114 lines
13 KiB
HTML
114 lines
13 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::view, std::ranges::enable_view, std::ranges::view_base</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_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>view, <span style="font-size:0.7em; line-height:130%">std::ranges::</span>enable_view, <span style="font-size:0.7em; line-height:130%">std::ranges::</span>view_base</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> T<span class="sy1">></span><br />
|
|
concept view <span class="sy1">=</span> ranges<span class="sy4">::</span><span class="me2">range</span><span class="sy1"><</span>T<span class="sy1">></span> <span class="sy3">&&</span> std<span class="sy4">::</span><span class="me2">movable</span><span class="sy1"><</span>T<span class="sy1">></span> <span class="sy3">&&</span> ranges<span class="sy4">::</span><span class="me2">enable_view</span><span class="sy1"><</span>T<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">template</span><span class="sy1"><</span><span class="kw1">class</span> T<span class="sy1">></span><br /></span>
|
|
<p><span class="kw1">inline</span> <span class="kw4">constexpr</span> <span class="kw4">bool</span> enable_view <span class="sy1">=</span><br /></p>
|
|
std<span class="sy4">::</span><span class="me2">derived_from</span><span class="sy1"><</span>T, view_base<span class="sy1">></span> <span class="sy3">||</span> <span class="coMULTI">/*is-derived-from-view-interface*/</span><span class="sy1"><</span>T<span class="sy1">></span><span class="sy4">;</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 t-since-cxx20">
|
|
<td>
|
|
<div><span class="mw-geshi cpp source-cpp"><span class="kw1">struct</span> view_base <span class="br0">{</span> <span class="br0">}</span><span class="sy4">;</span></span></div>
|
|
</td>
|
|
<td>(3)</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>
|
|
<div class="t-li1"><span class="t-li">1)</span> <code>view</code> 概念指定拥有适合用于构造范围适配器管道的语义属性的 <a href="cpp-ranges-range.html"><code>range</code></a> 类型（例如一对迭代器或按需创建其元素的生成器 <a href="cpp-ranges-range.html"><code>range</code></a> 。</div>
|
|
<div class="t-li1"><span class="t-li">2)</span> <code>enable_view</code> 变量模板用于指示 <a href="cpp-ranges-range.html"><code>range</code></a> 是否为 <code>view</code> 。 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="coMULTI">/*is-derived-from-view-interface*/</span><span class="sy1"><</span>T<span class="sy1">></span></span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> 当且仅当 <code>T</code> 准确拥有一个对于某类型 <code>U</code> 的公开基类 <span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">view_interface</span><span class="sy1"><</span>U<span class="sy1">></span></span></span> ，且 <code>T</code> 无任何对于其他类型 <code>V</code> 的基类 <span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">view_interface</span><span class="sy1"><</span>V<span class="sy1">></span></span></span> 。<br />
|
|
用户可对要实现 <code>view</code> 的无 cv 限定的程序定义类型特化 <code>enable_view</code> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">true</span></span></span> ，而对不实现的类型特化为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> 。这些特化应当<a href="cpp-language-constant_expression.html#.E5.8F.AF.E7.94.A8.E4.BA.8E.E5.B8.B8.E9.87.8F.E8.A1.A8.E8.BE.BE.E5.BC.8F">可用于常量表达式</a>并拥有 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">const</span> <span class="kw4">bool</span></span></span> 类型。</div>
|
|
<div class="t-li1"><span class="t-li">3)</span> 从 <code>view_base</code> 派生会允许 <a href="cpp-ranges-range.html"><code>range</code></a> 类型实现 <code>view</code> 。</div>
|
|
<h3><span class="mw-headline" id=".E8.AF.AD.E4.B9.89.E8.A6.81.E6.B1.82">语义要求</span></h3>
|
|
<div class="t-li1"><span class="t-li">1)</span> <code>T</code> 实现 <code>view</code> ，仅若：
|
|
<ul>
|
|
<li><code>T</code> 的移动构造拥有常数时间复杂度，且</li>
|
|
<li>若从保有 <span class="mjax" style="display:none">\(\scriptsize M\)</span><span class="mjax-fallback texhtml" style="white-space: nowrap;">M</span> 个元素的 <code>T</code> 对象作出 <span class="mjax" style="display:none">\(\scriptsize N\)</span><span class="mjax-fallback texhtml" style="white-space: nowrap;">N</span> 次复制与/或移动，则这 <span class="mjax" style="display:none">\(\scriptsize N\)</span><span class="mjax-fallback texhtml" style="white-space: nowrap;">N</span> 个对象拥有 <span class="mjax" style="display:none">\(\scriptsize \mathcal{O}{(N+M)}\)</span><span class="mjax-fallback texhtml" style="white-space: nowrap;">𝓞(N+M)</span> 时间复杂度析构（这蕴含被移动的 <code>view</code> 对象拥有 <span class="mjax" style="display:none">\(\scriptsize \mathcal{O}{(1)}\)</span><span class="mjax-fallback texhtml" style="white-space: nowrap;">𝓞(1)</span> 析构），且</li>
|
|
<li><span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">copy_constructible</span><span class="sy1"><</span>T<span class="sy1">></span></span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> ，或 <code>T</code> 的复制构造拥有常数时间复杂度，且</li>
|
|
<li><span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">copyable</span><span class="sy1"><</span>T<span class="sy1">></span></span></span> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw2">false</span></span></span> ，或 <code>T</code> 拥有不多于析构后随复制构造的时间复杂度。</li>
|
|
</ul>
|
|
</div>
|
|
<h3><span class="mw-headline" id=".E6.B3.A8.E8.A7.A3">注解</span></h3>
|
|
<p><code>view</code> 类型的示例为：</p>
|
|
<ul>
|
|
<li>包装一对迭代器的 <a href="cpp-ranges-range.html"><code>range</code></a> 类型，例如 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">ranges</span><span class="sy4">::</span><span class="me2">subrange</span><span class="sy1"><</span>I<span class="sy1">></span></span></span> 。</li>
|
|
<li>通过 <span class="t-lc"><a href="cpp-memory-shared_ptr.html">std::shared_ptr</a></span> 持有其元素并与所有副本共享所有权的 <a href="cpp-ranges-range.html"><code>range</code></a> 类型。</li>
|
|
<li>按需生成其元素的 <a href="cpp-ranges-range.html"><code>range</code></a> 类型，例如 <span class="t-lc">std::ranges::iota_view</span> 。</li>
|
|
</ul>
|
|
<p>诸如 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1"><</span><a href="cpp-string-basic_string.html"><span class="kw1230">std::<span class="me2">string</span></span></a><span class="sy1">></span></span></span> 的可复制容器通常不符合 <code>view</code> 的语义要求，因为复制容器会复制其所有元素，这无法在常数时间内完成。</p>
|
|
<p>视图原先被描述成可廉价复制并且为非占有的范围，不过不要求要实现 <code>view</code> 的类型为可复制或非占有。然而，它必须仍然有廉价的复制（凡在可能时）、移动及析构，以令<a href="cpp-ranges.html#.E8.8C.83.E5.9B.B4.E9.80.82.E9.85.8D.E5.99.A8">范围适配器</a>不会拥有不期待的复杂度。</p>
|
|
<p>实现 <a href="cpp-concepts-movable.html"><code>movable</code></a> 和 <a href="cpp-ranges-range.html"><code>range</code></a> 的类型默认被认为是视图，若它公开且无歧义地派生自 <code>view_base</code> 或恰好一个 <span class="t-lc">ranges::view_interface</span> 的特化。</p>
|
|
<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://wg21.link/P2325R3">P2325R3</a></td>
|
|
<td>C++20</td>
|
|
<td><code>view</code> 要求 <a href="cpp-concepts-default_initializable.html"><code>default_initializable</code></a></td>
|
|
<td>不要求</td>
|
|
</tr>
|
|
<tr>
|
|
<td><a rel="nofollow" class="external text" href="https://cplusplus.github.io/LWG/issue3549">LWG 3549</a></td>
|
|
<td>C++20</td>
|
|
<td><code>enable_view</code> 不检测来自 <code>view_interface</code> 的继承</td>
|
|
<td>检测</td>
|
|
</tr>
|
|
<tr>
|
|
<td><a rel="nofollow" class="external text" href="https://wg21.link/P2415R2">P2415R2</a></td>
|
|
<td>C++20</td>
|
|
<td>析构的时间复杂度上的限制过于严格</td>
|
|
<td>已放松</td>
|
|
</tr>
|
|
</table>
|
|
</div>
|
|
<div class="visualClear"></div>
|
|
</div>
|
|
</div>
|
|
</div>
|
|
</body>
|
|
</html>
|