gistfile1.rb

class Class
  def cattr_accessor(sym)
    class_eval("unless defined? @@#{sym}\n@@#{sym} = nil\nend\n\ndef self.#{sym}=(obj)\n@@#{sym} = obj\nend\n", __FILE__, __LINE__ + 1)
    class_eval("def self.#{sym}\n@@#{sym}\nend\n", __FILE__, __LINE__ + 1)
  end
end

class A
  cattr_accessor :attribute
end

class B
  cattr_accessor :attribute
end
A.attribute = "PASS"
puts "A.attribute == #{A.class_variable_get(:@@attribute).inspect}"
puts "B.attribute == #{B.class_variable_get(:@@attribute).inspect}"
puts
B.attribute = "FAIL"
puts "A.attribute == #{A.class_variable_get(:@@attribute).inspect}"
puts "B.attribute == #{B.class_variable_get(:@@attribute).inspect}"
puts A.attribute

# 1.9.2 output:
# A.attribute == "PASS"
# B.attribute == nil

# A.attribute == "PASS"
# B.attribute == "FAIL"
# PASS

# Rubinius 1.2.1dev output:
# A.attribute == nil
# B.attribute == "PASS"
# 
# A.attribute == nil
# B.attribute == "FAIL"
# FAIL