C++: Passing a linked list to a static function - c++

Question

I have written a set of functions which together build a list of "Hubs" from a .csv file. I wish to generate a linked-list of these hubs in my main function I have a "head" pointer that I can pass to other functions in main.
The code in main:
HubNode *hh = NULL;
HubNode **head;
*head = hh;
Tools::loadHubs(head);
cout << hh->name; /* HubNodes have a string called "name" */
The code in Tools:
void Tools::loadHubs(HubNode **head)
{
string line, name, location;
// headFlights = currFlights = prevFlights = NULL;
ifstream myfile("Hub.csv");
if (myfile.is_open())
{
while (getline(myfile, line)) {// Omit the Caption Line.
while (getline(myfile, name, ','))//Get every value in order.
{
getline(myfile, location, '\n');
// cout << line << "\n";
// cout << name << "\n";
// cout << location << "\n";
HubNode::AddHub(name, location, head);
}
}
myfile.close();
}
else { cout << "\nUnable to open file\n"; }
}
The code in HubNode:
void HubNode::AddHub(string sname, string slocation, HubNode **head)
{
HubNode* newNode = new HubNode;
HubNode *point;
newNode->next = NULL;
newNode->name = sname;
newNode->location = slocation;
if (*head != NULL)
{
HubNode *curr = *head;
while (curr->next != NULL)
{
curr = curr->next;
}
curr->next = newNode;
}
else
{
point = newNode;
*head = point;
}
}
I thought that using a double pointer to the head of the list in this way would work, so that from "hh" in main, I would have access to the whole linked-list.
When I compile and start debugging, I can see that AddHubs is successfully creating the HubNodes in its scope, But when I try to access any of the elements from main (e.g. by cout << hh->name), I get a segmentation fault.
What have I done wrong? (Let me know if I need to post more code...)

Answer 1

You wouldn't do this:
int value = 10;
int *p;
*p = value;
So why would you think this would work:
HubNode *hh = NULL;
HubNode **head;
*head = hh;
The indirection is the same, only the type has changed. And both snippets invoke undefined behavior. This code should do this:
HubNode *hh = NULL;
Tools::loadHubs(&hh);
cout << hh->name;
Further, your add function should be:
void HubNode::AddHub(const string& sname, const string& slocation, HubNode **head)
{
HubNode* newNode = new HubNode;
newNode->next = NULL;
newNode->name = sname;
newNode->location = slocation;
while (*head)
head = &(*head)->next;
*head = newNode;
}
It gets even more straight forward if you provide a proper constructor for HubNode that takes the sname and slocation as construction parameters and initializes the node next member to NULL. if you code that, the add becomes simply:
void HubNode::AddHub(const string& sname, const string& slocation, HubNode **head)
{
while (*head)
head = &(*head)->next;
*head = new HubNode(sname, slocation);
}