Question

I have some trouble with inheritance from a template class. The code below does not compile, showing this error : main.cpp : undefined reference to OBJ1<1000>::method()

parent.h

template <int nb>
class PARENT
{
  PARENT() {};
  ~PARENT() {};

  virtual void method() = 0;
  enum { nb_ = nb };
};

obj1.h

#include "parent.h"

template <int nb>
class OBJ1 : public PARENT<nb>
{
  virtual void method();
};

obj1.cpp

#include "obj1.h"

template <int nb>
void OBJ1<nb>::method()
{
  //code
}

main.cpp

#include "obj1.h"

int main()
{
  OBJ1<1000> toto;
  toto.method();
}

Where am I wrong ?

Answer 1

When dealing with templates you cannot split declaration and implementation into separate files. See this question for the reasons (and also for a more concise description to workaround this).

This needs to be merged (You can also #include the implementation file into the header to let the preprocessor do the merge.):

// obj1.hpp

#include "parent.h"

template <int nb>
class OBJ1 : public PARENT<nb>
{
  virtual void method();
};

template <int nb>
void OBJ1<nb>::method()
{
  //code
}