Which link — weak or unowned — provides higher performance when used in Swift?

In Swift, a weak reference is an optional reference that automatically becomes nil when the object it refers to is deallocated. An unowned reference is a non-optional reference that assumes the object always exists during access and does not become nil.

In terms of performance, unowned references work slightly faster because they do not require a nil check when accessed, unlike weak references, which are always optional and require checking. However, the performance difference is usually negligible and should not be a decisive factor in choosing.

Use unowned when you are sure that the reference will not become nil during use, and weak when the object might be deallocated, and you want to avoid dangling references.

Example:

class Owner {
    var pet: Pet?
}

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

Here, unowned is used because pet always exists together with owner, and owner cannot be nil during the lifetime of pet.