[go-nuts] json/xml Encode doesn't return number of bytes written

http://play.golang.org/p/5X9BD8jBQN

You don't have to 'wrap', you can create a type that implements io.Writer
and counts the bytes written to it. Then you use io.MultiWriter duplicate
writes to both.

If you were using an os.File, you can always f.Seek(0, os.SEEK_CUR) before
and after to compare the number of bytes written.

Bill

On Thursday, February 20, 2014 5:31:04 PM UTC-8, ja...@jakesandlund.com
wrote:

Thanks Bill. I think the MultiWriter approach is marginally better than
"wrapping". I still think it's rather annoying that json/xml encoders have
the bytes written information and just throw it
away: http://golang.org/src/pkg/encoding/json/stream.go?s=3669:3716#L173

What use case do you have for it?

In my case, I wanted to know how many bytes I was writing to a json file.
So yeah, I could do the seek trick, but that seems hacky, and I don't want
to make a syscall.

Wrapping a writer for counting is only 9 lines of code and can be reused
for any io.Writer.
http://play.golang.org/p/ckL_xrDSsC

https://github.com/ParsePlatform/go.counting may be helpful. Though it
is very little code like Jesse suggested.

I find it quite odd. I'm writing a storage abstraction package which
basically just encodes the data before to write it. Part of this I need to
implement the io.Writer interface. It seems that the encode method
deliberately masks the number of bytes written so I would be interested to
know why it doesn't return this data if it's already available.
http://golang.org/src/encoding/json/stream.go?s=3433:3470#L136

  173 if _, err = enc.w.Write(e.Bytes()); err != nil { 174 enc.err = err 175 } 176 encodeStatePool.Put(e) 177 return err 178 }