Basic linkedList

#include <iostream>
#include <utility>
#include <fstream>
#include <memory>

using namespace std;

template < typename T >
class List;

template <typename T >
struct Node
{
    friend class List<T>;

    Node()
    {
        next = nullptr;
    }

    explicit Node ( const T & element )
    {
        value = element;
        next = nullptr;
    }

    T value;
    Node<T> * next;
};

template < typename T >
class List{


private :
    Node<T> * firstElement;
    int size;

public:
    List()
    {
        size = 0;
        firstElement = nullptr;
    }

    explicit List( const T & element )
    {
        size = 1;
        firstElement = new Node<T>(element);
        firstElement->next = nullptr;
    }

    void AddElement( const T & element )
    {
        auto * iter = firstElement;
        while ( iter->next != nullptr ){
            iter = iter->next;
        }

        iter->next = new Node<T> (element);
    }

    friend ostream & operator <<( ostream & os , List<T> & l )
    {
        if( l.size == 0 ){
            return os;
        }

        auto * iter = l.firstElement;

        while(iter->next != nullptr)
        {
            os << iter->value << ",";
            iter = iter->next;
        }
        os << iter->value;
        return os;
    }
};

int main()
{
    List<int> l(5 );
    l.AddElement(6);
    cout << l << endl ;
}