Re: Decoupling classes

From:

"Victor Bazarov" <v.Abazarov@comAcast.net>

Newsgroups:

comp.lang.c++

Date:

Tue, 25 Apr 2006 09:30:26 -0400

Message-ID:

<e2l89i$hou$1@news.datemas.de>

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
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();

Did you mean

V *v = new W();

? You would probably need a virtual destructor in 'V', of course.
It doesn't really matter, though. What you're trying to do _is_
impossible.

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

No matter how you slice it, the type of 't' is A*. There is no way
for the program to learn where 't' came from. If 'A' or 'B' _were_
polymorphic types, one could try using 'dynamic_cast', yet it might
still fail (say, if 't' is part of another class deriving from 'A').

Double dispatch would help, if you allow that 'B' could know about 'W'.

return 0;
}

What problem are you trying to solve? Perhaps it's possible to do
using templates?

V
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