Creating a static const Vec

Question

I am trying to initialize a Vec<String> with some settings that can be reused over my code.

I am using const left: Vec<String> = vec![... but this doesn't work:

error[E0308]: mismatched types
  --> names-generator.rs:2:27
   |
2  | const left: Vec<String> = vec![
   |                           ^ expected slice, found array of 93 elements
   |
   = note: expected type `Box<[std::string::String]>`
   = note:    found type `Box<[&str; 93]>`
   = note: this error originates in a macro outside of the current crate

What is the recommended way of doing something like this?

String literals are of type `&'static str`, not `String`. `String` is a string on heap. — glebm, Mar 13 '17 at 12:53
They are not unrelated; in fact they are strongly related. The **only way** to make a `Vec` *or* a `String` is at runtime because they each require heap allocation. The only way to have a single instance of a runtime value like `Vec` or `String` is through the mechanism of somethign like `InitOnce` or `lazy_static!`. — Shepmaster, Mar 15 '17 at 12:37
the answer bellow works for me and I am not using any of those. — simao, Mar 15 '17 at 14:02

score 60 · Accepted Answer · edited Sep 23 '19 at 17:56

60

Do you want it to be mutable? Do the values have to be Strings? If the answer is "no" to both, you can use an array of string slices ([&str; N]) instead of a Vec<String>:

const LEFT: [&'static str; 3] = ["Hello", "World", "!"];
// or
const LEFT: &'static [&'static str] = &["Hello", "World", "!"];

consts are basically copied wherever they are used, so the second form may be preferable depending on the size of the array.

edited Sep 23 '19 at 17:56

Shepmaster

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answered Mar 13 '17 at 12:51

glebm

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The array is quite big. so probably I don't want it copied everywhere. Still trying to make sense of that signature though. Thanks – simao Mar 13 '17 at 13:46
5

The first one is an array of 3 references to static strings, the second one is a reference to a static array of references to static strings. Static here means the lifetime = lifetime of the program, so effectively stuff that's stored in the DATA section of the executable. I see that the array has 93 elements: 93 64-bit pointers is way smaller than a CPU cache line so the first signature might actually be faster, definitely worth benchmarking. – glebm Mar 13 '17 at 14:00
2

@glebm Not true. A CPU cache line (on typical desktop and mobile CPUs) is 64 bytes. 93 64-bit pointers would occupy 744 bytes in total, or 12 cache lines. – Brennan Vincent Nov 07 '19 at 09:37
1

@BrennanVincent is absolutely right. Also each slice is actually a pointer + length, so it's 16 bytes per slice. – glebm May 08 '20 at 19:38
And if the answer is "yes" to either... we do... what? – Mike 'Pomax' Kamermans Aug 06 '23 at 18:49
@Mike: If the answer is "yes" then initialization at runtime is required, possibly using [lazy_static](https://crates.io/crates/lazy_static) or similar tools to ensure that initialization is performed exactly once. – blerontin Aug 22 '23 at 08:50

Creating a static const Vec

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