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

int main()
{
constexpr auto monthsInYear{12};
auto ymd{std::chrono::day(1)/std::chrono::July/2020};
std::cout << "#1 " << ymd << '\n';

ymd -= std::chrono::years(10);
std::cout << "#2 " << ymd << '\n';
assert(ymd.month() == std::chrono::July);
assert(ymd.year() == std::chrono::year(2010));

ymd += std::chrono::months(10 * monthsInYear + 11);
std::cout << "#3 " << ymd << '\n';
assert(ymd.month() == std::chrono::month(6));
assert(ymd.year() == std::chrono::year(2021));

// Handling the ymd += months "overflow" case.
ymd = std::chrono::May/31/2021; // ok
std::cout << "#4 " << ymd << '\n';
assert(ymd.ok());

ymd += std::chrono::months{1}; // bad date: June has only 30 days
std::cout << "#5 " << ymd << '\n';
assert(not ymd.ok());
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 << "#6 " << ymd1 << '\n';
assert(ymd1.ok());
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 << "#7 " << ymd2 << '\n';
assert(ymd2.ok());
assert(ymd2 == std::chrono::July/1/2021);
}

Output:¶

#1 2020-07-01
#2 2010-07-01
#3 2021-06-01
#4 2021-05-31
#5 2021-06-31 is not a valid date
#6 2021/Jun/last
#7 2021-07-01

See also¶

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