Question

I am trying to write a code for a linked list but when I try to add a node the code gets stuck at the line immediately after the while loop in the add() function, reporting an access violation error. What’s wrong?

#include<iostream>

template <class T>
class linkedlist
{
    struct node
    {
        T data;
        node *lp;
    }*p;
public:
    linkedlist();
    void add(T t);
};

template<class T>
void linkedlist<T>::add(T t)
{
    node *r,*q;
    r = q = p;

    while(p!= NULL)
    {
        q = p;
        p = p->lp;
    }

    q->lp = new node;
    q->lp->data = t;
    p = r;
}

template<class T>
linkedlist<T>::linkedlist()
{
    p = NULL;
}

int main()
{
    linkedlist<int> l1;
    l1.add(3);
}

Answer 1

You initialize p to NULL in the constructor, then try to dereference it in add (via q):

r = q = p;
// the while loop will not be executed as p == NULL
q->lp = new node;

You must initialize p first – either during construction (in which case your “empty” list won’t be physically empty, so you must deal with this specifically, e.g. when iterating / removing elements), or you should check in add for the case when p == null and handle it differently.

Side note: p is supposed to point to your head element, so it is risky to use it also to iterate through the list in add, then restore its original value (stored in r). Why not simply leave it always intact, and use r for iterating? One less chance for bugs.