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std::chrono::duration<Rep,Period>::duration - cppreference.com

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constexpr duration() = default;
(1) (since C++11) 
duration( const duration& ) = default;
(2) (since C++11) 
template< class Rep2 >
constexpr explicit duration( const Rep2& r );
(3) (since C++11) 
template< class Rep2, class Period2 >
constexpr duration( const duration<Rep2, Period2>& d );
(4) (since C++11)

Constructs a new duration from one of several optional data sources.

1) The default constructor.

2) The copy constructor.

3) Constructs a duration with r ticks.

This overload participates in overload resolution only if all following conditions are satisfied:

  • is_convertible<const Rep2&, Rep>::value is true.
  • Any of the following conditions is satisfied:[1]
  • std::chrono::treat_as_floating_point<Rep>::value is true.
  • std::chrono::treat_as_floating_point<Rep2>::value is false.

4) Constructs a duration by converting d to an appropriate period and tick count, as if by std::chrono::duration_cast<duration>(d).count().

This overload participates in overload resolution only if no overflow is induced in the conversion, and any of the following conditions is satisfied:[2]

  • std::chrono::treat_as_floating_point<Rep>::value is true.
  • All following conditions are satisfied:
  • std::ratio_divide<Period2, Period>::den is 1.
  • std::chrono::treat_as_floating_point<Rep2>::value is false.
  1. ↑ 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.
  2. ↑ 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:

#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 2094 C++11 for overload (4), std::ratio_divide<Period2, period>::num
might overflow when evaluating
std::ratio_divide<Period2, period>::den 		overload (4) does not
participate in overload
resolution in this case
LWG 3050 C++11 convertibility constraint used non-const xvalue use const lvalues instead

See also