Use internal::MessageGlobalsBase and default_instance() API.

Using `MessageGlobalsBase::default_instance(const void*)` allows transparent
handling of returning default instance at an arbitrary (but constant) offset
from "message globals".

PiperOrigin-RevId: 868373777

Author: protobuf-github-bot

src/google/protobuf/compiler/cpp/field_generators/message_field.cc

@@ -44,16 +44,19 @@ std::vector<Sub> Vars(const FieldDescriptor* field, const Options& opts,
   bool is_foreign = IsCrossFileMessage(field);
   std::string field_name = FieldMemberName(field, split);
   std::string qualified_type = FieldMessageTypeName(field, opts);
-  std::string default_ref =
-      QualifiedMsgGlobalsInstanceName(field->message_type(), opts);
+
   std::string base = absl::StrCat(
       "::", ProtobufNamespace(opts), "::",
       HasDescriptorMethods(field->file(), opts) ? "Message" : "MessageLite");
 
   return {
       {"Submsg", qualified_type},
       {"MemberType", use_base_class ? base : qualified_type},
-      {"kDefault", default_ref},
+      {"kDefaultRef",
+       absl::Substitute(
+           "*::google::protobuf::internal::MessageGlobalsBase::default_instance<$0>(&$1)",
+           qualified_type,
+           QualifiedMsgGlobalsInstanceName(field->message_type(), opts))},
       Sub{"cast_to_field",
           use_base_class ? absl::Substitute("reinterpret_cast<$0*>", base) : ""}
           .ConditionalFunctionCall(),
@@ -193,7 +196,7 @@ void SingularMessage::GenerateInlineAccessorDefinitions(io::Printer* p) const {
       $TsanDetectConcurrentRead$;
       $StrongRef$;
       const $Submsg$* p = $cast_field_$;
-      return p != nullptr ? *p : reinterpret_cast<const $Submsg$&>($kDefault$);
+      return p != nullptr ? *p : $kDefaultRef$;
     }
     inline const $Submsg$& $Msg$::$name$() const ABSL_ATTRIBUTE_LIFETIME_BOUND {
       $WeakDescriptorSelfPin$;
@@ -558,7 +561,7 @@ void OneofMessage::GenerateInlineAccessorDefinitions(io::Printer* p) const {
     inline const $Submsg$& $Msg$::_internal_$name_internal$() const {
       $StrongRef$;
       return $has_field$ ? static_cast<const $Submsg$&>(*$cast_field_$)
-                         : reinterpret_cast<const $Submsg$&>($kDefault$);
+                         : $kDefaultRef$;
     }
   )cc");
   p->Emit(R"cc(

src/google/protobuf/compiler/cpp/file.cc

@@ -610,7 +610,7 @@ void FileGenerator::GenerateSourceDefaultInstance(int idx, io::Printer* p) {
   if (IsFileDescriptorProto(file_, options_)) {
     p->Emit(
         R"cc(
-          struct $type$ {
+          struct $type$ : ::_pbi::MessageGlobalsBase {
 #if defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
             constexpr $type$() : _default(::_pbi::ConstantInitialized{}) {}
 #else   // defined(PROTOBUF_CONSTINIT_DEFAULT_INSTANCES)
@@ -635,7 +635,7 @@ void FileGenerator::GenerateSourceDefaultInstance(int idx, io::Printer* p) {
                             generator->index_in_file_messages(), options_)},
         },
         R"cc(
-          struct $type$ {
+          struct $type$ : ::_pbi::MessageGlobalsBase {
             constexpr $type$() : _default(::_pbi::ConstantInitialized{}) {}
             ~$type$() {}
             //~ _default must be the first member.
@@ -653,7 +653,7 @@ void FileGenerator::GenerateSourceDefaultInstance(int idx, io::Printer* p) {
   } else {
     p->Emit(
         R"cc(
-          struct $type$ {
+          struct $type$ : ::_pbi::MessageGlobalsBase {
             constexpr $type$() : _default(::_pbi::ConstantInitialized{}) {}
             ~$type$() {}
             union {

src/google/protobuf/compiler/cpp/message.cc

@@ -2188,6 +2188,7 @@ void MessageGenerator::GenerateClassDefinition(io::Printer* p) {
         }},
        {"decl_impl", [&] { GenerateImplDefinition(p); }},
        {"classdata_type", ClassDataType(descriptor_, options_)},
+       {"msg_globals", MsgGlobalsInstanceName(descriptor_, options_)},
        {"split_friend",
         [&] {
           if (!ShouldSplit(descriptor_, options_)) return;
@@ -2251,7 +2252,8 @@ void MessageGenerator::GenerateClassDefinition(io::Printer* p) {
           $descriptor_accessor$;
           $get_descriptor$;
           $nodiscard $static const $classname$& default_instance() {
-            return *reinterpret_cast<const $classname$*>(&_$classname$_globals_);
+            return *$pbi$::MessageGlobalsBase::default_instance<$classname$>(
+                &$msg_globals$);
           }
           $decl_oneof$;
           static constexpr int kIndexInFileMessages = $index_in_file_messages$;