Generic Linked List error: LinkedList is not a class template

Question

// This is the Node.h file

#ifndef NODE
#define NODE                                                                            

template <typename T>
class Node
{
   private:
   T elem;
   Node *next;
   friend class LinkedList<T>;
};

#endif // NODE

This is the LinkedLilst.h file

#ifndef LINKED_LIST
#define LINKED_LIST

#include "Node.h"

template <typename T>
class LinkedList
{
public:
  LinkedList();
  ~LinkedList();
  bool empty() const;
  const T &front() const;
  void addFront(const T &e);
  void removeFront();

private:
  Node<T> *head;
};

#endif // LINKED_LIST

This is the LinkedList.cpp file

#include <iostream>
#include "LinkedList.h"
using namespace std;

template <typename T>
LinkedList<T>::LinkedList() : head(NULL) {}

template <typename T>
bool LinkedList<T>::empty() const // I don't want it to modify the data member of the function.
{
  return head == NULL;
}

template <typename T>
LinkedList<T>::~LinkedList()
{
  while (!empty())
    removeFront(); 
}
...
...
...

This is my main.cpp file

#include <iostream>
#include "LinkedList.h"
using namespace std;
int main()
{
  LinkedList<int> ls;
  ls.addFront(3);
  cout << ls.front();
  return 0;
}

I don't know why I am getting the error: 'LinkedList' is not a class template

   friend class LinkedList<T>; in Node.h

The problem is that Node.h file doesn't have anything related to LinkedList. I added LinkedList Declaration but it still showing errors. Please help.

Answer 1

You need to forward declare the LinkedList class template:

#ifndef NODE
#define NODE                                                                            

template<class> class LinkedList;   // <- forward declaration

template <typename T>
class Node
{
   private:
   T elem;
   Node *next;
   friend class LinkedList<T>;
};

#endif // NODE

The next problem you are going to run in to is probably a linking problem. I suggest moving the class member functions definitions into the header files. More on that here: Why can templates only be implemented in the header file?