Path: utzoo!utgpu!jarvis.csri.toronto.edu!clyde.concordia.ca!uunet!microsoft!jimad
From: jimad@microsoft.UUCP (JAMES ADCOCK)
Newsgroups: comp.lang.c++
Subject: Re: virtual assignment operator is possible?
Keywords: assignment operator, virtual function
Message-ID: <10137@microsoft.UUCP>
Date: 3 Jan 90 20:50:51 GMT
References: <1989Dec30.174745.8174@sdr.slb.com>
Reply-To: jimad@microsoft.UUCP (JAMES ADCOCK)
Organization: Microsoft Corp., Redmond WA
Lines: 17

Try defining the following:

 virtual  B& B::operator=(const B&)
 virtual  B& D::operator=(const B&)
[virtual?]D& D::operator=(const D&)

Then when using generic pointers to your base class B the second op=
will be used to assign to a D pointed to.  When using pointers of type
D the third op= will be used.  In the second op=, if you cannot create
a D from a B in general, but only when the B& is really to a D -- then
you'll have to resort to runtime checking, and complain at runtime if
someone passes you a non-D.  Make the third op= virtual if you want to
allow the same choices in subclasses of D.  If D won't be subclassed,
it would be faster to not make the 3rd op= virtual.  You'll probably
want to define a D::D(B&) too.  Alternately, make all your op='s return
void, and don't support the infrequent cases where assignment is used
for value.