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std::chrono::weekday::ok - cppreference.com

From cppreference.com

constexpr bool ok() const noexcept;

 (since C++20)

Checks if the weekday value stored in *this is in the valid range, i.e., [0, 6].

Return value

true if the weekday value stored in *this is in the range [0, 6]. Otherwise false.

Example

#include <chrono>
#include <iomanip>
#include <iostream>
#include <locale>
#include <string>

struct weekday_ok : std::numpunct<char>
{
    std::string do_truename()  const override { return " (is valid weekday)"; }
    std::string do_falsename() const override { return " (is not valid weekday)"; }
};

int main()
{
    std::cout.imbue(std::locale(std::cout.getloc(), new weekday_ok));
    std::cout << std::boolalpha;

    for (const unsigned u : {0 /* Sun */, 1 /* Mon */, 6, 7 /* Sun */, 8, 9})
    {
        const std::chrono::weekday wd{u};
        std::cout << "u: " << u << "; wd: " << wd.c_encoding() << wd.ok() << '\n';
    }
}

Output: 

u: 0; wd: 0 (is valid weekday)
u: 1; wd: 1 (is valid weekday)
u: 6; wd: 6 (is valid weekday)
u: 7; wd: 0 (is valid weekday)
u: 8; wd: 8 (is not valid weekday)
u: 9; wd: 9 (is not valid weekday)

See also