QSharedPointer dynamicCast and objectCast fail on ubuntu

Question

I am using Qt 5.0.1 under Ubuntu 10.04 and in my application I need to use QSharedPointer together with the appropriate dynamic_cast (object_cast) conversions at runtime. These conversions are called in a shared object which is properly loaded at runtime. I have to deploy my application to run it on an embedded computer with Ubuntu 10.04 operating system; that's why I'm using Qt under Ubuntu 10.04.

If I build the my application and I run it in the same machine where Qt libraries are installed, everything works as expected.

However if I run the application on the embedded computer, qSharedPointerDynamicCast (and the similar QSharedPointer::dynamicCast, qSharedPointerObjectCast and QSharedPointer::objectCast) fails and returns a null pointer. I have checked that both machines have the same configuration and, with the exception of dynamic_cast/object_cast issues, everything seems to work fine.

I think this may be a RTTI problem, however, after extensive googling, I did not find the solution of my problem.

qSharedPointerObjectCast use qobject_cast, The qobject_cast() function behaves similarly to the standard C++ dynamic_cast(), with the advantages that it doesn't require RTTI support and it works across dynamic library boundaries. So RTTI does not have to influence on the behaviour. — AnatolyS, Mar 19 '13 at 13:57
does the binary package contain all necessary qt libraries including qt plugins? — AnatolyS, Mar 19 '13 at 14:00
Thank you for your reply. I have tried to use both objectCast and dynamicCast. However I recognized that objectCast works fine for a class (say class A) but it doesn't work for another class (say class B) of my application; dynamicCast, instead, works fine with class B and fails with class A. — arms, Mar 19 '13 at 14:04
It seems that the built package contains all the required libraries and plugins since if I simply skip the "buggy code" using some "if-else" statement no error occurs. — arms, Mar 19 '13 at 14:07
Yes, both classes inherit from QObject and are marked with Q_OBJECT macro. Also the "non-Qt" members of these classes are inherit from QObject and are marked with Q_OBJECT. — arms, Mar 19 '13 at 14:07
also check slice effects in your code: struct A{}; struct B : A {}; B b; A a = b; // slice — AnatolyS, Mar 19 '13 at 14:13
In one shared object I have: QSharedPointer pA = QSharedPointer (new A); In another shared object I have: QSharedPointer p_Object = "..get the pointer from the other shared object..." QSharedPointer pA = qSharedPointerObjectCast(pObject) // after this line pa is a null QSharedPointer. I have verified that pObject.data() is the right pointer. — arms, Mar 19 '13 at 14:14
if it is possible please show how to "..get the pointer from the other shared object..." — AnatolyS, Mar 19 '13 at 14:23
It is not simple to do that since I use a complex class structure to manage several shared objects within a multi-thread application. However I verified that the "pure" pointer I get is the right one. I verified it by its value and by its metaObject data. I'm not able to replicate my problem with a simple code. — arms, Mar 19 '13 at 14:29
if you write QSharedPointer p1(new A); QSharedPointer p2 = qSharedPointerObjectCast (p1); does it work correctly? — AnatolyS, Mar 19 '13 at 14:32
I've just tried and within the same shared object it works fine. — arms, Mar 19 '13 at 14:43
yes, it must work correctly when QObject represents only the A class or inheritors of the A. Can you check what class pObject holds ( pObject->metaObject()->className() ) from your example before the casting? — AnatolyS, Mar 19 '13 at 14:50
I did that check and pObject holds the className A...and it is right...I guess... — arms, Mar 19 '13 at 14:53
qobject_cast uses QObject *QMetaObject::cast(QObject *obj) const { if (obj) { const QMetaObject *m = obj->metaObject(); do { if (m == this) return obj; } while ((m = m->d.superdata)); } return 0; } so your casting can be failed only if your objects have two different meta objects. But I don't understand how it can happen. Try to get addresses of metaObject()s from your pObject and for example from local: A local; — AnatolyS, Mar 19 '13 at 15:00
I don't know: both address and metaObject are right...however both qSharedPointerObjectCast and qobject_cast (using the "pure" pointer) within different shared objects do not work. — arms, Mar 19 '13 at 15:09
the final casting is QSharedPointer pA = qSharedPointerObjectCast (pObject) and after that pA.isNull() returns TRUE. it's strange situation. — AnatolyS, Mar 19 '13 at 15:21
just for clarify QSharedPointer s1(new A()); A local; const QMetaObject* mo1 = local.metaObject(); const QMetaObject* mo2 = s1.data()->metaObject(); assert(mo1 == mo2); // ok QSharedPointer s2 = qSharedPointerObjectCast (s1); assert(!s2.isNull()); // failed — AnatolyS, Mar 19 '13 at 15:36
Yes this is right. It seems that qobject_cast doesn't work among shared objects. Within the machine where Qt is installed I have also verified that if I create an object in one shared-object, and I use object_cast in another shared-object it doesn't work...however dynamic_cast works fine. In the target machine, instead, dynamic_cast doesn't work at all... — arms, Mar 19 '13 at 15:40
do you use qojbect_cast to cast one shared object to other or are you talking about qSharedPointerObjectCast? — AnatolyS, Mar 19 '13 at 15:44
I've tried both qSharedPointerObjectCast on QSharedPointer objects, and qobject_cast on pure pointers. — arms, Mar 19 '13 at 15:48
do you use multiple inheritance? it would be perfect to see definition of the A. — AnatolyS, Mar 19 '13 at 15:50
I don't use multiple inheritance. Class A is like this: class myObject : public QObject { Q_OBJECT }; class A : public myObject { Q_OBJECT } — arms, Mar 19 '13 at 15:53

score 0 · Answer 1 · answered Sep 24 '13 at 19:02

0

I think that the only explanation for your troubles is that you have memory bugs in your application and you're either corrupting your objects, or using freed objects. Use Valgrind and see what you get.

answered Sep 24 '13 at 19:02

Kuba hasn't forgotten Monica

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QSharedPointer dynamicCast and objectCast fail on ubuntu

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