From: Sean O'Halpin <sean.ohalpin@...>
Date: 2009-03-01T07:36:08+09:00
Subject: Re: Marshal.load does not create new instances?

On Sat, Feb 28, 2009 at 8:50 PM, Ian Trudel <ian.trudel@gmail.com> wrote:
[snip]
> We should perhaps see the problem as if it was extreme: let's imagine
> that we have multiple programs which have each a class Data but is
> completely different (no similar instance variables nor methods, nothing
> in common at all). No access to those programs and yet have to load all
> the files within a single Ruby program. What one would do?

Well, you could dynamically extend the loaded instances with modules
that add the specific required behaviour.
Something like this:

First file represents whatever created the data in the first place:

# file1
class MyData
  attr_accessor :kind
  attr_accessor :name
  def initialize(kind, name)
    @kind = kind
    @name = name
  end
end

instance = MyData.new("Greeting", "World")
data = Marshal.dump(instance)
File.open("data.dat", "wb") do |file|
  file.write(data)
end
# end of file1

Second file shows how you could load this data and dynamically decide
how it should behave as an instance:

# file2
# these modules will be used to extend the loaded instance depending
# on its @kind
module Hello
  def run
    puts "Hello #{ @name }"
  end
end

module Goodbye
  def run
    puts "Goodbye #{ @name }"
  end
end

# You need to define this if you're unmarshalling data that has been
# saved as MyData - no way round it as Marshal embeds the class name
# in the data
class MyData
end

# unmarshall data and extend depending on the @kind
data = File.read("data.dat")
instance = Marshal.load(data)
# this is shorthand for determining the nature of the data
if instance.instance_variable_defined?("@kind")
  kind = instance.instance_variable_get("@kind")
  if Object.const_defined?(kind)
    extension = Object.const_get(kind)
    instance.extend(extension)
    instance.run
  else
    puts "@kind not known: #{instance.inspect}"
  end
else
  puts "@kind not defined for: #{instance.inspect}"
end
# end of file2

I'm using @kind as shorthand to stand for something that distinguishes
between instances of your data. (BTW, you can't use Data as a class
name in Ruby - it's reserved for use with C extensions).

HTH,
Regards,
Sean