How Does `std::shared_ptr` Maintain Destructor Functionality Despite Type Erasure?

Type Erasure and the Functionality of std::shared_ptr

When working with the std::shared_ptr class, it's commonly observed that std::shared_ptr behaves in a surprising yet functional manner. This phenomenon stems from the underlying concept of type erasure employed by std::shared_ptr.

Type Erasure and Destructor Invocation

std::shared_ptr utilizes type erasure by internally storing a deleter function that is invoked when the pointer is destroyed. This function is responsible for deallocating the memory pointed to by the shared pointer. Importantly, the deleter function is type-specific, meaning that it knows how to destruct objects of a specific type.

std::shared_ptr and Type Erasure

Casting a std::shared_ptr to std::shared_ptr does not remove the type-specific deleter function. The deleter remains associated with the shared pointer and will still be invoked upon destruction, even though the explicit type information has been erased. This is because the deleter function is stored separately from the pointed-to object and is not affected by the type conversion.

Consequence for Destructor Invocation

When a std::shared_ptr is destroyed, the stored deleter function is invoked. If the deleter was previously associated with a std::shared_ptr, it will still attempt to destruct the pointed-to object using the destructor for type T. This is why code that populates a std::vector<:shared_ptr>> with std::shared_ptr objects will still invoke the destructor for test.

Guaranteed Behavior by the Standard

While the current behavior of std::shared_ptr in this regard is not explicitly stated by the C standard, it is widely assumed to be a side effect of the type erasure implementation used by the most common compilers. However, future changes to the internal implementation of std::shared_ptr could potentially break code that relies on this behavior. Therefore, it is not recommended to rely on the functionality of std::shared_ptr as it is not guaranteed by the standard.

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