title: What's the difference between "_" and "__" in Objective-C?
body: <p>I've read that <code>_</code> (underscore) is only used to tell that the variable/method/class is for the API use only and should not be used by other programmers.</p>
  
  <p>What about <code>__</code> (double underscore)?</p>
  
  <p>In this example <code>__block BOOL found = NO;</code></p>
  
  <p>From the Apple Docs about <strong>Block Objects</strong>.</p>
  
  <p>Also, can anyone help me understand the concept of <strong>pointer-to-pointers</strong>? 
  <code>**var</code>
  I've read some answers about it but didn't really get it. When and how should I use it?</p>