Have you used reference variables like Unowned in your projects?

Yes, in iOS projects I have used reference variables of the unowned type.

unowned is a type of weak reference that does not increase the reference count, but assumes that the object it references will always exist during its use.

This is useful for preventing strong reference cycles, for example, in closures or mutual references between objects, when one object is guaranteed to outlive the other.

An example of using unowned in a closure:

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

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

let john = Person(name: "John")
let apt = Apartment(unit: "4A", tenant: john)
john.apartment = apt

Here, tenant is declared as unowned because the apartment should not exist without a tenant, which prevents a strong reference cycle.