C++ How to prefix uint16_t in front of uint8_t *buffer

Question

Here is how I create my message with FlatBuffers and how I send it over TCP with Boost ASIO.

flatbuffers::FlatBufferBuilder builder;
auto email = builder.CreateString("test@asd.dg");
auto password = builder.CreateString("test");
auto loginRequest = Vibranium::CreateLoginRequest(builder,email,password);
builder.FinishSizePrefixed(loginRequest);
size_t size = builder.GetSize();
uint8_t *buf = builder.GetBufferPointer();
//And here I am sending it with Boost ASIO.
boost::asio::write(s, boost::asio::buffer(buf,size));

What I would like to do is get a value of this enum:

enum ServerOpcode : uint16_t{
    SMSG_AUTH_CONNECTION_RESPONSE                    = 0x001,
    SMSG_LOGIN_REQUEST                               = 0x002,
    SMSG_LOGIN_REQUEST_RESPONSE_TEST                 = 0xA99,
};

Let's say like so:

ServerOpcode opc;
opc = ServerOpcode::SMSG_LOGIN_REQUEST;

And prepend opc in front of buf* .

As uint16_t is fixed size than on the receiving end i'll know that the ServerOpcdode is always kept in the first two bytes.

So how can I prepend opc infron of buf is it even possible and how?

PS Note that the file_indetifier of FlatBuffers is not a solution here.

Answer 1

You can allocate an intermediate buffer.

flatbuffers::FlatBufferBuilder builder;
auto email = builder.CreateString("test@asd.dg");
auto password = builder.CreateString("test");
auto loginRequest = Vibranium::CreateLoginRequest(builder,email,password);
builder.FinishSizePrefixed(loginRequest);
size_t size = builder.GetSize();
uint8_t *buf = builder.GetBufferPointer();
uint8_t *actualBuffer = new uint8_t[size + 2];
actualBuffer[1] = (code >> 8);
actualBuffer[0] = (code&0xFF);
memcpy(actualBuffer + 2, buf, size);
//And here I am sending it with Boost ASIO.
boost::asio::write(s, boost::asio::buffer(actualBuffer, size + 2));
delete[] actualBuffer;

I've chosen to encode the uint16_t code as Big Endian, as is good form for any sort of network communication I am familiar with, however you can of course also just do a memcpy(actualBuffer, &code, 2); , at the risk of not knowing the endianess.