https://github.com/google/mozc/issues/460

--- /src/dictionary/user_dictionary.cc
+++ /src/dictionary/user_dictionary.cc
@@ -289,7 +289,7 @@
     }
 
     auto_register_mode_ = false;
-    dic_->Load(*(storage.get()));
+    dic_->Load(storage.get()->user_dictionary_storage_base);
   }
 
  private:
--- /src/dictionary/user_dictionary_session.cc
+++ /src/dictionary/user_dictionary_session.cc
@@ -69,11 +69,11 @@
   }
 
   virtual bool RunUndo(mozc::UserDictionaryStorage *storage) {
-    if (storage->dictionaries_size() == 0) {
+    if (storage->user_dictionary_storage_base.dictionaries_size() == 0) {
       return false;
     }
 
-    storage->mutable_dictionaries()->RemoveLast();
+    storage->user_dictionary_storage_base.mutable_dictionaries()->RemoveLast();
     return true;
   }
 
@@ -94,7 +94,7 @@
     }
 
     RepeatedPtrField<UserDictionary> *dictionaries =
-        storage->mutable_dictionaries();
+        storage->user_dictionary_storage_base.mutable_dictionaries();
     dictionaries->AddAllocated(dictionary_.release());
 
     // Adjust the position of the reverted dictionary.
@@ -120,10 +120,10 @@
   }
 
   virtual bool RunUndo(mozc::UserDictionaryStorage *storage) {
-    if (storage->dictionaries_size() != 1) {
+    if (storage->user_dictionary_storage_base.dictionaries_size() != 1) {
       return false;
     }
-    dictionary_->Swap(storage->mutable_dictionaries(0));
+    dictionary_->Swap(storage->user_dictionary_storage_base.mutable_dictionaries(0));
     return true;
   }
 
@@ -144,7 +144,7 @@
   virtual bool RunUndo(mozc::UserDictionaryStorage *storage) {
     UserDictionary *dictionary =
         UserDictionaryUtil::GetMutableUserDictionaryById(
-            storage, dictionary_id_);
+            &storage->user_dictionary_storage_base, dictionary_id_);
     if (dictionary == NULL) {
       return false;
     }
@@ -169,7 +169,7 @@
   virtual bool RunUndo(mozc::UserDictionaryStorage *storage) {
     UserDictionary *dictionary =
         UserDictionaryUtil::GetMutableUserDictionaryById(
-            storage, dictionary_id_);
+            &storage->user_dictionary_storage_base, dictionary_id_);
     if (dictionary == NULL || dictionary->entries_size() == 0) {
       return false;
     }
@@ -195,7 +195,7 @@
   virtual bool RunUndo(mozc::UserDictionaryStorage *storage) {
     UserDictionary *dictionary =
         UserDictionaryUtil::GetMutableUserDictionaryById(
-            storage, dictionary_id_);
+            &storage->user_dictionary_storage_base, dictionary_id_);
     if (dictionary == NULL ||
         index_ < 0 || dictionary->entries_size() <= index_) {
       return false;
@@ -240,7 +240,7 @@
   virtual bool RunUndo(mozc::UserDictionaryStorage *storage) {
     UserDictionary *dictionary =
         UserDictionaryUtil::GetMutableUserDictionaryById(
-            storage, dictionary_id_);
+            &storage->user_dictionary_storage_base, dictionary_id_);
     if (dictionary == NULL) {
       return false;
     }
@@ -306,7 +306,7 @@
   virtual bool RunUndo(mozc::UserDictionaryStorage *storage) {
     UserDictionary *dictionary =
         UserDictionaryUtil::GetMutableUserDictionaryById(
-            storage, dictionary_id_);
+            &storage->user_dictionary_storage_base, dictionary_id_);
     if (dictionary == NULL) {
       return false;
     }
@@ -345,7 +345,7 @@
 
 // TODO(hidehiko) move this to header.
 const UserDictionaryStorage &UserDictionarySession::storage() const {
-  return *storage_;
+  return storage_->user_dictionary_storage_base;
 }
 mozc::UserDictionaryStorage *UserDictionarySession::mutable_storage() {
   return storage_.get();
@@ -464,7 +464,7 @@
     const string &dictionary_name, uint64 *new_dictionary_id) {
   UserDictionaryCommandStatus::Status status =
       UserDictionaryUtil::CreateDictionary(
-          storage_.get(), dictionary_name, new_dictionary_id);
+          &storage_->user_dictionary_storage_base, dictionary_name, new_dictionary_id);
   if (status == UserDictionaryCommandStatus::USER_DICTIONARY_COMMAND_SUCCESS) {
     AddUndoCommand(new UndoCreateDictionaryCommand);
   }
@@ -488,7 +488,7 @@
   int original_index;
   UserDictionary *deleted_dictionary;
   if (!UserDictionaryUtil::DeleteDictionary(
-          storage_.get(), dictionary_id,
+          &storage_->user_dictionary_storage_base, dictionary_id,
           &original_index, &deleted_dictionary)) {
     // Failed to delete the dictionary.
     return UserDictionaryCommandStatus::UNKNOWN_DICTIONARY_ID;
@@ -510,7 +510,7 @@
     uint64 dictionary_id, const string &dictionary_name) {
   string original_name;
   const UserDictionary *dictionary =
-      UserDictionaryUtil::GetUserDictionaryById(*storage_, dictionary_id);
+      UserDictionaryUtil::GetUserDictionaryById(storage_->user_dictionary_storage_base, dictionary_id);
   if (dictionary != NULL) {
     // Note that if dictionary is null, it means the dictionary_id is invalid
     // so following RenameDictionary will fail, and error handling is done
@@ -547,7 +547,7 @@
     uint64 dictionary_id, const UserDictionary::Entry &entry) {
   UserDictionary *dictionary =
       UserDictionaryUtil::GetMutableUserDictionaryById(
-          storage_.get(), dictionary_id);
+          &storage_->user_dictionary_storage_base, dictionary_id);
   if (dictionary == NULL) {
     return UserDictionaryCommandStatus::UNKNOWN_DICTIONARY_ID;
   }
@@ -575,7 +575,7 @@
     uint64 dictionary_id, int index, const UserDictionary::Entry &entry) {
   UserDictionary *dictionary =
       UserDictionaryUtil::GetMutableUserDictionaryById(
-          storage_.get(), dictionary_id);
+          &storage_->user_dictionary_storage_base, dictionary_id);
   if (dictionary == NULL) {
     return UserDictionaryCommandStatus::UNKNOWN_DICTIONARY_ID;
   }
@@ -604,7 +604,7 @@
     uint64 dictionary_id, const std::vector<int> &index_list) {
   UserDictionary *dictionary =
       UserDictionaryUtil::GetMutableUserDictionaryById(
-          storage_.get(), dictionary_id);
+          &storage_->user_dictionary_storage_base, dictionary_id);
   if (dictionary == NULL) {
     return UserDictionaryCommandStatus::UNKNOWN_DICTIONARY_ID;
   }
@@ -644,7 +644,7 @@
     uint64 dictionary_id, const string &data) {
   UserDictionary *dictionary =
       UserDictionaryUtil::GetMutableUserDictionaryById(
-          storage_.get(), dictionary_id);
+          &storage_->user_dictionary_storage_base, dictionary_id);
   if (dictionary == NULL) {
     return UserDictionaryCommandStatus::UNKNOWN_DICTIONARY_ID;
   }
@@ -699,7 +699,7 @@
     uint64 *new_dictionary_id) {
   UserDictionaryCommandStatus::Status status =
       UserDictionaryUtil::CreateDictionary(
-          storage_.get(), dictionary_name, new_dictionary_id);
+          &storage_->user_dictionary_storage_base, dictionary_name, new_dictionary_id);
   if (status != UserDictionaryCommandStatus::USER_DICTIONARY_COMMAND_SUCCESS) {
     return status;
   }
@@ -709,7 +709,7 @@
 
   UserDictionary *dictionary =
       UserDictionaryUtil::GetMutableUserDictionaryById(
-          storage_.get(), *new_dictionary_id);
+          &storage_->user_dictionary_storage_base, *new_dictionary_id);
   if (dictionary == NULL) {
     // The dictionary should be always found.
     return UserDictionaryCommandStatus::UNKNOWN_ERROR;
@@ -719,7 +719,7 @@
 }
 
 bool UserDictionarySession::EnsureNonEmptyStorage() {
-  if (storage_->dictionaries_size() > 0) {
+  if (storage_->user_dictionary_storage_base.dictionaries_size() > 0) {
     // The storage already has at least one dictionary. Do nothing.
     return false;
   }
@@ -728,7 +728,7 @@
   uint64 new_dictionary_id;
   UserDictionaryCommandStatus::Status status =
       UserDictionaryUtil::CreateDictionary(
-          storage_.get(), default_dictionary_name_, &new_dictionary_id);
+          &storage_->user_dictionary_storage_base, default_dictionary_name_, &new_dictionary_id);
   CHECK_EQ(
       status, UserDictionaryCommandStatus::USER_DICTIONARY_COMMAND_SUCCESS);
   return true;
@@ -755,7 +755,7 @@
 
 void UserDictionarySession::ClearDictionariesAndUndoHistory() {
   ScopedUserDictionaryLocker l(storage_.get());
-  storage_->clear_dictionaries();
+  storage_->user_dictionary_storage_base.clear_dictionaries();
   ClearUndoHistory();
 }
 
--- /src/dictionary/user_dictionary_session_test.cc
+++ /src/dictionary/user_dictionary_session_test.cc
@@ -95,12 +95,12 @@
 
   ASSERT_EQ(UserDictionaryCommandStatus::FILE_NOT_FOUND, session.Load());
 
-  session.mutable_storage()->set_version(10);
+  session.mutable_storage()->user_dictionary_storage_base.set_version(10);
   ASSERT_EQ(UserDictionaryCommandStatus::USER_DICTIONARY_COMMAND_SUCCESS,
             session.Save());
 
   // Clear once, in order to make sure that Load is actually working.
-  session.mutable_storage()->Clear();
+  session.mutable_storage()->user_dictionary_storage_base.Clear();
   ASSERT_EQ(UserDictionaryCommandStatus::USER_DICTIONARY_COMMAND_SUCCESS,
             session.Load());
 
@@ -130,7 +130,7 @@
 
   // Create huge dummy data.
   {
-    UserDictionaryStorage *storage = session.mutable_storage();
+    UserDictionaryStorage *storage = &session.mutable_storage()->user_dictionary_storage_base;
     for (int i = 0; i < 100; ++i) {
       UserDictionary *dictionary = storage->add_dictionaries();
       for (int j = 0; j < 1000; ++j) {
@@ -147,7 +147,7 @@
   ASSERT_EQ(UserDictionaryCommandStatus::FILE_SIZE_LIMIT_EXCEEDED,
             session.Save());
 
-  session.mutable_storage()->Clear();
+  session.mutable_storage()->user_dictionary_storage_base.Clear();
   ASSERT_EQ(UserDictionaryCommandStatus::USER_DICTIONARY_COMMAND_SUCCESS,
             session.Load());
 
@@ -191,7 +191,7 @@
 
   while (session.storage().dictionaries_size() <
          ::mozc::UserDictionaryStorage::max_dictionary_size()) {
-    session.mutable_storage()->add_dictionaries();
+    session.mutable_storage()->user_dictionary_storage_base.add_dictionaries();
   }
   EXPECT_EQ(UserDictionaryCommandStatus::DICTIONARY_SIZE_LIMIT_EXCEEDED,
             session.CreateDictionary("dictionary 2", &dummy_dictionary_id));
@@ -204,7 +204,7 @@
   const uint64 kDummyId = 10;
   {
     UserDictionary *user_dictionary =
-        session.mutable_storage()->add_dictionaries();
+        session.mutable_storage()->user_dictionary_storage_base.add_dictionaries();
     user_dictionary->set_id(kDummyId);
   }
 
@@ -229,7 +229,7 @@
   const uint64 kDummyId = 10;
   {
     UserDictionary *user_dictionary =
-        session.mutable_storage()->add_dictionaries();
+        session.mutable_storage()->user_dictionary_storage_base.add_dictionaries();
     user_dictionary->set_id(kDummyId);
   }
 
--- /src/dictionary/user_dictionary_storage.cc
+++ /src/dictionary/user_dictionary_storage.cc
@@ -109,7 +109,7 @@
   mozc::protobuf::io::IstreamInputStream zero_copy_input(&ifs);
   mozc::protobuf::io::CodedInputStream decoder(&zero_copy_input);
   decoder.SetTotalBytesLimit(kDefaultTotalBytesLimit, -1);
-  if (!ParseFromCodedStream(&decoder)) {
+  if (!user_dictionary_storage_base.ParseFromCodedStream(&decoder)) {
     LOG(ERROR) << "Failed to parse";
     if (!decoder.ConsumedEntireMessage() || !ifs.eof()) {
       LOG(ERROR) << "ParseFromStream failed: file seems broken";
@@ -137,11 +137,11 @@
   }
 
   // Check dictionary id here. if id is 0, assign random ID.
-  for (int i = 0; i < dictionaries_size(); ++i) {
-    const UserDictionary &dict = dictionaries(i);
+  for (int i = 0; i < user_dictionary_storage_base.dictionaries_size(); ++i) {
+    const UserDictionary &dict = user_dictionary_storage_base.dictionaries(i);
     if (dict.id() == 0) {
-      mutable_dictionaries(i)->set_id(
-          UserDictionaryUtil::CreateNewDictionaryId(*this));
+      user_dictionary_storage_base.mutable_dictionaries(i)->set_id(
+          UserDictionaryUtil::CreateNewDictionaryId(this->user_dictionary_storage_base));
     }
   }
 
@@ -171,7 +171,7 @@
       return false;
     }
 
-    if (!SerializeToOstream(&ofs)) {
+    if (!user_dictionary_storage_base.SerializeToOstream(&ofs)) {
       LOG(ERROR) << "SerializeToString failed";
       last_error_type_ = SYNC_FAILURE;
       return false;
@@ -227,7 +227,7 @@
     return false;
   }
 
-  const UserDictionary &dic = dictionaries(index);
+  const UserDictionary &dic = user_dictionary_storage_base.dictionaries(index);
   for (size_t i = 0; i < dic.entries_size(); ++i) {
     const UserDictionaryEntry &entry = dic.entries(i);
     ofs << entry.key() << "\t" << entry.value() << "\t"
@@ -241,7 +241,7 @@
 bool UserDictionaryStorage::CreateDictionary(
     const string &dic_name, uint64 *new_dic_id) {
   UserDictionaryCommandStatus::Status status =
-      UserDictionaryUtil::CreateDictionary(this, dic_name, new_dic_id);
+      UserDictionaryUtil::CreateDictionary(&this->user_dictionary_storage_base, dic_name, new_dic_id);
   // Update last_error_type_
   switch (status) {
     case UserDictionaryCommandStatus::DICTIONARY_NAME_EMPTY:
@@ -273,7 +273,7 @@
 }
 
 bool UserDictionaryStorage::DeleteDictionary(uint64 dic_id) {
-  if (!UserDictionaryUtil::DeleteDictionary(this, dic_id, NULL, NULL)) {
+  if (!UserDictionaryUtil::DeleteDictionary(&this->user_dictionary_storage_base, dic_id, NULL, NULL)) {
     // Failed to delete dictionary.
     last_error_type_ = INVALID_DICTIONARY_ID;
     return false;
@@ -304,8 +304,8 @@
     return true;
   }
 
-  for (int i = 0; i < dictionaries_size(); ++i) {
-    if (dic_name == dictionaries(i).name()) {
+  for (int i = 0; i < user_dictionary_storage_base.dictionaries_size(); ++i) {
+    if (dic_name == user_dictionary_storage_base.dictionaries(i).name()) {
       last_error_type_ = DUPLICATED_DICTIONARY_NAME;
       LOG(ERROR) << "duplicated dictionary name";
       return false;
@@ -318,14 +318,14 @@
 }
 
 int UserDictionaryStorage::GetUserDictionaryIndex(uint64 dic_id) const {
-  return UserDictionaryUtil::GetUserDictionaryIndexById(*this, dic_id);
+  return UserDictionaryUtil::GetUserDictionaryIndexById(this->user_dictionary_storage_base, dic_id);
 }
 
 bool UserDictionaryStorage::GetUserDictionaryId(const string &dic_name,
                                                 uint64 *dic_id) {
-  for (size_t i = 0; i < dictionaries_size(); ++i) {
-    if (dic_name == dictionaries(i).name()) {
-      *dic_id = dictionaries(i).id();
+  for (size_t i = 0; i < user_dictionary_storage_base.dictionaries_size(); ++i) {
+    if (dic_name == user_dictionary_storage_base.dictionaries(i).name()) {
+      *dic_id = user_dictionary_storage_base.dictionaries(i).id();
       return true;
     }
   }
@@ -335,7 +335,7 @@
 
 user_dictionary::UserDictionary *UserDictionaryStorage::GetUserDictionary(
     uint64 dic_id) {
-  return UserDictionaryUtil::GetMutableUserDictionaryById(this, dic_id);
+  return UserDictionaryUtil::GetMutableUserDictionaryById(&this->user_dictionary_storage_base, dic_id);
 }
 
 UserDictionaryStorage::UserDictionaryStorageErrorType
@@ -352,8 +352,8 @@
   }
 
   int auto_index = -1;
-  for (int i = 0; i < dictionaries_size(); ++i) {
-    if (dictionaries(i).name() == kAutoRegisteredDictionaryName) {
+  for (int i = 0; i < user_dictionary_storage_base.dictionaries_size(); ++i) {
+    if (user_dictionary_storage_base.dictionaries(i).name() == kAutoRegisteredDictionaryName) {
       auto_index = i;
       break;
     }
@@ -361,17 +361,17 @@
 
   UserDictionary *dic = NULL;
   if (auto_index == -1) {
-    if (UserDictionaryUtil::IsStorageFull(*this)) {
+    if (UserDictionaryUtil::IsStorageFull(this->user_dictionary_storage_base)) {
       last_error_type_ = TOO_MANY_DICTIONARIES;
       LOG(ERROR) << "too many dictionaries";
       UnLock();
       return false;
     }
-    dic = add_dictionaries();
-    dic->set_id(UserDictionaryUtil::CreateNewDictionaryId(*this));
+    dic = user_dictionary_storage_base.add_dictionaries();
+    dic->set_id(UserDictionaryUtil::CreateNewDictionaryId(this->user_dictionary_storage_base));
     dic->set_name(kAutoRegisteredDictionaryName);
   } else {
-    dic = mutable_dictionaries(auto_index);
+    dic = user_dictionary_storage_base.mutable_dictionaries(auto_index);
   }
 
   if (dic == NULL) {
@@ -410,13 +410,13 @@
 }
 
 bool UserDictionaryStorage::ConvertSyncDictionariesToNormalDictionaries() {
-  if (CountSyncableDictionaries(*this) == 0) {
+  if (CountSyncableDictionaries(this->user_dictionary_storage_base) == 0) {
     return false;
   }
 
-  for (int dictionary_index = dictionaries_size() - 1;
+  for (int dictionary_index = user_dictionary_storage_base.dictionaries_size() - 1;
        dictionary_index >= 0; --dictionary_index) {
-    UserDictionary *dic = mutable_dictionaries(dictionary_index);
+    UserDictionary *dic = user_dictionary_storage_base.mutable_dictionaries(dictionary_index);
     if (!dic->syncable()) {
       continue;
     }
@@ -433,10 +433,10 @@
 
     // Delete removed or unused sync dictionaries.
     if (dic->removed() || dic->entries_size() == 0) {
-      for (int i = dictionary_index + 1; i < dictionaries_size(); ++i) {
-        mutable_dictionaries()->SwapElements(i - 1, i);
+      for (int i = dictionary_index + 1; i < user_dictionary_storage_base.dictionaries_size(); ++i) {
+        user_dictionary_storage_base.mutable_dictionaries()->SwapElements(i - 1, i);
       }
-      mutable_dictionaries()->RemoveLast();
+      user_dictionary_storage_base.mutable_dictionaries()->RemoveLast();
       continue;
     }
 
@@ -445,7 +445,7 @@
           kDictionaryNameConvertedFromSyncableDictionary;
       int index = 0;
       while (UserDictionaryUtil::ValidateDictionaryName(
-                 *this, new_dictionary_name)
+                 this->user_dictionary_storage_base, new_dictionary_name)
              != UserDictionaryCommandStatus::USER_DICTIONARY_COMMAND_SUCCESS) {
         ++index;
         new_dictionary_name = Util::StringPrintf(
@@ -456,7 +456,7 @@
     dic->set_syncable(false);
   }
 
-  DCHECK_EQ(0, CountSyncableDictionaries(*this));
+  DCHECK_EQ(0, CountSyncableDictionaries(this->user_dictionary_storage_base));
 
   return true;
 }
@@ -487,7 +487,7 @@
 bool UserDictionaryStorage::IsValidDictionaryName(const string &name) {
   UserDictionaryCommandStatus::Status status =
       UserDictionaryUtil::ValidateDictionaryName(
-          UserDictionaryStorage::default_instance(), name);
+          user_dictionary::UserDictionaryStorage::default_instance(), name);
 
   // Update last_error_type_.
   switch (status) {
--- /src/dictionary/user_dictionary_storage.h
+++ /src/dictionary/user_dictionary_storage.h
@@ -72,13 +72,15 @@
 class Mutex;
 class ProcessMutex;
 
-// Inherit from ProtocolBuffer
-// TODO(hidehiko): Get rid of this implementation.
-class UserDictionaryStorage : public user_dictionary::UserDictionaryStorage {
+class UserDictionaryStorage {
  public:
   typedef user_dictionary::UserDictionary UserDictionary;
   typedef user_dictionary::UserDictionary::Entry UserDictionaryEntry;
 
+  // Instance of base class generated by Protocol Buffers compiler.
+  // Regular inheritance strongly discouraged.
+  user_dictionary::UserDictionaryStorage user_dictionary_storage_base;
+
   enum UserDictionaryStorageErrorType {
     USER_DICTIONARY_STORAGE_NO_ERROR = 0,  // default
     FILE_NOT_EXISTS,
--- /src/dictionary/user_dictionary_storage_test.cc
+++ /src/dictionary/user_dictionary_storage_test.cc
@@ -118,13 +118,13 @@
   const size_t kDictionariesSize = 3;
   uint64 id[kDictionariesSize];
 
-  const size_t dict_size = storage.dictionaries_size();
+  const size_t dict_size = storage.user_dictionary_storage_base.dictionaries_size();
 
   for (size_t i = 0; i < kDictionariesSize; ++i) {
     EXPECT_TRUE(storage.CreateDictionary(
         "test" + std::to_string(static_cast<uint32>(i)),
         &id[i]));
-    EXPECT_EQ(i + 1 + dict_size, storage.dictionaries_size());
+    EXPECT_EQ(i + 1 + dict_size, storage.user_dictionary_storage_base.dictionaries_size());
   }
 
   for (size_t i = 0; i < kDictionariesSize; ++i) {
@@ -133,7 +133,7 @@
   }
 
   for (size_t i = 0; i < kDictionariesSize; ++i) {
-    EXPECT_EQ(storage.mutable_dictionaries(i + dict_size),
+    EXPECT_EQ(storage.user_dictionary_storage_base.mutable_dictionaries(i + dict_size),
               storage.GetUserDictionary(id[i]));
     EXPECT_EQ(NULL, storage.GetUserDictionary(id[i] + 1));
   }
@@ -165,7 +165,7 @@
   EXPECT_FALSE(storage.DeleteDictionary(0));
 
   EXPECT_TRUE(storage.DeleteDictionary(id[1]));
-  EXPECT_EQ(kDictionariesSize + dict_size - 1, storage.dictionaries_size());
+  EXPECT_EQ(kDictionariesSize + dict_size - 1, storage.user_dictionary_storage_base.dictionaries_size());
 }
 
 TEST_F(UserDictionaryStorageTest, DeleteTest) {
@@ -174,7 +174,7 @@
 
   // repeat 10 times
   for (int i = 0; i < 10; ++i) {
-    storage.Clear();
+    storage.user_dictionary_storage_base.Clear();
     std::vector<uint64> ids(100);
     for (size_t i = 0; i < ids.size(); ++i) {
       EXPECT_TRUE(storage.CreateDictionary(
@@ -191,10 +191,10 @@
       alive.push_back(ids[i]);
     }
 
-    EXPECT_EQ(alive.size(), storage.dictionaries_size());
+    EXPECT_EQ(alive.size(), storage.user_dictionary_storage_base.dictionaries_size());
 
     for (size_t i = 0; i < alive.size(); ++i) {
-      EXPECT_EQ(alive[i], storage.dictionaries(i).id());
+      EXPECT_EQ(alive[i], storage.user_dictionary_storage_base.dictionaries(i).id());
     }
   }
 }
@@ -268,7 +268,7 @@
         const size_t entry_size = Util::Random(100) + 1;
         for (size_t j = 0; j < entry_size; ++j) {
           UserDictionaryStorage::UserDictionary *dic =
-              storage1.mutable_dictionaries(i);
+              storage1.user_dictionary_storage_base.mutable_dictionaries(i);
           UserDictionaryStorage::UserDictionaryEntry *entry =
               dic->add_entries();
           entry->set_key(GenRandomString(10));
@@ -288,7 +288,7 @@
       EXPECT_TRUE(storage2.Load());
     }
 
-    EXPECT_EQ(storage1.DebugString(), storage2.DebugString());
+    EXPECT_EQ(storage1.user_dictionary_storage_base.DebugString(), storage2.user_dictionary_storage_base.DebugString());
   }
 }
 
@@ -352,7 +352,7 @@
     uint64 dict_id = 0;
     ASSERT_TRUE(storage.CreateDictionary(data.dictionary_name, &dict_id));
     UserDictionaryStorage::UserDictionary *dict =
-        storage.mutable_dictionaries(storage.GetUserDictionaryIndex(dict_id));
+        storage.user_dictionary_storage_base.mutable_dictionaries(storage.GetUserDictionaryIndex(dict_id));
     dict->set_syncable(data.is_sync_dictionary);
     dict->set_removed(data.is_removed_dictionary);
     if (data.has_normal_entry) {
@@ -369,7 +369,7 @@
       entry->set_removed(true);
     }
   }
-  EXPECT_EQ(9, UserDictionaryStorage::CountSyncableDictionaries(storage));
+  EXPECT_EQ(9, UserDictionaryStorage::CountSyncableDictionaries(storage.user_dictionary_storage_base));
 
   ASSERT_TRUE(storage.ConvertSyncDictionariesToNormalDictionaries());
 
@@ -385,12 +385,12 @@
     { true, kDictionaryNameConvertedFromSyncableDictionary },
   };
 
-  EXPECT_EQ(0, UserDictionaryStorage::CountSyncableDictionaries(storage));
-  ASSERT_EQ(arraysize(expected_data), storage.dictionaries_size());
+  EXPECT_EQ(0, UserDictionaryStorage::CountSyncableDictionaries(storage.user_dictionary_storage_base));
+  ASSERT_EQ(arraysize(expected_data), storage.user_dictionary_storage_base.dictionaries_size());
   for (size_t i = 0; i < arraysize(expected_data); ++i) {
     SCOPED_TRACE(Util::StringPrintf("verify %d", static_cast<int>(i)));
     const ExpectedData &expected = expected_data[i];
-    const UserDictionaryStorage::UserDictionary &dict = storage.dictionaries(i);
+    const UserDictionaryStorage::UserDictionary &dict = storage.user_dictionary_storage_base.dictionaries(i);
 
     EXPECT_EQ(expected.dictionary_name, dict.name());
     EXPECT_FALSE(dict.syncable());
@@ -404,42 +404,42 @@
   }
 
   // Test duplicated dictionary name.
-  storage.Clear();
+  storage.user_dictionary_storage_base.Clear();
   {
     uint64 dict_id = 0;
     storage.CreateDictionary(
         UserDictionaryStorage::default_sync_dictionary_name(), &dict_id);
     storage.CreateDictionary(
         kDictionaryNameConvertedFromSyncableDictionary, &dict_id);
-    ASSERT_EQ(2, storage.dictionaries_size());
+    ASSERT_EQ(2, storage.user_dictionary_storage_base.dictionaries_size());
     UserDictionaryStorage::UserDictionary *dict;
-    dict = storage.mutable_dictionaries(0);
+    dict = storage.user_dictionary_storage_base.mutable_dictionaries(0);
     dict->set_syncable(true);
     dict->add_entries()->set_key("0");
-    dict = storage.mutable_dictionaries(1);
+    dict = storage.user_dictionary_storage_base.mutable_dictionaries(1);
     dict->set_syncable(false);
     dict->add_entries()->set_key("1");
   }
   ASSERT_TRUE(storage.ConvertSyncDictionariesToNormalDictionaries());
-  EXPECT_EQ(0, UserDictionaryStorage::CountSyncableDictionaries(storage));
-  EXPECT_EQ(2, storage.dictionaries_size());
+  EXPECT_EQ(0, UserDictionaryStorage::CountSyncableDictionaries(storage.user_dictionary_storage_base));
+  EXPECT_EQ(2, storage.user_dictionary_storage_base.dictionaries_size());
   EXPECT_EQ(Util::StringPrintf("%s_1",
                                kDictionaryNameConvertedFromSyncableDictionary),
-            storage.dictionaries(0).name());
+            storage.user_dictionary_storage_base.dictionaries(0).name());
   EXPECT_EQ(kDictionaryNameConvertedFromSyncableDictionary,
-            storage.dictionaries(1).name());
+            storage.user_dictionary_storage_base.dictionaries(1).name());
 }
 
 TEST_F(UserDictionaryStorageTest, AddToAutoRegisteredDictionary) {
   {
     UserDictionaryStorage storage(GetUserDictionaryFile());
-    EXPECT_EQ(0, storage.dictionaries_size());
+    EXPECT_EQ(0, storage.user_dictionary_storage_base.dictionaries_size());
     EXPECT_TRUE(storage.AddToAutoRegisteredDictionary(
         "key1", "value1", UserDictionary::NOUN));
-    EXPECT_EQ(1, storage.dictionaries_size());
-    EXPECT_EQ(1, storage.dictionaries(0).entries_size());
+    EXPECT_EQ(1, storage.user_dictionary_storage_base.dictionaries_size());
+    EXPECT_EQ(1, storage.user_dictionary_storage_base.dictionaries(0).entries_size());
     const UserDictionaryStorage::UserDictionaryEntry &entry1 =
-        storage.dictionaries(0).entries(0);
+        storage.user_dictionary_storage_base.dictionaries(0).entries(0);
     EXPECT_EQ("key1", entry1.key());
     EXPECT_EQ("value1", entry1.value());
     EXPECT_EQ(UserDictionary::NOUN, entry1.pos());
@@ -447,10 +447,10 @@
 
     EXPECT_TRUE(storage.AddToAutoRegisteredDictionary(
         "key2", "value2", UserDictionary::NOUN));
-    EXPECT_EQ(1, storage.dictionaries_size());
-    EXPECT_EQ(2, storage.dictionaries(0).entries_size());
+    EXPECT_EQ(1, storage.user_dictionary_storage_base.dictionaries_size());
+    EXPECT_EQ(2, storage.user_dictionary_storage_base.dictionaries(0).entries_size());
     const UserDictionaryStorage::UserDictionaryEntry &entry2 =
-        storage.dictionaries(0).entries(1);
+        storage.user_dictionary_storage_base.dictionaries(0).entries(1);
     EXPECT_EQ("key2", entry2.key());
     EXPECT_EQ("value2", entry2.value());
     EXPECT_EQ(UserDictionary::NOUN, entry2.pos());
@@ -474,7 +474,7 @@
   {
     UserDictionaryStorage storage(GetUserDictionaryFile());
     {
-      UserDictionary *dictionary = storage.add_dictionaries();
+      UserDictionary *dictionary = storage.user_dictionary_storage_base.add_dictionaries();
       dictionary->set_id(kDummyDictionaryId);
       UserDictionary::Entry *entry = dictionary->add_entries();
       entry->set_key("key");
--- /src/dictionary/user_dictionary_test.cc
+++ /src/dictionary/user_dictionary_test.cc
@@ -334,9 +334,9 @@
     std::istringstream is(contents);
     CHECK(is.good());
 
-    storage->Clear();
+    storage->user_dictionary_storage_base.Clear();
     UserDictionaryStorage::UserDictionary *dic
-        = storage->add_dictionaries();
+        = storage->user_dictionary_storage_base.add_dictionaries();
     CHECK(dic);
 
     string line;
@@ -389,7 +389,7 @@
   {
     UserDictionaryStorage storage("");
     UserDictionaryTest::LoadFromString(kUserDictionary0, &storage);
-    dic->Load(storage);
+    dic->Load(storage.user_dictionary_storage_base);
   }
 
   // A normal lookup operation.
@@ -425,7 +425,7 @@
   {
     UserDictionaryStorage storage("");
     LoadFromString(kUserDictionary1, &storage);
-    dic->Load(storage);
+    dic->Load(storage.user_dictionary_storage_base);
   }
 
   // A normal lookup again.
@@ -450,7 +450,7 @@
   {
     UserDictionaryStorage storage("");
     LoadFromString(kUserDictionary0, &storage);
-    dic->Load(storage);
+    dic->Load(storage.user_dictionary_storage_base);
   }
 
   // A normal lookup operation.
@@ -480,7 +480,7 @@
   {
     UserDictionaryStorage storage("");
     LoadFromString(kUserDictionary1, &storage);
-    dic->Load(storage);
+    dic->Load(storage.user_dictionary_storage_base);
   }
 
   // A normal lookup.
@@ -504,7 +504,7 @@
   {
     UserDictionaryStorage storage("");
     LoadFromString(kUserDictionary0, &storage);
-    dic->Load(storage);
+    dic->Load(storage.user_dictionary_storage_base);
   }
 
   // A normal lookup operation.
@@ -540,7 +540,7 @@
     uint64 id = 0;
     EXPECT_TRUE(storage.CreateDictionary("test", &id));
     UserDictionaryStorage::UserDictionary *dic =
-        storage.mutable_dictionaries(0);
+        storage.user_dictionary_storage_base.mutable_dictionaries(0);
 
     // "名詞"
     UserDictionaryStorage::UserDictionaryEntry *entry =
@@ -555,7 +555,7 @@
     entry->set_value("suggest_only");
     entry->set_pos(user_dictionary::UserDictionary::SUGGESTION_ONLY);
 
-    user_dic->Load(storage);
+    user_dic->Load(storage.user_dictionary_storage_base);
   }
 
   // "suggestion_only" should not be looked up.
@@ -577,7 +577,7 @@
   {
     UserDictionaryStorage storage("");
     UserDictionaryTest::LoadFromString(kUserDictionary0, &storage);
-    dic->Load(storage);
+    dic->Load(storage.user_dictionary_storage_base);
   }
 
   TestLookupPrefixHelper(nullptr, 0, "start", 4, *dic);
@@ -612,7 +612,7 @@
     uint64 id = 0;
     EXPECT_TRUE(storage.CreateDictionary("test", &id));
     UserDictionaryStorage::UserDictionary *dic =
-        storage.mutable_dictionaries(0);
+        storage.user_dictionary_storage_base.mutable_dictionaries(0);
     for (size_t j = 0; j < 10000; ++j) {
       UserDictionaryStorage::UserDictionaryEntry *entry =
           dic->add_entries();
@@ -681,15 +681,15 @@
     UserDictionaryStorage storage(filename);
     EXPECT_TRUE(storage.Load());
     int index = 0;
-    EXPECT_EQ(1, storage.dictionaries_size());
-    EXPECT_EQ(100, storage.dictionaries(index).entries_size());
+    EXPECT_EQ(1, storage.user_dictionary_storage_base.dictionaries_size());
+    EXPECT_EQ(100, storage.user_dictionary_storage_base.dictionaries(index).entries_size());
     for (int i = 0; i < 100; ++i) {
       EXPECT_EQ("key" + std::to_string(i),
-                storage.dictionaries(index).entries(i).key());
+                storage.user_dictionary_storage_base.dictionaries(index).entries(i).key());
       EXPECT_EQ("value" + std::to_string(i),
-                storage.dictionaries(index).entries(i).value());
+                storage.user_dictionary_storage_base.dictionaries(index).entries(i).value());
       EXPECT_EQ(user_dictionary::UserDictionary::NOUN,
-                storage.dictionaries(index).entries(i).pos());
+                storage.user_dictionary_storage_base.dictionaries(index).entries(i).pos());
     }
   }
 
@@ -722,12 +722,12 @@
   {
     UserDictionaryStorage storage(filename);
     EXPECT_TRUE(storage.Load());
-    EXPECT_EQ(1, storage.dictionaries_size());
-    EXPECT_EQ(1, storage.dictionaries(0).entries_size());
-    EXPECT_EQ("key", storage.dictionaries(0).entries(0).key());
-    EXPECT_EQ("value", storage.dictionaries(0).entries(0).value());
+    EXPECT_EQ(1, storage.user_dictionary_storage_base.dictionaries_size());
+    EXPECT_EQ(1, storage.user_dictionary_storage_base.dictionaries(0).entries_size());
+    EXPECT_EQ("key", storage.user_dictionary_storage_base.dictionaries(0).entries(0).key());
+    EXPECT_EQ("value", storage.user_dictionary_storage_base.dictionaries(0).entries(0).value());
     EXPECT_EQ(user_dictionary::UserDictionary::NOUN,
-              storage.dictionaries(0).entries(0).pos());
+              storage.user_dictionary_storage_base.dictionaries(0).entries(0).pos());
   }
 }
 
@@ -746,7 +746,7 @@
     uint64 id = 0;
     EXPECT_TRUE(storage.CreateDictionary("test", &id));
     UserDictionaryStorage::UserDictionary *dic =
-        storage.mutable_dictionaries(0);
+        storage.user_dictionary_storage_base.mutable_dictionaries(0);
     for (size_t j = 0; j < 10000; ++j) {
       UserDictionaryStorage::UserDictionaryEntry *entry =
           dic->add_entries();
@@ -770,7 +770,7 @@
 
     suppression_dictionary_->Lock();
     EXPECT_TRUE(suppression_dictionary_->IsLocked());
-    user_dic->Load(storage);
+    user_dic->Load(storage.user_dictionary_storage_base);
     EXPECT_FALSE(suppression_dictionary_->IsLocked());
 
     for (size_t j = 0; j < 10; ++j) {
@@ -782,11 +782,11 @@
 
   // Remove suppression entry
   {
-    storage.Clear();
+    storage.user_dictionary_storage_base.Clear();
     uint64 id = 0;
     EXPECT_TRUE(storage.CreateDictionary("test", &id));
     UserDictionaryStorage::UserDictionary *dic =
-        storage.mutable_dictionaries(0);
+        storage.user_dictionary_storage_base.mutable_dictionaries(0);
     for (size_t j = 0; j < 10000; ++j) {
       UserDictionaryStorage::UserDictionaryEntry *entry =
           dic->add_entries();
@@ -798,7 +798,7 @@
     }
 
     suppression_dictionary_->Lock();
-    user_dic->Load(storage);
+    user_dic->Load(storage.user_dictionary_storage_base);
     EXPECT_FALSE(suppression_dictionary_->IsLocked());
 
     for (size_t j = 0; j < 10; ++j) {
@@ -825,7 +825,7 @@
     uint64 id = 0;
     EXPECT_TRUE(storage.CreateDictionary("test", &id));
     UserDictionaryStorage::UserDictionary *dic =
-        storage.mutable_dictionaries(0);
+        storage.user_dictionary_storage_base.mutable_dictionaries(0);
 
     for (size_t j = 0; j < 10; ++j) {
       UserDictionaryStorage::UserDictionaryEntry *entry =
@@ -845,7 +845,7 @@
       entry->set_pos(user_dictionary::UserDictionary::SUGGESTION_ONLY);
     }
 
-    user_dic->Load(storage);
+    user_dic->Load(storage.user_dictionary_storage_base);
   }
 
   {
@@ -878,7 +878,7 @@
   UserDictionaryStorage storage("");
 
   {
-    UserDictionaryStorage::UserDictionary *dic1 = storage.add_dictionaries();
+    UserDictionaryStorage::UserDictionary *dic1 = storage.user_dictionary_storage_base.add_dictionaries();
     CHECK(dic1);
     UserDictionaryStorage::UserDictionaryEntry *entry;
     entry = dic1->add_entries();
@@ -893,7 +893,7 @@
     entry->set_pos(user_dictionary::UserDictionary::NOUN);
   }
   {
-    UserDictionaryStorage::UserDictionary *dic2 = storage.add_dictionaries();
+    UserDictionaryStorage::UserDictionary *dic2 = storage.user_dictionary_storage_base.add_dictionaries();
     CHECK(dic2);
     UserDictionaryStorage::UserDictionaryEntry *entry;
     entry = dic2->add_entries();
@@ -912,7 +912,7 @@
     entry->set_value("value5");
     entry->set_pos(user_dictionary::UserDictionary::NOUN);
   }
-  dic->Load(storage);
+  dic->Load(storage.user_dictionary_storage_base);
 
   EXPECT_INTEGER_STATS("UserRegisteredWord", 5);
 }
@@ -925,7 +925,7 @@
   {
     UserDictionaryStorage storage("");
     UserDictionaryTest::LoadFromString(kUserDictionary0, &storage);
-    dic->Load(storage);
+    dic->Load(storage.user_dictionary_storage_base);
   }
 
   // Entry is in user dictionary but has no comment.
--- /src/gui/word_register_dialog/word_register_dialog.cc
+++ /src/gui/word_register_dialog/word_register_dialog.cc
@@ -298,7 +298,7 @@
   }
 
   UserDictionary *dic =
-      session_->mutable_storage()->mutable_dictionaries(index);
+      session_->mutable_storage()->user_dictionary_storage_base.mutable_dictionaries(index);
   CHECK(dic);
 
   if (dic->name() != DictionarycomboBox->currentText().toStdString()) {
--- /src/prediction/user_history_predictor.cc
+++ /src/prediction/user_history_predictor.cc
@@ -291,23 +291,23 @@
     return false;
   }
 
-  if (!ParseFromString(input)) {
+  if (!user_history_base.ParseFromString(input)) {
     LOG(ERROR) << "ParseFromString failed. message looks broken";
     return false;
   }
 
-  VLOG(1) << "Loaded user histroy, size=" << entries_size();
+  VLOG(1) << "Loaded user histroy, size=" << user_history_base.entries_size();
   return true;
 }
 
 bool UserHistoryStorage::Save() const {
-  if (entries_size() == 0) {
+  if (user_history_base.entries_size() == 0) {
     LOG(WARNING) << "etries size is 0. Not saved";
     return false;
   }
 
   string output;
-  if (!AppendToString(&output)) {
+  if (!user_history_base.AppendToString(&output)) {
     LOG(ERROR) << "AppendToString failed";
     return false;
   }
@@ -494,12 +494,12 @@
     return false;
   }
 
-  for (size_t i = 0; i < history.entries_size(); ++i) {
-    dic_->Insert(EntryFingerprint(history.entries(i)),
-                 history.entries(i));
+  for (size_t i = 0; i < history.user_history_base.entries_size(); ++i) {
+    dic_->Insert(EntryFingerprint(history.user_history_base.entries(i)),
+                 history.user_history_base.entries(i));
   }
 
-  VLOG(1) << "Loaded user histroy, size=" << history.entries_size();
+  VLOG(1) << "Loaded user histroy, size=" << history.user_history_base.entries_size();
 
   return true;
 }
@@ -521,13 +521,13 @@
 
   UserHistoryStorage history(filename);
   for (const DicElement *elm = tail; elm != nullptr; elm = elm->prev) {
-    history.add_entries()->CopyFrom(elm->value);
+    history.user_history_base.add_entries()->CopyFrom(elm->value);
   }
 
   // Updates usage stats here.
   UsageStats::SetInteger(
       "UserHistoryPredictorEntrySize",
-      static_cast<int>(history.entries_size()));
+      static_cast<int>(history.user_history_base.entries_size()));
 
   if (!history.Save()) {
     LOG(ERROR) << "UserHistoryStorage::Save() failed";
--- /src/prediction/user_history_predictor.h
+++ /src/prediction/user_history_predictor.h
@@ -61,11 +61,15 @@
 class UserHistoryPredictorSyncer;
 
 // Added serialization method for UserHistory.
-class UserHistoryStorage : public mozc::user_history_predictor::UserHistory {
+class UserHistoryStorage {
  public:
   explicit UserHistoryStorage(const string &filename);
   ~UserHistoryStorage();
 
+  // Instance of base class generated by Protocol Buffers compiler.
+  // Regular inheritance strongly discouraged.
+  mozc::user_history_predictor::UserHistory user_history_base;
+
   // Loads from encrypted file.
   bool Load();
 
--- /src/prediction/user_history_predictor_test.cc
+++ /src/prediction/user_history_predictor_test.cc
@@ -2222,7 +2222,7 @@
 
   UserHistoryStorage storage1(filename);
 
-  UserHistoryPredictor::Entry *entry = storage1.add_entries();
+  UserHistoryPredictor::Entry *entry = storage1.user_history_base.add_entries();
   CHECK(entry);
   entry->set_key("key");
   entry->set_key("value");
@@ -2230,7 +2230,7 @@
   UserHistoryStorage storage2(filename);
   storage2.Load();
 
-  EXPECT_EQ(storage1.DebugString(), storage2.DebugString());
+  EXPECT_EQ(storage1.user_history_base.DebugString(), storage2.user_history_base.DebugString());
   FileUtil::Unlink(filename);
 }
 
--- /src/rewriter/usage_rewriter_test.cc
+++ /src/rewriter/usage_rewriter_test.cc
@@ -312,7 +312,7 @@
   // Load mock data
   {
     UserDictionaryStorage storage("");
-    UserDictionaryStorage::UserDictionary *dic = storage.add_dictionaries();
+    UserDictionaryStorage::UserDictionary *dic = storage.user_dictionary_storage_base.add_dictionaries();
 
     UserDictionaryStorage::UserDictionaryEntry *entry = dic->add_entries();
     entry->set_key("うま");
@@ -320,7 +320,7 @@
     entry->set_pos(user_dictionary::UserDictionary::NOUN);
     entry->set_comment("アルパカコメント");
 
-    user_dictionary_->Load(storage);
+    user_dictionary_->Load(storage.user_dictionary_storage_base);
   }
 
   // Emulates the conversion of key="うま".