title: Different behavior of __weak on NSURL and NSString
body: <p>Whats the difference between NSString and NSURL on __weak ?</p>
  <p>My example:</p>
  <pre><code>__weak NSURL *myURL = [NSURL fileURLWithPath:@&quot;/tmp&quot;];
  __weak NSString *myString = @&quot;123&quot;;
  
  NSLog(@&quot;myURL: %@&quot;, myURL);
  NSLog(@&quot;myString: %@&quot;, myString);
  </code></pre>
  <p><strong>The result:</strong></p>
  <blockquote>
  <p>2012-07-10 19:23:49.858 myApp[56093:303] myURL: (null)</p>
  <p>2012-07-10 19:23:49.859 myApp[56093:303] myString: 123</p>
  </blockquote>
  <p>Why isn't the result myString == (null)</p>