# std::chrono::round(std::chrono::duration)

From cppreference.com
< cpp‎ | chrono‎ | duration

Defined in header `<chrono>` |  |
template <class ToDuration, class Rep, class Period> constexpr ToDuration round( const std::chrono::duration<Rep, Period>& d ); |  | (since C++17)

Returns the value `t` representable in `ToDuration` that is the closest to `d`. If there are two such values, returns the even value (that is, the value `t` such that t % 2 == 0).

The function does not participate in the overload resolution unless `ToDuration` is a specialization of std::chrono::duration and std::chrono::treat_as_floating_point_v<typename ToDuration::rep> is false

### Parameters

d | - | duration to convert

### Return value

`d` rounded to the nearest duration of type `ToDuration`, rounding to even in halfway cases.

### Possible implementation

    namespace detail {
    template<class> inline constexpr bool is_duration_v = false;
    template<class Rep, class Period> inline constexpr bool is_duration_v<
        std::chrono::duration<Rep, Period>> = true;
    }

    template <class To, class Rep, class Period,
              class = std::enable_if_t<detail::is_duration_v<To> &&
                     !std::chrono::treat_as_floating_point_v<typename To::rep>>>
    constexpr To round(const std::chrono::duration<Rep, Period>& d)
    {
        To t0 = std::chrono::floor<To>(d);
        To t1 = t0 + To{1};
        auto diff0 = d - t0;
        auto diff1 = t1 - d;
        if (diff0 == diff1) {
            if (t0.count() & 1)
                return t1;
            return t0;
        } else if (diff0 < diff1) {
            return t0;
        }
        return t1;
    }

### Example

Run this code

    #include <iostream>
    #include <iomanip>
    #include <chrono>

    int main()
    {
        using namespace std::chrono_literals;
        using Sec = std::chrono::seconds;
        for (std::cout << "Duration\tFloor\tRound\tCeil\n"
                          "(ms)\t\t(sec)\t(sec)\t(sec)\n";
            auto const d: {
                +4999ms, +5000ms, +5001ms, +5499ms, +5500ms, +5999ms,
                -4999ms, -5000ms, -5001ms, -5499ms, -5500ms, -5999ms, }) {
            std::cout << std::showpos << d.count() << "\t\t"
                      << std::chrono::floor<Sec>(d).count() << '\t'
                      << std::chrono::round<Sec>(d).count() << '\t'
                      << std::chrono::ceil <Sec>(d).count() << '\n';
        }
    }

Output:

    Duration   Floor   Round   Ceil
    (ms)       (sec)   (sec)   (sec)
    +4999      +4      +5      +5
    +5000      +5      +5      +5
    +5001      +5      +5      +6
    +5499      +5      +5      +6
    +5500      +5      +6      +6
    +5999      +5      +6      +6
    -4999      -5      -5      -4
    -5000      -5      -5      -5
    -5001      -6      -5      -5
    -5499      -6      -5      -5
    -5500      -6      -6      -5
    -5999      -6      -6      -5

### See also

duration_cast (C++11) | converts a duration to another, with a different tick interval (function template)
floor(std::chrono::duration) (C++17) | converts a duration to another, rounding down (function template)
ceil(std::chrono::duration) (C++17) | converts a duration to another, rounding up (function template)
round(std::chrono::time_point) (C++17) | converts a time_point to another, rounding to nearest, ties to even (function template)
round roundf roundl lround lroundf lroundl llround llroundf llroundl (C++11)(C++11)(C++11)(C++11)(C++11)(C++11)(C++11)(C++11)(C++11) | nearest integer, rounding away from zero in halfway cases (function)

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