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From: mittle@blinn.watson.ibm.com (Josh Mittleman)
Newsgroups: comp.lang.c++
Subject: Re: Can I make overloaded operator functions virtual?
Message-ID: <1991Mar21.184133.14506@watson.ibm.com>
Date: 21 Mar 91 18:41:33 GMT
References: <1991Mar20.074256.334@csis.dit.csiro.au>
Sender: @watson.ibm.com
Reply-To: mittle@ibm.com
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Organization: IBM T. J. Watson Research
Lines: 45

The problem you describe is precisely the same one addressed earliest this
week under the subject "Seeking neat way to do binary "virtual" functions.
A followup post by cok@islsun.kodak.com suggests some ways around the
problem, but also ntoes that a more fundmanetal solution is really needed.
I've run across the same problem in another context, and I have what might
be a reasonable solution.  I would appreciate comments.

What we need is a new kind of template or prototype for a class, so that
the compiler can be told to treat certain class names as parameters.  To
work with the most recent example:

class Base 
{
public:
  virtual int operator==(const Base&);
};

class Derived : public Base
{
public:
  int operator==(const Derived&);
};

We would really like Derived::operator== to match the virtual
Base::operator==, but we need to be able to treat the argument as a
Derived&.  My solution is somehow to tell the compiler that Base& is to be
treated as a parameter of the parameterized class Base in certain
circumstances.  A possible syntax:

class Base(parameter Base)     // Defines formal parameters
{
public:
  virtual int operator==(parameter const Base&);  // parameterized virtual
						  // function. 
};

This seems to solve the problems suggested, and makes a great deal of sense
to me.  It requires the designer of class Base to forsee which members will
need to be parameterized; I'm not sure if that is a good thing or not.

Comments?

===========================================================================
Josh Mittleman (mittle@ibm.com or joshua@paul.rutgers.edu)
J2-C28 T.J. Watson Research Center, PO Box 704, Yorktown Heights, NY  10598