As with model inheritance, by inheriting ultimately from ActionController:: Base both the Users and Microposts controllers gain a large amount of functionality,

such as the ability to manipulate model objects, filter inbound HTTP requests, and render views as HTML. Since all Rails controllers inherit from ApplicationController, rules defined in the Application controller automatically apply to every action in the application. For example, in Section 8.2.1 we’ll see how to include helpers for signing in and signing out of all of the sample application’s controllers.