Re: Decoupling classes

From:
Tom Widmer <tom_usenet@hotmail.com>
Newsgroups:
comp.lang.c++
Date:
Tue, 25 Apr 2006 14:48:27 +0100
Message-ID:
<e2l993$n6t$1@emma.aioe.org>
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&).


I don't think those two requirements are compatible - either you need V
to know about both A and B (in which case you have the normal, cyclic
visitor pattern), or you need to use dynamic_cast (in which case you
have the acyclic visitor pattern).

Google for "cyclic visitor" and "acyclic visitor". A good implementation
of both is available as part of the Loki library. See
http://sourceforge.net/projects/loki-lib/

Here's your code in acyclic form:

struct V {
   virtual ~V(){}
};

struct Base {
   virtual ~Base(){}
   virtual void accept(V&) = 0;
};

struct A;

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

struct A: public Base {
   virtual void accept(V& v)
   {
     if (AV* av = dynamic_cast<AV*>(&v))
       av->visit(*this);
   }
};

struct B;

struct BV: public virtual V {
   virtual void visit(B& b) = 0;
};

struct B : public A {
   virtual void accept(V& v)
   {
     if (BV* bv = dynamic_cast<BV*>(&v))
       bv->visit(*this);
   }
};

#include <iostream>

struct W : public AV, public BV {
    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->accept(*v);
    t = &b;
    t->accept(*v);
    std::cin.get();
    return 0;
}

Tom

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