Question

The nature of pointers being NULL in C++ seems to feel arbitrary. I’m sure there’s a method to it that I’m missing, but the following makes sense to me, but doesn’t seem to work. I have the following method for adding a node to a linked list:

LLNode *ll; // set to NULL in constructor.
void addToLL(Elem *e)
{
    LLNode *current = ll;
    while(true)
    {
        // edge case of an empty list.
        if (ll == NULL)
        {

            ll = new LLNode(e);
            break;
        }
        else if (current == NULL)
        {
            current = new LLNode(e);
            break;
        }
        else {
            current = current->next;
        }

    }
}

When adding a 2nd node to the list, the case for current == NULL does not get caught, so it tries to call current = current->next and crashes do to accessing invalid memory. Why would this be the case? A LLNode has a pointer to an Elem, and a pointer called next to another LLNode.

Answer 1

You probably didn’t set the next pointer to NULL in the LLNode constructor.

Objects of the basic types in C++ (pointer types, numeric types, etc.) have indeterminate initial values: they don’t get initialized by default. You need to explicitly initialize such objects before you use them.