ios程序在ios5下出现黑屏的问题

用xcode4.2生成的程序是可以在ios5.1和ios6上运行的，但是在ios5下黑屏：

报错信息为：

 2013-01-16 19:03:55.969 HelloCoco2d[5934:c07] Applications are expected to have a root view controller at the end of application launch

经查询原因是ios5.1以上默认使用RootViewController，而在ios5以下默认不使用RootViewController

需要在AppDelegate手动添加代码

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{
  self.window = [[UIWindow alloc] initWithFrame:[[UIScreen mainScreen] bounds]];  
  self.rootViewController=[[RootViewController alloc]init];
  self.window.rootViewController =self.rootViewController;
  [self.window makeKeyAndVisible];
  return YES;
}