From: "R. Mark Volkmann" <mark@...>
Date: 2005-06-20T23:18:55+09:00
Subject: Re: preventing instantiation

Quoting Steven Jenkins <steven.jenkins@ieee.org>:

> >>On 6/19/05, R. Mark Volkmann <mark@ociweb.com> wrote:
> >>
> >>>What is the recommended way in Ruby to prevent other classes from creating
> >>>instances of a given class?  The class isn't a singleton.  It creates
> >>>several
> >>>instances of itself and no additional instances should be able to be
> >>>created.
> >
> > When I put the following in my class, it seems that even that class can't
> > create instances.
> >
> > private_class_method :new
> >
> > I'm confused.  How can I create a few objects within the class and then
> > prevent other classes from creating additional instances?
>
> Does this do it?
>
> sjenkins@tidal ~ $ cat t.rb
> require 'singleton'
>
> class A; end
>
> a0 = A.new
> a1 = A.new
> a2 = A.new
>
> class A
>   include Singleton
> end
>
> a3 = A.new
> sjenkins@tidal ~ $ ruby t.rb
> t.rb:13: private method `new' called for A:Class (NoMethodError)
>
> Remember, 'include' is exectuable.

That does work, but I see two issues with that approach.
1) it adds an "instance" method that isn't useful and is misleading
2) someone might think the class is a singleton when in fact it isn't

--
R. Mark Volkmann
Partner, Object Computing, Inc.