downcast-rs

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, associated types, and constraints.

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

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

// or

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

// or

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

// or

trait TraitGenericAssociatedConstrained: Downcast { type H: Clone; } impl_downcast!(TraitGenericAssociatedConstrained assoc H where T: Copy, H: Clone);

// or

// Use these variants when specifying concrete type parameters. trait TraitGenericConcrete: Downcast {} impl_downcast!(concrete TraitGenericConcrete);

trait TraitGenericAssociatedConcrete: Downcast { type H; } impl_downcast!(concrete TraitGenericAssociatedConcrete assoc H=f64); ```

Example without generics

```rust

[macro_use]

extern crate downcastrs; use downcastrs::Downcast;

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

// Concrete types implementing Base. struct Foo(u32); impl Base for Foo {} struct Bar(f64); impl Base for Bar {}

fn main() { // Create a trait object. let mut base: Box = 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>());

} ```

Example with a generic trait with associated types and constraints

```rust

[macro_use]

extern crate downcastrs; use downcastrs::Downcast;

// To create a trait with downcasting methods, extend Downcast and run // impldowncast!() on the trait. trait Base: Downcast { type H: Copy; } impldowncast!(Base assoc H where T: Clone, H: Copy); // or: impl_downcast!(concrete Base assoc H=f32)

// Concrete types implementing Base. struct Foo(u32); impl Base for Foo { type H = f32; } struct Bar(f64); impl Base for Bar { type H = f32; }

fn main() { // Create a trait object. let mut base: Box> = 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 2015, Ashish Myles. This software is dual-licensed under the MIT and Apache 2.0 licenses.