class_var.rb

module Foo
  def self.included(base)
    base.extend(ClassMethods)
  end

  module ClassMethods
    def metaclass; class << self; self; end; end

    def hello(name = nil)
      if name
        self.class_variable_set(:@@name, name)
      else
        self.class_variable_defined?(:@@name) ? self.class_variable_get(:@@name) : nil
      end
    end
  end
end

class Bar
  include Foo
end

class Baz
  include Foo
end

class Bleh < Bar
end

Bar.hello("heer")

puts Bar.hello()
puts Bleh.hello

p Baz.hello