default.rs - source
Description: Source of the Rust file `library/core/src/default.rs`.
Keywords: rust, rustlang, rust-lang

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    //! The `Default` trait for types which may have meaningful default values.

    #![stable(feature = "rust1", since = "1.0.0")]

    /// 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 `struct`s
    /// that define a set of options:
    ///
    /// ```
    /// # #[allow(dead_code)]
    /// struct SomeOptions {
    ///     foo: i32,
    ///     bar: f32,
    /// }
    /// ```
    ///
    /// How can we define some default values? You can use `Default`:
    ///
    /// ```
    /// # #[allow(dead_code)]
    /// #[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:
    ///
    /// ```
    /// # #[allow(dead_code)]
    /// # #[derive(Default)]
    /// # struct SomeOptions {
    /// #     foo: i32,
    /// #     bar: f32,
    /// # }
    /// 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 `derive`d, 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:
    ///
    /// ```
    /// # #![allow(dead_code)]
    /// enum Kind {
    ///     A,
    ///     B,
    ///     C,
    /// }
    ///
    /// impl Default for Kind {
    ///     fn default() -> Self { Kind::A }
    /// }
    /// ```
    ///
    /// # Examples
    ///
    /// ```
    /// # #[allow(dead_code)]
    /// #[derive(Default)]
    /// struct SomeOptions {
    ///     foo: i32,
    ///     bar: f32,
    /// }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub trait Default: Sized {
        /// 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:
        ///
        /// ```
        /// # #[allow(dead_code)]
        /// enum Kind {
        ///     A,
        ///     B,
        ///     C,
        /// }
        ///
        /// impl Default for Kind {
        ///     fn default() -> Self { Kind::A }
        /// }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        fn default() -> Self;
    }

    /// Return the default value of a type according to the `Default` trait.
    ///
    /// The type to return is inferred from context; this is equivalent to
    /// `Default::default()` but shorter to type.
    ///
    /// For example:
    /// ```
    /// #![feature(default_free_fn)]
    ///
    /// use std::default::default;
    ///
    /// #[derive(Default)]
    /// struct AppConfig {
    ///     foo: FooConfig,
    ///     bar: BarConfig,
    /// }
    ///
    /// #[derive(Default)]
    /// struct FooConfig {
    ///     foo: i32,
    /// }
    ///
    /// #[derive(Default)]
    /// struct BarConfig {
    ///     bar: f32,
    ///     baz: u8,
    /// }
    ///
    /// fn main() {
    ///     let options = AppConfig {
    ///         foo: default(),
    ///         bar: BarConfig {
    ///             bar: 10.1,
    ///             ..default()
    ///         },
    ///     };
    /// }
    /// ```
    #[unstable(feature = "default_free_fn", issue = "73014")]
    #[inline]
    pub fn default<T: Default>() -> T {
        Default::default()
    }

    /// Derive macro generating an impl of the trait `Default`.
    #[rustc_builtin_macro]
    #[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
    #[allow_internal_unstable(core_intrinsics)]
    pub macro Default($item:item) {
        /* compiler built-in */
    }

    macro_rules! default_impl {
        ($t:ty, $v:expr, $doc:tt) => {
            #[stable(feature = "rust1", since = "1.0.0")]
            impl Default for $t {
                #[inline]
                #[doc = $doc]
                fn default() -> $t { $v }
            }
        }
    }

    default_impl! { (), (), "Returns the default value of `()`" }
    default_impl! { bool, false, "Returns the default value of `false`" }
    default_impl! { char, '\x00', "Returns the default value of `\\x00`" }

    default_impl! { usize, 0, "Returns the default value of `0`" }
    default_impl! { u8, 0, "Returns the default value of `0`" }
    default_impl! { u16, 0, "Returns the default value of `0`" }
    default_impl! { u32, 0, "Returns the default value of `0`" }
    default_impl! { u64, 0, "Returns the default value of `0`" }
    default_impl! { u128, 0, "Returns the default value of `0`" }

    default_impl! { isize, 0, "Returns the default value of `0`" }
    default_impl! { i8, 0, "Returns the default value of `0`" }
    default_impl! { i16, 0, "Returns the default value of `0`" }
    default_impl! { i32, 0, "Returns the default value of `0`" }
    default_impl! { i64, 0, "Returns the default value of `0`" }
    default_impl! { i128, 0, "Returns the default value of `0`" }

    default_impl! { f32, 0.0f32, "Returns the default value of `0.0`" }
    default_impl! { f64, 0.0f64, "Returns the default value of `0.0`" }