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

In a Nutshell

Defining operator<=> allows the compiler to automatically generate <, >, <=, >=, ==, and !=. Say goodbye to writing comparison code manually.

Header

<compare> (when using predefined comparison categories)

Core API Cheat Sheet

Operation	Signature	Description
Three-way comparison	auto operator<=>(const T&) const = default;	Compiler automatically generates comparison logic
Manual three-way comparison	std::strong_ordering operator<=>(const T&) 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 this alone automatically generates !=

Minimal Example

#include <compare>
#include <iostream>

struct Point {
    int x, y;

    // Compiler auto-generates <, <=, >, >=, ==, !=
    std::strong_ordering operator<=>(const Point&) const = default;
};

int main() {
    Point p1{1, 2}, p2{1, 5};

    if (p1 < p2) {
        std::cout << "p1 is less than p2\n";
    }
    // p1 == p1, p2 != p1 also work
}

Embedded Applicability: Medium

• Compile-time feature, zero runtime overhead—defaulted comparison code is equivalent to handwritten code.
• Suitable for structs requiring lexicographical comparison, such as sensor data or protocol headers.
• Requires C++20 support (GCC 10+); some embedded toolchains are not yet fully ready.
• Comparison categories (strong/weak/partial) are abstract concepts; teams need a unified understanding.

Compiler Support

GCC	Clang	MSVC
10	10	19.20

See Also

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

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