title: Adding a weak ivar to a Objective-C class using the runtime
body: <p>Is it possible to add a zeroing weak ref (in the ARC sense, not GC) instance variable to a class created using the runtime? If so how?</p>
  
  <p>There is a function called <code>class_setWeakIvarLayout()</code> which seems to be related to this but the documentation (both for this function and the non-weak counterpart) is <em>very</em> thin unfortunately.</p>