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

Understanding Object Lifecycle in C++

📖 Scenario: Imagine you are managing a simple library system. Each book is represented as an object in your program. You want to understand how objects are created, used, and destroyed in C++.

🎯 Goal: You will create a Book class, create an object of this class, and observe the messages printed during the object's lifecycle: creation, usage, and destruction.

📋 What You'll Learn

Create a class named Book with a constructor and destructor that print messages.

Create an object of class Book named myBook.

Print a message inside the main function to show the object is being used.

Observe the order of printed messages to understand the object lifecycle.

💡 Why This Matters

🌍 Real World

Understanding object lifecycle helps manage resources like memory and files safely in programs.

💼 Career

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

1

Create the Book class with constructor and destructor

Write a class named Book with a constructor that prints "Book created" and a destructor that prints "Book destroyed".

C++

class Book {
public:
    Book() {
        // Your code here
    }
    ~Book() {
        // Your code here
    }
};

Hint

The constructor is a function with the same name as the class and no return type. The destructor has the same name but with a tilde (~) before it.

2

Create an object named myBook of class Book

In the main function, create an object named myBook of type Book.

C++

int main() {
    // Your code here
    return 0;
}

Hint

Inside main(), declare an object by writing the class name followed by the object name and a semicolon.

3

Print a message showing the object is being used

Inside the main function, after creating myBook, add a line to print "Using the book object".

C++

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

Hint

Use std::cout to print the message inside main() after creating the object.

4

Run the program and observe the output

Run the program and observe the order of printed messages. The output should show Book created, then Using the book object, and finally Book destroyed.

C++

int main() {
    Book myBook;
    std::cout << "Using the book object" << std::endl;
    // Your code here
    return 0;
}

Hint

Compile and run the program. Notice the order of messages shows when the object is created, used, and destroyed.

Practice

1. Which of the following best describes when a destructor is called for an automatic (local) object in C++?

easy

A. When the object is created

B. When the object goes out of scope

C. When the program starts

D. Only when the object is deleted manually

Solution

1. Step 1: Understand automatic object lifecycle

   Automatic objects are created when their scope starts and destroyed when their scope ends.

2. Step 2: Identify destructor call timing

   The destructor is called automatically when the object goes out of scope to clean up resources.

3. Final Answer:

   When the object goes out of scope -> Option B

4. Quick Check:

   Destructor called at scope end = When object goes out of scope [OK]

Hint: Destructor runs automatically at scope end for local objects [OK]

Common Mistakes:

• Thinking destructor runs at object creation
• Believing manual deletion is needed for local objects
• Confusing program start with object destruction

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

easy

A. void Car();

B. destructor Car();

C. Car::~Car();

D. ~Car();

Solution

1. Step 1: Recall destructor syntax

   Destructor syntax uses a tilde (~) followed by the class name and parentheses.

2. Step 2: Match correct option

   ~Car(); matches the correct syntax: ~Car();

3. Final Answer:

   ~Car(); -> Option D

4. Quick Check:

   Destructor syntax = ~ClassName() [OK]

Hint: Destructor always starts with ~ followed by class name [OK]

Common Mistakes:

• Using void return type for destructor
• Writing destructor with class scope resolution in declaration
• Using keyword 'destructor' which doesn't exist

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

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

int main() {
  Test t1;
  {
    Test t2;
  }
  return 0;
}

medium

A. Created Created Destroyed Destroyed

B. Created Destroyed Created Destroyed

C. Created Created Destroyed

D. Destroyed Created Created Destroyed

Solution

1. Step 1: Trace object creation

   First, t1 is created: prints "Created ". Then inside inner scope, t2 is created: prints another "Created ".

2. Step 2: Trace object destruction

   When inner scope ends, t2 is destroyed: prints "Destroyed ". Then at program end, t1 is destroyed: prints "Destroyed ".

3. Final Answer:

   Created Created Destroyed Destroyed -> Option A

4. Quick Check:

   Constructor and destructor calls match output [OK]

Hint: Objects destroyed in reverse order of creation at scope end [OK]

Common Mistakes:

• Assuming destruction happens immediately after creation
• Forgetting inner scope destruction before outer
• Mixing order of constructor and destructor prints

4. Consider the following code snippet:

class Sample {
public:
  Sample() { std::cout << "Start "; }
  ~Sample() { std::cout << "End "; }
};

int main() {
  Sample* p = new Sample();
  // Missing delete p;
  return 0;
}

What is the problem with this code?

medium

A. Syntax error due to missing delete

B. Destructor is called twice causing error

C. Memory leak because destructor is not called

D. No problem, code runs fine

Solution

1. Step 1: Identify dynamic object creation

   The object is created with new, so it lives on the heap and needs manual deletion.

2. Step 2: Check for destructor call

   Since delete p; is missing, destructor is never called, causing a memory leak.

3. Final Answer:

   Memory leak because destructor is not called -> Option C

4. Quick Check:

   Missing delete on new object = memory leak [OK]

Hint: Always delete dynamically allocated objects to call destructor [OK]

Common Mistakes:

• Assuming destructor runs automatically for new objects
• Thinking missing delete causes syntax error
• Believing destructor is called twice

5. You want to track how many objects of class Box currently exist during program execution. Which approach correctly uses constructors and destructors to maintain this count?

hard

A. Increment a static counter in constructor and decrement it in destructor

B. Increment a global variable in constructor only

C. Decrement a static counter in constructor and increment in destructor

D. Use constructor to print count and destructor to reset count

Solution

1. Step 1: Understand object counting logic

   To track live objects, increase count when object is created and decrease when destroyed.

2. Step 2: Use static counter in class

   A static variable shared by all objects can hold the count, incremented in constructor and decremented in destructor.

3. Final Answer:

   Increment a static counter in constructor and decrement it in destructor -> Option A

4. Quick Check:

   Static counter + constructor increment + destructor decrement = correct count [OK]

Hint: Use static variable incremented in constructor, decremented in destructor [OK]

Common Mistakes:

• Changing count only in constructor or destructor, not both
• Using global variable instead of static class member
• Reversing increment and decrement roles