Part 1: The C Language (Rapid Fire)Keywords & Basics
Q: What is the auto keyword in C?
A: It is the default storage class for local variables. Rarely used explicitly.
Q: What is the register keyword?
A: Suggests the compiler store the variable in a CPU register for faster access. You cannot get the address (&) of a register variable.
Q: What is signed vs unsigned?
A: signed uses one bit for the sign (+/-). unsigned uses all bits for magnitude, effectively doubling the positive range.
Q: What is typedef?
A: It creates an alias for an existing data type (e.g., typedef unsigned long ulong;) to improve code readability.
Q: What is an enum?
A: A user-defined type consisting of a set of named integer constants (e.g., enum Color { RED, GREEN };).
Q: What is the size of an empty struct in C?
A: It is technically undefined (or 0) in C, but in C++, the size of an empty struct/class is 1 byte to ensure unique addresses.
Q: What is a token in C?
A: The smallest individual unit in a program (keywords, identifiers, constants, strings, operators).
Q: What is the scope of a function?
A: By default, functions have extern scope (global). If marked static, they are limited to the defining file.
Q: What is the return type of printf?
A: It returns an int: the total number of characters successfully written to the output.
Q: What is the return type of scanf?
A: It returns an int: the number of items successfully read and assigned.
Q: What is type promotion?
A: Implicitly converting a smaller type (like char or short) to int during arithmetic operations.
Q: What is the difference between i++ and ++i?
A: i++ (Post-increment) uses the value then increments. ++i (Pre-increment) increments then uses the value.
Q: What are bitwise operators?
A: Operators that manipulate individual bits: & (AND), | (OR), ^ (XOR), ~ (NOT), << (Left Shift), >> (Right Shift).
Q: How do you check if a number is odd using bitwise?
A: if (num & 1) returns true for odd numbers.
Q: How do you check if a number is a power of 2?
A: if ((n > 0) && ((n & (n - 1)) == 0))
Q: What is Short Circuit evaluation?
A: In A && B, if A is false, B is not evaluated. In A || B, if A is true, B is not evaluated.
Pointers & Arrays (The "Tricky" Stuff)
Q: What is *ptr++ vs ++*ptr?
A: *ptr++ returns the value, then moves the pointer. ++*ptr increments the value pointed to.
Q: What is a generic pointer?
A: A void*. It can hold the address of any type but must be cast before dereferencing.
Q: What is a Null Pointer?
A: A pointer that points to address 0 ((void*)0). It is used to signify "no value".
Q: What is a Wild Pointer?
A: An uninitialized pointer that points to an arbitrary memory location (dangerous).
Q: Can you subtract two pointers?
A: Yes, if they point to elements of the same array. The result is the number of elements between them (ptrdiff_t).
Q: Can you add two pointers?
A: No, adding addresses makes no logical sense in memory mapping.
Q: What is an Array of Pointers?
A: int *arr[5]; - An array where every element is a pointer to an integer.
Q: What is a Pointer to an Array?
A: int (*ptr)[5]; - A single pointer that points to an entire array of 5 integers.
Q: What is argv and argc?
A: argc: Argument count. argv: Argument vector (array of strings). Used for Command Line Arguments.
Q: Is arr same as &arr?
A: Value-wise, yes (same address). Type-wise, no. arr is int* (first element), &arr is int(*)[size] (whole array).
Q: What is a near, far, and huge pointer?
A: Legacy concepts (16-bit DOS). Near = 16-bit offset. Far = Segment+Offset. Huge = Normalized Far pointer. Not used in modern systems.
Q: How to free a block of memory without free()?
A: You can use realloc(ptr, 0).
Q: What happens if you free a pointer twice?
A: "Double Free" error. It corrupts the heap management data structures and usually crashes the program.
Q: What is a flexible array member?
A: C99 feature: An array [] at the end of a struct allows allocating variable-sized structures.
Advanced C & System
Q: What is #pragma pack(1)?
A: It forces the compiler to align structure members on 1-byte boundaries (disabling padding).
Q: What is #define?
A: A preprocessor directive to create macros or constants.
Q: What is the # and ## operator in macros?
A: # converts an argument to a string. ## concatenates two tokens.
Q: What is a segmentation fault (segfault)?
A: Occurs when a program tries to access memory it doesn't have permission to access (e.g., dereferencing NULL).
Q: What is the difference between Text and Binary files?
A: Text files process newlines (\n) and encoding. Binary files read raw bytes exactly as stored.
Q: What is fseek()?
A: Moves the file pointer to a specific location in a file.
Q: What is Little Endian vs Big Endian?
A: Little: LSB stored at lowest address. Big: MSB stored at lowest address.
Q: What is setjmp and longjmp?
A: The C equivalent of "throw/catch". It allows jumping across functions (non-local goto).
Q: What is memory layout of a C program?
A: Text (Code), Data (Global/Static), Heap (Dynamic), Stack (Local/Functions).
Q: What is a constant pointer to constant data?
A: const int * const ptr; - Neither the pointer address nor the value can be changed.
Part 2: C++ Language (Rapid Fire)OOP & Core
Q: What is an Object?
A: An instance of a class that holds state (variables) and behavior (methods).
Q: What is Encapsulation?
A: Wrapping data and functions into a single unit (class) and restricting access using modifiers.
Q: What is Abstraction?
A: Hiding complex implementation details and showing only the essential features to the user.
Q: What is Polymorphism?
A: The ability of a message to be displayed in more than one form (Compile-time vs Runtime).
Q: What is Inheritance?
A: A mechanism where one class acquires the properties and behaviors of another class.
Q: What is struct vs class inheritance default?
A: struct inherits public by default. class inherits private by default.
Q: What is Multiple Inheritance?
A: A class derived from more than one base class (class C : public A, public B).
Q: What is a friend class?
A: A class that can access the private and protected members of another class.
Q: Can constructors be virtual?
A: No, because the VTABLE is not initialized until the constructor finishes executing.
Q: Can destructors be virtual?
A: Yes, and they should be in base classes to ensure proper cleanup of derived objects.
Q: What is an explicit constructor?
A: A constructor marked explicit prevents implicit type conversion (e.g., Class c = 5;).
Q: What is this?
A: A const pointer pointing to the current object instance.
Q: What is static member function?
A: A function that belongs to the class, not an object. It cannot access this or non-static members.
Q: What is Method Hiding?
A: If a child defines a non-virtual method with the same name as the parent, the parent's method is hidden.
Q: What is slicing?
A: When a derived object is assigned to a base object by value, the derived part is "sliced" off.
Q: What is a local class?
A: A class defined inside a function. It cannot have static data members.
Q: What is a nested class?
A: A class defined inside another class. It is a member of the enclosing class.
Q: What is the scope resolution operator?
A: :: used to define methods outside class, access static members, or access global variables.
Q: What is the mutable keyword?
A: Allows a member variable to be modified even inside a const member function.
Q: What is a Copy Constructor?
A: ClassName(const ClassName &obj). Creates a new object as a copy of an existing one.
Memory & Pointers in C++
Q: malloc vs new return type?
A: malloc returns void*. new returns the exact object pointer type.
Q: free vs delete?
A: free releases memory. delete calls the destructor AND releases memory.
Q: What is delete[]?
A: Used to delete arrays allocated with new[]. Using delete (without []) on an array causes UB.
Q: What is std::unique_ptr?
A: A smart pointer with exclusive ownership. Cannot be copied, only moved.
Q: What is std::shared_ptr?
A: A smart pointer that allows multiple pointers to own the same resource (Ref-counting).
Q: What is std::make_shared?
A: A helper function that allocates memory for the object and the control block in one go (optimization).
Q: What is a circular dependency in smart pointers?
A: Two shared_ptrs pointing to each other, preventing the ref-count from reaching 0. Solved with weak_ptr.
Q: What is placement new?
A: Allows constructing an object in a pre-allocated memory buffer. new (address) ClassName();.
Q: What is a Reference?
A: An alias for an existing variable. Must be initialized upon declaration and cannot be null.
Q: Pointer vs Reference?
A: Pointers can be null/reassigned; references cannot. Pointers have their own address; references share the address.
Standard Template Library (STL)
Q: What is a Container?
A: An object that stores a collection of other objects (Vector, List, Map).
Q: What is an Iterator?
A: An object that points to an element in a container, allowing traversal (like a smart pointer).
Q: vector::size() vs vector::capacity()?
A: size is the number of elements. capacity is the allocated memory space before it needs to resize.
Q: What is vector::reserve()?
A: Requests that the vector capacity be at least enough to contain n elements (prevents reallocations).
Q: Time complexity of vector access?
A: O(1) - Constant time.
Q: Time complexity of list access?
A: O(n) - Linear time (must traverse).
Q: map vs set?
A: set stores unique keys. map stores unique Key-Value pairs.
Q: Internal structure of std::map?
A: Red-Black Tree (Self-balancing BST).
Q: Internal structure of std::unordered_map?
A: Hash Table.
Q: What is a Functor?
A: A class/struct that overloads the () operator, allowing objects to be used like functions.
Q: What is a Predicate?
A: A functor or function that returns a boolean (used in sort, find_if).
Q: What is std::sort complexity?
A: O(N log N) (Introsort: Mix of QuickSort, HeapSort, and InsertionSort).
Q: What is std::pair?
A: A struct holding two heterogeneous objects (first, second).
Q: What is std::array (C++11)?
A: A wrapper around static C-style arrays. It doesn't decay to a pointer automatically and knows its size.
Q: What is std::deque?
A: Double-ended queue. Elements can be added to front or back efficiently.
Modern C++ (11/14/17/20)
Q: What is nullptr?
A: A type-safe pointer literal representing a null pointer, replacing NULL macro.
Q: What is auto deduction?
A: Compiler infers type from initializer. auto x = 5; -> int.
Q: What is decltype?
A: Inspects the declared type of an entity or expression (e.g., decltype(x) y = x;).
Q: What is a Lambda capture list []?
A: Defines what outside variables are available inside the lambda. [=] (by value), [&] (by ref).
Q: What is std::move?
A: Casts an l-value to an r-value, enabling move semantics (transferring ownership instead of copying).
Q: What is a move constructor?
A: Class(Class&& other). It steals resources from other and nulls out other.
Q: What is constexpr?
A: Evaluated at compile time.
Q: What is std::tuple?
A: A generalization of std::pair holding a collection of heterogeneous elements.
Q: What is std::optional (C++17)?
A: A wrapper that may or may not contain a value, eliminating the need for magic numbers or null pointers.
Q: What is std::variant (C++17)?
A: A type-safe union. It can hold one of several specified types.
Q: What is std::any (C++17)?
A: A type-safe container for single values of any type.
Q: What is std::string_view (C++17)?
A: A lightweight, non-owning reference to a string. Avoids string copying.
Q: What are Concepts (C++20)?
A: A way to specify constraints on template arguments (e.g., template
Q: What are Coroutines (C++20)?
A: Functions that can be suspended and resumed (using co_await, co_yield).
Q: What is if constexpr?
A: Compile-time if. The false branch is discarded during compilation (good for templates).