# 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;
```

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, `false` is returned.
1. See 1. This overload is only available if `KeyType` is comparable with `typename object_t::key_type` and `typename object_comparator_t::is_transparent` denotes a type.
1. 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`](https://json.nlohmann.me/api/json_pointer/index.md) that is comparable with [`string_t`](https://json.nlohmann.me/api/basic_json/string_t/index.md) using [`object_comparator_t`](https://json.nlohmann.me/api/basic_json/object_comparator_t/index.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. `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, `false` is returned.
1. See 1.
1. `true` if the JSON pointer can be resolved to a stored value, `false` otherwise.

## Exception safety

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

## Exceptions

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

## Complexity

Logarithmic in the size of the JSON object.

## Notes

- This method always returns `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, `contains(0)`) does not compile: such an argument would otherwise implicitly convert to a null `const char*` and, from there, cause undefined behavior when constructing a `std::string` for the object key. To check for an array element instead, use [`at`](https://json.nlohmann.me/api/basic_json/at/index.md), [`operator[]`](https://json.nlohmann.me/api/basic_json/operator%5B%5D/index.md), or compare against [`size`](https://json.nlohmann.me/api/basic_json/size/index.md).

Postconditions

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

## Examples

Example: (1) check with key

The example shows how `contains()` is used.

```
#include <iostream>
#include <nlohmann/json.hpp>

using json = nlohmann::json;
using namespace nlohmann::literals;

int main()
{
    // create some JSON values
    json j_object = R"( {"key": "value"} )"_json;
    json j_array = R"( [1, 2, 3] )"_json;

    // call contains
    std::cout << std::boolalpha <<
              "j_object contains 'key': " << j_object.contains("key") << '\n' <<
              "j_object contains 'another': " << j_object.contains("another") << '\n' <<
              "j_array contains 'key': " << j_array.contains("key") << std::endl;
}
```

Output:

```
j_object contains 'key': true
j_object contains 'another': false
j_array contains 'key': false
```

Example: (2) check with key using string_view

The example shows how `contains()` is used.

```
#include <iostream>
#include <string_view>
#include <nlohmann/json.hpp>

using namespace std::string_view_literals;
using json = nlohmann::json;
using namespace nlohmann::literals;

int main()
{
    // create some JSON values
    json j_object = R"( {"key": "value"} )"_json;
    json j_array = R"( [1, 2, 3] )"_json;

    // call contains
    std::cout << std::boolalpha <<
              "j_object contains 'key': " << j_object.contains("key"sv) << '\n' <<
              "j_object contains 'another': " << j_object.contains("another"sv) << '\n' <<
              "j_array contains 'key': " << j_array.contains("key"sv) << std::endl;
}
```

Output:

```
j_object contains 'key': true
j_object contains 'another': false
j_array contains 'key': false
```

Example: (3) check with JSON pointer

The example shows how `contains()` is used.

```
#include <iostream>
#include <nlohmann/json.hpp>

using json = nlohmann::json;
using namespace nlohmann::literals;

int main()
{
    // create a JSON value
    json j =
    {
        {"number", 1}, {"string", "foo"}, {"array", {1, 2}}
    };

    std::cout << std::boolalpha
              << j.contains("/number"_json_pointer) << '\n'
              << j.contains("/string"_json_pointer) << '\n'
              << j.contains("/array"_json_pointer) << '\n'
              << j.contains("/array/1"_json_pointer) << '\n'
              << j.contains("/array/-"_json_pointer) << '\n'
              << j.contains("/array/4"_json_pointer) << '\n'
              << j.contains("/baz"_json_pointer) << std::endl;

    try
    {
        // try to use an array index with leading '0'
        j.contains("/array/01"_json_pointer);
    }
    catch (const json::parse_error& e)
    {
        std::cout << e.what() << '\n';
    }

    try
    {
        // try to use an array index that is not a number
        j.contains("/array/one"_json_pointer);
    }
    catch (const json::parse_error& e)
    {
        std::cout << e.what() << '\n';
    }
}
```

Output:

```
true
true
true
true
false
false
false
```

## See also

- [find](https://json.nlohmann.me/api/basic_json/find/index.md) find a value in an object
- [count](https://json.nlohmann.me/api/basic_json/count/index.md) returns the number of occurrences of a key

## Version history

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