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From: dlw@odi.com (Dan Weinreb)
Newsgroups: comp.lang.c++
Subject: Re: Implementing virtual functions that return reference to self
Message-ID: <403@odi.ODI.COM>
Date: 27 Jul 89 20:17:37 GMT
References: <8975@thorin.cs.unc.edu>
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Reply-To: dlw@odi.com
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In-reply-to: kelleymt@luther.cs.unc.edu's message of 26 Jul 89 16:13:42 GMT

In article <8975@thorin.cs.unc.edu> kelleymt@luther.cs.unc.edu (Michael T. Kelley) writes:

   In this case, I know I'm dealing with a Derived, so I'd like to be able
   to use goodbye().  Can someone explain the harm in allowing hello()
   to return a Derived& in the Derived class?  Or is there an 
   implementation issue lurking underneath?

My colleagues and I have seen this problem several times, in slightly
different guises.  As far as we can tell, there is no good solution;
you have to use explicit casts.  In my experience so far with C++,
this is the biggest problem caused by C++'s mixture of run-time and
compile-time type checking.  (More accurately, its mixture of
run-time-polymorphic objects and typed variables.)  There's no such
problem in languages like Smalltalk-80 and Lisp/CLOS, in which
variables are untyped.  (I recognize that typed variables have
advantages too, and I'm not trying to provoke a general discussion of
the virtues and drawbacks of typed variables!)

The underlying philosophical problem is that d.hello().goodbye(); is
doing something that is perfectly meaningful, but the C++ compile-time
type checking system cannot prove in advance that the code meaningful,
so the compiler must tag the code as an error.  You have to put in an
explicit cast into the code in order to assert to C++ that
everything's really all legal.

I don't see any way to fix this that is consonant with the general C++
language design.  It looks like a tough problem. Is there anybody out
there who can tell us whether Trellis/Owl has a way to deal with this?
Are there other object-oriented languages with typed variables in
which an analogous construct can be written?

Dan Weinreb		Object Design, Inc.