From: 7stud -- <bbxx789_05ss@...>
Date: 2011-04-15T09:01:01+09:00
Subject: Re: What do you do when you need to attach data to an object instance?

Aaron D. Gifford wrote in post #992841:
> What do you do when you see a need to be able to attach some data to
> an object instance for later use somewhere else in a body of code?
> Lately I've resorted to this:
>
> ## Generic utility to allow one to attach data with a getter/setter to
> ## any instance of any object so long as there isn't a method name
> ## collision:
> def attach_data(obj, name, data)
>   getter = name.to_sym
>   setter = (name.to_s + '=').to_sym
>   raise "method name collision for #{obj.class} instance" if
> obj.respond_to?(getter) || obj.respond_to?(setter)
>   ## The 'value' local variable will remain in existence in the lambda
> closures below:
>   value = data
>   meta = class << obj ; self ; end
>   meta.send(:define_method, getter, lambda { value })
>   meta.send(:define_method, setter, lambda {|val| value = val })
>

1) Note that you don't need to use send() there -- but maybe that's your 
preferred closure?  Once you have the singleton class, you can define 
methods on it like this:

def attach_data(obj, name, data)
  getter = name.to_sym
  setter = (name.to_s + '=').to_sym

  raise "method name collision for #{obj.class} instance" if
    obj.respond_to?(getter) || obj.respond_to?(setter)

  #value = data

  singleton = class <<obj
    self
  end

  singleton.class_eval do
    define_method(getter) do
      data
    end

    define_method(setter) do |x|
      data = x
    end
  end


  #meta.send(:define_method, getter, lambda { value })
  #meta.send(:define_method, setter, lambda {|val| value = val })

  #value
  obj
end


2) I don't understand why you are creating a new local variable called 
value and closing over that?  data is also a local variable and you can 
create a closure over that.  Additional calls to attach_data() will 
create new local variables--including data, so if you call attach_data() 
twice data will not be shared.

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