How do i use non-QObject classes from QtCore like QStandardPaths?

[machineonamission]

I cannot for the life of me figure out how to link QStandardPaths so that it can be called from rust. The class has static methods that I'd like to call.

[ahayzen-kdab]

Hey, so as QStandardPaths is a normal C++ class an not a QObject / class with signals you can use pure CXX extern blocks for this. So you would use an extern "C++" block to represent it https://cxx.rs/extern-c++.html

Then recently CXX has added support for defining static member functions via the Self attribute see here https://cxx.rs/attributes.html#self (note before this support was merged you had to create a normal function that then called the static method to be able to reach it in CXX).

So with this new support it'd likely look something like

#[cxx::bridge]
mod ffi {
    unsafe extern "C++" {
        include!("cxx-qt-lib/qstring.h");
        type QString = cxx_qt_lib::QString;
    }
    unsafe extern "C++" {
        include!(<QtCore/QStandardPaths>);
        type QStandardPaths;

        #[Self = "QStandardPaths"]  // <- the Self here means that it is a static method
        fn displayName(type: QStandardPathsStandardLocation) -> QString;
    }

    // defining the enum is slightly fun as it's within the QStandardPaths rather than a namespace
    // we define an extern enum https://cxx.rs/shared.html#extern-enums
    #[repr(i32)]
    #[derive(Copy, Clone, Debug, ...)] // <- choose your derives
    enum QStandardPathsStandardLocation {
        DesktopLocation,
        DocumentsLocation,
        ... // <- match these to the Qt ones
    }

    unsafe extern "C++" {
        include!("myheader.h");  // <- have a header in your include path where we define the below
        type QStandardPathsStandardLocation;  // <- this then points to the alias in C++
    }
}

For the enum to work you'd need a small bit of C++ in the myheader.h;

// As CXX cannot reach the enum inside the class we define an alias
using QStandardPathsStandardLocation = QStandardPaths::StandardLocation;

I have not tested this but hopefully this gives you an idea, for the enum parts consider looking at what has been done in cxx-qt-lib for other similar enums. Normally it's a matter of getting your C++ into types and structure that can be represented in CXX, which sometimes means adding extra C++ code to bridge between 😅