pub trait IndexMut<Idx: ?Sized>: Index<Idx> {
const 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);