Borrowing

Shared & borrows

A shared borrow reads data without taking ownership. Many shared borrows may exist at once because none can change the value:

let s = String::from("hi"); let r = &s; println!("{} {}", s, r);

One &mut

A mutable borrow allows change but only one may exist at a time. It must not overlap with any shared borrow:

let mut s = String::from("hi"); let r = &mut s; r.push_str("!");

The two rules

Rule one is many shared borrows or exactly one mutable borrow, never both together. Rule two is that every borrow must stay valid while in use:

let mut s = String::from("hi"); let a = &s; println!("{}", a); let b = &mut s; b.push_str("!");

No dangling (compiler stops you)

A borrow can never outlive its owner. Return owned data from a function instead of a reference to a local value:

fn make() -> String { String::from("hi") }

Next: Lifetimes

Article author: Arthur Isaev