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 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
):
-
Symmetry: if
A: PartialEq<B>
andB: PartialEq<A>
, thena == b
impliesb == a
; and -
Transitivity: if
A: PartialEq<B>
andB: PartialEq<C>
andA: PartialEq<C>
, thena == b
andb == c
impliesa == c
. This must also work for longer chains, such as whenA: PartialEq<B>
,B: PartialEq<C>
,C: PartialEq<D>
, andA: PartialEq<D>
all exist.
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.
§Cross-crate considerations
Upholding the requirements stated above can become tricky when one crate implements PartialEq
for a type of another crate (i.e., to allow comparing one of its own types with a type from the
standard library). The recommendation is to never implement this trait for a foreign type. In
other words, such a crate should do impl PartialEq<ForeignType> for LocalType
, but it should
not do impl PartialEq<LocalType> for ForeignType
.
This avoids the problem of transitive chains that criss-cross crate boundaries: for all local
types T
, you may assume that no other crate will add impl
s that allow comparing T == U
. In
other words, if other crates add impl
s that allow building longer transitive chains U1 == ... == T == V1 == ...
, then all the types that appear to the right of T
must be types that the
crate defining T
already knows about. This rules out transitive chains where downstream crates
can add new impl
s that “stitch together” comparisons of foreign types in ways that violate
transitivity.
Not having such foreign impl
s also avoids forward compatibility issues where one crate adding
more PartialEq
implementations can cause build failures in downstream crates.
§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);
RunBy 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);
RunRequired Methods§
Provided Methods§
Implementors§
impl PartialEq for AsciiChar
impl PartialEq for Infallible
impl PartialEq for core::fmt::Alignment
impl PartialEq for IpAddr
impl PartialEq for Ipv6MulticastScope
impl PartialEq for SocketAddr
impl PartialEq for FpCategory
impl PartialEq for IntErrorKind
impl PartialEq for SearchStep
impl PartialEq for core::sync::atomic::Ordering
impl PartialEq for core::cmp::Ordering
impl PartialEq for bool
impl PartialEq for char
impl PartialEq for f16
impl PartialEq for f32
impl PartialEq for f64
impl PartialEq for f128
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 AllocError
impl PartialEq for Layout
impl PartialEq for LayoutError
impl PartialEq for TypeId
impl PartialEq for CpuidResult
impl PartialEq for CharTryFromError
impl PartialEq for DecodeUtf16Error
impl PartialEq for ParseCharError
impl PartialEq for TryFromCharError
impl PartialEq for FromBytesUntilNulError
impl PartialEq for FromBytesWithNulError
impl PartialEq for CStr
impl PartialEq for Error
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 ParseFloatError
impl PartialEq for ParseIntError
impl PartialEq for TryFromIntError
impl PartialEq for RangeFull
impl PartialEq for core::ptr::Alignment
impl PartialEq for ParseBoolError
impl PartialEq for Utf8Error
impl PartialEq for RawWaker
impl PartialEq for RawWakerVTable
impl PartialEq for Duration
impl PartialEq for TryFromFloatSecsError
impl PartialEq<IpAddr> for Ipv4Addr
impl PartialEq<IpAddr> for Ipv6Addr
impl PartialEq<Ipv4Addr> for IpAddr
impl PartialEq<Ipv6Addr> for IpAddr
impl<'a> PartialEq for Location<'a>
impl<'a> PartialEq for Utf8Chunk<'a>
impl<A, B: ?Sized> PartialEq<&B> for &A
impl<A, B: ?Sized> PartialEq<&B> for &mut A
impl<A, B: ?Sized> PartialEq<&mut B> for &A
impl<A, B: ?Sized> PartialEq<&mut B> for &mut A
impl<B: PartialEq, C: PartialEq> PartialEq for ControlFlow<B, C>
impl<Dyn: ?Sized> PartialEq for DynMetadata<Dyn>
impl<F: FnPtr> PartialEq for F
impl<H> PartialEq for BuildHasherDefault<H>
impl<Idx: PartialEq> PartialEq for core::ops::Range<Idx>
impl<Idx: PartialEq> PartialEq for core::ops::RangeFrom<Idx>
impl<Idx: PartialEq> PartialEq for core::ops::RangeInclusive<Idx>
impl<Idx: PartialEq> PartialEq for RangeTo<Idx>
impl<Idx: PartialEq> PartialEq for RangeToInclusive<Idx>
impl<Idx: PartialEq> PartialEq for core::range::Range<Idx>
impl<Idx: PartialEq> PartialEq for core::range::RangeFrom<Idx>
impl<Idx: PartialEq> PartialEq for core::range::RangeInclusive<Idx>
impl<Ptr: Deref, Q: Deref> PartialEq<Pin<Q>> for Pin<Ptr>
impl<T> PartialEq for (T₁, T₂, …, Tₙ)
This trait is implemented for tuples up to twelve items long.