using sister inheritance

Question

let us suppose some (legacy) code, which cannot be touched, declare

struct B{
 public:
  void f(){}
};

and let us suppose to have

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

is it possible to make an A subclass call B::f without explicitly calling f(), i.e. instead of

  struct C: public A, public B{
   void f(){
   B::f();
  }
 };

having something like

 struct C:virtual public A,virtual public B{

 };

(note that this last class is abstract, for the compiler A::f is not defined)

Answer 1

Directly in C++, it is impossible to dispatch polymorphically based on some implicit matching of B's functions to A's. You could resort to some kind of code generation using gccxml or other similar products, but if there's only a hundred functions a macro can reduce the forwarding to a one-liner anyway - not worth introducing extra tools unless you've got thousands and thousands of these to do.

Answer 2

Make an implementation of A which delegates to B:

class A_Impl : public A
{
public:
    virtual void f()
    {
        b.f();
    }
private:
    B b;
}

Implement C by deriving from A_Impl:

class C: public A_Impl
{
};

Or, if you only want to show A in the inheritance hierarchy, derive publicly from A and privately from A_Impl:

class C: public A, private virtual A_Impl
{
};