From: David Flanagan <david@...>
Date: 2007-09-21T02:49:49+09:00
Subject: Re: class variables and singleton classes

Eric,

I think this is just one manifestation of the fact that class variables 
are no longer shared between classes and subclasses. That is: I don't 
think the behavior you're seeing is specific to singletons.

Here's what I write about class variables and inheritance in the book 
I'm working on.  These particular paragraphs have not been through 
technical review, but I think I've got it right.  (Also note that the 
info on eigenclass.org about class variables is out-of-date and the code 
snippets there no longer do what they are said to do:

--- begin book excerpt ---

       <para>
	Class variables are inherited by subclasses.  If a class
	<literal>A</literal> defines a variables <literal>@@a</literal>,
	then a subclass <literal>B</literal> can use that variable.
       </para>

       <para>
	In Ruby 1.9 and later, class variables behave like constants.
	That is, when a subclass assigns a new value to a class variable
	defined by a superclass, it is simply creating a new class
	variable in the subclass, and it does not affect the
	superclass's copy of the variable.
       </para>

       <para>
	In Ruby 1.8 and earlier, however, class variables are actually
	shared between classes and their subclasses. If a subclass
	assigns a value to a class variable already in use by a
	superclass, it alters the value seen by the superclass.  The
	mere presence of the subclass can alter the behavior of any
	class methods in the superclass that use the class
	variable. This is a strong argument for the use of class
	instance variables instead of class variables.
       </para>

       <para>
	The following code illustrates the differences between versions
	of Ruby.  It outputs "123" in Ruby 1.8 and outputs "111" in Ruby
         1.9:
       </para>

<programlisting><![CDATA[
class A
   @@value = 1                   # class variable
   def A.value; @@value; end     # class variable accessor method
end
print A.value                   # display value of A's class variable
class B < A; @@value = 2; end   # subclass defines class variable
print A.value                   # display value of A's class variable
class C < A; @@value = 3; end   # another subclass defines class variable
print A.value                   # display value of A's class variable
]]></programlisting>

--- end book excerpt --

	David


Eric Hodel wrote:
> Class variables in singleton classes are separate from class variables 
> in classes in 1.9 now.  Is this on purpose?
> 
> $ cat class_variables.rb
> module X
>   @@cv1 = nil
> 
>   class << self
>     def cv1() @@cv1 ||= 'cv1' end
>   end
> 
>   def self.cv1=(obj) @@cv1 = obj end
> 
>   @@cv2 = nil
> 
>   def self.cv2() @@cv2 ||= 'cv2' end
>   def self.cv2=(obj) @@cv2 = obj end
> end
> 
> p X.cv1
> X.cv1 = 'new1'
> p X.cv1
> 
> p X.cv2
> X.cv2 = 'new2'
> p X.cv2
> 
> $ ruby18 -v class_variables.rb
> ruby 1.8.6 (2007-06-30 patchlevel 5000) [i686-darwin8.10.1]
> "cv1"
> "new1"
> "cv2"
> "new2"
> $ ruby19 -v class_variables.rb
> ruby 1.9.0 (2007-09-15 patchlevel 0) [i686-darwin8.10.1]
> syck.so: warning: method redefined; discarding old value
> "cv1"
> "cv1"
> "cv2"
> "new2"
> 
> -- 
> Poor workers blame their tools. Good workers build better tools. The
> best workers get their tools to do the work for them. -- Syndicate Wars
> 
> 
>