Sending raw data with write() in boost::asio

I've recently decided to use boost::asio for my sockets, but now I'm running into a problem: documentation seems to be lacking.

What I want to do is write a function that will send a message consisting of the following structure:

• 2 bytes of an unsigned integer (uint16_t) for an opcode all bytes
• all bytes after that (a flexible amount) being any type of data (cast to void*). This data will be operated upon based on the opcode

For example, if the opcode is 1, perhaps defined as OPCODE_LOGIN, then the bytes following the opcode could contain a string containing login information, etc.

bool sendMessage(tcp::socket* sock, uint16_t opcode, void* data)
{
    void* fullData = malloc(sizeof(uint16_t) + sizeof(data));
    memcpy(fullData, (void*)opcode, sizeof(opcode));
    memcpy(&fullData + sizeof(uint16_t), data, sizeof(data));
    boost::asio::write(sock, boost::asio::buffer(fullData, sizeof(fullData)));
    // by the way, at this point, is it safe to delete fullData to prevent memory leaks?
    return true;

}

This does not compile, however. I get a cryptic compilation error regarding the call to write:

1>------ Build started: Project: client, Configuration: Debug Win32 ------
1>  main.cpp
1>c:\boost\boost_1_47\boost\asio\impl\write.hpp(46): error C2228: left of '.write_some' must have class/struct/union
1>          type is 'boost::asio::basic_stream_socket<Protocol> '
1>          with
1>          [
1>              Protocol=boost::asio::ip::tcp
1>          ]
1>          did you intend to use '->' instead?
1>          c:\boost\boost_1_47\boost\asio\impl\write.hpp(59) : see reference to function template instantiation 'size_t boost::asio::write<SyncWriteStream,ConstBufferSequence,boost::asio::detail::transfer_all_t>(SyncWriteStream &,const ConstBufferSequence &,Completi