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Trait std::default::Default ​

一个用于给类型提供有用的默认值的 trait。

rust
pub trait Default: Sized {
    // Required method
    fn default() -> Self;
}

derive宏 ​

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：

rust
struct SomeOptions {
    foo: i32,
    bar: f32,
}

我们如何定义一些默认值？ 您可以使用 Default：

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

fn main() {
    let options: SomeOptions = Default::default();
}

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：

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

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 ​

在 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。

rust
#[derive(Default)]
enum Kind {
    #[default]
    A,
    B,
    C,
}

不能在非单元或非详尽变体上使用 #[default] 属性。

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：

rust
enum Kind {
    A,
    B,
    C,
}

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

Required Methods ​

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。

rust
fn default() -> Self

使用内置的默认值：

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let i: i8 = Default::default();
let (x, y): (Option<String>, f64) = Default::default();
let (a, b, (c, d)): (i32, u32, (bool, bool)) = Default::default();

制作自己的：

rust
enum Kind {
    A,
    B,
    C,
}

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

​

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> ​

rust
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> ​

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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> ​

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

impl<B> Default for Cow<'_, B> ​

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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> ​

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

impl<I> Default for Copied<I> ​

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

impl<I> Default for Enumerate<I> ​

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

impl<I> Default for Flatten<I> ​

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

impl<I> Default for Fuse<I> ​

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

impl<I> Default for Rev<I> ​

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

impl<Idx> Default for std::ops::Range<Idx> ​

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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> ​

rust
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> ​

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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> ​

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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> ​

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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] ​

impl<T> Default for [T; 1] ​

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

impl<T> Default for [T; 2] ​

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

impl<T> Default for [T; 3] ​

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

impl<T> Default for [T; 4] ​

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

impl<T> Default for [T; 5] ​

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

impl<T> Default for [T; 6] ​

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

impl<T> Default for [T; 7] ​

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

impl<T> Default for [T; 8] ​

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

impl<T> Default for [T; 9] ​

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

impl<T> Default for [T; 10] ​

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

impl<T> Default for [T; 11] ​

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

impl<T> Default for [T; 12] ​

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

impl<T> Default for [T; 13] ​

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

impl<T> Default for [T; 14] ​

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

impl<T> Default for [T; 15] ​

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

impl<T> Default for [T; 16] ​

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

impl<T> Default for [T; 17] ​

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

impl<T> Default for [T; 18] ​

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

impl<T> Default for [T; 19] ​

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

impl<T> Default for [T; 20] ​

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

impl<T> Default for [T; 21] ​

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

impl<T> Default for [T; 22] ​

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

impl<T> Default for [T; 23] ​

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

impl<T> Default for [T; 24] ​

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

impl<T> Default for [T; 25] ​

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

impl<T> Default for [T; 26] ​

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

impl<T> Default for [T; 27] ​

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

impl<T> Default for [T; 28] ​

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

impl<T> Default for [T; 29] ​

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

impl<T> Default for [T; 30] ​

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

impl<T> Default for [T; 31] ​

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

impl<T> Default for [T; 32] ​

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

impl<T> Default for (T₁, T₂, …, Tₙ) ​

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

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

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

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

impl<T> Default for Cell<T> ​

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

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

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

impl<T> Default for OnceCell<T> ​

impl<T> Default for RefCell<T> ​

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

impl<T> Default for SyncUnsafeCell<T> ​

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

impl<T> Default for UnsafeCell<T> ​

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

impl<T> Default for Reverse<T> ​

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

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

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

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

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

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

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

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

impl<T> Default for BinaryHeap<T> ​

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

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

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

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

impl<T> Default for PhantomData<T> ​

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

impl<T> Default for ManuallyDrop<T> ​

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

impl<T> Default for Saturating<T> ​

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

impl<T> Default for Wrapping<T> ​

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

impl<T> Default for AssertUnwindSafe<T> ​

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

impl<T> Default for Rc<T> ​

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

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

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

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

impl<T> Default for AtomicPtr<T> ​

impl<T> Default for Arc<T> ​

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

impl<T> Default for Exclusive<T> ​

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

impl<T> Default for OnceLock<T> ​

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

impl<T> Default for Vec<T, Global> ​

impl<T, A> Default for std::collections::btree_set::IntoIter<T, A> ​

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

impl<T, A> Default for std::vec::IntoIter<T, A> ​

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

impl<T, S> Default for HashSet<T, S> ​

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

impl<T, const LANES: usize> Default for Mask<T, LANES> ​

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

impl<T, const N: usize> Default for Simd<T, N> ​

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

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

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

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

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

impl Default for TokenStream ​

impl Default for TestTimeOptions ​

impl Default for OutputFormat ​

impl Default for ColorConfig ​

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