The Standard Template Library (STL)

Ready-made containers and algorithms, so you don't reinvent the wheel

Big Idea: The STL is a giant toolbox that already ships with C++. Instead of writing your own list, dictionary, or sorting code from scratch, you reuse containers like std::vector and std::map and algorithms like std::sort — tested, fast, and ready to use.

By the end you will be able to:

store a growing list of values in a std::vector,
look things up by key with a std::map,
use ready-made algorithms like sort, find, and count,
loop over any container with a clean range-based for.

1. What is the STL?

The Standard Template Library is a collection of ready-to-use tools built into C++. You get it for free — you just #include the right header. It has three main kinds of tools:

Containers — objects that hold your data (like vector and map).
Algorithms — ready-made operations that work on your data (like sort, find, count).
Iterators — the "pointers" that connect algorithms to containers (you'll see .begin() and .end()).

Think of the STL as a kitchen full of appliances. You could chop everything by hand, but why not use the blender that's already there?

2. std::vector — a list that grows

A plain C++ array has a fixed size. A std::vector is a smarter array that can grow and shrink while your program runs. You add items with push_back, ask how many there are with size(), and read any item with square brackets [i] (starting at index 0).

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

int main() {
    vector<int> scores;      // an empty, resizable list of ints
    scores.push_back(90);    // add items to the end
    scores.push_back(75);
    scores.push_back(88);

    cout << "We have " << scores.size() << " scores\n";
    cout << "First score: " << scores[0] << "\n";
    return 0;
}

Output:

We have 3 scores
First score: 90

Notice we never said how big the vector was — it grew by itself as we pushed items in.

The Standard Template Library (STL)