Add book entry for instantiating in Rust

Author: BenFordTytherington

book/src/SUMMARY.md

@@ -22,6 +22,7 @@ SPDX-License-Identifier: MIT OR Apache-2.0
   - [Nested Objects](./concepts/nested_objects.md)
   - [Inheritance & Overriding](./concepts/inheritance.md)
   - [Casting](./concepts/casting.md)
+  - [Instantiating in Rust](./concepts/instantiating_in_rust.md)
 - [Reference: the bridge module](./bridge/index.md)
   - [`extern "RustQt"`](./bridge/extern_rustqt.md)
   - [`extern "C++Qt"`](./bridge/extern_cppqt.md)

book/src/concepts/instantiating_in_rust.md

@@ -0,0 +1,57 @@
+<!--
+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,ignore
+#[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.
+The included header file contains some wrapper templates for constructing `unique_ptr<T>`, `shared_ptr<T>` and `*T`.
+By exposing this to the bridge with the correct namespace, constructing these structs is possible in Rust.
+These helper methods live in cxx-qt-lib, and thus need to be included, and this is also why the namespace is necessary.
+These are helper functions defined by CXX-Qt, and are ***Not the same as*** `std::make_unique`, etc...
+
+## Passing Parameters
+
+You can also supply the constructor with parameters via these helper methods,
+but you should ensure that any constructors with different arguments have different names in Rust,  via renaming.
+
+```rust,ignore
+#[namespace = "rust::cxxqtlib1"]
+unsafe extern "C++" {
+    include!("cxx-qt-lib/common.h");
+
+    #[rust_name = "new_my_object_with_parent"]
+    fn make_unique(parent: *mut QObject) -> UniquePtr<MyObject>;
+    
+    // Overloading without the parent parameter
+    #[rust_name = "new_my_object"]
+    fn make_unique() -> UniquePtr<MyObject>;
+}
+```
+
+
+
+## Possible Methods
+
+| Name          | C++ Return Type | Rust Return Type |
+|---------------|-----------------|------------------|
+| `make_unique` | `unique_ptr<T>` | `UniquePtr<T>`   |
+| `make_shared` | `shared_ptr<T>` | `SharedPtr<T>`   |
+| `new_ptr`     | `*T`            | `*mut T`         |

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`.
+The `QObject`s Rust struct can also be directly constructed via a `UniquePtr<YourObject>` 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.