QByteArray to QObject typecasting

ddze

It seemed a trivial problem but somehow I did not find an example of casting from QByteArray to a QObject. I have tried several ways like QObject *temp = qobject_cast<QObject *>(QByteArray) .

kshegunov

@ddze
Well, you can't! These classes are not in the same hierarchy, so neither qobject_cast, nor dynamic_cast will work. In fact QByteArray is a base class, and doesn't depend on the Qt's meta-object system in comparison to QObject.

Kind regards.

ddze

I thought so , thanks kshegunov. Looks like that I will have to overload the operators << and >>.

SGaist

Hi and welcome to devnet,

In addition to what @kshegunov wrote. What are you trying to serialize exactly ?

ddze

@SGaist

I am trying to serialize an object to into the QMimeData instance. Still need to get around the the missing operators << >> in the QGraphicsItem class.

SGaist

What would you like to do with that object ?

ddze

Transfer it from one place to another.

kshegunov

@ddze
Usually you don't do that, for the same reason that you don't copy QObject instances. If you have some data put into a QObject one usually extracts that data by a method and serializes the data itself, not the QObject. Consider what you're supposed to do to serialize the object hierarchy (with the parents and children) for a QObject instance? Also signals, slots and connections can't be trivially expanded in means of serialization ... there are tons of problems that prevent you from actually copying/serializing (which is basically the same thing) a QObject instance.

Kind regards.

ddze

@kshegunov and @SGaist ,

Thank you.

I will probably go with the plan of storing a pointer into the QMimeData instead of streaming in/out the entire structure of the object.

Hamed.Masafi

Your class must implement this operators:

QDataStream &operator<<
QDataStream &operator>>

After that you can use this code:

MyClass c;
QDataStream reader(byteArray);
reader << c;

ddze

@Hamed.Masafi

I know that and it would be easy if there was no a pointer to a QGraphicItem in the object structure (which has no << , >> ) operators.

Also , my class provides the insertion and retraction operators but seems impossible to use them in the MIme as they fail to populate the members of the class such as QByteArray , QGraphicItem ... since these classes do not provide <<>> operators on their own.

Looks pretty limited staff that can be transferred with the QMimeData , basic texts, urls, qt-class objects , but the objects from non Qt classes containing the Qt class members with no << >> operators, it is hard or even impossible - since it is requirement that they have << >> in order to use the QDataStream object.