title: Creating and using a dummy NSData subclass doesn't work
body: <p>I have a problem with creating my own subclass of <code>NSData</code>, which I want to have a custom <code>description</code> method. Even creating a dummy <code>NSData</code> subclass:</p>
  
  <pre><code>@interface MyData : NSData {}
  @end
  </code></pre>
  
  <p>and</p>
  
  <pre><code>@implementation MyData
  @end
  </code></pre>
  
  <p>and using it results in weird bugs (the function that uses it never exits, and control somehow returns to the run loop). I thought that maybe I am responsible for rewriting the designated initializers of <code>NSData</code> (calling the <code>super</code> implementation), but none is mentioned in the doc. So:</p>
  
  <ul>
  <li>what are the designated initializers of <code>NSData</code>?</li>
  <li>what is the bare minimum I need to write for a dummy subclass of <code>NSData</code>?</li>
  </ul>