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());

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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.

Trait std::hash::Hash

§Implementing Hash

§Hash and Eq

§Prefix collisions

§Portability

Required Methods§

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

§Examples

Provided Methods§

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

§Examples

Object Safety§

Implementors§

impl Hash for AsciiChar

impl Hash for std::cmp::Ordering

impl Hash for Infallible

impl Hash for ErrorKind

impl Hash for IpAddr

impl Hash for Ipv6MulticastScope

impl Hash for SocketAddr

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

impl Hash for Format

impl Hash for SectionId

impl Hash for RunTimeEndian

impl Hash for bool

impl Hash for char

impl Hash for i8

impl Hash for i16

impl Hash for i32

impl Hash for i64

impl Hash for i128

impl Hash for isize

impl Hash for !

impl Hash for str

impl Hash for u8

impl Hash for u16

impl Hash for u32

impl Hash for u64

impl Hash for u128

impl Hash for ()

impl Hash for usize

impl Hash for Layout

impl Hash for TypeId

impl Hash for CStr

impl Hash for CString

impl Hash for OsStr

impl Hash for OsString

impl Hash for Error

impl Hash for FileType

impl Hash for PhantomPinned

impl Hash for Ipv4Addr

impl Hash for Ipv6Addr

impl Hash for SocketAddrV4

impl Hash for SocketAddrV6

impl Hash for RangeFull

impl Hash for UCred

impl Hash for Path

impl Hash for PathBuf

impl Hash for PrefixComponent<'_>

impl Hash for Alignment

impl Hash for String

impl Hash for ThreadId

impl Hash for Duration

impl Hash for Instant

impl Hash for SystemTime

impl Hash for DebugTypeSignature

impl Hash for DwoId

impl Hash for Encoding

impl Hash for LineEncoding

impl Hash for Register

impl Hash for DwAccess

impl Hash for DwAddr

impl Hash for DwAt

impl Hash for DwAte

impl Hash for DwCc

impl Hash for DwCfa

impl Hash for DwChildren

impl Hash for DwDefaulted

impl Hash for DwDs

impl Hash for DwDsc

§Implementing `Hash`

§`Hash` and `Eq`

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

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

impl<'input, Endian> Hash for EndianSlice<'input, Endian>
where Endian: Hash + Endianity,

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

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

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

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

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

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

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

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

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

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

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

impl<R> Hash for LocationListEntry<R>
where R: Hash + Reader,

impl<R> Hash for Expression<R>
where R: Hash + Reader,

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

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

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

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

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

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

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

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

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

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

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

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

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

impl<T> Hash for NonZero<T>
where T: ZeroablePrimitive + Hash,

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

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

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

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

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

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

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