What does `default operator ` generate?

A consistent set of comparison operations based on memberwise lexicographic comparison rules in C++20.

Strong vs weak ordering?

Use `std::strong_ordering` for total orders; `weak_ordering` when distinct values can compare equivalent; `partial_ordering` for partial orders (e.g. floating-point with NaN).

Migrating from `operator<` only?

Add `operator `/`default` where appropriate, or hand-write ` ` if memberwise default is wrong—verify callers.

When use `auto` return for ` `?

Handy when synthesized from member comparisons; spell out the category when you need a specific ordering type.

C++ Three-Way Comparison | The Spaceship Operator `<=>` (C++20)

Q: Migrating from `operator<` only?

Add `operator `/`default` where appropriate, or hand-write ` ` if memberwise default is wrong—verify callers.

Q: When use `auto` return for ` `?

Handy when synthesized from member comparisons; spell out the category when you need a specific ordering type.

2026년 3월 12일 · 10분 읽기 · 수정 2026년 3월 12일 Intermediate Tutorial

이 글의 핵심

The spaceship operator generates consistent `<`, `>`, `<=`, `>=`, and often `==`/`!=` when `default`ed—if you model ordering correctly.

Spaceship operator

C++20 operator<=> replaces hand-written ==, !=, <, <=, >, >= in many types.

struct Point {
    int x, y;
    auto operator<=>(const Point&) const = default;
};

Return types

std::strong_ordering — total order (typical arithmetic-like types)
std::weak_ordering — distinct values may compare equivalent (e.g. case-insensitive string)
std::partial_ordering — unordered results possible (e.g. NaN)

`default` comparison

Memberwise lexicographic comparison on declared members—great for simple aggregates.

Custom `<=>`

Implement custom ordering (e.g. sort Student by score only) and sometimes operator== separately when equality semantics differ from <=> results.

Composite comparison

Chain if (auto c = field <=> other.field; c != 0) return c; across fields.

Comparison categories

Illustrates strong_ordering::less, weak_ordering::equivalent, partial_ordering::unordered with NaN.

Pitfalls

Custom <=> without == when default equality would be wrong—define operator== explicitly.
Comparing unrelated types without conversions.
default comparing pointer members by address when value semantics matter.

FAQ

C++20+ feature. Performance comparable to hand-written operators—compiler can optimize aggressively.

Range adaptors
Template lambda
Concepts & constraints

Keywords

C++, spaceship operator, three-way comparison, <=>, C++20.