nlohmann::basic_json::find

// (1)
iterator find(const typename object_t::key_type& key);
const_iterator find(const typename object_t::key_type& key) const;

// (2)
template<typename KeyType>
iterator find(KeyType&& key);
template<typename KeyType>
const_iterator find(KeyType&& key) const;

1. Finds an element in a JSON object with a key equivalent to key. If the element is not found or the JSON value is not an object, end() 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.

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 of the element to search for.

Return value

Iterator to an element with a key equivalent to key. If no such element is found or the JSON value is not an object, a past-the-end iterator (see end()) is returned.

Exception safety

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

Complexity

Logarithmic in the size of the JSON object.

Notes

This method always returns end() when executed on a JSON type that is not an object.

Examples

Example: (1) find object element by key

The example shows how find() is used.

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

using json = nlohmann::json;

int main()
{
    // create a JSON object
    json j_object = {{"one", 1}, {"two", 2}};

    // call find
    auto it_two = j_object.find("two");
    auto it_three = j_object.find("three");

    // print values
    std::cout << std::boolalpha;
    std::cout << "\"two\" was found: " << (it_two != j_object.end()) << '\n';
    std::cout << "value at key \"two\": " << *it_two << '\n';
    std::cout << "\"three\" was found: " << (it_three != j_object.end()) << '\n';
}

Output:

"two" was found: true
value at key "two": 2
"three" was found: false

Example: (2) find object element by key using string_view

The example shows how find() is used.

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

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

int main()
{
    // create a JSON object
    json j_object = {{"one", 1}, {"two", 2}};

    // call find
    auto it_two = j_object.find("two"sv);
    auto it_three = j_object.find("three"sv);

    // print values
    std::cout << std::boolalpha;
    std::cout << "\"two\" was found: " << (it_two != j_object.end()) << '\n';
    std::cout << "value at key \"two\": " << *it_two << '\n';
    std::cout << "\"three\" was found: " << (it_three != j_object.end()) << '\n';
}

Output:

"two" was found: true
value at key "two": 2
"three" was found: false

See also

• count returns the number of occurrences of a key
• contains checks whether a key exists

Version history

1. Added in version 3.11.0.
2. Added in version 1.0.0. Changed to support comparable types in version 3.11.0.