title: How to cast an object from its base class into its subclass
body: <p>I have a class <code>User</code> which is a subclass of class <code>PFUser</code>:</p>
  
  <pre><code>class User: PFUser {
   var isManager = false
  }
  </code></pre>
  
  <p>In one of my methods I receive a <code>PFUser</code> object and I want to cast it to a <code>User</code> object</p>
  
  <pre><code>func signUpViewController(signUpController: PFSignUpViewController!, didSignUpUser user: PFUser!) {
   currentUser = user
  }
  </code></pre>
  
  <p>Is this possible?</p>