title: Subscripting syntax for NSMapTable in iOS 6
body: <p>I'm using <code>NSMapTable</code> in a number of places in an iOS 6 project, and would like to be able to use the new dictionary subscripting style for accessing objects. (<code>NSMapTable</code> behaves mostly like an <code>NSMutableDictionary</code>, but can be configured with various memory management options for the keys and values it stores. More background in <a href="https://stackoverflow.com/questions/6904533/nsmaptable-and-nsmutabledictionary-differences">this StackOverflow question</a>.)</p>
  
  <p>The compiler reports this when attempting to use subscripting syntax on an <code>NSMapTable</code> instance:</p>
  
  <blockquote>
    <p>Expected method to read dictionary element not found on object of type 'NSMapTable *'.</p>
  </blockquote>
  
  <p>How can I use a category to extend <code>NSMapTable</code> to allow the new <code>NSDictionary</code>-style subscripting?</p>