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 following conditions:

• Each of the following qualified identifiers is valid and denotes a type:
• T::rep
• T::period
• T::duration
• T::time_point
• Each of the following expressions is well-formed when treated as an unevaluated operand:
• T::is_steady
• T::now()

If the program adds specializations for std::is_clock or std::is_clock_v, the behavior is undefined.

Template parameters

Helper variable template

template< class T >
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; // type is not checked
            T::now();     // return type is not checked
        }
struct is_clock<T> : std::true_type {};

Notes

If T otherwise meets the Clock requirements, but T::is_steady is not of type constbool, or T::now() is not of type T::time_point, the result of is_clock_v<T> is unspecified.

Example

#include <chrono>
#include <ratio>

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

int main() {}