How to return a field from a static structure

January 18, 2022

I have code:

static mut SETTINGS: Option<Settings> = None;

pub struct Settings {
    pub(crate) db_path: String,
    pub(crate) is_debug: bool
}

pub fn create_settings(settings: Settings) {
    unsafe {
        SETTINGS = Option::from(settings)
    }
}

pub mod get {
    pub fn db_path() -> String {
        unsafe {super::SETTINGS.expect("Settings not definition").db_path}
    }
}

And I get errors

error[E0507]: cannot move out of static item `SETTINGS`
  --> src/settings.rs:16:17
   |
16 |         unsafe {super::SETTINGS.expect("Settings not definition").db_path}
   |                 ^^^^^^^^^^^^^^^ move occurs because `SETTINGS` has type `std::option::Option<Settings>`, which does not implement the `Copy` trait

I need a static structure and get fields from it. How do I write getter functions so I can just call functions and get settings instead of passing a reference to a struct. Settings are global and alone

>Solution :

You could use once_cell::OnceCell for that (notice that it can be set just once):

use once_cell::sync::OnceCell;


static SETTINGS: OnceCell<Settings> = OnceCell::new();

#[derive(Debug)]
pub struct Settings {
    pub(crate) db_path: String,
    pub(crate) is_debug: bool
}

pub fn create_settings(settings: Settings) {
    SETTINGS.set(settings).expect("Set should occur only once")
}

pub mod get {
    pub fn db_path<'a>() -> &'a str {
        &super::SETTINGS.get().expect("Settings not definition").db_path
    }
}

Playground

Also notice that the return type of db_path changed, it is now &'a str, you can return a reference to the string. If you really want it to return a String then you would have to Clone it.

Most probably you would need also just plain OnceCell, your methods would be just wrapper around it (access and setting).