Trait std::default::Default

1.0.0 · 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,
}

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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();
}

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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() };
}

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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,
}

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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 }
}

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Examples

#[derive(Default)]
struct SomeOptions {
    foo: i32,
    bar: f32,
}

Run

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();

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Making your own:

enum Kind {
    A,
    B,
    C,
}

impl Default for Kind {
    fn default() -> Self { Kind::A }
}

Trait std::default::Default

Required Methods§

fn default() -> Self

Implementors§

impl Default for &str

impl Default for &CStr

impl Default for &OsStr

impl Default for &mut str

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 Global

impl Default for System

impl Default for Box<str, Global>

impl Default for Box<CStr, Global>

impl Default for Box<OsStr>

impl Default for DefaultHasher

impl Default for RandomState

impl Default for CString

impl Default for OsString

impl Default for Error

impl Default for FileTimes

impl Default for SipHasher

impl Default for std::io::Empty

impl Default for Sink

impl Default for PhantomPinned

impl Default for RangeFull

impl Default for PathBuf

impl Default for String

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

impl Default for AtomicU32

impl Default for AtomicU64

impl Default for AtomicUsize

impl Default for Condvar

impl Default for Duration

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

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

impl<H> Default for BuildHasherDefault<H>

impl<I> Default for Cloned<I>where I: Default,

impl<I> Default for Copied<I>where I: Default,

impl<I> Default for Enumerate<I>where I: Default,

impl<I> Default for Flatten<I>where I: Default + Iterator, <I as Iterator>::Item: IntoIterator,

impl<I> Default for Fuse<I>where I: Default,

impl<I> Default for Rev<I>where I: Default,

impl<Idx> Default for std::ops::Range<Idx>where Idx: Default,

impl<K, V> Default for Keys<'_, K, V>

impl<K, V> Default for std::collections::btree_map::Range<'_, K, V>

impl<K, V> Default for Values<'_, K, V>

impl<K, V> Default for BTreeMap<K, V, Global>

impl<K, V, A> Default for std::collections::btree_map::IntoIter<K, V, A>where A: Allocator + Default + Clone,

impl<K, V, A> Default for IntoKeys<K, V, A>where A: Allocator + Default + Clone,

impl<K, V, A> Default for IntoValues<K, V, A>where A: Allocator + Default + Clone,

impl<K, V, S> Default for HashMap<K, V, S>where S: Default,

impl<T> Default for &[T]

impl<T> Default for &mut [T]

impl<T> Default for Option<T>

impl<T> Default for [T; 0]