Path: utzoo!attcan!uunet!aplcen!haven!ncifcrf!lhc!nih-csl!sparkler.dcrt.nih.gov!kgorlen
From: kgorlen@sparkler.dcrt.nih.gov (Keith Gorlen)
Newsgroups: comp.lang.c++
Subject: Re: Copying objects
Keywords: Object copy constructor
Message-ID: <210@nih-csl.nih.gov>
Date: 27 Jun 90 19:43:36 GMT
References: <18753@well.sf.ca.us>
Sender: news@nih-csl.nih.gov
Reply-To: kgorlen@sparkler.dcrt.nih.gov (Keith Gorlen)
Distribution: comp
Lines: 35

In article <18753@well.sf.ca.us>, mccarthy@well.sf.ca.us (Patrick
McCarthy) writes:
|>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)?

You might take a look at how the NIH Class Library implements the
shallowCopy() and deepCopy() member functions.  Basically, each class X
defines the virtual function

	Object* X::shallowCopy() const	{ return new X(*this); }

which will create a shallow copy of an X object by using the copy
constructor for class X.

Object::deepCopy() is a non-virtual member function that first uses
shallowCopy() to make a shallow copy of an object, then invokes the
virtual member function deepenShallowCopy().  Each class that has member
variables that are pointers to objects or class instances implements
deepenShallowCopy() to convert the shallow copy to a deep copy by
applying deepenShallowCopy() recursively to the pointers and objects it
contains.

All this is described in detail in Section 7.3 of our new book "Data
Abstraction and Object-Oriented Programming in C++", John Wiley & Sons,
ISBN 047192346X.  (I understand the first copies are probably running
off the presses as I type.)
                                
The software is available via anonymous FTP in the file pub/nihcl.tar.Z
on host alw.nih.gov (128.231.128.251).

	Keith Gorlen			phone: (301) 496-1111
	Building 12A, Room 2033		uucp: uunet!nih-csl!kgorlen
	National Institutes of Health	Internet: kgorlen@alw.nih.gov
	Bethesda, MD 20892