QSharedPointer<const QObject> results in compiler errors when used in metatyping (Q_DECLARE_METATYPE)

[Franz Hirschbeck]

I am currently porting Qt 5 to Qt 6 using Visual Studio 2022 and run into compiler errors.

When using QSharedPointer to a const object that is derived from QObject the metatyping is trying to register a conversion from const to non-const. This looks to me as a Bug in Qt but I couldn't find an issue and I am not sure if some compiler settings are wrong.

A minimal example:

Q_DECLARE_METATYPE(QSharedPointer<const QObject>);

I know that there is no need to explicitly declare this but this is the minimal example to run into this situation

The error message:

1>E:\ExternalLibs\windows_vs17_x64-Qt-6.2.3_r41935\include\QtCore/qmetatype.h(1196,1): error C2440: 'return': cannot convert from 'T *' to 'QObject *'
1>        with
1>        [
1>            T=const QObject
1>        ]
1>E:\ExternalLibs\windows_vs17_x64-Qt-6.2.3_r41935\include\QtCore/qmetatype.h(1196,32): message : Conversion loses qualifiers
1>E:\ExternalLibs\windows_vs17_x64-Qt-6.2.3_r41935\include\QtCore/qmetatype.h(1195): message : while compiling class template member function 'QObject *QtPrivate::QSmartPointerConvertFunctor<QSharedPointer<const QObject>>::operator ()(const SmartPointer &) const'
1>        with
1>        [
1>            SmartPointer=QSharedPointer<const QObject>
1>        ]
1>E:\ExternalLibs\windows_vs17_x64-Qt-6.2.3_r41935\include\QtCore/qmetatype.h(613): message : see reference to function template instantiation 'QObject *QtPrivate::QSmartPointerConvertFunctor<QSharedPointer<const QObject>>::operator ()(const SmartPointer &) const' being compiled
1>        with
1>        [
1>            SmartPointer=QSharedPointer<const QObject>
1>        ]
1>E:\ExternalLibs\windows_vs17_x64-Qt-6.2.3_r41935\include\QtCore/qmetatype.h(1579): message : see reference to class template instantiation 'QtPrivate::QSmartPointerConvertFunctor<QSharedPointer<const QObject>>' being compiled
1>E:\ExternalLibs\windows_vs17_x64-Qt-6.2.3_r41935\include\QtCore/qmetatype.h(1579): message : while compiling class template member function 'bool QtPrivate::MetaTypeSmartPointerHelper<T,void>::registerConverter(void)'
1>        with
1>        [
1>            T=QSharedPointer<const QObject>
1>        ]
1>E:\ExternalLibs\windows_vs17_x64-Qt-6.2.3_r41935\include\QtCore/qmetatype.h(1233): message : see reference to function template instantiation 'bool QtPrivate::MetaTypeSmartPointerHelper<T,void>::registerConverter(void)' being compiled
1>        with
1>        [
1>            T=QSharedPointer<const QObject>
1>        ]
1>E:\ExternalLibs\windows_vs17_x64-Qt-6.2.3_r41935\include\QtCore/qmetatype.h(1233): message : see reference to class template instantiation 'QtPrivate::MetaTypeSmartPointerHelper<T,void>' being compiled
1>        with
1>        [
1>            T=QSharedPointer<const QObject>
1>        ]
1>E:\Tests_Lib_Integration\Src\Libs\Basics\Basics\Array1d.cpp(17): message : see reference to function template instantiation 'int qRegisterNormalizedMetaType<QSharedPointer<const QObject>>(const QByteArray &)' being compiled

Qt version: 6.2.3
Visual Studio: 17.0.7 (2022 LTSC 17.0)
Compiler Options:

/permissive- /Yu"Basics/Precompiled.h" /GS /W3 /Zc:wchar_t  /Gm- /Od /Ob0 /FI"Basics/Precompiled.h" /Zc:inline /fp:precise /D "_UNICODE" /D "UNICODE"  /D "_ENABLE_EXTENDED_ALIGNED_STORAGE" /D "QT_GUI_LIB" /D "QT_WIDGETS_LIB" /D "QT_PRINTSUPPORT_LIB" /D "QT_XML_LIB" /D "QT_NETWORK_LIB" /D "QT_TESTLIB_LIB" /errorReport:prompt /WX- /Zc:forScope /RTC1 /GR /Gd /MDd /openmp /std:c++17 /EHa /diagnostics:column 

Is there something wrong with my setup?
Am I the only one using QSharedPointer to const object with signal/slot?

[kshegunov]

@Franz-Hirschbeck said in QSharedPointer<const QObject> results in compiler errors when used in metatyping (Q_DECLARE_METATYPE):

Is there something wrong with my setup?

Yes, but not syntax-wise.

Am I the only one using QSharedPointer to const object with signal/slot?

QSharedPointer doesn't really make any sense here, as you're reference counting (externally) an object, that is never supposed to be detached (copied). What should be the purpose of having a QSharedPointer instead of say const QObject *?

Edit: Well, I'd missed that this is an old-ish topic when I replied