std::chrono...r_month_day)(3)

💖 NAME 💖

std::chrono::operator==,(std::chrono::year_month_day) — bestie, it's literally std::chrono::operator==,(std::chrono::year_month_day). That's the whole name. Iconic, honestly. This is a page from the C++ Standard Library docs (the (3) manpage vibe), and we're about to make date comparisons make sense, no cap.

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✨ Synopsis ✨

So this all lives in the header <chrono> 🕰️ — don't forget to include it or nothing works and you'll be sad.

Overload (1) — the equality one, (since C++20):

constexpr bool operator==( const std::chrono::year_month_day& x,
                           const std::chrono::year_month_day& y )
    noexcept;

Overload (2) — the spaceship one 🚀, (since C++20):

constexpr std::strong_ordering
    operator<=>( const std::chrono::year_month_day& x,
                 const std::chrono::year_month_day& y ) noexcept;

🦋 What it actually does

It compares the two year_month_day values x and y. That's it, that's the vibe. 💅

But okay, the way it compares them is lowkey the main character energy here: it's a lexicographical comparison. That means year() gets checked first, then month(), then day(). Ordered, hierarchal, very organized of it. 👑

And here's the cheat code: the <, <=, >, >=, and != operators are all synthesized from operator<=> and operator== respectively. So you get all five for free — no extra code, they just appear. Slay. ✨

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🎀 Return value 🎀

1. x.year() == y.year() && x.month() == y.month() && x.day() == y.day()
2. If x.year() <=> y.year != 0, then x.year() <=> y.year; otherwise if x.month() <=> y.month() != 0, then x.month() <=> y.month(); otherwise x.day() <=> y.day().

Fr fr, read that second one like a little flowchart: year first, then month if the year tied, then day if the month tied. No skipping steps. 🫶

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🌸 Notes 🌸

Here's the important bit 🚨 — if both x and y represent valid dates (that's x.ok() && y.ok() == true), then the result of the lexicographical comparison is consistent with the calendar order. So valid dates sort like actual calendar dates. Beautiful. Chef's kiss. 😭💖

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🌈 Example 🌈

// Run this code

#include <chrono>

int main()
{
    constexpr auto ymd1{std::chrono::day(13)/7/1337};
    constexpr auto ymd2{std::chrono::year(1337)/7/13};
    static_assert(ymd1 == ymd2);
    static_assert(ymd1 <= ymd2);
    static_assert(ymd1 >= ymd2);
    static_assert(ymd1 <=> ymd2 == 0);
}

Not me noticing that ymd1 and ymd2 are written in a different order but still come out equal 😭😭 because the year is 1337, the month is 7 and the day is 13 in both. The lexicographical order ate. 🔥

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⭐ Sign-off ⭐

Okay bestie, that's the whole page! 💜 This one came from cppreference.com, dated 2024.06.10, and the source line was std::chrono...r_month_day)(3) — go forth and compare your year_month_days with confidence, you absolute icon. 🌟🎀🧚