What is the role of weak and unowned references in memory management in programming languages?

Weak and unowned references are used for memory management and preventing cyclic references that can lead to memory leaks.

• Weak references do not increase the reference count of the object. If the object they refer to is deallocated, the weak reference automatically becomes nil (in Swift). This is useful to avoid strong reference cycles, for example, between a delegate and its owner.

• Unowned references also do not increase the reference count, but they assume the object always exists during the reference's lifetime. If the object is already deallocated, accessing an unowned reference will cause a crash. They are used when the lifecycle of the referenced object is guaranteed to be longer than the reference.

Example in Swift:

class Person {
    var apartment: Apartment?
}

class Apartment {
    weak var tenant: Person?  // weak to avoid cycle
}

Here, the weak reference prevents holding the Person object in memory if it is no longer needed.