ruby cookbook -- Making a Copy of an Object

Recipe 8.16. Making a Copy of an Object
The downside of dup is that it creates a new instance of the object's original class. If you open up a specific object and give it a singleton method, you implicitly create a metaclass, an anonymous subclass of the original class. Calling dup on the object will yield a copy that lacks the singleton methods. The other object-copy method,  Object#clone, makes a copy of the metaclass and instantiates the copy, instead of instantiating the object's original class.

	material = 'cotton'
	class << material
	  def definition
	     puts 'The better half of velour.'
	  end
	end
	
	material.definition
	# The better half of velour.

	'cotton'.definition
	# NoMethodError: undefined method 'definition' for "cotton":String

	material.clone.definition
	# The better half of velour.

	material.dup.definition
	# NoMethodError: undefined method 'definition' for "cotton":String

Object#clone and Object#dup both perform shallow copies: they make copies of an object without also copying its instance variables. You'll end up with two objects whose instance variables point to the same objects. Modifications to one object's instance variables will be visible in the other object. This can cause problems if you're not expecting it:

	class StringHolder
	  attr_reader :string
	  def initialize(string)
	    @string = string
	  end
	end

	s1 = StringHolder.new('string')
	s2 = s1.dup
	s3 = s1.clone

	s1.string[1] = 'p'
	s2.string                              # => "spring"
	s3.string                              # => "spring"

If you want to do a deep copy, an easy (though not particularly quick) way is to serialize the object to a binary string with Marshal, then load a new object from the string:

	class Object
	  def deep_copy
  	    Marshal.load(Marshal.dump(self))
	  end
	end

	s1 = StringHolder.new('string')
	s2 = s1.deep_copy
	s1.string[1] = 'p'
	s1.string                              # => "spring"
	s2.string                              # => "string"

Note that this will only work on an object that has no singleton methods:

	class << s1
	  def definition
	     puts "We hold strings so you don't have to."
	  end
	end
	s1.deep_copy
	# TypeError: singleton can't be dumped

When an object is cloned or duplicated, Ruby creates a new instance of its class or superclass, but without calling the initialize method. If you want to define some code to run when an object is cloned or duplicated, define an initialize_copy method. This is a hook method that gives you a chance to modify the copy before Ruby passes it back to whoever called clone or dup. If you want to simulate a deep copy without using Marshal, this is your chance to modify the copy's instance variables:

	class StringHolder
	  def initialize_copy(from)
	   @string = from.string.dup
	  end
	end

	s1 = StringHolder.new('string')
	s2 = s1.dup
	s3 = s1.clone
	s1.string[1] = "p"
	s2.string                              # => "string"
	s3.string                              # => "string"