What is the difference between Strong and Weak reference types in memory management and referencing model?

A strong reference is a regular strong reference to an object that increases the reference count (retain count) of that object. As long as at least one strong reference exists, the object will not be deallocated from memory.

A weak reference is a weak reference that does not increase the reference count of the object. It is used to prevent retain cycles. If the object referenced by a weak reference is deallocated, the weak reference automatically becomes nil.

Example in Swift:

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

class Apartment {
    let unit: String
    weak var tenant: Person?  // weak reference to avoid retain cycle
    init(unit: String) { self.unit = unit }
}

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

Here, the weak reference tenant does not retain the Person object, which helps prevent memory leaks.