<!DOCTYPE html><html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta name="generator" content="rustdoc"><meta name="description" content="Source to the Rust file `libcore/default.rs`."><meta name="keywords" content="rust, rustlang, rust-lang"><title>default.rs.html -- source</title><link rel="stylesheet" type="text/css" href="../../normalize.css"><link rel="stylesheet" type="text/css" href="../../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" type="text/css" href="../../dark.css"><link rel="stylesheet" type="text/css" href="../../light.css" id="themeStyle"><script src="../../storage.js"></script><link rel="shortcut icon" href="https://doc.rust-lang.org/favicon.ico"></head><body class="rustdoc source"><!--[if lte IE 8]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><div class="sidebar-menu">☰</div><a href='../../core/index.html'><img src='https://www.rust-lang.org/logos/rust-logo-128x128-blk-v2.png' alt='logo' width='100'></a></nav><div class="theme-picker"><button id="theme-picker" aria-label="Pick another theme!"><img src="../../brush.svg" width="18" alt="Pick another theme!"></button><div id="theme-choices"></div></div><script src="../../theme.js"></script><nav class="sub"><form class="search-form js-only"><div class="search-container"><input class="search-input" name="search" autocomplete="off" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><a id="settings-menu" href="../../settings.html"><img src="../../wheel.svg" width="18" alt="Change settings"></a></div></form></nav><section id="main" class="content"><pre class="line-numbers"><span id="1"> 1</span>
<span id="2"> 2</span>
<span id="3"> 3</span>
<span id="4"> 4</span>
<span id="5"> 5</span>
<span id="6"> 6</span>
<span id="7"> 7</span>
<span id="8"> 8</span>
<span id="9"> 9</span>
<span id="10"> 10</span>
<span id="11"> 11</span>
<span id="12"> 12</span>
<span id="13"> 13</span>
<span id="14"> 14</span>
<span id="15"> 15</span>
<span id="16"> 16</span>
<span id="17"> 17</span>
<span id="18"> 18</span>
<span id="19"> 19</span>
<span id="20"> 20</span>
<span id="21"> 21</span>
<span id="22"> 22</span>
<span id="23"> 23</span>
<span id="24"> 24</span>
<span id="25"> 25</span>
<span id="26"> 26</span>
<span id="27"> 27</span>
<span id="28"> 28</span>
<span id="29"> 29</span>
<span id="30"> 30</span>
<span id="31"> 31</span>
<span id="32"> 32</span>
<span id="33"> 33</span>
<span id="34"> 34</span>
<span id="35"> 35</span>
<span id="36"> 36</span>
<span id="37"> 37</span>
<span id="38"> 38</span>
<span id="39"> 39</span>
<span id="40"> 40</span>
<span id="41"> 41</span>
<span id="42"> 42</span>
<span id="43"> 43</span>
<span id="44"> 44</span>
<span id="45"> 45</span>
<span id="46"> 46</span>
<span id="47"> 47</span>
<span id="48"> 48</span>
<span id="49"> 49</span>
<span id="50"> 50</span>
<span id="51"> 51</span>
<span id="52"> 52</span>
<span id="53"> 53</span>
<span id="54"> 54</span>
<span id="55"> 55</span>
<span id="56"> 56</span>
<span id="57"> 57</span>
<span id="58"> 58</span>
<span id="59"> 59</span>
<span id="60"> 60</span>
<span id="61"> 61</span>
<span id="62"> 62</span>
<span id="63"> 63</span>
<span id="64"> 64</span>
<span id="65"> 65</span>
<span id="66"> 66</span>
<span id="67"> 67</span>
<span id="68"> 68</span>
<span id="69"> 69</span>
<span id="70"> 70</span>
<span id="71"> 71</span>
<span id="72"> 72</span>
<span id="73"> 73</span>
<span id="74"> 74</span>
<span id="75"> 75</span>
<span id="76"> 76</span>
<span id="77"> 77</span>
<span id="78"> 78</span>
<span id="79"> 79</span>
<span id="80"> 80</span>
<span id="81"> 81</span>
<span id="82"> 82</span>
<span id="83"> 83</span>
<span id="84"> 84</span>
<span id="85"> 85</span>
<span id="86"> 86</span>
<span id="87"> 87</span>
<span id="88"> 88</span>
<span id="89"> 89</span>
<span id="90"> 90</span>
<span id="91"> 91</span>
<span id="92"> 92</span>
<span id="93"> 93</span>
<span id="94"> 94</span>
<span id="95"> 95</span>
<span id="96"> 96</span>
<span id="97"> 97</span>
<span id="98"> 98</span>
<span id="99"> 99</span>
<span id="100">100</span>
<span id="101">101</span>
<span id="102">102</span>
<span id="103">103</span>
<span id="104">104</span>
<span id="105">105</span>
<span id="106">106</span>
<span id="107">107</span>
<span id="108">108</span>
<span id="109">109</span>
<span id="110">110</span>
<span id="111">111</span>
<span id="112">112</span>
<span id="113">113</span>
<span id="114">114</span>
<span id="115">115</span>
<span id="116">116</span>
<span id="117">117</span>
<span id="118">118</span>
<span id="119">119</span>
<span id="120">120</span>
<span id="121">121</span>
<span id="122">122</span>
<span id="123">123</span>
<span id="124">124</span>
<span id="125">125</span>
<span id="126">126</span>
<span id="127">127</span>
<span id="128">128</span>
<span id="129">129</span>
<span id="130">130</span>
<span id="131">131</span>
<span id="132">132</span>
<span id="133">133</span>
<span id="134">134</span>
<span id="135">135</span>
<span id="136">136</span>
<span id="137">137</span>
<span id="138">138</span>
<span id="139">139</span>
<span id="140">140</span>
<span id="141">141</span>
<span id="142">142</span>
<span id="143">143</span>
<span id="144">144</span>
<span id="145">145</span>
<span id="146">146</span>
<span id="147">147</span>
<span id="148">148</span>
<span id="149">149</span>
<span id="150">150</span>
<span id="151">151</span>
<span id="152">152</span>
<span id="153">153</span>
<span id="154">154</span>
<span id="155">155</span>
<span id="156">156</span>
<span id="157">157</span>
<span id="158">158</span>
</pre><pre class="rust ">
<span class="comment">// Copyright 2013 The Rust Project Developers. See the COPYRIGHT</span>
<span class="comment">// file at the top-level directory of this distribution and at</span>
<span class="comment">// http://rust-lang.org/COPYRIGHT.</span>
<span class="comment">//</span>
<span class="comment">// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or</span>
<span class="comment">// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license</span>
<span class="comment">// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your</span>
<span class="comment">// option. This file may not be copied, modified, or distributed</span>
<span class="comment">// except according to those terms.</span>
<span class="doccomment">//! The `Default` trait for types which may have meaningful default values.</span>
<span class="attribute">#![<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="doccomment">/// A trait for giving a type a useful default value.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Sometimes, you want to fall back to some kind of default value, and</span>
<span class="doccomment">/// don't particularly care what it is. This comes up often with `struct`s</span>
<span class="doccomment">/// that define a set of options:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// # #[allow(dead_code)]</span>
<span class="doccomment">/// struct SomeOptions {</span>
<span class="doccomment">/// foo: i32,</span>
<span class="doccomment">/// bar: f32,</span>
<span class="doccomment">/// }</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// How can we define some default values? You can use `Default`:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// # #[allow(dead_code)]</span>
<span class="doccomment">/// #[derive(Default)]</span>
<span class="doccomment">/// struct SomeOptions {</span>
<span class="doccomment">/// foo: i32,</span>
<span class="doccomment">/// bar: f32,</span>
<span class="doccomment">/// }</span>
<span class="doccomment">///</span>
<span class="doccomment">/// fn main() {</span>
<span class="doccomment">/// let options: SomeOptions = Default::default();</span>
<span class="doccomment">/// }</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Now, you get all of the default values. Rust implements `Default` for various primitives types.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// If you want to override a particular option, but still retain the other defaults:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// # #[allow(dead_code)]</span>
<span class="doccomment">/// # #[derive(Default)]</span>
<span class="doccomment">/// # struct SomeOptions {</span>
<span class="doccomment">/// # foo: i32,</span>
<span class="doccomment">/// # bar: f32,</span>
<span class="doccomment">/// # }</span>
<span class="doccomment">/// fn main() {</span>
<span class="doccomment">/// let options = SomeOptions { foo: 42, ..Default::default() };</span>
<span class="doccomment">/// }</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ## Derivable</span>
<span class="doccomment">///</span>
<span class="doccomment">/// This trait can be used with `#[derive]` if all of the type's fields implement</span>
<span class="doccomment">/// `Default`. When `derive`d, it will use the default value for each field's type.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ## How can I implement `Default`?</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Provide an implementation for the `default()` method that returns the value of</span>
<span class="doccomment">/// your type that should be the default:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// # #![allow(dead_code)]</span>
<span class="doccomment">/// enum Kind {</span>
<span class="doccomment">/// A,</span>
<span class="doccomment">/// B,</span>
<span class="doccomment">/// C,</span>
<span class="doccomment">/// }</span>
<span class="doccomment">///</span>
<span class="doccomment">/// impl Default for Kind {</span>
<span class="doccomment">/// fn default() -> Kind { Kind::A }</span>
<span class="doccomment">/// }</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// # #[allow(dead_code)]</span>
<span class="doccomment">/// #[derive(Default)]</span>
<span class="doccomment">/// struct SomeOptions {</span>
<span class="doccomment">/// foo: i32,</span>
<span class="doccomment">/// bar: f32,</span>
<span class="doccomment">/// }</span>
<span class="doccomment">/// ```</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="kw">pub</span> <span class="kw">trait</span> <span class="ident">Default</span>: <span class="ident">Sized</span> {
<span class="doccomment">/// Returns the "default value" for a type.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Default values are often some kind of initial value, identity value, or anything else that</span>
<span class="doccomment">/// may make sense as a default.</span>
<span class="doccomment">///</span>
<span class="doccomment">/// # Examples</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Using built-in default values:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// let i: i8 = Default::default();</span>
<span class="doccomment">/// let (x, y): (Option<String>, f64) = Default::default();</span>
<span class="doccomment">/// let (a, b, (c, d)): (i32, u32, (bool, bool)) = Default::default();</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">///</span>
<span class="doccomment">/// Making your own:</span>
<span class="doccomment">///</span>
<span class="doccomment">/// ```</span>
<span class="doccomment">/// # #[allow(dead_code)]</span>
<span class="doccomment">/// enum Kind {</span>
<span class="doccomment">/// A,</span>
<span class="doccomment">/// B,</span>
<span class="doccomment">/// C,</span>
<span class="doccomment">/// }</span>
<span class="doccomment">///</span>
<span class="doccomment">/// impl Default for Kind {</span>
<span class="doccomment">/// fn default() -> Kind { Kind::A }</span>
<span class="doccomment">/// }</span>
<span class="doccomment">/// ```</span>
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="kw">fn</span> <span class="ident">default</span>() <span class="op">-></span> <span class="self">Self</span>;
}
<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">default_impl</span> {
(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">t</span>:<span class="ident">ty</span>, <span class="macro-nonterminal">$</span><span class="macro-nonterminal">v</span>:<span class="ident">expr</span>, <span class="macro-nonterminal">$</span><span class="macro-nonterminal">doc</span>:<span class="ident">tt</span>) <span class="op">=></span> {
<span class="attribute">#[<span class="ident">stable</span>(<span class="ident">feature</span> <span class="op">=</span> <span class="string">"rust1"</span>, <span class="ident">since</span> <span class="op">=</span> <span class="string">"1.0.0"</span>)]</span>
<span class="kw">impl</span> <span class="ident">Default</span> <span class="kw">for</span> <span class="macro-nonterminal">$</span><span class="macro-nonterminal">t</span> {
<span class="attribute">#[<span class="ident">inline</span>]</span>
<span class="attribute">#[<span class="ident">doc</span> <span class="op">=</span> <span class="macro-nonterminal">$</span><span class="macro-nonterminal">doc</span>]</span>
<span class="kw">fn</span> <span class="ident">default</span>() <span class="op">-></span> <span class="macro-nonterminal">$</span><span class="macro-nonterminal">t</span> { <span class="macro-nonterminal">$</span><span class="macro-nonterminal">v</span> }
}
}
}
<span class="macro">default_impl</span><span class="macro">!</span> { (), (), <span class="string">"Returns the default value of `()`"</span> }
<span class="macro">default_impl</span><span class="macro">!</span> { <span class="ident">bool</span>, <span class="bool-val">false</span>, <span class="string">"Returns the default value of `false`"</span> }
<span class="macro">default_impl</span><span class="macro">!</span> { <span class="ident">char</span>, <span class="string">'\x00'</span>, <span class="string">"Returns the default value of `\\x00`"</span> }
<span class="macro">default_impl</span><span class="macro">!</span> { <span class="ident">usize</span>, <span class="number">0</span>, <span class="string">"Returns the default value of `0`"</span> }
<span class="macro">default_impl</span><span class="macro">!</span> { <span class="ident">u8</span>, <span class="number">0</span>, <span class="string">"Returns the default value of `0`"</span> }
<span class="macro">default_impl</span><span class="macro">!</span> { <span class="ident">u16</span>, <span class="number">0</span>, <span class="string">"Returns the default value of `0`"</span> }
<span class="macro">default_impl</span><span class="macro">!</span> { <span class="ident">u32</span>, <span class="number">0</span>, <span class="string">"Returns the default value of `0`"</span> }
<span class="macro">default_impl</span><span class="macro">!</span> { <span class="ident">u64</span>, <span class="number">0</span>, <span class="string">"Returns the default value of `0`"</span> }
<span class="macro">default_impl</span><span class="macro">!</span> { <span class="ident">u128</span>, <span class="number">0</span>, <span class="string">"Returns the default value of `0`"</span> }
<span class="macro">default_impl</span><span class="macro">!</span> { <span class="ident">isize</span>, <span class="number">0</span>, <span class="string">"Returns the default value of `0`"</span> }
<span class="macro">default_impl</span><span class="macro">!</span> { <span class="ident">i8</span>, <span class="number">0</span>, <span class="string">"Returns the default value of `0`"</span> }
<span class="macro">default_impl</span><span class="macro">!</span> { <span class="ident">i16</span>, <span class="number">0</span>, <span class="string">"Returns the default value of `0`"</span> }
<span class="macro">default_impl</span><span class="macro">!</span> { <span class="ident">i32</span>, <span class="number">0</span>, <span class="string">"Returns the default value of `0`"</span> }
<span class="macro">default_impl</span><span class="macro">!</span> { <span class="ident">i64</span>, <span class="number">0</span>, <span class="string">"Returns the default value of `0`"</span> }
<span class="macro">default_impl</span><span class="macro">!</span> { <span class="ident">i128</span>, <span class="number">0</span>, <span class="string">"Returns the default value of `0`"</span> }
<span class="macro">default_impl</span><span class="macro">!</span> { <span class="ident">f32</span>, <span class="number">0.0f32</span>, <span class="string">"Returns the default value of `0.0`"</span> }
<span class="macro">default_impl</span><span class="macro">!</span> { <span class="ident">f64</span>, <span class="number">0.0f64</span>, <span class="string">"Returns the default value of `0.0`"</span> }
</pre>
</section><section id="search" class="content hidden"></section><section class="footer"></section><aside id="help" class="hidden"><div><h1 class="hidden">Help</h1><div class="shortcuts"><h2>Keyboard Shortcuts</h2><dl><dt><kbd>?</kbd></dt><dd>Show this help dialog</dd><dt><kbd>S</kbd></dt><dd>Focus the search field</dd><dt><kbd>↑</kbd></dt><dd>Move up in search results</dd><dt><kbd>↓</kbd></dt><dd>Move down in search results</dd><dt><kbd>↹</kbd></dt><dd>Switch tab</dd><dt><kbd>⏎</kbd></dt><dd>Go to active search result</dd><dt><kbd>+</kbd></dt><dd>Expand all sections</dd><dt><kbd>-</kbd></dt><dd>Collapse all sections</dd></dl></div><div class="infos"><h2>Search Tricks</h2><p>Prefix searches with a type followed by a colon (e.g. <code>fn:</code>) to restrict the search to a given type.</p><p>Accepted types are: <code>fn</code>, <code>mod</code>, <code>struct</code>, <code>enum</code>, <code>trait</code>, <code>type</code>, <code>macro</code>, and <code>const</code>.</p><p>Search functions by type signature (e.g. <code>vec -> usize</code> or <code>* -> vec</code>)</p><p>Search multiple things at once by splitting your query with comma (e.g. <code>str,u8</code> or <code>String,struct:Vec,test</code>)</p></div></div></aside><script>window.rootPath = "../../";window.currentCrate = "core";</script><script src="../../aliases.js"></script><script src="../../main.js"></script><script defer src="../../search-index.js"></script></body></html>
distrib
>
Mageia
>
6
>
armv7hl
>
media
>
core-updates
>
by-pkgid
>
4e2dbb669434a7691662cb2f0ad38972
>
files
>
11542
