Decoupling classes

Hi, I've looked through the FAQ, and I can't seem to find an answer to
this one. Can anyone point me to a design pattern that will produce the
desired behaviour illustrated below please? I know why it doesn't print
"W visiting B", and I've read about double dispatch now too. What I'd
really like though is to keep A and V completely unaware of the
existence of B and W and still end up with "W visiting B" getting
printed. Ideally too I'd like to avoid putting a dynamic_cast in
W::visit(A&).

Is there a nice design pattern for doing this? Or an I searching for the
impossible.

Thanks for any advice,
Alan

class A {
};

class V {
public:
   virtual void visit(A& a) = 0;
};

class B : public A {
};

class W : public V {
public:
   virtual void visit(A& a) {
      std::cout << "W visiting A" << std::endl;
   }

   virtual void visit(B& b) {
      std::cout << "W visiting B" << std::endl;
   }
};

int main(void) {
   B b;
   A a;
   A *t = &a;

   W *v = new W();

   v->visit(*t);
   t = &b;
   v->visit(*t);

   return 0;
}