add API to visit union

Summary:
This diff added `visit_union` that applies the callable to active member of thrift union. Example:

  visit_union(thriftUnion, [](const ThriftField& meta, auto&& value) {
    LOG(INFO) << meta.name << " --> " << value;
  })

ThriftField schema is defined here: https://git.io/JJQpY

# `visit_union` vs `std::visit`

1. Parameter order is different. `visit_union` takes callable as last argument for API readability and consistency. [`boost::hana::unpack`](https://www.boost.org/doc/libs/1_74_0/libs/hana/doc/html/group__group-Foldable.html#ga7b0c23944364ce61136e10b978ae2170) chose the same design with explanation:
2. It doesn't allow callable to return a value. Reasons
    1. Prevent misusing when union is `__EMPTY__` (should it be default initialized, value initialized or throw exception?)
    2. This is redundant, user can use lambda-captured reference to return value from callable.
3. It doesn't allow multiple union objects. Using multiple objects are not common, and user can achieve it by nested lambda.

Reviewed By: vitaut

Differential Revision: D23252286

fbshipit-source-id: b053d67605dfaa408b9e07d281a3ea02018642c9

thrift/compiler/generate/t_mstch_cpp2_generator.cc

@@ -2042,6 +2042,9 @@ void t_mstch_cpp2_generator::generate_visitation(const t_program* program) {
       cache_->programs_[id],
       "module_for_each_field.h",
       name + "_for_each_field.h");
+
+  render_to_file(
+      cache_->programs_[id], "module_visit_union.h", name + "_visit_union.h");
 }
 
 void t_mstch_cpp2_generator::generate_structs(t_program const* program) {

thrift/compiler/generate/templates/cpp2/module_visit_union.h.mustache

@@ -0,0 +1,48 @@
+<%!
+
+  Copyright (c) Facebook, Inc. and its affiliates.
+
+  Licensed under the Apache License, Version 2.0 (the "License");
+  you may not use this file except in compliance with the License.
+  You may obtain a copy of the License at
+
+      http://www.apache.org/licenses/LICENSE-2.0
+
+  Unless required by applicable law or agreed to in writing, software
+  distributed under the License is distributed on an "AS IS" BASIS,
+  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+  See the License for the specific language governing permissions and
+  limitations under the License.
+
+%><% > Autogen%>
+#pragma once
+
+#include "<%program:include_prefix%><%program:name%>_metadata.h"
+#include <thrift/lib/cpp2/visitation/visit_union.h>
+
+namespace apache {
+namespace thrift {
+namespace detail {
+
+<%#program:structs%>
+<%#struct:union?%>
+template <>
+struct VisitUnion<<% > common/namespace_cpp2%><%struct:name%>> {
+  template <typename F, typename T>
+  void operator()(F&& f, T&& t) const {
+    using Union = std::remove_reference_t<T>;
+    constexpr auto get_metadata = get_field_metadata<<% > common/namespace_cpp2%><%struct:name%>>;
+    switch (t.getType()) {
+    <%#struct:fields%>
+    case Union::Type::<%field:cpp_name%>:
+      return f(get_metadata(<%field:index%>), *static_cast<T&&>(t).<%field:cpp_name%>_ref());
+    <%/struct:fields%>
+    case Union::Type::__EMPTY__: ;
+    }
+  }
+};
+<%/struct:union?%>
+<%/program:structs%>
+} // namespace detail
+} // namespace thrift
+} // namespace apache

thrift/compiler/test/fixtures/visitation/gen-cpp2/module_visit_union.h

@@ -0,0 +1,163 @@
+/**
+ * Autogenerated by Thrift
+ *
+ * DO NOT EDIT UNLESS YOU ARE SURE THAT YOU KNOW WHAT YOU ARE DOING
+ *  @generated
+ */
+#pragma once
+
+#include "thrift/compiler/test/fixtures/visitation/gen-cpp2/module_metadata.h"
+#include <thrift/lib/cpp2/visitation/visit_union.h>
+
+namespace apache {
+namespace thrift {
+namespace detail {
+
+template <>
+struct VisitUnion<::test_cpp2::cpp_reflection::union1> {
+  template <typename F, typename T>
+  void operator()(F&& f, T&& t) const {
+    using Union = std::remove_reference_t<T>;
+    constexpr auto get_metadata = get_field_metadata<::test_cpp2::cpp_reflection::union1>;
+    switch (t.getType()) {
+    case Union::Type::ui:
+      return f(get_metadata(0), *static_cast<T&&>(t).ui_ref());
+    case Union::Type::ud:
+      return f(get_metadata(1), *static_cast<T&&>(t).ud_ref());
+    case Union::Type::us:
+      return f(get_metadata(2), *static_cast<T&&>(t).us_ref());
+    case Union::Type::ue:
+      return f(get_metadata(3), *static_cast<T&&>(t).ue_ref());
+    case Union::Type::__EMPTY__: ;
+    }
+  }
+};
+template <>
+struct VisitUnion<::test_cpp2::cpp_reflection::union2> {
+  template <typename F, typename T>
+  void operator()(F&& f, T&& t) const {
+    using Union = std::remove_reference_t<T>;
+    constexpr auto get_metadata = get_field_metadata<::test_cpp2::cpp_reflection::union2>;
+    switch (t.getType()) {
+    case Union::Type::ui_2:
+      return f(get_metadata(0), *static_cast<T&&>(t).ui_2_ref());
+    case Union::Type::ud_2:
+      return f(get_metadata(1), *static_cast<T&&>(t).ud_2_ref());
+    case Union::Type::us_2:
+      return f(get_metadata(2), *static_cast<T&&>(t).us_2_ref());
+    case Union::Type::ue_2:
+      return f(get_metadata(3), *static_cast<T&&>(t).ue_2_ref());
+    case Union::Type::__EMPTY__: ;
+    }
+  }
+};
+template <>
+struct VisitUnion<::test_cpp2::cpp_reflection::union3> {
+  template <typename F, typename T>
+  void operator()(F&& f, T&& t) const {
+    using Union = std::remove_reference_t<T>;
+    constexpr auto get_metadata = get_field_metadata<::test_cpp2::cpp_reflection::union3>;
+    switch (t.getType()) {
+    case Union::Type::ui_3:
+      return f(get_metadata(0), *static_cast<T&&>(t).ui_3_ref());
+    case Union::Type::ud_3:
+      return f(get_metadata(1), *static_cast<T&&>(t).ud_3_ref());
+    case Union::Type::us_3:
+      return f(get_metadata(2), *static_cast<T&&>(t).us_3_ref());
+    case Union::Type::ue_3:
+      return f(get_metadata(3), *static_cast<T&&>(t).ue_3_ref());
+    case Union::Type::__EMPTY__: ;
+    }
+  }
+};
+template <>
+struct VisitUnion<::test_cpp2::cpp_reflection::unionA> {
+  template <typename F, typename T>
+  void operator()(F&& f, T&& t) const {
+    using Union = std::remove_reference_t<T>;
+    constexpr auto get_metadata = get_field_metadata<::test_cpp2::cpp_reflection::unionA>;
+    switch (t.getType()) {
+    case Union::Type::i:
+      return f(get_metadata(0), *static_cast<T&&>(t).i_ref());
+    case Union::Type::d:
+      return f(get_metadata(1), *static_cast<T&&>(t).d_ref());
+    case Union::Type::s:
+      return f(get_metadata(2), *static_cast<T&&>(t).s_ref());
+    case Union::Type::e:
+      return f(get_metadata(3), *static_cast<T&&>(t).e_ref());
+    case Union::Type::a:
+      return f(get_metadata(4), *static_cast<T&&>(t).a_ref());
+    case Union::Type::__EMPTY__: ;
+    }
+  }
+};
+template <>
+struct VisitUnion<::test_cpp2::cpp_reflection::union_with_special_names> {
+  template <typename F, typename T>
+  void operator()(F&& f, T&& t) const {
+    using Union = std::remove_reference_t<T>;
+    constexpr auto get_metadata = get_field_metadata<::test_cpp2::cpp_reflection::union_with_special_names>;
+    switch (t.getType()) {
+    case Union::Type::get:
+      return f(get_metadata(0), *static_cast<T&&>(t).get_ref());
+    case Union::Type::getter:
+      return f(get_metadata(1), *static_cast<T&&>(t).getter_ref());
+    case Union::Type::lists:
+      return f(get_metadata(2), *static_cast<T&&>(t).lists_ref());
+    case Union::Type::maps:
+      return f(get_metadata(3), *static_cast<T&&>(t).maps_ref());
+    case Union::Type::name:
+      return f(get_metadata(4), *static_cast<T&&>(t).name_ref());
+    case Union::Type::name_to_value:
+      return f(get_metadata(5), *static_cast<T&&>(t).name_to_value_ref());
+    case Union::Type::names:
+      return f(get_metadata(6), *static_cast<T&&>(t).names_ref());
+    case Union::Type::prefix_tree:
+      return f(get_metadata(7), *static_cast<T&&>(t).prefix_tree_ref());
+    case Union::Type::sets:
+      return f(get_metadata(8), *static_cast<T&&>(t).sets_ref());
+    case Union::Type::setter:
+      return f(get_metadata(9), *static_cast<T&&>(t).setter_ref());
+    case Union::Type::str:
+      return f(get_metadata(10), *static_cast<T&&>(t).str_ref());
+    case Union::Type::strings:
+      return f(get_metadata(11), *static_cast<T&&>(t).strings_ref());
+    case Union::Type::type:
+      return f(get_metadata(12), *static_cast<T&&>(t).type_ref());
+    case Union::Type::value:
+      return f(get_metadata(13), *static_cast<T&&>(t).value_ref());
+    case Union::Type::value_to_name:
+      return f(get_metadata(14), *static_cast<T&&>(t).value_to_name_ref());
+    case Union::Type::values:
+      return f(get_metadata(15), *static_cast<T&&>(t).values_ref());
+    case Union::Type::id:
+      return f(get_metadata(16), *static_cast<T&&>(t).id_ref());
+    case Union::Type::ids:
+      return f(get_metadata(17), *static_cast<T&&>(t).ids_ref());
+    case Union::Type::descriptor:
+      return f(get_metadata(18), *static_cast<T&&>(t).descriptor_ref());
+    case Union::Type::descriptors:
+      return f(get_metadata(19), *static_cast<T&&>(t).descriptors_ref());
+    case Union::Type::key:
+      return f(get_metadata(20), *static_cast<T&&>(t).key_ref());
+    case Union::Type::keys:
+      return f(get_metadata(21), *static_cast<T&&>(t).keys_ref());
+    case Union::Type::annotation:
+      return f(get_metadata(22), *static_cast<T&&>(t).annotation_ref());
+    case Union::Type::annotations:
+      return f(get_metadata(23), *static_cast<T&&>(t).annotations_ref());
+    case Union::Type::member:
+      return f(get_metadata(24), *static_cast<T&&>(t).member_ref());
+    case Union::Type::members:
+      return f(get_metadata(25), *static_cast<T&&>(t).members_ref());
+    case Union::Type::field:
+      return f(get_metadata(26), *static_cast<T&&>(t).field_ref());
+    case Union::Type::fields:
+      return f(get_metadata(27), *static_cast<T&&>(t).fields_ref());
+    case Union::Type::__EMPTY__: ;
+    }
+  }
+};
+} // namespace detail
+} // namespace thrift
+} // namespace apache

thrift/lib/cpp2/visitation/visit_union.h

@@ -0,0 +1,50 @@
+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#include <folly/Traits.h>
+#include <thrift/lib/cpp2/visitation/metadata.h>
+
+namespace apache {
+namespace thrift {
+namespace detail {
+template <class T>
+struct VisitUnion {
+  static_assert(sizeof(T) < 0, "Must include visitation header");
+};
+} // namespace detail
+
+/**
+ * Applies the callable to active member of thrift union. Example:
+ *
+ *   visit_union(thriftUnion, [](const ThriftField& meta, auto&& value) {
+ *     LOG(INFO) << *meta.name_ref() << " --> " << value;
+ *   })
+ *
+ * ThriftField schema is defined here: https://git.io/JJQpY
+ * If `no_metadata` thrift option is enabled, ThriftField will be empty.
+ * If union is empty, callable won't be called.
+ *
+ * @param t thrift union
+ * @param f a callable that accepts all member types from union
+ */
+template <typename T, typename F>
+void visit_union(T&& t, F f) {
+  return detail::VisitUnion<folly::remove_cvref_t<T>>()(f, static_cast<T&&>(t));
+}
+} // namespace thrift
+} // namespace apache

thrift/test/UnionFieldRef.thrift

@@ -17,9 +17,9 @@
 namespace cpp2 apache.thrift.test
 
 union Basic {
-  1: string str
-  2: i64 int64
-  3: list<i32> list_i32
+  2: string str
+  1: i64 int64
+  4: list<i32> list_i32
 }
 
 union DuplicateType {

thrift/test/visitation_visit_union_test.cpp

@@ -0,0 +1,73 @@
+/*
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <folly/portability/GTest.h>
+#include <thrift/test/gen-cpp2/UnionFieldRef_visit_union.h> // @manual=:union_field_ref-cpp2-visitation
+using namespace std;
+
+namespace apache {
+namespace thrift {
+namespace test {
+TEST(UnionFieldTest, basic) {
+  Basic a;
+  visit_union(a, [&](auto&&, auto&&) { FAIL(); });
+
+  static const string str = "foo";
+  a.str_ref() = str;
+  visit_union(a, [](auto&& meta, auto&& v) {
+    EXPECT_EQ(meta.name, "str");
+    EXPECT_EQ(meta.type.getType(), meta.type.t_primitive);
+    EXPECT_EQ(meta.id, 2);
+    EXPECT_EQ(meta.is_optional, false);
+    if constexpr (std::is_same_v<decltype(v), string&>) {
+      EXPECT_EQ(v, str);
+    } else {
+      FAIL();
+    }
+  });
+
+  static const int64_t int64 = 42LL << 42;
+  a.int64_ref() = int64;
+  visit_union(a, [](auto&& meta, auto&& v) {
+    EXPECT_EQ(meta.name, "int64");
+    EXPECT_EQ(meta.type.getType(), meta.type.t_primitive);
+    EXPECT_EQ(meta.id, 1);
+    EXPECT_EQ(meta.is_optional, false);
+    EXPECT_EQ(typeid(v), typeid(int64_t));
+    if constexpr (std::is_same_v<decltype(v), int64_t&>) {
+      EXPECT_EQ(v, int64);
+    } else {
+      FAIL();
+    }
+  });
+
+  static const vector<int32_t> list_i32 = {3, 1, 2};
+  a.list_i32_ref() = list_i32;
+  visit_union(a, [](auto&& meta, auto&& v) {
+    EXPECT_EQ(meta.name, "list_i32");
+    EXPECT_EQ(meta.type.getType(), meta.type.t_list);
+    EXPECT_EQ(meta.id, 4);
+    EXPECT_EQ(meta.is_optional, false);
+    if constexpr (std::is_same_v<decltype(v), vector<int32_t>&>) {
+      EXPECT_EQ(v, list_i32);
+    } else {
+      FAIL();
+    }
+  });
+}
+} // namespace test
+} // namespace thrift
+} // namespace apache