From: Dan Doel <djd15@...>
Date: 2003-08-22T15:43:31+09:00
Subject: Re: Class variables - a surprising result

Patrick Bennett wrote:

> Geesh - sneaky stuff.. 


It can get sneakier.  There's more than one way to make a class instance 
variable in ruby;
two useful, and two useless.

class Foo
    @class_instance_var1 = "I am variable 1"

    class << self
       @class_instance_var2 = "I am variable 2"
    end

    def make3
       class << self
          class << self
             @class_instance_var3a = "I am variable 3a"
          end
          @class_instance_var3b = "I am variable 3b"
       end
    end

    self.class.class_eval { @class_instance_var4 = "I am variable 4" }
end

Okay:

#1 is an instance variable of the Foo class object. It's accessed from 
instance methods like so:
    self.class.class_eval { @class_instance_var1 }
Also, you can use #attr methods to make accessors for you like so:

class Foo
    class << self
       attr_accessor :class_instance_var1
    end
end

#2 is an instance variable of the singleton class of Foo.  This is the 
type of class instance variable
created when you do:

    class << self.class
       ...
    end

In an instance method.  These are separate from #1 type class instance 
variables, so you can't mix
and match the two. To make an accessor for this, you do:

class Foo
    class << self
       class << self
          attr_accessor :class_instance_var2
       end
    end
end

#3 has two variables, but they're equally useless.  Essentially they're 
the two different flavors shown
above, but instead of using the Foo class object, you're using the 
singleton class of an instance of
Foo.  This essentially makes them no better than instance variables, 
unless you capture the singleton
class and make new instances with it, in which case you should be able 
to access them using the
methods in #1 and #2.

#4 is the same as #2, except that it uses Foo's formal class, which is 
Class. This makes
@class_instance_var4 available to all classes, which is probably 
undesireable.

I'm not sure which of #1 or #2 is more accepted/useful/whatever.  There 
probably isn't a concensus,
since I don't think this comes up a whole lot.

Are we confused yet? :)

- Dan