Destructor role in C++ - Mini Project: Build & Apply

Destructor role

📖 Scenario: Imagine you have a simple program that manages a book in a library. When the book object is no longer needed, it should clean up by announcing it is being removed.

🎯 Goal: You will create a class Book with a destructor that prints a message when the object is destroyed. This shows how destructors help clean up or notify when an object is removed.

📋 What You'll Learn

Create a class named Book

Add a constructor that sets the book's title

Add a destructor that prints "Destroying book: <title>"

Create a Book object with the title "C++ Basics"

Allow the destructor message to show when the object goes out of scope

💡 Why This Matters

🌍 Real World

Destructors help manage resources like memory or files by cleaning up automatically when objects are no longer needed.

💼 Career

Understanding destructors is important for writing safe and efficient C++ programs, especially in systems programming and software development.

1

Create the Book class with a title

Write a class named Book with a public string variable title. Add a constructor that takes a string parameter bookTitle and sets title to it.

C++

// Your code here

Hint

Use class Book and add a constructor that sets the title variable.

2

Add a destructor to the Book class

Inside the Book class, add a destructor ~Book() that prints "Destroying book: " followed by the title and a newline.

C++

#include 
#include 

class Book {
public:
    std::string title;
    Book(std::string bookTitle) {
        title = bookTitle;
    }
    // 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 with the title "C++ Basics".

C++

#include 
#include 

class Book {
public:
    std::string title;
    Book(std::string bookTitle) {
        title = bookTitle;
    }
    ~Book() {
        std::cout << "Destroying book: " << title << std::endl;
    }
};

int main() {
    // Your code here
}

Hint

Create the myBook object inside main() using the constructor.

4

Run the program to see the destructor message

Add a return 0; statement at the end of main(). Run the program and observe the output showing the destructor message.

C++

#include 
#include 

class Book {
public:
    std::string title;
    Book(std::string bookTitle) {
        title = bookTitle;
    }
    ~Book() {
        std::cout << "Destroying book: " << title << std::endl;
    }
};

int main() {
    Book myBook("C++ Basics");
    // Your code here
}

Hint

The destructor message appears when myBook goes out of scope at the end of main().

Practice

1. What is the main role of a destructor in C++?

easy

A. To clean up resources when an object is destroyed

B. To create new objects

C. To initialize variables

D. To overload operators

Solution

1. Step 1: Understand the purpose of a destructor

   A destructor is a special function that runs when an object is destroyed to free resources like memory or files.

2. Step 2: Compare with other options

   Creating objects, initializing variables, and overloading operators are not roles of destructors.

3. Final Answer:

   To clean up resources when an object is destroyed -> Option A

4. Quick Check:

   Destructor role = cleanup [OK]

Hint: Destructor always cleans up when object ends [OK]

Common Mistakes:

• Confusing destructor with constructor
• Thinking destructor creates objects
• Believing destructor initializes variables

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

easy

A. Car~() {}

B. void ~Car() {}

C. ~Car() {}

D. void Car() {}

Solution

1. Step 1: Recall destructor syntax

   A destructor has the same name as the class with a tilde (~) prefix and no return type.

2. Step 2: Check each option

   ~Car() {} matches the correct syntax. void ~Car() {} wrongly includes 'void'. Car~() {} has wrong order. void Car() {} lacks tilde and looks like a constructor.

3. Final Answer:

   ~Car() {} -> Option C

4. Quick Check:

   Destructor syntax = ~ClassName() [OK]

Hint: Destructor name = ~ + class name, no return type [OK]

Common Mistakes:

• Adding return type to destructor
• Placing tilde after class name
• Confusing destructor with constructor syntax

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

class Box {
public:
  Box() { std::cout << "Created\n"; }
  ~Box() { std::cout << "Destroyed\n"; }
};

int main() {
  Box b;
  return 0;
}

medium

A. Destroyed\nCreated\n

B. Created\nDestroyed\n

C. Created\n

D. No output

Solution

1. Step 1: Trace object creation

   When b is created, constructor prints "Created".

2. Step 2: Trace object destruction

   At end of main(), b is destroyed, destructor prints "Destroyed".

3. Final Answer:

   Created\nDestroyed\n -> Option B

4. Quick Check:

   Constructor then destructor output [OK]

Hint: Constructor runs first, destructor runs last [OK]

Common Mistakes:

• Assuming destructor runs before constructor
• Expecting no output
• Confusing order of prints

4. Identify the error in this destructor code:

class Sample {
public:
  ~Sample(int x) { }
};

medium

A. Destructor cannot have parameters

B. Destructor must return int

C. Destructor name should not have ~

D. Destructor must be static

Solution

1. Step 1: Check destructor parameters

   Destructors cannot have parameters; they must have empty parentheses.

2. Step 2: Verify other rules

   Destructor has no return type, must have ~ prefix, and cannot be static.

3. Final Answer:

   Destructor cannot have parameters -> Option A

4. Quick Check:

   Destructor params = none [OK]

Hint: Destructor always has empty parentheses [OK]

Common Mistakes:

• Adding parameters to destructor
• Adding return type
• Removing ~ from destructor name

5. Consider a class FileHandler that opens a file in its constructor. Which is the best way to ensure the file is closed properly when the object is no longer needed?

hard

A. Do nothing; files close automatically when program ends

B. Manually call a close method before deleting the object

C. Rely on the operating system to close the file automatically

D. Write a destructor that closes the file

Solution

1. Step 1: Understand resource management

   Files opened by an object should be closed when the object is destroyed to avoid resource leaks.

2. Step 2: Use destructor for cleanup

   Destructor runs automatically when object ends, so closing the file there ensures proper cleanup.

3. Final Answer:

   Write a destructor that closes the file -> Option D

4. Quick Check:

   Destructor cleans resources automatically [OK]

Hint: Use destructor to free resources automatically [OK]

Common Mistakes:

• Forgetting to close files causing leaks
• Relying on manual close calls
• Assuming OS always cleans up immediately