Introduction

Geant4 is a powerful simulation toolkit widely used in high-energy physics, medical physics, and space science. It allows users to simulate the interactions of particles with matter. The toolkit is designed using Object-Oriented Programming (OOP) in C++, making it modular, reusable, and scalable.

To effectively use Geant4, understanding OOP is essential. OOP helps organize code into structured components (objects), making it easier to model detector geometries, particle interactions, and physics processes.

This blog introduces key OOP concepts in C++ and how they are applied in Geant4, helping beginners develop well-structured and maintainable simulation code.

Key OOP Concepts in C++ Used in Geant4

1. Classes and Objects

A class is a blueprint for creating objects. Objects are instances of a class.

class Detector
{
public:
    void Construct();  // Method to construct the detector
};

int main()
{
    Detector myDetector;  // Create an object of Detector
    myDetector.Construct();  // Call the method
}
      

2. Inheritance

Inheritance allows a class to inherit properties and methods from another class.

class Animal  // Base class
{
public:
    void Eat() { std::cout << "Eating..." << std::endl; }
};

class Dog : public Animal  // Derived class
{
public:
    void Bark() { std::cout << "Barking..." << std::endl; }
};

int main()
{
    Dog myDog;
    myDog.Eat();  // Inherited from Animal
    myDog.Bark(); // Defined in Dog
}
      

In Geant4:

• G4VUserDetectorConstruction is an abstract base class.

• You inherit from it and define your own detector in MyDetectorConstruction.

class MyDetectorConstruction : public G4VUserDetectorConstruction
{
public:
    G4VPhysicalVolume* Construct() override;
};
      

3. Polymorphism (Virtual Functions)

Polymorphism allows derived classes to override functions from the base class while keeping a common interface.

class Base
{
public:
    virtual void Show() { std::cout << "Base class" << std::endl; }  // Virtual function
};

class Derived : public Base
{
public:
    void Show() override { std::cout << "Derived class" << std::endl; } // Overriding
};

int main()
{
    Base* obj = new Derived(); // Pointer to base class, but calling derived class method
    obj->Show();  // Output: Derived class
}
      

4. Constructors & Initialization Lists

Constructors initialize objects, and initialization lists help optimize member initialization.

class Detector
{
private:
    int id;
public:
    Detector(int val) : id(val) {}  // Initialization list
};
      

In Geant4, you will see initialization lists in constructors:

class MyActionInitialization : public G4VUserActionInitialization
{
private:
    MyDetectorConstruction* fDetector;

public:
    MyActionInitialization(MyDetectorConstruction* detector) : fDetector(detector) {}
};