Swift Class Extension Example

In this short Swift code example, we will learn how new functionality to using Swift class using the class extension.

  • Declare an object of String data type and add to it our own custom function,
  • Add our own function to an existing Int class in Swift

If you are interested in video lessons on how to write Unit tests and UI tests to test your Swift mobile app, check out this page: Unit Testing Swift Mobile App

import UIKit

class ViewController: UIViewController {
    override func viewDidLoad() {
        // Declare String value
        let firstName:String = "Sergey"

        // Declare Int value
        let intNumber:Int=5
        // Call the "takeAway()" function we have extended the Int class with: 
        print("5 take away 4 equals \(intNumber.takeAway(4))")
        // Call the greatTheWorld() function we have extended the String class with

// Extend the String class in Swift
extension String {
    func greatTheWorld() {
         print("Hello world")

// Extend the Int class in Swift
extension Int {
    func takeAway(a:Int)->Int {
        return self-a;

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