List Class implementation Memory allocation error with a pointer

Question

So I trying to do an implementation of the List class but when trying to do the erase function I keep getting a pointer being freed was not allocated error.

Here is the code of my function erase():

template <class T> void List<T>::erase(ListIterator<T> & start, ListIterator<T> & stop)
{
    while (start<= stop) {
    ListIterator<T> * temp = &start;
    ++temp;
    delete start.currentLink;
    start.currentLink = temp->currentLink;
    }
}

Now I have already tested my link and listiterator class and they work I have done everything else like insert() and push_back(). I have not been able to find out where I the memory allocation goes wrong. is any ideas that might point me in the right direction.

Answer 1

According to your implementation you will be freeing same memory space in third iteration of while loop (i,e if you are freeing more than 2 element then you will probably get an error).

template <class T> void List<T>::erase(ListIterator<T> & start, ListIterator<T> & stop)
{
          while (start<= stop) {
              ListIterator<T> * temp = &start;
Line:1        ++temp;
Line:2        delete start.currentLink;
Line:3        start.currentLink = temp->currentLink;
    }
}

Consider ListClass = {A, B, C} with A.link = 100, B.link = 101 and C.link = 102

1st Iteration:

Line1: temp points to B

Line2: you free 100

Line3: you assign A.link = temp.link (i,e B.link) = 101

2nd Iteration:

Line1: temp still points to B

Line2: you free 101

Line3: you assign A.link = temp.link (i,e B.link) = 101 which is already freed

3rd Iteration

Line1: temp still points to same darn B

Line2: you free 101 which is already freed <--- probably gdb complains here

Easy Fix:

template <class T> void List<T>::erase(ListIterator<T> & start, ListIterator<T> & stop)
{
    ListIterator<T> * temp = &start;
    while (start<= stop) {
        ++temp;
        delete start.currentLink;
        start.currentLink = temp->currentLink;
    }
}

Better Fix:

template <class T> void List<T>::erase(ListIterator<T> & start, ListIterator<T> & stop)
{
    ListIterator<T> * temp = &start;
    while (start<= stop) {
        delete start.currentLink;
        start++;
    }
    temp->currentLink = start.currentLink;
}

Although I am confused regarding the erase function as last 2 element will have same currentLink (as far as I can guess from your implementation).

Hope this helps.