unit and acceptance testing, automation, productivity

Implicitly vs Force Unwrapping Swift Optionals

This post is a followup to a "Why Implicitly Unwrapping Swift Optionals Is Dangerous". Daniel H Steinberg , on Twitter as @dimsumthinking, pointed out to me that the terminology used in the poost wasn't accurate:

He was right. I went back on the Swift book, did more reading, and understood that what I kept referring to as implicitly unwrapping is a technique actually called force unwrapping, and that to implicitly unwrap an Optional value means something slighltly different.

Let's have a look.

Implicitly Unwrapping

You define an Optional as impliticly unwrapped when you define its type like this:

let x: String!

This technique allows you to tell the compiler to automatically unwrap that value, as if it wasn't optional at all.

Simlarly to Type? which is a syntactic sugar for Optional<Type>, Type! is equivalent to ImplicitlyUnwrappedOptional<Type>.

A common example of implicitly unwrapped optionals is how view controller define their IBOutlets:

@IBOutlet var messageLabel: UILabel!
@IBOutlet var actionButton: UIButton!

It makes sense to define the outlets as implicitly unwrapped optionals because their are going to be instantiated by Interface Builder. It would be cumbersome to always unwrap each view outlet inside view controllers.

Because of their implicitly unwrapped nature, if you forget to connect an outlet to it Interface Builder view an run the app you'll get a runtime error when you try to access it in the view controller.

Force Unwrap

Force unwrap, or force-unwrap, or forced unwrapping, is the technique most of the previous post was actually about. It consist in adding a ! after an Optional value, to automatically unwrap it, without having to check whether it is nil or not.

let strings = ["mokacoding", "is", "a", "blog"]
let firstLength: Int = strings.first!.length

Like implicitly unwrapping, force unwrap uses a ! and makes the compiler treat an otherwise optional value as the type it wraps. Unlike implicitly unwrapping though, this technique is used on existing values.

You define a type as implicitly unwrapped, ImplicitlyUnwrappedOptional<T>, and you force unwrap a value which has Optional type.

They are both dangerous

As said in the IBOutlet example and in the previous post, unwrapping an Optional value without taking into account its nullability is dangerous, it can in fact crash your app.

There are some scenarios in which implictly or forced unwrapping an Optional can make sense, such as the outlets scenario. It is up to us as developers to take into account the tradeoff between code that is easier to consume, versus code that leverages the compiler to guarantee its safety.

My take on this? Never use ! apart for IBOutlets. I'd rather sprinkle if lets and guard letss all around my code then access a nil value and crash my apps because something changed an the assumption "its never going to be nil" which I made when I wrote that ! is no longer valid, but I forgot about having written it in the first place.

How do you feel about impliticly unwrapping or force unwrap Optional values? Get in touch on Twitter @mokagio or leave a comment below.

Thanks again Daniel for pointing my mistake out, leading to me learning more about Swift, an writing this post 😄.

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