#dup and #clone are so rude!

nodup.rb

class Object
  private :dup
  private :clone
  private :freeze
end

module ValueObject
  def self.included(into)
    into.send :public, :dup
    into.send :public, :clone
    into.send :public, :freeze
  end
end

[Array, Hash, String].each do |value_class|
  value_class.send :include, ValueObject
end

module ValueStruct

  def initialize(*a)
    super(*a)
    include ValueObject
  end

end

Struct.send :include, ValueStruct

if $0 == __FILE__
  
  require 'test/unit'

  class NoDupTest < Test::Unit::TestCase
    
    def test_object_nodup
      o = Object.new
      assert_raise NoMethodError do
        o.dup
      end
      assert_raise NoMethodError do
        o.clone
      end
    end

    def test_own_class_nodup
      c = Class.new
      o = c.new
      assert_raise NoMethodError do
        o.dup
      end
      assert_raise NoMethodError do
        o.clone
      end
    end

    def test_value_class_dup
      c = Class.new do
        include ValueObject
      end
      assert_nothing_raised do
        c.dup
      end
    end

    def test_array_dup
      a = [:yes]
      assert_equal a.dup.first, :yes
    end

    def test_hash_dup
      h = {:yes => true}
      assert h.dup[:yes]
    end

    def test_string_dup
      s = "abc"
      assert_equal s.dup, "abc"
    end

    # FIXME: How can I get ValueObject into every generated Struct class?
    def test_struct_dup
      s = Struct.new(:a, :b, :c)
      o = s.new(1, 2, 3)
      assert_equal o.dup.a, 1
    end

  end

end

why-nodup.rb

# #dup and #clone break encapsulation


class Box

  def initialize(size)
    @size = size
    @items = []
  end

  def <<(item)
    if full?
      raise "box is full"
    else
      @items << item
    end
  end

  def full?
    @items.length == @size
  end

end

box1 = Box.new 3
box1 << :fork
box1 << :knife
box2 = box1.dup
box2 << :spoon
box1 << :napkin # raises "box is full"