code line is killing my program - c++

Question

i made this linked list class in c++ and it works fine except after i run it the program goes unresponsive. i have located the line that's causing the problem but i have no idea why. Even when i type it differently it still does the same thing.
Here's my list class:
#include <string>
template<class T>
class List : public Object{
private:
Node<T>* first;
Node<T>* last;
int length;
public:
List() : Object(new std::string("List")) {
first = NULL;
last = NULL;
length = 0;
}
~List() {
delete first;
delete last;
}
void Add(T value) {
if(first==NULL)
first = new Node<T>(NULL, value);
else if(last==NULL)
---->last = new Node<T>(first, value);<-----
else
last = new Node<T>(last, value);
length++;
}
T Remove(T value) {
Node<T>* temp = first;
while(temp!=NULL) {
if(temp->GetValue()==value) {
temp->GetPrev()->SetNext(temp->GetNext());
temp->GetNext()->SetPrev(temp->GetPrev());
delete temp;
length--;
return value;
}
temp = temp->GetNext();
}
return 0;
}
T Get(int index) {
Node<T>* temp = first;
int i = 0;
while(temp!=NULL) {
if(i==index)
return temp->GetValue();
i++;
temp = temp->GetNext();
}
return 0;
}
};
when i remove the marked line above the program go unresponsive. This is my Node constructor:
#include <string>
template<class T>
class Node : public Object{
private:
Node* next;
Node* prev;
T value;
public:
Node(Node* prev, T value) : Object(new std::string("Node")){
if(prev!=NULL) {
prev->next = this;
this->prev = next;
}
next = NULL;
this->value = value;
}
~Node() {
delete next;
}
T GetValue() {
return value;
}
Node* GetNext() {
return next;
}
Node* GetPrev() {
return next;
}
};
my object class:
#include <string>
class Object {
private:
std::string* type;
public:
Object() {
type = new std::string("Object");
}
Object(std::string* type) {
this->type = type;
}
~Object() {
delete type;
}
std::string* GetType() {
return type;
}
};
my Test.cpp
#include <iostream>
#include <string>
#include "Object.h"
#include "Node.h"
#include "List.h"
using namespace std;
int main () {
List<int> l;
l.Add(5);
l.Add(93);
l.Add(17);
l.Add(7789);
l.Add(60);
cout << "node 4 is:" << l.Get(3) << endl;
return 0;
}
error image http://i50.tinypic.com/2mw5phi.png
thanks for reading and please help as soon as you can, comment if you need me to supply more info.

Answer 1

Edit: There are many problems with your program, but what might be causing your crash: Your Add-function does not work correctly. It should be something like this:
if(first==NULL) {
first = new Node<T>(NULL, value);
last = first;
} else {
last = new Node<T>(last, value);
}
length++;
Otherwise, it will not correctly insert the second element. Why? With your original code, after the first add, your last is still NULL because of the else. So on the second add, you set last to new Node<T>(NULL, value). Therefore, it will not assign the first element's next pointer. And your list will be inconsistent.
Apart from that, there are double-frees, unnecessary heap-allocation of the string field in your Object class, ownership issues etc. To give you just one more example: Your List destructor will cause a heap corruption due to a double free. Calling delete first will delete all nodes due to the delete next in Node's destructor, as long as the list is consistent. Then you call delete last, but that object was already freed. This will corrupt your program's memory management and can also cause a crash at program exit.

Answer 2

Does this function seem correct to you??
It says GetPrev, but its actually getting next.
Node* GetPrev() {
return next;
}

Answer 3

I found that if I comment out this line in the Node constructor the code compiles:
if (next != NULL) {
// next->next = this;
prev = next;
}
Edit 1:
I also realized that you were doing this in your Node class:
private:
Node* next;
Node* prev;
T value;
Since these objects are declared in the Node class, they are at this time incomplete types. I managed to replicate that problem down to a simple one like this:
template <class T>
struct S {
S* s = new S();
~S() { delete s; }
};
int main() {
S<int> s; // Segmentation fault (core dumped) ./test > .stdout
}
This causes a crash because S is an incomplete type within itself.
I'm getting the same segementation fault as I got in your code. I'm pretty sure it's because the pointers in the Node class are built upon incomplete types; and accessing the data from them is looking into memory that isn't yours, hence the crash.