What is the purpose and use of weak references with the unowned keyword in programming?

In Swift, the keyword unowned is used to create weak references that do not increase the reference count of an object, but it is assumed that the object will exist for the lifetime of the reference. This helps to avoid strong reference cycles (retain cycles), which lead to memory leaks.

The difference between unowned and weak is that unowned is not an optional reference — it always assumes the object exists. If the object is deallocated and an unowned reference is accessed, the application will crash with an error.

unowned is used in cases where one entity owns another, and the second references the first but should not keep it in memory. For example, delegates are often declared as unowned.

Example:

class Owner {
    var pet: Pet?
}

class Pet {
    unowned let owner: Owner
    init(owner: Owner) {
        self.owner = owner
    }
}

let owner = Owner()
owner.pet = Pet(owner: owner)

Here, Pet references Owner through unowned to avoid a strong reference cycle.