std::chrono::clock_time_conversion(3)

NAME
std::chrono::clock_time_conversion - std::chrono::clock_time_conversion

Synopsis
   Defined in header <chrono>
   template< class Dest, class Source > (since C++20)
   struct clock_time_conversion {};
   std::chrono::clock_time_conversion is a trait that specifies how to convert a
   std::chrono::time_point of the Source clock to that of the Dest clock. It does so by
   providing a const-callable operator() that accepts an argument of type
   std::chrono::time_point<Source, Duration> and returns a
   std::chrono::time_point<Dest, OtherDuration> that represents an equivalent point in
   time. The duration of the returned time point is computed from the source duration
   in a manner that varies for each specialization. clock_time_conversion is normally
   only used indirectly, via std::chrono::clock_cast.
   A program may specialize clock_time_conversion if at least one of the template
   parameters is a user-defined clock type.
   The primary template is an empty struct. The standard defines the following
   specializations:
   template< class Clock > (1) (since C++20)
   struct clock_time_conversion<Clock, Clock>;
   template<>
   struct clock_time_conversion<std::chrono::system_clock, (2) (since C++20)
   std::chrono::system_clock>;
   template<>
   struct clock_time_conversion<std::chrono::utc_clock, (3) (since C++20)
   std::chrono::utc_clock>;
   template<>
   struct clock_time_conversion<std::chrono::system_clock, (4) (since C++20)
   std::chrono::utc_clock>;
   template<>
   struct clock_time_conversion<std::chrono::utc_clock, (5) (since C++20)
   std::chrono::system_clock>;
   template< class Clock > (6) (since C++20)
   struct clock_time_conversion<Clock, std::chrono::system_clock>;
   template< class Clock > (7) (since C++20)
   struct clock_time_conversion<std::chrono::system_clock, Clock>;
   template< class Clock > (8) (since C++20)
   struct clock_time_conversion<Clock, std::chrono::utc_clock>;
   template< class Clock > (9) (since C++20)
   struct clock_time_conversion<std::chrono::utc_clock, Clock>;
   1-3) Identity conversion: operator() returns a copy of the argument.
   4,5) Conversions between std::chrono::sys_time and std::chrono::utc_time: operator()
   calls std::chrono::utc_clock::to_sys and std::chrono::utc_clock::from_sys,
   respectively.
   6,7) Conversions to and from std::chrono::sys_time when Clock supports from_sys and
   to_sys: operator() calls Clock::to_sys and Clock::from_sys, respectively.
   8,9) Conversions to and from std::chrono::utc_time when Clock supports from_utc and
   to_utc: operator() calls Clock::to_utc and Clock::from_utc, respectively.

Member functions
   Each specialization has an implicitly-declared default constructor, copy
   constructor, move constructor, copy assignment operator, move assignment operator,
   and destructor. std::chrono::clock_time_conversion::operator()
   template< class Duration >
   std::chrono::time_point<Clock, Duration> (1) (member of
                        specialization (1))
       operator()( const std::chrono::time_point<Clock,
   Duration>& t ) const;
   template< class Duration >
   std::chrono::sys_time<Duration> (2) (member of
                        specialization (2))
       operator()( const std::chrono::sys_time<Duration> & t )
   const;
   template< class Duration >
   std::chrono::utc_time<Duration> (3) (member of
                        specialization (3))
       operator()( const std::chrono::utc_time<Duration>& t )
   const;
   template< class Duration >
   std::chrono::sys_time<Duration> (4) (member of
                        specialization (4))
       operator()( const std::chrono::utc_time<Duration>& t )
   const;
   template< class Duration >
   std::chrono::utc_time<Duration> (5) (member of
                        specialization (5))
       operator()( const std::chrono::sys_time<Duration>& t )
   const;
   template< class Duration >
   auto operator()( const std::chrono::sys_time<Duration>& t ) (6) (member of
   const specialization (6))
       -> decltype(Clock::from_sys(t));
   template< class Duration >
   auto operator()( const std::chrono::time_point<SourceClock, (7) (member of
   Duration>& t ) const specialization (7))
       -> decltype(Clock::to_sys(t));
   template< class Duration >
   auto operator()( const std::chrono::utc_time<Duration>& t ) (8) (member of
   const specialization (8))
       -> decltype(Clock::from_utc(t));
   template< class Duration >
   auto operator()( const std::chrono::time_point<Clock, (9) (member of
   Duration>& t ) const specialization (9))
       -> decltype(Clock::to_utc(t));
   Converts the argument std::chrono::time_point to the destination clock.
   1-3) Identity conversion. Returns t unchanged.
   4) Returns std::chrono::utc_clock::to_sys(t).
   5) Returns std::chrono::utc_clock::from_sys(t).
   6) Returns Clock::from_sys(t). This overload participates in overload resolution
   only if the expression Clock::from_sys(t) is well-formed. The program is ill-formed
   if Clock::from_sys(t) does not return std::chrono::time_point<Clock, Duration> where
   Duration is some valid specialization of std::chrono::duration.
   7) Returns Clock::to_sys(t). This overload participates in overload resolution only
   if the expression Clock::to_sys(t) is well-formed. The program is ill-formed if
   Clock::to_sys(t) does not return std::chrono::sys_time<Duration> where Duration is
   some valid specialization of std::chrono::duration.
   8) Returns Clock::from_utc(t). This overload participates in overload resolution
   only if the expression Clock::from_utc(t) is well-formed. The program is ill-formed
   if Clock::from_utc(t) does not return std::chrono::time_point<Clock, Duration> where
   Duration is some valid specialization of std::chrono::duration.
   9) Returns Clock::to_utc(t). This overload participates in overload resolution only
   if the expression Clock::to_utc(t) is well-formed. The program is ill-formed if
   Clock::to_utc(t) does not return std::chrono::utc_time<Duration> where Duration is
   some valid specialization of std::chrono::duration.

Parameters
   t - time point to convert

Return value
   The result of the conversion as described above:
   1-3) t.
   4) std::chrono::utc_clock::to_sys(t).
   5) std::chrono::utc_clock::from_sys(t).
   6) Clock::from_sys(t).
   7) Clock::to_sys(t).
   8) Clock::from_utc(t).
   9) Clock::to_utc(t).

See also
   clock_cast convert time points of one clock to another
   (C++20) (function template)