Testing whether TestChild is going to be able to modify TestParent's string1 and array1, or, more generally, how to avoid dumb pass-by-reference errors.

pass_by_reference.rb

# Testing whether TestChild is going to be able to modify TestParent's
# string1 and array1, or, overall, which objects are really pass-by-reference
# and which are not [in this case, testing strings and arrays]
class TestParent
  attr_accessor :string1, :array1
	def initialize(string1, array1)
		@string1 = string1
		@array1 = array1
		puts
		puts "The parent's starting string is: #{@string1}"
		puts "The parent's starting array is: #{@array1.inspect}"
		@t2 = TestChild.new(@string1, @array1, self)
	end
end

class TestChild
	def initialize(string, array, test_parent)
		@string2 = string
		@string2 << " dude!" # will this modify TestParent's string???
		@array2 = array
		@array2 << ["cookies", "are", "awesome"] # will this modify TestParent's array???
		@test_parent = test_parent

		# ... let's find out!
		puts
		puts "The parent's string is now: #{@test_parent.string1}"
		puts "The child's string is now: #{@string2}"
		puts
		puts "The parent's array is now: #{@test_parent.array1.inspect}"
		puts "The child's array is now: #{@array2}"
		puts
	end
end
t = TestParent.new("howdy", [1,2])
# RESULTS: 
# YES! BOTH the parent's string1 AND array1 WERE modified when we
# used the shovel operator in the TestChild.  
# What does this mean?  BE CAREFUL how you treat the objects you pass around!
# If you use destructive operations on them (eg. the shovel <<), it WILL mess
# up the parent's objects as well.  
# To fix this, you either need to use non-destructive operations OR pass in
# duplicates of the original objects instead of (references to) the real deal.
# In this case, if instead of @string2 << " dude!" we had used 
# @string2 += " dude!" or instead of @array2 << [stuff], we had used
# @array2 += [stuff], we would have been fine.  Lesson learned.