Should the destructor for a base class that you won't use to delete an object, be virtual?

Question

Say I have two base classes,

struct A {}; 
struct B {}; 

and a derived one that uses multiple inheritance

struct D : A, B {}; 

If my use scenario goes something like this:

A *obj = new D; 

i.e. I'll never use the B base class to refer to the derived object, do I have to make the destructor of both bases virtual? I'm currently declaring the destructor of B as protected to forbid other users from doing this, but is it enough?

What about the destructor of D?

Answer 1

It is not necessary for B's destructor to be virtual as long as a B* is never used to delete derived objects. See [expr.delete]/3:

... if the static type of the object to be deleted is different from its dynamic type, the static type shall be a base class of the dynamic type of the object to be deleted and the static type shall have a virtual destructor or the behavior is undefined.

In this case the "static type" is the type T cv where the operand to delete has type T cv *. So the requirements in your case are imposed on A, not on B.