How to define a class variable on a singleton class

问题

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?

回答1:

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/

回答2:

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

来源：https://stackoverflow.com/questions/33921185/how-to-define-a-class-variable-on-a-singleton-class