## 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" ~~~