Rust enums are great for types where all variations are known beforehand. But in the case where you want to implement a container of user-defined types, an open-ended type like a trait object is needed. In some cases, it is useful to cast the trait object back into its original concrete type to access additional functionality and performant inlined implementations.
downcast is an exercise in adding basic down-casting support to trait objects
just as MOPA and
downcast do while avoiding unsafe code and
without replicating the behavior in the standard library. This is at the
(negligible) expense of adding two methods to the down-castable trait's vtable.
To make a trait downcastable, make it extend the downcast::Downcast trait and
invoke downcast_impl! on it as follows:
rust
trait Trait: Downcast {}
downcast_impl!(Trait);
```rust
extern crate downcast; use downcast::Downcast;
// To create a trait with downcasting methods, extend Downcast and run
// downcastimpl!() on the trait.
trait Base: Downcast {}
downcastimpl!(Base);
// Concrete type implementing Base. struct Foo(u32); impl Base for Foo {}
fn main() {
// Create a trait object.
let mut base: Box
// Downcast to Foo.
assert_eq!(base.downcast_ref::<Foo>().unwrap().0, 42);
} ```
Copyright 2015, Ashish Myles. This software is dual-licensed under the MIT and Apache 2.0 licenses.