< cpp‎ | chrono‎ | duration
Utilities library
General utilities
Date and time
Function objects
Formatting library (C++20)
Relational operators (deprecated in C++20)
Integer comparison functions
Swap and type operations
Common vocabulary types
Elementary string conversions
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.


[edit] Parameters

d - duration to convert

[edit] Return value

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

[edit] 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;

[edit] Example

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


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

[edit] See also

converts a duration to another, with a different tick interval
(function template) [edit]
converts a duration to another, rounding down
(function template) [edit]
converts a duration to another, rounding up
(function template) [edit]
converts a time_point to another, rounding to nearest, ties to even
(function template) [edit]
nearest integer, rounding away from zero in halfway cases
(function) [edit]