Path: utzoo!attcan!uunet!decwrl!apple!well!mccarthy
From: mccarthy@well.sf.ca.us (Patrick McCarthy)
Newsgroups: comp.lang.c++
Subject: Copying objects
Keywords: Object copy constructor
Message-ID: <18753@well.sf.ca.us>
Date: 27 Jun 90 03:01:01 GMT
Distribution: comp
Lines: 32

Given a pointer to a base type, what is the best way to copy the object
pointed to (bearing in mind that it may actually point to an object of a
derived type)?
 
In other words, suppose you have a class called Base, and two classes
Derived_1 and Derived_2 which inherit from Base.  Suppose further that p
and q are of type Base *.  In order to make p point to a copy of q using
q's copy constructor, you would have know q's type in advance as
follows:
 
    p = (Base *) new Derived_2(q);
 
I am searching for a solution which does not require knowlege of the
type of the object being copied.
 
The best way I can think of to do this is to define a virtual function,
Base *Base::CloneSelf(), in the base class.  This function would then be
overridden by all inherited classes.  The wierdism here is that in order
to override the function, Derived_1::CloneSelf() would have to return
a pointer of type Base *.
 
It seems as though there should be a better way of doing this.	Any and
all suggestions are welcome (as long as they're _constructive_ - pun
intended :-)
 
Thanks!
 
------------------------------------------------------------------------
Pat McCarthy
mccarthy@well.uucp
 
".signature? I ain't got to show you no stinking .signature!"