Currently, this is not directly supported, though downcasting in
general is something we have contemplated as a feature. It might be
possible to create some kind of horrible hack based on objects. A
trait like:
trait Dynamic { }
impl<T> Dynamic for T { }
would allow any value to be cast to an object. The type descriptor can
then be extracted from the vtable of the object using some rather
fragile unsafe code that will doubtless break when we change the
vtable format. The real question is what you can do with the type
descriptor; they are not canonicalized, after all. Still, it's
... very close. This is basically how dynamic downcasting would work,
in any case.
Niko
On Fri, Aug 23, 2013 at 07:49:57AM +0300, Oren Ben-Kiki wrote:
> Is it possible to implement something like Haskell's Dynamic value holder
> in Rust? (This would be similar to supporting C++'s dynamic_cast).
> Basically, something like this:
>
> pub struct Dynamic { ... }
> impl Dynamic {
> pub fn put(value: ~T) { ... }
> pub fn get() -> Option<T> { ... }
> }
>
> I guess this would require unsafe code... even so, it seems to me that Rust
> pointers don't carry sufficient meta-data for the above to work. A possible
> workaround would be something like:
>
> pub struct Dynamic { type_name: ~str, ... }
> impl Dynamic {
> pub fn put(type_name: &str, value: ~T) { Dynamic { type_name:
> type_name, ... } }
> pub fn get(&'a self, type_name: &str) -> Option<&'a T> {
> assert_eq!(type_name, self.type_name); ... } }
> }
>
> And placing the burden on the caller to always use the type name "int" when
> putting or getting `int` values, etc. This would still require some sort of
> unsafe code to cast the `~T` pointer into something and back, while
> ensuring that the storage for the `T` (whatever its size is) is not
> released until the `Dynamic` itself is.
>
> (Why do I need such a monstrosity? Well, I need it to define a
> `Configuration` container, which holds key/value pairs where whoever sets a
> value knows its type, whoever gets the value should ask for the same type,
> and the configuration can hold values of "any" type - not from a predefined
> list of types).
>
> Is such a thing possible, and if so, how?
>
> Thanks,
>
> Oren Ben-Kiki
> _______________________________________________
> Rust-dev mailing list
> [email protected]
> https://mail.mozilla.org/listinfo/rust-dev
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