Include vs. Extend
Include and Extend allow us to take methods from a module and add them to an object. They work slightly differently from each other though. Let’s take a look…
Extend adds methods directly to an object
Extend adds methods to an object. It extends that object by adding new features to it.
class Hamster end module PetSkills def snuggle;end end Hamster.extend PetSkills
If you extend a class, you create a class method.
h = Hamster.new Hamster.methods.include? :snuggle # => true h.methods.include? :snuggle # => false
If you extend an instance of a class, you create an instance method, but only on that instance. You can extend any object like this.
h.extend PetSkills h.methods.include? :snuggle # => true i = Hamster.new; i.methods.include? :snuggle # => false
You can call extend on any object to add methods to that object alone.
Include adds instance methods to a class
Include takes a more traditional approach. If you include a module in a class, the methods in the module will be added as instance methods, and will be available to all instances of that class.
class Gerbil include PetSkills end g = Gerbil.new Gerbil.methods.include? :snuggle # => false g.methods.include? :snuggle # => true
Extend will add methods to an object, and only to that object. If we extend a class we get class methods.
Include will include methods from a module into a class, those methods become instance methods for objects of that type.