Introduction

Geant4 is a powerful simulation toolkit used in high‑energy physics, medical physics, and space science. It simulates particle interactions with matter and is designed with Object‑Oriented Programming (OOP) in C++, which makes it modular and extensible. This post introduces the minimum OOP you need to feel comfortable navigating Geant4 classes and patterns.

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();
}

2. Inheritance

Inheritance allows a class to reuse and extend another class.

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

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

int main() {
    Dog d;
    d.Eat();  // Inherited
    d.Bark(); // Defined in Dog
}

In Geant4, you inherit from abstract interfaces such as G4VUserDetectorConstruction and implement your geometry:

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

3. Polymorphism (Virtual Functions)

Polymorphism lets you call derived overrides through base pointers while sharing a common interface.

class Base {
public:
    virtual void Show() { std::cout << "Base class\n"; }
};

class Derived : public Base {
public:
    void Show() override { std::cout << "Derived class\n"; }
};

int main() {
    Base* obj = new Derived();
    obj->Show(); // Output: Derived class
    delete obj;
}

4. Constructors & Initialization Lists

Constructors set up objects; initialization lists are efficient for member initialization.

class Detector {
    int id;
public:
    explicit Detector(int v) : id(v) {}
};

Common in Geant4 action classes:

class MyActionInitialization : public G4VUserActionInitialization {
    MyDetectorConstruction* fDetector;
public:
    explicit MyActionInitialization(MyDetectorConstruction* det)
      : fDetector(det) {}
};