C++ Interview Questions and Answers

C++ not only preserves all elements of the C language, but it also simplifies memory management and introduces many new features, such as:

We know that when we generate a class object dynamically from heap memory with the new keyword, we must specifically delete it after we are finished with its operations to prevent memory leaks in C++ programs.

As an example, consider the class Pen below.

class Pen {
  
public:
      Pen() { 
         cout << "Constructor..." <<endl; 
      }
		
      ~Pen() { 
         cout << "Destructor...!" <<endl; 
      }
	  void write(){
		  cout << "Writing...!" <<endl; 
	  }
};

Below is an example, of how we create an object of the class Pen dynamically and delete it after we are done with operations.

int main()
{
	//create an object dynamically
   Pen* pen = new Pen();
   pen->write();//operations
   
   delete pen;//de-allocate the memory
   
   return 0;
}

How to create an array of objects of a class in C?

Let’s see how we create an array of objects in the below C++ program example. In this example, we will create an array of 3 objects, perform operations and delete a dynamic array of pointers properly.

int main()
{
	//create an array of objects
   Pen* pen = new Pen[3];

   pen[0].write();
   pen[1].write();
   pen[2].write();
   
   delete [] pen;//de-allocate array of objects

   return 0;
}

Output:

Constructor…
Constructor…
Constructor…
Writing…!
Writing…!
Writing…!
Destructor…!
Destructor…!
Destructor…!

Proof of proper deletion of an array of objects in C++

First, note that if we create an object of a class and delete the object then class constructor and destructor will be called respectively.

In the above example, we have created an array of 3 objects and deleted it using statement delete [] pen; if we look at the output the class constructor and destructor were called 3 times each. means, we have deleted objects properly.

Now, If we delete the object using the statement “delete pen” ( This is a general mistake a programmer makes) then the destructor will be called only once, and for the rest 2 objects destructor will not be called and the program will crash. and this will prove that objects are not deleted properly.

Let’s see the output if we use the statement “delete pen”

int main()
{
	//create an array of objects
   Pen* pen = new Pen[3];

   pen[0].write();
   pen[1].write();
   pen[2].write();
   
   //this is not a prper way to delete
   //array of objects
   delete pen;

   return 0;
}

Output:

Constructor…
Constructor…
Constructor…
Writing…!
Writing…!
Writing…!
Destructor…!

and then the program will crash at run time.

Conclusion:

In C++, a single object of a class created dynamically is deleted using the statement “delete pointer” and an array of objects are deleted using the statement “delete [ ] pointer”.