C++ practice questions with answers

Practise C++ questions at your own pace: pointers, references, the STL, virtual functions, smart pointers and move semantics. Pick an answer to see whether you are right and why. Nothing is timed or ranked here; for that, play the Daily Quiz at 4 PM IST.

1. What does this print?

   easy

   int a = 7, b = 2;
   std::cout << a / b << " " << a % b;

   1. 3.5 1
   2. 3 1
   3. 3 0
   4. 4 1

   Answer: B. 3 1

   Explanation: Dividing two ints truncates towards zero: 7 / 2 = 3, and the remainder is 1.

2. Which container keeps its keys sorted?

   easy

   1. std::unordered_map
   2. std::map
   3. std::vector
   4. std::queue

   Answer: B. std::map

   Explanation: std::map is a balanced binary search tree (usually red-black): sorted keys, O(log n) operations. unordered_map is a hash table.

3. What does the & in void f(int& x) mean?

   easy

   1. x is a pointer
   2. x is a reference: changes inside f change the caller's variable
   3. x is passed by value
   4. x is the address of an int

   Answer: B. x is a reference: changes inside f change the caller's variable

   Explanation: Pass-by-reference avoids copying and lets the function modify the original. const int& gives no-copy read-only access.

4. What is the size of the vector after these lines?

   easy

   std::vector<int> v(3, 5);
   v.push_back(1);
   v.pop_back();
   v.push_back(2);

   1. 3
   2. 4
   3. 5
   4. 6

   Answer: B. 4

   Explanation: v(3, 5) holds three 5s. push_back then pop_back cancel out, and the last push_back makes it 4.

5. What does this print?

   medium

   int x = 5;
   int* p = &x;
   *p += 3;
   std::cout << x;

   1. 5
   2. 8
   3. The address of x
   4. Undefined

   Answer: B. 8

   Explanation: p points to x, so *p += 3 changes x itself.

6. Which function is a virtual call resolved at run time?

   medium

   1. A non-virtual member function
   2. A virtual function called through a base-class pointer or reference
   3. A static member function
   4. A function template

   Answer: B. A virtual function called through a base-class pointer or reference

   Explanation: Dynamic dispatch uses the object's vtable, so the derived override runs even through a Base*. Non-virtual calls are bound at compile time.

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

   medium

   1. To make the class abstract
   2. So that deleting a derived object through a base pointer runs the derived destructor too
   3. It makes construction faster
   4. It is required to compile

   Answer: B. So that deleting a derived object through a base pointer runs the derived destructor too

   Explanation: Without it, delete basePtr only runs ~Base, which is undefined behaviour and usually leaks the derived part's resources.

8. What is the time complexity of std::sort?

   medium

   1. O(n)
   2. O(n log n) on average and in the worst case (introsort)
   3. O(n²) worst case
   4. O(log n)

   Answer: B. O(n log n) on average and in the worst case (introsort)

   Explanation: Since C++11 the standard requires O(n log n) comparisons. Implementations use introsort: quicksort that falls back to heapsort.

9. What problem does std::unique_ptr solve?

   hard

   1. Sharing ownership between many owners
   2. Exclusive ownership: the object is deleted automatically when the unique_ptr goes out of scope, and it can be moved but not copied
   3. Making objects thread-safe
   4. Garbage collection

   Answer: B. Exclusive ownership: the object is deleted automatically when the unique_ptr goes out of scope, and it can be moved but not copied

   Explanation: This is RAII. shared_ptr is for shared ownership with reference counting; weak_ptr breaks cycles.

10. What does std::move do?

    hard

    1. Moves memory to a new address
    2. Casts its argument to an rvalue reference so a move constructor or move assignment can be chosen
    3. Deletes the source object
    4. Copies the object faster

    Answer: B. Casts its argument to an rvalue reference so a move constructor or move assignment can be chosen

    Explanation: std::move itself moves nothing; it only allows overloads that take T&& to "steal" resources. The moved-from object is left valid but unspecified.

11. After these lines, what is true about r?

    hard

    std::vector<int> v = {1, 2, 3};
    int& r = v[0];
    v.reserve(1000);

    1. r is still a valid reference to v[0]
    2. r may now dangle: reallocation invalidates references, pointers and iterators
    3. r becomes 0
    4. It does not compile

    Answer: B. r may now dangle: reallocation invalidates references, pointers and iterators

    Explanation: Growing capacity moves the elements to new memory. Any reference or iterator taken before the reallocation is invalid.

12. Which statements about const in C++ are true? (Select all that apply)

    hard

    1. A const member function promises not to modify the object
    2. const int* p means the int cannot be changed through p
    3. int* const p means p cannot point elsewhere
    4. const variables are always stored in read-only memory

    Answer: A. A const member function promises not to modify the object; B. const int* p means the int cannot be changed through p; C. int* const p means p cannot point elsewhere

    Explanation: Read const from right to left: const int* is a pointer to const int; int* const is a const pointer. Where a const object is stored is up to the compiler.

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