Explicit lifetimes
- Rust course
- Intermediate
- Lesson 14/115
- 4 min
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 enoughEven though the result here is actually a, the compiler doesn't look at runtime values; it decides by the contract in the signature.
Practice 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").
Starter code
fn main() {
// buraya yaz
}Sample 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"));
}Expected output
karpuz
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Does a lifetime parameter change how long a value lives?
- It decides at runtime
- No; it only describes the relationship between references Correct answer
- Yes, it extends it
- Yes, it shortens it
Explanation: Lifetime annotations are a contract; the compiler checks at call sites whether the contract is kept.
2 Lifetime parameter
fn en_uzun____(x: &'a str, y: &'a str) -> &'a str {
if x.len() >= y.len() { x } else { y }
}fn en_uzun<'a>(x: &'a str, y: &'a str) -> &'a str {
if x.len() >= y.len() { x } else { y }
}Explanation: Lifetime parameters are also declared in angle brackets, with an apostrophe.
3 Syntax and meaning
| Term | Meaning |
|---|---|
<'a> |
Lifetime parameter |
&'a str |
Reference living for 'a |
-> &'a str |
Return tied to the inputs |
E0106 |
Missing lifetime specifier |