Structs vs classes in iOS Swift - When to Use Which

The Big Idea

Discover why choosing structs or classes can save your app from hidden bugs and confusion!

The Scenario

Imagine you are building a contact list app. You try to keep track of each contact's details manually by copying and updating information everywhere in your code.

When you change a contact's phone number, you must find and update every place that holds that contact's data.

The Problem

This manual way is slow and confusing. You might forget to update some places, causing wrong or outdated info to show.

It's easy to make mistakes and hard to keep your app working well as it grows.

The Solution

Using structs and classes helps organize your data smartly. Structs make copies of data, so changes don't affect others unexpectedly.

Classes let you share data easily by reference, so updates happen everywhere automatically.

This clear difference helps you choose the right tool to keep your app's data safe and easy to manage.

Before vs After

✗ Before

var contact1 = ["name": "Alice", "phone": "123"]
var contact2 = contact1
contact2["phone"] = "456"  // manual copy, easy to mess up

✓ After

struct Contact {
  var name: String
  var phone: String
}
var contact1 = Contact(name: "Alice", phone: "123")
var contact2 = contact1
contact2.phone = "456"  // struct copy, safe and clear

What It Enables

It enables you to manage data safely and clearly, avoiding bugs and making your app easier to build and maintain.

Real Life Example

Think of a photo editing app: using classes for shared filters means changing a filter updates all photos using it, while using structs for individual photo settings keeps each photo's edits separate.

Key Takeaways

Manual data copying is error-prone and hard to maintain.

Structs create safe copies of data; classes share data by reference.

Choosing between structs and classes helps keep your app's data organized and bug-free.

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