Trait Default

1.6.0 (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§

impl Default for &str

1.10.0 · 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 &CStr

impl Default for &OsStr

impl Default for &mut str

impl Default for EulerRot

impl Default for AsciiChar

impl Default for BigEndian

impl Default for LittleEndian

impl Default for Sample

impl Default for Repeat

impl Default for CompressionType

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

impl Default for DynamicImage

impl Default for PrefilterConfig

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 ravif::av1encoder::BitDepth

impl Default for DeflateLevel

impl Default for Compressor

impl Default for ChromaSampling

impl Default for zune_core::bit_depth::BitDepth

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 Affine2

impl Default for Affine3A

impl Default for BVec2

impl Default for BVec3

impl Default for BVec3A

impl Default for BVec4

impl Default for BVec4A

impl Default for DAffine2

impl Default for DAffine3

impl Default for DMat2

impl Default for DMat3

impl Default for DMat4

impl Default for DQuat

impl Default for DVec2

impl Default for DVec3

impl Default for DVec4

impl Default for FontConfig

impl Default for FontGlyphRanges

Trait Default

§`enum`s

§How can I implement `Default`?