Attr_accessor on class variables

Question

attr_accessor does not work on the following code. The error says "undefined method 'things' for Parent:Class (NoMethodError)":

class Parent   @@things = []   attr_accessor :things end Parent.things << :car  p Parent.things 

However the following code works

class Parent   @@things = []   def self.things     @@things   end   def things     @@things   end end Parent.things << :car  p Parent.things 

Answer 1

attr_accessor defines accessor methods for an instance. If you want class level auto-generated accessors you could use it on the metaclass

class Parent   @things = []    class << self     attr_accessor :things   end end  Parent.things #=> [] Parent.things << :car Parent.things #=> [:car] 

but note that this creates a class level instance variable not a class variable. This is likely what you want anyway, as class variables behave differently then you might expect when dealing w/ inheritence. See "Class and Instance Variables In Ruby".

Answer 2

attr_accessor generates accessors for instance variables. Class variables in Ruby are a very different thing, and they are usually not what you want. What you probably want here is a class instance variable. You can use attr_accessor with class instance variables like so:

class Something   class <<self     attr_accessor :things   end end 

Then you can write Something.things = 12 and it will work.