TITLE: virtual assignment and derivation

[ There are a bunch of other tips relating to the assignment
  operator. See the link from http://www.ses.com/~clarke
]

PROBLEM: Ross Smith <alien@central.co.nz>

How do you get a virtual assignment operator to work properly?


RESPONSE: kanze@lts.sel.alcatel.de (James Kanze), 7 Dec 95

I'm not sure that this is the answer Ross Smith was looking for.
Consider the following (B is base, D1 and D2 derive from B):

	B*              pd1 = new D1 ;
	B*              pd2 = new D2 ;
	*pd1 = *pd2 ;

How would you make this work?  (IMHO, it basically requires double
dispatch, and even then, it apparently involves changing the type of
*pd1, which is not allowed.)

In practice, I've found the best solution for classes with inheritance
is simply to ban assignment, making operator= private.  I then provide
a clone virtual function, and let the user decide exactly what he
wants:

	B*              pd1 = new D1 ;
	B*              pd2 = new D2 ;

	pd1 = pd2 ;                     //  shallow copy
	pd1 = pd2->clone() ;            //  deep copy, including
	                                //      type change.

(Obviously, in such cases, I either link in the Boehm garbage
collector or use reference counted pointers instead of built-in
pointers.)

Assignment and polymorphism don't seem to work very well together.