'=' : cannot convert from 'CircularDoubleDirectedList<int>::Node *' to 'Node *' - c++

Question

I have Node* current where I store a pointer to what node that is current at "top" of the list. When I set a new node as current I get the error:
'=' : cannot convert from 'CircularDoubleDirectedList<int>::Node *' to 'Node *'
while compiling class template member function 'void CircularDoubleDirectedList<int>::addAtCurrent(const T &)' with [ T=int ]
It is the three rows with //Problem comment that generates those errors if take them away everything works fine.
#include "ICircularDoubleDirectedList.h"
template <typename T> class CircularDoubleDirectedList;
class Node;
template <typename T>
class CircularDoubleDirectedList :
public ICircularDoubleDirectedList<T>{
public:
//Variables
Node* current;
int nrOfElements;
direction currentDirection;
//Functions
CircularDoubleDirectedList();
~CircularDoubleDirectedList();
void addAtCurrent(const T& element) override;
private:
class Node
{
public:
T data;
Node* forward;
Node* backward;
Node(const T& element);
};
};
template <typename T>
void CircularDoubleDirectedList<T>::addAtCurrent(const T& element){
Node* newNode = new Node(element);
newNode->data = element;
if (this->nrOfElements == 0){
newNode->forward = newNode;
newNode->backward = newNode;
}
else{
this->current->forward = newNode; // Problem
this->current->forward->backward = newNode; // Problem
}
this->current = newNode; //Problem
}

Answer 1

When you forward declare Node as being outside of the class here:
template <typename T> class CircularDoubleDirectedList;
class Node;
That is declaring a type Node in the global namespace. It is ::Node. Then, within your class declaration, current takes on that type:
template <typename T>
class CircularDoubleDirectedList
: public ICircularDoubleDirectedList<T>
{
public:
Node* current; // this is a pointer to ::Node.
};
Then you provide a declaration of CircularDoubleDirectedList<T>::Node. This is not the same type as ::Node. It also gets looked up first by name resolution rules. So in here:
template <typename T>
void CircularDoubleDirectedList<T>::addAtCurrent(const T& element){
Node* newNode = new Node(element); // newNode is a pointer to
// CircularDoubleDirectedList<T>::Node
But current is a pointer to the still-incomplete type ::Node. Hence the error - you have inadvertently created two types named Node.
If you're going to forward-declare Node, you have to do it inside the class:
template <typename T>
class CircularDoubleDirectedList
: public ICircularDoubleDirectedList<T>
{
class Node; // NOW it's CircularDoubleDirectedList<T>::Node
};