About multiple inheritance and ambiguity

Question

In the following example:

class A {
public:
    virtual void f() { cout << "a" << endl; }
    virtual void h() { cout << "A" << endl; }
};
class s1 : public A {
public:
    virtual void f() { cout << "s1" << endl; }
};
class s2 : public A {
public:
    virtual void h() { cout << "s2" << endl; }
};
class GS : public s1, public s2 {
public:
};
int main()
{
    s1 *q = new GS;
    q->h();//no problem

    GS a;
    a.h();//error

}

Why does a.h(); give an ambiguity error yet q->h(); doesn't?

Doesn't *q have an instance of GS which should cause the same ambiguity problem?

Answer 1

Your use of multiple inheritance causes two instances of A to appear in GS. When you use S1 *q to access the GS instance, it follows the A instance associated with S1. Since S1 does not implement h(), the output of q->h() will be the implementation provided by A itself.

If you want q->h() to use the implementation provided by S2, then you need to create a diamond using virtual inheritance. Doing so will also remove the ambiguity when using a.h(), since virtual inheritance will cause only one instance of A to appear in GS.

class s1 : virtual public A {
public:
    virtual void f() { cout << "s1" << endl; }
};
class s2 : virtual public A {
public:
    virtual void h() { cout << "s2" << endl; }
};