[Development] QVariant container API
Jedrzej.Nowacki at qt.io
Mon Apr 1 12:33:16 CEST 2019
On Monday, April 1, 2019 11:09:27 AM CEST Vasily Pupkin wrote:
> I would like to submit a patch. Since it is probably going to break binary
> compatibility and is mostly about coding conventions, I would like to have
> some feedback before investing time.
> The general idea is to create a playground project for automatic
> (de)serialization for Qt (not manual, i.e. overriding operators). The only
> stopper is container deserialization.
> To operate containers without staticaly knowing their types, one should be
> able to make conversions from QVariant containing a container to container
> of QVariants and vice versa.
> Seriazation is quite straightforward via QSequentialIterable and
> QAssociativeIterable. But deserialization is currently possible only by
> QMetaType::registerConverter. I think, that this is would be so nice to
> have an ability to create a container via QVariant from collection of
> QVariant instances without a need for custom converters.
> QList<QVariant> variantContainer;
> QVariant variant(QVariant::fromValue(variantContainer));
> QVariant containerVariant(variant.convert(qMetaTypeId<QList<int> >()));
You can be sure that the conversion from QList<int> to QSequentialIterable
will be successful, but the other way around is not given. QSequentialIterable
can contain QVariants of different types. I'm curious what is the use case for
I think it could be implemented by extending:
which registers conversion from a container to QSequentialIterable. You could
add the opposite function. That way no binary compatibility would be broken
nor major re-factoring would be needed.
> And so I have stumbled into a problem. QSequentialIterable and
> QAssociativeIterable related code is scattered across <qvariant.h> and
> <qmetatype.h>. All related conversions are conveyed in templates, instead
> 387 .
> I just don't feel, that further
> 2 and
> 121 development
> is the way to go.
Well, it is a bit of a mess. Conceptually, conversion code should be in
qmetatype.cpp/qmetatype.h. QVariant should use QMetaType to access / modify /
create / convert instances of different types. The code is not well split
because QMetaType class is a late addition, so as usual historical reasons
:-). If you want to cleanup it a bit I would start on moving
QSequentialIterable and QAssociativeIterable out of qvariant.h to own headers.
The fact that the conversion happens through QVariantValueHelperInterface in
my opinion is a mistake, this code should be de-inlined as it is hard to
> And so the question is: should I try to refactor sequential and associative
> iterators, so templates are used only to staticaly gather information about
> containers and conversions are made in qvariant.cpp, where they belong?
Sure, try, you can add me to review :-)
> It also would be nice to hear, if someone acknowledges this conversion
> feature to be of use.
Yeah, there are not so many users of QXXXIterable. So as long as we can keep
cost of it low it is fine.
> Thanks for reading this far.
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