# std::chrono::duration<Rep,Period>::duration

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< cpp‎ | chrono‎ | duration

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 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 `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
    }

### See also

operator= | assigns the contents (public member function)

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