default.rs.html -- source
Description: Source to the Rust file `src/libcore/default.rs`.
Keywords: rust, rustlang, rust-lang

      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
    159
    160
    161
    162
    163
    164
    165
    166
    167
    168
    169
    170
    171
    172
    173
    174
    175
    176
    177
    178
    179
    180
    181
    182
    183
    184
    185
    186
    187
    188
    189
    190
    191
    192
    193
    194
    195
    196
    197
    198
    199
    200

    //! 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() -> 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;
    }

    /// Return the default value of a type according to the `Default` trait.
    ///
    /// The type to return is inferred from context; this is equivalent to
    /// `Default::default()` but shorter to type.
    ///
    /// For example:
    /// ```
    /// #![feature(default_free_fn)]
    ///
    /// use std::default::default;
    ///
    /// #[derive(Default)]
    /// struct AppConfig {
    ///     foo: FooConfig,
    ///     bar: BarConfig,
    /// }
    ///
    /// #[derive(Default)]
    /// struct FooConfig {
    ///     foo: i32,
    /// }
    ///
    /// #[derive(Default)]
    /// struct BarConfig {
    ///     bar: f32,
    ///     baz: u8,
    /// }
    ///
    /// fn main() {
    ///     let options = AppConfig {
    ///         foo: default(),
    ///         bar: BarConfig {
    ///             bar: 10.1,
    ///             ..default()
    ///         },
    ///     };
    /// }
    /// ```
    #[unstable(feature = "default_free_fn", issue = "73014")]
    #[inline]
    pub fn default<T: Default>() -> T {
        Default::default()
    }

    /// Derive macro generating an impl of the trait `Default`.
    #[rustc_builtin_macro]
    #[stable(feature = "builtin_macro_prelude", since = "1.38.0")]
    #[allow_internal_unstable(core_intrinsics)]
    pub macro Default($item:item) {
        /* compiler built-in */
    }

    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`" }