moc-generated code error at Qt6 when not at Qt5

[JonB]

@Axel-Spoerl
Hi Axel, thank you for testing.

All is clean, and reproducible from scratch by me.

This is a compilation issue, so main and creating instances doesn't matter. I don't know why you don't see the same as I do. As I move across I am still using qmake; you use cmake, I cannot see why that would cause a difference but will try tomorrow. I will also get exact Qt versions.

[Axel Spoerl]

Hi Jon,
Let us see your .pro file.
We’ll fix this together!

[JonB]

@Axel-Spoerl
I am looking at moving this test over to cmake to see whether it makes any difference/works like you showed. Will take me a while as I discover what that involves!

Meanwhile the .pro file you asked for, which I am using when the compiler generates the error:

QT = core

CONFIG += c++17 cmdline

# You can make your code fail to compile if it uses deprecated APIs.
# In order to do so, uncomment the following line.
#DEFINES += QT_DISABLE_DEPRECATED_BEFORE=0x060000    # disables all the APIs deprecated before Qt 6.0.0

SOURCES += \
        bar.cpp \
        foo.cpp \
        main.cpp

# Default rules for deployment.
qnx: target.path = /tmp/$${TARGET}/bin
else: unix:!android: target.path = /opt/$${TARGET}/bin
!isEmpty(target.path): INSTALLS += target

HEADERS += \
    bar.h \
    foo.h

[JonB]

@Axel-Spoerl
Before I try to move to cmake, in case that makes any difference. With qmake if you refer to the error in my first post you see it happens on moc_foo.cpp. The relevant moc/compilation lines executed are:

g++ -pipe -g -std=gnu++1z -Wall -Wextra -fPIC -dM -E -o moc_predefs.h /usr/lib/x86_64-linux-gnu/qt6/mkspecs/features/data/dummy.cpp
/usr/lib/qt6/libexec/moc -DQT_QML_DEBUG -DQT_CORE_LIB --include /home/jon/QtTests/mocerror/build/Desktop-Debug/moc_predefs.h -I/usr/lib/x86_64-linux-gnu/qt6/mkspecs/linux-g++ -I/home/jon/QtTests/mocerror -I/usr/include/x86_64-linux-gnu/qt6 -I/usr/include/x86_64-linux-gnu/qt6/QtCore -I. -I/usr/include/c++/13 -I/usr/include/x86_64-linux-gnu/c++/13 -I/usr/include/c++/13/backward -I/usr/lib/gcc/x86_64-linux-gnu/13/include -I/usr/local/include -I/usr/include/x86_64-linux-gnu -I/usr/include ../../foo.h -o moc_foo.cpp
g++ -c -pipe -g -std=gnu++1z -Wall -Wextra -fPIC -D_REENTRANT -DQT_QML_DEBUG -DQT_CORE_LIB -I../../../mocerror -I. -I/usr/include/x86_64-linux-gnu/qt6 -I/usr/include/x86_64-linux-gnu/qt6/QtCore -I. -I/usr/lib/x86_64-linux-gnu/qt6/mkspecs/linux-g++ -o moc_foo.o moc_foo.cpp

My generated moc_foo.cpp file from my pasted code is 151 lines long. The error message includes

required from ‘struct QMetaTypeId<QList<Bar*> >’
/usr/include/x86_64-linux-gnu/qt6/QtCore/qmetatype.h:2364:42: recursively required by substitution of ‘template<class T> struct QtPrivate::BuiltinMetaType<T, typename std::enable_if<QMetaTypeId2<T>::IsBuiltIn, void>::type> [with T = QList<Bar*>]’
QtPrivate::TypeAndForceComplete<QList<Bar*>, std::integral_constant<bool, false>
moc_foo.cpp:95:5: required from here

There I see function:

Q_CONSTINIT const QMetaObject Foo::staticMetaObject = { {
    QMetaObject::SuperData::link<QObject::staticMetaObject>(),
    qt_meta_stringdata_Foo.offsetsAndSizes,
    qt_meta_data_Foo,
    qt_static_metacall,
    nullptr,
    qt_incomplete_metaTypeArray<qt_meta_stringdata_Foo_t,
        // Q_OBJECT / Q_GADGET
        QtPrivate::TypeAndForceComplete<Foo, std::true_type>,
        // method 'this_works'
        QtPrivate::TypeAndForceComplete<void, std::false_type>,
        QtPrivate::TypeAndForceComplete<Bar *, std::false_type>,
        // method 'this_errors'
        QtPrivate::TypeAndForceComplete<void, std::false_type>,
        QtPrivate::TypeAndForceComplete<QList<Bar*>, std::false_type>
    >,
    nullptr
} };

The line generating the error is the

QtPrivate::TypeAndForceComplete<QList<Bar*>

Something in evaluating that is what is complaining that it does not have the "complete" definition of Bar class/type (because of how I wish to arrange my #include "bar.h"). But see how there is a also a line for QtPrivate::TypeAndForceComplete<Bar *, std::false_type>. That does not error. Hence the question: apart from why it has changed at Qt6, why does a slot parameter of QList<Bar *> need to know the full definition of Bar when a slot parameter of Bar * does not?

Ultimately the error comes from the moc-generated code causing the compiler to try to evaluate sizeof(Bar)/whether Bar has a Q_OBJECT (I think)/whatever it is doing with Q_PROPERTY or Q_DECLARE_METATYPE(T*). Which of course fails on an incomplete definition of Bar. I know about copying of signal/slot parameters (e.g. it certainly would complain if I tried to pass Bar or QList<Bar>) but I am passing pointers and I do not see why QList<Bar *> (fails) needs to know any more about Bar than Bar * (succeeds) does.

Do you not have similar in your generated moc_foo.cpp for your build using cmake?

[JonB]

@Axel-Spoerl
I have now changed over to cmake (project mocerror2). FWIW my CMakeLists.txt:

cmake_minimum_required(VERSION 3.14)

project(mocerror2 LANGUAGES CXX)

set(CMAKE_AUTOUIC ON)
set(CMAKE_AUTOMOC ON)
set(CMAKE_AUTORCC ON)

set(CMAKE_CXX_STANDARD 17)
set(CMAKE_CXX_STANDARD_REQUIRED ON)

find_package(QT NAMES Qt6 Qt5 REQUIRED COMPONENTS Core)
find_package(Qt${QT_VERSION_MAJOR} REQUIRED COMPONENTS Core)

add_executable(mocerror2
  main.cpp
  bar.cpp bar.h foo.cpp foo.h
)
target_link_libraries(mocerror2 Qt${QT_VERSION_MAJOR}::Core)

include(GNUInstallDirs)
install(TARGETS mocerror2
    LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}
    RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR}
)

As I expected it makes no difference and I get same error:

[ 14%] Automatic MOC and UIC for target mocerror2
[ 14%] Built target mocerror2_autogen
[ 28%] Building CXX object CMakeFiles/mocerror2.dir/mocerror2_autogen/mocs_compilation.cpp.o
In file included from /usr/include/x86_64-linux-gnu/qt6/QtCore/qobject.h:18,
                 from /usr/include/x86_64-linux-gnu/qt6/QtCore/QObject:1,
                 from /home/jon/QtTests/mocerror2/build/Desktop-Debug/mocerror2_autogen/EWIEGA46WW/../../../../foo.h:5,
                 from /home/jon/QtTests/mocerror2/build/Desktop-Debug/mocerror2_autogen/EWIEGA46WW/moc_foo.cpp:10,
                 from /home/jon/QtTests/mocerror2/build/Desktop-Debug/mocerror2_autogen/mocs_compilation.cpp:2:
/usr/include/x86_64-linux-gnu/qt6/QtCore/qmetatype.h: In instantiation of ‘struct QtPrivate::IsPointerToTypeDerivedFromQObject<Bar*>’:
/usr/include/x86_64-linux-gnu/qt6/QtCore/qmetatype.h:1102:60:   required from ‘struct QMetaTypeId<Bar*>’
/usr/include/x86_64-linux-gnu/qt6/QtCore/qmetatype.h:1122:38:   required from ‘struct QMetaTypeId2<Bar*>’
/usr/include/x86_64-linux-gnu/qt6/QtCore/qmetatype.h:1585:1:   required from ‘struct QMetaTypeId<QList<Bar*> >’
/usr/include/x86_64-linux-gnu/qt6/QtCore/qmetatype.h:2364:42:   recursively required by substitution of ‘template<class T> struct QtPrivate::BuiltinMetaType<T, typename std::enable_if<QMetaTypeId2<T>::IsBuiltIn, void>::type> [with T = QList<Bar*>]’
/usr/include/x86_64-linux-gnu/qt6/QtCore/qmetatype.h:2364:42:   required from ‘constexpr const QtPrivate::QMetaTypeInterface QtPrivate::QMetaTypeInterfaceWrapper<QList<Bar*> >::metaType’
/usr/include/x86_64-linux-gnu/qt6/QtCore/qmetatype.h:2486:16:   required from ‘constexpr const QtPrivate::QMetaTypeInterface* QtPrivate::qTryMetaTypeInterfaceForType() [with Unique = {anonymous}::qt_meta_stringdata_Foo_t; TypeCompletePair = TypeAndForceComplete<QList<Bar*>, std::integral_constant<bool, false> >]’
/usr/include/x86_64-linux-gnu/qt6/QtCore/qmetatype.h:2537:55:   required from ‘constexpr const QtPrivate::QMetaTypeInterface* const qt_incomplete_metaTypeArray [5]<{anonymous}::qt_meta_stringdata_Foo_t, QtPrivate::TypeAndForceComplete<Foo, std::integral_constant<bool, true> >, QtPrivate::TypeAndForceComplete<void, std::integral_constant<bool, false> >, QtPrivate::TypeAndForceComplete<Bar*, std::integral_constant<bool, false> >, QtPrivate::TypeAndForceComplete<void, std::integral_constant<bool, false> >, QtPrivate::TypeAndForceComplete<QList<Bar*>, std::integral_constant<bool, false> > >’
/home/jon/QtTests/mocerror2/build/Desktop-Debug/mocerror2_autogen/EWIEGA46WW/moc_foo.cpp:95:5:   required from here
/usr/include/x86_64-linux-gnu/qt6/QtCore/qmetatype.h:842:23: error: invalid application of ‘sizeof’ to incomplete type ‘Bar’
  842 |         static_assert(sizeof(T), "Type argument of Q_PROPERTY or Q_DECLARE_METATYPE(T*) must be fully defined");
      |                       ^~~~~~~~~
gmake[2]: *** [CMakeFiles/mocerror2.dir/build.make:82: CMakeFiles/mocerror2.dir/mocerror2_autogen/mocs_compilation.cpp.o] Error 1
gmake[1]: *** [CMakeFiles/Makefile2:86: CMakeFiles/mocerror2.dir/all] Error 2
gmake: *** [Makefile:136: all] Error 2
10:38:41: The process "/usr/bin/cmake" exited with code 2.

I do not understand why you do not get same error as I do.

My gcc is 13.2.0. My moc is 6.4.2 as is my Qt6.

[Axel Spoerl]

I am using gcc 13.3.0, moc 6.9.0.
The only difference in our CMake files is that I require 3.16, you require 3.14.

[JonB]

@Axel-Spoerl
Thank you. So what would you like me to do about our difference in behaviour? :)
[I fetch all my Qt stuff from apt as supplied with Ubuntu 24.04 distro.]

[J.Hilk]

I actually remember something similar, early when QVector was nuked for QList to prosper.

I think if you change it from QList<Bar *>bars to QList<Bar *> *bars it actually works. Axel is using a way later Qt6 version ( 6.8 vs 6.4.2) QList was probably touched again in between those versions, to fix this :D

[JonB]

@J-Hilk said in moc-generated code error at Qt6 when not at Qt5:

I think if you change it from QList<Bar *>bars to QList<Bar *> *bars it actually works.

But those are quite different types! :) Are you just showing me that to say what it could cope with?

Axel is using a way later Qt6 version ( 6.8 vs 6.4.2) QList was probably touched again in between those versions, to fix this :D

Ah! Well in that case it's obviously not to be reported. Thanks for the heads-up.

With Qt5 I always had Qt 5.15 (or maybe 5.12) via apt under various Ubuntu releases over the years. And I have to say that was just great, I didn't encounter problems and I didn't need later versions. I suspected Qt6 would be problematic, looks like I get a quite older version and am going to have issues which really need a later version from TQtC :( Ho-hum....

[J.Hilk]

@JonB what was the reason again, why you don't fetch the latest version from the installer or the repo (to compile yourself) ?

[JonB]

@J-Hilk

• I have never even attempted to download from TQtC, having seen the endless problems with downloads/mirrors. Fills me with dread....
• ...As does when I see people who for whatever reason (e.g. maybe to do with configuration, whatever it happens) get an error during compilation of the whole of Qt....
• I don't have the disk space in VM for Qt sources + compilation outputs.
• I don't have the memory in VM to compile it. Not to mention the requirements for QWebEngine.
• I share (development) with other people using Qt from Ubuntu distro. They are not going to want to fetch/compile/install or take from me.

Am I being difficult? ;-) It was all so simple with Qt5, everything worked. Now beginning to think of backing out of the whole of Qt6 and reverting to Qt5. But that would be a shame for my answering questions on this forum.

[J.Hilk]

@JonB I Understand, I myself are still on 5.15.17 and I stay with 5 as long as possible.

But Android support is apparently already dropped for Qt5. So I have at most a year until I have to switch over :(

[JonB]

@J-Hilk said in moc-generated code error at Qt6 when not at Qt5:

@JonB I Understand, I myself are still on 5.15.17 and I stay with 5 as long as possible.

OMG! What platform/OS version are you on?

[J.Hilk]

@JonB windows, macOS, iOS, iPadOS & Android. Still so far all the latest available (not betas)

[JonB]

@J-Hilk Right, but not my Ubuntu 24.04 which I am moving to, and doing Qt as part of it.

[J.Hilk]

@JonB non of our customers use Linux, only reason why we support macOS is because its my development environment and its only a few steps more to deploy for it as well ;)