199 lines
16 KiB
HTML
199 lines
16 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::distance</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_iterator_ranges_distance 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::</span>distance</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-iterator.html">iterator</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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<div>定义于头文件 <code><a href="cpp-header-iterator.html"><iterator></a></code></div>
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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> std<span class="sy4">::</span><span class="me2">input_or_output_iterator</span> I, std<span class="sy4">::</span><span class="me2">sentinel_for</span><span class="sy1"><</span>I<span class="sy1">></span> S <span class="sy1">></span><br />
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<span class="kw4">constexpr</span> std<span class="sy4">::</span><span class="me2">iter_difference_t</span><span class="sy1"><</span>I<span class="sy1">></span> distance<span class="br0">(</span> I first, S last <span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td>(1)</td>
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<td><span class="t-mark-rev t-since-cxx20">(C++20 起)</span></td>
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<div><span class="mw-geshi cpp source-cpp"><span class="kw1">template</span><span class="sy1"><</span> ranges<span class="sy4">::</span><span class="me2">range</span> R<span class="sy1">></span><br />
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<span class="kw4">constexpr</span> ranges<span class="sy4">::</span><span class="me2">range_difference_t</span><span class="sy1"><</span>R<span class="sy1">></span> distance<span class="br0">(</span>R<span class="sy3">&&</span> r<span class="br0">)</span><span class="sy4">;</span></span></div>
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</td>
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<td>(2)</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 class="t-li1"><span class="t-li">1)</span> 返回从 <code>first</code> 到 <code>last</code> 的自增次数。</div>
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<div class="t-li1"><span class="t-li">2)</span> 同 <span class="t-v">(1)</span> ，但以 <code>r</code> 为源范围，若 <code>R</code> 实现 <span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">sized_range</span></span></span> 则使用 <span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">size</span><span class="br0">(</span>r<span class="br0">)</span></span></span> ，否则如同以 <span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">begin</span><span class="br0">(</span>r<span class="br0">)</span></span></span> 为 <code>first</code> 并以 <span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">end</span><span class="br0">(</span>r<span class="br0">)</span></span></span> 为 <code>last</code> 。</div>
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<p>此页面上描述的仿函数实体是 <i>niebloid</i> ，即：</p>
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<ul>
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<li>调用它们中的任何一者时不可指定显式模板实参。</li>
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<li>它们无一对<a href="cpp-language-adl.html">实参依赖查找</a>可见。</li>
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<li>在<a href="cpp-language-unqualified_lookup.html">通常无限定查找</a>寻找函数调用运算符左侧的名称找到它们之一时，它禁止<a href="cpp-language-adl.html">实参依赖查找</a>。</li>
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</ul>
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<p>实际上，它们能以函数对象，或以某些特殊编译器扩展实现。</p>
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<h3><span class="mw-headline" id=".E5.8F.82.E6.95.B0">参数</span></h3>
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<table class="t-par-begin">
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<tr class="t-par">
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<td>first</td>
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<td>-</td>
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<td>指向首元素的迭代器</td>
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</tr>
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<tr class="t-par">
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<td>last</td>
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<td>-</td>
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<td>代表 <code>first</code> 所指向的范围结尾的哨位</td>
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</tr>
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<tr class="t-par">
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<td>r</td>
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<td>-</td>
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<td>要计算距离的范围</td>
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</tr>
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</table>
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<h3><span class="mw-headline" id=".E8.BF.94.E5.9B.9E.E5.80.BC">返回值</span></h3>
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<p>需要从 <code>first</code> 到 <code>last</code> 的自增次数。若 <code>I</code> 与 <code>S</code> 为同一类型并实现 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">sized_sentinel_for</span><span class="sy1"><</span>S, I<span class="sy1">></span></span></span> 而且 <code>first</code> 从 <code>last</code> 可达则值可为负。</p>
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<h3><span class="mw-headline" id=".E5.A4.8D.E6.9D.82.E5.BA.A6">复杂度</span></h3>
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<div class="t-li1"><span class="t-li">1)</span> 若 <code>S</code> 实现 <span class="t-c"><span class="mw-geshi cpp source-cpp">std<span class="sy4">::</span><span class="me2">sized_sentinel_for</span><span class="sy1"><</span>S, I<span class="sy1">></span></span></span> 则为常数，否则为线性。</div>
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<div class="t-li1"><span class="t-li">2)</span> 若 <code>R</code> 实现 <span class="t-c"><span class="mw-geshi cpp source-cpp">ranges<span class="sy4">::</span><span class="me2">sized_range</span></span></span> 则为常数，否则同 <span class="t-v">(1)</span> 。</div>
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<h3><span class="mw-headline" id=".E5.8F.AF.E8.83.BD.E7.9A.84.E5.AE.9E.E7.8E.B0">可能的实现</span></h3>
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<table class="eq-fun-cpp-table">
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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="kw1">struct</span> distance_fn <span class="br0">{</span>
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<span class="kw1">template</span><span class="sy1"><</span>std<span class="sy4">::</span><span class="me2">input_or_output_iterator</span> I, std<span class="sy4">::</span><span class="me2">sentinel_for</span><span class="sy1"><</span>I<span class="sy1">></span> S<span class="sy1">></span>
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<span class="kw4">constexpr</span> std<span class="sy4">::</span><span class="me2">iter_difference_t</span><span class="sy1"><</span>I<span class="sy1">></span> operator<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span>I first, S last<span class="br0">)</span> <span class="kw4">const</span>
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<span class="br0">{</span>
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<span class="kw1">if</span> <span class="kw4">constexpr</span> <span class="br0">(</span>std<span class="sy4">::</span><span class="me2">sized_sentinel_for</span><span class="sy1"><</span>S, I<span class="sy1">></span><span class="br0">)</span> <span class="br0">{</span>
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<span class="kw1">return</span> last <span class="sy2">-</span> first<span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw1">else</span> <span class="br0">{</span>
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std<span class="sy4">::</span><span class="me2">iter_difference_t</span><span class="sy1"><</span>I<span class="sy1">></span> result <span class="sy1">=</span> <span class="nu0">0</span><span class="sy4">;</span>
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<span class="kw1">while</span> <span class="br0">(</span>first <span class="sy3">!</span><span class="sy1">=</span> last<span class="br0">)</span> <span class="br0">{</span>
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<span class="sy2">++</span>first<span class="sy4">;</span>
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<span class="sy2">++</span>result<span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw1">return</span> result<span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="br0">}</span>
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<span class="kw1">template</span><span class="sy1"><</span>ranges<span class="sy4">::</span><span class="me2">range</span> R<span class="sy1">></span>
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<span class="kw4">constexpr</span> ranges<span class="sy4">::</span><span class="me2">range_difference_t</span><span class="sy1"><</span>R<span class="sy1">></span> operator<span class="br0">(</span><span class="br0">)</span><span class="br0">(</span>R<span class="sy3">&&</span> r<span class="br0">)</span> <span class="kw4">const</span>
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<span class="br0">{</span>
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<span class="kw1">if</span> <span class="kw4">constexpr</span> <span class="br0">(</span>ranges<span class="sy4">::</span><span class="me2">sized_range</span><span class="sy1"><</span>std<span class="sy4">::</span><span class="me2">remove_cvref_t</span><span class="sy1"><</span>R<span class="sy1">>></span><span class="br0">)</span> <span class="br0">{</span>
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<span class="kw1">return</span> <span class="kw1">static_cast</span><span class="sy1"><</span>ranges<span class="sy4">::</span><span class="me2">range_difference_t</span><span class="sy1"><</span>R<span class="sy1">>></span><span class="br0">(</span>ranges<span class="sy4">::</span><span class="me2">size</span><span class="br0">(</span>r<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="kw1">else</span> <span class="br0">{</span>
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<span class="kw1">return</span> ranges<span class="sy4">::</span><span class="me2">distance</span><span class="br0">(</span>ranges<span class="sy4">::</span><span class="me2">begin</span><span class="br0">(</span>r<span class="br0">)</span>, ranges<span class="sy4">::</span><span class="me2">end</span><span class="br0">(</span>r<span class="br0">)</span><span class="br0">)</span><span class="sy4">;</span>
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<span class="br0">}</span>
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<span class="br0">}</span>
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<span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">inline</span> <span class="kw4">constexpr</span> <span class="kw4">auto</span> distance <span class="sy1">=</span> distance_fn<span class="br0">{</span><span class="br0">}</span><span class="sy4">;</span></pre></div>
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</div>
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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 <iostream></span>
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<span class="co2">#include <iterator></span>
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<span class="co2">#include <vector></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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<a href="cpp-container-vector.html"><span class="kw1269">std::<span class="me2">vector</span></span></a><span class="sy1"><</span><span class="kw4">int</span><span class="sy1">></span> v<span class="br0">{</span> <span class="nu0">3</span>, <span class="nu0">1</span>, <span class="nu0">4</span> <span class="br0">}</span><span class="sy4">;</span>
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<span class="kw1">namespace</span> ranges <span class="sy1">=</span> std<span class="sy4">::</span><span class="me2">ranges</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">"distance(first, last) = "</span>
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<span class="sy1"><<</span> ranges<span class="sy4">::</span><span class="me2">distance</span><span class="br0">(</span>v.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span>, v.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
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<span class="sy1"><<</span> <span class="st0">"distance(last, first) = "</span>
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<span class="sy1"><<</span> ranges<span class="sy4">::</span><span class="me2">distance</span><span class="br0">(</span>v.<span class="me1">end</span><span class="br0">(</span><span class="br0">)</span>, v.<span class="me1">begin</span><span class="br0">(</span><span class="br0">)</span><span class="br0">)</span> <span class="sy1"><<</span> <span class="st0">'<span class="es1">\n</span>'</span>
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<span class="sy1"><<</span> <span class="st0">"distance(v) = "</span> <span class="sy1"><<</span> ranges<span class="sy4">::</span><span class="me2">distance</span><span class="br0">(</span>v<span class="br0">)</span> <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">distance(first, last) = 3
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distance(last, first) = -3
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distance(v) = 3</pre></div>
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</div>
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</div>
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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-iterator-ranges-advance.html"><span class="t-lines"><span>ranges::advance</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>令迭代器前进给定的距离或到给定的边界<br />
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<span class="t-mark">(niebloid)</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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<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-distance.html"><span class="t-lines"><span>distance</span></span></a></div>
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</div>
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</td>
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<td>返回两个迭代器间的距离<br />
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<span class="t-mark">(函数模板)</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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