FlatBuffers: Write to and read from binary file?

Question

I have basic knowledge of file streams in C++ and Google FlatBuffers . The Schema file is quite simple, also creating a buffer and reading from a buffer pointer. The thing that I don't get is how to save multiple buffers into a binary file, and then read that binary file to get any random buffer.

Here is a simple Schema with two arrays of floats:

table Car {
    field_a:[float];
    field_b:[float];
}

.

A function for building the buffer (although without the file saving):

bool save_flatbuf(string file_path, vector<double> vec_a, vector<double> vec_b) {
    flatbuffers::FlatBufferBuilder builder;

    auto vec_floats_a = builder.CreateVector(vec_a, vec_a.size());
    auto vec_floats_b = builder.CreateVector(vec_b, vec_b.size());

    auto mloc = CreateCar(builder, &vec_floats_a, &vec_floats_b);

    builder.Finish(mloc);

    // How to save it into a binary file with a list of "Cars"?
}

.

And a function for reading the buffer after it was read from the binary file (without the file reading):

bool read_flatbuf(string file_path) {

    // How to get the buffer pointer to a "Car" from a binary file with a "list of Cars" ? .

    vector<double> final_vec_a;
    vector<double> final_vec_b;

    auto car = GetCar(buffer_pointer);

    auto fa = car->field_a();
    auto fb = car->field_b();

    final_vec_a.resize(fa->size());
    for (int i = 0; i < fa->size(); i++) {
        final_vec_a[i] = fa->Get(i);
    }

    final_vec_b.resize(fb->size());
    for (int i = 0; i < fb->size(); i++) {
        final_vec_b[i] = fb->Get(i);
    }
}

Not sure if the way to access the buffer's info is correct. For instance the way to get the length of the array fields.

Code examples for file interaction (write/read multiple buffers in one file) would be welcome.

Answer 1

The best way to do this to add that list of cars to your schema:

table Garage {
  cars:[Car];
}

Then you can collect multiple car offsets (from CreateCar ), call CreateVector on them, call CreateGarage on that, and then feed the result of that to Finish .

To read, start similarly from GetGarage(buffer_pointer) .

Answer 2

My solution is adding additional size information.

for writer ::

for (item : flatbuffer_list) {
   int size = item.GetSize();
   write (file, &size, sizeof(size));
   write (file, item.GetBufferPointer(), item.GetSize());
}

for reader ::

while(!eof(file)) {
   int size;
   read (file, &size, sizeof(size));
   read (file, buffer, size);
   auto item = GetItem(buffer);
}

Answer 3

Quick reference to store buffer into binary file.

builder.Finish(mloc);
uint8_t *buf = builder.GetBufferPointer();
int size = builder.GetSize();

std::ofstream ofile("data.bin", std::ios::binary);
ofile.write((char *)buf, size);
ofile.close();

To read from file:

const std::string inputFile = "data.bin";
std::ifstream infile(inputFile, std::ios_base::binary);
std::vector<char> buffer( std::istreambuf_iterator<char>(infile),
                      std::istreambuf_iterator<char>());

Answer 4

"Code examples for file interaction (write/read multiple buffers in one file) would be welcome."

I use fbs and json like this for my test game. (generate to out_cpp folder : gamedata.bin, gamedata.h)

flatc -b -c -o out_cpp gamedata.fbs gamedata.json

and I found this flatbuffers sample which is quite useful at first time.

https://github.com/gene-hightower/fb

In my case, the git sample is not working correctly unless you use flatbuffers::LoadFile() instead of the sample provided load_file().