C + + empty class size

Class A {};
Class b{};
Class C:public a{
virtual void fun () = 0;
};
Class D:public B,public c{};

Class A, B is obviously empty class, its size should be 0, why the compiler output is 1? This is the reason we have just described the instantiation (empty class can also be instantiated), each instance has a unique address in memory, in order to achieve this, the compiler will often give an empty class implicitly add a byte, so that the empty class in memory after instantiation has a unique address. So a, B has a size of 1.

and Class C is derived from the Class A, it has a pure virtual function, because of the reason for the virtual function, there is a pointer to the virtual function (VPTR), the 32-bit system is allocated to the size of the pointer is 4 bytes, so finally the size of Class C is 4.

C + + empty class size