Structs vs classes in iOS Swift

Introduction

Structs and classes help you organize data and behavior in your app. They let you create your own types to keep your code clean and easy to understand.

When you want to group related data together, like a point with x and y coordinates.

When you need to create reusable blueprints for objects in your app, like a user profile.

When you want to decide if your data should be copied or shared when used in different places.

When you want to use simple data containers without inheritance.

When you want to use features like inheritance or reference sharing.

Syntax

iOS Swift

struct MyStruct {
  var property: Int
  func method() {
    // code
  }
}

class MyClass {
  var property: Int
  func method() {
    // code
  }
}

Structs use the keyword struct, classes use class.

Structs are value types, classes are reference types.

Examples

Changing p2.x does not affect p1.x because structs are copied.

iOS Swift

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

let p1 = Point(x: 10, y: 20)
var p2 = p1
p2.x = 30

Changing person2.name also changes person1.name because classes share the same instance.

iOS Swift

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

let person1 = Person(name: "Alice")
let person2 = person1
person2.name = "Bob"

Sample App

This example shows that Garage is a class (reference type), so garage1 and garage2 share the same instance. Changing garage2.car.model affects garage1.car.model. But Car is a struct (value type), so the car inside the class is not copied when assigned; both garages share the same car instance.

iOS Swift

struct Car {
  var model: String
}

class Garage {
  var car: Car
  init(car: Car) {
    self.car = car
  }
}

var car1 = Car(model: "Toyota")
var garage1 = Garage(car: car1)
var garage2 = garage1
garage2.car.model = "Honda"

print("garage1 car model: \(garage1.car.model)")
print("garage2 car model: \(garage2.car.model)")

Important Notes

Use structs for simple data that you want to copy when assigned or passed around.

Use classes when you need shared, mutable state or inheritance.

Structs come with automatic memberwise initializers, classes do not.

Summary

Structs are value types and copied on assignment.

Classes are reference types and shared on assignment.

Choose based on whether you want copying or shared behavior.

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