[Interest] qobject_cast

[Ch'Gans]

On 20 April 2017 at 18:10, Igor Mironchik <igor.mironchik at gmail.com> wrote:
>
>
> 20.04.2017 9:01, Ch'Gans пишет:
>
> On 20 April 2017 at 17:56, Igor Mironchik <igor.mironchik at gmail.com> wrote:
>
> 20.04.2017 8:47, Ch'Gans пишет:
>
> On 20 April 2017 at 17:03, Igor Mironchik <igor.mironchik at gmail.com>
> wrote:
>
> Hi,
>
>
> 20.04.2017 7:52, Ch'Gans пишет:
>
> On 20 April 2017 at 16:38, Igor Mironchik <igor.mironchik at gmail.com>
> wrote:
>
> Hi,
>
> What the benefits of qobject_cast<> on simple static_cast<>? Or there
> is
> no
>
> qobject_cast
> - works only on QObject (obviously)
> - returns nullptr if the object cannot be casted to the required type
>
> And it can fail if QObject derived pointer was returned from plugin.
>
> Is this a statement or a question?
>
> if I'm not wrong, qobject_cast relies on QMetaObject
> (object->metaObject() and Class::staticMetaObject()).
>
> Actually, i've just checked the sources and qobject_cast<T*>(object)
> returns "static_cast<T*>(T::staticMetaObject().cast(object))" and
> QMetaObject::cast(obj) returns "(obj &&
> obj->metaObject()->inherits(this)) ? obj : nullptr"
>
> QMetaObject allows you to walk the class inheritance hierarchy, and
> inherits() does exactly this.
> I don't think that the fact that the object comes from a plugin can
> make inherits() fail.
> If you are able to load the plugin successfully, it means that you
> have no unresolved symbols.
>
> Exactly! qobject_cast<> will fail.
>
> Look in application you have:
>
> class Channel
>   :  public QObject
> {};
>
> In plugin you have:
>
> class MyChannel
>   :  public Channel
> {};
>
> Channel * create();
>
> In application you call:
>
> Channel * ch = create();
>
> And use this object in your application.
>
> Then in one slot in application you can have:
>
> void mySlot()
> {
>   Channel * ch = qobject_cast< Channel* > ( sender() );
>   // You are 100% sure that sender() is Channel!!!
>   // And this cast will fail!!!
>
> Try to call dumpObjectInfo(sender());
>
>
> OBJECT Globe::ComoChannel::Channel

If sender() is a Channel*, then qobject_cast should not fail.

Try to add this to your slot, this will print the inheritance list

    qDebug() << "Looking for" << &Channel::staticMetaObject <<
Channel::staticMetaObject.className() << "on" << sender() <<
qobject_cast< Channel* > ( sender() )
    const QMetaObject *mo = sender->metaObject();
   while (mo != nullptr) {
        qDebug() << mo << mo->className() << (mo ==
&Channel::staticMetaObject ? "<-" : "");
        mo = mo->superClass();
    }

This is roughly how QMetaObject::inherits() works. if you get the
"<-", then qobject_cast should work as expected.

Chris

>
> SIGNALS OUT
>
> signal: destroyed(QObject*)
>
> signal: destroyed()
>
> signal: objectNameChanged(QString)
>
> signal: sourceUpdated(Como::Source)
>
> <functor or function pointer>
>
> signal: sourceDeregistered(Como::Source)
>
> <functor or function pointer>
>
> signal: connected()
>
> <functor or function pointer>
>
> <functor or function pointer>
>
> signal: disconnected()
>
> <functor or function pointer>
>
> <functor or function pointer>
>
> <functor or function pointer>
>
> signal: messagesRate(int)
>
> <functor or function pointer>
>
> signal: aboutToConnectToHost(QHostAddress,ushort)
>
> <functor or function pointer>
>
> signal: aboutToDisconnectFromHost()
>
> <functor or function pointer>
>
> signal: aboutToSendGetListOfSources()
>
> <functor or function pointer>
>
> SIGNALS IN
>
> <-- QTimer::unnamed <unknown>
>
> <-- QTimer::unnamed <unknown>
>
> <-- Como::ClientSocket::unnamed <unknown>
>
> <-- Como::ClientSocket::unnamed <unknown>
>
> <-- Como::ClientSocket::unnamed <unknown>
>
> <-- Como::ClientSocket::unnamed <unknown>
>
> <-- Como::ClientSocket::unnamed <unknown>
>
>
>
>
>
> Chris
>
> }
>
> AFAIK, the only problems with plugins is meta-type registration that
> has to be done explicitly by calling qRegisterMataType(...);
> I don't think that QMetaObject suffers from this. Maybe someone can
> confirm this.
>
> - doesn't relies on RTTI, and is way faster than dynamic cast
>
> This is understandable. But with comparison with static cast there are no
> benefits?
>
> My point was: dynamic_cast is safer than static_cast, and qobject_cast
> is faster than dynamic_cast.
>
> So qobject_cast is a "double win" over static_cast and dynamic_cast.
>
> Chris
>
> Chris
>
> one?
>
> _______________________________________________
> Interest mailing list
> Interest at qt-project.org
> http://lists.qt-project.org/mailman/listinfo/interest
>
>