Question

Can anyone explain to me why I am getting an access violation when I try to add notes? It occurs when I try to set the "pLast" pointer to the previous node (for a doubly linked list). The code worked as singly linked list. When I added a "pLast" pointer and the needed code to make it a doubly linked list everything broke. 

#include <iostream>
#include <stdlib.h>

using namespace std;

//build class that has a private function to inc count. 
class LinkedListCount {
private:
    struct CountNode {
        int data;
        int countVar; //Make the variable "countVar" private to protect integrity. 
        CountNode* pNext = NULL;
        CountNode* pLast = NULL; //Needed for bubbling back through list. 

    };
    //Keep track of head
    CountNode* head;
    CountNode* current;
    CountNode* temp;



public:
    //Constructor Function (Set default values for head, current, and temp) 
    LinkedListCount() {
        head = NULL;
        current = NULL;
        temp = NULL;
    }
    void AddNode(int dataIn) { //Addnode Function
        //Create and populate list. 
        CountNode* newNode = new CountNode; 
        newNode->pNext = NULL; 
        newNode->pLast = NULL;
        newNode->data = dataIn;
        temp = head;
        newNode->countVar = 0;
        if (temp != NULL) { //We already have data entery.
            if (temp->pNext == NULL) {
                newNode = temp->pNext;
                newNode->pLast = head; //****THIS IS WHERE ACCESS VIOLATION OCCURES
            }
            //Set variables with the understanding that the head is the only data point. 
            else {
                current = temp->pNext; //Set it equal to head. 
            }
            while (current->pNext != NULL) {//This could be eliminated with keeping track of a tail. 
                current = current->pNext; //Attach this to the end of the list. 
            }
            current->pNext = newNode; //And newMode->pNext = to Null so next time I add data I'll get to the end of the list. 
            newNode->pLast = current;
        }
        else if (head == NULL) {
            head = newNode;
        }

    }
};

void addNodes(LinkedListCount &DataList) { //Populates list. 

    for (int i = 0; i < 20; i++) {
        DataList.AddNode(i);
    }
}



int main(void)
{       
addNodes(DataList);   
}

Answer 1

Solution:

Program modified by solving the access violation issue. The issue is with assigning the node to temp.

Highlighted code contains the modified code.

---------------------------------------------

Source Code:

#include <iostream>
#include <stdlib.h>

using namespace std;

//build class that has a private function to inc count.
class LinkedListCount {
private:
    struct CountNode {
        int data;
        int countVar; //Make the variable "countVar" private to protect integrity.
        CountNode* pNext = NULL;
        CountNode* pLast = NULL; //Needed for bubbling back through list.

    };
    //Keep track of head
    CountNode* head;
    CountNode* current;
    CountNode* temp;

public:
    //Constructor Function (Set default values for head, current, and temp)
    LinkedListCount() {
        head = NULL;
        current = NULL;
        temp = NULL;
    }
    void AddNode(int dataIn) { //Addnode Function
        //Create and populate list.
        CountNode* newNode = new CountNode;
        newNode->pNext = NULL;
        newNode->pLast = NULL;
        newNode->data = dataIn;
        temp = head;
        newNode->countVar = 0;
        if (temp != NULL) { //We already have data entery.
            if (temp->pNext == NULL) {

               temp->pNext = newNode;
               newNode->pNext = temp; //Modified....
               return;
            }
            //Set variables with the understanding that the head is the only data point.
            else {
               current = temp; //Set it equal to head.
            }
            while (current->pNext != head) {//This could be eliminated with keeping track of a tail.
                current = current->pNext; //Attach this to the end of the list.
            }
            current->pNext = newNode; //And newMode->pNext = to Null so next time I add data I'll get to the end of the list.
            newNode->pNext = head;
        }
        else if (head == NULL) {
            head = newNode;
       }

    }
};

void addNodes(LinkedListCount &DataList) { //Populates list.

    for (int i = 0; i < 20; i++) {
       DataList.AddNode(i);
    }
}

int main(void)
{
   LinkedListCount DataList;   
   addNodes(DataList);
}

---------------------------------------------------------------

Output:

------------------------------------------------

Kindly up-vote if you are satisfied with this solution.

Node data: 0 Node data: 1 Node data: 2 Node data: 3 Node data: 4 Node data: 5 Node data: 6 Node data: 7 Node data: 8 Node data: 9 Node data: 10 Node data: 11 Node data: 12 Node data: 13 Node data: 14 Node data: 15 Node data: 16 Node data: 17 Node data: 18 Node data: 19