How Does `std::shared_ptr` Preserve Object Destruction Behavior Despite Type Erasure?

std::shared_ptr Functionality Unraveled

The inquiry revolves around the puzzling functionality of std::shared_ptr. Initially dismissed as implausible, the behavior observed in the following code snippet brought skepticism:

<code class="cpp">#include <memory>
#include <iostream>
#include <vector>

int main() {
  std::cout << "At begin of main.\ncreating std::vector<std::shared_ptr<void>>" << std::endl;
  std::vector<std::shared_ptr<void>> v;
  {
    std::cout << "Creating test" << std::endl;
    v.push_back(std::shared_ptr<test>(new test()));
    std::cout << "Leaving scope" << std::endl;
  }
  std::cout << "Leaving main" << std::endl;
  return 0;
}</code>

The code exhibits the following output:

At begin of main.
creating std::vector<std::shared_ptr<void>>
Creating test
Test created
Leaving scope
Leaving main
Test destroyed

Understanding Type Erasure

The key to this behavior lies in the type erasure performed by std::shared_ptr. When initializing a new instance, std::shared_ptr stores an internal deleter function. This function, by default, invokes the delete operator upon destruction of the shared_ptr. This mechanism ensures that the destructor of the object pointed to is invoked at the appropriate time, regardless of the type of the shared_ptr.

Consequence of Casting

Casting a std::shared_ptr to std::shared_ptr does not alter the stored deleter function, which still references the original test destructor. Thus, when the shared_ptr is destroyed, the correct destructor is still called, resulting in the expected behavior.

Standard Compliance and Future Implications

Regarding the guaranteed behavior of this technique, it's crucial to note that the internal implementation of std::shared_ptr may vary across different compilers and platforms. While type erasure has been a fundamental aspect of shared_ptrs, future changes to its implementation may potentially impact the described functionality.

Therefore, relying solely on this behavior is not a recommended practice and should be avoided in production code. Instead, alternative approaches, such as using std::function or a custom deleter class, are more robust and will provide consistent behavior across various implementations.

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