Object creation and destruction flow in C++ - Mini Project: Build & Apply

Object Creation and Destruction Flow

📖 Scenario: Imagine you are managing a small library system. Each book is represented as an object. You want to see when a book is created and when it is removed from the system.

🎯 Goal: You will create a simple C++ class Book that prints messages when a book object is created and destroyed. This will help you understand the flow of object creation and destruction.

📋 What You'll Learn

Create a class named Book with a constructor and destructor

Constructor should print "Book created"

Destructor should print "Book destroyed"

Create a main function that creates a Book object

Observe the order of printed messages to understand object lifecycle

💡 Why This Matters

🌍 Real World

Understanding object creation and destruction is important when managing resources like files, memory, or network connections in real applications.

💼 Career

Many software jobs require knowledge of how objects are created and destroyed to write efficient and bug-free code, especially in C++.

1

Create the Book class with constructor

Write a class named Book with a public constructor that prints "Book created" when called.

C++

// Define class Book with a constructor that prints "Book created"
// Your code here

Hint

Use std::cout inside the constructor to print the message.

2

Add destructor to Book class

Add a public destructor to the Book class that prints "Book destroyed" when called.

C++

#include 

class Book {
public:
    Book() {
        std::cout << "Book created" << std::endl;
    }
    // Add destructor here that prints "Book destroyed"
    // Your code here
};

Hint

Destructor has the same name as the class but with a tilde ~ before it.

3

Create a Book object in main

Write a main function that creates a Book object named myBook.

C++

#include 

class Book {
public:
    Book() {
        std::cout << "Book created" << std::endl;
    }
    ~Book() {
        std::cout << "Book destroyed" << std::endl;
    }
};

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

Hint

Inside main, declare Book myBook; to create the object.

4

Run and observe output

Run the program and print the output to see the messages from the constructor and destructor.

C++

#include 

class Book {
public:
    Book() {
        std::cout << "Book created" << std::endl;
    }
    ~Book() {
        std::cout << "Book destroyed" << std::endl;
    }
};

int main() {
    Book myBook;
    // Print output by running the program
    // Your code here
    return 0;
}

Hint

When the program ends, the destructor is called automatically for myBook.

Practice

1. Which of the following statements about object creation and destruction in C++ is correct?

easy

A. Constructors run only when an object is destroyed.

B. Destructors are called before an object is created.

C. Constructors are called automatically when an object is created.

D. Destructors run only when an object is created.

Solution

1. Step 1: Understand constructor timing

   Constructors are special functions that run automatically when an object is created to initialize it.

2. Step 2: Understand destructor timing

   Destructors run when an object is destroyed or goes out of scope, not before creation.

3. Final Answer:

   Constructors are called automatically when an object is created. -> Option C

4. Quick Check:

   Constructor = on creation [OK]

Hint: Remember: constructor = create, destructor = destroy [OK]

Common Mistakes:

• Confusing when constructors and destructors run
• Thinking destructors run before creation
• Believing constructors run on destruction

2. Which of the following is the correct syntax to declare a destructor for a class named Car in C++?

easy

A. ~Car();

B. Car();

C. void ~Car();

D. destructor Car();

Solution

1. Step 1: Recall destructor syntax

   In C++, a destructor is declared with a tilde (~) followed by the class name and parentheses, no return type.

2. Step 2: Check options

   ~Car(); matches the correct syntax: ~Car();. Others are invalid syntax.

3. Final Answer:

   ~Car(); -> Option A

4. Quick Check:

   Destructor syntax = ~ClassName() [OK]

Hint: Destructor always starts with ~ and no return type [OK]

Common Mistakes:

• Adding return type to destructor
• Using wrong keywords like 'destructor'
• Confusing constructor syntax with destructor

3. Consider the following C++ code:

class Test {
public:
  Test() { std::cout << "C"; }
  ~Test() { std::cout << "D"; }
};

int main() {
  Test t1;
  {
    Test t2;
  }
  std::cout << "E";
  return 0;
}

What is the output when this program runs?

medium

A. CCDEE

B. CCDED

C. CDEC

D. CCED

Solution

1. Step 1: Trace object creation

   t1 is created first, prints 'C'. Then t2 is created inside inner block, prints another 'C'.

2. Step 2: Trace object destruction and output

   When inner block ends, t2 is destroyed, prints 'D'. Then 'E' is printed. Finally, t1 is destroyed at program end, prints 'D'.

3. Final Answer:

   CCDED -> Option B

4. Quick Check:

   Creation C twice, destruction D twice, E in middle [OK]

Hint: Destructors run when objects go out of scope [OK]

Common Mistakes:

• Forgetting destructor runs at block end
• Mixing order of destruction
• Ignoring output inside main

4. Identify the error in the following C++ code snippet related to object destruction:

class Sample {
public:
  Sample() { std::cout << "Created"; }
  void ~Sample() { std::cout << "Destroyed"; }
};

int main() {
  Sample s;
  return 0;
}

medium

A. Destructor has a return type which is not allowed.

B. Destructor name is incorrect; it should be 'destruct'.

C. Constructor is missing a return type.

D. Destructor should be public static.

Solution

1. Step 1: Check destructor declaration

   Destructor must not have a return type. Here, 'void' before '~Sample()' is invalid syntax.

2. Step 2: Verify other options

   Destructor name is correct with '~Sample'. Constructors have no return type but that is correct here. Destructors cannot be static.

3. Final Answer:

   Destructor has a return type which is not allowed. -> Option A

4. Quick Check:

   Destructor = no return type [OK]

Hint: Destructor never has a return type, not even void [OK]

Common Mistakes:

• Adding void or other return types to destructor
• Misnaming destructor
• Making destructor static

5. You have a class Resource that opens a file in its constructor and closes it in its destructor. Which of the following code snippets correctly ensures the file is closed automatically when the object goes out of scope?

hard

A. void useFile() {
Resource* r = new Resource();
// use file
delete r;
} // file closed here automatically

B. void useFile() {
Resource* r = new Resource();
// use file
} // file closed here automatically

C. void useFile() {
Resource r;
// use file
delete &r;
} // file closed here

D. void useFile() {
Resource r;
// use file
} // file closed here automatically

Solution

1. Step 1: Understand automatic destruction

   Objects created on stack (like 'Resource r;') are destroyed automatically when going out of scope, calling destructor to close file.

2. Step 2: Analyze dynamic allocation

   Objects created with 'new' must be deleted manually; otherwise destructor won't run automatically, risking resource leaks.

3. Final Answer:

   void useFile() { Resource r; // use file } // file closed here automatically -> Option D

4. Quick Check:

   Stack objects auto-destroyed, heap objects need delete [OK]

Hint: Use stack objects for automatic destructor calls [OK]

Common Mistakes:

• Forgetting to delete heap objects
• Trying to delete stack objects
• Assuming destructor runs without delete on heap