cppdraft_translate/cppdraft/time/clock/cast.md

[time.clock.cast]

30 Time library [time]

30.7 Clocks [time.clock]

30.7.10 time_point conversions [time.clock.cast]

30.7.10.1 Class template clock_time_conversion [time.clock.conv]

namespace std::chrono {template<class DestClock, class SourceClock>struct clock_time_conversion {};}

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clock_time_conversion serves as a trait which can be used to specify how to convert a source time_point of typetime_point<SourceClock, Duration> to a destination time_point of typetime_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.

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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].

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

templatestruct clock_time_conversion<Clock, Clock> {template time_point<Clock, Duration>operator()(const time_point<Clock, Duration>& t) const;};

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

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Returns: t.

template<>struct clock_time_conversion<system_clock, system_clock> {template sys_timeoperator()(const sys_time& t) const;};

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

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Returns: t.

template<>struct clock_time_conversion<utc_clock, utc_clock> {template utc_timeoperator()(const utc_time& t) const;};

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

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Returns: t.

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

template<>struct clock_time_conversion<utc_clock, system_clock> {template utc_time<common_type_t<Duration, seconds>>operator()(const sys_time& t) const;};

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template<class Duration> utc_time<common_type_t<Duration, seconds>> operator()(const sys_time<Duration>& t) const;

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Returns: utc_clock​::​from_sys(t).

template<>struct clock_time_conversion<system_clock, utc_clock> {template sys_time<common_type_t<Duration, seconds>>operator()(const utc_time& t) const;};

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template<class Duration> sys_time<common_type_t<Duration, seconds>> operator()(const utc_time<Duration>& t) const;

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Returns: utc_clock​::​to_sys(t).

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

templatestruct clock_time_conversion<system_clock, SourceClock> {templateauto operator()(const time_point<SourceClock, Duration>& t) const-> decltype(SourceClock::to_sys(t));};

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

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Constraints: SourceClock​::​to_sys(t) is well-formed.

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Mandates: SourceClock​::​to_sys(t) returns a sys_time for some type Duration2 ([time.point.general]).

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Returns: SourceClock​::​to_sys(t).

templatestruct clock_time_conversion<DestClock, system_clock> {templateauto operator()(const sys_time& t) const-> decltype(DestClock::from_sys(t));};

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

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Constraints: DestClock​::​from_sys(t) is well-formed.

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Mandates: DestClock​::​from_sys(t) returns a time_point<DestClock, Duration2> for some type Duration2 ([time.point.general]).

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Returns: DestClock​::​from_sys(t).

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

templatestruct clock_time_conversion<utc_clock, SourceClock> {templateauto operator()(const time_point<SourceClock, Duration>& t) const-> decltype(SourceClock::to_utc(t));};

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

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Constraints: SourceClock​::​to_utc(t) is well-formed.

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Mandates: SourceClock​::​to_utc(t) returns a utc_time for some type Duration2 ([time.point.general]).

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Returns: SourceClock​::​to_utc(t).

templatestruct clock_time_conversion<DestClock, utc_clock> {templateauto operator()(const utc_time& t) const-> decltype(DestClock::from_utc(t));};

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

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Constraints: DestClock​::​from_utc(t) is well-formed.

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Mandates: DestClock​::​from_utc(t) returns a time_point<DestClock, Duration2> for some type Duration2 ([time.point.general]).

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Returns: DestClock​::​from_utc(t).

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

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template<class DestClock, class SourceClock, class Duration> auto clock_cast(const time_point<SourceClock, Duration>& t);

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Constraints: At least one of the following clock time conversion expressions is well-formed:

• (1.1)

clock_time_conversion<DestClock, SourceClock>{}(t)

• (1.2)

clock_time_conversion<DestClock, system_clock>{}( clock_time_conversion<system_clock, SourceClock>{}(t))

• (1.3)

clock_time_conversion<DestClock, utc_clock>{}( clock_time_conversion<utc_clock, SourceClock>{}(t))

• (1.4)

clock_time_conversion<DestClock, utc_clock>{}( clock_time_conversion<utc_clock, system_clock>{}( clock_time_conversion<system_clock, SourceClock>{}(t)))

• (1.5)

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 feweroperator() calls on clock_time_conversion specializations.

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Mandates: Among the well-formed clock time conversion expressions from the above list, there is a unique best expression.

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Returns: The best well-formed clock time conversion expression in the above list.