clock_time_conversion struct

A trait that specifies how to convert a time_point from one type of clock to another, while maintaining the equivalent point in time.

Syntax

// C++20
1) template<class Dest, class Source> struct clock_time_conversion {};
2) template<class Clock> struct clock_time_conversion<Clock, Clock>;
3) template<> struct clock_time_conversion<system_clock, system_clock>;
4) template<> struct clock_time_conversion<utc_clock, utc_clock>;
5) template<> struct clock_time_conversion<system_clock, utc_clock>;
6) template<> struct clock_time_conversion<utc_clock, system_clock>;
7) template<class Clock> struct clock_time_conversion<Clock, system_clock>;
8) template<class Clock> struct clock_time_conversion<system_clock, Clock>;
9) template<class Clock> struct clock_time_conversion<Clock, utc_clock>;
10) template<class Clock> struct clock_time_conversion<utc_clock, Clock>;

Template parameters

Clock
A clock type to convert from/to.

Dest
The clock type to convert to.

Source
The clock type to convert from.

The traits provide the following conversions:

1) An empty struct that is defined only so that it can be specialized.
2-4) Identity conversions. Returns the same clock you pass in.
5-6) Converting between sys_time and utc_time calls utc_clock::to_sys or utc_clock::from_sys depending on the direction of the conversion.
7-8) Converting between sys_time and the specified clock, when the specified clock supports to_sys and from_sys, results in a call to Clock::to_sys or Clock::from_sys, depending on the direction of the conversion.
9-10) Converting between utc_timeand the specified clock, when the specified clock supports from_utc and to_sys, results in a call to Clock::to_utc or Clock::from_utc, depending on the direction of the conversion.

Members

Name	Description
operator ()	Converts a time_point from one clock to another.

Remarks

You usually won't use this trait directly in your code. It's used by the clock_cast conversion function.

Requirements

Header: <chrono>

Namespace: std::chrono

Compiler Option: /std:c++latest

operator()

Converts a time_point from one clock type to another while maintaining the equivalent point in time.

Syntax

1)
template <class Duration>
time_point<Clock, Duration> operator()(const time_point<Clock, Duration>& t) const;

2)
template <class Duration>
sys_time<Duration> operator()(const sys_time<Duration> & t) const;

3)
template <class Duration>
utc_time<Duration> operator()(const utc_time<Duration>& t) const;

4)
template <class Duration>
sys_time<Duration> operator()(const utc_time<Duration>& t) const;

5)
template <class Duration>
utc_time<Duration> operator()(const sys_time<Duration>& t) const;

Parameters

t The time_point to convert.

Return value

1-3) Identity conversions. No conversion. Returns t without any changes.
4) Returns utc_clock::to_sys(t).
5) Returns utc_clock::from_sys(t).

Deduction guides

The following deduction guides are provided for template <class Duration> operator():

1)
template <class Duration> auto operator()(const sys_time<Duration>& t) const
    -> decltype(Clock::from_sys(t));

2)
template <class Duration> auto operator()(const time_point<SourceClock, Duration>& t) const
    -> decltype(Clock::to_sys(t));

3)
template <class Duration> auto operator()(const utc_time<Duration>& t) const
    -> decltype(Clock::from_utc(t));

4)
template <class Duration> auto operator()(const time_point<Clock, Duration>& t) const
    -> decltype(Clock::to_utc(t));

1) Participates in overload resolution only when Clock supports from_sys() and returns time_point<Clock, Duration>.
2) Participates in overload resolution only when Clock supports to_sys() and returns sys_time<Duration>.
3) Participates in overload resolution only when Clock supports from_utc() and returns time_point<Clock, Duration>.
4) Participates in overload resolution only when Clock supports to_utc() and returns utc_time<Duration>.

Example: clock_time_conversion

#include <iostream>
#include <chrono>

using namespace std::chrono;

int main()
{
    auto sd = sys_days{ 2021y / July / 26 };
    auto time = clock_time_conversion<utc_clock, system_clock>{}(sd);
    std::cout << time << "\n";
    return 0;
}

2021-07-26 00:00:00

See also

<chrono>
clock_cast
Header Files Reference