引发异常：写访问冲突。 newNode为nullptr

Question

I'm just getting back into coding. It's been a few years. I can not figure out why this is throwing a nullptr access violation. I've condensed it into a single line of code. The violation is thrown when I try to set my second entry (newNode's pLast pointer) to head.

I'm trying to create a doubly linked list with a bubble sort based off of searches performed (aka countVar). Any help would be great.

#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

            if (temp->pNext == NULL) { // temp->pNext is NULL
            newNode = temp->pNext; // newNode is now NULL too
            newNode->pLast = head; // attempt to use pLast of NULL
        }

I've put comments in your code to see why you have the access violation.