Handling Projections with Pointers to Objects in C++

When working with collections containing pointers to objects, handling projections requires a slightly different approach. Instead of accessing the object directly, you need to dereference the pointer to access the object it points to.

Consider a collection of Player pointers that you want to sort based on their Level attribute using a projection function. Here's an example:

1#include <vector>
2#include <iostream>
3#include <algorithm>
4#include <memory>
5
6struct Player {
7  std::string Name;
8  int Level;
9};
10
11int main() {
12  std::vector<std::shared_ptr<Player>> Party {
13    std::make_shared<Player>(Player{"Legolas", 49}),
14    std::make_shared<Player>(Player{"Gimli", 47}),
15    std::make_shared<Player>(Player{"Gandalf", 53})
16  };
17
18  std::ranges::sort(Party, {},
19    [](const std::shared_ptr<Player>& P) {
20      return P->Level;
21  });
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, the projection function is a lambda that takes a shared_ptr<Player> and returns the Level of the Player object by dereferencing the pointer (P->Level). This approach ensures that the algorithm works correctly with pointers to objects.

Key Points:

• Use the -> operator to access members of the object pointed to by the pointer.
• Ensure the projection function handles the dereferencing properly.
• This technique can be applied to different pointer types (shared_ptr, unique_ptr, raw pointers, etc.).

By understanding how to handle pointers within projection functions, you can effectively use projections with collections containing pointers, enabling more flexible and powerful data manipulation.