What is a lifetime?
- Rust course
- Intermediate
- Lesson 13/115
- 4 min
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 enoughLifetimes 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.
Practice Static text
Write fn karsilama() -> &'static str returning "merhaba". Print the result and its length separated by a space.
Starter code
fn main() {
// buraya yaz
}Sample solution
fn karsilama() -> &'static str {
"merhaba"
}
fn main() {
let s = karsilama();
println!("{} {}", s, s.len());
}Expected output
merhaba 7
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What does <code>&'static str</code> mean?
- Text on the stack
- A global variable
- A text reference valid until the end of the program Correct answer
- An unchangeable String
Explanation: String literals are embedded in the program binary and are always valid, so their type is &'static str.
2 Static lifetime
fn karsilama() -> &____ str {
"merhaba"
}fn karsilama() -> &'static str {
"merhaba"
}Explanation: String literals live until the end of the program: &'static str.
3 Value and lifetime
| Term | Meaning |
|---|---|
"merhaba" |
'static |
fn f() -> &'static str |
Can return a literal |
let x = 5; &x |
Scope of x |
String::from("a") |
Owned, no lifetime |