1 //! The `Default` trait for types with a default value.
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.
57 /// When using `#[derive(Default)]` on an `enum`, you need to choose which unit variant will be
58 /// default. You do this by placing the `#[default]` attribute on the variant.
61 /// #[derive(Default)]
70 /// You cannot use the `#[default]` attribute on non-unit or non-exhaustive variants.
72 /// ## How can I implement `Default`?
74 /// Provide an implementation for the `default()` method that returns the value of
75 /// your type that should be the default:
78 /// # #![allow(dead_code)]
85 /// impl Default for Kind {
86 /// fn default() -> Self { Kind::A }
93 /// # #[allow(dead_code)]
94 /// #[derive(Default)]
95 /// struct SomeOptions {
100 #[cfg_attr(not(test), rustc_diagnostic_item = "Default")]
101 #[stable(feature = "rust1", since = "1.0.0")]
103 pub trait Default: Sized {
104 /// Returns the "default value" for a type.
106 /// Default values are often some kind of initial value, identity value, or anything else that
107 /// may make sense as a default.
111 /// Using built-in default values:
114 /// let i: i8 = Default::default();
115 /// let (x, y): (Option<String>, f64) = Default::default();
116 /// let (a, b, (c, d)): (i32, u32, (bool, bool)) = Default::default();
122 /// # #[allow(dead_code)]
129 /// impl Default for Kind {
130 /// fn default() -> Self { Kind::A }
133 #[stable(feature = "rust1", since = "1.0.0")]
134 fn default() -> Self;
137 /// Return the default value of a type according to the `Default` trait.
139 /// The type to return is inferred from context; this is equivalent to
140 /// `Default::default()` but shorter to type.
144 /// #![feature(default_free_fn)]
146 /// use std::default::default;
148 /// #[derive(Default)]
149 /// struct AppConfig {
154 /// #[derive(Default)]
155 /// struct FooConfig {
159 /// #[derive(Default)]
160 /// struct BarConfig {
166 /// let options = AppConfig {
175 #[unstable(feature = "default_free_fn", issue = "73014")]
178 pub fn default<T: Default>() -> T {
182 /// Derive macro generating an impl of the trait `Default`.
183 #[rustc_builtin_macro(Default, attributes(default))]
184 #[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
185 #[allow_internal_unstable(core_intrinsics)]
186 pub macro Default($item:item) {
187 /* compiler built-in */
190 macro_rules! default_impl {
191 ($t:ty, $v:expr, $doc:tt) => {
192 #[stable(feature = "rust1", since = "1.0.0")]
193 #[rustc_const_unstable(feature = "const_default_impls", issue = "87864")]
194 impl const Default for $t {
204 default_impl! { (), (), "Returns the default value of `()`" }
205 default_impl! { bool, false, "Returns the default value of `false`" }
206 default_impl! { char, '\x00', "Returns the default value of `\\x00`" }
208 default_impl! { usize, 0, "Returns the default value of `0`" }
209 default_impl! { u8, 0, "Returns the default value of `0`" }
210 default_impl! { u16, 0, "Returns the default value of `0`" }
211 default_impl! { u32, 0, "Returns the default value of `0`" }
212 default_impl! { u64, 0, "Returns the default value of `0`" }
213 default_impl! { u128, 0, "Returns the default value of `0`" }
215 default_impl! { isize, 0, "Returns the default value of `0`" }
216 default_impl! { i8, 0, "Returns the default value of `0`" }
217 default_impl! { i16, 0, "Returns the default value of `0`" }
218 default_impl! { i32, 0, "Returns the default value of `0`" }
219 default_impl! { i64, 0, "Returns the default value of `0`" }
220 default_impl! { i128, 0, "Returns the default value of `0`" }
222 default_impl! { f32, 0.0f32, "Returns the default value of `0.0`" }
223 default_impl! { f64, 0.0f64, "Returns the default value of `0.0`" }