RefCell

A RefCell is another way to change values without needing to declare mut. It means "reference cell", and is like a Cell but uses references instead of copies.

We will create a User struct. So far you can see that it is similar to Cell:

use std::cell::RefCell;

#[derive(Debug)]
struct User {
    id: u32,
    year_registered: u32,
    username: String,
    active: RefCell<bool>,
    // Many other fields
}

fn main() {
    let user_1 = User {
        id: 1,
        year_registered: 2020,
        username: "User 1".to_string(),
        active: RefCell::new(true),
    };

    println!("{:?}", user_1.active);
}

This prints RefCell { value: true }.

There are many methods for RefCell. Two of them are .borrow() and .borrow_mut(). With these methods, you can do the same thing you do with & and &mut. The rules are the same:

Many borrows is fine,
one mutable borrow is fine,
but mutable and immutable together is not fine.

So changing the value in a RefCell is very easy:


#![allow(unused)]
fn main() {
// 🚧
user_1.active.replace(false);
println!("{:?}", user_1.active);
}

And there are many other methods like replace_with that uses a closure:


#![allow(unused)]
fn main() {
// 🚧
let date = 2020;

user_1
    .active
    .replace_with(|_| if date < 2000 { true } else { false });
println!("{:?}", user_1.active);
}

But you have to be careful with a RefCell, because it checks borrows at runtime, not compilation time. Runtime means when the program is actually running (after compilation). So this will compile, even though it is wrong:

use std::cell::RefCell;

#[derive(Debug)]
struct User {
    id: u32,
    year_registered: u32,
    username: String,
    active: RefCell<bool>,
    // Many other fields
}

fn main() {
    let user_1 = User {
        id: 1,
        year_registered: 2020,
        username: "User 1".to_string(),
        active: RefCell::new(true),
    };

    let borrow_one = user_1.active.borrow_mut(); // first mutable borrow - okay
    let borrow_two = user_1.active.borrow_mut(); // second mutable borrow - not okay
}

But if you run it, it will immediately panic.

thread 'main' panicked at 'already borrowed: BorrowMutError', C:\Users\mithr\.rustup\toolchains\stable-x86_64-pc-windows-msvc\lib/rustlib/src/rust\src\libcore\cell.rs:877:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
error: process didn't exit successfully: `target\debug\rust_book.exe` (exit code: 101)

already borrowed: BorrowMutError is the important part. So when you use a RefCell, it is good to compile and run to check.