<mysterious but solved> Trouble using qobject_cast with const QObject*

Asperamanca

I am using Q_DECLARE_INTERFACE / Q_INTERFACES in order to be able to use qobject_cast for objects which are not QObjects. The following works fine:

@class ITsc_SnapEnabledContainer
{

public:
// Some pure virtual methods that don't matter here

// Helper to cast QObject to this interface
static ITsc_SnapEnabledContainer* castToSnapEnabledContainer(QObject* pObject);

};

Q_DECLARE_INTERFACE(ITsc_SnapEnabledContainer, "ITsc_SnapEnabledContainer")@

...and in the cpp:

@ITsc_SnapEnabledContainer* ITsc_SnapEnabledContainer::castToSnapEnabledContainer(QObject* pObject)
{
return qobject_cast<ITsc_SnapEnabledContainer*>(pObject);
}@

I have been using these types of cast operations for a long time, and never had any trouble.
Until I tried to add this code:

Header:
@
// Const overload
static const ITsc_SnapEnabledContainer* castToSnapEnabledContainer(const QObject* pObject);
@

cpp:
@
const ITsc_SnapEnabledContainer* ITsc_SnapEnabledContainer::castToSnapEnabledContainer(const QObject* pObject)
{
return qobject_cast<const ITsc_SnapEnabledContainer*>(pObject);
}
@

As soon as I add this, I get errors:
@Error 1 error C2039: 'qt_check_for_QOBJECT_macro' : is not a member of 'ITsc_SnapEnabledContainer' c:\qtvs2010\4.8.1\src\corelib\kernel\qobject.h 368
Error 2 error C2039: 'staticMetaObject' : is not a member of 'ITsc_SnapEnabledContainer' c:\qtvs2010\4.8.1\src\corelib\kernel\qobject.h 370
Error 3 error C2228: left of '.cast' must have class/struct/union c:\qtvs2010\4.8.1\src\corelib\kernel\qobject.h 370
@

Any ideas?

Zlatomir

Add Q_OBJECT macro in your class (don't forget to run qmake), qobject_cast needs that macro, see the documentation "here":http://qt-project.org/doc/qt-5/qobject.html#qobject_cast

Asperamanca

In this case, it is not a QObject-derived class. The qobject_cast should work based on the Q_DECLARE_INTERFACE / Q_INTERFACES macros.

See "docs on qobject_cast":http://qt-project.org/doc/qt-4.8/qobject.html#qobject_cast

"qobject_cast() can also be used in conjunction with interfaces; see the Plug & Paint example for details."

Asperamanca

The following works, but I wonder why:

@const ITsc_SnapEnabledContainer* ITsc_SnapEnabledContainer::castToSnapEnabledContainer(const QObject* pObject)
{
return qobject_cast<ITsc_SnapEnabledContainer*>(pObject);
}@

It looks like I can take a const-QObject, make a non-const qobject_cast to return a const interface. Strange.