Trait gstd::prelude::Default

pub trait Default: Sized {
    // Required method
    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.

enums

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

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 }
}

Object Safety

This trait is not object safe.

Implementors

impl Default for &str

impl Default for &CStr

impl Default for &mut str

impl Default for ErrorReplyReason

impl Default for ReplyCode

impl Default for SignalCode

impl Default for SimpleExecutionError

impl Default for SimpleProgramCreationError

impl Default for SuccessReplyReason

impl Default for AsciiChar

impl Default for BigEndian

impl Default for LittleEndian

impl Default for PollNext

impl Default for bool

impl Default for char

impl Default for f16

impl Default for f32

impl Default for f64

impl Default for f128

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 gstd::ActorId

impl Default for gstd::CodeId

impl Default for gstd::MessageId

impl Default for Reservations

impl Default for CString

impl Default for Error

impl Default for SipHasher

impl Default for alloc::alloc::Global

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 AHasher

Provides a default Hasher with fixed keys. This is typically used in conjunction with BuildHasherDefault to create AHashers in order to hash the keys of the map.

Generally it is preferable to use RandomState instead, so that different hashmaps will have different keys. However if fixed keys are desirable this may be used instead.

Example

use std::hash::BuildHasherDefault;
use ahash::{AHasher, RandomState};
use std::collections::HashMap;

let mut map: HashMap<i32, i32, BuildHasherDefault<AHasher>> = HashMap::default();
map.insert(12, 34);

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

impl Default for AtomicWaker

impl Default for gcore::general::ActorId

impl Default for gcore::general::CodeId

impl Default for gcore::general::MessageId

impl Default for ReservationId

impl Default for BlockNumberWithHash

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 HashWithValue

impl Default for TwoHashes

impl Default for TwoHashesWithValue

impl Default for OnceBool

impl Default for OnceNonZeroUsize

impl Default for H128

impl Default for H160

impl Default for H256

impl Default for H384

impl Default for H512

impl Default for H768

impl Default for U128

impl Default for U256

impl Default for U512

impl Default for PortableRegistryBuilder

impl Default for Registry

impl Default for PhantomPinned

impl Default for RangeFull

impl Default for Alignment

Returns Alignment::MIN, which is valid for any type.

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

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

impl Default for String

impl Default for Duration

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, T> Default for OnceRef<'a, T>

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

where A: Allocator + Default,

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<F> Default for OptionFuture<F>

impl<F> Default for Variants<F>

where F: Default + Form,

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,

impl<Fut> Default for FuturesOrdered<Fut>

where Fut: Future,

impl<Fut> Default for FuturesUnordered<Fut>

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 gstd::prelude::ops::Range<Idx>

where Idx: Default,

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

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

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

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

impl<K, V, A> Default for gstd::prelude::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, A> Default for HashMap<K, V, S, A>

where S: Default, A: Default + Allocator,

impl<St> Default for SelectAll<St>

where St: Stream + Unpin,

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]

where T: Default,

impl<T> Default for [T; 2]

where T: Default,

impl<T> Default for [T; 3]

where T: Default,

impl<T> Default for [T; 4]

where T: Default,

impl<T> Default for [T; 5]

where T: Default,

impl<T> Default for [T; 6]

where T: Default,

impl<T> Default for [T; 7]

where T: Default,

impl<T> Default for [T; 8]

where T: Default,

impl<T> Default for [T; 9]

where T: Default,

impl<T> Default for [T; 10]

where T: Default,

impl<T> Default for [T; 11]

where T: Default,

impl<T> Default for [T; 12]

where T: Default,

impl<T> Default for [T; 13]

where T: Default,

impl<T> Default for [T; 14]

where T: Default,

impl<T> Default for [T; 15]

where T: Default,

impl<T> Default for [T; 16]

where T: Default,

impl<T> Default for [T; 17]

where T: Default,

impl<T> Default for [T; 18]

where T: Default,

impl<T> Default for [T; 19]

where T: Default,

impl<T> Default for [T; 20]

where T: Default,

impl<T> Default for [T; 21]

where T: Default,

impl<T> Default for [T; 22]

where T: Default,

impl<T> Default for [T; 23]

where T: Default,

impl<T> Default for [T; 24]

where T: Default,

impl<T> Default for [T; 25]

where T: Default,

impl<T> Default for [T; 26]

where T: Default,

impl<T> Default for [T; 27]

where T: Default,

impl<T> Default for [T; 28]

where T: Default,

impl<T> Default for [T; 29]

where T: Default,

impl<T> Default for [T; 30]

where T: Default,

impl<T> Default for [T; 31]

where T: Default,

impl<T> Default for [T; 32]

where T: Default,

impl<T> Default for (T₁, T₂, …, Tₙ)

where T: Default,

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

impl<T> Default for Cell<T>

where T: Default,

impl<T> Default for LazyCell<T>

where T: Default,

impl<T> Default for gstd::prelude::cell::OnceCell<T>

impl<T> Default for RefCell<T>

where T: Default,

impl<T> Default for SyncUnsafeCell<T>

where T: Default,

impl<T> Default for UnsafeCell<T>

where T: Default,

impl<T> Default for Reverse<T>

where T: Default,

impl<T> Default for gstd::prelude::collections::binary_heap::IntoIter<T>

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

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

impl<T> Default for gstd::prelude::collections::linked_list::IntoIter<T>

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

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

impl<T> Default for BTreeSet<T>

impl<T> Default for BinaryHeap<T>

where T: Ord,

impl<T> Default for LinkedList<T>

impl<T> Default for VecDeque<T>

impl<T> Default for Arc<T>

where T: Default,

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

impl<T> Default for AtomicPtr<T>

impl<T> Default for Exclusive<T>

where T: Default + ?Sized,

impl<T> Default for allocator_api2::stable::boxed::Box<T>

where T: Default,

impl<T> Default for allocator_api2::stable::vec::Vec<T>

impl<T> Default for OnceBox<T>

impl<T> Default for Lazy<T>

where T: Default,

impl<T> Default for once_cell::unsync::OnceCell<T>

impl<T> Default for Interner<T>

where T: Ord,

impl<T> Default for Path<T>

where T: Form,

impl<T> Default for Unalign<T>

where T: Default,

impl<T> Default for Empty<T>

impl<T> Default for PhantomData<T>

where T: ?Sized,

impl<T> Default for ManuallyDrop<T>

where T: Default + ?Sized,

impl<T> Default for Saturating<T>

where T: Default,

impl<T> Default for Wrapping<T>

where T: Default,

impl<T> Default for AssertUnwindSafe<T>

where T: Default,

impl<T> Default for Rc<T>

where T: Default,

impl<T> Default for gstd::prelude::rc::Weak<T>

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

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

impl<T> Default for gstd::prelude::Box<[T]>

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

where T: Default,

impl<T> Default for gstd::prelude::Vec<T>

impl<T, A> Default for gstd::prelude::collections::btree_set::IntoIter<T, A>

where A: Allocator + Default + Clone,

impl<T, A> Default for allocator_api2::stable::boxed::Box<[T], A>

where A: Allocator + Default,

impl<T, A> Default for HashTable<T, A>

where A: Allocator + Default,

impl<T, A> Default for gstd::prelude::vec::IntoIter<T, A>

where A: Allocator + Default,

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

where S: Default, A: Default + Allocator,

impl<T, const CAP: usize> Default for ArrayVec<T, CAP>

impl<T, const N: usize> Default for Mask<T, N>

where T: MaskElement, LaneCount<N>: SupportedLaneCount,

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

where LaneCount<N>: SupportedLaneCount, T: SimdElement + Default,

impl<T: Default> Default for Mutex<T>

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

impl<const CAP: usize> Default for ArrayString<CAP>