How do I properly serialize and deserialize a QList class in QT using QDatastream?

[JonB]

@twodee
You seem to be talking about changing between pointers or not, that's not what I meant. And I trust you used your layer variable and not your Layer type (note the capitalizations, you must get that right, I did mean literally out << layer;). I'm surprised at your finding as now I don't understand how the cast could make any difference at runtime, but I'm not a C++ expert so we'll leave it at that. Sorry.

[twodee]

@JonB Yes, I did use my layer variable, I did comment out the raster part and that didn't seem to work. My findings look a bit odd to me as well, thanks for trying. :D

[VRonin]

The normal way to do it:

• create 2 pure virtual protected members in Layer that takes a QDataStream& argument and saves/loads the layer from it
• create QDataStream stream operators for Layer (not Layer*) that do nothing but calling those protected methods
• serialise something that will tell you what kind of Layer it will be (something like QVariant::userType does)
• serialise directly by dereferencing a Layer* no need to cast anything

RasterLayer *layer2 = new RasterLayer; in >> *layer2; is perfectly valid, as long as paintWidget owns the allocated memory

[twodee]

@VRonin Hi, thanks for responding. I am getting this error which I usually can avoid using a const somewhere: error: passing ‘const RasterLayer’ as ‘this’ argument discards qualifiers [-fpermissive] : setName(name);

As you can see in the layer.h, it is an inline method inline void setName(QString &name) { _name = name; } , what should I be doing in this case?

[VRonin]

Looks like you are deserialising on a const method. What is the code around that call?

[twodee]

This post is deleted!

[twodee]

@VRonin I figured the serialization out, now I have to do something like out << *layer; to get it serialized. But how will that allow me to serialize a QList<Layer*> to be serialized as a whole?

[JonB]

@twodee
Read through threadin this forum https://forum.qt.io/topic/58701/how-to-serialize-deserialize-a-qlist-myclass

As per http://doc.qt.io/qt-5/datastreamformat.html, QList<T> can be de/serialized. You need to provide the de/serialization for your class T, which is what you have been working on here.

[twodee]

This post is deleted!

[twodee]

As the post said it's about one line with the operator new, could you tell me what that line is?

 QList<Layer*> layers = paintWidget->getItems(); 
out << layers;

This doesn't seem to do the trick. I have been scratching my head for too long, sorry if I am being stupid.

Also for some reason overloading QDataStream& operator<<(QDataStream& ds, const Layer *layer) doesn't work anymore, which did work earlier.

[VRonin]

out << qint32(layers.size());
for(Layer* layer : qAsConst(layers))
out << qint32(layer->type()) << *layer;

---

qint32 size;
qint32 type;
in >> size;
while(size-- >0 ){
in >> type;
Layer* layer = nullptr;
switch(type){
case Raster:
layer = new RasterLayer;
break;
default:
Q_UNREACHABLE();
}
in >> *layer;
}

[JonB]

@VRonin
I don't want to muddy the waters for the OP here, but may I ask: why do you explicitly serialize the layers list yourself in a loop? I thought that the point of the link I mentioned, http://doc.qt.io/qt-5/datastreamformat.html, is that it shows:

The QDataStream allows you to serialize some of the Qt data types. The table below lists the data types that QDataStream can serialize

QList<T>	
    The number of items (quint32)
    The items (T)

so why can't you just out << layers ?

Is this because you want to know where layer->type() is in the serialization, so that you can look at it during deserialization and do your own newing? If there were no sub-classing going on in the list elements then you wouldn't need to do that and could just do the list directly? Could it then just be deserialized with in >> layers? Or will deserializing never do any newing of elements for you?

[VRonin]

@JonB That would imply having datastream operators acting on pointers and that's dangerous:

• What if you pass a null pointer
• What if you pass a dangling pointer
• Who own the memory allocated by the pointer? the operator?

[JonB]

@VRonin
So are you saying: "Yes, you can de/serialize QList<T> directly as per that link, but while that's fine for simple types it's not suitable for pointers"?

I may be confusing myself. In my C# we don't have "pointers" and we just de/serialize lists directly without a care. Deserializing does whatever newing is necessary behind the scenes.

[J.Hilk]

@JonB said in How do I properly serialize and deserialize a QList class in QT using QDatastream?:

I may be confusing myself. In my C# we don't have "pointers" and we just de/serialize lists directly without a care.

I beg to differ!

you can use pointers in c# and manage your memory by hand, but you have to explicitly tell the compiler to allow it with -unsafe, IIRC

Quick google search:
https://www.tutorialspoint.com/csharp/csharp_unsafe_codes.htm

[VRonin]

take this example:

// QList<int*> m_ownerList;
m_ownerList.append(new int(5));
m_ownerList.append(new int(3));

m_myInt1 = new  int(5);
m_myInt2 = new  int(3);
m_nonOwnerList = QList<int*>{{m_myInt1 ,m_myInt2} };

What should QDataStream& operator>>(QDataStream& , const QList<int*> ) do? free the memory already allocated or not?

[JonB]

@J.Hilk
But I would never want to use pointers in C# or manage memory by hand, that's (more than) half the point of using C#!?

Maybe there's a misunderstanding. I don't use C# with Qt (I use Python). I'm just familiar with C# compared to C++. I am trying to understand @VRonin's explanation of de/serializing this QList<T>, where he is saying he does it explicitly to manage pointers, when I know I would just de/serialize a list from C# without iterating the elements myself, and trying to understand why.

[twodee]

@VRonin Hi, thanks for the extended example. I have come up with a working solutin that looks similar to yours, could you let me know if this is a good way to deal with it?

QDataStream& operator >>(QDataStream& stream, QList<Layer*>& layers){
    layers.clear();
    int size;
    int type;

    stream>>size;

    QString name;
    QPixmap pixmap;

    Layer* layer = nullptr;
    for(int i =0; i<size; ++i){

        stream >> name >> type;
        switch (type) {
        case Layer::RASTER:
            stream >> pixmap;
            layer = new RasterLayer(name, pixmap.toImage());
            break;
        default:
            Q_UNREACHABLE();
            break;
        }
        layer->read(stream);

        layers.push_back(layer);
    }
    return stream;
}

[VRonin]

The very first line of your code is a memory leak. This is exactly my point. Don't handle memory inside serialising/deserialising it's not the right place

[twodee]

I felt wrong in doing so too as I mentioned in my original question, but I still am not sure where would I be creating the new RasterLayer in my case. Could you give me some clues on how to go about it?