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

From cppreference.net

C++
Date and time library
std::chrono::year_month
constexpr std:: chrono :: year_month &
operator + = ( const std:: chrono :: years & dy ) const noexcept ;
(1) (自 C++20 起)
constexpr std:: chrono :: year_month &
operator + = ( const std:: chrono :: months & dm ) const noexcept ;
(2) (自 C++20 起)
constexpr std:: chrono :: year_month &
operator - = ( const std:: chrono :: years & dy ) const noexcept ;
(3) (自 C++20 起)
constexpr std:: chrono :: year_month &
operator - = ( const std:: chrono :: months & dm ) const noexcept ;
(4) (自 C++20 起)

修改 * this 所表示的时间点，增减时长为 dy 或 dm 。

1) 等价于 * this = * this + dy ; 。
2) 等价于 * this = * this + dm ; 。
3) 等价于 * this = * this - dy ; 。
4) 等价于 * this = * this - dm ; 。

对于可同时转换为 std::chrono::years 和 std::chrono::months 的时间段，若调用存在歧义，则优先选择 years 重载 (1,3) 。

示例

运行此代码 
#include <cassert>
#include <chrono>
int main()
{
    auto ym{std::chrono::day(1)/7/2023};
    ym -= std::chrono::years{2};
    assert(ym.month() == std::chrono::July);
    assert(ym.year() == std::chrono::year(2021));
    ym += std::chrono::months{7};
    assert(ym.month() == std::chrono::month(2));
    assert(ym.year() == std::chrono::year(2022));
}

参见

(C++20)
对 year_month 执行算术运算
(函数)