27 Time library [time]

27.3 Cpp17Clock requirements [time.clock.req]

A clock is a bundle consisting of a duration, a time_­point, and a function now() to get the current time_­point.
The origin of the clock's time_­point is referred to as the clock's epoch.
A clock shall meet the requirements in Table 86.
In Table 86 C1 and C2 denote clock types.
t1 and t2 are values returned by C1::now() where the call returning t1 happens before ([intro.multithread]) the call returning t2 and both of these calls occur before C1::time_­point::max().
This means C1 did not wrap around between t1 and t2.
end note
Table 86: Cpp17Clock requirements   [tab:time.clock]
Return type
Operational semantics
An arithmetic type or a class emulating an arithmetic type
The representation type of C1::duration.
a specialization of ratio
The tick period of the clock in seconds.
chrono::duration<C1::rep, C1::period>
The duration type of the clock.
chrono::time_­point<C1> or chrono::time_­point<C2, C1::duration>
The time_­point type of the clock.
C1 and C2 shall refer to the same epoch.
const bool
true if t1 <= t2 is always true and the time between clock ticks is constant, otherwise false.
Returns a time_­point object representing the current point in time.
The relative difference in durations between those reported by a given clock and the SI definition is a measure of the quality of implementation.
end note
A type TC meets the Cpp17TrivialClock requirements if: