Trait std::default::Default

1.0.0 · source ·
pub trait Default: Sized {
    // Required method
    fn default() -> Self;
}
Expand description

一个用于给类型提供有用的默认值的 trait。

有时,您希望回退到某种默认值,而不必特别关心它是什么。 这经常出现在定义一组选项的 struct 中:

struct SomeOptions {
    foo: i32,
    bar: f32,
}
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我们如何定义一些默认值? 您可以使用 Default

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

fn main() {
    let options: SomeOptions = Default::default();
}
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现在,您将获得所有默认值。Rust 为各种原始类型实现 Default

如果要覆盖特定选项,但仍保留其他默认值:

fn main() {
    let options = SomeOptions { foo: 42, ..Default::default() };
}
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Derivable

如果类型的所有字段都实现 Default,则此 trait 可以与 #[derive] 一起使用。 当 derived 时,它将为每个字段的类型使用默认值。

enums

enum 上使用 #[derive(Default)] 时,您需要选择默认的单元变体。 您可以通过将 #[default] 属性放在变体上来执行此操作。

#[derive(Default)]
enum Kind {
    #[default]
    A,
    B,
    C,
}
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您不能在非单元或非详尽变体上使用 #[default] 属性。

如何实现 Default

提供 default() 方法的实现,该实现返回您类型的值,该值应为默认值:

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§

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fn default() -> Self

返回类型的 “默认值”。

默认值通常是某种初始值,标识值或其他可能有意义的默认值。

Examples

使用内置的默认值:

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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制作自己的:

enum Kind {
    A,
    B,
    C,
}

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

Implementors§

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impl Default for &str

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impl Default for &CStr

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impl Default for &OsStr

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impl Default for &mut str

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impl Default for bool

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impl Default for char

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impl Default for f32

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impl Default for f64

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impl Default for i8

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impl Default for i16

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impl Default for i32

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impl Default for i64

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impl Default for i128

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impl Default for isize

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impl Default for u8

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impl Default for u16

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impl Default for u32

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impl Default for u64

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impl Default for u128

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impl Default for ()

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impl Default for usize

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impl Default for Global

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impl Default for System

1.17.0 · source§

impl Default for Box<str, Global>

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impl Default for Box<CStr, Global>

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impl Default for Box<OsStr>

1.13.0 · source§

impl Default for DefaultHasher

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impl Default for RandomState

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impl Default for CString

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impl Default for OsString

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impl Default for Error

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impl Default for FileTimes

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impl Default for SipHasher

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impl Default for std::io::Empty

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impl Default for Sink

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impl Default for PhantomPinned

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impl Default for RangeFull

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impl Default for PathBuf

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impl Default for String

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impl Default for AtomicBool

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impl Default for AtomicI8

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impl Default for AtomicI16

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impl Default for AtomicI32

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impl Default for AtomicI64

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impl Default for AtomicIsize

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impl Default for AtomicU8

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impl Default for AtomicU16

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impl Default for AtomicU32

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impl Default for AtomicU64

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impl Default for AtomicUsize

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impl Default for Condvar

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impl Default for Duration

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

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

1.70.0 · source§

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

1.11.0 · source§

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

1.7.0 · source§

impl<H> Default for BuildHasherDefault<H>

1.70.0 · source§

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

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

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

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

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

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

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

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

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

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

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

1.70.0 · source§

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

1.70.0 · source§

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

1.70.0 · source§

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

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

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impl<T> Default for &[T]

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impl<T> Default for &mut [T]

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impl<T> Default for Option<T>

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impl<T> Default for [T; 0]

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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impl<T> Default for (T₁, T₂, …, Tₙ)where T: Default,

This trait is implemented for tuples up to twelve items long.

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impl<T> Default for Box<[T], Global>

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impl<T> Default for Box<T, Global>where T: Default,

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impl<T> Default for Cell<T>where T: Default,

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impl<T> Default for LazyCell<T, fn() -> T>where T: Default,

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impl<T> Default for OnceCell<T>

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impl<T> Default for RefCell<T>where T: Default,

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impl<T> Default for SyncUnsafeCell<T>where T: Default,

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impl<T> Default for UnsafeCell<T>where T: Default,

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impl<T> Default for Reverse<T>where T: Default,

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impl<T> Default for std::collections::binary_heap::IntoIter<T>

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impl<T> Default for std::collections::btree_set::Iter<'_, T>

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impl<T> Default for std::collections::btree_set::Range<'_, T>

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impl<T> Default for std::collections::linked_list::IntoIter<T, Global>

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impl<T> Default for std::collections::linked_list::Iter<'_, T>

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impl<T> Default for std::collections::linked_list::IterMut<'_, T>

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impl<T> Default for BTreeSet<T, Global>

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impl<T> Default for BinaryHeap<T>where T: Ord,

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impl<T> Default for LinkedList<T, Global>

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impl<T> Default for VecDeque<T, Global>

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impl<T> Default for std::iter::Empty<T>

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impl<T> Default for PhantomData<T>where T: ?Sized,

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impl<T> Default for ManuallyDrop<T>where T: Default + ?Sized,

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impl<T> Default for Saturating<T>where T: Default,

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impl<T> Default for Wrapping<T>where T: Default,

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impl<T> Default for AssertUnwindSafe<T>where T: Default,

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impl<T> Default for Rc<T>where T: Default,

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impl<T> Default for std::rc::Weak<T>

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impl<T> Default for std::slice::Iter<'_, T>

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impl<T> Default for std::slice::IterMut<'_, T>

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impl<T> Default for AtomicPtr<T>

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impl<T> Default for Arc<T>where T: Default,

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impl<T> Default for Exclusive<T>where T: Default + ?Sized,

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impl<T> Default for OnceLock<T>

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impl<T> Default for std::sync::Weak<T>

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impl<T> Default for Vec<T, Global>

1.70.0 · source§

impl<T, A> Default for std::collections::btree_set::IntoIter<T, A>where A: Allocator + Default + Clone,

1.70.0 · source§

impl<T, A> Default for std::vec::IntoIter<T, A>where A: Allocator + Default,

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impl<T, S> Default for HashSet<T, S>where S: Default,

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impl<T, const LANES: usize> Default for Mask<T, LANES>where T: MaskElement, LaneCount<LANES>: SupportedLaneCount,

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impl<T, const N: usize> Default for Simd<T, N>where LaneCount<N>: SupportedLaneCount, T: SimdElement + Default,

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impl<T: Default> Default for Cursor<T>

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impl<T: Default> Default for LazyLock<T>

1.10.0 · source§

impl<T: Default> Default for RwLock<T>

1.10.0 · source§

impl<T: ?Sized + Default> Default for Mutex<T>