unowned-unsafe.swift

class Person {
    let name: String
    init(name: String) { self.name = name }
    func printName() { print("name = \(name)") }
    deinit { print("\(name) is being deinited") }
}

var p: Person? = Person(name: "Hoshi")
p?.printName()  // name = Hoshi

unowned(unsafe) let q = p!
q.printName()   // name = Hoshi

p = nil
//q.printName() // incorrect checksum for freed object - object was probably modified after being freed.

let pp = Person(name: "Takanori")
q.printName()   // name = Takanori

http://stackoverflow.com/questions/26553924/what-is-the-difference-in-swift-between-unownedsafe-and-unownedunsafe