C++ Linked List confusion

Question

I've been stuck trying to figure out how I can implement a NODE based link list that can hold anything (via templates).

I've got the Node class ready and working(tested with my Stack class).

I was curious where I could be going wrong when making the function called insertAtIndex where I take in the data to store and the index at where it should be stored.

My Node class

template <class T>
class Node
{
    public:
    T *data; //the object information
    Node<T> *next; //pointer to the next node element

    Node()
    {
        next = 0;
        data = 0;
    }
};

Here's my Linked List class so far

template <class T>
class LinkedList
{
private:
    Node<T> *head;
    int count;
public:
    LinkedList()
    {
        head = 0;
        count = 0;
    }
    int getCount()
    {
        return count;
    }

    void insertAtIndex(T* dat, int index)
    {   
        Node<T> * temp = new Node<T>(dat);

        if(index == 0)
        {

            temp = head;
            temp->data  = dat;
            temp->next = temp->next;
            temp->next = temp;
            delete temp;
            count++;
        }

        else if(index <= count)
        {
            Node<T> *cursor = new Node<T>();

            for(int i = 0; i < index - 1; i++)
            {
                cursor = cursor->next;
            }

            Node<T> * temp = new Node<T>();
            temp->data  = dat;
            temp->next = cursor->next;
            cursor->next = temp;
            count++;
        }

    }

    void Print()
    {

        // Temp pointer
        Node<T> *tmp = head;

        // No nodes
        if ( tmp == NULL ) {
        cout << "EMPTY" << endl;
        return;
        }

        // One node in the list
        if ( tmp->next == NULL ) {
        cout << *tmp->data;
        cout << " --> ";
        cout << "NULL" << endl;
        }
        else 
        {
        // Parse and print the list
        do
        {
            cout << *tmp->data;
            cout << " --> ";
            tmp = tmp->next;
        }
        while ( tmp != NULL );

        cout << "NULL" << endl;
        }
    }
};

Answer 1

You should add Node(T*) constructor to your Node class:

template <class T>
class Node
{
    public:
    T *data; //the object information
    Node<T> *next; //pointer to the next node element

    Node()
    {
        next = 0;
        data = 0;
    }

    Node(T* newData) { // <<---------- Here it is
        next = 0;
        data = newData;
    }
};

Answer 2

You need to rethink how you are adding nodes to the list. Here is your code with comments on what it is actually doing.

   Node<T> * temp = new Node<T>(dat);  // Create new node, good

    if(index == 0)
    {

        temp = head;                  // Throw away pointer to new node, bad. temp is now head.
        temp->data  = dat;            // Overwrite head's data, bad
        temp->next = temp->next;      // Set a variable to itself, does nothing.
        temp->next = temp;            // Change head's next to point to itself, bad
        delete temp;                  // delete the head node, bad
        count++;
    }

What you actually want to do is:

• Create a new node with the right data (you already do this)
• If you are putting it at the start of the list:
  • Point the new node's next pointer to head
  • Change head to point to the new node

That's all - don't delete anything.

Once you have that working, move on to the more complicated part of adding a node to the middle or the list (your else part).

A debugger (or at least output statements) is essential for learning where your code is going wrong. You can look at the state of your variables at each point.

Answer 3

You might want to consider a base node class that only has a next pointer as a member, then an inherited class that adds a template data member. Common list functions could be used for the base class.