ReferenceCycle.swift

class Person {
    let name: String
    var apartment: Apartment?
    init(name: String) { self.name = name }
}

class Apartment {
    let unit: String
    var tenant: Person?
    init(unit: String) { self.unit = unit }
}

...

func someFunction(){
    let john = Person(name: "John Appleseed")
    let unit4A = Apartment(unit: "4A")

    john.apartment = unit4A
    unit4A.tenant = john
}