS - toggle implicit conversions
        "},{"location":"features/creating_values/","title":"Creating JSON values","text":"

        There are several ways to create a JSON value in memory. This page gives an overview; to read a value from JSON text instead, see parsing.

        "},{"location":"features/creating_values/#from-c-values","title":"From C++ values","text":"

        Any value of a supported C++ type can be assigned to or used to construct a json:

        json j_number  = 42;\njson j_float   = 3.141;\njson j_string  = \"Hello\";\njson j_boolean = true;\njson j_null    = nullptr;\njson j_vector  = std::vector<int>{1, 2, 3};   // array\n

        See converting values for the full set of supported types.

        "},{"location":"features/creating_values/#with-initializer-lists","title":"With initializer lists","text":"

        Objects and arrays can be written concisely with brace-enclosed initializer lists:

        // an array\njson array = {1, 2, 3, 4};\n\n// an object (a list of key/value pairs)\njson object = {\n    {\"pi\", 3.141},\n    {\"happy\", true},\n    {\"name\", \"Niels\"},\n    {\"nothing\", nullptr},\n    {\"list\", {1, 0, 2}},\n    {\"object\", {{\"currency\", \"USD\"}, {\"value\", 42.99}}}\n};\n

        The library decides between an array and an object based on the content: a list whose elements are all two-element lists with a string as the first element is treated as an object, everything else as an array.

        Ambiguous cases: {} vs. []

        Because the same {} syntax is used for both arrays and objects, some cases are ambiguous. To force a particular type, use the explicit factory functions json::array and json::object:

        json empty_array_explicit = json::array();               // []\njson empty_object_explicit = json::object();             // {}\n\n// a JSON array with one object, not an object with one member\njson array_of_objects = json::array({{\"key\", \"value\"}}); // [{\"key\":\"value\"}]\n

        Related to this, single-element brace initialization such as json j{value}; wraps the element in a single-element array by default, and its behavior even differs between compilers. See the FAQ for details and the opt-in JSON_BRACE_INIT_COPY_SEMANTICS macro.

        "},{"location":"features/creating_values/#building-incrementally","title":"Building incrementally","text":"

        A value can also be built up piece by piece. Accessing a non-existing object key or array index with operator[] creates the element on the fly:

        json j;                    // null\nj[\"answer\"][\"everything\"] = 42;   // becomes an object\nj[\"list\"] = {1, 0, 2};\nj[\"list\"].push_back(3);           // [1,0,2,3]\n

        See modifying values for push_back, emplace, and related functions.

        "},{"location":"features/creating_values/#with-the-_json-literal","title":"With the _json literal","text":"

        The _json user-defined literal parses a string at the call site and is a convenient way to write a JSON value inline:

        Example 
        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    json j = R\"( {\"hello\": \"world\", \"answer\": 42} )\"_json;\n\n    std::cout << std::setw(2) << j << '\\n';\n}\n

        Output:

        {\n  \"answer\": 42,\n  \"hello\": \"world\"\n}\n

        Note this parses the string, so \"42\"_json is the number 42, whereas json(\"42\") is the JSON string \"42\".

        "},{"location":"features/creating_values/#see-also","title":"See also","text":"
        • basic_json constructors - all ways to construct a value
        • array / object - force array or object type
        • operator\"\"_json - the _json literal
        • Converting values - which C++ types can be used
        • Parsing - create a value from JSON text
        "},{"location":"features/enum_conversion/","title":"Specializing enum conversion","text":"

        By default, enum values are serialized to JSON as integers. In some cases, this could result in undesired behavior. If the integer values of any enum values are changed after data using those enum values has been serialized to JSON, then deserializing that JSON would result in a different enum value being restored, or the value not being found at all.

        It is possible to more precisely specify how a given enum is mapped to and from JSON as shown below:

        // example enum type declaration\nenum TaskState {\n    TS_STOPPED,\n    TS_RUNNING,\n    TS_COMPLETED,\n    TS_INVALID=-1,\n};\n\n// map TaskState values to JSON as strings\nNLOHMANN_JSON_SERIALIZE_ENUM( TaskState, {\n    {TS_INVALID, nullptr},\n    {TS_STOPPED, \"stopped\"},\n    {TS_RUNNING, \"running\"},\n    {TS_COMPLETED, \"completed\"},\n})\n

        The NLOHMANN_JSON_SERIALIZE_ENUM() macro declares a set of to_json() / from_json() functions for type TaskState while avoiding repetition and boilerplate serialization code.

        "},{"location":"features/enum_conversion/#usage","title":"Usage","text":"

        Serialization converts an enum value to its mapped string, deserialization does the reverse, and an unrecognized JSON value deserializes to the first pair in the map:

        // enum to JSON as string\njson j = TS_STOPPED;\nassert(j == \"stopped\");\n\n// json string to enum\njson j3 = \"running\";\nassert(j3.get<TaskState>() == TS_RUNNING);\n\n// undefined json value to enum (where the first map entry above is the default)\njson jPi = 3.14;\nassert(jPi.get<TaskState>() == TS_INVALID );\n
        Example: serializing/deserializing enums, including a second enum type 
        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nnamespace ns\n{\nenum TaskState\n{\n    TS_STOPPED,\n    TS_RUNNING,\n    TS_COMPLETED,\n    TS_INVALID = -1\n};\n\nNLOHMANN_JSON_SERIALIZE_ENUM(TaskState,\n{\n    { TS_INVALID, nullptr },\n    { TS_STOPPED, \"stopped\" },\n    { TS_RUNNING, \"running\" },\n    { TS_COMPLETED, \"completed\" }\n})\n\nenum class Color\n{\n    red, green, blue, unknown\n};\n\nNLOHMANN_JSON_SERIALIZE_ENUM(Color,\n{\n    { Color::unknown, \"unknown\" }, { Color::red, \"red\" },\n    { Color::green, \"green\" }, { Color::blue, \"blue\" }\n})\n} // namespace ns\n\nint main()\n{\n    // serialization\n    json j_stopped = ns::TS_STOPPED;\n    json j_red = ns::Color::red;\n    std::cout << \"ns::TS_STOPPED -> \" << j_stopped\n              << \", ns::Color::red -> \" << j_red << std::endl;\n\n    // deserialization\n    json j_running = \"running\";\n    json j_blue = \"blue\";\n    auto running = j_running.get<ns::TaskState>();\n    auto blue = j_blue.get<ns::Color>();\n    std::cout << j_running << \" -> \" << running\n              << \", \" << j_blue << \" -> \" << static_cast<int>(blue) << std::endl;\n\n    // deserializing undefined JSON value to enum\n    // (where the first map entry above is the default)\n    json j_pi = 3.14;\n    auto invalid = j_pi.get<ns::TaskState>();\n    auto unknown = j_pi.get<ns::Color>();\n    std::cout << j_pi << \" -> \" << invalid << \", \"\n              << j_pi << \" -> \" << static_cast<int>(unknown) << std::endl;\n}\n

        Output:

        ns::TS_STOPPED -> \"stopped\", ns::Color::red -> \"red\"\n\"running\" -> 1, \"blue\" -> 2\n3.14 -> -1, 3.14 -> 3\n
        "},{"location":"features/enum_conversion/#maps-with-enum-keys","title":"Maps with enum keys","text":"

        By default, maps with enum keys, such as std::map<TaskState, std::string>, are stored as arrays of [key, value] pairs, because JSON object keys must be strings. Define JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS before including the library to store them as objects, with the keys converted by the enum's to_json() function:

        std::map<TaskState, std::string> m = {{TS_STOPPED, \"aa\"}, {TS_COMPLETED, \"bb\"}};\n\njson j = m;\n// default:                                   [[\"stopped\",\"aa\"],[\"completed\",\"bb\"]]\n// with JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS: {\"completed\":\"bb\",\"stopped\":\"aa\"}\n

        Either form can be read back, with or without the macro.

        "},{"location":"features/enum_conversion/#notes","title":"Notes","text":"

        Just as in Arbitrary Type Conversions above,

        • NLOHMANN_JSON_SERIALIZE_ENUM() MUST be declared in your enum type's namespace (which can be the global namespace), or the library will not be able to locate it, and it will default to integer serialization.
        • It MUST be available (e.g., proper headers must be included) everywhere you use the conversions.

        Other Important points:

        • When using get<ENUM_TYPE>(), undefined JSON values will default to the first pair specified in your map. Select this default pair carefully. If you desire an exception in this circumstance use NLOHMANN_JSON_SERIALIZE_ENUM_STRICT() which behaves identically except for throwing an out_of_range.410 exception on unrecognized values, both when serializing an enum value not listed in the map and when deserializing a JSON value that matches none of the map's entries.
        • If an enum or JSON value is specified more than once in your map, the first matching occurrence from the top of the map will be returned when converting to or from JSON.
        • To disable the default serialization of enumerators as integers and force a compiler error instead, see JSON_DISABLE_ENUM_SERIALIZATION.
        Example: NLOHMANN_JSON_SERIALIZE_ENUM_STRICT throwing on unrecognized values 
        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nnamespace ns\n{\n\nenum class Color\n{\n    red,\n    green,\n    blue,\n    unknown // not mapped in JSON_SERIALIZE_ENUM_STRICT\n};\n\nNLOHMANN_JSON_SERIALIZE_ENUM_STRICT(Color,\n{\n    {Color::red, \"red\"},\n    {Color::green, \"green\"},\n    {Color::blue, \"blue\"}\n})\n\n} // namespace ns\n\n\nint main()\n{\n    // invalid serialization\n    try\n    {\n        // ns::color::unknown was not mapped in macro\n        json invalid_serialization = ns::Color::unknown;\n    }\n    catch (const json::exception e)\n    {\n        std::cout << \"deserialization failed: \" << e.what() << std::endl;\n    }\n\n    // invalid deserialization\n    try\n    {\n        // what does not map to an enum\n        json invalid_deserialization(\"what\");\n        ns::Color color = invalid_deserialization.get<ns::Color>();\n    }\n    catch (const json::exception e)\n    {\n        std::cout << \"deserialization failed: \" << e.what() << std::endl;\n    }\n\n    return 0;\n}\n

        Output:

        deserialization failed: [json.exception.out_of_range.410] enum value out of range for Color\ndeserialization failed: [json.exception.out_of_range.410] enum value out of range for Color: \"what\"\n
        "},{"location":"features/iterators/","title":"Iterators","text":""},{"location":"features/iterators/#overview","title":"Overview","text":"

        A basic_json value is a container and allows access via iterators. Depending on the value type, basic_json stores zero or more values.

        As for other containers, begin() returns an iterator to the first value and end() returns an iterator to the value following the last value. The latter iterator is a placeholder and cannot be dereferenced. In case of null values, empty arrays, or empty objects, begin() will return end().

        "},{"location":"features/iterators/#iteration-order-for-objects","title":"Iteration order for objects","text":"

        When iterating over objects, values are ordered with respect to the object_comparator_t type which defaults to std::less. See the types documentation for more information.

        Example: iteration order of object values 
        // create JSON object {\"one\": 1, \"two\": 2, \"three\": 3}\njson j;\nj[\"one\"] = 1;\nj[\"two\"] = 2;\nj[\"three\"] = 3;\n\nfor (auto it = j.begin(); it != j.end(); ++it)\n{\n    std::cout << *it << std::endl;\n}\n

        Output:

        1\n3\n2\n

        The reason for the order is the lexicographic ordering of the object keys \"one\", \"three\", \"two\".

        "},{"location":"features/iterators/#access-object-keys-during-iteration","title":"Access object keys during iteration","text":"

        The JSON iterators have two member functions, key() and value() to access the object key and stored value, respectively. When calling key() on a non-object iterator, an invalid_iterator.207 exception is thrown.

        Example: access object keys with key() and value() 
        // create JSON object {\"one\": 1, \"two\": 2, \"three\": 3}\njson j;\nj[\"one\"] = 1;\nj[\"two\"] = 2;\nj[\"three\"] = 3;\n\nfor (auto it = j.begin(); it != j.end(); ++it)\n{\n    std::cout << it.key() << \" : \" << it.value() << std::endl;\n}\n

        Output:

        one : 1\nthree : 3\ntwo : 2\n
        "},{"location":"features/iterators/#range-based-for-loops","title":"Range-based for loops","text":"

        C++11 allows using range-based for loops to iterate over a container.

        for (auto it : j_object)\n{\n    // \"it\" is of type json::reference and has no key() member\n    std::cout << \"value: \" << it << '\\n';\n}\n

        For this reason, the items() function allows accessing iterator::key() and iterator::value() during range-based for loops. In these loops, a reference to the JSON values is returned, so there is no access to the underlying iterator.

        for (auto& el : j_object.items())\n{\n    std::cout << \"key: \" << el.key() << \", value:\" << el.value() << '\\n';\n}\n

        The items() function also allows using structured bindings (C++17):

        for (auto& [key, val] : j_object.items())\n{\n    std::cout << \"key: \" << key << \", value:\" << val << '\\n';\n}\n

        Note

        When iterating over an array, key() will return the index of the element as string. For primitive types (e.g., numbers), key() returns an empty string.

        Warning

        Using items() on temporary objects is dangerous. Make sure the object's lifetime exceeds the iteration. See #2040 for more information.

        "},{"location":"features/iterators/#reverse-iteration-order","title":"Reverse iteration order","text":"

        rbegin() and rend() return iterators in the reverse sequence.

        Example: reverse iteration with rbegin() and rend() 
        json j = {1, 2, 3, 4};\n\nfor (auto it = j.rbegin(); it != j.rend(); ++it)\n{\n    std::cout << *it << std::endl;\n}\n

        Output:

        4\n3\n2\n1\n
        "},{"location":"features/iterators/#iterating-strings-and-binary-values","title":"Iterating strings and binary values","text":"

        Note that \"value\" means a JSON value in this setting, not values stored in the underlying containers. That is, *begin() returns the complete string or binary array and is also safe if the underlying string or binary array is empty.

        Example: iterate over a string value 
        json j = \"Hello, world\";\nfor (auto it = j.begin(); it != j.end(); ++it)\n{\n    std::cout << *it << std::endl;\n}\n

        Output:

        \"Hello, world\"\n
        "},{"location":"features/iterators/#iterator-invalidation","title":"Iterator invalidation","text":"Operations invalidated iterators clear all"},{"location":"features/json_patch/","title":"JSON Patch and Diff","text":""},{"location":"features/json_patch/#patches","title":"Patches","text":"

        JSON Patch (RFC 6902) defines a JSON document structure for expressing a sequence of operations to apply to a JSON document. Operations address locations in the document using JSON Pointer paths. With the patch function, a JSON Patch is applied to the current JSON value by executing all operations from the patch, yielding the patched document as a new value.

        Applying a patch without copying

        patch leaves the original value unchanged and returns the patched result as a copy. If the document is large and the original value is no longer needed, patch_inplace applies the same operations in place instead.

        Example: apply a JSON Patch

        The following code shows how a JSON patch is applied to a value.

        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // the original document\n    json doc = R\"(\n        {\n          \"baz\": \"qux\",\n          \"foo\": \"bar\"\n        }\n    )\"_json;\n\n    // the patch\n    json patch = R\"(\n        [\n          { \"op\": \"replace\", \"path\": \"/baz\", \"value\": \"boo\" },\n          { \"op\": \"add\", \"path\": \"/hello\", \"value\": [\"world\"] },\n          { \"op\": \"remove\", \"path\": \"/foo\"}\n        ]\n    )\"_json;\n\n    // apply the patch\n    json patched_doc = doc.patch(patch);\n\n    // output original and patched document\n    std::cout << std::setw(4) << doc << \"\\n\\n\"\n              << std::setw(4) << patched_doc << std::endl;\n}\n

        Output:

        {\n    \"baz\": \"qux\",\n    \"foo\": \"bar\"\n}\n\n{\n    \"baz\": \"boo\",\n    \"hello\": [\n        \"world\"\n    ]\n}\n
        "},{"location":"features/json_patch/#diff","title":"Diff","text":"

        The library can also calculate a JSON patch (i.e., a diff) given two JSON values with the diff function.

        flowchart LR\n    S[\"source\"] -->|\"diff(source, target)\"| P[\"patch\"]\n    S -->|\"source.patch(patch)\"| T[\"target\"]\n    P -.->|\"applied to source, yields\"| T

        Invariant

        For two JSON values source and target, the following code yields always true:

        source.patch(diff(source, target)) == target;\n
        Example: create a JSON Patch from the difference of two values

        The following code shows how a JSON patch is created as a diff for two JSON values.

        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // the source document\n    json source = R\"(\n        {\n            \"baz\": \"qux\",\n            \"foo\": \"bar\"\n        }\n    )\"_json;\n\n    // the target document\n    json target = R\"(\n        {\n            \"baz\": \"boo\",\n            \"hello\": [\n                \"world\"\n            ]\n        }\n    )\"_json;\n\n    // create the patch\n    json patch = json::diff(source, target);\n\n    // roundtrip\n    json patched_source = source.patch(patch);\n\n    // output patch and roundtrip result\n    std::cout << std::setw(4) << patch << \"\\n\\n\"\n              << std::setw(4) << patched_source << std::endl;\n}\n

        Output:

        [\n    {\n        \"op\": \"replace\",\n        \"path\": \"/baz\",\n        \"value\": \"boo\"\n    },\n    {\n        \"op\": \"remove\",\n        \"path\": \"/foo\"\n    },\n    {\n        \"op\": \"add\",\n        \"path\": \"/hello\",\n        \"value\": [\n            \"world\"\n        ]\n    }\n]\n\n{\n    \"baz\": \"boo\",\n    \"hello\": [\n        \"world\"\n    ]\n}\n
        "},{"location":"features/json_patch/#see-also","title":"See also","text":"
        • JSON Pointer - the addressing scheme used for patch paths
        • JSON Merge Patch - a simpler, less expressive alternative patch format
        • patch - apply a JSON Patch, returning the result as a copy
        • patch_inplace - apply a JSON Patch without copying
        • diff - compute a JSON Patch from two values
        "},{"location":"features/json_pointer/","title":"JSON Pointer","text":""},{"location":"features/json_pointer/#introduction","title":"Introduction","text":"

        The library supports JSON Pointer (RFC 6901) as an alternative means to address structured values. A JSON Pointer is a string that identifies a specific value within a JSON document.

        Consider the following JSON document

        {\n    \"array\": [\"A\", \"B\", \"C\"],\n    \"nested\": {\n        \"one\": 1,\n        \"two\": 2,\n        \"three\": [true, false]\n    }\n}\n

        Then every value inside the JSON document can be identified as follows:

        JSON Pointer JSON value `` {\"array\":[\"A\",\"B\",\"C\"],\"nested\":{\"one\":1,\"two\":2,\"three\":[true,false]}} /array [\"A\",\"B\",\"C\"] /array/0 A /array/1 B /array/2 C /nested {\"one\":1,\"two\":2,\"three\":[true,false]} /nested/one 1 /nested/two 2 /nested/three [true,false] /nested/three/0 true /nested/three/1 false

        Note / does not identify the root (i.e., the whole document), but an object entry with empty key \"\". See RFC 6901 for more information.

        "},{"location":"features/json_pointer/#json-pointer-creation","title":"JSON Pointer creation","text":"

        JSON Pointers can be created from a string:

        json::json_pointer p(\"/nested/one\");\n

        Furthermore, a user-defined string literal can be used to achieve the same result:

        auto p = \"/nested/one\"_json_pointer;\n

        The escaping rules of RFC 6901 are implemented. See the constructor documentation for more information.

        "},{"location":"features/json_pointer/#value-access","title":"Value access","text":"

        JSON Pointers can be used in the at, operator[], and value functions just like object keys or array indices.

        // the JSON value from above\nauto j = json::parse(R\"({\n    \"array\": [\"A\", \"B\", \"C\"],\n    \"nested\": {\n        \"one\": 1,\n        \"two\": 2,\n        \"three\": [true, false]\n    }\n})\");\n\n// access values\nauto val = j[\"\"_json_pointer];                              // {\"array\":[\"A\",\"B\",\"C\"],...}\nauto val1 = j[\"/nested/one\"_json_pointer];                  // 1\nauto val2 = j.at(json::json_pointer(\"/nested/three/1\"));    // false\nauto val3 = j.value(json::json_pointer(\"/nested/four\"), 0); // 0\n

        To read a value with a fallback, use value with a JSON Pointer; to test for existence, use contains. Neither needs intermediate checks, see Nested values.

        Creating intermediate levels that don't exist

        See the operator[] notes for how array vs. object is decided when a pointer creates intermediate levels that don't exist yet.

        "},{"location":"features/json_pointer/#flatten-unflatten","title":"Flatten / unflatten","text":"

        The library implements a function flatten to convert any JSON document into a JSON object where each key is a JSON Pointer and each value is a primitive JSON value (i.e., a string, boolean, number, or null).

        // the JSON value from above\nauto j = json::parse(R\"({\n    \"array\": [\"A\", \"B\", \"C\"],\n    \"nested\": {\n        \"one\": 1,\n        \"two\": 2,\n        \"three\": [true, false]\n    }\n})\");\n\n// create flattened value\nauto j_flat = j.flatten();\n

        The resulting value j_flat is:

        {\n  \"/array/0\": \"A\",\n  \"/array/1\": \"B\",\n  \"/array/2\": \"C\",\n  \"/nested/one\": 1,\n  \"/nested/two\": 2,\n  \"/nested/three/0\": true,\n  \"/nested/three/1\": false\n}\n

        The reverse function, unflatten recreates the original value.

        auto j_original = j_flat.unflatten();\n
        "},{"location":"features/json_pointer/#see-also","title":"See also","text":"
        • Class json_pointer
        • Functions value, contains, and at accept JSON Pointers; see Nested values
        • Function flatten
        • Function unflatten
        • JSON Patch - paths inside a patch are JSON Pointers
        • JSON Merge Patch - an alternative patch format that does not use JSON Pointer
        "},{"location":"features/json_view/","title":"Zero-copy JSON views","text":"

        <nlohmann/json_view.hpp> adds a read-only, non-owning way to look at a parsed JSON text, as an alternative to building a basic_json tree with parse().

        "},{"location":"features/json_view/#the-problem","title":"The problem","text":"

        basic_json::parse() builds a tree of basic_json values: one allocation for every array and object, and every string copied into its own std::string. That is the right trade-off when the program goes on to read and write the value freely, but it does more work than necessary when only a small part of a large JSON text is actually needed, or when the same text is parsed over and over (many small messages, for instance) and most of the resulting tree is thrown away almost immediately.

        "},{"location":"features/json_view/#the-idea","title":"The idea","text":"

        basic_json_document::parse() parses the same JSON grammar, with the same options, but instead of a tree it builds a flat index of the values it found: one 16-byte entry per value (and one per object key), in document order. Integers are converted while parsing and stored in their entry. Floating-point numbers and strings are referenced in place in the source text: a float is converted when it is read, and only strings that contain escape sequences are decoded, into one shared buffer owned by the document.

        basic_json_view is a small, trivially copyable handle (two pointers) into that index. It gives you the read-only, type-inspection part of the basic_json interface -- type() and the is_*() predicates, size()/empty() -- as well as element access (operator[], at, front/back), lookup (find, contains, count), and iteration (begin/end, items) -- without ever allocating a basic_json value. When you do need an actual basic_json value for a subtree, materialize() builds exactly the one parse() would have produced for it.

        "},{"location":"features/json_view/#how-to-use-it","title":"How to use it","text":"

        Include <nlohmann/json_view.hpp> in addition to (or instead of) <nlohmann/json.hpp>. Parse into a json_document, inspect its root(), and materialize() when you need a real value. materialize() works on any view, not just the root, so you can turn just the subtree you need into a value:

        Example: parse a document, inspect its root, and materialize it 
        #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // json_document is basic_json_document<nlohmann::json>: same value types\n    // and containers as the ordinary json specialization\n    json_document doc = json_document::parse(R\"({\"pi\": 3.14, \"numbers\": [1, 2, 3]})\");\n\n    std::cout << doc.root().is_object() << '\\n';\n    std::cout << doc.root().materialize().dump() << '\\n';\n}\n

        Output:

        true\n{\"numbers\":[1,2,3],\"pi\":3.14}\n

        ordered_json_document is the equivalent for ordered_json, just as ordered_json is to json.

        "},{"location":"features/json_view/#ownership-and-lifetime","title":"Ownership and lifetime","text":"

        A document either borrows the text it was parsed from, or owns its own copy of it; call owns_source() to find out which happened. parse() decides this from the value category and type of its argument (an lvalue std::string is borrowed; an rvalue std::string is moved in, owned without a copy; a stream is read into an owned buffer; and so on -- see parse's Notes for the full table). parse_copy() always owns a copy, regardless of the input.

        A borrowed document depends on your buffer

        If a document borrows its text, that text must outlive the document (and every view taken from it) and must stay unmodified for as long as the document exists, since floating-point values and unescaped strings are read from it lazily. Reading or writing through a view after the underlying buffer is gone is undefined behavior, exactly as it would be for a dangling std::string_view.

        A view is valid only while all of the following hold:

        • the document is alive,
        • the document has not been re-parsed since the view was taken (with read() or parse() into it),
        • the document has not had shrink_to_fit() called on it since, and
        • if the document borrows its source text, that text is still alive and unmodified.

        Moving the basic_json_document<> itself is fine and does not invalidate its views: the index is a separate heap allocation that keeps its address across the move. Take a fresh view from root() whenever any of the other conditions above was not met.

        A view can dangle, exactly like a std::string_view: nothing stops you from using it after its document is gone. This is deliberate. A view is two pointers and trivially copyable; making it safe (a std::shared_ptr in the document and a std::weak_ptr in each view) would make it larger, add reference counting to every copy, and a lock() to every access -- exactly the costs the view exists to avoid. What is caught is the obvious case: because a view dies with its document, root() is not callable on a temporary document: auto v = json_document::parse(text).root(); does not compile. Give the document a name first.

        Example: borrowed and owned documents, and when views become invalid 
        #include <iostream>\n#include <string>\n#include <utility>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // BORROWED: doc only points into `text`; `text` must outlive `doc`\n    std::string text = R\"({\"a\": 1})\";\n    json_document doc = json_document::parse(text);\n    std::cout << doc.owns_source() << '\\n'; // false\n\n    // a view is valid as long as the document is alive, has not been\n    // re-parsed (read()) or shrunk (shrink_to_fit()), and -- if borrowed --\n    // the source text is alive\n    nlohmann::json_view v = doc.root();\n    std::cout << v.is_object() << '\\n';\n\n    // re-parsing the SAME document invalidates views taken before the call;\n    // `v` above must not be used after this line\n    doc.read(R\"([1, 2, 3])\");\n    v = doc.root(); // take a fresh view instead\n    std::cout << v.is_array() << '\\n';\n\n    // moving the document does not invalidate views: the node index is\n    // heap-allocated and does not move with the document object\n    json_document moved = std::move(doc);\n    std::cout << v.is_array() << '\\n';\n}\n

        Output:

        false\ntrue\ntrue\ntrue\n
        "},{"location":"features/json_view/#what-is-the-same-as-parse","title":"What is the same as parse()","text":"
        • Accept/reject. accept() and parse() accept and reject exactly the same inputs as basic_json::accept()/basic_json::parse(), with the same ignore_comments and ignore_trailing_commas options.
        • Errors. A failing parse throws the same exception -- the same id, message, and position -- because on failure the library's own parser is run on the same bytes to produce the diagnostic. Valid input never pays for this, and the replay builds no values: it costs about a lexing pass up to the error. allow_exceptions == false skips the replay and gives a discarded document instead of throwing, just as it gives a discarded value for basic_json::parse().
        • Number classification. An integer literal that does not fit into the 64-bit integer type becomes a floating-point value, exactly as it does for basic_json::parse().
        • Macros. JSON_STRICT_NUL_HANDLING and JSON_NOEXCEPTION/JSON_THROW_USER behave the same way they do for <nlohmann/json.hpp>. These are the macros that change what the parser accepts or how it reports errors. The others either do not apply (JSON_DIAGNOSTICS needs parent pointers in basic_json values) or only affect the values materialize() produces, through BasicJsonType.
        "},{"location":"features/json_view/#what-is-different","title":"What is different","text":"
        • Only 64-bit integers. basic_json_document<BasicJsonType> requires BasicJsonType::number_integer_t and number_unsigned_t to both be 64 bits wide; this is a compile-time static_assert.
        • A 4 GiB input limit. An input of 4294967280 bytes (4 GiB minus 16 bytes) or more throws out_of_range.416, a limit basic_json::parse() does not have. It keeps an index entry at 16 bytes: offsets and counts are 32 bits wide. The limit is 16 bytes below 2^32, because the parser reads in 16-byte steps, and a position one step past the end must still fit.
        • A stream is always read to its end. There is no partial/streaming read of an std::istream: the stream is read to its end into a buffer the document owns, and parsing starts after that.
        • No source positions on materialize(). Even with JSON_DIAGNOSTIC_POSITIONS enabled, materialize() does not set them. The index has the start offset of every value, but not the end of arrays, objects, and escaped strings, which the positions also need.
        • Objects iterate in document order. begin()/ end() and items() visit an object's members in the order they appear in the source text. basic_json's default object_t is a std::map, which sorts by key, so iterating a materialize()d value can print members in a different order than iterating the view they came from.
        • Chained access is safe. operator[] with a missing key, an index out of range, or an unresolvable JSON pointer returns a discarded view, and operator[] on a discarded view returns a discarded view without throwing: v[\"a\"][\"b\"][0] can be tested once at the end. Type errors on values that exist (a key on an array, an index on an object) still throw, and at throws for every missing value.
        • Duplicate keys: lookups find the first member, parse() keeps the last. See Duplicate keys below.
        • No JSON_DIAGNOSTICS path. Exceptions thrown by basic_json_view's own element access and lookup functions never carry the JSON Pointer path JSON_DIAGNOSTICS would otherwise add: the view has no basic_json value to point at, so the exception is created without one, regardless of how BasicJsonType was built.
        • Ordering comparisons are not provided by basic_json_view -- there is no operator<. operator== and operator!= are provided, though: two views, or a view and a BasicJsonType value, compare equal exactly when materialize() or parse() would produce equal values for them, without ever building a tree to do it. For ordering, too, materialize() is the way to get a value you can compare.
        "},{"location":"features/json_view/#duplicate-keys","title":"Duplicate keys","text":"

        A lookup in a view and in the parsed value can give different answers

        If an object in the source text repeats a key, operator[], at, find, value, contains, and JSON pointer resolution all return the first member with that key. basic_json::parse() instead keeps the last value of a repeated key, so for {\"a\":1,\"a\":2}, view[\"a\"] is 1 while parse(text)[\"a\"] is 2. count counts every member with the key, so it is 2 here.

        begin()/end() and items() visit every occurrence, in document order, and size() counts all of them, so view.size() can be larger than view.materialize().size().

        Why the first? A lookup can stop as soon as it finds a match. Returning the last member would force every lookup to scan all members of the object, even when the key is found at the very first one: this made lookups in small objects 1.6 to 3.4 times slower. The hash index of the keys of objects with 128 members or more leads to the first member of a key as well. Other zero-copy parsers that index the source text, such as yyjson and simdjson, also return the first member. RFC 8259 only says that names within an object SHOULD be unique and that the behavior of a receiver that sees duplicates is unpredictable, so neither choice is wrong.

        What stays the same as parse()? materialize() and get<std::map<...>>() replay every member in order, so they keep the last value exactly like basic_json::parse() (at the position of the first occurrence of the key, for an ordered_json).

        How do I get the value parse() would give? Either call view.materialize() and look the key up in the result, or iterate the members and keep the last match:

        // the last member with the key \"a\", as parse() would keep it\njson_view last;\nfor (const auto item : view.items())\n{\n    if (item.key() == \"a\")\n    {\n        last = item.value();\n    }\n}\n

        If you cannot trust the source text to have unique keys, check size() against the number of distinct keys, or reject duplicates when iterating.

        "},{"location":"features/json_view/#getting-values-out-without-copying","title":"Getting values out without copying","text":"

        get<T>() converts many T directly from the flat index, without ever building a basic_json value for the conversion: bool, arithmetic types, std::nullptr_t, std::string/other std::basic_strings (copied once), basic_json/ordered_json (via materialize()), basic_json_view itself, std::vector<U>, and std::map/std::unordered_map with string-like keys. Every other type -- std::list, std::pair, std::array, enumerations, user types with a from_json() -- goes through materialize().get<T>() instead: the subtree is built into a real basic_json value first, exactly as parse() would, and converted from there.

        Two conversions never copy at all:

        • get_string() (equivalently, get<string_view_t>()) returns a string as a string_view_t pointing into the document's source() text -- or, for a string that contains escape sequences, into the document's own buffer of decoded strings -- instead of allocating a new std::string.
        • number_token() returns a number exactly as it was written in the source, e.g. \"1.50\", \"1E2\", or an integer with more digits than any number type holds, instead of rounding it into a double/int64_t the way get<T>() (and basic_json::parse()) would.

        Both results are only valid as long as the view -- and, for a string with no escapes, the borrowed source text -- is.

        "},{"location":"features/json_view/#writing-a-view-back","title":"Writing a view back","text":"

        dump() serializes a view directly from the flat index, without ever building a basic_json value. An object's members are written in document order, not sorted by key, and every occurrence of a repeated key is written, not only the last one -- the same two ways iteration already differs from a materialize()d value, see Duplicate keys. materialize().dump() gives a different result in both respects for a json_view.

        By default, numbers are written the way basic_json::dump() would. number_format::source instead copies every number exactly as it was written in the source text -- a price like 19.90, a long order or account ID with more digits than any number type holds, or a high-precision coordinate -- something basic_json cannot do at all, since parsing already reduces a number to its parsed double/int64_t value.

        operator<< writes a view to a stream the way basic_json's does, using the stream's width/fill for indentation.

        "},{"location":"features/json_view/#editing-a-document","title":"Editing a document","text":"

        Everything above is read-only: a json_document/json_view lets you look at a parsed text without copying it, but not change it. basic_json_document<BasicJsonType, true> -- more conveniently spelled json_editable_document or ordered_json_editable_document -- also lets you set a value, push_back onto or insert into an array, and erase an object member or an array element, still without ever building a basic_json tree for parts you do not touch.

        Editable defaults to false, so json_document/ordered_json_document are unaffected -- they carry none of the bookkeeping edits need, and calling set/push_back/insert/erase on one is a compile error, not a runtime one.

        "},{"location":"features/json_view/#why-editing-without-reformatting","title":"Why: editing without reformatting","text":"

        The 19.90 price from above is exactly the kind of value that makes editing a json or ordered_json value in place lossy. Say you parse a configuration file, patch one field, and write it back:

        • json re-sorts every key on the way in (object_t is a std::map) and rewrites every number to its shortest round-trip form on the way out -- a one-field patch turns into a diff that reorders the whole file and rewrites 19.90 to 19.9.
        • ordered_json keeps the key order, but still rewrites every number the same way: parsing has already reduced it to a double/int64_t, and there is no way back to how it was spelled in the source text.

        An editable document keeps both. dump() of an edited document writes members in document order -- a member set added goes at the end, exactly where it was inserted, and an erased member simply leaves a gap: everything around it keeps its place -- and number_format::source keeps the exact spelling of every number an edit did not itself touch; a number an edit did touch is written the way BasicJsonType::dump() would write it, since there is no source spelling for a brand new value.

        Example: patch a configuration, keeping member order and number spellings 
        #include <iostream>\n#include <nlohmann/json_view.hpp>\n\nusing json = nlohmann::json;\nusing json_editable_document = nlohmann::json_editable_document;\nusing json_editable_view = nlohmann::json_editable_view;\n\nint main()\n{\n    // a configuration file, as it might be read from disk\n    const std::string text = R\"({\"name\": \"cache\", \"host\": \"db1\", \"port\": 6379, \"price\": 19.90})\";\n    std::cout << text << \"\\n\\n\";\n\n    // patch two fields -- \"price\" is never touched\n    json_editable_document doc = json_editable_document::parse(text);\n    doc.set(doc.root(), \"host\", \"db2\");\n    doc.set(doc.root(), \"retries\", 3);\n\n    // member order (the new member at the end) and the untouched number's\n    // exact spelling survive\n    std::cout << doc.root().dump(-1, ' ', false, json_editable_view::number_format::source) << '\\n';\n\n    // the same patch on a plain json value: keys are sorted (object_t is a\n    // std::map), and \"price\" is rewritten even though the patch never\n    // touched it\n    json plain = json::parse(text);\n    plain[\"host\"] = \"db2\";\n    plain[\"retries\"] = 3;\n    std::cout << plain.dump() << '\\n';\n}\n

        Output:

        {\"name\": \"cache\", \"host\": \"db1\", \"port\": 6379, \"price\": 19.90}\n\n{\"name\":\"cache\",\"host\":\"db2\",\"port\":6379,\"price\":19.90,\"retries\":3}\n{\"host\":\"db2\",\"name\":\"cache\",\"port\":6379,\"price\":19.9,\"retries\":3}\n
        "},{"location":"features/json_view/#what-stays-valid-and-what-an-edit-costs","title":"What stays valid, and what an edit costs","text":"

        The source text itself is never written, and the parsed index never moves -- a value keeps the node it was parsed into for as long as it is not itself replaced. So every view taken before an edit, including a previously obtained root(), stays valid and, if it still refers to the edited value, sees the edit; a view of a value a later edit drops or replaces just keeps showing what it last held. New values go to storage the document allocates and owns on demand. The one thing an edit does invalidate is the iterators taken over an edited array or object: the first time one of its elements is set, appended to, inserted into, or erased, its elements move from the parsed, fixed layout to a growable block of links so that push_back can later grow it in amortized constant time -- existing elements are not touched, but an iterator that was walking the old layout no longer matches. A string obtained with get_string() is unaffected either way and stays valid across further edits. See basic_json_document's Edits for the details, and set's Exception safety for what an edit guarantees if it throws (the basic guarantee, not the strong one dump() and the read-only functions provide). How edits are kept in the index is described in the architecture overview.

        "},{"location":"features/json_view/#images","title":"Images","text":"

        save() writes a document as an image: a byte buffer that load() reads back into a document without parsing -- no lexing, no building the node index, nothing but copying the nodes and pointing the text and the decoded strings at the image. Where parse_copy() still has to scan the whole input, load() turns that scan into a copy of the node index alone.

        Why. A document that is parsed once and then read many times -- a configuration loaded at startup, a template rendered on every request, a large reference dataset a worker process needs in memory -- pays for parsing once but can amortize save()'s cost across every later load. That makes images useful for a cache: save a document the first time it is parsed (to a file, a shared-memory segment, an in-process cache), and load() it on every later use instead of parsing the source text again. They are just as useful for handing a parsed document to another process (or a forked worker) running the same build of the library, since load() turns the transfer into a copy of the node index plus pointers into the received bytes, not a re-parse.

        Choosing a check. load() takes an image_check that trades validation against speed: image_check::full (the default) checks everything the parser itself guarantees, so a checked image is exactly as safe to read and serialize as a freshly parsed document -- the right choice whenever the image did not come straight from this process's own save(), such as a file or a network peer. image_check::bounds only checks structure and bounds -- cheaper, since it skips scanning the text and the decoded strings -- and fits a cache the process trusts, one it wrote and reads back itself. image_check::none skips validation entirely, for an image trusted as much as the process's own memory. See load()'s Notes for exactly what each level does and does not guarantee.

        Example: cache a parsed configuration as an image 
        #include <cstdint>\n#include <iostream>\n#include <string>\n#include <vector>\n#include <nlohmann/json_view.hpp>\n\nusing json_document = nlohmann::json_document;\n\nint main()\n{\n    std::cout << std::boolalpha;\n\n    // a configuration a service parses once and then caches as an image, so\n    // that later requests can load() it instead of parsing the text again\n    const std::string text = R\"({\"name\": \"cache\", \"host\": \"db1\", \"port\": 6379, \"replicas\": [\"db2\", \"db3\"]})\";\n    const json_document config = json_document::parse(text);\n\n    // save() turns the parsed document into a byte buffer: a 64-byte header,\n    // the node index, the source text, and the decoded strings\n    const std::vector<std::uint8_t> image = config.save();\n    std::cout << image.size() << '\\n';\n\n    // the same document always saves to the same bytes\n    std::cout << (image == json_document::parse(text).save()) << '\\n';\n\n    // loading the image back needs no parsing, yet dumps exactly what\n    // parsing the text produced\n    const json_document reloaded = json_document::load(image);\n    std::cout << (reloaded.root().dump() == config.root().dump()) << '\\n';\n}\n

        Output:

        316\ntrue\ntrue\n

        Experimental

        The image format is versioned but not yet stable, and may change in an incompatible way before it is declared stable. It is little-endian only, and tied to the library build that wrote it -- use it to cache a document or to hand one to another process running the same build, not as a long-term storage format; keep the original JSON text if a saved document needs to be readable by a future library version.

        The idea of a document you can read without parsing comes from zero-copy formats such as FlatBuffers and YaFF; the check load() runs follows the idea of FlatBuffers' Verifier. No code is taken from either.

        "},{"location":"features/json_view/#choosing-between-json-ordered_json-the-sax-interface-and-json_view","title":"Choosing between json, ordered_json, the SAX interface, and json_view","text":"json / ordered_json SAX interface json_document / json_view json_editable_document / json_editable_view Ownership owns every value owns nothing; you decide what to keep, in your handler borrows or owns the text; the index is always owned by the document same as json_document; edits go to storage the document owns Mutability freely mutable not applicable (a one-shot event stream) read-only set/push_back/insert/erase edit in place; the source text is never rewritten What you get a full tree you can read, write, and keep as long as you like a sequence of callbacks; whatever your handler builds from them a flat index plus, on demand, materialize()d json/ordered_json values for the parts you actually use the same, plus dump() of an edited document that keeps the member order and, with number_format::source, the spelling of every untouched number Typical use general-purpose JSON handling: config, request/response bodies you build or modify, anything you hold onto validating or projecting a text into your own data structure without ever holding the whole thing as JSON large or high-volume input where you only need part of it, or need it repeatedly, and can keep the source text (or a copy) alive for as long as the document lives a document you read, patch a few fields of, and write back -- a configuration file, for instance -- where the rest of it should come back exactly as it was Caching/reload not applicable -- re-parse, or roll your own serialization not applicable save()/load(): cache the parsed index as an image and reload it without parsing same, saving the document's current -- possibly edited -- state"},{"location":"features/json_view/#version-history","title":"Version history","text":"
        • Added in version 3.13.0 unreleased.
        "},{"location":"features/macros/","title":"Supported Macros","text":"

        Some aspects of the library can be configured by defining preprocessor macros before including the json.hpp header. See also the API documentation for macros for examples and more information.

        "},{"location":"features/macros/#json_assertx","title":"JSON_ASSERT(x)","text":"

        This macro controls which code is executed for runtime assertions of the library.

        See full documentation of JSON_ASSERT(x).

        "},{"location":"features/macros/#json_brace_init_copy_semantics","title":"JSON_BRACE_INIT_COPY_SEMANTICS","text":"

        When defined to 1, single-element brace initialization of a basic_json value (e.g., json j{value};) is treated as a copy/move of the element rather than wrapping it in a single-element array. The default value is 0, which preserves the existing behavior.

        See full documentation of JSON_BRACE_INIT_COPY_SEMANTICS.

        "},{"location":"features/macros/#json_catch_userexception","title":"JSON_CATCH_USER(exception)","text":"

        This macro overrides catch calls inside the library.

        See full documentation of JSON_CATCH_USER(exception).

        "},{"location":"features/macros/#json_delete_deprecated_functions","title":"JSON_DELETE_DEPRECATED_FUNCTIONS","text":"

        When defined to 1, all deprecated functions are declared as deleted instead of only being marked as deprecated, so code that still calls them no longer compiles. This way, you can find all calls that need to be replaced before version 4.0.0 removes these functions.

        The macro can also be set with the CMake option JSON_DeleteDeprecatedFunctions (OFF by default).

        See full documentation of JSON_DELETE_DEPRECATED_FUNCTIONS.

        "},{"location":"features/macros/#json_diagnostics","title":"JSON_DIAGNOSTICS","text":"

        This macro enables extended diagnostics for exception messages. Possible values are 1 to enable or 0 to disable (default).

        When enabled, exception messages contain a JSON Pointer to the JSON value that triggered the exception, see Extended diagnostic messages for an example. Note that enabling this macro increases the size of every JSON value by one pointer and adds some runtime overhead.

        The diagnostics messages can also be controlled with the CMake option JSON_Diagnostics (OFF by default) which sets JSON_DIAGNOSTICS accordingly.

        See full documentation of JSON_DIAGNOSTICS.

        "},{"location":"features/macros/#json_diagnostic_positions","title":"JSON_DIAGNOSTIC_POSITIONS","text":"

        When enabled, two new member functions start_pos() and end_pos() are added to basic_json values. If the value was created by calling theparse function, then these functions allow querying the byte positions of the value in the input it was parsed from. The byte positions are also used in exceptions to help locate errors.

        The diagnostics positions can also be controlled with the CMake option JSON_Diagnostic_Positions (OFF by default) which sets JSON_DIAGNOSTIC_POSITIONS accordingly.

        See full documentation of JSON_DIAGNOSTIC_POSITIONS

        "},{"location":"features/macros/#json_has_cpp_11-json_has_cpp_14-json_has_cpp_17-json_has_cpp_20-json_has_cpp_23-json_has_cpp_26","title":"JSON_HAS_CPP_11, JSON_HAS_CPP_14, JSON_HAS_CPP_17, JSON_HAS_CPP_20, JSON_HAS_CPP_23, JSON_HAS_CPP_26","text":"

        The library targets C++11, but also supports some features introduced in later C++ versions (e.g., std::string_view support for C++17). For these new features, the library implements some preprocessor checks to determine the C++ standard. By defining any of these symbols, the internal check is overridden and the provided C++ version is unconditionally assumed. This can be helpful for compilers that only implement parts of the standard and would be detected incorrectly.

        See full documentation of JSON_HAS_CPP_11, JSON_HAS_CPP_14, JSON_HAS_CPP_17, JSON_HAS_CPP_20, JSON_HAS_CPP_23, and JSON_HAS_CPP_26.

        "},{"location":"features/macros/#json_has_filesystem-json_has_experimental_filesystem","title":"JSON_HAS_FILESYSTEM, JSON_HAS_EXPERIMENTAL_FILESYSTEM","text":"

        When compiling with C++17, the library provides conversions from and to std::filesystem::path. As compiler support for filesystem is limited, the library tries to detect whether <filesystem>/std::filesystem (JSON_HAS_FILESYSTEM) or <experimental/filesystem>/std::experimental::filesystem (JSON_HAS_EXPERIMENTAL_FILESYSTEM) should be used. To override the built-in check, define JSON_HAS_FILESYSTEM or JSON_HAS_EXPERIMENTAL_FILESYSTEM to 1.

        See full documentation of JSON_HAS_FILESYSTEM and JSON_HAS_EXPERIMENTAL_FILESYSTEM.

        "},{"location":"features/macros/#json_has_ranges","title":"JSON_HAS_RANGES","text":"

        The library uses std::ranges (and concepts) where available, for example, to parse from C++20 ranges and to construct JSON arrays from range views. The library detects whether the standard library supports ranges and disables the support on toolchains with an incomplete implementation. To override the built-in check, define JSON_HAS_RANGES to 1 or 0.

        See full documentation of JSON_HAS_RANGES.

        "},{"location":"features/macros/#json_has_range_view_conversion","title":"JSON_HAS_RANGE_VIEW_CONVERSION","text":"

        When JSON_HAS_RANGES is enabled (and the compiler is not MinGW), a JSON array can be constructed directly from a C++20 range view such as std::views::filter(...). To override the built-in check, define JSON_HAS_RANGE_VIEW_CONVERSION to 1 or 0.

        See full documentation of JSON_HAS_RANGE_VIEW_CONVERSION.

        "},{"location":"features/macros/#json_has_static_rtti","title":"JSON_HAS_STATIC_RTTI","text":"

        The library detects whether the compiler supports run time type information (RTTI), which it needs, for instance, to exclude std::any from the candidate types of the implicit conversion on C++17. To override the built-in check, define JSON_HAS_STATIC_RTTI to 1 or 0.

        See full documentation of JSON_HAS_STATIC_RTTI.

        "},{"location":"features/macros/#json_has_std_format","title":"JSON_HAS_STD_FORMAT","text":"

        When compiling with C++20 and a standard library that provides <format>, the library provides a std::formatter specialization for JSON values. To override the built-in check, define JSON_HAS_STD_FORMAT to 1 or 0.

        See full documentation of JSON_HAS_STD_FORMAT.

        "},{"location":"features/macros/#json_has_three_way_comparison","title":"JSON_HAS_THREE_WAY_COMPARISON","text":"

        When the compiler and standard library support 3-way comparison (the spaceship operator <=>), the library provides it for JSON values. To override the built-in check, define JSON_HAS_THREE_WAY_COMPARISON to 1 or 0.

        See full documentation of JSON_HAS_THREE_WAY_COMPARISON.

        "},{"location":"features/macros/#json_noexception","title":"JSON_NOEXCEPTION","text":"

        Exceptions can be switched off by defining the symbol JSON_NOEXCEPTION.

        See full documentation of JSON_NOEXCEPTION.

        "},{"location":"features/macros/#json_disable_enum_serialization","title":"JSON_DISABLE_ENUM_SERIALIZATION","text":"

        When defined, default parse and serialize functions for enums are excluded and have to be provided by the user, for example, using NLOHMANN_JSON_SERIALIZE_ENUM.

        See full documentation of JSON_DISABLE_ENUM_SERIALIZATION.

        "},{"location":"features/macros/#json_disable_stream_bulk_scan","title":"JSON_DISABLE_STREAM_BULK_SCAN","text":"

        When defined to 1, parsing from a std::istream reads one character at a time instead of scanning the characters that the stream buffer holds in bulk. Results are the same either way; the macro is an escape hatch for stream buffers that do not follow the usual std::streambuf contract. The default value is 0.

        See full documentation of JSON_DISABLE_STREAM_BULK_SCAN.

        "},{"location":"features/macros/#json_disable_tuple_reference_conversion","title":"JSON_DISABLE_TUPLE_REFERENCE_CONVERSION","text":"

        When defined to 1, a JSON value can no longer be created from a one-element std::tuple holding a reference to a JSON value, such as the result of std::forward_as_tuple(j). This lets std::tuple convert such tuples element-wise. This is planned to become the default in version 4.0.0.

        See full documentation of JSON_DISABLE_TUPLE_REFERENCE_CONVERSION.

        "},{"location":"features/macros/#json_disable_unlocked_file_io","title":"JSON_DISABLE_UNLOCKED_FILE_IO","text":"

        When defined to 1, parsing from a FILE* reads every character with std::fgetc, which locks the file each time, instead of locking the file once and reading it with the unlocked stdio functions. Results are the same either way. The default value is 0.

        See full documentation of JSON_DISABLE_UNLOCKED_FILE_IO.

        "},{"location":"features/macros/#json_no_automatic_udls","title":"JSON_NO_AUTOMATIC_UDLS","text":"

        When defined, <nlohmann/json.hpp> does not include <nlohmann/json_literals.hpp> with the user-defined string literals operator\"\"_json and operator\"\"_json_pointer. This reduces the compile time of translation units that do not use them, because the literals instantiate the parser in every translation unit that includes them. Include <nlohmann/json_literals.hpp> where the literals are needed.

        See full documentation of JSON_NO_AUTOMATIC_UDLS.

        "},{"location":"features/macros/#json_no_io","title":"JSON_NO_IO","text":"

        When defined, headers <cstdio>, <ios>, <iosfwd>, <istream>, and <ostream> are not included and parse functions relying on these headers are excluded. This is relevant for environment where these I/O functions are disallowed for security reasons (e.g., Intel Software Guard Extensions (SGX)).

        See full documentation of JSON_NO_IO.

        "},{"location":"features/macros/#json_no_thread_local","title":"JSON_NO_THREAD_LOCAL","text":"

        When defined, the library does not use thread_local storage. Copying a value and comparing two values then always avoid the call stack rather than descending into a bounded number of levels first, which is slower but yields the same values and the same comparisons.

        See full documentation of JSON_NO_THREAD_LOCAL.

        "},{"location":"features/macros/#json_precise_stream_position","title":"JSON_PRECISE_STREAM_POSITION","text":"

        When defined to 1, operator>> and non-strict sax_parse leave an input stream positioned right after the parsed value, instead of also consuming the character that terminates a number. The default value is 0, which preserves the existing behavior; this is planned to become the default in version 4.0.0.

        See full documentation of JSON_PRECISE_STREAM_POSITION.

        "},{"location":"features/macros/#json_skip_library_version_check","title":"JSON_SKIP_LIBRARY_VERSION_CHECK","text":"

        When defined, the library will not create a compiler warning when a different version of the library was already included.

        See full documentation of JSON_SKIP_LIBRARY_VERSION_CHECK.

        "},{"location":"features/macros/#json_skip_unsupported_compiler_check","title":"JSON_SKIP_UNSUPPORTED_COMPILER_CHECK","text":"

        When defined, the library will not create a compile error when a known unsupported compiler is detected. This allows using the library with compilers that do not fully support C++11 and may only work if unsupported features are not used.

        See full documentation of JSON_SKIP_UNSUPPORTED_COMPILER_CHECK.

        "},{"location":"features/macros/#json_strict_binary_utf8","title":"JSON_STRICT_BINARY_UTF8","text":"

        When defined to 1, to_cbor, to_ubjson, to_bjdata, and to_bson throw type_error.316 for a string value or object key that is not valid UTF-8. The default value is 0, which writes the bytes unchanged as before version 3.13.0 unreleased; this is planned to become the default in version 4.0.0.

        The check can also be enabled with the CMake option JSON_StrictBinaryUTF8 (OFF by default) which sets JSON_STRICT_BINARY_UTF8 accordingly.

        See full documentation of JSON_STRICT_BINARY_UTF8.

        "},{"location":"features/macros/#json_strict_nul_handling","title":"JSON_STRICT_NUL_HANDLING","text":"

        When defined to 1, a '\\0' (NUL) byte anywhere in the input is rejected with parse_error.101, like any other unexpected byte, instead of being silently treated as end of input (see the FAQ entry for background). The default value is 0, which preserves the existing behavior; this is planned to become the default in version 4.0.0.

        The strict handling can also be enabled with the CMake option JSON_StrictNulHandling (OFF by default) which sets JSON_STRICT_NUL_HANDLING accordingly.

        See full documentation of JSON_STRICT_NUL_HANDLING.

        "},{"location":"features/macros/#json_throw_userexception","title":"JSON_THROW_USER(exception)","text":"

        This macro overrides throw calls inside the library. The argument is the exception to be thrown.

        See full documentation of JSON_THROW_USER(exception).

        "},{"location":"features/macros/#json_try_user","title":"JSON_TRY_USER","text":"

        This macro overrides try calls inside the library.

        See full documentation of JSON_TRY_USER.

        "},{"location":"features/macros/#json_use_implicit_conversions","title":"JSON_USE_IMPLICIT_CONVERSIONS","text":"

        When defined to 0, implicit conversions are switched off. By default, implicit conversions are switched on.

        See full documentation of JSON_USE_IMPLICIT_CONVERSIONS.

        "},{"location":"features/macros/#json_use_global_udls","title":"JSON_USE_GLOBAL_UDLS","text":"

        When defined to 1 (default), the user-defined string literals operator\"\"_json and operator\"\"_json_pointer are placed into the global namespace instead of nlohmann::literals::json_literals.

        See full documentation of JSON_USE_GLOBAL_UDLS.

        "},{"location":"features/macros/#json_use_legacy_discarded_value_comparison","title":"JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON","text":"

        When defined to 1, the library restores the legacy behavior in which comparing a discarded value with <= or >= yields true instead of false. This behavior is deprecated and switched off (0) by default.

        See full documentation of JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON.

        "},{"location":"features/macros/#json_use_objects_for_enum_keyed_maps","title":"JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS","text":"

        When defined to 1, maps with enum keys (e.g., std::map<E, T>) are stored as objects, using the enum's conversion for the keys, instead of arrays of [key, value] pairs. It is switched off (0) by default.

        See full documentation of JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS.

        "},{"location":"features/macros/#json_use_simdutf","title":"JSON_USE_SIMDUTF","text":"

        When defined, UTF-8 validation of JSON strings read from contiguous byte input is delegated to the simdutf library instead of the built-in scalar validator. This is an opt-in external dependency and is not defined by default.

        See full documentation of JSON_USE_SIMDUTF.

        "},{"location":"features/macros/#nlohmann_define_type_-nlohmann_define_derived_type_","title":"NLOHMANN_DEFINE_TYPE_*(...), NLOHMANN_DEFINE_DERIVED_TYPE_*(...)","text":"

        The library defines 12 macros to simplify the serialization/deserialization of types. See the page on arbitrary type conversion for a detailed discussion.

        "},{"location":"features/macros/#nlohmann_json_namespace-nlohmann_json_namespace_begin-nlohmann_json_namespace_end-nlohmann_json_namespace_no_version","title":"NLOHMANN_JSON_NAMESPACE, NLOHMANN_JSON_NAMESPACE_BEGIN, NLOHMANN_JSON_NAMESPACE_END, NLOHMANN_JSON_NAMESPACE_NO_VERSION","text":"

        These macros relate to the versioned, inline nlohmann namespace:

        • NLOHMANN_JSON_NAMESPACE evaluates to the full name of the nlohmann namespace (including the inline ABI namespace).
        • NLOHMANN_JSON_NAMESPACE_BEGIN / NLOHMANN_JSON_NAMESPACE_END open and close the namespace (for example, to add specializations).
        • NLOHMANN_JSON_NAMESPACE_NO_VERSION, when defined to 1, omits the version component from the inline namespace.

        See the nlohmann Namespace page, and the full documentation of NLOHMANN_JSON_NAMESPACE, NLOHMANN_JSON_NAMESPACE_BEGIN / NLOHMANN_JSON_NAMESPACE_END, and NLOHMANN_JSON_NAMESPACE_NO_VERSION.

        "},{"location":"features/macros/#nlohmann_json_serialize_enumtype","title":"NLOHMANN_JSON_SERIALIZE_ENUM(type, ...)","text":"

        This macro simplifies the serialization/deserialization of enum types. See Specializing enum conversion for more information.

        See full documentation of NLOHMANN_JSON_SERIALIZE_ENUM.

        A strict variant NLOHMANN_JSON_SERIALIZE_ENUM_STRICT throws an exception on undefined input instead of falling back to the first mapping.

        "},{"location":"features/macros/#nlohmann_json_version_major-nlohmann_json_version_minor-nlohmann_json_version_patch","title":"NLOHMANN_JSON_VERSION_MAJOR, NLOHMANN_JSON_VERSION_MINOR, NLOHMANN_JSON_VERSION_PATCH","text":"

        These macros are defined by the library and contain the version numbers according to Semantic Versioning 2.0.0.

        See full documentation of NLOHMANN_JSON_VERSION_MAJOR, NLOHMANN_JSON_VERSION_MINOR, and NLOHMANN_JSON_VERSION_PATCH.

        "},{"location":"features/merge_patch/","title":"JSON Merge Patch","text":"

        The library supports JSON Merge Patch (RFC 7396) as a patch format. The merge patch format is primarily intended for use with the HTTP PATCH method as a means of describing a set of modifications to a target resource's content. This function applies a merge patch to the current JSON value.

        Instead of using JSON Pointer to specify values to be manipulated, it describes the changes using a syntax that closely mimics the document being modified. Unlike JSON Patch, a JSON Merge Patch cannot express every kind of change (e.g., it cannot reorder array elements or remove a specific array element), but it is easier to read and write for object-shaped documents.

        The function merge_diff creates a merge patch that changes one JSON value into another.

        Not a general deep merge

        A merge patch is not a general deep merge: a null value in the patch deletes the key from the target. To merge two objects recursively (e.g., defaults and user settings), use update with merge_objects set to true; see Merging objects.

        Example

        The following code shows how a JSON Merge Patch is applied to a JSON document.

        #include <iostream>\n#include <nlohmann/json.hpp>\n#include <iomanip> // for std::setw\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // the original document\n    json document = R\"({\n                \"title\": \"Goodbye!\",\n                \"author\": {\n                    \"givenName\": \"John\",\n                    \"familyName\": \"Doe\"\n                },\n                \"tags\": [\n                    \"example\",\n                    \"sample\"\n                ],\n                \"content\": \"This will be unchanged\"\n            })\"_json;\n\n    // the patch\n    json patch = R\"({\n                \"title\": \"Hello!\",\n                \"phoneNumber\": \"+01-123-456-7890\",\n                \"author\": {\n                    \"familyName\": null\n                },\n                \"tags\": [\n                    \"example\"\n                ]\n            })\"_json;\n\n    // apply the patch\n    document.merge_patch(patch);\n\n    // output original and patched document\n    std::cout << std::setw(4) << document << std::endl;\n}\n

        Output:

        {\n    \"author\": {\n        \"givenName\": \"John\"\n    },\n    \"content\": \"This will be unchanged\",\n    \"phoneNumber\": \"+01-123-456-7890\",\n    \"tags\": [\n        \"example\"\n    ],\n    \"title\": \"Hello!\"\n}\n
        "},{"location":"features/merge_patch/#see-also","title":"See also","text":"
        • JSON Patch and Diff - a more expressive alternative that describes a sequence of operations
        • JSON Pointer - the addressing scheme used by JSON Patch
        • Function merge_patch
        • Function merge_diff
        • Function update - merge objects, optionally recursively
        "},{"location":"features/modifying_values/","title":"Modifying values","text":"

        Once a JSON value exists, its content can be changed: elements can be added, replaced, merged, and removed. This page gives an overview of the available operations. For read access, see element access.

        "},{"location":"features/modifying_values/#adding-to-arrays","title":"Adding to arrays","text":"

        New elements are appended to an array with push_back or constructed in place with emplace_back. If the value is null, it is converted to an array first, so these functions can also be used to build an array from scratch.

        json j;                 // null\nj.push_back(1);         // [1]\nj.push_back(2);         // [1,2]\nj.emplace_back(3);      // [1,2,3]\n\n// operator+= is a shorthand for push_back\nj += 4;                 // [1,2,3,4]\n
        "},{"location":"features/modifying_values/#adding-to-objects","title":"Adding to objects","text":"

        The most common way to add or replace a member is operator[], which inserts the key if it does not exist yet:

        json j;\nj[\"name\"] = \"Mary\";     // {\"name\":\"Mary\"}\nj[\"name\"] = \"John\";     // {\"name\":\"John\"}  (replaced)\n

        emplace inserts a member only if the key is not already present, and reports whether the insertion happened \u2014 useful for \"add if absent\" semantics.

        "},{"location":"features/modifying_values/#merging-objects","title":"Merging objects","text":"

        To merge one object into another, update copies all members from another object (similar to Python's dict.update). This is the idiomatic way to combine two objects. It has two modes:

        • By default, the merge is shallow: existing keys are overwritten, even if both values are objects.
        • With merge_objects = #!cpp true, keys whose values are objects in both JSON values are merged recursively. Everything else is overwritten. In particular, arrays are replaced, not concatenated.
        Example 
        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // create two JSON objects\n    json o1 = R\"( {\"color\": \"red\", \"price\": 17.99, \"names\": {\"de\": \"Flugzeug\"}} )\"_json;\n    json o2 = R\"( {\"color\": \"blue\", \"speed\": 100, \"names\": {\"en\": \"plane\"}} )\"_json;\n    json o3 = o1;\n\n    // add all keys from o2 to o1 (updating \"color\", replacing \"names\")\n    o1.update(o2);\n\n    // add all keys from o2 to o1 (updating \"color\", merging \"names\")\n    o3.update(o2, true);\n\n    // output updated object o1 and o3\n    std::cout << std::setw(2) << o1 << '\\n';\n    std::cout << std::setw(2) << o3 << '\\n';\n}\n

        Output:

        {\n  \"color\": \"blue\",\n  \"names\": {\n    \"en\": \"plane\"\n  },\n  \"price\": 17.99,\n  \"speed\": 100\n}\n{\n  \"color\": \"blue\",\n  \"names\": {\n    \"de\": \"Flugzeug\",\n    \"en\": \"plane\"\n  },\n  \"price\": 17.99,\n  \"speed\": 100\n}\n

        A common use of the recursive mode is combining defaults with user settings. Nested defaults that the user did not set are kept:

        json defaults = {{\"log\", {{\"level\", \"info\"}, {\"file\", \"app.log\"}}}, {\"retries\", 3}};\njson user_settings = {{\"log\", {{\"level\", \"debug\"}}}};\n\njson config = defaults;\nconfig.update(user_settings, true);\n// {\"log\":{\"file\":\"app.log\",\"level\":\"debug\"},\"retries\":3}\n

        JSON Merge Patch (RFC 7396) also merges objects recursively, but it is a different tool: a null in the patch means \"remove this key\". It is meant for applying merge patch documents (e.g., received via HTTP PATCH). To merge configuration-like objects, use update(..., true). To apply a sequence of well-defined edit operations, see JSON Patch.

        "},{"location":"features/modifying_values/#removing-elements","title":"Removing elements","text":"

        Elements are removed with erase, which accepts an object key, an array index, or an iterator. clear empties a value while keeping its type, and operator[] combined with assignment can overwrite a value entirely.

        json j = {{\"a\", 1}, {\"b\", 2}, {\"c\", 3}};\nj.erase(\"b\");           // {\"a\":1,\"c\":3}\n\njson a = {1, 2, 3, 4};\na.erase(1);             // [1,3,4]  (erase by index)\n
        "},{"location":"features/modifying_values/#see-also","title":"See also","text":"
        • push_back / emplace_back - append to an array
        • emplace - insert into an object if the key is absent
        • update - merge objects (shallow, or recursive with merge_objects)
        • merge_patch - apply an RFC 7396 merge patch
        • erase / clear - remove elements
        • JSON Patch and Diff and JSON Merge Patch - structured modifications
        "},{"location":"features/modules/","title":"Modules","text":"

        This library has experimental support for C++ modules, introduced in C++20. The library can be imported by writing import nlohmann.json; instead of #include <nlohmann/json.hpp>.

        Please be aware that the module is experimental and a full test is outstanding, and the exported symbols are subject to change.

        "},{"location":"features/modules/#requirements","title":"Requirements","text":"

        The nlohmann.json module requires that the build system is configured to build and resolve modules when imported. Obviously, as modules were introduced in C++20, this feature can only be used in C++20 and subsequent versions.

        To enable building the nlohmann.json module (which is not done by default), the macro NLOHMANN_JSON_BUILD_MODULES must be passed to the build system.

        "},{"location":"features/modules/#example","title":"Example","text":"

        When using modules rather than headers, the previous example for creating a json object through a JSON file, would instead be: 

        import std;\nimport nlohmann.json;\n\nusing json = nlohmann::json;\n\n// ...\n\nstd::ifstream f(\"example.json\");\njson data = json::parse(f);\n

        "},{"location":"features/modules/#modules-do-not-export-macros","title":"Modules do not export macros","text":"

        It should be noted that as modules do not export macros, the nlohmann.json module will not export any macros.

        "},{"location":"features/modules/#exported-symbols","title":"Exported symbols","text":"

        Only the following symbols are exported from nlohmann.json:

        • nlohmann::adl_serializer
        • nlohmann::basic_json
        • nlohmann::basic_json_document
        • nlohmann::basic_json_view
        • nlohmann::json
        • nlohmann::json_document
        • nlohmann::json_pointer
        • nlohmann::json_view
        • nlohmann::ordered_json
        • nlohmann::ordered_json_document
        • nlohmann::ordered_json_view
        • nlohmann::ordered_map
        • nlohmann::to_string
        • nlohmann::literals::json_literals::operator\"\"_json
        • nlohmann::literals::json_literals::operator\"\"_json_pointer

        The module always exports the two user-defined string literals, even if JSON_NO_AUTOMATIC_UDLS is defined when building it.

        Additionally, the following nlohmann::detail symbols are exported, solely to work around an MSVC compilation issue (#3970). They are implementation details, not part of the public API, and should not be used directly:

        • nlohmann::detail::json_sax_dom_callback_parser
        • nlohmann::detail::unknown_size
        "},{"location":"features/modules/#known-issues","title":"Known issues","text":"

        C++20 modules support is exercised in CI against current GCC and Clang on Ubuntu, and the default MSVC toolset on Windows Server 2022 \u2014 there is no documented minimum compiler version, unlike feature-test-macro-gated features such as JSON_HAS_RANGES.

        Known compiler issues

        • GCC may emit \"redefinition\" errors when #include <nlohmann/json.hpp> appears in a module preamble together with other imports. This is an upstream GCC bug, not yet resolved as of GCC 16. Workarounds: include nlohmann/json.hpp before other #includes, use import nlohmann.json; instead, or upgrade GCC. (issue #5103)
        • MSVC could fail with C2039: 'json_sax_dom_callback_parser' is not a member of ... detail; fixed by exporting the required internal symbols from json.cppm (see Exported symbols above). (issue #3970)

        If you hit a different module-related build failure, search existing issues before filing a new one.

        "},{"location":"features/namespace/","title":"nlohmann Namespace","text":"

        The 3.11.0 release introduced an inline namespace to allow different parts of a codebase to safely use different versions of the JSON library as long as they never exchange instances of library types.

        "},{"location":"features/namespace/#structure","title":"Structure","text":"

        The complete default namespace name is derived as follows:

        • The root namespace is always nlohmann.
        • The inline namespace starts with json_abi and is followed by several optional ABI tags according to the value of these ABI-affecting macros, in order:
          • JSON_DIAGNOSTICS defined non-zero appends _diag.
          • JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON defined non-zero appends _ldvcmp.
          • JSON_DIAGNOSTIC_POSITIONS defined non-zero appends _dp.
          • JSON_BRACE_INIT_COPY_SEMANTICS defined non-zero appends _bics.
          • JSON_PRECISE_STREAM_POSITION defined non-zero appends _psp.
          • JSON_STRICT_NUL_HANDLING defined non-zero appends _snul.
          • JSON_STRICT_BINARY_UTF8 defined non-zero appends _sbu8.
          • JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS defined non-zero appends _ekmo.
          • JSON_DISABLE_STREAM_BULK_SCAN defined non-zero appends _dsbs.
          • JSON_DISABLE_UNLOCKED_FILE_IO defined non-zero appends _dufio.
          • JSON_VIEW_NO_SIMD defined appends _vns.
          • JSON_VIEW_USE_SSSE3 defined appends _vssse3.
        • The inline namespace ends with the suffix _v followed by the 3 components of the version number separated by underscores. To omit the version component, see Disabling the version component below.

        For example, the namespace name for version 3.11.2 with JSON_DIAGNOSTICS defined to 1 is:

        nlohmann::json_abi_diag_v3_11_2\n
        "},{"location":"features/namespace/#purpose","title":"Purpose","text":"

        Several incompatibilities have been observed. Amongst the most common ones is linking code compiled with different definitions of JSON_DIAGNOSTICS. This is illustrated in the diagram below.

        graph\n    json[\"<strong>nlohmann_json (v3.10.5)</strong><br>JSON_DIAGNOSTICS=0\"]\n    json_diag[\"<strong>nlohmann_json (v3.10.5)</strong><br>JSON_DIAGNOSTICS=1\"]\n    library[\"<strong>some library</strong>\"]\n    app[\"<strong>application</strong>\"]\n\n    library --> json\n    app --> json_diag\n    app --> library

        In releases prior to 3.11.0, mixing any version of the JSON library with different JSON_DIAGNOSTICS settings would result in a crashing application. If some_library never passes instances of JSON library types to the application, this scenario became safe in version 3.11.0 and above due to the inline namespace yielding distinct symbol names.

        "},{"location":"features/namespace/#limitations","title":"Limitations","text":"

        Neither the compiler nor the linker will issue as much as a warning when translation units \u2013 intended to be linked together and that include different versions and/or configurations of the JSON library \u2013 exchange and use library types.

        There is an exception when forward declarations are used (i.e., when including json_fwd.hpp) in which case the linker may complain about undefined references.

        "},{"location":"features/namespace/#disabling-the-version-component","title":"Disabling the version component","text":"

        Different versions are not necessarily ABI-incompatible, but the project does not actively track changes in the ABI and recommends that all parts of a codebase exchanging library types be built with the same version. Users can, at their own risk, disable the version component of the inline namespace, allowing different versions \u2013 but not configurations \u2013 to be used in cases where the linker would otherwise output undefined reference errors.

        To do so, define NLOHMANN_JSON_NAMESPACE_NO_VERSION to 1.

        This applies to version 3.11.2 and above only; versions 3.11.0 and 3.11.1 can apply the technique described in the next section to emulate the effect of the NLOHMANN_JSON_NAMESPACE_NO_VERSION macro.

        Use at your own risk

        Disabling the namespace version component and mixing ABI-incompatible versions will result in crashes or incorrect behavior. You have been warned!

        "},{"location":"features/namespace/#disabling-the-inline-namespace-completely","title":"Disabling the inline namespace completely","text":"

        When interoperability with code using a pre-3.11.0 version of the library is required, users can, at their own risk restore the old namespace layout by redefining NLOHMANN_JSON_NAMESPACE_BEGIN, NLOHMANN_JSON_NAMESPACE_END as follows:

        #define NLOHMANN_JSON_NAMESPACE_BEGIN  namespace nlohmann {\n#define NLOHMANN_JSON_NAMESPACE_END    }\n

        Use at your own risk

        Overriding the namespace and mixing ABI-incompatible versions will result in crashes or incorrect behavior. You have been warned!

        "},{"location":"features/namespace/#version-history","title":"Version history","text":"
        • Introduced inline namespace (json_v3_11_0[_abi-tag]*) in version 3.11.0.
        • Changed structure of inline namespace in version 3.11.2.
        • Added ABI tag _dp (JSON_DIAGNOSTIC_POSITIONS) in version 3.12.0.
        • Added ABI tags _bics (JSON_BRACE_INIT_COPY_SEMANTICS), _psp (JSON_PRECISE_STREAM_POSITION), _snul (JSON_STRICT_NUL_HANDLING), _sbu8 (JSON_STRICT_BINARY_UTF8), _ekmo (JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS), _dsbs (JSON_DISABLE_STREAM_BULK_SCAN), _dufio (JSON_DISABLE_UNLOCKED_FILE_IO), _vns (JSON_VIEW_NO_SIMD), and _vssse3 (JSON_VIEW_USE_SSSE3) in version 3.13.0 unreleased.
        "},{"location":"features/object_order/","title":"Object Order","text":"

        The JSON standard defines objects as \"an unordered collection of zero or more name/value pairs\". As such, an implementation does not need to preserve any specific order of object keys.

        "},{"location":"features/object_order/#default-behavior-sort-keys","title":"Default behavior: sort keys","text":"

        The default type nlohmann::json uses a std::map to store JSON objects, and thus stores object keys sorted alphabetically.

        Example: json sorts object keys 
        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    json j;\n    j[\"one\"] = 1;\n    j[\"two\"] = 2;\n    j[\"three\"] = 3;\n\n    std::cout << j.dump(2) << '\\n';\n}\n

        Output:

        {\n  \"one\": 1,\n  \"three\": 3,\n  \"two\": 2\n}\n
        "},{"location":"features/object_order/#alternative-behavior-preserve-insertion-order","title":"Alternative behavior: preserve insertion order","text":"

        If you do want to preserve the insertion order, you can use the type nlohmann::ordered_json.

        Example: ordered_json preserves insertion order 
        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing ordered_json = nlohmann::ordered_json;\n\nint main()\n{\n    ordered_json j;\n    j[\"one\"] = 1;\n    j[\"two\"] = 2;\n    j[\"three\"] = 3;\n\n    std::cout << j.dump(2) << '\\n';\n}\n

        Output:

        {\n  \"one\": 1,\n  \"two\": 2,\n  \"three\": 3\n}\n

        Alternatively, nlohmann::fifo_map also preserves the insertion order and, unlike ordered_map, keeps a lookup index, so it does not have the quadratic cost described below. It is used through a small adapter (integration).

        If the order does not matter and you only want faster lookup, boost::unordered_flat_map, absl::flat_hash_map, absl::node_hash_map, and several other hash maps work through an adapter that restores the template argument order basic_json expects; see Template Parameter Requirements. Note these are unordered, not insertion-ordered.

        tsl::ordered_map cannot be used: its iterators expose the mapped value as const, while basic_json needs to modify it in place.

        The ordered_map behind nlohmann::ordered_json is deliberately minimal and has no lookup index, so every key access is a linear scan and building an object of n keys costs O(n\u00b2). This is unnoticeable at typical object sizes but becomes significant for objects with many thousands of keys; see ordered_map complexity. The alternatives above keep a lookup index and do not have this cost.

        "},{"location":"features/object_order/#notes-on-parsing","title":"Notes on parsing","text":"

        Note that you also need to call the right parse function when reading from a file. Assume file input.json contains the JSON object above:

        {\n  \"one\": 1,\n  \"two\": 2,\n  \"three\": 3\n}\n

        Right way

        The following code correctly calls the parse function from nlohmann::ordered_json:

        std::ifstream i(\"input.json\");\nauto j = nlohmann::ordered_json::parse(i);\nstd::cout << j.dump(2) << std::endl;\n

        The output will be:

        {\n  \"one\": 1,\n  \"two\": 2,\n  \"three\": 3\n}\n
        Wrong way

        The following code incorrectly calls the parse function from nlohmann::json which does not preserve the insertion order, but sorts object keys. Assigning the result to nlohmann::ordered_json compiles, but does not restore the order from the input file.

        std::ifstream i(\"input.json\");\nnlohmann::ordered_json j = nlohmann::json::parse(i);\nstd::cout << j.dump(2) << std::endl;\n

        The output will be:

        {\n  \"one\": 1,\n  \"three\": 3,\n  \"two\": 2\n}\n
        "},{"location":"features/performance/","title":"Performance","text":"

        Speed was never the primary goal of this library. The design goals page says so plainly: \"There are certainly faster JSON libraries out there.\" Intuitive syntax, trivial integration, and thorough testing came first. If a hard real-time budget or the last percent of throughput matters more than convenience, a faster, more specialized library may be a better fit.

        That said, how you use this library still makes a measurable difference. This page collects practical, code-verified techniques for reducing time, memory, and compile-time cost -- without repeating the detailed pages it links to.

        "},{"location":"features/performance/#parsing-input","title":"Parsing input","text":"

        parse accepts a string, a pair of iterators, a container, a std::istream, or a FILE* (see Parsing). Internally, every input is wrapped in an input adapter, and not all adapters are equally fast.

        For inputs backed by contiguous, single-byte memory -- a std::string, a std::vector<char>, a string literal, or a pointer range -- the library uses iterator_input_adapter, wrapped in a raw pointer so the fast paths below apply on every supported standard. This adapter exposes two optimizations the lexer detects at compile time:

        • it can reconstruct already-consumed input on demand for error messages, instead of copying every character as it is read, and
        • the lexer can scan ordinary string characters, numbers, and indentation directly out of the buffer, several bytes at a time, rather than one character (and one function call) at a time.

        A std::istream (including std::ifstream) gets the same bulk scanning: input_stream_adapter scans the characters its stream buffer already holds (the get area) and asks the stream buffer for more only when they are used up. Nothing is read ahead, so the stream is left exactly where it would be after reading one character at a time, and operator>> can still read several values from one stream. A stream buffer without a get area is read one character at a time. A FILE* is still read one character at a time, but file_input_adapter locks the file once per parse instead of once per character. See input sources and performance for the full table. According to the description of #5828, parsing from a stream is within about 10 to 20 % of parsing an equivalent std::string.

        Inputs that are still read one character at a time: iterator ranges that are not contiguous (e.g. std::list<char> or std::istreambuf_iterator<char>), wide strings, and user-defined adapters. An iterator pair over non-contiguous but random-access storage (e.g. std::deque<char>::iterator) gets the first optimization but not the second, since the byte-scanning fast path requires contiguous storage.

        Practically: a file can be passed directly to parse, which also avoids the memory for a copy of the file. Reading it into a string first is no longer necessary for speed:

        // fast: the stream buffer's get area is scanned in bulk\nstd::ifstream f(\"example.json\");\njson j = json::parse(f);\n

        Two macros switch the stream optimizations off without changing any result: JSON_DISABLE_STREAM_BULK_SCAN for stream buffers that do not follow the usual std::streambuf contract, and JSON_DISABLE_UNLOCKED_FILE_IO for programs in which another thread must use the same FILE object while it is parsed.

        For contiguous input with many non-ASCII characters, JSON_USE_SIMDUTF can additionally speed up UTF-8 validation by using the simdutf library instead of the built-in scalar validator; streaming inputs (files, std::istream, wide strings, user-defined adapters) always use the scalar path regardless of this macro.

        "},{"location":"features/performance/#large-documents","title":"Large documents","text":"

        Parsing always produces SAX events internally; parse simply feeds them to a consumer that builds a complete basic_json value tree (a DOM) in memory. For documents too large to comfortably hold as a DOM, three alternatives avoid building it:

        • Implement the SAX interface directly and pass it to sax_parse; only the parts of the input you choose to keep ever become basic_json values.
        • Pass a parser callback to parse. This still builds a DOM, but the callback can discard finished elements as soon as they are handled, so memory usage stays bounded by one element (plus the unparsed remainder of the input) instead of the whole document -- see the recipe for streaming a large homogeneous array.
        • Parse into a json_document (<nlohmann/json_view.hpp>) instead of a basic_json. It keeps the input text and builds a flat index of 16 bytes per value; strings and floating-point numbers are not copied, but read from the text when needed. Read-only views give the familiar element access, and only the parts you materialize() become basic_json values. A document that borrows the text instead of owning a copy needs the text to outlive it; see choosing between json, the SAX interface, and json_view.

        If the data is naturally record-oriented, consider JSON Lines instead of one large JSON document: reading and parsing it line by line with std::getline means only one line's value is ever in memory at a time, and a malformed line does not invalidate lines already processed.

        "},{"location":"features/performance/#binary-formats","title":"Binary formats","text":"

        JSON text is not a compact format. If the data is only exchanged between programs (not read by humans), the binary formats -- BJData, BON8, BSON, CBOR, MessagePack, and UBJSON -- encode the same values more compactly, which reduces both the bytes transferred and, for most of them, the work needed to parse them back. The size comparison on that page, measured against minified JSON for four reference documents, shows the effect varies a lot by document shape: CBOR and MessagePack come out at 50.5% of the minified JSON size for the numeric-array-heavy canada.json, but only around 87-88% for the string-heavy jeopardy.json, where there is less numeric data to encode more compactly. BON8 is the most compact option in that comparison for text-heavy documents (63.5%-87.5%), at the cost of an incomplete serializer (no unsigned integers above int64). Which format -- and whether it is worth the loss of human readability at all -- depends on the actual data; see the comparison tables before choosing one.

        "},{"location":"features/performance/#object-type-json-vs-ordered_json","title":"Object type: json vs. ordered_json","text":"

        The default json type stores object keys in a std::map, giving logarithmic-time lookup, insertion, and erasure, at the cost of sorting keys alphabetically rather than preserving insertion order (see Object Order). ordered_json uses nlohmann::ordered_map instead, a std::vector-backed container with no lookup index: every key-based operation is a linear scan, so building an object of n distinct keys costs O(n\u00b2) in total -- this applies equally to inserting keys one by one and to parsing an object, since the parser inserts each key as it is read. The measurements on the ordered_map page show this is negligible at typical object sizes (2000 keys: 0.7 ms for json vs. 3.6 ms for ordered_json, a 5x factor) but grows steeply for large, machine-generated objects (16 000 keys: 3.3 ms vs. 181.6 ms, a 54x factor).

        If insertion order matters and an object routinely has many thousands of keys, ordered_json's quadratic build cost may not be acceptable. The library's ObjectType template parameter can be set to a different container instead: nlohmann::fifo_map keeps insertion order with a real lookup index (avoiding the quadratic cost), while std::unordered_map, boost::unordered_flat_map, absl::flat_hash_map, and similar hash maps trade insertion order for average-case constant-time lookup (through an adapter, since their template argument order does not match what basic_json expects) -- see Object Order for the full list.

        "},{"location":"features/performance/#avoiding-copies","title":"Avoiding copies","text":"
        • Move instead of copy. Constructing a basic_json from an existing one is linear in its size for the copy constructor but constant for the move constructor. The same applies to assigning a large std::string, std::vector, or other container into a value: pass it as std::move(x) rather than x whenever x is no longer needed afterwards.
        • Access without copying. get<T>() returns a copy of the stored value converted to T. When a reference or pointer to the value already stored inside the basic_json is enough, get_ref() and get_ptr() access it directly: both pages state, word for word, \"No copies are made.\" -- at the cost of that reference or pointer becoming invalid once the underlying value changes.
        • Iterate by reference. basic_json::iterator::operator*() returns a reference (an alias for basic_json&), but a range-based for loop with a by-value loop variable (for (auto el : j)) still copies each element, because plain auto drops the reference. Write for (const auto& el : j) (or auto& for a mutable loop), and use items() the same way when the key is needed too -- its own examples use for (auto& el : j.items()).
        • Construct in place. emplace_back() (arrays, amortized constant time) and emplace() (objects, logarithmic in the size of the container for json) forward their arguments directly to a basic_json constructor, rather than requiring a temporary value to be constructed and then copied or moved in. push_back() has an rvalue overload (push_back(basic_json&&)) for a value that already exists: j.push_back(std::move(value)) moves it in instead of copying it.
        • Skip the bounds check when it is redundant. at() and operator[] have the same complexity (constant for a valid array index, logarithmic for an object key in json) -- the difference is that at() additionally checks the key or index and throws if it is invalid, while operator[] does not (see unchecked access and checked access). Prefer operator[] when the surrounding code has already established that the access is valid.
        • Reserve array capacity. basic_json has no public reserve(), but when building a large array incrementally with a known final size, get_ref() exposes the underlying array_t so it can be reserved directly -- see \"reserving array capacity\" for the one-line recipe.
        "},{"location":"features/performance/#serialization","title":"Serialization","text":"

        dump() with the default indent = -1 selects \"the most compact representation\" (word for word from the page); any non-negative indent pretty-prints instead, which is more readable but produces more bytes and more work. dump() builds and returns a complete string_t containing the whole serialization. operator<< writes directly to a std::ostream instead, through the same serializer, but without ever materializing that intermediate string -- so if the destination is a stream (a file, or std::cout), os << j; avoids the allocation and copy that os << j.dump(); would incur for large values.

        "},{"location":"features/performance/#diagnostics-overhead","title":"Diagnostics overhead","text":"

        Two opt-in macros add diagnostic information to exceptions and to every value, at a cost that is only worth paying while it is in use:

        • JSON_DIAGNOSTICS adds a JSON Pointer to exception messages, pointing at the value that triggered the exception. Quoting the page directly: \"enabling this macro increases the size of every JSON value by one pointer and adds some runtime overhead\" -- every value gains a parent pointer that has to be kept up to date as the document is built and modified.
        • JSON_DIAGNOSTIC_POSITIONS adds start_pos() and end_pos(), the byte offsets a value occupied in its parsed input. Quoting the page: \"enabling this macro increases the size of every JSON value by two std::size_t fields and adds slight runtime overhead to parsing, copying JSON value objects, and the generation of error messages for exceptions.\"

        Both default to off. Enable them where better diagnostics are worth the overhead (for example, while validating untrusted input, or in a debug build), and keep them off in a release build that does not need them.

        "},{"location":"features/performance/#compile-time","title":"Compile time","text":"

        <nlohmann/json_fwd.hpp> forward-declares basic_json, json, ordered_json, json_pointer, and adl_serializer, pulling in only a handful of lightweight standard headers instead of the full json.hpp. A header that only needs to name nlohmann::json -- in a function signature or a class member declaration, for instance -- can include json_fwd.hpp and leave #include <nlohmann/json.hpp> to the source files that actually parse, build, or serialize values, the same way a project would forward-declare any other heavy class to keep it out of widely-included headers:

        // my_type.hpp\n#include <nlohmann/json_fwd.hpp>\n\nclass my_type\n{\n    nlohmann::json config() const;\n};\n\n// my_type.cpp\n#include <nlohmann/json.hpp>\n#include \"my_type.hpp\"\n\nnlohmann::json my_type::config() const { /* ... */ }\n

        One caveat: ABI-affecting macros such as JSON_DIAGNOSTICS and JSON_DIAGNOSTIC_POSITIONS are encoded into the library's inline namespace name. Every translation unit -- whether it includes json_fwd.hpp or the full header -- must define them the same way, or linking fails with undefined references instead of a compile error.

        If I/O support is not needed at all, JSON_NO_IO excludes <cstdio>, <ios>, <iosfwd>, <istream>, and <ostream> outright and drops the std::istream/FILE* parse overloads and operator<< that depend on them (dump() itself is unaffected, since it only returns a string); it exists for environments where those headers are unavailable (such as Intel SGX), and as a side effect those headers are then never processed by the compiler at all.

        "},{"location":"features/performance/#see-also","title":"See also","text":"
        • Design goals - why this library does not optimize for speed first
        • Architecture - how input adapters, the lexer, and the serializer fit together
        • Parsing - the available parsing functions and inputs
        • SAX interface - parse without building a DOM
        • Zero-copy JSON views - parse into a flat index of the text and read it without building a DOM
        • Binary formats - compact alternatives to JSON text
        • Object Order - json vs. ordered_json and other ObjectType choices
        • Template Parameter Requirements - custom container and allocator types
        • Supported macros - overview of all configuration macros, including the diagnostics ones above
        "},{"location":"features/serialization/","title":"Serialization","text":"

        Serialization is the process of turning a JSON value back into JSON text. It is the counterpart to parsing. The central function is dump, which returns the JSON text as a string.

        json j = {{\"pi\", 3.141}, {\"happy\", true}};\n\nstd::string s = j.dump();   // {\"happy\":true,\"pi\":3.141}\n

        To write a value directly to a stream (for example, a file or std::cout), the operator<< is provided:

        std::cout << j << std::endl;\n

        String, not raw value

        dump always returns a JSON text. Serializing a JSON string therefore includes the surrounding quotes and escapes special characters. To obtain the contained string value without quotes, use get<std::string>() instead of dump. See the converting values page.

        "},{"location":"features/serialization/#pretty-printing","title":"Pretty-printing","text":"

        By default, dump produces the most compact representation without any superfluous whitespace. Passing a non-negative indent argument pretty-prints the output with the given number of spaces per level:

        Example: pretty-print JSON values with dump() 
        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON values\n    json j_object = {{\"one\", 1}, {\"two\", 2}};\n    json j_array = {1, 2, 4, 8, 16};\n    json j_string = \"Hell\u00f6 \ud83d\ude00!\";\n\n    // call dump()\n    std::cout << \"objects:\" << '\\n'\n              << j_object.dump() << \"\\n\\n\"\n              << j_object.dump(-1) << \"\\n\\n\"\n              << j_object.dump(0) << \"\\n\\n\"\n              << j_object.dump(4) << \"\\n\\n\"\n              << j_object.dump(1, '\\t') << \"\\n\\n\";\n\n    std::cout << \"arrays:\" << '\\n'\n              << j_array.dump() << \"\\n\\n\"\n              << j_array.dump(-1) << \"\\n\\n\"\n              << j_array.dump(0) << \"\\n\\n\"\n              << j_array.dump(4) << \"\\n\\n\"\n              << j_array.dump(1, '\\t') << \"\\n\\n\";\n\n    std::cout << \"strings:\" << '\\n'\n              << j_string.dump() << '\\n'\n              << j_string.dump(-1, ' ', true) << '\\n';\n\n    // create JSON value with invalid UTF-8 byte sequence\n    json j_invalid = \"\u00e4\\xA9\u00fc\";\n    try\n    {\n        std::cout << j_invalid.dump() << std::endl;\n    }\n    catch (const json::type_error& e)\n    {\n        std::cout << e.what() << std::endl;\n    }\n\n    std::cout << \"string with replaced invalid characters: \"\n              << j_invalid.dump(-1, ' ', false, json::error_handler_t::replace)\n              << \"\\nstring with ignored invalid characters: \"\n              << j_invalid.dump(-1, ' ', false, json::error_handler_t::ignore)\n              << '\\n';\n}\n

        Output:

        objects:\n{\"one\":1,\"two\":2}\n\n{\"one\":1,\"two\":2}\n\n{\n\"one\": 1,\n\"two\": 2\n}\n\n{\n    \"one\": 1,\n    \"two\": 2\n}\n\n{\n    \"one\": 1,\n    \"two\": 2\n}\n\narrays:\n[1,2,4,8,16]\n\n[1,2,4,8,16]\n\n[\n1,\n2,\n4,\n8,\n16\n]\n\n[\n    1,\n    2,\n    4,\n    8,\n    16\n]\n\n[\n    1,\n    2,\n    4,\n    8,\n    16\n]\n\nstrings:\n\"Hell\u00f6 \ud83d\ude00!\"\n\"Hell\\u00f6 \\ud83d\\ude00!\"\n[json.exception.type_error.316] invalid UTF-8 byte at index 2: 0xA9\nstring with replaced invalid characters: \"\u00e4\ufffd\u00fc\"\nstring with ignored invalid characters: \"\u00e4\u00fc\"\n

        The indentation character can be changed with the second argument (e.g., a tab '\\t'). An indent of 0 inserts newlines but no leading spaces, and the default of -1 selects the compact single-line form.

        "},{"location":"features/serialization/#non-ascii-characters","title":"Non-ASCII characters","text":"

        Strings are stored and serialized as UTF-8 (see types). By default, dump copies valid non-ASCII characters as-is. Setting the third argument ensure_ascii to true escapes all non-ASCII characters with \\uXXXX sequences, so that the output contains only ASCII characters:

        json j = \"\u82f9\u679c\";\nj.dump();               // \"\u82f9\u679c\"\nj.dump(-1, ' ', true);  // \"\\u82f9\\u679c\"\n
        "},{"location":"features/serialization/#handling-invalid-utf-8","title":"Handling invalid UTF-8","text":"

        If a string contains invalid UTF-8 sequences (for example, because it holds data in another encoding such as Latin-1), serialization fails by default. The fourth argument of dump selects an error_handler:

        • strict (default) \u2014 throw a type_error.316 exception.
        • replace \u2014 replace invalid bytes with the Unicode replacement character U+FFFD (\ufffd).
        • ignore \u2014 silently drop invalid bytes.
        • keep \u2014 copy invalid bytes to the output unchanged; the result is not valid UTF-8.
        Example: serialize invalid UTF-8 with different error handlers 
        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create JSON value with invalid UTF-8 byte sequence\n    json j_invalid = \"\u00e4\\xA9\u00fc\";\n    try\n    {\n        std::cout << j_invalid.dump() << std::endl;\n    }\n    catch (const json::type_error& e)\n    {\n        std::cout << e.what() << std::endl;\n    }\n\n    std::cout << \"string with replaced invalid characters: \"\n              << j_invalid.dump(-1, ' ', false, json::error_handler_t::replace)\n              << \"\\nstring with ignored invalid characters: \"\n              << j_invalid.dump(-1, ' ', false, json::error_handler_t::ignore)\n              << \"\\nstring with the invalid byte kept as is (\" << j_invalid.dump(-1, ' ', false, json::error_handler_t::keep).size()\n              << \" bytes, not valid UTF-8 itself)\\n\";\n}\n

        Output:

        [json.exception.type_error.316] invalid UTF-8 byte at index 2: 0xA9\nstring with replaced invalid characters: \"\u00e4\ufffd\u00fc\"\nstring with ignored invalid characters: \"\u00e4\u00fc\"\nstring with the invalid byte kept as is (7 bytes, not valid UTF-8 itself)\n

        Avoiding invalid UTF-8

        The best fix is to ensure that all strings are UTF-8 encoded before storing them. See the FAQ on non-ASCII characters for how to convert wide or Latin-1 strings.

        "},{"location":"features/serialization/#numbers-nan-and-binary-values","title":"Numbers, NaN, and binary values","text":"
        • Numbers are serialized with enough precision to round-trip; see number serialization.
        • NaN and infinity cannot be represented in JSON and are serialized as null; see NaN handling. The binary formats can preserve them.
        • Binary values have no JSON representation and are serialized as a helper object for debugging only; see binary values.
        "},{"location":"features/serialization/#using-stdformat-stdprint-and-fmt","title":"Using std::format, std::print, and fmt","text":"

        Since version 3.12.0, JSON values can be formatted directly with C++20's std::format whenever the standard library provides the <format> header (controlled by JSON_HAS_STD_FORMAT). This is enabled by the std::formatter<basic_json> specialization, which also makes JSON values work with std::format_to and with C++23's std::print/std::println:

        std::print(\"{}\", j);      // compact, like j.dump()\nstd::print(\"{:2}\", j);    // pretty-printed with indent 2 (like j.dump(2))\nstd::println(\"{:#}\", j);  // pretty-printed with the default indent\n

        The format spec mirrors the dump parameters: \"{:#}\" pretty-prints, a width such as \"{:2}\" sets the indent, and a fill-and-align prefix such as \"{:.>#}\" sets the indent character.

        For the {fmt} library, the library ships a format_as helper. Note its behavior depends on the fmt version; see the FAQ entry for the details and a recipe for a full fmt::formatter specialization.

        "},{"location":"features/serialization/#serializing-to-other-formats","title":"Serializing to other formats","text":"

        Besides JSON text, a value can also be serialized to the more compact binary formats (BJData, BON8, BSON, CBOR, MessagePack, UBJSON).

        "},{"location":"features/serialization/#see-also","title":"See also","text":"
        • dump - serialize to a JSON-formatted string
        • operator<< - serialize to a stream
        • to_string - user-defined-conversion helper
        • std::formatter<basic_json> - use JSON values with std::format and std::print
        • format_as - use JSON values with the {fmt} library
        • Parsing - the reverse operation
        "},{"location":"features/trailing_commas/","title":"Trailing Commas","text":"

        Like comments, this library does not support trailing commas in arrays and objects by default.

        You can set parameter ignore_trailing_commas to true in the parse function to allow trailing commas in arrays and objects. Note that a single comma as the only content of the array or object ([,] or {,}) is not allowed, and multiple trailing commas ([1,,]) are not allowed either.

        This library does not add trailing commas when serializing JSON data.

        For more information, see JSON With Commas and Comments (JWCC).

        Example

        Consider the following JSON with trailing commas.

        {\n    \"planets\": [\n        \"Mercury\",\n        \"Venus\",\n        \"Earth\",\n        \"Mars\",\n        \"Jupiter\",\n        \"Uranus\",\n        \"Neptune\",\n    ]\n}\n

        When calling parse without additional argument, a parse error exception is thrown. If ignore_trailing_commas is set to true, the trailing commas are ignored during parsing:

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::string s = R\"(\n    {\n        \"planets\": [\n            \"Mercury\",\n            \"Venus\",\n            \"Earth\",\n            \"Mars\",\n            \"Jupiter\",\n            \"Uranus\",\n            \"Neptune\",\n        ]\n    }\n    )\";\n\n    try\n    {\n        json j = json::parse(s);\n    }\n    catch (json::exception& e)\n    {\n        std::cout << e.what() << std::endl;\n    }\n\n    json j = json::parse(s,\n                         /* callback */ nullptr,\n                         /* allow exceptions */ true,\n                         /* ignore_comments */ false,\n                         /* ignore_trailing_commas */ true);\n    std::cout << j.dump(2) << '\\n';\n}\n

        Output:

        [json.exception.parse_error.101] parse error at line 11, column 9: syntax error while parsing value - unexpected ']'; expected '[', '{', or a literal\n{\n  \"planets\": [\n    \"Mercury\",\n    \"Venus\",\n    \"Earth\",\n    \"Mars\",\n    \"Jupiter\",\n    \"Uranus\",\n    \"Neptune\"\n  ]\n}\n
        "},{"location":"features/binary_formats/","title":"Binary Formats","text":"

        Though JSON is a ubiquitous data format, it is not a very compact format suitable for data exchange, for instance, over a network. Hence, the library supports

        • BJData (Binary JData),
        • BON8 (Binary Object Notation 8),
        • BSON (Binary JSON),
        • CBOR (Concise Binary Object Representation),
        • MessagePack, and
        • UBJSON (Universal Binary JSON)

        to efficiently encode JSON values to byte vectors and to decode such vectors.

        "},{"location":"features/binary_formats/#comparison","title":"Comparison","text":""},{"location":"features/binary_formats/#completeness","title":"Completeness","text":"Format Serialization Deserialization BJData complete complete BON8 incomplete: no unsigned integers above int64 complete BSON incomplete: top-level value must be an object incomplete, but all JSON types are supported CBOR complete incomplete, but all JSON types are supported MessagePack complete complete UBJSON complete complete"},{"location":"features/binary_formats/#binary-values","title":"Binary values","text":"Format Binary values Binary subtypes BJData not supported not supported BON8 not supported not supported BSON supported supported CBOR supported supported MessagePack supported supported UBJSON not supported not supported

        See binary values for more information.

        "},{"location":"features/binary_formats/#sizes","title":"Sizes","text":"Format canada.json twitter.json citm_catalog.json jeopardy.json BJData 53.2 % 91.1 % 78.1 % 96.6 % BJData (size) 58.6 % 92.1 % 86.7 % 97.4 % BJData (size+type) 58.6 % 92.1 % 86.5 % 97.4 % BON8 50.5 % 83.8 % 63.5 % 87.5 % BSON 85.8 % 95.2 % 95.8 % 106.7 % CBOR 50.5 % 86.3 % 68.4 % 88.0 % MessagePack 50.5 % 86.0 % 68.5 % 87.9 % UBJSON 53.2 % 91.3 % 78.2 % 96.6 % UBJSON (size) 58.6 % 92.3 % 86.8 % 97.4 % UBJSON (size+type) 55.9 % 92.3 % 85.0 % 95.0 %

        Sizes compared to minified JSON value.

        "},{"location":"features/binary_formats/bjdata/","title":"BJData","text":"

        The BJData format was derived from and improved upon Universal Binary JSON(UBJSON) specification (Draft 12). Specifically, it introduces an optimized array container for efficient storage of N-dimensional packed arrays (ND-arrays); it also adds 5 new type markers - [u] - uint16, [m] - uint32, [M] - uint64, [h] - float16 and [B] - byte - to unambiguously map common binary numeric types; furthermore, it uses little-endian (LE) to store all numerics instead of big-endian (BE) as in UBJSON to avoid unnecessary conversions on commonly available platforms.

        Compared to other binary JSON-like formats such as MessagePack and CBOR, both BJData and UBJSON demonstrate a rare combination of being both binary and quasi-human-readable. This is because all semantic elements in BJData and UBJSON, including the data-type markers and name/string types, are directly human-readable. Data stored in the BJData/UBJSON format is not only compact in size, fast to read/write, but also can be directly searched or read using simple processing.

        References

        • BJData Specification
        "},{"location":"features/binary_formats/bjdata/#serialization","title":"Serialization","text":"

        The library uses the following mapping from JSON values types to BJData types according to the BJData specification:

        JSON value type value/range BJData type marker null null null Z boolean true true T boolean false false F number_integer -9223372036854775808..-2147483649 int64 L number_integer -2147483648..-32769 int32 l number_integer -32768..-129 int16 I number_integer -128..127 int8 i number_integer 128..255 uint8 U number_integer 256..32767 int16 I number_integer 32768..65535 uint16 u number_integer 65536..2147483647 int32 l number_integer 2147483648..4294967295 uint32 m number_integer 4294967296..9223372036854775807 int64 L number_integer 9223372036854775808..18446744073709551615 uint64 M number_unsigned 0..127 int8 i number_unsigned 128..255 uint8 U number_unsigned 256..32767 int16 I number_unsigned 32768..65535 uint16 u number_unsigned 65536..2147483647 int32 l number_unsigned 2147483648..4294967295 uint32 m number_unsigned 4294967296..9223372036854775807 int64 L number_unsigned 9223372036854775808..18446744073709551615 uint64 M number_float any value float64 D string with shortest length indicator string S array see notes on optimized format/ND-array array [ object see notes on optimized format map { binary see notes on binary values array [$B

        Complete mapping

        The mapping is complete in the sense that any JSON value type can be converted to a BJData value.

        Any BJData output created by to_bjdata can be successfully parsed by from_bjdata.

        Size constraints

        The following values can not be converted to a BJData value:

        • strings with more than 18446744073709551615 bytes, i.e., 264-1 bytes (theoretical)

        UTF-8 validation of string values and object keys

        BJData strings must use UTF-8 encoding. By default (the error_handler parameter left at keep), to_bjdata() writes the bytes of string values and object keys unchanged, even if they are not valid UTF-8. With error_handler_t::strict, it throws type_error.316 for ill-formed UTF-8 instead; replace/ignore sanitize the string. JSON_STRICT_BINARY_UTF8 makes strict the default.

        Unused BJData markers

        The following markers are not used in the conversion:

        • Z: no-op values are not created.
        • C: single-byte strings are serialized with S markers.

        NaN/infinity handling

        If NaN or Infinity are stored inside a JSON number, they are serialized properly. This behavior differs from the dump() function which serializes NaN or Infinity to null.

        Endianness

        A breaking difference between BJData and UBJSON is the endianness of numerical values. In BJData, all numerical data types (integers UiuImlML and floating-point values hdD) are stored in the little-endian (LE) byte order as opposed to big-endian as used by UBJSON. Adopting LE to store numeric records avoids unnecessary byte swapping on most modern computers where LE is used as the default byte order.

        Optimized formats

        Optimized formats for containers are supported via two parameters of to_bjdata:

        • Parameter use_size adds size information to the beginning of a container and removes the closing marker.
        • Parameter use_type further checks whether all elements of a container have the same type and adds the type marker to the beginning of the container. The use_type parameter must only be used together with use_size = true.

        Note that use_size = true alone may result in larger representations - the benefit of this parameter is that the receiving side is immediately informed of the number of elements in the container.

        ND-array optimized format

        BJData extends UBJSON's optimized array size marker to support ND-arrays of uniform numerical data types (referred to as packed arrays). For example, the 2-D uint8 integer array [[1,2],[3,4],[5,6]], stored as nested optimized array in UBJSON [ [$U#i2 1 2 [$U#i2 3 4 [$U#i2 5 6 ], can be further compressed in BJData to [$U#[$i#i2 2 3 1 2 3 4 5 6 or [$U#[i2 i3] 1 2 3 4 5 6.

        To maintain type and size information, ND-arrays are converted to JSON objects following the annotated array format (defined in the JData specification (Draft 3)), when parsed using from_bjdata. For example, the above 2-D uint8 array can be parsed and accessed as

        {\n    \"_ArrayType_\": \"uint8\",\n    \"_ArraySize_\": [2,3],\n    \"_ArrayData_\": [1,2,3,4,5,6]\n}\n

        Likewise, when a JSON object in the above form is serialized using to_bjdata, it is automatically converted into a compact BJData ND-array.

        When parsing, an ND-array whose dimension vector is empty, contains a single integer, contains two integers with the first being 1, or contains a 0 is returned as a regular (possibly empty) array rather than an annotated object.

        An object is only converted if the annotation describes a packed array that is parsed back into the same annotated object; otherwise it is serialized as a regular JSON object, so the annotation is never lost in a round trip. This requires all of the following:

        • \"_ArrayType_\" is one of uint8, int8, uint16, int16, uint32, int32, uint64, int64, single, double, char, or byte,
        • \"_ArraySize_\" is an array, since the dimensions are written as the ND-array header's length,
        • \"_ArraySize_\" has at least two entries and is not a 1\u00d7N row vector (first entry 1), since other shapes are parsed back as a regular array,
        • every entry of \"_ArraySize_\" is a positive integer, and their product is representable as a std::size_t,
        • \"_ArrayData_\" is an array holding exactly that many elements, and
        • every element of \"_ArrayData_\" is a number of the kind named by \"_ArrayType_\": for the integer types, a value that fits the named width; for double, any value; for single, a value that survives narrowing to float and back without change (for instance, 0.1 does not, since it is not exactly representable as float).

        An annotated object is always read back with its keys in the order shown above, \"_ArrayType_\", \"_ArraySize_\", \"_ArrayData_\", regardless of the order the ND-array's header stores them in on the wire. This matters for ordered_json, whose comparison takes key order into account.

        The current version of this library does not yet support automatic detection of and conversion from a nested JSON array input to a BJData ND-array.

        Restrictions in optimized data types for arrays and objects

        Due to diminished space saving, hampered readability, and increased security risks, in BJData, the allowed data types following the $ marker in an optimized array and object container are restricted to non-zero-fixed-length data types. Therefore, the valid optimized type markers can only be one of UiuImlMLhdDCB. This also means other variable ([{SH) or zero-length types (TFN) can not be used in an optimized array or object in BJData.

        Binary values

        BJData provides a dedicated B marker (defined in the BJData specification (Draft 3)) that is used in optimized arrays to designate binary data. This means that, unlike UBJSON, binary data can be both serialized and deserialized.

        To preserve compatibility with BJData Draft 2, the Draft 3 optimized binary array must be explicitly enabled using the version parameter of to_bjdata.

        In Draft2 mode (default), if the JSON data contains the binary type, the value stored as a list of integers, as suggested by the BJData documentation. In particular, this means that the serialization and the deserialization of JSON containing binary values into BJData and back will result in a different JSON object.

        Example: serialize JSON values to BJData, with and without size/type optimization 
        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\n// function to print BJData's diagnostic format\nvoid print_byte(uint8_t byte)\n{\n    if (32 < byte and byte < 128)\n    {\n        std::cout << (char)byte;\n    }\n    else\n    {\n        std::cout << (int)byte;\n    }\n}\n\nint main()\n{\n    // create a JSON value\n    json j = R\"({\"compact\": true, \"schema\": false})\"_json;\n\n    // serialize it to BJData\n    std::vector<std::uint8_t> v = json::to_bjdata(j);\n\n    // print the vector content\n    for (auto& byte : v)\n    {\n        print_byte(byte);\n    }\n    std::cout << std::endl;\n\n    // create an array of numbers\n    json array = {1, 2, 3, 4, 5, 6, 7, 8};\n\n    // serialize it to BJData using default representation\n    std::vector<std::uint8_t> v_array = json::to_bjdata(array);\n    // serialize it to BJData using size optimization\n    std::vector<std::uint8_t> v_array_size = json::to_bjdata(array, true);\n    // serialize it to BJData using type optimization\n    std::vector<std::uint8_t> v_array_size_and_type = json::to_bjdata(array, true, true);\n\n    // print the vector contents\n    for (auto& byte : v_array)\n    {\n        print_byte(byte);\n    }\n    std::cout << std::endl;\n\n    for (auto& byte : v_array_size)\n    {\n        print_byte(byte);\n    }\n    std::cout << std::endl;\n\n    for (auto& byte : v_array_size_and_type)\n    {\n        print_byte(byte);\n    }\n    std::cout << std::endl;\n}\n

        Output:

        {i7compactTi6schemaF}\n[i1i2i3i4i5i6i7i8]\n[#i8i1i2i3i4i5i6i7i8\n[$i#i812345678\n
        "},{"location":"features/binary_formats/bjdata/#deserialization","title":"Deserialization","text":"

        The library maps BJData types to JSON value types as follows:

        BJData type JSON value type marker no-op no value, next value is read N null null Z false false F true true T float16 number_float h float32 number_float d float64 number_float D uint8 number_unsigned U int8 number_integer i uint16 number_unsigned u int16 number_integer I uint32 number_unsigned m int32 number_integer l uint64 number_unsigned M int64 number_integer L byte number_unsigned B string string S char string C array array (optimized values are supported) [ ND-array object (in JData annotated array format) [$.#[. object object (optimized values are supported) { binary binary (strongly-typed byte array) [$B

        Complete mapping

        The mapping is complete in the sense that any BJData value can be converted to a JSON value.

        Ill-formed UTF-8 in string values and object keys

        BJData strings must use UTF-8 encoding, but checking it on read is opt-in: with the error_handler parameter left at keep (the default), from_bjdata() accepts a string value or object key whose bytes are not valid UTF-8 and hands them back unchanged. Passing error_handler_t::strict makes from_bjdata() check and throw parse_error.113 for ill-formed UTF-8, and replace/ignore sanitize the string instead of keeping it. However, dump() still requires valid UTF-8 and throws type_error.316 for a value read with the default keep handler, unless an error handler is passed that replaces or ignores the ill-formed bytes. to_bjdata()'s own error_handler parameter defaults to keep (see above), so such a value is written back unchanged.

        Round trips

        A value returned by from_bjdata can be serialized with to_bjdata using any combination of options and parsed back into an equal value, and serializing that value again with the same options produces the same bytes. The exception is binary values: they are only written as an optimized binary array ([$B) if Draft 3 is enabled and both use_size and use_type are set. Otherwise, they are written as arrays of integers and parsed back as such (see the notes on binary values above), and serializing such an array again may choose different, but equally valid, type markers. The bytes can then differ, but parsing them again yields the same value.

        Example: deserialize a JSON value from BJData 
        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create byte vector\n    std::vector<std::uint8_t> v = {0x7B, 0x69, 0x07, 0x63, 0x6F, 0x6D, 0x70, 0x61,\n                                   0x63, 0x74, 0x54, 0x69, 0x06, 0x73, 0x63, 0x68,\n                                   0x65, 0x6D, 0x61, 0x69, 0x00, 0x7D\n                                  };\n\n    // deserialize it with BJData\n    json j = json::from_bjdata(v);\n\n    // print the deserialized JSON value\n    std::cout << std::setw(2) << j << std::endl;\n}\n

        Output:

        {\n  \"compact\": true,\n  \"schema\": 0\n}\n
        "},{"location":"features/binary_formats/bon8/","title":"BON8","text":"

        BON8 (Binary Object Notation 8) is a compact binary serialization format for JSON values. It uses the byte values that cannot begin a UTF-8 character as type markers, so strings are stored as plain UTF-8 without a length prefix: a string ends at the first byte that cannot continue it. Integers from -10 to 39, true, false, null, and the floating-point values -1.0, 0.0, and 1.0 take a single byte, and arrays and objects with up to four elements need no terminator.

        References

        • BON8 specification
        • Reference implementation in HikoGUI
        "},{"location":"features/binary_formats/bon8/#serialization","title":"Serialization","text":"

        The library uses the following mapping from JSON values types to BON8 types according to the BON8 specification:

        JSON value type value/range BON8 type first byte null null null 0xFA boolean true true 0xF9 boolean false false 0xF8 number_integer -9223372036854775808..-2147483649 int64 0x8D number_integer -2147483648..-33818507 int32 0x8C number_integer -33818506..-264075 4-byte negative integer 0xF0..0xF7 number_integer -264074..-1931 3-byte negative integer 0xE0..0xEF number_integer -1930..-11 2-byte negative integer 0xC2..0xDF number_integer -10..-1 1-byte negative integer 0xB8..0xC1 number_integer 0..39 1-byte positive integer 0x90..0xB7 number_integer 40..3879 2-byte positive integer 0xC2..0xDF number_integer 3880..528167 3-byte positive integer 0xE0..0xEF number_integer 528168..67637031 4-byte positive integer 0xF0..0xF7 number_integer 67637032..2147483647 int32 0x8C number_integer 2147483648..9223372036854775807 int64 0x8D number_unsigned 0..39 1-byte positive integer 0x90..0xB7 number_unsigned 40..3879 2-byte positive integer 0xC2..0xDF number_unsigned 3880..528167 3-byte positive integer 0xE0..0xEF number_unsigned 528168..67637031 4-byte positive integer 0xF0..0xF7 number_unsigned 67637032..2147483647 int32 0x8C number_unsigned 2147483648..9223372036854775807 int64 0x8D number_float -1.0 -1.0 0xFB number_float 0.0 0.0 0xFC number_float 1.0 1.0 0xFD number_float any other value representable by a float binary32 0x8E number_float any value NOT representable by a float binary64 0x8F string empty end of string 0xFF string non-empty UTF-8 string 0x00..0x7F, 0xC2..0xF4 array size: 0..4 array with count 0x80..0x84 array size: 5 or more array (terminated by 0xFE) 0x85 object size: 0..4 object with count 0x86..0x8A object size: 5 or more object (terminated by 0xFE) 0x8B binary size: 0..4 array with count 0x80..0x84 binary size: 5 or more array (terminated by 0xFE) 0x85

        An integer that takes 2 to 4 bytes starts with a UTF-8 lead byte (0xC2..0xF7) that is followed by a byte that cannot continue a UTF-8 character: 0x00..0x7F for positive and 0xC0..0xFF for negative integers. A string is terminated by 0xFF only if it is empty, if another string follows it, or if nothing follows it in the message; otherwise, the byte after it ends it: the first byte of the next value, or the 0xFE that ends an array or object. For example, [\"e\"] is serialized as 0x81 0x65 0xFF, but [1,2,3,4,\"e\"] as 0x85 0x91 0x92 0x93 0x94 0x65 0xFE.

        Complete mapping

        Except for the values listed below, any JSON value can be converted to a BON8 value.

        Any BON8 output created by to_bon8 can be successfully parsed by from_bon8.

        Unsupported values

        The following values can not be converted to a BON8 value:

        • unsigned integers above 9223372036854775807, because BON8 has no unsigned 64-bit integer type (out_of_range.407)
        • strings that are not valid UTF-8, because the end of a string is determined from its encoding (type_error.316)

        NaN/infinity handling

        -0.0, Infinity, and -Infinity are serialized as binary32 (type 0x8E, 5 bytes total). NaN is serialized as the binary32 value 0x7F800001 that the specification recommends. This is in contrast to the dump function which serializes NaN or Infinity to null.

        Binary values

        BON8 has no binary type. Binary values are serialized as arrays of integers (0..255), so they are read back as arrays. The subtype is not serialized.

        Canonical representation

        The output follows the specification's canonical representation rules: every value uses the shortest encoding, floating-point numbers use binary32 whenever that loses no precision, and object keys are sorted by their UTF-8 code units. There are two exceptions:

        • Strings are not normalized to Unicode Normalization Form C (NFC).
        • Object keys are written in the order of the object type, which is sorted for json, but not for ordered_json.
        Example: serialize a JSON value to BON8 
        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // create a JSON value\n    json j = R\"({\"compact\": true, \"format\": \"BON8\", \"schema\": 0})\"_json;\n\n    // serialize it to BON8\n    std::vector<std::uint8_t> v = json::to_bon8(j);\n\n    // print the vector content\n    for (auto& byte : v)\n    {\n        std::cout << \"0x\" << std::hex << std::setw(2) << std::setfill('0') << (int)byte << \" \";\n    }\n    std::cout << std::endl;\n}\n

        Output:

        0x89 0x63 0x6f 0x6d 0x70 0x61 0x63 0x74 0xf9 0x66 0x6f 0x72 0x6d 0x61 0x74 0xff 0x42 0x4f 0x4e 0x38 0xff 0x73 0x63 0x68 0x65 0x6d 0x61 0x90 \n
        "},{"location":"features/binary_formats/bon8/#deserialization","title":"Deserialization","text":"

        The library maps BON8 types to JSON value types as follows:

        BON8 type JSON value type first byte UTF-8 string string 0x00..0x7F array with count array 0x80..0x84 array (terminated by 0xFE) array 0x85 object with count object 0x86..0x8A object (terminated by 0xFE) object 0x8B int32 number_unsigned or number_integer 0x8C int64 number_unsigned or number_integer 0x8D binary32 number_float 0x8E binary64 number_float 0x8F 1-byte positive integer number_unsigned 0x90..0xB7 1-byte negative integer number_integer 0xB8..0xC1 UTF-8 string string 0xC2..0xF4, followed by 0x80..0xBF 2- to 4-byte positive integer number_unsigned 0xC2..0xF7, followed by 0x00..0x7F 2- to 4-byte negative integer number_integer 0xC2..0xF7, followed by 0xC0..0xFF false false 0xF8 true true 0xF9 null null 0xFA -1.0 number_float 0xFB 0.0 number_float 0xFC 1.0 number_float 0xFD empty string string 0xFF

        Non-negative integers are read as number_unsigned, negative integers as number_integer.

        Info

        Values that do not use the canonical representation, such as integers with a longer encoding than necessary, arrays and objects with up to four elements that are terminated by 0xFE, unsorted object keys, or a 0xFF after a string that would also end without it, are accepted. A second 0xFF is not a terminator but an empty string.

        Strings must be valid UTF-8, and a string at the very end of a message must be terminated by 0xFF.

        Info

        Any BON8 output created by to_bon8 can be successfully parsed by from_bon8.

        Example: deserialize a JSON value from BON8 
        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create byte vector\n    std::vector<std::uint8_t> v = {0x89, 0x63, 0x6f, 0x6d, 0x70, 0x61, 0x63, 0x74,\n                                   0xf9, 0x66, 0x6f, 0x72, 0x6d, 0x61, 0x74, 0xff,\n                                   0x42, 0x4f, 0x4e, 0x38, 0xff, 0x73, 0x63, 0x68,\n                                   0x65, 0x6d, 0x61, 0x90\n                                  };\n\n    // deserialize it with BON8\n    json j = json::from_bon8(v);\n\n    // print the deserialized JSON value\n    std::cout << std::setw(2) << j << std::endl;\n}\n

        Output:

        {\n  \"compact\": true,\n  \"format\": \"BON8\",\n  \"schema\": 0\n}\n
        "},{"location":"features/binary_formats/bson/","title":"BSON","text":"

        BSON, short for Binary JSON, is a binary-encoded serialization of JSON-like documents. Like JSON, BSON supports the embedding of documents and arrays within other documents and arrays. BSON also contains extensions that allow representation of data types that are not part of the JSON spec. For example, BSON has a Date type and a BinData type.

        References

        • BSON Website - the main source on BSON
        • BSON Specification - the specification
        "},{"location":"features/binary_formats/bson/#serialization","title":"Serialization","text":"

        The library uses the following mapping from JSON values types to BSON types:

        JSON value type value/range BSON type marker null null null 0x0A boolean true, false boolean 0x08 number_integer -9223372036854775808..-2147483649 int64 0x12 number_integer -2147483648..2147483647 int32 0x10 number_integer 2147483648..9223372036854775807 int64 0x12 number_unsigned 0..2147483647 int32 0x10 number_unsigned 2147483648..9223372036854775807 int64 0x12 number_unsigned 9223372036854775808..18446744073709551615 uint64 0x11 number_float any value double 0x01 string any value string 0x02 array any value document 0x04 object any value document 0x03 binary any value binary 0x05

        Incomplete mapping

        The mapping is incomplete, since only JSON-objects (and things contained therein) can be serialized to BSON. Also, keys may not contain U+0000, since they are serialized a zero-terminated c-strings.

        BSON type 0x11 interoperability

        The BSON specification defines type 0x11 as a Timestamp. This library uses marker 0x11 when serializing number_unsigned values in the range 9223372036854775808..18446744073709551615. Other BSON implementations may therefore interpret these values as Timestamps instead of unsigned integers.

        Binary values without a subtype

        BSON requires every binary value to have a subtype. If a binary value has no subtype, this library serializes it with the generic subtype 0x00. After deserialization, has_subtype() returns true and subtype() returns 0. As a result, serializing and deserializing a JSON object containing such a value produces a different JSON object, even though the binary data is unchanged.

        Example: serialize a JSON value to BSON 
        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // create a JSON value\n    json j = R\"({\"compact\": true, \"schema\": 0})\"_json;\n\n    // serialize it to BSON\n    std::vector<std::uint8_t> v = json::to_bson(j);\n\n    // print the vector content\n    for (auto& byte : v)\n    {\n        std::cout << \"0x\" << std::hex << std::setw(2) << std::setfill('0') << (int)byte << \" \";\n    }\n    std::cout << std::endl;\n}\n

        Output:

        0x1b 0x00 0x00 0x00 0x08 0x63 0x6f 0x6d 0x70 0x61 0x63 0x74 0x00 0x01 0x10 0x73 0x63 0x68 0x65 0x6d 0x61 0x00 0x00 0x00 0x00 0x00 0x00 \n
        "},{"location":"features/binary_formats/bson/#deserialization","title":"Deserialization","text":"

        The library maps BSON record types to JSON value types as follows:

        BSON type BSON marker byte JSON value type double 0x01 number_float string 0x02 string document 0x03 object array 0x04 array binary 0x05 binary undefined 0x06 unsupported ObjectId 0x07 unsupported boolean 0x08 boolean UTC Date-Time 0x09 unsupported null 0x0A null Regular Expr. 0x0B unsupported DB Pointer 0x0C unsupported JavaScript Code 0x0D unsupported Symbol 0x0E unsupported JavaScript Code w/ scope 0x0F unsupported int32 0x10 number_integer uint64(Timestamp) 0x11 number_unsigned int64 0x12 number_integer 128-bit decimal float 0x13 unsupported Max Key 0x7F unsupported Min Key 0xFF unsupported

        Incomplete mapping

        The mapping is incomplete. The unsupported mappings are indicated in the table above.

        Handling of BSON type 0x11

        This library deserializes BSON type 0x11 (Timestamp) as a number_unsigned value. The 64-bit value is preserved, but the Timestamp type information is not.

        Lenient BSON input handling

        The BSON reader is lenient in a few areas where the BSON specification is more restrictive:

        • array element keys are not checked against the required decimal sequence (0, 1, 2, ...),
        • any non-zero byte is accepted as true for the boolean type, and
        • the payload for binary subtype 0x02 is returned as-is, including its inner length prefix.

        If BSON input must be validated for strict specification compliance, validate it separately before passing it to from_bson().

        Ill-formed UTF-8 in string values

        The BSON specification requires string values (type 0x02) to be valid UTF-8, but this is not required of a decoder, so checking is opt-in: with the error_handler parameter left at keep (the default), from_bson() accepts a string value whose bytes are not valid UTF-8 and hands them back unchanged. Passing error_handler_t::strict makes from_bson() check and throw parse_error.113 for ill-formed UTF-8, and replace/ignore sanitize the string instead of keeping it. However, dump() still requires valid UTF-8 and throws type_error.316 for a value read with the default keep handler, unless an error handler is passed that replaces or ignores the ill-formed bytes. to_bson()'s own error_handler parameter defaults to keep, so such a string value or element (key) name is written unchanged; with strict (the default if JSON_STRICT_BINARY_UTF8 is enabled), it throws the same exception instead. Element (key) names are never validated on read, since they are read byte-by-byte as a C string. binary values (type 0x05) are unaffected, since they are not required to hold text.

        Example: deserialize a JSON value from BSON 
        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create byte vector\n    std::vector<std::uint8_t> v = {0x1b, 0x00, 0x00, 0x00, 0x08, 0x63, 0x6f, 0x6d,\n                                   0x70, 0x61, 0x63, 0x74, 0x00, 0x01, 0x10, 0x73,\n                                   0x63, 0x68, 0x65, 0x6d, 0x61, 0x00, 0x00, 0x00,\n                                   0x00, 0x00, 0x00\n                                  };\n\n    // deserialize it with BSON\n    json j = json::from_bson(v);\n\n    // print the deserialized JSON value\n    std::cout << std::setw(2) << j << std::endl;\n}\n

        Output:

        {\n  \"compact\": true,\n  \"schema\": 0\n}\n
        "},{"location":"features/binary_formats/cbor/","title":"CBOR","text":"

        The Concise Binary Object Representation (CBOR) is a data format whose design goals include the possibility of extremely small code sizes, fairly small message size, and extensibility without the need for version negotiation.

        References

        • CBOR Website - the main source on CBOR
        • CBOR Playground - an interactive webpage to translate between JSON and CBOR
        • RFC 8949 - the CBOR specification
        "},{"location":"features/binary_formats/cbor/#serialization","title":"Serialization","text":"

        The library uses the following mapping from JSON values types to CBOR types according to the CBOR specification (RFC 8949):

        JSON value type value/range CBOR type first byte null null Null 0xF6 boolean true True 0xF5 boolean false False 0xF4 number_integer -9223372036854775808..-2147483649 Negative integer (8 bytes follow) 0x3B number_integer -2147483648..-32769 Negative integer (4 bytes follow) 0x3A number_integer -32768..-129 Negative integer (2 bytes follow) 0x39 number_integer -128..-25 Negative integer (1 byte follow) 0x38 number_integer -24..-1 Negative integer 0x20..0x37 number_integer 0..23 Integer 0x00..0x17 number_integer 24..255 Unsigned integer (1 byte follow) 0x18 number_integer 256..65535 Unsigned integer (2 bytes follow) 0x19 number_integer 65536..4294967295 Unsigned integer (4 bytes follow) 0x1A number_integer 4294967296..18446744073709551615 Unsigned integer (8 bytes follow) 0x1B number_unsigned 0..23 Integer 0x00..0x17 number_unsigned 24..255 Unsigned integer (1 byte follow) 0x18 number_unsigned 256..65535 Unsigned integer (2 bytes follow) 0x19 number_unsigned 65536..4294967295 Unsigned integer (4 bytes follow) 0x1A number_unsigned 4294967296..18446744073709551615 Unsigned integer (8 bytes follow) 0x1B number_float any value representable by a float Single-Precision Float 0xFA number_float any value NOT representable by a float Double-Precision Float 0xFB string length: 0..23 UTF-8 string 0x60..0x77 string length: 24..255 UTF-8 string (1 byte follow) 0x78 string length: 256..65535 UTF-8 string (2 bytes follow) 0x79 string length: 65536..4294967295 UTF-8 string (4 bytes follow) 0x7A string length: 4294967296..18446744073709551615 UTF-8 string (8 bytes follow) 0x7B array size: 0..23 array 0x80..0x97 array size: 24..255 array (1 byte follow) 0x98 array size: 256..65535 array (2 bytes follow) 0x99 array size: 65536..4294967295 array (4 bytes follow) 0x9A array size: 4294967296..18446744073709551615 array (8 bytes follow) 0x9B object size: 0..23 map 0xA0..0xB7 object size: 24..255 map (1 byte follow) 0xB8 object size: 256..65535 map (2 bytes follow) 0xB9 object size: 65536..4294967295 map (4 bytes follow) 0xBA object size: 4294967296..18446744073709551615 map (8 bytes follow) 0xBB binary size: 0..23 byte string 0x40..0x57 binary size: 24..255 byte string (1 byte follow) 0x58 binary size: 256..65535 byte string (2 bytes follow) 0x59 binary size: 65536..4294967295 byte string (4 bytes follow) 0x5A binary size: 4294967296..18446744073709551615 byte string (8 bytes follow) 0x5B

        Binary values with subtype are mapped to tagged values (0xD8..0xDB) depending on the subtype, followed by a byte string, see \"binary\" cells in the table above.

        Complete mapping

        The mapping is complete in the sense that any JSON value type can be converted to a CBOR value.

        NaN/infinity handling

        NaN, Infinity, and -Infinity are serialized as a CBOR half-precision float (type 0xF9, 3 bytes total): NaN as 0xF9 0x7E 0x00, Infinity as 0xF9 0x7C 0x00, and -Infinity as 0xF9 0xFC 0x00. This behavior differs from the normal JSON serialization which serializes NaN or Infinity to null.

        Note

        Prior to version 3.13.0 unreleased, NaN and Infinity were instead serialized as a CBOR double-precision float (type 0xFB, 9 bytes total), because the check used to select a smaller encoding compared magnitudes with NaN, which is always false and caused the intended half-precision path to be skipped.

        Unused CBOR types

        The following CBOR types are not used in the conversion:

        • UTF-8 strings terminated by \"break\" (0x7F)
        • arrays terminated by \"break\" (0x9F)
        • maps terminated by \"break\" (0xBF)
        • byte strings terminated by \"break\" (0x5F)
        • date/time (0xC0..0xC1)
        • bignum (0xC2..0xC3)
        • decimal fraction (0xC4)
        • bigfloat (0xC5)
        • expected conversions (0xD5..0xD7)
        • simple values (0xE0..0xF3, 0xF8)
        • undefined (0xF7)
        • half-precision floats (0xF9)
        • break (0xFF)

        Tagged items

        Binary subtypes will be serialized as tagged items. See binary values for an example.

        Example: serialize a JSON value to CBOR 
        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // create a JSON value\n    json j = R\"({\"compact\": true, \"schema\": 0})\"_json;\n\n    // serialize it to CBOR\n    std::vector<std::uint8_t> v = json::to_cbor(j);\n\n    // print the vector content\n    for (auto& byte : v)\n    {\n        std::cout << \"0x\" << std::hex << std::setw(2) << std::setfill('0') << (int)byte << \" \";\n    }\n    std::cout << std::endl;\n}\n

        Output:

        0xa2 0x67 0x63 0x6f 0x6d 0x70 0x61 0x63 0x74 0xf5 0x66 0x73 0x63 0x68 0x65 0x6d 0x61 0x00 \n
        "},{"location":"features/binary_formats/cbor/#deserialization","title":"Deserialization","text":"

        The library maps CBOR types to JSON value types as follows:

        CBOR type JSON value type first byte Integer number_unsigned 0x00..0x17 Unsigned integer number_unsigned 0x18 Unsigned integer number_unsigned 0x19 Unsigned integer number_unsigned 0x1A Unsigned integer number_unsigned 0x1B Negative integer number_integer 0x20..0x37 Negative integer number_integer 0x38 Negative integer number_integer 0x39 Negative integer number_integer 0x3A Negative integer number_integer 0x3B Byte string binary 0x40..0x57 Byte string binary 0x58 Byte string binary 0x59 Byte string binary 0x5A Byte string binary 0x5B Byte string binary 0x5F UTF-8 string string 0x60..0x77 UTF-8 string string 0x78 UTF-8 string string 0x79 UTF-8 string string 0x7A UTF-8 string string 0x7B UTF-8 string string 0x7F array array 0x80..0x97 array array 0x98 array array 0x99 array array 0x9A array array 0x9B array array 0x9F map object 0xA0..0xB7 map object 0xB8 map object 0xB9 map object 0xBA map object 0xBB map object 0xBF False false 0xF4 True true 0xF5 Null null 0xF6 Half-Precision Float number_float 0xF9 Single-Precision Float number_float 0xFA Double-Precision Float number_float 0xFB

        Indefinite-length UTF-8 strings (0x7F) and byte strings (0x5F) are supported. Each chunk must be a definite-length string of the same major type, as required by RFC 8949, Section 3.2.3.

        Incomplete mapping

        The mapping is incomplete in the sense that not all CBOR types can be converted to a JSON value. The following CBOR types are not supported and will yield parse errors:

        • simple values (0xE0..0xF3, 0xF8)
        • undefined (0xF7)

        Tagged items (0xC0..0xDB) are not interpreted either; see the note on tagged items below.

        Negative integer overflow

        CBOR negative integers (major type 1) are decoded as -1 - n. If the encoded magnitude n is too large for the result to fit into number_integer_t (std::int64_t by default), the result is stored as number_float_t, like a too small integer in JSON text. For example, -18446744073709551616 (0x3B followed by eight 0xFF bytes) is stored as -1.8446744073709552e+19.

        Object keys

        CBOR allows map keys of any type, whereas JSON only allows strings as keys in object values. Therefore, CBOR maps with keys other than text strings (major type 3) are rejected with a parse_error.113 exception (or, with allow_exceptions set to false, a discarded value) naming the type of the key that was found, for instance:

        [json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR object key: only string keys are supported, but found an unsigned integer; last byte: 0x01\n

        This applies to the SAX interface as well, as the key is read before it is passed on. This is a deliberate restriction of the library's JSON value model, not an oversight: formats built on CBOR maps with integer keys, such as COSE (RFC 9052) or CWT (RFC 8392), cannot be read with this library and need a general-purpose CBOR library instead.

        Ill-formed UTF-8 in text strings

        RFC 8949, Section 3.1 requires CBOR text strings (major type 3) to be valid UTF-8, but leaves it up to the decoder whether to enforce this, so checking is opt-in: with the error_handler parameter left at keep (the default), from_cbor() accepts a text string (object keys included) whose bytes are not valid UTF-8 and hands them back unchanged. Passing error_handler_t::strict makes from_cbor() check and throw parse_error.113 for ill-formed UTF-8, and replace/ignore sanitize the string instead of keeping it. However, dump() still requires valid UTF-8 and throws type_error.316 for a value read with the default keep handler, unless an error handler is passed that replaces or ignores the ill-formed bytes. to_cbor()'s own error_handler parameter defaults to keep, so such a value is written back unchanged; with strict (the default if JSON_STRICT_BINARY_UTF8 is enabled), it throws the same exception instead. Byte strings (major type 2) are unaffected, since they are not required to hold text.

        Tagged items

        Tagged items (0xC0..0xDB) will throw a parse error by default. They can be ignored by passing cbor_tag_handler_t::ignore to function from_cbor, in which case the tag is skipped and the enclosed data item is parsed on its own. Passing cbor_tag_handler_t::store to function from_cbor stores tagged byte strings (for bytes 0xd8..0xdb) as binary values with the tag as subtype; other tagged values are read as if the tag were ignored. If several tags precede a byte string, only the innermost one is stored. Note that no tag is ever interpreted: for instance, a text string tagged with tag 0 (date/time) stays a string.

        Example: deserialize a JSON value from CBOR 
        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create byte vector\n    std::vector<std::uint8_t> v = {0xa2, 0x67, 0x63, 0x6f, 0x6d, 0x70, 0x61, 0x63,\n                                   0x74, 0xf5, 0x66, 0x73, 0x63, 0x68, 0x65, 0x6d,\n                                   0x61, 0x00\n                                  };\n\n    // deserialize it with CBOR\n    json j = json::from_cbor(v);\n\n    // print the deserialized JSON value\n    std::cout << std::setw(2) << j << std::endl;\n}\n

        Output:

        {\n  \"compact\": true,\n  \"schema\": 0\n}\n
        "},{"location":"features/binary_formats/messagepack/","title":"MessagePack","text":"

        MessagePack is an efficient binary serialization format. It lets you exchange data among multiple languages like JSON. But it's faster and smaller. Small integers are encoded into a single byte, and typical short strings require only one extra byte in addition to the strings themselves.

        References

        • MessagePack website
        • MessagePack specification
        "},{"location":"features/binary_formats/messagepack/#serialization","title":"Serialization","text":"

        The library uses the following mapping from JSON values types to MessagePack types according to the MessagePack specification:

        JSON value type value/range MessagePack type first byte null null nil 0xC0 boolean true true 0xC3 boolean false false 0xC2 number_integer -9223372036854775808..-2147483649 int64 0xD3 number_integer -2147483648..-32769 int32 0xD2 number_integer -32768..-129 int16 0xD1 number_integer -128..-33 int8 0xD0 number_integer -32..-1 negative fixint 0xE0..0xFF number_integer 0..127 positive fixint 0x00..0x7F number_integer 128..255 uint 8 0xCC number_integer 256..65535 uint 16 0xCD number_integer 65536..4294967295 uint 32 0xCE number_integer 4294967296..18446744073709551615 uint 64 0xCF number_unsigned 0..127 positive fixint 0x00..0x7F number_unsigned 128..255 uint 8 0xCC number_unsigned 256..65535 uint 16 0xCD number_unsigned 65536..4294967295 uint 32 0xCE number_unsigned 4294967296..18446744073709551615 uint 64 0xCF number_float any value representable by a float float 32 0xCA number_float any value NOT representable by a float float 64 0xCB string length: 0..31 fixstr 0xA0..0xBF string length: 32..255 str 8 0xD9 string length: 256..65535 str 16 0xDA string length: 65536..4294967295 str 32 0xDB array size: 0..15 fixarray 0x90..0x9F array size: 16..65535 array 16 0xDC array size: 65536..4294967295 array 32 0xDD object size: 0..15 fix map 0x80..0x8F object size: 16..65535 map 16 0xDE object size: 65536..4294967295 map 32 0xDF binary size: 0..255 bin 8 0xC4 binary size: 256..65535 bin 16 0xC5 binary size: 65536..4294967295 bin 32 0xC6

        Complete mapping

        The mapping is complete in the sense that any JSON value type can be converted to a MessagePack value.

        Any MessagePack output created by to_msgpack can be successfully parsed by from_msgpack.

        Size constraints

        The following values can not be converted to a MessagePack value:

        • strings with more than 4294967295 bytes
        • byte strings with more than 4294967295 bytes
        • arrays with more than 4294967295 elements
        • objects with more than 4294967295 elements

        Serializing such a value throws out_of_range.412.

        NaN/infinity handling

        NaN, Infinity, and -Infinity are serialized as a MessagePack float 32 (type 0xCA, 5 bytes total), regardless of magnitude, in contrast to the dump function which serializes NaN or Infinity to null.

        Note

        Prior to version 3.13.0 unreleased, NaN and Infinity were instead serialized as a MessagePack float 64 (type 0xCB, 9 bytes total), because the check used to select the smaller float 32 encoding compared magnitudes with NaN, which is always false and caused the float 32 path to be skipped.

        Example: serialize a JSON value to MessagePack 
        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    // create a JSON value\n    json j = R\"({\"compact\": true, \"schema\": 0})\"_json;\n\n    // serialize it to MessagePack\n    std::vector<std::uint8_t> v = json::to_msgpack(j);\n\n    // print the vector content\n    for (auto& byte : v)\n    {\n        std::cout << \"0x\" << std::hex << std::setw(2) << std::setfill('0') << (int)byte << \" \";\n    }\n    std::cout << std::endl;\n}\n

        Output:

        0x82 0xa7 0x63 0x6f 0x6d 0x70 0x61 0x63 0x74 0xc3 0xa6 0x73 0x63 0x68 0x65 0x6d 0x61 0x00 \n
        "},{"location":"features/binary_formats/messagepack/#deserialization","title":"Deserialization","text":"

        The library maps MessagePack types to JSON value types as follows:

        MessagePack type JSON value type first byte positive fixint number_unsigned 0x00..0x7F fixmap object 0x80..0x8F fixarray array 0x90..0x9F fixstr string 0xA0..0xBF nil null 0xC0 false false 0xC2 true true 0xC3 float 32 number_float 0xCA float 64 number_float 0xCB uint 8 number_unsigned 0xCC uint 16 number_unsigned 0xCD uint 32 number_unsigned 0xCE uint 64 number_unsigned 0xCF int 8 number_integer 0xD0 int 16 number_integer 0xD1 int 32 number_integer 0xD2 int 64 number_integer 0xD3 str 8 string 0xD9 str 16 string 0xDA str 32 string 0xDB array 16 array 0xDC array 32 array 0xDD map 16 object 0xDE map 32 object 0xDF bin 8 binary 0xC4 bin 16 binary 0xC5 bin 32 binary 0xC6 ext 8 binary 0xC7 ext 16 binary 0xC8 ext 32 binary 0xC9 fixext 1 binary 0xD4 fixext 2 binary 0xD5 fixext 4 binary 0xD6 fixext 8 binary 0xD7 fixext 16 binary 0xD8 negative fixint number_integer 0xE0-0xFF

        Info

        Any MessagePack output created by to_msgpack can be successfully parsed by from_msgpack.

        Object keys

        MessagePack allows map keys of any type, whereas JSON only allows strings as keys in object values. Like the JSON-compatible profile sketched in the MessagePack specification, this library restricts map keys to str values. Maps with keys of any other type are rejected with a parse_error.113 exception (or, with allow_exceptions set to false, a discarded value) naming the type of the key that was found, for instance:

        [json.exception.parse_error.113] parse error at byte 2: syntax error while parsing MessagePack object key: only string keys are supported, but found nil; last byte: 0xC0\n

        This applies to the SAX interface as well, as the key is read before it is passed on. Such input needs a general-purpose MessagePack library instead.

        Ill-formed UTF-8 in string values

        The MessagePack specification explicitly allows a str value (fixstr, str 8, str 16, str 32) to contain a byte sequence that is not valid UTF-8, and expects a deserializer to hand the original bytes back unchanged. This library follows that by default: with its error_handler parameter left at keep (the default), from_msgpack() reads str bytes (object keys included) as-is, without validating them, so such a value round-trips through from_msgpack(to_msgpack(j)) byte for byte. Passing error_handler_t::strict makes from_msgpack() check anyway and throw parse_error.113 for ill-formed UTF-8, and replace/ignore sanitize the string instead of keeping it. to_msgpack() also writes str bytes as-is by default, since the specification permits it; its error_handler parameter can be set to strict to throw type_error.316 instead, or to replace/ignore to sanitize the string, for instance for a decoder that rejects ill-formed UTF-8. However, dump() still requires valid UTF-8 and throws type_error.316 for a value read this way with the default keep handler, unless an error handler is passed that replaces or ignores the ill-formed bytes.

        Example: deserialize a JSON value from MessagePack 
        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create byte vector\n    std::vector<std::uint8_t> v = {0x82, 0xa7, 0x63, 0x6f, 0x6d, 0x70, 0x61, 0x63,\n                                   0x74, 0xc3, 0xa6, 0x73, 0x63, 0x68, 0x65, 0x6d,\n                                   0x61, 0x00\n                                  };\n\n    // deserialize it with MessagePack\n    json j = json::from_msgpack(v);\n\n    // print the deserialized JSON value\n    std::cout << std::setw(2) << j << std::endl;\n}\n

        Output:

        {\n  \"compact\": true,\n  \"schema\": 0\n}\n
        "},{"location":"features/binary_formats/ubjson/","title":"UBJSON","text":"

        Universal Binary JSON (UBJSON) is a binary form directly imitating JSON, but requiring fewer bytes of data. It aims to achieve the generality of JSON, combined with being much easier to process than JSON.

        References

        • UBJSON Website
        "},{"location":"features/binary_formats/ubjson/#serialization","title":"Serialization","text":"

        The library uses the following mapping from JSON values types to UBJSON types according to the UBJSON specification:

        JSON value type value/range UBJSON type marker null null null Z boolean true true T boolean false false F number_integer -9223372036854775808..-2147483649 int64 L number_integer -2147483648..-32769 int32 l number_integer -32768..-129 int16 I number_integer -128..127 int8 i number_integer 128..255 uint8 U number_integer 256..32767 int16 I number_integer 32768..2147483647 int32 l number_integer 2147483648..9223372036854775807 int64 L number_unsigned 0..127 int8 i number_unsigned 128..255 uint8 U number_unsigned 256..32767 int16 I number_unsigned 32768..2147483647 int32 l number_unsigned 2147483648..9223372036854775807 int64 L number_unsigned 9223372036854775808..18446744073709551615 high-precision H number_float any value float64 D string with shortest length indicator string S array see notes on optimized format array [ object see notes on optimized format map {

        Complete mapping

        The mapping is complete in the sense that any JSON value type can be converted to a UBJSON value.

        Any UBJSON output created by to_ubjson can be successfully parsed by from_ubjson.

        Size constraints

        The following values can not be converted to a UBJSON value:

        • strings with more than 9223372036854775807 bytes (theoretical)

        UTF-8 validation of string values and object keys

        UBJSON's required string encoding is UTF-8. By default (the error_handler parameter left at keep), to_ubjson() writes the bytes of string values and object keys unchanged, even if they are not valid UTF-8. With error_handler_t::strict, it throws type_error.316 for ill-formed UTF-8 instead; replace/ignore sanitize the string. JSON_STRICT_BINARY_UTF8 makes strict the default.

        Unused UBJSON markers

        The following markers are not used in the conversion:

        • Z: no-op values are not created.
        • C: single-byte strings are serialized with S markers.

        NaN/infinity handling

        If NaN or Infinity are stored inside a JSON number, they are serialized properly. This behavior differs from the dump() function which serializes NaN or Infinity to null.

        Optimized formats

        The optimized formats for containers are supported: Parameter use_size adds size information to the beginning of a container and removes the closing marker. Parameter use_type further checks whether all elements of a container have the same type and adds the type marker to the beginning of the container. The use_type parameter must only be used together with use_size = true.

        Note that use_size = true alone may result in larger representations - the benefit of this parameter is that the receiving side is immediately informed on the number of elements of the container.

        An array whose type marker is Z (null), T (true) or F (false) stores no payload at all, because the marker already is the value. Its declared count is therefore the only thing that decides how much memory the receiving side allocates, and a handful of bytes can describe billions of elements. from_ubjson rejects such an array with out_of_range.408 when the count exceeds 1,048,576 (1 << 20), and to_ubjson writes longer arrays of these types without the annotation, so any value it produces can be read back.

        Binary values

        If the JSON data contains the binary type, the value stored is a list of integers, as suggested by the UBJSON documentation. In particular, this means that serialization and the deserialization of a JSON containing binary values into UBJSON and back will result in a different JSON object.

        Example: serialize JSON values to UBJSON, with and without size/type optimization 
        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\n// function to print UBJSON's diagnostic format\nvoid print_byte(uint8_t byte)\n{\n    if (32 < byte and byte < 128)\n    {\n        std::cout << (char)byte;\n    }\n    else\n    {\n        std::cout << (int)byte;\n    }\n}\n\nint main()\n{\n    // create a JSON value\n    json j = R\"({\"compact\": true, \"schema\": false})\"_json;\n\n    // serialize it to UBJSON\n    std::vector<std::uint8_t> v = json::to_ubjson(j);\n\n    // print the vector content\n    for (auto& byte : v)\n    {\n        print_byte(byte);\n    }\n    std::cout << std::endl;\n\n    // create an array of numbers\n    json array = {1, 2, 3, 4, 5, 6, 7, 8};\n\n    // serialize it to UBJSON using default representation\n    std::vector<std::uint8_t> v_array = json::to_ubjson(array);\n    // serialize it to UBJSON using size optimization\n    std::vector<std::uint8_t> v_array_size = json::to_ubjson(array, true);\n    // serialize it to UBJSON using type optimization\n    std::vector<std::uint8_t> v_array_size_and_type = json::to_ubjson(array, true, true);\n\n    // print the vector contents\n    for (auto& byte : v_array)\n    {\n        print_byte(byte);\n    }\n    std::cout << std::endl;\n\n    for (auto& byte : v_array_size)\n    {\n        print_byte(byte);\n    }\n    std::cout << std::endl;\n\n    for (auto& byte : v_array_size_and_type)\n    {\n        print_byte(byte);\n    }\n    std::cout << std::endl;\n}\n

        Output:

        {i7compactTi6schemaF}\n[i1i2i3i4i5i6i7i8]\n[#i8i1i2i3i4i5i6i7i8\n[$i#i812345678\n
        "},{"location":"features/binary_formats/ubjson/#deserialization","title":"Deserialization","text":"

        The library maps UBJSON types to JSON value types as follows:

        UBJSON type JSON value type marker no-op no value, next value is read N null null Z false false F true true T float32 number_float d float64 number_float D uint8 number_unsigned U int8 number_integer i int16 number_integer I int32 number_integer l int64 number_integer L string string S char string C array array (optimized values are supported) [ object object (optimized values are supported) {

        Complete mapping

        The mapping is complete in the sense that any UBJSON value can be converted to a JSON value.

        Ill-formed UTF-8 in string values and object keys

        UBJSON's required string encoding is UTF-8, but checking it on read is opt-in: with the error_handler parameter left at keep (the default), from_ubjson() accepts a string value or object key whose bytes are not valid UTF-8 and hands them back unchanged. Passing error_handler_t::strict makes from_ubjson() check and throw parse_error.113 for ill-formed UTF-8, and replace/ignore sanitize the string instead of keeping it. However, dump() still requires valid UTF-8 and throws type_error.316 for a value read with the default keep handler, unless an error handler is passed that replaces or ignores the ill-formed bytes. to_ubjson()'s own error_handler parameter defaults to keep (see above), so such a value is written back unchanged.

        Example: deserialize a JSON value from UBJSON 
        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // create byte vector\n    std::vector<std::uint8_t> v = {0x7B, 0x69, 0x07, 0x63, 0x6F, 0x6D, 0x70, 0x61,\n                                   0x63, 0x74, 0x54, 0x69, 0x06, 0x73, 0x63, 0x68,\n                                   0x65, 0x6D, 0x61, 0x69, 0x00, 0x7D\n                                  };\n\n    // deserialize it with UBJSON\n    json j = json::from_ubjson(v);\n\n    // print the deserialized JSON value\n    std::cout << std::setw(2) << j << std::endl;\n}\n

        Output:

        {\n  \"compact\": true,\n  \"schema\": 0\n}\n
        "},{"location":"features/element_access/","title":"Element Access","text":"

        There are many ways elements in a JSON value can be accessed:

        • unchecked access via operator[]
        • checked access via at
        • access with default value via value
        • iterators
        • JSON pointers

        Testing whether a key or index exists before accessing it is also possible, with contains or find (which returns an iterator to the value, or end() if it is not found).

        flowchart TD\n    A[\"accessing a value\"] --> B{\"must it exist?\"}\n    B -->|\"yes, missing is an error\"| C[\"at() -- throws\"]\n    B -->|\"yes, but checking is my job\"| D[\"operator[] -- unchecked\"]\n    B -->|\"no, a fallback is fine\"| E[\"value() -- default value\"]\n    A --> F{\"just testing first?\"}\n    F -->|\"yes\"| G[\"contains() / find()\"]
        "},{"location":"features/element_access/checked_access/","title":"Checked access: at","text":""},{"location":"features/element_access/checked_access/#overview","title":"Overview","text":"

        The at member function performs checked access; that is, it returns a reference to the desired value if it exists and throws a basic_json::out_of_range exception otherwise.

        Example: read access

        Consider the following JSON value:

        {\n    \"name\": \"Mary Smith\",\n    \"age\": 42,\n    \"hobbies\": [\"hiking\", \"reading\"]\n}\n

        Assume the value is parsed to a json variable j.

        expression value j {\"name\": \"Mary Smith\", \"age\": 42, \"hobbies\": [\"hiking\", \"reading\"]} j.at(\"name\") \"Mary Smith\" j.at(\"age\") 42 j.at(\"hobbies\") [\"hiking\", \"reading\"] j.at(\"hobbies\").at(0) \"hiking\" j.at(\"hobbies\").at(1) \"reading\"

        The return value is a reference, so it can be used to modify the original value.

        Example: write access 
        j.at(\"name\") = \"John Smith\";\n

        This code produces the following JSON value:

        {\n    \"name\": \"John Smith\",\n    \"age\": 42,\n    \"hobbies\": [\"hiking\", \"reading\"]\n}\n

        When accessing an invalid index (i.e., an index greater than or equal to the array size) or the passed object key is non-existing, an exception is thrown.

        Example: access via invalid index or missing key 
        j.at(\"hobbies\").at(3) = \"cooking\";\n

        This code produces the following exception:

        [json.exception.out_of_range.401] array index 3 is out of range\n

        When extended diagnostic messages are enabled by defining JSON_DIAGNOSTICS, the exception further gives information where the key or index is missing or out of range.

        [json.exception.out_of_range.401] (/hobbies) array index 3 is out of range\n
        "},{"location":"features/element_access/checked_access/#notes","title":"Notes","text":"

        Exceptions

        • at can only be used with objects (with a string argument) or with arrays (with a numeric argument). For other types, a basic_json::type_error is thrown.
        • basic_json::out_of_range exception exceptions are thrown if the provided key is not found in an object or the provided index is invalid.
        "},{"location":"features/element_access/checked_access/#summary","title":"Summary","text":"scenario non-const value const value access to existing object key reference to existing value is returned const reference to existing value is returned access to valid array index reference to existing value is returned const reference to existing value is returned access to non-existing object key basic_json::out_of_range exception is thrown basic_json::out_of_range exception is thrown access to invalid array index basic_json::out_of_range exception is thrown basic_json::out_of_range exception is thrown"},{"location":"features/element_access/default_value/","title":"Access with default value: value","text":""},{"location":"features/element_access/default_value/#overview","title":"Overview","text":"

        In many situations, such as configuration files, missing values are not exceptional, but may be treated as if a default value was present. For this case, use value(key, default_value) which takes the key you want to access and a default value in case there is no value stored with that key. This is equivalent to Python's dict.get(key, default).

        "},{"location":"features/element_access/default_value/#example","title":"Example","text":"Example

        Consider the following JSON value:

        {\n    \"logOutput\": \"result.log\",\n    \"append\": true\n}\n

        Assume the value is parsed to a json variable j.

        expression value j {\"logOutput\": \"result.log\", \"append\": true} j.value(\"logOutput\", \"logfile.log\") \"result.log\" j.value(\"append\", true) true j.value(\"append\", false) true j.value(\"logLevel\", \"verbose\") \"verbose\""},{"location":"features/element_access/default_value/#nested-values","title":"Nested values","text":"

        To read a value deep inside a document, pass a JSON Pointer instead of a key. The default value is returned if the value at the pointer does not exist, including the case that an intermediate key is missing. There is no need to check each level with contains first.

        json j = {{\"server\", {{\"port\", 8080}}}, {\"list\", {10, 20}}};\n\nint port = j.value(\"/server/port\"_json_pointer, 80);        // 8080\nint timeout = j.value(\"/server/limits/timeout\"_json_pointer, 30); // 30 (missing intermediate key)\nint second = j.value(\"/list/1\"_json_pointer, 0);            // 20 (numeric tokens index arrays)\nint third = j.value(\"/list/5\"_json_pointer, 0);             // 0 (index out of range)\n\nbool has_port = j.contains(\"/server/port\"_json_pointer);    // true\nbool has_host = j.contains(\"/server/host/name\"_json_pointer); // false\n

        If the path is only available as a dotted string such as \"server.port\", do not build the pointer by concatenating \"/\" and the parts: keys containing / or ~ would be misinterpreted. Append each part as a reference token with operator/= instead. It escapes the token for you.

        json::json_pointer to_pointer(const std::string& dotted)\n{\n    json::json_pointer ptr;\n    std::istringstream in(dotted);\n    for (std::string token; std::getline(in, token, '.');)\n    {\n        ptr /= token;\n    }\n    return ptr;\n}\n\nint port = j.value(to_pointer(\"server.port\"), 80);          // 8080\nint first = j.value(to_pointer(\"list.0\"), 0);               // 10\n

        The key \"a/b\" yields the pointer \"/a~1b\"; the dot-splitting itself is up to the caller, so keys containing . need a different separator.

        "},{"location":"features/element_access/default_value/#notes","title":"Notes","text":"

        Exceptions

        • With string keys, value can only be used with objects. For other types, a basic_json::type_error is thrown.
        • With JSON Pointers, value can be used with both objects and arrays. For other types (null, boolean, number, string), a basic_json::type_error is thrown.

        null and mistyped members are not missing

        value returns the default value only if the key is absent. If the member exists, it is converted to the type of the default value, even if it is null. For {\"k\": null}, the call j.value(\"k\", 0) throws a basic_json::type_error, and so does a member of another type such as a string where a number is expected. The same holds for JSON Pointers.

        To treat null like a missing value, check for it explicitly:

        int n = (j.contains(\"k\") && !j[\"k\"].is_null()) ? j[\"k\"].get<int>() : 0;\n

        With C++17, get<std::optional<T>>() maps null to an empty optional. As at throws out_of_range for an absent key, use find to cover both cases:

        std::optional<int> n;                        // empty if \"k\" is absent or null\nif (const auto it = j.find(\"k\"); it != j.end())\n{\n    n = it->get<std::optional<int>>();       // still throws for a non-number such as \"text\"\n}\n

        Return type

        The value function is a template, and the return type of the function is determined by the type of the provided default value unless otherwise specified. This can have unexpected effects. In the example below, we store a 64-bit unsigned integer. We get exactly that value when using operator[]. However, when we call value and provide 0 as default value, then -1 is returned. This occurs, because 0 has type int which overflows when handling the value 18446744073709551615.

        To address this issue, either provide a correctly typed default value or use the template parameter to specify the desired return type. Note that this issue occurs even when a value is stored at the provided key, and the default value is not used as the return value.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    json j = json::parse(R\"({\"uint64\": 18446744073709551615})\");\n\n    std::cout << \"operator[]:                \" << j[\"uint64\"] << '\\n'\n              << \"default value (int):       \" << j.value(\"uint64\", 0) << '\\n'\n              << \"default value (uint64_t):  \" << j.value(\"uint64\", std::uint64_t(0)) << '\\n'\n              << \"explicit return value type: \" << j.value<std::uint64_t>(\"uint64\", 0) << '\\n';\n}\n

        Output:

        operator[]:                18446744073709551615\ndefault value (int):       -1\ndefault value (uint64_t):  18446744073709551615\nexplicit return value type: 18446744073709551615\n
        "},{"location":"features/element_access/default_value/#see-also","title":"See also","text":"
        • value for access with default value
        • documentation on checked access
        • documentation on JSON Pointer
        • contains to check whether a key or JSON Pointer exists
        • json_pointer::operator/= to build a pointer token by token
        "},{"location":"features/element_access/unchecked_access/","title":"Unchecked access: operator[]","text":""},{"location":"features/element_access/unchecked_access/#overview","title":"Overview","text":"

        Elements in a JSON object and a JSON array can be accessed via operator[] similar to a std::map and a std::vector, respectively.

        Example: read access

        Consider the following JSON value:

        {\n    \"name\": \"Mary Smith\",\n    \"age\": 42,\n    \"hobbies\": [\"hiking\", \"reading\"]\n}\n

        Assume the value is parsed to a json variable j.

        expression value j {\"name\": \"Mary Smith\", \"age\": 42, \"hobbies\": [\"hiking\", \"reading\"]} j[\"name\"] \"Mary Smith\" j[\"age\"] 42 j[\"hobbies\"] [\"hiking\", \"reading\"] j[\"hobbies\"][0] \"hiking\" j[\"hobbies\"][1] \"reading\"

        The return value is a reference, so it can modify the original value. In case the passed object key is non-existing, a null value is inserted which can immediately be overwritten.

        Example: write access 
        j[\"name\"] = \"John Smith\";\nj[\"maidenName\"] = \"Jones\";\n

        This code produces the following JSON value:

        {\n    \"name\": \"John Smith\",\n    \"maidenName\": \"Jones\",\n    \"age\": 42,\n    \"hobbies\": [\"hiking\", \"reading\"]\n}\n

        When accessing an invalid index (i.e., an index greater than or equal to the array size), the JSON array is resized such that the passed index is the new maximal index. Intermediate values are filled with null.

        Example: filling up arrays with null values 
        j[\"hobbies\"][0] = \"running\";\nj[\"hobbies\"][3] = \"cooking\";\n

        This code produces the following JSON value:

        {\n    \"name\": \"John Smith\",\n    \"maidenName\": \"Jones\",\n    \"age\": 42,\n    \"hobbies\": [\"running\", \"reading\", null, \"cooking\"]\n}\n
        "},{"location":"features/element_access/unchecked_access/#notes","title":"Notes","text":"

        Design rationale

        The library behaves differently to std::vector and std::map:

        • std::vector::operator[] never inserts a new element.
        • std::map::operator[] is not available for const values.

        The type json wraps all JSON value types. It would be impossible to remove operator[] for const objects. At the same time, inserting elements for non-const objects is really convenient as it avoids awkward insert calls. To this end, we decided to have an inserting non-const behavior for both arrays and objects.

        Info

        The access is unchecked. In case the passed object key does not exist or the passed array index is invalid, no exception is thrown.

        Danger

        • It is undefined behavior to access a const object with a non-existing key.
        • It is undefined behavior to access a const array with an invalid index.
        • In debug mode, an assertion will fire in both cases. You can disable assertions by defining the preprocessor symbol NDEBUG or redefine the macro JSON_ASSERT(x). See the documentation on runtime assertions for more information.

        Exceptions

        operator[] can only be used with objects (with a string argument) or with arrays (with a numeric argument). For other types, a basic_json::type_error is thrown.

        "},{"location":"features/element_access/unchecked_access/#performance-reserving-array-capacity","title":"Performance: reserving array capacity","text":"

        There is no public reserve(count) member on basic_json for pre-allocating array capacity. If you are building a large array incrementally (e.g., via repeated push_back()) and know its final size ahead of time, you can reserve capacity via get_ref() to access the underlying array_t directly:

        json j = json::array();\nj.get_ref<json::array_t&>().reserve(1000);\nfor (int i = 0; i < 1000; ++i) {\n    j.push_back(i);\n}\n
        "},{"location":"features/element_access/unchecked_access/#summary","title":"Summary","text":"scenario non-const value const value access to existing object key reference to existing value is returned const reference to existing value is returned access to valid array index reference to existing value is returned const reference to existing value is returned access to non-existing object key reference to newly inserted null value is returned undefined behavior; runtime assertion in debug mode access to invalid array index reference to newly inserted null value is returned; any index between previous maximal index and passed index are filled with null undefined behavior; runtime assertion in debug mode"},{"location":"features/parsing/","title":"Parsing","text":"

        This library can create a JSON value from a wide range of inputs. This page gives an overview of the available parsing functions and how they behave; the linked pages go into more detail.

        flowchart LR\n    I[\"JSON input\"] --> P[\"parse()\"]\n    I --> S[\"sax_parse()\"]\n    I --> A[\"accept()\"]\n    P -->|\"optional parser callback filters values\"| D[\"basic_json value (DOM)\"]\n    S --> H[\"events delivered to a user SAX handler\"]\n    A --> V[\"bool: is the input valid JSON?\"]
        "},{"location":"features/parsing/#input","title":"Input","text":"

        The parse function reads a JSON value from an input. The input can be

        • a string (std::string, C string, or string literal),
        • a std::istream (e.g., an std::ifstream reading from a file),
        • a FILE* pointer,
        • a pair of iterators over a contiguous range (e.g., a std::vector<std::uint8_t>), or
        • a contiguous container.
        // parse from a string\njson j = json::parse(R\"({\"happy\": true, \"pi\": 3.141})\");\n\n// parse from a file\nstd::ifstream f(\"example.json\");\njson data = json::parse(f);\n

        The input must be encoded in UTF-8; other encodings are not supported. A single input may contain only one JSON value. Inputs consisting of multiple values separated by newlines are handled by the JSON Lines format.

        By default, the library rejects comments and trailing commas. Both can be enabled with parameters of the parse function \u2014 see comments and trailing commas.

        "},{"location":"features/parsing/#strictness-and-trailing-data","title":"Strictness and trailing data","text":"

        parse reads a single JSON value and requires the whole input to be consumed: any non-whitespace data after the value is reported as a parse error. Use it when you want to guarantee that an input is exactly one complete JSON document.

        operator>> follows relaxed std::istream semantics instead: it parses one JSON value and leaves the stream positioned right after it, without requiring the rest of the stream to be consumed. This is what makes it possible to read several concatenated values from the same stream, but it also means that \"a valid document followed by trailing bytes\" is accepted rather than rejected. If you are validating conformance, or need to reject any input that is not exactly one JSON document, prefer parse.

        When using operator>> to read several concatenated values this way, a value that is a number must be followed by whitespace, because operator>> consumes the character that terminates a number, unless JSON_PRECISE_STREAM_POSITION is defined to 1 \u2014 see the operator>> notes for details and examples.

        "},{"location":"features/parsing/#input-sources-and-performance","title":"Input sources and performance","text":"

        All inputs produce the same values and the same exceptions, but they are read in different ways, which makes some of them faster than others:

        input how it is read strings, string views, string literals, contiguous containers, pointer or iterator ranges over contiguous memory the whole input is scanned in bulk: runs of string characters, numbers, and indentation are recognized several bytes at a time std::istream (std::ifstream, std::istringstream, ...) the characters the stream buffer holds are scanned in bulk like a string; the stream buffer is asked for more when they are used up FILE* one character at a time, with the file locked once for the whole parse other iterator ranges (e.g., std::list<char>, std::istreambuf_iterator<char>), wide strings one character at a time

        Parsing a file from a std::ifstream is therefore only about 10 to 20 % slower than reading the whole file into a std::string first and parsing that, and it needs no memory for a copy of the file. The stream is never read ahead: after parsing, it is positioned exactly as if every character had been read one at a time, so operator>> can still read several values from one stream (see also JSON Lines).

        Two macros switch these optimizations off without changing any result: JSON_DISABLE_STREAM_BULK_SCAN for stream buffers that do not follow the usual std::streambuf contract, and JSON_DISABLE_UNLOCKED_FILE_IO for programs in which another thread must use the same FILE object while it is parsed.

        "},{"location":"features/parsing/#sax-vs-dom-parsing","title":"SAX vs. DOM parsing","text":"

        The library offers two parsing models:

        • DOM parsing (the default): the complete input is read and stored as an in-memory basic_json value that can be traversed and modified freely. This is what parse does, and it is the right choice for most use cases.
        • SAX parsing: instead of building a value, the parser reports events (such as \"a string was read\" or \"an object started\") to a handler that you implement. This avoids building the full value in memory and is useful for very large inputs or when you only need to extract parts of the input. See the SAX interface for details and sax_parse for the API.

        You can influence a DOM parse without switching to the SAX interface by passing a parser callback, which is called during parsing and can, for example, discard parts of the input.

        "},{"location":"features/parsing/#exceptions","title":"Exceptions","text":"

        When the input is not valid JSON, the parse function throws an exception by default. If exceptions are undesired or unavailable, the parser can instead return a discarded value, or accept can be used to only check whether an input is valid JSON. See parsing and exceptions for the available options.

        "},{"location":"features/parsing/#see-also","title":"See also","text":"
        • parse - deserialize from a compatible input
        • accept - check if the input is valid JSON
        • sax_parse - generate SAX events
        • JSON Lines - parse newline-delimited JSON
        • parser callbacks - influence the parsing by a callback function
        • SAX interface - implement a custom SAX handler
        • parsing and exceptions - control error handling
        • parsing untrusted input - what to consider when parsing input from untrusted sources
        "},{"location":"features/parsing/json_lines/","title":"JSON Lines","text":"

        The JSON Lines format is a text format of newline-delimited JSON. In particular:

        1. The input must be UTF-8 encoded.
        2. Every line must be a valid JSON value.
        3. The line separator must be \\n. As \\r is silently ignored, \\r\\n is also supported.
        4. The final character may be \\n, but is not required to be one.

        JSON Text example

        {\"name\": \"Gilbert\", \"wins\": [[\"straight\", \"7\u2663\"], [\"one pair\", \"10\u2665\"]]}\n{\"name\": \"Alexa\", \"wins\": [[\"two pair\", \"4\u2660\"], [\"two pair\", \"9\u2660\"]]}\n{\"name\": \"May\", \"wins\": []}\n{\"name\": \"Deloise\", \"wins\": [[\"three of a kind\", \"5\u2663\"]]}\n

        JSON Lines input with more than one value is treated as invalid JSON by the parse or accept functions. To process it line by line, functions like std::getline can be used:

        Example: Parse JSON Text input line by line

        The example below demonstrates how JSON Lines can be processed.

        #include <sstream>\n#include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // JSON Lines (see https://jsonlines.org)\n    std::stringstream input;\n    input << R\"({\"name\": \"Gilbert\", \"wins\": [[\"straight\", \"7\u2663\"], [\"one pair\", \"10\u2665\"]]}\n{\"name\": \"Alexa\", \"wins\": [[\"two pair\", \"4\u2660\"], [\"two pair\", \"9\u2660\"]]}\n{\"name\": \"May\", \"wins\": []}\n{\"name\": \"Deloise\", \"wins\": [[\"three of a kind\", \"5\u2663\"]]}\n)\";\n\n    std::string line;\n    while (std::getline(input, line))\n    {\n        std::cout << json::parse(line) << std::endl;\n    }\n}\n

        Output:

        {\"name\":\"Gilbert\",\"wins\":[[\"straight\",\"7\u2663\"],[\"one pair\",\"10\u2665\"]]}\n{\"name\":\"Alexa\",\"wins\":[[\"two pair\",\"4\u2660\"],[\"two pair\",\"9\u2660\"]]}\n{\"name\":\"May\",\"wins\":[]}\n{\"name\":\"Deloise\",\"wins\":[[\"three of a kind\",\"5\u2663\"]]}\n

        Note

        Using operator>> like

        json j;\nwhile (input >> j)\n{\n    std::cout << j << std::endl;\n}\n

        with a JSON Lines input does not work, because the parser will try to parse one value after the last one and throw a parse_error.101 exception. The same happens for a stream of concatenated (non-newline-delimited) JSON values: operator>> reads them one at a time, but the loop above throws after the last value. To read either format with operator>>, check for the end of the stream before each read:

        json j;\nwhile (input >> std::ws && input.peek() != std::char_traits<char>::eof())\n{\n    input >> j;\n    std::cout << j << std::endl;\n}\n

        A value that is a number must be followed by whitespace -- see the notes of operator>> for details.

        "},{"location":"features/parsing/parse_exceptions/","title":"Parsing and Exceptions","text":"

        When the input is not valid JSON, an exception of type parse_error is thrown. This exception contains the position in the input where the error occurred, together with a diagnostic message and the last read input token. The exceptions page contains a list of examples for parse error exceptions. In case you process untrusted input, always enclose your code with a try/catch block, like

        json j;\ntry\n{\n    j = json::parse(my_input);\n}\ncatch (json::parse_error& ex)\n{\n    std::cerr << \"parse error at byte \" << ex.byte << std::endl;\n}\n

        In case exceptions are undesired or not supported by the environment, there are different ways to proceed:

        "},{"location":"features/parsing/parse_exceptions/#switch-off-exceptions","title":"Switch off exceptions","text":"

        The parse() function accepts a bool parameter allow_exceptions which controls whether an exception is thrown when a parse error occurs (true, default) or whether a discarded value should be returned (false).

        json j = json::parse(my_input, nullptr, false);\nif (j.is_discarded())\n{\n    std::cerr << \"parse error\" << std::endl;\n}\n

        Note there is no diagnostic information available in this scenario.

        "},{"location":"features/parsing/parse_exceptions/#use-accept-function","title":"Use accept() function","text":"

        Alternatively, function accept() can be used which does not return a json value, but a bool indicating whether the input is valid JSON.

        if (!json::accept(my_input))\n{\n    std::cerr << \"parse error\" << std::endl;\n}\n

        Again, there is no diagnostic information available.

        "},{"location":"features/parsing/parse_exceptions/#user-defined-sax-interface","title":"User-defined SAX interface","text":"

        Finally, you can implement the SAX interface and decide what should happen in case of a parse error.

        This function has the following interface:

        bool parse_error(std::size_t position,\n                 const std::string& last_token,\n                 const json::exception& ex);\n

        The return value indicates whether the parsing should continue, so the function should usually return false.

        Example: report parse errors without exceptions

        The example derives from the library's DOM parser and overrides parse_error to print the error instead of throwing. Note the DOM parser is an implementation detail (nlohmann::detail) and may change between releases; see Do not use the detail namespace.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\n// a DOM parser that reports parse errors instead of throwing\nclass sax_no_exception : public nlohmann::detail::json_sax_dom_parser<json, nlohmann::detail::string_input_adapter_type>\n{\n  public:\n    explicit sax_no_exception(json& j)\n        : nlohmann::detail::json_sax_dom_parser<json, nlohmann::detail::string_input_adapter_type>(j, false)\n    {}\n\n    bool parse_error(std::size_t position,\n                     const std::string& last_token,\n                     const json::exception& ex)\n    {\n        std::cout << \"parse error at input byte \" << position << \"\\n\"\n                  << ex.what() << \"\\n\"\n                  << \"last read: \\\"\" << last_token << \"\\\"\"\n                  << std::endl;\n        return false;\n    }\n};\n\nint main()\n{\n    std::string myinput = \"[1,2,3,]\";\n\n    json result;\n    sax_no_exception sax(result);\n\n    bool parse_result = json::sax_parse(myinput, &sax);\n    if (!parse_result)\n    {\n        std::cout << \"parsing unsuccessful!\" << std::endl;\n    }\n\n    std::cout << \"parsed value: \" << result << std::endl;\n}\n

        Output:

        parse error at input byte 8\n[json.exception.parse_error.101] parse error at line 1, column 8: syntax error while parsing value - unexpected ']'; expected '[', '{', or a literal\nlast read: \"3,]\"\nparsing unsuccessful!\nparsed value: [1,2,3]\n
        "},{"location":"features/parsing/parser_callbacks/","title":"Parser Callbacks","text":""},{"location":"features/parsing/parser_callbacks/#overview","title":"Overview","text":"

        With a parser callback function, the result of parsing a JSON text can be influenced. When passed to parse, it is called on certain events (passed as parse_event_t via parameter event) with a set recursion depth depth and context JSON value parsed. The return value of the callback function is a boolean indicating whether the element that emitted the callback shall be kept or not.

        The type of the callback function is:

        template<typename BasicJsonType>\nusing parser_callback_t =\n    std::function<bool(int depth, parse_event_t event, BasicJsonType& parsed)>;\n
        "},{"location":"features/parsing/parser_callbacks/#callback-event-types","title":"Callback event types","text":"

        We distinguish six scenarios (determined by the event type) in which the callback function can be called. The following table describes the values of the parameters depth, event, and parsed.

        parameter event description parameter depth parameter parsed parse_event_t::object_start the parser read { and started to process a JSON object depth of the parent of the JSON object a JSON value with type discarded parse_event_t::key the parser read a key of a value in an object depth of the currently parsed JSON object a JSON string containing the key parse_event_t::object_end the parser read } and finished processing a JSON object depth of the parent of the JSON object the parsed JSON object parse_event_t::array_start the parser read [ and started to process a JSON array depth of the parent of the JSON array a JSON value with type discarded parse_event_t::array_end the parser read ] and finished processing a JSON array depth of the parent of the JSON array the parsed JSON array parse_event_t::value the parser finished reading a JSON value depth of the value the parsed JSON value Example: sequence of callback events

        When parsing the following JSON text,

        {\n    \"name\": \"Berlin\",\n    \"location\": [\n        52.519444,\n        13.406667\n    ]\n}\n

        these calls are made to the callback function:

        event depth parsed object_start 0 discarded key 1 \"name\" value 1 \"Berlin\" key 1 \"location\" array_start 1 discarded value 2 52.519444 value 2 13.406667 array_end 1 [52.519444,13.406667] object_end 0 {\"location\":[52.519444,13.406667],\"name\":\"Berlin\"}

        No built-in nesting depth limit

        The library has no built-in limit on recursion/nesting depth while parsing. A parser callback can only discard content it has already parsed (by returning false); it cannot make parsing fail once a nesting limit is exceeded partway through reading a deeply nested value. If you need to reject over-deep untrusted input outright, track depth in a callback and throw from it once your limit is exceeded (a thrown exception propagates out of parse() as usual).

        "},{"location":"features/parsing/parser_callbacks/#return-value","title":"Return value","text":"

        Discarding a value (i.e., returning false) has different effects depending on the context in which the function was called:

        • Discarded values in structured types are skipped. That is, the parser will behave as if the discarded value was never read.
        • In case a value outside a structured type is skipped, it is replaced with null. This case happens if the top-level element is skipped.
        Example: skip an object key while parsing

        The example below demonstrates the parse() function with and without callback function.

        #include <iostream>\n#include <iomanip>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    // a JSON text\n    auto text = R\"(\n    {\n        \"Image\": {\n            \"Width\":  800,\n            \"Height\": 600,\n            \"Title\":  \"View from 15th Floor\",\n            \"Thumbnail\": {\n                \"Url\":    \"http://www.example.com/image/481989943\",\n                \"Height\": 125,\n                \"Width\":  100\n            },\n            \"Animated\" : false,\n            \"IDs\": [116, 943, 234, 38793]\n        }\n    }\n    )\";\n\n    // parse and serialize JSON\n    json j_complete = json::parse(text);\n    std::cout << std::setw(4) << j_complete << \"\\n\\n\";\n\n    // define parser callback\n    json::parser_callback_t cb = [](int depth, json::parse_event_t event, json & parsed)\n    {\n        // skip object elements with key \"Thumbnail\"\n        if (event == json::parse_event_t::key and parsed == json(\"Thumbnail\"))\n        {\n            return false;\n        }\n        else\n        {\n            return true;\n        }\n    };\n\n    // parse (with callback) and serialize JSON\n    json j_filtered = json::parse(text, cb);\n    std::cout << std::setw(4) << j_filtered << '\\n';\n}\n

        Output:

        {\n    \"Image\": {\n        \"Animated\": false,\n        \"Height\": 600,\n        \"IDs\": [\n            116,\n            943,\n            234,\n            38793\n        ],\n        \"Thumbnail\": {\n            \"Height\": 125,\n            \"Url\": \"http://www.example.com/image/481989943\",\n            \"Width\": 100\n        },\n        \"Title\": \"View from 15th Floor\",\n        \"Width\": 800\n    }\n}\n\n{\n    \"Image\": {\n        \"Animated\": false,\n        \"Height\": 600,\n        \"IDs\": [\n            116,\n            943,\n            234,\n            38793\n        ],\n        \"Title\": \"View from 15th Floor\",\n        \"Width\": 800\n    }\n}\n
        "},{"location":"features/parsing/parser_callbacks/#recipe-rejecting-duplicate-object-keys","title":"Recipe: rejecting duplicate object keys","text":"

        The JSON specification leaves the handling of objects with repeated keys up to the implementation. As described in object_t, it is unspecified which value for a repeated key ends up in the resulting json value -- once parsing has produced that value, the duplicate is already gone, because object storage maps each key to a single value. If duplicate keys should instead be treated as an error, a parser callback can detect them while the object is still being read, before that ambiguity ever applies.

        Example: reject duplicate object keys 
        #include <iostream>\n#include <nlohmann/json.hpp>\n#include <stdexcept>\n#include <string>\n#include <unordered_set>\n#include <vector>\n\nusing json = nlohmann::json;\n\njson parse_strict(const std::string& input)\n{\n    // one key set per nesting depth, reused across sibling objects\n    std::vector<std::unordered_set<std::string>> keys;\n\n    auto reject_duplicate_keys = [&](int depth, json::parse_event_t event, json & parsed)\n    {\n        if (event == json::parse_event_t::object_start)\n        {\n            // keys of this object are reported at depth+1 (see the event table above)\n            const auto child_depth = static_cast<std::size_t>(depth) + 1;\n            if (keys.size() <= child_depth)\n            {\n                keys.resize(child_depth + 1);\n            }\n            keys[child_depth].clear();\n            return true;\n        }\n\n        if (event == json::parse_event_t::key)\n        {\n            auto& seen = keys[static_cast<std::size_t>(depth)];\n            const auto& key = parsed.get_ref<const std::string&>();\n            if (!seen.insert(key).second)\n            {\n                throw std::runtime_error(\"duplicate JSON object key: \" + key);\n            }\n            return true;\n        }\n\n        return true;\n    };\n\n    return json::parse(input, reject_duplicate_keys);\n}\n\nint main()\n{\n    // parsing succeeds when all keys are unique\n    json j = parse_strict(R\"({\"one\": 1, \"two\": 2})\");\n    std::cout << j << '\\n';\n\n    // parsing throws when a key is repeated\n    try\n    {\n        parse_strict(R\"({\"one\": 1, \"one\": 2})\");\n    }\n    catch (const std::exception& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

        Output:

        {\"one\":1,\"two\":2}\nduplicate JSON object key: one\n

        This approach has three limitations:

        • The depth-indexed bookkeeping must account for the fact that object_start reports the depth of the parent of the object, while the key events inside that object are reported one depth deeper (see the event table above); it is easy to get this off by one for nested objects.
        • The thrown exception cannot carry a parse_error-style byte offset, because position tracking only exists inside the parser and lexer, not at the callback layer.
        • The exception only names the repeated key, not where it occurs in the document. Reporting its full path requires maintaining a stack of the enclosing keys and array indices in the callback as well.

        A SAX interface does not lift the position limitation: its key function receives no position either -- only parse_error is passed the byte position.

        "},{"location":"features/parsing/parser_callbacks/#recipe-streaming-a-large-homogeneous-array","title":"Recipe: streaming a large homogeneous array","text":"

        A common use case is a huge top-level array of many similarly-shaped objects, too large to hold entirely in memory as a json value. A parser callback can hand off each completed element to a user function and then discard it, so memory usage stays bounded by a single element (plus the not-yet-parsed tail of the input) rather than the whole document. Since the top-level array's array_start/array_end are reported at depth == 0 (its parent is the document root), the object elements it contains are reported at depth == 1:

        Example: stream a large top-level array 
        std::ifstream input(\"large_array.json\");\n\nauto callback = [](int depth, json::parse_event_t event, json& parsed) -> bool {\n    if (depth == 1 && event == json::parse_event_t::object_end) {\n        handle_element(parsed); // process the element, e.g. write it elsewhere\n        return false;           // discard it -- frees its memory before the next one is parsed\n    }\n    return true; // keep everything else, including the (by then empty) top-level array\n};\n\njson::parse(input, callback);\n

        If the array's elements are scalars or nested arrays instead of objects, check for parse_event_t::value or parse_event_t::array_end at depth == 1 instead. The same approach works for a top-level object of many homogeneous values by checking object_end/value events at depth == 1 there too.

        "},{"location":"features/parsing/parser_callbacks/#recipe-max-nesting-depth-via-a-callback","title":"Recipe: max nesting depth via a callback","text":"

        Since there is no built-in nesting-depth limit (see the note above), a callback can enforce one manually by tracking the maximum depth seen and throwing once it is exceeded:

        Example: limit the nesting depth 
        constexpr int max_depth = 32;\n\nauto callback = [](int depth, json::parse_event_t /*event*/, json& /*parsed*/) -> bool {\n    if (depth > max_depth) {\n        throw std::runtime_error(\"maximum nesting depth exceeded\");\n    }\n    return true;\n};\n\njson::parse(input, callback);\n
        "},{"location":"features/parsing/sax_interface/","title":"SAX Interface","text":"

        The library uses a SAX-like interface with the following functions:

        classDiagram\n\nclass sax_t [\"json::sax_t\"] {\n    <<interface>>\n    +bool null()*\n\n    +bool boolean(bool val)*\n\n    +bool number_integer(number_integer_t val)*\n    +bool number_unsigned(number_unsigned_t val)*\n\n    +bool number_float(number_float_t val, const string_t& s)*\n\n    +bool string(string_t& val)*\n    +bool binary(binary_t& val)*\n\n    +bool start_object(std::size_t elements)*\n    +bool end_object()*\n    +bool start_array(std::size_t elements)*\n    +bool end_array()*\n    +bool key(string_t& val)*\n\n    +bool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex)*\n}
        // called when null is parsed\nbool null();\n\n// called when a boolean is parsed; value is passed\nbool boolean(bool val);\n\n// called when a signed or unsigned integer number is parsed; value is passed\nbool number_integer(number_integer_t val);\nbool number_unsigned(number_unsigned_t val);\n\n// called when a floating-point number is parsed; value and original string is passed\nbool number_float(number_float_t val, const string_t& s);\n\n// called when a string is parsed; value is passed and can be safely moved away\nbool string(string_t& val);\n// called when a binary value is parsed; value is passed and can be safely moved away\nbool binary(binary_t& val);\n\n// called when an object or array begins or ends, resp. The number of elements is passed (or -1 if not known)\nbool start_object(std::size_t elements);\nbool end_object();\nbool start_array(std::size_t elements);\nbool end_array();\n// called when an object key is parsed; value is passed and can be safely moved away\nbool key(string_t& val);\n\n// called when a parse error occurs; byte position, the last token, and an exception is passed\nbool parse_error(std::size_t position, const std::string& last_token, const json::exception& ex);\n

        The return value of each function determines whether parsing should proceed.

        To implement your own SAX handler, proceed as follows:

        1. Implement the SAX interface in a class. You can use class nlohmann::json_sax<json> as base class, but you can also use any class where the functions described above are implemented and public.
        2. Create an object of your SAX interface class, e.g. my_sax.
        3. Call bool json::sax_parse(input, &my_sax); where the first parameter can be any input like a string or an input stream and the second parameter is a pointer to your SAX interface.

        Note the sax_parse function only returns a bool indicating the result of the last executed SAX event. It does not return json value - it is up to you to decide what to do with the SAX events. Furthermore, no exceptions are thrown in case of a parse error - it is up to you what to do with the exception object passed to your parse_error implementation. Internally, the SAX interface is used for the DOM parser (class json_sax_dom_parser) as well as the acceptor (json_sax_acceptor), see file json_sax.hpp.

        "},{"location":"features/parsing/sax_interface/#see-also","title":"See also","text":"
        • json_sax - documentation of the SAX interface
        • sax_parse - SAX parser
        "},{"location":"features/parsing/untrusted_input/","title":"Parsing Untrusted Input","text":"

        This page is for applications that parse JSON -- or one of the supported binary formats (BJData, BON8, BSON, CBOR, MessagePack, UBJSON) -- from a source they do not fully control, such as a network connection, an uploaded file, or another process. It summarizes what the library already does for such input and what remains the caller's responsibility, linking to the pages that cover each aspect in detail rather than repeating them.

        For the project's threat model and the countermeasures behind these behaviors, see the assurance case; to report a vulnerability, see the security policy.

        "},{"location":"features/parsing/untrusted_input/#errors-without-exceptions","title":"Errors without exceptions","text":"

        By default, parse() throws a parse_error (for instance parse_error.101 for a syntax error) when the input is not valid. If your environment cannot use exceptions for untrusted input, the library offers several alternatives; see Parsing and exceptions for the full comparison:

        • Pass false as the third argument to parse() to get a discarded value (checked with is_discarded()) instead of a thrown exception, with no diagnostic information.
        • Use accept() to only check whether the input is valid JSON, without building a value.
        • Implement the SAX interface and override parse_error() to react to an error yourself, with the byte position and the exception that would otherwise have been thrown; see the example that overrides it to print instead of throw.

        If exceptions are unavailable entirely (-fno-exceptions, or JSON_NOEXCEPTION defined), every throw in the library becomes a call to std::abort() -- there is no way to recover from a parse error of untrusted input in that configuration; see Switch off exceptions for the details and for overriding this with JSON_THROW_USER.

        "},{"location":"features/parsing/untrusted_input/#nesting-depth","title":"Nesting depth","text":"

        The JSON parser and the binary readers are iterative: they keep the containers they are currently inside of on a heap-allocated stack instead of calling themselves once per nesting level, so the native call stack does not grow with the nesting depth of the input. A deeply nested document is therefore bounded by available memory, not by the call stack, however deeply it is nested.

        No built-in depth limit while parsing

        Neither the parser nor the binary readers impose a limit on how deep the input may nest. An attacker can still exhaust memory (though not the call stack) with a sufficiently deep document. If you need to reject over-deep untrusted input outright, track the depth yourself, either with a parser callback for the JSON parser, or by counting start_object/start_array and end_object/end_array calls in a SAX handler (for the JSON parser or a binary format alike) and throwing once your limit is exceeded.

        Once a value has been parsed, operations that walk it recursively -- serializing it with dump, hashing it, copying it, comparing two values with ==, <, or (in C++20) <=>, merging with update, applying a merge_patch, creating a patch with diff or merge_diff, flatten, and the binary writers (to_cbor, to_msgpack, ...) -- descend at most 128 levels on the call stack and continue below that with an explicit stack instead, so none of them can exhaust the stack either, however deeply the value is nested. Destroying a value (its destructor) never recurses at all, regardless of nesting depth, for the same reason.

        "},{"location":"features/parsing/untrusted_input/#input-size","title":"Input size","text":"

        The library does not limit the overall size of a JSON text; a value nested or wide enough will use memory proportional to the input. If you parse untrusted input of unbounded size, check the size of the file or stream yourself before -- or while -- handing it to parse().

        For the binary formats, an announced size is never trusted outright:

        • Reading a string or binary value copies the input in bounded 4096-byte chunks and grows the result as bytes are actually consumed, rather than allocating the announced length up front -- a truncated input runs out of bytes (reported as a parse error) instead of triggering an oversized allocation.
        • When an array announces its number of elements and the array container supports reserve() (as std::vector, the default, does), the library reserves storage for at most 16384 of them upfront, regardless of how large the announced count is; further elements still grow the container normally as they are read.
        • An announced array or object size that exceeds what the target container could ever hold (its max_size()) is rejected immediately as out_of_range.408, without attempting to allocate anything.
        "},{"location":"features/parsing/untrusted_input/#strings","title":"Strings","text":"

        Invalid UTF-8 is rejected while parsing, not just while serializing:

        • In JSON text, an ill-formed UTF-8 byte in a string is a parse_error.101 (\"invalid string: ill-formed UTF-8 byte\").
        • In a binary format, a string that is not valid UTF-8 is a parse_error.113.

        A '\\0' (NUL) byte inside a quoted JSON string is always rejected (it must be escaped as \\u0000). A NUL byte outside of a string is different: by default it is silently treated as the end of the input, so trailing bytes after it -- including further, otherwise well-formed JSON -- are silently ignored rather than rejected. Since untrusted input that happens to embed a NUL is a way to make part of it disappear without a parse error, see the FAQ entry and consider defining JSON_STRICT_NUL_HANDLING to 1 to reject a NUL byte like any other unexpected byte instead.

        Parsing is not the only place invalid UTF-8 matters: a string that reached a json value some other way (for example, constructed by application code, or, before JSON_STRICT_NUL_HANDLING existed, read from a binary format that does not validate strings) still has to round-trip back to JSON text. By default, dump() throws type_error.316 if the string is not valid UTF-8; passing error_handler_t::replace or error_handler_t::ignore avoids the exception instead of crashing an application that forgot to catch it. See Handling invalid UTF-8 for the options and an example.

        "},{"location":"features/parsing/untrusted_input/#duplicate-object-keys","title":"Duplicate object keys","text":"

        The JSON specification leaves the handling of repeated keys in an object up to the implementation, and this library does too: as described in object_t, it is unspecified which of the values for a repeated key ends up in the parsed object. If your application must reject duplicate keys instead of silently resolving them one way or another, see the parser callback recipe for rejecting duplicate keys.

        "},{"location":"features/parsing/untrusted_input/#numbers","title":"Numbers","text":"

        A number whose value cannot be represented -- for instance 1E1000, which overflows double -- is rejected while parsing as out_of_range.406 rather than silently becoming infinity. An integer that is syntactically valid but does not fit the 64-bit integer types is not rejected; it is instead stored as a double, which may lose precision for very large values. See number limits for the exact ranges and an example.

        "},{"location":"features/parsing/untrusted_input/#comments-and-trailing-commas","title":"Comments and trailing commas","text":"

        Both comments and trailing commas are rejected by default, matching the JSON specification; they must be explicitly enabled per call with the ignore_comments and ignore_trailing_commas parameters of parse() or accept(). Do not enable either for input whose conformance you cannot otherwise control, since interoperability with strictly conforming JSON consumers is exactly what the default rejects.

        "},{"location":"features/parsing/untrusted_input/#checklist","title":"Checklist","text":"
        • Wrap parsing in a try/catch block, or use allow_exceptions=false/accept() if your environment cannot use exceptions; never let JSON_NOEXCEPTION's abort() be the first time you think about error handling.
        • If the input's nesting depth matters to you, enforce your own limit with a parser callback or a SAX handler; the library bounds the call stack but not memory use.
        • Bound the size of the input itself before parsing, independent of the library's own bounded allocations for binary format lengths.
        • Decide up front how you want a string with invalid UTF-8 -- from any source, not only parsing -- to be serialized (strict, replace, or ignore), rather than discovering it from an uncaught type_error.316.
        • If a stray NUL byte silently truncating trailing input is a problem for your input format, define JSON_STRICT_NUL_HANDLING.
        • Decide whether duplicate object keys should be an error for your application, and add a callback if so.
        • Do not enable ignore_comments or ignore_trailing_commas for input that must be strictly conforming JSON.

        For the broader design rationale and how it is tested (fuzzing, sanitizers, static analysis), see the assurance case and quality assurance. To report a security issue in the library itself, follow the security policy.

        "},{"location":"features/parsing/untrusted_input/#see-also","title":"See also","text":"
        • Parsing - overview of the parsing functions
        • Parsing and exceptions - error handling without exceptions
        • Parser callbacks - depth limits, duplicate-key rejection, and streaming recipes
        • SAX interface - implement a custom handler with access to parse errors and positions
        • Serialization - handling invalid UTF-8 when dumping
        • Number handling - number ranges and overflow behavior
        • Assurance case - the library's threat model and countermeasures
        • Security policy - how to report a vulnerability
        "},{"location":"features/types/","title":"Types","text":"

        This page gives an overview of how JSON values are stored and how this can be configured.

        "},{"location":"features/types/#overview","title":"Overview","text":"

        By default, JSON values are stored as follows:

        JSON type C++ type object std::map<std::string, basic_json> array std::vector<basic_json> null std::nullptr_t string std::string boolean bool number std::int64_t, std::uint64_t, and double

        Note there are three different types for numbers - when parsing JSON text, the best fitting type is chosen.

        "},{"location":"features/types/#storage","title":"Storage","text":"
        classDiagram\n\nclass value_t {\n    <<enumeration>>\n    null\n    object\n    array\n    string\n    boolean\n    number_integer\n    number_unsigned\n    number_float\n    binary\n    discarded\n}\n\nclass json_value {\n    <<union>>\n    object_t* object\n    array_t* array\n    string_t* string\n    binary_t* binary\n    boolean_t boolean\n    number_integer_t number_integer\n    number_unsigned_t number_unsigned\n    number_float_t number_float\n}\n\nclass basic_json {\n    -value_t m_type\n    -json_value m_value\n    +typedef object_t\n    +typedef array_t\n    +typedef binary_t\n    +typedef boolean_t\n    +typedef number_integer_t\n    +typedef number_unsigned_t\n    +typedef number_float_t\n}\n\nbasic_json .. json_value\nbasic_json .. value_t
        "},{"location":"features/types/#template-arguments","title":"Template arguments","text":"

        The data types to store a JSON value are derived from the template arguments passed to class basic_json:

        template<\n    template<typename U, typename V, typename... Args> class ObjectType = std::map,\n    template<typename U, typename... Args> class ArrayType = std::vector,\n    class StringType = std::string,\n    class BooleanType = bool,\n    class NumberIntegerType = std::int64_t,\n    class NumberUnsignedType = std::uint64_t,\n    class NumberFloatType = double,\n    template<typename U> class AllocatorType = std::allocator,\n    template<typename T, typename SFINAE = void> class JSONSerializer = adl_serializer,\n    class BinaryType = std::vector<std::uint8_t>,\n    class CustomBaseClass = void\n>\nclass basic_json;\n

        Type json is an alias for basic_json<> and uses the default types.

        From the template arguments, the following types are derived:

        using object_comparator_t = std::less<>;\nusing object_t = ObjectType<StringType, basic_json, object_comparator_t,\n                   AllocatorType<std::pair<const StringType, basic_json>>>;\n\nusing array_t = ArrayType<basic_json, AllocatorType<basic_json>>;\n\nusing string_t = StringType;\n\nusing boolean_t = BooleanType;\n\nusing number_integer_t = NumberIntegerType;\nusing number_unsigned_t = NumberUnsignedType;\nusing number_float_t = NumberFloatType;\n\nusing binary_t = nlohmann::byte_container_with_subtype<BinaryType>;\n

        Not every type can be passed for these template arguments: the library uses the resulting types in ways that imply a number of requirements, for instance that StringType is char-based or that ArrayType is vector-like. These requirements are collected in Template Parameter Requirements.

        "},{"location":"features/types/#objects","title":"Objects","text":"

        RFC 8259 describes JSON objects as follows:

        An object is an unordered collection of zero or more name/value pairs, where a name is a string and a value is a string, number, boolean, null, object, or array.

        "},{"location":"features/types/#default-type","title":"Default type","text":"

        With the default values for ObjectType (std::map), StringType (std::string), and AllocatorType (std::allocator), the default type for object_t is:

        std::map<\n  std::string, // key_type\n  basic_json, // value_type\n  std::less<>, // key_compare\n  std::allocator<std::pair<const std::string, basic_json>> // allocator_type\n>\n
        "},{"location":"features/types/#behavior","title":"Behavior","text":"

        The choice of object_t influences the behavior of the JSON class. With the default type, objects have the following behavior:

        • When all names are unique, objects will be interoperable in the sense that all software implementations receiving that object will agree on the name-value mappings.
        • When the names within an object are not unique, it is unspecified which one of the values for a given key will be chosen. For instance, {\"key\": 2, \"key\": 1} could be equal to either {\"key\": 1} or {\"key\": 2}. To reject duplicate keys instead of silently resolving them one way or another, see this parsing recipe.
        • Internally, name/value pairs are stored in lexicographical order of the names. Objects will also be serialized (see dump) in this order. For instance, both {\"b\": 1, \"a\": 2} and {\"a\": 2, \"b\": 1} will be stored and serialized as {\"a\": 2, \"b\": 1}.
        • When comparing objects, the order of the name/value pairs is irrelevant. This makes objects interoperable in the sense that they will not be affected by these differences. For instance, {\"b\": 1, \"a\": 2} and {\"a\": 2, \"b\": 1} will be treated as equal.
        "},{"location":"features/types/#key-order","title":"Key order","text":"

        The order in which name/value pairs are added to the object is not preserved by the library. Therefore, iterating an object may return name/value pairs in a different order than they were originally stored. In fact, keys will be traversed in alphabetical order as std::map with std::less is used by default. Please note this behavior conforms to RFC 8259, because any order implements the specified \"unordered\" nature of JSON objects.

        "},{"location":"features/types/#limits","title":"Limits","text":"

        RFC 8259 specifies:

        An implementation may set limits on the maximum depth of nesting.

        In this class, the object's limit of nesting is not explicitly constrained. However, a maximum depth of nesting may be introduced by the compiler or runtime environment. A theoretical limit can be queried by calling the max_size function of a JSON object.

        "},{"location":"features/types/#storage_1","title":"Storage","text":"

        Objects are stored as pointers in a basic_json type. That is, for any access to object values, a pointer of type object_t* must be dereferenced.

        "},{"location":"features/types/#converting-maps-with-non-string-keys","title":"Converting maps with non-string keys","text":"

        A std::map/std::unordered_map whose key type is not string-like (e.g., std::map<int, std::string>) is converted to a JSON array of 2-element [key, value] arrays rather than a JSON object, because JSON object keys must be strings:

        std::map<int, std::string> m{{1, \"one\"}, {2, \"two\"}};\njson j = m;\n// j is [[1,\"one\"],[2,\"two\"]], not {\"1\":\"one\",\"2\":\"two\"}\n
        "},{"location":"features/types/#arrays","title":"Arrays","text":"

        RFC 8259 describes JSON arrays as follows:

        An array is an ordered sequence of zero or more values.

        "},{"location":"features/types/#default-type_1","title":"Default type","text":"

        With the default values for ArrayType (std::vector) and AllocatorType (std::allocator), the default type for array_t is:

        std::vector<\n  basic_json, // value_type\n  std::allocator<basic_json> // allocator_type\n>\n
        "},{"location":"features/types/#limits_1","title":"Limits","text":"

        RFC 8259 specifies:

        An implementation may set limits on the maximum depth of nesting.

        In this class, the array's limit of nesting is not explicitly constrained. However, a maximum depth of nesting may be introduced by the compiler or runtime environment. A theoretical limit can be queried by calling the max_size function of a JSON array.

        "},{"location":"features/types/#storage_2","title":"Storage","text":"

        Arrays are stored as pointers in a basic_json type. That is, for any access to array values, a pointer of type array_t* must be dereferenced.

        "},{"location":"features/types/#strings","title":"Strings","text":"

        RFC 8259 describes JSON strings as follows:

        A string is a sequence of zero or more Unicode characters.

        Unicode values are split by the JSON class into byte-sized characters during deserialization.

        "},{"location":"features/types/#default-type_2","title":"Default type","text":"

        With the default values for StringType (std::string), the default type for string_t is std::string.

        "},{"location":"features/types/#encoding","title":"Encoding","text":"

        Strings are stored in UTF-8 encoding. Therefore, functions like std::string::size() or std::string::length() return the number of bytes in the string rather than the number of characters or glyphs.

        "},{"location":"features/types/#string-comparison","title":"String comparison","text":"

        RFC 8259 states:

        Software implementations are typically required to test names of object members for equality. Implementations that transform the textual representation into sequences of Unicode code units and then perform the comparison numerically, code unit by code unit are interoperable in the sense that implementations will agree in all cases on equality or inequality of two strings. For example, implementations that compare strings with escaped characters unconverted may incorrectly find that \"a\\\\b\" and \"a\\u005Cb\" are not equal.

        This implementation is interoperable as it does compare strings code unit by code unit.

        "},{"location":"features/types/#storage_3","title":"Storage","text":"

        String values are stored as pointers in a basic_json type. That is, for any access to string values, a pointer of type string_t* must be dereferenced.

        "},{"location":"features/types/#booleans","title":"Booleans","text":"

        RFC 8259 implicitly describes a boolean as a type which differentiates the two literals true and false.

        "},{"location":"features/types/#default-type_3","title":"Default type","text":"

        With the default values for BooleanType (bool), the default type for boolean_t is bool.

        "},{"location":"features/types/#storage_4","title":"Storage","text":"

        Boolean values are stored directly inside a basic_json type.

        "},{"location":"features/types/#numbers","title":"Numbers","text":"

        See the number handling article for a detailed discussion on how numbers are handled by this library.

        RFC 8259 describes numbers as follows:

        The representation of numbers is similar to that used in most programming languages. A number is represented in base 10 using decimal digits. It contains an integer component that may be prefixed with an optional minus sign, which may be followed by a fraction part and/or an exponent part. Leading zeros are not allowed. (...) Numeric values that cannot be represented in the grammar below (such as Infinity and NaN) are not permitted.

        This description includes both integer and floating-point numbers. However, C++ allows more precise storage if it is known whether the number is a signed integer, an unsigned integer, or a floating-point number. Therefore, three different types, number_integer_t, number_unsigned_t, and number_float_t are used.

        "},{"location":"features/types/#default-types","title":"Default types","text":"

        With the default values for NumberIntegerType (std::int64_t), the default type for number_integer_t is std::int64_t. With the default values for NumberUnsignedType (std::uint64_t), the default type for number_unsigned_t is std::uint64_t. With the default values for NumberFloatType (double), the default type for number_float_t is double.

        "},{"location":"features/types/#default-behavior","title":"Default behavior","text":"
        • The restrictions about leading zeros are not enforced in C++. Instead, leading zeros in integer literals lead to an interpretation as an octal number. Internally, the value will be stored as a decimal number. For instance, the C++ integer literal 010 will be serialized to 8. During deserialization, leading zeros yield an error.
        • Not-a-number (NaN) values will be serialized to null.
        "},{"location":"features/types/#limits_2","title":"Limits","text":"

        RFC 8259 specifies:

        An implementation may set limits on the range and precision of numbers.

        When the default type is used, the maximal integer number that can be stored is 9223372036854775807 (INT64_MAX) and the minimal integer number that can be stored is -9223372036854775808 (INT64_MIN). Integer numbers that are out of range will yield over/underflow when used in a constructor. During deserialization, too large or small integer numbers will automatically be stored as number_unsigned_t or number_float_t.

        When the default type is used, the maximal unsigned integer number that can be stored is 18446744073709551615 (UINT64_MAX) and the minimal integer number that can be stored is 0. Integer numbers that are out of range will yield over/underflow when used in a constructor. During deserialization, too large or small integer numbers will automatically be stored as number_integer_t or number_float_t.

        RFC 8259 further states:

        Note that when such software is used, numbers that are integers and are in the range [-253+1, 253-1] are interoperable in the sense that implementations will agree exactly on their numeric values.

        As this range is a subrange of the exactly supported range [INT64_MIN, INT64_MAX], this class's integer type is interoperable.

        RFC 8259 states:

        This specification allows implementations to set limits on the range and precision of numbers accepted. Since software that implements IEEE 754-2008 binary64 (double precision) numbers is generally available and widely used, good interoperability can be achieved by implementations that expect no more precision or range than these provide, in the sense that implementations will approximate JSON numbers within the expected precision.

        This implementation does exactly follow this approach, as it uses double precision floating-point numbers by default (see NumberFloatType above). Note that with the default double, numbers smaller than -1.79769313486232e+308 or greater than 1.79769313486232e+308 are rejected during parsing with out_of_range.406. Non-finite values (NaN and infinity) can only be stored by assigning them directly and are serialized to null.

        "},{"location":"features/types/#storage_5","title":"Storage","text":"

        Integer number values, unsigned integer number values, and floating-point number values are stored directly inside a basic_json type.

        "},{"location":"features/types/number_handling/","title":"Number Handling","text":"

        This document describes how the library is handling numbers.

        "},{"location":"features/types/number_handling/#background","title":"Background","text":"

        This section briefly summarizes how the JSON specification describes how numbers should be handled.

        "},{"location":"features/types/number_handling/#json-number-syntax","title":"JSON number syntax","text":"

        JSON defines the syntax of numbers as follows:

        RFC 8259, Section 6

        The representation of numbers is similar to that used in most programming languages. A number is represented in base 10 using decimal digits. It contains an integer component that may be prefixed with an optional minus sign, which may be followed by a fraction part and/or an exponent part. Leading zeros are not allowed.

        A fraction part is a decimal point followed by one or more digits.

        An exponent part begins with the letter E in uppercase or lowercase, which may be followed by a plus or minus sign. The E and optional sign are followed by one or more digits.

        The following railroad diagram from json.org visualizes the number syntax:

        "},{"location":"features/types/number_handling/#number-interoperability","title":"Number interoperability","text":"

        On number interoperability, the following remarks are made:

        RFC 8259, Section 6

        This specification allows implementations to set limits on the range and precision of numbers accepted. Since software that implements IEEE 754 binary64 (double precision) numbers [IEEE754] is generally available and widely used, good interoperability can be achieved by implementations that expect no more precision or range than these provide, in the sense that implementations will approximate JSON numbers within the expected precision. A JSON number such as 1E400 or 3.141592653589793238462643383279 may indicate potential interoperability problems, since it suggests that the software that created it expects receiving software to have greater capabilities for numeric magnitude and precision than is widely available.

        Note that when such software is used, numbers that are integers and are in the range [-253+1, 253-1] are interoperable in the sense that implementations will agree exactly on their numeric values.

        "},{"location":"features/types/number_handling/#library-implementation","title":"Library implementation","text":"

        This section describes how this library implements the above number specification.

        "},{"location":"features/types/number_handling/#number-storage","title":"Number storage","text":"

        In the default json type, numbers are stored as std::uint64_t, std::int64_t, and double, respectively. Thereby, std::uint64_t and std::int64_t are used only if they can store the number without loss of precision. If this is impossible (e.g., if the number is too large), the number is stored as double.

        Positive integers are stored as std::uint64_t, while negative integers are stored as std::int64_t. This distinction is determined at parse time: if the JSON number has a leading minus sign, it uses signed integer storage; otherwise, it uses unsigned integer storage.

        flowchart TD\n    A[\"number literal\"] --> B{\"has a fraction (.) or exponent (e/E)?\"}\n    B -->|\"yes\"| F[\"number_float_t\"]\n    B -->|\"no\"| C{\"has a leading minus sign?\"}\n    C -->|\"yes\"| D[\"try number_integer_t\"]\n    C -->|\"no\"| E[\"try number_unsigned_t\"]\n    D -->|\"overflow\"| F\n    E -->|\"overflow\"| F

        Notes

        • Numbers with a decimal digit or scientific notation are always stored as double.
        • The number types can be changed, see Template number types.
        • The library converts integers and floating-point numbers itself, independent of the locale. Floating-point numbers are correctly rounded (to nearest, ties to even). Only a long double that is not IEEE 754 binary64 (e.g., the 80-bit x87 format) is converted with std::from_chars where available, or else with std::strtold. For that call, the library temporarily replaces the . with the decimal point of the current locale (which may be longer than one byte, e.g., in fa_IR.UTF-8), so the result does not depend on the locale either. Changing the locale in another thread during parsing is undefined behavior of the C library, though. Before version 3.13.0 unreleased, the conversion was realized by std::strtoull, std::strtoll, and std::strtod, respectively.

        Examples

        • Integer -12345678912345789123456789 is smaller than INT64_MIN and will be stored as floating-point number -1.2345678912345788e+25.
        • Integer 1E3 will be stored as floating-point number 1000.0.
        "},{"location":"features/types/number_handling/#number-limits","title":"Number limits","text":"
        • Any 64-bit signed or unsigned integer can be stored without loss of precision.
        • Numbers exceeding the limits of double (i.e., numbers whose rounded value is not satisfying std::isfinite such as 1E400) will throw exception json.exception.out_of_range.406 during parsing. Numbers too small for double (such as 1E-400) become zero, with the sign of the number.
        • Floating-point numbers are rounded to the next number representable as double. For instance 3.141592653589793238462643383279 is stored as 0x400921fb54442d18. This is the same behavior as the code double x = 3.141592653589793238462643383279;.

        Interoperability

        • The library is interoperable with respect to the specification, because its supported range [-263, 264-1] is larger than the described range [-253+1, 253-1].
        • All integers outside the range [-263, 264-1], as well as floating-point numbers are stored as double. This also concurs with the specification above.
        "},{"location":"features/types/number_handling/#zeros","title":"Zeros","text":"

        The JSON number grammar allows for different ways to express zero, and this library will store zeros differently:

        Literal Stored value and type Serialization 0 std::uint64_t(0) 0 -0 std::int64_t(0) 0 0.0 double(0.0) 0.0 -0.0 double(-0.0) -0.0 0E0 double(0.0) 0.0 -0E0 double(-0.0) -0.0

        That is, -0 is stored as a signed integer, but the serialization does not reproduce the -.

        "},{"location":"features/types/number_handling/#number-serialization","title":"Number serialization","text":"
        • Integer numbers are serialized as is; that is, no scientific notation is used.
        • Floating-point numbers are serialized with the fewest digits that read back as the same value (the closest such digits if there are several), in the layout of the %g printf modifier: 1.5, 100.0, 1e+100. Doubles are converted with the algorithm of \u017bmij, floats with Grisu2, which can write more digits than necessary.

        Notes regarding precision of floating-point numbers

        As described above, floating-point numbers are rounded to the nearest double and serialized with the shortest representation to allow round-tripping. This can yield confusing examples:

        • The serialization can have fewer decimal places than the input: 2555.5599999999999 will be serialized as 2555.56. The reverse can also be true.
        • The serialization can be in scientific notation even if the input is not: 0.0000972439793401814 will be serialized as 9.72439793401814e-05. The reverse can also be true: 12345E-5 will be serialized as 0.12345.
        • Conversions from float to double can also introduce rounding errors: 
          float f = 0.3;\njson j = f;\nstd::cout << j << '\\n';\n
          yields 0.30000001192092896.

        All examples here can be reproduced by passing the original double value to

        std::printf(\"%.*g\\n\", std::numeric_limits<double>::max_digits10, double_value);\n
        "},{"location":"features/types/number_handling/#nan-handling","title":"NaN handling","text":"

        NaN (not-a-number) cannot be expressed with the number syntax described above and are in fact explicitly excluded:

        RFC 8259, Section 6

        Numeric values that cannot be represented in the grammar below (such as Infinity and NaN) are not permitted.

        That is, there is no way to parse a NaN value. However, assignments can store NaN values in a JSON value.

        This library serializes NaN values as null. This corresponds to the behavior of JavaScript's JSON.stringify function.

        Example

        The following example shows how a NaN value is stored in a json value.

        int main()\n{\n    double val = std::numeric_limits<double>::quiet_NaN();\n    std::cout << \"val=\" << val << std::endl;\n    json j = val;\n    std::cout << \"j=\" << j.dump() << std::endl;\n    val = j;\n    std::cout << \"val=\" << val << std::endl;\n}\n

        output:

        val=nan\nj=null\nval=nan\n
        "},{"location":"features/types/number_handling/#number-comparison","title":"Number comparison","text":"

        Floating-point numbers inside JSON values are compared with json::number_float_t::operator== which is double::operator== by default.

        Alternative comparison functions

        To compare floating-point while respecting an epsilon, an alternative comparison function could be used, for instance

        template<typename T, typename = typename std::enable_if<std::is_floating_point<T>::value, T>::type>\ninline bool is_same(T a, T b, T epsilon = std::numeric_limits<T>::epsilon()) noexcept\n{\n    return std::abs(a - b) <= epsilon;\n}\n
        Or you can self-define an operator equal function like this:

        bool my_equal(const_reference lhs, const_reference rhs)\n{\n    const auto lhs_type = lhs.type();\n    const auto rhs_type = rhs.type();\n    if (lhs_type == rhs_type)\n    {\n        switch(lhs_type)\n        {\n            // self_defined case\n            case value_t::number_float:\n                return std::abs(lhs - rhs) <= std::numeric_limits<float>::epsilon();\n\n            // other cases remain the same with the original\n            ...\n        }\n    }\n    ...\n}\n

        (see #703 for more information.)

        Note

        NaN values never compare equal to themselves or to other NaN values. See #514.

        "},{"location":"features/types/number_handling/#number-conversion","title":"Number conversion","text":"

        Just like the C++ language itself, the get family of functions allows conversions between unsigned and signed integers, and between integers and floating-point values. This behavior may be surprising.

        Unconditional number conversions

        double d = 42.3;                          // non-integer double value 42.3\njson jd = d;                              // stores double value 42.3\nstd::int64_t i = jd.get<std::int64_t>();  // now i==42; no warning or error is produced\n

        Note the last line with throw a json.exception.type_error.302 exception if jd is not a numerical type, for instance a string.

        Numeric conversions are performed according to the corresponding C++ conversion rules. The library does not perform range checks when converting between numeric types.

        In particular, conversions from floating-point values to integer types, or conversions to integer types with a smaller range than the stored value, may produce implementation-defined or undefined behavior if the source value cannot be represented by the target type.

        Applications requiring checked conversions should inspect the stored number type with is_number_float(), is_number_integer(), is_number_unsigned(), or type(), and perform explicit range checks before converting to a narrower type.

        The rationale is twofold:

        1. JSON does not define a number type or precision (see above).
        2. C++ also allows silently converting between number types.

        Conditional number conversion

        The code above can be solved by explicitly checking the nature of the value with members such as is_number_integer() or is_number_unsigned():

        // check if jd is really integer-valued\nif (jd.is_number_integer())\n{\n    // if so, do the conversion and use i\n    std::int64_t i = jd.get<std::int64_t>();\n    // ...\n}\nelse\n{\n    // otherwise, take appropriate action\n    // ...\n}\n

        Note this approach also has the advantage that it can react on non-numerical JSON value types such as strings.

        (Example taken from #777.)

        "},{"location":"features/types/number_handling/#determine-number-types","title":"Determine number types","text":"

        As the example in Number conversion shows, there are different functions to determine the type of the stored number:

        • is_number() returns true for any number type
        • is_number_integer() returns true for signed and unsigned integers
        • is_number_unsigned() returns true for unsigned integers only
        • is_number_float() returns true for floating-point numbers
        • type_name() returns \"number\" for any number type
        • type() returns a different enumerator of value_t for all number types
        function unsigned integer signed integer floating-point string is_number() true true true false is_number_integer() true true false false is_number_unsigned() true false false false is_number_float() false false true false type_name() \"number\" \"number\" \"number\" \"string\" type() number_unsigned number_integer number_float string"},{"location":"features/types/number_handling/#template-number-types","title":"Template number types","text":"

        The number types can be changed with template parameters.

        position number type default type possible values 5 signed integers std::int64_t std::int32_t, std::int16_t, etc. 6 unsigned integers std::uint64_t std::uint32_t, std::uint16_t, etc. 7 floating-point double float, long double

        Constraints on number types

        • The type for signed integers must be convertible from long long. The type for floating-point numbers is used in case of overflow.
        • The type for unsigned integers must be convertible from unsigned long long. The type for floating-point numbers is used in case of overflow.
        • The type for unsigned integers can also be the same signed type as the type for signed integers (e.g., std::int64_t for both). Then every integer is stored as a signed integer, and is_number_unsigned() is always false, see Template Parameter Requirements.
        • The type for floating-point numbers must be distinct from the integer types.
        • Only double, float, and long double are supported for floating-point numbers.

        Example

        A basic_json type that uses long double as floating-point type.

        using json_ld = nlohmann::json::with_float_t<long double>;\n

        Note values should then be parsed with json_ld::parse rather than json::parse as the latter would parse floating-point values to double before then converting them to long double.

        "},{"location":"features/types/template_parameters/","title":"Template Parameter Requirements","text":"

        Class basic_json is configurable through eleven template parameters. The library never formally states what a type passed for one of these parameters has to provide -- the requirements are implied by the way the library uses the resulting object_t, array_t, string_t, etc. This page collects these requirements so they do not have to be discovered by trial and error. Each section lists the concrete types that are known to work for that parameter and the ones that do not, checked against Boost 1.83, Abseil 20250127.0, Folly, EASTL 3.21, ankerl::unordered_dense, phmap, gtl, robin_hood, tsl::ordered_map, and Qt 6.

        To change a single template parameter and keep the others, use the member alias templates with_*_t; for instance, nlohmann::json::with_float_t<long double> is json with long double as number_float_t.

        "},{"location":"features/types/template_parameters/#how-to-read-this-page","title":"How to read this page","text":"

        Requirements are split into two groups:

        • Always required -- needed to instantiate basic_json at all, or needed by functions that virtually every program uses (construction, element access, dump).
        • Required for ... -- only needed when a particular part of the API is instantiated. Member function templates are only instantiated when they are used, so a type may be perfectly usable even though it does not satisfy these requirements, as long as the corresponding functions are never called.

        Requirements are not checked

        Three requirements are checked with a static_assert: the array iterator category, the width of BinaryType's value_type, and NumberUnsignedType being at least as wide as NumberIntegerType. The rest are not diagnosed with dedicated error messages, and violating most of them results in a compiler error somewhere inside the library. Four violations are not caught at compile time at all:

        • A StringType whose data() is not null-terminated compiles and silently misparses numbers stored as a long double that is not IEEE 754 binary64 (e.g., the 80-bit x87 format), because the lexer hands the buffer to std::strtold.
        • A stateful AllocatorType compiles and silently ignores its state: allocation, deallocation, and get_allocator() each use a different default-constructed instance.
        • The two cross-specialization conversions below. These abort on an assertion in a normal build, and only fail silently under NDEBUG.
        "},{"location":"features/types/template_parameters/#overview","title":"Overview","text":"Template parameter Default Notable substitutes ObjectType std::map nlohmann::ordered_map, Abseil hash maps ArrayType std::vector std::deque StringType std::string std::string-like types over char BooleanType bool none worth using NumberIntegerType std::int64_t any signed integer type NumberUnsignedType std::uint64_t any unsigned integer type at least as wide as NumberIntegerType, or NumberIntegerType itself NumberFloatType double float (long double: no binary formats) AllocatorType std::allocator stateless allocators JSONSerializer adl_serializer serializers with the same interface BinaryType std::vector<std::uint8_t> std::vector<char> CustomBaseClass void any default-constructible class

        Third-party containers and incomplete types

        object_t is instantiated inside the definition of basic_json -- it is probed for a key_compare member to form object_comparator_t -- i.e. while basic_json is still an incomplete type. std::map is required by the standard to support incomplete mapped types; most third-party maps are not, and inspecting the mapped type at class scope (for instance with std::is_trivially_move_assignable) makes them unusable as ObjectType, no matter how their template arguments are adapted. This rules out absl::btree_map, phmap::btree_map, gtl::btree_map, robin_hood::unordered_node_map, folly::F14FastMap, and eastl::hash_map.

        array_t is only named in the class definition and is not instantiated until basic_json is complete, so an ArrayType that inspects its value type at class scope is generally fine -- boost::container::small_vector and static_vector both reject incomplete value types yet work here. absl::InlinedVector is the exception: the std::is_trivially_move_assignable<basic_json> it evaluates while instantiating itself re-enters the library's own trait machinery mid-instantiation.

        Folly requires C++20

        Folly's headers use consteval and std::type_identity, so any basic_json specialization that names a Folly type has to be compiled as C++20 or later, whatever the rest of the library supports.

        "},{"location":"features/types/template_parameters/#objecttype","title":"ObjectType","text":"

        ObjectType is instantiated as

        using object_t = ObjectType<StringType,                                    // key_type\n                            basic_json,                                    // mapped_type\n                            default_object_comparator_t,                   // key_compare\n                            AllocatorType<std::pair<const StringType,\n                                                    basic_json>>>;         // allocator_type\n

        i.e., the template arguments follow the order and meaning of std::map.

        "},{"location":"features/types/template_parameters/#always-required","title":"Always required","text":"
        • The template must be usable with four type arguments in the order shown above. The third argument is a comparator; containers that expect something else in this position (e.g., a hash function) need an alias template or wrapper -- see Notes.
        • An optional member type key_compare. If it is present it becomes object_comparator_t; otherwise default_object_comparator_t is used.
        • Member types key_type, mapped_type, value_type, and iterator.
        • value_type must behave like std::pair<const key_type, mapped_type>; the library accesses .first and .second on it.
        • iterator must be default-constructible and satisfy LegacyBidirectionalIterator. The type returned by cbegin()/cend() must satisfy the same requirements.
        • Constructors: default, copy, move, and from an iterator range (first, last).
        • Member functions begin(), end(), cbegin(), cend(), empty(), size(), max_size(), clear(), find(key), count(key), emplace(key, value), insert(value_type), insert(first, last), operator[](key), erase(iterator), and erase(first, last). erase(iterator) may return the following iterator or void; in the latter case the library computes the successor itself, before erasing.
        • erase(key) is optional: if the container does not provide one, the library falls back to find(key) followed by erase(iterator).
        • at(key) is required only by to_ubjson and to_bjdata, but every container tried here provides it.
        • emplace and insert(value_type) must return std::pair<iterator, bool> and must have unique-key semantics; multimaps cannot be used.
        • The type must be swappable (via std::swap or an ADL swap).
        • The comparison operators == and <; !=, <=, >, and >= are derived from them. Where the library uses three-way comparison (C++20), == and <=> are required instead -- the six two-way operators do not satisfy it. They implement basic_json's comparison operators.
        "},{"location":"features/types/template_parameters/#required-for-heterogeneous-key-lookup","title":"Required for heterogeneous key lookup","text":"

        The overloads of at, operator[], find, contains, count, erase, and value that accept a key type other than object_t::key_type require

        • a transparent comparator, i.e. object_comparator_t has a member type is_transparent (this is why the default comparator is std::less<> since C++14), and
        • corresponding heterogeneous find, count, erase, and operator[] overloads on the container.
        "},{"location":"features/types/template_parameters/#notes","title":"Notes","text":""},{"location":"features/types/template_parameters/#stdunordered_map-needs-an-adapter","title":"std::unordered_map needs an adapter","text":"

        std::unordered_map cannot be passed directly: its third template parameter is a hash function, but basic_json passes a comparator in that position. An alias template or wrapper that restores the expected argument order makes it usable:

        template<class Key, class T, class IgnoredCompare, class Allocator>\nstruct unordered_map_object\n    : std::unordered_map<Key, T, std::hash<Key>, std::equal_to<Key>, Allocator>\n{\n    using base_t = std::unordered_map<Key, T, std::hash<Key>, std::equal_to<Key>, Allocator>;\n    using base_t::base_t;\n};\n\nusing unordered_json = nlohmann::json::with_object_t<unordered_map_object>;\n

        Whether std::unordered_map can be instantiated at all depends on the standard library: object_t is formed while basic_json is still incomplete (see the warning above), and libstdc++ 9 needs the size of the mapped type to instantiate the hash map's node type, so the adapter does not compile there. Newer libstdc++ versions, and the hash maps listed below, do not have that problem.

        The adapter above works verbatim for Abseil's, Boost's, phmap's and gtl's hash maps, which all place the hash function third and take a std::pair<const Key, T> allocator fifth. Two need a different adapter:

        • ankerl::unordered_dense expects an allocator over std::pair<Key, T> (non-const key), so the allocator has to be rebound to that or dropped.
        • robin_hood's fifth parameter is the non-type MaxLoadFactor100, so its adapter must drop the allocator entirely.

        None of these hash maps defines key_compare, so all of them additionally rely on object_comparator_t falling back to default_object_comparator_t; see object_comparator_t.

        "},{"location":"features/types/template_parameters/#abseil-hash-maps","title":"Abseil hash maps","text":"

        absl::flat_hash_map and absl::node_hash_map tolerate an incomplete value type, but they take a hash function as their third template argument. The same adapter as for std::unordered_map makes them usable:

        template<class Key, class T, class IgnoredCompare, class Allocator>\nstruct flat_hash_object\n    : absl::flat_hash_map<Key, T, absl::Hash<Key>, std::equal_to<Key>, Allocator>\n{\n    using base_t = absl::flat_hash_map<Key, T, absl::Hash<Key>, std::equal_to<Key>, Allocator>;\n    using base_t::base_t;\n};\n\nusing flat_hash_json = nlohmann::json::with_object_t<flat_hash_object>;\n

        absl::node_hash_map keeps references to the mapped values valid across insertions; absl::flat_hash_map does not, which makes it behave like ordered_json with respect to iterator invalidation. Both expose a capacity() member function, so JSON_DIAGNOSTICS treats them conservatively and keeps the parent pointers correct either way.

        "},{"location":"features/types/template_parameters/#iteration-order","title":"Iteration order","text":"

        The library never relies on the container's iteration order for correctness; it does determine the order in which object keys are serialized by dump and visited by items. See Object Order.

        "},{"location":"features/types/template_parameters/#capacity-marks-a-container-as-insertion-ordered","title":"capacity() marks a container as insertion-ordered","text":"

        With JSON_DIAGNOSTICS enabled, the library detects insertion-ordered maps by probing for a capacity() member function (nlohmann::ordered_map inherits it from std::vector) and refreshes all parent pointers after every insertion. An ObjectType that happens to have a capacity() member is therefore treated conservatively -- this is correct, but slower.

        "},{"location":"features/types/template_parameters/#key-order-and-duplicate-keys","title":"Key order and duplicate keys","text":"

        The library does not sort or de-duplicate keys itself; the behavior described in object_t is entirely the behavior of the chosen container.

        Reference implementation

        docs/mkdocs/docs/examples/custom_object_type.hpp wraps a private std::map and satisfies every requirement above. It does not define key_compare, so object_comparator_t falls back to default_object_comparator_t -- a good starting point for a custom ObjectType.

        #pragma once\n\n#include <map>\n#include <utility>\n\n// A minimal, self-contained ObjectType built around a private std::map.\n// key_compare is deliberately not exposed: when an ObjectType has no\n// key_compare member, the library falls back to its own default comparator.\n// See https://json.nlohmann.me/features/types/template_parameters/#objecttype\ntemplate<class Key, class T, class Compare, class Allocator>\nclass custom_object_type\n{\n    using map_t = std::map<Key, T, Compare, Allocator>;\n    map_t data_;\n\n  public:\n    using key_type = typename map_t::key_type;\n    using mapped_type = typename map_t::mapped_type;\n    using value_type = typename map_t::value_type;\n    using size_type = typename map_t::size_type;\n    using iterator = typename map_t::iterator;\n    using const_iterator = typename map_t::const_iterator;\n\n    custom_object_type() = default;\n    custom_object_type(const custom_object_type&) = default;\n    custom_object_type(custom_object_type&&) = default;\n    custom_object_type& operator=(const custom_object_type&) = default;\n    custom_object_type& operator=(custom_object_type&&) = default;\n\n    template<class InputIt>\n    custom_object_type(InputIt first, InputIt last) : data_(first, last) {}\n\n    iterator begin()\n    {\n        return data_.begin();\n    }\n    iterator end()\n    {\n        return data_.end();\n    }\n    const_iterator begin() const\n    {\n        return data_.begin();\n    }\n    const_iterator end() const\n    {\n        return data_.end();\n    }\n    const_iterator cbegin() const\n    {\n        return data_.cbegin();\n    }\n    const_iterator cend() const\n    {\n        return data_.cend();\n    }\n\n    bool empty() const\n    {\n        return data_.empty();\n    }\n    size_type size() const\n    {\n        return data_.size();\n    }\n    size_type max_size() const\n    {\n        return data_.max_size();\n    }\n    void clear()\n    {\n        data_.clear();\n    }\n\n    iterator find(const key_type& key)\n    {\n        return data_.find(key);\n    }\n    const_iterator find(const key_type& key) const\n    {\n        return data_.find(key);\n    }\n    size_type count(const key_type& key) const\n    {\n        return data_.count(key);\n    }\n\n    std::pair<iterator, bool> emplace(const key_type& key, const mapped_type& value)\n    {\n        return data_.emplace(key, value);\n    }\n\n    std::pair<iterator, bool> insert(const value_type& value)\n    {\n        return data_.insert(value);\n    }\n\n    template<class InputIt>\n    void insert(InputIt first, InputIt last)\n    {\n        data_.insert(first, last);\n    }\n\n    mapped_type& operator[](const key_type& key)\n    {\n        return data_[key];\n    }\n\n    mapped_type& at(const key_type& key)\n    {\n        return data_.at(key);\n    }\n    const mapped_type& at(const key_type& key) const\n    {\n        return data_.at(key);\n    }\n\n    iterator erase(iterator pos)\n    {\n        return data_.erase(pos);\n    }\n    iterator erase(iterator first, iterator last)\n    {\n        return data_.erase(first, last);\n    }\n    size_type erase(const key_type& key)\n    {\n        return data_.erase(key);\n    }\n\n    void swap(custom_object_type& other)\n    {\n        data_.swap(other.data_);\n    }\n\n    friend bool operator==(const custom_object_type& lhs, const custom_object_type& rhs)\n    {\n        return lhs.data_ == rhs.data_;\n    }\n    friend bool operator<(const custom_object_type& lhs, const custom_object_type& rhs)\n    {\n        return lhs.data_ < rhs.data_;\n    }\n};\n
        Example: use the custom ObjectType 
        #include <iostream>\n#include <type_traits>\n\n#include <nlohmann/json.hpp>\n\n#include \"custom_object_type.hpp\"\n\nusing custom_json = nlohmann::json::with_object_t<custom_object_type>;\n\nint main()\n{\n    custom_json j;\n    j[\"pi\"] = 3.141;\n    j[\"happy\"] = true;\n    j[\"list\"] = {1, 2, 3};\n\n    std::cout << j.dump(2) << std::endl;\n    std::cout << std::boolalpha << (custom_json::parse(j.dump()) == j) << std::endl;\n\n    // custom_object_type has no key_compare member, so object_comparator_t\n    // falls back to its default\n    std::cout << std::boolalpha\n              << std::is_same<custom_json::object_comparator_t, custom_json::default_object_comparator_t>::value\n              << std::endl;\n}\n

        Output:

        {\n  \"happy\": true,\n  \"list\": [\n    1,\n    2,\n    3\n  ],\n  \"pi\": 3.141\n}\ntrue\ntrue\n
        "},{"location":"features/types/template_parameters/#compatible-containers","title":"Compatible containers","text":"Container Notes std::map (default) nlohmann::ordered_map used by ordered_json; keeps insertion order nlohmann::fifo_map keeps insertion order; adapter puts fifo_map_compare in the comparator slot boost::container::map, boost::container::flat_map no adapter needed std::unordered_map through the adapter above; not with libstdc++ 9, see the note boost::unordered_map, boost::unordered_flat_map, boost::unordered_node_map through the adapter above absl::flat_hash_map, absl::node_hash_map through the adapter above; flat_hash_map moves mapped values on rehash phmap::flat_hash_map, phmap::node_hash_map, gtl::flat_hash_map through the adapter above ankerl::unordered_dense::map and segmented_map adapter must rebind or drop the allocator robin_hood::unordered_flat_map adapter must drop the allocator folly::F14NodeMap through the adapter above; requires C++20, see the note above folly::sorted_vector_map alias must drop the allocator, whose value type it disagrees on"},{"location":"features/types/template_parameters/#containers-that-cannot-be-used","title":"Containers that cannot be used","text":"Container Reason absl::btree_map, phmap::btree_map, gtl::btree_map require a complete mapped type robin_hood::unordered_node_map, folly::F14FastMap, eastl::hash_map require a complete mapped type eastl::map EASTL iterators do not work with std::iterator_traits tsl::ordered_map its iterators expose the mapped value as const QMap no value_type member type QHash its value_type is the mapped type rather than a key/value pair, and its iterators dereference to the mapped value std::multimap, std::unordered_multimap emplace does not return std::pair<iterator, bool>"},{"location":"features/types/template_parameters/#arraytype","title":"ArrayType","text":"

        ArrayType is instantiated as

        using array_t = ArrayType<basic_json, AllocatorType<basic_json>>;\n
        "},{"location":"features/types/template_parameters/#always-required_1","title":"Always required","text":"
        • The template must be usable with two type arguments (value type and allocator).
        • Member types value_type and iterator.
        • Constructors: default, copy, and move; and from an iterator range (first, last).
        • Member functions begin(), end(), cbegin(), cend(), empty(), size(), max_size(), clear(), operator[](size_type), back(), push_back(), emplace_back(), pop_back(), resize(), insert() (single element, count, and range), erase(pos), and erase(first, last). basic_json::insert(pos, initializer_list) goes through the range overload, so no initializer-list insert is needed. at(size_type) is not required: basic_json::at(size_type) checks the index itself and then uses operator[].
        • iterator must be default-constructible, and it as well as the type returned by cbegin()/cend() must satisfy LegacyRandomAccessIterator. A static_assert only checks for LegacyBidirectionalIterator, but dump (cend() - 1), erase(idx) (begin() + idx), and the random-access operations of basic_json::iterator require random access.
        • The comparison operators, as for ObjectType: == and <, or == and <=> under C++20.
        "},{"location":"features/types/template_parameters/#required-for-individual-functions","title":"Required for individual functions","text":"
        • A member type value_type, for to_bson of an array.
        • A constructor from (count, value), for basic_json(size_type, const basic_json&).
        • Swappability, via std::swap or an ADL swap, for swap(array_t&).

        capacity() is optional

        With JSON_DIAGNOSTICS enabled, the library reads array_t::capacity() to find out whether adding an element reallocated the array and moved its elements, which would invalidate the parent pointers. An array type without a capacity() member function is handled conservatively: the parent pointers of all elements are refreshed after every insertion, which makes adding n elements cost O(*n*\u00b2). Only diagnostics builds pay this; without them capacity() is never called.

        Reference implementation

        docs/mkdocs/docs/examples/custom_array_type.hpp wraps a private std::vector and satisfies every requirement above -- a good starting point for a custom ArrayType.

        #pragma once\n\n#include <memory>\n#include <utility>\n#include <vector>\n\n// A minimal, self-contained ArrayType built around a private std::vector.\n// See https://json.nlohmann.me/features/types/template_parameters/#arraytype\ntemplate<class T, class Allocator = std::allocator<T>>\nclass custom_array_type\n{\n    using vector_t = std::vector<T, Allocator>;\n    vector_t data_;\n\n  public:\n    using value_type = typename vector_t::value_type;\n    using size_type = typename vector_t::size_type;\n    using iterator = typename vector_t::iterator;\n    using const_iterator = typename vector_t::const_iterator;\n\n    custom_array_type() = default;\n    custom_array_type(const custom_array_type&) = default;\n    custom_array_type(custom_array_type&&) = default;\n    custom_array_type& operator=(const custom_array_type&) = default;\n    custom_array_type& operator=(custom_array_type&&) = default;\n\n    template<class InputIt>\n    custom_array_type(InputIt first, InputIt last) : data_(first, last) {}\n\n    custom_array_type(size_type count, const T& value) : data_(count, value) {}\n\n    iterator begin()\n    {\n        return data_.begin();\n    }\n    iterator end()\n    {\n        return data_.end();\n    }\n    const_iterator begin() const\n    {\n        return data_.begin();\n    }\n    const_iterator end() const\n    {\n        return data_.end();\n    }\n    const_iterator cbegin() const\n    {\n        return data_.cbegin();\n    }\n    const_iterator cend() const\n    {\n        return data_.cend();\n    }\n\n    bool empty() const\n    {\n        return data_.empty();\n    }\n    size_type size() const\n    {\n        return data_.size();\n    }\n    size_type max_size() const\n    {\n        return data_.max_size();\n    }\n    void clear()\n    {\n        data_.clear();\n    }\n    void resize(size_type n)\n    {\n        data_.resize(n);\n    }\n\n    T& operator[](size_type pos)\n    {\n        return data_[pos];\n    }\n    const T& operator[](size_type pos) const\n    {\n        return data_[pos];\n    }\n\n    T& back()\n    {\n        return data_.back();\n    }\n    const T& back() const\n    {\n        return data_.back();\n    }\n\n    void push_back(const T& value)\n    {\n        data_.push_back(value);\n    }\n    void push_back(T&& value)\n    {\n        data_.push_back(std::move(value));\n    }\n\n    template<class... Args>\n    void emplace_back(Args&& ... args)\n    {\n        data_.emplace_back(std::forward<Args>(args)...);\n    }\n\n    void pop_back()\n    {\n        data_.pop_back();\n    }\n\n    iterator insert(const_iterator pos, const T& value)\n    {\n        return data_.insert(pos, value);\n    }\n    iterator insert(const_iterator pos, size_type count, const T& value)\n    {\n        return data_.insert(pos, count, value);\n    }\n    template<class InputIt>\n    iterator insert(const_iterator pos, InputIt first, InputIt last)\n    {\n        return data_.insert(pos, first, last);\n    }\n\n    iterator erase(const_iterator pos)\n    {\n        return data_.erase(pos);\n    }\n    iterator erase(const_iterator first, const_iterator last)\n    {\n        return data_.erase(first, last);\n    }\n\n    void swap(custom_array_type& other)\n    {\n        data_.swap(other.data_);\n    }\n\n    friend bool operator==(const custom_array_type& lhs, const custom_array_type& rhs)\n    {\n        return lhs.data_ == rhs.data_;\n    }\n    friend bool operator<(const custom_array_type& lhs, const custom_array_type& rhs)\n    {\n        return lhs.data_ < rhs.data_;\n    }\n};\n
        Example: use the custom ArrayType 
        #include <iostream>\n\n#include <nlohmann/json.hpp>\n\n#include \"custom_array_type.hpp\"\n\nusing custom_json = nlohmann::json::with_array_t<custom_array_type>;\n\nint main()\n{\n    custom_json j = custom_json::array();\n    j.push_back(1);\n    j.push_back(2);\n    j.push_back(3);\n\n    std::cout << j.dump() << std::endl;\n    std::cout << std::boolalpha << (custom_json::parse(j.dump()) == j) << std::endl;\n}\n

        Output:

        [1,2,3]\ntrue\n
        "},{"location":"features/types/template_parameters/#compatible-containers_1","title":"Compatible containers","text":"Container Notes std::vector (default) std::deque references survive appends, but not insertions elsewhere; see the capacity() note above std::pmr::vector through an alias, as the allocator comes from AllocatorType instead boost::container::vector, deque, devector boost::container::stable_vector the only one tried that keeps references valid across every insertion boost::container::small_vector, folly::small_vector through an alias that fixes the inline capacity boost::container::static_vector through the same kind of alias, for arrays that stay within the fixed capacity folly::fbvector requires C++20, see the note above"},{"location":"features/types/template_parameters/#containers-that-cannot-be-used_1","title":"Containers that cannot be used","text":"Container Reason std::list no operator[], and no random-access iterators eastl::vector, QList, QVector no max_size(); they handle the incomplete value type fine absl::InlinedVector requires a complete value type, see the note above absl::FixedArray the size is fixed at construction, so resize, push_back, insert and erase are missing"},{"location":"features/types/template_parameters/#stringtype","title":"StringType","text":"

        StringType is used both for JSON string values and for the keys of JSON objects (string_t and object_t::key_type).

        "},{"location":"features/types/template_parameters/#always-required_2","title":"Always required","text":"
        • A member type value_type that is one byte wide and char-compatible. The library stores and processes UTF-8 encoded char data and passes data() to functions that take a const char*, such as std::strtold (only used to parse a long double that is not IEEE 754 binary64, see NumberFloatType). std::wstring, std::u16string, and std::u32string are not valid choices; see the FAQ on wide string handling.
        • Constructors: default, copy, move, from const char* (which must not be explicit), from (const char*, size_type), and from (size_type, char); and copy or move assignment.
        • Member functions size(), clear(), resize(n, c), data(), push_back(char), and operator[] (const and non-const, returning references). c_str() and back() are not required.
        • data() must return a pointer to a contiguous, null-terminated buffer. float, double, and a long double that is IEEE 754 binary64 are converted by the library itself and do not depend on this. For any other NumberFloatType (a long double of another format), the parser falls back to std::strtold when std::from_chars is not available or declines the token, and std::strtold reads up to the null character. A type whose data() is not null-terminated does not fail to compile; with such a NumberFloatType it can silently misparse floating-point numbers.
        • append(const char*, size_type), used by dump, and append(const StringType&), used by the CBOR reader for indefinite-length strings. The library's internal string concatenation additionally has to append a char and a const char*; for each it selects between append(arg), operator+=, append(first, last), and append(data, size).
        • The comparison operator == against another StringType, and < for use as a key of the chosen ObjectType (with the default comparator, std::less<> must be able to compare two StringType values, and a StringType with the key types used for lookup). != is never applied to a StringType, and == against const char* is resolved by the implicit const char* constructor.
        "},{"location":"features/types/template_parameters/#required-for-the-binary-formats","title":"Required for the binary formats","text":"
        • resize(n), used by the readers to make room for a block of bytes.
        • Non-const operator[], into which the readers std::memcpy those bytes. A non-const data() would serve just as well, but std::string has only had one since C++17, and the library still supports C++11.
        "},{"location":"features/types/template_parameters/#required-for-json-pointer-flatten-and-diff","title":"Required for JSON Pointer, flatten, and diff","text":"
        • A static member npos and the member function find_first_of(char, size_type) -- together with data(), reserve(n), and append(const char*, size_type) they implement the escaping and unescaping of reference tokens described in RFC 6901. Neither find(const StringType&, size_type), nor substr(pos, count), nor replace(pos, count, const StringType&) is required.
        • empty().
        • begin() and end() -- used by operator[](const json_pointer&) to decide whether a reference token denotes an array index.
        "},{"location":"features/types/template_parameters/#required-for-other-functionality","title":"Required for other functionality","text":"Functionality Additional requirement diff, items, std::hash conversion of a std::size_t to StringType: either assignability from the result of std::to_string, or an ADL overload void int_to_string(StringType&, std::size_t) operator/(std::size_t) the same conversion of a std::size_t to StringType as diff, items, and std::hash above std::hash<basic_json> additionally a specialization of std::hash<StringType> to_bson find(value_type) and npos parse from a string_t the input adapters must accept it; otherwise pass a character range operator<<(std::ostream&, const json_pointer&) streamability to std::ostream to_string conversion of StringType to std::string (the function returns a std::string) exception messages data() and size(), or begin() and end()"},{"location":"features/types/template_parameters/#compatible-types","title":"Compatible types","text":"Type Notes std::string (default) std::basic_string with a custom stateless allocator std::pmr::string see the warning below before relying on the memory resource boost::container::string needs a user-supplied std::hash specialization (Boost provides boost::hash instead) folly::fbstring requires C++20, see the note above eastl::string needs a user-supplied std::hash and an ADL int_to_string (it is not assignable from a std::string); parse does not accept it directly -- pass a character range or a std::string a custom string class in a user-defined namespace if the requirements above are met"},{"location":"features/types/template_parameters/#types-that-cannot-be-used","title":"Types that cannot be used","text":"Type Reason std::wstring, std::u16string, std::u32string the character type is not one byte wide std::u8string one byte wide, but char8_t is not char-compatible absl::Cord no value_type, and the storage is not contiguous QString no append(const char*, size_type); its QChar is also two bytes wide, though that is never diagnosed

        A std::pmr::string mostly does not use the memory resource you choose

        basic_json cannot be given an allocator or a memory resource. AllocatorType is default-constructed at every allocation and has to be stateless (see AllocatorType), and string values the library creates are constructed with their own default allocator. So:

        • Every string the library itself produces -- from parse, from dump, or by default construction -- allocates from std::pmr::get_default_resource().
        • Copying an arena-backed string into a value silently drops its memory resource: the copy lands on the default resource, because std::pmr::polymorphic_allocator does not propagate on copy construction. Nothing warns about this.
        • Moving one in does keep it, and later growth still allocates from that arena -- but it does not survive a copy of the enclosing basic_json.
        • Passing std::pmr::polymorphic_allocator as AllocatorType does not work around any of this; it does not compile.

        Apart from moving a string in, the only way to redirect these allocations is the process-global std::pmr::set_default_resource().

        Reference implementation

        docs/mkdocs/docs/examples/custom_string_type.hpp wraps a private std::string and satisfies every requirement above -- a good starting point for a custom StringType. The unit test tests/src/unit-alt-string.cpp contains a more thorough variant, alt_string, exercised against a larger part of the API.

        #pragma once\n\n#include <cstddef>\n#include <ostream>\n#include <string>\n\n// A minimal, self-contained StringType built around a private std::string.\n// Wraps rather than inherits, so it exposes exactly what the library needs\n// and nothing more of std::string's interface.\n//\n// Covers the \"Always required\" members, the extras needed for the binary\n// formats, JSON Pointer / flatten / unflatten, and the int_to_string overload\n// needed for diff and items. Extending it further (e.g. for\n// std::hash<basic_json> or to_bson) is a matter of adding the extra members\n// listed in the \"Required for other functionality\" table.\n//\n// See https://json.nlohmann.me/features/types/template_parameters/#stringtype\nclass custom_string_type\n{\n    std::string data_;\n\n  public:\n    using value_type = char;\n    using size_type = std::string::size_type;\n    using iterator = std::string::iterator;\n    using const_iterator = std::string::const_iterator;\n\n    static constexpr size_type npos = std::string::npos;\n\n    custom_string_type() = default;\n    custom_string_type(const custom_string_type&) = default;\n    custom_string_type(custom_string_type&&) = default;\n    custom_string_type& operator=(const custom_string_type&) = default;\n    custom_string_type& operator=(custom_string_type&&) = default;\n\n    // not explicit: the library relies on being able to hand it a string literal\n    custom_string_type(const char* s) : data_(s) {}\n    custom_string_type(const char* s, size_type count) : data_(s, count) {}\n    custom_string_type(size_type count, char ch) : data_(count, ch) {}\n\n    size_type size() const\n    {\n        return data_.size();\n    }\n    bool empty() const\n    {\n        return data_.empty();\n    }\n    void clear()\n    {\n        data_.clear();\n    }\n    void resize(size_type n)\n    {\n        data_.resize(n);\n    }\n    void resize(size_type n, char c)\n    {\n        data_.resize(n, c);\n    }\n    void reserve(size_type n)\n    {\n        data_.reserve(n);\n    }\n\n    // must stay null-terminated -- the parser hands this to std::strtoull &\n    // friends; std::string::data() has guaranteed that since C++11\n    const char* data() const\n    {\n        return data_.data();\n    }\n\n    void push_back(char c)\n    {\n        data_.push_back(c);\n    }\n\n    char& operator[](size_type pos)\n    {\n        return data_[pos];\n    }\n    char operator[](size_type pos) const\n    {\n        return data_[pos];\n    }\n\n    custom_string_type& append(const char* s, size_type count)\n    {\n        data_.append(s, count);\n        return *this;\n    }\n    custom_string_type& append(const custom_string_type& other)\n    {\n        data_.append(other.data_);\n        return *this;\n    }\n    custom_string_type& operator+=(char c)\n    {\n        data_.push_back(c);\n        return *this;\n    }\n\n    size_type find_first_of(char c, size_type pos = 0) const\n    {\n        return data_.find_first_of(c, pos);\n    }\n\n    iterator begin()\n    {\n        return data_.begin();\n    }\n    iterator end()\n    {\n        return data_.end();\n    }\n    const_iterator begin() const\n    {\n        return data_.begin();\n    }\n    const_iterator end() const\n    {\n        return data_.end();\n    }\n\n    // found by ADL; converts array indices to keys in diff and items\n    friend void int_to_string(custom_string_type& target, std::size_t value)\n    {\n        target.data_ = std::to_string(value);\n    }\n\n    friend bool operator==(const custom_string_type& lhs, const custom_string_type& rhs)\n    {\n        return lhs.data_ == rhs.data_;\n    }\n    friend bool operator<(const custom_string_type& lhs, const custom_string_type& rhs)\n    {\n        return lhs.data_ < rhs.data_;\n    }\n\n    // not required by the library itself, but dump() returns a custom_string_type\n    // and this makes `std::cout << j.dump()` work as expected\n    friend std::ostream& operator<<(std::ostream& os, const custom_string_type& s)\n    {\n        return os << s.data_;\n    }\n};\n
        Example: use the custom StringType 
        #include <iostream>\n\n#include <nlohmann/json.hpp>\n\n#include \"custom_string_type.hpp\"\n\nusing custom_json = nlohmann::json::with_string_t<custom_string_type>;\n\nint main()\n{\n    custom_json j;\n    j[\"pi\"] = 3.141;\n    j[\"happy\"] = true;\n    j[\"list\"] = {1, 2, 3};\n\n    std::cout << j.dump(2) << std::endl;\n    std::cout << std::boolalpha << (custom_json::parse(j.dump()) == j) << std::endl;\n}\n

        Output:

        {\n  \"happy\": true,\n  \"list\": [\n    1,\n    2,\n    3\n  ],\n  \"pi\": 3.141\n}\ntrue\n
        "},{"location":"features/types/template_parameters/#booleantype","title":"BooleanType","text":"

        boolean_t is stored directly inside basic_json, as a member of an anonymous union.

        "},{"location":"features/types/template_parameters/#always-required_3","title":"Always required","text":"
        • A literal type that is trivially default-constructible, trivially copyable, and trivially destructible; otherwise the union's special member functions are deleted.
        • Implicitly convertible from bool -- an explicit constructor is not enough, because the to_json overload for a custom BooleanType is constrained on std::is_convertible -- and contextually convertible to bool (here an explicit operator bool is fine).
        • Comparison operators ==, !=, <, <=, >, >= (or <=>).
        • Convertible from and to bool through the serializer, because get<bool>() is used internally.

        There is little reason to use anything other than bool here.

        "},{"location":"features/types/template_parameters/#compatible-types_1","title":"Compatible types","text":"

        bool is the only usable choice. Another trivially copyable type that is implicitly convertible to and from bool -- std::uint8_t, say -- does compile, and JSON booleans still round-trip, but the type then serves as both boolean_t and an ordinary integer: basic_json can no longer be constructed or assigned from a std::uint8_t at all (the boolean and unsigned-integer to_json overloads become ambiguous), and get<std::uint8_t>() on a number throws type_error.302 instead of returning the value.

        "},{"location":"features/types/template_parameters/#numberintegertype-and-numberunsignedtype","title":"NumberIntegerType and NumberUnsignedType","text":"

        Both types are stored directly inside basic_json's union.

        "},{"location":"features/types/template_parameters/#always-required_4","title":"Always required","text":"
        • std::is_integral must be satisfied: NumberIntegerType must be a signed integer type, NumberUnsignedType an unsigned integer type or the same type as NumberIntegerType (see below). Class types are not supported -- among others, the constructors taking integer values are constrained on std::is_integral.
        • Trivially default-constructible, trivially copyable, and trivially destructible (union member).
        • std::numeric_limits must be specialized for both types.
        • An unsigned NumberUnsignedType must be able to represent the absolute value of every NumberIntegerType value; serialization of negative numbers converts the value to NumberUnsignedType. A static_assert requires it to be at least as wide as NumberIntegerType, which is what that amounts to for the standard integer types.
        • Both types must fit into the internal 64-character number buffer used by dump, which is the case for all standard integer types.
        • std::hash<basic_json> additionally requires std::hash specializations.
        "},{"location":"features/types/template_parameters/#notes_1","title":"Notes","text":"

        The number types influence what the parser accepts: an integer literal that does not round-trip through the chosen type is stored as number_float_t instead. Choosing types narrower than 64 bits therefore silently changes parse results rather than raising an error. See Number Handling for details.

        "},{"location":"features/types/template_parameters/#same-type-for-both","title":"Same type for both","text":"

        NumberUnsignedType may be the same type as NumberIntegerType, e.g. nlohmann::json::with_integers_t<std::int64_t, std::int64_t> (see with_integers_t), for code that only wants to handle one integer type. Such a basic_json stores every integer as number_integer_t:

        • is_number_unsigned() is always false, and type() never returns value_t::number_unsigned. Constructing a value from value_t::number_unsigned gives a value_t::number_integer value.
        • Values of unsigned C++ types (e.g., 5u) are converted to NumberIntegerType, so values above its maximum wrap around, as values outside the range of a narrower NumberIntegerType already do.
        • The parsers store a non-negative integer above the maximum of NumberIntegerType (e.g., 18446744073709551615 for std::int64_t) as number_float_t, like any other integer that does not fit.
        • Converting from a basic_json with distinct types turns its unsigned values into number_integer_t values.
        "},{"location":"features/types/template_parameters/#compatible-types_2","title":"Compatible types","text":"Type pair Support std::int64_t / std::uint64_t (default) full std::int32_t / std::uint32_t, long long / unsigned long long full; narrower types change which literals the parser can represent any other pair of standard signed/unsigned integer types full the same signed integer type for both, e.g. std::int64_t / std::int64_t full; every integer is stored as number_integer_t, see above class types, enumerations not usable; std::is_integral must hold bool, number_float_t, or the type of another member of the union not usable; std::is_integral<bool> is in fact true, but the get_impl_ptr overloads for boolean_t, number_integer_t, number_unsigned_t and number_float_t would collide"},{"location":"features/types/template_parameters/#numberfloattype","title":"NumberFloatType","text":"

        number_float_t is stored directly inside basic_json's union.

        "},{"location":"features/types/template_parameters/#always-required_5","title":"Always required","text":"
        • Trivially default-constructible, trivially copyable, and trivially destructible (union member).
        • std::numeric_limits must be specialized; max_digits10 is used to size the conversion.
        • std::isfinite must be applicable to the type.
        "},{"location":"features/types/template_parameters/#required-for-parsing-and-serialization","title":"Required for parsing and serialization","text":"

        NumberFloatType must be one of float, double, or long double:

        • The parser converts number literals to float, double, and a long double that is IEEE 754 binary64 itself; other long double formats are converted with std::from_chars where available, or with std::strtold. The library provides overloads for exactly these three types.
        • dump falls back to std::snprintf with the %g and %Lg conversion specifiers, for which the library likewise provides only double and long double overloads (float is promoted to double).

        If std::numeric_limits<NumberFloatType> describes an IEEE 754 binary64 number, dump uses the algorithm of \u017bmij, which produces the shortest representation that round-trips. For IEEE 754 binary32 numbers, it uses Grisu2, which produces a short representation that round-trips. Otherwise the snprintf fallback with max_digits10 digits is used.

        "},{"location":"features/types/template_parameters/#required-for-the-binary-formats_1","title":"Required for the binary formats","text":"

        NumberFloatType must be float or double. The writers for CBOR, MessagePack, UBJSON, BJData, BON8, and BSON map a floating-point value onto an IEEE 754 binary32 or binary64 field and have no encoding for long double.

        "},{"location":"features/types/template_parameters/#compatible-types_3","title":"Compatible types","text":"Type Support double (default) full; shortest round-trip output through \u017bmij float full; short round-trip output through Grisu2 long double dump and parse only; the binary format writers do not compile, as they only handle IEEE 754 binary32 and binary64 any other type not usable"},{"location":"features/types/template_parameters/#allocatortype","title":"AllocatorType","text":"

        AllocatorType is instantiated with one argument, for each of object_t, array_t, string_t, binary_t, basic_json, std::pair<const StringType, basic_json>, and std::pair<StringType, basic_json>.

        AllocatorType is not the only allocator a basic_json uses. It allocates the JSON values themselves, but most temporary storage is allocated with std::allocator. This includes the parser's stacks and the stacks that process deeply nested values without recursion.

        "},{"location":"features/types/template_parameters/#always-required_6","title":"Always required","text":"
        • The template must be usable with exactly one type argument. The library instantiates AllocatorType<T> directly and never uses std::allocator_traits<...>::rebind_alloc.
        • It must satisfy the Allocator named requirement so that std::allocator_traits can be used with it.
        • It must be default-constructible and stateless. Objects are allocated with a default-constructed allocator and deallocated with a different default-constructed allocator, and get_allocator() returns a default-constructed instance. Allocators carrying state are not supported, so there is no way to tell a basic_json where to allocate from; see the note under StringType for what that means in practice. A stateful allocator is not diagnosed: it compiles and silently ignores the state.
        • It must support incomplete types: AllocatorType<basic_json> is instantiated inside the definition of basic_json itself.
        • std::allocator_traits<AllocatorType<basic_json>>::pointer becomes basic_json::pointer, and iterators are constructed from raw basic_json* values. The pointer type must therefore be a plain pointer; fancy pointers are not supported.
        "},{"location":"features/types/template_parameters/#compatible-types_4","title":"Compatible types","text":"Type Support std::allocator (default) full a custom stateless allocator template full stateful allocators, e.g. std::pmr::polymorphic_allocator not usable; see the requirements above"},{"location":"features/types/template_parameters/#jsonserializer","title":"JSONSerializer","text":"

        JSONSerializer is instantiated as JSONSerializer<T, void> and defaults to adl_serializer.

        "},{"location":"features/types/template_parameters/#always-required_7","title":"Always required","text":"
        • The template must accept two type arguments. It does not have to give the second one a default -- basic_json declares the parameter as template<typename T, typename SFINAE = void> class JSONSerializer, so uses such as JSONSerializer<T> inside the library supply void themselves. The second parameter exists so that partial specializations can be constrained by SFINAE.
        • For every type T that is converted to a JSON value, a static member function static void to_json(basic_json&, T) must exist.
        • For every type T that is converted from a JSON value, either static void from_json(const basic_json&, T&) or static T from_json(const basic_json&) must exist. The latter form is required for types that are not default-constructible; see Arbitrary Types Conversions.
        • To support the converting constructor between different basic_json specializations, to_json must be available for boolean_t, number_integer_t, number_unsigned_t, number_float_t, string_t, object_t, array_t, and binary_t of the source specialization.
        "},{"location":"features/types/template_parameters/#compatible-types_5","title":"Compatible types","text":"Type Support nlohmann::adl_serializer (default) full a class template deriving from adl_serializer full; the usual way to change behavior while keeping the defaults an unrelated template with the same interface full, but it has to handle every type the library converts"},{"location":"features/types/template_parameters/#binarytype","title":"BinaryType","text":"

        BinaryType is not a JSON type; it is used for the byte strings of the binary formats. It is wrapped as

        using binary_t = nlohmann::byte_container_with_subtype<BinaryType>;\n
        "},{"location":"features/types/template_parameters/#always-required_8","title":"Always required","text":"
        • A non-final class type -- byte_container_with_subtype derives from it publicly.
        • A member type value_type that is exactly one byte wide (e.g., std::uint8_t, char, or std::byte). Readers and writers reinterpret the container's storage as raw bytes, so a wider value_type is rejected with a static_assert.
        • Contiguous storage: the binary readers std::memcpy into &binary[n], the writers reinterpret_cast data(). data() + n would do for the readers too, but they share one helper with StringType, whose non-const data() is C++17 and later only.
        • Default-constructible, copy-constructible, and move-constructible.
        • Member functions size(), empty(), data(), resize(), operator[], back(), begin(), end(), cbegin(), and cend() with random-access iterators, and insert(pos, first, last), which the CBOR reader uses to join the chunks of an indefinite-length byte string. push_back() is not required.
        • Comparison operators: == is used by byte_container_with_subtype, the relational operators by basic_json's comparison operators.
        "},{"location":"features/types/template_parameters/#required-for-individual-functions_1","title":"Required for individual functions","text":"
        • clear(), for basic_json::clear().

        max_size(), at(), reserve(), erase(), pop_back(), and emplace_back() are not used at all.

        See binary_t for how a non-default BinaryType changes the meaning of assigning such a container to a basic_json value.

        Reference implementation

        docs/mkdocs/docs/examples/custom_binary_type.hpp wraps a private std::vector<std::uint8_t> and satisfies every requirement above -- a good starting point for a custom BinaryType.

        #pragma once\n\n#include <cstdint>\n#include <initializer_list>\n#include <vector>\n\n// A minimal, self-contained BinaryType built around a private std::vector.\n// See https://json.nlohmann.me/features/types/template_parameters/#binarytype\nclass custom_binary_type\n{\n    using vector_t = std::vector<std::uint8_t>;\n    vector_t data_;\n\n  public:\n    using value_type = vector_t::value_type;\n    using size_type = vector_t::size_type;\n    using iterator = vector_t::iterator;\n    using const_iterator = vector_t::const_iterator;\n\n    custom_binary_type() = default;\n    custom_binary_type(const custom_binary_type&) = default;\n    custom_binary_type(custom_binary_type&&) = default;\n    custom_binary_type& operator=(const custom_binary_type&) = default;\n    custom_binary_type& operator=(custom_binary_type&&) = default;\n\n    template<class InputIt>\n    custom_binary_type(InputIt first, InputIt last) : data_(first, last) {}\n\n    // so basic_json::binary({0x01, 0x02}) can build one directly\n    custom_binary_type(std::initializer_list<std::uint8_t> init) : data_(init) {}\n\n    size_type size() const\n    {\n        return data_.size();\n    }\n    bool empty() const\n    {\n        return data_.empty();\n    }\n    void clear()\n    {\n        data_.clear();\n    }\n    void resize(size_type n)\n    {\n        data_.resize(n);\n    }\n\n    // read-only is enough: the writers only ever read from a binary value\n    const std::uint8_t* data() const\n    {\n        return data_.data();\n    }\n\n    std::uint8_t& operator[](size_type pos)\n    {\n        return data_[pos];\n    }\n    std::uint8_t operator[](size_type pos) const\n    {\n        return data_[pos];\n    }\n\n    std::uint8_t& back()\n    {\n        return data_.back();\n    }\n    std::uint8_t back() const\n    {\n        return data_.back();\n    }\n\n    iterator begin()\n    {\n        return data_.begin();\n    }\n    iterator end()\n    {\n        return data_.end();\n    }\n    const_iterator begin() const\n    {\n        return data_.begin();\n    }\n    const_iterator end() const\n    {\n        return data_.end();\n    }\n    const_iterator cbegin() const\n    {\n        return data_.cbegin();\n    }\n    const_iterator cend() const\n    {\n        return data_.cend();\n    }\n\n    template<class InputIt>\n    iterator insert(const_iterator pos, InputIt first, InputIt last)\n    {\n        return data_.insert(pos, first, last);\n    }\n\n    friend bool operator==(const custom_binary_type& lhs, const custom_binary_type& rhs)\n    {\n        return lhs.data_ == rhs.data_;\n    }\n    friend bool operator<(const custom_binary_type& lhs, const custom_binary_type& rhs)\n    {\n        return lhs.data_ < rhs.data_;\n    }\n};\n
        Example: use the custom BinaryType 
        #include <iostream>\n\n#include <nlohmann/json.hpp>\n\n#include \"custom_binary_type.hpp\"\n\nusing custom_json = nlohmann::json::with_binary_t<custom_binary_type>;\n\nint main()\n{\n    const auto j = custom_json::binary({0x01, 0x02, 0x03});\n\n    std::cout << j.dump() << std::endl;\n    std::cout << std::boolalpha << (custom_json::from_cbor(custom_json::to_cbor(j)) == j) << std::endl;\n}\n

        Output:

        {\"bytes\":[1,2,3],\"subtype\":null}\ntrue\n
        "},{"location":"features/types/template_parameters/#compatible-containers_2","title":"Compatible containers","text":"Container Notes std::vector<std::uint8_t> (default) std::vector<char>, std::vector<std::byte> dump() writes the bytes as 0..255 whichever is used boost::container::vector<std::uint8_t>, boost::container::small_vector<std::uint8_t, N> absl::InlinedVector<std::uint8_t, N> usable here, unlike as an ArrayType, because the value type is complete eastl::vector<std::uint8_t> usable here, unlike as an ArrayType, because max_size() is not needed folly::fbvector<std::uint8_t> requires C++20, see the note above"},{"location":"features/types/template_parameters/#containers-that-cannot-be-used_2","title":"Containers that cannot be used","text":"Container Reason QByteArray no empty() (it spells that isEmpty()); its insert takes an index rather than an iterator; and it converts to string_t, which makes to_json ambiguous between a string and a binary value std::string binary_t::container_type and string_t would be the same type, so the two swap overloads collide and basic_json cannot be instantiated at all std::deque<std::uint8_t> storage is not contiguous, so there is no data() containers whose value_type is wider than one byte see above -- accepted by the compiler, wrong at runtime"},{"location":"features/types/template_parameters/#custombaseclass","title":"CustomBaseClass","text":"

        CustomBaseClass is an extension point: unless it is void (the default, which selects the empty nlohmann::json_default_base), basic_json publicly derives from it.

        "},{"location":"features/types/template_parameters/#always-required_9","title":"Always required","text":"
        • A non-final, default-constructible class type.
        • basic_json is copy-/move-constructible and copy-/move-assignable only if CustomBaseClass is.
        "},{"location":"features/types/template_parameters/#notes_2","title":"Notes","text":"

        basic_json is documented to be a StandardLayoutType. Because basic_json has non-static data members of its own, a CustomBaseClass with non-static data members forfeits this guarantee.

        Note the namespace of CustomBaseClass becomes an associated namespace of basic_json for the purpose of argument-dependent lookup.

        See json_base_class_t for an example.

        "},{"location":"features/types/template_parameters/#compatible-types_6","title":"Compatible types","text":"Type Support void (default) an empty base class is used; no effect on basic_json any default-constructible, non-final class full; see json_base_class_t"},{"location":"features/types/template_parameters/#cross-specialization-conversions","title":"Cross-specialization conversions","text":"

        Converting a value from one basic_json specialization into another (see the converting constructor) imposes two additional requirements that are not diagnosed at compile time. With assertions enabled they abort on the JSON_ASSERT at the end of the converting constructor; under NDEBUG they fail silently at runtime:

        • The target string_t must be directly constructible from the source string_t. Otherwise the string is converted to an array of character codes.
        • The target object_t::key_type must be directly constructible from the source object's key type. Otherwise the object is converted to an array of key/value pairs.

        See issue #3425, string_t, and object_t.

        "},{"location":"features/types/template_parameters/#see-also","title":"See also","text":"
        • Types -- overview of how JSON values are stored
        • Number Handling -- how the number types affect parsing and serialization
        • Object Order -- using an insertion-ordered ObjectType
        • basic_json -- API documentation of the class template
        "},{"location":"home/architecture/","title":"Architecture","text":"

        This page gives a high-level overview of the library's architecture. It should help new contributors to get an idea of the used concepts and where to make changes.

        "},{"location":"home/architecture/#overview","title":"Overview","text":"

        The library is built around a single class template, nlohmann::basic_json. A basic_json value is a node in a tree of JSON values. All other components either create such a tree from an input (parsing), write a tree to an output (serialization), or give access to it (iterators, JSON Pointer, conversions).

        flowchart LR\n    input[/\"input<br>(string, stream,<br>iterator range, file)\"/]\n    ia[\"input adapter\"]\n    lexer[\"lexer\"]\n    parser[\"parser\"]\n    breader[\"binary_reader\"]\n    sax[\"SAX interface\"]\n    value[(\"basic_json<br>value tree\")]\n    serializer[\"serializer\"]\n    bwriter[\"binary_writer\"]\n    oa[\"output adapter\"]\n    output[/\"output<br>(string, stream,<br>vector)\"/]\n\n    input --> ia\n    ia --> lexer --> parser --> sax\n    ia --> breader --> sax\n    sax --> value\n    value --> serializer --> oa\n    value --> bwriter --> oa\n    oa --> output
        • JSON text is read by an input adapter, tokenized by the lexer, and turned into SAX events by the parser.
        • Binary formats (BJData, BON8, BSON, CBOR, MessagePack, UBJSON) are read by an input adapter and turned into the same SAX events by the binary_reader.
        • A SAX consumer receives the events. The one used by parse builds a basic_json value tree.
        • The serializer (JSON text) or the binary_writer (binary formats) writes a value tree to an output adapter.
        "},{"location":"home/architecture/#source-layout","title":"Source layout","text":"

        The public headers are in include/nlohmann:

        • json.hpp defines class basic_json.
        • json_fwd.hpp contains forward declarations.
        • adl_serializer.hpp, byte_container_with_subtype.hpp, and ordered_map.hpp define adl_serializer, byte_container_with_subtype, and ordered_map.
        • json_view.hpp is a separate, optional header that defines basic_json_document and basic_json_view, a flat-index, read-only, non-owning way to look at a parsed JSON text; see Zero-copy JSON views. It builds on json.hpp internals (it requires the same library version) and has its own detail/view/ subdirectory.

        Everything else lives in detail/ and namespace nlohmann::detail, which is not part of the public API. Paths below are relative to include/nlohmann.

        Component Location Value type enumeration detail/value_t.hpp Input adapters detail/input/input_adapters.hpp Lexer detail/input/lexer.hpp, detail/input/number_parse.hpp, detail/input/string_scan.hpp Parser detail/input/parser.hpp SAX interface and DOM builders detail/input/json_sax.hpp Binary format readers detail/input/binary_reader.hpp JSON serializer detail/output/serializer.hpp, detail/conversions/to_chars.hpp Binary format writers detail/output/binary_writer.hpp Output adapters detail/output/output_adapters.hpp Iterators detail/iterators/ Conversions from/to arbitrary types detail/conversions/from_json.hpp, detail/conversions/to_json.hpp JSON Pointer detail/json_pointer.hpp Exceptions detail/exceptions.hpp Type traits and C++ feature backports detail/meta/ Macros detail/macro_scope.hpp, detail/macro_unscope.hpp, detail/abi_macros.hpp

        The single-header version single_include/nlohmann/json.hpp is generated from these files with make amalgamate and must not be edited by hand. The same command also generates single_include/nlohmann/json_view.hpp from json_view.hpp and detail/view/.

        "},{"location":"home/architecture/#template-parameters","title":"Template parameters","text":"

        basic_json is parameterized by the types it uses to store values and to convert from and to other types:

        Template parameter Default Used for ObjectType std::map objects, see object_t ArrayType std::vector arrays, see array_t StringType std::string strings and object keys, see string_t BooleanType bool Booleans, see boolean_t NumberIntegerType std::int64_t signed integers, see number_integer_t NumberUnsignedType std::uint64_t unsigned integers, see number_unsigned_t NumberFloatType double floating-point numbers, see number_float_t AllocatorType std::allocator allocating objects, arrays, strings, and binary values JSONSerializer adl_serializer conversions from/to other types, see adl_serializer BinaryType std::vector<std::uint8_t> binary values, see binary_t CustomBaseClass void an optional base class, see json_base_class_t

        The library provides two specializations:

        • json uses all default template arguments.
        • ordered_json uses ordered_map as ObjectType to keep the insertion order of object keys.

        The requirements on the template arguments are listed in Template Parameter Requirements.

        "},{"location":"home/architecture/#value-storage","title":"Value storage","text":"

        Each basic_json value stores its content as a tagged union: an enumeration value_t names the type of the value, and a union json_value holds the value itself. Both are members of the nested struct data, which is the only data member m_data of basic_json:

        struct data\n{\n    /// the type of the current element\n    value_t m_type = value_t::null;\n\n    /// the value of the current element\n    json_value m_value = {};\n};\n\ndata m_data = {};\n

        with

        enum class value_t : std::uint8_t\n{\n    null,             ///< null value\n    object,           ///< object (unordered set of name/value pairs)\n    array,            ///< array (ordered collection of values)\n    string,           ///< string value\n    boolean,          ///< boolean value\n    number_integer,   ///< number value (signed integer)\n    number_unsigned,  ///< number value (unsigned integer)\n    number_float,     ///< number value (floating-point)\n    binary,           ///< binary array (ordered collection of bytes)\n    discarded         ///< discarded by the parser callback function\n};\n\nunion json_value {\n  /// object (stored with pointer to save storage)\n  object_t *object;\n  /// array (stored with pointer to save storage)\n  array_t *array;\n  /// string (stored with pointer to save storage)\n  string_t *string;\n  /// binary (stored with pointer to save storage)\n  binary_t *binary;\n  /// boolean\n  boolean_t boolean;\n  /// number (integer)\n  number_integer_t number_integer;\n  /// number (unsigned integer)\n  number_unsigned_t number_unsigned;\n  /// number (floating-point)\n  number_float_t number_float;\n};\n

        Objects, arrays, strings, and binary values are allocated on the heap with AllocatorType, and the union only stores a pointer to them. This keeps a basic_json value small: one pointer-sized union and one byte for the type. The class maintains the invariant that the pointer matching m_type is never null; assert_invariant() checks it with runtime assertions.

        "},{"location":"home/architecture/#node-index-of-json-views","title":"Node index of JSON views","text":"

        A basic_json_document (see Zero-copy JSON views) does not build a tree of values. Its parser (detail/view/builder.hpp) writes a flat array of 16-byte nodes, one per value and one per object key, in document order. A basic_json_view is a pointer to the document and a pointer to one node. The layout is struct node in detail/view/node.hpp (the numbers are bit offsets, 32 bits per row):

        packet-beta\n  0-7: \"kind\"\n  8-15: \"flags\"\n  16-31: \"extra\"\n  32-63: \"off\"\n  64-95: \"len\"\n  96-127: \"next\"
        Bytes Field Type Contents 0 kind uint8_t the type, numbered as value_t: 0 null, 1 object, 2 array, 3 string, 4 boolean, 5 signed integer, 6 unsigned integer, 7 float; 10 for a link (see below). There is no node for a discarded value: a discarded view has none 1 flags uint8_t bits 0-1: where a string's bytes (or a number's token) are (0: the source text, 1: the buffer of decoded strings, for strings with escapes, 2: the edit buffer); bit 2: the value of a boolean; bit 3: moved, bit 4: new, bit 5: linked (see below) 2-3 extra uint16_t numbers: the number of integer digits (low byte) and fraction digits (high byte), 255 for more; objects: the number of their key hash index (1-based), or 0; otherwise 0 4-7 off uint32_t where the value starts: the first byte after a string's opening quote (or its position in the buffer of decoded strings), the first byte of a number or literal, the bracket of an array or object 8-11 len uint32_t strings: the length after decoding; floats and literals: the length of the token; arrays and objects: the number of elements 12-15 next uint32_t arrays and objects: the number of nodes of the subtree, including the node itself
        • Integers keep their converted value in bytes 8-15 instead of len and next; the length of their token follows from the number of digits in extra (and the sign). Non-negative integers are unsigned integers, as with parse.
        • Floats keep only their token. The digit layout in extra lets the conversion read the digits without scanning the token again, and only when the value is read.
        • Object members are the node of the key (a string) followed by the nodes of the value.
        • Navigation needs no pointers: the elements of an array or object follow its node, and the node after a value's subtree is next nodes further for an array or object, and the next node otherwise (document_data::after). Views step from element to element this way and skip whole subtrees in constant time.
        • Offsets are 32 bits wide, so a document is limited to 4294967279 bytes, 4 GiB minus 16 bytes (a margin below 2^32 for positions one scanner step past the end of the text; out_of_range.416).
        • Large objects (128 members or more) get a hash index for their keys after parsing (detail/view/object_index.hpp): an open-addressing table whose slots hold the distance from the object's node to a key's node, so that a lookup does not compare every key. Of duplicate keys the table leads to the first, as a linear search does. The object's extra holds the number of its table. Only 65,535 tables fit into extra; objects beyond them are searched linearly.

        For example, {\"a\": [1, 2.5]} becomes five nodes. Each node's elements follow it, and next leads from an array or object past its subtree:

        flowchart LR\n    n0[\"0: object<br>len 1, next 5\"]\n    n1[\"1: key a\"]\n    n2[\"2: array<br>len 2, next 3\"]\n    n3[\"3: unsigned integer 1\"]\n    n4[\"4: float 2.5\"]\n    e([\"end\"])\n    n0 --> n1 --> n2 --> n3 --> n4 --> e\n    n0 -. next .-> e\n    n2 -. next .-> e
        Node kind extra off len next 0 1 (object) 0 0 1 5 1 3 (string) 0 2 1 0 2 2 (array) 0 6 2 3 3 6 (unsigned integer) 0x0001 7 - - 4 7 (float) 0x0101 10 3 0

        All flags are 0. The integer's bytes 8-15 hold its value, 1; its extra says it has one digit. The float's extra says it has one integer and one fraction digit, and its len is that of the token 2.5.

        Editable documents (json_editable_document, detail/view/edit.hpp and detail/view/edit_storage.hpp) never write the source text and never move or resize the parsed index, so views stay valid while the document is edited:

        • A new scalar is written over its node. Its text (a string, or the token of a number as dump() writes it) goes to the edit buffer, which flags bits 0-1 then name.
        • An array or object whose elements change gets the flag moved (bit 3): its elements then live in a separate sequence (a header node, then the entries), whose number is in off. The entries are links (kind 10), whose bytes 8-15 hold the address of the value's node, so values never move.
        • A node written by an edit gets the flag new (bit 4): it has no position in the source text.
        • A value that an entry of such a sequence links to gets the flag linked (bit 5): its extent in the parsed layout (next) no longer matters, so a scalar can replace an array or object there in place.
        • Views of read-only documents compile without any of this: how views walk the index is a template parameter (navigation<Editable>).

        Images (save and load, detail/view/image.hpp) store the nodes as they are: a 64-byte header (the magic bytes NJVI, a format version, the sizes, and reserved bytes that must be zero), the nodes, the text, and the decoded strings. An edited document is first written in document order, as the parser would have written it (without links), and the numbers of the hash indexes are cleared, since load rebuilds the indexes. So a change of the node layout is a change of the image format: it must raise image_version, and load then rejects images of other versions (parse_error.116) instead of misreading them.

        "},{"location":"home/architecture/#input-adapters","title":"Input adapters","text":"

        Input is read via input adapters that abstract a source. Every input adapter provides this interface:

        /// the type of the characters in the input\nusing char_type = ...;\n\n/// read a single character; returns std::char_traits<char_type>::eof() at the end of the input\ntypename std::char_traits<char_type>::int_type get_character();\n\n/// read up to count * sizeof(T) bytes into dest and return the number of bytes read\n/// (used by the binary readers)\ntemplate<class T>\nstd::size_t get_elements(T* dest, std::size_t count = 1);\n

        The lexer detects two optional extensions at compile time. Only iterator_input_adapter provides them, and only for random-access input of single-byte characters:

        • supports_seek, get_consumed_count(), and copy_consumed_range() let the lexer reconstruct already consumed input for error messages instead of copying every character it reads. With get_read_count() and count_consumed_lines(), the lexer also derives the read position from the adapter and counts lines only when an error needs them, instead of counting characters and lines on every read.
        • supports_bulk_scan, bulk_data(), bulk_remaining(), and bulk_skip() let the lexer scan strings directly in contiguous memory, several bytes at a time, and skip the spaces that indent a line in 8-byte steps.

        The function input_adapter picks the right adapter for the argument passed to parse, accept, sax_parse, or the from_* functions:

        • iterator_input_adapter reads from an iterator range, which also covers strings, containers, and pointers.
        • wide_string_input_adapter reads from ranges of wchar_t, char16_t, or char32_t and converts them to UTF-8. It cannot be used for binary formats; its get_elements() throws.
        • input_stream_adapter reads from a std::istream.
        • file_input_adapter reads from a std::FILE*.
        "},{"location":"home/architecture/#sax-interface","title":"SAX interface","text":"

        The parser does not build values itself. It reports what it reads as events to a SAX consumer, which implements the interface json_sax: null, boolean, number_integer, number_unsigned, number_float, string, binary, start_object, key, end_object, start_array, end_array, and parse_error.

        The library comes with two consumers in detail/input/json_sax.hpp:

        • json_sax_dom_parser builds a basic_json value tree. parse uses it.
        • json_sax_dom_callback_parser does the same, but calls a parser callback for each event, which can skip values. parse uses it when a callback is given.

        The binary_reader emits the same events for binary formats, so sax_parse works with a user-defined consumer for JSON and for all binary formats alike.

        "},{"location":"home/architecture/#output-adapters","title":"Output adapters","text":"

        Output is written via output adapters:

        void write_character(CharType c);\n\nvoid write_characters(const CharType* s, std::size_t length);\n

        The serializer (used by dump and operator<<) and the binary_writer (used by the to_* functions) write to one of these adapters:

        • output_vector_adapter appends to a std::vector.
        • output_stream_adapter writes to a std::ostream.
        • output_string_adapter appends to a string.
        "},{"location":"home/architecture/#value-conversion","title":"Value conversion","text":"

        Values are converted from and to other types with the JSONSerializer template parameter. The default, adl_serializer, calls the free functions

        template<class T>\nvoid to_json(basic_json& j, const T& t);\n\ntemplate<class T>\nvoid from_json(const basic_json& j, T& t);\n

        found by argument-dependent lookup. The library defines them for standard types in detail/conversions; users add them for their own types, see Arbitrary Type Conversions. The serialization macros generate these functions.

        "},{"location":"home/architecture/#additional-features","title":"Additional features","text":"
        • JSON Pointer (class json_pointer) addresses values inside a tree. It is also the basis of JSON Patch.
        • Binary formats are read by binary_reader and written by binary_writer.
        • A custom base class can add members to every basic_json value.
        • Serialization macros generate to_json and from_json functions for user-defined types.
        "},{"location":"home/architecture/#details-namespace","title":"Details namespace","text":"

        Namespace nlohmann::detail contains all implementation details. It is not part of the public API and may change in any release. Besides the components above, it contains:

        • type traits to detect the capabilities of user-defined types (detail/meta/type_traits.hpp),
        • backports of C++14/17 features to C++11 (detail/meta/cpp_future.hpp), and
        • helpers such as string_concat and string_escape.
        "},{"location":"home/customers/","title":"Customers","text":"

        The library is used in multiple projects, applications, operating systems, etc. The list below is not exhaustive, but the result of an internet search. If you know further customers of the library, please let me know.

        "},{"location":"home/customers/#space-exploration","title":"Space Exploration","text":"
        • Peregrine Lunar Lander Flight 01 - The library was used for payload management in the Peregrine Moon Lander, developed by Astrobotic Technology and launched as part of NASA's Commercial Lunar Payload Services (CLPS) program. After six days in orbit, the spacecraft was intentionally redirected into Earth's atmosphere, where it burned up over the Pacific Ocean on January 18, 2024.
        • NASA Unsteady Pressure-Sensitive Paint Processing, NASA software for processing high-speed video recordings of wind tunnel tests on launch vehicle and aircraft models
        • Terma TEMU, an emulator of spacecraft on-board computers used to develop and validate flight software for European space missions
        "},{"location":"home/customers/#automotive","title":"Automotive","text":"
        • Alexa Auto SDK, a software development kit enabling the integration of Alexa into automotive systems
        • Apollo, a framework for building autonomous driving systems
        • Automotive Grade Linux (AGL), a collaborative open-source platform for automotive software development
        • Autoware, an open-source software stack for autonomous driving built on ROS 2
        • Eclipse S-CORE, an open-source software platform for the software-defined vehicle backed by major automotive manufacturers and suppliers
        • Genesis Motor (infotainment), a luxury automotive brand
        • Hyundai (infotainment), a global automotive brand
        • Kia (infotainment), a global automotive brand
        • Mercedes-Benz Operating System (MB.OS), a core component of the vehicle software ecosystem from Mercedes-Benz
        • NVIDIA DRIVE OS, the operating system and DriveWorks SDK powering NVIDIA's platform for autonomous vehicles
        • Rivian (infotainment), an electric vehicle manufacturer
        • Suzuki (infotainment), a global automotive and motorcycle manufacturer
        "},{"location":"home/customers/#gaming-and-entertainment","title":"Gaming and Entertainment","text":"
        • Anno 117: Pax Romana, a city-building strategy game set in the Roman Empire
        • Assassin's Creed: Mirage, a stealth-action game set in the Middle East, focusing on the journey of a young assassin with classic parkour and stealth mechanics
        • Battlefield 6, a military first-person shooter known for its large-scale multiplayer battles
        • Battlefield: REDSEC, a free-to-play battle royale experience set in the Battlefield universe
        • BioMenace: Remastered, a remaster of the classic side-scrolling platform shooter
        • Chasm: The Rift, a first-person shooter blending horror and adventure, where players navigate dark realms and battle monsters
        • College Football 25, a college football simulation game featuring gameplay that mimics real-life college teams and competitions
        • College Football 26, a college football simulation game featuring licensed teams and stadiums
        • College Football 27, the latest installment of the college football simulation series
        • Concepts, a digital sketching app designed for creative professionals, offering flexible drawing tools for illustration, design, and brainstorming
        • Depthkit, a tool for creating and capturing volumetric video, enabling immersive 3D experiences and interactive content
        • Dune: Awakening, an open-world survival MMO set on the desert planet Arrakis
        • EA Sports FC 25, an association football simulation with club, career, and online modes
        • EA Sports FC 26, the latest installment of the association football simulation series
        • EA Sports UFC 6, a mixed martial arts fighting simulation
        • FiveM, a modification framework for Grand Theft Auto V that powers custom multiplayer servers
        • FLUX:: Immersive, a suite of professional audio processing and immersive mixing plugins used in music and post-production
        • IMG.LY, a platform offering creative tools and SDKs for integrating advanced image and video editing in applications
        • immersivetech, a technology company focused on immersive experiences, providing tools and solutions for virtual and augmented reality applications
        • Kodi, a home theater and media center application
        • LOOT, a tool for optimizing the load order of game plugins, commonly used in The Elder Scrolls and Fallout series
        • LunaTranslator, a real-time translation tool for visual novels
        • MaaAssistantArknights, an automation assistant for the mobile game Arknights
        • Madden NFL 25, a sports simulation game capturing the excitement of American football with realistic gameplay and team management features
        • Madden NFL 26, an American football simulation with franchise and team management modes
        • Madden NFL 27, the latest installment of the American football simulation series
        • Marne, an unofficial private server platform for hosting custom Battlefield 1 game experiences
        • Minecraft, a popular sandbox video game
        • Mumble, a low-latency, open-source voice chat application widely used by gaming communities
        • NHL 22, a hockey simulation game offering realistic gameplay, team management, and various modes to enhance the hockey experience
        • OBS Studio, a free and open-source suite for video recording and live streaming
        • OpenRCT2, an open source re-implementation of RollerCoaster Tycoon 2
        • Pixelpart, a 2D animation and video compositing software that allows users to create animated graphics and visual effects with a focus on simplicity and ease of use
        • Razer Cortex, a gaming performance optimizer and system booster designed to enhance the gaming experience
        • Red Dead Redemption II, an open-world action-adventure game following an outlaw's story in the late 1800s, emphasizing deep storytelling and immersive gameplay
        • RetroArch, a frontend for emulators, game engines, and media players built on the libretro API
        • shadPS4, a PlayStation 4 emulator for Windows, Linux and macOS
        • skate., a free-to-play skateboarding game set in an open world
        • Snapchat, a multimedia messaging and augmented reality app for communication and entertainment
        • Steel Century Groove, an action game released in 2026
        • Sunshine, a self-hosted game streaming host compatible with Moonlight clients
        • Tactics Ogre: Reborn, a tactical role-playing game featuring strategic battles and deep storytelling elements
        • Throne and Liberty, an MMORPG that offers an expansive fantasy world with dynamic gameplay and immersive storytelling
        • Unity Vivox, a communication service that enables voice and text chat functionality in multiplayer games developed with Unity
        • xemu, an emulator of the original Xbox console
        • Zool: Redimensioned, a modern reimagining of the classic platformer featuring fast-paced gameplay and vibrant environments
        "},{"location":"home/customers/#consumer-electronics","title":"Consumer Electronics","text":"
        • Audinate, a provider of networked audio solutions specializing in Dante technology, which facilitates high-quality digital audio transport over IP networks
        • Canon CanoScan LIDE, a series of flatbed scanners offering high-resolution image scanning for home and office use
        • Canon PIXMA Printers, a line of all-in-one inkjet printers known for high-quality printing and wireless connectivity
        • Cisco Webex Desk Camera, a video camera designed for professional-quality video conferencing and remote collaboration
        • DJI Edge SDK, the reference applications for DJI's Edge SDK, used to build edge computing services on DJI drone docks
        • Elgato Stream Deck, a family of programmable control surfaces for content creators and their plugin ecosystem
        • Instagrid, a manufacturer of portable, high-performance battery systems for professional mobile power supply
        • iRobot, a manufacturer of autonomous home robots including the Roomba vacuum cleaner range
        • Logitech Logi Bolt, the management application for Logitech's secure wireless connectivity technology
        • Novitus, a manufacturer of fiscal cash registers and point-of-sale devices
        • Philips Hue Personal Wireless Lighting, a smart lighting system for customizable and wireless home illumination
        • Ray-Ban Meta Smart glasses, a pair of smart glasses designed for capturing photos and videos with integrated connectivity and social features
        • Razer Synapse, a unified configuration software enabling hardware customization for Razer devices
        • Sharp Professional Displays, a range of large-format interactive displays for business and education
        • Siemens SINEMA Remote Connect, a remote connectivity solution for monitoring and managing industrial networks and devices securely
        • Skydio, a manufacturer of autonomous drones for inspection, public safety, and defense applications
        • Sony PlayStation 4, a gaming console developed by Sony that offers a wide range of games and multimedia entertainment features
        • Sony Spatial Reality Display, a glasses-free stereoscopic 3D display and its plugins for Blender, 3ds Max, and ZBrush
        • Sony Virtual Webcam Driver for Remote Camera, a software driver that enables the use of Sony cameras as virtual webcams for video conferencing and streaming
        • Yamaha Clavinova, a series of digital pianos combining acoustic piano feel with digital sound technology
        "},{"location":"home/customers/#operating-systems-and-platforms","title":"Operating Systems and Platforms","text":"
        • Apple iOS and macOS, a family of operating systems developed by Apple, including iOS for mobile devices and macOS for desktop computers
        • Chromium, the open-source browser project that Google Chrome, Microsoft Edge, and many other browsers are built on, where the library is used as data container for on-device model execution
        • Google Fuchsia, an open-source operating system developed by Google, designed to be secure, updatable, and adaptable across various devices
        • LG webOS, a Linux-based operating system used in LG smart TVs, signage, and embedded devices
        • Microsoft Azure Linux, a Linux distribution developed by Microsoft for Azure infrastructure and edge workloads
        • OpenHarmony, an open-source operating system for smart devices and the foundation of HarmonyOS
        • SerenityOS, an open-source operating system that aims to provide a simple and beautiful user experience with a focus on simplicity and elegance
        • Windows Subsystem for Linux, a compatibility layer that runs Linux environments natively on Windows
        • Yocto, a Linux-based build system for creating custom operating systems and software distributions, tailored for embedded devices and IoT applications
        "},{"location":"home/customers/#development-tools-and-ides","title":"Development Tools and IDEs","text":"
        • Accentize SpectralBalance, an adaptive speech analysis tool designed to enhance audio quality by optimizing frequency balance in recordings
        • Airbus Ghidralligator, a Ghidra-based emulator from Airbus CyberSecurity used to fuzz and analyse embedded firmware
        • Apache brpc, an industrial-grade remote procedure call framework for C++
        • Arm Compiler for Linux, a software development toolchain for compiling and optimizing applications on Arm-based Linux systems
        • BBEdit, a professional text and code editor for macOS
        • CoderPad, a collaborative coding platform that enables real-time code interviews and assessments for developers; the library is included in every CoderPad instance and can be accessed with a simple #include \"json.hpp\"
        • Codon, an ahead-of-time compiler for a Python-like language
        • Compiler Explorer, a web-based tool that allows users to write, compile, and visualize the assembly output of code in various programming languages; the library is readily available and accessible with the directive #include <nlohmann/json.hpp>.
        • Flutter, a UI toolkit for building natively compiled applications for mobile, web, and desktop from a single codebase
        • Fraunhofer VVenC, a fast and efficient encoder for the Versatile Video Coding (H.266/VVC) standard
        • GitHub CodeQL, a code analysis tool used for identifying security vulnerabilities and bugs in software through semantic queries
        • GoPro ngfx, a low-level graphics abstraction and profiling framework developed by GoPro
        • gRPC, a high-performance universal remote procedure call framework
        • Hex-Rays, a reverse engineering toolset for analyzing and decompiling binaries, primarily used for security research and vulnerability analysis
        • ImHex, a hex editor designed for reverse engineering, providing advanced features for data analysis and manipulation
        • Intel GITS, a tool for capturing and replaying graphics API calls for debugging and performance analysis
        • Intel GPA Framework, a suite of cross-platform tools for capturing, analyzing, and optimizing graphics applications across different APIs
        • Intopix, a provider of advanced image processing and compression solutions used in software development and AV workflows
        • Java SE, the core Java platform that provides the libraries and runtime needed to build and run general-purpose Java applications
        • Meta Yoga, a layout engine that facilitates flexible and efficient user interface design across multiple platforms
        • MKVToolNix, a set of tools for creating, editing, and inspecting MKV (Matroska) multimedia container files
        • MRTech IFF SDK, an image processing SDK for machine vision applications with GPU-accelerated pipelines
        • Nix, a purely functional package manager
        • Notepad++, a free source code editor that supports various programming languages
        • NVIDIA Nsight Compute, a performance analysis tool for CUDA applications that provides detailed insights into GPU performance metrics
        • openFrameworks, a community-developed C++ toolkit for creative coding
        • OpenRGB, an open source RGB lighting control that doesn't depend on manufacturer software
        • OpenTelemetry C++, a library for collecting and exporting observability data in C++, enabling developers to implement distributed tracing and metrics in their application
        • Oracle GraalVM, a high-performance JDK distribution with ahead-of-time compilation and polyglot runtime support
        • Philips amp-cucumber-cpp-runner, a behaviour-driven development test runner for embedded C++ software developed at Philips
        • Qt Creator, an IDE for developing applications using the Qt application framework
        • Qt for MCUs, a graphics framework for building fluid user interfaces on microcontrollers
        • React Native, a framework for building native mobile applications using React
        • Scanbot SDK, a software development kit (SDK) that provides tools for integrating advanced document scanning and barcode scanning capabilities into applications
        • STMicroelectronics TouchGFX, a graphical user interface framework shipped with STM32 microcontrollers for building embedded HMIs
        • swagger-codegen, a template-driven engine that generates API clients and server stubs from an OpenAPI specification
        • Swoole, a coroutine-based concurrency engine for PHP
        • Tracy Profiler, a real-time frame profiler for games and other applications
        • WasmEdge, a lightweight WebAssembly runtime for edge and cloud workloads
        • x64dbg, an open source user mode debugger for Windows, aimed at reverse engineering and malware analysis
        "},{"location":"home/customers/#machine-learning-and-ai","title":"Machine Learning and AI","text":"
        • Alibaba MNN, a lightweight deep learning inference engine for mobile and embedded devices
        • AMD Gaia, an open-source framework for running generative AI applications locally on AMD hardware
        • AMD Vitis AI (VAIP), the execution provider stack that runs AI models on AMD Ryzen AI and adaptive computing devices
        • Apple Core ML Tools, a set of tools for converting and configuring machine learning models for deployment in Apple's Core ML framework
        • Avular Mobile Robotics, a platform for developing and deploying mobile robotics solutions
        • FunASR, a speech recognition toolkit for training and deploying end-to-end models
        • Google gemma.cpp, a lightweight C++ inference engine designed for running AI models from the Gemma family
        • Google Magenta The Infinite Crate, an open-source generative AI plugin for digital audio workstations from Google's Magenta research team
        • GPT4All, a desktop application for running local large language models on consumer hardware
        • Huawei MindSpore, a deep learning framework for training and inference across device, edge, and cloud
        • KTransformers, a framework for heterogeneous large language model inference
        • llama.cpp, a C++ library designed for efficient inference of large language models (LLMs), enabling streamlined integration into applications
        • LocalAI, a self-hosted inference engine that exposes local models through an OpenAI-compatible API
        • MLX, an array framework for machine learning on Apple Silicon
        • Mozilla llamafile, a tool designed for distributing and executing large language models (LLMs) efficiently using a single file format
        • NVIDIA ACE, a suite of real-time AI solutions designed for the development of interactive avatars and digital human applications, enabling scalable and sophisticated user interactions
        • NVIDIA Instant NGP, an implementation of instant neural graphics primitives for rapid scene reconstruction
        • NVIDIA TensorRT, an SDK for high-performance deep learning inference, including its TensorRT-LLM extension for large language models
        • NVIDIA TensorRT-LLM, a toolkit for optimizing and serving large language model inference on GPUs
        • ONNX Runtime, a cross-platform inference and training accelerator for machine learning models
        • OpenVINO, Intel's toolkit for optimizing and deploying deep learning inference across CPUs, GPUs, and NPUs
        • PaddleOCR, an optical character recognition toolkit that turns documents and images into structured data
        • PaddlePaddle, a deep learning framework for distributed training and inference
        • Peer, a platform offering personalized AI assistants for interactive learning and creative collaboration
        • PyTorch, a machine learning framework for building and training neural networks, widely used in research and production
        • Qualcomm AI Engine Direct, a toolchain for building and running generative AI applications on Snapdragon devices
        • sherpa-onnx, a speech toolkit for on-device recognition, synthesis and speaker diarization
        • stable-diffusion.cpp, a C++ implementation of the Stable Diffusion image generation model
        • TanvasTouch, a software development kit (SDK) that enables developers to create tactile experiences on touchscreens, allowing users to feel textures and physical sensations in a digital environment
        • TensorFlow, a machine learning framework that facilitates the development and training of models, supporting data serialization and efficient data exchange between components
        • whisper.cpp, a C++ implementation of OpenAI's Whisper automatic speech recognition model
        "},{"location":"home/customers/#scientific-research-and-analysis","title":"Scientific Research and Analysis","text":"
        • BLACK, a bounded linear temporal logic (LTL) satisfiability checker
        • CERN ALICE O2, the online-offline computing framework of the ALICE heavy-ion experiment at the Large Hadron Collider
        • CERN Atlas Athena, a software framework used in the ATLAS experiment at the Large Hadron Collider (LHC) for performance monitoring
        • CERN CMSSW, the offline software framework of the CMS experiment at the Large Hadron Collider
        • CERN Gaudi, the event-processing framework used by the LHCb and ATLAS experiments at the Large Hadron Collider
        • ICU, the International Components for Unicode, a mature library for software globalization and multilingual support
        • KAMERA, a platform for synchronized data collection and real-time deep learning to map marine species like polar bears and seals, aiding Arctic ecosystem research
        • KiCad, a free and open-source software suite for electronic design automation
        • LLNL ROSE, a compiler infrastructure from Lawrence Livermore National Laboratory for building source-to-source program analysis and transformation tools
        • Maple, a symbolic and numeric computing environment for advanced mathematical modeling and analysis
        • MeVisLab, a software framework for medical image processing and visualization.
        • MITK, the Medical Imaging Interaction Toolkit, a framework for developing interactive medical image processing software
        • OpenPMD API, a versatile programming interface for accessing and managing scientific data, designed to facilitate the efficient storage, retrieval, and sharing of simulation data across various applications and platforms
        • ORNL DataFed, a federated scientific data management system developed at Oak Ridge National Laboratory
        • ParaView, an open-source tool for large-scale data visualization and analysis across various scientific domains
        • QGIS, a free and open-source geographic information system (GIS) application that allows users to create, edit, visualize, and analyze geospatial data across a variety of formats
        • Sandia InterSpec, spectral radiation analysis software from Sandia National Laboratories for identifying radioactive isotopes
        • VolView, a lightweight application for interactive visualization and analysis of 3D medical imaging data.
        • VTK, a software library for 3D computer graphics, image processing, and visualization
        "},{"location":"home/customers/#business-and-productivity-software","title":"Business and Productivity Software","text":"
        • ArcGIS PRO, a desktop geographic information system (GIS) application developed by Esri for mapping and spatial analysis
        • Autodesk Desktop, a software platform developed by Autodesk for creating and managing desktop applications and services
        • Check Point, a cybersecurity company specializing in threat prevention and network security solutions, offering a range of products designed to protect enterprises from cyber threats and ensure data integrity
        • EasyEffects, an audio effects processor for PipeWire offering limiting, compression and equalization
        • espanso, a cross-platform text expander
        • Karabiner-Elements, a keyboard customizer for macOS
        • MacType, a font rendering engine for Windows
        • magicplan, a mobile application for creating floor plans and interior designs using augmented reality
        • Microsoft Office for Mac, a suite of productivity applications developed by Microsoft for macOS, including tools for word processing, spreadsheets, and presentations
        • Microsoft Teams, a team collaboration application offering workspace chat and video conferencing, file storage, and integration of proprietary and third-party applications and services
        • MuseScore, a free and open-source music notation and composition application
        • NanaZip, a 7-Zip derivative built for modern Windows
        • Nexthink Infinity, a digital employee experience management platform for monitoring and improving IT performance
        • Sophos Connect Client, a secure VPN client from Sophos that allows remote users to connect to their corporate network, ensuring secure access to resources and data
        • Stonebranch, a cloud-based cybersecurity solution that integrates backup, disaster recovery, and cybersecurity features to protect data and ensure business continuity for organizations
        • Tablecruncher, a data analysis tool that allows users to import, analyze, and visualize spreadsheet data, offering interactive features for better insights and decision-making
        • VNote, a Markdown-based note-taking application written in C++
        "},{"location":"home/customers/#databases-and-big-data","title":"Databases and Big Data","text":"
        • ADIOS2, a data management framework designed for high-performance input and output operations
        • Apache Doris, a real-time analytical database for high-concurrency queries
        • Claris FileMaker Server, the server platform hosting FileMaker custom apps and databases, developed by Apple subsidiary Claris
        • ClickHouse, a column-oriented database management system for real-time analytical queries
        • Cribl Stream, a real-time data processing platform that enables organizations to collect, route, and transform observability data, enhancing visibility and insights into their systems
        • DB Browser for SQLite, a visual open-source tool for creating, designing, and editing SQLite database files
        • Manticore Search, a database for search, offering full-text and vector queries
        • Milvus, a cloud-native vector database built for embedding similarity search
        • MongoDB, a general-purpose document database
        • MySQL Connector/C++, a C++ library for connecting and interacting with MySQL databases
        • MySQL NDB Cluster, a distributed database system that provides high availability and scalability for MySQL databases
        • MySQL Shell, an advanced client and code editor for interacting with MySQL servers, supporting SQL, Python, and JavaScript
        • PrestoDB, a distributed SQL query engine designed for large-scale data analytics, originally developed by Facebook
        • ROOT Data Analysis Framework, an open-source data analysis framework widely used in high-energy physics and other fields for data processing and visualization
        • Typesense, an open source typo-tolerant search engine
        • Vearch, a distributed vector database developed at JD.com for similarity search and retrieval-augmented generation
        • WiredTiger, a high-performance storage engine for databases, offering support for compression, concurrency, and checkpointing
        "},{"location":"home/customers/#simulation-and-modeling","title":"Simulation and Modeling","text":"
        • Adobe Lagrange, a geometry processing library developed by Adobe for mesh manipulation and analysis
        • Arcturus HoloSuite, a software toolset for capturing, editing, and streaming volumetric video, featuring advanced compression technologies for high-quality 3D content creation
        • azul, a fast and efficient 3D city model viewer designed for visualizing urban environments and spatial data
        • Bambu Studio, a slicing and print management application for Bambu Lab 3D printers
        • Blender, a free and open-source 3D creation suite for modeling, animation, rendering, and more
        • cpplot, a library for creating interactive graphs and charts in C++, which can be viewed in web browsers
        • Foundry Nuke, a powerful node-based digital compositing and visual effects application used in film and television post-production
        • FreeCAD, a free and open-source parametric 3D CAD modeler for product design and engineering
        • GAMS, a high-performance mathematical modeling system for optimization and decision support
        • Keysight WirelessPro, a simulation platform for 5G, 5G-Advanced, and 6G cellular network research
        • Kitware SMTK, a software toolkit for managing simulation models and workflows in scientific and engineering applications
        • M-Star, a computational fluid dynamics software for simulating and analyzing fluid flow
        • MapleSim CAD Toolbox, a software extension for MapleSim that integrates CAD models, allowing users to import, manipulate, and analyze 3D CAD data within the MapleSim environment for enhanced modeling and simulation
        • Microsoft AirSim, a simulator for autonomous vehicles and drones built on Unreal Engine
        • NVIDIA Omniverse, a platform for 3D content creation and collaboration that enables real-time simulations and interactive experiences across various industries
        • OpenSCAD, a script-driven solid 3D CAD modeller
        • OrcaSlicer, an open-source slicer supporting a wide range of consumer 3D printers
        • Pixar Renderman, a photorealistic 3D rendering software developed by Pixar, widely used in the film industry for creating high-quality visual effects and animations
        • PrusaSlicer, the slicing software developed by Prusa Research for its 3D printers
        • ROS - Robot Operating System, a set of software libraries and tools that assist in developing robot applications
        • UBS, a multinational financial services and banking company
        "},{"location":"home/customers/#enterprise-and-cloud-applications","title":"Enterprise and Cloud Applications","text":"
        • Acronis Cyber Protect Cloud, an all-in-one data protection solution that combines backup, disaster recovery, and cybersecurity to safeguard business data from threats like ransomware
        • Baereos, a backup solution that provides data protection and recovery options for various environments, including physical and virtual systems
        • Bitdefender Home Scanner, a tool from Bitdefender that scans devices for malware and security threats, providing a safeguard against potential online dangers
        • Cisco MLS++, an implementation of the Messaging Layer Security protocol for end-to-end encrypted group messaging
        • Citrix Provisioning, a solution that streamlines the delivery of virtual desktops and applications by allowing administrators to manage and provision resources efficiently across multiple environments
        • Citrix Virtual Apps and Desktops, a solution from Citrix that delivers virtual apps and desktops
        • CyberArk, a security solution that specializes in privileged access management, enabling organizations to control and monitor access to critical systems and data, thereby enhancing overall cybersecurity posture
        • Deutsche Telekom sysrepo-plugins, a collection of YANG datastore plugins used to manage network devices
        • Egnyte Desktop, a secure cloud storage solution designed for businesses, enabling file sharing, collaboration, and data management across teams while ensuring compliance and data protection
        • Elster, a digital platform developed by German tax authorities for secure and efficient electronic tax filing and management using secunet protect4use
        • Envoy, a cloud-native edge and service proxy that forms the data plane of many service meshes
        • Ethereum Solidity, a high-level, object-oriented programming language designed for implementing smart contracts on the Ethereum platform
        • gVisor, an application kernel that provides a secure sandbox for running untrusted containers
        • IBM Storage Virtualize, the software powering IBM FlashSystem enterprise storage arrays
        • Inciga, a monitoring tool for IT infrastructure, designed to provide insights into system performance and availability through customizable dashboards and alerts
        • Intel Accelerator Management Daemon for VMware ESXi, a management tool designed for monitoring and controlling Intel hardware accelerators within VMware ESXi environments, optimizing performance and resource allocation
        • Juniper Identity Management Service
        • Meta FBOSS, the software stack that controls the network switches in Meta's data centers
        • Microsoft Azure IoT SDK, a collection of tools and libraries to help developers connect, build, and deploy Internet of Things (IoT) solutions on the Azure cloud platform
        • Microsoft Confidential Consortium Framework, a framework for building secure, highly available applications on trusted execution environments
        • Microsoft WinGet, a command-line utility included in the Windows Package Manager
        • Mitsubishi Electric SECS/GEM, the semiconductor equipment communication software running on Mitsubishi Electric C Controller and C intelligent function modules
        • Moxa, a provider of industrial networking, computing, and automation infrastructure
        • plexusAV, a high-performance AV-over-IP transceiver device capable of video encoding and decoding using the IPMX standard
        • Pointr, a platform for indoor positioning and navigation solutions, offering tools and SDKs for developers to create location-based applications
        • secunet protect4use, a secure, passwordless multifactor authentication solution that transforms smartphones into digital keyrings, ensuring high security for online services and digital identities
        • Sencore MRD 7000, a professional multi-channel receiver and decoder supporting UHD and HD stream decoding
        • Siemens SINEC, a family of network management and infrastructure services for industrial networks
        • Toshiba Industrial Servers, the FS20000R series of industrial servers for factory automation and control systems
        • Wazuh, a security platform for threat detection, integrity monitoring and incident response
        • ZeroTier, a software-defined networking service that creates virtual Ethernet networks
        "},{"location":"home/debugging/","title":"Debugging","text":"

        This page collects the library's built-in debugger integrations and other debugging-related features. They are not linked from a single place elsewhere in the docs, so are collected here.

        "},{"location":"home/debugging/#visual-studio-natvis","title":"Visual Studio (natvis)","text":"

        The repository ships nlohmann_json.natvis at its root, a Natvis file that gives json/ordered_json values a friendly, key/value debugger view instead of showing raw internal fields, when debugging with the MSVC debug engine (cppvsdbg) in Visual Studio or VS Code.

        Debug engines that wrap LLDB instead of the MSVC debug engine (for example, codelldb in VS Code) only have partial/experimental Natvis support, and commonly fall back to showing raw internal fields even with the .natvis file present. Switching to cppvsdbg where available, or checking your debug extension's own Natvis support/version, are the next things to try if this happens. There is currently no bundled LLDB-native pretty-printer script in this repository.

        "},{"location":"home/debugging/#gdb","title":"GDB","text":"

        The repository ships a GDB Python pretty printer under tools/gdb_pretty_printer, with its own usage instructions in that directory's README.md.

        "},{"location":"home/debugging/#extended-exception-diagnostics","title":"Extended exception diagnostics","text":"

        Defining JSON_DIAGNOSTICS before including the library augments type_error/out_of_range-style exceptions with a JSON Pointer to the offending value, which can help pinpoint where in a large document a runtime error occurred. This only applies to exceptions thrown after a value exists (e.g. during element access); parse errors, which happen before any value exists to point at, are not covered by this mechanism -- see Parsing and exceptions for how parse errors report their own location instead.

        "},{"location":"home/design_goals/","title":"Design goals","text":"

        There are myriads of JSON libraries out there, and each may even have its reason to exist. Our class had these design goals:

        • Intuitive syntax. In languages such as Python, JSON feels like a first-class data type. We used all the operator magic of modern C++ to achieve the same feeling in your code.

        • Trivial integration. Our whole code consists of a single header file json.hpp. That's it. No library, no subproject, no dependencies, no complex build system. The class is written in vanilla C++11. All in all, everything should require no adjustment of your compiler flags or project settings.

        • Serious testing. Our class is heavily unit-tested and covers 100% of the code, including all exceptional behavior. Furthermore, we checked with Valgrind and the Clang Sanitizers that there are no memory leaks. Google OSS-Fuzz additionally runs fuzz tests against all parsers 24/7, effectively executing billions of tests so far. To maintain high quality, the project is following the OpenSSF Best Practices.

        Other aspects were not so important to us:

        • Memory efficiency. Each JSON object has an overhead of one pointer (the maximal size of a union) and one enumeration element (1 byte). The default generalization uses the following C++ data types: std::string for strings, int64_t, uint64_t or double for numbers, std::map for objects, std::vector for arrays, and bool for Booleans. However, you can template the generalized class basic_json to your needs.

        • Speed. There are certainly faster JSON libraries out there. However, if your goal is to speed up your development by adding JSON support with a single header, then this library is the way to go. If you know how to use a std::vector or std::map, you are already set.

        See the contribution guidelines for more information.

        "},{"location":"home/exceptions/","title":"Exceptions","text":""},{"location":"home/exceptions/#overview","title":"Overview","text":""},{"location":"home/exceptions/#base-type","title":"Base type","text":"

        All exceptions inherit from class json::exception (which in turn inherits from std::exception). It is used as the base class for all exceptions thrown by the basic_json class. This class can hence be used as \"wildcard\" to catch exceptions.

        classDiagram\n  direction LR\n    class `std::exception` {\n        <<interface>>\n    }\n\n    class `json::exception` {\n        +const int id\n        +const char* what() const\n    }\n\n    class `json::parse_error` {\n        +const std::size_t byte\n    }\n\n    class `json::invalid_iterator`\n    class `json::type_error`\n    class `json::out_of_range`\n    class `json::other_error`\n\n    `std::exception` <|-- `json::exception`\n    `json::exception` <|-- `json::parse_error`\n    `json::exception` <|-- `json::invalid_iterator`\n    `json::exception` <|-- `json::type_error`\n    `json::exception` <|-- `json::out_of_range`\n    `json::exception` <|-- `json::other_error`
        "},{"location":"home/exceptions/#switch-off-exceptions","title":"Switch off exceptions","text":"

        Exceptions are used widely within the library. They can, however, be switched off with either using the compiler flag -fno-exceptions or by defining the symbol JSON_NOEXCEPTION. In this case, exceptions are replaced by abort() calls. You can further control this behavior by defining JSON_THROW_USER (overriding throw), JSON_TRY_USER (overriding try), and JSON_CATCH_USER (overriding catch).

        Note that JSON_THROW_USER should leave the current scope (e.g., by throwing or aborting), as continuing after it may yield undefined behavior.

        Example: switch off exceptions and log errors before aborting

        The code below switches off exceptions and creates a log entry with a detailed error message in case of errors.

        #include <iostream>\n\n#define JSON_TRY_USER if(true)\n#define JSON_CATCH_USER(exception) if(false)\n#define JSON_THROW_USER(exception)                           \\\n    {std::clog << \"Error in \" << __FILE__ << \":\" << __LINE__ \\\n               << \" (function \" << __FUNCTION__ << \") - \"    \\\n               << (exception).what() << std::endl;           \\\n     std::abort();}\n\n#include <nlohmann/json.hpp>\n

        Note the explanatory what() string of exceptions is not available for MSVC if exceptions are disabled, see #2824.

        See documentation of JSON_TRY_USER, JSON_CATCH_USER and JSON_THROW_USER for more information.

        "},{"location":"home/exceptions/#extended-diagnostic-messages","title":"Extended diagnostic messages","text":"

        Exceptions in the library are thrown in the local context of the JSON value they are detected. This makes detailed diagnostics messages, and hence debugging, difficult.

        Example: standard diagnostic message 
        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    json j;\n    j[\"address\"][\"street\"] = \"Fake Street\";\n    j[\"address\"][\"housenumber\"] = \"12\";\n\n    try\n    {\n        int housenumber = j[\"address\"][\"housenumber\"];\n    }\n    catch (const json::exception& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

        Output:

        [json.exception.type_error.302] type must be number, but is string\n

        This exception can be hard to debug if storing the value \"12\" and accessing it is further apart.

        To create better diagnostics messages, each JSON value needs a pointer to its parent value such that a global context (i.e., a path from the root value to the value that led to the exception) can be created. That global context is provided as JSON Pointer.

        As this global context comes at the price of storing one additional pointer per JSON value and runtime overhead to maintain the parent relation, extended diagnostics are disabled by default. They can, however, be enabled by defining the preprocessor symbol JSON_DIAGNOSTICS to 1 before including json.hpp.

        Example: extended diagnostic message with JSON_DIAGNOSTICS 
        #include <iostream>\n\n# define JSON_DIAGNOSTICS 1\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    json j;\n    j[\"address\"][\"street\"] = \"Fake Street\";\n    j[\"address\"][\"housenumber\"] = \"12\";\n\n    try\n    {\n        int housenumber = j[\"address\"][\"housenumber\"];\n    }\n    catch (const json::exception& e)\n    {\n        std::cout << e.what() << '\\n';\n    }\n}\n

        Output:

        [json.exception.type_error.302] (/address/housenumber) type must be number, but is string\n

        Now the exception message contains a JSON Pointer /address/housenumber that indicates which value has the wrong type.

        See documentation of JSON_DIAGNOSTICS for more information.

        "},{"location":"home/exceptions/#parse-errors","title":"Parse errors","text":"

        The library throws this exception when a parse error occurs. Parse errors can occur during the deserialization of JSON text or of one of the binary formats (BJData, BON8, BSON, CBOR, MessagePack, UBJSON), as well as when using JSON Patch.

        Exceptions have ids 1xx.

        Byte index

        Member byte holds the byte index of the last read character in the input file.

        For an input with n bytes, 1 is the index of the first character and n+1 is the index of the terminating null byte or the end of file. This also holds true when reading a byte vector (CBOR or MessagePack).

        Example: catch a parse_error exception

        The following code shows how a parse_error exception can be caught.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    try\n    {\n        // parsing input with a syntax error\n        json j = json::parse(\"[1,2,3,]\");\n    }\n    catch (const json::parse_error& e)\n    {\n        // output exception information\n        std::cout << \"message: \" << e.what() << '\\n'\n                  << \"exception id: \" << e.id << '\\n'\n                  << \"byte position of error: \" << e.byte << std::endl;\n    }\n}\n

        Output:

        message: [json.exception.parse_error.101] parse error at line 1, column 8: syntax error while parsing value - unexpected ']'; expected '[', '{', or a literal\nexception id: 101\nbyte position of error: 8\n
        "},{"location":"home/exceptions/#jsonexceptionparse_error101","title":"json.exception.parse_error.101","text":"

        This error indicates a syntax error while deserializing a JSON text. The error message describes that an unexpected token (character) was encountered, and the member byte indicates the error position.

        Example message

        Input ended prematurely:

        [json.exception.parse_error.101] parse error at 2: unexpected end of input; expected string literal\n

        No input:

        [json.exception.parse_error.101] parse error at line 1, column 1: attempting to parse an empty input; check that your input string or stream contains the expected JSON\n

        Control character was not escaped:

        [json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid string: control character U+0009 (HT) must be escaped to \\u0009 or \\\\; last read: '\"<U+0009>'\"\n

        String was not closed:

        [json.exception.parse_error.101] parse error at line 1, column 2: syntax error while parsing value - invalid string: missing closing quote; last read: '\"'\n

        Invalid number format:

        [json.exception.parse_error.101] parse error at line 1, column 3: syntax error while parsing value - invalid number; expected '+', '-', or digit after exponent; last read: '1E'\n

        \\u was not be followed by four hex digits:

        [json.exception.parse_error.101] parse error at line 1, column 6: syntax error while parsing value - invalid string: '\\u' must be followed by 4 hex digits; last read: '\"\\u01\"'\n

        Invalid UTF-8 surrogate pair:

        [json.exception.parse_error.101] parse error at line 1, column 13: syntax error while parsing value - invalid string: surrogate U+DC00..U+DFFF must follow U+D800..U+DBFF; last read: '\"\\uD7FF\\uDC00'\"\n

        Invalid UTF-8 byte:

        [json.exception.parse_error.101] parse error at line 3, column 24: syntax error while parsing value - invalid string: ill-formed UTF-8 byte; last read: '\"vous \\352t'\n

        Tip

        • Make sure the input is correctly read. Try to write the input to standard output to check if, for instance, the input file was successfully opened.
        • Paste the input to a JSON validator like https://jsonlint.com or a tool like jq.
        "},{"location":"home/exceptions/#jsonexceptionparse_error102","title":"json.exception.parse_error.102","text":"

        JSON uses the \\uxxxx format to describe Unicode characters. Code points above 0xFFFF are split into two \\uxxxx entries (\"surrogate pairs\"). This error indicates that the surrogate pair is incomplete or contains an invalid code point.

        Example message

        parse error at 14: missing or wrong low surrogate\n

        Note

        This exception is not used any more. Instead json.exception.parse_error.101 with a more detailed description is used.

        "},{"location":"home/exceptions/#jsonexceptionparse_error103","title":"json.exception.parse_error.103","text":"

        Unicode supports code points up to 0x10FFFF. Code points above 0x10FFFF are invalid.

        Example message

        parse error: code points above 0x10FFFF are invalid\n

        Note

        This exception is not used any more. Instead json.exception.parse_error.101 with a more detailed description is used.

        "},{"location":"home/exceptions/#jsonexceptionparse_error104","title":"json.exception.parse_error.104","text":"

        RFC 6902 requires a JSON Patch document to be a JSON document that represents an array of objects.

        Example message

        [json.exception.parse_error.104] parse error: JSON patch must be an array of objects\n
        "},{"location":"home/exceptions/#jsonexceptionparse_error105","title":"json.exception.parse_error.105","text":"

        An operation of a JSON Patch document must contain exactly one \"op\" member, whose value indicates the operation to perform. Its value must be one of \"add\", \"remove\", \"replace\", \"move\", \"copy\", or \"test\"; other values are errors.

        Example message

        [json.exception.parse_error.105] parse error: operation 'add' must have member 'value'\n
        [json.exception.parse_error.105] parse error: operation 'copy' must have string member 'from'\n
        [json.exception.parse_error.105] parse error: operation value 'foo' is invalid\n

        "},{"location":"home/exceptions/#jsonexceptionparse_error106","title":"json.exception.parse_error.106","text":"

        An array index in a JSON Pointer (RFC 6901) may be 0 or any number without a leading 0.

        Example message

        [json.exception.parse_error.106] parse error: array index '01' must not begin with '0'\n
        "},{"location":"home/exceptions/#jsonexceptionparse_error107","title":"json.exception.parse_error.107","text":"

        A JSON Pointer must be a Unicode string containing a sequence of zero or more reference tokens, each prefixed by a / character.

        Example message

        [json.exception.parse_error.107] parse error at byte 1: JSON pointer must be empty or begin with '/' - was: 'foo'\n
        "},{"location":"home/exceptions/#jsonexceptionparse_error108","title":"json.exception.parse_error.108","text":"

        In a JSON Pointer, only ~0 and ~1 are valid escape sequences.

        Example message

        [json.exception.parse_error.108] parse error: escape character '~' must be followed with '0' or '1'\n
        "},{"location":"home/exceptions/#jsonexceptionparse_error109","title":"json.exception.parse_error.109","text":"

        A JSON Pointer array index must be a number. This exception is thrown for an array index that does not begin with a digit, except for - and the empty reference token, which throw out_of_range.404 where they cannot be resolved.

        Example messages

        [json.exception.parse_error.109] parse error: array index 'one' is not a number\n
        [json.exception.parse_error.109] parse error: array index '+1' is not a number\n

        Note

        Before version 3.13.0 unreleased, a one-character array index that is not a digit (e.g., x) threw out_of_range.404 instead.

        "},{"location":"home/exceptions/#jsonexceptionparse_error110","title":"json.exception.parse_error.110","text":"

        When parsing a binary format, the byte vector ends before the complete value has been read.

        Example messages

        [json.exception.parse_error.110] parse error at byte 5: syntax error while parsing CBOR string: unexpected end of input\n
        [json.exception.parse_error.110] parse error at byte 2: syntax error while parsing UBJSON value: expected end of input; last byte: 0x5A\n
        [json.exception.parse_error.110] parse error at byte 8: syntax error while parsing BSON number: unexpected end of input\n

        "},{"location":"home/exceptions/#jsonexceptionparse_error112","title":"json.exception.parse_error.112","text":"

        An unexpected byte was read in a binary format or length information is invalid (BSON).

        Example messages

        [json.exception.parse_error.112] parse error at byte 1: syntax error while parsing CBOR value: invalid byte: 0x1C\n
        [json.exception.parse_error.112] parse error at byte 1: syntax error while parsing MessagePack value: invalid byte: 0xC1\n
        [json.exception.parse_error.112] parse error at byte 4: syntax error while parsing BJData size: expected '#' after type information; last byte: 0x02\n
        [json.exception.parse_error.112] parse error at byte 4: syntax error while parsing UBJSON size: expected '#' after type information; last byte: 0x02\n
        [json.exception.parse_error.112] parse error at byte 10: syntax error while parsing BSON string: string length must be at least 1, is -2147483648\n
        [json.exception.parse_error.112] parse error at byte 15: syntax error while parsing BSON binary: byte array length cannot be negative, is -1\n
        [json.exception.parse_error.112] parse error at byte 5: syntax error while parsing BSON document: document size 6 does not match the number of bytes read (5)\n

        "},{"location":"home/exceptions/#jsonexceptionparse_error113","title":"json.exception.parse_error.113","text":"

        A string could not be read from a binary format: either a value that is not a string was read where one was required (for instance as a map key), the string's length specification is invalid, or the string's bytes are not valid UTF-8 and the error_handler parameter of the corresponding from_* function is set to strict. By default (error_handler_t::keep), the bytes of a string are not checked for valid UTF-8 on read; see the ill-formed UTF-8 notes on the individual binary format pages for how such a string is handled depending on error_handler.

        CBOR and MessagePack allow map keys of any type, but JSON object keys are always strings. Maps with keys of any other type (for instance integers or null) are therefore not supported; see the notes on CBOR and MessagePack.

        Example messages

        [json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR object key: only string keys are supported, but found an unsigned integer; last byte: 0x01\n
        [json.exception.parse_error.113] parse error at byte 2: syntax error while parsing MessagePack object key: only string keys are supported, but found nil; last byte: 0xC0\n
        [json.exception.parse_error.113] parse error at byte 2: syntax error while parsing CBOR string: expected length specification (0x60-0x7B) or indefinite string type (0x7F); last byte: 0x7C\n
        [json.exception.parse_error.113] parse error at byte 2: syntax error while parsing UBJSON char: byte after 'C' must be in range 0x00..0x7F; last byte: 0x82\n
        [json.exception.parse_error.113] parse error at byte 3: syntax error while parsing BJData string: string length must not be negative\n
        [json.exception.parse_error.113] parse error at byte 3: syntax error while parsing CBOR string: invalid string: ill-formed UTF-8 byte\n

        "},{"location":"home/exceptions/#jsonexceptionparse_error114","title":"json.exception.parse_error.114","text":"

        The parsing of the corresponding BSON record type is not implemented (yet).

        Example message

        [json.exception.parse_error.114] parse error at byte 5: Unsupported BSON record type 0xFF\n
        "},{"location":"home/exceptions/#jsonexceptionparse_error115","title":"json.exception.parse_error.115","text":"

        A UBJSON high-precision number could not be parsed.

        Example message

        [json.exception.parse_error.115] parse error at byte 5: syntax error while parsing UBJSON high-precision number: invalid number text: 1A\n
        "},{"location":"home/exceptions/#jsonexceptionparse_error116","title":"json.exception.parse_error.116","text":"

        basic_json_document::load() rejected an image: either the bytes are not one save() could have written (too short, an unknown magic number or format version, or sizes that do not fit the buffer), or they are, but fail the requested image_check.

        Example message

        [json.exception.parse_error.116] parse error: invalid json_document image: the check failed\n

        Note

        This exception was added in version 3.13.0 unreleased, together with images.

        "},{"location":"home/exceptions/#iterator-errors","title":"Iterator errors","text":"

        This exception is thrown if iterators passed to a library function do not match the expected semantics.

        Exceptions have ids 2xx.

        Example: catch an invalid_iterator exception

        The following code shows how an invalid_iterator exception can be caught.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    try\n    {\n        // calling iterator::key() on non-object iterator\n        json j = \"string\";\n        json::iterator it = j.begin();\n        auto k = it.key();\n    }\n    catch (const json::invalid_iterator& e)\n    {\n        // output exception information\n        std::cout << \"message: \" << e.what() << '\\n'\n                  << \"exception id: \" << e.id << std::endl;\n    }\n}\n

        Output:

        message: [json.exception.invalid_iterator.207] cannot use key() for non-object iterators\nexception id: 207\n
        "},{"location":"home/exceptions/#jsonexceptioninvalid_iterator201","title":"json.exception.invalid_iterator.201","text":"

        The iterators passed to constructor basic_json(InputIT first, InputIT last) are not compatible, meaning they do not belong to the same container. Therefore, the range (first, last) is invalid.

        Example message

        [json.exception.invalid_iterator.201] iterators are not compatible\n
        "},{"location":"home/exceptions/#jsonexceptioninvalid_iterator202","title":"json.exception.invalid_iterator.202","text":"

        In the erase or insert function, the passed iterator pos does not belong to the JSON value for which the function was called. It hence does not define a valid position for the deletion/insertion.

        Example messages

        [json.exception.invalid_iterator.202] iterator does not fit current value\n
        [json.exception.invalid_iterator.202] iterators first and last must point to objects\n

        "},{"location":"home/exceptions/#jsonexceptioninvalid_iterator203","title":"json.exception.invalid_iterator.203","text":"

        Either iterator passed to function erase(IteratorType first, IteratorType last) does not belong to the JSON value from which values shall be erased. It hence does not define a valid range to delete values from.

        Example message

        [json.exception.invalid_iterator.203] iterators do not fit current value\n
        "},{"location":"home/exceptions/#jsonexceptioninvalid_iterator204","title":"json.exception.invalid_iterator.204","text":"

        When an iterator range for a primitive type (number, boolean, or string) is passed to a constructor or an erase function, this range has to be exactly (begin(), end()), because this is the only way the single stored value is expressed. All other ranges are invalid.

        Example message

        [json.exception.invalid_iterator.204] iterators out of range\n
        "},{"location":"home/exceptions/#jsonexceptioninvalid_iterator205","title":"json.exception.invalid_iterator.205","text":"

        When an iterator for a primitive type (number, boolean, or string) is passed to an erase function, the iterator has to be the begin() iterator, because it is the only way to address the stored value. All other iterators are invalid.

        Example message

        [json.exception.invalid_iterator.205] iterator out of range\n
        "},{"location":"home/exceptions/#jsonexceptioninvalid_iterator206","title":"json.exception.invalid_iterator.206","text":"

        The iterators passed to constructor basic_json(InputIT first, InputIT last) belong to a JSON null value and hence to not define a valid range.

        Example message

        [json.exception.invalid_iterator.206] cannot construct with iterators from null\n
        "},{"location":"home/exceptions/#jsonexceptioninvalid_iterator207","title":"json.exception.invalid_iterator.207","text":"

        The key() member function can only be used on iterators belonging to a JSON object, because other types do not have a concept of a key.

        Example message

        [json.exception.invalid_iterator.207] cannot use key() for non-object iterators\n
        "},{"location":"home/exceptions/#jsonexceptioninvalid_iterator208","title":"json.exception.invalid_iterator.208","text":"

        The operator[] to specify a concrete offset cannot be used on iterators belonging to a JSON object, because JSON objects are unordered.

        Example message

        [json.exception.invalid_iterator.208] cannot use operator[] for object iterators\n
        "},{"location":"home/exceptions/#jsonexceptioninvalid_iterator209","title":"json.exception.invalid_iterator.209","text":"

        The offset operators (+, -, +=, -=) cannot be used on iterators belonging to a JSON object, because JSON objects are unordered.

        Example message

        [json.exception.invalid_iterator.209] cannot use offsets with object iterators\n
        "},{"location":"home/exceptions/#jsonexceptioninvalid_iterator210","title":"json.exception.invalid_iterator.210","text":"

        The iterator range passed to the insert function is not compatible, meaning they do not belong to the same container. Therefore, the range (first, last) is invalid.

        Example message

        [json.exception.invalid_iterator.210] iterators do not fit\n
        "},{"location":"home/exceptions/#jsonexceptioninvalid_iterator211","title":"json.exception.invalid_iterator.211","text":"

        The iterator range passed to the insert function must not be a subrange of the container to insert to.

        Example message

        [json.exception.invalid_iterator.211] passed iterators may not belong to container\n
        "},{"location":"home/exceptions/#jsonexceptioninvalid_iterator212","title":"json.exception.invalid_iterator.212","text":"

        When two iterators are compared, they must belong to the same container.

        Example message

        [json.exception.invalid_iterator.212] cannot compare iterators of different containers\n
        "},{"location":"home/exceptions/#jsonexceptioninvalid_iterator213","title":"json.exception.invalid_iterator.213","text":"

        The order of object iterators cannot be compared, because JSON objects are unordered.

        Example message

        [json.exception.invalid_iterator.213] cannot compare order of object iterators\n
        "},{"location":"home/exceptions/#jsonexceptioninvalid_iterator214","title":"json.exception.invalid_iterator.214","text":"

        Cannot retrieve value from iterator: The iterator either refers to a null value, or it refers to a primitive type (number, boolean, or string), but does not match the iterator returned by begin().

        Example message

        [json.exception.invalid_iterator.214] cannot get value\n
        "},{"location":"home/exceptions/#type-errors","title":"Type errors","text":"

        This exception is thrown in case of a type error; that is, a library function is executed on a JSON value whose type does not match the expected semantics.

        Exceptions have ids 3xx.

        Example: catch a type_error exception

        The following code shows how a type_error exception can be caught.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    try\n    {\n        // calling push_back() on a string value\n        json j = \"string\";\n        j.push_back(\"another string\");\n    }\n    catch (const json::type_error& e)\n    {\n        // output exception information\n        std::cout << \"message: \" << e.what() << '\\n'\n                  << \"exception id: \" << e.id << std::endl;\n    }\n}\n

        Output:

        message: [json.exception.type_error.308] cannot use push_back() with string\nexception id: 308\n
        "},{"location":"home/exceptions/#jsonexceptiontype_error301","title":"json.exception.type_error.301","text":"

        To create an object from an initializer list, the initializer list must consist only of a list of pairs whose first element is a string. When this constraint is violated, an array is created instead.

        Example message

        [json.exception.type_error.301] cannot create object from initializer list\n
        "},{"location":"home/exceptions/#jsonexceptiontype_error302","title":"json.exception.type_error.302","text":"

        During implicit or explicit value conversion, the JSON type must be compatible with the target type. For instance, a JSON string can only be converted into string types, but not into numbers or boolean types.

        Example messages

        [json.exception.type_error.302] type must be object, but is null\n
        [json.exception.type_error.302] type must be string, but is object\n

        This exception is also thrown with JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS if a key of a map with enum keys is not converted to a string, for instance, because the enum is stored as an integer.

        "},{"location":"home/exceptions/#jsonexceptiontype_error303","title":"json.exception.type_error.303","text":"

        To retrieve a reference to a value stored in a basic_json object with get_ref, the type of the reference must match the value type. For instance, for a JSON array, the ReferenceType must be array_t &.

        Example messages

        [json.exception.type_error.303] incompatible ReferenceType for get_ref, actual type is object\n
        [json.exception.type_error.303] incompatible ReferenceType for get_ref, actual type is number\"\n

        "},{"location":"home/exceptions/#jsonexceptiontype_error304","title":"json.exception.type_error.304","text":"

        The at() member functions can only be executed for certain JSON types.

        Example messages

        [json.exception.type_error.304] cannot use at() with string\n
        [json.exception.type_error.304] cannot use at() with number\n

        "},{"location":"home/exceptions/#jsonexceptiontype_error305","title":"json.exception.type_error.305","text":"

        The operator[] member functions can only be executed for certain JSON types.

        Example messages

        [json.exception.type_error.305] cannot use operator[] with a string argument with array\n
        [json.exception.type_error.305] cannot use operator[] with a numeric argument with object\n

        "},{"location":"home/exceptions/#jsonexceptiontype_error306","title":"json.exception.type_error.306","text":"

        The value() member functions can only be executed for certain JSON types.

        Example message

        [json.exception.type_error.306] cannot use value() with number\n
        "},{"location":"home/exceptions/#jsonexceptiontype_error307","title":"json.exception.type_error.307","text":"

        The erase() member functions can only be executed for certain JSON types.

        Example message

        [json.exception.type_error.307] cannot use erase() with string\n
        "},{"location":"home/exceptions/#jsonexceptiontype_error308","title":"json.exception.type_error.308","text":"

        The push_back() and operator+= member functions can only be executed for certain JSON types.

        Example message

        [json.exception.type_error.308] cannot use push_back() with string\n
        "},{"location":"home/exceptions/#jsonexceptiontype_error309","title":"json.exception.type_error.309","text":"

        The insert() member functions can only be executed for certain JSON types.

        Example messages

        [json.exception.type_error.309] cannot use insert() with array\n
        [json.exception.type_error.309] cannot use insert() with number\n

        "},{"location":"home/exceptions/#jsonexceptiontype_error310","title":"json.exception.type_error.310","text":"

        The swap() member functions can only be executed for certain JSON types.

        Example message

        [json.exception.type_error.310] cannot use swap() with number\n
        "},{"location":"home/exceptions/#jsonexceptiontype_error311","title":"json.exception.type_error.311","text":"

        The emplace() and emplace_back() member functions can only be executed for certain JSON types.

        Example messages

        [json.exception.type_error.311] cannot use emplace() with number\n
        [json.exception.type_error.311] cannot use emplace_back() with number\n

        "},{"location":"home/exceptions/#jsonexceptiontype_error312","title":"json.exception.type_error.312","text":"

        The update() member functions can only be executed for certain JSON types.

        Example message

        [json.exception.type_error.312] cannot use update() with array\n
        "},{"location":"home/exceptions/#jsonexceptiontype_error313","title":"json.exception.type_error.313","text":"

        The unflatten() function converts an object whose keys are JSON Pointers back into an arbitrary nested JSON value. The JSON Pointers must not overlap, because then the resulting value would not be well-defined.

        Example message

        [json.exception.type_error.313] invalid value to unflatten\n
        "},{"location":"home/exceptions/#jsonexceptiontype_error314","title":"json.exception.type_error.314","text":"

        The unflatten() function only works for an object whose keys are JSON Pointers.

        Example message

        Calling unflatten() on an array [1,2,3]:

        [json.exception.type_error.314] only objects can be unflattened\n
        "},{"location":"home/exceptions/#jsonexceptiontype_error315","title":"json.exception.type_error.315","text":"

        The unflatten() function only works for an object whose keys are JSON Pointers and whose values are primitive.

        Example message

        Calling unflatten() on an object {\"/1\", [1,2,3]}:

        [json.exception.type_error.315] values in object must be primitive\n
        "},{"location":"home/exceptions/#jsonexceptiontype_error316","title":"json.exception.type_error.316","text":"

        The dump() function only works with UTF-8 encoded strings; that is, if you assign a std::string to a JSON value, make sure it is UTF-8 encoded. See the FAQ entry on serializing untrusted or invalid UTF-8 for background and the recommended fix.

        The binary writers to_cbor(), to_ubjson(), to_bjdata(), and to_bson() throw this exception as well for a string value or object key that is not valid UTF-8 if their error_handler is strict (the default if JSON_STRICT_BINARY_UTF8 is enabled). So does to_msgpack() if error_handler_t::strict is passed.

        Example message

        Calling dump() on a JSON value containing an ISO 8859-1 encoded string: 

        [json.exception.type_error.316] invalid UTF-8 byte at index 15: 0x6F\n

        Tip

        • Store the source file with UTF-8 encoding.
        • Pass an error handler as last parameter to the dump() function to avoid this exception:
          • json::error_handler_t::replace will replace invalid bytes sequences with U+FFFD
          • json::error_handler_t::ignore will silently ignore invalid byte sequences
          • json::error_handler_t::keep will copy invalid byte sequences to the output unchanged
        "},{"location":"home/exceptions/#jsonexceptiontype_error317","title":"json.exception.type_error.317","text":"

        The dynamic type of the object cannot be represented in the requested serialization format (e.g., a raw true or null JSON object cannot be serialized to BSON)

        Example messages

        Serializing null to BSON: 

        [json.exception.type_error.317] to serialize to BSON, top-level type must be object, but is null\n
        Serializing [1,2,3] to BSON: 
        [json.exception.type_error.317] to serialize to BSON, top-level type must be object, but is array\n

        Tip

        Encapsulate the JSON value in an object. That is, instead of serializing true, serialize {\"value\": true}

        "},{"location":"home/exceptions/#jsonexceptiontype_error318","title":"json.exception.type_error.318","text":"

        With JSON_USE_OBJECTS_FOR_ENUM_KEYED_MAPS, a map with enum keys is stored as an object. This exception is thrown if two of its keys are converted to the same string, so one of the entries would be lost. This happens, for instance, if NLOHMANN_JSON_SERIALIZE_ENUM does not list an enumerator and it is therefore converted like the first listed one.

        Example message

        [json.exception.type_error.318] duplicate object key 'red'\n
        "},{"location":"home/exceptions/#jsonexceptiontype_error319","title":"json.exception.type_error.319","text":"

        basic_json_document::set, basic_json_document::push_back, and basic_json_document::insert can store any basic_json value except a binary one: a json_document has no representation for binary values, which only ever arise from parsing a binary format or from an explicit json::binary value, not from JSON text.

        Example message

        [json.exception.type_error.319] cannot store a binary value in a json_document\n

        Note

        This exception was added in version 3.13.0 unreleased, together with editable json_documents.

        "},{"location":"home/exceptions/#jsonexceptiontype_error320","title":"json.exception.type_error.320","text":"

        basic_json_document::save() cannot write an image of a discarded document. save() and load() also throw this exception on a big-endian target, since the image format is little-endian only.

        Example messages

        [json.exception.type_error.320] cannot save a discarded json_document\n
        [json.exception.type_error.320] json_document images need a little-endian target\n

        Note

        This exception was added in version 3.13.0 unreleased, together with images.

        "},{"location":"home/exceptions/#jsonexceptiontype_error321","title":"json.exception.type_error.321","text":"

        A discarded value (one created by parse() with a callback that returns false for the value, or by default-constructing a basic_json with value_t::discarded) was passed to a binary serialization function, either directly or nested in an array or object. There is no way to represent a discarded value in CBOR, MessagePack, UBJSON, BJData, BSON, or BON8.

        Example message

        Serializing [1, 2] to CBOR, where the second element was discarded by a parser callback: 

        [json.exception.type_error.321] cannot serialize discarded value to CBOR\n

        "},{"location":"home/exceptions/#out-of-range","title":"Out of range","text":"

        This exception is thrown in case a library function is called on an input parameter that exceeds the expected range, for instance, in the case of array indices or nonexisting object keys.

        Exceptions have ids 4xx.

        Example: catch an out_of_range exception

        The following code shows how an out_of_range exception can be caught.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    try\n    {\n        // calling at() for an invalid index\n        json j = {1, 2, 3, 4};\n        j.at(4) = 10;\n    }\n    catch (const json::out_of_range& e)\n    {\n        // output exception information\n        std::cout << \"message: \" << e.what() << '\\n'\n                  << \"exception id: \" << e.id << std::endl;\n    }\n}\n

        Output:

        message: [json.exception.out_of_range.401] array index 4 is out of range\nexception id: 401\n
        "},{"location":"home/exceptions/#jsonexceptionout_of_range401","title":"json.exception.out_of_range.401","text":"

        The provided array index i is larger than size-1.

        Example message

        array index 3 is out of range\n
        "},{"location":"home/exceptions/#jsonexceptionout_of_range402","title":"json.exception.out_of_range.402","text":"

        The special array index - in a JSON Pointer never describes a valid element of the array, but the index past the end. That is, it can only be used to add elements at this position, but not to read it.

        Example message

        array index '-' (3) is out of range\n
        "},{"location":"home/exceptions/#jsonexceptionout_of_range403","title":"json.exception.out_of_range.403","text":"

        The provided key was not found in the JSON object.

        Example message

        key 'foo' not found\n
        "},{"location":"home/exceptions/#jsonexceptionout_of_range404","title":"json.exception.out_of_range.404","text":"

        A reference token in a JSON Pointer could not be resolved, for instance an array index that begins with a digit but contains other characters (e.g., 1a), or - where it cannot be used.

        Example message

        unresolved reference token 'foo'\n
        "},{"location":"home/exceptions/#jsonexceptionout_of_range405","title":"json.exception.out_of_range.405","text":"

        The JSON Patch operations 'remove' and 'move' cannot be applied to the root element of the JSON value. The exception is also thrown when the parent or a reference token of an empty JSON Pointer is requested, for instance by back or pop_back.

        Example message

        JSON pointer has no parent\n
        "},{"location":"home/exceptions/#jsonexceptionout_of_range406","title":"json.exception.out_of_range.406","text":"

        A parsed number could not be stored without changing it to NaN or INF. For the binary formats, this happens when a finite floating-point number does not fit into number_float_t, for example a double-precision number when number_float_t is float.

        Example messages

        number overflow parsing '10E1000'\n
        [json.exception.out_of_range.406] syntax error while parsing CBOR value: number overflow\n

        "},{"location":"home/exceptions/#jsonexceptionout_of_range407","title":"json.exception.out_of_range.407","text":"

        An integer number cannot be represented by the binary format it is serialized to:

        • BON8 only stores integers that fit into int64, so to_bon8 throws for unsigned integers greater than 9223372036854775807.
        • CBOR, MessagePack, and BSON store integers in at most 64 bits. With the default number types, every integer fits, but a number_integer_t or number_unsigned_t wider than 64 bits (e.g., __int128) can hold values outside the range of the format: [-2^64, 2^64-1] for CBOR, [-2^63, 2^64-1] for MessagePack, and the range of int64 (signed integers) or uint64 (unsigned integers) for BSON.

        UBJSON and BJData never throw this exception, because they serialize integers beyond 64 bits as high-precision numbers.

        Former occurrences

        The UBJSON and BSON writers used to throw this exception as well:

        • Since version 3.9.0, integer numbers beyond int64 are serialized as high-precision UBJSON numbers.
        • Since version 3.12.0, integer numbers beyond int64 are serialized as uint64 BSON numbers.

        Before version 3.13.0 unreleased, CBOR, MessagePack, and BSON silently truncated integers wider than 64 bits, and BJData truncated unsigned integers wider than 64 bits.

        Example messages

        integer number 18446744073709551615 cannot be represented by BON8 as it does not fit int64\n
        integer number 1267650600228229401496703205376 cannot be represented by CBOR as it does not fit [-2^64, 2^64-1]\n

        "},{"location":"home/exceptions/#jsonexceptionout_of_range408","title":"json.exception.out_of_range.408","text":"

        The size of an array or object in a binary format exceeds the maximal capacity: the size following # for UBJSON/BJData, or the encoded length for CBOR.

        The exception is also thrown for a UBJSON array of a type that is encoded by its marker alone (Z, T or F) whose declared count exceeds 1,048,576 (1 << 20). Such an array has no payload, so its count alone decides how much memory is allocated, and a handful of bytes would otherwise describe billions of values. to_ubjson writes longer arrays of these types without the size and type annotation, so any value it produces can still be read back.

        Example messages

        excessive array size: 8658170730974374167\n
        [json.exception.out_of_range.408] syntax error while parsing CBOR size: excessive array size\n
        [json.exception.out_of_range.408] syntax error while parsing CBOR size: excessive map size\n
        [json.exception.out_of_range.408] syntax error while parsing UBJSON size: excessive array size\n

        "},{"location":"home/exceptions/#jsonexceptionout_of_range409","title":"json.exception.out_of_range.409","text":"

        Key identifiers to be serialized to BSON cannot contain code point U+0000, since the key is stored as zero-terminated c-string.

        Example message

        BSON key cannot contain code point U+0000 (at byte 2)\n
        "},{"location":"home/exceptions/#jsonexceptionout_of_range410","title":"json.exception.out_of_range.410","text":"

        This exception is thrown when an undefined value is used with NLOHMANN_JSON_SERIALIZE_ENUM_STRICT, or when an array index in a JSON pointer exceeds the range of size_type (e.g., on 32-bit platforms).

        Example message

        enum value out of range\narray index 18446744073709551616 exceeds size_type\n
        "},{"location":"home/exceptions/#jsonexceptionout_of_range411","title":"json.exception.out_of_range.411","text":"

        A JSON Patch add operation cannot be applied because the target location's parent is neither an object nor an array. Per RFC 6902, an add target must reference a member of an existing object or an element of an existing array; a primitive value (string, number, boolean, etc.) cannot receive a new member or element.

        Example message

        cannot add value: the JSON Patch 'add' target's parent is of type string, but must be an object or array\n

        Note

        This exception was added in version 3.13.0 unreleased. Before that, this situation hit an internal assertion (aborting the program in debug builds) or was silently ignored when assertions were disabled.

        "},{"location":"home/exceptions/#jsonexceptionout_of_range412","title":"json.exception.out_of_range.412","text":"

        BSON stores the length of documents, arrays, strings, and binary values in a signed 32-bit integer, and MessagePack stores the length of strings, binary values, arrays, and objects in at most an unsigned 32-bit integer. This exception is thrown when a value is too large to be described by such a length field.

        Example messages

        BSON length 2147483661 exceeds maximum of 2147483647\n
        MessagePack length 4294967296 exceeds maximum of 4294967295\n

        Note

        This exception was added in version 3.13.0 unreleased. Before that, the BSON length was silently truncated, and to_bson produced documents with negative length prefixes that from_bson rejected; to_msgpack wrote such a value without any length, producing output that could not be read back.

        "},{"location":"home/exceptions/#jsonexceptionout_of_range413","title":"json.exception.out_of_range.413","text":"

        A JSON Patch remove operation cannot be applied because the target location's parent is neither an object nor an array. Per RFC 6902, a remove target must reference a member of an existing object or an element of an existing array; a primitive value (string, number, boolean, etc.) or null has no members or elements to remove.

        Example message

        cannot remove value: the JSON Patch 'remove' target's parent is of type number, but must be an object or array\n

        Note

        This exception was added in version 3.13.0 unreleased. Before that, this situation was silently ignored (the remove operation had no effect).

        "},{"location":"home/exceptions/#jsonexceptionout_of_range414","title":"json.exception.out_of_range.414","text":"

        A JSON Patch move operation's \"from\" location is a proper prefix of its \"path\" location. Per RFC 6902 (section 4.4), a location cannot be moved into one of its own children.

        Example message

        cannot move value: 'from' path '/0' is a proper prefix of 'path' '/0/0'\n

        Note

        This exception was added in version 3.13.0 unreleased. Before that, this situation could succeed with a corrupted result: for an array target, removing the \"from\" element before the \"add\" step shifted subsequent indices, so \"path\" silently re-resolved to a different element than intended.

        "},{"location":"home/exceptions/#jsonexceptionout_of_range415","title":"json.exception.out_of_range.415","text":"

        MessagePack's ext type and BSON's binary subtype are each stored in a single byte. This exception is thrown when serializing a byte_container_with_subtype whose subtype exceeds 255.

        Example message

        [json.exception.out_of_range.415] subtype 70000 is too large for the MessagePack ext type (max 255)\n

        Note

        This exception was added in version 3.13.0 unreleased. Before that, subtypes above 255 were silently truncated modulo 256 instead of raising an error.

        "},{"location":"home/exceptions/#jsonexceptionout_of_range416","title":"json.exception.out_of_range.416","text":"

        basic_json_document::parse() and the other parsing functions of basic_json_document index a value's position in the source text in 32 bits, so they do not support an input of 4294967280 bytes (4 GiB minus 16 bytes) or more. The same 32-bit limit applies to an editable document's own storage: set and push_back throw this exception once the strings and number tokens written by edits reach 4 GiB in total, or once more than 4294967295 arrays/objects have had an element set or appended to them. The same limit applies to an image: save() throws it if the node count, the text, or the decoded strings it would write would individually reach 4 GiB.

        Example messages

        [json.exception.out_of_range.416] input of 4294967280 bytes or more is not supported by json_document\n
        [json.exception.out_of_range.416] edits of 4 GiB or more are not supported by json_document\n
        [json.exception.out_of_range.416] images of 4 GiB or more are not supported by json_document\n

        Note

        This exception was added in version 3.13.0 unreleased, together with <nlohmann/json_view.hpp>. basic_json::parse() has no such limit.

        "},{"location":"home/exceptions/#further-exceptions","title":"Further exceptions","text":"

        This exception is thrown in case of errors that cannot be classified with the other exception types.

        Exceptions have ids 5xx.

        Example: catch an other_error exception

        The following code shows how an other_error exception can be caught.

        #include <iostream>\n#include <nlohmann/json.hpp>\n\nusing json = nlohmann::json;\nusing namespace nlohmann::literals;\n\nint main()\n{\n    try\n    {\n        // executing a failing JSON Patch operation\n        json value = R\"({\n            \"best_biscuit\": {\n                \"name\": \"Oreo\"\n            }\n        })\"_json;\n        json patch = R\"([{\n            \"op\": \"test\",\n            \"path\": \"/best_biscuit/name\",\n            \"value\": \"Choco Leibniz\"\n        }])\"_json;\n        value.patch(patch);\n    }\n    catch (const json::other_error& e)\n    {\n        // output exception information\n        std::cout << \"message: \" << e.what() << '\\n'\n                  << \"exception id: \" << e.id << std::endl;\n    }\n}\n

        Output:

        message: [json.exception.other_error.501] unsuccessful: {\"op\":\"test\",\"path\":\"/best_biscuit/name\",\"value\":\"Choco Leibniz\"}\nexception id: 501\n
        "},{"location":"home/exceptions/#jsonexceptionother_error501","title":"json.exception.other_error.501","text":"

        A JSON Patch operation 'test' failed. The unsuccessful operation is also printed.

        Example message

        Executing {\"op\":\"test\", \"path\":\"/baz\", \"value\":\"bar\"} on {\"baz\": \"qux\"}:

        [json.exception.other_error.501] unsuccessful: {\"op\":\"test\",\"path\":\"/baz\",\"value\":\"bar\"}\n
        "},{"location":"home/exceptions/#jsonexceptionother_error502","title":"json.exception.other_error.502","text":"

        to_ubjson and to_bjdata were called with use_type = true but use_size = false. UBJSON requires a size marker (#) after a type marker ($).

        Example message

        [json.exception.other_error.502] use_type requires use_size = true\n

        Note

        This exception was added in version 3.13.0 unreleased. Before that, debug builds aborted on an assertion and release builds wrote a $ marker without #, which from_ubjson then rejected.

        "},{"location":"home/faq/","title":"Frequently Asked Questions (FAQ)","text":""},{"location":"home/faq/#known-bugs","title":"Known bugs","text":""},{"location":"home/faq/#brace-initialization-yields-arrays","title":"Brace initialization yields arrays","text":"

        Question

        Why does

        json j{true};\n

        and

        json j(true);\n

        yield different results ([true] vs. true)?

        This is a known issue, and -- even worse -- the behavior differs between GCC and Clang. The \"culprit\" for this is the library's constructor overloads for initializer lists to allow syntax like

        json array = {1, 2, 3, 4};\n

        for arrays and

        json object = {{\"one\", 1}, {\"two\", 2}}; \n

        for objects.

        Tip

        To avoid any confusion and ensure portable code, do not use brace initialization with the types basic_json, json, or ordered_json unless you want to create an object or array as shown in the examples above.

        To explicitly create a single-element array, use json::array({value}):

        json j = json::array({true});  // [true]\n

        Opt-in copy semantics (since version 3.13.0 unreleased)

        If you define JSON_BRACE_INIT_COPY_SEMANTICS to 1 before including the library, single-element brace initialization is treated as copy/move instead of creating a single-element array:

        #define JSON_BRACE_INIT_COPY_SEMANTICS 1\n#include <nlohmann/json.hpp>\n\njson obj = {{\"key\", \"value\"}};\njson j{obj};   // -> {\"key\":\"value\"}  (copy, not array)\n

        Without the macro (default behavior), json j{obj} creates [{\"key\":\"value\"}]. This opt-in macro fixes issue #5074 while preserving backwards compatibility for existing code.

        "},{"location":"home/faq/#limitations","title":"Limitations","text":""},{"location":"home/faq/#relaxed-parsing","title":"Relaxed parsing","text":"

        Question

        Can you add an option to ignore trailing commas?

        Yes, trailing commas in arrays and objects can be allowed by setting parameter ignore_trailing_commas of parse to true; see trailing commas. They are rejected by default, and the library never adds them when serializing.

        "},{"location":"home/faq/#parse-errors-reading-non-ascii-characters","title":"Parse errors reading non-ASCII characters","text":"

        Questions

        • Why is the parser complaining about a Chinese character?
        • Does the library support Unicode?
        • I get an exception [json.exception.parse_error.101] parse error at line 1, column 53: syntax error while parsing value - invalid string: ill-formed UTF-8 byte; last read: '\"Test\u00e9$')\"

        The library supports Unicode input as follows:

        • Only UTF-8 encoded input is supported, which is the default encoding for JSON, according to RFC 8259.
        • std::u16string and std::u32string can be parsed, assuming UTF-16 and UTF-32 encoding, respectively. These encodings are not supported when reading from files or other input containers.
        • Other encodings such as Latin-1 or ISO 8859-1 are not supported and will yield parse or serialization errors.
        • The library will not replace Unicode noncharacters.
        • Invalid surrogates (e.g., incomplete pairs such as \\uDEAD) will yield parse errors.
        • The strings stored in the library are UTF-8 encoded. When using the default string type (std::string), note that its length/size functions return the number of stored bytes rather than the number of characters or glyphs.
        • When you store strings with different encodings in the library, calling dump() may throw an exception unless json::error_handler_t::replace, json::error_handler_t::ignore, or json::error_handler_t::keep are used as error handlers.

        In most cases, the parser is right to complain, because the input is not UTF-8 encoded. This is especially true for Microsoft Windows, where Latin-1 or ISO 8859-1 is often the standard encoding.

        "},{"location":"home/faq/#nul-bytes-in-the-input","title":"NUL bytes in the input","text":"

        Questions

        • Why does json::parse() silently ignore part of my input?
        • Why does a std::string/buffer with extra data after the JSON text parse without error, while a similar-looking string with extra text does not?

        A '\\0' (NUL) byte anywhere in the input is treated the same as the real end of the input, rather than as an ordinary (and, outside of a string, invalid) byte. Everything from that byte onward is silently ignored, without a parse error \u2014 including further, otherwise well-formed JSON:

        json::parse(std::string(\"123\") + '\\0');          // == 123, no error\njson::parse(std::string(\"123\") + '\\0' + \"true\"); // == 123, the \"true\" is silently ignored too\n

        This is different from any other unexpected trailing byte, which does raise parse_error.101:

        json::parse(\"123x\"); // throws parse_error.101: unexpected additional data\n

        This falls out of the same convention used when no explicit input length is given at all: json::parse(const char*) already stops at the first NUL byte via strlen(), since a bare pointer has no length of its own. The library applies that same NUL-terminated-C-string convention uniformly, rather than only when a length is genuinely unavailable \u2014 so a std::string, iterator range, or container whose content happens to include a NUL byte is affected the same way a raw const char* would be.

        If your input may contain a trailing or embedded NUL that is not meant to signal the end of the JSON text \u2014 for instance, a fixed-size, zero-padded buffer \u2014 trim it yourself before calling parse(), since the library will otherwise silently stop there instead of raising an error:

        s.resize(s.find('\\0')); // drop everything from the first NUL onward, if any\njson::parse(s);\n

        Opt-in strict handling (since version 3.13.0 unreleased)

        Manually trimming every input is easy to forget. If you define JSON_STRICT_NUL_HANDLING to 1 before including the library, a '\\0' byte is instead rejected like any other unexpected byte and raises parse_error.101, instead of being treated as end of input:

        #define JSON_STRICT_NUL_HANDLING 1\n#include <nlohmann/json.hpp>\n\njson::parse(std::string(\"123\") + '\\0'); // throws parse_error.101 instead of silently returning 123\n

        This macro defaults to 0 (disabled, preserving the behavior described above) to avoid breaking existing code that may depend on it, even unknowingly; it is planned to become the default in version 4.0.0. See its documentation for details, including how it also affects char arrays such as string literals.

        Note that this is unrelated to an unescaped NUL byte occurring inside a quoted JSON string, which is a different, already-invalid case and is correctly rejected either way:

        json::parse(std::string(\"\\\"\") + '\\0' + \"\\\"\"); // throws parse_error.101: control character U+0000 (NUL) must be escaped to \\u0000\n
        "},{"location":"home/faq/#wide-string-handling","title":"Wide string handling","text":"

        Question

        Why are wide strings (e.g., std::wstring) dumped as arrays of numbers?

        As described above, the library assumes UTF-8 as encoding. To store a wide string, you need to change the encoding.

        Example

        #include <codecvt> // codecvt_utf8\n#include <locale>  // wstring_convert\n\n// encoding function\nstd::string to_utf8(std::wstring& wide_string)\n{\n    static std::wstring_convert<std::codecvt_utf8<wchar_t>> utf8_conv;\n    return utf8_conv.to_bytes(wide_string);\n}\n\njson j;\nstd::wstring ws = L\"\u8ecaB1234 \u3053\u3093\u306b\u3061\u306f\";\n\nj[\"original\"] = ws;\nj[\"encoded\"] = to_utf8(ws);\n\nstd::cout << j << std::endl;\n

        The result is:

        {\n  \"encoded\": \"\u8ecaB1234 \u3053\u3093\u306b\u3061\u306f\",\n  \"original\": [36554, 66, 49, 50, 51, 52, 32, 12371, 12435, 12395, 12385, 12399]\n}\n
        "},{"location":"home/faq/#usage","title":"Usage","text":""},{"location":"home/faq/#thread-safety","title":"Thread safety","text":"

        Question

        Is basic_json thread-safe?

        No. basic_json provides no built-in synchronization, the same as std::map or std::vector. Concurrent reads of the same value from multiple threads are safe, as are concurrent (non-overlapping) accesses to independent json objects. However, any concurrent write to a json object -- or a concurrent read while another thread writes to the same object -- is a data race and requires external synchronization (e.g., a std::mutex) by the caller.

        "},{"location":"home/faq/#schema-validation","title":"Schema validation","text":"

        Question

        Does this library support JSON Schema validation?

        Not directly, but the companion project json-schema-validator builds JSON Schema (draft 7; draft 4 in its older, now-superseded 1.x releases) validation on top of this library and is a common recommendation for this use case.

        "},{"location":"home/faq/#exceptions","title":"Exceptions","text":""},{"location":"home/faq/#parsing-without-exceptions","title":"Parsing without exceptions","text":"

        Question

        Is it possible to indicate a parse error without throwing an exception?

        Yes, see Parsing and exceptions.

        "},{"location":"home/faq/#key-name-in-exceptions","title":"Key name in exceptions","text":"

        Question

        Can I get the key of the object item that caused an exception?

        Yes, you can. Please define the symbol JSON_DIAGNOSTICS to get extended diagnostics messages.

        "},{"location":"home/faq/#serialization-issues","title":"Serialization issues","text":""},{"location":"home/faq/#number-precision","title":"Number precision","text":"

        Question

        • It seems that precision is lost when serializing a double.
        • Can I change the precision for floating-point serialization?

        The library serializes floating-point numbers with the shortest representation that parses back to the same value, using at most std::numeric_limits<number_float_t>::max_digits10 (17 for IEEE doubles) significant digits. This guarantees that value -> string -> value round-trips. The reverse direction, string -> value -> string, is only guaranteed for numbers with at most std::numeric_limits<number_float_t>::digits10 (15 for IEEE doubles) significant digits; a number with more digits may not be representable exactly and is then serialized as the closest double.

        cppreference.com

        The value of std::numeric_limits<T>::max_digits10 is the number of base-10 digits that are necessary to uniquely represent all distinct values of the type T, such as necessary for serialization/deserialization to text.

        Tip

        The website https://float.exposed gives a good insight into the internal storage of floating-point numbers.

        See this section on the library's number handling for more information.

        "},{"location":"home/faq/#serializing-untrusted-or-invalid-utf-8","title":"Serializing untrusted or invalid UTF-8","text":"

        Questions

        • Why does dump() throw when I serialize data that came from the network?
        • Is CVE-2024-34363 a vulnerability in this library?

        Crashes reported against this library that stem from an uncaught type_error.316 while serializing unvalidated input (e.g., CVE-2024-34363) are a usage issue, not a library vulnerability: dump() throws in its default strict mode because RFC 8259 requires JSON text to be valid UTF-8.

        The recommended pattern is to pass a non-strict error_handler or to handle the exception:

        // replace invalid sequences with U+FFFD instead of throwing\nconst auto s = j.dump(-1, ' ', false, json::error_handler_t::replace);\n
        "},{"location":"home/faq/#using-json-values-with-stdformat-or-fmt","title":"Using JSON values with std::format or fmt","text":"

        Question

        • Can I use std::format(\"{}\", j) on a JSON value?
        • Can I use fmt::format(\"{}\", j) or fmt::print(\"{}\", j) (the {fmt} library) on a JSON value?

        std::format works out of the box since version 3.13.0 unreleased, as long as the standard library provides <format> (see JSON_HAS_STD_FORMAT); see std::formatter<basic_json> for details, including the \"{:#}\" pretty-print spec, indent widths (\"{:2}\"), and custom indent characters (\"{:.>#}\").

        For fmt, the library ships format_as, a small customization point fmt looks for via argument-dependent lookup. It only has an effect on fmt 10.0.0 through 11.0.2 \u2014 from fmt 11.1.0 onwards, fmt no longer picks up a format_as overload that returns a std::string. On such versions (or any version, if you also want the same \"{:#}\"/width/fill-and-align spec support that std::formatter<basic_json> has), define your own fmt::formatter specialization; see format_as for a recipe that mirrors it.

        If you get ambiguous-overload errors when passing a JSON value to fmt::format/fmt::print without any fmt::formatter<json> specialization in scope, that's fmt picking up basic_json's implicit operator ValueType() conversion operator (see #964 and #958); disabling it via JSON_USE_IMPLICIT_CONVERSIONS 0 avoids the ambiguity.

        "},{"location":"home/faq/#compilation-issues","title":"Compilation issues","text":""},{"location":"home/faq/#android-sdk","title":"Android SDK","text":"

        Question

        Why does the code not compile with Android SDK?

        Since NDK r18 (2018), GCC and the gnustl/stlport C++ libraries have been removed from the Android NDK; Clang and libc++ are now the only compiler and C++ library, and they support C++11 and later out of the box. With a current NDK, no special configuration is needed to use this library.

        Only very old NDKs (before r18), which defaulted to GCC and gnustl, lacked C++11 library features such as std::to_string. If you run into this, update to a current NDK.

        "},{"location":"home/faq/#incomplete-detector-type-with-gcc-11","title":"Incomplete detector type with GCC < 11","text":"

        Question

        Why does GCC 10 or older fail with invalid use of incomplete type 'struct nlohmann::detail::detector<..., to_json_function, ...>' for a type that holds an optional member?

        This happens with GCC 10 and older in C++11/C++14 mode when all of these hold:

        • a class Holder has an optional<Dummy> member (e.g., boost::optional),
        • Dummy has a constructor taking a json value, and
        • to_json for Holder is a free function in the namespace of Dummy.
        class Dummy {\n  public:\n    explicit Dummy(const nlohmann::json& j);\n};\n\nclass Holder {\n    boost::optional<Dummy> d;\n};\n\nvoid to_json(nlohmann::json& j, const Holder& h);  // triggers the error\n

        To decide whether Dummy is copyable, the compiler checks whether a Dummy can be converted to json. That check looks up to_json via argument-dependent lookup, finds the unrelated to_json for Holder, and eventually asks again whether Dummy is copyable. GCC before version 11 turns this cycle into a hard error; GCC 11 and later, Clang, and C++17 mode compile the code. The same error shows up without this library whenever a constrained converting constructor is involved, so the library can't avoid it.

        To work around this, define to_json (and from_json) as a hidden friend inside the class. That way, argument-dependent lookup only finds it for Holder:

        class Holder {\n    boost::optional<Dummy> d;\n\n    friend void to_json(nlohmann::json& j, const Holder& h) { /* ... */ }\n};\n

        The NLOHMANN_DEFINE_TYPE_INTRUSIVE macros define hidden friends as well. See #3669 for details.

        "},{"location":"home/faq/#missing-stl-function","title":"Missing STL function","text":"

        Questions

        • Why do I get a compilation error 'to_string' is not a member of 'std' (or similarly, for strtod or strtof)?
        • Why does the code not compile with MinGW or Android SDK?

        This is not an issue with the code, but rather with the compiler itself. On Android, use a current NDK (see above). For MinGW, please refer to this site and this discussion for information on how to fix this bug.

        "},{"location":"home/license/","title":"License","text":"

        The class is licensed under the MIT License:

        Copyright \u00a9 2013-2026 Niels Lohmann

        Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the \u201cSoftware\u201d), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

        The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

        THE SOFTWARE IS PROVIDED \u201cAS IS\u201d, WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

        The class contains the UTF-8 Decoder from Bjoern Hoehrmann which is licensed under the MIT License (see above). Copyright \u00a9 2008-2009 Bj\u00f6rn Hoehrmann bjoern@hoehrmann.de

        The class contains a slightly modified version of the Grisu2 algorithm from Florian Loitsch which is licensed under the MIT License (see above). Copyright \u00a9 2009 Florian Loitsch

        The class contains a port of the shortest double-to-decimal conversion of \u017bmij by Victor Zverovich, including the conversion of the digits to text by Xiang JunBo and the SIMD instruction sequence of Dougall Johnson, which is licensed under the MIT License (see above). Copyright \u00a9 2025 Victor Zverovich

        The class contains a copy of Hedley from Evan Nemerson which is licensed as CC0-1.0.

        The class contains an adapted version of the Eisel-Lemire algorithm, its table of powers of five, and its digit comparison for long numbers from fast_float by Daniel Lemire and contributors, which is available under the MIT License (used here), the Apache 2.0 License, and the Boost Software License. Copyright \u00a9 2021 The fast_float authors

        The view's parser (<nlohmann/json_view.hpp>) contains techniques and code adapted from yyjson by YaoYuan, which is licensed under the MIT License (see above): table-driven decoding of \\u escapes and fixed-offset unrolled checks.

        The view's parser (<nlohmann/json_view.hpp>) validates non-ASCII strings with the vector UTF-8 check of simdjson by Daniel Lemire, Geoff Langdale, John Keiser, and contributors (its \"lookup4\" algorithm and tables, after J. Keiser and D. Lemire, \"Validating UTF-8 In Less Than One Instruction Per Byte\", 2021), which is available under the MIT License (used here) and the Apache 2.0 License. Copyright \u00a9 2018-2025 The simdjson authors

        "},{"location":"home/releases/","title":"Releases","text":"

        This page summarizes the notable changes of every release and links to the relevant documentation. The complete release notes \u2014 including all changes, the download files, and their checksums \u2014 are published on the GitHub releases page.

        Unreleased changes

        This documentation is built from the develop branch and may describe changes that are not part of a release yet. Their version numbers are followed by an unreleased badge.

        "},{"location":"home/releases/#unreleased","title":"Unreleased","text":"
        • Converting an rvalue container to JSON (json j = std::move(vec);, json j = std::map<std::string, std::string>{...};) now moves the elements instead of copying them (e.g., std::vector<std::string> with 100,000 strings: 4.7 ms to 2.3 ms). Two consequences: the source container is left in a valid but unspecified (moved-from) state, and if an exception is thrown during the conversion, the source may be partially moved-from, so the strong exception guarantee no longer holds for the source of an rvalue conversion (the JSON value itself is unaffected). Lvalues and const objects are still copied. See basic_json.
        "},{"location":"home/releases/#v3120-2025-04-11","title":"v3.12.0 (2025-04-11)","text":"

        Fixes bugs found in 3.11.3 and adds several features. All changes are backward-compatible.

        • Adds diagnostic byte positions via JSON_DIAGNOSTIC_POSITIONS, exposed through the new start_pos and end_pos member functions.
        • Makes the conversion macros templated (so they also work with ordered_json) and adds NLOHMANN_DEFINE_DERIVED_TYPE for derived classes.
        • Adds std::optional support (C++17) and lets patch, diff, and flatten work with arbitrary string types.
        • Extends the binary formats: BJData draft 3 and unsigned 64-bit integers for BSON.
        • Adds multidimensional C-array conversion and UTF-8 encoded std::filesystem::path conversions, and raises the minimum CMake version to 3.5 and supports CMake 4.0.

        Full release notes.

        "},{"location":"home/releases/#v3113-2023-11-28","title":"v3.11.3 (2023-11-28)","text":"

        Adds features and fixes bugs found in 3.11.2. All changes are backward-compatible.

        • Adds a custom base class as a node customization point.
        • Adds serialization-only conversion macros (NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE_ONLY_SERIALIZE and NLOHMANN_DEFINE_TYPE_INTRUSIVE_ONLY_SERIALIZE) and a clearer parse error for empty input.
        • Adds Bazel and Swift Package Manager build support.
        • Fixes custom allocators, a memory leak in adl_serializer's to_json, initializer-list construction when size_type is not int, and many compiler warnings.

        Full release notes.

        "},{"location":"home/releases/#v3112-2022-08-12","title":"v3.11.2 (2022-08-12)","text":"

        Fixes bugs found in 3.11.1 and restructures the namespace. All changes are backward-compatible.

        • Fixes the value function (broken for strings, size types, and nullptr in 3.11.0) and makes json_fwd.hpp self-contained.
        • Restores using json_pointer as a key in associative containers and comparing it with strings.
        • Restructures the inline namespace and allows disabling the version component, and avoids heap allocations in the BJData parser.

        Full release notes.

        "},{"location":"home/releases/#v3111-2022-08-01","title":"v3.11.1 (2022-08-01)","text":"

        Fixes a regression from 3.11.0. All changes are backward-compatible.

        • Restores the global user-defined string literals operator\"\"_json and operator\"\"_json_pointer, which 3.11.0 had moved into a namespace by default.

        Full release notes.

        "},{"location":"home/releases/#v3110-2022-08-01","title":"v3.11.0 (2022-08-01)","text":"

        One of the largest releases ever. All changes are backward-compatible.

        • Allows std::string_view as object keys in at, operator[], value, erase, find, contains, and count.
        • Adds the BJData binary format (the fifth supported format).
        • Improves C++20 support, including operator<=> and <ranges>-compatible iterators.
        • Adds a versioned, ABI-tagged inline namespace (NLOHMANN_JSON_NAMESPACE) and the option to move the UDLs out of the global namespace (JSON_USE_GLOBAL_UDLS).
        • Adds patch_inplace, default values for the conversion macros, and an option to disable enum serialization (JSON_DISABLE_ENUM_SERIALIZATION).

        This release introduced a UDL regression that was fixed in 3.11.1. Full release notes.

        "},{"location":"home/releases/#v3105-2022-01-03","title":"v3.10.5 (2022-01-03)","text":"

        Bug-fix release. All changes are backward-compatible.

        • Guards the std::filesystem conversions behind compiler-support checks (JSON_HAS_FILESYSTEM), which can be set to 0 to disable them altogether.

        Full release notes.

        "},{"location":"home/releases/#v3104-2021-10-16","title":"v3.10.4 (2021-10-16)","text":"

        Fixes regressions introduced in 3.10.0. All changes are backward-compatible.

        • Fixes the std::filesystem::path conversion (which could trigger a stack overflow and broke compilation on Windows).
        • Fixes compilation for types with an explicit defaulted constructor and for code relying on the return values of std::find and std::remove.

        Full release notes.

        "},{"location":"home/releases/#v3103-2021-10-08","title":"v3.10.3 (2021-10-08)","text":"

        Fixes more regressions from 3.10.0. All changes are backward-compatible.

        • Fixes extended-diagnostics assertions triggered by update and by inserting into arrays.
        • Supports custom allocators when writing binary formats into a std::vector, and allows conversion from types that only provide begin()/end().

        Full release notes.

        "},{"location":"home/releases/#v3102-2021-08-26","title":"v3.10.2 (2021-08-26)","text":"

        Re-release of 3.10.1, whose Git tag pointed at the wrong commit due to a bug in the release script. All changes are backward-compatible. Full release notes.

        "},{"location":"home/releases/#v3101-2021-08-24","title":"v3.10.1 (2021-08-24)","text":"

        Fixes a regression from 3.10.0. All changes are backward-compatible.

        • Fixes an extended-diagnostics assertion triggered when used with ordered_json, and hardens the GDB pretty-printer.

        Full release notes.

        "},{"location":"home/releases/#v3100-2021-08-17","title":"v3.10.0 (2021-08-17)","text":"

        Feature release. All changes are backward-compatible.

        • Adds extended diagnostic messages (JSON_DIAGNOSTICS) that prepend a JSON pointer to exception messages to pinpoint the offending value.
        • Adds a GDB pretty-printer and a cbor_tag_handler_t store option to keep CBOR tags as binary subtypes.
        • Supports containers with non-default-constructible types and parsing from std::byte.
        • Adds JSON_NO_IO to exclude the I/O headers and the JSON_HAS_CPP_* macros to override the detected C++ standard.

        Full release notes.

        "},{"location":"home/releases/#v391-2020-08-06","title":"v3.9.1 (2020-08-06)","text":"

        Fixes two regressions from 3.9.0. All changes are backward-compatible.

        • Accepts consecutive comments and completes the ordered_json interface (e.g. ordered_json::parse).

        Full release notes.

        "},{"location":"home/releases/#v390-2020-07-27","title":"v3.9.0 (2020-07-27)","text":"

        Feature release adding four long-requested features. All changes are backward-compatible.

        • Optional comment parsing in parse via the ignore_comments parameter.
        • ordered_json to preserve the insertion order of object keys.
        • An option to switch off implicit conversions.
        • The NLOHMANN_DEFINE_TYPE_* convenience macros, plus high-precision-number support for UBJSON and CBOR tag handling.

        Full release notes.

        "},{"location":"home/releases/#v380-2020-06-14","title":"v3.8.0 (2020-06-14)","text":"

        Feature release. All changes are backward-compatible.

        • Introduces a binary value type that is read from and written to CBOR, BSON, and MessagePack, and can be shared between formats.
        • Generalizes the input adapters to read from any LegacyInputIterator container (3\u201310 % faster parsing).
        • Fixes contains for JSON pointers and makes the binary from_cbor/from_msgpack/etc. functions respect allow_exceptions.

        Full release notes.

        "},{"location":"home/releases/#v373-2019-11-17","title":"v3.7.3 (2019-11-17)","text":"

        Fixes a regression from 3.7.2 that could yield quadratic complexity in destructor calls. All changes are backward-compatible. Full release notes.

        "},{"location":"home/releases/#v372-2019-11-10","title":"v3.7.2 (2019-11-10)","text":"

        Fixes a stack overflow for deeply nested input by making the destructor iterative; parsing is now bounded only by available memory. All changes are backward-compatible. Full release notes.

        "},{"location":"home/releases/#v371-2019-11-06","title":"v3.7.1 (2019-11-06)","text":"

        Bug-fix release. All changes are backward-compatible.

        • Fixes a segmentation fault when serializing the std::int64_t minimum value and fixes contains for JSON pointers.
        • Allows items with a custom string type and makes json_pointer::back const.

        Full release notes.

        "},{"location":"home/releases/#v370-2019-07-28","title":"v3.7.0 (2019-07-28)","text":"

        Convenience features and house-keeping. All changes are backward-compatible.

        • Adds a contains overload that checks a JSON pointer without throwing, a generic to_string, and a return value for emplace_back.

        Full release notes.

        "},{"location":"home/releases/#v361-2019-03-20","title":"v3.6.1 (2019-03-20)","text":"

        Fixes a regression (GCC 7/8 compilation) and a <Windows.h> build error introduced in 3.6.0. All changes are backward-compatible. Full release notes.

        "},{"location":"home/releases/#v360-2019-03-20","title":"v3.6.0 (2019-03-20)","text":"

        Feature release. All changes are backward-compatible.

        • Reworks the JSON pointer interface (operator/, push_back, parent_pointer, \u2026).
        • Adds a contains function to test for an object key and greatly improves the performance of integer serialization.

        Full release notes.

        "},{"location":"home/releases/#v350-2018-12-22","title":"v3.5.0 (2018-12-22)","text":"

        Feature release. All changes are backward-compatible.

        • Adds structured-binding support via the items function and reading from FILE* in the parse function.
        • Fixes the eofbit handling on input streams and a bug in the BSON SAX parser.

        Full release notes.

        "},{"location":"home/releases/#v340-2018-10-30","title":"v3.4.0 (2018-10-30)","text":"

        Feature release. All changes are backward-compatible.

        • Adds BSON read/write support.
        • Adds configurable Unicode error handlers to dump (throw, replace with U+FFFD, or ignore) and the NLOHMANN_JSON_SERIALIZE_ENUM macro for enum conversion.
        • Improves parse-error messages with line/column positions and context.

        Full release notes.

        "},{"location":"home/releases/#v330-2018-10-05","title":"v3.3.0 (2018-10-05)","text":"

        Feature release. All changes are backward-compatible.

        • Adds GCC 4.8 support, the get_to function, and an overhauled and documented CMake integration.

        Full release notes.

        "},{"location":"home/releases/#v320-2018-08-20","title":"v3.2.0 (2018-08-20)","text":"

        Feature release. All changes are backward-compatible.

        • Adds a SAX interface and a non-recursive parser.
        • Adds parsing from wide-string types (std::wstring, std::u16string, std::u32string) and std::string_view (C++17), and round-tripping of std::map/std::unordered_map with non-string keys.

        Full release notes.

        "},{"location":"home/releases/#v312-2018-03-14","title":"v3.1.2 (2018-03-14)","text":"

        Bug-fix release. All changes are backward-compatible.

        • Fixes a memory leak in the parser callback and adds user-defined string-type support to the parser and serializer.

        Full release notes.

        "},{"location":"home/releases/#v311-2018-02-13","title":"v3.1.1 (2018-02-13)","text":"

        Bug-fix release. All changes are backward-compatible.

        • Fixes parsing of indefinite-length CBOR strings, a user-defined conversion to vector types, and overflow detection for UBJSON containers.

        Full release notes.

        "},{"location":"home/releases/#v310-2018-02-01","title":"v3.1.0 (2018-02-01)","text":"

        Feature release. All changes are backward-compatible.

        • Adds UBJSON read/write support and JSON Merge Patch via merge_patch.
        • Switches to the Grisu2 algorithm for short, round-trippable floating-point output, and splits the header into multiple files with a forward-declaration header.

        Full release notes.

        "},{"location":"home/releases/#v301-2017-12-29","title":"v3.0.1 (2017-12-29)","text":"

        Fixes small issues in the JSON Pointer and JSON Patch implementations (invalid \"copy\" targets and non-integer array indices). All changes are backward-compatible. Full release notes.

        "},{"location":"home/releases/#v300-2017-12-17","title":"v3.0.0 (2017-12-17)","text":"

        First 3.x release \u2014 a major release with breaking changes (see the migration guide).

        • Introduces user-defined exceptions (json::exception and subtypes, each with an identifier).
        • Adds a non-throwing accept function and an allow_exceptions flag for parse, and an update function to merge objects.
        • Adds streaming for CBOR and MessagePack and allows storing NaN/infinity.
        • Non-UTF-8 strings now throw on serialization, and the iterator category changed to bidirectional.

        Full release notes.

        "},{"location":"home/releases/#v211-2017-02-25","title":"v2.1.1 (2017-02-25)","text":"

        Bug-fix release. All changes are backward-compatible.

        • Makes number parsing and serialization locale-independent with correct floating-point round-tripping; released files are now GPG-signed.

        Full release notes.

        "},{"location":"home/releases/#v210-2017-01-28","title":"v2.1.0 (2017-01-28)","text":"

        Feature release. All changes are backward-compatible.

        • Adds conversions from and to arbitrary user-defined types via to_json/from_json, the meta function, and the option to switch off exceptions (JSON_NOEXCEPTION).

        Full release notes.

        "},{"location":"home/releases/#v2010-2017-01-02","title":"v2.0.10 (2017-01-02)","text":"

        Fixes several security-relevant bugs in the CBOR and MessagePack parsers found by OSS-Fuzz. All changes are backward-compatible. Full release notes.

        "},{"location":"home/releases/#v209-2016-12-16","title":"v2.0.9 (2016-12-16)","text":"

        Adds the CBOR and MessagePack binary formats. All changes are backward-compatible. Full release notes.

        "},{"location":"home/releases/#v208-2016-12-02","title":"v2.0.8 (2016-12-02)","text":"

        Adds the emplace and emplace_back functions and improves parsing and serialization performance. All changes are backward-compatible. Full release notes.

        "},{"location":"home/releases/#v207-2016-11-02","title":"v2.0.7 (2016-11-02)","text":"

        Fixes several parser bugs found through the \"Parsing JSON is a Minefield\" study (short files, encoding detection, surrogate pairs). All changes are backward-compatible. Full release notes.

        "},{"location":"home/releases/#v206-2016-10-15","title":"v2.0.6 (2016-10-15)","text":"

        Fixes operator[] for JSON pointers so that it creates missing values like the other overloads. All changes are backward-compatible. Full release notes.

        "},{"location":"home/releases/#v205-2016-09-14","title":"v2.0.5 (2016-09-14)","text":"

        Fixes a remaining stream end-of-file detection bug in the parser. All changes are backward-compatible. Full release notes.

        "},{"location":"home/releases/#v204-2016-09-11","title":"v2.0.4 (2016-09-11)","text":"

        Fixes stream end-of-file detection in the parser. All changes are backward-compatible. Full release notes.

        "},{"location":"home/releases/#v203-2016-08-31","title":"v2.0.3 (2016-08-31)","text":"

        Generalizes the parser to accept any contiguous sequence of one-byte elements and deprecates the input-stream constructor in favor of the parse function. All changes are backward-compatible. Full release notes.

        "},{"location":"home/releases/#v202-2016-07-31","title":"v2.0.2 (2016-07-31)","text":"

        Overhauls the parser (now rejecting unescaped control characters), tightens the class invariants, and cleans up the code. All changes are backward-compatible. Full release notes.

        "},{"location":"home/releases/#v201-2016-06-28","title":"v2.0.1 (2016-06-28)","text":"

        Fixes a performance regression in the dump function by adjusting the stream locale once per serialization. All changes are backward-compatible. Full release notes.

        "},{"location":"home/releases/#v200-2016-06-24","title":"v2.0.0 (2016-06-24)","text":"

        Feature release with a minor (potentially non-backward-compatible) API change from added noexcept and constexpr specifiers.

        • Adds JSON Pointer support in at and operator[], plus flatten and unflatten.
        • Adds JSON Patch via diff and patch, unsigned 64-bit integer support, and locale-independent serialization.

        Full release notes.

        "},{"location":"home/releases/#v110-2016-01-24","title":"v1.1.0 (2016-01-24)","text":"

        Bug-fix and feature release. All changes are backward-compatible.

        • Improves floating-point round-tripping, adds a get_ref accessor for stored values, and introduces runtime assertions.

        Full release notes.

        "},{"location":"home/releases/#v100-2015-12-28","title":"v1.0.0 (2015-12-28)","text":"

        First official release. Full release notes.

        "},{"location":"home/releases/#see-also","title":"See also","text":"
        • Migration Guide \u2014 how to future-proof your code for the next major version and replace deprecated functions.
        "},{"location":"home/sponsors/","title":"Sponsors","text":"

        You can sponsor this library at GitHub Sponsors.

        "},{"location":"home/sponsors/#priority-sponsor","title":"Priority Sponsor","text":"
        • Martti Laine
        • Paul Harrington
        "},{"location":"home/sponsors/#named-sponsors","title":"Named Sponsors","text":"
        • Michael Hartmann
        • Stefan Hagen
        • Steve Sperandeo
        • Robert Jefe Lindst\u00e4dt
        • Steve Wagner

        Thanks everyone!

        "},{"location":"integration/","title":"Integration","text":"

        There are several ways to add this header-only library to a C++ project. The following flowchart summarizes how to pick one:

        flowchart TD\n    A[Add the library to a C++ project] --> B{Already using CMake?}\n    B -- no --> C{Using pkg-config or plain Makefiles?}\n    C -- yes --> D[pkg-config]\n    C -- no --> E[Copy the single header]\n    B -- yes --> F{Library installed system-wide?}\n    F -- yes --> G[\"find_package()\"]\n    F -- no --> H{Use a package manager?}\n    H -- yes --> I[Package manager]\n    H -- no --> J[\"add_subdirectory() or FetchContent\"]
        • Copy the single header, as described below \u2014 no build-system integration required.
        • CMake: use find_package() if the library is already installed, add_subdirectory() to embed the source tree, or FetchContent to download it at configure time; see CMake.
        • Package managers: install the library with a package manager such as Homebrew, Conan, or vcpkg; see Package Managers.
        • pkg-config: if you use bare Makefiles instead of CMake, pkg-config can supply the include flags for an already-installed library.

        Once the library is integrated, see the Migration Guide for how to keep your code future-proof across releases.

        "},{"location":"integration/#header-only","title":"Header only","text":"

        json.hpp is the single required file in single_include/nlohmann or released here. You need to add

        #include <nlohmann/json.hpp>\n\n// for convenience\nusing json = nlohmann::json;\n

        to the files you want to process JSON and set the necessary switches to enable C++11 (e.g., -std=c++11 for GCC and Clang).

        You can further use file single_include/nlohmann/json_fwd.hpp for forward declarations (see Compile times), and file single_include/nlohmann/json_literals.hpp for the user-defined string literals if you define JSON_NO_AUTOMATIC_UDLS.

        For the read-only, non-owning basic_json_document/basic_json_view types, additionally include single_include/nlohmann/json_view.hpp; see Zero-copy JSON views.

        "},{"location":"integration/cmake/","title":"CMake","text":""},{"location":"integration/cmake/#integration","title":"Integration","text":"

        You can use the nlohmann_json::nlohmann_json interface target in CMake. This target populates the appropriate usage requirements for INTERFACE_INCLUDE_DIRECTORIES to point to the appropriate include directories and INTERFACE_COMPILE_FEATURES for the necessary C++11 flags. Most package managers that provide a CMake package configuration for this library expose this same target.

        "},{"location":"integration/cmake/#external","title":"External","text":"

        To use this library from a CMake project, you can locate it directly with find_package() and use the namespaced imported target from the generated package configuration:

        Example

        CMakeLists.txt
        cmake_minimum_required(VERSION 3.5)\nproject(ExampleProject LANGUAGES CXX)\n\nfind_package(nlohmann_json 3.12.0 REQUIRED)\n\nadd_executable(example example.cpp)\ntarget_link_libraries(example PRIVATE nlohmann_json::nlohmann_json)\n

        The package configuration file, nlohmann_jsonConfig.cmake, can be used either from an install tree or directly out of the build tree.

        "},{"location":"integration/cmake/#embedded","title":"Embedded","text":"

        To embed the library directly into an existing CMake project, place the entire source tree in a subdirectory and call add_subdirectory() in your CMakeLists.txt file.

        Example

        CMakeLists.txt
        cmake_minimum_required(VERSION 3.5)\nproject(ExampleProject LANGUAGES CXX)\n\n# If you only include this third party in PRIVATE source files, you do not need to install it\n# when your main project gets installed.\nset(JSON_Install OFF CACHE INTERNAL \"\")\n\nadd_subdirectory(nlohmann_json)\n\nadd_executable(example example.cpp)\ntarget_link_libraries(example PRIVATE nlohmann_json::nlohmann_json)\n

        Note

        Do not use include(nlohmann_json/CMakeLists.txt), since that carries with it unintended consequences that will break the build. It is generally discouraged (although not necessarily well documented as such) to use include(...) for pulling in other CMake projects anyways.

        "},{"location":"integration/cmake/#supporting-both","title":"Supporting Both","text":"

        To allow your project to support either an externally supplied or an embedded JSON library, you can use a pattern akin to the following.

        Example

        CMakeLists.txt
        project(ExampleProject LANGUAGES CXX)\n\noption(EXAMPLE_USE_EXTERNAL_JSON \"Use an external JSON library\" OFF)\n\nadd_subdirectory(thirdparty)\n\nadd_executable(example example.cpp)\n\n# Note that the namespaced target will always be available regardless of the import method\ntarget_link_libraries(example PRIVATE nlohmann_json::nlohmann_json)\n
        thirdparty/CMakeLists.txt
        if(EXAMPLE_USE_EXTERNAL_JSON)\n    find_package(nlohmann_json 3.12.0 REQUIRED)\nelse()\n    set(JSON_BuildTests OFF CACHE INTERNAL \"\")\n    add_subdirectory(nlohmann_json)\nendif()\n

        thirdparty/nlohmann_json is then a complete copy of this source tree.

        "},{"location":"integration/cmake/#fetchcontent","title":"FetchContent","text":"

        Since CMake v3.14, FetchContent can be used to automatically download a release as a dependency at configure time.

        Example

        CMakeLists.txt
        cmake_minimum_required(VERSION 3.14)\nproject(ExampleProject LANGUAGES CXX)\n\ninclude(FetchContent)\n\nFetchContent_Declare(json URL https://github.com/nlohmann/json/releases/download/v3.12.0/json.tar.xz)\nFetchContent_MakeAvailable(json)\n\nadd_executable(example example.cpp)\ntarget_link_libraries(example PRIVATE nlohmann_json::nlohmann_json)\n

        Note

        It is recommended to use the URL approach described above which is supported as of version 3.10.0. It is also possible to pass the Git repository like

        FetchContent_Declare(json\n    GIT_REPOSITORY https://github.com/nlohmann/json\n    GIT_TAG v3.12.0\n)\n

        However, the repository https://github.com/nlohmann/json download size is quite large.

        "},{"location":"integration/cmake/#cmake-options","title":"CMake Options","text":""},{"location":"integration/cmake/#json_buildtests","title":"JSON_BuildTests","text":"

        Build the unit tests when BUILD_TESTING is enabled. This option is ON by default if the library's CMake project is the top project and the tests directory exists (the release archive json.tar.xz does not contain it). That is, when integrating the library as described above, the test suite is not built unless explicitly switched on with this option.

        "},{"location":"integration/cmake/#json_ci","title":"JSON_CI","text":"

        Enable CI build targets. The exact targets are used during the several CI steps and are subject to change without notice. This option is OFF by default.

        "},{"location":"integration/cmake/#json_deletedeprecatedfunctions","title":"JSON_DeleteDeprecatedFunctions","text":"

        Delete the deprecated functions instead of only deprecating them by defining the macro JSON_DELETE_DEPRECATED_FUNCTIONS. This option is OFF by default.

        "},{"location":"integration/cmake/#json_diagnostics","title":"JSON_Diagnostics","text":"

        Enable extended diagnostic messages by defining macro JSON_DIAGNOSTICS. This option is OFF by default.

        Does not apply to a pre-installed package

        This option only takes effect when building nlohmann/json from source as part of your own CMake project (e.g. via FetchContent or add_subdirectory). It has no effect on a package that was already built and installed elsewhere (Homebrew, vcpkg, a system package, etc.) \u2014 the resulting compile definition is baked into the exported nlohmann_jsonTargets.cmake at install time, and set(JSON_Diagnostics ON) before find_package() does not change it (verified against the Homebrew-installed package: the exported target still carries a fixed $<$<BOOL:OFF>:JSON_DIAGNOSTICS=1>, regardless of any variable set in the consuming project).

        To enable extended diagnostics for a pre-installed package, override the imported target's property directly after find_package():

        find_package(nlohmann_json REQUIRED)\nset_target_properties(nlohmann_json::nlohmann_json PROPERTIES\n    INTERFACE_COMPILE_DEFINITIONS \"JSON_DIAGNOSTICS=1\")\n

        This only works cleanly when your project is the sole consumer of that imported target. If nlohmann_json is pulled in from more than one place in your dependency graph with different JSON_DIAGNOSTICS values, you may see a \"JSON_DIAGNOSTICS\" redefined compiler error, since conflicting -D flags can end up on the same compile command line.

        "},{"location":"integration/cmake/#json_diagnostic_positions","title":"JSON_Diagnostic_Positions","text":"

        Enable position diagnostics by defining macro JSON_DIAGNOSTIC_POSITIONS. This option is OFF by default.

        "},{"location":"integration/cmake/#json_disableenumserialization","title":"JSON_DisableEnumSerialization","text":"

        Disable default enum serialization by defining the macro JSON_DISABLE_ENUM_SERIALIZATION. This option is OFF by default.

        "},{"location":"integration/cmake/#json_disabletuplereferenceconversion","title":"JSON_DisableTupleReferenceConversion","text":"

        Disable the conversion from a one-element std::tuple holding a reference to a JSON value by defining the macro JSON_DISABLE_TUPLE_REFERENCE_CONVERSION. This option is OFF by default.

        "},{"location":"integration/cmake/#json_fasttests","title":"JSON_FastTests","text":"

        Skip expensive/slow test suites. This option is OFF by default. Depends on JSON_BuildTests.

        "},{"location":"integration/cmake/#json_globaludls","title":"JSON_GlobalUDLs","text":"

        Place user-defined string literals in the global namespace by defining the macro JSON_USE_GLOBAL_UDLS. This option is ON by default; see the migration guide for how to prepare code for the next major release, where the literals are removed from the global namespace.

        "},{"location":"integration/cmake/#json_implicitconversions","title":"JSON_ImplicitConversions","text":"

        Enable implicit conversions by defining macro JSON_USE_IMPLICIT_CONVERSIONS. This option is ON by default; see the migration guide for how to prepare code for the next major release, where implicit conversions are switched off by default.

        "},{"location":"integration/cmake/#json_install","title":"JSON_Install","text":"

        Install CMake targets during install step. This option is ON by default if the library's CMake project is the top project. Installing also generates a pkg-config file for tools that rely on pkg-config instead of CMake.

        "},{"location":"integration/cmake/#json_legacydiscardedvaluecomparison","title":"JSON_LegacyDiscardedValueComparison","text":"

        Enable the (incorrect) legacy comparison behavior of discarded JSON values by defining macro JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON. This option is OFF by default.

        "},{"location":"integration/cmake/#json_multipleheaders","title":"JSON_MultipleHeaders","text":"

        Use the non-amalgamated version of the library. This option is ON by default.

        "},{"location":"integration/cmake/#json_systeminclude","title":"JSON_SystemInclude","text":"

        Treat the library headers like system headers (i.e., adding SYSTEM to the target_include_directories call) to check for this library by tools like Clang-Tidy. This option is OFF by default.

        "},{"location":"integration/cmake/#json_strictbinaryutf8","title":"JSON_StrictBinaryUTF8","text":"

        Check string values and object keys for valid UTF-8 in the CBOR, UBJSON, BJData, and BSON writers, by defining the macro JSON_STRICT_BINARY_UTF8. This option is OFF by default.

        "},{"location":"integration/cmake/#json_strictnulhandling","title":"JSON_StrictNulHandling","text":"

        Reject a '\\0' (NUL) byte in the input instead of treating it as end of input, by defining the macro JSON_STRICT_NUL_HANDLING. This option is OFF by default.

        "},{"location":"integration/cmake/#json_testsimdutf","title":"JSON_TestSimdutf","text":"

        Build the unit tests against the simdutf UTF-8 validation backend by defining JSON_USE_SIMDUTF for every test target. simdutf is fetched during configuration; its version is set by the cache variable JSON_SIMDUTF_VERSION. This option is OFF by default. Depends on JSON_BuildTests.

        "},{"location":"integration/cmake/#json_valgrind","title":"JSON_Valgrind","text":"

        Execute the test suite with Valgrind. This option is OFF by default. Depends on JSON_BuildTests.

        "},{"location":"integration/cmake/#nlohmann_json_build_modules","title":"NLOHMANN_JSON_BUILD_MODULES","text":"

        Build the experimental C++ module nlohmann.json (requires CMake 3.28 or later and C++20). This option is OFF by default.

        A consuming project must link the dedicated nlohmann_json_modules CMake target (not just nlohmann_json::nlohmann_json) for import nlohmann.json; to resolve:

        set(NLOHMANN_JSON_BUILD_MODULES ON)\nadd_subdirectory(path/to/json)\n\nadd_executable(myproject main.cpp)\ntarget_link_libraries(myproject PRIVATE nlohmann_json_modules)\ntarget_compile_definitions(myproject PRIVATE NLOHMANN_JSON_BUILD_MODULES)\n
        "},{"location":"integration/compile_times/","title":"Compile times","text":"

        The library is header-only and makes heavy use of templates, so every translation unit that includes <nlohmann/json.hpp> pays for parsing the header and instantiating what it uses. This page lists the options to reduce that cost, ordered by how much they typically save.

        Measurements

        The numbers below are medians of nine runs compiling a single translation unit with -std=c++17 -c against the single-header version, with Apple clang and GCC 16 on macOS (Apple silicon). They show the order of magnitude to expect; measure your own code before and after a change.

        "},{"location":"integration/compile_times/#include-json_fwdhpp-in-headers","title":"Include json_fwd.hpp in headers","text":"

        Header files that only need to name the json type \u2014 for function declarations, members held by pointer or reference, or friend declarations \u2014 can include <nlohmann/json_fwd.hpp> instead of <nlohmann/json.hpp>. It only forward-declares basic_json, json_pointer, adl_serializer, and ordered_map, and defines the aliases json and ordered_json. The translation units that actually use the values then include <nlohmann/json.hpp>.

        person.hpp
        #pragma once\n#include <nlohmann/json_fwd.hpp>\n\nstruct person;\nvoid to_json(nlohmann::json& j, const person& p);\nvoid from_json(const nlohmann::json& j, person& p);\n
        person.cpp
        #include \"person.hpp\"\n#include <nlohmann/json.hpp>\n\nvoid to_json(nlohmann::json& j, const person& p) { /* ... */ }\nvoid from_json(const nlohmann::json& j, person& p) { /* ... */ }\n
        Compiler json.hpp (-O0) json_fwd.hpp (-O0) Change Apple clang 704 ms 329 ms \u221253% GCC 16 779 ms 242 ms \u221269%

        This is the most effective option, because it avoids the full header in every translation unit that includes your headers.

        "},{"location":"integration/compile_times/#opt-out-of-the-automatic-user-defined-string-literals","title":"Opt out of the automatic user-defined string literals","text":"

        The user-defined string literals operator\"\"_json and operator\"\"_json_pointer are ordinary inline functions whose bodies call the parser. As <nlohmann/json.hpp> includes them by default, every translation unit instantiates the parser, even if it never parses anything itself.

        Define JSON_NO_AUTOMATIC_UDLS for the whole project and include <nlohmann/json_literals.hpp> instead of <nlohmann/json.hpp> in the files that use the literals (it includes <nlohmann/json.hpp> itself):

        target_compile_definitions(my_target PRIVATE JSON_NO_AUTOMATIC_UDLS)\n
        #include <nlohmann/json_literals.hpp> // only where \"...\"_json is used; includes <nlohmann/json.hpp>\n

        The saving applies to translation units that do not parse JSON, for example ones that define types and their conversions or only pass json values around:

        Compiler Translation unit Default (-O0 / -O2) JSON_NO_AUTOMATIC_UDLS (-O0 / -O2) Change Apple clang model 776 ms / 846 ms 629 ms / 692 ms \u221219% / \u221218% GCC 16 model 1022 ms / 1120 ms 882 ms / 965 ms \u221214% / \u221214% Apple clang parsing 992 ms / 1815 ms 1006 ms / 1823 ms +1% / 0% GCC 16 parsing 2018 ms / 3420 ms 1990 ms / 3454 ms \u22121% / +1%

        Translation units that include only the header save up to a third. Translation units that parse anyway instantiate the parser regardless and see no difference.

        "},{"location":"integration/compile_times/#instantiate-basic_json-once","title":"Instantiate basic_json once","text":"

        Each translation unit instantiates the member functions of nlohmann::json it uses. An explicit instantiation declaration tells the compiler that the non-template members are instantiated elsewhere, so it can skip them:

        json_instance.hpp
        #pragma once\n#include <nlohmann/json.hpp>\n\nextern template class nlohmann::basic_json<>;\n
        json_instance.cpp
        #include \"json_instance.hpp\"\n\ntemplate class nlohmann::basic_json<>;\n

        Include json_instance.hpp instead of <nlohmann/json.hpp> and compile and link json_instance.cpp once.

        Compiler Translation unit Default (-O0 / -O2) extern template (-O0 / -O2) Change Apple clang parsing 992 ms / 1815 ms 953 ms / 1625 ms \u22124% / \u221210% GCC 16 parsing 2018 ms / 3420 ms 1522 ms / 2728 ms \u221225% / \u221220% Apple clang json_instance.cpp \u2014 2166 ms / 4660 ms \u2014 GCC 16 json_instance.cpp \u2014 5085 ms / 10616 ms \u2014

        Notes:

        • The saving grows with the number of translation units that use json, while the instantiation translation unit is compiled only once (and is rarely recompiled, as it does not depend on your code).
        • Member function templates (such as get<T>(), parse(InputType&&), or value(key, default)) are not covered by the explicit instantiation and are still instantiated where they are used.
        • The declaration covers exactly nlohmann::json. Add the same lines for nlohmann::ordered_json (nlohmann::basic_json<nlohmann::ordered_map>) or your own basic_json specializations if you use them.
        "},{"location":"integration/compile_times/#use-c20-modules","title":"Use C++20 modules","text":"

        With a toolchain that supports named modules, import nlohmann.json; compiles the library once into a module and avoids parsing the header in every translation unit. See Modules for requirements and known issues. Module support is experimental and currently depends heavily on the compiler version.

        "},{"location":"integration/compile_times/#use-precompiled-headers","title":"Use precompiled headers","text":"

        Build systems can precompile <nlohmann/json.hpp> together with other stable headers, for example with CMake's target_precompile_headers:

        target_precompile_headers(my_target PRIVATE <nlohmann/json.hpp>)\n

        This removes the cost of parsing the header, but not of instantiating templates in each translation unit, so it combines well with the options above.

        "},{"location":"integration/compile_times/#options-without-effect-on-compile-times","title":"Options without effect on compile times","text":"

        Some configuration macros change what the library declares, but do not measurably change compile times:

        Macro Apple clang, model (-O0 / -O2) GCC 16, model (-O0 / -O2) default 776 ms / 846 ms 1022 ms / 1120 ms JSON_NO_IO 764 ms / 836 ms 1022 ms / 1117 ms JSON_USE_GLOBAL_UDLS=0 763 ms / 852 ms 1019 ms / 1106 ms

        JSON_USE_GLOBAL_UDLS only controls where the literals are declared; to avoid their cost, use JSON_NO_AUTOMATIC_UDLS instead.

        "},{"location":"integration/compile_times/#see-also","title":"See also","text":"
        • JSON_NO_AUTOMATIC_UDLS - do not include the user-defined string literals automatically
        • Modules - C++20 module support
        • Header only - including the library
        "},{"location":"integration/migration_guide/","title":"Migration Guide","text":"

        This page collects some guidelines on how to future-proof your code for future versions of this library. For how to add the library to your project in the first place, see Integration, CMake, or Package Managers. The roadmap lists what will change in version 4.0, including the macros that let you try its behavior with a 3.x release; this page describes how to adjust your code.

        "},{"location":"integration/migration_guide/#replace-deprecated-functions","title":"Replace deprecated functions","text":"

        The following functions have been deprecated and will be removed in the next major version (i.e., 4.0.0), see the roadmap for an overview. All deprecations are annotated with HEDLEY_DEPRECATED_FOR to report which function to use instead.

        Find all calls of deprecated functions

        Define JSON_DELETE_DEPRECATED_FUNCTIONS to 1 (or set the CMake option JSON_DeleteDeprecatedFunctions) to delete all deprecated functions. Every remaining call then fails to compile, even if deprecation warnings are disabled, so your code is ready for version 4.0.0 unreleased once it compiles with the macro.

        "},{"location":"integration/migration_guide/#parsing","title":"Parsing","text":"

        • Function friend std::istream& operator<<(basic_json&, std::istream&) is deprecated since 3.0.0. Please use friend std::istream& operator>>(std::istream&, basic_json&) instead.

          DeprecatedFuture-proof 
          nlohmann::json j;\nstd::stringstream ss(\"[1,2,3]\");\nj << ss;\n
          nlohmann::json j;\nstd::stringstream ss(\"[1,2,3]\");\nss >> j;\n

        • Passing iterator pairs or pointer/length pairs to parsing functions (parse, accept, sax_parse, from_cbor, from_msgpack, from_ubjson, and from_bson) via initializer lists is deprecated since 3.8.0. Instead, pass two iterators; for instance, call from_cbor(ptr, ptr+len) instead of from_cbor({ptr, len}). Likewise, passing a pointer and a length as two separate arguments to from_cbor, from_msgpack, from_ubjson, and from_bson is deprecated since 3.8.0, and to from_bjdata and from_bon8 since 3.13.0; call from_cbor(ptr, ptr+len) instead of from_cbor(ptr, len). These overloads will not be removed in version 4.0.0, but deleted, so a call like from_cbor(ptr, len) cannot compile and convert len to the strict parameter.

          DeprecatedFuture-proof 
          const char* s = \"[1,2,3]\";\nbool ok = nlohmann::json::accept({s, s + std::strlen(s)});\n
          const char* s = \"[1,2,3]\";\nbool ok = nlohmann::json::accept(s, s + std::strlen(s));\n
        "},{"location":"integration/migration_guide/#json-pointers","title":"JSON Pointers","text":"

        • Comparing JSON Pointers with strings via operator== and operator!= is deprecated since 3.11.2. To compare a json_pointer p with a string s, convert s to a json_pointer first and use json_pointer::operator== or json_pointer::operator!=.

          DeprecatedFuture-proof 
          nlohmann::json::json_pointer lhs(\"/foo/bar/1\");\nassert(lhs == \"/foo/bar/1\");\n
          nlohmann::json::json_pointer lhs(\"/foo/bar/1\");\nassert(lhs == nlohmann::json::json_pointer(\"/foo/bar/1\"));\n

        • The implicit conversion from JSON Pointers to string (json_pointer::operator string_t) is deprecated since 3.11.0. Use json_pointer::to_string instead.

          DeprecatedFuture-proof 
          nlohmann::json::json_pointer ptr(\"/foo/bar/1\");\nstd::string s = ptr;\n
          nlohmann::json::json_pointer ptr(\"/foo/bar/1\");\nstd::string s = ptr.to_string();\n

        • Passing a basic_json specialization as template parameter RefStringType to json_pointer is deprecated since 3.11.0. The string type can now be directly provided. This also applies to passing such a JSON pointer to at, contains, operator[], and value.

          DeprecatedFuture-proof 
          using my_json = nlohmann::json::with_string_t<my_string_type>;\nnlohmann::json_pointer<my_json> ptr(\"/foo/bar/1\");\n
          nlohmann::json_pointer<my_string_type> ptr(\"/foo/bar/1\");\n

          Thereby, my_json::json_pointer is an alias for nlohmann::json_pointer<my_string_type>; in general, basic_json::json_pointer is always an alias to the json_pointer with the appropriate string type for all specializations of basic_json.

        "},{"location":"integration/migration_guide/#miscellaneous-functions","title":"Miscellaneous functions","text":"

        • The function iterator_wrapper is deprecated since 3.1.0. Please use the member function items instead.

          DeprecatedFuture-proof 
          for (auto &x : nlohmann::json::iterator_wrapper(j))\n{\n    std::cout << x.key() << \":\" << x.value() << std::endl;\n}\n
          for (auto &x : j.items())\n{\n    std::cout << x.key() << \":\" << x.value() << std::endl;\n}\n

        • Function friend std::ostream& operator>>(const basic_json&, std::ostream&) is deprecated since 3.0.0. Please use friend operator<<(std::ostream&, const basic_json&) instead.

          DeprecatedFuture-proof 
          j >> std::cout;\n
          std::cout << j;\n

        • The legacy comparison behavior for discarded values is deprecated since 3.11.0. It is already disabled by default and can still be enabled by defining JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON to 1.

          DeprecatedFuture-proof 
          #define JSON_USE_LEGACY_DISCARDED_VALUE_COMPARISON 1\n#include <nlohmann/json.hpp>\n
          #include <nlohmann/json.hpp>\n
        "},{"location":"integration/migration_guide/#replace-implicit-conversions","title":"Replace implicit conversions","text":"

        Implicit conversions via operator ValueType will be switched off by default in the next major release of the library.

        You can prepare existing code by already defining JSON_USE_IMPLICIT_CONVERSIONS to 0 and replace any implicit conversions with calls to get, get_to, get_ref, or get_ptr.

        Automatic migration

        The community-maintained clang-tidy check modernize-nlohmann-json-explicit-conversions rewrites most implicit conversions into calls to get. It is not part of clang-tidy itself; see discussion #4610 for how to build and use it.

        DeprecatedFuture-proofFuture-proof (alternative) 
        nlohmann::json j = \"Hello, world!\";\nstd::string s = j;\n
        nlohmann::json j = \"Hello, world!\";\nauto s = j.get<std::string>();\n
        nlohmann::json j = \"Hello, world!\";\nstd::string s;\nj.get_to(s);\n
        "},{"location":"integration/migration_guide/#import-namespace-literals-for-udls","title":"Import namespace literals for UDLs","text":"

        The user-defined string literals operator\"\"_json and operator\"\"_json_pointer will be removed from the global namespace in the next major release of the library.

        DeprecatedFuture-proof 
        nlohmann::json j = \"[1,2,3]\"_json;\n
        using namespace nlohmann::literals;\nnlohmann::json j = \"[1,2,3]\"_json;\n

        To prepare existing code, define JSON_USE_GLOBAL_UDLS to 0 and bring the string literals into scope where needed.

        "},{"location":"integration/migration_guide/#do-not-hard-code-the-complete-library-namespace","title":"Do not hard-code the complete library namespace","text":"

        The nlohmann namespace contains a sub-namespace to avoid problems when different versions or configurations of the library are used in the same project. Always use nlohmann as namespace or, when the exact version and configuration is relevant, use macro NLOHMANN_JSON_NAMESPACE to denote the namespace.

        DangerousFuture-proofFuture-proof (alternative) 
        void to_json(nlohmann::json_abi_v3_11_2::json& j, const person& p)\n{\n    j[\"age\"] = p.age;\n}\n
        void to_json(nlohmann::json& j, const person& p)\n{\n    j[\"age\"] = p.age;\n}\n
        void to_json(NLOHMANN_JSON_NAMESPACE::json& j, const person& p)\n{\n    j[\"age\"] = p.age;\n}\n
        "},{"location":"integration/migration_guide/#do-not-use-the-detail-namespace","title":"Do not use the detail namespace","text":"

        The nlohmann::detail namespace is not part of the public API of the library and can change in any version without an announcement. Do not rely on any function or type in the detail namespace.

        "},{"location":"integration/package_managers/","title":"Package Managers","text":"

        Homebrew nlohmann-json Meson nlohmann_json Bazel nlohmann_json Conan nlohmann_json Spack nlohmann-json Hunter nlohmann_json vcpkg nlohmann-json cget nlohmann/json Swift Package Manager nlohmann/json NuGet nlohmann.json Conda nlohmann_json MacPorts nlohmann-json CPM.cmake gh:nlohmann/json xmake nlohmann_json

        "},{"location":"integration/package_managers/#running-example","title":"Running example","text":"

        Throughout this page, we will describe how to compile the example file example.cpp below.

        #include <nlohmann/json.hpp>\n#include <iostream>\n#include <iomanip>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::setw(4) << json::meta() << std::endl;\n}\n

        When executed, this program should create output similar to

        {\n    \"compiler\": {\n        \"c++\": \"201103\",\n        \"family\": \"gcc\",\n        \"version\": \"12.4.0\"\n    },\n    \"copyright\": \"(C) 2013-2026 Niels Lohmann\",\n    \"name\": \"JSON for Modern C++\",\n    \"platform\": \"apple\",\n    \"url\": \"https://github.com/nlohmann/json\",\n    \"version\": {\n        \"major\": 3,\n        \"minor\": 12,\n        \"patch\": 0,\n        \"string\": \"3.12.0\"\n    }\n}\n

        Many of the package managers below install a CMake package configuration that exposes the same nlohmann_json::nlohmann_json interface target described in CMake; their CMake examples below link against that target.

        "},{"location":"integration/package_managers/#homebrew","title":"Homebrew","text":"

        Summary

        formula: nlohmann-json

        • Available versions: current version and development version (with --HEAD parameter)
        • The formula is updated with every release.
        • Maintainer: Niels Lohmann
        • File issues at the Homebrew issue tracker
        • Homebrew website

        If you are using Homebrew, you can install the library with

        brew install nlohmann-json\n

        The header can be used directly in your code or via CMake.

        Example: Raw compilation

        1. Create the following file:

          example.cpp
          #include <nlohmann/json.hpp>\n#include <iostream>\n#include <iomanip>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::setw(4) << json::meta() << std::endl;\n}\n

        2. Install the package:

          brew install nlohmann-json\n

        3. Compile the code and pass the Homebrew prefix to the include path such that the library can be found:

          c++ example.cpp -I$(brew --prefix nlohmann-json)/include -std=c++11 -o example\n
        Example: CMake

        1. Create the following files:

          example.cpp
          #include <nlohmann/json.hpp>\n#include <iostream>\n#include <iomanip>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::setw(4) << json::meta() << std::endl;\n}\n
          CMakeLists.txt
          cmake_minimum_required(VERSION 3.15)\nproject(json_example)\n\nfind_package(nlohmann_json CONFIG REQUIRED)\n\nadd_executable(json_example example.cpp)\ntarget_link_libraries(json_example PRIVATE nlohmann_json::nlohmann_json)\n

        2. Install the package:

          brew install nlohmann-json\n

        3. Compile the code and pass the Homebrew prefix to CMake to find installed packages via find_package:

          CMAKE_PREFIX_PATH=$(brew --prefix) cmake -S . -B build\ncmake --build build\n
        "},{"location":"integration/package_managers/#meson","title":"Meson","text":"

        Summary

        wrap: nlohmann_json

        • Available versions: current version and select older versions (see WrapDB)
        • The package is updated automatically from file meson.build.
        • File issues at the library issue tracker
        • Meson website

        If you are using the Meson Build System, add this source tree as a meson subproject. You may also use the include.zip published in this project's Releases to reduce the size of the vendored source tree. Alternatively, you can get a wrap file by downloading it from Meson WrapDB, or use

        meson wrap install nlohmann_json\n

        Please see the Meson project for any issues regarding the packaging.

        The provided meson.build can also be used as an alternative to CMake for installing nlohmann_json system-wide in which case a pkg-config file and the CMake package config files are installed. To use it, have your build system require the nlohmann_json pkg-config dependency, or use find_package(nlohmann_json) in CMake. In Meson, it is preferred to use the dependency() object with a subproject fallback, rather than using the subproject directly.

        The options that change the library's configuration are available in Meson as well, named like the CMake options without the JSON_ prefix: MultipleHeaders, GlobalUDLs, ImplicitConversions, DisableEnumSerialization, DisableTupleReferenceConversion, Diagnostics, Diagnostic_Positions, LegacyDiscardedValueComparison, StrictNulHandling, StrictBinaryUTF8, and DeleteDeprecatedFunctions. They have the same defaults as in CMake, except that MultipleHeaders is false. Set them with -D when setting up the build, or with the subproject name as prefix when the library is used as a subproject:

        meson setup build -Dnlohmann_json:Diagnostics=true\n

        The resulting compile definitions are part of the Meson dependency, the pkg-config file, and the CMake target.

        Example: Wrap

        1. Create the following files:

          meson.build
          project('json_example', 'cpp',\n  version: '1.0',\n  default_options: ['cpp_std=c++11']\n)\n\ndependency_json = dependency('nlohmann_json', required: true)\n\nexecutable('json_example',\n  sources: ['example.cpp'],\n  dependencies: [dependency_json],\n  install: true\n)\n
          example.cpp
          #include <nlohmann/json.hpp>\n#include <iostream>\n#include <iomanip>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::setw(4) << json::meta() << std::endl;\n}\n

        2. Use the Meson WrapDB to fetch the nlohmann/json wrap:

          mkdir subprojects\nmeson wrap install nlohmann_json\n

        3. Build:

          meson setup build\nmeson compile -C build\n
        "},{"location":"integration/package_managers/#bazel","title":"Bazel","text":"

        Summary

        use bazel_dep, git_override, or local_path_override

        • Any version, that is available via Bazel Central Registry
        • File issues at the library issue tracker
        • Bazel website

        This repository provides a Bazel MODULE.bazel and a corresponding BUILD.bazel file. Therefore, this repository can be referenced within a MODULE.bazel by rules such as archive_override, git_override, or local_path_override. To use the library, you need to depend on the target @nlohmann_json//:json (i.e., via deps attribute).

        Example: Bazel module with bazel_dep

        1. Create the following files:

          BUILD
          cc_binary(\n    name = \"main\",\n    srcs = [\"example.cpp\"],\n    deps = [\"@nlohmann_json//:json\"],\n)\n
          MODULE.bazel
          bazel_dep(name = \"nlohmann_json\", version = \"3.12.0.bcr.2\")\n
          example.cpp
          #include <nlohmann/json.hpp>\n#include <iostream>\n#include <iomanip>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::setw(4) << json::meta() << std::endl;\n}\n

        2. Build and run:

          bazel build //:main\nbazel run //:main\n
        "},{"location":"integration/package_managers/#conan","title":"Conan","text":"

        Summary

        recipe: nlohmann_json

        • Available versions: current version and older versions (see Conan Center)
        • The package is updated automatically via this recipe.
        • File issues at the Conan Center issue tracker
        • Conan website

        If you are using Conan to manage your dependencies, merely add nlohmann_json/x.y.z to your conanfile's requires, where x.y.z is the release version you want to use.

        Example: CMake with the Conan toolchain

        1. Create the following files:

          Conanfile.txt
          [requires]\nnlohmann_json/3.12.0\n\n[generators]\nCMakeToolchain\nCMakeDeps\n
          CMakeLists.txt
          cmake_minimum_required(VERSION 3.15)\nproject(json_example)\n\nfind_package(nlohmann_json REQUIRED)\n\nadd_executable(json_example example.cpp)\ntarget_link_libraries(json_example PRIVATE nlohmann_json::nlohmann_json)\n
          example.cpp
          #include <nlohmann/json.hpp>\n#include <iostream>\n#include <iomanip>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::setw(4) << json::meta() << std::endl;\n}\n

        2. Call Conan:

          conan install . --output-folder=build --build=missing\n

        3. Build:

          cmake -S . -B build -DCMAKE_TOOLCHAIN_FILE=\"conan_toolchain.cmake\" -DCMAKE_BUILD_TYPE=Release\ncmake --build build\n
        "},{"location":"integration/package_managers/#spack","title":"Spack","text":"

        Summary

        package: nlohmann-json

        • Available versions: current version and older versions (see Spack package)
        • The package is updated with every release.
        • Maintainer: Axel Huebl
        • File issues at the Spack issue tracker
        • Spack website

        If you are using Spack to manage your dependencies, you can use the nlohmann-json package via

        spack install nlohmann-json\n

        Please see the Spack project for any issues regarding the packaging.

        Example: CMake with a Spack-installed package

        1. Create the following files:

          CMakeLists.txt
          cmake_minimum_required(VERSION 3.15)\nproject(json_example)\n\nfind_package(nlohmann_json REQUIRED)\n\nadd_executable(json_example example.cpp)\ntarget_link_libraries(json_example PRIVATE nlohmann_json::nlohmann_json)\n
          example.cpp
          #include <nlohmann/json.hpp>\n#include <iostream>\n#include <iomanip>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::setw(4) << json::meta() << std::endl;\n}\n

        2. Install the library:

          spack install nlohmann-json\n

        3. Load the environment for your Spack-installed packages:

          spack load nlohmann-json\n

        4. Build the project with CMake:

          cmake -S . -B build -DCMAKE_PREFIX_PATH=$(spack location -i nlohmann-json)\ncmake --build build\n
        "},{"location":"integration/package_managers/#hunter","title":"Hunter","text":"

        Summary

        package: nlohmann_json

        • Available versions: current version and older versions (see Hunter package)
        • The package is updated with every release.
        • File issues at the Hunter issue tracker
        • Hunter website

        If you are using Hunter on your project for external dependencies, then you can use the nlohmann_json package via

        hunter_add_package(nlohmann_json)\n

        Please see the Hunter project for any issues regarding the packaging.

        Example: CMake with HunterGate

        1. Create the following files:

          CMakeLists.txt
          cmake_minimum_required(VERSION 3.15)\n\ninclude(\"cmake/HunterGate.cmake\")\nHunterGate(\n    URL \"https://github.com/cpp-pm/hunter/archive/v0.26.12.tar.gz\"\n    SHA1 \"6498c5d0dec25d7fffb2b0574ebaf265179b894d\"\n)\n\nproject(json_example)\n\nhunter_add_package(nlohmann_json)\nfind_package(nlohmann_json CONFIG REQUIRED)\n\nadd_executable(json_example example.cpp)\ntarget_link_libraries(json_example PRIVATE nlohmann_json::nlohmann_json)\n
          example.cpp
          #include <nlohmann/json.hpp>\n#include <iostream>\n#include <iomanip>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::setw(4) << json::meta() << std::endl;\n}\n

        2. Download required files

          mkdir cmake\nwget https://raw.githubusercontent.com/cpp-pm/gate/master/cmake/HunterGate.cmake -O cmake/HunterGate.cmake\n

        3. Build the project with CMake:

          cmake -S . -B build\ncmake --build build\n
        "},{"location":"integration/package_managers/#vcpkg","title":"vcpkg","text":"

        Summary

        package: nlohmann-json

        • Available versions: current version
        • The package is updated with every release.
        • File issues at the vcpkg issue tracker
        • vcpkg website

        If you are using vcpkg on your project for external dependencies, then you can install the nlohmann-json package with

        vcpkg install nlohmann-json\n

        and follow the then displayed descriptions. Please see the vcpkg project for any issues regarding the packaging.

        Example: CMake with the vcpkg toolchain

        1. Create the following files:

          CMakeLists.txt
          cmake_minimum_required(VERSION 3.15)\nproject(json_example)\n\nfind_package(nlohmann_json CONFIG REQUIRED)\n\nadd_executable(json_example example.cpp)\ntarget_link_libraries(json_example PRIVATE nlohmann_json::nlohmann_json)\n
          example.cpp
          #include <nlohmann/json.hpp>\n#include <iostream>\n#include <iomanip>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::setw(4) << json::meta() << std::endl;\n}\n

        2. Install package:

          vcpkg install nlohmann-json\n

        3. Build:

          cmake -S . -B build -DCMAKE_TOOLCHAIN_FILE=$VCPKG_ROOT/scripts/buildsystems/vcpkg.cmake\ncmake --build build\n
        "},{"location":"integration/package_managers/#cget","title":"cget","text":"

        Summary

        package: nlohmann/json

        • Available versions: current version and older versions
        • The package is updated with every release.
        • File issues at the cget issue tracker
        • cget website

        If you are using cget, you can install the latest master version with

        cget install nlohmann/json\n

        A specific version can be installed with cget install nlohmann/json@v3.12.0. Also, the multiple header version can be installed by adding the -DJSON_MultipleHeaders=ON flag (i.e., cget install nlohmann/json -DJSON_MultipleHeaders=ON).

        Example: CMake with the cget toolchain

        1. Create the following files:

          CMakeLists.txt
          cmake_minimum_required(VERSION 3.15)\nproject(json_example)\n\nfind_package(nlohmann_json REQUIRED)\n\nadd_executable(json_example example.cpp)\ntarget_link_libraries(json_example PRIVATE nlohmann_json::nlohmann_json)\n
          example.cpp
          #include <nlohmann/json.hpp>\n#include <iostream>\n#include <iomanip>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::setw(4) << json::meta() << std::endl;\n}\n

        2. Initialize cget

          cget init\n

        3. Install the library

          cget install nlohmann/json\n

        4. Build

          cmake -S . -B build -DCMAKE_TOOLCHAIN_FILE=cget/cget/cget.cmake\ncmake --build build\n
        "},{"location":"integration/package_managers/#swift-package-manager","title":"Swift Package Manager","text":"

        Summary

        package: nlohmann/json

        • Available versions: current version and older versions
        • The package is updated with every release.
        • File issues at the library issue tracker
        • Xcode documentation

        If you are using the Swift Package Manager, add this repository as a package dependency and depend on its json product:

        dependencies: [\n    .package(url: \"https://github.com/nlohmann/json\", from: \"3.12.0\")\n],\ntargets: [\n    .target(name: \"MyTarget\", dependencies: [.product(name: \"json\", package: \"json\")])\n]\n

        The library's own Package.swift publishes single_include/nlohmann (not single_include) as the public headers directory, so include the header without the nlohmann/ prefix:

        #include <json.hpp>\n
        Example: a minimal executable package

        1. Create the following files (the source file goes into Sources/json_example/, following Swift Package Manager's directory layout convention):

          Package.swift
          // swift-tools-version: 5.9\nimport PackageDescription\n\nlet package = Package(\n    name: \"json_example\",\n    dependencies: [\n        .package(url: \"https://github.com/nlohmann/json\", from: \"3.12.0\")\n    ],\n    targets: [\n        .executableTarget(\n            name: \"json_example\",\n            dependencies: [\n                .product(name: \"json\", package: \"json\")\n            ]\n        )\n    ]\n)\n
          Sources/json_example/example.cpp
          #include <json.hpp>\n#include <iostream>\n#include <iomanip>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::setw(4) << json::meta() << std::endl;\n}\n

        2. Build and run:

          swift run --build-system native\n

        Warning

        On some toolchains, swift run/swift build fail to link an executable target against the header-only json product with an error such as Build input file cannot be found: '.../json.o', because the product itself has no compiled sources; see #4650 and the upstream Swift Package Manager issue. Passing --build-system native (shown above) selects Swift Package Manager's legacy build system, which does not have this problem; depending on the library from a library target instead of an executable is not affected either.

        You can also add the dependency from within Xcode via File \u2192 Add Package Dependencies\u2026 and the same repository URL; see Apple's documentation.

        "},{"location":"integration/package_managers/#nuget","title":"NuGet","text":"

        Summary

        package: nlohmann.json

        • Available versions: current and previous versions
        • The package is updated with every release.
        • Maintainer: Hani Kaabi
        • File issues at the maintainer's issue tracker
        • NuGet website

        If you are using NuGet, you can use the package nlohmann.json with

        dotnet add package nlohmann.json\n

        NuGet integrates with C++ projects through MSBuild, so it is mainly useful for Visual Studio/MSBuild projects; using it as a dependency from other build systems, such as CMake, is possible but more cumbersome than the other package managers on this page.

        Example: Visual Studio project

        1. Right-click the project (any C++ project) in \"Solution Explorer\" and select \"Manage NuGet Packages\u2026\"

        2. Switch to the \"Browse\" tab.

        3. Search for nlohmann.json, select it, and click \"Install\".

        4. #include <nlohmann/json.hpp> in your code and build the project. The package's build/native/nlohmann.json.targets file adds $(MSBuildThisFileDirectory)include to the project's AdditionalIncludeDirectories, so no further include path configuration is needed.

        For further details, see the original discussion this section is based on.

        "},{"location":"integration/package_managers/#conda","title":"Conda","text":"

        Summary

        package: nlohmann_json

        • Available versions: current and previous versions
        • The package is updated with every release.
        • File issues at the feedstock's issue tracker
        • Conda documentation

        If you are using conda, you can use the package nlohmann_json from conda-forge executing

        conda install -c conda-forge nlohmann_json\n
        Example: Raw compilation

        1. Create the following file:

          example.cpp
          #include <nlohmann/json.hpp>\n#include <iostream>\n#include <iomanip>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::setw(4) << json::meta() << std::endl;\n}\n

        2. Create and activate an environment json:

          conda create -n json\nconda activate json\n

        3. Install the package:

          conda install -c conda-forge nlohmann_json\n

        4. Build the code:

          g++ -std=c++11 -I$(conda info --base)/envs/json/include example.cpp -o example\n
        "},{"location":"integration/package_managers/#msys2","title":"MSYS2","text":"

        Summary

        package: mingw-w64-nlohmann-json

        • The package is updated automatically.
        • File issues at the MINGW-packages issue tracker
        • MSYS2 website

        If you are using MSYS2, you can use the mingw-w64-nlohmann-json package; type pacman -S mingw-w64-i686-nlohmann-json or pacman -S mingw-w64-x86_64-nlohmann-json for installation.

        Example: Raw compilation

        1. Create the following file:

          example.cpp
          #include <nlohmann/json.hpp>\n#include <iostream>\n#include <iomanip>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::setw(4) << json::meta() << std::endl;\n}\n

        2. Install the package (from an MSYS2 MinGW 64-bit shell):

          pacman -S mingw-w64-x86_64-nlohmann-json\n

        3. Compile the code:

          g++ example.cpp -std=c++11 -o example\n
        "},{"location":"integration/package_managers/#macports","title":"MacPorts","text":"

        Summary

        port: nlohmann-json

        • Available versions: current version
        • The port is updated with every release.
        • File issues at the MacPorts issue tracker
        • MacPorts website

        If you are using MacPorts, execute 

        sudo port install nlohmann-json\n

        to install the nlohmann-json package.

        Example: Raw compilation

        1. Create the following file:

          example.cpp
          #include <nlohmann/json.hpp>\n#include <iostream>\n#include <iomanip>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::setw(4) << json::meta() << std::endl;\n}\n

        2. Install the package:

          sudo port install nlohmann-json\n

        3. Compile the code and pass the MacPorts prefix to the include path such that the library can be found:

          c++ example.cpp -I/opt/local/include -std=c++11 -o example\n
        Example: CMake

        1. Create the following files:

          example.cpp
          #include <nlohmann/json.hpp>\n#include <iostream>\n#include <iomanip>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::setw(4) << json::meta() << std::endl;\n}\n
          CMakeLists.txt
          cmake_minimum_required(VERSION 3.15)\nproject(json_example)\n\nfind_package(nlohmann_json CONFIG REQUIRED)\n\nadd_executable(json_example example.cpp)\ntarget_link_libraries(json_example PRIVATE nlohmann_json::nlohmann_json)\n

        2. Install the package:

          sudo port install nlohmann-json\n

        3. Compile the code:

          cmake -S . -B build\ncmake --build build\n
        "},{"location":"integration/package_managers/#build2","title":"build2","text":"

        Summary

        package: nlohmann-json library target: nlohmann-json%lib{json} available in package repositories: - cppget.org (recommended) - package's sources (for advanced users)

        • Available versions: current version and older versions since 3.7.3 (see cppget.org)
        • The package is maintained and published by the build2 community in this repository.
        • File issues at the package source repository
        • build2 website

        Note: build2 should not be considered as a standalone package-manager. It is a build-system + package manager + project manager, a set of tools that work hand-in-hand. build2-based projects do not rely on existing CMake scripts and the build scripts defining the project's targets are specific to build2.

        To use this package in an existing build2 project, the general steps are:

        1. Make the package available to download from a package repository that provides it.

          Your project's repositories.manifest specifies where the package manager will try to acquire packages by default. Make sure one of the repositories specified in this file provides nlohmann-json package. The recommended open-source repository is cppget.org.

          If the project has been created using bdep new, cppget.org is already specified in repositories.manifest but commented, just uncomment these lines: 

          :\nrole: prerequisite\nlocation: https://pkg.cppget.org/1/stable\n

        2. Add this package as dependency of your project.

          In your project's manifest add the dependency to the package using depends: nlohmann-json. You could also add some version constraints. For example, to depend on the latest version available: 

          depends: nlohmann-json\n

        3. Add this library as dependency of your target that uses it.

          In the buildfile defining the target that will use this library:

          - import the target `lib{json}` from the `nlohmann-json` package, for example:\n    ```\n    import nljson = nlohmann-json%lib{json}\n    ```\n\n- then add the library's target as requirement for your target using it, for example:\n    ```\n    exe{example} : ... $nljson\n    ```\n

        4. Use the library in your project's code and build it.

          At this point, assuming your project is initialized in a build-configuration, any b or bdep update command that will update/build the project will also acquire the missing dependency automatically, then build it and link it with your target.

          If you just want to synchronize dependencies for all your configurations to download the ones you just added: 

          bdep sync -af\n

          Example: from scratch, using build2's bdep new command

          1. Create a new executable project \"example\" (see bdep new command details for the various options to create a new project):

            bdep new example\n

          2. Edit these files by replacing their content:

            • example/repositories.manifest: Enable acquiring packages from https://cppget.org by uncommenting the related lines:

              project's `repositories.manifest`
              : 1\nsummary: example project repository\n\n:\nrole: prerequisite\nlocation: https://pkg.cppget.org/1/stable\n#trust: ...\n\n#:\n#role: prerequisite\n#location: https://git.build2.org/hello/libhello.git\n

            • example/manifest: Add the latest version of the nlohmann-json package as dependency to the project:

              project's `manifest`
              name: example\nversion: 0.1.0-a.0.z\nlanguage: c++\nsummary: example C++ executable\nlicense: other: proprietary ; Not free/open source.\ndescription-file: README.md\nurl: https://example.org/example\nemail: your@emailprovider.com\n#build-error-email: your@emailprovider.com\ndepends: * build2 >= 0.16.0\ndepends: * bpkg >= 0.16.0\n#depends: libhello ^1.0.0\n\ndepends: nlohmann-json\n

            • example/example/buildfile: import the library's target to be used as requirement for building the executable target exe{example}:

              project's `buildfile`
              libs =\nimport libs = nlohmann-json%lib{json}\n\nexe{example}: {hxx ixx txx cxx}{**} $libs testscript\n\ncxx.poptions =+ \"-I$out_root\" \"-I$src_root\"\n

            • example/example/example.cxx: bdep new generates a \"hello world\" by default, replace it by this:

              example.cxx
              #include <nlohmann/json.hpp>\n#include <iostream>\n#include <iomanip>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::setw(4) << json::meta() << std::endl;\n}\n

            • example/example/testscript: (optional) if you want to be able to test that executable's output is correct using b test:

              `testscript` checking the output of the program
              : json basics\n:\n$* >>~/EOO/\n{\n    \"compiler\": {\n/        \"c\\+\\+\": \"\\d+\",/\n/        \"family\": \"[\\w\\d]+\",/\n/        \"version\": \\d+/\n    },\n/    \"copyright\": \"\\(C\\) 2013-\\d+ Niels Lohmann\",/\n    \"name\": \"JSON for Modern C++\",\n/    \"platform\": \"[\\w\\d]+\",/\n    \"url\": \"https://github.com/nlohmann/json\",\n    \"version\": {\n        \"major\": 3,\n/        \"minor\": \\d+,/\n/        \"patch\": \\d+,/\n/        \"string\": \"3\\.\\d+\\.\\d+\"/\n    }\n}\nEOO\n

          3. Initialize the project in a default C/C++ build configuration directory, then build and test:

            cd example/\n\n# create default C/C++ build configuration in ../example-myconfig/, initialize the project in it (downloads it's dependencies in it too)\nbdep init -C @myconfig cc\n\n# build only,\nb\n\n# or build and test the executable's output, will only work if the `testscript` is correct\nb test\n
          "},{"location":"integration/package_managers/#cpmcmake","title":"CPM.cmake","text":"

          Summary

          package: gh:nlohmann/json

          • Available versions: current and previous versions
          • The package is updated with every release.
          • File issues at the CPM.cmake issue tracker
          • CPM.cmake website

          If you are using CPM.cmake, add the CPM.cmake script and the following snippet to your CMake project:

          CPMAddPackage(\"gh:nlohmann/json@3.12.0\")\n
          Example: CMake with CPMAddPackage

          1. Create the following files:

            example.cpp
            #include <nlohmann/json.hpp>\n#include <iostream>\n#include <iomanip>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::setw(4) << json::meta() << std::endl;\n}\n
            CMakeLists.txt
            cmake_minimum_required(VERSION 3.15)\nproject(json_example)\n\ninclude(${CMAKE_SOURCE_DIR}/cmake/CPM.cmake)\n\nCPMAddPackage(\"gh:nlohmann/json@3.12.0\")\n\nadd_executable(json_example example.cpp)\ntarget_link_libraries(json_example PRIVATE nlohmann_json::nlohmann_json)\n

          2. Download CPM.cmake

            mkdir -p cmake\nwget -O cmake/CPM.cmake https://github.com/cpm-cmake/CPM.cmake/releases/latest/download/get_cpm.cmake\n

          3. Build

            cmake -S . -B build\ncmake --build build\n
          "},{"location":"integration/package_managers/#xmake","title":"xmake","text":"

          Summary

          package: nlohmann_json

          • Available versions: current and previous versions
          • The package is updated with every release.
          • File issues at the xmake issue tracker
          • xmake website
          Example: xmake project

          1. Create the following files:

            example.cpp
            #include <nlohmann/json.hpp>\n#include <iostream>\n#include <iomanip>\n\nusing json = nlohmann::json;\n\nint main()\n{\n    std::cout << std::setw(4) << json::meta() << std::endl;\n}\n
            xmake.lua
            add_requires(\"nlohmann_json\")\n\nadd_rules(\"mode.debug\", \"mode.release\")\ntarget(\"xm\")\n    set_kind(\"binary\")\n    add_files(\"example.cpp\")\n    add_packages(\"nlohmann_json\")\n    set_languages(\"cxx11\")\n

          2. Build

            xmake\n

          3. Run

            xmake run\n
          "},{"location":"integration/package_managers/#other-package-managers","title":"Other package managers","text":"

          The library is also contained in many other package repositories; Repology tracks the packaged versions across repositories.

          "},{"location":"integration/package_managers/#buckaroo","title":"Buckaroo","text":"

          If you are using Buckaroo, you can install this library's module with buckaroo add github.com/buckaroo-pm/nlohmann-json. There is a demo repo here.

          Warning

          The module is outdated as the respective repository has not been updated in years.

          "},{"location":"integration/package_managers/#cocoapods","title":"CocoaPods","text":"

          If you are using CocoaPods, you can use the library by adding pod \"nlohmann_json\", '~>3.1.2' to your podfile (see an example). Please file issues at the repository, as its issue tracker is no longer reachable.

          Warning

          The module is outdated as the respective pod has not been updated in years.

          "},{"location":"integration/package_managers/#npm","title":"npm","text":"

          This project does not publish an official npm package. The npm package nlohmann-json (or similarly named packages) is not maintained or endorsed by this project. Use one of the package managers listed above, or integrate the single header directly.

          "},{"location":"integration/package_managers/#esp-idf-and-platformio","title":"ESP-IDF and PlatformIO","text":"

          There is no official package published to the ESP-IDF Component Registry or the PlatformIO Registry. A community-maintained fork, Johboh/nlohmann-json, publishes this library to both registries on each new release and can be used as an unofficial component/package for ESP-IDF and PlatformIO projects. As the library is header-only, it can otherwise be used directly by adding its include/ directory to your component's/project's include paths, like any other integration method described on this page.

          "},{"location":"integration/pkg-config/","title":"Pkg-config","text":"

          If you are using bare Makefiles, you can use pkg-config to generate the include flags that point to where the library is installed:

          pkg-config nlohmann_json --cflags\n

          A pkg-config file is installed by CMake (when the JSON_Install option is enabled, which is the default for a top-level build) as well as by several package managers.

          Users of the Meson build system will also be able to use a system-wide library, which will be found by pkg-config:

          json = dependency('nlohmann_json', required: true)\n
          "}]} \ No newline at end of file