Virtual destructor and undefined behavior

Question

This question is different than \'When/why should I use a virtual destructor?\'.

struct B {
  virtual void foo ();
  ~B() {}  // <--         


        

Answer 1

As reaffirmed by others this is totally undefined because the Base's destructor is not virtual, and no statements can be made by anybody. See this thread for a reference to the standard and further discussion.

(Of course, individual compilers are entitled to make certain promises, but I haven't heard anything about that in this case.)

I find it interesting though, that in this case I think that malloc and free are better defined in some cases than new and delete. Perhaps we should be using those instead :-)

Given a base class and a derived class, neither of which have any virtual methods, the following is defined:

Base * ptr = (Base*) malloc(sizeof(Derived)); // No virtual methods anywhere
free(ptr); // well-defined

You might get a memory leak if D had complex extra members, but apart from this is is defined behaviour.