# Trait std::ops::IndexMut 1.0.0 [−] [src]

    #[lang = "index_mut"]
    pub trait IndexMut<Idx>: Index<Idx> where
        Idx: ?Sized, {
        fn index_mut(&mut self, index: Idx) -> &mut Self::Output;
    }

The `IndexMut` trait is used to specify the functionality of indexing operations like `container[index]` when used in a mutable context.

`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` if `value` implements `Copy`.

# 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<'a>(&'a self, index: Side) -> &'a Weight {
            println!("Accessing {:?}-side of balance immutably", index);
            match index {
                Side::Left => &self.left,
                Side::Right => &self.right,
            }
        }
    }

    impl IndexMut<Side> for Balance {
        fn index_mut<'a>(&'a mut self, index: Side) -> &'a mut Weight {
            println!("Accessing {:?}-side of balance mutably", index);
            match index {
                Side::Left => &mut self.left,
                Side::Right => &mut self.right,
            }
        }
    }

    fn main() {
        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);
    }Run

## Required Methods

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

The method for the mutable indexing (`container[index]`) operation

## Implementors

- `impl IndexMut<Range<usize>> for String`
- `impl IndexMut<RangeTo<usize>> for String`
- `impl IndexMut<RangeFrom<usize>> for String`
- `impl IndexMut<RangeFull> for String`
- `impl IndexMut<RangeInclusive<usize>> for String`
- `impl IndexMut<RangeToInclusive<usize>> for String`
- `impl<T> IndexMut<usize> for Vec<T>`
- `impl<T> IndexMut<Range<usize>> for Vec<T>`
- `impl<T> IndexMut<RangeTo<usize>> for Vec<T>`
- `impl<T> IndexMut<RangeFrom<usize>> for Vec<T>`
- `impl<T> IndexMut<RangeFull> for Vec<T>`
- `impl<T> IndexMut<RangeInclusive<usize>> for Vec<T>`
- `impl<T> IndexMut<RangeToInclusive<usize>> for Vec<T>`
- `impl<A> IndexMut<usize> for VecDeque<A>`
- `impl<T, I> IndexMut<I> for [T] where
  I: SliceIndex<[T]>,`
- `impl IndexMut<Range<usize>> for str`
- `impl IndexMut<RangeTo<usize>> for str`
- `impl IndexMut<RangeFrom<usize>> for str`
- `impl IndexMut<RangeFull> for str`
- `impl IndexMut<RangeInclusive<usize>> for str`
- `impl IndexMut<RangeToInclusive<usize>> for str`

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