Object lifecycle in C++ - Cheat Sheet & Quick Revision

Recall & Review

beginner

What are the main stages of an object's lifecycle in C++?

The main stages are: creation (when the object is made), usage (when the object is used), and destruction (when the object is removed and its memory freed).

beginner

What happens during the construction of an object?

During construction, the object's memory is allocated and its constructor function runs to set up initial values or states.

beginner

What is the role of a destructor in C++?

A destructor is a special function that runs automatically when an object is destroyed. It cleans up resources like memory or files the object used.

intermediate

How does scope affect an object's lifecycle?

An object created inside a scope (like inside a function) is destroyed automatically when the scope ends, meaning its destructor is called then.

intermediate

What is the difference between automatic and dynamic object lifecycles?

Automatic objects are created and destroyed automatically based on scope. Dynamic objects are created with new and must be manually destroyed with delete.

When is an object's destructor called in C++?

A Only when the program ends

B When the object is created

C When the program starts

D When the object goes out of scope or is deleted

The destructor runs when the object is no longer needed, either when it goes out of scope or when deleted if created dynamically.

Which keyword is used to create a dynamic object in C++?

A new

B create

C malloc

D construct

The new keyword allocates memory and calls the constructor for dynamic objects.

What happens if you forget to delete a dynamic object?

A Program crashes immediately

B Object is destroyed automatically

C Memory leak occurs

D Destructor runs twice

Not deleting dynamic objects causes memory to stay allocated, leading to memory leaks.

Which function is automatically called when an object is created?

A Copy

B Constructor

C Main

D Destructor

The constructor sets up the object when it is created.

If an object is declared inside a function, when is it destroyed?

A When the function ends

B At program start

C When the object is used

D Never

Objects declared inside a function are destroyed when the function finishes and the scope ends.

Explain the lifecycle of an object in C++ from creation to destruction.

Think about what happens when you make an object and when it disappears.

Describe the difference between automatic and dynamic object lifecycles in C++.

Consider how and when objects are created 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