From: "David A. Black" <dblack@...>
Date: 2008-09-02T03:24:15+09:00
Subject: Re: included() vs extended()

Hi --

On Tue, 2 Sep 2008, Maciej Tomaka wrote:

>
>> So my question is: given that extend() is implemented (I think) using
>> metclass.include(), and that the end result of both is identical, how
>> come
>> Hooks.included is not called when using extend()? It seems reasonable
>> that
>> both the included() and extended() hooks should be called in this
>> situation,
>> though I can just as well see the argument against doing this. Anyone
>> care
>> to enlighten/persuade me as to why this behaviour exists?
>
> In ruby everything is an object. So even class is an object.
> Still we have extend and include which are not the same methods:
>
> include => includes code into class.
>
> extend  => extends using module, appends class methods.

include and extend both do the same thing: they add a module to the
method lookup path of an object or set of objects. So, for example:

   class C
   end

   c = C.new
   c.talk     # looks in C, Object, Kernel
              # no 'talk' method found: error

   module M
     def talk
       puts "hi"
     end
   end

   class C
     include M
   end

   c.talk     # looks in C, M ...stops there because it finds 'talk'

No code is added to C. When you use extend, you're doing an "include"
operation on the object's singleton class. So:

   str = "hello"
   str.extend(M)
   str.talk    # looks in its singleton class, then M...
               # and finds 'talk'

If you do, for example:

   C.extend(M)

then you're adding M to the lookup path of C. That means you can do:

   C.talk

But note that it's exactly the same pattern as before.

> bar = Bar.new
>
> bar            - object of class Bar
> bar.class      - class of object bar
> Bar            - class Bar, object of class Class
> Bar.metaclass  - class of class Bar :)

The class of class Bar is Class. The singleton class of Bar isn't the
same as the class of Bar.


David

-- 
Rails training from David A. Black and Ruby Power and Light:
   Intro to Ruby on Rails  January 12-15   Fort Lauderdale, FL
   Advancing with Rails    January 19-22   Fort Lauderdale, FL *
   * Co-taught with Patrick Ewing!
See http://www.rubypal.com for details and updates!