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std::chrono::is_clock(3)       C++ Standard Libary	std::chrono::is_clock(3)

NAME
     std::chrono::is_clock - std::chrono::is_clock

Synopsis
	Defined in header <chrono>
	template< class T >	    (since C++20)
	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 de-
     termines 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
	T - a type to check

	Helper variable template

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

     Inherited from std::integral_constant

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

Member functions
	operator bool converts the object to bool, returns value
		      (public member function)
	operator()    returns value
	(C++14)       (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; // not required to be bool
		    T::now();	  // not required to be T::time_point
		}
	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>>
	  );
      }

http://cppreference.com 	   2024.06.10		std::chrono::is_clock(3)
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