From: David Pollak <pollak@...>
Date: 2006-06-23T14:10:13+09:00
Subject: Re: Quick singleton question

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Ben,

Try:
class Dude
  attr parent
  attr kids

  def initialize(parent)
    @parent = parent
    @kids = []
    parent.add_kid(self) unless parent.nil?
  end

  def do_something_in_the_tree(depth = 0, &block)
    @kids.each {|kid| kid.do_something_in_the_tree(depth + 1, &block)}
    block.call(self, depth)
  end

  protected
  def add_kid(the_kid)
    @kids << the_kid
  end
end

Kids know their parent.  Parents know their kids.  I threw in a way to
recursively walk the tree... so you can do something like:
root.do_something_in_the_tree {|node, depth| puts "#{' '*depth}#{node}" }

Thanks,

David


On 6/22/06, ben@iagu.net <ben@iagu.net > wrote:
>
> Hi,
>
> In a structure I'm building, I need to be able to add children (of the
> same
> class) to an object, and have the child object know who its parent is. So
> I
> can do obj.instance_variable_set(:@parent,parentobj), but that doesn't
> create the accessor for me, so I still need to do
>
> class << child
>     def parent
>         @parent
>     end
> end
>
> That's not really all that onerous, and I can always predefine attr_reader
> for :parent in the main class to save the second step, but it lead me to
> wondering if there is a 'nice' way to add an instance variable and its
> accessor all at once to an object, because that will probably come in
> handy
> one day.
>
> Initially I wanted to do
>
> class << child
>     def parent
>         parentobj
>     end
> end
>
> but obviously that will fail since parentobj is out of scope.
>
> Anyway, that's it. :)
>
> ben
>
>
>


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--------
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http://dppruby.com

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