From: Chuck Remes <cremes.devlist@...>
Date: 2008-08-05T08:30:40+09:00
Subject: Re: Is it ellegant to use a global variable to store a Logger object?


On Aug 4, 2008, at 5:38 PM, Chuck Remes wrote:
>
> For lurkers, here is some slightly corrected code (the above doesn't  
> compile).
>
> require 'logger'
>
> module Namespace
> module Logger
>   def Logger.key= *key
>     @key = key.first unless key.empty?
>   end
>
>   def Logger.key
>    @key
>   end
>
>   def Logger.instance
>     @instances ||= Hash.new { |h,k| h[key || :default ]  
> = ::Logger.new(STDOUT) }
>     @instances[@key]
>   end
>
>    %w( debug info warn error fatal ).each do |method|
>     	module_eval <<-code
>         def Logger.#{ method }(*a, &b)
>           instance.#{ method }(*a, &b)
>         end
>       code
>     end
>  end
> end

Hmmm... I puzzled through this all the way home and came to the  
conclusion that I don't know why, or if, this works.

If we are saving information in an instance variable in a class  
method, clearly that instance variable can be overwritten by another  
writer, yes? These class methods are visible to anyone who can this  
module in scope, therefore the @key variable can be pretty volatile.

This whole setup is predicated on WriterA calling  
Namespace::Logger.key = <unique val> which is later used by  
Namespace::Logger.instance as an index into a hash to lookup the  
correct ::Logger object in @instances. However, there is no guarantee  
that @key will remain <unique val> for WriterA. If WriterB comes along  
and makes the same call, that writer replaces <unique val> with its  
own value.

If that didn't happen, then how would @instances store references to  
everyone's ::Logger objects? These variables must be shared for that  
to work.

Right?

If true, I'm back to square one. Each writer would have to pass in  
their own unique key so they lookup the right ::Logger object. Now  
instead of injecting the ::Logger object into all of my classes, I  
have to make sure the @key value is known by all.

Please correct me where I am wrong.

cr