Trait druid::piet::cairo::glib::bitflags::_core::default::Default

1.0.0 · source ·

pub trait Default: Sized {
    // Required method
    fn default() -> Self;
}

Expand description

A trait for giving a type a useful default value.

Sometimes, you want to fall back to some kind of default value, and don’t particularly care what it is. This comes up often with structs that define a set of options:

struct SomeOptions {
    foo: i32,
    bar: f32,
}

How can we define some default values? You can use Default:

#[derive(Default)]
struct SomeOptions {
    foo: i32,
    bar: f32,
}

fn main() {
    let options: SomeOptions = Default::default();
}

Now, you get all of the default values. Rust implements Default for various primitives types.

If you want to override a particular option, but still retain the other defaults:

fn main() {
    let options = SomeOptions { foo: 42, ..Default::default() };
}

Derivable

This trait can be used with #[derive] if all of the type’s fields implement Default. When derived, it will use the default value for each field’s type.

`enum`s

When using #[derive(Default)] on an enum, you need to choose which unit variant will be default. You do this by placing the #[default] attribute on the variant.

#[derive(Default)]
enum Kind {
    #[default]
    A,
    B,
    C,
}

You cannot use the #[default] attribute on non-unit or non-exhaustive variants.

How can I implement `Default`?

Provide an implementation for the default() method that returns the value of your type that should be the default:

enum Kind {
    A,
    B,
    C,
}

impl Default for Kind {
    fn default() -> Self { Kind::A }
}

Examples

#[derive(Default)]
struct SomeOptions {
    foo: i32,
    bar: f32,
}

Required Methods§

source

fn default() -> Self

Returns the “default value” for a type.

Default values are often some kind of initial value, identity value, or anything else that may make sense as a default.

Examples

Using built-in default values:

let i: i8 = Default::default();
let (x, y): (Option<String>, f64) = Default::default();
let (a, b, (c, d)): (i32, u32, (bool, bool)) = Default::default();

Making your own:

enum Kind {
    A,
    B,
    C,
}

impl Default for Kind {
    fn default() -> Self { Kind::A }
}

Implementors§

const: unstable · source§

impl Default for &str

§

impl Default for &GStr

1.10.0 · source§

impl Default for &CStr

1.9.0 · source§

impl Default for &OsStr

1.28.0 · source§

impl Default for &mut str

Trait druid::piet::cairo::glib::bitflags::_core::default::Default

Required Methods§

fn default() -> Self

Implementors§

impl Default for &str

impl Default for &GStr

impl Default for &CStr

impl Default for &OsStr

impl Default for &mut str

impl Default for Code

impl Default for Location

impl Default for ScrollbarsEnabled

impl Default for FillStrat

impl Default for KnobStyle

impl Default for TabsEdge

impl Default for TabsTransition

impl Default for FontStyle

impl Default for druid::piet::LineCap

impl Default for druid::piet::LineJoin

impl Default for TextAlignment

impl Default for DynamicImage

impl Default for FluentNumberCurrencyDisplayStyle

impl Default for FluentNumberStyle

impl Default for BlendMode

impl Default for tiny_skia::painter::FillRule

impl Default for SpreadMode

impl Default for ImageRendering

impl Default for usvg::PaintOrder

impl Default for ShapeRendering

impl Default for TextRendering

impl Default for Visibility

impl Default for ColorMatrixKind

impl Default for ColorInterpolation

impl Default for SpreadMethod

impl Default for usvg::style::FillRule

impl Default for usvg::style::LineCap

impl Default for usvg::style::LineJoin

impl Default for bool

impl Default for char

impl Default for f32

impl Default for f64

impl Default for i8

impl Default for i16

impl Default for i32

impl Default for i64

impl Default for i128

impl Default for isize

impl Default for u8

impl Default for u16

impl Default for u32

impl Default for u64

impl Default for u128

impl Default for ()

impl Default for usize

impl Default for DebugState

impl Default for Unit

impl Default for ScrollComponent

impl Default for druid::Affine

impl Default for druid::FileDialogOptions

impl Default for ImageBuf

impl Default for druid::Insets

impl Default for KeyEvent

impl Default for druid::Modifiers

impl Default for MouseButtons

impl Default for druid::Point

impl Default for druid::Rect

impl Default for druid::RoundedRectRadii

impl Default for druid::Scale

impl Default for ScaledArea

impl Default for druid::Size

impl Default for druid::Vec2

impl Default for WindowConfig

impl Default for WindowHandle

impl Default for Recording

impl Default for AttributeSpans

impl Default for FontDescriptor

impl Default for LayoutMetrics

impl Default for RichTextBuilder

impl Default for Selection

impl Default for druid::widget::ProgressBar