QVariant with custom class pointer does not return same address

Question

I need to assign a pointer to a custom class in qml using QQmlContext::setContextProperty(). Another qml object has Q_PROPERTY of the same type to retrieve it again.

A simple test showed me that the conversion does not work like i thought.

#include <QCoreApplication>
#include <QDebug>
#include <QMetaType>

class TestClass
{
public: TestClass() { qDebug() << "TestClass()::TestClass()"; }
};

Q_DECLARE_METATYPE(TestClass*)

int main(int argc, char *argv[])
{
    QCoreApplication app(argc, argv);

    qDebug() << "metaTypeId =" << qMetaTypeId<TestClass*>();

    auto testObject = new TestClass;
    QVariant variant(qMetaTypeId<TestClass*>(), testObject);
    auto test = variant.value<TestClass*>();

    qDebug() << testObject << variant << test;

    return 0;
}

This tiny test application gives me an output like this:

metaTypeId = 1024
TestClass::TestClass()
0x1b801e0 QVariant(TestClass*, ) 0x0

I would really like to get the same pointer out again after converting it down to a QVariant. Later I will assign it to a qml context and then the conversation must work correctly.

QVariant::value: If the pointer stored in the QVariant can be qobject_cast to T, then that result is returned. Otherwise a null pointer is returned. Note that this only works for QObject subclasses which use the Q_OBJECT macro. If you want to store a pointer in QVariant, you have to store it as void*. — thuga
If I would use void* as QVariant-Type, can the Q_PROPERTY for interfacing with qml still be a TestClass*? I cannot add the Q_OBJECT macro to the class i want to transport through qml, because the class was not written by me. — feedc0de
What is the goal of passing an object to QML which is not designed for it? — m7913d
I need to pass it to multiple custom c++ QuickItem. It is the pointer to my data model. — feedc0de

thuga thuga · Accepted Answer · 2017-06-13T07:40:17

This works for me using Qt 5.9:

#include <QVariant>
#include <QDebug>

class CustomClass
{
public:
    CustomClass()
    {
    }
};    
Q_DECLARE_METATYPE(CustomClass*)

class OtherClass
{
public:
    OtherClass()
    {
    }
};
Q_DECLARE_METATYPE(OtherClass*)

int main()
{
    CustomClass *c = new CustomClass;
    OtherClass *o = new OtherClass;
    QVariant v;
    v.setValue(c);
    QVariant v2;
    v2.setValue(o);

    qDebug() << v.userType() << qMetaTypeId<CustomClass*>() << v2.userType() << qMetaTypeId<OtherClass*>();
    qDebug() << v.value<CustomClass*>() << c << v2.value<OtherClass*>() << o;

    return 0;
}

And the output i got:

1024 1024 1025 1025
0x81fca50 0x81fca50 0x81fca60 0x81fca60

QVariant with custom class pointer does not return same address

2 Answers