Object persistency is provided by Geant4 as an optional category, so that the user may run Geant4 with or without an 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 to exist when the application terminates. Persistent objects, on the other hand, live beyond the termination of the application process and may then be accessed by other processes (in some cases, by processes on other machines).
C++ does not have, as an intrinsic part of the language, the ability to store and retrieve persistent objects. Geant4 provides an abstract framework for persistency of hits, digits and events.
An example which demonstrates an implementation of object
persistency using one of the tools accessible through the available
interface, is provided in
Object persistency of Geant4 objects is also possible by the
mean of the
Reflex provides, in a non-intrusive way, reflection capabilities to C++
classes by generating "dictionary information" for them. Those
dictionaries can then be loaded in memory allowing direct
persistency of the given objects without any instrumentation of the
code. The Reflex library is also part of
ROOT (since release
The basic steps that one needs to do in order to use Reflex with ROOT I/O for arbitrary C++ classes is:
Generate the dictionary for the given classes using the
genreflex tool from ROOT (this usually is done by adding
the appropriate command to the makefile)
Add initialization of ROOT I/O and loading of the generated dictionary for the given classes in the appropriate part of the code
Whenever the objects to be persistified are available, call the
WriteObject method of
the pointer to the appropriate object as argument (usually it is some sort
of container, like
std::vector containing the collection
of objects to be persistified)
The example provided in
demonstrates an implementation of object persistency using the
Reflex mechanism in ROOT I/O.