[Development] unified data model API in QtCore => thin wrapper proposal

[Arnaud Clère]

Hi,
Thiago, did you decide on something regarding QCborValue in Qt5.12?

For structured traces, we have to deal with any user-defined data types so, we can do with QCborValue, QJsonValue or QFoo too. Now, the value of exposing the common backend of QJson and QCbor as a QFoo binary format is not clear to me, and the "lowest common denominator" approach is inherently limited.

In my latest QBind prototype, we can write any user-defined type in various formats, and we can even take a copy of the user-defined type to serialize it later. So, QFoo is not required as an intermediate storage and would probably be slower. The latest benchmark shows the QBind approach can be *very* efficient:

(usecs)  |QDebug|Json|Cbor|Writable|Writable>Cbor
MSVC 17
<<atoms__|     8|   6|   4|        0|  3
<<doubles|    15|  19|   4|        0|  5
GCC 5.3
<<atoms__|     4|   3|   2|        0|  4
<<doubles|    29|  28|   4|        0|  4

You can test for yourself, it is 2 source files and a .pro
https://gricad-gitlab.univ-grenoble-alpes.fr/modmed/modmedLog/tree/master/tests/QBind

If you are afraid of template code bloat in the QBind approach, I think a disciplined use can mitigate that, i.e. use QBind to just write to a single format. Actually, I think the compiler has more opportunities to factor out similar code in our QBind approach than with hand-written serialization code. It removes so much boiler plate code with anecdotal differences...

Please let me know what you think. 
Arnaud

-----Original Message-----
From: Thiago Macieira <thiago.macieira at intel.com>
Sent: jeudi 14 juin 2018 02:08

> This email is to start that discussion and answer these questions:
>   a) should we have this API?
>   b) if so, what would this API look like?
>   c) if not, should we unify at least JSON and CBOR?
>   c) either way, should remove QCborValue until we have it?
...
> This API would also be the replacement of the JSON and CBOR parsers, 
> for which we'd have a unified API, something like:
>   QFoo json = QJson::fromJson(jcontent);
>   QFoo cbor = QCbor::fromCbor(ccontent);
>   qDebug() << QCbor::toDiagnosticFormat(json);	// yes, json