title: What is the difference between a weak reference and an unowned reference?
body: <p>Swift has:</p>
  
  <ul>
  <li>Strong References</li>
  <li>Weak References</li>
  <li>Unowned References</li>
  </ul>
  
  <p>How is an unowned reference different from a weak reference?</p>
  
  <p>When is it safe to use an unowned reference?</p>
  
  <p>Are unowned references a security risk like <a href="http://en.wikipedia.org/wiki/Dangling_pointer">dangling pointers</a> in C/C++?</p>