[ntuple] Implement RField<TObject>::ReadInClusterImpl

Otherwise RFieldBase::ReadInClusterImpl tries to use the principal
column to translate to a global index, which results in a nullptr
dereference.

Fixes #18404

Author: hahnjo

Diff for: tree/ntuple/inc/ROOT/RField.hxx

@@ -473,6 +473,7 @@ protected:
    std::size_t AppendImpl(const void *from) final;
    void ReadTObject(void *to, UInt_t uniqueID, UInt_t bits);
    void ReadGlobalImpl(ROOT::NTupleSize_t globalIndex, void *to) final;
+   void ReadInClusterImpl(RNTupleLocalIndex localIndex, void *to) final;
 
    void AfterConnectPageSource() final;
 

Diff for: tree/ntuple/src/RFieldMeta.cxx

@@ -1005,6 +1005,14 @@ void ROOT::RField<TObject>::ReadGlobalImpl(ROOT::NTupleSize_t globalIndex, void
    ReadTObject(to, uniqueID, bits);
 }
 
+void ROOT::RField<TObject>::ReadInClusterImpl(RNTupleLocalIndex localIndex, void *to)
+{
+   UInt_t uniqueID, bits;
+   CallReadOn(*fSubfields[0], localIndex, &uniqueID);
+   CallReadOn(*fSubfields[1], localIndex, &bits);
+   ReadTObject(to, uniqueID, bits);
+}
+
 void ROOT::RField<TObject>::AfterConnectPageSource()
 {
    if (GetOnDiskTypeVersion() != 1) {

Diff for: tree/ntuple/test/rfield_vector.cxx

@@ -1,5 +1,7 @@
 #include "ntuple_test.hxx"
 
+#include <TObject.h>
+
 // A layer of indirection to hide std::vector's second template parameter.
 // This way we can generalize tests over RVec and std::vector using a template template parameter (see below).
 template <typename T>
@@ -494,3 +496,40 @@ TEST(RNTuple, VectorOfBitset)
    EXPECT_EQ("111", ptrVecOfBitset->at(0).to_string());
    EXPECT_EQ("000", ptrVecOfBitset->at(1).to_string());
 }
+
+TEST(RNTuple, VectorOfTObject)
+{
+   FileRaii fileGuard("test_ntuple_vector_of_tobject.root");
+   {
+      auto model = RNTupleModel::Create();
+      auto objs = model->MakeField<std::vector<TObject>>("objs");
+      auto writer = RNTupleWriter::Recreate(std::move(model), "ntpl", fileGuard.GetPath());
+
+      TObject obj;
+      obj.SetUniqueID(42);
+      objs->push_back(obj);
+      obj.SetUniqueID(43);
+      objs->push_back(obj);
+      writer->Fill();
+      writer->CommitCluster();
+
+      objs->clear();
+      obj.SetUniqueID(44);
+      objs->push_back(obj);
+      writer->Fill();
+   }
+
+   auto reader = RNTupleReader::Open("ntpl", fileGuard.GetPath());
+   ASSERT_EQ(2u, reader->GetNEntries());
+   auto &entry = reader->GetModel().GetDefaultEntry();
+   auto objs = entry.GetPtr<std::vector<TObject>>("objs");
+
+   reader->LoadEntry(0);
+   ASSERT_EQ(2u, objs->size());
+   EXPECT_EQ(42u, objs->at(0).GetUniqueID());
+   EXPECT_EQ(43u, objs->at(1).GetUniqueID());
+
+   reader->LoadEntry(1);
+   ASSERT_EQ(1u, objs->size());
+   EXPECT_EQ(44u, objs->at(0).GetUniqueID());
+}