C++ interview questions

1. What is a C++ program?

A C++ program is a collection of objects that call each other's methods. Each object has state and behavior. A class is the template for these objects.

Full lesson: Cpp basic syntax

2. What is the main function in C++?

The main function is the entry point of every C++ program. The program starts execution at main. You can define other functions, but main is required.

int main() {
  return 0;
}

Full lesson: Cpp functions

3. How do you declare a variable in C++?

You declare a variable by writing its type and name. The type sets the size and range of the variable. The name must start with a letter or underscore.

int age = 25;

Full lesson: Cpp variable types

4. What does the unsigned modifier do?

The unsigned modifier stops a variable from storing negative numbers. For integers, it doubles the positive range. You place it before the type.

unsigned int count = 4000000000;

Full lesson: Cpp modifier types

5. How does an if statement work in C++?

An if statement runs a code block when a condition is true. If the condition is false, the program skips the block. A non-zero value counts as true.

if (a < 20) {
  cout << "a is small";
}

Full lesson: Cpp if

6. How does an if...else statement work?

An if...else statement picks one of two code blocks. The if block runs when the condition is true. The else block runs when the condition is false.

if (a < 20) {
  cout << "small";
} else {
  cout << "large";
}

Full lesson: Cpp if else

7. How does a for loop work in C++?

A for loop repeats a block a set number of times. It has three parts: init, condition, and increment. The init part runs only once.

for (int i = 0; i < 5; i++) {
  cout << i << endl;
}

Full lesson: Cpp for loop

8. How does a while loop work in C++?

A while loop repeats a block while a condition is true. The program checks the condition before each pass. The loop stops when the condition becomes false.

int i = 0;
while (i < 5) {
  cout << i << endl;
  i++;
}

Full lesson: Cpp while loop

9. How do you declare an array in C++?

You declare an array with a type, a name, and a size. The size must be an integer constant greater than zero. Elements sit in contiguous memory.

int numbers[5] = {1, 2, 3, 4, 5};

Full lesson: Cpp arrays

10. What is a C-style string in C++?

A C-style string is a character array that ends with a null character. The null character marks the end of the string. The array size must include this extra character.

char site[7] = {'R','U','N','O','O','B','\0'};

Full lesson: Cpp strings

11. How do you define a class in C++?

You define a class with the class keyword, a name, and a body. The body holds member variables and member functions. A class is a template for objects.

class Box {
public:
  int width;
};

Full lesson: Cpp classes objects

12. How do you define a function in C++?

A function definition gives the name, return type, parameters, and body. The body holds the statements that do the task. You call the function by its name.

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

Full lesson: Cpp functions

13. What is a common mistake with a for loop condition?

A common mistake is a condition that never becomes false. The loop then runs forever. Check that the increment moves the variable toward the exit condition.

for (int i = 0; i < 5; i--) { // i moves away from 5
  cout << i;
}

Full lesson: Cpp for loop

14. What is a common mistake with array indexes?

A common mistake is an index outside the array bounds. C++ does not check array bounds for you. An out-of-range index can corrupt memory.

int a[3] = {1,2,3};
cout << a[3]; // out of bounds

Full lesson: Cpp arrays

15. What is a common mistake with C-style strings?

A common mistake is to forget the null character at the end. Without it, string functions read past the array. Always reserve one extra element for the null character.

char s[5] = {'H','e','l','l','o'}; // no null terminator

Full lesson: Cpp strings

16. What is C++?

C++ is a high-level object-oriented programming language. Bjarne Stroustrup began its design at Bell Labs in 1979. C++ expands the C language and runs on many platforms.

Full lesson: Cpp tutorial

17. What are the basic built-in data types in C++?

C++ provides seven basic built-in data types. These types include integer, floating-point, double floating-point, character, wide character, and boolean types. The operating system allocates memory based on the data type.

Full lesson: Cpp data types

18. How do you use a nested if statement in C++?

You place one if statement inside another if statement. The inner if runs only when the outer condition is true. This tests multiple conditions more precisely.

if (a > 0) {
  if (b > 0) {
    // both are positive
  }
}

Full lesson: Cpp nested if

19. How do you use a nested loop in C++?

You place one loop inside another loop. C++ allows at least 256 nesting levels. The inner loop runs completely for each outer loop iteration.

for (int i = 0; i < 3; i++) {
  for (int j = 0; j < 2; j++) {
    // runs 6 times total
  }
}

Full lesson: Cpp nested loops

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

An abstract class has at least one pure virtual function. A pure virtual function uses '= 0' in its declaration. You cannot create objects from an abstract class.

class Box {
public:
  virtual double getVolume() = 0;
};

Full lesson: Cpp interfaces

21. How does inheritance work in C++?

Inheritance lets you define a new class based on an existing class. The existing class is the base class. The new class is the derived class and reuses base class members.

class Dog : public Animal {
  // Dog inherits from Animal
};

Full lesson: Cpp inheritance

22. What is the <string> library in C++?

The <string> header provides the std::string class. This class encapsulates C-style strings. It offers safer and easier string operations.

#include <string>
std::string name = "Alice";

Full lesson: Cpp libs string

23. What is the <list> container in C++?

The <list> container stores a collection of elements. It supports bidirectional iterators. You can insert and delete elements quickly at any position.

#include <list>
std::list<int> mylist;
mylist.push_back(10);

Full lesson: Cpp libs list

24. What is the <array> container in C++?

The <array> container provides a fixed-size array. The size is set at compile time. It offers better type safety than C arrays.

#include <array>
std::array<int, 5> arr = {1, 2, 3, 4, 5};

Full lesson: Cpp libs array

25. What is the STL in C++?

The STL is the Standard Template Library. It provides template classes and functions for common data structures and algorithms. The STL supports generic programming.

Full lesson: Cpp stl tutorial

26. How do you write to a file in C++?

Use the ofstream class from the <fstream> library. Create an ofstream object and open a file. Then write data with the stream insertion operator.

#include <fstream>
std::ofstream file("data.txt");
file << "Hello";
file.close();

Full lesson: Cpp files streams

27. What is a common mistake with a while loop condition?

A common mistake is a condition that never becomes false. This creates an infinite loop. Always update the loop variable inside the loop body.

int i = 0;
while (i < 5) {
  // forgot i++
}

Full lesson: Cpp while loop

28. What is a common mistake with nested if statements?

A common mistake is a missing brace. Without braces, the else binds to the nearest if. This changes the logic and causes wrong results.

if (a)
  if (b)
    x = 1;
else
  x = 2; // else binds to inner if

Full lesson: Cpp nested if

29. What is a common mistake with inheritance?

A common mistake is forgetting the access specifier. The default access for a class is private. Use public inheritance when you want an is-a relationship.

class Dog : Animal { // private inheritance
};

Full lesson: Cpp inheritance

30. What is a common mistake with the <array> container?

A common mistake is using a runtime size. The size of std::array must be a compile-time constant. You cannot change the size after creation.

int n = 5;
std::array<int, n> arr; // error: n is not constant

Full lesson: Cpp libs array

31. How do you add a comment in C++?

Use two slashes for a single-line comment. Use a slash and an asterisk to start a block comment. Close the block comment with an asterisk and a slash.

// This is a single-line comment
/* This is a
   block comment */

Full lesson: Cpp basic syntax

32. What is a C++ modifier type?

A modifier changes the meaning of a base type. Common modifiers are signed, unsigned, long, and short. You place a modifier before the type name.

unsigned int count = 10;
long int total = 100000L;

Full lesson: Cpp modifier types

33. What is the difference between a declaration and a definition in C++?

A declaration tells the compiler the name and type of a variable. A definition also reserves memory for the variable. You can declare a variable many times but define it only once.

extern int value; // declaration
int value = 5;    // definition

Full lesson: Cpp variable types

34. How do you create an object from a class in C++?

Write the class name and then the object name. You can also pass arguments to a constructor. Use a dot to access the public members of the object.

class Box { public: int size; };
Box b;
b.size = 3;

Full lesson: Cpp classes objects

35. How do you use a string object in C++?

Include the <string> header and declare a std::string variable. The class manages memory for you. You can join strings with the plus operator.

#include <string>
std::string a = "Hello";
std::string b = a + " World";

Full lesson: Cpp libs string

36. How do you read a line of text from a file in C++?

Include the <fstream> header and create an ifstream object. Open the file and check that the open call succeeds. Use getline to read one line at a time.

#include <fstream>
#include <string>
std::ifstream in("data.txt");
std::string line;
std::getline(in, line);

Full lesson: Cpp files streams

37. How do you find the type of an object at runtime in C++?

Include the <typeinfo> header and use the typeid operator. The operator returns a reference to a type_info object. Call the name member to get a type name.

#include <typeinfo>
int x = 0;
const std::type_info& t = typeid(x);

Full lesson: Cpp libs typeinfo

38. How do you do element-wise math on a numeric array in C++?

Use the valarray class template from the <valarray> header. You can assign one value to all elements. You can also add two valarray objects element by element.

#include <valarray>
std::valarray<double> a(3);
a = 1.0;
std::valarray<double> b = a + a;

Full lesson: Cpp libs valarray

39. What is a common mistake with the <string> library in C++?

A common mistake is to forget the <string> header. Another mistake is to mix a C-style string and a std::string in one expression. Include the header and use std::string for both values.

#include <string>
std::string s = "text";

Full lesson: Cpp libs string

40. What is a common mistake with file streams in C++?

A common mistake is to use a file before you check that it opened. Another mistake is to forget to close the file. Always test the stream and close it when you finish.

std::ifstream in("data.txt");
if (!in) { return 1; }
// use the file
in.close();

Full lesson: Cpp files streams

41. What is a common mistake with the typeid operator in C++?

A common mistake is to expect the same type name from every compiler. The name member can return different text on different systems. Do not compare the name string to a fixed value.

#include <typeinfo>
#include <iostream>
std::cout << typeid(1).name();

Full lesson: Cpp libs typeinfo

42. What is a common mistake with the valarray class in C++?

A common mistake is to use an index that is outside the array size. Another mistake is to forget that valarray has no bounds check. Check the size before you use an index.

#include <valarray>
std::valarray<int> v(3);
v[3] = 1; // wrong: index out of range

Full lesson: Cpp libs valarray

43. What is a common mistake with the STL in C++?

A common mistake is to use an iterator after the container changes. Another mistake is to forget the header for a container. Include the correct header and get a new iterator after a change.

#include <vector>
std::vector<int> v = {1, 2};
v.push_back(3);

Full lesson: Cpp stl tutorial

44. What is a common mistake with a function return type in C++?

A common mistake is to forget the return statement in a function that returns a value. Another mistake is to return a type that does not match the declaration. Match the return value to the declared type.

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

Full lesson: Cpp functions

45. What is a common mistake with a class member in C++?

A common mistake is to access a private member from outside the class. Another mistake is to forget the semicolon after the class body. Keep data private and add the semicolon.

class Box {
    int size;
}; // do not forget the semicolon

Full lesson: Cpp classes objects