Object creation and destruction flow in C++ - Step-by-Step Execution

Concept Flow - Object creation and destruction flow

Start Program

↓

Call Constructor

↓

Object Created

↓

Use Object

↓

Call Destructor

↓

Object Destroyed

↓

End Program

This flow shows how a C++ object is created by calling its constructor, used, then destroyed by calling its destructor when it goes out of scope.

Execution Sample

C++

#include <iostream>

class MyClass {
public:
  MyClass() { std::cout << "Created\n"; }
  ~MyClass() { std::cout << "Destroyed\n"; }
};

int main() {
  MyClass obj;
  return 0;
}

This code creates an object of MyClass, prints "Created" when constructed, and "Destroyed" when the object goes out of scope at program end.

Execution Table

Step	Action	Output	Object State
1	Enter main function		No object
2	Call MyClass constructor	Created	Object obj created
3	Object obj in use		Object obj alive
4	Exit main function scope		Object obj about to be destroyed
5	Call MyClass destructor	Destroyed	Object obj destroyed
6	Program ends		No object

💡 Program ends after object destruction when main function scope ends

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 5	Final
obj	Not created	Created	Alive	Destroyed	No object

Key Moments - 3 Insights

Why do we see "Created" before "Destroyed" in the output?

The constructor runs first when the object is created (Step 2), printing "Created". The destructor runs last when the object goes out of scope (Step 5), printing "Destroyed".

When exactly is the destructor called?

The destructor is called automatically when the object goes out of scope, here at the end of main (Step 4 and 5), as shown in the execution_table.

What happens if we create the object inside a block {}?

The object is destroyed as soon as the block ends, calling the destructor immediately after the block, similar to Step 4 and 5.

Visual Quiz - 3 Questions

Look at the execution table, what is the state of 'obj' after Step 3?

A Object obj alive

B Object obj destroyed

C No object

D Object obj about to be destroyed

💡 Hint

Check the 'Object State' column for Step 3 in the execution_table

At which step is the destructor called according to the execution table?

A Step 2

B Step 5

C Step 3

D Step 6

💡 Hint

Look for the action 'Call MyClass destructor' in the execution_table

If we remove the object creation line, what would be the output?

A Only Destroyed

B Only Created

C No output

D Created and Destroyed

💡 Hint

Without object creation, constructor and destructor are not called, so no output lines appear

Concept Snapshot

Object creation calls the constructor.
Object destruction calls the destructor.
Constructor runs when object is made.
Destructor runs when object goes out of scope.
Output shows creation then destruction messages.

Full Transcript

This visual trace shows how a C++ object is created and destroyed. The program starts and enters main. At Step 2, the constructor runs, printing "Created" and creating the object. The object is alive and can be used. When main ends, the object goes out of scope at Step 4, and the destructor runs at Step 5, printing "Destroyed". Finally, the program ends with no objects alive. This flow helps understand when constructors and destructors run automatically in C++.

Practice

1. Which of the following statements about object creation and destruction in C++ is correct?

easy

A. Constructors run only when an object is destroyed.

B. Destructors are called before an object is created.

C. Constructors are called automatically when an object is created.

D. Destructors run only when an object is created.

Solution

1. Step 1: Understand constructor timing

   Constructors are special functions that run automatically when an object is created to initialize it.

2. Step 2: Understand destructor timing

   Destructors run when an object is destroyed or goes out of scope, not before creation.

3. Final Answer:

   Constructors are called automatically when an object is created. -> Option C

4. Quick Check:

   Constructor = on creation [OK]

Hint: Remember: constructor = create, destructor = destroy [OK]

Common Mistakes:

• Confusing when constructors and destructors run
• Thinking destructors run before creation
• Believing constructors run on destruction

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

easy

A. ~Car();

B. Car();

C. void ~Car();

D. destructor Car();

Solution

1. Step 1: Recall destructor syntax

   In C++, a destructor is declared with a tilde (~) followed by the class name and parentheses, no return type.

2. Step 2: Check options

   ~Car(); matches the correct syntax: ~Car();. Others are invalid syntax.

3. Final Answer:

   ~Car(); -> Option A

4. Quick Check:

   Destructor syntax = ~ClassName() [OK]

Hint: Destructor always starts with ~ and no return type [OK]

Common Mistakes:

• Adding return type to destructor
• Using wrong keywords like 'destructor'
• Confusing constructor syntax with destructor

3. Consider the following C++ code:

class Test {
public:
  Test() { std::cout << "C"; }
  ~Test() { std::cout << "D"; }
};

int main() {
  Test t1;
  {
    Test t2;
  }
  std::cout << "E";
  return 0;
}

What is the output when this program runs?

medium

A. CCDEE

B. CCDED

C. CDEC

D. CCED

Solution

1. Step 1: Trace object creation

   t1 is created first, prints 'C'. Then t2 is created inside inner block, prints another 'C'.

2. Step 2: Trace object destruction and output

   When inner block ends, t2 is destroyed, prints 'D'. Then 'E' is printed. Finally, t1 is destroyed at program end, prints 'D'.

3. Final Answer:

   CCDED -> Option B

4. Quick Check:

   Creation C twice, destruction D twice, E in middle [OK]

Hint: Destructors run when objects go out of scope [OK]

Common Mistakes:

• Forgetting destructor runs at block end
• Mixing order of destruction
• Ignoring output inside main

4. Identify the error in the following C++ code snippet related to object destruction:

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

int main() {
  Sample s;
  return 0;
}

medium

A. Destructor has a return type which is not allowed.

B. Destructor name is incorrect; it should be 'destruct'.

C. Constructor is missing a return type.

D. Destructor should be public static.

Solution

1. Step 1: Check destructor declaration

   Destructor must not have a return type. Here, 'void' before '~Sample()' is invalid syntax.

2. Step 2: Verify other options

   Destructor name is correct with '~Sample'. Constructors have no return type but that is correct here. Destructors cannot be static.

3. Final Answer:

   Destructor has a return type which is not allowed. -> Option A

4. Quick Check:

   Destructor = no return type [OK]

Hint: Destructor never has a return type, not even void [OK]

Common Mistakes:

• Adding void or other return types to destructor
• Misnaming destructor
• Making destructor static

5. You have a class Resource that opens a file in its constructor and closes it in its destructor. Which of the following code snippets correctly ensures the file is closed automatically when the object goes out of scope?

hard

A. void useFile() {
Resource* r = new Resource();
// use file
delete r;
} // file closed here automatically

B. void useFile() {
Resource* r = new Resource();
// use file
} // file closed here automatically

C. void useFile() {
Resource r;
// use file
delete &r;
} // file closed here

D. void useFile() {
Resource r;
// use file
} // file closed here automatically

Solution

1. Step 1: Understand automatic destruction

   Objects created on stack (like 'Resource r;') are destroyed automatically when going out of scope, calling destructor to close file.

2. Step 2: Analyze dynamic allocation

   Objects created with 'new' must be deleted manually; otherwise destructor won't run automatically, risking resource leaks.

3. Final Answer:

   void useFile() { Resource r; // use file } // file closed here automatically -> Option D

4. Quick Check:

   Stack objects auto-destroyed, heap objects need delete [OK]

Hint: Use stack objects for automatic destructor calls [OK]

Common Mistakes:

• Forgetting to delete heap objects
• Trying to delete stack objects
• Assuming destructor runs without delete on heap