Trait Default

1.0.0 (const: unstable) · 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§

This trait is not dyn compatible.

In older versions of Rust, dyn compatibility was called "object safety", so this trait is not object safe.

Implementors§

1.0.0 (const: unstable) · Source§

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 &DataLocale

impl Default for &DataMarkerAttributes

impl Default for &StyledStr

impl Default for &rustmax::clap::builder::Styles

impl Default for &CStr

impl Default for &rustmax::std::ffi::OsStr

impl Default for &mut str

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

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

impl Default for StartKind

impl Default for Action

impl Default for anstyle_parse::state::definitions::State

impl Default for BigEndian

impl Default for LittleEndian

impl Default for Tz

impl Default for colorchoice::ColorChoice

impl Default for Sample

impl Default for Repeat

impl Default for BaseUnit

impl Default for Kilo

impl Default for BidiPairedBracketType

impl Default for UpsamplingMethod

impl Default for IpNet

impl Default for MfType

impl Default for CheckType

impl Default for png::common::Compression

impl Default for AdaptiveFilterType

impl Default for png::filter::FilterType

impl Default for Channels

impl Default for ColorSpace

impl Default for ChromaSamplePosition

impl Default for ColorPrimaries

impl Default for MatrixCoefficients

impl Default for PixelRange

impl Default for TransferCharacteristics

impl Default for CpuFeatureLevel

impl Default for Tune

impl Default for FrameTypeOverride

impl Default for PredictionMode

impl Default for WhichCaptures

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

impl Default for ExtractKind

impl Default for CompressionCache

impl Default for rustls::quic::Version

impl Default for DeflateLevel

impl Default for tiff::encoder::compression::Compressor

impl Default for IntegerRadix

impl Default for GraphemeClusterBreak

impl Default for SentenceBreak

impl Default for WordBreak

impl Default for ChromaSampling

impl Default for zlib_rs::deflate::Method

impl Default for Strategy

impl Default for DeflateFlush

impl Default for InflateFlush

impl Default for BlockType

impl Default for BitDepth

impl Default for CanDerive

impl Default for MacroParsingBehavior

impl Default for AliasVariation

impl Default for EnumVariation

impl Default for FieldVisibilityKind

impl Default for Formatter

impl Default for MacroTypeVariation

impl Default for NonCopyUnionStyle

impl Default for RustEdition

impl Default for rustmax::clap::ColorChoice

impl Default for ValueHint

impl Default for ListStyleType

Trait Default

§`enum`s

§How can I implement `Default`?