Using ? (optionals) in Swift variables

Question

What exactly is the difference between a property definition in Swift of:

var window: UIWindow?

vs

var window: UIWindow

I've read it's basically an "optional" but I don't understand what it's for.

This is creating a class property called window right? so what's the need for the '?'

Read about it in the documentation: https://developer.apple.com/library/prerelease/ios/documentation/Swift/Conceptual/Swift_Programming_Language/TheBasics.html#//apple_ref/doc/uid/TP40014097-CH5-XID_428. It's a fundamental concept to Swift — Jack, Jun 03 '14 at 21:48
Also all the down votes on this question for no reason make me not want to help out around here as much as I try to. — Woodstock, Jun 03 '14 at 21:52
I think in this case, a window might not be initialized/available. Thus, by defining it as an `optional`, if you call `window.addSubView()`, it will not unwrap and just return `nil` — Jack, Jun 03 '14 at 21:53
A better example of an `optional` would be for a `delegate` property. This would allow you to call `delegate.protocolFunction()` without checking if `delegate` is `nil`. Please do go read about it in the docs though it does a great job explaining! — Jack, Jun 03 '14 at 21:56
After answering and editing, realised that this has already been asked. — nevan king, Jun 03 '14 at 22:36

score 2 · Answer 1 · answered Jun 03 '14 at 21:55

2

UIWindow? means that the value may be absent. It's either an UIWindow instance or nothing.

answered Jun 03 '14 at 21:55

Rudolf Adamkovič

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score 2 · Accepted Answer · answered Jun 03 '14 at 21:59

The ? identifier means that the variable is optional, meaning its value can be nil. If you have a value in your code, declaring it as non-optional allows the compiler to check at build time if it has a chance to become nil.

You can check whether it is nil in an if statement:

var optionalName: String? = "John Appleseed"

if optionalName {
    // <-- here we know it's not nil, for sure!
}

Many methods that require a parameter to be non-nil will declare that they explicitly need a non-optional value. If you have an optional value, you can convert it into non-optional (for example, UIWindow? -> UIWindow) by unwrapping it. One of the primary methods for unwrapping is an if let statement:

var greeting = "Hello!"

// at this point in the code, optionalName could be nil

if let name = optionalName {
    // here, we've unwrapped it into name, which is not optional and can't be nil
    greeting = "Hello, \(name)"
}

See The Swift Programming Language, page 11 for a brief introduction, or page 46 for a more detailed description.

Using ? (optionals) in Swift variables

2 Answers2