Class declaration syntax in Swift - Mini Project: Build & Apply

Class declaration syntax

📖 Scenario: You are creating a simple app to keep track of pets in a pet store. Each pet has a name and an age.

🎯 Goal: Build a Swift class called Pet with properties for name and age.

📋 What You'll Learn

Create a class named Pet

Add a property name of type String

Add a property age of type Int

Create an instance of Pet with name "Buddy" and age 3

Print the pet's name and age

💡 Why This Matters

🌍 Real World

Classes help organize data and behavior for real-world objects like pets, users, or products in apps.

💼 Career

Understanding class syntax is essential for building apps in Swift, especially for iOS development.

1

Create the Pet class with properties

Create a class called Pet with two properties: name of type String and age of type Int. Do not add any methods yet.

Swift

# Your code here

Hint

Use the class keyword to create a class. Declare properties with var and specify their types.

2

Add an initializer to the Pet class

Add an initializer init(name: String, age: Int) to the Pet class that sets the name and age properties.

Swift

class Pet {
    var name: String
    var age: Int

    // Add initializer here
    
}

Hint

Use init to create a constructor. Use self to refer to the current object's properties.

3

Create an instance of Pet

Create a variable called myPet and assign it a new Pet object with name set to "Buddy" and age set to 3.

Swift

class Pet {
    var name: String
    var age: Int

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

// Create instance myPet here

Hint

Use the class name Pet followed by parentheses with arguments to create an object.

4

Print the pet's name and age

Print the name and age of myPet using print. Format the output exactly as: Buddy is 3 years old.

Swift

class Pet {
    var name: String
    var age: Int

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

var myPet = Pet(name: "Buddy", age: 3)

// Print pet details here

Hint

Use string interpolation with \(variable) inside the print statement.

Practice

1. What is the correct way to start declaring a class named Car in Swift?

easy

A. function Car() { }

B. def Car() { }

C. class Car { }

D. struct Car { }

Solution

1. Step 1: Recognize Swift class syntax

   In Swift, classes start with the keyword class followed by the class name and curly braces.

2. Step 2: Match the correct option

   class Car { } uses class Car { }, which is the correct syntax for declaring a class named Car.

3. Final Answer:

   class Car { } -> Option C

4. Quick Check:

   Swift class declaration = class ClassName { } [OK]

Hint: Look for 'class' keyword followed by name and braces [OK]

Common Mistakes:

• Using 'def' or 'function' which are not Swift keywords
• Confusing struct with class
• Missing curly braces

2. Which of the following is the correct syntax to declare a class with an initializer in Swift?

easy

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

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

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

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

Solution

1. Step 1: Understand property declaration order

   In Swift, properties should be declared before the initializer for clarity and correctness.

2. Step 2: Check initializer syntax

   The initializer must assign the parameter to the property using self.name = name.

3. Final Answer:

   class Person { var name: String; init(name: String) { self.name = name } } -> Option B

4. Quick Check:

   Properties before init, use self.property = parameter [OK]

Hint: Declare properties before init; assign with self.property = parameter [OK]

Common Mistakes:

• Declaring properties after init causing errors
• Assigning parameter to self incorrectly
• Missing semicolon or braces

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

class Animal {
    var name: String
    init(name: String) {
        self.name = name
    }
    func speak() -> String {
        return "Hello, I am \(name)"
    }
}

let dog = Animal(name: "Buddy")
print(dog.speak())

medium

A. Hello, I am name

B. Error: Missing initializer

C. Hello, I am

D. Hello, I am Buddy

Solution

1. Step 1: Understand class initialization

   The class Animal has a property name initialized via init(name: String). The instance dog is created with name "Buddy".

2. Step 2: Analyze the speak() method output

   The speak() method returns a string with the name property interpolated. So it returns "Hello, I am Buddy".

3. Final Answer:

   Hello, I am Buddy -> Option D

4. Quick Check:

   String interpolation with property = Hello, I am Buddy [OK]

Hint: Check property value passed to init and used in method [OK]

Common Mistakes:

• Confusing property name with string literal
• Forgetting to call speak() method
• Assuming default values without init

4. Identify the error in this Swift class declaration:

class Book {
    var title: String
    init(title: String) {
        title = title
    }
}

medium

A. Property 'title' is not initialized correctly

B. Missing class keyword

C. Initializer has wrong name

D. No error, code is correct

Solution

1. Step 1: Check property assignment in initializer

   The initializer assigns title = title, which assigns the parameter to itself, not to the property.

2. Step 2: Correct assignment syntax

   It should be self.title = title to assign the parameter value to the property.

3. Final Answer:

   Property 'title' is not initialized correctly -> Option A

4. Quick Check:

   Use self.property = parameter to assign [OK]

Hint: Use self.property = parameter to avoid confusion [OK]

Common Mistakes:

• Assigning parameter to itself instead of property
• Forgetting self keyword
• Assuming implicit assignment

5. You want to create a Swift class Rectangle with properties width and height, and a method area() that returns the area. Which of the following is the correct complete class declaration?

hard

A. class Rectangle {
var width: Double
var height: Double
init(width: Double, height: Double) {
self.width = width
self.height = height
}
func area() -> Double {
return width * height
}
}

B. class Rectangle {
var width: Double
var height: Double
func area() -> Double {
return width * height
}
init(width: Double, height: Double) {
width = width
height = height
}
}

C. class Rectangle {
var width: Double
var height: Double
init(width: Double, height: Double) {
width = width
height = height
}
func area() -> Double {
return width * height
}
}

D. class Rectangle {
var width: Double
var height: Double
init(width: Double, height: Double) {
self.width = width
self.height = height
}
func area() {
width * height
}
}

Solution

1. Step 1: Check property initialization

   Properties width and height must be assigned using self.width = width and self.height = height in the initializer.

2. Step 2: Verify method return type and body

   The area() method must return a Double and use return width * height.

3. Final Answer:

   class Rectangle { var width: Double var height: Double init(width: Double, height: Double) { self.width = width self.height = height } func area() -> Double { return width * height } } -> Option A

4. Quick Check:

   Use self.property in init and return in methods [OK]

Hint: Assign properties with self. and return values in methods [OK]

Common Mistakes:

• Assigning parameters to themselves without self.
• Missing return statement in method
• Incorrect method return type