crash in QListWidget::dropEvent when I add a UserRole to my QListItemWidget :'(

[mbruel]

@VRonin said in crash in QListWidget::dropEvent when I add a UserRole to my QListItemWidget :'(:

http://lengthily.blogspot.com/2017/02/one-size-fits-all.html
Also keep in mind that Qt-parent child relationship does not know about your design so it might delete stuff you didn't intend to

Well, I said manager to not enter in the details... but basically it's a subclass of QApplication that owns my Model (which own all the Elements). It also owns the HMI if there is one.

class MyApplication : public QApplication
{
private:
    const AppMode _mode;
    AppLang       _lang;

    Model        *_model;
    MainWindow   *_hmi;

    static const QMap<AppLang, QString> sAppLanguages;
    static const QString sTranslationsFileName;

    static const QString sVersion;
    static const QString sAppName;
...
};

However I use singleton for some thread safe objects as a convenient way to be able to get the instance from anywhere in the code. Mainly for to get my LogService, the IconFactory, the UndoStack and the IOService.
I think it has some sense and it is practical to be able to do such calls:

Iconfactory::getInstance()->getElementTypeIcon(elem->getElementType);
UndoStack::getInstance()->beginMacro(macroLabel);

Here is what the Singleton class I use:

#ifndef SINGLETON_H
#define SINGLETON_H

template <typename T> class Singleton
{
protected:
    // Constructor /Destructor
    Singleton() = default;
    virtual ~Singleton() = default;

public:
    Singleton(const Singleton &other) = delete;
    Singleton(const Singleton &&other) = delete;
    Singleton & operator=(const Singleton &other) = delete;
    Singleton & operator=(const Singleton &&other) = delete;

    // Public Interface
    static T *getInstance(){
        if (!_singleton) {
            _singleton = new T;
        }
        return (static_cast<T*> (_singleton));
    }

    static void kill()
    {
        if (_singleton)
        {
            delete _singleton;
            _singleton = nullptr;
        }
    }

private:
    // Unique instance
    static T *_singleton;
};

template <typename T> T *Singleton<T>::_singleton;
#endif // SINGLETON_H

Rq: for Services, I generally derive from this PureStaticClass

class PureStaticClass
{
    PureStaticClass() = delete;
    PureStaticClass(const PureStaticClass &other) = delete;
    PureStaticClass(const PureStaticClass &&other) = delete;

    PureStaticClass & operator=(const PureStaticClass &other) = delete;
    PureStaticClass & operator=(const PureStaticClass &&other) = delete;
};

@VRonin said in crash in QListWidget::dropEvent when I add a UserRole to my QListItemWidget :'(:

Also keep in mind that Qt-parent child relationship does not know about your design so it might delete stuff you didn't intend to

Yes I'm aware of that, I do it only for my Data Object that aren't updated directly by any QT objects. Those objects will have a pointer on them to extract some data but never delete it.

[mbruel]

@mpergand
I've tried that and it doesn't work with work on my env. I'm still getting the issue:

QVariant::save: unable to save type 'Element*' (type id: 1025).

Here is my code:

#include <QDataStream>
class Element;
inline QDataStream &operator>>(QDataStream &in, Element*& elem) {

    in >> elem;
    return in;
}
inline QDataStream &operator<<(QDataStream &in, Element*& elem) {

    in << elem;
    return in;
}

if I override the startDrag event like this:

void ItemOfferingListWidget::startDrag(Qt::DropActions supportedActions)
{
//   return QListWidget::startDrag(supportedActions);
    Q_UNUSED(supportedActions)
    QMimeData       *mimeData = new QMimeData();
    QByteArray       encodedData;
    QDataStream stream(&encodedData, QIODevice::WriteOnly);
    for (QListWidgetItem *item  : selectedItems())
    {
        QMap<int, QVariant> data;
        data[Qt::UserRole] =  item->data(Qt::UserRole);
//        Element *elem = item->data(Qt::UserRole).value<Element*>();
//        qulonglong ptrval(*reinterpret_cast<qulonglong *>(&elem));
        stream << row(item) << data;
//        stream << row(item) << elem->getId() << elem->getElementTypeName();
    }
    mimeData->setData("application/x-qabstractitemmodeldatalist", encodedData);
    QDrag *drag = new QDrag(this);
    drag->setMimeData(mimeData);
    drag->exec(Qt::CopyAction | Qt::MoveAction, Qt::CopyAction);
}

The error message kicks in when I try to write the data in the stream.
I'm jumping in

template <class Key, class T>
inline QDataStream &operator<<(QDataStream &s, const QMap<Key, T> &map)
{
    return QtPrivate::writeAssociativeContainer(s, map);
}

but never in my overriden function :(

[VRonin]

My fault, I totally forgot you also need qRegisterMetaTypeStreamOperators

P.S.
I still don't like your design but at the end of the day if you are happy with it and it works, go for it

[mbruel]

@VRonin said in crash in QListWidget::dropEvent when I add a UserRole to my QListItemWidget :'(:

My fault, I totally forgot you also need qRegisterMetaTypeStreamOperators

Ok, now I don't have the "QVariant::save: unable to save type 'Element*' (type id: 1025).
" message but the app is crashing in an infinite loop in:

inline QDataStream &operator>>(QDataStream &in, Element*& elem) {

    in >> elem;
    return in;
}

on the first line...

Thread 1 (Thread 0x7ffff7fb8140 (LWP 16621)):
#0  0x000055555557fd65 in operator>> (in=..., elem=@0x7fffffffa280: 0x0) at ../src_cosi7/Model/Element.h:370
No locals.
#1  0x000055555557fd6a in operator>> (in=..., elem=@0x7fffffffa280: 0x0) at ../src_cosi7/Model/Element.h:370
No locals.
#2  0x000055555557fd6a in operator>> (in=..., elem=@0x7fffffffa280: 0x0) at ../src_cosi7/Model/Element.h:370
No locals.
#3  0x000055555557fd6a in operator>> (in=..., elem=@0x7fffffffa280: 0x0) at ../src_cosi7/Model/Element.h:370
No locals.
#4  0x000055555557fd6a in operator>> (in=..., elem=@0x7fffffffa280: 0x0) at ../src_cosi7/Model/Element.h:370
No locals.
#5  0x000055555557fd6a in operator>> (in=..., elem=@0x7fffffffa280: 0x0) at ../src_cosi7/Model/Element.h:370
No locals.
#6  0x000055555557fd6a in operator>> (in=..., elem=@0x7fffffffa280: 0x0) at ../src_cosi7/Model/Element.h:370
No locals.
#7  0x000055555557fd6a in operator>> (in=..., elem=@0x7fffffffa280: 0x0) at ../src_cosi7/Model/Element.h:370
No locals.
#8  0x000055555557fd6a in operator>> (in=..., elem=@0x7fffffffa280: 0x0) at ../src_cosi7/Model/Element.h:370

... 

#261515 0x000055555557fd6a in operator>> (in=..., elem=@0x7fffffffa280: 0x0) at ../src_cosi7/Model/Element.h:370
No locals.
#261516 0x000055555557fd6a in operator>> (in=..., elem=@0x7fffffffa280: 0x0) at ../src_cosi7/Model/Element.h:370
No locals.
#261517 0x0000555555580182 in QtMetaTypePrivate::QMetaTypeFunctionHelper<Element*, true>::Load (stream=..., t=0x7fffffffa280) at /opt/Qt/5.10.1/gcc_64/include/QtCore/qmetatype.h:777
No locals.
#261518 0x00007ffff66c596a in QMetaType::load(QDataStream&, int, void*) () from /opt/Qt/5.10.1/gcc_64/lib/libQt5Core.so.5
No symbol table info available.
#261519 0x00007ffff66ed12c in QVariant::load(QDataStream&) () from /opt/Qt/5.10.1/gcc_64/lib/libQt5Core.so.5
No symbol table info available.
#261520 0x00007ffff66ed25f in operator>>(QDataStream&, QVariant&) () from /opt/Qt/5.10.1/gcc_64/lib/libQt5Core.so.5
No symbol table info available.
#261521 0x00005555555ef34e in QtPrivate::readAssociativeContainer<QMap<int, QVariant> > (s=..., c=...) at /opt/Qt/5.10.1/gcc_64/include/QtCore/qdatastream.h:285
        k = 256
        t = {d = {data = {c = 0 '\000', uc = 0 '\000', s = 0, sc = 0 '\000', us = 0, i = 0, u = 0, l = 0, ul = 0, b = false, d = 0, f = 0, real = 0, ll = 0, ull = 0, o = 0x0, ptr = 0x0, shared = 0x0}, type = 1025, is_shared = 0, is_null = 0}}
        i = 0
        stateSaver = {stream = 0x7fffffffa310, oldStatus = QDataStream::Ok}
        n = 3
#261522 0x00005555555eebff in operator>><int, QVariant> (s=..., map=...) at /opt/Qt/5.10.1/gcc_64/include/QtCore/qdatastream.h:436
No locals.
#261523 0x00005555555ed678 in ItemOfferingListWidget::dropEvent (this=0x5555562138d0, event=0x7fffffffa8f0) at ../src_cosi7/hmi/GUI/widget/ItemOfferingListWidget.cpp:103
        userRoleIter = {i = 0x7fffffffa480}
        mimeData = {d = 0x5555563d7f70}
        modelData = {d = 0x7ffff67e4940 <QMapDataBase::shared_null>}
        mimeReader = {d = {d = 0x5555563a35c0}, dev = 0x55555617f260, owndev = true, noswap = false, byteorder = QDataStream::BigEndian, ver = 17, q_status = QDataStream::Ok}
        row = 0
        col = 0
        __PRETTY_FUNCTION__ = "virtual void ItemOfferingListWidget::dropEvent(QDropEvent*)"
#261524 0x00007ffff7735018 in QWidget::event(QEvent*) () from /opt/Qt/5.10.1/gcc_64/lib/libQt5Widgets.so.5
No symbol table info available.

[VRonin]

That's because your code is an obvious infinite recursion. You have to fix it

[mpergand]

Yes, there're something wrong with the >> operator.

Anyway, I think it's a oversized process just to save a single pointer ...

QDataStream allows to save/load a pointer to char:

operator<<(const char *s)
operator>>(char *&s)

So, you simply need to use reinterpret_cast, back and forth for your custom class.
Et voilà

[mbruel]

Well I nearly lost a day on this so I think I'm just going to stay with my working version where I create my own QDrag and put by hand the reinterpret_cast of the address of my pointer.
It works well and I don't need the register and registerforstream of my metatype (neither to reimplement the stream operator on DataStream)

[mbruel]

@mpergand said in crash in QListWidget::dropEvent when I add a UserRole to my QListItemWidget :'(:

Yes, there're something wrong with the >> operator.

Anyway, I think it's a oversized process just to save a single pointer ...

QDataStream allows to save/load a pointer to char:

operator<<(const char *s)
operator>>(char *&s)

So, you simply need to use reinterpret_cast, back and forth for your custom class.
Et voilà

Yep that is what I'm doing but through a qulonglong instead of the char*

@VRonin

That's because your code is an obvious infinite recursion. You have to fix it

I pasted your code from the other post thinking the break would end the for loop. (I don't know the transaction objects you're using)
I've updated to be:

            const QByteArray mimeData = event->mimeData()->data("application/x-qabstractitemmodeldatalist");
            QDataStream mimeReader(mimeData);
            int row, col;
            QMap<int,QVariant> modelData;
            while (!mimeReader.atEnd())
            {
                mimeReader >> row >> col >> modelData;
                qDebug() << "Receiving row: " << row <<  " modelData.size: " << modelData.size();
                const auto userRoleIter = modelData.constFind(Qt::UserRole);
                if(userRoleIter != modelData.cend())
                {
                    Element *elem = userRoleIter->value<Element*>();
                    qDebug() << "Receiving elem: " << elem->getName()  << " (row in original list: " << row << ")";
                }
                else{
                    qDebug() << "No UserRole...";
                }
            }

but I still have the same issue...
I don't see the obvious recursive loop :(
this code is working well if I don't use the Qmap<int, QVariant> but build the QDrag myself.
Could you let me know what's wrong plz

[VRonin]

@mbruel said in crash in QListWidget::dropEvent when I add a UserRole to my QListItemWidget :'(:

I don't see the obvious recursive loop

inside

inline QDataStream &operator>>(QDataStream &in, Element*& elem) {
    in >> elem;
    return in;
}

in >> elem; calls QDataStream &operator>>(QDataStream &in, Element*& elem). that's the infinite recursion.

Serialising pointers to objects as numerics tickles my evil detector, in any case, Qt has qintptr type that is designed for this

[mbruel]

@VRonin
ah yeah indeed... I've just copied the code without thinking cause without it and only the add of the qRegisterMetaTypeStreamOperators it wasn't working so I guess I need to reimplement the stream operator on DataStream .

anyway, if I've to do that, and cast the pointer address, I don't see the point of doing all this. I'm just gonna cast directly in the dragStart in my own QDrag and decode it like I was doing in my working solution above.

Thanks for the qintptr, I'm going to use this instead of the qulonglong.