title: unsafe_unretained and weak and assign under ARC?
body: <blockquote>
    <p><strong>Possible Duplicate:</strong><br>
    <a href="https://stackoverflow.com/questions/9784762/strong-weak-retain-unsafe-unretained-assign">strong / weak / retain / unsafe_unretained / assign</a>  </p>
  </blockquote>
  
  
  
  <p>what's difference between unsafe_unretained and weak and assign under ARC?
  thanks a lot.</p>