ARC in Swift: Basics and beyond

• prefer value types to avoid the dangers of unintended sharing that comes with reference types

Object lifetimes and ARC

• An object’s lifetime in Swift begins at initialization (init()) and ends at last use

• ARC automatically manages memory, by deallocating an object after its lifetime ends

• ARC determines an object’s lifetime by keeping track of its reference counts

• ARC is mainly driven by the Swift compiler which inserts retain and release operations

• At runtime, retain increments the reference count and release decrements it

• When the reference count drops to zero, the object will be deallocated

Example

Let’s say that we have the following code:

class Traveler {
  var name: String
  var destination: String?
}

func test() {
  let traveler1 = Traveler(name: "Lily")
  let traveler2 = traveler1
  traveler2.destination = "Big Sur"
  print("Done traveling")
}

In test(), first, a Traveler object is created, then its reference is copied, and finally, its destination is updated

In order to automatically manage the memory of the Traveler object, the Swift compiler inserts:

• a retain operation when a reference begins

• a release operation after the last use of the reference

Note how in test():

1. traveler1 is the first reference to the Traveler object, and its last use is the copy

2. traveler2 is another reference to the Traveler object, and its last use is the destination update

In test()’s body the Swift compiler:

• inserts a release operation immediately after the last use of the traveler1 reference

• it does not insert a retain operation when the reference begins, because initialization sets the reference count to 1

• inserts a retain operation when the traveler2 reference begins

• inserts a release operation immediately after the last use of the traveler2 reference

Let’s step through the code and see what happens at runtime:

• the Traveler object is created on the heap and initialized with a reference count of 1

• in preparation of the new traveler2 reference, the retain operation (added by the compiler) executes, incrementing the reference count to 2

• after the last use of the traveler1 reference, the release operation executes, decrementing the reference count to 1

• after the last use of the traveler2 reference, the release operation executes, decrementing the reference count to 0

• Once the reference count drops to zero, the object can be deallocated

Observable object lifetimes

• Object lifetimes in Swift are use-based

• An object’s guaranteed minimum lifetime begins at initialization and ends at last use

This is different from languages like C++, in which an object’s lifetime is guaranteed to end at the closing brace

• in the example above, we saw the object was deallocated immediately after the last use, however, in practice, object lifetimes are determined by the retain and release operations inserted by the Swift compiler

• depending on the ARC optimizations, the observed object lifetimes may differ from their guaranteed minimum, ending beyond the last use of the object

  • In such cases, the object is deallocated at a program point beyond its last use

Deinitialization side effects

In most cases, it doesn’t matter what the exact lifetime of an object is. However, with language features like weak and unowned references and deinitializer side effects, it is possible to observe object lifetimes:

• if you have programs that rely on observed object lifetimes instead of guaranteed object lifetimes, you can end up with problems in the future

• because relying on observed object lifetimes may work today, but it is only a coincidence

• Observed object lifetimes are an emergent property of the Swift compiler and can change as implementation details change

Safe techniques for using weak and unowned references

• Swift’s withExtendedLifetime() - explicitly extends the lifetime of an object

  • With this approach, you should ensure withExtendedLifetime() is used every time a weak reference has a potential to cause bugs

• Redesign to access via strong reference

• Redesign to avoid weak/unowned reference

Reference cycles can often be avoided by rethinking algorithms and transforming cyclic class relationships to tree structures.

Avoiding the need for weak and unowned references may have additional implementation cost, but this is a definite way to eliminate all potential object lifetime bugs.

Safe techniques for using deinitializer side-effects

• Swift’s withExtendedLifetime()

• Redesign to limit visibility of internal class details

• Redesign to avoid deinitializer side-effects (e.g., use defer instead of deinit)

Optimize Object Lifetimes

• New in Xcode 13, a new experimental Optimize Object Lifetimes build setting is available for the Swift compiler

• This enables powerful lifetime shortening ARC optimizations

• With this build setting turned on, you may see objects being deallocated immediately after last use much more consistently, bringing observed object lifetimes closer to their guaranteed minimum

• This may expose hidden object lifetime bugs