Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!tut.cis.ohio-state.edu!att!cbnewsc!jwd
From: jwd@cbnewsc.ATT.COM (joseph.w.davison)
Newsgroups: comp.object
Subject: Re: "Messaging" (Re: Reentrant code)
Message-ID: <4115@cbnewsc.ATT.COM>
Date: 26 Oct 89 20:06:56 GMT
References: <135300010@p.cs.uiuc.edu>
Reply-To: jwd@cbnewsc.ATT.COM (joseph.w.davison,55515,ih,6n503,312 979 4920)
Organization: AT&T Bell Laboratories
Lines: 34

In article <135300010@p.cs.uiuc.edu> johnson@p.cs.uiuc.edu writes:
>
>
>I said
>>> There is no essential difference between Smalltalk messages and C++
>>> virtual functions.
>
...
>The big advantage of Smalltalk untyped messaging is that it is much
>better suited to experimental programming.  Virtual functions are
>closely tied to C++'s type system, which forces the programmer to
>decide interfaces early.  When the interfaces turn out to be wrong,
>as usually happens, the programmer has to make lots of revisions.
>However, from a high-level design point of view, I don't think that
>there is much difference between them.
>

Basically, I agree, C++ virtual functions are pretty much the same
as Smalltalk messages.  However, one cannot escape the static typing/
dynamic typing distinction.  The first time I came to see the big
difference between C++ and Smalltalk was when I tried to implement
ComplexNumber.  In Smalltalk, when  I operated on a ComplexNumber
and the result had a 0 imaginary component, I returned only the 
real component, whatever type it was.  That works nicely, to simplify
many methods, and is similar to the way I use complex numbers.  When
I tried to write ComplexNumber::operator+(...) in C++, it became
very clear what the difference was.

Static typing not only forces early decisions about interfaces, it also
precludes some approaches to problems.  Some people think that's an
asset; I tend to think some of the decisions it forces early are better
made later.
-- 
 Joe Davison      jwd@ihlts.att.com