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TTreeReader Class Reference

TTreeReader is a simple, robust and fast interface to read values from a TTree, TChain or TNtuple.

It uses TTreeReaderValue<T> and TTreeReaderArray<T> to access the data.

Example code can be found in tutorials/tree/hsimpleReader.C and tutorials/trees/h1analysisTreeReader.h and tutorials/trees/h1analysisTreeReader.C for a TSelector.

Roottest contains an example showing the full power.

A simpler analysis example - the one from the tutorials - can be found below: it histograms a function of the px and py branches.

// A simple TTreeReader use: read data from hsimple.root (written by hsimple.C)
#include "TFile.h
#include "TH1F.h
#include "TTreeReader.h
#include "TTreeReaderValue.h
void hsimpleReader() {
// Create a histogram for the values we read.
TH1F("h1", "ntuple", 100, -4, 4);
// Open the file containing the tree.
TFile *myFile = TFile::Open("$ROOTSYS/tutorials/hsimple.root");
// Create a TTreeReader for the tree, for instance by passing the
// TTree's name and the TDirectory / TFile it is in.
TTreeReader myReader("ntuple", myFile);
// The branch "px" contains floats; access them as myPx.
TTreeReaderValue<Float_t> myPx(myReader, "px");
// The branch "py" contains floats, too; access those as myPy.
TTreeReaderValue<Float_t> myPy(myReader, "py");
// Loop over all entries of the TTree or TChain.
while (myReader.Next()) {
// Just access the data as if myPx and myPy were iterators (note the '*'
// in front of them):
myHist->Fill(*myPx + *myPy);

A more complete example including error handling and a few combinations of TTreeReaderValue and TTreeReaderArray would look like this:

#include <TFile.h>
#include <TH1.h>
#include <TTreeReader.h>
#include "TriggerInfo.h"
#include "Muon.h"
#include "Tau.h"
#include <vector>
#include <iostream>
if (value->GetSetupStatus() < 0) {
std::cerr << "Error " << value->GetSetupStatus()
<< "setting up reader for " << value->GetBranchName() << '\n';
return false;
return true;
// Analyze the tree "MyTree" in the file passed into the function.
// Returns false in case of errors.
bool analyze(TFile* file) {
// Create a TTreeReader named "MyTree" from the given TDirectory.
// The TTreeReader gives access to the TTree to the TTreeReaderValue and
// TTreeReaderArray objects. It knows the current entry number and knows
// how to iterate through the TTree.
TTreeReader reader("MyTree", file);
// Read a single float value in each tree entries:
TTreeReaderValue<float> weight(reader, "event.weight");
if (!CheckValue(weight)) return false;
// Read a TriggerInfo object from the tree entries:
TTreeReaderValue<TriggerInfo> triggerInfo(reader, "triggerInfo");
if (!CheckValue(triggerInfo)) return false;
//Read a vector of Muon objects from the tree entries:
if (!CheckValue(muons)) return false;
//Read the pT for all jets in the tree entry:
TTreeReaderArray<double> jetPt(reader, "jets.pT");
if (!CheckValue(jetPt)) return false;
// Read the taus in the tree entry:
if (!CheckValue(taus)) return false;
// Now iterate through the TTree entries and fill a histogram.
TH1F("hist", "TTreeReader example histogram", 10, 0., 100.);
while (reader.Next()) {
// Access the TriggerInfo object as if it's a pointer.
if (!triggerInfo->hasMuonL1())
// Ditto for the vector<Muon>.
if (!muons->size())
// Access the jetPt as an array, whether the TTree stores this as
// a std::vector, std::list, TClonesArray or Jet* C-style array, with
// fixed or variable array size.
if (jetPt.GetSize() < 2 || jetPt[0] < 100)
// Access the array of taus.
if (!taus.IsEmpty()) {
// Access a float value - need to dereference as TTreeReaderValue
// behaves like an iterator
float currentWeight = *weight;
for (const Tau& tau: taus) {
hist->Fill(tau.eta(), currentWeight);
} // TTree entry / event loop

Definition at line 48 of file TTreeReader.h.


class  Iterator_t
 Iterate through the entries of a TTree. More...

Public Types

enum  EEntryStatus {
  kEntryValid = 0, kEntryNotLoaded, kEntryNoTree, kEntryNotFound,
  kEntryChainSetupError, kEntryChainFileError, kEntryDictionaryError, kEntryLast
typedef Iterator_t iterator
- Public Types inherited from TObject
enum  { kIsOnHeap = 0x01000000, kNotDeleted = 0x02000000, kZombie = 0x04000000, kBitMask = 0x00ffffff }
enum  { kSingleKey = BIT(0), kOverwrite = BIT(1), kWriteDelete = BIT(2) }
enum  EStatusBits {
  kCanDelete = BIT(0), kMustCleanup = BIT(3), kObjInCanvas = BIT(3), kIsReferenced = BIT(4),
  kHasUUID = BIT(5), kCannotPick = BIT(6), kNoContextMenu = BIT(8), kInvalidObject = BIT(13)

Public Member Functions

 TTreeReader ()
 TTreeReader (TTree *tree)
 Access data from tree. More...
 TTreeReader (const char *keyname, TDirectory *dir=NULL)
 Access data from the tree called keyname in the directory (e.g. More...
 ~TTreeReader ()
 Tell all value readers that the tree reader does not exist anymore. More...
Iterator_t begin ()
 Return an iterator to the 0th TTree entry. More...
Iterator_t end () const
Long64_t GetCurrentEntry () const
 Returns the index of the current entry being read. More...
Long64_t GetEntries (Bool_t force) const
EEntryStatus GetEntryStatus () const
TTreeGetTree () const
Bool_t IsChain () const
Bool_t Next ()
void Restart ()
 Restart a Next() loop from the first entry. More...
void SetChain (const char *, TFileCollection *)
EEntryStatus SetEntriesRange (Long64_t beginEntry, Long64_t endEntry)
 Set the range of entries to be processed. More...
EEntryStatus SetEntry (Long64_t entry)
void SetLastEntry (Long64_t entry)
EEntryStatus SetLocalEntry (Long64_t entry)
void SetTree (TTree *tree)
 Set (or update) the which tree to reader from. More...
void SetTree (const char *, TDirectory *)
- Public Member Functions inherited from TObject
 TObject ()
 TObject constructor. More...
 TObject (const TObject &object)
 TObject copy ctor. More...
virtual ~TObject ()
 TObject destructor. More...
void AbstractMethod (const char *method) const
 Use this method to implement an "abstract" method that you don't want to leave purely abstract. More...
virtual void AppendPad (Option_t *option="")
 Append graphics object to current pad. More...
virtual void Browse (TBrowser *b)
 Browse object. May be overridden for another default action. More...
virtual const char * ClassName () const
 Returns name of class to which the object belongs. More...
virtual void Clear (Option_t *="")
virtual TObjectClone (const char *newname="") const
 Make a clone of an object using the Streamer facility. More...
virtual Int_t Compare (const TObject *obj) const
 Compare abstract method. More...
virtual void Copy (TObject &object) const
 Copy this to obj. More...
virtual void Delete (Option_t *option="")
 Delete this object. More...
virtual Int_t DistancetoPrimitive (Int_t px, Int_t py)
 Computes distance from point (px,py) to the object. More...
virtual void Draw (Option_t *option="")
 Default Draw method for all objects. More...
virtual void DrawClass () const
 Draw class inheritance tree of the class to which this object belongs. More...
virtual TObjectDrawClone (Option_t *option="") const
 Draw a clone of this object in the current pad. More...
virtual void Dump () const
 Dump contents of object on stdout. More...
virtual void Error (const char *method, const char *msgfmt,...) const
 Issue error message. More...
virtual void Execute (const char *method, const char *params, Int_t *error=0)
 Execute method on this object with the given parameter string, e.g. More...
virtual void Execute (TMethod *method, TObjArray *params, Int_t *error=0)
 Execute method on this object with parameters stored in the TObjArray. More...
virtual void ExecuteEvent (Int_t event, Int_t px, Int_t py)
 Execute action corresponding to an event at (px,py). More...
virtual void Fatal (const char *method, const char *msgfmt,...) const
 Issue fatal error message. More...
virtual TObjectFindObject (const char *name) const
 Must be redefined in derived classes. More...
virtual TObjectFindObject (const TObject *obj) const
 Must be redefined in derived classes. More...
virtual Option_tGetDrawOption () const
 Get option used by the graphics system to draw this object. More...
virtual const char * GetIconName () const
 Returns mime type name of object. More...
virtual const char * GetName () const
 Returns name of object. More...
virtual char * GetObjectInfo (Int_t px, Int_t py) const
 Returns string containing info about the object at position (px,py). More...
virtual Option_tGetOption () const
virtual const char * GetTitle () const
 Returns title of object. More...
virtual UInt_t GetUniqueID () const
 Return the unique object id. More...
virtual Bool_t HandleTimer (TTimer *timer)
 Execute action in response of a timer timing out. More...
virtual ULong_t Hash () const
 Return hash value for this object. More...
virtual void Info (const char *method, const char *msgfmt,...) const
 Issue info message. More...
virtual Bool_t InheritsFrom (const char *classname) const
 Returns kTRUE if object inherits from class "classname". More...
virtual Bool_t InheritsFrom (const TClass *cl) const
 Returns kTRUE if object inherits from TClass cl. More...
virtual void Inspect () const
 Dump contents of this object in a graphics canvas. More...
void InvertBit (UInt_t f)
virtual Bool_t IsEqual (const TObject *obj) const
 Default equal comparison (objects are equal if they have the same address in memory). More...
virtual Bool_t IsFolder () const
 Returns kTRUE in case object contains browsable objects (like containers or lists of other objects). More...
Bool_t IsOnHeap () const
virtual Bool_t IsSortable () const
Bool_t IsZombie () const
virtual void ls (Option_t *option="") const
 The ls function lists the contents of a class on stdout. More...
void MayNotUse (const char *method) const
 Use this method to signal that a method (defined in a base class) may not be called in a derived class (in principle against good design since a child class should not provide less functionality than its parent, however, sometimes it is necessary). More...
virtual Bool_t Notify ()
 This method must be overridden to handle object notification. More...
void Obsolete (const char *method, const char *asOfVers, const char *removedFromVers) const
 Use this method to declare a method obsolete. More...
void operator delete (void *ptr)
 Operator delete. More...
void operator delete[] (void *ptr)
 Operator delete []. More...
voidoperator new (size_t sz)
voidoperator new (size_t sz, void *vp)
voidoperator new[] (size_t sz)
voidoperator new[] (size_t sz, void *vp)
TObjectoperator= (const TObject &rhs)
 TObject assignment operator. More...
virtual void Paint (Option_t *option="")
 This method must be overridden if a class wants to paint itself. More...
virtual void Pop ()
 Pop on object drawn in a pad to the top of the display list. More...
virtual void Print (Option_t *option="") const
 This method must be overridden when a class wants to print itself. More...
virtual Int_t Read (const char *name)
 Read contents of object with specified name from the current directory. More...
virtual void RecursiveRemove (TObject *obj)
 Recursively remove this object from a list. More...
void ResetBit (UInt_t f)
virtual void SaveAs (const char *filename="", Option_t *option="") const
 Save this object in the file specified by filename. More...
virtual void SavePrimitive (std::ostream &out, Option_t *option="")
 Save a primitive as a C++ statement(s) on output stream "out". More...
void SetBit (UInt_t f, Bool_t set)
 Set or unset the user status bits as specified in f. More...
void SetBit (UInt_t f)
virtual void SetDrawOption (Option_t *option="")
 Set drawing option for object. More...
virtual void SetUniqueID (UInt_t uid)
 Set the unique object id. More...
virtual void SysError (const char *method, const char *msgfmt,...) const
 Issue system error message. More...
Bool_t TestBit (UInt_t f) const
Int_t TestBits (UInt_t f) const
virtual void UseCurrentStyle ()
 Set current style settings in this object This function is called when either TCanvas::UseCurrentStyle or TROOT::ForceStyle have been invoked. More...
virtual void Warning (const char *method, const char *msgfmt,...) const
 Issue warning message. More...
virtual Int_t Write (const char *name=0, Int_t option=0, Int_t bufsize=0)
 Write this object to the current directory. More...
virtual Int_t Write (const char *name=0, Int_t option=0, Int_t bufsize=0) const
 Write this object to the current directory. More...

Protected Member Functions

void DeregisterValueReader (ROOT::Internal::TTreeReaderValueBase *reader)
 Remove a value reader for this tree. More...
ROOT::Internal::TNamedBranchProxyFindProxy (const char *branchname) const
TCollectionGetProxies ()
void Initialize ()
 Initialization of the director. More...
Bool_t RegisterValueReader (ROOT::Internal::TTreeReaderValueBase *reader)
 Add a value reader for this tree. More...
EEntryStatus SetEntryBase (Long64_t entry, Bool_t local)
 Load an entry into the tree, return the status of the read. More...
- Protected Member Functions inherited from TObject
virtual void DoError (int level, const char *location, const char *fmt, va_list va) const
 Interface to ErrorHandler (protected). More...
void MakeZombie ()

Private Types

enum  EPropertyBits { kBitIsChain = BIT(14) }

Private Attributes

 proxying director, owned More...
 directory (or current file for chains) More...
EEntryStatus fEntryStatus
 status of most recent read request More...
Long64_t fLastEntry
 The last entry to be processed. More...
Int_t fMostRecentTreeNumber
 TTree::GetTreeNumber() of the most recent tree. More...
THashTable fProxies
 attached ROOT::TNamedBranchProxies; owned More...
Bool_t fProxiesSet
 True if the proxies have been set, false otherwise. More...
 tree that's
std::deque< ROOT::Internal::TTreeReaderValueBase * > fValues
 readers that use our director More...


class ROOT::Internal::TTreeReaderArrayBase
class ROOT::Internal::TTreeReaderValueBase

Additional Inherited Members

- Static Public Member Functions inherited from TObject
static Long_t GetDtorOnly ()
 Return destructor only flag. More...
static Bool_t GetObjectStat ()
 Get status of object stat flag. More...
static void SetDtorOnly (void *obj)
 Set destructor only flag. More...
static void SetObjectStat (Bool_t stat)
 Turn on/off tracking of objects in the TObjectTable. More...

#include <TTreeReader.h>

Inheritance diagram for TTreeReader:

Member Typedef Documentation

◆ iterator

Definition at line 127 of file TTreeReader.h.

Member Enumeration Documentation

◆ EEntryStatus


data read okay


no entry has been loaded yet


the tree does not exist


the tree entry number does not exist


problem in accessing a chain element, e.g. file without the tree


problem in opening a chain's file


problem reading dictionary info from tree


last entry was reached

Definition at line 129 of file TTreeReader.h.

◆ EPropertyBits


our tree is a chain

Definition at line 200 of file TTreeReader.h.

Constructor & Destructor Documentation

◆ TTreeReader() [1/3]

TTreeReader::TTreeReader ( )

Definition at line 140 of file TTreeReader.h.

◆ TTreeReader() [2/3]

TTreeReader::TTreeReader ( TTree tree)

Access data from tree.

Definition at line 167 of file TTreeReader.cxx.

◆ TTreeReader() [3/3]

TTreeReader::TTreeReader ( const char *  keyname,
TDirectory dir = NULL 

Access data from the tree called keyname in the directory (e.g.

TFile) dir, or the current directory if dir is NULL. If keyname cannot be found, or if it is not a TTree, IsZombie() will return true.

Definition at line 188 of file TTreeReader.cxx.

◆ ~TTreeReader()

TTreeReader::~TTreeReader ( )

Tell all value readers that the tree reader does not exist anymore.

Definition at line 205 of file TTreeReader.cxx.

Member Function Documentation

◆ begin()

Iterator_t TTreeReader::begin ( )

Return an iterator to the 0th TTree entry.

Definition at line 182 of file TTreeReader.h.

◆ DeregisterValueReader()

void TTreeReader::DeregisterValueReader ( ROOT::Internal::TTreeReaderValueBase reader)

Remove a value reader for this tree.

Definition at line 406 of file TTreeReader.cxx.

◆ end()

Iterator_t TTreeReader::end ( ) const

Definition at line 185 of file TTreeReader.h.

◆ FindProxy()

ROOT::Internal::TNamedBranchProxy* TTreeReader::FindProxy ( const char *  branchname) const

Definition at line 189 of file TTreeReader.h.

◆ GetCurrentEntry()

Long64_t TTreeReader::GetCurrentEntry ( ) const

Returns the index of the current entry being read.

Definition at line 267 of file TTreeReader.cxx.

◆ GetEntries()

Long64_t TTreeReader::GetEntries ( Bool_t  force) const

Definition at line 178 of file TTreeReader.h.

◆ GetEntryStatus()

EEntryStatus TTreeReader::GetEntryStatus ( ) const

Definition at line 175 of file TTreeReader.h.

◆ GetProxies()

TCollection* TTreeReader::GetProxies ( )

Definition at line 191 of file TTreeReader.h.

◆ GetTree()

TTree* TTreeReader::GetTree ( ) const

Definition at line 177 of file TTreeReader.h.

◆ Initialize()

void TTreeReader::Initialize ( )

Initialization of the director.

Definition at line 218 of file TTreeReader.cxx.

◆ IsChain()

Bool_t TTreeReader::IsChain ( ) const

Definition at line 159 of file TTreeReader.h.

◆ Next()

Bool_t TTreeReader::Next ( )

Definition at line 161 of file TTreeReader.h.

◆ RegisterValueReader()

Bool_t TTreeReader::RegisterValueReader ( ROOT::Internal::TTreeReaderValueBase reader)

Add a value reader for this tree.

Definition at line 391 of file TTreeReader.cxx.

◆ Restart()

void TTreeReader::Restart ( )

Restart a Next() loop from the first entry.

Definition at line 258 of file TTreeReader.cxx.

◆ SetChain()

void TTreeReader::SetChain ( const char *  ,

Definition at line 157 of file TTreeReader.h.

◆ SetEntriesRange()

TTreeReader::EEntryStatus TTreeReader::SetEntriesRange ( Long64_t  beginEntry,
Long64_t  endEntry 

Set the range of entries to be processed.

This call is usually followed by an iteration of the range using TTreeReader::Next(), which will visit the the entries from begiNEntry to endEntry - 1. If endEntry > beginEntry, this call is equivalent to SetEntry(beginEntry - 1); SetLastEntry(endEntry);. Otherwise endEntry is ignored and only beginEntry is set.

kEntryValid (more useful since ROOT v6.10).

Definition at line 246 of file TTreeReader.cxx.

◆ SetEntry()

EEntryStatus TTreeReader::SetEntry ( Long64_t  entry)

Definition at line 162 of file TTreeReader.h.

◆ SetEntryBase()

TTreeReader::EEntryStatus TTreeReader::SetEntryBase ( Long64_t  entry,
Bool_t  local 

Load an entry into the tree, return the status of the read.

For chains, entry is the global (i.e. not tree-local) entry number, unless local is true, in which case entry specifies the entry number within the current tree. This is needed for instance for TSelector::Process().

Definition at line 282 of file TTreeReader.cxx.

◆ SetLastEntry()

void TTreeReader::SetLastEntry ( Long64_t  entry)
This is a very misleading interface, please read carefully:

SetLastEntry() really sets the entry that Next() will not read, i.e. the first entry number for which Next() will return kFALSE. It is equivalant to end() for iterators. This interface has been deprecated in ROOT 6.10 and will be removed in ROOT 6.12. Please use SetEntriesRange() instead.

Definition at line 171 of file TTreeReader.h.

◆ SetLocalEntry()

EEntryStatus TTreeReader::SetLocalEntry ( Long64_t  entry)

Definition at line 163 of file TTreeReader.h.

◆ SetTree() [1/2]

void TTreeReader::SetTree ( TTree tree)

Set (or update) the which tree to reader from.

tree can be a TTree or a TChain.

Definition at line 367 of file TTreeReader.cxx.

◆ SetTree() [2/2]

void TTreeReader::SetTree ( const char *  ,

Definition at line 156 of file TTreeReader.h.

Friends And Related Function Documentation

◆ ROOT::Internal::TTreeReaderArrayBase

Definition at line 219 of file TTreeReader.h.

◆ ROOT::Internal::TTreeReaderValueBase

Definition at line 218 of file TTreeReader.h.

Member Data Documentation

◆ fDirector

ROOT::Internal::TBranchProxyDirector* TTreeReader::fDirector

proxying director, owned

Definition at line 208 of file TTreeReader.h.

◆ fDirectory

TDirectory* TTreeReader::fDirectory

directory (or current file for chains)

Definition at line 205 of file TTreeReader.h.

◆ fEntryStatus

EEntryStatus TTreeReader::fEntryStatus

status of most recent read request

Definition at line 206 of file TTreeReader.h.

◆ fLastEntry

Long64_t TTreeReader::fLastEntry

The last entry to be processed.

When set (i.e. >= 0), it provides a way to stop looping over the TTree when we reach a certain entry: Next() returns kEntryLast when GetCurrentEntry() reaches fLastEntry

Definition at line 215 of file TTreeReader.h.

◆ fMostRecentTreeNumber

Int_t TTreeReader::fMostRecentTreeNumber

TTree::GetTreeNumber() of the most recent tree.

Definition at line 207 of file TTreeReader.h.

◆ fProxies

THashTable TTreeReader::fProxies

attached ROOT::TNamedBranchProxies; owned

Definition at line 210 of file TTreeReader.h.

◆ fProxiesSet

Bool_t TTreeReader::fProxiesSet

True if the proxies have been set, false otherwise.

Definition at line 216 of file TTreeReader.h.

◆ fTree

TTree* TTreeReader::fTree

tree that's read

Definition at line 204 of file TTreeReader.h.

◆ fValues

std::deque<ROOT::Internal::TTreeReaderValueBase*> TTreeReader::fValues

readers that use our director

Definition at line 209 of file TTreeReader.h.

The documentation for this class was generated from the following files: