27 Time library [time]

27.5 Class template duration [time.duration]

27.5.2 Constructors [time.duration.cons]

template<class Rep2> constexpr explicit duration(const Rep2& r);
Constraints: is_­convertible_­v<const Rep2&, rep> is true and
  • treat_­as_­floating_­point_­v<rep> is true or
  • treat_­as_­floating_­point_­v<Rep2> is false.
[Example 1: duration<int, milli> d(3); // OK duration<int, milli> d(3.5); // error — end example]
Postconditions: count() == static_­cast<rep>(r).
template<class Rep2, class Period2> constexpr duration(const duration<Rep2, Period2>& d);
Constraints: No overflow is induced in the conversion and treat_­as_­floating_­point_­v<rep> is true or both ratio_­divide<Period2, period>​::​den is 1 and treat_­as_­floating_­point_­v<Rep2> is false.
[Note 1:
This requirement prevents implicit truncation error when converting between integral-based duration types.
Such a construction could easily lead to confusion about the value of the duration.
— end note]
[Example 2: duration<int, milli> ms(3); duration<int, micro> us = ms; // OK duration<int, milli> ms2 = us; // error — end example]
Effects: Initializes rep_­ with duration_­cast<duration>(d).count().