char buffer[10];
async_read(socket_, asio::buffer(buffer, 8)
        , [](boost::system::error_code const & error, size_t bytes_xfer){});

streambuf buf;
    async_read(socket_, buf
    , [](
      boost::system::error_code const & error, size_t bytes_xfer){});

void commit(std::size_t n)
  {
    if (pptr() + n > epptr()) //тут
      n = epptr() - pptr();
    pbump(static_cast<int>(n));
    setg(eback(), gptr(), pptr());
  }

class TCPSession
        {
        ...
        private:
            std::unique_ptr<char> receive_buffer_;
        }
        void TCPSession::read_packet_len()
        {

            receive_buffer_.reset(new char[2]());
            async_read(socket_, asio::buffer(receive_buffer_.get(), 2)
                , [my = shared_from_this()](
                  boost::system::error_code const & error, size_t bytes_xfer){
                my->read_packet();
            });
        }

        void TCPSession::read_packet()
        {

            uShort datalen = 0;
            memcpy((char*)&datalen, receive_buffer_.get(), 2);
            boost::endian::endian_reverse_inplace<uShort>(datalen);
            std::cout << "datalen: " << datalen << endl; // результат 8
            receive_buffer_.reset(new char[datalen -2]());
                async_read(socket_, asio::buffer(receive_buffer_.get(), datalen - 2),
    [my=shared_from_this()](boost::system::error_code const & error, size_t bytes_xfer)
       {/*проверяю считанные символы*/});
        }

receiveBytes();