How to use type casting in Swift

Type casting is a method used to check the type of a variable or to treat it as a different superclass/subclass. Swift implements type casting using is and as operators.

Type checking

The keyword is can be used to check the type of a variable.

Code

Explanation

Line 1: We assign a value of 4 to a variable of type Any.
Lines 3–7: We check if aVariable is an integer and print the appropriate message.
Lines 9–13: We check if aVariable is a string and print the appropriate message.

Upcasting/downcasting

A variable of a class may refer to an instance of its subclass or superclass. The keyword as can be used to upcast or downcast a variable. Upcasting is treating that variable as a superclass instance, while downcasting is treating it as a subclass instance.

There are two cast operators used for this:

as?: This operator is used when we are not sure if the downcast will succeed. If the downcast succeeds, it will return an optional value of the type we're trying to downcast to. Otherwise, it will just return nil.
as!: This operator is used when we know that the downcast will always succeed. If it does not succeed, it will throw a runtime error.

Code

class Person {
  var name: String
  init(name: String) {
    self.name = name
  }
}
class Employee: Person {
  var salary: Int
  init(name: String, salary: Int) {
    self.salary = salary
    super.init(name:name)
  }
}
class Student: Person {
  var rollNumber: Int
  init(name: String, rollNumber: Int) {
    self.rollNumber = rollNumber
    super.init(name:name)
  }
}
let anEmployee = Employee(name: "Ali", salary: 400)
let aStudent = Student(name: "Educative", rollNumber: 500)
// Forcing upcast to person
let personOne = anEmployee as Person
let personTwo = aStudent as Person
// checking downcasting with as?
print(personOne as? Student ?? "Person One is not a student")
print(personTwo as? Student ?? "Person Two is not a student")

Explanation

Lines 1–22: We declare three classes, namely Person, Student, and Employee. The classes Student and Employee inherit from the Person class.
Lines 24–29: We initialize instances of the Employee and Student classes and then upcast them to the Person class using the as operator.
Lines 32–33: We use the keyword as? to check if personOne and personTwo are instances of the Student class. The hard-coded string is the default value returned by ?? in case it gets a Nil. Otherwise, it returns the downcasted optional value of the class, which we can see when we run the code.