C for Geant4 users

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

2
Introduction

• Geant4 users 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
• Inheritance
• Singleton

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Class
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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 stdvectorltTgt, when increasing vector
capacity
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Class method arguments

• Class declaration
• class AClass
• public
• int fun1(int i)
• // default argument
• double fun2(int n0)
• // argument type
• double fun2(double)
• // !! Ambiguity here !!
• double fun2(double w3.14)
• // !! 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 n0 or w3.14 ?
• double v objA.fun2()

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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

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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()

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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
• AClassAClass(int i)
• number i
• // or
• AClassAClass(int i)
• number(i)
• // here no choice, as const
• AClassAClass(double p)
• pi(p)

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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
• AClassAClass(int i)
• number(i), b(0,0)
• // here as well
• AClassAClass(double p)
• pi(p), b(0,0)
• AClassAClass(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 ! -)
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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

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Inheritance

• The main feature that makes a language object
  oriented

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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 !
HumanBeingAnimal - locomotion() walk -
feeding() everything
FishAnimal - locomotion() swim -
feeding() fish,insects
BirdAnimal - locomotion() fly -
feeding() insects
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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,

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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
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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,

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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

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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
• public
• G4VPhysicalVolume Construct()

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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

?
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Singleton

• One word about the technique used for the various
  managers in Geant4

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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

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Singleton an example

• In G4SDManager.hh
• class G4SDManager
• public
• static G4SDManager GetSDMpointer()
• private
• G4SDManager()
• private
• static G4SDManager fSDManager

• In G4SDManager.cc
• G4SDManager G4SDManagerfSDManager 0
• G4SDManager G4SDManagerGetSDMpointer()
• 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 G4SDManagerGetSDMpointer(
  )

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Thats all