Trait std::cmp::PartialEq

1.0.0 · source ·

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

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

Expand description

Trait for comparisons using the equality operator.

Implementing this trait for types provides the == and != operators for those types.

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 comparisons using the equality operator, 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 and B: PartialEq<A>, then a == b implies b == a; and
Transitive: if A: PartialEq 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.

Violating these requirements 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.

§Derivable

This trait can be used with #[derive]. When derived on structs, two instances are equal if all fields are equal, and not equal if any fields are not equal. When derived 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 BookFormats to be compared with Books.

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§

source

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

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

Provided Methods§

source

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.

Trait std::cmp::PartialEq

§Derivable

§How can I implement PartialEq?

§How can I compare two different types?

§Examples

Required Methods§

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

Provided Methods§

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

Implementors§

impl PartialEq for AsciiChar

impl PartialEq for BacktraceStatus

impl PartialEq for TryReserveErrorKind

impl PartialEq for Infallible

impl PartialEq for VarError

impl PartialEq for std::fmt::Alignment

impl PartialEq for ErrorKind

impl PartialEq for SeekFrom

impl PartialEq for IpAddr

impl PartialEq for Ipv6MulticastScope

impl PartialEq for Shutdown

impl PartialEq for SocketAddr

impl PartialEq for FpCategory

impl PartialEq for IntErrorKind

impl PartialEq for BacktraceStyle

impl PartialEq for SearchStep

impl PartialEq for std::sync::atomic::Ordering

impl PartialEq for RecvTimeoutError

impl PartialEq for TryRecvError

impl PartialEq for std::cmp::Ordering

impl PartialEq for bool

impl PartialEq for char

impl PartialEq for f32

impl PartialEq for f64

impl PartialEq for i8

impl PartialEq for i16

impl PartialEq for i32

impl PartialEq for i64

impl PartialEq for i128

impl PartialEq for isize

impl PartialEq for !

impl PartialEq for str

impl PartialEq for u8

impl PartialEq for u16

impl PartialEq for u32

impl PartialEq for u64

impl PartialEq for u128

impl PartialEq for ()

impl PartialEq for usize

impl PartialEq for CpuidResult

impl PartialEq for AllocError

impl PartialEq for Layout

impl PartialEq for LayoutError

impl PartialEq for TypeId

impl PartialEq for CharTryFromError

impl PartialEq for DecodeUtf16Error

impl PartialEq for ParseCharError

impl PartialEq for TryFromCharError

impl PartialEq for UnorderedKeyError

impl PartialEq for TryReserveError

impl PartialEq for CStr

impl PartialEq for CString

impl PartialEq for FromBytesUntilNulError

impl PartialEq for FromBytesWithNulError

impl PartialEq for FromVecWithNulError

impl PartialEq for IntoStringError

impl PartialEq for NulError

impl PartialEq for OsStr

impl PartialEq for OsString

impl PartialEq for Error

impl PartialEq for FileType

impl PartialEq for Permissions

impl PartialEq for PhantomPinned

impl PartialEq for Assume

impl PartialEq for AddrParseError

impl PartialEq for Ipv4Addr

impl PartialEq for Ipv6Addr

impl PartialEq for SocketAddrV4

impl PartialEq for SocketAddrV6

impl PartialEq for NonZero<i8>

§How can I implement `PartialEq`?