[Development] QDataStream: blackbox or document all versions?

Simon Hausmann Simon.Hausmann at qt.io
Mon Sep 26 11:34:59 CEST 2016


I'm very much in favor of using a proper schema based system such as protocol buffers if we decide

to remove the black box from serialization. They don't appear to be connected, but the moment you

need to deal with changes in the format, the protobuf approach wins IMO. The other advantage is interoperability

with the world outside of Qt.

But this isn't so much a question of what to do, I I think it's more of a question of man power. Until then I'm in favor

of QDataStream remaining a black box.


From: Development <development-bounces+simon.hausmann=qt.io at qt-project.org> on behalf of Michael Winkelmann <michael.winkelmann at qt.io>
Sent: Monday, September 26, 2016 10:40:02 AM
To: development at qt-project.org
Subject: Re: [Development] QDataStream: blackbox or document all versions?

I think what Qt is lacking is a good serialization module like cereal or
boost serialization.
With this pattern, QDataStream would be just another archive format,
like JSON or XML.
Cereal has portable and unportable binary for endianness.

The developer should be able specify how the data is serialized and
select an arbitrary archive type that takes streams operators as default
serialization method.

PS: For wire protocols, I would also consider Protobuf as an option.

On 26.09.2016 10:11, Ulf Hermann wrote:
> On 09/25/2016 05:58 AM, Thiago Macieira wrote:
>> A thread[1] on the interest mailing list started when someone asked
>> for the
>> docs for the current format of the QDataStream wire protocol, to which I
>> replied that it doesn't exist as we don't maintain such docs.
>> Long story short, we have two options: [...]
> I see another option: Make QDataStream a good interchange format.
> From your reply on the interest mailing list:
>> I also don't think the QDataStream binary format is particularly good
>> for non-
>> Qt purposes. It passes internals that are really about how Qt works,
>> not meant
>> for interchange of information. For example, QDate passes the Julian
>> Day in
>> 64-bit format -- who in the world exchanges data like that? One would
>> expect
>> the serialisation format of a date and time to be something like
>> milliseconds
>> since 1970, milliseconds since 1601 or a string format, like ISO
>> 3337. It
>> passes strings as UTF-16 encoded instead of encoding with UTF-8,
>> which would
>> in turn allow interchanging QString and QByteArray serialisation
>> formats.
>> It's also by default quite inefficient since it marshalls everything
>> as big-
>> endian, while most CPUs are little-endian. It has no support for
>> indexing or
>> random-access seeking. It can't be used as a DOM storage, like
>> QJsonDocument
>> can.
> All of that, except for the default endianness, could be fixed on a
> per operator base. Changing the default endianness can also be done
> with minimal effort and would only require incrementing the data
> stream version. Considering this, QDataStream may at the moment not be
> very efficient, but it does have some potential. We could probably
> even change all the operators to read and write CBOR, and that would
> also only require a version change. The versioning allows us to adapt
> the wire format to whatever is needed now or in the future. I don't
> know any other data interchange format which can do this.
> This doesn't change the implicit conversion problem that Marc
> mentioned, but at the moment I don't quite see a way to fix it without
> removing a lot of convenience for the user (that is, have the user
> specify the exact intended type for each serialization operation).
> regards,
> Ulf
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