Inheritance is a very common aspect of Object-Oriented Programming Languages. It allows you to define properties or behavior in a single class, and inherit them in another class without having to redefine them. This is shown in the following example:
class Person
let name: String
let age: int
func greet() -> void = print "Hello, \(name)!"
// constructors omitted
class Student extends Person
let email: String
func send(message: String) -> void = ...
// constructors omitted
The Student class extends, i.e. inherits from the Person class. This means that a Student is also a Person, and also has a name and an age.
let student = Student(name: "John", age: 22, email: "[email protected]")
let person = Person(name: "Alex", age: 32)
student.greet() // prints 'Hello, John!' (1)
print student.age // prints '22' (2)
print // prints '[email protected]'
person.greet() // prints 'Hello, Alex!'
print person.age // prints '32'
print // error, because Person does not have an email field
person.send("!!!") // error, because Person does not have a send(String) method
Notice how the last two statements cause a compiler error: person is not a Student, and thus does not necessarily have an email property. Neither do they have the sendMail method available for use.
The statements (1) and (2) however, are perfectly legal. The Student inherits the age property as well as the greet method from its parent class Person.