Weak references to break cycles in Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Swift code with weak references?

Consider the following Swift code that uses weak references to avoid retain cycles. What will be printed?

Swift

class Person {
    let name: String
    weak var apartment: Apartment?
    init(name: String) { self.name = name }
    deinit { print("Person \(name) is being deinitialized") }
}

class Apartment {
    let unit: String
    var tenant: Person?
    init(unit: String) { self.unit = unit }
    deinit { print("Apartment \(unit) is being deinitialized") }
}

var john: Person? = Person(name: "John")
var unit4A: Apartment? = Apartment(unit: "4A")
john!.apartment = unit4A
unit4A!.tenant = john

john = nil
unit4A = nil

A

Person John is being deinitialized
Apartment 4A is being deinitialized

B No output (memory leak due to retain cycle)

C Person John is being deinitialized only

D Apartment 4A is being deinitialized only

💡 Hint

Think about how weak references affect object lifetime and deinitialization.

Because apartment is a weak reference, when john and unit4A are set to nil, both objects are deinitialized. This avoids a retain cycle.

❓ Predict Output

intermediate

What happens if weak is replaced by strong in this Swift example?

Given the same classes as before, if weak var apartment is changed to var apartment (strong reference), what will be the output after setting john and unit4A to nil?

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

var john: Person? = Person(name: "John")
var unit4A: Apartment? = Apartment(unit: "4A")
john!.apartment = unit4A
unit4A!.tenant = john

john = nil
unit4A = nil

A

Person John is being deinitialized
Apartment 4A is being deinitialized

B No output (retain cycle prevents deinitialization)

C Person John is being deinitialized only

D Apartment 4A is being deinitialized only

💡 Hint

Consider what happens when two objects strongly reference each other.

Without the weak reference, john and unit4A strongly reference each other, causing a retain cycle. Neither is deinitialized when set to nil.

🔧 Debug

advanced

Identify the error causing a retain cycle in this Swift code

Examine the code below. It is intended to avoid retain cycles using weak references. However, a retain cycle still occurs. What is the mistake?

Swift

class Owner {
    let name: String
    weak var pet: Pet?
    init(name: String) { self.name = name }
    deinit { print("Owner \(name) deinitialized") }
}

class Pet {
    let name: String
    var owner: Owner?
    init(name: String) { self.name = name }
    deinit { print("Pet \(name) deinitialized") }
}

var alice: Owner? = Owner(name: "Alice")
var fluffy: Pet? = Pet(name: "Fluffy")
alice!.pet = fluffy
fluffy!.owner = alice

alice = nil
fluffy = nil

A The owner property in Pet should be weak to break the cycle

B Both properties should be weak to avoid retain cycles

C The pet property should not be weak; owner should be weak instead

D No retain cycle exists; code works correctly

💡 Hint

Think about which side should hold a weak reference to break the cycle.

The retain cycle occurs because owner in Pet is a strong reference. Making it weak breaks the cycle.

📝 Syntax

advanced

Which option correctly declares a weak reference in Swift?

Choose the correct syntax to declare a weak reference to an instance of class Car in Swift.

A weak let myCar: Car?

B var weak myCar: Car?

C var myCar: weak Car?

D weak var myCar: Car?

💡 Hint

Remember the order of keywords in Swift property declarations.

The correct syntax places weak before var or let. Since weak references must be variables, weak var is correct.

🚀 Application

expert

How many objects remain in memory after this Swift code runs?

Analyze the following Swift code. After setting manager and employee to nil, how many objects remain in memory?

Swift

class Manager {
    let name: String
    var employee: Employee?
    init(name: String) { self.name = name }
    deinit { print("Manager \(name) deinitialized") }
}

class Employee {
    let name: String
    weak var manager: Manager?
    init(name: String) { self.name = name }
    deinit { print("Employee \(name) deinitialized") }
}

var manager: Manager? = Manager(name: "Mia")
var employee: Employee? = Employee(name: "Eli")
manager!.employee = employee
employee!.manager = manager

manager = nil
employee = nil

A 2 objects remain; retain cycle prevents deinitialization

B 1 object remains; Manager is retained

C 0 objects remain; both are deinitialized

D 1 object remains; Employee is retained

💡 Hint

Consider which references are weak and which are strong.

The manager holds a strong reference to employee, but employee holds a weak reference back. Setting both to nil breaks the cycle and deinitializes both.

Practice

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

easy

A. To increase the performance of the program by caching values

B. To prevent strong reference cycles and allow objects to be deallocated

C. To make variables constant and unchangeable

D. To create multiple copies of an object automatically

Solution

1. Step 1: Understand what strong reference cycles are

   Strong reference cycles happen when two objects hold strong references to each other, preventing deallocation.

2. Step 2: Role of weak references

   Weak references do not increase the reference count, so they help break these cycles and allow memory to be freed.

3. Final Answer:

   To prevent strong reference cycles and allow objects to be deallocated -> Option B

4. Quick Check:

   Weak references prevent cycles = C [OK]

Hint: Weak references avoid memory cycles by not owning objects [OK]

Common Mistakes:

• Thinking weak makes variables constant
• Confusing weak with performance optimization
• Assuming weak creates copies

2. Which of the following is the correct syntax to declare a weak reference to an instance of class Person in Swift?

easy

A. weak var friend: Person?

B. weak let friend: Person?

C. weak var friend: Person

D. var weak friend: Person?

Solution

1. Step 1: Understand weak reference syntax

   Weak references must be declared with weak var and must be optional because the referenced object can be nil.

2. Step 2: Check each option

   weak var friend: Person? uses weak var friend: Person?, which is correct syntax.

3. Final Answer:

   weak var friend: Person? -> Option A

4. Quick Check:

   Weak references use 'weak var' and optional type = A [OK]

Hint: Weak references are always optional vars [OK]

Common Mistakes:

• Using 'let' with weak
• Not making the type optional
• Placing 'weak' after 'var'

3. Consider the following Swift code snippet:

class Owner {
    var pet: Pet?
}

class Pet {
    weak var owner: Owner?
}

let owner = Owner()
let pet = Pet()
owner.pet = pet
pet.owner = owner

print(owner.pet!.owner! === owner)

What is the output of this code?

medium

A. nil

B. false

C. true

D. Compilation error

Solution

1. Step 1: Understand the references

   The Owner has a strong reference to Pet, and Pet has a weak reference back to Owner.

2. Step 2: Evaluate the print statement

   The expression owner.pet!.owner! === owner checks if the pet's owner is the same instance as owner. Since pet.owner weakly references owner, this is true.

3. Final Answer:

   true -> Option C

4. Quick Check:

   Weak reference points back correctly = true [OK]

Hint: Weak references still point to original object if alive [OK]

Common Mistakes:

• Thinking weak reference becomes nil immediately
• Confusing identity check with equality
• Expecting compilation error due to weak

4. Given the following code, why does it cause a strong reference cycle?

class A {
    var b: B?
}

class B {
    var a: A?
}

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

How can you fix this cycle?

medium

A. Change a in class B to a weak reference

B. Change b in class A to a weak reference

C. Make both references strong

D. Use unowned for both references

Solution

1. Step 1: Identify the cycle

   Class A holds a strong reference to B, and B holds a strong reference back to A, causing a cycle.

2. Step 2: Break the cycle with weak reference

   Changing the reference from B to A to weak var a: A? breaks the cycle because B no longer strongly owns A.

3. Final Answer:

   Change a in class B to a weak reference -> Option A

4. Quick Check:

   Break cycle by making one side weak = D [OK]

Hint: Make one side weak to break cycles [OK]

Common Mistakes:

• Making both references weak (loses ownership)
• Making the wrong reference weak
• Using unowned incorrectly causing crashes

5. You have two classes, Teacher and Student, where each Teacher has many Students and each Student has one Teacher. How should you declare the references to avoid strong reference cycles?

hard

A. Teacher holds weak references to Students; Student holds a strong reference to Teacher

B. Both hold weak references

C. Both hold strong references

D. Teacher holds strong references to Students; Student holds a weak reference to Teacher

Solution

1. Step 1: Analyze ownership

   A Teacher owns many Students, so it should hold strong references to them.

2. Step 2: Avoid cycle by weak reference

   Each Student should hold a weak reference back to its Teacher to avoid a strong cycle.

3. Final Answer:

   Teacher holds strong references to Students; Student holds a weak reference to Teacher -> Option D

4. Quick Check:

   Owner strong, back weak = B [OK]

Hint: Owner holds strong, back reference is weak [OK]

Common Mistakes:

• Making owner reference weak
• Making both references strong causing cycles
• Making both references weak losing ownership