Oneof example

[John Rogers]

Hi,

I am planning on using a "oneof" message as my top level wrapper.

I have read the manual's partial example, but I am confused by this section:

> The user is expected to set the filed manually using the correct field tag::
>  MyMessage msg = MyMessage_init_zero;
>  msg.payload.msg2.value = true;
>  msg.which_payload = MyMessage_msg2_tag;
>
> Notice that neither ``which_payload`` field nor the unused fileds in ``payload``
> will consume any space in the resulting encoded message.

If the "which_payload" is not sent as part of the message, what is its purpose, and how does the decoder know which payload is used?

Is there a full example showing the encode and decode?

If not, please give me a few hints, and I will be very happy to share my example once its done.

Thanks!!!
- John

[Petteri Aimonen]

Hi,

> If the "which_payload" is not sent as part of the message, what is its
> purpose, and how does the decoder know which payload is used?

It is encoded as part of the tag number - the same way normal fields are
distinguished from eachother. I agree that the documentation is a bit
misleading on this part, but it is technically correct in that it
doesn't take any extra space compared to what every field takes anyway.

> Is there a full example showing the encode and decode?

This could help:
https://github.com/nanopb/nanopb/tree/master/tests/oneof

--
Petteri

[John Rogers]

The tests/oneof looks excellent! I will check it out.

Question:
The "using unions" example uses all optional fields to implement a union...

"oneof" seems to be better, since optional fields don't seem to be forced to be mutually exclusive...

So no I am a bit confused about which should be used and when?

Or to put it another way... since "oneof" seems to be the more logical/appropriate choice... what circumstance would require or benefit from an optional field based "union"?

Thanks!!!

[Petteri Aimonen]

Hi,

> The "using unions" example uses all optional fields to implement a union...

I should probably remove that example in 0.4.0. It dates from 2013, back
when oneofs didn't exist yet.

--
Petteri

[John Rogers]

If it really is obsolete removing it makes sense, but if it still has some potential benefit, a comment in the readme would be enough.

But Thanks for clearing this up for me!

I figured one was the better way to go, and I thought that the 'oneof' was the preferred way, but very nice to know for sure!

Thanks again!!

-John

[dvdve...@gmail.com]

so in proto 3 everything is "optional". Does this include the "one of"?

So it could more accurately be described as "one of or none" technically speaking.

Is this correct?

[Petteri Aimonen]

Hi,

> So it could more accurately be described as "one of or none" technically speaking.

Yes. Nanopb will set the tag field as 0 if the oneof field is missing
completely.

--
Petteri