Because I was tired to always answer this question:

Dear rooters,

sometimes, while filling a TTree, I encountered this message:

Error in <TTree::Fill>: Failed filling branch:matches.matches, nbytes=-1

can someone explain me what does it mean? What mistake did I do while creating the code to fill that TTree?

I decided to systematically print this message in addition to the existing message
 This error is symptomatic of a Tree created as a memory-resident Tree
 Instead of doing:
    TTree *T = new TTree(...)
    TFile *f = new TFile(...)
 you should do:
    TFile *f = new TFile(...)
    TTree *T = new TTree(...)


git-svn-id: http://root.cern.ch/svn/root/trunk@15603 27541ba8-7e3a-0410-8455-c3a389f83636

tree/src/TTree.cxx

-// @(#)root/tree:$Name:  $:$Id: TTree.cxx,v 1.289 2006/06/25 14:14:11 pcanal Exp $
+// @(#)root/tree:$Name:  $:$Id: TTree.cxx,v 1.290 2006/06/27 14:36:28 brun Exp $
 // Author: Rene Brun   12/01/96
 
 /*************************************************************************
@@ -2753,6 +2753,15 @@ Int_t TTree::Fill()
       nbytes += (nwrite = branch->Fill());
       if ( nwrite < 0 )  {
          Error("Fill","Failed filling branch:%s.%s, nbytes=%d",GetName(),branch->GetName(),nwrite);
+         if (nerror < 2) {
+            printf(" This error is symptomatic of a Tree created as a memory-resident Tree\n");
+            printf(" Instead of doing:\n");
+            printf("    TTree *T = new TTree(...)\n");
+            printf("    TFile *f = new TFile(...)\n");
+            printf(" you should do:\n");
+            printf("    TFile *f = new TFile(...)\n");
+            printf("    TTree *T = new TTree(...)\n");
+         }
          nerror++;
       }
    }