Custom Comparison Logic

Implementing custom comparison logic using the spaceship operator <=> in C++20 allows you to define how your custom types should be compared in a consistent and efficient manner. Here’s a step-by-step guide to implementing this:

Step 1: Define the Custom Type

First, define your custom type. For example, let’s create a Player class that compares players based on their scores and, if the scores are the same, by their names.

1#include <compare>
2#include <iostream>
3#include <string>
4
5class Player {
6 public:
7  Player(std::string name, int score)
8    : Name{name}, Score{score} {}
9
10  std::strong_ordering operator<=>(
11    const Player& Other) const {
12    if (auto cmp = Score <=> Other.Score; cmp != 0) {
13      return cmp;
14    }
15    return Name <=> Other.Name;
16  }
17
18  bool operator==(const Player& Other) const {
19    return Score == Other.Score && Name == Other.Name;
20  }
21
22  std::string Name;
23  int Score;
24};
25
26int main() {
27  Player Player1{"Alice", 10};
28  Player Player2{"Bob", 10};
29  Player Player3{"Alice", 20};
30
31  if (Player1 < Player2) {
32    std::cout << Player1.Name
33      << " is ranked lower than "
34      << Player2.Name << "\n";
35  }
36  if (Player1 > Player3) {
37    std::cout << Player1.Name
38      << " is ranked higher than "
39      << Player3.Name << "\n";
40  }
41  if (Player1 == Player1) {
42    std::cout << Player1.Name
43      << " is equal to "
44      << Player1.Name << "\n";
45  }
46}

1Alice is ranked lower than Bob
2Alice is equal to Alice

Step 2: Implement Custom Comparison Logic

In the operator<=>, we first compare the scores. If they are not equal, we return the result of that comparison.

If the scores are equal, we then compare the names. This secondary comparison ensures that players with the same score are ordered by their names.

Step 3: Use the Custom Comparison

You can now use all the comparison operators with your custom type. The <=> operator handles the ordering, and the == operator ensures that equality is checked correctly.

In this example, the comparison logic considers both the Score and Name fields of the Player class, demonstrating how you can implement and use custom comparison logic effectively with the spaceship operator in C++20.