Here is a complete, concise guide to the C++ Standard Template Library
(STL)—its components, use-cases, and how/when to use each part, all in
one place:
C++ STL: Overview
The Standard Template Library (STL) is a collection of C++ template
classes and functions providing ready-to-use data structures and
algorithms. STL makes code reusable, efficient, and robust by
abstracting common patterns in data manipulation135689.
STL is built on four pillars:
Containers: Store and manage collections of objects.
Algorithms: Perform operations (search, sort, etc.) on containers.
Iterators: Traverse and access container elements.
Function Objects (Functors): Objects that behave like functions,
often used with algorithms.
1. Containers
Containers are objects that store data. They are divided into:
a. Sequence Containers (Maintain order)
vector: Dynamic array; use for random access, fast insert/remove
at end.
deque: Double-ended queue; fast insert/remove at both ends.
list: Doubly linked list; fast insert/remove anywhere, slow random
access.
array: Fixed-size array.
forward_list: Singly linked list; less common.
b. Associative Containers (Sorted, unique
keys)
set: Stores unique elements in sorted order; use for fast lookup and
uniqueness.
map: Key-value pairs, sorted by key; use for dictionaries.
multiset: Like set, but allows duplicates.
multimap: Like map, but allows duplicate keys.
c. Unordered Associative Containers (Hash-
based, no order)
unordered_set: Unique elements, fast lookup, no order.
unordered_map: Key-value, fast lookup, no order.
unordered_multiset/multimap: Allow duplicates.
d. Container Adapters
stack: LIFO; use for undo, recursion simulation.
queue: FIFO; use for BFS, scheduling.
priority_queue: Always access largest/smallest; use for heaps,
greedy.
2. Algorithms
STL algorithms are generic functions that operate on containers using
iterators2356789.
Common Algorithms:
Sorting: sort(begin, end), stable_sort
Searching: find, binary_search, lower_bound, upper_bound
Counting: count, count_if
Modifying: reverse, rotate, unique, remove
Set Operations: set_union, set_intersection
Permutation: next_permutation, prev_permutation
Others: accumulate (sum), for_each
When to use:
Any time you need to process, search, or modify data in containers
—prefer algorithms over manual loops for clarity and speed.
3. Iterators
Iterators are like pointers that let you traverse containers23567.
Types:
begin()/end(): Start and end positions.
rbegin()/rend(): Reverse traversal.
cbegin()/cend(): Constant iterators.
When to use:
When you need to traverse, search, or modify elements in a
container, especially with algorithms.
4. Function Objects (Functors)
Objects that act like functions, often used to customize STL algorithms
(e.g., custom sorting)369.
Example:
cpp
struct Compare {
bool operator()(int a, int b) { return a > b; }
};
std::sort(v.begin(), v.end(), Compare());
STL: When and Where to Use Each
Component
Container/
Use Case Example Syntax
Algorithm
Dynamic array, random
vector vector<int> v;
access
Fast insert/remove at
deque deque<int> d;
ends
Frequent insert/remove
list list<int> l;
anywhere
Unique, sorted elements set set<int> s;
Key-value mapping
map map<int, string> m;
(sorted)
Key-value mapping (fast, unordered_map<int,
unordered_map
no sort) string> um;
LIFO (undo, recursion) stack stack<int> s;
FIFO (BFS, scheduling) queue queue<int> q;
Always access
priority_queue priority_queue<int> pq;
largest/smallest
Sorting sort sort(v.begin(), v.end());
binary_search, binary_search(v.begin(),
Search (sorted) v.end(), x);
lower_bound
Count occurrences count count(v.begin(), v.end(), x);
Traverse/modify elements Iterators, for_each for(auto it = v.begin(); ... )
Custom operation in sort(v.begin(), v.end(),
Functor/lambda
algorithm myComp);
References for Further Reading
[Simplilearn STL Guide]1
[Codedamn STL Guide]2
[LinkedIn STL Guide]3
[Programiz STL]5
[Scaler STL Guide]6
[cppreference STL PDF]7
[Wikipedia STL]8
[PW Skills STL]9
Summary:
STL is essential for efficient C++ programming. Master containers for
data storage, algorithms for operations, iterators for traversal, and
functors for customization. Use STL to write faster, cleaner, and more
reliable code136.
Citations:
1. https://www.simplilearn.com/tutorials/cpp-tutorial/cpp-standard-
template-library
2. https://codedamn.com/news/c/c-complete-guide-to-standard-
template-library-stl
3. https://www.linkedin.com/pulse/understanding-stl-c-
comprehensive-guide-aritra-pain-nk2tc
4. https://learn.microsoft.com/en-us/cpp/standard-library/cpp-
standard-library-reference?view=msvc-170
5. https://www.programiz.com/cpp-programming/standard-template-
library
6. https://www.scaler.com/topics/stl-in-cpp/
7. https://www.cppreference.com/Cpp_STL_ReferenceManual.pdf
8. https://en.wikipedia.org/wiki/Standard_Template_Library
9. https://pwskills.com/blog/components-of-stl-in-c/
Answer from Perplexity: pplx.ai/share
Here’s a comprehensive list of all important C++ STL containers with
their key functions, when to use them, and especially the most-used
functions for hashmaps (unordered_map), sets, vectors, and more. This is
designed for quick reference and practical use in DSA and CP.
STL Containers, Use-Cases, and Key
Functions
1. Sequence Containers
vector
Use when: You need a dynamic array with fast random access and
frequent insertions/removals at the end.
Key functions:
o push_back(val), pop_back()
o size(), empty()
o clear(), resize(n), reserve(n)
o front(), back()
o insert(pos, val), erase(pos)
o begin(), end(), rbegin(), rend()
o operator[], at(i)
deque
Use when: You need fast insertions/removals at both ends.
Key functions: Same as vector plus push_front(val), pop_front()
list
Use when: You need frequent insertions/removals anywhere
(doubly linked list).
Key functions: push_front(), push_back(), pop_front(), pop_back(), insert(),
erase(), begin(), end()
2. Associative Containers
set
Use when: You need unique, sorted elements with fast lookup.
Key functions:
o insert(val), erase(val)
o find(val)
o count(val) (returns 0 or 1)
o lower_bound(val), upper_bound(val)
o begin(), end()
map
Use when: You need key-value pairs, keys are unique and sorted.
Key functions:
o insert({key, val}), erase(key)
o find(key)
o operator[] (access or insert by key)
o count(key)
o begin(), end()
multiset, multimap
Use when: You need duplicates (otherwise same as set/map).
Key functions: Same as set/map, but count(val) can be >1.
3. Unordered Associative Containers (Hash-
based, O(1) average time)
unordered_set
Use when: You need unique elements, don't care about order,
want faster average lookup.
Key functions: Same as set.
unordered_map (Hashmap)
Use when: You need key-value pairs, don't care about order, want
fast average lookup.
Key functions:
o insert({key, val}), erase(key)
o find(key) → returns iterator or end()
o operator[] (access or insert by key)
o count(key)
o begin(), end()
o clear(), size(), empty()
Typical Usage Example:
cpp
unordered_map<int, string> mp;
mp[1] = "one";
mp.insert({2, "two"});
if (mp.count(2)) { /* exists */ }
mp.erase(1);
for (auto &p : mp) cout << p.first << " " << p.second << endl;
4. Container Adapters
stack
Use when: LIFO (last-in, first-out) needed.
Key functions: push(val), pop(), top(), size(), empty()
queue
Use when: FIFO (first-in, first-out) needed.
Key functions: push(val), pop(), front(), back(), size(), empty()
priority_queue
Use when: Always need largest (default, max-heap) or smallest
(min-heap with custom comparator).
Key functions: push(val), pop(), top(), size(), empty()
5. String
Use when: Manipulating text, substrings, searching, etc.
Key functions: size(), length(), substr(pos, len), find(str), replace(), insert(),
erase(), push_back(), pop_back(), c_str()
6. STL Algorithms (from <algorithm> )
Sorting: sort(v.begin(), v.end())
Searching: find(), binary_search()
Counting: count(), count_if()
Removing: remove(), unique()
Others: reverse(), next_permutation(), lower_bound(), upper_bound(),
accumulate() (from <numeric>)
Summary Table: STL Containers and Key
Functions
Container Key Functions (most used) When to Use
push_back, pop_back, size, insert, Dynamic array,
vector
erase, at, [], begin, end random access
Container Key Functions (most used) When to Use
push_front, push_back, pop_front,
deque Double-ended queue
pop_back, size, begin, end
Frequent
push_front, push_back, pop_front,
list insert/erase
pop_back, insert, erase, begin, end
anywhere
insert, erase, find, count, lower_bound, Unique, sorted
set
upper_bound, begin, end elements
unordered_s Unique, fast lookup,
insert, erase, find, count, begin, end
et no order
Key-value, sorted
map insert, erase, find, count, [], begin, end
keys
unordered_ Key-value, fast
insert, erase, find, count, [], begin, end
map lookup, no order
stack push, pop, top, size, empty LIFO
queue push, pop, front, back, size, empty FIFO
Always
priority_que
push, pop, top, size, empty largest/smallest
ue
element
size, length, substr, find, insert, erase,
string Text manipulation
push_back, pop_back, c_str
References
[WsCube Tech STL Guide]2
[Codedamn STL Guide]7
[Wikipedia STL]8
cppreference STL
Tip:
For DSA/CP, focus especially on vector, unordered_map, set, priority_queue,
and STL algorithms. Practice using iterators (begin(), end()) with
algorithms for maximum flexibility and speed.
If you want code examples for any specific container or function, just ask!
Citations:
1. https://teuder.github.io/rcpp4everyone_en/300_STL.html
2. https://www.wscubetech.com/resources/cpp/stl
3. https://stackoverflow.com/questions/7448514/how-can-i-know-how-
much-memory-an-stl-object-takes
4. https://en.cppreference.com/w/cpp/header/memory
5. https://cfd.university/learn/what-every-cfd-developer-needs-to-
know-about-c/the-power-of-the-standard-template-library-stl-in-c
6. https://www.design-reuse.com/articles/25090/dynamic-memory-
allocation-fragmentation-c.html
7. https://codedamn.com/news/c/c-complete-guide-to-standard-
template-library-stl
8. https://en.wikipedia.org/wiki/Standard_Template_Library
Answer from Perplexity: pplx.ai/share
