Classes are reference types in Swift - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a new instance of the class.

Swift

class Dog {
    var name: String
    init(name: String) {
        self.name = name
    }
}

let myDog = [1](name: "Buddy")

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

ADog()

BDog

Cdog

Dclass Dog

Attempts:

3 left

💡 Hint

Common Mistakes

Using lowercase 'dog' instead of 'Dog'.

Adding parentheses after the class name when not needed.

To create a new instance of a class, use the class name followed by parentheses with any required parameters.

2fill in blank

medium

Complete the code to change the name property of the Dog instance.

Swift

let myDog = Dog(name: "Buddy")
myDog.[1] = "Max"

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

Aname

BName()

CsetName

DName

Attempts:

3 left

💡 Hint

Common Mistakes

Using uppercase 'Name' instead of lowercase 'name'.

Trying to call a method 'setName' which does not exist.

Properties are accessed using their exact name with correct case. Here, 'name' is the property to change.

3fill in blank

hard

Fix the error in the code to make both variables refer to the same Dog instance.

Swift

let dog1 = Dog(name: "Buddy")
let dog2 = [1]
dog2.name = "Charlie"
print(dog1.name)

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

Adog1

BDog(name: "Charlie")

CDog()

Ddog2

Attempts:

3 left

💡 Hint

Common Mistakes

Creating a new Dog instance instead of assigning dog1.

Assigning dog2 to itself before initialization.

Assigning dog2 = dog1 makes both variables point to the same instance, so changes to dog2 affect dog1.

4fill in blank

hard

Fill both blanks to complete the class method that changes the dog's name.

Swift

class Dog {
    var name: String
    init(name: String) {
        self.name = name
    }
    func [1](newName: String) {
        self.[2] = newName
    }
}

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

AchangeName

Bname

CupdateName

DnewName

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'newName' as the property name instead of 'name'.

Using a method name that is not descriptive.

The method name can be 'changeName', and inside it, we assign the newName to the property 'name'.

5fill in blank

hard

Fill all three blanks to create a copy of a Dog instance and change the copy's name without affecting the original.

Swift

class Dog {
    var name: String
    init(name: String) {
        self.name = name
    }
    func copy() -> Dog {
        return Dog(name: self.[1])
    }
}

let dog1 = Dog(name: "Buddy")
let dog2 = dog1.[2]()
dog2.[3] = "Max"
print(dog1.name)

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

Aname

Bcopy

DchangeName

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning dog2 = dog1 instead of copying.

Changing dog2's name property incorrectly.

The copy method returns a new Dog with the same name. Calling dog1.copy() creates a new instance. Changing dog2.name does not affect dog1.

Practice

1. What happens when you assign an instance of a class to another variable in Swift?

easy

A. A new copy of the object is created.

B. Both variables refer to the same object in memory.

C. The original object is deleted.

D. The variables become constants automatically.

Solution

1. Step 1: Understand class assignment behavior

   In Swift, classes are reference types, so assigning one instance to another variable copies the reference, not the object.

2. Step 2: Effect of assignment on variables

   Both variables point to the same memory location, so changes via one variable affect the other.

3. Final Answer:

   Both variables refer to the same object in memory. -> Option B

4. Quick Check:

   Classes are reference types = A [OK]

Hint: Class assignment copies reference, not object [OK]

Common Mistakes:

• Thinking a new object is created on assignment
• Confusing classes with structs (value types)
• Assuming variables become constants automatically

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

easy

A. class Dog: String { var name }

B. struct Dog { var name: String }

C. class Dog { var name: String }

D. func Dog() { var name: String }

Solution

1. Step 1: Identify correct class syntax

   In Swift, a class is declared using the keyword 'class' followed by the class name and curly braces containing properties.

2. Step 2: Check each option

   class Dog { var name: String } correctly declares a class with a property. struct Dog { var name: String } declares a struct, not a class. class Dog: String { var name } has invalid syntax with inheritance from String and missing property type. func Dog() { var name: String } uses 'func' which is for functions, not classes.

3. Final Answer:

   class Dog { var name: String } -> Option C

4. Quick Check:

   Correct class syntax = A [OK]

Hint: Use 'class' keyword and braces for class declaration [OK]

Common Mistakes:

• Using 'struct' instead of 'class'
• Incorrect inheritance syntax
• Using 'func' instead of 'class'

3. What will be printed by the following Swift code?

class Counter {
  var count = 0
}

let a = Counter()
let b = a
b.count = 5
print(a.count)

medium

A. 0

B. Compilation error

C. nil

D. 5

Solution

1. Step 1: Understand reference assignment

   Variable 'b' is assigned to 'a', so both refer to the same Counter instance.

2. Step 2: Effect of changing 'b.count'

   Changing 'b.count' to 5 changes the shared object's property, so 'a.count' also becomes 5.

3. Final Answer:

   5 -> Option D

4. Quick Check:

   Class references share data = 5 [OK]

Hint: Changing one reference changes all references [OK]

Common Mistakes:

• Assuming 'a.count' stays 0 after changing 'b.count'
• Expecting a compilation error
• Confusing with value type behavior

4. Identify the error in this Swift code snippet:

class Person {
  var name: String
}

let p1 = Person()
p1.name = "Alice"

medium

A. Class Person cannot be instantiated with 'let'.

B. Cannot assign value to 'name' because it is a constant.

C. Missing initializer for 'name' property in class Person.

D. No error; code runs fine.

Solution

1. Step 1: Check class property initialization

   The class Person has a non-optional property 'name' without an initial value or initializer.

2. Step 2: Understand Swift's requirement for property initialization

   Swift requires all non-optional properties to be initialized before use, either with a default value or in an initializer.

3. Final Answer:

   Missing initializer for 'name' property in class Person. -> Option A

4. Quick Check:

   Non-optional properties need initialization = C [OK]

Hint: Non-optional properties must be initialized [OK]

Common Mistakes:

• Thinking 'let' prevents property assignment
• Assuming classes can't be instantiated with 'let'
• Ignoring property initialization rules

5. You want to create a shared settings object that multiple parts of your app can modify and see updates immediately. Which approach best uses Swift classes to achieve this?

hard

A. Create a single instance of a Settings class and pass its reference around.

B. Create multiple instances of a Settings struct and copy them as needed.

C. Use global variables instead of classes for shared data.

D. Use a function that returns a new Settings object every time.

Solution

1. Step 1: Understand shared mutable state with classes

   Classes are reference types, so a single instance shared by multiple parts allows changes to be seen everywhere.

2. Step 2: Evaluate options for shared settings

   Create a single instance of a Settings class and pass its reference around. uses one shared instance, which fits the requirement. Create multiple instances of a Settings struct and copy them as needed. uses structs (value types), so copies won't reflect changes. Use global variables instead of classes for shared data. uses globals, which is not recommended for maintainability. Use a function that returns a new Settings object every time. creates new objects each time, so no shared state.

3. Final Answer:

   Create a single instance of a Settings class and pass its reference around. -> Option A

4. Quick Check:

   Shared mutable object = B [OK]

Hint: Use one shared class instance for shared mutable data [OK]

Common Mistakes:

• Using structs which copy data instead of sharing
• Relying on global variables for sharing
• Creating new instances instead of sharing one