QML/C++/QObject Struct Copy

Question

my Qt/QML program uses classes declared in C++ to structure information in QML and pass it as a bundle to C++ functions.

The structs are derived from QObject as required by QML. Example:

class ResultStorageOptionsStruct : public QObject {
    Q_OBJECT

public:
    explicit ResultStorageOptionsStruct(QObject * parent = nullptr) : QObject(parent) {}
    ~ResultStorageOptionsStruct() = default;
    ResultStorageOptionsStruct(const ResultStorageOptionsStruct& blob) {
    }

    bool folderValid;
    QString folderURL;

    Q_PROPERTY(bool folderValid MEMBER folderValid)
    Q_PROPERTY(QString folderURL MEMBER folderURL)


};

Q_DECLARE_METATYPE(ResultStorageOptionsStruct);

Using this method, passing information from QML to C++ works without a problem, but handling of that information on the C++ side is very bothersome due to the fact that you cannot copy/assign the class(derived of QObject).

I would like to copy/serialize/deserialize my classes for easy handling once they are on the c++ side. I do not need the features QObject gives me once we are on the c++ side as those classes are just containers for the information.

Is there some kind of trick or data structure to make this happen?

That would work, but has the downside that the struct can not be instantiate in QML itself (as far as I know). This would make bindings to UI elements impossible, which is a feature I would be loathe to miss. — ghst
I guess you don't want to make a "creator" function (like Qt.point(...)). Maybe a cast operator is an option? This does duplicate the structs but sounds like you might like it — Amfasis

e8johan e8johan · Accepted Answer · 2020-06-03T19:24:33

You can serialize and deserialize to a QIODevice, but this would be for file storage. To me, it sounds more like you want a QObject wrapper around a plain C++ object.

I would create a C++ class with the data that you need. Then create a QObject derived class with a (weak) reference to the C++ class that you wrap the original class in when passing it to QML.

class MyData
{
public:
    int x;
};

class MyDataWrapped : QObject
{
    Q_OBJECT
public:
    MyDataWrapped(MyData *obj) : md(obj) {}

    void x() const { return md->x; }

    Q_PROPERTY(...)

private:
    MyData *md; // You might want to make this a weak pointer
};

What is potentially tricky is if the data is changed from QML or C++, while the wrapper lives. How can QML tell that a property is changed if you change it from C++, and the other way around. Here, QObject and signals and slots come in handy. However, if the data is read-only while shared, I guess you should be fine.

QML/C++/QObject Struct Copy

1 Answers