std::chrono...:time_point)(3)

🎀 std::chrono::round(std::chrono::time_point) 🎀

Hey bestie 💖 this is the <std::chrono...:time_point)(3) page, straight outta the C++ Standard Libary manual (yes, that spelling is exactly what's up there, and we are not fixing it, periodt). This one is giving main character energy for rounding a time point to the nearest representable value, no cap.

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💅 NAME 💅

std::chrono::round(std::chrono::time_point) — std::chrono::round(std::chrono::time_point)

Yes, the NAME section literally lists it twice. We keep both because details are sacred. 🫶

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🌸 Synopsis 🌸

Defined in header <chrono>:

template< class ToDuration, class Clock, class Duration >

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

since C++17 ✨

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.

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🦋 Parameters 🦋

• tp — time point to round to nearest

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✨ Return value ✨

tp rounded to nearest time point using duration of type ToDuration, rounding to even in halfway cases.

That halfway-tie behavior is the whole vibe check: ties go to even, bestie. 💅

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🧚 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())};
}

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🍓 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

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👑 See also 👑

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

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Source/version + date, bestie: http://cppreference.com · 2024.06.10 💋✨