How use global static const string?

Question

There is a ALPHABET for arithmetic, it's const and used in a lot of functions. I want make the ALPHABET global, static, and const. Some functions need the ALPHABET type as &String, but str.to_string() function will happen once memory copy. this function is called frequently, so it's not good.

The used function is a library, can't change it.

static ALPHABET: &str = "xxxxxxxxxxxxxxxxxxxxxxx";

fn some1(value: &String, alphabet: &String) -> String {
    xxxx
}

fn main() {
    some1("test", ALPHABET); // Can't build
    some1("test", &ALPHABET.to_string()); // OK, but will happend memory copy.
}

How use the ALPHABET without to_string() function?

See also why [it is discouraged for functions to accept `&String` or `&Vec<_>` parameters](https://stackoverflow.com/questions/40006219). A well designed function would not require that. — E_net4, Nov 25 '20 at 08:56

score 2 · Accepted Answer · edited Nov 25 '20 at 14:53

2

It would be better if your function uses &str instead:

fn some1(value: &str, alphabet: &str) -> String {
    xxxx
}

Playground

Since this is in a library you cannot modify, you could use lazy_static to instantiate a static reference to the String:

use lazy_static::lazy_static; // 1.4.0

lazy_static! {
    static ref APLHABET: String = "xxxxxxxxxxxxxxxxxxxxxxx".to_string();
}

Playground

edited Nov 25 '20 at 14:53

Shepmaster

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answered Nov 25 '20 at 08:51

Netwave

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I know, but the function is a library, can't change it. – Sloong Nov 25 '20 at 09:26

How use global static const string?

1 Answers1