这是我的代码：

import UIKit

class ViewController: UIViewController {

     var myString: NSString?

    override func viewDidLoad() {
        super.viewDidLoad()
        myString = "himanth"
        print(myString)
        NSLog("%@",myString)
        // Do any additional ssetup after loading the view,typically from a nib.
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }


}

如果我这样声明myString

var myString: NSString!

可以使用NSLog,我也可以看到控制台.

但是声明这样的字符串会导致问题

var myString: NSString?

它反映在NSLog并显示错误.

这是什么问题？