Trait std::ops::IndexMut

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

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<'a>(&'a self, index: Side) -> &'a Self::Output {
        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 Self::Output {
        println!("Accessing {:?}-side of balance mutably", index);
        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

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

Performs the mutable indexing (container[index]) operation.

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Implementors

impl IndexMut<Range<usize>> for String

fn index_mut(&mut self, index: Range<usize>) -> &mut str

impl IndexMut<RangeFrom<usize>> for String

fn index_mut(&mut self, index: RangeFrom<usize>) -> &mut str

impl IndexMut<RangeFull> for String

fn index_mut(&mut self, _index: RangeFull) -> &mut str

impl IndexMut<RangeInclusive<usize>> for String

fn index_mut(&mut self, index: RangeInclusive<usize>) -> &mut str

impl IndexMut<RangeTo<usize>> for String

fn index_mut(&mut self, index: RangeTo<usize>) -> &mut str

impl IndexMut<RangeToInclusive<usize>> for String

fn index_mut(&mut self, index: RangeToInclusive<usize>) -> &mut str

impl<A> IndexMut<usize> for VecDeque<A>

Important traits for &'_ mut F

impl<'_, F> Future for &'_ mut F where
    F: Unpin + Future + ?Sized,  type Output = <F as Future>::Output;impl<'_, I> Iterator for &'_ mut I where
    I: Iterator + ?Sized,  type Item = <I as Iterator>::Item;impl<'_, R: Read + ?Sized> Read for &'_ mut Rimpl<'_, W: Write + ?Sized> Write for &'_ mut W

fn index_mut(&mut self, index: usize) -> &mut A

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

fn index_mut(&mut self, index: I) -> &mut <I as SliceIndex<str>>::Output

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

fn index_mut(&mut self, index: I) -> &mut <I as SliceIndex<[T]>>::Output

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

Important traits for Vec<u8>

impl Write for Vec<u8>

fn index_mut(&mut self, index: I) -> &mut <Vec<T> as Index<I>>::Output

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