Path: utzoo!attcan!uunet!taumet!steve
From: steve@taumet.com (Stephen Clamage)
Newsgroups: comp.lang.c++
Subject: Re: Copying objects
Message-ID: <304@taumet.com>
Date: 4 Jul 90 17:27:35 GMT
References: <18753@well.sf.ca.us> <4853@darkstar.ucsc.edu> <55612@microsoft.UUCP>
Reply-To: steve@taumet.UUCP (Stephen Clamage)
Distribution: comp
Organization: Taumetric Corporation, San Diego
Lines: 26

In article <55612@microsoft.UUCP> jimad@microsoft.UUCP (Jim ADCOCK) writes:
>Interestingly, operator= has the unique characteristic of being
>virtual but not inherited!  Can someone explain what this is suppose
>to mean?

If a derived class inherits an operator=, what happens to the extra
fields in the derived class?  If the programmer goes to the trouble
of writing operator=, the compiler must assume that something special,
beyond memberwise assignment, is required.  The compiler cannot guess
what this is, so leaves it to the programmer.  Hence, operator= is not
inherited.  (We could argue that if the derived class has no data
members it is OK to inherit the operator=, but it is simpler to go
with the ARM and say it is not inherited.)

Virtual is a separate concept, and a virtual operator= merely assures
that the correct operator is called in the case of polymorphic objects.

The ARM states that the default operator= (memberwise assignment) is
generated if (among other things) its address is taken.  If a derived
class needs a virtual operator= and the programmer does not declare one,
the compiler should generate the default version and put it in the
virtual table.  If this didn't happen, it may be a bug in the compiler,
or a compiler written to an earlier version of C++.
-- 

Steve Clamage, TauMetric Corp, steve@taumet.com