[ntuple] Allow inline definition of RNTuple model in RNTupleWriter::Recreate

Fixes root-project#8711

Author: maszyman

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

@@ -473,6 +473,10 @@ public:
                                                   std::string_view ntupleName,
                                                   std::string_view storage,
                                                   const RNTupleWriteOptions &options = RNTupleWriteOptions());
+   static std::unique_ptr<RNTupleWriter> Recreate(std::initializer_list<std::pair<std::string_view, std::string_view>> fields,
+                                                  std::string_view ntupleName,
+                                                  std::string_view storage,
+                                                  const RNTupleWriteOptions &options = RNTupleWriteOptions());
    /// Throws an exception if the model is null.
    static std::unique_ptr<RNTupleWriter> Append(std::unique_ptr<RNTupleModel> model, std::string_view ntupleName,
                                                 TFile &file,

tree/ntuple/v7/src/RNTuple.cxx

@@ -365,6 +365,21 @@ ROOT::Experimental::RNTupleWriter::Recreate(std::unique_ptr<RNTupleModel> model,
    return Create(std::move(model), std::move(sink), options);
 }
 
+std::unique_ptr<ROOT::Experimental::RNTupleWriter>
+ROOT::Experimental::RNTupleWriter::Recreate(std::initializer_list<std::pair<std::string_view, std::string_view>> fields, std::string_view ntupleName,
+                                            std::string_view storage, const RNTupleWriteOptions &options)
+{
+   auto sink = Detail::RPagePersistentSink::Create(ntupleName, storage, options);
+   auto model = RNTupleModel::Create();
+   for ( const auto& fieldDesc : fields ) {
+      std::string typeName(fieldDesc.first);
+      std::string fieldName(fieldDesc.second);
+      auto field = RFieldBase::Create(fieldName, typeName);
+      model->AddField(field.Unwrap());
+   }
+   return Create(std::move(model), std::move(sink), options);
+}
+
 std::unique_ptr<ROOT::Experimental::RNTupleWriter>
 ROOT::Experimental::RNTupleWriter::Append(std::unique_ptr<RNTupleModel> model, std::string_view ntupleName, TFile &file,
                                           const RNTupleWriteOptions &options)

tree/ntuple/v7/test/ntuple_basics.cxx

@@ -147,6 +147,47 @@ TEST(RNTuple, WriteRead)
    EXPECT_STREQ("abc", rdKlass->s.c_str());
 }
 
+TEST(RNTuple, WriteReadInlinedModel)
+{
+   FileRaii fileGuard("test_ntuple_writeread_inlinedmodel.root");
+
+   {
+      auto writer = RNTupleWriter::Recreate({
+         {"std::uint32_t", "id"},
+         {"std::vector<float>", "vpx"},
+         {"std::vector<float>", "vpy"},
+         {"std::vector<float>", "vpz"},
+      }, "NTuple", fileGuard.GetPath());
+
+      auto entry = writer->CreateEntry();
+      *entry->GetPtr<std::uint32_t>("id") = 1;
+      *entry->GetPtr<std::vector<float>>("vpx") = {1.0, 1.1, 1.2};
+      *entry->GetPtr<std::vector<float>>("vpy") = {2.0, 2.1, 2.2};
+      *entry->GetPtr<std::vector<float>>("vpz") = {3.0, 3.1, 3.2};
+
+      writer->Fill(*entry);
+   }
+
+   auto reader = RNTupleReader::Open("NTuple", fileGuard.GetPath());
+   EXPECT_EQ(1U, reader->GetNEntries());
+   reader->LoadEntry(0);
+   auto readid = reader->GetModel().GetDefaultEntry().GetPtr<std::uint32_t>("id");
+   EXPECT_EQ(1, *readid);
+   auto readvpx = reader->GetModel().GetDefaultEntry().GetPtr<std::vector<float>>("vpx");
+   EXPECT_FLOAT_EQ(1.0, (*readvpx)[0]);
+   EXPECT_FLOAT_EQ(1.1, (*readvpx)[1]);
+   EXPECT_FLOAT_EQ(1.2, (*readvpx)[2]);
+   auto readvpy = reader->GetModel().GetDefaultEntry().GetPtr<std::vector<float>>("vpy");
+   EXPECT_FLOAT_EQ(2.0, (*readvpy)[0]);
+   EXPECT_FLOAT_EQ(2.1, (*readvpy)[1]);
+   EXPECT_FLOAT_EQ(2.2, (*readvpy)[2]);
+   auto readvpz = reader->GetModel().GetDefaultEntry().GetPtr<std::vector<float>>("vpz");
+   EXPECT_FLOAT_EQ(3.0, (*readvpz)[0]);
+   EXPECT_FLOAT_EQ(3.1, (*readvpz)[1]);
+   EXPECT_FLOAT_EQ(3.2, (*readvpz)[2]);
+
+}
+
 TEST(RNTuple, FileAnchor)
 {
    FileRaii fileGuard("test_ntuple_file_anchor.root");