Add test for std::variant

Closes #4

Author: hahnjo

types/README.md

@@ -2,4 +2,5 @@
 
  * [`fundamental`](fundamental): fundamental column 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/fundamental/README.md

@@ -6,7 +6,7 @@
 
 __Covered under a different category:__
  * `[Split]Index{32,64}`: with [`std::string`](../string) and [`std::vector`](../vector)
- * `Switch`
+ * `Switch`: with [`std::variant`](../variant)
 
 __Missing:__
  * `Real32Trunc`

types/variant/README.md

@@ -0,0 +1,17 @@
+# `std::variant`
+
+## Fields
+
+ * `f` of type `std::variant<std::int32_t, std::string, std::vector<std::int32_t>>`
+
+## Entries
+
+1. `std::int32_t` with value `1`
+2. `std::string` with value `"abc"`
+3. `std::vector<std::int32_t>` with value `{1, 2, 3}`
+4. Empty `std::string`
+5. Empty `std::vector`
+
+__Missing:__
+A `std::variant` that is `valueless_by_exception`.
+This requires a type that throws during move assignment.

types/variant/read.C

@@ -0,0 +1,76 @@
+#include <ROOT/REntry.hxx>
+#include <ROOT/RNTupleReader.hxx>
+
+using ROOT::Experimental::REntry;
+using ROOT::Experimental::RNTupleReader;
+
+#include <cstdint>
+#include <fstream>
+#include <ostream>
+#include <string>
+#include <string_view>
+#include <variant>
+#include <vector>
+
+using Vector = std::vector<std::int32_t>;
+using Variant = std::variant<std::int32_t, std::string, Vector>;
+
+static void PrintVariantValue(const REntry &entry, std::string_view name,
+                              std::ostream &os, bool last = false) {
+  Variant &value = *entry.GetPtr<Variant>(name);
+  os << "    \"" << name << "\": ";
+  if (value.index() == 0) {
+    os << std::get<std::int32_t>(value);
+  } else if (value.index() == 1) {
+    os << "\"" << std::get<std::string>(value) << "\"";
+  } else if (value.index() == 2) {
+    Vector &vectorValue = std::get<Vector>(value);
+    os << "[";
+    bool first = true;
+    for (auto element : vectorValue) {
+      if (first) {
+        first = false;
+      } else {
+        os << ",";
+      }
+      os << "\n      " << element;
+    }
+    if (!vectorValue.empty()) {
+      os << "\n    ";
+    }
+    os << "]";
+  }
+
+  if (!last) {
+    os << ",";
+  }
+  os << "\n";
+}
+
+void read(std::string_view input = "types.variant.root",
+          std::string_view output = "types.variant.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";
+
+    PrintVariantValue(entry, "f", os, /*last=*/true);
+
+    os << "  }";
+    // Newline is intentionally missing, may need to print a comma before the
+    // next entry.
+  }
+  os << "\n";
+  os << "]\n";
+}

types/variant/write.C

@@ -0,0 +1,47 @@
+#include <ROOT/RNTupleModel.hxx>
+#include <ROOT/RNTupleWriteOptions.hxx>
+#include <ROOT/RNTupleWriter.hxx>
+
+using ROOT::Experimental::RNTupleModel;
+using ROOT::Experimental::RNTupleWriteOptions;
+using ROOT::Experimental::RNTupleWriter;
+
+#include <cstdint>
+#include <string>
+#include <string_view>
+#include <variant>
+#include <vector>
+
+using Vector = std::vector<std::int32_t>;
+using Variant = std::variant<std::int32_t, std::string, Vector>;
+
+void write(std::string_view filename = "types.variant.root") {
+  auto model = RNTupleModel::Create();
+
+  auto value = model->MakeField<Variant>("f");
+
+  RNTupleWriteOptions options;
+  options.SetCompression(0);
+  auto writer =
+      RNTupleWriter::Recreate(std::move(model), "ntpl", filename, options);
+
+  // First entry: std::int32_t
+  *value = 1;
+  writer->Fill();
+
+  // Second entry: std::string
+  *value = "abc";
+  writer->Fill();
+
+  // Third entry: std::vector<std::int32_t>
+  *value = Vector{1, 2, 3};
+  writer->Fill();
+
+  // Fourth entry: empty std::string
+  *value = "";
+  writer->Fill();
+
+  // Fifth entry: empty std::vector
+  *value = {};
+  writer->Fill();
+}