Separating Header and Method Implementation

[dickyDick]

I used to put my methods implementation together with class definition in the header file. I would like to separate them. The following are my new code:

// File: MyClass.h
#ifndef __MYCLASS_HEADER__
#define __MYCLASS_HEADER__

template <class t>
class MyClass {
  public:
  void doNothing(t var);
};

#endif	// __MYCLASS_HEADER__

// File: MyClass.cpp
#include "MyClass.h"

template <class t>
void MyClass<t>::doNothing(t var) {
  var = var;
}

// File: MyMain.cpp
#include "MyClass.h"

int main() {
  MyClass<int> myClass;

  return 0;
}

I got the following linking error,
error LNK2001: unresolved external symbol "public: void __thiscall MyClass<int>::doNothing(int)" (?doNothing@?$MyClass@H@@QAEXH@Z)

I must have missed something. Anyone can help?

[pianorain]

Functions of template classes have to be defined in the header. What I usually do (just to preserve the nice header-implementation seperation) is create a .tmpl file, put my template implementation in there, and then #include it inside the .h inclusion guards.

[dickyDick]

I used to combine them even for non-template classes. A while ago, as I developed my program I encountered some problem as I included a header file (non-template class) in more than 1 cpp file. Someone told me it is a bad habit to combine. As from your reply, I understand that for template classes, they have to be combined. So combine them if it is template class and separate them if it is non-template class. Is that right?

Could you also give me some example for your suggestion above?

[pianorain]

Sure. I implement my template class and inline functions in exactly the same manner, since the implementation has be included in the header file. So, for a template class, I'd do something like this:

//in template_class.h
#ifndef TEMPLATE_CLASS_H_JNSDF_092839
#define TEMPLATE_CLASS_H_JNSDF_092839

template <class T>
class MyClass
{
  public:
    void doNothing();
};

#include "template_class.tmpl"

#endif

//in template_class.tmpl
template <class T>
void MyClass<T>::doNothing()
{
  //do stuff
}

//in main.cpp
#include "template_class.h"

int main()
{
  MyClass<int> myClass;
  myClass.doNothing();
  return 0;
}

Similarly with inline code, except I use a .inl extension so that I know what kind of code I have in each file.