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§

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 · 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 &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 Tz

impl Default for colorchoice::ColorChoice

impl Default for DwarfFileType

impl Default for RunTimeEndian

impl Default for Pointer

impl Default for BaseUnit

impl Default for Kilo

impl Default for LocaleFallbackPriority

impl Default for IpNet

impl Default for PrefilterConfig

impl Default for Endianness

impl Default for WhichCaptures

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

impl Default for ExtractKind

impl Default for Item

impl Default for GraphemeClusterBreak

impl Default for SentenceBreak

impl Default for WordBreak

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::byteorder::BigEndian

impl Default for rustmax::byteorder::LittleEndian

impl Default for rustmax::clap::ColorChoice

impl Default for ValueHint

impl Default for Target

impl Default for TimestampPrecision

impl Default for WriteStyle

impl Default for PollNext

impl Default for Disambiguation

impl Default for OffsetConflict

impl Default for FileFailurePersistence

impl Default for RngAlgorithm

impl Default for BellStyle

impl Default for Behavior

impl Default for Direction

impl Default for PathArguments

impl Default for rustmax::tera::Value

§Examples

impl Default for rustmax::termcolor::ColorChoice

impl Default for MissedTickBehavior

impl Default for AsciiChar

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

impl Default for i128

Trait Default

§`enum`s

§How can I implement `Default`?