Trait Default

1.0.0 (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.

Implementors§

1.0.0 (const: unstable) · Source§

impl Default for &str

1.10.0 · Source§

impl Default for &CStr

1.9.0 · Source§

impl Default for &OsStr

1.28.0 (const: unstable) · 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 PathArguments

impl Default for Item

impl Default for IntegerRadix

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

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 Eager

impl Default for Lazy

impl Default for InvalidBufferSize

impl Default for InvalidOutputSize

impl Default for DefaultHashBuilder

impl Default for BoundLifetimes

impl Default for Generics

impl Default for Array

impl Default for ArrayOfTables

impl Default for Document<&'static str>

impl Default for DocumentMut

impl Default for InlineTable

impl Default for RawString

impl Default for Decor

impl Default for Table

impl Default for Span

impl Default for B0

impl Default for B1

impl Default for Z0

impl Default for Equal

impl Default for Greater

impl Default for Less

impl Default for UTerm

impl Default for AdhocAttr

impl Default for TokenAccumulator

impl Default for Accum

impl Default for IsUsed

impl Default for Recognised

impl Default for ImportedDefinitions

impl Default for DdOptions

impl Default for UnprocessedOptions

impl Default for AngleBrackets

impl Default for RepeatAnalysisVisitor

impl Default for ExplicitErrorNamedArgs

impl Default for ErrorAccumulator

impl Default for SyntheticMacroModule

impl Default for SyntheticMacroTemplate

impl Default for Error

impl Default for FormattingOptions

impl Default for Global

impl Default for alloc::boxed::Box<str>

impl Default for alloc::boxed::Box<CStr>

Trait Default

§`enum`s

§How can I implement `Default`?