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From: tomas@inmic.se (Tomas Lundstrom)
Newsgroups: comp.lang.c++
Subject: type casting
Message-ID: <TOMAS.91Feb6103918@basi.inmic.se>
Date: 6 Feb 91 09:39:18 GMT
Sender: news@sics.se
Distribution: comp
Organization: Swedish Institute of Microelectronics, Kista
Lines: 59


Hello,

I'm not sure that my first posting made it, so I try again (sorry if
it is a duplicate).

Suppose I have a class hierarchy

					A
				       / \
				      /   \
				     B     C
					  / \
					 /   \
					D     E

The descendants have no additional data, only additional functions.

What I want is a 'universal object' that can be used for objects of
all classes in the tree. The question is: Can I use an 'A' object and
just cast it to the descendant classes ? It works on our system, but I
suspect it might be implementation dependent. (In other words, is it
guaranteed that the layout of an A object and an E object is
identical, if C and E only adds functions to A ?)


If the answer to these questions is 'no', I was thinking of creating a
"universal" class A' like this:

					A
				       / \
				      /   \
				     B     C
				     |    / \
				     |   /   \
				     |  D     E
				     |  |    /
				     |  |   /
				     |  |  /
				      \ | /
					A'

I guess that an A' object always could be safely cast to any other
class in the tree. I also suspect that this is not what multiple
inheritance was intended for ... will I be condemned as a heretic ? ;-)

I'd appreciate constructive comments,

thanks /Tomas

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Tomas Lundstrom                                
NMP-CAD                                       /^^^\_/^^^^>
Swedish Institute of Microelectronics        /          (_____________ # # #
P.O. Box 1084                                |  II | | _______________|||||||
S-164 21 Kista                               \  o  o  (                # # #
Tel. +46-8-7521126                            \___/^\__>
email tomas@nmpcad.se			Rockus et rollare necesse est
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