Object lifecycle in C++ - Time & Space Complexity

Time Complexity: Object lifecycle

O(n)

Understanding Time Complexity

When we create and destroy objects in C++, the time it takes depends on how many objects we handle.

We want to know how the time grows as we make more objects.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

class MyObject {
public:
    MyObject() { /* constructor work */ }
    ~MyObject() { /* destructor work */ }
};

void createObjects(int n) {
    MyObject* arr = new MyObject[n];
    delete[] arr;
}

This code creates an array of n objects and then deletes them, calling constructors and destructors.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Calling the constructor and destructor for each object.
• How many times: Exactly n times each, once per object.

How Execution Grows With Input

As the number of objects n increases, the total work grows in direct proportion.

Input Size (n)	Approx. Operations
10	About 20 (10 constructions + 10 destructions)
100	About 200 (100 constructions + 100 destructions)
1000	About 2000 (1000 constructions + 1000 destructions)

Pattern observation: The total work doubles the number of objects because each object is constructed and then destroyed.

Final Time Complexity

Time Complexity: O(n)

This means the time to create and destroy objects grows linearly with the number of objects.

Common Mistake

[X] Wrong: "Creating many objects takes the same time as creating just one because constructors are fast."

[OK] Correct: Even if each constructor is fast, doing it many times adds up, so total time grows with the number of objects.

Interview Connect

Understanding how object creation and destruction scale helps you write efficient code and explain performance clearly in real projects.

Self-Check

"What if the destructor does extra work that depends on the size of the object? How would the time complexity change?"

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