diff --git a/BUILD b/BUILD
index 64d1faf..bfb89c2 100644
--- a/BUILD
+++ b/BUILD
@@ -33,7 +33,7 @@ cc_library(
         "-Woverloaded-virtual",
         "-Wnon-virtual-dtor",
         "-Wno-missing-field-initializers",
-        "-std=c++11",
+        "-std=c++17",
         "-DGFLAGS_NS=google",
     ],
     linkopts = [
diff --git a/src/braft/ballot.h b/src/braft/ballot.h
index bff9f94..6e5a58e 100644
--- a/src/braft/ballot.h
+++ b/src/braft/ballot.h
@@ -16,8 +16,8 @@
 
 #pragma once
 #include <algorithm>
-#include <vector>
 #include <optional>
+#include <vector>
 
 #include "braft/configuration.h"
 
@@ -32,12 +32,15 @@ class Ballot {
     };
 
     Ballot() : _quorum(0), _old_quorum(0){};
-    //FIXME(ehds): Remove optional.
-    // void init(const Configuration& conf, const Configuration& old_conf);
-    void init(const Configuration& conf, std::optional<const Configuration> old_conf);
+    // FIXME(ehds): Remove optional.
+    //  void init(const Configuration& conf, const Configuration& old_conf);
+    void init(const Configuration& conf,
+              std::optional<const Configuration> old_conf);
     PosHint grant(const PeerId& peer, PosHint hint);
     void grant(const PeerId& peer);
-    bool granted() const { return _quorum <= 0 && _old_quorum <= 0; }
+    bool granted() const { return quorum() <= 0 && old_quorum() <= 0; }
+    int quorum() const { return _quorum; }
+    int old_quorum() const { return _old_quorum; }
 
    private:
     struct UnfoundPeerId {
diff --git a/test/common.h b/test/common.h
new file mode 100644
index 0000000..3cedf60
--- /dev/null
+++ b/test/common.h
@@ -0,0 +1,6 @@
+#pragma once
+#ifdef private
+#undef private
+#endif
+#include <gtest/gtest.h>
+#define private public
diff --git a/test/test_ballot.cpp b/test/test_ballot.cpp
index 5b10772..7a2e737 100644
--- a/test/test_ballot.cpp
+++ b/test/test_ballot.cpp
@@ -14,8 +14,10 @@
 
 // Authors: Zhangyi Chen(chenzhangyi01@baidu.com)
 
-#include <gtest/gtest.h>
+#include <optional>
+
 #include "braft/ballot.h"
+#include "common.h"
 
 class BallotTest : public testing::Test {};
 
@@ -29,17 +31,17 @@ TEST(BallotTest, sanity) {
     conf.add_peer(peer2);
     conf.add_peer(peer3);
     braft::Ballot bl;
-    bl.init(conf, NULL);
-    ASSERT_EQ(2, bl._quorum);
-    ASSERT_EQ(0, bl._old_quorum);
+    bl.init(conf, std::nullopt);
+    ASSERT_EQ(2, bl.quorum());
+    ASSERT_EQ(0, bl.old_quorum());
     bl.grant(peer1);
-    ASSERT_EQ(1, bl._quorum);
+    ASSERT_EQ(1, bl.quorum());
     braft::Ballot::PosHint hint = bl.grant(peer1, braft::Ballot::PosHint());
-    ASSERT_EQ(1, bl._quorum);
+    ASSERT_EQ(1, bl.quorum());
     hint = bl.grant(peer1, hint);
-    ASSERT_EQ(1, bl._quorum);
+    ASSERT_EQ(1, bl.quorum());
     hint = bl.grant(peer4, hint);
-    ASSERT_EQ(1, bl._quorum);
+    ASSERT_EQ(1, bl.quorum());
     hint = bl.grant(peer2, hint);
     ASSERT_TRUE(bl.granted());
 }
@@ -54,27 +56,27 @@ TEST(BallotTest, joint_consensus_same_conf) {
     conf.add_peer(peer2);
     conf.add_peer(peer3);
     braft::Ballot bl;
-    bl.init(conf, &conf);
-    ASSERT_EQ(2, bl._quorum);
-    ASSERT_EQ(2, bl._old_quorum);
+    bl.init(conf, conf);
+    ASSERT_EQ(2, bl.quorum());
+    ASSERT_EQ(2, bl.old_quorum());
     bl.grant(peer1);
-    ASSERT_EQ(1, bl._quorum);
-    ASSERT_EQ(1, bl._old_quorum);
+    ASSERT_EQ(1, bl.quorum());
+    ASSERT_EQ(1, bl.old_quorum());
     braft::Ballot::PosHint hint = bl.grant(peer1, braft::Ballot::PosHint());
-    ASSERT_EQ(1, bl._quorum);
-    ASSERT_EQ(1, bl._old_quorum);
+    ASSERT_EQ(1, bl.quorum());
+    ASSERT_EQ(1, bl.old_quorum());
     hint = bl.grant(peer1, hint);
-    ASSERT_EQ(1, bl._quorum);
-    ASSERT_EQ(1, bl._old_quorum);
+    ASSERT_EQ(1, bl.quorum());
+    ASSERT_EQ(1, bl.old_quorum());
     hint = bl.grant(peer4, hint);
-    ASSERT_EQ(1, bl._quorum);
-    ASSERT_EQ(1, bl._old_quorum);
+    ASSERT_EQ(1, bl.quorum());
+    ASSERT_EQ(1, bl.old_quorum());
     ASSERT_FALSE(bl.granted());
     hint = bl.grant(peer2, hint);
     ASSERT_TRUE(bl.granted());
     hint = bl.grant(peer3, hint);
-    ASSERT_EQ(-1, bl._quorum);
-    ASSERT_EQ(-1, bl._old_quorum);
+    ASSERT_EQ(-1, bl.quorum());
+    ASSERT_EQ(-1, bl.old_quorum());
 }
 
 TEST(BallotTest, joint_consensus_different_conf) {
@@ -92,12 +94,12 @@ TEST(BallotTest, joint_consensus_different_conf) {
     conf2.add_peer(peer3);
     conf2.add_peer(peer4);
     braft::Ballot bl;
-    bl.init(conf, &conf2);
+    bl.init(conf, conf2);
     bl.grant(peer1);
     bl.grant(peer2);
     ASSERT_FALSE(bl.granted());
-    ASSERT_EQ(0, bl._quorum);
-    ASSERT_EQ(1, bl._old_quorum);
+    ASSERT_EQ(0, bl.quorum());
+    ASSERT_EQ(1, bl.old_quorum());
     bl.grant(peer4);
     ASSERT_TRUE(bl.granted());
 }
diff --git a/test/test_ballot_box.cpp b/test/test_ballot_box.cpp
index 4b907cf..efc0f02 100644
--- a/test/test_ballot_box.cpp
+++ b/test/test_ballot_box.cpp
@@ -3,12 +3,14 @@
 // Author: Zhangyi Chen (chenzhangyi01@baidu.com)
 // Date: 2016/02/03 15:59:18
 
-#include <algorithm>
-#include <gtest/gtest.h>
 #include <butil/string_printf.h>
+
+#include <algorithm>
+
 #include "braft/ballot_box.h"
 #include "braft/configuration.h"
 #include "braft/fsm_caller.h"
+#include "common.h"
 
 class BallotBoxTest : public testing::Test {
 protected:
diff --git a/test/test_checksum.cpp b/test/test_checksum.cpp
index 110274c..5666c0b 100644
--- a/test/test_checksum.cpp
+++ b/test/test_checksum.cpp
@@ -3,10 +3,11 @@
 // Author: Zhangyi Chen (chenzhangyi01@baidu.com)
 // Date: 2016/04/11 12:15:37
 
-#include <gtest/gtest.h>
 #include <braft/util.h>
 #include <butil/crc32c.h>
 
+#include "common.h"
+
 class ChecksumTest : public testing::Test {
 protected:
     void SetUp() {}
diff --git a/test/test_cli.cpp b/test/test_cli.cpp
index ed816ab..5cd250a 100644
--- a/test/test_cli.cpp
+++ b/test/test_cli.cpp
@@ -3,13 +3,14 @@
 // Author: Zhangyi Chen (chenzhangyi01@baidu.com)
 // Date: 2018/01/12 12:56:17
 
-#include <gtest/gtest.h>
-#include <gflags/gflags.h>
-#include <butil/unique_ptr.h>
 #include <brpc/server.h>
-#include "braft/raft.h"
+#include <butil/unique_ptr.h>
+#include <gflags/gflags.h>
+
 #include "braft/cli.h"
 #include "braft/node.h"
+#include "braft/raft.h"
+#include "common.h"
 
 class CliTest : public testing::Test {
 public:
diff --git a/test/test_configuration.cpp b/test/test_configuration.cpp
index 9461128..b933367 100644
--- a/test/test_configuration.cpp
+++ b/test/test_configuration.cpp
@@ -16,10 +16,11 @@
  * =====================================================================================
  */
 
-#include <gtest/gtest.h>
 #include <butil/logging.h>
-#include "braft/raft.h"
+
 #include "braft/configuration_manager.h"
+#include "braft/raft.h"
+#include "common.h"
 
 class TestUsageSuits : public testing::Test {
 protected:
diff --git a/test/test_file_service.cpp b/test/test_file_service.cpp
index 0f805b2..7703c76 100644
--- a/test/test_file_service.cpp
+++ b/test/test_file_service.cpp
@@ -3,16 +3,16 @@
 // Author: Zhangyi Chen (chenzhangyi01@baidu.com)
 // Date: 2015/11/06 15:40:45
 
-#include <gtest/gtest.h>
-#include <gflags/gflags.h>
-#include <butil/logging.h>
+#include <brpc/server.h>
 #include <butil/file_util.h>
+#include <butil/logging.h>
+#include <gflags/gflags.h>
 
-#include <brpc/server.h>
 #include "braft/file_service.h"
-#include "braft/util.h"
-#include "braft/remote_file_copier.h"
 #include "braft/file_system_adaptor.h"
+#include "braft/remote_file_copier.h"
+#include "braft/util.h"
+#include "common.h"
 
 namespace braft {
 DECLARE_bool(raft_file_check_hole);
diff --git a/test/test_file_system_adaptor.cpp b/test/test_file_system_adaptor.cpp
index f5da03c..9670504 100644
--- a/test/test_file_system_adaptor.cpp
+++ b/test/test_file_system_adaptor.cpp
@@ -4,8 +4,8 @@
 // Author: ZhengPengFei (zhengpengfei@baidu.com)
 // Date: 2017/06/16 10:29:05
 
-#include <gtest/gtest.h>
 #include "braft/file_system_adaptor.h"
+#include "common.h"
 
 class TestFileSystemAdaptorSuits : public testing::Test {
 protected:
diff --git a/test/test_fsm_caller.cpp b/test/test_fsm_caller.cpp
index bb3c186..cd2a0ff 100644
--- a/test/test_fsm_caller.cpp
+++ b/test/test_fsm_caller.cpp
@@ -3,14 +3,15 @@
 // Author: Zhangyi Chen (chenzhangyi01@baidu.com)
 // Date: 2015/12/01 17:03:46
 
-#include <gtest/gtest.h>
-#include <butil/string_printf.h>
 #include <butil/memory/scoped_ptr.h>
+#include <butil/string_printf.h>
+
+#include "braft/configuration.h"
 #include "braft/fsm_caller.h"
-#include "braft/raft.h"
 #include "braft/log.h"
-#include "braft/configuration.h"
 #include "braft/log_manager.h"
+#include "braft/raft.h"
+#include "common.h"
 
 class FSMCallerTest : public testing::Test {
 protected:
diff --git a/test/test_fsync.cpp b/test/test_fsync.cpp
index 96bbdbf..604ef9a 100644
--- a/test/test_fsync.cpp
+++ b/test/test_fsync.cpp
@@ -3,14 +3,15 @@
 // Author: Zhangyi Chen (chenzhangyi01@baidu.com)
 // Date: 2016/02/23 16:22:15
 
-#include <sys/types.h>
-#include <sys/stat.h>
-#include <fcntl.h>
-#include <unistd.h>
-#include <gtest/gtest.h>
 #include <butil/fd_guard.h>
-#include <butil/time.h>
 #include <butil/logging.h>
+#include <butil/time.h>
+#include <fcntl.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+#include "common.h"
 
 class FsyncTest : public testing::Test {
 };
diff --git a/test/test_leader_lease.cpp b/test/test_leader_lease.cpp
index d296b8b..005c9d1 100644
--- a/test/test_leader_lease.cpp
+++ b/test/test_leader_lease.cpp
@@ -14,12 +14,13 @@
 
 // Authors: Pengfei Zheng (zhengpengfei@baidu.com)
 
-#include <gtest/gtest.h>
 #include <butil/logging.h>
-#include "braft/util.h"
-#include "braft/node.h"
-#include "braft/lease.h"
+
 #include "../test/util.h"
+#include "braft/lease.h"
+#include "braft/node.h"
+#include "braft/util.h"
+#include "common.h"
 
 namespace braft {
 DECLARE_bool(raft_enable_leader_lease);
diff --git a/test/test_log.cpp b/test/test_log.cpp
index 25d046f..537b492 100644
--- a/test/test_log.cpp
+++ b/test/test_log.cpp
@@ -4,20 +4,21 @@
 // Author: WangYao (fisherman), wangyao02@baidu.com
 // Date: 2015/10/08 17:00:05
 
-#include <gtest/gtest.h>
-#include <sys/types.h>
-#include <sys/stat.h>
-#include <fcntl.h>
 #include <butil/atomicops.h>
 #include <butil/file_util.h>
-#include <butil/files/file_path.h>
-#include <butil/files/file_enumerator.h>
 #include <butil/files/dir_reader_posix.h>
-#include <butil/string_printf.h>
+#include <butil/files/file_enumerator.h>
+#include <butil/files/file_path.h>
 #include <butil/logging.h>
-#include "braft/util.h"
+#include <butil/string_printf.h>
+#include <fcntl.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+
 #include "braft/log.h"
 #include "braft/storage.h"
+#include "braft/util.h"
+#include "common.h"
 
 namespace braft {
 DECLARE_bool(raft_trace_append_entry_latency);
diff --git a/test/test_log_entry.cpp b/test/test_log_entry.cpp
index 600e274..bae8a5d 100644
--- a/test/test_log_entry.cpp
+++ b/test/test_log_entry.cpp
@@ -16,10 +16,11 @@
  * =====================================================================================
  */
 
-#include <gtest/gtest.h>
-#include <butil/logging.h>
 #include <butil/iobuf.h>
+#include <butil/logging.h>
+
 #include "braft/log_entry.h"
+#include "common.h"
 
 class TestUsageSuits : public testing::Test {
 protected:
diff --git a/test/test_log_manager.cpp b/test/test_log_manager.cpp
index e555d4a..bc8883c 100644
--- a/test/test_log_manager.cpp
+++ b/test/test_log_manager.cpp
@@ -3,16 +3,15 @@
 // Author: Zhangyi Chen (chenzhangyi01@baidu.com)
 // Date: 2015/11/24 16:30:49
 
-#include <gtest/gtest.h>
-
+#include <bthread/countdown_event.h>
+#include <butil/macros.h>
 #include <butil/memory/scoped_ptr.h>
 #include <butil/string_printf.h>
-#include <butil/macros.h>
 
-#include <bthread/countdown_event.h>
-#include "braft/log_manager.h"
 #include "braft/configuration.h"
 #include "braft/log.h"
+#include "braft/log_manager.h"
+#include "common.h"
 
 class LogManagerTest : public testing::Test {
 protected:
diff --git a/test/test_memory_storage.cpp b/test/test_memory_storage.cpp
index 88e16ce..779abdb 100644
--- a/test/test_memory_storage.cpp
+++ b/test/test_memory_storage.cpp
@@ -3,8 +3,8 @@
 // Author: qinduohao@baidu.com
 // Date: 2017/05/23
 
-#include <gtest/gtest.h>
 #include "braft/memory_log.h"
+#include "common.h"
 
 namespace braft {
 extern void global_init_once_or_die();
diff --git a/test/test_meta.cpp b/test/test_meta.cpp
index 903a645..268b538 100644
--- a/test/test_meta.cpp
+++ b/test/test_meta.cpp
@@ -1,8 +1,9 @@
-#include <gflags/gflags.h>
-#include <gtest/gtest.h>
 #include <butil/status.h>
+#include <gflags/gflags.h>
+
 #include "braft/raft.h"
 #include "braft/raft_meta.h"
+#include "common.h"
 
 namespace braft {
 extern void global_init_once_or_die();
diff --git a/test/test_node.cpp b/test/test_node.cpp
index 10cc01a..11668d4 100644
--- a/test/test_node.cpp
+++ b/test/test_node.cpp
@@ -4,22 +4,24 @@
 // Author: WangYao (fisherman), wangyao02@baidu.com
 // Date: 2015/10/08 17:00:05
 
-#include <algorithm>
-#include <gflags/gflags.h>
-#include <gtest/gtest.h>
-#include <sys/types.h>
-#include <sys/stat.h>
-#include <fcntl.h>
 #include <braft/sync_point.h>
-#include <butil/logging.h>
-#include <butil/files/file_path.h>
-#include <butil/file_util.h>
-#include <butil/fast_rand.h>
 #include <brpc/closure_guard.h>
 #include <bthread/bthread.h>
 #include <bthread/countdown_event.h>
+#include <butil/fast_rand.h>
+#include <butil/file_util.h>
+#include <butil/files/file_path.h>
+#include <butil/logging.h>
+#include <fcntl.h>
+#include <gflags/gflags.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+
+#include <algorithm>
 #include <vector>
+
 #include "../test/util.h"
+#include "common.h"
 
 namespace braft {
 extern bvar::Adder<int64_t> g_num_nodes;
diff --git a/test/test_protobuf_file.cpp b/test/test_protobuf_file.cpp
index fdb6830..a130f3a 100644
--- a/test/test_protobuf_file.cpp
+++ b/test/test_protobuf_file.cpp
@@ -3,7 +3,7 @@
  *
  *       Filename:  test_protobuf_file.cpp
  *
- *    Description:  
+ *    Description:
  *
  *        Version:  1.0
  *        Created:  2015/09/22 19:48:31
@@ -15,8 +15,8 @@
  *
  * =====================================================================================
  */
+#include "common.h"
 
-#include <gtest/gtest.h>
 #include "braft/local_storage.pb.h"
 #include "braft/protobuf_file.h"
 
diff --git a/test/test_repeated_timer_task.cpp b/test/test_repeated_timer_task.cpp
index 2e15710..df29c9d 100644
--- a/test/test_repeated_timer_task.cpp
+++ b/test/test_repeated_timer_task.cpp
@@ -4,7 +4,8 @@
 // Author: Zhangyi Chen (chenzhangyi01@baidu.com)
 // Date: 2016/11/02 21:12:26
 
-#include <gtest/gtest.h>
+#include "common.h"
+
 #include "braft/repeated_timer_task.h"
 
 class RepeatedTimerTaskTest : public testing::Test {
diff --git a/test/test_route_table.cpp b/test/test_route_table.cpp
index a05b26b..e8eb788 100644
--- a/test/test_route_table.cpp
+++ b/test/test_route_table.cpp
@@ -1,9 +1,11 @@
-#include "braft/configuration.h"
-#include "braft/route_table.h"
 #include <gflags/gflags.h>
-#include <gtest/gtest.h>
+
 #include <vector>
+
 #include "../test/util.h"
+#include "braft/configuration.h"
+#include "braft/route_table.h"
+#include "common.h"
 
 class RouteTableTest : public testing::Test {
   void SetUp() {
diff --git a/test/test_snapshot.cpp b/test/test_snapshot.cpp
index 3b3a98c..23648a0 100644
--- a/test/test_snapshot.cpp
+++ b/test/test_snapshot.cpp
@@ -4,17 +4,18 @@
 // Author: WangYao (fisherman), wangyao02@baidu.com
 // Date: 2015/10/08 17:00:05
 
-#include <gtest/gtest.h>
-#include <gflags/gflags.h>
-#include <butil/logging.h>
+#include <brpc/server.h>
 #include <butil/file_util.h>
+#include <butil/logging.h>
 #include <errno.h>
-#include <brpc/server.h>
-#include "braft/snapshot.h"
-#include "braft/raft.h"
-#include "braft/util.h"
+#include <gflags/gflags.h>
+
 #include "braft/local_file_meta.pb.h"
+#include "braft/raft.h"
+#include "braft/snapshot.h"
 #include "braft/snapshot_throttle.h"
+#include "braft/util.h"
+#include "common.h"
 #include "memory_file_system_adaptor.h"
 
 namespace logging {
diff --git a/test/test_snapshot_executor.cpp b/test/test_snapshot_executor.cpp
index 7d63bb0..b564215 100644
--- a/test/test_snapshot_executor.cpp
+++ b/test/test_snapshot_executor.cpp
@@ -3,13 +3,14 @@
 // Author: Zhangyi Chen (chenzhangyi01@baidu.com)
 // Date: 2016/07/27 20:13:42
 
-#include <gtest/gtest.h>
-#include <butil/atomicops.h>
 #include <brpc/server.h>
-#include "braft/snapshot_executor.h"
+#include <butil/atomicops.h>
+
 #include "braft/fsm_caller.h"
-#include "braft/util.h"
 #include "braft/raft.h"
+#include "braft/snapshot_executor.h"
+#include "braft/util.h"
+#include "common.h"
 
 namespace braft {
 
diff --git a/test/test_storage.cpp b/test/test_storage.cpp
index c9f7aea..7ea0967 100644
--- a/test/test_storage.cpp
+++ b/test/test_storage.cpp
@@ -3,7 +3,8 @@
 // Author: Zhangyi Chen (chenzhangyi01@baidu.com)
 // Date: 2016/03/29 10:35:06
 
-#include <gtest/gtest.h>
+#include <common.h>
+
 #include "braft/storage.h"
 
 namespace braft {
diff --git a/test/test_throttle.cpp b/test/test_throttle.cpp
index 94b35a9..7248b36 100644
--- a/test/test_throttle.cpp
+++ b/test/test_throttle.cpp
@@ -4,11 +4,12 @@
 // Author: Xiong Kai (xiongkai@baidu.com)
 // Date: 2017/09/07 14:06:13
 
-#include <gtest/gtest.h>
 #include <butil/logging.h>
+
 #include "braft/raft.h"
-#include "braft/util.h"
 #include "braft/snapshot_throttle.h"
+#include "braft/util.h"
+#include "common.h"
 
 class TestUsageSuits : public testing::Test {
 protected:
diff --git a/test/test_util.cpp b/test/test_util.cpp
index 731d9fa..6ac9a78 100644
--- a/test/test_util.cpp
+++ b/test/test_util.cpp
@@ -4,14 +4,14 @@
 // Author: WangYao (fisherman), wangyao02@baidu.com
 // Date: 2015/10/08 17:00:05
 
-#include <gtest/gtest.h>
+#include <butil/files/scoped_temp_dir.h>
 #include <butil/logging.h>
-#include <sys/types.h>
-#include <sys/stat.h>
 #include <fcntl.h>
-#include <butil/files/scoped_temp_dir.h>
+#include <sys/stat.h>
+#include <sys/types.h>
 
 #include "braft/util.h"
+#include "common.h"
 
 class TestUsageSuits : public testing::Test {
 protected:
diff --git a/test/util.h b/test/util.h
index 4730fbd..836065d 100644
--- a/test/util.h
+++ b/test/util.h
@@ -18,11 +18,13 @@
 #define PUBLIC_RAFT_TEST_UTIL_H
 
 #include <gflags/gflags.h>
-#include "braft/node.h"
+
 #include "braft/enum.pb.h"
 #include "braft/errno.pb.h"
-#include "braft/snapshot_throttle.h" 
+#include "braft/node.h"
 #include "braft/snapshot_executor.h"
+#include "braft/snapshot_throttle.h"
+#include "common.h"
 
 using namespace braft;
 bool g_dont_print_apply_log = false;