Calling a function of sister class C++

Question

Consider the following code:

#include <iostream>

class A
{
public:
    virtual void f() = 0;
    virtual void g() = 0;
};

class B : virtual public A
{
public:
    virtual void f()
    {
        g();
    }
};

class C : virtual public A
{
public:
    virtual void g()
    {
        std::cout << "C::g" << std::endl;
    }
};

class D : public C, public B
{
};

int main()
{
    B* b = new D;
    b->f();
}

The output of the following program is C::g.

How does the compiler invoke a function of a sister class of class B??

Answer 1

N3337 10.3/9

[ Note: The interpretation of the call of a virtual function depends on the type of the object for which it is called (the dynamic type), whereas the interpretation of a call of a non-virtual member function depends only on the type of the pointer or reference denoting that object (the static type) (5.2.2). — end note ]

The dynamic type is type to which pointer really points, not type that was declared as pointed type.

Therefore:

D d;
d.g(); //this results in C::g as expected

is same as:

B* b = new D;
b->g();

And because inside your B::f call to g() is (implicitly) called on this pointer whose dynamic type is D, call resolves to D::f, which is C::f.

If you look closely, it's the (exactly) same behaviour as shown in code above, only that b is now implicit this instead.

That's the whole point of virtual functions.