In what situations should the `unowned` keyword be used in Swift?

The unowned keyword in Swift is used to declare a weak reference that is not optional and does not increase the reference count of an object.

It is used in situations where there is a cyclic reference between objects, but it is guaranteed that the object being referenced will exist longer than the unowned reference.

For example, in a parent-child relationship, where the child holds an unowned reference to the parent to avoid a cycle:

class Parent {
    var child: Child?
}

class Child {
    unowned let parent: Parent
    init(parent: Parent) {
        self.parent = parent
    }
}

let dad = Parent()
let kid = Child(parent: dad)
dad.child = kid

If the object referenced by unowned is deallocated, attempting to access this reference will lead to a runtime error. Therefore, unowned is suitable when the lifecycle of the object is guaranteed to be longer than the reference to it.