C++ Static_cast over a pointer to virtual base class

Question

After about 6 o 7 years programming nearly exclusively on C++, I've discovered that:

struct A
{
   virtual ~A() = 0; // Abstract class
};

A::~A() {}

struct B : virtual A
{};

int main()
{
    A* a = new B;
    (void)static_cast<B*>(a);

    return 0;
}

throws a compiler error, because the standard doesn't allow to make a static cast from a pointer to a virtual base class. I assume that that's related to the memory layout associated to a virtual base class, but I would like to know the details.

your code compiles; didn't you mean `struct B : public virtual A`? — lorro, Jun 24 '16 at 13:40

score 0 · Answer 1 · answered Jun 24 '16 at 13:48

The reason is, the memory image of virtual A within B and that of A might differ. Without allowing it to differ, the well-known 'dared diamond' could not be solved (well, at least not easily, since we cannot request A to be aware of this and set up an ABI that's preferable to us). Think of virtual A as having a virtual function for each function and member access of A in B. It's not necessarily implemented like that, but you might model it that way.

C++ Static_cast over a pointer to virtual base class

1 Answers1