弱引用----weak、unowned

weak var person: Person?
unowned let person: Person
unowned var person: Person

　　weak: 必须为var，必须为可选性(一定得有'?')。对象释放后，weak指针会自动置为nil。

　　unowned：不可以是可选性(一定不可以有'?')。对象释放后，unowned指针不会自动置为nil。

class Person {
    
    var name: String
    var dog: Dog?
    
    init(name: String) {
        self.name = name
        print("person initializa")
    }
    
    deinit{
        print("person deinitialize")
    }
}

class Dog {

    var name: String
    weak var person: Person?
    
    init(name: String) {
        self.name = name
        print("dog initializa")
    }
    
    deinit{
        print("dog deinitialize")
    }
}

var person: Person? = Person(name: "Tom")
var dog: Dog? = Dog(name: "Taidi")
dog.person = person

person = nil
dog?.person // nil
dog = nil

class Person {
    
    var name: String
    var dog: Dog?
    
    init(name: String) {
        self.name = name
        print("person initializa")
    }
    
    deinit{
        print("person deinitialize")
    }
}

class Dog {

    var name: String
    unowned var person: Person
    
    init(name: String, person: Person) {
        self.name = name
        self.person = person
        print("dog initializa")
    }
    
    deinit{
        print("dog deinitialize")
    }
}

var person: Person? = Person(name: "Tom")
var dog: Dog? = Dog(name: "Taidi", person: person!)

person = nil
dog?.person　　// 会崩溃
dog = nil