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From: moose@svc.portal.com
Newsgroups: comp.lang.objective-c
Subject: Re: Requirements for Class Variables for Objective-C
Message-ID: <1990Oct17.184431.14195@svc.portal.com>
Date: 17 Oct 90 18:44:31 GMT
References: <5638@stpstn.UUCP> <270A66D0.14263@ics.uci.edu> <kenneth.655324438@exklusiv> <GREGOR.90Oct14231438@spade.parc.xerox.com> <1990Oct15.185718.8342@svc.portal.com> <10777@pt.cs.cmu.edu>
Organization: Software Venture
Lines: 68

In article <10777@pt.cs.cmu.edu> dandb@k.gp.cs.cmu.edu (Dean Rubine) writes:
[let's leave these choices up for all future posts]
The choices:

 #1 -- each subclass has its own copy of its parent's class variables
 
 #2 -- all subclasses share a single copy of their parent's class variables

>
>     As I read Mr. Rutman's reasoning, given choice #2, all I'd have to do to
>associate an event handler with every subclass of View is declare a variable
>(class or static) in EVERY subclass, and provide appropriate methods in EVERY
>subclass.  Yeech.  Note that I'd likely be repeating the same code in every
>subclass - this seems contrary to the object-oriented paradigm, in which
>inheritance is supposed to be used to avoid such of repetition of code.  This
>problem doesn't come up with class variables of kind #1.    

A bit of code is going to follow.


int myGlobal;

+ saveMyGlobal:(int)number
{
	myGlobal = number;
	return self;
}

+ (int)getMyGlobal
{
	return myGlobal;
}

now, all references to myGlobal are through these lovely factory methods.
(Instance methods would work fine, but who cares :->)

Subclass now.
We want to override myGlobal with

int myOwnGlobal;

We override saveMyGlobal and getMyGlobal with

+ saveMyGlobal:(int)number
{
	myOwnGlobal = number;
	return self;
}

+ (int)getMyGlobal	{return myOwnGlobal;}

We could even go so far as to have something like

+ saveMyGlobal:(int)number
{
	myGlobal -= myOwnGlobal;	//to remove the old value
	myGlobal += number;		//keep a total for all subclasses
	myOwnGlobal = number;
	return self;
}

Now, show me how to do that in #1, a case where I actually want the subclass
to have its own copy of the variable and a copy of the superclasses variable.
-- 
Michael Rutman				|	moose@svc.portal.com
Cubist					|	makes me a NeXT programmer
Software Ventures			|	That's in Berkeley
smile, you're on standard disclaimer	|	<fill in with cute saying>