dynamic dispatch in swift

A_and_B.swift

class A {
    func foo() -> String {
        return "A foo"
    }
}

class B: A {
    override func foo() -> String {
        return "B foo"
    }
}

let a = A()
let aFoo = a.foo()    // "A foo"

let b = B()
let bFoo = b.foo()    // "B foo"

let b2 = B() as A
let b2Foo = b2.foo()  // "B foo"