Can Projection Functions Return References in C++?

Yes, projection functions in C++ can return references instead of values.

Returning references can be particularly useful for avoiding unnecessary copies, especially when working with large objects or when the original object needs to be modified.

Example with References:

Consider a collection of Player objects where the projection function returns a reference to a member variable:

1#include <vector>
2#include <iostream>
3#include <algorithm>
4
5struct Player {
6  std::string Name;
7  int Level;
8};
9
10int& getLevel(Player& P) {
11  return P.Level;
12}
13
14int main() {
15  std::vector<Player> Party {
16    {"Legolas", 49},
17    {"Gimli", 47},
18    {"Gandalf", 53}
19  };
20
21  std::ranges::sort(Party, {}, getLevel);
22
23  for (const auto& P : Party) {
24    std::cout << "[" << P.Level << "] "
25      << P.Name << "\n";
26  }
27}

1[47] Gimli
2[49] Legolas
3[53] Gandalf

In this example, getLevel() is a projection function that returns a reference to the Level attribute of a Player object. The std::ranges::sort() algorithm uses this reference for sorting.

When to Use References:

• Performance: Returning references can improve performance by avoiding copies, especially for large objects.
• Modifications: When you need the algorithm to modify the original objects based on the projection.

Caveats:

• Lifespan: Ensure the reference remains valid for the duration of its use. Avoid returning references to local variables.
• Const-correctness: If the projection should not modify the object, use const references.

Example with const References:

To ensure the projection function does not modify the original object, return a const reference:

1#include <vector>
2#include <iostream>
3#include <algorithm>
4
5struct Player {
6  std::string Name;
7  int Level;
8};
9
10const int& getLevel(const Player& P) {
11  return P.Level;
12}
13
14int main() {
15  std::vector<Player> Party {
16    {"Legolas", 49},
17    {"Gimli", 47},
18    {"Gandalf", 53}
19  };
20
21  std::ranges::sort(Party, {}, getLevel);
22
23  for (const auto& P : Party) {
24    std::cout << "[" << P.Level << "] "
25      << P.Name << "\n";
26  }
27}

1[47] Gimli
2[49] Legolas
3[53] Gandalf

In this example, getLevel() returns a const reference, ensuring the std::ranges::sort() algorithm does not modify the Level attribute.

Using references in projection functions can provide performance benefits and allow for more flexible manipulation of the original data. Ensure you manage the lifespan and const-correctness of the references properly to avoid potential issues.