std::chrono::weekday::weekday

weekday() = default;	(1)	(since C++20)
constexpr explicit weekday( unsigned wd ) noexcept;	(2)	(since C++20)
constexpr weekday( const std::chrono::sys_days& dp ) noexcept;	(3)	(since C++20)
constexpr explicit weekday( const std::chrono::local_days& dp ) 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 dp 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 dp corresponds to, as if by weekday(std::chrono::sys_days(dp.time_since_epoch())).

Example

#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 <= 2023; ++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

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https://en.cppreference.com/w/cpp/chrono/weekday/weekday