Classes

Remarks#

The init() is a special method in classes which is used to declare an initializer for the class. More information can be found here: Initializers

Defining a Class

You define a class like this:

class Dog {}

A class can also be a subclass of another class:

class Animal {}
class Dog: Animal {}

In this example, Animal could also be a protocol that Dog conforms to.

Reference Semantics

Classes are reference types, meaning that multiple variables can refer to the same instance.

class Dog {
    var name = ""
}

let firstDog = Dog()
firstDog.name = "Fido"

let otherDog = firstDog  // otherDog points to the same Dog instance
otherDog.name = "Rover"  // modifying otherDog also modifies firstDog

print(firstDog.name)  // prints "Rover"

Because classes are reference types, even if the class is a constant, its variable properties can still be modified.

class Dog {
    var name: String // name is a variable property.
    let age: Int // age is a constant property.
    init(name: String, age: Int) {
        self.name = name
        self.age = age
    }
}

let constantDog = Dog(name: "Rover", age: 5)// This instance is a constant.
var variableDog = Dog(name: "Spot", age 7)// This instance is a variable.

constantDog.name = "Fido" // Not an error because name is a variable property.
constantDog.age = 6 // Error because age is a constant property.
constantDog = Dog(name: "Fido", age: 6)
/* The last one is an error because you are changing the actual reference, not
just what the reference points to. */

variableDog.name = "Ace" // Not an error because name is a variable property.
variableDog.age = 8 // Error because age is a constant property.
variableDog = Dog(name: "Ace", age: 8)
/* The last one is not an error because variableDog is a variable instance and
therefore the actual reference can be changed. */

Test whether two objects are identical (point to the exact same instance) using ===:

class Dog: Equatable {
    let name: String
    init(name: String) { self.name = name }
}

// Consider two dogs equal if their names are equal.
func ==(lhs: Dog, rhs: Dog) -> Bool {
    return lhs.name == rhs.name
}

// Create two Dog instances which have the same name.
let spot1 = Dog(name: "Spot")
let spot2 = Dog(name: "Spot")

spot1 == spot2   // true, because the dogs are equal
spot1 != spot2   // false

spot1 === spot2  // false, because the dogs are different instances
spot1 !== spot2  // true

Properties and Methods

Classes can define properties that instances of the class can use. In this example, Dog has two properties: name and dogYearAge:

class Dog {
    var name = ""
    var dogYearAge = 0
}

You can access the properties with dot syntax:

let dog = Dog()
print(dog.name)
print(dog.dogYearAge)

Classes can also define methods that can be called on the instances, they are declared similar to normal functions, just inside the class:

class Dog {
    func bark() {
        print("Ruff!")
    }
}

Calling methods also uses dot syntax:

dog.bark()

Classes and Multiple Inheritance

Swift does not support multiple inheritance. That is, you cannot inherit from more than one class.

class Animal { ... }
class Pet { ... }

class Dog: Animal, Pet { ... } // This will result in a compiler error.

Instead you are encouraged to use composition when creating your types. This can be accomplished by using protocols.

Remarks#

Defining a Class

Reference Semantics

Properties and Methods

Classes and Multiple Inheritance

deinit