The contents of main()
will vary according to the
needs of a given simulation application and therefore must be supplied
by the user. The Geant4 toolkit does not provide a main()
method, but a sample is provided here as a guide to the beginning
user. Example 2.1 is the simplest example of
main()
required to build a simulation program.
Example 2.1.
Simplest example of main()
#include "G4RunManager.hh" #include "G4UImanager.hh" #include "ExN01DetectorConstruction.hh" #include "ExN01PhysicsList.hh" #include "ExN01PrimaryGeneratorAction.hh" int main() { // construct the default run manager G4RunManager* runManager = new G4RunManager; // set mandatory initialization classes runManager->SetUserInitialization(new ExN01DetectorConstruction); runManager->SetUserInitialization(new ExN01PhysicsList); // set mandatory user action class runManager->SetUserAction(new ExN01PrimaryGeneratorAction); // initialize G4 kernel runManager->initialize(); // get the pointer to the UI manager and set verbosities G4UImanager* UI = G4UImanager::GetUIpointer(); UI->ApplyCommand("/run/verbose 1"); UI->ApplyCommand("/event/verbose 1"); UI->ApplyCommand("/tracking/verbose 1"); // start a run int numberOfEvent = 3; runManager->BeamOn(numberOfEvent); // job termination delete runManager; return 0; }
The main()
method is implemented by two toolkit
classes, G4RunManager and G4UImanager,
and three classes, ExN01DetectorConstruction,
ExN01PhysicsList and
ExN01PrimaryGeneratorAction, which are derived from
toolkit classes. Each of these are explained in the following sections.
The first thing main()
must do is create an instance of
the G4RunManager class. This is the only manager class in
the Geant4 kernel which should be explicitly constructed in the
user's main()
. It controls the flow of the program and
manages the event loop(s) within a run. When G4RunManager is
created, the other major manager classes are also created. They are
deleted automatically when G4RunManager is deleted. The run
manager is also responsible for managing initialization procedures,
including methods in the user initialization classes. Through these
the run manager must be given all the information necessary to
build and run the simulation, including
how the detector should be constructed,
all the particles and all the physics processes to be simulated,
how the primary particle(s) in an event should be produced and
any additional requirements of the simulation.
In the sample main()
the lines
runManager->SetUserInitialization(new ExN01DetectorConstruction); runManager->SetUserInitialization(new ExN01PhysicsList);
create objects which specify the detector geometry and physics processes, respectively, and pass their pointers to the run manager. ExN01DetectorConstruction is an example of a user initialization class which is derived from G4VUserDetectorConstruction. This is where the user describes the entire detector setup, including
its geometry,
the materials used in its construction,
a definition of its sensitive regions and
the readout schemes of the sensitive regions.
Similarly ExN01PhysicsList is derived from G4VUserPhysicsList and requires the user to define
the particles to be used in the simulation,
the range cuts for these particles and
all the physics processes to be simulated.
The next instruction in main()
runManager->SetUserAction(new ExN01PrimaryGeneratorAction);
creates an instance of a particle generator and passes its pointer
to the run manager. ExN01PrimaryGeneratorAction is an
example of a user action class which is derived from
G4VUserPrimaryGeneratorAction. In this class the user must
describe the initial state of the primary event. This class has a
public virtual method named generatePrimaries()
which will
be invoked at the beginning of each event. Details will be given in
Section 2.6.
Note that Geant4 does not provide any default behavior for generating a primary event.
The next instruction
runManager->initialize();
performs the detector construction, creates the physics processes,
calculates cross sections and otherwise sets up the run. The final
run manager method in main()
int numberOfEvent = 3; runManager->beamOn(numberOfEvent);
begins a run of three sequentially processed events. The
beamOn()
method may be invoked any number of times within
main()
with each invocation representing a separate run.
Once a run has begun neither the detector setup nor the physics
processes may be changed. They may be changed between runs,
however, as described in Section 3.4.4.
More information on G4RunManager in general is found in
Section 3.4.
As mentioned above, other manager classes are created when the
run manager is created. One of these is the user interface manager,
G4UImanager. In main()
a pointer to
the interface manager must be obtained
G4UImanager* UI = G4UImanager::getUIpointer();
in order for the user to issue commands to the program. In the
present example the applyCommand()
method is called three
times to direct the program to print out information at the run,
event and tracking levels of simulation. A wide range of commands
is available which allows the user detailed control of the
simulation. A list of these commands can be found in
Section 7.1.
There are three classes which must be defined by the user. Two
of them are user initialization classes, and the other is a user
action class. They must be derived from the abstract base classes
provided by Geant4: G4VUserDetectorConstruction,
G4VuserPhysicsList and
G4VuserPrimaryGeneratorAction.
Geant4 does not provide default behavior for these classes.
G4RunManager checks for the existence of these mandatory
classes when the initialize()
and BeamOn()
methods are invoked.
As mentioned in the previous section, G4VUserDetectorConstruction requires the user to define the detector and G4VUserPhysicsList requires the user to define the physics. Detector definition will be discussed in Sections
Section 2.2 and Section 2.3. Physics definition will be discussed in Sections Section 2.4 and Section 2.5. The user action G4VuserPrimaryGeneratorAction requires that the initial event state be defined. Primary event generation will be discussed in Section 2.7.
Geant4 provides five user hook classes:
G4UserRunAction
G4UserEventAction
G4UserStackingAction
G4UserTrackingAction
G4UserSteppingAction
There are several virtual methods in each of these classes which allow the specification of additional procedures at all levels of the simulation application. Details of the user initialization and action classes are provided in Chapter 6.
Geant4 provides a category named intercoms. G4UImanager is the manager class of this category. Using the functionalities of this category, you can invoke set methods of class objects of which you do not know the pointer. In Example 2.2, the verbosities of various Geant4 manager classes are set. Detailed mechanism description and usage of intercoms will be given in the next chapter, with a list of available commands. Command submission can be done all through the application.
Example 2.2.
An example of main()
using interactive
terminal and visualization. Code modified from the previous
example are shown in blue.
#include "G4RunManager.hh" #include "G4UImanager.hh" #include "G4UIterminal.hh" #include "G4VisExecutive.hh" #include "N02DetectorConstruction.hh" #include "N02PhysicsList.hh" #include "N02PrimaryGeneratorAction.hh" #include "N02RunAction.hh" #include "N02EventAction.hh" #include "N02SteppingAction.hh" #include "g4templates.hh" int main(int argc,char** argv) { // construct the default run manager G4RunManager * runManager = new G4RunManager; // set mandatory initialization classes N02DetectorConstruction* detector = new N02DetectorConstruction; runManager->SetUserInitialization(detector); runManager->SetUserInitialization(new N02PhysicsList); // visualization manager G4VisManager* visManager = new G4VisExecutive; visManager->initialize(); // set user action classes runManager->SetUserAction(new N02PrimaryGeneratorAction(detector)); runManager->SetUserAction(new N02RunAction); runManager->SetUserAction(new N02EventAction); runManager->SetUserAction(new N02SteppingAction); // get the pointer to the User Interface manager G4UImanager* UI = G4UImanager::GetUIpointer(); if(argc==1) // Define (G)UI terminal for interactive mode { G4UIsession * session = new G4UIterminal; UI->ApplyCommand("/control/execute prerun.g4mac"); session->sessionStart(); delete session; } else // Batch mode { G4String command = "/control/execute "; G4String fileName = argv[1]; UI->ApplyCommand(command+fileName); } // job termination delete visManager; delete runManager; return 0; }
Although not yet included in the above examples, output streams will be needed. G4cout and G4cerr are iostream objects defined by Geant4. The usage of these objects is exactly the same as the ordinary cout and cerr, except that the output streams will be handled by G4UImanager. Thus, output strings may be displayed on another window or stored in a file. Manipulation of these output streams will be described in Section 7.2.4. These objects should be used instead of the ordinary cout and cerr.