Functions and ownership

See how passing a value into a function affects ownership.

Functions and ownership

Passing a value to a function is like assignment: non-Copy types are moved into the function and the caller loses them:

fn yazdir(s: String) {
    println!("{s}");
}   // s is dropped here

let ad = String::from("Ada");
yazdir(ad);
// println!("{ad}");   // error: ad was moved

let n = 5;
kare(n);             // the i32 is copied, n is still valid

If the function only reads the value, it should take a reference (&String or &str) instead of ownership.

When to take ownership

If a function will store the value (put it in a struct field, push it to a vector), taking ownership is natural: fn ekle(&mut self, ad: String). If it transforms the value and returns a new one, taking ownership also avoids a needless copy: fn buyut(mut s: String) -> String. Otherwise borrowing is preferred.

Exercise A borrowing function

Write fn uzunluk(s: &str) -> usize. Call uzunluk(&ad) for let ad = String::from("Ada Lovelace");, then print println!("{} {}", ad, n); to show you can still use ad.

Show example solution

fn uzunluk(s: &str) -> usize {
    s.len()
}

fn main() {
    let ad = String::from("Ada Lovelace");
    let n = uzunluk(&ad);
    println!("{} {}", ad, n);
}

Test yourself

What is the problem with the signature <code>fn uzunluk(s: String) -> usize</code>?
- It takes ownership just to read; the caller loses the String
- usize is the wrong type
- A String's length can't be measured
- There is no problem
Show answer

It takes ownership just to read; the caller loses the String — fn uzunluk(s: &str) -> usize is better: it borrows and accepts both String and &str.

Borrowed parameter

fn uzunluk(s: ____) -> usize {
    s.len()
}

Show answer

fn uzunluk(s: &str) -> usize {
    s.len()
}

Signature and meaning

fn f(s: String)

Takes ownership
fn f(s: &str)

Borrows
fn f(n: i32)

Takes a copy
fn f(mut s: String) -> String

Transforms and returns it

Next lesson: Return and ownership

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies