Unowned references for guaranteed lifetime in Swift

Introduction

Unowned references let you refer to another object without keeping it alive, assuming it will always exist while you use it.

When two objects refer to each other but one always outlives the other.

To avoid memory leaks caused by strong reference cycles.

When you want to keep a reference without increasing the object's lifetime.

In delegate patterns where the delegate is guaranteed to exist while the delegator exists.

Syntax

Swift

class SomeClass {
    unowned var other: OtherClass

    init(other: OtherClass) {
        self.other = other
    }
}

An unowned reference must always point to an object that exists while you use it.

If the object is deallocated, accessing an unowned reference will cause a runtime crash.

Examples

A CreditCard has an unowned reference to its Customer because the card cannot exist without the customer.

Swift

class Customer {
    let name: String
    var card: CreditCard?

    init(name: String) {
        self.name = name
    }
}

class CreditCard {
    let number: UInt64
    unowned let customer: Customer

    init(number: UInt64, customer: Customer) {
        self.number = number
        self.customer = customer
    }
}

The Department has an unowned reference to Company because the company always exists while the department exists.

Swift

class Department {
    let name: String
    unowned let company: Company

    init(name: String, company: Company) {
        self.name = name
        self.company = company
    }
}

class Company {
    let name: String
    var department: Department?

    init(name: String) {
        self.name = name
    }
}

Sample Program

This example shows a Person and an Apartment. The apartment has an unowned reference to the person because the person always exists while the apartment exists. When john is set to nil, both objects are deinitialized without a memory leak.

Swift

class Person {
    let name: String
    var apartment: Apartment?

    init(name: String) {
        self.name = name
    }

    deinit {
        print("Person \(name) is being deinitialized")
    }
}

class Apartment {
    let unit: String
    unowned let tenant: Person

    init(unit: String, tenant: Person) {
        self.unit = unit
        self.tenant = tenant
    }

    deinit {
        print("Apartment \(unit) is being deinitialized")
    }
}

var john: Person? = Person(name: "John")
john!.apartment = Apartment(unit: "4B", tenant: john!)

john = nil

Important Notes

Use unowned only when you are sure the referenced object will not be nil during the reference's lifetime.

If the referenced object might become nil, use weak instead to avoid crashes.

Accessing an unowned reference after the object is deallocated causes a runtime error.

Summary

Unowned references let you refer to another object without increasing its lifetime.

They are used when the referenced object is guaranteed to exist while you use it.

Using unowned helps avoid memory leaks from strong reference cycles.

Practice

1. What is the main purpose of using unowned references in Swift?

easy

A. To automatically nil out the reference when the object is deallocated

B. To create a strong reference that keeps the object alive

C. To make a copy of the referenced object

D. To refer to another object without increasing its lifetime

Solution

1. Step 1: Understand what unowned means

   unowned references do not increase the lifetime of the object they point to.

2. Step 2: Compare with other reference types

   Unlike strong references, unowned does not keep the object alive, and unlike weak references, it assumes the object will always exist during use.

3. Final Answer:

   To refer to another object without increasing its lifetime -> Option D

4. Quick Check:

   Unowned references = no lifetime increase [OK]

Hint: Unowned means no lifetime increase, but object must exist [OK]

Common Mistakes:

• Confusing unowned with strong references
• Thinking unowned automatically nils out
• Assuming unowned copies the object

2. Which of the following is the correct syntax to declare an unowned reference to an instance of class Person named friend?

easy

A. unowned var friend: Person

B. var friend: unowned Person

C. var friend: Person unowned

D. var unowned friend: Person

Solution

1. Step 1: Recall Swift syntax for unowned references

   The correct syntax places unowned before the variable declaration: unowned var variableName: Type.

2. Step 2: Match the options

   unowned var friend: Person matches this syntax exactly: unowned var friend: Person.

3. Final Answer:

   unowned var friend: Person -> Option A

4. Quick Check:

   Unowned keyword before var = correct syntax [OK]

Hint: Put 'unowned' before 'var' for correct syntax [OK]

Common Mistakes:

• Placing 'unowned' after the variable name
• Using 'var' after 'unowned' incorrectly
• Writing 'unowned' after the type

3. Consider the following Swift code snippet:

class Owner {
    var pet: Pet?
}

class Pet {
    unowned var owner: Owner
    init(owner: Owner) {
        self.owner = owner
    }
}

let owner = Owner()
owner.pet = Pet(owner: owner)
print(owner.pet?.owner === owner)

What will be the output?

medium

A. true

B. false

C. nil

D. Runtime error

Solution

1. Step 1: Understand the relationship between Owner and Pet

   Owner has an optional Pet, and Pet has an unowned reference back to Owner, meaning Pet expects Owner to exist while Pet exists.

2. Step 2: Analyze the print statement

   The print checks if the owner property of pet is the same instance as owner using identity operator ===. Since pet.owner was set to owner, this is true.

3. Final Answer:

   true -> Option A

4. Quick Check:

   Unowned reference points to existing object = true [OK]

Hint: Unowned references assume object exists, so identity check is true [OK]

Common Mistakes:

• Thinking unowned makes owner nil
• Confusing identity operator with equality
• Expecting runtime error due to unowned

4. Given the code below, what is the main issue causing a runtime crash?

class A {
    unowned var b: B
    init(b: B) {
        self.b = b
    }
}

class B {
    var a: A?
}

var bInstance: B? = B()
var aInstance: A? = A(b: bInstance!)
bInstance?.a = aInstance

bInstance = nil
print(aInstance!.b)
aInstance = nil

medium

A. Optional unwrapping failure

B. Unowned reference b is accessed after B is deallocated

C. Syntax error in unowned declaration

D. Strong reference cycle causing memory leak

Solution

1. Step 1: Understand the unowned reference lifetime

   The unowned reference b in class A assumes B instance exists as long as A exists.

2. Step 2: Analyze object deallocation order

   When bInstance is set to nil, B is deallocated, but A remains alive (held by aInstance). Thus b in A points to deallocated B, and print(aInstance!.b) accesses it, causing crash.

3. Final Answer:

   Unowned reference b is accessed after B is deallocated -> Option B

4. Quick Check:

   Unowned must not outlive referenced object [OK]

Hint: Unowned crashes if referenced object is gone first [OK]

Common Mistakes:

• Confusing unowned with weak references
• Ignoring object deallocation order
• Assuming unowned prevents crashes automatically

5. You have two classes, Teacher and Student, where each Student has an unowned reference to their Teacher, and each Teacher has a strong array of Student instances. Which of the following best explains why using unowned for the Student to Teacher reference is appropriate?

hard

A. Because unowned references automatically nil out when the Teacher is deallocated

B. Because the Student might outlive the Teacher, so unowned avoids nil references

C. Because the Teacher always exists as long as the Student exists, preventing strong reference cycles

D. Because unowned references increase the lifetime of the Teacher to match the Student

Solution

1. Step 1: Understand the lifetime relationship

   The Teacher owns the Student strongly, so the Teacher will exist as long as the Student does.

2. Step 2: Why use unowned here?

   Using unowned for the Student to Teacher reference avoids a strong reference cycle because Student does not keep Teacher alive.

3. Final Answer:

   Because the Teacher always exists as long as the Student exists, preventing strong reference cycles -> Option C

4. Quick Check:

   Unowned = guaranteed lifetime, no cycle [OK]

Hint: Use unowned when referenced object lifetime is guaranteed [OK]

Common Mistakes:

• Thinking unowned nils out automatically
• Assuming unowned increases lifetime
• Using unowned when referenced object may be nil