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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. //! //! 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 have your own type, you need to implement `Default` yourself: //! //! ``` //! # #![allow(dead_code)] //! enum Kind { //! A, //! B, //! C, //! } //! //! impl Default for Kind { //! fn default() -> Kind { Kind::A } //! } //! //! #[derive(Default)] //! struct SomeOptions { //! foo: i32, //! bar: f32, //! baz: Kind, //! } //! //! //! fn main() { //! let options: SomeOptions = Default::default(); //! } //! ``` //! //! 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() }; //! } //! ``` #![stable(feature = "rust1", since = "1.0.0")] use marker::Sized; /// A trait for giving a type a useful default value. /// /// A struct can derive default implementations of `Default` for basic types using /// `#[derive(Default)]`. /// /// # 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 } default_impl! { isize, 0 } default_impl! { i8, 0 } default_impl! { i16, 0 } default_impl! { i32, 0 } default_impl! { i64, 0 } default_impl! { f32, 0.0f32 } default_impl! { f64, 0.0f64 }