What types of references exist in Swift and how do they differ?.

In Swift, there are three main types of object references:

1. Strong references — these are the default references. An object will not be deallocated from memory as long as there is at least one strong reference to it. This is the primary mechanism for managing object lifetimes.

2. Weak references — references that do not increase the object's reference count. They are used to prevent retain cycles, for example, in delegates. Weak references are always declared as optional (weak var delegate: SomeDelegate?) because the object they refer to can be deallocated, making the reference nil.

3. Unowned references — also do not increase the reference count, but assume that the object exists for the lifetime of the reference. They are used when the owner guarantees the existence of the related object. These references are non-optional and are not automatically set to nil, so accessing them after the object has been deallocated will result in a runtime error.

Example:

class Person {
    var name: String
    weak var apartment: Apartment? // weak reference
    init(name: String) { self.name = name }
}

class Apartment {
    let unit: String
    unowned let tenant: Person // unowned reference
    init(unit: String, tenant: Person) {
        self.unit = unit
        self.tenant = tenant
    }
}

Here, Person has a weak reference to Apartment, and Apartment has an unowned reference to Person, which prevents retain cycles and ensures proper memory management.