Dangling reference

See how Rust prevents dangling references.

Dangling references

A dangling reference is a reference pointing at a value that no longer exists. This bug, common in C and C++, is prevented at compile time in Rust:

fn sarkan() -> &String {     // error: missing lifetime
    let s = String::from("geçici");
    &s                        // s is dropped here!
}

The fix is to return the value itself instead of a reference: fn yeni() -> String { String::from("geçici") }. Ownership then passes to the caller.

The same bug is caught in inner blocks too:

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

The error message says "borrowed value does not live long enough" and shows where x is dropped. Moving the value to the outer scope fixes it.

Write fn mesaj_uret(ad: &str) -> String; it returns format!("Hoş geldin, {ad}") (not a reference). Print the result of mesaj_uret("Ece").

Show example solution

fn mesaj_uret(ad: &str) -> String {
    format!("Hoş geldin, {ad}")
}

fn main() {
    println!("{}", mesaj_uret("Ece"));
}

Return the value

fn yeni_mesaj() -> ____ {
    String::from("merhaba")
}

Show answer

fn yeni_mesaj() -> String {
    String::from("merhaba")
}