# Functions

Ignoring elision, function signatures with lifetimes have a few constraints:

- any reference must have an annotated lifetime.
- any reference being returned must have the same lifetime as an input or be `static`.

Additionally, note that returning references without input is banned if it would result in returning references to invalid data. The following example shows off some valid forms of functions with lifetimes:

    // One input reference with lifetime `'a` which must live
    // at least as long as the function.
    fn print_one<'a>(x: &'a i32) {
        println!("`print_one`: x is {}", x);
    }

    // Mutable references are possible with lifetimes as well.
    fn add_one<'a>(x: &'a mut i32) {
        *x += 1;
    }

    // Multiple elements with different lifetimes. In this case, it
    // would be fine for both to have the same lifetime `'a`, but
    // in more complex cases, different lifetimes may be required.
    fn print_multi<'a, 'b>(x: &'a i32, y: &'b i32) {
        println!("`print_multi`: x is {}, y is {}", x, y);
    }

    // Returning references that have been passed in is acceptable.
    // However, the correct lifetime must be returned.
    fn pass_x<'a, 'b>(x: &'a i32, _: &'b i32) -> &'a i32 { x }

    //fn invalid_output<'a>() -> &'a String { &String::from("foo") }
    // The above is invalid: `'a` must live longer than the function.
    // Here, `&String::from("foo")` would create a `String`, followed by a
    // reference. Then the data is dropped upon exiting the scope, leaving
    // a reference to invalid data to be returned.

    fn main() {
        let x = 7;
        let y = 9;

        print_one(&x);
        print_multi(&x, &y);

        let z = pass_x(&x, &y);
        print_one(z);

        let mut t = 3;
        add_one(&mut t);
        print_one(&t);
    }

### See also:

functions