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system_clock structure

A clock based on the system's real-time clock.

Syntax

struct system_clock;

Remarks

This clock represents the system-wide wall clock.

The system_clock isn't monotonic or steady.

A clock is monotonic if the value that is returned by a first call to now() is always less than or equal to the value that is returned by a later call to now(). The system_clock isn't monotonic because the system time may be adjusted between calls to now().

A clock is steady if it is monotonic and if the time between clock ticks is constant. Because the system_clock isn't monotonic, it isn't steady, although the time between clock ticks is constant. You can only use a steady clock to take the time before an event, the time after an event, and reliably subtract them to get the duration of the event because a steady clock won't be adjusted during that time.

Members

Convenience type aliases

Name Description
system_clock::duration In Microsoft's implementation, it's a synonym for duration<long long, ratio<1, 10'000'000>. It represents a duration of time measured in units of 100 nanoseconds.
system_clock::period In Microsoft's implementation, it's a synonym for ratio<1, 10'000'000>. Represents the time in fractions of a second (100 nanoseconds) between each tick in the duration.
system_clock::rep A synonym for long long. It's the type used to represent the integral units in this clock's system_clock::duration.
system_clock::time_point A synonym for time_point<system_clock>. Represents a time_point for this clock.

Related

Name Description
sys_days A synonym for using sys_days = sys_time<days>. A count of days, represented by a time_point that is associated with a system_clock. Defined in std::chrono
sys_seconds A synonym for using sys_seconds = sys_time<seconds>. A count of seconds, represented by a time_point that is associated with a system_clock. Defined in std::chrono
sys_time A synonym for template <class Duration> using sys_time = time_point<system_clock, Duration>. Represents a time_point for a system_clock. You specify the duration. Defined in std::chrono.

Public methods

Name Description
from_time_t Static. Returns a time_point for a system_clock that most closely approximates the specified time_t.
now Static. Returns the current time.
to_time_t Static. Returns a time_t object that most closely approximates the specified time_point.

Public constants

Name Description
system_clock::is_steady Indicates whether the clock type is steady. It's false

Requirements

Header: <chrono>

Namespace: std::chrono

from_time_t

Static method that returns a time_point that most closely approximates the time that is represented by Tm.

static time_point from_time_t(time_t Tm) noexcept;

Parameters

Tm
A time_t object.

is_steady

A static value that specifies whether the clock type is steady. Because the system_clock isn't steady, you can't use this clock to take the time before an event, the time after an event, and reliably subtract them to get the duration of the event because the clock may be adjusted during the timing interval.

static const bool is_steady = false;

Return Value

In this implementation, system_clock::is_steady always returns false.

Remarks

A clock is steady if it is monotonic and if the time between clock ticks is constant. The system_clock isn't monotonic because the value that is returned by a first call to now() isn't always less than or equal to the value that is returned by a later call to now() because the system time can be adjusted without notice.

now

Static method that returns the system's current wall-clock time.

static time_point now() noexcept;

Return Value

A time_point that represents the current time.

to_time_t

Static method that returns a time_t that most closely approximates the time that is represented by Time.

static time_t to_time_t(const time_point& Time) noexcept;

Parameters

Time
A time_point object.

See also

<chrono>
file_clock class
high_resolution_clock
steady_clock struct
tai_clock class
utc_clock class
Header Files Reference