Default constructor in C++ - Mini Project: Build & Apply

Default Constructor in C++

📖 Scenario: Imagine you are creating a simple program to represent a book in a library. Each book has a title and a number of pages. You want to create a class to hold this information and learn how to use a default constructor to set initial values.

🎯 Goal: Build a C++ class called Book with a default constructor that sets the title to "Unknown" and pages to 0. Then create an object of this class and print its details.

📋 What You'll Learn

Create a class named Book

Add two member variables: title (string) and pages (int)

Write a default constructor that sets title to "Unknown" and pages to 0

Create an object of Book using the default constructor

Print the title and pages of the object

💡 Why This Matters

🌍 Real World

Default constructors help create objects with safe starting values, useful in many programs like managing library books or user profiles.

💼 Career

Understanding constructors is essential for C++ programming jobs, especially in software development and systems programming.

1

Create the Book class with member variables

Write a class called Book with two public member variables: title of type std::string and pages of type int.

C++

#include 
#include 

// Define class Book with public members title and pages
// Your code here

Hint

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

2

Add a default constructor to Book

Inside the Book class, add a default constructor named Book() that sets title to "Unknown" and pages to 0.

C++

#include 
#include 

class Book {
public:
    std::string title;
    int pages;

    // Add default constructor here
    // Your code here
};

Hint

Write a constructor like Book() { title = "Unknown"; pages = 0; }.

3

Create a Book object using the default constructor

In the main() function, create an object named myBook of type Book using the default constructor.

C++

#include 
#include 

class Book {
public:
    std::string title;
    int pages;

    Book() {
        title = "Unknown";
        pages = 0;
    }
};

int main() {
    // Create Book object myBook
    // Your code here
    return 0;
}

Hint

Write Book myBook; inside main().

4

Print the title and pages of myBook

Add code in main() to print the title and pages of the myBook object. Use std::cout to print exactly: Title: Unknown and Pages: 0 on separate lines.

C++

#include 
#include 

class Book {
public:
    std::string title;
    int pages;

    Book() {
        title = "Unknown";
        pages = 0;
    }
};

int main() {
    Book myBook;
    // Print title and pages of myBook
    // Your code here
    return 0;
}

Hint

Use std::cout << "Title: " << myBook.title << std::endl; and similarly for pages.

Practice

1. What is a default constructor in C++?

easy

A. A constructor that requires at least one argument.

B. A constructor with no parameters that initializes objects with default values.

C. A function that deletes objects automatically.

D. A special function that runs only when the program ends.

Solution

1. Step 1: Understand constructor basics

   A constructor is a special function to create objects.

2. Step 2: Identify default constructor meaning

   A default constructor has no parameters and sets default values.

3. Final Answer:

   A constructor with no parameters that initializes objects with default values. -> Option B

4. Quick Check:

   Default constructor = no parameters, default values [OK]

Hint: Default constructor has zero parameters [OK]

Common Mistakes:

• Thinking default constructor requires arguments
• Confusing destructor with constructor
• Assuming default constructor runs at program end

2. Which of the following is the correct syntax for a default constructor in C++?

easy

A. class MyClass { MyClass() {} };

B. class MyClass { MyClass(int x) {} };

C. class MyClass { void MyClass() {} };

D. class MyClass { MyClass() int {} };

Solution

1. Step 1: Check constructor syntax

   Constructor has the same name as the class and no return type.

2. Step 2: Identify default constructor syntax

   Default constructor has empty parentheses and no parameters.

3. Final Answer:

   class MyClass { MyClass() {} }; -> Option A

4. Quick Check:

   Constructor syntax = class name + () + no return type [OK]

Hint: Constructor name = class name, no return type, empty () [OK]

Common Mistakes:

• Adding void return type to constructor
• Using parameters in default constructor
• Incorrect placement of braces

3. What will be the output of this code?

class Box {
public:
  int length;
  Box() { length = 5; }
};
int main() {
  Box b;
  std::cout << b.length;
  return 0;
}

medium

A. Compilation error

B. 0

C. 5

D. Random value

Solution

1. Step 1: Understand the default constructor

   The constructor sets length to 5 when object b is created.

2. Step 2: Check output statement

   Printing b.length outputs the value set by constructor, which is 5.

3. Final Answer:

   5 -> Option C

4. Quick Check:

   Constructor sets length=5, output = 5 [OK]

Hint: Default constructor sets values before output [OK]

Common Mistakes:

• Assuming uninitialized int defaults to 0
• Expecting compilation error due to missing initialization
• Thinking output is random without constructor

4. Identify the error in this class definition:

class Sample {
public:
  int x;
  Sample() {
    x = 10
  }
};

medium

A. Class missing closing brace

B. Constructor name does not match class name

C. Missing return type in constructor

D. Missing semicolon after x = 10

Solution

1. Step 1: Check constructor body syntax

   Each statement inside constructor must end with a semicolon.

2. Step 2: Locate missing semicolon

   The line 'x = 10' lacks a semicolon at the end.

3. Final Answer:

   Missing semicolon after x = 10 -> Option D

4. Quick Check:

   Statements need semicolons [OK]

Hint: Check for missing semicolons inside constructor [OK]

Common Mistakes:

• Adding return type to constructor
• Ignoring missing semicolon errors
• Confusing constructor name with return type

5. Given this class:

class Point {
public:
  int x, y;
  Point() { x = 0; y = 0; }
  Point(int a, int b) { x = a; y = b; }
};

// In main:
Point p1;
Point p2(3, 4);
std::cout << p1.x << "," << p1.y << " " << p2.x << "," << p2.y;

What is the output and why?

hard

A. 0,0 3,4 because p1 uses default constructor, p2 uses parameterized constructor.

B. 3,4 0,0 because constructors are swapped.

C. Compilation error due to multiple constructors.

D. 0,0 0,0 because default constructor overrides parameterized one.

Solution

1. Step 1: Understand constructor calls

   p1 is created with no arguments, so default constructor sets x=0, y=0.

2. Step 2: Understand parameterized constructor

   p2 is created with (3,4), so x=3, y=4 from parameterized constructor.

3. Step 3: Check output statement

   Output prints p1's coordinates then p2's coordinates separated by space.

4. Final Answer:

   0,0 3,4 because p1 uses default constructor, p2 uses parameterized constructor. -> Option A

5. Quick Check:

   Default and parameterized constructors work together [OK]

Hint: Default constructor for no args, parameterized for args [OK]

Common Mistakes:

• Assuming default constructor overrides parameterized
• Expecting compilation error with multiple constructors
• Mixing up which constructor is called