pub trait IndexMut<Idx>: Index<Idx> where
Idx: ?Sized, {
fn index_mut(&mut self, index: Idx) -> &mut Self::Output;
}
Expand description
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
Implementations on Foreign Types
1.50.0 (const: unstable) · sourceimpl<T, I, const N: usize> IndexMut<I> for [T; N] where
[T]: IndexMut<I>,
impl<T, I, const N: usize> IndexMut<I> for [T; N] where
[T]: IndexMut<I>,
const: unstable · sourceimpl<I> IndexMut<I> for str where
I: SliceIndex<str>,
impl<I> IndexMut<I> for str where
I: SliceIndex<str>,
fn index_mut(&mut self, index: I) -> &mut <I as SliceIndex<str>>::Output
1.26.0 · sourceimpl IndexMut<RangeToInclusive<usize>> for String
impl IndexMut<RangeToInclusive<usize>> for String
fn index_mut(&mut self, index: RangeToInclusive<usize>) -> &mut str
1.26.0 · sourceimpl IndexMut<RangeInclusive<usize>> for String
impl IndexMut<RangeInclusive<usize>> for String
fn index_mut(&mut self, index: RangeInclusive<usize>) -> &mut str
sourceimpl<'_, K, V, Q, S> IndexMut<&'_ Q> for IndexMap<K, V, S> where
Q: Hash + Equivalent<K> + ?Sized,
K: Hash + Eq,
S: BuildHasher,
impl<'_, K, V, Q, S> IndexMut<&'_ Q> for IndexMap<K, V, S> where
Q: Hash + Equivalent<K> + ?Sized,
K: Hash + Eq,
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!
sourceimpl<K, V, S> IndexMut<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!
Implementors
impl<I, T, const LANES: usize> IndexMut<I> for Simd<T, LANES> where
T: SimdElement,
I: SliceIndex<[T]>,
LaneCount<LANES>: SupportedLaneCount,
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.