Structs are value types (copy on assign) in Swift - Mini Project: Build & Apply

Understanding Structs as Value Types in Swift

📖 Scenario: Imagine you have a simple app that keeps track of a book's details. You want to see how changing one copy of a book's information does not affect another copy.

🎯 Goal: You will create a Book struct, make a copy of it, change the copy, and then print both to see that the original stays the same. This shows how structs in Swift are value types and copy on assign.

📋 What You'll Learn

Create a struct called Book with two properties: title and author, both of type String.

Create a variable originalBook of type Book with title "Swift Basics" and author "John Appleseed".

Create a variable copiedBook and assign it the value of originalBook.

Change the title of copiedBook to "Advanced Swift".

Print the title of both originalBook and copiedBook to show they are different.

💡 Why This Matters

🌍 Real World

Structs are used in apps to represent simple data like user profiles, settings, or items. Understanding value types helps keep data safe from accidental changes.

💼 Career

Many Swift jobs require knowing when to use structs versus classes. This knowledge helps write safer and more efficient code.

1

Create the Book struct

Create a struct called Book with two String properties: title and author.

Swift

# Your code here

Hint

Use the struct keyword and define two variables inside it.

2

Create the original Book variable

Create a variable called originalBook of type Book with title set to "Swift Basics" and author set to "John Appleseed".

Swift

struct Book {
    var title: String
    var author: String
}

# Your code here

Hint

Create a variable and use the struct's initializer with the exact values.

3

Copy originalBook to copiedBook and change the title

Create a variable called copiedBook and assign it the value of originalBook. Then change the title of copiedBook to "Advanced Swift".

Swift

struct Book {
    var title: String
    var author: String
}

var originalBook = Book(title: "Swift Basics", author: "John Appleseed")

# Your code here

Hint

Assign originalBook to copiedBook and then update the title property of copiedBook.

4

Print titles of both books

Print the title of originalBook and copiedBook on separate lines to show they are different.

Swift

struct Book {
    var title: String
    var author: String
}

var originalBook = Book(title: "Swift Basics", author: "John Appleseed")

var copiedBook = originalBook
copiedBook.title = "Advanced Swift"

# Your code here

Hint

Use two print statements, one for each book's title.

Practice

1. What happens when you assign one struct variable to another in Swift?

easy

A. A new copy of the struct data is created.

B. Both variables point to the same data in memory.

C. The original struct is deleted.

D. The struct becomes a class automatically.

Solution

1. Step 1: Understand struct assignment behavior

   Structs in Swift are value types, so assigning one struct to another copies the data.

2. Step 2: Compare with reference types

   Unlike classes, structs do not share references; each variable has its own copy.

3. Final Answer:

   A new copy of the struct data is created. -> Option A

4. Quick Check:

   Struct assignment = copy data [OK]

Hint: Structs copy data on assignment, not references [OK]

Common Mistakes:

• Thinking structs share data like classes
• Assuming assignment links variables
• Confusing structs with reference types

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

easy

A. struct Person = { var name: String }

B. struct Person { var name: String }

C. class Person { var name: String }

D. struct Person() { var name: String }

Solution

1. Step 1: Recall Swift struct syntax

   Structs are defined using the keyword struct followed by the name and braces containing properties.

2. Step 2: Identify correct syntax

   The correct syntax is struct Person { var name: String }. Using = after the name is invalid, using class defines a class instead of a struct, and adding parentheses like Person() is incorrect.

3. Final Answer:

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

4. Quick Check:

   Struct syntax = struct Name { properties } [OK]

Hint: Use 'struct Name { }' to define structs [OK]

Common Mistakes:

• Using class keyword instead of struct
• Adding parentheses after struct name
• Using equals sign in struct definition

3. Consider this Swift code:

struct Point { var x: Int; var y: Int }
var p1 = Point(x: 5, y: 10)
var p2 = p1
p2.x = 20
print(p1.x)

What is the output?

medium

A. 20

B. Compilation error

C. 5

D. 10

Solution

1. Step 1: Understand struct copying on assignment

   Assigning p1 to p2 copies the data, so p2 is a separate copy.

2. Step 2: Analyze mutation effect

   Changing p2.x to 20 does not affect p1.x, which remains 5.

3. Final Answer:

   5 -> Option C

4. Quick Check:

   Struct copy means original unchanged [OK]

Hint: Changing copy doesn't affect original struct [OK]

Common Mistakes:

• Assuming p1.x changes to 20
• Confusing structs with reference types
• Expecting runtime error on assignment

4. This Swift code tries to update a struct inside a function:

struct Counter { var count: Int }
func increment(counter: Counter) {
    var counter = counter
    counter.count += 1
}
var c = Counter(count: 0)
increment(counter: c)
print(c.count)

What is the output and why?

medium

A. 0, because the function modifies a copy, not the original

B. 1, because the function updates the original struct

C. Compilation error due to modifying parameter

D. Runtime error due to immutable struct

Solution

1. Step 1: Understand parameter passing for structs

   Structs are passed by value, so the function receives a copy of c.

2. Step 2: Analyze mutation inside function

   The function modifies its local copy, but c outside remains unchanged.

3. Final Answer:

   0, because the function modifies a copy, not the original -> Option A

4. Quick Check:

   Structs passed by value = original unchanged [OK]

Hint: Structs passed to functions are copies, not originals [OK]

Common Mistakes:

• Expecting original struct to change
• Thinking function parameter is reference
• Assuming compilation or runtime error

5. You want to create a Swift struct that tracks a score and allows updating it inside a method. Which approach correctly updates the struct's property?

hard

A. Use a global variable to hold the struct and update it.

B. Use a non-mutating method and assign a new struct to self.

C. Declare the struct as a class to allow property changes.

D. Use a mutating method inside the struct to change the property.

Solution

1. Step 1: Recall how to modify struct properties

   Structs are value types; to modify properties inside methods, the method must be marked mutating.

2. Step 2: Evaluate options

   Use a mutating method inside the struct to change the property. correctly uses a mutating method. Use a non-mutating method and assign a new struct to self. is incorrect because non-mutating methods cannot change properties. Declare the struct as a class to allow property changes. changes the type to class, which is different. Use a global variable to hold the struct and update it. uses a global variable, which is not ideal.

3. Final Answer:

   Use a mutating method inside the struct to change the property. -> Option D

4. Quick Check:

   Mutating methods update struct properties [OK]

Hint: Mark methods mutating to change struct properties [OK]

Common Mistakes:

• Trying to modify properties in non-mutating methods
• Confusing structs with classes for mutability
• Using global variables instead of methods