Memory Ownership and Smart Pointers

Deleting Raw Pointers from Unique Pointers

What happens if I try to delete the raw pointer obtained from the get() method of a unique pointer?

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Deleting a raw pointer obtained from the get() method of a std::unique_ptr is a dangerous operation that leads to undefined behavior. It's important to understand why this is problematic and how to avoid such situations.

Let's break this down with an example:

#include <iostream>
#include <memory>

class Character {
public:
  Character(std::string name)
    : name(std::move(name)) {
    std::cout << "Creating " << name << '\n';
  }

  ~Character() {
    std::cout << "Destroying " << name << '\n';
  }

private:
  std::string name;
};

int main() {
  auto frodo = std::make_unique<Character>("Frodo");

  Character* rawPtr = frodo.get(); 
  delete rawPtr;                   

  // frodo still thinks it owns the object!
}

If we were to run this code, here's what would happen:

  1. The Character object "Frodo" is created and owned by the std::unique_ptr.
  2. We get the raw pointer using get().
  3. We delete the raw pointer, which destroys the Character object.
  4. At the end of main(), frodo (the std::unique_ptr) goes out of scope and tries to delete the object again.

This results in a double deletion, which is undefined behavior. It could crash your program, corrupt memory, or seemingly do nothing while causing subtle bugs elsewhere.

Here's why this is dangerous:

  1. Ownership Violation: std::unique_ptr is designed to have exclusive ownership of the object it points to. By deleting the raw pointer, we're violating this contract.
  2. Double Deletion: The std::unique_ptr will try to delete the object again when it goes out of scope, leading to undefined behavior.
  3. Dangling Pointer: After deleting the raw pointer, the std::unique_ptr becomes a dangling pointer, pointing to memory that has been freed.

To avoid these issues:

  1. Never delete raw pointers obtained from get().
  2. Use get() only when you need to pass the raw pointer to functions that expect a raw pointer, and ensure those functions don't delete the pointer.
  3. If you need to transfer ownership, use std::move() instead:
#include <iostream>
#include <memory>

class Character {
public:
  Character(std::string name)
    : mName(std::move(name)) {
    std::cout << "Creating " << mName << '\n';
  }

  ~Character() {
    std::cout << "Destroying " << mName << '\n';
  }

private:
  std::string mName;
};

void takeOwnership(std::unique_ptr<Character> ptr) {
  // This function now owns the Character object
  std::cout << "Taking ownership\n";
}

int main() {
  auto FrodoPtr = std::make_unique<Character>("Frodo");
  takeOwnership(std::move(FrodoPtr)); 

  if (!FrodoPtr) {
    std::cout << "FrodoPtr is now null";
  }
}
Creating Frodo
Taking ownership
Destroying Frodo
FrodoPtr is now null

In this safe version, we transfer ownership of the Character object to the takeOwnership function using std::move(). The std::unique_ptr in main() becomes null after the move, preventing any double deletion issues.

Remember, the whole point of std::unique_ptr is to manage the lifetime of dynamically allocated objects automatically. By manually deleting the raw pointer, we're defeating this purpose and introducing potential bugs. Stick to using the std::unique_ptr interface, and let it handle the deletion for you.

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

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