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From: allan_w@my-dejanews.com (Allan W)
Newsgroups: comp.std.c++
Subject: Re: automatic downcast support through function parameters
Date: Fri, 17 Jan 2003 00:23:54 +0000 (UTC)
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danielgutson@hotmail.com (danielgutson@hotmail.com) wrote
> allan_w@my-dejanews.com (Allan W) wrote
> 
> Hello Alan:
Allan.

> First, I'm afraid that virtual functions can only be member functions,
> as far as there is a 'virtual table OF a class' (there are no 'global'
> virtual tables).
> (I strongly suggest to compile or test your code before posting).

I accidentally left the word "virtual" in the two free-standing
functions. I also used the wrong notation for calling "callg."
Try this version, which may do what you want without any language
changes:
> >     struct Base {
            ~Base();
> >         virtual void callg() { g(this); }
> >     };
> >     struct Der : public Base {
> >         virtual void callg() { g(this); }
> >     };
        void g(Base*) { std::cout << "g(Base*)"; }
        void g(Der*)  { std::cout << "g(Der *)"; }
> >     void callg(Base*b) { b->callg(); }
> >     int main() {
            Base *b = new Base;
            Base *d = new Der;
            //g(*d) would call the wrong version, but callg() figures it out:
            b->callg(); // Displays "g(Base*)"
            d->callg(); // Displays "g(Der *)"
            delete b;
            delete d;
> >     }

> Second, I'm not trying to do anything similar with virtual functions,
> just hold abstraction of the received parameter:
> a function receives an 'upcasted' parameter (contravariant), ignoring
> the original derived class, and simply recover it when used.
> As many people posted in this thread, RTTI is needed.
> They are also right about compiling units: the linker should solve
> this.

I'm trying to show that you can accomplish the same thing today, without
a language change ('upcasted parameter'). Once you've called the right
virtual function (in this case callg()), the virtual function "knows" the
right type and can call the correct free-standing function (g).

> Finally, the Visitor pattern does not apply to this situation.

I didn't think it would, directly. I just meant that you could
apply some of the same principals to call the correct code
depending on what data type was used.

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