Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!microsoft!jimad
From: jimad@microsoft.UUCP (Jim ADCOCK)
Newsgroups: comp.lang.c++
Subject: Re: Whatever happened to virtual data?
Message-ID: <71205@microsoft.UUCP>
Date: 11 Mar 91 21:43:54 GMT
References: <1991Mar7.204853.22289@watmath.waterloo.edu>
Reply-To: jimad@microsoft.UUCP (Jim ADCOCK)
Organization: Microsoft Corp., Redmond WA
Lines: 36

In article <1991Mar7.204853.22289@watmath.waterloo.edu> gjditchfield@watmsg.uwaterloo.ca (Glen Ditchfield) writes:
|This may be all a caffeine dream, but I think that, a while back, serious
|consideration was being given to the idea of "virtual data".  I think the
|idea was that a base class would define a virtual data member, and that
|derived classes could redefine its value.  Like a virtual function, a
|virtual data member would be shared by all instances of a class and any
|derived classes that did not redefine it.  Unlike static members, it would
|not be shared with instances of derived classes that did redefine it.

Today one would do this something like:

class X
{
	static int _virt_data;
public:
	inline virtual int& virt_data() { return _virt_data; }
	// or get_virt_data(), set_virt_data(int) ....
};

And that's all there is to it until one implements another class that
wants a distinct _virt_data.  Then a programmer has to re-implement
the two lines that have _virt_data's on them, or alternatively, invoke
a macro hack to create the two lines from one macro.

There seems to be two issues here: 1) the ugliness required to implement
this each time.  2) The speed of the resulting "inline virtual" call.

Issue #2 can be solved, if a particular compiler cares enough, through the
use of fat tables and/or customization instead of vtables.  Thus issue
#2 is not a language issue, but rather a quality-of-implementation issue.

Issue #1 could be solved, or at least greatly improved, if the capabilities
of templates were improved to allow greater applicability to member functions.

Thus, I claim its not necessary to add virtual member variables,
rather we should tweak the capabilities of templates.