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 primitive types.
If you want to override a particular option, but still retain the other defaults:
fn main() {
let options = SomeOptions { foo: 42, ..Default::default() };
}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.
enumsWhen 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.
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 }
}#[derive(Default)]
struct SomeOptions {
foo: i32,
bar: f32,
}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.
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 }
}This trait is not dyn compatible.
In older versions of Rust, dyn compatibility was called "object safety", so this trait is not object safe.
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 ExitCodeThe default value is ExitCode::SUCCESS
impl Default for ExitStatusThe default value is one which indicates successful completion.
impl Default for AlignmentReturns 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 AtomicBoolAvailable on target_has_atomic_load_store=8 only.
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 std::sync::nonpoison::Condvar
impl Default for Arc<str>
impl Default for Arc<CStr>
impl Default for std::sync::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 *const Twhere
T: Thin + ?Sized,impl<T> Default for *mut Twhere
T: Thin + ?Sized,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 Pin<Box<T>>where
Box<T>: Default,
T: ?Sized,impl<T> Default for Pin<Rc<T>>where
Rc<T>: Default,
T: ?Sized,impl<T> Default for Pin<Arc<T>>where
Arc<T>: Default,
T: ?Sized,impl<T> Default for UnsafePinned<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>Available on target_has_atomic_load_store=ptr only.
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 std::array::IntoIter<T, N>
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 std::sync::nonpoison::RwLock<T>
impl<T: Default> Default for LazyLock<T>
impl<T: Default> Default for ReentrantLock<T>
impl<T: Default> Default for std::sync::RwLock<T>
impl<T: ?Sized + Default> Default for std::sync::nonpoison::Mutex<T>
impl<T: ?Sized + Default> Default for std::sync::Mutex<T>
© 2010 The Rust Project Developers
Licensed under the Apache License, Version 2.0 or the MIT license, at your option.
https://doc.rust-lang.org/std/default/trait.Default.html