Swift - Inheritance and Polymorphism

Consider the following hierarchy:

classDiagram
    Person <|-- Student
    Student <|-- SchoolBandMember
    class Person {
        +firstName: String
        +lastName: String
    }
    class Student {
        +grades: [Double]
    }
    class SchoolBandMember {
        +minimumPracticeTime: Int
    }

Inheritance

Translates in Swift:

class Person {
  var firstName: String
  var lastName: String
  
  init(firstName: String, lastName: String) {
    self.firstName = firstName
    self.lastName = lastName
  }
}

class Student: Person {
  var grades: [Double] = []

  init(firstName: String, lastName: String, grades: [Double]) {
    self.grades = grades
    super.init(firstName, lastName)
  }
}

class SchoolBandMember: Student {
  var minimumPracticeTime = 2
}

(!) In swift a Class can only inherits from one Superclass

Definitions:

• Subclass: a Class that is inheriting form another Class (e.g. Student, SchoolBandMember)
• Superclass: a Class that has another Class inheriting from it (e.g. Person, Student)
• Baseclass: a Class that is NOT inheriting from another class (e.g. Person). A Baseclass has no Superclass

Class initialization

• Designated initializer: the one you implement

  • A deginated initializer must call the superclass designated initializer with super.init()

• Convenience initializer: is an initializer that calls another initalizer from the same class with self.init().

  • A Convenience initializer has the convenience keyword.
  • Ultimately, the chain of conveniente initializers must call the designated initializer from the same class.

• Two-phases initialization

  • Phase 1. initialize the stored properties.
    • call super.init() to continue Phase 1 for superclasses
  • Phase 2. self is now ready and you can access its methods.

• required

  • A required init() methods requires it to be implemented in all subclasses.

Polymorphism

Instantiate some objects:

let alice = Person(firstName: "Alice", lastName: "Red")
let bob = Student(firstName: "Bob", lastName: "White")
let carl = SchoolBandMember(firstName: "Carl", lastName: "Black")

Polymorphism: in Swift it is possible to cast from a subclass to a superclass and vice-versa.

Upcasting:

let student = carl as Student

Forced Downcasting:

let bandMember = student as! SchoolBandMember

The above downcasting may produce errors. We may do it in a safe way:

Conditional Downcasting operator:

if let bandMember = student as? SchoolBandMember {
  print("Student is a SchoolBandMember")
} else {
  print("Student is a NOT SchoolBandMember")
}

Alternative: use a switch statement.