weak.swift

class Person {
    
    let name: String
    var apartment: Apartment?
    
    init(name: String) {
        self.name = name
    }
    deinit {
        print("\(name) is being deinitialized, memory deallocated")
    }
}

class Apartment {
    
    let unit: String
    weak var tenant : Person?
    
    init(unit:String) {
        self.unit = unit
    }
    
    deinit {
        print("Apartment \(unit) is being deinitialized, memory deallocated")
    }
}

var person: Person? = Person(name: "Sasha")
var apartment: Apartment? = Apartment(unit: "4f")

person?.apartment = apartment
apartment?.tenant = person
person = nil
// prints "Sasha is being deinitialized, memory deallocated"