From: justin@...
Date: 2019-01-14T17:37:31+00:00
Subject: [ruby-core:91085] [Ruby trunk Bug#8297] extend & inherited class	variable issue

Issue #8297 has been updated by jsc (Justin Collins).


This appears to still be an issue with Ruby 2.6.0 (ruby 2.6.0p0 (2018-12-25 revision 66547) [x86_64-linux]):

Example:
~~~
module M
  @@xyz = 123
end

puts "M.class_variables: #{M.class_variables.inspect}"
puts "M.class_variable_get :@@xyz: #{M.class_variable_get :@@xyz}"

class C
  extend M
end

puts "C.class_variables: #{C.class_variables.inspect}"
puts "C.class_variable_get :@@xyz: #{C.class_variable_get :@@xyz}"
~~~

Output:

~~~
M.class_variables: [:@@xyz]
M.class_variable_get :@@xyz: 123
C.class_variables: []
Traceback (most recent call last):
	1: from 8297.rb:13:in `<main>'
8297.rb:13:in `class_variable_get': uninitialized class variable @@xyz in C (NameError)
~~~

----------------------------------------
Bug #8297: extend & inherited class variable issue
https://bugs.ruby-lang.org/issues/8297#change-76313

* Author: dunric (David Unric)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: 2.0.0p0
* Backport: 1.9.3: UNKNOWN, 2.0.0: UNKNOWN
----------------------------------------
=begin
By the current documentation, (({Object#extend})) method has to (only) add instance methods of a module given as a parameter.
In the following example, the class ((|C|)) is extended with module ((|M|)).
By (({class_variables})) method sent to singleton class of C also did inherit class variable ((|@@xyz|)).
However when inherited ((|@@xyz|)) is accessed, (({NameError})) exception is raised as it is was not initialized.

  module M
    @@xyz = 123
  end
  M.class_variables    # [:@@xyz]
  M.class_variable_get :@@xyz    # 123 , so far so good

  class C
    extend M
  end
  p C.singleton_class.class_variables    # [:@@xyz]
  p C.singleton_class.class_variable_get :@@xyz # NameError exception

Either (({class_variables})) returns invalid array - ie. ((|@@xyz|)) was not inherited at all or (({class_variable_get})) ignores class variables inherited from module (when sent to a singleton).
=end
Prior Ruby versions like 1.9.3p392 does not suffer this issue as return with Module#class_variables returns an empty array.




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