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Bug #8297

extend & inherited class variable issue

Added by dunric (David Unric) over 6 years ago. Updated 12 days ago.

Status:
Open
Priority:
Normal
Assignee:
-
Target version:
-
ruby -v:
2.0.0p0
Backport:
[ruby-core:54463]

Description

=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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Updated by dunric (David Unric) over 6 years ago

=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).

Prior Ruby versions like 1.9.3p392 does not suffer this issue as Module#class_variables returns an empty array.
=end

Updated by nobu (Nobuyoshi Nakada) over 6 years ago

  • Description updated (diff)

Updated by jsc (Justin Collins) 7 months ago

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)

Updated by jeremyevans0 (Jeremy Evans) 12 days ago

This bug is still present in the master branch. I think the best way to fix it is to modify Module#class_variables for singleton classes of classes/modules to use the same lookup order as Module#class_variable_get:

  • Singleton Class
  • Class
  • All Ancestors of Class

Note that this does not include modules included in the singleton class.

Attached is a patch that implements this behavior.

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