title: Swift vs. +[NSValue valueWithNonretainedObject:]
body: <p>In Objective-C, I might use <code>+[NSValue valueWithNonretainedObject:]</code> to keep a unique ID of an object, where I don't wish to retain the object itself. Seems like that's deprecated for Swift.</p>
  
  <p>How to do in Swift?</p>