9 The C++ Interpreter Cling

ROOT has a C++ interpreter called cling built in. It is used for the prompt, both C++ and Python. It also serves as a source of information to store C++ objects, and provides the back-end for ROOT’s signal/slot and plug-in mechanisms.

This chapter focuses on the parts of cling that you will encounter while interacting with ROOT.

9.1 The ROOT Prompt

Start up a ROOT session by typing root at the system prompt.

$ root
  | Welcome to ROOT 6.00/00                http://root.cern.ch |
  |                               (c) 1995-2014, The ROOT Team |
  | Built for linuxx8664gcc                                    |
  | From tag v6-00-00, 30 May 2014                             |
  | Try '.help', '.demo', '.license', '.credits', '.quit'/'.q' |

root [0]

Now we create a TLine object:

root [1] TLine l;
root [2] l.Print()
TLine  X1=0.000000 Y1=0.000000 X2=0.000000 Y2=0.000000
root [3] l.SetX1(10)
root [4] l.SetY1(11)
root [5] l.Print()
TLine  X1=10.000000 Y1=11.000000 X2=0.000000 Y2=0.000000
root [6] .g l
.g l
ROOT_prompt_0     1 (address: NA) class TLine l, size = 72
root [7] l.GetX1();
root [8] l.GetX1()
(Double_t) 1.000000e+01

Note some of the features of the ROOT prompt: - Terminating with ‘;‘ is not required, see “C++ Extensions To Ease Scripting” below. - Emacs style command line editing. - Raw interpreter commands start with a dot; .g l for instance shows the interpreter information on the global called l. - To show the result of an expression just do not type the trailing ;.

For the further examples we will “abbreviate” root [0] etc by root [].

root [] .class TLine
class TLine
SIZE: 72 FILE: TLine.h LINE: 39
Base classes: --------------------------------------------------------
0x20       public TAttBBox2D
List of member variables --------------------------------------------------
TLine.h          42 0x28       protected: Double_t fX1
TLine.h          43 0x30       protected: Double_t fY1
TLine.h          44 0x38       protected: Double_t fX2
TLine.h          45 0x40       protected: Double_t fY2
TLine.h          50 0x0      public: enum TLine::<anonymous at /home/axel/build/root/trunk/obj/include/TLine.h:49:4> kLineNDC
TLine.h          51 0x0      public: enum TLine::<anonymous at /home/axel/build/root/trunk/obj/include/TLine.h:49:4> kVertical
TLine.h          52 0x0      public: enum TLine::<anonymous at /home/axel/build/root/trunk/obj/include/TLine.h:49:4> kHorizontal
TLine.h          94 0x0      private: static class TClass *fgIsA
List of member functions :---------------------------------------------------
filename     line:size busy function type and name
(compiled)     (NA):(NA) 0 public: TLine();
(compiled)     (NA):(NA) 0 public: TLine(Double_t x1, Double_t y1, Double_t x2, Double_t y2);
(compiled)     (NA):(NA) 0 public: TLine(const TLine &line);
(compiled)     (NA):(NA) 0 public: virtual ~TLine() noexcept;
(compiled)     (NA):(NA) 0 public: void Copy(class TObject &line) const;
(compiled)     (NA):(NA) 0 public: virtual Int_t DistancetoPrimitive(Int_t px, Int_t py);
(compiled)     (NA):(NA) 0 public: virtual class TLine *DrawLine(Double_t x1, Double_t y1, Double_t x2, Double_t y2);
(compiled)     (NA):(NA) 0 public: virtual class TLine *DrawLineNDC(Double_t x1, Double_t y1, Double_t x2, Double_t y2);
(compiled)     (NA):(NA) 0 public: virtual void ExecuteEvent(Int_t event, Int_t px, Int_t py);
(compiled)     (NA):(NA) 0 public: Double_t GetX1() const;
(compiled)     (NA):(NA) 0 public: Double_t GetX2() const;
(compiled)     (NA):(NA) 0 public: Double_t GetY1() const;
(compiled)     (NA):(NA) 0 public: Double_t GetY2() const;
(compiled)     (NA):(NA) 0 public: Bool_t IsHorizontal();
(compiled)     (NA):(NA) 0 public: Bool_t IsVertical();
(compiled)     (NA):(NA) 0 public: virtual void ls(Option_t *option = "") const;
(compiled)     (NA):(NA) 0 public: virtual void Paint(Option_t *option = "");
(compiled)     (NA):(NA) 0 public: virtual void PaintLine(Double_t x1, Double_t y1, Double_t x2, Double_t y2);
(compiled)     (NA):(NA) 0 public: virtual void PaintLineNDC(Double_t u1, Double_t v1, Double_t u2, Double_t v2);
(compiled)     (NA):(NA) 0 public: virtual void Print(Option_t *option = "") const;
(compiled)     (NA):(NA) 0 public: virtual void SavePrimitive(std::ostream &out, Option_t *option = "");
(compiled)     (NA):(NA) 0 public: virtual void SetNDC(Bool_t isNDC = kTRUE);
(compiled)     (NA):(NA) 0 public: void SetHorizontal(Bool_t set = kTRUE);
(compiled)     (NA):(NA) 0 public: void SetVertical(Bool_t set = kTRUE);
(compiled)     (NA):(NA) 0 public: virtual void SetX1(Double_t x1);
(compiled)     (NA):(NA) 0 public: virtual void SetX2(Double_t x2);
(compiled)     (NA):(NA) 0 public: virtual void SetY1(Double_t y1);
(compiled)     (NA):(NA) 0 public: virtual void SetY2(Double_t y2);
(compiled)     (NA):(NA) 0 public: virtual struct Rectangle_t GetBBox();
(compiled)     (NA):(NA) 0 public: virtual class TPoint GetBBoxCenter();
(compiled)     (NA):(NA) 0 public: virtual void SetBBoxCenter(const class TPoint &p);
(compiled)     (NA):(NA) 0 public: virtual void SetBBoxCenterX(const Int_t x);
(compiled)     (NA):(NA) 0 public: virtual void SetBBoxCenterY(const Int_t y);
(compiled)     (NA):(NA) 0 public: virtual void SetBBoxX1(const Int_t x);
(compiled)     (NA):(NA) 0 public: virtual void SetBBoxX2(const Int_t x);
(compiled)     (NA):(NA) 0 public: virtual void SetBBoxY1(const Int_t y);
(compiled)     (NA):(NA) 0 public: virtual void SetBBoxY2(const Int_t y);
(compiled)     (NA):(NA) 0 public: static class TClass *Class();
(compiled)     (NA):(NA) 0 public: static const char *Class_Name();
(compiled)     (NA):(NA) 0 public: static Version_t Class_Version();
(compiled)     (NA):(NA) 0 public: static void Dictionary();
(compiled)     (NA):(NA) 0 public: virtual class TClass *IsA() const;
(compiled)     (NA):(NA) 0 public: virtual void ShowMembers(class TMemberInspector &insp) const;
(compiled)     (NA):(NA) 0 public: virtual void Streamer(class TBuffer &);
(compiled)     (NA):(NA) 0 public: void StreamerNVirtual(class TBuffer &ClassDef_StreamerNVirtual_b);
(compiled)     (NA):(NA) 0 public: static const char *DeclFileName();
(compiled)     (NA):(NA) 0 public: static int ImplFileLine();
(compiled)     (NA):(NA) 0 public: static const char *ImplFileName();
(compiled)     (NA):(NA) 0 public: static int DeclFileLine();
root [] .> test.log
root [] l.Dump();
root [] .>
root [] ?

Here we see:

Now let us execute a multi-line command:

root [] {
root [] ? TLine l;
root [] ? for (int i = 0; i < 5; i++) {
root [] ?    l.SetX1(i);
root [] ?    l.SetY1(i+1);
root [] ?    l.Print();
root [] ? }
root [] ? }
TLine  X1=0.000000 Y1=1.000000 X2=0.000000 Y2=0.000000
TLine  X1=1.000000 Y1=2.000000 X2=0.000000 Y2=0.000000
TLine  X1=2.000000 Y1=3.000000 X2=0.000000 Y2=0.000000
TLine  X1=3.000000 Y1=4.000000 X2=0.000000 Y2=0.000000
TLine  X1=4.000000 Y1=5.000000 X2=0.000000 Y2=0.000000
root [] .q

Here we note:

9.2 Feeding Sources Files To ROOT: C++ Scripts

ROOT script files (often called “Macros”) contain pure C++ code. They can contain a simple sequence of statements like in the multi command line example given above, but also arbitrarily complex class and function definitions.

The most frequent interaction with the ROOT prompt uses .x to “run” a file:

root [] .x myScript.C

This loads myScript.C into the interpreter and calls the function myScript(). You can pass arguments using .x myScript.C(12, "A String").

Alternatively you can load the script and then run a function explicitly:

root [] .L myScript.C
root [] myScript()

The above is equivalent to .x myScript.C.

In a named script, the objects created on the stack are deleted when the function exits. In a common scenario you create a histogram in a named script on the stack. You draw the histogram, but when the function exits the canvas is empty and the histogram has disappeared. To avoid the histogram from disappearing you can create it on the heap (by using new). This will leave the histogram object intact, but the pointer in the named script scope will be “gone”. Since histograms (and trees) are added to the list of objects in the current directory, you can always retrieve them to delete them if needed.

root[] TH1F *h = (TH1F*)gDirectory->Get("myHist");              // or
root[] TH1F *h = (TH1F*)gDirectory->GetList()->FindObject("myHist");

In addition, histograms and trees are automatically deleted when the current directory is closed. This will automatically take care of the clean up. See “Input/Output”.

9.2.1 Executing a Script From a Script

You may want to execute a script conditionally inside another script. To do it you need to call the interpreter and you can do that with TROOT::ProcessLine(). The example $ROOTSYS/tutorials/tree/cernstaff.C calls a script to build the root file if it does not exist:

void cernstaff() {
   if (gSystem->AccessPathName("cernstaff.root")) {
      gROOT->ProcessLine(".x cernbuild.C");

ProcessLine takes a parameter, which is a pointer to an int or to a TInterpreter::EErrorCode to let you access the interpreter error code after an attempt to interpret. This will contain the error as defined in enum TInterpreter::EErrorCode with TInterpreter::kSuccess being the value for a successful execution.

9.2.2 Executing a Script From the Invocation

Instead if starting ROOT and running a script on the prompt you can also pass it to ROOT in its invocation:

$ root -l -b 'myCode.C("some String", 12)'

The exact kind of quoting depends on your shell; the one shown here works for bash-like shells.

ROOT can evaluate any expression as part of the invocation; another version of the previous example can be spelled like this:

$ root -l -b -e 'gROOT->ProcessLine(".x myCode.C(\"some String\", 12)");'

9.3 C++ Extensions To Ease Scripting

In the next example, we demonstrate three of the most important extensions ROOT and Cling make to C++. Start ROOT in the directory $ROOTSYS/tutorials (make sure to have first run .x hsimple.C):

root [0] f = new TFile("hsimple.root")
(class TFile *) 0x4045e690
root [1] f->ls()
TFile**         hsimple.root
TFile*         hsimple.root
KEY: TH1F     hpx;1   This is the px distribution
KEY: TH2F     hpxpy;1 py ps px
KEY: THProfile        hprof;1 Profile of pz versus px
KEY: TNtuple  ntuple;1        Demo ntuple
root [2] hpx->Draw()
Warning in <MakeDefCanvas>: creating a default canvas with name c1
root [3] .q

The root [0] command shows the first extension; the declaration of f may be omitted as a shortcut for auto. Cling will correctly create f as pointer to object of class TFile. Nonetheless we recommend to use auto f = new TFile("hsimple.root").

The second extension is more important. In case Cling cannot find an object being referenced, it will ask ROOT to search for an object with an identical name in the search path defined by TROOT::FindObject(). If ROOT finds the object, it returns a pointer to this object to Cling and a pointer to its class definition and Cling will execute the requested member function. This shortcut is quite natural for an interactive system and saves much typing. In this example, ROOT searches for hpx and finds it in hsimple.root.

The next, fundamental extension is shown below. There is no need to put a semicolon at the end of a line. When you leave it off the value of the expression will be printed on the next line. For example:

root[] 23+5
root[] 23+5;
root[] TMath::Sin
(Double_t (*)(Double_t)) Function @0x7ffff7ebb090
  at include/TMath.h:418:
inline Double_t TMath::Sin(Double_t x)
   { return sin(x); }

Be aware that these extensions do not work when a compiler replaces the interpreter. Your code will not compile, hence when writing large scripts, it is best to stay away from these shortcuts. It will save you from having problems compiling your scripts using a real C++ compiler.

9.4 ACLiC: Compiling Scripts Into Libraries

Instead of having Cling interpret your script there is a way to have your scripts compiled, linked and dynamically loaded using the C++ compiler and linker. The advantage of this is that your scripts will run with the speed of compiled C++ and that you can use language constructs that are not fully supported by Cling. On the other hand, you cannot use any Cling shortcuts (see “C++ Extensions To Ease Scripting” above) and for small scripts, the overhead of the compile/link cycle might be larger than just executing the script in the interpreter.

ACLiC will build a dictionary and a shared library from your C++ script, using the compiler and the compiler options that were used to compile the ROOT executable. You do not have to write a Makefile remembering the correct compiler options, and you do not have to exit ROOT.

9.4.1 Usage

Before you can compile your interpreted script you need to add include statements for the classes used in the script. Once you did that, you can build and load a shared library containing your script. To load it use the command .L and append the file name with a +.

root[] .L MyScript.C+

The + option generates the shared library and names it by taking the name of the file “filename” but replacing the dot before the extension by an underscore and by adding the shared library extension for the current platform. For example on most platforms, hsimple.cxx will generate hsimple_cxx.so.

The + command rebuild the library only if the script or any of the files it includes are newer than the library. When checking the timestamp, ACLiC generates a dependency file which name is the same as the library name, just replacing the ‘so’ extension by the extension ‘d’. For example on most platforms, hsimple.cxx will generate hsimple_cxx.d.

To ensure that the shared library is rebuilt you can use the ++ syntax:

root[] .L MyScript.C++

To build, load, and execute the function with the same name as the file you can use the .x command. This is the same as executing a named script; you can also provide parameters. The only difference is you need to append a + or a ++.

root[] .x MyScript.C+(4000)
Creating shared library /home/./MyScript_C.so

You can select whether the script in compiled with debug symbol or with optimization by appending the letter ‘g’ or ‘O’ after the ‘+’ or ‘++’. Without the specification, the script is compiled with the same level of debugging symbol and optimization as the currently running ROOT executable. For example:

root[] .L MyScript.C++g

will compile MyScript.C with debug symbols; usually this means giving the -g option to compiler.

root[] .L MyScript.C++O

will compile MyScript.C with optimizations; usually this means giving the -O option to compiler. The syntax:

root[] .L MyScript.C++

is using the default optimization level. The initial default is to compile with the same level of optimization as the root executable itself. The default can be changed by:

root[] gSystem->SetAclicMode(TSystem::kDebug);
root[] gSystem->SetAclicMode(TSystem::kOpt);

Note that the commands:

root[] .L MyScript.C+g
root[] .L MyScript.C+O

respectively compile MyScript.C with debug and optimization if the library does not exist yet; they will not change the debug and the optimization level if the library already exist and it is up to date. To use ACLiC from compiled code or from inside another macro, we recommend using gROOT->ProcessLine(). For example, in one script you can use ACLiC to compile and load another script.

gROOT->ProcessLine(".L MyScript.C+")
gROOT->ProcessLine(".L MyScript.C++")

9.4.2 Setting the Include Path

You can get the include path by typing:

root[] .include

You can append to the include path by typing:

root[] .include $HOME/mypackage/include

In a script you can append to the include path:

gSystem->AddIncludePath(" -I$HOME/mypackage/include ")

You can also overwrite the existing include path:

gSystem->SetIncludePath(" -I$HOME/mypackage/include ")

The $ROOTSYS/include directory is automatically appended to the include path, so you do not have to worry about including it. To add library that should be used during linking of the shared library use something like:

gSystem->AddLinkedLibs("-L/my/path -lanylib");

This is especially useful for static libraries. For shared ones you can also simply load them before trying to compile the script:


ACLiC uses the directive fMakeSharedLibs to create the shared library. If loading the shared library fails, it tries to output a list of missing symbols by creating an executable (on some platforms like OSF, this does not HAVE to be an executable) containing the script. It uses the directive fMakeExe to do so. For both directives, before passing them to TSystem::Exec(), it expands the variables $SourceFiles, $SharedLib, $LibName, $IncludePath, $LinkedLibs, $ExeNameand$ObjectFiles. See SetMakeSharedLib() for more information on those variables. When the file being passed to ACLiC is on a read only file system, ACLiC warns the user and creates the library in a temporary directory:

root[] .L readonly/t.C++
Warning in <ACLiC>: /scratch/aclic/subs/./readonly is not writable!
Warning in <ACLiC>: Output will be written to /tmp
Info in <TUnixSystem::ACLiC>: creating shared library

To select the temporary directory ACLiC looks at $TEMP, $TEMP_DIR, $TEMPDIR, $TMP, $TMPDIR, $TMP_DIRor uses /tmp (or C:/). Also, a new interface TSystem::Get/SetBuildDir is introduced to let users select an alternative ‘root’ for building of the ACLiC libraries. For filename/full/path/name/macro.C, the library is created as fBuildDir/full/path/name/macro_C.so.

9.4.3 Dictionary Generation

You can direct what is added to the dictionary generated by ACLiC in two ways. The simplest way is to add at the end of script (i.e. after the symbols have been defined) something like:

#if defined(__ROOTCLING__)
#pragma link C++ class MyOtherClass;

You can also write this portion of code in a file name MyScript_linkdef.h where the suffix '_linkdef'is the prefix defined by the key ‘ACLiC.Linkdef‘ in the currently used resource file (usually .rootrcor$ROOTSYS/etc/system.rootrc) and the prefix is the name of your script.

The default behavior of rootcling is to not link in (i.e. generate the dictionary for) any of the symbols. In particular, this means that the following lines are, in the general case, unnecessary.

#pragma link off all globals;
#pragma link off all classes;
#pragma link off all functions;

This also means that linking the instantiation of a class template:

#pragma link C++ class mytemplate<int>;

ONLY links this specific class. You need to request the generation of the iterators explicitly.

See the documentation of rootcling for details how pragma can be used.

NOTE: You should not call ACLiC with a script that has a function called main().

9.4.4 Intermediate Steps and Files

ACLiC executes two steps and a third one if needed. These are:

ACLiC makes a shared library with a dictionary containing the classes and functions declared in the script. It also adds the classes and functions declared in included files with the same name as the script file and any of the following extensions: .h, .hh, .hpp, .hxx, .hPP, .hXX. This means that, by default, you cannot combine scripts from different files into one library by using #include statements; you will need to compile each script separately. In a future release, we plan to add the global variables declared in the script to the dictionary also. If you are curious about the specific calls, you can raise the ROOT debug level: gDebug=3 and ACLiC will print these steps. If you need to keep the intermediate files around, for example when debugging the script using gdb, use gDebug=7.

9.4.5 Moving between Interpreter and Compiler

The best way to develop portable scripts is to make sure you can always run them with both, the interpreter and with ACLiC. To do so, do not use the Cling extensions and program around the Cling limitations. When it is not possible or desirable to program around the Cling limitations, you can use the C preprocessor symbols defined for Cling and rootcling.

The preprocessor symbol __CLING__ is defined for both ROOT and rootcling. The symbol __ROOTCLING__ (and __MAKECINT__ for backward compatibility) is only defined in rootcling.

Use !defined(__CLING__) || defined(__ROOTCLING__) to bracket code that needs to be seen by the compiler and rootcling, but will be invisible to the interpreter.

Use !defined(__CLING__)to bracket code that should be seen only by the compiler and not by Cling nor rootcling.For example, the following will hide the declaration and initialization of the array gArray from both Cling and rootcling.

#if !defined(__CLING__)
int gArray[] = { 2, 3, 4};

Because ACLiC calls rootcling to build a dictionary, the declaration of gArray will not be included in the dictionary, and consequently, gArray will not be available at the command line even if ACLiC is used. Cling and rootcling will ignore all statements between the "#if !defined (__CLING__)" and “#endif". If you want to use gArray in the same script as its declaration, you can do so. However, if you want use the script in the interpreter you have to bracket the usage of gArray between #if's, since the definition is not visible. If you add the following preprocessor statements:

#if !defined(__CLING__)
int gArray[] = { 2, 3, 4};
#elif defined(__ROOTCLING__)
int gArray[];

gArray will be visible to rootcling but still not visible to Cling. If you use ACLiC, gArray will be available at the command line and be initialized properly by the compiled code.

We recommend you always write scripts with the needed include statements. In most cases, the script will still run with the interpreter. However, a few header files are not handled very well by Cling.

These types of headers can be included in interpreted and compiled mode:

Hiding header files from rootcling that are necessary for the compiler but optional for the interpreter can lead to a subtle but fatal error. For example:

#ifndef __CLING__
#include "TTree.h"
class TTree;

class subTree : public TTree {

In this case, rootcling does not have enough information about the TTree class to produce the correct dictionary file. If you try this, rootcling and compiling will be error free, however, instantiating a subTree object from the Cling command line will cause a fatal error. In general, it is recommended to let rootcling see as many header files as possible.

9.5 Classes Defined By Scripts

Lets create a small class TMyClass and a derived class TChild. The virtual method TMyClass::Print()is overridden in TChild. Save this in file called script4.C.

#include <iostream.h>

class TMyClass {
      float   fX;     //x position in centimeters
      float   fY;     //y position in centimeters
      TMyClass() { fX = fY = -1; }
      virtual void Print() const;
      void SetX(float x) { fX = x; }
      void SetY(float y) { fY = y; }
void TMyClass::Print() const  // parent print method
   cout << "fX = " << fX << ", fY = " << fY << endl;
class TChild : public TMyClass {
      void Print() const;
void TChild::Print() const  // child print metod
   cout << "This is TChild::Print()" << endl;

To execute script4.C do:

root[] .L script4.C
root[] TMyClass *a = new TChild
root[] a->Print()
This is TChild::Print()
fX = -1, fY = -1
root[] a->SetX(10)
root[] a->SetY(12)
root[] a->Print()
This is TChild::Print()
fX = 10, fY = 12
root[] .class TMyClass
class TMyClass
size=0x8 FILE:script4.C LINE:3
List of base class-----------------------------------
List of member variable------------------------------
Defined in TMyClass
0x0        private: float fX
0x4        private: float fY
List of member function------------------------------
Defined in TMyClass
filename       line:size busy function type and name
script4.C         16:5    0 public: class TMyClass TMyClass(void);
script4.C         22:4    0 public: void Print(void);
script4.C         12:1    0 public: void SetX(float x);
script4.C         13:1    0 public: void SetY(float y);
root[] .q

As you can see, an interpreted class behaves just like a compiled class. See “Adding a Class” for ways how to add a class with a shared library and with ACLiC.

9.6 Inspecting Objects

An object of a class inheriting from TObject can be inspected, with the Inspect() method. The TObject::Inspect method creates a window listing the current values of the objects members. For example, the next picture is of TFile.

root[] TFile f("staff.root")
root[] f.Inspect()

You can see the pointers are in red and can be clicked on to follow the pointer to the object. If you clicked on fList, the list of objects in memory and there were none, no new canvas would be shown. On top of the page are the navigation buttons to see the previous and next screen.

ROOT object inspector of TFile

ROOT object inspector of TFile

The object inspector of fKeys, the list of keys in the memory

The object inspector of fKeys, the list of keys in the memory