      1
      2
      3
      4
      5
      6
      7
      8
      9
     10
     11
     12
     13
     14
     15
     16
     17
     18
     19
     20
     21
     22
     23
     24
     25
     26
     27
     28
     29
     30
     31
     32
     33
     34
     35
     36
     37
     38
     39
     40
     41
     42
     43
     44
     45
     46
     47
     48
     49
     50
     51
     52
     53
     54
     55
     56
     57
     58
     59
     60
     61
     62
     63
     64
     65
     66
     67
     68
     69
     70
     71
     72
     73
     74
     75
     76
     77
     78
     79
     80
     81
     82
     83
     84
     85
     86
     87
     88
     89
     90
     91
     92
     93
     94
     95
     96
     97
     98
     99
    100
    101
    102
    103
    104
    105
    106
    107
    108
    109
    110
    111
    112
    113
    114
    115
    116
    117
    118
    119
    120
    121
    122
    123
    124
    125
    126
    127
    128
    129
    130
    131
    132
    133
    134
    135
    136
    137
    138
    139
    140
    141
    142
    143
    144
    145
    146
    147
    148
    149
    150
    151
    152
    153
    154
    155
    156
    157
    158

    // 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.

    //! 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`?
    ///
    /// Provide 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() -> Kind { 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() -> Kind { 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`" }

# Help

## Keyboard Shortcuts

?
  Show this help dialog
S
  Focus the search field
⇤
  Move up in search results
⇥
  Move down in search results
⏎
  Go to active search result
+
  Collapse/expand all sections

## Search Tricks

Prefix searches with a type followed by a colon (e.g. `fn:`) to restrict the search to a given type.

Accepted types are: `fn`, `mod`, `struct`, `enum`, `trait`, `type`, `macro`, and `const`.

Search functions by type signature (e.g. `vec -> usize` or `* -> vec`)