Can you explain what is meant by 'Unowned' in memory management?

In the context of memory management in iOS (and Swift), the term "Unowned" refers to a weak reference to an object that does not increase the reference count (retain count), but assumes that the object exists during the use of this reference.

Differences from weak:

• A weak reference can become nil if the object is deallocated, and is therefore declared as optional.
• An unowned reference is not optional and does not automatically become nil. If the object referenced by unowned is deallocated, attempting to access it will lead to a runtime error (crash).

It is used when it is guaranteed that the lifetime of the object being referenced is longer than or equal to the lifetime of the reference.

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 holds an unowned reference to Owner, assuming that Owner lives longer than Pet.