starnix_uapi::arch32::__static_assertions::_core::default

Trait Default

pub 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.

§`enum`s

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 · Source

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.

Trait Default Copy item path

§Derivable

§enums

§How can I implement Default?

§Examples

Required Methods§

fn default() -> Self

§Examples

Dyn Compatibility§

Implementors§

impl Default for &Value

impl Default for &str

impl Default for &CStr

impl Default for &OsStr

impl Default for &mut str

impl Default for FileLeaseType

impl Default for AsciiChar

impl Default for Value

§Examples

impl Default for zx_packet_guest_vcpu_type_t

impl Default for zx_packet_type_t

impl Default for zx_page_request_command_t

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 UserAndOrGroupId

impl Default for DeviceType

impl Default for AccessCheck

impl Default for FileMode

impl Default for MountFlags

impl Default for PersonalityFlags

impl Default for ResourceLimits

impl Default for SigSet

impl Default for __kernel_fd_set

impl Default for __kernel_fsid_t

impl Default for __kernel_itimerspec

impl Default for __kernel_old_timespec

impl Default for __kernel_sigaction

impl Default for __kernel_sock_timeval

impl Default for __kernel_timespec

impl Default for __old_kernel_stat

impl Default for __sifields__bindgen_ty_1

impl Default for __sifields__bindgen_ty_2

impl Default for __sifields__bindgen_ty_3

impl Default for __sifields__bindgen_ty_4

impl Default for __sifields__bindgen_ty_5

impl Default for __sifields__bindgen_ty_5__bindgen_ty_1__bindgen_ty_1

impl Default for __sifields__bindgen_ty_5__bindgen_ty_1__bindgen_ty_2

impl Default for __sifields__bindgen_ty_5__bindgen_ty_1__bindgen_ty_3

impl Default for __sifields__bindgen_ty_6

impl Default for __sifields__bindgen_ty_7

impl Default for __sk_buff

impl Default for __user_cap_data_struct

impl Default for __user_cap_header_struct

impl Default for _fpreg

impl Default for _fpstate_32

impl Default for _fpstate_64

impl Default for _fpx_sw_bytes

impl Default for _fpxreg

impl Default for _header

impl Default for _xmmreg

impl Default for _xstate

impl Default for _xt_align

impl Default for _ymmh_state

Trait Default

§`enum`s

§How can I implement `Default`?

impl Default for sifieldsbindgen_ty_1

impl Default for sifieldsbindgen_ty_2

impl Default for sifieldsbindgen_ty_3

impl Default for sifieldsbindgen_ty_4

impl Default for sifieldsbindgen_ty_5

impl Default for sifieldsbindgen_ty_5__bindgen_ty_1__bindgen_ty_1

impl Default for sifieldsbindgen_ty_5__bindgen_ty_1__bindgen_ty_2

impl Default for sifieldsbindgen_ty_5__bindgen_ty_1__bindgen_ty_3

impl Default for sifieldsbindgen_ty_6

impl Default for sifieldsbindgen_ty_7