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diff --git a/README.md b/README.md new file mode 100644 index 0000000..2c462f7 --- /dev/null +++ b/README.md @@ -0,0 +1,165 @@ +# 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 = "1.2.0" +``` + +This crate is `no_std` compatible. To use it without `std`: + +```toml +[dependencies] +downcast-rs = { version = "1.2.0", 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 1.1.0, the minimum supported Rust version is 1.33 to support `Rc` and `Arc` +in the receiver position. + +```rust +trait Trait: Downcast {} +impl_downcast!(Trait); + +// Also supports downcasting `Arc`-ed trait objects by extending `DowncastSync` +// and starting `impl_downcast!` with `sync`. +trait TraitSync: DowncastSync {} +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); +``` + +## Example without generics + +```rust +// Import macro via `macro_use` pre-1.30. +#[macro_use] +extern crate downcast_rs; +use downcast_rs::DowncastSync; + +// To create a trait with downcasting methods, extend `Downcast` or `DowncastSync` +// and run `impl_downcast!()` on the trait. +trait Base: DowncastSync {} +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<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<Base>` into `Box<Bar>`. + let res = base.downcast::<Bar>(); + assert!(res.is_err()); + let base = res.unwrap_err(); + // Convert `Box<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<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. + let mut arc: Arc<Base> = Arc::new(Foo(42)); + 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 +// Can call macro via namespace since rust 1.30. +extern crate downcast_rs; +use downcast_rs::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; } +downcast_rs::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<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>()); +} +``` + +## 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. |