Trait 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 }
}

Dyn Compatibility§

impl Default for ExitStatus

The default value is one which indicates successful completion.

TokenStream::default() returns an empty stream, i.e. this is equivalent with TokenStream::new().

§Example

use std::hash::BuildHasherDefault;
use ahash::{AHasher, RandomState};
use std::collections::HashMap;

let mut map: HashMap<i32, i32, BuildHasherDefault<AHasher>> = HashMap::default();
map.insert(12, 34);

The defaults are that of beStrict=false in the WHATWG URL Standard

This is the same as [RandomState::new()]

NOTE: For safety this trait impl is only available available if either of the flags runtime-rng (on by default) or compile-time-rng are enabled. This is to prevent weakly keyed maps from being accidentally created. Instead one of constructors for [RandomState] must be used.

Instantiate the default transformations, the identity transform.

Returns a Uri representing /

impl<T> Default for AHashSet<T>

NOTE: For safety this trait impl is only available available if either of the flags runtime-rng (on by default) or compile-time-rng are enabled. This is to prevent weakly keyed maps from being accidentally created. Instead one of constructors for [RandomState] must be used.

Trait DefaultCopy item path

§Derivable

§enums

§How can I implement Default?

§Examples

Required Methods§

fn default() -> Self

§Examples

Dyn Compatibility§

Implementors§

impl Default for &str

impl Default for &CStr

impl Default for &OsStr

impl Default for &DataLocale

impl Default for &DataMarkerAttributes

impl Default for &mut str

impl Default for AttributeMask

impl Default for AttributeMaskBuilder<'_>

impl Default for AspectRatio

impl Default for BorderAlignment

impl Default for Fill

impl Default for Focusable

impl Default for HighlightMode

impl Default for ImageCover

impl Default for OverflowMode

impl Default for SamplingMode

impl Default for ShadowPosition

impl Default for SvgPaint

impl Default for TextOverflow

impl Default for AccordionStatus

impl Default for ButtonStatus

impl Default for DropdownItemStatus

impl Default for DropdownStatus

impl Default for HandleStatus

impl Default for InputMode

impl Default for InputStatus

impl Default for MenuItemStatus

impl Default for OrderDirection

impl Default for ScrollDirection

impl Default for ScrollPosition

impl Default for SelectableTextStatus

impl Default for SliderStatus

impl Default for SwitchStatus

impl Default for TabStatus

impl Default for TileStatus

impl Default for freya::prelude::Alignment

impl Default for Code

impl Default for Content

impl Default for CursorIcon

impl Default for Direction

impl Default for freya::prelude::Key

impl Default for NavigationMode

impl Default for Position

impl Default for PreferredTheme

impl Default for RenderError

impl Default for freya::prelude::Size

impl Default for VisibleSize

impl Default for AssetAge

impl Default for CameraIndex

impl Default for Ease

impl Default for EditableMode

impl Default for Function

impl Default for OnCreation

impl Default for OnDepsChange

impl Default for OnFinish

impl Default for RequestedFormatType

impl Default for SurfaceThemeIndicator

impl Default for AsciiChar

impl Default for IpNet

impl Default for PathArguments

impl Default for bool

impl Default for char

impl Default for f16

impl Default for f32

impl Default for f64

impl Default for f128

impl Default for i8

impl Default for i16

impl Default for i32

impl Default for i64

Trait Default

§`enum`s

§How can I implement `Default`?