A demo of Ruby passing by reference and the way it can mess up duplication of nested objects.

pass-by-ref-demo

#the outer object

class Pen

	attr_accessor :animals

	def initialize
		# Animals can be in 
		# one of three rows
		# in the pen.
		@animals = [[],
		            [],
		            []]
	end

	def add_animal(animal, row)
		@animals[row] << animal
	end

end

#the inner object type

class Animal

	attr_reader :name, :sounds

	def initialize(name)
		@name = name
		@sounds = []
	end

	def new_sound(sound)
		@sounds << sound
	end

	def to_s
      str = " #{name}:"
      sounds.each { |sound| str += " #{sound}" }
      str
    end

end

#make our first pen with cows that go "moo"

moo_pen = Pen.new

moo_cow = Animal.new("cow")

moo_cow.new_sound("moo")

moo_pen.add_animal(moo_cow, 0)

# Now we've added a cow to row 1 of our pen.

puts moo_pen.animals

#returns 
#  cow: moo

# What if we want to copy moo_pen and 
# teach all the copy_cows to talk
# without making the moo_pen cows
# any smarter?

# FIRST we need a dup method
# which we will first do 
# the WRONG WAY:

# Make a 'deep dup' method which
# works on nested arrays.
module MakeCopies

	def deep_dup(array)
	  new_arr = []
	  array.each do |item|
	  	if item.is_a?(Array)
	  	  new_arr << item.dup
	  	else
	  	  new_arr << item
	  	end
	  end
	  new_arr
	end

end
	
# Then use it to dup the animals array.
class Pen
	include MakeCopies

	def dup
	  new_pen = Pen.new
	  #attempt to 'deep-dup' the animals
	  old_animals = animals
	  new_animals = deep_dup(old_animals)
	  new_pen.animals = new_animals
	  new_pen
	end

end

# Finally, let's copy our moo_pen
# and teach the cows to talk.

experimental_pen = moo_pen.dup

experimental_pen.animals.each do |row|
	row.each do |animal|
		# Teach it to talk
		animal.new_sound("Hello")
	end
end

# Test the copy pen to see how it's animals sound:

puts experimental_pen.animals

# Returns 
# cow: moo Hello

# BUT look at our original moo_pen:

puts moo_pen.animals

# Returns
# cow: moo Hello

# Do you see what happened?
# Both pens have a talking cow
# Because it is still the same cow.
# Ruby is referencing the same place
# in memory, which was altered
# when we added the sound "Hello".

# I'll post again about the possible solutions
# to properly duping nested objects, which 
# my partner and I did eventually figure out.