C/C++ Interview Questions and Answers

​

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

Answer:

• malloc() is a C function that allocates raw memory but does not call the constructor.

• new allocates memory and calls the constructor.

int* p1 = (int*)malloc(sizeof(int)); // C-style int* p2 = new int; // C++-style

2. What is the difference between delete and delete[] in C++?

Answer:

• delete is used for a single object.

• delete[] is used for an array of objects.

int* p1 = new int; delete p1; int* p2 = new int[10]; delete[] p2;

3. What are volatile and mutable keywords in C++?

Answer:

• volatile: Prevents compiler optimizations, ensuring the value is read from memory.

• mutable: Allows modification of class members even if the object is const.

4. What is placement new in C++?

Answer: Placement new allows constructing an object at a specific memory location.

#include <new> char buffer[sizeof(int)]; int* p = new (buffer) int(42); // Constructs an int at buffer

5. What is RAII (Resource Acquisition Is Initialization)?

Answer: RAII ensures resource management (memory, file handles) using constructors/destructors.

class File { FILE* f; public: File(const char* filename) { f = fopen(filename, "r"); } ~File() { fclose(f); } };

6. What is a memory leak, and how do you detect it?

Answer: A memory leak occurs when allocated memory is not freed. Use Valgrind or smart pointers (std::unique_ptr, std::shared_ptr) to detect/prevent it.

7. What are smart pointers in C++?

Answer:

• unique_ptr: Sole ownership, automatically deletes memory.

• shared_ptr: Shared ownership, deleted when last reference is gone.

• weak_ptr: Avoids circular references in shared_ptr.

#include <memory> std::unique_ptr<int> ptr = std::make_unique<int>(10);

8. What is the "Rule of Five" in C++?

Answer: If a class defines one of Destructor, Copy Constructor, Copy Assignment, Move Constructor, Move Assignment, it should define all five to manage resources properly.

9. What is the difference between struct and class in C++?

Answer:

• struct has public members by default.

• class has private members by default.

10. What is function overloading and operator overloading?

Answer:

• Function Overloading: Same function name, different parameters.

• Operator Overloading: Defines how operators work with user-defined types.

class Complex { public: int real, imag; Complex operator+(const Complex& c) { return {real + c.real, imag + c.imag}; } };

11. What is a pure virtual function and an abstract class?

Answer: A pure virtual function is declared with = 0, making a class abstract.

class Abstract { virtual void show() = 0; // Pure virtual function };

12. What is the Diamond Problem in C++?

Answer: It occurs in multiple inheritance when a class inherits from two classes that share a common base.
Use virtual inheritance to solve it.

13. What are std::move and std::forward?

Answer:

• std::move: Converts an lvalue to an rvalue, enabling move semantics.

• std::forward: Preserves value category in template functions.

14. What is the difference between const_cast, static_cast, dynamic_cast, and reinterpret_cast?

Answer:

• const_cast: Removes const from a variable.

• static_cast: Compile-time cast.

• dynamic_cast: Casts polymorphic objects safely.

• reinterpret_cast: Converts one type to another without safety checks.

15. What is a lambda function in C++?

Answer: An anonymous function that captures variables from its enclosing scope.

auto sum = [](int a, int b) { return a + b; }; std::cout << sum(2, 3);

16. What is a virtual destructor?

Answer: A virtual destructor ensures derived class destructors are called when deleting via a base class pointer.

class Base { virtual ~Base() {} };

17. What is a move constructor?

Answer: A constructor that moves resources instead of copying them.

class Example { int* data; public: Example(Example&& other) noexcept : data(other.data) { other.data = nullptr; } };

18. How does std::unordered_map work internally?

Answer: It uses hash tables for fast lookups, averaging O(1) complexity.

19. What is SFINAE (Substitution Failure Is Not An Error)?

Answer: It allows overload resolution to fail silently, used in template metaprogramming.

20. What is perfect forwarding in C++?

Answer: Perfect forwarding passes arguments to another function while preserving their type.

template <typename T> void wrapper(T&& arg) { func(std::forward<T>(arg)); }

21. What is the difference between deep copy and shallow copy?

Answer:

• Shallow Copy: Copies only the memory address (default behavior).

• Deep Copy: Allocates new memory and copies values.

22. What are constexpr and consteval in C++?

Answer:

• constexpr: Evaluates expressions at compile-time.

• consteval: Forces compile-time evaluation (C++20).

23. How does exception handling work in C++?

Answer: Uses try, catch, and throw.

try { throw std::runtime_error("Error"); } catch (const std::exception& e) { std::cout << e.what(); }

24. How does memory alignment affect performance in C++?

Answer: Misaligned data structures cause CPU performance penalties. Use alignas to align memory.

struct alignas(16) Vector { float x, y, z, w; };

25. What is a coroutine in C++20?

Answer: A coroutine allows functions to be paused and resumed using co_await.

#include <coroutine> std::suspend_always suspend;