Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!taumet!steve
From: steve@taumet.com (Stephen Clamage)
Newsgroups: comp.lang.c++
Subject: Re: C++ binary classes (was: typeof() and virtuals and overloading, oh my!)
Message-ID: <662@taumet.com>
Date: 9 Apr 91 15:38:47 GMT
References: <19093@milton.u.washington.edu> <157@devnull.mpd.tandem.com> <162@devnull.mpd.tandem.com> <27FA1D8B.50C3@tct.com> <1317@appli.se>
Organization: Taumetric Corporation, San Diego
Lines: 30

niklas@appli.se (Niklas Hallqvist) writes:

>So, what is required for distribution of binary only C++ classes that are
>to be extensible by inheritance and adding virtual functions?  Source code
|for the constructors?  The class definition of course (in source form, but
|I believe no (or almost none) C++ compiler supports precompiled headers). 
|I understand that extending the class with extra data members won't work,
|but that's what inheritance are for, isn't it?  As I see it, this should
|be of little concern to C++ class developpers, since no work worth
|protecting should reside in constructors.
|...
|Another problem is that no constructors
|from the original library could be inline expanded in the binary components,
|but that is of course easily dealt with as long as you can ensure that an
|optimizing compiler won't do that without your knowledge.  Have I missed
|something important in my reasoning here? Supposing I'm correct, is this
|change to the language rules wortwhile?

There are still problems.  If a class does not have an explicit non-inline
null constructor X::X(), copy constructor X::X(X&), and assignment
X::operator=(X&), the compiler may have used default inline versions of
these in internal library code, which would become invalid if the class
definition changed.

It is just not a good idea to modify class definitions without recompiling
all code which uses those definitions.  This probably limits the
usefulness of binary-only libraries.
-- 

Steve Clamage, TauMetric Corp, steve@taumet.com