Trait Default

1.0.0 (const: unstable) · Source

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

Expand description

🌱 core A trait for giving a type a useful default value. (Derivable)

^📍^code ^{re-exported from core::default}

📜
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 &CStr

1.9.0 · Source§

impl Default for &OsStr

1.28.0 (const: unstable) · Source§

impl Default for &mut str

1.0.0 (const: unstable) · Source§

impl Default for &str

1.0.0 (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 &CStr

impl Default for &OsStr

impl Default for &mut str

impl Default for &str

impl Default for ()

impl Default for Abi

impl Default for Adler32

impl Default for Alignment

impl Default for AngleDirection

impl Default for AngleKind

impl Default for AnsiColor3

impl Default for AppControlSet

impl Default for alloc::sync::Arc<CStr>

impl Default for alloc::sync::Arc<str>

impl Default for devela::zall::Arc<str>

impl Default for AsciiChar

impl Default for AsciiSet

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

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

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

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

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

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

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

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

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

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

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

impl Default for AtomicBool

impl Default for AtomicF32

impl Default for AtomicF64

impl Default for AtomicI8

impl Default for AtomicI16

impl Default for AtomicI32

impl Default for AtomicI64

impl Default for AtomicI128

impl Default for AtomicIsize

impl Default for AtomicU8

impl Default for AtomicU16

impl Default for AtomicU32

impl Default for AtomicU64

impl Default for AtomicU128

impl Default for AtomicUsize

impl Default for AudioChannel

impl Default for AudioChannels

impl Default for AudioDeviceDir

impl Default for AudioStreamDir

impl Default for BinTag4

impl Default for BitfieldExample

impl Default for Box<CStr>

impl Default for Box<OsStr>

impl Default for Box<str>

impl Default for Boxed

impl Default for BrokenDownTime

impl Default for ByteString

impl Default for CString

impl Default for CharAscii

impl Default for ColorDepth

impl Default for std::sync::nonpoison::condvar::Condvar

impl Default for devela::zall::Condvar

impl Default for CpuId<CpuIdReaderNative>

impl Default for Date

impl Default for DateTime

impl Default for DateTimeRound

impl Default for DefaultCustom

impl Default for DefaultHashBuilder

impl Default for DefaultHasher

impl Default for DefaultRandomSource

impl Default for DeviceId

Trait Default

§`enum`s

§How can I implement `Default`?