Why classes exist alongside structs in Swift - Performance Analysis

Time Complexity: Why classes exist alongside structs

O(n)

Understanding Time Complexity

When we look at why Swift has both classes and structs, it helps to think about how their performance changes as we use them more.

We want to understand how the cost of using classes or structs grows when we work with many objects.

Scenario Under Consideration

Analyze the time complexity of creating and copying structs versus classes.

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

class Circle {
  var center: Point
  var radius: Int
  init(center: Point, radius: Int) {
    self.center = center
    self.radius = radius
  }
}

// Creating many instances
var points = [Point]()
var circles = [Circle]()
for i in 0..

This code creates many Point structs and Circle class instances, showing how they behave when copied or referenced.

Identify Repeating Operations

Look at what repeats as we create many objects.

• Primary operation: Creating and storing each Point struct and Circle class instance.
• How many times: Exactly n times, once per loop iteration.

How Execution Grows With Input

As n grows, the number of creations grows the same way.

Input Size (n)	Approx. Operations
10	10 creations of structs and classes
100	100 creations of structs and classes
1000	1000 creations of structs and classes

Pattern observation: The work grows directly with n, doubling n doubles the work.

Final Time Complexity

Time Complexity: O(n)

This means the time to create and store these objects grows in a straight line with the number of items.

Common Mistake

[X] Wrong: "Classes and structs always have the same performance because they both hold data."

[OK] Correct: Structs copy all their data when assigned or passed, which can be costly for large data, while classes use references, so copying is cheaper but managing references adds overhead.

Interview Connect

Understanding why Swift has both classes and structs helps you explain design choices clearly and shows you know how performance can change with different data types.

Self-Check

"What if we changed the struct Point to have many properties or nested structs? How would the time complexity of copying change?"

Practice

1. Why do classes exist alongside structs in Swift?

easy

A. Because classes cannot store data, only methods

B. Because structs are slower than classes in all cases

C. Because classes support shared references and inheritance, while structs do not

D. Because structs allow inheritance but classes do not

Solution

1. Step 1: Understand the difference between classes and structs

   Structs are value types, meaning each copy is independent. Classes are reference types, meaning multiple variables can share the same instance.

2. Step 2: Identify why classes are needed

   Classes allow shared data through references and support inheritance, which structs do not.

3. Final Answer:

   Because classes support shared references and inheritance, while structs do not -> Option C

4. Quick Check:

   Classes = shared references + inheritance [OK]

Hint: Classes share data and support inheritance; structs do not [OK]

Common Mistakes:

• Thinking structs support inheritance
• Believing classes cannot store data
• Assuming structs are always slower

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

easy

A. struct MyClass { var name: String }

B. class MyClass { var name: String }

C. func MyClass() { var name: String }

D. let MyClass = class { var name: String }

Solution

1. Step 1: Recall Swift syntax for class declaration

   Classes in Swift are declared using the keyword class followed by the class name and braces.

2. Step 2: Check each option

   class MyClass { var name: String } correctly uses class MyClass { var name: String }. struct MyClass { var name: String } uses struct, not class. func MyClass() { var name: String } uses func, which is incorrect. let MyClass = class { var name: String } uses invalid syntax.

3. Final Answer:

   class MyClass { var name: String } -> Option B

4. Quick Check:

   Class declaration uses 'class' keyword [OK]

Hint: Use 'class' keyword to declare a class in Swift [OK]

Common Mistakes:

• Using 'struct' instead of 'class'
• Trying to declare class inside a function
• Incorrect syntax with 'let' or 'func'

3. What will be the output of this Swift code?

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

var p1 = Person(name: "Alice")
var p2 = p1
p2.name = "Bob"
print(p1.name)

medium

A. Bob

B. Alice

C. Compilation error

D. nil

Solution

1. Step 1: Understand class reference behavior

   Classes are reference types, so p1 and p2 point to the same object.

2. Step 2: Analyze the code changes

   Changing p2.name to "Bob" changes the shared object. Printing p1.name shows "Bob".

3. Final Answer:

   Bob -> Option A

4. Quick Check:

   Class variables share references, so name = Bob [OK]

Hint: Classes share instances; changing one changes all references [OK]

Common Mistakes:

• Thinking p1 and p2 are independent copies
• Expecting 'Alice' as output
• Confusing class with struct behavior

4. Identify the error in this Swift code that tries to use inheritance with a struct:

struct Vehicle {
  var speed: Int
}

struct Car: Vehicle {
  var brand: String
}

medium

A. Cannot declare variables inside structs

B. Missing initializer in Car struct

C. Syntax error: colon should be replaced with comma

D. Structs cannot inherit from other structs or classes

Solution

1. Step 1: Understand inheritance rules in Swift

   In Swift, only classes support inheritance. Structs cannot inherit from other structs or classes.

2. Step 2: Analyze the code

   The code tries to make Car inherit from Vehicle, both structs, which is invalid.

3. Final Answer:

   Structs cannot inherit from other structs or classes -> Option D

4. Quick Check:

   Inheritance = classes only, structs no [OK]

Hint: Only classes support inheritance, not structs [OK]

Common Mistakes:

• Thinking structs can inherit
• Confusing initializer errors with inheritance
• Misreading colon syntax as error

5. You want to create a data model where multiple parts of your app share and modify the same user profile data. Which Swift type should you choose and why?

hard

A. Use a class because it allows shared references and identity

B. Use a class because structs cannot store properties

C. Use a struct because it supports inheritance for flexibility

D. Use a struct because it copies data safely between parts

Solution

1. Step 1: Identify the need for shared data

   The requirement is that multiple parts share and modify the same user profile data, meaning shared references are needed.

2. Step 2: Choose the correct Swift type

   Classes are reference types that allow shared references and identity, so changes in one place reflect everywhere. Structs copy data, so changes do not propagate.

3. Final Answer:

   Use a class because it allows shared references and identity -> Option A

4. Quick Check:

   Shared data needs class reference type [OK]

Hint: Shared mutable data needs classes, not structs [OK]

Common Mistakes:

• Choosing structs for shared mutable data
• Thinking structs support inheritance
• Believing classes cannot store properties