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std::chrono::operator+, std::chrono::operator- (std::chrono::year_month_weekday)

From cppreference.com
 
 
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C-style date and time
 
 
constexpr std::chrono::year_month_weekday
operator+(const std::chrono::year_month_weekday& ymwd, const std::chrono::months& dm) noexcept;
(1) (since C++20)
constexpr std::chrono::year_month_weekday
operator+(const std::chrono::months& dm, const std::chrono::year_month_weekday& ymwd) noexcept;
(2) (since C++20)
constexpr std::chrono::year_month_weekday
operator+(const std::chrono::year_month_weekday& ymwd, const std::chrono::years& dy) noexcept;
(3) (since C++20)
constexpr std::chrono::year_month_weekday
operator+(const std::chrono::years& dy, const std::chrono::year_month_weekday& ymwd) noexcept;
(4) (since C++20)
constexpr std::chrono::year_month_weekday
operator-(const std::chrono::year_month_weekday& ymwd, const std::chrono::months& dm) noexcept;
(5) (since C++20)
constexpr std::chrono::year_month_weekday
operator-(const std::chrono::year_month_weekday& ymwd, const std::chrono::years& dy) noexcept;
(6) (since C++20)
1-2) Adds dm.count() months to the date represented by ymwd. The result has the same year() and month() as std::chrono::year_month(ymwd.year(), ymwd.month()) + dm and the same weekday() and index() as ymwd.
3-4) Adds dy.count() years to the date represented by ymwd. The result is equivalent to std::chrono::year_month_weekday(ymwd.year() + dy, ymwd.month(), ymwd.weekday_indexed()).
5) Subtracts dm.count() months from the date represented by ymwd. Equivalent to ymwd + -dm.
6) Subtracts dy.count() years from the date represented by ymwd. Equivalent to ymwd + -dy.

For durations that are convertible to both std::chrono::years and std::chrono::months, the years overloads (3,4,6) are preferred if the call would otherwise be ambiguous.

[edit] Notes

Even if ymwd.ok() is true, the resulting year_month_weekday may not represent a valid date if ymwd.index() is 5.

[edit] Example

#include <iostream>
#include <chrono>
 
int main()
{
    std::cout << std::boolalpha;
 
    auto ymwdi {1/std::chrono::Wednesday[1]/2021};
    ymwdi = std::chrono::years(5) + ymwdi;
    // First Wednesday in January, 2026
    std::cout << (static_cast<std::chrono::year_month_day>(ymwdi) == 
                  std::chrono::January/7/2026) << ' ';
    ymwdi = ymwdi - std::chrono::months(6);
    // First Wednesday in July, 2025
    std::cout << (static_cast<std::chrono::year_month_day>(ymwdi) ==
                  std::chrono::July/2/2025) << '\n';
}

Output:

true true