Exception thrown: write access violation (this->tail was nullptr) - c++

Question

I have compiled my code and it seemed to work correctly. But out of no-where I get the error (this->tail was nullptr). I have tried changing the creation of the new node. But nothing seems to work. I cannot tell where tail is being set to nullptr and messing up the code.
How would I go about fixing this problem? Is there any way to set tail to non-nullptr without ruining every other function? I am not too familiar with exception throwing, so if you could explain the situation it would help a lot.
#ifndef MYDLL_H
#define MYDLL_H
#include <iostream>
#include <new>
using namespace std;
class MyDLL
{
struct Node
{
int i;
Node* next;
Node* prev;
};
Node* head;
Node* tail;
public:
MyDLL();
~MyDLL();
void append(int);
void remove(int);
bool find(int) const;
void clear();
void print() const;
void reverse() const;
};
MyDLL::MyDLL()
{
head = nullptr;
tail = nullptr;
}
MyDLL::~MyDLL()
{
clear();
}
void MyDLL::append(int i)
{
Node *n = new Node{ i, nullptr, nullptr };
if (head = nullptr)
{
head = n;
tail = n;
}
else
{
n->prev = tail;
tail->next = n; **<--- This is where the exception thrown error is showing up**
tail = n;
}
}
void MyDLL::remove(int i)
{
Node* p = head;
Node* q = tail;
while (p != nullptr && p->i != i)
{
q = p;
p = p->next;
}
if (p = nullptr)
{
return;
}
if (q = nullptr)
{
head = p->next;
}
else
{
q->next = p->next;
}
if (p->next = 0)
{
tail = q;
}
else
{
p->next->prev = q;
}
delete(p);
}
bool MyDLL::find(int i) const
{
Node* p = tail;
while (p != nullptr)
{
if (p->i = i)
{
return (true);
}
p = p->prev;
}
return (false);
}
void MyDLL::clear()
{
while (tail != nullptr)
{
Node* p = tail;
tail = p->prev;
delete (p);
}
head = nullptr;
}
void MyDLL::print() const
{
Node* p = head;
while (p)
{
cout << p->i << "\t";
p = p->next;
}
cout << "\n";
}
void MyDLL::reverse() const
{
Node* p = tail;
while (p)
{
cout << p->i << "\t";
p = p->prev;
}
cout << "\n";
}
#endif
int main()
{
MyDLL list;
list.append(5);
list.append(6);
list.append(7);
list.append(8);
list.print();
list.reverse();
cout << system("pause");
}

Answer 1

This code is exactly why you want to use nullptr = tail instead of tail = nullptr. In particular, every time you "check" for tail to be a null pointer, you are assigning nullptr to tail, and then assign operator returns a value which gets then implicitly casted to a boolean value, giving you no errors. But the error is actually there. Replace the "=" operator with "==" when performing a comparison, unless you actually have a reason for assigning and checking the return value.

Answer 2

Please fix = with == in your code.
void MyDLL::append(int i)
{
Node *n = new Node{ i, nullptr, nullptr };
if (head == nullptr)
It is always recommended to do like the reverse comparison (nullptr == head)