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From: chewy@apple.com (Paul Snively)
Newsgroups: comp.object
Subject: Persistent Objects Was: Re: Pre-computing objects
Message-ID: <4908@internal.Apple.COM>
Date: 26 Oct 89 16:52:16 GMT
References: <4671be53.20b6d@apollo.HP.COM>
Sender: usenet@Apple.COM
Organization: Apple Computer, Inc.
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In article <4671be53.20b6d@apollo.HP.COM> vinoski@apollo.HP.COM (Stephen 
Vinoski) writes:
>   Some time ago a discussion arose in comp.lang.c++ about reading 
objects in
> from, say, a disk file.  There were several answers, but none that 
seemed all
> that great, so I hope to start a discussion of the subject here.

Good!  The discussion that you're trying to start essentially revolves 
around what are usually called "persistent objects."

Persistent Objects are almost as hard to define (let alone implement) as 
Object-Oriented Programming is.  When I think of a persistent object, I 
think of an object that can be stored on disk (that includes both instance 
variables AND methods, folks) and, if it's the first object of its class 
to be so stored, the appropriate class information will be stored as well 
(out of necessity).  Persistent objects should also be able to be read 
back in from disk, of course.

This isn't by any means a trivial set of requirements.  First of all, 
saving methods can be anywhere from tricky to impossible.  As if that 
weren't bad enough, what happens if you save some object (and its 
associated class structure), and then try to read it back into a running 
system where the class structure is perhaps superficially the same (e.g. 
the class has the same name) but differs semantically--that is, it has new 
class/instance variables or new/different methods?

(Picture a poor programmer running into the night, screaming...)

Ok, so these are some of the more obvious challenges.  I'd recommend 
looking at lots of OOPSLA proceedings to get some idea as to where to look 
next.  I'd also recommend reading up on distributed object systems.

By the way, C++ is one of the harder languages to do persistence in, in my 
opinion (compared to, say, Smalltalk or Common Lisp with CLOS).

__________________________________________________________________________
Just because I work for Apple Computer, Inc. doesn't mean that they 
believe what I believe or vice-versa.
__________________________________________________________________________
C++ -- The language in which only friends can access your private members.
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