TwoPers converter used to write old Pers versions of some objects

InnerDetector/InDetEventCnv/InDetEventAthenaPool/src/InDetSimDataCollectionCnv.cxx

 /*
-  Copyright (C) 2002-2022 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2023 CERN for the benefit of the ATLAS collaboration
 */
 
 #include "InDetSimDataCollectionCnv.h"
+#include "InDetSimDataCollectionCnv_p1.h"
+#include "InDetSimDataCollectionCnv_p2.h"
+#include "InDetSimDataCollectionCnv_p3.h"
+// Gaudi
+#include "GaudiKernel/MsgStream.h"
+
+
+bool InDetSimDataCollectionCnv::writeOldPers() {
+   return true;
+}
+
+
+InDetSimDataCollection_p2*
+InDetSimDataCollectionCnv::createOldPersistent(InDetSimDataCollection* transCont) {
+   MsgStream mlog(msgSvc(), "InDetSimDataCollectionCnvOld" );
+   InDetSimDataCollectionCnv_p2   converter;
+   return converter.createPersistent( transCont, mlog );
+}
 
 
 InDetSimDataCollection_PERS* InDetSimDataCollectionCnv::createPersistent(InDetSimDataCollection* transCont) {
     MsgStream mlog(msgSvc(), "InDetSimDataCollectionCnv" );
-    InDetSimDataCollectionCnv_PERS   converter;
+    InDetSimDataCollectionCnv_p3   converter;
     InDetSimDataCollection_PERS *persObj = converter.createPersistent( transCont, mlog );
     return persObj;
 }

InnerDetector/InDetEventCnv/InDetEventAthenaPool/src/InDetSimDataCollectionCnv.h

 /*
-  Copyright (C) 2002-2017 CERN for the benefit of the ATLAS collaboration
+  Copyright (C) 2002-2023 CERN for the benefit of the ATLAS collaboration
 */
 
 #ifndef INDETSIMDATACOLLECTIONCNV_H
 #define INDETSIMDATACOLLECTIONCNV_H
-                                                                                                                                                             
-#include "AthenaPoolCnvSvc/T_AthenaPoolCustomCnv.h"
-                                                                                                                                                             
+
 #include "InDetSimData/InDetSimDataCollection.h"
-#include "InDetSimDataCollectionCnv_p1.h"
+#include "AthenaPoolCnvSvc/T_AthenaPoolTwoPersCnv.h"
+// full includes needed for unique_ptr in "TwoPers" converter
 #include "InDetSimDataCollectionCnv_p2.h"
 #include "InDetSimDataCollectionCnv_p3.h"
 
-// Gaudi
-#include "GaudiKernel/MsgStream.h"
+
 // typedef to the latest persistent version
 typedef  InDetSimDataCollection_p3     InDetSimDataCollection_PERS;
-typedef  InDetSimDataCollectionCnv_p3  InDetSimDataCollectionCnv_PERS;
 
 // base class 
-typedef  T_AthenaPoolCustomCnv<InDetSimDataCollection, InDetSimDataCollection_PERS >   InDetSimDataCollectionCnvBase;
+typedef T_AthenaPoolTwoPersCnv< InDetSimDataCollection,
+                                InDetSimDataCollection_PERS,
+                                InDetSimDataCollection_p2 >  InDetSimDataCollectionCnvBase;
 
-class InDetSimDataCollectionCnv : public InDetSimDataCollectionCnvBase {
+class InDetSimDataCollectionCnv : public InDetSimDataCollectionCnvBase
+{
   friend class CnvFactory<InDetSimDataCollectionCnv >;
-                                                                                                                                                             
-protected:
+
 public:
-  InDetSimDataCollectionCnv (ISvcLocator* svcloc) : InDetSimDataCollectionCnvBase(svcloc) {}
+   using InDetSimDataCollectionCnvBase::InDetSimDataCollectionCnvBase;
+   virtual bool         writeOldPers() override final; 
+
 protected:
-  virtual InDetSimDataCollection_PERS*   createPersistent (InDetSimDataCollection* transCont);
-  virtual InDetSimDataCollection* createTransient ();
+  virtual InDetSimDataCollection_PERS* createPersistent( InDetSimDataCollection* transCont ) override final;
+  virtual InDetSimDataCollection_p2*   createOldPersistent( InDetSimDataCollection* transCont ) override final;
+  virtual InDetSimDataCollection*      createTransient () override final;
 };
-                                                                                                                                                             
+   
+
 #endif
 
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