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# 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.
2. 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.
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` 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. `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, `false` is returned.
2. See 1.
3. `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.
2. The function does not throw exceptions.
3. The function can throw the following exceptions:
   - Throws `parse_error.106` if an array index begins with `0`.
   - Throws `parse_error.109` if an array index was not a number.

## 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.

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 (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 (json::parse_error& e)
        {
            std::cout << e.what() << '\n';
        }
    }

Output:

    true
    true
    true
    true
    false
    false
    false

## 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.

Last update: July 31, 2022
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