cppListFwInsertAtDeleteAt

#include <iostream>

template <typename ElemT>
class ListFw
{
    class Node
    {
        ElemT data;
        Node* next;
    public:
        Node(const ElemT& dataP, Node* nextP = nullptr) : data{ dataP }, next{ nextP }{}
        friend ListFw;
    };

    Node* head;
    Node* tail;
    size_t size;
    Node* getNode(size_t idx);
public:
    ListFw() : head{ nullptr }, tail{ nullptr }, size{ 0 } {}
    bool empty()const { return size == 0; }
    size_t get_size()const { return size; }
    ElemT& front() { return head->data; }
    const ElemT& front()const { return head->data; }
    ElemT& back() { return tail->data; }
    const ElemT& back()const { return tail->data; }
    void push_front(const ElemT& data);
    void push_back(const ElemT& data);
    bool pop_front();
    bool pop_back();

    void insert_at(size_t idx, const ElemT& data);
    bool delete_at(size_t idx);

    void print()
    {
        if (!head) { std::cout << "[empty]"; }
        else
        {
            for (auto curr{ head }; curr; curr = curr->next)
            {
                std::cout << curr->data << ' ';
            }
        }
        std::cout << '\n';
    }

    void clear();
    ~ListFw();
};

template <typename ElemT>
void ListFw<ElemT>::push_front(const ElemT& data)
{
    head = new Node{ data, head };

    if (!tail) { tail = head; }
    ++size;
}

template <typename ElemT>
void ListFw<ElemT>::push_back(const ElemT& data)
{
    if (!head)
    {
        head = new Node{ data, head };
        tail = head;
    }
    else
    {
        tail->next = new Node{ data, nullptr };
        tail = tail->next;
    }
    ++size;
}

template <typename ElemT>
bool ListFw<ElemT>::pop_front()
{
    if (head)
    {
        Node* oldHead{ head };
        head = head->next;
        delete oldHead;
        --size;
        if (!size) { tail = nullptr; }
        return true;
    }
    return false;
}

template <typename ElemT>
bool ListFw<ElemT>::pop_back()
{
    if (tail)
    {
        if (tail == head)
        {
            delete tail;
            head = nullptr;
            tail = nullptr;
            size = 0;
            return true;
        }

        Node* curr{ head };
        for (; !(curr->next == tail); curr = curr->next);

        delete tail;
        curr->next = nullptr;
        tail = curr;
        --size;
        return true;
    }
    return false;
}

template <typename ElemT>
void ListFw<ElemT>::clear()
{
    while (head)
    {
        std::cout << "Clear for: " << front() << '\n';
        pop_front();
    }
}

template <typename ElemT>
ListFw<ElemT>::~ListFw()
{
    while (tail)
    {
        std::cout << "Destruct for: " << back() << '\n';
        pop_back();
    }
}

template <typename ElemT>
void printListState(const ListFw<ElemT>& l)
{
    std::cout << "Size: " << l.get_size() << '\n';
    std::cout << "Front: " << l.front() << '\n';
    std::cout << "Back: " << l.back() << '\n';
}

template <typename ElemT>
typename ListFw<ElemT>::Node* ListFw<ElemT>::getNode(size_t idx)
{
    Node* curr{ nullptr };
    size_t currIdx;

    if (idx >= 0 and idx < size)
    {
        for (curr = head, currIdx = 0; currIdx < idx; curr = curr->next, ++currIdx);
    }
    return curr;
}

template <typename ElemT>
void ListFw<ElemT>::insert_at(size_t idx, const ElemT& data)
{
    Node* prev{ getNode(idx - 1) };
    Node* curr{ prev ? prev->next : nullptr};

    if (!curr or curr == head) { push_front(data); return; }
    else
    {
        prev->next = new Node{ data, curr };
    }
    ++size;
}

template <typename ElemT>
bool ListFw<ElemT>::delete_at(size_t idx)
{
    if (idx == 0) { pop_front(); return true; }
    Node* prev{ getNode(idx - 1) };
    Node* curr{ prev ? prev->next : nullptr };

    if (curr)
    {
        if (curr == head) { head = curr->next; }
        else
        {
            if (curr->next) { prev->next = curr->next; }
            else { tail = prev; prev->next = nullptr; }
        }
        --size;
        delete curr;
        return true;
    }
    return false;
}

int main()
{
    ListFw<int> l;

    std::cout << "Size: " << l.get_size() << '\n';

    l.push_back(42);
    l.push_back(33);

    printListState(l);

    l.pop_front();
    l.pop_back();
    std::cout << "Size: " << l.get_size() << '\n';

    for (int i{ 0 }; i < 5; l.push_front(i++));

    printListState(l);
    l.clear();

    for (int i{ 0 }; i < 5; l.push_back(i++));
    printListState(l);

    l.insert_at(3, 42);
    l.print();

    l.delete_at(3);
    l.print();

    l.clear();
    l.print();

    l.insert_at(42, 33);
    l.print();
    l.delete_at(0);
    l.print();

    l.insert_at(-33, 42);
    l.print();
    l.delete_at(0);
    l.print();

    return 0;
}