RE: Access objects inheriting from TAttLine via TAttLine pointer

I do not know, what's inside TObjArray::Print, but I know, that TAttLine does not inherit TObject,
so what you do in your cast is equal to reinterpret_cast, which is wrong in case of multiple inheritance. So, this is the code with undefined behavior.
________________________________________
From: owner-roottalk@root.cern.ch [owner-roottalk@root.cern.ch] on behalf of Thomas Lauf [thl@hll.mpg.de]
Sent: 06 February 2012 11:15
To: roottalk (Mailing list discussing all aspects of the ROOT system.)
Subject: [ROOT] Access objects inheriting from TAttLine via TAttLine pointer

Hello ROOTers,

I like to access different objects which all inherit from TAttLine. As a
simple example of what I want to do, take the following script where all
objects are actually TLine instances:

void TLineTest()
{
   TObjArray Array;

   Array.Add( new TLine( 0, 0, 1, 1 ) );
   Array.Add( new TLine( 0, 0, 1, 1 ) );
   Array.Add( new TLine( 0, 0, 1, 1 ) );

   TLine* Line;
   Int_t i=0;

   Array.Print();

   TIter Next( &Array );

   while( (Line = (TLine*) Next()) ) {
     cout << Line->GetLineColor() << endl;
   }

   Array.Print();

}

The output is, as expected, the content of the list, then three times 1
(the line color) and, again, the output of the list.

Now I want to apply this scheme to a list containing different objects
which all inherit from the TAttLine class:

void TAttLineTest()
{
   TObjArray Array;

   Array.Add( new TH1I( "foo", "bar", 0, 0, 0 ) );
   Array.Add( new TLine( 0, 0, 1, 1 ) );
   Array.Add( new TGraph(1) );

   TAttLine* AttLine;
   Int_t i=0;

   Array.Print();

   TIter Next( &Array );

   while( (AttLine = (TAttLine*) Next()) ) {
     cout << AttLine->GetLineColor() << endl;
   }

   Array.Print();

}

The script is analog to the example above, but now I use a TAttLine
pointer to access the objects in the array. Running this script leads to
a segmentation fault with the following output (with ROOT 5.32):

bash-3.2$ root -x -q TAttLineTest.C
   *******************************************
   *                                         *
   *        W E L C O M E  to  R O O T       *
   *                                         *
   *   Version   5.32/00   2 December 2011   *
   *                                         *
   *  You are welcome to visit our Web site  *
   *          http://root.cern.ch            *
   *                                         *
   *******************************************

ROOT 5.32/00 (tags/v5-32-00@42375, Jan 24 2012, 13:07:59 on linuxx8664gcc)

CINT/ROOT C/C++ Interpreter version 5.18.00, July 2, 2010
Type ? for help. Commands must be C++ statements.
Enclose multiple statements between { }.
root [0]
Processing TAttLineTest.C...
Collection name='TObjArray', class='TObjArray', size=16
  TH1.Print Name  = foo, Entries= 0, Total sum= 0
  TLine  X1=0.000000 Y1=0.000000 X2=1.000000 Y2=1.000000
  x[0]=0, y[0]=0
0
0
0
Collection name='TObjArray', class='TObjArray', size=16

  *** Break *** segmentation violation

[...]

===========================================================
#10 0x00000032d70bdb65 in __dynamic_cast () from /usr/lib64/libstdc++.so.6
#11 0x00002ab7e684f42e in TCollection::PrintCollectionEntry
(this=0x171e3200,
     entry=0x17266360, option=0x2ab7e6cd039b "", recurse=0)
     at /sirius/thl/root/core/cont/src/TCollection.cxx:307
#12 0x00002ab7e684efef in TCollection::Print (this=0x171e3200,
     option=0x2ab7e6cd039b "", recurse=1)
     at /sirius/thl/root/core/cont/src/TCollection.cxx:357
#13 0x00002ab7e684df13 in TCollection::Print (this=0x171e3200,
     option=0x2ab7e6cd039b "")
     at /sirius/thl/root/core/cont/src/TCollection.cxx:334
===========================================================

Somehow, accessing the objects via the TAttList pointer corrupts the
array. Does anyone know why? I also wonder why the color of the TLine
object in the list is now '0'...

I've tested this script also on ROOT 5.30 and 5.26 with the same result.

Regards, Thomas