std::chrono:: operator+, std::chrono:: operator- (std::chrono::weekday)

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C-style date and time
std::chrono::weekday
Defined in header <chrono>
constexpr std:: chrono :: weekday operator + ( const std:: chrono :: weekday & wd,
const std:: chrono :: days & d ) noexcept ;
(1) (since C++20)
constexpr std:: chrono :: weekday operator + ( const std:: chrono :: days & d,
const std:: chrono :: weekday & wd ) noexcept ;
(2) (since C++20)
constexpr std:: chrono :: weekday operator - ( const std:: chrono :: weekday & wd,
const std:: chrono :: days & d ) noexcept ;
(3) (since C++20)
constexpr std:: chrono :: days operator - ( const std:: chrono :: weekday & wd1,
const std:: chrono :: weekday & wd2 ) noexcept ;
(4) (since C++20)
1,2) Adds d. count ( ) days to wd . The weekday value held in the result is computed by first evaluating static_cast < long long > ( wd. c_encoding ( ) ) + d. count ( ) and reducing it modulo 7 to an integer in the range [ ​ 0 ​ , 6 ] .
3) Subtracts d. count ( ) days from wd . Equivalent to return wd + - d ; .
4) If wd1. ok ( ) and wd2. ok ( ) are both true , returns a std::chrono::days value d such that d. count ( ) is in the range [ ​ 0 ​ , 6 ] and wd2 + d == wd1 . Otherwise the returned value is unspecified.

Return value

1-3) A std::chrono::weekday holding a weekday value calculated as described above.
4) A std::chrono::days representing the distance between wd1 and wd2 .

Notes

As long as the computation doesn't overflow, (1-3) always return a valid weekday even if wd. ok ( ) is false .

Example

Run this code 
#include <chrono>
#include <iostream>
 
int main()
{
    std::cout << std::boolalpha;
 
    std::chrono::weekday wd{4};
    wd = wd + std::chrono::days(2);
    std::cout << (wd == std::chrono::weekday(6)) << ' '
              << (wd == std::chrono::Saturday) << ' ';
 
    wd = wd - std::chrono::days(3);
    std::cout << (wd == std::chrono::weekday(3)) << ' '
              << (wd == std::chrono::Wednesday) << ' ';
 
    wd = std::chrono::Tuesday;
    wd = wd + std::chrono::days{8}; // (((2 + 8) == 10) % 7) == 3;
    std::cout << (wd == std::chrono::Wednesday) << ' ';
 
    wd = wd + (std::chrono::Sunday - std::chrono::Thursday); // (3 + 3) == 6
    std::cout << (wd == std::chrono::Saturday) << '\n';
}

Output: 

true true true true true true

See also

increments or decrements the weekday
(public member function)
adds or subtracts a number of days
(public member function)
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