TRef.cxx

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// @(#)root/cont:$Id$
// Author: Rene Brun   28/09/2001

/*************************************************************************
 * Copyright (C) 1995-2000, Rene Brun and Fons Rademakers.               *
 * All rights reserved.                                                  *
 *                                                                       *
 * For the licensing terms see $ROOTSYS/LICENSE.                         *
 * For the list of contributors see $ROOTSYS/README/CREDITS.             *
 *************************************************************************/

/** \class TRef
\ingroup Base

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
~~~ {.cpp}
    TRef  fRef;
~~~
one can do:
~~~ {.cpp}
    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:
~~~ {.cpp}
  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:
~~~ {.cpp}
      TExec *myExec = gROOT->GetExec(execName);
      myExec->SetAction(actionCommand);
~~~
     where actionCommand is a string containing a C++ instruction. Examples:
~~~ {.cpp}
      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:
~~~ {.cpp}
     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:
~~~ {.cpp}
     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
~~~ {.cpp}
   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:
~~~ {.cpp}
     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.
*/

#include "TRef.h"
#include "TROOT.h"
#include "TClass.h"
#include "TProcessUUID.h"
#include "TRefTable.h"
#include "TObjArray.h"
#include "TExec.h"
#include "TSystem.h"
#include "TObjString.h"

TObjArray  *TRef::fgExecs  = 0;
TObject    *TRef::fgObject = 0;

ClassImp(TRef);

////////////////////////////////////////////////////////////////////////////////
/// Create a ref to obj.

TRef::TRef(TObject *obj)
{
   *this = obj;
}

////////////////////////////////////////////////////////////////////////////////
/// TRef copy ctor.

TRef::TRef(const TRef &ref) : TObject(ref)
{
   *this = ref;
}

////////////////////////////////////////////////////////////////////////////////
/// Assign object to reference.

void TRef::operator=(TObject *obj)
{
   UInt_t uid = 0;
   fPID = 0;
   if (obj) {
      if (obj->IsA()->CanIgnoreTObjectStreamer()) {
         Error("operator= ","Class: %s IgnoreTObjectStreamer. Cannot reference object",obj->ClassName());
         return;
      }
      if (obj->TestBit(kHasUUID)) {
         fPID = gROOT->GetUUIDs();
         obj->SetBit(kIsReferenced);
         SetBit(kHasUUID);
         uid = obj->GetUniqueID();
      } else {
         if (!obj->TestBit(kIsReferenced)) {
            TProcessID::AssignID(obj);
         }
         uid = obj->GetUniqueID();
         fPID = TProcessID::GetProcessWithUID(uid,obj);
         ResetBit(kHasUUID);
      }
   }
   SetUniqueID(uid);
}

////////////////////////////////////////////////////////////////////////////////
/// TRef assignment operator.

TRef &TRef::operator=(const TRef &ref)
{
   if (this != &ref) {
      SetUniqueID(ref.GetUniqueID());
      fPID = ref.fPID;
      SetBit(kHasUUID,ref.TestBit(kHasUUID));
   }
   return *this;
}

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

Bool_t operator==(const TRef &r1, const TRef &r2)
{
   if (r1.GetPID() == r2.GetPID() && r1.GetUniqueID() == r2.GetUniqueID()) return kTRUE;
   else return kFALSE;
}

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

Bool_t operator!=(const TRef &r1, const TRef &r2)
{
   if (r1.GetPID() == r2.GetPID() && r1.GetUniqueID() == r2.GetUniqueID()) return kFALSE;
   else return kTRUE;
}

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

Int_t TRef::AddExec(const char *name)
{
#ifdef R__COMPLETE_MEM_TERMINATION
   if (!fgExecs) GetListOfExecs();
#else
   if (!fgExecs) fgExecs = new TObjArray(10);
#endif

   TExec *exec = (TExec*)fgExecs->FindObject(name);
   if (!exec) {
      // we register this Exec to the list of Execs.
      exec = new TExec(name,"");
      fgExecs->Add(exec);
   }
   return fgExecs->IndexOf(exec);
}

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

TObjArray *TRef::GetListOfExecs()
{
#ifdef R__COMPLETE_MEM_TERMINATION
   static TObjArray listOfExecs(10);
   if (!fgExecs) {
      listOfExecs.SetOwner(kTRUE);
      fgExecs = &listOfExecs;
   }
#else
   if (!fgExecs) fgExecs = new TObjArray(10);
#endif
   return fgExecs;

}

////////////////////////////////////////////////////////////////////////////////
/// Return a pointer to the referenced object.

TObject *TRef::GetObject() const
{
   //TObject *obj = 0;
   if (!fPID) return 0;
   if (!TProcessID::IsValid(fPID)) return 0;
   UInt_t uid = GetUniqueID();

   //the reference may be in the TRefTable
   TRefTable *table = TRefTable::GetRefTable();
   if (table) {
      table->SetUID(uid, fPID);
      table->Notify();
   }

   //Try to find the object from the table of the corresponding PID
   TObject *obj = fPID->GetObjectWithID(uid);

   //if object not found, then exec action if an action has been defined
   if (!obj) {
      //execid in the first 8 bits
      Int_t execid = TestBits(0xff0000);
      if (execid > 0) {
         execid = execid>>16;
         TExec *exec = (TExec*)fgExecs->At(execid-1);
         if (exec) {
            //we expect the object to be returned via TRef::SetStaticObject
            fgObject = const_cast<TRef*>(this);
            exec->Exec();
            if ((const TRef*)fgObject != this)
               obj = fgObject;
            else obj=0;
            if (obj){
               uid = TProcessID::AssignID(obj);
               ((TRef*)this)->SetUniqueID(uid);
               fPID->PutObjectWithID(obj,uid);
            } else {
               //well may be the Exec has loaded the object
               obj = fPID->GetObjectWithID(uid);
            }
         }
      }
   }

   return obj;
}

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

void TRef::SetAction(const char *name)
{
   TExec *exec = (TExec*)GetListOfExecs()->FindObject(name);
   if (!exec) {
      Error("SetAction","Unknown TExec: %s",name);
      return;
   }
   Int_t execid = 1 + fgExecs->IndexOf(exec);
   SetBit(execid << 16);
}

////////////////////////////////////////////////////////////////////////////////
/// 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.

void TRef::SetAction(TObject *parent)
{
   if (!parent) return;
   if (gDirectory) gDirectory->SetTRefAction(this,parent);
}

 ///////////////////////////////////////////////////////////////////////////////
 /// Returns the static object.

TObject *TRef::GetStaticObject() {
   return fgObject;
}

////////////////////////////////////////////////////////////////////////////////
/// static Obsolete function kept for back compatibility.
/// In the near future will print a Warning, then will be deleted.

void TRef::SetObject(TObject *obj)
{
   SetStaticObject(obj);
}

////////////////////////////////////////////////////////////////////////////////
/// 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().

void TRef::SetStaticObject(TObject *obj)
{
   fgObject = obj;
}

////////////////////////////////////////////////////////////////////////////////
/// Stream an object of class TRef.

void TRef::Streamer(TBuffer &R__b)
{
   UShort_t pidf;
   if (R__b.IsReading()) {
      TObject::Streamer(R__b);
      if (TestBit(kHasUUID)) {
         TString s;
         s.Streamer(R__b);
         TProcessUUID *pid = gROOT->GetUUIDs();
         UInt_t number = pid->AddUUID(s.Data());
         fPID = pid;
         SetUniqueID(number);
         if (gDebug > 1) {
            printf("Reading TRef (HasUUID) uid=%d, obj=%lx\n",GetUniqueID(),(Long_t)GetObject());
         }
      } else {
         R__b >> pidf;
         pidf += R__b.GetPidOffset();
         fPID = R__b.ReadProcessID(pidf);
         //The execid has been saved in the unique id of the TStreamerElement
         //being read by TStreamerElement::Streamer
         //The current element (fgElement) is set as a static global
         //by TStreamerInfo::ReadBuffer (Clones) when reading this TRef
         Int_t execid = R__b.GetTRefExecId();
         if (execid) SetBit(execid<<16);
         if (gDebug > 1) {
            printf("Reading TRef, pidf=%d, fPID=%lx, uid=%d, obj=%lx\n",pidf,(Long_t)fPID,GetUniqueID(),(Long_t)GetObject());
         }
      }
   } else {
      TObject::Streamer(R__b);

      if (TestBit(kHasUUID)) {
         TObjString *objs = gROOT->GetUUIDs()->FindUUID(GetUniqueID());
         objs->String().Streamer(R__b);
         if (gDebug > 1) {
            printf("Writing TRef (HasUUID) uid=%d, obj=%lx\n",GetUniqueID(),(Long_t)GetObject());
         }
      } else {
         pidf = R__b.WriteProcessID(fPID);
         R__b << pidf;
         if (gDebug > 1) {
            printf("Writing TRef, pidf=%d, fPID=%lx, uid=%d, obj=%lx\n",pidf,(Long_t)fPID,GetUniqueID(),(Long_t)GetObject());
         }
      }
   }
}