Error in My Queue datastructres implementation - c++

Question

I'm trying to implement a Queue using a linked list, but I get this error when I pass an integer to the enqueue function in main
Exception thrown: write access violation.
this->tail was nullptr.
here is my Queue.h
#include <iostream>
using namespace std;
template <class T >
class Node
{
public:
T data;
Node* next;
Node();
Node(T _data);
};
template <class T>
class Queue
{
public:
Node<T> *head, *tail;
int elemsCnt;
Queue();
~Queue();
int count();
void clear();
void enqueue(T);
T dequeue();
T front();
T back();
bool empty();
};
and this is my Queue.cpp which i wrote the functions ..
#include "Queue.h"
template <class T>
Node<T>::Node()
{
next = NULL;
}
template <class T>
Node<T>::Node(T _data)
{
data = _data;
next = NULL;
}
template <class T>
Queue<T>::Queue()
{
head = tail = NULL;
elemsCnt = 0;
}
template <class T>
Queue<T>::~Queue()
{
clear();
}
template <class T>
int Queue<T>::count()
{
return elemsCnt;
}
template <class T>
bool Queue<T>::empty()
{
return(elemsCnt == 0);
}
template <class T>
void Queue<T>::enqueue(T val)
{
Node<T>* inserted = new Node<T>(val);
tail->next = inserted;
tail = inserted;
++elemsCnt;
}
template <class T>
T Queue<T>::dequeue()
{
Node<T>* deleted = head;
T val = head->data;
head = head->next;
delete deleted;
--elemsCnt;
if (empty())
tail = NULL;
return val;
}
template <class T>
T Queue<T>::front()
{
return head->data;
}
template <class T>
T Queue<T>::back()
{
return tail->data;
}
template <class T>
void Queue<T>::clear()
{
while (!empty())
{
Node<T>* deleted = head;
head = head->next;
delete deleted;
--elemsCnt;
}
tail = NULL;
}

Answer 1

because initially your Queue is empty. when you have:
void Queue<T>::enqueue(T val)
{
Node<T>* inserted = new Node<T>(val);
tail->next = inserted; //at this point tail is still NULL
//it is illegal to try to access * and . on NULL
tail = inserted;
++elemsCnt;
}
if you want to fix it try this:
void Queue<T>::enqueue(T val)
{
Node<T>* inserted = new Node<T>(val);
if(this->elemsCnt==0)
{
//empty Queue
this->head=inserted;
this->tail=inserted;
}
else
{
//non empty
inserted->next=this->head; //make sure the new head can point to older head
this->head=inserted; update the pointer for head.
}
++elemsCnt;
}

Answer 2

just like the exception says, tail is NULL. Your enqueue method is not handling the case when the list is empty (head == tail == null).
template <class T>
void Queue<T>::enqueue(T val)
{
Node<T>* inserted = new Node<T>(val);
if (head == NULL)
{
head = tail = inserted;
}
else
{
tail->next = inserted;
tail = inserted;
}
++elemsCnt;
}