Add test for std::optional

Closes #15

Author: hahnjo

types/README.md

@@ -1,6 +1,7 @@
 # Types
 
  * [`fundamental`](fundamental): fundamental column types
+ * [`optional`](optional): `std::optional` with different element types
  * [`string`](string): `std::string` with all `[Split]Index{32,64}` column types
  * [`variant`](variant): `std::variant` with `Switch` column type
  * [`vector`](vector): `std::vector` with all `[Split]Index{32,64}` column types

types/optional/README.md

@@ -0,0 +1,15 @@
+# `std::optional`
+
+## Fields
+
+ * `Int32`
+ * `String`
+ * `Vector`
+
+with the corresponding element type and the default column types.
+
+## Entries
+
+1. Simple values
+2. No value
+3. Null / empty values

types/optional/read.C

@@ -0,0 +1,88 @@
+#include <ROOT/REntry.hxx>
+#include <ROOT/RNTupleReader.hxx>
+
+using ROOT::Experimental::REntry;
+using ROOT::Experimental::RNTupleReader;
+
+#include <cstdint>
+#include <fstream>
+#include <optional>
+#include <ostream>
+#include <string>
+#include <string_view>
+#include <vector>
+
+using VectorTy = std::vector<std::int32_t>;
+
+template <typename T> static void PrintValue(const T &value, std::ostream &os);
+
+template <> void PrintValue(const std::int32_t &value, std::ostream &os) {
+  os << value;
+}
+
+template <> void PrintValue(const std::string &value, std::ostream &os) {
+  os << "\"" << value << "\"";
+}
+
+template <> void PrintValue(const VectorTy &value, std::ostream &os) {
+  os << "[";
+  bool first = true;
+  for (auto element : value) {
+    if (first) {
+      first = false;
+    } else {
+      os << ",";
+    }
+    os << "\n      " << element;
+  }
+  if (!value.empty()) {
+    os << "\n    ";
+  }
+  os << "]";
+}
+
+template <typename T>
+static void PrintOptionalValue(const REntry &entry, std::string_view name,
+                               std::ostream &os, bool last = false) {
+  auto &value = *entry.GetPtr<std::optional<T>>(name);
+  os << "    \"" << name << "\": ";
+  if (!value.has_value()) {
+    os << "null";
+  } else {
+    PrintValue(*value, os);
+  }
+  if (!last) {
+    os << ",";
+  }
+  os << "\n";
+}
+
+void read(std::string_view input = "types.optional.root",
+          std::string_view output = "types.optional.json") {
+  std::ofstream os(std::string{output});
+  os << "[\n";
+
+  auto reader = RNTupleReader::Open("ntpl", input);
+  auto &entry = reader->GetModel().GetDefaultEntry();
+  bool first = true;
+  for (auto index : *reader) {
+    reader->LoadEntry(index);
+
+    if (first) {
+      first = false;
+    } else {
+      os << ",\n";
+    }
+    os << "  {\n";
+
+    PrintOptionalValue<std::int32_t>(entry, "Int32", os);
+    PrintOptionalValue<std::string>(entry, "String", os);
+    PrintOptionalValue<VectorTy>(entry, "Vector", os, /*last=*/true);
+
+    os << "  }";
+    // Newline is intentionally missing, may need to print a comma before the
+    // next entry.
+  }
+  os << "\n";
+  os << "]\n";
+}

types/optional/write.C

@@ -0,0 +1,51 @@
+#include <ROOT/RField.hxx>
+#include <ROOT/RNTupleModel.hxx>
+#include <ROOT/RNTupleUtil.hxx>
+#include <ROOT/RNTupleWriteOptions.hxx>
+#include <ROOT/RNTupleWriter.hxx>
+
+using ROOT::Experimental::EColumnType;
+using ROOT::Experimental::RField;
+using ROOT::Experimental::RNTupleModel;
+using ROOT::Experimental::RNTupleWriteOptions;
+using ROOT::Experimental::RNTupleWriter;
+
+#include <cstdint>
+#include <memory>
+#include <optional>
+#include <string>
+#include <string_view>
+#include <vector>
+
+using VectorTy = std::vector<std::int32_t>;
+
+void write(std::string_view filename = "types.optional.root") {
+  auto model = RNTupleModel::Create();
+
+  auto Int32 = model->MakeField<std::optional<std::int32_t>>("Int32");
+  auto String = model->MakeField<std::optional<std::string>>("String");
+  auto Vector = model->MakeField<std::optional<VectorTy>>("Vector");
+
+  RNTupleWriteOptions options;
+  options.SetCompression(0);
+  auto writer =
+      RNTupleWriter::Recreate(std::move(model), "ntpl", filename, options);
+
+  // First entry: simple values
+  *Int32 = 1;
+  *String = "abc";
+  *Vector = {1, 2, 3};
+  writer->Fill();
+
+  // Second entry: no value
+  Int32->reset();
+  String->reset();
+  Vector->reset();
+  writer->Fill();
+
+  // Third entry: null / empty value
+  *Int32 = 0;
+  *String = "";
+  *Vector = VectorTy{};
+  writer->Fill();
+}