Default inheritance access specifier

Question

If I have for example two classes A and B, such that class B inherits A as follows:

class B: public A

Answer 1

AS other casting problem you have

class A { virtual void test() = 0; };
class B : virtual public A { virtual void testb() {} };
class C : virtual public A { virtual void testc() {} };
class D : public B, public C {
 virtual void test() override {}
}

void main() {
   D d;
   void* v = &d;
   A* a = &d;

   ((D*)A)->test(); //OK
   ((D*)v)->test(); //undefined behavior (that call testb() in vtable logic at 1st inheritance position)
   dynamic_cast(v)->test(); //compile error cast from void* not permitted

   //resolution
   void* x = a;
   ((D*)x)->test(); //OK but as you can see, you must to store a* in x*

}