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# nlohmann::basic_json::accept

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

    // (2)
    template<typename IteratorType>
    static bool accept(IteratorType first, IteratorType last,
                       const bool ignore_comments = 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 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 (must not be null)
  - a C-style array of characters
  - a pointer to a null-terminated string of single byte characters
  - 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)
`first` (in)
  iterator to start of character range
`last` (in)
  iterator to end of 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.

## Complexity

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

## Notes

(1) A UTF-8 byte order mark is silently ignored.

Runtime assertion

The precondition that a passed `FILE` pointer must not be null is enforced with a runtime assertion.

## 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.

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.

Last update: July 28, 2022
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