Adding a class to ROOT

You can extend ROOT with your own classes, or rather, you can use your own classes with ROOT.

Using a class in the interpreter (cling or PyROOT)

To load a C++ class definition into the interpreter, it is sufficient to #include the header of the class - and probably load the appropriate library.

root [0] .L libMyClass
root [1] #include "MyClass.h"
root [2] MyClass obj; obj.DoSomething();
import ROOT
ROOT.gInterpreter.Declare('#include "MyClass.h"')
obj = ROOT.MyClass()

This loads on Linux or macOS, while libMyClass.dll is loaded on Windows. On macOS, also libMyClass.dylib is tried.

You can simplify both the C++ and the PyROOT version by adding the following line to the MyClass.h file.


This causes the library to be loaded automatically and allows you to skip the .L / ROOT.gSystem.Load() line.

Storing your class in ROOT files or in a TTree

You want to either create a TTree branch of your class or store an object of your class in a ROOT file.

   MyClass obj;
   auto file = TFile::Open("out.root", "RECREATE");
   file->WriteObject(&obj, "myObj");

For this to work, ROOT needs to know about the MyClass type: its data members, base classes, how to construct such an object when reading it back, etc. This is provided through a dictionary that can be easily generated with CMake.


ROOT’s I/O function requires that each class have one of the following constructors:

  • Default constructor
    A public constructor with zero parameters or with one or more parameters that all have default values: MyClass(int = 42);.

  • I/O constructor
    A constructor with exactly one parameter of type “pointer to ioctortype”.

   struct ioctortype;
   class MyKlass {
     MyClass() = default;    // Protected: Cannot be used by ROOT I/O.

Member initialization

When you set the values of your object, ROOT also sets pointers. If a pointer is non-null, it must assume that the object is pointing to something and will delete object it is pointing to. To avoid a crash here, make sure you initialize your pointers in the constructor used by the I/O - the default constructor or the I/O constructor.

   class MyClass {
      std::string *fStr = nullptr;

The crucial part here is = nullptr, which tells the ROOT I/O that it does not need to delete anything in a freshly constructed MyClass object.