NAME¶

std::chrono::round(std::chrono::time_point) - std::chrono::round(std::chrono::time_point)

Synopsis¶

Defined in header <chrono>
template< class ToDuration, class Clock, class Duration >

constexpr std::chrono::time_point<Clock, ToDuration> (since C++17)

round( const std::chrono::time_point<Clock, Duration>& tp );

Returns the nearest time point to tp representable in ToDuration, rounding to even
in halfway cases.

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¶

tp - time point to round to nearest

Return value¶

tp rounded to nearest time point using 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 Clock, class FromDuration,
class = std::enable_if_t<detail::is_duration_v<To> &&
!std::chrono::treat_as_floating_point_v<typename To::rep>>>
constexpr std::chrono::time_point<Clock, To> round(
const std::chrono::time_point<Clock, FromDuration>& tp)
{
return std::chrono::time_point<Clock, To>{
std::chrono::round<To>(tp.time_since_epoch())};
}

Example¶

// Run this code

#include <chrono>
#include <iostream>
#include <string>

template<typename TimePoint>
std::string to_string(const TimePoint& time_point)
{
return std::to_string(time_point.time_since_epoch().count());
}

int main()
{
using namespace std::literals::chrono_literals;
using Sec = std::chrono::seconds;

std::cout << "Time point\t" "Cast\t" "Floor\t" "Round\t" "Ceil\n";
std::cout << "(ms)\t\t" "(s)\t" "(s)\t" "(s)\t" "(s)\n";
for (const auto value_ms : {5432ms, 5678ms})
{
std::chrono::time_point<std::chrono::system_clock, std::chrono::milliseconds>
time_point_ms(value_ms);

std::cout
<< to_string(time_point_ms) << "\t\t"
<< to_string(std::chrono::time_point_cast<Sec>(time_point_ms)) << '\t'
<< to_string(std::chrono::floor<Sec>(time_point_ms)) << '\t'
<< to_string(std::chrono::round<Sec>(time_point_ms)) << '\t'
<< to_string(std::chrono::ceil<Sec>(time_point_ms)) << '\n';
}
}

Output:¶

Time point Cast Floor Round Ceil
(ms) (s) (s) (s) (s)
5432 5 5 5 6
5678 5 5 6 6

See also¶

time_point_cast converts a time point to another time point on the
(C++11) same clock, with a different duration
(function template)
ceil(std::chrono::time_point) converts a time_point to another, rounding up
(C++17) (function template)
floor(std::chrono::time_point) converts a time_point to another, rounding down
(C++17) (function template)
round(std::chrono::duration) converts a duration to another, rounding to nearest,
(C++17) 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)