How to define a class variable on a singleton class

Question

I want to define a class variable on a singleton class. I checked this program's result:

class C
  class << self
    @@val = 100
  end
end

C.singleton_class.class_variables #=> [], I expect [:@@val]
C.class_variables #=> [:@@val]

I expect the scope of @@val to be the singleton class, isn't it?

Would you tell me how to define a class variable on a singleton class using class << self, and the reason why this program is not correct?

sawa · Accepted Answer · 2015-11-25 16:39:43Z

3

It is because when Ruby parser meets a class variable, the current class is resolved according to the lexical scope.

Cf. http://blog.honeybadger.io/lexical-scoping-and-ruby-class-variables/

answered Nov 25, 2015 at 16:39

sawa

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Comments

Aleksei Matiushkin · Accepted Answer · 2015-11-25 16:25:31Z

1

To set the singleton class variable, one might use:

class C
  class << self
    class_variable_set :@@cv, 42
  end
end
C.singleton_class.class_variables #⇒ [:@@cv]
C.singleton_class.class_variable_get :@@cv #⇒ 42

answered Nov 25, 2015 at 16:25

Aleksei Matiushkin

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3 Comments

Akira Noguchi Over a year ago

thanks. would you tell me why above program is not correct?. that scope is singleton class, isn't it ?

Aleksei Matiushkin Over a year ago

The more I think about it, the more I am convinced this is ruby parser glitch. The scope is C.singleton_class, yes. Even class C ; class << self ; class << self ; @@ccv = 42 ; end ; end ; end results in setting class variable on C. Probably this is how @ and @@ are handled. It responds to the class opened despite whether it appears inside class method, instance method, or any declaration.

Aleksei Matiushkin Over a year ago

@CarySwoveland I have it on purpose.

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