Geant4 User's Guide for Application Developers - Geant4 - CERN

Detector Definition and ResponseG4int myDigiCollID = fDM->GetDigiCollectionID( "digi_collection_name" );Then, you can get the pointer to your concrete G4THitsCollection object or G4TDigiCollection object of thecurrently processing event.MyHitsCollection * HC = fDM->GetHitsCollection( myHitsCollID );MyDigiCollection * DC = fDM->GetDigiCollection( myDigiCollID );In case you want to access to the hits or digits collection of previous events, add the second argument.MyHitsCollection * HC = fDM->GetHitsCollection( myHitsCollID, n );MyDigiCollection * DC = fDM->GetDigiCollection( myDigiCollID, n );where, n indicates the hits or digits collection of the n th previous event.4.6. Object Persistency4.6.1. Persistency in Geant4Object persistency is provided by Geant4 as an optional category, so that the user may run Geant4 with or withoutan object database management system (ODBMS).When a usual (transient) object is created in C++, the object is placed onto the application heap and it ceases toexist when the application terminates. Persistent objects, on the other hand, live beyond the termination of theapplication process and may then be accessed by other processes (in some cases, by processes on other machines).Figure 4.9. Persistent object.C++ does not have, as an intrinsic part of the language, the ability to store and retrieve persistent objects. Geant4provides an abstract framework for persistency of hits, digits and events.Two examples demonstrating an implementation of object persistency using one of the tools accessible throughthe available interface, is provided in examples/extended/persistency.4.6.2. Using Reflex for persistency of Geant4 objectsObject persistency of Geant4 objects is also possible by the mean of the Reflex library. Reflex provides, in anon-intrusive way, reflection capabilities to C++ classes by generating "dictionary information" for them. Thosedictionaries can then be loaded in memory allowing direct persistency of the given objects without any instrumentationof the code. The Reflex library is also part of ROOT (since release v5.08).The basic steps that one needs to do in order to use Reflex with ROOT I/O for arbitrary C++ classes is:1. Generate the dictionary for the given classes using the genreflex tool from ROOT (this usually is done byadding the appropriate command to the makefile)2. Add initialization of ROOT I/O and loading of the generated dictionary for the given classes in the appropriatepart of the code137