template<typename KeyT> bool contains(KeyT && key) const;
Check whether an element exists in a JSON object with key equivalent to key. If the element is not found or the JSON value is not an object, #!cpp false is returned.
KeyT : A type for an object key other than basic_json::json_pointer.
key (in) : key value to check its existence.
#!cpp true if an element with specified key exists. If no such element with such key is found or the JSON value is not an object, #!cpp false is returned.
Strong exception safety: if an exception occurs, the original value stays intact.
Logarithmic in the size of the JSON object.
This method always returns #!cpp false when executed on a JSON type that is not an object.
??? example
The example shows how `contains()` is used. ```cpp --8<-- "examples/contains.cpp" ``` Output: ```json --8<-- "examples/contains.output" ```