[Interest] QDatastream, QMap, QImage serialization

[Thiago Macieira]

On Wednesday, 16 May 2018 13:51:39 PDT Jason H wrote:
> I'm trying to write a GUI utility to provide a simple UI. I'm wanting a
> no-frills database of several Qt types, and I'm trying to use QVariantMap
> for that. It's going well except that QImages aren't being serialized
> correctly. Yes, I know there is a QVariant limitation with "GUI types" (
> http://doc.qt.io/qt-5/qvariant.html#a-note-on-gui-types ) however QImage
> has an operator QVariant(). I was expecting to see pixel data get stored in
> the file, but it's just being stored as an empty QString. So it seems that
> QDataStream can't serialize a QImage? I know that's not true, so it looks
> like QDataStream can't serialize a QVariant that is a QImage. I think
> that's what I'm running into. I would expect some QByteArray fall-back, but
> the bytes aren't making it to disk. It would be ok for me if I had to
> manually convert the image later from an opaque byte array, so long as it
> was written and read properly.

QImage has an operator<< for QDataStream that writes PNG content to the 
stream. However QVariant cannot serialise non-QtCore types to QDataStream.

> What's the best way to accomplish this serialization?

Manually serialise your QVariantMap: for each value that is an image, call 
that operator<< directly. That implies manually deserialising too, which means 
detecting when the stream contains an image.

-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel Open Source Technology Center