ARC / Strong References / Retain Cycles

// Strong reference (default)
var a: MyClass? = MyClass() // Reference count: 1
var b = a // Reference count: 2
a = nil // Reference count: 1 (b still holds it)
b = nil // Reference count: 0 → released

// Pattern that causes a retain cycle
class A {
    var b: B? // A holds a strong reference to B
}
class B {
    var a: A? // B holds a strong reference to A → retain cycle!
}

sample_arc_strong_reference.swift

class Person {
    let name: String

    init(name: String) {
        self.name = name
        print("\(name) created")
    }

    // 'deinit' is a method called just before the instance is freed from memory.
    deinit {
        print("\(name) released")
    }
}

// Basic strong reference behavior
var ref1: Person? = Person(name: "user_x") // Reference count: 1
var another = ref1 // Reference count: 2

ref1 = nil // Reference count: 1 (another still holds it)
print("Set ref1 to nil")
another = nil // Reference count: 0 → released
print("Set another to nil")

// Retain cycle example (memory leak)
class Apartment {
    let unit: String
    var tenant: Resident? // Strong reference

    init(unit: String) {
        self.unit = unit
        print("Room \(unit) created")
    }
    deinit { print("Room \(unit) released") }
}

class Resident {
    let name: String
    var apartment: Apartment? // Strong reference → retain cycle!

    init(name: String) {
        self.name = name
        print("\(name) created")
    }
    deinit { print("\(name) released") }
}

// Creating a retain cycle
var apt: Apartment? = Apartment(unit: "101A")
var res1: Resident? = Resident(name: "user_y")

apt!.tenant = res1 // Apartment → Resident (strong reference)
res1!.apartment = apt // Resident → Apartment (strong reference)

apt = nil // Not released (res1 still holds a reference)
res1 = nil // Not released (apt still holds a reference)
print("--- Neither was released due to the retain cycle ---")

Running the above produces the following output:

swift arc_strong_reference.swift
user_x created
Set ref1 to nil
user_x released
Set another to nil
Room 101A created
user_y created
--- Neither was released due to the retain cycle ---

ARC does not apply to structs, enums, or primitive types. Only instances of classes (reference types) are managed by ARC.

When a retain cycle occurs, the reference count never reaches zero and deinit is never called. Retain cycles cause memory leaks. Use weak or unowned to break the cycle.

For details on resolving retain cycles with weak and unowned, see weak / unowned.