From: Rick DeNatale <rick.denatale@...>
Date: 2006-10-27T11:13:35+09:00
Subject: Re: why is my singleton method called before the class is initialize?

On 10/26/06, matt neuburg <matt@tidbits.com> wrote:
> bachase@gmail.com <bachase@gmail.com> wrote:
>
> > Consider:
> >
> > class BigGuy
> >    def initialize
> >       @favorites = ["a","b","c"]
> >    end
> >
> >    class << self
> >      def display_favs
> >       class_eval "p @favorites"
> >      end
> >    end
> > end
> >
> > class LittleGuy < BigGuy
> >      display_favs
> > end
> >
> > Why does this display "nil"?  I expected the initialize of BigGuy to
> > occur prior to invoking display_favs.  Any other construction I should
> > be using to get my desire behavior of the constructor of the base class
> > getting called prior to singleton methods called from the child class?
>
> "Initialize" is called when the class is instantiated (i.e. with new).
> You are never instantiating this class, so "initialize" is never called.

True, but kind of beside the point.  The initialize method if and when
it is called will set the instance variable @favorites in a new
INSTANCE of BigGuy.

class BigGuy
 ...
  class << self
    def display_favs
     class_eval "p @favorites"
    end
  end
end

This makes display_favs a class method of BigGuy.
In the method display_favs self is the class object BigGuy.

so
   class_eval "p @favorites"
is the same as
   BigGuy.class_eval "p @favorites"

which is printing a class instance variable of BigGuy which doesn't exist.

And...

class LittleGuy < BigGuy
    display_favs
end

inside this class definition, self is LittleGuy so

    display_favs
is equivalent to:
    LittleGuy.display_favs

which executes
   BigGuy::display_favs

by inheritance

which prints the uninitialized class instance variable @favorites.

Note also that the class instance variable @favorite in BigGuy is
different from the clas instance variable @favorite in LittleGuy,
although neither one is initialized.


-- 
Rick DeNatale

My blog on Ruby
http://talklikeaduck.denhaven2.com/