std::chrono::month::operator+=, std::chrono::month::operator-=

constexpr std::chrono::month& operator+=( const std::chrono::months& m ) noexcept;
 (1) (since C++20) 
constexpr std::chrono::month& operator-=( const std::chrono::months& m ) noexcept;
 (2) (since C++20)

Adds or subtracts m.count() from the month value, reducing the result modulo 12 to an integer in the range [1, 12].

1) Performs *this = *this + m;.
2) Performs *this = *this - m;.

Return value

A reference to this month after modification.

Notes

After a call to one of these functions, ok() is always true if no overflow occurred during the operation.

Example

#include <chrono>
#include <iostream>
 
int main()
{
    std::cout << std::boolalpha;
 
    std::chrono::month m{6};
    m += std::chrono::months(2);
    std::cout << (m == std::chrono::month(8)) << ' '
              << (m == std::chrono::August) << ' ';
 
    m -= std::chrono::months(3);
    std::cout << (m == std::chrono::month(5)) << ' '
              << (m == std::chrono::May) << ' ';
 
    m = std::chrono::October;
    m += std::chrono::months{8}; // ((10 += 8 == 18) % 12) == 6;
    std::cout << (m == std::chrono::June) << ' ';
 
    m -= std::chrono::months{std::chrono::December - std::chrono::February}; // -= 10
    // (6 -= 10) == -4; -4 % 12 == (12 - 4) == 8
    std::cout << (m == std::chrono::August) << '\n';
}

Output:

true true true true true true

See also

increments or decrements the month
(public member function)
(C++20)
 performs arithmetic on months
(function)

© cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.
https://en.cppreference.com/w/cpp/chrono/month/operator_arith