Unhandled exception thrown: read access violation. "this" was 0x8 - Using Shared_ptr - c++

Question

Node.h
#pragma once
#include <iostream>
#include<memory>
class Node
{
public:
Node();
Node(int k, int d);
int key;
int data;
std::shared_ptr<Node> next;
std::shared_ptr<Node> previous;
//Node* next;
//Node* previous;
};
doubleLinkedList.h
#pragma once
/*! \class Double Linked List
\brief A double linked list data structure
*/
#include <iostream>
#include "../Node.h"
#include <string>
#include<memory>
class DoubleLinkedList : public Node
{
private :
std::shared_ptr<Node> head; //Node* head;
std::shared_ptr<Node> temp; //Node* temp;
std::shared_ptr<Node> mypointer; //Node* ptr;
std::shared_ptr<Node> nextNode;
std::shared_ptr<Node> prevNode;
public :
DoubleLinkedList();
DoubleLinkedList(std::shared_ptr<Node> n);
std::shared_ptr<Node> checkNodeExsits(int k); //Node*
void addNodeToFront(std::shared_ptr<Node> n); //Node*
void addNodeToEnd(std::shared_ptr<Node> n); //Node*
void addNodeAfter(int k, std::shared_ptr<Node> n); //Node*
void UpdateNode(int k , int d);
void deleteNode(int k);
void printList();
void printInfo(std::string Info);
};
Node.cpp
#include "Node.h"
#include <iostream>
Node::Node()
{
key = 0;
data = 0;
next = nullptr;
previous = nullptr;
}
Node::Node(int k, int d)
{
key = k;
data = d;
}
doubleLinkedList.cpp
#include <iostream>
#include "include\doubleLinkedList.h"
DoubleLinkedList::DoubleLinkedList()
{
head = nullptr;
}
DoubleLinkedList::DoubleLinkedList(std::shared_ptr<Node> n)
{
head = n;
}
std::shared_ptr<Node> DoubleLinkedList::checkNodeExsits(int k)
{
temp = nullptr;
mypointer = head;
while (mypointer != nullptr) {
if (mypointer -> key == k) {
temp = mypointer;
}
mypointer = mypointer-> next;
}
return temp;
}
void DoubleLinkedList::addNodeToFront(std::shared_ptr<Node> n)
{
if (checkNodeExsits(n->key) != nullptr)
{
printInfo("Node Already exist with key Number ");
}
else {
if (head == nullptr) {
head = n;
printInfo("Node Added as Head Node");
}
else {
head->previous = n;
n->next = head;
head = n;
printInfo("Node Added To The Begining");
}
}
}
void DoubleLinkedList::addNodeToEnd(std::shared_ptr<Node> n)
{
if (checkNodeExsits(n->key) != nullptr)
{
printInfo("Node Already exist with key Number");
}
else {
if (head == nullptr)
{
head = n; // if there isnt any node in the list.
printInfo("Node Has Been Added As Head Node");
}
else {
mypointer = head;
while (mypointer ->next != nullptr)
{
mypointer = mypointer->next;
}
mypointer->next = n;
n->previous = mypointer;
printInfo("Node Has Been Added To The End");
}
}
}
void DoubleLinkedList::addNodeAfter(int k, std::shared_ptr<Node> n)
{
mypointer = checkNodeExsits(k);
if (mypointer == nullptr) {
printInfo("No Node Exists With The Key Value");
}
else {
if (checkNodeExsits(n -> key) != nullptr) {
printInfo("Node Already exist with key Number");
}
else {
nextNode = mypointer-> next;
// inserting at the end
if (nextNode == nullptr) {
mypointer-> next = n;
n -> previous = mypointer;
printInfo("Node Inserted at the END");
}
//inserting in between
else {
n -> next = nextNode;
nextNode -> previous = n;
n -> previous = mypointer;
mypointer-> next = n;
printInfo("Node Inserted in Between");
}
}
}
}
void DoubleLinkedList::UpdateNode(int k, int d)
{
mypointer = checkNodeExsits(k);
if (mypointer != nullptr) {
mypointer-> data = d;
std::cout << "Node Data Updated Successfully" << std::endl;
}
else {
std::cout << "Node Doesn't exist with key value : " << k << std::endl;
}
}
void DoubleLinkedList::deleteNode(int k)
{
mypointer = checkNodeExsits(k);
if (mypointer == nullptr) {
std::cout << "No node exists with key value: " << k << std::endl;
}
else {
if (head -> key == k) {
head = head -> next;
std::cout << "Node UNLINKED with keys value : " << k << std::endl;
}
else {
nextNode = mypointer-> next;
prevNode = mypointer-> previous;
// deleting at the end
if (nextNode == nullptr) {
prevNode -> next = nullptr;
std::cout << "Node Deleted at the END" << std::endl;
}
//deleting in between
else {
prevNode -> next = nextNode;
nextNode -> previous = prevNode;
std::cout << "Node Deleted in Between" << std::endl;
}
}
}
}
void DoubleLinkedList::printList()
{
if (head == nullptr) {
std::cout << "No Nodes in Doubly Linked List";
}
else {
std::cout << std::endl << "Doubly Linked List Values : ";
temp = head;
while (temp != nullptr) {
std::cout << "[Key: " << temp->key << ", Data: " << temp->data << "] <___> " << std::endl;
temp = temp -> next;
}
}
}
void DoubleLinkedList::printInfo(std::string Info)
{
std::cout << Info << std::endl;
}
main.cpp
#include <iostream>
#include "../include/doubleLinkedList.h"
#include"../Node.h"
void Print(std::string info)
{
std::cout << info << std::endl;
}
int main() {
DoubleLinkedList myNode;
//Node* newNode = new Node(2,7);
std::shared_ptr<Node> newNode = std::make_shared<Node>(2, 7); // enter key number and data number
std::shared_ptr<Node> newNode1 = std::make_shared<Node>(3, 9);// enter key number and data number
newNode->key;
newNode->data;
myNode.addNodeToFront(newNode);
newNode->key;
newNode->data;
myNode.addNodeAfter(2, newNode1); // enter the key number of existing node and then to add new key number and new data
myNode.printList();
system("pause");
return 0;
}
The error I'm getting is:
unhandled exception thrown: read access violation. "this" was 0x8
To elaborate -
( Access violation reading location 0x0000000000000010. Unhandled exception thrown: read access violation).
So, the code would work when I use raw pointers. The only thing I can deduce from this is that there's a chain of shared pointers in the method of void DoubleLinkedList::addNodeAfter() nextNode = mypointer-> next;
or that shared pointers can't have nullptr assigned to it.
I'm clueless as to why this is happening.

Answer 1

Let's take a walk through addNodeAfter
mypointer = checkNodeExsits(k);
We've checked to make sure value k exists in the list and gotten a pointer to it if it does. If it doesn't, we get a null pointer.
if (mypointer == nullptr) {
Tests whether or not k was found. Let's assume it was, mypointer isn't null, and we enter the else
else {
if (checkNodeExsits(n -> key) != nullptr) {
Here we check to make sure the node we're inserting isn't a duplicate. Again let's take the else branch
else {
nextNode = mypointer-> next;
Should be safe, right? We know that mypointer was not null because we tested it earlier. But when we look... Holy smurf! The program crashed! It was null. How did it become null?
The answer lies in another question: Where did mypointer come from? It's not defined within this function, so it has wider scope. Turns out it is a DoubleLinkedList member variable. Is someone else messing with it? We don't have multiple threads, so it must be another function that was called by addNodeAfter.
That would have to be checkNodeExsits, so let's take a look at it.
std::shared_ptr<Node> DoubleLinkedList::checkNodeExsits(int k)
{
temp = nullptr;
mypointer = head; // well lookie here!
while (mypointer != nullptr) {
if (mypointer -> key == k) {
temp = mypointer;
}
mypointer = mypointer-> next; // and here!
}
return temp;
}
We can see that if the inserted node's value does not exist, thus putting us in the else case back in addNodeAfter where we're going to insert the new node, mypointer can only be null!
The overly broad scope of mypointer turns the member variable into a boobytrap. It needs to be a variable local to checkNodeExsits and addNodeAfter to prevent these functions from trashing the state of the functions that use them. This leads to questioning whether mypointer should be a member variable anywhere it is found. That leads to wondering about temp, nextnode and prevnode. They all sound like temporary holders of local state information. Likely the only DoubleLinkedList member that should be a member is head.
To fix: Remove the definition of mypointer from the class and resolve the compiler errors that result. If a use of mypointer can be easily replaced with a local variable, do so. For any that remain and are needing of longer-term storage for resuse, you'll have to get a bit more creative. Make the call on how long-lived and how wide a scope is necessary to get the behaviour you want and then give this new variable an appropriately descriptive name to help you remember the circumstances in which it should be used.
I recommend repeating this process for the other member variables except for head where the necessary scope and lifetime is obvious.
General rule of thumb: Keep the scope of variables as tight as possible. Only widen the scope of a variable if you absolutely need to reuse its state later, and if you find you must be very careful with it to ensure you don't cause unintended side-effects.