title: What does the "strong" keyword do
body: <p>I downloaded the Xcode 4.2 developer preview version and I created a cocoa application. But I found a very weird syntax in the delegate class:</p>
  
  <pre><code>@property (strong) IBOutlet NSWindow *window;
  </code></pre>
  
  <p>What does this mean? And the compiler can't even compile it.</p>
  
  <p>Thanks in advance!</p>