30 OOPs Interview Questions and Answer in C++ (2023)

C++ is a powerful programming language known for its extensive use of Object-Oriented Programming (OOP) principles. In the world of software development, mastering OOP concepts is crucial for building scalable, modular, and maintainable code. Whether you're a beginner looking to grasp the fundamentals or an experienced developer aiming to refine your knowledge, this article provides detailed answers to the top 30 OOP interview questions in C++.

1. What is Object-Oriented Programming (OOP)?

Answer: Object-oriented programming is a programming paradigm that uses objects to model real-world entities and organizes code into classes and objects. It promotes the use of concepts like encapsulation, inheritance, and polymorphism to enhance code modularity and reusability.

2. What are the four fundamental principles of OOP?

Answer: The four fundamental principles of OOP are:

• Encapsulation: This principle bundles data (attributes) and the methods (functions) that operate on that data into a single unit, the class.

• Abstraction: Abstraction involves hiding complex implementation details and exposing only the necessary features of an object.

• Inheritance: Inheritance allows a class to inherit properties and behaviors from another class. It promotes code reuse and the creation of a hierarchy of classes.

• Polymorphism: Polymorphism enables objects of different classes to be treated as objects of a common base class. It includes function overloading and function overriding.

3. What is a class in C++?

Answer: A class in C++ is a blueprint for creating objects. It defines the structure (attributes or properties) and behavior (methods or functions) that objects of that class will have.

4. What is an object in C++?

Answer: An object in C++ is an instance of a class. It represents a specific real-world entity with its own data (attributes) and behavior (methods).

Example:

#include <iostream>

// Define a class named 'Person'
class Person {
public:
    // Public members of the class
    std::string name;
    int age;

    // Member function to introduce the person
    void introduce() {
        std::cout << "I am " << name << " and I am " << age << " years old.";
    }
};

int main() {
    // Create an object of the 'Person' class
    Person person1;

    // Assign values to the object's members
    person1.name = "Alice";
    person1.age = 30;

    // Call the member function to introduce the person
    person1.introduce();

    return 0;
}

Output:

I am Alice and I am 30 years old.

In the above code, we define a class named Person that has two public data members: name and age, and a public member function introduced to introduce the person. In the main function, we create an object of the Person class named persons.

5. What is the difference between a class and an object?

Answer: A class is a template or blueprint for creating objects. It defines the structure and behavior of objects but doesn't occupy memory. An object, on the other hand, is an instance of a class, representing real data and taking up memory.

6. What is Encapsulation in OOP?

Answer: Encapsulation is the principle of bundling data and the methods that operate on that data into a single unit (a class). It provides data hiding, protecting the integrity of an object's data from external access.

7. What is a constructor in C++?

Answer: A constructor in C++ is a special member function of a class that is automatically called when an object of the class is created. It is used for initializing object properties.

8. What is the destructor in C++?

Answer: A destructor in C++ is a special member function that is called when an object goes out of scope or is explicitly destroyed. It's used for cleaning up resources and deallocating memory.

9. What is the "this" pointer in C++?

Answer: The "this" pointer in C++ is a pointer that points to the current instance of an object. It is used within class methods to access the object's members and avoid naming conflicts with local variables.

10. What is inheritance in C++?

Answer: Inheritance is a mechanism in C++ that allows a class to inherit the properties and behaviors of another class. It promotes code reuse and the creation of a hierarchy of classes.

11. Explain the types of inheritance in C++.

Answer: There are several types of inheritance in C++:

• Single Inheritance: A class inherits from a single base class.
• Multiple Inheritance: A class can inherit from multiple base classes.
• Multilevel Inheritance: A class derives from a class that, in turn, derives from another class.
• Hierarchical Inheritance: Multiple derived classes inherit from a single base class.

12. What is a base class and a derived class in C++?

Answer: A base class is the class being inherited from, and a derived class is the class that inherits from a base class. The derived class inherits the properties and behaviors of the base class.

• Base Class: A base class, often referred to as a parent class or superclass, is a class from which other classes (derived classes) inherit properties and behaviors. It serves as a template or blueprint for creating derived classes.

• Derived Class: A derived class, often referred to as a child class or subclass, is a class that inherits attributes and behaviors from a base class. The derived class extends or specializes the functionality of the base class by adding new attributes and methods or by modifying the inherited ones.

13. What is polymorphism in C++?

Answer: Polymorphism is a key concept in OOP. It allows objects of different classes to be treated as objects of a common base class. There are two main types of polymorphism in object-oriented programming:

1. Compile Time Polymorphism.

Compile Time Polymorphism also known as static binding or early binding occurs at compile time when the program is being compiled. It is achieved through function overloading and operator overloading.

Function Overloading: Function overloading is the ability of a class to have multiple functions with the same name, provided that they have different parameter lists. The appropriate function to call is determined at compile time based on the number and types of arguments passed. (alert-passed)

Example:

// Example of Function overloading in C++
class MathOperations {
public:
    int add(int a, int b) {
        return a + b;
    }

    double add(double a, double b) {
        return a + b;
    }
};

2. Run Time Polymorphism.

Run Time Polymorphism also known as late binding or dynamic binding occurs at runtime when the program is executed, it is achieved through method overriding, virtual functions, and interfaces/abstract classes.

Method Overriding: Method overriding is the ability of a derived class to provide a specific implementation for a method that is already defined in its base class. The appropriate function to call is determined at runtime, based on the actual type of the object.(alert-passed)

Example:

// Example of Method overriding
class Shape {
public:
    virtual double area() {
        return 0.0; // Base class provides a default implementation
    }
};

class Circle : public Shape {
public:
    double area() override {
        return 3.14 * radius * radius;
    }
};

14. Explain function overloading in C++.

Answer: Function overloading is the ability to define multiple functions with the same name but different parameter lists in the same class. The appropriate function is selected at compile time based on the number and types of arguments.

15. What is function overriding in C++?

Answer: Function overriding is the process of providing a specific implementation for a method defined in a base class. It allows a derived class to provide its own implementation of a method with the same name and parameters as the base class.

16. What is an abstract class in C++?

Answer: An abstract class in C++ is a class that cannot be instantiated. It is typically used as a base class and may contain pure virtual functions, which must be implemented by derived classes.

17. What is a virtual function in C++?

Answer: A virtual function is a member function declared in a base class with the `virtual` keyword. It allows derived classes to provide their own implementations, enabling dynamic binding and polymorphism.

18. What is a pure virtual function in C++?

Answer: A pure virtual function is a virtual function that is declared in a base class but has no implementation.

It is defined with the `= 0` syntax, and derived classes must provide an implementation.

19. What is an interface class in C++?

Answer: C++ doesn't have a built-in "interface" keyword like some other languages. Instead, an interface is often implemented using an abstract base class with pure virtual functions.

20. What is operator overloading in C++?

Answer: Operator overloading allows you to define how C++ operators should work with user-defined types. For example, you can define custom behavior for operators like `+`, `-`, or `==` for your classes.

21. What are the new and delete operators in C++?

Answer: `new` is used to dynamically allocate memory for objects, and `delete` is used to deallocate memory. They are used to manage dynamic memory allocation.

22. What are templates in C++?

Answer: Templates allow you to define generic types and functions that can work with different data types without code duplication. They facilitate code reuse and type safety, especially when working with collections and algorithms.

23. Explain the role of the "friend" keyword in C++.

Answer: The "friend" keyword allows a function or class to access the private and protected members of another class. It promotes encapsulation while providing exceptions.

24. What is a smart pointer in C++?

Answer: A smart pointer is a C++ object that manages the memory allocated for another object. Types include shared_ptr, unique_ptr, and weak_ptr. They help prevent memory leaks and provide automatic memory management.

25. What is the RAII (Resource Acquisition Is Initialization) principle in C++?

Answer: RAII is a programming paradigm where resource management is tied to the lifetime of objects. Resources are acquired in constructors and released in destructors. This principle helps ensure that resources are properly managed and released, even in the presence of exceptions.

26. What is dynamic binding (late binding) in C++?

Answer: Dynamic binding allows the selection of the appropriate function to be delayed until runtime. It enables polymorphism, where the actual function to be called is determined based on the runtime type of an object.

27. What is a vtable in C++?

Answer: A vtable, short for virtual function table, is a data structure used for dynamic dispatch in C++. It maps virtual functions to their implementations. Each class with virtual functions has its own vtable.

28. What is the C++ Standard Template Library (STL)?

Answer: The C++ Standard Template Library (STL) is a set of C++ template classes to provide general-purpose classes and functions with templates to implement many popular and commonly used algorithms and data structures. It includes containers like vectors, maps, and algorithms like sorting and searching.

29. How does C++ handle multiple inheritance, and what is the diamond problem?

Answer: C++ supports multiple inheritance, allowing a class to inherit from multiple base classes. The "diamond problem" occurs when a class inherits from two or more classes that share a common base class. It can lead to ambiguity. To resolve the diamond problem, C++ uses the "virtual" keyword to specify virtual inheritance, ensuring that there's only one shared base class instance.

30. How do you implement an interface in C++?

Answer: In C++, interfaces are implemented using abstract classes with pure virtual functions. A class can derive from the abstract class and must provide implementations for all the pure virtual functions to satisfy the interface requirements.

These detailed answers should help you prepare for C++ OOP interviews by better understanding the concepts and principles involved.

Difference Between Cluster and Non-Cluster Index in SQL.

In Relational Databases, indexes are crucial in optimizing data retrieval operations. Clustered and non-clustered indexes are two common types of indexes used in database management systems. They serve similar purposes but differ in their structures and functionality. Here in this article, we are going to understand the difference between Cluster and Non-Cluster Index.

What is an Index?

An index is a data structure that improves the speed of data retrieval operations on a database table. It's essentially a copy of a portion of the table data, organized in a way that allows for faster lookup and retrieval of specific rows.

Cluster Index in SQL.

A clustered index determines the physical order of the data rows in a table. Here are the key characteristics of a clustered index:

• Unique: There can be only one clustered index per table because it defines the physical order of rows.

• Data Storage: The actual data rows are stored in the order of the clustered index.

• Primary Key: By default, the primary key of a table is used to define the clustered index. This enforces a unique key constraint on the primary key column.

Example of a Clustered Index.

Let's consider a simplified table of student records:

StudentID	Name	Age	GPA
101	John	23	9.2
102	Mohit	21	8.5
103	Alice	20	9.5
104	Charlie	22	8.4

In this case, if the StudentID column is defined as the primary key, it becomes the clustered index. The rows are physically stored in the order of StudentID.

Non-Cluster Index in SQL.

A non-clustered index does not dictate the physical order of the data rows but instead provides a separate structure for fast data retrieval. Here are the key characteristics of a non-clustered index:

• Multiple Indexes: You can have multiple non-clustered indexes on a single table.

• Data Storage: The data rows are not stored in the order of the non-clustered index.

• Fast Data Retrieval: Non-clustered indexes improve the speed of SELECT, JOIN, and WHERE clause queries.

Example of a Non-Clustered Index.

Continuing with our student records example, if you want to quickly retrieve students by their Name, you can create a non-clustered index on the Name column. This index would store a sorted list of student names and their corresponding StudentID values. When you query for a student by name, the database can efficiently look up the StudentID using the non-clustered index and then use that ID to locate the actual data row.

Difference Between Cluster and Non-Cluster Index.

Cluster Index	Non-Cluster Index
Cluster Index Dictates the physical order of data rows in the table.	Non-Cluster Index Does not dictate the physical order of data rows.
Only one clustered index per table.	Multiple non-clustered indexes per table.
Actual data rows are stored in the order of the clustered index.	Data rows are not stored in the order of the non-clustered index.
By default, the primary key is often used as the clustered index.	Not necessarily associated with the primary key.
Data modifications (INSERT, UPDATE, DELETE) can be slower when affecting the order defined by the clustered index.	Data modifications do not directly impact data order.
Generally faster for range-based queries (e.g., date ranges) or specific lookup by the clustered key.	Improves SELECT, JOIN, and WHERE clause queries on indexed columns.
Automatically enforces a unique constraint on the clustered key.	Can be used to enforce unique constraints, but it's not automatic.

Clustered and non-clustered indexes are fundamental tools for optimizing data retrieval operations in a relational database. Understanding their differences and use cases is essential for designing efficient database schemas. When used correctly, these indexes can significantly improve the performance of your database-driven applications.

C# Program to Remove Spaces in a String.

Problem: Given a string str, you need to remove all the whitespace from the given string and return a new string without spaces. Write a C# code to solve this problem of removing all spaces from the given string.

Example:

Input: Hello AlgoLesson
Output: HelloAlgoLesson

Input: Good Morning
Output: GoodMorning

We can solve this problem with multiple approaches and here we are going to discuss three different approaches to solve this.

Approach 1: Using string.Replace Method.

In this approach, we use the built-in string.Replace method to replace spaces with an empty string. We will call the Replace method on the input string and provide two arguments: the first argument is the character or substring to be replaced (in this case, a space), and the second argument is the replacement string (an empty string ""). 

Example C# Code:

// C# code implementation to remove whitespace from string
using System;

public class HelloWorld
{
    public static string RemoveSpacesUsingReplace(string input)
    {
        return input.Replace(" ", "");
    }
    public static void Main(string[] args)
    {
        string str = "Welcome to AlgoLesson";
        string result = RemoveSpacesUsingReplace(str);
        
        Console.WriteLine ("With Whitespace: " + str);
        Console.WriteLine ("Without Whitespace: " + result);
    }
}

Output:

With Whitespace: Welcome to AlgoLesson
Without Whitespace: WelcometoAlgoLesson

Approach 2: Using string.Join Method.

This approach splits the input string into an array of substrings using spaces as separators and then joins the substrings together without spaces. We use the Split method to split the input string into an array of substrings. The new[] { ' ' } argument specifies that we want to split the string at spaces. StringSplitOptions.RemoveEmptyEntries removes any empty substrings resulting from consecutive spaces. We then use string.Join to concatenate the array of substrings into a single string without spaces.

Example C# Code:

// C# code implementation to remove whitespace from string using Join method
using System;

public class AlgoLesson
{
    public static string RemoveSpacesUsingSplitAndJoin(string input)
    {
        string[] words = input.Split(new[] { ' ' }, StringSplitOptions.RemoveEmptyEntries);
        return string.Join("", words);
    }
    public static void Main(string[] args)
    {
        string str = "Welcome to AlgoLesson";
        string result = RemoveSpacesUsingSplitAndJoin(str);
        
        Console.WriteLine ("With Whitespace: " + str);
        Console.WriteLine ("Without Whitespace: " + result);
    }
}

Output:

With Whitespace: Welcome to AlgoLesson
Without Whitespace: WelcometoAlgoLesson

Approach 3: Using string Builder.

This approach uses a StringBuilder to efficiently remove spaces by appending non-space characters. We initialize a StringBuilder (sb) to build the resulting string. We iterate through each character in the input string. If a character is not a space, we append it to the StringBuilder. After processing all characters, we convert the StringBuilder to a string using the ToString method and return the cleaned string.

Example C# Code:

// C# code implementation to remove whitespace from string using string builder
using System;

public class HelloWorld
{
    public static string RemoveSpacesUsingStringBuilder(string input)
    {
        StringBuilder sb = new StringBuilder();
    
        foreach (char c in input)
        {
            if (c != ' ')
            {
                sb.Append(c);
            }
        }
    
        return sb.ToString();
    }
    public static void Main(string[] args)
    {
        string str = "Welcome to AlgoLesson";
        string result = RemoveSpacesUsingSplitAndJoin(str);
        
        Console.WriteLine ("With Whitespace: " + str);
        Console.WriteLine ("Without Whitespace: " + result);
    }
}

Output:

With Whitespace: Welcome to AlgoLesson
Without Whitespace: WelcometoAlgoLesson

These are four different approaches to removing spaces from a string in C#. You can choose the one that best suits your specific requirements and coding style.

C# Program Remove Special Characters from String.

In C#, multiple ways exist to remove special characters from a given string. This is a significant problem that we often face when we try to compare two different strings. In this article, we are going to discuss three different ways to remove special characters from a given string.

Approach 1: Using Regular Expressions.

You can utilize regular expressions (Regex) to remove special characters from a string in C#. This method is flexible and allows you to specify which characters to keep or remove.

Example Code: 

// C# program to remove special character using regular expression

using System;
using System.Text.RegularExpressions;

public class HelloWorld
{
    static string RemoveSpecialCharactersRegex(string input)
    {
        // Regular expression pattern to match special characters
        string pattern = @"[^a-zA-Z0-9\s]";
        
        // Regex.Replace to remove special characters from the input string
        return Regex.Replace(input, pattern, "");
    }
    
    public static void Main(string[] args)
    {
        string inputString = "Hello, @World! 123";
        
        // Remove special characters from the input string using Regex
        string result = RemoveSpecialCharactersRegex(inputString);
        
        Console.WriteLine("Original String: " + inputString);
        Console.WriteLine("String with Special Characters Removed: " + result);
    }
}

Output:

Original String: Hello, @World! 123
String with Special Characters Removed: Hello World 123

Approach 2: Using LINQ and Char.IsLetterOrDigit.

You can use LINQ to filter out characters that are letters or digits, effectively removing special characters. This approach will also remove spaces between two words if exist.

Example Code: 

// C# code to remove speical characters from string using LINQ

using System;
using System.Linq;

public class Program
{
    static string RemoveSpecialCharactersLinq(string input)
    {
        // Use LINQ to filter out non-alphanumeric characters and spaces
        string cleanedString = new string(input.Where(char.IsLetterOrDigit).ToArray());
        return cleanedString;
    }

    static void Main(string[] args)
    {
        string inputString = "Hello, @World! 123";
        
        // Remove special characters from the input string using LINQ
        string result = RemoveSpecialCharactersLinq(inputString);
        
        Console.WriteLine("Original String: " + inputString);
        Console.WriteLine("String with Special Characters Removed: " + result);
    }
}

Output:

Original String: Hello, @World! 123
String with Special Characters Removed: HelloWorld123

Approach 3: Using a String Builder.

Using a StringBuilder is a memory-efficient approach for removing special characters from a string. The key advantage of this approach is that it minimizes memory overhead compared to creating multiple string objects. 

Example Code:

// C# code to remove special character using stringbuider function
using System;
using System.Text;

public class Program
{
    static string RemoveSpecialCharactersStringBuilder(string input)
    {
        StringBuilder sb = new StringBuilder();
        
        foreach (char c in input)
        {
            if (char.IsLetterOrDigit(c) || char.IsWhiteSpace(c))
            {
                sb.Append(c);
            }
        }
        
        return sb.ToString();
    }

    static void Main(string[] args)
    {
        string inputString = "Hello, @World! 123";
        
        // Remove special characters from the input string using a StringBuilder
        string result = RemoveSpecialCharactersStringBuilder(inputString);
        
        Console.WriteLine("Original String: " + inputString);
        Console.WriteLine("String with Special Characters Removed: " + result);
    }
}

Output:

Original String: Hello, @World! 123
String with Special Characters Removed: Hello World 123

All three approaches will remove special characters and return a string containing only alphanumeric characters and spaces. You can choose the method that best fits your requirements and coding style.