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, 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 Default for Chain

where A: Default, B: Default,

impl Default for Cow<'_, B>

where B: ToOwned + ?Sized, ToOwned>::Owned: Default,

impl Default for BuildHasherDefault

impl Default for Cloned

where I: Default,

impl Default for Copied

where I: Default,

impl Default for Enumerate

where I: Default,

impl Default for Flatten

where I: Default + Iterator, Iterator>::Item: IntoIterator,

impl Default for Fuse

where I: Default,

impl Default for Rev

where I: Default,

impl Default for std::ops::Range

where Idx: Default,

impl Default for std::range::Range

where Idx: Default,

impl Default for std::collections::hash_set::IntoIter

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

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

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

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

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

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

impl Default for std::collections::hash_map::IntoIter

impl Default for std::collections::hash_map::IntoKeys

impl Default for std::collections::hash_map::IntoValues

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

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

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

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

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

impl Default for BTreeMap

impl Default for std::collections::btree_map::IntoIter

where A: Allocator + Default + Clone,

impl Default for std::collections::btree_map::IntoKeys

where A: Allocator + Default + Clone,

impl Default for std::collections::btree_map::IntoValues

where A: Allocator + Default + Clone,

impl Default for HashMap

where S: Default,

impl Default for &[T]

impl Default for &mut [T]

impl Default for Option

impl Default for [T; 0]

impl Default for [T; 1]

where T: Default,

impl Default for [T; 2]

where T: Default,

impl Default for [T; 3]

where T: Default,

impl Default for [T; 4]

where T: Default,

impl Default for [T; 5]

where T: Default,

impl Default for [T; 6]

where T: Default,

impl Default for [T; 7]

where T: Default,

impl Default for [T; 8]

where T: Default,

impl Default for [T; 9]

where T: Default,

impl Default for [T; 10]

where T: Default,

impl Default for [T; 11]

where T: Default,

impl Default for [T; 12]

where T: Default,

impl Default for [T; 13]

where T: Default,

impl Default for [T; 14]

where T: Default,

impl Default for [T; 15]

where T: Default,

impl Default for [T; 16]

where T: Default,

impl Default for [T; 17]

where T: Default,

impl Default for [T; 18]

where T: Default,

impl Default for [T; 19]

where T: Default,

impl Default for [T; 20]

where T: Default,

impl Default for [T; 21]

where T: Default,

impl Default for [T; 22]

where T: Default,

impl Default for [T; 23]

where T: Default,

impl Default for [T; 24]

where T: Default,

impl Default for [T; 25]

where T: Default,

impl Default for [T; 26]

where T: Default,

impl Default for [T; 27]

where T: Default,

impl Default for [T; 28]

where T: Default,

impl Default for [T; 29]

where T: Default,

impl Default for [T; 30]

where T: Default,

impl Default for [T; 31]

where T: Default,

impl Default for [T; 32]

where T: Default,

impl Default for (T₁, T₂, …, Tₙ)

where T: Default,

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

impl Default for Box<[T]>

impl Default for Box

where T: Default,

impl Default for Cell

where T: Default,

impl Default for LazyCell

where T: Default,

impl Default for OnceCell

impl Default for RefCell

where T: Default,

impl Default for SyncUnsafeCell

where T: Default,

impl Default for UnsafeCell

where T: Default,

impl Default for Reverse

where T: Default,

impl Default for std::collections::binary_heap::IntoIter

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

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

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

impl Default for std::collections::linked_list::IntoIter

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

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

impl Default for BTreeSet

impl Default for BinaryHeap

where T: Ord,

impl Default for LinkedList

impl Default for VecDeque

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

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

impl Default for std::iter::Empty

impl Default for PhantomContravariant

where T: ?Sized,

impl Default for PhantomCovariant

where T: ?Sized,

impl Default for PhantomData

where T: ?Sized,

impl Default for PhantomInvariant

where T: ?Sized,

impl Default for ManuallyDrop

where T: Default + ?Sized,

impl Default for Saturating

where T: Default,

impl Default for Wrapping

where T: Default,

impl Default for AssertUnwindSafe

where T: Default,

impl Default for Rc<[T]>

impl Default for Rc

where T: Default,

impl Default for std::rc::Weak

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

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

impl Default for AtomicPtr

impl Default for Arc<[T]>

impl Default for Arc

where T: Default,

impl Default for Exclusive

where T: Default + ?Sized,

impl Default for OnceLock

impl Default for std::sync::Weak

impl Default for Vec

impl Default for std::collections::btree_set::IntoIter

where A: Allocator + Default + Clone,

impl Default for std::vec::IntoIter

where A: Allocator + Default,

impl Default for HashSet

where S: Default,

implusize> Default for Mask

where T: MaskElement, LaneCount: SupportedLaneCount,

implusize> Default for Simd

where LaneCount: SupportedLaneCount, T: SimdElement + Default,

implDefault> Default for Cursor

implDefault> Default for LazyLock

implDefault> Default for ReentrantLock

implDefault> Default for RwLock

implSized + Default> Default for Mutex