20 General utilities library [utilities]

20.12 Time utilities [time]

20.12.7 Clocks [time.clock]

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The types defined in this subclause shall satisfy the TrivialClock requirements ([time.clock.req]).

20.12.7.1 Class system_clock [time.clock.system]

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Objects of class system_clock represent wall clock time from the system-wide realtime clock.

class system_clock {
public:
  typedef see below                           rep;
  typedef ratio<unspecified, unspecified>   period;
  typedef chrono::duration<rep, period>       duration;
  typedef chrono::time_point<system_clock>    time_point;
  static constexpr bool is_steady =        unspecified;

  static time_point now() noexcept;

  // Map to C API
  static time_t      to_time_t  (const time_point& t) noexcept;
  static time_point  from_time_t(time_t t) noexcept;
};

typedef unspecified system_clock::rep;

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Requires: system_clock::duration::min() < system_clock::duration::zero() shall be true. [ Note: This implies that rep is a signed type.  — end note ]

static time_t to_time_t(const time_point& t) noexcept;

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Returns: A time_t object that represents the same point in time as t when both values are restricted to the coarser of the precisions of time_t and time_point. It is implementation defined whether values are rounded or truncated to the required precision.

static time_point from_time_t(time_t t) noexcept;

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Returns: A time_point object that represents the same point in time as t when both values are restricted to the coarser of the precisions of time_t and time_point. It is implementation defined whether values are rounded or truncated to the required precision.

20.12.7.2 Class steady_clock [time.clock.steady]

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Objects of class steady_clock represent clocks for which values of time_point never decrease as physical time advances and for which values of time_point advance at a steady rate relative to real time. That is, the clock may not be adjusted.

class steady_clock {
public:
  typedef unspecified                               rep;
  typedef ratio<unspecified, unspecified>          period;
  typedef chrono::duration<rep, period>              duration;
  typedef chrono::time_point<unspecified, duration>  time_point;
  static constexpr bool is_steady =               true;

  static time_point now() noexcept;
};

20.12.7.3 Class high_resolution_clock [time.clock.hires]

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Objects of class high_resolution_clock represent clocks with the shortest tick period. high_resolution_clock may be a synonym for system_clock or steady_clock.

class high_resolution_clock {
public:
  typedef unspecified                               rep;
  typedef ratio<unspecified, unspecified>          period;
  typedef chrono::duration<rep, period>              duration;
  typedef chrono::time_point<unspecified, duration> time_point;
  static constexpr bool is_steady =               unspecified;

  static time_point now() noexcept;
};