Class definition syntax in C++ - Mini Project: Build & Apply

Class definition syntax

📖 Scenario: You are creating a simple program to represent a book in a library system.

🎯 Goal: Build a C++ class called Book with basic information and display it.

📋 What You'll Learn

Create a class named Book

Add two public member variables: title and author of type std::string

Create an object of class Book with specific values

Print the book's title and author

💡 Why This Matters

🌍 Real World

Classes like <code>Book</code> help organize data in programs such as library management systems or book stores.

💼 Career

Understanding class syntax is essential for software development jobs that use C++ for building applications with structured data.

1

Create the Book class with public member variables

Write a class definition named Book with two public std::string variables: title and author.

C++

#include 
#include 

// Define class Book with public variables title and author
// Your code here

Hint

Use class Book { public: std::string title; std::string author; };

2

Create a Book object with specific values

Create a Book object named myBook. Set myBook.title to "The Great Gatsby" and myBook.author to "F. Scott Fitzgerald".

C++

#include 
#include 

class Book {
public:
    std::string title;
    std::string author;
};

// Create Book object myBook and set title and author
// Your code here

Hint

Use Book myBook; then assign values with myBook.title = "The Great Gatsby"; and myBook.author = "F. Scott Fitzgerald";

3

Add code to print the book's title and author

Inside main(), add std::cout statements to print myBook.title and myBook.author on separate lines.

C++

#include 
#include 

class Book {
public:
    std::string title;
    std::string author;
};

int main() {
    Book myBook;
    myBook.title = "The Great Gatsby";
    myBook.author = "F. Scott Fitzgerald";
    // Print title and author
    // Your code here
    return 0;
}

Hint

Use std::cout << myBook.title << std::endl; and similarly for author.

4

Run the program to display the book details

Run the program. It should print the title The Great Gatsby on the first line and the author F. Scott Fitzgerald on the second line.

C++

#include 
#include 

class Book {
public:
    std::string title;
    std::string author;
};

int main() {
    Book myBook;
    myBook.title = "The Great Gatsby";
    myBook.author = "F. Scott Fitzgerald";
    std::cout << myBook.title << std::endl;
    std::cout << myBook.author << std::endl;
    // Run the program to see output
    // Your code here
    return 0;
}

Hint

Make sure you run the program and check the output matches exactly.

Practice

1. What is the correct way to start defining a class named Car in C++?

easy

A. class Car { };

B. define Car { };

C. Car class { };

D. class = Car { };

Solution

1. Step 1: Recognize the class keyword usage

   In C++, a class is defined using the keyword class followed by the class name and curly braces.

2. Step 2: Check the syntax correctness

   The correct syntax is class Car { }; with a semicolon at the end.

3. Final Answer:

   class Car { }; -> Option A

4. Quick Check:

   class keyword + name + braces + semicolon = correct [OK]

Hint: Use 'class ClassName { };' to define a class [OK]

Common Mistakes:

• Omitting the semicolon after the class definition
• Using 'define' instead of 'class'
• Placing '=' between class and name

2. Which of the following is the correct syntax to declare a public member function start() inside a class?

easy

A. public void start();

B. void public start();

C. void start() public;

D. public: void start();

Solution

1. Step 1: Understand access specifier placement

   In C++, access specifiers like public: are labels followed by a colon, not keywords before functions.

2. Step 2: Identify correct function declaration

   The function declaration inside the class is void start(); placed under the public: label.

3. Final Answer:

   public: void start(); -> Option D

4. Quick Check:

   Access specifier label + function declaration = correct [OK]

Hint: Use 'public:' label before members, not before return type [OK]

Common Mistakes:

• Writing 'public' before return type without colon
• Placing 'public' after function parentheses
• Using 'void public' which is invalid

3. What will be the output of this code?

class Box {
public:
  int length;
};

int main() {
  Box b;
  b.length = 10;
  std::cout << b.length << std::endl;
  return 0;
}

medium

A. Compilation error

B. 10

C. 0

D. Undefined behavior

Solution

1. Step 1: Analyze class and member access

   The class Box has a public integer member length, so it can be accessed and assigned outside the class.

2. Step 2: Trace the main function execution

   An object b is created, b.length is set to 10, then printed. This will output 10.

3. Final Answer:

   10 -> Option B

4. Quick Check:

   Public member assigned 10, printed 10 [OK]

Hint: Public members can be accessed and printed directly [OK]

Common Mistakes:

• Assuming private by default causes error
• Forgetting to assign value before printing
• Expecting zero because of uninitialized variable

4. Identify the error in this class definition:

class Person {
  string name;
public:
  void setName(string n) {
    name = n;
  }
};

medium

A. Missing semicolon after class definition

B. Member variable 'name' should be public

C. Missing #include <string> and std:: prefix

D. Function setName should return string

Solution

1. Step 1: Check for syntax errors

   The class ends with a semicolon, so no missing semicolon error.

2. Step 2: Check usage of string type

   The code uses string without including <string> header and without std:: prefix, which causes compilation error.

3. Final Answer:

   Missing #include <string> and std:: prefix -> Option C

4. Quick Check:

   Use #include and std::string for string type [OK]

Hint: Always include <string> and use std::string in C++ [OK]

Common Mistakes:

• Assuming string is built-in without include
• Thinking private members must be public to work
• Expecting setName to return a value

5. You want to define a class Rectangle with private members width and height, and a public function area() that returns the area. Which is the correct class definition?

hard

A. class Rectangle {
private:
int width, height;
public:
int area() { return width * height; }
};

B. class Rectangle {
public:
int width, height;
int area() { return width * height; }
};

C. class Rectangle {
int width, height;
int area() { return width * height; }
};

D. class Rectangle {
private:
int width, height;
int area() { return width * height; }
};

Solution

1. Step 1: Identify private members declaration

   Private members must be declared under private: label to restrict access.

2. Step 2: Confirm public function declaration

   The function area() should be public to allow access from outside.

3. Step 3: Check for correct syntax and placement

   class Rectangle { private: int width, height; public: int area() { return width * height; } }; correctly places width and height as private and area() as public with proper syntax.

4. Final Answer:

   class Rectangle { private: int width, height; public: int area() { return width * height; } }; -> Option A

5. Quick Check:

   Private data + public method = correct class design [OK]

Hint: Use 'private:' for data, 'public:' for functions in class [OK]

Common Mistakes:

• Forgetting access specifiers, making members public by default
• Placing function inside private when it should be public
• Omitting semicolon after class definition