diff --git a/upbc/generator.cc b/upbc/generator.cc
index dbbccf0a48..08f488fbea 100644
--- a/upbc/generator.cc
+++ b/upbc/generator.cc
@@ -704,25 +704,38 @@ int TableDescriptorType(const protobuf::FieldDescriptor* field) {
 }
 
 struct SubmsgArray {
-  std::vector<const protobuf::Descriptor*> messages;
-  absl::flat_hash_map<const protobuf::Descriptor*, int> indexes;
-};
-
-SubmsgArray GetSubmsgArray(const protobuf::Descriptor* message) {
-  SubmsgArray ret;
-  MessageLayout layout(message);
-  std::vector<const protobuf::FieldDescriptor*> sorted_submsgs =
-      SortedSubmessages(message);
-  int i = 0;
-  for (auto submsg : sorted_submsgs) {
-    if (ret.indexes.find(submsg->message_type()) != ret.indexes.end()) {
-      continue;
+ public:
+  SubmsgArray(const protobuf::Descriptor* message) : message_(message) {
+    MessageLayout layout(message);
+    std::vector<const protobuf::FieldDescriptor*> sorted_submsgs =
+        SortedSubmessages(message);
+    int i = 0;
+    for (auto submsg : sorted_submsgs) {
+      if (indexes_.find(submsg->message_type()) != indexes_.end()) {
+        continue;
+      }
+      submsgs_.push_back(submsg->message_type());
+      indexes_[submsg->message_type()] = i++;
     }
-    ret.messages.push_back(submsg->message_type());
-    ret.indexes[submsg->message_type()] = i++;
   }
-  return ret;
-}
+
+  const std::vector<const protobuf::Descriptor*>& submsgs() const {
+    return submsgs_;
+  }
+
+  int GetIndex(const protobuf::FieldDescriptor* field) {
+    (void)message_;
+    assert(field->containing_type() == message_);
+    auto it = indexes_.find(field->message_type());
+    assert(it != indexes_.end());
+    return it->second;
+  }
+
+ private:
+  const protobuf::Descriptor* message_;
+  std::vector<const protobuf::Descriptor*> submsgs_;
+  absl::flat_hash_map<const protobuf::Descriptor*, int> indexes_;
+};
 
 void WriteSource(const protobuf::FileDescriptor* file, Output& output) {
   EmitFileWarning(file, output);
@@ -748,17 +761,17 @@ void WriteSource(const protobuf::FileDescriptor* file, Output& output) {
     std::string fields_array_ref = "NULL";
     std::string submsgs_array_ref = "NULL";
     MessageLayout layout(message);
-    SubmsgArray submsg_array = GetSubmsgArray(message);
+    SubmsgArray submsg_array(message);
 
-    if (!submsg_array.messages.empty()) {
+    if (!submsg_array.submsgs().empty()) {
       // TODO(haberman): could save a little bit of space by only generating a
       // "submsgs" array for every strongly-connected component.
       std::string submsgs_array_name = msgname + "_submsgs";
       submsgs_array_ref = "&" + submsgs_array_name + "[0]";
       output("static const upb_msglayout *const $0[$1] = {\n",
-             submsgs_array_name, submsg_array.messages.size());
+             submsgs_array_name, submsg_array.submsgs().size());
 
-      for (auto submsg : submsg_array.messages) {
+      for (auto submsg : submsg_array.submsgs()) {
         output("  &$0,\n", MessageInit(submsg));
       }
 
@@ -777,7 +790,7 @@ void WriteSource(const protobuf::FileDescriptor* file, Output& output) {
         std::string presence = "0";
 
         if (field->cpp_type() == protobuf::FieldDescriptor::CPPTYPE_MESSAGE) {
-          submsg_index = submsg_array.indexes[field->message_type()];
+          submsg_index = submsg_array.GetIndex(field);
         }
 
         if (MessageLayout::HasHasbit(field)) {