Downcast

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-rs adds basic downcasting support to trait objects, supporting type parameters and constraints.

To make a trait downcastable, make it extend the downcast::Downcast trait and invoke impl_downcast! on it as follows:

trait Trait: Downcast {}
impl_downcast!(Trait);

// or

trait TraitGeneric<T>: Downcast {}
impl_downcast!(TraitGeneric<T>);

// or

trait TraitGenericConstrained<T: Copy>: Downcast {}
impl_downcast!(TraitGenericConstrained<T> where T: Copy);

// or

// Use this variant when specifying concrete type parameters.
trait TraitGenericConcrete<T: Copy>: Downcast {}
impl_downcast!(concrete TraitGenericConcrete<u32>);

Example without generics

#[macro_use]
extern crate downcast;
use downcast::Downcast;

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

// 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);
}

Example with a generic trait

#[macro_use]
extern crate downcast_rs;
use downcast_rs::Downcast;

// To create a trait with downcasting methods, extend `Downcast` and run
// impl_downcast!() on the trait.
trait Base<T>: Downcast {}
impl_downcast!(Base<T>);   // or: impl_downcast!(concrete Base<u32>);

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

fn main() {
    // Create a trait object.
    let mut base: Box<Base<u32>> = Box::new(Foo(42));

    // Downcast to Foo.
    assert_eq!(base.downcast_ref::<Foo>().unwrap().0, 42);
}

downcast-rs 0.1.1

Downcast

Example without generics

Example with a generic trait

License