From: Steve Tuckner <STUCKNER@...>
Date: 2003-12-03T01:33:17+09:00
Subject: Re: Attempted roadmap of future instance variables....

So what is the relationship between @_ vars and @vars that are defined in a
class context (if any)? Is this just to allow easier access to class local
vars (that you can already define)? Does below describe the situation
accurately?

class A
	@a = 1		# a is a class local variable
	@_a = 2		# a is also a class local variable
	@@a = 3		# a is a global variable whose visibility is itself and its
sub-classes

	def A.a		# define to allow access to class's instance var
		@a
	end

	def display
		puts "a (class local - current) = #{self.class.a}"
		puts "a (class local - new) = #{@_a}"
		puts "a (class global) = #{@@a}"
	end
end

class B < A
end

A.new.display
=> a (class local - current) = 1
=> a (class local - new) = 2
=> a (class global) = 3

B.new.display
=> a (class local - current) =
=> a (class local - new) =
=> a (class global) = 3

Steve Tuckner

> -----Original Message-----
> From: David A. Black [mailto:dblack@wobblini.net]
> Sent: Tuesday, December 02, 2003 9:24 AM
> To: ruby-talk ML
> Subject: Attempted roadmap of future instance variables....
>
>
> Hi --
>
> I figure that I owe the world at least a shot at clearing the cobwebs
> (of my own creation, that is :-) on this instance variable thing.
>
> So... for those still reading... here's what I *really* think is being
> proposed:
>
>
> class A
>   def m
>     @var  = "hi"          # regular instance variable
>     @_var = "hello"       # class-local instance variable
>   end
> end
>
> class B < A
>   def n
>     puts @var             # same as @var, above
>     puts @_var            # not same as @_var above
>   end
> end
>
> o = B.new
> o.m
> o.n
>
> => Output:
>
>    hi      # the @var that was set in A#m is used here
>    nil     # the @_var that was in A#m is *not* used here
>            # (because we're now in a method defined in B,
>            # a subclass of A, and class local instance
>            # variables are not shared by subclasses)
>
> In other words:
>
>   * Regular instance variables live per-name per-object
>     (o has exactly one @var).
>
>   * Class-local instance variables live per-name per-object per-class
>     (o's methods defined in A have a @_var, and o's methods defined in
>     B have a different @_var).
>
>
> David
>
> --
> David A. Black
> dblack@wobblini.net
>