From: Jacob Fugal <lukfugl@...>
Date: 2006-06-21T07:19:53+09:00
Subject: Re: Achieve pure object oriented design in Ruby

On 6/20/06, Robert Dober <robert.dober@gmail.com> wrote:
> The real problem is  requirement 2) of the OP,  instanciation of class B
> shall only be possible in class A, e.g. in an instance of A, I do not think
> this is possible, but I would not bet on it (not much).
> It would already be nice to have some code which defends against the vanilla
> instanciation of B outside of an A instance.
> I did not yet come up with such a solution though.
> Any ideas?

One way would be to make B.new protected/private (similar to the way
it's done in the Singleton mixin). But once you've done that, I'm not
sure if there's a clean way to allow A to use that method -- Ruby
doesn't have "friends" ala C++ (and I'm glad for it). Of course, once
it's protected you *could* have A use send explicitly:

  class B
    class << self
      protected :new
    end
  end

  class A
    def create_b
      @b_objs << B.send(:new)
    end
  end

  a = A.new
  a.create_b # works fine
  B.new # raises NoMethodError

This works, but it doesn't feel very clean :/

Jacob Fugal