What can't I access class variable from object in ruby?

Question

I wan't to set a class variable of a class from the outside(via attr_accessor), and then access it from inside one of its objects. I'm using ruby 1.9.2. This is my code:

class Service
  def initialize(id)
    @my_id = id   
  end

  class << self
    attr_accessor :shared_id
  end

  def system_id
    @my_id + @@shared_id
  end
end

If I set Service.shared_id = "A2", and then call Service.new("A").system_id, this doesn't return "AA2". It displays the following error:

uninitialized class variable @@shared_id in Service

The behaviour is like if I didn't set the Service.service_id. Can someone please explain why this happens?

Answer 1

attr_accessor creates methods to manipulate instance variables — it does not create instance or class variables. To create a class variable, you must set it to something:

@@shared_id = something

There's no helper method to generate accessor for class variables, so you have to write them yourself.

However, class variables, because of their weird lookup rules, are rarely used — avoided, even. Instead, instance variables at class-level are used.

class Service
  @shared_id = thing

  class << self
    attr_accessor :shared_id
  end

  def system_id
     # use self.class.shared_id; you could add a shared_id helper to generate it, too.
  end
end