Automatic Reference Counting

class Person {
    let name: String
    init(name: String) { self.name = name }
    var apartment: Apartment?
    deinit { println("(name) is being deinitialized") }
}

class Apartment {
    let number: Int
    init(number: Int) { self.number = number }
    var tenant: Person?
    deinit { println("Apartment #(number) is being deinitialized") }
}

var john: Person?
var number73: Apartment?

john = Person(name: "John Appleseed")
number73 = Apartment(number: 73)

john!.apartment = number73
number73!.tenant = john    

Resolving Strong Reference Cycles Between Class Instances: Weak References

class Person {
    let name: String
    init(name: String) { self.name = name }
    var apartment: Apartment?
    deinit { println("(name) is being deinitialized") }
}

class Apartment {
    let number: Int
    init(number: Int) { self.number = number }
    weak var tenant: Person?
    deinit { println("Apartment #(number) is being deinitialized") }
}

var john: Person?
var number73: Apartment?

john = Person(name: "John Appleseed")
number73 = Apartment(number: 73)

john!.apartment = number73
number73!.tenant = john  

Resolving Strong Reference Cycles Between Class Instances: Unowned References

class Customer {
    let name: String
    var card: CreditCard?
    init(name: String) {
        self.name = name
    }
    deinit { println("(name) is being deinitialized") }
}

class CreditCard {
    let number: Int
    unowned let customer: Customer
    init(number: Int, customer: Customer) {
        self.number = number
        self.customer = customer
    }
    deinit { println("Card #(number) is being deinitialized") }
}

var john: Customer?
john = Customer(name: "John Appleseed")
john!.card = CreditCard(number: 1234_5678_9012_3456, customer: john!)   

Unowned References and Implicitly Unwrapped Optional Properties

class Country {
    let name: String
    let capitalCity: City!
    init(name: String, capitalName: String) {
        self.name = name
        self.capitalCity = City(name: capitalName, country: self)
    }
}

class City {
    let name: String
    unowned let country: Country
    init(name: String, country: Country) {
        self.name = name
        self.country = country
    }
} 

var country = Country(name: "Canada", capitalName: "Ottawa")
println("(country.name)"s capital city is called (country.capitalCity.name)")
// prints "Canada"s capital city is called Ottawa" 

The initializer for City is called from within the initializer for Country. However, the initializer for Country cannot pass self to the City initializer until a new Country instance is fully initialized, as described in Two-Phase Initialization.

To cope with this requirement, you declare the capitalCity property of Country as an implicitly unwrapped optional property, indicated by the exclamation mark at the end of its type annotation (City!). This means that the capitalCity property has a default value of nil, like any other optional, but can be accessed without the need to unwrap its value as described in Implicitly Unwrapped Optionals.

Resolving Strong Reference Cycles for Closures

class HTMLElement {

    let name: String
    let text: String?

    @lazy var asHTML: () -> String = {
        if let text = self.text {
            return "<(self.name)>(text)</(self.name)>"
        } else {
            return "<(self.name) />"
        }
    }

    init(name: String, text: String? = nil) {
        self.name = name
        self.text = text
    }

    deinit {
        println("(name) is being deinitialized")
    }

} 

var paragraph: HTMLElement? = HTMLElement(name: "p", text: "hello, world")
println(paragraph!.asHTML())
// prints "<p>hello, world</p>” 
paragraph = nil  // the message in the HTMLElement deinitializer is not printed 

class HTMLElement {

    let name: String
    let text: String?

    @lazy var asHTML: () -> String = {
        [unowned self] in
        if let text = self.text {
            return "<(self.name)>(text)</(self.name)>"
        } else {
            return "<(self.name) />"
        }
    }

    init(name: String, text: String? = nil) {
        self.name = name
        self.text = text
    }

    deinit {
        println("(name) is being deinitialized")
    }

}

var paragraph: HTMLElement? = HTMLElement(name: "p", text: "hello, world")
println(paragraph!.asHTML())
// prints "<p>hello, world</p>”

paragraph = nil
// prints "p is being deinitialized"