Dragging std::shared_ptr

[cardio63]

Greetings. I want to use drag-and-drop to drag a QTreeWidgetItem that has std::shared_ptr user data. Qt is throwing an exception because it doesn't know how to store the shared_ptr onto a QDataStream. I read a bit and it seems that Q_DECLARE_SMART_POINTER_METATYPE is supposed to allow this to work. However, I have not had any success. I have built a tiny example showing the problem. Does anyone know how to do this properly?

In case you're wondering, I need to use shared_ptr (as opposed to other Qt shared pointers) because the pointer comes from a non-Qt part of the application (which I cannot control). I'm not comfortable converting the shared pointer to a raw pointer because it could go out of scope during my operation.

Thank you.

// DragSharedPtr.cpp

#include <QtGlobal>
#include <QMetaType>
#include <QVariant>
#include <QDataStream>

#include <memory>

Q_DECLARE_SMART_POINTER_METATYPE(std::shared_ptr)
Q_DECLARE_METATYPE(std::shared_ptr<int>);


int main()
{
    qRegisterMetaType< std::shared_ptr<int> >();

    // create variant holding the shared pointer (this works)
    auto spint = std::make_shared<int>(42);
    QVariant varspint = QVariant::fromValue(spint);

    // get the shared pointer back (this works)
    auto spint2 = varspint.value<std::shared_ptr<int>>();
    Q_ASSERT(spint2 == spint);

    // Try to encode the variant into a stream.  This is
    // needed when, for example, dragging the variant.
    //
    // This code compiles, but fails with a console warning:
    //
    //     QVariant::save: unable to save type  
    //                     'std:;shard_ptr<int>' (type id: 1024)
    //
    // and an exception:
    //    
    //     Program: <path to Qt>\bin\Qt5Cored.dll
    //     Module: 5.5.1
    //     File: <path to Qt>\src\corelib\global\qglobal.cpp
    //     Line: 2974
    //     ASSERT failure in QVariant::save: "Invalid type to save", 
    //            file <path...>\qvariant.cpp, line 2124
    //    
    QByteArray encoded;
    QDataStream stream(&encoded, QIODevice::WriteOnly);
    stream << varspint;

    return 0;
}

[jsulm]

You have to implement QDataStream & QDataStream::operator<<(std::shared_ptr<int>) and QDataStream & QDataStream::operator>>(std::shared_ptr<int>&). I don't see any other possibility, because QDataStream does not know how to serialize/deserialize a std::sgared_ptr.

[cardio63]

Thanks. Yes, I ended up doing just that. I'm a bit concerned about lifetime of ownership, but I don't see a better way.