clocks

Overview

A clock is a device that has a starting time or epoch time and a resolution or period. The epoch time can vary based on the clock type. For a wall clock, epoch time starts from  1970.1.1 12:00 GMT, Chrono library defines three clocks as defined below. 

Details

Topics
system_clock
steady_clock
high_resolution_clock

system_clock 

system_clock represents the system-wide real time wall clock. Its attributes are:

• It is intended to represent the real time, and thus it can be translated in some way to and from calendar representations. 
• Its time_point values can refer to times before the epoch (with negative values).
• All processes running on the system shall retrieve the same time_point values by using this clock.

member types

Name	Description
rep	signed arithmetic type representing the number of ticks in the clock's duration
period	ratio type representing the tick period of the clock, in seconds.
duration	duration<rep, period>, capable of representing negative durations.
time_point	time_point<system_clock>

member constants

Name	Description
bool is_steady	A bool value specifying whether the clock always advances. true if the time between ticks is always constant and increasing.
Example /* prints Monotonic :false Resolution :ratio<1,1000000000> Now :1740224214 seconds */ cout << "Monotonic\t:" << boolalpha << system_clock::is_steady << endl; cout << "Resolution\t:" << "ratio<" << system_clock::period::num << "," << system_clock::period::den << ">" << endl; cout << "Now\t\t:" << duration_cast<seconds>(system_clock::now().time_since_epoch()).count() << " seconds" << endl;

member functions

Name	Description
time_point system_clock::now()	Returns a time_point representing the current point in time. Example //prints:[1740146546277738489, <1,1000000000>] cout << system_clock::time_point(system_clock::now()).time_since_epoch();
time_t to_time_t (const time_point& tp)	Converts tp into its equivalent of type time_t.
system_clock::time_point system_clock::from_time_t (time_t t)	Converts t to a time point type, using the coarser precision of the two types.
Example time_t today = system_clock::to_time_t(system_clock::now()); //prints:[1740223822000000000, <1,1000000000>] cout << system_clock::from_time_t(today).time_since_epoch() << endl;

steady_clock 

steady_clock represents a monotonic clock. The time points of this clock cannot decrease as physical time moves forward and the time between ticks of this clock is constant. This clock is not related to wall clock time (for example, it can be time since last reboot), and is most suitable for measuring intervals.

It's monotonic as its now() never returns a lower value than in a previous call.

member types

Name	Description
rep	signed arithmetic type representing the number of ticks in the clock's duration
period	ratio type representing the tick period of the clock, in seconds.
duration	duration<rep, period>, capable of representing negative durations.
time_point	time_point<steady_clock>

member constants

Name	Description
bool is_steady	A bool value specifying whether the clock always advances. true if the time between ticks is always constant and increasing.
Example /* prints Monotonic :true Resolution :ratio<1,1000000000> Now :6025161 seconds */ cout << "Monotonic\t:" << boolalpha << steady_clock::is_steady << endl; cout << "Resolution\t:" << "ratio<" << steady_clock::period::num << "," << steady_clock::period::den << ">" << endl; cout << "Now\t\t:" << duration_cast<seconds>(steady_clock::now().time_since_epoch()).count() << " seconds" << endl;

member functions

Name	Description
time_point steady_clock::now()	Returns a time_point representing the current point in time. Example //prints:[6025308175775127, <1,1000000000>] cout << steady_clock::time_point(steady_clock::now()).time_since_epoch();

high_resolution_clock 

high_resolution_clock is the clock with the shortest tick period. It may be a synonym for system_clock or steady_clock.

The clock represents the clock with the smallest tick period provided by the implementation. It may be an alias of system_clock or steady_clock, or a third, independent clock.

The members of clock classes provide access to the current time_point.

member types

Name	Description
rep	signed arithmetic type representing the number of ticks in the clock's duration
period	ratio type representing the tick period of the clock, in seconds.
duration	duration<rep, period>, capable of representing negative durations.
time_point	time_point<high_resolution_clock >

member constants

Name	Description
bool is_steady	A bool value specifying whether the clock always advances. true if the time between ticks is always constant and increasing.
Example /* prints Monotonic :false Resolution :ratio<1,1000000000> Now :1740247378 seconds */ cout << "Monotonic\t:" << boolalpha << high_resolution_clock ::is_steady << endl; cout << "Resolution\t:" << "ratio<" << high_resolution_clock ::period::num << "," << high_resolution_clock ::period::den << ">" << endl; cout << "Now\t\t:" << duration_cast<seconds>(high_resolution_clock ::now().time_since_epoch()).count() << " seconds" << endl;

member functions

Name	Description
time_point steady_clock::now()	Returns a time_point representing the current point in time. Example //prints:[6025308175775127, <1,1000000000>] cout << high_resolution_clock ::time_point(high_resolution_clock ::now()).time_since_epoch();