From: dblack@...
Date: 2006-01-21T10:38:06+09:00
Subject: Re: Inserting

Hi --

On Sat, 21 Jan 2006, James Hughes wrote:

> On 1/20/06, dblack@wobblini.net <dblack@wobblini.net> wrote:
>> Hi --
>>
>> On Sat, 21 Jan 2006, James Hughes wrote:
>>
>>> On 1/19/06, Paul Brannan <pbrannan@atdesk.com> wrote:
>>>>
>>>> You can also create a mixin and extend Foo::Bar with the mixin.  If you
>>>> do this, instance methods in the mixin will be available as class
>>>> methods in Foo::Bar and derived classes.
>>>
>>> Ok, I'm doing something like the following:
>>>
>>> module FooBarHelper
>>>
>>>  class << Foo::Bar
>>>     # some instance methods, which reference class vars from Foo::Bar
>>>  end
>>> end
>>>
>>> Then, in classes A and B I do "include FooBarHelper".  When I run this
>>> I get "uninitialized class variable" errors for the Foo::Bar's class
>>> vars.
>>>
>>> So, to state my question again, how can I extend the functionality of
>>> the Foo::Bar library (short of hacking it's source directly), while
>>> still maintaining access to the class variables defined in Foo::Bar?
>>> Is this even possible?
>>
>> Does this help at all?

[...]

> Hmm. Not sure yet. While I try to wrap my head around what's going on
> with the const_missing and const_get magic above, here is the short
> script I wrote to try and simulate the real-world problem I'm trying
> to solve. Note that the derived class  (A) is trying to access a class
> variable and not a constant as in your example.

Whoops, I misread.

> module Foo
>  class Bar
>    @@classvar = "Boo!"
>  end
> end
>
> module FooBarHelper
>  class << Foo::Bar
>    def amethod
>      puts  @@classvar

The class variables of Foo::Bar's singleton class are not the same as
those of Foo::Bar, so you'd want to avoid opening up the singleton
class.  You can do this instead:

class Foo::Bar
   def self.amethod
     puts @@classvar
   end
end

As things stand there's no need for the FooBarHelper module at all:

class A < Foo::Bar
end

A.amethod    # Boo!


David

-- 
David A. Black
dblack@wobblini.net

"Ruby for Rails", from Manning Publications, coming April 2006!
http://www.manning.com/books/black