Downcasting Box<Any> to T, or working around



As part of a crazy loosely typed system I have a function that returns a Box<Any +'static> that I can cast to a type I know it is. Unfortunately when I try and do that cast I get this exception:

src/components/ 35:83 error: the trait `core::marker::Reflect` is not implemented for the type `T` [E0277]
src/components/ 	let component_list : &'static SharedMutex<T> = *component_list_any.downcast_ref().expect("Invalid type for components list");

Seems like an easy fix, add Reflect to the type constraint of the Any, but that yields this exception:

src/components/ 12:25 error: use of unstable library feature 'reflect_marker': requires RFC and more experience (see issue #27749)
src/components/ use std::marker::Reflect;

That issue #27749 looks like it won’t soon be solved, some new system that isn’t done yet will first be implemented after which it is a bit unclear for me.

So that leaves me with the question: is there a workaround for this?

Alternatively, maybe I could redesign my system a bit, it’s totally in an exploratory fase, so I’m very much open to suggestions. I have included the full source of the file my problem is in, the problem function is called get_components_lock.

/// Components are types that associate an entity with a set of properties
/// for each type a list of all its instances is kept so they can be iterated
/// over efficiently by systems.
/// Whenever a new component is created it is added to a list.
/// component! { physics, body: physics.RigidBody, physics_id: physics.ID }

use std::collections::{ HashSet, HashMap };
use std::any::{ Any, TypeId };
use shared_mutex::{ SharedMutex, SharedMutexReadGuard, SharedMutexWriteGuard };

pub mod example;

pub struct Component {
	pub name: TypeId,
	pub get_component_list: fn () -> Box<Any + 'static>

lazy_static! {
	pub static ref COMPONENTS: SharedMutex<HashMap<TypeId, Component>> = SharedMutex::new(HashMap::new());

pub fn register(component : Component) {
	let mut components = COMPONENTS.write().expect("COMPONENTS lock corrupted");
	components.insert(, component);

pub fn get_components_lock<'mutex, T>(id : TypeId) -> SharedMutexReadGuard<'mutex, T> where T : 'static {
	let components ="COMPONENTS lock corrupted");
	let component = components.get(&id).expect("Unknown component type requested");
	let component_list_any = (component.get_component_list)();
	let component_list : &'static SharedMutex<T> = *component_list_any.downcast_ref().expect("Invalid type for components list");
	return"Component LIST lock corrupted"); 


What’s the motivation for the design? So you have a known type Component that’s basically just a wrapper for a heap allocated thing that you want to share around?

Instead of externalising the cast from the Box<Any> to T could it be possible to introduce some stronger typing by making Component a trait so you have a RigidBodyPhysics component and a SoftBodyPhysics component for instance, instead of a single definition of Component that needs to do somersaults to abstract types?

Then you wouldn’t need get_component_list because you can downcast from Component to T. Unless there’s something fundamental I’m missing :smile:


Adding T : Any+'static seems to work for your example.


Ahh of course. That totally works, I don’t know why I was thrown off by the error message so much, I constrain to Any in various other places. It’s what I get for late night coding…

Thanks :slight_smile:


Hi KodrAus,

I’m definitely not ruling out there’s a better design possible, but at the moment the components lists are statically defined and I don’t know where, so I need some kind of accessor or generic reference. I went with accessor here, I forgot why.