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Most Askable C++ Interview Questions and Answers

VINNER TECH July 23, 2023



1. What is C++?

C++ is a high-level, general-purpose programming language developed as an extension of the C programming language. It was created by Bjarne Stroustrup in the early 1980s. C++ is known for its efficiency, flexibility, and object-oriented features, making it suitable for various applications, including systems software, game development, embedded systems, and more.

2. Differences between C and C++

C:

  • Procedural programming language
  • Does not support classes and objects
  • Lacks built-in support for object-oriented programming
  • No function overloading
  • Pointers are used extensively
  • Standard I/O using printf() and scanf() functions

C++:

  • Object-oriented programming language
  • Supports classes and objects
  • Provides features like inheritance, encapsulation, and polymorphism
  • Allows function overloading
  • Pointers are used along with references
  • Standard I/O using cin and cout objects from the iostream library

3. Features of C++

  • Object-Oriented: C++ supports classes, objects, inheritance, polymorphism, and encapsulation.
  • Efficient: Provides low-level memory manipulation with pointers and direct hardware access.
  • Standard Library: Offers a rich set of functions and classes for various operations.
  • Portable: Code written in C++ can be compiled and run on different platforms.
  • Extensible: Allows the creation of user-defined types and functions.
  • High Performance: C++ enables direct hardware manipulation and optimized code execution.
  • Exception Handling: Provides a mechanism to handle runtime errors gracefully.
  • Template Support: Supports generic programming through templates.

4. What is an Object-Oriented Programming Language?

Object-oriented programming (OOP) is a programming paradigm that focuses on organizing code into objects, which are instances of classes. OOP encourages concepts like encapsulation, inheritance, polymorphism, and abstraction to structure programs and simplify development.

5. The Four Pillars of Object-Oriented Programming (OOP)

1. Abstraction:
Abstraction allows complex systems to be represented by their essential characteristics, hiding unnecessary details. It helps in creating user-defined data types (classes) that define the object's properties and behavior.

2. Encapsulation:
Encapsulation bundles data (attributes) and functions (methods) that operate on the data, restricting access to the internal implementation details. It protects data from unauthorized access and modification.

3. Inheritance:
Inheritance enables a new class (subclass or derived class) to inherit properties and behaviors from an existing class (base class or superclass). It promotes code reusability and hierarchy in class relationships.

4. Polymorphism:
Polymorphism allows objects of different classes to be treated as objects of a common base class. It enables a single interface to represent multiple data types, providing flexibility and dynamic behavior.

6. What is a Class in C++?

In C++, a class is a user-defined data type that serves as a blueprint for creating objects. It encapsulates data (attributes) and functions (methods) that operate on that data, defining the properties and behaviors of the objects instantiated from the class.

7. What is an Object in C++?

An object is an instance of a class, created based on the blueprint provided by the class. It represents a real-world entity with its attributes and behavior. Objects interact with each other through methods defined in their class.

8. Difference between a Class and an Object

Class:

  • A blueprint that defines the properties and behaviors of objects.
  • It is a user-defined data type.
  • Does not consume memory space when defined.

Object:

  • An instance of a class, representing a real-world entity.
  • Occupies memory space when created.
  • Can have multiple instances of the same class.

9. Access Specifiers in C++ (public, private, protected)

Access specifiers determine the visibility and accessibility of class members (attributes and methods) from different parts of the program.

1. public:

  • Members declared as public are accessible from any part of the program.
  • They can be accessed by objects and derived classes.

2. private:

  • Members declared as private are only accessible within the class itself.
  • They cannot be accessed by objects or derived classes.

3. protected:

  • Members declared as protected are accessible within the class and its derived classes.
  • They are not accessible outside the class hierarchy.

10. What is a Constructor, and How Many Types of Constructors Are There?

A constructor is a special member function in a class that is automatically called when an object is created. It is used to initialize the object's data members and allocate resources if needed.

Types of Constructors in C++:

1. Default Constructor:

  • Takes no arguments and is automatically called if no other constructor is defined.
  • Initializes the object's data members to default values.

2. Parameterized Constructor:

  • Takes parameters and allows custom initialization of data members based on provided values.

3. Copy Constructor:

  • Creates a new object by copying the values of another object of the same class.

4. Destructor:

  • A special type of constructor used to deallocate resources and perform cleanup before an object is destroyed.

11. What is a Destructor, and When Is It Called?

A destructor is a special member function with the same name as the class, prefixed with a tilde (~). It is automatically called when an object goes out of scope or is explicitly deleted using the delete keyword. The destructor is responsible for freeing allocated resources and performing necessary cleanup before the object's memory is deallocated.

12. What is Function Overloading in C++?

Function overloading allows multiple functions with the same name but different parameter lists to coexist in a class. The compiler distinguishes between these functions based on the number or type of parameters, allowing for flexibility in function calls.

13. What is Operator Overloading in C++?

Operator overloading enables custom definitions of operators for user-defined data types (classes). It allows objects of a class to behave like built-in types by redefining the behavior of operators such as +, -, *, /, etc.

14. Concept of Inheritance and Its Types in C++

Inheritance is a fundamental OOP concept where a new class (derived class) can inherit properties and behaviors from an existing class (base class). This promotes code reusability and establishes a hierarchical relationship between classes.

Types of Inheritance in C++:

1. Single Inheritance:

  • A derived class inherits from a single base class.

2. Multiple Inheritance:

  • A derived class inherits from multiple base classes.

3. Multilevel Inheritance:

  • A derived class is derived from another derived class.

4. Hierarchical Inheritance:

  • Multiple derived classes are created from a single base class.

5. Hybrid Inheritance:

  • A combination of two or more types of inheritance.

15. What is Multiple Inheritance, and How is it Handled in C++?

Multiple inheritance is a situation where a class can inherit properties and behaviors from more than one base class. In C++, it is handled by specifying all the base classes in the derived class declaration, separated by commas. The derived class will have access to members of all the base classes.

16. What is Virtual Inheritance?

Virtual inheritance is used to address the "diamond problem" that arises when multiple base classes have a common base class. It ensures that only one instance of the common base class is inherited by the most derived class, avoiding duplication of data.

17. Difference between private, public, and protected Inheritance

Private Inheritance:

  • The derived class has private access to the base class members.
  • It is used when the derived class needs to implement the base class interface but should not be seen as a subtype of the base class.

Public Inheritance:

  • The derived class has public access to the base class members.
  • It is used when the derived class should be seen as a subtype of the base class and can be used interchangeably.

Protected Inheritance:

  • The derived class has protected access to the base class members.
  • It is rarely used and provides a restricted form of inheritance.

18. Explain Polymorphism and Its Types in C++

Polymorphism allows objects of different classes to be treated as objects of a common base class. It enables a single interface to represent multiple data types, providing flexibility and dynamic behavior during runtime.

Types of Polymorphism in C++:

1. Compile-time Polymorphism (Static Polymorphism):

  • Achieved through function overloading and operator overloading.
  • The decision is made at compile time.

2. Runtime Polymorphism (Dynamic Polymorphism):

  • Achieved through function overriding using virtual functions.
  • The decision is made at runtime.

19. What is a Virtual Function?

A virtual function is a function declared in the base class with the virtual keyword. It allows a derived class to provide its own implementation of the function, enabling runtime polymorphism.

20. What is a Pure Virtual Function, and How is it Different from a Virtual Function?

A pure virtual function is declared in the base class using the syntax virtual returnType functionName() = 0;. Unlike a regular virtual function, it does not have any implementation in the base class. Its purpose is to make the base class abstract, meaning it cannot be instantiated, and any derived class must override the pure virtual function to become concrete.

21. How is Runtime Polymorphism Achieved in C++?

Runtime polymorphism is achieved by using virtual functions in C++. When a base class function is marked as virtual, any derived class can provide its own implementation for that function using function overriding. The function called during runtime depends on the actual object type rather than the pointer or reference type.

22. What is an Abstract Class?

An abstract class is a class that contains one or more pure virtual functions. It serves as an interface or blueprint for other classes, providing a common set of methods that must be overridden in derived classes.

23. Can an Abstract Class Have a Constructor?

Yes, an abstract class can have a constructor. The constructor of an abstract class is called when an object of a derived class is created. However, the constructor cannot be used to instantiate an object of the abstract class itself since it is incomplete due to the pure virtual functions.

24. What is a Friend Function in C++?

A friend function is a function that is granted access to the private and protected members of a class even though it is not a member of that class. It is declared using the friend keyword inside the class.

25. What is the Use of this Pointer in C++?

The this pointer is a special pointer in C++ that points to the current instance of the class. It is used to access the members of the current object, especially when there is a naming conflict between a parameter and a member variable. The this pointer is automatically passed to non-static member functions.


26. Explain Static and Dynamic Binding

Static Binding:

  • Also known as Early Binding.
  • The determination of which function to call is done at compile-time.
  • It occurs when the function call is bound to the function definition based on the static type of the object.
  • Mostly associated with functions in the base class being called.

Dynamic Binding:

  • Also known as Late Binding or Runtime Binding.
  • The determination of which function to call is done at runtime.
  • It occurs when the function call is bound to the function definition based on the actual type of the object pointed to or referenced.
  • Achieved through the use of virtual functions in C++.

27. Difference between new and malloc() for Memory Allocation

  • new is an operator in C++ used for dynamic memory allocation and object construction.
  • malloc() is a function in C used for dynamic memory allocation, and it doesn't call constructors.
  • new returns a pointer to the constructed object of the requested type.
  • malloc() returns a void pointer (void*) and requires explicit type casting.
  • new automatically calculates the size based on the data type.
  • malloc() requires the user to specify the size of memory to allocate.

28. Deallocating Memory in C++

  • In C++, memory allocated using new should be deallocated using delete, and memory allocated using malloc() should be deallocated using free().
  • Failing to deallocate memory can lead to memory leaks, where the allocated memory remains inaccessible and consumes resources.
  • It's essential to release memory to the operating system when it's no longer needed.

29. Smart Pointer in C++

  • A smart pointer is a class template in C++ that acts like a regular pointer but provides additional features like automatic memory management and ownership semantics.
  • It automatically deallocates the memory when the smart pointer goes out of scope, avoiding memory leaks.
  • Three types of smart pointers in C++ are unique_ptr, shared_ptr, and weak_ptr.

30. Differences between unique_ptr, shared_ptr, and weak_ptr

Unique_ptr:

  • Represents exclusive ownership of the dynamically allocated object.
  • Cannot be copied but can be moved.
  • When the unique_ptr goes out of scope, it automatically deletes the associated object.

Shared_ptr:

  • Represents shared ownership of the dynamically allocated object.
  • Keeps track of the number of shared_ptrs pointing to the same object.
  • Automatically deletes the object when the last shared_ptr referring to it goes out of scope.

Weak_ptr:

  • Does not participate in ownership and does not affect the lifetime of the object.
  • Used in combination with shared_ptr to break potential circular references.
  • It allows accessing the shared object, but it doesn't prevent it from being deleted.

31. RAII (Resource Acquisition Is Initialization)

  • RAII is a C++ programming idiom that ties the lifetime of a resource (e.g., memory, file handle) to the lifetime of an object.
  • Resources are acquired during the object's construction and released during its destruction.
  • It ensures that resources are properly managed and released, even in the presence of exceptions or early returns.
  • RAII helps avoid resource leaks and simplifies resource management.

32. Namespace in C++

  • A namespace is a C++ feature that allows the organization of code elements (variables, functions, classes) into logical groups to prevent naming conflicts.
  • It helps avoid naming collisions, especially when different libraries or code modules are used together.
  • Namespace declarations start with the keyword namespace followed by the namespace name and enclosed in curly braces.

33. Handling Exceptions in C++

  • Exceptions are used in C++ to handle error conditions and exceptional situations that disrupt the normal flow of the program.
  • The try, catch, and throw keywords are used for exception handling.
  • Code that may raise an exception is placed within the try block.
  • If an exception is thrown, it is caught and handled in one or more catch blocks.
  • The throw keyword is used to explicitly throw an exception when an error occurs.

34. Common STL Containers in C++

  • STL (Standard Template Library) in C++ provides various container classes to store and manipulate data efficiently.
  • Some common containers include:
    • Vector: A dynamic array that grows and shrinks automatically.
    • List: A doubly-linked list that allows fast insertion and deletion at any position.
    • Map: A container that stores key-value pairs sorted by the key.
    • Set: A container that stores unique elements in a sorted order.
    • Queue: A container that follows the FIFO (First In, First Out) principle.
    • Stack: A container that follows the LIFO (Last In, First Out) principle.

35. Difference between vector and list

Vector:

  • Implemented as a dynamic array that resizes as elements are added or removed.
  • Provides fast access to elements using random access iterators ([] operator or at() function).
  • Insertion and deletion in the middle of the vector are relatively slow as elements may need to be shifted.

List:

  • Implemented as a doubly-linked list where each element contains pointers to the previous and next elements.
  • Provides fast insertion and deletion at any position, as it involves updating pointers.
  • Accessing elements in the list requires traversing from the beginning, making it slower than vector's random access.

36. Map Container and Its Implementation

  • map is an associative container in C++ that stores key-value pairs sorted by the keys.
  • Implemented as a balanced binary search tree (usually Red-Black Tree).
  • Allows fast search, insertion, and deletion of elements in O(log n) time complexity.
  • The keys are unique, and the map ensures they are always sorted.

37. Iterators in C++

  • Iterators are objects that allow traversal of elements in a container (like arrays, vectors, lists, etc.).
  • They act as pointers to elements in the container and provide a uniform way to access elements regardless of the container's type or implementation.
  • C++ offers several types of iterators, including:
    • Input iterators: Read-only access and forward traversal.
    • Output iterators: Write-only access and forward traversal.
    • Forward iterators: Read and write access, forward traversal.
    • Bidirectional iterators: Read and write access, bidirectional traversal (forward and backward).
    • Random-access iterators: Read and write access, random access to elements.

38. Reversing a String in C++

cpp
#include <iostream>
#include <algorithm> // For std::reverse

int main() {
    std::string str = "Hello, World!";
    std::reverse(str.begin(), str.end());
    std::cout << str << std::endl; // Output: "!dlroW ,olleH"
    return 0;
}

39. Keyword const in C++

  • The const keyword in C++ is used to declare constants or indicate that an object or function is not allowed to modify the data it points to or represents.
  • When used with variables, it means the variable's value cannot be changed once it's initialized.
  • When used with pointers or references, it indicates that the data being pointed to or referred to is read-only.

40. References in C++

  • References in C++ provide an alias (an alternative name) for an existing object or variable.
  • They are declared using the & symbol and must be initialized when declared.
  • References act as a "nickname" for the original object, and any changes made to the reference will affect the original object.
  • They are often used to pass variables to functions by reference, avoiding unnecessary copying.

41. Function Template Specialization

  • Function templates in C++ allow writing generic functions that work with different data types.
  • Function template specialization is a feature that allows providing a specific implementation for a particular data type.
  • When the compiler encounters a function call with the specialized type, it will use the specialized implementation instead of the generic template.

42. The mutable Keyword in C++

  • The mutable keyword is used in C++ to modify the behavior of a class member marked as const.
  • When a member variable is declared as mutable, it can be modified even within a const member function of the class.
  • This is helpful when a member variable needs to be modified for caching or other internal purposes within a const member function.

43. Difference between delete and delete[] Operators

  • In C++, the delete operator is used to deallocate memory that was dynamically allocated using new.
  • The delete[] operator is specifically used to deallocate memory that was allocated using new[] for arrays.
  • Using the wrong form of deletion for the corresponding allocation can lead to undefined behavior, such as memory leaks or program crashes.

44. The C++ auto Keyword

  • The auto keyword in C++ is used for type inference, allowing the compiler to automatically deduce the data type of a variable based on its initializer.
  • It simplifies code and makes it more maintainable, especially when dealing with complex type names or iterator declarations.

45. Difference between nullptr and NULL

  • nullptr is a keyword introduced in C++11 that represents a null pointer and is used for pointer types.
  • NULL is a macro that was traditionally used in C and early versions of C++ to represent a null pointer.
  • Using nullptr is preferred in modern C++ code as it is type-safe and avoids potential ambiguity.

46. Lambda Function in C++

  • A lambda function is an anonymous, inline function defined using the lambda expression syntax introduced in C++11.
  • It allows creating small, throwaway functions without explicitly defining a named function.
  • Lambda functions are often used as arguments to higher-order functions like std::for_each, std::sort, etc.
  • They can capture variables from their surrounding scope by reference or by value, allowing for flexible and concise coding.

47. Move Semantics in C++11

  • Move semantics in C++11 is a feature that enables efficient transfer of resources (e.g., memory) from one object to another.
  • It avoids unnecessary deep copying and improves performance for objects with expensive-to-copy data.
  • Move semantics are achieved using move constructors and move assignment operators, implemented using r-value references (&&).

48. Difference between emplace_back() and push_back() in a Vector

  • Both emplace_back() and push_back() are used to add elements to the end of a vector in C++.
  • emplace_back() constructs the element in-place at the end of the vector, forwarding its arguments directly to the element's constructor. It avoids unnecessary copying or moving of elements.
  • push_back() adds a copy of the provided element to the end of the vector. If the element is an r-value, it is moved.

49. Preventing Inheritance in C++

  • To prevent inheritance in C++, a class can be marked as final.
  • The final keyword, when added after the class declaration, indicates that the class cannot be used as a base class for inheritance.
  • If another class tries to derive from a final class, it will result in a compilation error.

50. The volatile Keyword in C++

  • The volatile keyword in C++ is used to indicate that a variable can be changed by external factors not directly related to the current execution flow.
  • It informs the compiler not to optimize the access to the variable and always read its latest value from memory, even if the compiler thinks the value has not changed.
  • volatile is often used for variables that can be modified by hardware or concurrently by multiple threads.

Please note that the above information is intended to be a concise overview. More detailed explanations and examples can be found in C++ reference documentation and textbooks.

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