Inheritance, overrides and safe casting

A school has a general person description and a specialized teacher description. Class inheritance can express a specialization, and an override replaces an inherited implementation. It should model a genuine substitutable relationship rather than merely avoid copying a few lines.

A subclass can inherit members from one superclass. Calling an overridden method through a superclass reference uses the instance’s implementation. A conditional cast with as? checks whether a reference can be viewed as a particular subtype and returns an optional instead of asserting success.

Save this complete program as main.swift and run it with the Swift toolchain. No separate input file is needed.

Swift · complete program

class Person {
  func role() -> String { "Person" }
}
class Teacher: Person {
  override func role() -> String { "Teacher" }
}
let person: Person = Teacher()
print(person.role())
print(person is Teacher)
print((person as? Teacher)?.role() ?? "Other")

Expected output

Teacher
true
Teacher

Follow the execution

The reference is declared as Person but points at a Teacher instance, so the override returns Teacher. The is check is true, and the conditional cast succeeds. The reference declaration does not replace the instance with a new superclass object.

Try this program in the Swift compiler. Paste the complete source above.

Your task: What does as? Teacher return for an ordinary Person instance?

Swift · complete program

class Person {
  func role() -> String { "Person" }
}
class Teacher: Person {
  override func role() -> String { "Teacher" }
}
let person: Person = Teacher()
print(person.role())
print(person is Teacher)
print((person as? Teacher)?.role() ?? "Other")

Worked answer

It returns nil. Handle that result rather than using a forced cast that can trap when the actual instance does not match.

Structs do not use class inheritance. Protocols can express shared capabilities without a superclass and often fit unrelated types better. Mark a class or member final when further overriding is not part of its intended contract.

It returns nil. Handle that result rather than using a forced cast that can trap when the actual instance does not match. Structs do not use class inheritance.

Further reading: official Swift language guide.