# why I don't like @@attributes class Base @@shared = 'cattr from base' class << self attr_accessor :unshared end def self.shared @@shared end self.unshared = 'attr from base' end class Derived < Base @@shared = 'cattr from derived' self.unshared = 'attr from derived' end class Ouch class << self attr_accessor :unshared end def self.shared @@shared end end puts Base.unshared puts Base.shared puts Derived.unshared puts Derived.shared puts Ouch.unshared puts Ouch.shared
Can you guess what this program will do? It demonstrates three of the ways that class variables cause trouble:
- The behave differently from instance variables if they are missing.
- They are shared across an inheritance hierarchy in a counterintuitive way.
- They require a new syntax, when the instance variable syntax is perfect for the task.
But maybe I am wrong. Ola Bini, who says many wise things, appears to be arguing exactly the opposite. Does anybody have a use case where class variables are the best solution?