Swift 中“无主（安全）”和“无主（不安全）”之间有什么区别

无主（安全）和无主（不安全）的区别

class Person {
    var name: String
    var car: Car?
    
    init(name: String) {
        self.name = name
    }
}
class Car {
    var brand: String
    weak var owner: Person?
    
    init(brand: String) {
        self.brand = brand
    }
}
var john: Person? = Person(name: "John")
var bmw: Car? = Car(brand: "BMW")
john?.car = bmw
bmw?.owner = john
john = nil
print(bmw?.owner) // 输出nil

class Country {
    var name: String
    var capital: City
    
    init(name: String, capital: City) {
        self.name = name
        self.capital = capital
    }
}
class City {
    var name: String
    unowned var country: Country
    
    init(name: String, country: Country) {
        self.name = name
        self.country = country
    }
}
var usa: Country? = Country(name: "USA", capital: City(name: "Washington D.C.", country: usa!))
usa = nil // 这里会导致运行时错误