1 //! The `Default` trait for types which may have meaningful default values.
3 #![stable(feature = "rust1", since = "1.0.0")]
5 /// A trait for giving a type a useful default value.
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:
12 /// # #[allow(dead_code)]
13 /// struct SomeOptions {
19 /// How can we define some default values? You can use `Default`:
22 /// # #[allow(dead_code)]
23 /// #[derive(Default)]
24 /// struct SomeOptions {
30 /// let options: SomeOptions = Default::default();
34 /// Now, you get all of the default values. Rust implements `Default` for various primitives types.
36 /// If you want to override a particular option, but still retain the other defaults:
39 /// # #[allow(dead_code)]
40 /// # #[derive(Default)]
41 /// # struct SomeOptions {
46 /// let options = SomeOptions { foo: 42, ..Default::default() };
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.
55 /// ## How can I implement `Default`?
57 /// Provide an implementation for the `default()` method that returns the value of
58 /// your type that should be the default:
61 /// # #![allow(dead_code)]
68 /// impl Default for Kind {
69 /// fn default() -> Self { Kind::A }
76 /// # #[allow(dead_code)]
77 /// #[derive(Default)]
78 /// struct SomeOptions {
83 #[cfg_attr(not(test), rustc_diagnostic_item = "Default")]
84 #[stable(feature = "rust1", since = "1.0.0")]
85 pub trait Default: Sized {
86 /// Returns the "default value" for a type.
88 /// Default values are often some kind of initial value, identity value, or anything else that
89 /// may make sense as a default.
93 /// Using built-in default values:
96 /// let i: i8 = Default::default();
97 /// let (x, y): (Option<String>, f64) = Default::default();
98 /// let (a, b, (c, d)): (i32, u32, (bool, bool)) = Default::default();
104 /// # #[allow(dead_code)]
111 /// impl Default for Kind {
112 /// fn default() -> Self { Kind::A }
115 #[stable(feature = "rust1", since = "1.0.0")]
116 fn default() -> Self;
119 /// Return the default value of a type according to the `Default` trait.
121 /// The type to return is inferred from context; this is equivalent to
122 /// `Default::default()` but shorter to type.
126 /// #![feature(default_free_fn)]
128 /// use std::default::default;
130 /// #[derive(Default)]
131 /// struct AppConfig {
136 /// #[derive(Default)]
137 /// struct FooConfig {
141 /// #[derive(Default)]
142 /// struct BarConfig {
148 /// let options = AppConfig {
157 #[unstable(feature = "default_free_fn", issue = "73014")]
160 pub fn default<T: Default>() -> T {
164 /// Derive macro generating an impl of the trait `Default`.
165 #[rustc_builtin_macro(Default, attributes(default))]
166 #[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
167 #[allow_internal_unstable(core_intrinsics)]
168 pub macro Default($item:item) {
169 /* compiler built-in */
172 macro_rules! default_impl {
173 ($t:ty, $v:expr, $doc:tt) => {
174 #[stable(feature = "rust1", since = "1.0.0")]
175 #[rustc_const_unstable(feature = "const_default_impls", issue = "87864")]
176 impl const Default for $t {
186 default_impl! { (), (), "Returns the default value of `()`" }
187 default_impl! { bool, false, "Returns the default value of `false`" }
188 default_impl! { char, '\x00', "Returns the default value of `\\x00`" }
190 default_impl! { usize, 0, "Returns the default value of `0`" }
191 default_impl! { u8, 0, "Returns the default value of `0`" }
192 default_impl! { u16, 0, "Returns the default value of `0`" }
193 default_impl! { u32, 0, "Returns the default value of `0`" }
194 default_impl! { u64, 0, "Returns the default value of `0`" }
195 default_impl! { u128, 0, "Returns the default value of `0`" }
197 default_impl! { isize, 0, "Returns the default value of `0`" }
198 default_impl! { i8, 0, "Returns the default value of `0`" }
199 default_impl! { i16, 0, "Returns the default value of `0`" }
200 default_impl! { i32, 0, "Returns the default value of `0`" }
201 default_impl! { i64, 0, "Returns the default value of `0`" }
202 default_impl! { i128, 0, "Returns the default value of `0`" }
204 default_impl! { f32, 0.0f32, "Returns the default value of `0.0`" }
205 default_impl! { f64, 0.0f64, "Returns the default value of `0.0`" }