Need to be able to choose the parent when deriving from different bases

[ahayzen-kdab]

Currently we assume that it'll be QObject* parent = nullptr, but for some Qt types (eg QQuickPaintedItem or QQuickItem) the parent is different.

We need to have a way in the cxx_qt::qobject macro to be able to specify a different parent other than QObject.

There seems to be these options

• If we can assume all parent types will be a raw pointer then just have #[cxx_qt::qobject(base = "QQuickPaintedItem", parent = "QQuickItem"]
• If we can't assume this then we might need something like #[cxx_qt::qobject(base = "QQuickPaintedItem", parent_ptr = "QQuickItem"] or #[cxx_qt::qobject(base = "QQuickPaintedItem", parent = "QQuickItem*", parent_default = "nullptr"]
• Have a separate macro for items that aren't directly derived from QObject, eg have #[cxx_qt::qquickitem(base = "QQuickPaintedItem")] this then means that we can generate the different asserts and parent etc and it's clear to the developer which base the type is deriving from

[LeonMatthesKDAB]

Could this be fixed by specifying our own constructor API 🤔 : #339

Would be a more general solution to the problem, right?

[ahayzen-kdab]

Potentially 🤔

So we then might have to get the following things into the same constructor feature

• Picking a different parent type
• Triggering code in the context of C++ from the constructor
• How do we represent that QObject* parent has a default value of nullptr ?
• Adding more args to the constructor (what happens with QML here though?)

[ahayzen-kdab]

We could make Option<*mut T> mean that it is a pointer that defaults to nullptr, but slightly odd syntax 🤔

[ahayzen-kdab]

Or do we use an attribute? As this appears to be supported by Syn in PatType https://docs.rs/syn/2.0.12/syn/struct.PatType.html

fn construct(#[default = "nullptr"] parent: *mut QQuickItem)

[ahayzen-kdab]

I've created an experiment in #513 of using the attribute route for the parent as the constructor route is much more complex. This then allows you to choose a different parent then allowing you to create a QQuickPaintedItem in Rust with no C++ :-)