NAME¶

std::chrono::round(std::chrono::duration) - std::chrono::round(std::chrono::duration)

Synopsis¶

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

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