C++ STL

So far, we have written many things ourselves.

But C++ already provides many useful tools that we can use instead of creating everything from scratch.

These tools are part of the STL, which stands for Standard Template Library.

C++ STL gives us ready-made containers and functions that make programming easier and faster.

What is STL in C++?

STL is a collection of ready-to-use tools in C++.

It includes:

• Containers
• Iterators
• Algorithms

For example, if you want to store many numbers, you can use a vector instead of creating everything yourself.

STL
├── Containers
├── Iterators
└── Algorithms

STL is used in many real C++ programs.

C++ STL Containers

A container is used to store and organize data.

Some commonly used STL containers are:

• vector
• list
• stack
• queue
• set
• map

Each container works differently.

For example:

vector → Stores items in a sequence
stack  → Last item comes out first
queue  → First item comes out first
set    → Stores unique values
map    → Stores key-value pairs

Let’s look at them one by one.

C++ STL Iterators

An iterator is used to move through the elements of an STL container.

Think of an iterator like a pointer that helps us move from one item to another.

For example:

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

int main() {
    vector<int> numbers = {10, 20, 30, 40, 50};

    vector<int>::iterator it;

    for (it = numbers.begin(); it != numbers.end(); ++it) {
        cout << *it << "\n";
    }

    return 0;
}

Output:

10
20
30
40
50

Here:

numbers.begin()

points to the first element.

And:

numbers.end()

points to the position just after the last element.

The * is used to get the value at the iterator’s current position.

C++ STL Algorithms

STL also provides ready-made algorithms for common tasks.

For example, we can use sort() to sort numbers.

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

int main() {
    vector<int> numbers = {50, 10, 40, 20, 30};

    sort(numbers.begin(), numbers.end());

    cout << "Sorted numbers:\n";

    for (int number : numbers) {
        cout << number << " ";
    }

    return 0;
}

Output:

Sorted numbers:
10 20 30 40 50

Instead of writing our own sorting logic, we can simply use:

sort(numbers.begin(), numbers.end());

This is one of the useful features of STL.

vector

A vector is a container that stores multiple values.

It is similar to an array, but a vector can grow or shrink when needed.

To use a vector, include:

#include <vector>

Let’s see a complete example:

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

int main() {
    vector<int> numbers;

    numbers.push_back(10);
    numbers.push_back(20);
    numbers.push_back(30);

    cout << "Numbers in the vector:\n";

    for (int number : numbers) {
        cout << number << "\n";
    }

    return 0;
}

Output:

Numbers in the vector:
10
20
30

The function:

numbers.push_back(10);

adds 10 to the vector.

We can add as many values as we need.

Getting the Size of a Vector

We can use size() to find how many elements are stored.

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

int main() {
    vector<string> fruits;

    fruits.push_back("Apple");
    fruits.push_back("Mango");
    fruits.push_back("Banana");

    cout << "Number of fruits: " << fruits.size() << "\n";

    cout << "Fruits:\n";

    for (string fruit : fruits) {
        cout << fruit << "\n";
    }

    return 0;
}

Output:

Number of fruits: 3
Fruits:
Apple
Mango
Banana

list

A list stores multiple elements like a sequence.

One useful feature of a list is that adding or removing elements from the beginning or middle can be efficient.

To use it, include:

#include <list>

Example:

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

int main() {
    list<int> numbers;

    numbers.push_back(20);
    numbers.push_back(30);
    numbers.push_front(10);

    cout << "Numbers in the list:\n";

    for (int number : numbers) {
        cout << number << " ";
    }

    return 0;
}

Output:

Numbers in the list:
10 20 30

Here:

numbers.push_back(20);

adds an element at the end.

And:

numbers.push_front(10);

adds an element at the beginning.

stack

A stack works like a stack of plates.

Imagine putting plates one on top of another.

The last plate you put on is the first plate you take off.

This is called LIFO:

Last In, First Out

To use a stack:

#include <stack>

Example:

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

int main() {
    stack<int> numbers;

    numbers.push(10);
    numbers.push(20);
    numbers.push(30);

    cout << "Top element: " << numbers.top() << "\n";

    numbers.pop();

    cout << "Top element after pop: " << numbers.top();

    return 0;
}

Output:

Top element: 30
Top element after pop: 20

Here:

numbers.push(30);

puts 30 on top.

And:

numbers.pop();

removes the top element.

queue

A queue works like a line of people waiting at a ticket counter.

The person who comes first gets served first.

This is called FIFO:

First In, First Out

To use a queue:

#include <queue>

Example:

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

int main() {
    queue<string> students;

    students.push("Rahul");
    students.push("Aman");
    students.push("Priya");

    cout << "First student: " << students.front() << "\n";

    students.pop();

    cout << "Next student: " << students.front();

    return 0;
}

Output:

First student: Rahul
Next student: Aman

Here:

students.push("Rahul");

adds Rahul to the queue.

And:

students.pop();

removes the first student.

set

A set stores values where duplicate values are not allowed.

For example, if we add 10 twice, the set will keep only one 10.

To use a set:

#include <set>

Example:

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

int main() {
    set<int> numbers;

    numbers.insert(10);
    numbers.insert(20);
    numbers.insert(10);
    numbers.insert(30);

    cout << "Numbers in the set:\n";

    for (int number : numbers) {
        cout << number << " ";
    }

    return 0;
}

Output:

Numbers in the set:
10 20 30

Notice that 10 was added twice, but it appears only once.

A set also keeps its elements sorted by default.

map

A map stores data in key-value pairs.

Think of a dictionary.

You have:

Word → Meaning

In a C++ map, you can have:

Key → Value

For example:

Name → Age
Rahul → 14
Priya → 15

To use a map:

#include <map>

Example:

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

int main() {
    map<string, int> students;

    students["Rahul"] = 14;
    students["Priya"] = 15;
    students["Aman"] = 13;

    cout << "Rahul's age: " << students["Rahul"] << "\n";
    cout << "Priya's age: " << students["Priya"];

    return 0;
}

Output:

Rahul's age: 14
Priya's age: 15

Here:

students["Rahul"] = 14;

means:

Key   → Rahul
Value → 14

We can also display all the values:

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

int main() {
    map<string, int> students;

    students["Rahul"] = 14;
    students["Priya"] = 15;
    students["Aman"] = 13;

    cout << "Student Information:\n";

    for (auto student : students) {
        cout << student.first << " - " << student.second << "\n";
    }

    return 0;
}

Output:

Student Information:
Aman - 13
Priya - 15
Rahul - 14

Here:

student.first

gives the key.

And:

student.second

gives the value.

Quick Comparison of C++ STL Containers

Why Use C++ STL?

STL saves us from writing common data structures and algorithms from scratch.

For example, instead of creating our own dynamic array, we can use:

vector<int>

Instead of writing our own sorting algorithm, we can use:

sort()

This makes C++ programs easier and faster to write.

Key Points to Remember

• STL stands for Standard Template Library.
• STL provides ready-to-use containers, iterators, and algorithms.
• vector stores a dynamic collection of values.
• list stores elements in a linked sequence.
• stack follows LIFO.
• queue follows FIFO.
• set stores unique values.
• map stores key-value pairs.
• Iterators help us move through containers.
• Algorithms provide ready-made operations such as sorting.

Frequently Asked Questions (FAQs)

Written by Shubhranshu Shekhar, who has trained 20000+ students in coding.

Nidhi Srivastav