165 lines
3.8 KiB
Rust
165 lines
3.8 KiB
Rust
// Copyright 2013 The Rust Project Developers. See the COPYRIGHT
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// file at the top-level directory of this distribution and at
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// http://rust-lang.org/COPYRIGHT.
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//
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// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
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// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
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// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
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// option. This file may not be copied, modified, or distributed
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// except according to those terms.
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//! The `Default` trait for types which may have meaningful default values.
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//!
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//! Sometimes, you want to fall back to some kind of default value, and
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//! don't particularly care what it is. This comes up often with `struct`s
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//! that define a set of options:
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//!
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//! ```
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//! # #[allow(dead_code)]
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//! struct SomeOptions {
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//! foo: i32,
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//! bar: f32,
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//! }
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//! ```
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//!
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//! How can we define some default values? You can use `Default`:
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//!
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//! ```
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//! # #[allow(dead_code)]
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//! #[derive(Default)]
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//! struct SomeOptions {
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//! foo: i32,
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//! bar: f32,
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//! }
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//!
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//!
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//! fn main() {
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//! let options: SomeOptions = Default::default();
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//! }
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//! ```
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//!
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//! Now, you get all of the default values. Rust implements `Default` for various primitives types.
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//! If you have your own type, you need to implement `Default` yourself:
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//!
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//! ```
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//! # #![allow(dead_code)]
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//! enum Kind {
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//! A,
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//! B,
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//! C,
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//! }
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//!
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//! impl Default for Kind {
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//! fn default() -> Kind { Kind::A }
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//! }
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//!
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//! #[derive(Default)]
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//! struct SomeOptions {
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//! foo: i32,
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//! bar: f32,
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//! baz: Kind,
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//! }
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//!
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//!
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//! fn main() {
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//! let options: SomeOptions = Default::default();
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//! }
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//! ```
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//!
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//! If you want to override a particular option, but still retain the other defaults:
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//!
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//! ```
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//! # #[allow(dead_code)]
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//! # #[derive(Default)]
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//! # struct SomeOptions {
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//! # foo: i32,
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//! # bar: f32,
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//! # }
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//! fn main() {
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//! let options = SomeOptions { foo: 42, ..Default::default() };
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//! }
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//! ```
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#![stable(feature = "rust1", since = "1.0.0")]
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use marker::Sized;
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/// A trait for giving a type a useful default value.
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///
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/// A struct can derive default implementations of `Default` for basic types using
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/// `#[derive(Default)]`.
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///
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/// # Examples
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///
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/// ```
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/// # #[allow(dead_code)]
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/// #[derive(Default)]
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/// struct SomeOptions {
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/// foo: i32,
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/// bar: f32,
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/// }
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/// ```
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#[stable(feature = "rust1", since = "1.0.0")]
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pub trait Default: Sized {
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/// Returns the "default value" for a type.
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///
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/// Default values are often some kind of initial value, identity value, or anything else that
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/// may make sense as a default.
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///
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/// # Examples
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///
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/// Using built-in default values:
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///
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/// ```
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/// let i: i8 = Default::default();
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/// let (x, y): (Option<String>, f64) = Default::default();
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/// let (a, b, (c, d)): (i32, u32, (bool, bool)) = Default::default();
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/// ```
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///
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/// Making your own:
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///
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/// ```
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/// # #[allow(dead_code)]
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/// enum Kind {
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/// A,
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/// B,
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/// C,
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/// }
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///
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/// impl Default for Kind {
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/// fn default() -> Kind { Kind::A }
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/// }
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/// ```
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#[stable(feature = "rust1", since = "1.0.0")]
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fn default() -> Self;
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}
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macro_rules! default_impl {
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($t:ty, $v:expr) => {
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#[stable(feature = "rust1", since = "1.0.0")]
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impl Default for $t {
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#[inline]
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fn default() -> $t { $v }
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}
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}
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}
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default_impl! { (), () }
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default_impl! { bool, false }
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default_impl! { char, '\x00' }
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default_impl! { usize, 0 }
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default_impl! { u8, 0 }
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default_impl! { u16, 0 }
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default_impl! { u32, 0 }
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default_impl! { u64, 0 }
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default_impl! { isize, 0 }
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default_impl! { i8, 0 }
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default_impl! { i16, 0 }
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default_impl! { i32, 0 }
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default_impl! { i64, 0 }
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default_impl! { f32, 0.0f32 }
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default_impl! { f64, 0.0f64 }
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