Top 10 C/C++ Interview Questions and Answers

This article presents a set of C and C++ interview questions, along with detailed answers, designed to help you ace your next job interview for a C/C++ programming role. These questions are also valuable for college viva examinations. Let’s dive in!

1. Structure vs. Union: What’s the Difference?

Question: Differentiate between a Structure and a Union, providing an example for each.

Answer: Structures and Unions are both user-defined data types in C and C++, but they differ in how they allocate memory.

• Structure: Allocates memory for each member individually. Members of a structure can exist simultaneously in memory.
• Union: Allocates only enough memory to hold the largest member. All members of a union share the same memory location. Only one member can be active at a time.

2. i++ vs. ++i: Understanding Increment Operators

Question: Explain the difference between i++ and ++i with an example.

Answer: Both i++ and ++i increment the value of the variable i. However, the key difference lies in when the increment occurs and what value is returned .

• i++ (Post-increment): Returns the current value of i before incrementing it. The increment happens as a side effect.
• ++i (Pre-increment): Increments the value of i first , and then returns the new value of i.

For example:

int i = 5;
int a = i++; // a will be 5, i will be 6
int b = ++i; // b will be 7, i will be 7

3. What is a Volatile Variable?

Question: What is a volatile variable?

Answer: A volatile variable is a variable whose value can be changed by external factors outside the control of the program during its execution. This tells the compiler that the value of this variable might change at any time, so the compiler should not optimize its usage (e.g., by caching the value in a register). This is crucial in situations like:

• Multi-threaded applications where variables are accessed by multiple threads.
• Hardware interaction, where a variable might represent a hardware register that changes asynchronously.
• Interrupt handlers.

For a detailed explanation, see Answer-3 (Replace ‘Answer3Link’ with an actual link if available) .

4. Initializing a Two-Dimensional Array with a For Loop

Question: How do you initialize a two-dimensional array using for loops in C/C++?

Answer: You typically use nested for loops to iterate through the rows and columns of the array.

int myArray[3][4]; // A 3x4 array

for (int i = 0; i < 3; i++) {
  for (int j = 0; j < 4; j++) {
    myArray[i][j] = i * 4 + j; // Example initialization logic
  }
}

5. Benefits of C++ over C

Question: Explain the benefits of using C++ over C.

Answer: C++ builds upon C, adding several features that make it more powerful and suitable for complex projects. Key advantages include:

• Object-Oriented Programming (OOP): C++ supports OOP principles like encapsulation, inheritance, and polymorphism, leading to better code organization, reusability, and maintainability.
• Classes and Objects: C++ introduces classes, which allow you to create custom data types (objects) that combine data and functions (methods) that operate on that data.
• Operator Overloading: Allows you to define the meaning of operators for user-defined types.
• Function Overloading: Allows you to define multiple functions with the same name but different parameters.
• Templates: Enable generic programming, allowing you to write code that works with different data types without having to write separate code for each type.
• Exception Handling: Provides a structured way to handle errors and exceptions, making code more robust.
• Standard Template Library (STL): Offers a rich set of data structures and algorithms.

6. Public, Private, and Protected Declaration Types

Question: Explain the meaning and usage of public, private, and protected access specifiers in C++.

Answer: These access specifiers control the visibility and accessibility of class members (variables and functions):

• public: Members declared as public are accessible from anywhere, both inside and outside the class.
• private: Members declared as private are only accessible from within the class itself.
• protected: Members declared as protected are accessible from within the class itself and from derived classes (classes that inherit from this class).

7. Friend Class and Friend Function in C++

Question: Explain the concept of friend classes and friend functions in C++.

Answer: A friend class or function is given special access to the private and protected members of another class. It’s a way to bypass the normal access restrictions of encapsulation, but it should be used sparingly, as it can weaken the benefits of encapsulation.

• Friend Function: A non-member function (or a member function of another class) that is declared as a friend within a class.
• Friend Class: A class declared as a friend within another class. All members of the friend class can access the private and protected members of the class that declared it as a friend.

8. The ‘Enum’ Data Type

Question: Explain the purpose of the enum data type.

Answer: The enum (enumeration) data type allows you to define a set of named integer constants. It enhances code readability and maintainability by providing meaningful names for integer values.

enum Color {
  RED,   // By default, RED = 0
  GREEN, // GREEN = 1
  BLUE  // BLUE = 2
};

Color myColor = GREEN;

9. The ‘typedef’ Keyword in C

Question: Explain the purpose of the typedef keyword in the C language.

Answer: The typedef keyword is used to create an alias (a new name) for an existing data type. This can simplify code, improve readability, and make it easier to change data types later.

typedef int myInteger; // myInteger is now an alias for int

myInteger x = 10; // Equivalent to int x = 10;

10. Double Pointer

Question: Explain the concept of a double pointer.

Answer: A double pointer (also known as a pointer to a pointer) is a pointer variable that stores the memory address of another pointer variable. It’s often used to work with arrays of pointers or to modify pointers passed as arguments to a function.

int x = 10;
int *ptr = &x;   // ptr stores the address of x
int **ptr2 = &ptr; // ptr2 stores the address of ptr

// To access the value of x through ptr2:
int value = **ptr2; // value will be 10

This collection of C/C++ interview questions and answers is beneficial for both freshers and experienced professionals. Good luck with your interview!