Object lifecycle in C++ - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of constructor and destructor calls

What is the output of this C++ program regarding constructor and destructor calls?

C++

#include <iostream>
class Test {
public:
    Test() { std::cout << "Constructed "; }
    ~Test() { std::cout << "Destructed "; }
};

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

A Destructed Constructed Constructed Destructed

B Constructed Destructed Constructed Destructed

C Constructed Destructed Destructed Constructed

D Constructed Constructed Destructed Destructed

💡 Hint

Remember that destructors are called when objects go out of scope.

The first object t1 is constructed, then inside the inner block t2 is constructed. When the inner block ends, t2 is destructed. Finally, when main ends, t1 is destructed.

❓ Predict Output

intermediate

Output of copy constructor and assignment operator

What is the output of this C++ program that uses copy constructor and assignment operator?

C++

#include <iostream>
class Sample {
public:
    Sample() { std::cout << "Default "; }
    Sample(const Sample&) { std::cout << "Copy "; }
    Sample& operator=(const Sample&) { std::cout << "Assign "; return *this; }
};

int main() {
    Sample a;
    Sample b = a;
    Sample c;
    c = a;
    return 0;
}

A Default Default Copy Assign

B Default Copy Default Assign

C Copy Default Assign Default

D Default Assign Copy Default

💡 Hint

Copy constructor is called during initialization, assignment operator during assignment.

Object a is created with default constructor. Object b is initialized from a, so copy constructor is called. Object c is default constructed, then assigned from a, so assignment operator is called.

🔧 Debug

advanced

Identify the error in object lifecycle management

What error does this C++ code produce related to object lifecycle?

C++

#include <iostream>
class Demo {
public:
    Demo() { std::cout << "Created "; }
    ~Demo() { std::cout << "Destroyed "; }
};

int main() {
    Demo* d = new Demo();
    // Missing delete
    return 0;
}

A Memory leak due to missing delete

B Double free error

C Segmentation fault

D No error, program runs fine

💡 Hint

Think about what happens when you use new without delete.

The object created with new is never deleted, so its destructor is never called and memory is leaked.

📝 Syntax

advanced

Identify the syntax error in constructor initialization

Which option contains the correct constructor initialization syntax for member variable 'x'?

C++

class Point {
    int x;
public:
    Point(int val) : x(val) {}
};

A Point(int val) : x(val) {}

B Point(int val) { x = val; }

C Point(int val) : x = val {}

D Point(int val) { x(val); }

💡 Hint

Constructor initialization list uses colon and parentheses without assignment.

Option A uses correct member initializer list syntax. Option A is valid but uses assignment inside constructor body, not initialization list. Option A and D have syntax errors.

🚀 Application

expert

Number of times destructor is called in complex object lifecycle

How many times is the destructor called when this program runs?

C++

#include <iostream>
class Obj {
public:
    Obj() { std::cout << "C"; }
    ~Obj() { std::cout << "D"; }
};

Obj globalObj;

int main() {
    Obj localObj1;
    {
        Obj localObj2;
        Obj localObj3 = localObj2;
    }
    return 0;
}

A 6

B 3

C 4

D 5

💡 Hint

Count all objects created and when they go out of scope, including global.

Objects created: globalObj (1), localObj1 (2), localObj2 (3), localObj3 (4, copy of localObj2). Destructors called for localObj3 and localObj2 at inner block end, localObj1 at main end, and globalObj at program end. Total 4 destructor calls.

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