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ROOT 6.08/07 Reference Guide |
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ROOT 6.08/07 Reference Guide |
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
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
one can do:
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.
A TRef is itself a TObject with an additional transient pointer fPID. When the statement fRef = robj is executed, the following actions happen:
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:
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.
If MyClass is the class of the referenced object, The TObject part of MyClass must be Streamed. One should not call MyClass::Class()->IgnoreTObjectStreamer()
A TRef cannot point to another TRef.
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.
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:
How to select this option? In the definition of the TRef data member in the original class, do:
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:
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:
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:
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
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:
then, the compiled or interpreted function GetWebHistogram() would have been called instead of the C++ script GetWebHistogram.C
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.
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.
Public Types | |
| enum | { kNotComputed = BIT(12) } |
 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 | |
| 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... | |
| 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 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 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... | |
| 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... | |
| 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... | |
| 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 | |
| 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>
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| TObject * TRef::GetObject | ( | ) | const |
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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.
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().
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ROOT 6.08/07 - Reference Guide Generated on Thu May 31 2018 21:47:59 using Doxygen 1.8.13.