Nested template class constructor - c++

Question

I the compiler can't find the definition of my constructor for the nested class.
My nested class Node is in the middle and the constructor is at the end.
Errors:
error C2244: 'CircularDoubleDirectedList::Node::Node' : unable
to match function definition to an existing declaration see
declaration of 'CircularDoubleDirectedList::Node::Node'
definition
'CircularDoubleDirectedList::Node::Node(const T &)'
existing declarations
'CircularDoubleDirectedList::Node::Node(const T &)'
Code:
#ifndef CIRCULARDOUBLEDIRECTEDLIST_H
#define CIRCULARDOUBLEDIRECTEDLIST_H
#include "ICircularDoubleDirectedList.h"
template <typename T> class CircularDoubleDirectedList;
template <typename T> class Node;
template <typename T>
class CircularDoubleDirectedList :
public ICircularDoubleDirectedList<T>{
public:
//Variabels
Node<T>* current;
int nrOfElements;
direction currentDirection;
//Functions
CircularDoubleDirectedList();
~CircularDoubleDirectedList();
void addAtCurrent(const T& element) override;
private:
template <typename T>
class Node
{
public:
T data;
Node<T>* forward;
Node<T>* backward;
Node(const T& element);// The constructor
};
};
template <typename T>
CircularDoubleDirectedList<T>::CircularDoubleDirectedList(){
this->nrOfElements = 0;
this->current = nullptr;
this->currentDirection = FORWARD;
}
template <typename T>
CircularDoubleDirectedList<T>::~CircularDoubleDirectedList(){
//TODO: Destroy all nodes
}
template <typename T>
void CircularDoubleDirectedList<T>::addAtCurrent(const T& element){
Node<T>* newNode = new Node<T>(element);
newNode->data = element;
if (this->nrOfElements == 0){
newNode->forward = newNode;
newNode->backward = newNode;
}
else{
//this->current->forward = newNode;
//this->current->forward->backward = newNode;
}
//this->current = newNode;
}
template <typename T>
CircularDoubleDirectedList<T>::Node<T>::Node(const T& element){
this->data = element;
}
#endif

Answer 1

First, the forward-declared template <typename T> class Node; is not the same as CircularDoubleDirectedList::Node - the former is a global class template, the latter is a nested class.
Second, you don't need to declare CircularDoubleDirectedList::Node as a template (and if you do, you have to use another template parameter name for it, not T). But as I understand, for this case you should just make it non-template, so:
template <typename T>
class CircularDoubleDirectedList :
public ICircularDoubleDirectedList<T>{
private:
class Node
{
public:
T data;
Node* forward;
Node* backward;
Node(const T& element);// The constructor
};
public:
Node* current;
//...
};
template <typename T>
CircularDoubleDirectedList<T>::Node::Node(const T& element){
this->data = element;
}

Answer 2

You have two class templates named Node, while in reality you want one non-template class named Node. You have forward-declared ::Node<T>, and you have the nested ::CircularDoubleDirectedList<T>::Node<U>.
If you really want it like that, you'll have to add another template keyword to the constructor definition:
template <typename T> //because CircularDoubleDirectedList is a template
template <typename U> //because Node is a template
CircularDoubleDirectedList<T>::Node<U>::Node(const T& element) : data(element)
{}
However, I can't see a single reason to have Node be a template. Inside CircularDoubleDirectedList<T>, do you want to use nodes with type other than T? If not, make Node a normal non-template class:
template <typename T>
class CircularDoubleDirectedList :
public ICircularDoubleDirectedList<T>{
public:
//Variabels
Node<T>* 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);// The constructor
};
};
template <typename T>
CircularDoubleDirectedList<T>::Node::Node(const T& element) : data(element)
{}