Convert Vec<String> into a slice of &str in Rust?
Per Steve Klabnik's writeup in the pre-Rust 1.0 documentation on the difference between String
and &str
, in Rust you should use &str
unless you really need to have ownership over a String
. Similarly, it's recommended to use references to slices (&[]
) instead of Vec
s unless you really need ownership over the Vec
.
I have a Vec<String>
and I want to write a function that uses this sequence of strings and it doesn't need ownership over the Vec
or String
instances, should that function take &[&str]
? If so, what's the best way to reference the Vec<String>
into &[&str]
? Or, is this coercion overkill?
Solution 1:
You can create a function that accepts both &[String]
and &[&str]
using the AsRef
trait:
fn test<T: AsRef<str>>(inp: &[T]) {
for x in inp { print!("{} ", x.as_ref()) }
println!("");
}
fn main() {
let vref = vec!["Hello", "world!"];
let vown = vec!["May the Force".to_owned(), "be with you.".to_owned()];
test(&vref);
test(&vown);
}
Solution 2:
This is actually impossible without memory allocation1.
Going from String
to &str
is not just viewing the bits in a different light; String
and &str
have a different memory layout, and thus going from one to the other requires creating a new object. The same applies to Vec
and &[]
Therefore, whilst you can go from Vec<T>
to &[T]
, and thus from Vec<String>
to &[String]
, you cannot directly go from Vec<String>
to &[&str]
. Your choices are:
- either accept
&[String]
- allocate a new
Vec<&str>
referencing the firstVec
, and convert that into a&[&str]
As an example of the allocation:
fn usage(_: &[&str]) {}
fn main() {
let owned = vec![String::new()];
let half_owned: Vec<_> = owned.iter().map(String::as_str).collect();
usage(&half_owned);
}
1The conversion required is impossible, however using generics and the AsRef<str>
bound as shown in @aSpex's answer you get a slightly more verbose function declaration with the flexibility you were asking for.