What is a garbage collector?

iOS does not have a traditional garbage collector. Instead, it uses the Automatic Reference Counting (ARC) mechanism.

• Automatic mechanism: ARC automatically counts strong references to objects.
• Memory release: When the number of strong references to an object drops to zero, ARC releases the allocated memory.
• Preventing leaks: Proper use of strong, weak, and unowned references helps prevent memory leaks and strong reference cycles.

Types of references in ARC:

• Strong: Increases the reference count. Default.
• Weak: Does not increase the reference count. Automatically becomes nil when the object is deallocated. Used to prevent strong reference cycles. Declared as an optional type.
• Unowned: Does not increase the reference count. It is guaranteed that the object it references exists for the entire lifetime of the unowned reference. Not optional. Used when two objects have a mutual strong relationship, but one has a shorter lifetime.

Advantages of ARC:

• Predictability: Memory is released deterministically as soon as the object is no longer used.
• Performance: No overhead from background garbage collection, which can cause pauses.
• Simplicity: Developers do not need to manually manage memory in most cases, but understanding reference counting principles is necessary to prevent leaks.

Example of a strong reference cycle:

class Person {
    let name: String
    var apartment: Apartment? // strong reference to Apartment

    init(name: String) { self.name = name }
    deinit { print("\(name) is being deinitialized") }
}

class Apartment {
    let unit: String
    weak var tenant: Person? // weak reference to Person to prevent cycle

    init(unit: String) { self.unit = unit }
    deinit { print("Apartment \(unit) is being deinitialized") }
}

var john: Person?
var unit4A: Apartment?

john = Person(name: "John Appleseed")
unit4A = Apartment(unit: "4A")

john!.apartment = unit4A
unit4A!.tenant = john // Using weak here is critical

john = nil
unit4A = nil
// Without weak tenant: Both john and unit4A would not be deinitialized.
// With weak tenant: Both john and unit4A are deinitialized correctly.