From: Robert Klemme <shortcutter@...>
Date: 2007-06-06T00:00:17+09:00
Subject: Re: Deep Copy

On 05.06.2007 16:45, Michael Artz wrote:
> I'm needing to implement a deep copy operation on a couple of objects,
> and was wondering if there was a "best practices" for this sort of
> thing.  I've seen the Marshal.load(Marshal.dump()) idiom, but I'm using
> singleton classes which can't be dumped.

It seems they can be dumped - but you get another instance:

irb(main):001:0> require 'singleton'
=> true
irb(main):002:0> class Foo
irb(main):003:1> include Singleton
irb(main):004:1> end
=> Foo
irb(main):005:0> Foo.instance
=> #<Foo:0x7ef573a4>
irb(main):006:0> Foo.instance
=> #<Foo:0x7ef573a4>
irb(main):007:0> Marshal.dump(Foo.instance)
=> "\004\bo:\bFoo\000"
irb(main):008:0> Marshal.load(Marshal.dump(Foo.instance))
=> #<Foo:0x10410b08>
irb(main):009:0> Marshal.load(Marshal.dump(Foo.instance)).object_id
=> 1065153200
irb(main):010:0> Foo.instance.object_id
=> 1065007570
irb(main):011:0>

>  So I'm implementing my deep
> copy similar to:
> 
> def deep_copy
>   other = self.clone
>   other.instance_var_1 = self.instance_var_1.clone
>   other.instance_var_2 = self.instance_var_2.deep_copy
>   other
> end
> 
> where some of the instance variables are objects for which I've written
> a deep_copy method.
> 
> The biggest problem with this method (that I can see), is that all of
> the attributes of the object need to be publically writable, which is
> undesirable.  Is there a better way to do this sort of thing?

You can use #instance_variable_get and #instance_variable_set to access 
  them.

Did you consider overriding #dup and / or #clone for this?  Typically 
you would not want a cloned / duped instance to refer to the original 
instance's instance variables (at least if they are mutable like 
collections, strings and other objects).

Kind regards

	robert