# Trait scale_info::prelude::cmp::PartialEq

1.0.0 · source · []
``````pub trait PartialEq<Rhs = Self> where    Rhs: ?Sized, {
fn eq(&self, other: &Rhs) -> bool;

fn ne(&self, other: &Rhs) -> bool { ... }
}``````
Expand description

Trait for equality comparisons which are partial equivalence relations.

`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`.

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

• `a != b` if and only if `!(a == b)` (ensured by the default implementation).

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);``````

### 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 `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
}``````

## Examples

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

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

## Required Methods

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

## Provided Methods

This method tests for `!=`.

## Implementors

This trait is implemented for function pointers with up to twelve arguments.

This trait is implemented for function pointers with up to twelve arguments.

This trait is implemented for function pointers with up to twelve arguments.

This trait is implemented for function pointers with up to twelve arguments.

This trait is implemented for function pointers with up to twelve arguments.

This trait is implemented for function pointers with up to twelve arguments.

Tests if two `BitSlice`s are semantically — not representationally — equal.

It is valid to compare slices of different ordering or memory types.

The equality condition requires that they have the same length and that at each index, the two slices have the same bit value.

Original

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