trouble using qobject_cast (MOC not seeing Q_OBJECT)

[devDawg]

Hi Everyone,

I've got a class that I am defining as a subclass of QAbstractProxyModel, which I will be instantiating inside of QML.

Here's some info I pulled from DataFilter.h:

class DataFilter: public QAbstractProxyModel {

    Q_OBJECT
 Q_PROPERTY(QObject* source READ getSource WRITE setSourceModel NOTIFY sourceChanged)
explicit DataFilter(QObject * parent = nullptr);

And here are my setter and constructor definitions for reference:

DataFilter::DataFilter(QObject *parent):
    QAbstractProxyModel(parent),
    choice(-1),
    source(nullptr),
    mOutputs()
    
{ }

void DataFilter::setSourceModel(QObject *model)
{

    QObject * tester = qobject_cast<DataModel*>(model);
    if (tester) {
        source = qobject_cast<DataModel&>(model);
    }
    else return;

}

This setter is what I am struggling with. BOTH DataFilter and DataModel have been defined with Q_OBJECT (I've been down this road before, so I know the errors), but for some reason, the moc is still not happy. My reason for this approach is to avoid failed casts.

/home/ghguest/Qt5.9.3/5.9.3/gcc_64/include/QtCore/qglobal.h:738: error: static assertion failed: qobject_cast requires the type to have a Q_OBJECT macro #define Q_STATIC_ASSERT_X(Condition, Message) static_assert(bool(Condition), Message)
^
On this same thought, the source member variable I am setting here is a reference (DataModel& source;). I am unsure if I am choosing the right cast target.

Overall, I am very confused by this. MOC is not happy, even though I've defined the target of the cast with Q_OBJECT. Here is a code chunk for you non-believers:

class DataModel: public QAbstractListModel {

Q_OBJECT
    explicit DataModel(const DataModel &model);

    explicit DataModel(DataModel &&);

    DataModel& operator=(const DataModel &);

    DataModel& operator=(DataModel &&);

As you can see, I've also defined my copy & move constructors/assignment operators.

Any ideas of things to try would be greatly appreciated.

Thanks in advance!

[devDawg]

I have solved the issue, and the result makes sense. First I'm going to reveal what I changed to complete an error-free build, then I'm going to explain why this makes sense.

I fixed my error by changing source; instead of storing a reference, I am now storing a pointer.
Meaning I went from this:

DataModel &source;

to this:

DataModel *source;

When a person stores a reference, they are storing the literal address in memory of that object. Now, because the address of the object in memory is probably tied to a bunch of other locations in memory, it is a time-costly and undesirable operation to copy by reference, because that would include making all the necessary ties to that new memory location.

Storing a pointer completely avoids the heavy operation of copying all of the arguments data. So instead of copying the object's address and mimicking all of its ties, you just pass a pointer that points to that object's address in memory.

I hope I was clear and also correct. C++ is a relatively new language to me, but I have done my fair share of reading about move and copy semantics.

If I have made an error in what I've said here, PLEASE tell me! Eager learner here.

[SGaist]

Hi,

There's one main major point you missed: you can not copy QObject based classes.

See the QObject documentation on the why.

[devDawg]

@SGaist Makes total sense.

Thanks man.