What is a lifetime?

Learn what lifetimes do for references.

What is a lifetime?

Every reference has a lifetime: the region of code where it's valid. The compiler checks that a reference doesn't outlive the value it points to by comparing lifetimes. Most of the time lifetimes are inferred and never written:

let r;                 // ---------+-- lifetime of r
{                      //          |
    let x = 5;         // -+-- x   |
    r = &x;            //  |       |
}                      // -+ x ends
// println!("{r}");    // error E0597: x doesn't live long enough

Lifetimes do nothing at runtime; they are purely a compile-time check.

The 'static lifetime

'static is the longest lifetime. String literals and static variables have it, so they can be returned safely from a function:

fn karsilama() -> &'static str {
    "merhaba"
}

static SURUM: &str = "1.0";

Even if a compiler error suggests 'static, the right fix is often to return an owned value (String) instead of a reference.

Exercise Static text

Write fn karsilama() -> &'static str returning "merhaba". Print the result and its length separated by a space.

Show example solution

fn karsilama() -> &'static str {
    "merhaba"
}

fn main() {
    let s = karsilama();
    println!("{} {}", s, s.len());
}

Test yourself

What does <code>&'static str</code> mean?
- Text on the stack
- A global variable
- A text reference valid until the end of the program
- An unchangeable String
Show answer

A text reference valid until the end of the program — String literals are embedded in the program binary and are always valid, so their type is &'static str.

Static lifetime

fn karsilama() -> &____ str {
    "merhaba"
}

Show answer

fn karsilama() -> &'static str {
    "merhaba"
}

Value and lifetime

"merhaba"

'static
fn f() -> &'static str

Can return a literal
let x = 5; &x

Scope of x
String::from("a")

Owned, no lifetime

Next lesson: Explicit lifetimes

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