Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!sun-barr!decwrl!nsc!voder!decvax!roger
From: roger@decvax.UUCP (Roger H. Scott)
Newsgroups: comp.lang.c++
Subject: Re: Problems with Cfront v2
Message-ID: <5@espol.decvax.UUCP>
Date: 20 Dec 89 21:56:15 GMT
References: <959@sl10c.concurrent.co.uk>
Reply-To: roger@procase.UUCP (Roger H. Scott)
Organization: proCASE Corporation, Santa Clara, CA
Lines: 38

In article <959@sl10c.concurrent.co.uk> asc@concurrent.co.uk (Andy Chittenden) writes:
>We have recently purchased version 2 of C++ and have managed to
>get it ported to our system V.2 UNIX system.
>...
>In addition, we would like to use virtual inheritence with v2.  However,
>we have hit upon a snag.  In v 2.0, objects derived from virtual base
>class objects contain pointers to the base class objects.  Our objects
>will be saved and retrieved from disk.  Such persistent objects cannot
>contain pointers, only relative addresses or offsets.

We have a similar problem at PROCASE.  Our system (as CASE system for C
programs) is built on a persistent C++ object database.  With a tolerable
amount of grundge we know how to passivate and activate "simple" (1.2 and
2.0 with single inheritance) C++ objects.  The mechanisms used to implement
MI in AT&T's 2.0 are *much* more difficult to passivate and activate - so much
so that we quickly decided that we could not use MI in our work either.  While
this is a significant issue for us, it is far from the most important reason
for our avoiding MI - the outrageous cost in both space and time.  I am
preparing a scathing indictment of AT&T's MI implementation for publication
in this news group sometime soon.  Just to whet your appetites, see if you can
guess how big a single instance of class Der<i> is as a function of <i>, below:

    // all are abstract classes - no data members whatsoever

    struct Base {
	virtual void f();
    };

    struct Der1 : virtual public Base {
    };

    struct Der2 : virtual public Der1 {
    };

    ...

    struct Der<i> : virtual public Der<i-1> {
    };