Casting
With the base attribute, it is possible to inherit from another type.
In order to access this parent class, we provide an API to cast up or down.
Currently, this is supported for objects in both extern "RustQt" and extern "C++Qt" blocks, which have either a #[qobject] attribute,
or a #[base = T] attribute. See the attributes documentation for more details on these attributes.
Note: Types in "C++Qt" blocks are required to have the
#[qobject]attribute
Accessing the base class
To access the methods of a base class in Rust, use the Upcast trait like so use cxx_qt::casting::Upcast;.
Objects with base classes can then be accessed with the following methods
| Self Type | Method |
|---|---|
&self | upcast() |
&mut self | upcast_mut() |
Pin<&mut self> | upcast_pin() |
This will then return a reference to the base in the same format as the self type, e.g. upcast() returns &Base, etc...
Accessing the child class
This also works in the opposite direction, allowing access to the child a base class was obtained from.
To do this, use the Downcast trait like so use cxx_qt::Downcast;.
The child can then be accessed in the same manner, with the following methods
| Self Type | Method |
|---|---|
&self | downcast() |
&mut self | downcast_mut() |
Pin<&mut self> | downcast_pin() |
These will return an Option<T>, as it is possible that downcasting will fail,
if the type is not actually of the given subclass,
and these also return in the same format as the self type, e.g. downcast() returns Option<&Sub>, etc...
Transitive casting
Given 3 types, where there is a grandparent relationship, e.g. that using 2 casts, you can go from A -> B -> C, CXX-Qt inlcudes a macro for automatically implementing a cast between A and C. This property also extends for longer chains. For example, if you have a deeply nested set of inheritance, you can quickly generate helpers to cast from your child type to any of its ancestors.
use cxx_qt::impl_transitive_cast;
impl_transitive_cast!(A, B, C, D);Will generate casting from A -> C, and A -> D, provided A -> B -> C -> D is already implemented.