Question

As mentioned in this answer simply calling the destructor for the second time is already undefined behavior 12.4/14(3.8).

For example:

class Class {
public:
    ~Class() {}
};
// somewhere in code:
{
    Class* object = new Class();
    object->~Class();
    delete object; // UB because at this point the destructor call is attempted again
}

In this example the class is designed in such a way that the destructor could be called multiple times – no things like double-deletion can happen. The memory is still allocated at the point where delete is called – the first destructor call doesn’t call the ::operator delete() to release memory.

For example, in Visual C++ 9 the above code looks working. Even C++ definition of UB doesn’t directly prohibit things qualified as UB from working. So for the code above to break some implementation and/or platform specifics are required.

Why exactly would the above code break and under what conditions?

Answer 1

Destructors are not regular functions. Calling one doesn’t call one function, it calls many functions. Its the magic of destructors. While you have provided a trivial destructor with the sole intent of making it hard to show how it might break, you have failed to demonstrate what the other functions that get called do. And neither does the standard. Its in those functions that things can potentially fall apart.

As a trivial example, lets say the compiler inserts code to track object lifetimes for debugging purposes. The constructor [which is also a magic function that does all sorts of things you didn’t ask it to] stores some data somewhere that says “Here I am.” Before the destructor is called, it changes that data to say “There I go”. After the destructor is called, it gets rid of the information it used to find that data. So the next time you call the destructor, you end up with an access violation.

You could probably also come up with examples that involve virtual tables, but your sample code didn’t include any virtual functions so that would be cheating.