ios - Can't unwrap 'Optional.None'

Question

When confronting the error fatal error:

Can't unwrap Optional.None

It is not that easy to trace this. What causes this error?

Code:

import UIKit

class WelcomeViewController: UIViewController {
    let cornerRad:CGFloat = 10.0
    @IBOutlet var label:UILabel
    @IBOutlet var lvl1:UIButton
    @IBOutlet var lvl2:UIButton
    @IBOutlet var lvl3:UIButton
    init(nibName nibNameOrNil: String?, bundle nibBundleOrNil: NSBundle?) {
        super.init(nibName: nibNameOrNil, bundle: nibBundleOrNil)
    }
    override func viewDidLoad() {
        super.viewDidLoad()
        lvl1.layer.cornerRadius = cornerRad
        lvl2.layer.cornerRadius = cornerRad
        lvl3.layer.cornerRadius = cornerRad
    }
}

Answer 1

You get this error, because you try to access a optional variable, which has no value. Example:

// This String is an optional, which is created as nil.
var optional: String?

var myString = optional! // Error. Optional.None

optional = "Value"
if optional {
    var myString = optional! // Safe to unwrap.
}

You can read more about optionals in the official Swift language guide.