From: "David A. Black" <dblack@...>
Date: 2004-12-10T20:38:06+09:00
Subject: Re: adding object attributes

Hi --

On Fri, 10 Dec 2004, Artur Merke wrote:

> Hi
>
> class A
>  attr_accessor :a
> end
>
> a= A.new
> b= A.new
>
> #this adds a new attribute to the object 'b'
> def b.dummy
>  @dummy
> end
> def b.dummy=(val)
>  @dummy= val
> end
>
> b.dummy= "hello world"
> print "\n", b.dummy
>
> Is there a shorthand notation for adding attributes to an object?
>
> For example one could think of the equivalent of adding an attribute to a
> class, like
>
> b.attr_accessor(:dummy)  #illegal in ruby
>
> Of course I could define a sort of public 'attr_accessor' for the class
> 'A' by myself, but I do NOT want to change/extend the class, only some objects.
>
> any suggestions?

What you want to do is use attr_accessor -- but, as you say, not in
'A', but rather in the singleton class of 'b'.  You would do this like
this:

    class << b
      attr_accessor :dummy
    end

(A singleton class is anonymous, so you have to use the "<< object"
notation to "fish" for it from the object whose singleton class it
is.)

In fact, when you do:

   def b.dummy ...

you are inserting a method into exactly the same class.  Opening up
the class, however, makes it possible to use class methods, like
attr_accessor.


David

-- 
David A. Black
dblack@wobblini.net