How Does std::shared_ptr Ensure Proper Destructor Calls During Cleanup?

Why std::shared_ptr Works

Introduction

The use of std::shared_ptr for arbitrary cleanup at shutdown may seem unusual, but it's a valid technique that exploits the fundamentals of shared pointers.

Type Erasure in std::shared_ptr

The key to understanding this behavior lies in the way std::shared_ptr performs type erasure. When created, a shared pointer stores a deleter function that determines how its managed object should be destroyed. This deleter can be explicitly specified or defaults to deleting the object using delete.

Preserving Destructor Information

When a shared_ptr is copied or constructed from another, the deleter function is preserved. This means that even when casting from std::shared_ptr to std::shared_ptr, the original deleter remains intact. As a result, when the std::shared_ptr is destroyed, the appropriate destructor is invoked based on the information stored in the deleter.

Example Illustration

The code provided in the question demonstrates this functionality. The following lines explain the behavior:

<code class="cpp">v.push_back(std::shared_ptr<test>(new test()));</code>

Here, a shared_ptr to an instance of class test is created and added to the vector v.

<code class="cpp">v.push_back(static_cast<std::shared_ptr<void>>(v.back()));</code>

The shared_ptr to test is then explicitly cast to std::shared_ptr and added to the vector again.

Compliance with Standard

The behavior of std::shared_ptr is guaranteed by the C standard. The standard specifies that the deleter function is preserved throughout all operations, including casting. However, implementations may differ in the specific mechanism used to achieve this behavior.

Conclusion

While std::shared_ptr can be used for arbitrary cleanup, it's important to note that its primary intended purpose is managing shared ownership of objects. Using it for other purposes, such as destruction cleanup, should be done with caution and with an understanding of the possible implementation details.

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