Rust Foundations·Ownership as a design tool

Ownership and borrowing

Every value has one owner; references let you borrow without taking ownership.

Intermediate 15 min read Lesson 6 of 300 Challenge included

What you will be able to do

Moving a value transfers ownership; the original binding can no longer be used.
Borrow with & for reading and &mut for mutation.
Keep borrows short so the compiler can prove safety.

In plain English

Ownership prevents use-after-free and double-free at compile time. A value can have many immutable references or one mutable reference, not both. The borrow checker is enforcing a rule about aliasing and mutation.

ownership.rsRust

fn longest<'a>(a: &'a str, b: &'a str) -> &'a str {
    if a.len() >= b.len() { a } else { b }
}

fn main() {
    let title = String::from("Rust Foundations");
    let borrowed = &title;
    println!("{}", longest(borrowed, "Go"));
    println!("{}", title);
}

Your turn

Borrow a string

Easy

Write a function that returns the length of a &str without taking ownership.

RUST · starter

fn length(value: String) -> usize {
    value.len()
}

Check your understanding before moving on.

Knowledge checkQuestion 1

How many mutable references can exist to a value at once?

Select an answer to continue

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