Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!swrinde!cs.utexas.edu!uunet!pdn!tscs!tct!chip
From: chip@tct.com (Chip Salzenberg)
Newsgroups: comp.std.c++
Subject: Re: Objects already contain type information => binary virtual functions
Message-ID: <27F63CE7.D89@tct.com>
Date: 31 Mar 91 19:47:51 GMT
References: <NGO.91Mar28093648@tammy.harvard.edu>
Organization: Teltronics/TCT, Sarasota, FL
Lines: 33

According to ngo@tammy.harvard.edu (Tom Ngo):
>The problem:
>
>    class B {
>        virtual void foo(B*);
>    };
>    class D : public B {
>        virtual void foo(D*);
>    };
>
>    void bar(B* b1, B* b2) {
>        b1.foo(b2);
>    }
>
>    D d1;
>    D d2;
>    bar(&d1,&d2);            // I want D::foo to be invoked

To suppose that D::foo() should override B::foo() is to misunderstand
the meaning of virtual functions, namely:

    A virtual function is _one_ function with potentially many
    implementations.

The code quoted above shows that D::foo() is prepared to work on a D,
while B::foo() is prepared only for a B.  Thus they are not the same
function.  They may be equivalent in one way or another; they may even
have the same body.  But they are not the _same_ function.  So the use
of "virtual" for these two functions is a design error.
-- 
Chip Salzenberg at Teltronics/TCT     <chip@tct.com>, <uunet!pdn!tct!chip>
   "All this is conjecture of course, since I *only* post in the nude.
    Nothing comes between me and my t.b.  Nothing."   -- Bill Coderre