From: Ben Giddings <ben@...>
Date: 2003-07-29T11:19:28+09:00
Subject: Singletons as globals and GC

Although Ruby allows the use of global variables, I still prefer not to 
use them.  I am wary that someone else might have chosen the same 
variable name as me, and besides, they just don't feel properly OO.  
Singletons seem like the best solution to this problem, but I'm 
concerned about garbage collection.

Say I have two classes that each do a lot of work.  I instantiate one, 
do a bunch of calculations and return a result.  I then instantiate a 
second class and do a bunch more work on that result.  Say both classes 
want to use this global / singleton, and want to make sure that it 
doesn't get garbage collected between calls, what's the best way to do 
this?

Example code:

class Stats
   include Singleton
   def initialize
     @count = 0
   end

   def sawAnother
     @count += 1
   end
end

class Worker1
   def doStuff
     @stats = Stats.instance
     ...
   end
end

class Worker2
   def doOtherStuff
     @stats = Stats.instance
     ...
   end

   def showResult
     @stats.dumpStats
   end
end

w1 = Worker1.new
w1.doStuff

w2 = Worker2.new
w2.doStuff
w2.showResult

What I want to avoid is having the Stats singleton get garbage 
collected between calls to w1.doStuff and the creation of Worker2.new.  
Any comments?

Ben