NAME¶

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

Synopsis¶

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

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¶

// Run this code

#include <iostream>
#include <cassert>
#include <chrono>

int main()
{
std::cout << std::boolalpha;

constexpr auto monthsInYear {12};
auto ymd {std::chrono::day(1)/std::chrono::July/2020};

ymd -= std::chrono::years(10);
std::cout << (ymd.month() == std::chrono::July) << ' ' // true
<< (ymd.year() == std::chrono::year(2010)) << ' '; // true

ymd += std::chrono::months(10 * monthsInYear + 11);
std::cout << (ymd.month() == std::chrono::month(6)) << ' ' // true
<< (ymd.year() == std::chrono::year(2021)) << ' '; // true

// Handling the ymd += months "overflow" case.

ymd = std::chrono::May/31/2021; // ok
std::cout << ymd.ok() << ' '; // true
ymd += std::chrono::months{1}; // bad date: June has only 30 days
std::cout << ymd.ok() << ' '; // false
assert(ymd == std::chrono::June/31/2021);

// Snap to the last day of the month, June 30:
const auto ymd1 = ymd.year()/ymd.month()/std::chrono::last;
std::cout << ymd1.ok() << ' '; // true
assert(ymd1 == std::chrono::June/30/2021);

// Overflow into the next month, July 1 (via converting to/from sys_days):
const std::chrono::year_month_day ymd2 = std::chrono::sys_days{ymd};
std::cout << ymd2.ok() << ' '; // true
assert(ymd2 == std::chrono::July/1/2021);
}

Output:¶

true true true true true false true true

See also¶

operator+ adds or subtracts a year_month_day and some number of years or months
operator- (function)
(C++20)