C++ linked list help (pointers)?

Question

template <typename T>
class LinkedNode
{
public:
    T data;
    LinkedNode<T> *next;
    LinkedNode<T> *prev;
    LinkedNode<T>();
    LinkedNode<T>(T);
};

// Default constructor
template <typename T>
LinkedNode<T>::LinkedNode()
{
    next = NULL;
    prev = NULL;
}



template <typename T>
class LinkedList
{
private:
    LinkedNode<T> *head;
public:
    LinkedList<T>();
    ~LinkedList<T>();
    void addFront(T);
    void addBack(T);
    void addAt(T, int);
    void removeFront();
    void removeBack();
    void removeAt(int);
    void printList();
};

// Constructor
template <typename T>
LinkedList<T>::LinkedList()
{
    head = NULL;
}

// Add new node to front
template <typename T>
void LinkedList<T>::addFront(T d)
{
    LinkedNode<T> temp;
    temp.data = d;

    if (head == NULL)
    {
        head = &temp;
    }
    else
    {
        temp.next = head;
        head->prev = &temp;
        head = &temp;
    }
}

// Add new node to back
template <typename T>
void LinkedList<T>::addBack(T d)
{
    // Find the back of this list
    LinkedNode<T> *ptr;
    ptr = head;

    while (ptr->next != NULL) // <------- DIES HERE, MEMORY ACCESS VIOLATION
    {
        ptr = ptr->next;
    }

    // Make a new node and join it to the back
    LinkedNode<T> temp;
    temp.data = d;
    temp.prev = ptr;
    ptr->next = &temp;
}

Here is a snippet from my linked list system. The issue is that it throws an error on the indicated line. The debugger says that the "head" pointer is pointing to a legitimate memory address of a LinkedNode with no "next" or "prev", but the "ptr" pointer points to address 0xcccccc, not the address of head? I'm really confused, I thought I understood pointers!

In general, 0xcccccc is the debugger telling you the memory being pointed to is uninitialized garbage. If the debugger is right, since ptr=head is correct, the error is in the loop. ptr became junk after ptr = ptr->next. So somehow there is a mistake in the chain. — VoidStar

K-ballo K-ballo · Accepted Answer · 2011-10-13T05:17:59

void LinkedList<T>::addFront(T d)
{
    LinkedNode<T> temp;
    temp.data = d;
    ...
    head = &temp;
    ...
}

temp is a variable of automatic storage, its lifetime will end as soon as the addFront function returns and you will get a pointer to an invalid object. You have to allocate nodes in the heap, do this instead:

    LinkedNode<T>* temp = new LinkedNode<T>;
    temp->data = d;
    ...
    head = temp;

C++ linked list help (pointers)?

2 Answers