Swift Structs and Classes

Structs and classes let you create your own types that group data and actions together.

Structs

A struct groups related properties and methods. Swift gives every struct a free initializer, called the memberwise initializer.

Example

swift

struct Student {
    var name: String
    var marks: Int

    func report() -> String {
        marks >= 50 ? "\(name) passed" : "\(name) needs a retake"
    }
}

let ali = Student(name: "Ali", marks: 82)
let zara = Student(name: "Zara", marks: 41)
print(ali.report())
print(zara.report())
print(ali.name)

Output

Ali passed
Zara needs a retake
Ali

Mutating Methods

A struct method cannot change the struct's properties unless it is marked mutating. The struct must also be stored in a var.

Example

swift

struct Wallet {
    var balance: Int

    mutating func spend(_ amount: Int) {
        if amount <= balance {
            balance -= amount
        } else {
            print("Not enough money")
        }
    }
}

var myWallet = Wallet(balance: 500)
myWallet.spend(200)
myWallet.spend(400)
print(myWallet.balance)

Output

Not enough money
300

Classes

A class is like a struct, but it has no free initializer, so you write init yourself. Classes can also inherit from other classes.

Example

swift

class Vehicle {
    var name: String

    init(name: String) {
        self.name = name
    }

    func describe() -> String {
        "\(name) is a vehicle"
    }
}

class Bike: Vehicle {
    override func describe() -> String {
        "\(name) has two wheels"
    }
}

let car = Vehicle(name: "Suzuki Mehran")
let bike = Bike(name: "Honda 125")
print(car.describe())
print(bike.describe())

Output

Suzuki Mehran is a vehicle
Honda 125 has two wheels

self.name means "the property of this object", to tell it apart from the parameter name.
override replaces a method from the parent class.

Value Types and Reference Types

This is the most important difference between the two:

A struct is a value type. Assigning it to a new variable makes a copy.
A class is a reference type. Assigning it shares the same object.

Example

swift

struct PointS { var x = 0 }
class PointC { var x = 0 }

var s1 = PointS()
var s2 = s1
s2.x = 10
print(s1.x, s2.x)

let c1 = PointC()
let c2 = c1
c2.x = 10
print(c1.x, c2.x)

Output

0 10
10 10

Changing s2 did not change s1, because s2 is a copy. Changing c2 also changed c1, because both names point to the same object.

Struct or Class?

Feature	Struct	Class
Type	Value (copied)	Reference (shared)
Free initializer	Yes	No
Inheritance	No	Yes
Changing properties in methods	Needs `mutating`	Allowed

Apple recommends using a struct by default. Choose a class when you need inheritance or when many parts of your app must share one object.

Computed Properties

A computed property calculates its value every time you read it:

Example

swift

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

let room = Rectangle(width: 12, height: 10)
print(room.area)

Output

120.0

Exercise

What is printed? struct Box { var n = 1 } var a = Box() var b = a b.n = 5 print(a.n)

156An error