Structs and Classes

Structs and classes are the main building blocks for modelling data in Swift. This lesson covers their similarities, key differences, when to use each, and related concepts like properties, methods, and initialisers.

---

Defining Structs and Classes

struct Point {
    var x: Double
    var y: Double
}

class Vehicle {
    var make: String
    var model: String
    var year: Int

    init(make: String, model: String, year: Int) {
        self.make = make
        self.model = model
        self.year = year
    }
}

Creating Instances

// Structs get a free memberwise initialiser
var origin = Point(x: 0, y: 0)

// Classes require an explicit init
let car = Vehicle(make: "Tesla", model: "Model 3", year: 2024)

---

Value Types vs Reference Types

This is the most important distinction between structs and classes:

Aspect	Struct (Value Type)	Class (Reference Type)
Assignment	Creates a copy	Creates a shared reference
Mutability	Requires var to modify	Modifiable even through let
Inheritance	Not supported	Supported
Deinitialiser	Not available	Available (deinit)
Identity	Compared by value	Compared by reference (===)
Memory	Typically stack	Heap (ARC managed)

Value Type Behaviour

var pointA = Point(x: 1, y: 2)
var pointB = pointA     // copies the value

pointB.x = 10
print(pointA.x)  // 1 — unchanged
print(pointB.x)  // 10

Reference Type Behaviour

let carA = Vehicle(make: "Toyota", model: "Camry", year: 2023)
let carB = carA          // shares the same instance

carB.model = "Corolla"
print(carA.model)  // "Corolla" — both point to the same object
print(carA === carB)  // true — same reference

---

Properties

Stored Properties

struct Rectangle {
    var width: Double
    var height: Double
}

Computed Properties

struct Rectangle {
    var width: Double
    var height: Double

    var area: Double {
        width * height
    }

    var perimeter: Double {
        get { 2 * (width + height) }
        set { width = newValue / 4; height = newValue / 4 }
    }
}

Property Observers

class StepCounter {
    var totalSteps: Int = 0 {
        willSet {
            print("About to set totalSteps to \(newValue)")
        }
        didSet {
            print("Changed from \(oldValue) to \(totalSteps)")
        }
    }
}

Lazy Properties