std::compare_three_way(3)

💅 std::compare_three_way — the C++ Standard Library page, but make it cute

okay so bestie, this one is std::compare_three_way, and it lives in the C++ Standard Library (the std:: namespace, obviously). it's a little function object that does the three-way comparison thing, and honestly? it's giving "quietly iconic." let me walk you through it 🫶

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✨ SYNOPSIS ✨

so it's defined in two headers, yes two, don't @ me:

Defined in header <compare>
Defined in header <functional>

and the struct itself:

struct compare_three_way;       (since C++20)

like the title says, this is a Function object for performing comparisons 💖 and here's the cute part — it deduces the parameter types and the return type of the function call operator. it just knows. main character energy fr.

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🎀 MEMBER FUNCTIONS 🎀

• operator() — obtains the result of three-way comparison on both arguments (public member function)

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💅 std::compare_three_way::operator() 💅

here's the signature, exactly as written:

template< class T, class U >
constexpr auto operator()( T&& t, U&& u ) const;

okay so, given the expression std::forward<T>(t) <=> std::forward<U>(u) as expr:

• if expr results in a call to built-in operator<=> comparing pointers, given the composite pointer type of t and u as P:
  • it compares the two converted pointers (of type P) in the implementation-defined strict total order over pointers:
    • if t precedes u, returns std::strong_ordering::less.
    • if u precedes t, returns std::strong_ordering::greater.
    • otherwise, returns std::strong_ordering::equal.
  • if the conversion sequence from T to P or the conversion sequence from U to P is not equality-preserving, the behavior is undefined 💀 (yes, undefined. be careful bestie.)
• otherwise:
  • returns the result of expr.
  • if std::three_way_comparable_with<T, U> is not modeled, the behavior is undefined 😭

also, important vibe check: this overload participates in overload resolution only if std::three_way_comparable_with<T, U> is satisfied.

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🌈 EXAMPLE 🌈

run this code, it's so good:

#include <compare>
#include <iostream>

struct Rational
{
    int num;
    int den; // > 0

    // Although the comparison X <=> Y will work, a direct call
    // to std::compare_three_way{}(X, Y) requires the operator==
    // be defined, to satisfy the std::three_way_comparable_with.
    constexpr bool operator==(Rational const&) const = default;
};

constexpr std::weak_ordering operator<=>(Rational lhs, Rational rhs)
{
    return lhs.num * rhs.den <=> rhs.num * lhs.den;
}

void print(std::weak_ordering value)
{
    value < 0 ? std::cout << "less\n" :
    value > 0 ? std::cout << "greater\n" :
                std::cout << "equal\n";
}

int main()
{
    Rational a{6, 5};
    Rational b{8, 7};
    print(a <=> b);
    print(std::compare_three_way{}(a, b));
}

Output:

greater
greater

both lines match, we love consistency ✨

ngl the comment in that example is the whole tea — X <=> Y works fine, but calling std::compare_three_way{}(X, Y) directly needs operator== defined so std::three_way_comparable_with is happy. iykyk 🧚

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🦋 DEFECT REPORTS 🦋

The following behavior-changing defect reports were applied retroactively to previously published C++ standards. (yes, they went back and fixed the past, iconic behavior 👑)

• LWG 3530 — applied to C++20 — behavior as published: syntactic checks were relaxed while comparing pointers — correct behavior: only semantic requirements are relaxed

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⭐ SEE ALSO ⭐

• ranges::equal_to (C++20) — constrained function object implementing x == y (class)
• ranges::not_equal_to (C++20) — constrained function object implementing x != y (class)
• ranges::less (C++20) — constrained function object implementing x < y (class)
• ranges::greater (C++20) — constrained function object implementing x > y (class)
• ranges::less_equal (C++20) — constrained function object implementing x <= y (class)
• ranges::greater_equal (C++20) — constrained function object implementing x >= y (class)

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okay that's the whole page!! every flag, every warning, every undefined-behavior jumpscare, all present and accounted for 💖 keep slaying those comparisons, bestie. mwah 🎀

source: http://cppreference.com — version 2024.06.10 — page: std::compare_three_way(3) 🦋✨