Re: Decoupling classes

"mlimber" <>
25 Apr 2006 06:34:26 -0700
Alan Woodland wrote:

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

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

Thanks for any advice,

class A {

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

class B : public A {

class W : public V {
   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();

   t = &b;

   return 0;

The "problem" is that you have used a polymorphic reference to treat
your B object as an A object. Thus, the best function overload for
W::visit() is the one that accepts an A-reference. If you instead
passed a B-reference directly, it would happily use the other overload.
If you can't make such a change, you'll have to either downcast using
dynamic_cast to regain the lost type information or redesign. The
dynamic_cast, BTW, would not need to be present in V since W can
override the virtual function that accepts an A-reference. (The
B-reference overload obviously cannot be called with a V object except
where the B object is treated as an A, so V would still be ignorant of
B and W types.)

Cheers! --M

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