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From: jbuck@galileo.berkeley.edu (Joe Buck)
Newsgroups: comp.lang.c++
Subject: Re: Whatever happened to virtual data?
Message-ID: <11833@pasteur.Berkeley.EDU>
Date: 9 Mar 91 23:21:31 GMT
References: <1991Mar7.204853.22289@watmath.waterloo.edu>
Sender: news@pasteur.Berkeley.EDU
Reply-To: jbuck@galileo.berkeley.edu (Joe Buck)
Lines: 40

In article <1991Mar7.204853.22289@watmath.waterloo.edu>, gjditchfield@watmsg.uwaterloo.ca (Glen Ditchfield) writes:
[ In reference to the dungeons-and-dragons game problem where the programmer
   wants a static varible for each derived class ]

|> Virtual data might also help programmers who want to implement their own
|> type tests.
|>    My guess is that the proposal was dropped because virtual functions
|> could provide the facility without much ugliness.

Yes, let's say you want a flag associated with each class derived from class
Thing, but you want a separate word for each class.  You can get the effect
of a virtual static data variable by using a virtual function that returns
a reference.  That is,

class Thing : {
	...
	virtual int& known() = 0;
};

class SomeKindOfThing : public Thing {
	...
	int& known() {
		static int knownFlag;
		return knownFlag;
	}
};

Notice that the knownFlag static variable inside the function works
very much like a virtual static class variable.

This is another case that makes it appear that a more powerful template
ability is needed, one that can specify how a virtual method is to
be overridden in each class: you need to write the above for each
derived class, because even though the function is exactly the same,
the knownFlag variable is different for each class.


--
Joe Buck
jbuck@galileo.berkeley.edu	 {uunet,ucbvax}!galileo.berkeley.edu!jbuck