Change static string value

Question

I have an static mutable value:

static mut ADDRESS: &'static str = "";

It is global, not in any function.

And now i have a problem:
I have functions that read this value, them working correctly, but when i tried to make function that change this value from arguments - it start crying that value "does't live long enough".

#[no_mangle]
pub unsafe extern fn Java_Rust_setAddress(
    env: JNIEnv,
    _: JClass,
    address: JString
) {
    let input: String = env.get_string(address).expect("Couldn't get java string!").into();
    ADDRESS = &input /Error there/
}

What can i do with it? Transfering address in each case is not a variant, i really need a simple global variable.

ADDRESS = input.as_str()

not works too.

But

ADDRESS = "Something"

works good.

Your `String` will get dropped after your function is finished. A reference to it can't go in a static variable with a static lifetime. — Aplet123, Dec 12 '20 at 15:49
Does this answer your question? [How do I create a global, mutable singleton?](https://stackoverflow.com/questions/27791532/how-do-i-create-a-global-mutable-singleton) — pretzelhammer, Dec 12 '20 at 15:49
@Aplet123 it's not critical for me make ADDRESS value from variable or from variable value. I can't believe that programming language made in 2010 and with so big community don't have so simple functionality as i need. — Gohryt, Dec 12 '20 at 15:57

score 2 · Answer 1 · answered Dec 12 '20 at 16:09

Your String will get dropped after the function is finished. Therefore, you cannot store a reference to in a static variable:

ADDRESS = &input;
       // ^^^^^^ input does not live long enough
       // ADDRESS requires that `input` is borrowed for `'static`
}
// < `input` dropped here while still borrowed by ADDRESS

Instead, you need to store a global heap-allocated String. There are a couple ways to have a global heap-allocated variable as explained here. In your case, you can use a thread_local global static variable and wrap the string in a Cell, which provides mutability:

use std::cell::Cell;

thread_local!(static ADDRESS: Cell<String> = Cell::new("".to_string()));

#[no_mangle]
pub unsafe extern fn Java_Rust_setAddress(
    env: JNIEnv,
    _: JClass,
    address: JString
) {
    let input: String = env.get_string(address).expect("Couldn't get java string!").into();
    ADDRESS.with(|address| address.set(input));
}

Change static string value

1 Answers1