Override << for an array instance variable in class

Question

I need to override the operator << for one single attribute in my class.

Basically, what I want is allow to push only unique integers to my array attribute.

Here's what I have:

class Player
  attr_accessor :moves

  def initialize
    @moves = []
  end

  def moves<<(value)
    raise Exception if @moves.include?(value)
    @moves.push(value)
  end
end

Unfortunately, this code doesn't work.

How can I improve that or maybe there are better ways to implement such functionality?

Answer 1

class Player
  attr_accessor :moves

  def initialize
    @moves = []
    @moves.define_singleton_method(:<<) do |value|
      raise Exception if include?(value)
      push(value)
    end
  end
end

You can add methods only specific to a given object using the Object#define_singleton_method. Ruby is really flexible when it comes to meta programming.

However such tools should be used sparingly. I don't know your specific situation, but you are probably better off not giving direct access to @moves. The best approach might be to define methods in Player that create an indirect and more restrictive interface to the internal representation and give you more control.