Data members and member functions in C++ - Mini Project: Build & Apply

Data members and member functions

📖 Scenario: You are creating a simple program to manage a book's information in a library system.

🎯 Goal: Build a C++ class called Book with data members and member functions to store and display book details.

📋 What You'll Learn

Create a class named Book

Add data members title and author of type std::string

Add a member function displayInfo() that prints the book's title and author

Create an object of Book and call displayInfo()

💡 Why This Matters

🌍 Real World

Managing book information is common in library software, bookstore inventory, and reading apps.

💼 Career

Understanding classes, data members, and member functions is fundamental for software development in C++.

1

Create the Book class with data members

Create a class called Book with two public data members: title and author, both of type std::string.

C++

#include 
#include 

// Define the Book class with title and author data members

int main() {
    return 0;
}

Hint

Use class Book { public: std::string title; std::string author; }; to define the class.

2

Add a member function to display book info

Inside the Book class, add a public member function called displayInfo() that prints the title and author separated by a comma and a space.

C++

#include 
#include 

class Book {
public:
    std::string title;
    std::string author;
    // Add displayInfo() member function here
};

int main() {
    return 0;
}

Hint

Define void displayInfo() { std::cout << title << ", " << author << std::endl; } inside the class.

3

Create a Book object and set data members

In main(), create an object called myBook of type Book. 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;
    void displayInfo() {
        std::cout << title << ", " << author << std::endl;
    }
};

int main() {
    // Create myBook object and set title and author
    
    // Your code here
}

Hint

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

4

Call the displayInfo() function to show book details

In main(), call the displayInfo() member function on the myBook object to print the book's title and author.

C++

#include 
#include 

class Book {
public:
    std::string title;
    std::string author;
    void displayInfo() {
        std::cout << title << ", " << author << std::endl;
    }
};

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

Hint

Call myBook.displayInfo(); to print the book details.

Practice

1. Which of the following best describes a data member in a C++ class?

easy

A. A keyword used to define a class

B. A function that performs actions on an object

C. A variable that holds information about an object

D. A special type of loop inside a class

Solution

1. Step 1: Understand what data members are

   Data members are variables inside a class that store information about each object created from that class.

2. Step 2: Differentiate from member functions

   Member functions are actions or behaviors, not storage. Keywords and loops are unrelated to data members.

3. Final Answer:

   A variable that holds information about an object -> Option C

4. Quick Check:

   Data member = variable storing object info [OK]

Hint: Data members store data, member functions do actions [OK]

Common Mistakes:

• Confusing data members with member functions
• Thinking data members are keywords
• Mixing data members with control structures

2. Which of the following is the correct way to declare a member function named display inside a C++ class?

easy

A. void display();

B. display void();

C. void display {}

D. function void display();

Solution

1. Step 1: Recall correct member function syntax

   Member functions are declared with return type, name, and parentheses, ending with a semicolon inside the class.

2. Step 2: Check each option

   void display(); matches correct syntax: return type 'void', function name 'display', parentheses, and semicolon. Others are invalid syntax.

3. Final Answer:

   void display(); -> Option A

4. Quick Check:

   Correct function declaration = void display(); [OK]

Hint: Member functions: returnType name(); inside class [OK]

Common Mistakes:

• Omitting parentheses after function name
• Using wrong order of return type and name
• Missing semicolon after declaration

3. What will be the output of the following C++ code?

#include <iostream>
class Box {
public:
  int length;
  void setLength(int len) {
    length = len;
  }
  int getLength() {
    return length;
  }
};

int main() {
  Box box1;
  box1.setLength(10);
  std::cout << box1.getLength();
  return 0;
}

medium

A. 0

B. Compilation error

C. length

D. 10

Solution

1. Step 1: Understand member function calls

   The function setLength(10) sets the data member length to 10 for object box1.

2. Step 2: Check output from getLength()

   The function getLength() returns the current value of length, which was set to 10.

3. Final Answer:

   10 -> Option D

4. Quick Check:

   Set length to 10, get length returns 10 [OK]

Hint: Setter sets value, getter returns it [OK]

Common Mistakes:

• Assuming default value 0 is printed
• Confusing function names
• Expecting compilation error due to missing constructor

4. Identify the error in the following C++ class code:

#include <iostream>
class Person {
  int age;
public:
  void setAge(int a) {
    age = a;
  }
  int getAge() {
    return age;
  }
};

int main() {
  Person p;
  p.age = 25;
  std::cout << p.getAge();
  return 0;
}

medium

A. Error: 'age' is private and cannot be accessed directly

B. No error, output is 25

C. Error: Missing return type for getAge

D. Error: setAge function is not called

Solution

1. Step 1: Check access to data member 'age'

   The data member 'age' is private by default because no access specifier is given before it.

2. Step 2: Analyze main function usage

   In main, code tries to access 'p.age = 25;' directly, which is not allowed for private members, causing a compilation error.

3. Final Answer:

   Error: 'age' is private and cannot be accessed directly -> Option A

4. Quick Check:

   Private members can't be accessed outside class [OK]

Hint: Private members need public functions to access [OK]

Common Mistakes:

• Trying to access private data members directly
• Ignoring default private access in classes
• Assuming no error if setter is not called

5. Given the class below, which code snippet correctly increases the count data member by 1 for an object obj?

class Counter {
private:
  int count = 0;
public:
  void increment() {
    count++;
  }
};

hard

A. obj.count = obj.count + 1;

B. obj.increment();

C. Counter::increment();

D. obj.count++; obj.increment();

Solution

1. Step 1: Understand the class design

   The class has a private data member count = 0 and provides a public member function increment() that increases count by 1.

2. Step 2: Check each option for correctness

   obj.count = obj.count + 1; tries to access private count directly, compilation error. obj.increment(); calls the member function correctly. Counter::increment(); tries to call non-static member function without an object, invalid. obj.count++; obj.increment(); first accesses private count, error.

3. Final Answer:

   obj.increment(); -> Option B

4. Quick Check:

   Use member function to change data member [OK]

Hint: Use object's member function to update data members [OK]

Common Mistakes:

• Calling member function without object
• Manually changing data member instead of using function
• Incrementing count twice accidentally