Instance variables in modules?

Question

How is it possible that I can have instance variables in a module even though I cannot create an instance of the module? What would be the purpose of @stack in module Stacklike below?

module Stacklike   def stack     @stack ||= []   end end 

Answer 1

Think of the instance variable as something which will exist in any class that includes your module, and things make a bit more sense:

module Stacklike   def stack     @stack ||= []   end    def add_to_stack(obj)     stack.push(obj)   end    def take_from_stack     stack.pop   end end  class ClownStack   include Stacklike    def size     @stack.length   end end  cs = ClownStack.new cs.add_to_stack(1) puts cs.size 

will output "1"

Answer 2

See the below:

p RUBY_VERSION module Stacklike   def stack     @stack ||= []   end    def add_to_stack(obj)     stack.push(obj)   end    def take_from_stack     stack.pop   end end  class A include Stacklike end  a = A.new p a.instance_variables #<~~ E p a.instance_variable_defined?(:@stack) #<~~ A a.add_to_stack(10) #<~~ B p a.instance_variable_defined?(:@stack) #<~~ C p a.instance_variables #<~~ D 

Output:

"1.9.3" [] false true [:@stack] 

Explanation: Yes, Module instance variables are present in the class when you would include them inside the class. But you can see that p a.instance_variable_defined?(:@stack) is showing false as @stack is still not defined till A. At point B I defined the instance variable @stack. Thus statement in point C, outputs as true. Means module instance variables are not being created by the module itself,but that can be done by the class instances if the class included that module. Statement in E outputs [] as still that point the instance variable was not defined, but if you see the output for the line D, it is proved the @stack is inside the object a of class A.

Why such design?

This is the design or sometimes come from the requirements. Say you have been asked to write a stack operation code which will be used by two ticket booking companies,Say A and B. Now A are stack policy for their customers to serve but also they have any more formalities with that. B company also using stack policy with their own formalities which is different from A. Thus in case of designing Stack operation inside class A and class B, better idea to write it in a common place,as Both A and B have this functionality common within them. In future if another company C comes to you you can also use that module into their class, without rewriting the same functionality for each A,B and C. There can be more thoughts but hope this will help you to answer your self for your last part of the questions.

That's all about the concept. Hope it helps.

Cheers!!