Consider the following code:

var defaults = NSUserDefaults.standardUserDefaults()
var userPref = defaults.stringForKey("userPref")!

func printString(string: String) {

Where is the bug? What does this bug cause? What’s the proper way to fix it?


The second line uses the stringForKey method of NSUserDefaults, which returns an optional, to account for the key not being found, or for the corresponding value not being convertible to a string.

During its execution, if the key is found and the corresponding value is a string, the above code works correctly. But if the key doesn’t exist, or the corresponding value is not a string, the app crashes with the following error:

fatal error: unexpectedly found nil while unwrapping an Optional value

The reason is that the forced unwrapping operator ! is attempting to force unwrap a value from a nil optional. The forced unwrapping operator should be used only when an optional is known to contain a non-nil value.

The solution consists of making sure that the optional is not nil before force-unwrapping it:

let userPref = defaults.stringForKey("userPref")
if userPref != nil {

An even better way is by using optional binding:

if let userPref = defaults.stringForKey("userPref") {


© 2017