# std::chrono::round(std::chrono::time_point)

< cpp‎ | chrono‎ | time point

定义于头文件 `<chrono>` |  |
template <class ToDuration, class Clock, class Duration> constexpr std::chrono::time_point<Clock, ToDuration> round(const std::chrono::time_point<Clock, Duration>& tp); |  | (C++17 起)

返回能以 `ToDuration` 表示的最接近 `tp` 的时间点，中点情况下向偶数舍入。

函数不参与重载决议，除非 `ToDuration` 是 std::chrono::duration 的特化且 std::chrono::treat_as_floating_point_v<typename ToDuration::rep> 为 false 。

### 参数

tp | - | 要舍入到最近值的时间点

### 返回值

舍入到使用 `ToDuration` 类型时长的最近时间点的 `tp` ，中点情况下向偶数舍入。

### 可能的实现

    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())};
    }

### 示例

    #include <iostream>
    #include <chrono>
    #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";
        }
    }

输出：

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

### 参阅

time_point_cast (C++11) | 转换时间点为同一时钟上拥有不同时长的另一时间点 (函数模板)
ceil(std::chrono::time_point) (C++17) | 转换 time_point 到另一个，向上取整 (函数模板)
floor(std::chrono::time_point) (C++17) | 转换 time_point 到另一个，向下取整 (函数模板)
round(std::chrono::duration) (C++17) | 转换时长到另一个时长，就近取整，偶数优先 (函数模板)
round roundf roundl lround lroundf lroundl llround llroundf llroundl (C++11)(C++11)(C++11)(C++11)(C++11)(C++11)(C++11)(C++11)(C++11) | 最接近整数，中间情况下向远离零舍入 (函数)