I'd like to create an Rc<str>
because I want reduce the indirection from following the 2 pointers that accessing an Rc<String>
requires. I need to use an Rc
because I truly have shared ownership. I detail in another question more specific issues I have around my string type.
Rc has a ?Sized
bound:
pub struct Rc<T: ?Sized> { /* fields omitted */ }
I've also heard that Rust 1.2 will come with proper support for storing unsized types in an Rc
, but I'm unsure how this differs from 1.1.
Taking the str
case as example, my naive attempt (also this for building from a String
) fails with:
use std::rc::Rc;
fn main() {
let a: &str = "test";
let b: Rc<str> = Rc::new(*a);
println!("{}", b);
}
error[E0277]: the trait bound `str: std::marker::Sized` is not satisfied
--> src/main.rs:5:22
|
5 | let b: Rc<str> = Rc::new(*a);
| ^^^^^^^ `str` does not have a constant size known at compile-time
|
= help: the trait `std::marker::Sized` is not implemented for `str`
= note: required by `<std::rc::Rc<T>>::new`
It's clear that in order to create an Rc<str>
, I need to copy the whole string: RcBox
would be itself an unsized type, storing the string itself alongside the weak and strong pointers — the naive code above doesn't even make sense.
I've been told that one can not instantiate such type, but instead instantiate an Rc<T>
with a sized T
and then coerce it to an unsized type. The example given is for the storing a trait object: first create Rc<ConcreteType>
and then coerce to Rc<Trait>
. But this doesn't make sense either: neither this nor this work (and you can't coerce from &str
or String
to str
anyway).
As of Rust 1.21.0 and as mandated by RFC 1845, creating an Rc<str>
or Arc<str>
is now possible:
use std::rc::Rc;
use std::sync::Arc;
fn main() {
let a: &str = "hello world";
let b: Rc<str> = Rc::from(a);
println!("{}", b);
// or equivalently:
let b: Rc<str> = a.into();
println!("{}", b);
// we can also do this for Arc,
let a: &str = "hello world";
let b: Arc<str> = Arc::from(a);
println!("{}", b);
}
(Playground)
See <Rc as From<&str>>
and <Arc as From<&str>>
.
Creating an Rc<[T]>
can be done via coercions and as
-casts from fixed sized arrays, e.g. coercions can be done as follows:
use std::rc::Rc;
fn main() {
let x: Rc<[i32; 4]> = Rc::new([1, 2, 3, 4]);
let y: Rc<[i32]> = x;
println!("{:?}", y);
}
However, this doesn't work for strings, since they have no raw fixed-sized equivalent to create the first value. It is possible to do unsafely, e.g. by creating a UTF-8 encoded Rc<[u8]>
and transmuting that to Rc<str>
. Theoretically there could be a crate on crates.io for it, but I can't find one at the moment.
An alternative is owning_ref
, which isn't quite std::rc::Rc
itself, but should allow, for example, getting an RcRef<..., str>
pointing into an Rc<String>
. (This approach will work best if one uses RcRef
uniformly in place of Rc
, except for construction.)
extern crate owning_ref;
use owning_ref::RcRef;
use std::rc::Rc;
fn main() {
let some_string = "foo".to_owned();
let val: RcRef<String> = RcRef::new(Rc::new(some_string));
let borrowed: RcRef<String, str> = val.map(|s| &**s);
let erased: RcRef<owning_ref::Erased, str> = borrowed.erase_owner();
}
The erasing means that RcRef<..., str>
s can come from multiple different sources, e.g. a RcRef<Erased, str>
can come from a string literal too.
NB. at the time of writing, the erasure with RcRef
requires a nightly compiler, and depending on owning_ref
with the nightly
feature:
[dependencies]
owning_ref = { version = "0.1", features = ["nightly"] }
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