Trait std::ops::IndexMut1.0.0 [] [src]

pub trait IndexMut<Idx>: Index<Idx> where Idx: ?Sized {
    fn index_mut(&mut self, index: Idx) -> &mut Self::Output;

The IndexMut trait is used to specify the functionality of indexing operations like container[index] when used in a mutable context.

container[index] is actually syntactic sugar for *container.index_mut(index), but only when used as a mutable value. If an immutable value is requested, the Index trait is used instead. This allows nice things such as v[index] = value if value implements Copy.


A very simple implementation of a Balance struct that has two sides, where each can be indexed mutably and immutably.

use std::ops::{Index,IndexMut};

enum Side {

#[derive(Debug, PartialEq)]
enum Weight {

struct Balance {
    pub left: Weight,
    pub right:Weight,

impl Index<Side> for Balance {
    type Output = Weight;

    fn index<'a>(&'a self, index: Side) -> &'a Weight {
        println!("Accessing {:?}-side of balance immutably", index);
        match index {
            Side::Left => &self.left,
            Side::Right => &self.right,

impl IndexMut<Side> for Balance {
    fn index_mut<'a>(&'a mut self, index: Side) -> &'a mut Weight {
        println!("Accessing {:?}-side of balance mutably", index);
        match index {
            Side::Left => &mut self.left,
            Side::Right => &mut self.right,

fn main() {
    let mut balance = Balance {
        right: Weight::Kilogram(2.5),
        left: Weight::Pound(1.5),

    // In this case balance[Side::Right] is sugar for
    // *balance.index(Side::Right), since we are only reading
    // balance[Side::Right], not writing it.

    // However in this case balance[Side::Left] is sugar for
    // *balance.index_mut(Side::Left), since we are writing
    // balance[Side::Left].
    balance[Side::Left] = Weight::Kilogram(3.0);

Required Methods

The method for the mutable indexing (container[index]) operation