- (IBAction)logOutButton:(id)sender
{
   //show confirmation message to user
   CustomAlert* alert = [[CustomAlert alloc] initWithTitle:@"Confirmation" message:@"Do you want  to exit?" delegate:self cancelButtonTitle:@"Cancel" otherButtonTitles:@"OK", nil];
   alert.style = AlertStyleWhite;
   [alert setFontName:@"Helvetica" fontColor:[UIColor blackColor] fontShadowColor:[UIColor clearColor]];
   [alert show];
}
- (void)alertView:(UIAlertView *)alertView clickedButtonAtIndex:(NSInteger)buttonIndex
{

   if (buttonIndex != 0)  // 0 == the cancel button
   {
      //home button press programmatically
      UIApplication *app = [UIApplication sharedApplication];
      [app performSelector:@selector(suspend)];
      //wait 2 seconds while app is going background
      [NSThread sleepForTimeInterval:2.0];
      //exit app when app is in background
      NSLog(@"exit(0)");
      exit(0);
  }
}

嗯，如果你的应用需要网络连接，你可能“不得不”退出应用程序。你可以显示一个警告，然后做这样的事情:

if ([[UIApplication sharedApplication] respondsToSelector:@selector(terminate)]) {
    [[UIApplication sharedApplication] performSelector:@selector(terminate)];
} else {
    kill(getpid(), SIGINT); 
}

[[UIApplication sharedApplication] terminateWithSuccess];

它工作正常，自动调用

- (void)applicationWillTerminateUIApplication *)application delegate.

要删除编译时警告，请添加以下代码

@interface UIApplication(MyExtras)
  - (void)terminateWithSuccess;
@end 

如果一个长期存在的应用程序也在后台执行，退出它可能是合适的，例如获取位置更新(使用位置更新后台功能)。

例如，假设用户退出了基于位置的应用程序，并使用home键将应用程序推到后台。在这种情况下，你的应用程序可能会继续运行，但完全退出它是有意义的。这将有利于用户(释放内存和其他不需要使用的资源)，也有利于应用程序的稳定性(即确保应用程序在可能的情况下定期重新启动是防止内存泄漏和其他低内存问题的安全网)。

这可以(虽然可能不应该，见下面:-)实现如下:

- (void)applicationDidEnterBackground:(UIApplication *)application
{
    if (/* logged out */) {
        exit(0);
    } else {
       // normal handling.
    }
}

由于应用程序将退出后台，它不会看起来错误的用户，也不会像崩溃，提供用户界面是恢复下次他们运行的应用程序。换句话说，对用户来说，它不会看起来有任何不同的系统启动终止应用程序时，应用程序是在后台。

Still, it would be preferable to use a more standard approach to let the system know that the app can be terminated. For example in this case, by making sure the GPS is not in use by stopping requesting location updates, including turning off show current location on a map view if present. That way the system will take care of terminating the app a few minutes (i.e. [[UIApplication sharedApplication] backgroundTimeRemaining]) after the app enters the background. This would get all the same benefits without having to use code to terminate the app.

- (void)applicationDidEnterBackground:(UIApplication *)application
{
    if (/* logged out */) {
       // stop requesting location updates if not already done so
       // tidy up as app will soon be terminated (run a background task using beginBackgroundTaskWithExpirationHandler if needed).
    } else {
       // normal handling.
    }
}

当然，使用exit(0)对于在前台运行的普通生产应用程序是不合适的，就像其他参考http://developer.apple.com/iphone/library/qa/qa2008/qa1561.html的答案一样

除了上面的，很好，答案我只想补充一下，想想清理你的记忆。

应用程序退出后，iPhone操作系统将自动清理应用程序遗留的所有内容，因此手动释放所有内存只会增加应用程序退出所需的时间。

点击这里查看问答:https://developer.apple.com/library/content/qa/qa1561/_index.html

Q: How do I programmatically quit my iOS application? There is no API provided for gracefully terminating an iOS application. In iOS, the user presses the Home button to close applications. Should your application have conditions in which it cannot provide its intended function, the recommended approach is to display an alert for the user that indicates the nature of the problem and possible actions the user could take — turning on WiFi, enabling Location Services, etc. Allow the user to terminate the application at their own discretion. WARNING: Do not call the exit function. Applications calling exit will appear to the user to have crashed, rather than performing a graceful termination and animating back to the Home screen. Additionally, data may not be saved, because -applicationWillTerminate: and similar UIApplicationDelegate methods will not be invoked if you call exit. If during development or testing it is necessary to terminate your application, the abort function, or assert macro is recommended