pub(crate) trait Default: Sized {
// Required method
fn default() -> Self;
}Expand description
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() };
}§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§
1.0.0 (const: unstable) · Sourcefn default() -> Self
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".
Implementors§
impl Default for &CStr
impl Default for &mut str
impl Default for &str
impl Default for ()
impl Default for Abi
impl Default for Alignment
Returns Alignment::MIN, which is valid for any type.
impl Default for AsciiChar
impl Default for Atomic<bool>
Available on
target_has_atomic_load_store=8 only.impl Default for Atomic<i8>
impl Default for Atomic<i16>
impl Default for Atomic<i32>
impl Default for Atomic<i64>
impl Default for Atomic<isize>
impl Default for Atomic<u8>
impl Default for Atomic<u16>
impl Default for Atomic<u32>
impl Default for Atomic<u64>
impl Default for Atomic<usize>
impl Default for Duration
impl Default for core::io::util::Empty
impl Default for Error
impl Default for FormattingOptions
impl Default for PhantomPinned
impl Default for RangeFull
impl Default for Sink
impl Default for SipHasher
impl Default for __c_anonymous_sockaddr_can_j1939
impl Default for __c_anonymous_sockaddr_can_tp
impl Default for bool
impl Default for can_filter
impl Default for can_frame
impl Default for canfd_frame
impl Default for canxl_frame
impl Default for char
impl Default for f16
impl Default for f32
impl Default for f64
impl Default for f128
impl Default for group
impl Default for hostent
Available on non-Cygwin and non-Horizon only.
impl Default for i8
impl Default for i16
impl Default for i32
impl Default for i64
impl Default for i128
impl Default for in6_addr
impl Default for iovec
impl Default for ipv6_mreq
Available on non-NuttX only.
impl Default for isize
impl Default for itimerval
impl Default for linger
Available on non-Cygwin only.
impl Default for pollfd
Available on non-Horizon only.
impl Default for protoent
impl Default for rlimit
impl Default for rusage
impl Default for servent
impl Default for sigval
impl Default for sockaddr_can
impl Default for statvfs
impl Default for statvfs64
impl Default for timespec
impl Default for timeval
impl Default for tms
impl Default for u8
impl Default for u16
impl Default for u32
impl Default for u64
impl Default for u128
impl Default for usize
impl Default for utimbuf
impl Default for winsize
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, B> Default for Chain<A, B>
impl<A> Default for RepeatN<A>
Creates an empty iterator, like repeat_n(value, 0)
but without needing any such value at hand.
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>
impl<I> Default for Fuse<I>where
I: Default,
impl<I> Default for Rev<I>where
I: Default,
impl<Idx> Default for core::ops::range::Range<Idx>where
Idx: Default,
impl<Idx> Default for core::range::Range<Idx>where
Idx: Default,
impl<P> Default for MaybeDangling<P>
impl<T, const N: usize> Default for IntoIter<T, N>
impl<T, const N: usize> Default for Mask<T, N>where
T: MaskElement,
impl<T, const N: usize> Default for Simd<T, N>where
T: SimdElement + Default,
impl<T, const VARIANT: u32, const FIELD: u32> Default for FieldRepresentingType<T, VARIANT, FIELD>where
T: ?Sized,
impl<T: Copy> Default for Padding<T>
impl<T> Default for &[T]
impl<T> Default for &mut [T]
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 *const T
impl<T> Default for *mut T
impl<T> Default for AssertUnwindSafe<T>where
T: Default,
impl<T> Default for Atomic<*mut T>
Available on
target_has_atomic_load_store=ptr only.