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.
Two examples demonstrating 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 using the Root-I/O
Root (since release
The basic steps that one needs to do in order to use Root-I/O for arbitrary C++ classes is:
Generate the dictionary for the given classes 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
TFilewith the pointer to the appropriate object as argument (usually it is some sort of container, like
std::vectorcontaining the collection of objects to be persistified)
The two examples (
demonstrate how to perform object persistency using the
Root-I/O mechanism for storing hits and geometry description.