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".

Implementors§

1.10.0 (const: unstable) · Source§

impl Default for &CStr

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 &mut str

impl Default for &str

impl Default for ()

impl Default for AHasher

§Example

impl Default for Abi

impl Default for ActorId

impl Default for Alignment

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 AtomicWaker

impl Default for B0

impl Default for B1

impl Default for BigEndian

impl Default for Bits

impl Default for BlockNumberWithHash

impl Default for gstd::prelude::Box<CStr>

impl Default for gstd::prelude::Box<str>

impl Default for ByteString

impl Default for CString

impl Default for CodeId

impl Default for scale_decode::error::context::Context

impl Default for scale_encode::error::context::Context

impl Default for Duration

impl Default for Eager

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

impl Default for Equal

impl Default for Error

impl Default for ErrorBytes

impl Default for ErrorWithGas

impl Default for ErrorWithHandle

impl Default for ErrorWithHash

impl Default for ErrorWithReplyCode

impl Default for ErrorWithSignalCode

impl Default for ErrorWithTwoHashes

impl Default for FormattingOptions

impl Default for alloc::alloc::Global

impl Default for allocator_api2::stable::alloc::global::Global

impl Default for Greater

impl Default for primitive_types::H128

impl Default for primitive_types::H128

impl Default for primitive_types::H160

impl Default for primitive_types::H160

impl Default for primitive_types::H256

impl Default for primitive_types::H256

impl Default for primitive_types::H384

impl Default for primitive_types::H384

impl Default for primitive_types::H512

impl Default for primitive_types::H512

impl Default for primitive_types::H768

impl Default for primitive_types::H768

impl Default for HashWithValue

impl Default for InvalidBufferSize

impl Default for InvalidOutputSize

impl Default for block_buffer::Lazy

impl Default for Less

Trait Default

§`enum`s

§How can I implement `Default`?

impl<'a, K, V> Default for gstd::prelude::collections::btree_map::Iter<'a, K, V>
where K: 'a, V: 'a,

impl<'a, K, V> Default for gstd::prelude::collections::btree_map::IterMut<'a, K, V>
where K: 'a, V: 'a,

impl<A, B> Default for Chain<A, B>
where A: Default, B: Default,

impl<A> Default for allocator_api2::stable::boxed::Box<str, A>
where A: Allocator + Default,

impl<A> Default for SmallVec<A>
where A: Array,

impl<B> Default for Cow<'_, B>
where B: ToOwned + ?Sized, <B as ToOwned>::Owned: Default,

impl<F, N, T> Default for FieldBuilder<F, N, T>
where F: Form,

impl<F, S> Default for TypeBuilder<F, S>
where F: Form,

impl<F, T> Default for FieldsBuilder<F, T>
where F: Form,