Trait druid::piet::cairo::glib::bitflags::_core::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 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§

§

impl PartialEq<&str> for GString

1.29.0 · source§

impl PartialEq<&GStr> for GString

Trait druid::piet::cairo::glib::bitflags::_core::cmp::PartialEq

Required Methods§

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

Provided Methods§

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

Implementors§

impl PartialEq<&str> for GString

impl PartialEq<&str> for OsString

impl PartialEq<&str> for Language

impl PartialEq<&str> for LanguageIdentifier

impl PartialEq<&str> for Region

impl PartialEq<&str> for Script

impl PartialEq<&str> for Variant

impl PartialEq<&GStr> for GString

impl PartialEq<Code> for Code

impl PartialEq<Color> for druid::Color

impl PartialEq<Cursor> for druid::Cursor

impl PartialEq<Handled> for Handled

impl PartialEq<Location> for druid::Location

impl PartialEq<MouseButton> for MouseButton

impl PartialEq<RawMods> for RawMods

impl PartialEq<RawMods> for druid::Modifiers

impl PartialEq<RawWindowHandle> for RawWindowHandle

impl PartialEq<SysMods> for druid::Modifiers

impl PartialEq<Target> for Target

impl PartialEq<WindowLevel> for WindowLevel

impl PartialEq<WindowSizePolicy> for WindowSizePolicy

impl PartialEq<WindowState> for druid::WindowState

impl PartialEq<Direction> for druid::text::Direction

impl PartialEq<Event> for druid::text::ImeInvalidation

impl PartialEq<Movement> for Movement

impl PartialEq<Action> for druid::text::TextAction

impl PartialEq<VerticalMovement> for VerticalMovement

impl PartialEq<WritingDirection> for WritingDirection

impl PartialEq<Axis> for Axis

impl PartialEq<CrossAxisAlignment> for CrossAxisAlignment

impl PartialEq<FillStrat> for FillStrat

impl PartialEq<LineBreaking> for LineBreaking

impl PartialEq<MainAxisAlignment> for MainAxisAlignment

impl PartialEq<TabsEdge> for TabsEdge

impl PartialEq<TabsTransition> for TabsTransition

impl PartialEq<ColorParseError> for ColorParseError

impl PartialEq<FontStyle> for FontStyle

impl PartialEq<ImageFormat> for druid::piet::ImageFormat

impl PartialEq<InterpolationMode> for InterpolationMode

impl PartialEq<LineCap> for druid::piet::LineCap

impl PartialEq<LineJoin> for druid::piet::LineJoin

impl PartialEq<TextAlignment> for TextAlignment

impl PartialEq<ColorType> for druid::piet::image_crate::ColorType

impl PartialEq<DynamicImage> for DynamicImage

impl PartialEq<ExtendedColorType> for ExtendedColorType

impl PartialEq<ImageFormat> for druid::piet::image_crate::ImageFormat

impl PartialEq<ImageOutputFormat> for ImageOutputFormat

impl PartialEq<ImageFormatHint> for ImageFormatHint

impl PartialEq<LimitErrorKind> for LimitErrorKind

impl PartialEq<ParameterErrorKind> for ParameterErrorKind

impl PartialEq<UnsupportedErrorKind> for UnsupportedErrorKind

impl PartialEq<Error> for druid::piet::image_crate::flat::Error

impl PartialEq<NormalForm> for NormalForm

impl PartialEq<FilterType> for druid::piet::image_crate::imageops::FilterType

impl PartialEq<PathEl> for druid::piet::kurbo::PathEl

impl PartialEq<PathSeg> for druid::piet::kurbo::PathSeg

impl PartialEq<Antialias> for Antialias

impl PartialEq<Content> for Content

impl PartialEq<DeviceType> for druid::piet::cairo::DeviceType

impl PartialEq<Error> for druid::piet::cairo::Error

impl PartialEq<Extend> for Extend

impl PartialEq<FillRule> for druid::piet::cairo::FillRule

impl PartialEq<Filter> for Filter

impl PartialEq<FontSlant> for FontSlant

impl PartialEq<FontType> for FontType

impl PartialEq<FontWeight> for druid::piet::cairo::FontWeight

impl PartialEq<Format> for Format

impl PartialEq<HintMetrics> for HintMetrics

impl PartialEq<HintStyle> for HintStyle

impl PartialEq<LineCap> for druid::piet::cairo::LineCap

impl PartialEq<LineJoin> for druid::piet::cairo::LineJoin

impl PartialEq<MeshCorner> for MeshCorner

impl PartialEq<Operator> for Operator

impl PartialEq<PathDataType> for PathDataType