Generating dictionaries

Generating dictionaries

A dictionary (“reflection database”) contains information about the types and functions that are available in a library.

With a dictionary you can call functions inside libraries. Dictionaries are also needed to write a class into a ROOT file (→ see ROOT files).

A dictionary consists of a source file, which contains the type information needed by Cling and ROOT’s I/O subsystem. This source file needs to be generated from the library’s headers and then compiled, linked and loaded. Only then does Cling and ROOT know what is inside a library.

There are two ways to generate a dictionary:

  • using ACLiC

  • using rootcling

Using ACLiC to generate dictionaries

With a given header file MyHeader.h, ACliC automatically generates a dictionary:

      root[] .L MyHeader.h+

Using rootcling to generate dictionaries manually

You can manually create a dictionary by using rootcling:

   rootcling -f DictOutput.cxx -c OPTIONS Header1.h Header2.h ... Linkdef.h
  • DictOutput.cxx Specifies the output file that will contain the dictionary. It will be accompanied by a header file DictOutput.h.

  • OPTIONS are:

    • Isomething: Adding an include path, so that rootcling can find the files included in Header1.h, Header2.h, etc.

    • DSOMETHING: Define a preprocessor macro, which is sometimes needed to parse the header files.

  • Header1.h Header2.h...: The headers files.

  • Linkdef.h: Tells rootcling, which classes should be added to the dictionary, → see Selecting dictionary entries: Linkdef.h.


Dictionaries that are used within the same project must have unique names.

Compiled object files relative to dictionary source files cannot reside in the same library or in two libraries loaded by the same application if the original source files have the same name. Example

In the first step, a TEvent and a TTrack class is defined. Next an event object is created to add tracks to it. The track objects have a pointer to their event. This shows that the I/O system correctly handles circular references.

In the second step, a TEvent and a TTrack call are implemented.
After that you can use rootcling to manually generate a directory. This generates the eventdict.cxx file.

The TEvent.h header

#ifndef __TEvent__
#define __TEvent__
#include "TObject.h"
class TCollection;
class TTrack;

class TEvent : public TObject {
   Int_t fId; // Event sequential id
   Float_t fTotalMom;       // Total momentum.
   TCollection *fTracks;    // Collection of tracks.
   TEvent() { fId = 0; fTracks = 0; }
   TEvent(Int_t id);
   void AddTrack(TTrack *t);
   Int_t GetId() const { return fId; }
   Int_t GetNoTracks() const;
   void Print(Option_t *opt="");
   Float_t TotalMomentum();
   ClassDef(TEvent,1);     //Simple event class.

The TTrack.h header

#ifndef __TTrack __
#define __TTrack__
#include "TObject.h"

class TEvent;
class TTrack : public TObject {
   Int_t fId;       // Track sequential id.
   TEvent *fEvent;  // TEvent to which track belongs.
   Float_t fPx;     // x part of track momentum.
   Float_t fPy;     // y part of track momentum.
   Float_t fPz;     // z part of track momentum.
   TTrack() { fId = 0; fEvent = 0; fPx = fPy = fPz = 0; }
   TTrack(Int_t id, Event *ev, Float_t px,Float_t py,Float_t pz);
   Float_t Momentum() const;
   TEvent *GetEvent() const { return fEvent; }
   void Print(Option_t *opt="");
   ClassDef (TTrack,1);    //Simple track class.

Implementation of TEvent and TTrack class

#include <iostream.h>
#include "TOrdCollection.h"
#include "TEvent.h"
#include "TTrack.h"

#include <iostream.h>
#include "TMath.h"
#include "Track.h"
#include "Event.h"

Using rootcling to generate the dictionary

rootcling eventdict.cxx -c TEvent.h TTrack.h

eventdict.cxx - the generated dictionary

void TEvent::Streamer(TBuffer &R__b) {

// Stream an object of class TEvent.
   if (R__b.IsReading()) {
      Version_t R__v = R__b.ReadVersion();
      R__b >> fId;
      R__b >> fTotalMom;
      R__b >> fTracks;
   } else {
      R__b << fId;
      R__b << fTotalMom;
      R__b << fTracks;

Selecting dictionary entries: Linkdef.h

To select which types and functions should go into a dictionary, create a Linkdef.h file that you use when you call rootcint manually. The Linkdef.h file is passed as the last argument to rootcint. It must end on Linkdef.h, LinkDef.h, or linkdef.h. For example, My_Linkdef.h is correct, Linkdef_mine.h is not.

The Linkdef.h file contains directives for rootcint, for what a dictionary should be created: select the types and functions that will be accessible from the prompt (or in general through CINT) and for I/O.

Preamble: deselection

A Linkdef.h file starts with the following preamble:

#ifdef __CINT__
#pragma link off all globals;
#pragma link off all classes;
#pragma link off all functions;
#pragma link C++ nestedclasses;

The first line protects the compiler from seeing the rootcint directives. The rootcint directives are in the form of #pragma statements. A #pragma link of all something says that by default, rootcint should not generate the dictionary for anything it sees.

The nested classes directive tells rootcint not to ignore nestedclasses, this is, classes defined inside classes like here:

class Outer {
  class Inner {
    // we want a dictionary for this one, too!


In the next step, tell rootcint for which objects the dictionary should be generated for:

#pragma link C++ class AliEvent+;
#pragma link C++ function StrDup;
#pragma link C++ function operator+(const TString&amp;,const TString&amp;);
#pragma link C++ global gROOT;
#pragma link C++ global gEnv;
#pragma link C++ enum EMessageTypes;


**The + after the class name: This enables an essential feature for rootcint. It is not a default setting, so you must add +at the end.

Selection by file name

Sometimes it is easier to say: Create a dictionary for everything defined in the MyHeader.h file.
Write the following statement into the Linkdef.h file:

#pragma link C++ defined_in "subdir/MyHeader.h";

Make sure that subdir/MyHeader.h corresponds to one of the header files that is passed to rootcint.


Add the following line at the end of the Linkdef.h file:

#endif /* __CINT__ */.

Example of a Linkdef.h file

#ifdef __CINT__
#pragma link off all globals;
#pragma link off all classes;
#pragma link off all functions;
#pragma link C++ nestedclasses;
#pragma link C++ global gHtml;
#pragma link C++ class THtml;

Embedding the rootcling call into a GNU Makefile

Use the following statement to compile and run the code.

.L MyCode.C+

If you need to use a Makefile, there is the following rule for generating a dictionary (see code snippet below). It will create a new source file, which you should compile like all the other sources in your library. In addition, you need to add the include path for ROOT, and you might have to link against ROOT’s libraries: libCore.

This rule generates the rootcling dictionary for the headers $(HEADERS) and a library containing the dictionary and the compiled $(SOURCES):

MyDict.cxx: $(HEADERS) Linkdef.h
[TAB]     rootcling -f $@ -c $(CXXFLAGS) -p $^ MyDict.cxx $(SOURCES)
[TAB]     g++ -shared -o$@ `root-config --ldflags` $(CXXFLAGS) -I$(ROOTSYS)/include $^

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.


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.