struct Example {
 var strongView = UIView()
 weak var weakView = UIView()
}

class Book {
 private var pages = [Page]()
 
 func add(_ page : Page) {
  pages.append(page)
 }
}

class Page {
 private var book : Book

 required init(book : Book) {
  self.book = book
 }
}

let book = Book()
let page = Page(book: book)
book.add(page)

struct Element {
 var name : String
 var number : Int
}

var firstElement = Element(name: "A", number: 1)

var secondElement = firstElement
secondElement.name = "B"
secondElement.number = 2

print(firstElement)
print(secondElement)

Element(name: “A”, number: 1)
Element(name: “B”, number: 2)

class Element {
 var name : String
 var number : Int
 
 required init(name : String, number : Int) {
  self.name = name
  self.number = number
 }
}

extension Element : CustomStringConvertible {
 var description : String {
  return "Element(name: \(name), number: \(number))"
 }
}

var firstElement = Element(name: "A", number: 1)

var secondElement = firstElement
secondElement.name = "B"
secondElement.number = 2

print(firstElement)
print(secondElement)

Element(name: B, number: 2)
Element(name: B, number: 2)

struct Book {
 private var pages = [Page]()
 
 mutating func add(_ page : Page) {
  pages.append(page)
 }
}

struct Page {
 private var book : Book
 
 init(book : Book) {
  self.book = book
 }
}

var book = Book()
let page = Page(book: book)
book.add(page)

class Book {
 private var pages = [Page]()
 
 func add(_ page : Page) {
  pages.append(page)
 }
}

class Page {
 private weak var book : Book?
 
 required init(book : Book) {
  self.book = book
 }
}

let book = Book()
let page = Page(book: book)
book.add(page)

protocol ListViewControllerDelegate {
 func configure(with list : [Any])
}

class ListViewController : UIViewController {
 
 var delegate : ListViewControllerDelegate?
 
 override func viewDidLoad() {
  super.viewDidLoad()
 }
 
}

protocol ListViewControllerDelegate : class {
 func configure(with list : [Any])
}

class ListViewController : UIViewController {
 
 weak var delegate : ListViewControllerDelegate?
 
 override func viewDidLoad() {
  super.viewDidLoad()
 }
 
}

class Example {

 private var counter = 0
 
 private var closure : (() -> ()) = { }
 
 init() {
  closure = {
   self.counter += 1
   print(self.counter)
  }
 }
 
 func foo() {
  closure()
 }
 
}

class Example {

 private var counter = 1
 
 private var closure : (() -> ()) = { }
 
 init() {
  closure = { [unowned self] in
   self.counter += 1
   print(self.counter)
  }
 }
 
 func foo() {
  closure()
 }
 
}

class Example {

 private var counter = 1
 
 private var closure : (() -> ()) = { }
 
 init() {
  closure = { [weak self] in
   self?.counter += 1
   print(self?.counter ?? "")
  }
 }
 
 func foo() {
  closure()
 }
 
}