Return and ownership
- Rust course
- Beginner
- Lesson 46/115
- 4 min
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.
Practice 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.
Starter code
fn main() {
// buraya yaz
}Sample solution
fn ekle_unlem(mut s: String) -> String {
s.push('!');
s
}
fn main() {
let s = ekle_unlem(String::from("Merhaba"));
println!("{}", s);
}Expected output
Merhaba!
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 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 Correct answer
Explanation: The compiler rejects the dangling reference; the solution is to return the String itself and transfer ownership.
2 Give ownership back
fn buyut(mut s: String) -> String {
s.make_ascii_uppercase();
____
}fn buyut(mut s: String) -> String {
s.make_ascii_uppercase();
s
}Explanation: Writing s as the last expression moves the String's ownership to the caller.
3 Code and result
| Term | Meaning |
|---|---|
-> String (local value) |
Ownership goes to the caller |
-> &String (local value) |
Compile error |
Returning (s, n) |
Giving ownership back |
&s parametresi |
Borrowing |