Bang.swift

gistfile1.txt

class Card {}
class Customer {
    var optionalCard: Card?
    var card: Card
    
    init() {
       self.card = Card() // required to avoid compiler error (card2 would be nil)
    }
}
let john : Customer? = Customer()
let jane : Customer = Customer()

john!.optionalCard = Card() // We need the ! just because john is Optional

if let someJohn = john {
    someJohn.card = Card()
}

jane.optionalCard = Card()
jane.card = Card()

If L10 would not have the ? the whole example wouldn't need a !.