The Swift Programming Language--

Type Casting

1. Type check operator: is
2. Type cast operator: as? or as!

• Defining a Class Hierarchy for Type Casting
  class Person {
  var name: String
  init(name: String){
  self.name = name
  }
  }

  class Teacher: Person {
      var class: String
      init(name: String, class: String){
          self.class = class
          super.init(name)
      }
  }
  
  class Student: Person {
      var studentId: Int
      init(name: String, studentId: Int){
           self.studentId = studentId
           super.init(name)
      }
  }
  
  //campus is of type Person
  let campus = [Teacher("Ted", "Math"), Teacher("Ruth", "English"), Teacher("Paul", "Nature"), Student("Amy", 001), Student("Andy", 002)]
  

• Checking Type
  for person in campus {
  if person is Teacher {
  print("(person.name) is a teacher.")
  }
  else if person is Student {
  print("(person.name) is a student.")
  }
  }

• Down Casting
  1.as? return an optional value
  2.as! will trigger runtime error of cast fail
  3.switch case use as(forced cast operator), not as?

  //optional type cast
  for person in campus {
       if let teacher = person as? Teacher {
            print("\(teacher.name) is a \(teacher.class) teacher.")
       }
       else if let student = person as? Student {
            print("\(student.name)'s ID is \(student.studentId).")
       }
  }
  
   //forced type cast
   let teachers: [AnyObject] = [Teacher("Ted", "Math"), Teacher("Ruth", "English"), Teacher("Paul", "Nature")]
   
    for obj in teachers {
        let teacher = obj as! Teacher
        print("\(teacher.name) is a \(teacher.class) teacher.")
    }
  

The cases of a switch statement use the forced version of the type cast operator (as, not as?) to check and cast to a specific type. This check is always safe within the context of a switch case statement.