Structs vs classes in iOS Swift - Practice Questions

Challenge - 5 Problems

🧠 Conceptual

intermediate

Difference in memory behavior between struct and class

Consider this Swift code where a struct and a class are assigned to new variables. What will be the output after modifying the new variables?

iOS Swift

struct Point {
  var x: Int
  var y: Int
}

class Location {
  var x: Int
  var y: Int
  init(x: Int, y: Int) {
    self.x = x
    self.y = y
  }
}

var p1 = Point(x: 1, y: 1)
var p2 = p1
p2.x = 5

var l1 = Location(x: 1, y: 1)
var l2 = l1
l2.x = 5

print(p1.x, l1.x)

A 5 5

B 1 1

C 1 5

D 5 1

💡 Hint

Think about how structs and classes are copied or referenced in Swift.

Structs are value types, so p2 is a copy of p1. Changing p2.x does not affect p1.x.

Classes are reference types, so l2 and l1 refer to the same object. Changing l2.x changes l1.x.

❓ ui_behavior

intermediate

Effect of mutating method in struct vs class

Given these Swift definitions, what will be printed after calling change() on each instance?

iOS Swift

struct CounterStruct {
  var count = 0
  mutating func change() {
    count += 1
  }
}

class CounterClass {
  var count = 0
  func change() {
    count += 1
  }
}

var cs = CounterStruct()
var cc = CounterClass()
cs.change()
cc.change()
print(cs.count, cc.count)

A 0 1

B 1 1

C 1 0

D 0 0

💡 Hint

Remember that structs need mutating keyword to change properties inside methods.

Struct's change() method is marked mutating, so it can modify count.

Class methods can modify properties without mutating keyword.

Both increase count by 1.

❓ lifecycle

advanced

Deinitialization behavior difference

Which statement about deinitializers in Swift structs and classes is true?

A Neither structs nor classes have deinitializers.

B Both structs and classes can have deinitializers.

C Structs have deinitializers but classes do not.

D Only classes can have deinitializers; structs cannot.

💡 Hint

Think about which types manage memory with reference counting.

Only classes support deinitializers because they are reference types and need cleanup when instances are destroyed.

Structs are value types and do not have deinitializers.

❓ navigation

advanced

Passing struct vs class between view controllers

You pass a struct instance and a class instance from one view controller to another. What happens if you modify the struct in the second view controller?

A The original struct in the first view controller remains unchanged.

B The original struct in the first view controller is changed.

C Both struct and class instances are changed in the first view controller.

D Neither struct nor class instances can be modified in the second view controller.

💡 Hint

Consider how value and reference types behave when passed between screens.

Structs are copied when passed, so modifying the copy does not affect the original.

Classes are passed by reference, so changes affect the original.

📝 Syntax

expert

Which code snippet correctly defines a struct with a computed property?

Select the Swift code that correctly defines a struct with a computed property named 'area' that returns width * height.

A

struct Rectangle {
  var width: Int
  var height: Int
  var area: Int {
    return width * height
  }
}

B

struct Rectangle {
  var width: Int
  var height: Int
  var area {
    return width * height
  }
}

C

struct Rectangle {
  var width: Int
  var height: Int
  var area: Int() {
    return width * height
  }
}

D

struct Rectangle {
  var width: Int
  var height: Int
  func area() -> Int {
    width * height
  }
}

💡 Hint

Computed properties use var with a type and a getter block.

Option A correctly defines a computed property with type Int and a getter returning width * height.

Option A defines a method, not a computed property.

Option A has invalid syntax with Int() after property name.

Option A misses the type declaration for area.

Practice

1. Which statement correctly describes the difference between struct and class in Swift?

easy

A. class is copied on assignment but struct is shared.

B. struct is a value type and class is a reference type.

C. struct supports inheritance but class does not.

D. struct can only have stored properties, class cannot.

Solution

1. Step 1: Understand value vs reference types

   In Swift, struct is a value type, meaning it is copied when assigned or passed around. class is a reference type, meaning instances share the same reference.

2. Step 2: Evaluate each option

   struct is a value type and class is a reference type. correctly states the difference. struct supports inheritance but class does not. is wrong because struct does not support inheritance. class is copied on assignment but struct is shared. reverses the behavior. struct can only have stored properties, class cannot. is false because class can have stored properties.

3. Final Answer:

   struct is a value type and class is a reference type. -> Option B

4. Quick Check:

   Value type = struct, Reference type = class [OK]

Hint: Remember: structs copy, classes share references [OK]

Common Mistakes:

• Confusing value and reference types
• Thinking structs support inheritance
• Assuming classes are copied on assignment

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

easy

A. class Person { var name: String }

B. struct Person: String { var name }

C. struct Person = { var name: String }

D. struct Person { var name: String }

Solution

1. Step 1: Recall Swift struct syntax

   A struct is declared with the keyword struct, followed by the name and curly braces containing properties.

2. Step 2: Check each option

   struct Person { var name: String } matches the correct syntax: struct Person { var name: String }. class Person { var name: String } uses class, not struct. struct Person = { var name: String } uses an invalid assignment syntax. struct Person: String { var name } incorrectly tries to inherit from String and misses type for name.

3. Final Answer:

   struct Person { var name: String } -> Option D

4. Quick Check:

   Struct declaration uses struct Name { } [OK]

Hint: Use 'struct Name { var property: Type }' for structs [OK]

Common Mistakes:

• Using class keyword instead of struct
• Incorrect syntax with equals sign
• Missing type annotation for properties

3. Consider the following Swift code:

struct Point {
  var x: Int
  var y: Int
}

var p1 = Point(x: 1, y: 2)
var p2 = p1
p2.x = 5
print(p1.x)

What will be printed?

medium

A. 1

B. 5

C. 0

D. Compilation error

Solution

1. Step 1: Understand struct copying behavior

   Since Point is a struct (value type), assigning p1 to p2 copies the data. They are independent.

2. Step 2: Analyze mutation and print

   Changing p2.x to 5 does not affect p1.x. So p1.x remains 1. The print statement outputs 1.

3. Final Answer:

   1 -> Option A

4. Quick Check:

   Structs copy on assignment, so original stays unchanged [OK]

Hint: Structs copy on assignment; changes don't affect original [OK]

Common Mistakes:

• Assuming p1.x changes when p2.x changes
• Confusing structs with classes (reference types)
• Expecting runtime error on mutation

4. The following Swift code tries to modify a property of a struct instance inside a function but causes a compile error:

struct Counter {
  var count: Int
}

func increment(counter: Counter) {
  counter.count += 1
}

What is the main reason for the error?

medium

A. Function parameter 'counter' is a constant by default

B. Struct properties cannot be modified inside functions

C. You cannot use '+=' operator on Int

D. Counter must be a class to modify properties

Solution

1. Step 1: Understand function parameter mutability

   In Swift, function parameters are constants by default. You cannot modify them inside the function.

2. Step 2: Analyze the error cause

   The parameter counter is a struct instance passed by value but immutable inside the function. Trying to change counter.count causes a compile error.

3. Final Answer:

   Function parameter 'counter' is a constant by default -> Option A

4. Quick Check:

   Function parameters are immutable unless marked inout [OK]

Hint: Function parameters are constants unless 'inout' is used [OK]

Common Mistakes:

• Thinking structs can't have mutable properties
• Believing '+=' is invalid for Int
• Assuming classes are required for mutation

5. You want to create a Swift type to represent a user profile that can be shared and updated across your app without copying. Which should you choose and why?

struct UserProfile {
  var name: String
  var age: Int
}

class UserProfileClass {
  var name: String
  var age: Int
  init(name: String, age: Int) {
    self.name = name
    self.age = age
  }
}

hard

A. Use struct because it copies data and prevents accidental changes.

B. Use class because structs cannot have properties.

C. Use class because it shares references and updates reflect everywhere.

D. Use struct because classes cannot have initializers.

Solution

1. Step 1: Identify sharing vs copying needs

   If you want updates to reflect everywhere (shared state), you need a reference type.

2. Step 2: Match type to behavior

   class is a reference type, so instances are shared. struct copies data, so changes won't reflect across copies.

3. Final Answer:

   Use class because it shares references and updates reflect everywhere. -> Option C

4. Quick Check:

   Shared mutable state = class [OK]

Hint: Choose class for shared mutable data, struct for copies [OK]

Common Mistakes:

• Choosing struct when shared updates are needed
• Thinking classes can't have initializers
• Believing structs can't have properties