Trait 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 }
}

Dyn Compatibility

This trait is not dyn compatible.

In older versions of Rust, dyn compatibility was called "object safety", so this trait is not object safe.

Implementors

impl Default for &str

impl Default for &CStr

impl Default for &OsStr

impl Default for &mut str

impl Default for AsciiChar

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 Global

impl Default for System

impl Default for Box<str>

impl Default for Box<CStr>

impl Default for Box<OsStr>

impl Default for ByteString

impl Default for CString

impl Default for OsString

impl Default for Error

impl Default for FormattingOptions

impl Default for FileTimes

impl Default for DefaultHasher

impl Default for RandomState

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 ExitCode

The default value is ExitCode::SUCCESS

impl Default for ExitStatus

The default value is one which indicates successful completion.

impl Default for Alignment

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

impl Default for DefaultRandomSource

impl Default for Rc<str>

impl Default for Rc<CStr>

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 Arc<str>

impl Default for Arc<CStr>

impl Default for Condvar

impl Default for Duration

impl<'a> Default for &'a ByteStr

impl<'a> Default for &'a mut ByteStr

impl<'a> Default for PhantomContravariantLifetime<'a>

impl<'a> Default for PhantomCovariantLifetime<'a>

impl<'a> Default for PhantomInvariantLifetime<'a>

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<Idx> Default for std::range::Range<Idx>

where Idx: Default,

impl<K> Default for std::collections::hash_set::IntoIter<K>

impl<K> Default for std::collections::hash_set::Iter<'_, K>

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

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

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

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

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

impl<K, V> Default for std::collections::hash_map::IntoIter<K, V>

impl<K, V> Default for std::collections::hash_map::IntoKeys<K, V>

impl<K, V> Default for std::collections::hash_map::IntoValues<K, V>

impl<K, V> Default for std::collections::hash_map::Iter<'_, K, V>

impl<K, V> Default for std::collections::hash_map::IterMut<'_, K, V>

impl<K, V> Default for std::collections::hash_map::Keys<'_, K, V>

impl<K, V> Default for std::collections::hash_map::Values<'_, K, V>

impl<K, V> Default for std::collections::hash_map::ValuesMut<'_, K, V>

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

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 std::collections::btree_map::IntoKeys<K, V, A>

where A: Allocator + Default + Clone,

impl<K, V, A> Default for std::collections::btree_map::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]

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 Box<[T]>

impl<T> Default for Box<T>

where T: Default,

impl<T> Default for Cell<T>

where T: Default,

impl<T> Default for LazyCell<T>

where T: Default,

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

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

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>

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 std::collections::vec_deque::Iter<'_, T>

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

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

impl<T> Default for PhantomContravariant<T>

where T: ?Sized,

impl<T> Default for PhantomCovariant<T>

where T: ?Sized,

impl<T> Default for PhantomData<T>

where T: ?Sized,

impl<T> Default for PhantomInvariant<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]>

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]>

impl<T> Default for Arc<T>

where T: Default,

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>

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>

where A: Allocator + Default,

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

where S: Default,

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 Cursor<T>

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

impl<T: Default> Default for ReentrantLock<T>

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

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