# std::chrono::is_clock

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< cpp‎ | chrono

Defined in header `<chrono>` |  |
template< class T > struct is_clock; |  | (since C++20)

If `T` satisfies the Clock requirements, provides the member constant `value` equal true. For any other type, `value` is false.

For the purpose of this trait, the extent to which an implementation determines that a type cannot meet the Clock requirements is unspecified, except that a minimum `T` shall not qualify as a Clock unless it meets all of the following conditions:

- The qualified-ids `T::rep`, `T::period`, `T::duration`, and `T::time_point` are all valid and each denotes a type;
- The expressions `T::is_steady` and `T::now()` are each well-formed when treated as an unevaluated operand.

The behavior of a program that adds specializations for `std::is_clock` or `std::is_clock_v` is undefined.

### Template parameters

T | - | a type to check

### Helper variable template

template< class T > inline constexpr bool is_clock_v = is_clock<T>::value; |  | (since C++20)

## Inherited from std::integral_constant

### Member constants

value [static] | true if `T` satisfies the Clock requirements, false otherwise (public static member constant)

### Member functions

operator bool | converts the object to bool, returns value (public member function)
operator() (C++14) | returns value (public member function)

### Member types

Type | Definition
`value_type` | bool
`type` | std::integral_constant<bool, value>

### Possible implementation

    template<class>
    struct is_clock : std::false_type {};

    template<class T>
        requires
            requires {
                typename T::rep;
                typename T::period;
                typename T::duration;
                typename T::time_point;
                T::is_steady;
                T::now();
            }
    struct is_clock<T> : std::true_type {};

### Example

Run this code

    #include <chrono>
    #include <ratio>

    int main()
    {
        static_assert
        (
            std::chrono::is_clock_v<std::chrono::utc_clock>
            and
            not std::chrono::is_clock_v<std::chrono::duration<int, std::exa>>
        );
    }

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