downcast-rs 0.1.0

Trait object downcasting support implemented with safe code.


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:

trait Trait: Downcast {}


extern crate downcast;
use downcast::Downcast;

// To create a trait with downcasting methods, extend `Downcast` and run
// downcast_impl!() on the trait.
trait Base: Downcast {}

// Concrete type implementing Base.
struct Foo(u32);
impl Base for Foo {}

fn main() {
    // Create a trait object.
    let mut base: Box<Base> = Box::new(Foo(42));

    // 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.