Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!ames!sun-barr!rutgers!att!cbnewsl!dog
From: dog@cbnewsl.ATT.COM (edward.n.schiebel)
Newsgroups: comp.lang.c++
Subject: Re: Objectifying incoming messages?
Message-ID: <1567@cbnewsl.ATT.COM>
Date: 22 Aug 89 11:35:17 GMT
References: <828@otc.otca.oz>
Organization: AT&T Bell Laboratories
Lines: 44

From article <828@otc.otca.oz>, by gregw@otc.otca.oz.au (Greg Wilkins):

[stuff deleted]

> However I think you have invented an extra problem for yourself.  
> If you cannot completely construct an object until a message is sent and
> a reply recieved, then
> I see the the solution is to construct a base class, this bass class
> then sends the message to find out
> which particular specialization it is and coerces itself to a derived
> class when it recieves the reply.
> 
You cannot do this.  The memory allocated at the time of the call to
the constructor is for a base class. By coercing it to a derived class
you
	1. are probable going to walk of the end of your memory
	2. are requiring a base class to know of its derived classes,
	   something which you may want to avoid.
	
One possible way around this is to define a base class which defines
the interface to your hierarchy of objects which manages only
a pointer to another base class, the real base class of your hierarchy.
The interface-base's constructor can figure out what type of object
is needed, allocate/construct it, and set its member pointer-to-base-class
to the new object.  It costs you a level of indirection, but it works.
Another win with this scheme, is you can change the apparent type of the 
object at run-type by deleting the current poniter and re-creating a
new different one from the hierrchy.
i.e.:
	class interface_base {
	public:
	   // interface functions
	   void interface_function1() {ptr->interface_function1();}
	private:
	   abstract_base* ptr; // points to one of many possibilities
	                       // in the hierarchy
	};
	
	Ed Schiebel
	AT&T Bell Laboratories
	...att!vilya!dog
	
ps.
thanks to jonathan shopiro for this idea a couple years ago!