OpenStudio SDK: when should the _Impl header files be included?

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I know this is a bit outside the scope of UnmetHours, but I still don't see a better place to post this question, and I cannot find the answer in the wiki/documentation/developper documentation of OpenStudio.

As far as I understand OpenStudio uses the  Pointer to Implemention (PIMPL, or Cheshire Cat idiom) for a variety of reasons including having a more stable API, hidding the implementation from the public eye, and - what's of interest to me here - reducing recompilation time.

In OpenStudio, the current coding standard is to have one class.hpp header file for the public side, and class_Impl.hpp for the private side. Both are implemented in the same class.cpp source file, and the public class only forwards to the Impl one.

To use an example: let's consider the class ThermalZone and the class Space. Let's assume that Space only has a single method called setThermalZone() that returns a std::vector<Space> of all spaces.

Space.cpp:

#include "Space.hpp"
#include "Space_Impl.hpp"

#include "ThermalZone.hpp"
// THIS? #include "ThermalZone_Impl.hpp"

 // Impl method  (declared in `Space_Impl.hpp`), that actually does the work
  bool Space_Impl::setThermalZone(ThermalZone& thermalZone)
  {
    bool result = this->setPointer(OS_SpaceFields::ThermalZoneName, thermalZone.handle());
    if (result){
      thermalZone.checkDaylightingControlsAndIlluminanceMaps();
    }
    return result;
  }

// Public method (declared in `Space.hpp`), that simply forwards
bool Space::setThermalZone(ThermalZone& thermalZone)
{
  return getImpl<detail::Space_Impl>()->setThermalZone(thermalZone);
}

If I'm following correctly, the gain in recompilation time arises if I modify the implementation of ThermalZone (ThermalZone_Impl), Space doesn't have to be recompiled because it's linking only to the ThermalZone.hpp header file.

The thing is, I very often see both the class.hpp and class_Impl.hpp being included in various classes (see the actual Space.cpp#L49 where ThermalZone_Impl.hpp is indeed included), when I don't see any of the class_Impl methods being used anywhere.

So my question is: when must/should I include the class.hpp? And more importantly, when must/should I also include the class_Impl.hpp?