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

#[lang = "index_mut"]
pub trait IndexMut<Idx>: Index<Idx> where
    Idx: ?Sized
{ fn index_mut(&mut self, index: Idx) -> &mut Self::Output; }

Used for indexing operations (container[index]) in mutable contexts.

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.


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,

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.
assert_eq!(balance[Side::Right], Weight::Kilogram(2.5));

// 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);Run

Required Methods

Performs the mutable indexing (container[index]) operation.