在下一个代码中，我尝试调用deinit方法以释放对该Person Class实例的所有引用，Mark但从未调用过deinit。为什么？

class Person{

    let name:String

    init(name:String){
        self.name = name
        println("Person created")
    }

    deinit {

        println("Person \(name) deinit")
    }
}

var Mark:Person? = Person(name:"Mark")
Mark = nil // Shouldn't the person deinit method be called here? It doesn't.