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api/basic_json/count.md

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+# <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.