C/C++ Interview Questions and Answers

1. What is the difference between global and static variables?

• Global variables: Accessible throughout the program.

• Static variables: Retain their value across function calls but have limited scope.

static int count = 0; // Remembers its value across function calls

2. What is the purpose of volatile keyword in C/C++?

It tells the compiler that a variable's value may change unexpectedly (e.g., in multithreading or hardware operations).

volatile int flag; // Compiler won't optimize this variable

3. What is the difference between typedef and #define?

• typedef defines a new name for an existing type.

• #define creates a macro (preprocessor directive).

typedef unsigned int uint; #define PI 3.14

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

• Shallow Copy: Copies memory addresses (default behavior).

• Deep Copy: Allocates new memory and copies data manually.

class A { int* ptr; A(const A& obj) { ptr = new int(*obj.ptr); } // Deep Copy };

5. What is function pointer in C?

A pointer that stores the address of a function.

void hello() { cout << "Hello"; } void (*funcPtr)() = hello; funcPtr(); // Calls hello()

6. What is a virtual destructor in C++?

A destructor that ensures proper cleanup of derived class objects when deleted via a base class pointer.

class Base { public: virtual ~Base() { cout << "Base Destructor"; } };

7. What is an inline function?

An inline function suggests the compiler to replace function calls with the function code to optimize performance.

inline int add(int a, int b) { return a + b; }

8. What is the difference between new and malloc()?

• new calls the constructor, malloc() does not.

• new returns the correct type, malloc() requires typecasting.

int* ptr = new int; int* ptr2 = (int*)malloc(sizeof(int));

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

A class with at least one pure virtual function.

class Shape { public: virtual void draw() = 0; // Pure virtual function };

10. What is a friend function in C++?

A function that can access private members of a class.

class A { friend void show(A obj); };

11. What is multiple inheritance in C++?

A class inheriting from multiple base classes.

class A {}; class B {}; class C : public A, public B {};

12. What is function overloading?

Defining multiple functions with the same name but different parameters.

void print(int x); void print(double x); void print(string x);

13. What is operator overloading?

Redefining operators for user-defined types.

class A { A operator+(const A& obj); };

14. What is dynamic polymorphism?

Method overriding where function resolution happens at runtime.

class Base { virtual void show() { cout << "Base"; } };

15. What is static polymorphism?

Method overloading where function resolution happens at compile time.

void print(int a); void print(double a);

16. What is a namespace in C++?

Used to avoid name conflicts.

namespace myNamespace { int x; }

17. What is a template in C++?

Allows writing generic code for multiple data types.

template <typename T> T add(T a, T b) { return a + b; }

18. What is the static_cast operator in C++?

Used for implicit type conversion.

int a = 10; double b = static_cast<double>(a);

19. What is the reinterpret_cast operator in C++?

Used to convert one pointer type into another.

int* a; char* b = reinterpret_cast<char*>(a);

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

A wrapper over raw pointers to manage memory automatically.

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

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

A compact function that can be defined inline.

auto add = [](int a, int b) { return a + b; };

22. What is the difference between map and unordered_map in C++?

• map stores key-value pairs in sorted order.

• unordered_map stores elements in any order for faster access.

std::map<int, string> m; std::unordered_map<int, string> um;

23. What is the purpose of the explicit keyword in C++?

Prevents implicit constructor calls.

class A { public: explicit A(int x); };

24. What is std::move() in C++?

Transfers ownership of an object (used for move semantics).

std::string s1 = "Hello"; std::string s2 = std::move(s1);

25. What is RAII in C++?

Resource Acquisition Is Initialization – ensures resource cleanup using constructors and destructors.

class File { std::ofstream file; public: File(const string& name) { file.open(name); } ~File() { file.close(); } };

1. What is the difference between stack and heap memory?

• Stack: Stores local variables, function calls, and grows automatically.

• Heap: Stores dynamically allocated memory and must be managed manually.

int x; // Stored in stack int* ptr = new int; // Stored in heap

2. What is the mutable keyword in C++?

Allows modification of a class member even if the object is const.

class A { mutable int x; };

3. What are rvalue and lvalue references in C++?

• lvalue: Has a memory location.

• rvalue: Temporary object with no memory location.

int x = 10; // x is lvalue int&& r = 20; // rvalue reference

4. What is the difference between std::unique_ptr and std::shared_ptr?

• std::unique_ptr has sole ownership.

• std::shared_ptr allows multiple ownership.

std::unique_ptr<int> uptr = std::make_unique<int>(10); std::shared_ptr<int> sptr = std::make_shared<int>(10);

5. What is a move constructor?

Transfers ownership from a temporary object to a new object.

class A { A(A&& obj) { cout << "Move constructor"; } };

6. What is an explicit constructor?

Prevents implicit conversions of single-parameter constructors.

class A { public: explicit A(int x) {} };

7. What is a functor in C++?

A class that overloads operator() to behave like a function.

class Functor { public: void operator()(int x) { cout << x; } };

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

A class should define the following to manage resources properly:

1. Destructor

2. Copy Constructor

3. Copy Assignment Operator

4. Move Constructor

5. Move Assignment Operator

class A { ~A(); A(const A&); A& operator=(const A&); A(A&&); A& operator=(A&&); };

9. What is Virtual Table (VTable) in C++?

A table storing function pointers to resolve virtual function calls at runtime.

10. What is Return Value Optimization (RVO)?

An optimization where the compiler eliminates unnecessary temporary object creation.

A func() { return A(); } // Compiler may optimize this

11. What is two-phase name lookup in C++?

Used in templates to resolve dependent names in two phases (declaration and instantiation).

12. What is a memory leak in C++?

Occurs when dynamically allocated memory is not freed properly.

int* ptr = new int(10); // Memory leak occurs if delete ptr is missing

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

Uses try, catch, and throw blocks to handle errors.

try { throw "Error"; } catch (const char* e) { cout << e; }

14. What is SFINAE in C++?

"Substitution Failure Is Not An Error" – a rule in template resolution.

template <typename T> auto func(T x) -> decltype(x + 1) { return x + 1; }

15. What is the std::enable_if in C++?

Used to enable/disable template functions based on conditions.

template <typename T, typename std::enable_if<std::is_integral<T>::value, int>::type = 0> void func(T x) { cout << x; }

16. What is std::forward in C++?

Preserves value category (lvalue/rvalue) when passing arguments.

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

17. What is std::any, std::optional, and std::variant?

• std::any: Stores values of any type.

• std::optional: Represents optional values.

• std::variant: Holds one of several types.

std::any val = 10; std::optional<int> opt; std::variant<int, double> var;

18. What is template metaprogramming?

Uses templates to compute values at compile time.

template <int N> struct Factorial { static constexpr int value = N * Factorial<N-1>::value; };

19. What is the Curiously Recurring Template Pattern (CRTP)?

A technique where a class inherits from a template instantiation of itself.

template <typename Derived> class Base {}; class Derived : public Base<Derived> {};

20. What is copy elision in C++?

An optimization that eliminates unnecessary object copying.

A func() { return A(); } // Compiler may optimize

21. What is type erasure in C++?

A technique that allows using different types without exposing them.

class Base { virtual void execute() = 0; };

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

Ensures resource cleanup using destructors.

class File { std::ofstream file; public: File(const std::string& name) { file.open(name); } ~File() { file.close(); } };

23. What is an intrusive pointer in C++?

A smart pointer that stores reference counts inside the object itself.

class Intrusive { int refCount; public: void addRef() { ++refCount; } };

24. What is memory alignment in C++?

Ensures that data structures are properly aligned for CPU efficiency.

struct alignas(16) Data { int x; };

25. What is the difference between emplace_back() and push_back() in C++?

• push_back() copies or moves an object into a container.

• emplace_back() constructs an object in place.

std::vector<std::string> v; v.push_back("Hello"); v.emplace_back(5, 'a'); // Constructs in place

1. What is a memory model in C++?

A memory model defines how memory operations (reads/writes) are ordered across different threads.

2. What is the difference between paging and segmentation in memory management?

• Paging: Divides memory into fixed-size blocks.

• Segmentation: Divides memory into variable-size segments based on logical divisions.

3. What is the difference between process and thread?

• Process: Independent execution unit with its own memory.

• Thread: A lightweight process that shares memory with other threads in the same process.

4. What are the different types of IPC (Inter-Process Communication)?

• Shared Memory

• Message Queues

• Pipes

• Sockets

5. What is the purpose of std::atomic in C++?

Ensures atomic operations on variables to prevent race conditions in multithreading.

std::atomic<int> count(0); count.fetch_add(1);

6. What is the difference between mutex and spinlock?

• Mutex: A lock that makes a thread wait (blocking) until it gets the resource.

• Spinlock: A lock where a thread continuously checks for resource availability (non-blocking).

7. What is deadlock, and how can it be prevented?

A situation where two or more threads wait indefinitely for resources held by each other.
Prevention methods:

• Avoid circular wait.

• Use a timeout mechanism.

• Use lock hierarchy.

8. What is a thread-safe function?

A function that can be safely executed by multiple threads concurrently without causing race conditions.

9. What is the difference between fork() and exec() in C?

• fork(): Creates a new process (child).

• exec(): Replaces the current process with a new one.

10. What are soft and hard real-time systems?

• Soft real-time: Delays are acceptable (e.g., video streaming).

• Hard real-time: Strict timing constraints (e.g., airbag deployment).

11. What is the use of memcpy() and memmove()?

• memcpy(): Copies memory but has issues with overlapping regions.

• memmove(): Safe for overlapping regions.

memmove(dest, src, size);

12. What is a virtual memory system?

A system where the OS provides the illusion of more memory than physically available using disk space (swap).

13. How does C++ handle memory fragmentation?

• Use pools/arenas for allocation.

• Use custom allocators.

• Use compacting garbage collection (in managed environments).

14. What is the difference between malloc() and brk()/sbrk()?

• malloc(): Allocates memory from the heap.

• brk()/sbrk(): Directly manipulates the program’s data segment size.

15. What are system calls in C?

Functions that request services from the OS kernel (e.g., read(), write(), open()).

int fd = open("file.txt", O_RDONLY); read(fd, buffer, size);

16. What is the difference between file descriptor and file pointer?

• File Descriptor: Low-level integer handle (open(), read(), write()).

• File Pointer: High-level buffered I/O (fopen(), fread(), fwrite()).

17. How does dynamic linking work?

Libraries are loaded at runtime instead of compile time to save memory and allow updates without recompiling.

void* handle = dlopen("libm.so", RTLD_LAZY); double (*cosine)(double) = (double (*)(double)) dlsym(handle, "cos");

18. What is the difference between monolithic and microkernel OS architecture?

• Monolithic: Entire OS runs in kernel mode (e.g., Linux).

• Microkernel: OS services run in user mode, reducing crash impact (e.g., QNX).

19. How does a kernel handle interrupts?

• Saves process state (context switch).

• Calls the appropriate interrupt handler.

• Restores process state and resumes execution.

20. What is the difference between context switching and thread switching?

• Context Switching: Switching between processes (high overhead).

• Thread Switching: Switching between threads within the same process (lower overhead).

21. What is an ELF file in Linux?

Executable and Linkable Format (ELF) is a common binary format for Linux executables and shared libraries.

22. What is the difference between big-endian and little-endian memory storage?

• Big-endian: Stores the most significant byte first.

• Little-endian: Stores the least significant byte first.

23. What is a kernel module?

A loadable component of the kernel that extends its functionality without modifying the core OS.

#include <linux/module.h> MODULE_LICENSE("GPL");

24. How does a virtual function call work internally?

A virtual function uses a vtable (virtual table) to resolve function calls at runtime instead of compile time.

class Base { virtual void show(); };

25. What is process scheduling, and what are its types?

Process scheduling determines the order in which processes execute.

Types:

• FCFS (First Come, First Serve)

• Round Robin

• Priority Scheduling

• Multilevel Queue Scheduling