json/docs/mkdocs/docs/api/basic_json/contains.md

nlohmann::basic_json::contains

// (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;

// (4)
template<typename T>
bool contains(T) const = delete;

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.
4. Deleted: this overload is only available if T is an integral type and is declared as deleted, so that a call with an integer key (for example, #!cpp j.contains(0)) fails to compile. Otherwise, the integer literal 0 would convert to a null #!cpp const char* and, from there, to the key type, causing undefined behavior at runtime.

Template parameters

KeyType

A type for an object key other than json_pointer that is comparable with string_t using object_comparator_t. 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.
4. Deleted; a call with an integral argument does not compile.

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.
4. Deleted; a call with an integral argument does not compile.

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.
• Calling this function with an integer argument (for example, #!cpp contains(0)) does not compile: such an argument would otherwise implicitly convert to a null #!cpp const char* and, from there, cause undefined behavior when constructing a #!cpp std::string for the object key. To check for an array element instead, use at, operator[], or compare against size.

!!! 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]`.

!!! warning "Deprecation"

Overload (3) also accepts a [`json_pointer`](../json_pointer/index.md) whose template argument is a `basic_json`
specialization (e.g., `nlohmann::json_pointer<nlohmann::json>`) instead of a string type. This is deprecated since
version 3.11.0 and will be removed in a future major version; use `basic_json::json_pointer` (for `json`,
`nlohmann::json_pointer<std::string>`) instead.

You should be warned by your compiler with a `-Wdeprecated-declarations` warning if you are using a deprecated
function.

See the [migration guide](../../integration/migration_guide.md#json-pointers) for how to update existing code.

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 a value in an object
• count 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. Fixed in version 3.13.0 to consistently accept std::string_view-convertible keys, as already supported by operator[], at, value, and other lookup functions.
3. Added in version 3.7.0.
4. Deleted overloads for integral key types added in version 3.13.0 to reject such calls at compile time instead of causing undefined behavior at runtime.