At the Rails Edge this morning, PragDave spoke out against class variables, and in favor of class instance variables. I totally agree. Look at this code:

# 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:

1. The behave differently from instance variables if they are missing.
2. They are shared across an inheritance hierarchy in a counterintuitive way.
3. 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?