Swift - class method which must be overridden by subclass

Question

Is there a standard way to make a "pure virtual function" in Swift, ie. one that must be overridden by every subclass, and which, if it is not, causes a compile time error?

Answer 1

You have two options:

1. Use a Protocol

Define the superclass as a Protocol instead of a Class

Pro: Compile time check for if each "subclass" (not an actual subclass) implements the required method(s)

Con: The "superclass" (protocol) cannot implement methods or properties

2. Assert in the super version of the method

Example:

class SuperClass {     func someFunc() {         fatalError("Must Override")     } }  class Subclass : SuperClass {     override func someFunc() {     } } 

Pro: Can implement methods and properties in superclass

Con: No compile time check

Answer 2

The following allows to inherit from a class and also to have the protocol's compile time check :)

protocol ViewControllerProtocol {     func setupViews()     func setupConstraints() }  typealias ViewController = ViewControllerClass & ViewControllerProtocol  class ViewControllerClass : UIViewController {      override func viewDidLoad() {         self.setup()     }      func setup() {         guard let controller = self as? ViewController else {             return         }          controller.setupViews()         controller.setupConstraints()     }      //.... and implement methods related to UIViewController at will  }  class SubClass : ViewController {      //-- in case these aren't here... an error will be presented     func setupViews() { ... }     func setupConstraints() { ... }  }