C++ Template Class method implementation within a cpp file

Question

i am struggling with the C++ template concept. What i have is something like that:

abstractClass.h:

namespace a {
  namespace b {
    template <typename RETURNTYPE, typename DATATYPE>
    class AbstractClass {

      protected:
        RETURNTYPE rtype;
        DATATYPE dtype;
        std::size_t countValues;

      public:
        explicit AbstractClass(DATATYPE _dtype, size_t _countValues):
          rtype(0),
          dtype(_data), 
          countValues(_countValues)  {

          }           
          virtual void process() = 0;
    };
  }   // namespace worker
}   // namespace eetneon

derivedClass.h

#include "abstractClass.h"

namespace a {
  namespace b {
    template <typename RETURNTYPE, typename DATATYPE>
    class DerivedClass : public AbstractClass<RETURNTYPE, DATATYPE> {

      public:
        DerivedClass(DATATYPE _data, size_t _countValues): 
          AbstractClass<RETURNTYPE, DATATYPE>(_data, _countValues) {
        }

        void process() override;

    };
  }   // namespace worker
}   // namespace eetneon

Now my question is: Is it possible to implement the void process() method within a *.cpp file or do I have to put the implementation in the header file, so the compiler can see it (I read something like that lately), because shit just got real, when i tried to put the implementation stuff away from the *.h file. Are there any sophisticated approaches to solve this problem? Everything i found until now seems a bit rough around the edges!

Thanks for your time and please excuse my horrible english.

Sincerely Hymir