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//! The `Default` trait for types which may have meaningful default values. #![stable(feature = "rust1", since = "1.0.0")] /// A trait for giving a type a useful default value. /// /// Sometimes, you want to fall back to some kind of default value, and /// don't particularly care what it is. This comes up often with `struct`s /// that define a set of options: /// /// ``` /// # #[allow(dead_code)] /// struct SomeOptions { /// foo: i32, /// bar: f32, /// } /// ``` /// /// How can we define some default values? You can use `Default`: /// /// ``` /// # #[allow(dead_code)] /// #[derive(Default)] /// struct SomeOptions { /// foo: i32, /// bar: f32, /// } /// /// fn main() { /// let options: SomeOptions = Default::default(); /// } /// ``` /// /// Now, you get all of the default values. Rust implements `Default` for various primitives types. /// /// If you want to override a particular option, but still retain the other defaults: /// /// ``` /// # #[allow(dead_code)] /// # #[derive(Default)] /// # struct SomeOptions { /// # foo: i32, /// # bar: f32, /// # } /// fn main() { /// let options = SomeOptions { foo: 42, ..Default::default() }; /// } /// ``` /// /// ## Derivable /// /// This trait can be used with `#[derive]` if all of the type's fields implement /// `Default`. When `derive`d, it will use the default value for each field's type. /// /// ## How can I implement `Default`? /// /// Provides an implementation for the `default()` method that returns the value of /// your type that should be the default: /// /// ``` /// # #![allow(dead_code)] /// enum Kind { /// A, /// B, /// C, /// } /// /// impl Default for Kind { /// fn default() -> Self { Kind::A } /// } /// ``` /// /// # Examples /// /// ``` /// # #[allow(dead_code)] /// #[derive(Default)] /// struct SomeOptions { /// foo: i32, /// bar: f32, /// } /// ``` #[stable(feature = "rust1", since = "1.0.0")] pub trait Default: Sized { /// Returns the "default value" for a type. /// /// Default values are often some kind of initial value, identity value, or anything else that /// may make sense as a default. /// /// # Examples /// /// Using built-in default values: /// /// ``` /// let i: i8 = Default::default(); /// let (x, y): (Option<String>, f64) = Default::default(); /// let (a, b, (c, d)): (i32, u32, (bool, bool)) = Default::default(); /// ``` /// /// Making your own: /// /// ``` /// # #[allow(dead_code)] /// enum Kind { /// A, /// B, /// C, /// } /// /// impl Default for Kind { /// fn default() -> Self { Kind::A } /// } /// ``` #[stable(feature = "rust1", since = "1.0.0")] fn default() -> Self; } macro_rules! default_impl { ($t:ty, $v:expr, $doc:tt) => { #[stable(feature = "rust1", since = "1.0.0")] impl Default for $t { #[inline] #[doc = $doc] fn default() -> $t { $v } } } } default_impl! { (), (), "Returns the default value of `()`" } default_impl! { bool, false, "Returns the default value of `false`" } default_impl! { char, '\x00', "Returns the default value of `\\x00`" } default_impl! { usize, 0, "Returns the default value of `0`" } default_impl! { u8, 0, "Returns the default value of `0`" } default_impl! { u16, 0, "Returns the default value of `0`" } default_impl! { u32, 0, "Returns the default value of `0`" } default_impl! { u64, 0, "Returns the default value of `0`" } default_impl! { u128, 0, "Returns the default value of `0`" } default_impl! { isize, 0, "Returns the default value of `0`" } default_impl! { i8, 0, "Returns the default value of `0`" } default_impl! { i16, 0, "Returns the default value of `0`" } default_impl! { i32, 0, "Returns the default value of `0`" } default_impl! { i64, 0, "Returns the default value of `0`" } default_impl! { i128, 0, "Returns the default value of `0`" } default_impl! { f32, 0.0f32, "Returns the default value of `0.0`" } default_impl! { f64, 0.0f64, "Returns the default value of `0.0`" }