Object lifecycle in C++

Introduction

Objects in C++ have a life from creation to destruction. Understanding this helps you manage memory and program behavior well.

When you create a variable or object in your program.

When you want to know when resources like memory are allocated or freed.

When debugging why an object's data changes or disappears.

When writing classes with constructors and destructors.

When managing dynamic memory with pointers.

Syntax

C++

class ClassName {
public:
    ClassName() { /* constructor code */ }
    ~ClassName() { /* destructor code */ }
};

// Creating an object
ClassName obj;

// Object is destroyed automatically when it goes out of scope

The constructor runs when an object is created.

The destructor runs when the object is destroyed (goes out of scope or deleted).

Examples

This example shows messages when the object is created and destroyed automatically.

C++

#include <iostream>

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

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

The object b is destroyed when the function ends.

C++

#include <iostream>

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

void createBox() {
    Box b;
    std::cout << "Inside function\n";
}

int main() {
    createBox();
    std::cout << "Back in main\n";
    return 0;
}

When using new, you must delete to destroy the object and free memory.

C++

#include <iostream>

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

int main() {
    Box* b = new Box();
    // use b
    delete b; // manually destroy the object
    return 0;
}

Sample Program

This program shows when the Person object is created and destroyed inside a block.

C++

#include <iostream>

class Person {
public:
    Person() { std::cout << "Person created\n"; }
    ~Person() { std::cout << "Person destroyed\n"; }
};

int main() {
    std::cout << "Start main\n";
    {
        Person p;
        std::cout << "Inside block\n";
    }
    std::cout << "End main\n";
    return 0;
}

Important Notes

Objects created inside a block are destroyed when the block ends.

Dynamic objects created with new must be deleted manually to avoid memory leaks.

Constructors and destructors help manage setup and cleanup automatically.

Summary

Objects have a lifecycle: creation (constructor) and destruction (destructor).

Automatic objects are destroyed when they go out of scope.

Dynamic objects need manual deletion to free memory.

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