title: using ARC, lifetime qualifier assign and unsafe_unretained
body: <p>i'm a little confused about these two qualifiers... 
  With ARC instead of using <code>weak</code> (i.e. if I need support iOS 4) I can use <code>unsafe_unretained</code> losing the auto-nil features... the final result seems to be similar to <code>assign</code>.</p>
  
  <ul>
  <li>Can I exchange <code>unsafe_unretained</code> with <code>assign</code> ? </li>
  <li>Are these qualifiers the same thing ? </li>
  </ul>
  
  <p>It would be really interesting any link of Apple documentation on this argument... I can find only a few rows <a href="http://developer.apple.com/library/ios/#releasenotes/ObjectiveC/RN-TransitioningToARC/_index.html" rel="nofollow noreferrer">here</a></p>