Trait freya::prelude::Default

1.0.0 · source ·

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 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.

§`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.

§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§

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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 }
}

impl Default for ExitStatus

The default value is one which indicates successful completion.

use chrono::NaiveDateTime;

assert_eq!(NaiveDateTime::default(), NaiveDateTime::UNIX_EPOCH);

impl Default for proc_macro2::TokenStream

TokenStream::default() returns an empty stream, i.e. this is equivalent with TokenStream::new().

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impl Default for ryu::buffer::Buffer

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impl Default for BuildMetadata

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impl Default for VersionReq

impl Default for UnsyncStorage

§

impl Default for AHasher

Provides a default Hasher with fixed keys. This is typically used in conjunction with BuildHasherDefault to create [AHasher]s in order to hash the keys of the map.

Generally it is preferable to use [RandomState] instead, so that different hashmaps will have different keys. However if fixed keys are desireable this may be used instead.

§Example

use std::hash::BuildHasherDefault;
use ahash::{AHasher, RandomState};
use std::collections::HashMap;

let mut map: HashMap<i32, i32, BuildHasherDefault<AHasher>> = HashMap::default();
map.insert(12, 34);

§

impl Default for AHasher

Provides a default Hasher with fixed keys. This is typically used in conjunction with BuildHasherDefault to create [AHasher]s in order to hash the keys of the map.

Generally it is preferable to use [RandomState] instead, so that different hashmaps will have different keys. However if fixed keys are desirable this may be used instead.

§Example

use std::hash::BuildHasherDefault;
use ahash::{AHasher, RandomState};
use std::collections::HashMap;

let mut map: HashMap<i32, i32, BuildHasherDefault<AHasher>> = HashMap::default();
map.insert(12, 34);

The defaults are that of https://url.spec.whatwg.org/#idna

Creates an instance of RandomState using keys obtained from the random number generator. Each instance created in this way will have a unique set of keys. (But the resulting instance can be used to create many hashers each or which will have the same keys.)

This is the same as [RandomState::new()]

NOTE: For safety this trait impl is only available available if either of the flags runtime-rng (on by default) or compile-time-rng are enabled. This is to prevent weakly keyed maps from being accidentally created. Instead one of constructors for [RandomState] must be used.

NOTE: For safety this trait impl is only available available if either of the flags runtime-rng (on by default) or compile-time-rng are enabled. This is to prevent weakly keyed maps from being accidentally created. Instead one of constructors for [RandomState] must be used.

impl<N, E, Ty, Ix> Default for Csr<N, E, Ty, Ix>
where Ty: EdgeType, Ix: IndexType,

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impl<N, E, Ty, Ix> Default for StableGraph<N, E, Ty, Ix>
where Ty: EdgeType, Ix: IndexType,

Create a new empty StableGraph.

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impl<N, E, Ty, Ix> Default for Graph<N, E, Ty, Ix>
where Ty: EdgeType, Ix: IndexType,

Create a new empty Graph.

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impl<N, E, Ty, Null, Ix> Default for MatrixGraph<N, E, Ty, Null, Ix>
where Ty: EdgeType, Null: Nullable<Wrapped = E>, Ix: IndexType,

impl<T> Default for [T; 32]
where T: Default,

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impl<T> Default for (T₁, T₂, …, Tₙ)
where T: Default,

The default value returned is one with all flags unset, i. e. [empty][Self::empty], unless customized.

§Derivable

§enums

§How can I implement Default?

§Examples

Required Methods§

fn default() -> Self

§Examples

Object Safety§

Implementors§

impl Default for &str

impl Default for &CStr

impl Default for &OsStr

impl Default for &mut str

impl Default for Code

impl Default for freya::events::Key

impl Default for AccordionStatus

impl Default for freya::prelude::Alignment

impl Default for ButtonStatus

impl Default for Content

impl Default for CursorIcon

impl Default for DirectionMode

impl Default for DropdownItemStatus

impl Default for DropdownStatus

impl Default for Ease

impl Default for EditableMode

impl Default for Function

impl Default for InputMode

impl Default for InputStatus

impl Default for MenuItemStatus

impl Default for OnFinish

impl Default for OrderDirection

impl Default for Position

impl Default for ScrollThumbState

impl Default for freya::prelude::Size

impl Default for SliderStatus

impl Default for SwitchStatus

impl Default for TileStatus

impl Default for IpNet

impl Default for Value

§Examples

impl Default for PathArguments

impl Default for bool

impl Default for char

impl Default for f32

impl Default for f64

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 Layers

impl Default for NodeReferenceLayout

impl Default for ParagraphElements

impl Default for freya::events::Modifiers

impl Default for PerformanceOverlayPlugin

impl Default for Global

impl Default for alloc::boxed::Box<str>

impl Default for alloc::boxed::Box<CStr>

impl Default for alloc::boxed::Box<OsStr>

impl Default for CString

impl Default for String

impl Default for core::fmt::Error

impl Default for SipHasher

impl Default for PhantomPinned

impl Default for RangeFull

impl Default for core::ptr::alignment::Alignment

impl Default for AtomicBool

impl Default for AtomicI8

impl Default for AtomicI16

impl Default for AtomicI32

impl Default for AtomicI64

§`enum`s

§How can I implement `Default`?