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

Parameterized constructor

📖 Scenario: Imagine you are creating a simple program to store information about books in a library. Each book has a title and a number of pages.

🎯 Goal: You will create a class called Book with a parameterized constructor to set the title and number of pages when creating a new book object.

📋 What You'll Learn

Create a class named Book

Add a parameterized constructor to Book that takes title and pages

Store the title and pages in member variables

Create a printDetails method to display the book's title and pages

Create a Book object with title "C++ Basics" and pages 250

Call printDetails to show the book information

💡 Why This Matters

🌍 Real World

Classes with parameterized constructors are used to create objects with specific initial data, like books in a library system.

💼 Career

Understanding constructors is essential for object-oriented programming jobs, enabling you to build flexible and reusable code.

1

Create the Book class with member variables

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

C++

// Your code here

Hint

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

2

Add a parameterized constructor to Book

Add a public parameterized constructor to the Book class that takes std::string title and int pages as parameters and sets the member variables accordingly.

C++

#include 
#include 

class Book {
private:
    std::string title;
    int pages;
public:
    // Add a parameterized constructor here
    // Your code here
};

int main() {
    return 0;
}

Hint

Use Book(std::string title, int pages) { this->title = title; this->pages = pages; } inside the class.

3

Add a method to print book details

Add a public method called printDetails to the Book class that prints the book's title and number of pages in the format: Title: [title], Pages: [pages].

C++

#include 
#include 

class Book {
private:
    std::string title;
    int pages;
public:
    Book(std::string title, int pages) {
        this->title = title;
        this->pages = pages;
    }
    // Add printDetails method here
    // Your code here
};

int main() {
    return 0;
}

Hint

Define void printDetails() { std::cout << "Title: " << title << ", Pages: " << pages << std::endl; }.

4

Create a Book object and print details

In main(), create a Book object named myBook using the parameterized constructor with title "C++ Basics" and pages 250. Then call myBook.printDetails() to display the book information.

C++

#include 
#include 

class Book {
private:
    std::string title;
    int pages;
public:
    Book(std::string title, int pages) {
        this->title = title;
        this->pages = pages;
    }
    void printDetails() {
        std::cout << "Title: " << title << ", Pages: " << pages << std::endl;
    }
};

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

Hint

Create the object with Book myBook("C++ Basics", 250); and call myBook.printDetails();.

Practice

1.

What is the main purpose of a parameterized constructor in C++?

easy

A. To initialize an object with specific values when it is created

B. To delete an object from memory

C. To copy one object to another

D. To create a function outside the class

Solution

1. Step 1: Understand constructor role

   A constructor sets up an object when it is created.

2. Step 2: Identify parameterized constructor use

   Parameterized constructors allow passing values to set object properties immediately.

3. Final Answer:

   To initialize an object with specific values when it is created -> Option A

4. Quick Check:

   Parameterized constructor = initialize with values [OK]

Hint: Think: constructor sets initial values when making an object [OK]

Common Mistakes:

• Confusing constructor with destructor
• Thinking constructors copy objects
• Believing constructors are separate functions

2.

Which of the following is the correct syntax for a parameterized constructor in C++?

class Box {
public:
    Box(int w, int h) { width = w; height = h; }
private:
    int width, height;
};

easy

A. Box(int w, int h) { width = w; height = h; }

B. void Box(int w, int h) { width = w; height = h; }

C. Box() { width = w; height = h; }

D. Box(int w, int h) -> void { width = w; height = h; }

Solution

1. Step 1: Check constructor name and return type

   Constructor must have the same name as the class and no return type.

2. Step 2: Verify parameter list and body

   Box(int w, int h) { width = w; height = h; } matches correct syntax: name matches class, parameters present, no return type.

3. Final Answer:

   Box(int w, int h) { width = w; height = h; } -> Option A

4. Quick Check:

   Constructor = class name + no return type [OK]

Hint: Constructor has no return type and matches class name [OK]

Common Mistakes:

• Adding void return type to constructor
• Using different name than class
• Missing parameters in parameterized constructor

3.

What will be the output of this code?

#include <iostream>
class Point {
public:
    int x, y;
    Point(int a, int b) { x = a; y = b; }
};

int main() {
    Point p(3, 4);
    std::cout << p.x << ", " << p.y << std::endl;
    return 0;
}

medium

A. Compilation error

B. 3, 4

C. 0, 0

D. Random values

Solution

1. Step 1: Understand constructor call

   The object p is created with values 3 and 4 passed to the constructor.

2. Step 2: Check member variable assignment and output

   The constructor sets x=3 and y=4, so printing p.x and p.y outputs "3, 4".

3. Final Answer:

   3, 4 -> Option B

4. Quick Check:

   Constructor sets values correctly = 3, 4 [OK]

Hint: Constructor sets values; output matches passed parameters [OK]

Common Mistakes:

• Assuming default values instead of passed ones
• Expecting compilation error due to missing default constructor
• Confusing output format

4.

Identify the error in this code snippet:

#include <iostream>
class Car {
public:
    int speed;
    Car(int s) { speed = s; }
};

int main() {
    Car c;
    std::cout << c.speed << std::endl;
    return 0;
}

medium

A. Variable speed is not declared

B. Incorrect syntax in constructor definition

C. Missing default constructor causes error when creating Car c;

D. Output will be garbage value

Solution

1. Step 1: Check object creation

   The line Car c; tries to create an object without parameters.

2. Step 2: Verify constructors available

   Only a parameterized constructor Car(int s) exists; no default constructor is defined.

3. Step 3: Understand compilation error

   Because no default constructor exists, Car c; causes a compilation error.

4. Final Answer:

   Missing default constructor causes error when creating Car c; -> Option C

5. Quick Check:

   No default constructor + no args object = error [OK]

Hint: If no default constructor, you must pass arguments when creating object [OK]

Common Mistakes:

• Assuming default constructor exists automatically
• Thinking variable declaration is missing
• Expecting garbage output instead of compile error

5.

Given this class, how can you create an object r with width 5 and height 10 using the parameterized constructor?

class Rectangle {
public:
    int width, height;
    Rectangle(int w, int h) { width = w; height = h; }
};

hard

A. Rectangle r = Rectangle{5};

B. Rectangle r();

C. Rectangle r = Rectangle();

D. Rectangle r = Rectangle(5, 10);

Solution

1. Step 1: Identify constructor parameters

   The constructor requires two integers: width and height.

2. Step 2: Match object creation syntax

   Rectangle r = Rectangle(5, 10); correctly calls the constructor with two arguments (5, 10) to create object r.

3. Step 3: Check other options

   Rectangle r(); is a function declaration, not object creation. Rectangle r = Rectangle(); calls default constructor which doesn't exist. Rectangle r = Rectangle{5}; passes only one argument, which is invalid.

4. Final Answer:

   Rectangle r = Rectangle(5, 10); -> Option D

5. Quick Check:

   Pass all required parameters to constructor [OK]

Hint: Call constructor with all parameters to create object [OK]

Common Mistakes:

• Using empty parentheses which declare a function
• Calling constructor without required arguments
• Passing wrong number of arguments