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#[macro_export]
macro_rules! impl_dyn_casting {
($trait:ident) => {
impl dyn $trait + '_ {
/// Downcast to the backend concrete type.
///
/// Returns `None` if the object was not from that backend.
pub fn cast_ref<T: $trait>(&self) -> Option<&T> {
let this: &dyn std::any::Any = self;
this.downcast_ref::<T>()
}
/// Mutable downcast to the backend concrete type.
///
/// Returns `None` if the object was not from that backend.
pub fn cast_mut<T: $trait>(&mut self) -> Option<&mut T> {
let this: &mut dyn std::any::Any = self;
this.downcast_mut::<T>()
}
/// Owned downcast to the backend concrete type.
///
/// Returns `Err` with `self` if the object was not from that backend.
pub fn cast<T: $trait>(self: Box<Self>) -> Result<Box<T>, Box<Self>> {
if self.cast_ref::<T>().is_some() {
let this: Box<dyn std::any::Any> = self;
// Unwrap is okay, we just checked the type of `self` is `T`.
Ok(this.downcast::<T>().unwrap())
} else {
Err(self)
}
}
}
};
}
pub use impl_dyn_casting;
#[cfg(test)]
mod tests {
use std::any::Any;
struct Foo;
trait FooTrait: Any {}
impl FooTrait for Foo {}
impl_dyn_casting!(FooTrait);
#[test]
fn dyn_casting() {
let foo_owned: Box<dyn FooTrait> = Box::new(Foo);
assert!(foo_owned.cast::<Foo>().is_ok());
let mut foo = Foo;
let foo_ref: &mut dyn FooTrait = &mut foo;
assert!((foo_ref).cast_ref::<Foo>().is_some());
assert!((&&&&foo_ref).cast_ref::<Foo>().is_some());
assert!(foo_ref.cast_mut::<Foo>().is_some());
}
}