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From: kgorlen@helix.nih.gov (Keith Gorlen)
Newsgroups: comp.lang.c++
Subject: Re: Nonvirtual base classes in MI
Message-ID: <911@nih-csl.nih.gov>
Date: 1 Feb 91 19:14:02 GMT
References: <1159@zinn.MV.COM> <63176@brunix.UUCP>
Sender: news@nih-csl.nih.gov
Reply-To: kgorlen@helix.nih.gov (Keith Gorlen)
Lines: 56

In article <63176@brunix.UUCP>, sdm@cs.brown.edu (Scott Meyers) writes:
|> In reponse to my posting, Michael J. Vilot described his uses of nonvirtual
|> base classe with MI.  His examples were interesting, but I didn't see any
|> uses of a DAG.  Perhaps I wasn't explicit enough in what I was looking for.
|> What I want to see is an example that can be boiled down to this:
|> 
|>         A       class A { ... };
|>        / \
|>       /   \
|>      B     C    class B: public (or private) A { ... };
|>       \   /     class C: public (or private) A { ... };
|>        \ /
|>         D       class D: (optionally virtual) public (or private) B, 
|>                          (optionally virtual) public (or private) C { ... };
|> 
|> What's important here is that neither B nor C inherits from A as a virtual
|> base.  That results in the memory layout of a D object looking like this,
|> at least conceptually:
|> 
|>     +------------+
|>     | B's A Part |
|>     +------------+
|>     | B Part     |
|>     +------------+
|>     | C's A Part |
|>     +------------+
|>     | C Part     |
|>     +------------+
|>     | D Part     |
|>     +------------+
|> 
|> Notice that there are two A parts in a D object.
|> 

Then I would draw the DAG like this:

	A     A
	|     |
	B     C
	 \   /
	   D	


Examples 13-1 and 13-2 in "Data Abstraction and Object-Oriented Programming
in C++" show multiply inheriting from class Link so that derived classes can
be threaded on multiple linked lists; i.e. class A = Link, class B = AllLink,
class C = QLink, and class D = Vehicle.

However, Tom Cargill's article in the most recent issue of "The C++ Journal"
shows how to do this example without using MI.

-- 
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