Trait freya::prelude::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.

The #[default] attribute was stabilized in Rust 1.62.0.

§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§

1.0.0 · 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 }
}

Object Safety§

This trait is not object safe.

Implementors§

1.0.0 · source§

impl Default for &str

1.10.0 · source§

impl Default for &CStr

1.9.0 · source§

impl Default for &OsStr

1.28.0 · source§

Trait freya::prelude::DefaultCopy item path

§Derivable

§enums

§How can I implement Default?

§Examples

Required Methods§

fn default() -> Self

§Examples

Object Safety§

Implementors§

impl Default for &str

impl Default for &CStr

impl Default for &OsStr

impl Default for &mut str

impl Default for Code

impl Default for Key

impl Default for AccordionStatus

impl Default for freya::prelude::Alignment

impl Default for AssetAge

impl Default for ButtonStatus

impl Default for Content

impl Default for CursorIcon

impl Default for DirectionMode

impl Default for DropdownItemStatus

impl Default for DropdownStatus

impl Default for Ease

impl Default for EditableMode

impl Default for Function

impl Default for InputMode

impl Default for InputStatus

impl Default for MenuItemStatus

impl Default for OnFinish

impl Default for OrderDirection

impl Default for Position

impl Default for freya::prelude::Size

impl Default for SliderStatus

impl Default for SwitchStatus

impl Default for TileStatus

impl Default for AsciiChar

impl Default for Role

impl Default for aho_corasick::packed::api::MatchKind

impl Default for aho_corasick::util::search::MatchKind

impl Default for StartKind

impl Default for BigEndian

impl Default for LittleEndian

impl Default for DependencyKind

impl Default for Edition

impl Default for ArtifactDebuginfo

impl Default for DynamicNode

impl Default for RenderReturn

impl Default for MouseButton

impl Default for Sample

impl Default for RectHeightStyle

impl Default for RectWidthStyle

impl Default for TextAlign

impl Default for TextDecorationMode

impl Default for TextDecorationStyle

impl Default for AttributeMask

impl Default for AttributeMaskBuilder<'_>

impl Default for BorderAlignment

impl Default for BorderStyle

impl Default for Fill

impl Default for TextOverflow

impl Default for OverflowMode

impl Default for ShadowPosition

impl Default for PollNext

impl Default for Repeat

impl Default for ApiPreference

impl Default for ContextApi

impl Default for ReleaseBehavior

impl Default for Robustness

impl Default for CompressionType

impl Default for image::codecs::png::FilterType

impl Default for DynamicImage

impl Default for SignalHandler

impl Default for IpNet

impl Default for KeyState

impl Default for Location

impl Default for PrefilterConfig

impl Default for EventKind

Trait freya::prelude::Default

§`enum`s

§How can I implement `Default`?