Bug #7475

Unexpected behavior of Module#append_features on singleton class

Added by Ernie Miller about 3 years ago. Updated almost 3 years ago.

Assignee:Yukihiro Matsumoto
ruby -v:ruby 1.9.3p327 (2012-11-10 revision 37606) [x86_64-darwin12.2.0] Backport:


A more involved explanation is available at ((URL:http://erniemiller.org/2012/11/29/ruby-tidbit-include-vs-extend-with-module-class-variables/))

In short, the handling of class variables (and constants) when a module is extended vs included is not as expected.


module Foo
  @@foo = 'foo'

class Bar
  include Foo

class Baz
  extend Foo

Bar.class_variable_get :@@foo # => "foo"
Baz.singleton_class.class_variable_get :@@foo # => NameError: uninitialized class variable @@foo in Class

We would expect constant and class variable lookup on the singleton class to work, but it doesn't. Both Rubinius and JRuby seem to behave as expected in this case.



#1 Updated by Ernie Miller almost 3 years ago

Another quick set of observations from this morning:

class Baz
  class << self
    Const = 'Const'
    @@foo = 'foo'

Baz.class_variables.inspect # => [:@@foo]
Baz.singleton_class.class_variables.inspect # => []
Baz.singleton_class.class_variable_get :@@foo # => 'foo' ???
Baz.const_get(:Const, false) rescue "Nope." # => 'Nope.'
Baz.singleton_class.const_get(:Const, false) rescue "Nope." # => Const

# Let's try setting it explicitly.
Baz.singleton_class.class_variable_set :@@foo, 'foo'
Baz.singleton_class.class_variables.inspect # => [] -- still "empty"

However, if we extend Foo on Baz, vs opening the singleton class with "class << self", we can class_variable_set on the singleton and see it show up in the list of class variables, vs being empty.

All of this is to say that I think some unexpected weirdness is going on in rb_include_module and/or include_class_new.

#2 Updated by Koichi Sasada almost 3 years ago

  • Assignee set to Yukihiro Matsumoto
  • Target version changed from 1.9.3 to next minor

Matz, could you check it?

#3 Updated by Nobuyoshi Nakada almost 3 years ago

  • Description updated (diff)

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