Implementing a Custom Swap Function

Implementing a custom swap() function can optimize standard library algorithms such as std::ranges::reverse(), especially if your type has specific requirements or optimizations for swapping elements. Here's how you can do it:

Custom swap Function

First, define your custom type and implement the swap function:

1#include <algorithm>
2#include <iostream>
3#include <vector>
4
5struct Player {
6  std::string name;
7  int score;
8
9  Player(std::string n, int s)
10    : name(n), score(s) {}
11
12  // Custom swap function
13  friend void swap(Player& a, Player& b) {
14    std::cout << "Swapping " << a.name
15      << " with " << b.name << "\n";
16    std::swap(a.name, b.name);
17    std::swap(a.score, b.score);
18  }
19};
20
21int main() {
22  std::vector<Player> players{
23    {"Alice", 30}, {"Bob", 20}, {"Charlie", 40}
24  };
25
26  std::ranges::reverse(players);
27
28  for (const auto& player : players) {
29    std::cout << player.name << ": "
30      << player.score << "\n";
31  }
32}

1Swapping Alice with Charlie
2Charlie: 40
3Bob: 20
4Alice: 30

How It Works

1. Define the Custom Type: We created a Player struct with a name and score.
2. Implement the swap() function: The swap() function swaps the name and score of two Player objects. Note the use of std::swap() within the custom swap().
3. Call the std::ranges::reverse() Algorithm: : When std::ranges::reverse() is called, it uses the custom swap() function to swap elements.

Benefits

• Optimization: Custom swap logic can be more efficient than the default std::swap(), especially if the type has complex move semantics.
• Debugging: Adding print statements in the swap() function helps in debugging by showing when and how elements are swapped.

Practical Example

Here's another example with a more complex type:

1#include <algorithm>
2#include <iostream>
3#include <vector>
4
5struct ComplexType {
6  int data[100];
7
8  friend void swap(ComplexType& a, ComplexType& b) {
9    std::cout << "Swapping ComplexType objects\n";
10    for (int i = 0; i < 100; ++i) {
11      std::swap(a.data[i], b.data[i]);
12    }
13  }
14};
15
16int main() {
17  std::vector<ComplexType> vec(2);
18  std::ranges::reverse(vec);
19}

1Swapping ComplexType objects

Conclusion

Implementing a custom swap() function allows you to tailor the swapping behavior to your specific type, which can optimize operations like std::ranges::reverse().

This is especially useful for types that have complex or costly move operations, ensuring efficient and correct element manipulation.