Include an instance method from a module and then outputting a class variable that was defined in the base class

Question

I'm attempting to include a method from a module in a class, and have the method that is included access a class variable of the base class that's doing the including, but it's not working like I assumed. Code:

class Awesome
    @@name = "ME!"
end

module MyGem
    module Namespace
        def typist
            puts @@name
        end
    end
end

if defined? Awesome
    Awesome.class_eval do
        include MyGem::Namespace
    end
end

Awesome.new.typist # Test our new 'injected' instance method!
#=> NameError: uninitialized class variable @@name in MyGem::Namespace

Clearly my understanding of Ruby's include behaviour is wonky, I thought include would incorporate the method(s) into the base class and the execution context would be the base class, but the error message seems to imply the execution context of included methods is in the original method's module.

So how can I achieve what I'd like to do as illustrated in my code snippet? Mind you, I'm more than happy to replace the class variable with any other implementation, such as using instance variables, or however which it takes to make it work :).

Zach Kemp · Accepted Answer · 2013-05-22 22:47:57Z

1

If you make the class variable lookup the result of a method call, it works:

def typist
  puts self.class.class_variable_get(:@@name)
end

I'm afraid I'm not familiar enough with class variables to explain why it works this way but not the other. I do know that class variables are relatively unpopular in the Ruby world, mostly for the reason that their behavior can be unpredictable. My gut feeling is that since you can define class variables in a module (classes inherit from Module, after all), referring to the variable in a module automatically searches within that module. Explicitly sending the lookup message to the class of the instance to which you are sending :typist ensures the correct variable is found.

edited May 22, 2013 at 22:47

answered May 22, 2013 at 18:45

Zach Kemp

11.9k1 gold badge35 silver badges47 bronze badges

Sign up to request clarification or add additional context in comments.

5 Comments

sunny1304 Over a year ago

your answer is not addressing the problem of the topic.

Zach Kemp Over a year ago

@sunny1304, Can you be more specific?

sunny1304 Over a year ago

I mean the code u provided inlcudes class_variale_get method...using that the code will work for sure....but we need to know why without class_variable_get the code is not working.

Peter Over a year ago

Okay, so the first occurrence of class should actually be replaced with "self.class", or else you will get an exception telling you that the "." after class is misplaced.

Zach Kemp Over a year ago

@TahitiPetey, my interpreter worked without it, but you're right - in general that probably good practice.

Collectives™ on Stack Overflow

Include an instance method from a module and then outputting a class variable that was defined in the base class

1 Answer 1

5 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

5 Comments

Your Answer

Sign up or log in

Post as a guest

Related