How to implement private inner class in Ruby

Question

Coming from Java, I am trying to implement LinkedList in Ruby. The usual way I would implement this in Java is have an class called LinkedList and private inner class called Node with each object of LinkedList as Node object.

class LinkedList
  private
  class Node
    attr_accessor :val, :next
  end
end

I do not want to expose the Node class to external world. However with this setup in Ruby, I can access the private Node class object outside the LinkedList class using this -

node = LinkedList::Node.new

I know, with Ruby 1.9, we can use private_constant method to designate Node as private constant. But I am wondering if this is the right way to accomplish this? Also why am I able to create Node objects outside the LinkedList class even though it is declared as private?

Answer 1

why am I able to create Node objects outside the LinkedList class even though it is declared as private?

Because in ruby constants ignore "regular" visibility modifiers. They're always public, regardless of which section they're in. To make them private, use private_constant. Call this inelegant design or whatever, but that's how it is.

Also, be warned that even with private_constant, the privateness means very little. Basically, the only thing it does is hide the constant from lists (LinkedList.constants) and direct resolution (LinkedList::Node). If one knows the name, they will be able to access it.

class LinkedList
  class Node
    attr_accessor :val, :next
  end

  private_constant :Node
end

LinkedList.const_get('Node') # => LinkedList::Node