std::chrono::is_clock

template< class T >
struct is_clock;

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 is_clock or is_clock_v is undefined.

Template parameters

Helper variable template

template< class T >
inline constexpr bool is_clock_v = is_clock<T>::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

#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>>
    );
}