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From: gaa@castle.ed.ac.uk (Gerard A. Allan)
Newsgroups: comp.lang.c++,comp.std.c++
Subject: Adding multiple inheritance to a single inheritance class library.
Keywords: multiple inheritance, virtual base
Message-ID: <10971@castle.ed.ac.uk>
Date: 11 Jun 91 18:55:12 GMT
Organization: Edinburgh Microfabrication Facility
Lines: 55


Many C++ libraries have a common root class and as a consequence they
contain many functions and methods that take the root class as an
argument requiring a cast to the desired type inside the function.
This can cause a problem when attempting to use multiple inheritance and
a virtual base class.
eg. 
	class Root {
		virtual void function( Root *r)=0;
	};
	class Derived : public Root {
		int x;
		virtual void function( Root *r);
	};
	void Derived::function( Root *r)
	{
		Derived *n=(Derived *)r;
		n->x++;
	}

if I now define Derived as,
			class Derived : virtual public Root {
			int x;
			virtual void function( Root *r);
			};

In preparation to make,
			class mine : virtual public Root {};
			class multi : public mine, public Derived {};

The function Derived::function(Root *r) no longer works as there is an
error   "cannot cast up from virtual baseclass Root"

This is of course perfectly true (ARM 10.6c) "Casting form a virtual
base class to a derived class is disallowed to avoid requiring an
implementation to maintain pointers to enclosing objects". This
makes it very difficult to use multiple inheritance and virtual bases 
from a library with a single root class. 

Is there some way round this problem so that I can combine classes in a
"natural" way ? After all, the cast is not ambiguous since there is only
one Derived. What solutions to this problem have C++ programmers
developed or is multiple inheritance incompatible with the single
inheritance methodology ? And finally, is there a case for requiring
"an implementation to maintain pointers to enclosing objects" ?

I'd be interested in hearing how programmers have dealt with this
problem. 

Gerard A. Allan                              | Post:  EMF
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