C++20 three way comparison operator — ensure backward compatibility: Part 8

In part one till seven of the tutorial series, we looked at how to use the C++20’s three way comparison operator. In this part of the tutorial series, we’ll look at the compatibility issues when using objects that were constructed before C++20 with the three way comparison operator and how to resolve them

1. Introduction

Consider the code below:

struct Cpp17{
    int num_;
    bool operator==(const Cpp17& rhs)const{
        return (num_ == rhs.num_)
    }
   
    bool operator <(const Cpp17& rhs)const{
        return (num_ < rhs.num_);
    }
    bool operator >(const Cpp17& rhs)const{
        return (num_ > rhs.num_);
    }
} 
struct Cpp20{
    Cpp17 obj_;
    auto operator <=>(const Cpp20& rhs) const = default;
};

Writing such code may be fine until one tries to use and compare them:

int main(){
    Cpp20 a, b;
    auto result = a <=> b;
}

This results in a compiler error. On my machine with gcc 10.2, I get the following compiler error (only the relevant error information is shown):

...
...
no match for 'operator<=>' (operand types are 'Cpp17' and 'Cpp17')
|     Cpp17 obj_;
|…