[ntuple] prevent writable access to RValues in REntry

tree/ntuple/v7/inc/ROOT/REntry.hxx

@@ -40,7 +40,10 @@ that are associated to values are managed.
 */
 // clang-format on
 class REntry {
+   friend class RCollectionNTupleWriter;
    friend class RNTupleModel;
+   friend class RNTupleReader;
+   friend class RNTupleWriter;
 
    /// The entry must be linked to a specific model (or one if its clones), identified by a model ID
    std::uint64_t fModelId = 0;
@@ -66,8 +69,24 @@ class REntry {
       return ptr;
    }
 
+   void Read(NTupleSize_t index)
+   {
+      for (auto &v : fValues) {
+         v.Read(index);
+      }
+   }
+
+   std::size_t Append()
+   {
+      std::size_t bytesWritten = 0;
+      for (auto &v : fValues) {
+         bytesWritten += v.Append();
+      }
+      return bytesWritten;
+   }
+
 public:
-   using Iterator_t = decltype(fValues)::iterator;
+   using ConstIterator_t = decltype(fValues)::const_iterator;
 
    REntry(const REntry &other) = delete;
    REntry &operator=(const REntry &other) = delete;
@@ -101,8 +120,8 @@ public:
 
    std::uint64_t GetModelId() const { return fModelId; }
 
-   Iterator_t begin() { return fValues.begin(); }
-   Iterator_t end() { return fValues.end(); }
+   ConstIterator_t begin() const { return fValues.cbegin(); }
+   ConstIterator_t end() const { return fValues.cend(); }
 };
 
 } // namespace Experimental

tree/ntuple/v7/inc/ROOT/RNTuple.hxx

@@ -253,11 +253,7 @@ public:
       LoadEntry(index, *fModel->GetDefaultEntry());
    }
    /// Fills a user provided entry after checking that the entry has been instantiated from the ntuple model
-   void LoadEntry(NTupleSize_t index, REntry &entry) {
-      for (auto& value : entry) {
-         value.Read(index);
-      }
-   }
+   void LoadEntry(NTupleSize_t index, REntry &entry) { entry.Read(index); }
 
    /// Returns an iterator over the entry indices of the RNTuple.
    ///
@@ -412,10 +408,7 @@ public:
       if (R__unlikely(entry.GetModelId() != fModel->GetModelId()))
          throw RException(R__FAIL("mismatch between entry and model"));
 
-      std::size_t bytesWritten = 0;
-      for (auto& value : entry) {
-         bytesWritten += value.Append();
-      }
+      const std::size_t bytesWritten = entry.Append();
       fUnzippedClusterSize += bytesWritten;
       fNEntries++;
       if ((fUnzippedClusterSize >= fMaxUnzippedClusterSize) || (fUnzippedClusterSize >= fUnzippedClusterSizeEst))
@@ -487,9 +480,7 @@ public:
 
    void Fill() { Fill(fDefaultEntry.get()); }
    void Fill(REntry *entry) {
-      for (auto &value : *entry) {
-         value.Append();
-      }
+      entry->Append();
       fOffset++;
    }