Qt programming: How to use custom data type in QVariantMap?

Question

I am writing a Qt app that maps a C++ class to Javascript object in QtWebkit. Firstly let me explain what I am trying to do:

I have a class inherited from QObject:

class myobj : public QObject {
    Q_OBJECT
public:
    myobj();
    ~myobj();

pulbic slots:
    void getData();
}

And in another class I tried to add myobj instances to QVariantMap:

QVariantMap anotherClass::getObj() {
    myobj* obj1 = new myobj();
    myobj* obj2 = new myobj();

    QVariantMap items;

    items.insert(QString("0"), QVariant(*obj1));
    items.insert(QString("1"), QVariant(*obj2));

    return items;
}

And then I got the following error:

error: no matching function for call to ‘QVariant::QVariant(myobj&)’

So I tried to add declarations:

Q_DECLARE_METATYPE(myobj);

But I got:

error: ‘QObject::QObject(const QObject&)’ is private

Any idea about this?

score 3 · Accepted Answer · answered Nov 29 '10 at 13:19

3

Like the compiler said, no constructor of QVariant exists that take a myobj as parameter. Have you tried to use the qVariantFromValue function instead?

I think this is what you are searching for.

answered Nov 29 '10 at 13:19

Patrice Bernassola

14,136
6
46
59

I also tried this: http://stackoverflow.com/questions/956335/subclass-of-qobject-qregistermetatype-and-the-private-copy-constructor but seemd still could not work – Mickey Shine Nov 29 '10 at 13:28
1

As explains here (http://doc.trolltech.com/4.7/qmetatype.html#Q_DECLARE_METATYPE) you have to declare a public default constructor, a public copy constructor and a public destructor. You do not have any copy constructor. So compiler tries to use the one of the mother class (QObject) which has been declared as private – Patrice Bernassola Nov 29 '10 at 13:57
2

QObjects and QVariant doesn't go together well, as variant _values_ are supposed to be copied, while QObjects are not. – Frank Osterfeld Nov 29 '10 at 14:15
The example in the description of the qVariantFromValue function is done with a QObject. So I guess it's not a problem – Patrice Bernassola Nov 29 '10 at 14:20

score 2 · Answer 2 · answered Nov 29 '10 at 14:06

2

If you register your custom type with Q_DECLARE_METATYPE(myobj), your class needs a public default constuctor (ok), a public destructor (ok) and a public copy constructor (MISSING which the error message is telling you), see the documentation.

answered Nov 29 '10 at 14:06

hmuelner

8,093
1
28
39

Qt programming: How to use custom data type in QVariantMap?

2 Answers2