json/api/basic_json/contains/index.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;

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 a 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 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, operator[], or compare against size.

Postconditions

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

Deprecation

Overload (3) also accepts a json_pointer 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 for how to update existing code.

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 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.
3. Added in version 3.7.0.
4. Deleted overloads for integral key types added in version 3.13.0 unreleased to reject such calls at compile time instead of causing undefined behavior at runtime.