multiple inheritance in C++, it looks like diamond but it is not

Question

A multiple inheritance problem in C++:

It looks like a diamond problem, but it is not. It cannot be solved by adding virtual inheritance.

struct A {
   virtual void f() { cout << "A::f()" << endl; };
};

struct B : public virtual A {
   void f() { cout << "B::f()" << endl;};
};

struct C :  public virtual A {
  void f() { cout << "C::f()" << endl;};
};

struct D : B, C { };

int main() {
  D d;

  B* bp = &d;
  A* ap = bp;
  D* dp = &d;

  ap->f();
  dp->f(); // compile error here because D has two candidates f() to override. 
        // error: request for member f is ambiguous
        // error: candidates are: virtual void C::f()
        // error:                 virtual void B::f()

  }

Am I right?

Answer 1

It looks like a diamond problem, but it is not.

Yes it is; that's the classic "diamond" pattern. Perhaps you mean, the problem isn't caused by the presence of the diamond? That's certainly true, so I wonder why you included that red herring in the question. You'll get the same issue from a simpler example:

struct B {
   virtual void f() { cout << "B::f()" << endl;};
};

struct C {
   virtual void f() { cout << "C::f()" << endl;};
};

struct D : B, C { };

compile error here because D has two candidates f() to override.

No, the error is because D doesn't override them, so there are two candidates for the function call. The virtual function is overridden in both direct base classes, but not in the derived class, so there is no unique final overrider.

You need to decide what D::f() is supposed to do, and implement it accordingly.