title: If ARC is handled by the compiler, why is weak not supported on iOS 4 and Mac OS X 10.6?
body: <p>Supposedly <code>weak</code> is like <code>assign</code> or <code>unsafe_unretained</code>, except that a <code>weak</code> variable is "zeroed" when the object that it points to is <code>dealloc</code>'ed.  But ARC is handled by the compiler, so why is <code>weak</code> not supported on iOS 4 and Mac OS X 10.6?</p>
  
  <p>(for this question please don't give a guess as an answer, but something more supported by facts/reference/docs)</p>