std::chrono::duration::duration(3)

NAME
std::chrono::duration::duration - std::chrono::duration::duration

Synopsis
   constexpr duration() = default; (1) (since C++11)
   duration( const duration& ) = default; (2) (since C++11)
   template< class Rep2 > (3) (since C++11)
   constexpr explicit duration( const Rep2& r );
   template< class Rep2, class Period2 > (4) (since C++11)
   constexpr duration( const duration<Rep2,Period2>& d );
   Constructs a new duration from one of several optional data sources.
   1) The default constructor is defaulted.
   2) The copy constructor is defaulted (makes a bitwise copy of the tick count).
   3) Constructs a duration with r ticks. Note that this constructor only participates
   in overload resolution if const Rep2& (the argument type) is implicitly convertible
   to rep (the type of this duration's ticks) and
                        * std::chrono::treat_as_floating_point<rep>::value is true,
                        or
                        * std::chrono::treat_as_floating_point<Rep2>::value is
                        false.
   (that is, a duration with an integer tick count cannot be constructed from a
   floating-point value, but a duration with a floating-point tick count can be
   constructed from an integer value)
   4) Constructs a duration by converting d to an appropriate period and tick count, as
   if by std::chrono::duration_cast<duration>(d).count(). In order to prevent
   truncation during conversion, this constructor only participates in overload
   resolution if computation of the conversion factor (by std::ratio_divide<Period2,
   Period>) does not overflow and:
                        * std::chrono::treat_as_floating_point<rep>::value == true
   or both:
                        * std::ratio_divide<Period2, period>::den == 1, and
                        * std::chrono::treat_as_floating_point<Rep2>::value ==
                        false.
   (that is, either the duration uses floating-point ticks, or Period2 is exactly
   divisible by period)

Parameters
   r - a tick count
   d - a duration to copy from

Example
   The following code shows several examples (both valid and invalid) of how to
   construct durations: // Run this code
 #include <chrono>
 int main()
 {
     std::chrono::hours h(1); // one hour
     std::chrono::milliseconds ms{3}; // 3 milliseconds
     std::chrono::duration<int, std::kilo> ks(3); // 3000 seconds
     // error: treat_as_floating_point<int>::value == false,
     // This duration allows whole tick counts only
 // std::chrono::duration<int, std::kilo> d3(3.5);
     // 30Hz clock using fractional ticks
     std::chrono::duration<double, std::ratio<1, 30>> hz30(3.5);
     // 3000 microseconds constructed from 3 milliseconds
     std::chrono::microseconds us = ms;
     // error: 1/1000000 is not divisible by 1/1000
 // std::chrono::milliseconds ms2 = us
     std::chrono::duration<double, std::milli> ms2 = us; // 3.0 milliseconds
 }
  Defect reports
   The following behavior-changing defect reports were applied retroactively to
   previously published C++ standards.
      DR Applied to Behavior as published Correct behavior
   LWG 3050 C++11 convertibility constraint used non-const use const lvalues
                       xvalue instead

See also
   operator= assigns the contents
             (public member function)