Explicit lifetimes

Specify explicit lifetime parameters with <'a>.

Explicit lifetimes

If a function takes two references and returns one of them, the compiler can't know which one the returned reference depends on. A lifetime parameter says this explicitly:

fn en_uzun<'a>(x: &'a str, y: &'a str) -> &'a str {
    if x.len() >= y.len() { x } else { y }
}

It reads: "the returned reference lives as long as both x and y are valid". Lifetime names are short and lowercase: 'a, 'b.

Checking at the call site

The compiler picks 'a as the shorter of the two arguments' lifetimes. If the result is used longer than that, it errors:

let a = String::from("uzun metin");
let sonuc;
{
    let b = String::from("kısa");
    sonuc = en_uzun(&a, &b);
}
// println!("{sonuc}");   // error: b doesn't live long enough

Even though the result here is actually a, the compiler doesn't look at runtime values; it decides by the contract in the signature.

Exercise Longest text

Write the function fn en_uzun<'a>(x: &'a str, y: &'a str) -> &'a str above. Print the result of en_uzun("elma", "karpuz").

Show example solution

fn en_uzun<'a>(x: &'a str, y: &'a str) -> &'a str {
    if x.len() >= y.len() {
        x
    } else {
        y
    }
}

fn main() {
    println!("{}", en_uzun("elma", "karpuz"));
}

Test yourself

Does a lifetime parameter change how long a value lives?
- It decides at runtime
- No; it only describes the relationship between references
- Yes, it extends it
- Yes, it shortens it
Show answer

No; it only describes the relationship between references — Lifetime annotations are a contract; the compiler checks at call sites whether the contract is kept.

Lifetime parameter

fn en_uzun____(x: &'a str, y: &'a str) -> &'a str {
    if x.len() >= y.len() { x } else { y }
}

Show answer

fn en_uzun<'a>(x: &'a str, y: &'a str) -> &'a str {
    if x.len() >= y.len() { x } else { y }
}

Syntax and meaning

<'a>

Lifetime parameter
&'a str

Reference living for 'a
-> &'a str

Return tied to the inputs
E0106

Missing lifetime specifier

Next lesson: Function lifetimes

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