From: dblack@...
Date: 2003-05-09T09:32:57+09:00
Subject: Re: Can a global be a constant?

Hi --

On Fri, 9 May 2003, Jim Freeze wrote:

> On Friday,  9 May 2003 at  9:16:35 +0900, dblack@superlink.net wrote:
> > Hi --
> >
> > On Fri, 9 May 2003, Jim Freeze wrote:
> >
> > > On Friday,  9 May 2003 at  8:57:15 +0900, dblack@superlink.net wrote:
> > >
> > > > You can use instance variables (of the object State) instead of class
> > > > variables, and then State becomes:
> > > >
> > > >   class State
> > > >     class << self
> > > >       attr_accessor :debug, :verbose
> > > >     end
> > > >   end
> > >
> > > Why does this not define the class method State.debug to
> > > set the instance variable @debug?
> >
> > It does; it creates #debug and #debug= in State's singleton class, and
> > those methods get/set State's instance variable @debug.
>
> So, this is something I don't understand. State is never instantiated.
> How does it have an instance variable?

It *itself* has an instance variable.  State is an instance (of
Class), so it can have instance variables.

Defining these methods in State's singleton class is actually a
variation of defining them generally for all Class objects.  If one
did this:

  class Class
    attr_accessor :debug
  end

then every Class object would have that method:

  class String
    self.debug = true
    puts debug         # true
  end

(Note that instance *of* String don't have the methods; String
*itself* has them, in its capacity as an instance of Class, to which
the methods have been added.)

The way I did it was a variant which put the accessor methods only in
State's singleton class (since other Class objects don't need those
methods).


David

-- 
David Alan Black
home: dblack@superlink.net
work: blackdav@shu.edu
Web:  http://pirate.shu.edu/~blackdav