Trait std::ops::IndexMut

#[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);

Required Methods

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


impl IndexMut<Range<usize>> for str [src]

Implements mutable substring slicing with syntax &mut self[begin .. end].

Returns a mutable slice of the given string from the byte range [begin..end).

This operation is O(1).


Panics if begin or end does not point to the starting byte offset of a character (as defined by is_char_boundary). Requires that begin <= end and end <= len where len is the length of the string.

impl IndexMut<Range<usize>> for String [src]

impl IndexMut<RangeFrom<usize>> for str [src]

Implements mutable substring slicing with syntax &mut self[begin ..].

Returns a mutable slice of the string from byte offset begin to the end of the string.

Equivalent to &mut self[begin .. len].

impl IndexMut<RangeFrom<usize>> for String [src]

impl IndexMut<RangeFull> for str [src]

Implements mutable substring slicing with syntax &mut self[..].

Returns a mutable slice of the whole string. This operation can never panic.

Equivalent to &mut self[0 .. len].

impl IndexMut<RangeFull> for String [src]

impl IndexMut<RangeInclusive<usize>> for str [src]

impl IndexMut<RangeInclusive<usize>> for String [src]

impl IndexMut<RangeTo<usize>> for str [src]

Implements mutable substring slicing with syntax &mut self[.. end].

Returns a mutable slice of the string from the beginning to byte offset end.

Equivalent to &mut self[0 .. end].

impl IndexMut<RangeTo<usize>> for String [src]

impl IndexMut<RangeToInclusive<usize>> for str [src]

impl IndexMut<RangeToInclusive<usize>> for String [src]

impl<A> IndexMut<usize> for VecDeque<A> [src]

ⓘImportant traits for &'a mut I
impl<'a, I> Iterator for &'a mut I where
    I: Iterator + ?Sized, 
    type Item = <I as Iterator>::Item;
impl<'a, R: Read + ?Sized> Read for &'a mut R
impl<'a, W: Write + ?Sized> Write for &'a mut W

impl<T, I> IndexMut<I> for [T] where
    I: SliceIndex<[T]>, 

impl<T, I> IndexMut<I> for Vec<T> where
    I: SliceIndex<[T]>, 

ⓘImportant traits for Vec<u8>
impl Write for Vec<u8>

© 2010 The Rust Project Developers
Licensed under the Apache License, Version 2.0 or the MIT license, at your option.