Custom class serialize with QDataStream

pvt.peter

Hi Everybody,

I have got a TreeItem custom class (not derived from QObject) which is "wrapped" into QVariant and i would like to serialize and deserialize it with QDataStream.
TreeItem represents a multi-level Tree with a list of child pointers and a parent pointer.
I placed the implemented operator<< and operator>> methods before main() method and qRegisterMetaTypeStreamOperators<TreeItem>("TreeItem"); in the first line of main() method.
I have defined a Q_DECLARE_METATYPE(TreeItem*) macro too.

See above:

#include "TheApp.h"
#include <QtWidgets/QApplication>
#include "TreeItem.h"

QDataStream &operator<<(QDataStream &out, const TreeItem &rhs) {
   out.writeRawData(reinterpret_cast<const char*>(&rhs), sizeof(rhs));
   return out;
}

QDataStream &operator>>(QDataStream &in, TreeItem &rhs) {
   in.readRawData(reinterpret_cast<char*>(&rhs), sizeof(rhs));
   return in;
}

int main(int argc, char *argv[])
{
   QApplication a(argc, argv);
   qRegisterMetaTypeStreamOperators<TreeItem>("TreeItem");
   QCoreApplication::setApplicationName(QString("TheApp"));
   TheApp w;
   w.show();
   return a.exec();
}

I would like to encode it with the following:

QByteArray encodedData;
QDataStream stream(&encodedData, QIODevice::WriteOnly);
TreeItem* treeItem = new TreeItem();
QVariant treeItemQVariant = QVariant::fromValue(treeItem);
stream << treeItemQVariant;

When i execute "stream << treeItemQVariant;" i got this error message:

   QVariant::save: unable to save type 'TreeItem*' (type id: 1037).
   ASSERT failure in QVariant::save: "Invalid type to save", file kernel\qvariant.cpp, line 2144
   Debug Error!

What is the problem?
How can i (de)serialize a custom class with QDataStream which is "wrapped" into a QVariant?

Thanks in advance,
It should be a big help for me,
BR,
Peter

VRonin

I don't like your serialisation with reinterpret_cast but the main point here is that you are getting confused by the pointer.

change Q_DECLARE_METATYPE(TreeItem*) to Q_DECLARE_METATYPE(TreeItem)
and change QVariant treeItemQVariant = QVariant::fromValue(treeItem); to QVariant treeItemQVariant = QVariant::fromValue(*treeItem); (be carefull not to leak treeItem)

@pvt.peter said in Custom class serialize with QDataStream:

How can i (de)serialize a custom class with QDataStream which is "wrapped" into a QVariant?

QVariant TempVariant;
stream >> TempVariant;
*treeItem = TempVariant.value<TreeItem>();

P.S.
in your code snippet you don't really need to pass through QVariant btw, you can just use stream << *treeItem;

pvt.peter

@VRonin Thanks for your reply, i did your modifications and it works with QVariant.

But how can i use stream << *treeItem; ?
Now i use the following codes:

Q_DECLARE_METATYPE(TreeItem)

QDataStream &operator<<(QDataStream &out, const TreeItem& rhs) {
   out.writeRawData(reinterpret_cast<const char*>(&rhs), sizeof(rhs));
   return out;
}
QDataStream &operator>>(QDataStream &in, TreeItem &rhs) {
   in.readRawData(reinterpret_cast<char*>(&rhs), sizeof(rhs));
   return in;
}

qRegisterMetaTypeStreamOperators<TreeItem>("TreeItem");

TreeItem* treeItem = new TreeItem();
stream << *treeItem;

TreeItem* data2 = nullptr;
while (!stream.atEnd()) {
   stream >> *data2;
}

There are some trouble with operator << and operator >>.
No 'operator <<' matches arguments of type 'QDataStream' and 'TreeItem'
No 'operator >>' matches arguments of type 'QDataStream' and 'TreeItem'
How can i fix them?

VRonin

You just have to declare the prototype QDataStream &operator<<(QDataStream &, const TreeItem& ) in the header file of TreeItem

pvt.peter

Hi @VRonin,

Thanks for your reply. Your answer was the solution.

michalmonday

@VRonin said in Custom class serialize with QDataStream:

I don't like your serialisation with reinterpret_cast

What is wrong with it? Is there any real risk associatied with using it in such situation? Or is it just a prejuidice because it is generally regarded to be unsafe?