Connecting link lists

[PcCopat]

Hello, I want to make a linked list program consisting of one main link, and each main links have sub link.

Main 1 -> Main 2 -> Main 3 -> ... -> Main N -> NULL
   |                |              |                        |
Sub 1-1     Sub 2-1     ...                      ...
   |                |
Sub 1-2     Sub 2-2
   |               |
  ...             ...

I have two structs for these:

struct NodeSmall
{
	int documentNum;
	NodeSmall *nextNum;
	NodeSmall()
	{
		documentNum = 0;
		nextNum = NULL;
	}
};

struct NodeBig
{
	string keyword;
	NodeBig *nxtkyw; //Link to next keyword
	NodeSmall *docNum; //Link to document number

	NodeBig()
	{
		keyword = "";
		nxtkyw = NULL;
	}
	
};

And I have a test main which reads first line from text file:

int main()
{

	string line, key;
	int number;
	ifstream dbfile;
	dbfile.open("docdb.txt");

	dbfile >> key >> number;

	NodeBig *head = new NodeBig;
	NodeSmall *docNum = new NodeSmall;
	
	head->keyword = key;
	head->nxtkyw = NULL;
	head->docNum->documentNum = number;

	cout << "keyword is " << head->keyword << " and it is on " << head->docNum->documentNum;
}

It does not give compiler error (vs2008) but it gives stopped working error after running the program. I think I've made a mistake while connecting link lists together. But I cannot find similar probles on internet. What should I do for fixing this error? Thank you.

[kbshibukumar]

Instead of

NodeSmall *docNum = new NodeSmall;

you have to use

head->docNum = new NodeSmall;

[PcCopat]

Thank you! It worked. Can you explain the reason?

[kbshibukumar]

When you say NodeSmall *docNum = new NodeSmall; a new NodeSmall is created in memory and head->docNum will not point to the same. Hence, head->docNum will be NULL and as you tried to access an element of NULL, the program crashed.