A reference cycle (or retain cycle) is so named because it indicates a cycle (opens new window) in the object graph (opens new window):

Each arrow indicates one object retaining (opens new window) another (a strong reference). Unless the cycle is broken, the memory for these objects will never be freed.

A retain cycle is created when two instances of classes reference each other:

class A { var b: B? = nil }
class B { var a: A? = nil }

let a = A()
let b = B()

a.b = b  // a retains b
b.a = a  // b retains a -- a reference cycle

Both instances they will live on until the program terminates. This is a retain cycle.

Weak References

To avoid retain cycles, use the keyword weak or unowned when creating references to break retain cycles.

class B { **weak** var a: A? = nil }

Weak or unowned references will not increase the reference count of an instance. These references don't contribute to retain cycles. The weak reference becomes nil when the object it references is deallocated.

a.b = b  // a retains b
b.a = a  // b holds a weak reference to a -- not a reference cycle

When working with closures, you can also use weak and unowned in capture lists (opens new window).

When interfacing with C APIs, one might want to back off Swift reference counter. Doing so is achieved with unmanaged objects.

If you need to supply a type-punned pointer to a C function, use toOpaque method of the Unmanaged structure to obtain a raw pointer, and fromOpaque to recover the original instance:

setupDisplayLink() {
  let pointerToSelf: UnsafeRawPointer = Unmanaged.passUnretained(self).toOpaque()
  CVDisplayLinkSetOutputCallback(self.displayLink, self.redraw, pointerToSelf)
}

func redraw(pointerToSelf: UnsafeRawPointer, /* args omitted */) {
  let recoveredSelf = Unmanaged<Self>.fromOpaque(pointerToSelf).takeUnretainedValue()
  recoveredSelf.doRedraw()
}

Note that, if using passUnretained and counterparts, it's necessary to take all precautions as with unowned references.

To interact with legacy Objective-C APIs, one might want to manually affect reference count of a certain object. For that Unmanaged has respective methods retain and release. Nonetheless, it is more desired to use passRetained and takeRetainedValue, which perform retaining before returning the result:

func preferredFilenameExtension(for uti: String) -> String! {
  let result = UTTypeCopyPreferredTagWithClass(uti, kUTTagClassFilenameExtension)
  guard result != nil else { return nil }

  return result!.takeRetainedValue() as String
}

These solutions should always be the last resort, and language-native APIs sould always be preferred.

Remarks

When to use the weak-keyword:

When to use the unowned-keyword:

Pitfalls

Example:

class A : CLLocationManagerDelegate
{
    init()
    {
        let locationManager = CLLocationManager()
        locationManager.delegate = self
        locationManager.startLocationUpdates()
    }
}

This example will not work properly, as the location manager is deallocated after the initializer returns. The proper solution is to create a strong reference as an instance variable:

class A : CLLocationManagerDelegate
{
    let locationManager:CLLocationManager

    init()
    {
        locationManager = CLLocationManager()
        locationManager.delegate = self
        locationManager.startLocationUpdates()
    }
}