What does an exclamation mark mean in the Swift language?

Question

The Swift Programming Language guide has the following example:

class Person {
    let name: String
    init(name: String) { self.name = name }
    var apar         


        

Answer 1

ASK YOURSELF

• Does the type person? have an apartment member/property? OR
• Does the type person have an apartment member/property?

If you can't answer this question, then continue reading:

_{To understand you may need super-basic level of understanding of Generics. See here. A lot of things in Swift are written using Generics. Optionals included}

The code below has been made available from this Stanford video. Highly recommend you to watch the first 5 minutes

An Optional is an enum with only 2 cases

enum Optional{
    case None
    case Some(T)
}

let x: String? = nil //actually means:

let x = Optional.None

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let x :String? = "hello" //actually means:

let x = Optional.Some("hello")

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var y = x! // actually means:

switch x {
case .Some(let value): y = value
case .None: // Raise an exception
}

Optional binding:

let x:String? = something
if let y = x {
    // do something with y
}
//Actually means:

switch x{
case .Some(let y): print)(y) // or whatever else you like using 
case .None: break
}

when you say var john: Person? You actually mean such:

enum Optional{
case .None
case .Some(Person)
}

Does the above enum have any property named apartment? Do you see it anywhere? It's not there at all! However if you unwrap it ie do person! then you can ... what it does under the hood is : Optional.Some(Person(name: "John Appleseed"))

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Had you defined var john: Person instead of: var john: Person? then you would have no longer needed to have the ! used, because Person itself does have a member of apartment

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As a future discussion on why using ! to unwrap is sometimes not recommended see this Q&A