Trouble with insertion function for ordered linked list class in C++

Question

I have a template class OList that is an ordered linked list (elements are ordered in ascending order). It has a function called void insert(const T & val) that inserts an element into the correct place in the list. For example, If I had an OList of ints with the values { 1,3,5 } and called insert(4) , the 4 would be inserted between the 3 and the 5, making OList { 1,3,4,5 } .

Now, what I have works fine when inserting elements into EMPTY OLists. However, when I use the following code:

OList<char> list;
for (int i = 0; i < 3; i++) {
    list.insert('C'); 
    list.insert('A');
}
printInfo(list);

printList(list) should output:

List = { A,A,A,C,C,C }  Size = 6        Range = A...C

Instead, it outputs:

List = { A,C,C,C, 

followed by a runtime error.

I have been messing with this for about 5 hours now, but I don't seem to be making any progress (aside from getting DIFFERENT wrong outputs and errors).

There are three relevant pieces of code: OList's default constructor, operator<<, printInfo(), insert(), and a helper function for insert that finds the node to insert the element. I don't see any reason to provide operator<< nor printInfo() since these seem to work fine elsewhere.

// default constructor
OList() {
    size = 0;
    headNode = new Node<T>;
    lastNode = new Node<T>;
    headNode->next = lastNode;
    lastNode->next = NULL;
}


void insert(const T & val) {
    if ( isEmpty() ) {
        lastNode->data = val;
    }
    else {
        Node<T> * pre = headNode;
        Node<T> * insertPoint = findInsertPoint(pre, val);
        Node<T> * insertNode = new Node<T>;
        insertNode->data = val;
        insertNode->next = insertPoint;
        pre->next = insertNode;

        // why is pre equal to headNode? 
        // I thought I changed that when using it
        // with findInsertPoint()
        cout << (pre == headNode) << endl;
    }

    size++;
}

// returns the node AFTER the insertion point
// pre is the node BEFORE the insertion point
Node<T> * findInsertPoint(Node<T> * pre, const T & val) {
    Node<T> * current = pre->next;

    for (int i = 0; (i < getSize()) && (val > current->data); i++) {
        pre = current;
        current = current->next;
    }

    return current;
}

lastNode is simply the last node in the list. headNode is a "dummy node" that contains no data and is only used as a starting place for the list.

Thanks in advanced. I'm really embarrassed to be asking for homework help on the internet, especially since I'm sure the main problem is my lack of a thorough understanding of pointers.

Answer 1

You are passing the pointer to pre by value into findInsertPoint, so it is copied, and the function changes the copy of pointer, and when the function returns, it is still the old pre, not the pre from inside the function.

If you want to change the pointer, you must pass pointer to the pointer to the function (or reference to pointer).