Swift is a fantastic way to write software, whether it’s for phones, desktops, servers, or anything else that runs code. It’s a safe, fast, and interactive programming language that combines the best in modern language thinking with wisdom from the wider Apple engineering culture and the diverse contributions from its open-source community. The compiler is optimized for performance and the language is optimized for development, without compromising on either.

Swift is friendly to new programmers. It’s an industrial-quality programming language that’s as expressive and enjoyable as a scripting language. Writing Swift code in a playground lets you experiment with code and see the results immediately, without the overhead of building and running an app.

Swift defines away large classes of common programming errors by adopting modern programming patterns:

  • Variables are always initialized before use.
  • Array indices are checked for out-of-bounds errors.
  • Integers are checked for overflow.
  • Optionals ensure that nil values are handled explicitly.
  • Memory is managed automatically.
  • Error handling allows controlled recovery from unexpected failures.

Swift code is compiled and optimized to get the most out of modern hardware. The syntax and standard library have been designed based on the guiding principle that the obvious way to write your code should also perform the best. Its combination of safety and speed make Swift an excellent choice for everything from “Hello, world!” to an entire operating system.

Swift combines powerful type inference and pattern matching with a modern, lightweight syntax, allowing complex ideas to be expressed in a clear and concise manner. As a result, code is not just easier to write, but easier to read and maintain as well.

Swift has been years in the making, and it continues to evolve with new features and capabilities. Our goals for Swift are ambitious. We can’t wait to see what you create with it.

Version Compatibility

This book describes Swift 5.2, the default version of Swift that’s included in Xcode 11.4. You can use Xcode 11.4 to build targets that are written in either Swift 5.2, Swift 4.2, or Swift 4.

When you use Xcode 11.4 to build Swift 4 and Swift 4.2 code, most Swift 5.2 functionality is available. That said, the following changes are available only to code that uses Swift 5.2 or later:

  • Functions that return an opaque type require the Swift 5.1 runtime.
  • The try? expression doesn’t introduce an extra level of optionality to expressions that already return optionals.
  • Large integer literal initialization expressions are inferred to be of the correct integer type. For example, UInt64(0xffff_ffff_ffff_ffff) evaluates to the correct value rather than overflowing.

A target written in Swift 5.2 can depend on a target that’s written in Swift 4.2 or Swift 4, and vice versa. This means, if you have a large project that’s divided into multiple frameworks, you can migrate your code from Swift 4 to Swift 5.2 one framework at a time.

A Swift Tour

Tradition suggests that the first program in a new language should print the words “Hello, world!” on the screen. In Swift, this can be done in a single line:

  1. print(“Hello, world!”)
  2. // Prints “Hello, world!”

If you have written code in C or Objective-C, this syntax looks familiar to you—in Swift, this line of code is a complete program. You don’t need to import a separate library for functionality like input/output or string handling. Code written at global scope is used as the entry point for the program, so you don’t need a main() function. You also don’t need to write semicolons at the end of every statement.

This tour gives you enough information to start writing code in Swift by showing you how to accomplish a variety of programming tasks. Don’t worry if you don’t understand something—everything introduced in this tour is explained in detail in the rest of this book.


For the best experience, open this chapter as a playground in Xcode. Playgrounds allow you to edit the code listings and see the result immediately.

Download Playground

Simple Values

Use let to make a constant and var to make a variable. The value of a constant doesn’t need to be known at compile time, but you must assign it a value exactly once. This means you can use constants to name a value that you determine once but use in many places.

  1. var myVariable = 42
  2. myVariable = 50
  3. let myConstant = 42

A constant or variable must have the same type as the value you want to assign to it. However, you don’t always have to write the type explicitly. Providing a value when you create a constant or variable lets the compiler infer its type. In the example above, the compiler infers that myVariable is an integer because its initial value is an integer.

If the initial value doesn’t provide enough information (or if there is no initial value), specify the type by writing it after the variable, separated by a colon.

  1. let implicitInteger = 70
  2. let implicitDouble = 70.0
  3. let explicitDouble: Double = 70


Create a constant with an explicit type of Float and a value of 4.

Values are never implicitly converted to another type. If you need to convert a value to a different type, explicitly make an instance of the desired type.

  1. let label = “The width is “
  2. let width = 94
  3. let widthLabel = label + String(width)


Try removing the conversion to String from the last line. What error do you get?

There’s an even simpler way to include values in strings: Write the value in parentheses, and write a backslash (\) before the parentheses. For example:

  1. let apples = 3
  2. let oranges = 5
  3. let appleSummary = “I have \(apples) apples.”
  4. let fruitSummary = “I have \(apples + oranges) pieces of fruit.”


Use \() to include a floating-point calculation in a string and to include someone’s name in a greeting.

Use three double quotation marks (""") for strings that take up multiple lines. Indentation at the start of each quoted line is removed, as long as it matches the indentation of the closing quotation marks. For example:

  1. let quotation = “””
  2. I said “I have \(apples) apples.”
  3. And then I said “I have \(apples + oranges) pieces of fruit.”
  4. “””

Create arrays and dictionaries using brackets ([]), and access their elements by writing the index or key in brackets. A comma is allowed after the last element.

  1. var shoppingList = [“catfish”, “water”, “tulips”]
  2. shoppingList[1] = “bottle of water”
  3. var occupations = [
  4. “Malcolm”: “Captain”,
  5. “Kaylee”: “Mechanic”,
  6. ]
  7. occupations[“Jayne”] = “Public Relations”

Arrays automatically grow as you add elements.

  1. shoppingList.append(“blue paint”)
  2. print(shoppingList)

To create an empty array or dictionary, use the initializer syntax.

  1. let emptyArray = [String]()
  2. let emptyDictionary = [String: Float]()

If type information can be inferred, you can write an empty array as [] and an empty dictionary as [:]—for example, when you set a new value for a variable or pass an argument to a function.

  1. shoppingList = []
  2. occupations = [:]

Control Flow

Use if and switch to make conditionals, and use forinwhile, and repeatwhile to make loops. Parentheses around the condition or loop variable are optional. Braces around the body are required.

  1. let individualScores = [75, 43, 103, 87, 12]
  2. var teamScore = 0
  3. for score in individualScores {
  4. if score > 50 {
  5. teamScore += 3
  6. } else {
  7. teamScore += 1
  8. }
  9. }
  10. print(teamScore)
  11. // Prints “11”

In an if statement, the conditional must be a Boolean expression—this means that code such as if score { ... } is an error, not an implicit comparison to zero.

You can use if and let together to work with values that might be missing. These values are represented as optionals. An optional value either contains a value or contains nil to indicate that a value is missing. Write a question mark (?) after the type of a value to mark the value as optional.

  1. var optionalString: String? = “Hello”
  2. print(optionalString == nil)
  3. // Prints “false”
  4. var optionalName: String? = “John Appleseed”
  5. var greeting = “Hello!”
  6. if let name = optionalName {
  7. greeting = “Hello, \(name)”
  8. }


Change optionalName to nil. What greeting do you get? Add an else clause that sets a different greeting if optionalName is nil.

If the optional value is nil, the conditional is false and the code in braces is skipped. Otherwise, the optional value is unwrapped and assigned to the constant after let, which makes the unwrapped value available inside the block of code.

Another way to handle optional values is to provide a default value using the ?? operator. If the optional value is missing, the default value is used instead.

  1. let nickName: String? = nil
  2. let fullName: String = “John Appleseed”
  3. let informalGreeting = “Hi \(nickName ?? fullName)”

Switches support any kind of data and a wide variety of comparison operations—they aren’t limited to integers and tests for equality.

  1. let vegetable = “red pepper”
  2. switch vegetable {
  3. case “celery”:
  4. print(“Add some raisins and make ants on a log.”)
  5. case “cucumber”, “watercress”:
  6. print(“That would make a good tea sandwich.”)
  7. case let x where x.hasSuffix(“pepper”):
  8. print(“Is it a spicy \(x)?”)
  9. default:
  10. print(“Everything tastes good in soup.”)
  11. }
  12. // Prints “Is it a spicy red pepper?”


Try removing the default case. What error do you get?

Notice how let can be used in a pattern to assign the value that matched the pattern to a constant.

After executing the code inside the switch case that matched, the program exits from the switch statement. Execution doesn’t continue to the next case, so there is no need to explicitly break out of the switch at the end of each case’s code.

You use forin to iterate over items in a dictionary by providing a pair of names to use for each key-value pair. Dictionaries are an unordered collection, so their keys and values are iterated over in an arbitrary order.

  1. let interestingNumbers = [
  2. “Prime”: [2, 3, 5, 7, 11, 13],
  3. “Fibonacci”: [1, 1, 2, 3, 5, 8],
  4. “Square”: [1, 4, 9, 16, 25],
  5. ]
  6. var largest = 0
  7. for (kind, numbers) in interestingNumbers {
  8. for number in numbers {
  9. if number > largest {
  10. largest = number
  11. }
  12. }
  13. }
  14. print(largest)
  15. // Prints “25”


Add another variable to keep track of which kind of number was the largest, as well as what that largest number was.

Use while to repeat a block of code until a condition changes. The condition of a loop can be at the end instead, ensuring that the loop is run at least once.

  1. var n = 2
  2. while n < 100 {
  3. n *= 2
  4. }
  5. print(n)
  6. // Prints “128”
  7. var m = 2
  8. repeat {
  9. m *= 2
  10. } while m < 100
  11. print(m)
  12. // Prints “128”

You can keep an index in a loop by using ..< to make a range of indexes.

  1. var total = 0
  2. for i in 0..<4 {
  3. total += i
  4. }
  5. print(total)
  6. // Prints “6”

Use ..< to make a range that omits its upper value, and use ... to make a range that includes both values.

Functions and Closures

Use func to declare a function. Call a function by following its name with a list of arguments in parentheses. Use -> to separate the parameter names and types from the function’s return type.

  1. func greet(person: String, day: String) -> String {
  2. return “Hello \(person), today is \(day).”
  3. }
  4. greet(person: “Bob”, day: “Tuesday”)


Remove the day parameter. Add a parameter to include today’s lunch special in the greeting.

By default, functions use their parameter names as labels for their arguments. Write a custom argument label before the parameter name, or write _ to use no argument label.

  1. func greet(_ person: String, on day: String) -> String {
  2. return “Hello \(person), today is \(day).”
  3. }
  4. greet(“John”, on: “Wednesday”)

Use a tuple to make a compound value—for example, to return multiple values from a function. The elements of a tuple can be referred to either by name or by number.

  1. func calculateStatistics(scores: [Int]) -> (min: Int, max: Int, sum: Int) {
  2. var min = scores[0]
  3. var max = scores[0]
  4. var sum = 0
  5. for score in scores {
  6. if score > max {
  7. max = score
  8. } else if score < min {
  9. min = score
  10. }
  11. sum += score
  12. }
  13. return (min, max, sum)
  14. }
  15. let statistics = calculateStatistics(scores: [5, 3, 100, 3, 9])
  16. print(statistics.sum)
  17. // Prints “120”
  18. print(statistics.2)
  19. // Prints “120”

Functions can be nested. Nested functions have access to variables that were declared in the outer function. You can use nested functions to organize the code in a function that is long or complex.

  1. func returnFifteen() -> Int {
  2. var y = 10
  3. func add() {
  4. y += 5
  5. }
  6. add()
  7. return y
  8. }
  9. returnFifteen()

Functions are a first-class type. This means that a function can return another function as its value.

  1. func makeIncrementer() -> ((Int) -> Int) {
  2. func addOne(number: Int) -> Int {
  3. return 1 + number
  4. }
  5. return addOne
  6. }
  7. var increment = makeIncrementer()
  8. increment(7)

A function can take another function as one of its arguments.

  1. func hasAnyMatches(list: [Int], condition: (Int) -> Bool) -> Bool {
  2. for item in list {
  3. if condition(item) {
  4. return true
  5. }
  6. }
  7. return false
  8. }
  9. func lessThanTen(number: Int) -> Bool {
  10. return number < 10
  11. }
  12. var numbers = [20, 19, 7, 12]
  13. hasAnyMatches(list: numbers, condition: lessThanTen)

Functions are actually a special case of closures: blocks of code that can be called later. The code in a closure has access to things like variables and functions that were available in the scope where the closure was created, even if the closure is in a different scope when it is executed—you saw an example of this already with nested functions. You can write a closure without a name by surrounding code with braces ({}). Use in to separate the arguments and return type from the body.

  1. numbers.map({ (number: Int) -> Int in
  2. let result = 3 * number
  3. return result
  4. })


Rewrite the closure to return zero for all odd numbers.

You have several options for writing closures more concisely. When a closure’s type is already known, such as the callback for a delegate, you can omit the type of its parameters, its return type, or both. Single statement closures implicitly return the value of their only statement.

  1. let mappedNumbers = numbers.map({ number in 3 * number })
  2. print(mappedNumbers)
  3. // Prints “[60, 57, 21, 36]”

You can refer to parameters by number instead of by name—this approach is especially useful in very short closures. A closure passed as the last argument to a function can appear immediately after the parentheses. When a closure is the only argument to a function, you can omit the parentheses entirely.

  1. let sortedNumbers = numbers.sorted { $0 > $1 }
  2. print(sortedNumbers)
  3. // Prints “[20, 19, 12, 7]”

Objects and Classes

Use class followed by the class’s name to create a class. A property declaration in a class is written the same way as a constant or variable declaration, except that it is in the context of a class. Likewise, method and function declarations are written the same way.

  1. class Shape {
  2. var numberOfSides = 0
  3. func simpleDescription() -> String {
  4. return “A shape with \(numberOfSides) sides.”
  5. }
  6. }


Add a constant property with let, and add another method that takes an argument.

Create an instance of a class by putting parentheses after the class name. Use dot syntax to access the properties and methods of the instance.

  1. var shape = Shape()
  2. shape.numberOfSides = 7
  3. var shapeDescription = shape.simpleDescription()

This version of the Shape class is missing something important: an initializer to set up the class when an instance is created. Use init to create one.

  1. class NamedShape {
  2. var numberOfSides: Int = 0
  3. var name: String
  4. init(name: String) {
  5. self.name = name
  6. }
  7. func simpleDescription() -> String {
  8. return “A shape with \(numberOfSides) sides.”
  9. }
  10. }

Notice how self is used to distinguish the name property from the name argument to the initializer. The arguments to the initializer are passed like a function call when you create an instance of the class. Every property needs a value assigned—either in its declaration (as with numberOfSides) or in the initializer (as with name).

Use deinit to create a deinitializer if you need to perform some cleanup before the object is deallocated.

Subclasses include their superclass name after their class name, separated by a colon. There is no requirement for classes to subclass any standard root class, so you can include or omit a superclass as needed.

Methods on a subclass that override the superclass’s implementation are marked with override—overriding a method by accident, without override, is detected by the compiler as an error. The compiler also detects methods with override that don’t actually override any method in the superclass.

  1. class Square: NamedShape {
  2. var sideLength: Double
  3. init(sideLength: Double, name: String) {
  4. self.sideLength = sideLength
  5. super.init(name: name)
  6. numberOfSides = 4
  7. }
  8. func area() -> Double {
  9. return sideLength * sideLength
  10. }
  11. override func simpleDescription() -> String {
  12. return “A square with sides of length \(sideLength).”
  13. }
  14. }
  15. let test = Square(sideLength: 5.2, name: “my test square”)
  16. test.area()
  17. test.simpleDescription()


Make another subclass of NamedShape called Circle that takes a radius and a name as arguments to its initializer. Implement an area() and a simpleDescription() method on the Circle class.

In addition to simple properties that are stored, properties can have a getter and a setter.

  1. class EquilateralTriangle: NamedShape {
  2. var sideLength: Double = 0.0
  3. init(sideLength: Double, name: String) {
  4. self.sideLength = sideLength
  5. super.init(name: name)
  6. numberOfSides = 3
  7. }
  8. var perimeter: Double {
  9. get {
  10. return 3.0 * sideLength
  11. }
  12. set {
  13. sideLength = newValue / 3.0
  14. }
  15. }
  16. override func simpleDescription() -> String {
  17. return “An equilateral triangle with sides of length \(sideLength).”
  18. }
  19. }
  20. var triangle = EquilateralTriangle(sideLength: 3.1, name: “a triangle”)
  21. print(triangle.perimeter)
  22. // Prints “9.3”
  23. triangle.perimeter = 9.9
  24. print(triangle.sideLength)
  25. // Prints “3.3000000000000003”

In the setter for perimeter, the new value has the implicit name newValue. You can provide an explicit name in parentheses after set.

Notice that the initializer for the EquilateralTriangle class has three different steps:

  1. Setting the value of properties that the subclass declares.
  2. Calling the superclass’s initializer.
  3. Changing the value of properties defined by the superclass. Any additional setup work that uses methods, getters, or setters can also be done at this point.

If you don’t need to compute the property but still need to provide code that is run before and after setting a new value, use willSet and didSet. The code you provide is run any time the value changes outside of an initializer. For example, the class below ensures that the side length of its triangle is always the same as the side length of its square.

  1. class TriangleAndSquare {
  2. var triangle: EquilateralTriangle {
  3. willSet {
  4. square.sideLength = newValue.sideLength
  5. }
  6. }
  7. var square: Square {
  8. willSet {
  9. triangle.sideLength = newValue.sideLength
  10. }
  11. }
  12. init(size: Double, name: String) {
  13. square = Square(sideLength: size, name: name)
  14. triangle = EquilateralTriangle(sideLength: size, name: name)
  15. }
  16. }
  17. var triangleAndSquare = TriangleAndSquare(size: 10, name: “another test shape”)
  18. print(triangleAndSquare.square.sideLength)
  19. // Prints “10.0”
  20. print(triangleAndSquare.triangle.sideLength)
  21. // Prints “10.0”
  22. triangleAndSquare.square = Square(sideLength: 50, name: “larger square”)
  23. print(triangleAndSquare.triangle.sideLength)
  24. // Prints “50.0”

When working with optional values, you can write ? before operations like methods, properties, and subscripting. If the value before the ? is nil, everything after the ? is ignored and the value of the whole expression is nil. Otherwise, the optional value is unwrapped, and everything after the ? acts on the unwrapped value. In both cases, the value of the whole expression is an optional value.

  1. let optionalSquare: Square? = Square(sideLength: 2.5, name: “optional square”)
  2. let sideLength = optionalSquare?.sideLength

Enumerations and Structures

Use enum to create an enumeration. Like classes and all other named types, enumerations can have methods associated with them.

  1. enum Rank: Int {
  2. case ace = 1
  3. case two, three, four, five, six, seven, eight, nine, ten
  4. case jack, queen, king
  5. func simpleDescription() -> String {
  6. switch self {
  7. case .ace:
  8. return “ace”
  9. case .jack:
  10. return “jack”
  11. case .queen:
  12. return “queen”
  13. case .king:
  14. return “king”
  15. default:
  16. return String(self.rawValue)
  17. }
  18. }
  19. }
  20. let ace = Rank.ace
  21. let aceRawValue = ace.rawValue


Write a function that compares two Rank values by comparing their raw values.

By default, Swift assigns the raw values starting at zero and incrementing by one each time, but you can change this behavior by explicitly specifying values. In the example above, Ace is explicitly given a raw value of 1, and the rest of the raw values are assigned in order. You can also use strings or floating-point numbers as the raw type of an enumeration. Use the rawValue property to access the raw value of an enumeration case.

Use the init?(rawValue:) initializer to make an instance of an enumeration from a raw value. It returns either the enumeration case matching the raw value or nil if there is no matching Rank.

  1. if let convertedRank = Rank(rawValue: 3) {
  2. let threeDescription = convertedRank.simpleDescription()
  3. }

The case values of an enumeration are actual values, not just another way of writing their raw values. In fact, in cases where there isn’t a meaningful raw value, you don’t have to provide one.

  1. enum Suit {
  2. case spades, hearts, diamonds, clubs
  3. func simpleDescription() -> String {
  4. switch self {
  5. case .spades:
  6. return “spades”
  7. case .hearts:
  8. return “hearts”
  9. case .diamonds:
  10. return “diamonds”
  11. case .clubs:
  12. return “clubs”
  13. }
  14. }
  15. }
  16. let hearts = Suit.hearts
  17. let heartsDescription = hearts.simpleDescription()


Add a color() method to Suit that returns “black” for spades and clubs, and returns “red” for hearts and diamonds.

Notice the two ways that the hearts case of the enumeration is referred to above: When assigning a value to the hearts constant, the enumeration case Suit.hearts is referred to by its full name because the constant doesn’t have an explicit type specified. Inside the switch, the enumeration case is referred to by the abbreviated form .hearts because the value of self is already known to be a suit. You can use the abbreviated form anytime the value’s type is already known.

If an enumeration has raw values, those values are determined as part of the declaration, which means every instance of a particular enumeration case always has the same raw value. Another choice for enumeration cases is to have values associated with the case—these values are determined when you make the instance, and they can be different for each instance of an enumeration case. You can think of the associated values as behaving like stored properties of the enumeration case instance. For example, consider the case of requesting the sunrise and sunset times from a server. The server either responds with the requested information, or it responds with a description of what went wrong.

  1. enum ServerResponse {
  2. case result(String, String)
  3. case failure(String)
  4. }
  5. let success = ServerResponse.result(“6:00 am”, “8:09 pm”)
  6. let failure = ServerResponse.failure(“Out of cheese.”)
  7. switch success {
  8. case let .result(sunrise, sunset):
  9. print(“Sunrise is at \(sunrise) and sunset is at \(sunset).”)
  10. case let .failure(message):
  11. print(“Failure… \(message)”)
  12. }
  13. // Prints “Sunrise is at 6:00 am and sunset is at 8:09 pm.”


Add a third case to ServerResponse and to the switch.

Notice how the sunrise and sunset times are extracted from the ServerResponse value as part of matching the value against the switch cases.

Use struct to create a structure. Structures support many of the same behaviors as classes, including methods and initializers. One of the most important differences between structures and classes is that structures are always copied when they are passed around in your code, but classes are passed by reference.

  1. struct Card {
  2. var rank: Rank
  3. var suit: Suit
  4. func simpleDescription() -> String {
  5. return “The \(rank.simpleDescription()) of \(suit.simpleDescription())”
  6. }
  7. }
  8. let threeOfSpades = Card(rank: .three, suit: .spades)
  9. let threeOfSpadesDescription = threeOfSpades.simpleDescription()


Write a function that returns an array containing a full deck of cards, with one card of each combination of rank and suit.

Protocols and Extensions

Use protocol to declare a protocol.

  1. protocol ExampleProtocol {
  2. var simpleDescription: String { get }
  3. mutating func adjust()
  4. }

Classes, enumerations, and structs can all adopt protocols.

  1. class SimpleClass: ExampleProtocol {
  2. var simpleDescription: String = “A very simple class.”
  3. var anotherProperty: Int = 69105
  4. func adjust() {
  5. simpleDescription += ” Now 100% adjusted.”
  6. }
  7. }
  8. var a = SimpleClass()
  9. a.adjust()
  10. let aDescription = a.simpleDescription
  11. struct SimpleStructure: ExampleProtocol {
  12. var simpleDescription: String = “A simple structure”
  13. mutating func adjust() {
  14. simpleDescription += ” (adjusted)”
  15. }
  16. }
  17. var b = SimpleStructure()
  18. b.adjust()
  19. let bDescription = b.simpleDescription


Add another requirement to ExampleProtocol. What changes do you need to make to SimpleClass and SimpleStructure so that they still conform to the protocol?

Notice the use of the mutating keyword in the declaration of SimpleStructure to mark a method that modifies the structure. The declaration of SimpleClass doesn’t need any of its methods marked as mutating because methods on a class can always modify the class.

Use extension to add functionality to an existing type, such as new methods and computed properties. You can use an extension to add protocol conformance to a type that is declared elsewhere, or even to a type that you imported from a library or framework.

  1. extension Int: ExampleProtocol {
  2. var simpleDescription: String {
  3. return “The number \(self)”
  4. }
  5. mutating func adjust() {
  6. self += 42
  7. }
  8. }
  9. print(7.simpleDescription)
  10. // Prints “The number 7”


Write an extension for the Double type that adds an absoluteValue property.

You can use a protocol name just like any other named type—for example, to create a collection of objects that have different types but that all conform to a single protocol. When you work with values whose type is a protocol type, methods outside the protocol definition are not available.

  1. let protocolValue: ExampleProtocol = a
  2. print(protocolValue.simpleDescription)
  3. // Prints “A very simple class. Now 100% adjusted.”
  4. // print(protocolValue.anotherProperty) // Uncomment to see the error

Even though the variable protocolValue has a runtime type of SimpleClass, the compiler treats it as the given type of ExampleProtocol. This means that you can’t accidentally access methods or properties that the class implements in addition to its protocol conformance.

Error Handling

You represent errors using any type that adopts the Error protocol.

  1. enum PrinterError: Error {
  2. case outOfPaper
  3. case noToner
  4. case onFire
  5. }

Use throw to throw an error and throws to mark a function that can throw an error. If you throw an error in a function, the function returns immediately and the code that called the function handles the error.

  1. func send(job: Int, toPrinter printerName: String) throws -> String {
  2. if printerName == “Never Has Toner” {
  3. throw PrinterError.noToner
  4. }
  5. return “Job sent”
  6. }

There are several ways to handle errors. One way is to use docatch. Inside the do block, you mark code that can throw an error by writing try in front of it. Inside the catch block, the error is automatically given the name error unless you give it a different name.

  1. do {
  2. let printerResponse = try send(job: 1040, toPrinter: “Bi Sheng”)
  3. print(printerResponse)
  4. } catch {
  5. print(error)
  6. }
  7. // Prints “Job sent”


Change the printer name to "Never Has Toner", so that the send(job:toPrinter:) function throws an error.

You can provide multiple catch blocks that handle specific errors. You write a pattern after catch just as you do after case in a switch.

  1. do {
  2. let printerResponse = try send(job: 1440, toPrinter: “Gutenberg”)
  3. print(printerResponse)
  4. } catch PrinterError.onFire {
  5. print(“I’ll just put this over here, with the rest of the fire.”)
  6. } catch let printerError as PrinterError {
  7. print(“Printer error: \(printerError).”)
  8. } catch {
  9. print(error)
  10. }
  11. // Prints “Job sent”


Add code to throw an error inside the do block. What kind of error do you need to throw so that the error is handled by the first catch block? What about the second and third blocks?

Another way to handle errors is to use try? to convert the result to an optional. If the function throws an error, the specific error is discarded and the result is nil. Otherwise, the result is an optional containing the value that the function returned.

  1. let printerSuccess = try? send(job: 1884, toPrinter: “Mergenthaler”)
  2. let printerFailure = try? send(job: 1885, toPrinter: “Never Has Toner”)

Use defer to write a block of code that is executed after all other code in the function, just before the function returns. The code is executed regardless of whether the function throws an error. You can use defer to write setup and cleanup code next to each other, even though they need to be executed at different times.

  1. var fridgeIsOpen = false
  2. let fridgeContent = [“milk”, “eggs”, “leftovers”]
  3. func fridgeContains(_ food: String) -> Bool {
  4. fridgeIsOpen = true
  5. defer {
  6. fridgeIsOpen = false
  7. }
  8. let result = fridgeContent.contains(food)
  9. return result
  10. }
  11. fridgeContains(“banana”)
  12. print(fridgeIsOpen)
  13. // Prints “false”


Write a name inside angle brackets to make a generic function or type.

  1. func makeArray<Item>(repeating item: Item, numberOfTimes: Int) -> [Item] {
  2. var result = [Item]()
  3. for _ in 0..<numberOfTimes {
  4. result.append(item)
  5. }
  6. return result
  7. }
  8. makeArray(repeating: “knock”, numberOfTimes: 4)

You can make generic forms of functions and methods, as well as classes, enumerations, and structures.

  1. // Reimplement the Swift standard library’s optional type
  2. enum OptionalValue<Wrapped> {
  3. case none
  4. case some(Wrapped)
  5. }
  6. var possibleInteger: OptionalValue<Int> = .none
  7. possibleInteger = .some(100)

Use where right before the body to specify a list of requirements—for example, to require the type to implement a protocol, to require two types to be the same, or to require a class to have a particular superclass.

  1. func anyCommonElements<T: Sequence, U: Sequence>(_ lhs: T, _ rhs: U) -> Bool
  2. where T.Element: Equatable, T.Element == U.Element
  3. {
  4. for lhsItem in lhs {
  5. for rhsItem in rhs {
  6. if lhsItem == rhsItem {
  7. return true
  8. }
  9. }
  10. }
  11. return false
  12. }
  13. anyCommonElements([1, 2, 3], [3])


Modify the anyCommonElements(_:_:) function to make a function that returns an array of the elements that any two sequences have in common.

The Basics

Swift is a new programming language for iOS, macOS, watchOS, and tvOS app development. Nonetheless, many parts of Swift will be familiar from your experience of developing in C and Objective-C.

Swift provides its own versions of all fundamental C and Objective-C types, including Int for integers, Double and Float for floating-point values, Bool for Boolean values, and String for textual data. Swift also provides powerful versions of the three primary collection types, ArraySet, and Dictionary, as described in Collection Types.

Like C, Swift uses variables to store and refer to values by an identifying name. Swift also makes extensive use of variables whose values can’t be changed. These are known as constants, and are much more powerful than constants in C. Constants are used throughout Swift to make code safer and clearer in intent when you work with values that don’t need to change.

In addition to familiar types, Swift introduces advanced types not found in Objective-C, such as tuples. Tuples enable you to create and pass around groupings of values. You can use a tuple to return multiple values from a function as a single compound value.

Swift also introduces optional types, which handle the absence of a value. Optionals say either “there is a value, and it equals x” or “there isn’t a value at all”. Using optionals is similar to using nil with pointers in Objective-C, but they work for any type, not just classes. Not only are optionals safer and more expressive than nil pointers in Objective-C, they’re at the heart of many of Swift’s most powerful features.

Swift is a type-safe language, which means the language helps you to be clear about the types of values your code can work with. If part of your code requires a String, type safety prevents you from passing it an Int by mistake. Likewise, type safety prevents you from accidentally passing an optional String to a piece of code that requires a non-optional String. Type safety helps you catch and fix errors as early as possible in the development process.

Constants and Variables

Constants and variables associate a name (such as maximumNumberOfLoginAttempts or welcomeMessage) with a value of a particular type (such as the number 10 or the string "Hello"). The value of a constant can’t be changed once it’s set, whereas a variable can be set to a different value in the future.

Declaring Constants and Variables

Constants and variables must be declared before they’re used. You declare constants with the let keyword and variables with the var keyword. Here’s an example of how constants and variables can be used to track the number of login attempts a user has made:

  1. let maximumNumberOfLoginAttempts = 10
  2. var currentLoginAttempt = 0

This code can be read as:

“Declare a new constant called maximumNumberOfLoginAttempts, and give it a value of 10. Then, declare a new variable called currentLoginAttempt, and give it an initial value of 0.”

In this example, the maximum number of allowed login attempts is declared as a constant, because the maximum value never changes. The current login attempt counter is declared as a variable, because this value must be incremented after each failed login attempt.

You can declare multiple constants or multiple variables on a single line, separated by commas:

  1. var x = 0.0, y = 0.0, z = 0.0


If a stored value in your code won’t change, always declare it as a constant with the let keyword. Use variables only for storing values that need to be able to change.

Type Annotations

You can provide a type annotation when you declare a constant or variable, to be clear about the kind of values the constant or variable can store. Write a type annotation by placing a colon after the constant or variable name, followed by a space, followed by the name of the type to use.

This example provides a type annotation for a variable called welcomeMessage, to indicate that the variable can store String values:

  1. var welcomeMessage: String

The colon in the declaration means “…of type…,” so the code above can be read as:

“Declare a variable called welcomeMessage that is of type String.”

The phrase “of type String” means “can store any String value.” Think of it as meaning “the type of thing” (or “the kind of thing”) that can be stored.

The welcomeMessage variable can now be set to any string value without error:

  1. welcomeMessage = “Hello”

You can define multiple related variables of the same type on a single line, separated by commas, with a single type annotation after the final variable name:

  1. var red, green, blue: Double


It’s rare that you need to write type annotations in practice. If you provide an initial value for a constant or variable at the point that it’s defined, Swift can almost always infer the type to be used for that constant or variable, as described in Type Safety and Type Inference. In the welcomeMessage example above, no initial value is provided, and so the type of the welcomeMessage variable is specified with a type annotation rather than being inferred from an initial value.

Naming Constants and Variables

Constant and variable names can contain almost any character, including Unicode characters:

  1. let π = 3.14159
  2. let 你好 = “你好世界”
  3. let 🐶🐮 = “dogcow”

Constant and variable names can’t contain whitespace characters, mathematical symbols, arrows, private-use Unicode scalar values, or line- and box-drawing characters. Nor can they begin with a number, although numbers may be included elsewhere within the name.

Once you’ve declared a constant or variable of a certain type, you can’t declare it again with the same name, or change it to store values of a different type. Nor can you change a constant into a variable or a variable into a constant.


If you need to give a constant or variable the same name as a reserved Swift keyword, surround the keyword with backticks (`) when using it as a name. However, avoid using keywords as names unless you have absolutely no choice.

You can change the value of an existing variable to another value of a compatible type. In this example, the value of friendlyWelcome is changed from "Hello!" to "Bonjour!":

  1. var friendlyWelcome = “Hello!”
  2. friendlyWelcome = “Bonjour!”
  3. // friendlyWelcome is now “Bonjour!”

Unlike a variable, the value of a constant can’t be changed after it’s set. Attempting to do so is reported as an error when your code is compiled:

  1. let languageName = “Swift”
  2. languageName = “Swift++”
  3. // This is a compile-time error: languageName cannot be changed.

Printing Constants and Variables

You can print the current value of a constant or variable with the print(_:separator:terminator:) function:

  1. print(friendlyWelcome)
  2. // Prints “Bonjour!”

The print(_:separator:terminator:) function is a global function that prints one or more values to an appropriate output. In Xcode, for example, the print(_:separator:terminator:) function prints its output in Xcode’s “console” pane. The separator and terminator parameter have default values, so you can omit them when you call this function. By default, the function terminates the line it prints by adding a line break. To print a value without a line break after it, pass an empty string as the terminator—for example, print(someValue, terminator: ""). For information about parameters with default values, see Default Parameter Values.

Swift uses string interpolation to include the name of a constant or variable as a placeholder in a longer string, and to prompt Swift to replace it with the current value of that constant or variable. Wrap the name in parentheses and escape it with a backslash before the opening parenthesis:

  1. print(“The current value of friendlyWelcome is \(friendlyWelcome)”)
  2. // Prints “The current value of friendlyWelcome is Bonjour!”


All options you can use with string interpolation are described in String Interpolation.


Use comments to include nonexecutable text in your code, as a note or reminder to yourself. Comments are ignored by the Swift compiler when your code is compiled.

Comments in Swift are very similar to comments in C. Single-line comments begin with two forward-slashes (//):

  1. // This is a comment.

Multiline comments start with a forward-slash followed by an asterisk (/*) and end with an asterisk followed by a forward-slash (*/):

  1. /* This is also a comment
  2. but is written over multiple lines. */

Unlike multiline comments in C, multiline comments in Swift can be nested inside other multiline comments. You write nested comments by starting a multiline comment block and then starting a second multiline comment within the first block. The second block is then closed, followed by the first block:

  1. /* This is the start of the first multiline comment.
  2. /* This is the second, nested multiline comment. */
  3. This is the end of the first multiline comment. */

Nested multiline comments enable you to comment out large blocks of code quickly and easily, even if the code already contains multiline comments.


Unlike many other languages, Swift doesn’t require you to write a semicolon (;) after each statement in your code, although you can do so if you wish. However, semicolons are required if you want to write multiple separate statements on a single line:

  1. let cat = “🐱”; print(cat)
  2. // Prints “🐱”


Integers are whole numbers with no fractional component, such as 42 and -23. Integers are either signed (positive, zero, or negative) or unsigned (positive or zero).

Swift provides signed and unsigned integers in 8, 16, 32, and 64 bit forms. These integers follow a naming convention similar to C, in that an 8-bit unsigned integer is of type UInt8, and a 32-bit signed integer is of type Int32. Like all types in Swift, these integer types have capitalized names.

Integer Bounds

You can access the minimum and maximum values of each integer type with its min and max properties:

  1. let minValue = UInt8.min // minValue is equal to 0, and is of type UInt8
  2. let maxValue = UInt8.max // maxValue is equal to 255, and is of type UInt8

The values of these properties are of the appropriate-sized number type (such as UInt8 in the example above) and can therefore be used in expressions alongside other values of the same type.


In most cases, you don’t need to pick a specific size of integer to use in your code. Swift provides an additional integer type, Int, which has the same size as the current platform’s native word size:

  • On a 32-bit platform, Int is the same size as Int32.
  • On a 64-bit platform, Int is the same size as Int64.

Unless you need to work with a specific size of integer, always use Int for integer values in your code. This aids code consistency and interoperability. Even on 32-bit platforms, Int can store any value between -2,147,483,648 and 2,147,483,647, and is large enough for many integer ranges.


Swift also provides an unsigned integer type, UInt, which has the same size as the current platform’s native word size:

  • On a 32-bit platform, UInt is the same size as UInt32.
  • On a 64-bit platform, UInt is the same size as UInt64.


Use UInt only when you specifically need an unsigned integer type with the same size as the platform’s native word size. If this isn’t the case, Int is preferred, even when the values to be stored are known to be nonnegative. A consistent use of Int for integer values aids code interoperability, avoids the need to convert between different number types, and matches integer type inference, as described in Type Safety and Type Inference.

Floating-Point Numbers

Floating-point numbers are numbers with a fractional component, such as 3.141590.1, and -273.15.

Floating-point types can represent a much wider range of values than integer types, and can store numbers that are much larger or smaller than can be stored in an Int. Swift provides two signed floating-point number types:

  • Double represents a 64-bit floating-point number.
  • Float represents a 32-bit floating-point number.


Double has a precision of at least 15 decimal digits, whereas the precision of Float can be as little as 6 decimal digits. The appropriate floating-point type to use depends on the nature and range of values you need to work with in your code. In situations where either type would be appropriate, Double is preferred.

Type Safety and Type Inference

Swift is a type-safe language. A type safe language encourages you to be clear about the types of values your code can work with. If part of your code requires a String, you can’t pass it an Int by mistake.

Because Swift is type safe, it performs type checks when compiling your code and flags any mismatched types as errors. This enables you to catch and fix errors as early as possible in the development process.

Type-checking helps you avoid errors when you’re working with different types of values. However, this doesn’t mean that you have to specify the type of every constant and variable that you declare. If you don’t specify the type of value you need, Swift uses type inference to work out the appropriate type. Type inference enables a compiler to deduce the type of a particular expression automatically when it compiles your code, simply by examining the values you provide.

Because of type inference, Swift requires far fewer type declarations than languages such as C or Objective-C. Constants and variables are still explicitly typed, but much of the work of specifying their type is done for you.

Type inference is particularly useful when you declare a constant or variable with an initial value. This is often done by assigning a literal value (or literal) to the constant or variable at the point that you declare it. (A literal value is a value that appears directly in your source code, such as 42 and 3.14159 in the examples below.)

For example, if you assign a literal value of 42 to a new constant without saying what type it is, Swift infers that you want the constant to be an Int, because you have initialized it with a number that looks like an integer:

  1. let meaningOfLife = 42
  2. // meaningOfLife is inferred to be of type Int

Likewise, if you don’t specify a type for a floating-point literal, Swift infers that you want to create a Double:

  1. let pi = 3.14159
  2. // pi is inferred to be of type Double

Swift always chooses Double (rather than Float) when inferring the type of floating-point numbers.

If you combine integer and floating-point literals in an expression, a type of Double will be inferred from the context:

  1. let anotherPi = 3 + 0.14159
  2. // anotherPi is also inferred to be of type Double

The literal value of 3 has no explicit type in and of itself, and so an appropriate output type of Double is inferred from the presence of a floating-point literal as part of the addition.

Numeric Literals

Integer literals can be written as:

  • decimal number, with no prefix
  • binary number, with a 0b prefix
  • An octal number, with a 0o prefix
  • hexadecimal number, with a 0x prefix

All of these integer literals have a decimal value of 17:

  1. let decimalInteger = 17
  2. let binaryInteger = 0b10001 // 17 in binary notation
  3. let octalInteger = 0o21 // 17 in octal notation
  4. let hexadecimalInteger = 0x11 // 17 in hexadecimal notation

Floating-point literals can be decimal (with no prefix), or hexadecimal (with a 0x prefix). They must always have a number (or hexadecimal number) on both sides of the decimal point. Decimal floats can also have an optional exponent, indicated by an uppercase or lowercase e; hexadecimal floats must have an exponent, indicated by an uppercase or lowercase p.

For decimal numbers with an exponent of exp, the base number is multiplied by 10exp:

  • 1.25e2 means 1.25 x 102, or 125.0.
  • 1.25e-2 means 1.25 x 10-2, or 0.0125.

For hexadecimal numbers with an exponent of exp, the base number is multiplied by 2exp:

  • 0xFp2 means 15 x 22, or 60.0.
  • 0xFp-2 means 15 x 2-2, or 3.75.

All of these floating-point literals have a decimal value of 12.1875:

  1. let decimalDouble = 12.1875
  2. let exponentDouble = 1.21875e1
  3. let hexadecimalDouble = 0xC.3p0

Numeric literals can contain extra formatting to make them easier to read. Both integers and floats can be padded with extra zeros and can contain underscores to help with readability. Neither type of formatting affects the underlying value of the literal:

  1. let paddedDouble = 000123.456
  2. let oneMillion = 1_000_000
  3. let justOverOneMillion = 1_000_000.000_000_1

Numeric Type Conversion

Use the Int type for all general-purpose integer constants and variables in your code, even if they’re known to be nonnegative. Using the default integer type in everyday situations means that integer constants and variables are immediately interoperable in your code and will match the inferred type for integer literal values.

Use other integer types only when they’re specifically needed for the task at hand, because of explicitly sized data from an external source, or for performance, memory usage, or other necessary optimization. Using explicitly sized types in these situations helps to catch any accidental value overflows and implicitly documents the nature of the data being used.

Integer Conversion

The range of numbers that can be stored in an integer constant or variable is different for each numeric type. An Int8 constant or variable can store numbers between -128 and 127, whereas a UInt8 constant or variable can store numbers between 0 and 255. A number that won’t fit into a constant or variable of a sized integer type is reported as an error when your code is compiled:

  1. let cannotBeNegative: UInt8 = -1
  2. // UInt8 cannot store negative numbers, and so this will report an error
  3. let tooBig: Int8 = Int8.max + 1
  4. // Int8 cannot store a number larger than its maximum value,
  5. // and so this will also report an error

Because each numeric type can store a different range of values, you must opt in to numeric type conversion on a case-by-case basis. This opt-in approach prevents hidden conversion errors and helps make type conversion intentions explicit in your code.

To convert one specific number type to another, you initialize a new number of the desired type with the existing value. In the example below, the constant twoThousand is of type UInt16, whereas the constant one is of type UInt8. They can’t be added together directly, because they’re not of the same type. Instead, this example calls UInt16(one) to create a new UInt16 initialized with the value of one, and uses this value in place of the original:

  1. let twoThousand: UInt16 = 2_000
  2. let one: UInt8 = 1
  3. let twoThousandAndOne = twoThousand + UInt16(one)

Because both sides of the addition are now of type UInt16, the addition is allowed. The output constant (twoThousandAndOne) is inferred to be of type UInt16, because it’s the sum of two UInt16 values.

SomeType(ofInitialValue) is the default way to call the initializer of a Swift type and pass in an initial value. Behind the scenes, UInt16 has an initializer that accepts a UInt8 value, and so this initializer is used to make a new UInt16 from an existing UInt8. You can’t pass in any type here, however—it has to be a type for which UInt16 provides an initializer. Extending existing types to provide initializers that accept new types (including your own type definitions) is covered in Extensions.

Integer and Floating-Point Conversion

Conversions between integer and floating-point numeric types must be made explicit:

  1. let three = 3
  2. let pointOneFourOneFiveNine = 0.14159
  3. let pi = Double(three) + pointOneFourOneFiveNine
  4. // pi equals 3.14159, and is inferred to be of type Double

Here, the value of the constant three is used to create a new value of type Double, so that both sides of the addition are of the same type. Without this conversion in place, the addition would not be allowed.

Floating-point to integer conversion must also be made explicit. An integer type can be initialized with a Double or Float value:

  1. let integerPi = Int(pi)
  2. // integerPi equals 3, and is inferred to be of type Int

Floating-point values are always truncated when used to initialize a new integer value in this way. This means that 4.75 becomes 4, and -3.9 becomes -3.


The rules for combining numeric constants and variables are different from the rules for numeric literals. The literal value 3 can be added directly to the literal value 0.14159, because number literals don’t have an explicit type in and of themselves. Their type is inferred only at the point that they’re evaluated by the compiler.

Type Aliases

Type aliases define an alternative name for an existing type. You define type aliases with the typealias keyword.

Type aliases are useful when you want to refer to an existing type by a name that is contextually more appropriate, such as when working with data of a specific size from an external source:

  1. typealias AudioSample = UInt16

Once you define a type alias, you can use the alias anywhere you might use the original name:

  1. var maxAmplitudeFound = AudioSample.min
  2. // maxAmplitudeFound is now 0

Here, AudioSample is defined as an alias for UInt16. Because it’s an alias, the call to AudioSample.min actually calls UInt16.min, which provides an initial value of 0 for the maxAmplitudeFound variable.


Swift has a basic Boolean type, called Bool. Boolean values are referred to as logical, because they can only ever be true or false. Swift provides two Boolean constant values, true and false:

  1. let orangesAreOrange = true
  2. let turnipsAreDelicious = false

The types of orangesAreOrange and turnipsAreDelicious have been inferred as Bool from the fact that they were initialized with Boolean literal values. As with Int and Double above, you don’t need to declare constants or variables as Bool if you set them to true or false as soon as you create them. Type inference helps make Swift code more concise and readable when it initializes constants or variables with other values whose type is already known.

Boolean values are particularly useful when you work with conditional statements such as the if statement:

  1. if turnipsAreDelicious {
  2. print(“Mmm, tasty turnips!”)
  3. } else {
  4. print(“Eww, turnips are horrible.”)
  5. }
  6. // Prints “Eww, turnips are horrible.”

Conditional statements such as the if statement are covered in more detail in Control Flow.

Swift’s type safety prevents non-Boolean values from being substituted for Bool. The following example reports a compile-time error:

  1. let i = 1
  2. if i {
  3. // this example will not compile, and will report an error
  4. }

However, the alternative example below is valid:

  1. let i = 1
  2. if i == 1 {
  3. // this example will compile successfully
  4. }

The result of the i == 1 comparison is of type Bool, and so this second example passes the type-check. Comparisons like i == 1 are discussed in Basic Operators.

As with other examples of type safety in Swift, this approach avoids accidental errors and ensures that the intention of a particular section of code is always clear.


Tuples group multiple values into a single compound value. The values within a tuple can be of any type and don’t have to be of the same type as each other.

In this example, (404, "Not Found") is a tuple that describes an HTTP status code. An HTTP status code is a special value returned by a web server whenever you request a web page. A status code of 404 Not Found is returned if you request a webpage that doesn’t exist.

  1. let http404Error = (404, “Not Found”)
  2. // http404Error is of type (Int, String), and equals (404, “Not Found”)

The (404, "Not Found") tuple groups together an Int and a String to give the HTTP status code two separate values: a number and a human-readable description. It can be described as “a tuple of type (Int, String)”.

You can create tuples from any permutation of types, and they can contain as many different types as you like. There’s nothing stopping you from having a tuple of type (Int, Int, Int), or (String, Bool), or indeed any other permutation you require.

You can decompose a tuple’s contents into separate constants or variables, which you then access as usual:

  1. let (statusCode, statusMessage) = http404Error
  2. print(“The status code is \(statusCode)”)
  3. // Prints “The status code is 404”
  4. print(“The status message is \(statusMessage)”)
  5. // Prints “The status message is Not Found”

If you only need some of the tuple’s values, ignore parts of the tuple with an underscore (_) when you decompose the tuple:

  1. let (justTheStatusCode, _) = http404Error
  2. print(“The status code is \(justTheStatusCode)”)
  3. // Prints “The status code is 404”

Alternatively, access the individual element values in a tuple using index numbers starting at zero:

  1. print(“The status code is \(http404Error.0)”)
  2. // Prints “The status code is 404”
  3. print(“The status message is \(http404Error.1)”)
  4. // Prints “The status message is Not Found”

You can name the individual elements in a tuple when the tuple is defined:

  1. let http200Status = (statusCode: 200, description: “OK”)

If you name the elements in a tuple, you can use the element names to access the values of those elements:

  1. print(“The status code is \(http200Status.statusCode)”)
  2. // Prints “The status code is 200”
  3. print(“The status message is \(http200Status.description)”)
  4. // Prints “The status message is OK”

Tuples are particularly useful as the return values of functions. A function that tries to retrieve a web page might return the (Int, String) tuple type to describe the success or failure of the page retrieval. By returning a tuple with two distinct values, each of a different type, the function provides more useful information about its outcome than if it could only return a single value of a single type. For more information, see Functions with Multiple Return Values.


Tuples are useful for simple groups of related values. They’re not suited to the creation of complex data structures. If your data structure is likely to be more complex, model it as a class or structure, rather than as a tuple. For more information, see Structures and Classes.


You use optionals in situations where a value may be absent. An optional represents two possibilities: Either there is a value, and you can unwrap the optional to access that value, or there isn’t a value at all.


The concept of optionals doesn’t exist in C or Objective-C. The nearest thing in Objective-C is the ability to return nil from a method that would otherwise return an object, with nil meaning “the absence of a valid object.” However, this only works for objects—it doesn’t work for structures, basic C types, or enumeration values. For these types, Objective-C methods typically return a special value (such as NSNotFound) to indicate the absence of a value. This approach assumes that the method’s caller knows there’s a special value to test against and remembers to check for it. Swift’s optionals let you indicate the absence of a value for any type at all, without the need for special constants.

Here’s an example of how optionals can be used to cope with the absence of a value. Swift’s Int type has an initializer which tries to convert a String value into an Int value. However, not every string can be converted into an integer. The string "123" can be converted into the numeric value 123, but the string "hello, world" doesn’t have an obvious numeric value to convert to.

The example below uses the initializer to try to convert a String into an Int:

  1. let possibleNumber = “123”
  2. let convertedNumber = Int(possibleNumber)
  3. // convertedNumber is inferred to be of type “Int?”, or “optional Int”

Because the initializer might fail, it returns an optional Int, rather than an Int. An optional Int is written as Int?, not Int. The question mark indicates that the value it contains is optional, meaning that it might contain some Int value, or it might contain no value at all. (It can’t contain anything else, such as a Bool value or a String value. It’s either an Int, or it’s nothing at all.)


You set an optional variable to a valueless state by assigning it the special value nil:

  1. var serverResponseCode: Int? = 404
  2. // serverResponseCode contains an actual Int value of 404
  3. serverResponseCode = nil
  4. // serverResponseCode now contains no value


You can’t use nil with non-optional constants and variables. If a constant or variable in your code needs to work with the absence of a value under certain conditions, always declare it as an optional value of the appropriate type.

If you define an optional variable without providing a default value, the variable is automatically set to nil for you:

  1. var surveyAnswer: String?
  2. // surveyAnswer is automatically set to nil


Swift’s nil isn’t the same as nil in Objective-C. In Objective-C, nil is a pointer to a nonexistent object. In Swift, nil isn’t a pointer—it’s the absence of a value of a certain type. Optionals of any type can be set to nil, not just object types.

If Statements and Forced Unwrapping

You can use an if statement to find out whether an optional contains a value by comparing the optional against nil. You perform this comparison with the “equal to” operator (==) or the “not equal to” operator (!=).

If an optional has a value, it’s considered to be “not equal to” nil:

  1. if convertedNumber != nil {
  2. print(“convertedNumber contains some integer value.”)
  3. }
  4. // Prints “convertedNumber contains some integer value.”

Once you’re sure that the optional does contain a value, you can access its underlying value by adding an exclamation point (!) to the end of the optional’s name. The exclamation point effectively says, “I know that this optional definitely has a value; please use it.” This is known as forced unwrapping of the optional’s value:

  1. if convertedNumber != nil {
  2. print(“convertedNumber has an integer value of \(convertedNumber!).”)
  3. }
  4. // Prints “convertedNumber has an integer value of 123.”

For more about the if statement, see Control Flow.


Trying to use ! to access a nonexistent optional value triggers a runtime error. Always make sure that an optional contains a non-nil value before using ! to force-unwrap its value.

Optional Binding

You use optional binding to find out whether an optional contains a value, and if so, to make that value available as a temporary constant or variable. Optional binding can be used with if and while statements to check for a value inside an optional, and to extract that value into a constant or variable, as part of a single action. if and while statements are described in more detail in Control Flow.

Write an optional binding for an if statement as follows:

  1. if let constantName = someOptional {
  2. statements
  3. }

You can rewrite the possibleNumber example from the Optionals section to use optional binding rather than forced unwrapping:

  1. if let actualNumber = Int(possibleNumber) {
  2. print(“The string \”\(possibleNumber)\” has an integer value of \(actualNumber)”)
  3. } else {
  4. print(“The string \”\(possibleNumber)\” could not be converted to an integer”)
  5. }
  6. // Prints “The string “123” has an integer value of 123″

This code can be read as:

“If the optional Int returned by Int(possibleNumber) contains a value, set a new constant called actualNumber to the value contained in the optional.”

If the conversion is successful, the actualNumber constant becomes available for use within the first branch of the if statement. It has already been initialized with the value contained within the optional, and so there’s no need to use the ! suffix to access its value. In this example, actualNumber is simply used to print the result of the conversion.

You can use both constants and variables with optional binding. If you wanted to manipulate the value of actualNumber within the first branch of the if statement, you could write if var actualNumber instead, and the value contained within the optional would be made available as a variable rather than a constant.

You can include as many optional bindings and Boolean conditions in a single if statement as you need to, separated by commas. If any of the values in the optional bindings are nil or any Boolean condition evaluates to false, the whole if statement’s condition is considered to be false. The following if statements are equivalent:

  1. if let firstNumber = Int(“4”), let secondNumber = Int(“42”), firstNumber < secondNumber && secondNumber < 100 {
  2. print(“\(firstNumber) < \(secondNumber) < 100”)
  3. }
  4. // Prints “4 < 42 < 100”
  5. if let firstNumber = Int(“4”) {
  6. if let secondNumber = Int(“42”) {
  7. if firstNumber < secondNumber && secondNumber < 100 {
  8. print(“\(firstNumber) < \(secondNumber) < 100”)
  9. }
  10. }
  11. }
  12. // Prints “4 < 42 < 100”


Constants and variables created with optional binding in an if statement are available only within the body of the if statement. In contrast, the constants and variables created with a guard statement are available in the lines of code that follow the guard statement, as described in Early Exit.

Implicitly Unwrapped Optionals

As described above, optionals indicate that a constant or variable is allowed to have “no value”. Optionals can be checked with an if statement to see if a value exists, and can be conditionally unwrapped with optional binding to access the optional’s value if it does exist.

Sometimes it’s clear from a program’s structure that an optional will always have a value, after that value is first set. In these cases, it’s useful to remove the need to check and unwrap the optional’s value every time it’s accessed, because it can be safely assumed to have a value all of the time.

These kinds of optionals are defined as implicitly unwrapped optionals. You write an implicitly unwrapped optional by placing an exclamation point (String!) rather than a question mark (String?) after the type that you want to make optional. Rather than placing an exclamation point after the optional’s name when you use it, you place an exclamation point after the optional’s type when you declare it.

Implicitly unwrapped optionals are useful when an optional’s value is confirmed to exist immediately after the optional is first defined and can definitely be assumed to exist at every point thereafter. The primary use of implicitly unwrapped optionals in Swift is during class initialization, as described in Unowned References and Implicitly Unwrapped Optional Properties.

An implicitly unwrapped optional is a normal optional behind the scenes, but can also be used like a non-optional value, without the need to unwrap the optional value each time it’s accessed. The following example shows the difference in behavior between an optional string and an implicitly unwrapped optional string when accessing their wrapped value as an explicit String:

  1. let possibleString: String? = “An optional string.”
  2. let forcedString: String = possibleString! // requires an exclamation point
  3. let assumedString: String! = “An implicitly unwrapped optional string.”
  4. let implicitString: String = assumedString // no need for an exclamation point

You can think of an implicitly unwrapped optional as giving permission for the optional to be force-unwrapped if needed. When you use an implicitly unwrapped optional value, Swift first tries to use it as an ordinary optional value; if it can’t be used as an optional, Swift force-unwraps the value. In the code above, the optional value assumedString is force-unwrapped before assigning its value to implicitString because implicitString has an explicit, non-optional type of String. In code below, optionalString doesn’t have an explicit type so it’s an ordinary optional.

  1. let optionalString = assumedString
  2. // The type of optionalString is “String?” and assumedString isn’t force-unwrapped.

If an implicitly unwrapped optional is nil and you try to access its wrapped value, you’ll trigger a runtime error. The result is exactly the same as if you place an exclamation point after a normal optional that doesn’t contain a value.

You can check whether an implicitly unwrapped optional is nil the same way you check a normal optional:

  1. if assumedString != nil {
  2. print(assumedString!)
  3. }
  4. // Prints “An implicitly unwrapped optional string.”

You can also use an implicitly unwrapped optional with optional binding, to check and unwrap its value in a single statement:

  1. if let definiteString = assumedString {
  2. print(definiteString)
  3. }
  4. // Prints “An implicitly unwrapped optional string.”


Don’t use an implicitly unwrapped optional when there’s a possibility of a variable becoming nil at a later point. Always use a normal optional type if you need to check for a nil value during the lifetime of a variable.

Error Handling

You use error handling to respond to error conditions your program may encounter during execution.

In contrast to optionals, which can use the presence or absence of a value to communicate success or failure of a function, error handling allows you to determine the underlying cause of failure, and, if necessary, propagate the error to another part of your program.

When a function encounters an error condition, it throws an error. That function’s caller can then catch the error and respond appropriately.

  1. func canThrowAnError() throws {
  2. // this function may or may not throw an error
  3. }

A function indicates that it can throw an error by including the throws keyword in its declaration. When you call a function that can throw an error, you prepend the try keyword to the expression.

Swift automatically propagates errors out of their current scope until they’re handled by a catch clause.

  1. do {
  2. try canThrowAnError()
  3. // no error was thrown
  4. } catch {
  5. // an error was thrown
  6. }

do statement creates a new containing scope, which allows errors to be propagated to one or more catch clauses.

Here’s an example of how error handling can be used to respond to different error conditions:

  1. func makeASandwich() throws {
  2. // …
  3. }
  4. do {
  5. try makeASandwich()
  6. eatASandwich()
  7. } catch SandwichError.outOfCleanDishes {
  8. washDishes()
  9. } catch SandwichError.missingIngredients(let ingredients) {
  10. buyGroceries(ingredients)
  11. }

In this example, the makeASandwich() function will throw an error if no clean dishes are available or if any ingredients are missing. Because makeASandwich() can throw an error, the function call is wrapped in a try expression. By wrapping the function call in a do statement, any errors that are thrown will be propagated to the provided catch clauses.

If no error is thrown, the eatASandwich() function is called. If an error is thrown and it matches the SandwichError.outOfCleanDishes case, then the washDishes() function will be called. If an error is thrown and it matches the SandwichError.missingIngredients case, then the buyGroceries(_:) function is called with the associated [String] value captured by the catch pattern.

Throwing, catching, and propagating errors is covered in greater detail in Error Handling.

Assertions and Preconditions

Assertions and preconditions are checks that happen at runtime. You use them to make sure an essential condition is satisfied before executing any further code. If the Boolean condition in the assertion or precondition evaluates to true, code execution continues as usual. If the condition evaluates to false, the current state of the program is invalid; code execution ends, and your app is terminated.

You use assertions and preconditions to express the assumptions you make and the expectations you have while coding, so you can include them as part of your code. Assertions help you find mistakes and incorrect assumptions during development, and preconditions help you detect issues in production.

In addition to verifying your expectations at runtime, assertions and preconditions also become a useful form of documentation within the code. Unlike the error conditions discussed in Error Handling above, assertions and preconditions aren’t used for recoverable or expected errors. Because a failed assertion or precondition indicates an invalid program state, there’s no way to catch a failed assertion.

Using assertions and preconditions isn’t a substitute for designing your code in such a way that invalid conditions are unlikely to arise. However, using them to enforce valid data and state causes your app to terminate more predictably if an invalid state occurs, and helps make the problem easier to debug. Stopping execution as soon as an invalid state is detected also helps limit the damage caused by that invalid state.

The difference between assertions and preconditions is in when they’re checked: Assertions are checked only in debug builds, but preconditions are checked in both debug and production builds. In production builds, the condition inside an assertion isn’t evaluated. This means you can use as many assertions as you want during your development process, without impacting performance in production.

Debugging with Assertions

You write an assertion by calling the assert(_:_:file:line:) function from the Swift standard library. You pass this function an expression that evaluates to true or false and a message to display if the result of the condition is false. For example:

  1. let age = -3
  2. assert(age >= 0, “A person’s age can’t be less than zero.”)
  3. // This assertion fails because -3 is not >= 0.

In this example, code execution continues if age >= 0 evaluates to true, that is, if the value of age is nonnegative. If the value of age is negative, as in the code above, then age >= 0 evaluates to false, and the assertion fails, terminating the application.

You can omit the assertion message—for example, when it would just repeat the condition as prose.

  1. assert(age >= 0)

If the code already checks the condition, you use the assertionFailure(_:file:line:) function to indicate that an assertion has failed. For example:

  1. if age > 10 {
  2. print(“You can ride the roller-coaster or the ferris wheel.”)
  3. } else if age >= 0 {
  4. print(“You can ride the ferris wheel.”)
  5. } else {
  6. assertionFailure(“A person’s age can’t be less than zero.”)
  7. }

Enforcing Preconditions

Use a precondition whenever a condition has the potential to be false, but must definitely be true for your code to continue execution. For example, use a precondition to check that a subscript is not out of bounds, or to check that a function has been passed a valid value.

You write a precondition by calling the precondition(_:_:file:line:) function. You pass this function an expression that evaluates to true or false and a message to display if the result of the condition is false. For example:

  1. // In the implementation of a subscript…
  2. precondition(index > 0, “Index must be greater than zero.”)

You can also call the preconditionFailure(_:file:line:) function to indicate that a failure has occurred—for example, if the default case of a switch was taken, but all valid input data should have been handled by one of the switch’s other cases.


If you compile in unchecked mode (-Ounchecked), preconditions aren’t checked. The compiler assumes that preconditions are always true, and it optimizes your code accordingly. However, the fatalError(_:file:line:) function always halts execution, regardless of optimization settings.

You can use the fatalError(_:file:line:) function during prototyping and early development to create stubs for functionality that hasn’t been implemented yet, by writing fatalError("Unimplemented") as the stub implementation. Because fatal errors are never optimized out, unlike assertions or preconditions, you can be sure that execution always halts if it encounters a stub implementation.

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