NetNews Usenet Archive 1992 #26/NN_1992_26.iso/spool/comp/lang/cplus/16174

Text File | 1992-11-12 | 2.2 KB | 49 lines

Newsgroups: comp.lang.c++
Path: sparky!uunet!munnari.oz.au!metro!extro.ucc.su.OZ.AU!maxtal
From: [e-mail] (John MAX Skaller)
Subject: Re: Discriminating Variant Records Without switch() ??
Message-ID: <[e-mail]>
Sender: [e-mail]
Nntp-Posting-Host: extro.ucc.su.oz.au
Organization: MAXTAL P/L C/- University Computing Centre, Sydney
References: <[e-mail]> <[e-mail]> <[e-mail]>
Date: Thu, 12 Nov 1992 22:37:35 GMT
Lines: 36

In article <[e-mail]> [e-mail] (Brian Button) writes:
>The point I'm trying to make is that there is no common set of
>operations which may be performed on these arbitrary objects.
>
>Since the set of operations is not common, we need to know the type of the
>object before proceeding with processing. Have we painted ourselves
>into a corner? It seems that we need a switch() to do this, but in a
>proper OOP design, it shouldn't be necessary.
>

    Yes, it should be necessary. To me the implications of
a common base with arbitrary derived classes are that there is indeed
a single set of common operations that are known statically.
Then polymorphism allows this homogenous interface to be used
to access (via virtual functions) different derived objects.

    In the case of many unrelated classes one cannot use
polymorphism. To access each object you must know its type
statically, and the switch is the only way to do this, you must
select physically different code for processing each object,
and the number of said objects must be known and fixed
and embodied in separate physical code.

    The key problem here is to have an arbitrary set of
application programs P1, P2 .... each with a finite and fixed
set of classes interfacing with a single server which can store
arbitrary objects without losing the type information.

    One way to do that is to set up a global registry.


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        JOHN (MAX) SKALLER,         [e-mail]
    Maxtal Pty Ltd, 6 MacKay St ASHFIELD, NSW 2131, AUSTRALIA
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