1 // Copyright 2013 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
11 //! The `Default` trait for types which may have meaningful default values.
13 #![stable(feature = "rust1", since = "1.0.0")]
15 /// A trait for giving a type a useful default value.
17 /// Sometimes, you want to fall back to some kind of default value, and
18 /// don't particularly care what it is. This comes up often with `struct`s
19 /// that define a set of options:
22 /// # #[allow(dead_code)]
23 /// struct SomeOptions {
29 /// How can we define some default values? You can use `Default`:
32 /// # #[allow(dead_code)]
33 /// #[derive(Default)]
34 /// struct SomeOptions {
40 /// let options: SomeOptions = Default::default();
44 /// Now, you get all of the default values. Rust implements `Default` for various primitives types.
46 /// If you want to override a particular option, but still retain the other defaults:
49 /// # #[allow(dead_code)]
50 /// # #[derive(Default)]
51 /// # struct SomeOptions {
56 /// let options = SomeOptions { foo: 42, ..Default::default() };
62 /// This trait can be used with `#[derive]` if all of the type's fields implement
63 /// `Default`. When `derive`d, it will use the default value for each field's type.
65 /// ## How can I implement `Default`?
67 /// Provide an implementation for the `default()` method that returns the value of
68 /// your type that should be the default:
71 /// # #![allow(dead_code)]
78 /// impl Default for Kind {
79 /// fn default() -> Kind { Kind::A }
86 /// # #[allow(dead_code)]
87 /// #[derive(Default)]
88 /// struct SomeOptions {
93 #[stable(feature = "rust1", since = "1.0.0")]
94 pub trait Default: Sized {
95 /// Returns the "default value" for a type.
97 /// Default values are often some kind of initial value, identity value, or anything else that
98 /// may make sense as a default.
102 /// Using built-in default values:
105 /// let i: i8 = Default::default();
106 /// let (x, y): (Option<String>, f64) = Default::default();
107 /// let (a, b, (c, d)): (i32, u32, (bool, bool)) = Default::default();
113 /// # #[allow(dead_code)]
120 /// impl Default for Kind {
121 /// fn default() -> Kind { Kind::A }
124 #[stable(feature = "rust1", since = "1.0.0")]
125 fn default() -> Self;
128 macro_rules! default_impl {
129 ($t:ty, $v:expr) => {
130 #[stable(feature = "rust1", since = "1.0.0")]
131 impl Default for $t {
133 fn default() -> $t { $v }
138 default_impl! { (), () }
139 default_impl! { bool, false }
140 default_impl! { char, '\x00' }
142 default_impl! { usize, 0 }
143 default_impl! { u8, 0 }
144 default_impl! { u16, 0 }
145 default_impl! { u32, 0 }
146 default_impl! { u64, 0 }
147 default_impl! { u128, 0 }
149 default_impl! { isize, 0 }
150 default_impl! { i8, 0 }
151 default_impl! { i16, 0 }
152 default_impl! { i32, 0 }
153 default_impl! { i64, 0 }
154 default_impl! { i128, 0 }
156 default_impl! { f32, 0.0f32 }
157 default_impl! { f64, 0.0f64 }