Trait 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.

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.

Implementors§

1.0.0 · Source§

impl Default for &str

1.10.0 · Source§

impl Default for &CStr

1.9.0 · Source§

impl Default for &OsStr

1.28.0 · Source§

Trait DefaultCopy item path

§Derivable

§enums

§How can I implement Default?

§Examples

Required Methods§

fn default() -> Self

§Examples

Dyn Compatibility§

Implementors§

impl Default for &str

impl Default for &CStr

impl Default for &OsStr

impl Default for &mut str

impl Default for Actor

impl Default for BlueprintType

impl Default for FunctionAuth

impl Default for Level

impl Default for MessageV1

impl Default for MessageV2

impl Default for MethodAuthTemplate

impl Default for NodeStateUpdates

impl Default for OnApplyCost

impl Default for OuterObjectInfo

impl Default for OwnerRole

impl Default for PackageRoyaltyConfig

impl Default for PartitionDatabaseUpdates

impl Default for PartitionStateUpdates

impl Default for PrimaryRoleBadgeWithdrawAttemptState

impl Default for PrimaryRoleLockingState

impl Default for PrimaryRoleRecoveryAttemptState

impl Default for RecoveryRoleBadgeWithdrawAttemptState

impl Default for RecoveryRoleRecoveryAttemptState

impl Default for SystemLockData

impl Default for TipSpecifier

impl Default for UpdateNumberOfMinRoundsPerEpochSettings

impl Default for WithdrawStrategy

impl Default for AsciiChar

impl Default for Edition

impl Default for MaintenanceStatus

impl Default for OptionalFile

impl Default for Publish

impl Default for Resolver

impl Default for PrefilterConfig

impl Default for DeliveryMode

impl Default for ManifestObjectNames

impl Default for UnspecifiedResources

impl Default for WhichCaptures

impl Default for MatchKind

impl Default for ExtractKind

impl Default for Profile

impl Default for Value

§Examples

impl Default for PathArguments

impl Default for Item

impl Default for FeatureBaseline

impl Default for OptimizeLevel

impl Default for ShrinkLevel

impl Default for CompilationMode

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 TestEnvironment<InMemorySubstateDatabase>

impl Default for TestEnvironmentBuilder<InMemorySubstateDatabase>

Trait Default

§`enum`s

§How can I implement `Default`?