On Thursday, December 15, 2016 at 6:05:53 PM UTC-8, Peng Wang wrote:
>
> Actually I don't really care about that, just don't want break the
> interface requirement
> in the doc it says "WriteAt writes len(p) bytes from p to the underlying
> data stream at offset off. It returns the number of bytes written from p (0
> <= n <= len(p)) and any error encountered that caused the write to stop
> early."
> but using binary.Write I lost that infomation. So what's the right way to
> do it ? or just ignore it and return 0?
>
> 在 2016年12月16日星期五 UTC+8上午4:56:12，Tamás Gulácsi写道：
>>
>> 2016. december 15., csütörtök 17:36:47 UTC+1 időpontban 彭望 a következőt
>> írta:
>>>
>>> Hi all, I'm doing a simple message serialization, message defines like
>>> below:
>>> type Envelope struct {
>>> Magic uint32
>>> DataSize uint32
>>> }
>>>
>>> type Message struct {
>>> Envelope
>>> Data []byte
>>> }
>>>
>>> and I want to implement the io.WriterTo interface for Message
>>> func (m *Message) WriteTo(w io.Writer) (n int64, err error) {
>>> err = binary.Write(w, binary.BigEndian, &m.Envelope)
>>> if err != nil {
>>> // here, how should I return a proper n? Since w maybe
>>> *net.TCPConn and binary.Write maybe write 1 bytes and peer has dropped the
>>> connection.
>>> return 0, fmt.Errorf("Write message envelope error: %s", err)
>>> }
>>>
>>> // multiple lines skipped
>>> }
>>>
>>
>> If you really want to know the number of bytes written, wrap w in a
>> counting writer, which counts the bytes written.
>>
>>> type countingWriter struct {
>>> w io.Writer
>>> N int64
>>> }
>>> func (w *countingWriter) Write(p []byte) (int, error) {
>>> n, err := w.w.Write(p)
>>> w.N += int64(n)
>>> return n, err
>>> }
>>>
>>
>>
>
You know the size of Envelope, so you can write it to a local byte slice
first and then pass that byte slice to w.Write.
https://play.golang.org/p/WZmxCjgQVm
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