From c6ffeadd2b3a1b6d8101c1eeb25b36b9d4605465 Mon Sep 17 00:00:00 2001
From: Oliver Lantwin <oliver.lantwin@cern.ch>
Date: Wed, 22 Jul 2026 11:52:46 +0200
Subject: [PATCH] Fix FORM's RNTuple backend build against ROOT 6.40

ROOT 6.40 moved RRawPtrWriteEntry from ROOT::Experimental::Detail to
ROOT::Detail. Select the namespace with a ROOT version check so the
RNTuple storage backend (FORM_USE_RNTUPLE_STORAGE=ON) compiles against
both ROOT 6.38 and 6.40.

Fixes #712.
---
 .../root_rntuple_write_container.hpp            | 17 ++++++++++++++++-
 1 file changed, 16 insertions(+), 1 deletion(-)

diff --git a/form/root_storage/root_rntuple_write_container.hpp b/form/root_storage/root_rntuple_write_container.hpp
index 8a582fb21..e386e120a 100644
--- a/form/root_storage/root_rntuple_write_container.hpp
+++ b/form/root_storage/root_rntuple_write_container.hpp
@@ -5,6 +5,8 @@
 
 #include "storage/storage_write_association.hpp"
 
+#include "RVersion.h"
+
 #include <memory>
 #include <string>
 
@@ -14,15 +16,28 @@ namespace ROOT {
   class RNTupleWriter;
   class RNTupleModel;
 
+#if ROOT_VERSION_CODE >= ROOT_VERSION(6, 40, 0)
+  namespace Detail {
+    class RRawPtrWriteEntry;
+  }
+#else
   namespace Experimental {
     namespace Detail {
       class RRawPtrWriteEntry;
     }
   }
+#endif
 }
 
 namespace form::detail::experimental {
 
+  //ROOT 6.40 moved RRawPtrWriteEntry from ROOT::Experimental::Detail to ROOT::Detail.
+#if ROOT_VERSION_CODE >= ROOT_VERSION(6, 40, 0)
+  using RRawPtrWriteEntry = ROOT::Detail::RRawPtrWriteEntry;
+#else
+  using RRawPtrWriteEntry = ROOT::Experimental::Detail::RRawPtrWriteEntry;
+#endif
+
   class ROOT_RNTuple_Write_ContainerImp : public Storage_Write_Association {
   public:
     ROOT_RNTuple_Write_ContainerImp(std::string const& name);
@@ -43,7 +58,7 @@ namespace form::detail::experimental {
     //State shared by ROOT_RField_ContainerImps
     std::unique_ptr<ROOT::RNTupleWriter> m_writer;
     std::unique_ptr<ROOT::RNTupleModel> m_model;
-    std::unique_ptr<ROOT::Experimental::Detail::RRawPtrWriteEntry> m_entry;
+    std::unique_ptr<RRawPtrWriteEntry> m_entry;
   };
 }