Custom C++ type from a QML module is not defined when used inside QML file

[monkeber]

In my project I'm trying to expose a singleton type to QML from C++. Initially I registered it with qmlRegisterSingletonInstance and it worked fine. But when I tried to replace this way with QML_ELEMENT and QML_SINGLETON macros it compiles fine but when QML tries to use the class it complains that the type is not defined.

My project has two modules, one consists solely of qml files and the second - of C++ files, where I try to expose the class from and use it in the first module. The structure is a bit complicated so I reproduced the issue with a simpler test project. I noticed that QML cannot recognize neither the singleton type nor a non-singleton type.

The project has a following structure:

│   CMakeLists.txt
│   main.cpp
│
└───logicmodule
        CMakeLists.txt
        Main.qml
        Message.h
        MySingleton.h

The code:

// logicmodule/MySingleton.h
#pragma once

#include <QObject>
#include <QQmlEngine>
#include <QString>

class MySingleton : public QObject
{
    Q_OBJECT
    QML_ELEMENT
    QML_SINGLETON
public:
    MySingleton(QObject *parent = nullptr)
        : QObject{parent}
    {}

public slots:
    QString exec(const QString &str) { return "somestring"; }
};

// logicmodule/Message.h
#pragma once

#include <QObject>
#include <QQmlEngine>
#include <QString>

class Message : public QObject
{
    Q_OBJECT
    QML_ELEMENT
    Q_PROPERTY(QString author MEMBER m_author NOTIFY authorChanged)
public:
    Message() {}

signals:
    void authorChanged();

private:
    QString m_author;
};

// logicmodule/Main.qml
import QtQuick
import QtQuick.Controls

Window {
    width: 640
    height: 480
    visible: true
    title: qsTr("Hello World")

    // property Message msg: Message {}
    Button {
        id: button
        anchors.centerIn: parent
        text: "Click me"
        onClicked: {
            label.text = MySingleton.exec(label.text)
        }
    }
    Label {
        id: label
        anchors.top: button.bottom
    }
}

// logicmodule/CMakeLists.txt
project(logicmodule VERSION 0.1 LANGUAGES CXX)

qt_add_library(${PROJECT_NAME} STATIC)
qt6_add_qml_module(${PROJECT_NAME}
    URI Main
    VERSION 1.0
    NO_PLUGIN
    RESOURCE_PREFIX "/qt/qml"
    QML_FILES
        Main.qml
    SOURCES
        MySingleton.h
        Message.h
)

target_link_libraries(${PROJECT_NAME} PUBLIC
    Qt6::Core
    Qt6::Gui
    Qt6::Qml
    Qt6::Quick
)

target_include_directories(${PROJECT_NAME}
    PUBLIC
        .
)

// main.cpp
#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include "Message.h"
#include "MySingleton.h"

int main(int argc, char *argv[])
{
    QGuiApplication app(argc, argv);

    QQmlApplicationEngine engine;

    const QUrl url(u"qrc:/qt/qml/Main/Main.qml"_qs);
    QObject::connect(
        &engine,
        &QQmlApplicationEngine::objectCreated,
        &app,
        [url](QObject *obj, const QUrl &objUrl) {
            if (!obj && url == objUrl)
                QCoreApplication::exit(-1);
        },
        Qt::QueuedConnection);

    engine.addImportPath(QCoreApplication::applicationDirPath() + "/qml");
    engine.addImportPath(":/");

    engine.load(url);

    if (engine.rootObjects().isEmpty()) {
        return -1;
    }

    return app.exec();
}

// CMakeLists.txt
cmake_minimum_required(VERSION 3.16)

project(typetest VERSION 0.1 LANGUAGES CXX)

set(CMAKE_CXX_STANDARD_REQUIRED ON)

find_package(Qt6 6.5 REQUIRED COMPONENTS Quick)
set(QT_QML_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/qml)

qt_standard_project_setup(REQUIRES 6.5)

add_subdirectory(logicmodule)

qt_add_executable(apptypetest
    main.cpp
)

target_link_libraries(apptypetest PUBLIC
    logicmodule
)

# No effect
# target_include_directories(apptypetest
#   PUBLIC
#       "${PROJECT_SOURCE_DIR}/logicmodule"
#       logicmodule
# )
# include_directories(logicmodule)

It all compiles fine but at runtime I get an error when trying to call MySingleton:

qrc:/qt/qml/Main/Main.qml:16: ReferenceError: MySingleton is not defined

I tried to research this issue but cannot find exactly my case, the closest I got is this one: qmltyperegistration include path does not acknowledge subdirectories

But the files with custom types seems to be included just fine without any prefixes.

I also noticed that in the build folder I have a file build\Debug\logicmodule\logicmodule_qmltyperegistrations.cpp and it contains calls to qmlRegisterTypesAndRevisions but it seems that it doesn't have an effect?

Also tried a project structure without any additional libraries (so without logicmodule lib, everything is in the project root and I have only one CMakeLists) and it seem to be working fine and all the custom types are recognized by QML.

Could somebody point me to what am I doing wrong here with the library? Why can't QML recognize the type when it's defined inside a lib?

I also had this question posted on SO here for a few days already, but unfortunately no solutions so far.

[Anumas]

I guess your problem comes down to NO_PLUGIN. The documation says the following: The STATIC QML modules also generate the static QML plugins if NO_PLUGIN is not specified. Targets that import such STATIC QML modules also need to explicitly link to corresponding QML plugins. https://doc.qt.io/qt-6/qt-add-qml-module.html#target-structure

I think that's why @dheerendra suggested you to make it shared lib.

Try removing NO_PLUGIN from qt_add_qml_module() & link against logicmoduleplugin instead of logicmodule in your root's CMakeLists.txt. And make a clean build as well

[Anumas]

Hi @monkeber ,

Did you try importing Main module? import Main from your Main.qml. You need to import it in order to use the C++ types defined in your Main module.

[dheerendra]

@monkeber said in Custom C++ type from a QML module is not defined when used inside QML file:

qt_add_library(${PROJECT_NAME} STATIC)

You can make it SHARED instead of STATIC.
qt_add_library(${PROJECT_NAME} SHARED)

This should work. Not sure why static is not working.

[monkeber]

Hi @Anumas

Yes, tried this, Qt Creator complains that such module is not found (I guess because Main.qml is a part of Main module and can't resolve the path) but the application compiles fine and there is still the same error:

qrc:/qt/qml/Main/Main.qml:18: ReferenceError: MySingleton is not defined

[Anumas]

I guess your problem comes down to NO_PLUGIN. The documation says the following: The STATIC QML modules also generate the static QML plugins if NO_PLUGIN is not specified. Targets that import such STATIC QML modules also need to explicitly link to corresponding QML plugins. https://doc.qt.io/qt-6/qt-add-qml-module.html#target-structure

I think that's why @dheerendra suggested you to make it shared lib.

Try removing NO_PLUGIN from qt_add_qml_module() & link against logicmoduleplugin instead of logicmodule in your root's CMakeLists.txt. And make a clean build as well

[monkeber]

Hi @dheerendra

Thank you for an interesting suggestion. After changing STATIC to SHARED I now have an error at the application start:

QQmlApplicationEngine failed to load component
qrc:/qt/qml/Main/Main.qml: No such file or directory

and the app exits immediately after this.

I tried to play around with the path in main.cpp and tried "qrc:/qml/Main/Main.qml", ":/qml/Main/Main.qml" or ":/qt/qml/Main/Main.qml" but none of them fixed this.

I also tried to google this and re-read the doc https://doc.qt.io/qt-6/qt-add-qml-module.htm and somewhat similar issues here and here.
And tried to make qt to generate a plugin:

# logicmodule/CMakeLists.txt
# ...
qt_add_library(${PROJECT_NAME} SHARED)
qt6_add_qml_module(${PROJECT_NAME}
	URI Main
	VERSION 1.0
	# NO_PLUGIN                                 #  <- commented out
	RESOURCE_PREFIX "/qt/qml"
	QML_FILES
		Main.qml
	SOURCES
		MySingleton.h
		Message.h
)
# ... other code is unchanged

# CMakeLists.txt
# ...
set(CMAKE_AUTORCC ON)                     # <- tried this just in case 
# ...
target_link_libraries(apptypetest PUBLIC
	Qt6::Core
	Qt6::Gui
	Qt6::Qml
	Qt6::Quick
	# logicmoduleplugin              # <- can't link to this because it's a MODULE_LIBRARY
	logicmodule                             # <- also tried to comment this one, no effect
)
# ... other code is unchanged

But I still get this error:

QQmlApplicationEngine failed to load component
qrc:/qt/qml/Main/Main.qml: No such file or directory

But in STATIC configuration from my original post it works and application at least starts (while still having issue with MySingleton is not being defined).

I suppose I've missed something while trying to convert to SHARED lib, maybe you could point out what I did wrong here please?

I was thinking maybe this issue has something to do with resource files? But I don't have explicitly defined resource files and registered everything through CMake assuming that it should handle it. Maybe I don't understand something here.

Also additional info I forgot to add to the original post:
I'm using Qt 6.7.0 and Qt Creator 13.0.0 if it helps.

[Anumas]

@monkeber after doing what I said you should get your app working

To fix Qt Creator complaining about unknown types, you have to modify your module definition:

project(logicmodulelib VERSION 0.1 LANGUAGES CXX)

qt_add_library(${PROJECT_NAME} STATIC)
qt_add_qml_module(${PROJECT_NAME}
    URI logicmodule
    VERSION 1.0
    RESOURCE_PREFIX "/qt/qml"
    QML_FILES
        Main.qml
    SOURCES
        MySingleton.h
        Message.h
)
...

In the URI you MUST follow your project structure. https://doc.qt.io/qt-6/qtqml-writing-a-module.html

Update your root's CMakeLists.txt to link to the plugin correctly: target_link_libraries(apptypetest PUBLIC logicmodulelibplugin)

[dheerendra]

You can see the example in my repository.
https://github.com/QtDheeru/QtExamples/tree/master/QML/QMLSingleTonIssue

[monkeber]

@Anumas Thank you very much, your suggestion indeed helped me and now qml properly recognizes custom types!

A note regarding Qt Creator complaining about unknown types - for some reason implementing fixes as you suggested didn't help, but what helped is setting QML_IMPORT_PATH in root CMakeLists.txt, do you think this is a proper fix for this or is something wrong here?

CMakeLists files now look like this:

# logicmodule/CMakeLists.txt
project(logicmodulelib VERSION 0.1 LANGUAGES CXX)

qt_add_library(${PROJECT_NAME} STATIC)
qt_add_qml_module(${PROJECT_NAME}
	URI logicmodule
	VERSION 1.0
	RESOURCE_PREFIX "/qt/qml"
	QML_FILES
		Main.qml
	SOURCES
		MySingleton.h
		Message.h
)

target_link_libraries(${PROJECT_NAME} PUBLIC
	Qt6::Core
	Qt6::Gui
	Qt6::Qml
	Qt6::Quick
)

target_include_directories(${PROJECT_NAME}
	PUBLIC
		.
)

# CmakeLists.txt
cmake_minimum_required(VERSION 3.16)

project(typetest VERSION 0.1 LANGUAGES CXX)

set(CMAKE_CXX_STANDARD_REQUIRED ON)

find_package(Qt6 6.5 REQUIRED COMPONENTS Quick)
qt_policy(SET QTP0001 NEW)
set(QT_QML_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/qt/qml)
set(QML_IMPORT_PATH "${CMAKE_BINARY_DIR}/qt/qml" CACHE STRING "Qt Creator extra qml import paths"
	FORCE)

qt_standard_project_setup(REQUIRES 6.5)

add_subdirectory(logicmodule)

qt_add_executable(apptypetest
	main.cpp
)

target_link_libraries(apptypetest PUBLIC
	Qt6::Gui
	Qt6::Qml
	logicmodulelibplugin
)

[monkeber]

@dheerendra Sorry, I'm not sure I understood your example correctly. When I start your example I get an error:

QQmlApplicationEngine failed to load component
qrc:/qt/qml/dheeru/Main/Main.qml: No such file or directory

Does it work for you? Maybe I'm doing something wrong.

I also tried to tweak it to make it work for me, but with no success so far.

[dheerendra]

Yes. It is working example. Did you make any changes to the program ?
I have failed case with StaticPlugin/CMakeLists.txt -
qt_add_library(${PROJECT_NAME} STATIC). Change this line to ->qt_add_library(${PROJECT_NAME} SHARED)

Everything should work.

[Anumas]

@monkeber Sorry, I forgot to add that you needed to reset your code model: Go to Tools -> QML/JS -> Reset Code Model. Always try to reset it when Qt Creator complains about unknown types.

[monkeber]

@Anumas Yes, I've tried to reset the code model, clean rebuild and also I've updated Qt Creator to 13.0.2 (from 13.0.0) just in case. Not sure what's wrong, but after commenting out setting QML_IMPORT_PATH again, rebuilding and resetting the code model - Qt Creator can't recognize the module:

QML_IMPORT_PATH is indeed not in CMakeCache.txt unless I set it from CMake.

Maybe I indeed need to set it manually:
https://doc.qt.io/qtcreator/creator-qml-modules-with-plugins.html#importing-qml-modules

I'm also on Windows, not sure if it's relevant or not.

[monkeber]

@dheerendra
Sorry, I'm not sure what am I doing wrong here, but even after changing qt_add_library(${PROJECT_NAME} STATIC) to qt_add_library(${PROJECT_NAME} SHARED) as you suggested I still get this error while trying to run the example:

23:42:38: Starting E:\Projects\QtExamples-master\QML\QMLSingleTonIssue\build\Debug-Desktop_Qt_6_7_0_MSVC2019_64bit_MSVC2022\appQMLSingleTonIssue.exe...
QML debugging is enabled. Only use this in a safe environment.
QQmlApplicationEngine failed to load component
qrc:/qt/qml/dheeru/Main/Main.qml: No such file or directory

What I did:

1. Download your repo in zip archive.
2. Unpack.
3. In Qt Creator Open File or Project -> QtExamples-master\QML\QMLSingleTonIssue\CMakeLists.txt
4. Build and Run in Debug configuration.

I did not change anything in the code, just retried it.

[dheerendra]

@monkeber How did you solve the problem ? It helps if you post the same here.

[monkeber]

Hi @dheerendra

I think the issue was in NO_PLUGIN as were suggested by @Anumas here:
https://forum.qt.io/post/802964

Seems to be working fine with static library now. I marked that message as a solution.