# Trait core::cmp::PartialEq

1.0.0 · source ·

    pub trait PartialEq<Rhs: ?Sized = Self> {
        // Required method
        fn eq(&self, other: &Rhs) -> bool;

        // Provided method
        fn ne(&self, other: &Rhs) -> bool { ... }
    }

Expand description

Trait for equality comparisons.

`x.eq(y)` can also be written `x == y`, and `x.ne(y)` can be written `x != y`. We use the easier-to-read infix notation in the remainder of this documentation.

This trait allows for partial equality, for types that do not have a full equivalence relation. For example, in floating point numbers `NaN != NaN`, so floating point types implement `PartialEq` but not `Eq`. Formally speaking, when `Rhs == Self`, this trait corresponds to a partial equivalence relation.

Implementations must ensure that `eq` and `ne` are consistent with each other:

- `a != b` if and only if `!(a == b)`.

The default implementation of `ne` provides this consistency and is almost always sufficient. It should not be overridden without very good reason.

If `PartialOrd` or `Ord` are also implemented for `Self` and `Rhs`, their methods must also be consistent with `PartialEq` (see the documentation of those traits for the exact requirements). It’s easy to accidentally make them disagree by deriving some of the traits and manually implementing others.

The equality relation `==` must satisfy the following conditions (for all `a`, `b`, `c` of type `A`, `B`, `C`):

- Symmetric: if `A: PartialEq<B>` and `B: PartialEq<A>`, then `a == b` implies `b == a`; and

- Transitive: if `A: PartialEq<B>` and `B: PartialEq<C>` and `A: PartialEq<C>`, then `a == b` and `b == c` implies `a == c`.

Note that the `B: PartialEq<A>` (symmetric) and `A: PartialEq<C>` (transitive) impls are not forced to exist, but these requirements apply whenever they do exist.

### Derivable

This trait can be used with `#[derive]`. When `derive`d on structs, two instances are equal if all fields are equal, and not equal if any fields are not equal. When `derive`d on enums, two instances are equal if they are the same variant and all fields are equal.

### How can I implement `PartialEq`?

An example implementation for a domain in which two books are considered the same book if their ISBN matches, even if the formats differ:

    enum BookFormat {
        Paperback,
        Hardback,
        Ebook,
    }

    struct Book {
        isbn: i32,
        format: BookFormat,
    }

    impl PartialEq for Book {
        fn eq(&self, other: &Self) -> bool {
            self.isbn == other.isbn
        }
    }

    let b1 = Book { isbn: 3, format: BookFormat::Paperback };
    let b2 = Book { isbn: 3, format: BookFormat::Ebook };
    let b3 = Book { isbn: 10, format: BookFormat::Paperback };

    assert!(b1 == b2);
    assert!(b1 != b3);

Run

### How can I compare two different types?

The type you can compare with is controlled by `PartialEq`’s type parameter. For example, let’s tweak our previous code a bit:

    // The derive implements <BookFormat> == <BookFormat> comparisons
    #[derive(PartialEq)]
    enum BookFormat {
        Paperback,
        Hardback,
        Ebook,
    }

    struct Book {
        isbn: i32,
        format: BookFormat,
    }

    // Implement <Book> == <BookFormat> comparisons
    impl PartialEq<BookFormat> for Book {
        fn eq(&self, other: &BookFormat) -> bool {
            self.format == *other
        }
    }

    // Implement <BookFormat> == <Book> comparisons
    impl PartialEq<Book> for BookFormat {
        fn eq(&self, other: &Book) -> bool {
            *self == other.format
        }
    }

    let b1 = Book { isbn: 3, format: BookFormat::Paperback };

    assert!(b1 == BookFormat::Paperback);
    assert!(BookFormat::Ebook != b1);

Run

By changing `impl PartialEq for Book` to `impl PartialEq<BookFormat> for Book`, we allow `BookFormat`s to be compared with `Book`s.

A comparison like the one above, which ignores some fields of the struct, can be dangerous. It can easily lead to an unintended violation of the requirements for a partial equivalence relation. For example, if we kept the above implementation of `PartialEq<Book>` for `BookFormat` and added an implementation of `PartialEq<Book>` for `Book` (either via a `#[derive]` or via the manual implementation from the first example) then the result would violate transitivity:

ⓘ

    #[derive(PartialEq)]
    enum BookFormat {
        Paperback,
        Hardback,
        Ebook,
    }

    #[derive(PartialEq)]
    struct Book {
        isbn: i32,
        format: BookFormat,
    }

    impl PartialEq<BookFormat> for Book {
        fn eq(&self, other: &BookFormat) -> bool {
            self.format == *other
        }
    }

    impl PartialEq<Book> for BookFormat {
        fn eq(&self, other: &Book) -> bool {
            *self == other.format
        }
    }

    fn main() {
        let b1 = Book { isbn: 1, format: BookFormat::Paperback };
        let b2 = Book { isbn: 2, format: BookFormat::Paperback };

        assert!(b1 == BookFormat::Paperback);
        assert!(BookFormat::Paperback == b2);

        // The following should hold by transitivity but doesn't.
        assert!(b1 == b2); // <-- PANICS
    }

Run

## Examples

    let x: u32 = 0;
    let y: u32 = 1;

    assert_eq!(x == y, false);
    assert_eq!(x.eq(&y), false);

Run

## Required Methods§

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#### fn eq(&self, other: &Rhs) -> bool

This method tests for `self` and `other` values to be equal, and is used by `==`.

## Provided Methods§

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#### fn ne(&self, other: &Rhs) -> bool

This method tests for `!=`. The default implementation is almost always sufficient, and should not be overridden without very good reason.

## Implementors§

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### impl PartialEq<AsciiChar> for AsciiChar

1.34.0 · source§

### impl PartialEq<Infallible> for Infallible

1.28.0 · source§

### impl PartialEq<Alignment> for core::fmt::Alignment

1.7.0 · source§

### impl PartialEq<IpAddr> for IpAddr

1.16.0 · source§

### impl PartialEq<IpAddr> for Ipv4Addr

1.16.0 · source§

### impl PartialEq<IpAddr> for Ipv6Addr

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### impl PartialEq<Ipv6MulticastScope> for Ipv6MulticastScope

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### impl PartialEq<SocketAddr> for SocketAddr

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### impl PartialEq<FpCategory> for FpCategory

1.55.0 · source§

### impl PartialEq<IntErrorKind> for IntErrorKind

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### impl PartialEq<Which> for Which

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### impl PartialEq<SearchStep> for SearchStep

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### impl PartialEq<Ordering> for core::sync::atomic::Ordering

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### impl PartialEq<Ordering> for core::cmp::Ordering

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### impl PartialEq<bool> for bool

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### impl PartialEq<char> for char

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### impl PartialEq<f32> for f32

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### impl PartialEq<f64> for f64

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### impl PartialEq<i8> for i8

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### impl PartialEq<i16> for i16

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### impl PartialEq<i32> for i32

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### impl PartialEq<i64> for i64

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### impl PartialEq<i128> for i128

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### impl PartialEq<isize> for isize

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### impl PartialEq<!> for !

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### impl PartialEq<str> for str

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### impl PartialEq<u8> for u8

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### impl PartialEq<u16> for u16

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### impl PartialEq<u32> for u32

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### impl PartialEq<u64> for u64

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### impl PartialEq<u128> for u128

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### impl PartialEq<()> for ()

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### impl PartialEq<usize> for usize

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### impl PartialEq<AllocError> for AllocError

1.28.0 · source§

### impl PartialEq<Layout> for Layout

1.50.0 · source§

### impl PartialEq<LayoutError> for LayoutError

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### impl PartialEq<TypeId> for TypeId

1.27.0 · source§

### impl PartialEq<CpuidResult> for CpuidResult

Available on x86 or x86-64 only.
1.34.0 · source§

### impl PartialEq<CharTryFromError> for CharTryFromError

1.9.0 · source§

### impl PartialEq<DecodeUtf16Error> for DecodeUtf16Error

1.20.0 · source§

### impl PartialEq<ParseCharError> for ParseCharError

1.59.0 · source§

### impl PartialEq<TryFromCharError> for TryFromCharError

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### impl PartialEq<CStr> for CStr

1.69.0 · source§

### impl PartialEq<FromBytesUntilNulError> for FromBytesUntilNulError

1.64.0 · source§

### impl PartialEq<FromBytesWithNulError> for FromBytesWithNulError

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### impl PartialEq<Error> for Error

1.33.0 · source§

### impl PartialEq<PhantomPinned> for PhantomPinned

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### impl PartialEq<Assume> for Assume

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### impl PartialEq<AddrParseError> for AddrParseError

1.16.0 · source§

### impl PartialEq<Ipv4Addr> for IpAddr

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### impl PartialEq<Ipv4Addr> for Ipv4Addr

1.16.0 · source§

### impl PartialEq<Ipv6Addr> for IpAddr

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### impl PartialEq<Ipv6Addr> for Ipv6Addr

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### impl PartialEq<SocketAddrV4> for SocketAddrV4

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### impl PartialEq<SocketAddrV6> for SocketAddrV6

1.34.0 · source§

### impl PartialEq<NonZeroI8> for NonZeroI8

1.34.0 · source§

### impl PartialEq<NonZeroI16> for NonZeroI16

1.34.0 · source§

### impl PartialEq<NonZeroI32> for NonZeroI32

1.34.0 · source§

### impl PartialEq<NonZeroI64> for NonZeroI64

1.34.0 · source§

### impl PartialEq<NonZeroI128> for NonZeroI128

1.34.0 · source§

### impl PartialEq<NonZeroIsize> for NonZeroIsize

1.28.0 · source§

### impl PartialEq<NonZeroU8> for NonZeroU8

1.28.0 · source§

### impl PartialEq<NonZeroU16> for NonZeroU16

1.28.0 · source§

### impl PartialEq<NonZeroU32> for NonZeroU32

1.28.0 · source§

### impl PartialEq<NonZeroU64> for NonZeroU64

1.28.0 · source§

### impl PartialEq<NonZeroU128> for NonZeroU128

1.28.0 · source§

### impl PartialEq<NonZeroUsize> for NonZeroUsize

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### impl PartialEq<ParseFloatError> for ParseFloatError

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### impl PartialEq<ParseIntError> for ParseIntError

1.34.0 · source§

### impl PartialEq<TryFromIntError> for TryFromIntError

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### impl PartialEq<RangeFull> for RangeFull

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### impl PartialEq<Alignment> for core::ptr::Alignment

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### impl PartialEq<ParseBoolError> for ParseBoolError

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### impl PartialEq<Utf8Error> for Utf8Error

1.36.0 · source§

### impl PartialEq<RawWaker> for RawWaker

1.36.0 · source§

### impl PartialEq<RawWakerVTable> for RawWakerVTable

1.3.0 · source§

### impl PartialEq<Duration> for Duration

1.66.0 · source§

### impl PartialEq<TryFromFloatSecsError> for TryFromFloatSecsError

1.10.0 · source§

### impl<'a> PartialEq<Location<'a>> for Location<'a>

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### impl<'a> PartialEq<Utf8Chunk<'a>> for Utf8Chunk<'a>

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### impl<A, B> PartialEq<[B]> for [A]where A: PartialEq<B>,

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### impl<A, B, const N: usize> PartialEq<&[B]> for [A; N]where A: PartialEq<B>,

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### impl<A, B, const N: usize> PartialEq<&mut [B]> for [A; N]where A: PartialEq<B>,

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### impl<A, B, const N: usize> PartialEq<[A; N]> for &[B]where B: PartialEq<A>,

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### impl<A, B, const N: usize> PartialEq<[A; N]> for &mut [B]where B: PartialEq<A>,

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### impl<A, B, const N: usize> PartialEq<[A; N]> for [B]where B: PartialEq<A>,

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### impl<A, B, const N: usize> PartialEq<[B; N]> for [A; N]where A: PartialEq<B>,

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### impl<A, B, const N: usize> PartialEq<[B]> for [A; N]where A: PartialEq<B>,

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### impl<A, B: ?Sized> PartialEq<&B> for &A where A: PartialEq<B> + ?Sized,

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### impl<A, B: ?Sized> PartialEq<&B> for &mut A where A: PartialEq<B> + ?Sized,

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### impl<A, B: ?Sized> PartialEq<&mut B> for &A where A: PartialEq<B> + ?Sized,

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### impl<A, B: ?Sized> PartialEq<&mut B> for &mut A where A: PartialEq<B> + ?Sized,

1.55.0 · source§

### impl<B: PartialEq, C: PartialEq> PartialEq<ControlFlow<B, C>> for ControlFlow<B, C>

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### impl<Dyn: ?Sized> PartialEq<DynMetadata<Dyn>> for DynMetadata<Dyn>

1.4.0 · source§

### impl<F: FnPtr> PartialEq<F> for F

1.29.0 · source§

### impl<H> PartialEq<BuildHasherDefault<H>> for BuildHasherDefault<H>

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### impl<Idx: PartialEq> PartialEq<Range<Idx>> for Range<Idx>

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### impl<Idx: PartialEq> PartialEq<RangeFrom<Idx>> for RangeFrom<Idx>

1.26.0 · source§

### impl<Idx: PartialEq> PartialEq<RangeInclusive<Idx>> for RangeInclusive<Idx>

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### impl<Idx: PartialEq> PartialEq<RangeTo<Idx>> for RangeTo<Idx>

1.26.0 · source§

### impl<Idx: PartialEq> PartialEq<RangeToInclusive<Idx>> for RangeToInclusive<Idx>

1.41.0 · source§

### impl<P: Deref, Q: Deref> PartialEq<Pin<Q>> for Pin<P>where P::Target: PartialEq<Q::Target>,

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

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

1.21.0 · source§

### impl<T> PartialEq<Discriminant<T>> for Discriminant<T>

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### impl<T, const LANES: usize> PartialEq<Mask<T, LANES>> for Mask<T, LANES>where T: MaskElement + PartialEq, LaneCount<LANES>: SupportedLaneCount,

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### impl<T, const N: usize> PartialEq<Simd<T, N>> for Simd<T, N>where LaneCount<N>: SupportedLaneCount, T: SimdElement + PartialEq,

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### impl<T: PartialEq + Copy> PartialEq<Cell<T>> for Cell<T>

1.20.0 · source§

### impl<T: PartialEq + ?Sized> PartialEq<ManuallyDrop<T>> for ManuallyDrop<T>

1.17.0 · source§

### impl<T: PartialEq> PartialEq<Bound<T>> for Bound<T>

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### impl<T: PartialEq> PartialEq<Option<T>> for Option<T>

1.36.0 · source§

### impl<T: PartialEq> PartialEq<Poll<T>> for Poll<T>

1.70.0 · source§

### impl<T: PartialEq> PartialEq<OnceCell<T>> for OnceCell<T>

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### impl<T: PartialEq> PartialEq<Saturating<T>> for Saturating<T>

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### impl<T: PartialEq> PartialEq<Wrapping<T>> for Wrapping<T>

1.19.0 · source§

### impl<T: PartialEq> PartialEq<Reverse<T>> for Reverse<T>

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### impl<T: PartialEq, E: PartialEq> PartialEq<Result<T, E>> for Result<T, E>

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### impl<T: ?Sized + PartialEq> PartialEq<RefCell<T>> for RefCell<T>

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### impl<T: ?Sized> PartialEq<*const T> for *const T

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### impl<T: ?Sized> PartialEq<*mut T> for *mut T

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### impl<T: ?Sized> PartialEq<PhantomData<T>> for PhantomData<T>

1.25.0 · source§

### impl<T: ?Sized> PartialEq<NonNull<T>> for NonNull<T>

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### impl<Y: PartialEq, R: PartialEq> PartialEq<GeneratorState<Y, R>> for GeneratorState<Y, R>