set in .rootrc
Root.ObjectStat:          1


http://www-numi.fnal.gov/offline_software/srt_public_context/WebDocs/howto_rootmodule.html
Using the ROOT object statistics facility
  /Root/ObjectStat/Set [1|0]
  /Root/ObjectStat/Print
     TObject statistics collection (via gObjectTable):
        Turn [on|off] the statistics collection (default: on)
        Print the gObjectTable

gObjectTable->Print() shows the number of objects per class currently
in memory (only if you have activated Root.ObjectStat to 1 in .rootrc)