weak.swift

class Person {
    let name: String
    init(name: String) { self.name = name }
    var apartment: Apartment?
}
class Apartment {
    let unit: String
    init(unit: String) { self.unit = unit }
    weak var tenant: Person?
}
var john = Person(name: "John Appleseed")
var unit4A = Apartment(unit: "4A")
john.apartment = unit4A
unit4A.tenant = john