std::chrono...year_month)(3)

💖 NAME 💖

bestie, welcome to the C++ Standard Libary manual page for std::chrono...year_month)(3) 🫶 it's std::chrono::operator+,std::chrono::operator-(std::chrono::year_month) — literally the std::chrono::operator+ and std::chrono::operator- overloads for std::chrono::year_month, and yes the vibes are immaculate.

✨ Synopsis ✨

okay so here's the whole overload lineup, all constexpr, all noexcept, all (since C++20) 💅

(1) (since C++20)

constexpr std::chrono::year_month operator+( const std::chrono::year_month& ym,
                                             const std::chrono::years& dy ) noexcept;

(2) (since C++20)

constexpr std::chrono::year_month operator+( const std::chrono::years& dy,
                                             const std::chrono::year_month& ym ) noexcept;

(3) (since C++20)

constexpr std::chrono::year_month operator+( const std::chrono::year_month& ym,
                                             const std::chrono::months& dm ) noexcept;

(4) (since C++20)

constexpr std::chrono::year_month operator+( const std::chrono::months& dm,
                                             const std::chrono::year_month& ym ) noexcept;

(5) (since C++20)

constexpr std::chrono::year_month operator-( const std::chrono::year_month& ym,
                                             const std::chrono::years& dy ) noexcept;

(6) (since C++20)

constexpr std::chrono::year_month operator-( const std::chrono::year_month& ym,
                                             const std::chrono::months& dm ) noexcept;

(7) (since C++20)

constexpr std::chrono::months operator-( const std::chrono::year_month& ym1,
                                         const std::chrono::year_month& ym2 ) noexcept;

Here's what they actually do, no cap:

• 1,2) — Adds dy.count() years to ym.
• 3,4) — Adds dm.count() months to ym.
• 5) — Subtracts dy.count() years from ym.
• 6) — Subtracts dm.count() months from ym.
• 7) — Returns the difference in months between the two time points represented by ym1 and ym2.

For durations that are convertible to both std::chrono::years and std::chrono::months, the years overloads (1,2,5) are preferred if the call would otherwise be ambiguous.

🔥 Return value 🔥

• 1,2) — std::chrono::year_month(ym.year() + dy, ym.month())
• 3,4) — A year_month value z such that z - ym == dm and z.ok() == true.
• 5) — ym + -dy
• 6) — ym + -dm
• 7) —

ym1.year() - ym2.year() +
std::chrono::months(int(unsigned(ym1.month())) -
                     int(unsigned(ym2.month())))

🌸 Notes 🌸

The result of subtracting two year_month values is a duration of type std::chrono::months. This duration unit represents the length of the average Gregorian month (30.436875 days), and the resulting duration bears no relationship to the actual number of days in the time period at issue. For example, the result of 2017y/3 - 2017y/2 is std::chrono::months(1), even though February 2017 only contains 28 days.

lowkey important: that std::chrono::months math is average-month math, not "count the actual days on the calendar" math. 💀

🎀 Example 🎀

// Run this code

#include <cassert>
#include <chrono>

int main()
{
    auto ym{std::chrono::year(2021)/std::chrono::July};

    ym = ym + std::chrono::months(14);
    assert(ym.month() == std::chrono::September);
    assert(ym.year() == std::chrono::year(2022));

    ym = ym - std::chrono::years(3);
    assert(ym.month() == std::chrono::month(9));
    assert(ym.year() == std::chrono::year(2019));

    ym = ym + (std::chrono::September - std::chrono::month(2));
    assert(ym.month() == std::chrono::April);
    assert(ym.year() == std::chrono::year(2020));
}

🦋 See also 🦋

• operator+= modifies the year_month by some number of months or years
• operator-= (public member function)

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💖 that's the whole page, bestie! source: http://cppreference.com · version/date: 2024.06.10 · page: std::chrono...year_month)(3) ✨🫶