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1a4d82fc | 1 | //! The `Default` trait for types which may have meaningful default values. |
1a4d82fc | 2 | |
85aaf69f | 3 | #![stable(feature = "rust1", since = "1.0.0")] |
1a4d82fc | 4 | |
e9174d1e | 5 | /// A trait for giving a type a useful default value. |
1a4d82fc | 6 | /// |
3157f602 XL |
7 | /// Sometimes, you want to fall back to some kind of default value, and |
8 | /// don't particularly care what it is. This comes up often with `struct`s | |
9 | /// that define a set of options: | |
10 | /// | |
11 | /// ``` | |
12 | /// # #[allow(dead_code)] | |
13 | /// struct SomeOptions { | |
14 | /// foo: i32, | |
15 | /// bar: f32, | |
16 | /// } | |
17 | /// ``` | |
18 | /// | |
19 | /// How can we define some default values? You can use `Default`: | |
20 | /// | |
21 | /// ``` | |
22 | /// # #[allow(dead_code)] | |
23 | /// #[derive(Default)] | |
24 | /// struct SomeOptions { | |
25 | /// foo: i32, | |
26 | /// bar: f32, | |
27 | /// } | |
28 | /// | |
3157f602 XL |
29 | /// fn main() { |
30 | /// let options: SomeOptions = Default::default(); | |
31 | /// } | |
32 | /// ``` | |
33 | /// | |
34 | /// Now, you get all of the default values. Rust implements `Default` for various primitives types. | |
35 | /// | |
36 | /// If you want to override a particular option, but still retain the other defaults: | |
37 | /// | |
38 | /// ``` | |
39 | /// # #[allow(dead_code)] | |
40 | /// # #[derive(Default)] | |
41 | /// # struct SomeOptions { | |
42 | /// # foo: i32, | |
43 | /// # bar: f32, | |
44 | /// # } | |
45 | /// fn main() { | |
46 | /// let options = SomeOptions { foo: 42, ..Default::default() }; | |
47 | /// } | |
48 | /// ``` | |
49 | /// | |
50 | /// ## Derivable | |
51 | /// | |
52 | /// This trait can be used with `#[derive]` if all of the type's fields implement | |
53 | /// `Default`. When `derive`d, it will use the default value for each field's type. | |
54 | /// | |
55 | /// ## How can I implement `Default`? | |
56 | /// | |
ba9703b0 | 57 | /// Provide an implementation for the `default()` method that returns the value of |
3157f602 XL |
58 | /// your type that should be the default: |
59 | /// | |
60 | /// ``` | |
61 | /// # #![allow(dead_code)] | |
62 | /// enum Kind { | |
63 | /// A, | |
64 | /// B, | |
65 | /// C, | |
66 | /// } | |
67 | /// | |
68 | /// impl Default for Kind { | |
a1dfa0c6 | 69 | /// fn default() -> Self { Kind::A } |
3157f602 XL |
70 | /// } |
71 | /// ``` | |
1a4d82fc JJ |
72 | /// |
73 | /// # Examples | |
74 | /// | |
75 | /// ``` | |
92a42be0 | 76 | /// # #[allow(dead_code)] |
1a4d82fc JJ |
77 | /// #[derive(Default)] |
78 | /// struct SomeOptions { | |
85aaf69f | 79 | /// foo: i32, |
1a4d82fc JJ |
80 | /// bar: f32, |
81 | /// } | |
82 | /// ``` | |
6a06907d | 83 | #[cfg_attr(not(test), rustc_diagnostic_item = "Default")] |
85aaf69f | 84 | #[stable(feature = "rust1", since = "1.0.0")] |
b039eaaf | 85 | pub trait Default: Sized { |
1a4d82fc JJ |
86 | /// Returns the "default value" for a type. |
87 | /// | |
88 | /// Default values are often some kind of initial value, identity value, or anything else that | |
89 | /// may make sense as a default. | |
90 | /// | |
91 | /// # Examples | |
92 | /// | |
93 | /// Using built-in default values: | |
94 | /// | |
95 | /// ``` | |
1a4d82fc JJ |
96 | /// let i: i8 = Default::default(); |
97 | /// let (x, y): (Option<String>, f64) = Default::default(); | |
85aaf69f | 98 | /// let (a, b, (c, d)): (i32, u32, (bool, bool)) = Default::default(); |
1a4d82fc JJ |
99 | /// ``` |
100 | /// | |
101 | /// Making your own: | |
102 | /// | |
103 | /// ``` | |
92a42be0 | 104 | /// # #[allow(dead_code)] |
1a4d82fc JJ |
105 | /// enum Kind { |
106 | /// A, | |
107 | /// B, | |
108 | /// C, | |
109 | /// } | |
110 | /// | |
111 | /// impl Default for Kind { | |
a1dfa0c6 | 112 | /// fn default() -> Self { Kind::A } |
1a4d82fc JJ |
113 | /// } |
114 | /// ``` | |
85aaf69f | 115 | #[stable(feature = "rust1", since = "1.0.0")] |
1a4d82fc JJ |
116 | fn default() -> Self; |
117 | } | |
118 | ||
f035d41b XL |
119 | /// Return the default value of a type according to the `Default` trait. |
120 | /// | |
121 | /// The type to return is inferred from context; this is equivalent to | |
122 | /// `Default::default()` but shorter to type. | |
123 | /// | |
124 | /// For example: | |
125 | /// ``` | |
126 | /// #![feature(default_free_fn)] | |
127 | /// | |
128 | /// use std::default::default; | |
129 | /// | |
130 | /// #[derive(Default)] | |
131 | /// struct AppConfig { | |
132 | /// foo: FooConfig, | |
133 | /// bar: BarConfig, | |
134 | /// } | |
135 | /// | |
136 | /// #[derive(Default)] | |
137 | /// struct FooConfig { | |
138 | /// foo: i32, | |
139 | /// } | |
140 | /// | |
141 | /// #[derive(Default)] | |
142 | /// struct BarConfig { | |
143 | /// bar: f32, | |
144 | /// baz: u8, | |
145 | /// } | |
146 | /// | |
147 | /// fn main() { | |
148 | /// let options = AppConfig { | |
149 | /// foo: default(), | |
150 | /// bar: BarConfig { | |
151 | /// bar: 10.1, | |
152 | /// ..default() | |
153 | /// }, | |
154 | /// }; | |
155 | /// } | |
156 | /// ``` | |
157 | #[unstable(feature = "default_free_fn", issue = "73014")] | |
158 | #[inline] | |
159 | pub fn default<T: Default>() -> T { | |
160 | Default::default() | |
161 | } | |
162 | ||
416331ca | 163 | /// Derive macro generating an impl of the trait `Default`. |
416331ca | 164 | #[rustc_builtin_macro] |
416331ca XL |
165 | #[stable(feature = "builtin_macro_prelude", since = "1.38.0")] |
166 | #[allow_internal_unstable(core_intrinsics)] | |
60c5eb7d XL |
167 | pub macro Default($item:item) { |
168 | /* compiler built-in */ | |
169 | } | |
416331ca | 170 | |
1a4d82fc | 171 | macro_rules! default_impl { |
3b2f2976 | 172 | ($t:ty, $v:expr, $doc:tt) => { |
85aaf69f | 173 | #[stable(feature = "rust1", since = "1.0.0")] |
1a4d82fc JJ |
174 | impl Default for $t { |
175 | #[inline] | |
041b39d2 | 176 | #[doc = $doc] |
5869c6ff XL |
177 | fn default() -> $t { |
178 | $v | |
179 | } | |
1a4d82fc | 180 | } |
5869c6ff | 181 | }; |
1a4d82fc JJ |
182 | } |
183 | ||
041b39d2 XL |
184 | default_impl! { (), (), "Returns the default value of `()`" } |
185 | default_impl! { bool, false, "Returns the default value of `false`" } | |
186 | default_impl! { char, '\x00', "Returns the default value of `\\x00`" } | |
1a4d82fc | 187 | |
041b39d2 XL |
188 | default_impl! { usize, 0, "Returns the default value of `0`" } |
189 | default_impl! { u8, 0, "Returns the default value of `0`" } | |
190 | default_impl! { u16, 0, "Returns the default value of `0`" } | |
191 | default_impl! { u32, 0, "Returns the default value of `0`" } | |
192 | default_impl! { u64, 0, "Returns the default value of `0`" } | |
193 | default_impl! { u128, 0, "Returns the default value of `0`" } | |
1a4d82fc | 194 | |
041b39d2 XL |
195 | default_impl! { isize, 0, "Returns the default value of `0`" } |
196 | default_impl! { i8, 0, "Returns the default value of `0`" } | |
197 | default_impl! { i16, 0, "Returns the default value of `0`" } | |
198 | default_impl! { i32, 0, "Returns the default value of `0`" } | |
199 | default_impl! { i64, 0, "Returns the default value of `0`" } | |
200 | default_impl! { i128, 0, "Returns the default value of `0`" } | |
1a4d82fc | 201 | |
041b39d2 XL |
202 | default_impl! { f32, 0.0f32, "Returns the default value of `0.0`" } |
203 | default_impl! { f64, 0.0f64, "Returns the default value of `0.0`" } |