From: Jean Helou <jean.helou@...>
Date: 2006-10-27T18:10:01+09:00
Subject: Re: why is my singleton method called before the class is initialize?

If you want to do it at the class level instead of the instance level,
it looks like what is done quite a lot in rails

for example in active record :

class Toto<<ActiveRecord::Base
end

would use totos as table name by default (define by AR) but you can
chage it to something else like this :

class Toto<<ActiveRecord::Base
  set_table_name "mice"
end

now it will use mice instead of toto

if this is what you are trying to do, have a look at rails
implementation, which relies on rails define_attr_method :
http://railsmanual.org/class/ActiveRecord::Base/define_attr_method

hope this helps

On 10/27/06, bachase@gmail.com <bachase@gmail.com> wrote:
> I'll say thanks to both of you.  I think I was confused on all
> accounts, but the biggest issue is probably that I thought the
> @favorites was the same for both classes.  I guess I will need to
> restructure things then.  Basically, I want the base class to provide
> some default settings that a child class could override using a class
> method.  Apparently, I am unclear on the way to do so.
>
> matt neuburg wrote:
> > Rick DeNatale <rick.denatale@gmail.com> wrote:
> >
> > > 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.
> >
> > Perhaps it's a matter of pedagogy. What *is* "the point"? That depends
> > on where one thinks the OP has ultimately gone wrong. The OP might have
> > any of several misconceptions (it is hard to be certain); the OP might
> > not be aware:
> >
> > (1) that initialize is called at instantiation time.
> >
> > (2) that a class and an instance of that class are different things.
> >
> > (3) that the singleton method is a class method.
> >
> > (4) that the @favorites referred to in the class method is not the
> > @favorites referred to in "initialize".
> >
> > Each of us is trying to teach, so each of us, like a good teacher, made
> > a guess about where the heart of the OP's misunderstanding might lie. I
> > guessed (1), with a little bit of (2) thrown in. You are leaning more
> > toward (3) and (4). Your guess is reasonable, but my guess is not
> > unreasonable, especially since the OP explicitly said: "I expected the
> > initialize ... to occur". I spoke to that issue, showing why that
> > expectation was wrong. So what I said is hardly "beside the point".
> >
> > m.
> >
> > > 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.
> >
> >
> > --
> > matt neuburg, phd = matt@tidbits.com, http://www.tidbits.com/matt/
> > Tiger - http://www.takecontrolbooks.com/tiger-customizing.html
> > AppleScript - http://www.amazon.com/gp/product/0596102119
> > Read TidBITS! It's free and smart. http://www.tidbits.com
>
>
>