Trait nom::lib::std::prelude::v1::rust_2018::Default 1.0.0[−][src]
pub trait Default {
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 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.
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
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 }
}Implementations on Foreign Types
Creates a new empty cell.
Example
#![feature(once_cell)]
use std::lazy::SyncOnceCell;
fn main() {
assert_eq!(SyncOnceCell::<()>::new(), SyncOnceCell::default());
}Creates an AtomicBool initialized to false.
Creates an UnsafeCell, with the Default value for T.
Return an empty ArrayString
Implementors
1.11.0[src]