pub trait IndexMut<Idx>: Index<Idx>where
Idx: ?Sized,{
// Required method
fn index_mut(&mut self, index: Idx) -> &mut Self::Output;
}Available on crate feature
std only.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§
Implementors§
impl IndexMut<Range<usize>> for UninitSlice
impl IndexMut<RangeFrom<usize>> for UninitSlice
impl IndexMut<RangeFull> for OsString
impl IndexMut<RangeFull> for UninitSlice
impl IndexMut<RangeInclusive<usize>> for UninitSlice
impl IndexMut<RangeTo<usize>> for UninitSlice
impl IndexMut<RangeToInclusive<usize>> for UninitSlice
impl<I> IndexMut<I> for strwhere
I: SliceIndex<str>,
impl<I> IndexMut<I> for Stringwhere
I: SliceIndex<str>,
impl<I, T, const N: usize> IndexMut<I> for Simd<T, N>
impl<K, V> IndexMut<(Bound<usize>, Bound<usize>)> for Slice<K, V>
impl<K, V> IndexMut<usize> for Slice<K, V>
impl<K, V> IndexMut<Range<usize>> for Slice<K, V>
impl<K, V> IndexMut<RangeFrom<usize>> for Slice<K, V>
impl<K, V> IndexMut<RangeFull> for Slice<K, V>
impl<K, V> IndexMut<RangeInclusive<usize>> for Slice<K, V>
impl<K, V> IndexMut<RangeTo<usize>> for Slice<K, V>
impl<K, V> IndexMut<RangeToInclusive<usize>> for 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>
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>
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!