std::chrono

C++ includes support for two types of time manipulation:

• The chrono library, a flexible collection of types that track time with varying degrees of precision (e.g. std::chrono::time_point).
• C-style date and time library (e.g. std::time)

std::chrono library

The chrono library defines three main types as well as utility functions and common typedefs.

• clocks
  • A clock consists of a starting point (or epoch) and a tick rate.
  • For example, a clock may have an epoch of January 1, 1970 and tick every second.
• time points
  • A time point is a duration of time that has passed since the epoch of a specific clock.
• durations
  • A duration consists of a span of time, defined as some number of ticks of some time unit.
  • For example, "42 seconds" could be represented by a duration consisting of 42 ticks of a 1-second time unit.
• Time of day (since C++20)
  • hh_mm_ss splits a duration representing time elapsed since midnight into hours, minutes, seconds, and fractional seconds, as applicable.
  • It is primarily a formatting tool.
  • gcc-10 not supported yet
• Calendar (since C++20)
• Time zone (since C++20)

std::duration and std::ratio

template<
    class Rep,  // an arithmetic type representing the number of ticks
    class Period = std::ratio<1> // a std::ratio representing the tick period
                                 // (the number of second's fractions per tick)
> class duration;

• The actual time interval (in seconds) that is held by a duration object d is roughly equal to d.count() * D::period::num / D::period::den, where D is of type chrono::duration<> and d is an object of such type.

where #include <ratio> defines:

template<
    std::intmax_t Num,       // numerator
    std::intmax_t Denom = 1  // denominator
> class ratio;

• The static data members num and den representing the numerator and denominator are calculated by dividing Num and Denom by their greatest common divisor.
  • e.g. std::ratio<3, 6>::type is std::ratio<1, 2>.
• Convenient type are defined:
  • nano: std::ratio<1, 1000000000>
  • micro: std::ratio<1, 1000000>
  • ...

Note that std::nano is a ratio, and std::nanoseconds is a duration!