SLN SLN - 1 year ago 75
Swift Question

Difference between forced unwrapping and implicitly unwrapped optionals

I was very confused about the forced and implicitly unwrapping in the beginning. Now the following understandings come from my self-study

There is no such an action named implicitly unwrapping but the implicitly unwrapped Optionals. implicitly unwrapped Optionals and the normal Optionals are both Optionals, the different is, when access an implicitly unwrapped Optional you confidently know that there is a valid value under the hood and ready for use. The normal Optionals need if-let binding or forced unwrapping action to access the possible values behinds the optional variables.


Forced unwrapping is an action acts on the normal Optionals ;

Implicitly unwrapped Optionals are Optionlas, usually used for class initialization and will pass values without exclamation mark when using.


Am I right? If my understanding is not precisely , I would appreciate that you could correct me.


Answer Source

First of all let's define an Optional

An Optional value is a container of some type (Int, String, UIColor, ...), it could contain the value (1, "Hello world", .greenColor(), ...) or nil.

let anOptionalInt: Int? = 1
let anotherOptionalInt: Int? = nil

enter image description here

When in Swift we see an Optional value we think: ok this could contain the actual value or nil.

Force unwrapping

It's the action of extracting the value contained inside an Optional. This operation is dangerous because you are telling the compiler: I am sure this Optional value does contain a real value, extract it!

let anOptionalInt: Int? = 1
let anInt: Int = anOptionalInt!

Now anInt contains the value 1.

enter image description here

If we perform a force unwrapping on an Optional value that happens to contain nil we get a fatalError, the app does crash and there no way to recover it.

let anotherOptionalInt: Int? = nil
let anotherInt = anotherOptionalInt!

fatal error: unexpectedly found nil while unwrapping an Optional value

enter image description here

Implicitly unwrapped optionals

When we define an Implicitly unwrapped optional, we define a container that will automatically perform a force unwrap each time we read it.

var text: String! = "Hello"

If now we read text

let name = text

we don't get an Optional String but a plain String because text automatically unwrapped it's content.

However text is still an optional so we can put a nil value inside it

text = nil

But as soon as we read it (and it contains nil) we get a fatal error because we are unwrapping an optional containing nil

let anotherName = text

fatal error: unexpectedly found nil while unwrapping an Optional value