Overloading operator ++ for links in a linked list (c++)

Maybe you should refactor your design and the code.

The link, or better said the Node, is normally implemented as an own class. An this class is embedded in the LinkedList class. And that Node class should be completely encapsulated and not shown to the outside world.

The user of the class will just deal with the data value. All the pointer stuff should be hidden.

And to be able to iterate over your class, which is similar to a std::forward_list, you can add ultra simple iterator functionality. Most functions can be implemented using a one liner.

I will show you below an ultra simple implementation example. This may be extended easily according to your needs.

From that you may take over some ideas to improve your design.

With the added iterator functionality, you may use range based for loops and std:: algorithms and all the like.

Please have a look and ask questions, if there should be something unclear.

#include <iostream>
#include <iterator>
#include <initializer_list>
#include <algorithm>

// Very simple implementation of a forward list
template <class T>
class SinglyLinkedList {

    // The node
    struct Node {
        T data{};     // Data. Would normally be a templated argument
        Node* next{};   // And the pointer to the next node

        Node(const T& i, Node* n = nullptr) : data(i), next(n) {}; // Simple constructor to set a value and next pointer
    };

    Node* head{};       // This is the start of the list
    // It would be advisable to have a tail pointer. We use the more inefficient approach here
    Node* getLast() const { Node* n{ head }; while (n and n->next) n = n->next; return n; }

public:
    // Constructor / Destructor --------------------------------------------------------------------------------------------------------
    ~SinglyLinkedList() { clear(); }

    // Default constuctor
    SinglyLinkedList() {}   // Default

    // From an initialization list
    SinglyLinkedList(const std::initializer_list<T>& il) { clear();  for (const T& i : il) push_back(i); } // From initializer list

    // Copy constructor
    SinglyLinkedList(const SinglyLinkedList& other) { clear(); for (const T &i : other) push_back(i); }

    // Move constructor. Will steal the elements from the other 
    SinglyLinkedList(SinglyLinkedList&& other) noexcept { head = other.head; other.head = nullptr; }

    // Assignment operator
    SinglyLinkedList& operator = (const SinglyLinkedList& other) { clear(); for (const T &i : other) push_back(i); }

    // Move assignment operator 
    SinglyLinkedList& operator = (SinglyLinkedList&& other) { head = other.head; other.head = nullptr; }

    // Housekeeping --------------------------------------------------------------------------------------------------------------
    void clear() { Node* tmp{ head }; while (tmp) { Node* toDelete{ tmp }; tmp = tmp->next; delete toDelete; } head = nullptr; }
    int empty() const { return head == nullptr; }
    int size() const { int k{}; Node* n{ head }; while (n) { ++k; n = n->next; } return k; }

    // Modify content --------------------------------------------------------------------------------------------------------------
    void push_front(const T& i) { Node* n = new Node(i); n->next = head; head = n; };
    void push_back(const T& i) { Node* n = new Node(i); Node* l = getLast(); if (l) l->next = n; else head = n; }
    void pop_front() { if (head) { Node* tmp = head->next; delete head; head = tmp; } }
    void pop_back() { // This is a little bit more difficult in a singly linked list
        if (head) {
            Node* n{ head }, * previous{};
            while (n and n->next) {
                previous = n;
                n = n->next;
            }
            delete n;
            if (previous)
                previous->next = nullptr;
            else
                head->next = nullptr;
        }
    }

    // Access elements --------------------------------------------------------------------------------
    T front() const { return head ? head->data : 0; };
    T back() const { Node* n = getLast(); return n ? n->data : 0; }

    // Add iterator properties to class ---------------------------------------------------------------
    struct iterator {                           // Local class for iterator
        Node* iter{};                           // Iterator is basically a pointer to the node

        // Define alias names necessary for the iterator functionality
        using iterator_category = std::forward_iterator_tag;
        using difference_type = std::ptrdiff_t;
        using value_type = T;
        using pointer = T*;
        using reference = T&;

        // Constructor
        iterator() {}
        iterator(Node* n) : iter(n) {}

        // Dereferencing
        reference operator *() const { return iter->data; }
        pointer operator ->() const { return &iter->data; }

        // Aithmetic operations
        iterator& operator ++() { if (iter) iter = iter->next; return *this; }
        iterator operator ++(int) { iterator temp{ *this }; ++* this; return temp; }
        iterator operator +(const difference_type& n) const { iterator temp{ *this };  difference_type k{ n }; while (k--)++temp; return temp; }
        iterator& operator +=(const difference_type& n) { difference_type k{ n }; while (k--)++* this; return *this; };

        // Comparison
        bool operator != (const iterator& other) const { return iter != other.iter; }
        bool operator == (const iterator& other) const { return iter == other.iter; }
        bool operator < (const iterator& other) const { return iter < other.iter; }
        bool operator > (const iterator& other) const { return iter > other.iter; }
        bool operator <= (const iterator& other) const { return iter <= other.iter; }
        bool operator >= (const iterator& other) const { return iter >= other.iter; }

        // Difference. Also complicated, because no random access
        difference_type operator-(const iterator& other) const {
            difference_type result{};
            Node* n{ iter };
            while (n and n != other.iter) {
                ++result;
                n = n->next;
            }
            return result;
        }
    };

    // Begin and end function to initialize an iterator
    iterator begin() const { return iterator(head); }
    iterator end() const { return iterator(); }

    // Functions typcical for forward lists ----------------------------------------------------------------------
    // Easy, becuase we can operate form the current iterator and do not need the "previous" element
    iterator insertAfter(iterator& pos, const T& i) {
        iterator result{};
        if (pos.iter and pos.iter->next) {
            Node* n = new Node(i, pos.iter->next);
            pos.iter->next = n;
            result = n;
        }
        return result;
    }
    iterator eraseAfter(iterator& pos) {
        iterator result{};
        if (pos.iter and pos.iter->next) {
            Node* tmp = pos.iter->next->next;
            delete pos.iter->next;
            pos.iter->next = tmp;
            result = pos.iter->next;
        }
        return result;
    }
};
// Test/Driver Code
int main() {

    // Example for initilizer list
    SinglyLinkedList<int> sllbase{ 5,6,7,8,9,10,11,12,13,14,15 };
    // Show move constructor
    SinglyLinkedList<int> sll(std::move(sllbase));

    // Add some values in the front
    sll.push_front(4);
    sll.push_front(3);
    sll.push_front(2);
    sll.push_front(1);

    // Delete 1st element (Number 1)
    sll.pop_front();
    // Delete last element
    sll.pop_back();

    // Use a std::algorithm on our custom linked list. Works because we have an interator
    SinglyLinkedList<int>::iterator iter = std::find(sll.begin(), sll.end(), 8);

    // Now add an element after 8
    iter = sll.insertAfter(iter, 88);
    // End delete the 9
    iter = sll.eraseAfter(iter);

    // Use range based for loop. Works because, we have iterators
    for (int i : sll)
        std::cout << i << ' ';
}