Trait Default

1.6.0 · 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§

impl Default for &str

1.10.0 · Source§

impl Default for &CStr

1.9.0 · Source§

impl Default for &OsStr

1.28.0 · Source§

impl Default for IoringRestrictionOp

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 &mut str

impl Default for Disambiguation

impl Default for OffsetConflict

impl Default for DistortionKind

impl Default for Easing

impl Default for StartTime

impl Default for EndPosition

impl Default for PlaybackPosition

impl Default for PrefilterConfig

impl Default for Equation

impl Default for PassAction

impl Default for VertexStep

impl Default for IOURING_OP

impl Default for IoringOp

impl Default for IoringRestrictionOp

impl Default for Limit

impl Default for UpdateKind

impl Default for AngleDirection

impl Default for AngleKind

impl Default for Sign

impl Default for UnitBi

impl Default for UnitSi

impl Default for devela::text::AsciiChar

impl Default for devela::_core::ascii::Char

impl Default for EulerRot

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

impl Default for isize

impl Default for u8

impl Default for u16

impl Default for u32

impl Default for u64

impl Default for u128

impl Default for ()

impl Default for usize

impl Default for Global

impl Default for devela::_dep::_std::alloc::System

impl Default for FileTimes

impl Default for DefaultHasher

impl Default for devela::_dep::_std::io::Empty

impl Default for ExitCode

impl Default for ExitStatus

impl Default for DefaultRandomSource

impl Default for Bump

impl Default for Attributes

impl Default for ContentStyle

impl Default for Date

impl Default for DateTime

impl Default for DateTimeRound

impl Default for ISOWeekDate

impl Default for devela::_dep::jiff::civil::Time

impl Default for TimeRound

impl Default for SpanParser

impl Default for SpanPrinter

impl Default for BrokenDownTime

impl Default for SignedDuration

impl Default for SignedDurationRound

impl Default for Span

Trait Default

§`enum`s

§How can I implement `Default`?