23 General utilities library [utilities]

23.17 Time utilities [time]

23.17.7 Clocks [time.clock]

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

23.17.7.1 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:
  using rep        = see below;
  using period     = ratio<unspecified, unspecified>;
  using duration   = chrono::duration<rep, period>;
  using time_point = chrono::time_point<system_clock>;
  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;
};
using system_clock::rep = unspecified;
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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.

23.17.7.2 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:
  using rep        = unspecified;
  using period     = ratio<unspecified, unspecified>;
  using duration   = chrono::duration<rep, period>;
  using time_point = chrono::time_point<unspecified, duration>;
  static constexpr bool is_steady = true;

  static time_point now() noexcept;
};

23.17.7.3 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:
  using rep        = unspecified;
  using period     = ratio<unspecified, unspecified>;
  using duration   = chrono::duration<rep, period>;
  using time_point = chrono::time_point<unspecified, duration>;
  static constexpr bool is_steady = unspecified;

  static time_point now() noexcept;
};