Rust Tutorial · Ownership and Borrowing · Lesson 10 of 29

Rust References and Borrowing

Borrow values with & and &mut instead of moving them, and the rules that prevent data races.

Create a free account to save this page, take notes and track your progress. Sign in

Borrowing

A reference lets you use a value without taking ownership: &value (shared, read-only) or &mut value (exclusive, can modify).

Shared and mutable references

fn total_length(words: &[String]) -> usize {
    words.iter().map(|w| w.len()).sum()
}

fn add_word(words: &mut Vec<String>, word: &str) {
    words.push(word.to_string());
}

fn main() {
    let mut words = vec![String::from("safe"), String::from("fast")];
    add_word(&mut words, "concurrent");
    println!("{} words, {} letters", words.len(), total_length(&words));
    println!("still owned here: {words:?}");
}

Output

3 words, 18 letters
still owned here: ["safe", "fast", "concurrent"]

Run this on your computer — see “Get Started” for setup.

The borrowing rules

At any time you can have either any number of shared references or exactly one mutable reference, and references must always be valid. The compiler (the "borrow checker") enforces this, which is how Rust prevents data races and dangling pointers.

The borrow checker in action

fn main() {
    let mut stock = vec![3, 1, 4];
    let first = &stock[0];          // shared borrow
    println!("first item: {first}");
    // stock.push(9);               // error while `first` is still used later
    stock.push(9);                  // fine: `first` is no longer used
    let biggest = stock.iter().max().unwrap();
    println!("biggest: {biggest}, all: {stock:?}");
}

Output

first item: 3
biggest: 9, all: [3, 1, 4, 9]

Run this on your computer — see “Get Started” for setup.

Test yourself

Which is allowed at the same time?

Many mutable references
Many shared references, or one mutable reference
One shared and one mutable reference
Anything, if you're careful

Show answer

Answer: Many shared references, or one mutable reference

Was this page helpful?