How I can mutate a struct's field from a method?

Question

I want to do this:

struct Point {
    x: i32,
    y: i32,
}

impl Point {
    fn up(&self) {
        self.y += 1;
    }
}

fn main() {
    let p = Point          


        

Answer 1

By using Cell you can emulate field-level mutability:

use std::cell::Cell;

struct Point {
    x: i32,
    y: Cell,
}

impl Point {
    fn up(&self) {
        self.y.set(self.y.get() + 1);
    }
}

fn main() {
    let p = Point { x: 0, y: Cell::new(0) };
    p.up();
    println!("y: {:?}", p.y);
}

This will print y: Cell { value: 7 } and we've successfully updated y.

Additionally, if you are using nightly channel, you can declare #![feature(cell_update)] on top of your .rs file and use the following syntax inside your up() method:

impl Point {
    fn up(&self) {
        self.y.update(|x| x + 1);
    }
}

Note: This feature above is a nightly-only experimental API.

From The Rust Programming Language at Rust 1.7.