Nesting vectors (With wrapping the inner vector in a table)

[Alexey Rybalchenko]

Hi,

I am trying to fill the following flatbuffer:

table DigiBuffer {
  data:[ubyte];
}

table Digi {
  x:int;
  bigBuffer:DigiBuffer;
}

table DigiPayload {
  digis:[Digi];
}

root_type DigiPayload;

using this code:

flatbuffers::FlatBufferBuilder builder = new flatbuffers::FlatBufferBuilder();
flatbuffers::Offset<TestDetectorFlat::Digi> digis[numEntries]; // numEntries ~= 20-30

for (int i = 0; i < numEntries; ++i) {
    DigiBufferBuilder dbb(builder);
    // fBigBuffer is array<unsigned char, 100000>
    auto bigBuffer = builder->CreateVector(fBigBuffer.data(), sizeof(fBigBuffer)); // here assertion `!nested' fails.
    dbb.add_data(bigBuffer);
    auto dbloc = dbb.Finish();

    DigiBuilder db(builder);
    db.add_x(4); // x:int
    db.add_bigBuffer(dbloc); // bigBuffer:DigiBuffer

    digis[i] = db.Finish();
}

auto dvector = builder->CreateVector(digis, nDigis);
auto mloc = CreateDigiPayload(builder, dvector); // digis:[Digi]
FinishDigiPayloadBuffer(builder, mloc);

It compiles fine, but when running it fails on assertion:
flatbuffers.h:664: void flatbuffers::FlatBufferBuilder::NotNested(): Assertion `!nested' failed.

The docs mention: "Nesting vectors is not supported, instead you can wrap the inner vector in a table.", which is why I added the DigiBuffer table in the first place (having only bigBuffer:[ubyte] in Digi originally). But I guess the way I do the nesting is wrong.

Can you please explain what I am doing wrong or give an example on how to nest vectors-tables properly.

[Maxim Zaks]

I guess you have to move this line up

auto bigBuffer = builder->CreateVector(fBigBuffer.data(), sizeof(fBigBuffer)); // here assertion `!nested' fails.

before

DigiBufferBuilder dbb(builder);

this is due to fact that you first have to create a vector, get its offset, than create the DigiBuffer Table and than add vectors offset to the table.

When you start a table you "reserve" an offset. You will "release" this reservation when you call finish. A vector string or a table can be created/started only when there is no previous reservation.

The explanation is a bit higher level, if you are interested in the details, why it is the case you would have to look in the implementation.

[Alexey Rybalchenko]

This helped, thank you!

[Daniel D]

Hi all,
I created the following flatbuffer:

namespace flatbuffers_tbl1;

table Info{
    num:int;
}

table Tbl1 {
    age: int = 22;
    name: string;
    vec:[Info];
}

root_type Tbl1;

I am trying create buffer but I don't know how to CreateVector vec:[Info]  and add vec to tbl1Builder. Anyone can give me an tutorial:

flatbuffers::FlatBufferBuilder fbb;
    auto nameData = fbb.CreateString("AAAA");

    InfoBuilder infoBuilder(fbb);
    infoBuilder.add_num(11);
    auto infoloc = infoBuilder.Finish();

    Tbl1Builder tbl1Builder(fbb);
    tbl1Builder.add_age(33);
    tbl1Builder.add_name(nameData);

    auto maloc = tbl1Builder.Finish();
    FinishTbl1Buffer(fbb, maloc);

[Wouter van Oortmerssen]

https://google.github.io/flatbuffers/flatbuffers_guide_tutorial.html

search for `weapons_vector` as an example how to write an vector of tables.

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