From: MikkelFJ <mikkelfj-anti-spam@...>
Date: 2002-12-17T05:03:37+09:00
Subject: Re: Deep copy in Rub?


<christopher.j.meisenzahl@citicorp.com> wrote in message
news:H00019f009c894c4@MHS...
>
> I remember when using C++ for the first time and running into the
shallow/deep
> copy problem. "I copied my object, freed something in obj_b, but it's also
gone
> in obj_a, what gives!?!"   :-)
>
> I'm a Ruby Newbie now and wonder how this issue is addressed in Ruby. I've
seen
> references to a clone() method, is this related?

Someone here knows more about this than I and I've never really worked with
deep copy, anyway ...

clone is a flat copy.
In Ruby, any value deep or flat, integer or complex object, is stored in an
object as a reference.
All references are simply copied using clone, not the content of the
references. Not what you want.

The marshal interface serializes an object to string, including references.
A deep copy can be made using marshal, search google groups for marshal and
copy. There are limitations to using marshal - there could be circular
references and you could be referencing too much data.

Are you really sure you need deep copy?
In all the time I've developed software I haven't needed deep copy ever,
although it is some of those things you initially wonder about.
I'd claim the need for deep copy indicates a design problem except for a few
cases:
1) Object creation by prototyping - for example copying drawings in a
drawing program
2) Remote object creation in distributed computing (see marshal, drb).

Actually there was a pretty ugly deep copy bug related to 1) in an early
Ration Rose version. Delete one of the carefully designed boxes, and all the
copies disappeared like magic.

Mikkel