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 the QMetaType documentation states that a type should have a public default constructor, a public copy constructor, and a public destructor.

I can override QObject's private copy constructor and declare a public copy constructor, but is this safe/ok/right?

class MyClass : public QObject {
  Q_OBJECT
  public:
    MyClass();
    MyClass(const MyClass &other);
    ~MyClass();
}
Q_DECLARE_METATYPE(MyClass);

Answer 1

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 2

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.