From: "Ara.T.Howard" <Ara.T.Howard@...>
Date: 2005-09-26T10:34:12+09:00
Subject: Re: Concerning Marshalling

On Sun, 25 Sep 2005, christophe.poucet@gmail.com wrote:

> Hello,
>
> I was trying to marshal some objects, however some of these objects
> contain references to singletons which of course can't be marshalled.
> That's why I tried something as follows:
>
> class Container
> end
>
> class SpecialContainer < Container
>  include Singleton
> end
>
> class Test
>  attr_accessor :container
>  def _dump(depth)
>    old = @container
>    if old == SpecialContainer.instance
>      @container = nil
>    end
>    result = super
>    @container = old
>    return result
>  end
>  def self._load(str)
>    super
>    if @container == nil
>      @container = SpecialContainer.instance
>    end
>    self
>  end
> end
>
> Sadly, however, it seems there is no default implementation for _dump,
> which means that in this case the super call results in a
> NoMethodError.  Is there any other way to resolve this problem without
> custom-coding a full _dump method?
>
> Regards,
> Christophe

for some reason the mailing list seemed to have eaten my post about this so
here it is again - sorry if it's a dup:


harp:~ > cat dumpable_singleton.rb

   module DumpableSingleton
     module InstanceMethods
       attr_accessor 'ikey'
       def _dump *a, &b
         Marshal::dump ikey, *a, &b
       end
       def == other
         self.ikey == other.ikey
       end
       alias_method 'eql?', '=='
     end
     module ClassMethods
       def new *a, &b
         ikey = []
         ikey.push *a
         ikey.push b if b
         if instances.has_key? ikey
           instances[ ikey ]
         else
           begin
             ikey.each{|k| Marshal::dump k}
           rescue TypeError, Exception
             raise ArgumentError, "all arguments must be dumpable!"
           end
           instances[ (obj = super(*a, &b)).ikey = ikey ] = obj
         end
       end
       def _load buf, *a, &b
         new(*(ikey = Marshal::load(buf, *a, &b)))
       end
       attr_accessor 'instances'
     end
     def self::included other
       other.module_eval{ include InstanceMethods }
       other.extend ClassMethods
       other.instances = {}
       other
     end
   end

   class C
     include DumpableSingleton
     def initialize(*a, &b); end
   end

   x = C::new 42
   y = C::new 42
   z = C::new 43

   puts '-' * 42
   C::instances.each {|k,c| p k => c}
   p x == y
   p x == z
   p y == z

   xd, yd, zd = [x, y, z].map{|c| Marshal::load(Marshal::dump(c))}

   puts '-' * 42
   C::instances.each {|k,c| p k => c}
   p xd == yd
   p xd == zd
   p yd == zd

   puts '-' * 42
   C::instances.each {|k,c| p k => c}
   p x == xd
   p y == yd
   p z == zd


harp:~ > ruby dumpable_singleton.rb

   ------------------------------------------
   {[43]=>#<C:0xb75cc3c4 @ikey=[43]>}
   {[42]=>#<C:0xb75cc4b4 @ikey=[42]>}
   true
   false
   false
   ------------------------------------------
   {[43]=>#<C:0xb75cc3c4 @ikey=[43]>}
   {[42]=>#<C:0xb75cc4b4 @ikey=[42]>}
   true
   false
   false
   ------------------------------------------
   {[43]=>#<C:0xb75cc3c4 @ikey=[43]>}
   {[42]=>#<C:0xb75cc4b4 @ikey=[42]>}
   true
   true
   true

i've used something like this in in a few small projects.

hth.

-a
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