title: Getting an unique identifier for a UIViewController
body: <p>I have a fully implemented iOS App consisting of several view controllers.</p>
  
  <p>Some of the view controllers are subclasses of e.g. UIViewController or UITableViewController, but some of them just use the original class directly, such as UINavigationController.</p>
  
  <p>Is there any way to get or generate a unique identifier for each instance, so I can reference to the same specific controller next time the app is opened?</p>