Declare list<vector2d> in QML and pass it to a C++ function which appends to the list?

Tom asso

How to declare a QList<QVector2D> in QML, then pass as an argument to an INVOKABLE C++ function, which appends to the QList? Fails at run-time.

C++ pseudocode:

class TopoDataItem : public QObject {
  Q_OBJECT
  Q_INVOKABLE bool runTest(QList<QVector2D> *qprofile)
}

main.qml declares a list<vector2d> then passes that to the C++ when a button is pressed:

// Declare bathymetry profile
property list<vector2d> profile
[...]
Button {
  text: qsTr('run test')
  onPressed: {topoDataItem.runTest(profile)}   
}

The call to topoDataItem.runTest(profile) generates this error:

"Could not convert argument 0 at"
	 "expression for onPressed@qrc:/main.qml:174"
qrc:/main.qml:174: TypeError: Passing incompatible arguments to C++ functions from JavaScript is not allowed.

What am I doing wrong? How should TopoDataItem::runTest() be declared, and how should it be invoked from QML?
Thanks!

Tom asso

@jeremy_k - thanks for that clarification.
I ended up defining runTest() as follows:

INVOKABLE QList<QVector2D> TopoDataItem::runTest(void ) {
  QList<QVector2D> profile;
  // Add content to profile
  [...]
  return profile;
}

The QML invokes the method as follows:

Button {
  text: qsTr('run test')
  onPressed: {
    var profile = topoDataItem.runTest();
    // Do stuff with profile data
    [...]
  }   
}

This seems to work with my application, without need to register additional stuff with the metatype system.

jeremy_k

Have you tried QList<QVector2D>, rather than a pointer to QList?

#include <QGuiApplication>
#include <QQmlApplicationEngine>

int main(int argc, char *argv[])
{
    QGuiApplication app(argc, argv);
    QQmlApplicationEngine engine;

    QObject::connect(
        &engine,
        &QQmlApplicationEngine::objectCreated,
        [&engine](QObject *obj, const QUrl & url) {
            qDebug() << obj->property("s");
        });

    engine.loadData("import QtQuick; QtObject { property list<vector2d> s }");

    return app.exec();
}

Output with Qt 6.5:

QVariant(QList<QVector2D>, QList())

Tom asso

@jeremy_k I change the function signature to:

bool TopoDataItem::runTest(QList<QVector2D> &qProfile)

Now when I invoke from QML I get this error:

Error: Unknown method parameter type: QList<QVector2D>&

If I make the function argument const like this:

bool TopoDataItem::runTest(const QList<QVector2D> &qProfile)

Now QML successfully calls the function - but I cannot append to the QList, since it is declared const. But runTest() needs to add content to the QList<QVector2D> profile argument. What am I missing here?
Thanks!

jeremy_k

@Tom-asso said in Declare list<vector2d> in QML and pass it to a C++ function which appends to the list?:

@jeremy_k I change the function signature to:
bool TopoDataItem::runTest(QList<QVector2D> &qProfile)
Now when I invoke from QML I get this error:
Error: Unknown method parameter type: QList<QVector2D>&

The bridge between QML and C++ uses the metatype system.

Background reading:
https://doc.qt.io/qt-6/qmetatype.html#Q_DECLARE_METATYPE

in particular:

Some types are registered automatically and do not need this macro:

Pointers to classes derived from QObject
QList<T>, QQueue<T>, QStack<T> or QSet<T> where T is a registered meta type
QHash<T1, T2>, QMap<T1, T2> or std::pair<T1, T2> where T1 and T2 are registered meta types
QPointer<T>, QSharedPointer<T>, QWeakPointer<T>, where T is a class that derives from QObject
Enumerations registered with Q_ENUM or Q_FLAG
Classes that have a Q_GADGET macro

Note that pointer and reference to QList of registered types is not in that list. I'm mildly surprised that a const reference works. QList is implicitly shared, making copy construction relatively cheap.

If I make the function argument const like this:
bool TopoDataItem::runTest(const QList<QVector2D> &qProfile)
Now QML successfully calls the function - but I cannot append to the QList, since it is declared const. But runTest() needs to add content to the QList<QVector2D> profile argument. What am I missing here?
Thanks!

See above concerning the metatype system.

Additionally, consider the ownership of objects in the list. The minimal non-working example in the first post suggests that the non-QObject objects are created by QML/javascript. Removing them from C++ can be a problem. For QObject instances, there's QJSEngine::setObjectOwnership(). For QVector2D, I have no idea.

Tom asso

@jeremy_k - thanks for that clarification.
I ended up defining runTest() as follows:

INVOKABLE QList<QVector2D> TopoDataItem::runTest(void ) {
  QList<QVector2D> profile;
  // Add content to profile
  [...]
  return profile;
}

The QML invokes the method as follows:

Button {
  text: qsTr('run test')
  onPressed: {
    var profile = topoDataItem.runTest();
    // Do stuff with profile data
    [...]
  }   
}

This seems to work with my application, without need to register additional stuff with the metatype system.