0

My program compiles with no errors but when I run the program it asks "Enter the number to be deleted:" if I enter any number other than the first number listed the program crashes with the following error:

"An unhandled exception of type 'System.NullReferenceException' occurred in doublyLinkedList.exe

Additional information: Object reference not set to an instance of an object."

```
#ifndef H_doublyLinkedList
#define H_doublyLinkedList
#include <iostream>
#include <cassert>
using namespace std;
template <class Type>
struct nodeType
{
Type info;
nodeType<Type> *next;
nodeType<Type> *back;
};
template <class Type>
class doublyLinkedList
{
friend ostream& operator<<(ostream&,
const doublyLinkedList<Type>&);
public:
const doublyLinkedList<Type>& operator=
(const doublyLinkedList<Type> &);
void initializeList();
bool isEmptyList();
void destroy();
void printList() const;
void reversePrint();
int length();
Type front();
Type back();
bool search(const Type& searchItem);
void insertNode(const Type& insertItem);
void deleteNode(const Type& deleteItem);
doublyLinkedList();
doublyLinkedList(const doublyLinkedList<Type>& otherList);
~doublyLinkedList();
protected:
int count;
nodeType<Type> *first;
nodeType<Type> *last;
private:
void copyList(const doublyLinkedList<Type>& otherList);
};
template<class Type>
doublyLinkedList<Type>::doublyLinkedList()
{
first= NULL;
last = NULL;
count = 0;
}
template<class Type>
bool doublyLinkedList<Type>::isEmptyList()
{
return(first == NULL);
}
template<class Type>
void doublyLinkedList<Type>::destroy()
{
nodeType<Type> *temp;
while(first != NULL)
{
temp = first;
first = first->next;
delete temp;
}
last = NULL;
count = 0;
}
template<class Type>
void doublyLinkedList<Type>::initializeList()
{
destroy();
}
template<class Type>
int doublyLinkedList<Type>::length()
{
return count;
}
template<class Type>
ostream& operator<<(ostream& osObject,
const doublyLinkedList<Type>& list)
{
nodeType<Type> *current;
current = list.first;
while(current != NULL)
{
cout<<current->info<<" ";
current = current->next;
}
return osObject;
}
template<class Type>
void doublyLinkedList<Type>::printList() const
{
nodeType<Type> *current;
current = first;
while (current != NULL)
{
cout << current->info << " ";
current = current->next;
}
}
template<class Type>
void doublyLinkedList<Type>::reversePrint()
{
nodeType<Type> *current;
current = last;
while(current != NULL)
{
cout<<current->info<<" ";
current = current->back;
}
}
template<class Type>
bool doublyLinkedList<Type>::search(const Type& searchItem)
{
bool found;
nodeType<Type> *current;
found = false;
current = first;
while(current != NULL && !found)
if(current->info >= searchItem)
found = true;
else
current = current->next;
if(found)
found = (current->info == searchItem);
return found;
}
template<class Type>
Type doublyLinkedList<Type>::front()
{
assert(first != NULL);
return first->info;
}
template<class Type>
Type doublyLinkedList<Type>::back()
{
assert(last != NULL);
return last->info;
}
template<class Type>
void doublyLinkedList<Type>::insertNode(const Type& insertItem)
{
nodeType<Type> *current;
nodeType<Type> *trailCurrent;
nodeType<Type> *newNode;
bool found;
newNode = new nodeType<Type>;
assert(newNode != NULL);
newNode->info = insertItem;
newNode->next = NULL;
newNode->back = NULL;
if(first == NULL)
{
first = newNode;
last = newNode;
count++;
}
else
{
found = false;
current = first;
while(current != NULL && !found)
if(current->info >= insertItem)
found = true;
else
{
trailCurrent = current;
current = current->next;
}
if(current == first)
{
first->back = newNode;
newNode->next = first;
first = newNode;
count++;
}
else
{
if(current != NULL)
{
trailCurrent->next = newNode;
newNode->back = trailCurrent;
newNode->next = current;
current->back = newNode;
}
else
{
trailCurrent->next = newNode;
newNode->back = trailCurrent;
last = newNode;
}
count++;
}
}
}
template<class Type>
void doublyLinkedList<Type>::deleteNode(const Type& deleteItem)
{
nodeType<Type> *current;
nodeType<Type> *trailCurrent;
bool found;
if(first == NULL)
cerr<<"Cannot delete from an empty list"<<endl;
else
if(first->info == deleteItem)
{
current = first;
first = first->next;
if(first != NULL)
first->back = NULL;
else
last = NULL;
count--;
delete current;
}
else
{
found = false;
current = first;
while(current != NULL && !found)
if(current->info >= deleteItem)
found = true;
else
current = current->next;
if(current == NULL)
cout<<"The item to be deleted is not in the list"
<<endl;
else
if(current->info == deleteItem)
{
trailCurrent = current->back;
trailCurrent->next = current->next;
if(current->next != NULL)
current->next->back = trailCurrent;
if(current == last)
last = trailCurrent;
count--;
delete current;
}
else
cout<<"The item to be deleted is not in list."
<<endl;
}
}
template<class Type>
void doublyLinkedList<Type>::copyList(const doublyLinkedList<Type>& otherList)
{
nodeType<Type> *newNode;
nodeType<Type> *current;
if(first != NULL)
destroy();
if(otherList.first == NULL)
{
first = NULL;
last = NULL;
count = 0;
}
else
{
current = otherList.first;
count = otherList.count;
first = new nodeType<Type>;
assert(first != NULL);
first->info = current->info;
first->next = NULL;
last = first;
current = current->next;
while(current != NULL)
{
newNode = new nodeType<Type>;
assert(newNode!= NULL);
newNode->info = current->info;
newNode->next = NULL;
last->next = newNode;
last = newNode;
current = current->next;
}
}
}
template<class Type>
doublyLinkedList<Type>::doublyLinkedList(const doublyLinkedList<Type>& otherlist)
{
first = NULL;
copyList(otherList);
}
template<class Type>
const doublyLinkedList<Type>& doublyLinkedList<Type>::operator=
(const doublyLinkedList<Type> & otherList)
{
if(this != &otherList)
copyList(otherList);
return *this;
}
template<class Type>
doublyLinkedList<Type>::~doublyLinkedList()
{
destroy();
}
#endif
#include <iostream>
#include "doublyLinkedList.h"
using namespace std;
int main()
{
doublyLinkedList<int> list1, list2;
int num;
cout << "Enter numbers ending with -999" << endl;
cin >> num;
while(num != -999)
{
list1.insertNode(num);
cin >> num;
}
cout << endl;
cout << "List 1: ";
list1.printList();
cout << endl;
cout << "Length List 1: " << list1.length() << endl;
list2 = list1;
cout << "List 2: ";
list2.printList();
cout << endl;
cout << "Length List 2: " << list2.length() << endl;
cout << "Enter the number to be deleted: ";
cin >> num;
list2.deleteNode(num);
cout << endl;
cout << "After deleting the node, List 2: ";
list2.printList();
cout << endl;
cout << "Length List 2: " << list2.length() << endl;
return 0;
}
```

any ideas as to what is causing this error?