Strong reference cycles between classes in Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a class named Person.

Swift

class [1] {
    var name: String
    init(name: String) {
        self.name = name
    }
}

Drag options to blanks, or click blank then click option'

AHuman

BPerson

CAnimal

DObject

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different class name like Human or Animal.

The class is named Person, so the blank should be filled with 'Person'.

2fill in blank

medium

Complete the code to declare a strong reference property 'apartment' in Person class.

Swift

class Person {
    var name: String
    var [1]: Apartment?
    init(name: String) {
        self.name = name
    }
}

Drag options to blanks, or click blank then click option'

Aroom

Bbuilding

Capartment

Dhouse

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different property name like house or room.

The property name is 'apartment' to represent the relationship.

3fill in blank

hard

Fix the error in the Apartment class to avoid a strong reference cycle by making the tenant reference weak.

Swift

class Apartment {
    let unit: String
    [1] var tenant: Person?
    init(unit: String) {
        self.unit = unit
    }
}

Drag options to blanks, or click blank then click option'

Astrong

Blazy

Cunowned

Dweak

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'strong' or 'unowned' which do not break the cycle.

Using 'lazy' which is unrelated here.

The 'tenant' property should be declared weak to avoid a strong reference cycle.

4fill in blank

hard

Fill both blanks to complete the code that creates a strong reference cycle between Person and Apartment.

Swift

let john = Person(name: "John")
let unit4A = Apartment(unit: "4A")
john.[1] = unit4A
unit4A.[2] = john

Drag options to blanks, or click blank then click option'

Aapartment

Btenant

Cowner

Dresident

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect property names like owner or resident.

The Person's apartment property is set to unit4A, and Apartment's tenant property is set to john, creating a strong reference cycle.

5fill in blank

hard

Fill all three blanks to complete the code that breaks the strong reference cycle using weak reference and optional chaining.

Swift

class Apartment {
    let unit: String
    [1] var tenant: Person?
    init(unit: String) {
        self.unit = unit
    }
}

let john = Person(name: "John")
let unit4A = Apartment(unit: "4A")
john.[2] = unit4A
unit4A.tenant = john
unit4A.tenant?.[3] = nil

Drag options to blanks, or click blank then click option'

Aweak

Bapartment

Cname

Dstrong

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'strong' instead of 'weak' for tenant.

Using wrong property names for john's apartment or tenant's property.

The tenant property is weak, john's apartment is set, and the tenant's apartment is accessed with optional chaining to set it to nil, breaking the cycle.

Practice

1.

What causes a strong reference cycle between two Swift classes?

Consider two classes holding references to each other.

easy

A. Neither class holds any reference to the other.

B. One class holds a weak reference and the other a strong reference.

C. Both classes hold strong references to each other, preventing deallocation.

D. Both classes hold weak references to each other.

Solution

1. Step 1: Understand strong references

   Strong references keep objects alive by increasing their reference count.

2. Step 2: Identify cycle condition

   If two objects hold strong references to each other, neither can be deallocated, causing a cycle.

3. Final Answer:

   Both classes hold strong references to each other, preventing deallocation. -> Option C

4. Quick Check:

   Strong references + mutual hold = cycle [OK]

Hint: Strong references both ways cause cycles; use weak/unowned [OK]

Common Mistakes:

• Thinking weak references cause cycles
• Assuming one strong and one weak causes cycle
• Confusing unowned with strong references

2.

Which of the following is the correct way to declare a weak reference in Swift to avoid a strong reference cycle?

class Person {
    var apartment: Apartment?
}

class Apartment {
    // What should go here?
    var tenant: ???
}

easy

A. var tenant: Person

B. weak var tenant: Person?

C. unowned var tenant: Person

D. var tenant: Person?

Solution

1. Step 1: Identify weak reference syntax

   In Swift, weak references are declared with the keyword weak and must be optional.

2. Step 2: Apply to property

   So, weak var tenant: Person? is correct to avoid strong cycles.

3. Final Answer:

   weak var tenant: Person? -> Option B

4. Quick Check:

   Weak + optional = correct syntax [OK]

Hint: Weak references must be optional and use 'weak' keyword [OK]

Common Mistakes:

• Declaring weak without optional type
• Using unowned when weak is needed
• Forgetting 'weak' keyword

3.

What will be the output of the following Swift code?

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

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

medium

A. Person John is being deinitialized
Apartment 4A is being deinitialized

B. No output, memory leak occurs

C. Only Person John is being deinitialized

D. Only Apartment 4A is being deinitialized

Solution

1. Step 1: Analyze references

   Person holds strong reference to Apartment; Apartment holds weak reference to Person.

2. Step 2: Check deallocation

   Setting both variables to nil breaks strong references, so both objects deinit and print messages.

3. Final Answer:

   Person John is being deinitialized Apartment 4A is being deinitialized -> Option A

4. Quick Check:

   Weak reference avoids cycle, both deinit [OK]

Hint: Weak reference breaks cycle; both objects deinit when nil [OK]

Common Mistakes:

• Expecting memory leak output
• Thinking only one object deinitializes
• Ignoring weak reference effect

4.

Identify the error in the following Swift code that causes a strong reference cycle:

class Customer {
    let name: String
    var card: CreditCard?
    init(name: String) { self.name = name }
    deinit { print("Customer \(name) is being deinitialized") }
}

class CreditCard {
    let number: UInt64
    var customer: Customer?
    init(number: UInt64) { self.number = number }
    deinit { print("CreditCard \(number) is being deinitialized") }
}

var alice: Customer? = Customer(name: "Alice")
var card: CreditCard? = CreditCard(number: 1234567890123456)
alice!.card = card
card!.customer = alice
alice = nil
card = nil

medium

A. CreditCard's customer property should be declared weak to avoid cycle.

B. Customer's card property should be declared weak to avoid cycle.

C. Both properties should be unowned to avoid cycle.

D. No error; code works fine without changes.

Solution

1. Step 1: Identify reference types

   Both Customer.card and CreditCard.customer are strong references by default.

2. Step 2: Find cycle cause

   Since both hold strong references, they create a cycle preventing deallocation.

3. Step 3: Fix cycle

   Declaring CreditCard.customer as weak var customer: Customer? breaks the cycle.

4. Final Answer:

   CreditCard's customer property should be declared weak to avoid cycle. -> Option A

5. Quick Check:

   Weak reference breaks cycle [OK]

Hint: Make one side weak to break cycle, usually the back reference [OK]

Common Mistakes:

• Making both references weak
• Making the wrong property weak
• Ignoring the cycle causing deallocation failure

5.

You have two classes, Owner and Car. The Owner has a strong reference to Car, and Car has a strong reference to Owner. You want to avoid a strong reference cycle but also ensure that Car always has an Owner during its lifetime. How should you declare the reference from Car to Owner?

hard

A. Declare the reference as weak var owner: Owner (non-optional)

B. Declare the reference as unowned var owner: Owner

C. Declare the reference as var owner: Owner (strong reference)

D. Declare the reference as weak var owner: Owner?

Solution

1. Step 1: Understand ownership requirements

   Since Car must always have an Owner during its lifetime, the reference cannot be optional.

2. Step 2: Choose reference type

   unowned references are non-optional and do not increase reference count, avoiding cycles.

3. Step 3: Apply declaration

   Declare unowned var owner: Owner in Car to avoid cycle and ensure non-optional reference.

4. Final Answer:

   Declare the reference as unowned var owner: Owner -> Option D

5. Quick Check:

   Unowned + non-optional = correct for guaranteed lifetime [OK]

Hint: Use unowned for non-optional back references to avoid cycles [OK]

Common Mistakes:

• Using weak with non-optional type (invalid)
• Using strong reference causing cycle
• Using weak optional when non-optional needed