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1a4d82fc JJ |
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. | |
4 | // | |
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. | |
10 | ||
11 | //! The `Default` trait for types which may have meaningful default values. | |
1a4d82fc | 12 | |
85aaf69f | 13 | #![stable(feature = "rust1", since = "1.0.0")] |
1a4d82fc | 14 | |
e9174d1e | 15 | /// A trait for giving a type a useful default value. |
1a4d82fc | 16 | /// |
3157f602 XL |
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: | |
20 | /// | |
21 | /// ``` | |
22 | /// # #[allow(dead_code)] | |
23 | /// struct SomeOptions { | |
24 | /// foo: i32, | |
25 | /// bar: f32, | |
26 | /// } | |
27 | /// ``` | |
28 | /// | |
29 | /// How can we define some default values? You can use `Default`: | |
30 | /// | |
31 | /// ``` | |
32 | /// # #[allow(dead_code)] | |
33 | /// #[derive(Default)] | |
34 | /// struct SomeOptions { | |
35 | /// foo: i32, | |
36 | /// bar: f32, | |
37 | /// } | |
38 | /// | |
3157f602 XL |
39 | /// fn main() { |
40 | /// let options: SomeOptions = Default::default(); | |
41 | /// } | |
42 | /// ``` | |
43 | /// | |
44 | /// Now, you get all of the default values. Rust implements `Default` for various primitives types. | |
45 | /// | |
46 | /// If you want to override a particular option, but still retain the other defaults: | |
47 | /// | |
48 | /// ``` | |
49 | /// # #[allow(dead_code)] | |
50 | /// # #[derive(Default)] | |
51 | /// # struct SomeOptions { | |
52 | /// # foo: i32, | |
53 | /// # bar: f32, | |
54 | /// # } | |
55 | /// fn main() { | |
56 | /// let options = SomeOptions { foo: 42, ..Default::default() }; | |
57 | /// } | |
58 | /// ``` | |
59 | /// | |
60 | /// ## Derivable | |
61 | /// | |
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. | |
64 | /// | |
65 | /// ## How can I implement `Default`? | |
66 | /// | |
67 | /// Provide an implementation for the `default()` method that returns the value of | |
68 | /// your type that should be the default: | |
69 | /// | |
70 | /// ``` | |
71 | /// # #![allow(dead_code)] | |
72 | /// enum Kind { | |
73 | /// A, | |
74 | /// B, | |
75 | /// C, | |
76 | /// } | |
77 | /// | |
78 | /// impl Default for Kind { | |
79 | /// fn default() -> Kind { Kind::A } | |
80 | /// } | |
81 | /// ``` | |
1a4d82fc JJ |
82 | /// |
83 | /// # Examples | |
84 | /// | |
85 | /// ``` | |
92a42be0 | 86 | /// # #[allow(dead_code)] |
1a4d82fc JJ |
87 | /// #[derive(Default)] |
88 | /// struct SomeOptions { | |
85aaf69f | 89 | /// foo: i32, |
1a4d82fc JJ |
90 | /// bar: f32, |
91 | /// } | |
92 | /// ``` | |
85aaf69f | 93 | #[stable(feature = "rust1", since = "1.0.0")] |
b039eaaf | 94 | pub trait Default: Sized { |
1a4d82fc JJ |
95 | /// Returns the "default value" for a type. |
96 | /// | |
97 | /// Default values are often some kind of initial value, identity value, or anything else that | |
98 | /// may make sense as a default. | |
99 | /// | |
100 | /// # Examples | |
101 | /// | |
102 | /// Using built-in default values: | |
103 | /// | |
104 | /// ``` | |
1a4d82fc JJ |
105 | /// let i: i8 = Default::default(); |
106 | /// let (x, y): (Option<String>, f64) = Default::default(); | |
85aaf69f | 107 | /// let (a, b, (c, d)): (i32, u32, (bool, bool)) = Default::default(); |
1a4d82fc JJ |
108 | /// ``` |
109 | /// | |
110 | /// Making your own: | |
111 | /// | |
112 | /// ``` | |
92a42be0 | 113 | /// # #[allow(dead_code)] |
1a4d82fc JJ |
114 | /// enum Kind { |
115 | /// A, | |
116 | /// B, | |
117 | /// C, | |
118 | /// } | |
119 | /// | |
120 | /// impl Default for Kind { | |
121 | /// fn default() -> Kind { Kind::A } | |
122 | /// } | |
123 | /// ``` | |
85aaf69f | 124 | #[stable(feature = "rust1", since = "1.0.0")] |
1a4d82fc JJ |
125 | fn default() -> Self; |
126 | } | |
127 | ||
128 | macro_rules! default_impl { | |
129 | ($t:ty, $v:expr) => { | |
85aaf69f | 130 | #[stable(feature = "rust1", since = "1.0.0")] |
1a4d82fc JJ |
131 | impl Default for $t { |
132 | #[inline] | |
1a4d82fc JJ |
133 | fn default() -> $t { $v } |
134 | } | |
135 | } | |
136 | } | |
137 | ||
138 | default_impl! { (), () } | |
139 | default_impl! { bool, false } | |
140 | default_impl! { char, '\x00' } | |
141 | ||
85aaf69f SL |
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 } | |
1a4d82fc | 147 | |
85aaf69f SL |
148 | default_impl! { isize, 0 } |
149 | default_impl! { i8, 0 } | |
150 | default_impl! { i16, 0 } | |
151 | default_impl! { i32, 0 } | |
152 | default_impl! { i64, 0 } | |
1a4d82fc JJ |
153 | |
154 | default_impl! { f32, 0.0f32 } | |
155 | default_impl! { f64, 0.0f64 } |