164 lines
18 KiB
HTML
164 lines
18 KiB
HTML
<!DOCTYPE html>
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<html lang="zh" dir="ltr" class="client-nojs" xmlns="http://www.w3.org/1999/xhtml" xml:lang="zh">
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<head><meta http-equiv="x-ua-compatible" content="ie=edge">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
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<title>std::ranges::views::counted</title>
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<link rel="stylesheet" href="ext.css" />
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<link rel="stylesheet" href="site_modules.css" />
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<body class="mediawiki ltr sitedir-ltr ns-0 ns-subject page-cpp_ranges_view_counted skin-cppreference2 action-view cpp-navbar">
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<div id="cpp-content-base">
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<div id="content"><a id="top"></a>
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<h1 id="firstHeading" class="firstHeading"><span style="font-size:0.7em; line-height:130%">std::ranges::views::</span>counted</h1>
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<div id="bodyContent">
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<div id="contentSub"><span class="subpages">< <a href="cpp.html">cpp</a>‎ | <a href="cpp-ranges.html">ranges</a></span></div>
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<div id="mw-content-text" lang="zh" dir="ltr" class="mw-content-ltr" xml:lang="zh">
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<table class="t-dcl-begin">
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<td>
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<div>定义于头文件 <code><a href="cpp-header-ranges.html"><ranges></a></code></div>
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</td>
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<td></td>
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<td></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">inline</span> <span class="kw4">constexpr</span> <span class="coMULTI">/*unspecified*/</span> counted <span class="sy1">=</span> <span class="coMULTI">/*unspecified*/</span><span class="sy4">;</span></span></div>
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<td class="t-dcl-nopad"></td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<td></td>
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<div>调用签名</div>
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<td></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> <span class="kw1">class</span> Iterator, <span class="kw1">class</span> DifferenceType <span class="sy1">></span><br /></span>
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<p> requires <span class="coMULTI">/* see below */</span><br /></p>
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<span class="kw4">constexpr</span> <span class="coMULTI">/*span-or-subrange*/</span> counted<span class="br0">(</span> Iterator<span class="sy3">&&</span> it, DifferenceType<span class="sy3">&&</span> count <span class="br0">)</span><span class="sy4">;</span></div>
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<p><code>counted</code> 视图是表示某迭代器 <code>i</code> 与非负整数 <code>n</code> 的<i>计数范围</i> <code>[i, n)</code> 的元素的 <a href="cpp-ranges-view.html"><code>view</code></a> 。</p>
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<p>计数范围 <code>[i, n)</code> 是始于以 <code>i</code> 所指向的元素，到若存在则为应用 <code>n</code> 次 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="sy2">++</span>i</span></span> 的结果所指向的元素为止，但不包含该元素的 <code>n</code> 个元素。</p>
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<p>若 <span class="t-c"><span class="mw-geshi cpp source-cpp">n <span class="sy1">==</span> <span class="nu0">0</span></span></span> ，则计数范围合法且为空。否则，计数范围仅若 <code>n</code> 为正， <code>i</code> 可解引用，且 <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="br0">[</span><span class="sy2">++</span>i, <span class="sy2">--</span>n<span class="br0">)</span></span></span> 为合法的计数范围才合法。</p>
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<p>正式而言，若 <span class="t-c"><span class="mw-geshi cpp source-cpp">it</span></span> 与 <span class="t-c"><span class="mw-geshi cpp source-cpp">count</span></span> 为表达式， <code>T</code> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-types-decay.html"><span class="kw629">std::<span class="me2">decay_t</span></span></a><span class="sy1"><</span>decltype<span class="br0">(</span><span class="br0">(</span>it<span class="br0">)</span><span class="br0">)</span><span class="sy1">></span></span></span> ，而 <code>D</code> 为 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">iter_difference_t</span><span class="sy1"><</span>T<span class="sy1">></span></span></span> ，则</p>
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<dl>
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<dd>若 <code>T</code> 实现 <a href="cpp-iterator-input_or_output_iterator.html"><code>input_or_output_iterator</code></a> 且 <span class="t-c"><span class="mw-geshi cpp source-cpp">decltype<span class="br0">(</span><span class="br0">(</span>count<span class="br0">)</span><span class="br0">)</span></span></span> 实现 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">convertible_to</span><span class="sy1"><</span>D<span class="sy1">></span></span></span> ，
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<ul>
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<li>否则，若 <code>T</code> 实现 <a href="cpp-iterator-contiguous_iterator.html"><code>contiguous_iterator</code></a> ，则 <span class="t-c"><span class="mw-geshi cpp source-cpp">views<span class="sy4">::</span><span class="me2">counted</span><span class="br0">(</span>it, count<span class="br0">)</span></span></span> 表达式等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">span</span><span class="br0">(</span>it, <span class="kw1">static_cast</span><span class="sy1"><</span><a href="cpp-types-size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a><span class="sy1">></span><span class="br0">(</span><span class="kw1">static_cast</span><span class="sy1"><</span>D<span class="sy1">></span><span class="br0">(</span>count<span class="br0">)</span><span class="br0">)</span><span class="br0">)</span></span></span> ，否则</li>
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<li>若 <code>T</code> 实现 <a href="cpp-iterator-random_access_iterator.html"><code>random_access_iterator</code></a> ，则 <span class="t-c"><span class="mw-geshi cpp source-cpp">views<span class="sy4">::</span><span class="me2">counted</span><span class="br0">(</span>it, count<span class="br0">)</span></span></span> 表达式等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">subrange</span><span class="br0">(</span>it, it <span class="sy2">+</span> <span class="kw1">static_cast</span><span class="sy1"><</span>D<span class="sy1">></span><span class="br0">(</span>count<span class="br0">)</span><span class="br0">)</span></span></span> ，否则</li>
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<li><span class="t-c"><span class="mw-geshi cpp source-cpp">views<span class="sy4">::</span><span class="me2">counted</span><span class="br0">(</span>it, count<span class="br0">)</span></span></span> 表达式等价于 <span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">subrange</span><span class="br0">(</span>std<span class="sy4">::</span><span class="me2">counted_iterator</span><span class="br0">(</span>it, count<span class="br0">)</span>, std<span class="sy4">::</span><span class="me2">default_sentinel</span><span class="br0">)</span></span></span></li>
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</ul>
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</dd>
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<dd>否则 <span class="t-c"><span class="mw-geshi cpp source-cpp">views<span class="sy4">::</span><span class="me2">counted</span><span class="br0">(</span>it, count<span class="br0">)</span></span></span> 非良构。</dd>
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</dl>
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<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>
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<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>
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<h3><span class="mw-headline" id=".E5.AE.9A.E5.88.B6.E7.82.B9.E5.AF.B9.E8.B1.A1">定制点对象</span></h3>
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<p>名字 <code>views::counted</code> 代表一个<i>定制点对象</i>，它是<a href="cpp-named_req-LiteralType.html">字面</a> <a href="cpp-concepts-semiregular.html"><code>semiregular</code></a> 类类型的 const <a href="cpp-named_req-FunctionObject.html">函数对象</a>。为说明目的，以 <code><i>__counted_fn</i></code> 表示其类型的 cv 无限定版本。</p>
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<p><code><i>__counted_fn</i></code> 的所有实例均相等。在相同参数上调用类型 <code><i>__counted_fn</i></code> 的不同实例的效果是等价的，无关乎指代该实例的表达式是左值还是右值，以及是否为 const 限定（然而不要求 volatile 限定的实例可调用）。从而能自由地复制 <code>views::counted</code> 并且能彼此替代地使用其副本。</p>
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<p>给定类型集合 <code>Args...</code> ，若 <span class="t-c"><span class="mw-geshi cpp source-cpp"><a href="cpp-utility-declval.html"><span class="kw1120">std::<span class="me2">declval</span></span></a><span class="sy1"><</span>Args<span class="sy1">></span><span class="br0">(</span><span class="br0">)</span>...</span></span> 满足上面对于 <code>views::counted</code> 的参数要求，则 <code><i>__counted_fn</i></code> 实现 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">invocable</span><span class="sy1"><</span>__counted_fn, Args...<span class="sy1">></span></span></span>、 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">invocable</span><span class="sy1"><</span><span class="kw4">const</span> __counted_fn, Args...<span class="sy1">></span></span></span>、 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">invocable</span><span class="sy1"><</span>__counted_fn<span class="sy3">&</span>, Args...<span class="sy1">></span></span></span> 及 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">invocable</span><span class="sy1"><</span><span class="kw4">const</span> __counted_fn<span class="sy3">&</span>, Args...<span class="sy1">></span></span></span> 。否则， <code><i>__counted_fn</i></code> 的函数调用运算符不参与重载决议。</p>
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<h3><span class="mw-headline" id=".E7.A4.BA.E4.BE.8B">示例</span></h3>
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<div class="t-example">
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<div class="t-example-live-link"></div>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="cpp source-cpp">
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<pre class="de1"><span class="co2">#include <ranges></span>
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<span class="co2">#include <iostream></span>
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<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
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<span class="br0">{</span>
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<span class="kw4">const</span> <span class="kw4">int</span> a<span class="br0">[</span><span class="br0">]</span> <span class="sy1">=</span> <span class="br0">{</span><span class="nu0">1</span>, <span class="nu0">2</span>, <span class="nu0">3</span>, <span class="nu0">4</span>, <span class="nu0">5</span>, <span class="nu0">6</span>, <span class="nu0">7</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">for</span><span class="br0">(</span><span class="kw4">int</span> i <span class="sy4">:</span> std<span class="sy4">::</span><span class="me2">views</span><span class="sy4">::</span><span class="me2">counted</span><span class="br0">(</span>a, <span class="nu0">3</span><span class="br0">)</span><span class="br0">)</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> i <span class="sy1"><<</span> <span class="st0">' '</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<span class="kw4">const</span> <span class="kw4">auto</span> il <span class="sy1">=</span> <span class="br0">{</span><span class="nu0">1</span>, <span class="nu0">2</span>, <span class="nu0">3</span>, <span class="nu0">4</span>, <span class="nu0">5</span><span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">for</span> <span class="br0">(</span><span class="kw4">int</span> i <span class="sy4">:</span> std<span class="sy4">::</span><span class="me2">views</span><span class="sy4">::</span><span class="me2">counted</span><span class="br0">(</span>il.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span> <span class="sy2">+</span> <span class="nu0">1</span>, <span class="nu0">3</span><span class="br0">)</span><span class="br0">)</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> i <span class="sy1"><<</span> <span class="st0">' '</span><span class="sy4">;</span>
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<a href="cpp-io-cout.html"><span class="kw1758">std::<span class="me2">cout</span></span></a> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
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<span class="br0">}</span></pre></div>
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</div>
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<p>输出：</p>
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<div dir="ltr" class="mw-geshi" style="text-align: left;">
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<div class="text source-text">
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<pre class="de1">1 2 3
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2 3 4</pre></div>
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</div>
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</div>
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<h3><span class="mw-headline" id=".E7.BC.BA.E9.99.B7.E6.8A.A5.E5.91.8A">缺陷报告</span></h3>
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<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>
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<table class="dsctable" style="font-size:0.8em">
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<tr>
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<th>DR</th>
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<th>应用于</th>
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<th>出版时的行为</th>
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<th>正确行为</th>
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</tr>
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<tr>
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<td><a rel="nofollow" class="external text" href="https://wg21.link/P2393R1">P2393R1</a></td>
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<td>C++20</td>
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<td>从整数类类型到 <code>size_t</code> 的隐式转换可能非法</td>
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<td>使之为显式</td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E5.8F.82.E9.98.85">参阅</span></h3>
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<table class="t-dsc-begin">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-ranges-subrange.html"><span class="t-lines"><span>ranges::subrange</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>将迭代器/哨位对结合为一个 <a href="cpp-ranges-view.html"><code>view</code></a><br />
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<span class="t-mark">(类模板)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-iterator-counted_iterator.html"><span class="t-lines"><span>counted_iterator</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>对到范围结尾距离进行跟踪的迭代器适配器<br />
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<span class="t-mark">(类模板)</span></td>
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</tr>
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<tr class="t-dsc">
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<td>
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<div class="t-dsc-member-div">
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<div><a href="cpp-algorithm-ranges-count.html"><span class="t-lines"><span>ranges::count</span><span>ranges::count_if</span></span></a></div>
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<div><span class="t-lines"><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span><span><span class="t-mark-rev t-since-cxx20">(C++20)</span></span></span></div>
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</div>
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</td>
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<td>返回满足指定判别标准的元素数<br />
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<span class="t-mark">(niebloid)</span></td>
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</tr>
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</table>
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</div>
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<div class="visualClear"></div>
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</div>
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</div>
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</div>
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</body>
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</html>
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