# 1.0.0[−][src]Trait core::ops::Index

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

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

## Associated Types

### `type Output: ?Sized`

The returned type after indexing.

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## Required methods

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

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

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## Implementors

### `impl<I> Index<I> for str where
I: SliceIndex<str>,` [src]

#### `type Output = I::Output`

#### `fn index(&self, index: I) -> &I::Output`[src]

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

#### `type Output = I::Output`

#### `fn index(&self, index: I) -> &I::Output`[src]

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