json/api/basic_json/accept.md

nlohmann::basic_json::accept

// (1)
template<typename InputType>
static bool accept(InputType&& i,
                   const bool ignore_comments = false,
                   const bool ignore_trailing_commas = false);

// (2)
template<typename IteratorType, typename SentinelType = IteratorType>
static bool accept(IteratorType first, SentinelType last,
                   const bool ignore_comments = false,
                   const bool ignore_trailing_commas = false);

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!=.

   Ill-formed UTF-16 and UTF-32 sequences are rejected: a lone surrogate, a high surrogate that is not followed by its low surrogate, and a code point above U+10FFFF all make this function return false.

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.

Template parameters

InputType

A compatible input, for instance:

• an std::istream object
• a #!c 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 or a std::string_view (C++17)
• 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)
• wide characters (wchar_t, char16_t, char32_t), which are converted to UTF-8: a wide string object (std::wstring, std::u16string, std::u32string) or an array (e.g. a wide string literal); a pointer to a null-terminated wide string (e.g. std::u16string::c_str()) is not supported

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

Parameters

i (in)

Input to parse from.

ignore_comments (in)

whether comments should be ignored and treated like whitespace (#!cpp true) or yield a parse error (#!cpp false); (optional, #!cpp false by default)

ignore_trailing_commas (in)

whether trailing commas in arrays or objects should be ignored and treated like whitespace (#!cpp true) or yield a parse error (#!cpp false); (optional, #!cpp 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!=

Return value

Whether the input is valid JSON.

Exception safety

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

Exceptions

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

Complexity

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

Notes

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.

Examples

??? example "Example: (1) reading from a string"

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

```cpp
#include <iostream>
#include <iomanip>
#include <nlohmann/json.hpp>

using json = nlohmann::json;

int main()
{
    // a valid JSON text
    auto valid_text = R"(
    {
        "numbers": [1, 2, 3]
    }
    )";

    // an invalid JSON text
    auto invalid_text = R"(
    {
        "strings": ["extra", "comma", ]
    }
    )";

    std::cout << std::boolalpha
              << json::accept(valid_text) << ' '
              << json::accept(invalid_text) << '\n';
}

```

Output:

```json
true false

```

??? example "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.

```cpp
#include <iostream>
#include <vector>
#include <nlohmann/json.hpp>

using json = nlohmann::json;

int main()
{
    // a buffer containing a JSON text followed by more data
    std::vector<std::uint8_t> input = {'[', '1', ',', '2', ',', '3', ']', 'o', 't', 'h', 'e', 'r'};

    std::cout << std::boolalpha
              << json::accept(input.begin(), input.begin() + 7) << ' '
              << json::accept(input.begin(), input.end()) << '\n';
}

```

Output:

```json
true false

```

??? example "Example: (3) reading from UTF-16 strings"

The encoding is deduced from the character type of the input. String literals like `u"..."`, `U"..."`, and
`L"..."` work as well.

```cpp
#include <iostream>
#include <string>
#include <nlohmann/json.hpp>

using json = nlohmann::json;

int main()
{
    // a valid JSON text, given as UTF-16
    const std::u16string valid_text = u"{\"name\":\"\u00e9\"}";

    // an invalid JSON text, given as UTF-16
    const std::u16string invalid_text = u"{\"name\":\"\u00e9\",}";

    std::cout << std::boolalpha
              << json::accept(valid_text) << ' '
              << json::accept(invalid_text) << '\n';
}

```

Output:

```json
true false

```

See also

• 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

Version history

• 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.
• 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.
• JSON_STRICT_NUL_HANDLING added in version 3.13.0 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 (JSON_DISABLE_STREAM_BULK_SCAN).
• Parsing from a FILE* locks the file once instead of once per character in version 3.13.0 (JSON_DISABLE_UNLOCKED_FILE_IO).
• Ill-formed UTF-16 and UTF-32 input (a lone surrogate, a high surrogate that is not followed by its low surrogate, or a code point above U+10FFFF) is rejected in version 3.13.0; before that, some such sequences were accepted or ended the input silently.

!!! warning "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
`#!cpp accept({ptr, ptr+len}, ...);` with `#!cpp 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](../../integration/migration_guide.md#parsing) for how to update existing code.