What is the difference between `weak` and `strong` references in Swift?

In Swift, strong and weak references relate to memory management and help avoid retain cycles.

• Strong reference — this is a regular reference that increases the reference count of an object. As long as there is at least one strong reference, the object will not be deallocated.

• Weak reference — this is a reference that does not increase the reference count. It is used for references that should not keep the object in memory to prevent retain cycles.

If the object referenced by a weak reference is deallocated, the weak reference automatically becomes nil.

Example:

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

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

var john: Person? = Person(name: "John")
var unit4A: Apartment? = Apartment(unit: "4A")

john!.apartment = unit4A
unit4A!.tenant = john

john = nil  // Person object will be deallocated, weak reference tenant becomes nil

Thus, weak references help prevent memory leaks by allowing objects to reference each other without creating retain cycles.