Dot notation

Internal methods

struct Person {
  var name: String
  func sayHello() {
    print("Hello, there! My name is \(name)!")


Whenever you define a new type, you must consider how you'll create new instances. This lesson covers different approaches to initializing property values.

Memberwise Initialisers

Custom Initialisers

Instance Methods


Mutating Methods

struct Odometer {
  var count: Int = 0 // Assigns a default value to the `count` 
  mutating func increment() {
    count += 1
  mutating func increment(by amount: Int) {
    count += amount
  mutating func reset() {
    count = 0
var odometer = Odometer() // odometer.count defaults to 0
odometer.increment() // odometer.count is incremented to 1
odometer.increment(by: 15) // odometer.count is incremented to
odometer.reset() // odometer.count is reset to 0

Computed Properties

With computed properties, you can create properties that can compute their value based on other instance properties or logic.  

struct Temperature {
  var celsius: Double
  var fahrenheit: Double {
    return celsius * 1.8 + 32
  var kelvin: Double {
    return celsius + 273.15

To add a computed property, you declare the property as a variable (because its value can change). You must also explicitly declare the type. Then you use an open curly brace ({) and closing curly brace (}) to define the logic that calculates the value to return. You can access computed properties using dot syntax, just as you would with any other property.

let currentTemperature = Temperature(celsius: 0.0)
Console Output:

Property Observers

willSet and didSet

struct StepCounter {
    var totalSteps: Int = 0 {
        willSet {
            print("About to set totalSteps to \(newValue)")
        didSet {
            if totalSteps > oldValue  {
                print("Added \(totalSteps - oldValue) steps")

Static (Type) Properties and Methods

These are defined on the type, not on the instances and can be got at with dot notation.


structures are passed by copying

Self and Shadowing

The self keyword refers to the current instance of the type. It's usually possible for the Swift compiler to infer it.

The use of self is required within initializers that have parameter names that match property names.

struct Temperature {
  var celsius: Double
  init(celsius: Double) {
    self.celsius = celsius

This concept is called shadowing, and you will learn more about it in a future lesson.

Let vs Var

As a general rule, you should use let whenever possible to define an instance of a structure, use var if the instance needs to be mutated, and use var when defining the properties of a structure.

Excerpt From: Apple Education. “App Development with Swift.” Apple Inc. - Education, 2017. iBooks.

Swift Classes and Inheritance