api/basic_json/contains.md

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

```cpp
// (1)
bool contains(const typename object_t::key_type& key) const;

// (2)
template<typename KeyType>
bool contains(KeyType&& key) const;

// (3)
bool contains(const json_pointer& ptr) const;
```

1. Check whether an element exists in a JSON object with a key equivalent to `key`. If the element is not found or the 
   JSON value is not an object, `#!cpp false` is returned.
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.
3. Check whether the given JSON pointer `ptr` can be resolved in the current JSON value.

## 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 to check its existence.

`ptr` (in)
:   JSON pointer to check its existence.

## Return value

1. `#!cpp true` if an element with specified `key` exists. If no such element with such a key is found or the JSON value
   is not an object, `#!cpp false` is returned.
2. See 1.
3. `#!cpp true` if the JSON pointer can be resolved to a stored value, `#!cpp false` otherwise.

## Exception safety

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

## Exceptions

1. The function does not throw exceptions.
2. The function does not throw exceptions.
3. The function does not throw exceptions.

## Complexity

Logarithmic in the size of the JSON object.

## Notes

- This method always returns `#!cpp false` when executed on a JSON type that is not an object.
- This method can be executed on any JSON value type.

!!! info "Postconditions"

    If `#!cpp j.contains(x)` returns `#!c true` for a key or JSON pointer `x`, then it is safe to call `j[x]`.

## Examples

??? example "Example: (1) check with key"

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

??? example "Example: (2) check with key using string_view"

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

??? example "Example: (3) check with JSON pointer"

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

## See also

- [find](find.md) find a value in an object
- [count](count.md) returns the number of occurrences of a key

## Version history

1. Added in version 3.11.0.
2. Added in version 3.6.0. Extended template `KeyType` to support comparable types in version 3.11.0.
3. Added in version 3.7.0.