WIP add book entry for instantiating in Rust

Author: BenFordTytherington

book/src/concepts/instantiating_in_rust.md

@@ -0,0 +1,24 @@
+<!--
+SPDX-FileCopyrightText: 2025 Klarälvdalens Datakonsult AB, a KDAB Group company <[email protected]>
+SPDX-FileContributor: Ben Ford <[email protected]>
+
+SPDX-License-Identifier: MIT OR Apache-2.0
+-->
+
+# Instantiating `QObject`s directly in Rust
+
+Your `QObject` types will most likely be instantiated via QML, but it is possible to create them in Rust via a template.
+By adding
+
+```rust
+#[namespace = "rust::cxxqtlib1"]
+unsafe extern "C++" {
+    include!("cxx-qt-lib/common.h");
+
+    #[cxx_name = "make_unique"]
+    #[doc(hidden)]
+    fn myobject_make_unique() -> UniquePtr<MyObject>;
+}
+```
+
+You can directly create an instance of your object wrapped in a `UniquePtr` within Rust, should you wish.

book/src/getting-started/1-qobjects-in-rust.md

@@ -70,6 +70,8 @@ In comparison to a normal opaque CXX class, the mapping between the `QObject` su
 
 The `QObject` subclass is its own type in Rust, as well as in C++.
 When such a `QObject` is instantiated, it will always also construct an instance of the Rust `struct`.
+These `QObject`s can also be constructed via a `UniquePtr<YourObject>` directly in rust, but this is a less common use case,
+see [here](../concepts/instantiating_in_rust.md) for more information.
 The `QObject` can then refer to the underlying Rust `struct` for property access.
 Any behavior of this `QObject` subclass will also be defined in Rust, e.g. using the [`#[qinvokable]`](../bridge/extern_rustqt.html#invokables) attribute.
 The Rust implementation also has access to the underlying Rust `struct` to modify any Rust data.

examples/qml_minimal/rust/src/cxxqt_object.rs

@@ -33,6 +33,15 @@ pub mod qobject {
         #[namespace = "my_object"]
         type MyObject = super::MyObjectRust;
     }
+
+    #[namespace = "rust::cxxqtlib1"]
+    unsafe extern "C++" {
+        include!("cxx-qt-lib/common.h");
+
+        #[cxx_name = "make_unique"]
+        #[doc(hidden)]
+        fn myobject_make_unique() -> UniquePtr<MyObject>;
+    }
     // ANCHOR_END: book_rustobj_struct_signature
 
     // ANCHOR: book_rustobj_invokable_signature
@@ -50,6 +59,7 @@ pub mod qobject {
 }
 
 // ANCHOR: book_use
+use crate::cxxqt_object::qobject::myobject_make_unique;
 use core::pin::Pin;
 use cxx_qt_lib::QString;
 // ANCHOR_END: book_use