1# basic_json::contains 2 3```cpp 4template<typename KeyT> 5bool contains(KeyT && key) const; 6``` 7 8Check whether an element exists in a JSON object with key equivalent to `key`. If the element is not found or the JSON 9value is not an object, `#!cpp false` is returned. 10 11## Template parameters 12 13`KeyT` 14: A type for an object key other than `basic_json::json_pointer`. 15 16## Parameters 17 18`key` (in) 19: key value to check its existence. 20 21## Return value 22 23`#!cpp true` if an element with specified `key` exists. If no such element with such key is found or the JSON value is 24not an object, `#!cpp false` is returned. 25 26## Exception safety 27 28Strong exception safety: if an exception occurs, the original value stays intact. 29 30## Complexity 31 32Logarithmic in the size of the JSON object. 33 34## Notes 35 36This method always returns `#!cpp false` when executed on a JSON type that is not an object. 37 38## Example 39 40??? example 41 42 The example shows how `contains()` is used. 43 44 ```cpp 45 --8<-- "examples/contains.cpp" 46 ``` 47 48 Output: 49 50 ```json 51 --8<-- "examples/contains.output" 52 ``` 53 54## Version history 55 56- Added in version 3.6.0. 57