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§

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 Order

impl Default for ResponseResultType

impl Default for ibc_core::channel::types::proto::v1::State

impl Default for HashOp

impl Default for LengthOp

impl Default for ibc_core::connection::types::proto::v1::State

impl Default for AsciiChar

impl Default for ibc_proto::cosmos::gov::v1::ProposalStatus

impl Default for ibc_proto::cosmos::gov::v1::VoteOption

impl Default for ibc_proto::cosmos::gov::v1beta1::ProposalStatus

impl Default for ibc_proto::cosmos::gov::v1beta1::VoteOption

impl Default for AuthorizationType

impl Default for BondStatus

impl Default for Infraction

impl Default for ibc_proto::cosmos::staking::v1beta1::InfractionType

impl Default for SignMode

impl Default for BroadcastMode

impl Default for OrderBy

impl Default for Edition

impl Default for VerificationState

impl Default for EnumType

impl Default for FieldPresence

impl Default for JsonFormat

impl Default for MessageEncoding

impl Default for RepeatedFieldEncoding

impl Default for Utf8Validation

impl Default for Label

impl Default for ibc_proto::google::protobuf::field_descriptor_proto::Type

impl Default for CType

impl Default for JsType

impl Default for OptionRetention

impl Default for OptionTargetType

impl Default for OptimizeMode

impl Default for Semantic

impl Default for IdempotencyLevel

impl Default for ibc_proto::ibc::applications::interchain_accounts::v1::Type

impl Default for ConsumerPacketDataType

impl Default for ibc_proto::interchain_security::ccv::v1::InfractionType

impl Default for PrefilterConfig

impl Default for Value

§Examples

impl Default for tendermint_proto::tendermint::v0_34::abci::CheckTxType

impl Default for EvidenceType

impl Default for tendermint_proto::tendermint::v0_34::abci::response_apply_snapshot_chunk::Result

impl Default for tendermint_proto::tendermint::v0_34::abci::response_offer_snapshot::Result

impl Default for tendermint_proto::tendermint::v0_34::privval::Errors

impl Default for tendermint_proto::tendermint::v0_34::types::BlockIdFlag

impl Default for tendermint_proto::tendermint::v0_34::types::SignedMsgType

impl Default for tendermint_proto::tendermint::v0_37::abci::CheckTxType

impl Default for tendermint_proto::tendermint::v0_37::abci::MisbehaviorType

impl Default for tendermint_proto::tendermint::v0_37::abci::response_apply_snapshot_chunk::Result

impl Default for tendermint_proto::tendermint::v0_37::abci::response_offer_snapshot::Result

impl Default for tendermint_proto::tendermint::v0_37::abci::response_process_proposal::ProposalStatus

impl Default for tendermint_proto::tendermint::v0_37::privval::Errors

impl Default for tendermint_proto::tendermint::v0_37::types::BlockIdFlag

impl Default for tendermint_proto::tendermint::v0_37::types::SignedMsgType

impl Default for tendermint_proto::tendermint::v0_38::abci::CheckTxType

impl Default for tendermint_proto::tendermint::v0_38::abci::MisbehaviorType

impl Default for tendermint_proto::tendermint::v0_38::abci::response_apply_snapshot_chunk::Result

impl Default for tendermint_proto::tendermint::v0_38::abci::response_offer_snapshot::Result

impl Default for tendermint_proto::tendermint::v0_38::abci::response_process_proposal::ProposalStatus

impl Default for VerifyStatus

impl Default for tendermint_proto::tendermint::v0_38::privval::Errors

impl Default for tendermint_proto::tendermint::v0_38::types::BlockIdFlag

impl Default for tendermint_proto::tendermint::v0_38::types::SignedMsgType

impl Default for Code

Trait Default

§`enum`s

§How can I implement `Default`?