default.rs.html -- source
Description: Source to the Rust file `/usr/obj/ports/rust-1.16.0/rustc-1.16.0-src/src/libcore/default.rs`.
Keywords: rust, rustlang, rust-lang

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    // Copyright 2013 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    //! 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() -> Kind { 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() -> Kind { Kind::A }
        /// }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        fn default() -> Self;
    }

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

    default_impl! { (), () }
    default_impl! { bool, false }
    default_impl! { char, '\x00' }

    default_impl! { usize, 0 }
    default_impl! { u8, 0 }
    default_impl! { u16, 0 }
    default_impl! { u32, 0 }
    default_impl! { u64, 0 }
    #[cfg(not(stage0))]
    default_impl! { u128, 0 }

    default_impl! { isize, 0 }
    default_impl! { i8, 0 }
    default_impl! { i16, 0 }
    default_impl! { i32, 0 }
    default_impl! { i64, 0 }
    #[cfg(not(stage0))]
    default_impl! { i128, 0 }

    default_impl! { f32, 0.0f32 }
    default_impl! { f64, 0.0f64 }

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