Trait sgx_tstd::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);

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);

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
}

Examples

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

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

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 sgx_tstd::cmp::PartialEq

Required Methods§

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

Provided Methods§

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

Implementors§

impl PartialEq for hashbrown_tstd::TryReserveError

impl PartialEq for OCallError

impl PartialEq for AddressKind

impl PartialEq for MapAddr

impl PartialEq for FutexClockId

impl PartialEq for FutexOp

impl PartialEq for PageType

impl PartialEq for Feature

impl PartialEq for Version

impl PartialEq for ExceptionType

impl PartialEq for ExceptionVector

impl PartialEq for HandleResult

impl PartialEq for PceError

impl PartialEq for QcnlError

impl PartialEq for Quote3Error

impl PartialEq for SgxStatus

impl PartialEq for DirIndex

impl PartialEq for EcResult

impl PartialEq for RsaKeyType

impl PartialEq for RsaResult

impl PartialEq for PckCertFlag

impl PartialEq for ProdType

impl PartialEq for QeType

impl PartialEq for QlAttestationAlgorithmId

impl PartialEq for QlCertKeyType

impl PartialEq for QlConfigVersion

impl PartialEq for QlLogLevel

impl PartialEq for QlPathType

impl PartialEq for QlQvResult

impl PartialEq for QlRequestPolicy

impl PartialEq for QvPathType

impl PartialEq for DhSessionRole

impl PartialEq for EnclaveMode

impl PartialEq for KeyName

impl PartialEq for ProtectPerm

impl PartialEq for RaKeyType

impl PartialEq for QuoteSignType

impl PartialEq for SwitchlessWokerEvent

impl PartialEq for SwitchlessWokerType

impl PartialEq for SgxDeviceStatus

impl PartialEq for AsciiChar

impl PartialEq for TryReserveErrorKind

impl PartialEq for Infallible

impl PartialEq for VarError

impl PartialEq for sgx_tstd::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 SearchStep

impl PartialEq for sgx_tstd::sync::atomic::Ordering

impl PartialEq for TcsPolicy

impl PartialEq for sgx_tstd::cmp::Ordering

impl PartialEq for Which

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