Subclass of QObject, qRegisterMetaType, and the private copy constructor

Question

I have a class that is a subclass of QObject that I would like to register as a meta-type. The QObject documentation states that the copy-constructor should be private, but

Answer 1

I use a separate copyValue(const MyClass & other) function to copy the data members that define the "values" of the MyClass instance. That ensures that I don't break the assumption of QObject unique identity, while still being able to duplicate the parts of the class that are defined at compile time.

Answer 2

It is not safe to make a QObject's copy constructor public. You can register a class pointer as the metatype, though. i.e.:

Q_DECLARE_METATYPE(MyClass*);

That's how Qt handles it with QObject and QWidget.

Answer 3

And the cpp:

QScriptValue QTFruitConstructor(QScriptContext * /* context */,
                            QScriptEngine *interpreter)
{
    //return interpreter->toScriptValue(new QTFruit());
    //or
    return interpreter->toScriptValue(QTFruit()); //but then you need the public copy contructor 
}

 QTFruit::QTFruit( const QTFruit & other )
 : QObject()
{
} 

QTFruit::~QTFruit()
{
}

QTFruit::QTFruit()
{
}

void QTFruit::fromJScript(eLedState state)
{
    int t = 0;
}

Answer 4

QTFruit fruit;
QScriptValue scriptedFruitObject = engine.newQObject(&fruit);
engine.globalObject().setProperty("fruit", scriptedFruitObject);

 engine.setDefaultPrototype(qMetaTypeId<QTFruit>(),
                                scriptedFruitObject);

 QScriptValue qsMetaObject =
        engine.newQMetaObject(fruit.metaObject());
 engine.globalObject().setProperty("eLedState",
                                       qsMetaObject);

int t = engine.evaluate("eLedState.On").toInteger();

engine.evaluate("fruit.fromJScript(1)");
engine.evaluate("fruit.fromJScript(eLedState.On)");

engine.evaluate("fruit.fromJScript(eLedState.TriState)");

//Create the ctor funtion
QScriptValue qsFruitCtor =
        engine.newFunction(QTFruitConstructor,
                                scriptedFruitObject);
//Expose ctor to javascript
 engine.globalObject().setProperty("QTFruit", qsFruitCtor);

 //Create the QTFruit object
engine.evaluate("var res = new QTFruit()");
engine.evaluate("res.fromJScript(eLedState.TriState)");


class QTFruit : public QObject
{
    Q_OBJECT

public:
    enum eLedState { Off, On , TriState};
    Q_ENUMS( eLedState )    
    QTFruit();
    ~QTFruit();
     QTFruit( const QTFruit & other );

    //QTFruit(const QTFruit& other);

public slots:
    void fromJScript(eLedState state);
     //void on_inputSpinBox1_valueChanged(int value);
     //void on_buttonClicked();
//   void fromJScript();
//private:

};
Q_DECLARE_METATYPE(QTFruit*)
Q_DECLARE_METATYPE(QTFruit)

QScriptValue QTFruitConstructor(QScriptContext * /* context */,
                            QScriptEngine *interpreter);

Answer 5

What you're asking for is perfectly ok. You can't use QObjects copy constructor (it's private) in the implementation of your copy constructor, but then again, no-one forces you to:

class MyClass : public QObject {
    Q_OBJECT
public:
    // ...
    MyClass( const MyClass & other )
        : QObject(), i( other.i ) {} // NOTE: calling QObject default ctor
    // ...
private:
    int i;
};

Depending on what services you need from QObject, you need to copy some properties over from other, in both the copy ctor and the copy assignment operator. E.g., if you use QObject for it's dynamic properties feature, you'd need to copy those, too:

    MyClass( const MyClass & other )
        : QObject(), i( other.i )
    {
        Q_FOREACH( const QByteArray & prop, other.dynamicPropertyNames() )
            setProperty( prop.constData(), other.property( prop.constData() ) );
    }

Likewise, if you want to maintain signal/slot connections.