object_instances_and_references_to_them.rb

class Child

  attr_accessor :age

  def initialize(age: nil)
    @age = age
  end

end

class Mother

  attr_accessor :child

  def initialize(child: nil)
    @child = child
  end

end

class Father

  attr_accessor :child

  def initialize(child: nil)
    @child = child
  end

end

################################################################################

# margaret and ronald, which reference different Parent objects,
# both have @child properties that are references to the same Child object,
# also referenced by the susie variable.

susie = Child.new age: 8
margaret = Mother.new child: susie
ronald = Father.new child: susie

puts susie.age #=> 8
puts margaret.child.age #=> 8
puts ronald.child.age #=> 8

puts "Happy Birthday!"

susie.age = 9
puts susie.age #=> 9
puts margaret.child.age #=> 9
puts ronald.child.age #=> 9

# The Child object to which susie, margaret.child and ronald.child are
# references can be changed through any of the three references.

################################################################################

# timmy is a new reference to the same object susie references.

timmy = susie

susie.age = 10

puts susie.age #=> 10
puts timmy.age #=> 10

# This is an example of the same behavior, without other classes complicating it

################################################################################

# sarah is not a reference to the same object;
# sarah is a reference to an independent duplicate of the Child object susie
# references.

sarah = susie.dup

sarah.age = 11

puts susie.age #=> 10
puts sarah.age #=> 11