Program has encountered a problem and needs to close

I'm trying to get this linked list project to work, but when I try to run it I get this error:

Lab6 has encountered a problem and needs to close

Lab6 being my project.

Code:

------------------node.h------------------

struct Node
{
	int value;
	Node *next;
};

class linkedList
{
public:
	linkedList() {first=0; length=0;}
	bool operator > (int a); //insert an integer at the end of the linked list using the operator new
	void displayList(); //display linked list
	bool Insert(int i, int a); // insert an integer by creating a new node in the location designated by i

	~linkedList();
private:
	Node *first;
	int length;
};

bool linkedList::operator > (int a)
{
	Node *newn, *current;
	newn = new Node;
	newn->value = a;
	newn->next = 0;
	current = first;
	while (current->next != 0)
		current = current->next;
	current->next = newn;
	length++;
	
	return true;
}

void linkedList::displayList()
{
	Node *current;
	current = first;
	for (int i=0; i<length; i++)
	{
		current = current->next;
		std::cout << current->value;
		if ((i+1) < length)
			std::cout << ", ";
	}
}

bool linkedList::Insert(int i, int a)
{
	if ((i > (length+1)) || (i < 1))
		return false;
	Node *newn, *current;
	newn = new Node;
	newn->value = a;
	current = first;
	for (int j=1; j<i; j++)
		current = current->next;
	newn->next = current->next;
	current->next = newn;
	length++;
	
	return true;
}

linkedList::~linkedList()
{
	Node *current = first;
	while (current->next != 0)
	{
		Node *next = current->next;
		delete current;
		current = next;
	}
delete current;
}



------------------usingnode.cpp------------------

#include <iostream>
#include "node.h"

using namespace std;

int main()
{
	linkedList L_int;
	L_int.Insert(1,34);
	L_int.Insert(1,23);
	L_int.Insert(3,12);
	L_int.Insert(2,50);
	L_int.Insert(1,54);
	L_int.Insert(7,11);

	L_int.displayList();

	return 0;
}

I don't get any errors while compiling, so I'm unsure what to do.

Its a run-time error. What you can do is run it with a debugger. Or put lines like

Code:

std::cout << "OK Here" << std::endl;

and see at what point it crashes. For example

Code:

	L_int.Insert(1,54);
        std::cout << "OK Here" << std::endl;
	L_int.Insert(7,11);
        std::cout << "OK Here" << std::endl;
	L_int.displayList();
        std::cout << "OK Here" << std::endl;

If you get one "OK Here", then you know that it crashes at L_int.Insert(7,11). If you get two, it crashes on L_int.displayList() and so on.
If you cannot spot the problem yourself post again specifying where it crashes

It crashes at this insert function.

Code:

bool linkedList::Insert(int i, int a)
{
	if ((i > (length+1)) || (i < 1))
		return false;
	Node *newn, *current;
	newn = new Node;
	newn->value = a;
	current = first;
	for (int j=1; j<i; j++)
		current = current->next;
	newn->next = current->next;	                //crashes here
	current->next = newn;
	length++;
	return true;
}

The first insert into the list, current is NULL. The line after the for-loop then tries to dereference a NULL pointer.

Well now is the time to read all about the debugger which comes with your IDE.

Rather than "There's a problem, sorry, gotta close"
You get "Whoops, there's a problem and it has something to do with this line of code".

Not only that, you can print any variable you please, and set breakpoints to stop the code at points you suspect the trouble is starting to happen.

It's an essential and indispensable tool - learn how to use it.