Question

I keep getting the following:

[Linker error] undefined reference to `ComponentClass::POSIZIONENULLA’
ld returned 1 exit status
[Build Error] [main.exe] Error 1

can somebody help me with this. Here is my code:

ComponentClass.h

#ifndef _ComponentClass_H
#define _ComponentClass_H

template< class T>
class ComponentClass
{
        public:

               typedef ComponentClass* posizione;
               static const posizione POSIZIONENULLA;
               ComponentClass();
};

template< class T>
ComponentClass<T>::ComponentClass()
{
const posizione POSIZIONENULLA=(ComponentClass*)-1;
}
#endif

ProjectClass.h

#ifndef _ProjectClass_H
#define _ProjectClass_H

#include "ComponentClass.h"

template<class T>
class ProjectClass
{
  public:
        typedef typename ComponentClass<T>::posizione posizione;



         ProjectClass();
         posizione dummyFunction();

         private:
          posizione dummyposition;

};

          template<class T>
          ProjectClass<T>::ProjectClass()
          {

           dummyposition=   (posizione)ComponentClass<T>::POSIZIONENULLA;                  
          }

          template<class T>
          typename ProjectClass<T>::posizione ProjectClass<T>::dummyFunction()
          {
           posizione tempPosition;
           tempPosition=(posizione)ComponentClass<T>::POSIZIONENULLA;
           return tempPosition;
          } 
#endif

main.cpp

#include <cstdlib>
#include <iostream>
#include <string>

#include "ProjectClass.h"

using std::cout;
using std::endl;
using std::string;

int main(int argc, char *argv[])
{  
ProjectClass<int> pc;
}

Answer 1

This code isn’t actually doing anything useful:

template< class T>
ComponentClass<T>::ComponentClass()
{
    const posizione POSIZIONENULLA=(ComponentClass*)-1;
}

It’s just declaring a local variable in the constructor, initializing it and then not using it. It’s not doing anything with your static const posizione POSIZIONENULLA.

I think you meant:

template< class T>
const typename ComponentClass<T>::posizione ComponentClass<T>::POSIZIONENULLA=(ComponentClass*)-1;

That ought to fix your link error. As a side note, using -1 as a “special” pointer value isn’t a great idea, as others have pointed out in the comments above