The reason you can't put a templated class into a .cpp file is because in order to "compile" a .cpp file you need to know what the type that is being used in place of T. As it stands a templated class (like your class J) doesn't have enough information to compile. Thus it must be all in headers.
If you want the break up the implementation into another file for cleanliness, the best practice is to use an .hxx file. Like this: inside your header file, J.h, put:
#ifndef _J_H__
#define _J_H__
template <class T> class J{ // member definitions };
#include "j.hxx"
#endif // _J_H__
and then, in j.hxx you'll have
template <class T> J<T>::J() { // constructor implementation }
template <class T> J<T>::~J() { // destructor implementation }
template <class T> void J<T>::memberFunc() { // memberFunc implementation }
// etc.
Finally in your .cpp file that uses the templated class, let's call it K.cpp you'll have:
#include "J.h" // note that this always automatically includes J.hxx
void f(void)
{
J<double> jinstance; // now the compiler knows what the exact type is.
}
