From: Joel VanderWerf <vjoel@...>
Date: 2005-10-01T09:33:47+09:00
Subject: Re: Concerning Marshalling

christophe (dot) poucet (at) gmail (dot) com wrote:
> Concerning tracking references.
> 
> I want to store each object as a separate value.  I can assure that
> each object has a unique id (all the objects that need to be stored
> separately are instance_of? Managed), however if I marshal the objects
> one by one, then the references they might hold internally to each
> other will of course no long work.  How would I solve this problem?
> Obviously I need to replace the references by some sort value that
> refers to it, or otherwise anything that is referred will be stored
> multiple times.  However I'm not quite sure how to put the hooks in
> place for this.
> 
> Christophe.
> 

Here's one way:

class Node
  @@nodes = Hash.new {|h,k| raise ArgumentError, "Missing node
#{k.inspect}"}

  # The id of a node can be anything but +nil+.
  attr_reader :id

  def initialize id
    @id = id
    @@nodes[id] = self
  end

  attr_accessor :prev_node, :next_node

  attr_accessor :temp_data
    # don't want this to persist (maybe it's a cache, or simply
    # undumpable, like a proc, or a file, or a singleton).

  def marshal_dump
    [prev_node, next_node].map {|node| node && node.id}
  end

  def marshal_load(dumped_obj)
    @prev_node, @next_node = dumped_obj.map do |node_id|
      node_id.nil? ? nil : @@nodes[node_id]
    end
  end

  def self.delete id
    @@nodes.delete id
  end
end

node1 = Node.new "One"
node2 = Node.new 2

node1.next_node = node2
node2.prev_node = node1

node1.temp_data = STDOUT
node2.temp_data = proc {"foo"}

p node1
node1_dumped = Marshal.dump(node1)
p Marshal.load(Marshal.dump(node1))

Node.delete 2
p Marshal.load(node1_dumped)
  # This should raise an exception, to show that the referred node is not
  # stored in node1_dumped, and not in the @@nodes hash.

__END__

#<Node:0xb7dfe260 @temp_data=#<IO:0xb7e4c078>, @id="One",
@next_node=#<Node:0xb7dfe24c
@temp_data=#<Proc:0xb7dfe9e0@marshal-refs2.rb:40>, @id=2,
@prev_node=#<Node:0xb7dfe260 ...>>>
#<Node:0xb7dfdeb4 @next_node=#<Node:0xb7dfe24c
@temp_data=#<Proc:0xb7dfe9e0@marshal-refs2.rb:40>, @id=2,
@prev_node=#<Node:0xb7dfe260 @temp_data=#<IO:0xb7e4c078>, @id="One",
@next_node=#<Node:0xb7dfe24c ...>>>, @prev_node=nil>
marshal-refs2.rb:2: Missing node 2 (ArgumentError)
        from marshal-refs2.rb:2:in `call'
        from marshal-refs2.rb:24:in `default'
        from marshal-refs2.rb:24:in `[]'
        from marshal-refs2.rb:24:in `marshal_load'
        from marshal-refs2.rb:23:in `map'
        from marshal-refs2.rb:23:in `marshal_load'
        from marshal-refs2.rb:47:in `load'
        from marshal-refs2.rb:47


-- 
      vjoel : Joel VanderWerf : path berkeley edu : 510 665 3407