Can you explain what data types are used to represent object references in memory?

In iOS, object references in memory are represented using pointers, but with features depending on the data type and memory management.

Main types of references:

• Strong reference — a standard reference that keeps the object in memory. As long as there is at least one strong reference, the object will not be deallocated.

• Weak reference — does not keep the object in memory. If the object is deallocated, the weak reference automatically becomes nil, preventing "dangling" pointers.

• Unowned reference — similar to weak, but assumes that the object exists for the lifetime of the reference. If the object is deallocated, accessing the unowned reference will cause a crash.

Example in Swift:

class Person {
    var name: String
    init(name: String) { self.name = name }
}

var strongRef: Person? = Person(name: "Alice") // strong reference
weak var weakRef = strongRef                 // weak reference
unowned var unownedRef = strongRef!          // unowned reference

strongRef = nil // the object will be deallocated, weakRef will become nil, unownedRef will become invalid

Thus, reference types help manage object lifetime and prevent memory leaks and retain cycles.