Trait kvarn::prelude::networking::prelude::utils::prelude::compact_str::core::default::Default

1.0.0 · source ·

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

Available on non-crate feature miri-test-libstd only.

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.

§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§

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 }
}

Object Safety§

This trait is not object safe.

Trait kvarn::prelude::networking::prelude::utils::prelude::compact_str::core::default::Default

§Derivable

§enums

§How can I implement Default?

§Examples

Required Methods§

fn default() -> Self

§Examples

Object Safety§

Implementors§

impl Default for &str

impl Default for &CStr

impl Default for &OsStr

impl Default for &mut str

impl Default for PreferredCompression

impl Default for ContextType

impl Default for PollNext

impl Default for PrefilterConfig

impl Default for HpkeAead

impl Default for HpkeKdf

impl Default for rustls::quic::Version

impl Default for rustls::quic::Version

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 YearRepr

impl Default for MissedTickBehavior

impl Default for bool

impl Default for char

impl Default for f32

impl Default for f64

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 CompressionOptions

impl Default for AllowList

impl Default for Rule

impl Default for ValueSet

impl Default for Plugins

impl Default for kvarn::extensions::Extensions

impl Default for Options

impl Default for LimitManager

impl Default for RunConfig

impl Default for Settings

impl Default for kvarn::prelude::fs::OpenOptions

impl Default for Bytes

impl Default for BytesMut

impl Default for CompactString

impl Default for kvarn::prelude::Duration

impl Default for Method

impl Default for PathBuf

impl Default for StatusCode

impl Default for Uri

impl Default for kvarn::prelude::Version

impl Default for WriteableBytes

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

impl Default for AtomicI32

impl Default for AtomicI64

impl Default for AtomicIsize

impl Default for AtomicU8

impl Default for AtomicU16

§`enum`s

§How can I implement `Default`?