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

constexpr std::chrono::year_month&
          operator+=( const std::chrono::years& dy ) const noexcept;
 (1) (since C++20) 
constexpr std::chrono::year_month&
          operator+=( const std::chrono::months& dm ) const noexcept;
 (2) (since C++20) 
constexpr std::chrono::year_month&
          operator-=( const std::chrono::years& dy ) const noexcept;
 (3) (since C++20) 
constexpr std::chrono::year_month&
          operator-=( const std::chrono::months& dm ) const noexcept;
 (4) (since C++20)

Modifies the time point *this represents by the duration dy or dm.

1) Equivalent to *this = *this + dy;
2) Equivalent to *this = *this + dm;
3) Equivalent to *this = *this - dy;
4) Equivalent to *this = *this - dm;

For durations that are convertible to both std::chrono::years and std::chrono::months, the years overloads (1,3) are preferred if the call would otherwise be ambiguous.

Example

#include <iostream>
#include <chrono>
 
int main()
{
    std::cout << std::boolalpha;
 
    auto ym {std::chrono::day(1)/7/2023};
 
    ym -= std::chrono::years {2};
    std::cout << (ym.month() == std::chrono::July) << ' '
              << (ym.year() == std::chrono::year(2021)) << ' ';
 
    ym += std::chrono::months {7};
    std::cout << (ym.month() == std::chrono::month(2)) << ' '
              << (ym.year() == std::chrono::year(2022)) << '\n';
}

Output:

true true true true

See also

(C++20)
 performs arithmetic on year_month
(function)

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