title: ViewController in ios 5
body: <p>I have a problem in ViewController. And I'm using xcode 4.3.3, trying to implement iPhone 5.1 apps. Every time I write some code about ViewController, it reports the same error: use of undeclared identifier "viewController". And if I declare it with type of <code>UIViewController</code>, it has runtime error.</p>
  
  <p>Some codes are like this:</p>
  
  <pre><code>- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions{   
      self.window.rootViewController = [[UINavigationController alloc]initWithRootViewController:viewController];
      // Override point for customization after application launch.
      return YES;
  }
  </code></pre>
  
  <p>It is in <code>AppDelegate</code>.m file.</p>
  
  <p>Is there any difference in using viewController in ios 4 and 5?</p>