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# nlohmann::basic_json::count

    // (1)
    size_type count(const typename object_t::key_type& key) const;

    // (2)
    template<typename KeyType>
    size_type count(KeyType&& key) const;

1. Returns the number of elements with key `key`. If `ObjectType` is the default `std::map` type, the return value will always be `0` (`key` was not found) or `1` (`key` was found).
2. See 1. This overload is only available if `KeyType` is comparable with `typename object_t::key_type` and `typename object_comparator_t::is_transparent` denotes a type.

## Template parameters

`KeyType`
  A type for an object key other than `json_pointer` that is comparable with `string_t` using `object_comparator_t`. This can also be a string view (C++17).

## Parameters

`key` (in)
  key value of the element to count.

## Return value

Number of elements with key `key`. If the JSON value is not an object, the return value will be `0`.

## Exception safety

Strong exception safety: if an exception occurs, the original value stays intact.

## Complexity

Logarithmic in the size of the JSON object.

## Notes

This method always returns `0` when executed on a JSON type that is not an object.

## Examples

Example: (1) count number of elements

The example shows how `count()` is used.

    #include <iostream>
    #include <nlohmann/json.hpp>

    using json = nlohmann::json;

    int main()
    {
        // create a JSON object
        json j_object = {{"one", 1}, {"two", 2}};

        // call count()
        auto count_two = j_object.count("two");
        auto count_three = j_object.count("three");

        // print values
        std::cout << "number of elements with key \"two\": " << count_two << '\n';
        std::cout << "number of elements with key \"three\": " << count_three << '\n';
    }

Output:

    number of elements with key "two": 1
    number of elements with key "three": 0

Example: (2) count number of elements using string_view

The example shows how `count()` is used.

    #include <iostream>
    #include <string_view>
    #include <nlohmann/json.hpp>

    using namespace std::string_view_literals;
    using json = nlohmann::json;

    int main()
    {
        // create a JSON object
        json j_object = {{"one", 1}, {"two", 2}};

        // call count()
        auto count_two = j_object.count("two"sv);
        auto count_three = j_object.count("three"sv);

        // print values
        std::cout << "number of elements with key \"two\": " << count_two << '\n';
        std::cout << "number of elements with key \"three\": " << count_three << '\n';
    }

Output:

    number of elements with key "two": 1
    number of elements with key "three": 0

## Version history

1. Added in version 3.11.0.
2. Added in version 1.0.0. Changed parameter `key` type to `KeyType&&` in version 3.11.0.

Last update: July 31, 2022
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