CODEWITHSUNDEEP
arraysstringrust
camccar
camccar

Reputation: 730

How do I return a fixed size array of string literals from a function?

This function will work for the type i32, but for the type str:

fn getValues() -> [str; 2] {
    [
        "37107287533902102798797998220837590246510135740250",
        "46376937677490009712648124896970078050417018260538",
    ]
}

I get the error:

error[E0277]: the size for values of type `str` cannot be known at compilation time
 --> src/lib.rs:1:1
  |
1 | / fn getValues() -> [str; 2] {
2 | |     [
3 | |         "37107287533902102798797998220837590246510135740250",
4 | |         "46376937677490009712648124896970078050417018260538",
5 | |     ]
6 | | }
  | |_^ doesn't have a size known at compile-time
  |
  = help: the trait `std::marker::Sized` is not implemented for `str`
  = note: to learn more, visit <https://doc.rust-lang.org/book/second-edition/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
  = note: slice and array elements must have `Sized` type

This error makes me think I need to add the size but I did: a size of 2. What is Rust wanting?

Upvotes: 5

Views: 5839

Answers (2)

gimpf
gimpf

Reputation: 4511

str is the str primitive type, which is an unsized type, having some limitations -- you encountered one of them.

To solve your issue, instead of returning a plain str, you need to return a &str, a borrowed string slice. In this specific case you could even use &'static str, because string literals always have a static lifetime.

Also, because a primitive array is also a DST (dynamically sized type, another way to size unsized type), you need to specify the number of elements (which you did correctly).

So, the complete declaration would be fn getValues()-> [&'static str; 100].

If you do not know the size of the array at compile time, you might want to use a Vec, if you have some non-literal strings you likely want to use a String. That would look like

fn getValues() -> Vec<String> { vec!["shoten".into()] }

P.S.: Do not shy away from those heap-allocated, owned types -- they do make programming easier. Fighting with references and lifetimes is good style, but you don't need to do all that upfront, but instead optimize and refactor later.

Upvotes: 10

DK.
DK.

Reputation: 59015

It's saying str doesn't have a fixed size, not that the array doesn't. You can't have values of type str, you can only have them behind some kind of pointer.

Use [&'static str; 100].

Upvotes: 2

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