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 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 (const: unstable) · 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".

Implementors§

1.10.0 (const: unstable) · Source§

impl Default for &DataMarkerAttributes

1.9.0 · 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 &CStr

impl Default for &DataLocale

impl Default for &DataMarkerAttributes

impl Default for &rustmax::std::ffi::OsStr

impl Default for &StyledStr

impl Default for &rustmax::clap::builder::Styles

impl Default for &rustmax::serde_json::Value

impl Default for &mut str

impl Default for &str

impl Default for ()

impl Default for AHasher

§Example

impl Default for ASN1_ITEM_st

impl Default for ASN1_TEMPLATE_st

impl Default for AUTHORITY_KEYID_st

impl Default for Abi

impl Default for syn::token::Abstract

impl Default for rustmax::syn::token::Abstract

impl Default for Acceptor

impl Default for Action

impl Default for Adam7Variant

impl Default for Adapter

impl Default for Adler32

impl Default for AhoCorasickBuilder

impl Default for AlertStyleType

impl Default for AlertType

impl Default for AliasVariation

impl Default for Alignment

impl Default for rustmax::rand::distr::Alphabetic

impl Default for rand::distr::other::Alphabetic

impl Default for rustmax::rand::distr::Alphanumeric

impl Default for rand::distr::other::Alphanumeric

impl Default for syn::token::And

impl Default for rustmax::syn::token::And

impl Default for syn::token::AndAnd

impl Default for rustmax::syn::token::AndAnd

impl Default for syn::token::AndEq

impl Default for rustmax::syn::token::AndEq

impl Default for AnyDelimiterCodec

impl Default for Arc<CStr>

impl Default for Arc<str>

impl Default for Arch

impl Default for ArchiveOffset

impl Default for Arg

impl Default for ArgGroup

impl Default for ArgMatches

impl Default for syn::token::As

impl Default for rustmax::syn::token::As

impl Default for AsciiChar

impl Default for AsciiParser

impl Default for syn::token::Async

impl Default for rustmax::syn::token::Async

impl Default for syn::token::At

impl Default for rustmax::syn::token::At

impl Default for rustmax::core::sync::atomic::Atomic<bool>

impl Default for rustmax::core::sync::atomic::Atomic<i8>

impl Default for rustmax::core::sync::atomic::Atomic<i16>

impl Default for rustmax::core::sync::atomic::Atomic<i32>

impl Default for rustmax::core::sync::atomic::Atomic<i64>

impl Default for rustmax::core::sync::atomic::Atomic<isize>

impl Default for rustmax::core::sync::atomic::Atomic<u8>

impl Default for rustmax::core::sync::atomic::Atomic<u16>

impl Default for rustmax::core::sync::atomic::Atomic<u32>

impl Default for rustmax::core::sync::atomic::Atomic<u64>

impl Default for rustmax::core::sync::atomic::Atomic<usize>

impl Default for AtomicBool

impl Default for AtomicI8

impl Default for AtomicI16

impl Default for AtomicI32

Trait Default

§`enum`s

§How can I implement `Default`?