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 (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".

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 &Bytes

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 Alignment

impl Default for AllocationHistory

impl Default for Arc<CStr>

impl Default for Arc<str>

impl Default for AsciiChar

impl Default for Atomic<bool>

impl Default for Atomic<i8>

impl Default for Atomic<i16>

impl Default for Atomic<i32>

impl Default for Atomic<i64>

impl Default for Atomic<isize>

impl Default for Atomic<u8>

impl Default for Atomic<u16>

impl Default for Atomic<u32>

impl Default for Atomic<u64>

impl Default for Atomic<usize>

impl Default for B0

impl Default for B1

impl Default for BigEndian

impl Default for BigInt

impl Default for BigUint

impl Default for icydb_core::types::Blob

impl Default for Box<Bytes>

impl Default for Box<CStr>

impl Default for Box<OsStr>

impl Default for Box<str>

impl Default for Buffer

impl Default for ByteBuf

impl Default for ByteString

impl Default for CString

impl Default for anyhow::Chain<'_>

impl Default for CompactEntityMetrics

impl Default for CompactEventCounters

impl Default for CompactMetric

impl Default for CompactMetricsReport

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

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

impl Default for Converter

impl Default for DataStoreSnapshot

impl Default for Date

impl Default for Day

impl Default for Decimal

impl Default for DeclarationCollector

impl Default for DecoderConfig

impl Default for DefaultHasher

impl Default for DeleteSpec

impl Default for DocComments

impl Default for DualCommitStore

impl Default for core::time::Duration

impl Default for time::duration::Duration

impl Default for icydb_core::types::Duration

impl Default for Eager

impl Default for core::io::util::Empty

impl Default for End

impl Default for Endian

impl Default for EntitySnapshot

impl Default for EntitySummary

impl Default for Equal

impl Default for Error

impl Default for EventCounters

impl Default for EventOps

impl Default for EventReport

Trait Default

§`enum`s

§How can I implement `Default`?