Projection Functions

Handling Projections with Pointers to Objects in C++

How do I handle projections when the collection contains pointers to objects?

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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:

#include <vector>
#include <iostream>
#include <algorithm>
#include <memory>

struct Player {
  std::string Name;
  int Level;
};

int main() {
  std::vector<std::shared_ptr<Player>> Party {
    std::make_shared<Player>(Player{"Legolas", 49}),
    std::make_shared<Player>(Player{"Gimli", 47}),
    std::make_shared<Player>(Player{"Gandalf", 53})
  };

  std::ranges::sort(Party, {},
    [](const std::shared_ptr<Player>& P) {
      return P->Level;
  });

  for (const auto& P : Party) {
    std::cout << "[" << P->Level << "] "
      << P->Name << "\n";
  }
}
[47] Gimli
[49] Legolas
[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.

Answers to questions are automatically generated and may not have been reviewed.

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