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The All-in-One Guide to C++20

The Compiler-generated Spaceship Operator

Explore the compiler-generated three-way comparison operator in C++20, known as the spaceship operator. Learn how it performs lexicographical comparisons implicitly as constexpr and noexcept. Understand its use with built-in and user-defined types, and discover performance optimizations for equality operators on string-like and vector-like types.

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The compiler-generated three-way comparison operator needs the header <compare>, is implicitly constexpr and noexcept, and it performs a lexicographical comparison. You can even directly use the three-way comparison operator.

Direct use of the three-way comparison operator

The following program directly uses the spaceship operator.

C++
#include <compare>
#include <iostream>
#include <string>
#include <vector>
int main() {
    std::cout << '\n';
    int a(2011);
    int b(2014);
    auto res = a <=> b;
    if(res < 0) std::cout << "a < b" << '\n';
    else if (res == 0) std::cout << "a == b" << '\n';
    else if (res > 0) std::cout << "a > b" << '\n';
    std::string str1("2014");
    std::string str2("2011");
    auto res2 = str1 <=> str2;
    if (res2 < 0) std::cout << "str1 < str2" << '\n';
    else if (res2 == 0) std::cout << "str1 == str2" << '\n';
    else if (res2 > 0) std::cout << "str1 > str2" << '\n';
    
    std::vector<int> vec1{1, 2, 3};
    std::vector<int> vec2{1, 2, 3};
    auto res3 = vec1 <=> vec2;
    if (res3 < 0) std::cout << "vec1 < vec2" << '\n';
    else if (res3 == 0) std::cout << "vec1 == vec2" << '\n';
    else if (res3 > 0) std::cout << "vec1 > vec2" << '\n';
    std::cout << '\n';
}

The program uses the spaceship operator for int (line 11), for string (line 18 ...