Path: utzoo!attcan!uunet!cs.utexas.edu!usc!apple!lsr
From: lsr@Apple.COM (Larry Rosenstein)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Multiple Inheritance for HandleObjects in C++
Message-ID: <8925@goofy.Apple.COM>
Date: 29 Jun 90 16:15:47 GMT
References: <15132@reed.UUCP>
Organization: Apple Computer, Inc.
Lines: 28

In article <15132@reed.UUCP> bowman@reed.UUCP (Eric Bowman) writes:
>I've just discovered, to my horror, that you can't create HandleObjects
>with multiple base classes in MPW C++.  Are there any work arounds for this?
>Anyone know why this is the case?

The reason is that the standard implementation of C++ multiple inheritance
requires that you pass a pointer to the middle of an object's storage.
That's because each class' method is compiled to access fields using fixed
offsets.  (If you look at the generated C code for a native C++ class, you
will see that it adds an offset to the object pointer before calling a
method.)

Methods of HandleObject subclasses expect to be passed a handle,
and it's not possible to pass a handle that refers to the middle of an
object.  Plus you have to worry about the handle moving if the heap
compacts.  

One workaround is to inherit from a primary class, and delegate some
functions to to instances of the secondary classes.  In other words, instead
of inheriting from A & B, you inherit only from A, and keep an instance of B
around.  Methods that need to access B's functionality do so by sending a
message to the instance of B.

-- 
		 Larry Rosenstein,  Object Specialist
 Apple Computer, Inc.  20525 Mariani Ave, MS 46-B  Cupertino, CA 95014
	    AppleLink:Rosenstein1    domain:lsr@Apple.COM
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