rust/src/libcore/default.rs

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// Copyright 2013 The Rust Project Developers. See the COPYRIGHT
// file at the top-level directory of this distribution and at
// http://rust-lang.org/COPYRIGHT.
//
// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
// option. This file may not be copied, modified, or distributed
// except according to those terms.
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//! The `Default` trait for types which may have meaningful default values.
//!
//! 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:
//!
//! ```
//! struct SomeOptions {
//! foo: int,
//! bar: f32,
//! }
//! ```
//!
//! How can we define some default values? You can use `Default`:
//!
//! ```
//! use std::default::Default;
//!
//! #[deriving(Default)]
//! struct SomeOptions {
//! foo: int,
//! 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 have your own type, you need to implement `Default` yourself:
//!
//! ```
//! use std::default::Default;
//!
//! enum Kind {
//! A,
//! B,
//! C,
//! }
//!
//! impl Default for Kind {
//! fn default() -> Kind { Kind::A }
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//! }
//!
//! #[deriving(Default)]
//! struct SomeOptions {
//! foo: int,
//! bar: f32,
//! baz: Kind,
//! }
//!
//!
//! fn main() {
//! let options: SomeOptions = Default::default();
//! }
//! ```
//!
//! If you want to override a particular option, but still retain the other defaults:
//!
//! ```
//! # use std::default::Default;
//! # #[deriving(Default)]
//! # struct SomeOptions {
//! # foo: int,
//! # bar: f32,
//! # }
//! fn main() {
//! let options = SomeOptions { foo: 42, ..Default::default() };
//! }
//! ```
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#![stable]
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/// A trait that types which have a useful default value should implement.
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///
/// A struct can derive default implementations of `Default` for basic types using
/// `#[deriving(Default)]`.
///
/// # Examples
///
/// ```
/// #[deriving(Default)]
/// struct SomeOptions {
/// foo: int,
/// bar: f32,
/// }
/// ```
#[stable]
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pub trait Default {
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/// Returns the "default value" for a type.
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///
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/// 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:
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///
/// ```
/// use std::default::Default;
///
/// let i: i8 = Default::default();
/// let (x, y): (Option<String>, f64) = Default::default();
/// let (a, b, (c, d)): (int, uint, (bool, bool)) = Default::default();
/// ```
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///
/// Making your own:
///
/// ```
/// use std::default::Default;
///
/// enum Kind {
/// A,
/// B,
/// C,
/// }
///
/// impl Default for Kind {
/// fn default() -> Kind { Kind::A }
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/// }
/// ```
#[stable]
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fn default() -> Self;
}
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macro_rules! default_impl(
($t:ty, $v:expr) => {
#[stable]
impl Default for $t {
#[inline]
#[stable]
fn default() -> $t { $v }
}
}
)
default_impl!((), ())
default_impl!(bool, false)
default_impl!(char, '\x00')
default_impl!(uint, 0u)
default_impl!(u8, 0u8)
default_impl!(u16, 0u16)
default_impl!(u32, 0u32)
default_impl!(u64, 0u64)
default_impl!(int, 0i)
default_impl!(i8, 0i8)
default_impl!(i16, 0i16)
default_impl!(i32, 0i32)
default_impl!(i64, 0i64)
default_impl!(f32, 0.0f32)
default_impl!(f64, 0.0f64)