Implicit instantiation of undefined template - c++

Question

Here's a template (queue) I'm trying to write:
#include <iostream>
using namespace std;
template <typename T>
class Queue
{
friend ostream& operator<< (ostream &, const Queue<T> & );
private:
template<class> class Node;
Node<T> *front;
Node<T> *back;
public:
Queue() : front(0), back(0) {}
~Queue();
bool Empty()
{
return front == 0;
}
void Push(const T& NewEl)
{
Node<T&> *El = new Node<T> (NewEl);
if (Empty())
front=back=El;
else
{
back-> next = El;
back = El;
}
}
void Pop()
{
if (Empty())
cout << "Очередь пуста." << endl;
else
{
Node<T> *El = front;
front = front -> next;
delete El;
}
}
void Clear()
{
while (! Empty())
Pop();
}
};
template <typename T>
class Node
{
friend class Queue<T>;
public:
Node() {next = 0;}
Node(T nd) {nd=node; next=0;}
T& getsetnode(){return node;}
Node<T>*& getsetnext(){return next;}
private:
T front;
T back;
T node;
Node<T> *next;
};
template <class T> ostream& operator<< (ostream &, const Queue<T> & );
int main()
{
Queue<int> *queueInt = new Queue<int>;
for (int i = 0; i<10; i++)
{
queueInt->Push(i);
cout << "Pushed " << i << endl;
}
if (!queueInt->Empty())
{
queueInt->Pop();
cout << "Pop" << endl;
}
queueInt->Front();
queueInt->Back();
queueInt->Clear();
cout << "Clear" << endl;
return 0;
}
At these lines:
Node<T&> *El = new Node<T> (NewEl);
front = front -> next;
delete El;
I get Implicit instantiation of undefined template 'Queue<int>::Node<int>'. What am I doing wrong? After reading this post I tried changing int to const int to see if that was the problem, but apparently it isn't, because I get the same error.
I'm using XCode with LLVM compiler 4.2. When I switch to GCC I get more errors:
template<class> class Node; gets Declaration of 'struct Queue<int>::Node<int>',
Node<T&> *El = new Node<T> (NewEl); gets Invalid use of incomplete type,
and anything dealing with assignment of anything to El can't convert <int&>* to <int>* (but deleting reference doesn't change anything for LLVM).

Answer 1

template <typename T>
class Queue
{
private:
template<class> class Node;
/* ... */
This is a forward declaration of Queue::Node. The latter defined class Node is in the global namespace, so they aren't the same and any usage of Queue::Node will result in an incomplete-type error. Since you don't provide an interface to the interior nodes anyway just scrap the global definition of Node and stick it into Queue:
template <typename T>
class Queue
{
private:
class Node
{
public:
Node() {next = 0;}
/* ... */
};
/* ... */
};