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Trait std::default::Default

pub trait Default {
    fn default() -> Self;
}

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.

How can I implement `Default`?

Provides 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

`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 }
}

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Implementations on Foreign Types

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

`fn default() -> HashMap<K, V, S>`

Creates an empty HashMap<K, V, S>, with the Default value for the hasher.

`impl<T, S> Default for HashSet<T, S> where S: BuildHasher + Default, T: Eq + Hash,`

`fn default() -> HashSet<T, S>`

Creates an empty HashSet<T, S> with the Default value for the hasher.

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Implementors

`impl Default for bool`[src]

`fn default() -> bool`[src]

Returns the default value of false

`impl Default for char`[src]

`fn default() -> char`[src]

Returns the default value of \x00

`impl Default for f32`[src]

`fn default() -> f32`[src]

Returns the default value of 0.0

`impl Default for f64`[src]

`fn default() -> f64`[src]

Returns the default value of 0.0

`impl Default for i128`[src]

`fn default() -> i128`[src]

Returns the default value of 0

`impl Default for i16`[src]

`fn default() -> i16`[src]

Returns the default value of 0

`impl Default for i32`[src]

`fn default() -> i32`[src]

Returns the default value of 0

`impl Default for i64`[src]

`fn default() -> i64`[src]

Returns the default value of 0

`impl Default for i8`[src]

`fn default() -> i8`[src]

Returns the default value of 0

`impl Default for isize`[src]

`fn default() -> isize`[src]

Returns the default value of 0

`impl Default for u128`[src]

`fn default() -> u128`[src]

Returns the default value of 0

`impl Default for u16`[src]

`fn default() -> u16`[src]

Returns the default value of 0

`impl Default for u32`[src]

`fn default() -> u32`[src]

Returns the default value of 0

`impl Default for u64`[src]

`fn default() -> u64`[src]

Returns the default value of 0

`impl Default for u8`[src]

`fn default() -> u8`[src]

Returns the default value of 0

`impl Default for ()`[src]

`fn default()`[src]

Returns the default value of ()

`impl Default for usize`[src]

`fn default() -> usize`[src]

Returns the default value of 0

`impl Default for Global`[src]

`fn default() -> Global`[src]

`impl Default for System`[src]

`fn default() -> System`[src]

`impl Default for Box<str>`[src]

`fn default() -> Box<str>`[src]

ⓘImportant traits for Box<I>

impl<I> Iterator for Box<I> where
    I: Iterator + ?Sized, 
    type Item = <I as Iterator>::Item;
impl<F> Future for Box<F> where
    F: Unpin + Future + ?Sized, 
    type Output = <F as Future>::Output;
impl<R: Read + ?Sized> Read for Box<R>
impl<W: Write + ?Sized> Write for Box<W>

`impl Default for Box<CStr>`[src]

`fn default() -> Box<CStr>`[src]

ⓘImportant traits for Box<I>

impl<I> Iterator for Box<I> where
    I: Iterator + ?Sized, 
    type Item = <I as Iterator>::Item;
impl<F> Future for Box<F> where
    F: Unpin + Future + ?Sized, 
    type Output = <F as Future>::Output;
impl<R: Read + ?Sized> Read for Box<R>
impl<W: Write + ?Sized> Write for Box<W>

`impl Default for Box<OsStr>`[src]

`fn default() -> Box<OsStr>`[src]

ⓘImportant traits for Box<I>

impl<I> Iterator for Box<I> where
    I: Iterator + ?Sized, 
    type Item = <I as Iterator>::Item;
impl<F> Future for Box<F> where
    F: Unpin + Future + ?Sized, 
    type Output = <F as Future>::Output;
impl<R: Read + ?Sized> Read for Box<R>
impl<W: Write + ?Sized> Write for Box<W>

`impl Default for DefaultHasher`[src]

`fn default() -> DefaultHasher`[src]

Creates a new DefaultHasher using new. See its documentation for more.

`impl Default for RandomState`[src]

`fn default() -> RandomState`[src]

Constructs a new RandomState.

`impl Default for CString`[src]

`fn default() -> CString`[src]

Creates an empty CString.

`impl Default for OsString`[src]

`fn default() -> OsString`[src]

Constructs an empty OsString.

`impl Default for Error`[src]

`fn default() -> Error`[src]

`impl Default for SipHasher`[src]

`fn default() -> SipHasher`[src]

`impl Default for PathBuf`[src]

`fn default() -> Self`[src]

`impl Default for String`[src]

`fn default() -> String`[src]

Creates an empty String.

`impl Default for AtomicBool`[src]

`fn default() -> AtomicBool`[src]

Creates an AtomicBool initialized to false.

`impl Default for AtomicI16`[src]

`fn default() -> AtomicI16`[src]

`impl Default for AtomicI32`[src]

`fn default() -> AtomicI32`[src]

`impl Default for AtomicI64`[src]

`fn default() -> AtomicI64`[src]

`impl Default for AtomicI8`[src]

`fn default() -> AtomicI8`[src]

`impl Default for AtomicIsize`[src]

`fn default() -> AtomicIsize`[src]

`impl Default for AtomicU16`[src]

`fn default() -> AtomicU16`[src]

`impl Default for AtomicU32`[src]

`fn default() -> AtomicU32`[src]

`impl Default for AtomicU64`[src]

`fn default() -> AtomicU64`[src]

`impl Default for AtomicU8`[src]

`fn default() -> AtomicU8`[src]

`impl Default for AtomicUsize`[src]

`fn default() -> AtomicUsize`[src]

`impl Default for Condvar`[src]

`fn default() -> Condvar`[src]

Creates a Condvar which is ready to be waited on and notified.

`impl Default for Duration`[src]

`fn default() -> Duration`[src]

`impl<'_> Default for &'_ str`[src]

`fn default() -> &'_ str`[src]

Creates an empty str

`impl<'_> Default for &'_ CStr`[src]

`fn default() -> Self`[src]

`impl<'_> Default for &'_ OsStr`[src]

`fn default() -> Self`[src]

Creates an empty OsStr.

`impl<'_> Default for &'_ mut str`[src]

`fn default() -> &'_ mut str`[src]

Creates an empty mutable str

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

`fn default() -> Cow<'_, B>`[src]

Creates an owned Cow<'a, B> with the default value for the contained owned value.

`impl<'_, T> Default for &'_ [T]`[src]

`fn default() -> &'_ [T]`[src]

ⓘImportant traits for &'_ [u8 ]

impl<'_> Read for &'_ [u8]
impl<'_> Write for &'_ mut [u8]

Creates an empty slice.

`impl<'_, T> Default for &'_ mut [T]`[src]

`fn default() -> &'_ mut [T]`[src]

ⓘImportant traits for &'_ [u8 ]

impl<'_> Read for &'_ [u8]
impl<'_> Write for &'_ mut [u8]

Creates a mutable empty slice.

`impl<A> Default for (A,) where A: Default,` [src]

`fn default() -> (A,)`[src]

`impl<A, B> Default for (A, B) where A: Default, B: Default,` [src]

`fn default() -> (A, B)`[src]

`impl<A, B, C> Default for (A, B, C) where A: Default, B: Default, C: Default,` [src]

`fn default() -> (A, B, C)`[src]

`impl<A, B, C, D> Default for (A, B, C, D) where A: Default, B: Default, C: Default, D: Default,` [src]

`fn default() -> (A, B, C, D)`[src]

`impl<A, B, C, D, E> Default for (A, B, C, D, E) where A: Default, B: Default, C: Default, D: Default, E: Default,` [src]

`fn default() -> (A, B, C, D, E)`[src]

`impl<A, B, C, D, E, F> Default for (A, B, C, D, E, F) where A: Default, B: Default, C: Default, D: Default, E: Default, F: Default,` [src]

`fn default() -> (A, B, C, D, E, F)`[src]

`impl<A, B, C, D, E, F, G> Default for (A, B, C, D, E, F, G) where A: Default, B: Default, C: Default, D: Default, E: Default, F: Default, G: Default,` [src]

`fn default() -> (A, B, C, D, E, F, G)`[src]

`impl<A, B, C, D, E, F, G, H> Default for (A, B, C, D, E, F, G, H) where A: Default, B: Default, C: Default, D: Default, E: Default, F: Default, G: Default, H: Default,` [src]

`fn default() -> (A, B, C, D, E, F, G, H)`[src]

`impl<A, B, C, D, E, F, G, H, I> Default for (A, B, C, D, E, F, G, H, I) where A: Default, B: Default, C: Default, D: Default, E: Default, F: Default, G: Default, H: Default, I: Default,` [src]

`fn default() -> (A, B, C, D, E, F, G, H, I)`[src]

`impl<A, B, C, D, E, F, G, H, I, J> Default for (A, B, C, D, E, F, G, H, I, J) where A: Default, B: Default, C: Default, D: Default, E: Default, F: Default, G: Default, H: Default, I: Default, J: Default,` [src]

`fn default() -> (A, B, C, D, E, F, G, H, I, J)`[src]

`impl<A, B, C, D, E, F, G, H, I, J, K> Default for (A, B, C, D, E, F, G, H, I, J, K) where A: Default, B: Default, C: Default, D: Default, E: Default, F: Default, G: Default, H: Default, I: Default, J: Default, K: Default,` [src]

`fn default() -> (A, B, C, D, E, F, G, H, I, J, K)`[src]

`impl<A, B, C, D, E, F, G, H, I, J, K, L> Default for (A, B, C, D, E, F, G, H, I, J, K, L) where A: Default, B: Default, C: Default, D: Default, E: Default, F: Default, G: Default, H: Default, I: Default, J: Default, K: Default, L: Default,` [src]

`fn default() -> (A, B, C, D, E, F, G, H, I, J, K, L)`[src]

`impl<H> Default for BuildHasherDefault<H>`[src]

`fn default() -> BuildHasherDefault<H>`[src]

`impl<K, V> Default for BTreeMap<K, V> where K: Ord,` [src]

`fn default() -> BTreeMap<K, V>`[src]

Creates an empty BTreeMap<K, V>.

`impl<K, V, S> Default for std::collections::HashMap<K, V, S> where K: Eq + Hash, S: BuildHasher + Default,` [src]

`fn default() -> HashMap<K, V, S>`[src]

Creates an empty HashMap<K, V, S>, with the Default value for the hasher.

`impl<T> Default for Option<T>`[src]

`fn default() -> Option<T>`[src]

Returns None.

`impl<T> Default for [T; 0]`[src]

`fn default() -> [T; 0]`[src]

`impl<T> Default for [T; 1] where T: Default,` [src]

`fn default() -> [T; 1]`[src]

`impl<T> Default for [T; 2] where T: Default,` [src]

`fn default() -> [T; 2]`[src]

`impl<T> Default for [T; 3] where T: Default,` [src]

`fn default() -> [T; 3]`[src]

`impl<T> Default for [T; 4] where T: Default,` [src]

`fn default() -> [T; 4]`[src]

`impl<T> Default for [T; 5] where T: Default,` [src]

`fn default() -> [T; 5]`[src]

`impl<T> Default for [T; 6] where T: Default,` [src]

`fn default() -> [T; 6]`[src]

`impl<T> Default for [T; 7] where T: Default,` [src]

`fn default() -> [T; 7]`[src]

`impl<T> Default for [T; 8] where T: Default,` [src]

`fn default() -> [T; 8]`[src]

`impl<T> Default for [T; 9] where T: Default,` [src]

`fn default() -> [T; 9]`[src]

`impl<T> Default for [T; 10] where T: Default,` [src]

`fn default() -> [T; 10]`[src]

`impl<T> Default for [T; 11] where T: Default,` [src]

`fn default() -> [T; 11]`[src]

`impl<T> Default for [T; 12] where T: Default,` [src]

`fn default() -> [T; 12]`[src]

`impl<T> Default for [T; 13] where T: Default,` [src]

`fn default() -> [T; 13]`[src]

`impl<T> Default for [T; 14] where T: Default,` [src]

`fn default() -> [T; 14]`[src]

`impl<T> Default for [T; 15] where T: Default,` [src]

`fn default() -> [T; 15]`[src]

`impl<T> Default for [T; 16] where T: Default,` [src]

`fn default() -> [T; 16]`[src]

`impl<T> Default for [T; 17] where T: Default,` [src]

`fn default() -> [T; 17]`[src]

`impl<T> Default for [T; 18] where T: Default,` [src]

`fn default() -> [T; 18]`[src]

`impl<T> Default for [T; 19] where T: Default,` [src]

`fn default() -> [T; 19]`[src]

`impl<T> Default for [T; 20] where T: Default,` [src]

`fn default() -> [T; 20]`[src]

`impl<T> Default for [T; 21] where T: Default,` [src]

`fn default() -> [T; 21]`[src]

`impl<T> Default for [T; 22] where T: Default,` [src]

`fn default() -> [T; 22]`[src]

`impl<T> Default for [T; 23] where T: Default,` [src]

`fn default() -> [T; 23]`[src]

`impl<T> Default for [T; 24] where T: Default,` [src]

`fn default() -> [T; 24]`[src]

`impl<T> Default for [T; 25] where T: Default,` [src]

`fn default() -> [T; 25]`[src]

`impl<T> Default for [T; 26] where T: Default,` [src]

`fn default() -> [T; 26]`[src]

`impl<T> Default for [T; 27] where T: Default,` [src]

`fn default() -> [T; 27]`[src]

`impl<T> Default for [T; 28] where T: Default,` [src]

`fn default() -> [T; 28]`[src]

`impl<T> Default for [T; 29] where T: Default,` [src]

`fn default() -> [T; 29]`[src]

`impl<T> Default for [T; 30] where T: Default,` [src]

`fn default() -> [T; 30]`[src]

`impl<T> Default for [T; 31] where T: Default,` [src]

`fn default() -> [T; 31]`[src]

`impl<T> Default for [T; 32] where T: Default,` [src]

`fn default() -> [T; 32]`[src]

`impl<T> Default for Box<[T]>`[src]

`fn default() -> Box<[T]>`[src]

ⓘImportant traits for Box<I>

impl<I> Iterator for Box<I> where
    I: Iterator + ?Sized, 
    type Item = <I as Iterator>::Item;
impl<F> Future for Box<F> where
    F: Unpin + Future + ?Sized, 
    type Output = <F as Future>::Output;
impl<R: Read + ?Sized> Read for Box<R>
impl<W: Write + ?Sized> Write for Box<W>

`impl<T> Default for Box<T> where T: Default,` [src]

`fn default() -> Box<T>`[src]

ⓘImportant traits for Box<I>

impl<I> Iterator for Box<I> where
    I: Iterator + ?Sized, 
    type Item = <I as Iterator>::Item;
impl<F> Future for Box<F> where
    F: Unpin + Future + ?Sized, 
    type Output = <F as Future>::Output;
impl<R: Read + ?Sized> Read for Box<R>
impl<W: Write + ?Sized> Write for Box<W>

Creates a Box<T>, with the Default value for T.

`impl<T> Default for Cell<T> where T: Default,` [src]

`fn default() -> Cell<T>`[src]

Creates a Cell<T>, with the Default value for T.

`impl<T> Default for RefCell<T> where T: Default,` [src]

`fn default() -> RefCell<T>`[src]

Creates a RefCell<T>, with the Default value for T.

`impl<T> Default for UnsafeCell<T> where T: Default,` [src]

`fn default() -> UnsafeCell<T>`[src]

Creates an UnsafeCell, with the Default value for T.

`impl<T> Default for Reverse<T> where T: Default,` [src]

`fn default() -> Reverse<T>`[src]

`impl<T> Default for BinaryHeap<T> where T: Ord,` [src]

`fn default() -> BinaryHeap<T>`[src]

Creates an empty BinaryHeap<T>.

`impl<T> Default for BTreeSet<T> where T: Ord,` [src]

`fn default() -> BTreeSet<T>`[src]

Makes an empty BTreeSet<T> with a reasonable choice of B.

`impl<T> Default for LinkedList<T>`[src]

`fn default() -> LinkedList<T>`[src]

Creates an empty LinkedList<T>.

`impl<T> Default for VecDeque<T>`[src]

`fn default() -> VecDeque<T>`[src]

Creates an empty VecDeque<T>.

`impl<T> Default for Empty<T>`[src]

`fn default() -> Empty<T>`[src]

ⓘImportant traits for Empty<T>

impl<T> Iterator for Empty<T>
    type Item = T;

`impl<T> Default for PhantomData<T> where T: ?Sized,` [src]

`fn default() -> PhantomData<T>`[src]

`impl<T> Default for ManuallyDrop<T> where T: Default + ?Sized,` [src]

`fn default() -> ManuallyDrop<T>`[src]

`impl<T> Default for Wrapping<T> where T: Default,` [src]

`fn default() -> Wrapping<T>`[src]

`impl<T> Default for Rc<T> where T: Default,` [src]

`fn default() -> Rc<T>`[src]

Creates a new Rc<T>, with the Default value for T.

Examples

use std::rc::Rc;

let x: Rc<i32> = Default::default();
assert_eq!(*x, 0);

`impl<T> Default for std::rc::Weak<T>`[src]

`fn default() -> Weak<T>`[src]

Constructs a new Weak<T>, allocating memory for T without initializing it. Calling upgrade on the return value always gives None.

Examples

use std::rc::Weak;

let empty: Weak<i64> = Default::default();
assert!(empty.upgrade().is_none());

`impl<T> Default for AtomicPtr<T>`[src]

`fn default() -> AtomicPtr<T>`[src]

Creates a null AtomicPtr<T>.

`impl<T> Default for Arc<T> where T: Default,` [src]

`fn default() -> Arc<T>`[src]

Creates a new Arc<T>, with the Default value for T.

Examples

use std::sync::Arc;

let x: Arc<i32> = Default::default();
assert_eq!(*x, 0);

`impl<T> Default for std::sync::Weak<T>`[src]

`fn default() -> Weak<T>`[src]

Constructs a new Weak<T>, without allocating memory. Calling upgrade on the return value always gives None.

Examples

use std::sync::Weak;

let empty: Weak<i64> = Default::default();
assert!(empty.upgrade().is_none());

`impl<T> Default for Vec<T>`[src]

`fn default() -> Vec<T>`[src]

ⓘImportant traits for Vec<u8>

impl Write for Vec<u8>

Creates an empty Vec<T>.

`impl<T, S> Default for std::collections::HashSet<T, S> where T: Eq + Hash, S: BuildHasher + Default,` [src]

`fn default() -> HashSet<T, S>`[src]

Creates an empty HashSet<T, S> with the Default value for the hasher.

`impl<T: Default> Default for Cursor<T>`[src]

`fn default() -> Cursor<T>`[src]

ⓘImportant traits for Cursor<T>

impl<T> Read for Cursor<T> where
    T: AsRef<[u8]>, 
impl<'_> Write for Cursor<&'_ mut [u8]>
impl<'_> Write for Cursor<&'_ mut Vec<u8>>
impl Write for Cursor<Vec<u8>>
impl Write for Cursor<Box<[u8]>>

`impl<T: Default> Default for RwLock<T>`[src]

`fn default() -> RwLock<T>`[src]

Creates a new RwLock<T>, with the Default value for T.

`impl<T: ?Sized + Default> Default for Mutex<T>`[src]

`fn default() -> Mutex<T>`[src]

Creates a Mutex<T>, with the Default value for T.

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© 2010 The Rust Project Developers
Licensed under the Apache License, Version 2.0 or the MIT license, at your option.
https://doc.rust-lang.org/std/default/trait.Default.html