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!=.

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 `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`

Parameters

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!=

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 FILE* or 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.

Examples

Example

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

#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:

true false

See also

• parse - deserialize from a compatible input
• sax_parse - parse input using the SAX interface
• operator>> - deserialize from stream

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.

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.