27 Time library [time]

27.4 Time-related traits [time.traits]

27.4.3 Specializations of common_­type [time.traits.specializations]

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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>; };
1
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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]
2
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[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>>; };
3
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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.