From: 7stud -- <bbxx789_05ss@...>
Date: 2008-03-07T00:39:26+09:00
Subject: Re: Cloning a module and isolating it from the clone

Daniel Lucraft wrote:
> Earle Clubb wrote:
>> The
>> result I'm trying to achieve is the same as if I had copied the module
>> source code and renamed the module.  
> 
> I think stud number 7 was onto something. How about this:
> 
> CommonDef = %q{
>   def self.hello
>     puts "hello from #{self}"
>   end
> }
> 
> module A
>   module_eval CommonDef
> end
> 
> module B
>   module_eval CommonDef
> end
> 
> A.hello
> B.hello
> 
> module B
>   def self.hello
>     puts "different hello"
>   end
> end
> 
> A.hello
> B.hello
> 
> ===>
> 
> hello from A
> hello from B
> hello from A
> different hello
> 
> _______
> 
> Seems to work right.
> 
> best,
> Dan

As far as I can tell, your code is no different than this:

module A
  def self.hello
    puts "hello from #{self}"
  end
end

module B
  def self.hello
    puts "hello from #{self}"
  end
end

A.hello
B.hello

module B
  def self.hello
    puts "different hello"
  end
end

A.hello
B.hello


1) You never cloned A.  All you did was create two separate modules 
which have similar methods.

2) Then you manually  overwrote the method in B to do something else.

I don't see how that addresses anything the op's question.  The problem 
has to do with the fact that clone makes a shallow copy.  In other 
words, clone copies references to objects, so you end up with two 
references to the same object.  Therefore, a cloned object has 
references to the same values as the original object.

However, I can't figure out a way to make a 'deep copy' where the object 
that a reference refers to is also copied.  I tried deleting all the 
class variables in the cloned module and then adding them back in with 
new values--but they still refer to the same objects as the original 
module, which is puzzling.

module A
  @@num = 10
  @@arr = [1, 2, 3]

  def A.get_num
    @@num
  end

  def A.set_num(x)
    @@num = x
  end
end

puts A.get_num

new_meth = %q{
  def B.remove(arr)
    arr.each do |name|
      remove_class_variable(name.to_sym)
    end
  end
}

B = A.clone
B.module_eval(new_meth)

class_vars = B.class_variables
B.remove(class_vars)

puts "class variables in B:"
p B.class_variables

class_vars.each do |name|
  B.module_eval("#{name} = 0")
end

B.set_num("hello")
puts 'After cloning:'
puts A.get_num
puts B.get_num

A.set_num("goodbye")
puts A.get_num
puts B.get_num

--output:--
10
class variables in B:
[]
After cloning:
hello
hello
goodbye
goodbye

The Ruby Way says there is a hack to make deep copies using Marshal, but 
it doesn't work for me:

module A
  @@num = 10
  @@arr = [1, 2, 3]

  def A.get_num
    @@num
  end

  def A.set_num(x)
    @@num = x
  end
end


puts A.get_num

B = Marshal.load(Marshal.dump(A))
B.set_num(30)
puts "After deep copying:"
puts A.get_num
puts B.get_num

--output:--
10
After deep copying:
30
30

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