Trait frame_support::dispatch::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 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 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.

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.

Implementors§

1.29.0 · source§

Trait frame_support::dispatch::PartialEq

Required Methods§

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

Provided Methods§

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

Implementors§

impl PartialEq<&str> for OsString

impl PartialEq<ArithmeticError> for ArithmeticError

impl PartialEq<DeriveError> for DeriveError

impl PartialEq<DeriveJunction> for DeriveJunction

impl PartialEq<PublicError> for PublicError

impl PartialEq<SecretStringError> for SecretStringError

impl PartialEq<Void> for Void

impl PartialEq<HttpError> for HttpError

impl PartialEq<HttpRequestStatus> for HttpRequestStatus

impl PartialEq<OffchainOverlayedChange> for OffchainOverlayedChange

impl PartialEq<StorageKind> for StorageKind

impl PartialEq<CallContext> for CallContext

impl PartialEq<StorageEntryModifierIR> for StorageEntryModifierIR

impl PartialEq<StorageHasherIR> for StorageHasherIR

impl PartialEq<MultiSignature> for MultiSignature

impl PartialEq<MultiSigner> for MultiSigner

impl PartialEq<TokenError> for TokenError

impl PartialEq<TransactionalError> for TransactionalError

impl PartialEq<DigestItem> for DigestItem

impl PartialEq<Era> for Era

impl PartialEq<DispatchError> for sp_runtime::legacy::byte_sized_error::DispatchError

impl PartialEq<Error> for sp_runtime::offchain::http::Error

impl PartialEq<Method> for Method

impl PartialEq<StorageRetrievalError> for StorageRetrievalError

impl PartialEq<RuntimeString> for RuntimeString

impl PartialEq<DisableStrategy> for DisableStrategy

impl PartialEq<OffenceError> for OffenceError

impl PartialEq<ExecutionError> for ExecutionError

impl PartialEq<Error> for sp_version::embed::Error

impl PartialEq<ReturnValue> for ReturnValue

impl PartialEq<Value> for sp_wasm_interface::Value

impl PartialEq<ValueType> for sp_wasm_interface::ValueType

impl PartialEq<Never> for Never

impl PartialEq<DispatchError> for frame_support::pallet_prelude::DispatchError

impl PartialEq<InvalidTransaction> for InvalidTransaction

impl PartialEq<TransactionSource> for TransactionSource

impl PartialEq<TransactionValidityError> for TransactionValidityError

impl PartialEq<UnknownTransaction> for UnknownTransaction

impl PartialEq<ChildInfo> for ChildInfo

impl PartialEq<ChildType> for ChildType

impl PartialEq<StateVersion> for StateVersion

impl PartialEq<ExecuteOverweightError> for ExecuteOverweightError

impl PartialEq<ProcessMessageError> for ProcessMessageError

impl PartialEq<LookupError> for frame_support::traits::schedule::LookupError

impl PartialEq<BalanceStatus> for BalanceStatus

impl PartialEq<DepositConsequence> for DepositConsequence

impl PartialEq<ExistenceRequirement> for ExistenceRequirement

impl PartialEq<Fortitude> for Fortitude

impl PartialEq<Precision> for Precision

impl PartialEq<Preservation> for Preservation

impl PartialEq<Provenance> for Provenance

impl PartialEq<Restriction> for Restriction

impl PartialEq<PaymentStatus> for PaymentStatus

impl PartialEq<DispatchClass> for DispatchClass

impl PartialEq<Pays> for Pays

impl PartialEq<Alignment> for frame_support::dispatch::fmt::Alignment

impl PartialEq<TryReserveErrorKind> for TryReserveErrorKind

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

impl PartialEq<Infallible> for Infallible

impl PartialEq<Which> for Which

impl PartialEq<IpAddr> for IpAddr

impl PartialEq<IpAddr> for Ipv4Addr

impl PartialEq<IpAddr> for Ipv6Addr

impl PartialEq<Ipv6MulticastScope> for Ipv6MulticastScope

impl PartialEq<SocketAddr> for SocketAddr

impl PartialEq<FpCategory> for FpCategory

impl PartialEq<IntErrorKind> for IntErrorKind

impl PartialEq<SearchStep> for SearchStep

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

impl PartialEq<Delimiter> for proc_macro::Delimiter

impl PartialEq<Spacing> for proc_macro::Spacing

impl PartialEq<BacktraceStatus> for BacktraceStatus

impl PartialEq<VarError> for VarError

impl PartialEq<SeekFrom> for std::io::SeekFrom