title: Which is the right one, nil or NULL, to mark "no Objective-C block"?
body: <p>If I want to pass nothing for an Objective-C block, what keyword should I use, <code>NULL</code> or <code>nil</code>? I'm asking this because an Objective-C block is an Objective-C object (as I know), but represented as a function pointer.</p>
  
  <p><code>NULL</code> and <code>nil</code> both indicate a <code>0x0</code> pointer, however they are different semantically. So I'm concerned about this.</p>