# Inconsistent behavior of singleton methods that access class variables: 1.8.7 vs 1.9.2

**URL:** <https://www.ruby-forum.com/t/inconsistent-behavior-of-singleton-methods-that-access-class-variables-1-8-7-vs-1-9-2/176242>\
**Category:** Ruby\
**Created:** [September 27, 2009, 9:09pm UTC](https://www.ruby-forum.com/t/inconsistent-behavior-of-singleton-methods-that-access-class-variables-1-8-7-vs-1-9-2/176242 "2009-09-27T21:09:04Z")\
**Posts on this page:** 2\
**Page:** 1

<div class="post-metadata">

**Author:** ![Nikolai\_L](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/n/2acd7d/32.png) [@Nikolai\_L](https://www.ruby-forum.com/u/Nikolai_L)\
**Post date:** [September 27, 2009, 9:09pm UTC](https://www.ruby-forum.com/t/inconsistent-behavior-of-singleton-methods-that-access-class-variables-1-8-7-vs-1-9-2/176242/1 "2009-09-27T21:09:04Z")

</div>

Sample code (based on fragment from  
rails/actionpack/test/abstract\_unit.rb):

module A  
class Sample; end  
module Incorrect; end  
module Correct; end

class \<\< Sample; def list; @@list ||= []; end; end  
class \<\< Incorrect; def list; @@list ||= []; end; end # original  
pattern  
module Correct; def self.list; @@list ||= []; end; end  
end

def test\_m(obj)  
2.times do |i|  
l = obj.list  
l \<\< “test”  
printf “%d) %s %d %s\n”, i, obj.name, l.object\_id, l.inspect  
end  
end

[A::Incorrect, A::Correct, A::Sample].each {|mod| test\_m(mod) }

Output:

ruby 1.8.7 (2009-09-11 patchlevel 202) [i686-linux]:  
0) A::Incorrect -603619398 [“test”]

1. A::Incorrect -603619398 [“test”, “test”]
2. A::Correct -603619548 [“test”]
3. A::Correct -603619548 [“test”, “test”]
4. A::Sample -603619398 [“test”, “test”, “test”]
5. A::Sample -603619398 [“test”, “test”, “test”, “test”]

ruby 1.9.2dev (2009-09-28 trunk 25126) [i686-linux]:  
0) A::Incorrect 79250400 [“test”]

1. A::Incorrect 79250300 [“test”]
2. A::Correct 79250220 [“test”]
3. A::Correct 79250220 [“test”, “test”]
4. A::Sample 79250060 [“test”]
5. A::Sample 79249980 [“test”]

So, the question is, what is the correct way to declare and use such  
constructs, and what caused the problem – bug in ruby or incorrect  
use of class variables?

---

<div class="post-metadata">

**Author:** ![Nikolai\_L](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/n/2acd7d/32.png) [@Nikolai\_L](https://www.ruby-forum.com/u/Nikolai_L)\
**Post date:** [September 29, 2009, 6:32am UTC](https://www.ruby-forum.com/t/inconsistent-behavior-of-singleton-methods-that-access-class-variables-1-8-7-vs-1-9-2/176242/2 "2009-09-29T06:32:54Z")

</div>

Hi,

In message “Re: Inconsistent behavior of singleton methods that access  
class variables: 1.8.7 vs 1.9.2”  
on Mon, 28 Sep 2009 04:06:42 +0900, Nikolai L.  
[removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) writes:

|So, the question is, what is the correct way to declare and use such  
|constructs, and what caused the problem – bug in ruby or incorrect  
|use of class variables?

The basic rules for class variables are:

(1) initialize at class variables at the top of the scope (in this  
case, A).

(2) avoid name conflict of class variables in the same top-level  
scope; in the case of the example, you used @@list several times  
in the scope of A.

(3) and lastly, and most importantly, do not use class variables,  
unless you really really need them; they are fundamentally global  
variables.

You’ve violated all of above rules.

```
          matz.

```