How do I set a static lifetime to a returned tuple in Rust?

Question

Assume I have the following constructor that returns a tuple:

pub struct WebCam {
    id: u8
}

impl WebCam {
    fn new() -> (Self, bool) {
        let w = WebCam {id: 1 as u8};
        return (w, false);
    }
}

pub fn main() {
    static (cam, isRunning): (WebCam, bool) = WebCam::new();
}

The above code does not compile. However, if I change static to let it compiles just fine. I'm not sure how to set the lifetime on the individual values of the returned tuple (in one line)?

Answer 1

https://doc.rust-lang.org/reference/items/static-items.html

It can be confusing whether or not you should use a constant item or a static item. Constants should, in general, be preferred over statics unless one of the following are true:

Large amounts of data are being stored
The single-address property of statics is required.
Interior mutability is required.

A static item is similar to a constant, except that it represents a precise memory location in the program. All references to the static refer to the same memory location. Static items have the static lifetime, which outlives all other lifetimes in a Rust program. Non-mut static items that contain a type that is not interior mutable may be placed in read-only memory. Static items do not call drop at the end of the program.

All access to a static is safe, but there are a number of restrictions on statics:

The type must have the Sync trait bound to allow thread-safe access. Statics allow using paths to statics in the constant expression used to initialize them, but statics may not refer to other statics by value, only through a reference. Constants cannot refer to statics.

I would rewrite you code like this:

pub struct WebCam {
    id: u8,
}

impl WebCam {
    fn new() -> (Self, bool) {
        (WebCam { id: 1u8 }, false)
    }
}

pub fn main() {
    let (cam, is_running) = WebCam::new();
    println!("{} {}", cam.id, is_running);
}

This works too:

pub struct WebCam {
    id: u8,
}

impl WebCam {
    fn new() -> (Self, bool) {
        (WebCam { id: 1u8 }, false)
    }
}
static mut IS_RUNNING: bool = false;
static mut WEB_CAM: WebCam = WebCam { id: 0u8 };

pub fn main() {
    let (cam, is_running) = WebCam::new();

    unsafe {
        IS_RUNNING = is_running;
        WEB_CAM.id = cam.id;
    }

    println!("{} {}", cam.id, is_running);
}