Persistent Reference link to a TObject A TRef is a lightweight object pointing to any TObject.

This object can be used instead of normal C++ pointers in case

the referenced object R and the pointer P are not written to the same file
P is read before R
R and P are written to different Tree branches

When a top level object (eg Event *event) is a tree/graph of many objects, the normal ROOT Streaming mechanism ensures that only one copy of each object in the tree/graph is written to the output buffer to avoid circular dependencies.

However if the object event is split into several files or into several branches of one or more Trees, normal C++ pointers cannot be used because each I/O operation will write the referenced objects.

When a TRef is used to point to a TObject *robj, for example in a class with

TRef fRef;

one can do:

fRef = robj; //to set the pointer

This TRef and robj can be written with two different I/O calls in the same or different files, in the same or different branches of a Tree.

If the TRef is read and the referenced object has not yet been read, the TRef will return a null pointer. As soon as the referenced object will be read, the TRef will point to it. If the referenced object is contained in a TTree it can be auto-loaded using the TBranchRef mechanism, which is set up by simply calling TTree::BranchRef().

TRef also supports the complex situation where a TFile is updated multiple times on the same machine or a different machine.

How does it work

A TRef is itself a TObject with an additional transient pointer fPID. When the statement fRef = robj is executed, the following actions happen:

The pointer fPID is set to the current TProcessID.
The current ObjectNumber (see below) is incremented by one.
robj::fUniqueID is set to ObjectNumber.
In the fPID object, the element fObjects[ObjectNumber] is set to robj
ref::fUniqueID is also set to ObjectNumber.

After having set fRef, one can immediately return the value of robj using fRef.GetObject(). This function returns directly fObjects[fUniqueID] from the fPID object.

When the TRef is written, the process id number pidf of fPID is written in addition to the TObject part of TRef (fBits,fUniqueID).

When the TRef is read, its pointer fPID is set to the value stored in the TObjArray of TFile::fProcessIDs (fProcessIDs[pidf]). The pidf is stored as a UShort_t limiting a file to 65535 distinct ProcessID objects.

The pidf is stored in the bits 24->31 of the fUniqueID of the TRef. This implies that the number of TRefs in a single ProcessID should not exceed 2**24 = 16777216. For pidf greater than 254, the value 0xff is stored in those bits and we use the table TProcessID::fgObjPIDs which links the referenced object's address to its ProcessID.

See section "ObjectNumber" below for a recipe to minimize the object count. If the object-number exceeds this limit, it could be the sign that:

The object count is never reset (see below)
TRef is misused.

When a referenced object robj is written, TObject::Streamer writes in addition to the standard (fBits,fUniqueID) the pidf. When this robj is read by TObject::Streamer, the pidf is read. At this point, robj is entered into the table of objects of the TProcessID corresponding to pidf.

WARNING1:

If MyClass is the class of the referenced object, The TObject part of MyClass must be Streamed. One should not call MyClass::Class()->IgnoreTObjectStreamer()

WARNING2:

A TRef cannot point to another TRef.

ObjectNumber

When an object is referenced (see TRef assignment operator or TRefArray::Add) a unique identifier is computed and stored in both the fUniqueID of the referenced and referencing object. This uniqueID is computed by incrementing by one the static global in TProcessID::fgNumber. fUniqueID is some sort of serial object number in the current session. One can retrieve at any time the current value of fgNumber by calling the static function TProcessID::GetObjectCount or set this number via TProcessID::SetObjectCount.

To avoid a growing table of fObjects in TProcessID, in case, for example, one processes many events in a loop, it might be necessary to reset the ObjectNumber at the end of processing of one event. See an example in $ROOTSYS/test/Event.cxx (look at function Build).

The value of ObjectNumber (say saveNumber=TProcessID::GetObjectCount()) may be saved at the beginning of one event and reset to this original value at the end of the event via TProcessID::SetObjectCount(saveNumber). These actions may be stacked.

Action on Demand

The normal behaviour of a TRef has been described above. In addition, TRef supports also "Actions on Demand". It may happen that the object referenced is not yet in memory, on a separate file or not yet computed. In this case TRef is able to automatically execute an action:

call to a compiled function (static function of member function)
call to an interpreted function
execution of a C++ script

How to select this option? In the definition of the TRef data member in the original class, do:

TRef fRef; //EXEC:execName. points to something

When the special keyword "EXEC:" is found in the comment field of the member, the next string is assumed to be the name of a TExec object. When a file is connected, the dictionary of the classes on the file is read in memory (see TFile::ReadStreamerInfo). When the TStreamerElement object is read, a TExec object is automatically created with the name specified after the keyword "EXEC:" in case a TExec with a same name does not already exist.

The action to be executed via this TExec can be specified with:

a call to the TExec constructor, if the constructor is called before opening the file.
a call to TExec::SetAction at any time. One can compute a pointer to an existing TExec with a name with:
TExec *myExec = gROOT->GetExec(execName);
myExec->SetAction(actionCommand);
where actionCommand is a string containing a C++ instruction. Examples:
myExec->SetAction("LoadHits()");
myExec->SetAction(".x script.C");

When a TRef is dereferenced via TRef::GetObject, its TExec will be automatically executed. In the function/script being executed, one or more of the following actions can be executed:

load a file containing the referenced object. This function typically looks in the file catalog (GRID).
compute a pointer to the referenced object and communicate this pointer back to the calling function TRef::GetObject via:
TRef::SetStaticObject(object).
When the TExec is called, it has access to the dereferencing TRef by calling GetStaticObject() (TRef::GetObject() sets fgObject to "this" before the call to TExec). This can be useful for accessing the TRef's fUniqueID.

As soon as an object is returned to GetObject, the fUniqueID of the TRef is set to the fUniqueID of the referenced object. At the next call to GetObject, the pointer stored in fPid:fObjects[fUniqueID] will be returned directly.

An example of action on demand is shown in $ROOTSYS/test/Event.h with the member:

TRef fWebHistogram; //EXEC:GetWebHistogram

When calling fWebHistogram.GetObject(), the function GetObject will automatically invoke a script GetWebHistogram.C via the interpreter.

An example of a GetWebHistogram.C script is shown below

void GetWebHistogram() {
   TFile *f= TFile::Open("http://root.cern.ch/files/pippa.root");
   f->cd("DM/CJ");
   TH1 *h6 = (TH1*)gDirectory->Get("h6");
   h6->SetDirectory(0);
   delete f;
   TRef::SetStaticObject(h6);
}

In the above example, a call to fWebHistogram.GetObject() executes the script with the function GetWebHistogram. This script connects a file with histograms: pippa.root on the ROOT Web site and returns the object h6 to TRef::GetObject.

Note that if the definition of the TRef fWebHistogram had been:

TRef fWebHistogram; //EXEC:GetWebHistogram()

then, the compiled or interpreted function GetWebHistogram() would have been called instead of the C++ script GetWebHistogram.C

Special case of a TRef pointing to an object with a TUUID

If the referenced object has a TUUID, its bit kHasUUID has been set. This case is detected by the TRef assignment operator. (For example, TFile and TDirectory have a TUUID) The TRef fPID points directly to the single object TProcessUUID (deriving from TProcessID) and managing the list of TUUIDs for a process. The TRef kHasUUID bit is set and its fUniqueID is set to the fUniqueID of the referenced object.

When the TRef is streamed to a buffer, the corresponding TUUID is also streamed with the TRef. When a TRef is read from a buffer, the corresponding TUUID is also read and entered into the global list of TUUIDs (if not already there). The TRef fUniqueID is set to the UUIDNumber. see TProcessUUID for more details.

Array of TRef

The special class TRefArray should be used to store multiple references. A TRefArray has one single pointer fPID for all objects in the array. It has a dynamic compact table of fUniqueIDs. Use a TRefArray rather then a collection of TRefs if all TRefs stem from the same process.

Example:

Suppose a TObjArray *mytracks containing a list of Track objects Suppose a TRefArray *pions containing pointers to the pion tracks in mytracks. This list is created with statements like: pions->Add(track); Suppose a TRefArray *muons containing pointers to the muon tracks in mytracks. The 3 arrays mytracks,pions and muons may be written separately.

Definition at line 32 of file TRef.h.

Public Member Functions
	TRef ()

	TRef (TObject *obj)
	Create a ref to obj. More...

	TRef (const TRef &ref)
	TRef copy ctor. More...

virtual	~TRef ()

TObject *	GetObject () const
	Return a pointer to the referenced object. More...

TProcessID *	GetPID () const

Bool_t	IsValid () const

void	operator= (TObject *obj)
	Assign object to reference. More...

TRef &	operator= (const TRef &ref)
	TRef assignment operator. More...

virtual void	SetAction (const char *name)
	Store the exec number (in the ROOT list of Execs) into the fBits of this TRef. More...

virtual void	SetAction (TObject *parent)
	Find the action to be executed in the dictionary of the parent class and store the corresponding exec number into fBits. More...

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...

ULong_t	CheckedHash ()
	Checked and record whether for this class has a consistent Hash/RecursiveRemove setup (*) and then return the regular Hash value for this object. More...

virtual const char *	ClassName () const
	Returns name of class to which the object belongs. More...

virtual void	Clear (Option_t *="")

virtual TObject *	Clone (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 TObject *	DrawClone (Option_t *option="") const
	Draw a clone of this object in the current selected pad for instance with: `gROOT->SetSelectedPad(gPad)`. 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 TObject *	FindObject (const char *name) const
	Must be redefined in derived classes. More...

virtual TObject *	FindObject (const TObject *obj) const
	Must be redefined in derived classes. More...

virtual Option_t *	GetDrawOption () 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_t *	GetOption () 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...

Bool_t	HasInconsistentHash () const
	Return true is the type of this object is known to have an inconsistent setup for Hash and RecursiveRemove (i.e. 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...

R__ALWAYS_INLINE Bool_t	IsOnHeap () const

virtual Bool_t	IsSortable () const

R__ALWAYS_INLINE 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...

void *	operator new (size_t sz)

void *	operator new (size_t sz, void *vp)

void *	operator new[] (size_t sz)

void *	operator new[] (size_t sz, void *vp)

TObject &	operator= (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...

R__ALWAYS_INLINE 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...

Static Public Member Functions
static Int_t	AddExec (const char *name)
	If Exec with name does not exist in the list of Execs, it is created. More...

static TObjArray *	GetListOfExecs ()
	Return a pointer to the static TObjArray holding the list of Execs. More...

static TObject *	GetStaticObject ()
	Returns the static object. More...

static void	SetObject (TObject *obj)
	static Obsolete function kept for back compatibility. More...

static void	SetStaticObject (TObject *obj)
	Static function to set the object found on the Action on Demand function. More...

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...

Protected Attributes
TProcessID *	fPID

Static Protected Attributes
static TObjArray *	fgExecs = 0
	Pointer to ProcessID when TRef was written. More...

static TObject *	fgObject = 0

Friends
Bool_t	operator!= (const TRef &r1, const TRef &r2)
	Return kTRUE if r1 and r2 do not point to the same object. More...

Bool_t	operator== (const TRef &r1, const TRef &r2)
	Return kTRUE if r1 and r2 point to the same object. More...

Additional Inherited Members
Public Types inherited from TObject
enum	{ kIsOnHeap = 0x01000000, kNotDeleted = 0x02000000, kZombie = 0x04000000, kInconsistent = 0x08000000, kBitMask = 0x00ffffff }

enum	{ kSingleKey = BIT(0), kOverwrite = BIT(1), kWriteDelete = BIT(2) }

enum	EDeprecatedStatusBits { kObjInCanvas = BIT(3) }

enum	EStatusBits { kCanDelete = BIT(0), kMustCleanup = BIT(3), kIsReferenced = BIT(4), kHasUUID = BIT(5), kCannotPick = BIT(6), kNoContextMenu = BIT(8), kInvalidObject = BIT(13) }

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 ()

#include <TRef.h>

Inheritance diagram for TRef:

[legend]

Constructor & Destructor Documentation

◆ TRef() [1/3]

TRef::TRef ( )

inline

Definition at line 42 of file TRef.h.

◆ TRef() [2/3]

TRef::TRef ( TObject * obj )

Create a ref to obj.

Definition at line 262 of file TRef.cxx.

◆ TRef() [3/3]

TRef::TRef ( const TRef & ref )

TRef copy ctor.

Definition at line 270 of file TRef.cxx.

◆ ~TRef()

virtual TRef::~TRef ( )

inlinevirtual

Definition at line 47 of file TRef.h.

Member Function Documentation

◆ AddExec()

Int_t TRef::AddExec ( const char * name )

static

If Exec with name does not exist in the list of Execs, it is created.

returns the index of the Exec in the list.

Definition at line 339 of file TRef.cxx.

◆ GetListOfExecs()

TObjArray * TRef::GetListOfExecs ( )

static

Return a pointer to the static TObjArray holding the list of Execs.

Definition at line 359 of file TRef.cxx.

◆ GetObject()

TObject * TRef::GetObject ( ) const

Return a pointer to the referenced object.

Definition at line 377 of file TRef.cxx.

◆ GetPID()

TProcessID* TRef::GetPID ( ) const

inline

Definition at line 50 of file TRef.h.

◆ GetStaticObject()

TObject * TRef::GetStaticObject ( )

static

Returns the static object.

Definition at line 458 of file TRef.cxx.

◆ IsValid()

Bool_t TRef::IsValid ( ) const

inline

Definition at line 51 of file TRef.h.

◆ operator=() [1/2]

void TRef::operator= ( TObject * obj )

Assign object to reference.

Definition at line 278 of file TRef.cxx.

◆ operator=() [2/2]

TRef & TRef::operator= ( const TRef & ref )

TRef assignment operator.

Definition at line 307 of file TRef.cxx.

◆ SetAction() [1/2]

void TRef::SetAction ( const char * name )

virtual

Store the exec number (in the ROOT list of Execs) into the fBits of this TRef.

Definition at line 429 of file TRef.cxx.

◆ SetAction() [2/2]

void TRef::SetAction ( TObject * parent )

virtual

Find the action to be executed in the dictionary of the parent class and store the corresponding exec number into fBits.

This function searches a data member in the class of parent with an offset corresponding to this. If a comment "TEXEC:" is found in the comment field of the data member, the function stores the exec identifier of the exec statement following this keyword.

Definition at line 449 of file TRef.cxx.

◆ SetObject()

void TRef::SetObject ( TObject * obj )

static

static Obsolete function kept for back compatibility.

In the near future will print a Warning, then will be deleted.

Definition at line 466 of file TRef.cxx.

◆ SetStaticObject()

void TRef::SetStaticObject ( TObject * obj )

static

Static function to set the object found on the Action on Demand function.

This function may be called by the user in the function called when a "EXEC:" keyword is specified in the data member field of the TRef. The function can get access to the dereferencing TRef (i.e. this)using the static function GetStaticObject().

Definition at line 478 of file TRef.cxx.

Friends And Related Function Documentation

◆ operator!=

Bool_t operator!=	(	const TRef &	r1,
		const TRef &	r2
	)

friend

Return kTRUE if r1 and r2 do not point to the same object.

Definition at line 329 of file TRef.cxx.

◆ operator==

Bool_t operator==	(	const TRef &	r1,
		const TRef &	r2
	)

friend

Return kTRUE if r1 and r2 point to the same object.

Definition at line 320 of file TRef.cxx.

Member Data Documentation

◆ fgExecs

TObjArray * TRef::fgExecs = 0

staticprotected

Pointer to ProcessID when TRef was written.

Definition at line 37 of file TRef.h.

◆ fgObject

TObject * TRef::fgObject = 0

staticprotected

Definition at line 38 of file TRef.h.

◆ fPID

TProcessID* TRef::fPID

protected

Definition at line 35 of file TRef.h.

Libraries for TRef:

[legend]

How does it work

WARNING1:

WARNING2:

ObjectNumber

Action on Demand

Special case of a TRef pointing to an object with a TUUID

Array of TRef

Public Member Functions

Static Public Member Functions

Protected Attributes

Static Protected Attributes

Friends

Additional Inherited Members

Constructor & Destructor Documentation

◆ TRef() [1/3]

◆ TRef() [2/3]

◆ TRef() [3/3]

◆ ~TRef()

Member Function Documentation

◆ AddExec()

◆ GetListOfExecs()

◆ GetObject()

◆ GetPID()

◆ GetStaticObject()

◆ IsValid()

◆ operator=() [1/2]

◆ operator=() [2/2]

◆ SetAction() [1/2]

◆ SetAction() [2/2]

◆ SetObject()

◆ SetStaticObject()

Friends And Related Function Documentation

◆ operator!=

◆ operator==

Member Data Documentation

◆ fgExecs

◆ fgObject

◆ fPID