std::chrono::is_clock

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

IfT satisfies theClock requirements, provides the member constantvalue equaltrue. For any other type,value isfalse.

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

Each of the followingqualified 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 anunevaluated operand:

T::is_steady
T::now()

If the program adds specializations forstd::is_clock orstd::is_clock_v, the behavior is undefined.

[edit]Template parameters

a type to check

[edit]Helper variable template

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

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Inherited fromstd::integral_constant

Member constants

value

[static]

true ifT satisfies theClock requirements,false otherwise
(public static member constant)

Member functions

operator bool	converts the object tobool, returnsvalue (public member function)
operator() (C++14)	returnsvalue (public member function)

Member types

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

[edit]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{};

[edit]Notes

IfT otherwise meets theClock requirements, butT::is_steady is not of typeconstbool, orT::now() is not of typeT::time_point, the result ofis_clock_v<T> is unspecified.

[edit]Example

Run this code

#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(){}