讨论后发现的问题是标签栏控制器的子项嵌入在导航控制器中，所以代码需要更改为，

The problem, as discovered after a discussion, was that the tab bar controller's children were embedded in navigation controllers, so the code needed to be changed to this,

override func prepareForSegue(segue: UIStoryboardSegue, sender: AnyObject?) { 
    let barViewControllers = segue.destinationViewController as! UITabBarController 
    let nav = barViewControllers.viewControllers![2] as! UINavigationController 
    let destinationViewController = nav.topviewcontroller as ProfileController 
    destinationViewController.firstName = self.firstName
    destinationViewController.lastName = self.lastName 
}