otter_nodejs_tests::inventory::core::default

Trait Default

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 primitive 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 (const: unstable) · 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§

This trait is not dyn compatible.

In older versions of Rust, dyn compatibility was called "object safety".

Default

Trait Default Copy item path

§Derivable

§enums

§How can I implement Default?

§Examples

Required Methods§

fn default() -> Self

§Examples

Dyn Compatibility§

Implementors§

impl Default for &'static HtmlStr

impl Default for &otter_nodejs_tests::tera::Value

impl Default for &str

impl Default for &OsStr

impl Default for &CStr

impl Default for &DataLocale

impl Default for &DataMarkerAttributes

impl Default for &mut str

impl Default for AggregatedIE

impl Default for AssetUrlKey

impl Default for otter_nodejs_tests::BigEndian

impl Default for otter_nodejs_tests::LittleEndian

impl Default for OccultationKindGeneral<(OccDisplacement, ZCoord)>

impl Default for PathResolveContext

impl Default for PathResolveMethod

impl Default for PieceAngle

impl Default for PieceMoveable

impl Default for PresentationLayout

impl Default for ProgressUpdateMode

impl Default for Target

impl Default for TimestampPrecision

impl Default for WriteStyle

impl Default for ErrorChannel

impl Default for Abi

impl Default for PieceLabelPlace

impl Default for otter_nodejs_tests::progress::Value

impl Default for BytesMode

impl Default for otter_nodejs_tests::tera::Value

§Examples

impl Default for AsciiChar

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

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

impl Default for StartKind

impl Default for LineEnding

impl Default for Tz

impl Default for PollNext

impl Default for DwarfFileType

impl Default for RunTimeEndian

impl Default for Pointer

impl Default for BaseUnit

impl Default for Kilo

impl Default for CollationCaseFirst

impl Default for CollationNumericOrdering

impl Default for CurrencyFormatStyle

impl Default for EmojiPresentationStyle

impl Default for SentenceBreakSupressions

impl Default for BidiPairedBracketType

impl Default for CompressionType

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

impl Default for DynamicImage

impl Default for IpNet

impl Default for PrefilterConfig

impl Default for Endianness

impl Default for SpecLabels

impl Default for Encoding

impl Default for png::common::Compression

impl Default for AdaptiveFilterType

impl Default for png::filter::FilterType

impl Default for WhichCaptures

impl Default for regex_automata::util::search::MatchKind

impl Default for ExtractKind

impl Default for ColorChoice

impl Default for MonthRepr

impl Default for Padding

impl Default for SubsecondDigits

impl Default for WeekNumberRepr

impl Default for WeekdayRepr

impl Default for YearRepr

impl Default for MissedTickBehavior

Trait Default

§`enum`s

§How can I implement `Default`?