Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!yale!cmcl2!rna!kc
From: kc@rna.UUCP (Kaare Christian)
Newsgroups: comp.lang.c++
Subject: Cloning derived classes
Keywords: inheritance, cloning, derived class
Message-ID: <1019@rna.UUCP>
Date: 2 May 90 14:20:25 GMT
Organization: Rockefeller University - Neurobiology
Lines: 28

I recently needed to make a copy (a clone) of a class object, given only a
base class pointer to the object.  C++ lets you do some things with a class,
knowing only its base type, but it doesn't provide an easy way to make
a copy.  My solution was to make a virtual member function that
returned the size of each of the derived classes, and then make an
operator= function (left arg, ref pointer to base, right arg, ref to
base) that did the work - it called the size fn of the right operand to
determine how much space was needed, allocated it, did a bitwise copy,
then did some housekeeping.

Shouldn't this be easier? Given a (base *) pointer to a derived object,
I think it should be easy to make a copy of the thing pointed at.

One way might be to add a clone facility, that would be used equivalently
to new, but that would duplicate an existing object.
	Base *p = ... ;
	Base *b;
	b = clone *p; // new takes a type, clone takes an object

Another method might be to have a typeof builtin, perhaps similar to the
following:
	Base *p = .... ;
	Base *b;
	b = new typeof(*p);	// allocates space
	*(typeof(*p) *)b = *(typeof(*p) *)p;	// ugly, but copies whole thing

Kaare Christian
kc@rna.rockefeller.edu