include : Mixes in specified module methods as instance methods in the target class

extend : Mixes in specified module methods as class methods in the target class

Example:

Now, on executing the below commands, you will get

ClassThatIncludes.new.module_method

=> “Module Method: Hi there!”

ClassThatExtends.module_method

=> “Module Method: Hi there!”

If you include module ReusableModule in class ClassThatIncludes, the methods, constants, classes, submodules, and other declarations gets referenced.

include is a private method, because it’s intended to be called from within the container class/module.

If you extend class ClassThatExtends with module ReusableModule, then the methods and constants gets copied. Obviously, if you are not careful, you can waste a lot of memory by dynamically duplicating definitions.

extend is a public method.

extend - adds the specified module’s methods and constants to the target’s metaclass (i.e. the singleton class). For ex :

  • if you call Klazz.extend(Mod), now Klazz has Mod’s methods (as class methods)

  • if you call obj.extend(Mod), now obj has Mod’s methods (as instance methods), but no other instance of of obj.class has those methods.