Added helper methods to access struct through reflection. (#4120)

* Added helper methods to access struct through reflection. Also added unit test for it.

* Added a TODO comment to check for the is_struct flag.

include/flatbuffers/reflection.h

@@ -105,6 +105,22 @@ inline Table *GetFieldT(const Table &table,
   return table.GetPointer<Table *>(field.offset());
 }
 
+// Get a field, if you know it's a struct.
+inline const Struct *GetFieldStruct(const Table &table,
+                                    const reflection::Field &field) {
+  // TODO: This does NOT check if the field is a table or struct, but we'd need
+  // access to the schema to check the is_struct flag.
+  assert(field.type()->base_type() == reflection::Obj);
+  return table.GetStruct<const Struct *>(field.offset());
+}
+
+// Get a structure's field, if you know it's a struct.
+inline const Struct *GetFieldStruct(const Struct &structure,
+                                    const reflection::Field &field) {
+  assert(field.type()->base_type() == reflection::Obj);
+  return structure.GetStruct<const Struct *>(field.offset());
+}
+
 // Raw helper functions used below: get any value in memory as a 64bit int, a
 // double or a string.
 // All scalars get static_cast to an int64_t, strings use strtoull, every other

tests/test.cpp

@@ -511,6 +511,20 @@ void ReflectionTest(uint8_t *flatbuf, size_t length) {
   auto hp_string = flatbuffers::GetAnyFieldS(root, hp_field, &schema);
   TEST_EQ_STR(hp_string.c_str(), "80");
 
+  // Get struct field through reflection
+  auto pos_struct = flatbuffers::GetFieldStruct(root, *pos_field_ptr);
+  TEST_NOTNULL(pos_struct);
+  TEST_EQ(flatbuffers::GetAnyFieldF(
+    *pos_struct, *pos_table_ptr->fields()->LookupByKey("z")), 3.0f);
+
+  auto test3_field = pos_table_ptr->fields()->LookupByKey("test3");
+  auto test3_struct = flatbuffers::GetFieldStruct(*pos_struct, *test3_field);
+  TEST_NOTNULL(test3_struct);
+  auto test3_object = schema.objects()->Get(test3_field->type()->index());
+
+  TEST_EQ(flatbuffers::GetAnyFieldF(
+    *test3_struct, *test3_object->fields()->LookupByKey("a")), 10);
+
   // We can also modify it.
   flatbuffers::SetField<uint16_t>(&root, hp_field, 200);
   hp = flatbuffers::GetFieldI<uint16_t>(root, hp_field);