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From: vinoski@apollo.hp.com (Stephen Vinoski)
Newsgroups: comp.lang.c++
Subject: Re: About Lists and things...
Message-ID: <1991Jun27.152553.12762@apollo.hp.com>
Date: 27 Jun 91 15:25:53 GMT
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In article <1991Jun27.095856.2@minerva.inesc.pt> fmhv@minerva.inesc.pt (Fernando Manuel Vasconcelos) writes:
>In article <28692A4A.59B7@tct.com> chip@tct.com (Chip Salzenberg) writes:
>>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.
>
>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 .

The answer to your objections is that you shouldn't be designing your software
so that you lose the "compile-time type" as Chip calls it.

If you're putting a Circle onto a ShapeList, it effectively becomes a Shape.  In
a sense, it is no longer a Circle (though the use of virtual functions allow it
to keep some of its "circle-ness").  Its interface becomes a Shape interface,
not a Circle interface.

If you need a Circle interface, design your software so that you always put your
Circle onto a CircleList.  Only put your Circles onto ShapeLists when you're
dealing with Shapes.

I agree with Chip 300%.


-steve

| Steve Vinoski  (508)256-0176 x5904       | Internet: vinoski@apollo.hp.com  |
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-- 
| Steve Vinoski  (508)256-0176 x5904       | Internet: vinoski@apollo.hp.com  |
| HP Apollo Division, Chelmsford, MA 01824 | UUCP: ...!apollo!vinoski         |