using C++ classes and structs in QML

[J.Hilk]

@mzimmers a struct and a class are literally the same in c++ 😉

did you forget to add Q_GADGET in your struct ?

[Bob64]

@mzimmers said in using C++ class in QML:

@SGaist I looked at the doc you referenced, and tried a few things. It appears that there are two ways to instantiate a C++ object in QML:

    property MyClass myClass1: MyClass {
        classInt: 44
    }
    MyClass {
        id: myClass2
        classInt: 33
    }

I don't know if these are identical "under the hood," but preliminary testing indicates that they both work.

As I understand it, they are both essentially the same in terms of instantiating the object. The difference is that in the second case the instance is instantiated and added as a child of your containing component (much like any other nested QML component), whereas in the first case you are assigning the instance to a property of your component.

[mzimmers]

@J-Hilk said in using C++ class in QML:

did you forget to add Q_GADGET in your struct ?

No, but I must be doing something just as silly. I even converted my struct to a class, so now I have:

class MyStruct {
    Q_GADGET
    QML_ELEMENT
    Q_PROPERTY(int myInt MEMBER m_myInt)
    int m_myInt = 55;
public:
    MyStruct() {}
};
Q_DECLARE_METATYPE(MyStruct)

along with:

class MyClass : public QObject {
    Q_OBJECT
    QML_ELEMENT
    Q_PROPERTY(int classInt MEMBER m_classInt NOTIFY classIntChanged)
    int m_classInt = 55;
signals:
    Q_INVOKABLE void classIntChanged();
public:
    MyClass() {}
};
Q_DECLARE_METATYPE(MyClass)

and in main.cpp:

    qmlRegisterType<MyStruct>("MyStruct", 1, 0, "MyStruct");
    qmlRegisterType<MyClass>("MyClass", 1, 0, "MyClass");

The first line produces a runtime error: "Invalid QML element name "MyStruct"; value type names should begin with a lowercase letter" while the second line works fine.

Somewhere, I'm overlooking one of the magic Qt macros, but I sure can't see it.

EDIT:

It works if I subclass MyStruct from QObject. This is OK for this little example, but has implications for my real project. Somehow I'm not getting Q_GADGET properly set up.

[SGaist]

@J-Hilk said in using C++ classes and structs in QML:

@mzimmers a struct and a class are literally the same in c++ 😉

Nope, there's key difference: the former has everything public by default while the latter has everything private.

[mzimmers]

@SGaist true enough, but I think I've obviated that difference with my use of the "public" keyword:
EDIT: I neglected to point out that the Q_GADGET macro causes everything declared after it to be private unless explicitly declared to the contrary, hence my use of public.

struct MyStruct {
    Q_GADGET
public:
    QML_ELEMENT
    QML_VALUE_TYPE(myStruct)
    Q_PROPERTY(int myInt MEMBER m_myInt)
    int m_myInt = 55;
    MyStruct() {}
};
Q_DECLARE_METATYPE(MyStruct)

With this declaration, I get a build error on this line:

property myStruct myStruct1: myStruct {

"error: Expected type name"

[jsulm]

@mzimmers Isn't the name of the struct MyStruct and not myStruct?

[mzimmers]

@jsulm if you're referring to the QML line of code, I get an error if I try to use "MyStruct:"

[J.Hilk]

@mzimmers can you show the full (copy pasted) content of your qml file ?
we haven't seen that and maybe its a simple error there

@SGaist said in using C++ classes and structs in QML:

Nope, there's key difference: the former has everything public by default while the latter has everything private

mäh, tomato/tomato, it changes the default but you can still declare private /prublic manually

[mzimmers]

@J-Hilk sure - I'll post from the top to the point of errors (I'm skipping the Labels that I use for telltales):

import QtQuick
import QtQuick.Controls
import QtQuick.Layouts
import QtQuick.Window

import MyStruct

Window {
    id: mainWindow
    width: 640
    height: 480
    visible: true

    property MyStruct myStruct1: MyStruct {
        myInt: 100
    }

    MyStruct {
        id: myStruct2
        myInt: 200
    }

While I'm at it, here's my main.cpp ("struct" is the project name):

#include <QGuiApplication>
#include <QQmlContext>
#include <QQmlApplicationEngine>

#include "mystruct.h"
#include "myclass.h"

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

    qmlRegisterType<MyStruct>("MyStruct", 1, 0, "MyStruct"); // tried "myStruct" too
    qmlRegisterType<MyClass>("MyClass", 1, 0, "MyClass");

    QObject::connect(&engine, &QQmlApplicationEngine::objectCreationFailed,
        &app, []() { QCoreApplication::exit(-1); },
        Qt::QueuedConnection);

    engine.loadFromModule("struct", "Main");

    return app.exec();
}

[J.Hilk]

don't know if that changed with Qt6 but shouldn't this

@mzimmers said in using C++ classes and structs in QML:

import MyStruct

be

import MyStruct 1.0

?

[mzimmers]

@J-Hilk that doesn't seem to matter any more. If you try to import a version number that's higher than what you registered, you'll get a runtime error, but that's about it.

This is just nuts, though. In main.cpp, this line:

qmlRegisterType<MyStruct>("myStruct", 1, 0, "myStruct");

Produces a warning from the editor/code model/whatever that QML types must begin with uppercase. Plus it won't build, giving an error at my QML declaration "error: Expected type name".

But if I modify that line in main.cpp to:

qmlRegisterType<MyStruct>("myStruct", 1, 0, "MyStruct"); // tried "myStruct" too

and then I have to change my Main.qml to:

    property MyStruct myStruct1: MyStruct {

I get a runtime error: "qt.qml.typeregistration: Invalid QML element name "MyStruct"; value type names should begin with a lowercase letter"

It's almost as though I shouldn't be registering my struct, though I don't know what the alternative might be.

[lemons]

@mzimmers
I thought the Q_GADGET macro is used for meta-types only, which can't be instantiated from QML.
To be used e.g. if the struct is a property of an QObject derived class, which is somehow accessible in QML (either passed through C++ or created in QML).
Allows you to use e.g. Q_PROPERTY macros, without the QObject overhead.

To be creatable from QML you need a derived class from QObject, so the Q_GADGET macro is not sufficient, as it is missing e.g. the signals and slots of the QObject class.

Note: I can be totally wrong / outdated, but this is how I was thinking and using it all the time :D

[JoeCFD]

@lemons said in using C++ classes and structs in QML:

Q_GADGET macro

Lemons seems right.

In QML (Qt Meta-Object Language), the Q_GADGET macro is typically used with C++ classes to create non-instantiable classes that can be registered with the Qt meta-object system. These classes are similar to Q_OBJECT classes but cannot have signals, slots, or properties. They are often used for data-only structures that need to be exposed to QML.

I use upper case for my class registration in qmlRegisterType without issues. I guess lower case is needed in app_engine->rootContext()->setContextProperty( ... );

[GrecKo]

You can now instantiate Gadgets from QML with undocumented macros (so maybe don't use it).

Use QML_VALUE_TYPE(typeName) with a lower case name like you currently do.
Then you can add QML_CONSTRUCTIBLE_VALUE or QML_STRUCTURED_VALUE (even both).

QML_CONSTRUCTIBLE_VALUE will call a user defined constructor

if you have Q_INVOKABLE MyStruct(int foo) : m_myInt{foo} {}
then doing property myStruct myStruct1: 42 will call the constructor with 42 as the foo param

QML_STRUCTURED_VALUE will assign properties depending on the js object you passed in QML.

property myStruct myStruct1: ({myInt: 42}) will create a default constructed MyStruct and assign its myInt property to 42.

Note that the type should always be default constructible.

EDIT: Note that you don't have to use QML_ELEMENT, Q_DECLARE_METATYPE or qmlRegisterType if you use Q_GADGET and QML_VALUE_TYPE.

[mzimmers]

@GrecKo curiouser and curiouser.

My modified struct:

struct MyStruct {
    QML_STRUCTURED_VALUE
    QML_VALUE_TYPE(myStruct)
    Q_PROPERTY(int myInt MEMBER m_myInt)
    int m_myInt = 55;
    MyStruct() {}
};

my registration (in main.cpp):

    qmlRegisterType<MyStruct>("myStruct", 1, 0, "MyStruct");

and my QML:

property myStruct myStruct1: myStruct ({myInt: 100})

produces a runtime error: "myStruct is not a type."

Isn't using QML_VALUE_TYPE intended to allow me to use "myStruct" in the QML?

[GrecKo]

As I said : you don't have to use qmlRegisterType. You do have to use Q_GADGET though.

And Q_GADGET adds a private: so you need to add back public:

struct MyStruct {
  Q_GADGET
  QML_VALUE_TYPE(myStruct)
  QML_STRUCTURED_VALUE

  Q_PROPERTY(int myInt MEMBER m_myInt)

public:
  int m_myInt = 55;
  MyStruct() {};
};

and my QML:
property myStruct myStruct1: myStruct ({myInt: 100})

Did I write that? nope. The syntax for structured values is :

property myStruct myStruct1: ({myInt: 100})

Ditch the myStruct on the right-hand side.

[mzimmers]

@GrecKo oh, that is a thing of beauty.

Now...about QML_STRUCTURED_VALUE being an undocumented macro, as you said...is it safe to use? I did notice mention of it in a bug report, so maybe it's OK.

Thanks for the help.

EDIT: everything above works, but if possible, I'd like to directly access enums defined in the struct (actually, this was the original point behind this entire thread).

struct MyStruct {
    Q_GADGET
    QML_STRUCTURED_VALUE
    QML_VALUE_TYPE(myStruct)
    Q_PROPERTY(int myInt MEMBER m_myInt)
public:
    enum MyEnums {
        Enum0,
        Enum1,
        Enum2,
        Enum3
    } m_myEnums;
    Q_ENUM(MyEnums)
    int m_myInt = 55;
    MyStruct() {}
};

This attempt doesn't work:

Label { text: "myStruct.Enum3: " + myStruct.Enum3 }

(it doesn't work with "MyStruct" either.)

Can this be made to work? Thanks...

EDIT 2:

I have a workaround, which is to define my enums in a separate class and expose them to QML using the guidelines here, but if possible I'd prefer to avoid this level of indirection, and keep the enums in the struct.

[GrecKo]

@mzimmers I don't believe it is possible no. Instead of defining your enums in a separate class I would do it in a namespace with Q_ENUM_NS instead.

[lemons]

@GrecKo said in using C++ classes and structs in QML:

You can now instantiate Gadgets from QML with undocumented macros (so maybe don't use it).

Use QML_VALUE_TYPE(typeName) with a lower case name like you currently do.
Then you can add QML_CONSTRUCTIBLE_VALUE or QML_STRUCTURED_VALUE (even both).

QML_CONSTRUCTIBLE_VALUE will call a user defined constructor

if you have Q_INVOKABLE MyStruct(int foo) : m_myInt{foo} {}
then doing property myStruct myStruct1: 42 will call the constructor with 42 as the foo param

QML_STRUCTURED_VALUE will assign properties depending on the js object you passed in QML.

property myStruct myStruct1: ({myInt: 42}) will create a default constructed MyStruct and assign its myInt property to 42.

Note that the type should always be default constructible.

EDIT: Note that you don't have to use QML_ELEMENT, Q_DECLARE_METATYPE or qmlRegisterType if you use Q_GADGET and QML_VALUE_TYPE.

This is amazing !!
Gonna have to explore the possibilities...

[GrecKo]

Coming back on what I said:

It is possible to expose an enum from a Q_GADGET.

If you add pragma ValueTypeBehavior: Addressable at the top of your QML file you can use the lower case gadget type name in QML and then do property int enumValue: myStruct.TestEnum.D

Alternatively you could do it very verbosely with QML_FOREIGN_NAMESPACE like explained here : https://codereview.qt-project.org/c/qt/qtdeclarative/+/510832 (fresh out of the oven)