Flatbuffers struct in union not working (C++)

Question

I am trying to get going with Flatbuffers in C++, but I'm already failing to write and read a struct in a union. I have reduced my original problem to an anonymous, minimal example.

Example Schema (favorite.fbs)

// favorite.fbs
struct FavoriteNumbers
{
    first: uint8;
    second: uint8;
    third: uint8;
}

union Favorite
{ FavoriteNumbers }

table Data
{ favorite: Favorite; }

root_type Data;

I compiled the schema using Flatbuffers 1.11.0 downloaded from the release page (I'm on Windows so to be safe I used the precompiled binaries).

flatc --cpp favorite.fbs

This generates the file favorite_generated.h.

Example Code (fav.cpp)

#include <iostream>
#include "favorite_generated.h"

int main(int, char**)
{
    using namespace flatbuffers;
    FlatBufferBuilder builder;

    // prepare favorite numbers and write them to the buffer
    FavoriteNumbers inFavNums(17, 42, 7);
    auto inFav{builder.CreateStruct(&inFavNums)};
    auto inData{CreateData(builder, Favorite_FavoriteNumbers, inFav.Union())};
    builder.Finish(inData);

    // output original numbers from struct used to write (just to be safe)
    std::cout << "favorite numbers written: "
        << +inFavNums.first() << ", "
        << +inFavNums.second() << ", "
        << +inFavNums.third() << std::endl;

    // output final buffer size
    std::cout << builder.GetSize() << " B written" << std::endl;

    // read from the buffer just created
    auto outData{GetData(builder.GetBufferPointer())};
    auto outFavNums{outData->favorite_as_FavoriteNumbers()};

    // output read numbers
    std::cout << "favorite numbers read: "
        << +outFavNums->first() << ", "
        << +outFavNums->second() << ", "
        << +outFavNums->third() << std::endl;

    return 0;
}

I'm using unary + to force numerical output instead of characters. An answer to another question here on StackOverflow told me I had to use CreateStruct to achieve what I want. I compiled the code using g++ 9.1.0 (by MSYS2).

g++ -std=c++17 -Ilib/flatbuffers/include fav.cpp -o main.exe

This generates the file main.exe.

Output

favorite numbers written: 17, 42, 7
32 B written
favorite numbers read: 189, 253, 34

Obviously this is not the desired outcome. What am I doing wrong?

Answer 1

Remove the & in front of inFavNums and it will work.

CreateStruct is a template function, which sadly in this case it means it will also take pointers without complaining about it. Would be nice to avoid that, but that isn't that easy in C++.