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

From cppreference.net
C++
Date and time library
std::chrono::weekday
constexpr std:: chrono :: weekday & operator ++ ( ) noexcept ;
(1) (since C++20)
constexpr std:: chrono :: weekday operator ++ ( int ) noexcept ;
(2) (since C++20)
constexpr std:: chrono :: weekday & operator -- ( ) noexcept ;
(3) (since C++20)
constexpr std:: chrono :: weekday operator -- ( int ) noexcept ;
(4) (since C++20)

Adds or subtracts 1 from the weekday value, reducing the result modulo 7 to an integer in the range [ ​ 0 ​ , 6 ] .

1,2) Performs * this + = std:: chrono :: days { 1 } ; .
3,4) Performs * this - = std:: chrono :: days { 1 } ; .

Contents

Parameters

(none)

Return value

1,3) A reference to this weekday after modification.
2,4) A copy of the weekday made before modification.

Notes

After a call to one of these functions, ok() is always true .

Example

Run this code 
#include <cassert>
#include <chrono>
#include <iostream>
int main()
{
    std::cout << std::boolalpha;
    std::chrono::weekday wd{0}; // Sunday is 0 or 7
    --wd;
    std::cout << (wd == std::chrono::Saturday) << ' ';
    ++wd;
    std::cout << (wd == std::chrono::Sunday) << '\n';
    wd = std::chrono::weekday{13};
    assert(!wd.ok());
    wd++;
    assert(wd.ok());
}

Output: 

true true

See also

adds or subtracts a number of days
(public member function)
(C++20)
performs arithmetic on weekday s
(function)