C++ Read Access Violation in Singly Linked List - c++

Question

I'm writing a program about creating a list of country, there seems to no error in my code but when debugging, I get the error: Unhandled exception thrown: Read access violation p was 0xFFFFFFFFFFFFFFFF, after I input some values.
Can anyone give me a hint or find me the mistake?
#include <cstring>
#include <string>
using namespace std;
this is my struct Provice.
struct Province
{
int Code;
string Name;
int Pop;
float Area;
};
struct node
{
struct Province data;
node* next;
};
struct List
{
node* head;
node* tail;
};
void Init(List &l)
{
l.head = NULL;
l.tail = NULL;
}
void add_tail(List& l, node* p)
{
if (l.head == NULL)
{
l.head = p;
l.tail = p;
}
else
{
l.tail->next = p;
l.tail = p;
}
}
I think I got some problem when I create a node here without initializing value, is this right ?
void inputListProvinces(List& l)
{
int n;
cin >> n;
int i = 0;
while(i<n)
{
node* p = new node;
cin >> p->data.Code;
cin.ignore();
getline(cin, p->data.Name);
cin.ignore();
cin >> p->data.Pop;
cin >> p->data.Area;
add_tail(l, p);
i++;
}
}
And the error happens here, but I dont know how to fix.
void outputListProvinces(List& l)
{
node* p = l.head;
while (p != NULL)
{
cout << p->data.Code << '\t'; /*Unhandled exception thrown: read access violation.
p was 0xFFFFFFFFFFFFFFFF*/
cout << p->data.Name << '\t';
cout << p->data.Pop << '\t';
cout << p->data.Area << '\t';
cout << endl;
p = p->next;
}
}
void outputProvince(node* p)
{
cout << p->data.Code << '\t';
cout << p->data.Name << '\t';
cout << p->data.Pop << '\t';
cout << p->data.Area << '\t';
}
void outputProvincesMore1MillionPop(List& l)
{
node* p = l.head;
while (p != NULL)
{
if (p->data.Pop > 1000)
{
outputProvince(p);
cout << endl;
}
p = p->next;
}
}
node* findProMaxArea(List& l)
{
node* n = l.head;
node* p = l.head;
while (p != NULL)
{
if (p->data.Area > n->data.Area)
{
n = p;
}
p = p->next;
}
return n;
}
int main()
{
List L;
Init(L);
inputListProvinces(L);
cout << "List of provinces:" << endl;
cout << "ID\t|Province\t|Population\t|Area" << endl;
outputListProvinces(L);
cout << "Provinces with a population of more than 1 million:" << endl;
outputProvincesMore1MillionPop(L);
cout << "The largest province:" << endl;
node* p = findProMaxArea(L);
if (p) outputProvince(p);
return 0;
}

Answer 1

Mistake is node never initializes its next pointer. You can only count on it being NULL if you set it to NULL, and the last node in the list MUST be NULLor the program can't find the end of the List and marches off into the wacky world of Undefined Behaviour.
Safest Fix: Add a constructor to node to make sure next is always initialized.
struct node
{
struct Province data;
node* next;
node(node* n = NULL): next(n)
{
}
};
There are other fixes, like making sure l.tail->next = NULL; at the end of inputListProvinces, but I don't think it's really worth the reduced overhead given the slow-ness of the console IO.
And if you do that, then you should also roll Init into List as a constructor:
struct List
{
node* head;
node* tail;
List(): head(NULL), tail(NULL)
{
}
};
This should leave you with the problem of a poorly-placed cin.ignore() consuming a character you don't want consumed.
Side note: Replace NULL with nullptr if available to your compiler and target C++ Standard revision. nullptr removes the bugs that can result from NULL being a glorified 0.