From: "David A. Black" <dblack@...>
Date: 2009-09-22T21:36:44+09:00
Subject: Re: start_with

Hi --

On Tue, 22 Sep 2009, Rong wrote:

> I tried changing the array to an instance variable but the code won't
> work.

You'd have to make it an instance variable of the class itself, like
this:

   class Person
     @people = []
     def self.people
       @people
     end

     def initialize
       self.class.people << self
     end
   end

You were very considerate in using a tweet instead of this mailing
list to point out that, in spite of my saying I had reservations about
using a constant the way your code did, I had done exactly the same
thing on page 108 of my book :-) So let me try to return the kindness
by explaining my reservations.

The main issue at least to be aware of is what happens when you
inherit:

   class Person
     P = []
     def initialize
       P << self
     end
   end

   class Rubyist < Person
   end

   r = Rubyist.new
   p Person::P   # => [#<Rubyist:0x2202a4>]

In other words, you end up using the same constant for all the
subclasses, and thus getting what you might consider false positives
(depending on exactly what you want to count). This is also why using
a class variable is a problem in this kind of role: it will also be
the same variable down the class hierarchy.

The other issue is just the question of using a "constant" for
something that changes, like this array. I don't consider that
to be automatically bad (and it's very different from re-assigning to
a constant, which is *really* non-constant), but it feels perhaps like
a looser fit than using an instance variable, as in my first example
above. The instance variable is designed to maintain state on a
per-object (in this case, per class-object) basis, which a constant
can do but which isn't their main or most "natural" role.


David

-- 
David A. Black, Director
Ruby Power and Light, LLC (http://www.rubypal.com)
Ruby/Rails training, consulting, mentoring, code review
Book: The Well-Grounded Rubyist (http://www.manning.com/black2)