The C++ Standard Template Library (STL) is a powerful set of template classes that provides general-purpose structures and functions for handling data efficiently. It is built around three core components: containers (which store data), algorithms (which process data), and iterators (which bridge the gap between containers and algorithms). Containers like std::vector, std::map, and std::set manage collections of objects using under-the-hood implementations like dynamic arrays and red-black trees. Iterators allow developers to traverse these collections safely without exposing their internal memory layouts. Mastering the STL is essential for writing optimized, production-ready C++ code and is a heavily tested topic in software engineering interviews.
1. What is the STL in C++?
Correct Answer: a) A library of generic containers, algorithms, and iterators
Explanation:
The STL provides reusable generic data structures and algorithms.
2. Which header is used for std::vector?
Correct Answer: b) <vector>
Explanation:
The vector container is declared in the vector header.
3. Which of the following is a sequence container?
Correct Answer: c) vector
Explanation:
Sequence containers store elements in a linear order.
4. Which of the following is an associative container?
Correct Answer: d) map
Explanation:
Associative containers store elements by key.
5. Which container is an unordered associative container?
Correct Answer: a) unordered_map
Explanation:
Unordered containers use hashing instead of sorted order.
6. Which container adapter uses deque as its default underlying container?
Correct Answer: b) queue
Explanation:
std::queue uses std::deque by default.
7. Which container adapter uses vector as its default underlying container?
Correct Answer: c) priority_queue
Explanation:
std::priority_queue uses std::vector by default.
8. Which container adapter can use deque as its default underlying container?
Correct Answer: d) stack
Explanation:
std::stack uses std::deque by default.
9. What does begin() return?
Correct Answer: a) An iterator to the first element
Explanation:
begin points to the first element of a container.
10. What does end() return?
Correct Answer: b) An iterator past the last element
Explanation:
end is a sentinel position after the last element.
11. What does rbegin() return?
Correct Answer: c) A reverse iterator to the last element
Explanation:
rbegin starts reverse traversal.
12. What does rend() return?
Correct Answer: d) A reverse iterator before the first element
Explanation:
rend marks the end of reverse traversal.
13. What is std::vector best described as?
Correct Answer: a) A dynamic array
Explanation:
vector provides contiguous, resizable storage.
14. What is std::list?
Correct Answer: b) A doubly linked list
Explanation:
list supports fast insertion and deletion at known positions.
15. What is std::forward_list?
Correct Answer: c) A singly linked list
Explanation:
forward_list stores elements in a one-way linked sequence.
16. Which container allows efficient insertion and deletion at both ends?
Correct Answer: d) deque
Explanation:
deque supports efficient operations at the front and back.
17. Which function adds an element to the end of a vector?
Correct Answer: a) push_back()
Explanation:
push_back appends an element at the end.
18. Which function removes the last element from a vector?
Correct Answer: b) pop_back()
Explanation:
pop_back removes the final element.
19. Which function constructs an element at the end of a vector?
Correct Answer: c) emplace_back()
Explanation:
emplace_back forwards arguments to construct the element in place.
20. Which function requests extra storage for a vector without changing size?
Correct Answer: d) reserve()
Explanation:
reserve increases capacity without adding elements.
21. What does size() return?
Correct Answer: a) The number of elements
Explanation:
size gives the current element count.
22. What does empty() return when a container has no elements?
Correct Answer: b) true
Explanation:
empty is true when size is zero.
23. Which function removes all elements from a container?
Correct Answer: a) clear()
Explanation:
clear destroys or removes all elements.
24. Which member function removes elements from many STL containers?
Correct Answer: d) erase()
Explanation:
erase deletes elements at positions or ranges.
25. Which function inserts elements into a container?
Correct Answer: a) insert()
Explanation:
insert places elements at a specified position.
26. What does std::map store?
Correct Answer: b) Key-value pairs
Explanation:
map associates each key with a mapped value.
27. What does std::set store?
Correct Answer: c) Unique keys
Explanation:
set contains unique sorted elements.
28. Which container allows duplicate keys?
Correct Answer: d) multiset
Explanation:
multiset can store repeated values.
29. What is true about keys in std::map?
Correct Answer: a) They are unique
Explanation:
Each key appears once in a map.
30. What is a typical advantage of std::unordered_map?
Correct Answer: b) Average constant-time lookup
Explanation:
Hash tables provide fast average access.
31. What does std::pair hold?
Correct Answer: c) Two values named first and second
Explanation:
pair groups two related values.
32. What does std::tuple hold?
Correct Answer: d) A fixed number of heterogeneous values
Explanation:
tuple can store multiple values of different types.
33. Which function creates a pair?
Correct Answer: a) std::make_pair
Explanation:
make_pair constructs a pair from two values.
34. Which function accesses the first element of a tuple?
Correct Answer: b) std::get<0>(t)
Explanation:
std::get uses a zero-based index.
35. What does std::sort do by default?
Correct Answer: c) Sorts using operator<
Explanation:
sort arranges elements in ascending order by default.
36. What does std::binary_search require?
Correct Answer: d) A sorted range
Explanation:
Binary search only works on sorted data.
37. What does std::find return?
Correct Answer: a) An iterator to the found element or end
Explanation:
find searches a range and returns an iterator.
38. What does std::count do?
Correct Answer: b) Counts matching elements
Explanation:
count returns how many elements match a value.
39. What does std::reverse do?
Correct Answer: c) Reverses the order of elements
Explanation:
reverse swaps elements to invert their order.
40. Which algorithm returns an iterator to the smallest element?
Correct Answer: d) min_element()
Explanation:
min_element locates the minimum element.
41. Which header provides std::accumulate?
Correct Answer: a) <numeric>
Explanation:
accumulate is declared in the numeric header.
42. What does std::for_each do?
Correct Answer: b) Applies a function to each element
Explanation:
for_each invokes a callable over a range.
43. What does std::transform do?
Correct Answer: c) Applies a function and stores results
Explanation:
transform can map a function over one or two ranges.
44. What does std::remove do?
Correct Answer: d) It shifts elements and returns a new logical end iterator
Explanation:
Use the erase-remove idiom to actually remove elements.
45. What does std::unique do?
Correct Answer: a) It removes consecutive equal elements logically
Explanation:
unique returns a new logical end iterator; erase may be needed.
46. What does std::lower_bound return?
Correct Answer: b) The first element not less than the value
Explanation:
lower_bound finds the insertion point for a value.
47. What does std::upper_bound return?
Correct Answer: c) The first element greater than the value
Explanation:
upper_bound finds the first strictly greater element.
48. What does std::equal_range return?
Correct Answer: d) A pair of iterators representing lower and upper bounds
Explanation:
equal_range combines lower_bound and upper_bound.
49. What iterator category do vector iterators usually support?
Correct Answer: a) Random access
Explanation:
vector iterators support constant-time jumps.
50. What can happen to vector iterators and references after reallocation?
Correct Answer: b) They can become invalidated
Explanation:
Reallocation may move elements to new storage.
