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From: fmhv@minerva.inesc.pt (Fernando Manuel Vasconcelos)
Newsgroups: comp.lang.c++
Subject: Re: About Lists and things...
Message-ID: <1991Jun27.095856.2@minerva.inesc.pt>
Date: 27 Jun 91 10:00:52 GMT
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In article <28692A4A.59B7@tct.com> chip@tct.com (Chip Salzenberg) writes:

>By "static type" I mean "compile-time type".  Perhaps an example will
>clarify my point.
>
>If you put a Circle into a ShapeList, you lose the Circle's
>compile-time type, because ShapeList.first() returns |Shape*|.
>Suppose that you decide to call a Circle-specific function on a Circle
>stored in a ShapeList.  Some would cast the |Shape*| to a |Circle*|
>and carry on.  My coding practice would require you to replace the
>ShapeList with a CircleList.
>
>Of course, that change may not be practical, perhaps because there are
>other non-Circle objects in the ShapeList.  If so, congratulations!
>You have just discovered that the function in question is actually a
>Shape function in disguise.  You should therefore add it to the Shape
>interface, presumably as a new virtual function.
>
>Clearer now?

I may be missing the point, however consider two objections to your proposal:

1. A pratical one: You may not be able to change shape.h because it belongs
to a library which you have only in binary ( of course you have the .h's but
you can't change them ... )

2. A conceptual one: That means the interface of a base class depends on the
interface of the derived classes. Using your example it is normal that a 
circle should know to answer it's radius. If I keep all my graphical
objects in a list of shapes* , I'll have to add a getRadius message to
the Shape class, only because a circle knows what that means ... But a GENERAL
shape doesn't .

Only my two cents ...


--
Fernando Manuel Hourtiguet de Vasconcelos  INESC - Instituto de Engenharia de
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