1. C++ for Geant4 users
A collection of most useful and/or common features used in and to use Geant4
Marc Verderi
LLR – Ecole polytechnique
14/11/2018
Marc Verderi - LLR Ecole polytechnique

2. Marc Verderi - LLR Ecole polytechnique
Introduction
Geant4 user’s interfaces do not involve “expert” C++ features.
The most advanced features exposed to the user:
Inheritance
The feature that makes a programming language “Object Oriented”
And which makes Geant4 versatile and extendable
Singletons
Which is the technique used for the many “managers” in Geant4
A little of templates
The so-called “generic programming”
Hope here is to make novice users a bit more comfortable with the above:
Assuming basic syntax is known (int, double, i++, for(…){..}, etc…)
Speak about
Class
constructor/destructor, methods & arguments, const, initialization and operator overloading
Singleton
14/11/2018
Marc Verderi - LLR Ecole polytechnique

3. Marc Verderi - LLR Ecole polytechnique
Class
14/11/2018
Marc Verderi - LLR Ecole polytechnique

4. Class : constructor/destructor
Class declaration:
class AClass {
public:
// default constructor:
AClass();
// copy constructor:
AClass(const AClass &);
// other constructor:
AClass([arguments]);
// destructor
~AClass(); … };
At use:
#include “AClass.hh” …
[some main, method…]{
AClass objA;
AClass objB(objA);
AClass objC([arg. values]);
AClass *p_obj = new AClass;
} // here, destructor called
// for objA, objB, objC,
// but not for *p_obj that
// requires delete p_obj;
Used when object passed by value.
Heavily used by STL:
eg std::vector<T>,
when increasing
vector capacity
14/11/2018
Marc Verderi - LLR Ecole polytechnique

5. Class : method & arguments
Class declaration:
class AClass { …
public:
int fun1(int i);
// default argument:
double fun2(int n=0);
// argument type:
double fun2(double);
// !! Ambiguity here !!
double fun2(double w=3.14);
// !! not allowed !!
int fun2(double); };
At use:
#include “AClass.hh” …
[some main, method…]{
AClass objA;
int i = objA.fun1(5);
double x = objA.fun2();
double z = objA.fun2(3.14);
// is it n=0 or w=3.14 ?
double v = objA.fun2();
14/11/2018
Marc Verderi - LLR Ecole polytechnique

6. Class : “publicity” levels
Class declaration:
class AClass { …
public:
// access to everyone:
int publicFun();
protected:
// access to derived classes:
double protectFun();
private:
// access to this class only
// (and “friend” classes)
void privateFun();
int someNumber; };
At use:
#include “AClass.hh” …
[some main, method…]{
AClass objA;
int i = objA.publicFun();
// valid only in derived
// classes code (see later):
double a = objA.protectFun();
// valid only in AClass code:
objA.privateFun();
int m = objA.someNumber;
14/11/2018
Marc Verderi - LLR Ecole polytechnique

7. Marc Verderi - LLR Ecole polytechnique
Class : keyword const
Class declaration:
class AClass { …
public:
// pointing to constant int
const int* add(int i)
{someNumber += i;
return &someNumber;}
// object itself unaffected:
int number() const
{return someNumber;}
// !! Wrong !!
void increment() const
{someNumber++;}
private:
int someNumber; };
At use:
#include “AClass.hh” …
[some main, method…]{
AClass objA;
const int* j = objA.add(5);
// !! Wrong !!
int *n = objA.add(5);
int k = objA.number();
14/11/2018
Marc Verderi - LLR Ecole polytechnique

8. Class : initialization
Class declaration:
class AClass { …
public:
AClass(int);
AClass(double);
AClass(int, int);
private:
int number;
const double pi; };
Class implementation:
#include “AClass.hh”
// could be:
AClass::AClass(int i) {
number = i; }
// or
: number(i)
{;}
// here no choice, as const:
AClass::AClass(double p)
: pi(p)
14/11/2018
Marc Verderi - LLR Ecole polytechnique

9. Class : initialization
Class declaration:
class AClass { …
public:
AClass(int);
AClass(double);
AClass(int, int);
private:
int number;
const double pi;
BClass b; };
With only constructor of BClass being with two int as arguments.
Class implementation:
#include “AClass.hh”
// must construct b in all
// cases:
AClass::AClass(int i)
: number(i), b(0,0)
{;}
// here as well:
AClass::AClass(double p)
: pi(p), b(0,0)
AClass::AClass(int i, int j)
: b(i,j)
Instructions are executed following the ordering passed after the “:”
Note that all members are not initialized properly here ! ;-)
14/11/2018
Marc Verderi - LLR Ecole polytechnique

10. Class : operator overloading
Class declaration:
Syntax for operator overloading cryptic 
class AClass { …
public:
// operator “=”:
AClass& operator =
(const AClass& toCopy);
// remember that all operators
// can be overloaded, even the
// “new” and “delete” ones. };
Not operator overloading:
// Global functions:
AClass operator +
(const AClass&, const AClass&);
At use:
#include “AClass.hh” …
[some main, method…]{
AClass toCopy, objA;
objA = toCopy;
AClass objI, objJ, objK;
objI = objJ + objK;
14/11/2018
Marc Verderi - LLR Ecole polytechnique

11. The main feature that makes a language object oriented
Inheritance
The main feature that makes a language object oriented
14/11/2018
Marc Verderi - LLR Ecole polytechnique

12. the actual behavior is the one of the derived class
Inheritance
You define a base class
(base class, interface, abstract interface, etc…)
Specifying what this class should be able to do
and from there derived classes
Which are of the type of the base class
But which specializes it : ie
the actual behavior is the one of the derived class
Animal:
- locomotion()
- feeding()
Too complicated operations to be described by only a few parameters common to all Animal categories !
HumanBeing:Animal
- locomotion() : walk
- feeding() : everything
Fish:Animal
- locomotion() : swim
- feeding() : fish,insects
Bird:Animal
- locomotion() : fly
- feeding() : insects
14/11/2018
Marc Verderi - LLR Ecole polytechnique

13. Marc Verderi - LLR Ecole polytechnique
Inheritance in Geant4 ?
Used in many places (as it must !)
For example:
Geometry:
G4VSolid:
Abstract interface to describe all geometrical shapes
G4Box, G4Tubs, etc… are derived from G4VSolid
(actually G4VCSGSolid, itself derived from G4VSolid)
Physics:
G4VProcess:
Abstract interface common to all physical processes:
Gamma conversion, multiple scattering, photo-fission, etc…
Sensitivity:
G4VSensitiveDetector, G4VHit, etc…
User interfaces:
Detector construction: G4VUserDetectorConstruction
User actions: G4UserTrackingAction, G4UserSteppingAction,… …
14/11/2018
Marc Verderi - LLR Ecole polytechnique

14. Marc Verderi - LLR Ecole polytechnique
Inheritance in Geant4 ?
Used in many places (as it must !)
For example:
Geometry:
G4VSolid:
Abstract interface to describe all geometrical shapes
G4Box, G4Tubs, etc… are derived from G4VSolid
(actually G4VCSGSolid, itself derived from G4VSolid)
Physics:
G4VProcess:
Abstract interface common to all physical processes:
Gamma conversion, multiple scattering, photo-fission, etc…
Sensitivity:
G4VSensitiveDetector, G4VHit, etc…
User interfaces:
Detector construction: G4VUserDetectorConstruction
User actions: G4UserTrackingAction, G4UserSteppingAction,… …
Navigation in Geant4 never asks:
distanceToBox if solid is a box
distanceToTubs if solid is a tub …
But only:
distanceTo the solid
C++ compiler does the rest:
Use method for box if solid is a box,
Use method for tub, if solid is a tub,
Makes the navigation generic
If a new solid shape is created, it will be accommodated for free, as inheriting from G4VSolid class. X X X X
14/11/2018
Marc Verderi - LLR Ecole polytechnique

15. Marc Verderi - LLR Ecole polytechnique
Inheritance in Geant4 ?
Used in many places (as it must !)
For example:
Geometry:
G4VSolid:
Abstract interface to describe all geometrical shapes
G4Box, G4Tubs, etc… are derived from G4VSolid
(actually G4VCSGSolid, itself derived from G4VSolid)
Physics:
G4VProcess:
Abstract interface common to all physical processes:
Gamma conversion, multiple scattering, photo-fission, etc…
Sensitivity:
G4VSensitiveDetector, G4VHit, etc…
User interfaces:
Detector construction: G4VUserDetectorConstruction
User actions: G4UserTrackingAction, G4UserSteppingAction,… …
14/11/2018
Marc Verderi - LLR Ecole polytechnique

16. Inheritance : the keyword virtual
A class is a « base class/abstract class/… » if at least one of its methods is declared « virtual »
Makes the compiler aware that the code to call is that of the daughter classes
Example in G4UserSteppingAction:
virtual void UserSteppingAction(const G4Step*) {;}
In this case, it has a default implementation…
Which does nothing (but in general, can be more than that)
The class is an « interface »
You can create an object of this type in memory:
G4UserSteppingAction dummySteppingAction;
Example in G4VUserDetectorConstruction:
virtual G4VPhysicalVolume* Construct() = 0;
It is a so-called « pure virtual method »:
It does not propose a default implementation
The class is an « abstract interface »
Creating such an object in memory is not possible
Only pointers on it can be declared:
G4VUserDetectorConstruction* detector;
14/11/2018
Marc Verderi - LLR Ecole polytechnique

17. Syntax for inheritance
Example of detector construction, in example N03
In header file G4VUserDetectorConstruction.hh :
class G4VUserDetectorConstruction { …
public:
virtual G4VPhysicalVolume* Construct() = 0; };
In header file ExN03DectectorConstruction.hh :
class ExN03DectectorConstruction : public G4VUserDetectorConstruction
G4VPhysicalVolume* Construct();
14/11/2018
Marc Verderi - LLR Ecole polytechnique

18. Inheritance: a few more things
Remember about publicity keywords:
“public:” fields are accessible to all
“protected:”
These fields are accessible to daughter classes
“private:”: fields accessible to the class only (and to “friend” classes)
Destructor of a base class is declared virtual
In general, to allow your stuff to be deleted when the destructor of the base class is called
Initialization:
Base class constructor to be called as case “BClass” before
At execution time:
A construction of a daughter class proceeds as follows:
First the constructor of base class is called
Then the constructor of daughter class is called
Automatically 
14/11/2018
Marc Verderi - LLR Ecole polytechnique

19. One word about the technique used for the various “managers” in Geant4
Singleton
One word about the technique used for the various “managers” in Geant4
14/11/2018
Marc Verderi - LLR Ecole polytechnique

20. Marc Verderi - LLR Ecole polytechnique
Singleton
A singleton is a class for which only one instance can be created in memory
By construction
ie: no choice !
Technique very useful to gather informations:
Keep track of all volumes created
Of all particles types declared in the simulation
Etc…
Provide a unique entity to which you can ask
What exist as volumes, or particles, etc…
But also, ask to add or remove a volume, a particle,…
Technique makes use of the keyword static
If in a class declaration a data member is declared static, all objects in memory of that class will share the same data member
14/11/2018
Marc Verderi - LLR Ecole polytechnique

21. G4SDManager* manager = G4SDManager::GetSDMpointer();
Singleton: an example
In G4SDManager.hh:
class G4SDManager {
public:
static G4SDManager* GetSDMpointer(); …
private:
G4SDManager();
static G4SDManager* fSDManager; };
In G4SDManager.cc:
G4SDManager* G4SDManager::fSDManager = 0;
G4SDManager* G4SDManager::GetSDMpointer() {
if (!fSDManager)
fSDManager = new G4SDManager(); }
return fSDManager;
Since constructor is private, only the class can create a G4SDManager
Indeed, I lie: in the code it is actually protected, but this is to make things simplier 
The static pointer (and thus unique) is initialized to zero.
Then, upon first call to GetSDMpointer(), the unique instance is created:
G4SDManager* manager = G4SDManager::GetSDMpointer();
14/11/2018
Marc Verderi - LLR Ecole polytechnique

22. Marc Verderi - LLR Ecole polytechnique
That’s all…
14/11/2018
Marc Verderi - LLR Ecole polytechnique