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std::chrono::year_month::operator+=, std::chrono::year_month::operator-= - cppreference.com

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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 <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));
}

See also