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C++ Standard Template Library (STL) - Complete Overview

What is STL?

The Standard Template Library (STL) is a powerful library in C++ that provides ready-to-use classes and functions for common data structures and algorithms.
It saves time by providing generic containers, iterators, and algorithms which can be reused for different data types.

Main Components of STL

  1. Containers — Data structures to store objects
  2. Algorithms — Functions that operate on containers
  3. Iterators — Objects that point to container elements and allow traversal
  4. Function Objects (Functors) — Objects acting like functions
  5. Adapters — Containers with restricted interfaces (stack, queue, etc.)

1. Containers

Containers store collections of objects. They can be:

Container Description Header Example Declaration
vector Dynamic array (resizable) <vector> vector<int> v;
list Doubly linked list <list> list<int> l;
deque Double-ended queue (fast front/back) <deque> deque<int> d;
set Sorted unique elements <set> set<int> s;
map Key-value pairs (sorted by key) <map> map<string, int> m;
unordered_map Key-value pairs, no order <unordered_map> unordered_map<int, string> um;

Example: vector

#include <vector>
#include <iostream>
using namespace std;

int main() {
    vector<int> v = {1, 2, 3};
    v.push_back(4);               // Add element at end
    cout << v[0] << endl;         // Access element by index
    cout << v.size() << endl;     // Get size of vector
}

2. Algorithms

Algorithms are predefined functions to perform operations on containers:

Algorithm Description Header
sort() Sort elements <algorithm>
reverse() Reverse elements <algorithm>
find() Find element <algorithm>
count() Count occurrences <algorithm>
max_element() Find max element <algorithm>
min_element() Find min element <algorithm>

Example: sort and find

#include <vector>
#include <algorithm>
#include <iostream>
using namespace std;

int main() {
    vector<int> v = {4, 2, 5, 1, 3};

    sort(v.begin(), v.end());  // Sort the vector

    for (int x : v) cout << x << " ";  // Print sorted vector
    cout << endl;

    // Find element 3
    auto it = find(v.begin(), v.end(), 3);
    if (it != v.end()) cout << "Found 3 at index " << (it - v.begin()) << endl;
}

3. Iterators

Iterators are pointers or objects that traverse containers. They allow algorithms to work on different containers uniformly.

Iterator Description
begin() Points to first element
end() Points just past the last element
rbegin() Reverse iterator at last element
rend() Reverse iterator before first element

Example: Using iterators

#include <vector>
#include <iostream>
using namespace std;

int main() {
    vector<int> v = {10, 20, 30};

    for (auto it = v.begin(); it != v.end(); ++it) {
        cout << *it << " ";  // Dereference iterator to get element
    }
    cout << endl;
}

4. Function Objects (Functors)

Function objects are objects that behave like functions by overloading the operator(). Used for custom behavior in algorithms.

5. Adapters

Adapters provide restricted interfaces on top of other containers:

Adapter Description Header
stack Last-in-first-out (LIFO) <stack>
queue First-in-first-out (FIFO) <queue>
priority_queue Elements with priority <queue>

Example: stack

#include <stack>
#include <iostream>
using namespace std;

int main() {
    stack<int> s;

    s.push(1);
    s.push(2);

    cout << s.top() << endl;  // Output: 2
    s.pop();
    cout << s.top() << endl;  // Output: 1
}

Summary Table

Component Purpose Common Headers
Containers Store data <vector>, <list>, <map>, <set>
Algorithms Perform operations <algorithm>
Iterators Traverse containers Provided by container headers
Function Objects Custom callable objects <functional>
Adapters Restricted interface containers <stack>, <queue>

Additional Tips

  • STL makes code cleaner, faster, and easier to maintain.
  • Use auto keyword with iterators for simplicity.
  • Algorithms use iterators, so they work with any container supporting those iterators.
  • Learn containers and algorithms gradually with practice.

References & Further Reading

Happy coding with STL! 🚀

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Ask me anytime for more examples or explanations!