Double Linked List, insert before current

[alex067]

I need to insert a node before the current pointer, however when I try to insert a node, it will produce an unhandled exception error @ line 117

#include <iostream>

using namespace std;
typedef int ItemType;
class NodeType
{
private:
	ItemType item;
	NodeType *next;
	NodeType *prev;
	friend class Double;
};
class Double
{
private:
	NodeType *head;
	NodeType *current;
	int position;
public:
	Double();
	void makeEmpty();
	bool isempty() const;
	bool isAtEnd(); 
	void goToStart();
	void goToNext();
	ItemType CurrentItem();
	void insert(ItemType);
	void deleteCurrentItem();
	int currentPosition();
};
Double::Double()
{
	head = NULL;
	current = NULL;
	position = 1;
}
void Double::makeEmpty()
{
	if (isempty())
	{
		cout << "List is empty!" << endl;
	}
	else
	{
		do
		{
			NodeType *garbage = current;
			current = current->next;
			current->prev = NULL;
			garbage->next = NULL;
			delete garbage;
			head = current;
			position--;
		} while (!isempty());
		//when list is empty
		position = 1;
	}
	current = NULL;
}
bool Double::isempty() const
{
	return (head == NULL); 
}
bool Double::isAtEnd()
{
	return (isempty() || current->next == NULL); //true if empty, or if next pointer is null
}
void Double::goToStart()
{
	if (isempty())
	{
		cout << "List is empty!" << endl;
	}
	else
	{
		current = head; //move to the start
		position = 1;
	}
}
void Double::goToNext()
{
	while (current)
	{
		if (!isAtEnd() && !isempty())
		{
			current = current->next;
			position++;
		}
		else if (isAtEnd())
		{
			return;
		}
	}
}
ItemType Double::CurrentItem()
{
	if (!isAtEnd() && !isempty())
	{
		return current->item;
	}
	
}
void Double::insert(ItemType newitem)
{

	NodeType *ptr = new NodeType;
	ptr->item = newitem;
	ptr->prev = NULL;
	ptr->next = NULL;
	if (isempty())
	{	
		head = ptr;
		goToStart();
	}
	else
	{
		(current->prev)->next = ptr;
		ptr->prev = current->prev;
		ptr->next = current;
		current->prev = ptr;
		position++;
	}
	ptr = NULL;
}
void Double::deleteCurrentItem()
{
	if (!isempty()) //if not empty and not at the end
	{
		NodeType *garbage = current;
		goToNext(); //moves current to next
		current->prev = garbage->prev;
		(garbage->prev)->next = current;
		garbage->prev = NULL;
		garbage->next = NULL;
		garbage = NULL;
		delete garbage;
		position--;
	}
	else
	{
		cout << "Empty list, nothing to delete!" << endl;
	}
}
int Double::currentPosition()
{
	return position;
}

void print(Double test);


int main()
{

	Double one;
	one.goToStart();
	cout << one.currentPosition() << endl; //to check if pointer works
	one.insert(5);
	one.insert(6);
	one.insert(10);
	one.insert(50);
	print(one);

	system("PAUSE");
	return 0;

}

void print(Double test)
{
	cout << "Current items in the list:" << endl;
	while (!test.isempty() || !test.isAtEnd())
	{
		test.goToStart();
		cout << test.CurrentItem() << endl;
		test.goToNext();
	}
}

[coder777]

current is not a good idea, because you don't know what it is.

You can find a reason for a crash on line 117. What happens if current->prev == NULL?

That happens when exactly one node exists and you try to add another.