Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!samsung!uunet!brunix!sdm
From: sdm@brunix (Scott Meyers)
Newsgroups: comp.lang.c++
Subject: Re: Design Problem
Message-ID: <22211@brunix.UUCP>
Date: 5 Dec 89 01:49:10 GMT
References: <22137@brunix.UUCP> <11258@csli.Stanford.EDU> <RICH.89Dec4144121@kalliope.Rice.edu>
Sender: news@brunix.UUCP
Reply-To: sdm@norton.UUCP (Scott Meyers)
Organization: Brown University Department of Computer Science
Lines: 39

In article <RICH.89Dec4144121@kalliope.Rice.edu> rich@Rice.edu (Carey R. Murphey) writes:
>Would it be reasonable to set the virtual memeber function  pointer
>to NULL (0) by default to indicate there is no function defined by
>default?  Here's an idea of how that might work:
>
>class ARC
>{
>  virtual void execute() = 0; // there is no execute method by default.
>}
>
>ARC an_arc;
>
>if (an_arc.execute)	
>  an_arc.execute(); // execute the method if it exists.
>
>I dunno... I haven't actually tried this to see if it works.  I think
>libg++ uses this in it's `Bag' container classes.

This is a good idea, and in fact it works with g++, but the AT&T compiler
won't take it.  (Actually, CC generates a warning -- "address of bound
function" -- and then cc chokes.)  I played around with this a bit, and it
doesn't look like there's any obvious way to dynamically determine the
address of a member function for a polymorphic object.  That is, given a
pointer to an object p and a virtual member function mf, there doesn't seem
to be any way to get the address of p->mf.  If anybody comes up with a
way to do this, I'd like to hear about it!

Scott
sdm@cs.brown.edu

S
i
g
h

S
i
g
h