Re: Access objects inheriting from TAttLine via TAttLine pointer

Hi Thomas,

What Timur meant (but because it's obvious to him didn't mention :-) is that you should use dynamic_cast<TAttLine*>. You might run into problems with CINT, though - so alternatively you can try to get TClass::DynamicCast
<http://root.cern.ch/root/html/TClass#TClass:DynamicCast> to do the same job:

   TObject* obj = 0;
   while( (obj = Next()) ) {
     TAttLine* attLine = (TAttLine*)
obj->IsA()->DynamicCast(TAttLine::Class(), obj)

     cout << attLine->GetLineColor() << endl;    }

Cheers, Axel.

Timur Pocheptsov wrote on 02/06/2012 11:23 AM:
> 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_at_root.cern.ch [owner-roottalk_at_root.cern.ch] on behalf of Thomas Lauf [thl_at_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_at_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
>
>
>