Object interaction in C++ - Mini Project: Build & Apply

Object interaction

📖 Scenario: You are creating a simple program to manage a library. The library has books, and each book has a title and an author. You want to show how two objects, Library and Book, can work together.

🎯 Goal: Build two classes, Book and Library, where the Library holds a list of Book objects. You will add books to the library and then print the details of all books in the library.

📋 What You'll Learn

Create a class Book with two public string members: title and author.

Create a class Library with a public vector of Book objects called books.

Add a method addBook to Library that takes a Book object and adds it to books.

Add a method printBooks to Library that prints all books with their titles and authors.

In main(), create a Library object, add two Book objects with exact titles and authors, then print all books.

💡 Why This Matters

🌍 Real World

Managing collections of objects is common in software like libraries, stores, or contact lists. This project shows how objects can hold and manage other objects.

💼 Career

Understanding object interaction is key for software development jobs, especially when designing systems with multiple related data types.

1

Create the Book class

Create a class called Book with two public string members: title and author. Then create two Book objects named book1 and book2 with these exact values: book1.title = "1984", book1.author = "George Orwell", book2.title = "To Kill a Mockingbird", book2.author = "Harper Lee".

C++

#include 
#include 

using namespace std;

// Create class Book with public members title and author

int main() {

    // Create book1 and book2 with given titles and authors

    return 0;
}

Hint

Define the Book class with two public string variables. Then create two Book objects and assign the exact titles and authors.

2

Create the Library class with a vector of books

Add a class called Library with a public vector<Book> member named books. Include #include <vector> at the top. In main(), create a Library object named library.

C++

#include 
#include 
#include 

using namespace std;

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

// Create class Library with public vector books

int main() {
    Book book1;
    book1.title = "1984";
    book1.author = "George Orwell";

    Book book2;
    book2.title = "To Kill a Mockingbird";
    book2.author = "Harper Lee";

    // Create Library object named library

    return 0;
}

Hint

Include the vector header. Define Library with a public vector of Book. Then create a Library object in main().

3

Add addBook method to Library

Add a public method addBook to the Library class that takes a Book parameter named book and adds it to the books vector using push_back. In main(), call library.addBook(book1) and library.addBook(book2).

C++

#include 
#include 
#include 

using namespace std;

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

class Library {
public:
    vector books;

    // Add addBook method here
};

int main() {
    Book book1;
    book1.title = "1984";
    book1.author = "George Orwell";

    Book book2;
    book2.title = "To Kill a Mockingbird";
    book2.author = "Harper Lee";

    Library library;

    // Add books to library here

    return 0;
}

Hint

Define addBook inside Library to add a Book to the books vector. Then call it twice in main().

4

Add printBooks method and print all books

Add a public method printBooks to Library that uses a for loop with variables book to iterate over books and prints each book's title and author in the format: Title: [title], Author: [author]. In main(), call library.printBooks().

C++

#include 
#include 
#include 

using namespace std;

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

class Library {
public:
    vector books;

    void addBook(Book book) {
        books.push_back(book);
    }

    // Add printBooks method here
};

int main() {
    Book book1;
    book1.title = "1984";
    book1.author = "George Orwell";

    Book book2;
    book2.title = "To Kill a Mockingbird";
    book2.author = "Harper Lee";

    Library library;

    library.addBook(book1);
    library.addBook(book2);

    // Call printBooks here

    return 0;
}

Hint

Use a for loop to go through each Book in books and print the title and author. Then call printBooks() in main().

Practice

1. What does it mean when one object calls a method on another object in C++?

easy

A. The first object is asking the second object to perform an action.

B. The first object copies the second object's data.

C. The first object deletes the second object.

D. The first object creates a new object inside the second object.

Solution

1. Step 1: Understand method calls between objects

   When one object calls a method on another, it requests that object to do something or provide information.

2. Step 2: Differentiate from other actions

   Copying data, deleting, or creating objects are different operations, not the same as calling a method.

3. Final Answer:

   The first object is asking the second object to perform an action. -> Option A

4. Quick Check:

   Object method call = request action [OK]

Hint: Method call means asking another object to do something [OK]

Common Mistakes:

• Confusing method call with copying data
• Thinking method call deletes objects
• Assuming method call creates new objects inside others

2. Which of the following is the correct syntax to pass an object obj of class Car to a method drive of another object driver?

easy

A. driver.drive(obj);

B. drive.driver(obj);

C. driver->drive(obj);

D. drive(obj).driver;

Solution

1. Step 1: Identify correct method call syntax

   For an object driver, calling method drive with argument obj uses dot notation: driver.drive(obj);

2. Step 2: Eliminate incorrect syntax

   drive.driver(obj); reverses object and method, driver->drive(obj); is for pointers, and drive(obj).driver; is invalid.

3. Final Answer:

   driver.drive(obj); -> Option A

4. Quick Check:

   Object.method(argument) = driver.drive(obj) [OK]

Hint: Use object.method(argument) syntax for calling methods [OK]

Common Mistakes:

• Using arrow operator on non-pointer objects
• Swapping method and object names
• Incorrect punctuation or order

3. What is the output of this code?

#include <iostream>

class Light {
public:
    void turnOn() { std::cout << "Light is ON" << std::endl; }
};

class Switch {
public:
    void press(Light &l) { l.turnOn(); }
};

int main() {
    Light roomLight;
    Switch wallSwitch;
    wallSwitch.press(roomLight);
    return 0;
}

medium

A. Compilation error

B. Light is OFF

C. No output

D. Light is ON

Solution

1. Step 1: Understand method call inside press()

   The press method calls turnOn() on the passed Light object, which prints "Light is ON".

2. Step 2: Trace main function calls

   wallSwitch.press(roomLight); calls turnOn() on roomLight, so output is "Light is ON".

3. Final Answer:

   Light is ON -> Option D

4. Quick Check:

   Method call prints "Light is ON" [OK]

Hint: Method calls inside other methods print output [OK]

Common Mistakes:

• Assuming no output because press() returns void
• Confusing reference passing with copying
• Thinking code causes compilation error

4. Identify the error in this code snippet:

#include <iostream>

class Engine {
public:
    void start() { std::cout << "Engine started" << std::endl; }
};

class Car {
public:
    void ignite(Engine e) { e.start(); }
};

int main() {
    Engine myEngine;
    Car myCar;
    myCar.ignite(myEngine);
    return 0;
}

medium

A. Method start() is not declared correctly.

B. Car class should inherit from Engine.

C. Passing Engine by value causes unnecessary copying.

D. Cannot call start() on Engine object.

Solution

1. Step 1: Analyze parameter passing in ignite()

   The method ignite takes Engine e by value, which copies the object unnecessarily.

2. Step 2: Suggest improvement

   Passing by reference (Engine &e) avoids copying and is more efficient.

3. Final Answer:

   Passing Engine by value causes unnecessary copying. -> Option C

4. Quick Check:

   Pass objects by reference to avoid copying [OK]

Hint: Pass objects by reference to avoid copies [OK]

Common Mistakes:

• Thinking method start() is incorrect
• Assuming inheritance is required
• Believing start() cannot be called on Engine

5. Given classes Teacher and Student, how can you design a method in Teacher that lets a teacher assign a grade to a student object? Choose the best approach.

hard

A. Make Student inherit from Teacher and override a grade method.

B. Define a method in Teacher that takes a Student reference and an int grade, then calls a Student method to set the grade.

C. Store grades only inside Teacher without involving Student objects.

D. Use global variables to store grades for all students.

Solution

1. Step 1: Understand object interaction for assigning grades

   The Teacher should interact with Student by calling a method on Student to set the grade, passing the grade value.

2. Step 2: Evaluate options

   Define a method in Teacher that takes a Student reference and an int grade, then calls a Student method to set the grade. correctly uses a method in Teacher that takes a Student reference and grade, then calls Student's method to set the grade. Other options misuse inheritance, ignore object interaction, or use bad global state.

3. Final Answer:

   Define a method in Teacher that takes a Student reference and an int grade, then calls a Student method to set the grade. -> Option B

4. Quick Check:

   Teacher calls Student method to assign grade [OK]

Hint: Teacher calls Student method with grade parameter [OK]

Common Mistakes:

• Misusing inheritance for unrelated roles
• Ignoring encapsulation by using globals
• Storing grades only in Teacher without Student update