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

User-defined and Auto-generated Comparison Operators

Explore the interaction between user-defined and compiler-generated comparison operators in C++20, focusing on the spaceship operator's auto-generation process and operator precedence rules to write cleaner and more efficient comparison code.

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When you can define one of the six comparison operators and also auto-generate all of them using the spaceship operator, there is one question: Which one has the higher priority? For example, the implementation MyInt has a user-defined less than and equal to operator and also the compiler-generated six comparison operators. Let’s see what happens.

C++
#include <compare>
#include <iostream>
class MyInt { 
    public:
        constexpr explicit MyInt(int val): value{val} { } 
        bool operator == (const MyInt& rhs) const {
            std::cout << "==  " << '\n';
            return value == rhs.value; 
        }
        bool operator < (const MyInt& rhs) const { 
            std::cout << "< " << '\n';
            return value < rhs.value;
        }
        auto operator<=>(const MyInt& rhs) const = default;
    private:
        int value;
}; 
int main() {
    MyInt myInt2011(2011);
    MyInt myInt2014(2014);
    
    myInt2011 == myInt2014;
    myInt2011 != myInt2014;
    myInt2011 < myInt2014;
    myInt2011 <= myInt2014;
    myInt2011 > myInt2014;
    myInt2011 >= myInt2014;
}

To see the user-defined == and < operator in action, I ...