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From: beard@ux5.lbl.gov (Patrick C Beard)
Newsgroups: comp.lang.c++
Subject: Re: multiple inheritance bug
Keywords: multiple inheritance, virtual base classes
Message-ID: <9592@dog.ee.lbl.gov>
Date: 6 Feb 91 04:54:01 GMT
References: <3553@island.COM>
Reply-To: beard@ux5.lbl.gov (Patrick C Beard)
Organization: Berkeley Systems, Inc.
Lines: 47
X-Local-Date: Tue, 5 Feb 91 20:54:01 PST

In article <3553@island.COM> lars@island.COM (Lars Nyman) writes:
#
#I have a problem with multiple inheritance using virtual base classes, and
#suspect it's a bug in the C++ compiler.

Don't be so quick to call this a bug.  It's actually a feature.  C++ does
not officially support the calling of virtual functions from a constructor.
The reason?  If you are in the base class constructor of a derived class,
and you call a virtual function, if it called a function that is defined in
the derived class, that function would be passed a pointer to an incompletely
constructed derived object.

#
#Three level hierarchy: Base, Derived, MostDerived.
#
#class Base { ... }
#class Derived : virtual Base { ... }
#class MostDerived : Derived, Mixin { ... }
#
#The problem occurs when executing inside Derived's constructor.   If
#Derived() calls a function in Base, which in turns calls a virtual 
#function implemented in Derived, the 'this' pointer will be incorrect
#(see below code for more technical info).
#Basically, makes it impossible to use multiple inheritance...

No it doesn't.  It just makes it impossible to have a constructor call
virtual functions of classes that are farther down in the inheritance
tree.  This shouldn't pose much of a problem.  My philosophy is that
constructors are for initialization.  They should perform no work.  That
is deferred until the object is completely initialized.  I find that this
makes my designs much more robust.

#
#I have tested it under Sun C++ 2.0 and MPW C++ 3.1, both which are 
#based on CFront 2.0.
#
#Is this fixed in CFront 2.1 ?
#Can somebody who have a C++ implementation based on CFront > 2.0 check
#whether the same problem exist ?

I would predict that this "feature" will remain in C++ always.

--
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|  Patrick C. Beard, Software Engineer, Berkeley Systems, Inc.   |
|                    "Heroes of technology."                     |
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