Trait Default

1.55.0 (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 primitive 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.

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 &serde_json::value::Value

impl Default for &str

impl Default for &CStr

impl Default for &OsStr

impl Default for &DataLocale

impl Default for &DataMarkerAttributes

impl Default for &mut str

impl Default for BuildPattern

impl Default for DateTime

impl Default for AsciiChar

impl Default for DependencyKind

impl Default for Edition

impl Default for ArtifactDebuginfo

impl Default for RemoteRedirect

impl Default for BidiPairedBracketType

impl Default for PrefilterConfig

impl Default for serde_json::value::Value

§Examples

impl Default for MonthRepr

impl Default for Padding

impl Default for SubsecondDigits

impl Default for UnixTimestampPrecision

impl Default for WeekNumberRepr

impl Default for WeekdayRepr

impl Default for YearRange

impl Default for YearRepr

impl Default for Item

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 Error

impl Default for FormattingOptions

impl Default for Git

impl Default for Project

impl Default for SystemEnv

impl Default for Box<str>

impl Default for Box<CStr>

impl Default for Box<OsStr>

impl Default for String

impl Default for Global

impl Default for ByteString

impl Default for CString

impl Default for Rc<str>

impl Default for Rc<CStr>

impl Default for Arc<str>

impl Default for Arc<CStr>

impl Default for SipHasher

impl Default for PhantomPinned

impl Default for RangeFull

impl Default for Alignment

impl Default for AtomicBool

impl Default for AtomicI8

impl Default for AtomicI16

Trait Default

§`enum`s

§How can I implement `Default`?