From: Dave Thomas <Dave@...>
Date: 2001-09-04T11:21:52+09:00
Subject: [ruby-talk:20833] Re: singleton help

Albert Wagner <alwagner@tcac.net> writes:


> > There is a logical problem with having 'instance' take parameters
> > (which is why is doesn't, I guess :). You could have
> >
> >    o1 = Fred.instance("barney")
> >    o2 = Fred.instance("wilma")
> >
> > What would that mean?
> 
> Actually, I do need to pass an argument to the singleton.  It needs
> access to the class that created it (owner?)  I handled it with a
> setup(owner) method, but it seems inelegant.

But the point there is that although you've created a single instance,
but what would it's state be if you passed different values to
#instance?

For example

   class A
     include Singleton

     def instance(ownerClass)   # NOT ALLOWED...
        @ownerClass = ownerClass
     end
   end

   class X
     def initialize
       @single = A.instance(self)
     end
   end

   class Y
     def initialize
       @single = A.instance(self)
     end
   end


What would the value of A.ownerClass be after

   X.new
   Y.new

If the singleton is specific to a particular class, perhaps nesting it
within that class might be a better solution. If it's shared between
classes and each wants to pass a different value to the #instance
method, then perhaps it isn't a singleton...


Regards


Dave