Protect against failed cast

[gondiaz]

In interactive mode, you might want to use DefaultEventAction without DefaultTrackingAction. Since you are in interactive mode, Geant4 will provide a not empty G4TrajectoryContainer, which is a vector of G4VTrajectory. The dowcast fails since Trajectory is a nexus class.

[paolafer]

I would say that in the interactive mode there is no need to use DefaultEventAction either, since one is not concerned with how many events are saved in the output file. One could simply use the default actions by not specifying any custom actions.
In fact, if one wanted to save only events with energy deposition above a threshold, and at the same time is not using DefaultTrackingAction, it wouldn't work, because the calculated energy deposited would be always zero. I will document the use of each action better, once the PR on the clean-up of the actions is approved.

[paolafer]

What we could do is, instead of adding a protection, adding a meaningful error message and let the program crash.

[gondiaz]

Agree. What do you think about placing such an error before the SetUserAction in NexusApp.cc? This would assure that you always choose a compatible set of user actions

[paolafer]

Agree. What do you think about placing such an error before the SetUserAction in NexusApp.cc?

I don't like it vey much, I would prefer to keep the code in the NexusApp class as simple as possible. I would rather put the protection in DefaultEventAction, which is the class that accesses the Trajectory object.

[paolafer]

@gondiaz, what about this PR? Are you going to add the comment to the crash or shall we close it?

[gondiaz]

At least not in the next two weeks

[paolafer]

At least not in the next two weeks

This doesn't answer my question :-).

[gondiaz]

The answer is yes then, but I don't know when.
If it is annoying to have this pending, it can be closed. If anyone wants to contribute and finish it, also works for me.

[paolafer]

It's not annoying, I just want to know if you forgot about it or you are going to finish it at some point.

[paolafer]

In the nexus convention, the second argument of G4Exception should be the function, so () should be added.

[paolafer]

I would say explicitly which one is the correct action, something like "DefaultTrackingAction is required when using DefaultEventAction".

[paolafer]

This PR adds a useful error message when DefaultTrackingAction is not used together with DefaultEventAction. Approved!