C++ Interview Questions and Answers (OOP, STL, Pointers)

Question 1

Which STL container keeps unique elements in sorted order?

Accepted Answer

set. std::set stores unique keys sorted; unordered_set is unsorted but faster on average.

Question 2

What is the main difference between a class and a struct in C++?

Accepted Answer

Default access: private for class, public for struct. Otherwise they are the same; both can have methods, constructors and inheritance.

Question 3

What is a reference in C++?

Accepted Answer

Another name (alias) for an existing variable. int& r = x; makes r refer to x; it must be initialised and can't be reseated.

Question 4

Which keyword enables runtime polymorphism in C++?

Accepted Answer

virtual. virtual functions are resolved through the vtable at run time.

Question 5

What does a pure virtual function (= 0) make a class?

Accepted Answer

Abstract. An abstract class can't be instantiated; derived classes must override the function.

Question 6

Which operator is used to free memory allocated with new[]?

Accepted Answer

delete[]. Pair new with delete and new[] with delete[].

Question 7

What does RAII stand for?

Accepted Answer

Resource Acquisition Is Initialization. Resources are tied to object lifetime and released in the destructor.

Question 8

Which smart pointer allows only one owner of an object?

Accepted Answer

unique_ptr. unique_ptr can be moved but not copied; shared_ptr counts owners.

Question 9

What is function overloading?

Accepted Answer

Several functions with the same name but different parameters. The compiler picks the right version from the argument types.

Question 10

What is the time complexity of push_back on a vector (amortised)?

Accepted Answer

O(1). Occasionally the vector grows and copies, but on average push_back is constant time.

Question 11

When is a destructor called?

Accepted Answer

When an object goes out of scope or is deleted. Stack objects are destroyed at the end of their block; heap objects on delete.

Question 12

Why should a base class with virtual functions have a virtual destructor?

Accepted Answer

So deleting through a base pointer also destroys the derived part. Without it, delete basePtr only runs the base destructor (undefined behaviour).

Question 13

What does the const after a member function, like int get() const, mean?

Accepted Answer

The function doesn't modify the object. Const member functions can be called on const objects.

Question 14

Which header provides sort(), reverse() and max_element()?

Accepted Answer

. accumulate lives in .

Question 15

What is a template in C++?

Accepted Answer

A way to write code that works with any type. template <typename T> T max(T a, T b) works for ints, doubles, strings and more.

Question 16

Which container gives the largest element first?

Accepted Answer

priority_queue. std::priority_queue is a max-heap by default.

Question 17

What does the 'auto' keyword do?

Accepted Answer

Lets the compiler deduce the variable's type. auto x = 3.5; makes x a double.

Question 18

Which access specifier lets derived classes use a member but hides it from other code?

Accepted Answer

protected. protected members are visible in the class and its subclasses.

Question 19

What happens when you read m["key"] from a std::map and the key is missing?

Accepted Answer

The key is inserted with a default value. Use find() or count() to check without inserting.

Question 20

What is the difference between a pointer and a reference?

Accepted Answer

A pointer can be null and re-pointed; a reference can't. References must be bound when created and always refer to the same object.

Question 21

What does this code print?
#include <iostream>
using namespace std;

void set(int& v) { v = 20; }

int main() {
    int x = 10;
    set(x);
    cout << x << endl;
    return 0;
}

Accepted Answer

20. The parameter is a reference, so the function changes x itself.

Question 22

What does this code print? #include using namespace std; struct Animal { virtual void speak() { cout << "Animal" << endl; } virtual ~Animal() = default; }; struct Dog : Animal { void speak() override { cout << "Dog" << endl; } }; int main() { Animal* a = new Dog(); a->speak(); delete a; return 0; }

Accepted Answer

Dog. speak() is virtual, so the Dog version runs through the base pointer.

Question 23

What does this code print? #include using namespace std; struct Animal { void speak() { cout << "Animal" << endl; } }; struct Dog : Animal { void speak() { cout << "Dog" << endl; } }; int main() { Dog d; Animal* a = &d; a->speak(); return 0; }

Accepted Answer

Animal. Without virtual, the call is decided by the pointer's type (Animal) at compile time.

Question 24

What does this code print?
set<int> s = {1, 2, 2, 3, 3};
cout << s.size() << endl;

Accepted Answer

3. A set ignores duplicates: {1, 2, 3}.

Question 25

What does this code print?
vector<int> v = {5, 2, 9, 1};
sort(v.begin(), v.end());
for (size_t i = 0; i < v.size(); i++) cout << v[i] << (i + 1 < v.size() ? " " : "
");

Accepted Answer

1 2 5 9. sort arranges the vector in ascending order.

Question 26

What does this code print?
#include <iostream>
using namespace std;

struct T {
    T() { cout << "Construct "; }
    ~T() { cout << "Destruct" << endl; }
};

int main() {
    { T t; }
    return 0;
}

Accepted Answer

Construct Destruct. The object is created inside the block and destroyed when the block ends.

Question 27

What does this code print? stack st; st.push(1); st.push(2); st.push(3); while (!st.empty()) { cout << st.top() << (st.size() > 1 ? " " : " "); st.pop(); }

Accepted Answer

3 2 1. A stack is last-in, first-out.

Question 28

What does this code print?
priority_queue<int> pq;
for (int x : {4, 9, 1}) pq.push(x);
cout << pq.top() << endl;

Accepted Answer

9. priority_queue keeps the largest element on top.

Question 29

What does this code print?
map<string, int> m;
m["a"] = 1;
int x = m["b"];
cout << m.size() << endl;

Accepted Answer

2. m["b"] doesn't exist yet, so reading it inserts "b" with 0, making 2 keys.

Question 30

What does this code print?
#include <iostream>
using namespace std;

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

int main() {
    cout << add(3, 4) << " " << add(1.5, 2.0) << endl;
    return 0;
}

Accepted Answer

7 3.5. Overloading picks add(int, int) for ints and add(double, double) for doubles.

Question 31

What does this code print?
string s = "Skill Park";
cout << s.substr(0, 5) << endl;

Accepted Answer

Skill. substr(0, 5) takes 5 characters starting at index 0.

Question 32

What does this code print? vector a = {10}; vector b = a; b[0] = 15; cout << a[0] << " " << b[0] << endl;

Accepted Answer

10 15. b is a copy of a, so changing b doesn't change a.

Question 33

What does this code print? vector a = {10}; vector& r = a; r[0] = 15; cout << a[0] << " " << r[0] << endl;

Accepted Answer

15 15. r is a reference to the vector a, so the change is visible through both names.

Question 34

What does this code print?
auto a = make_shared<int>(5);
auto b = a;
cout << a.use_count() << endl;

Accepted Answer

2. Two shared_ptrs own the same object.

Question 35

What does this code print?
try {
    throw runtime_error("bad");
} catch (const exception& e) {
    cout << "Caught: " << e.what() << endl;
}

Accepted Answer

Caught: bad. The thrown runtime_error is caught by the matching catch block.

Question 36

What does this code print?
int k = 4;
auto mul = [k](int x) { return x * k; };
cout << mul(3) << endl;

Accepted Answer

12. The lambda captures k = 4 by value and multiplies: 3 × 4 = 12.

Question 37

What does this code print?
vector<int> v = {1, 2, 3, 4, 5};
cout << accumulate(v.begin(), v.end(), 0) << endl;

Accepted Answer

15. accumulate adds every element to the starting value 0.

Question 38

What does this code print?
#include <iostream>
using namespace std;

struct Base { Base() { cout << "Base "; } };
struct Derived : Base { Derived() { cout << "Derived" << endl; } };

int main() {
    Derived d;
    return 0;
}

Accepted Answer

Base Derived. The base class part is always constructed before the derived part.

Question 39

What is the difference between new and malloc?

Accepted Answer

new calls constructors and is type-safe; malloc only allocates raw memory. Pair new with delete, malloc with free.

Question 40

What is a namespace?

Accepted Answer

A named scope that prevents name clashes. std is the standard library's namespace.

Question 41

What is an inline function?

Accepted Answer

A function the compiler may expand at the call site. Avoids call overhead for small functions.

Question 42

What is the 'this' pointer?

Accepted Answer

A pointer to the object a member function was called on. Used to tell members apart from parameters.

Question 43

What is a friend function?

Accepted Answer

A non-member function allowed to access private members. Declared with the friend keyword inside the class.

Question 44

What is the rule of three?

Accepted Answer

If a class needs a destructor, copy constructor or copy assignment, it likely needs all three. The rule of five adds move constructor and move assignment.

Question 45

What is move semantics?

Accepted Answer

Transferring resources from a temporary object instead of copying them. std::move and && (rvalue references), since C++11.

Question 46

What is a lambda capture [&]?

Accepted Answer

Capture all used variables by reference. [=] captures by value.

Question 47

What is the difference between std::map and std::unordered_map?

Accepted Answer

map is sorted (tree, O(log n)); unordered_map is hashed (average O(1)). Pick unordered_map unless you need order.

Question 48

What does std::vector::reserve do?

Accepted Answer

Allocates capacity in advance without changing size. Avoids repeated reallocations.

Question 49

What is iterator invalidation?

Accepted Answer

Iterators becoming unusable after the container changes, e.g. vector reallocation. Erasing or inserting into a vector can invalidate iterators.

Question 50

What is an abstract class in C++?

Accepted Answer

A class with at least one pure virtual function. virtual void draw() = 0;

Question 51

What does the override keyword do?

Accepted Answer

Makes the compiler check that the function really overrides a base virtual function. Catches signature mistakes.

Question 52

What does the final keyword do on a virtual function?

Accepted Answer

Prevents further overriding in derived classes. On a class, it prevents inheritance.

Question 53

What is a const reference parameter (const T&) good for?

Accepted Answer

Passing large objects without copying and without allowing changes. Common for strings and vectors.

Question 54

What is the STL?

Accepted Answer

The Standard Template Library of containers, iterators and algorithms. vector, map, sort, find…

Question 55

What is a deque?

Accepted Answer

A double-ended queue with fast insert/remove at both ends. std::deque supports random access too.

Question 56

What does std::pair hold?

Accepted Answer

Two values, accessed as first and second. make_pair(1, "a").

Question 57

What is the purpose of std::sort's third argument?

Accepted Answer

A custom comparison function or lambda. sort(v.begin(), v.end(), greater<int>()).

Question 58

What is undefined behaviour?

Accepted Answer

Code whose result the C++ standard doesn't specify, like reading out of bounds. Anything may happen; avoid it.

Question 59

What does nullptr replace?

Accepted Answer

NULL / 0 as a null pointer. nullptr has its own type and avoids overload ambiguities.

Question 60

What is a static member variable?

Accepted Answer

A variable shared by all objects of the class. Defined once outside the class (before C++17 inline).

Question 61

What happens if an exception is thrown and not caught?

Accepted Answer

std::terminate is called and the program ends. Catch exceptions you can handle.

Question 62

What is a template specialization?

Accepted Answer

A custom implementation of a template for a specific type. template<> class Box { … };

Question 63

What is the difference between struct and class inheritance by default?

Accepted Answer

struct inherits publicly by default; class inherits privately. Write public explicitly to be clear.

Question 64

What is object slicing?

Accepted Answer

Copying a derived object into a base object, losing the derived part. Use references or pointers to avoid it.

C++ Interview Questions and Answers (OOP, STL, Pointers)

Q1. Which STL container keeps unique elements in sorted order?

Q2. What is the main difference between a class and a struct in C++?

Q3. What is a reference in C++?

Q4. Which keyword enables runtime polymorphism in C++?

Q5. What does a pure virtual function (= 0) make a class?

Q6. Which operator is used to free memory allocated with new[]?

Q7. What does RAII stand for?

Q8. Which smart pointer allows only one owner of an object?

Q9. What is function overloading?

Q10. What is the time complexity of push_back on a vector (amortised)?

Q11. When is a destructor called?

Q12. Why should a base class with virtual functions have a virtual destructor?

Q13. What does the const after a member function, like int get() const, mean?

Q14. Which header provides sort(), reverse() and max_element()?

Q15. What is a template in C++?

Q16. Which container gives the largest element first?

Q17. What does the 'auto' keyword do?

Q18. Which access specifier lets derived classes use a member but hides it from other code?

Q19. What happens when you read m["key"] from a std::map and the key is missing?

Q20. What is the difference between a pointer and a reference?

Q21. What does this code print?

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Q39. What is the difference between new and malloc?

Q40. What is a namespace?

Q41. What is an inline function?

Q42. What is the 'this' pointer?

Q43. What is a friend function?

Q44. What is the rule of three?

Q45. What is move semantics?

Q46. What is a lambda capture [&]?

Q47. What is the difference between std::map and std::unordered_map?

Q48. What does std::vector::reserve do?

Q49. What is iterator invalidation?

Q50. What is an abstract class in C++?

Q51. What does the override keyword do?

Q52. What does the final keyword do on a virtual function?

Q53. What is a const reference parameter (const T&) good for?

Q54. What is the STL?

Q55. What is a deque?

Q56. What does std::pair hold?

Q57. What is the purpose of std::sort's third argument?

Q58. What is undefined behaviour?

Q59. What does nullptr replace?

Q60. What is a static member variable?

Q61. What happens if an exception is thrown and not caught?

Q62. What is a template specialization?

Q63. What is the difference between struct and class inheritance by default?

Q64. What is object slicing?

Q65. Which smart pointer breaks reference cycles with shared_ptr?

Q66. What is constexpr?

Q67. What does std::string::npos represent?

Q68. Why use emplace_back instead of push_back?

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