std::chrono::weekday:: weekday

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std::chrono::weekday
weekday ( ) = default ;
(1) (since C++20)
constexpr explicit weekday ( unsigned wd ) noexcept ;
(2) (since C++20)
constexpr weekday ( const std:: chrono :: sys_days & sd ) noexcept ;
(3) (since C++20)
constexpr explicit weekday ( const std:: chrono :: local_days & ld ) noexcept ;
(4) (since C++20)

Constructs a weekday object.

1) Default constructor leaves the weekday value uninitialized.
2) Constructs a weekday object holding the weekday value wd . If wd == 7 , the value held is ​ 0 ​ . If wd > 255 , the value held is unspecified.
3) Constructs a weekday object representing the day of the week sd corresponds to. This constructor defines an implicit conversion from std::chrono::sys_days to weekday .
4) Constructs a weekday object representing the day of the week ld corresponds to, as if by weekday ( std:: chrono :: sys_days ( ld. time_since_epoch ( ) ) ) .

Parameters

wd - a weekday value
sd - a system days object
ld - a local days object

Example

Run this code 
#include <chrono>
#include <iomanip>
#include <iostream>
 
int main()
{
    constexpr std::chrono::weekday friday{5}; // uses overload (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';
        }
    }
}

Output: 

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
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