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>
static bool accept(IteratorType first, IteratorType 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

   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.

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
• an object obj for which begin(obj) and end(obj) produces a valid pair of iterators.

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

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

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
• 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 3.12.1.

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.