29 Time library [time]

29.4 Time-related traits [time.traits]

29.4.3 Specializations of common_type [time.traits.specializations]

template<class Rep1, class Period1, class Rep2, class Period2>
  struct common_type<chrono::duration<Rep1, Period1>, chrono::duration<Rep2, Period2>> {
    using type = chrono::duration<common_type_t<Rep1, Rep2>, see below>;
  };

The period of the duration indicated by this specialization of common_type is the greatest common divisor of Period1 and Period2.

[Note 1:

This can be computed by forming a ratio of the greatest common divisor of Period1::num and Period2::num and the least common multiple of Period1::den and Period2::den.

— end note]

[Note 2:

The typedef name type is a synonym for the duration with the largest tick period possible where both duration arguments will convert to it without requiring a division operation.

The representation of this type is intended to be able to hold any value resulting from this conversion with no truncation error, although floating-point durations can have round-off errors.

— end note]

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template<class Clock, class Duration1, class Duration2>
  struct common_type<chrono::time_point<Clock, Duration1>, chrono::time_point<Clock, Duration2>> {
    using type = chrono::time_point<Clock, common_type_t<Duration1, Duration2>>;
  };

The common type of two time_point types is a time_point with the same clock as the two types and the common type of their two durations.