Path: utzoo!attcan!uunet!lll-winken!ames!sun-barr!apple!rutgers!att!westmark!mole-end!mat
From: mat@mole-end.UUCP (Mark A Terribile)
Newsgroups: comp.lang.c++
Subject: Re: address of virtual function (revisited)
Summary: Virtual consts?  A syntactic and semantic change ...
Message-ID: <182@mole-end.UUCP>
Date: 11 May 89 11:23:31 GMT
References: <904@garya.Solbourne.COM> <6590118@hplsla.HP.COM>
Organization: mole-end--private system.  admin: mole-end!newtnews
Lines: 39

> > >Again, for OOP to be affordable, these simple, everyday "get" and
> > >"put" type functionalities must be handled at minimal cost.

> > I get a frustrated by assertions of this kind.  By any standard C++
> > is "affordable", ...

> Sorry, I meant my comments in the context of the relative cost of
> virtual functions, compared to simple lookups as you discuss below:
  
	[Virtual function calls when all that's needed is a stored
	 value representing ``sizeof'']
 
> ...  Unfortunately, at least when using K&R compilers for the back end,
> cfront knows a lot more about the code to be produced than the backend
> compiler.  So cfront would be the place that a lot of the optimzations
> would have to occur.


I think that this is wrong.  Remember that anywhere the class is visible,
a programmer may derive a new class with a replacement function that can't
be implemented as a constant lookup.

The only way to avoid this is to make the original base class promise
that this won't be done, and then to enforce the promise.  This means a
*declaration* to that effect.

What you really want, I think, is a virtual const.  I think it's a neat
idea and useful for certain purposes, *but* it's a new language feature
and implementing every feature that might be composed at the fringe of the
language smacks of PL/I.  Have we forgotten wher *that* leads?

Allowing the constructor to initialize a const member with the sizeof is, all
in all, a reasonable idea, except that it only works for the base that owns
that member.  It has to be written as an assignment, I believe.  (Still not
too bad, though not quite as safe in the face of program changes.)
-- 

(This man's opinions are his own.)
From mole-end				Mark Terribile