Return and ownership

See how returning a value transfers ownership back.

Returning and ownership

When a function returns a value, ownership moves to the caller. This lets values created inside a function be handed out safely:

fn yeni_liste() -> Vec<i32> {
    let v = vec![1, 2, 3];
    v                  // ownership goes to the caller, not dropped
}

fn ekle_unlem(mut s: String) -> String {
    s.push('!');
    s                  // the same String is handed back, no copy
}

Giving ownership and taking it back

To give a value to a function and get it back afterwards, you can return it, but it is clumsy:

fn uzunluk_ver(s: String) -> (String, usize) {
    let n = s.len();
    (s, n)
}
let (s, n) = uzunluk_ver(String::from("Rust"));

That's why Rust uses references for this: the function borrows the value and ownership never changes hands. The next group is about this.

Exercise Transform and give back

Write fn ekle_unlem(mut s: String) -> String; it appends '!' and returns s. Call it with let s = ekle_unlem(String::from("Merhaba")); and print it.

Show example solution

fn ekle_unlem(mut s: String) -> String {
    s.push('!');
    s
}

fn main() {
    let s = ekle_unlem(String::from("Merhaba"));
    println!("{}", s);
}

Test yourself

Can a function return a reference (<code>&String</code>) to its local <code>String</code>?
- Yes, always
- Only with mut
- Only with clone
- No; the value is dropped at the end of the function and the reference would dangle
Show answer

No; the value is dropped at the end of the function and the reference would dangle — The compiler rejects the dangling reference; the solution is to return the String itself and transfer ownership.

Give ownership back

fn buyut(mut s: String) -> String {
    s.make_ascii_uppercase();
    ____
}

Show answer

fn buyut(mut s: String) -> String {
    s.make_ascii_uppercase();
    s
}

Code and result

-> String (local value)

Ownership goes to the caller
-> &String (local value)

Compile error
Returning (s, n)

Giving ownership back
&s parametresi

Borrowing

Next lesson: Summary - Ownership

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