std::chrono::year_month::operator+=,std::chrono::year_month::operator-= (3) - Linux Manuals

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

NAME

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

Synopsis

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;

Notes

Durations that are convertible to std::chrono::months, but not std::chrono::years, can be directly added to or subtracted from a year_month. Durations convertible to std::chrono::years cannot because such durations are also convertible to std::chrono::months, resulting in an ambiguity:

  using namespace std::chrono;

  using decades = duration<int, std::ratio_multiply<std::ratio<10>, years::period>>;
  using kilomonths = duration<int, std::ratio_multiply<std::kilo, months::period>>;

  auto ym = 2001y/April;
  ym += decades{1}; // error, ambiguous
  ym += kilomonths{1}; // OK