json/docs/mkdocs/docs/api/basic_json/from_ubjson.md

nlohmann::basic_json::from_ubjson

// (1)
template<typename InputType>
static basic_json from_ubjson(InputType&& i,
                              const bool strict = true,
                              const bool allow_exceptions = true,
                              const error_handler_t error_handler = error_handler_t::keep);
// (2)
template<typename IteratorType, typename SentinelType = IteratorType>
static basic_json from_ubjson(IteratorType first, SentinelType last,
                              const bool strict = true,
                              const bool allow_exceptions = true,
                              const error_handler_t error_handler = error_handler_t::keep);

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.

Template parameters

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

Parameters

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 (#!cpp true by default)

allow_exceptions (in)

whether to throw exceptions in case of a parse error (optional, #!cpp 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

Return value

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

Exception safety

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

Exceptions

• 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 an invalid type marker is read (including a type marker after $ of an optimized container, even if its count is 0), or if $ is not followed by #
• Throws parse_error.113 if a string could not be parsed successfully (negative or non-integer length, C followed by a byte above 0x7F), if the count of an optimized container is negative, or if a string value or object key is not valid UTF-8 and error_handler is strict
• Throws parse_error.115 if the text of a high-precision number (H) is not a valid JSON number
• Throws out_of_range.406 if a high-precision number, or a float64 when number_float_t is float, does not fit into number_float_t
• Throws out_of_range.408 if the count of an optimized container exceeds the capacity of the container (or of std::size_t; an array of type N is empty, so only the latter applies to it), if an optimized array of Z, T, or F has more than 1,048,576 elements, or if the optimized arrays of Z, T, and F in the input together have more than 1,048,576 elements plus JSON_UBJSON_VALUELESS_ELEMENTS_PER_BYTE (4 by default) for every byte read so far

Complexity

Linear in the size of the input.

Examples

??? example

The example shows the deserialization of a byte vector in UBJSON format to a JSON value.
 
```cpp
--8<-- "examples/from_ubjson.cpp"
```

Output:

```json
--8<-- "examples/from_ubjson.output"
```

See also

• UBJSON - the format, its byte layout and how values are mapped
• Binary formats - overview and comparison of the supported binary formats
• Parsing untrusted input - limits and safe handling of hostile data
• Binary values - how binary values and subtypes are handled
• to_ubjson - create a UBJSON serialization of a JSON value
• JSON_UBJSON_VALUELESS_ELEMENTS_PER_BYTE - set how many elements of arrays of null, true, or false the input may announce per byte
• 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

Version history

• 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.
• Extended overload (2) to accept heterogeneous iterator+sentinel pairs (C++20 ranges support) in version 3.13.0.
• Added error_handler parameter in version 3.13.0.
• Report start_array(0) to a SAX handler for an optimized array of type N ([$N#n), whatever its count, in version 3.13.0.
• Limited the total number of elements of optimized arrays of Z, T, and F in one input in version 3.13.0; the number of elements per byte can be set with JSON_UBJSON_VALUELESS_ELEMENTS_PER_BYTE, for example to read files written by version 3.12.0 and earlier that exceed the default.

!!! warning "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, this overload
  will be deleted (`= delete`) rather than removed. Please replace all calls like
  `#!cpp from_ubjson(ptr, len, ...);` with `#!cpp 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
  `#!cpp from_ubjson({ptr, ptr+len}, ...);` with `#!cpp 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](../../integration/migration_guide.md#parsing) for how to update existing code.