Swift uses ARC to track and deallocate unused objects. Learn about the three types of reference counts and how ARC works — in this detailed post. Post: https://alexdremov.me/dive-into-swifts-memory-management/

code_16.swift

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

class Apartment {
    let unit: String
    init(unit: String) { self.unit = unit }
    
    weak var tenant: Person?
    
    deinit { print("Apartment \(unit) is being deinitialized") }
}

var john: Person? = Person(name: "John Appleseed")
var unit4A: Apartment? = Apartment(unit: "4A")

john!.apartment = unit4A // Person -> Apartment: strong reference
unit4A!.tenant = john // Apartment -> Person: weak reference

john = nil
unit4A = nil

code_22.swift

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

class CreditCard {
    let number: UInt64
    unowned let customer: Customer
    init(number: UInt64, customer: Customer) {
        self.number = number
        self.customer = customer
    }
    deinit { print("Card #\(number) is being deinitialized") }
}

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

john = nil // No retain cycle, both objects are deallocated

code_27.swift

class User {
	var id: Int
    var name: String
    
    init(id: Int, name: String) {
    	self.id = id
        self.name = name
    }
}

let user = User(id: 0, name: "John")

code_3.swift

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

class Apartment {
    let unit: String
    init(unit: String) { self.unit = unit }
    var tenant: Person?
    deinit { print("Apartment \(unit) is being deinitialized") }
}

var john: Person? = Person(name: "John Appleseed")
var unit4A: Apartment? = Apartment(unit: "4A")

john!.apartment = unit4A // Person -> Apartment: strong reference
unit4A!.tenant = john // Apartment -> Person: strong reference

john = nil // Person is no longer needed
unit4A = nil // Apartment is no longer needed

code_44.swift

class Person {
	var name: String
	var voice: Voice? = nil

    init(name: String) {
    	self.name = name
    }
    func say() { voice?.say() }
    deinit {
    	print("Person deallocated")
    }
}

class Voice {
	var say: () -> ()
    init(say: @escaping () -> ()) { self.say = say }
    deinit {
    	print("Voice deallocated")
    }
}

var person: Person? = Person(name: "Alex")
person!.say()

person = nil

code_48.swift

self.voice = Voice {
	print("My name is \(self.name)")
}

code_52.swift

self.voice = Voice {[weak self] in
	guard let self = self else { return; }
	print("My name is \(self.name)")
}

code_53.txt

My name is Alex
Person deallocated
Voice deallocated