# Trait IndexMut

1.6.0 (const: unstable) · Source

    pub trait IndexMut<Idx: ?Sized>: Index<Idx> {
        // Required method
        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§

1.0.0 · Source

#### 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§

1.0.0 (const: unstable) · Source§

### impl<I> IndexMut<I> for str
where I: SliceIndex<str>,

Source§

### impl<I> IndexMut<I> for ByteStr
where I: SliceIndex<ByteStr>,

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### impl<I, T, const N: usize> IndexMut<I> for Simd<T, N>
where T: SimdElement, LaneCount<N>: SupportedLaneCount, I: SliceIndex<[T]>,

1.0.0 (const: unstable) · Source§

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

1.50.0 (const: unstable) · Source§

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