Ownership and Borrowing

Rust References and Borrowing

Let a function use a value without taking it: & to read, &mut to change.

Intermediate
3 min read
1 runnable example

What you'll learn

& and &mut
Borrowing in function calls
Many readers or one writer

Run code →

Borrow → what it allows

length(&message)read only

add_exclamation(&mut message)can change it

let first = &messagemany readers are fine

Use a value without owning it

A reference lets code use a value without owning it. This is called borrowing. &value borrows for reading and &mut value borrows for changing.

length only reads the text, so it takes &str and message can still be used afterwards. add_exclamation needs &mut String to change it. Rust allows many readers at once or one writer, but never both, which rules out data races at compile time.

Key points

&value borrows for reading.
&mut value borrows for changing.
Many readers or one writer, never both.

Runnable Rust example

Rust

Run code →

main.rs

fn length(text: &str) -> usize {
    text.len()
}

fn add_exclamation(text: &mut String) {
    text.push('!');
}

fn main() {
    let mut message = String::from("Borrow me");

    let size = length(&message); // immutable borrow
    println!("{message} has {size} bytes");

    add_exclamation(&mut message); // mutable borrow
    println!("{message}");

    let first = &message;
    let second = &message; // many readers are fine
    println!("{first} {second}");
}

Get a Run button for this example Result of running the code

Borrow me has 9 bytes
Borrow me!
Borrow me! Borrow me!

Practice

Open the example in the online compiler, change a value or add a line, and predict the output before you run it. Then break the code on purpose and read the compiler's message.

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