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c++destruction

Undefined behavior during destrction?

Let's consider the following code:

#include <iostream>

struct A{ virtual void foo(){ } };

struct B : A { virtual void foo(){ } };

A *a = new B;


int main()
{
    delete a; //UB?
}

I deliberately didn't define a virtual destructor. The compiler printed a message about causing UB, is it true?

Solution

  • Formally you have UB if you delete through a pointer to T, which is not the most derived type, and T doesn't have a virtual destructor.

    In practice you can get away with it if you don't have any data mambers, but it's still very ungood and unnecessary practice.

    Note: when you use a shared_ptr it creates a deleter function at the point of initialization, and that deleter function can remember the original type, which, if that type is the most derived type, ensures well-defined deletion. E.g. in your case shared_ptr<A> p( new B ); would be OK.