137 lines
12 KiB
HTML
137 lines
12 KiB
HTML
<!DOCTYPE html>
|
|
<html lang="zh" dir="ltr" class="client-nojs" xmlns="http://www.w3.org/1999/xhtml" xml:lang="zh">
|
|
<head><meta http-equiv="x-ua-compatible" content="ie=edge">
|
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
|
|
<title>std::ranges::views::values, std::ranges::values_view</title>
|
|
<link rel="stylesheet" href="ext.css" />
|
|
<link rel="stylesheet" href="site_modules.css" />
|
|
</head>
|
|
<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_ranges_values_view skin-cppreference2 action-view cpp-navbar">
|
|
<div id="cpp-content-base">
|
|
<div id="content"><a id="top"></a>
|
|
<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::ranges::</span><span style="font-size:0.7em; line-height:130%">views::</span>values, <span style="font-size:0.7em; line-height:130%">std::ranges::</span>values_view</h1>
|
|
<div id="bodyContent">
|
|
<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-ranges.html">ranges</a></span></div>
|
|
<div id="mw-content-text" lang="zh" dir="ltr" class="mw-content-ltr" xml:lang="zh">
|
|
<table class="t-dcl-begin">
|
|
<tbody>
|
|
<tr class="t-dsc-header">
|
|
<td>
|
|
<div>定义于头文件 <code><a href="cpp-header-ranges.html"><ranges></a></code></div>
|
|
</td>
|
|
<td></td>
|
|
<td></td>
|
|
</tr>
|
|
<tr class="t-dcl t-since-cxx20">
|
|
<td>
|
|
<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span><span class="kw1">class</span> R<span class="sy1">></span><br />
|
|
<span class="kw1">using</span> values_view <span class="sy1">=</span> ranges<span class="sy4">::</span><span class="me2">elements_view</span><span class="sy1"><</span>R, <span class="nu0">1</span><span class="sy1">></span><span class="sy4">;</span></span></div>
|
|
</td>
|
|
<td>(1)</td>
|
|
<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
|
|
</tr>
|
|
<tr class="t-dcl t-since-cxx20">
|
|
<td>
|
|
<div><span class="mw-geshi cpp source-cpp"><span class="kw1">namespace</span> views <span class="br0">{</span><br /></span>
|
|
<p> <span class="kw1">inline</span> <span class="kw4">constexpr</span> <span class="kw4">auto</span> values <span class="sy1">=</span> ranges<span class="sy4">::</span><span class="me2">elements</span><span class="sy1"><</span><span class="nu0">1</span><span class="sy1">></span><span class="sy4">;</span><br /></p>
|
|
<span class="br0">}</span></div>
|
|
</td>
|
|
<td>(2)</td>
|
|
<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
|
|
</tr>
|
|
<tr class="t-dcl-sep">
|
|
<td></td>
|
|
<td></td>
|
|
<td></td>
|
|
</tr>
|
|
</tbody>
|
|
</table>
|
|
<p>接收<i>元组式</i>值（例如 <span class="t-lc"><a href="cpp-utility-tuple.html">std::tuple</a></span> 或 <span class="t-lc"><a href="cpp-utility-pair.html">std::pair</a></span> ）的 <a href="cpp-ranges-view.html"><code>view</code></a> ，并产生<i>值类型</i>为被适配的视图的值的<i>第二</i>个元素的视图。</p>
|
|
<div class="t-li1"><span class="t-li">1)</span> <span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">elements_</span>­view<span class="sy1"><</span>R, <span class="nu0">1</span><span class="sy1">></span></span></span> 的别名。</div>
|
|
<div class="t-li1"><span class="t-li">2)</span> <a href="cpp-ranges.html#.E8.8C.83.E5.9B.B4.E9.80.82.E9.85.8D.E5.99.A8.E5.AF.B9.E8.B1.A1">范围适配器对象</a>。表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp">views​<span class="sy4">::</span><span class="me2">values</span><span class="br0">(</span>e<span class="br0">)</span></span></span> 表达式对任何适合的子表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp">e</span></span> <i>表达式等价</i>于 <span class="t-c"><span class="mw-geshi cpp source-cpp">values_­view<span class="sy1"><</span>views​<span class="sy4">::</span>​all_­t<span class="sy1"><</span>decltype<span class="br0">(</span><span class="br0">(</span>e<span class="br0">)</span><span class="br0">)</span><span class="sy1">>></span><span class="br0">{</span>e<span class="br0">}</span></span></span> 。</div>
|
|
<h3><span class="mw-headline" id=".E8.A1.A8.E8.BE.BE.E5.BC.8F.E7.AD.89.E4.BB.B7">表达式等价</span></h3>
|
|
<p>表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp">e</span></span> <i>表达式等价</i>于表达式 <span class="t-c"><span class="mw-geshi cpp source-cpp">f</span></span> ，若 <span class="t-c"><span class="mw-geshi cpp source-cpp">e</span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp">f</span></span> 拥有相同效果，均为<a href="cpp-language-noexcept_spec.html">潜在抛出</a>或均非潜在抛出（即 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">noexcept</span><span class="br0">(</span>e<span class="br0">)</span> <span class="sy1">==</span> <span class="kw1">noexcept</span><span class="br0">(</span>f<span class="br0">)</span></span></span> ），且均为<a href="cpp-language-constant_expression.html">常量子表达式</a>或均非常量子表达式。</p>
|
|
<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
|
|
<p><code>values_view</code> 能用于从关联容器提取<i>值</i>，例如 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw1">for</span> <span class="br0">(</span><span class="kw4">auto</span> <span class="kw4">const</span><span class="sy3">&</span> value <span class="sy4">:</span> std<span class="sy4">::</span><span class="me2">views</span><span class="sy4">::</span><span class="me2">values</span><span class="br0">(</span>map<span class="br0">)</span><span class="br0">)</span> <span class="br0">{</span> <span class="coMULTI">/*...*/</span> <span class="br0">}</span></span></span> 。</p>
|
|
<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
|
|
<div class="t-example">
|
|
<div class="t-example-live-link"></div>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="cpp source-cpp">
|
|
<pre class="de1"><span class="co2">#include <iostream></span>
|
|
<span class="co2">#include <ranges></span>
|
|
<span class="co2">#include <map></span>
|
|
|
|
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
|
|
<span class="br0">{</span>
|
|
<a href="cpp-container-map.html"><span class="kw1281">std::<span class="me2">map</span></span></a><span class="sy1"><</span><span class="kw4">char</span>, <span class="kw4">int</span><span class="sy1">></span> map<span class="br0">{</span> <span class="br0">{</span><span class="st0">'A'</span>, <span class="nu0">1</span><span class="br0">}</span>, <span class="br0">{</span><span class="st0">'B'</span>, <span class="nu0">2</span><span class="br0">}</span>, <span class="br0">{</span><span class="st0">'C'</span>, <span class="nu0">3</span><span class="br0">}</span>, <span class="br0">{</span><span class="st0">'D'</span>, <span class="nu0">4</span><span class="br0">}</span>, <span class="br0">{</span><span class="st0">'E'</span>, <span class="nu0">5</span><span class="br0">}</span> <span class="br0">}</span><span class="sy4">;</span>
|
|
|
|
<span class="kw4">auto</span> odd <span class="sy1">=</span> <span class="br0">[</span><span class="br0">]</span><span class="br0">(</span><span class="kw4">int</span> x<span class="br0">)</span> <span class="br0">{</span> <span class="kw1">return</span> <span class="nu0">0</span> <span class="sy3">!</span><span class="sy1">=</span> <span class="br0">(</span>x <span class="sy3">&</span> <span class="nu0">1</span><span class="br0">)</span><span class="sy4">;</span> <span class="br0">}</span><span class="sy4">;</span>
|
|
|
|
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">"Odd values in the map: "</span><span class="sy4">;</span>
|
|
<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">int</span> value <span class="sy4">:</span> map <span class="sy3">|</span> std<span class="sy4">::</span><span class="me2">views</span><span class="sy4">::</span><span class="me2">values</span> <span class="sy3">|</span> std<span class="sy4">::</span><span class="me2">views</span><span class="sy4">::</span><span class="me2">filter</span><span class="br0">(</span>odd<span class="br0">)</span><span class="br0">)</span>
|
|
<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> value <span class="sy1"><<</span> <span class="st0">' '</span><span class="sy4">;</span>
|
|
<span class="br0">}</span></pre></div>
|
|
</div>
|
|
<p>输出：</p>
|
|
<div dir="ltr" class="mw-geshi" style="text-align: left;">
|
|
<div class="text source-text">
|
|
<pre class="de1">Odd values in the map: 1 3 5</pre></div>
|
|
</div>
|
|
</div>
|
|
<h3><span class="mw-headline" id=".E7.BC.BA.E9.99.B7.E6.8A.A5.E5.91.8A">缺陷报告</span></h3>
|
|
<p>下列更改行为的缺陷报告追溯地应用于以前出版的 C++ 标准。</p>
|
|
<table class="dsctable" style="font-size:0.8em">
|
|
<tr>
|
|
<th>DR</th>
|
|
<th>应用于</th>
|
|
<th>出版时的行为</th>
|
|
<th>正确行为</th>
|
|
</tr>
|
|
<tr>
|
|
<td><a rel="nofollow" class="external text" href="https://cplusplus.github.io/LWG/issue3563">LWG 3563</a></td>
|
|
<td>C++20</td>
|
|
<td><code>values_view</code> 由于使用 <span class="t-lc">views::all_t</span> 无法参与 CTAD</td>
|
|
<td>移除 <span class="t-lc">views::all_t</span></td>
|
|
</tr>
|
|
</table>
|
|
<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
|
|
<table class="t-dsc-begin">
|
|
<tr class="t-dsc">
|
|
<td>
|
|
<div class="t-dsc-member-div">
|
|
<div><a href="cpp-ranges-keys_view.html"><span class="t-lines"><span>ranges::keys_view</span><span>views::keys</span></span></a></div>
|
|
<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
|
|
</div>
|
|
</td>
|
|
<td>选取 pair 式值组成的 <a href="cpp-ranges-view.html"><code>view</code></a> 并产生每个 pair 的第一元素的 <a href="cpp-ranges-view.html"><code>view</code></a><br />
|
|
<span class="t-mark">(类模板)</span> <span class="t-mark">(范围适配器对象)</span></td>
|
|
</tr>
|
|
<tr class="t-dsc">
|
|
<td>
|
|
<div class="t-dsc-member-div">
|
|
<div><a href="cpp-ranges-elements_view.html"><span class="t-lines"><span>ranges::elements_view</span><span>views::elements</span></span></a></div>
|
|
<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
|
|
</div>
|
|
</td>
|
|
<td>选取元组式值组成的 <a href="cpp-ranges-view.html"><code>view</code></a> 和数值 N ，产生每个 tuple 的第 N 个元素的 <a href="cpp-ranges-view.html"><code>view</code></a><br />
|
|
<span class="t-mark">(类模板)</span> <span class="t-mark">(范围适配器对象)</span></td>
|
|
</tr>
|
|
<tr class="t-dsc">
|
|
<td>
|
|
<div class="t-dsc-member-div">
|
|
<div><a href="cpp-numeric-valarray-slice.html"><span class="t-lines"><span>slice</span></span></a></div>
|
|
</div>
|
|
</td>
|
|
<td>valarray 的 BLAS 式切片：起始下标、长度、跨度<br />
|
|
<span class="t-mark">(类)</span></td>
|
|
</tr>
|
|
</table>
|
|
</div>
|
|
<div class="visualClear"></div>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
</body>
|
|
</html>
|