From: Jim Freeze <jim@...>
Date: 2001-07-07T04:30:58+09:00
Subject: [ruby-talk:17423] How to copy an object

Hi:

I have a class that contains a class (possibly a list of classes).
The container class can also be contained by an other class.
Essentially, the classes are just used as fancy structs to
collect data in.

The problem is, I want to copy objects and have unique instances
of these objects. I can do this by modifying #dup in the parent class,
but I would like it to be more automatic. Maybe I can make this
more clear with an example:

Here is a normal case:

class Box
  attr_accessor :item
  def initialize
    @item = "item"
  end
end

class Package
  def initialize
    @b = Box.new
  end
  def change_item
    @b.item = "changed"
  end
  def to_s
    @b.item
  end
end

c1 = Package.new
c2 = c1.dup
puts "c1:#{c1}"
puts "c2:#{c2}"
c1.change_item
puts "c1:#{c1}"
puts "c2:#{c2}"

--> Output
c1:item
c2:item
c1:changed
c2:changed

This is not what I wanted. I want
c2 to have its own copy of the data.
I can get this by I modifying Package#clone thusly:

class Package
  def clone  ## called by #dup
    copy = super
    @b = @b.dup
    copy
  end
end


--> Output
c1:item
c2:item
c1:changed
c2:item


Thanks works fine, but it puts the responsibility
on the container class to modify #clone.
And if I have alot of different classes/object,
it is a problem waiting to happen.

Is there a way to modify the Box class
to make unique copies automatically?

Or, is there a different way to look at the problem
and collect data differently?

Thanks

=========================================================
Jim Freeze
jim@freeze.org
---------------------------------------------------------
No comment at this time.
http://www.freeze.org
=========================================================