borrow and borrow_mut

Use RefCell's borrow and borrow_mut methods.

borrow and borrow_mut

RefCell counts how many readers and writers exist at any moment. Calling borrow_mut() while a reader is alive panics: "already borrowed: BorrowMutError". To check instead of panicking, try_borrow and try_borrow_mut return a Result:

let c = RefCell::new(5);
let r1 = c.borrow();
println!("{}", c.try_borrow_mut().is_err());   // true: a reader exists
drop(r1);
*c.borrow_mut() += 1;                         // free now
println!("{}", c.borrow());                   // 6

A common mistake

Keeping a Ref in a variable and trying to write in the same scope produces code that compiles but crashes at runtime:

let liste = RefCell::new(vec![1]);
let ilk = liste.borrow();          // the reader is still alive
// liste.borrow_mut().push(2);     // panic!
let n = ilk[0];
drop(ilk);
liste.borrow_mut().push(n + 1);    // safe

Rule: copy the value, or limit the borrow with a braced block.

Exercise Safe attempt

Create RefCell::new(5). In an inner block take borrow() and print the value and the result of try_borrow_mut().is_err() separated by a space. After the block, increment the value by 1 with borrow_mut and print it.

Show example solution

use std::cell::RefCell;

fn main() {
    let c = RefCell::new(5);
    {
        let r1 = c.borrow();
        let cakisma = c.try_borrow_mut().is_err();
        println!("{} {}", *r1, cakisma);
    }
    *c.borrow_mut() += 1;
    println!("{}", c.borrow());
}

Test yourself

What happens with <code>let a = c.borrow(); let b = c.borrow();</code>?
- No problem; multiple readers are allowed
- Panic
- Compile error
- b becomes None
Show answer

No problem; multiple readers are allowed — The rules match the compile-time rules: many readers are fine, a writer must be alone.

Try without panic

let sonuc = c.____();   // Err(BorrowMutError)

Show answer

let sonuc = c.try_borrow_mut();   // Err(BorrowMutError)

Case and result

borrow + borrow

Allowed
borrow + borrow_mut

Panic
borrow + try_borrow_mut

Err
drop(r) + borrow_mut

Works

Next lesson: Rc<RefCell<T>>

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