Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!bionet!agate!ucbvax!hplabs!hp-ses!hpcuhb!hpindda!hardin
From: hardin@hpindda.HP.COM (John Hardin)
Newsgroups: comp.lang.c++
Subject: Re: Objectifying incoming messages?
Message-ID: <6170009@hpindda.HP.COM>
Date: 28 Jul 89 16:35:08 GMT
References: <444@cimshop.UUCP>
Organization: HP Information Networks, Cupertino, CA
Lines: 27

dog@cbnewsl.ATT.COM (edward.n.schiebel) writes:

> From article <444@cimshop.UUCP>, by davidm@cimshop.UUCP (David Masterson):
> > The fundamental question hinges around the lack of context of the incoming
> > message.  Therefore, the message user must look at the data in order to
> > determine what to coerce (definition?) the message into.  Is this appropriate
> > or is there a better way?
> > 
> The same problem arises reading objects from disk.  Until you look at
> the data, you don't know what class of object to instance so it
> can read itself.  It seems to me, this is one place you still
> need a header/type field and a switch statement.
>----------

Reading objects from disk is exactly the subject of one of the demo
programs that comes with Borland's Turbo Pascal 5.5 (with object oriented
extentions).  I haven't looked at it in detail, but I believe they
derive classes from their stream class and then the derived classes
are somehow "registered" with the stream class.  A get from the stream
then first reads the next object type and calls the "get" method of that
class.  This embeds the switch within the steam's "get" method.
BTW, I believe the demo is called CARDFILE.  Has anyone else looked at
this?  If you have, what do you think of it?

John Hardin
hardin%hpindgh@hplabs.hp.com
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