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Question

I'm watching all the swift tutorials from Apple, but I'm having problems with one of the examples:

class Person {
    var residence: Residence?
}

class Residence {
    var address: Address?
}

class Address {
    var buildingNumber: String? = "234"
    var streetName: String? = "Main St."
    var appartmentNumber: String?
}

let paul = Person()
var addressNumber: Int?

addressNumber = paul.residence?.address?.buildingNumber?.toInt()

if let number = addressNumber {
    "correct"
} else {
    "fault"
}

It's always printing out "fault". Is there something painfully obvious I'm missing?

Answer 1

let paul = Person()

You do nothing else to paul including setting his optional residence variable. So in the next line of code, you are accessing paul 's residence which is nil .

addressNumber = paul.residence?.address?.buildingNumber?.toInt()
                      ^ failing right here, residence? returns nil

So with optional chaining, this entire expression returns nil , so when compared in your next if let statement, it is false. This is why the else clause is being executed.

Answer 2

Think it through, one step at a time. Particularly, your optional chaining:

addressNumber = paul.residence?.address?.buildingNumber?.toInt()

Ask yourself:

1. What is paul ?

2. What is paul 's residence property set to?

3. What is the address of paul 's residence ?

Hint: You shouldn't make it past step 2.

Answer 3

Well you are never actually creating a valid Residence or Address , if you change your code to:

class Person {
    // actually create a residence object
    var residence: Residence? = Residence()
}

class Residence {
    // actually create a address object
    var address: Address? = Address()
}

class Address {
    // ...
}

let paul = Person()
var addressNumber: Int?

// Before Paul was never assigned a valid residence, now one will be created
addressNumber = paul.residence?.address?.buildingNumber?.toInt()

if let number = addressNumber {
    "correct"
} else {
    "fault"
}
// gives you `correct`

then it should work great!

Answer 4

You never initialize anything except Person() , so the residence property on paul is nil .

So, in the following optional chain:

addressNumber = paul.residence?.address?.buildingNumber?.toInt()
                              ^ you get nil here and don't continue

This means that addressNumber is nil .