# Trait std::hash::BuildHasher 1.7.0 [−] [src]

    pub trait BuildHasher {
        type Hasher: Hasher;
        fn build_hasher(&self) -> Self::Hasher;
    }

A trait for creating instances of `Hasher`.

A `BuildHasher` is typically used (e.g. by `HashMap`) to create `Hasher`s for each key such that they are hashed independently of one another, since `Hasher`s contain state.

For each instance of `BuildHasher`, the `Hasher`s created by `build_hasher` should be identical. That is, if the same stream of bytes is fed into each hasher, the same output will also be generated.

# Examples

    use std::collections::hash_map::RandomState;
    use std::hash::{BuildHasher, Hasher};

    let s = RandomState::new();
    let mut hasher_1 = s.build_hasher();
    let mut hasher_2 = s.build_hasher();

    hasher_1.write_u32(8128);
    hasher_2.write_u32(8128);

    assert_eq!(hasher_1.finish(), hasher_2.finish());Run

## Associated Types

### `type Hasher: Hasher`

Type of the hasher that will be created.

## Required Methods

### `fn build_hasher(&self) -> Self::Hasher`

Creates a new hasher.

Each call to `build_hasher` on the same instance should produce identical `Hasher`s.

# Examples

    use std::collections::hash_map::RandomState;
    use std::hash::BuildHasher;

    let s = RandomState::new();
    let new_s = s.build_hasher();Run

## Implementors

- `impl<H> BuildHasher for BuildHasherDefault<H> where
  H: Default + Hasher, type Hasher = H;`
- `impl BuildHasher for RandomState type Hasher = DefaultHasher;`

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