Add Transitive casting and testing code

Author: BenFordTytherington

crates/cxx-qt-gen/src/generator/rust/fragment.rs

@@ -89,12 +89,6 @@ impl GeneratedRustFragment {
                         }
                     }
                 },
-                // Add back once we figure out the bug with QObject, for automatic transitive casts
-                // parse_quote! {
-                //     unsafe impl ::cxx_qt::MainCast for #struct_name {
-                //        type Base = #base_qualified;
-                //     }
-                // }
             ],
         })
     }

crates/cxx-qt/src/lib.rs

@@ -178,32 +178,6 @@ pub unsafe trait Upcast<T> {
     }
 }
 
-/// Docs
-pub unsafe trait MainCast: Upcast<Self::Base> {
-    /// The first parent of the Type
-    type Base;
-}
-
-unsafe impl<T, A, B> Upcast<B> for T
-where
-    T: MainCast<Base = A>,
-    A: MainCast<Base = B>,
-{
-    unsafe fn upcast_ptr(this: *const Self) -> *const B {
-        let base = Self::upcast_ptr(this);
-        A::upcast_ptr(base)
-    }
-
-    unsafe fn from_base_ptr(base: *const B) -> *const Self {
-        let base = A::from_base_ptr(base);
-        if base.is_null() {
-            std::ptr::null()
-        } else {
-            Self::from_base_ptr(base)
-        }
-    }
-}
-
 /// This trait is automatically implemented by CXX-Qt and you most likely do not need to manually implement it.
 /// Trait for downcasting to a subclass, provided the subclass implements [Upcast] to this type.
 /// Returns `None` in cases where `Sub` isn't a child class of `Self`.

examples/qml_features/rust/src/custom_base_class.rs

@@ -10,9 +10,10 @@
 #[cxx_qt::bridge]
 pub mod qobject {
     // ANCHOR: book_base_include
-    unsafe extern "C++" {
+    unsafe extern "C++Qt" {
         include!(<QtCore/QAbstractListModel>);
         /// Base for Qt type
+        #[qobject]
         type QAbstractListModel;
     }
     // ANCHOR_END: book_base_include
@@ -272,11 +273,44 @@ pub mod qobject {
     }
 }
 
-use crate::custom_base_class::qobject::CustomBaseClass;
+macro_rules! impl_transitive_cast {
+    ($first:ty, $second:ty, $third:ty) => {
+        unsafe impl cxx_qt::Upcast<$third> for $first {
+            unsafe fn upcast_ptr(this: *const Self) -> *const $third {
+                let base = Self::upcast_ptr(this);
+                <$second as Upcast<$third>>::upcast_ptr(base)
+            }
+
+            unsafe fn from_base_ptr(base: *const $third) -> *const Self {
+                let base = <$second as Upcast<$third>>::from_base_ptr(base);
+                if base.is_null() {
+                    std::ptr::null()
+                } else {
+                    Self::from_base_ptr(base)
+                }
+            }
+        }
+    };
+}
+
+macro_rules! chain_cast {
+    ($first:ty, $second:ty, $third:ty) => {
+        impl_transitive_cast!($first, $second, $third);
+    };
+
+    ($first:ty, $second:ty, $third:ty, $($rest:ty),*) => {
+        impl_transitive_cast!($first, $second, $third);
+        $(impl_transitive_cast!($first, $third, $rest);)*
+    };
+}
+
+use crate::custom_base_class::qobject::{CustomBaseClass, AbstractBaseClass, QAbstractListModel, QObject};
 use core::pin::Pin;
-use cxx_qt::{CxxQtType, Threading};
+use cxx_qt::{CxxQtType, Threading, Upcast};
 use cxx_qt_lib::{QByteArray, QHash, QHashPair_i32_QByteArray, QModelIndex, QVariant, QVector};
 
+chain_cast!(CustomBaseClass, AbstractBaseClass, QAbstractListModel, QObject);
+
 impl Default for qobject::State {
     fn default() -> Self {
         Self::Idle
@@ -297,9 +331,16 @@ pub struct CustomBaseClassRust {
     pub(crate) vector: Vec<(u32, f64)>,
 }
 
+
 impl qobject::CustomBaseClass {
     /// Virtual method for logging type
     pub fn log(self: &CustomBaseClass) {
+        let _abs: &AbstractBaseClass = self.upcast();
+        let _qabs: &QAbstractListModel = self.upcast();
+        let qobj: &QObject = self.upcast();
+
+        qobj.dump_object_info();
+
         println!(
             "state: {}\npending adds: {}\nid: {}\nvector: {:?}\n",
             self.state.repr, self.pending_adds, self.id, self.vector

examples/qml_minimal/qml/main.qml

@@ -28,11 +28,6 @@ ApplicationWindow {
         string: qsTr("My String with my number: %1").arg(myObject.number)
     }
 
-    A {
-        id: a
-        number: 10
-    }
-
     Column {
         anchors.fill: parent
         anchors.margins: 10
@@ -60,12 +55,6 @@ ApplicationWindow {
             onClicked: myObject.sayHi(myObject.string, myObject.number)
         }
 
-        Button {
-            text: qsTr("Test A")
-
-            onClicked: a.test()
-        }
-
         Button {
             text: qsTr("Quit")