FlatBufferTest.cpp

//DISCLAIMER: If something looks trivially wrong, it probably is. I'm a java developer, not yet a C++ dev.

//====================Schema====================
union FinalEventUnion
{
  CharactersLoadedEvent,
  SomeOtherEvent,
  AnotherEvent,
  ...
}
table FinalEvent
{
  id: uint;
  input_time: ulong;
  final_event: FinalEventUnion;
}

table CharactersLoadedEvent
{
  characters: [string];//max string size 16, max array size 256
}

//====================C++ code (encoder)====================
std::tuple<size_t, uint8_t *> convert_characters_loaded_event(internal_event_format::characters_loaded_event &ple, uint64_t time){
	flatbuffers::FlatBufferBuilder fbb;//Looks like the problem may be here! fbb goes out of scope at the end of this method, but is still used in the main function!

	std::vector<flatbuffers::Offset<flatbuffers::String>> characters_;
	for (auto & element : ple.characters_)//there is a single entry in characters_ {"AOL"}
	{
		auto raw_str = element.c_str(); //element contains "AOL"
		auto fb_str = fbb.CreateString(raw_str);//after this, fbb.buf_.buf_ is [...,'\x3','\0','\0','\0','A','O','L','\0'] (the end of 1024 byte array)
		characters_.push_back(fb_str);
	}

	auto characters = fbb.CreateVector(characters_);//after this, fbb.buf_.buf_ is [...,'\x1','\0','\0','\0','\x4','\0','\0','\0','\x3','\0','\0','\0','A','O','L','\0'] (the end of 1024 byte array)

	external_event_format::CharactersLoadedEventBuilder e(fbb);
	e.add_characters(characters);
	auto inner_event = e.Finish();
	//after this, fbb.buf_.buf_ is [...,'\x6','\0','\b','\0','\x4','\0','\x6','\0','\0','\0','\x4','\0','\0','\0',\x1','\0','\0','\0','\x4','\0','\0','\0','\x3','\0','\0','\0','A','O','L','\0'] (the end of 1024 byte array)
	auto inner_event_type = external_event_format::FinalEventUnion_CharactersLoadedEvent;
	std::tuple<size_t, uint8_t *> buffer_tuple = build_output_event(fbb, inner_event_type, inner_event.Union(), time);
	return buffer_tuple;
}

std::tuple<size_t, uint8_t *> build_output_event(flatbuffers::FlatBufferBuilder &fbb,
		const FinalEventUnion output_event_type,
		const flatbuffers::Offset<void> &inner_event,
		uint64_t ns_time)
{
	FinalEventBuilder u(fbb);
	u.add_input_time(ns_time);
	u.add_final_event_type(final_event_type);
	u.add_final_event(inner_event);
	//u.Finish();//Removed after Wouter's comment regarding fbb.Finish()
	FinishFinalEventBuffer(fbb, u.Finish());//Added after Wouter's comment regarding fbb.Finish()
	uint64_t buffer_size = fbb.GetSize();
	auto buffer = fbb.GetBufferPointer();
	//all previous operations add to fbb's buffer, while "AOL" remains at bottom (of 1024 byte buffer fbb.buf_.buf_)
	//buffer seems to have same contents, but shifted, "AOL\0" now shows up at byte 68 in buffer, while it was at byte 1020 in fbb.buf_.buf_

	return std::make_tuple(buffer_size, buffer);
}


struct data_chunk{ char data[5000]; }

void decode(data_chunk &data){
  auto event = GetFinalEvent(data.data);
  switch(event->final_event_type()){
    case FinalEventUnion::FinalEventUnion_CharactersLoadedEvent:
      process(reinterpret_cast<const CharactersLoadedEvent*>(event->final_event());
      break;
    ...
  }
}

void process(const CharactersLoadedEvent* ev){
	auto v = convert_to_std_vector(ev->characters());
	...
}

void convert_to_std_vector(const flatbuffers::Vector<flatbuffers::Offset<flatbuffers::String>> *e){
	std::vector<std::string> v;
	for (flatbuffers::uoffset_t i = 0; i < e->size(); i++)
	{
		auto element = e->Get(i);
		v.push_back(element->str());
	}
	return v;
}

//====================C++ code (main)====================
//main
//Put on the wire
...
std::tuple<size_t, uint8_t*> buffer_tuple = convert_characters_loaded_event(ple, time_stamp);
std::get<0>(buffer_tuple);//size of the byte buffer
//by this time, the contents of the byte buffer seem to have completely changed and I don't see "AOL" anywhere! <==============PROBLEM!
std::get<1>(buffer_tuple);//byte buffer to put on the wire

//Take off the wire
...
data_chunk data = ...

decode(data);
...