List in C++ STL

A list in C++ is a sequence container that allows you to store elements one after another.

• Implemented as a doubly linked list and maintains both front and back for fast operations on both the ends.
• Data is stored in non-contiguous memory, allowing fast insertion and deletion anywhere in the list (beginning, middle, or end).

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

int main(){
    list<int> myList;

    myList.push_back(10);
    myList.push_back(20);
    myList.push_front(5);

    cout << "List elements: ";
    for (int n : myList){
        cout << n << " ";
    }
    cout << endl;

    return 0;
}

List elements: 5 10 20 

Syntax

A list is defined as the std::list class template inside the <list> header file.

list<T> l;

where,

• T: Type of elements in the list.
• l: Name assigned to the list.

Basic Operations

The basic operations of list are shown below:

1. Inserting Elements

• insert() is used for fast insertion if the position iterator is known; otherwise, traverse the list to reach the position.
• push_front() is used to insert at the beginning and push_back() to insert at the end.

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

int main(){
    list<int> l = {3, 2};

    l.push_back(5);
    l.push_front(1);
    auto it = l.begin();
    advance(it, 2);
    l.insert(it, 4);

    for (auto i : l)
        cout << i << " ";
    return 0;
}

1 3 4 2 5 

2. Accessing Elements

• Lists do not allow random access, so to get an element at a specific position, you need to go through the list one by one from the start or end.
• The first and last elements can be accessed quickly using front() and back() methods.

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

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

    cout << l.front() << endl;
    cout << l.back() << endl;
    cout << *next(l.begin(), 2);

    return 0;
}

1
5
4

Explanation: "next(l.begin(), 2)" moves the iterator two positions forward from the start of the list and * dereferences it to print the element at index 2 (the third element).

3. Updating Elements

• List elements can be updated by accessing them with an iterator and using the assignment operator (=) to set a new value.
• Since lists do not support random access, you must use an iterator to reach the element you want to update.

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

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

    l.front() = 11;
    auto it = l.begin();
    advance(it, 2);
    
    *it = 10;

    for (auto i : l)
        cout << i << " ";
    return 0;
}

11 3 10 2 5 

4. Finding Elements

• To find an element in a list, you can use the find() function from the <algorithm> library.
• find() returns an iterator to the element if it is found, or the end iterator if it is not found.

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

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

    // Finding 4
    auto it = find(l.begin(), l.end(), 4);
    if (it != l.end())
        cout << *it;
    else
        cout << "Element Not Found!";
    return 0;
}

4

5. Traversing

• A list can be traversed using begin() and end() iterators in a loop.
• Start from begin() and keep moving the iterator until it reaches end(), accessing each element along the way.

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

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

    // Traversing using iterators
    for (auto it = l.begin(); it != l.end(); ++it)
        cout << *it << " ";
        
    return 0;
}

1 3 4 2 5 

6. Deleting Elements

• erase() deletes an element from the list using an iterator to its position.
• pop_front() and pop_back() quickly delete the first and last elements of the list.

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

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

    l.pop_back();
    l.pop_front();
    
    auto it = l.begin();
    advance(it, 2);
    l.erase(it);

    for (auto i : l)
        cout << i << " ";
    return 0;
}

3 4 

Other Member Functions of List

Following is the list of all member functions of std::list class in C++:

Forward List vs List