Trait IndexMut

pub trait IndexMut<Idx>: Index<Idx>
where
    Idx: ?Sized,
{
    // Required method
    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.

Examples

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};

#[derive(Debug)]
enum Side {
    Left,
    Right,
}

#[derive(Debug, PartialEq)]
enum Weight {
    Kilogram(f32),
    Pound(f32),
}

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

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

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

impl IndexMut<Side> for Balance {
    fn index_mut(&mut self, index: Side) -> &mut Self::Output {
        println!("Accessing {index:?}-side of balance mutably");
        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

fn index_mut(&mut self, index: Idx) -> &mut Self::Output

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

Panics

May panic if the index is out of bounds.

Implementors

impl IndexMut<usize> for AllocVec

impl IndexMut<RangeFull> for OsString

impl<'a> IndexMut<usize> for postcard::ser::flavors::Slice<'a>

impl<A, I> IndexMut<I> for SmallVec<A>

where A: Array, I: SliceIndex<[<A as Array>::Item]>,

impl<I> IndexMut<I> for str

where I: SliceIndex<str>,

impl<I> IndexMut<I> for String

where I: SliceIndex<str>,

impl<I, T, const N: usize> IndexMut<I> for Simd<T, N>

where T: SimdElement, LaneCount<N>: SupportedLaneCount, I: SliceIndex<[T]>,

impl<K, V> IndexMut<(Bound<usize>, Bound<usize>)> for indexmap::map::slice::Slice<K, V>

impl<K, V> IndexMut<usize> for indexmap::map::slice::Slice<K, V>

impl<K, V> IndexMut<Range<usize>> for indexmap::map::slice::Slice<K, V>

impl<K, V> IndexMut<RangeFrom<usize>> for indexmap::map::slice::Slice<K, V>

impl<K, V> IndexMut<RangeInclusive<usize>> for indexmap::map::slice::Slice<K, V>

impl<K, V> IndexMut<RangeFull> for indexmap::map::slice::Slice<K, V>

impl<K, V> IndexMut<RangeTo<usize>> for indexmap::map::slice::Slice<K, V>

impl<K, V> IndexMut<RangeToInclusive<usize>> for indexmap::map::slice::Slice<K, V>

impl<K, V> IndexMut<K> for BoxedSlice<K, V>

where K: EntityRef,

Mutable indexing into a BoxedSlice.

impl<K, V> IndexMut<K> for PrimaryMap<K, V>

where K: EntityRef,

Mutable indexing into an PrimaryMap.

impl<K, V> IndexMut<K> for SecondaryMap<K, V>

where K: EntityRef, V: Clone,

Mutable indexing into an SecondaryMap.

The map grows as needed to accommodate new keys.

impl<K, V, Q, S> IndexMut<&Q> for IndexMap<K, V, S>

where Q: Hash + Equivalent<K> + ?Sized, S: BuildHasher,

Access IndexMap values corresponding to a key.

Mutable indexing allows changing / updating values of key-value pairs that are already present.

You can not insert new pairs with index syntax, use .insert().

Examples

use indexmap::IndexMap;

let mut map = IndexMap::new();
for word in "Lorem ipsum dolor sit amet".split_whitespace() {
    map.insert(word.to_lowercase(), word.to_string());
}
let lorem = &mut map["lorem"];
assert_eq!(lorem, "Lorem");
lorem.retain(char::is_lowercase);
assert_eq!(map["lorem"], "orem");

use indexmap::IndexMap;

let mut map = IndexMap::new();
map.insert("foo", 1);
map["bar"] = 1; // panics!

impl<K, V, S> IndexMut<(Bound<usize>, Bound<usize>)> for IndexMap<K, V, S>

impl<K, V, S> IndexMut<usize> for IndexMap<K, V, S>

Access IndexMap values at indexed positions.

Mutable indexing allows changing / updating indexed values that are already present.

You can not insert new values with index syntax – use .insert().

Examples

use indexmap::IndexMap;

let mut map = IndexMap::new();
for word in "Lorem ipsum dolor sit amet".split_whitespace() {
    map.insert(word.to_lowercase(), word.to_string());
}
let lorem = &mut map[0];
assert_eq!(lorem, "Lorem");
lorem.retain(char::is_lowercase);
assert_eq!(map["lorem"], "orem");

use indexmap::IndexMap;

let mut map = IndexMap::new();
map.insert("foo", 1);
map[10] = 1; // panics!

impl<K, V, S> IndexMut<Range<usize>> for IndexMap<K, V, S>

impl<K, V, S> IndexMut<RangeFrom<usize>> for IndexMap<K, V, S>

impl<K, V, S> IndexMut<RangeInclusive<usize>> for IndexMap<K, V, S>

impl<K, V, S> IndexMut<RangeFull> for IndexMap<K, V, S>

impl<K, V, S> IndexMut<RangeTo<usize>> for IndexMap<K, V, S>

impl<K, V, S> IndexMut<RangeToInclusive<usize>> for IndexMap<K, V, S>

impl<T, A> IndexMut<usize> for VecDeque<T, A>

where A: Allocator,

impl<T, I> IndexMut<I> for [T]

where I: SliceIndex<[T]>,

impl<T, I, A> IndexMut<I> for Vec<T, A>

where I: SliceIndex<[T]>, A: Allocator,

impl<T, I, const N: usize> IndexMut<I> for [T; N]

where [T]: IndexMut<I>,