invalid use of template name without an argument list

Question

I am facing a problem with my linked list class, I have created the interface and implementation files of the class, but when I build it, this error occurs: "invalid use of template name 'LinkedList' without an argument list". here's my interface file:

#ifndef LINKEDLIST_H
#define LINKEDLIST_H

template <typename T>
struct Node{
    T info;
    Node<T> *next;
};

template <typename T>
class LinkedList
{
    Node<T> *start;
    Node<T> *current;
public:
    LinkedList();
    ~LinkedList();
};

#endif // LINKEDLIST_H

and this is my implementation code:

#include "LinkedList.h"

LinkedList::LinkedList()
{
   start = nullptr;
   current = nullptr;
}

LinkedList::~LinkedList()
  {

  }

Solution

Write it like this:

template<typename T>
LinkedList<T>::LinkedList()
{
   start = nullptr;
   current = nullptr;
}

And similarly for other member functions. But you'll run into another problem - declarations and definitions of a template can't be separated to different files.

Comments

[Alladin]

I have done it, but as you said, there is an error with the declaration of 'NULL', it says that NULL was not declared in this scope. how can I solve this issue??

[jrok]

@Alladinsaoudi NULL is a macro that's defined in several different headers listed here. You'll have to include at least one of them where you want to use it.

[Alladin]

@jrol thanks a looooooooooot , i do really appreciate your help. :))))

[Walter]

@Alladinsaoudi With C++11 better use nullptr instead of NULL. Moreover, you can always simply use 0 instead of NULL (but nullptr is better as the compiler can make some checks).

[Isaac Woods]

Anyone using C++11 or above should now use nullptr.

[ignacio]

Maybe it is worth mentioning that sometimes the library already provides a specialization for the class you want to use, like a Vector3 that could be used as Vector3<double> or Vector3d