Running QtApp on native c++ engine + rust backend w/o QML-engine

[Kuraga13]

Hello,
I am trying to do something similar to the example demo_thread.
Idea is backend in rust(finally in a separate thread as in the example), and frontend-application in Qt.I mean with a ready-made working Qt application.
Now I'm trying to collect at least something minimal demo.

use core::pin::Pin;

#[cxx_qt::bridge(cxx_file_stem = "backend_interface", namespace = "cxx_qt::backend_interface")]
pub mod qobject {

    extern "RustQt" {
        #[qobject]
        type BackendInterface = super::BackendInterfaceRust;
    }

    unsafe extern "RustQt" {
        #[qinvokable]
        fn test_backend(self: Pin<&mut BackendInterface>);
    }
}

#[derive(Default)]
pub struct BackendInterfaceRust;

impl qobject::BackendInterface {
    pub fn test_backend(self: Pin<&mut Self>) {

        println!("Hi from Rust! test_backend" );

    }
}

#include "cxx-qt-gen/backend_interface.cxxqt.h"
MainWindow::MainWindow(QWidget *parent, Qt::WindowFlags flags):
QMainWindow(parent, flags)
{ 
         .......
	cxx_qt::backend_interface::BackendInterface test;
        test.testBackend();
}

By the output of the compiler, does it necessarily need a qml engine to work?

[build] C:/msys64/mingw32/bin/../lib/gcc/i686-w64-mingw32/13.2.0/../../../../i686-w64-mingw32/bin/ld.exe: libtest_qt_rust.a(qqmlapplicationengine.o):qqmlapplicationengine.cpp:(.text$_ZN4rust9cxxqtlib124qqmlapplicationengineNewEv+0x24): undefined reference to 

1. Is it supported, is it possible?
2. If yes, can you provide a minimal example of binding or suggest how to do it. thanks in advance

[Be-ing]

If you don't want to link to QtQml, disable the qt_qml feature of the cxx-qt-lib crate. It's enabled by default, so you'll need default-features = false in Cargo.toml.

Regardless, that link error is curious. It looks like the error got cut off? Did you copy and paste the whole error?

[ahayzen-kdab]

Note that in addition to demo_threading there is a short example in https://github.com/KDAB/cxx-qt/blob/main/examples/qml_features/rust/src/threading.rs of how to do threading.

And is this a CMake build or a Cargo build you are doing ? this should be possible it just sounds like either you have QML features enabled or we have a bug somewhere that is trying to bind to the QML engine.

[Kuraga13]

@Be-ing & @ahayzen-kdab thanks for the quick replies. Yes, I didn't have default-features = false in cxx-qt-lib, now all is work, thanks.

To build my rather old project from Qt5, I had to take into account some little things. For example setting Q_SLOTS instead of slots.
Now it remains to connect qt_thread, I will keep this discussion open - if something works out, I will create a small-repository with an example.

[ahayzen-kdab]

For the Q_SLOTS vs slots, slots can still work in C++ code if you haven't set QT_NO_KEYWORDS in CMake, we just set that to ensure we don't accidentally use signals or slots in our generated C++ code which can collide with other frameworks / libraries. Or is this not what you are referring too ?

[Kuraga13]

Yes, that's it, thanks.