diff --git a/DDCore/include/DD4hep/AlignmentData.h b/DDCore/include/DD4hep/AlignmentData.h
index f4f5fa51e9bb959d92c6f94fc04b367f2c8ce11a..1c5517547ede822b8cbe6e2f30eb64e56e80679c 100644
--- a/DDCore/include/DD4hep/AlignmentData.h
+++ b/DDCore/include/DD4hep/AlignmentData.h
@@ -196,4 +196,5 @@ namespace dd4hep {
     {  return detectorToLocal(Position(det[0],det[1],det[2]));                        }
   };
 }         /* End namespace dd4hep               */
+std::ostream& operator << (std::ostream& s, const dd4hep::AlignmentData& data);
 #endif    /* DD4HEP_ALIGMENTS_ALIGNMENTDATA_H   */