App crashes using gadgets defined in a library

[mjakubowski]

Given two CMake subprojects: foo and bar, I want to define Q_GADGET in bar and use it in foo, but the application crashes when accessing a QObject property of type Bar (defined in bar project).

foo CMakeLists, created by QtCreator, I just added SOURCES / HEADER and added bar to target_link_libraries:

cmake_minimum_required(VERSION 3.16)

project(foo VERSION 0.1 LANGUAGES CXX)

set(CMAKE_CXX_STANDARD_REQUIRED ON)

find_package(Qt6 6.5 REQUIRED COMPONENTS Quick)

qt_standard_project_setup(REQUIRES 6.5)

set(SOURCES
    # cmake-format: sort
    main.cpp
    foo.cpp
    fooobjectwithbarproperty.cpp
)

set(HEADER
    # cmake-format: sort
    fooobjectwithbarproperty.h
    foo.h
)

qt_add_executable(appfoo
    main.cpp
)

qt_add_qml_module(appfoo
    URI foo
    VERSION 1.0
    QML_FILES Main.qml
    SOURCES
        ${SOURCES}
        ${HEADER}
)

set_target_properties(appfoo PROPERTIES
    MACOSX_BUNDLE_GUI_IDENTIFIER my.example.com
    MACOSX_BUNDLE_BUNDLE_VERSION ${PROJECT_VERSION}
    MACOSX_BUNDLE_SHORT_VERSION_STRING ${PROJECT_VERSION_MAJOR}.${PROJECT_VERSION_MINOR}
    MACOSX_BUNDLE TRUE
    WIN32_EXECUTABLE TRUE
)

target_link_libraries(appfoo
    PUBLIC Qt6::Quick bar
)

install(TARGETS appfoo
    BUNDLE DESTINATION .
    LIBRARY DESTINATION ${CMAKE_INSTALL_LIBDIR}
    RUNTIME DESTINATION ${CMAKE_INSTALL_BINDIR}

)

bar CMakeLists:

cmake_minimum_required(VERSION 3.14)

project(bar 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_library(bar SHARED
  bar_global.h
  bar.cpp
  bar.h
)

target_link_libraries(bar PRIVATE Qt${QT_VERSION_MAJOR}::Core)

target_compile_definitions(bar PRIVATE BAR_LIBRARY)

top-level CMakeLists:

cmake_minimum_required(VERSION 3.26.0)

project(
    FooBar
    VERSION 0.0.1
    DESCRIPTION "Demo for Qt issue"
    LANGUAGES CXX)

set(CMAKE_CXX_STANDARD 20)

add_subdirectory(foo)
add_subdirectory(bar)

My QObject, defined in foo, looks like this:

class FooObjectWithBarProperty : public QObject
{
    Q_OBJECT
    Q_PROPERTY(Bar bar READ getBar WRITE setBar NOTIFY barChanged)
    Q_PROPERTY(Foo foo READ getFoo WRITE setFoo NOTIFY fooChanged)

public:
    const Bar getBar() const {
        return _bar;
    }

    void setBar(const Bar& bar) {
        _bar = bar;
        emit barChanged(_bar);
    }

    const Foo getFoo() const {
        return _foo;
    }

    void setFoo(const Foo& foo) {
        _foo = foo;
        emit fooChanged(_foo);
    }

private:
    Bar _bar;
    Foo _foo;

signals:
    void barChanged(const Bar&);
    void fooChanged(const Foo&);
};

Foo is defined in foo, while Bar is defined in Bar, they both look exactly the same:

struct Foo
{
    Q_GADGET
    Q_PROPERTY(double x MEMBER x)
    Q_PROPERTY(double y MEMBER y)

public:
    double x, y;
};

Given the following setup in main.cpp:

QQmlApplicationEngine engine;
engine.rootContext()->setContextProperty("obj", new FooObjectWithBarProperty());

I added a Text item into my Main.qml that looks like this:

Text {
    anchors.centerIn: parent
    text: obj.foo.x
}

This correctly displays 0. If I change the binding too obj.bar.x the app crashes:

Stacktrace here suggest the metatype for Bar is not correctly registered, but I don't know how to correctly register it. I've tried qRegisterMetaType<Bar>, Q_DECLARE_METATYPE(Bar) in both foo or bar, I experimented a bit with QML_FOREIGN, but the docs are very sparse on it. All to no avail.

What is the necessary change here, and why?

Repo for this is available at https://github.com/marcinjakubowski/foo_bar_qt_issue

[mjakubowski]

As it turns out, it's because of this line in bar's CMakeLists:

find_package(QT NAMES Qt6 Qt5 REQUIRED COMPONENTS Core)

It looks for Qt6 and Qt5, and, for some reason, picks Qt5 to generate moc files, as evidenced by the resulting moc_bar.cpp:

/****************************************************************************
** Meta object code from reading C++ file 'bar.h'
**
** Created by: The Qt Meta Object Compiler version 67 (Qt 5.15.9)

Removing Qt5 from that find_package call and rebuilding the project makes it all work fine.