25 Time library [time]

25.7 Clocks [time.clock]

25.7.9 time_­point conversions [time.clock.cast]

25.7.9.1 Class template clock_­time_­conversion [time.clock.conv]

namespace std::chrono {
  template<class DestClock, class SourceClock>
  struct clock_time_conversion {};
}
clock_­time_­conversion serves as a trait which can be used to specify how to convert a source time_­point of type time_­point<SourceClock, Duration> to a destination time_­point of type time_­point<DestClock, Duration> via a specialization: clock_­time_­conversion<DestClock, SourceClock>.
A specialization of clock_­time_­conversion<DestClock, SourceClock> shall provide a const-qualified operator() that takes a parameter of type time_­point<SourceClock, Duration> and returns a time_­point<DestClock, OtherDuration> representing an equivalent point in time.
OtherDuration is a chrono::duration whose specialization is computed from the input Duration in a manner which can vary for each clock_­time_­conversion specialization.
A program may specialize clock_­time_­conversion if at least one of the template parameters is a user-defined clock type.
Several specializations are provided by the implementation, as described in [time.clock.cast.id], [time.clock.cast.sys.utc], [time.clock.cast.sys], and [time.clock.cast.utc].

25.7.9.2 Identity conversions [time.clock.cast.id]

template<typename Clock>
struct clock_time_conversion<Clock, Clock> {
  template<class Duration>
    time_point<Clock, Duration>
      operator()(const time_point<Clock, Duration>& t) const;
};
template<class Duration> time_point<Clock, Duration> operator()(const time_point<Clock, Duration>& t) const;
Returns: t.
template<>
struct clock_time_conversion<system_clock, system_clock> {
  template<class Duration>
    sys_time<Duration>
      operator()(const sys_time<Duration>& t) const;
};
template<class Duration> sys_time<Duration> operator()(const sys_time<Duration>& t) const;
Returns: t.
template<>
struct clock_time_conversion<utc_clock, utc_clock> {
  template<class Duration>
    utc_time<Duration>
      operator()(const utc_time<Duration>& t) const;
};
template<class Duration> utc_time<Duration> operator()(const utc_time<Duration>& t) const;
Returns: t.

25.7.9.3 Conversions between system_­clock and utc_­clock [time.clock.cast.sys.utc]

template<>
struct clock_time_conversion<utc_clock, system_clock> {
  template<class Duration>
    utc_time<common_type_t<Duration, seconds>>
      operator()(const sys_time<Duration>& t) const;
};
template<class Duration> utc_time<common_type_t<Duration, seconds>> operator()(const sys_time<Duration>& t) const;
Returns: utc_­clock::from_­sys(t).
template<>
struct clock_time_conversion<system_clock, utc_clock> {
  template<class Duration>
    sys_time<common_type_t<Duration, seconds>>
      operator()(const utc_time<Duration>& t) const;
};
template<class Duration> sys_time<common_type_t<Duration, seconds>> operator()(const utc_time<Duration>& t) const;
Returns: utc_­clock::to_­sys(t).

25.7.9.4 Conversions between system_­clock and other clocks [time.clock.cast.sys]

template<class SourceClock>
struct clock_time_conversion<system_clock, SourceClock> {
  template<class Duration>
    auto operator()(const time_point<SourceClock, Duration>& t) const
      -> decltype(SourceClock::to_sys(t));
};
template<class Duration> auto operator()(const time_point<SourceClock, Duration>& t) const -> decltype(SourceClock::to_sys(t));
Remarks: This function does not participate in overload resolution unless SourceClock::to_­sys(t) is well-formed.
If SourceClock::to_­sys(t) does not return sys_­time<Duration>, where Duration is a valid chrono::duration specialization, the program is ill-formed.
Returns: SourceClock::to_­sys(t).
template<class DestClock>
struct clock_time_conversion<DestClock, system_clock> {
  template<class Duration>
    auto operator()(const sys_time<Duration>& t) const
      -> decltype(DestClock::from_sys(t));
};
template<class Duration> auto operator()(const sys_time<Duration>& t) const -> decltype(DestClock::from_sys(t));
Remarks: This function does not participate in overload resolution unless DestClock::from_­sys(t) is well-formed.
If DestClock::from_­sys(t) does not return time_­point<DestClock, Duration>, where Duration is a valid chrono::duration specialization, the program is ill-formed.
Returns: DestClock::from_­sys(t).

25.7.9.5 Conversions between utc_­clock and other clocks [time.clock.cast.utc]

template<class SourceClock>
struct clock_time_conversion<utc_clock, SourceClock> {
  template<class Duration>
    auto operator()(const time_point<SourceClock, Duration>& t) const
      -> decltype(SourceClock::to_utc(t));
};
template<class Duration> auto operator()(const time_point<SourceClock, Duration>& t) const -> decltype(SourceClock::to_utc(t));
Remarks: This function does not participate in overload resolution unless SourceClock::to_­utc(t) is well-formed.
If SourceClock::to_­utc(t) does not return utc_­time<Duration>, where Duration is a valid chrono::duration specialization, the program is ill-formed.
Returns: SourceClock::to_­utc(t).
template<class DestClock>
struct clock_time_conversion<DestClock, utc_clock> {
  template<class Duration>
    auto operator()(const utc_time<Duration>& t) const
      -> decltype(DestClock::from_utc(t));
};
template<class Duration> auto operator()(const utc_time<Duration>& t) const -> decltype(DestClock::from_utc(t));
Remarks: This function does not participate in overload resolution unless DestClock::from_­utc(t) is well-formed.
If DestClock::from_­utc(t) does not return time_­point<DestClock, Duration>, where Duration is a valid chrono::duration specialization, the program is ill-formed.
Returns: DestClock::from_­utc(t).

25.7.9.6 Function template clock_­cast [time.clock.cast.fn]

template<class DestClock, class SourceClock, class Duration> auto clock_cast(const time_point<SourceClock, Duration>& t);
Remarks: This function does not participate in overload resolution unless at least one of the following clock time conversion expressions is well-formed:
  • clock_time_conversion<DestClock, SourceClock>{}(t)
    
  • clock_time_conversion<DestClock, system_clock>{}(
        clock_time_conversion<system_clock, SourceClock>{}(t))
    
  • clock_time_conversion<DestClock, utc_clock>{}(
        clock_time_conversion<utc_clock, SourceClock>{}(t))
    
  • clock_time_conversion<DestClock, utc_clock>{}(
        clock_time_conversion<utc_clock, system_clock>{}(
            clock_time_conversion<system_clock, SourceClock>{}(t)))
    
  • clock_time_conversion<DestClock, system_clock>{}(
        clock_time_conversion<system_clock, utc_clock>{}(
            clock_time_conversion<utc_clock, SourceClock>{}(t)))
    
A clock time conversion expression is considered better than another clock time conversion expression if it involves fewer operator() calls on clock_­time_­conversion specializations.
If, among the well-formed clock time conversion expressions from the above list, there is not a unique best expression, the clock_­cast is ambiguous and the program is ill-formed.
Returns: The best well-formed clock time conversion expression in the above list.