C++ STL Interview Questions 2026: 28 Q&A With Code

Last Updated: June 2026 | Level: Freshers to Mid-Level | Read Time: ~16 min

The Standard Template Library is the most queried part of C++ in interviews because it shows you can write correct, efficient code without reinventing data structures. Candidates report that container choice, iterators, and the algorithm header are guaranteed topics for product-company and competitive-programming rounds. This guide covers 28 STL questions with full answers, complexity tables, and predict-the-output traps. Behaviour reflects the official cppreference documentation; confirm any complexity guarantee on the standard.

Pair this with C++ Programming Placement Questions and C++ OOPs Interview Questions 2026.

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Table of Contents

1. Containers (Q1 to Q12)
2. Iterators (Q13 to Q18)
3. Algorithms (Q19 to Q24)
4. Advanced (Q25 to Q28)
5. Predict the Output
6. Complexity Cheat Sheet
7. Frequently Asked Questions

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Containers

Q1. What are the categories of STL containers? Easy

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Q2. What is the difference between vector and array? Easy

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Q3. How does vector grow internally? Medium

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Q4. What is the difference between vector and list? Medium

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Q5. What is the difference between map and unordered_map? Medium

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Q6. What is the difference between set and multiset? Easy

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Q7. What is a priority_queue and its default order? Medium

priority_queue<int, vector<int>, greater<int>> minHeap;
minHeap.push(3); minHeap.push(1);
minHeap.top();   // 1

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Q8. What is the difference between deque and vector? Medium

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Q9. How do you remove elements from a vector efficiently? Hard

v.erase(std::remove(v.begin(), v.end(), 5), v.end());

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Q10. What is emplace_back vs push_back? Medium

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Q11. Can you store custom objects as map keys? Medium

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Q12. What does reserve do for a vector? Medium

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Iterators

Q13. What are the categories of iterators? Medium

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Q14. What is iterator invalidation? Hard

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Q15. What is the difference between begin/end and rbegin/rend? Easy

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Q16. What does auto do with iterators? Easy

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Q17. How do you safely erase while iterating a map? Hard

for (auto it = m.begin(); it != m.end(); )
  if (cond(*it)) it = m.erase(it); else ++it;

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Q18. What is a const_iterator? Medium

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Algorithms

Q19. How does std::sort work and what is its complexity? Medium

sort(v.begin(), v.end(), [](int a, int b){ return a > b; }); // descending

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Q20. What is the difference between sort and stable_sort? Medium

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Q21. What do lower_bound and upper_bound return? Hard

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Q22. What does std::find do and when is it O(n)? Easy

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Q23. What does accumulate do? Medium

int sum = accumulate(v.begin(), v.end(), 0);

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Q24. What is the difference between count and count_if? Easy

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Advanced

Q25. What are move semantics and how does the STL use them? Hard

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Q26. What is the difference between at() and operator[] for containers? Medium

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Q27. Why can operator[] on a map surprise you? Hard

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Q28. What is a span and a string_view? Hard

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Predict the Output

Snippet 1

map<int,int> m;
cout << m[5] << "\n";
cout << m.size();

Output:

0
1

Why: m[5] inserts a default-constructed int (0) and returns it, so the map now has size 1. The silent-insert trap.

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Snippet 2

vector<int> v = {1,2,3};
auto it = v.begin();
v.push_back(4);
cout << *it;

Output:

(undefined behaviour)

Why: push_back may reallocate, invalidating it. Dereferencing it is undefined. Re-acquire iterators after structural changes.

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Snippet 3

set<int> s = {3,1,2,1};
for (int x : s) cout << x << " ";

Output:

1 2 3 

Why: set stores unique keys in sorted order, so the duplicate 1 is dropped and iteration is ascending.

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Complexity Cheat Sheet

Container	access	insert	erase	ordered
vector	O(1)	O(1) back amortised	O(n) middle	no
list	O(n)	O(1) with iterator	O(1) with iterator	no
map / set	O(log n)	O(log n)	O(log n)	yes
unordered_map / set	O(1) avg	O(1) avg	O(1) avg	no
priority_queue	O(1) top	O(log n)	O(log n)	heap order

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Frequently Asked Questions

What STL topic is asked most in C++ interviews in 2026?

Candidates report that vector versus list, how map and unordered_map differ in ordering and complexity, and the use of common algorithms like sort and lower_bound are the most repeated STL questions.

Do I need to know container complexity for C++ interviews?

Yes. Interviewers expect O(1) amortised push_back for vector, O(log n) for map operations, and O(1) average for unordered_map, which drives the choice between ordered and hash containers.

Is the STL important for competitive programming interviews?

Critically. Fluent use of vector, set, map, priority_queue, and the algorithm header lets you write correct fast solutions, and interviewers expect that rather than reimplementing data structures.

Why does map[key] sometimes grow the map unexpectedly?

Accessing a missing key with operator[] default-constructs and inserts a value, silently increasing the size. Use find or count when you only want to check whether a key exists.

What is iterator invalidation and why does it matter?

It is when a container operation makes existing iterators unsafe to use, such as a vector reallocation on push_back. Dereferencing an invalidated iterator is undefined behaviour, so re-acquire iterators after structural changes.

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Related Articles

• C++ Programming Placement Questions, the master set
• C++ OOPs Interview Questions 2026, classes and inheritance
• C++ Pointers Interview Questions 2026, memory and pointers
• Data Structures Interview Questions 2026, the DSA behind the STL

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Confirm any complexity guarantee on the official cppreference documentation before your interview. This guide reflects candidate-reported patterns and public preparation resources as of June 2026.