Defining the == Operator

The == operator is necessary even if we already have the <=> (spaceship) operator for a couple of reasons. Primarily, it comes down to performance and specific use cases.

Firstly, == is usually more efficient than <=>. When checking for equality, == can directly compare values, often using simpler operations.

On the other hand, <=> has to determine whether one value is less than, equal to, or greater than another, which can involve more complex logic.

For instance, comparing two strings for equality might only require a single operation if their lengths differ, whereas a three-way comparison would need to perform additional checks to determine the ordering.

Secondly, == is semantically clearer in many contexts. When you see == in code, you immediately understand that the check is for equality.

Using <=> for equality checks can make the code less readable and harder to understand at a glance, especially for those who may not be familiar with the spaceship operator.

Here's an example:

1#include <compare>
2#include <iostream>
3
4class Number {
5 public:
6  std::strong_ordering operator<=>(
7    const Number& Other) const {
8    return Value <=> Other.Value;
9  }
10
11  bool operator==(const Number& Other) const {
12    return Value == Other.Value;
13  }
14
15  int Value;
16};
17
18int main() {
19  Number A{1};
20  Number B{2};
21
22  if (A == B) {
23    std::cout << "A is equal to B";
24  } else {
25    std::cout << "A is not equal to B";
26  }
27}

1A is not equal to B

In this example, operator== directly compares the values, while operator<=> handles the three-way comparison. By defining both, we ensure that equality checks are efficient and our intentions in the code are clear.