As I mentioned previously in #885 (comment), I'm currently working on adding the NRLMSIS2.0 atmospheric model to JSBSim which is kind of a revival of the existing src/models/atmosphere/FGMSIS.cpp code (which was based on the previous version 1.0 of NRLMSIS)¹.

In order for this feature to work, we will need to modify the atmosphere model that is currently created by default in FGFDMExec:

Basically, the idea is to call at some point the following code:

if (some_XML_instructions_are_found) {
  // Delete the FGStandardAtmosphere instance and replace it by an FGMSIS instance
  Models[eAtmosphere] = std::make_shared<FGMSIS>(this);
  // ERROR messages : Failed to tie property XXX to object methods.
}

The trick is that if no special care is taken, the properties atmosphere/T-R, atmosphere/P-Psf, etc. will still be bound to the old instance of FGStandardAtmosphere even after the above code has been executed. With the current code, FGMSIS would fail to bind itself to the aforementioned properties as they would already be tied to FGStandardAtmosphere and JSBSim would complain with the infamous error message Failed to tie property atmosphere/T-R to object methods. Even worse, if one would try to access atmosphere/T-R and the likes, it would cause a SEGFAULT as the property would try to call the method FGAtmosphere::GetTemperature() on a deleted FGStandardAtmopshere instance.

To avoid this issue, we need to tell JSBSim to unbind FGStandardAtmosphere before an instance of FGMSIS is created. Currently there is no generic code in JSBSim that allows to unbind a single FGModel instance (you'd need to untie the properties one by one which is less than ideal from a maintenance standpoint).

To address this issue the current PR adds a new method FGPropertyManager::Unbind(void* instance) to unbind the properties of a given class instance. With this new method, the code above will become something like:

if (some_XML_instructions_are_found) {
  PropertyManager->Unbind(Models[eAtmosphere]);
  Models[eAtmosphere] = std::make_shared<FGMSIS>(this);
  // SUCCESS! No "Failed to tie..." error messages.
}

The new Unbind method also restores the read/write attributes of each properties as the newly bound instance might want to set them differently.

Finally, the unit test FGAtmosphereTest is amended by the PR to test the new feature and checks that the properties such atmosphere/T-R are tied to the correct instance (see the method testRun() in FGAmosphereTest).