Trait Default

1.0.0 (const: unstable) · 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 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.

Default

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 &serde_json::value::Value

impl Default for &str

impl Default for &CStr

impl Default for &OsStr

impl Default for &DataLocale

impl Default for &DataMarkerAttributes

impl Default for &mut str

impl Default for Formatting

impl Default for ScopeType

impl Default for datex_core::global::protocol_structures::block_header::BlockType

impl Default for UserAgent

impl Default for EncryptionType

impl Default for ReceiverType

impl Default for SignatureType

impl Default for InterfacePriority

impl Default for ResponseResolutionStrategy

impl Default for ResponseTimeout

impl Default for ReconnectionConfig

impl Default for RTCSdpTypeDX

impl Default for EndpointInstance

impl Default for EndpointType

impl Default for datex_core::values::core_values::map::Map

impl Default for AsciiChar

impl Default for aho_corasick::packed::api::MatchKind

impl Default for aho_corasick::util::search::MatchKind

impl Default for StartKind

impl Default for LineEnding

impl Default for RoundingMode

impl Default for BigEndian

impl Default for LittleEndian

impl Default for TagMode

impl Default for ErrorChannel

impl Default for PollNext

impl Default for BidiPairedBracketType

impl Default for IpNet

impl Default for libudev::monitor::EventType

impl Default for PrefilterConfig

impl Default for nu_ansi_term::style::Color

impl Default for ElementParser

impl Default for WhichCaptures

impl Default for regex_automata::util::search::MatchKind

impl Default for ExtractKind

impl Default for rtcp::extended_report::BlockType

impl Default for TTLorHopLimitType

impl Default for PacketType

impl Default for SdesType

impl Default for StatusChunkTypeTcc

impl Default for SymbolSizeTypeTcc

impl Default for SymbolTypeTcc

impl Default for H265Payload

impl Default for CameraDirection

impl Default for VideoRotation

impl Default for CompressionCache

impl Default for rustls::quic::Version

impl Default for Direction

impl Default for ConnectionRole

impl Default for serde_json::value::Value

§Examples

impl Default for stun::agent::EventType

impl Default for UnderlineOption

impl Default for MonthRepr

impl Default for Padding

impl Default for SubsecondDigits

impl Default for UnixTimestampPrecision

impl Default for WeekNumberRepr

impl Default for WeekdayRepr

impl Default for YearRange

impl Default for YearRepr

impl Default for MissedTickBehavior

Trait Default

§`enum`s

§How can I implement `Default`?