From: "David A. Black" <dblack@...>
Date: 2005-07-12T19:42:07+09:00
Subject: Re: class_attr_accessor

Hi --

On Tue, 12 Jul 2005 gwtmp01@mac.com wrote:

>
> On Jul 11, 2005, at 11:20 PM, David A. Black wrote:
>> I sometimes wonder whether all the "class << self" examples obscure the
>> fact that this is a general-purpose thing.  In case that's the case,
>> here's an example:
>> 
>>    str = "I am a string"
>>    class << str
>>      def x
>>        puts "method illustrating singleton class"
>>      end
>>    end
>
> It always seems clearer to me when written as:
>
>   str = "I am a string"
>   def str.x
>     puts "method illustrating singleton class"
>   end
>
> I understand that the method is really defined in the
> anonymous singleton class associated with str but the
> 'def object.method' syntax is more self-explanatory
> (to me) than the 'class <<obj' syntax.

"def obj.meth" is definitely the most direct way to add a method to an
object.  But it's not a drop-in replacement for opening up the
object's singleton class.  Consider:

   class << obj
     attr_accessor :x
   end

which you could do without opening the class but it would be a pain.
Also, there's a difference between the two techniques in the scope of
constants:

   X = 1
   obj = Object.new

   class << obj
     X = 2
     def b
       X
     end
   end

   def obj.a
     X
   end

   p obj.a     # 1
   p obj.b     # 2

(Thanks to Guy Decoux for pointing that out a couple of years ago.)

So it can be important to have the "class interface" to the object's
singleton operations.


David

-- 
David A. Black
dblack@wobblini.net