• A C++ library providing generic, reusable data structures and algorithms
• Containers: vector (dynamic array), list (doubly linked list), map (key-value pairs, typically balanced-tree based), set (unique sorted elements), stack, queue
• Iterators: generalized pointers used to traverse container elements uniformly
• Algorithms: pre-built functions (e.g., sort(), find(), count()) that operate generically over containers via iterators

#include <vector>
#include <algorithm>
vector<int> nums = {5, 2, 8, 1, 9};
sort(nums.begin(), nums.end());   // 1 2 5 8 9

Why STL exists

Before STL, every C++ programmer wrote their own linked list, their own sort function, etc. — repetitive and error-prone. STL provides pre-built, well-tested, generic (template-based) versions of the data structures and algorithms used in almost every program.

The three pieces, working together

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

int main()
{
    vector<int> nums = {5, 2, 8, 1, 9};   // CONTAINER

    sort(nums.begin(), nums.end());       // ALGORITHM, operating via ITERATORS (begin/end)

    for (int n : nums)                    // range-based loop, uses iterators internally
        cout << n << " ";                 // 1 2 5 8 9

    auto it = find(nums.begin(), nums.end(), 8);   // ALGORITHM again
    if (it != nums.end())
        cout << "\nFound 8 at position " << (it - nums.begin());   // position 3

    return 0;
}

map — key-value pairs, worked example

#include <map>
map<string, int> ages;
ages["Ali"] = 25;
ages["Sara"] = 30;
cout << ages["Ali"];   // 25
for (auto& pair : ages)
    cout << pair.first << ": " << pair.second << endl;   // iterates in sorted key order

Why iterators matter — the generalization

An iterator behaves like a pointer (*it dereferences, ++it advances) but works uniformly across very different containers (vector, list, map...). This is why sort(nums.begin(), nums.end()) can work on a vector while the same style of call works on other containers too — the algorithm doesn't need to know the container's internal structure, only how to move an iterator.

Important MCQs

1. Which STL container provides fast random access and dynamic resizing?
   • a) list b) vector c) map d) set
2. Which STL container stores unique, sorted elements?
   • a) vector b) list c) set d) queue
3. STL algorithms like sort() operate generically over containers using:
   • a) Pointers only b) Iterators c) Static members d) Templates alone, without iterators
4. Which container is best suited for key-value lookups with sorted keys?
   • a) vector b) map c) stack d) queue

Self-Test Questions