[C++] Additional accessors for fields of union types. (#4184)

* Additional accessors for fields of union types.

* Reveal the hidden parameter.

* External explicit specializations for templated accessors.

* Changes according to code style. Generated code included as well.

src/idl_gen_cpp.cpp

@@ -1118,6 +1118,38 @@ class CppGenerator : public BaseGenerator {
       code_ += "    return {{FIELD_VALUE}};";
       code_ += "  }";
 
+      if (field.value.type.base_type == BASE_TYPE_UNION) {
+        auto u = field.value.type.enum_def;
+
+        code_ += "  template<typename T> "
+                "const T *{{FIELD_NAME}}_as() const;";
+
+        for (auto u_it = u->vals.vec.begin();
+             u_it != u->vals.vec.end(); ++u_it) {
+          if (!(*u_it)->struct_def) {
+            continue;
+          }
+          auto arg_struct_def = (*u_it)->struct_def;
+          auto full_struct_name = WrapInNameSpace(*arg_struct_def);
+
+          // @TODO: Mby make this decisions more universal? How?
+          code_.SetValue("U_GET_TYPE", field.name + UnionTypeFieldSuffix());
+          code_.SetValue("U_ELEMENT_TYPE", WrapInNameSpace(
+                         u->defined_namespace, GetEnumValUse(*u, **u_it)));
+          code_.SetValue("U_FIELD_TYPE", "const " + full_struct_name + " *");
+          code_.SetValue("U_ELEMENT_NAME", full_struct_name);
+          code_.SetValue("U_FIELD_NAME",
+                         field.name + "_as_" + (*u_it)->name);
+
+          // `const Type *union_name_asType() const` accessor.
+          code_ += "  {{U_FIELD_TYPE}}{{U_FIELD_NAME}}() const {";
+          code_ += "    return ({{U_GET_TYPE}}() == {{U_ELEMENT_TYPE}})? "
+                  "static_cast<{{U_FIELD_TYPE}}>({{FIELD_NAME}}()) "
+                  ": nullptr;";
+          code_ += "  }";
+        }
+      }
+
       if (parser_.opts.mutable_buffer) {
         if (is_scalar) {
           code_.SetValue("OFFSET_NAME", offset_str);
@@ -1222,6 +1254,44 @@ class CppGenerator : public BaseGenerator {
     code_ += "};";  // End of table.
     code_ += "";
 
+    // Explicit specializations for union accessors
+    for (auto it = struct_def.fields.vec.begin();
+         it != struct_def.fields.vec.end(); ++it) {
+      const auto &field = **it;
+      if (field.deprecated ||
+          field.value.type.base_type != BASE_TYPE_UNION) {
+        continue;
+      }
+
+      auto u = field.value.type.enum_def;
+      code_.SetValue("FIELD_NAME", field.name);
+
+      for (auto u_it = u->vals.vec.begin();
+           u_it != u->vals.vec.end(); ++u_it) {
+        if (!(*u_it)->struct_def) {
+          continue;
+        }
+
+        auto arg_struct_def = (*u_it)->struct_def;
+        auto full_struct_name = WrapInNameSpace(*arg_struct_def);
+
+        code_.SetValue("U_ELEMENT_TYPE", WrapInNameSpace(
+                       u->defined_namespace, GetEnumValUse(*u, **u_it)));
+        code_.SetValue("U_FIELD_TYPE", "const " + full_struct_name + " *");
+        code_.SetValue("U_ELEMENT_NAME", full_struct_name);
+        code_.SetValue("U_FIELD_NAME",
+                       field.name + "_as_" + (*u_it)->name);
+
+        // `template<> const T *union_name_as<T>() const` accessor.
+        code_ += "template<> "
+                "inline {{U_FIELD_TYPE}}{{STRUCT_NAME}}::{{FIELD_NAME}}_as"
+                "<{{U_ELEMENT_NAME}}>() const {";
+        code_ += "  return {{U_FIELD_NAME}}();";
+        code_ += "}";
+        code_ += "";
+      }
+    }
+
     GenBuilders(struct_def);
 
     if (parser_.opts.generate_object_based_api) {

tests/monster_test_generated.h

@@ -582,6 +582,16 @@ struct Monster FLATBUFFERS_FINAL_CLASS : private flatbuffers::Table {
   const void *test() const {
     return GetPointer<const void *>(VT_TEST);
   }
+  template<typename T> const T *test_as() const;
+  const Monster *test_as_Monster() const {
+    return (test_type() == Any_Monster)? static_cast<const Monster *>(test()) : nullptr;
+  }
+  const TestSimpleTableWithEnum *test_as_TestSimpleTableWithEnum() const {
+    return (test_type() == Any_TestSimpleTableWithEnum)? static_cast<const TestSimpleTableWithEnum *>(test()) : nullptr;
+  }
+  const MyGame::Example2::Monster *test_as_MyGame_Example2_Monster() const {
+    return (test_type() == Any_MyGame_Example2_Monster)? static_cast<const MyGame::Example2::Monster *>(test()) : nullptr;
+  }
   void *mutable_test() {
     return GetPointer<void *>(VT_TEST);
   }
@@ -762,6 +772,18 @@ struct Monster FLATBUFFERS_FINAL_CLASS : private flatbuffers::Table {
   static flatbuffers::Offset<Monster> Pack(flatbuffers::FlatBufferBuilder &_fbb, const MonsterT* _o, const flatbuffers::rehasher_function_t *_rehasher = nullptr);
 };
 
+template<> inline const Monster *Monster::test_as<Monster>() const {
+  return test_as_Monster();
+}
+
+template<> inline const TestSimpleTableWithEnum *Monster::test_as<TestSimpleTableWithEnum>() const {
+  return test_as_TestSimpleTableWithEnum();
+}
+
+template<> inline const MyGame::Example2::Monster *Monster::test_as<MyGame::Example2::Monster>() const {
+  return test_as_MyGame_Example2_Monster();
+}
+
 struct MonsterBuilder {
   flatbuffers::FlatBufferBuilder &fbb_;
   flatbuffers::uoffset_t start_;