copy_example.rb

class ShallowCopy
  def initialize
    @var = [1, 2, 3]
  end

  attr_reader :var
end

obj = ShallowCopy.new
cpy = obj.dup
puts cpy.var.equal? obj.var # => true, i.e. modifying @var in cpy affects obj

class DeepCopy
  def initialize
    @var = [1, 2, 3]
  end

  def initialize_copy(other)
    @var = other.var.dup
  end

  attr_reader :var
end

obj = DeepCopy.new
cpy = obj.dup
puts cpy.var.equal? obj.var # => false, i.e. modifying @var in cpy doesn't
                            # afffect obj