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std::chrono::round(std::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

#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

(C++11)
 converts a duration to another, with a different tick interval
(function template)
(C++17)
 converts a duration to another, rounding down
(function template)
(C++17)
 converts a duration to another, rounding up
(function template)
(C++17)
 converts a time_point to another, rounding to nearest, ties to even
(function template)
(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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