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From: "Warren W. Gay VE3WWG" <ve3wwg@home.com>
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Newsgroups: comp.lang.ada
Subject: Re: C++ virtual function mechanism in Ada ?
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Date: Wed, 25 Jul 2001 16:10:11 GMT
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Date: 2001-07-25T16:10:11+00:00
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Jean-Pierre Rosen wrote:
> 
> "Didier.pieroux" <dpieroux.no-spam@pi.be> a �crit dans le message news: IDz77.6315$ar1.18443@www.newsranger.com...
> [snipped for brievity]
> 
> > One way to get the job done is to use:
> > procedure F(A: in T1) is begin Put("F1:");     E(T1'Class(A)); end F;
> > That switches the dynamical dispatching on and everybody is happy. But the bad
> > point is that I have the impression that extending a base type can require
> > modifications in that base type implementation. Too bad for code reliability,
> > isn't it ?
> This is the right way to do it.
> 
> >
> > So, my first question is: knowing that a characteristic feature of primitive
> > operations is to call them with objects of a derived type, why did the Ada
> > designers choose not to use dynamical dispatching when calling a primitive
> > function ?  I guess that most of the time Ada programmers (as C++ programmers)
> > want to really call the primitive operation corresponding to the type of the
> > actual parameter, no ?
> A fundamental question, so let's go.
> There is a fundamental difference between C++ and Ada: in C++, dispatching is a property of the function (virtual or not).
> Second: Ada is a language driven by types. It is always the *type* of the parameters that determines what is called. 

Don't forget that in Ada95, the return value type can also be important ;-)

C++ does not dispatch on the return type.

-- 
Warren W. Gay VE3WWG
http://members.home.net/ve3wwg