One question: Do you only want to retrieve the exact type that was passed in, or would you want to be able to extract an impl that matches the type actually contained ?
The latter is more difficult to implement (dynamic_cast goes through hoops to check those things), but it is doable if sufficient information is encoded in the v-table. On Fri, Aug 23, 2013 at 5:04 PM, Oren Ben-Kiki <[email protected]> wrote: > Yes, this would be similar to the `Typeable` type class in Haskell. It > queries the vtable-equivalent, which contains stuff like the name of the > type and allows doing `typeof(x)`, dynamic casts, etc. This is heavily > magical (that is, depends on the hidden internal representation) and > properly belongs in the standard platform and not in a user-level library. > > > On Fri, Aug 23, 2013 at 4:40 PM, Niko Matsakis <[email protected]> wrote: > >> 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 >> >> > > _______________________________________________ > Rust-dev mailing list > [email protected] > https://mail.mozilla.org/listinfo/rust-dev > >
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