api/basic_json/count.md

# <small>nlohmann::basic_json::</small>count

```cpp
// (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 `#!cpp typename object_t::key_type` and
   `#!cpp typename object_comparator_t::is_transparent` denotes a type.

## Template parameters

`KeyType`
:   A type for an object key other than [`json_pointer`](../json_pointer/index.md) that is comparable with
    [`string_t`](string_t.md) using  [`object_comparator_t`](object_comparator_t.md).
    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 "Example: (1) count number of elements"

    The example shows how `count()` is used.
    
    ```cpp
    --8<-- "examples/count__object_t_key_type.cpp"
    ```
    
    Output:
    
    ```json
    --8<-- "examples/count__object_t_key_type.output"
    ```

??? example "Example: (2) count number of elements using string_view"

    The example shows how `count()` is used.
    
    ```cpp
    --8<-- "examples/count__keytype.c++17.cpp"
    ```
    
    Output:
    
    ```json
    --8<-- "examples/count__keytype.c++17.output"
    ```

## See also

- [find](find.md) find a value in an object
- [contains](contains.md) checks whether a key exists

## 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.