class Object
  def self.attribute(*arg,&block)
    (name,value) = arg
    self.send(:define_method, name) {
      if instance_variables.include? "@#{name}"
           self.instance_eval "@#{name}"
      else
        if block_given?
          instance_eval &block
        else
          value
        end
      end
    }
    self.send(:define_method, "#{name}="){ |value|
      self.instance_eval "@#{name} = value"
    }
  end
end

class Foo
  attribute :bar
  attribute(:fud) {instance_variables.include?("@bar") ? @bar : ' '}
  attribute :fi, 10
end

>>   t=Foo.new
=> #<Foo:0xb7b9cadc>
>> t.bar
=> nil
>> t.fud
=> " "
>> t.fi
=> 10
>> t.bar = 5
=> 5
>> t.fud
=> 5
>> t.fud = 20
=> 20
>> t.fud
=> 20
>>