Three-Way Comparison Operator <=> (C++20)

In a Nutshell

Define operator<=> and the compiler auto-generates all six comparison operators (<, <=, >, >=, ==, !=) — goodbye boilerplate comparison code.

Header

#include <compare> (when using predefined comparison categories)

Core API Quick Reference

Operation	Signature	Description
Three-way comparison	auto operator<=>(const T&) const = default;	Compiler auto-generates comparison logic
Custom three-way	std::strong_ordering operator<=>(const T& rhs) const;	Custom comparison semantics
Strong ordering	std::strong_ordering	Equivalent elements are indistinguishable (e.g., int)
Weak ordering	std::weak_ordering	Equivalent elements are distinguishable but compare equal (e.g., case-insensitive strings)
Partial ordering	std::partial_ordering	Incomparable values exist (e.g., NaN)
Equality operator	bool operator==(const T&) const = default;	Defaulting == alone auto-generates !=

Minimal Example

// Standard: C++20
#include <compare>
#include <iostream>

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

int main() {
    Point a{1, 2}, b{1, 3};
    std::cout << (a < b)  << "\n"; // true  (auto-generated)
    std::cout << (a == b) << "\n"; // false (auto-generated)
    std::cout << (a != b) << "\n"; // true  (auto-generated)

    auto cmp = a <=> b;
    std::cout << (cmp < 0) << "\n"; // true (strong_ordering::less)
}

Embedded Applicability: Medium

• Compile-time feature with zero runtime overhead — default-generated comparisons are equivalent to hand-written code
• Suitable for structs requiring lexicographic comparison (sensor data, protocol headers)
• Requires C++20 support (GCC 10+); some embedded toolchains are not yet fully ready
• Comparison categories (strong/weak/partial) are conceptually abstract; teams need shared understanding

Compiler Support

GCC	Clang	MSVC
10	10	19.20

See Also

• cppreference: Default comparisons
• cppreference: std::strong_ordering

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