json/api/basic_json/count.md

nlohmann::basic_json::count

// (1)
size_type count(const typename object_t::key_type& key) const;

// (2)
template<typename KeyType>
size_type count(KeyType&& key) const;

// (3)
template<typename T>
size_type count(T) const = delete;

1. Returns the number of elements with key key. If ObjectType is the default std::map type, the return value will always be 0 (key was not found) or 1 (key was found).
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. 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.count(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 of the element to count.

Return value

Number of elements with key key. If the JSON value is not an object, the return value will be 0.

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 0 when executed on a JSON type that is not an object.
• Calling this function with an integer argument (for example, #!cpp count(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.

Examples

??? example "Example: (1) count number of elements"

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

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

using json = nlohmann::json;

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

    // call count()
    auto count_two = j_object.count("two");
    auto count_three = j_object.count("three");

    // print values
    std::cout << "number of elements with key \"two\": " << count_two << '\n';
    std::cout << "number of elements with key \"three\": " << count_three << '\n';
}

```

Output:

```json
number of elements with key "two": 1
number of elements with key "three": 0

```

??? example "Example: (2) count number of elements using string_view"

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

```cpp
#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 count()
    auto count_two = j_object.count("two"sv);
    auto count_three = j_object.count("three"sv);

    // print values
    std::cout << "number of elements with key \"two\": " << count_two << '\n';
    std::cout << "number of elements with key \"three\": " << count_three << '\n';
}

```

Output:

```json
number of elements with key "two": 1
number of elements with key "three": 0

```

See also

• find find a value in an object
• contains checks whether a key exists

Version history

1. Added in version 3.11.0.
2. Added in version 1.0.0. Changed parameter key type to KeyType&& 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. Deleted overload for integral key types added in version 3.13.0 to reject such calls at compile time instead of causing undefined behavior at runtime.