cpp core · medium

The Spaceship Operator <=>

The C++20 three-way comparison operator <=> ('spaceship') returns an ordering object rather than a bool: comparing that result against literal 0 recovers <, <=, >, >=. Defaulting operator<=> compares members in declaration order and synthesizes all four relational operators, and a defaulted operator== adds == and !=, so two lines replace six. The return type names a comparison category: strong_ordering means equal values are fully substitutable (int, pointers); weak_ordering means values can be equivalent without being identical (case-insensitive strings); and partial_ordering allows 'unordered', as with floating-point NaN, where every comparison except != is false. Pick the category by what equality should mean and let the compiler generate the rest.

The code

struct Point {
    int x, y;
    auto operator<=>(const Point&) const = default; // synthesizes < <= > >=
    bool operator==(const Point&) const = default;  // and == / !=
};

(a <=> b) < 0 // means a < b   — compare the ordering against 0
        (a <=> b) ==
    0 // means a == b

        1 <=> 2 // std::strong_ordering::less
                  0.0 /
                  0.0 <=>
        1.0 // std::partial_ordering::unordered   (NaN)

What this lesson walks through

1. 01<=> returns an ordering, not a bool
2. 02Each relation is the ordering vs 0
3. 03= default gives you all six
4. 04strong_ordering: equal means identical
5. 05weak_ordering: equivalent, not identical
6. 06partial_ordering: some pairs are unordered
7. 07Pick the category, default the operators

The three-way comparison operator <=> evaluates both operands once and returns an ordering object describing their relationship: less, equal, or greater. To get a yes/no answer you compare that ordering with literal 0.