Expection Unhandled Error when implementing a LinkedList in c++ - c++

Question

Alright so I am attempting to implement a LinkedList data structure but when I try to loop through my list (printNodes and insert functions) I run into an error that says: "Unhandled exception thrown: read access violation. tmpNode was 0xCDCDCDCD." I feel like it has something to do with my pointers not behaving in the manner I think they should but I am not sure. Some assistance would be very much appreciated.
#include<iostream>;
using namespace std;
struct Node {
int data;
Node* next;
Node(int el) {data = el; } //constructor
Node(int el, Node* ptr) { data = el; next = ptr; } //constructor
};
class LinkedList {
public:
Node* head = NULL, * tail = NULL;
void addToHead(int el) {
head = new Node(el, head);
}
void insert(int el) {
Node* newNode = new Node(el);
if (head == nullptr) {
head = newNode;
}
else {
Node* tmpNode = head;
while (tmpNode->next != nullptr) {
tmpNode = tmpNode->next;
}tmpNode->next = newNode;
}
}
void printNodes() {
Node* tmpNode = head;
cout << tmpNode->data;
while (tmpNode->next != nullptr) {
std::cout << tmpNode->data;
tmpNode = tmpNode->next;
}
}
};
int main() {
LinkedList myList = LinkedList();
myList.insert(10);
myList.addToHead(20);
myList.insert(10);
myList.printNodes();
}

Answer 1

Your iteration is correct, but there is a problem with your printNodes function. It dereference tmpNode without checking for null:
void printNodes() {
Node* tmpNode = head;
cout << tmpNode->data; // <-- here
while (tmpNode->next != nullptr) {
std::cout << tmpNode->data;
tmpNode = tmpNode->next;
}
}
I would change it to the following:
void printNodes() {
Node* tmpNode = head;
while (tmpNode != nullptr) {
std::cout << tmpNode->data << ", ";
tmpNode = tmpNode->next;
}
}
Apart from that, as said in comments, if you set next member to null in Node constructor it should work fine.
To search, it is the same thing but checking for the data:
Node* findNode(int el) {
Node* tmpNode = head;
Node* ret = nullptr;
while (tmpNode != nullptr) {
if (tmpNode->data == el) {
ret = tmpNode;
break;
}
tmpNode = tmpNode->next;
}
return ret;
}
And in main:
Node* n = myList.findNode(10);
if (n)
std::cout << "N 10: " << n->data << "\n";
n = myList.findNode(30);
if (n)
std::cout << "N 30: " << n->data << "\n";
else
std::cout << "There is no N 30\n";
There are memory leak problems also, as specified by #RikusHoney in the comments.