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From: rmartin@clear.com (Bob Martin)
Newsgroups: comp.lang.c++
Subject: Re: type casting (ARM Experts needed.)
Message-ID: <1991Feb11.184333.23382@clear.com>
Date: 11 Feb 91 18:43:33 GMT
References: <TOMAS.91Feb6103918@basi.inmic.se>
Distribution: comp
Organization: Clear Communications, Inc.
Lines: 53

In article <TOMAS.91Feb6103918@basi.inmic.se> tomas@inmic.se (Tomas Lundstrom) writes:
>
>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 ? 

ARM 5.4 says on this subject:
	
	A pointer to a class B may be explicitly converted to a pointer to a
	class D that has B as a direct or indirect base class if an unambiguous
	conversion from D to B exists (%4.6, %10.1.1) and if B is not a virtual
	base class (%10.1).  Such a cast from a base to a derived class assumes
	that the object of the base class is a sub-object of an object of the
	derived class; the resulting pointer points to the enclosing object of
	the derived class.  If the object of the base class is not a sub-object
	of an object of the derived, the cast may cause an exception.

OK, what does this mean?  ARM experts please check my interpretation...

First the single inheritance case:  
It would appear that there is no problem casting a B* to a D* so long as 
the object being cast was initially created as a D or something derived
from D.  If you created the object as a B, then you may get an exception
when you cast the B* to the D*.  ARM does not indicate the conditions of
when, or under what circumstances the exception does _not_ occur.  It simply
may cause an exception.

The Multiple Inheritance case:
In this case there may be no clear inheritance pathway to convert the B*
to the D*.  D may be reachable via many inheritance paths from B.  In
such a case the conversion is ambiguous and cannot be done.
-- 
+-Robert C. Martin-----+:RRR:::CCC:M:::::M:| Nobody is responsible for |
| rmartin@clear.com    |:R::R:C::::M:M:M:M:| my words but me.  I want  |
| uunet!clrcom!rmartin |:RRR::C::::M::M::M:| all the credit, and all   |
+----------------------+:R::R::CCC:M:::::M:| the blame.  So there.     |