ARC overview for memory management in Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a strong reference to an instance.

Swift

var person: Person? = [1] Person()

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

Aunowned

Bweak

Cstrong

Dlazy

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'weak' or 'unowned' when declaring a variable without initialization.

By default, variables hold strong references to instances in Swift.

2fill in blank

medium

Complete the code to declare a weak reference to avoid retain cycles.

Swift

class Person {
    weak var friend: [1]?
}

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

APerson

BString

CInt

DBool

Attempts:

3 left

💡 Hint

Common Mistakes

Using value types like String or Int with 'weak'.

Weak references must be optional and refer to class types to avoid retain cycles.

3fill in blank

hard

Fix the error in the code to prevent a strong reference cycle.

Swift

class Customer {
    var card: [1]?
}

class CreditCard {
    var owner: Customer?
}

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

ACreditCard

Bunowned CreditCard

Cweak CreditCard

DString

Attempts:

3 left

💡 Hint

Common Mistakes

Using strong references causing retain cycles.

Using 'weak' when the reference should not be optional.

Using 'unowned' breaks the strong reference cycle when the referenced instance is expected to always exist.

4fill in blank

hard

Fill both blanks to create a dictionary with keys as names and values as their lengths, filtering names longer than 3 characters.

Swift

let names = ["Anna", "Bob", "Catherine", "Dave"]
let nameLengths = [[1]: [2] for name in names if name.count > 3]

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

Aname

Bname.count

Cname.length

Dlen(name)

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'name.length' which is not valid in Swift.

Using 'len(name)' which is Python syntax.

Use 'name' as key and 'name.count' as value to create the dictionary comprehension.

5fill in blank

hard

Fill all three blanks to create a closure that captures self weakly and prints a message.

Swift

class Printer {
    func printMessage() {
        let closure = { [[1] self] in
            print("Message from [2]")
        }
        closure()
    }
}

let printer = Printer()
printer.printMessage()

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

Aweak

Bstrong

Cself

DPrinter

Attempts:

3 left

💡 Hint

Common Mistakes

Not capturing self weakly causing retain cycles.

Using 'strong' which is not a capture keyword.

Use '[weak self]' to avoid retain cycles, refer to 'self' inside closure, and print class name 'Printer'.

Practice

1. What does ARC stand for in Swift's memory management?

easy

A. Active Reference Control

B. Automatic Resource Control

C. Asynchronous Runtime Code

D. Automatic Reference Counting

Solution

1. Step 1: Understand the acronym ARC

   ARC in Swift stands for Automatic Reference Counting, which manages memory automatically.

2. Step 2: Eliminate incorrect options

   Other options do not match the correct term used in Swift memory management.

3. Final Answer:

   Automatic Reference Counting -> Option D

4. Quick Check:

   ARC = Automatic Reference Counting [OK]

Hint: Remember ARC counts references automatically [OK]

Common Mistakes:

• Confusing ARC with manual memory management
• Thinking ARC stands for resource control
• Mixing ARC with asynchronous code concepts

2. Which of the following is the correct way to declare a class in Swift that ARC will manage?

easy

A. struct Person {}

B. class Person {}

C. func Person() {}

D. var Person = {}

Solution

1. Step 1: Identify ARC-managed types

   ARC manages memory for class instances, so classes are declared with class.

2. Step 2: Check syntax correctness

   class Person {} is the correct syntax for a class in Swift.

3. Final Answer:

   class Person {} -> Option B

4. Quick Check:

   Class declaration = class Person {} [OK]

Hint: ARC manages classes, not structs or functions [OK]

Common Mistakes:

• Using struct instead of class for ARC examples
• Trying to declare a class with func or var
• Confusing function and class syntax

3. Consider the following Swift code:

class Dog {}
var dog1: Dog? = Dog()
var dog2 = dog1
 dog1 = nil
print(dog2 == nil)

What will be printed?

medium

A. true

B. Compile error

C. false

D. nil

Solution

1. Step 1: Analyze reference assignments

   dog1 is assigned a Dog instance, dog2 copies the reference from dog1, so both point to the same object.

2. Step 2: Effect of setting dog1 to nil

   Setting dog1 to nil removes one reference, but dog2 still holds the reference, so the object remains alive.

3. Step 3: Evaluate print statement

   dog2 is not nil, so dog2 == nil is false, so it prints false.

4. Final Answer:

   false -> Option C

5. Quick Check:

   dog2 still references object = false [OK]

Hint: Setting one reference nil doesn't free object if others exist [OK]

Common Mistakes:

• Assuming dog2 becomes nil when dog1 is nil
• Confusing reference assignment with copying object
• Expecting a compile error due to optional usage

4. What is the problem with this Swift code snippet related to ARC?

class Person {
  var friend: Person?
}

var alice: Person? = Person()
alice?.friend = alice
alice = nil

medium

A. Memory leak due to strong reference cycle

B. Compile error because of optional chaining

C. No problem, memory is freed correctly

D. Runtime crash due to nil assignment

Solution

1. Step 1: Identify reference cycle

   alice references a Person instance, and that instance's friend property points back to alice, creating a strong cycle.

2. Step 2: Effect on ARC

   Because both references are strong, ARC cannot free the Person instance even after alice is set to nil, causing a memory leak.

3. Final Answer:

   Memory leak due to strong reference cycle -> Option A

4. Quick Check:

   Strong cycle prevents deallocation = memory leak [OK]

Hint: Strong references pointing to each other cause leaks [OK]

Common Mistakes:

• Thinking optional chaining causes compile error
• Assuming ARC frees memory despite cycles
• Believing nil assignment crashes program

5. You want to avoid a strong reference cycle between two classes Owner and Pet. Which is the best way to declare the reference from Pet to Owner to prevent memory leaks?

hard

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

B. Declare the reference as var owner: Owner

C. Declare the reference as unowned var owner: Owner?

D. Declare the reference as let owner: Owner

Solution

1. Step 1: Understand strong vs weak references

   Strong references increase ARC count and can cause cycles; weak references do not increase ARC count.

2. Step 2: Choose weak reference to break cycle

   Declaring owner as weak var owner: Owner? allows ARC to free objects properly by not increasing reference count.

3. Final Answer:

   Declare the reference as weak var owner: Owner? -> Option A

4. Quick Check:

   Use weak to avoid strong reference cycles [OK]

Hint: Use weak references to break cycles safely [OK]

Common Mistakes:

• Using unowned incorrectly causing crashes
• Using strong references causing memory leaks
• Forgetting to make weak references optional