Exception thrown: read access violation. Doubly linked list

[HeavyDrugs]

I'm getting this error. I don't understand why temp->next is nullptr if temp=head.

Exception thrown: read access violation.
temp->**next** was nullptr. occurred

#include "pch.h"
#include <iostream>

using namespace std;

struct node {
	int data;
	struct node *next;
	struct node *prev;
};

void create_list(node *&head, node *&tail, int num);

void display_dlist(node *head);
void reverse_display_dlist(node *head);

void insert_position_dlist(node *current, node *tail, int value, int position);
void insert_before_dlist(node *&head, int key);

void delete_element_dlist(node *&head, int key);
void delete_position_dlist(node *&head, int pos);
void delete_whole_dlist(node *&head);

void search_dlist(node *head, int key);



int Size = 0;


int main()
{
	int choice, value, position;

	node *head = NULL;
	node *tail = NULL;

	cout << "1.Sukuria dvikrypti sarasa (INSERTS A NODE)" << endl;
	cout << "2.Spausdinimas nuo pradziu" << endl;
	cout << "3.Spausdinimas nuo galo" << endl;
	cout << "4.Iterpia nauja elementa i nurodyta saraso pozicija" << endl;
	cout << "5.Iterpia elementa pries tam tikra elementa" << endl;
	cout << "6.Istrina pasirinkta reiksme (temp->next nullptr" << endl;
	cout << "7.Istrina pasirinkta pozicija" << endl;
	cout << "8.Istrina visa sarasa" << endl;
	cout << "9.Elemento paieskat" << endl;


	while (1) {

		cout << "Iveskite pasirinkima: ";
		cin >> choice;
		cout << endl;

		switch (choice) {
		case 1:
			cout << "Iveskite elementa: ";
			cin >> value;
			create_list(head, tail, value);
			cout << endl;
			break;
		case 2:
			cout << "Sarasas is priekio- ";
			display_dlist(head);
			cout << endl;
			break;
		case 3:
			cout << "Sarasas is galo- ";
			reverse_display_dlist(head);
			cout << endl;
			break;
		case 4:
			cout << "Iveskite norima pozicija ";
			cin >> position;
			cout << endl;
			cout << "Iveskite elementa ";
			cin >> value;
			insert_position_dlist(head, tail, value, position);
			cout << endl;
			break;
		case 5:
			cout << "Iveskite elementa pries kuri norite iterpti ";
			int key;
			cin >> key;
			cout << endl;
			insert_before_dlist(head, key);
			break;
		case 6:
			cout << "Kuria reiksme norite istrinti ";
			cin >> key;
			cout << endl;
			delete_element_dlist(head, key);
			break;
		case 7:
			cout << "Kuria pozicija norite istrinti ";
			cin >> position;
			delete_position_dlist(head, position);
			cout << endl;
			break;
		case 8:
			cout << "Sarasas istrintas";
			delete_whole_dlist(head);
			cout << endl;
			break;
		case 9:
			cout << "Kokio elemento ieskote? ";
			cin >> key;
			search_dlist(head, key);
			cout << endl;
			break;
		default:
			cout << "Tokio pasirinkimo nera" << endl;
			break;
		}
	}

	return 0;
}
void create_list(node *&head, node *&tail, int num) {
	struct node *temp, *temp1;
	temp = new node;
	temp->data = num;
	temp->next = NULL;
	if (head == NULL) {
		temp->prev = NULL;
		head = temp;
	}
	else {
		temp1 = head;
		while (temp1->next != NULL)
			temp1 = temp1->next;
		temp1->next = temp;
		temp->prev = temp1;
		tail = temp;
	}
	Size++;
}


void display_dlist(node *head) {
	node *temp = head;

	if (temp == NULL) {
		cout << "Sarasas tuscias" << endl;
		return;
	}

	while (temp != NULL) {
		cout << temp->data << "  ";
		temp = temp->next;
	}
	cout << endl;
}

void reverse_display_dlist(node *head) {
	node *temp = head;

	if (temp == NULL) {
		cout << "Sarasas tuscias" << endl;
		return;
	}

	while (temp->next != NULL) {
		temp = temp->next;
	}
	while (temp != head) {
		cout << temp->data << "  ";
		temp = temp->prev;
	}
	cout << temp->data << endl;
}


void insert_position_dlist(node *head, node *tail, int value, int position) { //kazkas su head insertion negerai
	node *temp = new node;
	node *temp1;
	node *temp2;

	temp->data = value;
	if (position == 1 || head == NULL)
	{
		if (head == NULL && position == 1)
		{
			temp->prev = temp->next = NULL;
			head = temp;
			return;
		}
		head->prev = temp;
		temp->prev = NULL;
		temp->next = head;
		head = temp;
		return;
	}
	temp1 = head;
	for (int i = 0; i < position - 2; i++)
	{
		temp1 = temp1->next;

	}
	temp->next = temp1->next;
	temp->prev = temp1;
	temp1->next = temp;
	if (temp->next != NULL)
	{
		temp2 = temp->next;
		temp2->prev = temp;
	}
	Size++;
}

void insert_before_dlist(node *&head, int key) {
	node *temp = new node;
	node *temp1 = head;
	int num;
	cout << "Iveskite elementa ";
	cin >> num;
	cout << endl;
	temp->data = num;

	while (temp1 != NULL) {
		if (temp1->data == key) {
			break;
		}
		temp1 = temp1->next;
		if (temp1->data == key) {
			break;
		}
	}
	temp->prev = temp1->prev;
	temp->next = temp1;
	temp1->prev = temp;

	if (temp->prev != NULL)
		temp->prev->next = temp;
	else {
		head = temp;
	}
	Size++;
}


void delete_element_dlist(node *&head, int key) { //kazkodel pradejo neveikt EYOOOOO
	node *temp;
	temp = head;
	if (temp == NULL)
	{
		cout << "Sarasas tuscias" << endl;
		return;
	}
	else if (temp->data == key) //+++++++++++
	{
		head = temp->next;
		delete temp;
	}
	else
	{
		while (temp->data != key)
			temp = temp->next;
		node* temp2 = NULL;
		temp2 = temp->next;
		while (temp->next->data != key)
		{
			temp = temp->next;
			temp2 = temp->next;
		}
		temp->next = temp2->next;
		temp2->prev = temp;

		delete temp;
	}
	Size--;
}

void delete_position_dlist(node *&head, int pos) { // 1 element-crash
	node *temp;
	temp = head;
	if (pos > Size + 1) {
		cout << "Blogai ivesta pozicija" << endl;
		return;
	}
	if (temp == NULL) {
		cout << "Sarasas tuscias" << endl;
		return;
	}
	if (temp->next == NULL) {
		delete temp;
		return;
	}

	for (int i = 1; i < pos&&temp != NULL; i++)
		temp = temp->next;

	if (pos == 1) {
		temp = head;
		head = head->next;
		head->prev = NULL;
		delete temp;
	}

	else if (temp->next == NULL) {
		node *temp;
		temp = head;
		while (temp->next->next != NULL)
			temp = temp->next;
		delete temp->next;
		temp->next = NULL;
	}

	else if (temp != NULL) {
		temp->prev->next = temp->next;
		temp->next->prev = temp->prev;
		delete temp;
	}
	Size--;
}

void delete_whole_dlist(node *&head) {
	if (head == NULL) {
		cout << "Sarasas jau yra tuscias" << endl;
		return;
	}

	node *current = head;
	node *temp;

	while (current != NULL)
	{
		temp = current->next;
		delete current;
		current = temp;
	}
	head = NULL;

	Size = 0;
}


void search_dlist(node *current, int key) {
	int pos = 0;

	if (current == NULL) {
		cout << "Sarasas tuscias" << endl;
		return;
	}
	cout << "Elemento pozicijos- ";
	while (current != NULL) {
		pos++;
		if (current->data == key) {
			cout << pos << "  ";
		}
		if (current->next != NULL)
			current = current->next;
		else
			break;
	}
	cout << endl;
}

[jonnin]

what input did you type in to crash it? Or is it dying before it gets that far?

you don't need struct in front of node when making variables of type node in c++, that is a C thing.

I suspect it is blowing up in create list, is that right?

[HeavyDrugs]

Input: 1 1 1 2 1 3 (inserts 3 nodes).
press 2 to print the list.
press 6 (this is a function to delete given key).
then press 2.

It will crash. It says that temp->next is NULL. But I don't know why becauseI did write "temp = head".

This only happens in this function.

[jonnin]

I will look later but look again at the logic in the else block at line 255.
it has to be in that chunk...

[coder777]

Line 258: At the end temp becomes null. You need to break the loop when that happens otherwise line 257 will crash.

[dhayden]

In create_list (which should probably be called append()), you don't set tail when appending to an empty list.

Also, when adding to the end, or printing in reverse, use the tail pointer.

[dhayden]

Some more comments:
When coding a linked list with a tail pointer, you're pretty much guaranteed that anywhere you can change the head pointer, you can change the tail pointer too. That means all of your insert and delete functions require a node * &tail argument.

You'll find the much easier if you start by writing these two routines:

// Insert "val" before "where". If "where" is null then insert at end.
void insert_before(node * &head, node * &tail, node *where, int val);

// Delete "where" from the list.  "Where" must be a node in
// the list
void delete_node(node * &head, node * &tail, node *where);

Then all the routines to modify the list simply find the place to insert/delete and call insert_before() or delete_node().

You will also save a lot of heartache if you create a temporary function to validate the list. Call this at the top of the while loop so you validate the list after each function. That way you'll immediately know when the list gets messed up.

void validate_list(node *head, node *tail)
{
    int sz=0;
    node *prev, *next;

    for (prev = NULL, next = head;
         next;
         prev = next, next = next->next) {
        if (( (sz==0 && prev == NULL) || prev->next == next)
            && next->prev == prev) {
            // okay
        } else {
            cout << "prev/next pointers aren't right\n";
        }
        ++sz;
    }
    if (prev != tail) {
        cout << "tail is bad\n";
    }
    if (sz != Size) {
        cout << "Size is " << Size << " instead of " << sz << '\n';
    }
}