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From: wmm@world.std.com (William M Miller)
Newsgroups: comp.lang.c++
Subject: Re: Can I make overloaded operator functions virtual?
Message-ID: <1991Mar22.044801.26365@world.std.com>
Date: 22 Mar 91 04:48:01 GMT
References: <1991Mar20.074256.334@csis.dit.csiro.au> <1991Mar21.184133.14506@watson.ibm.com>
Distribution: usa
Organization: The World Public Access UNIX, Brookline, MA
Lines: 36

mittle@blinn.watson.ibm.com (Josh Mittleman) writes:
> 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&.

No, you really *don't* want that.  Allowing something like that would open a
major hole in the type system.  Assuming Derived::operator==() actually did
override Base::operator==(), code like the following would be possible:

        Base b;
        Derived d;
        Base& br = d;

        if (br == b) ...

Since "br" actually refers to a Derived object, the "==" invokes
Derived::operator==() -- but it passes a Base object as the argument instead
of the Derived object expected by Derived::operator==().  Boom!

This is a frequent request, but it just doesn't make sense when you look at
the ramifications.

-- William M. Miller, Glockenspiel, Ltd.
   wmm@world.std.com