Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!rice!brazos!rich
From: rich@Rice.edu (Carey R. Murphey)
Newsgroups: comp.lang.c++
Subject: Re: Design Problem
Message-ID: <RICH.89Dec4144121@kalliope.Rice.edu>
Date: 4 Dec 89 20:41:21 GMT
References: <22137@brunix.UUCP> <11258@csli.Stanford.EDU>
Sender: root@rice.edu
Organization: Department of Electrical and Computer Engineering, Rice
	University
Lines: 36
In-reply-to: keith@csli.Stanford.EDU's message of 4 Dec 89 17:25:40 GMT

In article <11258@csli.Stanford.EDU> keith@csli.Stanford.EDU (Keith Nishihara) writes:

   >What I really want to be able to say is "if function f() is defined for
   >object o, then call o.f()".  Is there a good way to acheive this effect?

   It seems to me that the virtual function defined on ARC with no ops
   for non executable arcs is what you need.  However, one other
   approach which might be considered if you have lots of functions
   like execute, and especially if the functions applicable to executable arcs
   are from an open ended set, is as below:

[code followed...]

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.

--
Rich@rice.edu