Error while qmlRegisterType with multiple inheritance class

Yujeonja

I have a class called View which inherits from QQuickItem.
And now I create class Box which inherits QQuickPaintedItem and View.

class Box : public QQuickPaintedItem, public View
{
    // Q_OBJECT  <- this also make errors so I removed it.
    ....
}

The problem is when I register this class with qmlRegisterType, there are some errors.

qmlRegisterType<Box>(uri, 0, 1, "Box");

First is

/usr/include/qt/QtQml/qqmlprivate.h:71:49: error: reference to ‘Box::staticMetaObject’ is ambiguous
   71 |     const char *className = T::staticMetaObject.className(); \
      |                                ~~~~~~~~~~~~~~~~~^~~~~~~~~

And second is

/usr/include/qt/QtCore/qmetatype.h:1529:40: error: ‘QObject’ is an ambiguous base of ‘Box’
 1529 |         enum { Value = sizeof(checkType(static_cast<T*>(nullptr))) == sizeof(yes_type) };
      |                              ~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~

And last one

/usr/include/qt/QtQml/qqml.h:349:51: error: reference to ‘Box::staticMetaObject’ is ambiguous
  349 |         uri, versionMajor, versionMinor, qmlName, &T::staticMetaObject,
      |                                                   ^~~~~~~~~~~~~~~~~~~~

Is it impossible to register QML C++ extension that multiple inheritance class?

SGaist

Hi,

@Yujeonja said in Error while qmlRegisterType with multiple inheritance class:

class Box : public QQuickPaintedItem, public View

Any chances that View also inherits QObject like QQuickPaintedItem ?

Yujeonja

@SGaist
Of course it's possible(View is also my code). But my concern is whether there will be any performance issues.

SGaist

You can't have double inheritance from QObject based class. That is the current issue you are hitting.

Does you view class really need to be QObject based ?
Do you really need to inherit from it rather than use it (i.e. composition) ?

Yujeonja

I discard QQuickPaintedItem and using updatePaintNode with QImage and QSGTexture.
The idea from https://stackoverflow.com/a/58597344/5902108

Below is my code.

QSGNode* Box::updatePaintNode(QSGNode *oldNode, QQuickItem::UpdatePaintNodeData *)
{
    QImage canvas(View::width(), View::height(), QImage::Format_ARGB32);
    QBrush brush;

    canvas.fill(QColor("transparent"));

    QPainter painter(&canvas);
    brush.setStyle(Qt::SolidPattern);
    brush.setColor(this->color());
    painter.setPen(Qt::NoPen);
    painter.setBrush(brush);
    painter.drawRect(0, 0, View::width(), View::height());

    QSGTexture *texture = this->window()->createTextureFromImage(canvas);
    QSGSimpleTextureNode *node = static_cast<QSGSimpleTextureNode*>(oldNode);
    if (!node) {
        node = new QSGSimpleTextureNode();
    }
    node->setRect(0, 0, View::width(), View::height());
    node->setTexture(texture);

    return node;
}