# Trait std::ops::Index

1.0.0 · source ·

    pub trait Index<Idx>where
        Idx: ?Sized,{
        type Output: ?Sized;

        // Required method
        fn index(&self, index: Idx) -> &Self::Output;
    }

Expand description

Used for indexing operations (`container[index]`) in immutable contexts.

`container[index]` is actually syntactic sugar for `*container.index(index)`, but only when used as an immutable value. If a mutable value is requested, `IndexMut` is used instead. This allows nice things such as `let value = v[index]` if the type of `value` implements `Copy`.

## Examples

The following example implements `Index` on a read-only `NucleotideCount` container, enabling individual counts to be retrieved with index syntax.

    use std::ops::Index;

    enum Nucleotide {
        A,
        C,
        G,
        T,
    }

    struct NucleotideCount {
        a: usize,
        c: usize,
        g: usize,
        t: usize,
    }

    impl Index<Nucleotide> for NucleotideCount {
        type Output = usize;

        fn index(&self, nucleotide: Nucleotide) -> &Self::Output {
            match nucleotide {
                Nucleotide::A => &self.a,
                Nucleotide::C => &self.c,
                Nucleotide::G => &self.g,
                Nucleotide::T => &self.t,
            }
        }
    }

    let nucleotide_count = NucleotideCount {a: 14, c: 9, g: 10, t: 12};
    assert_eq!(nucleotide_count[Nucleotide::A], 14);
    assert_eq!(nucleotide_count[Nucleotide::C], 9);
    assert_eq!(nucleotide_count[Nucleotide::G], 10);
    assert_eq!(nucleotide_count[Nucleotide::T], 12);

Run

## Required Associated Types§

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#### type Output: ?Sized

The returned type after indexing.

## Required Methods§

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

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

##### Panics

May panic if the index is out of bounds.

## Implementors§

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### impl Index<Range<usize>> for String

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#### type Output = str

1.47.0 · source§

### impl Index<RangeFrom<usize>> for CStr

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#### type Output = CStr

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### impl Index<RangeFrom<usize>> for String

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#### type Output = str

1.7.0 · source§

### impl Index<RangeFull> for CString

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#### type Output = CStr

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### impl Index<RangeFull> for OsString

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#### type Output = OsStr

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### impl Index<RangeFull> for String

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#### type Output = str

1.26.0 · source§

### impl Index<RangeInclusive<usize>> for String

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#### type Output = str

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### impl Index<RangeTo<usize>> for String

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#### type Output = str

1.26.0 · source§

### impl Index<RangeToInclusive<usize>> for String

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#### type Output = str

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

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#### type Output = <I as SliceIndex<str>>::Output

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

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#### type Output = <I as SliceIndex<[T]>>::Output

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### impl<K, Q, V, A> Index<&Q> for BTreeMap<K, V, A>where A: Allocator + Clone, K: Borrow<Q> + Ord, Q: Ord + ?Sized,

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#### type Output = V

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### impl<K, Q, V, S> Index<&Q> for HashMap<K, V, S>where K: Eq + Hash + Borrow<Q>, Q: Eq + Hash + ?Sized, S: BuildHasher,

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#### type Output = V

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### impl<T, A> Index<usize> for VecDeque<T, A>where A: Allocator,

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#### type Output = T

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

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#### type Output = <I as SliceIndex<[T]>>::Output

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### impl<T, I, A, FH> Index<I> for Vec<T, A, FH>where I: SliceIndex<[T]>, A: Allocator, FH: FailureHandling,

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#### type Output = <I as SliceIndex<[T]>>::Output

1.50.0 · source§

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

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#### type Output = <[T] as Index<I>>::Output