C++ doesn't save the changes by other method - c++

Question

I'm trying to implement a linked list class in C++ and I got problem. I have the += operator who adds new node.
the linked list class interface:
template <typename Type>
class LinkedList {
public:
LinkedList<Type>* head;
// linked list stracture
Type data;
LinkedList<Type>* next;
// others ....
size_t length;
public:
LinkedList();
~LinkedList();
void initializeHead(LinkedList<Type>* headPtr);
size_t size() const;
LinkedList& operator+=(const Type& add);
void operator-=(const Type& remove);
LinkedList<Type>& operator[] (const size_t index) const;
bool operator== (const LinkedList<Type> &versus) const;
friend ostream& operator<< (ostream& out,LinkedList& obj);
};
and here i have the += overload implement:
template <typename Type> LinkedList<Type>& LinkedList<Type>::operator +=(const Type& add) {
// head ptr - :)
LinkedList<Type>* p = head->next;
// go to the end
while(p) p = p->next;
// now on end - create new..!!!
try {
p = new LinkedList<Type>;
} catch (bad_alloc& e) {
cout << "There\'s an allocation error....";
} catch (...) {
cout << "An unknown error.." << endl;
}// fill and done
p->data = add;
p->next = NULL;
// increment length .........
++head->length;
// done ............
return *p;
}
Additionally , I have "array" access overload method:
template <typename Type> LinkedList<Type>& LinkedList<Type>::operator [](const size_t index) const {
if(index < 0 || index >= length) // invaild argument
throw exception();
// continue
LinkedList<Type>* p = head;
for(size_t i = 0; i < index; ++i) p = p->next; // we are at what we want
return *p;
}
All works correctly - I checked on the dibugger,
the problem is - += doesn't save the new node in "head->next", for some reason, after finish += method, head->next equal to null.
Do someone know why the new allocation don't link to head->next?
Thanks a lot!!

Answer 1

after while(p) p = p->next; p is NULL
and next you do p = new LinkedList<Type>; but you don't link the p into the head.

Answer 2

Instead of:
// go to the end
while(p) p = p->next;
You need:
head->next = p;

Answer 3

As the other answers say, you go beyond the list when you try to add. Try something like this:
template <typename Type> LinkedList<Type>& LinkedList<Type>::operator +=(const Type& add)
{
LinkedList<Type> *last;
// Find the last node in the list
for (last = head; last != 0 && last->next != 0; last = last->next)
{
}
// `last` now points to the last node in the list, or is zero
// If zero (i.e. NULL) then list is empty
if (last == 0)
{
head = new LinkedList<Type>;
head->next = 0;
head->data = add;
head->length = 0;
}
else
{
last->next = new LinkedList<Type>;
last->next->next = 0;
last->next->data = add;
}
// We can safely use `head` as we are sure it won't be zero
head->length++;
// Return the added node
return (last != 0 ? *last->next : *head);
}

Answer 4

You can also use temporary variable to store last node and then the last node will point to new node.
This is sample code. You need to take care of some situations like adding first node etc.
LinkedList<Type>* temp = NULL;
while(p)
{
temp = p;
p = p->next;
}
try
{
p = new LinkedList<Type>;
temp->next = p;
}