I am getting the error "invalid null pointer" - c++

Question

I want to read in student names from a file and insert them into my linked-list, but I am having this problem with an error box. The error reads "Expression: Invalid Null Pointer."
I've googled with no such luck. I think I have an idea where I've went wrong, but I don't know how to fix it.
If you could help, that would be great!
Here is my code:
P.S I'm not nearly done so my code might be incomplete, I'm just trying to weed out all my errors now so I don't have triple my errors at the end.
LList.h
#include <iostream>
#include <iomanip>
#include <string>
#ifndef LLIST_H
#define LLIST_H
typedef int ElementType;
class LList
{
public:
LList();
~LList();
void insert(std::string new_data);
void display();
void remove(std::string delete_data);
private:
class Node
{
public:
std::string data;
Node *next;
Node(std::string data_value = NULL);
};
Node *head;
int mySize;
};
#endif LLIST_H
LList.cpp
#include <iomanip>
#include <iostream>
#include <string>
#include "LList.h"
using namespace std;
LList::Node::Node (string data_value)
{
this -> data = data_value;
this -> next = NULL;
}
LList::LList()
{
this -> head = new Node(0);
mySize = 0;
string data = "";
}
LList::~LList()
{
delete this -> head;
}
void LList::insert(string new_data)
{
Node *tempHolder;
tempHolder = this->head;
while (tempHolder->next != NULL)
tempHolder = tempHolder -> next;
Node *newNode = new Node(new_data);
tempHolder ->next = newNode;
this->mySize++;
}
void LList::display()
{
Node *temp;
temp = head->next;
while(temp -> next != NULL)
{
cout << temp -> data << endl;
temp = temp -> next ;
}
}
void LList::remove(string delete_data)
{
Node *tempHolder;
tempHolder = head;
while (tempHolder->next != NULL )
{
if (tempHolder->next->data == delete_data)
{
Node *delete_ptr = tempHolder->next;
tempHolder->next = tempHolder->next->next;
delete delete_ptr;
mySize-- ;
break;
} else
tempHolder = tempHolder->next;
}
}
Main.cpp
#include <iostream>
#include <iomanip>
#include <string>
#include "LList.h"
#include <fstream>
using namespace std;
int main()
{
LList student;
ifstream infile;
char readLine[500];
infile.open ("names.txt");
if(infile.is_open())
{
while (!infile.eof())
{
infile.getline(readLine,sizeof(readLine)); // read a line from file
student.insert(readLine);
}
}
else
{
cout << "Can't open file!" << endl;
}
}

Answer 1

I found my problem.
In:
LList::LList()
{
this -> head = new Node(0);
mySize = 0;
string data = "";
}
Node(0);
is calling my
LList::Node::Node (string data_value)
{
this -> data = data_value;
this -> next = NULL;
}
which is initialized as a string.
I changed
Node(0);
to
Node("");
and it worked flawlessly.

Answer 2

I wonder could you give the reference where you read that you may to write?
Node(std::string data_value = NULL);
Class std::string has no constructor that converts NULL to an object of type std::string.
it would be much better to declare the constructor without a default argument
Node( std::string data_value );
There is no any sense to create a node without data.
In fact there is no any need to declare a constructor of Node. It could be used as an aggregate.
Also change the constructor of LList as
LList::LList() : head( 0 ), mySize( 0 ){}
Also the destructor is invalied
LList::~LList()
{
delete this -> head;
}
You have to delete not only head but all nodes in the LList.
Also nodes in a single linked list are inserted in the beginning of the list that is before the head.
I would write method insert the following way provided that the constructor of Node is removed bacause it is not needed.
void LList::insert( const std::string &new_data )
{
head = new Node { new_data, head };
}
If your compiler does not support the initializer list then you indeed need to define constructor in class Node.
Node( const std::string &data_value, next_node = NULL );
In this case method insert will look as
void LList::insert( const std::string &new_data )
{
head = new Node( new_data, head );
}