# downcast-rs
[](https://github.com/marcianx/downcast-rs/actions)
[](https://crates.io/crates/downcast-rs)
[](https://docs.rs/downcast-rs)
Rust enums are great for types where all variations are known beforehand. But a
container of user-defined types requires an open-ended type like a **trait
object**. Some applications may want to cast these trait objects back to the
original concrete types to access additional functionality and performant
inlined implementations.
`downcast-rs` adds this downcasting support to trait objects using only safe
Rust. It supports **type parameters**, **associated types**, and **constraints**.
## Usage
Add the following to your `Cargo.toml`:
```toml
[dependencies]
downcast-rs = "2.0.1"
```
This crate is `no_std` compatible. To use it without `std`:
```toml
[dependencies]
downcast-rs = { version = "2.0.1", default-features = false }
```
To make a trait downcastable, make it extend either `downcast::Downcast` or
`downcast::DowncastSync` and invoke `impl_downcast!` on it as in the examples
below.
Since 2.0.0, the minimum supported Rust version is 1.56.
```rust
# use downcast_rs::{Downcast, impl_downcast};
# #[cfg(feature = "sync")]
# use downcast_rs::DowncastSync;
trait Trait: Downcast {}
impl_downcast!(Trait);
// Also supports downcasting `Arc`-ed trait objects by extending `DowncastSync`
// and starting `impl_downcast!` with `sync`.
# #[cfg(feature = "sync")]
trait TraitSync: DowncastSync {}
# #[cfg(feature = "sync")]
impl_downcast!(sync TraitSync);
// With type parameters.
trait TraitGeneric1<T>: Downcast {}
impl_downcast!(TraitGeneric1<T>);
// With associated types.
trait TraitGeneric2: Downcast { type G; type H; }
impl_downcast!(TraitGeneric2 assoc G, H);
// With constraints on types.
trait TraitGeneric3<T: Copy>: Downcast {
type H: Clone;
}
impl_downcast!(TraitGeneric3<T> assoc H where T: Copy, H: Clone);
// With concrete types.
trait TraitConcrete1<T: Copy>: Downcast {}
impl_downcast!(concrete TraitConcrete1<u32>);
trait TraitConcrete2<T: Copy>: Downcast { type H; }
impl_downcast!(concrete TraitConcrete2<u32> assoc H=f64);
# fn main() {}
```
## Example without generics
```rust
# use std::rc::Rc;
# #[cfg(feature = "sync")]
# use std::sync::Arc;
# use downcast_rs::impl_downcast;
# #[cfg(not(feature = "sync"))]
# use downcast_rs::Downcast;
# #[cfg(feature = "sync")]
use downcast_rs::DowncastSync;
// To create a trait with downcasting methods, extend `Downcast` or `DowncastSync`
// and run `impl_downcast!()` on the trait.
# #[cfg(not(feature = "sync"))]
# trait Base: Downcast {}
# #[cfg(not(feature = "sync"))]
# impl_downcast!(Base);
# #[cfg(feature = "sync")]
trait Base: DowncastSync {}
# #[cfg(feature = "sync")]
impl_downcast!(sync Base); // `sync` => also produce `Arc` downcasts.
// Concrete types implementing Base.
#[derive(Debug)]
struct Foo(u32);
impl Base for Foo {}
#[derive(Debug)]
struct Bar(f64);
impl Base for Bar {}
fn main() {
// Create a trait object.
let mut base: Box<dyn Base> = Box::new(Foo(42));
// Try sequential downcasts.
if let Some(foo) = base.downcast_ref::<Foo>() {
assert_eq!(foo.0, 42);
} else if let Some(bar) = base.downcast_ref::<Bar>() {
assert_eq!(bar.0, 42.0);
}
assert!(base.is::<Foo>());
// Fail to convert `Box<dyn Base>` into `Box<Bar>`.
let res = base.downcast::<Bar>();
assert!(res.is_err());
let base = res.unwrap_err();
// Convert `Box<dyn Base>` into `Box<Foo>`.
assert_eq!(42, base.downcast::<Foo>().map_err(|_| "Shouldn't happen.").unwrap().0);
// Also works with `Rc`.
let mut rc: Rc<dyn Base> = Rc::new(Foo(42));
assert_eq!(42, rc.downcast_rc::<Foo>().map_err(|_| "Shouldn't happen.").unwrap().0);
// Since this trait is `Sync`, it also supports `Arc` downcasts.
# #[cfg(feature = "sync")]
let mut arc: Arc<dyn Base> = Arc::new(Foo(42));
# #[cfg(feature = "sync")]
assert_eq!(42, arc.downcast_arc::<Foo>().map_err(|_| "Shouldn't happen.").unwrap().0);
}
```
## Example with a generic trait with associated types and constraints
```rust
use downcast_rs::{Downcast, impl_downcast};
// To create a trait with downcasting methods, extend `Downcast` or `DowncastSync`
// and run `impl_downcast!()` on the trait.
trait Base<T: Clone>: Downcast { type H: Copy; }
impl_downcast!(Base<T> assoc H where T: Clone, H: Copy);
// or: impl_downcast!(concrete Base<u32> assoc H=f32)
// Concrete types implementing Base.
struct Foo(u32);
impl Base<u32> for Foo { type H = f32; }
struct Bar(f64);
impl Base<u32> for Bar { type H = f32; }
fn main() {
// Create a trait object.
let mut base: Box<dyn Base<u32, H=f32>> = Box::new(Bar(42.0));
// Try sequential downcasts.
if let Some(foo) = base.downcast_ref::<Foo>() {
assert_eq!(foo.0, 42);
} else if let Some(bar) = base.downcast_ref::<Bar>() {
assert_eq!(bar.0, 42.0);
}
assert!(base.is::<Bar>());
}
```
## Why no changes in a while?
This library is a thoroughly-tested boilerplate generator, is code complete, has
no unsafe, and is vanishingly unlikely to have any security issues to patch.
## License
Copyright 2020, Ashish Myles (maintainer) and contributors.
This software is dual-licensed under the [MIT](LICENSE-MIT) and
[Apache 2.0](LICENSE-APACHE) licenses.
### Contribution
Unless you explicitly state otherwise, any contribution intentionally submitted
for inclusion in the work by you, as defined in the Apache-2.0 license, shall be
dual licensed as above, without any additional terms or conditions.