1. Some words on classes

Code in ROOT / AliROOT / AliPhysics has the form of classes, generally one class per file

• Our analysis task will also be a c++ class
• A class contains both variables and functions (called `methods') as members
• Often the member variables must be accessed through the methods
• A bit abstract, but not so difficult when you see it

class Rectangle
{
    private :
        int width , height ;
    public :
        Rectangle (int , int );
        int GetArea () { return height * width ;}
}

Classes are nice, because they can be derived from one-another:

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• CRectangle is derived from base class CPolyogon, and inherits its members

This avoids having to repeat common code for all classes, but can sometimes make understanding the code a bit tricky. Have a look at the following code snippets:

class Polygon
{
    private :
        int width , height ;
}

class Rectangle : public Polygon
{
    public :
        Rectangle (int , int );
        int GetArea () { return height * width ;}
}

class Triangle : public Polygon
{
    public :
        Triangle (int , int );
        int GetArea () { return ( height * width ) /2;}
}

many of the functions we'll use, are inherited from an analysis base class!