uniffi::deps::static_assertions::_core::prelude::v1

Trait Default

1.6.0 · 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 · 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 &StyledStr

impl Default for &Styles

impl Default for &CStr

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

impl Default for &mut str

impl Default for Action

impl Default for State

impl Default for DependencyKind

impl Default for Edition

impl Default for ValueHint

impl Default for clap_builder::util::color::ColorChoice

impl Default for colorchoice::ColorChoice

impl Default for Container

impl Default for PrefilterConfig

impl Default for StrCtx

impl Default for Endian

impl Default for Value

§Examples

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 StripBytes

impl Default for StripStr

impl Default for WinconBytes

impl Default for Effects

impl Default for Reset

impl Default for Style

impl Default for Params

impl Default for AsciiParser

impl Default for Utf8Parser

impl Default for Utf8PathBuf

impl Default for MetadataCommand

impl Default for Arg

impl Default for ArgGroup

impl Default for Command

impl Default for clap_builder::builder::os_str::OsStr

impl Default for PossibleValue

impl Default for ValueRange

impl Default for Str

impl Default for StyledStr

impl Default for Styles

impl Default for BoolValueParser

impl Default for BoolishValueParser

impl Default for FalseyValueParser

impl Default for NonEmptyStringValueParser

impl Default for OsStringValueParser

impl Default for PathBufValueParser

impl Default for StringValueParser

impl Default for ArgMatches

impl Default for Indices<'_>

impl Default for RawValues<'_>

impl Default for Id

Trait Default

§`enum`s

§How can I implement `Default`?