Trait std::hash::Hash

1.0.0 · source ·
pub trait Hash {
    // Required method
    fn hash<H>(&self, state: &mut H)
       where H: Hasher;

    // Provided method
    fn hash_slice<H>(data: &[Self], state: &mut H)
       where H: Hasher,
             Self: Sized { ... }
}
Expand description

A hashable type.

Types implementing Hash are able to be hashed with an instance of Hasher.

Implementing Hash

You can derive Hash with #[derive(Hash)] if all fields implement Hash. The resulting hash will be the combination of the values from calling hash on each field.

#[derive(Hash)]
struct Rustacean {
    name: String,
    country: String,
}
Run

If you need more control over how a value is hashed, you can of course implement the Hash trait yourself:

use std::hash::{Hash, Hasher};

struct Person {
    id: u32,
    name: String,
    phone: u64,
}

impl Hash for Person {
    fn hash<H: Hasher>(&self, state: &mut H) {
        self.id.hash(state);
        self.phone.hash(state);
    }
}
Run

Hash and Eq

When implementing both Hash and Eq, it is important that the following property holds:

k1 == k2 -> hash(k1) == hash(k2)

In other words, if two keys are equal, their hashes must also be equal. HashMap and HashSet both rely on this behavior.

Thankfully, you won’t need to worry about upholding this property when deriving both Eq and Hash with #[derive(PartialEq, Eq, Hash)].

Violating this property is a logic error. The behavior resulting from a logic error is not specified, but users of the trait must ensure that such logic errors do not result in undefined behavior. This means that unsafe code must not rely on the correctness of these methods.

Prefix collisions

Implementations of hash should ensure that the data they pass to the Hasher are prefix-free. That is, values which are not equal should cause two different sequences of values to be written, and neither of the two sequences should be a prefix of the other.

For example, the standard implementation of Hash for &str passes an extra 0xFF byte to the Hasher so that the values ("ab", "c") and ("a", "bc") hash differently.

Portability

Due to differences in endianness and type sizes, data fed by Hash to a Hasher should not be considered portable across platforms. Additionally the data passed by most standard library types should not be considered stable between compiler versions.

This means tests shouldn’t probe hard-coded hash values or data fed to a Hasher and instead should check consistency with Eq.

Serialization formats intended to be portable between platforms or compiler versions should either avoid encoding hashes or only rely on Hash and Hasher implementations that provide additional guarantees.

Required Methods§

source

fn hash<H>(&self, state: &mut H)
where H: Hasher,

Feeds this value into the given Hasher.

Examples
use std::hash::{DefaultHasher, Hash, Hasher};

let mut hasher = DefaultHasher::new();
7920.hash(&mut hasher);
println!("Hash is {:x}!", hasher.finish());
Run

Provided Methods§

1.3.0 · source

fn hash_slice<H>(data: &[Self], state: &mut H)
where H: Hasher, Self: Sized,

Feeds a slice of this type into the given Hasher.

This method is meant as a convenience, but its implementation is also explicitly left unspecified. It isn’t guaranteed to be equivalent to repeated calls of hash and implementations of Hash should keep that in mind and call hash themselves if the slice isn’t treated as a whole unit in the PartialEq implementation.

For example, a VecDeque implementation might naïvely call as_slices and then hash_slice on each slice, but this is wrong since the two slices can change with a call to make_contiguous without affecting the PartialEq result. Since these slices aren’t treated as singular units, and instead part of a larger deque, this method cannot be used.

Examples
use std::hash::{DefaultHasher, Hash, Hasher};

let mut hasher = DefaultHasher::new();
let numbers = [6, 28, 496, 8128];
Hash::hash_slice(&numbers, &mut hasher);
println!("Hash is {:x}!", hasher.finish());
Run

Object Safety§

This trait is not object safe.

Implementors§

source§

impl Hash for AsciiChar

source§

impl Hash for std::cmp::Ordering

1.44.0 · source§

impl Hash for Infallible

source§

impl Hash for ErrorKind

1.7.0 · source§

impl Hash for IpAddr

source§

impl Hash for Ipv6MulticastScope

source§

impl Hash for SocketAddr

source§

impl Hash for std::sync::atomic::Ordering

source§

impl Hash for bool

source§

impl Hash for char

source§

impl Hash for i8

source§

impl Hash for i16

source§

impl Hash for i32

source§

impl Hash for i64

source§

impl Hash for i128

source§

impl Hash for isize

1.29.0 · source§

impl Hash for !

source§

impl Hash for str

source§

impl Hash for u8

source§

impl Hash for u16

source§

impl Hash for u32

source§

impl Hash for u64

source§

impl Hash for u128

source§

impl Hash for ()

source§

impl Hash for usize

1.28.0 · source§

impl Hash for Layout

source§

impl Hash for TypeId

1.64.0 · source§

impl Hash for CStr

1.64.0 · source§

impl Hash for CString

source§

impl Hash for OsStr

source§

impl Hash for OsString

source§

impl Hash for Error

1.1.0 · source§

impl Hash for FileType

1.33.0 · source§

impl Hash for PhantomPinned

source§

impl Hash for Ipv4Addr

source§

impl Hash for Ipv6Addr

source§

impl Hash for SocketAddrV4

source§

impl Hash for SocketAddrV6

1.34.0 · source§

impl Hash for NonZeroI8

1.34.0 · source§

impl Hash for NonZeroI16

1.34.0 · source§

impl Hash for NonZeroI32

1.34.0 · source§

impl Hash for NonZeroI64

1.34.0 · source§

impl Hash for NonZeroI128

1.34.0 · source§

impl Hash for NonZeroIsize

1.28.0 · source§

impl Hash for NonZeroU8

1.28.0 · source§

impl Hash for NonZeroU16

1.28.0 · source§

impl Hash for NonZeroU32

1.28.0 · source§

impl Hash for NonZeroU64

1.28.0 · source§

impl Hash for NonZeroU128

1.28.0 · source§

impl Hash for NonZeroUsize

source§

impl Hash for RangeFull

source§

impl Hash for UCred

Available on Unix only.
source§

impl Hash for Path

source§

impl Hash for PathBuf

source§

impl Hash for PrefixComponent<'_>

source§

impl Hash for Alignment

source§

impl Hash for String

1.19.0 · source§

impl Hash for ThreadId

1.3.0 · source§

impl Hash for Duration

1.8.0 · source§

impl Hash for Instant

1.8.0 · source§

impl Hash for SystemTime

source§

impl<'a> Hash for Component<'a>

source§

impl<'a> Hash for Prefix<'a>

1.10.0 · source§

impl<'a> Hash for Location<'a>

source§

impl<B> Hash for Cow<'_, B>
where B: Hash + ToOwned + ?Sized,

1.55.0 · source§

impl<B, C> Hash for ControlFlow<B, C>
where B: Hash, C: Hash,

source§

impl<Dyn> Hash for DynMetadata<Dyn>
where Dyn: ?Sized,

1.4.0 · source§

impl<F> Hash for F
where F: FnPtr,

source§

impl<Idx> Hash for Range<Idx>
where Idx: Hash,

source§

impl<Idx> Hash for RangeFrom<Idx>
where Idx: Hash,

1.26.0 · source§

impl<Idx> Hash for RangeInclusive<Idx>
where Idx: Hash,

source§

impl<Idx> Hash for RangeTo<Idx>
where Idx: Hash,

1.26.0 · source§

impl<Idx> Hash for RangeToInclusive<Idx>
where Idx: Hash,

source§

impl<K, V, A> Hash for BTreeMap<K, V, A>
where K: Hash, V: Hash, A: Allocator + Clone,

1.41.0 · source§

impl<P> Hash for Pin<P>
where P: Deref, <P as Deref>::Target: Hash,

1.17.0 · source§

impl<T> Hash for Bound<T>
where T: Hash,

source§

impl<T> Hash for Option<T>
where T: Hash,

1.36.0 · source§

impl<T> Hash for Poll<T>
where T: Hash,

source§

impl<T> Hash for *const T
where T: ?Sized,

source§

impl<T> Hash for *mut T
where T: ?Sized,

source§

impl<T> Hash for &T
where T: Hash + ?Sized,

source§

impl<T> Hash for &mut T
where T: Hash + ?Sized,

source§

impl<T> Hash for [T]
where T: Hash,

source§

impl<T> Hash for (T₁, T₂, …, Tₙ)
where T: Hash + ?Sized,

This trait is implemented for tuples up to twelve items long.

1.19.0 · source§

impl<T> Hash for Reverse<T>
where T: Hash,

source§

impl<T> Hash for PhantomData<T>
where T: ?Sized,

1.21.0 · source§

impl<T> Hash for Discriminant<T>

1.20.0 · source§

impl<T> Hash for ManuallyDrop<T>
where T: Hash + ?Sized,

1.74.0 · source§

impl<T> Hash for Saturating<T>
where T: Hash,

source§

impl<T> Hash for Wrapping<T>
where T: Hash,

1.25.0 · source§

impl<T> Hash for NonNull<T>
where T: ?Sized,

source§

impl<T, A> Hash for Box<T, A>
where T: Hash + ?Sized, A: Allocator,

source§

impl<T, A> Hash for BTreeSet<T, A>
where T: Hash, A: Allocator + Clone,

source§

impl<T, A> Hash for LinkedList<T, A>
where T: Hash, A: Allocator,

source§

impl<T, A> Hash for VecDeque<T, A>
where T: Hash, A: Allocator,

source§

impl<T, A> Hash for Rc<T, A>
where T: Hash + ?Sized, A: Allocator,

source§

impl<T, A> Hash for Arc<T, A>
where T: Hash + ?Sized, A: Allocator,

source§

impl<T, A> Hash for Vec<T, A>
where T: Hash, A: Allocator,

The hash of a vector is the same as that of the corresponding slice, as required by the core::borrow::Borrow implementation.

use std::hash::BuildHasher;

let b = std::hash::RandomState::new();
let v: Vec<u8> = vec![0xa8, 0x3c, 0x09];
let s: &[u8] = &[0xa8, 0x3c, 0x09];
assert_eq!(b.hash_one(v), b.hash_one(s));
Run
source§

impl<T, E> Hash for Result<T, E>
where T: Hash, E: Hash,

source§

impl<T, const N: usize> Hash for [T; N]
where T: Hash,

The hash of an array is the same as that of the corresponding slice, as required by the Borrow implementation.

use std::hash::BuildHasher;

let b = std::hash::RandomState::new();
let a: [u8; 3] = [0xa8, 0x3c, 0x09];
let s: &[u8] = &[0xa8, 0x3c, 0x09];
assert_eq!(b.hash_one(a), b.hash_one(s));
Run
source§

impl<T, const N: usize> Hash for Simd<T, N>

source§

impl<Y, R> Hash for CoroutineState<Y, R>
where Y: Hash, R: Hash,