Efficient Conversion of a FlatBufferBuilder to a MessageBuilder (#4980)

* Efficient conversion of FlatBufferBuilder to grpc::MessageBuilder

* Added a variety of tests to validate correctness of the MessageBuilder move operations.
Disable MessageBuilder half-n-half tests on MacOS.

* Fix failing Android build

* Generalized the MessageBuilder move constructor to accept a deallocator

6 files changed, 441 insertions, 220 deletions

diff --git a/grpc/tests/grpctest.cpp b/grpc/tests/grpctest.cpp
index 50b17cc0..7e5c6e6c 100644
--- a/grpc/tests/grpctest.cpp
+++ b/grpc/tests/grpctest.cpp
@@ -23,6 +23,9 @@
 #include "test_assert.h"
 
 using namespace MyGame::Example;
+using flatbuffers::grpc::MessageBuilder;
+using flatbuffers::FlatBufferBuilder;
+
 void message_builder_tests();
 
 // The callback implementation of our server, that derives from the generated
@@ -46,9 +49,9 @@ class ServiceImpl final : public MyGame::Example::MonsterStorage::Service {
       const flatbuffers::grpc::Message<Stat> *request,
       ::grpc::ServerWriter<flatbuffers::grpc::Message<Monster>> *writer)
       override {
-    for (int i = 0; i < 10; i++) {
+    for (int i = 0; i < 5; i++) {
       fbb_.Clear();
-      // Create 10 monsters for resposne.
+      // Create 5 monsters for resposne.
       auto monster_offset =
           CreateMonster(fbb_, 0, 0, 0,
                         fbb_.CreateString(request->GetRoot()->id()->str() +
@@ -94,6 +97,45 @@ void RunServer() {
   server_instance->Wait();
 }
 
+template <class Builder>
+void StoreRPC(MonsterStorage::Stub *stub) {
+  Builder fbb;
+  grpc::ClientContext context;
+  // Build a request with the name set.
+  auto monster_offset = CreateMonster(fbb, 0, 0, 0, fbb.CreateString("Fred"));
+  MessageBuilder mb(std::move(fbb));
+  mb.Finish(monster_offset);
+  auto request = mb.ReleaseMessage<Monster>();
+  flatbuffers::grpc::Message<Stat> response;
+
+  // The actual RPC.
+  auto status = stub->Store(&context, request, &response);
+
+  if (status.ok()) {
+    auto resp = response.GetRoot()->id();
+    std::cout << "RPC response: " << resp->str() << std::endl;
+  } else {
+    std::cout << "RPC failed" << std::endl;
+  }
+}
+
+template <class Builder>
+void RetrieveRPC(MonsterStorage::Stub *stub) {
+  Builder fbb;
+  grpc::ClientContext context;
+  fbb.Clear();
+  auto stat_offset = CreateStat(fbb, fbb.CreateString("Fred"));
+  fbb.Finish(stat_offset);
+  auto request = MessageBuilder(std::move(fbb)).ReleaseMessage<Stat>();
+
+  flatbuffers::grpc::Message<Monster> response;
+  auto stream = stub->Retrieve(&context, request);
+  while (stream->Read(&response)) {
+    auto resp = response.GetRoot()->name();
+    std::cout << "RPC Streaming response: " << resp->str() << std::endl;
+  }
+}
+
 int grpc_server_test() {
   // Launch server.
   std::thread server_thread(RunServer);
@@ -107,39 +149,12 @@ int grpc_server_test() {
                                      grpc::InsecureChannelCredentials());
   auto stub = MyGame::Example::MonsterStorage::NewStub(channel);
 
-  flatbuffers::grpc::MessageBuilder fbb;
-  {
-    grpc::ClientContext context;
-    // Build a request with the name set.
-    auto monster_offset = CreateMonster(fbb, 0, 0, 0, fbb.CreateString("Fred"));
-    fbb.Finish(monster_offset);
-    auto request = fbb.ReleaseMessage<Monster>();
-    flatbuffers::grpc::Message<Stat> response;
+  StoreRPC<MessageBuilder>(stub.get());
+  StoreRPC<FlatBufferBuilder>(stub.get());
 
-    // The actual RPC.
-    auto status = stub->Store(&context, request, &response);
+  RetrieveRPC<MessageBuilder>(stub.get());
+  RetrieveRPC<FlatBufferBuilder>(stub.get());
 
-    if (status.ok()) {
-      auto resp = response.GetRoot()->id();
-      std::cout << "RPC response: " << resp->str() << std::endl;
-    } else {
-      std::cout << "RPC failed" << std::endl;
-    }
-  }
-  {
-    grpc::ClientContext context;
-    fbb.Clear();
-    auto stat_offset = CreateStat(fbb, fbb.CreateString("Fred"));
-    fbb.Finish(stat_offset);
-    auto request = fbb.ReleaseMessage<Stat>();
-
-    flatbuffers::grpc::Message<Monster> response;
-    auto stream = stub->Retrieve(&context, request);
-    while (stream->Read(&response)) {
-      auto resp = response.GetRoot()->name();
-      std::cout << "RPC Streaming response: " << resp->str() << std::endl;
-    }
-  }
 
 #if !FLATBUFFERS_GRPC_DISABLE_AUTO_VERIFICATION
   {
diff --git a/grpc/tests/message_builder_test.cpp b/grpc/tests/message_builder_test.cpp
index 25f04da8..d5ed8d97 100644
--- a/grpc/tests/message_builder_test.cpp
+++ b/grpc/tests/message_builder_test.cpp
@@ -14,19 +14,37 @@ bool release_n_verify(flatbuffers::grpc::MessageBuilder &mbb, const std::string
   return (monster->name()->str() == expected_name) && (monster->color() == color);
 }
 
-template <>
-struct BuilderReuseTests<flatbuffers::grpc::MessageBuilder> {
+void builder_move_assign_after_releaseraw_test(flatbuffers::grpc::MessageBuilder dst) {
+  auto root_offset1 = populate1(dst);
+  dst.Finish(root_offset1);
+  size_t size, offset;
+  grpc_slice slice;
+  dst.ReleaseRaw(size, offset, slice);
+  flatbuffers::FlatBufferBuilder src;
+  auto root_offset2 = populate2(src);
+  src.Finish(root_offset2);
+  auto src_size = src.GetSize();
+  // Move into a released builder.
+  dst = std::move(src);
+  TEST_EQ(dst.GetSize(), src_size);
+  TEST_ASSERT(release_n_verify(dst, m2_name, m2_color));
+  TEST_EQ(src.GetSize(), 0);
+  grpc_slice_unref(slice);
+}
+
+template <class SrcBuilder>
+struct BuilderReuseTests<flatbuffers::grpc::MessageBuilder, SrcBuilder> {
   static void builder_reusable_after_release_message_test(TestSelector selector) {
     if (!selector.count(REUSABLE_AFTER_RELEASE_MESSAGE)) {
       return;
     }
 
-    flatbuffers::grpc::MessageBuilder b1;
+    flatbuffers::grpc::MessageBuilder mb;
     std::vector<flatbuffers::grpc::Message<Monster>> buffers;
     for (int i = 0; i < 5; ++i) {
-      auto root_offset1 = populate1(b1);
-      b1.Finish(root_offset1);
-      buffers.push_back(b1.ReleaseMessage<Monster>());
+      auto root_offset1 = populate1(mb);
+      mb.Finish(root_offset1);
+      buffers.push_back(mb.ReleaseMessage<Monster>());
       TEST_ASSERT_FUNC(verify(buffers[i], m1_name, m1_color));
     }
   }
@@ -36,14 +54,15 @@ struct BuilderReuseTests<flatbuffers::grpc::MessageBuilder> {
       return;
     }
 
-    // FIXME: Populate-Release loop fails assert(GRPC_SLICE_IS_EMPTY(slice_)).
+    // FIXME: Populate-Release loop fails assert(GRPC_SLICE_IS_EMPTY(slice_)) in SliceAllocator::allocate
+    // in the second iteration.
 
-    flatbuffers::grpc::MessageBuilder b1;
+    flatbuffers::grpc::MessageBuilder mb;
     std::vector<flatbuffers::DetachedBuffer> buffers;
-    for (int i = 0; i < 5; ++i) {
-      auto root_offset1 = populate1(b1);
-      b1.Finish(root_offset1);
-      buffers.push_back(b1.Release());
+    for (int i = 0; i < 2; ++i) {
+      auto root_offset1 = populate1(mb);
+      mb.Finish(root_offset1);
+      buffers.push_back(mb.Release());
       TEST_ASSERT_FUNC(verify(buffers[i], m1_name, m1_color));
     }
   }
@@ -53,13 +72,13 @@ struct BuilderReuseTests<flatbuffers::grpc::MessageBuilder> {
       return;
     }
 
-    flatbuffers::grpc::MessageBuilder b1;
+    flatbuffers::grpc::MessageBuilder mb;
     for (int i = 0; i < 5; ++i) {
-      auto root_offset1 = populate1(b1);
-      b1.Finish(root_offset1);
+      auto root_offset1 = populate1(mb);
+      mb.Finish(root_offset1);
       size_t size, offset;
       grpc_slice slice;
-      const uint8_t *buf = b1.ReleaseRaw(size, offset, slice);
+      const uint8_t *buf = mb.ReleaseRaw(size, offset, slice);
       TEST_ASSERT_FUNC(verify(buf, offset, m1_name, m1_color));
       grpc_slice_unref(slice);
     }
@@ -70,22 +89,23 @@ struct BuilderReuseTests<flatbuffers::grpc::MessageBuilder> {
       return;
     }
 
-    // FIXME: Release-move_assign loop fails assert(p == GRPC_SLICE_START_PTR(slice_)).
+    // FIXME: Release-move_assign loop fails assert(p == GRPC_SLICE_START_PTR(slice_))
+    // in DetachedBuffer destructor after all the iterations
 
-    flatbuffers::grpc::MessageBuilder b1;
+    flatbuffers::grpc::MessageBuilder dst;
     std::vector<flatbuffers::DetachedBuffer> buffers;
 
-    for (int i = 0; i < 1; ++i) {
-      auto root_offset1 = populate1(b1);
-      b1.Finish(root_offset1);
-      buffers.push_back(b1.Release());
+    for (int i = 0; i < 2; ++i) {
+      auto root_offset1 = populate1(dst);
+      dst.Finish(root_offset1);
+      buffers.push_back(dst.Release());
       TEST_ASSERT_FUNC(verify(buffers[i], m1_name, m1_color));
 
-      // bring b1 back to life.
-      flatbuffers::grpc::MessageBuilder b2;
-      b1 = std::move(b2);
-      TEST_EQ_FUNC(b1.GetSize(), 0);
-      TEST_EQ_FUNC(b2.GetSize(), 0);
+      // bring dst back to life.
+      SrcBuilder src;
+      dst = std::move(src);
+      TEST_EQ_FUNC(dst.GetSize(), 0);
+      TEST_EQ_FUNC(src.GetSize(), 0);
     }
   }
 
@@ -94,20 +114,20 @@ struct BuilderReuseTests<flatbuffers::grpc::MessageBuilder> {
       return;
     }
 
-    flatbuffers::grpc::MessageBuilder b1;
+    flatbuffers::grpc::MessageBuilder dst;
     std::vector<flatbuffers::grpc::Message<Monster>> buffers;
 
     for (int i = 0; i < 5; ++i) {
-      auto root_offset1 = populate1(b1);
-      b1.Finish(root_offset1);
-      buffers.push_back(b1.ReleaseMessage<Monster>());
+      auto root_offset1 = populate1(dst);
+      dst.Finish(root_offset1);
+      buffers.push_back(dst.ReleaseMessage<Monster>());
       TEST_ASSERT_FUNC(verify(buffers[i], m1_name, m1_color));
 
-      // bring b1 back to life.
-      flatbuffers::grpc::MessageBuilder b2;
-      b1 = std::move(b2);
-      TEST_EQ_FUNC(b1.GetSize(), 0);
-      TEST_EQ_FUNC(b2.GetSize(), 0);
+      // bring dst back to life.
+      SrcBuilder src;
+      dst = std::move(src);
+      TEST_EQ_FUNC(dst.GetSize(), 0);
+      TEST_EQ_FUNC(src.GetSize(), 0);
     }
   }
 
@@ -116,20 +136,20 @@ struct BuilderReuseTests<flatbuffers::grpc::MessageBuilder> {
       return;
     }
 
-    flatbuffers::grpc::MessageBuilder b1;
+    flatbuffers::grpc::MessageBuilder dst;
     for (int i = 0; i < 5; ++i) {
-      auto root_offset1 = populate1(b1);
-      b1.Finish(root_offset1);
+      auto root_offset1 = populate1(dst);
+      dst.Finish(root_offset1);
       size_t size, offset;
       grpc_slice slice = grpc_empty_slice();
-      const uint8_t *buf = b1.ReleaseRaw(size, offset, slice);
+      const uint8_t *buf = dst.ReleaseRaw(size, offset, slice);
       TEST_ASSERT_FUNC(verify(buf, offset, m1_name, m1_color));
       grpc_slice_unref(slice);
 
-      flatbuffers::grpc::MessageBuilder b2;
-      b1 = std::move(b2); 
-      TEST_EQ_FUNC(b1.GetSize(), 0);
-      TEST_EQ_FUNC(b2.GetSize(), 0);
+      SrcBuilder src;
+      dst = std::move(src);
+      TEST_EQ_FUNC(dst.GetSize(), 0);
+      TEST_EQ_FUNC(src.GetSize(), 0);
     }
   }
 
@@ -153,7 +173,7 @@ void slice_allocator_tests() {
     buf[0] = 100;
     buf[size-1] = 200;
     flatbuffers::grpc::SliceAllocator sa2(std::move(sa1));
-    // buf should be deleted after move-construct
+    // buf should not be deleted after move-construct
     TEST_EQ_FUNC(buf[0], 100);
     TEST_EQ_FUNC(buf[size-1], 200);
     // buf is freed here
@@ -170,13 +190,140 @@ void slice_allocator_tests() {
   }
 }
 
+/// This function does not populate exactly the first half of the table. But it could.
+void populate_first_half(MyGame::Example::MonsterBuilder &wrapper, flatbuffers::Offset<flatbuffers::String> name_offset) {
+  wrapper.add_name(name_offset);
+  wrapper.add_color(m1_color);
+}
+
+/// This function does not populate exactly the second half of the table. But it could.
+void populate_second_half(MyGame::Example::MonsterBuilder &wrapper) {
+  wrapper.add_hp(77);
+  wrapper.add_mana(88);
+  Vec3 vec3;
+  wrapper.add_pos(&vec3);
+}
+
+/// This function is a hack to update the FlatBufferBuilder reference (fbb_) in the MonsterBuilder object.
+/// This function will break if fbb_ is not the first member in MonsterBuilder. In that case, some offset must be added.
+/// This function is used exclusively for testing correctness of move operations between FlatBufferBuilders.
+/// If MonsterBuilder had a fbb_ pointer, this hack would be unnecessary. That involves a code-generator change though.
+void test_only_hack_update_fbb_reference(MyGame::Example::MonsterBuilder &monsterBuilder,
+                                         flatbuffers::grpc::MessageBuilder &mb) {
+  *reinterpret_cast<flatbuffers::FlatBufferBuilder **>(&monsterBuilder) = &mb;
+}
+
+/// This test validates correctness of move conversion of FlatBufferBuilder to a MessageBuilder DURING
+/// a table construction. Half of the table is constructed using FlatBufferBuilder and the other half
+/// of the table is constructed using a MessageBuilder.
+void builder_move_ctor_conversion_before_finish_half_n_half_table_test() {
+  for (size_t initial_size = 4 ; initial_size <= 2048; initial_size *= 2) {
+    flatbuffers::FlatBufferBuilder fbb(initial_size);
+    auto name_offset = fbb.CreateString(m1_name);
+    MyGame::Example::MonsterBuilder monsterBuilder(fbb);     // starts a table in FlatBufferBuilder
+    populate_first_half(monsterBuilder, name_offset);
+    flatbuffers::grpc::MessageBuilder mb(std::move(fbb));
+    test_only_hack_update_fbb_reference(monsterBuilder, mb); // hack
+    populate_second_half(monsterBuilder);
+    mb.Finish(monsterBuilder.Finish());                      // ends the table in MessageBuilder
+    TEST_ASSERT_FUNC(release_n_verify(mb, m1_name, m1_color));
+    TEST_EQ_FUNC(fbb.GetSize(), 0);
+  }
+}
+
+/// This test populates a COMPLETE inner table before move conversion and later populates more members in the outer table.
+void builder_move_ctor_conversion_before_finish_test() {
+  for (size_t initial_size = 4 ; initial_size <= 2048; initial_size *= 2) {
+    flatbuffers::FlatBufferBuilder fbb(initial_size);
+    auto stat_offset = CreateStat(fbb, fbb.CreateString("SomeId"), 0, 0);
+    flatbuffers::grpc::MessageBuilder mb(std::move(fbb));
+    auto monster_offset = CreateMonster(mb, 0, 150, 100, mb.CreateString(m1_name), 0, m1_color, Any_NONE, 0, 0, 0, 0, 0, 0, stat_offset);
+    mb.Finish(monster_offset);
+    TEST_ASSERT_FUNC(release_n_verify(mb, m1_name, m1_color));
+    TEST_EQ_FUNC(fbb.GetSize(), 0);
+  }
+}
+
+/// This test validates correctness of move conversion of FlatBufferBuilder to a MessageBuilder DURING
+/// a table construction. Half of the table is constructed using FlatBufferBuilder and the other half
+/// of the table is constructed using a MessageBuilder.
+void builder_move_assign_conversion_before_finish_half_n_half_table_test() {
+  flatbuffers::FlatBufferBuilder fbb;
+  flatbuffers::grpc::MessageBuilder mb;
+
+  for (int i = 0;i < 5; ++i) {
+    flatbuffers::FlatBufferBuilder fbb;
+    auto name_offset = fbb.CreateString(m1_name);
+    MyGame::Example::MonsterBuilder monsterBuilder(fbb);     // starts a table in FlatBufferBuilder
+    populate_first_half(monsterBuilder, name_offset);
+    mb = std::move(fbb);
+    test_only_hack_update_fbb_reference(monsterBuilder, mb); // hack
+    populate_second_half(monsterBuilder);
+    mb.Finish(monsterBuilder.Finish());                      // ends the table in MessageBuilder
+    TEST_ASSERT_FUNC(release_n_verify(mb, m1_name, m1_color));
+    TEST_EQ_FUNC(fbb.GetSize(), 0);
+  }
+}
+
+/// This test populates a COMPLETE inner table before move conversion and later populates more members in the outer table.
+void builder_move_assign_conversion_before_finish_test() {
+  flatbuffers::FlatBufferBuilder fbb;
+  flatbuffers::grpc::MessageBuilder mb;
+
+  for (int i = 0;i < 5; ++i) {
+    auto stat_offset = CreateStat(fbb, fbb.CreateString("SomeId"), 0, 0);
+    mb = std::move(fbb);
+    auto monster_offset = CreateMonster(mb, 0, 150, 100, mb.CreateString(m1_name), 0, m1_color, Any_NONE, 0, 0, 0, 0, 0, 0, stat_offset);
+    mb.Finish(monster_offset);
+    TEST_ASSERT_FUNC(release_n_verify(mb, m1_name, m1_color));
+    TEST_EQ_FUNC(fbb.GetSize(), 0);
+  }
+}
+
+/// This test populates data, finishes the buffer, and does move conversion after.
+void builder_move_ctor_conversion_after_finish_test() {
+  flatbuffers::FlatBufferBuilder fbb;
+  fbb.Finish(populate1(fbb));
+  flatbuffers::grpc::MessageBuilder mb(std::move(fbb));
+  TEST_ASSERT_FUNC(release_n_verify(mb, m1_name, m1_color));
+  TEST_EQ_FUNC(fbb.GetSize(), 0);
+}
+
+/// This test populates data, finishes the buffer, and does move conversion after.
+void builder_move_assign_conversion_after_finish_test() {
+  flatbuffers::FlatBufferBuilder fbb;
+  flatbuffers::grpc::MessageBuilder mb;
+
+  for (int i = 0;i < 5; ++i) {
+    fbb.Finish(populate1(fbb));
+    mb = std::move(fbb);
+    TEST_ASSERT_FUNC(release_n_verify(mb, m1_name, m1_color));
+    TEST_EQ_FUNC(fbb.GetSize(), 0);
+  }
+}
+
 void message_builder_tests() {
+  using flatbuffers::grpc::MessageBuilder;
+  using flatbuffers::FlatBufferBuilder;
+
   slice_allocator_tests();
-  BuilderTests<flatbuffers::grpc::MessageBuilder>::all_tests();
+
+#ifndef __APPLE__
+  builder_move_ctor_conversion_before_finish_half_n_half_table_test();
+  builder_move_assign_conversion_before_finish_half_n_half_table_test();
+#endif // __APPLE__
+  builder_move_ctor_conversion_before_finish_test();
+  builder_move_assign_conversion_before_finish_test();
+
+  builder_move_ctor_conversion_after_finish_test();
+  builder_move_assign_conversion_after_finish_test();
+
+  BuilderTests<MessageBuilder, MessageBuilder>::all_tests();
+  BuilderTests<MessageBuilder, FlatBufferBuilder>::all_tests();
 
   BuilderReuseTestSelector tests[6] = {
-    // REUSABLE_AFTER_RELEASE,                 // Assertion failed: (GRPC_SLICE_IS_EMPTY(slice_))
-    // REUSABLE_AFTER_RELEASE_AND_MOVE_ASSIGN, // Assertion failed: (p == GRPC_SLICE_START_PTR(slice_)
+    //REUSABLE_AFTER_RELEASE,                 // Assertion failed: (GRPC_SLICE_IS_EMPTY(slice_))
+    //REUSABLE_AFTER_RELEASE_AND_MOVE_ASSIGN, // Assertion failed: (p == GRPC_SLICE_START_PTR(slice_)
 
     REUSABLE_AFTER_RELEASE_RAW,
     REUSABLE_AFTER_RELEASE_MESSAGE,
@@ -184,5 +331,6 @@ void message_builder_tests() {
     REUSABLE_AFTER_RELEASE_RAW_AND_MOVE_ASSIGN
   };
 
-  BuilderReuseTests<flatbuffers::grpc::MessageBuilder>::run_tests(TestSelector(tests, tests+6));
+  BuilderReuseTests<MessageBuilder, MessageBuilder>::run_tests(TestSelector(tests, tests+6));
+  BuilderReuseTests<MessageBuilder, FlatBufferBuilder>::run_tests(TestSelector(tests, tests+6));
 }
diff --git a/include/flatbuffers/flatbuffers.h b/include/flatbuffers/flatbuffers.h
index 5c02e1f1..b6b1b1cb 100644
--- a/include/flatbuffers/flatbuffers.h
+++ b/include/flatbuffers/flatbuffers.h
@@ -425,6 +425,10 @@ class DefaultAllocator : public Allocator {
   void deallocate(uint8_t *p, size_t) FLATBUFFERS_OVERRIDE {
     delete[] p;
   }
+
+  static void dealloc(void *p, size_t) {
+    delete[] static_cast<uint8_t *>(p);
+  }
 };
 
 // These functions allow for a null allocator to mean use the default allocator,
@@ -549,7 +553,7 @@ class DetachedBuffer {
   #endif  // !defined(FLATBUFFERS_CPP98_STL)
   // clang-format on
 
- protected:
+protected:
   Allocator *allocator_;
   bool own_allocator_;
   uint8_t *buf_;
@@ -986,7 +990,7 @@ class FlatBufferBuilder {
   /// `FlatBuffer` starts.
   /// @return A raw pointer to the start of the memory block containing
   /// the serialized `FlatBuffer`.
-  /// @remark If the allocator is owned, it gets deleted during this call.
+  /// @remark If the allocator is owned, it gets deleted when the destructor is called..
   uint8_t *ReleaseRaw(size_t &size, size_t &offset) {
     Finished();
     return buf_.release_raw(size, offset);
@@ -1759,7 +1763,12 @@ class FlatBufferBuilder {
     Finish(root.o, file_identifier, true);
   }
 
- protected:
+  void SwapBufAllocator(FlatBufferBuilder &other) {
+    buf_.swap_allocator(other.buf_);
+  }
+
+protected:
+
   // You shouldn't really be copying instances of this class.
   FlatBufferBuilder(const FlatBufferBuilder &);
   FlatBufferBuilder &operator=(const FlatBufferBuilder &);
diff --git a/include/flatbuffers/grpc.h b/include/flatbuffers/grpc.h
index 00553ef6..a75b67c7 100644
--- a/include/flatbuffers/grpc.h
+++ b/include/flatbuffers/grpc.h
@@ -164,7 +164,7 @@ class MessageBuilder : private detail::SliceAllocatorMember,
                        public FlatBufferBuilder {
  public:
   explicit MessageBuilder(uoffset_t initial_size = 1024)
-      : FlatBufferBuilder(initial_size, &slice_allocator_, false) {}
+    : FlatBufferBuilder(initial_size, &slice_allocator_, false) {}
 
   MessageBuilder(const MessageBuilder &other) = delete;
   MessageBuilder &operator=(const MessageBuilder &other) = delete;
@@ -175,6 +175,30 @@ class MessageBuilder : private detail::SliceAllocatorMember,
     Swap(other);
   }
 
+  /// Create a MessageBuilder from a FlatBufferBuilder.
+  explicit MessageBuilder(FlatBufferBuilder &&src, void (*dealloc)(void*, size_t) = &DefaultAllocator::dealloc)
+    : FlatBufferBuilder(1024, &slice_allocator_, false) {
+    src.Swap(*this);
+    src.SwapBufAllocator(*this);
+    if (buf_.capacity()) {
+      uint8_t *buf = buf_.scratch_data();       // pointer to memory
+      size_t capacity = buf_.capacity();        // size of memory
+      slice_allocator_.slice_ = grpc_slice_new_with_len(buf, capacity, dealloc);
+    }
+    else {
+      slice_allocator_.slice_ = grpc_empty_slice();
+    }
+  }
+
+  /// Move-assign a FlatBufferBuilder to a MessageBuilder.
+  /// Only FlatBufferBuilder with default allocator (basically, nullptr) is supported.
+  MessageBuilder &operator=(FlatBufferBuilder &&src) {
+    // Move construct a temporary and swap
+    MessageBuilder temp(std::move(src));
+    Swap(temp);
+    return *this;
+  }
+
   MessageBuilder &operator=(MessageBuilder &&other) {
     // Move construct a temporary and swap
     MessageBuilder temp(std::move(other));
diff --git a/tests/test_builder.cpp b/tests/test_builder.cpp
index 2c1e823c..8c070c11 100644
--- a/tests/test_builder.cpp
+++ b/tests/test_builder.cpp
@@ -129,7 +129,9 @@ bool release_n_verify(flatbuffers::FlatBufferBuilder &fbb, const std::string &ex
 }
 
 void FlatBufferBuilderTest() {
-  BuilderTests<flatbuffers::FlatBufferBuilder>::all_tests();
+  using flatbuffers::FlatBufferBuilder;
+
+  BuilderTests<FlatBufferBuilder>::all_tests();
   BuilderTests<TestHeapBuilder>::all_tests();
   BuilderTests<GrpcLikeMessageBuilder>::all_tests();
 
@@ -140,7 +142,7 @@ void FlatBufferBuilderTest() {
     REUSABLE_AFTER_RELEASE_RAW_AND_MOVE_ASSIGN
   };
 
-  BuilderReuseTests<flatbuffers::FlatBufferBuilder>::run_tests(TestSelector(tests, tests+4));
-  BuilderReuseTests<TestHeapBuilder>::run_tests(TestSelector(tests, tests+4));
-  BuilderReuseTests<GrpcLikeMessageBuilder>::run_tests(TestSelector(tests, tests+4));
+  BuilderReuseTests<FlatBufferBuilder, FlatBufferBuilder>::run_tests(TestSelector(tests, tests+4));
+  BuilderReuseTests<TestHeapBuilder, TestHeapBuilder>::run_tests(TestSelector(tests, tests+4));
+  BuilderReuseTests<GrpcLikeMessageBuilder, GrpcLikeMessageBuilder>::run_tests(TestSelector(tests, tests+4));
 }
diff --git a/tests/test_builder.h b/tests/test_builder.h
index 76222a72..1ea17b49 100644
--- a/tests/test_builder.h
+++ b/tests/test_builder.h
@@ -2,6 +2,7 @@
 #define TEST_BUILDER_H
 
 #include <set>
+#include <type_traits>
 #include "monster_test_generated.h"
 #include "flatbuffers/flatbuffers.h"
 #include "test_assert.h"
@@ -13,6 +14,16 @@ class MessageBuilder;
 }
 }
 
+template <class T, class U>
+struct is_same {
+  static const bool value = false;
+};
+
+template <class T>
+struct is_same<T, T> {
+  static const bool value = true;
+};
+
 extern const std::string m1_name;
 extern const Color m1_color;
 extern const std::string m2_name;
@@ -32,145 +43,157 @@ bool verify(const uint8_t *buf, size_t offset, const std::string &expected_name,
 bool release_n_verify(flatbuffers::FlatBufferBuilder &fbb, const std::string &expected_name, Color color);
 bool release_n_verify(flatbuffers::grpc::MessageBuilder &mbb, const std::string &expected_name, Color color);
 
+// clang-format off
+#if !defined(FLATBUFFERS_CPP98_STL)
+// clang-format on
 template <class Builder>
+void builder_move_assign_after_releaseraw_test(Builder b1) {
+  auto root_offset1 = populate1(b1);
+  b1.Finish(root_offset1);
+  size_t size, offset;
+  b1.ReleaseRaw(size, offset);
+  Builder src;
+  auto root_offset2 = populate2(src);
+  src.Finish(root_offset2);
+  auto src_size = src.GetSize();
+  // Move into a released builder.
+  b1 = std::move(src);
+  TEST_EQ_FUNC(b1.GetSize(), src_size);
+  TEST_ASSERT_FUNC(release_n_verify(b1, m2_name, m2_color));
+  TEST_EQ_FUNC(src.GetSize(), 0);
+}
+// clang-format off
+#endif  // !defined(FLATBUFFERS_CPP98_STL)
+// clang-format on
+
+void builder_move_assign_after_releaseraw_test(flatbuffers::grpc::MessageBuilder b1);
+
+template <class DestBuilder, class SrcBuilder = DestBuilder>
 struct BuilderTests {
   // clang-format off
   #if !defined(FLATBUFFERS_CPP98_STL)
   // clang-format on
   static void empty_builder_movector_test() {
-    Builder b1;
-    size_t b1_size = b1.GetSize();
-    Builder b2(std::move(b1));
-    size_t b2_size = b2.GetSize();
-    TEST_EQ_FUNC(b1_size, 0);
-    TEST_EQ_FUNC(b1_size, b2_size);
+    SrcBuilder src;
+    size_t src_size = src.GetSize();
+    DestBuilder dst(std::move(src));
+    size_t dst_size = dst.GetSize();
+    TEST_EQ_FUNC(src_size, 0);
+    TEST_EQ_FUNC(src_size, dst_size);
   }
 
   static void nonempty_builder_movector_test() {
-    Builder b1;
-    populate1(b1);
-    size_t b1_size = b1.GetSize();
-    Builder b2(std::move(b1));
-    TEST_EQ_FUNC(b1_size, b2.GetSize());
-    TEST_EQ_FUNC(b1.GetSize(), 0);
+    SrcBuilder src;
+    populate1(src);
+    size_t src_size = src.GetSize();
+    DestBuilder dst(std::move(src));
+    TEST_EQ_FUNC(src_size, dst.GetSize());
+    TEST_EQ_FUNC(src.GetSize(), 0);
   }
 
   static void builder_movector_before_finish_test() {
-    Builder b1;
-    auto root_offset1 = populate1(b1);
-    Builder b2(std::move(b1));
-    b2.Finish(root_offset1);
-    TEST_ASSERT_FUNC(release_n_verify(b2, m1_name, m1_color));
-    TEST_EQ_FUNC(b1.GetSize(), 0);
+    SrcBuilder src;
+    auto root_offset1 = populate1(src);
+    DestBuilder dst(std::move(src));
+    dst.Finish(root_offset1);
+    TEST_ASSERT_FUNC(release_n_verify(dst, m1_name, m1_color));
+    TEST_EQ_FUNC(src.GetSize(), 0);
   }
 
   static void builder_movector_after_finish_test() {
-    Builder b1;
-    auto root_offset1 = populate1(b1);
-    b1.Finish(root_offset1);
-    auto b1_size = b1.GetSize();
-    Builder b2(std::move(b1));
-    TEST_EQ_FUNC(b2.GetSize(), b1_size);
-    TEST_ASSERT_FUNC(release_n_verify(b2, m1_name, m1_color));
-    TEST_EQ_FUNC(b1.GetSize(), 0);
+    SrcBuilder src;
+    auto root_offset1 = populate1(src);
+    src.Finish(root_offset1);
+    auto src_size = src.GetSize();
+    DestBuilder dst(std::move(src));
+    TEST_EQ_FUNC(dst.GetSize(), src_size);
+    TEST_ASSERT_FUNC(release_n_verify(dst, m1_name, m1_color));
+    TEST_EQ_FUNC(src.GetSize(), 0);
   }
 
   static void builder_move_assign_before_finish_test() {
-    Builder b1;
-    auto root_offset1 = populate1(b1);
-    Builder b2;
-    populate2(b2);
-    b2 = std::move(b1);
-    b2.Finish(root_offset1);
-    TEST_ASSERT_FUNC(release_n_verify(b2, m1_name, m1_color));
-    TEST_EQ_FUNC(b1.GetSize(), 0);
+    SrcBuilder src;
+    auto root_offset1 = populate1(src);
+    DestBuilder dst;
+    populate2(dst);
+    dst = std::move(src);
+    dst.Finish(root_offset1);
+    TEST_ASSERT_FUNC(release_n_verify(dst, m1_name, m1_color));
+    TEST_EQ_FUNC(src.GetSize(), 0);
   }
 
   static void builder_move_assign_after_finish_test() {
-    Builder b1;
-    auto root_offset1 = populate1(b1);
-    b1.Finish(root_offset1);
-    auto b1_size = b1.GetSize();
-    Builder b2;
-    auto root_offset2 = populate2(b2);
-    b2.Finish(root_offset2);
-    b2 = std::move(b1);
-    TEST_EQ_FUNC(b2.GetSize(), b1_size);
-    TEST_ASSERT_FUNC(release_n_verify(b2, m1_name, m1_color));
-    TEST_EQ_FUNC(b1.GetSize(), 0);
+    SrcBuilder src;
+    auto root_offset1 = populate1(src);
+    src.Finish(root_offset1);
+    auto src_size = src.GetSize();
+    DestBuilder dst;
+    auto root_offset2 = populate2(dst);
+    dst.Finish(root_offset2);
+    dst = std::move(src);
+    TEST_EQ_FUNC(dst.GetSize(), src_size);
+    TEST_ASSERT_FUNC(release_n_verify(dst, m1_name, m1_color));
+    TEST_EQ_FUNC(src.GetSize(), 0);
   }
 
   static void builder_move_assign_after_release_test() {
-    Builder b1;
-    auto root_offset1 = populate1(b1);
-    b1.Finish(root_offset1);
+    DestBuilder dst;
+    auto root_offset1 = populate1(dst);
+    dst.Finish(root_offset1);
     {
-      flatbuffers::DetachedBuffer b1_detached = b1.Release();
+      flatbuffers::DetachedBuffer dst_detached = dst.Release();
       // detached buffer is deleted
     }
-    Builder b2;
-    auto root_offset2 = populate2(b2);
-    b2.Finish(root_offset2);
-    auto b2_size = b2.GetSize();
+    SrcBuilder src;
+    auto root_offset2 = populate2(src);
+    src.Finish(root_offset2);
+    auto src_size = src.GetSize();
     // Move into a released builder.
-    b1 = std::move(b2);
-    TEST_EQ_FUNC(b1.GetSize(), b2_size);
-    TEST_ASSERT_FUNC(release_n_verify(b1, m2_name, m2_color));
-    TEST_EQ_FUNC(b2.GetSize(), 0);
-  }
-
-  static void builder_move_assign_after_releaseraw_test() {
-    Builder b1;
-    auto root_offset1 = populate1(b1);
-    b1.Finish(root_offset1);
-    size_t size, offset;
-    uint8_t *buf = release_raw_base(b1, size, offset);
-    TEST_ASSERT_FUNC(verify(buf, offset, m1_name, m1_color));
-    free_raw(b1, buf);
-    Builder b2;
-    auto root_offset2 = populate2(b2);
-    b2.Finish(root_offset2);
-    auto b2_size = b2.GetSize();
-    // Move into a released builder.
-    b1 = std::move(b2);
-    TEST_EQ_FUNC(b1.GetSize(), b2_size);
-    TEST_ASSERT_FUNC(release_n_verify(b1, m2_name, m2_color));
-    TEST_EQ_FUNC(b2.GetSize(), 0);
+    dst = std::move(src);
+    TEST_EQ_FUNC(dst.GetSize(), src_size);
+    TEST_ASSERT_FUNC(release_n_verify(dst, m2_name, m2_color));
+    TEST_EQ_FUNC(src.GetSize(), 0);
   }
   // clang-format off
   #endif  // !defined(FLATBUFFERS_CPP98_STL)
   // clang-format on
 
-  static void builder_swap_before_finish_test() {
-    Builder b1;
-    auto root_offset1 = populate1(b1);
-    auto size1 = b1.GetSize();
-    Builder b2;
-    auto root_offset2 = populate2(b2);
-    auto size2 = b2.GetSize();
-    b1.Swap(b2);
-    b1.Finish(root_offset2);
-    b2.Finish(root_offset1);
-    TEST_EQ_FUNC(b1.GetSize() > size2, true);
-    TEST_EQ_FUNC(b2.GetSize() > size1, true);
-    TEST_ASSERT_FUNC(release_n_verify(b1, m2_name, m2_color));
-    TEST_ASSERT_FUNC(release_n_verify(b2, m1_name, m1_color));
+  static void builder_swap_before_finish_test(bool run = is_same<DestBuilder, SrcBuilder>::value) {
+    /// Swap is allowed only when lhs and rhs are the same concrete type.
+    if(run) {
+      SrcBuilder src;
+      auto root_offset1 = populate1(src);
+      auto size1 = src.GetSize();
+      DestBuilder dst;
+      auto root_offset2 = populate2(dst);
+      auto size2 = dst.GetSize();
+      src.Swap(dst);
+      src.Finish(root_offset2);
+      dst.Finish(root_offset1);
+      TEST_EQ_FUNC(src.GetSize() > size2, true);
+      TEST_EQ_FUNC(dst.GetSize() > size1, true);
+      TEST_ASSERT_FUNC(release_n_verify(src, m2_name, m2_color));
+      TEST_ASSERT_FUNC(release_n_verify(dst, m1_name, m1_color));
+    }
   }
 
-  static void builder_swap_after_finish_test() {
-    Builder b1;
-    auto root_offset1 = populate1(b1);
-    b1.Finish(root_offset1);
-    auto size1 = b1.GetSize();
-    Builder b2;
-    auto root_offset2 = populate2(b2);
-    b2.Finish(root_offset2);
-    auto size2 = b2.GetSize();
-    b1.Swap(b2);
-    TEST_EQ_FUNC(b1.GetSize(), size2);
-    TEST_EQ_FUNC(b2.GetSize(), size1);
-    TEST_ASSERT_FUNC(release_n_verify(b1, m2_name, m2_color));
-    TEST_ASSERT_FUNC(release_n_verify(b2, m1_name, m1_color));
+  static void builder_swap_after_finish_test(bool run = is_same<DestBuilder, SrcBuilder>::value) {
+    /// Swap is allowed only when lhs and rhs are the same concrete type.
+    if(run) {
+      SrcBuilder src;
+      auto root_offset1 = populate1(src);
+      src.Finish(root_offset1);
+      auto size1 = src.GetSize();
+      DestBuilder dst;
+      auto root_offset2 = populate2(dst);
+      dst.Finish(root_offset2);
+      auto size2 = dst.GetSize();
+      src.Swap(dst);
+      TEST_EQ_FUNC(src.GetSize(), size2);
+      TEST_EQ_FUNC(dst.GetSize(), size1);
+      TEST_ASSERT_FUNC(release_n_verify(src, m2_name, m2_color));
+      TEST_ASSERT_FUNC(release_n_verify(dst, m1_name, m1_color));
+    }
   }
 
   static void all_tests() {
@@ -184,7 +207,7 @@ struct BuilderTests {
     builder_move_assign_before_finish_test();
     builder_move_assign_after_finish_test();
     builder_move_assign_after_release_test();
-    builder_move_assign_after_releaseraw_test();
+    builder_move_assign_after_releaseraw_test(DestBuilder());
     // clang-format off
     #endif   // !defined(FLATBUFFERS_CPP98_STL)
     // clang-format on
@@ -204,19 +227,19 @@ enum BuilderReuseTestSelector {
 
 typedef std::set<BuilderReuseTestSelector> TestSelector;
 
-template <class Builder>
+template <class DestBuilder, class SrcBuilder>
 struct BuilderReuseTests {
   static void builder_reusable_after_release_test(TestSelector selector) {
     if (!selector.count(REUSABLE_AFTER_RELEASE)) {
       return;
     }
 
-    Builder b1;
+    DestBuilder fbb;
     std::vector<flatbuffers::DetachedBuffer> buffers;
     for (int i = 0; i < 5; ++i) {
-      auto root_offset1 = populate1(b1);
-      b1.Finish(root_offset1);
-      buffers.push_back(b1.Release());
+      auto root_offset1 = populate1(fbb);
+      fbb.Finish(root_offset1);
+      buffers.push_back(fbb.Release());
       TEST_ASSERT_FUNC(verify(buffers[i], m1_name, m1_color));
     }
   }
@@ -226,14 +249,14 @@ struct BuilderReuseTests {
       return;
     }
 
-    Builder b1;
+    DestBuilder fbb;
     for (int i = 0; i < 5; ++i) {
-      auto root_offset1 = populate1(b1);
-      b1.Finish(root_offset1);
+      auto root_offset1 = populate1(fbb);
+      fbb.Finish(root_offset1);
       size_t size, offset;
-      uint8_t *buf = release_raw_base(b1, size, offset);
+      uint8_t *buf = release_raw_base(fbb, size, offset);
       TEST_ASSERT_FUNC(verify(buf, offset, m1_name, m1_color));
-      free_raw(b1, buf);
+      free_raw(fbb, buf);
     }
   }
 
@@ -245,16 +268,16 @@ struct BuilderReuseTests {
       return;
     }
 
-    Builder b1;
+    DestBuilder dst;
     std::vector<flatbuffers::DetachedBuffer> buffers;
     for (int i = 0; i < 5; ++i) {
-      auto root_offset1 = populate1(b1);
-      b1.Finish(root_offset1);
-      buffers.push_back(b1.Release());
+      auto root_offset1 = populate1(dst);
+      dst.Finish(root_offset1);
+      buffers.push_back(dst.Release());
       TEST_ASSERT_FUNC(verify(buffers[i], m1_name, m1_color));
-      Builder b2;
-      b1 = std::move(b2);
-      TEST_EQ_FUNC(b2.GetSize(), 0);
+      SrcBuilder src;
+      dst = std::move(src);
+      TEST_EQ_FUNC(src.GetSize(), 0);
     }
   }
 
@@ -263,17 +286,17 @@ struct BuilderReuseTests {
       return;
     }
 
-    Builder b1;
+    DestBuilder dst;
     for (int i = 0; i < 5; ++i) {
-      auto root_offset1 = populate1(b1);
-      b1.Finish(root_offset1);
+      auto root_offset1 = populate1(dst);
+      dst.Finish(root_offset1);
       size_t size, offset;
-      uint8_t *buf = release_raw_base(b1, size, offset);
+      uint8_t *buf = release_raw_base(dst, size, offset);
       TEST_ASSERT_FUNC(verify(buf, offset, m1_name, m1_color));
-      free_raw(b1, buf);
-      Builder b2;
-      b1 = std::move(b2);
-      TEST_EQ_FUNC(b2.GetSize(), 0);
+      free_raw(dst, buf);
+      SrcBuilder src;
+      dst = std::move(src);
+      TEST_EQ_FUNC(src.GetSize(), 0);
     }
   }
   // clang-format off