Trait Default

1.0.0 (const: unstable) · Source

pub trait Default: Sized {
    // Required method
    fn default() -> Self;
}

Expand description

core 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§

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 &str

impl Default for &CStr

impl Default for &OsStr

impl Default for &mut str

impl Default for AngleDirection

impl Default for AngleKind

impl Default for AnsiColor3b

impl Default for Dither

impl Default for Key

impl Default for KeyFfi

impl Default for KeyState

impl Default for PixelFormat

impl Default for Sign

impl Default for SixelMean

impl Default for SixelQuality

impl Default for SixelSplit

impl Default for TimeGranularity

impl Default for WebEventKind

impl Default for WebKeyLocation

impl Default for UnitBi

impl Default for UnitSi

impl Default for devela::text::AsciiChar

impl Default for core::ascii::ascii_char::AsciiChar

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 ExampleBitfield

impl Default for ExampleBitfieldCustom

impl Default for ExampleBitfieldExtra

impl Default for ExampleEnumSet

impl Default for devela::all::Arc<str>

impl Default for devela::all::AtomicBool

impl Default for AtomicF32

impl Default for AtomicF64

impl Default for devela::all::AtomicI8

impl Default for devela::all::AtomicI16

impl Default for devela::all::AtomicI32

impl Default for devela::all::AtomicI64

impl Default for AtomicI128

impl Default for devela::all::AtomicIsize

impl Default for devela::all::AtomicU8

impl Default for devela::all::AtomicU16

impl Default for devela::all::AtomicU32

impl Default for devela::all::AtomicU64

impl Default for AtomicU128

impl Default for devela::all::AtomicUsize

impl Default for devela::all::Box<str>

impl Default for devela::all::Box<CStr>

impl Default for devela::all::Box<OsStr>

impl Default for devela::all::Box<dyn Modulator>

impl Default for Boxed

impl Default for CString

Trait Default

§`enum`s

§How can I implement `Default`?