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std::chrono::weekday:: weekday

From cppreference.net
C++
Date and time library
std::chrono::weekday
weekday ( ) = default ;
(1) (C++20 起)
constexpr explicit weekday ( unsigned wd ) noexcept ;
(2) (C++20 起)
constexpr weekday ( const std:: chrono :: sys_days & sd ) noexcept ;
(3) (C++20 起)
constexpr explicit weekday ( const std:: chrono :: local_days & ld ) noexcept ;
(4) (C++20 起)

构造一个 weekday 对象。

1) 默认构造函数不初始化星期值。
2) 构造一个持有星期值 wd 的 weekday 对象。若 wd == 7 ，则持有的值为 ​ 0 ​ 。若 wd > 255 ，则持有的值未指定。
3) 构造一个表示 sd 对应星期几的 weekday 对象。该构造函数定义了从 std::chrono::sys_days 到 weekday 的隐式转换。
4) 构造一个 weekday 对象，表示 ld 对应的星期几，如同通过 weekday ( std:: chrono :: sys_days ( ld. time_since_epoch ( ) ) ) 实现的那样。

参数

wd - 工作日数值
sd - 系统天数对象
ld - 本地天数对象

示例

运行此代码 
#include <chrono>
#include <iomanip>
#include <iostream>
int main()
{
    constexpr std::chrono::weekday friday{5}; // 使用重载 (2)
    static_assert(friday == std::chrono::Friday);
    for (int y{2020}; y <= 2024; ++y)
    {
        const std::chrono::year cur_year{y};
        for (int cur_month{1}; cur_month != 13; ++cur_month)
        {
            const std::chrono::year_month_day ymd{cur_year/cur_month/13};
            const std::chrono::weekday cur_weekday{std::chrono::sys_days(ymd)}; // (3)
            if (cur_weekday == friday)
                std::cout << ymd << " is " << friday << '\n';
        }
    }
}

输出： 

2020-03-13 is Fri
2020-11-13 is Fri
2021-08-13 is Fri
2022-05-13 is Fri
2023-01-13 is Fri
2023-10-13 is Fri
2024-09-13 is Fri
2024-12-13 is Fri