Saving Qlist data in a mysql database

SMF-Qt

This is my data setup (simplified):

Class Animation
{
...
typedef struct
{
QImage I;
QTime ts;
} Anim_t;
...
};

The QList can have between 0 and 10 items in it at any one time.
The QImage's are small 512x512 pixels.
I would like to save the whole Qlist in a mysql table as a single entity.
Has anyone got a suggested solution for this problem ?

Thanks

SGaist

Hi,

What do you mean by "as a single entity" ? The whole QList as one binary blob ?

If so you can use QDataStream to serialize your list into a QByteArray and store that array as a blob.

SMF-Qt

Yes if possible.
Thanks

SMF-Qt

Looking at the documentation QDataStream takes a QIODevice but I see no way to use a QList as a source for the QIODevice?

Christian Ehrlicher

Please take a look at the examples on how to use QDataStream. It clearly shows how to put a QString into a QIODevice (in this case a QFile, but for QByteArray/QBuffer it's the same).

JonB

@SMF-Qt
Be aware that in order to stream your struct to/from a QDataStream you will need it to implement << & >> operators (not hard, just needs to be done).

Note also that the image will be streamed as a PNG image (https://doc.qt.io/qt-5/qimage.html#operator-lt-lt-21), you don't have a choice over this, so that needs to be acceptable to you.

SGaist

In addition to @JonB, the operators shall be implemented for your custom structure, not QList.

SMF-Qt

Tried this for the << operator:

QDataStream &operator<<(QDataStream &stream,const Anim_t &anim)
{
	stream << anim.ts.msecsSinceStartOfDay() << anim.I;
	return  stream;
}

Got this error, what have I missed ?

anim.h:35:15: error: 'QDataStream& Anim::operator<<(QDataStream&, const Anim::Anim_t&)' must have exactly one argument
35 | QDataStream &operator<<(QDataStream &stream,const Anim_t &anim)

SMF-Qt

For completeness I have done this for the ">>" operator and this gives a similar error:

QDataStream &operator>>(QDataStream &stream,const Anim_t &anim)
{
	int t;
	stream >> t >> anim.I;
	anim.ts = QTime::fromMSecsSinceStartOfDay(t);

	return  stream;
}

Christian Ehrlicher

It must be a free function, if it's a member of your struct then you have to remove const Anim_t &anim

JonB

@SMF-Qt
(In addition to @Christian-Ehrlicher) I thought you were going to implementing the << & >> streaming operators for your Anim_t struct itself.

SMF-Qt

Ok implimented operators as free functions and everything now compiles, however on running the application the QByteArray is empty (example with one element in QList):

...

QDataStream &operator<<(QDataStream &stream, const Anim::Anim_t &anim)
{
stream << anim.ts.msecsSinceStartOfDay() << anim.I;
return stream;
}

QDataStream &operator>>(QDataStream &stream, Anim::Anim_t &anim)
{
int t;
stream >> t >> anim.I;
anim.ts = QTime::fromMSecsSinceStartOfDay(t);
return stream;
}

...

	QByteArray array;
        QDataStream stream(array);
	for(int i=0;i<Animations.size();i++)
	{
	    stream << Animations.at(i);
	}
	QByteArray tmp=qCompress(array,9);
	fprintf(stderr,"Binary Blob Animations (%lld) size %lld compressed size %lld\n",Animations.size(),array.size(),tmp.size());

...

Binary Blob Animations (1) size 0 compressed size 4

Any suggestions ?
Thanks

jsulm

@SMF-Qt said in Saving Qlist data in a mysql database:

QDataStream stream(array);

Please read: https://doc.qt.io/qt-5/qdatastream.html#QDataStream-3
"Use QDataStream(QByteArray*, int) if you want to write to a byte array."

SMF-Qt

Got it thanks. Problem solved.

Christian Ehrlicher

@SMF-Qt Then also please mark this topic as solved, thx.

SGaist

If I may, there's no need for the loop, you should be able to directly stream the list object and not go manually with that loop.