patterns/idioms/pass-var-to-closure.md

Pass variables to closure

Description

By default, closures capture their environment by borrowing. Or you can use move-closure to move whole environment. However, often you want to move just some variables to closure, give it copy of some data, pass it by reference, or perform some other transformation.

Use variable rebinding in separate scope for that.

Example

Use

use std::rc::Rc;

let num1 = Rc::new(1);
let num2 = Rc::new(2);
let num3 = Rc::new(3);
let closure = {
    // `num1` is moved
    let num2 = num2.clone();  // `num2` is cloned
    let num3 = num3.as_ref();  // `num3` is borrowed
    move || {
        *num1 + *num2 + *num3;
    }
};

instead of

use std::rc::Rc;

let num1 = Rc::new(1);
let num2 = Rc::new(2);
let num3 = Rc::new(3);

let num2_cloned = num2.clone();
let num3_borrowed = num3.as_ref();
let closure = move || {
    *num1 + *num2_cloned + *num3_borrowed;
};

Advantages

Copied data are grouped together with closure definition, so their purpose is more clear and they will be dropped immediately even if they are not consumed by closure.

Closure uses same variable names as surrounding code whether data are copied or moved.

Disadvantages

Additional indentation of closure body.