Top 20 C++ interview questions and answers in 2024

Top 20 C++ Asked Interview Questions and Answers in 2024

I still remember my first C++ developer interview—fresh out of college, confident, and completely blindsided by the depth of the questions. I walked in thinking I knew enough because I’d been coding for years. But when the interviewer started digging into memory management and object-oriented programming, I quickly realized there was much more to learn. Concepts like polymorphism and virtual functions tripped me up, and I wished I had prepared more thoroughly.

That experience taught me two critical lessons. First, C++ demands a deep understanding and can’t be approached casually. Second, thorough interview preparation is essential, especially with a language as complex and powerful as C++.

What interviewers love to test

From my interview experiences, I’ve learned that preparing for a C++ interview means diving deeper than just basic syntax. Interviewers often focus on core principles like memory management, stack vs. heap, and how polymorphism works internally. Mastering these advanced topics can be the key difference between landing the job and missing out.

Structure of the blog

This blog is designed to help you avoid my mistakes and take your next interview prepared and confident. It’ll cover 20 of the most commonly asked C++ interview questions, breaking down each answer so you understand the “what” and the “why.” From the basics of object-oriented programming to advanced topics like smart pointers and memory management, this guide will be your go-to resource as you prepare to ace your next C++ interview.

Key objectives:

  • Grasp basic C++ concepts: Understand core features like classes, structures, inheritance, and pointers, which form the foundation of C++.
  • Master advanced features: Learn about templates, smart pointers (unique_ptr, shared_ptr, weak_ptr), and RAII principles to manage resources efficiently.
  • Understand memory management: Gain insights into destructors, dynamic memory allocation, and the differences between malloc() and new.
  • Grasp polymorphism and virtual functions: Learn how virtual functions work and the importance of vtables and vpointer in enabling runtime polymorphism.
  • Master type casting techniques: Differentiate between static_cast, dynamic_cast, const_cast, and reinterpret_cast with practical coding examples.

1. Basic C++ questions

Let’s start with some fundamental questions that will help you understand the core features of C++ and set a solid foundation for more advanced topics.

Question 1: What are the basic features of C++?

FeatureDescription
EncapsulationBundles data and methods into a single unit, ensuring better data protection.
Classes and objectsClasses are blueprints for creating objects, enabling organized code.
Object-oriented programmingEmphasizes using objects and classes for structuring code.
InheritanceEnables new classes to be created from existing ones, promoting reusability.
PolymorphismAllows treating objects of different classes as objects of a common base.

Question 2: What is the difference between C and C++?

AspectCC++
Programming paradigmProceduralSupports both procedural and object-oriented programming (OOP)
Standard libraryLimited standard functionsExtensive standard template library (STL)
Memory managementManual memory managementSupports constructors, destructors, and smart pointers
OverloadingNot supportedFunction and operator overloading supported
Type checkingLess strictStronger type checking mechanisms

Question 3: What are the data types in C++?

Data Type CategoryExamplesDescription
Primitiveint, float, char, boolBasic types representing fundamental data
DerivedArrays, pointers, referencesTypes derived from primitive types
User-definedStructures, unions, classesCustom types, which are created by users for specific use cases

Question 4: Explain the concept of pointers in C++.

Pointers are variables that store the memory address of another variable. They are a powerful feature of C++ that enables efficient memory management and facilitates complex data structures like linked lists, trees, and graphs. Here’s why pointers are important:

  • Memory access: Pointers allow direct access to memory locations, which can be essential for certain applications, such as working with hardware or optimizing performance.
  • Dynamic memory allocation: By using pointers with functions like new and delete, you can dynamically allocate and deallocate memory, giving you control over resource usage.
  • Pass by reference/pointer: In C++, you can pass variables to functions by reference or pointer instead of by value. This approach improves performance, particularly when dealing with large data structures, by avoiding unnecessary data copying.

The following illustration will show you how a pointer holds the address of a variable:

Pointers in C++

Question 5: What is the this pointer in C++?

The this pointer is a special pointer in C++ that is automatically passed to every non-static member function of a class. It points to the object for which the member function is called. Here’s what it does:

  • Access object members: Inside a member function, this can be used to access the object’s data members and member functions.
  • Handle ambiguity: It helps resolve ambiguity when member names conflict with function parameters or local variables.
  • Return current object: The this pointer can be used to return the current object from a member function, enabling method chaining in C++ (e.g., return *this).

Let’s dive into some key OOP-related questions to deepen your understanding of how C++ uses object-oriented principles.

2. Object-oriented programming (OOP) related questions

Let’s dive into some key OOP-related questions to deepen your understanding of how C++ uses object-oriented principles.

Question 6: What is the difference between structure and class in C++?

In C++, the difference between structures and classes lies in their access specifiers:

  • Structure: Members of a structure are public by default.
  • Class: Members of a class are private by default.

Code example

C++

Real-world example

The Employee structure can be compared to a company directory where basic information, such as employee ID and name, is publicly accessible, allowing anyone to view or modify these details without restrictions, as all struct members are public by default. On the other hand, the Manager class resembles an HR system that stores sensitive data, like performance reviews or salary details. Access to this information is restricted, and only authorized personnel (specific functions) can modify or view these details because class members are private by default and can only be accessed through controlled functions like setId and getId.

Question 7: What is inheritance and its types in C++?

Inheritance allows a class to inherit properties and methods from another class. There are four types of inheritance in C++:

  • Single inheritance: Inheritance from one base class
  • Multiple inheritance: Inheritance from more than one base class
  • Multilevel inheritance: A class inherits from another derived class
  • Hybrid inheritance: A combination of multiple and multilevel inheritance

Code example

Real-world example:

Imagine an ElectricCar class inheriting basic functionality from a base Vehicle class, such as start() and stop(). This represents single inheritance, where ElectricCar reuses common vehicle behaviors. Now, consider a FlyingCar class that inherits from both Car and Aircraft classes. Here, the FlyingCar would gain functionalities of both driving on roads (from Car, like drive()) and flying in the air (from Aircraft, like fly()), showcasing multiple inheritance where a single class can inherit characteristics from more than one base class.

Question 8: What is polymorphism in C++?

Polymorphism allows the same function or operator to behave differently based on the object it’s acting upon. It comes in two forms:

  • Compile-time polymorphism (Static polymorphism): Achieved through function or operator overloading.
  • Runtime polymorphism (Dynamic polymorphism): Achieved through inheritance and virtual functions.

Code example

Real-world example

Compile-time polymorphism could be used in a payment system to handle data types (e.g., credit card numbers, PayPal IDs) with overloaded functions. Runtime polymorphism ensures that the right processPayment() method is invoked depending on whether the object is a CreditCardPayment or PayPalPayment object.

Question 9: What is encapsulation in C++?

Encapsulation is the bundling of data and methods that operate on that data within a single unit (class) while restricting direct access to some components, making data more secure.

Code example

Real-world example

Think of a bank account system. The balance is private, meaning it cannot be directly modified. Only authorized methods, such as deposit() and withdraw(), can change the balance, protecting sensitive data from unauthorized access.

Question 10: What is abstraction in C++?

Abstraction is the concept of hiding the internal details of a system and exposing only essential functionalities. In C++, abstraction is achieved through abstract classes and interfaces (pure virtual functions).

Code example

Real-world example

In graphics software, the user doesn’t need to know how a shape is drawn internally. They only use the draw() function to render the shape on the screen, with the actual implementation hidden behind the abstraction.

3. Advanced C++ questions

Let’s explore advanced C++ concepts crucial for mastering the language and excelling in technical interviews.

Question 11: What are templates in C++?

Templates in C++ allow you to write generic and reusable code. They enable functions and classes to operate with different data types without rewriting code for each type. There are two main types of templates:

  • Function templates: Allows creating a function that can operate on different data types.

Example

Function template

  • Class templates: Allow classes to operate with any data type.

Example

Class template

Real-world example

In an actual e-commerce platform, such a design can handle carts with any type of product (books, electronics, groceries, etc.) without needing separate implementations for each product type. Templates allow the system to scale and adapt as more types of items are added, reducing code duplication while maintaining flexibility.

Question 12: What is the difference between deep copy and shallow copy?

A shallow copy duplicates an object but not the objects it points to, meaning both copies will share the same memory addresses for any pointer data members. On the other hand, a deep copy creates a new copy of the object and all the objects it references, ensuring complete separation.

Shallow copy example

Shallow copy

Deep copy example

Deep copy

Real-world example

Deep copying is crucial when handling dynamic memory allocation to avoid unexpected modifications when working with object copies, especially in data processing or graphics rendering cases.

Question 13: What is a virtual function in C++?

A virtual function is a member function in a base class declared with the virtual keyword, allowing it to be overridden in derived classes. It ensures that the correct function is called for an object, regardless of the type of reference (or pointer) used to call the function. Virtual functions enable runtime polymorphism.

Code example

Virtual function in C++

  • VTable (Virtual table): A table that stores addresses of the class’s virtual functions.
  • VPointer (Virtual pointer): A hidden pointer within each class object points to the VTable.

Real-world example

Virtual functions are commonly used in software design patterns like the Strategy pattern, where an object’s behavior can be changed at runtime based on which derived class it points to.

Question 14: What is the use of the mutable keyword in C++?

The mutable keyword allows a class member to be modified even if it is part of an object declared as const. This is useful for members that are meant to be updated internally without affecting the logical constancy of the object.

Code example

‘mutable’ keyword in C++

Real-world example

A logger class might use mutable to keep track of the number of times a log is printed, even if the logger object itself is declared const.

Question 15: What is the RAII principle in C++?

RAII (Resource acquisition is initialization) is a programming idiom that ensures resources are properly acquired and released. It ties the resource life cycle to the object’s lifetime, meaning resources are acquired in the constructor and released in the destructor.

Code example

RAII principle in C++

Real-world example: RAII is commonly used in managing resources such as file handles, database connections, and mutex locks in multi-threaded programming. It helps prevent resource leaks by ensuring that resources are properly cleaned up when the associated object goes out of scope.

4. Memory Management and Performance Questions

Let’s delve into some crucial questions about memory management and performance optimization techniques in C++, which are essential for writing efficient and safe code.

Question 16: What are smart pointers in C++?

Smart pointers are a safer alternative to traditional pointers in C++. They help manage the lifetime of dynamically allocated objects and automatically deallocate memory when the object is no longer in use, reducing the risk of memory leaks.

  • unique_ptr: Owns an object exclusively. It cannot be copied, only moved, ensuring only one pointer can manage the resource at any time.

Code example

unique_ptr in C++

  • shared_ptr: Allows multiple pointers to share ownership of an object. It maintains a reference count and deallocates the object when the count reaches zero.

Code example

shared_ptr in C++

  • weak_ptr: Used with shared_ptr to hold a non-owning reference to the object, preventing circular references that could cause memory leaks.

Code example

weak_ptr in C++

Real-world example

To ensure proper cleanup, smart pointers are commonly used in resource management, such as managing database connections, file handles, and network sockets.

Question 17: What is the difference between malloc() and newin C++?

  • malloc(): It’s a C library function that allocates memory on the heap but does not call constructors for object initialization. Memory allocation must be manually deallocated using free().

Code example

malloc() in C++

  • new: It’s a C++ operator that allocates memory and calls the constructor to initialize objects. Memory is deallocated using the delete operator.

Code example

new operator in C++

Real-world example

new is preferred in C++ for object-oriented programming as it handles allocation and initialization, whereas malloc() is primarily used in legacy C codebases.

Question 18: What are destructors in C++?

A destructor is a special member function of a class that is automatically called when an object goes out of scope or is explicitly deleted. It releases resources, such as closing file streams or deallocating memory.

Code example

Destructors in C++

Real-world example

Destructors are vital in managing resources like file handles, database connections, and sockets, ensuring they are properly closed when no longer needed.

Question 19: How does dynamic memory allocation work in C++?

Dynamic memory allocation allows programs to allocate memory at runtime from the heap, unlike stack memory, which is limited and automatically managed.

  • Heap: Large pool of memory managed manually. Used for objects that need to persist beyond the function call.
  • Stack: Fast but limited memory used for function call management. Automatically deallocates memory when the function exits.

Code example

Dynamic memory allocation work in C++

Real-world example

Dynamic memory allocation is commonly used in applications that require large and flexible data storage, such as graphics rendering engines, where the exact amount of memory needed is only known at runtime.

Question 20: What is the use of static_cast, dynamic_cast, const_cast, and reinterpret_cast?

C++ provides four types of type casting operators:

  • static_cast: Used for converting between related types, such as converting int to float or downcasting pointers within the same class hierarchy.

Code example

static_cast in C++

  • dynamic_cast: Used for safely casting pointers and references within an inheritance hierarchy. It checks the validity of the cast at runtime.

Code example

dynamic_cast in C++

  • const_cast: Used to add or remove the const qualifier from a variable.

Code example

const_cast in C++

Note: Modifying a const variable using const_cast leads to undefined behavior. While the code will compile and run, it’s generally unsafe and should be avoided in real applications.

  • reinterpret_cast: Reinterprets the bits of a type as another type without any safety checks. It’s used for low-level, unsafe conversions.

Code example

reinterpret_cast in C++

Real-world example

These casts are essential in low-level programming, such as systems development, graphics programming, or when interfacing with hardware where specific data reinterpretation is required.

5. Tips to ace C++ interviews in 2024

Let’s explore some effective tips to help you excel in C++ interviews and stand out as a top candidate.

Research the company’s tech stack

Understanding the company’s tech stack is crucial for tailoring your interview preparation. Research how the company uses C++—for system development, embedded systems, game development, or finance. For instance, if the company focuses on embedded systems, you should be well-versed in low-level memory management, real-time constraints, and performance optimization in C++. Tailoring your preparation to their specific domain will show your proactive approach and make you stand out as a candidate who understands the practical application of C++ in their business.

Example tip

If interviewing at a game development company, emphasize your knowledge of C++’s performance aspects, multithreading, and graphics programming libraries like DirectX or OpenGL.

Brush up on data structures and algorithms

A solid understanding of Data Structures and Algorithms (DSA) is just as important as knowing C++ syntax. Many interview questions will test your problem-solving skills and ability to use the right data structure for a given scenario. You’ll often encounter questions about arrays, linked lists, trees, stacks, queues, hash maps, and algorithms like sorting, searching, and dynamic programming.

Example tip

Focus on optimizing your code’s time and space complexity, and practice implementing algorithms in C++. Understanding how to manipulate pointers, manage memory, and optimize recursive functions is crucial, especially in competitive coding environments.

Practice coding challenges

Hands-on practice is essential for mastering C++ interview questions. We include many problems in the course below to help you sharpen your C++ skills. Start with easy problems to build confidence and gradually move on to more complex challenges.

Grokking the Coding Interview Patterns in C++: The ultimate guide to coding interviews in C++, Grokking the Coding Interview Patterns in C++ teaches proven strategies developed by FAANG engineers to get interview-ready in just a few hours.

Example tip

Regularly participate in coding problems to improve your problem-solving speed and learn how to tackle new and unexpected challenges. This will prepare you for the technical aspects of the interview and train you to think quickly under pressure.

6. Conclusion

Throughout this blog, we’ve covered the essential C++ concepts and questions you will likely encounter in interviews. From basic C++ features like classes and pointers to advanced topics such as smart pointers, polymorphism, and memory management, this guide equips you with the knowledge to tackle beginner and advanced-level questions. We also highlighted the importance of understanding object-oriented programming principles, dynamic memory allocation, and the various types of casting in C++—all critical for mastering the language and impressing interviewers.

If you found this guide helpful, don’t stop here! Explore our other blogs on data structures, algorithms, and advanced C++ techniques to deepen your understanding.

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Frequently Asked Questions

What is the difference between malloc() and new in C++?

malloc() is a C library function used for memory allocation on the heap, which does not initialize the allocated memory or call constructors, making it unsuitable for C++ objects. It requires the use of free() to deallocate memory. On the other hand, new is a C++ operator that allocates memory and initializes objects by calling their constructors. new also provides type safety and throws exceptions on failure, unlike malloc(), which returns NULL. delete is used to deallocate memory allocated by new, making it a more integrated and safer option in C++.

What are smart pointers in C++, and why are they used?

Smart pointers (unique_ptr, shared_ptr, and weak_ptr) are C++ objects that manage the lifetime of dynamically allocated resources, providing automatic memory management to avoid memory leaks. unique_ptr provides exclusive ownership of an object, meaning only one pointer can point to the resource at a time. shared_ptr allows multiple pointers to share ownership of an object with reference counting, while weak_ptr provides a non-owning reference that prevents cyclic references. Smart pointers automatically release resources when no longer needed, reducing manual memory management errors.

Explain the concept of RAII (Resource acquisition is initialization) in C++.

RAII is a C++ programming idiom where resource management, such as memory allocation or file handling, is tied to the lifespan of objects. Resources are acquired in a class constructor and automatically released in the destructor, ensuring that resources are cleaned up properly when an object goes out of scope. This approach helps prevent resource leaks and ensures robust exception safety. RAII is commonly used in C++ to manage dynamic memory, file streams, and locks, providing a safe and efficient way to handle resources.

What is the difference between shallow copy and deep copy in C++?

A shallow copy duplicates an object and its pointers but not the objects pointed to, resulting in shared memory between the original and the copy. This can lead to unintended side effects, such as one object modifying the shared resource. In contrast, a deep copy duplicates the object and the data it points to, creating independent copies. Deep copies are essential when dealing with dynamic memory to ensure that changes in one object do not affect another, preserving data integrity.

What are virtual functions, and why are they important in C++?

Virtual functions are member functions in a base class that can be overridden in derived classes to achieve runtime polymorphism. They allow the most derived function version to be called through a base class pointer or reference, ensuring the correct function implementation is executed based on the actual object type. Virtual functions rely on a mechanism called the virtual table (vtable) and virtual pointer (vptr) to dynamically resolve the function call. They are crucial for designing flexible and extensible software, enabling derived classes to modify or extend base class behaviors.