[Development] D-Bus and (de-)marshalling of complex types (QDBusArgument)

[Stephen Kelly]

On Thursday, April 26, 2012 12:19:14 Thiago Macieira wrote:
> On quinta-feira, 26 de abril de 2012 11.49.02, Alberto Mardegan wrote:
> > Hi all!
> > 
> >  The current QtDBus implementation automatically marshals only the
> > 
> > basic D-Bus types, and puts all the complex types into a QDBusArgument.
> 
> Yeah, I called it the "Pattern Buffer" when I wrote that trick in 2006.
> Because I like Star Trek and I wanted a codename, since other technologies
> had them (Scribe, Interview, etc.).
> 
> > I've found a reason for this behaviour here:
> > http://permalink.gmane.org/gmane.comp.freedesktop.dbus/5920
> > 
> > However, unless I'm missing something, this only explains why we cannot
> > marshal for aggregate types (structures); we should still be able to
> > marshal array and dictionary types.
> 
> Not exactly... there are two reasons why this can't be done.
> 
> The first is that QtDBus doesn't choose a container type for arrays and
> dictionaries. It works just fine with QList, QVector, QLinkedList,
> std::list, std::vector, etc. for arrays, as well as QHash, QMap, std::map,
> std::hash_map, std::unordered_map, etc. for dictionaries. It's up to the
> user of QtDBus to choose which one they want to use.
> 
> The second is that each container is a different C++ type. QList<int> is
> wholly different from QList<uint> from C++'s perspective and from ours,
> even though they happen to have the exact same memory layout (which isn't
> true for other, more different types). We'd have to register as metatypes
> every single QList of the basic types, plus every single QMap of the
> permutations of possible basic type-dictionaries in D-Bus.
> 
> So even if we don't include more complex types like arrays of arrays, arrays
> of dictionaries, dictionaries of arrays, we'd have 15 QList-types
> (including QStringList, QVariantList and QByteArray), 12 * 14 QMap types
> (including QVariantMap). I'm not going to register 179 types at QtDBus
> start up -- that's twice as many types than a typical Qt application has
> today, including the builtins!
> 
> Still, QtDBus *does* register the basic QList types for convenience. And I
> suppose we can add some convenience elsewhere too. I've seen many projects
> that need to force an inclusion of a Q_DECLARE_METATYPE(QList<int>) to their
> generated C++ sources.

Note that that will no longer be needed with Qt 5. You don't need to declare 
QList<T> as a metatype if T is a metatype (but you would still need to 
register it). This recurses too:

https://codereview.qt-
project.org/#patch,unified,24588,1,tests/auto/corelib/kernel/qmetatype/tst_qmetatype_compiletest.cpp

Thanks,

-- 
Stephen Kelly <stephen.kelly at kdab.com> | Software Engineer
KDAB (Deutschland) GmbH & Co.KG, a KDAB Group Company
www.kdab.com || Germany +49-30-521325470 || Sweden (HQ) +46-563-540090
KDAB - Qt Experts - Platform-Independent Software Solutions
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.qt-project.org/pipermail/development/attachments/20120430/870d1d58/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: signature.asc
Type: application/pgp-signature
Size: 198 bytes
Desc: This is a digitally signed message part.
URL: <http://lists.qt-project.org/pipermail/development/attachments/20120430/870d1d58/attachment.sig>